Sorption behavior of selenide on montmorillonite

Batch sorption experiments were performed to investigate the sorption mechanism of Se on montmorillonite under reducing conditions in deep geological environments. Based on E_h–pH diagrams and ultraviolet–visible spectra, Se was dissolved as selenide (Se(–II)) anions under the experimental conditions. The distribution coefficients (K_d; m³ kg^?1) of Se(–II) indicated ionic strength independence and slight pH dependence. The K_d values of Se(–II) were higher than those of Se(IV), which also exists as an anionic species. X-ray absorption near edge spectroscopy showed that the oxidation state of Se-sorbed on montmorillonite was zero even though selenide remained in the solution. These results suggest that Se(–II) was oxidized and precipitated on the montmorillonite surface. Therefore, it is implied that a redox reaction on the montmorillonite surface contributed to high K_d values for Se(–II).