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

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.