(659b) Development and Application of Detailed Kinetics Mechanisms for PFAS Incineration

Newly developed detailed kinetic mechanisms are presented for PFAS pyrolysis and combustion, and their accuracy for modeling incineration reactors is demonstrated. Manufacturing and use of "per- and polyfluoroalkyl substances," once widely embraced for their outstanding properties, are now restricted because of their sub-ppt-level toxicity and bioaccumulation throughout the environment. Clean incineration is urgently needed to destroy PFAS into small molecules like HF, CO, and CO₂ without forming persistent, large-molecule byproducts.

Recent data provide the basis for testing detailed kinetic mechanisms that we have predicted for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS). The necessary chemical intermediates, thermochemistry, reactions, rate expressions, and transport properties have been developed using computational quantum chemistry (Gaussian at B3LYP/6-31(G(d,p) and M06-2x/6-311+G(d,p) levels), ideal-gas statistical mechanics and reaction theory (Arkane), and correlations. Testing for pyrolysis, flame, and post-flame reactor performance has primarily used the reactive-flow models of ANSYS Chemkin Pro.

Modeling predictions show that pericyclic decompositions dominate pyrolysis, not radical chemistry, mainly because of the strong C-F bonds and lack of abstractable Hs. The lowest-energy decompositions from PFOA are HF elimination to make a perfluoroalkane that ends in a "lactone" three-membered -(CF-CO-O-) ring, decomposing to CO or CO₂; a perfluoro alpha-olefin; or a 1H-perfluoroalkane. In a flame, radicals can abstract the lone H in PFOA or PFOS, respectively generating a perfluorocarboxylate or sulfonate that decomposes to release CO₂ or SO₃, although the PFAS decompositions still compete. In conventional post-flame incineration, PFAS decompositions appear to dominate.

Predictions will be tested and refined by comparison to experiments in the EPA Rainbow pilot-plant incinerator, recognizing that practical incinerators do not feed PFOA or PFOS per se but instead ionic solutions of these compounds. These findings point to control strategies for making incineration effective for the PFAS feed and for combustion intermediates.