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

Whether superoxide radical anion (O-2(center dot-)) 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 1,4benzoquinone (BQ) as redox mediators, the photochemical oxidation of As(III) and reduction of nitrate (NO3-) was carefully investigated. O-2(center dot-) singlet oxygen (O-1(2)), H2O2, and semiquinone radical (BQH(center dot)) 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)-O-center dot) was proved as the key species that led to the oxidation of As(III) in the UV/BQH(2) system. It should be noted that the (OH)-O-center dot radicals were generated from the photolysis of H2O2, which came from the disproportionation of O-2(center dot-) and the reaction of O-2(center dot-) with BQH(2). Both the photoejected e(aq)(-) from (1)(BQH(2))* and the direct electron transfer with (3)(BQH(2))* contributed to the reduction of NO3- in the UV/BQH(2) process. No synergistic effect was observed in the redox conversion of As(III) and NO3-, further demonstrating that the role of BQH(center dot) 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.