Adsorption of cesium on Czech smectite-rich clays—A comparative study

The adsorption of cesium on various smectite-rich clays was studied as a function of the initial concentration of cesium and in the presence of competitive ions (Na+, K+ and Ca2+) using the radiotracer method, examining the effects of the clay structure and comparing the behaviour of bentonites with that of sedimentary clays. The experiments were performed under laboratory conditions, at 22–25 °C, in ambient atmosphere and using synthetic average granite groundwater as the liquid phase. Reference smectites (STx−1, SAz−1), illite (IMt−1) and kaolinite (KGa−2), three Czech smectite-rich clays (bentonite from Rokle, sedimentary clays from Skalná and Maršov) and three Spanish bentonites from Cabo de Gata were used as the natural sorbents. The adsorption data were fitted by the Freundlich isotherm. Smectites exhibited the highest values of the adsorption capacity and the highest affinity for cesium within the studied concentration range, 10− 6 to 10− 1 M. The Kd values disclosed two different trends—a relatively constant adsorption rate at Cs concentrations below 10− 2 M (typical for smectites–bentonites from Spain), or a continuous decrease in the adsorption rate over the whole concentration range (“non-smectites”, such as illite and kaolinite-sedimentary clays (Skalná, Maršov). Therefore, the higher contents of illite and kaolinite (48 wt.% in Skalná and 25 wt.% in Maršov) significantly influence the cesium adsorption trend and the cation exchange capacities of the bulk material. Surprisingly, the Rokle bentonite, containing more than 60 wt.% of expandable clay structures, shows the “non-smectite” trend. This can be explained by the possible presence of expandable vermiculite in addition to smectite. The cesium adsorption behaviour of vermiculite is considered similar to that of illite. For trace cesium concentrations (10− 6 M), the major competitive ion is K+, followed by Ca2+ and finally by Na+. Bentonites are more affected than sedimentary clays containing illite—this is due to the ion exchange mechanism predominating in the adsorption on smectites, compared to specific selective adsorption on illite and mica-type clay minerals.