DFT study of Se(-II) sorption on biotite in reducing conditions

Abstract Crystalline rock is considered a potential host rock for a deep geological repository (DGR) for storing nuclear waste. Biotite is a common accessory mineral found in crystalline rocks such as granite and tonalite that plays an important role in the absorption of selenium (Se). In particular, the radioisotope Se-79 is widely present in spent nuclear fuel, and it is crucial for evaluating the suitability of a DGR for storing nuclear waste. In the anticipated reducing conditions of a DGR, the prevalent oxidation state would be Se (-II). However, the low solubility of Se under reducing conditions means that traditional spectrometry methods are not effective at studying the sorption mechanisms of Se (-II). Therefore, this study investigated the sorption mechanisms of Se (-II) on biotite under reducing conditions by using density functional theory (DFT) calculations to investigate the inner-sphere complex and outer-sphere complex reactions. Both complexes were found to contribute to sorption of Se (-II) on biotite with the outer-sphere complexes being the primary sorption mechanism under the naturally neutral groundwater conditions. The results of the DFT calculations were consistent with the sorption experimental data and the results of the surface complexation modeling, which further confirmed the significance of the inner-sphere and outer-sphere complexes to the sorption of Se (-II) on biotite under a wider range of pH conditions.