Exploring sorption behaviors of Se(IV) and Se(VI) on Beishan granite: Batch, ATR-FTIR, and XPS investigations

Abstract

As the important anions in the high-level radioactive waste (HLRW), and the sorption behaviors of Se(VI) and Se(IV) on Beishan granite are important to the safe evaluation and performance assessment of the deep geological repository for HLRW in China. In this study, the sorption behaviors of Se(IV) and Se(VI) on Beishan granite were investigated under different environmental factors combining batch and spectroscopic approaches, such as X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transformed infrared (ATR-FTIR). The results showed that Se(IV) sorption on Beishan granite was much higher than that of Se(VI) mainly due to their molecule charge densities and speciation in solution. The sorption of Se(IV) was independent of ionic strength, suggesting that the inner-sphere complexes (ISCs) was dominant for Se(IV) sorption on granite. However, Se(VI) sorption on Beishan granite was strongly dependent on ionic strength, which indicated that ion exchange and outer-sphere complexes (OSCs) dominated Se(VI) sorption. ATR-FTIR confirmed that Se(IV) and humic acid (HA) could form a soluble complex of HA-Se(IV) in solution. Therefore, under low pH and HA concentration conditions, the presence of HA could enhance Se(IV) sorption; while inhibited Se(IV) sorption on granite to some extent under high pH and HA concentration conditions. In case of Se(VI), the presence of HA greatly reduced the surface charge density of the Beishan granite, which in turn enhanced the electrostatic repulsion of Se(VI) and granite surfaces, thereby inhibiting Se(VI) sorption on granite surface. When Se(IV) coexisted with Eu (III), the sorption of Se(IV) on granite could be enhanced to a large extent, which might be due to the formation of Eu2(SeO3)3 complex with Se(IV) under alkaline condition (consistent with ATR-FTIR), or surface charge modifications of Beishan granite caused by Eu(III) sorption. Owing to the ISCs of Se(IV) on granite, the presence of anions almost no obvious effect on Se(IV) sorption except for SO32− and HPO42− in terms of the competitive sorption, which inhibited Se(IV) sorption to some extent. It was interesting that CO32− could enhance Se(IV) sorption to a large extent, however the mechanism was still not clear. Furthermore, XPS further confirmed that Se(IV) sorption on granite was dominated by Fe(II) and Fe(III), and the host mineral was biotite.