An Interpretation of Cation Selectivity Variations in M+—M+ Exchange on Clays1

Murray B. McBride
Department of Agronomy, Cornell University, Ithaca, New York 14853
1 Agronomy Paper No. 1331.

Abstract: The decreasing preference of montmorillonite for K+ relative to Na+ as the clay adsorbs increasing amounts of K+ is shown to be the general rule for the exchange of strongly hydrating ions by weakly hydrating ions. Variability in the mass-action selectivity coefficient is interpreted in terms of a composition-dependent surface entropy, which is a function of the chemical properties of the exchanging ion as well as the nature of the adsorption sites. The generally used mass-action form of exchange equation may only be applicable to exchange systems in which both ions have solution-like mobility at the exchanger surface. It is suggested that experimental variables such as ionic strength can greatly influence the degree of fit of data to a given ion-exchange equation.

Key Words: Adsorption • Cation exchange • Montmorillonite • Selectivity • Statistical mechanics

Clays and Clay Minerals; December 1979 v. 27; no. 6; p. 417-422; DOI: 10.1346/CCMN.1979.0270604
© 1979, The Clay Minerals Society
Clay Minerals Society (www.clays.org)