Sorption of selenite on Tamusu clay in simulated groundwater with high salinity under aerobic/anaerobic conditions

Highlights: • Selenite sorption was investigated in simulated groundwater with high salinity. • K_d values in aerobic conditions were higher than those in anaerobic conditions. • The change trend of the K_d values was not affected by oxygen. • Selenite sorption was promoted at alkaline conditions. - Abstract: The sorption behavior of selenite onto the Tamusu clay from a preselected high-level radioactive waste disposal site in Inner Mongolia, China, was first investigated in simulated groundwater with high salinity by batch sorption experiments under aerobic/anaerobic conditions. The results demonstrated that the K_d values rapidly decreased and then remained steady in the pH range of 2.0–8.0. However, selenite sorption was promoted when pH exceeded 8.0, which might be attributed to the coprecipitation between Ca²⁺ and SeO₃²⁻. Besides, the change trend of the K_d values as functions of various parameters was not affected by oxygen. The sorption kinetics and isotherms could be well fitted by the pseudo-second-order kinetic model and the Freundlich model for both aerobic and anaerobic conditions, and the calculated thermodynamic parameters (△G and △H) suggested that the selenite sorption process was a spontaneous and endothermic process. Additionally, the XPS results revealed that Se(IV) could be reduced to Se (0) only in anaerobic conditions and that the different amounts of Fe on the clay surface led to the discrepancy of the Se(IV) K_d values under aerobic and anaerobic conditions even in high-salt simulated groundwater. Overall, our findings in this study are significant in regards to the retardation of selenite on the host rock under high salinity conditions.