Molecular Characterization of the Thermal Degradation of Per- and Polyfluoroalkyl Substances in Aqueous Film-Forming Foams via Temperature-Programmed Thermal Desorption–Pyrolysis–Direct Analysis in Real Time–Mass Spectrometry
Christopher P. West, Hilary M. Brown, Patrick W. Fedick
Abstract
The release of aqueous film-forming foam (AFFF), containing per- and polyfluoroalkyl substances (PFAS), from Department of Defense activities has received attention over the years due to the environmental persistence and bioaccumulation of PFAS. As a result, the National Defense Act established that the removal of PFAS-containing waste is critical. Thermal destruction methods are commonly used techniques, yet the fate of degraded PFAS remains poorly understood. In this study, we employ thermal desorption–pyrolysis–direct analysis in real time–mass spectrometry (TD-pyro-DART-MS) to characterize products of pyrolysis and determine the extent of degradation from 25 to 600 °C. PFAS ranging from 4–14 carbon atoms were monitored in situ, followed by legacy AFFF. Headgroup scission was observed, followed by carbon–carbon bond cleavages in the structures resulting in [C x F y ] − fragments differing by -CF 2 (50 Da) and -C 2 F 4 (100 Da). High-molecular weight PFAS resulted in more detectable pyrolytic fragments than low-molecular weight counterparts. AFFF concentrate thermal degradation analysis was more complex and was determined to require higher-resolution mass spectrometers for molecular assignment. This study demonstrates the development of a robust analytical methodology for in situ characterization of the products of thermal degradation of PFAS related to thermal remediation or when PFAS are used to extinguish fuel fires.