A critical review on photocatalytic reductive conversion of uranium by CdS-based catalysts

The conversion of soluble hexavalent uranium (U(VI)) to insoluble tetravalent uranium (U(IV)) facilitates uranium recovery. However, current efficient uranium conversion technologies face challenges such as high energy consumption and/or limited selectivity. Photocatalytic technology has emerged as one of the most effective methods for U(VI)/U(IV) conversion, but the necessity of sacrificial agents restrains its real application. CdS has gained significant attention in recent years since CdS-based catalysts avoid sacrificial agent usage to some extent, benefiting from their advantageous conduction band position and narrow bandgap. However, their application remains debated due to potential photocorrosion during treatment processes. The current work conducts a critical review on photocatalytic reductive conversion of U(VI) by CdS-based catalysts. Properties and design strategies of CdS-based materials are discussed based on the structural and physicochemical properties of CdS, including element doping, defect engineering, hetero/homojunction, biohybrid and morphology control. The electron transfer mechanisms and their impact on U(VI) reduction is also elucidated, together with the analysis of intermediates. The effects of various factors, such as oxygen environment, coexisting metal ions, pH, biological contamination, electron sacrificial agents and photocorrosion on the reduction of U(VI) by CdS-based photocatalysts are highlighted. Then, available strategies to overcome complex environmental challenges are summarized. Finally, the challenges and prospects of CdS-based materials for its future applications are discussed.