Defluorination of perfluorooctanoic acid in alkaline dimethyl sulfoxide/water solutions: kinetics and carbon-focused redox mechanism insights.

The kinetics and mechanisms of perfluorooctanoic acid (PFOA, CF3-(CF2)6-COOH) defluorination were investigated in alkaline dimethyl sulfoxide/water (DMSO/H2O) solutions under various conditions. PFOA and its by-products were characterized and relatively quantified using ultra-high-pressure liquid chromatography coupled with mass spectrometry, while degradation efficiency was assessed by potentiometric fluoride measurements with an F⁻-selective electrode, complemented by scanning electron microscopy and energy-dispersive spectroscopy. The redox degradation mechanisms of PFOA were projected through a detailed description of redox reactions involving only carbon atoms within PFOA and its by-products, driven by OH·/OH⁻ and H2O/H· redox couples, into which OH· and H· are transient redox intermediates associated with electron-transfer events, rather than freely diffusing radical species; OH· and H· originated from the redox reactions between the electron donor OH- and the electron acceptors O2, PFOA and H2O. The nucleophilic and/or electrophilic nature of redox reactions depends on the carbon oxidation state. The most favorable PFOA degradation pathway involves the stepwise oxidation of carbon driven solely by OH·/OH⁻ redox couple, leading to the elimination of a -CF2- unit as 2 F⁻ and CO32⁻. The OH·/OH⁻ and H·/H2O redox couples mediate secondary electron-transfer pathways, resulting in the simultaneous oxidation and reduction of two different carbon atoms within the same molecule. Water content and its continuous formation during the redox reactions limit the process by enhancing the solvation of OH⁻, thereby decreasing the oxidizing capacity of the OH·/OH⁻ redox couple. The higher the NaOH/PFOA molar ratio (≥ 62:1), the lower the H2O content (DMSO/H2O volume ratio ≥ 3.87:1) and the higher the temperature (≥ 120 °C), the greater the PFOA defluorination.