water 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, * 1 2
3
*
ID
Ewha Girls High School, 26 Jeongdong-gil, Jung-gu, Seoul 04516, Korea;
[email protected] Carbon Mineralization Center, Climate Change Mitigation and Sustainability Division, Korea Institute of Geosciences and Mineral Resources (KIGAM), 124, Gwahak-ro, Yuseong-gu, Daejeon 34132, Korea;
[email protected] 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. Hard water can cause many problems around the world, including increased scaling on water pipes, boilers, atopic eczema and odd-tasting drinking water. Hardness in natural water is caused by dissolved minerals, mainly calcium and magnesium compounds. According to the Water Quality Association (WQA) and the United States Geological Survey (USGS), hard water is classified based on the Ca2+ and Mg2+ ion concentration in waters, 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+ ions 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 concentrations 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 lime 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 lead to other issues as well. It also affects soap and detergent 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
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Several studies have indicated a link between hardness concentrations (particularly calcium and Several have indicated a link between hardness concentrations (particularly and magnesium) andstudies cardiovascular diseases, Alzheimer’s disease and atopic eczema calcium [3,4]. However, magnesium) and cardiovascular diseases, Alzheimer’s disease and atopic eczema [3,4]. However, the link is tenuous, and many confounding variables exist in the studies [5]. Hard water isthe 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, iron 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, forwaters, most waters, total hardness is caused by major and associated anions for most the totalthe hardness is caused by major cations and cations associated anions as shown in as shown in Figure 1. Figure 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− alkalinity ions and
− , CO 2− − number The cations are usually associated of HCO3hardness andto OH alkalinity acidity-associated ions such as SO42−, with NO3−, the andanions Cl−. Carbonate the of ions 3 refers 2 − − − hardness ions associated with alkalinity ions. When the solution is super-saturated, the cations in the and acidity-associated ions such as SO4 , NO3 , and Cl . Carbonate hardness refers to the number water can precipitate as with a hard scale. Carbonate hardness is sensitive to temperature and of hardness ions associated alkalinity ions. When the solution is super-saturated, thecan cations precipitate easily. At high temperatures, Mg salts can also produce hard scales in boilers. in the water can precipitate as a hard scale. Carbonate hardness is sensitive to 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 temperatures, 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 calcite process inthus a closed-pressure In this the crystallization process of precipitated PCC obtained couldreactor. be utilized as apaper, filler material to enhance the mechanical propertiescalcium of carbonate (PCC) and magnesium carbonate from hard water samples were investigated. The calcite composites in the plastics and paper industries. PCC thus obtained could be utilized as a filler material to enhance the mechanical properties of 2. Materials Methods composites in theand plastics and paper industries.
The existence of Ca2+ and Mg2+ ions in water causes hardness. A water supply with a hardness level of 100 parts per million (100 ppm) contains the equivalent of 100 g of CaCO3 in one liter of water. The total hardness is 2+ equal the2+ sum of in magnesium hardness and calcium hardness. The existence of Ca andtoMg ions water causes hardness. A water supply with a hardness In the present experiment, total hardness was measured by the EDTA method hard water level of 100 parts per million (100 ppm) contains the equivalent of 100 g of CaCO3 infor one liter of water. samples collected from different areas of South Korea and artificial hard water samples that were The total hardness is equal to the sum of magnesium hardness and calcium hardness. 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 samples collected from different areas of South Korea and artificial hard water samples that were samples were tested, as shown in Table 1.
