Oxygen-Dependent Cascade Defluorination of GenX via Synergistic Mechanism of Singlet Oxygen and Nitrogen Oxide Radicals in Fe ³⁺ /Fe ²⁺ Redox Cycling

Hexafluoropropylene oxide dimer acid (GenX), a perfluorooctanoic acid substitute, poses environmental risks due to its persistence and toxicity. Effective defluorination methods are crucial, especially in clarifying reactive species' roles in carbon-skeleton disruption versus C-F bond cleavage. We present a novel UV/Fe(NO₃)₃ system that integrates ligand-to-metal charge transfer (LMCT), reactive nitrogen species (RNS), and singlet oxygen (¹O₂) to achieve 90% GenX degradation in 1 h and 99% defluorination in 6 h. Mechanistic studies under controlled atmospheres reveal a hierarchical pathway: Fe³⁺-triggered LMCT initiates C-C bond disruption, decarboxylation, and ether-oxygen bond cleavage, while nitrate-photolysis-derived RNS accelerates this process via electron transfer (ET). Fe²⁺-mediated conversion of O₂ to ¹O₂ drives C-F bond scission, revealing an oxygen-dependent cascade defluorination involving Fe³⁺/Fe²⁺ redox cycling and RNS. Oxygen availability raises defluorination from 8 to 99%. Density functional theory calculations confirm that the Fe²⁺ activation of triplet O₂ to ¹O₂ lowers the defluorination barrier by 118.8 kcal/mol. This approach works across diverse water matrices, achieving >88% defluorination of eight per-/polyfluoroalkyl substances (PFAS) within 12 h and remains robust in treating fluorochemical effluents. Using acidic iron-containing pickling wastewater as an in situ reagent enables >90% GenX defluorination, offering a closed-loop, additive-free, scalable solution for sustainable PFAS remediation.