Electrochemical Oxidation of Hexafluoropropylene Oxide Dimer Acid (GenX): Mechanistic Insights and Efficient Treatment Train with Nanofiltration
By Nasim E. Pica, Joanna Funkhouser, Yiming Yin, Zuoyou Zhang, Donato M. Ceres, Tiezheng Tong, and Jens Blotevogel
Environ. Sci. Technol.
October 13, 2019
Hexafluoropropylene oxide dimer acid (HFPO-DA, trade name GenX) is a perfluoroalkyl ether carboxylic acid (PFECA) that has been detected in watersheds around the world. Similar to other per- and polyfluoroalkyl substances (PFASs), few processes are able to break HFPO-DA's persistent carbon-fluorine bonds. This study provides both experimental and computational lines of evidence for HFPO-DA mineralization during electrochemical oxidation at a boron-doped diamond anode with a low potential for the generation of stable organofluorine intermediates. Our density functional theory calculations consider the major operative mechanism, direct electron transfer, throughout the entire pathway. Initial oxidative attack does not break the ether bond, but leads to stepwise mineralization of the acidic side chain. Our mechanistic investigations reveal that hydroxyl radicals are unreactive towards HFPO-DA, while electrochemically activated sulfate facilitates its oxidation. Furthermore, we demonstrate that a NF90 membrane is capable of removing 99.5% of HFPO-DA from contaminated water. Electrochemical treatment of the nanofiltration rejectate is shown to reduce both energy and electrode costs by more than one order of magnitude compared to direct electrochemical treatment of the raw water. Overall, a nanofiltration - electrochemical oxidation treatment train is a sustainable destructive approach for cost-effective elimination of HFPO-DA and other PFASs from contaminated water.