Sodium/Calcium Exchange in Montmorillonite and Illite Suspensions

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 Ca 3+ 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.