The objective of this project is to demonstrate in situ thermal treatment (ISTT) for per and polyfluoroalkyl substance (PFAS) removal from unsaturated soil coupled with destruction of PFAS condensate using ultraviolet (UV)-sulfite and hydrothermal destruction technologies. The demonstraton will determine remediation effectiveness including attainment of desired operating temperature, site removal effectiveness, treatment effectiveness for the vapors and condensate, energy efficiency and ease of use. Field demonstration of ISTT will confirm the temperature threshold for PFAS removal, quantify removal rates, expand the knowledge of PFAS thermal desorption characteristics and mechanisms, and determine the extent of PFAS destruction in the heating process. 

The FlexHeater® thermal conduction heating (TCH) remediation system will be implemented to heat, desorb and volatilize PFAS and co-contaminants present in unsaturated soil. The FlexHeater® system uses a coiled single wire, which is heated to about 900° Celsius (C) and can deliver varying amounts of heat vertically in a steel heater well. As the soil is heated, PFAS and co-contaminants are vaporized and captured through a conventional, high temperature soil vapor extraction technology. The effluent vapors are cooled, extracted steam condensed, and all phases are generally treated ex situ using granular activated carbon (GAC) on-site.

Beyond treating vapor condensate samples with GAC, which is a mature technology, the feasibility of direct PFAS destruction will be assessed using hydrothermal destruction, an emerging technology being developed at the Colorado School of Mines (CSM) under SERDP funding. Findings from this SERDP project demonstrate rapid and complete destruction of PFAS in both dilute and concentrated mixtures of AFFF, including undiluted AFFF stockpiles. Solutions are amended with sodium hydroxide (1-5 M) and heated to subcritical conditions (300-350°C) to promote base-catalyzed decomposition and defluorination within short reaction times (e.g., 15-90 min). Although counter-intuitive, it requires less energy to heat condensed water to hydrothermal conditions than it does to vaporize water at ambient temperatures. Thus, it is anticipated that this will be an energy efficient technology for treating streams like vapor phase condensates.

Successful application of ISTT for PFAS-impacted soil will provide an on-site, in situ treatment option where none presently exists. The technology reduces the DoD’s current and future liabilities through removal and ultimate destruction of PFAS from source area soils, which represent an ongoing contribution of PFAS to groundwater, reducing the groundwater treatment life cycle cost. Although in situ thermal treatment is relatively expensive when compared to capping or other physical containment, it is less expensive than excavation and off-site incineration. Additionally, removal of the source area results in a reduction in life cycle cost for groundwater treatment systems which would otherwise have to operate in perpetuity. Demonstrating this innovative and cost-competitive technology will provide the DoD an in situ alternative to stabilization, landfilling or incineration, which represent the state of the industry for addressing PFAS in soils. (Anticipated Project Completion - 2024)