Study On The Performance And Mechanism Of As(Ⅲ) Oxidation By UV/TiO₂ And UV/Na₂SiO₃/H₂O₂ Systems Under Alkaline Condition

Arsenic（As）is a toxic contaminant widely distributed in groundwater and poses a serious threat to human health.Since As（Ⅲ）has higher migration capacity and toxicity than As（V）,treatment of arsenic pollution usually requires first oxidation of As（Ⅲ）to As（V）.Photocatalysis is a new catalytic technology driven by solar energy,and the photocatalytic oxidation of As（Ⅲ）in water by TiO₂ is a complex process,especially the mechanism of catalytic oxidation of As（Ⅲ）by TiO₂ in alkaline water and the presence of active species are still controversial.In this study,we reveal a novel ligand-metal charge transfer（LMCT）mechanism for the TiO₂ photocatalytic oxidation of As（Ⅲ）under alkaline conditions with the help of kinetic experiments and characterization analysis.The As（Ⅲ）photo-oxidation rate gradually increased with the increase of p H from 8.0 to 11.0,which indicated that the alkaline environment enhanced the activity of TiO₂ photocatalytic oxidation of As（Ⅲ）.Although h⁺,·OH,¹O₂,O₂^·-and-OOH were considered as the main active oxide species in the UV/TiO₂ system,we found that superoxide and peroxide formed on the TiO₂ surface were the main active species for the oxidation of As（Ⅲ）.The LMCT pathway contributes to the photocatalytic oxidation of As（Ⅲ）under alkaline conditions by（i）adsorption of As（Ⅲ）on the TiO₂ surface and subsequent formation of As（Ⅲ）-O-Ti（IV）surface complexes with accelerated electron transfer;（ii）direct electron reduction of dissolved oxygen on the TiO₂ surface to superoxide,followed by extraction of As（Ⅲ）or H₂O H to form peroxides;（iii）the adsorbed peroxides extracts H from the hydroxyl group of As（Ⅲ）to form H₂O₂,accompanied by As（Ⅲ）oxidation.This LMCT mechanism enriches our understanding of the photocatalytic oxidation of As（Ⅲ）and other contaminants rich in-OH.In addition,Na₂SiO₃ enhances the performance of the UV/TiO₂ system for the oxidation of As（Ⅲ）.k_obs value of the UV/TiO₂/Na₂SiO₃ system for the oxidation of As（Ⅲ）at p H 8.0 is 1.57×10^-2 min^-1.The oxidation rate of As（Ⅲ）was accelerated by the addition of H₂O₂ to the UV/Na₂SiO₃ system,and the oxidation rate constant of As（Ⅲ）was 2.64×10^-2 min^-1 after 120 min.The dark state reaction of Na₂SiO₃/H₂O₂system resulted in 90.80%oxidation of As（Ⅲ）after 30 min,meaning that the co-oxidation of As（Ⅲ）by Na₂SiO₃ and H₂O₂ could achieve better results.In the UV/Na₂SiO₃/H₂O₂ system,H₂O₂ is less stable and more likely to decompose to produce O₂^·-,which in turn oxidizes As（Ⅲ）.Scavenger experiments and characterization analysis confirmed that O₂^·-was the main active species for the oxidation of As（Ⅲ）in the UV/Na₂SiO₃/H₂O₂ system.This study provides a new idea for the oxidation of As（Ⅲ）and other pollutants rich in-OH.