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Sodium/Calcium Exchange in Montmorillonite and Illite Suspensions¹

ABSTRACT

The effect of salt concentration and exchanger composition on Na/Ca exchange in montmorillonite and illite suspensions (0.02 to 0.08 g clay/g water) was studied, where the equivalent fraction of exchangeable Ca, E_Ca, ranged from 0.6 to 1. A theoretical evaluation based on double-layer theory for montmorillonite indicates that the formation of tactoids and internal surfaces at the Ca³⁺ end of the exchange isotherm can cause the Vanselow selectivity coefficient, K_V, to depend on exchangeable cation composition and total electrolyte concentration. Whereas for montmorillonite, K_V increased with E_Ca (0.6 < E_Ca < 1), as predicted for internal surfaces, the reverse was true for illite, where external surfaces predominate. Likewise, for a given Na adsorption ratio, dilution of the equilibrium solution for illite (0.6 < E_Ca < 1) and for montmorillonite (E_Ca 0.6) decreased K_V as predicted for external surfaces.

The average Gapon selectivity coefficient for the montmorillonite (0.011) and illite clays (0.023) with E_Ca 0.8, suggests that the value for soils, (0.0148) reflects their mixed mineralogy.

¹ Contribution from U. S. Salinity Laboratory, USDA, SEA-AR, 4500 Glenwood Drive, Riverside, CA 92501.

² Visiting Soil Scientist and Soil Scientists, respectively. Dr. Shainberg's permanent address is the Institute of Soil and Water, The Volcani Center, ARO, Bet-Dagan, Israel.

Received for publication February 5, 1980. Accepted for publication June 19, 1980.

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