2. Materials and Methods
prepared in the lab. The hardness was then reduced by the carbonation process. In all, five different artificial hard water samples, different drinking samples, and eight different natural water Tablefive 1. Different water samples water names with given numbers. samples were tested, as shown in Table 1. Artificial Hard Water Drinking Water Natural Water Sample Name
Number
Sample Name
Number
Sample Name
CHOICE 1. Different water samples Artificial Soft water TableA-(1) D-(1) names with given Softenernumbers. pass water water
Artificial Slightly Hard Artificial Hard Water Natural Water A-(2) ViO water Drinking D-(2) Water Seoul Arisu water water Sample Name Number Sample Name Number Sample Name Artificial Moderately Sam Da Soo A-(3) D-(3) Seoul Park Tap water Artificial Soft water A-(1) water CHOICE water D-(1) Softener pass water Hard water Artificial VeryHard Hardwater Artificial Slightly ViO water D-(2) Seoul Arisu water A-(4) A-(2) O-EAU water D-(4) Seoul Outdoor Swimming pool (1) water water Sam Da Soo Artificial Moderately Hard water A-(3) D-(3) Seoul Parkpool Tap water Seoul Outdoor Swimming (2) water water Seoul Outdoor Swimming pool Toilet Seoul Outdoor water Swimming pool Artificial Very Hard water A-(4) O-EAU water D-(4) (1) water (Korea Institute of Geoscience and Seoul Outdoor Swimming pool Mineral Resources(KIGAM)-Waste water (2) water
Number N-(1) N-(2) Number N-(3) N-(1) N-(2) N-(4) N-(3) N-(5)
N-(6) N-(4) N-(7) N-(5)
Seoul Outdoor Swimming pool Toilet water
N-(6)
(Korea Institute of Geoscience and Mineral Resources(KIGAM)-Waste water
N-(7)
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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 Synthetic artificially water was prepared withat major ions mixed intointhe proper amounts hardness and on the user hard requirements. CO 32− is present low concentrations most natural water 2+ , Ca2+ , Na+ , K+ and Cl− , SO 2− depending on their of fresh water. Most compositions include Mg 4 and hard water and samples at a pH of seven. The alkalinity was measured frequently for the soft hardness and on user requirements. CO3 2− isofpresent at low concentrations in mostartificial natural water was assumed to the be equal to the concentration the HCO 3− ions [7]. The different a pH of seven. The alkalinity was moderately measured frequently the softwater and hard and samples at were denoted as soft, slightly hard, hard and for very hard withwater respect towas the − ions [7]. The different artificial water samples assumed to be equal to the concentration of the HCO chemical compositions [8], as shown in Table 2. 3 were denoted as soft, slightly hard, moderately hard and very hard water with respect to the chemical compositions [8], as shown in Table composition 2. Table 2. Chemical for the preparation of hard water. Water Samples
NaHCO3
Soft Water Samples Slightly hard ModeratelySoft hard Slightly hard Hard Moderately hard Very hard Hard Very hard
Reagent Added (mg/L) CaSO4·2H2O MgSO4
Approximate Final Water Quality pH Hardness (mg/L)
Table 2. Chemical composition for the preparation of hard water.
12.0 48.0 3 NaHCO 96.0 12.0 48.0 192.0 96.0 384.0 192.0 384.0
7.5 30.0 CaSO4 ·2H2 O 7.560.0 30.0 120.0 60.0 240.0 120.0
7.5 30.0 MgSO4 7.5 60.0 30.0 120.0 60.0 240.0 120.0
Reagent Added (mg/L)
240.0
240.0
KCl
0.5 Approximate 6.4–6.8 Final Water Quality 10–13 2.0 7.2–7.6 40–48 KCl pH Hardness (mg/L) 7.4–7.8 0.54.0 6.4–6.8 10–1380–100 2.08.0 7.2–7.6 40–48 7.6–8.0 160–180 4.0 7.4–7.8 80–100 16.0 8.0–8.4 280–320 8.0 7.6–8.0 160–180 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 Water hardness be readily by titration with the chelating EDTA (ethylene diamine tetra acetic can acid). In the determined sample solution containing calcium and agent magnesium ions, the diamine tetra acetic acid). In the sample solution containing calcium and magnesium ions, the reaction reaction takes place with an excess of EDTA by a titration process the indicator added remains 2+ 2+ takes place withblack an excess of EDTA by a titration theMg indicator as eriochrome as eriochrome T (EBT)-blue because all ofprocess the Caand and ions added presentremains are complexed with 2+ ions present are black T (EBT)-blue because all of thethe Ca2+ and Mg complexed with [9]. the EDTA [9]. During this process, EDTA reacts with the Ca2+ and Mg2+ ions andthe to EDTA form the 2+ and Mg2+ ions and to form the EDTA-Ca and During this process, the EDTA reacts with the Ca EDTA-Ca and EDTA-Mg complex (Figure 2), and after the completion of this reaction, the wine red EDTA-Mg complex (Figure 2), andthe after the completion of this theand wine redCa-EDTA color disappears color disappears slowly because EBT-Ca and EBT-Mg arereaction, replaced form stable, slowly because the EBT-Ca and EBT-Mg are replaced and form Ca-EDTA stable, colourless complex, colourless complex, while simultaneously a blue color appears due to the effect of the EBT indicator while [10]. simultaneously a blue color appears due to the effect of the EBT indicator [10].
