Role of Carbonate Species on General Acid Catalysis of Bromide Oxidation by Hypochlorous Acid (HOCl) and Oxidation by Molecular Chlorine (Cl2)

Kinetic models for disinfectant decay and disinfection byproduct (DBP) formation are necessary for predicting water quality from the treatment plant to the tap. A kinetic model for conditions relevant to chloramine disinfection of drinking water (pH 6-9 and carbonate-buffered) was developed to simulate incomplete bromide (Br^-) oxidation during short prechlorination periods because it is the first step in a complex system of reactions that leads to disinfectant loss and DBP formation. Hypochlorous acid (HOCl+Br-→kHOClHOBr+Cl-) and molecular chlorine (Cl2+Br-+H2O→kCl2HOBr+2Cl-+H+) were the free chlorine species relevant to Br^- oxidation, and Cl₂ hydrolysis and formation reactions (Cl2+H2O+A-⇌k-4k4HOCl+HA+Cl-) were necessary to accurately simulate Cl₂ concentrations instead of assuming equilibrium. Previous work has shown that Br^- oxidation by HOCl and Cl₂ formation are acid-catalyzed and Cl₂ hydrolysis is base-catalyzed, but the impact of carbonate species had not been studied. This work showed that the carbonate species have an enhanced catalytic impact with rate constants up to 1000 times larger than would be estimated by the Brønsted relationship for similar acids, which causes the oxidation by HOCl rate constant (k_HOCl) to nearly double and oxidation by Cl₂ to occur above pH 7 in high-alkalinity waters.