Probing the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter: Insights into the H₂O₂-Dependent Pathway

This study quantifies the contribution of the H₂O₂-dependent pathway to hydroxyl radical (^•OH) production from the photolysis of dissolved organic matter (DOM). ^•OH formation rates were cross-validated using benzoate and terephthalate as probe compounds for diverse DOM sources (reference isolates and whole waters). Catalase addition revealed that the H₂O₂-dependent pathway accounts for 10-20% of the total ^•OH production in DOM isolate materials, but no significant correlation was observed between ambient iron (Fe) concentrations and H₂O₂-dependent ^•OH formation. This lack of correlation was likely due to lower total Fe levels in isolated materials, thus limiting the concentration of photochemically produced Fe(II) available for reaction with H₂O₂. Notably, the H₂O₂-dependent pathway contributed 11 ± 3% to ^•OH formation from Pony Lake fulvic acid, which had the lowest Fe content, implicating additional H₂O₂-driven formation mechanisms independent of Fe. Experiments with the DOM model compounds acetophenone and p-benzoquinone indicated no ^•OH production from triplet DOM reactions with H₂O₂. However, ^•OH formation rate increased 6-fold when H₂O₂ was reduced by ketyl radicals formed from the reaction between excited triplet acetophenone and 2,4,6-trimethylphenol. This study advances the knowledge of ^•OH production mechanisms from DOM photolysis, providing insight into the role of H₂O₂ in aquatic photochemical processes.