Reactivity of Bromine Radical with Dissolved Organic Matter Moieties and Monochloramine: Effect on Bromate Formation...

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).