(110b) Detailed Kinetic Model and Mechanism for Incinerating Perfluorocarboxylic Acids, Perfluorosulfonic Acids, and Their Mixtures

A detailed kinetic model has been created and tested for incineration of perfluorocarboxylic acids (PFCAs), perfluorosulfonic acids (PFSAs), and their mixtures. Although these PFAS compounds have proven valuable technically, they and associated contaminated media are now often being incinerated because of their health hazards. Tests against literature and new EPA Rainbow-reactor data help assess its value in modeling and managing incineration.

Reaction steps have been proposed; thermochemistry for the reactants and products have been calculated using computational quantum chemistry (G4 up to C4 species, M06-2X-D3(0)/def2-QZVPP for larger species using Gaussian 16 Revision C.01) and statistical mechanics;^1,2 and rate coefficients for proposed reactions have been calculated using ωB97XD/def2-TZVP computational chemistry and reaction theory, adopted from the literature, or estimated by analogies. Concentrations are modeled for reactants, intermediates, and products using ChemkinPro. Experimentally, aqueous PFCAs/PFSAs are injected into the plug-flow post-combustion gases of the Rainbow incinerator.

Pathways are strikingly different from hydrocarbon combustion. PFSAs principally decompose by pericyclic reactions to HF+sultones, which quickly form SO2+acyl fluorides; pericyclic elimination of SO₂ yielding alcohols that form HF+acyl fluorides; or homolytically scission to perfluoroalkyls+HOCO or CF₂COOH. After sulfur is lost, the chemistry is identical to PFCA steps, which make HF+lactones (and then acyl fluorides) or perfluoroalkyls. The acyl fluorides scission to perfluoroalkyls plus resonantly stabilized CF₂CFO radical, and perfluoroalkyls decompose two carbons at a time, making C₂F₄ by beta-scission, or one at a time, making singlet CF₂ until CF₃ is reached. Alkyls can also combine with HOCO and CF₂COOH to make smaller PFCAs.

1. H. Ram et al., "Thermochemistry of Species in Gas-Phase Thermal Oxidation of C2 to C8 Perfluorinated Carboxylic Acids," J. Phys. Chem. A 128:7 (2024) 1313–1326. DOI: 10.1021/acs.jpca.3c06937

2. H. Ram et al., "Thermochemistry for Gas-Phase Thermal Oxidation of PFAS: Perfluorinated Sulfonic Acids," J. Phys. Chem. A. (accepted).