This study investigated the UV254 photolysis of free available chlorine and bromine species in water. The intrinsic quantum yields for •OH and X• (X = Cl or Br) generation were determined by model fitting of formaldehyde formation using a tert-butanol assay to be 0.61/0.45 for HOCl/OCl– and 0.32/0.43 for HOBr/OBr–. The steady-state •OH concentration in UV/HOX was higher than that in UV/OX– by a factor of 23.3 and 7.8 for Cl and Br, respectively. This was attributed to the different •OH consumption rate by HOCl versus OCl–, while for HOBr/OBr–, both the •OH formation and consumption rates were implied. This was supported by a k of 1.4 × 108 M–1 s–1 for the •OH reaction with HOCl, which was >14 times less than the k for •OH reactions with OCl–, HOBr, and OBr–. Formation of ClO3 – and BrO3 – was found to be significant with apparent quantum yields of 0.12–0.23. A detailed mechanistic study on the formation of XO3 – including a new pathway involving XO• is presented, which has important implications as the level of XO3 – can exceed the regulation (BrO3 –) or guideline (ClO3 –) values during UV/halogen oxidant water treatment. Our new kinetic models well simulate the experimental results for the halogen oxidant decomposition, probe compound degradation, and formation of ClO3 – and BrO3 –.

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