Ammonium-Carbamate-Rich Organogels for the Preparation of Amorphous Calcium Carbonates

minerals

Article Ammonium-Carbamate-Rich Organogels for the Preparation of Amorphous Calcium Carbonates

Zoltán Bacsik, Peng Zhang and Niklas Hedin *

Department of Materials and Environmental Chemistry, Stockholm University, SE 106 91 Stockholm, Sweden; [email protected] (Z.B.); [email protected] (P.Z.) * Correspondence: [email protected]; Tel.: +46-8-162417

Received: 31 May 2017; Accepted: 18 June 2017; Published: 27 June 2017

Abstract: Amine-CO2 chemistry is important for a range of different chemical processes, including carbon dioxide capture. Here, we studied how aspects of this chemistry could be used to prepare calcium carbonates. Chemically crosslinked organogels were ﬁrst prepared by reacting hyperbranched polyethylene imine (PEI) dissolved in DMSO with carbon dioxide. The crosslinks of the organogel consisted of ammonium-carbamate ion pairs as was shown by IR spectroscopy. These carbamate-rich organogels were subsequently subjected to aqueous solutions of calcium acetate, and amorphous calcium carbonate (ACC) precipitated. The ACC did not crystalize during the mixing for up to 20 h, as was shown by a combination of IR spectroscopy, X-ray diffraction, scanning electron microscopy, and thermal analysis. Some PEI had been included or adsorbed on the ACC particles. Traces of calcite were observed in one sample that had been subjected to water in a work-up procedure.

Keywords: amine; carbon dioxide; ammonium-carbamate ion pairs; organogel; amorphous calcium carbonate (ACC)

1. Introduction The understanding of the details of precipitation and crystallization of calcium carbonates has changed. Now it commonly involves aspects of non-traditional mechanisms of crystallization where clusters and amorphous calcium carbonates (ACCs) are important objects [1]. An enhanced understanding of the formation of calcium carbonates is important as they contribute to the carbon balance globally, and there are, for example, indications that ocean acidification is already affecting the growth of coral reefs negatively [2]. In addition, calcium carbonates are important in various technical applications. For example, calcite can be added to a certain degree in Portland cement [3] and be used as a filler in papers [4]. Calcium carbonates are very important for many organisms in their biomineralized tissues, where the calcium carbonate contributes with a functional toughening [5,6]. ACCs occur in certain biomineralized tissues [5,7–9] but also in synthetic calcium carbonates [10–12] where they have been shown to display both structural and chemical differences [13–15]. Polymers have been shown to moderate the precipitation and crystallization of calcium carbonates by affecting the time scales of aggregation and crystallization, as well as the polymorphism and the morphologies of the particles [16–19]. Specifically, amines and polyamines have been shown to influence the precipitation and crystallization of calcium carbonates by various interactions with carbonate ions, affecting the buffering capacity, and their tendencies to adsorb to carbonate-rich interfaces [20–22]. Amines and polyamines have been studied to a large extent when it comes to their abilities to capture carbon dioxide from flue gas or air [23–26]. The CO2-amine chemistry is surprisingly rich and involves coupled reaction networks with bicarbonate, carbamates, and carbamic acid

