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

This study investigated the UV 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 -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 of 1.4 × 10 M s for the OH reaction with HOCl, which was >14 times less than the for OH reactions with OCl, HOBr, and OBr. Formation of ClO and BrO was found to be significant with apparent quantum yields of 0.12-0.23. A detailed mechanistic study on the formation of XO including a new pathway involving XO is presented, which has important implications as the level of XO can exceed the regulation (BrO) or guideline (ClO) 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 ClO and BrO.