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The simultaneous removal of calcium and chloride ions from calcium chloride solution using magnesium–aluminum oxide
ARTICLE IN PRESS
Water Research 37 (2003) 4045–4050
Technical note
The simultaneous removal of calcium and chloride ions from calcium chloride solution using magnesium–aluminum oxide Tomohito Kamedaa,b,*, Toshiaki Yoshiokaa, Teruhisa Mitsuhashia, Miho Uchidaa, Akitsugu Okuwakia a
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba 07, Aoba-ku, Sendai 980-8579, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan Received 6 November 2002; received in revised form 24 April 2003; accepted 1 May 2003
Abstract We investigated the removal of Ca2+ and Cl from CaCl2 solution at 20–60 C, using magnesium–aluminum oxide, Mg0.80Al0.20O1.10, prepared by the thermal decomposition of a hydrotalcite-like compound, Mg0.80Al0.20(OH)2 (CO3)0.10 0.78H2O. The degree of Ca2+ and Cl removal from the solution increased with increasing initial CaCl2 concentration, temperature, and quantity of Mg0.80Al0.20O1.10 added. When Mg0.80Al0.20O1.10 was added to 0.25 M CaCl2 solution in a Mg0.80Al0.20O1.10/CaCl2 molar ratio of 20, the degree of Ca2+ and Cl removal from the solution at 60 C after 0.5 h was 93.0% and 98.2%, respectively. These results reveal that Mg0.80Al0.20O1.10 has the capacity to remove Ca2+ and Cl simultaneously from aqueous solution. r 2003 Elsevier Ltd. All rights reserved. Keywords: Hydrotalcite-like compound; Magnesium–aluminum oxide; CaCl2 solution; Removal; Simultaneous; Buffering action
1. Introduction Chlorofluorocarbons (CFCs) are chemically stable, and diffuse into the stratosphere where they convert ozone molecules into oxygen. Furthermore, CFCs contribute significantly to the greenhouse effect. The use of CFCs is being reduced as a result of the United Nations Environmental Protection Protocol for CFC regulation, which was adopted in Montreal, Canada, in 1987. Since remaining stocks of CFCs must be converted into harmless compounds, many researchers have developed processes to detoxify CFCs [1]. One process decomposes CFCs in the presence of H2O, according to
*Corresponding author. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81-22-217-5148; fax: +81-22-217-5148. E-mail address: [email protected] (T. Kameda).
the reaction: CCl3 F þ 2H2 O-CO2 þ HF þ 3HCl:
ð1Þ
Then, the resulting HCl and HF gases are neutralized with milk of lime, in the reaction: HF þ 3HClþ2CaðOHÞ2 -0:5CaF2 þ 1:5CaCl2 þ 4H2 O:
ð2Þ
However, a particularly frustrating aspect of this process is the CaCl2 wastewater that is produced. Unless this calcium- and chloride-rich wastewater is treated, the salt load may cause severe damage to aquatic environments [2]. The wastewater is typically diluted with large amounts of water, and then discharged into rivers [3]. This may be a low-cost method, but it can be affected by water shortages, depending on the season or the site of the CFC treatment plant. In addition, depending on the season, the discharged salt may cause severe damage to river ecosystems. In short, the stable treatment of the
0043-1354/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00311-7
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wastewater is not provided by using this method. After all, the CaCl2 wastewater must be treated at public wastewater treatment plants such as sewage-works to complete the treatment. However, this wastewater cannot be flushed down drains, due to the high CaCl2 concentration, which can corrode drainpipes and cause severe scaling in the pipes with calcium precipitation as gypsum, phosphate, and carbonate. Therefore, the wastewater must be transported to public wastewater treatment plants directly. To facilitate this, the wastewater should be converted into small amounts of CaCl2 wastes. These wastes can be produced by the uptake of Ca2+ and Cl from the CaCl2 wastewater. The produced Ca2+ and Cl wastes should be transported to public plants. Hydrotalcite-like compounds (HTs) are layered double hydroxides with anion exchange properties [4–6]. The basic chemical composition of HT is 3þ n ½Mg2þ 1x Alx ðOHÞ2 ðA Þx=n mH2 O; where An=CO2 3 , Cl , etc., and 0.20%x%0.33. Its crystal structure consists of positively charged brucite-like octahedral hydroxide layers, which are neutralized by the interlayer anion, and water molecules occupy the remaining interlayer space. 2 CO2 3 -HT (HT in which CO3 is the interlayer anion) is transformed into magnesium–aluminum oxide (Mg–Al oxide) when heated to 450–800 C, as expressed in Eq. (3) [7–10].
Mg1x Alx ðOHÞ2 ðCO3 Þx=2 -Mg1x Alx O1þx=2 þ x=2CO2 þ H2 O:
ð3Þ
The resulting Mg–Al oxide can rehydrate and combine with anions, reconstructing the original HT structure in the presence of water and anions, as expressed in Eq. (4) [7–10]. Mg1x Alx O1þx=2 þ x=nAn þ ð1 þ x=2ÞH2 O -Mg1x Alx ðOHÞ2 Ax=n þ xOH :
ð4Þ
The important feature of this reaction is the release of OH by the rehydration and combination of the Mg–Al oxide with anions in solution. Considering this property, we propose the utilization of Mg–Al oxide as both a precipitant of Ca2+ and a fixative of Cl for the uptake of Ca2+ and Cl from CaCl2 wastewater. Some studies have examined the treatment of inorganic salt solutions using Mg–Al oxide. For example, Yurimoto et al. investigated the use of Mg– Al oxide to treat solutions of cations, such as Pb2+, Cd2+, and Zn2+, and found that the cations were removed from the solutions when they were precipitated as the respective hydroxide or carbonate [11]. However, they did not examine the removal of the counter anions from the solutions by the Mg–Al oxide. Conversely, several studies have examined the removal of Cl from
NaCl solution [8–10] or hydrochloric acid [12–14], but none have examined the removal of Cl from a chloride solution containing divalent or trivalent cations. No study has examined the removal of both Cl and the counter cation from solution by Mg–Al oxide. In this study, we investigated the simultaneous removal of Ca2+ and Cl from CaCl2 solution by Mg–Al oxide. The effects of the initial CaCl2 concentration, temperature, and quantity of Mg–Al oxide on Ca2+ and Cl removal were examined in detail. The Cl removal from CaCl2 and NaCl solutions was also compared.
