Bromamines (i.e., monobromamine NH2Br, dibromamine NHBr2 and tribromamine NBr3) can be formed during oxidative water treatment containing bromide and ammonia. Bromide (Br-) is present in all water sources at concentrations ranging from 0.01 to 1 mg L-1 in fresh waters and about 67 mg L-1 in seawater. Knowledge about the formation of bromamines and their subsequent reactions with phenolic compounds is relevant for the control of the disinfection by-products formation. In this study, the rate constants of NH2Br with phenol and 4 bromophenols were determined in an excess of phenolic compounds for pH values from 5 to 9. Decomposition of NH2Br was controlled by the reaction of monobromoammonium ion NH3Br+ with phenolate species with specific rate constants ranging from 2.3 x 103 for 2,4,6-tribromophenol to 1.22 x 107 M-1 s-1 for phenol. At pH 8.1, bromophenol formation was modelled for an excess of NH2Br over phenol. Under this condition, no formation of bromoform CHBr3 was observed due to the low reactivity of NH2Br with 2,4,6-tribromophenol at pH 8.1. For NHBr2, pseudo-first order rate constants of NHBr2 decomposition were similar for all tested phenols and did not vary with the concentration of phenols, suggesting that the reaction would be controlled by the decomposition of NHBr2 into hypobromous acid HOBr, with HOBr reacting rapidly with phenols. In contrast to NH2Br, a significant amount of CHBr3 was produced in excess of NHBr2 at pH 8.1, which would further highlight the role of HOBr in the overall reactivity of NHBr2 with phenols. A comprehensive kinetic model including the formation and decomposition of NH2Br and NHBr2 and the reactivity of HOBr and NH2Br with phenolic compounds was developed in order to better understand bromine demand and formation of CHBr3 during disinfection of saline waters containing bromide and ammonia.

Reactivity of monobromamine and dibromamine with phenolic compounds: Rate constants and formation of bromoform

ACS Fall 2021

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