Strategies for boosting the photocatalytic reduction of toxic metal ions: Progress and prospects

With rapid industrialization and a growing population, toxic metal ions, such as Cr(VI), U(VI), Hg(II), U(VI), and Ni(II), are inevitably discharged into aquatic systems, seriously threatening human health and environmental sustainability. Among the various wastewater treatment technologies, the photocatalytic reduction of metal ions is considered a promising metal ion removal method due to its flexible operation, light energy utilization, no (or less) additional chemicals, and no sludge production. Many photocatalysts have been developed to reduce metal ions, but their practical feasibility is often limited by their low visible/near-infrared light absorption ability and high recombination rate of photogenerated charge carriers. In light of such limitations, various significant modification strategies, including heterojunction construction (e.g., type-I heterojunction, type-II heterojunction, p-n heterojunction, Z-scheme heterojunction, S-scheme heterojunction, and Schottky heterojunction), vacancy engineering (e.g., oxygen vacancies, sulfur vacancies, and bismuth vacancies), facet engineering, and the introduction of an oxalic acid/oxalate, are systematically discussed in this review. To enhance environmental sustainability, economic factors, and practicability, it is worth developing efficient solar-light-responsive photocatalysts and continuous-flow reactors in the future. Overall, this review aims to provide a thorough understanding and rational guidance for the design of semiconductor photocatalysts for the photocatalytic reduction of toxic metal ions.