Minerals 2017, 7, 110; doi:10.3390/min7070110 www.mdpi.com/journal/minerals Minerals 2017, 7, 110 2 of 8 moieties [27–29]. In many solvents, ammonium-carbamate ion pairs are the dominant species and form rapidly [30]. From a technological point of view, carbamates are typically preferred over bicarbonates in carbon-capture systems as they form and decompose relatively rapidly [27,31]. For polyamines in selected solvents, organogels have been shown to form when being contacted with CO2(g). For example, Carretti et al. have derived and studied such organogels that had been prepared from polyallylamine and CO2(g), and they related the gelation to ammonium-carbamate-based crosslinks [32]. Carretti et al. also extended these studies to include organogels of polyethyleneimine (PEI) induced by reactions with CO2, and studied these ammonium-carbamate crosslinked gels in the context of art preservation [33,34]. We got inspired by the option to prepare ammonium-carbamate crosslinked organogels of PEI and CO2 and wanted to investigate if such could be used to prepare calcium carbonates. The case of using such gels as sources of CO2 was strengthened by the study of Prah et al. [35], who had shown that molecular ammonium carbamates could be used in the crystallization of calcium carbonates using solutions of calcium acetate. The novelty of this study is that we use polymeric carbamates that were prepared from reactive CO2 absorption in PEI solutions. It could be relevant to combined CO2 capture and mineralization. The organogels were prepared from hyperbranched and low molecular weight PEI and CO2, and we studied aspects of the precipitation of ACC when the organogels were mixed with aqueous solutions of calcium acetate.

2. Materials and Methods A solution of hyperbranched PEI (Sigma-Aldrich, Saint Louis, MO, USA, CAS number: 25987-06-8, average Mw = 800) in dimethyl sulfoxide (DMSO) was prepared, by adding and mixing 0.3677 g (2.76 mmol) of PEI in 2 mL DMSO. The organogel was prepared by bubbling CO2 through the PEI solution with a flow rate of 2.3 mL/min for 1 h. Subsequently, the gel was centrifuged to reduce the amount of DMSO. A solution of calcium acetate hydrate (Sigma-Aldrich, CAS number: 114460-21-8) was prepared in water; by adding and mixing 5 g of calcium acetate hydrate in 50 mL of deionized water. 4.6 mL of the calcium acetate solution was added at a flow rate of 0.75 mL/min to the CO2-induced organogel of PEI using a syringe pump. Such reactive mixtures were stirred (500 rpm) for 1 h (samples SW and SE) or 20 h (samples LW and LE) at room temperature. The formed solids were filtered off and washed carefully with 5 mL of deionized water and 2 × 5 mL of ethanol (samples SW and LW) or with 3 × 5 mL of ethanol (samples SE and LE). The white solids were dried at room temperature. The sample nomenclature used is: S for short mixing and L for long mixing; W for washed with water and E for washed with ethanol. The samples were studied with infrared (IR) spectroscopy using a Varian 670-IR spectrometer (Varian, Mulgrave, Australia) equipped with a single reflection ATR device (Specac, Orpington, UK) with a diamond ATR element. X-ray diffraction was used to study the crystallinity of the solids formed using a XPERT-PRO PANalytical powder diffractometer (PANalytical B.V., Almelo, The Netherlands) (reflection mode with an X’Celerator detector (Cu Kα1 radiation, λ = 1.5418 Å) from 5.0◦ to 85.0◦ (2θ). The step size was set to 0.0131◦, and the step time was 0.0104 ◦/min, which resulted in a total accumulation time of 126 min. Scanning electron microscopy (SEM) was used to study the morphologies of the solids using a JEOL JSM-7401F scanning electron microscope (JEOL USA Inc., Peabody, MA, USA) that operated at a voltage of 1 kV. The amount of organics in the solids was estimated by thermal analysis using a TA Instruments gravimetric thermal analyzer. The samples were subjected to an air flow 20 mL/min, the temperature range was 25–900 ◦C, and the temperature rate was 5 ◦C/min.

3. Results and Discussion

Gels formed rapidly when CO2 was bubbled through solutions of PEI in DMSO, and Figure1 displays a corresponding image. For the gelation, we presumed that all the amine groups had reacted Minerals 2017, 7, 110 3 of 8

with CO2 forming ammonium-carbamate ion pairs following the stoichiometry of amine: CO2 = 2:1. InMinerals analogyMinerals 2017, 2017 to7, 110 the, 7, 110 study of Carretti et al. [32], we hypothesized that chemical crosslinks had3 of formed 8 3 of 8 among the PEI chains when the amino groups had reacted with CO2. This hypothesis of chemical amongamong the PEIthe PEIchains chains when when the the am aminoino groups groups had reactedreacted with with CO CO2. This2. This hypothesis hypothesis of chemical of chemical crosslinks with alkylammonium-alkylcarbamate ion pairs was supported by IR analyzes. crosslinkscrosslinks with with alkylammonium-alkylcarbamate alkylammonium-alkylcarbamate ion pairspairs was was supported supported by byIR analyzes.IR analyzes.

