Hydroxyl radicals (^●OH) play a role in the environmental fate of contaminants due to their high reaction rates with both organic and inorganic compounds. There are several sources of ^●OH in surface waters including the photolysis of nitrate and nitrite, the oxidation of ferrous iron in the presence of hydrogen peroxide (H₂O₂; the photo-Fenton reaction), and the photolysis of dissolved organic matter (DOM). For systems with low nitrate, nitrite, and iron, the photoproduction of ^●OH by DOM will be most relevant. The production of ^●OH by DOM is known to occur through two pathways: 1) the generation of H₂O₂ by DOM followed by H₂O₂ photolysis (known as the H₂O₂-dependent pathway) and 2) the direct generation of ^●OH by DOM without involvement of H₂O₂ (the H₂O₂-independent pathway). However, the mechanisms of the H₂O₂-independent pathway remain unclear. Prior work has shown that ^●OH is produced during the irradiation of quinones and hydroxybenzoic acids, which are often used as model photosensitizers for DOM, but several challenges have hindered progress in understanding the H₂O₂-independent pathway. The generation of a lower-energy hydroxylating species (^●OH-like species) has been observed during the irradiation of DOM and photosensitizers, however, most probe compounds used to measure ^●OH are unable to distinguish free ^●OH from ^●OH-like species. There are several hypotheses to the identity of ^●OH-like species during the photolysis of quinones and hydroxybenzoic acids, but further work is needed to confirm these identities. This work aims to overcome the challenges in measuring free ^●OH and clarifying the H₂O₂-independent pathway. Methane is the only quencher thought to be selective for free ^●OH due to its high C-H bond strength, so it was used to determine whether free ^●OH or ^●OH-like species are produced during the photolysis of quinones, hydroxybenzoic acids, and DOM isolates. Preliminary work has shown the production of free ^●OH does not occur during the photolysis of 1,4-benzoquinone, though this has previously been debated. Examining the mechanisms of ^●OH production by DOM and model photosensitizers will expound the role of ^●OH in the degradation of contaminants in both natural and engineered systems.

Kari Norris

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