Sustained Fe3+/Fe2+ Cycling Enables Photochemical Defluorination of Perfluoroalkyl Carboxylic Acids via Ligand-to-Metal Charge Transfer

Abstract

Per- and polyfluoroalkyl substances (PFASs) are widespread persistent pollutants that threaten environmental and human health. Although Fe3+-mediated ligand-to-metal charge transfer (LMCT) has been shown to promote PFAS photochemical defluorination, existing systems rely predominantly on high-energy UV irradiation, often achieve incomplete defluorination, and generally exhibit poor performance toward short-chain and ultrashort-chain PFASs. Herein, we report a near-UV- to visible-light-driven photodefluorination strategy for perfluoroalkyl carboxylic acids (PFCAs) using commercial Fe3+ salts in acetonitrile under 405 nm irradiation, which achieved complete defluorination of long-chain perfluorooctanoic acid (PFOA) within 420 min and ultrashort-chain trifluoroacetic acid (TFA) within 240 min. Mechanistic studies combining density functional theory calculations and experimental evidence revealed that LMCT initiated decarboxylation, generating perfluoroalkyl radicals that subsequently degraded through oxidative chain-shortening reactions. The Fe3+/Fe2+ redox cycle was sustained by molecular oxygen, with hydroxyl and superoxide radicals playing auxiliary roles in reoxidizing Fe2+ and oxidizing intermediates rather than directly attacking intact PFOA. This strategy is broadly applicable to C2–C9 PFCAs, where the degradation rates are inversely correlated with chain length and are effective against perfluoroalkyl ether carboxylic acids. These findings establish a mechanistic framework for Fe3+-mediated PFAS photodefluorination in nonaqueous media and clarify the fundamental chemistry governing this process.