Low-temperature, ambient-pressure and rapid mineralization of per- and polyfluoroalkyl substances by molten alkali

Per- and polyfluoroalkyl substances (PFAS) exhibit extreme environmental persistence due to the high strength of C-F bonds. This study presents an innovative NaOH-KOH binary eutectic molten system capable of achieving rapid and complete PFAS mineralization under mild conditions. The broad-spectrum efficacy was primarily assessed for representative perfluoroalkyl sulfonic/carboxylic acids. All the test compounds, spanning different carbon chain lengths and head groups, showed the consistent defluorination approaching 100 % in molten alkali (NaOH:KOH = 3:7 molar ratio, 200 °C, ambient pressure). The detailed mechanism investigation was further performed on the typical refractory perfluoroalkyl sulfonic acid, i.e., perfluorooctanesulfonate (PFOS). The formate, carbonate, oxalate, and fluoride were identified as the major end products of PFOS degradation, while 1H-perfluorohept-1-ene, 1H-perfluorohexane, perfluorooct-2-enoic acid and perfluorohept-1-en-1-ol were detected as the transient intermediates by high-resolution time-of-flight mass spectrometer. An initial OH⁻-catalyzed cleavage of sulfonate head group, leading to the formation of per- and polyfluoroalkenes, followed by sequential hydroxylation and decarboxylation reactions, is proposed as the dominant mechanism responsible for complete mineralization of PFOS in molten alkali. All the findings here open up a novel way for refractory PFAS destruction with potentials in treating PFAS-laden wastes (e.g., spent activated carbon, industrial residues, sewage sludge).