The photodegradation of polybrominated diphenyl ethers (PBDEs) in various environmental matrices: Kinetics and mechanisms

• The photodegradation rates of PBDEs in various matrices are compared.
• Factors affecting the photodegradation kinetics of PBDEs in various matrices are summarized.
• The photodegradation products and mechanisms of PBDEs in various matrices are summarized.
• Both direct and indirect photodegradation pathways are included.

The fate and transport of polybrominated diphenyl ethers (PBDEs) have raised a great deal of concern due to their persistence, bioaccumulative nature and adverse effects. Photodegradation is one of the important transformation processes for organic contaminants in the environment. In order to understand the photochemical behaviors of PBDEs, this study critically reviews the kinetics and mechanisms of PBDEs photodegradation in different matrices: (i) organic solvent phase; (ii) aqueous phase; (iii) gas phase; (iv) solid phase, and (v) TiO2-mediated phase. The half-lives of PBDEs in solid and gas phases are significantly longer than those in liquids. The degradation kinetics are affected by the number and positions of the bromines in PBDEs, solvent effects, the composition of any solid matrix, and the presence or absence of common environmental species such as humic substances, metal cations and halide anions. Regarding the photodegradation mechanisms, reductive debromination and intermolecular elimination of HBr are the two predominant degradation pathways in direct photolysis to produce lower brominated BDEs and polybrominated dibenzofurans (PBDFs), respectively. A variety of derivatives can be formed depending on the medium. Hydroxylated PBDEs (OH-PBDEs) and bromophenols are produced through the reaction of PBDEs with OH radicals in aqueous and atmospheric systems. Chlorinated BDEs form in the presence of chloride in water, and hydroxylated PBDFs (OH-PBDFs) and methoxylated PBDFs (MeO-PBDFs) form in methanol. Thus, it is important to scrutinize the photodegradation products and photochemical mechanisms of PBDEs under field-relevant conditions in environmental health monitoring and cleaning up contaminated sites.

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