New Insights into the Degradation Mechanism of Perfluorooctanoic Acid by Persulfate from Density Functional Theory and Experimental Data

Thermally activated persulfate is a promising oxidant for in situ remediation of perfluorooctanoic acid (PFOA), yet a comprehensive understanding of the degradation mechanism is still lacking. In this study, we used density functional theory (DFT) calculations and experimental data to map entire reaction pathways for the degradation of PFOA by persulfate, with specific considerations on the influence of pH. The DFT results showed that the rate-limiting step was the first electron abstraction from PFOA, yet the generation of SO₄^•- from the decomposition of persulfate contributed a large part of the free energy of activation (ΔG^‡) for the overall reaction. The subsequent steps did not contribute to the ΔG^‡. For the electron abstraction from PFOA, we investigated reactions using protonated and deprotonated species of PFOA and SO₄^•- and showed that the reaction of anionic PFOA with HSO₄^• was most favorable with a ΔG^‡ of 7.2 kJ/mol. This explains why low pH (<3.5) is a sine qua non condition for the degradation of PFOA by persulfate. The overall ΔG^‡ derived theoretically based on the pathway involved HSO₄^• was consistent with the ΔG^‡ determined experimentally. This study provides valuable insight into remediation strategies that include persulfate as an oxidizing agent for perfluoroalkyl carboxylic acids.