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 (center dot OH) production from the photolysis of dissolved organic matter (DOM). center dot 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 center dot OH production in DOM isolate materials, but no significant correlation was observed between ambient iron (Fe) concentrations and H2O2-dependent center dot 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 center dot 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 center dot OH production from triplet DOM reactions with H2O2. However, center dot 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 center dot OH production mechanisms from DOM photolysis, providing insight into the role of H2O2 in aquatic photochemical processes.