Exploration of PFAS degradation over lower titanium suboxides: Ti₆O₁₁ case

Herein, the Magneli phase is synthesised and applied to the electrochemical oxidation of PFAS. Rietveld refinement, together with FTIR and Raman spectroscopy, confirmed Ti₆O₁₁ as the predominant phase. EPR spectroscopy revealed the formation of SO₄^⦁-, OH^⦁, and O₂^⦁- radicals at the Ti₆O₁₁ adlayer, with the additional appearance of spin-trap adducts with carbon, attributed to PFAS degradation products. Quenching experiments point to SO₄^⦁- driving the initial step of the degradation, with subsequent degradation involving concerted action with the abundant OH^⦁, as confirmed by UPLC-MS/MS, with 94% degradation of PFOS and 67% of PFOA. Degradation was monitored under a potentiostatic regime, where electric energy per order was 0.8 kWh m^-3 and 1.7 kWh m^-3 for PFOS and PFOA, respectively, indicating easier PFOS degradation. Theoretical calculations predicted a 0.69 eV barrier for the decarboxylation of PFOA, after the initial radical formation, while PFOS degradation proceeds barrierlessly. Degradation efficiency in the river water with mixed PFAS panel (PFCAs C4-C14, C16, C18, and PFSAs C4-C10, C12) was excellent (∼95%) for long-chain PFAS, while lowered to ∼40% for PFSAs (C4-C5) and ∼20% for PFCAs (C7-C8). Ti₆O₁₁ presents an alternative to the widely studied Ti₄O₇ for the electrochemical oxidation of PFAS.