High quantum yield (1.4 mol Es−1) of hydroxyl radicals (∙OH) from photolysis of chlorine under typical disinfection conditions indicates the potential of UV/chlorine coexposure in serving as both disinfection and advanced oxidation processes (AOP). In this study, photolysis of chlorine and bromine was explored in buffer and simulated natural water solutions under low-pressure UV (LPUV) and medium-pressure UV (MPUV) lamps. At pH 6.5 and 8.5, the quantum yields of bromine photolysis were 3.8 and 0.6 for MPUV, and 4.4 and 0.8 for LPUV, respectively. At pH 6.5, the photolysis of bromine was faster than that of chlorine under either MPUV or LPUV source, while at a higher pH of 8.5, the contrary was found. For all conditions tested, the presence of bromide did not significantly change the observed photolysis rate of total free halogen during the UV/chlorine process in the presence or absence of natural organic matter. Upon UV irradiation, chlorine always produces higher ∙OH concentration than bromine does. The presence of bromide results in considerable decrease of ∙OH concentration at pH 6.5, compared to that obtained from the UV/chlorine process in the absence of bromide.

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