Formatted Title
Partitioning and Storage of Per- and Polyfluoroalkyl Substances at Fire Training Areas Considering Supramolecular Assemblies
Background/Objectives
There are current uncertainties regarding why aqueous film forming foam (AFFF)-impacted soils and concrete surfaces retain a significant mass of amphiphillic perfluoroalkly acids (PFAAs) such as perfluorooctane sulfonate (PFOS), which continues to leach for decades following cessation of AFFF use. Multiple site investigations where assessment of per- and polyfluoroalkyl substances (PFAS) at fire training areas using advanced characterization tools, such as the total oxidiseable precursor (TOP) assay, have determined that high concentrations of PFAS, including elevated concentrations of cationic and zwitterionic precursors are present in superficial soil and vadose zone and can represent an ongoing source of PFAAs. However, this does not explain the elevated levels of PFOS detected on or near concrete surfaces. The objectives are to demonstrate that the formation of supramolecular forms of PFAS can represent an additional storage mechanism for PFAS at fire training areas (FTAs).
Approach/Activities
A detailed literature review of publications considering the behavior of fluorosurfactants was performed to shed light on their partitioning behavior. Self assembly phenomena exhibited by amphiphillic fluorosurfactants at higher concentrations, driven by both enthalpic and entropic forces, was assessed as the potential driving mechanism behind the observed multilayerd sorption. PFAS are known to self-assemble to form large supramolecular assemblies, especially at interfaces where they accumulate. From examination of the physical chemistry of PFAS it is considered that when they are dispensed in AFFF they coat solid surfaces with SA-PFAS, via the repeat formation of Langmuir-Blodgett films and that this mechanism is potentially responsible for storage of PFAS at interfaces in the vadose zone. In the saturated zone at FTAs relatively high concentrations (mg/L) of dissolved PFAS will concentrate at solid-liquid interfaces leading to the potential for SA-PFAS to form. The supramolecular assemblies formed by SA-PFAS are considered to be an important reservoir of PFAS at FTAs, as they can potentially comprise millions of liquid crystalline layers, which slowly release PFAS.
Results/Lessons Learned
The vertical and horizontal delineation of PFAS at this site will be presented in relation to the site’s hydrogeology and lithology. The results from site investigations of FTAs using advanced surface characterization methods including electron microscopy, ion beam analyses and surface mass spectrometry will be compared to provide evidence of the formation of supramolecular forms of PFAS. This will be combined with a detailed understanding of the partitioning and self-assembly behavior of fluorosurfactants to explain why unsaturated zone soils and concrete surfaces can comprises a long-term reservoir of amphiphilic PFAAs.
The physical chemistry exhibited by fluorosurfactants will be described to explain their aggregation properties. The often frequent and repeat, historical applications of Class B firefighting foams such as AFFFs to open ground are considered to be responsible for this mass distribution, where amphiphilic PFAS are primarily stored in the vadose zone. When assessing the distribution of PFAS in soil and groundwater as a result of repeat fire training or equipment testing, the fate and behavior of all of the components of AFFF, such as glycols, hydrocarbon surfactants need to be considered, in addition to the PFAS.
Self-assembly of C8 PFAS into contiguous bilayers has been reported to occur at concentrations as low as 50 mg/L, meaning that the g/L concentrations encountered in AFFF as discharged to ground, is far greater than that needed for growth of these lamellar, vesicles and microtubules on soil surfaces. A model will be described which details the interaction of PFAS in AFFF with soils, explaining why FTAs remain as continuing sources of PFAS to groundwater plumes for many decades. The formation of multiple bilayers where cationic and zwitterionic PFAS facilitate and stabilize the formation of bilayers will be discussed, which can also account for the slow rates of biotransformation of these precursors, as they are potentially not bioavailable when located internally within multiple layers.