PFAS are present in AFFF used by the DoD and other organizations to extinguish hydrocarbon fires. Different AFFF formulations have been used, but all contain a complex mixture of PFAS, including those of greatest regulatory concern - the perfluoroalkyl acids (PFAAs) and potential PFAA precursors. The Environmental Protection Agency (EPA) has recommended a Health Advisory Level for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), and several states have promulgated standards for PFOA, PFOS, and some of the related PFAAs.
It is critical to elucidate the mechanisms and pathways that underlie conventional and innovative thermal technologies for PFAS destruction. Thermal processes are defined as processes in which the temperature of the bulk medium (e.g., water, air, soil) is purposefully raised to a temperature exceeding 100°C. Example technologies include incineration, pyrolysis, supercritical water oxidation, and hydrothermal alkaline treatment. Both experimental and theoretical studies are needed to better understand the mechanisms, rates, and products of the most important pathways by which PFAS transform and mineralize under process-relevant operating conditions (temperature, residence time, reactant concentrations, waste stream composition, gaseous/ condensed phases). Important PFAS thermal decomposition mechanisms include combustion, pyrolysis, oxidations, reactions with radicals such as H abstractions, hydrolysis, elimination, and other reactions. Information on the formation and stability of the PIDs and other byproducts is particularly important to support the design and operation of thermal technologies and ensure that treatment targets are met and undesired product formation is minimized. Understanding PID and byproduct formation may also inform the characterization of source zones at firefighting training sites and inhalation exposure of firefighters using AFFF.
PFAS represent a highly diverse chemical family with a significant range in physiochemical properties and, thus, multiple processes are likely to simultaneously occur across a range of temperatures and other operating conditions. In general, studies have demonstrated that desorption and volatilization of PFAS occurs at relatively lower temperatures (e.g., <700oC), but the possibility exists for some PFAS to transform within the thermal desorption range of others. For a specific suite of conditions, understanding the temperature range where more recalcitrant PFAS become volatile and desorb relative to where other more labile PFAS begin to transform is of critical importance since most AFFF-impacted sites are often characterized by the presence of highly complex mixtures of many PFAS.