By Ramona Darlington, Ph.D., Edwin Barth, Ph.D., P.E., and John McKernan, Ph.D.
Since the 1960s, Aqueous Film-Forming Foams (AFFFs) have been the standard for fighting petroleum fires and conducting firefighting training at U.S. military bases. The synthetic foam’s chemical properties make it ideal for quickly smothering petroleum fires. Recent studies, however, have raised concerns about the long-term environmental and human health impact of contamination in soil and groundwater around sites where the foams were heavily used.
AFFFs contain per- and polyfluoroalkyl substances (PFAS), a class of chemicals that has come under increasing regulatory and public pressure. A 2016 report, “Detection of Poly- and Perfluoroalkyl Substances in U.S. Drinking Water Linked to Industrial Sites, Military Fire Training Areas, and Wastewater Treatment Plants,” documented PFAS contamination in drinking water in communities in 33 states. Most of these communities are near military bases where numerous firefighting exercises were conducted. Given this concern, new answers are needed to help the military clean up or contain PFAS contamination on these sites.
DIAGNOSING THE PROBLEM
PFAS encompasses a family of thousands of individual chemicals that are used in industrial and commercial products. As a group, PFAS chemicals are highly resistant to heat, water and oil, making them highly useful for industrial applications and consumer products ranging from non-stick cookware to stain-resistant fabrics.
AFFFs were developed by the Naval Research Laboratory in the 1960s to extinguish fuel fires faster. At military sites, PFAS accumulation is often found near fire fighting training areas, hangers, runways and crash sites. High accumulation of PFAS is also associated with sludge disposal areas and oil-water separators.
The same chemical properties that make PFAS so effective in firefighting foams and other products make them hard to remediate. PFAS chemicals have very limited reactivity. As a result, PFAS is highly persistent in the environment and bioaccumulative in human and animal tissues—meaning it is absorbed at a faster rate than it is removed, therefore accumulating within the organism. This is especially true for “long-chain” PFAS chemicals such as perfluorooctane sulfonate (PFOS), which was until recently commonly used in firefighting foams. Some PFAS chemicals also have the potential to travel through the environment. Contamination in soil can sometimes make its way into groundwater.
Toxicology studies have raised concern about potential toxic effects for humans exposed to PFAS, including possible developmental effects for fetuses and young children. Other studies have pointed to possible links to cancer, immune system disorders and fertility problems. For these reasons, the Environmental Protection Agency (EPA) has set lifetime health advisory levels for PFOS and perfluorooctanoic acid (PFOA), another long-chain PFAS chemical, at just 70-ppt.
SOIL SORPTION TECHNOLOGIES
The persistence and mobility of PFAS in the environment make it a serious long-term concern for military bases. Relying on natural attenuation for reduction in potency and long-term monitoring is not an effective strategy for PFAS. Active treatment strategies must be used to either clean up the contamination or contain it so that it cannot leach from contaminated soil into groundwater or drinking water reserves.
Most of these treatment strategies are in their infancy and need more research and validation in the field. One promising approach currently in use is sorption, in which a material with PFAS-adsorbent properties is introduced to the contaminated area to immobilize the PFAS (although not destroying it). While there are sorption technologies for both water and soil, because PFAS releases tend to occur on the surface and some PFAS chemicals have a strong affinity to soil, sorption technologies could be highly beneficial. The goal of these treatments is to immobilize the PFAS in the soil to prevent it from leaching into groundwater.
Battelle and the EPA recently completed a literature review to determine which soil sorption technologies—carbon based, resins, minerals, biomaterials and molecular imprinted polymers—have the most promise for PFAS-contaminated sites.
Biomaterials, as a class, may be less effective for PFAS contamination than other options, as they are likely to (or have the potential to) biodegrade over time. Resins also have significant limitations for use in soil treatments and are cost prohibitive compared to other options. Carbon and mineral treatment show more promise for treatment of PFAS-contaminated soils.
One promising approach currently in use is sorption, in which a material with PFAS-adsorbent properties is introduced to the contaminated area to immobilize the PFAS (although not destroying it).
Carbon sorbents can be classified into three broad groups: granular activated carbon, powdered activated carbon, and multiwalled carbon nanotubes. Of these, carbon nanotubes has the highest sorption capacity, followed by powdered activated carbon. However, granular activated has been shown to be the most applicable for treatment. Carbon’s non-polar functional groups make it highly useful for hydrophobic contaminants like PFAS. All forms do tend to be fouled by high concentrations of organic matter in sediments or soils. Studies also suggest that they work better for low concentrations of PFAS than for higher concentrations.
ANALYZING MINERAL OPTIONS
Mineral options for PFAS sorption include organoclays, organically modified phyllosilicates (such as montmorillonite, kaolinite and palygorskite), iron oxides (goethite, hematite), and silica.
Organoclays have been the mineral most widely studied. They have a high sorption capacity and modifications may further enhance their capacity. Unmodified organoclays have a surface that is hydrophilic, making them ineffective for hydrophobic compounds like PFAS. However, modification with surfactants and amine or amino groups enhances its ability to adsorb PFAS compounds like PFOA and PFOS.
Silica-based sorbents, like organoclays, can be modified with surfactants to enhance sorption of PFAS. Due to their high cost, their use is mainly limited to solid phase extraction for laboratory analysis of PFAS compounds.
Iron oxide minerals have been shown to have a strong affinity to PFAS compounds. Some studies have even demonstrated that they outperform modified organoclays in their sorption capacity, but more research is needed in this area.
CONSIDERATIONS IN SELECTION
The overall effectiveness of a sorption treatment for PFAS compounds will be impacted by several variables.
Media Characteristics. The efficacy of a particular sorbent is impacted by the media (soil) in which it is used. The pH and the presence of inorganic and organic ions each can affect sorption efficiency. PFAS sorption decreases in more alkaline environments. Natural organic matter present in the soil will greatly reduce the sorption capacity of activated carbon. Inorganic ions impact sorption capacity of organoclays by changing the charge of the sorbent.
PFAS Characteristics. PFAS chemicals are broadly categorized by the length of the carbon-fluoride chain that forms their backbone. Longchain compounds (with six or more carbon atoms) tend to adsorb more strongly than shorter-chain compounds. The functional groups attached to the chain also have an impact; sulfonate functional groups lead to stronger adsorption.
Sorbent Characteristics. The physical and chemical characteristics of the sorbent material have a large effect on sorption capacity. Smaller particles or highly porous materials have higher specific surface area and higher sorption rates. More basic or positively charged sorbents tend to have better performance than more acidic or negatively charged materials.
HEALTH AT INSTALLATIONS
Finding answers for military bases that are contaminated with PFAS is urgent. There are an estimated 26,000 PFAS-contaminated sites across the United States, and six million Americans are believed to be impacted by PFAS-contaminated drinking water. The vast majority of these communities are around military sites where AFFFs were used.
Though AFFFs are far from the only source of PFAS contamination in the environment, their heavy concentrations on and around the bases and the specific chemical makeup of PFAS used in firefighting foams makes these sites a priority. Stabilizing PFAS in the soil will significantly reduce the potential for harm to surrounding communities.
Ramona Darlington, Ph.D., is Senior Research Scientist, Battelle; firstname.lastname@example.org.
Edwin Barth, Ph.D., P.E., is Senior Engineer, and John McKernan, Ph.D., is Acting Branch Chief, National Risk Management Research Laboratory, Environmental Protection Agency. They can be reached at email@example.com; and firstname.lastname@example.org.
[Article first published in the January-February 2018 issue of The Military Engineer.]