The Chemical Modelling of Clay/Electrolyte Interactions for Montmorillonite

A B S T R A C T: A study of the ion-exchange properties of montmorillonite has been performed in order to facilitate computer predictions of the chemical properties of natural fluids and mineral assemblies. Clay/electrolyte interactions can be described using a technique based on the concept of hypothetical surface complex formation. This technique, which is compatible with ion-association models uch as GEOCHEM, can be used to simulate simultaneous ion-exchange, hydrolysis of clay edges and anion adsorption on clay surfaces. Effects such as variable cation-exchange capacity and compositionally dependent exchange constants, normally indicating non-ideal behaviour, can be simulated using different combinations ofideal reactions involving charged surfaces and complexing roups representing clay edges. The modelling procedures are flexible and thermodynamically self-consistent. The techniques were appliexl to data on the ion-exchange characteristics of Wyoming bentonite to yield thermodynamic data for the reactivity of this clay with alkali metals, alkaline arths and a range of firsbrow transition metals at 25~ It is common in the application of the classical thermodynamic treatment of ion-exchange (Argersinger, 1950; Gaines & Thomas, 1953; Hogfeld, 1953) to assume that reactivity is