2. Materials and methods All the reagents used were purchased from Kanto Chemical Co. Inc. CO2 3 -HT was prepared using a coprecipitation reaction described previously [15]. A mixed Mg(NO3)2 and Al(NO3)3 solution (2.0 M) with a Mg/Al molar ratio of 4.0 was added to 1.0 M Na2CO3 solution with stirring. When the pH of the reaction mixture fell below 10, a solution of 2.0 M NaOH was added along with the mixed Mg–Al solution to maintain the pH at 10. The reaction mixture was then stirred continuously at 40 C for 4 h and then at 70 C for 40 h. Finally, CO2 3 -HT was isolated by filtering the resulting suspension, washing it extensively with deionized water, and drying it at 105 C for 24 h. The chemical composition of the CO2 was Mg0.80Al0.20(OH)2 3 -HT (CO3)0.10 0.78H2O. Mg–Al oxide was prepared by calcining the prepared CO2 3 -HT at 500 C for 1 h, and the chemical composition of the Mg–Al oxide was Mg0.80Al0.20O1.10. Ten ml of 0.05–0.25 M CaCl2 solution and Mg0.80Al0.20O1.10 were placed in 50-ml screw-top tubes and shaken at 20–60 C for 0.5–24 h. The quantity of Mg0.80Al0.20O1.10 added was 0.33–2.0 times the stoichiometric quantity according to Eq. (5). Mg0:80 Al0:20 O1:10 þ 0:10CaCl2 þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 Cl0:20 þ 0:10CaðOHÞ2 :
ð5Þ
Mg0.80Al0.20O1.10 was also added to 10 ml of 0.5 M NaCl solution and shaken at 20 C for 6 h. The quantity of Mg0.80Al0.20O1.10 added was 2.0 times the stoichiometric quantity according to Eq. (6). Mg0:80 Al0:20 O1:10 þ 0:20NaCl þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 Cl0:20 þ 0:20NaOH:
ð6Þ
The suspended solutions were filtered without washing with water, and the pH of the filtrate was measured. The Cl concentration of the filtrate was determined using a Dionex QIC ion chromatograph and a Dionex model AS4A column (eluent: 1.8 mM sodium carbonate and 1.7 mM sodium bicarbonate, flow rate: 1.5 ml min1).
ARTICLE IN PRESS T. Kameda et al. / Water Research 37 (2003) 4045–4050
The Ca2+ concentration of the filtrate was determined using a Dionex DX-100 ion chromatograph and a Dionex model CS12A column (eluent: methanesulfonic acid, flow rate: 1.0 ml min1). The precipitates were identified by X-ray diffraction (XRD) using a Rigaku Denki Geiger-flex 2013 diffractometer employing Nifiltered CuKa radiation at 30 kV and 15 mA (scan rate: 2 min1).
3. Results and discussion First, the effect of the cation on Cl removal from solution was examined. Table 1 lists the pH of the CaCl2 and NaCl solutions before and after the removal of Cl, and the degree of Cl removal from both solutions by Mg0.80Al0.20O1.10. Mg0.80Al0.20O1.10 was shown to remove Cl from both solutions. In addition, the degree of Ca2+ removal from the CaCl2 solution was 60.6%. These results indicate that Mg0.80Al0.20O1.10 has uptake capacity for Ca2+ and Cl in aqueous solution. The Cl removal is attributed to the rehydration and combination of the Mg0.80Al0.20O1.10 with Cl in solution, as expressed in Eq. (7). Mg0:80 Al0:20 O1:10 þ 0:20Cl þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 Cl0:20 þ 0:20OH : 2+
ð7Þ
The Ca removal is attributed to the precipitation of Ca(OH)2 due to the release of OH, as shown in Eq. (7). This is confirmed by the fact that the precipitate was a mixture of HT and a slight amount of Ca(OH)2, and the final pH was 12.4. Near this pH, Ca2+ is known to precipitate as Ca(OH)2 from aqueous solution [16]. The formation of Ca(OH)2 as one component of the precipitate is attributed to filtering the suspended solutions without washing through the experimental operation. Since Ca(OH)2 is slightly soluble in aqueous solution, it generally does not remain as a precipitate [17]. Filtering without washing might also have effect on the Cl removal; namely, Cl might be removed by adhering to the surface of the precipitate in addition to the combination with Mg0.80Al0.20O1.10. The degree of Cl removal from NaCl solution was lower than that from CaCl2 solution, although the initial
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Cl concentrations were the same. Approximately half of the Cl remained in the NaCl solution after 6 h. Sato et al. investigated the removal of Cl from NaCl and NaNO3 solutions by Mg–Al oxide, and found that the selectivity with which the Mg–Al oxide combined with the anions in solution was in the order OH>Cl>NO 3 [10]. That is, the selectivity increased with decreasing anion size. Similarly, the quantitative removal of Cl from the NaCl solution in our experiment was prevented by the formation of OH– HT (HT in which OH is the interlayer anion), as expressed in Eq. (8), due to the release of OH as shown in Eq. (7). Mg0:80 Al0:20 O1:10 þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 ðOHÞ0:20 :
ð8Þ
By contrast, the high degree of Cl removal from the CaCl2 solution is attributed to the low amount of Mg0.80Al0.20O1.10 that combined with OH due to the precipitation of Ca(OH)2. The effect of the initial CaCl2 concentration on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 is shown in Fig. 1. Ca2+ and Cl were removed from CaCl2 solutions at any initial concentration. The degree of Ca2+ and Cl removal increased with increasing initial CaCl2 concentration. The increased degree of Cl removal is attributed to the higher concentration of Cl relative to OH in the solution during the reaction. Although the OH concentration is increased by the reaction shown in Eq. (7), the increase is offset by the precipitation of Ca(OH)2 due to the buffering action of Ca2+ in solution. That is, the OH concentration remains constant at the pH at which Ca(OH)2 is saturated in solution. Therefore, the Cl concentration increasingly exceeds the OH concentration with increasing the initial CaCl2 concentration. Combination of Mg0.80 Al0.20O1.10 with Cl is easy in a solution with a high Cl concentration and a low OH concentration. The increased degree of Ca2+ removal is attributed to the increased amount of OH released via Eq. (7) with increasing Cl removal. As the amount of OH released increases, the amount of Ca2+ precipitated as Ca(OH)2 increases.
