S-Scheme Heterojunction D-ZnO@FexOy Derived From ZIF-8 Organic Framework for Photoassisted Reduction of U(Ⅵ)

Abstract

With the rapid development of nuclear energy, uranium as the main element of nuclear energy production, its source has been widely explored. At present, uranium resources mainly come from uranium mining, and a lot of radioactive uranium containing wastewater is produced in the process of uranium mining. Until now, the treatment technology of uranium mine wastewater is still in the research and development stage in China, which leads to the storage of a large amount of uranium mine wastewater and the discharge of nuclear waste liquid, which poses a threat to human health and the environment due to the comprehensive effect of chemical toxicity and radiation toxicity. On the other hand, other metal cations in uranium wastewater, such as Ca2+ and Mg2+, compete with U(Ⅵ) for active sites, making it difficult for most treatment methods to effectively recover uranium resources from uranium wastewater. Therefore, how to effectively treat radioactive uranium mine wastewater is full of challenges. Photocatalysis is a kind of green environmental protection technology with natural selectivity to non-variable valence ions, which can precipitate the soluble U(Ⅵ) in wastewater. The photocatalysis technology is expected to overcome the competition between metal cations and uranium for active sites, and realize the efficient extraction of U(Ⅵ) in uranium mine wastewater. In this study, ZIF-8-derived Zinc@iron oxide(D-ZnO@FexOy) heterostructures were prepared by impregnation of metal salt solution and high-temperature pyrolysis using ZIF-8 as a precursor for photoassisted reduction of U(Ⅵ) in uranium ore wastewater. The electron transport mode of D-ZnO@FexOy in S-scheme heterojunction is deduced by calculating the work function and band structure. Under photoexcitation, the direction of electron flow changes, the useless electrons and holes recombine at the interface, and the electrons and holes with strong redox ability are retained and participate in the photocatalytic reaction. Compared with D-ZnO and FexOy, this kind of spatially separated photogenerated electrons and holes have stronger redox ability and longer electron lifetime. Meanwhile, the internal electric field formed at the heterojunction interface can effectively enhance the directional electron migration, providing a strong power for promoting the photoassisted reduction of uranium. Therefore, compared with iron oxide(FexOy) and ZIF-8-derived ZnO(D-ZnO), the S-type heterostructure D-ZnO@FexOy exhibits stronger photoassisted uranium reduction ability(>95%) in the presence of multiple competing ions, and the material has good anti-interference and stability. High U(Ⅵ) removal performance(above 86%) is maintained after 5 cycles. The mechanism study shows that hydroxyl radical and superoxide radical are the main active species of photocatalytic reduction of U(Ⅵ). U(Ⅵ) is deposited on the surface of the material in the form of (UO2)O2•2H2O crystals. This study provides a valuable way to explore the photocatalyst with high efficiency photoelectron separation to reduce U(Ⅵ).

Keywords

• s-scheme heterojunction
• photocatalysis
• zif-8
• reduction of uranium