Modeling U(VI) sorption on bentonite under thermal and geochemical conditions relevant to high-level radioactive waste repositories

Ensuring the long-term safety of high-level radioactive waste repositories requires reliable buffer materials. Bentonite, primarily composed of montmorillonite, is widely used due to its high sorption capacity for radionuclides. However, the long-term sorption behavior of bentonite under evolving repository conditions is not yet fully understood. Previous studies have mainly focused on static laboratory conditions and have rarely considered coupled physicochemical changes over time. In this study, a coupled surface complexation and cation exchange model was integrated with illitization kinetics to quantitatively assess the long-term evolution of U(VI) sorption over 1,000 years. This approach enables a direct comparison of the relative effects of thermal, geochemical, and mineralogical factors on bentonite performance. The results showed that U(VI) sorption strongly depended on pH, with distinct shifts in dominant sorption mechanisms. Elevated temperatures, high cation concentrations, and the presence of carbonate significantly reduced sorption. Although illitization decreased the montmorillonite fraction under high-temperature, potassium-rich conditions, its overall effect on U(VI) sorption was limited, with less than a 10% reduction in over 1,000 years. These findings provide quantitative insight into the relative importance of the key processes and support the application of time-dependent geochemical models to assess bentonite buffer performance in deep geological disposal systems.