Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7e. Ex Situ PFAS Treatment: Soils/Solids and Other Waste Streams

Formatted Title:

Lessons Learned for Ex Situ PFAS-Impacted Soil Treatment Using Thermal Conduction Heating

Background/Objectives:

The release of per- and polyfluoroalkyl substances (PFAS) has occurred at Department of Defense (DoD) sites across the United States. PFAS-impacted soil can result in persistent groundwater plumes, and more stringent state and federal regulatory requirements are foreseen for soil containing PFAS for groundwater protection. The highest PFAS concentrations are typically observed in shallow soils, which DoD military installations are increasingly excavating and stockpiling. Once PFAS-impacted soil is excavated, offsite disposal is currently the most developed option for removal from DoD sites. Due to concerns associated with off-site disposal, a significant need exists for alternative on-site treatment approaches.

Laboratory and field testing have shown significant PFAS removal at 350ºC-400ºC. These demonstrations were completed to document PFAS reductions achievable at the field scale using thermal conduction heating (TCH). Under the ESTCP program, Paragon and TRS implemented an ex situ demonstration at Eielson Air Force Base. TRS is currently implementing ex situ PFAS soil treatment on a larger scale through the Defense Innovation Unit at Joint Base Elmendorf-Richardson (JBER).

Approach/Activities:

At Eielson, a small soil pile was chosen for the demonstration, containing approximately 134 cubic yards of soil and was approximately 4.5 feet tall. The soil contained PFOS at an estimated starting concentration of 230 µg/kg (two orders of magnitude higher than the Alaska cleanup objective of 3.0 µg/kg). Vertical heater borings were installed through the soil pile and the surface was covered and insulated. Heating was performed for 100 days, reaching temperatures in the pile between 350 and 450ºC. Vapors were extracted from dedicated vacuum extraction wells, cooled, and treated on site using a wet scrubber and granular activated carbon. Sampling of the soil and extracted fluids was performed to document the performance. This included vapor sampling using EPA Method OTM-45 and paired soil samples for comparison of pre- and post-heating soil concentrations.

At JBER, the TCH ex situ field demonstration will be performed on 2,000 cubic yards of PFAS-impacted soil. The soil will be heated to 400°C using electrically powered horizontal heaters arranged in triangular arrays. Once the target temperature is achieved, heating will continue until the PFAS soil concentration performance objectives are met. The heating duration is anticipated to require approximately 120 days, with a similar vapor and process water treatment system similar to Eielson AFB. Soil samples distributed across the soil pile will be collected before and after heating, with temperature monitoring at each soil sample location throughout system operations. Vapor and process water samples will also be collected during system operations and analyzed for PFAS concentrations.

Results/Lessons Learned:

Significant reductions of PFAS compounds were demonstrated at Eielson. All soil samples from locations which reached 400ºC were below laboratory detection limits for PFOA and PFOS. Soils heated to 340-360ºC for less than one week had PFOS concentrations in the 0-10 µg/kg range. One such soil sample was heated for an additional week in the laboratory at 350ºC resulting in concentrations below detection limits. Many of the lessons learned at Eielson are being implemented at JBER. Construction at JBER is currently underway, with operations anticipated to be completed by Spring 2024.

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Phillip Stallings