Cation selectivity in sodium-calcium, sodium-magnesium, and calcium-magnesium exchange on Wyoming bentonite at 298 K

Exchange isotherms were prepared for Na in Na—»Ca and Na^Mg exchange reactions, and for Mg in Ca—»Mg exchange reactions, at 298 K on Wyoming bentonite suspended in a 0.05M perchlorate background. These isotherms were essentially congruent with the appropriate thermodynamic nonpreference exchange isotherms. It was concluded from this fact that there is essentially no difference in the affinity of montmorillonite clay for Ca vs. Mg. However, exchange isotherms prepared for Na in Na(I) — Ca(II) and Na(I) — Mg(II) exchange, as well as published ones for Mg(II) in Ca(II) — Mg(II) exchange on montmorillonite suspended in a 0.05M chloride background consistently indicate a slight preference on the clay for Ca(II) over Mg(II). This preference for Ca(II) is concluded to be the result of the formation of CaCl complexes, which are more stable thermodynamically and have a greater affinity for the clay than MgCl complexes. Additional Index Words: cation exchange, chloride complexes, saline soils, thermodynamics of cation exchange. Sposito, G. , K.M. Holtzclaw, C. Jouany, and L. Charlet. 1983. Cation selectivity in sodium-calcium, sodium-magnesium, and calcium-magnesium exchange on Wyoming bentonite at 298 K. Soil Sci. Soc. Am. J. 47:917-921. O OF THE CENTRAL PROBLEMS in the chemistry of arid-zone soils is the quantitative characterization of an exchange complex bearing sodium, calcium, and magnesium ions. Rahman and Rowell (1979) and Bresler et al. (1982, part 1) have reviewed this problem recently and have brought into clear focus the two objectives of understanding the patterns of Na-Ca-Mg exchange reactions and of predicting the effect of exchanger composition on soil physical properties. The consensus of the literature is that exchangeable sodium percentage (ESP), electrolyte concentration, and soil mineralogy are the three master chemical variables on which the water-conducting properties of arid-zone soils depend. What remains is to develop a unified, quantitative description of the interrelations among these variables that is of predictive value. Much of the research on Na-Ca-Mg exchange reactions has involved montmorillonite clay or montmorillonitic soils. In respect to exchange selectivity, the trends that have emerged from the results of published studies are: (i) for a given distribution of Na and bivalent cation in the aqueous solution phase, the resulting ESP on the clay is somewhat higher in NaMg exchange than in Na-Ca exchange, and (ii) in CaMg exchange, a small preference for Ca exists on the exchange complex (Schwertmann, 1962; Clark, 1966; Dolcater et al., 1968; Levy and Hillel, 1968; Hunsaker 1 Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521. Received 10 Nov. 1982. Approved 19 Apr. 1983. 2 Professor of Soil Science, Research Associate IV, Visiting Postdoctoral Fellow, and Graduate Research Assistant, respectively. and Pratt, 1971; Levy and Shainberg, 1972; Levy et al., 1972; van Bladel et al., 1973; Jensen and Babcock, 1973; Gheyi and van Bladel, 1975; Macs and Cremers, 1977; Rahman and Rowell, 1979; Rowell and Shainberg, 1979; van Bladel and Gheyi, 1980; Shainberg et al., 1980). The extent to which Na-Mg exchange results in larger ESP values than Na-Ca exchange is not pronounced, the differences being the order of 3 to 5 in ESP (Rahman and Rowell, 1979). The same is true of the selectivity of montmorillonite for calcium over magnesium. For example, van Bladel and Gheyi (1980) have reported a mean value of 0.8 ± 0.1 for the overall Vanselow selectivity coefficient, KTV = (xTMg CaT)/(xTCa MgT) [1] which describes the stoichiometric cation exchange reaction: CaX2(s) + MgCl2(aq) = MgX2(s) + CaCl2(aq), [2] where XT is the mole fraction of all species of a metal in the exchanger phase, MT (M = Ca or Mg) is the total molarity of all species of a metal in the aqueous solution phase, and X refers to 1 mol of negative charge on Camp Berteau montmorillonite clay. This mean value of KTV is only a little smaller than 1.0, the value that indicates no overall preference for calcium vs. magnesium on the clay. A common feature in all of the experimental studies cited above [except one, that of Macs and Cremers (1977)] is the use of a chloride background ionic medium in which to carry out the exchange reactions. Sposito et al. (1983) have shown recently that, in chloride media, Na-Ca and Na-Mg exchange reactions on montmorillonite (Wyoming bentonite) actually are ternary cation exchange reactions, involving Na, M, and the complex, MC1+ (M = Ca or Mg). According to the data presented by Sposito et al. (1983), as the charge fraction of Ca(II) increases beyond 0.3 in mixed NaCl/CaQ2 solutions reacting with a Na-montmorillonite, the species of Ca(II) that competes effectively with Na on the external surfaces of the quasicrystals that have formed is the complex, CaCl, not the free cation, Ca. In the case of Na-Mg exchange, the same process was shown to occur, but to a lesser extent because montmorillonite apparently exhibits a lesser affinity for MgCl than for CaCl (Sposito et al., 1983, Fig. 1). The significance of the results of Sposito et al. (1983) for cation selectivity in Na-Ca-Mg exchanges on montmorillonite is the possibility that the small overall preference of Ca(II) over Mg(II) that has been observed repeatedly in chloride background media may, in fact, be produced by the greater affinity of the species CaCl for the clay relative to the species MgCl+. It is possible that, without the presence of monovalent chloride complexes, montmorillonite would exhibit no differences in Ca(II) vs. Mg(II) selectivity in exchange reactions involving solely the free cations, Na, Ca, and Mg. In this paper, the first measurements of Ca 917 Published online September, 1983