(654g) Present Status and New Results for Pfas Destruction Chemistry

A combustion/pyrolysis mechanism is presented for the destruction chemistry for perfluorooctanoic acid (PFOA), an important PFAS compound. Per- and Polyfluoroalkyl Substances (PFAS) have rapidly become a high national environmental priority. PFAS are durable chemicals whose valuable properties have led them to be widely used in cookware, outdoor wear, food wrappers, and firefighting; unfortunately, they bio-accumulate and can have adverse health effects. Furthermore, fluorocarbons have flame-suppressant characteristics as understood through our mechanistic modeling since 1994 [1,2], making incineration more difficult.

PFOA incineration chemistry occurs in the presence of hydrocarbon combustion. The first step in alkane combustion is abstraction of the weakest-held hydrogen to form an alkyl radical that unzips (beta-scissions) to generate olefin degradation products. In contrast, carbon-fluorine bonds are 20-40 kJ/mol stronger. Fluorine atoms are thus harder to abstract. The acidic H is abstracted more easily, forming an unstable Criegee radical that can rapidly eliminate CO₂ to make perfluorohept-1-yl, which can unzip. Thermochemistry and kinetics are generated using computational quantum chemistry (G16 DFT and ONIOM), statistical mechanics, and transition-state theory; compared to literature where possible, including group-additivity estimates from the RMG/Arkane package; combined with the current NIST kinetics model; tested in plug-flow reactor calculations to establish consistency and dominant mechanistic pathways; compared and contrasted with the recent model-compound mechanism of Altarawneh et al. [3]; and further tested against data from a lab incinerator.

References

1. P.R. Westmoreland, D. R. F. Burgess, Jr., W. Tsang, M. R. Zachariah, "Fluoromethane Chemistry and Its Role in Flame Suppression," Proceedings of the Combustion Institute 25, 1505-1611 (1994).
2. C.D. Needham, P.R. Westmoreland, “Combustion and Flammability Chemistry for the Refrigerant HFO-1234yf (2,3,3,3-tetrafluroropropene)," Combustion and Flame 184, 176-185 (2017).
3. M. Altarawneh, M.H. Almatarneh, B.Z. Dlugogorski, "Thermal decomposition of perfluorinated carboxylic acids: Kinetic model and theoretical requirements for PFAS incineration," Chemosphere 286, 131685 (2022).