PFAS and LNAPL/PFAS Microemulsions at NAPL Remediation Sites
Volume 10, Issue 5 | August 2022


Applied NAPL Science Review

PFAs and LNAPL/PFAS Microemulsions at NAPL Remediation Sites

Editor: Lisa Reyenga, PE

ANSR Scientific Advisory Board
J. Michael Hawthorne, PG, Board Chairman, GEI Consultants, Inc.
Andrew J. Kirkman, PE, BP Corporation North America
Robert Frank, RG, Jacobs
Paul Cho, PG, CA Regional Water Quality Control Board-LA
Randy St. Germain, Dakota Technologies, Inc.
Dr. Terrence Johnson, USEPA
Brent Stafford, Shell Oil Co.
Douglas Blue, Ph.D., Imperial Oil Environmental & Property Solutions (Retired)
Natasha Sihota, Ph.D., Chevron
Kyle Waldron, Marathon Petroleum
Danny D. Reible, Professor at Texas Tech University
Reeti Doshi, National Grid
Mahsa, Shayan, Ph.D., PE, AECOM Technical Services


Applied NAPL Science Review (ANSR) is a scientific ejournal that provides insight into the science behind the characterization and remediation of Non-Aqueous Phase Liquids (NAPLs) using plain English. We welcome feedback, suggestions for future topics, questions, and recommended links to NAPL resources. All submittals should be sent to the editor.


DISCLAIMER: This article was prepared by the author(s) in their personal capacity. The opinions expressed in this article are the author’s own and do not necessarily reflect the views of Applied NAPL Science Review (ANSR) or of the ANSR Review Board members.


PFAS and LNAPL/PFAS Microemulsions at NAPL Remediation Sites


Sid Park, Jacobs 

Where historical NAPL fires were extinguished using AFFF, PFAS may be retained in the subsurface in residual NAPL mass, at the air- or NAPL-water interface, or as a viscous LNAPL/PFAS microemulsion (LPME). PFAS retention in-situ and potential presence of LPME with phase behavior that is very different from both the LNAPL and PFAS require special consideration for management or treatment. This article summarizes the current understanding PFAS partitioning to NAPL and potential LPME formation for consideration in-situ characterization and remedial design.

Introduction

Aqueous film-forming foam (AFFF) contains high concentrations (grams per liter) of per- and polyfluoroalkyl substances (PFAS) and was historically used to combat fires where the fuel source was non-aqueous phase liquids (NAPLs) including light non-aqueous phase liquids (LNAPLs) such as diesel, gasoline, or waste oil and/or dense non-aqueous phase liquids (DNAPLs) such as trichloroethene (TCE) (Figure 1). These events occurred at fire-training areas, fuel spill locations, in hangars or on runways, and at bulk fuel storage areas. Conceptually, after being sprayed on the fire, AFFF foam eventually collapsed and AFFF liquid infiltrated the subsurface. PFAS dissolved in water also infiltrated the subsurface. Although several studies showed preferential association of PFAS to NAPL and at the air-water interface (AWI), evidence from one study indicated PFAS can form an immobile LNAPL/PFAS microemulsion (LPME) with milligrams per liter (mg/L) concentrations of PFAS (Figure 2) at the NAPL-water interface (NWI). PFAS retained in NAPL, at the AWI, or as LPME may represent an ongoing source of PFAS to groundwater and limit disposal options for recovered LNAPL, which was previously treated through traditional means that are likely ineffective for PFAS treatment. PFAS persistence in the subsurface and potential treatment difficulty can increase remedial durations and associated costs. Therefore, understanding the presence and behavior of PFAS at sites with NAPL or where LPME may have formed is necessary for site characterization, remediation, and revitalization.


Figure 1. Conceptual Drawing of PFAS and LNAPL co-release location.


Figure 2. Photograph of LPME formation from Kostarelos et al. (2021) shows the viscosity of LPME
where the interface between the upper and middle phases remains when the pipette is tilted.

