Photochemical Transformation Of Emerging Pollutants Methoxylated Polybrominated Diphenyl Ethers In Water

In recent years, methoxylated polybrominated diphenyl ethers (MeO-PBDEs) have frequently been detected in non-biological environmental media, e.g., air, seawater and sediment. To assess the environmental risks of MeO-PBDEs, it is essential to understand their transformation in the environment. Previous studies proposed that photodegradation is a crucial way to remove polybrominated diphenyl ethers (PBDEs) and hydroxylated PBDEs. Therefore, MeO-PBDEs are hypothesized to photodegrade as well. To date, it is still unclear about the photodegradation kinetics, products and mechanisms of MeO-PBDEs. Based on these questions, relevant investigations were carried out and summarized as follows:(1) 4'-MeO-BDE-17,5-MeO-BDE-47,5'-MeO-BDE-99,6-MeO-BDE-47 and 6-MeO-BDE-85 frequently detected in the natural waters were selected to investigate their photodegradation. Meanwhile, the effects of dissolved oxygen in water on the photoreactions of MeO-PBDEs were also unveiled. Simulated sunlight experiments indicate that 6-MeO-BDE-47 resisted photodegradation for 20 h, while other MeO-PBDEs underwent relatively fast photodegradation with half-lives to be 67 ± 8?225 ± 22 min. Photo-excited MeO-PBDEs (except 6-MeO-BDE-47) can sensitize dissolved oxygen to generate singlet oxygen (1O2) and superoxide anion radical (O2-). O2 was difficult to degrade the MeO-PBDEs, whereas O2- was reactive with MeO-PBDEs. Less-brominated MeO-PBDEs products were detected for 5-MeO-BDE-47,5'-MeO-BDE-99 and 6-MeO-BDE-85, indicating hydrodebromination was a crucial photodegradation pattern for some MeO-PBDEs in natural waters.(2) 5-MeO-BDE-47,5'-MeO-BDE-99 and 6-MeO-BDE-85 were employed to investigate their photocyclization in water and methanol by simulated photochemical experiments and density functional theory (DFT) calculation. Experimental results show that photocyclization product methoxylated polybrominated dibenzofurans (MeO-PBDFs) can only be formed in the methanol solution of 5-MeO-BDE-47. DFT results indicate that the lowest excited triplet state (T1) of 5-MeO-BDE-47 can form MeO-PBDFs via direct cyclization pathway. Intra-annular H-elimination was the rate-determining step for all the cyclization pathways (excluding Pathway 1-4) with high reaction barriers (>19.7 kcal/mol); while the Pathway 1-4 in the methanol solution of 5-MeO-BDE-47 makes ring closure the rate-determining step with a lower barrier (13.8 kcal/mol), indicating this pathway is kinetically favorable. For the Pathway 1-4, H-elimination assisted by a Br cleaved from an ortho-C-Br bond was observed with a 2.0 kcal/mol barrier. Thus, the DFT results reasonably explained the experimental findings, and clarified the reason why photocyclization of MeO-PBDEs was dependent on substitution patterns of Br and environmental media.(3) Competition kinetics method was employed to determine the second-order reaction rate constants with·OH (kOH) for five MeO-PBDEs, including 4'-MeO-BDE-17,5-MeO-BDE-47, 5'-MeO-BDE-99,6-MeO-BDE-47 and 6-MeO-BDE-85. Meanwhile, the products were also analyzed. kOH values range between (0.531 ± 0.015) × 1010 and (1.030 ± 0.015) × 1010 M-1 s-1 5'-MeO-BDE-99 and 6-MeO-BDE-85 have higher ·OH reactivity than 5-MeO-BDE-47 and 6-MeO-BDE-47, respectively.5-MeO-BDE-47 has higher kOH than BDE-47, while 6-MeO-BDE-47 shows weaker ·OH reactivity than BDE-47. Only a hydroxylated MeO-PBDEs (HO-MeO-PBDEs) product arising from HO-addition was observed in the 5-MeO-BDE-47 solution, implying a underlying source of HO-MeO-PBDEs in natural waters. DFT calculation indicates the most favorable HO-addition product for 5-MeO-BDE-47 was 5-MeO-5'-HO-BDE-47. In the end, the relative contribution by direct photolysis and reaction with OH to the degradation of MeO-PBDEs in surface waters at 40°N latitude was estimated, indicating direct photolysis was the dominant degradation pathway for MeO-PBDEs with relative contribution to be above 80%.(4) Dissolved organic matter (DOM) from the Suwannee River including fulvic acid, humic acid and natural organic matter were employed to investigate their influences on the photodegradation of 5-MeO-BDE-47. Results indicate that DOM impeded the photodegradation of 5-MeO-BDE-47 through light screening (relative contribution to be 44% ?88%) and other quenching effects. The phenolic moieties of DOM as antioxidants effectively hampered the degradation of 5-MeO-BDE-47 by T1 state of DOM. DOM can also significantly keep 5-MeO-BDE-47 from oxidation by ·OH.