Computational Study of the Gas-Phase Thermal Degradation and the Reaction Rate Coefficients of Chlorinated Perfluoro-Alkyl Carboxylic Acids

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a class of ubiquitous pollutants that do not appreciatively biodegrade. A ban on the most commonly used PFAS, perfluoroalkyl carboxylic acids (PFCA) and perfluoroalkyl sulfonic acids, created a gap that was filled by several similarly heavily fluorinated compounds. While chemical and biological studies on several subclasses of PFAS have progressed significantly, testing on the chlorinated PFAS is still lagging. For most PFAS, the main treatment consists of filtration followed by pyrolysis of the laden adsorbents. Herein, the pyrolytic degradation pathways of mono- and polychlorinated PFCA (ClxPFCA) are analyzed. This study showed that the bond dissociation energies (BDEs) are slightly lower in the acid form compared to the carboxylate form. The C–Cl bonds are generally the weakest (∼70 kcal/mol) while the strongest are the C–F bonds (105-110 kcal/mol). After studying their BDEs, the secondary degradation pathways of the radicals formed by the first bond cleavage were analyzed. Five reaction pathways were studied for the chlorinated radicals generated by the first bond cleavage. The activations energies (Ea) and the enthalpies (ΔH) in the cases of a barrierless reaction are similar among the group of radicals. The lowest Ea are those of the Cl-shifts (∼10–25 kcal/mol) while the most energetic pathways are the F-losses (50–65 kcal/mol). Finally, a kinetic study was performed to evaluate the reaction rate coefficients of each pathway. It showed a preference for the β-scission pathways at the highest temperatures analyzed and the F-shift followed by Cl-loss at the lowest temperatures. The lifetimes of the radicals are generally in the ms-min for the perfluorinated alkyl radicals and min-hr for the perfluorocarboxyalkyl radicals.