Sorption Mechanisms of Se(-II) on Biotite in Reducing Conditions by DFT Simulation

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Jieci Yang
Satoru Tsushima

Abstract

Crystalline rock is considered a suitable potential host rock for a deep geological repository (DGR). Biotite, as a common accessory mineral found in crystalline rocks such as granite and tonalite, has been identified as an important mineral for selenium (Se) sorption. Se-79, being an element of importance in the safety assessment of the DGR for used nuclear fuel waste, has its primary oxidation state as Se(-II) under the reducing conditions expected at repository depths. The present study therefore aims to explore the sorption mechanisms of Se(-II) on biotite under reducing conditions.

Due to the low solubility of Se under reducing conditions, traditional spectrometry methods are not applicable for studying the sorption mechanisms of Se(-II). Instead, Density Functional Theory (DFT) calculation is employed to investigate the sorption mechanisms of Se(-II) on biotite by investigating the inner-sphere complex and outer-sphere complex reactions as the main adsorption mechanisms. It is found that the outer-sphere complexes are the primary sorption mechanism, while the inner-sphere complexes also contribute to the sorption process. The result of the DFT calculation compares well with the sorption experimental data and the surface complexation model.

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