Effect of Ultrasound on Calcium Carbonate Crystallization

Effect of ultrasound on calcium carbonate crystallization

Martijn Wagterveld

Effect of ultrasound on calcium carbonate crystallization

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft; op gezag van de Rector Magniﬁcus Prof. ir. K.C.A.M. Luyben; voorzitter van het College voor Promoties,

in het openbaar te verdedigen op vrijdag 15 maart 2013 om 12:30 uur

door

Rene´ Martijn WAGTERVELD

Elektrotechnisch ingenieur geboren te IJsselmuiden Dit proefschrift is goedgekeurd door de promotor

Prof. dr. G. J. Witkamp

Samenstelling promotiecommissie Rector Magniﬁcus, voorzitter Prof. dr. G. J. Witkamp, Technische Universiteit Delft, promotor Prof. dr. ir. M. T. Kreutzer, Technische Universiteit Delft Prof. dr. A. Schmidt-Ott, Technische Universiteit Delft Prof. dr. E. Vlieg, Radboud Universiteit Nijmegen Prof. dr. ir. A. B. de Haan, Technische Universiteit Eindhoven Dr. ir. M. M. Mayer, EasyMeasure b.v. Prof. dr. P. D. E. M. Verhaert, Technische Universiteit Delft, reservelid

ISBN: 978-94-6108-411-8

Martijn Wagterveld, 2013 Eﬀect of Ultrasound on Calcium Carbonate Crystallization

PhD thesis Delft University of Technology, Delft, The Netherlands printed by: Gildeprint Drukkerijen - The Netherlands Summary

Scaling, also known as inorganic or precipitation fouling, comprises the formation of hard mineral deposits i.e. on process or membrane equipment. Calcium carbonate is one of the most abundant minerals on earth and is the most common scaling salt. Especially in reverse osmosis (RO) membrane systems, scale formation has always been a serious limitation. Scaling causes flux decline, membrane degradation, loss of production and elevated operating costs. Prediction of scale formation tendency, together with implementing suitable scale prevention measures, is therefore essential for optimal operation. In this work a novel concept is proposed for the prediction of scale formation tendency. By enhancing the crystallization (kinetics) locally and monitoring the process itself, scaling can be predicted accurately before it occurs in the bulk solution. If successful, this will result in a typical proactive sensor because precautions can be made (based on the measurement - prediction) before damage has occurred (as in most if not all existing scaling monitoring devices). The better scaling risk assessment improves chemical dosage (preventing overdosing) and prevents the necessity of cleaning or membrane replacement. Ultrasound is selected as possible method for crystallization enhancement. Consequently, the topic of this thesis is the effect of ultrasound on crystallization of calcium carbonate. After the introduction in chapter 1, the first part of the thesis considers the effect of ultrasound on CaCO3 crystallization from solutions without additives (chapter 2 - chapter 4). Chapter 2 describes the effect of ultrasound on seeded calcite (polymorph of CaCO3) crystallization. The crystallization rate of calcite was enhanced by 46% through ultrasonic irradiation (42,150 Hz, 17 W dm−3). It was shown that this effect was related to the alteration of the seed crystals’ habit and size. During ultrasonic irradiation disruption of conglomerates and erosion of single crystals occurred,

v Summary accompanied by the production of many fines. The increased surface area available for crystal growth resulted in the observed crystallization rate enhancement. In chapter 3 the interaction of acoustic cavitation with suspended calcite crystals, as discussed in chapter 2, is visualized using high speed imaging. Cavitation clusters, evolved from cavitation inception and collapse, caused attrition, disruption of aggregates and deagglomeration, whereas streamer cavitation was observed to cause deagglomeration only. The appearance of voluminous fragments with large planes of fracture indicated that acoustic cavitation can also cause the breakage of single crystal structures. Cavitation on the surface gave the crystals momentum yet it was shown that breakage of accelerated crystals by interparticle collisions is unrealistic because of their small sizes and low velocities. Chapter 4 delineates the way ultrasound exerts its effect by applying ultrasound in different treatment periods (time windows). Applying ultrasound during the first 10 minutes of the experiment did not result in any significant effect which rules out an influence on primary nucleation. The application of ultrasound starting later in the experiment enhanced precipitation of CaCO3. The proposed dominant mechanism responsible is deaggregation during the early growth phase (nuclei to crystals conversion regime). This effect is attributed to shear induced by micromixing and / or shear / stress induced by (supersonic) shockwaves, as a result of cavitation. Scan- ning electron microscope analysis shows that ultrasound increases the total number of particles that has, in addition, a more uniform size distribution compared with the untreated experiment. Consequently the available surface area for growth is higher resulting in a higher volumetric precipitation rate. The effect of ultrasound in solutions without additives has now been determined. In practical applications, water contains more components than calcium and carbonate. For instance in reverse osmosis membrane processes antiscalants are added to the water to prevent scaling. Moreover, other ions present, such as magnesium and sulfate, influence the crystallization of CaCO3. Organic substances known as humic substances or humic acids, can also reduce the crystallization rate of CaCO3. Chapter 5 covers the effect of ultrasound on the growth of calcite in the presence antiscalant nitrilotris(methylene phosphonic acid) (NTMP). During seeded growth experiments and in the presence of NTMP, ultrasound induced an approximately twofold increase in volumetric growth rate. In addition, after inhibition by NTMP ultrasound restored the growth more quickly. The results could be explained in part by the physical effect of ultrasound that causes breakage and attrition of poisoned crystals, which resulted in an increase in fresh surface area. Mass spectroscopy anal-

vi ysis of sonicated NTMP solutions revealed that there is also a chemical effect of ultrasound that plays an important role. Several breakdown products were identified with mass spectrometry, which showed that ultrasound caused the progressive loss of phosphonate groups from NTMP, probably by means of physicochemically generated free radicals and/or pyrolysis in the hot bubble-bulk interface. Chapter 6 presents the effect of ultrasound on free-drift precipitation of calcium carbonate in the presence of antiscalants NTMP and HEDP, humic substances, or foreign ions such as magnesium and sulfate. The applied concentrations of CaCO3 and foreign ions, were based on reverse osmosis drinking water concentrate. The largest effect was found for solutions with antiscalant NTMP and subscribed to degradation of NTMP (see chapter 5). The presence of magnesium in addition to NTMP extended the inhibition time but ultrasound resulted in a similar effect as for the solution with NTMP only. For antiscalant HEDP the experiments were inconclusive. A smaller, but significant effect was seen for the experiments with foreign ions sulfate and magnesium (as a reduction in induction time). The experiments with humic substances did not result in any measurable effect on free-drift precipitation of CaCO3. Chapter 7 discusses the effect of sulfate (and to a lesser extent magnesium) on the precipitation of CaCO3 in artificial drinking water reverse osmosis concentrate

(without ultrasound). CaCO3 formation slows down with increasing sulfate concentration and the preferential polymorph shifts from vaterite to aragonite. With this polymorphic change, a new combined habit is observed where (presumably) aragonite spikes grow on top of vaterite (“morning star” habit). Chapter 8 describes an extension to the software Minteq to systems in which the total amount of salt is a function of time. The model-based estimation of solid CaCO3 corresponds well with the experimental observations in the early growth stage. The estimate of CO2 exchange is evaluated by determining the rate-limiting step. For the sulfate experiments (without precipitation) the diﬀusion constant and equilibrium partial CO2 pressure correspond well with the expected values.

Electric fields might influence the crystallization of CaCO3 by affecting, for instance, the energetics of nucleation or the kinetics of either nucleation or growth. Chapter 9 discusses the application of an electric field as actuation method in CaCO3 crystallization. Two new measurement configurations were developed to increase the electric field strength compared to the methods described in literature. The first, the modified sitting drop method, had low reproducibility and was therefore abandoned. The second method, the glass plate sandwich, was miniaturized using microfluidics. As for a possible effect of electric field strength on CaCO3 crystallization, these microflu-

vii Summary idic experiments were inconclusive. Studying electric field effects on crystallization requires a modification of either the detection technique or microfluidic design. Finally, chapter 10 discusses the implications of the gained knowledge in this work with respect to a sensor for scaling tendency prediction, and perspectives are outlined.

viii Samenvatting

Mineralen kunnen een harde aanslag vormen op membranen en in leidingen zoals toegepast in de proces industrie. Deze aanslag wordt kalkaanslag of ketelsteen ge- noemd en staat ook bekent onder de Engelse term “scaling”. Calcium carbonaat (kalksteen) is éénvan de meest voorkomende mineralen op aarde, en komt als zout het meest voor in dergelijke aanslag. Scaling veroorzaakt significante beperkingen, met name in water productie faciliteiten die gebruik maken van omgekeerde osmose (RO) membranen. Het leidt tot vermindering van de vloeistof doorstroom, kwali- teit van membranen en capaciteit van waterproductie. Doordat scaling moeilijk te voorspellen en te verwijderen is, heeft het hogere (onderhouds)kosten tot gevolg. Het waarnemen en voorspellen van de neiging tot aangroei, het risico op scaling, is dus noodzakelijk voor optimale bedrijfsvoering. In dit proefschrift wordt een innovatief concept gepresenteerd om risico op scaling te voorspellen. Door lokaal (de kinetiek van) de beginstappen van scaling te versnellen, i.e. de kiemvorming en kristalgroei (kristallisatie) en dus het scaling proces zelf te bestuderen, kan mogelijke vorming van scaling accuraat worden voorspeld voordat het plaats heeft in de bulk. Als dit succesvol is, dan zal dit resulteren in een proactieve sensor omdat voorzorgsmaatregelen (op basis van de meting-voorspelling) genomen kunnen worden voordat er schade is ontstaan. De betere beoordeling van het risico op scaling zorgt voor een verbeterde afstemming van de dosering van chemicaliën. Hierdoor wordt overdosering voorkomen en is het schoonmaken en vervangen van membranen niet langer noodzakelijk. Toepassing van ultrageluid is geselecteerd als mogelijke methode om de kristallisatie te versnellen. Daardoor is het effect van ultrageluid op kristallisatie van calcium carbonaat het hoofdonderwerp van dit proefschrift. Na de introductie in hoofdstuk 1, wordt in het eerste deel van dit proefschrift

ix Samenvatting

(hoofdstuk 2 tot en met hoofdstuk 4) het effect van ultrageluid op de kristallisatie van calcium carbonaat uit een oplossing zonder additieven beschreven. Hoofdstuk 2 laat zien dat ultrageluid de kristallisatiesnelheid van calciet (polymorf van CaCO3) significant doet toenemen met 46% (42.150 Hz, 17 W dm−3). Het verkregen effect wordt toegeschreven aan de verandering van de vorm en grootte van de bestudeerde entkristallen. Wanneer ultrageluid wordt toegepast zorgen imploderende bellen (cavitatie) voor erosie van kristallen en het opbreken van vergroeide kristallen, waarbij gruis vrijkomt. Dit resulteert in toename van het totale kristaloppervlak, wat leidt tot een grotere volumetrische kristalgroei. In hoofdstuk 3 wordt hoge snelheid fotografie gebruikt om de interactie tussen caviterende bellen en calciet entkristallen te bestuderen, zoals beschreven in hoofdstuk 2. Clusters van imploderende bellen, ontstaan uit de initiële vorming en implosie van bellen, veroorzaken attritie, disruptie van aggregaten en deagglomeratie van kristallen. Stromen van cavitatie zorgden alleen voor deagglomeratie. Het in twee delen breken van kristallen werd ook waargenomen. Cavitatie op of vlakbij het kristaloppervlak zorgde voor hevige versnelling van deeltjes, maar het werd aangetoond dat opbreken van kristallen door botsingen zeer onwaarschijnlijk is door de kleine afmeting en lage snelheid van de kristallen. In hoofdstuk 4 wordt ultrageluid in verschillende periodes toegepast (tijdvensters) tijdens vrije val precipitatie (ongecontroleerde pH) van CaCO3 in een oplossing zonder additieven. Tijdens de eerste 10 minuten van het experiment resulteerde ultrageluid in geen enkel meetbaar effect. Een effect van ultrageluid op primaire kiemvorming is daardoor onwaarschijnlijk. Toepassing van ultrageluid later in het experiment resulteerde wel in versnelde precipitatie van CaCO3. Het veronderstelde mechanisme is deaggregatie tijdens de eerste groeifase, waar kiemen uitgroeien tot volwaardige kristallen. Dit effect wordt toegeschreven aan schuifspanningen ontstaan door micromixing en interne mechanische spanning veroorzaakt door (supersonische) schokgol- ven tijdens cavitatie. Elektronenmicroscopie laat zien dat het totaal aantal deeltjes groter is wanneer ultrageluid wordt toegepast en dat deze deeltjes meer uniform zijn verdeeld in vergelijking met het onbehandelde experiment. Dit resulteert eveneens in toename van het totale kristaloppervlak, wat leidt tot een grotere volumetrische kristalgroei.

Het eﬀect van ultrageluid in oplossingen van CaCO3 zonder additieven is nu on- derzocht. Water zal in de praktijk veel meer componenten bevatten dan calcium en carbonaat alleen. In het geval van omgekeerde osmose worden bijvoorbeeld antiscalants toegevoegd aan het water om scaling te voorkomen. Daarnaast kunnen andere

x ionen, zoals magnesium en sulfaat, de kristallisatie van CaCO3 be¨ınvloeden. Ook organische componenten, bekend staand als humuszuren, kunnen de kristallisatie van

CaCO3 veranderen. Hoofdstuk 5 bespreekt de toepassing van ultrageluid tijdens de groei van calciet in aanwezigheid van antiscalant nitrilotris(methylene phosphonic acid) (NTMP). In geënte groei experimenten werd een toename van de volumetrische groeisnelheid waargenomen bij toepassing van ultrageluid. Ook het herstel van groei, na inhibitie van NTMP, werd versneld door toepassing van ultrageluid. Ook hier zorgt ultrageluid voor opbreken en vergruizen van NTMP vergiftigde kristallen, wat zorgt voor vers kristaloppervlak voor groei. Massaspectroscopie van met ultrageluid behandeld NTMP laat zien dat dit molecuul kan worden afgebroken. Het afbreken is mogelijk veroorzaakt door radicalen gevormd tijdens cavitatie of door pyrolyse door cavitatie. Hoofdstuk 6 behandelt het effect van ultrageluid op vrije val precipitatie van calcium carbonaat in aanwezigheid van antiscalants NTMP en HEDP, humuszuren, en de ionen magnesium en sulfaat. De toegepaste concentraties van CaCO3, magnesium en sulfaat zijn gebaseerd op praktijkwaarden van omgekeerde osmose concentraat (drinkwater productie). Het grootste effect werd waargenomen voor NTMP (zonder andere toevoegingen) en toegeschreven aan het afbreken van het molecuul (zie hoofdstuk 5). De aanwezigheid van magnesium in deze oplossing (met NTMP) verlengde de inhibi- tietijd, maar ultrageluid zorgde voor een vergelijkbaar effect als in de oplossing met NTMP (zonder magnesium). De resultaten van de experimenten met HEDP waren niet afdoende. Een klein, maar significant effect werd waargenomen in de experimenten met sulfaat en magnesium (als een verkorte inductietijd). De experimenten met humuszuren resulteerden niet in een meetbaar effect. In hoofdstuk 7 wordt het effect van sulfaat (en in mindere mate magnesium) on- derzocht op precipitatie van CaCO3 in omgekeerde osmose drinkwater concentraat

(zonder ultrageluid). De vorming van CaCO3 gaat langzamer als de concentratie sulfaat wordt verhoogd. De voorkeurs-polymorf verschuift hierbij van vateriet naar aragoniet. Met deze verandering werd een nieuwe gecombineerde kristalvorm waargenomen waarbij (vermoedelijk) stekels van aragoniet bovenop vateriet kristallen groeien (“morgenster” kristalvorm). Hoofdstuk 8 beschrijft een uitbreiding van de software Minteq voor systemen waarbij de totale hoeveelheid opgelost zout een functie van tijd is. De schatting van de hoeveelheid vaste stof CaCO3 berekend met het model, komt goed overeen met de experimentele schatting in de vroege groeifase. De schatting van CO2 uitwisseling is ge¨evalueerd door de limiterende stap te bepalen. Voor de experimenten met sulfaat

xi Samenvatting

(zonder precipitatie) komen de diﬀusie constante en evenwichts-partieeldruk van CO2 goed overeen met de verwachte waardes.

Toepassing van een elektrisch veld zou wellicht ook de kristallisatie van CaCO3 kunnen be¨ınvloeden, bijvoorbeeld de kinetiek van kiemvorming of kristalgroei, en wordt besproken in hoofdstuk 9. Twee nieuwe configuraties zijn ontwikkeld om de veldsterkte van het elektrisch veld te verhogen (ten opzichte van methodes toegepast in de literatuur). De eerste methode, de aangepaste “zittende druppel”methode, was slecht reproduceerbaar en daarom niet langer toegepast. De tweede methode, een ingeklemd kanaal tussen twee glazen platen, was met behulp van microfluidica geminiaturiseerd. Deze tweede methode heeft nog niet afdoende resultaten opgeleverd om te kunnen concluderen dat het elektrisch veld een effect op CaCO3 heeft. Voor een succesvol vervolg is een aanpassing van de detectie techniek of het microfluidisch ontwerp noodzakelijk. Als laatste wordt in hoofdstuk 10 de implicaties en perspectieven van de opgedane kennis vertaald naar de toepassing van een sensor voor de voorspelling van het risico op scaling.

xii Contents

Summary v

Samenvatting ix

1. Introduction 1 1.1. Motivation ...... 2 1.1.1. Introduction ...... 2 1.1.2. Driving force for crystallization/scaling ...... 3 1.1.3. Scaling mitigation ...... 5 1.1.4. Scaling tendency estimation models ...... 5 1.1.5. Current (scaling) monitoring methods ...... 6 1.1.6. Proposed alternative scaling monitoring concept ...... 7 1.1.7. Ultrasound ...... 8 1.2. Research objectives ...... 10 1.3. Thesis outline ...... 11

2. Seeded calcite sonocrystallization 15 2.1. Introduction ...... 16 2.2. Experimental procedure ...... 17 2.3. Results and discussion ...... 20 2.4. Conclusion ...... 25

xiii Contents

3. Visualization of acoustic cavitation eﬀects on suspended calcite crystals 29 3.1. Introduction ...... 30 3.2. Experimental ...... 32 3.2.1. Materials ...... 32 3.2.2. Experimental set-up and procedures ...... 32 3.3. Results and discussion ...... 34 3.3.1. Seed characterization ...... 34 3.3.2. Bubble structures ...... 35 3.3.3. Disruption of aggregates and deagglomeration ...... 35 3.3.4. Seed acceleration by bubble expansion and collapse ...... 37 3.3.5. Eﬀect of cavitation on crystal habit ...... 41 3.4. Conclusion ...... 46

4. Eﬀect of ultrasonic treatment on early growth during CaCO3 precipitation 51 4.1. Introduction ...... 52 4.2. Experimental Section ...... 53 4.2.1. Chemicals ...... 53 4.2.2. Experimental setup ...... 53 4.2.3. Experimental procedures ...... 55 4.3. Results ...... 56 4.3.1. Ultrasound during entire experiment (0 - 4500 s) ...... 57 4.3.2. Ultrasound during primary nucleation (0 - 600 s) ...... 62 4.3.3. Ultrasound during early growth (600 - 1200 s) ...... 62 4.3.4. Ultrasound during crystal outgrowth (1800 - 2400 s) ...... 63 4.4. Discussion ...... 64 4.5. Conclusion ...... 67

5. Ultrasonic reactivation of phosphonate (NTMP) poisoned calcite during crystal growth 73 5.1. Introduction ...... 74 5.2. Experimental ...... 76 5.2.1. Chemicals ...... 76 5.2.2. Calcite seed crystals ...... 76 5.2.3. Experimental set-up ...... 77 5.2.4. Experimental procedures ...... 77

xiv Contents

5.3. Results and discussion ...... 79

5.3.1. CaCO3 supersaturation and growth mechanism ...... 79 5.3.2. Seeded calcite growth experiments ...... 80 5.4. Conclusion ...... 86

6. Free-drift precipitation of CaCO3 in the presence of foreign ions, antiscalants and ultrasound. 91 6.1. Introduction ...... 92 6.2. Experimental ...... 94 6.2.1. Chemicals ...... 94 6.2.2. Experimental setup ...... 95 6.2.3. Experimental procedures ...... 96 6.3. Results and discussion ...... 99 6.3.1. Antiscalants: NTMP, HEDP ...... 99 6.3.2. Foreign ions: Sulfate and magnesium ...... 108 6.3.3. Humic substances ...... 110 6.4. Conclusion ...... 112

7. Polymorphic change from vaterite to aragonite under inﬂuence of sulfate: the “morning star” habit 119 7.1. Introduction ...... 120 7.2. Experimental ...... 121 7.2.1. Chemicals ...... 121 7.2.2. Experimental setup ...... 121 7.2.3. Experimental procedures ...... 121 7.3. Results and discussion ...... 122 7.4. Conclusion ...... 131

8. Modeling the carbonate system 137 8.1. Introduction ...... 138 8.2. Newton-Raphson ...... 138 8.3. Calcium carbonate precipitation and pH ...... 142

8.3.1. Modeling solid CaCO3 formation from pH: Newton-Raphson . 142 8.3.2. Calcium carbonate solid formation from measured pH . . . . . 144

8.4. CO2 exchange and pH ...... 147

8.4.1. Modeling CO2 exchange based on pH: Newton-Raphson . . . . 147

xv Contents

8.4.2. CO2 exchange from measured pH ...... 148 8.5. Conclusion ...... 151

9. External electric field in calcium carbonate crystallization 155 9.1. Introduction ...... 156 9.2. Experimental ...... 158 9.2.1. Chemicals ...... 158 9.2.2. Experimental setup and procedures ...... 159 9.3. Results and discussion ...... 161 9.3.1. Modified sitting drop ...... 161 9.3.2. Microfluidics ...... 162 9.4. Conclusion ...... 168

10.Discussion and perspectives 173 10.1. External energy sources inﬂuencing crystallization ...... 174 10.1.1. Ultrasound ...... 174 10.1.2. Electromagnetic ﬁelds ...... 175 10.2. Scaling ...... 176 10.3. Device for monitoring scaling risk ...... 177 10.3.1. Miniaturization ...... 178 10.3.2. Dynamic model ...... 178 10.4. Additional process optimization ...... 179

A. Experiments sonoluminescence reactor 181 A.1. Experimental ...... 182 A.1.1. Chemicals ...... 182 A.1.2. Experimental setup ...... 182 A.1.3. Solution preparation ...... 183 A.1.4. Experimental procedures ...... 183 A.2. Results ...... 185

B. Supporting information for: Eﬀect of US on early growth 187

C. Matlab-code: Solid CaCO3 as function of pH 191

Acknowledgements 195

Curriculum Vitae 197

xvi 1 Introduction

Author: R.M. Wagterveld

1 1. Introduction

1.1. Motivation

1.1.1. Introduction

Calcium carbonate is one of the most abundant minerals on earth. Its low solubility in water creates excellent conditions for the formation of the well known stalagmites and stalactites in limestone caves. Due to its abundance, and low solubility in water, it is not surprising that this mineral emerges in every day life. When one boils some water in a kettle to make tea, “scales” of limestone will appear. The same scales are present on the heating element of your washing machine (fig. 1.1a), thereby reducing the efficiency. That scaling is very persistent and hard to remove becomes certainly evident when cleaning the bathroom. The use of an acidic cleaning product is usually required. The term scaling, also known as inorganic or precipitation fouling, comprises the formation of hard mineral deposits. Typical examples of equipment that suffer from scaling are process (fig. 1.1b) or membrane equipment (fig. 1.1c) such as boiler systems, cooling towers, mining process water, oil wells and membrane water purification systems. Besides calcium carbonate, common scaling minerals are calcium sulfate

(CaSO4 · H2O), barium sulfate (BaSO4), strontium sulphate (SrSO4), silicates (forms of silicon oxide, SiO2), calcium phosphate (Ca3PO4), ferric- and aluminum hydroxides

(Fe(OH)3 and Al(OH)3). Integrated water supply systems are increasingly utilizing non-traditional water sources such as seawater, excessively hard or brackish groundwater, poorer quality surface waters, and wastewater. These sources commonly require treatment with

(a) (b) (c) Domestic: Washing machine Industrial: Scaling in Industrial: Scaling on mem- heating element [1] pipe [2] brane [3]

Figure 1.1.: Typical examples of scaling

2 1.1. Motivation membrane technologies before use in water supply systems [4]. In 2009, 44% of the total global water production was achieved by reverse osmosis, and an additional 8% with other membrane technologies [5]. Especially in reverse osmosis (RO) membrane systems, scale formation has always been a serious limitation. The high operating water product recoveries and salt re- jection efficiencies of RO desalination processes cause at the feed side a 4 to 10 times increase of the relative dissolved salt concentration [5] which will lead to a driving force for scaling. In particular for brackish water RO, scaling is the main limiting factor (due to low biofouling risk) [5]. Chapters 6 and 7 discuss such brackish groundwater with a concentration factor of 5. Scaling causes flux decline, membrane degradation, loss of production and elevated operating costs. The operating costs of a large brackish RO plant producing 700,000 m3 per day have been estimated in 2006 to be $20,000,000 per year (Yun et al. [6]), of which 40% can be ascribed to prevention of scaling, so $8,000,000 per year. Membrane replacement takes half of this sum, the other half is due to chemicals added. Improving the operating costs for scale prevention with only 5% will already save $400,000 per year. Given the damage-related costs mentioned above, process optimization demands scaling prevention based on accurate prediction. In general, scaling leads to blockage of water flow (or heat flux) through pipes and membranes causing:

Higher energy consumption Lower production capacity Higher maintenance Shorter lifetime of the process installation

1.1.2. Driving force for crystallization/scaling

The process of scaling can only commence when the condition of supersaturation is achieved. The fundamental dimensionless driving force for crystallization (start of the scaling process) as function of supersaturation is deﬁned as follows, where S (-), ∆µ (J mol−1), R (J mol−1 K−1), T (K) and v (-), are the supersaturation ratio, change in chemical potential, gas constant, absolute temperature, and number of diﬀerent ion species in the formula unit (v = 2 for CaCO3), respectively [7]:

∆µ = v ln S (1.1) RT

3 1. Introduction

Figure 1.2.: Schematic representation of the metastable zone. The thick line represents Ksp. Be- 2+ 2– low this line the solution is undersaturated and stable, implying Ca and CO3 remain permanently in solution. Far above the line the solution is supersaturated and crystallization takes place. Between the dotted and thick line the solution is supersaturated but metastable due to kinetics. The arrow indicates the eﬀect of antiscalants, shifting the metastable limit further from the solubility line.

The driving force is positive when S > 1, or supersaturated. The supersaturation ratio is then deﬁned in terms of activities, where IAP is the ion activity product and

Ksp the thermodynamic equilibrium solubility product:

1 IAP v S = (1.2) Ksp A region exists, just above saturation, where kinetics are so slow that the system is considered to be metastable. This region is also referred to as metastable zone and is schematically depicted in fig. 1.2. The position of the “metastable limit” (dashed line), which together with the solubility defines the metastable zone, is not well defined and depends on the measurement method and application. Above the “metastable limit” crystallization is “spontaneous” and scaling tendency is high. Scale formation comprises complex phenomena involving both crystallization and transport mechanisms. In addition to supersaturation conditions, kinetics of crystallization should also be considered. Nucleation (critical nucleus formation), followed by crystal growth can result in membrane surface (pore) blockage, the first form of scaling. The crystals might agglomerate and form layers which leads to cake formation, the second form of scaling. Whether supersaturation, through crystallization, leads to scaling, is ruled by various operating conditions such as pH, temperature, operating pressure, permeation

4 1.1. Motivation rate, flow velocity, and presence of other salts or metal ions [8]. Small variations in water chemistry may have a large influence on precipitation kinetics. For example far substoichiometric concentrations of chemicals known as antiscalants increase the apparent metastability significantly.

1.1.3. Scaling mitigation

The most common scale mitigating techniques can be grouped into three categories [8]:

Altering feed water characteristics, i.e. acidiﬁcation Optimization of operating parameters and system design i.e. limiting product recovery Antiscalant addition

Application depends on the nature of the feed water, membrane compatibility with acid or scale inhibitor and cost. In practice the use of antiscalants is almost inevitable in reverse osmosis membrane systems. Antiscalants inhibit growth of crystals and thereby increase the apparent metastability (fig. 1.2). The economic benefit of the use (costs) of antiscalants eventually arises from higher product recovery [9–12]. Currently antiscalants are highly overdosed since the scaling tendency is difficult to assess [13, 14]. Besides the high cost, due to stricter environmental regulations, a reduction of antiscalant dosage is necessary. Moreover with the use of antiscalants the risk of (bio)fouling is 4-10x higher [5], all implying an improvement of scaling tendency determination [4]. With better scaling risk assessment, processes can operate closer to the metastable limit, implying a higher efficiency at lower costs.

1.1.4. Scaling tendency estimation models

Most research on scale formation and control focuses on synthetic (or analytical) solutions. Although this helps in gaining insight in individual components, it fails to mimic the chemistry of water of more complex composition. In an attempt to account for the complexity of water chemistry empirical models have been developed. All practiced scale prediction models use the theoretical concept of saturation [15] and do not include, for instance, that antiscalants eﬀectiveness is not equal for all scalants [5]. Langelier saturation index (LSI) was the ﬁrst introduced scaling index and is based on the comparison measured pH and an theoretical “pH”, based on water composition

5 1. Introduction analysis. This latter pH is calculated with information on total dissolved solids, calcium concentration, temperature and alkalinity. LSI does not work very well for high salinity systems which led to the development of the Stiﬀ-Davis index (SDI or S&DI). This index is as LSI but with an additional empirical salinity correction. The Ryznar stability index (RSI) takes into account measured data from previously scaled water and transforms LSI into an empirical relation. A fourth index, the Puckorius scaling Index (PSI) is similar to RSI but uses the equilibrium pH (calculated from the alkalinity) instead of the measured pH, and is introduced to include the buﬀer capacity of water.

1.1.5. Current (scaling) monitoring methods

The dynamic behavior of the water chemistry forces an on-line scaling tendency estimation. Successful application of scaling tendency models just discussed would comprise the online measurement of every component in the system, which is very impractical. An alternative would be to measure a dominant component, for instance the “consumption rate” of antiscalant. Since the concentration of antiscalant is very low, additional labeling is necessary for accurate measurement, as is applied in the Trasar technology [16]. In this technology the antiscalant is labeled with ﬂuorescent dye and measured with optical means. However, besides being more expensive, the variation in antiscalant is not a direct measure of scaling tendency. The dosage of antiscalant is better controlled, but is still based on (static) scaling tendency models. Since scaling cannot be precluded, scaling monitors have been applied. To date, scaling monitoring methods rely on the detection of scaling (and not on scaling tendency) and the following methods have been researched:

Permeate ﬂux decline [17] Ultrasonic time-domain reﬂectometry analysis [18] Surface acoustic wave [19] Quartz crystal [20] Visual observation [21] Electrical impedance spectroscopy analysis [22]

All methods (except for the permeate ﬂux decline according to the authors) share the same drawback: They monitor scaling, but not the risk of scaling. In other words, scaling already takes place and mitigation such as cleaning or membrane replacement is still a necessity. The monitoring method based on permeate ﬂux decline takes place

6 1.1. Motivation

Figure 1.3.: Artist impression of the proposed concept. Part of the (supersaturated) bulk solution is directed to the measurement device, either flow through or batch sampled. An actuator enhances the crystallization kinetics and a sensor detects the course of crystallization. Suggested actuation is by application of ultrasound or a large electric field. in a bypass stream and is said to monitor scaling locally before it occurs in the main process [17]. However, flux decline can have many other causes [23] which may lead to erroneous scaling risk assessment.

1.1.6. Proposed alternative scaling monitoring concept

Current scaling prediction is based on empirical models which are not very accurate. Scaling measurement is based on the actual scaling, which makes the response inher- ently too late. The scaling prediction and scaling measurement can be improved by measuring the scaling process and detect scaling before it occurs in the bulk. Pro- posed concept: By enhancing the crystallization (kinetics or thermodynamics) locally and monitoring the process itself, scaling can be predicted accurately before it occurs in the bulk solution. If successful, this will result in a typical proactive sensor because precautions can be made (based on the measurement - prediction) before damage has occurred. The better scaling risk assessment improves chemical dosage (preventing overdosing) and prevents the necessity of cleaning or membrane replacement. Figure 1.3 provides an artist impression of the proposed concept. Part of the bulk solution is directed to the measurement device, either ﬂow through or batch sampled. An actuator enhances the crystallization kinetics and a sensor detects the course of crystallization. The ideal actuator should enhance crystallization in a generic way and should be additional to the conditions present, e.g. keep the bulk conditions constant.

7 1. Introduction

Two methods were selected as possible actuator, ultrasound and electric fields [24– 26]. Both methods have been reported to affect crystallization but the results were ambiguous. The sensor should capture the course of crystallization, since this is the beginning of scaling. This gives understanding of the bulk conditions but cannot directly be ex- trapolated to scaling, since geometrical conditions and housing / membrane material also affect the resulting scaling. This should be taken into account in the interpretation of the data, but these parameters do not change over time and can easily be incorporated in the scaling prediction. As a sensor one can think of a pH measurement or conductivity detection but both parameters might not give enough sensitivity when complex solutions are applied. An optical measurement seems to be the best candidate as long as the bulk solution is relatively transparent, otherwise an acousti- cal measurement is an option. With optical detection one can apply light scattering / transmission, laser diffraction, optical attenuation for particle detection, or even infrared or Raman detection for identification of newly formed material.

In this work the focus will be on CaCO3 crystallization, with application of ultrasound and pH measurements, sometimes assisted with optical scattering measurements.

1.1.7. Ultrasound

Ideally, enhancing the precipitation kinetics is done by adding external energy to the system. Applying ultrasound might be a good candidate as such an external source. Ultrasound can help controlling the course of precipitation and crystallization processes and is also referred to as sonocrystallization [27]. The positive effects seen in sonocrystallization are usually ascribed to cavitation that appears in high power ultrasound. Cavitation is the interaction of (acoustic) pressure waves with cavities (microbubbles), caused by the rupture of the fluid in the negative pressure cycle. Microscopic bubbles oscillate or grow and collapse under the varying pressure field inside the treated liquid. Several effects (can) occur during this process; the formation of radicals, generation of shockwaves and microjets, local hotspots of high pressure (up to 200 MPa or 2000 bar) and temperature (up to 6000 K), micromixing, macromixing and rise of bulk temperature [28]. These effects by cavitation might influence the thermodynamics (local hotspots of high pressure and temperature) or kinetics of the crystallization / scaling process, and thereby shifting, or reducing, the metastable

8 1.1. Motivation zone (ﬁg. 1.2). Inside the cavitating bubble, vaporized molecules can undergo homolytic cleavage, due to the extreme conditions of high temperature (and pressure), to give hydrogen atoms and hydroxide radicals [28]. These radicals might come into play in the crystallization process.

Two forms of cavitation can be distinguished, non-inertial (or stable) and inertial (or transient) cavitation [29]. Non-inertial cavitation is the stable oscillation of a bubble. Surface oscillations can cause interesting bubble geometries [30], as shown in fig. 1.4. Mixing is enhanced around these bubbles but they do not exhibit the other effects found during cavitation. Inertial cavitation is the growth and collapse of bubbles, as depicted in fig. 1.5. In the collapse phase a local hotspot of high pressure and temperature is created. Shockwaves are generated, and for asymmetric collapse, microjets are created. The fast expansion and collapse and presence of shockwaves lead to enhanced mixing. Shockwaves and microjets might cause surface erosion, like pitting or attrition, or even breakage. Furthermore particle (de)agglomeration and (de)aggregation have been observed previously [27]. During application of ultrasound the bulk temperature will rise. Controlling the bulk temperature is therefore a necessity.

Figure 1.4.: Examples of non-inertial cavitation e.g. ultrasound causing bubble oscillations. A) Surface oscillation causing transitions between circular and square bubble geometry projection. Time between consecutive images is 11 µs, frequency of ultrasound 42.2 kHz. B) Surface oscillation causing transitions between circular and hexagonal bubble geometry projection. Time between consecutive images is 22 µs, frequency of ultrasound is 20.7 kHz

9 1. Introduction

Figure 1.5.: Example of inertial cavitation e.g. ultrasound causing bubble growth and collapse. Time between consecutive images is 8 µs, frequency of ultrasound 42.2 kHz. The series shows exactly one period. The sine wave shows a typical sequence of inertial cavitation. In a negative pressure cycle a cavity is formed. It eﬀectively grows faster in size in consecutive negative pressure cycles than it shrinks in positive cycles. At a certain critical size the bubble completely collapses and a local hotspot is created.

1.2. Research objectives

By enhancing the crystallization (kinetics) locally and monitoring the process itself, scaling can be predicted accurately before it occurs in the bulk solution. Ultrasound is selected as principle method for (possible) crystallization enhancement. Application of an electric ﬁeld is also brieﬂy addressed. Consequently, the main research objectives are:

Characterize the eﬀect of ultrasound on crystallization of CaCO3 in:

– Solutions of CaCO3 without additives.

– Solutions of CaCO3 with antiscalant added. – Solutions with (variations on) components and concentrations based on drinking water RO concentrate.

Clarify the underlying mechanisms in CaCO3 sonocrystallization. Explore experimental methods to investigate the eﬀect of an electric ﬁeld on

CaCO3 crystallization.

