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/S₂O₈^2–, 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 mg_C L^–1, respectively. Overall, the five-probe method is a practical and easily accessible method to determine multiple radicals simultaneously.