Development of a Five-Chemical-Probe Method to Determine Multiple Radicals Simultaneously in Hydroxyl and Sulfate Radical-Mediated Advanced Oxidation Processes

Advanced oxidation processes (AOPs), such as hydroxyl radical (HO•)- and sulfate radical (SO4•–)-mediated oxidation, are attractive technologies used in water and wastewater treatments. To evaluate the treatment efficiencies of AOPs, monitoring the primary radicals (HO• and SO4•–) as well as the secondary radicals generated from the reaction of HO•/SO4•– with water matrices is necessary. Therefore, we developed a novel chemical probe method to examine five key radicals simultaneously, including HO•, SO4•–, Cl•, Cl2•–, and CO3•–. Five probes, including nitrobenzene, para-chlorobenzoic acid, benzoic acid, 2,4,6-trimethylbenzoic acid, and 2,4,6-trimethylphenol, were selected in this study. Their bimolecular reaction rate constants with diverse radicals were first calibrated under the same conditions to minimize systematic errors. Three typical AOPs (UV/H2O2, UV/S2O82–, and UV/HSO5–) were tested to obtain the radical steady-state concentrations. The effects of dissolved organic matter, Br–, and the probe concentration were inspected. Our results suggest that the five-probe method can accurately measure radicals in the HO•- and SO4•–-mediated AOPs when the concentration of Br– and DOM are less than 4.0 μM and 15 mgC L–1, respectively. Overall, the five-probe method is a practical and easily accessible method to determine multiple radicals simultaneously.