(10a) Thermal Degradation Pathways of Perfluorobutane Sulfonic Acid (PFBS) to Perfluorobutanoicacid (PFBA) and Smaller Perfluoro Carboxylic Acids (PFCAs)

Per- and polyfluoroalkyl substances (PFAS) pose significant environmental concerns due to their persistence and toxicity. Incineration is the most commonly used method for PFAS destruction. This study focuses on developing the reaction kinetics involved in the thermal degradation of perfluorobutane sulfonic acid (PFBS) as a surrogate for larger perfluoro sulfonic acids, such as perfluorooctane sulfonic acid (PFOS). One interesting feature of perfluoro sulfonic acid decomposition is that it leads to the formation of perfluoro carboxylic acids (PFCAs), such as perfluorobutanoic acid (PFBA). Our investigation reveals two distinct reaction pathways for PFBS degradation leading to PFBA, as well as an additional pathway for PFBA generating smaller PFCAs. One pathway involves the formation of an α-sultone intermediate after HF removal. The α-sultone, inherently unstable, undergoes SO₂ removal to yield an aldehyde-containing PFAS species (C₃F₇CFO). Another pathway proceeds through the formation of an alcoholic group with subsequent SO₂ removal, resulting in the C₄F₉OH species, which can further undergo HF removal to yield C₃F₇CFO. Irrespective of the pathway, C₃F₇CFO species is always formed from these two mechanisms, which upon reaction with water molecules, yields PFBA and HF. PFBA may undergo α-lactone formation by HF removal. Followed by CO removal from the α-lactone intermediate, C₂F₅CFO is formed. This aldehyde group can undergo a reaction with H₂O molecules to regenerate CF₃CF₂CO(OH) and HF, and this carboxylic acid regeneration cycle can continue to form smaller chain PFCAs. Computational calculations were conducted using Gaussian09 at the B2PLYPD3/cc-pVTZ level of theory, with single-point energies performed at the DLPNO/aug-cc-pVQZ level. Observations from the Environmental Protection Agency’s experiments on PFOS incineration suggest the formation of PFOA and other smaller PFCAs providing valuable validation for our computational findings. Our research aims to develop comprehensive reaction pathways and a kinetic mechanism to enhance our understanding of PFOS degradation processes in collaboration with EPA.