Electroreductive Defluorination of Unsaturated PFAS by a Quaternary Ammonium Surfactant-Modified Cathode via Direct Cathodic Reduction

Abstract

Remediation of per- and polyfluoroalkyl substances (PFAS) in groundwater remains a technological challenge due to the trace concentrations of PFAS and the strength of their C–F bonds. This study investigated an electroreductive system with a quaternary ammonium surfactant-modified cathode for degrading (E)-perfluoro(4-methylpent-2-enoic acid) (PFMeUPA) at a low cathodic potential. A removal efficiency of 99.81% and defluorination efficiency of 78.67% were achieved under −1.6 V (vs Ag/AgCl) at the cathode modified by octadecyltrimethylammonium bromide (OTAB). The overall degradation procedure started with the adsorption of PFMeUPA onto the modified cathode. This adsorption process was promoted by hydrophobic and electrostatic interactions between the surfactants and PFMeUPA, of which the binding percentage, binding mode, and binding energy were determined via molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The step-wise degradation pathway of PFMeUPA, including reductive defluorination and hydrogenation, was derived. Meanwhile, C–F bond breaking with direct electron transfer only was achieved for the first time in this study, which also showed that the CC bond structure of PFAS facilitates the C–F cleavage. Overall, this study highlights the crucial role of quaternary ammonium surfactants in electron transfer and electrocatalytic activities in the electroreductive system and provides insights into novel remediation approaches on PFAS-contaminated groundwater.

Electrochemical degradation of a C6-perfluoroalkyl substance (PFAS) using a simple activated carbon cathode

“…These results show a 27.1% increased defluorination percentage in 86.7% less time compared to the only other study investigating PFMeUPA electrochemical reduction. 27 Note that the 85 °C control experiments achieved at least 95% recovery of added PFMeUPA after desorption (Fig. S6†) indicating these experiments were not impacted by sorptive or volatile losses.…”

“…26 An electrode material that has a high surface area and functional groups that encourage sorption of PFAS, such as GAC, should increase the likelihood of reaction between any reducible PFAS and advanced reductive radical species produced, thereby minimizing any competing reactions with aqueous contaminants, and increasing the possibility of PFAS reduction as shown by using a quaternary ammonium surfactant modified cathode. 27…”

“…anodes), with very few studies focused on electrochemical reduction. 27,31–34 There are, however, a large number of studies focused on advanced reductive processes, such as the use of UV-photocatalysts for PFAS reduction, 23 or chemical reductive processes involving zinc as an electron donor. 32,35–37…”

“…34 Lastly, Wang et al investigated PFMeUPA reductive defluorination using a surface modified carbon sheet as the cathode and platinum, an expensive material, as the anode. 27…”

“…26 An electrode material that has a high surface area and functional groups that encourage sorption of PFAS, such as GAC, should increase the likelihood of reaction between any reducible PFAS and advanced reductive radical species produced, thereby minimizing any competing reactions with aqueous contaminants, and increasing the possibility of PFAS reduction as shown by using a quaternary ammonium surfactant modified cathode. 27 Whilst there are a multitude of effective electrochemical treatments for PFAS reported in the literature, the majority of these are based on expensive electrode materials such as boron-doped-diamond, 28 or mixed metal oxide materials containing toxic metals such as antimony, tin or nickel. 29 Of the few inexpensive electrodes that display low toxicity that are able to effectively mineralize PFAS, their synthesis is complex and involves sintering at temperatures of 1300 °C or higher.…”