abstract
- 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.