Sorption, desorption and exchange of cesium on glaciofluvial deposits

Distribution ratios and isotherms for sorption, desorption and isotope-exchange of cesium (labelled with Cs-137) were measured on grain size fractions (<2 mm) of quaternary glaciofluvial deposits. Sediment materials from two locations within Switzerland and synthetic groundwaters of different compositions were used. The investigated concentration range for cesium was 10-11-10-3 M. Cesium introduced into the system with the solid phase may influence the measurements at the lowest concentrations. Depending on the experimental conditions, the distribution ratios vary between 3 and 30'000 ml/g. The isotherms are nonlinear. Normalization of the cesium concentration in the solid with the cation-exchange capacity leads to nearly identical isotherms for all size fractions of the two geographic locations. Desorption and exchange are retarded at the higher cesium concentrations. This can be explained by structural changes in clay minerals which dominate the sorption of cesium on this material. Variations in the composition of the groundwater influence the sorption of cesium only slightly; potassium and hydrogen ions are the main competitors. (orig.)