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).
Kinetics and transformation products for the reactions of ozone with pyrrole, imidazole, and pyrazole were determined. For the imidazole–ozone reaction, all possible transformation products were identified, completing the mass balance.
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