Thermal decomposition of perfluorinated carboxylic acids: Kinetic model and theoretical requirements for PFAS...

Thermal decomposition of high-fluorine content PFAS streams for the disposal of old generations of concentrates of firefighting foams, exhausted ion-exchanged resins and granular activated carbon, constitutes the preferred method for destruction of these materials. This contribution studies the thermal transformation of perfluoropentanoic acid (C₄F₉C(O)OH, PFPA), as a model PFAS species, in gas-phase reactions over broad ranges of temperature and residence time, which characterise incinerators and cement kilns. Our focus is only on gas-phase reactions, to formulate a gas-phase submodel that, in future, could be used in comprehensive simulation of thermal destruction of PFAS; such comprehensive models will need to comprise fluorine mineralisation on flyash and in clinker material. Our submodel consists of 56 reactions and 45 species, and includes new pathways that cover the initial decomposition channels of PFPA, including those that lead to the formation of the n-C₄F₉ radical, the abstraction of hydroxyl H by O/H radicals, the fragmentation of the n-C₄F₉ radical, reactions between HF and perfluoropentanoic acid, as well as between HF and heptafluorobutanoyl fluoride (C₃F₇COF), and the cyclisation reactions. The model illustrates the formation of a wide spectrum of small C_nF_m and C_nHF_m compounds in the temperature window of 800-1500 K, 2 and 25 s residence time in a plug flow reactor, providing theoretical estimates for the operating conditions of PFAS thermal destruction systems. The initiation reactions involve the loss of HF and formation of the transition α-lactone species that converts to C₃F₇COF, with C₄F₉C(O)OH completely decomposed at 1020 K for 2 s residence time. At 1500 K, we predict the emission of ꞉CF₂ (biradical difluorocarbene), HF, CO₂, CO, CF₄, C₂F₆, and C₂F₄, but at < 1400 K, we note the formation of 1H-nonafluorobutane (C₄HF₉), phosgene (COF₂), and heptafluorobutanoyl fluoride (C₃F₇COF), with 1-C₄F₈, 2-C₄F₈ and C₃HF₇ persisting to 1500 K. We demonstrate that, the gas-phase pyrolysis processes by themselves convert PFAS to HF and short-chain fluorocarbons, with similar product distribution for short (2 s) and long (25 s) residence times, as long as the treatment temperature exceeds 1500 K. These residence times reflect those encountered in incinerators and cement kilns, respectively. Thermokinetic and mechanistic insights revealed herein shall assist to innovate PFAS thermal disposal technologies, and, from a fundamental perspective, to accelerate research progress in modelling of gas/solid reactions that mineralise PFAS-derived fluorine.