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 (O-3) with peroxymonosulfate (PMS; HSO5-) has been proposed and experimentally verified. We demonstrate that the reaction between the anion of PMS (i.e., SO52-) and O-3 is primarily responsible for driving O-3 consumption with a measured second order rate constant of (2.12 +/- 0.03) x 10(4) M-1 s(-1). The formation of both .OH and SO4.(-) from the reaction between SO52- and O-3 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 O-3 consumption, respectively. An adduct, O3SOO + O-3 (R) O3SO5-, is assumed as the first step, which further decomposes into SO5.(-) and O-3.(-). The subsequent reaction of SO5.(-) with O-3.(-) is proposed to generate SO4.(-), while O-3.(-) converts to OH. A definition of R-ct,OH and Rct,SO4.(-) (i.e., respective ratios of OH and SO4.(-) exposures to O-3.(-) exposure) is adopted to quantify relative contributions of OH and SO4. Increasing pH leads to increases in both values of R-ct,OH and R-ct,SO4.(-) but does not significantly affect the ratio of R-ct,SO4.(-) to R-ct,OH.(-) (i.e., R-ct,SO4.(-)/R-ct,.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 O-3 consumption to generate OH and thus leads to an increase in the R-ct,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 O-3/PMS process in real water is also confirmed.