Direct and dissolved oxygen involved photodegradation of MeO-PBDEs in water

Abstract

Photodegradation has been proved to be a crucial way of elimination for polybrominated diphenyl ethers (PBDEs) and hydroxylated PBDEs (HO-PBDEs). However, it is still unknown whether methoxylated PBDEs (MeO-PBDEs) can also undergo photodegradation. In this study, 4′-MeO-BDE-17, 5-MeO-BDE-47, 5′-MeO-BDE-99, 6-MeO-BDE-47 and 6-MeO-BDE-85 were selected as targets to investigate their photodegradation in water. Meanwhile, the effects of dissolved oxygen on the photoreactions of MeO-PBDEs were also unveiled. Simulated sunlight experiments indicate that 6-MeO-BDE-47 resisted photodegradation for 20h, while other MeO-PBDEs underwent relatively fast photodegradation, which was greatly susceptible to the substitution patterns of methoxyl and bromine. Photo-excited MeO-PBDEs (except 6-MeO-BDE-47) can sensitize dissolved oxygen to generate singlet oxygen (1O2) and superoxide anion radical (O2−). The generated 1O2 cannot degrade the MeO-PBDEs, whereas O2− was reactive with MeO-PBDEs. The contribution of dissolved oxygen to the photodegradation of 4′-MeO-BDE-17 and 6-MeO-BDE-85 was negligible; while the negative contribution was observed for 5-MeO-BDE-47 and 5′-MeO-BDE-99. Hydrodebromination was a crucial photodegradation pathway for MeO-PBDEs (excluding 4′-MeO-BDE-17 and 6-MeO-BDE-47). Eventually, direct photolysis half-lives of MeO-PBDEs except 6-MeO-BDE-47 in the surface waters at 40 N latitude were calculated to be 1.35–3.46d in midsummer and 6.39–17.47d in midwinter.