Table 1 pH of the CaCl2 and NaCl solutions before and after the removal of Cl, and the degree of Cl removal from both solutions by Mg0.80Al0.20O1.10 [Cl]=0.5 M
CaCl2 NaCl
Degree of Cl removal (%)
pH Initial
Final
6.1 5.3
12.4 13.4
68.7 51.4
Temp: 20 C, time: 6 h, quantity of Mg0.80Al0.20O1.10:2.0 times the stoichiometric quantity according to Eqs. (5) and (6).
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100 Degree of Ca2+ and Clremoval / %
80 60 40 ClCa2+
20 0
0
0.3 0.1 0.2 Initial CaCl2 concentration / M
Fig. 1. Effect of the initial CaCl2 concentration on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 (temp: 20 C, time: 6 h, quantity of Mg0.80Al0.20O1.10: 2.0 times the stoichiometric quantity according to Eq. (5)).
100 Degree of Ca2+ and Clremoval / %
(c) 80
(b)
60 (a)
40
ClCa2+
20 0
0
2
4 Time / h
6
the reaction under chemical reaction control. The increased degree of Ca2+ removal is attributable to the increased amount of OH released via Eq. (7) with increasing Cl removal. Fig. 2 shows that the rate of Cl removal at 20 C was relatively slow. The degree of Ca2+ and Cl removal at 20 C after 24 h was 76.1% and 89.3%, respectively. This means that the reaction does not attain equilibrium within 24 h at 20 C, whereas the reaction surely attains equilibrium within 3 h at 60 C. The Ca2+ and Cl concentrations at 20 C for 24 h were 0.06 and 0.06 M, whereas those at 60 C for 3 h were 0.02 and 0.01 M, respectively. This slow reaction suggests that the following two reactions occur. The first is the rehydration and combination of Mg0.80Al0.20O1.10 with Cl. The second is the anion exchange reaction between Cl and OH intercalated into the interlayer of OHHT formed via Eq. (8). This is attributed to the temperature dependence of the reaction of Mg0.80 Al0.20O1.10, i.e., the amount of Cl combined with Mg0.80Al0.20O1.10 decreased with decreasing temperature [13]. Therefore, OH-HT was formed at first at 20 C, however, the anion exchange reaction gradually occurred due to the high concentration of Cl in the CaCl2 solution. Fig. 3 shows the effect of the quantity of Mg0.80 Al0.20O1.10 on Ca2+ and Cl removal. The degree of the Ca2+ and Cl removal increased with increasing quantity of Mg0.80Al0.20O1.10. Even when Mg0.80Al0.20O1.10 was added at a stoichiometric quantity according to Eq. (5), Cl was not removed from the CaCl2 solution quantitatively, due to the formation of OH-HT after the release of OH, as expressed in Eqs. (7) and (8). At 2.0 times the stoichiometric quantity, more than 90% of the Ca2+ and a quantitative amount of the Cl were removed from the CaCl2 solution. At least 2.0 times the stoichiometric quantity of
Fig. 2. Effect of time on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 at (a) 20 C, (b) 40 C, and (c) 60 C (initial CaCl2 concentration: 0.25 M, quantity of Mg0.80Al0.20O1.10 : 2.0 times the stoichiometric quantity according to Eq. (5)).
Degree of Ca2+ and Clremoval / %
Fig. 2 shows the effect of time on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 at 20–60 C. The degree of Ca2+ and Cl removal increased with time at any temperature, and increased with increasing temperature for any time. At 60 C, the degree of removal increased very rapidly with time, and 93.0% of the Ca2+ and 98.2% of the Cl were removed from the CaCl2 solution after 0.5 h. The rehydration and combination of Mg–Al oxide with anions, such as S2O2 6 , Cl , and the benzenecarboxylate ion, proceed under chemical reaction control, i.e., they are temperature dependent [9,13,18]. Accordingly, the increased degree of Cl removal from the CaCl2 solution with increasing temperature is certainly attributed to the proceeding of
100 80 60 40 20 0
ClCa2+
0 1 2 Quantity of Mg0.80Al0.20O1.10 / times the stoichiometric quantity
Fig. 3. Effect of the quantity of Mg0.80Al0.20O1.10 on Ca2+ and Cl removal (initial CaCl2 concentration: 0.25 M, temp: 60 C, time: 3 h).
ARTICLE IN PRESS T. Kameda et al. / Water Research 37 (2003) 4045–4050
Mg0.80Al0.20O1.10 was found to be required in order to remove Ca2+ and Cl from solution quantitatively. Mg–Al oxide was found to have uptake capacity for Ca2+ and Cl in CaCl2 solution. We expect that Mg–Al oxide is applied to convert CaCl2 wastewater into a small amount of CaCl2 waste. Treating CaCl2 wastewater with Mg–Al oxide may create wastewater with a pH of approximately 12. However, the wastewater so created can be re-used to prepare milk of lime, which is used in order to neutralize the HCl and HF gases produced in the decomposition of CFCs. A mixture of HT and Ca(OH)2 is generated by the reaction of Mg–Al oxide and CaCl2 wastewater. The HT is certainly ClHT, in which Cl is intercalated into the interlayer of the HT. When such a mixture is brought to a public wastewater treatment plant, and washed with water, separate Cl-HT and Ca(OH)2 solutions can be produced. At the plant, the Ca(OH)2 solutions can be treated, while the Cl-HT produced can be used as an ion exchanger to remove phosphorus from other wastewater [19–21]. In addition, Mg–Al oxide can be regenerated with hydrochloric acid by calcining the Cl-HT at 450–800 C [22,23]. This Mg–Al oxide can then be re-used to treat additional CaCl2 wastewater.
