The redox behavior of uranium on Beishan granite: Effect of Fe2+ and Fe3+ content

Abstract The Beishan granitic area in Gansu Province is a site with the greatest potential for a repository of high-level radioactive waste (HLW) in China. In this study, the redox behavior of uranium on Beishan granite was investigated at pH values from ~4.4 to ~9.2. Due to the presence of Fe2+-containing fluorannite, results showed that U(VI) was partially reduced by the granites from boreholes 2 (486 m) and 28 (670 m) at a relatively low initial pH whether Na2CO3/NaCl or native groundwater was used as a background electrolyte. Partial oxidation of UO2 was observed when UO2 contacted Beishan granite directly. Therefore, this incomplete reduction of U(VI) was mainly attributed to minor Fe3+ that was either originally contained in the granite or generated during U(VI) reduction. Consequently, aliovalent oxides (e.g., U3O8, U3O7, U4O9, etc.) should be the thermodynamically stable phase in Beishan granite. A mechanism involving the dissolution of Fe2+ from the granite structure followed by interfacial adsorption/reaction was proposed for the U(VI) reduction. This study demonstrates that Beishan granite has a good reducing capacity, which is suitable for the immobilization of redox-sensitive radionuclides. However, potential oxidation of spent fuel by Fe3+ in the granite should also been taken into account.