Evaluation Of The Concentration And Contribution Of Superoxide Radical For Micropollutant Abatement During Ozonation

Due to the fast reaction of superoxide radical (O-2(center dot-)) with ozone (O-3), it has been suggested that O-2(center dot-) is present at very low concentrations during ozonation. Therefore, while O-2(center dot-) has been considered a critical chain carrier for promoting O-3 decomposition to hydroxyl radicals (center dot OH), the direct reactions of O-2(center dot-) with micropollutants have been assumed to be insignificant during ozonation. In this study, we monitored the exposures of O-3, center dot OH, and O-2(center dot-) by following the depletion of O-3, p-chlorobenzoic acid (pCBA, as center dot OH probe), and tetrachloromethane (CCl4, as O-2(center dot-) probe) during ozonation of various water matrices (surface water, groundwater, and secondary wastewater effluent). For a given water matrix, the ratio between center dot OH and O-3 exposures (R-ct), O-2(center dot-) and O-3 exposures (R-SO), as well as O-2(center dot-) and center dot OH exposures (R-SH) remained almost constant over the entire reaction time. This suggests that during ozonation, the ratios between the transient concentrations of center dot OH and O-3, O-2(center dot-) and O-3, and O-2(center dot-) and center dot OH were also constant and equaled to the R-ct, R-SO, and R-SH, respectively. Based on the O-3, center dot OH, and O-2(center dot-) 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(center dot-) than center dot OH (R-SH = similar to 5-8), the reactions with O-2(center dot-) played a non-negligible or even dominant role in the abatement of some micropollutants that have similar or higher O-2(center dot-) reactivity than center dot OH reactivity (e.g., tetrachloroethylene, chloroform, and PFOA). Compared with the previous model that neglected the contribution of O-2(center dot-) 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. (C) 2021 Elsevier Ltd. All rights reserved.