Efficient electro-oxidation-based degradation of per- and polyfluoroalkyl (PFAS) persistent pollutants by using plasma torch synthesized pure-Magnéli phase-Ti4O7 anodes

Pure Magnéli-phase Ti₄O₇ were prepared by means of a Plasma Torch (PT) coating method and integrated into an advanced electro-catalytic oxidation (AEO) process in order to degrade perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) persistent pollutants present in waters. The X-ray diffraction analysis confirmed the polycrystalline nature of the pure Magnéli phase PT-Ti₄O₇ coatings (∼100 μm thick)). The Raman spectra of the PT-Ti₄O₇ coatings also exhibited the two characteristic peaks (at 138 and 183 cm^-1) of the PT-Ti₄O₇ Magnéli phase. Scanning electron microscopy revealed the nanostructured hierarchical morphology of the PT-Ti₄O₇ thus conferring them high surface area. The PT-Ti₄O₇ anodes are shown to achieve higher degradation efficiencies towards PFOA and PFOS in comparison with the conventional boron-doped diamond anodes. By investigating several AEO parameters (including current density, treatment time, nature of the anode material), we were able to optimise the AEO process. Thus, for both PFOA and PFOS (at an initial concentration of 500 ppb in synthetic wastewaters), degradation efficiencies as high as 96.6% and 99.7% were achieved, respectively, with a current density of 20 mA/cm², a treatment time of 120 min and PT-Ti₄O₇ mesh-type anodes. PFOA and PFOS can be degraded by both direct anodic electrochemical oxidation (^•OH radicals) and indirect electrochemical oxidation via mediators, such as persulphate acid (H₂S₂O₈) generated by sulphate anodic oxidation. The degradation of both compounds followed pseudo-first-order kinetics. The reaction rate constant (k) for PFOS removal was 4.63 × 10^-2 min^-1, whereas 2.76 × 10^-2 min^-1 was recorded for PFOA removal. Subsequently, we have used the above optimal AEO operating conditions to treat real wastewater effluents (containing 17 types of PFAS molecules with a total content of 8500 ppb) and achieved a degradation rate of 39.1%-87.4% for eight of the 17 PFAS compounds. The degradation rate was found to be dependent on the chemical structure and chain length of each PFOA/PFOS component.