Reactivity of natural organic matter with aqueous chlorine and bromine

While both aqueous bromine (HOBr/OBr^-) and chlorine (HOCl/OCl^-) react with natural organic matter (NOM) during water treatment, limited direct parallel comparison of bromine versus chlorine has been conducted. Experiments with model compounds and natural waters indicated more efficient substitution reactions with bromine than chlorine. Kinetic experiments with NOM isolates with and without pre-ozonation were conducted to obtain second-order rate constants (k) with bromine and chlorine. Two-stage reaction kinetics (rapid initial and slower consumption stages) were observed. Bromine reacted about 10 times faster than chlorine with NOM isolates during both stages. The rapid initial stage reactions were too fast to quantify k values, but qualitative estimates ranged between 500 and 5000M ^-1s^-1. For the slower second stage k values for bromine were 15 to 167M^-1s^-1 over the pH range of 5-11, and lower for chlorine (k = 0.7-5M^-1s^-1). Values of k correlated with initial SUVA values of NOM (UVA measured at 254nm divided by DOC). Based upon UV/VIS and solid-state ¹³C-NMR spectroscopy, chlorine addition to a NOM isolate resulted in significant oxidation of aromatic and ketone groups while bromine had significantly less change in spectra. Overall, the improved knowledge that bromine reacts faster and substitutes more efficiently than chlorine will be useful in developing strategies to control disinfection by-product formation during water treatment.