Laboratory Studies

Four laboratory studies reviewed indicate the potential for PFAS retention to NAPL and at the AWI and NWI. A brief summary of each study and associated results relevant to this article are summarized in the following table.  More recent studies confirm these earlier findings (Brusseau and Van Glubt [2021], Brusseau [2021], Silva et al. [2019]).

ReferenceStudy Brief*Relevant Conclusions(s)*
Guelfo and Higgins (2013)Batch sorption experiments of transport potential of perfluoroalkyl acids (PFAAs) in different soils, and soil with TCE.TCE acted as a PFAS sorbent in soil with low organic carbon content.
McKenzie et al. (2016)1-D column experiments with vertical up-flow through loamy sand with TCE to assess PFAA transport.TCE provided additional adsorption capacity/increased PFAA retardation.
PFAA were transferred to the NAPL phase.
Brusseau (2018)Used data collected from literature model and assess the importance and magnitude of perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) retention processes.PFAS retardation includes multiple processes. Adsorption at the AWI in sandy soil contributed to 50 percent of the retention.
Retention at the NWI and partitioning to bulk NAPL were also significant.
Van Glubt and Brusseau (2021)Batch experiments of PFAS retention using PFOS and PFOA as representative PFAS and TCE and decane as representative NAPLs.
Column experiments studied PFAS retention and transport in absence and presence of NAPL.
Interfacial adsorption greatest for longer-chain PFAS (PFOS).
NAPL can contribute
significantly to PFAS retention; adsorption to the NWI accounted for ∼77% of the retention observe compared to bulk NAPL.

*Table provides only a nominal description of the activities and findings that are relevant to this article.

A study by Kostarelos et al. (2021) is the first to report the formation of a viscous, stable, microemulsion (i.e., LPME), shown on Figure 2, when application-strength AFFF (3 percent in water) with Jet Fuel A in batch experiments were mixed. Working with actual, field-obtained AFFF, they also conducted 1-D flow experiments (column) where LNAPL, at residual saturation within the sand, was contacted by AFFF. The experiments confirmed LPME formation under realistic field conditions that mimicked the scenario of AFFF infiltration into the subsurface following LNAPL. In addition to forming LPME with historical AFFF formulations containing PFAS, Kostarelos et al. (2021) confirmed that newer C6 MilSpec and fluorine-free AFFF also formed viscous LPME in the laboratory experiments.

The LPME was observed to be thermodynamically stable with transport properties very different from those of the LNAPL or AFFF alone (Dwarakanath et al., 2000). Kostarelos, et al. (2021) note that the LPME that formed had a viscosity 110 times that of water and significantly higher than those of Jet Fuel A and AFFF-solution as shown on Figure 3.


Figure 3. Viscosity of 3 percent AFFF-1, Jet Fuel A, and LPME (equilibrated for 10 minutes and several weeks)
(chart from Kostarelos et al. [2021])

Relevance

Because AFFF was routinely used to douse NAPL fires, these contaminants have interacted in the subsurface at many sites. Based on the laboratory evidence, PFAS retention in the subsurface at the AWI, NWI, within residual NAPL, or as LPME may be a relatively common occurrence. To date, remediation at these sites targeted NAPL recovery or treatment without consideration of PFAS. In particular, LNAPL skimming or enhanced oil recovery technologies were not designed to mobilize and extract LPME with physical properties different from the LNAPL alone. In addition, at sites where LNAPL is extracted and treated ex-situ, the presence of PFAS in the LNAPL likely affects treatability and disposal options. Therefore, site characterization and remedial designs need to understand and account for PFAS behavior in the subsurface at sites with co-located releases of NAPL.

