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Article Removal of Hardness from Water Samples by a Carbonation Process with a Closed Pressure Reactor

Min Kyung Ahn 1, Ramakrishna Chilakala 2, Choon Han 3 and Thriveni Thenepalli 2,* ID

1 Ewha Girls High School, 26 Jeongdong-gil, Jung-gu, Seoul 04516, Korea; [email protected] 2 Carbon Mineralization Center, Climate Change Mitigation and Sustainability Division, Korea Institute of Geosciences and Resources (KIGAM), 124, Gwahak-ro, Yuseong-gu, Daejeon 34132, Korea; [email protected] 3 Chemical Engineering Department, Kwangwoon University, 20 Kwangwoon-ro, Nowong-gu, Seoul 01897, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-42-868-3578

Received: 15 November 2017; Accepted: 8 January 2018; Published: 10 January 2018

Abstract: One of the undesirable characteristics of some ground and natural water sources is hardness. can cause many problems around the world, including increased scaling on water pipes, boilers, atopic eczema and odd-tasting . Hardness in natural water is caused by dissolved , mainly and compounds. According to the Association (WQA) and the United States Geological Survey (USGS), hard water is classified based on the Ca2+ and Mg2+ in , as follows: 0–60 ppm as soft; 61–120 ppm as moderately hard; 121–180 ppm as hard and more than 180 ppm as very hard water. Most water utilities consider a hardness level between 50 and 150 ppm of CaCO3 as publicly acceptable. The present study investigated the effects of a carbonation process on the removal of hardness in different water samples. Currently, a wide variety of hardness removal technologies are available. Among those conventional methods, carbonation is an inexpensive process which can be used for the removal of Ca2+ and Mg2+ from hard water. This study measured the hardness levels of 17 different water samples using the ethylene diamine tetra acetic acid (EDTA) method. Among these, Seoul outdoor swimming pool water (140 ppm) samples showed high of Ca2+ and Mg2+ ions. The hardness of the different water samples was reduced by 40–85% by a carbonation process with a closed pressure reactor for a 5 min reaction time.

Keywords: water hardness; removal; carbonation; closed pressure reactor

1. Introduction Hardness (hard water) is one of the common water quality problems throughout the world. A total of 70% of the earth’s surface is covered by water. Of this, 97% is only sea water (non-drinkable). Only 2.5% of fresh water is accessible for human use and of this only 0.5% is used as drinking water. Hard water is found at a rate exceeding 85%, as water picks up minerals such as magnesium and calcium ions from rocks and soil, leading to the hard water. Ground water contains more minerals than surface water, so it is harder than surface water. There are many exceptions, i.e., lakes in soil districts [1]. Knowing the hardness of water is important when evaluating its use as a domestic or industrial water supply. Hard water interferes with laundering, washing, bathing and personal grooming [2]. Clothes laundered in hard water may look dingy and harsh. Hard water utilization in the home can to other issues as . It also affects and used for cleaning because soap used in hard water combines with the high amount of minerals to form a sticky sludge. Bathing with soap in hard water can deposit this sticky residue onto the skin and can lead to irritation under slightly acid conditions.

Water 2018, 10, 54; doi:10.3390/w10010054 www.mdpi.com/journal/water Water 2018, 10, 54 2 of 10

Water 2018, 10, 54 2 of 10 Several studies have indicated a link between hardness concentrations (particularly calcium and magnesium)Several and studies cardiovascular have indicated diseases, a link between Alzheimer’s hardness disease concentrations and atopic (particularly eczema calcium [3,4]. However, and the linkmagnesium) is tenuous, and andcardiovascular many confounding diseases, Alzheimer’s variables disease exist in and the atopic studies eczema [5]. [3,4]. Hard However, water is the mainly link is tenuous, and many confounding variables exist in the studies [5]. Hard water is mainly caused caused by calcium and magnesium cations and to a lesser extent aluminum, and other cations. by calcium and magnesium cations and to a lesser extent aluminum, iron and other cations. Calcium Calcium and magnesium are the two major cations responsible for hardness in natural water [6]. and magnesium are the two major cations responsible for hardness in natural water [6]. Therefore, Therefore,for most for waters, most waters,the total thehardness total hardnessis caused by is causedmajor cations by major and cationsassociated and anions associated as shown anions in as shownFigure in Figure 1. 1.

Figure 1. Major cations and associated anions in water. Figure 1. Major cations and associated anions in water.

