Cesium sorption on mylonite

As part of the laboratory support program for the field migration experiment at the Grimsel Test Site (GTS) in the Swiss Alps, the sorption behaviour of cesium on Grimsel mylonite was studied. Batch sorption experiments were carried out in N₂ atmosphere (<3 ppm O₂). The adsorption isotherms were reversible and non-linear for cesium concentrations of between 3.2·10^–8 and 5.0·10^–4M. Two different sites appear to be involved in sorption depending on whether Cs loading was high (10^–6–10^–3 meq/g) or low (10^–7–10^–6 meq/g). At low Cs loadings adsorption was considered to occur mainly at the crystal edges of mica particles. Selectivity coefficients for exchange between cesium and potassium were calculated for different Cs loadings. It was suggestd that by varying the potassium concentration of the solution and by making some assumptions, a K_d value for cesium at the migration site could be estimated. Data were fitted to both Freundlich and Dubinin-Radushkevich isotherms. The empirical Freundlich parameters enabled a site distribution function to be calculated and a mean energy of sorption of about 12 kJ/mol was found using Dubinin-Radushkevich isotherms approach.