Discharge Regime Transition and Salinity-Tolerant PFOA Defluorination by Nanosecond Pulsed Gas-Liquid Interface Plasma

The degradation of perfluorooctanoic acid (PFOA) in high-salinity wastewater remains a critical challenge, as conventional advanced oxidation processes suffer from severe scavenging of reactive species by coexisting ions. This study investigates how carrier gas composition and solution conductivity affect PFOA defluorination (50 mg/L) in a nanosecond pulsed gas-liquid interface plasma reactor. By systematically evaluating the effects of carrier gases (N 2 and O 2 ) and solution conductivities (0.01 to 22.6 mS/cm using NaCl and Na 2 SO 4 ), we describe an observed shift in the dominant macroscopic controlling variable for PFOA defluorination as solution conductivity increases. At low conductivity (~ 0.01 mS/cm), the defluorination rate is sensitive to the carrier gas composition: an N 2 atmosphere produced roughly twice the fluoride yield of an O 2 atmosphere at 240 s, consistent with a reductive contribution that may involve hydrated electrons. At high conductivity (6.2–22.6 mS/cm, NaCl or Na 2 SO 4 ), the reduced liquid impedance raised the injected energy per pulse by a factor of 1.6–1.9, and the N 2 /O 2 difference largely disappeared. Defluorination data from all four gas/salt combinations converged onto a single energy-dose curve, indicating that within the parameter range studied, cumulative energy input becomes a sufficient macroscopic descriptor of the observed defluorination rate. The underlying C–F bond cleavage remains electron-driven throughout, and the effects of ionic strength, interfacial PFOA enrichment, dissolved O 2 reduction, and discharge morphology cannot be independently separated in the present experimental design. These results suggest that nanosecond pulsed plasma can maintain defluorination performance in high-salinity matrices where conventional AOPs are strongly inhibited by radical scavenging.