10 1.3. Thesis outline

1.3. Thesis outline

The main scope of this thesis is the effect of ultrasound on calcium carbonate crystallization. The common theme in all the chapters is the mechanism of acoustic cavitation and its effect on crystallization (nucleation and growth) and interaction with crystals. Possible interference of additives found in reverse osmosis drinking water concentrate is investigated by using synthetic solutions. Additionally, the speciation is modeled, specifically in case of time varying pH. Finally the application of an alternative method, electric field, is briefly addressed. Chapter 2 describes the effect of ultrasound on seeded calcite (polymorph of

CaCO3) crystallization. In a (seeded) constant composition experiment, the conditions during crystal growth, such as temperature, solution composition and thus pH, were kept constant. With this methodology, application of ultrasound should, if effective, directly lead to an affected (volumetric) crystallization rate. With scanning electron microscopy the size and habit of the crystals with and without treatment were investigated, as well as the particle size distributions. In chapter 3 the interaction of acoustic cavitation with suspended calcite crystals, as discussed in chapter 2, is visualized using high speed imaging. Possible breakage by high velocity interparticle collisions is investigated and particle acceleration and deceleration by cavity expansion and collapse is modeled. The cavitation phenomena responsible for disruption of agglomerates and aggregates are discussed, as well as the effect of cavitation on crystal habit (using SEM analysis). Chapter 4 delineates the way ultrasound exerts its effect by applying ultrasound in different treatment periods (time windows). For that, three stages in precipitation are distinguished: The first is primary nucleation, either homogeneous or heterogeneous. The second is “early growth”, when secondary nucleation can take place and crystals grow to detectable size. The last is “late growth” during which formed crystals continue to grow and the supersaturation is reducing to saturation. Calcium carbonate formation, using the free-drift method, is monitored by three independent parameters: pH, light scattering, and scanning electron microscopy (SEM). Chapter 5 covers the effect of ultrasound on the growth of calcite in the presence antiscalant nitrilotris(methylene phosphonic acid) (NTMP). The calcite crystal growth was measured using the constant composition method at various NTMP concentrations with and without ultrasonic irradiation. Mass spectrometry is used to detect break down products of NTMP. Chapter 6 presents the effect of ultrasound on free-drift precipitation of calcium

11 1. Introduction carbonate in the presence of NTMP, HEDP, humic substances, or foreign ions such as magnesium and sulfate. Since all additives modify the precipitation of calcium carbonate, the effect of ultrasound might be suppressed or enhanced. Chapter 7 discusses the effect of sulfate (and to a lesser extent magnesium) on the precipitation of CaCO3 in artificial drinking water reverse osmosis concentrate

(without ultrasound). The focus is on overall kinetics of CaCO3 formation and the types of polymorphs formed. A new habit, the “morning star”, is introduced in this chapter. Chapter 8 describes an extension to the software Minteq. In previous experiments this software has been applied to determine the initial speciation. In this chapter the model is applied to systems in which the total amount of salt is a function of time, e.g. due to CaCO3 deposition or gaseous CO2 exchange with the environment. Chapter 9 discusses the application of an electric ﬁeld as actuation method in

CaCO3 crystallization. Two experimental methods are explored, a variation of the sitting drop technique, and the glass plate sandwich in combination with microﬂuidics. Finally, Chapter 10 discusses the implications of the gained knowledge in this work with respect to a sensor for scaling tendency prediction, and perspectives are outlined.

References

[1] Unknown. Limescale is caused by Hard Water. 2012. url: http://www.plumber24hours. co.uk/plumber-blog/ (cit. on p. 2). [2] Unknown. Pipe full of scale formed by Calcium Carbonate and other salts solved in the water. 2012. url: http://www.merusonline.com/in-general/water-containing- calcium-carbonate (cit. on p. 2). [3] Unknown. Mixed sulphate scaling rapidly causes deterioration in RO membrane per- meability. 2012. url: http://www.environmentalthinking.com/et/environmental_ thinking / Reducing - the - environmental - impact - of - membrane - cleaning / 1 / 200336175636 (cit. on p. 2). [4] S. J. Khan, D. Murchland, M. Rhodes, and T. D. Waite. “Management of concentrated waste streams from high-pressure membrane water treatment systems”. In: Crit. Rev. Environ. Sci. Technol. 39.5 (2009), pp. 367–415 (cit. on pp. 3, 5). [5] L. F. Greenlee, D. F. Lawler, B. D. Freeman, B. Marrot, and P. Moulin. “Reverse osmosis desalination: Water sources, technology, and today’s challenges”. In: Water Res. 43.9 (2009), pp. 2317–2348 (cit. on pp. 3, 5). [6] T. I. Yun, C. J. Gabelich, M. R. Cox, A. A. Moﬁdi, and R. Lesan. “Reducing costs for large-scale desalting plants using large-diameter, reverse osmosis membranes”. In: Desalination 189.1 (2006), pp. 141–154 (cit. on p. 3). [7] J. W. Mullin. Crystallization. Butterworth-Heinemann, 2001 (cit. on p. 3).

12 1.3. References

[8] A. Antony, J. H. Low, S. Gray, A. E. Childress, P. Le-Clech, and G. Leslie. “Scale formation and control in high pressure membrane water treatment systems: A review”. In: J. Membr. Sci. 383.1 (2011), pp. 1–16 (cit. on p. 5). [9] S. He, A. T. Kan, and M. B. Tomson. “Inhibition of calcium carbonate precipitation in NaCl brines from 25 to 90 C”. In: Appl. Geochem. 14.1 (1999), pp. 17–25 (cit. on p. 5). [10] D. Hasson, A. Drak, and R. Semiat. “Inception of CaSO4 scaling on RO membranes at various water recovery levels”. In: Desalination 139.1-3 (2001), pp. 73–81 (cit. on p. 5). [11] J. Guo and S. J. Severtson. “Inhibition of Calcium Carbonate Nucleation with Amino- phosphonates at High Temperature, pH and Ionic Strength”. In: Ind. Eng. Chem. Res. 43 (2004), pp. 5411–5417 (cit. on p. 5). [12] L. F. Greenlee, F. Testa, D. F. Lawler, B. D. Freeman, and P. Moulin. “The effect of antiscalant addition on calcium carbonate precipitation for a simplified synthetic brackish water reverse osmosis concentrate”. In: Water Res. 44.9 (2010), pp. 2957– 2969 (cit. on p. 5). [13] R. A. Dawe and Y. Zhang. “Kinetics of calcium carbonate scaling using observations from glass micromodels”. In: J. Petrol. Sci. Eng. 18.3-4 (1997), pp. 179–187 (cit. on p. 5). [14] T. Chen, A. Neville, and M. Yuan. “Calcium carbonate scale formation–assessing the initial stages of precipitation and deposition”. In: J. Petrol. Sci. Eng. 46.3 (2005), pp. 185–194 (cit. on p. 5). [15] V. A. Prisyazhniuk. “Prognosticating scale-forming properties of water”. In: Appl. Therm. Eng. 27.8 (2007), pp. 1637–1641 (cit. on p. 5). [16] E. H. K. Zeiher, B. Ho, and K. D. Williams. “Novel antiscalant dosing control”. In: Desalination 157.1 (2003), pp. 209–216 (cit. on p. 6). [17] C. A. C Van de Lisdonk, B. M. Rietman, S. G. J. Heijman, G. R. Sterk, and J. C. Schippers. “Prediction of supersaturation and monitoring of scaling in reverse osmosis and nanofiltration membrane systems”. In: Desalination 138.1 (2001), pp. 259–270 (cit. on pp. 6, 7). [18] X. Lu, E. Kujundzic, G. Mizrahi, J. Wang, K. Cobry, M. Peterson, J. Gilron, and A. R. Greenberg. “Ultrasonic sensor control of flow reversal in ro desalination part 1: Mitigation of calcium sulfate scaling”. In: J. Membr. Sci. 419–420 (2012), pp. 20–32 (cit. on p. 6). [19] M. Löffelmannand A. Mersmann. “How to measure supersaturation?” In: Chem. Eng. Sci. 57.20 (2002), pp. 4301–4310 (cit. on p. 6). [20] O. J. Joung, Y. H. Kim, and K. Fukui. “Determination of metastable zone width in cooling crystallization with a quartz crystal sensor”. In: Sens. Act. B. 105.2 (2005), pp. 464–472 (cit. on p. 6). [21] M. Uchymiak, A. Rahardianto, E. Lyster, J. Glater, and Y. Cohen. “A novel RO ex situ scale observation detector (EXSOD) for mineral scale characterization and early detection”. In: J.Membr. Sci. 291.1 (2007), pp. 86–95 (cit. on p. 6).

13 1. Introduction

[22] A. Antony, T. Chilcott, H. Coster, and G. Leslie. “In situ structural and functional characterization of reverse osmosis membranes using electrical impedance spectroscopy”. In: J. Membr. Sci. 425–426 (2013), pp. 89 –97 (cit. on p. 6). [23] E. Alhseinat and R. Sheikholeslami. “A completely theoretical approach for assessing fouling propensity along a full-scale reverse osmosis process”. In: Desalination 301.0 (2012), pp. 1 –9 (cit. on p. 7). [24] M. D. Luque de Castro and F. Priego-Capote. “Ultrasound-assisted crystallization (sonocrystallization)”. In: Ultrason. Sonochem. 14.6 (2007), pp. 717–724 (cit. on p. 8). [25] E. Revalor, Z. Hammadi, J. P. Astier, R. Grossier, E. Garcia, C. Hoﬀ, K. Furuta, T. Okustu, R. Morin, and S. Veesler. “Usual and unusual crystallization from solution”. In: J. Cryst. Growth 312.7 (2010), pp. 939–946 (cit. on p. 8). [26] Z. Hammadi and S. Veesler. “New approaches on crystallization under electric ﬁelds”. In: Prog. Biophys. Mol. Biol. 101.1 (2009), pp. 38–44 (cit. on p. 8). [27] M. L. de Castro and F. Priego-Capote. “Ultrasound-assisted crystallization (sonocrystallization)”. In: Ultrason. Sonochem. 14 (2007), pp. 717–724 (cit. on pp. 8, 9). [28] Y. Shah, A. Pandit, and V. Moholkar. Cavitation reaction engineering. Plenum Pub Corp, 1999 (cit. on pp. 8, 9). [29] T. G. Leighton. “What is ultrasound?” In: Prog. Biophys. Mol. Biol. 93.1-3 (2007), pp. 3–83 (cit. on p. 9). [30] M. Versluis, S. M. van der Meer, D. Lohse, P. Palanchon, D. Goertz, C. T. Chin, and N. de Jong. “Microbubble surface modes [ultrasound contrast agents]”. In: Ultrason. Symp., 2004 IEEE. Vol. 1. IEEE. 2004, pp. 207–209 (cit. on p. 9).

14 2 Seeded calcite sonocrystallization

abstract The seeded sonocrystallization of calcite was investigated by measuring the volumetric crystallization rate at constant composition conditions. The crystallization rate of calcite was enhanced by 46% through ultrasonic irradiation (42,150 Hz, 17 W dm−3) of a supersaturated crystal suspension. It was shown that this eﬀect was related to the alteration of the seed crystals’ habit and size. During ultrasonic irradiation disruption of conglomerates and erosion of single crystals occurred, accompanied by the production of many ﬁnes. The increased surface area available for crystal growth resulted in the observed crystallization rate enhancement.

This chapter has been published as: L. Boels, R.M. Wagterveld, M.M. Mayer, G.J. Witkamp. Seeded calcite sonocrystallization. Journal of Crystal Growth 312 (2010) 961-966

15 2. Seeded calcite sonocrystallization

2.1. Introduction

Ultrasound can help controlling the course of crystallization processes. This is also referred to as sonocrystallization. Sonocrystallization received much attention the last decade due to its positive effects in crystallization processes for the synthesis of phar- maceuticals and proteins [1–9], organic compounds [10–12] and inorganic compounds [13–21]. Many effects are ascribed to the ultrasonic treatment. Both an increased and a retarded crystal growth rate were reported [2, 4–6, 10, 14, 15, 19]. The same holds for the increase [9] and decrease [3, 4, 6, 8, 9, 12, 16–19, 21, 22] of induction periods. A few papers mention a decrease of the metastable zone width [8, 10, 13, 14, 18] as well as an enhanced solubility of a sparingly soluble salt [20]. Furthermore, effects are seen on the mean particle size [1, 2, 5–7, 9, 11–13, 16, 21], the particle size distribution and crystal morphology [1, 2, 5–7, 10–14, 16]. Acoustic streaming and cavitation are the most important phenomena that occur when ultrasound is passed through a liquid-solid system. These phenomena can produce a series of unique chemical and physical effects. In acoustic streaming, acoustic waves produce a stirring effect. In the vicinity of the crystal surfaces, this stirring effect causes a reduction of the diffusion layers’ thickness, thereby increasing the liquid-solid mass transport [23]. In addition, ultrasound produces small imploding cavities that generate high-energy shock waves which impinge on the particle surface. This can create high velocity interparticle collisions that can alter the particle morphology and size dramatically. It was reported that these interparticle collisions occur with such a great force that even metal particles tend to melt together [24]. Microjet formation in the vicinity of the particle surface is another well-established mechanism for accounting the effect of cavitation in solid-liquid systems. However, in literature it is mentioned that this mechanism can only occur if the surface is several times larger than the resonant bubble size [25]. Calcium carbonate is one of the most abundant minerals, and the problem of its scaling propensity is encountered in many industrial water treatment processes. Al- though the topic of sonocrystallization has received much attention in the last decade, the effect of ultrasonic irradiation on the precipitation of calcium carbonate has hardly been investigated and therefore the literature on this topic is sparse. Dalas [15] reported a retarded crystal growth in the presence of ultrasonic irradiation. No effect of this irradiation on the nature, morphology or the size of the formed calcium carbonate crystals was observed. In the study conducted by Nishida [19], it was observed that the spontaneous precipitation of calcium carbonate was enhanced in the presence of

16 2.2. Experimental procedure ultrasonic irradiation. It was suggested that this was due to an enhanced nucleation rate during the ultrasonic treatment. Furthermore, this research confirmed the observations of Dalas that neither the morphology nor the size of the calcium carbonate crystals was affected by the ultrasonic irradiation. Berdonosov and co-workers [26] found that the transition of metastable vaterite to calcite can be enhanced by ultrasonic irradiation. It was suggested that this effect was related to energetic collisions between vaterite and calcite particles. Considering the physical effects that occur due to cavitation, it is reasonable to expect that the habit and size of calcium carbonate crystals can be altered through ultrasonic irradiation. The aim of the present work is to investigate whether or not this occurs. For this purpose the volumetric crystal growth rate (i.e. crystallization rate) of calcite seed crystals during ultrasonic irradiation was measured using the proven constant composition methodology [27, 28]. If the habit and/or size of the seed crystals can be indeed affected, the volumetric crystal growth rate should also change. Therefore, the crystallization rate after the irradiation period was also measured.

2.2. Experimental procedure

Only analytical grade reagents, high quality water (MilliQ Reagent Water System, resistivity >18 MΩ cm) and grade A glassware were used throughout the experiments. Calcite seed crystals were prepared, using the method described by Reddy and Nancollas [29], by slowly adding 2 dm3 of a 0.20 M calcium chloride solution to 2 dm3 0.20 M sodium carbonate solution at 25 ◦C. The freshly precipitated seed crystals were aged overnight in mother liquor and were subsequently washed with MilliQ water each day for 1 week. Afterwards, the washed seed crystals were aged for 3 weeks before filtering. The dried crystals were characterized by scanning electron microscopy (Jeol JSM-6480LV), nitrogen adsorption (Micromeritics Tristar 3000) and ATR-FT-IR spectroscopy (Shimadzu 4800). The specific surface area of the seed crystals was found to be 0.17 m2 g−1 as determined with a five-point BET method [30] on 3 replicate samples. The ATR-FT-IR spectra showed characteristic adsorptions for calcite at 1795, 1392, 871 and 711 cm−1. The characterization confirmed the crystals to be pure calcite and appeared as interpenetrated conglomerates. A double walled thermostatted glass reactor equipped with a floating magnetic stirrer bar to minimize any grinding effects, a by-pass loop and an ultrasonic transducer were used (fig. 2.1). The ultrasonic transducer was part of a dedicated homebuilt sys-

17 2. Seeded calcite sonocrystallization

Figure 2.1.: Scheme of the constant-composition experimental set-up consisting of (1) a double walled glass reactor, (2) ﬂoating magnetic stirrer bar, (3) membrane pump, (4) ultrasonic transducer, (5) free port for seed addition, (6) pH electrode and reference electrode, and (7) a temperature sensor. tem that could be controlled precisely in terms of shape, frequency and amplitude of the alternating current for driving the transducer. For the control of pH, a combined Pt-ring pH electrode and a shielded Ag/AgCl reference electrode in combination with two coupled automatic burettes (Metrohm Titrino 785 and Dosimat 665) were used. In order to avoid any possibility of damaging the electrodes by the ultrasonic irradiation, both electrodes and a temperature sensor were positioned in a glass cell in the by-pass stream. A positive displacement membrane pump was used to pump the crystal suspension with 1.5 dm3 min1 through the by-pass. Metastable working solutions were prepared by slow addition of 500 cm3 of a 4 mM calcium solution to 500 cm3 of a 4 mM bicarbonate solution in the reactor. Fresh titrant solutions were prepared every day as shown in table 2.1. Potassium chloride was added to all solutions to maintain the ionic strength constant at 0.1. The working solution was stirred at 500 rpm. The pH of the stable working solution

18 2.2. Experimental procedure

Table 2.1.: Composition of the titrant solutions. Reagent Calcium titrant Carbonate titrant (mM) (mM)

CaCl2 · 2 H2O 53.0 - NaHCO3 - 4.0 Na2CO3 - 49.0 KCl 38.0 46.0 was adjusted to the desired value of 8.500 using a 0.05 M KOH solution. The stability of the working solution was verified by observing a constant pH for at least 45 min. After the addition of 0.250 g dry seed crystals, the solution pH started to decrease as a result of calcite precipitation. This triggered the automatic burettes to add concurrently equivalent amounts of calcium chloride and sodium carbonate solutions in order to achieve and maintain the pH at the target value ( ± 0.002 pH units). In this way, a constant degree of supersaturation was maintained throughout the growth experiment. After 30 min of normal growth, the ultrasonic irradiation was initiated. The solution was treated with ultrasound for 45 min at a frequency of 42,150 Hz and with an intensity of 17 W (real output power intensity pre-determined by measuring the adiabatic temperature rise in time using a similar well isolated glass reactor filled with 1 dm3 of water). The volume of added titrant solution was monitored over time and this data represented the calcium carbonate precipitation rate. During the experiment, aliquots of solution were rapidly removed, filtered through a 0.2 µm filter and analyzed for the calcium content with inductive-coupled plasma spectrometry (Optima 3000XL, Perkin-Elmer) to verify the constancy of the degree of supersaturation. After the ultrasonic treatment the crystals were allowed to grow for another 75 min at the same supersaturation. All experiments were conducted at atmospheric pressure with ambient levels of CO2. By keeping the reactor lid closed and all ports sealed during the experiments, the exchange of atmospheric CO2 with the reactor solution was minimized. Seed addition and reactor sampling were performed as rapidly as possible. The temperature was maintained constant at 25 ±0.1 ◦C by circulating water from a thermo-bath through the jacket of the reactor. During the ultrasonic irradiation, the thermo-bath was set to cool in order to keep the reaction mixture temperature at 25 ±0.1 ◦C. In order to investigate the effect of ultrasound on the crystal habit, 0.250 g dry seed crystals were added to a 1 dm3 saturated calcium carbonate solution and the resulting

19 2. Seeded calcite sonocrystallization

Figure 2.2.: Titrated volume of Ca2+ vs. time of constant composition calcite growth experiments: three sets of control experiments and two sets with ultrasonic treatment (start after 30 min). Experimental conditions: S = 2.11, pH= 8.500, T = 25◦C. suspension was stirred and treated with ultrasound in the same way as during the growth experiments. Throughout the experiment, samples of 15 cm3 of solution were collected and filtered through a 0.2 µm filter. After drying, the samples were analyzed with SEM. For comparison, the experiment was repeated with a stirred saturated suspension without applying ultrasound (control).Afterwards, both suspensions were transferred to a particle size and shape (video) analyzer equipped with a liquid flow cell (Eyetech, Ankersmid) in order to measure the number distributions of the particle size. Three replicate analyses were performed in which 5000 particles were analyzed to reach a confidence level above 99%.

2.3. Results and discussion

The driving force for crystallization can be expressed as

∆µ = v ln S (2.1) RT where R (J mol −1 K −1) is the gas constant, T (K) is the absolute temperature, ∆µ (J mol−1) is the change in chemical potential, and S (-) is the supersaturation.

For CaCO3, S is best expressed in terms of the solubility product:

20 2.3. Results and discussion

Table 2.2.: Growth rate derived from a cubic ﬁt of three experimental datasets and their average. 2+ 2– ◦ Experimental conditions: S= 2.11, [Ca ] = [CO3 ]= 2 mM, pH= 8.500, T = 25 C. Experiment Rate, R SSE R2 (mol m−2 s−1) (-) (-) Control 01 1.77 × 10−6 0.1170 0.999996 Control 02 1.98 × 10−6 0.1275 0.999995 Control 03 1.70 × 10−6 0.1199 0.999995 Average 1.82 × 10−6 0.0388 0.999999

1 IAP v (2.2) Ksol −1 1 where IAP (mol kg ) is the ion activity product, Ksol (mol kg ) is the solubility product, and v (-) is the number of ions in the formula unit. The supersaturation, S, with respect to calcite of the working solution used here was calculated to be 2.11 (software: Visual Minteq v2.53, model: Davies) and the change in chemical potential, ∆µ, of transfer from supersaturated solution to equilibrium, measured 1.85 kJ mol−1. If the surface area of the used seed crystals is known, the constant linear calcite growth rate, R (mol m−2 s−1), can be easily obtained from the initial slope of the titrant addition versus time curve. However, growth leads to an increase of crystal volume and an accompanying increase of surface area. This causes the slope of the titrant addition versus time curve to increase over time. The first time derivative of the added volume of Ca2+ titrant is denoted here as the (volumetric) crystallization rate (cm3 s−1). fig. 2.2 shows three datasets of constant composition calcite growth experiments (control), and two datasets of similar experiments but with 45 min of ultrasonic treatment (ultrasound). A cubic regression was used to fit the control experiments and their average. The linear growth rate can be derived from the first order constant [31, 32]. The first 15 min show an initial growth surge. This period is excluded from the fit. The growth rate was estimated to be 1.82 mol m−2 s−1 (6.76 10−11) m s−1) (table 2.2). The results in fig. 2.2 and table 2.2 reveal that the experiments were highly reproducible. Furthermore, the measured growth rate corresponds well with values found in literature [33]. This suggests that the experimental set-up is suitable to study the effect of ultrasound on the crystallization of calcium carbonate. After the ultrasonic irradiation is started, the slope of the ultrasound curve starts to increase faster compared to the slope of the control curves fig. 2.2. This effect is

21 2. Seeded calcite sonocrystallization

Figure 2.3.: Crystallization rate (volumetric) defined as the time derivative of the added volume of Ca2+ titrant (cm3 s−1) (see fig. 2.2). A moving average (span = 15 of 448 data points) was used on all datasets. more clearly seen in fig. 2.3 where the crystallization rate of the average values of control and ultrasound datasets are plotted as function of time. At the start of the ultrasonic treatment at t =30 min, the crystallization rate of the ultrasonic treatment shows a clear increase compared to the control curve. After termination of the ultrasonic treatment (t = 75 min), when all conditions are exactly the same compared to the control experiments, the ratio of both crystallization rates remains the same. This is demonstrated in fig. 2.4 where the ratio between the ultrasound and control crystallization rates is shown. Before the ultrasonic treatment, this ratio is close to 1 during the first 30 min. Then, the figure shows a clear increase of precipitation rate during the treatment and approaches a final value of approximately 46% after the treatment. After termination of the ultrasonic treatment, there is no decline in precipitation rate. This indicates that the enhanced precipitation rate is caused by permanent physical changes of the seed crystals, and not by a temporarily enhancement of the liquid-solid mass transport. The retardation effect reported by Dalas [15] was not observed. The precipitation rate already increased during the ultrasonic treatment. It is possible that this suggested retardation effect was camouflaged because of the relatively large increase in crystal reactivity. The used calcite seeds and supersaturation levels in both studies are quite similar. However, the differences in the ultrasonic field applied and the way

22 2.3. Results and discussion

Figure 2.4.: Ratio of crystallization rates: ultrasound/control (-). The increase in crystallization rate caused by the ultrasonic irradiation is approx. 46%. the pH was measured during the ultrasonic irradiation differed markedly. Therefore, the causes of the contradictory observations remain unclear. The SEM pictures of the crystals at the end of the growth experiments (t = 150 min) showed no clear difference between the control and ultrasound experiments. Never- theless, the effect of ultrasonic irradiation on the crystals could easily be observed from the samples taken during the treatment period in a saturated solution (without growth). In figs. 2.5 and 2.6, SEM pictures of, respectively, untreated and treated seed crystals in a saturated solution are shown. These pictures are a cross-selection of 21 pictures taken from 7 samples. The appearance of some fines in fig. 2.5B can be explained in two ways. First, the introduction of the dry seed crystals probably led to some initial breeding. Second, the seeds may have experienced some attrition due to the pumping and stirring action in the set-up. Attrition may contribute to the increasing slope of the control growth curves. In Fig. 6, seed crystals treated with ultrasound are shown. It can be seen that in the first period the interpenetrated conglomerates were disrupted. At the end of the ultrasound treatment, numerous fine particles and many damaged crystals were observed. Because supersaturation was absent in these experiments, the appearance of these fine particles can be subscribed to the attrition and breakage of the parent crystals during the ultrasonic treatment. Although the same mechanism is expected to have played a pivotal role in the actual growth experiments, it is not possible to exclude nucleation mechanisms driven by supersaturation. The production of fines and the erosion of the parent seed crystals by the action of

23 2. Seeded calcite sonocrystallization

Figure 2.5.: SEM pictures of untreated seed crystals in a saturated solution: (A) starting seeds and (B) after 60 min stirring and pumping. the ultrasonic irradiation caused a rapid increase of the total surface area available for growth. It is likely that the rapid healing of the fractured parent crystals and the outgrowth of fines after the ultrasonic irradiation period in the growth experiments camouflaged any differences in crystal habit at the end. Furthermore, the wide crystal size distribution of the seed crystals makes it difficult to distinguish grown particle fragments from original small seeds. Besides calcite, other polymorphs of calcium carbonate are aragonite and vaterite. Although the preference for the calcite polymorph is thermodynamically favored under the experimental conditions of the bulk liquid (25 ◦C, 1 atm pressure) [29], the local conditions in the vicinity of an imploding cavity are significantly different [23]. It is questionable, however, if the change in local conditions is maintained for a sufficient time to allow the growth of a different polymorph. In addition, the presence of calcite seeds kinetically dictates the growth of this polymorph. Also, ATR-FT-IR spectra of the grown seeds did not show any characteristic adsorptions other than those for calcite. Therefore, it is reasonable to expect that no other polymorphs grew besides calcite. The particle size distributions by number of the treated and untreated seed crystals are shown in fig. 2.7. Note that the lower detection limit of the particle size measurement was 1.5 µm. Therefore, the large number of fines present in the treated samples according to the SEM pictures did not contribute to these distributions. Although the small seeds and seed fragments were not counted, there is a shift in the distributions. The arithmetic (number-length) mean of the number distribution (fig. 2.7A) shifts

24 2.4. Conclusion

Figure 2.6.: SEM pictures of seed crystals treated with ultrasound in a saturated solution: (A) after 5 min and (B) after 45 min of ultrasonic irradiation. from 10.3 to 8.9 µm. This indicates that there is a decrease in the number of larger particles (note the drop in the distribution between 10 and 20 µm) and the number of particles in the lower part of the distribution increases (1.5-3.2 µm). This shows that the seed crystals are fragmented into smaller parts. These observations are in line with the SEM analysis. The SEM and particle size analysis showed that the physical eﬀects that occurred during ultrasonic irradiation gave rise to a change in the habit and particle size of the calcite crystals. The resulting increment of the surface area is a major factor in the observed growth rate enhancement. In addition, it can be assumed that the density of active growth sites on the fractured crystal surfaces is much higher and facilitates a higher growth rate compared to the ripened smooth crystal surfaces. It is well known, however, that these fractured crystal surfaces rapidly heal and proceed to grow at a much lower rate [34]. This might explain why the ratio between the ultrasound and control crystallization rates in ﬁg. 2.4 is still increasing right after termination of the ultrasonic treatment before it reaches a constant value.

2.4. Conclusion

The eﬀect of ultrasound on the seeded calcite crystallization was investigated. A dedicated experimental set-up based on the constant composition method gave highly reproducible results. It was shown that the volumetric crystallization rate was increased by 46% after 45 min of ultrasonic irradiation (17 W dm−3). Control ex-

25 2. Seeded calcite sonocrystallization

Figure 2.7.: Particle size distribution (equivalent area diameter) by number of the untreated and treated seed crystals. periments without supersaturation showed that during the irradiation period, seed crystals were disrupted and many ﬁnes appeared. SEM and particle size analysis showed that it is possible to alter the habit and size of the calcite crystals through ultrasonic irradiation. The increased surface area available for crystal growth resulted in the observed crystallization rate enhancement.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development a Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Concentrates” and theme “Sensoring” for the discussions and their financial support.

References

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26 2.4. References

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[18] N. Lyczko, F. Espitalier, O. Louisnard, and J. Schwartzentruber. “Effect of ultrasound on the induction time and the metastable zone widths of potassium sulphate”. In: Chem. Eng. J. 86.3 (2002), pp. 233–241 (cit. on p. 16). [19] I. Nishida. “Precipitation of calcium carbonate by ultrasonic irradiation”. In: Ultrason. Sonochem. 11.6 (2004), pp. 423–428 (cit. on p. 16). [20] L. H. Thompson and L. K. Doraiswamy. “The rate enhancing effect of ultrasound by inducing supersaturation in a solid-liquid system”. In: Chem. Eng. Sci. 55.16 (2000), pp. 3085–3090 (cit. on p. 16). [21] C. Virone, H. J. M. Kramer, G. M. Van Rosmalen, A. H. Stoop, and T. W. Bakker. “Primary nucleation induced by ultrasonic cavitation”. In: J. Cryst. Growth 294.1 (2006), pp. 9–15 (cit. on p. 16). [22] A. Kordylla, T. Krawczyk, F. Tumakaka, and G. Schembecker. “Modeling ultrasound- induced nucleation during cooling crystallization”. In: Chem. Eng. Sci. 64.8 (2009), pp. 1635–1642 (cit. on p. 16). [23] Y. T. Shah, A. B. Pandit, and V. S. Moholkar. Cavitation reaction engineering. Springer, 1999 (cit. on pp. 16, 24). [24] S. J. Doktycz and K. S. Suslick. “Interparticle collisions driven by ultrasound”. In: Science 247.4946 (1990), pp. 1067–1069 (cit. on p. 16). [25] K. Suslick. “Sonochemistry”. In: Science 247.4949 (1990), pp. 1439–1445 (cit. on p. 16). [26] S. S. Berdonosov, I. V. Znamenskaya, and I. V. Melikhov. “Mechanism of the vaterite- to-calcite phase transition under sonication”. In: Inorg. Mater. 41.12 (2005), pp. 1308– 1312 (cit. on p. 17). [27] T. F. Kazmierczak, M. B. Tomson, and G. H. Nancollas. “Crystal growth of calcium carbonate. A controlled composition kinetic study”. In: J. Phys. Chem. 86.1 (1982), pp. 103–107 (cit. on p. 17). [28] M. B. Tomson and G. H. Nancollas. “Mineralization kinetics: A constant composition approach”. In: Science 200.4345 (1978), pp. 1059–1060 (cit. on p. 17). [29] M. M. Reddy and G. H. Nancollas. “The crystallization of calcium carbonate: I. Iso- topic exchange and kinetics”. In: J. Colloid Interface Sci. 36.2 (1971), pp. 166–172 (cit. on pp. 17, 24). [30] S. Brunauer, P. H. Emmett, and E. Teller. “Adsorption of gases in multimolecular layers”. In: J. Am. Chem. Soc. 60.2 (1938), pp. 309–319 (cit. on p. 17). [31] J. N`yvltand M. Matuchová.“Determination of linear growth rates of crystals (II). The shape factors method”. In: Krist. Tech. 11.3 (1976), pp. 245–253 (cit. on p. 21). [32] M. Matuchováand J. N`yvlt.“Determination of linear growth rates of crystals (I). Calculation of linear growth rates of individual crystal faces from overall rates”. In: Krist. Tech. 11.2 (1976), pp. 149–161 (cit. on p. 21). [33] A. R. Hoch, M. M. Reddy, and G. R. Aiken. “Calcite crystal growth inhibition by humic substances with emphasis on hydrophobic acids from the Florida Everglades”. In: Geochim. Cosmochimi. Acta 64.1 (2000), pp. 61–72 (cit. on p. 21). [34] J. Mullin. Crystallization. A Butterworth-Heinemann Title, 2001 (cit. on p. 25).

28 3 Visualization of acoustic cavitation eﬀects on suspended calcite crystals

abstract The acoustic cavitation (42,080 Hz, 7.1 W cm−2 or 17 W) effects on suspended calcite crystals, sized between 5 and 50 µm, have been visualized for the first time using high speed photography. High speed recordings with a duration of 1 s containing up to 300,000 frames per second, revealed the effect of cluster and streamer cavitation on several calcite crystals. Cavitation clusters, evolved from cavitation inception and collapse, caused attrition, disruption of aggregates and deagglomeration, whereas streamer cavitation was observed to cause deagglomeration only. Cavitation on the surface gave the crystals momentum. However, it is shown that breakage of accelerated crystals by interparticle collisions is unrealistic because of their small sizes and low velocities. Crystals that were accelerated by bubble expansion, subsequently experienced a deceleration much stronger than expected from drag forces, upon bubble collapse. Experiments with pre-dried crystals seemed to support the current theory on bubble nucleation through the presence of pre-existing gas pockets. However, experiments with fully wetted crystals also showed the nucleation of bubbles on the crystal surface. Although microjet impingement on the crystal surface could not be directly visualized with high speed photography, scanning electron microscopy (SEM) analysis of irradiated calcite seeds showed deep circular indentations. It was suggested that these indentations might be caused by shockwave induced jet impingement. Furthermore, the appearance of voluminous fragments with large planes of fracture indicated that acoustic cavitation can also cause the breakage of single crystal structures.

This chapter has been published as: R.M. Wagterveld, L. Boels, M.M. Mayer, G.J. Witkamp. Visual- ization of acoustic cavitation eﬀects on suspended calcite crystals. Ultrasonics Sonochemistry 18 (2011) 216-225.

29 3. Visualization of cavitation on calcite crystals

3.1. Introduction

When ultrasound is passed through a liquid it produces pressure waves which cause negative pressure during the rarefaction stage. This negative pressure can lead to rupture of the fluid and is accompanied by the generation of cavities (microbubbles). The rupture of fluid and the interaction of acoustic waves with these bubbles are called acoustic cavitation. The varying pressure makes the bubbles oscillate in size. These oscillations are low energetic and this phenomenon is called non-inertial cavitation (or stable cavitation). Above a certain negative pressure threshold, this will lead to inertial cavitation (or transient cavitation), which is the high energetic fast growth and collapse of bubbles [1]. Besides the pressure constraint, inertial cavitation can only occur if the radius of the initial cavity lies between a lower and upper critical value for cavitation. With decreasing frequency of the applied ultrasound, this size range increases which results in more inertial cavitation [2]. In many practical systems the inception of cavitation is observed at much lower tensile stress than calculated using the classical cavitation nucleation theory. The generally accepted theory is that the cavitation inception is mediated by cavities already present in the liquid. The existence of these nucleation sites is usually subscribed to gas pockets in crevices on surfaces and particles. The gas pockets arise by the introduction of water in the container, or by introduction of dry particles into liquid. Some reported experiments, however, cannot be explained by this theory [3]. Mørch developed a slightly modified theory based on surface structure [4]. In this theory it is assumed that in case of concave surface structures, stress arises at the liquid solid interface. Mørch stated that in this situation, the liquid ruptures at lower tensile stress, without the need of gas pockets. When ultrasound is passed through a liquid-solid system, bubble cavitation causes a series of unique physical phenomena that can affect the solid. Microjets and high energetic shockwaves are produced by inertial cavitation. Shockwaves are formed when cavities rapidly expand or collapse. Recently, it has been shown experimentally that spherical particles can be accelerated significantly by the shockwave that occurs during the explosive growth of a cavity on the particle surface (cavitation inception) [5, 6]. During the collapse of a cavity, high local temperatures and pressures arise which result in a pressure shockwave [7–12]. Shockwaves may cause mechanical damage to close objects and are known to cause material erosion [1]. This phenomenon is used in lithotripsy to fragment kidney stones [13–16]. The collapse of bubbles close to a large

30 3.1. Introduction rigid boundary will be asymmetric and leads to microjet formation in the direction of the surface. Microjets can result in erosion and pitting of the surface [17]. These phenomena make ultrasound a powerful tool to influence many physical and chemical processes. In sonocrystallization, ultrasound is used to help controlling the course of crystallization processes. Sonocrystallization received considerable attention in the last decade. Most of the reported positive effects of ultrasound on crystallization processes are related to the process of crystal nucleation [18]. Here, the focus is on the physical effects that ultra sound has on existing crystals. In this paper a distinction is made between agglomerates and aggregates using the following definitions. An agglomerate is an assemblage of particles which are loosely coherent, and an aggregate is an assemblage of particles rigidly joined together (ISO14887 [19]). Shockwaves can reduce agglomeration. Ultrasonic treatment of crystals might lead to erosion which can change the crystal habit [20, 21]. Recently, the authors reported that ultra sound can increase the volumetric crystal growth rate of calcite significantly [22] (chapter 2). It was shown that this effect was related to the alteration of the seed crystals’ habit and size. Seed crystals seemed to be subjected to attrition and breakage during ultrasonic irradiation. Although it was suggested that the rupture of agglomerates occurs in close proximity of a collapsing bubble [23], no direct visualization of this phenomenon has been reported yet. One attempt was made by Guo et al. who investigated the acoustic effects on large sugar crystals. It was suggested that both the collisions of crystals and vibration and implosion of cavitation bubbles lead to crystal deagglomeration and breakage [24]. However, the reported observations were recorded with a frame rate of only 4500 fps. This is far too low to capture the 20,000 Hz acoustic phenomena (4 cavitation cycles per frame) that actually caused the observed breakage and deagglomeration of the sugar crystals. Several in situ methods have been used to investigate the effect of cavitation on particles or surfaces, like high speed (HS) imaging (in combination with schlieren/shadow imaging [7–10]) and optical beam deflection (OBD) measurements [9, 11]. Further- more, Scanning Electron Microscopy (SEM) analysis can be used to investigate the resulting particles or surfaces after ultrasonic treatment, to show attrition or breakage of solids, deagglomeration and pitting [25–27]. In high speed photography, most of the research involved large surfaces [28–34] and relatively large (metal) particles [5, 6, 24, 35]. Also, high speed photography was used to study bubble-bubble interaction [36], cavitation clusters [37, 38] and resonant bubbles [39–43]. The aim of the present work is to identify the cavitation phenomena that are responsible for crystal deagglomeration, disruption of aggregates, attrition and breakage

31 3. Visualization of cavitation on calcite crystals that occurs during seeded calcite sonocrystallization. For this purpose, the physical interaction of acoustic cavitation with suspended calcite crystals was visualized using high speed photography. In addition, SEM analysis was used to investigate the habit of treated crystals.

