A THERMODYNAMIC SURFACE MODEL FOR CESIUM SORPTION ON BENTONITE

Caesium sorption on Wyoming bentonite MX-80 has been studied in soluti ons of NaCl, KCl, MgCl2, CaCl2, NaNO3 and Ca(NO3)(2) of concentrations varying between 0.025 and 1 mol/L, as well as in a weakly saline (I = 0.004 ml/L) and a strongly saline (I = 0.46 mol/L) natural groundwate r. These experiments have been used to derive a thermodynamic model fo r the interaction of caesium with the bentonite surface in accordance with a surface chemical model, including acid/base reactions developed recently for montmorillonite. The sorption behaviour of caesium on be ntonite can be described, within the experimental and model uncertaint ies, in terms of a one-site ion exchange model. The ion exchange const ant obtained for the reaction NaX + Cs+ reversible arrow CsX + Na+ (wh ere X represents the ion exchange sites on montmorillonite) is log(10) K-ex(0) = 1.6. Impurities in the bentonite, influencing the concentra tions of competing cations, such as Na+, K+, Mg2+ and Ca2+, have a cru cial impact on the sorption of caesium. This impact can be adequately quantified with the present model. The model predictions compare well with sorption data published in the open literature on both Wyoming be ntonite MX-80 and other types of bentonite. Distribution coefficients from the literature obtained from both batch and diffusion experiments and varying over four orders of magnitude are reproduced and explaine d successfully by the model.