Production of hydroxylated polybrominated diphenyl ethers (OH-PBDEs) from bromophenols in natural and engineered processes

Abstract

The emerging environmental pollutant hydroxylated polybrominated diphenyl ethers (OH-PBDEs) have been of particular concern in recent years due to their ubiquitous distribution in environmental matrices and adverse health effects. Since there is no industrial-scale manufacturing of OH-PBDEs, their origins are inconclusive and not fully understood, thus triggering extensive discussions among the academic community. Recently, the polymerization of simple bromophenols (BrPs) has been identified as an important origin of OH-PBDEs, which are widely reported in various oxidation processes. This review comprehensively surveyed previous studies about OH-PBDEs formation from BrPs precursors in both natural (e.g., photochemical processes, naturally occurring manganese oxides, enzymes-mediated oxidation) and engineered processes (e.g., ozonation, chlorination, permanganate/ferrate associated oxidation, radicals based oxidation, nonradically activated persulfates). Generally, oxidative coupling reactions of bromophenoxyl radicals resulting from BrPs were recognized as the main mechanism for OH-PBDEs formation, which were discussed emphatically by combining theoretical calculations with the experimental evidence. Some discrepancies were found in interpreting the preferential coupling pathway of bromophenoxyl radicals according to different calculation methods. So, further efforts are needed to use advanced analytical methods for sensitive identification of OH-PBDEs to better evaluate the formation potentials of diverse OH-PBDEs from BrPs. Carbon materials associated oxidation technologies (e.g., carbon nanotube activated ferrate or persulfates) may be applied as a promising method for controlling OH-PBDEs during treatment of BrPs-containing waters. Overall, this review reveals a prevalent phenomenon of BrPs transformation to OH-PBDEs and meanwhile advances the mechanistic understanding. Moreover, the knowledge gaps are identified and some recommendations are proposed for future studies.