4. Conclusions We showed that Mg–Al oxide could be used as both a precipitant of Ca2+ and a fixative of Cl for the uptake of the Ca2+ and the Cl from aqueous solution, i.e., Ca2+ and Cl could be removed from CaCl2 solution simultaneously by Mg–Al oxide. In addition, we found that the cations in the chloride solution affected the removal of Cl from solution. For example, the degree of Cl removal from CaCl2 solution was higher than that from NaCl solution when the initial Cl concentrations were the same. The degree of Ca2+ and Cl removal from aqueous solution by Mg–Al oxide increased with increasing initial CaCl2 concentration, temperature, and quantity of Mg–Al oxide. Ca2+ and Cl removal in excess of 90% from 0.25 M CaCl2 solution could be achieved within 0.5 h using 2.0 times the stoichiometric quantity of Mg0.80Al0.20O1.10 at 60 C.
References [1] Nihon Kagaku-kai. Furon no Kankyoukagaku to Gijyutsutaisaku, Gakkai Syuppan Senta. Tokyo, 1991. p. 144–58. [2] Horii Y, Higuchi S, Shimaoka T, Hanashima M. The treatment technology of landfill leachates containing condensed inorganic salts. Haikibutsu Gakkaishi 1997;8:529–39.
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[3] Takano F, Shoji T, Komatsu T. Management of sanitary landfills in Sendai city, having a high concentration of incinerator residue. Haikibutsu Gakkaishi 1997;8:523–8. [4] Ingram L, Taylor HFW. The crystal structures of sjogrenite and pyroaurite. Mineral Mag 1967;36:465–79. [5] Miyata S. Anion-exchange properties of hydrotalcite-like compounds. Clays Clay Miner 1983;31:305–11. [6] Cavani F, Trifiro F, Vaccari A. Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 1991;11:173–301. [7] Miyata S. Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner 1980;28:50–6. [8] Sato T, Wakabayashi T, Shimada M. Adsorption of various anions by magnesium aluminum oxide (Mg0.7Al0.3O1.15). Ind Eng Chem Prod Res Dev 1986;25:89–92. [9] Sato T, Tezuka M, Endo T, Shimada M. Kinetics of anion uptake by rock salt-type magnesium aluminum oxide solutions. React Solids 1987;3:287–95. [10] Sato T, Onai S, Yoshioka T, Okuwaki A. Causticization of sodium carbonate with rock-salt type magnesium aluminum oxide formed by the thermal decomposition of hydrotalcite-like layered double hydroxide. J Chem Tech Biotechnol 1993;57:137–40. [11] Yurimoto J, Miki M, Watanabe H, Shiraishi K. Suiyouekichyuu no Mukiion no Jyokyo Houhou. Japanese Patent 54/24993 1979. [12] Kameda T, Miyano Y, Yoshioka T, Uchida M, Okuwaki A. New treatment methods for waste water containing chloride ion using magnesium–aluminum oxide. Chem Lett 2000;2000:1136–7. [13] Kameda T, Yoshioka T, Uchida M, Miyano Y, Okuwaki A. New treatment method for dilute hydrochloric acid using magnesium–aluminum oxide. Bull Chem Soc Japan 2002;75:595–9. [14] Kameda T, Yabuuchi F, Yoshioka T, Uchida M, Okuwaki A. New method of treating dilute mineral acids using magnesium–aluminum oxide. Wat Res 2003;37:1545–50. [15] Yun SK, Pinnavaia TJ. Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides. Chem Mater 1995;7:348–54. [16] Pourbaix M. Atlas of electrochemical equilibria in aqueous solutions. National Association of Corrosion Engineers, Houston, 1974. p.150. [17] Kameda T, Yoshioka T, Uchida M, Okuwaki A. Synthesis of Hydrotalcite from Seawater and Its Application to Phosphorus Removal. Phosphorus, Sulfur and Silicon 2002;177:1503–6. [18] Sato T, Kato K, Yoshioka T, Okuwaki A. Uptake of benzenecarboxylate ions by magnesium aluminum oxides. J Chem Tech Biotechnol 1992;55:385–90. [19] Ookubo A, Ooi H, Hayashi K. Preparation and phosphate ion-exchange properties of a hydrotalcite-like compound. Langmuir 1993;9:1418–22. [20] Shin H, Kim M, Nam S, Moon H. Phosphorus removal by hydrotalcite-like compounds(HTLcs). Water Sci Tech 1996;34:161–8. [21] Kawamoto A, Ookubo A, Sato T, Suzuki T. Phosphorus removal from wastewater by layer structure inorganic ion exchanger with high selectivity to phosphate anion. Mizukankyou Gakkaishi 1999;22:875–81.
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[22] Miyata S. The syntheses of hydrotalcite-like compounds and their structures and physico-chemical properties-I: The 2+ systems Mg2+-Al3+-NO -Al3+-Cl, Mg2+-Al3+ 3 , Mg 2+ -ClO -Al3+-Cl and Zn2+-Al3+-Cl. Clays Clay 4 , Ni Miner 1975;23:369–75.
[23] Constantino VRL, Pinnavaia TJ. Basic properties of 3+ Mg2+ 1xAl x layered double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate anions. Inorg Chem 1995;34:883–92.