Figure 1. Photograph of the organogel formed by subjecting a solution of polyethylene imine in Figure 1. Photograph of the organogel formed by subjecting a solution of polyethylene imine in DMSO DMSO to CO2. to CO2. Figure 1. Photograph of the organogel formed by subjecting a solution of polyethylene imine in Figure 2 shows the IR spectra of the organogel and the corresponding PEI. Alkylammonium- DMSO to CO2. Figurealkylcarbamate2 shows ion pairs the were IR spectradetected by of analyzing the organogel the IR spectrum and theof the corresponding organogel. In this PEI. spectrum, presented in Figure 2a, the broad band at frequencies between 3350 and 2000 cm−1 and the Alkylammonium-alkylcarbamateFigure 2 shows the IR spectra ionof the pairs organogel were detectedand the corresponding by analyzing PEI. the Alkylammonium- IR spectrum of bands at 1636 cm−1 and 1465 cm−1 are typical for ammonium groups, and the band at a frequency of the organogel. In this spectrum, presented in Figure2a, the broad band at frequencies between 3350 alkylcarbamate−1 ion pairs were detected by analyzing the IR spectrum of the organogel. In this 1565 cm− 1is a clear signature of the C=O−1 group of carbamates−1 [36]. Bands were also observed for and 2000 cm and the bands at 1636 cm and 1465 cm are typical for ammonium groups,−1 and the spectrum,DMSO, presented of which thein Figureone with 2a, the the highest broad intensity band at appeared frequencies at a frequencybetween 3350of 1018 and cm 2000−1. cm and the −1 bandbands at at a 1636 frequency cm−1 and of 1565 1465 cm cm−1is are a clear typical signature for ammonium of the C=O groups, group and of carbamates the band at [36 a]. frequency Bands were of also1565 observedcm−1 is a forclear DMSO, signature of which of the theC=O one group with of the carbamates highest intensity [36]. Bands appeared were also at a frequencyobserved for of −1 1018DMSO, cm of .which the one with the highest intensity appeared at a frequency of 1018 cm−1.

Figure 2. IR spectra of the (a) polyethylene imine (PEI)/ammonium-carbamate/DMSO organogel and (b) PEI.

The organogels were centrifuged to reduce the amount of DMSO. Subsequently, aqueous solutions of calcium acetate were added and white solid precipitates formed, see the reactions in Scheme 1. We added a stoichiometric amount of Ca2+ ions, corresponding to a 1:1 ratio of FigureFigureCa 2+2.:carbamate. IR 2. spectraIR spectra ofFor the ofthis ( thea) addition, polyethylene (a) polyethylene we assumed imine imine (PEI)/ammonium-carbamate/DMSO that (PEI)/ammonium-carbamate/DMSO all of the amine groups of the organogel PEI organogel had andformed and(b) PEI. (ammonium-carbamatesb) PEI. giving an N–carbamate ratio of 2:1. Two different groups of samples were The organogels were centrifuged to reduce the amount of DMSO. Subsequently, aqueous solutions of calcium acetate were added and white solid precipitates formed, see the reactions in Scheme 1. We added a stoichiometric amount of Ca2+ ions, corresponding to a 1:1 ratio of Ca2+:carbamate. For this addition, we assumed that all of the amine groups of the PEI had formed ammonium-carbamates giving an N–carbamate ratio of 2:1. Two different groups of samples were

