Enhanced formation of trihalomethane disinfection byproducts from halobenzoquinones under combined UV/chlorine conditions

Halobenzoquinones (HBQs) are highly toxic disinfection byproducts (DBPs) and are also precursors of other DBPs such as trihalomethanes (THMs). The formation of THMs from HBQs during chlorine-only and UV/chlorine processes with or without bromide was investigated experimentally. Density functional theory (DFT) reactivity descriptors were also applied to predict the nucleophilic/electrophilic reactive sites on HBQs and intermediates. The results were combined to explain the different behaviors of 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ) and tetrachloro-1,4-benzoquinone (TCBQ) and to propose mechanism for the promoting roles of UV and hydroxylation of HBQs in THMs formation. Under UV/chlorine, UV significantly enhanced THMs formation from 2,6-DCBQ compared to chlorine-only, mainly due to the production of OH-DCBQ*. Excited 2,6-DCBQ* by UV benefited nucleophilic hydrolysis to produce OH-DCBQ*, which favored electrophilic attack by chlorine, thereby inducing more THMs formation. UV/chlorine modestly promoted THMs formation from TCBQ compared to chlorine-only. Hydroxylation of TCBQ and UV irradiation were both important in promoting THMs formation due to the high electrophilic property of OH-TCBQ and TCBQ*. Meanwhile, hydroxylation of HBQs and CHCl3 formation were enhanced at higher pH. This work suggested that enhanced formation of THMs from HBQs should be considered in the application of combined UV and chlorine processes.