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).

Keywords: bromate; bromine radical; dissolved organic matter; model compounds; ozone; reaction kinetics.

Figure 1

Quantitative structure–activity relationship of the measured second-order rate constants for the reactions of all selected model compounds (except p-benzoquinone) with Br^• and the computed free energies of activation for electron transfer reactions (see Table 1). The aromatic compounds with reported k_Br• in the literature^, are labeled as “(ref)” and are also shown for comparison. The literature values were not included in the regression.

Figure 2

Relative product yields per degraded phenol as a function of the γ-radiolysis time during the reaction of phenol with Br^•, for the condition with 22 μM phenol, 0.7 mM 1,2-dibromoethane, 40 mM t-butanol, and 50 mM phosphate buffer (pH 7.1). Concentrations of phenol and the products as a function of time are provided in Figure S10.

Scheme 1. Reaction Pathway for the Reaction of Phenol with Br^• Based on the Identified (in squares) and Suspected Products

The initial step of forming phenoxyl radical is described in detail in Scheme S1.

Figure 3

Calculated fractions (see text) of Br^• reacting with DOM (green line), ozone (red line), bromide (blue line), or NH₂Cl (orange line) as a function of ozone concentration in the (a) absence or (b) presence of NH₂Cl. The selected concentrations were 1 mgC/L DOC, 0.2 μM Br^– (16 μg/L Br^–), and 15 μM NH₂Cl. Shaded areas indicate a typical range of ozone doses applied in drinking water treatment in Switzerland (0.5–1.0 mgO₃/mgC).

Figure 4

(a) Bromate formation as a function of the ozone exposure for five ozonation conditions. Unaltered Lake Zurich water contained 1.4 mgC/L DOC, 2 μM bromide, 1 mM phosphate buffer (pH 7.6), 5 μM pCBA, and a 60 μM ozone dose (black four pointed stars). The other conditions additionally contained 10 μM formate (orange circles), 4 μM ammonium (blue diamonds), 7 μM NH₂Cl (green squares), or 15 μM NH₂Cl (red triangles), respectively. (b) A close-up for a low range of ozone exposures (dotted rectangle in a). The slopes of the first three data points are provided in Table 2.