Water Research 37 (2003) 4045–4050
Technical note
The simultaneous removal of calcium and chloride ions from calcium chloride solution using magnesium–aluminum oxide Tomohito Kamedaa,b,*, Toshiaki Yoshiokaa, Teruhisa Mitsuhashia, Miho Uchidaa, Akitsugu Okuwakia a
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba 07, Aoba-ku, Sendai 980-8579, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan Received 6 November 2002; received in revised form 24 April 2003; accepted 1 May 2003
Abstract We investigated the removal of Ca2+ and Cl from CaCl2 solution at 20–60 C, using magnesium–aluminum oxide, Mg0.80Al0.20O1.10, prepared by the thermal decomposition of a hydrotalcite-like compound, Mg0.80Al0.20(OH)2 (CO3)0.10 0.78H2O. The degree of Ca2+ and Cl removal from the solution increased with increasing initial CaCl2 concentration, temperature, and quantity of Mg0.80Al0.20O1.10 added. When Mg0.80Al0.20O1.10 was added to 0.25 M CaCl2 solution in a Mg0.80Al0.20O1.10/CaCl2 molar ratio of 20, the degree of Ca2+ and Cl removal from the solution at 60 C after 0.5 h was 93.0% and 98.2%, respectively. These results reveal that Mg0.80Al0.20O1.10 has the capacity to remove Ca2+ and Cl simultaneously from aqueous solution. r 2003 Elsevier Ltd. All rights reserved. Keywords: Hydrotalcite-like compound; Magnesium–aluminum oxide; CaCl2 solution; Removal; Simultaneous; Buffering action
1. Introduction Chlorofluorocarbons (CFCs) are chemically stable, and diffuse into the stratosphere where they convert ozone molecules into oxygen. Furthermore, CFCs contribute significantly to the greenhouse effect. The use of CFCs is being reduced as a result of the United Nations Environmental Protection Protocol for CFC regulation, which was adopted in Montreal, Canada, in 1987. Since remaining stocks of CFCs must be converted into harmless compounds, many researchers have developed processes to detoxify CFCs [1]. One process decomposes CFCs in the presence of H2O, according to
*Corresponding author. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1,1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81-22-217-5148; fax: +81-22-217-5148. E-mail address: [email protected] (T. Kameda).
the reaction: CCl3 F þ 2H2 O-CO2 þ HF þ 3HCl:
ð1Þ
Then, the resulting HCl and HF gases are neutralized with milk of lime, in the reaction: HF þ 3HClþ2CaðOHÞ2 -0:5CaF2 þ 1:5CaCl2 þ 4H2 O:
ð2Þ
However, a particularly frustrating aspect of this process is the CaCl2 wastewater that is produced. Unless this calcium- and chloride-rich wastewater is treated, the salt load may cause severe damage to aquatic environments [2]. The wastewater is typically diluted with large amounts of water, and then discharged into rivers [3]. This may be a low-cost method, but it can be affected by water shortages, depending on the season or the site of the CFC treatment plant. In addition, depending on the season, the discharged salt may cause severe damage to river ecosystems. In short, the stable treatment of the
0043-1354/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00311-7
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wastewater is not provided by using this method. After all, the CaCl2 wastewater must be treated at public wastewater treatment plants such as sewage-works to complete the treatment. However, this wastewater cannot be flushed down drains, due to the high CaCl2 concentration, which can corrode drainpipes and cause severe scaling in the pipes with calcium precipitation as gypsum, phosphate, and carbonate. Therefore, the wastewater must be transported to public wastewater treatment plants directly. To facilitate this, the wastewater should be converted into small amounts of CaCl2 wastes. These wastes can be produced by the uptake of Ca2+ and Cl from the CaCl2 wastewater. The produced Ca2+ and Cl wastes should be transported to public plants. Hydrotalcite-like compounds (HTs) are layered double hydroxides with anion exchange properties [4–6]. The basic chemical composition of HT is 3þ n ½Mg2þ 1x Alx ðOHÞ2 ðA Þx=n mH2 O; where An=CO2 3 , Cl , etc., and 0.20%x%0.33. Its crystal structure consists of positively charged brucite-like octahedral hydroxide layers, which are neutralized by the interlayer anion, and water molecules occupy the remaining interlayer space. 2 CO2 3 -HT (HT in which CO3 is the interlayer anion) is transformed into magnesium–aluminum oxide (Mg–Al oxide) when heated to 450–800 C, as expressed in Eq. (3) [7–10].
Mg1x Alx ðOHÞ2 ðCO3 Þx=2 -Mg1x Alx O1þx=2 þ x=2CO2 þ H2 O:
ð3Þ
The resulting Mg–Al oxide can rehydrate and combine with anions, reconstructing the original HT structure in the presence of water and anions, as expressed in Eq. (4) [7–10]. Mg1x Alx O1þx=2 þ x=nAn þ ð1 þ x=2ÞH2 O -Mg1x Alx ðOHÞ2 Ax=n þ xOH :
ð4Þ
The important feature of this reaction is the release of OH by the rehydration and combination of the Mg–Al oxide with anions in solution. Considering this property, we propose the utilization of Mg–Al oxide as both a precipitant of Ca2+ and a fixative of Cl for the uptake of Ca2+ and Cl from CaCl2 wastewater. Some studies have examined the treatment of inorganic salt solutions using Mg–Al oxide. For example, Yurimoto et al. investigated the use of Mg– Al oxide to treat solutions of cations, such as Pb2+, Cd2+, and Zn2+, and found that the cations were removed from the solutions when they were precipitated as the respective hydroxide or carbonate [11]. However, they did not examine the removal of the counter anions from the solutions by the Mg–Al oxide. Conversely, several studies have examined the removal of Cl from
NaCl solution [8–10] or hydrochloric acid [12–14], but none have examined the removal of Cl from a chloride solution containing divalent or trivalent cations. No study has examined the removal of both Cl and the counter cation from solution by Mg–Al oxide. In this study, we investigated the simultaneous removal of Ca2+ and Cl from CaCl2 solution by Mg–Al oxide. The effects of the initial CaCl2 concentration, temperature, and quantity of Mg–Al oxide on Ca2+ and Cl removal were examined in detail. The Cl removal from CaCl2 and NaCl solutions was also compared.
