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 such as GEOCHEM, can be used to simulate simultaneous ionexchange, 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 of ideal reactions involving charged surfaces and complexing groups 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 earths 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 dominated by a process of stoichiometric replacement of ions on the surfaces of a material of fixed charge. However, with clays this reaction may be accompanied by simultaneous anion uptake and hydrolysis, with the consequence that the effects of these additional reactions are embodied in the values of solid-phase activity coefficients. This feature may be undesirable, and it is the aim of this paper to offer a simple description of clay/electrolyte interactions which formally acknowledges the existence of multiple reactions. The approach developed here is compatible with existing multi-component equilibrium models (Nordstrom et aL, 1979) based on the principle of ion association. In essence, the methods herein involve transforming classical ion-exchange constants into sets of formation constants which can be used in the above speciation models. A preliminary attempt has been made to apply these techniques to the properties of Wyoming bentonite.