Probing the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter: Insights into the H<sub>2</sub>O<sub>2</sub>-Dependent Pathway
Kai Cheng, Hang Li, Juliana R. Laszakovits, Charles M. Sharpless, Fernando L. Rosario‐Ortiz, Garrett McKay
Abstract
High Resolution Image Download MS PowerPoint Slide This study quantifies the contribution of the H 2 O 2 -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 H 2 O 2 -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 H 2 O 2 -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 H 2 O 2 . Notably, the H 2 O 2 -dependent pathway contributed 11 ± 3% to • OH formation from Pony Lake fulvic acid, which had the lowest Fe content, implicating additional H 2 O 2 -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 H 2 O 2 . However, • OH formation rate increased 6-fold when H 2 O 2 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 H 2 O 2 in aquatic photochemical processes.