Curious Incidences of PFAS at Environmental Interfaces Impacted by Aqueous Film Forming Foams
Dorin Bodgan, Emerson Christie, Konstantinos Kostarelos, Charles Schaefer, Trever Schwichtenberg, and Jennifer A. Field
Many per- and polyfluoroalkyl substances (PFAS) are surface-active-agents, designed to impart properties of oil and water repellency to surfaces of consumer products and to lower the surface tension of aqueous commercial solutions. Given their surface-active nature, PFAS naturally accumulate at the air-water interface of surface waters and at the non-aqueous phase liquid (NAPL)-water interface in the subsurface. Recent attention is focused on interfaces as two-dimensional systems; however, interfaces where PFAS accumulate in natural surface water and groundwater have the potential to become three-dimensional. Under surface water conditions, PFAS accumulate at the air-water interface and the analysis of foams on a freshwater lake impacted by aqueous film forming foams (AFFF) and other PFAS sources, indicate the preferential enrichment of long chain PFAS and other source-specific PFAS, such as saturated fluorotelomer acids. Foams are comprised of hundreds of mg/L dissolved organic carbon (DOC), while PFAS concentrations are in ng/L-mg/L range. Thus, foaming may arise from the naturally occurring DOC in the region, which then entrains and enriches PFAS from underlying surface water (≤ ng/L concentrations). Enrichment factors varied spatially with PFOS (linear isomer) values reaching enrichment factors up to 4,000. Linear PFAS isomers exhibited higher enrichment factors compared to branched PFAS isomers. Partitioning of PFAS to interfaces in soils/sediments may also lead to the retention of PFAS in source zone areas associated with repeated AFFF applications over decades of fire-fighter training activities. At low concentrations, PFAS partition to the two-dimensional NAPL-water interface; however, at higher concentrations nearing the AFFF field application rate, three-dimensional microemulsions form. Microemulsions were also observed in bench-scale mixing experiments with Jet Fuel A and MilSpec AFFF, as well as for modern fluorine-free AFFF. Column experiments in which AFFF (applied at field AFFF application rates) infiltrated a sand pack with residual Jet Fuel A resulted in the formation of Winsor Type II microemulsions. Over 75% of the PFAS mass in the AFFF was retained by microemulsion and remained in the column, due to the viscosity of the microemulsion. At low and high concentrations PFAS partitioning in the presence of NAPL is influenced by salinity. Overall, the capacity of three-dimensional interfaces (e.g., foam/surface microlayer of surface water and microemulsions in the subsurface) for PFAS is greater than that of two-dimensional interfaces.
About the speaker:
Dr. Jennifer Field is a Professor in the Department of Environmental and Molecular Toxicology at Oregon State University and also serves as an Associate Editor for the journal Environmental Science and Technology. Her current research focuses on environmental analytical chemistry of PFAS, in particular in groundwater and leandfill leachates.