A thermodynamic surface model for caesium sorption on bentonite

Caesium sorption on Wyoming bentonite MX-80 has been studied in solutions 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 groundwater. These experiments have been used to derive a thermodynamic model for 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 bentonite can be described, within the experimental and model uncertainties, in terms of a one-site ion exchange model. The ion exchange constant obtained for the reaction NaX+Cs⁺CsX+Na⁺ (where X represents the ion exchange sites on montmorillonite) is log₁₀K⁰_ex=1.6. Impurities in the bentonite, influencing the concentrations of competing cations, such as Na⁺, K⁺, Mg²⁺ and Ca²⁺, have a crucial 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 bentonite 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 explained successfully by the model.