2+ and Mg 2+ ions. 2+ and Figure 2. EDTA complex thereaction reactionwith with Figure 2. EDTA complexformation formation by by the CaCa Mg2+ ions.
2.3. Reactor for for aa Carbonation Carbonation Process Process 2.3. The The KIGAM KIGAM Closed Closed Pressure Pressure Reactor The Korea Institute Institute of of Geosciences Geosciences and and Mineral Mineral resources resources (KIGAM), (KIGAM), has has introduced introduced aa closed closed The Korea pressure reactor for the removal of hardness. This can be accomplished in conjunction with the pressure reactor for the removal of hardness. This can be accomplished in conjunction with the coagulation a coagulation of of suspended suspended solids, solids, as as shown shown in in Figure Figure 3. 3. Here, Here, aa 11 L L closed closed pressure pressure reactor, reactor, with with a maximum working CO 2 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 60 L of carbonated water, as in shown 4. This closedreactor, pressure reactor, upon a of carbonated water, as shown Figurein4. Figure This closed pressure based uponbased a carbonation carbonation process, is used control the concentrations of magnesium calcium andions magnesium ions inInhard process, is used to control thetoconcentrations of calcium and in hard water. this water. In this carbonation process, 1 L of each water sample was brought into the reactor carbonation process, 1 L of each water sample was brought into the reactor chamber and chamber CO2 gas and CO2 gas was injected using a CO2 injector machine. The closed pressure reactor has two
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Water 2018, 10, 54 4 of 10for was injected using a CO2 injector machine. The closed pressure reactor has two alternative outlets Water 2018, 10, 54 4 of 10 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 bottom outletoutlets is for the pilot scalethe reactor. alternative for collecting 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. During the water carbonation by the closed pressure reactor, three different CO2 flow reactor (1–2 L)hard and the bottom outlet isprocess for the pilot scale reactor. During the hard water carbonation process by the closed pressure reactor, three different CO2 During the hard water carbonation closedinvolved pressure7reactor, CO2 rates were injected, giving different reactionprocess times. by Thethe process g/L COthree 3 min reaction, 2 for adifferent flow rates were injected, giving different reaction times. The process involved 7 g/L CO2 for a 3 min rates giving different reaction Thea process involvedThe 7 g/L CO2 for a 3reaction min 14 flow g/L of COwere gas injected, for a 4 min reaction and 21 g/L oftimes. CO2 for 5 min reaction. carbonation reaction, 142 g/L of CO2 gas for a 4 min reaction and 21 g/L of CO2 for a 5 min reaction. The carbonation 2+The 2+ ions to reaction, 14 g/L of CO2 gasoffor a 4 min reaction and 21 g/L ofwhich CO2 for a 5 minwith reaction. carbonation began with the hydration carbon dioxide in the water, reacted Ca and Mg 2+ and Mg2+ reaction began with the hydration of carbon dioxide in the water, which reacted with Ca2+ 2+ reaction began with the hydration of carbon dioxide in the water, which reacted with Ca dioxide and Mgflow form calcium carbonate and magnesium carbonate precipitates. The effects of the carbon ions to form calcium carbonate and magnesium carbonate precipitates. The effects of the carbon ions form calciumatcarbonate and and magnesium carbonate precipitates. The effects of the carbon rate, thetotemperature carbonation, the pH on thethe precipitation formation were investigated. dioxide flow rate, the temperature at carbonation, and pH on the precipitation formation were dioxide flow rate, the temperature at carbonation, and the pH on the precipitation formation were The pH values of all pH the samples measured and after the carbonation investigated. The values ofwere all the samplesbefore were carbonation measured before carbonation and afterprocess the investigated. The pH values of all theScientific, samples were measured USA). before carbonation and after the with a pH meter (A 214, Orion, Waltham, carbonation process with a pHThermo meter (A 214, Orion, Thermo MA, Scientific, Waltham, MA, USA). carbonation process with a pH meter (A 214, Orion, Thermo Scientific, Waltham, MA, USA).