3.2. Experimental

3.2.1. Materials

Two batches of calcite seed crystals were prepared. The ﬁrst batch was synthesized, using the method described by Nancollas and Reddy [44], by slowly adding 2 dm3 of 3 a 0.20 M CaCl2 (VWR, 100%) solution to 2 dm 0.20 M Na2CO3 (Boom, >99.5%) solution at 25 ◦C. The second batch was synthesized by adding both solutions simultaneously at 25 ◦C. The batches were denoted as A and B respectively. The freshly precipitated seed crystal batch A and part of batch B, hereafter B1, were aged overnight in mother liquor and were subsequently washed with Milli-Q water each day for one week. Afterwards, the washed seed crystals were aged for three weeks before ﬁltering. Seeds A were dried at 105 ◦C, and seeds B were stored in Milli-Q. The other part of batch B, B2, was also washed with Milli-Q by carefully replacing the mother liquid by Milli-Q water leaving the settled crystals always submerged in liquid. After washing the seeds each day for one week, the seeds were stored in Milli-Q water. The crystals were characterized by Scanning Electron Microscopy (Jeol JSM-6480 LV) and ATR-FTIR spectroscopy (Shimadzu 4800).

3.2.2. Experimental set-up and procedures

In a small square glass box, 0.05 g of calcite seeds were suspended in 100 cm3 of a saturated CaCO3 solution (1.6 mM NaHCO3 (Boom, 99.5%), 1.6 mM CaCl2, 95 mM KCl (VWR, 100%)). All experiments were performed at ambient conditions. First, the suspension was allowed to equilibrate with the atmosphere for one day at room temperature. The ultrasonic irradiation period did not exceed 10 s in order to minimize the degassing and heating effect of the ultrasound. After the experiments, samples of 15 cm3 of solution were collected and filtered through a 0.2 µm filter. After drying, the samples were analyzed with SEM. The interaction of cavitation with the calcite seeds was recorded with a high-speed camera (Photron Fastcam SA1.1 model 675K-M1), which can take up to 675,000 frames per second (fps). The camera was equipped with a 12x zoom lens (Navitar).

32 3.2. Experimental

The focal point was adjusted just below the transducer tip. A high powered light source (Karl Storz, Techno Light 270) served as back illumination for the high-speed camera fig. 3.1. The ultrasonic transducer was part of a dedicated homebuilt system that could be controlled precisely in terms of shape, frequency and amplitude of the alternating current for driving the transducer. The transducer was operated at 42,080 Hz. The real output power generated from the transducer was estimated by measuring the adiabatic temperature rise of water irradiated with ultrasound. The input power was determined to be 17 W or 7.1 W cm2. Just before the experiment was initiated, settled calcite seeds were mixed by filling a syringe with 50 ml of the saturated solution and injecting it back into the box. When the calcite suspension was irradiated, a 1 s record was made of the suspension. Afterwards, an image was taken of an object with known size (needle) in order to determine the scale of the images. Due to the applied frequency of 42,080 Hz the frame rates were chosen to be at least twice as high, starting with 100,000 fps. At this frame rate, the maximum resolution was 320x128 pixels. Increasing the frame rate resulted in a lower image resolution. In order to obtain enough detail of the cavitation effects, a magnifying lens was used. However this reduced the field of view and depth of field. Because the exact location of cavitation interacting with suspended crystals could not be controlled nor predicted, acquiring useful images was rather tedious and required a factor of luck. The particle velocity, v (m s−1), and acceleration, a (m s−2), were determined from the movies. For each frame the center position of the particle of interest was measured, the displacement from frame to frame was calculated and multiplied by the frame rate, fps (s−1), according to:

q 2 2 vN+0.5 = (xN+1 − xN ) + (yN+1 − yN ) fps (3.1)

here x (m) is the x-coordinate of the particle of interest and y (m) is the y- coordinate. In a similar fashion the acceleration was determined from the acquired velocities:

aN+1 = (vN+1.5 − vN+0.5) fps (3.2)

In order to investigate the eﬀect of ultrasound on the crystal habit, 0.5 g seed crystals were added to a 1 dm3 saturated calcium carbonate solution in a double walled thermostatted glass reactor equipped with a ﬂoating magnetic stirrer bar to

33 3. Visualization of cavitation on calcite crystals

Figure 3.1.: Scheme of the experimental set-up consisting of: (1) ultrasonic transducer, (2) high power light, (3) high-speed camera with zoom lens, and (4) glass box. minimize any grinding effects. The seed suspension was stirred at 400 rpm and treated with ultrasound for 45 min at 25 ◦C. After the treatment, samples of 15 cm3 of solution were collected and filtered through a 0.2 µm filter. After drying, the samples were analyzed with SEM.

3.3. Results and discussion

3.3.1. Seed characterization

The seeds of batch A are the same seeds that were used in the seeded calcite sonocrystallization experiments reported in our previous work [22] (chapter 2). These seeds appeared as interpenetrated conglomerates or intergrowths (here after aggregates) made of several small rhombohedral particles (fig. 3.2A). The seeds of the second batch had a quite different appearance (fig. 3.2B). In contrary to seeds A, seeds B hardly contained aggregates, but mainly consisted of separate, much larger particles. This proved to be advantageous in the high speed recordings, because of three reasons. First, it is possible to discriminate between deagglomeration and particle breakage/erosion. Secondly, the seeds could be better distinguished from the bubbles which allowed for better insight in the mechanism of cavity inception and bubble collapse. Thirdly, SEM analysis of larger seeds could reveal more detail on particle erosion. The ATR-FTIR spectra showed characteristic adsorptions for calcite at 1795, 1392, 871 and 711 cm−1 for all batches.

34 3.3. Results and discussion

Figure 3.2.: SEM pictures of seed crystals: (A) seed crystals A (from [22] with permission from Else- vier), and (B) seed crystals B.

3.3.2. Bubble structures

It was observed that bubbles can form different structures which are described in literature [37]. First, there were streamers, located at the edges of the source and near the surface of the source, accompanied by small clusters (fig. 3.3A). Secondly, large clusters appeared at some distance away from the source (fig. 3.3B). Mettin [37] mentions that these clusters probably exhibit large erosion potential but also concluded that there is a lack of experimental data supporting this. In every conducted experiment, it was observed that the formation of a large cluster always starts with the inception of a single cavity on the surface of a particle. Bubbles in streamers did not necessarily start on the surface of seed crystals and the cavitation was also less intense compared to large clusters. Therefore, the influence of cavitation on particles is expected to be much less in case of streamers.

3.3.3. Disruption of aggregates and deagglomeration

In our previous work [22] (chapter 2) it was shown that seeds A are easily disrupted upon ultrasonic treatment. The next image series shows the fragmentation of a calcite seed (type A) by a large cluster (ﬁg. 3.4). In the second frame (t = 8 µs), a trace of a cavity is visible, which is the actual inception of the bubble which evolves into a cluster. The third frame shows the appearance of a small fragment, which indicates that cavitation causes crystal fragmentation. After 32 µs, the imploded cavity splits

35 3. Visualization of cavitation on calcite crystals

Figure 3.3.: Streamer (A) and large clusters (B). Image A is recorded without seed crystals in the liquid, for better distinction of the bubbles from the seeds. Cluster formation always started on the surface of a particle. Part of the ultrasonic source is visible (dark silhouette) at the top of both images. Scale bar is 500 µm (both images). up, expands again, merges and collapses (t = 40 µs). Now two fragments appear, clearly broken off the crystal. In the next cavitation cycle the first indication of the cluster is visible (last frame) that appears shortly after. Occasionally, crystals were caught in the moving cluster and sometimes underwent interparticle collisions. While the velocity of the clusters was 1.5 m s−1 (determined by the displacement of the center of the cluster), the velocities of captured crystals inside the cluster ranged from 1.5 to 4.0 m s−1. As soon as the particles were no longer under the influence of cavitation, they lose velocity fast and collisions of these particles were no longer observed. The crystal shown in fig. 3.4, was dried before it was submerged in the saturated solution. Probably, small gas cavities already present on the crystal facilitated bubble inception during ultrasonic irradiation. It is interesting to see if this bubble inception also occurs on completely wetted crystals. Several experiments were conducted with crystals B2, and in each experiment much less large cluster formation was encountered (only a few in 1 s of recording) compared to the experiments with initially dry crystals A (hundreds in 1 s of recording). A possible explanation for the inception of clusters on these crystals could be found by the theory developed by Mørch [45] which was proven experimentally [5, 46]. Due to the large size of streamers, they were able to pick up seed crystals from the

36 3.3. Results and discussion

Figure 3.4.: Images of an oscillating cavitation bubble that nucleated on the surface of a type A calcite crystal. The single crystal fragmentizes as a result of the violent collapse of the bubble. Frame rate: 125,000 fps. This movie is available with the online version of the paper. solution and caused deagglomeration. The mechanism proposed by Kusters [23] has great similarities with the observations shown in fig. 3.5. A small agglomerate of a seed type B (arrow 1), was picked up by a streamer and moved into the field of view in the first frame. In the second frame (t = 8 µs) a bubble from the streamer expands on the agglomerate causing it to turn. The bubble (arrow 2) collapses between frame 3 and 4 (t = 16 µs and t = 24 µs) and separates the agglomerate in two single crystals. In the following cavitation cycle the bubbles hardly expand. In the subsequent cycle (t = 56 µs) another bubble expands and collapses on the surface of the agglomerate, causing the two parts to move away from each other (arrow 3). In order to capture the bubble dynamics in more frames, several recordings were made at a frame rate of 300,000 fps. One of these recordings is shown in fig. 3.6. The images reveal that cavitation does not necessarily have to start on the surface of a particle to cause deagglomeration. It is shown that a bubble (arrow in fig. 3.6) expands close to the surface of a seed B, touches it and collapses in close proximity of the surface. This causes the particle to separate into two large parts. Although the higher frame rate reveals more of the bubble dynamics, the lower resolution makes it difficult to judge whether the fragments were part of an agglomerate or a single crystal.

3.3.4. Seed acceleration by bubble expansion and collapse

Recently, the acceleration of particles by the inception of cavitation on the surface was reported [5, 6]. Figure 3.7 shows that particle acceleration can also occur by the attachment and detachment of a pre-existing cavitation bubble. In the ﬁrst frame,

37 3. Visualization of cavitation on calcite crystals

Figure 3.5.: Images of streamer cavitation causing deagglomeration of type B1 calcite crystal (arrow 1). The agglomerate (length: ≈ 49 µm) splits up (arrow 3) as a result of the bubble collapse (arrow 2). Frame rate: 125,000 fps; shutter time: 1 µs, scale bar is 50 µm. This movie is available with the online version of the paper.

38 3.3. Results and discussion

Figure 3.6.: Rupture of type B1 calcite crystal (length: ≈ 41 µm), shot at 300,000 fps, scale bar is 53 µm. This movie is available with the online version of the paper. a cavitating bubble (arrow 1) approaches a crystal (of type B1, arrow 2). Upon bubble expansion (frames t = 20 and t = 30) the bubble wall pushes the crystal away. The bubble collapses, and remains attached to the surface (frame t = 40). In the next frame, the bubble expands again and the crystal accelerates. After collapse, the bubble splits; part of the bubble moves away from the crystal and another part stays attached to the surface. Both bubbles continue to cavitate, and every expansion of the attached bubble leads to movement of the crystal. After the third expansion (frame t = 110), the bubble completely detaches (frame t = 120) and the crystal obtains its highest average velocity of 1.6 m s−1 between these frames, by accelerating with an average of approximately 60,000 m s−2. The particle immediately slows down in the next frame, with an average acceleration of approximately -90,000 m s−2, to an average velocity of 0.7 m s−1 (see also ﬁg. 3.8). It seems that the strong deceleration is not only caused by drag forces. The drag force on small particles moving through ﬂuids at relatively low speeds, where there is no turbulence, can be described by Stokes’s law. From this law, a general equation of motion can be derived, assuming the liquid velocity to be zero [47]:

− t v(t) = v0e τ (3.3)

−1 where v0 (m s ) is the initial velocity, t (s) time, and τ (s) the relaxation time.

The relaxation time is a constant, deﬁned by material and ﬂuid properties. Using ρs (kg m−3) the density of the solid, η (Pa s) the liquid viscosity and d (m) the spherical particle diameter gives:

ρ d2 τ = s (3.4) 18η

39 3. Visualization of cavitation on calcite crystals

Figure 3.7.: An example of crystal (B1, length: ≈ 36 µm, arrow 2) acceleration by bubble (arrow 1) expansion and collapse. Frame rate: 100,000 fps. Scale bar length is 100 µm. The cavitation cycle is 23 µs which is slightly more than two frames. One frame shows an instant of bubble growth followed by the next frame showing an instant of bubble collapse. This movie is available with the online version of the paper.

40 3.3. Results and discussion

Stokes’s law can only be used for Reynolds numbers smaller than 1. At maximum velocity the particle’s Reynolds number is 65 and Newton’s law has to be applied, resulting in the following relaxation time:

4 ρ d2 τ = s (3.5) 3 CDηRe

24 C = 1 + 0.15Re0.687 (3.6) D Re

where Re (-) is Reynolds number and CD (-) the drag coefficient, described by an empirical relation for 1 < Re < 1000 (eq. (3.6)). The new relaxation time is no longer constant because Re changes with velocity. However, an outer limit of maximum drag can be defined by assuming Re to be constant at its maximum value. In reality the velocity profile will be between Stokes’ law and the outer limit of maximum drag calculated with Newton’s law, here defined as the drag window. In fig. 3.8, the measured velocity profile of the accelerated crystal B1 particle (after frame t = 110 µs) and the profiles calculated according to Stokes and Newton are shown. The initial deceleration from 1.6 to 0.7 m s−1 is much faster than calculated and lies far outside the drag window. Therefore this deceleration is not (only) related to drag, but to an additional external force. Because the crystal is not fixed it may get a superimposed velocity towards the centre of the collapsing bubble within the surrounding liquid [48]. The detachment of the bubble is probably late in the growth cycle and the particle is prone to the influence of bubble collapse, causing the particle to decelerate fast, which explains the sudden decrease in velocity in fig. 3.8. After the initial drop in velocity, the profiles can be recalculated (fig. 3.8B). The experimental values lie inside the drag window, which shows that the gentle decrease in velocity can be subscribed to drag.

3.3.5. Eﬀect of cavitation on crystal habit

In several experiments the appearance of small fragments from single seeds B after a cavitation event could be recorded (attrition). Also, many fine fragments can be seen in the SEM images of the treated seeds (fig. 3.10A), which indicate crystal attrition. On the other hand, several large broken crystal fragments were identified with SEM analysis of seeds B that were irradiated for 45 min (fig. 3.9). These images indicate that “large breakage” occasionally occurs during ultrasonic irradiation. However, despite our efforts, none of the experiments with seeds B resulted in a recording in

41 3. Visualization of cavitation on calcite crystals

Figure 3.8.: Velocity profile of accelerated crystal B1 from fig. 3.7. The velocity at t = 0 µs corresponds to the velocity between t = 110 and 120 µs in fig. 3.7. The arrow indicates the large decrease in velocity right after bubble collapse. The dashed line is calculated with Stokes, the dotted line with Newton (outer limit of maximum drag). Calculation based on the (initial) velocity at, (A) t = 0 s (Re = 65), and (B) t = 10 µs (Re = 28). which single crystal structures were broken. Single crystal breakage can only occur, simply stated, if the energy input is higher than the energy required for maximum elastic deformation. Above this barrier, the stress in the crystal is released by plastic deformation and crack growth. In brittle materials, most of the energy goes to crack growth, and the plastic deformation volume is relatively small. During crack growth, material bonds break, and thereby two new surfaces are created. The energy necessary to create a new surface is defined as the fracture surface energy [50]. Less energy is required to create small fragments (e.g. breakage of crystal corners, attrition) compared to large fragments (i.e. total crystal breakage). A considerable discrepancy exists between theoretical surface fracture energies, defined by the material bond strength, and experimentally observed surface fracture energies. This is subscribed to Griffith cracks which are small flaws in the material. The number of Griffith cracks increases with increasing particle volume, causing large particles to break more easily than small particles [51]. Particle breakage by collision depends on impact velocity and particle size. The breakage of gypsum and limestone particles, 100 µm in diameter, was reported to occur at a minimum impact velocity of 13 and 23 m s−1 respectively. Furthermore, when the particle diameter decreases a factor two, the necessary impact velocity is doubled [51]. The maximum velocities measured in our experiments are well below

42 3.3. Results and discussion

Figure 3.9.: SEM pictures of voluminous fragments of seeds B1 with large planes of fracture. these reported values, and the used seeds are also much smaller than 100 µm. In addition, calcite is harder than gypsum (respectively 3 and 2 on Mohs scale [52]) and will be more difficult to break. Although flaws in the crystal structure may cause the crystals to break at reduced impact velocity, it is very unlikely that breakage occurred directly through collision. Two other possible mechanisms can be the cause of crystal breakage; interaction with shockwaves and interaction with microjets (or a combination of both). Shock- waves are produced during inception and at collapse [7–9]. If the collapse takes place in the vicinity of a large rigid surface the collapse will be asymmetric leading to microjet formation. When a bubble collapses on a surface, both a microjet and a shockwave are formed simultaneously. According to Shima [8], the mechanism con- tributing the most to erosion or breakage depends on the shape and the contact angle of the bubble at maximum expansion. Lithotripsy experiments with artificial kidney stones of gypsum showed that the induced shockwave is responsible for total breakage and that microjets, formed during cavitation, cause pitting [16]. The breakage mechanism (brittle breakage) of calcite and kidney stones should be similar. However, the differences in shock intensity and particle size between the reported lithotripsy experiments and the current experiment should not be neglected. In lithotripsy, the shockwave is externally generated and the intensity can be controlled. In the current work (acoustic cavitation), shockwaves were produced by the inception and implosion of cavitating bubbles and the intensity could not be determined. To capture microjet dynamics, very high frame rates are necessary, which consequently results in poor

43 3. Visualization of cavitation on calcite crystals

Figure 3.10.: SEM pictures of damaged calcite seeds. Possible broken aggregates (A, B); circular indentations caused by ultrasound (B, C) and by laser ablation (D) in calcite. (From [49], with kind permission from Springer Science + Business Media). quality recordings with the current experimental equipment. SEM analysis of treated seeds could provide more information on the breakage mechanisms. The images in ﬁg. 3.10 and the data that was reported in our previous work [22] (chapter 2) suggest that the interaction of calcite seeds with cavitation bubbles results in particle fragmentation. It is reasonable to expect that aggregates break at their weakest points, most likely the points where individual crystals are joined together. Figure 3.10A and B show possible aggregate breakage plains. What is striking in ﬁg. 3.10B is the circular indentation in the centre of the crystal, which was not subscribed to a growth defect due to its jagged appearance. Also, the morphology of this hole shows a close resemblance with holes produced by laser ablation of calcite

44 3.3. Results and discussion

Figure 3.11.: Microjets captured at >250,000 fps.

(fig. 3.10C and D respectively) [49]. In the latter case, a circular hole was made by a short, high intensity laser pulse which removed some material leaving a circular indentation. Although the sizes of the holes are different, the morphology inside the holes is remarkably similar. The appearance of the holes as seen in fig. 3.10B and C was not very common. Only a handful was noticed on several samples of the treated calcite suspension, containing hundreds of (broken) crystals and larger fragments. It was shown that high energetic collisions between small calcite particles are unlikely to occur, due to the low particle velocities. Therefore, it is plausible that these holes were caused by one of the inertial cavitation phenomena; shockwave or microjet impingement. Microjet impingement is known to leave behind characteristic microscopic holes in various surfaces [27, 53], resembling the holes in fig. 3.10B and C. The majority of these studies, however, comprise jetting on large rigid surfaces. The seed crystals under investigation are smaller than the resonant bubble size, and, therefore, the surface of the crystal cannot induce distortion of bubble collapse leading to jet formation [54]. However, it has been shown that jet formation can also occur when cavities collapse under influence of a shock wave [55, 56], irrespective of the presence of a surface [57] (and references therein). Jets produced in this way travel in the direction of the shock. In fig. 3.11, several jets are shown which were captured at high frame rates (>250,000 fps). In each case, no large surfaces or bubbles were present in the direction of the jet. This shows that jets were formed during the ultrasonic irradiation of the crystal suspensions, probably due to shockwaves that originated from other collapsing bubbles nearby. Based on this, it is reasonable to suggest that shockwave induced jetting can also occur from collapsing bubbles attached to small crystal surfaces. If the direction of the shock is towards the surface, the jet impinges on the crystal surface which might result in the characteristic holes shown in fig. 3.9. However, despite our efforts, a record of jet impingement on a suspended crystal could not be obtained. Therefore, no direct proof for jetting on the crystals can be presented in the current work, and

45 3. Visualization of cavitation on calcite crystals further research on this topic is required.

3.4. Conclusion

The cavitation phenomena that cause suspended calcite crystals to deagglomerate, disaggregate and accelerate were successfully visualized with high speed photography. Cavitation clusters, evolved from cavitation inception and collapse, caused attrition, disruption of aggregates and deagglomeration. Streamer cavitation was observed to cause deagglomeration and did not show crystal fragmentation potential. Seeds can be accelerated by cavitation on the surface. However, high energetic interparticle collisions between accelerated particles were not observed due to their limited velocity and size. Crystals that were accelerated by bubble expansion, subsequently experienced a deceleration much stronger than expected from drag forces, upon bubble collapse. Experiments with pre-dried crystals supported the theory of bubble nucleation through pre-existing gas pockets, however, experiments with fully wetted crystals also showed bubble nucleation on the crystal surface. The appearance of voluminous fragments with large planes of fracture, as shown by SEM, indicated that acoustic cavitation can cause the breakage of single crystal structures. Also, deep circular indentations were discovered. Although microjet impingement on the crystal surface could not be directly visualized, it is suggested that these indentations might be caused by shockwave induced jet impingement.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Sensoring” and theme “Concentrate” for the discussions and their financial support.

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50 4 Eﬀect of ultrasonic treatment on early growth during

CaCO3 precipitation

abstract The present study focuses on the effect of ultrasound (42 kHz, 7.1 W cm−2) on the free drift precipitation of CaCO3 from a clear, supersaturated solution. To delineate the way ultrasound exerts its effect, we applied different treatment periods (time windows). Applying ultrasound during the first 10 minutes of the experiment did not result in any significant effect which rules out an influence on primary nucleation. The application of ultrasound starting later in the experiment enhanced precipitation of CaCO3. The dominant mechanism responsible for enhanced precipitation is deaggregation during the early growth phase (nuclei to crystals conversion regime). This effect is attributed to shear induced by micromixing and / or shear / stress induced by (supersonic) shockwaves, as a result of cavitation. With ultrasound applied, on-line pH and scattering measurements displayed a reduction in induction time and increase in volumetric precipitation rate. Scanning electron microscope analysis shows that ultrasound increases the total number of particles that has, in addition, a more uniform size distribution compared with the untreated experiment. Consequently the available surface area for growth is higher resulting in a higher volumetric precipitation rate. With and without ultrasound the formed polymorph was predominantly vaterite with small amounts of calcite.

This chapter has been published as: R.M. Wagterveld, H. Miedema, G.J. Witkamp. Eﬀect of ultrasonic treatment on early growth during CaCO3 precipitation. Crystal Growth & Design 12 9 (2012) 4403-4410.

51 4. Eﬀect of US on early growth during CaCO3 precipitation 4.1. Introduction

Calcium carbonate is one of the most abundant minerals on earth, and its scaling propensity is a problem in many industrial water treatment processes. Moreover it is an important raw material in a wide range of industries. Controlling the formation of calcium carbonate is therefore of great interest, and as a result the crystallization of CaCO3 from clear solution (spontaneous precipitation) has been investigated extensively [1–15]. In the initial phase of precipitation, or nucleation, clusters of calcium carbonate obtain a critical size. In the past decade more evidence has pointed to the existence of a two-step nucleation mechanism [16] in which the nucleation of a crystal occurs within stable mesoscopic clusters of dense liquid. These prenucleation phases were also found in calcium carbonate experiments [17, 18]. Calcium carbonate nucleation, in this case the vaterite polymorph, consists of several steps. Prenucleation clusters are formed (∼ 1 nm) and aggregate to nanoparticles with a size distribution of around 30 nm [18]. These particles aggregate and grow out at the expense of others. The nanocrystalline domains share the same three-dimensional (3D) orientation, resulting in the development of a single crystalline structure [18, 19]. Mostly, some time elapses before a measurable amount of the newly formed material is detected, the induction time, or induction period, and it marks the ability of the solution to stay in a metastable state. This induction time is heavily dependent on the measurement method and various methods have been used by others [4–7, 20, 21]. The application of ultrasound during crystallization and precipitation processes receives increasing attention. Ultrasound can help controlling the course of these processes and is also referred to as sonocrystallization [22]. The positive effects obtained in sonocrystallization are usually ascribed to effects caused by cavitation, a phenomenon occurring when a liquid is exposed to high power ultrasound. Cavi- tation is the interaction of (acoustic) pressure waves with cavities (microbubbles), caused by the rupture of liquid in the negative pressure cycle. Microscopic bubbles grow and collapse under the varying pressure field inside the treated liquid. Several effects can occur during this process: the formation of radicals, the generation of shockwaves and microjets, creation of local hotspots of high pressure and temperature, micromixing, macromixing and rise of bulk temperature [23]. Several authors already investigated the effect of ultrasound on the precipitation of calcium carbonate from solution [24–30] but the results are not unambiguous [30]. Previously the authors reported on the effect of ultrasound on the growth phase of calcium carbonate

52 4.2. Experimental Section

[31] (chapter 2). The volumetric crystal growth rate of calcite in a constant composition experiment was increased by ultrasonic treatment. High speed recordings and scanning electron microscopy (SEM) revealed that ultrasound caused the suspended crystals to deagglomerate, disaggregate, and accelerate. Also particle breakage and attrition occurred in these experiments. This created a larger specific surface area available for growth, leading to larger volumetric growth rates [32] (chapter 3). The present study aims to shed light on the basic processes underlying the effects of ultrasound. A prerequisite to pursuit that goal requires a firm definition of terms. We therefore distinguish three stages in precipitation. The first one is primary nucleation, either homogeneous or heterogeneous. In this work this stage is also referred to as “nucleation”, characterized by high supersaturation. The second one is the “early growth” stage when secondary nucleation can take place and crystals grow out to detectable size. During this stage the supersaturation starts to decrease. The last stage is that of “late growth” during which formed crystals continue to grow and the supersaturation is reducing to saturation. In order to delineate the way ultrasound exerts its effect, we applied different treatment periods (time windows). Calcium carbonate formation is monitored by three independent parameters: pH, light scattering, and scanning electron microscopy (SEM).

4.2. Experimental Section

4.2.1. Chemicals

Only analytical grade reagents, grade A glassware and high quality water (MilliQ Reagent Water System, resistivity >18 MΩ·cm) were used throughout the experiments. Calcium chloride (CaCl2 · 2 H2O), sodium bicarbonate (NaHCO3) and sodium hydroxide (NaOH) were obtained from VWR (Amsterdam, the Netherlands) and potassium chloride (KCl) from Sigma-Aldrich (Zwijndrecht, the Netherlands).

4.2.2. Experimental setup

A schematic representation of the experimental setup used is presented in fig. 4.1. A double walled thermostatted glass reactor, equipped with a floating magnetic stirrer bar (Nalgene) to minimize any grinding effects, a by-pass loop and an ultrasonic transducer were used. The ultrasonic transducer was part of a dedicated homebuilt system that could be controlled precisely in terms of shape, frequency and amplitude of the alternating current for driving the transducer. Free drift experiments

53 4. Eﬀect of US on early growth during CaCO3 precipitation

[33] were conducted and nucleation / growth were determined on-line by recording pH (CPS11D, Endress+Hausser, buffer accuracy ±0.02 pH) and static light scattering using a homebuilt sensor. This sensor consists of 3 mW 850 nm diode laser (LDM850/RLJ, Roithner-laser) coupled into a quartz flow-through cuvette (Hellma) and a photodiode (SFH213FA, Osram) placed at an angle of 90◦. To prevent ambient light entering the photodiode and reduce the noise, the sensor, including flow-through cuvette, is placed in an electrically grounded metal box. The current generated by the scattered laser light is amplified (AD711, Analog Devices) and measured with dedicated LabVIEW software (LabVIEW 2010, NI PCI-6251 DAQ). In order to avoid any possible influence or damage by the ultrasonic irradiation, the pH electrode is placed in a glass cell in a by-pass stream. The pH electrode is equipped with a temperature sensor. A positive displacement membrane pump was used to pump the reactor liquid with 1.0 dm3 min−1 through the by-pass.

Figure 4.1.: Experimental setup used consisting of (1) double walled thermostatted glass reactor, (2) ﬂoating magnetic stirrer bar, (3) pump, (4) ultrasonic transducer, (5) light scattering sensor, (6) pH electrode with integrated temperature sensor, (7) sampling port

54 4.2. Experimental Section

Table 4.1.: Solution composition and conditions in the reactor (after mixing). Supersaturation ratio S is deﬁned in eq. (4.2), with respect to the vaterite polymorph at 298 K, pH 8.79 and an ionic strength of 0.10. Calculations based on software: Visual Minteq v3.0; model: Davies.

[CaCl2] [NaHCO3] [KCl] [NaOH] S (mM) (mM) (mM) (mM) (-) 4.0 2.0 86.0 0.2 2.10

4.2.3. Experimental procedures

3 Free drift experiments were started by simultaneously adding 500 cm of CaCl2 and 3 500 cm NaHCO3 solution to the reactor. The experiments were performed in an open reactor. Before addition, the pH was adjusted to the desired value (8.79) by adding NaOH to the former solution, and the ionic strength of the mixture was adjusted to 0.1 by adding KCl to the latter solution. The mixture (see table 4.1 for composition) was stirred at 400 rpm, and the temperature of the solution was maintained at 298 ± 0.1 K by circulating cooling water around the reactor. The nucleation and subsequent growth could now be monitored by a drop in pH and a rise in voltage of the static light scattering sensor. The newly formed crystals were allowed to grow for some time, and samples were taken by filtering 25 cm3 of solution over a 0.2 µm filter (Isopore, Millipore). The dried crystals were characterized by scanning electron microscopy (Jeol JSM-6480LV). Raman spectroscopy (Horiba Jobin-Yvon LabRAM HR) was performed on the dried crystals to confirm the obtained polymorph. Dur- ing the experiments, aliquots of solution were rapidly removed, filtered through a 0.2 µm filter, dried, and analyzed for the calcium content with inductive-coupled plasma spectrometry (Optima 3000XL, Perkin-Elmer) to measure the degree of supersaturation. In all cases the applied ultrasonic frequency was 42150 Hz and the intensity 7.1 W cm−2 or 17 W dm−3 (real output power predetermined by measuring the adiabatic temperature rise in time using a similar well isolated glass reactor filled with 1 dm3 of water). All experiments started with the same initial conditions as given in table 4.1. The fundamental dimensionless driving force for precipitation in electrolyte solutions is defined as follows [34]:

∆µ = v ln S (4.1) RT Here ∆µ (J mol−1) is the change in chemical potential, R (J mol−1 K−1 ) the gas

55 4. Eﬀect of US on early growth during CaCO3 precipitation constant, T (K) is the absolute temperature, v (-) is the number of ions in the formula unit (v = 2 for CaCO3), and S (-) is the supersaturation ratio, which is expressed in terms of activities:

1 IAP v S = (4.2) Ksp

Where IAP is the ion activity product and Ksp is the thermodynamic equilibrium solubility product. The supersaturation is pH- and temperature-dependent and polymorph specific [34]. Not included in the supersaturation ratio is the size-dependent solubility, defined by the Gibbs-Thomson (also known as Gibbs-Kelvin or Kelvin) relation [34]. This relation states that small crystals have a higher solubility than large crystals. As a result of supersaturation, created at the start of every experiment, critical clusters are being formed, a process called (primary) nucleation, followed by the cluster outgrowth and/or agglomeration and eventually resulting in detectable crystals (growth). Apart from SEM measurements, pH recordings are at the heart of this study and the conclusions drawn. A critical reflection on the precision and repeatability of the procedure followed is therefore mandatory. The precision of the pH electrode itself is 0.01 pH unit (n = 10). In addition, we determined the repeatability of the pH recording during the precipitation experiments employing two types of methods (criteria) and using two independent data sets (pH vs time) recorded under identical experimental conditions. The first method calculates the mean absolute error (ideally,

∆pH = 0) of the difference between the pH recorded at time t during the first experiment and the pH recorded at the corresponding time t during the duplicate experiment. For the blank (1) and ultrasound experiment (2), ∆pH (± s.d. with n = 500) is 0.009 ± 0.005 and 0.020 ± 0.017, respectively. The second method relies on the calculated correlation coefficient (ideally, R2 = 1) of the two pH data sets (pH vs pH). For the blank (1) and ultrasound experiment (2), R2 = 0.995 and 0.979, respectively. These results make us conclude that the pH recordings during the experiments are highly repeatable.

4.3. Results

Free drift experiments were performed to investigate the eﬀect of ultrasound on spontaneous precipitation of calcium carbonate. The ultrasonic treatment was started at diﬀerent moments in time and applied during several treatment periods (see bars

56 4.3. Results above the time axes in ﬁg. 4.2). Five regimes can be distinguished:

1. Blank: No ultrasonic treatment applied. In this experiment the ultrasonic source is in the solution, but there is no driving voltage.

2. Ultrasound: Complete ultrasonic treatment, starting when adding the CaCl2 /

NaHCO3 mixture until the end of the experiment after 75 minutes (t = 0 - 4500 s). 3. Ultrasonic treatment applied during the ﬁrst 10 minutes only (t = 0 - 600 s). 4. Ultrasonic treatment applied from 10 to 20 minutes only (t = 600 - 1200 s). 5. Ultrasonic treatment applied from 30 to 40 minutes only (t = 1800 - 2400 s).

Experiments 1 to 5, as described above, can be compared by the pH proﬁles (see section 8.3) as shown in ﬁg. 4.2A, since precipitation of CaCO3 leads to a decrease in pH under the chosen conditions. In addition, particles form and cause scattering of incident light. The amount of scattering per unit of time depends on both the size and number of particles present. The online measurement of scattering is therefore a

(qualitative) measure of CaCO3 precipitation. The scattering profiles of experiments 1 to 5 can be found in fig. 4.2B. It shows the average of scattered light intensity (of 1 second, sampling frequency 100 s−1) of the incident light measured at an angle of 90◦. Scanning electron microscope images (fig. 4.3) provide more information on size and habit of the precipitated crystals.

4.3.1. Ultrasound during entire experiment (0 - 4500 s)

From the onset of the experiments calcium carbonate is formed, releasing protons (see section 8.3). The induction time marks the interval between the onset of the experiment and the moment this pH decline becomes measurable. Apart from the initial deviation due to a slow response of the pH-electrode, note that until 1000 s the pH in both the blank and ultrasound experiment remains fairly constant. This observation indicates that possible (enhanced) CO2 exchange (see section 8.4) does not have a measurable effect on pH. We therefore conclude that CO2 exchange can safely be neglected in our experiments. After the induction time has elapsed, the steepness of the pH profile is related to the volumetric precipitation rate (the amount of CaCO3 precipitated per unit of time). Comparing the pH profiles of the blank and ultrasound experiments, experiments 1 and 2 as described above, reveals that the complete ultrasonic treatment decreases the induction time and increases the volumetric precipitation rate (fig. 4.2A), with much faster dropping of the supersaturation.

57 4. Eﬀect of US on early growth during CaCO3 precipitation

Figure 4.2.: Free drift experiments performed under five different conditions: (A) pH (-) vs. time (s) measurements, and (B) Scattering (A.U.) vs. time (s) measurements. The bars just above the time axis represent the ultrasonic treatment periods corresponding with the following time windows: 1. Blank: No ultrasonic treatment applied; 2. Ultrasound: Complete ultrasonic treatment (t = 0 - 4500 s); 3. Ultrasonic treatment applied during the first 10 minutes only (t = 0 - 600 s); 4. Ultrasonic treatment applied from 10 to 20 minutes only (t = 600 - 1200 s); 5. Ultrasonic treatment applied from 30 to 40 minutes only (t = 1800 - 2400 s).

