Insights into the Enhanced Photogeneration of Hydroxyl Radicals from Chlorinated Dissolved Organic Matter.

Free available chlorine has been and is being applied in global water treatment and readily reacts with dissolved organic matter (DOM) in aquatic environments, leading to the formation of chlorinated products. Chlorination enhances the photoreactivity of DOM, but the influence of chlorinated compounds on the photogeneration of hydroxyl radicals (OH) has remained unexplored. In this study, a range of chlorinated carboxylate-substituted phenolic model compounds were employed to assess their OH photogeneration capabilities. These compounds demonstrated a substantial capacity for OH production, exhibiting quantum yields of 0.1-5.9 × 10 through direct photolysis under 305 nm and 0.2-9.5 × 10 through a triplet sensitizer (4-benzoylbenzoic acid)-inducing reaction under 365 nm LED irradiation. Moreover, the chlorinated compounds exhibited higher light absorption and OH quantum yields compared to those of their unchlorinated counterparts. The OH photogeneration capacity of these compounds exhibited a positive correlation with their triplet state one-electron oxidation potentials. Molecular-level compositional analysis revealed that aromatic structures rich in hydroxyl and carboxyl groups (e.g., O/C > 0.5 with H/C < 1.5) within DOM serve as crucial sources of OH, and chlorination of these compounds significantly enhances their capacity to generate OH upon irradiation. This study provides novel insights into the enhanced photogeneration of OH from chlorinated DOM, which is helpful for understanding the fate of trace pollutants in chlorinated waters.