Minerals 2017, 7, 110 4 of 8

The organogels were centrifuged to reduce the amount of DMSO. Subsequently, aqueous solutions of calcium acetate were added and white solid precipitates formed, see the reactions in Scheme1. We added a stoichiometric amount of Ca2+ ions, corresponding to a 1:1 ratio of Ca2+:carbamate. For this addition,Minerals 2017 we, 7, assumed110 that all of the amine groups of the PEI had formed ammonium-carbamates4 of 8 giving an N–carbamate ratio of 2:1. Two different groups of samples were prepared and subjected to shortprepared (samples and subjected SE and SW) to short and long (samples (samples SE and LE and SW) LW) and mixing long (samples after the LE calcium and LW) acetate mixing solutions after hadthe calcium been added. acetate solutions had been added. InIn aa control experimentexperiment when only water had been added to the organogel, the gel disappeared, whichwhich waswas expectedexpected byby thethe instabilityinstability of ammonium-carbamateammonium-carbamate ion pairs in aqueous solutions. Bicarbonate moieties tend to form on addition of water, and thethe PEIPEI isis expectedexpected toto protonatedprotonated andand solublesoluble inin water.water.

Scheme 1. Chemical reactions in the synthesis of amorphous calcium carbonate by bubbling CO2 into Scheme 1. Chemical reactions in the synthesis of amorphous calcium carbonate by bubbling CO2 into solutionsolution ofof PEI PEI and and subjecting subjecting the formedthe formed organogel organogel (PEI–ammonium-carbamate) (PEI–ammonium-carbamate) to aqueous to solutionsaqueous ofsolutions calcium of acetate. calcium acetate.

The solid precipitates were either washed in ethanol (samples SE and LE) or washed in water The solid precipitates were either washed in ethanol (samples SE and LE) or washed in water and and ethanol (samples SW and LW). These experiments were performed to assess eventual ethanol (samples SW and LW). These experiments were performed to assess eventual contributions of contributions of water washing on the crystallization of calcium carbonate. The samples were studied water washing on the crystallization of calcium carbonate. The samples were studied with several with several methods. The XRD patterns revealed that the samples SE, SW, and LE were amorphous, methods. The XRD patterns revealed that the samples SE, SW, and LE were amorphous, and that and that the sample LW (20 h of mixing and water washing) contained a minor fraction of calcite with the sample LW (20 h of mixing and water washing) contained a minor fraction of calcite with broad broad diffraction lines. Figure 3 shows the diffractograms of the samples, and the low signal-to-noise diffraction lines. Figure3 shows the diffractograms of the samples, and the low signal-to-noise levels levels related to the amorphous nature of the samples. related to the amorphous nature of the samples. The XRD diffraction patterns in Figure3 indicated the absence of crystallinity but could not easily be used to detect ACCs, although the wide-angle scattering in the patterns is typical for ACC. IR spectroscopy, however, can be very useful for studying ACCs [14,37]. The IR spectra of samples SE, SW, LE, and LW are shown in Figure4. The spectra clearly showed that the calcium carbonates in samples SE, SW, and LE consisted of fully of ACC, and mainly of ACC in sample LW. The bands observed at a frequency of 1380 cm−1, with a shoulder at 1465 cm−1, belong to the asymmetric stretching of carbonates. The band at a frequency of 1071 cm−1 corresponds to the symmetric stretching, and the band at 861 cm−1 to the out-of-plane bending modes. These frequency values were typical for ACCs. The band for the in-plane bending mode of the carbonate ions were observed at frequencies of around 700 cm−1. This band is the most useful to distinguish ACCs and various polymorphs of crystalline calcium carbonates. For the ACCs of this study, a broad doublet was observed at a frequency of

Figure 3. X-ray diffraction patterns of the amorphous calcium-carbonate-rich samples (a) SE, (b) SW, (c) LE, and (d) LW that had been precipitated when subjecting organogels (PEI–ammonium- carbamate) to aqueous solutions of calcium acetate. S and L stand for mixing for 1 h and 20 h; E and W stand for ethanol and water-and-ethanol washing.

