An Investigation of Thermal Air Degradation and Pyrolysis of Per-and Polyfluoroalkyl Substances and Aqueous Film-Forming Foams in Soil

In this study, we found that thermal decomposition of per- and Thermal desorption and polyfluoroalkyl substances (PFAS) in soil was rapid at moderate temperatures of gas-phase decomposition 400-500 degrees 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 pertluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), short-chain homologues, cationic and zwitterionic precursors, and PFOA and PFOS alternatives, occurred in 30 min at 500 degrees C in both a sealed reactor in air and a horizontal reactor under a continuous flow of N-2. The effect of the initial PFAS level in soil (0.001-10 mu mol/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 degrees 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.