(635a) Destruction-Reaction Pathways for Perfluorocarboxylic and -Sulfonic Acids from a Detailed Kinetic Model

A detailed kinetic model, ncsuPFASmech, for pyrolysis/incineration of perfluorocarboxylic and perfluorosulfonic acids (PFCAs and PFSAs) is used to identify reaction paths for destroying these mixtures, as well as the intermediates that could be hazardous if not fully destroyed. These mixtures have been valuable for stain-proofing fabrics and putting out fires and are now ubiquitous in the environment. However, they must be destroyed. Their health hazards and the large existing quantities of PFCA/PFSA concentrates point to incineration as a practical means of converting the fluorine to HF and thus to benign CaF₂.

The model integrates kinetics, thermochemistry, and transport properties for 1572 reactions and 264 F-containing species and 45 other species for natural-gas chemistry. Its PFSAs range from FSO₃H to perfluorooctane sulfonic acid (PFOS, nC₈F₁₇-SO₃H, or PF8Sacid), covered by 96 reactions for PFSAs and their intermediates. PFCAs are from FCOOH (fluoroformic acid or PF1acid) to perfluorooctanoic acid (PFOA, nC₇F₁₅-COOH, or PF8acid), requiring 364 reactions. Most of the rest are from the 2022 version of the NIST model for methane and C0-C3 fluorocarbons [1] with some modifications. New reactions and parameters are computed by quantum chemistry or worked out by appropriate analogies, notably through interpreting and re-analyzing Russian perfluorocarbon literature. Thermochemistry computations [2,3] use G4 iwith homodesmic corrections for C0 to C3 PFS species and C0-C4 PFCs; for heavier species M06-2X-D3(0)/def2-QZVPP is used. For kinetics, transition states, reactant(s), and product(s) were calculated self-consistently with ωB97XD/def2-TZVP.

Reactor modeling with Chemkin and Cantera reveals the roles of homolytic molecule scissions, alpha- and beta-scissions, and perfluoroalkylsulfite isomerizations and decompositions. Temperature steers the dominant pathways, transitioning from lower-temperature bicyclic lactone/sultone pericyclic routes to homolytic loss of head groups. Little abstraction occurs due to the strong C-F bonds.

We gratefully acknowledge support by the US Environmental Protection Agency, Contract 68HERC22C0057, and use of the NCSU High-Performance-Computing-Services Core Facility (RRID:SCR_022168).

References
[1] D. R. F. Burgess et al., NIST Fluorocarbon Combustion Model, 2022 (provided directly).
[2] H. Ram, T.P. Sadej, C.C. Murphy, T.J. Mallo, P.R. Westmoreland, J. Phys. Chem. A 129:13 (2025) 3176–3182.
[3] H. Ram, C. Murphy DePompa, P.R. Westmoreland, J. Phys. Chem. A 129:11 (2025) 2823-2827.