A similar time course as seen in the pH-profile was observed during the scattering response, fig. 4.2B. Here, the initial high levels of scattering are caused by bubbles created during mixing. Over time these bubbles disappear from the solution leading to a decrease in scattering as observed over the first minutes. Note that in case of

58 4.3. Results the ultrasound experiment the bubbles disappear much faster. As seen in the pH measurement, during the induction time the level of scattering in both the blank and ultrasound experiment does not change. The ultrasound experiment deviates from the blank experiment by a shorter induction time (now defined as the time the scattering profile starts to increase). This confirms that the reduced induction time, as seen in the pH measurement, is related to enhanced precipitation and not to possible CO2 exchange. The ultrasound experiment also shows a faster increase in scattering, which might be seen as an increased volumetric precipitation rate, and a larger amount of overall scattering. This last observation can be caused by larger particles, more particles or a combination of both. Experiments performed under the same conditions showed some deviation in the induction time, but the precipitation profiles (steepness and overall scattering) were very similar and repeatable (appendix B, fig. B.1). fig. 4.3A,B show the scanning electron microscope (SEM) images of the blank and ultrasound experiment (complete treatment), respectively, of samples taken halfway in the experiment, at t = 2250 s. Before this moment in time, growth had started in both experiments. The ultrasound experiment has a lower pH, and thus lower supersaturation, but also a much higher level of scattering than the blank experiment. The precipitated polymorph appears to be vaterite in both cases (hexagonal shaped, polymorph confirmed with Raman spectroscopy [35], appendix B, fig. B.2a). fig. 4.4A,B show the particle size distribution (PSD) and relative particle size distribution of these images, respectively, presented as size-frequency histograms with a logarithmic x-axis. The SEM images show a representative portion assuming a uniform particle distribution in both the reactor and on the filter. The frequency PSD shows the amount of particles with a certain size range; the relative frequency PSD is normalized with the total number of particles. The frequency particle size distribution (fig. 4.4A) shows that there are many more particles present in case of the ultrasound experiment compared with the blank. However, the relative size distribution of fig. 4.4B shows that the size of the particles in both experiments are similar. The particles in the blank experiment are even slightly larger. The higher amount of scattering in the ultrasound experiment is thus predominantly caused by an increase in the number of particles in this particular case. The higher volumetric precipitation rate is therefore ascribed to the larger number of particles, implying an inherent larger total surface area available for growth. Interestingly the scattering profile of the ultrasound experiment, fig. 4.2B, shows a slight decrease toward the end. This suggests that particles dissolve. However, evidence obtained from the simultaneously recorded pH profiles, points to the conclu-

59 4. Eﬀect of US on early growth during CaCO3 precipitation

Figure 4.3.: Scanning Electron Microscope (SEM) images of samples taken at t = 2250: (A) Blank experiment (1), (B) Ultrasound experiment (2), complete treatment; Samples taken at t = 4500: (C) Blank experiment (1), (D) Ultrasound experiment (2), complete treatment. Sample volume: 25 cm3, ﬁltered over a 0.2 µm membrane ﬁlter (visible as background).

sion of calcium carbonate growth, displayed by the continuous pH decrease (fig. 4.2A). Several mechanisms, or a combination of those mechanisms, can be responsible for the decline in scattering while the pH decreases: agglomeration of crystals (resulting in less particles), scaling on the walls of the experimental equipment, dissolution of small crystals resulting in extra growth of large crystals (Ostwald ripening, driven by the Gibbs-Thomson relation) and the recrystallization of one polymorph to form another (Ostwald’s rule of stages) [34]. fig. 4.3C,D shows the SEM images of the blank and ultrasound experiment respectively at the end of the experiment. In both images some crystals of the calcite polymorph are visible (cubic shaped, polymorph confirmed with Raman spectroscopy [35], appendix B, fig. B.2b). The edges of the vaterite crystals, especially those in the ultrasound experiment, are more ragged compared with the

60 4.3. Results

Figure 4.4.: Particle size distribution (PSD) of blank and ultrasound experiments (complete treatment), based on the samples of fig. 4.3, presented as size-frequency histograms with a logarithmic x-axis. (A) Frequency and (B) Relative frequency PSD at time t = 2500 s of blank experiment (1) (gray line; from fig. 4.3A) and ultrasound experiment (2), complete treatment (black line; from fig. 4.3B); (C) Frequency and (D) Relative frequency PSD at time t = 4500 s of blank experiment (1) (gray line; from fig. 4.3C), and ultrasound experiment (2), complete treatment (black line; from fig. 4.3D). crystals halfway in the experiment. This points at dissolution of the edges [34]. The total number of crystals at the end of the ultrasound experiment is also a bit less than halfway the experiment, as can be derived from the frequency PSD (fig. 4.4C). The crystals have a wider distribution with both more larger and smaller crystals (the distributions becomes more bimodal). Probably the supersaturation of vaterite is so low (supersaturation ratio, S = 1.20, at the end of the experiment) that small vaterite crystals dissolve while large crystals still grow (Gibbs-Thomson effect) [36]. Also, dissolved vaterite will reprecipitate to form the calcite polymorph while this polymorph is still supersaturated (supersaturation ratio, S = 2.30) at the end of the experiment (t = 4500 s).

61 4. Eﬀect of US on early growth during CaCO3 precipitation

The blank experiment on the other hand does not show a drop in scattering before t = 4500. At the end of this experiment crystals are larger compared with crystals halfway. The relative PSD profiles of the blank experiment are similar (only the crystals grew larger) and the total number of crystals did increase for this experiment. The same drop in scattering is also present in the blank experiment though, but takes place after t = 4500 and, consequently, is not shown in fig. 4.2. To decipher the effect of ultrasound on precipitation, notably to study the stage during which ultrasound exerts its effect, the treatment periods were varied, as described above for experiments 3 - 5.

4.3.2. Ultrasound during primary nucleation (0 - 600 s)

To investigate the effect of ultrasound on the initial phase of precipitation, the formation of clusters or nuclei, ultrasound is applied during the first ten minutes of the experiment only (t = 0 - 600 s), regime 3. The assumption is made that the process of nucleation starts upon the first instance of supersaturation, in this case time t = 0. Because of the small size of a critical nucleus, the actual initial process of nucleus formation cannot be determined. Later processes in time can be followed however by looking at the pH and scattering profiles during the precipitation experiments. If ultrasound increases the kinetics of (critical) nucleus formation, this should result in a shorter induction time, visible in the pH and scattering profiles. Additionally, if ultrasound increases the concentration of possible nucleation sites this should be noticeable as an increase in volumetric precipitation rate and possibly a shorter induction time. The latter is valid when the number of particles influences the time of detection, which seems to be the case for both the pH and scattering measurements. As fig. 4.2 shows, both the pH and scattering profile of experiment 3 show no clear difference with the blank experiment. This finding makes us conclude that ultrasound has no measurable effect during the first 10 minutes of CaCO3 precipitation.

4.3.3. Ultrasound during early growth (600 - 1200 s)

In contrast to the unchanged profiles of experiment number 3, when ultrasound is applied a little later, from t = 600 − 1200 s (regime 4) there is a clear effect on both the pH and scattering profile compared with the blank experiment, fig. 4.2. The treatment is applied when, in the blank experiment, there is no detectable growth yet. This probably means that the treatment is in the region where there is a conversion

62 4.3. Results from nuclei to crystals. Scattering increases directly after the period of ultrasound application. Growth is a sequence of consecutive steps: adsorption onto the surface, diffusion on the surface, surface reaction and finally integration into the crystal lattice, and the growth rate of a crystal is determined by the slowest among these steps [37]. Besides the incorporation of ions, also aggregation of larger clusters can take place, all leading to the formation of single crystalline particles. As could be seen from fig. 4.4A, i.e. the frequency particle size distribution halfway through the blank (1) and ultrasound (full treatment, 2) experiments, there is a large increase in the number of particles. However, the relative particle size distribution, fig. 4.4B, shows little difference. There are mainly less larger particles and, in addition, the distribution is more uniform. If only the growth rate was positively affected by ultrasound, this should have resulted in the same number of particles, as seen in the blank experiment, but on average particles should have grown larger. Actually the opposite is true, there are many more but also smaller particles. This points to deaggregation during the outgrowth of crystals, especially since vaterite is known to aggregate during the early growth phase [38–41]. This also complies with other research where more uniform size distributions were observed when ultrasound is applied [26, 28, 42]. The effects during cavitation that can be responsible for deaggregation are shear induced by micromixing and/or shear/stress induced by shockwaves. This can prevent particles to form a bridging neck [40, 43]. When the experiments 2 and 4 in fig. 4.2 are compared, complete treatment versus treatment during t = 600 − 1200 s, respectively, some differences can be noticed. The moment in time that the pH drops and scattering rises is approximately the same, but the rate is much faster for the complete treatment. Apparently more crystals are created in the complete treatment compared with experiment 4. Presumably, the 600 - 1200 s period has to be extended to obtain the same profiles as for the complete treatment.

4.3.4. Ultrasound during crystal outgrowth (1800 - 2400 s)

To confirm that ultrasound also has an effect in a later stage, during the outgrowth of crystals, another experiment is performed, experiment 5. In this experiment the treatment is applied during t = 1800 − 2400 s, the region where the blank experiment starts to precipitate significantly. The induction times for the blank (1) and ultrasound experiment (5) are similar; however the rate of precipitation of the ultrasound

63 4. Eﬀect of US on early growth during CaCO3 precipitation experiment is much higher. In fact it approaches almost the rate of the complete treatment (2). The eﬀect seems to be larger in the interval of experiment 5 compared with experiment 4 and more particles are generated. In this region the particles are much larger than in region 3 making them more susceptible to breakage.

4.4. Discussion

Experimental conditions are of great inﬂuence on the outcome of precipitation studies.

This might explain the large variety of reported effects of ultrasound on CaCO3 precipitation [24–30]. From all possible effects, micromixing (including effects by shock waves), macromixing, and the rise of bulk temperature seem to be the likely cause of the observed changes in volumetric precipitation rates as reported elsewhere [24–30]. It is not expected that radicals change the course of calcium carbonate crystallization directly [30]. Nishida [25] noticed an enhanced effect on the precipitation kinetics and attributes this to the enhanced mixing, especially macrostreaming. They concluded that there was no direct effect of cavitation on the formed product. This corresponds well with the results in this study. The conditions as used by Nishida are also close to the CO2 equilibrium so any effect of CO2 would have been minor.

Interestingly it is also claimed that CaCO3 can nucleate on nanobubbles, as produced in high frequency ultrasound (MHz range), resulting in hollow spheroid nanoparticles [27]. These conditions have not been reproduced yet and most of the work is done at lower ultrasonic frequencies (kHz). Keeping the temperature constant by actively cooling rules out possible bulk temperature effects. In some reported experiments the temperature is not controlled [26, 28], which led to extreme temperature rises, especially in small volumes, causing much higher supersaturations and different precipitation pathways. In other temperature controlled experiments, only the cooling liquid temperature is maintained constant [25]. In that case the temperature inside the reactor might still rise when ultrasound is applied. The work in which the generation of aragonite by ultrasonic treatment, as opposed to vaterite in blank experiments, is described [26, 30], can most certainly be ascribed to rising bulk temperatures as is also recognized as such by Price et al. [30]. To rule out an effect of bulk temperature in our experiments, the reactor vessel is cooled actively. This does not prevent the existence of local hotspots during bubble collapse. However, although these hotspots are said to lead to an increased nucleation rate [26], the absence of a measurable effect when ultrasound is applied from t = 0 - 600

64 4.4. Discussion s (regime 3) shows that this phenomenon is not applicable for the chosen experimental conditions. Another possible explanation for enhanced primary nucleation: cluster segregation due to large pressure gradients in the vicinity of a collapsing bubbles [44] seems to be ruled out by these results. Although it is often assumed that ultrasound leads to a larger number of nucleation sites [22, 26, 30], our study does not validate the conclusion that ultrasound has an eﬀect during primary nucleation.

Breakage of crystals, as described previously [32] (chapter 3), could also lead to more and smaller particles. However, when particles are too small (< 100 nm), they cannot be accelerated by cavity expansion due to the high energy cost of neck formation before cavity detachment [45]. Additionally, the relaxation time (the time necessary to adapt to a new condition of forces) is proportional to the square of the particle diameter, so small particles will relax much faster than larger particles [46]. Accordingly, breakage due to energetic high velocity interparticle collisions is very unlikely, moreover, since this is also not observed under similar experimental conditions for larger crystals (∼ 25 µm)[32] (chapter 3). Breakage due to shockwave induced stress in the crystal lattice is also less likely for small particles because the specific fracture energy is larger when particles are smaller [47]. Breakage is easier when there are flaws (Griffith cracks) in the crystal lattice and since small crystals contain less flaws than large crystals, breakage is less likely for small crystals. Because, from t = 600 − 1200 s (regime 4), the nuclei and crystals are still very small, in the nanometer size range, the particles are tough to break. However, when aggregates have been formed there are more flaws in the crystal due to the different orientation of the aggregated particles [19]. Hence the specific fracture energy of an aggregate will be lower than that of a single crystal [48]. From t = 1200 s the crystals are measurable and larger in size making them susceptible to breakage. This additional breakage can explain the difference between experiment 4 (t = 600 − 1200 s) and experiment 5 (t = 1800 − 2400 s).

An alternative mechanism leading to more and smaller particles would be dissolution of one polymorph followed by growth of another, for instance, of amorphous calcium carbonate (ACC) nanoparticles to vaterite crystals [18]. Unfortunately it is unclear if ACC nanoparticles form before the outgrowth of vaterite crystals in our experiments. However ACC is undersaturated for the chosen experimental conditions so this seems unlikely. On the other hand, the possible contribution of ACC to the formation of vaterite cannot be ignored because ACC possibly exists as small domains in vaterite crystals [19]. Assuming that indeed ACC formed in this case, the only way

65 4. Eﬀect of US on early growth during CaCO3 precipitation to obtain more particles, other than by deaggregation, is by enhancing the primary nucleation of vaterite from dissolved ACC. Price et al. [30] also mention that ultrasound can lead to preferential polymorphs (vaterite versus calcite) even when the bulk temperature is maintained constant by active cooling. However they do not take into account the interaction with CO2 for this particular experiment. The carbonate concentration has a 5 times higher aqueous

CO2 concentration compared to conditions posed by the atmospheric pressure. In that case ultrasound might enhance release of CO2 which will lead to higher pH values (section 8.4). Although the total amount of carbonic species will decrease, it does not result in lower supersaturation. On the contrary, this counterintuitive result, obtained 2– by model calculations (VMinteq v3.0), can be explained by higher CO3 activity due to the pH increase and by that in enhanced supersaturation. This higher supersaturation might result in diﬀerent precipitation pathways. Since the dehydration of CO2 is much faster than the hydration [49], and the larger diﬀerence in aqueous CO2 compared to our experiments, this ultrasonically enhanced dehydration might become an issue. Moreover, it is possible that due to the application of ultrasound vaterite particles are formed earlier in time causing the transformation of vaterite to calcite to take place earlier in time as well, as discussed previously.

Ultrasonic liquid degassing below the CO2 equilibrium solubility can occur due to rectified diffusion of cavitation bubbles [50, 51]. This will lead to a lower apparent gas solubility, and it has been reported that with frequencies in the lower range (20 kHz) the apparent solubility of nitrogen gas lowered with maximally 12% [52]. However the amount of rectified diffusion in the low frequency range, especially in the transient cavitation regime, is low [50, 51]. It has been reported that nitrogen gas uptake takes place below the apparent gas solubility, related to ultrasonically enhanced mixing [52].

Radical formation by ultrasound might also lead to faster uptake of CO2, especially of the hydroxyl radical [12]. So if ultrasound has an eﬀect through the accelerated uptake of CO2 it will result in retardation of the precipitation process.

Price et al. investigated the role of CO2 in other ultrasound precipitation experiments. They found that bubbling the solution beforehand with CO2, while sparging with CO2 during the experiment, did not result in precipitation at all. They explained this by the diﬃculty to generate cavitation in CO2 saturated liquid. Although the cavitation will be less violent [53], they did not mention that bubbling with CO2 leads to uptake of CO2 under the applied conditions. This will cause the pH to drop, resulting in a signiﬁcantly lower supersaturation and may even lead to undersaturation.

They also mentioned that, when CO2 bubbling was applied beforehand and without

66 4.5. Conclusion bubbling during the experiment, it did result in precipitation. In that situation the pH might have risen again due to a release of CO2 under atmospheric pressure. This brings the solution back into supersaturation especially since the dehydration of CO2 is much faster than the hydration [49]. The current work shows that micromixing and shockwaves most likely cause deaggregation in the early growth stage of calcium carbonate precipitation, resulting in a faster volumetric precipitation rate and shorter induction time. The assumption that ultrasound produces a larger amount of primary nucleation sites [22, 26, 30] is rejected as the dominant mechanism and probably does not apply for the current conditions. Another often encountered assumption in the literature, that cavitation causes breakage of particles by energetic interparticle collisions [30], does not hold for our experiments. Nevertheless, breakage of large particles (micrometer size range) due to stress induced by shockwaves is a possibility.

4.5. Conclusion

Online pH and scattering measurements showed that application of ultrasound reduces the induction time and increases the volumetric precipitation rate. Scanning electron microscope analysis of multiple samples taken during the experiment shows that ultrasound increases the total number of particles having a more uniform distribution compared with the blank experiment. Applying ultrasound during the first 10 minutes of the experiment did not result in any significant effects. This rules out the influence of ultrasound on primary nucleation under the chosen experimental conditions, and with it, also the effect of high local pressures and temperatures. The application of ultrasound starting later in the experiment shows that ultrasound modifies the precipitation in the early growth phase, that is, where nuclei grow out to become crystals. We interpret our findings in terms of a deaggregation mechanism that is responsible for a larger number of crystals in this early growth phase. The inherent increase in available surface area for growth leads to faster volumetric precipitation rates. The attributed cavitation effects responsible for deaggregation are shear induced by micromixing or shear/stress induced by shockwaves. Breakage of particles by energetic interparticle collisions does not apply for our experiments. Nevertheless, breakage of large particles (micrometer size range) due to stress induced by shockwaves might contribute in the late growth stage of precipitation.

67 4. Eﬀect of US on early growth during CaCO3 precipitation Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, centre of excellence for sustainable water technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Develop- ment Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors thank the participants of the research theme “Sensoring” for the discussions and their financial support, and Ton van der Zande for the Raman analysis.

References

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[44] R. Grossier, O. Louisnard, and Y. Vargas. “Mixture segregation by an inertial cavitation bubble”. In: Ultrason. Sonochem. 14.4 (2007), pp. 431–437 (cit. on p. 65). [45] B. M. Borkent, M. Arora, C. D. Ohl, N. De Jong, M. Versluis, D. Lohse, K. A. Mørch, E. Klaseboer, and B. C. Khoo. “The acceleration of solid particles subjected to cavitation nucleation”. In: J. Fluid Mech. 610 (2008), pp. 157–182 (cit. on p. 65). [46] W. C. Hinds. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. Wiley-Interscience, 1998 (cit. on p. 65). [47] S. Yashima, Y. Kanda, and S. Sano. “Relationships between particle size and fracture energy or impact velocity required to fracture as estimated from single particle crushing”. In: Powder Technol. 51.3 (1987), pp. 277–282 (cit. on p. 65). [48] Y. Asakuma, T. Honda, K. Maeda, H. Miki, and K. Fukui. “Fragmentation behavior of aggregated crystal in suspension crystallization processes”. In: Powder Technol. 181.3 (2008), pp. 266–272 (cit. on p. 65). [49] X. Wang, W. Conway, R. Burns, N. McCann, and M. Maeder. “Comprehensive study of the hydration and dehydration reactions of carbon dioxide in aqueous solution”. In: J. Phys. Chem. A 114.4 (2010), pp. 1734–1740 (cit. on pp. 66, 67). [50] A. Eller and H. G. Flynn. “Rectified diffusion during nonlinear pulsations of cavitation bubbles”. In: J. Acoust. Soc. Am. 37 (1965), p. 493 (cit. on p. 66). [51] H. M. Hung and M. R. Hoffmann. “Kinetics and mechanism of the sonolytic degradation of chlorinated hydrocarbons: frequency effects”. In: J. Phys. Chem. A 103.15 (1999), pp. 2734–2739 (cit. on p. 66). [52] F. Laugier, C. Andriantsiferana, A. Wilhelm, and H. Delmas. “Ultrasound in gas– liquid systems: Effects on solubility and mass transfer”. In: Ultrason. Sonochem. 15.6 (2008), pp. 965–972 (cit. on p. 66). [53] J. González-Garc´ıa,V. Sáez,I. Tudela, M. D´ıez-Garcia,M. Deseada Esclapez, and O. Louisnard. “Sonochemical treatment of water polluted by chlorinated organocom- pounds. A review”. In: Water 2.1 (2010), pp. 28–74 (cit. on p. 66).

5 Ultrasonic reactivation of phosphonate (NTMP) poisoned calcite during crystal growth

abstract The effect of ultrasonic irradiation (42,150 Hz, 17 W dm3/7.1 W cm−2) on the growth of calcite in the presence of the inhibitor nitrilotris(methylene phosphonic acid) (NTMP) was investigated at constant composition conditions. In seeded growth experiments, it was found that the inhibiting effect of NTMP on crystal growth could be seriously mitigated under influence of ultrasonic irradiation. An approximately twofold increase in volumetric growth rate was achieved during ultrasonic irradiation, and recovery of the growth rate following inhibition was strongly enhanced compared to growth experiments without ultrasonic irradiation. The results could be explained in part by the physical effect of ultrasound that causes breakage and attrition of poisoned crystals, which resulted in an increase in fresh surface area. Mass spectroscopy analysis of sonicated NTMP solutions revealed that there is also a chemical effect of ultrasound that plays an important role. Several breakdown products were identified, which showed that ultrasound caused the progressive loss of phosphonate groups from NTMP, probably by means of physicochemically generated free radicals and/or pyrolysis in the hot bubble-bulk interface.

This chapter has been published as: L. Boels, R.M. Wagterveld, G.J. Witkamp. Ultrasonic reactivation of phosphonate poisoned calcite during crystal growth Ultrasonics Sonochemistry 18 (2011) 1225-1231.

73 5. Ultrasound during inhibited calcite growth by NTMP

5.1. Introduction

The presence of sparingly soluble inorganic compounds that tend to form scale layers on surfaces of, for example, membranes and heat exchange equipment, like calcium carbonate, imposes limitations to the efficiency of many industrial water treatment processes. With the aid of growth inhibitors (or antiscalants), primarily comprising polyelectrolytes such as phosphonates or polycarboxylates, the precipitation (scaling) of these inorganic compounds from aqueous solutions can be prevented or retarded at far substoichiometric inhibitor concentrations. One example of a phosphonate growth inhibitor is nitrilotris(methylene phosphonic acid) (NTMP), which is widely used in many technical applications for scale and corrosion inhibition [1–7]. Unfortunately, the presence of growth inhibitors also hinders the desirable downstream removal of scaling salts from waste brines [8–11]. Therefore, these inhibitors should be removed [11, 12] or degraded [13] in order to facilitate scaling salt removal. Recently, ozonation has been applied [13] to degrade growth inhibitors in reverse osmosis concentrates followed by the unseeded precipitation of calcium carbonate through sodium bicarbonate dosing. Removal of calcium carbonate by means of seeded crystallization offers several technical and economical benefits compared to the unseeded precipitation technique. In the first place, because a preferential surface area for heterogeneous nucleation and growth is provided, which can significantly improve crystallization kinetics [9, 10, 14]. In the second place, because selection of seeds with a suitable size can significantly improve the solid-liquid separation efficiency. The reactivity of the seed crystals, the degree of supersaturation and the solution composition dictate the growth rate and hence, the calcium carbonate removal efficiency. The presence of growth inhibitors like NTMP can markedly decrease the reactivity of seed crystals by blocking the active growth sites through adsorption of these compounds onto the crystal surface [2, 3, 6, 15]. The question is whether or not ultrasound can be used to improve the reactivity of poisoned seed crystals during growth. The unique physical phenomena that occur when ultrasound is passed through a liquid-solid system might help reactivate poisoned seed crystals in two ways. First, there is the physical effect of ultrasound which can enhance the reactivity of the crystals by enlarging the surface area. Previous research on seeded calcite crystallization has demonstrated that the volumetric growth rate of calcite can be enhanced significantly by the action of inertial cavitation [16] (chapter 2). High speed recordings revealed the mechanism of deagglomeration, attrition and crystal breakage

74 5.1. Introduction by acoustic cavitation [17] (chapter 3). Among others, potash alum [18], sugar [19], roxithromycin [20] and potassium dihydrogen phosphate [21] crystals were reported to erode upon ultrasonic treatment as well.

Second, there is the chemical effect of ultrasound that might cause degradation of the growth inhibitor. It is well known that inertial cavitation in water leads to the formation of highly reactive species including hydroxyl (·OH), hydrogen (·H) and hydroperoxyl (·HO2) radicals, and hydrogen peroxide [22, 23]. The most important of which are the hydroxyl radicals. This is due to the very short-lived extreme conditions that occur during bubble collapse. It has been stated that, locally, the temperature and pressure may reach up to 5000 K and 1000 atm, respectively [24–26]. In general, this so called “hot-spot” theory has been adopted in most sonochemical studies to explain experimental results. According to this theory, the harsh conditions inside a collapsing bubble provide the activation energy required for homolytic bond cleavage of solvent and substrate molecules that diffused into the bubble. The radicals generated in this way either react with each other or diffuse into the bulk liquid to serve as oxidants. In the hot interfacial zones of cavitation bubbles, which have been estimated to heat up to approximately 2000 K [26] during bubble collapse, both pyrolysis and free-radical reactions can occur.

Ultrasound has been extensively investigated as an advanced oxidation process for degrading various pollutants including aromatic compounds, chlorinated aliphatic hydrocarbons, explosives, herbicides and pesticides, organic dyes, organic and inorganic gaseous pollutants, organic sulfur compounds, oxygenates and alcohols, pharmaceu- ticals, personal care products, pathogens and bacteria in water [27]. Sonochemical oxidation works best for volatile hydrophobic compounds, which can diﬀuse and degrade inside the collapsing bubble. Only few reports deal with nonvolatile hydrophilic compounds, including natural organic matter in groundwater [28] and humic acids [29]. No reports are available on the sonochemical degradation of phosphonates.

In the present work, the eﬀect of ultrasound on the growth of calcite in the presence of nitrilotris(methylene phosphonic acid) (NTMP) was investigated. The aim was to improve in situ the reactivity of poisoned calcite seed crystals by using ultrasound.

The calcite crystal growth was measured using the constant composition method [30] at various NTMP concentrations with and without ultrasonic irradiation at low supersaturation. With this method, any change in reactivity of the calcite crystals by ultrasonic irradiation can be measured directly.

75 5. Ultrasound during inhibited calcite growth by NTMP

Figure 5.1.: Scanning electron microscopy picture of the employed calcite seed crystals. The crystals appeared as interpenetrated conglomerates with a size of around 10 µm.

5.2. Experimental

5.2.1. Chemicals

All solutions were prepared using high quality water (MilliQ Reagent Water System, resistivity >18 MΩ cm) saturated in air. Sodium bicarbonate (NaHCO3), calcium chloride (CaCl2 · 2 H2O) and potassium chloride (KCl) were of analytical grade and were obtained from Boom (Meppel, The Netherlands). Nitrilotris(methylene phosphonic acid) (NTMP) was obtained from Fluka in its acid form with a purity of 99%. From this, a stock solution with a concentration of 1.00 g dm3 (3.34 mM) and neutral pH (after KOH addition) was prepared and stored without exposure to light at 277 K.

5.2.2. Calcite seed crystals

The preparation of the calcite seed crystals has been described previously [16] (chapter 2). The dry crystals were characterized by scanning electron microscopy (Jeol JSM-6480 LV), ATR-FT-IR spectroscopy (Shimadzu 4800) and nitrogen adsorption (Micromeritics Tristar 3000). The specific surface area of the seed crystals was found to be 0.17 ± 0.04 m2 g−1 as determined with a five-point BET method [31] on 3 replicate samples. The ATR-FT-IR spectra confirmed that the crystals were pure calcite with characteristic adsorptions at 1795, 1392, 871 and 711 cm−1. The crystals appeared as interpenetrated conglomerates with a size of around 10 µm (fig. 5.1).

76 5.2. Experimental

5.2.3. Experimental set-up

A scheme of the constant composition experimental set-up used to measure the volumetric crystal growth rate of calcite during ultrasonic irradiation is shown in fig. 5.2. A double walled glass reactor was used, equipped with a floating magnetic stirrer (Nalgene) to minimize grinding of the seed crystals, a by-pass loop and an ultrasonic transducer. For the control of pH, a combined Pt-ring pH electrode and a shielded Ag/AgCl reference electrode (Syntrode, Metrohm) in combination with two coupled automatic burettes (Metrohm Titrino 785 and Dosimat 665) were used. In order to avoid damaging or influencing the electrodes by the ultrasonic irradiation, both electrodes and a temperature sensor were positioned in a glass cell in the by-pass stream. The ultrasonic transducer was part of a dedicated homebuilt system that could be controlled precisely in terms of shape, frequency and amplitude of the alternating current for driving the transducer. The transducer was operated at 42,150 Hz. The actual value of the ultrasound power delivered to the medium by the stainless steel horn was determined by measuring the adiabatic temperature rise of water irradiated with ultrasound. For this purpose a similar well isolated glass reactor filled with 1 kg water was used. The input power was determined to be 17 W or 7.1 W cm−2.

5.2.4. Experimental procedures

Metastable working solutions were prepared by slow addition of 500 cm3 of a 4 mM 3 CaCl2 solution to a stirred 500 cm of a 4 mM NaHCO3 solution in the reactor. Before addition of the calcium solution, an aliquot of the NTMP standard solution was injected to the carbonate solution to obtain the desired NTMP concentration in the range of 0.01-0.05 mg dm3. The working solution was stirred at 400 rpm. The solution pH was adjusted to the desired value of 8.650 ± 0.002 using a 0.05 M KOH solution. The stability of the working solution was veriﬁed by observing a constant pH for at least 45 min. Experiments were started by the addition of 0.250 ± 0.002 g seed crystals as a slurry: the dry seed crystals were weighed into a syringe and a small amount of working solution was drawn up into the syringe, and then the content was injected back into the growth solution with several rinses. After the addition of the seed crystals, the solution pH started to decrease as a result of calcite growth. This triggered the automatic burettes to add concurrently equivalent amounts of calcium chloride and sodium carbonate solutions in order to achieve and maintain the pH

77 5. Ultrasound during inhibited calcite growth by NTMP

Figure 5.2.: Scheme of the constant-composition experimental set-up consisting of (1) a double walled glass reactor, (2) a ﬂoating magnetic stirrer bar, (3) a pump, (4) an ultrasonic transducer, (5) a free port for seed addition, (6) a pH electrode and reference electrode and (7) a temperature sensor. at 8.650 ± 0.002. In this way, a constant degree of supersaturation was maintained throughout the growth experiment. Fresh titrant solutions were prepared every day with a sevenfold higher calcium and carbonate concentration compared to the working solution. Potassium chloride was added to all solutions to maintain the ionic strength constant at 0.1 M. Constant ultrasonic irradiation was initiated after 15 min of normal growth and lasted for 45 min. After that, the seeds were allowed to grow further until 25 cm3 of titrant was consumed. In this way, dilution of the NTMP inhibitor by the addition of NTMP free titrant was kept to a minimum (< 5%). Depending on the employed inhibitor concentration, total growth times ranged from 2 to 15 h. Also, experiments were conducted in which several fresh doses of NTMP (0.03 mg dm3) were added to the same growth solution. Ultrasonic irradiation was initiated 15 min after each fresh NTMP dose.

78 5.3. Results and discussion

The temperature was maintained constant at 298 ± 0.1 K by circulating water from a thermobath through the jacket of the reactor. During the ultrasonic irradiation, the thermobath was set to cool in order to keep the reaction mixture temperature at 298 ± 0.1 K. During the experiment, aliquots of solution were rapidly removed through a septum, ﬁltered by a 0.2 µm ﬁlter and analyzed for the calcium content with inductive- coupled plasma spectrometry (Optima 3000XL, Perkin-Elmer) to verify the constancy of the degree of supersaturation. All experiments were conducted at atmospheric pressure with ambient levels of

CO2. By keeping the reactor lid closed and all ports sealed during the experiments, the exchange of CO2 between the atmosphere and the reactor solution was minimized. Ultrasonic irradiation of metastable working solutions without the addition of seed crystals, did not result in a change in pH compared to the control. This indicates that any degassing eﬀects were neglectible in the sealed reactor. Also, no spontaneous

CaCO3 precipitation was observed within 24 h. In order to investigate whether or not the employed ultrasonic irradiation was able to aﬀect the NTMP molecules chemically, control experiments were carried out by irradiating a fresh 5.0 mg dm3 NTMP solution in pure water at neutral pH. Duplicate samples were injected immediately after the experiment into a triple-quad mass spectrometer (Agilent 6410).

5.3. Results and discussion

5.3.1. CaCO3 supersaturation and growth mechanism

For CaCO3, the supersaturation ratio, S, is best expressed in terms of activities:

1 IAP v (5.1) Ksp

where IAP is the ion activity product, Ksp is the thermodynamic solubility product, and v is the number of ions in the formula unit [32]. The supersaturation ratio, S, with respect to calcite of the working solution used here was calculated to be 2.48 (software: Visual Minteq v2.53, model: Davies). At this relatively low supersaturation ratio, a metastable solution can be obtained in which precipitation is thermodynamically possible, but spontaneous precipitation is improbable due to slow kinetics. The metastable solutions used in this study, showed a constant pH for at least 24 h, indicating that no spontaneous CaCO3 precipitation occurred.

79 5. Ultrasound during inhibited calcite growth by NTMP

Figure 5.3.: Eﬀect of ultrasonic irradiation on the inhibition of calcite growth by NTMP (0.03 mg dm3): (A) moles of added Ca2+ (n) versus time curves of an ultrasound experiment and two control experiments; (B) derivative plots of the same experiments. The inhibited growth rate Ri and normal growth rate R0 regions are shown; (C) second derivative plots used for determining the inhibition period ti. A smoothing spline method was applied for the curves in graphs B and C.

Preliminary growth experiments at diﬀerent supersaturation ratios (2.11 > S < 2.91) revealed a second-order dependence of growth rate on the supersaturation ratio, indicating a spiral growth mechanism [32].

5.3.2. Seeded calcite growth experiments

The inhibiting effect of NTMP on the crystal growth of calcite can be described in terms of the values of reduced (or inhibited) growth rates and the length of the inhibition period caused by the addition of NTMP to the growth solution. This is illustrated in fig. 5.3 where the amount of added Ca2+ (n) versus time (t) curves and the first and second derivative plots of these curves for one ultrasound and two control experiments are shown. Without the presence of NTMP, an almost linear curve was obtained (fig. 5.3A). The slope of this titrant addition versus time curve increases over time. Growth leads to an increase of crystal volume and surface and the volumetric growth rate increases with the size of the growth spirals, and hence the number of growth sites. Therefore, a cubic regression must be used to fit the control experiments. The linear growth rate can be derived from the first order constant [33, 34]. In presence of NTMP, the addition rate of titrant rapidly decreases to a minimum value due to NTMP adsorption, followed by a period of constant (inhibited) growth

80 5.3. Results and discussion

Table 5.1.: Seeded calcite growth in the presence of NTMP: eﬀect of ultrasound on Ri and ti as a 2+ – function of NTMP concentration. Conditions: [Ca ] = [HCO3] = 2 mM, S = 2.48, pH = 8.65, IS = 0.1 M, T = 298 K.

Exp. Ultrasound [NTMP] Ri Ri,l ti (45 min) (mg dm3) (mol g−1 (mol m−2 (min) min−1) min−1) 1 No 0 38.0 22.4 0 2 No 0 37.8 22.2 0 3 No 0 35.9 21.1 0 4 No 0 38.3 22.6 0 5 No 0 39.0 22.9 0 7 No 0.01 26.2 15.4 50.9 8 Yes 0.01 27.2 - 36.4 9 No 0.03 3.63 2.14 167 10 No 0.03 3.47 2.04 115 11 Yes, 95 min 0.03 6.40 - 70.4 12 No 0.03 2.65 1.56 164 No 0.03 4.94 2.91 75.0 No 0.03 3.50 2.06 98.0 No 0.03 3.45 2.03 88.0 13 Yes 0.03 4.77 - 125 No 0.03 4.11 - 95.0 No 0.03 3.01 - 112 No 0.03 2.56 - 131 14 Yes 0.03 5.84 - 116 Yes 0.03 8.13 - 57.0 Yes 0.03 6.74 - 66.0 Yes 0.03 7.14 - 60.0 15 No 0.04 0.49 0.290 741 16 No 0.04 0.57 0.335 786 17 Yes 0.04 1.33 - 171 18 Yes 0.04 1.33 - 177 19 No 0.05 0.54 0.318 805 20 No 0.05 0.38 0.224 819 21 Yes 0.05 0.92 - 306 22 Yes 0.05 1.04 -

81 5. Ultrasound during inhibited calcite growth by NTMP

Figure 5.4.: Eﬀect of ultrasonic irradiation on the growth rates (A) and inhibition times (B) as a function of NTMP concentration. (C) Insert showing the eﬀect of ultrasonic irradiation at an NTMP concentration above 0.03 mg dm3.

(Ri) (fig. 5.3B). This period is the inhibition period (ti), which was determined from the second order derivative plot (fig. 5.3C). A linear regression was used to fit this period. After a while, the growth rate increases rapidly again to a value (R0) similar to that expected for uninhibited growth (control). If ultrasound is applied for 45 min, however, the Ca2+ addition rate increases during the inhibition period, and strikingly, the rate recovers much faster to the uninhibited value. It is not possible to determine a linear growth rate after ultrasonic treatment owing to the increase of total surface area by deagglomeration, attrition and crystal breakage. For this reason, only the volumetric growth rate, R (mol g−1 min−1) can be determined. Hereafter, the term “growth rate” always refers to the volumetric growth rate. These experiments have been done over a range of NTMP concentrations at the same supersaturation ratio. In fig. 5.4, the length of the inhibition period and the values of the inhibited growth rates caused by addition of NTMP are shown (see table 5.1). From the strong decrease in growth rate, it can be deduced that the presence of NTMP markedly decreased the reactivity of calcium carbonate crystals (fig. 5.4A). At low NTMP concentrations (0.00-0.03 mg dm3), the decrease in growth rate is directly proportional to the NTMP concentration, while at higher concentrations, the growth rate approaches a minimum. A similar trend was observed for the growth inhibitors sodium triphosphate and polymaleic acid [35]. Ultrasonic irradiation causes a strong enhancement of growth rate. An approximately twofold increase can be seen compared to the control experiments for 0.03, 0.04 and 0.05 mg dm3 NTMP

82 5.3. Results and discussion

Figure 5.5.: Effect of multiple doses of NTMP and multiple ultrasonic irradiations on calcite growth: (A) raw data of an ultrasound and a control experiment. At 1-4, a fresh dose of NTMP (0.03 mg dm3) was added; (B) derivative plot for control experiment; (C) derivative plot for ultrasound experiment. A smoothing spline method was applied for the curves in graphs B and C. respectively. The inhibition period of 36.4 min for 0.01 mg dm3 NTMP is much shorter than the irradiation period of 45 min (fig. 5.4B). Therefore, the period of inhibited growth did not receive the full 45 min of irradiation. This explains why the growth rate enhancement is much smaller as opposed to the other experiments. The recovery of growth rate following inhibition was strongly enhanced by ultrasonic irradiation (fig. 5.4B). While the reduction of the inhibition period by ultrasonic irradiation is relatively small at low NTMP concentrations, this reduction is much more profound at higher NTMP concentrations. The observed enhancement of growth rate and reduction of inhibition period after ultrasonic irradiation, can be explained in part by the physical effect that ultrasound has on the crystals. From previous research using identical calcite crystals and equipment, it has been demonstrated that crystal deagglomeration, attrition and breakage occurs [16, 17] (chapters 2 and 3). Consequently, the total surface area available for growth increases, and therefore the growth rate increases. Although broken crystal planes tend to have a lower linear growth rate because of their higher surface energy and hence higher solubility, it is likely that newly created crystal faces grow much faster as opposed to faces poisoned with NTMP. In the experiments shown in fig. 5.5, fresh doses of NTMP (0.03 mg dm3) were added after the inhibition period. This was done three times with and without ultrasonic

83 5. Ultrasound during inhibited calcite growth by NTMP

Figure 5.6.: Eﬀect of multiple doses of NTMP and multiple ultrasonic irradiations on calcite growth rate Ri and inhibition time ti: (A) Ri as a function of NTMP dose for experiments with 0, 1 and 4 successive irradiation periods, respectively; (B) ti as a function of NTMP dose for experiments with 0, 1 and 4 successive irradiation periods, respectively. irradiation. The rate recovered to the uninhibited value even after four equal doses of NTMP. Also, the inhibition periods for successive equal doses were very similar after the second NTMP dose. Furthermore, after each successive ultrasound treatment, the growth rate for the ultrasound experiment was approximately twice as high compared to the control experiment.

