Comparative study on the reductive immobilization of Se(IV) by Beishan granite and Tamusu claystone

Abstract

Understanding the retardation of 79Se by the host rock is essential for the safety assessment of the high-level radioactive waste (HLRW) repository and thus the final site-selection process. In this work, we performed a comparative study on the interaction of Se(IV) with Beishan granite and Tamusu claystone, and multiple characterization techniques were used to identify the speciation of Se and to unravel the underlying mechanism. Results showed that Se(IV) can be reduced to Se(0) by the Beishan granite from borehole 16 that contains Fe(II)-bearing fluorannite, and the removal efficiency of Se(IV) is more favorable at acidic condition. We proposed that release of the structure Fe2+ in granite, followed by surface adsorption and reaction could be responsible for the Se(IV) reduction by this granite. In addition to Se(0) product, FeSe2 was also observed for Se(IV) reacted with the granite from borehole 28. This is the first time observed the formation of FeSe2 on natural Fe(II)-containing minerals, which is mainly ascribed to the simultaneous occurrence of aqueous sulfide dissolved from the ground granite powder. The reactive sulfide might be generated from the breakage of Fe(S)–S bonds of iron sulfides (e.g., pyrite) contained in the granite during the anoxic grinding process, and it could mainly account for the Se(IV) reduction by the granite from borehole 28. Due to the presence of pyrite, the Tamusu claystone from borehole Tzk2 was effective to remove aqueous Se(IV) via the formation of Se(0). Conversely, the Se(IV) removal efficiency was much lower for the claystone from borehole Tzk1 that did not contain pyrite, although there was a considerable amount of Fe(II)-bearing fluorannite. Hence, the Fe2+ contained in Tamusu claystone was less active for Se(IV) reduction in comparison with that in Beishan granite from borehole 16. The findings of this study provide important data for the safety assessment of China's future HLRW repository, but also improve our understanding of the geochemical behavior of selenium in the presence of Fe(II)- and sulfide-bearing minerals in the relevant anoxic environments.