Reactivity of Bromine Radical with Dissolved Organic Matter Moieties and Monochloramine: Effect on Bromate Formation during Ozonation
Sungeun Lim, Benjamin Barrios, Daisuke Minakata, Urs von Gunten
Abstract
High Resolution Image Download MS PowerPoint Slide 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 (NH 2 Cl) with Br • using γ-radiolysis. The k Br• of all model compounds were high ( k Br• > 10 8 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 NH 2 Cl to quench Br • was tested based on the fast reaction ( k Br•, NH2Cl = 4.4 × 10 9 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 NH 2 Cl can substantially suppress bromate formation, especially in waters containing low DOC concentrations (1–2 mgC/L).