Reactivity of BrCl, Br₂, BrOCl, Br₂O, and HOBr Toward Dimethenamid in Solutions of Bromide + Aqueous Free Chlorine

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br₂, BrOCl, and Br₂O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr]_T), [Cl^–], and [FAC]_o) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr]_T ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK_a of HOBr. This complex dependence on [HOBr]_T implicates Br₂O as an active brominating agent. That bromination rates increased with increasing [Cl^–], [FAC]_o (at constant [HOBr]_T), and excess bromide (where [Br^–]_o>[FAC]_o) implicate BrCl, BrOCl, and Br₂, respectively, as brominating agents. As equilibrium constants for the formation of Br₂O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD­(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br₂O < BrOCl ≈ Br₂ < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.