Evaluation of the concentration and contribution of superoxide radical for micropollutant abatement during ozonation

Abstract

Due to the fast reaction of superoxide radical (O2•−) with ozone (O3), it has been suggested that O2•− is present at very low concentrations during ozonation. Therefore, while O2•− has been considered a critical chain carrier for promoting O3 decomposition to hydroxyl radicals (•OH), the direct reactions of O2•− with micropollutants have been assumed to be insignificant during ozonation. In this study, we monitored the exposures of O3, •OH, and O2•− by following the depletion of O3, p-chlorobenzoic acid (pCBA, as •OH probe), and tetrachloromethane (CCl4, as O2•− probe) during ozonation of various water matrices (surface water, groundwater, and secondary wastewater effluent). For a given water matrix, the ratio between •OH and O3 exposures (Rct), O2•− and O3 exposures (RSO), as well as O2•− and •OH exposures (RSH) remained almost constant over the entire reaction time. This suggests that during ozonation, the ratios between the transient concentrations of •OH and O3, O2•− and O3, and O2•− and •OH were also constant and equaled to the Rct, RSO, and RSH, respectively. Based on the O3, •OH, and O2•− exposures observed during ozonation, a chemical kinetic model was proposed to simulate the abatement of ten ozone-resistant micropollutants in the three water matrices by ozonation. The results indicate that due to the higher concentrations of O2•− than •OH (RSH = ~5–8), the reactions with O2•− played a non-negligible or even dominant role in the abatement of some micropollutants that have similar or higher O2•− reactivity than •OH reactivity (e.g., tetrachloroethylene, chloroform, and PFOA). Compared with the previous model that neglected the contribution of O2•− to micropollutant abatement, the proposed model more accurately simulated the abatement efficiencies of the test micropollutants during ozonation. These results indicate that the proposed model can provide a useful tool for the generalized prediction of micropollutant abatement by ozonation.