Without ultrasonic irradiation, multiple doses of NTMP caused ti to increase and

Ri to decrease fig. 5.6 . This suggests that NTMP had some permanent effect on the growing crystals. This effect was not observed when ultrasound was applied after each NTMP dose. With ultrasound, the growth rate and inhibition period are quite similar for each successive irradiation period. This suggests that the ultrasonic irradiation affected not only the crystals, but also the NTMP molecules.

Mass spectroscopy: detection of breakdown products

The mass spectra of the treated and untreated NTMP solution revealed the appearance of two distinctive peaks that increased with irradiation time (ﬁg. 5.7). The peak with molar weight 204 corresponds to the weight of the well known breakdown product imino(dimethylene)phosphonic acid (MW 205) [36–38]. It is not clear however, if the peak at MW 109 corresponds to aminomethylphosphonic acid (MW 111) or hydroxymethylphosphonic acid (MW 112), because both have a similar molar mass. The spectra indicate that the stable C-P bond in NTMP can be cleaved by means of ultrasonic irradiation. Progressive loss of ligand-donor groups eventually

84 5.3. Results and discussion

Figure 5.7.: Mass spectra of MilliQ, control and sonicated samples, showing the appearance of two compounds with molecular weights (MW) of 109 and 204 only for the sonicated samples. For each sample, the average of two spectra was plotted. generates products that do not effectively coordinate surface Ca2+ ions, and consequently, the reactivity of the crystals increases. In fig. 5.8, a tentative scheme of the sonochemical NTMP degradation is shown. No significant decrease in the NTMP peak was detected, showing that not all NTMP molecules where oxidized, even after 120 min of irradiation. This is supported by the growth experiments. The 45 min irradiation at, for example, 0.04 mg dm3 NTMP, does not yield a similar or lower growth rate compared to the average growth rate measured at 0.03 mg dm3 NTMP. This indicates that only a small amount of NTMP has been degraded. In general, oxidation of dissolved species in the bulk solution by free radicals produced by sonochemical decomposition of water molecules, is not very efficient, espe- – cially in the presence of radical scavengers like HCO3 [28]. Only a small portion of the total energy supplied to the system results in useful free-radical reactions. On the other hand, pyrolysis in the hot bubble-bulk interface has been reported to be much less affected by other chemical components [39]. Such pyrolysis might have played an important role, all the more since the estimated temperature of the bubble- bulk interface is sufficient to degrade NTMP, which already hydrolytically degrades at 533 K [36]. Since cavitation primarily takes place on the particle surface [17, 40– 43] (chapter 3) where adsorbed NTMP is also present, most NTMP degradation is expected to occur on the crystal surface. To conclude, the use of ultrasound for the reactivation of poisoned calcite seed crystals in water has been shown to be technically feasible and could improve the

85 5. Ultrasound during inhibited calcite growth by NTMP

Figure 5.8.: Tentative scheme of sonolytical NTMP degradation into imino(dimethylene)phosphonic acid (IDMP), aminomethylphosphonic acid (AMP) and hydroxymethylphosphonic acid (HMP). MW = molar weight. downstream removal of calcium carbonate from waste brines containing phosphonate growth inhibitors. However, the limitation of using ultrasound for this application on a large scale is that it is relatively ineﬃcient with respect to the input energy. This drawback, which is encountered in basically all sonochemical waste water treatment applications, might be partially circumvented by using hybrid oxidation techniques [44]. For example, a synergistic eﬀect has been reported for the degradation of phenol in water by using ultrasound in combination with ozone [45]. Another option would be to implement less energy consuming techniques for producing inertial cavitation, such as hydrodynamic cavitation [46].

5.4. Conclusion

It has been demonstrated that the use of ultrasound for the reactivation of poisoned calcite crystals in water is technically feasible. The inhibiting effect of NTMP on calcite crystal growth could be markedly mitigated under influence of ultrasonic irradiation. Recovery of growth rate following inhibition was strongly enhanced by the applied ultrasonic irradiation. In addition, a twofold increase of the inhibited growth rate was observed when ultrasound was applied. The results could be explained in part by the physical effect of ultrasound that causes breakage and attrition of poisoned crystals, which results in fresh surface area. Mass spectroscopy analysis of treated NTMP revealed that there is also a chemical effect of ultrasound that plays

86 5.4. References an important role. Several breakdown products were identiﬁed in NTMP solutions treated with ultrasound. This showed that the C-P bonds in NTMP can be cleaved, probably by means of physicochemically generated free radicals and/or pyrolysis in the hot bubble-bulk interface.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme Concentrates for the discussions and their financial support. We would especially like to thank Ton van der Zande for the mass spectroscopy analysis.

References

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90 6 Free-drift precipitation of CaCO3 in the presence of foreign ions, antiscalants and ultrasound.

abstract The eﬀect of ultrasound on the free-drift precipitation of calcium carbonate in the presence of antiscalants, humic substances, or foreign ions such as magnesium and sulfate was investigated. The applied concentrations of CaCO3 and foreign ions, were based on reverse osmosis drinking water concentrate. The largest eﬀect was found for solutions with antiscalant NTMP. Degradation of NTMP by ultrasound reduced the inhibition period.

This can be used to predict the NTMP dosage effectiveness. In the CaCO3 solution without NTMP but with Mg2+, aragonite is the obtained polymorph. With NTMP, only calcite is observed and it is speculated that NTMP blocks the incorporation of magnesium in the structure of calcium carbonate, possibly by competition of adsorption sites. Application of ultrasound resulted in a decrease of inhibition time, similar to the solution without magnesium. For antiscalant HEDP the experiments were inconclusive. A smaller, but significant effect was seen for the experiments with foreign ions sulfate and magnesium (as a reduction in induction time). The experiments with humic substances did not result in any measurable effect on free-drift precipitation of CaCO3. The application of a constant partial CO2 pressure above the solution resulted in improved reproducibility of the experiments. For that reason an open system or a closed system with application of CO2 gas in the headspace is preferred over a closed system without partial pressure control.

Author: R.M. Wagterveld

91 6. Free-drift precipitation of CaCO3 in the presence of additives and US 6.1. Introduction

Calcium carbonate is one of the most abundant minerals, and the problem of its scaling propensity is encountered in many industrial water treatment processes. Un- wanted precipitation of minerals on process equipment and membranes forms hard layers (scales) which are challenging to remove. This leads to blockage of water flow through pipes and membranes causing higher energy consumption and the necessity of cleaning. In practice scaling of calcium carbonate (and other sparingly soluble salts) is prevented by adding antiscalants to the water [1–4]. These chemicals inhibit the growth of crystals and thereby extend the time of apparent metastability. Growth inhibition is primarily achieved by adding polyelectrolytes like phosphonates and polycarboxylates [5] (chapter 5). Currently existing antiscalants are highly overdosed since the scaling tendency is difficult to assess [6, 7]. However due to stricter environmental regulations a reduction of antiscalant dosage is necessary implying an improvement of scaling tendency determination. The scaling tendency is ruled by various factors. Water chemistry plays a major role, but also the physical nature of the material on which it is depositing and hydrodynamic conditions are important. Small changes in water chemistry can significantly influence precipitation kinetics. As an example: far substoichiometric concentrations of antiscalants increase the apparent metastability. Measuring the concentration of every component online is very impractical. A possible alternative method to determine the scaling tendency is by locally enhancing the precipitation kinetics and thereby recognizing the tendency of precipitation before it occurs in the bulk. The precipitation of calcium carbonate, and its initial step, nucleation, is a well studied subject [8–22]. Nucleation can occur when a solution is supersaturated, i.e. the ion activity product is higher than its solubility. The classical nucleation theory is based on the principle that, at supersaturation, metastable clusters form by stochastic solute clustering. As a critical size has been reached these “nuclei” grow out into crystals [23]. According to this theory, nucleation is the direct phase transformation which is thermodynamically hindered by an energy barrier. The kinetics, or nucleation rate, is then determined by the attachment / detachment frequency of monomers. Due to the threshold character of nucleation, some time may elapse before a measurable amount of the new phase is detected. This time is called the induction time, or induction period and it marks the ability of the solution to stay in a metastable state. This induction time is heavily dependent on the measurement method and various methods have been used by others [11–14, 24, 25]. In the last decade more

92 6.1. Introduction evidence points to the existence of a two step nucleation mechanism [26] in which the nucleation of a crystal occurs within stable mesoscopic clusters of dense liquid. These prenucleation phases were also found in calcium carbonate experiments [27, 28]. Ideally, enhancing the precipitation kinetics is done by adding an external energy source, maintaining the bulk conditions such as water chemistry, pH and temperature. Applying ultrasound might be a good candidate as such an external source. Ultrasound can help controlling the course of precipitation and crystallization processes and is also referred to as sonocrystallization [29]. The positive effects seen in sonocrystallization are usually subscribed to cavitation that appears in high power ultrasound. Cavitation is the interaction of (acoustic) pressure waves with cavities (microbubbles), caused by the rupture of the fluid in the negative pressure cycle. Mi- croscopic bubbles grow and collapse under the varying pressure field inside the treated liquid. Several effects (can) occur during this process; the formation of radicals, generation of shockwaves and microjets, local hotspots of high pressure and temperature, micromixing, macromixing and rise of bulk temperature [30]. Several authors have already investigated the effect of ultrasound on the nucleation of calcium carbonate from a solution without additives [31–37] but the results are not unambiguous. Recently the authors reported on the effect of ultrasound on free-drift precipitation [38] (chapter 4). Applying ultrasound during the first 10 minutes did not result in any significant effect, but application starting later in the experiment enhanced precipitation of CaCO3. A reduction in induction time and increase in volumetric precipitation rate was subscribed to deaggregation in the early growth phase (nuclei to crystals conversion regime). Previously the authors reported on the effect of ultrasound on the (late) growth phase of calcium carbonate [39] (chapter 2). The volumetric crystal growth rate of calcite in a constant composition experiment was increased by the ultrasonic treatment. High speed recordings and SEM analysis revealed that the ultrasound caused the suspended crystals to deagglomerate, disaggregate and accelerate. Also particle breakage and attrition occurred in these experiments [40] (chapter 3). Much research is performed on the spontaneous precipitation of calcium carbonate in the presence of growth inhibitors [1, 4, 41–44], and in this work we will consider two antiscalants: nitrilotris(methylene phosphonic acid) or NTMP (also known as ATMP) and 1-hydroxyethane-1, 1-diphosphonate or HEDP. Both NTMP and HEDP adsorb on the surface of calcium carbonate, thereby interfering with the mass transport towards the growing crystal. This leads to poisoned crystals, the growth rate will drop drastically and growth is inhibited. Although, the effect of ultrasound on growth of

93 6. Free-drift precipitation of CaCO3 in the presence of additives and US phosphonate poisoned calcite crystals has been investigated by the authors previously [5] (chapter 5), the effect of ultrasound on inhibited precipitation of calcium carbonate has not been investigated yet. Ultrasound has been applied in vaterite synthesis under influence of HEDP, with the purpose to prevent agglomeration [45], though, a discussion on ultrasound is lacking. Besides the presence of intentionally added growth inhibitors, the presence of ions, or other compounds such as humic substances, might also influence the crystallization of CaCO3. The influence of the magnesium ion is well documented and is known to reduce calcite growth rates [46–51], stabilize amorphous calcium carbonate (ACC) [52, 53] and favor aragonite formation [47, 49, 50, 54]. Sulfate stabilizes the formation of vaterite [55], promotes the formation of aragonite [56–59], reduces the rate of precipitation (rate of nucleation and growth) of CaCO3, enhances (spherical) aggregation

[60, 61] and even increases the thermodynamic solubility product of CaCO3 [62]. To 2+ 2– date, precipitation of CaCO3 in the presence of ultrasound and Mg or SO4 has not been reported. Humic substances can form complexes with Ca2+ [63], thereby effectively reducing the supersaturation. Moreover, surface growth sites can be blocked by adsorbing ligands [64], inhibiting the crystallization of CaCO3 [61, 65]. Oxidation by ultrasound in combination with hydrogen peroxide has been used to degrade humic substances [66]. In that study, hydrogen peroxide was an initiator of the oxidation of humic substances, and without H2O2 but with ultrasound no degradation was reported. Contradictory, it was also reported that ultrasound can affect a solution containing humic substances, measured as a reduction of TOC [67]. The aim of this work is to investigate the effect of ultrasound on the spontaneous, or free-drift, precipitation of calcium carbonate in the presence of NTMP, HEDP, humic substances, or foreign ions such as magnesium and sulfate. Since all mentioned additives modify the precipitation of CaCO3, it is important to know if, and how, these processes are affected by ultrasound for optimal scaling risk assessment.

6.2. Experimental

6.2.1. Chemicals

94 6.2. Experimental obtained from VWR (Amsterdam, the Netherlands); potassium chloride (KCl), magnesium chloride (MgCl2 · 6 H2O), 1-hydroxyethane-1, 1-diphosphonate (HEDP, purity 99%, acid form, see ﬁg. 6.1a), nitrilotris(methylene phosphonic acid) (NTMP, purity 99%, acid form, see ﬁg. 6.1b), and humic acid from Sigma-Aldrich (Zwijndrecht, the

Netherlands); sodium sulfate (Na2SO4) and manganese chloride (MnCl2 · 2 H2O) from Boom (Meppel, the Netherlands).

For controlling CO2 conditions, a mixture of CO2 (0.86 %) and N2 gas was obtained from Airproducts (Utrecht, the Netherlands). The CO2 concentration is ap- ◦ proximately 22.6 times higher than ambient CO2 at 25 C.

OH HO P O H C H O OH O H H O N OH C C HO P C P OH P P HO H H OH OH O OH CH3 OH

(a) NTMP (b) HEDP

Figure 6.1.: Chemical structure of applied antiscalants.

6.2.2. Experimental setup

A schematic representation of the experimental setup used is presented in figure fig. 6.2. A double walled thermostatted glass reactor, equipped with a floating magnetic stirrerbar (Nalgene) to minimize any grinding effects, a by-pass loop and an ultrasonic transducer were used. The ultrasonic transducer was part of a dedicated homebuilt system that could be controlled precisely in terms of shape, frequency and amplitude of the alternating current for driving the transducer. Free-drift experiments were conducted and the nucleation / growth was determined either by pH measurement (CPS11D, Endress+Hausser) only, or by pH measurement and static light scattering measurement (homebuilt) simultaneously. The static light scattering sensor consists of 3 mW 850 nm diode laser (LDM850/RLJ, Roithner-laser) coupled into a quartz flowthrough cuvette (Hellma) and a photodiode (SFH213FA, Osram) placed at an angle of 90◦. To prevent ambient light entering the photodiode the sen-

95 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.2.: Scheme of the experimental setup, consisting of (1) double walled glass reactor, (2) ﬂoating magnetic stirrer bar, (3) pump, (4) ultrasonic transducer, (5) light scattering sensor, (6)

pH electrode with integrated temperature sensor and (7) CO2 input. sor including flowthrough cuvette is placed in a box. The current generated by the scattered laser light is amplified (AD711, Analog Devices) and measured with dedicated LabVIEW software (LabVIEW 2010, NI PCI-6251 DAQ). In order to avoid any possible influence or damage to the pH electrode by the ultrasonic irradiation it is placed in a glass cell in the by-pass stream. The pH electrode is equipped with a temperature sensor. A positive displacement membrane pump was used to pump the reactor liquid with 1.0 dm3 min−1 through the by-pass.

6.2.3. Experimental procedures

Although the experiments described in this work were performed under a set of diﬀer- ent conditions, the ultrasound and corresponding reference experiment were always performed under the same experimental conditions. The initial concentrations for calcium and carbonate were based on drinking water reverse osmosis concentrate (From

Vitens, Zwolle, the Netherlands), resulting in 9 mM CaCl2 and 14 mM NaHCO3. The experimental conditions based on the additive concentrations, calculated with soft-

96 6.2. Experimental

Table 6.1.:

Solution composition in the reactor, with 9 mM CaCl2 and 14 mM NaHCO3 (after mixing). Supersaturation SC ,SV and SA are deﬁned in equation 6.1, with respect to the calcite, vaterite and aragonite polymorph, respectively, at 25◦C. The supersaturation ratio, initial pH and ionic strength were calculated using software: Visual Minteq v3.0; model: Davies. 2– 2+ [SO4 ] [Mg ] [KCl] SC SV SA pH I µM (mM) (mM) (mM) (-) (-) (-) (-) (-) NTMP 0.33 - - 63.0 5.68 2.93 4.82 7.89 0.100 NTMP/Mg 0.33 - 4.5 63.0 5.44 2.83 4.61 7.86 0.100 NTMP/Mg 0.33 - 9.0 38.0 5.24 2.73 4.44 7.83 0.100 NTMP/Mg 0.33 - 18.0 13.0 4.88 2.54 4.13 7.78 0.100 HEDP 0.24 - - - 6.55 3.42 5.56 7.87 0.039 2– 2+ SO4 /Mg - 10.0 1.8 - 5.54 2.88 4.69 7.89 0.039 Humics 0.75* - - 63.0 5.68 2.93 4.82 7.89 0.100 * mg dm−3 ware Visual Minteq v3.0; Model: Davies, are depicted in table 6.1. For precipitation of CaCO3, the supersaturation ratio S (-) is expressed in terms of activities:

1 IAP v S = (6.1) Ksp

Where IAP is the ion activity product, Ksp is the thermodynamic equilibrium solubility product, and v is the number of ions in the formula unit [68]. The variations for the respective experiments are given in table 6.2. The supersaturation ratio, initial pH and ionic strength were calculated using software: Visual Minteq v3.0; model: Davies. Here it is assumed that NTMP, HEDP and humic substances do not contribute to the equilibrium speciation since the added amount is low. 3 Every experiments was started by simultaneously adding 500 cm of CaCl2 and 3 500 cm of NaHCO3 solution to the reactor. For experiments with adjusted initial ionic strength, KCl was added to the latter solution. The solution was stirred at 400 rpm and the temperature was maintained at 298 ± 0.1 ◦K throughout the entire experiment. The nucleation and subsequent growth were monitored by a drop in pH and / or a rise in voltage of the static light scattering sensor. The nucleated crystals were allowed to grow for some time and a sample was taken by filtering 20-25 cm3 of solution over a 0.2 µm filter. The dried crystals were characterized by scanning electron microscopy (Jeol JSM-6480LV) and Raman spectroscopy (Horiba Jobin-Yvon LabRAM HR). During the experiments, aliquots of solution were rapidly removed, filtered through a 0.2 µm filter, dried and analyzed for the calcium content with

97 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Table 6.2.: Variations on the reference experimental procedure.

Experiment Scat. I Reactor CO2 Meas. Adj. flush NTMP Yes Yes Closed - NTMP and Mg2+ Yes Yes Closed - HEDP - - Open - 2– 2+ SO4 and Mg - - Open - Humics Yes Yes Closed Yes inductive-coupled plasma spectrometry (Optima 3000XL, Perkin-Elmer) to confirm the degree of supersaturation. In the experiments with NTMP, HEDP and humic substances, an aliquot (0.1 - 3 0.5 cm ) of the prepared stock solutions was added to the NaHCO3 solution. The concentrations of the stock solutions were 1 g dm−3 or 3.34 mM for NTMP and 4.85 mM for HEDP. The preparation of humic substances stock solution was as follows. First 10 gram of humic acid was dissolved in 100 cm3 MilliQ water. This solution was first filtered over a 0.45 µm filter and then filtered over a 0.20 µm filter to remove the solids. After filtration, the total carbon concentration was measured to be 5 g dm−3 3 (measured with TOC, Shimadzu TOC-VCPH). Of that solution, 0.15 cm was added to the crystallizing volume giving 0.75 mg dm−3 total carbon. The ultrasonic treatment was always started immediately after mixing at a frequency of 42,150 Hz and with an intensity of 17 W or 7.1 W·cm2 (real output power intensity pre-determined by measuring the adiabatic temperature rise in time using a similar well isolated glass reactor filled with 1 dm3 of water). For the experiments with HEDP, and sulfate and magnesium, the treatment start and duration were varied. In order to maintain the temperature at 298 ± 0.1 ◦K cooler water was recirculated around the reactor.

In the experiments with controlled CO2 environment the reactor is ﬁlled with the

CO2 /N2 gas mixture before starting the experiments. After adding the CaCl2 and

NaHCO3 solutions the lid of the reactor is closed and the air in the headspace is ﬂushed with the gas mixture. Statistical analysis of repeatability for pH is based on previous work [38] (chapter 4).

The first method calculates the mean absolute error (ideally, ∆pH = 0) of the difference between the pH recorded at time t during the first experiment and the pH recorded at the corresponding time t during the duplicate experiment. The second method relies on the calculated correlation coefficient (ideally, R2 = 1) of the two pH

98 6.3. Results and discussion data sets (pH vs pH). Moreover, the precision of the pH electrode itself is 0.01 pH unit (n = 10). Exactly the same statistical analysis was applied to asses the repeatability of the scattering recordings.

6.3. Results and discussion

The initial pH for consecutive experiments was hard to control. This was most probably caused by CO2 exchange during the preparation procedure (as described in section 8.4). The steady rise in pH during the ﬁrst part of the experiments is also ascribed to this phenomenon. Moreover, the initial pH seemed to vary more for a closed system, compared to an open system. These insights were obtained during the course of experiments described here. For that reason, some experiments were performed in a closed system (older experiments), and some in an open system (newer experiments). For the experiments with humic substances suppression of CO2 exchange was attempted. To prevent the supersaturation ratio to drop below its critical value saturation and ensure that CO2 release-induced pH increase did not overshadow the crystallization-induced pH drop, CO2 gas was ﬂushed in the reactor headspace to equilibrate the liquid / gas partial CO2 pressure. Moreover, some experiments were performed with an adjusted ionic strength. The applied experimental conditions were given in tables 6.1 and 6.2

6.3.1. Antiscalants: NTMP, HEDP

NTMP

When a relatively small (far substoichiometric) amount of antiscalants, such as nitrilotris(methylene phosphonic acid) or NTMP (also known as ATMP), and 1-hydroxyethane-1, 1-diphosphonate, or HEDP, is added to a supersaturated CaCO3 solution, the induction time is delayed (this delay is referred to as inhibition time) and the rate of crystallization (volumetric growth rate) is reduced. The effect of 0.33 µM of 2+ 2– NTMP to a solution with typical concentrations of Ca and CO3 found in drinking water reverse osmosis membrane concentrate, is given in fig. 6.3. Figure 6.3a depicts the pH measurement response, fig. 6.3b the scattering response. The white (open) symbols refer to the reference experiment without NTMP (without ultrasound) and the grey lines / symbols to the experiment with NTMP (without ultrasound). The induction time of the reference experiment without NTMP is very short. Within 300 s the pH drops drastically and the scattering increases accordingly. In

99 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.3.:

Free-drift experiments of CaCO3 precipitation in the presence of NTMP: a) pH (-) vs time (s) measurements, b) Scattering (-) vs time (s) measurements. White (open symbols) reference experiment: without NTMP and ultrasound, black with NTMP and ca. 3600 s (1 hour) of ultrasound applied (starting from t = 0 s), gray with NTMP no ultrasound. the presence of a small amount of NTMP (0.33 µm) the induction time is prolonged with the inhibition time. The pH response starts to decrease from t = 6500 s on. The simultaneously measured scattering profile shows a very similar induction time. The difference in slope of the spontaneous and inhibited precipitation profile (after the induction and inhibition time) suggest a reduced volumetric growth rate of CaCO3 in the presence of NTMP. These findings correspond with calcite seeds experiencing inhibited growth by NTMP [5] (chapter 5). Scanning electron microscope (SEM) images of samples taken just after the induction or inhibition period, fig. 6.4a and c respectively, show that the habit and preferred polymorph of the precipitated crystals change when NTMP is added. The reference experiment exhibits typical spherullitic formations of vaterite, whereas the inhibited experiment shows spheres of vaterite and twinned calcite crystals recognizable by the doubly terminated scalenohedron structures with a notch in the middle (polymorphs confirmed with Raman). The same crystal structures in free-drift precipitation of

CaCO3 under inﬂuence of NTMP have been seen by others (although at higher concentrations) [69]. Moreover the inhibited crystals seem to have grown larger in size than the spontaneous precipitated crystals.

Ultrasound is applied during the inhibited precipitation of CaCO3 by NTMP (fig. 6.3 black symbols and lines). The treatment is stopped when a clear drop in pH was observed during the experiment, typically after 3800 s (approximately one hour). Sam- ples for SEM were taken directly after shutting the ultrasound off, fig. 6.4b. The SEM

100 6.3. Results and discussion

Figure 6.4.:

Free-drift experiments of CaCO3 precipitation in the presence of NTMP, SEM images (25 cm3 sample volume): a) Reference experiment: without NTMP and ultrasound (sample at 600 s), b) with NTMP and ca. 3600 s (1 hour) of ultrasound applied (starting from t = 0 s) (sample at 3800 s), c) with NTMP, no ultrasound (sample at 6300 s) image after treatment with ultrasound shows that with reduced inhibition spherical vaterite is the dominant crystal habit. The effect of ultrasound on precipitation is similar to the NTMP inhibited growth experiments with ultrasonically treated calcite seeds [5] (chapter 5). The inhibition time is reduced upon ultrasonic treatment, However, the crystallization rate does not seem to be affected much. The slope of the pH and scattering profiles after inhibition is comparable for the ultrasonically treated and untreated experiments. Analyzing the repeatability reveals the pH experiment to be less repeatable than the scattering experiment, fig. 6.3a versus b. For the reference experiment, without NTMP and ultrasound, the pH repeatability is defined by: ∆pH (± s.d.)

= 0.012 ± 0.008, R2 = 0.956, versus ∆scat = 0.014 ± 0.011, R2 = 0.983 for the scattering curves, which is similar. However, with NTMP (without ultrasound) pH repeatability gives: ∆pH = 0.144±0.039, R2 = 0.686, whereas scattering gives very good repeatability: ∆scat = 0.005 ± 0.006, R2 = 0.993. The ultrasound experiment gives better repeatability for pH: ∆pH = 0.037 ± 0.013, R2 = 0.740 and for scattering: ∆scat = 0.005 ± 0.005, R2 = 0.875. It is remarkable that when pH is ill repeatable, scattering is not, whereas both measurements were carried out in the exact same experiment. The high scattering repeatability suggest similar conditions for the repeated experiments, hinting that the pH measurement is much more sensitive to experimental conditions than the scattering measurement (see discussion on CO2 exchange, chapter 8). Also, additional inﬂuence such as a slow response of the pH electrode cannot be excluded.

Besides a direct eﬀect of ultrasound on CaCO3 precipitation, degradation of NTMP, or crystal breakage, are possible mechanisms. Crystal breakage is very unlikely

101 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.5.:

Free-drift experiments of CaCO3 precipitation in the presence of NTMP and magnesium (18 mM): a) pH (-) vs time (s) measurements, b) Scattering (-) vs time (s) measurements. White (open symbols) reference experiment: without NTMP and ultrasound, black with NTMP and ca. 3600 s (1 hour) of ultrasound applied (starting from t = 0 s), gray with NTMP no ultrasound. though, since the slope of the precipitation profiles remain unchanged after ultrasonic treatment. Moreover, since the treatment is stopped as soon as growth is detected, most of the treatment will be in the region where the crystals are small and difficult to break [38] (chapter 4). Hence, the degradation of NTMP is the main mechanism [5] (chapter 5). The reactivated surfaces can take part in the growth process again, leading to shorter inhibition time. As will be clarified in the next section, the direct effect of ultrasound on CaCO3 precipitation is minimal under the current conditions. The reduced inhibition time by ultrasound can be exerted to predict the NTMP dosage effectiveness. In practice this means that ultrasound can be applied to probe the apparent metastable state of the liquid with NTMP as the applied antiscalant. To sketch the working of such method, consider the following situation. With the application of ultrasound reduced inhibition might cause the solution to become instable (precipitation) within a predefined time frame. In that case the antiscalant dosage should be increased in the bulk. Is the solution stable (no precipitation) within that time frame, the antiscalant dosage can be reduced in the bulk. Due to the reduced inhibition time, some time is created to perform this mitigation in the bulk solution.

NTMP and Magnesium

Additional to the experiments performed with a solution of CaCO3 and NTMP, the eﬀect of the presence of magnesium is investigated in this system (for solution com-

102 6.3. Results and discussion position see table 6.1). As an example, fig. 6.5 depicts the pH and scattering profiles with 18 mM Mg2+ added. When compared with the graphs of fig. 6.3 several difference can be noted. First, the induction and inhibition times were significantly prolonged under influence of 2+ magnesium. This effect of Mg on CaCO3 precipitation is well known and ascribed to the incorporation of Mg in the crystal lattice [47, 57] and the higher dehydration energy as opposed to calcium [47]. Another important effect is that Mg2+ influences the speciation, leading to a reduction in supersaturation for increased Mg2+ concentration (table 6.1). The induction time (with Mg2+, without NTMP) is extended to ca. 1200 s. When the pH profile is inspected the inhibition time was around 10,000 s, fig. 6.5a. However, the scattering profile shows a clear increase earlier in the experiment compared to the pH profile, around 9000 and 9500 s. This can be explained by the effect of CO2 exchange which influences the pH measurement. With ultrasound applied, again the inhibition time is greatly reduced to ca. 6000 s, similar to the experiment without magnesium. No experiments were performed to investigate the effect of ultrasound on CaCO3 in the presence of magnesium only, thus an additional effect of ultrasound caused on magnesium can therefore not be excluded (but is not expected). So, also in the presence of the “growth rate reducing” ion magnesium, ultrasound can be applied to determine the effectiveness of NTMP. In terms of repeatability, pH and scattering profiles show similarities. The reference experiment without NTMP and ultrasound has the lowest pH repeatability. For pH:

∆pH = 0.031 ± 0.022, R2 = 0.239, versus ∆scat = 0.044 ± 0.057, R2 = 0.541 for scattering. The experiment with NTMP (without ultrasound) has the lowest scattering repeatability. For pH: ∆pH = 0.011±0.009, R2 = 0.884, versus ∆scat = 0.006 ± 0.009, R2 = 0.328 for scattering. The repeatability with ultrasound appeared to be the best: pH: ∆pH = 0.008 ± 0.007, R2 = 0.956, versus ∆scat = 0.004 ± 0.002, R2 = 0.996 for scattering. Since the pH and scattering proﬁles were similar in terms of repeatability, the consecutive experiments of the least repeatable experiment (with NTMP, without ultrasound), seem to be performed under slightly diﬀerent experimental conditions.

Figure 6.6 shows the samples taken from the CaCO3 precipitate in the presence of magnesium only and in additional presence of NTMP. Without NTMP, ﬁg. 6.6b,d and f, the obtained polymorph is aragonite. Magnesium does not adsorb on, nor incorporate in, aragonite [47], but it does incorporate in vaterite [57]. This is the reason that with magnesium added, aragonite and not vaterite (as in the experiments without magnesium) is the much more stable polymorph. However, with NTMP applied,

103 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.6.:

Free-drift experiments of CaCO3 precipitation in the presence of NTMP and Magnesium ions, samples for SEM images were taken after signiﬁcant pH decrease (sample volume: 25 cm3). Left hand side of images with NTMP, right hand side without NTMP.: a) With NTMP and 4.5 mM Mg2+, b) 4.5 mM Mg2+ only (no NTMP), c) With NTMP and 9.0 mM Mg2+, d) 9.0 mM Mg2+ only (no NTMP), e) With NTMP and 18 mM Mg2+, f) 18 mM Mg2+ only (no NTMP)

104 6.3. Results and discussion

Figure 6.7.:

Free-drift experiments of CaCO3 precipitation in the presence of HEDP a) Increasing HEDP in the direction of the arrow: 0 µM (0 mg dm−3), 48.5 µM (0.01 mg dm−3), 146 µM (0.03 mg dm−3) and 243 µM (0.05 mg dm−3), respectively. b) Ultrasound experiments in the presence of 243 µM (0.05 mg dm−3) HEDP. Gray line: Blank experiment, dashed lines: 0 - 300 s, dotted lines: 300 - 1200 s, black line: 1200 - 2700 s. different crystal polymorphs and habits can be observed. In all cases the preferential polymorph is calcite. We can speculate that NTMP blocks the incorporation of magnesium in the structure of calcium carbonate, possibly by competition of adsorption sites. For the lowest magnesium concentration, the crystals resemble the calcite crystals in experiments with NTMP only, fig. 6.4c (twinned calcite crystals recognizable by the doubly terminated scalenohedron structures with a notch in the middle). With increasing magnesium concentration the volume around the notch seems to be deposited with calcium carbonate. The crystal habit shows similarities with parallel growth. Although the habit changes, the polymorph remains to be calcite. With the highest concentration of magnesium present, the volume around the notch is completely filled leading to rugged planes (parallel growth is not recognizable as such any more).

HEDP

In the previous section, the eﬀect of a relative small amount of antiscalant NTMP added to a supersaturated solution of CaCO3 was described. Similar experiments were performed with antiscalant 1-hydroxyethane-1, 1-diphosphonate, or HEDP. Fig- ure 6.7a depicts the eﬀect of increasing HEDP concentration (direction of the arrow). The higher the HEDP concentration, the longer it takes for precipitation to occur,

105 6. Free-drift precipitation of CaCO3 in the presence of additives and US thus the larger the inhibition time. The maximum applied concentration for HEDP (0.24 µM) is less than the concentration as applied for NTMP (0.33 µM). Analyzing the SEM images of the precipitate after 15 minutes of experiment, the flat vaterite plates become more spherical with increasing HEDP concentration (fig. 6.8a,b, samples at 900 s). With the highest HEDP concentration no (large) crystals are visible after 900 s, fig. 6.8c. However zooming in 8x reveals the existence of nanosized particles. With an undersaturated calcium carbonate solution, these particles were not encountered, but they were found in every experiment with SEM images taken just before the induction time. This includes the experiments in [38] 2+ 2– (chapter 4), and experiments with Mg and SO4 , chapter 7. The same samples were investigated with a different SEM, resulting in higher resolution images, fig. 6.8e,f. These images also show the grains of the gold coating (dotted, carpet like structure), and even cracks in the coating are visible (fig. 6.8f). The particles are in the order of 30 nm and share the same size distribution as aggregated prenucleation clusters [28]. It seems that HEDP cannot block the formation of these nanoaggregates, but prevents the nanoaggregates from further aggregation and growth into full sized crystals. The property of HEDP to prevent agglomeration has been reported by others [45]. When ultrasound is applied to the solution with the highest HEDP concentration (0.24 µM), no significant reduction of the inhibition time was observed fig. 6.7b. This does not match with the reduced inhibition time for NTMP, although the treatment periods were also much shorter and the concentration of inhibitor lower. Treatment in the first 300 seconds did not result in a significant effect. Treatment between 300 - 1200 and 1200 - 2100 s seems to result in reduced inhibition. The variation in repeated experiments were in the order of the reduction though, so a definite conclusion is not possible. For the ultrasound applied from 0 - 300 s the repeatability is: ∆pH = 0.064 ± 0.059, R2 = 0.914. For treatment during 300 - 1200 s: ∆pH = 0.065 ± 0.047, R2 = 0.928. The correlation between experiments is high but the variation in pH as well. Since scattering experiments are lacking, the conditions of repeated experiments might have been slightly different. The discrepancy between NTMP and HEDP treated ultrasound might be explained by the fact that the amine free HEDP is not expected to decompose as easily as NTMP under influence of radicals [70]. Ultrasound might have the same influence on the formation of crystals in the early growth phase, as described for solutions with CaCO3 only in [38] (chapter 4). Espe- cially since, if we conclude there is an effect, the largest effect is seen for the treatment in the early growth phase period (300 - 1200 s). However, as stated before, due to

106 6.3. Results and discussion

Figure 6.8.:

Free-drift experiments of CaCO3 precipitation in the presence of HEDP, samples taken at 900 s, sample volume: 25 cm3. a) 48.5 µM (0.01 mg dm−3); b) 146 µM (0.03 mg dm−3); c) 243 µM (0.05 mg dm−3); d) 8x zoom of c, e) 10x zoom of sample c) with another SEMa, f) 2x zoom of e).

aFESEM images (Jeol, JSM-6000F), by Herman Teunis of the membrane technology Group, Uni- versity of Twente, Enschede the Netherlands.

107 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.9.: 2– 2+ Free-drift experiments of CaCO3 precipitation in the presence of SO4 (10 mM), Mg (1.8 mM) and ultrasound. Gray line: Blank experiment (no ultrasound), dotted lines: 0 - 300 s, dashed dark gray lines: 300 - 1200 s, dashed black lines: 1200 - 2700 s. the large variation, strict conclusions are not possible. Since there is no clear reduced inhibition time in the HEDP experiments, possible eﬀects of ultrasound on the precipitation of calcium carbonate can therefore be ruled out as the cause to the reduced inhibition of NTMP (see previous section). We can therefore conclude that for HEDP ultrasound cannot be applied to determine the eﬀectiveness of inhibition.

6.3.2. Foreign ions: Sulfate and magnesium

The effect of sulfate on free-drift calcium carbonate precipitation will be discussed in detail in chapter 7. This chapter shows that the presence of sulfate and magnesium results in larger induction times and lower supersaturation. In this section the focus is on the effect of ultrasound on free-drift CaCO3 precipitation in the presence of foreign ions sulfate and magnesium. It is not expected that ultrasound has a direct effect on the sulfate or magnesium ion. The effect of sulfate on free-drift calcium carbonate precipitation will be discussed in detail in chapter 7. This chapter shows that the presence of sulfate and magnesium results in larger induction times and lower supersaturation. In this section the focus is on the effect of ultrasound on free-drift

CaCO3 precipitation in the presence of foreign ions sulfate and magnesium. It is not expected that ultrasound has a direct effect on the sulfate or magnesium ion. Ultrasonic treatment in the first 300 s does not result in a significant difference with the blank experiment (no ultrasound), fig. 6.9 gray line. This complies with the treatment in solutions of CaCO3 without additives as discussed in chapter 4. We

108 6.3. Results and discussion

Figure 6.10.:

SEM images at the end of free-drift experiment (4500 s) CaCO3 precipitation in the 2– 2+ 3 presence of SO4 (10 mM), Mg (1.8 mM) and ultrasound (sample volume: 20 cm ). a) 0 - 300 s of ultrasound (dotted curves in ﬁg. 6.9), b) 1200 - 2700 s of ultrasound (dashed black lines in ﬁg. 6.9).