The cations are usually associated with the anions of HCO3−, CO32− and OH− ions and 3− 2− − Theacidity-associated cations are usually ions such associated as SO42−, withNO3−, theand anions Cl−. of HCO hardness, CO3 refersand to OH the numberalkalinity of ions 2− − − and acidity-associatedhardness ions associat ionsed suchwith alkalinity as SO4 ions., NO When3 , and the Cl solution. Carbonate is super-saturated, hardness refersthe cations to the in numberthe of hardnesswater can ions precipitate associated as witha hard alkalinity scale. Carbonat ions. Whene hardness the solutionis sensitive is super-saturated,to and the can cations in theprecipitate water can easily. precipitate At high astemperatures, a hard scale. Mg Carbonate salts can also hardness produce is hard sensitive scales in to boilers. temperature and can The main objective of this study is to describe the removal of hardness in water by a carbonation precipitate easily. At high , Mg salts can also produce hard scales in boilers. process in a closed-pressure reactor. In this paper, the crystallization process of precipitated calcium The main objective of this study is to describe the removal of hardness in water by a carbonation carbonate (PCC) and magnesium carbonate from hard water samples were investigated. The processPCC in thus a closed-pressure obtained could reactor.be utilized In thisas a paper,filler material the crystallization to enhance the process mechanical of precipitated properties calcium of carbonatecomposites (PCC) in and the plasti magnesiumcs and paper carbonate industries. from hard water samples were investigated. The calcite PCC thus obtained could be utilized as a filler material to enhance the mechanical properties of composites2. Materials in the and plastics Methods and paper industries. The existence of Ca2+ and Mg2+ ions in water causes hardness. A water supply with a hardness 2. Materials and Methods level of 100 parts per million (100 ppm) contains the equivalent of 100 g of CaCO3 in one liter of water. TheThe total existence hardness of Ca is 2+equaland to Mg the2+ sumions of inmagnesium water causes hardness hardness. and calcium A water hardness. supply with a hardness In the present experiment, total hardness was measured by the EDTA method for hard water level of 100 parts per million (100 ppm) contains the equivalent of 100 g of CaCO3 in one liter of water. The totalsamples hardness collected is equalfrom different to the sum areas of magnesiumof South Korea hardness and artificial and calciumhard water hardness. samples that were prepared in the lab. The hardness was then reduced by the carbonation process. In all, five different In the present experiment, total hardness was measured by the EDTA method for hard water artificial hard water samples, five different drinking water samples, and eight different natural water samplessamples collected were tested, from differentas shown in areas Table of 1. South Korea and artificial hard water samples that were prepared in the lab. The hardness was then reduced by the carbonation process. In all, five different artificial hard water samples,Table five 1. Different different water drinking samples waternames with samples, given numbers. and eight different natural water samples wereArtificial tested, Hard as Water shown in Table Drinking1. Water Natural Water Sample Name Number Sample Name Number Sample Name Number CHOICE Artificial Soft water Table A-(1) 1. Different water samplesD-(1) names with given Softener numbers. pass water N-(1) water Artificial Slightly Hard Artificial Hard WaterA-(2) ViO water Drinking D-(2) Water Seoul Arisu water Natural Water N-(2) water ArtificialSample Moderately Name NumberSam Da Sample Soo Name Number Sample Name Number A-(3) D-(3) Seoul Park N-(3) ArtificialHard water Soft water A-(1)water CHOICE water D-(1) Softener pass water N-(1) Artificial Very Hard Artificial Slightly Hard waterA-(4) A-(2) O-EAU water ViO water D-(4) D-(2)Seoul Outdoor Swimming Seoul Arisu pool water (1) water N-(4) N-(2) water Sam Da Soo Artificial Moderately Hard water A-(3) D-(3) Seoul Outdoor Swimming Seoul Park pool Tap water(2) water N-(5) N-(3) water Seoul Outdoor Swimming pool Toilet Seoul Outdoor Swimming pool N-(6) Artificial Very Hard water A-(4) O-EAU water D-(4) water N-(4) (1) water (Korea Institute of Geoscience and Seoul Outdoor Swimming pool N-(7) Mineral Resources(KIGAM)-Waste water N-(5) (2) water Seoul Outdoor Swimming pool N-(6) Toilet water

(Korea Institute of Geoscience and Mineral N-(7) Resources(KIGAM)-Waste water Water 2018, 10, 54 3 of 10 Water 2018, 10, 54 3 of 10 2.1. Artificial Hard Water Samples Synthetic artificially hard water was prepared with major ions mixed into the proper amounts 2.1. Artificial Hard Water Samples of fresh water. Most compositions include Mg2+, Ca2+, Na+, K+ and Cl−, SO42− depending on their hardnessSynthetic and on artificially the user hardrequirements. water was CO prepared32− is present with at major low concentrations ions mixed into in the most proper natural amounts water 2+ 2+ + + − 2− samplesof fresh water.at a pH Most of seven. compositions The alkalinity include was Mg measured, Ca , Na frequently,K and for Cl the, SO soft4 anddepending hard water on theirand 2− washardness assumed and onto thebe equal user requirements. to the concentration CO3 is of present the HCO at low3− ions concentrations [7]. The different in most artificial natural water water samples atwere a pH denoted of seven. as soft, The alkalinityslightly hard, was moderately measured frequently hard and forvery the hard soft water and hard with water respect and to was the − chemicalassumed tocompositions be equal to the[8], concentrationas shown in Table of the 2. HCO 3 ions [7]. The different artificial water samples were denoted as soft, slightly hard, moderately hard and very hard water with respect to the chemical compositions [8], asTable shown 2. Chemical in Table 2composition. for the preparation of hard water.

