Parametric study on the degradation of perfluorooctane sulfonic acid (PFOS) using in-liquid and above-liquid non-thermal plasma reactors at atmospheric pressure

Non-thermal plasma (NTP) represents a promising technological approach for the treatment of water contaminated by recalcitrant pollutants, including per- and polyfluoroalkyl substances (PFAS). The main objective of this work was to examine the degradation of perfluorooctane sulfonic acid (PFOS) using two distinct NTP reactors: in-liquid plasma (ILP), and above-liquid plasma (ALP), operated at atmospheric pressure and ambient air. A parametric study examined the influence of key operational factors on PFOS degradation, including applied voltage, frequency, conductivity, and inter-electrode distance, followed by an evaluation of degradation as a function of treatment time under selected conditions. PFOS was routinely quantified using UV-Vis spectroscopy for rapid monitoring, while PFAS degradation byproducts were identified and quantified by LC-MS/MS. Results demonstrated significantly higher degradation efficiency in the ALP reactor (up to 93%) compared to ILP (max 22%) after 60 min of treatment. ICCD imaging revealed distinct discharge morphologies in both reactors. Trace amounts of several perfluoroalkyl carboxylic acids (PFCAs; C4–C8) and a short-chain perfluorosulfonic acid (PFSA; C4) were identified as degradation byproducts. A fluorine mass balance revealed that 34% of fluorine was attributed to residual PFOS and its byproducts by the end of treatment in the ALP reactor.