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The derailment of an unmanned train carrying crude oil and subsequent fire in the town of Lac-Mégantic, Quebec, led to the use of 33 000 L of aqueous film forming foam (AFFF) concentrate. While it is known that per- and polyfluoroalkyl substances (PFASs) contained in AFFFs pose a potential environmental and health risk, critical knowledge gaps remain as regards to their environmental fate after release. The accident in Lac-Mégantic provided valuable information regarding the identity and concentration of PFASs present in the soil after the AFFF deployment, as well as their possible transformation over time. The current study analyzed four sets of samples from Lac-Mégantic: soil collected days after the accident from a heavily impacted area, soil sampled two years later from the treatment biopiles, soil collected two years after the accident from downtown Lac-Mégantic, and nonimpacted soil from a nearby area. A total of 33 PFASs were quantified in the soils. The highest observed concentrations correspond to those of 6:2 fluorotelomer sulfonamidoalkyl betaine, 6:2 and 8:2 fluorotelomer sulfonates, and short chain perfluorocarboxylic acids. The soils collected in Lac-Mégantic two years after the accident show a total PFAS concentration that is ∼50 times lower than soils collected in 2013, while the proportion of perfluoroalkyl acids in those samples shows an increase. Qualitative analysis revealed the presence in soil of 55 additional PFASs that had been previously identified in AFFF formulations. The present study highlights the need to perform detailed analysis of AFFF impacted sites, instead of focusing solely on perfluoroalkyl acids.

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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.