Figure 3. Carbonation process process with the closed pressure reactor. Figure with the theclosed closedpressure pressurereactor. reactor. Figure3.3.Carbonation Carbonation process with
Figure 4. KIGAM bench scale carbonation reactor. Figure4.4.KIGAM KIGAM bench bench scale Figure scale carbonation carbonationreactor. 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. Re-crystallization on the surface of the particles to createwith a very thin single-crystal The crystallites formedtakes join place together to form polycrystalline particles a rhombohedral shape. shape. Re-crystallization takes place on the surface of the particles to create a very thin single-crystal shell, which thentakes extends inwards, leading of tothe true single crystals. Figure 5a shows the scanning Re-crystallization place on the surface particles to create a very thin single-crystal 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 which thenmicroscopy extends inwards, to true single crystals. showsofthe electron electron (SEM) leading (JEOL Company, Seoul, South Figure Korea) 5a results thescanning rhombohedral calcium carbonate crystals and Figure 5b shows the results X-ray diffraction (XRD) (Korea Institute of microscopy (SEM) (JEOL Company, Seoul, South Korea) of the rhombohedral calcium carbonate calcium carbonate crystals and Figure 5b shows the X-ray diffraction (XRD) (Korea Institute of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea) analysis, which indicated that crystals and Figure 5b shows the X-ray diffraction (XRD)South (Korea Institute of Geosciences and Mineral Geosciences and Mineral Resources (KIGAM), Daejeon, Korea) analysis, which indicated that pure calcite was formed. The magnesium carbonate peak did not appear in the XRD results due to a Resources (KIGAM), Daejeon, South Korea) analysis, which pure calcite was formed. pure calcite was formed. The magnesium carbonate peak did indicated not appearthat in the XRD results due to a low quantity of magnesium, but the energy dispersive x-ray spectroscopy (EDS) (Korea Institute of The magnesium carbonate peakbut didthe notenergy appeardispersive in the XRD results due to a low quantity magnesium, low quantity of magnesium, x-ray spectroscopy (EDS) (KoreaofInstitute of Geosciences and Mineral Resources (KIGAM), Daejeon, South Korea) results show a negligible butGeosciences the energyand dispersive spectroscopy (Korea Institute Geosciences Mineral Mineral x-ray Resources (KIGAM),(EDS) Daejeon, South Korea)ofresults show a and negligible quantity of magnesium. Resources Daejeon, South Korea) results show a negligible quantity of magnesium. quantity(KIGAM), of magnesium.
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(a)
(b)
Figure5.5.(a)(a) SEM EDS results of calcium and magnesium carbonate crystals formed by Figure SEM and and EDS results of calcium and magnesium carbonate crystals formed by carbonation; carbonation; (b) XRD results of calcite. (b) XRD results of calcite.
