The Role of Bromide in Oxidative Water Treatment

Bromide (Br-) levels in fresh waters range from a few ¿gL-1 to several mgL-1. In seawater its concentration is about 67 mgL-1. The goal of this thesis is to assess the fate of bromide during oxidative water treatment and to explore its influence on these treatment processes. During oxidative water treatment such as chlorination, ozonation or treatment with peracetic acid, bromide can be oxidized to reactive bromine; the most important species under water treatment conditions is hypobromous acid (HOBr, pKa 8.8). In the first part of this thesis, the reactivity of bromine with inorganic and organic compounds is discussed together with a comprehensive compilation of second-order rate constants. HOBr shows a high reactivity towards anionic inorganic compounds, ammonia, and organic compounds with electron-rich moieties, such as activated aromatic compounds, neutral amines, sulfamides, olefins, and reduced sulfur species (apparent second-order rate constants at pH 7 in the range of 103-109 M-1s-1). Second-order rate constants for these reactions are in general up to three orders of magnitude higher than corresponding rate constants for hypochlorous acid (HOCl). In the second part of the thesis, the kinetics of the formation of inorganic and organic chlor- and bromamines (monochloramine, N-chloromethylamine, N-chlorodimethylamine, monobromamine, dibromamine, N-bromomethylamine, N,N-dibromomethylamine, and N-bromodimethylamine) and their reactivity with phenolic compounds were investigated. Reactions of HOBr and HOCl with ammonia and organic amines lead to a rapid formation of inorganic and organic halamines with second-order rate constants in the range of 106-108 M-1s-1. Apparent second-order rate constants for the reactions of bromamines with phenol range from 2×101 to 4×102 M-1s-1 at pH 7. The corresponding chloramine rate constants are about three to four orders of magnitude lower. The main products from the reactions of halamines with phenol and resorcinol are halogenated compounds formed through electrophilic aromatic substitution, whereas for catechol and hydroquinone the corresponding benzoquinones are formed through electron transfer. In the third part of this thesis, the N-nitrosodimethylamine (NDMA) formation potentials from the reaction of mono- and dibromamine with the nitrogenous precursors dimethylamine, trimethylamine, N,N-dimethylisopropylamine, N,N-dimethylbenzylamine, ranitidine, and 5-dimethylaminofurfurylalcohol (DFUR) are evaluated. The reactions of chloramine with these precursors lead to high NDMA yields. In contrast, much lower NDMA formation is observed for mono- and dibromamine. For DFUR, quantum chemical computations show that the Gibbs free energies (¿G) are indeed more favorable for the reaction with chloramines than with bromamines. Based on model calculations, these results demonstrate that during chlorination with no or low ammonia present, HOBr is formed and can, despite its low concentration, outcompete HOCl reactions and substantially influence the formation of transformation products. In the presence of ammonia, HOBr formation is limited due to the high reactivity of HOCl with ammonia. Ozonation of waters containing ammonia and bromide leads to the formation of mainly inorganic bromamines. During ballast water treatment with peracetic acid, high levels of bromine and ¿ in the presence of ammonia and/or organic amines ¿ bromamine are generated, both of which can react further with natural organic matter.