Minerals 2017, 7, 110 4 of 8

prepared and subjected to short (samples SE and SW) and long (samples LE and LW) mixing after the calcium acetate solutions had been added. In a control experiment when only water had been added to the organogel, the gel disappeared, which was expected by the instability of ammonium-carbamate ion pairs in aqueous solutions. Bicarbonate moieties tend to form on addition of water, and the PEI is expected to protonated and soluble in water.

Scheme 1. Chemical reactions in the synthesis of amorphous calcium carbonate by bubbling CO2 into solution of PEI and subjecting the formed organogel (PEI–ammonium-carbamate) to aqueous solutions of calcium acetate.

The solid precipitates were either washed in ethanol (samples SE and LE) or washed in water and ethanol (samples SW and LW). These experiments were performed to assess eventual Minerals 2017, 7, 110 5 of 8 contributions of water washing on the crystallization of calcium carbonate. The samples were studied with several methods. The XRD patterns revealed that the samples SE, SW, and LE were amorphous, ~700and cmthat−1 the. In sample the spectrum LW (20 of h sampleof mixing LW, and a sharp water band washing) also appeared contained at a 711 minor cm− fraction1, which of is calcite typical with for calcitebroad and diffraction corroborated lines. theFigure ﬁndings 3 shows from the XRD diffractograms of a small fraction of the ofsamples, calcite inand this the sample. low signal-to-noise IR bands of thelevels PEI related were observed to the amorphous in the C-H nature stretching of the region samples. at frequencies of 2800–3000 cm−1.

Minerals 2017, 7, 110 5 of 8

The XRD diffraction patterns in Figure 3 indicated the absence of crystallinity but could not easily be used to detect ACCs, although the wide-angle scattering in the patterns is typical for ACC. IR spectroscopy, however, can be very useful for studying ACCs [14,37]. The IR spectra of samples SE, SW, LE, and LW are shown in Figure 4. The spectra clearly showed that the calcium carbonates in samples SE, SW, and LE consisted of fully of ACC, and mainly of ACC in sample LW. The bands observed at a frequency of 1380 cm−1, with a shoulder at 1465 cm−1, belong to the asymmetric stretching of carbonates. The band at a frequency of 1071 cm−1 corresponds to the symmetric stretching, and the band at 861 cm−1 to the out-of-plane bending modes. These frequency values were typical for ACCs. The band for the in-plane bending mode of the carbonate ions were observed at frequencies of around 700 cm−1. This band is the most useful to distinguish ACCs and various polymorphs of crystalline calcium carbonates. For the ACCs of this study, a broad doublet was FigureFigure 3. 3.X-ray X-ray diffraction diffraction patterns patterns of of the the amorphous amorphous calcium-carbonate-rich calcium-carbonate-rich samplessamples ((aa)) SE, SE, ( (bb)) SW, SW, observed at a frequency of ~700 cm−1. In the spectrum of sample LW, a sharp band also appeared at (c()c) LE, LE, and and (d) LW(d) thatLW hadthat been had precipitated been precipitated when subjecting when subjecting organogels organogels (PEI–ammonium-carbamate) (PEI–ammonium- 711 cm−1, which is typical for calcite and corroborated the findings from XRD of a small fraction of tocarbamate) aqueous solutions to aqueous of calciumsolutions acetate. of calcium S and acetate. L stand S forand mixing L stand for for 1 hmixing and 20 for h; E1 h and and W 20 stand h; E forand calcite in this sample. IR bands of the PEI were observed in the C-H stretching region at frequencies ethanolW stand and for water-and-ethanol ethanol and water-and-ethanol washing. washing. of 2800–3000 cm−1.

