Pyrolysis of Two Perfluoroalkanesulfonates (PFSAs) and PFSA-Laden Granular Activated Carbon (GAC): Decomposition Mechanisms and the Role of GAC.

Abstract

Thermal treatment of perfluoroalkyl and polyfluoroalkyl substances (PFASs) presents a promising opportunity to halt the PFAS cycle. However, how co-occurring materials such as granular activated carbon (GAC) influence thermal decomposition products of PFASs, and underlying mechanisms remain unclear. We studied the pyrolysis of two potassium salts of perfluoroalkanesulfonates (PFSAs, C_nF_2n+1SO₃K), perfluorobutanesulfonate (PFBS-K), and perfluorooctanesulfonate (PFOS-K), with or without GAC. PFBS-K is more stable than PFOS-K for pure standards, but when it is adsorbed onto GAC, its thermal stabilities and decomposition behaviors are similar. Temperatures and heating rates can significantly influence the decomposition mechanisms and products for pure standards, while these effects are less pronounced when PFSAs are adsorbed onto GAC. We further studied the underlying decomposition mechanisms. Pure standards of C_nF_2n+1SO₃K can decompose directly in their condense phase by reactions: F(CF₂)_nSO₃K → F(CF₂)_n-2CF═CF₂ + KFSO₃ or F(CF₂)_nSO₃K → F(CF₂)_n^- + K⁺ + SO₃. GAC appears to facilitate breakage of the C-S bond to release SO₂ at temperatures as low as 280 °C. GAC promotes fluorine mineralization through functional reactive sites. SiO₂ is particularly important for the surface-mediated mineralization of PFASs into SiF₄. These findings offer valuable insights into optimizing thermal treatment strategies for PFAS-contaminated waste.