Superfast degradation of refractory organic contaminants by ozone activated with thiosulfate: Efficiency and mechanisms
2020
Yang, Jingxin | Luo, Cheng | Li, Tingting | Cao, Jie | Dong, Wenyi | Li, Ji | Ma, Jun
Thiosulfate (S₂O₃²⁻) is frequently used as an ozone (O₃) quenching agent when investigating the ozonation of organic contaminants and the kinetics thereof. In this study, however, O₃ is activated by S₂O₃²⁻, resulting in a superfast degradation of O₃-refractory contaminants. Therefore, the focus of this study is the exploration into the enhancing role of S₂O₃²⁻ in the degradation of refractory organic contaminants by O₃, which has been overlooked thus far. Results obtained from scavenging experiments and electron paramagnetic resonance (EPR) spectra verify that •OH generated from the reaction of S₂O₃²⁻ with O₃ is mainly responsible for the superfast degradation of O₃-refractory contaminants. The •OH yield from the O₃/S₂O₃²⁻ process is determined to be 0.216. A plausible mechanism for the generation of •OH from the O₃/S₂O₃²⁻ process is proposed with the implementation of density functional theory (DFT). Initially, ozone reacts with a sulfur of S₂O₃²⁻ to form OOOSSO₃²⁻. The adduct then rearranges to OO(O)SSO₃²⁻ or HOO(O)SSO₃²⁻ in the presence of H⁺, which cleaves to give a sulfoxide radical cation and O₂•⁻/HO₂•. O₂•⁻/HO₂• is rapidly transformed into •OH by O₃ through a series of steps. Degradation efficiency of O₃-refractory contaminants of this process highly depends on the molar ratio of S₂O₃²⁻ and O₃ ([S₂O₃²⁻]:[O₃]). The optimal [S₂O₃²⁻]:[O₃] is pH dependent in synthetic water (e.g. 0.3 at pH 7). The presence of bicarbonate inhibits the degradation of refractory contaminants by the O₃/S₂O₃²⁻ process. Humic acid exhibits a slight enhancing effect at low concentrations (0.1–0.2 mg-C/L), and an inhibiting effect at higher concentrations (≥0.4 mg-C/L). In addition, the efficacy of the O₃/S₂O₃²⁻ process in real water matrices is also confirmed.
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