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

This study investigated the UV254 photolysis of free available chlorine and bromine species in water. The intrinsic quantum yields for (OH)-O-center dot and X-center dot (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)-O-center dot 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)-O-center dot consumption rate by HOCl versus OCl-, while for HOBr/OBr-, both the (OH)-O-center dot formation and consumption rates were implied. This was supported by a k of 1.4 x 10(8) M-1 s(-1) for the (OH)-O-center dot reaction with HOCl, which was >14 times less than the k for (OH)-O-center dot 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 center dot 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-.