New insights into the reaction mechanisms and kinetics of aromatic compounds degraded by ClO· and BrO· to produce disinfection by-products

The degradation of aromatic compounds by chlorine oxygen radicals (ClO·) and bromine oxygen radicals (BrO·) generated in the UV/chlorine process have been widely investigated. However, the transformation process and reaction mechanisms of the disinfection by-products (DBPs) generated during the degradation of aromatic compounds by ClO· and BrO· are still unclear. In this study, we used theoretical calculation methods investigated the reaction mechanisms and kinetics of DBPs generated during the degradation of seven phenolic compounds and two aromatic compounds by ClO· and BrO·. The most favorable types of initial reactions for these nine compounds reacting with ClO· (BrO·) were addition reactions and reactions that abstracted hydrogen atoms from the hydroxyl group. While being degraded by free radicals, these compounds were also photolyzed by UV light. Cl-transfer reactions and -CH3 group chain-breaking reactions were key reactions for the generation of phenolic DBPs and aliphatic DBPs, respectively. During degradation by free radicals, DBPs generated after 5 hours of degradation from aromatic compounds containing -CH3 groups were mainly monochloromethane and dichloromethane. The main DBPs generated from phenolic compounds containing -CH3 groups were halogenated phenolic DBPs. However, compounds only containing the electron-withdrawing functional groups generated very low concentrations of DBPs (less than 1 nM). These findings provided meaningful insights into the reaction mechanisms and kinetics of the generation of aliphatic DBPs and phenolic DBPs from aromatic compounds at the molecular level.