2– can therefore also conclude that cavitation has no direct (measurable) effect on SO4 and Mg2+. The treatment in the early growth phase (aggregation and outgrowth of nanoaggregates), 300 - 1200 s (fig. 6.9 dashed dark gray lines), does result in a significant effect, the largest effect of the experiments discussed here. Treatment in the later growth phase (growth of larger crystals with possible breakage), 1200 s - 2100 s (fig. 6.9 dashed black lines) results in a less pronounced effect, but is still significant. The results comply with the results for a system without additives with higher pH and lower supersaturation, see [38] (chapter 4), but the effects seem less pronounced. Unfortunately, not that many experiments were performed for the conditions described in this section. Since there is only data of one reference experiment, repeatability analysis is not possible for this experiment. The repeatability of the other experiments is very similar for the different conditions applied. For the ultrasound applied from 0 - 300 s the repeatability is: ∆pH = 0.017 ± 0.009, R2 = 0.963. For treatment during 300 - 1200 s: ∆pH = 0.020 ± 0.012, R2 = 0.965, and for 1200 -

2100 s: ∆pH = 0.018 ± 0.009, R2 = 0.971. Inspecting the SEM images of the samples taken at the end of the experiment (4500 s) reveals that applying ultrasound starting after 300 s, results in more crystals, but at ﬁrst glance, not in a diﬀerent size distribution. This again corresponds well with the experiments described in [38] (chapter 4). Since this case resembles experiments in solutions of CaCO3 without additives, an additional interaction of ultrasound with

109 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.11.:

Free-drift experiments of CaCO3 precipitation in the presence of humic substances: a) pH (-) vs time (s) measurements, b) Scattering (-) vs time (s) measurements. White (open symbols) reference experiment: without ultrasound, gray with ultrasound during ca. 3600 s (1 hour) of ultrasound applied (starting from t = 0 s), black with ultrasound during the complete experiment (ca. 9000 s or 2.5 hours).

2+ 2– Mg and / or SO4 , thereby additionally inﬂuencing the precipitation of CaCO3, does not seem to take place. More experiments are necessary to support the conclusions drawn here.

6.3.3. Humic substances

Humic substances are known to inhibit crystallization of CaCO3 as well [61, 65]. For the experiments with humic substances, a mixture of CO2 and N2 gas was ﬂushed through the reactor headspace, minimizing CO2 exchange between water and air.

Figure 6.11 depicts the pH curves for free-drift experiments of CaCO3 in the presence of humic substances. The experiment without humic substances, is not included in

fig. 6.11. Application of CO2 did not modify the course of crystallization for the solution without humic substances, thus the pH curve of that solution can be found in fig. 6.3, since all other conditions are the same. Comparing the pH curves of experiments with inhibitor, humic substances in fig. 6.11, with NTMP in fig. 6.3, reveals the effect of the application of CO2 gas. Instead of a steady rise of pH (before crystal growth) without application, the pH stabilizes (before crystal growth) with

CO2 applied.

Application of ultrasound during CaCO3 precipitation in the presence of humic substances did not result in a significant effect, when inspecting the pH curves (fig. 6.11).

Ultrasound does not seem to aﬀect the inhibition of CaCO3 precipitation by humic

110 6.3. Results and discussion substances. However, the scattering curves show a slight difference between the experiment with and without ultrasound applied. The addition of humic substances modify the optical path of the infrared laser. Addition of even this small amount of humic substances results in a slightly red colored solution. This results in more laser light adsorption, and probably also more scattering. When ultrasound is applied, the scattering curve gradually increases in the inhibition period (before crystal growth), which points to a reduced adsorption or increased scattering. Inspection with the naked eye during the experiment gave the impression that the red color be- came less pronounced when time passed. The humic substances seem to be affected by ultrasound, but this does not affect the inhibition of CaCO3 precipitation. In terms of repeatability, the pH and scattering profiles show similarities. The reference experiment without ultrasound had for pH: ∆pH = 0.012 ± 0.007, R2 =

0.950, versus ∆scat = 0.007 ± 0.007, R2 = 0.987 for scattering. The experiment with 1 hour of ultrasound has the lowest repeatability. For pH: ∆pH = 0.023 ±

0.026, R2 = 0.792, versus ∆scat = 0.016 ± 0.019, R2 = 0.905 for scattering. The repeatability with 2 hours of ultrasound was pH: ∆pH = 0.013 ± 0.011, R2 = 0.944, versus ∆scat = 0.005±0.004, R2 = 0.994 for scattering. In general the repeatability was very good for these experiments. So application of CO2 seems to be beneficial for the repeatability in a closed system, although this does not seem to apply for the resulting effect, since there is no significant difference between pH curves with and without ultrasound. The correspondence of an open system with CO2 applied in the headspace, is the constant partial CO2 pressure above the reactor. This seems to be the reason for the better reproducible results. In a closed system without partial pressure control, variations in partial pressure are more easily attained. This might lead to temporary (random) changes in supersaturation by dissolved CO2 gas exchange, resulting in less repeatability due to variation in precipitation paths. About the exact reason for the lack of measurable effect of ultrasound we can only speculate. It might have to do with the adsorption of wide variety of organic molecules present in the solution, reducing the ability of nanocrystals of CaCO3 to aggregate, removing the suggested deaggregation effect in the early growth phase. SEM images of samples taken at the end of the experiment, see fig. 6.12 show crystals similar to the crystals obtained with NTMP and magnesium (fig. 6.6c). The calcite crystals are similar to the twinned doubly terminated scalenohedron crystal structures with a notch in the middle, however for some crystals with parallel growth around the notch.

111 6. Free-drift precipitation of CaCO3 in the presence of additives and US

Figure 6.12.:

Free-drift experiments of CaCO3 precipitation in the presence of humic substances (sample volume: 25 cm3): a) pH (-) vs time (s) measurements, b) Scattering (-) vs time (s) measurements. White (open symbols) reference experiment: without ultrasound, gray with ultrasound during ca. 3600 s (1 hour) of ultrasound applied (starting from t = 0 s), black with ultrasound during the complete experiment (ca. 9000 s or 2.5 hours).

6.4. Conclusion

Application of ultrasound aﬀect under certain conditions the free-drift precipitation of

CaCO3 (drinking water RO concentration) in the presence of foreign ions, antiscalants and ultrasound. The largest effect of ultrasound can be found in experiments with antiscalant NTMP as a reduced inhibition period. Main mechanism responsible for the observed effect is the break down of NTMP by radicals when ultrasound is applied. The reduced inhibition time by ultrasound can be exerted to predict the NTMP dosage effectiveness. In practice this means that ultrasound can be applied to probe the apparent metastable state of the liquid with NTMP as the applied antiscalant. For antiscalant HEDP the experiments were inconclusive and ultrasound does not seem to be the right tool to measure and control dosage effectiveness. The addition of magnesium to the system with NTMP resulted in extended inhibition times, due to lower supersaturation (compared to the system without magnesium), incorporation of magnesium in the crystal lattice and higher dehydration energy of magnesium compared to calcium. In the CaCO3 solution without NTMP but with Mg2+, aragonite is the obtained polymorph. With NTMP, only calcite is observed and it is speculated that NTMP blocks the incorporation of magnesium in the structure of calcium carbonate, possibly by competition of adsorption sites. Application of ultrasound resulted in a decrease of inhibition time, similar to the so-

112 6.4. References lution without magnesium. So, also in the presence of magnesium, ultrasound can be applied to determine the effectiveness of NTMP. A smaller, but significant effect was seen for the experiments with foreign ions sulfate and magnesium (as a reduction in induction time). The results were very similar to the effects seen in experiments in a solution without additives. The presence of foreign ions, sulfate and magnesium, does not seem to influence the response to ultrasound compared with solutions without additives. Additional experiments are necessary to support this conclusion. The experiments with humic substances did not result in any measurable effect.

The application of a constant partial CO2 pressure above the solution resulted in better repeatable experiments. For that reason an open system or a closed system with application of CO2 gas in the headspace is preferred over a closed system without partial pressure control.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Sensoring” for the discussions and their financial support.

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118 7 Polymorphic change from vaterite to aragonite under inﬂuence of sulfate: the “morning star” habit

abstract The presence of sulfate in reverse osmosis drinking water concentrate and its eﬀect on calcium carbonate precipitation was studied, notably the overall kinetics of CaCO3 formation and the types of polymorphs formed. CaCO3 formation slows down with increasing sulfate concentration and the preferential polymorph shifts from vaterite to aragonite with increasing sulfate concentration. With this polymorphic change, a new combined habit is observed where (presumably) aragonite spikes grow on top of vaterite (“morning star” habit). The presence of a moderate magnesium concentration results in the shift of vaterite to aragonite at relatively low sulfate concentrations; where sulfate and magnesium appear to have an additive eﬀect. Without magnesium, spikes on top of vaterite were also observed, but only at relatively high sulfate concentration. Without the presence of magnesium, single crystals of aragonite were not found.

This chapter has been submitted as: R.M. Wagterveld, M. Yu, H. Miedema, G.J. Witkamp. Poly- morphic change from vaterite to aragonite under inﬂuence of sulfate: the “morning star” habit Journal of Crystal Growth.

119 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

7.1. Introduction

There is an increasing interest in understanding and controlling mineralogical and biological processes, especially processes determined by the composition of seawater. Calcium carbonate is such a mineral which receives much attention. Besides the occurrence in seawater, calcium carbonate is an important industrial product. Fur- thermore, the scaling propensity of calcium carbonate is a problem in many industrial water treatment processes. Controlling the formation of calcium carbonate is therefore of great interest and as a result the crystallization of CaCO3 from clear solution (spontaneous precipitation) has been investigated extensively [1–10].

CaCO3 is known to exist in six forms: amorphous (ACC), two hydrated (mono- hydrocalcite, or MHC, and ikaite) and three anhydrous polymorphic forms. The anhydrous polymorphs are calcite, aragonite and vaterite, in which vaterite is consid- 2+ 2– ered to be metastable. Apart from Ca and CO3 , seawater contains more ions, and after chloride, sodium and magnesium, sulfate is the most abundant species. Actually, sodium sulfate is the second most common water-soluble mineral in nature [11] which makes sulfate abundantly present in groundwater and surface water as well. The effect of cations, anions and other substances on the precipitation pathway of CaCO3 has been investigated extensively, but the documentation on the eﬀect of sulfate is sparse. A recent study addresses the role of sulfate in sea-water crystallization [12]. It has been documented that sulfate stabilizes the formation of the vaterite polymorph [13], but there is also evidence that sulfate promotes the formation of the aragonite polymorph [12, 14–16]. Furthermore it has been reported that sulfate reduces the rate of precipitation (rate of nucleation and growth) of CaCO3 and even increases the thermodynamic solubility product of CaCO3 [17]. Moreover it has been mentioned that sulfate enhances (spherical) aggregation [18, 19].

Even though the role of sulfate in CaCO3 formation is rather ambiguous, it seems an important parameter in water chemistry and composition. This is true not only for sea water, but also, for example, for concentrates that result from reverse osmosis (RO) processes during the production of drinking water. In the present study we investigate the eﬀect of sulfate (and to a lesser extent magnesium) on the precipitation of CaCO3 in drinking water, notably the overall kinetics of CaCO3 formation and the types of polymorphs formed.

120 7.2. Experimental

Table 7.1.: Reference solution composition (after mixing), based on drinking water RO concentrate (numbers from Vitens (Zwolle, the Netherlands)). Solution1 Solution 2 Compound (mM) Compound (mM)

CaCl2 · 2 H2O 8.96 NaHCO3 14.2 MgCl2 · 6 H2O 1.81 Na2SO4 2.66 −5 MnCl2 · 2 H2O 9.09 x 10

7.2. Experimental

7.2.1. Chemicals

Only analytical grade reagents, grade A glassware and high quality water (MilliQ Reagent Water System, resistivity >18 MΩ·cm) were used throughout the experiments. Calcium chloride (CaCl2 · 2 H2O), sodium bicarbonate (NaHCO3), sodium chloride (NaCl) and hydrochloric acid (HCl, 0.1M) were obtained from VWR (Am- sterdam, the Netherlands), magnesium chloride (MgCl2 · 6 H2O) from Sigma-Aldrich

(Zwijndrecht, the Netherlands) and sodium sulfate Na2SO4 and manganese chloride

MnCl2 · 2 H2O) from Boom (Meppel, the Netherlands).

7.2.2. Experimental setup

An open, double walled, thermostatted glass reactor was used, equipped with a floating magnetic stirrer bar (Nalgene) to minimize any grinding effects. Free drift experiments [20] were conducted and nucleation / growth were determined on-line by recording pH (CPS11D, Endress+Hausser, buffer accuracy ±0.02 pH). The pH electrode is equipped with a temperature sensor. A positive displacement membrane pump was used to recirculate the reactor liquid with 1.0 dm3 min−1.

7.2.3. Experimental procedures

The free drift method was applied to study precipitation in the present work. During a free drift experiment, the driving force for precipitation, i.e. the supersaturation, decreases as a result of CaCO3 formation. The fundamental dimensionless driving force for precipitation in electrolyte solutions is deﬁned as follows [21]:

∆µ = v ln S (7.1) RT

121 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

Here ∆µ (J mol−1) is the change in chemical potential, R (J mol−1 K−1) the gas constant, T (K) the absolute temperature, v (-) the number of ions in the formula unit (v = 2 for CaCO3) and S (-) the supersaturation ratio, which is expressed in terms of activities:

1 IAP v S = (7.2) Ksp 2+ 2– Where IAP is the ion activity product of Ca and CO3 and Ksp the thermodynamic equilibrium solubility product of the particular CaCO3 polymorph. The reference composition of the solutions used in the experiments is based on drinking water RO concentrate, table 7.1 (numbers from Vitens, Zwolle, the Netherlands). In 2– order to study the eﬀect of SO4 , its concentration in solution 2 was varied between 0 and 60 mM. Every experiment was started by adding (simultaneously) 500 cm3 of solution 1 and 500 cm3 of solution 2 (table 7.1) to the reactor, while stirring at 400 rpm. The temperature was maintained at 25◦ C throughout the entire experiment.

One of the main parameters to monitor CaCO3 precipitation is pH. Due to the chosen conditions, the carbonate consumption resulting from nucleation and ulterior growth could be monitored by the drop in pH, the precipitation rate. The nucleated crystals were allowed to grow over time and two samples were taken by filtering a specific amount of solution (20-25 cm3) over a 0.2 µm filter. The dried crystals of the first sample were characterized by scanning electron microscopy (Jeol JSM-6480LV) and Raman spectroscopy (Horiba Jobin-Yvon LabRAM HR). The dried crystals of the second sample were dissolved in HCl. The presence of sulfate and magnesium in the crystal structure was determined by measuring the calcium, sulfur and magnesium content of the dissolved crystals with ICP, inductive-coupled plasma spectrometry (Optima 3000XL, Perkin-Elmer). Moreover, during the experiments, aliquots of solution were rapidly removed, filtered through a 0.2 µm filter, and analyzed for the calcium, sulfur and magnesium content with ICP.

7.3. Results and discussion

Three series of experiments, A, B and C, were performed to investigate how the sulfate concentration aﬀects precipitation of CaCO3, based on drinking water RO concentrate:

2– 1. Variation of SO4 concentration, without additional adjustments (table 7.2).

122 7.3. Results and discussion

Table 7.2.: 2– Experiment series A: Variation in SO4 concentration with respect to the reference solution (experiment A.2 in this table, composition in table 7.1). Ionic strength, initial pH and initial supersaturation ratio for vaterite (Sv) and aragonite (Sa) vary accordingly. Su- persaturation ratio S is deﬁned in eq. (7.2), at 298 K. Calculations based on the measured pH, software: Visual Minteq v3.0; model: Davies. 2– Exp. [SO4 ] Sv Sa pH I # (mM) (-) (-) (-) (-) A.1 0.00 3.67 5.97 7.95 0.044 A.2 2.66 3.60 5.86 7.98 0.049 A.3 5.00 3.53 5.75 8.00 0.054 A.4 10.0 3.34 5.43 8.02 0.064 A.5 30.0 3.07 4.99 8.13 0.112 A.6 60.0 2.75 4.47 8.18 0.185

2– 2. Variation of SO4 concentration, with all solutions having the same (calculated) initial pH of 7.83, and ionic strength, I = 0.185, through addition of HCl and NaCl, respectively (table 7.3). 2– 3. Variation of SO4 concentration, with all solutions having the same (calculated) initial pH of 7.83, and ionic strength, I = 0.185, without the presence of

MgCl2 · 6 H2O and MnCl2 · 2 H2O (table 7.4).

Figure 7.1a describes the pH measurement of experiment series A. Table 7.2 gives the calculated conditions obtained with thermodynamic modeling software VMinteq V3.0 (model Davies) with the measured pH as master parameter. Increasing the sulfate concentration results in increasing ionic strength and pH but decreasing supersaturation ratios of the CaCO3 polymorphs. If CO2 exchange is not taken into account, the calculated pH lies between 7.86-7.90 for experiment series A. All measured initial pH values in ﬁg. 7.1 were higher, which seems to be related to the excess amount of dissolved CO2 in the solution. Release of CO2 gas to the atmosphere occurs already during solution preparation, resulting in this higher initial pH. Also, during the experiment, release of CO2 will have an eﬀect on the pH and activity of carbonate. As a result of this release, the supersaturation ratio, perhaps counter- intuitively, slightly increases during the experiment (section 8.4). In contrast to the

CaCO3 polymorphs, under all conditions applied, even at the highest sulfate levels, the solution remained unsaturated with respect to all polymorphs containing sulfate. Increasing the concentration of sulfate (from A.1 to A.6), leads to increasing initial pH, ionic strength and initial supersaturation ratio. The dissolved sulfate chelates a

123 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

Figure 7.1.: Eﬀect of sulfate on the precipitation rate of calcium carbonate, measured as a decrease 2– in pH. The arrows indicate increasing SO4 concentration: 0.00, 2.66, 5.00, 10.0, 30.0 and 2– 2– 60.0 mM SO4 , respectively. a) Experiment A: Variation of SO4 concentration causes varying initial supersaturation ratio, initial pH and ionic strength (table 7.2). b) As experiment A but with all solutions having the same calculated initial pH of 7.83, and ionic strength, I = 0.185, through addition of HCl and NaCl, respectively (table 7.3). c)

As experiment B but in the absence of MgCl2 · 6 H2O and MnCl2 · 2 H2O (table 7.4).

0 considerable part of the calcium and magnesium ions present, forming aqueous CaSO4 0 2– 2+ 2+ and MgSO4. This also causes less binding of CO3 to Ca and Mg thereby shifting 2– – the CO3 -HCO3 equilibrium, resulting in an increase in pH. This pH increase is only

124 7.3. Results and discussion

Figure 7.2.: Scanning electron microscope (SEM) images of experiment A. Sample volume, 25 cm3, filtered over a 0.2 µm membrane filter: Sulfate concentrations: a) 0.00 mM, b) 2.66 mM, c) 5.00 mM, d) 10.0 mM, e) 30.0 mM, f) 60.0 mM. The precipitation rate decreases from a) to f). partly compensated for by the higher ionic strength. Overall, increased sulfate levels result in lower supersaturation ratios of the CaCO3 polymorphs. Figure 7.2 shows the scanning electron microscope (SEM) images at the end of experiments A, t = 2700 s. The experiment without sulfate addition (A.1) resulted in the vaterite polymorph (fig. 7.2a). The image exhibits a large volume of particles since this is the fastest pH-dropping experiment (fig. 7.1a), thus displaying the highest precipitation rate. The higher the sulfate concentration, the lower the precipitation rate and total volume of crystals. From fig. 7.2c on, a clear change in the crystal habit can be observed. The hexagonal vaterite crystals start to exhibit spikes, and aragonite crystals become noticeable. Especially in fig. 7.2d, a vaterite crystal is visible with spikes growing out from its surface. We assume that these spikes are of the aragonite polymorph, since the habit of the measured aragonite crystals had a similar spiked shape. The promotion of aragonite formation by sulfate has been reported before [12, 14– 16]. The transformation of vaterite to aragonite under influence of (sodium)dodecylsulfate has been described previously [22–24]. Without additives, the transformation of vaterite to aragonite has been attributed to a dissolution recrystallization process at elevated temperatures [25]. The growth of one polymorph on top of another has

125 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

Figure 7.3.: Scanning electron microscope (SEM) images of the first 15 minutes of experiment A.4, sulfate concentration 10 mM, showing the development of the “morning star” habit. Sam- ple volume, 25 cm3, filtered over a 0.2 µm membrane filter: a) t = 300 s, b) t = 600 s, c) t = 900 s. Note that the first image has a larger magnification (a, 37000x), than the others (b, c, 25000x) already been found for the combination calcite and aragonite or calcite and vaterite with [26] and without [14] the presence of additives. The coexistent growth of aragonite and vaterite in the presence of dodecylsulfate and n-pentanol has been mentioned briefly in a previous study but without much detail [22]. To the best of our knowledge to date, the present study for the first time reports the polymorphic change of vaterite to aragonite under the influence of sulfate, and the typical combined habit of aragonite spikes on top of hexagonal vaterite. We will call the (combined) habit of this vaterite crystal with spikes of aragonite “morning star”, since it closely resembles the medieval weapon carrying this name. Figure 7.3 shows the formation of the two different polymorphs, vaterite and aragonite, over time, based on three consecutive samples taken at t = 5, 10 and 15 minutes 2– after starting the experiment (A.4, 10 mM SO4 ). The first 15 minutes show the development of the “morning star” crystals to its final habit, similar to the one in fig. 7.2d. Already after the first 5 minutes, tiny, thin spikes can be seen on the growing vaterite crystal, fig. 7.3a. In the following five minutes the spikes grew larger and thicker fig. 7.3b, and it is clear that the habit after 15 minutes (fig. 7.3c) is the same as after 45 minutes (fig. 7.2d). We presented the crystals with the largest spikes in this image series. Figure 7.4 shows examples of Raman spectra taken over time and under different ionic conditions. At intermediate sulfate levels (10 mM) both polymorphs vaterite (V) and aragonite (A) were observed. After 15 minutes most of the crystals were of the vaterite polymorph (fig. 7.4A). The highest aragonite contribution was measured at the end of the experiment (fig. 7.4B). At the highest sulfate concentration almost all crystals were of the aragonite polymorph (fig. 7.4C).

126 7.3. Results and discussion

Interestingly, the speciﬁc habit as observed in this work has been described previously. Chong et al. [27] ascribed the needle shape to CaSO4 and the hexagonal shape to CaCO3. Although we found exactly the same habit as described, the Ra- man spectra explicitly show the vaterite and aragonite polymorph only, without any

CaSO4 or calcite contribution, thereby contradicting the conclusion of Chong et al. Unfortunately, the lack of SEM and Raman data in the Chong study complicates a fair comparison of the two studies. In addition we performed our experiments at lower supersaturation ratios and lower temperature.

2– Figure 7.1A clearly shows an effect of SO4 on the CaCO3 precipitation rate. How- ever, it is not clear whether this is due to the lower supersaturation ratios at higher 2– 2– SO4 concentration or the high SO4 concentration itself. We therefore conducted 2– experiments at constant ionic strength (adjusted with NaCl) while changing the SO4 content of the solution. In addition, the initial pH of the solutions was calculated to be the same for all conditions, 7.83, adjusted with HCl, experiment series B, table 7.3. 2– As a result, with increasing SO4 concentration, the spread in initial supersaturation ratios (exp. B) was much less compared to experiment series A (table 7.2). Apart from constant pH and ionic strength in experiments B, the key difference between the solutions used for experiments A and B is the lower supersaturation ratios during experiments B. As seen in table 3, for experiment B the supersaturation ratios are also decreasing with increasing sulfate concentration. The reduced supersaturation ratio of exp B compared to exp A should result in a longer induction time and lower precipitation rate and this is also visible in fig. 7.1. Moreover it can be seen that experiment B.5 and B.6 did not start at the same pH as all other B experiments. We do not have an explanation for this discrepancy as e.g. glass and polymer pH electrodes, and changing the order in the solution preparation procedure all gave the same result. Also, using a different model, i.e. PhreeqcI v2.18; model: Pitzer, did not predict this behavior either and gave results similar to Visual Minteq v3.0; model: Davies.

SEM images of experiment B.4 and B.6 are shown in fig. 7.5a and b. Keeping the pH and ionic strength constant resulted in a less distinct “morning star” habit for the combined vaterite / aragonite polymorph. When we zoom in on the SEM image 2– of experiment B.4, 10.0 mM SO4 , fig. 7.5c, spiked vaterite crystals can still be seen. Moreover the aragonite polymorph was still present. The experiments A and B show the combined effect of sulfate and magnesium. The experiments without sulfate but with magnesium (A.1 and B.1) do not result in the aragonite polymorph. However, it

127 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

Figure 7.4.: Raman spectra of A) Experiment A.4, t = 900 s, predominantly vaterite (characteristic peaks, V, at: 210, 268, 302, 743, 751, 1075, 1091 cm−1) [28]; B) Experiment A.4 t = 2400 s, predominantly vaterite, but peaks from aragonite are distinguishable; C) Experiment A.6, t = 2700 s, predominantly aragonite (characteristic peaks, A, at: 155, 205, 705, 1085 cm−1) [28]. has been suggested that, in the presence of dodecylsulfate, magnesium (or n-pentanol) might be necessary to favor aragonite formation [22, 23]. To investigate if the same holds under sulfate conditions, another set of experiments was performed. In the third set of experiments the influence of the presence of magnesium, and to a lesser extent manganese, was investigated. The effect of manganese is expected to be minimal, since the concentration is very low. Actually, it has been reported already that manganese has no effect on the precipitation rate [29]. By excluding both magnesium and manganese from the conditions of experiments B the mere effect of sulfate could be determined. Exclusion of these species will lead to a higher pH (since 2– – magnesium and manganese form complexes with CO3 and OH ) and evidently a lower ionic strength. To compensate for that, the pH and ionic strength are adjusted to obtain the same calculated conditions as for experiment B: pH 7.83 and ionic strength of 0.185. The absence of magnesium and manganese does not result in any difference in supersaturation ratio, e.g. B.1 vs C.1 or B.5 vs C.5 (the small variations in table 7.4 compared to table 7.3 are caused by the difference in the experimental initial pH). The fraction of carbonate (0.8 % at most) and sulfate (4.0 % at most) binding to magne-

128 7.3. Results and discussion

Table 7.3.: 2– Experiment series B: Variation in SO4 concentration of the reference solution (experiment A.2 in this table, composition in table 7.1), with adjusted constant ionic strength (I = 0.185 which equals A.6) and pH (7.83, based on A.1 at high ionic strength, I = 0.185). This makes the initial conditions the same for the experiment series, except for the difference 2– in SO4 concentration. Supersaturation ratio for vaterite (Sv) and aragonite (Sa) change accordingly. Supersaturation ratio S is defined in eq. (7.2), at 298 K. Calculations based on the measured pH, software: Visual Minteq v3.0; model: Davies. 2– Exp. [SO4 ] [NaCl] [HCl] Sv Sa pH # (mM) (mM) (mM) (-) (-) (-) B.1 0.00 150 0.000 2.61 4.24 7.90 B.2 2.66 144 0.004 2.54 4.14 7.89 B.3 5.00 138 0.007 2.57 4.18 7.91 B.4 10.0 126 0.014 2.51 4.08 7.91 B.5 30.0 77.5 0.039 2.34 3.80 7.93 B.6 60.0 0.00 0.071 2.30 3.74 8.02 sium and manganese is too low for the applied conditions to influence the activity of 2+ 2– Ca and CO3 . However, although the supersaturation ratio was not different (e.g. B.1 vs C.1), the measured pH profiles over time were clearly different in the absence of magnesium and manganese, compare fig. 7.1b and c. The induction time was shorter and the pH dropped further in the case without magnesium and manganese (e.g. C.1 is ≈ 0.15 pH points lower than B.1 at t = 2700 ). As remarked before, fig. 7.1c shows the mere effect of sulfate addition on calcium carbonate precipitation since this ion species was the only added component. Clearly, sulfate alone also resulted in a longer induction time and lower precipitation rate. Comparing the pH-profiles of experiment B.1 and A.4 show very similar induction time and precipitation rate. Given the much larger supersaturation ratio during exp A.4 in the presence of sulfate, we conclude that the presence of sulfate alone does not suffice to reduce the induction time and precipitation rate. Inspecting the SEM images of experiment C.4 and C.6, fig. 7.5d and e, respectively, leads to the conclusion that single crystals of aragonite were absent. Whereas the presence of magnesium seems to be of great importance for preferential aragonite formation, the presence of sulfate is still necessary to obtain the aragonite polymorph. The effects of sulfate and magnesium appear to be additive. Zooming in on fig. 7.5e, fig. 7.5f shows that without magnesium there were still some spiked vaterite particles. However, these spikes were much less pronounced than for the experiments with magnesium. Apparently, sulfate can (somehow) invoke calcium carbonate to create spikes

129 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

Table 7.4.: Experiment series C: Similar conditions as experiment B, without the presence of Mg2+ (and Mn2+). With adjusted constant ionic strength (I = 0.185 which equals A.6) and pH (7.83, based on A.1 at high ionic strength, I = 0.185). Supersaturation ratio for vaterite (Sv) and aragonite (Sa) change accordingly. Supersaturation ratio S is deﬁned in eq. (7.2), at 298 K. Calculations based on the measured pH, software: Visual Minteq v3.0; model: Davies. 2– Exp. [SO4 ] [NaCl] [HCl] Sv Sa pH # (mM) (mM) (mM) (-) (-) (-) C.1 0.00 155 0.009 2.61 4.25 7.90 C.2 2.66 148 0.012 2.57 4.19 7.90 C.3 5.00 143 0.015 2.54 4.14 7.90 C.4 10.0 131 0.023 2.48 4.04 7.90 C.5 30.0 81.3 0.047 2.34 3.80 7.93 C.6 60.0 0.00 0.078 2.25 3.65 8.00 on the vaterite polymorph. Even though magnesium seems to play an important role to form aragonite on top of vaterite, its presence is not an absolute prerequisite for spikes formation. Tang et al. claim that aragonite is favored over calcite in the presence of sulfate [14] (without the presence of magnesium). They also mention that the precipitation rate decreases when the sulfate concentration is increasing like is shown in ﬁg. 7.1. They also state that part of the sulfate is taken up by the CaCO3 crystals

(≈ 1%), but do not form a separate CaSO4 phase. Finally, we will discuss briefly the possible mechanisms by which sulfate and magnesium may exert their effect on growth rate and type of polymorphs formed. The 2+ 2– presence of Mg and SO4 might cause the incorporation of these ions in the crystal lattice. The high dehydration energy of magnesium might play a role in the preferential formation of aragonite [30]. Magnesium does not adsorb on, nor incorporate in, aragonite [31], but it does incorporate in vaterite [12]. This is exactly the reason that at elevated magnesium levels, aragonite and not vaterite is the much more stable polymorph. On the other hand, with vaterite the dominant polymorph formed, magnesium might cause a reduction in growth rate due to the higher dehydration energy as opposed to calcium [31], an effect clearly visible when comparing fig. 7.1c and d. As for sulfate, it has been hypothesized that due to its tetrahedral shape it can replace planar carbonate, thereby destabilizing the c- and a-axis of the vaterite crystal. The pH and alkalinity might influence this incorporation [12, 17, 32]. As for magnesium, the incorporation of sulfate in aragonite is much less than for vaterite [12] and calcite

130 7.4. Conclusion

[33]. Recently, it has been reported that the incorporation of magnesium did not affect the incorporation of sulfate in vaterite [12]. Sulfate incorporation in vaterite and aragonite did not change the unit cell parameters [12] and even stabilizes the vaterite polymorph [13]. Moreover, it has been reported that the solubility of calcite can increase due to incorporation of the sulfate ion above 0.5 mol percent [17]. We did not measure any sulfate (detection limit of 0.1 mol percent) so a solubility change in our experiment is not expected. Incorporation of sulfate ions has also been measured with atomic force microscopy in the case of calcite growth [11]. Interestingly it is claimed that this effect starts to occur above 5 mM of added 2– SO4 , where the supersaturation ratio is similar (even a bit lower) to our conditions, but pH is much higher. Tang [14] also mentions that from a concentration of 10 2– mM SO4 added, there is a significant shift in the polymorphic presence, from calcite to vaterite. Sulfate adsorbs on the CaCO3 crystal surface, reducing the step velocity [11]. The surface free energy of vaterite is lower than for aragonite. Since at the initial crystallization stages surface energies predominate, vaterite is favored over aragonite (while aragonite is the more stable bulk crystal) [34, 35]. The strong interaction with sulfate results in the increase in surface free energy and therefore retards the development of that crystal surface [14]. Moreover it is possible that this increase in surface free energy of vaterite results in the preferential formation of aragonite, especially when the crystal is destabilized by magnesium incorporation.

7.4. Conclusion

The presence of sulfate in reverse osmosis drinking water concentrate modiﬁes the precipitation of CaCO3. Based on recorded pH proﬁles over time, CaCO3 formation slows down with increasing sulfate concentration. After 45 minutes of precipitation the preferential polymorph shifts from vaterite to aragonite with increasing sulfate concentration. With this polymorphic change, a new combined habit is observed where aragonite spikes grow on top of vaterite (“morning star” habit). Maintaining a constant pH and ionic strength with increasing sulfate concentration also results in decreasing supersaturation ratio. At the same supersaturation ratios, the higher sulfate concentration results in lower precipitation rates, so apparently a decreasing supersaturation ratio is not the only factor responsible for the reduced precipitation rates. The presence of a moderate magnesium concentration results in the shift of

131 7. Sulfate in calcium carbonate precipitation: “Morning star” habit

Figure 7.5.: Scanning electron microscope (SEM) images of experiments B and C. Sample volume, 20 cm3, filtered over a 0.2 µm membrane filter: Experiment B, pH 7.83, I = 0.185 (table 7.3): Sulfate concentrations: a) 10.0 mM (B.4), b) 60.0 mM (B.6), c) Zoom (factor 2x) of a) 10.0 mM (B.4). Experiment C, same conditions as B, without magnesium and manganese table 7.4. Sulfate concentrations: d) 10.0 mM (C.4), e) 60.0 mM (C.6), f) Zoom (factor 3x) of e) 60.0 mM (C.6). preferential polymorph formation of vaterite to aragonite at relatively low sulfate concentrations; where sulfate and magnesium appear to have an additive effect. Without magnesium, spikes on top of vaterite were also observed, but only at relatively high sulfate concentration. Without the presence of magnesium single crystals of aragonite were not found.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Sensoring” for the discussions and their financial support.

132 7.4. References

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135

8 Modeling the carbonate system

abstract The standard Newton-Raphson model, as used in speciation software, such as Minteq or Phreeq, was extended with a time dependent measurement entry. In the carbonate system, the time dependent pH was used to calculate one time dependent unknown, e.g.

CaCO3 formation or CO2 exchange, and with it, the complete speciation based on the thermodynamic equilibria. The model based estimation of solid CaCO3 corresponds well with the experimental observations in the early growth stage. The estimate of CO2 exchange is evaluated by determining the rate limiting step. For the sulfate experiments (without precipitation) the diﬀusion constant and equilibrium partial CO2 pressure correspond well with the expected values.

Author: R.M. Wagterveld

137 8. Modeling the carbonate system

8.1. Introduction

There is ample commercial software available on the market that allows the calculation of the composition of a solution. Since the ionic strength, and thus the activities, are a function of the concentration of the ionic species, and vice versa, an iterative calculation procedure must be applied to determine both simultaneously [1]. Minteq, for instance, uses the Newton-Raphson algorithm to calculate ionic strength, and thus the activities of all ion species, given the total amount of dissolved salt.

Irrespective the algorithm used, all software have in common that they are rather rigid in the sense that they cannot be applied to a system in which the amount of one of its constituents changes over time, e.g. due to CaCO3 deposition or gaseous CO2 exchange with the environment. Here we show that the application of Minteq can be extended to systems in which the total amount of salt is a function of time. The method outlined here is based on an iterative procedure to minimize the diﬀerence between (calculated) total sum of ion species and total amount of added salt corrected for the loss (formation solid CaCO3) or CO2 exchange with the environment. The calculation is solely based on the Newton-Raphson algorithm and Minteq’s chemical database. Even though exempliﬁed with the CaCl2/NaHCO3 system, the method introduced here can be applied to any system in which the total amount of dissolved ion species varies over time.

8.2. Newton-Raphson

The Newton-Raphson method can be applied to estimate the speciation in the carbonate system as used in the experiments described in chapters 4, 6 and 7. The formulation is based on Morel’s tableau concept [2]. A number of NX components,

Xk, are chosen among the NC species Ci. The formation of each species present in the system is then a combination of the components.

