Bromine radical (Br•) has been hypothesized to be a key intermediate of bromate formation during ozonation. Once formed, Br• further reacts with ozone to eventually form bromate. However, this reaction competes with the reaction of Br• with dissolved organic matter (DOM), of which reactivity and reaction mechanisms are less studied to date. To fill this gap, this study determined the second-order rate constant (k) of the reactions of selected organic model compounds, a DOM isolate, and monochloramine (NH2Cl) with Br• using γ-radiolysis. The k Br• of all model compounds were high (k Br• > 108 M–1 s–1) and well correlated with quantum-chemically computed free energies of activation, indicating a selectivity of Br• toward electron-rich compounds, governed by electron transfer. The reaction of phenol (a representative DOM moiety) with Br• yielded p-benzoquinone as a major product with a yield of 59% per consumed phenol, suggesting an electron transfer mechanism. Finally, the potential of NH2Cl to quench Br• was tested based on the fast reaction (k Br•, NH2Cl = 4.4 × 109 M–1 s–1, this study), resulting in reduced bromate formation of up to 77% during ozonation of bromide-containing lake water. Overall, our study demonstrated that Br• quenching by NH2Cl can substantially suppress bromate formation, especially in waters containing low DOC concentrations (1–2 mgC/L).

Singlet oxygen (1O2) is a selective reactive oxygen species that plays a key role for the fate of various organic compounds in the aquatic environment under sunlight irradiation, engineered water oxidation systems, atmospheric water droplets, and biomedical systems. While the initial rate-determining charge-transfer reaction mechanisms and kinetics of 1O2 have been studied extensively, no comprehensive studies have been performed to elucidate the reaction mechanisms with organic compounds that have various functional groups. In this study, we use density functional theory calculations to determine elementary reaction mechanisms with a wide variety of organic compounds. The theoretically calculated aqueous-phase free energies of activation of single electron transfer and 1O2 addition reactions are compared to the experimentally determined rate constants in the literature to determine linear free-energy relationships. The theoretically calculated free energies of activation for the groups of phenolates and phenols show excellent correlations with the Hammett constants that accept electron densities by through-resonance. The dominant elementary reaction mechanism is discussed for each group of compounds. As a practical implication, we demonstrate the fate of environmentally relevant organic compounds induced by photochemically produced intermediate species at different pH and evaluate the impact of predicting rate constants to the half-life.

Single-electron transfer is a major aqueous-phase reaction mechanism commonly used in environmental engineering and natural processes such as aquatic photochemistry and advanced oxidation processes. While the Marcus theory is frequently used to analyze single-electron transfers, many previous studies appear to have overlooked its application, with uncertain energy values being reported without validation. Herein, using the carbonate radical as the oxidant, we analyze the validity of the Marcus theory to aqueous-phase reactions involving aromatic compounds. We highlight the impact of charged targeted molecules by comparing the reactivity with phenolate and aniline. Further, we expand the validated methodology to a wide range of structurally diverse organic compounds and reveal the underlying reaction mechanisms, such as outer-/inner-sphere single-electron transfer and proton coupled electron transfer. Our research outlines the next steps to be taken in Marcus theory calculations to investigate aqueous-phase environmental reactions.

UV photolysis is an effective process to remove nitrosamines from contaminated water resources.

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