Degradation of UV-P mediated by hydroxyl radical, sulfate radical and singlet oxygen in aquatic solution: DFT and experimental studies

Abstract

2-(2′-hydroxy-5′-methylphenyl) benzotriazole (UV–P) is a type of emerging persistent organic pollutant that is reported harmful to organisms. However, its degradation mechanisms and transformation behaviors in aquatic environments are not yet clear, which are significant for better understanding its environmental fate and potential toxicological impacts. In present work, the degradation mechanisms, kinetics, half-life times and eco-toxicity assessment of UV-P initiated by hydroxyl radical (•OH), sulfate radical (SO4•‾), and singlet oxygen (1O2) are systematically studied using density functional theory (DFT) and experimental methods. The initiated reaction results show that benzene ring of UV-P is vulnerable to attack by •OH, while benzotriazole is easily attacked by SO4•‾. The kinetic calculations indicate that •OH-addition reaction R15 is dominant initial pathway. And the half-life (t1/2) of UV-P is calculated according to rate constants, t1/2 decreases rapidly with [ROS] increasing. UV-P exhibits environmental persistence when [•OH] ≤ 10−17 M. The subsequent degradation mechanisms of hydroxylated UV-P react with •OH and O2 are also calculated. A novel ring-opening reaction channel is proposed that O2-addition intermediate combines with hydroperoxyl radical (HO2•) to cleave aromatic ring. The rate-determining step is intramolecular dehydration reaction with the energy barrier of 32.98 kcal mol−1 and 41.13 kcal mol−1 to cleave benzene ring and benzotriazole ring, respectively. The degradation experiments of UV-P are conducted in Co3O4 activated potassium peroxymonosulfate (PMS) system, and liquid chromatograph-mass spectrometer (LC-MS) results identified that dihydroxylated species are main intermediates, which is consistent with theoretical calculation results. Furthermore, the eco-toxicity assessment shows that the acute and chronic toxicities of most degradation products are reduced compared with UV-P, however, their toxicity levels still keep at toxic and harmful. The environmental risk of UV-P deserves more attention.