Resultsand andDiscussion Discussion 3.3.Results Thewater watersample samplehardness hardnesslevels levelswere weremeasured measuredby bythe theEDTA EDTAmethod methodand andthe thehardness hardnesswas was The reduced by bythe thecarbonation carbonationprocess. process. In In addition, addition, the the pH pH levels levels before before and andafter aftercarbonation carbonation were were reduced measured in all of the water samples. During the carbonation process the CO 2 injection, temperature measured in all of the water samples. During the carbonation process the CO2 injection, temperature 2+ and pH pH effect role to precipitate the the Ca2+Ca and and to reduce hardness 2+ Mg and effect played playedaakey key role to precipitate and carbonates Mg2+ carbonates and tothe reduce the in different water samples. Table 3 Table shows3 shows the hardness of different waterwater samples before and after hardness in different water samples. the hardness of different samples before and carbonation. after carbonation. Table3.3.Hardness Hardnessof ofdifferent differentwater watersamples samplesbefore beforeand andafter aftercarbonation. carbonation. Table Sample Name
Water Hardness-before
Water Hardness-before Sample Name Carbonation (ppm) Carbonation (ppm)
A-(1) A-(2)A-(1) A-(3)A-(2) A-(3) A-(4) A-(4) A-(5)A-(5)
38 84 110 172 344
38 84 110 172 344
D-(1) D-(1) D-(2)D-(2) D-(3)D-(3) D-(4)D-(4)
96 60 22 52
96 60 22 52
N-(1)N-(1) N-(2)N-(2) N-(3)N-(3) N-(4) N-(4) N-(5) N-(5)N-(6) N-(6)N-(7) N-(7)N-(8) N-(8)
64 56 64 70 132 140 76 66
64 56 64 70 132 140 76 66
Water Hardness after Carbonation (mg/L) Water Hardness after Carbonation (mg/L) 7 g/L CO2 for 3 min 14 g/L CO2 for 4 min 21 g/L CO2 for 5 min 7 g/L CO for 3 minwater14 g/L CO2 for 4 min 21 g/L CO2 for 5 min 2 I. Artificial hard I. Artificial 24 hard water 17 12 24 17 39 12 52 22 52 39 50 22 65 24 65 50 24 108 79 43 108 79 43 210 90 210 158 158 90 II. Drinking water samples II. Drinking water samples 60 45 32 60 45 32 38 17 38 25 25 17 14 7 14 10 10 7 33 24 24 18 33 18 III. watersamples samples III. Natural Natural water 40 19 40 30 30 19 35 27 27 15 35 15 40 29 29 18 40 18 45 32 14 45 32 14 79 52 20 79 20 88 56 52 21 88 21 50 35 56 16 40 29 35 18 50 16 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. Theamounts carbonation process by the closed pressure reactor method was in an aqueous medium. Different of CO 2 gas were injected into the reaction bottle for different reaction times, such as Different amounts of CO 2 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 7 greaction CO2 pertimes. liter for 3 mindioxide reaction,was 14 g CO2 per liter 4 min reaction 21 ginCO 2 per liter for 5asmin Carbon injected into thefor reaction bottle as and shown Figure 4. 5 min reaction times. Carbon dioxide was injected into the reaction bottle as shown in Figure 4. as The basic two steps involved in the carbonation reaction are (i) hydration of carbon dioxide The basic two steps involved in the carbonation reaction are (i) hydration of carbon dioxide as shown in Formulas (1)–(3); (ii) the carbonate ions react with calcium ions to form calcium carbonate shown in Formulas precipitates (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)
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CO2(g) + H2 O(l) ↔ H2 CO3(aq) (Dissociation of aqueous CO2 into water)
(1)
H2 CO3(aq) ↔ H+ + HCO3 −
(2)
HCO3 − → H+ + CO3 2−
(3)
(Produces supersaturation when) (Ca2+ )(CO3 2− ) >1 S1 = Ksp
(4)
(Formation of nuclei or critical cluster) Ca2+ + CO3 2− → CaCO3 (nuclei) (Crystal growth occurs spontaneously)
(5)
Ca2+ + CO3 2− → CaCO3 (Calcite) (Finally, calcite formed)
(6)
