Finding Mechanisms and Conditions for Pyrolysis of Perfluorooctanoic Acid (PFOA) Using a Detailed Kinetic Model

Perfluorooctanoic acid (PFOA or PF8acid) pyrolysis reactions, intermediates, and products are identified for using the most comprehensive gas-phase kinetics model for per- and polyfluorinated alkyl substances (PFAS) to date, ncsuPFASmech, proving generally consistent with the available literature measurements. An important reason is thoroughly including C1 through C8 perfluorinated carboxylic acids (PFCAs) and their reaction intermediates. Quantitative predictions are made for pyrolysis mole-fraction profiles of these PFCAs in ideal plug-flow reactors at temperatures ranging from 500 to 1000 °C. Two classes of reactions are predicted to dominate PFOA destruction: (1) at T<680 °C, pericyclic eliminations of HF, principally making an α-lactone intermediate that mainly decomposes to CO and perfluoroheptanoyl fluoride, and (2) at T>680 °C, homolytic scissions of the n-perfluoroalkyl chain, yielding n-perfluoroalkyl radicals and rCOOH or PFCA radicals. Radical abstraction is not significant, unlike in hydrocarbon pyrolysis processes. Analysis shows that temperatures of 800 °C and above are needed to pyrolyze PFOA to near-completion in millisecond time scales; however, destruction of fluoro-organic intermediates is also necessary. Without added hydrogen sources such as H2, H2O, or hydrocarbons, full mineralization of PFOA to HF cannot be achieved by pyrolysis alone.