Fe2+-NTA synergized UV254 photolytic defluorination of perfluorooctane sulfonate (PFOS): Enhancing through intramolecular electron density perturbation via electron acquisition

Abstract

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant posing a risk in environmental persistence, bioaccumulation and biotoxicity. This study was to reach a comprehensive and deeper understanding of PFOS elimination in a UV₂₅₄ photolytic treatment with the co-presence of Fe²⁺ and nitrilotriacetic acid trisodium salt (NTA). PFOS defluorination was noticeably enhanced in the UV/Fe²⁺-NTA treatment compared with UV/NTA, UV/Fe²⁺ and our previously studied UV/Fe³⁺ treatments. UV-vis, FTIR, and UPLC/MS-MS results indicated the formation of PFOS-Fe²⁺-NTA complex in PFOS, Fe²⁺ and NTA mixture. The transition energy gap of PFOS-Fe²⁺-NTA decreased below the excitation energy supplied by UV₂₅₄ irradiation, corresponding with red shift appearing in UV-vis scanning spectrum. This favored intramolecular electron transfer from Fe²⁺-NTA to PFOS under UV₂₅₄ irradiation to form electron-accepting PFOS. Molecular electrostatic potential and atom charge distribution analyses suggested electron density rearrangement and perturbation in the perfluorinated carbon chain of electron-accepting PFOS, leading to the decrease in bond dissociation energies. Intermediate products detection suggested the parallel defluorination pathways of PFOS desulfonation, middle carbon chain scission and direct C-F cleavage. NTA exhibited crucial functions in the UV/Fe²⁺-NTA treatment by holding Fe²⁺/Fe³⁺ in soluble form as a chelant and favoring water activation to generate hydrated electrons (e_aq^-) under UV irradiation as a photosensitizer. Fe²⁺ acting as the conduit for electron transfer and the bridge of PFOS anion and NTA was thought functioning best at 200 µM in this study. The degree of UV/Fe²⁺-NTA -synergized PFOS defluorination also depended on e_aq^- yield and UV₂₅₄ photon flux. The structure dependence on the electron transfer process of PFOS and PFOA was explored incorporating molecular structure descriptors. Because of possessing greater potential to acquire electrons or less likeliness to donate its electrons than PFOA, PFOS exhibited faster defluorination kinetics in the published "reduction treatments" than "oxidation" ones. Whereas, PFOA defluorination kinetics were at similar level in both "reduction" and "oxidation" treatments.

Keywords: Electron density perturbation; PFOS defluorination; PFOS-Fe(2+)-NTA complexing; UV(254) irradiation.