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

Superoxide radicals (O 2 •− ) have been suggested as an important chain carrier in the radical chain reaction that promotes ozone (O 3 ) decomposition to hydroxyl radicals ( • OH) during ozonation. However, due to the difficulty in measuring transient O 2 •− 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 •− 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 •− probe), the O 2 •− exposure during ozonation was determined. Based on the measured O 2 •− exposures, the relative contribution of O 2 •− to O 3 decomposition, in comparison to OH − , • OH, 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 •− -promoted radical chain reaction during ozonation. In general, the reaction with O 2 •− accounted for ∼59‒70% and ∼45‒52% of the overall O 3 decomposition during ozonation of the selected synthetic solutions and natural waters, respectively. This confirms that O 2 •− plays a critical role in promoting O 3 decomposition to • OH. Overall, this study provides new insights on the controlling factors for ozone stability during ozonation processes.