As an example, consider an aqueous solution containing CaCl2. Three species can be distinguished as follows, which are deﬁned by chemical equilibrium constant K,

138 8.2. Newton-Raphson and activity γ, where γM is the activity of monovalent ions and γD of divalent ions:

[CaCl−] 1 KCaCl+ = (8.1a) [Ca2+][Cl−] γD + ∗ [CaOH ] 1 K + = (8.1b) CaOH [Ca2+][OH−] γD + − M 2 KOH− = [H ][OH ](γ ) (8.1c)

The concentration and activity of water are assumed constant. If the concentration of water molecules is included in the equilibrium constant K of species containing – KOH− the hydroxide ion, [OH ] can be written as , where K − is usually expressed [H+] OH as Kw. Based on these equations an equilibria constant vector K, species vector C, component vector X, and stoichiometry matrix A can be deﬁned as well in terms of an activity vector γ for the species (γC) and components (γX):

X = [Ca2+] [Cl−] [H+]

D M M γX = γ γ γ

 2+     D   [Ca ] KCa2+ γ 1 0 0  −     M     [Cl ]  KCl−  γ  0 1 0  +     M     [H ]  K +  γ  0 0 1 C =   K =  H  γ =   A =    +    C  M     [CaCl ]  KCaCl+  γ  1 1 0  +     M    [CaOH ] K +  γ  1 0 −1    CaOH      − M [OH ] KOH− γ 0 0 −1

∗ KCaOH+ in vector K is diﬀerent from KCaOH+ in eq. (8.1b). Kw was used to modify this equilibrium constant, so that [OH–] can be expressed as 1 . This is done by [H+] ∗ multiplying KCaOH+ with Kw and is applied to all equilibrium constants of components containing [OH–]. Using the vectors and matrices, the species concentration for th the i species (where i=1..NC ) is given by [2]:

K NX i Y Ai,k Ci = (γX,kXk) (8.2) γC,i k=1

th Where γC,i is the activity coeﬃcient belonging the i species, γX,k the coeﬃcient of

139 8. Modeling the carbonate system the kth component (either γM or γD in case of the example). The following equation must hold, making use of the total component concentration T, i.e., the total amount of salt added. For the jth component of T (where j=1..NX , the number of elements in the total vector which is equal to the number of components):

N XC Ai,jCi − Tj = 0 (8.3) i=1 Since concentrations and activity are interrelated by activity coefficients, and thus ionic strength, they all have to be calculated self-consistently using an iterative procedure. The first estimate for the concentration of components is based on the total concentration of that (added) component (the situation where there is no complexation), and an activity coefficient of unity. The error in the initial estimation and following iterations can now be calculated, resulting in the residual vector R, where th for the j component of R (where j=1..NX ) holds:

N XC Rj = Ai,jCi − Tj (8.4) i=1

In order to ﬁnd the roots of eq. (8.3), Rj in eq. (8.4) needs to be minimized to −16 an arbitrary small number with respect to each component Xk, e.g. 10 (actually the sum of all Rj is the criterion to be smaller than that number, see eq. (8.14). To do so Minteq uses the Newton-Raphson algorithm. In general, the Newton-Raphson method estimates the root of a function (f) using the derivative of that function (f 0). Given the estimate xn, the next iteration xn+1 is determined as follows:

f(xn) xn+1 = xn − (8.5) f 0(xn)

In this specific case, with x = Xk and f(x) = Rj, the gradient of eq. (8.4) has to be calculated. The resulting Jacobian matrix J (first-order partial derivative matrix of the function vector, R with respect to component vector X) is used to improve the estimation of the component vector in the next iteration, Xn+1. The activity coefficient is considered to be constant for the calculation of the Jacobian of the nth iteration (k=1..NX , j=1..NX ). Combining eqs. (8.2) and (8.5):

n NC n ∂Rj X (Ci) J n = = A A (8.6) j,k ∂(X )n i,k i,j (X )n k i=1 k

140 8.2. Newton-Raphson

Rearranging and including the Jacobian in eq. (8.5) results in:

Xn+1 − Xn = −(Jn)−1R ⇒ ∆X = −(Jn)−1R (8.7)

To prevent negative concentrations the following calculations are applied [3]:

∆Xk w = max (−2 n ) (8.8) k=1..NX (Xk)

 1  if w > 1 m = w (8.9) 1 if w ≤ 1

The new estimate becomes:

Xn+1 = Xn + m∆X (8.10)

Based on this estimation the ionic strength and new activity coefficients are determined. In principle any activity coefficient model can be used, such as the Pitzer model, as applied in software package PhreeqcI. Here, the activity coefficient of charged species is calculated with the Davies model, neutral species are calculated using the development of Helgeson [4, 5]. The ionic strength, I (mol dm−3) is calculated from the concentration of species and their valence z (-) :

N XC 1 I = C z2 (8.11) 2 i i i=1 th For the i component of γC (i=1..NC ) holds:  √ !  2 I −0.51zi √ − 0.3I if zi 6= 0 log γC,i = 1 + I (8.12)  0.1I if zi = 0

And the ﬁrst NX elements of γC equal γX (for k=1..NX ):

γX,k = γC,k (8.13)

The algorithm ends when the total error is below 10−16:

NX X −16 |Rj| ≤ 10 (8.14) j=1

141 8. Modeling the carbonate system

8.3. Calcium carbonate precipitation and pH

The previous section outlined the basic algorithm in the particular case that the total dissolved carbon remains constant over time. In open systems that allow CO2 exchange with the environment and / or the deposition of solid CaCO3, this prerequisite no longer holds. The next step therefore is to extend the algorithm to systems in which one or more of the components varies with time. First, we will discuss the implementation of the formation of solid CaCO3. To do so we brieﬂy elaborate on

CaCO3 precipitation. The formation of crystals, as a result of supersaturation, leads to a depletion of free calcium and free carbonate:

2+ 2− Ksp Ca + CO3 )−−−−−−* CaCO3 ↓ (8.15)

– Since HCO3 is the dominant species in these experiments, the depletion of carbonate – is accompanied by a decrease in pH due to the dissociation of H2CO3 and HCO3, respectively:

K1 + − H2CO3 )−−−−* H + HCO3 (8.16)

− K2 + 2− HCO3 )−−−−* H + CO3 (8.17)

The supersaturation of CaCO3 is pH dependent, a drop in pH will cause the solution to be less supersaturated and vice versa. Even though a decrease in pH points to precipitation of CaCO3, due to binding of dissociated protons to other species in solution, pH is a semi-quantitative measure of CaCO3 formation.

8.3.1. Modeling solid CaCO3 formation from pH: Newton-Raphson With the total dissolved carbon constant over time, the algorithm uses the residual vector R of total carbon (eq. (8.4)) representing the difference between (the CO3 known) total added and total calculated carbon present. Obviously, as soon as CaCO3 starts to precipitate, the total dissolved carbon present turns into an unknown parameter. By implication we need a slightly different approach. Therefore, in order to compensate for the lack of knowledge of constant total carbon, one of the other system components needs to be measured over time. As already mentioned, CaCO3 precipitation is accompanied by a pH decrease, reflecting the interconnection of activities of

142 8.3. Calcium carbonate precipitation and pH ionic species by chemical equilibria. Even though we are in no way bound to pH, pH seems an obvious choice as parameter to record. In addition, from the experimental point of view, pH is a relatively easy parameter to measure (even though the required accuracy as presented here might be challenging). With pH as a known parameter and total carbon deﬁned as the sum of total carbon dissolved and CaCO3 precipitated, a very similar iterative procedure as outlined before results in a consistent ion species speciﬁcation of the system. Since the pH is known (from the measurement) and thus the activity of H+, the + ∗ ∗ + H entry can be removed from vectors X and γX, yielding X and γX. The H column can be separated from matrix A resulting in A∗ and ApH. Equation (8.2) now becomes:

NX Ki Y ∗ pH ∗ ∗ Ai,k −pH Ai Ci = (γX,kXk ) · (10 ) (8.18) γC,i k=1 Additionally a new phase, solid CaCO (Y ), and a new stoichiometric matrix 3 CaCO3 B is introduced. With an adjusted total concentrations vector T∗, without total H + (since the activity of H is already known), TH, this leads to a new residual vector (see eq. (8.4)):

N XC R∗ = A∗ C∗ + B Y − T ∗ (8.19) j i,j i j CaCO3 j i=1

th Resulting in a similar Jacobian as in eq. (8.6), for the n iteration (k=1..NX , j=1..NX ):

N XC (C∗)n J ∗n = A∗ A∗ i (8.20) j,k i,j i,k (X∗)n i=1 k The starred Jacobian is used to calculate X∗ and subsequently the activity coef- ﬁcients for the next iteration. To determine the formation of CaCO solid, Y , 3 CaCO3 the pH residual has to be used (since the pH is the measured parameter):

NC X pH ∗ RH = Ai Ci − TH (8.21) i=1 The starred residual vector elements, eq. (8.19), looking at calcium and carbonate, only diﬀers from the original residual R by the solid phase Y : CaCO3

143 8. Modeling the carbonate system

R∗ = R + Y (8.22) Ca Ca CaCO3 ∗ R = RCO + YCaCO (8.23) CO3 3 3

Unfortunately, the concentration of H+ is not directly related to calcium carbonate (through the stoichiometric matrix B). However, when calcium carbonate precipitates, the pH drops, so the concentration H+ should rise. The original residuals of calcium and carbonate (eq. (8.4)), and the residual pH (eq. (8.21)), can now be used to calculate the new estimate of Y . CaCO3

R = R + Y + R + Y (8.24) H Ca CaCO3 CO3 CaCO3 R − R − R H Ca CO3 YCaCO = (8.25) 3 2

When a time varying pH is considered, the calculation of Y is performed for CaCO3 every measured pH.

8.3.2. Calcium carbonate solid formation from measured pH

The extended model described in the previous subsection can be used to determine the amount of calcium carbonate formed in a free drift experiment, by measuring pH. The equilibrium constants used for calculation were taken from Minteq’s database. The free drift experiment performed in chapter 4 resulted in a drop in pH over time, fig. 4.2. Two of these experiments are considered here, experiment 1. Blank: No ultrasonic treatment applied, and experiment 2. Ultrasound: Complete ultrasonic treatment (t = 0 - 4500 s). The raw measurement data is repeated in fig. 8.1a, gray lines. Due to possible deviations in the measurement and influence of CO2 exchange (see next section), the starting pH might differ from the calculated equilibrium pH. To prevent the calculation of a negative amount of solid, and to exclude the initial small decrease in pH (due to CO2 exchange, see chapter 4 for discussion), the pH vector is slightly adapted. Every pH larger than the equilibrium pH is set to the equilibrium pH (in this case 8.79). The resulting adapted pH vector is shown in fig. 8.1a, black lines. Figure 8.1a displays the calculated amount of solid CaCO3 as function of time. Halfway, at t = 2250 s, a large difference exists between the pH of experiment 1 and

2. The amount of solid CaCO3 formed in the ultrasound experiment was calculated

144 8.3. Calcium carbonate precipitation and pH

Figure 8.1.: Free drift experiments performed: (A) pH (-) vs. time (s), and (B) Calculated amount of formed calcium carbonate solid (mmol) vs. time (s). Solid line: Experiment 1. Blank: No ultrasonic treatment applied; Dashed line: experiment 2. Ultrasound: Complete ultrasonic treatment (t = 0 - 4500 s)

using eq. (8.25) as 0.071 mmol, versus 0.022 mmol in the blank experiment. Using the particle size distributions (ﬁg. 4.4), an estimate can be obtained of the actual precipitated volume. The fraction of photographed membrane area from the total crystal containing membrane area, and the fraction of sample volume of the total volume, were determined. Assuming spherical particles, and a vaterite density of 2.66 · 103 kg m−3, results in 0.074 and 0.015 mmol, which is reasonably close to the calculated values. At the end of the experiments there is a discrepancy between calculated and estimated values. At t = 4500 s the amount of solid CaCO3 was calculated to be 0.14 mmol for ultrasound (1) and 0.12 mmol for the blank (2) experiment. Using the particle size distributions, the estimated value for ultrasound (1) is 0.014 mmol and 0.054

145 8. Modeling the carbonate system for the blank (2) experiment. The largest difference can be seen for the ultrasound experiment (1), a factor 10. This discrepancy was also found in the response of the scattering measurement, fig. 4.2b. Possible explanations were given in the discussion of chapter 4. Hereof, scaling on the walls of the experimental equipment, dissolution of small crystals resulting in extra growth of large crystals (Ostwald ripening, driven by the Gibbs-Thomson relation) and the recrystallization of one polymorph to form another (Ostwald’s rule of stages) might explain the observed discrepancy. The dissolution might result in too small crystals, which are not retained by the 0.2 µm filter, and therefore do not contribute to the calculated total volume.

Figure 8.2.: Supersaturation of the vaterite polymorph SV (-) vs. pH (-) for the system of chapter 4.

There is also another way, and in fact internal control, to conﬁrm that the extended

Minteq algorithm calculates the formed solid amount of CaCO3 correctly. To that end we calculate the pH using the standard Minteq algorithm after subtracting this amount of precipitated CaCO3 from the initial total amount of added carbon. Ideally, the corresponding calculated pH by Minteq should be exactly the same as the pH at which we calculated the formed amount of solid CaCO3 using the extended algorithm (which is the case for our calculations). The model can also be used to determine the supersaturation as function of pH. Most crystals formed in the experiments of chapter 4 were of the vaterite polymorph. Figure 8.2 displays the supersaturation of vaterite versus pH obtained from the model. It shows that the relation of supersaturation and pH is close to linear. It conﬁrms that the pH might be used as indicator for supersaturation qualitatively, as suggested

146 8.4. CO2 exchange and pH in chapter 4.

8.4. CO2 exchange and pH

Not only precipitation will decrease the pH but CO2 uptake as well. CO2 release on the other hand, will increase the pH. This might be relevant and demands consider- ation because (some of) the measurements were performed in an open system. The partial pressure of CO2 in the air near the solution surface, PCO2 , and the concentration of dissolved CO2 are related through Henry’s constant KH :

PCO2 = KH · CO2 (8.26)

When the system of chapter 4 is considered, the liquid tends to take up gaseous CO2.

Under those conditions, taking into account the atmospheric partial CO2 pressure of 38.50 Pa e.g. 3.8 · 10−4 atm (calculated with VMinteq v3.0), within the experimental pH range (8.0-8.8) two kinetic reactions predominate [6]:

k1 CO2 CO2 + H2O −−−−→ H2CO3 (8.27)

k2 − CO2 − CO2 + OH −−−−→ HCO3 (8.28)

Because of the dissociation of H2CO3 eq. (8.16) and the shift in the water equilibrium, both reactions cause the pH to drop, reaching an equilibrium value of 8.43 under the applied conditions. Importantly, the kinetics of hydration, eq. (8.27), and conversion of CO2, eq. (8.28), are relatively slow, as is the diffusion of gaseous CO2 into the liquid. This causes the pH change due to CO2 uptake to be very slow which is confirmed by the results, as can be seen in fig. 4.2.

The opposite situation, release of CO2 from solution, is occurring for the system described in chapter 7. The kinetic equations of eqs. (8.27) and (8.28) are reversed, and the release is much faster than uptake of CO2 [6]. This might have a larger inﬂuence on the pH.

8.4.1. Modeling CO2 exchange based on pH: Newton-Raphson A similar approach, as for the amount of precipitated solids, is followed for calcu- lating the amount of exchanged CO2. Using again the residuals gives the amount of exchanged CO , Y : 2 CO2

147 8. Modeling the carbonate system

R + 2Y = R + Y (8.29) H CO2 CO3 CO2 Y = R − R (8.30) CO2 CO3 H

8.4.2. CO2 exchange from measured pH

To conﬁrm the eﬀect of CO2 release on pH, experiments were performed in a system without precipitation, but with similar pH as in the experiments of chapter 7. The composition of the solution is based on the solutions of chapter 7, experiment B, however, it contains less CaCl2 and more HCl. The reference solution is given in table 8.1. The experimental time was almost 10 hours. Apart from the concentrations, the experimental conditions were the same as for experiment B.2, chapter 7.

First the amount of exchanged CO2 was determined using eq. (8.30), see ﬁg. 8.3a. After the experimental time, as applied in chapter chapter 7, 2700 s, the fraction of carbonic species released is less than 4%. This would result in a minor reduction of the supersaturation. However, the accompanying pH increase leads to an increased supersaturation. After almost 10 hours the fraction of carbonic species released is over 10 %. The molar concentration released CO2 can be used to calculate the partial

CO2 pressure based on the remaining CO2 in the reactor. This partial pressure is given in fig. 8.3b. The assumption is made that the liquid is perfectly mixed, causing uniform concentration of dissolved CO2. Thin boundary layers at the gas/liquid interface were neglected. The diffusion problem can now be considered as an instantaneous point source diffusion problem. The concentration inside the reactor as function of time is given by equations eqs. (8.31a) to (8.31c) for a 1, 2 and 3 dimensional diffusion

Table 8.1.: Reference solution composition (after mixing), based on drinking water RO concentrate without precipitation (numbers from Vitens (Zwolle, the Netherlands)). Solution1 Solution 2 Compound (mM) Compound (mM)

CaCl2 · 2 H2O 0.02 NaHCO3 14.2 MgCl2 · 6 H2O 1.81 Na2SO4 2.66 −5 MnCl2 · 2 H2O 9.09 · 10 NaCl 23.0 HCl 0.20

148 8.4. CO2 exchange and pH

Figure 8.3.:

Model calculation of CO2 exchange from the solution of table 8.1 (using eq. (8.30)): a) Left y-axis: Amount of released CO2 (mol), right y-axis: Fraction released CO2 of total carbonate (%). b) Partial CO2 pressure in the reactor (-) obtained from model calculation of a). problem, respectively [7].

2ξC0 C(t) = − Ceq 1D Model (8.31a) p4πD (t + τ) 2αC C(t) = 0 − C 2D Model (8.31b) 4πD (t + τ) eq

2βC0 C(t) = − Ceq 3D Model (8.31c) (p4πD (t + τ))3

2 −1 Here D (m s ) is the diﬀusion constant, τ (s) the time correction for CO2 release between solution preparation and start of the experiment, C0 is the initial concentration of dissolved CO2 and Ceq the equilibrium concentration of CO2. Since the concentration is directly related to the partial pressure (eq. (8.26)), the partial pres-

149 8. Modeling the carbonate system

Figure 8.4.: One dimensional (1D) diffusion problem applied to experimental setup [7]. X-axis is normal to the reactor open surface. sure can be used for C causing the unit to be dimensionless (-). The factors ξ (m), α (m2) and β (m3) are geometrical parameters and the factor 2 appears because of a no-flux boundary in one direction [7]. Due to the mixing, there is some turbulence at the surface of the reactor. This causes the diffusion area (the area penetrated by the gas molecules) to be larger than the cross sectional area of the reactor. This complicates the modeling of diffusion in 2 or 3 dimensions. Due to this unknown area it is impossible to obtain a value for the diffusion constant. However, the case of 1 dimension, does not depend on the exact diffusion area [7]. Consider fig. 8.4, the 1D case, where the x-axis is normal to the reactor open surface. The factor ξ equals the reactor depth and the diffusion only takes place in upward direction. The no-flux boundary is now at the bottom of the reactor, causing the factor of 2 in eq. (8.31a).

The amount of exchanged CO2 gas can be expressed as a partial pressure using eqs. (8.26) to (8.28). The initial partial pressure was calculated from the speciation in equilibrium to be C0 = 0.01. The modeled partial pressure during the experiment, using the measured pH, is given in Figure 8.3b. Using a reactor depth of 0.12 m, the data of fig. 8.3b can be fitted using eq. (8.31a), to find D, Ceq and τ. The results of the fit are given in table 8.2. The expected value for the diffusion constant, D is 1.6 · 10−5 m2 s−1 and 3.8 · 10−4 for the equilibrium partial pressure,

Ceq, of CO2 gas in air. The equilibrium concentration seems to be underestimated in the 1D model, but the diffusion constant is in the order of magnitude of the expected value. The underestimation arises from the fact that the partial pressure (or concentration) scales with √1 in case of 1D diffusion. In reality the gas can diffuse t in much more directions and a 2D (C(t) ∼ 1 ) or 3D (C(t) ∼ √1 ) diffusion is much t t3 more likely.

150 8.5. Conclusion

Table 8.2.: Fit parameters instantaneous diﬀusion models eqs. (8.31a) to (8.31c) applied to drinking water RO concentrate without precipitation table 8.1 1D 2D 3D 2 −1 −5 −5 2 −5 D (m s ) 4.2 · 10 α · 6.2 · 10 β 3 · 2.3 · 10 −6 −4 −4 Ceq (-) 4.7 · 10 3.8 · 10 5.6 · 10 τ (s) 1081 5199 9468 R2 0.9878 0.9982 0.9957

As mentioned before, the diffusion constant cannot be obtained with the 2D and 3D model. However the 2D and 3D model might give a better estimation for the equilibrium partial pressure. The other estimates for the 2D and 3D model are also given in table 8.2, where the unknown geometrical paramaters α and β are considered to affect the diffusion constant only. As can be seen, the best fit, expressed by R2, is obtained with the 2D model. The equilibrium partial pressure obtained from this fit is exactly the same as the expected equilibrium partial pressure. The 3D equilibrium partial pressure is slightly overestimated. Summarizing, the results suggest that the release rate of CO2 is indeed limited by the diffusion constant of CO2 in air. This may seem a rather surprising conclusion because diffusion processes in fluids are known to proceed slowly. Therefore two remarks to elucidate this finding. First, we ignored the mass transfer resistance at the liquid-air interface. Secondly, the effect of stirring the solution should not be under estimated. In the experiment a magnetic stirrer bar of 6.3 cm in diameter was used, spinning at 400 rpm, or 6.7 rotations per second. It seems reasonable to assume that an upper limit for the induced convective flow equals the velocity of the stirrer bar, i.e. 1.31 m s−1. This convective component results in an apparent (or effective) diffusion coefficient nine orders of magnitude larger than the −9 2 −1 coefficient for purely diffusive transport of CO2 in water (10 m s ). Even though we over simplified matters considerably, this back-of-an-envelope calculation puts our conclusion in another perspective by showing that under the conditions applied CO2 exchange may be very well rate limited by CO2 diffusion in air.

8.5. Conclusion

The Newton-Raphson method used to calculate the speciation in thermodynamic equilibrium, has been extended with a time dependent input. A time dependent pH vector was used to calculate the amount of precipitated solid CaCO3 or the amount

151 8. Modeling the carbonate system

of exchanged CO2, and with it, the time dependent speciation. The model-based estimates of solid CaCO3 corresponds well with the experimental estimates in the early growth stage. In the late growth stage, the calculated amount of CaCO3 differs from the estimated amount, but shows similar discrepancy as noticed with scattering measurements. The model-based estimation of exchanged CO2 is evaluated by modeling the diffusion using a 1D, 2D and 3D diffusion models, which revealed air-diffusion to be the rate limiting step. For the sulfate experiments (without precipitation) the diffusion constant and equilibrium partial CO2 pressure correspond well with the expected values. The sulfate system without precipitation could be modeled best with an instantaneous 2D diffusion model. The release of CO2 was limited by the diffusion constant of CO2 in air.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Sensoring” for the discussions and their financial support. Henk Miedema and Karel Keesman are thanked for their discussions.

References

[1] H. N. S. Wiechers, P. Sturrock, and G. V. R. Marais. “Calcium carbonate crystallization kinetics”. In: Water Res. 9.9 (1975), pp. 835–845 (cit. on p. 138). [2] J. Carrayrou, R. Mos´e,and P. Behra. “New eﬃcient algorithm for solving thermodynamic chemistry”. In: AIChE J. 48.4 (2002), pp. 894–904 (cit. on pp. 138, 139). [3] D. Smith. Solution of simultaneous chemical equilibria in heterogeneous systems: implementation in Matlab. 2007. url: http : / / www . wilfridlaurieruniversity . ca / documents/24984/matlab_speciation_calculation.pdf (cit. on p. 141). [4] H. Helgeson. “Thermodynamics of hydrothermal systems at elevated temperatures and pressures”. In: Am. J. Sci. 267.7 (1969), pp. 729–804 (cit. on p. 141). [5] J. D. Allison, J. Kevin, N. Gradac, and D. S. Brown. MINTEQA2/PRODEFA2: A Geochemical Assessment Model for Environmental Systems: Version 3.0 User’s Man- ual. National Technical Information Service, 1991 (cit. on p. 141). [6] X. Wang, W. Conway, R. Burns, N. McCann, and M. Maeder. “Comprehensive study of the hydration and dehydration reactions of carbon dioxide in aqueous solution”. In: J. Phys. Chem. A 114.4 (2010), pp. 1734–1740 (cit. on p. 147).

152 8.5. References

[7] S. A. Socolofsky and G. H. Jirka. Environmental Fluid Mechanics Part I: Mass Transfer and Diﬀusion - Engineering Lectures. 2002. url: http://www.ifh.uni-karlsruhe. de / lehre / envflu _ i / Downloads / course _ script / ed2 / script _ ed2 . pdf (cit. on pp. 149, 150).

153

9 External electric ﬁeld in calcium carbonate crystallization

abstract Electric fields might influence the crystallization of CaCO3 by affecting, for instance, the energetics of nucleation or the kinetics of either nucleation or growth. Only external fields were considered, so electrochemical reactions could be excluded. Two new measurement configurations were developed to increase the field strength compared to the methods described in literature. The first, the sitting drop method, was modified containing only one insulated electrode (instead of two) and one contact electrode. Unfortunately, the reproducibility was low, possibly due to evaporation and the fact that the insulating layer on the electrode was not free of pinholes, which, in turn, may give rise to electrochemical reactions. The second method, the glass plate sandwich, was miniaturized using microfluidics. Even though this microfluidic design functioned properly, the detection of crystals was challenging due to low light conditions resulting from the specific design. As for a possible effect of electric field strength on CaCO3 crystallization, these microfluidic experiments were inconclusive. Studying electric field effects on crystallization requires a modification of either the detection technique or microfluidic design.

Author: R.M. Wagterveld

155 9. External electric ﬁeld in calcium carbonate crystallization

9.1. Introduction

In the previous chapters the focus has been on the application of an external energy field, ultrasound, in order to study the process of crystallization. Another external energy source which has been briefly investigated in this research, is the application of an electric field to enhance either nucleation or growth of CaCO3. A major advantage of the application of an electric field, is it’s relative simplicity. Compared to ultrasound, it is easy to generate an electric field in terms of shape, intensity and frequency. Moreover, it is much better suited for miniaturization, which is beneficial for the use in a device for monitoring scaling risk. The potential of an external electric field to influence crystallization has been recognized by Kashchiev [1]. The application of an electric field is expected to affect the energetics (nucleation work) and the kinetics (monomer attachment) in nucleation. According to the thermodynamic analysis in Kashchiev’s work, the permittivity of the cluster should differ from the permittivity of the medium for the external electric field to cause an effect. Moreover, the permittivity of the cluster should be larger than the permittivity of the medium to have an enhancing effect. For calcium carbonate in water that means that an inhibition is expected for DC-fields since the permittivity of calcium carbonate is much less than water (8 vs 80 F m−1) [2]. Kashchiev also states that low electric field strength hardly changes the nucleation rate. The nucleation rate is enhanced significantly (under the right conditions) from 1 MV m−1 and higher. Furthermore, the impact of the field will be more prominent if the monomer size is large [3]. By varying the frequency it is possible to change the permittivity of the medium (as experienced by the field), since the permittivity depends on frequency. In that way it is possible to switch from inhibition to enhancement by adapting the frequency of an AC-field [4], and, even to control the preferential polymorph formation [5]. Internal electric fields (as described in [6–9]), e.g. experiments involving (high) electrode currents, are not considered here, since these may involve electrochemical reactions. With these reactions the experimental conditions change significantly and depend highly on the type of chemicals used. The effects seen might be a direct result of the changing chemical conditions and might not be subscribed to the application of an electric field specifically.

Contrary to most ionic crystals, such as CaCO3, the monomer size is large for protein crystals. Not surprisingly, most published work on external electric ﬁelds applied on solution crystallization is on protein nucleation [4, 5, 10–17], but also the

156 9.1. Introduction crystallization of polymers [18], metal-organic compounds [3], amino-acids [19] and even ionic crystals (NaCl) [20] were investigated. Several techniques were applied in previous research. The sitting drop technique is applied by the majority. Here the crystallization is taking place inside a droplet, either in contact with a plane [3, 5, 10, 12, 14–18], or levitated [20]. The electrodes are placed outside the droplet, with air or oil between the droplet and the electrodes. In case of direct current (DC), most of the applied field will fall over this insulating layer of air or oil. This will result in a significantly lower field over the crystallizing volume. The numbers for the field strength mentioned in literature are usually based on the applied voltage and distance between the electrodes. The field strength is thus, in most of the cases, much lower over the crystallizing volume. Another technique is the glass plate sandwich with electrodes deposited on the outside wall of the main channel [13, 19]. Again, due the insulating glass layer, most of the field will fall over the glass, and the field in the crystallizing volume will be reduced. Interestingly, effects were reported at field strengths much lower than the predicted field strength of 1 MV m−1. An inhibition of the crystallization has been reported [18], a reduction of number of crystals with an increase in quality and crystallization rate (crystals nucleate / grow faster) [10–13, 19, 20], and a preferential orientation in the applied field [13, 19]. Even a change in preferential polymorph formation has been observed [5, 19]. In some cases a weak field led to the suppression of nucleation and strong fields to the enhancement of nucleation [14]. However, in the same work, some proteins showed an enhancement of nucleation at weak fields, so this does not seem to be a general mechanism. Another important observation in that work was that electrodes in a batch crystallizer did not result in any effect but for the same system, the sitting drop technique did [14]. This was attributed to the air-liquid interface, where fluid flow is induced when an external field is applied. So other electrically induced, most probably kinetic, effects might be responsible for the reported results, since most of the results were obtained with lower field strengths than suggested by Kashchiev. In another, more applied, research, an electrostatic treatment of water has been proposed to induce calcium carbonate nucleation / growth in the liquid instead of the surface [21]. In other words to prevent scaling by crystallizing CaCO3 in solution. As mentioned before, the miniaturization of a crystallizing system, with an external electric field applied, is relatively easy. It is even very beneficial to reduce the size of this system since the field strength is inversely related to the distance between the electrodes. In small systems it is therefore much easier to generate electric fields

157 9. External electric field in calcium carbonate crystallization with high field strength. The miniaturization step has already been performed for the sitting drop technique [12]. Moreover, miniaturization, specifically the use of microfluidics, attracts increasing attention in crystallization research. The small volume, high throughput and highly controllable conditions make microfluidics a perfect tool to research nucleation / growth kinetics [22]. Microfluidic crystallization studies can be divided in two groups. The first is droplet based [23–35] where a train of droplets (usually aqueous droplets in an oil environment) is generated inside a microchannel. Every single pi- coliter droplet serves as an individual crystallization reactor. The second is based on continuous flow, where the conditions can be controlled accurately, or very rapidly changed, such as controlled mixing by interface diffusion [36–40], or temperature steps along the microchannel [41, 42]. The crystallization of calcium carbonate has already been investigated using microfluidics systems [34, 38]. Yin et al. [38] applied microfluidics to investigate the in-

fluence of proteins on CaCO3 polymorph formation based on the continuous method with interface diffusion. Yashina et al. [34] applied the droplet based technology to investigate the preferential polymorph formation under several different conditions. The combination of microfluidics, crystallization and electric fields has been described before but that study focused exclusively on internal electric fields (in contact with the crystallizing volume) [7]. In this chapter variations of the sitting drop technique and the glass plate sandwich in combination with microfluidics were investigated.

9.2. Experimental

9.2.1. Chemicals

Only analytical grade reagents, grade A glassware and high quality water (MilliQ Reagent Water System, resistivity >18 MΩ·cm) were used throughout the experiments. Calcium chloride (CaCl2 · 2 H2O), sodium bicarbonate (NaHCO3), sodium chloride (NaCl) and hydrochloric acid (HCl, 0.1M) were obtained from VWR (Ams- terdam, the Netherlands).

158 9.2. Experimental

9.2.2. Experimental setup and procedures

Modiﬁed sitting drop

The first experimental setup was based on the sitting drop method. This method was slightly modified in an attempt to increase the field-strength inside the crystallizing volume. An artistic impression of the setup is given in fig. 9.1. To increase the field strength, the insulating layer on one of the electrodes was removed, resulting this electrode to be in direct contact with the fluid, with the other electrode insulated. A droplet was put on the insulated electrode. The counter electrode, a platinum / iridium wire, coated with insulating material but with the tip freely exposed to the solution. This noble electrode was grounded (0 V with respect to Ag / AgCl) to prevent electrochemical reactions. A voltage was applied on the insulated electrode to generate the field. The small tip versus the large insulated electrode results in a large electric field gradient near the wire tip because of the locally strong convergence of field lines (expressed by the gray lines in fig. 9.1).

Figure 9.1.: Artist impression of the modiﬁed sitting drop technique (not on scale).

Glass plate sandwich using microﬂuidics

The second experimental setup was based on the glass plate sandwich method. Both electrodes are insulated from the crystallizing volume by the glass resulting in relatively low field strength in the volume of interest. By reducing the distance between the electrodes, and keeping the glass as thin as possible, the field strength can be increased. The fabrication of a microfluidic chip makes these small dimensions possible.

159 9. External electric ﬁeld in calcium carbonate crystallization

Figure 9.2 shows the cross-section scheme of the microfluidic chip (fabricated by Lionix bv, Enschede, the Netherlands). The electrodes “sandwich” the main channel. The electric field is perpendicular to the fluid flow and the field strength is uniform over the entire crystallization volume. The lower electrode in fig. 9.2 contains holes so crystals could be detected using an optical microscope. The backside of the upper glass wafer is back-etched to reduce the thickness of the glass locally, and thus the distance between the electrodes. The inlets and outlets are fabricated on the side of the chip, so it can fit under the optical microscope (Leica MZ9.5) or Raman equipment (Horiba Jobin-Yvon LabRAM HR). Chip dimensions are given in the next section, table 9.1.

Figure 9.2.: Cross section of microﬂuidic chip. The chip consists of 2 bonded wafers. The top wafer is back-etched to reduce the distance between the electrodes.

Diffusive mixing is obtained using a T-junction, such as sketched in fig. 9.5. One inlet contains CaCl2 solution and the other NaHCO3. The pumping speed (Harvard Apparatus pump 33) and geometrical parameters determine the mixing profile and residence time. For constant pumping speed, the distance along the main channel is directly related to the crystallization time. The position where the first crystals are detected gives the induction time. If the electric field has an effect on the induction time, crystals should be detected closer to the inlet of the main channel.

160 9.3. Results and discussion

9.3. Results and discussion

9.3.1. Modiﬁed sitting drop

The first method applied was a slightly modified sitting drop method, where one electrode is in contact with the crystallizing volume, and the other electrode is insulated. With proper dimensioning of the system and read-out electronics, it is possible to determine the change in conductivity using the same electrodes which generate the field. The advantage of a conductivity measurement using the same electrodes is that monitoring crystallization does not influence the crystallization process itself. Ad- ditional electrodes, such as pH electrodes, will influence the electric field and might induce currents, possibly leading to electrochemical reactions (especially in the system designed and used here). The equivalent electrical model of the system is given in fig. 9.3a. The resulting frequency versus impedance relation is given in fig. 9.3b. The conductivity can be determined from the plateau, resulting from the cell resistance, when the appropriate frequency is selected. The left hand side diagonal is caused by the series capacitors, the right hand side diagonal by the parallel capacitor. Dimensions and material selection determine the exact profile of the bodeplot. For systems in which only a small amount of ions crystallize, and the activity changes considerably, the conductivity measurements are challenging.

Ccell

Cins Cdl Cdl Rcell

(a) (b)

Figure 9.3.: Electrical equivalent circuit of the modiﬁed sitting drop setup: a) Electrical equivalent circuit. Here Cins and Cdl are the capacitance of the insulating layer and the electrode double layer capacitance, respectively. Ccell is the (stray) capacitance of the electrodes and Rcell is the resistance caused by the conductivity of the medium. b) Bodeplot, or absolute impedance vs frequency plot, of the equivalent circuit.

161 9. External electric ﬁeld in calcium carbonate crystallization

Several insulating layers were used: Anodized aluminum, plastic foil (PP, PE), spin-coated photoresist on copper and brass, spin-coated Teflon on silicon wafers, silicon nitride on silicon, silicon oxide on silicon. It proved to be very difficult to obtain pinhole free insulating layers. It was therefore impossible to exclude the electrochemical reactions with this particular, one contact electrode, design. Moreover, the change in conductivity due to the crystallization was too small to measure with the used read-out electronics. An optical measurement would be the best alternative for determining the course of crystallization. The reproducibility of this measurement system was very low. The measurement cell was open at the top, causing evaporation, and thereby changing the concentration profile, and supersaturation, inside the crystallizing volume. This could be solved by using oil on top of the liquid, blocking the evaporation, but this proved to be very impractical. Moreover, the field intensity is very sensitive for the position of the wire-electrode, leading to different conditions for every experiment. For all reasons above, the adjusted sitting drop design was abandoned and an alternative microfluidic design was pursued.

9.3.2. Microﬂuidics

The second experimental setup was based on the glass plate sandwich method, scaled down using microfluidics. Since the dimensions are small, special care should be taken for the design. The electric field strength should be as large as possible, the mixing should not take too long and diffusion back into the inlets should be prevented.

Miniaturization and scaling laws

Electric field strength (with dimension V m−1) scales inverse with length scale, in this context the distance between the electrodes. This is important because it implies that a relatively modest applied potential (e.g. 1 V) can result in high electric field strength if the distance between the electrodes is short enough. Lower voltage is preferred for safety and practicality, especially for AC, since high voltage AC is difficult to obtain and can cause issues concerning electromagnetic compatibility. One volt applied with the electrodes 100 µm apart relates to a field strength of 10,000 V m−1, when insulating layers are not considered. However, to prevent possible electrochemical reactions, insulating layers were applied. Drawback of this design is the (unknown) voltage drop over the glass wall of the channel and the higher potentials that need to be applied in order to have a comparable field strength as obtained with internal

162 9.3. Results and discussion electrodes. Here, again, reducing the distance between the electrodes is beneficial to generate a large field at reduced voltage. For small dimensions the Reynold’s number, Re (-), eq. (9.1), drops considerably. −3 −1 Here ρ (kg m ), v (m s ), dH (m) and η (Pa s) are density, fluid velocity, hydraulic diameter and dynamic viscosity, respectively, with dH given for square channels (w (m) and d (m) the width and depth of the channel) [43]:

ρ · v · d Re = H (9.1) η 2 · w · d d = (9.2) H w + d

For Re < 1, the flow is laminar. Without turbulence, the mixing will take place by diffusion. The local supersaturation ratio will be much better predictable, and due to the small dimensions involved, diffusion will be relatively fast. Another advantage is that, due to the short dimensions, temperature can be controlled much more effectively. Possible large temperature gradients can be prevented by miniaturization.

Design issues

The most important factor is creating a large electric field between the electrodes. This means that the distance between the electrodes should be as small as possible, and that the glass between the electrodes should be as thin as possible. The electrodes are on the bottom and top of the channel since this configuration is by far the one favored from the point of view of fabrication. Considering the maximum expected size of the crystals (≈ 10 µm), the height of the channel is chosen 5 times larger, 50 µm. This will suffice to prevent blocking or clogging of the channel. The thickness of the glass is governed by the mechanical strength of the material. The glass should still be thick enough to prevent the structure to become too fragile. Two glass wafers were used to produce the microfluidic chip. A thin wafer of 100 µm and a thick wafer of approximately 1000 µm. The channels were etched in the thin wafer, leading to 50 µm glass between the electrode and main channel. The thick wafer is used as support. To decrease the thickness of the glass between electrode and main channel, this glass is back-etched at the position of the main channel. The resulting local thickness is ≈ 100 µm. Simulations were performed with the software package Comsol multiphysics v.4.0. The (DC) electric field strength is determined across the main channel, see fig. 9.4.

163 9. External electric ﬁeld in calcium carbonate crystallization

The simulation is based on the respective permittivities of glass and water and does not take into account the presence of ions. The simulation boundary conditions were zero charge on the non-electrode boundaries, applied voltage on the top electrode, ground on the bottom electrode and charge conservation in the entire volume. With 100 V applied, the resulting field strength in the crystallizing volume will be ≈ 1.7·104 V m−1. The field strength based on the distance between the electrodes, and not taking into account the different materials (ideal case), would be 0.5·106 V m−1. In case ions are present there will be charge build-up at the electrodes (ions start to collect), resulting in the double layer capacitance. This causes additional voltage drop at the glass / liquid interface (up to 1 µm into the liquid) resulting in a lower field strength inside the remaining crystallizing volume.

