Bromine Radical (Br• and Br₂•–) Reactivity with Dissolved Organic Matter and Brominated Organic Byproduct Formation

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Abstract

Dissolved organic matter (DOM) is a major scavenger of bromine radicals (e.g., Br• and Br₂•-) in sunlit surface waters and during oxidative processes used in water treatment. However, the literature lacks quantitative measurements of reaction rate constants between bromine radicals and DOM and lacks information on the extent to which these reactions form brominated organic byproducts. Based on transient kinetic analysis with different fractions and sources of DOM, we determined reaction rate constants for DOM with Br• ranging from 7 to (4.2 ± 1.3) × 108 M_C-1 s-1, which are comparable with those of HO• but higher than those with Br₂•- (k = (9.0 ± 2.0) × 104 to (12.4 ± 2.1) × 105 M_C-1 s-1). Br• and Br₂•- attack the aromatic and antioxidant moieties of DOM via the electron transfer mechanism, resulting in Br- release with minimal substitution of bromine into DOM. For example, the total organic bromine was less than 0.25 μM (as Br) at environmentally relevant bromine radicals' exposures of ∼10-9 M·s. The results give robust evidence that the scavenging of bromine radicals by DOM is a crucial step to prevent inorganic bromine radical chemistry from producing free bromine (HOBr/OBr-) and subsequent brominated byproducts.

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