Electrochemical Reduction of PFOA in Organic Media: A Step toward Environmental Remediation

Perfluorooctanoic acid (PFOA) is one of the most concerning perfluoroalkyl substances (PFAS), and several strategies have been proposed for its degradation in water. Among these, electrochemical reduction is a promising approach despite its inherent chemical limitations, mainly the electrochemical potential window of water. In this study, we explore the reductive behavior of PFOA in an aprotic water-free solvent (acetonitrile) to gain mechanistic insights into the electrochemical defluorination. Cyclic voltammetry (CV) of protonated PFOA reveals a chemically irreversible reduction process at a peak potential of −2.55 V vs SCE on a glassy carbon electrode (GCE). Besides, the strong adsorption of PFOA onto GCE was observed in the peak at −1.70 V vs SCE. The deprotonated form (PFO–) shows no electrochemical activity, except in a solvent mixture of acetonitrile and 8 M H2O. CV characterization suggests that both PFOA and PFO– (+ 8 M H2O) undergo a concerted C–F bond cleavage since the electron transfer coefficient (αexp) was below 0.5. The latter was consistent with a Marcus–Hush electron transfer approach, which showed no significant thermodynamic differences between both systems, giving similar activation free energies (ΔG‡) of about 2 eV. Density functional theory (DFT) calculations support the proposed concerted mechanism, with calculated standard reduction potentials (E0) in agreement with experimental values. Controlled-potential electrolysis shows that the byproducts obtained depend on the protonation state of PFOA. Spectroscopic characterization (via NMR, UV–Vis spectroscopy, and LC-MS/MS) exhibited multiple partially hydrogenated and unsaturated products during the PFOA reduction. In the case of PFO– in the acetonitrile/water mixture, a lower defluorination was observed, leading mainly to hydrogenated compounds.