Adsorption to granular activated carbon and ion exchange are treatment approaches commonly applied by the Department of Defense (DoD) to clean water impacted with per- and polyfluoroalkyl substances (PFAS). However, these technologies create solid and liquid waste streams that need to be treated destructively to avoid any potential future legacy. While all organic chemicals, including PFAS, are susceptible to thermal treatment, they decompose at different rates controlled by gas-phase chemical kinetic factors such as temperature, reaction atmosphere, and residence time. Incomplete treatment in incinerators or furnaces may lead to the emission of potentially toxic products of incomplete combustion. Thus, there is a critical need to help Remedial Program Managers (RPMs) at the DoD increase their understanding of PFAS incineration on materials of interest and PFAS fate during thermal reactivation of activated carbon and other carbon-based sorbents. In an effort to address the large number of PFAS species that occur in aqueous film-forming foam (AFFF)-impacted materials, the overarching objective of this project is to develop predictive tools that can achieve chemical accuracy (±1 kcal/mol) and aid in the assessment of thermal destruction performance for various PFAS of interest in both liquid and solid waste streams.

Within this project, highly accurate quantum chemical calculations will be combined with machine learning approaches to enable the rapid prediction of key thermochemical properties for a broad set of PFAS. Next, model PFAS with a focus on DoD-relevant species will be selected and investigated for their thermal destruction under incineration conditions in liquid waste streams, considering all relevant mechanisms such as unimolecular decomposition, reaction with radical species, and radical recombination to form high-molecular weight products. To investigate the fate of PFAS during granular activated carbon regeneration, a representative model of the sorbent surface will be developed and the impacts of adsorption on the thermal PFAS stability will be probed. Finally, practical models will be developed to estimate parameters relevant for future thermal stability prediction of previously uncharacterized PFAS.

Experimental investigations of thermal fate of PFAS are critically needed, but may experience limitations due to the vast number of individual PFAS species, absence of suitable analytical methods for incomplete combustion products, and scale effects of laboratory studies. This project aims to develop a novel predictive approach of similar or higher accuracy than experimental data that will reveal critical mechanistic information regarding required treatment temperatures and residence times for the complete destruction of PFAS as well as regarding the potential formation of incomplete combustion products. Ultimately, this project will provide straightforward models that can be used by DoD RPMs, scientists, and practitioners to predict the thermal stability of any PFAS of interest. Furthermore, it is envisioned that this project will be supportive of ongoing experimental studies, improving mechanistic insights and mass balances. (Anticipated Project Completion - 2025)

Blotevogel, J., R.J. Giraud, and A.K. Rappé. 2023. Incinerability of PFOA and HFPO-DA: Mechanisms, Kinetics, and Thermal Stability Ranking. Chemical Engineering Journal, 457:141235. doi.org/10.1016/j.cej.2022.141235.