Mechanistic and Kinetic Understanding of the UV254 Photolysis of Chlorine and Bromine Species in Water and Formation of Oxyhalides

Abstract

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^-.