Evaluation of the role of superoxide radical as chain carrier for the formation of hydroxyl radical during ozonation

Superoxide radicals (O-2(center dot-)) have been suggested as an important chain carrier in the radical chain reaction that promotes ozone (O-3) decomposition to hydroxyl radicals ((OH)-O-center dot) during ozonation. However, due to the difficulty in measuring transient O-2(center dot-) concentrations, this hypothesis has not been verified under realistic ozonation conditions during water treatment. In this study, a probe compound was used in combination with kinetic modeling to evaluate the role of O-2(center dot-) for O-3 decomposition during ozonation of synthetic solutions with model promotors and inhibitors (methanol and acetate or tert-butanol) and natural waters (one groundwater and two surface waters). By measurement of the abatement of spiked tetrachloromethane (as a O-2(center dot-) probe), the O-2(center dot-) exposure during ozonation was determined. Based on the measured O-2(center dot-) exposures, the relative contribution of O-2(center dot-) to O-3 decomposition, in comparison to OH, (OH)-O-center dot, and dissolved organic matter (DOM), was quantitatively evaluated using kinetic modeling. The results show that water compositions (e.g., the concentration of promotors and inhibitors, and the O-3 reactivity of DOM) have a considerable effect on the extent of the O-2(center dot-)-promoted radical chain reaction during ozonation. In general, the reaction with O-2(center dot-) accounted for similar to 59-70% and similar to 45-52% of the overall O-3 decomposition during ozonation of the selected synthetic solutions and natural waters, respectively. This confirms that O-2(center dot-) plays a critical role in promoting O-3 decomposition to similar to OH. Overall, this study provides new insights on the controlling factors for ozone stability during ozonation processes.