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

Topic Area: 7f2. In Situ PFAS Soil Treatment Approaches

Formatted Title:

In Situ Thermal Treatment of PFAS in Vadose Zone Soils

Background/Objectives:

Per- and polyfluorinated alkyl substances (PFAS)-impacted soil may act as a continuing source to large groundwater plumes, resulting in extensive containment or remediation costs and/or land use restrictions for those locations. Although technology options to address PFAS in soils are beginning to emerge, they either generate waste that requires disposal or present potential limitations in future land use. Additionally, most solutions require ex situ soil treatment, which will be logistically difficult at some sites with expensive infrastructure hindering excavation. There is a strong need for a cost-effective in situ soil treatment option that removes PFAS from soil and effectively mitigates liability at the site.

Approach/Activities:

A field-scale pilot test, funded through ESTCP (ER20-5250), to demonstrate in situ thermal desorption of PFAS from vadose zone soil was performed between February and July 2023. The pilot test took place at a former fire training area where PFAS impacts to soil exceeded screening levels by 2 orders of magnitude. The soil was heated to an average temperature of 403.6ºC during the treatment test period using an electrically powered thermal conduction heating system. Concentrations of PFAS were measured in soil before and after the pilot test and subsurface vapor collected during system operations was sampled using Other Test Method 45 (OTM-45) and OTM-50 at six time points throughout the heating process. The condensate generated was also routinely sampled for PFAS. The off-gas and condensate were treated using vapor and liquid phase granular activated carbon, respectively prior to discharge. Pre and post treatment soil samples were also subjected to supplemental analysis using leaching environmental assessment framework (LEAF) to identify whether heating of a contaminant source area could illustrate measurable reduction in PFAS leachability. Subsamples of the condensate were sent to Colorado School of Mines for hydrothermal alkaline and ultraviolet-sulfite destructive treatment.

Results/Lessons Learned:

Concentrations of PFAS measured in the vapor and condensate streams were used to construct a mass balance around treatment operations. Where a temperature of at least 350 °C was maintained for seven days, a 99.5% average reduction of perfluorooctanesulfonic acid (PFOS) in soil was observed. It was suspected that there was long-term shallow water incursion on the eastern side of the test area and the target temperatures were either not reached or not held for the duration of the test. As such, PFAS removal from soil varied spatially on the eastern side of the test area. Additionally, several PFAS were present on the eastern side after heating that were not present before heating, suggesting that transformation occurred during the pilot test. PFAS were measured in condensate throughout the pilot test. Concentrations in the condensate peaked 30 days after heating began and subsequently decreased as the heating progressed past 30 days. Granular activated carbon was demonstrated effective at treating the condensate and vapor to below the discharge limits.

Menu

Lynette Stauch