Occurrence Of Unknown Reactive Species In Uv/H(2)O2 System Leading To False Interpretation Of Hydroxyl Radical Probe Reactions

The UV/H2O2 process is a benchmark advanced oxidation process (AOP) that in situ generates highly reactive and nonselective hydroxyl radical (center dot OH) to oxidatively destroy a wide range of organic compounds. Accurately quantifying the concentration of short-lived center dot OH is essential to predict process performance, optimize the operation parameters, and compare with other process options. The center dot OH concentration is typically measured using organic probe molecules that react with center dot OH but not with other oxidants. In the extremely wellcharacterized UV/H2O2 system in which center dot OH is proven to be the dominant oxidant, using photolysis-resistant probes such as benzoic acid and its derivatives is a widely agreed and practiced norm. We herein report that certain center dot OH probe compounds can be degraded in UV/H2O2 system by unknown reactive species that has not been reported in the past. Several common organic probes, particularly p-substituted benzoic acid compounds (i. e., p-hydroxybenzoic acid, p-chlorobenzoic acid, and p-phthalic acid), were found to be vulnerable to attack by the unknown reactive species, leading to false quantification of center dot OH concentration under high radical scavenging conditions. Lines of evidence obtained from a series of center dot OH scavenging experiments performed under various conditions (i.e., different concentrations of H2O2, center dot OH probe compounds, and dissolved oxygen) point toward excited state H2O2. The results from this study suggest the importance of using appropriate center dot OH probe compounds in mechanistic studies and needs for considering the unidentified role of excited state of H2O2 on the UV/ H2O2 process and related AOPs.