2. Materials and methods All the reagents used were purchased from Kanto Chemical Co. Inc. CO2 3 -HT was prepared using a coprecipitation reaction described previously [15]. A mixed Mg(NO3)2 and Al(NO3)3 solution (2.0 M) with a Mg/Al molar ratio of 4.0 was added to 1.0 M Na2CO3 solution with stirring. When the pH of the reaction mixture fell below 10, a solution of 2.0 M NaOH was added along with the mixed Mg–Al solution to maintain the pH at 10. The reaction mixture was then stirred continuously at 40 C for 4 h and then at 70 C for 40 h. Finally, CO2 3 -HT was isolated by filtering the resulting suspension, washing it extensively with deionized water, and drying it at 105 C for 24 h. The chemical composition of the CO2 was Mg0.80Al0.20(OH)2 3 -HT (CO3)0.10 0.78H2O. Mg–Al oxide was prepared by calcining the prepared CO2 3 -HT at 500 C for 1 h, and the chemical composition of the Mg–Al oxide was Mg0.80Al0.20O1.10. Ten ml of 0.05–0.25 M CaCl2 solution and Mg0.80Al0.20O1.10 were placed in 50-ml screw-top tubes and shaken at 20–60 C for 0.5–24 h. The quantity of Mg0.80Al0.20O1.10 added was 0.33–2.0 times the stoichiometric quantity according to Eq. (5). Mg0:80 Al0:20 O1:10 þ 0:10CaCl2 þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 Cl0:20 þ 0:10CaðOHÞ2 :
ð5Þ
Mg0.80Al0.20O1.10 was also added to 10 ml of 0.5 M NaCl solution and shaken at 20 C for 6 h. The quantity of Mg0.80Al0.20O1.10 added was 2.0 times the stoichiometric quantity according to Eq. (6). Mg0:80 Al0:20 O1:10 þ 0:20NaCl þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 Cl0:20 þ 0:20NaOH:
ð6Þ
The suspended solutions were filtered without washing with water, and the pH of the filtrate was measured. The Cl concentration of the filtrate was determined using a Dionex QIC ion chromatograph and a Dionex model AS4A column (eluent: 1.8 mM sodium carbonate and 1.7 mM sodium bicarbonate, flow rate: 1.5 ml min1).
ARTICLE IN PRESS T. Kameda et al. / Water Research 37 (2003) 4045–4050
The Ca2+ concentration of the filtrate was determined using a Dionex DX-100 ion chromatograph and a Dionex model CS12A column (eluent: methanesulfonic acid, flow rate: 1.0 ml min1). The precipitates were identified by X-ray diffraction (XRD) using a Rigaku Denki Geiger-flex 2013 diffractometer employing Nifiltered CuKa radiation at 30 kV and 15 mA (scan rate: 2 min1).
3. Results and discussion First, the effect of the cation on Cl removal from solution was examined. Table 1 lists the pH of the CaCl2 and NaCl solutions before and after the removal of Cl, and the degree of Cl removal from both solutions by Mg0.80Al0.20O1.10. Mg0.80Al0.20O1.10 was shown to remove Cl from both solutions. In addition, the degree of Ca2+ removal from the CaCl2 solution was 60.6%. These results indicate that Mg0.80Al0.20O1.10 has uptake capacity for Ca2+ and Cl in aqueous solution. The Cl removal is attributed to the rehydration and combination of the Mg0.80Al0.20O1.10 with Cl in solution, as expressed in Eq. (7). Mg0:80 Al0:20 O1:10 þ 0:20Cl þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 Cl0:20 þ 0:20OH : 2+
ð7Þ
The Ca removal is attributed to the precipitation of Ca(OH)2 due to the release of OH, as shown in Eq. (7). This is confirmed by the fact that the precipitate was a mixture of HT and a slight amount of Ca(OH)2, and the final pH was 12.4. Near this pH, Ca2+ is known to precipitate as Ca(OH)2 from aqueous solution [16]. The formation of Ca(OH)2 as one component of the precipitate is attributed to filtering the suspended solutions without washing through the experimental operation. Since Ca(OH)2 is slightly soluble in aqueous solution, it generally does not remain as a precipitate [17]. Filtering without washing might also have effect on the Cl removal; namely, Cl might be removed by adhering to the surface of the precipitate in addition to the combination with Mg0.80Al0.20O1.10. The degree of Cl removal from NaCl solution was lower than that from CaCl2 solution, although the initial
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Cl concentrations were the same. Approximately half of the Cl remained in the NaCl solution after 6 h. Sato et al. investigated the removal of Cl from NaCl and NaNO3 solutions by Mg–Al oxide, and found that the selectivity with which the Mg–Al oxide combined with the anions in solution was in the order OH>Cl>NO 3 [10]. That is, the selectivity increased with decreasing anion size. Similarly, the quantitative removal of Cl from the NaCl solution in our experiment was prevented by the formation of OH– HT (HT in which OH is the interlayer anion), as expressed in Eq. (8), due to the release of OH as shown in Eq. (7). Mg0:80 Al0:20 O1:10 þ 1:10H2 O -Mg0:80 Al0:20 ðOHÞ2 ðOHÞ0:20 :
ð8Þ
By contrast, the high degree of Cl removal from the CaCl2 solution is attributed to the low amount of Mg0.80Al0.20O1.10 that combined with OH due to the precipitation of Ca(OH)2. The effect of the initial CaCl2 concentration on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 is shown in Fig. 1. Ca2+ and Cl were removed from CaCl2 solutions at any initial concentration. The degree of Ca2+ and Cl removal increased with increasing initial CaCl2 concentration. The increased degree of Cl removal is attributed to the higher concentration of Cl relative to OH in the solution during the reaction. Although the OH concentration is increased by the reaction shown in Eq. (7), the increase is offset by the precipitation of Ca(OH)2 due to the buffering action of Ca2+ in solution. That is, the OH concentration remains constant at the pH at which Ca(OH)2 is saturated in solution. Therefore, the Cl concentration increasingly exceeds the OH concentration with increasing the initial CaCl2 concentration. Combination of Mg0.80 Al0.20O1.10 with Cl is easy in a solution with a high Cl concentration and a low OH concentration. The increased degree of Ca2+ removal is attributed to the increased amount of OH released via Eq. (7) with increasing Cl removal. As the amount of OH released increases, the amount of Ca2+ precipitated as Ca(OH)2 increases.