Carbon dioxide species are important participants in reactions that control the pH of natural waters. Reactions among the alkalinity-related species, aqueous CO2 , H2 CO3 (aq), HCO3 − , and CO3 2− , and directly pH related species, OH− and H+ , are relatively fast and can be evaluated with chemical equilibrium 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 carbonate precipitates easier at increased water temperature. The equilibrium [Ca2+ ] + [CO3 2− ] = [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. The equilibrium can be computed by [Ca2+ ] × [CO3 2− ] = Ks , where Ks is the temperature-dependent solubility product for CaCO3 . As the aim is to remove the hardness, the actual Ca content should be decreased. It means that the content of CO3 2- must increase in order to overcome the solubility product and get a precipitation of CaCO3 . From the equilibrium, HCO3 − = H+ + CO3 2− and the connected equation [H+ ] × [CO3 2− ] = K2 × [HCO3− ], where K2 is a temperature-dependent constant, it follows that to increase the content of [CO3 2− ] it is necessary to increase the content of hydrogen carbonate and/or decrease the content of [H+ ]. As – log [H+ ] = pH, meaning that an increased pH gives more [CO3 2− ]. 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
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reaction, 1410,g 54 CO2 per liter for 4 min reaction and 21 g CO2 per liter for 5 min reaction times,7 of carbon Water 2018, 10 dioxide was injected into the reaction bottle as shown in Figure 4. thisstudy, study,three threedifferent different water samples (i)(i) artificial hard water samples, InInthis sampleswere wereused, used,including including artificial hard water samples, drinkingwater watersamples, samples, and and (iii) (iii) natural byby thethe carbonation (ii)(ii)drinking natural water watersamples, samples,totoreduce reducehardness hardness carbonation process.The Theresults resultsclearly clearlyindicated indicatedthat thatthe thehardness hardnessof ofthe the different different water water samples samples was was reduced process. reduced by by approximately 30–40% using 7 g CO 2/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 70–85% using 21 g CO2/L at 5 min carbonation at room 4 min carbonation, and reduced byreduced 70–85% by using 21 g CO 2 /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 Figure 6. temperature, as shown in Figure 6. When the CO2 pressure in the reactor increased, the bicarbonate 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 with2+Ca2+ and2+Mg2+ ions due to the dissociation of carbonic acid, and these carbonate ions reacted with Ca and Mg ions to toincrease increasethe thehardness hardness removal efficiency. Bicarbonate and carbonate ionsincreased was increased removal efficiency. Bicarbonate and carbonate ions was due to due the to 2+ 2+ the dissociation of carbonic and these carbonate ions reacted and to 2+ andCa dissociation of carbonic acid,acid, and these carbonate ions reacted with Cawith Mg2+ and ions Mg and to ions increase increase the hardness removal efficiency at a lower pH of 4–6 for 5 min reaction time. the hardness removal efficiency at a lower pH of 4–6 for 5 min 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 The five different artificial hardresults waterclearly samples were tested forhardness reducingremoval hardness with different temperatures: 25, 40 and 60 °C. The indicated that the efficiency was ◦ C. The results clearly indicated that the hardness removal efficiency temperatures: 25, 40 and 60 enhanced up to 60–70% by the carbonation process with 21 g of CO2/L for 5 min reaction at 40 °C was enhancedand up 80–85% to 60–70% by the carbonation withprocess 21 g ofwith CO221 /Lg for 5 min reaction temperature hardness was reduced byprocess carbonation of CO 2/L for 5 min at ◦ C temperature and 80–85% hardness was reduced by carbonation process with 21 g of CO /L for 40reaction at 60 °C temperature. These results clearly indicated that the water hardness gradually 2 5 min reaction at increasing 60 ◦ C temperature. These clearly indicated thatMg the2+ water gradually decreased with temperature dueresults to the reduction of Ca2+ and ions inhardness the carbonation process, as shown in Figure 7. decreased with increasing temperature due to the reduction of Ca2+ and Mg2+ ions in the carbonation process, as shown in Figure 7.