Water Samples Reagent Added (mg/L) Approximate Final Water Quality Table 2. Chemical composition for the preparation of hard water. NaHCO3 CaSO4·2H2OMgSO4 KCl pH Hardness (mg/L)

SoftWater Samples12.0 Reagent7.5 Added (mg/L)7.5 0.5 Approximate6.4–6.8 Final Water Quality10–13 Slightly hard 48.0 30.0 30.0 2.0 7.2–7.6 40–48 NaHCO3 CaSO4·2H2O MgSO4 KCl pH Hardness (mg/L) ModeratelySoft hard 96.0 12.0 7.5 60.0 7.5 60.0 0.5 4.0 6.4–6.8 7.4–7.8 10–13 80–100 HardSlightly hard192.0 48.0 30.0120.0 30.0120.0 2.08.0 7.2–7.67.6–8.0 40–48160–180 Moderately hard 96.0 60.0 60.0 4.0 7.4–7.8 80–100 Very hardHard 384.0 192.0 120.0 240.0 120.0 240.0 8.0 16.0 7.6–8.0 8.0–8.4 160–180 280–320 Very hard 384.0 240.0 240.0 16.0 8.0–8.4 280–320 2.2. Water Hardness as Measured by the EDTA Method 2.2. Water Hardness as Measured by the EDTA Method Water hardness can be readily determined by titration with the chelating agent EDTA (ethylene diamineWater tetra hardness acetic canacid). be readilyIn the determinedsample solution by titration containing with thecalcium chelating and agentmagnesium EDTA (ethyleneions, the reactiondiamine tetratakes acetic place acid).with an In theexcess sample of EDTA solution by containinga titration process calcium and magnesiumthe indicator ions, added the remains reaction astakes eriochrome place with black an excess T (EBT)-blue of EDTA bybecause a titration all of process the Ca and2+ and the Mg indicator2+ ions addedpresent remains are complexed as eriochrome with theblack EDTA T (EBT)-blue [9]. During because this proces all of thes, the Ca 2+EDTAand Mgreacts2+ ions with present the Ca are2+ and complexed Mg2+ ions with and the to EDTAform the [9]. EDTA-CaDuring this and process, EDTA-Mg the EDTAcomplex reacts (Figure with 2), the and Ca after2+ and the Mg completion2+ ions and of tothis form reaction, the EDTA-Ca the wine andred colorEDTA-Mg disappears complex slowly (Figure because2), and the after EBT-Ca the completion and EBT-Mg of this are reaction, replaced the and wine form red Ca-EDTA color disappears stable, colourlessslowly because complex, the EBT-Ca while simultaneously and EBT-Mg are a blue replaced color and appears form due Ca-EDTA to the stable,effect of colourless the EBT indicator complex, while[10]. simultaneously a blue color appears due to the effect of the EBT indicator [10].

2+ 2+ FigureFigure 2. EDTA 2. EDTA complex complex formation formation by thethe reactionreaction with with Ca Ca2+ and and Mg Mg2+ ions. ions.

2.3. The KIGAM Closed Pressure Reactor for a Carbonation Process The Korea Institute of Geosciences and Mineral resources (KIGAM), has introduced a closed pressure reactor for thethe removalremoval ofof hardness.hardness. This can be accomplished in conjunction with the coagulation of suspended solids, as shown in Figure 33.. Here,Here, aa 11 LL closedclosed pressurepressure reactor,reactor, withwith aa maximum working CO2 pressure of 10 bar is used. It is connected with a CO2 cylinder containing maximum working CO2 pressure of 10 bar is used. It is connected with a CO2 cylinder containing 60 L 60of carbonatedL of carbonated water, aswater, shown as inshown Figure in4. Figure This closed 4. This pressure closed reactor, pressure based reactor, upon based a carbonation upon a process,carbonation is used process, to control is used the to concentrations control the concen of calciumtrations and of magnesiumcalcium and ions magnesium in hardwater. ions in In hard this water. In this carbonation process, 1 L of each water sample was brought into the reactor chamber carbonation process, 1 L of each water sample was brought into the reactor chamber and CO2 gas and CO2 gas was injected using a CO2 injector machine. The closed pressure reactor has two

