Head Group Dependence and Kinetic Bottlenecks of Gas- phase Thermal PFAS Destruction

Varying and sometimes con[icting temperatures and products have been reported from studies addressing thermal PFAS destruction, often because decomposition pathways are highly dependent on the respective experimental system. Here we applied highest-level 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 per[uoroalkyl acids decompose via unimolecular head group loss, either through HF elimination or homolytic bond cleavage as a function of head group type. In contrast, all investigated [uorotelomers undergo initial hydrogen abstraction from the characteristic C2H4 moiety by hydroxyl radicals under representative incineration conditions, followed by radical decomposition. Subsequent formation of per[uoroalkanes including CF4 can then be prevented by supplying su`cient hydrogen donors such as hydrocarbon fuel and water as well as by scavenging released [uorine. This leads to the generation of stable 1H-per[uoroalkanes. While parent PFAS decomposition proceeds at gas-phase temperatures ≤700 °C, carbon-carbon cleavage of 1H-per[uoroalkanes requires up to ~950 °C at 2 seconds gas residence time, making this step the kinetic bottleneck on the way to complete thermal PFAS mineralization.