UV/Fe-facilitated PFOA defluorination was often reported and recognized to proceed through a "ligand-to-metal charge transfer (LMCT)" mechanism in the literatures. Sufficient Fe supply is important for sustaining the LMCT reaction pathway. In this study, an interesting "excessive defluorination" was observed, even the continuous Fe supply was cut off, implying a parallel mechanism strengthening PFOA defluorination. Based the results of intermediate products detection, F NMR analysis, and exploration of electron density alternation, transition energy evolution, and bonds characteristics, remarkable electron density perturbation in [PFOA-Fe] was revealed. This effect was triggered by the complexation between PFOA anion and Fe, diminishing electron shielding on the perfluorinated carbon chain. Hence, the dissociation energy of C-C bonds was reduced by maximally 53% (C4-C5). Once attacked by high-flux UV photons, the perfluorinated carbon chain underwent scission, and subsequent defluorination was achieved via hydrolysis reactions. This parallel mechanism cooperated with the LMCT mechanism, leading to the observed "excessive defluorination." The degree of UV/Fe-synergized PFOA defluorination depended on UV photon flux and Fe dosage. High UV intensity guaranteed fast defluorination kinetics. A [Fe]/[PFOA] molar ratio near 1 showed the best UV/Fe synergic effect on PFOA defluorination.
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| http://dx.doi.org/10.1016/j.jhazmat.2022.128865 | DOI Listing |
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