Water 2018, 10, 54 4 of 10

Water 2018, 10, 54 4 of 10 wasWater injected 2018, 10 using, 54 a CO2 injector machine. The closed pressure reactor has two alternative outlets4 of 10 for collecting the final precipitated solution. The upper outlet is for the bench scale reactor (1–2 L) and the alternative outlets for collecting the final precipitated solution. The upper outlet is for the bench scale bottomalternative outlet outlets is for thefor collecting pilot scale the reactor. final precipitated solution. The upper outlet is for the bench scale reactor (1–2 L) and the bottom outlet is for the pilot scale reactor. reactorDuring (1–2 the L)hard and the water bottom carbonation outlet is processfor the pilot by the scale closed reactor. pressure reactor, three different CO2 flow During the hard water carbonation process by the closed pressure reactor, three different CO2 rates wereDuring injected, the hard giving water different carbonatio reactionn process times. by The the process closed involved pressure 7 reactor, g/L CO three2 for adifferent 3 min reaction, CO2 flow rates were injected, giving different reaction times. The process involved 7 g/L CO2 for a 3 min 14flow g/L ofrates CO were2 gas injected, for a 4 min giving reaction different and 21reaction g/L of times. CO2 forThe a process 5 min reaction. involved The 7 g/L carbonation CO2 for a 3 reaction min reaction, 14 g/L of CO2 gas for a 4 min reaction and 21 g/L of CO2 for a 5 min reaction. The carbonation reaction, 14 g/L of CO2 gas for a 4 min reaction and 21 g/L of CO2 for a 5 min reaction.2+ The carbonation2+ beganreaction with began the hydration with the hydration of carbon of dioxide carbon indioxide the water, in the which water, reacted which reacted with Ca withand Ca2+ Mg and Mgions2+ to reaction began with the hydration of in the water, which reacted with Ca2+ and Mg2+ formions calcium to form carbonate and magnesium and magnesium carbonate ca precipitates.rbonate precipitates. The effects The of theeffects carbon of the dioxide carbon flow ions to form calcium carbonate and magnesium carbonate precipitates. The effects of the carbon rate,dioxide the temperature flow rate, the at temperature carbonation, at andcarbonation, the pH on and the the pH on the precipitation formation were formation investigated. were dioxide flow rate, the temperature at carbonation, and the pH on the precipitation formation were Theinvestigated. pH values ofThe all pH the samplesvalues of were all the measured samples before were carbonationmeasured before and aftercarbonation the carbonation and after process the investigated. The pH values of all the samples were measured before carbonation and after the withcarbonation a pH meter process (A 214, with Orion, a pH Thermometer (A Scientific, 214, Orion, Waltham, Thermo MA,Scientific, USA). Waltham, MA, USA). carbonation process with a pH meter (A 214, Orion, Thermo Scientific, Waltham, MA, USA).

Figure 3. Carbonation process with the closed pressure reactor. FigureFigure 3. 3.Carbonation Carbonation process with th thee closed closed pressure pressure reactor. reactor.

Figure 4. KIGAM bench scale carbonation reactor. FigureFigure 4.4. KIGAMKIGAM bench scale carbonation carbonation reactor. reactor. 2.4. Characterization of Crystals by SEM, XRD and EDS 2.4. Characterization of Crystals by SEM, XRD and EDS 2.4. Characterization of Crystals by SEM, XRD and EDS The crystallites formed join together to form polycrystalline particles with a rhombohedral The crystallites formed join together to form polycrystalline particles with a rhombohedral shape.The crystallitesRe-crystallization formed ta joinkes place together on the to formsurface polycrystalline of the particles particles to create with a very a rhombohedral thin single-crystal shape. shape. Re-crystallization takes place on the surface of the particles to create a very thin single-crystal Re-crystallizationshell, which then takes extends place inwards, on the surfaceleading ofto thetrue particles single crystals. to create Figure a very 5a thin shows single-crystal the scanning shell, shell, which then extends inwards, leading to true single crystals. Figure 5a shows the scanning electron microscopy (SEM) (JEOL Company, Seoul, South Korea) results of the rhombohedral whichelectron then microscopy extends inwards, (SEM) leading(JEOL Company, to true single Seou crystals.l, South FigureKorea)5 aresults shows of the the scanning rhombohedral electron calcium carbonate crystals and Figure 5b shows the X-ray diffraction (XRD) (Korea Institute of microscopycalcium carbonate (SEM) (JEOL crystals Company, and Figure Seoul, 5b South shows Korea) the resultsX-ray diffraction of the rhombohedral (XRD) (Korea calcium Institute carbonate of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea) analysis, which indicated that crystalsGeosciences and Figure and Mineral5b shows Resources the X-ray (KIGAM), diffraction Daejeo (XRD)n, South (Korea Korea) Institute analysis, of Geosciences which indicated and Mineral that pure calcite was formed. The magnesium carbonate peak did not appear in the XRD results due to a Resourcespure calcite (KIGAM), was formed. Daejeon, The magnesium South Korea) carbonate analysis, pe whichak did indicatednot appear that in the pure XRD calcite results was due formed. to a low quantity of magnesium, but the energy dispersive x-ray spectroscopy (EDS) (Korea Institute of Thelow magnesium quantity of carbonate magnesium, peak but did the not energy appear dispersi in the XRDve x-ray results spectroscopy due to a low (EDS) quantity (Korea of Institute magnesium, of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea) results show a negligible butGeosciences the energy and dispersive Mineral x-rayResources spectroscopy (KIGAM), (EDS) Daejeon, (Korea South Institute Korea) of results Geosciences show a andnegligible Mineral quantity of magnesium. Resourcesquantity (KIGAM),of magnesium. Daejeon, South Korea) results show a negligible quantity of magnesium.

