UVC photolysis of chlorine and bromine species in aqueous solution: formation yields and consumption rates of primary radicals and oxyhalide production

Abstract

UV photolysis of chlorine and bromine is increasingly relevant for various water quality control engineering, however, the underlying chemistry has not been fully understood. This study investigated the formation yields and consumption rates of •OH and X• (X = Cl or Br) radicals and oxyhalide (XO3-) formation during UV photolysis at 254 nm of aqueous halogen oxidant species at pHs 5 or 6 (HOX) and 10 (OX-). Intrinsic quantum yields for the radical formation from the photodecomposition of HOCl, OCl-, HOBr, and OBr- were 0.61, 0.46, 0.27, and 0.44 respectively, which could be determined by measuring formaldehyde formation from tert-butanol as the radical scavenger and kinetic modeling of the system. The first-order degradation rate of nitrobenzene as an ·OH probe was larger by a factor of ~16 for HOCl than OCl- and a factor of ~8 for HOBr than OBr-, respectively. The observed large difference in the oxidation efficacy of organic solute cannot be explained by the differed radical formation quantum yields. The second-order rate constant (k) for the reaction of ·OH with OCl- was 100-fold larger than the k for ·OH with HOCl based on our new measurements. Thus, the large difference in the scavenging rate of ·OH by the chlorine species mainly accounts for the oxidation efficacy difference. For the bromine species, both the different UV absorption rate and ·OH scavenging rate by HOBr vs OBr- contributes to the oxidation efficacy difference by a factor of ~4 and ~2, respectively. The molar yields of oxyhalides at the UV fluence of 2000 mJ/cm2 were 9.2%, 7.7%, 8.3%, and 3.4% from HOCl, OCl-, HOBr, and OBr-, respectively. The ClO3- formation was decreased upon addition of tert-butanol, more significantly for HOCl than OCl-, which could also be explained by the different ·OH scavenging rate by the chlorine species. Kinetic models were constructed based on the several key reaction parameters from this study and literature (intrinsic quantum yields, ·OH scavenging rates), which could well simulate the oxidant decomposition, oxidation of organic solutes, and oxyhalide formation in UV/chlorine and UV/bromine system. For UV/chlorine system, the reaction between ClO2· and ClO· was newly found to be important for the fate of ClO· and ClO3- formation. Overall, the kinetic and mechanistic information from this study can be useful for modeling of contaminant degradation and oxyhalide formation in UV/chlorine and UV/bromine processes.