TiO2-based catalysts for photocatalytic reduction of aqueous oxyanions: State-of-the-art and future prospects

Nowadays, an increasing discharge of oxyanions to the natural environment has been attracting worldwide attention. TiO2-based photocatalysis is regarded as one of the most promising technologies for the conversion of toxic oxyanions (such as chromate, nitrate, nitrite, bromate, perchlorate and selenate) to harmless and/or less toxic substances in contaminated waters. Various types of TiO2-based catalysts have been developed, and each of them exhibits its own advantages in catalytic reduction of oxyanions. However, the application of these nanostructured TiO2 in real water bodies remains a challenge, with limitations associated with sunlight harvesting abilities, production costs, reuse stability and exposure risks. Herein, we aim to present a critical review on reported TiO2-based photocatalytic reduction of aqueous oxyanions, provide a comprehensive understanding of the possible reaction pathways of formed active species, and evaluate the reduction performance of different types of TiO2-based catalysts. In addition, the impact of operating parameters (such as solution pH, temperature, dissolved oxygen and coexisting substances) on catalytic reduction performance is discussed. Furthermore, the perspectives of TiO2-based photocatalytic reduction of oxyanions are also proposed.