Water 2018, 10, 54 5 of 10 Water 2018, 10, 54 5 of 10

(a) (b)

FigureFigure 5. 5.(a )(a SEM) SEM and and EDS resultsEDS results of calcium of calcium and magnesium and magnesium carbonate crystalscarbonate formed crystals by carbonation; formed by (bcarbonation;) XRD results (b of) XRD calcite. results of calcite.

3.3. ResultsResults andand DiscussionDiscussion TheThe waterwater samplesample hardnesshardness levelslevels werewere measuredmeasured byby thethe EDTAEDTA methodmethod andand thethe hardnesshardness waswas reducedreduced byby thethe carbonationcarbonation process.process. InIn addition,addition, thethe pHpH levelslevels beforebefore andand afterafter carbonationcarbonation werewere measured in all of the water samples. During the carbonation process the CO2 injection, temperature measured in all of the water samples. During the carbonation process the CO2 injection, temperature 2+ 2+ andand pHpH effect effect played played a akey key role role to precipitate to precipitate the theCa Caand2+ Mgand Mg2+ carbonates and to reduce and to the reduce hardness the hardnessin different indifferent water samples. water samples. Table 3 Tableshows3 showsthe hard theness hardness of different of different water water samples samples before before and andafter aftercarbonation. carbonation.

TableTable 3. 3.Hardness Hardnessof ofdifferent different water water samples samples before before and and after after carbonation. carbonation.

Water Hardness-before Water Hardness after Carbonation (mg/L) Sample Name Water Hardness-before Water Hardness after Carbonation (mg/L) Sample Name Carbonation (ppm) 7 g/L CO2 for 3 min 14 g/L CO2 for 4 min 21 g/L CO2 for 5 min Carbonation (ppm) I.7 g/LArtificial CO2 for hard 3 min water 14 g/L CO2 for 4 min 21 g/L CO2 for 5 min A-(1) 38 I. Artificial 24 hard water 17 12 A-(2)A-(1) 84 38 52 24 17 39 12 22 A-(3)A-(2) 110 84 65 52 39 50 22 24 A-(3) 110 65 50 24 A-(4) 172 108 79 43 A-(4) 172 108 79 43 A-(5)A-(5) 344 344 210 210 158 158 90 90 II.II. Drinking Drinking water water samples samples D-(1) 96 60 45 32 D-(1) 96 60 45 32 D-(2) D-(2)60 60 38 38 25 25 17 17 D-(3) D-(3)22 22 14 14 10 10 7 7 D-(4) D-(4)52 52 33 33 24 24 18 18 III.III. Natural waterwater samples samples N-(1)N-(1) 64 64 40 40 30 30 19 19 N-(2)N-(2) 56 56 35 35 27 27 15 15 N-(3)N-(3) 64 64 40 40 29 29 18 18 N-(4)N-(4) 70 70 45 45 32 32 14 14 N-(5) 132 79 52 20 N-(5)N-(6) 132 140 79 88 56 52 21 20 N-(6)N-(7) 140 76 88 50 35 56 16 21 N-(7)N-(8) 76 66 50 40 29 35 18 16 N-(8) 66 40 29 18 3.1. Effect of Carbon Dioxide Flow Rates 3.1. Effect of Carbon Dioxide Flow Rates The carbonation process by the closed pressure reactor method was in an aqueous medium. The carbonation process by the closed pressure reactor method was in an aqueous medium. Different amounts of CO2 gas were injected into the reaction bottle for different reaction times, such as Different amounts of CO2 gas were injected into the reaction bottle for different reaction times, such 7 g CO2 per liter for 3 min reaction, 14 g CO2 per liter for 4 min reaction and 21 g CO2 per liter for 5as min 7 g reactionCO2 pertimes. liter for Carbon 3 min dioxidereaction, was 14 g injected CO2 per into liter the for reaction 4 min reaction bottle as and shown 21 g in CO Figure2 per4 liter. for 5 minThe reaction basic twotimes. steps Carbon involved dioxide in the was carbonation injected into reaction the reaction are (i) bottle hydration as shown of carbon in Figure dioxide 4. as shownThe in Formulasbasic two (1)–(3);steps involved (ii) the carbonatein the carbonation ions react re withaction calcium are (i) ionshydration to form of calcium carbon carbonatedioxide as precipitatesshown in Formulas (Formula (1)–(3); (4)). (ii) the carbonate ions react with calcium ions to form calcium carbonate precipitates (Formula (4)).

