Headgroup Dependence and Kinetic Bottlenecks of Gas-Phase Thermal PFAS Destruction

Abstract

Recent studies of thermal PFAS destruction have reported seemingly incoherent temperatures and products, often because decomposition pathways are highly dependent on the respective experimental system. Here, we applied highly accurate DLPNO–CCSD(T) coupled cluster calculations to isolate and identify the major processes during thermal PFAS destruction in the gas phase, with relevance to incineration, thermal oxidation, and other thermal treatment technologies in which PFAS and their volatile decomposition products desorb into the gas phase. All investigated perfluoroalkyl acids decompose via unimolecular headgroup loss, either through HF elimination or homolytic bond cleavage as a function of headgroup type. In contrast, all investigated fluorotelomers undergo initial hydrogen abstraction from the characteristic C₂H₄ moiety by hydroxyl radicals under representative incineration conditions, followed by radical decomposition. Subsequent formation of perfluoroalkanes, including CF₄, can then be prevented by supplying sufficient hydrogen donors such as hydrocarbon fuel and water as well as by scavenging released fluorine. This leads to the generation of stable 1H-perfluoroalkanes. While parent PFAS decomposition proceeds at gas-phase temperatures ≤700 °C, carbon–carbon cleavage of 1H-perfluoroalkanes requires up to ∼950 °C at 2 s gas residence time, making this step the kinetic bottleneck on the way to complete thermal PFAS mineralization.