Production of Sulfate Radical and Hydroxyl Radical by Reaction of Ozone with Peroxymonosulfate: A Novel Advanced Oxidation Process

In this work, simultaneous generation of hydroxyl radical (•OH) and sulfate radical (SO4•–) by the reaction of ozone (O3) with peroxymonosulfate (PMS; HSO5–) has been proposed and experimentally verified. We demonstrate that the reaction between the anion of PMS (i.e., SO52–) and O3 is primarily responsible for driving O3 consumption with a measured second order rate constant of (2.12 ± 0.03) × 104 M–1 s–1. The formation of both •OH and SO4•– from the reaction between SO52– and O3 is confirmed by chemical probes (i.e., nitrobenzene for •OH and atrazine for both •OH and SO4•–). The yields of •OH and SO4•– are determined to be 0.43 ± 0.1 and 0.45 ± 0.1 per mol of O3 consumption, respectively. An adduct, –O3SOO– + O3 → –O3SO5–, is assumed as the first step, which further decomposes into SO5•– and O3•–. The subsequent reaction of SO5•– with O3 is proposed to generate SO4•–, while O3•– converts to •OH. A definition of Rct,•OH and Rct,SO4•– (i.e., respective ratios of •OH and SO4•– exposures to O3 exposure) is adopted to quantify relative contributions of •OH and SO4•–. Increasing pH leads to increases in both values of Rct,•OH and Rct,SO4•– but does not significantly affect the ratio of Rct,SO4•– to Rct,•OH (i.e., Rct,SO4•–/Rct,•OH), which represents the relative formation of SO4•– to •OH. The presence of bicarbonate appreciably inhibits the degradation of probes and fairly decreases the relative contribution of •OH for their degradation, which may be attributed to the conversion of both •OH and SO4•– to the more selective carbonate radical (CO3•–). Humic acid promotes O3 consumption to generate •OH and thus leads to an increase in the Rct,•OH value in the O3/PMS process, while humic acid has negligible influence on the Rct,SO4•– value. This discrepancy is reasonably explained by the negligible effect of humic acid on SO4•– formation and a lower rate constant for the reaction of humic acid with SO4•– than with •OH. In addition, the efficacy of the O3/PMS process in real water is also confirmed.