Mechanistic Insights into Plasma-Liquid Interfacial Degradation of Perfluorooctanoic Acid (PFOA) in an Argon RF Plasma Jet

Radio frequency (RF) driven atmospheric pressure plasma jets (APPJs) operate in a non-equilibrium regime that enables highly localized energy deposition and reactive species delivery to gas-liquid interfaces. In this study, 10 mg L⁻¹ perfluorooctanoic acid (PFOA) was treated in a 13.5 mL recirculating, flow-through reactor contacted by an RF argon APPJ. The parent PFOA was removed to below detection within two hours, while fluoride release continued and reached up to ~ 80% after three hours. Identified short-chain perfluoroalkyl acids (C ₂ -C ₇ ) accounted for ~ 12% of the initial mass. Systematic variation of discharge power, liquid flowrate, radical and electron scavengers, and processing gas composition was used to elucidate the dominant degradation pathways. PFOA removal was unaffected by liquid flowrate but increased with discharge power, indicating that degradation is controlled by interfacial plasma processes rather than bulk hydrodynamic transport. Fluorine mass balance and scavenger experiments demonstrate that the majority of fluoride release (> 70%) occurs directly during the initial interfacial dissociation of parent PFOA, driven by plasma-generated species. Hydroxyl radical scavenging had no effect on PFOA degradation but significantly suppressed fluoride formation, indicating a secondary role in oxidizing partially fluorinated intermediates. In contrast, AgNO ₃ , a solvated-electron probe, produced minimal changes in degradation or defluorination trends. Gas substitution experiments (Ar, He, Ar/H ₂ , Ar/N ₂ ) revealed substantially reduced defluorination in helium, supporting a mechanistic picture in which dominant initiating steps occur near the plasma-liquid interface and depend on gas-phase discharge characteristics, particularly electron density and vacuum ultraviolet (VUV) photon flux generation.