Mechanisms and pathways of debromination of polybrominated diphenyl ethers (PBDEs) in various nano-zerovalent iron-based bimetallic systems.

This study investigated the relative significance of electron transfer (e-transfer) and H-atom transfer (H-transfer) in a nanoscale zerovalent iron (n-ZVI) system and six n-ZVI-based bimetallic systems (Fe/Cu, Fe/Ni, Fe/Pd, Fe/Ag, Fe/Pt, and Fe/Au) through a case study of the debromination of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47). The results revealed that the reactivities of these bimetallic particles to BDE-47 decreased in the following order: Fe/Pd>Fe/Ag>Fe/Cu>Fe/Ni>Fe/Au>Fe/Pt≈n-ZVI. Debromination of BDE-47 in metal-H2 systems suggested that Ni, Pd, Pt, Cu and Au can utilize H2 to debrominate BDE-47. In the H-transfer process, BDE-47 preferentially debrominated the para‑bromine substituent to generate BDE-17, whereas in the e-transfer process, BDE-47 preferentially debrominate ortho‑bromine substituent to generate BDE-28. The debromination pathways of BDE-47 in bimetallic and NaBH4-metal systems suggested that Fe/Ni, Fe/Pd and Fe/Pt debrominate polybrominated diphenyl ethers (PBDEs) through a H-transfer dominant mechanism, while Fe/Ag debrominate PBDEs through an e-transfer dominant mechanism. In the cases of Fe/Cu and Fe/Au, the e-transfer and H-transfer may be equally involved in the debromination of PBDEs. These results greatly improve our understanding of the relative significance of e-transfer and H-transfer in the dehalogenation of halogenated aromatic compounds (HACs) in various n-ZVI-based bimetallic systems.