Table 1 pH of the CaCl2 and NaCl solutions before and after the removal of Cl, and the degree of Cl removal from both solutions by Mg0.80Al0.20O1.10 [Cl]=0.5 M
CaCl2 NaCl
Degree of Cl removal (%)
pH Initial
Final
6.1 5.3
12.4 13.4
68.7 51.4
Temp: 20 C, time: 6 h, quantity of Mg0.80Al0.20O1.10:2.0 times the stoichiometric quantity according to Eqs. (5) and (6).
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100 Degree of Ca2+ and Clremoval / %
80 60 40 ClCa2+
20 0
0
0.3 0.1 0.2 Initial CaCl2 concentration / M
Fig. 1. Effect of the initial CaCl2 concentration on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 (temp: 20 C, time: 6 h, quantity of Mg0.80Al0.20O1.10: 2.0 times the stoichiometric quantity according to Eq. (5)).
100 Degree of Ca2+ and Clremoval / %
(c) 80
(b)
60 (a)
40
ClCa2+
20 0
0
2
4 Time / h
6
the reaction under chemical reaction control. The increased degree of Ca2+ removal is attributable to the increased amount of OH released via Eq. (7) with increasing Cl removal. Fig. 2 shows that the rate of Cl removal at 20 C was relatively slow. The degree of Ca2+ and Cl removal at 20 C after 24 h was 76.1% and 89.3%, respectively. This means that the reaction does not attain equilibrium within 24 h at 20 C, whereas the reaction surely attains equilibrium within 3 h at 60 C. The Ca2+ and Cl concentrations at 20 C for 24 h were 0.06 and 0.06 M, whereas those at 60 C for 3 h were 0.02 and 0.01 M, respectively. This slow reaction suggests that the following two reactions occur. The first is the rehydration and combination of Mg0.80Al0.20O1.10 with Cl. The second is the anion exchange reaction between Cl and OH intercalated into the interlayer of OHHT formed via Eq. (8). This is attributed to the temperature dependence of the reaction of Mg0.80 Al0.20O1.10, i.e., the amount of Cl combined with Mg0.80Al0.20O1.10 decreased with decreasing temperature [13]. Therefore, OH-HT was formed at first at 20 C, however, the anion exchange reaction gradually occurred due to the high concentration of Cl in the CaCl2 solution. Fig. 3 shows the effect of the quantity of Mg0.80 Al0.20O1.10 on Ca2+ and Cl removal. The degree of the Ca2+ and Cl removal increased with increasing quantity of Mg0.80Al0.20O1.10. Even when Mg0.80Al0.20O1.10 was added at a stoichiometric quantity according to Eq. (5), Cl was not removed from the CaCl2 solution quantitatively, due to the formation of OH-HT after the release of OH, as expressed in Eqs. (7) and (8). At 2.0 times the stoichiometric quantity, more than 90% of the Ca2+ and a quantitative amount of the Cl were removed from the CaCl2 solution. At least 2.0 times the stoichiometric quantity of
Fig. 2. Effect of time on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 at (a) 20 C, (b) 40 C, and (c) 60 C (initial CaCl2 concentration: 0.25 M, quantity of Mg0.80Al0.20O1.10 : 2.0 times the stoichiometric quantity according to Eq. (5)).
Degree of Ca2+ and Clremoval / %
Fig. 2 shows the effect of time on Ca2+ and Cl removal by Mg0.80Al0.20O1.10 at 20–60 C. The degree of Ca2+ and Cl removal increased with time at any temperature, and increased with increasing temperature for any time. At 60 C, the degree of removal increased very rapidly with time, and 93.0% of the Ca2+ and 98.2% of the Cl were removed from the CaCl2 solution after 0.5 h. The rehydration and combination of Mg–Al oxide with anions, such as S2O2 6 , Cl , and the benzenecarboxylate ion, proceed under chemical reaction control, i.e., they are temperature dependent [9,13,18]. Accordingly, the increased degree of Cl removal from the CaCl2 solution with increasing temperature is certainly attributed to the proceeding of
100 80 60 40 20 0
ClCa2+
0 1 2 Quantity of Mg0.80Al0.20O1.10 / times the stoichiometric quantity
Fig. 3. Effect of the quantity of Mg0.80Al0.20O1.10 on Ca2+ and Cl removal (initial CaCl2 concentration: 0.25 M, temp: 60 C, time: 3 h).
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Mg0.80Al0.20O1.10 was found to be required in order to remove Ca2+ and Cl from solution quantitatively. Mg–Al oxide was found to have uptake capacity for Ca2+ and Cl in CaCl2 solution. We expect that Mg–Al oxide is applied to convert CaCl2 wastewater into a small amount of CaCl2 waste. Treating CaCl2 wastewater with Mg–Al oxide may create wastewater with a pH of approximately 12. However, the wastewater so created can be re-used to prepare milk of lime, which is used in order to neutralize the HCl and HF gases produced in the decomposition of CFCs. A mixture of HT and Ca(OH)2 is generated by the reaction of Mg–Al oxide and CaCl2 wastewater. The HT is certainly ClHT, in which Cl is intercalated into the interlayer of the HT. When such a mixture is brought to a public wastewater treatment plant, and washed with water, separate Cl-HT and Ca(OH)2 solutions can be produced. At the plant, the Ca(OH)2 solutions can be treated, while the Cl-HT produced can be used as an ion exchanger to remove phosphorus from other wastewater [19–21]. In addition, Mg–Al oxide can be regenerated with hydrochloric acid by calcining the Cl-HT at 450–800 C [22,23]. This Mg–Al oxide can then be re-used to treat additional CaCl2 wastewater.
