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Dynamics of Ion Exchange in Supersaturated Solutions Dmitri Muraviev,*,† Ruslan Kh. Khamizov,‡ Nikolai A. Tikhonov,§ and Valery V. Kirshin§ Department of Analytical Chemistry, Autonomous University of Barcelona, E-08193 Bellaterra (Barcelona), Spain, Vernadsky Institute of Geological and Analytical Chemistry, Kosygin Str. 19, 117975 Moscow, Russia, and Department of Mathematics, Physical Faculty, Lomonosov Moscow State University, 119899 Moscow, Russia Received July 7, 1997. In Final Form: October 9, 1997X This paper reports the results on experimental and theoretical study of the new phenomenon called ion-exchange-isothermal-supersaturation (IXISS). This effect is observed for a number of ion-exchange systems where frontal or reverse frontal separation is accompanied by the formation of extremely stable supersaturated solutions of low solubility substances in the interbed space of ion-exchange columns. After leaving the column a supersaturated solution crystallizes spontaneously, which allows for designing a practically ideal ion-exchange process where a crystalline product is obtained directly after the ion-exchange treatment cycle. The paper is comprised of results on experimental investigation of IXISS of magnesium carbonate on carboxylic resins which is observed in desorption of Mg2+ from the resin in the Mg form with Na2CO3 solutions or with solutions of Na2CO3-NaHCO3 mixtures. The physical and mathematical models of the ion-exchange processes accompanied by the IXISS effect are proposed and their validity is experimentally confirmed.
Introduction The variety of known ion-exchange systems of great practical importance, where an ion-exchange reaction is coupled with the formation of low solubility substances, is very wide. The ion-exchange interaction with the formation of one or several low solubility substances either can be involved in the process purposely or can be an undesirable phenomenon. Process designs related to the first case are usually used for shifting the ion-exchange equilibrium.1-5 In the second case the formation a of low solubility substance may take place, for example, in water treatment processes (precipitation of iron hydroxide6), under the regeneration of a cation exchanger in the Ca form by concentrated H2SO4 solution (precipitation of CaSO47,8), and in some other situations. Although the ion-exchange processes accompanied by formation of slightly soluble substances are characterized by the serious drawback that precipitate formation inside a resin bed may occlude ion-exchange columns, the advantage of such a combination continues to attract the attention of scientists and engineers. Addition of precipitation inhibitors,9 and realization of the process in multisectional10 or countercurrent columns11 have been used to prevent * Author for correspondence. † Autonomous University of Barcelona. ‡ Vernadsky Institute of Geological and Analytical Chemistry. § Lomonosov Moscow State University. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Bogatyrev, V. L. Ion Exchange Resins in Mixed Bed; Khimia: Leningrad, 1968 (Russian). (2) Vulikh, A. I. Ion Exchange Synthesis; Khimia: Moscow, 1973 (Russian). (3) Glueck, A. R. Desalination 1968, 4, 32. (4) Ryabinin, A. I.; Afanasiev, U. A.; Lazareva, E. I.; Eremin, V. I. Zh. Prikl. Khim. 1975, 48, 35 (Russian). (5) Ryabinin, A. I.; Afanasiev, U. A.; Eremin, V. I.; Panjushkina, V. T.; Bukov, N. N. Dokl. Akad. Nauk SSSR 1975, 225, 1115 (Russian). (6) Calmon, C. In Ion Exchange for Pollution Control; Calmon, C., Gold, H., Eds.; CRC Press: Boca Raton, FL, 1979; Vol. 1, p 20. (7) Bolto, B. A.; Pawlowsky, L. Effluent Water Treat. J. 1983, 23, 371. (8) Calmon, C. In Ion Exchange for Pollution Control; Calmon, C., Gold, H., Eds.; CRC Press: Boca Raton, FL; 1979; Vol. 1, p 46. (9) Witmer, F. E.; Beitelshees, C. D.; Haugseth, L. A. AIChE Symp. Ser. 1974, 70, 170. (10) Van der Meer, A. P.; Weve, D. N. M. M.; Wesseling, J. A. In Ion Exchange Technology; Naden, D., Streat, M., Eds.; Ellis-Horwood: Chichester; 1984; p 284.