FigureFigure 4. IR 4. spectra IR spectra of theof the amorphous-calcium-carbonate-rich amorphous-calcium-carbonate-rich samples samples (a) (SE,a) SE,(b) SW, (b) SW,(c) LE, (c) and LE, ( andd) (d) LW. These samples had precipitated when subjecting the organogels (PEI–ammonium-carbamate) to LW. These samples had precipitated when subjecting the organogels (PEI–ammonium-carbamate) to aqueous solutions of calcium acetates. S and L stand for mixing for 1 h and 20 h; E and W stand for aqueous solutions of calcium acetates. S and L stand for mixing for 1 h and 20 h; E and W stand for ethanol and water-and-ethanol washing. ethanol and water-and-ethanol washing. The morphologies of the particles were studied by SEM, and the features observed in the images Thesupported morphologies the amorphous of the nature particles of samples. were studied Figure by 5 shows SEM, andSEM theimages features of the observed ACC-rich in samples the images supportedof SE and the LE amorphous and note that nature spherical of samples. particles Figure are typically5 shows observed SEM images for ACC of [14,38–40]. the ACC-rich samples of SE and LE and note that spherical particles are typically observed for ACC [14,38–40]. The amounts of amines in samples SE, SW, LE, and LW, were determined by thermogravimetric (TG) experiments. Figure6 shows the TG curves, and they were surprisingly different. All curves had mass losses in three major temperature regions. The mass loss in the temperature range between room temperature and 200 ◦C was attributed to water loss. The second region of mass losses occurred at temperatures between 200 and 450 ◦C. These losses were attributed to the combustion of PEI. Sample SE (red), SW (black), and LW (green) had similar trends in this region, but sample LE (blue) had a signiﬁcantly different tendency with a very signiﬁcant step of mass loss related to combustion

Figure 5. SEM image of the amorphous-calcium-carbonate-rich samples (a) SE and (b) LE. They had been precipitated when subjecting an organogel (PEI–ammonium-carbamate) to an aqueous solution of calcium acetate.

Minerals 2017, 7, 110 5 of 8

Minerals 2017, 7, 110 6 of 8 at a temperature of 370 ◦C. In the third regions, there were defined mass loss steps related to the decomposition of the carbonate groups (calcination) in all samples. From the mass losses observed in this region, the amounts of calcium carbonate were estimated. The amount of calcium carbonate in Figure 4. IR spectra of the amorphous-calcium-carbonate-rich samples (a) SE, (b) SW, (c) LE, and (d) the samples SE, SW, and LE were similar and about 50 wt %, and sample LW consisted of ~70 wt % LW. These samples had precipitated when subjecting the organogels (PEI–ammonium-carbamate) to calcium carbonate. We speculate that the higher amount of CaCO3 in sample LW can be explained by aqueous solutions of calcium acetates. S and L stand for mixing for 1 h and 20 h; E and W stand for the calcite formation and consequently a more effective washing of the sample. Considering that the Mineralsethanol 2017, 7 and, 110 water-and-ethanol washing. 6 of 8 amount of water appeared to be around 5 wt % in each sample, the amount of polymer and solvent in the samplesTheThe amountsmorphologies was between of amines of the 25 in andparticles samples 45 wt were SE, %. InSW, studied this LE, context, and by SEM, LW, note wereand that the determined the features molecular observed by thermogravimetric formula in the for images ACC ·x (TG)issupported typically experiments. the observed amorphous Figure to be 6 CaCOshowsnature3 theofH samples.TG2O; curves, however, Figu andre the they5 significantshows were SEM surprisingly amount images of ofdifferent. organics the ACC-rich All in the curves samples had massmadeof SE lossesand it very LE in difficultand three note major to that robustly sphericaltemperature establish particles regions. the compositionare typicallyThe mass ofobserved loss the in ACC the for in temperatureACC the studied [14,38–40]. samples.range between room temperature and 200 °C was attributed to water loss. The second region of mass losses occurred at temperatures between 200 and 450 °C. These losses were attributed to the combustion of PEI. Sample SE (red), SW (black), and LW (green) had similar trends in this region, but sample LE (blue) had a significantly different tendency with a very significant step of mass loss related to combustion at a temperature of 370 °C. In the third regions, there were defined mass loss steps related to the decomposition of the carbonate groups (calcination) in all samples. From the mass losses observed in this region, the amounts of calcium carbonate were estimated. The amount of calcium carbonate in the samples SE, SW, and LE were similar and about 50 wt %, and sample LW consisted of ~70 wt % calcium carbonate. We speculate that the higher amount of CaCO3 in sample LW can be explained by the calcite formation and consequently a more effective washing of the sample. Considering that the amount of water appeared to be around 5 wt % in each sample, the amount of polymer and solvent Figure 5. SEM image of the amorphous-calcium-carbonate-rich samples (a) SE and (b) LE. They had in theFigure samples 5. SEM was image between of the 25–45 amorphous-calcium-carbonate-rich wt %. In this context, note that samples the molecular (a) SE and (formulab) LE. They for had ACC is been precipitated when subjectingsubjecting an organogel (PEI–ammonium-carbamate)(PEI–ammonium-carbamate) to an aqueous solution typically observed to be CaCO3·xH2O; however, the significant amount of organics in the samples of calcium acetate. madeof it calcium very difficult acetate. to robustly establish the composition of the ACC in the studied samples.

