New insights into ferric iron-facilitated UV254 photolytic defluorination of perfluorooctanoic acid (PFOA): Combined experimental and theoretical study

UV/Fe3+-facilitated PFOA defluorination was often reported and recognized to proceed through a ligand-tometal charge transfer (LMCT) mechanism in the literatures. Sufficient Fe3+ supply is important for sustaining the LMCT reaction pathway. In this study, an interesting excessive defluorination was observed, even the continuous Fe(3+ )supply was cut off, implying a parallel mechanism strengthening PFOA defluorination. Based the results of intermediate products detection, 19F NMR analysis, and exploration of electron density alternation, transition energy evolution, and bonds characteristics, remarkable electron density perturbation in [PFOA-Fe]2+ was revealed. This effect was triggered by the complexation between PFOA anion and Fe3+, 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 UV254 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/Fe3+- synergized PFOA defluorination depended on UV254 photon flux and Fe3+ dosage. High UV254 intensity guaranteed fast defluorination kinetics. A [Fe3+]/[PFOA] molar ratio near 1 showed the best UV/Fe3+ synergic effect on PFOA defluorination.

Automated summary

This study reveals a parallel mechanism in addition to the LMCT mechanism for UV/Fe3+-synergized PFOA defluorination, depending on UV254 photon flux and Fe3+ dosage. Remarkable electron density perturbation in [PFOA-Fe]2+ is crucial for excessive defluorination.