UV/FeⅡNTA as a novel photoreductive system for the degradation of perfluorooctane sulfonate (PFOS) via a photoinduced intramolecular electron transfer mechanism

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant that is toxic and bio-accumulative. Previously, we used hydrated electrons (eaq–) generated by the UV photolysis of nitrilotriacetic acid (NTA) to initiate the photoreductive decomposition of PFOS. However, due to the protonation of NTA and the scavenging effect of H+ on eaq–, this process relies highly on alkaline conditions. Herein, we report on an enhanced UV photoreductive system based on FeⅡNTA, which results in the decomposition of PFOS at pH 8.0 under anoxic conditions. After 10 h of photolysis, the degradation and defluorination efficiencies of PFOS in the UV/FeⅡNTA system were ~ 60% and 29.5%, respectively, with a pseudo first-order degradation rate constant of kobs = 0.081 h−1. Laser flash photolysis results combined with time-dependent density functional theory (TDDFT) calculations indicate that PFOS, Fe(H2O)62+, and NTA form a penta-coordinated metal–ligand complex that undergoes a UV-induced directional electron transfer from FeⅡNTA to PFOS. PFOS decomposes via a mechanism that proceeds through a concerted photoinduced intramolecular charge transfer instead of direct attack by eaq–. Model chelate studies show that the inherent properties of the transition metal ion and the electron-donating capabilities of the complexing ligands determine the efficiency for photoreductive electron transfer. A low apparent activation energy of 4.74 kJ/mol over a broad pH range results in higher electron transfer efficiencies for UV/FeⅡNTA photolysis compared to photolysis initiated by un-complexed NTA.