The Department of Defense (DoD) has many fire training areas (FTAs) impacted by residual per- and polyfluoroalkyl substances (PFAS) as a result of the use of aqueous film-forming foam (AFFF). Environmentally sustainable and cost-effective remediation approaches for PFAS-impacted matrices are needed for such sites. There are current uncertainties regarding why AFFF-impacted soils and concrete surfaces retain a significant mass of PFAS that continues to leach for decades following the cessation of AFFF use. PFAS are known to self-assemble to form large supramolecular assemblies, comprising multiple bilayers at interfaces where they concentrate. Self-assembled PFAS (SA-PFAS) structures comprise lyotropic liquid crystals that have grown to form 500 microns long structures over four months. These assemblies have been demonstrated to form at solid-liquid interfaces at concentrations five orders of magnitude lower than PFAS critical micelle concentrations. It is hypothesized that PFAS in dispensed AFFF coats solid surfaces with SA-PFAS via the repeat formation of Langmuir-Blodgett type films, which is responsible for storing PFAS at interfaces in the vadose zone. In saturated zones at FTAs, relatively high concentrations (mg/L) of dissolved PFAS can concentrate at solid-liquid interfaces leading to SA-PFAS formation. These supramolecular assemblies are likely to be an important reservoir of PFAS at FTA. This project aims to characterize these supramolecular assemblies, their formation mechanisms, and kinetics, thus guiding remediation strategies.
The technical approach of this project aims to characterize SA-PFAS supramolecular assemblies using advanced PFAS characterization methods (e.g., high-resolution mass spectrometry, total organofluorine analysis) and complementary visualization tools (e.g., electron microscopy, X-ray microtomography). Impacted materials collected from the field will be subject to detailed characterization to survey the penetration of a large variety of PFAS species. AFFF field discharge will be simulated in laboratory experiments. For uptake of PFAS by concrete materials, surfaces with a range of capillary pores, air voids, and carbonation depth will be examined. Uptake kinetics will be determined under varying water chemistry conditions. For PFAS retention by sand columns, the surficial and subsurface horizons where SA-PFAS mainly accumulate will be identified. Aggressive extraction methods used to extract PFAS from solid matrices will be verified, and their applicability to delaminate and dissolve large supramolecular forms of SA-PFAS will be investigated.
The anticipated benefits include developing a detailed understanding of the distribution and behavior of SA-PFAS to effectively target remediation where these structures are located, potentially diminishing the volume of soil and concrete that requires treatment. Further benefits include contributions to more sustainable treatment technologies that can cost-effectively remove SA-PFAS from impacted FTAs. Significant savings can be made by the development of targeted treatment approaches to delaminate and dissolve SA-PFAS, or by encouraging their formation, so they can be removed or stabilized. (Anticipated Project Completion - 2025)