Per- and polyfluoroalkyl substances (PFAS) are thermally stable organofluorine compounds whose complete destruction by incineration is difficult due to strong C–F bonds, radical stabilization, and formation of toxic intermediate products. In this research, atomistic degradation pathways of perfluorobutanoic acid (C 3 F 7 COOH) were investigated using a combined molecular dynamics (MD) and density functional theory (DFT) to quantify bond dissociation, intermediate formation, and kinetic limits under regular incineration temperature (1200 K). High-temperature MD simulations revealed that thermal cleavage of the C–C bond between the perfluoroalkyl chain and carboxyl group is the dominant initial reaction, generating C 3 F 7 • radicals and CO 2 . Subsequent pathways involve β-scission, O•/O 2 addition, and radical recombination to produce intermediates such as C 2 F 4 , CF 3 • , CF 2 , CF 3 C(O)F and COF 2 . The DFT transition-state calculations provide activation energies from 40 to 513 kJ mol −1 , enabling estimation of rate constants and residence times from transition-state theory. At 1200 K temperature, early fragmentation step occurs over micro-to millisecond timescales, whereas C 2 F 4 defluorination, COF 2 decomposition, and CF 4 breakdown calculations show residence times exceeding seconds to hundreds of seconds well above typical incinerator gas-phase residence times. These findings provide quantitative evidence that PFAS incineration proceeds through complex multistep of degradation pathways and that kinetically persistent products of incomplete combustion result primarily from slow defluorination steps. The mechanistic and kinetic trends identified here provide molecular-level guidance for optimizing incineration conditions to improve PFAS mineralization. • PFBA degradation under incineration conditions were resolved by reactive MD and DFT. • DFT shows defluorination of C2F4, CF2O & CF4 is kinetically inaccessible at 1200 K. • Residence-time analysis shows, fluorocarbon intermediates outlast incinerator times. • Molecular-level insights explain incomplete combustion & guide PFAS thermal treatment. SynopsisThis research explains the chemical limits of PFAS incineration and highlights why current practices may be insufficient, informing the design of more effective thermal treatment strategies to protect the environment.
Meegoda et al. (Tue,) studied this question.