Plasma–Liquid Interfacial F/OH Exchange Drives PFAS Defluorination in a Piezoelectric Direct Discharge System

Per- and polyfluoroalkyl substances (PFAS) pose persistent environmental risks due to the extreme stability of C–F bonds, challenging conventional remediation technologies. Herein, a piezo direct discharge (PDD) plasma system is developed for rapid, energy-efficient PFAS defluorination at the plasma–liquid interface under ambient conditions. Leveraging interfacial enrichment and confined plasma activation, the PDD system achieved near-complete degradation within 10 min, with 97.1% removal and 86.6% defluorination for perfluorooctanoic acid (PFOA), and 99.7% removal with 48.5% defluorination for perfluorooctanesulfonic acid (PFOS). Combined multitechnique analysis (ion chromatography, LC-MS/MS with D2O isotope tracing, reactive species characterization, and fluoride mass balance) and density functional theory calculations suggest that F/OH exchange is a key initial pathway contributing to PFAS defluorination. This process involves synergistic C–F bond activation by energetic and solvated electrons (e–/eaq–), and subsequent hydroxyl substitution mediated by acidification-stabilized interfacial •OH radicals, confirmed by characteristic Cn–F + OH and Cn–2F + 2OH intermediates. The system exhibits promising applicability across various PFAS and scalability in the continuous liquid-film mode. Overall, this work establishes PDD plasma as a mechanistically resolved and energy-efficient platform for PFAS remediation, offering actionable insights for kinetic modeling and concentration-and-destroy treatment strategies.