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

Thiosulfate (S2O32-) is frequently used as an ozone (O-3) quenching agent when investigating the ozonation of organic contaminants and the kinetics thereof. In this study, however, O-3 is activated by S2O32-, resulting in a superfast degradation of O-3-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 O-3, which has been overlooked thus far. Results obtained from scavenging experiments and electron paramagnetic resonance (EPR) spectra verify that (OH)-O-center dot generated from the reaction of S2O32- with O-3 is mainly responsible for the superfast degradation of O-3-refractory contaminants. The (OH)-O-center dot yield from the O-3/S2O32- process is determined to be 0.216. A plausible mechanism for the generation of (OH)-O-center dot from the O-3/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 O-2(center dot-)/HO2 center dot. O-2(center dot-)/HO2 center dot is rapidly transformed into (OH)-O-center dot by O-3 through a series of steps. Degradation efficiency of O-3-refractory contaminants of this process highly depends on the molar ratio of S2O32- and O-3 ([S2O32-]:[O-3]). The optimal [S2O32-]:[O-3] 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 O-3/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 (similar to 0.4 mg-C/L). In addition, the efficacy of the O-3/S2O32- process in real water matrices is also confirmed. (C) 2020 Elsevier Ltd. All rights reserved.