Perfluorobutanesulfonate (PFBS) Treatment by Electrical Discharge Plasma: Reaction Mechanisms and Degradation Pathways

Electrical discharge plasmas rapidly degrade short-chain (SC) per- and polyfluoroalkyl substances (PFAS) in the presence of sacrificial surfactants. These surfactants facilitate the transport of PFAS to the plasma-liquid interface through electrostatic and hydrophobic interactions, where PFAS and surfactants are ultimately degraded. This study investigates the degradation of perfluorobutanesulfonate (PFBS) by nonthermal plasma, both in the absence and presence of quaternary alkyl trimethylammonium surfactants: octyl-, dodecyl-, and hexadecyl trimethylammonium bromide (C8TAB, C12TAB, and C16TAB, respectively). Advanced analytical techniques, including targeted, suspect, and nontargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS), gas chromatography (GC-MS headspace), and ion chromatography (IC), were employed to identify the degradation byproducts of PFBS and the surfactants. Suspect and nontargeted analyses (NTA) revealed the formation of shorter-chain perfluorocarboxylic acids (PFCAs), products from H/F and OH/F exchange reactions, fluorinated ketones, fluorinated alcohols, unsaturated fluorinated compounds, and shorter-chain perfluorosulfonic acids (PFSAs); however, the last has been ascribed to contamination in the PFBS reagent. Notably, the degradation of PFBS, both in the absence and presence of surfactants, produced a nearly identical set of byproducts. Based on these newly identified byproducts, a series of degradation pathways has been proposed, involving solvated electrons and OH radicals as the primary reactive species. This study provides critical insights into the complex mechanisms and pathways of PFBS degradation during nonthermal plasma treatment. The findings have significant implications for optimizing plasma technologies and other PFAS treatment methods, with the proposed pathways expected to be relevant for PFAS degradation by technologies that utilize oxidative and nonoxidative species.