April 18, 2024

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In Conversation: Why Do PFAS Stay in Soil?

In a groundbreaking study published in the journal Environmental Science & Technology, Geosyntec’s Thomas (Tom) Wanzek, PhD, investigates the factors influencing how per- and polyfluoroalkyl substances (PFAS) interact with soil particles. This research tackles the critical issue of PFAS-contamination persistence, particularly from aqueous-film-forming foams (AFFFs) used in firefighter training grounds.

In this discussion, Tom walks us through the study’s methods and describes the exciting potential their findings hold for future PFAS remediation tools and strategies.

About the Speaker

Tom is a Senior Staff Scientist based in Oregon who specializes in soil characterization, ecosystem restoration, and the fate and transport of PFAS in soil systems. He offers research and development experience on retention mechanisms of PFAS from AFFF in saturated soil systems and develops user-friendly tools to estimate soil sorptive capacity for PFAS.

The Conversation

What is the article about?

This article is connected to another I published a year ago. Both articles stem from research during my doctoral studies, which was funded by the Department of Defense's Strategic Environmental Research and Development Program (SERDP).

The goal of the project was to understand the factors that drive the interaction between PFAS molecules and soil particles, including PFAS molecular properties and soil physical and chemical properties, and why PFAS source zones can persist for decades. For example, at one of our research sites, the last known PFAS application was in the early 90s, yet high PFAS concentrations were still present in groundwater 20 to 30 years later.

Our research focused on what happens in these PFAS source zones, which are widespread around the world. By better understanding what happens in these zones, we can work to develop more effective remediation techniques.

What was your role?

I focused on PFAS interactions with soil under saturated conditions, which are common, especially in deeper groundwater zones. Doing so also allowed us to eliminate the variable of air-water interfaces and isolate the interactions between PFAS and soil surfaces.

PFAS are designed to be surfactants, meaning they are attracted to interfaces. Our approach was to simplify the system by focusing solely on interactions between solids and water at the interface between soil and water.

What is different about this study?

Many PFAS-soil-interaction studies rely on batch experiments. These involve mixing soil, water, and PFAS at potentially unrealistic ratios and simulating ideal, homogenized conditions. While this simplifies the results, it can also mask natural environmental dynamics.

A focus of our experimental design was to replicate, as best as possible, what happens in the environment and the real-world processes under PFAS that might interact with soil. We used columns to flow the contaminants through the system, mimicking what happens in real-world environments. Additionally, we used actual AFFF instead of isolated PFAS molecules.

What did using columns tell you that a batch experiment wouldn’t?

Batch experiments often use isolated PFAS standards purchased from companies. This approach removes variables but doesn't reflect reality. PFAS in the environment don't exist as single molecules; they arrive with a complex mixture of other chemicals.

For example, PFAS make up only a small portion of AFFF by weight, between 2 and 15%. The remaining components include organic solvents, hydrocarbon surfactants, polymers, salts, buffers, and preservatives. All of these components can potentially influence how PFAS interact with soil.

Tell us more about the experiment.

4 Applications of AFFF Image

Environ. Sci. Technol. 2024, 58, 3, 1659–1668
Publication Date: January 10, 2024
Copyright © 2024 American Chemical Society

We started with low concentrations of PFAS to focus on how individual PFAS molecules interact with soil. We then wanted to see how things changed when we used a more realistic scenario, like the concentration firefighters might use. So, in the next set of experiments, we applied PFAS at a much higher concentration, about the same concentration of AFFF firefighters use for training or in an actual emergency response event. We also kept the soil saturated to avoid any influence from air–water interfaces.

These higher concentrations led to the formation of micelles, which are clusters of PFAS molecules. As expected, the presence of micelles significantly increased the amount of PFAS retained in the soil, by about 25 to 30%. Interestingly, even with these micelles present, our original model, which was developed under the low-PFAS-concentration conditions, with just a minor adjustment, could still accurately predict the amount of PFAS retained by the soil.

Then, we took things a step further. We simulated what might happen at a fire training center where firefighters repeatedly practice. We did this by applying PFAS to the soil column four times in a row. This allowed us to see how the PFAS interacted with the soil under these more environmentally relevant conditions.

An interesting pattern emerged, and it depended on the specific type and chemical composition of the PFAS molecule. For what we call anionic PFAS, the amount retained simply kept increasing with each application. However, for a different type called zwitterionic PFAS, things were more complex.

Initially, some zwitterionic PFAS mass was retained. Then, with subsequent applications, we saw some of the mass from the first application being displaced, meaning more came out of the bottom of the column than was applied. This suggests that some of the previously retained zwitterionic PFAS might have been desorbed, or released, back into the water flow.

This research highlights the importance of considering the specific chemistry of each PFAS molecule. Depending on the type, the PFAS might be consistently retained by the soil, steadily released over time, or even show a combination of both behaviors.

What were your findings?

Organic carbon, previously considered the primary driver, wasn't the most significant factor in our research. The properties of the PFAS molecules themselves, including their size, fluorine content, and the number of nitrogen atoms, play a role in how they interact with the soil.

The soil properties are also important. The amount of iron oxide, organic carbon content, and the total surface area of the soil particles all contribute to PFAS retention.

The most interesting take-away from our model is a bit counterintuitive. We learned how readily water pushes PFAS molecules out of the solution and toward the soil and may be a highly significant factor for retention. So, molecule size and their aversion to water might be more critical than just how strongly they bind to or interact with the soil surface.

When we talk about mass retained versus mass loss of PFAS, what does that mean for how contaminated the soil is?

This gets tricky because current USEPA-approved testing methods only detect specific PFAS types, not the entire range that can be found in AFFF. Even if your tests show low PFAS mass, there could be more present that the tests miss. Essentially, not being able to see it doesn't guarantee it is not there.

And the contamination likely depends on the training center's age, number of training activities, and the types of AFFF and PFAS used. These factors will influence the types and amounts of PFAS coming out of the soil.

Is the tool you mentioned in study one being developed to track different PFAS types and link them to specific cleanup options?

Not yet. Currently, it's an academic tool demonstrating the key factors influencing PFAS retention. It could potentially replace extensive lab work needed in some PFAS cycling models to determine a soil's ability to hold PFAS.

More Information

Read more: Repeated Aqueous Film-Forming Foams Applications: Impacts on Polyfluoroalkyl Substances Retention in Saturated Soil | Environmental Science & Technology (acs.org)

Hear Tom on our YouTube channel:
PFAS: Data Sampling and What Does it Mean? | Tom Wanzek (youtube.com)
PFAS: Collecting Defensible Data | Tom Wanzek (youtube.com)

For PFAS Consultation | Contact Tom at This email address is being protected from spambots. You need JavaScript enabled to view it..