Electrochemical Oxidation of Perfluorooctanoic Acid on a Bi 2 O 3 –SnO 2 /Ti 4 O 7 Reactive Electrochemical Membrane: Application to Groundwater Treatment with Mechanistic Insights

This study investigated electrochemical oxidation of perfluorooctanoic acid (PFOA) using a Bi2O3–SnO2 catalyst supported on a Ti4O7 reactive electrochemical membrane (BTO/REM). In an inert NaClO4 electrolyte, the BTO/REM achieved 98.1 ± 0.4% PFOA removal, 85.6 ± 3.6% fluoride yield, and 94.4 ± 4.6% total fluorine balance at 4.2 V/SHE and a 60 s hydraulic residence time (tr). Perfluoroalkyl carboxylates (PFCAs) with three to six perfluoroalkyl carbons formed at a combined yield of 15.3 ± 2.6%. In real groundwater, PFOA removal was limited (∼1%) due to low ionic conductivity, but adding NaClO4 increased removal to ∼91% at 4.7 V/SHE with tr of 15 s. However, adding a Na2HP4 electrolyte to groundwater promoted electrode fouling via phosphate radical adsorption and CaHPO4/Mg­(OH)2 precipitation, which inhibited PFOA oxidation and increased short-chain PFCA formation. In contrast, Na2SO4 showed minimal surface adsorption and maintained high PFOA removal, suggesting it as a practical additive for enhancing conductivity in complex groundwater matrices. Mechanistic analysis indicated that electrode fouling shifted the dominant reaction pathway from surface-mediated defluorination to solution-mediated sequential PFCA formation. These findings provide actionable insights for optimizing electrochemical PFAS destruction and highlight the importance of electrolyte selection to maximize efficiency and minimize undesirable byproducts in real-world applications.