CO2(g) + H2O(l) ↔ H2CO3(aq) (1) (Dissociation of aqueous CO2 into water)

Water 2018, 10, 54 6 of 10

CO + H O ↔ H CO 2(g) 2 (l) 2 3(aq) (1) (Dissociation of aqueous CO2 into water) + − H2CO3(aq) ↔ H + HCO3 (2) − + 2− HCO3 → H + CO3 (3) (Produces supersaturation when) 2+ 2− (Ca )(CO3 ) (4) S1 = Ksp > 1 (Formation of nuclei or critical cluster) Ca2+ + CO 2− → CaCO (nuclei) 3 3 (5) (Crystal growth occurs spontaneously)

Ca2+ + CO 2− → CaCO (Calcite) 3 3 (6) (Finally, calcite formed) Carbon dioxide species are important participants in reactions that control the pH of natural − 2− waters. Reactions among the alkalinity-related species, aqueous CO2,H2CO3(aq), HCO3 , and CO3 , and directly pH related species, OH− and H+, are relatively fast and can be evaluated with models. Rates of equilibration between solute species and gaseous CO2 across a phase boundary are slower, and water bodies exposed to the atmosphere may not be in equilibrium with it at all times [11]. Here, we have presented the basic theory concept. The theory behind the calcium–carbon-dioxide equilibrium states that when gas carbon dioxide is pressed into the water, this takes place according to Henry’s law. The carbon dioxide combines with water and forms (diprotonic) carbonic acid, which is very difficult to find analytically, and for this reason is presented as aqueous carbon dioxide subsequently split into hydrogen ions and hydrogen carbonate ions. The hydrogen carbonate ion then may release a hydrogen ion and also a carbonate ion. The carbonate ions thus formed combine with calcium (and similarly magnesium) to form calcium carbonate. This substance is difficult to dissolve in water and precipitates quite easily. The calcium 2+ 2− carbonate precipitates easier at increased water temperature. The equilibrium [Ca ] + [CO3 ] = [CaCO3] tells at which concentrations precipitation takes place. The unique character of calcium carbonate is also present, namely that its precipitation increases with temperature instead of the opposite. Normally, it is easier to dissolve substances at increased temperature, but with CaCO3 it is the opposite, which explains the great practical interest paid to hardness on the heating of water in different kinds of boilers in households and industry. Thus, hardness removal increases with temperature. 2+ 2− The equilibrium can be computed by [Ca ] × [CO3 ] = Ks, where Ks is the temperature-dependent solubility product for CaCO3. As the aim is to remove the hardness, the actual 2- Ca content should be decreased. It means that the content of CO3 must increase in order to overcome − + 2− the solubility product and get a precipitation of CaCO3. From the equilibrium, HCO3 = H + CO3 + 2− 3− and the connected equation [H ] × [CO3 ] = K2 × [HCO ], where K2 is a temperature-dependent 2− constant, it follows that to increase the content of [CO3 ] it is necessary to increase the content of hydrogen carbonate and/or decrease the content of [H+]. As – log [H+] = pH, meaning that an 2− increased pH gives more [CO3 ]. Thus, a higher pH is favorable for Ca precipitation. There is a lowering of pH due to the dissociation of the carbonic acid into hydrogen ions and hydrogen carbonate. However, the increase in hydrogen carbonate concentration also increases the carbonate content, which contradicts the negative effect of a lower pH and creates a larger precipitation of CaCO3. CO2 hydration and complexes are possible with calcium ions and crystal formation. The carbonation process by the closed pressure reactor method with different amounts of CO2 gas was injected into the reaction bottle for different reaction times, such as 7 g CO2 per liter for 3 min Water 2018, 10, 54 7 of 10

reaction,Water 2018 14, 10 g, 54 CO 2 per liter for 4 min reaction and 21 g CO2 per liter for 5 min reaction times,7 of carbon 10 dioxide was injected into the reaction bottle as shown in Figure4. InIn this this study, study, three three differentdifferent water samples samples were were used, used, including including (i) (i) artificial artificial hard hard water water samples, samples, (ii)(ii) drinking drinking water water samples, samples, andand (iii)(iii) natural water water samples, samples, to to reduce reduce hardness hardness by by the the carbonation carbonation process.process. The The results results clearly clearly indicated indicated that that thethe hardnesshardness ofof thethe differentdifferent waterwater samplessamples was reduced by by approximately 30–40% using 7 g CO2/L for 3 min carbonation, reduced by 50–60% using 14 g approximately 30–40% using 7 g CO2/L for 3 min carbonation, reduced by 50–60% using 14 g CO2/L at CO2/L at 4 min carbonation, and reduced by 70–85% using 21 g CO2/L at 5 min carbonation at room 4 min carbonation, and reduced by 70–85% using 21 g CO2/L at 5 min carbonation at room temperature. temperature.However, the water hardness gradually decreased in different artificial (A), drinking (D) and However, the water hardness gradually decreased in different artificial (A), drinking (D) and natural (N) water samples with increasing CO2 gas flow rates from 7 g/L to 21 g/L at room temperature, natural (N) water samples with increasing CO2 gas flow rates from 7 g/L to 21 g/L at room as shown in Figure6. temperature, as shown in Figure 6. When the CO2 pressure in the reactor increased, the and carbonate ions increased When the CO2 pressure in the reactor increased, the bicarbonate and carbonate ions increased due to the dissociation of carbonic acid, and these carbonate ions reacted with Ca2+ and Mg2+ ions due to the dissociation of carbonic acid, and these carbonate ions reacted with Ca2+ and Mg2+ ions to toincrease increase the the hardness hardness removal removal efficiency. efficiency. Bicarbonate Bicarbonate and carbonate and carbonate ions was ions increased was increased due to duethe to 2+ 2+ thedissociation dissociation of carbonic of carbonic acid, acid, and these and thesecarbonate carbonate ions reacted ions reacted with Ca with2+ and Ca Mg2+ andions Mgand to ionsincrease and to increasethe hardness the hardness removal removal efficiency efficiency at a lower at pH a lower of 4–6 pH for of5 min 4–6 forreaction 5 min time. reaction time.