4. Conclusions We showed that Mg–Al oxide could be used as both a precipitant of Ca2+ and a fixative of Cl for the uptake of the Ca2+ and the Cl from aqueous solution, i.e., Ca2+ and Cl could be removed from CaCl2 solution simultaneously by Mg–Al oxide. In addition, we found that the cations in the chloride solution affected the removal of Cl from solution. For example, the degree of Cl removal from CaCl2 solution was higher than that from NaCl solution when the initial Cl concentrations were the same. The degree of Ca2+ and Cl removal from aqueous solution by Mg–Al oxide increased with increasing initial CaCl2 concentration, temperature, and quantity of Mg–Al oxide. Ca2+ and Cl removal in excess of 90% from 0.25 M CaCl2 solution could be achieved within 0.5 h using 2.0 times the stoichiometric quantity of Mg0.80Al0.20O1.10 at 60 C.
References [1] Nihon Kagaku-kai. Furon no Kankyoukagaku to Gijyutsutaisaku, Gakkai Syuppan Senta. Tokyo, 1991. p. 144–58. [2] Horii Y, Higuchi S, Shimaoka T, Hanashima M. The treatment technology of landfill leachates containing condensed inorganic salts. Haikibutsu Gakkaishi 1997;8:529–39.
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[3] Takano F, Shoji T, Komatsu T. Management of sanitary landfills in Sendai city, having a high concentration of incinerator residue. Haikibutsu Gakkaishi 1997;8:523–8. [4] Ingram L, Taylor HFW. The crystal structures of sjogrenite and pyroaurite. Mineral Mag 1967;36:465–79. [5] Miyata S. Anion-exchange properties of hydrotalcite-like compounds. Clays Clay Miner 1983;31:305–11. [6] Cavani F, Trifiro F, Vaccari A. Hydrotalcite-type anionic clays: preparation, properties and applications. Catal. Today 1991;11:173–301. [7] Miyata S. Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner 1980;28:50–6. [8] Sato T, Wakabayashi T, Shimada M. Adsorption of various anions by magnesium aluminum oxide (Mg0.7Al0.3O1.15). Ind Eng Chem Prod Res Dev 1986;25:89–92. [9] Sato T, Tezuka M, Endo T, Shimada M. Kinetics of anion uptake by rock salt-type magnesium aluminum oxide solutions. React Solids 1987;3:287–95. [10] Sato T, Onai S, Yoshioka T, Okuwaki A. Causticization of sodium carbonate with rock-salt type magnesium aluminum oxide formed by the thermal decomposition of hydrotalcite-like layered double hydroxide. J Chem Tech Biotechnol 1993;57:137–40. [11] Yurimoto J, Miki M, Watanabe H, Shiraishi K. Suiyouekichyuu no Mukiion no Jyokyo Houhou. Japanese Patent 54/24993 1979. [12] Kameda T, Miyano Y, Yoshioka T, Uchida M, Okuwaki A. New treatment methods for waste water containing chloride ion using magnesium–aluminum oxide. Chem Lett 2000;2000:1136–7. [13] Kameda T, Yoshioka T, Uchida M, Miyano Y, Okuwaki A. New treatment method for dilute hydrochloric acid using magnesium–aluminum oxide. Bull Chem Soc Japan 2002;75:595–9. [14] Kameda T, Yabuuchi F, Yoshioka T, Uchida M, Okuwaki A. New method of treating dilute mineral acids using magnesium–aluminum oxide. Wat Res 2003;37:1545–50. [15] Yun SK, Pinnavaia TJ. Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides. Chem Mater 1995;7:348–54. [16] Pourbaix M. Atlas of electrochemical equilibria in aqueous solutions. National Association of Corrosion Engineers, Houston, 1974. p.150. [17] Kameda T, Yoshioka T, Uchida M, Okuwaki A. Synthesis of Hydrotalcite from Seawater and Its Application to Phosphorus Removal. Phosphorus, Sulfur and Silicon 2002;177:1503–6. [18] Sato T, Kato K, Yoshioka T, Okuwaki A. Uptake of benzenecarboxylate ions by magnesium aluminum oxides. J Chem Tech Biotechnol 1992;55:385–90. [19] Ookubo A, Ooi H, Hayashi K. Preparation and phosphate ion-exchange properties of a hydrotalcite-like compound. Langmuir 1993;9:1418–22. [20] Shin H, Kim M, Nam S, Moon H. Phosphorus removal by hydrotalcite-like compounds(HTLcs). Water Sci Tech 1996;34:161–8. [21] Kawamoto A, Ookubo A, Sato T, Suzuki T. Phosphorus removal from wastewater by layer structure inorganic ion exchanger with high selectivity to phosphate anion. Mizukankyou Gakkaishi 1999;22:875–81.
ARTICLE IN PRESS 4050
T. Kameda et al. / Water Research 37 (2003) 4045–4050
[22] Miyata S. The syntheses of hydrotalcite-like compounds and their structures and physico-chemical properties-I: The 2+ systems Mg2+-Al3+-NO -Al3+-Cl, Mg2+-Al3+ 3 , Mg 2+ -ClO -Al3+-Cl and Zn2+-Al3+-Cl. Clays Clay 4 , Ni Miner 1975;23:369–75.
[23] Constantino VRL, Pinnavaia TJ. Basic properties of 3+ Mg2+ 1xAl x layered double hydroxides intercalated by carbonate, hydroxide, chloride, and sulfate anions. Inorg Chem 1995;34:883–92.