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

Due to the fast reaction of superoxide radical ( O 2 • − ) with ozone (O 3 ), it has been suggested that O 2 • − is present at very low concentrations during ozonation. Therefore, while O 2 • − has been considered a critical chain carrier for promoting O 3 decomposition to hydroxyl radicals (•OH), the direct reactions of O 2 • − with micropollutants have been assumed to be insignificant during ozonation. In this study, we monitored the exposures of O 3 , •OH, and O 2 • − by following the depletion of O 3 , p -chlorobenzoic acid ( p CBA, as •OH probe), and tetrachloromethane (CCl 4 , as O 2 • − probe) during ozonation of various water matrices (surface water, groundwater, and secondary wastewater effluent). For a given water matrix, the ratio between •OH and O 3 exposures (R ct ), O 2 • − and O 3 exposures (R SO ), as well as O 2 • − and •OH exposures (R SH ) remained almost constant over the entire reaction time. This suggests that during ozonation, the ratios between the transient concentrations of •OH and O 3 , O 2 • − and O 3 , and O 2 • − and •OH were also constant and equaled to the R ct , R SO , and R SH , respectively. Based on the O 3 , •OH, and O 2 • − 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 O 2 • − than •OH (R SH  = ~5–8), the reactions with O 2 • − played a non-negligible or even dominant role in the abatement of some micropollutants that have similar or higher O 2 • − reactivity than •OH reactivity (e.g., tetrachloroethylene, chloroform, and PFOA). Compared with the previous model that neglected the contribution of O 2 • − 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.