Adsorption behavior of Se(Ⅳ) and Se(Ⅵ) on sodium bentonite

Abstract Se is one of the radionuclides focused on in the safety evaluation of deep geological disposal repositories for high-level wastes, and elements with different valence states have different chemical properties and adsorption behaviors, which directly affects their adsorption efficiency and mechanism on bentonite. This study investigated the adsorption behavior of selenium in different valence states (Se(IV) and Se(VI)) on sodium bentonite through batch static adsorption experiments to assess its potential application in deep geological disposal repositories. The results indicate that the adsorption distribution coefficients ( K d ) of Se(IV) and Se(VI) on sodium-based bentonite decrease first, then increase, and finally decrease again with increasing pH. However, the adsorption of Se(IV) is less affected by ionic strength, while the adsorption of Se(VI) decreases with increasing ionic strength, temperature, and initial concentration. The adsorption of Se(IV) is an entropy-increasing, non-spontaneous, and endothermic chemical process influenced by environmental pH, which can be described by the Langmuir isotherm model and the pseudo-second-order kinetic model; the adsorption of Se(VI) is a pH-dependent, entropy-decreasing, spontaneous, and exothermic physical process, which can be described by the Langmuir isotherm model and the pseudo-first-order kinetic model. Both exhibit weak competitive adsorption on sodium-based bentonite, and the electrostatic repulsive forces between the negatively charged Se species and the sodium-based bentonite surface inhibit adsorption. FTIR and XRD spectral analyses show no significant changes in functional group characteristics before and after adsorption. The research results are of great significance for understanding the behavior of nuclides during geological migration.