Superfast degradation of refractory organic contaminants by ozone activated with thiosulfate: Efficiency and...

Thiosulfate (S₂O₃^2-) 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 S₂O₃^2-, resulting in a superfast degradation of O₃-refractory contaminants. Therefore, the focus of this study is the exploration into the enhancing role of S₂O₃^2- 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 S₂O₃^2- with O₃ is mainly responsible for the superfast degradation of O₃-refractory contaminants. The ^•OH yield from the O₃/S₂O₃^2- process is determined to be 0.216. A plausible mechanism for the generation of ^•OH from the O₃/S₂O₃^2- process is proposed with the implementation of density functional theory (DFT). Initially, ozone reacts with a sulfur of S₂O₃^2- to form OOOSSO₃^2-. The adduct then rearranges to OO(O)SSO₃^2- or HOO(O)SSO₃^2- 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 S₂O₃^2- and O₃ ([S₂O₃^2-]:[O₃]). The optimal [S₂O₃^2-]:[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/S₂O₃^2- 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/S₂O₃^2- process in real water matrices is also confirmed.