Mechanisms of Hydroxylating Species Production by Dissolved Organic Matter Model Photosensitizers

Hydroxyl radicals ( • OH) are a species of interest in the environmental fate of contaminants due to their fast and nonselective reactions with both organic and inorganic compounds. Sources of • OH in surface waters include nitrate and nitrite photolysis, the photo-Fenton reaction, and dissolved organic matter (DOM) photolysis. The production mechanisms of • OH by DOM are classified based on their involvement of hydrogen peroxide (H 2 O 2 ), known as the H 2 O 2 -dependent and H 2 O 2 -independent pathways. While the generation of • OH through the H 2 O 2 -dependent has been well-studied, the H 2 O 2 -independent pathway has remained unclear due to reactions between • OH (or other lower-energy hydroxylating species) and probe and quencher compounds. In this work, the pathways of • OH formation by six model quinones and hydroxybenzoic acids were investigated using methane and catalase to quench • OH and H 2 O 2, respectively. This work suggests that quinones primarily generate lower-energy hydroxylating species but may form • OH in the presence of an electron donor through the H 2 O 2 -dependent pathway. Hydroxybenzoic acids were shown to produce free • OH through both H 2 O 2 -dependent and -independent pathways. Based on these results, we estimate that only 10–20% of all hydroxylating species produced by DOM in natural surface waters are free • OH, implying that previous work has overestimated • OH steady-state concentrations and their contribution to contaminant fate.