Uranium typically occurs in the hexavalent form U(Ⅵ) as the mobile, aqueous uranyl ion in radioactive wastewater. The reduction of soluble U(Ⅵ) to insoluble U(Ⅳ) oxide is an effective approach to eliminate uranium pollution. Herein, the metal organic framework material MIL-53(Fe) was successfully synthesized by a solvothermal method, and its application as photocatalyst in the reduction of U(Ⅵ) under visible light was studied in detail using various types and concentrations of hole trapping agents, solution pH values, and catalyst dosages. The results show that the use of formic acid as the hole trapping agent greatly accelerates the catalytic reaction rate by improving the charge separation efficiency. When 1 mmol·L-1 formic acid was used and the initial concentration of U(Ⅵ) was 50 mg·L-1, MIL-53(Fe) achieved a high reduction rate of 80% after 2 hours of visible light exposure. Photoelectron spectroscopy (XPS) clearly suggested that U(Ⅳ) was generated during the reaction process. A possible mechanism is that formic acid reacted with the photogenerated hole, resulting in the formation of·COO-, which can reduce U(Ⅵ) to U(Ⅳ). Accordingly, the elimination of the uranium pollution from wastewater was achieved.
Keywords: MIL-53(Fe); U (Ⅵ); hole trapping agent; metal organic framework; photocatalytic reduction.