Figure 6. Reducing water hardness by carbonation in different water (a) artificial water; (b) drinking Figure 6. Reducing water hardness by carbonation in different water (a) artificial water; (b) drinking water; and (c) natural water samples. water; and (c) natural water samples. 3.2. Effect of Temperature 3.2. Effect of Temperature The five different artificial hard water samples were tested for reducing hardness with different temperatures:The five different 25, 40 and artificial 60 °C. Th harde results water clearly samples indicated were tested that the for hardness reducing removal hardness efficiency with different was ◦ temperatures:enhanced up to 25, 60–70% 40 and by 60 theC. carbonation The results process clearly with indicated 21 g of that CO2 the/L for hardness 5 min reaction removal at efficiency 40 °C wastemperature enhanced and up 80–85% to 60–70% hardness by the was carbonation reduced by process carbonation with process 21 g of with CO 221/L g forof CO 5 min2/L for reaction 5 min at ◦ 40reactionC temperature at 60 °C andtemperature. 80–85% hardness These results was reducedclearly indicated by carbonation that the process water withhardness 21 g ofgradually CO2/L for 5 mindecreased reaction with at increasing 60 ◦C temperature. temperature These due resultsto the reduction clearly indicated of Ca2+ and that Mg the2+ waterions in hardness the carbonation gradually decreasedprocess, as with shown increasing in Figure temperature 7. due to the reduction of Ca2+ and Mg2+ ions in the carbonation process, as shown in Figure7.

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Figure 7. Water hardness reduced by carbonation at different temperatures in artificial hard water Figure 7. Water hardness reduced by carbonation at different temperatures in artificial hard samples. water samples. Figure 7. Water hardness reduced by carbonation at different temperatures in artificial hard water The hardness removal studies were conducted at different temperatures of 25, 40, and 60 °C. samples. ◦ TemperatureThe hardness plays removal a major studiesrole in the were removal conducted of me attal differentions in the temperatures water. Figure of 7 25,reveals 40, andthat 60the C. TemperatureremovalThe of hardness plays a major removalincreases role studies as in temperature the were removal conducted increases, of metal at different this ions may in temperatures thepartly water. be due Figure of to 25, the 740, revealsincrease and 60 thatin°C. ion the removalmobilityTemperature of hardnessfor the plays CO increases 2a majorgas molecules. role as temperature in the Temperatureremoval increases, of me affectstal thisions maythein the interaction partly water. be Figure duebetween to7 reveals the the increase that CO the2 ingas ion mobilitymoleculesremoval for the andof hardness CO the2 gas metal increases molecules. ions, aswhich temperature Temperature influences increases, affects the stability the this interaction may of partly the betweenmetal-CObe due tothe 2the gas COincrease complex.2 gas in molecules ion The mobility for the CO2 gas molecules. Temperature affects the interaction between the CO2 gas andcarbonation the metal ions,of calcium which influencesions proceeds the stabilityat temper ofatures the metal-CO over 252 gas°C, complex.while the The carbonation carbonation of of calciummagnesiummolecules ions proceeds ionsand shouldthe atmetal temperatures be possibleions, which only over influences at 25 higher◦C, while thetemperatures. stability the carbonation of the metal-CO of magnesium2 gas complex. ions should The be carbonation of calcium ions proceeds at temperatures over 25 °C, while the carbonation of possible only at higher temperatures. 3.3.magnesium pH Effects ofions Carbonation should be possible only at higher temperatures. 3.3. pH Effects of Carbonation 3.3.The pH pH Effects of allof Carbonationsamples was measured before and after carbonation. The CO2 gas molecules react withThe water-producing pH of all samples carbonic was measured acid and the before pH andis lowered, after carbonation. see Figure 8, The where CO it gasalso moleculesis found that react The pH of all samples was measured before and after carbonation. The CO2 gas2 molecules react at higher CO2 flow rates the pH becomes lower. with water-producingwith water-producing carbonic carbonic acid acid and and the the pH pH is is lowered, lowered, seesee FigureFigure 88,, where it it also also is is found found that that at higherat COhigher2 flow CO2 rates flow therates pH the becomes pH becomes lower. lower.

FigureFigure 8. pH8. pH changes changes in in different different hard hard water water samples,samples, (a)) artificialartificial hard hard water, water, (b ()b drinking) drinking water water and and Figure 8. pH changes in different hard water samples, (a) artificial hard water, (b) drinking water and (c) (naturalc) natural water water samples samples by by carbonation carbonation at at differentdifferent CO22 flowflow ratesrates. . (c) natural water samples by carbonation at different CO2 flow rates.