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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. ◦ The hardness removal were conducted at different temperatures 40, and Temperature plays a majorstudies role in the removal of metal ions in the water. Figureof7 25, reveals that 60 the C. Temperature plays a major role intemperature thewere removal of metal ionsmay intemperatures the water. Figure 7 increase reveals that hardness removal studies conducted at different ofto25, and 60 in °C.ionthe removalThe of hardness increases as increases, this partly be due the40, Temperature role in the Temperature removal of metal ions may in the water.be Figure 7 reveals that removal of hardness increases as temperature increases, this partly due to the the increase mobility for theplays CO 2a major gas molecules. affects the interaction between COthe 2 in gasion removal of hardness increases as temperature increases, this may partly be due to the increase in ion mobility for the CO gas molecules. Temperature affects the interaction between the CO gas molecules molecules and the metal ions, which influences the stability of the metal-CO 2 gas complex. The 2 2 mobility CO2 gas affects the interaction between thecarbonation CO2 gas of of carbonation of the calcium ionsmolecules. proceeds at temperatures over 252 gas °C, complex. while theThe carbonation and the metalfor ions, which influences the Temperature stability of the metal-CO molecules and the metal ions, which influences the stability of the metal-CO 2 gas complex. The ◦ magnesium ions should be possible only higher temperatures. calcium ions proceeds at temperatures overat25 C, while the carbonation of magnesium ions should be carbonation of calcium ions proceeds at temperatures over 25 °C, while the carbonation of possible only at higher temperatures. ions should be possible only at higher temperatures. 3.3.magnesium pH Effects of Carbonation 3.3. pH The Effects pHofofCarbonation all samples was measured before and after carbonation. The CO2 gas molecules react 3.3. pH Effects of Carbonation
with carbonic acid and the pH is lowered, see Figure 8,The where it alsomolecules is found that Thewater-producing pH was measured aftercarbonation. carbonation. 2 gas The of pHall ofsamples all samples was measuredbefore before and and after The COCO 2 gas molecules react react at higher CO2 flow rates the pH becomes lower. with water-producing carbonic acid and the pH is lowered, see Figure 8, where it also is found that at with water-producing carbonic acid and the pH is lowered, see Figure 8, where it also is found that
higher lower. at CO higher CO2rates flow the ratespH thebecomes pH becomes lower. 2 flow
Figure 8. pH changes inindifferent (a) artificial artificialhard hardwater, water,(b) (b)drinking drinking water and Figure 8. pH changes differenthard hardwater water samples, samples, (a) water and Figure 8. pH changes in different hard water samples, (a) artificial hard water, (b) drinking water and (c) (c) natural water samples CO22 flow flowrates rates. . natural water samplesby bycarbonation carbonationat at different different CO (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 Efficiency Efficiency of of the the Carbonation Carbonation Process Process The The hardness hardness removal removal efficiency efficiency was was 70–80% 70–80% for for artificial artificial water water samples, samples, 60–70% 60–70% for for drinking drinking water 70–85% for natural water samples, respectively. The results are shown Figure9. samples and 70–85% for natural water samples, respectively. The results are shown water samples and 70–85% for natural water samples, respectively. The results are shown in ininFigure Figure 9.9.
Figure of the carbonation process. Figure 9. 9. Water Water hardness hardness removal removal efficiency removal efficiency efficiency of of the the carbonation carbonation process. process.
3.5. Growth Mechanism 3.5. Crystal Mechanism 3.5. Crystal Growth Mechanism In the carbonation reaction, the CO gas was and formed charged In the thecarbonation carbonation reaction, the2 gas CO2was 2 gas was hydrated hydrated andnegatively formed negatively negatively charged In reaction, the CO hydrated and formed charged bicarbonate 2+ and Mg2+ nucleated and formed calcium bicarbonate ions. Under super-saturated conditions the Ca 2+ 2+ 2+ 2+ bicarbonate Under super-saturated Mg nucleated and formed calcium ions. Underions. super-saturated conditionsconditions the Ca the andCa Mgandnucleated and formed calcium and and magnesium carbonates [12,13]. Consequently, small calcite crystallites appeared on the surface, and magnesium carbonates [12,13]. Consequently, small calcite crystallites appeared on the surface, magnesium carbonates [12,13]. Consequently, small calcite crystallites appeared on the as shown in Figure 10, which presents the crystal growth mechanism of the carbonation process. shown in in Figure Figure 10, 10, which which presents presents the the crystal crystalgrowth growthmechanism mechanismof ofthe thecarbonation carbonationprocess. process. as shown
Figure 10. Crystal growth mechanism of the carbonation process with closed pressure reactor Figure reactor.. Figure10. 10.Crystal Crystalgrowth growthmechanism mechanismofofthe thecarbonation carbonationprocess processwith withclosed closed pressure pressure reactor.
