Role of oxygen and superoxide radicals in promoting H2O2 production during VUV/UV radiation of water

Abstract

As a green and effective water treatment technology, vacuum UV (VUV) radiation could generate reactive species such as hydroxyl radicals (OH) and H2O2 via the homolysis and ionization of water. The formation mechanism of H2O2 in pure water was studied under irradiation of VUV/UV lamp emitting at 185 and 254 nm. The influence of dissolved oxygen was investigated. The production and the formation rate of H2O2 were promoted with enough dissolved oxygen, which demonstrated that the oxygen played an important role in H2O2 evolution. The UV254 radiation could induce the decomposing of H2O2 and had little effect on the synthesis of H2O2. Additionally, the alkaline condition and addition of some anions were unfavorable to the production of H2O2. Besides, 4-chloro-7-nitrobenzo-2-oxa-1,3-dizole (NBD-Cl) was used as the molecular probe to confirm the role of superoxide radicals (O2−/HO2) in the oxygen-containing condition. The possible formation mechanism of H2O2 was proposed. The H2O2 was produced only by the recombination of OH in the oxygen-free system, while related to O2−/HO2 besides the recombination of OH in the oxygen-containing system. This study brought new insights into the process that utilizing the in-situ generated H2O2 in VUV/UV water treatment for organic contaminants.