Dry thermal air degradation and pyrolysis of per-and polyfluoroalkyl substances in spent media and other solid materials
Authors:   , Alinezhad Ali 
Published: November 2024

Per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals that have been manufactured for decades, serving as processing aids and key components in numerous industrial and commercial products such as aqueous film-forming foams (AFFFs), non-stick cookware, and fastfood packaging. Extensive U.S. national surveys have detected perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in over 95 percent of blood samples, with concentrations that pose significant health risks. Recent studies, including those from our group, have shown that thermal treatment is an promising technology in the management and remediation of PFAS. In this research, our results reveal that the thermal decomposition of PFAS, including PFAS alternatives such as GenX, occurs at significantly lower temperatures than previously understood. We demonstrate that the thermolysis of perfluoroalkyl carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), and GenX can occur at temperatures of 150--200 [degrees] C. The addition of highly porous adsorbents like GAC notably accelerates PFAS decomposition, with the decomposition rate constant increasing up to 150-fold at 250 [degrees] C. The presence of non-activated charcoals/biochars with a low affinity for PFOA did not accelerate its thermal decomposition, suggesting that the [pi] electron-rich, polyaromatic surface of charcoal/GAC played an insignificant role compared to the adsorbent's porosity. In soil, thermal decomposition of PFAS is rapid at moderate temperatures (400-500 [degrees] C), achieving substantial degradation ([greater than] 99 percent) within 30 min. The degradation efficiency is consistent across various PFAS, including PFOA, PFOS, short-chain homologues, and their alternatives, regardless of initial concentration or soil texture. Notably, the presence of kaolinite in the soil significantly decreases the yield of fluorine radicals at temperatures above 300 [degrees] C, likely due to chemisorption. 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. Furthermore, the research elucidates the thermal decomposition mechanisms of perfluoroalkyl ether carboxylic acids (PFECAs) and short-chain PFCAs. Computational and experimental analyses indicate that the thermal transformation of hexafluoropropylene oxide dimer acid to trifluoroacetic acid (TFA) occurs predominantly through the cleavage of the C-O ether bond near the carboxyl group. This process produces intermediates such as perfluoropropionic acid (PFPeA) and TFA, with a minor pathway leading to perfluorobutanoic acid (PFBA). The weakest bonds in PFPeA and PFBA are the C-C bonds connecting the [alpha] - and [beta]-carbons, supporting the efficacy of C-C scission in PFCA thermal decomposition. Overall, our results indicate that conventional thermal treatment processes, including those utilizing GAC and other highly porous materials, can significantly enhance PFAS degradation. This research provides critical insights into optimizing thermal remediation strategies for PFAScontaminated environments, ensuring more effective and efficient pollutant management.