4. 4. Conclusions Conclusions 4. Conclusions The purpose this experimental study was to the of water The purpose purposeofof ofthis this experimental study to determine determine the hardness hardness of different different water The experimental study was was to determine the hardness of different water samples samples collected from different areas in South Korea, and to reduce the hardness by a simple samples collected from different areas in South Korea, and to reduce the hardness by a simple collected from different areas in South Korea, and to reduce the hardness by a simple carbonation carbonation process in pressure carbonation process in aa closed closed pressure reactor. reactor. process in a closed pressure reactor. (i) (i) The The carbonation carbonation process process effectively effectively reduced reduced the the hardness hardness in in all all artificial artificial water water samples samples by by (i) approximately The carbonation process effectively reduced the hardness in all artificial water samples by (70–80%). At room temperature, a 21 g/L CO 2 flow rate for 5 min was enough to approximately (70–80%). At room temperature, a 21 g/L CO2 flow rate for 5 min was enough to approximatelyof(70–80%). At room a 21 g/LThe CO2 flow ratereduction for 5 min was enough to 2+ and 2+ temperature, remove 2+ and Mg remove most most of the the Ca Ca2+ Mg2+2+ions ions from from the the water. water. The hardness hardness reduction increased increased with with remove most of the Ca and Mg ions from the water. The hardness reduction increased with 2+ 2+ ions with increasing 2+ ions with increasing temperature, temperature, because because carbon carbon dioxide dioxide effectively effectively reacts reacts with with Ca Ca2+2+ and and Mg Mg2+ increasing temperature, because carbon dioxide effectively reacts with Ca and Mg ions with increasing temperature. The results indicated that 85% of the hardness was possible increasing temperature. The results indicated that 85% of the hardness was possible to to remove remove by by aa simple simple carbonation carbonation 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.
References 1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11.
12.
13.
Water Review, Consumer Report. A Publication of the Water Quality; The South Korean Research Council: Chungcheongnam-do, Korea, 1990; Volume 5. Padmapriya, R.; Saranya, T.; Thirunalasundari, T. Phyllanthus emblica-A Biopotential for hard water treatment. Int. J. Pure Appl. Biosci. 2015, 3, 291–295. Neri, L.C.; Johansen, H.L. Water hardness and cardiovascular mortality. N. Y. Acad. Sci. 1978, 304, 203–221. [CrossRef] Sengupta, P. Potential health impacts of Hard Water. Int. J. Prev. Med. 2014, 4, 866–875. Comstock, G.W. Water hardness and cardiovascular disease. Am. J. Epidemiol. 1979, 110, 375–400. [CrossRef] [PubMed] Agostinho, L.C.L.; Nascimento, L.; Cavalcanti, B.F. Water hardness removal for industrial use: Application of the electrolysis process. Open Access Sci. Rep. 2012, 1, 460–465. Smith, E.J.; Davison, W.; Hamilton-Taylor, J. Methods for preparing synthetic freshwaters. Water Res. 2002, 36, 1286–1296. [CrossRef] Marking, L.L.; Dawson, V.K. Methods for assessment of toxicity or efficacy of mixtures of chemicals. In Investigations in Fish Control; Series Number 67; U.S. Fish and Wildlife Service (USFWS): La Crosse, WI, USA, 1975; pp. 1–8. Cash, D. Analysis of Calcium in a Supplement tablet; Analysis of Magnesium in Epsom Salt; Hardness of Water; Mohawk College of Applied Arts and Technology: Hamilton, ON, Canada, 2008. Kakisako, M.; Nishikawa, K.; Nakano, M.; Harada, K.S.; Tatsuoka, T.; Koga, N. Stepwise inquiry into hard water in a high school chemistry laboratory. J. Chem. Educ. 2016, 93, 1923–1928. [CrossRef] Hem, J.D. Study and Interpretation of the Chemical Characteristics of Natural Water. U.S Geological Survey Water Supply Paper 2254, 1-272; United States Government Printing Office, 1985. Available online: https://pubs.usgs.gov/wsp/wsp2254/pdf/wsp2254a.pdf (accessed on 15 August 2017). Kim, Y.Y.; Schenk, A.S.; Ihli, J.; Kulak, A.N.; Hetherington, N.B.J.; Tang, C.C.; Schmahl, W.W.; Griesshaber, E.; Hyett, G.; Meldrum, F.C. A critical analysis of calcium carbonate mesocrystals. Nat. Commun. 2014, 5, 1–14. [CrossRef] [PubMed] Mahvi, A.H.; Shafiee, F.; Naddafi, K. Feasibility study of crystallization process for water softening in pellet reactor. Int. J. Environ. Sci. Technol. 2005, 1, 301–304. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
