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 (H₂O₂), known as the H₂O₂-dependent and H₂O₂-independent pathways. While the generation of ^•OH through the H₂O₂-dependent has been well-studied, the H₂O₂-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 H₂O₂, 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 H₂O₂-dependent pathway. Hydroxybenzoic acids were shown to produce free ^•OH through both H₂O₂-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.