Rethinking Radical Quantification Methods in VUV/UV Water Treatment Process: Alcohol Scavenging and Probe-Model Combining Strategy

Abstract The use of vacuum ultraviolet (VUV)/ultraviolet (UV) technology for water pollutant degradation has grown rapidly in recent years; however, distinguishing the roles of the radicals generated in this process remains difficult and sometimes even methodologically unsound. To address this issue, this study evaluated three methods commonly used for radical determination in advanced oxidation processes (AOPs) and optimized them through rational design choices. The results show that the use of alcohol quenchers generated additional amounts of H 2 O 2 , which can perturb the chemistry of the scavenged system and bias the role of HO•. Increasing solution pH inhibited H 2 O 2 formation significantly from isopropanol and tert-butanol but mildly from methanol, and lowering dissolved oxygen to 2.5 mg/L by purging nitrogen gas continuously reduced H 2 O 2 formation dramatically. Collectively, these results indicate that using alcohols is not a reliable choice for radical quantification under practical conditions. Instead, the reliability of radical determination and contribution analysis can be improved by combining kinetic modeling with the probing method, since a model depends strongly on the quantity and quality of input reactions and rate constants and probing experiments can help calibrate those data used for modeling. By combining both, this study found that the VUV/UV process was essentially governed by scavenging of primary radicals and formation of secondary radicals in water matrices, and the types and concentrations of radicals evolved dynamically over time. Collectively, this study provides fundamental insights into methods for determining radicals in complex systems containing multiple radical species.