Assessing the potential contributions of additional retention processes to PFAS retardation in the subsurface

Abstract

A comprehensive understanding of the transport and fate of per- and poly-fluoroalkyl substances (PFAS) in the subsurface is critical for accurate risk assessments and design of effective remedial actions. A multi-process retention model is proposed to account for potential additional sources of retardation for PFAS transport in source zones. These include partitioning to the soil atmosphere, adsorption at air-water interfaces, partitioning to trapped organic liquids (NAPL), and adsorption at NAPL-water interfaces. An initial assessment of the relative magnitudes and significance of these retention processes was conducted for two PFAS of primary concern, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), and an example precursor (fluorotelomer alcohol, FTOH). The illustrative evaluation was conducted using measured porous-medium properties representative of a sandy vadose-zone soil. Data collected from the literature were used to determine measured or estimated values for the relevant distribution coefficients, which were in turn used to calculate retardation factors for the model system. The results showed that adsorption at the air-water interface was a primary source of retention for both PFOA and PFOS, contributing approximately 50% of total retention for the conditions employed. Adsorption to NAPL-water interfaces and partitioning to bulk NAPL were also shown to be significant sources of retention. NAPL partitioning was the predominant source of retention for FTOH, contributing ~98% of total retention. These results indicate that these additional processes may be, in some cases, significant sources of retention for subsurface transport of PFAS. The specific magnitudes and significance of the individual retention processes will depend upon the properties and conditions of the specific system of interest (e.g., PFAS constituent and concentration, porous medium, aqueous chemistry, fluid saturations, co-contaminants). In cases wherein these additional retention processes are significant, retardation of PFAS in source areas would likely be greater than what is typically estimated based on the standard assumption of solid-phase adsorption as the sole retention mechanism. This has significant ramifications for accurate determination of the migration potential and magnitude of mass flux to groundwater, as well as for calculations of contaminant mass residing in source zones. Both of which have critical implications for human-health risk assessments.

The influence of molecular structure on the adsorption of PFAS to fluid-fluid interfaces: Using QSPR to predict interfacial adsorption coefficients

“…As discussed by Brusseau (2018), fluid-fluid interfacial adsorption of PFAS is influenced by numerous factors, many of which may be spatially and temporally variable. The potential influence of solution-chemistry related factors on K i was noted above.…”

“…To date, research has focused primarily on two such processes, the sorption of PFAS to solids such as soil, sediment, and activated carbon, and air-water partitioning. However, given the molecular properties of PFAS, it is critical to also consider adsorption at fluid-fluid interfaces as a potential retention process for environmental systems (Brusseau, 2018).…”

Occurrence and behavior of per‐ and polyfluoroalkyl substances from aqueous film‐forming foam in groundwater systems

“…An assessment of PFAS retention processes in the subsurface, based on retention values (R) calculated using data available in the literature, lead to a number of interesting conclusions (Brusseau, ). First, micelle formation may not be that important in the subsurface since the critical micelle concentration of at least some of the PFAS of interest (PFOA and PFOS) is much higher than concentrations observed in source zones.…”

“…An assessment of PFAS retention processes in the subsurface, based on retention values (R) calculated using data available in the literature, lead to a number of interesting conclusions (Brusseau, 2018).…”

“…The results suggest that accumulation at liquid–liquid interfaces accounts for most of the retention (10.0) in an aquifer system and sorption and partitioning accounts for much less (6.3). Only three compounds were tested, so it is possible that some compounds in AFFF mixtures might behave somewhat differently; however, it appears that the dominant processes in PFAS retention are related to adsorption to surfaces as a result of their surfactant qualities (Brusseau, ). Some of the retention mechanisms cited earlier, like accumulation at the air–water surface and accumulation at air–product surfaces, are specific for a source zone/release point, and their contribution may decrease as contaminated groundwater migrates past the source zone and moves deeper into the aquifer system.…”