Summary

Laboratory evidence suggests that PFAS may be retained in the subsurface at sites where NAPL was also released likely resulting in a persistent source. In addition, the formation of LPME may complicate existing remediation strategies at LNAPL sites currently using methods such as extraction and offsite disposal. With the recent development of an analytical method at Oregon State University (Jennifer Field, 2022, Personal Communication) to quantify PFAS in NAPL, these sites may need to be re-evaluated for the presence of residual PFAS mass in NAPL that may be acting as a long-term source to groundwater. Further study is needed to identify the conditions that affect PFAS retention, how common those conditions are, better understand their retention mechanisms, and evaluate technologies to treat the PFAS-impacted media. In addition, at sites where NAPL has been extracted and later found to contain PFAS, ex situ technologies to separate PFAS from NAPL require study.

A Word of Caution

The laboratory studies evaluated simplified conditions and fewer NAPL and PFAS compounds than are likely present at release sites. In particular, AFFF is comprised of complex mixtures of PFAS, and NAPLs at release sites may include mixtures of different LNAPLs and DNAPLs in various degrees of weathering. The studies also do not account for the unique and often heterogenous conditions in situ. The use of AFFF to extinguish a NAPL-based fire does not necessitate the presence of LPME but the evidence suggests that residual PFAS mass may be present at the AWI, within NAPL, or at the NWI. Currently, there is no commercial method to quantify PFAS in NAPL. Care should be given to assumptions of LPME presence or absence based on soil and groundwater analytical data.

References 

Brusseau, M. L., Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Sci Total Environ. 2018 February 01; 613-614: 176–185. doi:10.1016/j.scitotenv.2017.09.065.

Brusseau, M. L., Examining the robustness and concentration dependency of PFAS air-water and NAPL-water interfacial adsorption coefficients. Water Res. 2021, 190.

Brusseau, M. L. and Van Glubt, S., The influence of molecular structure on PFAS adsorption at air-water interfaces in electrolyte solutions. Chemosphere, 2021, 281.

Dwarakanath, V.; Pope, G. A. Surfactant phase behavior with field degreasing solvent. Environ. Sci. Technol. 2000, 34 (22), 4842−4848.

Guelfo, J. L. and Higgins, C. P., Subsurface transport potential of perfluoroalkyl acids at aqueous film-forming foam (AFFF)-impacted sites. Environ. Sci. Technol. 2013, 47, (9), 4164-4171.

Kostarelos, K.; Sharma, P.; Christie, E.; Wanzek, T.; Field, J., Viscous microemulsions of aqueous film-forming foam (AFFF) and Jet Fuel A inhibit infiltration and subsurface transport. Environ. Sci. Technol. Lett. 2021, 8, (2), 142-147.

McKenzie, E. R.; Siegrist, R. L.; McCray, J. E.; Higgins, C. P., The influence of a non-aqueous phase liquid (NAPL) and chemical oxidant application on perfluoroalkyl acid (PFAA) fate and transport. Water Res. 2016, 92, 199-207.

Silva, J. A. K.; Martin, W. A.; Johnson, J. L.; McCray, J. E., Evaluating air-water and NAPL- water interfacial adsorption and retention of Perfluorocarboxylic acids within the Vadose zone. J Contam Hydrol. 2019, 223.

Van Glubt, S. and Brusseau, M. L., Contribution of nonaqueous-phase liquids to the retention and transport of per and polyfluoroalkyl substances (PFAS) in porous media. Environ. Sci. Technol. 2021, 55, (6), 3706-k.


Research Corner

Per- and Polyfluoroalkyl Substances (PFAS): Protein Binding and Partitioning and Sorption in Light Non-Aqueous Phase Liquids

Emerson C. Christie
Doctor of Philosophy
Oregon State University

Abstract:

