Thermal Decomposition of Two Gaseous Perfluorocarboxylic Acids: Products and Mechanisms.

The thermal decomposition products and mechanisms of per- and polyfluoroalkyl substances (PFASs) are poorly understood despite the use of thermal treatment to remediate PFAS-contaminated media. To identify the thermal decomposition products and mechanisms of perfluorocarboxylic acids (PFCAs), gaseous perfluoropropionic acid (PFPrA) and perfluorobutyric acid (PFBA) were decomposed in nitrogen and oxygen at temperatures from 200 to 780 °C. In nitrogen (i.e., pyrolysis), the primary products of PFPrA were CF═CF, CFCFH, and CFCOF. CFCF═CF was the dominant product of PFBA. These products are produced by HF elimination (detected as low as 200 °C). CF and CF were observed from both PFCAs, suggesting formation of perfluorocarbon radical intermediates. Pyrolysis products were highly thermally stable, resulting in poor defluorination. In oxygen (i.e., combustion), the primary product of both PFPrA and PFBA below 400 °C was COF, but the primary product was SiF above 600 °C due to reactions with the quartz reactor. Oxygen facilitated thermal defluorination by reacting with PFCAs and with pyrolysis products (i.e., fluoroolefins and fluorocarbon radicals). Platinum improved combustion of PFCAs to COF at temperatures as low as 200 °C, while quartz promoted the combustion of PFCAs into SiF at higher temperatures (>600 °C), highlighting the importance of surface reactions that are not typically incorporated into computational approaches.