Debromination of polybrominated diphenyl ethers (PBDEs) by zero valent zinc: Mechanisms and predicting descriptors

Polybrominated diphenyl ethers (PBDEs) are a class of brominated flame retardants that are ubiquitous in the environment. The physical and chemical properties of PBDEs make them difficult to degrade, with the conventional remediation methods being relatively inefficient. In this study, the reactivity of zero valent zinc (ZVZ) toward 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) was evaluated under aqueous solution. First-order rate constants (k_obs) for BDE-47 disappearance increased with decreased pH, which is attributed to the dissolution of surface zinc oxides that promote the contact between the active site on zinc surface and BDE molecules. The k_obs of ten investigated PBDEs in ZVZ system are positively correlated with the energy of lowest unoccupied orbitals (E_LUMO) of PBDEs (R² = 0.902). The debromination pathways of BDE-47 in ZVZ system are: BDE-47 → BDE-28 → BDE-15 → BDE-3 → DE, which is the same to the debromination pathways of BDE-47 in zero valent iron (ZVI) in previous study. In addition, the singly occupied molecular orbitals (SOMOs) of the BDE anions can well reflect the actual debromination pathways of PBDEs by comparing the size of the CBr antibonding characterized lobes. Our results suggest that the debromination of PBDEs by ZVZ is based on the electron transfer mechanism, and the SOMOs of BDE anions can be used to predict the debromination pathways of untested PBDEs.