Sunlight irradiation of dissolved organic matter (DOM) leads to the formation of photochemically produced reactive intermediates (PPRI), which can induce the indirect phototransformation of a variety of aquatic organic contaminants. Among these PPRI, long-lived photooxidants (LLPO) have been hypothesized to be responsible for the enhanced transformation of electron-rich phenols, anilines and phenylureas observed at a low initial concentration (0.1 μM) compared to a higher concentration (5.0 μM). LLPO, having estimated lifetimes τ~70 μs, occur concomitantly with the shorter-lived excited triplet states of DOM, 3DOM* (τ~2 μs). The goal of the present study was to enhance the knowledge about the chemical nature of LLPO, which are hypothesized to be DOM-derived phenoxyl radicals. Steady-state irradiations (λ &gt; 320 nm) were performed using DOM model systems, consisting of a photosensitizer and an electron-poor phenol, and chemically modified DOM. Model systems, in which well-defined phenoxyl radicals were produced by photosensitized oxidation of an electron-poor phenol, caused an analogous enhancement effect on the transformation rates of electron-rich target compounds. This supports the hypothesis that LLPO may comprise electron-poor phenoxyl radical moieties of the DOM. These transformation rates, measured at 0.1 μM initial concentration of a target compound, depended on the redox potentials of the excited triplet photosensitizers and the electron-poor phenols, and on the speciation of the latter. This could be rationalized in terms of the reaction between electron-poor phenoxyl radicals and the target compounds. Further investigations were conducted using DOM that was pre-treated by applying low doses (&lt;0.1 moloxidant/molC) of ozone or chlorine. Such treatments caused a decrease of the electron donating capacity of DOM. When treated DOM was used as a photosensitizer, a decrease in transformation rate constants of target compounds at 0.1 μM initial concentration was observed compared to untreated DOM. Since the electron donating capacity of DOM is mainly attributed to its phenolic content, these results suggest that DOM-derived phenoxyl radicals are relevant components of LLPO.

Long-lived photooxidants derived from dissolved organic matter: Effects and characterization

ACS Fall 2021

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