Electrochemical Debromination of Brominated Aromatic Flame Retardants Using Activated Carbon-Based Cathodes

Both legacy (e.g., polybrominated diphenyl ethers; PBDEs) and emerging (e.g., hexabromobenzene) flame retardants frequently feature brominated aromatic motifs, which are associated with persistent and bioaccumulative properties. While activated carbon has been used to treat brominated aromatics, it is a passive sorbent and does not degrade them. Using model bromobenzenes, this study illustrates debromination of brominated aromatics sorbed to activated carbon when the carbon is fashioned into cathodes and treated at a -1.3 V/SHE applied potential. Debromination rates fit a quantitative structure-activity relationship (QSAR), increasing with increasing free energy changes (ΔG) for a two-electron transfer to the brominated aromatic. Half-lives ranged from ∼4 min for hexabromobenzene to ∼15 d for bromobenzene, although the debromination half-life for bromobenzene decreased to ∼50 h at -1.8 V/SHE. The QSAR for bromobenzene debromination was also predictive for debromination of two PBDEs (BDE-99 and BDE-47), indicating that the QSAR was broadly applicable across brominated aromatic structures. Debromination released bromide to the catholyte, while lower-order brominated aromatic intermediates remained sorbed to the cathode. Debromination rates roughly correlated with the conductivity of the black carbon. The strong sorption capacity of carbon-based cathodes permits sequestration of brominated aromatics from contaminated waters within short hydraulic residence times, while an electric potential can be periodically applied to debrominate contaminants sequestered on the carbon.