Enhanced Bromate Formation during Chlorination of Bromide-Containing Waters in the Presence of CuO: Catalytic Disproportionation of Hypobromous Acid

Abstract

Bromate (BrO₃^–) in drinking water is traditionally seen as an ozonation byproduct from the oxidation of bromide (Br^–), and its formation during chlorination is usually not significant. This study shows enhanced bromate formation during chlorination of bromide-containing waters in the presence of cupric oxide (CuO). CuO was effective to catalyze hypochlorous acid (HOCl) or hypobromous acid (HOBr) decay (e.g., at least 10⁴ times enhancement for HOBr at pH 8.6 by 0.2 g L^–1 CuO). Significant halate concentrations were formed from a CuO-catalyzed hypohalite disproportionation pathway. For example, the chlorate concentration was 2.7 ± 0.2 μM (225.5 ± 16.7 μg L^–1) after 90 min for HOCl (<i>C</i>_o = 37 μM, 2.6 mg L^–1 Cl₂) in the presence of 0.2 g L^–1 CuO at pH 7.6, and the bromate concentration was 6.6 ± 0.5 μM (844.8 ± 64 μg L^–1) after 180 min for HOBr (<i>C</i>_o = 35 μM) in the presence of 0.2 g L^–1 CuO at pH 8.6. The maximum halate formation was at pHs 7.6 and 8.6 for HOCl or HOBr, respectively, which are close to their corresponding p<i>K</i>_a values. In a HOCl–Br^––CuO system, BrO₃^– formation increases with increasing CuO doses and initial HOCl and Br^– concentrations. A molar conversion (Br^– to BrO₃^–) of up to (90 ± 1)% could be achieved in the HOCl–Br^––CuO system because of recycling of Br^– to HOBr by HOCl, whereas the maximum BrO₃^– yield in HOBr–CuO is only 26%. Bromate formation is initiated by the formation of a complex between CuO and HOBr/OBr^–, which then reacts with HOBr to generate bromite. Bromite is further oxidized to BrO₃^– by a second CuO-catalyzed process. These novel findings may have implications for bromate formation during chlorination of bromide-containing drinking waters in copper pipes