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

Abstract

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).

Critical Review on Bromate Formation during Ozonation and Control Options for Its Minimization

“…The Br • /Br 2 •– concentration ratio is ∼1 at 500 μg/L bromide. Therefore, for low to moderate bromide levels, Br • will always dominate and therefore contribute to further reactions with ozone. Another important sink for Br • is DOM, with second-order rate constants ( k ) in the range of 1.4–4.2 × 10 8 M C –1 s –1 for DOM isolates and real waters. , Experiments with a preozonated DOM isolate showed that the second-order rate constant with Br • did not change significantly, which implies a constant consumption of Br • by DOM during ozonation …”

“…Reactions of Br • with organic compounds proceed mainly by electron transfer, − shown for DOM in eq : normalB normalr • + normalD normalO normalM → normalB normalr − + normalD normalO normalM •+ It has been demonstrated that a minor fraction may proceed by an addition of bromine to the ensuing brominated products …”

“…The apparent second-order rate constant at circumneutral pH for the formation of bromochloramine from the reaction of NH 2 Cl with Br – is low ( k = 1.4 × 10 –1 M –1 s –1 ): normalN normalH 2 normalC normall + normalB normalr − → products For a NH 2 Cl concentration of 5 mg/L as Cl 2 (∼130 μM NH 2 Cl), an ozone concentration of 1 mg/L, and an R ct of 10 –8 , the fractions of Br – reacting with NH 2 Cl, O 3 , and • OH are 0.3%, 94%, and 5.7%, respectively. This shows clearly that reaction is not an efficient sink for bromide. It has been shown that Br • reacts with a k of 4.4 × 10 9 M –1 s –1 with NH 2 Cl . An addition of 15 μM NH 2 Cl (∼1 mg/L as Cl 2 ) can reduce the contribution of the O 3 –Br • reaction (eq ) from 8–15% to 2–4%, and therefore, the NH 2 Cl–Br • and NH 2 Cl–HOBr reactions contribute roughly equally to the reduction of the level of bromate formation …”

“…It has been shown that Br • reacts with a k of 4.4 × 10 9 M –1 s –1 with NH 2 Cl . An addition of 15 μM NH 2 Cl (∼1 mg/L as Cl 2 ) can reduce the contribution of the O 3 –Br • reaction (eq ) from 8–15% to 2–4%, and therefore, the NH 2 Cl–Br • and NH 2 Cl–HOBr reactions contribute roughly equally to the reduction of the level of bromate formation …”

“…The Br M C −1 s −1 for DOM isolates and real waters. 66,67 Experiments with a preozonated DOM isolate showed that the second-order rate constant with Br • did not change significantly, which implies a constant consumption of Br • by DOM during ozonation. 67 Reactions of Br • with organic compounds proceed mainly by electron transfer, 66−68 shown for DOM in eq 19:…”