Electro-Oxidation and Electro-Fenton Degradation of PFASs Using a Grid-Shaped Ti4O7 Magnéli-Phase Anode: Effect of Concentration and Evidence of Defluorination

The persistence of per- and polyfluoroalkyl substances (PFASs) in aquatic environments requires efficient and sustainable treatment technologies. In this study, the electrochemical degradation of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) was investigated using a grid-shaped Ti4O7 Magnéli-phase anode under electro-oxidation (EO) and electro-oxidation coupled with electro-Fenton (EO-EF) conditions. Structural characterization confirmed the predominance of Ti4O7 in the electrode material. At an initial concentration of 2 ppm, PFOS was rapidly and almost completely removed under both EO and EO-EF, whereas PFOA exhibited slower degradation kinetics, identifying it as the kinetically limiting compound. Coupling EO with electro-Fenton mainly enhanced the degradation kinetics, particularly for PFOA, while final removal efficiencies remained comparable. The influence of initial concentration was further examined, showing that lowering the PFOA concentration to 0.2 ppm, representative of environmentally relevant levels, enabled nearly complete removal within 300 min. Fluoride ion monitoring under optimized EO-EF conditions confirmed partial defluorination, demonstrating that PFOA removal is accompanied by C-F bond cleavage. These findings highlight the respective roles of EO and EO-EF processes and support the potential of Ti4O7-based anodes for energy-competitive PFAS remediation.