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formation of precipitates of low solubility substance in the resin bed. In this context the phenomenon called ionexchange isothermal supersaturation (IXISS) is of particular interest. This effect is observed for a number of ion-exchange systems, where frontal (also known as frontal ion-exchange chromatography12,13) or reverse frontal separation14 is accompanied by the formation of supersaturated solutions of slightly (or sparingly) soluble substances. This supersaturated solution may remain stable within the column interstitial space for a long period (up to several days), while after leaving the column it crystallizes spontaneously, which allows for designing practically ideal ion-exchange process where a crystalline product is obtained right after the ion-exchange treatment cycle. This phenomenon has been discovered for the first time by Muraviev for low solubility amino acids such as glutamic, aspartic, and some others.15-17 Later, Khamisov et al. observed this effect for some low solubility calcium and magnesium salts.18-20 Several practical applications of the IXISS effect have been reported by Muraviev et al.21,22 and Khamizov et al.18-20 Nevertheless, the tailored application of the IXISS phenomenon for the design of highly efficient ion-exchange processes requires deeper understanding the nature of this phenomenon. Another problem which also needs to be solved refers to the (11) Vulliez-Sermet, P.; Zaganiaris, E. IWC Proc. 1984, 225. (12) Bobleter, O.; Bonn, G. In Ion Exchangers; Dorfner, K., Ed.; Walter de Gruyter: Berlin, 1991; p 1208. (13) Muraviev, D.; Chanov, A. V.; Denisov, A. M.; Omarova, F. M.; Tuikina, S. R. React. Polym. 1992, 17, 29. (14) Gorshkov, V. I.; Muraviev, D.; Warshawsky, A. Solvent Extr. Ion Exch., in press. (15) Muraviev, D. Zh. Fiz. Khim. 1979, 53 (2), 438 (Russian). (16) Muraviev, D.; Saurin, A. D. Zh. Fiz. Khim. 1980, 54, 1271 (Russian). (17) Muraviev, D.; Gorshkov, V. I.; Fesenko, S. A. Zh. Fiz. Khim. 1982, 56, 1567 (Russian). (18) Khamizov, R. Kh.; Muraviev, D.; Warshawsky, A. In Ion Exchange and Solvent Extraction; Marinsky, J., Marcus, Y., Eds.; Marcel Dekker: New York, 1995; Vol. 12, p 93. (19) Khamizov, R. Kh.; Mironova, L. I.; Tikhonov, N. A.; Bychkov, A. V.; Poezd, A. D. Sep. Sci. Technol. 1996, 31, 1. (20) Khamizov, R. Kh.; Novitsky, E. G.; Mironova, L. I.; Fokina, O. V.; Zhiguleva, T. I.; Krachak, A. N. Tekhn. Machin. 1996, 4, 112 (Russian). (21) Muraviev, D.; Gorshkov, V. I.; Medvedev, G. A.; Ferapontov, N. B.; Kovalenko, Ju. A. Zh. Prikl. Khim. 1979, 52, 1183 (Russian). (22) Muraviev, D. N.; Gorshkov, V. I. Zh. Fiz. Khim. 1982, 56, 1560 (Russian).
© 1997 American Chemical Society
Dynamics of Ion Exchange in Supersaturated Solutions
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Figure 1. Concentration-volume histories obtained by sorption of Mg2+ on KB-4 (1) and KB-4P2 (2) resins in Na form from solution of 0.45 NaCl and 0.12 equiv/dm3 MgCl2 mixture: height of resin bed, L ) 20 cm; column cross-section area, S ) 1.9 cm2; solution flow rate, v ) 0.33 dm3/h.
modeling of the ion-exchange processes accompanied by the IXISS effect. The present study was undertaken (1) to study the formation conditions and stability of supersaturated MgCO3 solutions in columns using IXISS technique and (2) to develop the physical and mathematical models of dynamics of ion exchange as a result of IXISS effect. Experimental Section Materials, Ion Exchangers, and Analytical Methods. Sodium carbonate, sodium bicarbonate, sodium chloride, and magnesium chloride of p.a. grade were used as received. Weakly acidic cation exchange resins KB-4 and KB-4P2 were commercial ion exchangers (Russian production) of methyl methacrylate type containing 6 and 2.5% cross-linking (divinylbenzene), respectively. The total ion exchange capacity of the resins equaled 9.0 (KB-4) and 9.6 (KB-4P2) mequiv/g. The concentrations of Na+ and Mg2+ were determined by atomic absorption using a Saturn-5 (Russia) photometer. The relative uncertainty of determination of metal ions was not more than 2%. Procedure. All experiments on IXISS of MgCO3 solutions were carried out under dynamic conditions in laboratory scale glass columns in a thermostated room at 298 ( 1 K. The ion exchangers underwent conventional conditioning by carrying out several ion-exchange cycles followed by their conversion into Mg form by rinsing resins in the Na form with the mixture 0.45 equiv/dm3 NaCl and 0.12 equiv/dm3 MgCl2 solutions. This solution was shown to be an adequate model of decalcinated seawater, which can be successfully used for recovery of magnesium compounds.19,20 Then the columns were rinsed with deionized water from the excess of Mg2+ ions and prepared for the magnesium desorption (stripping) cycle. The stripping of magnesium was carried out by passing a solution of either Na2CO3 or Na2CO3 + NaHCO3 mixture through the bed of resin in the Mg form at a constant flow rate. The concentration of Na2CO3 and the composition of Na2CO3 + NaHCO3 mixture were varied in a wide range to determine the optimal conditions for IXISS of magnesium carbonate solution. The supersaturated eluate was collected in portions where the concentrations of Mg2+ and Na+ were determined.