Figure 9.4.: Comsol simulation results: Cross section of the chip, simulation of the (DC) electric ﬁeld strength with 100 V applied. Field strength in the crystallizing volume ≈ 1.7·104 V m−1 V m−1.

The second factor is the residence time. The precipitation should take place within this residence time, and ideally, the crystals reach maximum size within this time- frame (to cover all nucleation / growth steps). Choosing a fluid velocity in the main channel of v = 50 µm s−1, and channel length of 5 cm, leads to a residence time of 1000 s, which should be sufficient for a typical precipitation experiment. The third factor is controlling the diffusive mixing. If the fluid velocity is too low, the ions will diffuse backwards into the inlet. If the fluid velocity is too high, there will be a large mixing region in the main channel, causing a large gradient in supersaturation. Moreover, at even higher fluid velocity, turbulence will evoke unpredictable mixing in the main channel. To obtain the right mixing profile the Pécletrelation can be used [43]:

164 9.3. Results and discussion

v · w P e = (9.3) D Here v (m s−1) is fluid velocity, w (m) the width of the channel and D (m s−2) the diffusion constant. With P e >> 1 the mixing region and mixing time will be too long. The width of the channels should also be at least 50 µm for the same reasons as the height. Choosing a main channel width of 400 µm with v = 50 µm s−1 results in P e = 20 (with D = 1 · 10−9 m s−1 a typical value for ions). Given the volumetric flow, in order to suppress the diffusion of ions from one inlet into the other (P e < 1), the inlet channel width is chosen to be smaller than the main channel. However, to obtain practical pump velocities this value is chosen not too small, in this case 100 µm. To sustain the fluid velocity in the main channel, the inlet fluid velocity should be v = 100 µm s−1, resulting in P e = 20 (with the characteristic diffusion length the penetration length of half the main channel, 200 µm).

Figure 9.5.: Comsol simulation results: Top view of the chip, simulation of the supersaturation ratio (steady state) in the channel at half the channel depth. Supersaturation is calculated with eq. (9.4). The dimensions applied were taken from table 9.1. The shown proﬁle represents steady-state conditions.

Due to the laminar flow conditions (main channel Re = 0.01, inlet channel Re = 0.007 see eq. (9.1)) in the microfluidic chip both streams run parallel in the main channel, where they mix by diffusion. Some time after fluid injection, there will be a steady-state where the diffusion profile is continuously the same due to the absence of turbulence. Another Comsol simulation was performed to determine the mixing profile inside the T-junction, expressed in supersaturation ratio, see fig. 9.5, presenting

165 9. External electric field in calcium carbonate crystallization the steady state. The first step in simulation is determining the steady state flow profile, using the laminar flow model (stationary incompressible flow, no inertial term). The boundaries are set to no slip and the velocity is set at the inlet boundaries. The outlet boundary is defined as ambient pressure. This steady state flow profile is used in the second step where the transport of diluted species is dynamically simulated (time dependent). The ions were considered as uncharged particles, so possible diffusion potential is not taken into account. A constant concentration was set at the inlets for calcium and carbonate, togehter with their respective diffusion constants. The outlet is set to outward flux, and for all remaining boundaries there is a no flux condition. Figure 9.5 depicts steady state reached after some time (100 s). The supersaturation ratio, which is expressed in terms of activities, is given by [44]:

1 IAP v S = (9.4) Ksp

Where IAP is the ion activity product and Ksp the thermodynamic equilibrium solubility product and v (-) the number of ions in the formula unit (v = 2 for CaCO3). 2+ 2– The diffusion constants of Ca and CO3 differ, leading to a non-symmetrical diffusion profile. As can be seen, supersaturation is achieved in the entire main channel, but does not extend into the inlet channels. The region on the left, the entrance of the main channel, shows the diffusion profile in terms of supersaturation. The supersat- 2+ 2– uration is with respect to the calcite polymorph and Ca and CO3 concentrations were based on the system of chapter 7. One of the electrodes is fabricated as grid, to make an opening for an optical characterization of the main channel. The diameter of the holes in the electrode should be smaller than the thickness of the glass layer to get a uniform field distribution in the crystallizing volume. However, the holes should be large enough for light to pen- etrate into the crystallizing volume to make the crystals visible with the microscope. Moreover the holes should be larger than the spot size of the Raman laser in case of Raman characterization. The holes were designed to be square and have dimensions of 35 x 35 µm, and are spaced with 25 µm. Finally, the length of the inlets can be chosen arbitrary, but long enough to have a fully developed plug flow. Another benefit of the current design is the absence of CO2 exchange (as discussed in 8), due to the closed system. An overview of the design and simulation parameters are given in table 9.1.

166 9.3. Results and discussion

Table 9.1.: Design and simulation parameters. Depth Width Length Velocity Thickness Electrode - (µm) (µm) (µm) (µm s−1) (µm) Main (µm) Main channel 50 400 5000 50 - - Inlets 50 100 - 100 - - Thick glass - - - - 1000 100 Thin glass - - - - 100 50

Discussion

Experiments were performed in the microfluidic chip, fabricated according to the design as proposed. Unfortunately, with this research, only experience was obtained. Even though we performed several successful pilot experiments with this microfluidic device, results did not allow to draw conclusions as for the effect of an electric field on crystallization. The major disadvantage of the current design is the difficulty to get light into the main channel. This makes it very challenging to obtain (optical) information on the crystallization process in the main channel. Combining the CaCl2 and NaHCO3 inflows at the main channel’s entrance works quite well and indeed crystals were detected (using a microscope), but due to the low density of crystals it was not possible to obtain a Raman spectrum. Moreover, due to the thickness of the glass, focusing of the laser beam was challenging. Detection of the scattered Raman signal was also more difficult due to a pinhole effect caused by the electrode grid. The position of the in- and outlet on the side of the chip allowed to study the chip microscopically but, unfortunately, compromised the strength of the reservoirs. Due to the glue procedure of tubes to the microchip, the glass broke easily upon usage. Moreover, the fused silica tubes could break, clogging the channels with pieces of silica. During the experiments that were successful it was possible to apply a DC field (60 V over the entire chip). Regrettably it was not possible to notice any difference between experiments with or without an electric field, since it was hard to detect the crystals at all. An improvement would be to make (small) crystals better visible, for instance by laser light scattering. The application of an AC-field, instead of DC, might result in larger field strengths in the main channel. Charge build-up at the glass-liquid interface is frustrated, reducing the voltage drop in the double layer. Moreover, there will be a smaller voltage drop over the insulating layer, since the impedance of a capacitor decreases with in-

167 9. External electric field in calcium carbonate crystallization creasing frequency. Applying AC instead of DC might help in the quest for positive results. The challenges encountered in the current microfluidic chip design could be handled by selecting different electrode materials and / or insulating layers, thus modifying the chip design. For instance, the use of an optically transparent electrode, such as indium tin oxide (ITO, or tin-doped indium oxide) would solve the problems with optical detection. Moreover, the electric field would be more uniform, since a grid electrode is not necessary. Such an electrode can be fabricated with similar properties as glass, so with an equal thickness for mechanical strength as is used in the current design. This electrode can be passivated by an insulating polymer layer, such as Poly(ethylene glycol) [45], or inorganic layer, such as molybdenum oxide [46]. The insulating layer can be much thinner, leading to larger field strengths in the crystallizing volume.

9.4. Conclusion

In this work the effect of an electric field on the crystallization of CaCO3, to date, could not be confirmed nor rejected. The proposed adjusted sitting drop method proved to be unsuccessful. The reproducibility was low, evaporation was an issue and the applied insulating electrodes were not free of pin-holes, so electrochemical reactions could not be excluded. The adjusted sitting drop method was abandoned in favor of the glass plate sandwich method using microfluidics. The microfluidic design seemed to work properly but the detection of crystals was an issue. The amount of light that can enter the chip was very low due to the specific design. Laser light scattering detection would be a possible improvement and might lead to results. Redesigning the microfluidic chip will solve most of the encountered challenges.

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân,the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank the participants of the research theme “Sensoring” for the discussions and their financial support.

168 9.4. References

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169 9. External electric ﬁeld in calcium carbonate crystallization

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170 9.4. References

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172 10 Discussion and perspectives

abstract This final chapter discusses the implications of the gained knowledge in this work with respect to a sensor for scaling tendency prediction. First, the actuation method is discussed. Since ultrasound does not seem to be generically applicable, alternative external energy sources for crystallization enhancement are discussed. The current research has been on applying external energy fields in crystallization, but there is still some information missing on the link between crystallization and scaling. This is briefly addressed, followed by an outlook on a device for monitor scaling risk. Finally some additional process optimization is suggested.

Author: R.M. Wagterveld

173 10. Discussion and perspectives

10.1. External energy sources inﬂuencing crystallization

External energy sources can be applied to influence crystallization, as was discussed in this thesis. Mechanical shock, friction and agitation are known to induce nucleation and can be introduced by various methods [1]. Another option is to vary temperature to influence supersaturation. An example can be found in sensors where a Peltier element causes a local temperature change, thereby inducing crystal growth on a surface [2]. Since the solubility of different salts has a different sensitivity towards temperature changes, this seems to be less useful as (generic) method to monitor scaling risk. The focus of the discussion will therefore be on external energy sources other than temperature.

10.1.1. Ultrasound

Cavitation

The application of ultrasound in CaCO3 solutions has been investigated in this work. Several mechanisms affecting the crystallization were identified such as deaggregation, attrition, deagglomeration and breakage. All these physical effects can be attributed to cavitation and require that crystals already have formed, but possibly only after they have grown out to a certain, critical size. Ultrasound did not seem to affect the linear growth kinetics directly, since after the time window of ultrasonic treatment the growth rate never decreased (stepwise). Additionally, for the solutions considered in this work, no significant effect could be identified on the initial stage where primary nucleation is considered to take place. Both of these findings suggest that, at least in synthetic solutions, there are no effects evoked by cavitation that affect crystallization other than physical effects. Especially in the presence of additives, the obtained results show that, in some cases, ultrasound did not have a measurable effect at all. A main conclusion arising from this study therefore is that ultrasound has not a generic effect on crystallization. In the end, experimental conditions determine to a large extent the effect of ultrasound on crystallization, and by implication the applicability of ultrasound in the sensor outlined here. It is therefore quite well possible that changing experimental conditions other than those explored here result in a stronger or more consistent effect of ultrasound. So far, the current research aimed at relatively low supersaturations as encountered in drinking water membrane systems. Higher supersaturations might lead to different results.

174 10.1. External energy sources inﬂuencing crystallization

Additionally, there is a huge knowledge gap on the eﬀect of ultrasound on all other scaling salts such is CaSO4 or BaSO4. Researching these salts might give a better understanding of the mechanisms involved.

Standing waves

Preliminary research hints on an effect of standing waves of ultrasound on the crystallization of calcium carbonate, appendix A. The experiments were performed in a sonoluminescence setup where the reactor is constructed as a high Q-factor resonator. To get sonoluminescence, the presence of a cavitating bubble is required and is introduced by dropping a small drop of solution in the reactor. Experiments with one sonoluminescent bubble present during crystallization resulted in an increase in number of crystals, fig. A.3A,B. Under the same conditions, but without the introduction of a cavitating bubble and thus sonoluminescence, also an increase in number of crystals was observed, fig. A.3C,D. Sonoluminescence, or even cavitation, does not seem to be a prerequisite for enhanced crystallization in this configuration (in contrast with the non standing wave configuration). We therefore conclude that under the conditions applied here apparently cavitation is not a prerequisite per se for crystallization enhancement. The exact mechanism is still unclear and additional research is necessary. This result might pave the way to better controllable application of ultrasound.

10.1.2. Electromagnetic ﬁelds

Optical methods

Another possibility is to use light as external energy source. Some successful experiments have been reported, but the literature on this topic is sparse. Examples include photomechanical action, such as cavitation, but also nonphotochemical laser-induced nucleation (NPLIN). In cavitation, temperature effects (which are much more important in optical cavitation than in ultrasonic cavitation) were linked to enhanced crystallization [3]. NPLIN in KCl solutions has been explained by the generation of transient electric fields during nanosecond laser pulses [4]. These electric fields were suggested to result in a reduction of the thermodynamical energy barrier associated with cluster formation. The temperature effects of photomechanical action seem to make this method less general applicable as actuator because the level of temperature-induced cavitation is ill-defined and hard to control. The nonphoto-

175 10. Discussion and perspectives chemical laser-induced nucleation is very promising, but the relation to scaling risk needs to be investigated.

Electric ﬁeld

Apart from ultrasound, other external energy sources may have an effect on CaCO3 crystallization rate as well, e.g., an electric field. Given the working mechanism of ultrasound (cavitation-induced crystal breakdown), it is expected that an electric field exerts its effect quite differently, e.g., by changing the energetics and/or kinetics of nucleation and/or growth. This is discussed in chapter 9 and already referred to in NPLIN experiments mentioned in the previous section. Whether or not this translates into a more accurate scaling prediction needs to be seen. In this work the microfluidic design used for application of large electric fields seemed to work properly but the detection of crystals turned out to be the main challenge. The amount of light entering the chip was very low due to the specific design with light passing the perforated bottom electrode. Laser light scattering detection might result in significant improvement. Redesigning the microfluidic chip will solve most of the encountered challenges.

Magnetic ﬁeld

Various electromagnetic implementations have been applied to crystallization processes. The best known example is application of a magnetic field to prevent scaling of CaCO3. In terms of working mechanism, the effect of magnetic field is thought to be indirect. Supposedly, the electric field generated by the magnetic field causes the crystallization to occur in solution rather than on surfaces [5]. The rate of crystallization is sometimes reported to increase but underlying mechanisms are still under debate [6]. Recently an attempt has been made to explain the mechanism by perturbations in loosely bound prenucleation clusters [7]. Because the effect of an magnetic field is associated with electric fields anyway, exposing the system directly to an electric field seems to be the better choice for application in a scaling risk sensor.

10.2. Scaling

The current research predominantly discusses the ways to aﬀect crystallization. The development of an actual scaling device, however, demands to bridge the knowledge gap between the two processes, crystallization and scaling.

176 10.3. Device for monitoring scaling risk

Firstly, more research is necessary to actually link the enhanced crystallization response to the risk of scaling. But in a more broader scope, it is still not clear what kind of structures are responsible for scaling, e.g. which polymorphs are involved, which foreign ions play a role and what are the conditions necessary for scaling. How does a layer of scale evolve, does it start with small nuclei, or is a certain trigger necessary? Which substrate material is most prone to scaling and does this relate to the crystal habit or polymorph? Answering these questions asks for further research. Another knowledge gap can be found in the working of antiscalants. It has been shown that antiscalants loose their ability to block growth after some time (chapter 5). The exact mechanism is still unknown and additional research on this might improve (i.e., lower) antiscalant dosage. In relation to this, reduction in potency over time of antiscalants might exhibit a long term scaling risk. At the membrane surface, due to concentration polarization, conditions for supersaturation are permanently present. Clusters of CaCO3, inhibited in their growth by antiscalant, may pose a rather low hazard to membrane functioning. However, as soon as the antiscalant has lost its anti-growth ability and because of the supersaturation conditions present, crystals will start growing and with that start to block the membrane. Timely flushing e.g. with MilliQ-water might remove these scaling hazards [8]. The presence of organic compounds, other ions, but also the interaction with gaseous CO2, affect crystallization and should be taken into account as discussed in chapters 6 and 7. Also the formation of other polymorphs or habits might influence the risk of scaling, some polymorphs or habits might lead to either harder or softer scales or attach more or less easy to equipment surfaces. .

10.3. Device for monitoring scaling risk

The ultimate goal, a device to monitor scaling risk, has not been achieved yet. Ul- trasound (by means of cavitation) does not result in a generic enhancement of crystallization and is therefore not the the best tool to use in order to probe the risk of scaling. However, for specific cases, ultrasound might be very suitable. An example where ultrasound can be effective, is in systems where ultrasonically degradable antiscalants are applied, as was shown for the antiscalant NTMP. The enhanced crystallization can be used to probe the effectiveness of these chemicals, which is otherwise very difficult to assess. Currently the best way to monitor the effectiveness is by monitoring the consumption rate of antiscalants. Since the concentration

177 10. Discussion and perspectives of antiscalant is very low, additional labeling is necessary for accurate measurement, as is applied in the Trasar technology [9]. Although this gives an impression of the concentration of antiscalants in solution, it does not give information how effective this concentration is to prevent scaling. Variations in the solution composition might change the effectiveness of the antiscalant. For instance, chapter 6 shows that the presence of magnesium ions enhances the inhibition time with the same NTMP concentration. Monitoring the process of (ultrasonically enhanced) crystallization itself, also reduces the necessity of additional measurements, such as magnesium concentration, to predict the effectiveness of the antiscalant. Since cavitation is poorly controllable and highly non-nonlinear, the application of electric fields seems to have a broader potential than applying ultrasound (at least when we restrict the effects of ultrasound to cavitation). If application of an electric field enhances crystallization in a generic way, this might be the best tool to probe the risk of scaling. The application of nonphotochemical laser-induced nucleation is also very promising for the same reason. But practically, a laser producing the necessary threshold power of mJ per pulse [4] is still too large and expensive to be applied as actuator, although prices and size might go down in the near future.

10.3.1. Miniaturization

In chapter 9, miniaturization was already mentioned to be beneficial for application of an electric field in a scaling risk monitor. But miniaturization of the device can als be very useful to reduce the sample volume and power consumed. Additionally, miniaturization can help for fabricating a handheld device. Recently it has been reported that cavitation by ultrasound can be induced in a microfluidic system. Moreover it has been suggested that cavitation in microfluidics is better controllable than in bulk solution [10]. Enhancement of the production of (highly reactive) radicals and sonoluminescence has been achieved but also physical effects, as the removal of a gold deposit on a glass slide, have been reported. Since cavitation seems to be better controllable with microfluidics, miniaturization might result in better reproducible results. So besides the geometrical advantage, and the higher sustainability, also the risk assessment is expected to be more accurate.

10.3.2. Dynamic model

Instead of forcing the system into CaCO3 deposition by means of energy supply, next we will discuss a totally diﬀerent type of pro-scaling sensor, one purely based on a

178 10.4. Additional process optimization combination of measuring ion species activity and modeling. Chapter 8 describes the modeling of the carbonate system in water. Even though the Minteq software has been extended to account for atmospheric CO2 exchange and CaCO3 deposition, the calculations are essentially based on the assumption of chemical equilibrium (i.e., ∂c/∂t = 0 for all ion species). A next step forward (but a much more demanding challenge) would be to describe a dynamic system (i.e., ∂c/∂t 6= 0). Theoretically, with a number of ion species measured over time, such a dynamic model should have the ability to predict future CaCO3 deposition as well.

10.4. Additional process optimization

A way to reduce the use unsustainable discharge of antiscalants is recovery of phosphonates [11, 12] or the use of alternative, green chemicals, such as carboxymethyl inulin biopolymers [13]. In both cases dosage optimization is still relevant to prevent scaling. The best way to reduce scaling risk is to prevent supersaturation in the system as much as possible. This can for instance be achieved by intermediate desupersaturation which might lead to higher water recovery [14, 15]. The scaling risk monitor can be applied to optimize this intermediate process as well. However, at the membrane interface, supersaturation will occur by deﬁnition since the process results in pure water and a concentrated (supersaturated) stream. Eutectic freeze crystallization of the resulting concentrate might be applied to achieve near zero liquid discharge [16, 17].

References

[1] J. W. Mullin. Crystallization. Butterworth-Heinemann, 2001 (cit. on p. 174). [2] M. Löffelmannand A. Mersmann. “How to measure supersaturation?” In: Chem. Eng. Sci. 57.20 (2002), pp. 4301–4310 (cit. on p. 174). [3] A. Soare, R. Dijkink, M. Rodriguez Pascual, C. Sun, P. W. Cains, D. Lohse, A. I. Stankiewicz, and J. M. Kramer. “Crystal Nucleation by Laser-Induced Cavitation”. In: Cryst. Growth Des. 11.6 (2011), pp. 2311–2316 (cit. on p. 175). [4] M. Ward and A. Alexander. “Non-photochemical laser-induced nucleation of potassium halides: effects of wavelength and temperature”. In: Cryst. Growth Des. 12.9 (2012), pp. 4554–4561 (cit. on pp. 175, 178). [5] W. N. Al Nasser, A. H. Al Ruwaie, M. J. Hounslow, and A. D. Salman. “Influence of electronic antifouling on agglomeration of calcium carbonate”. In: Powder Technol. 206.1 (2011), pp. 201–207 (cit. on p. 176).

179 10. Discussion and perspectives

[6] A. Fathi, T. Mohamed, G. Claude, G. Maurin, and B. A. Mohamed. “Effect of a magnetic water treatment on homogeneous and heterogeneous precipitation of calcium carbonate”. In: Water Res. 40.10 (2006), pp. 1941–1950 (cit. on p. 176). [7] J. M. D. Coey. “Magnetic water treatment–how might it work?” In: Philos. Mag. 92.31 (2012), pp. 3857–3865 (cit. on p. 176). [8] L. D. Nghiem and T. Cath. “A scaling mitigation approach during direct contact membrane distillation”. In: Sep. Purif. Technol. 80.2 (2011), pp. 315–322 (cit. on p. 177). [9] E. H. K. Zeiher, B. Ho, and K. D. Williams. “Novel antiscalant dosing control”. In: Desalination 157.1 (2003), pp. 209–216 (cit. on p. 178). [10] R. D. Fernandez, P. Cintas, and H. J. G. E. Gardeniers. “Merging microfluidics and sonochemistry: towards greener and more efficient micro-sono-reactors.” In: Chem. Commun. 48 (2012), pp. 10935–10947 (cit. on p. 178). [11] L. Boels, T. Tervahauta, and G. J. Witkamp. “Adsorptive removal of nitrilotris (methylenephosphonic acid) antiscalant from membrane concentrates by iron-coated waste filtration sand”. In: J. Hazard. Mat. 182.1 (2010), pp. 855–862 (cit. on p. 179). [12] L. Boels, K. Keesman, and G. J. Witkamp. “Adsorption of Phosphonate Antiscalant from RO Membrane Concentrate onto Granular Ferric Hydroxide”. In: Environ. Sci. Technol. (2012) (cit. on p. 179). [13] L. Boels and G. J. Witkamp. “Carboxymethyl Inulin Biopolymers: A Green Alternative for Phosphonate Calcium Carbonate Growth Inhibitors”. In: Cryst. Growth Des. 11.9 (2011), pp. 4155–4165 (cit. on p. 179). [14] B. C. McCool, A. Rahardianto, and Y. Cohen. “Antiscalant removal in accelerated desupersaturation of RO concentrate via chemically-enhanced seeded precipitation (CESP)”. In: Water Res. (2012) (cit. on p. 179). [15] C. J. Gabelich, M. D. Williams, A. Rahardianto, J. C. Franklin, and Y. Cohen. “High- recovery reverse osmosis desalination using intermediate chemical demineralization”. In: J. Membr. Sci. 301.1 (2007), pp. 131–141 (cit. on p. 179). [16] S. T. Reddy, A. E. Lewis, G. J. Witkamp, H. J. M. Kramer, and J. Van Spronsen. “Recovery of Na2SO4· 10H2O from a reverse osmosis retentate by eutectic freeze crystallisation technology”. In: Chem. Eng. Res. Des. 88.9 (2010), pp. 1153–1157 (cit. on p. 179). [17] D. G. Randall, J. Nathoo, and A. E. Lewis. “A case study for treating a reverse osmosis brine using Eutectic Freeze CrystallizationApproaching a zero waste process”. In: Desalination 266.1 (2011), pp. 256–262 (cit. on p. 179).

180 A Experiments sonoluminescence reactor

Author: R.M. Wagterveld, J.W. van Egmond

181 A. Experiments sonoluminescence reactor

A.1. Experimental

A.1.1. Chemicals

Figure A.1.: Scheme of the experimental reactor. A glass reactor with two ultrasonic transducers glued with epoxy, and a glued microphone. A sine wave is generated by a frequency generator and ampliﬁed. An electrical impedance matching electrical circuit with inductor improves the power transfer at the resonance frequency of the glass. A microphone at the bottom of the reactor is used to determine the optimal resonance frequency.

A.1.2. Experimental setup

A schematic representation of the experimental setup used is presented in ﬁgure ﬁg. A.1. A spherical glass reactor of 100 cm3 is used, with two ultrasonic transducers (SMR3010T60P8NA, Steiner & Martins Inc, USA) glued exactly opposite of each other. In this way the reactor is constructed as a high Q-factor resonator. Both ultrasonic transducers were part of a dedicated homebuilt system that could be controlled precisely in terms of shape, frequency and amplitude of the alternating current for driving the transducers. A microphone was used to determine the optimal

182 A.1. Experimental resonance frequency of the system.

A.1.3. Solution preparation

In order to have sonoluminescence, the liquid should be degassed. Degassing is achieved by boiling 500 cm3 of MilliQ water for at least 15 minutes. The bottle is taken of the hotplate, immediately capped, and cooled down to room temperature. The solutions are prepared by weighing the salt and bringing it in a volumetric flask by dissolving with an aliquot of (non-degassed) MilliQ water. To achieve a pH of 8.80 at the start of the experiments (to reduce exchange of CO2 with air), a small amount of 1M NaOH was added to the CaCl2 solution. Just before the start of the experiment, the volumetric flasks are filled with degassed water to achieve the desired concentration. Both solutions are carefully poured in a measuring cylinder and mixed without introducing bubbles. The created supersaturated solution has a concentration of 4 mM CaCl2 and 4 mM NaHCO3 and the pH was 8.80.

Figure A.2.: A) Sonoluminescence; one bubble is cavitating, producing a blue glow which is referred to as sonoluminescence. B) Reactor with ultrasound and crystallization (no sonoluminescence).

A.1.4. Experimental procedures

Part of this solution was poured in one spherical reactor for ultrasound experiments, the other part was poured in another exact same spherical reactor for the reference experiments. The ultrasonic treatment always started immediately after pouring at a frequency of 25.70 kHz. In case of the sonoluminescence experiments, Exp. 1, an

183 A. Experiments sonoluminescence reactor aliquot of solution was withdrawn with a pipette. One drop of solution was dropped back into the solution to create a bubble. Under the right conditions, in our setup around 48 Vpp over the transducers, the bubble settles in the center of the spherical reactor and starts to glow, producing sonoluminescence. This is shown in ﬁg. A.2A. In case of non-sonoluminescence experiments, Exp. 2, the exact same power is fed tot the transducers, 48 Vpp, but this time the bubble is not introduced. So the system is in resonance but there is no cavitation due to the absence of bubbles (because of the degassed liquid), ﬁg. A.2B. The electrical power consumed by the transducers was measured to be 0.76W for both experiments.

Figure A.3.: Free drift experiments Exp 1. A) Reference (no ultrasound) B) Sonoluminence; Exp 2. A) Reference (no ultrasound) B) Ultrasound (no sonoluminescence)

The nucleated crystals were allowed to grow for some time and a sample was taken by ﬁltering 25 cm3 of solution over a 0.2 µm ﬁlter. The dried crystals were characterized by scanning electron microscopy (Jeol JSM-6480LV) and Raman spectroscopy

184 A.2. Results

(Horiba Jobin-Yvon LabRAM HR).

A.2. Results

The SEM images reveal that experiments with ultrasound show a larger amount of crystals than experiments without ultrasound. Figure A.3A and B show the result of Exp. 1, a sonoluminescence experiment. Figure A.3A shows the reference, Figure A.3B the experiment with sonoluminescence. The experiment with sonoluminescence seems to have more crystals, and also shows some agglomeration. A similar result can be seen for Exp. 2 in fig. A.3C and D. The experiments with ultrasound (no sonoluminescence, fig. A.3C) show more crystals and agglomeration than the reference fig. A.3D.

185

B Supporting information for: Eﬀect of US on early growth

187 B. Supporting information for: Eﬀect of US on early growth

Figure B.1.: Free drift experiments performed under several diﬀerent conditions, explained in the leg- end on the right hand side. As can be seen there is some variation in the apparent induction times for experiments perfomed under the same conditions, however, the proﬁles are similar.

188 Figure B.2.: Raman spectra obtained from particles obtained after filtration over a 0.2 µm filter. a) hexagonal shaped vaterite (characteristic peaks at: 210, 268, 302, 743, 751, 1075, 1091 cm−1) b) cubic shaped calcite (characteristic peaks at: 155, 281, 710, 1085, 1435 cm−1). The dominant polymorph could be identified by combining their appearance with spectra from Raman (Characteristic peaks from: Wehrmeister, U. et al. Raman Spectrosc. 2010, 41, 193-201.).

189

C Matlab-code: Solid CaCO3 as function of pH

clear; clc; A=[1 0 0 0 0 0; 0 1 0 0 0 0; 0 0 1 0 0 0 ; 0 0 0 1 0 0; 0 0 0 0 1 0 0 0 0 0 0 1; 0 1 1 0 0 0; 1 1 0 0 0 0; 1 1 0 1 0 0; 0 1 0 −1 0 0 1 0 0 2 0 0; 1 0 0 1 0 0; 0 0 1 0 1 0; 0 0 0 −1 1 0; 0 0 1 0 0 1 1 0 0 0 0 1; 1 0 0 1 0 1; 0 0 0 −1 0 1; 0 0 0 −1 0 0]; K=[0; 0; 0; 0; 0; 0; 4; 3.22; 11.434; −12.697; 16.681; 10.329; −.3; −13.757 −.3; 1.27; 10.029; −13.897; −13.997]; z z=[4 4 1 1 1 1 1 0 1 1 0 1 0 0 0 1 0 0 1];%z squared vector T=[0.002 0.004 0.094 0.0018 0.086 0.0022]';% Total concentration vector iterations=1000;% maximum number of iterations per calculation cycle criteria=1e−15;% maximum error % preconditioning vectors, initial estimates Nx=size(A,2); Nc=size(A,1); X=T; gamma X=ones(Nx,1); gamma C=ones(Nc,1);

%PartI: Calculation of pH, pH original, in system without precipitation

forI I=1:iterations % calc species logC=(K−log10(gamma C))+A*log10(gamma X.*X); C=10.ˆ(logC); R=A'*C−T;% residual vector % calc the jacobian J=zeros(Nx,Nx);% preconditioning for j=1:Nx; for k=1:Nx; for i=1:Nc; J(j,k)=J(j,k)+A(i,j)*A(i,k)*C(i)/X(k); end end end deltaX=J\(−1*R);% DeltaX for next estimation %calculation to prevent negative concentrations w=max([1, −1*deltaX'./(0.5*X')]); m=1/w;

191 C. Matlab-code: Solid CaCO3 as function of pH

X=X+m*deltaX;% New estimation ofX

I=0.5*z z*C;% Ionic strength % Calculation activities gamma=10.ˆ(−(z z*0.51*((sqrt(I)/(1+sqrt(I)))−0.3*I))); forI II=1:Nc ifz z(I II)<1;gamma(I II)=10ˆ(0.1*I);end% neutral species end ifI >1; gamma=ones(1,Nc);end gamma X=gamma(1:Nx)';%Splitting activities inC andX vector gamma C=gamma';

error=sum(abs(R)); if error<=criteria; break; end% stop iteration when below error end pH original=−log10(C(4)*gamma C(4));% intial pH

%PartII, calculate solid formation from pH vector

B=[1 1 0 0 0]';% New stoichiometry matrix for Calcium Carbonate S X=0;% Initial guess for the formation of Calcium Carbonate solid

%Adjust the vectors and matrices: Astar=A; Astar(:,4)=[];% Removing theH column fromA Tstar=T; Tstar(4)=[];% Removing totalH gamma Xstar=gamma X; gamma Xstar(4)=[];% RemovingH from gamma X Xstar=X; Xstar(4)=[];% use the calculated values as initial guess, removeH+

% input pH vector pHin=[8.8150 8.8170 8.8170 8.8160 8.8160 8.8160 8.8160 8.8140 8.8130 etc

% input pH vector adjusted to get only positive numbers for the solid pH=pHin(100:(size(pHin,2)))−((max(pHin(100:(size(pHin,2)))))−pH original);

Nx=size(Astar,2); Nc=size(Astar,1); S XpH=zeros(1,size(pH,2));total iterations=zeros(1,size(pH,2)); forII I=1:size(pH,2);% Recalculate equilibrium for every pH forII II=1:iterations logCstar=(K−log10(gamma C))+Astar*log10(gamma Xstar.*Xstar)−A(:,4)*pH(II I); Cstar=10.ˆ(logCstar);% calc species Rstar=Astar'*Cstar+B*S X−Tstar;% starred residual RH=A(:,4)'*Cstar−T(4);%H+ residual

% calc the jacobian Jstar=zeros(Nx,Nx); for j=1:Nx; for k=1:Nx; for i=1:Nc; Jstar(j,k)=Jstar(j,k)+Astar(i,j)*Astar(i,k)*Cstar(i)/Xstar(k); end

192 end end

RS=Astar'*Cstar−Tstar;% Original residuals S X=0.5*(RH−RS(1)−RS(2));% Calculation newS

deltaXstar=Jstar\(−1*Rstar); w=max([1,−1*deltaXstar'./(0.5*Xstar')]); m=1/w; Xstar=Xstar+m*deltaXstar; Istar=0.5*z z*Cstar;

gamma=10.ˆ(−(z z*0.51*((sqrt(Istar)/(1+sqrt(Istar)))−0.3*Istar))); forII III=1:Nc ifz z(II III)<1;gamma(II III)=10ˆ(0.1*Istar);end end gamma Xstar=gamma(1:Nx)'; gamma C=gamma'; if Istar>1; G=ones(1,Nc);end error=sum(abs(Rstar)); if error<=criteria; break; end end S XpH(II I)=S X;% Amount of solid CaCO3 as function of pH end

193

Acknowledgements

This thesis could not have been in your hands without the help and contribution of many people. I owe my sincere gratitude to my promotor and supervisor, Geert-Jan Witkamp. Geert-Jan, thank you for your advise and giving me the freedom in this research. You have an excellent feel for the required supervision and your advise has been inevitable. In the first year Mateo Mayer had been my daily supervisor. This project originated from his mind, as well as many other projects within Wetsus. After one year in my project he decided to continue with his own business full time. This stopped the daily supervision, but not the engagement to this project. Mateo it has been, and still is, a pleasure to spar with you about the topic of this thesis, and many other subjects. After a while, Henk Miedema took over his task within the sensoring theme. Thank you Henk, for the many, many discussion we had. Your comments improved the quality of my articles a lot, and it is just a great pleasure to chat with you on the many topics as we did (and still do). As you as reader might have noticed, most of the published articles were in collab- oration with one additional person, my colleague Luciaan Boels. He started almost at the same time on a similar topic and with our different backgrounds we had a perfect synergy. Luciaan, I owe my greatest gratitude to you, it was a great pleasure working together. I think it improved the quality of both of our work and the discussions we had contributed for the largest part to my view on crystallization. The coffee breaks and joint trips we took to conferences and Delft have always been a great pleasure. During my project my students have helped out a great deal. I am indebted to Teofil Minea, Dorota Pigiel, Miao Yu and Jan Willem van Egmond. Thank you for all the work you have done. It did not only help the project, but also taught me much about myself. It was a great experience working with all of you.

195 Acknowledgements

Then I have to mention some persons helping out a lot at the start of the project. Petra van Dalfsen, thanks for being around in the first years. Helping me out in the wet chemical lab (as chemical newbie), but also for the time in the office and during theme meetings, dinners and our trip to the course in Udine. Luewton Lemos Fel´ıcioAgosthinho and Elmar Fuchs, thanks for the persistence in getting a high speed camera to Wetsus. It was an important attribute to the research, and I had lots of fun doing the exploring experiments in the “chicken house”. Karel Keesman, thanks for the advise on the modeling chapter and the company at the courses in Udine. The technical and analytical staff at Wetsus make the institute tick. Thank you Wim Borgonje, Jan Tuinstra, Harrie Bos, Harm van der Kooi, Ernst Panstra, Jelmer Dijkstra, Mieke Haan, Janneke Tempel and Marianne Heegstra for all the support. In special I would like to thank Ton van der Zande for his help with Raman analysis and Arie Zwijnenburg for the help with SEM. My colleagues at Wetsus and Delft University, Luciaan Boels, Camiel Jansen and Kamuran Yasadi, thanks for all the discussions and interesting Friday afternoon experiments we did in the lab. A four year project cannot succeed without a pleasant working environment. Thanks to all my office colleagues, Petra van Dalfsen, Agata Brzozowzka, Nienke Stein, Loes Fasotte, Marthe de Graaff, Lena Faust, Taina Tervahauta, Ana Marta Santos, Gerrit Tamminga, Pom Rungnapha Khiewwijit and Vytautas Abromaitis for creating this. All people in the Sensoring theme, especially Maurice Tax as theme coordinator and the PhD’s Petra van Dalfsen, Nienke Stein, Natalia Hoog, Thijs van Leest, Jeroen Heldens, Judith Staginus, Marjolein Woutersen and Jorick van ’t Oever, thanks for the interesting discussions and enjoyable meetings. For all other people that miss their name, it would be too much to mention all of you. But I would like to thank you for all the borrels, meetings, dinners and just for being around during the entire project. Finally I would like to thank my paranimfs, Doekle Yntema and Kamuran Yasadi, for supporting me through the final stage. And last but not least I owe my gratitude to my friends and family for supporting me, and especially Gea, to keep my head cool during stressful times.

Martijn Wagterveld Drachten, February 2013

196 Curriculum Vitae

Martijn Wagterveld was born on 19th of January 1983 in IJsselmuiden. In 2000, after finishing his secondary school (Gymnasium) at CSG Vincent van Gogh in Assen, he started to study electrical engineering at the University of Twente in Enschede. He did an internship at the company Scandinavian Micro Biodevices ApS in Farum, Denmark, with topic: Excimer laser 3D micromachining and surface modification for superhydrophobicity, in 2006. His master assignment took place at the company Medimate BV, En- schede, with topic: Plug formation by diffusion through a membrane for sample injection in microfluidic chips. In 2007 he received the title “ingenieur” by completing his studies, with major Microsystems and Microelectronics and minor Law. In January 2008 he started as a PhD-candidate of Delft University of Technology, at Wetsus in Leeuwarden, on the topic discussed in this thesis. From April 2009, he started to work as a researcher at Wetsus for 1 day a week, guiding and assisting students and PhD’s in the area of electrical engineering (measurements) and physics (optics). Since October 2012 he works as a (pre-)postdoctoral researcher at Wetsus.

197