Production of Hydroxylated Polybrominated Diphenyl Ethers (OH-PBDEs) from Bromophenols by Manganese Dioxide

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are of significant concern because of their enhanced toxicological effects compared to PBDEs. Research to date has attributed the origin of OH-PBDEs to biological metabolism of PBDEs and natural production in the environment. However, it is unclear how OH-PBDEs are formed naturally. In this study, we explored the formation of OH-PBbEs via the oxidative transformation of simple bromophenols (BPs, e.g., 4-BP, 2,4-DBP, and 2,4,6-TBP) by birnessite (delta-MnO2). Results showed that OH-PBDEs were readily produced by delta-MnO2 with BPs as precursors. For example, oxidation of 2,4-DBP by delta-MnO2 yielded 2'-OH-BDE-68 and 2',5'-OH-BDE-25. Other OH-PBDEs, such as 6-OH-BDE-13, 2',5'-OH-BDE-3, 4'-OH-BDE-121, and 2',5'-OH-BDE-69, were detected from the reaction with 4-BP and 2,4,6-TBP. The formation of OH-PBDEs likely resulted from the oxidative coupling of bromophenoxy radicals. Mild acidic conditions enhanced while coexisting cations (e.g., Na+, Mg2+, and Ca2+) suppressed the transformation. Given the ubiquity of BPs and delta-MnO2, oxidation of BPs by delta-MnO2 and other metal oxides is likely an abiotic route for the formation of OH-PBDEs in the environment.