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic surfactants that have recently been identified as persistent organic pollutants. These so called “Forever Chemicals” have been detected in drinking waters, ground waters, soils, and consumer and industrial products globally; with environmental impacts stretching into the artic, far from known PFAS sources. The increase in awareness regarding PFAS distribution in the environment has generated interest into how PFAS interact with humans, what PFAS specific properties may be involved, and what additional environmental compartments may they be found in. In Chapter 2 we discuss the use of molecular dynamics (MD) modeling to screen for protein – PFAS binding affinity to inform experimental measurements of binding affinity via equilibrium dialysis (Eq D). The equilibrium dissociation constants (KD) of six perfluoroalkyl carboxylates (PFCAs) and three perfluoroalkyl sulfonates (PFSAs) to liver and intestinal fatty acid binding proteins (L- and I-FABPs) and peroxisome proliferator activated nuclear receptors (PPAR-α, – δ and – γ) were determined via liquid chromatography mass spectrometry. The MD models were found to predict relative and not absolute binding for all protein – PFAS combinations. This research was the first to identify sub micromolar binding between short chain PFAS (6 or less carbons) and PPAR-α and δ, which may have implications for the assumed safety of shorter chain PFAS due to rapid clearance. Chain length dependent binding was observed for L-FABP but not observed for PPAR proteins which means that for these proteins binding affinity cannot be inferred by PFAS chain length. Additionally, a comparison was made between KDs derived from EqD and other in-vitro approaches, using these experimental results and results from literature. It was discovered that KDs derived from EqD were lower (i.e. higher binding affinity) than other in-vitro approaches which has implications for comparisons between methodologies and raises an important question regarding which KDs should be considered most relevant in-vivo. Research discussed in Chapter 3 surrounds the development of an extraction and analytical method to quantify PFAS in environmental non-aqueous phase liquids (NAPL). As mentioned above, PFAS are used in industrial products and one common group of industrial products that have been identified as the root cause for environmental PFAS contamination at U.S. military sites are aqueous film forming foams (AFFF). AFFF are complex mixtures known to contain high concentrations of many surfactants including PFAS. At U.S. military sites it is also common to encounter NAPL in the subsurface. Co-disposal of PFAS (AFFF) with NAPL has happened historically through intentional use (e.g. firefighting) or unintentionally at waste sites. In order to quantify PFAS within NAPL, a liquid-liquid extraction method was developed that could successfully extract anionic, cationic, and zwitterionic PFAS. This research discovered the presence of PFAS in recovered NAPL at microgram per liter concentrations at non-source zone sites. Concentrations of PFAS in NAPL are likely much higher at source zone sites and could have implications for NAPL remediation. Chapter 4 discusses the partitioning and interfacial adsorption of PFAS into NAPL at environmentally relevant concentrations (i.e. nano – microgram per liter). Given the discovery of low microgram per liter concentrations of PFAS in recovered NAPL discussed in Chapter 3, it is relevant to investigate what partitioning and sorption processes are occurring at these concentrations. Current research in this area has focused on the NAPL – water interface and has done so at high concentrations, milli – gram per liter. Here we performed batch equilibrium experiments at low concentrations (2,000 – 100,000 ng/L) between jet fuel A (NAPL) and synthetic freshwater. Single point partition coefficients (Kn) were calculated for PFAS of carbon chain length 8-14 across the concentration range. Values for Kn decreased with increasing PFAS concentration indicating non-ideal partitioning, which become more evident with increasing chain length. Partitioning into jet fuel A was not observed for PFAS below eight carbons. Interfacial sorption (Knw) was estimated by mass difference and found to be orders of magnitude higher than previously reported literature values.


Related Links

API LNAPL Resources
CL:AIRE Technical Guidance
Concawe LNAPL Toolbox
CRC CARE Technical Reports
CSAP MNA Toolkits
EPA NAPL Guidance
Groundwater Monitoring & Remediation
GWSDAT
ITRC LNAPL Resources
ITRC DNAPL Documents
SERDP-ESTCP
Sustainable Remediation Forum

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This article kicks off our series on PFAS and NAPL. We will also continue to intersperse articles on other topics of interest such as natural source zone remediation, bioremediation, NAPL forensics, and remediation case studies. Please contact us if you would like to submit an article or would like to see articles on other topics.

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Concawe (CONservation of Clean Air and Water in Europe) published the LNAPL Toolbox. The LNAPL Toolbox is “a unique collection of useful tools, calculators, data, and resources to help LNAPL scientists and engineers better understand how to manage LNAPL at their sites.”


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