Figure 6. Thermogravimetric curves of the amorphous-ca amorphous-calcium-carbonate-richlcium-carbonate-rich samples samples SE SE (red), (red), SW (black), LE LE (blue), (blue), and and LW LW (green). (green). These These samples samples had hadprecipitated precipitated when when subjecting subjecting organogels organogels (PEI– ammonium-carbamate)(PEI–ammonium-carbamate) to aqueous to aqueous solutions solutions of calciu of calciumm acetates. acetates. S and S L and stand L stand for mixing for mixing for 1 forh and 1 h 20and h; 20E and h; E W and stand W stand for ethanol for ethanol and water-and-ethanol and water-and-ethanol washing. washing.

4. Conclusions Conclusions Ammonium-carbamate-rich organogels organogels were were successfully successfully used used to to prepare prepare ACC. ACC. It It is our understanding thatthat thethe ammonium-carbamateammonium-carbamate ion ion pairs pairs are are coupled coupled chemically chemically to bicarbonatesto bicarbonates in the in theaqueous aqueous calcium calcium acetate acetate solution solution used used for the for precipitationthe precipitation of ACC. of ACC. The detailsThe details of this of chemistry this chemistry were werenot studied. not studied. A natural A natural extension extension of this studyof this could study be could to study be to this study chemistry this chemistry in some detail. in some ACC detail. was ACCstabilized was atstabilized the two mixingat the two times mixing used intimes this used study in (1 this h and study 20 h), (1 theh and extent 20 ofh), this the stabilizationextent of this is stabilizationrelevant for furtheris relevant studies for further as well studies the interfacial as well detailsthe interfacial of the PEI–ACC details of interactions. the PEI–ACC interactions.

Acknowledgments: Swedish Research Council (2016-03568) and the Carl Trygger Foundation are thanked.

Author Contributions: Z.B. and N.H. conceived and designed the experiments, analyzed data, and wrote the paper; P.Z. performed most of the experiments and analyzed data. Z.B. performed the IR experiments.

Conflicts of Interest: The authors declare no conflict of interest.

Minerals 2017, 7, 110 7 of 8

Acknowledgments: Swedish Research Council (2016-03568) and the Carl Trygger Foundation are thanked. Author Contributions: Z.B. and N.H. conceived and designed the experiments, analyzed data, and wrote the paper; P.Z. performed most of the experiments and analyzed data. Z.B. performed the IR experiments. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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