There are currently uncertainties regarding why soils impacted with aqueous film-forming foams (AFFF), non aqueous phase liquids (NAPL), and liquid interfaces retain a significant mass of per- and polyfluoroalkyl substances (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 layers at interfaces and on surfaces where they concentrate. These supramolecular structures have been reported to be very stable, so could potentially act as a long-term source of dissolved phase PFAS where AFFF has repeatedly been released. Self-assembled (SA)-PFAS have been described to form at air-water interfaces, be deposited on solid surfaces, and form at NAPL-air interfaces. These supramolecular assemblies are likely to be an important reservoir of PFAS at fire training areas and have also been reported to form on treatment media whilst PFAS are being removed from water. This project aims to develop and use tools to characterize SA-PFAS, define the conditions required for their formation, assess their stability, and apply treatment approaches to cause their dissolution.

The technical approach aims to characterize SA-PFAS supramolecular assemblies using a series of advanced surface and interface characterization methods (e.g., electron microscopy, X-ray diffraction, ion and subatomic particle beam and mass spectrometry tools), which are trialed by application to an increasingly complex number of matrices impacted by SA-PFAS. The nature of the PFAS layers can have an impact on their stability, so their detailed structure and stacking arrangement will be investigated. The work is planned to progress from using perfluorooctanesulfonic acid to combinations of PFAS and AFFF. Flat wafers of silicon dioxide surfaces (to represent quartz sand) will first be used to determine the PFAS concentrations and exposure times required for SA-PFAS formation. Experiments will be established to replicate the vadose and saturated zones at impacted sites. Porous rocks and unconsolidated soils will be assessed to determine their potential to accumulate SA-PFAS, and field-collected samples will also be investigated. Further experiments will focus on liquid interfaces and assess the formation of SA-PFAS at the air-water, NAPL-air and NAPL-water interfaces. The stability of these SA-PFAS and their potential for disassembly will be assessed using tensiometry and interfacial rheology techniques. Weathering of SA-PFAS will be examined and solvents which may cause their dissolution trialed. Samples of spent treatment media such as granular activated carbon and ion exchange resins will be characterized to determine their SA-PFAS loading.

The anticipated benefits of this project include the development of SA-PFAS characterization tools to identify these structures in multiple matrices. Understanding the conditions required for SA-PFAS to form, where they form, and their stability will provide information essential to the development of conceptual site models of mass flux of PFAS from source areas. Understanding the distribution and behavior of SA-PFAS will allow effective targeting of remediation to where these structures are located, potentially diminishing the volume of soil that requires treatment. Examining the stability of SA-PFAS and the conditions that cause their removal, will lead to the development of more effective remediation technologies and regeneration of treatment media, ultimately improving the cost effectiveness of PFAS treatment technologies, directly benefiting the warfighter and installation communities. (Anticipated Project Completion - 2027)