Fe(III)-mediated PFAS phototransformation under UV222 irradiation: LMCT initiation chemistry and performance in electroplating wastewater.

Per- and polyfluoroalkyl substances (PFAS) are highly resistant to photochemical degradation due to their weak UV absorption and strong C-F bonds. Fe(III) chelation can enhance PFAS photoreactivity via ligand-to-metal charge transfer (LMCT), yet the wavelength/PFAS structure dependence and underlying mechanisms under environmentally relevant conditions remain unresolved. Here, Fe(III)-mediated photochemical transformation of perfluorooctanoic acid (PFOA, 0.1 μM) was investigated under monochromatic irradiation from 222 to 405 nm. Electrochemical measurement indicates strong Fe(III)-PFOA complexation (conditional Kf = 6 × 107, pH = 3), extending PFOA photoreactivity to longer wavelengths and enabling highly efficient far-UVC transformation (99 % at 2500 mJ cm-2). Although •OH were detected by EPR, quenching and control experiments indicate that LMCT-triggered electron transfer initiates direct photodecarboxylation of the [C7F15COOFe]2+·5H2O complex (∆G≠ = 38 kJ mol-1), producing Fe(II), CO2, and •C7F15; while •OH mainly contributes to Fe(II)/Fe(III) cycling and further oxidation of PFOA intermediates. Transformation product analysis revealed chain-shortening, C-F cleavage, and hydroxylation/oxygenation pathways, leading to progressive defluorination (54 % after 2 h) and declining trend of cytotoxicity. LMCT-driven phototransformation by UV222 appears to be influenced more strongly by perfluorocarbon chain length and H-substitution than by headgroup among the studied PFAS: PFOA ≈ PFOS > PFHpA > 6:2 FTS > PFHxA ≈ PFHxS ≈ HFPO-DA ≫ PFPeA. In real electroplating wastewaters (pH 4-7), photo-responsive long-chain PFAS at high level could still be decomposed by UV222/Fe(III), while short-chain and fluorotelomer/ether PFAS persisted. This work provides mechanistic insight into Fe(III)-mediated PFAS photochemistry and informs the development of efficient, oxidant-free treatment strategies for PFAS.