Enhanced PFAS Defluorination through Control of Radical-Dependent Degradation Pathways

Per- and polyfluoroalkyl substances (PFAS) are persistent pollutants that are resistant to conventional water treatment. This study investigates the electrochemical degradation of perfluorooctanoic acid (PFOA) in perchlorate and sulfate electrolytes with boron-doped diamond. Quantification of PFOA degradation and fluoride (F–) and short-chain perfluorocarboxylic acid (PFCA) formation showed similar defluorination efficiency in both electrolytes at 4.0 VRHE but significantly higher PFCA yields with the sulfate electrolyte. Through electrochemistry-mass spectrometry, 19F and 1H NMR spectroscopy, potential-dependent investigation, and radical-scavenger experiments, we showed that both electrolytes exhibit similar decarboxylation rates of the carboxylic group to evolve CO2. However, HO• primarily drove C–F cleavage, releasing F– and leaving the remaining fluorocarbon backbone hydrogenated, while SO4•– promoted C–C cleavage to form fully fluorinated shorter-chain PFCAs. These results highlight the distinct roles of free radicals in PFOA degradation, offering mechanistic insights to design electrolytes to promote complete PFAS defluorination and suppress the formation of short-chain PFAS.