Sorption of Se(IV) on Gaomiaozi bentonite: Batch and spectroscopic studies

Se-79 is regarded as one of the scarce radionuclides with a protracted half-life. Exploring the sorption behaviors of selenium on bentonite is critical for the safe disposal of radioactive waste. The sorption of Se(IV) on Gaomiaozi bentonite at a low concentration (7 × 10^-8 M) was systematically studied to set reliable parameters in this work by batch sorption method and spectroscopic techniques. The factor of pH, ionic strength, initial Se(IV) concentrations, solid-to-liquid ratio and the addition of uranyl ions were investigated. The sorption results indicate that Gaomiaozi bentonite exhibited remarkable sorption capacity towards selenium under conditions of acidic pH and high ionic strength. The XPS spectra reveal that Se(IV) remains stable on bentonite surface and there is no redox reaction observed between Se(IV) and bentonite. The possible sorption mechanism is interpreted based on species calculation and photon correlation spectroscopy. In detail, the positively charged electric double layer surrounding the surface of the lamellae will "spill over" when the ionic strength is low, thereby encircling the edge site area of bentonite. Therefore, the selenium anion is difficult to enter, leading to a low sorption capacity. The aggregation state of bentonite particles does not primarily govern the sorption behavior of Se(IV). Overall, this work may provide valuable information on sorption mechanism of Se(IV) at the bentonite/water interface and help us understand the geochemical behaviors of Se(IV) in repository environments.