Results and Discussion The typical concentration-volume histories of magnesium sorption on KB-4 and KB-4P2 resins in the Na form are shown in Figure 1. As seen, the sorption of Mg2+ ions from a NaCl-MgCl2 mixture proceeds effectively, which testifies to sufficiently high selectivity of the resin toward Mg2+ over Na+. This conclusion is consistent with the results on determination of selectivity factors for Na+-
Figure 2. Concentration-volume histories of desorption of Mg2+ from KB-4 (1, 3) and KB-4P2 (2) resins with 1.5 M Na2CO3 + 0.6 M NaHCO3 (1, 2) and 1.5 M Na2CO3 (3). Curve 4 corresponds to Mg2+ concentration in supernatant after crystallization of supersaturated solution samples. Conditions were as follows (see Figure 1): L ) 20 cm; S ) 1.9 cm2; v ) 0.084 dm3/h.
Mg2+ exchange from artificial seawater on carboxylic resins reported by Muraviev et al.23 A variety of stripping agents have been tested, and the most efficient ones appear to be the mixtures of sodium carbonate and sodium bicarbonate at concentrations of 1.5-1.6 and 0.4-0.6 mol/dm3, respectively. Figure 2 shows the typical breakthrough curves of magnesium desorption from carboxylic resins by using different stripping agents. As seen in Figure 2, during elution the effective desorption of magnesium is observed, but MgCO3 does not precipitate in the column and remains as a stable 0.5 N solution (that corresponds to the supersaturation degree, γ, of ≈5 at 298 K) at least over a period of 72 h. Removal of this supersaturated solution from the column leads to spontaneous crystallization of MgCO3‚3H2O in the form of coarse well-shaped crystals. Curve 4 in Figure 2 corresponds to Mg2+ concentration in the supernatant over MgCO3‚3H2O precipitate measured 3 h after the crystallization has been started. The comparison of the desorption efficiency of the IXISS active (1.5 M Na2CO3 + 0.59 M NaHCO3) versus the conventional (4.3 M NaCl) stripping agent shown in Figure 3 testifies in favor of the former. The results presented in Figures 1 and 2 may seem to contradict each other. Indeed, the sharp sorption fronts of Mg2+ (see Figure 1) and sharp magnesium peaks obtained by desorption with IXISS active stripping agents (see Figure 2) indicate that the resins are more selective toward Mg2+ in the first case, while in the second they preferentially sorb Na+. Note that the same conclusion follows from the results shown in Figure 3. This conclusion becomes clearer after consideration of the ion-exchange equilibrium in the systems under study. The Mg2+-Na+ exchange reaction on a carboxylic resin (e.g., either KB-4 or KB-4P2) can be written as follows:
(R-COO-)2Mg2+ + 2Na+ h 2R-COO-Na+ + Mg2+ (1) (23) Muraviev, D.; Noguerol, J.; Valiente, M. React. Polym. 1996, 28, 111.
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Figure 3. Regeneration of KB-4 resin in Mg form with solutions of 1.5 M Na2CO3 + 0.6 M NaHCO3 mixture (1) and 4.3 M NaCl (2).
The equilibrium in reaction 1 can be quantitatively characterized by the selectivity coefficient, KMgNa:
KMgNa )
qNa(CMg)1/2 (qMg)1/2CNa
(2)
where C and q are the concentrations of metal ions in the solution and resin phases, respectively. For chloride or sulfate media, KMgNa is known to be
