Redox Conversion of Arsenite and Nitrate in the UV/Quinone Systems

Whether superoxide radical anion (O₂^•-) was a key reactive species in the oxidation of arsenite (As(III)) in photochemical processes has long been a controversial issue. With hydroquinone (BQH₂) and 1,4-benzoquinone (BQ) as redox mediators, the photochemical oxidation of As(III) and reduction of nitrate (NO₃^-) was carefully investigated. O₂^•-, singlet oxygen (¹O₂), H₂O₂, and semiquinone radical (BQH^•) were all possible reactive species in the irradiated system. However, since the formation of As(IV) is a necessary step in the oxidation of As(III), taking the standard reduction potentials into account, the reactions between the above species and As(III) were thermodynamically unfavorable. On the basis of radical scavenging experiments, hydroxyl radical (^•OH) was proved as the key species that led to the oxidation of As(III) in the UV/BQH₂ system. It should be noted that the ^•OH radicals were generated from the photolysis of H₂O₂, which came from the disproportionation of O₂^•- and the reaction of O₂^•- with BQH₂. Both the photoejected e_aq^- from ¹(BQH₂)* and the direct electron transfer with ³(BQH₂)* contributed to the reduction of NO₃^- in the UV/BQH₂ process. No synergistic effect was observed in the redox conversion of As(III) and NO₃^-, further demonstrating that the role of BQH^• was negligible in the studied systems. The results here are helpful for a better understanding of the photochemical behaviors of quinones in the aquatic environment.