Radical-promoted formation of dibenzofuran during combined UV-chlorine treatment on mono-substituted diphenyl ether

Polybrominated diphenyl ethers (PBDEs) are a group of flame retardants which are frequently detected in the aquatic environment. PBDEs would convert into intermediates during some natural or artificial processes. A simple PBDEs congener, 2-bromodiphenyl ether (BDE-1), was targeted, and its transformation characteristics during combined UV-chlorine treatment were investigated in this study. It was found that BDE-1 could rapidly transform in combined UV-chlorine process, and HO[rad] was the predominant radical that contributed to BDE-1 degradation in ultrapure water system. The formation of toxic dibenzofuran (DF) and 2-hydroxydibenzofuran (2-OH-DF) was significantly promoted in combined UV-chlorine treatment, whose total yield (2.78%) was 1.46 times to that in UV treatment alone. Radical scavenger tests revealed that enhanced formation of DF-type products in combined UV-chlorine treatment was attributed to radical reactions. Cl[rad] or HO[rad] generated from free available chlorine photolysis reacted with ortho-carbon radicals generated from BDE-1 photolysis, and produced intermediates 2-chlorodiphenyl ether (2-Cl-DE) or 2-phenoxyphenol (2-OH-DE), which underwent intramolecular elimination of HCl or H₂O and formed DF and 2-OH-DF. The treatment with alkaline conditions, high chlorine doses and strong irradiation would be beneficial to the less formation of DF-type products. Besides, the matrix effects of ambient water also inhibited the formation of DF-type products. Overall, this work would provide an insight into the role of free radicals in dibenzofuran formation during combined UV-chlorine treatment on PBDEs and a scientific reference for optimizing operational parameters in the water treatment.