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3.4.3.4. Water Water Hardness Hardness Removal Removal Effic EfficiencyEfficiencyiency ofof thethe CarbonationCarbonation ProcessProcess TheThe hardnesshardness removal removal efficiencyefficiencyefficiency waswas 70–80%70–80% forfor artificialartificialartificial waterwater samples,samples, 60–70%60–70% forfor drinkingdrinking waterwater samples samples and and 70–85% 70–85% forfor naturalnatural waterwater water samples,samples, samples, respectively.respectively. respectively. TheThe The resultsresults results areare are shownshown shown inin in FigureFigure Figure 9.9.9 .

FigureFigure 9. 9. WaterWater hardness hardness removal removal efficiency efficiencyefficiency ofof thethe carbonationcarbonation process.process.

3.5.3.5. Crystal Crystal Growth Growth Mechanism Mechanism

In the carbonation reaction, the CO2 gas was hydrated and formed negatively charged InIn thethe carbonation carbonation reaction, reaction, the COthe2 gasCO was2 gas hydrated was hydrated and formed and negatively formed chargednegatively bicarbonate charged bicarbonate ions. Under super-saturated conditions the Ca2+ and Mg2+ nucleated and formed calcium ions.bicarbonate Under ions. super-saturated Under super- conditionssaturated conditions the Ca2+ theand Ca Mg2+ and2+ nucleated Mg2+ nucleated and formed and formed calcium calcium and and magnesium carbonates [12,13]. Consequently, small calcite crystallites appeared on the surface, magnesiumand magnesium carbonates carbonates [12, 13[12,13].]. Consequently, Consequently, small small calcite calcite crystallites crystallites appeared appeared onon thethe surface, as shown in Figure 10, which presents the crystal growth mechanism of the carbonation process. as shown inin FigureFigure 1010,, whichwhich presentspresents thethe crystalcrystal growthgrowth mechanismmechanism ofof thethe carbonationcarbonation process.process.

Figure 10. Crystal growth mechanism of the carbonation process with closed pressure reactor. FigureFigure 10. 10. CrystalCrystal growth growth mechanism mechanism of of the the carbonation carbonation process process with with closed closed pressure pressure reactor.reactor. 4. Conclusions 4. Conclusions The purpose of this experimental study was to determine the hardness of different water The purposepurpose of of this this experimental experimental study study was was to determine to determine the hardness the hardness of different of different water samples water samples collected from different areas in South Korea, and to reduce the hardness by a simple collectedsamples collected from different from areasdifferent in South areas Korea,in South and Korea, to reduce and the to hardnessreduce the by hardness a simple carbonationby a simple carbonation process in a closed pressure reactor. processcarbonation in a closedprocess pressure in a closed reactor. pressure reactor.

(i)(i) TheThe carbonationcarbonation processprocess effectivelyeffectively reducedreduced thethe hardnesshardness inin allall artificialartificial waterwater samplessamples byby (i) approximatelyThe carbonation (70–80%). process At effectively room temperature, reduced the a 21 hardness g/L CO2 flow in all rate artificial for 5 min water was samples enough byto approximately (70–80%). At room temperature, a 21 g/L CO2 flow rate for 5 min was enough to approximately (70–80%).2+ At room2+ temperature, a 21 g/L CO flow rate for 5 min was enough to remove most of the Ca 2+ and Mg 2+ ions from the water. The 2hardness reduction increased with remove most of the Ca2+ and Mg 2+ ions from the water. The hardness reduction increased with remove most of the Ca and Mg ions from the water. The hardness reduction2+ increased2+ with increasingincreasing temperature,temperature, becausebecause carboncarbon dioxidedioxide effectivelyeffectively reactsreacts withwith CaCa2+ andand MgMg2+ ionsions withwith increasing temperature, because carbon dioxide effectively reacts with Ca2+ and Mg2+ ions with increasingincreasing temperature.temperature. TheThe resultsresults indicatedindicated thatthat 85%85% ofof thethe hardnesshardness waswas possiblepossible toto removeremove byby aa simplesimple carbonationcarbonation method.method.

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increasing temperature. The results indicated that 85% of the hardness was possible to remove by a simple carbonation method. (ii) In drinking water samples the hardness was also reduced effectively using the carbonation process, with 21 g/L CO2 at room temperature for 5 min reaction reducing the water sample hardness by approximately 60 to 70%.

(iii) For the natural water samples collected from different areas of South Korea, 21 g/L of CO2 carbonation for 5 min at room temperature reduced the hardness by 70 to 85%. The results revealed that the carbonation process in a closed pressure reactor effectively control the water hardness of different kinds of water by relatively simple means.

Acknowledgments: This research was supported by a research grant of the “Research and Education Program”, Chemical Engineering Department, Kwangwoon University, Seoul, Korea. Author Contributions: Min Kyung Ahn collected the water samples from various places and performed the experiments. Choon Han, Ramakrishna Chilakala and Thriveni Thenepalli conceived and designed the experiments; analyzed the data and wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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