The aims of this study were two-fold: (1) to improve our understanding of the thermal stability of per- and polyfluoroalkyl substances and (2) to investigate their decomposition mechanisms on spent granular activated carbon (GAC) during thermal reactivation. We studied seven perfluoroalkyl carboxylic acids (PFCAs), three perfluoroalkyl sulfonic acids (PFSAs), and one perfluoroalkyl ether carboxylic acid (PFECA) in different atmospheres (N2, O2, CO2, and air). The destabilization of studied compounds during thermal treatment followed first-order kinetics. The temperature needed for thermally destabilizing PFCAs increased with the number of perfluorinated carbons (n CF2). Decomposition of PFCAs such as perfluorooctanoic acid (PFOA) on GAC initiated at temperatures as low as 200 °C. The PFECA was even more readily decomposed than PFCA with the same n CF2. PFSAs such as perfluorooctanesulfonic acid (PFOS), on the other hand, required a much higher temperature (≥450 °C) to decompose. Volatile organofluorine species were the main thermal decomposition product of PFOA and PFOS at low to moderate temperatures (≤600 °C). Efficient mineralization to fluoride ions (>80%) of PFOA and PFOS on GAC occurred at 700 °C or higher, accompanied by near complete PFOA and PFOS decomposition (>99.9%). Thermal decomposition pathways of PFOA were proposed.

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In this study, we investigated thermal decomposition mechanisms of cationic, zwitterionic, and anionic polyfluoroalkyl substances, including those present in aqueous film-forming foam (AFFF) samples. We present novel evidence that polyfluoroalkyl substances gave quantitative yields of perfluoroalkyl substances of different chain lengths during thermal treatment. The results support a radical-mediated transformation mechanism involving random-chain scission and end-chain scission, leading to the formation of perfluoroalkyl carboxylic acids such as perfluorooctanoic acid (PFOA) from certain polyfluoroalkyl amides and sulfonamides. Our results also support a direct thermal decomposition mechanism (chain stripping) on the nonfluorinated moiety of polyfluoroalkyl sulfonamides, resulting in the formation of perfluorooctanesulfonic acid (PFOS) and other structurally related polyfluoroalkyl compounds. Thermal decomposition of 8:2 fluorotelomer sulfonate occurred through end-chain scission and recombination reactions, successively yielding PFOS. All of the studied polyfluoroalkyl substances began to degrade at 200–300 °C, exhibiting near-complete decomposition at ≥400 °C. Using a high-resolution parent ion search method, we demonstrated for the first time that low-temperature thermal treatments of AFFF samples led to the generation of anionic fluoroalkyl substances, including perfluoroheptanesulfonamide, 8:2 fluorotelomer sulfonic acid, N-methyl perfluorooctane sulfonamide, and a previously unreported compound N-2-propenyl-perfluorohexylsulfonamide. This study provides key insights into the fate of polyfluoroalkyl substances in thermal processes.

In this study, we found that thermal decomposition of per- and polyfluoroalkyl substances (PFAS) in soil was rapid at moderate temperatures of 400–500 °C, regardless of whether the soil was contaminated by a single PFAS compound or a PFAS mixture in aqueous film-forming foams. Substantial degradation (>99%) of PFAS in soil, including perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), short-chain homologues, cationic and zwitterionic precursors, and PFOA and PFOS alternatives, occurred in 30 min at 500 °C in both a sealed reactor in air and a horizontal reactor under a continuous flow of N2. The effect of the initial PFAS level in soil (0.001–10 μmol/g) and soil texture was insignificant, provided a sufficiently high temperature was applied. Furthermore, this study showed, for the first time, that kaolinite dramatically decreased the apparent yield of F from PFAS heated at >300 °C, likely due to the chemisorption of F radicals on kaolinite. This phenomenon was not observed when kaolinite and an inorganic fluoride salt (NaF) were thermally treated. Lastly, various nonpolar thermal degradation products of PFOA and PFOS were reported for the first time. The profile of fluorinated volatiles, particularly perfluoroalkenes, was similar between these two chemicals. The results support a radical-mediated degradation pathway of PFAS.

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In this study, we have developed an innovative thermal degradation strategy for treating per- and polyfluoroalkyl substance (PFAS)-containing solid materials. Our strategy satisfies three criteria: the ability to achieve near-complete degradation of PFASs within a short timescale, nonselectivity, and low energy cost. In our method, a metallic reactor containing a PFAS-laden sample was subjected to electromagnetic induction that prompted a rapid temperature rise of the reactor via the Joule heating effect. We demonstrated that subjecting PFASs (0.001–12 μmol) to induction heating for a brief duration (e.g., <40 s) resulted in substantial degradation (>90%) of these compounds, including recalcitrant short-chain PFASs and perfluoroalkyl sulfonic acids. This finding prompted us to conduct a detailed study of the thermal phase transitions of PFASs using thermogravimetric analysis and differential scanning calorimetry (DSC). We identified at least two endothermic DSC peaks for anionic, cationic, and zwitterionic PFASs, signifying the melting and evaporation of the melted PFASs. Melting and evaporation points of many PFASs were reported for the first time. Our data suggest that the rate-limiting step in PFAS thermal degradation is linked with phase transitions (e.g., evaporation) occurring on different time scales. When PFASs are rapidly heated to temperatures similar to those produced during induction heating, the evaporation of melted PFAS slows down, allowing for the degradation of the melted PFAS.