Selenium isotope fractionation during adsorption onto montmorillonite and kaolinite

Adsorption is an important geochemical process constraining the global cycling of selenium (Se) in the environment. However, Se isotope fractionation during adsorption onto clay minerals has been rarely reported. In this study, Se isotope fractionation during adsorption onto montmorillonite and kaolinite was investigated by a combination of adsorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Results showed that a small adsorption and negligible isotope fractionation were observed during Se(VI) adsorption on montmorillonite and kaolinite at pH 4.5. By contrast, Se isotope fractionations (Δ^82/76Se_{dissolved-adsorbed}) during the adsorption of Se(IV) on montmorillonite and kaolinite were ≤ 0.23‰, suggesting that lighter Se(IV) isotopes are preferentially adsorbed onto clay minerals. In addition, different effect of ionic strength on Se(IV) isotope fractionation was found between kaolinite and montmorillonite, which is likely ascribed to distinct surface charge and structure between the two clay minerals. These little or no Se isotope fractionations could be related to the fact that Se oxyanions are mainly adsorbed on montmorillonite and kaolinite via the outer-sphere complexation, as revealed by Se K-edge EXAFS analysis. The findings from this study would help further identify and constrain the key geochemical processes causing Se isotope variations, providing a foundation to develop Se isotope as a proxy for biogeochemical cycling of Se in the natural environment.