Kinetic modelling of the bromine-ammonia system: Formation and decomposition of bromamines

Bromamines i.e. monobromamine (NH₂Br), dibromamine (NHBr₂), and tribromamine (NBr₃) can be formed during oxidative treatment of waters containing bromide and ammonia. The formation and decomposition of bromamines in aqueous solution was investigated and a comprehensive kinetic model of the bromine-ammonia system was developed at 23 ± 1 °C. Determination of rate constants and model validation were primarily performed at pH 8.0 - 8.3 for subsequent application to seawater disinfection. The rate constant of NHBr₂ self-decomposition was determined by second-order rate law linearization with k₉ = 5.5 (± 0.8) M^-1s^-1 at pH 8.10. The rate constant of NBr₃ self-decomposition increased proportionately to the concentration of hydroxide ions (OH^-) according to the equation k₁₀ = 4.4 (± 0.1) × 10⁷. [OH^-] over the pH range 6.0 - 8.5, which gave k₁₀ = 56 (± 1) M^-1s^-1 at pH 8.10. The rate constants of NHBr₂ and NBr₃ formation were obtained by fitting model-predicted data to the experimental results and were found to be k₃ = 2.3 (± 0.2) × 10⁴M^-1s^-1 and k₅ = 4.0 (± 0.6) × 10³M^-1s^-1, respectively at pH 8.10. NBr₃ was also found to react with NHBr₂ with k₁₁ = 3.4 (± 0.2) × 10³M^-1s^-1 at pH 8.10. A kinetic model was proposed based on these experimental rate constants and literature values, which provided a good prediction of bromamines formation and decomposition for various initial bromine and ammonia concentrations. The kinetic model was also used to accurately predict the total oxidant concentration and the speciation of bromamines during breakpoint bromination. This study provides kinetic data to model more complex oxidative systems such as seawater chlorination in the presence of ammonia.