Subsurface injection of colloidal activated carbon (CAC) is an in situ remedial approach for the treatment of groundwater impacted by per- and polyfluoroalkyl substances (PFAS); however, CAC does not provide PFAS mass destruction. Installed CAC barriers may provide an opportunity for cost-effective in situ PFAS destruction because they concentrate PFAS into a small area that can be efficiently treated by in situ chemical oxidation with activated persulfate (PS). This approach will provide both in situ PFAS and PFAS precursor mass destruction and CAC barrier regeneration.

The overall objective of this proof-of-concept project is to develop an effective CAC sorbent that can be regenerated in situ while providing rapid and extensive defluorination of selected PFAS and precursors utilizing activated PS. Specific technical objectives in this project are as follows:

• Engineer a CAC that provides efficient PFAS defluorination at low temperature through rapid PS activation and minimal sulfate radical quenching.
• Quantify the potential impact of aquifer geochemistry on the PS activation and PFAS degradation processes for a range of PFAS and PFAS-precursors.
• Perform a feasibility assessment using the data collected (Go/No Go decision gate 1).

The first approach will use batch studies to evaluate the ability of partially oxidized and N-doped CAC to activate PS at lower temperature without quenching sulfate radicals. A second approach will decorate inert CAC with inexpensive iron-based materials (FeS, FeCuS₂) that activate PS at low temperature to provide sulfate radicals near adsorbed PFAS and PFAS precursors. These materials will have higher steady state concentration of sulfate radicals in the system compared to unmodified CAC, rapidly and completely defluorination selected adsorbed PFAS and PFAS precursors. The project team will then assess the performance of these materials in real groundwaters collected from impacted sites to determine the magnitude of the impact of groundwater constituents on PS activation and PFAS reactivity. Finally, the project team will use this data set to conduct a preliminary assessment of the technical and economic feasibility of performing this operation on CAC barriers installed in the field, considering the relevant tradeoffs between approaches (e.g., energy, PS, and material costs versus mass of PFAS destroyed).

The findings from the project will determine if selected PFAS and PFAS precursors adsorbed to CAC can be defluorinated at ambient groundwater temperatures using activated persulfate. It will identify technical barriers to this approach, such as scavenging of sulfate radicals by CAC or groundwater constituents present at AFFF-impacted sites, and develop the means of overcoming these barriers to develop sorbent materials that can provide significant in situ PFAS mass destruction and regenerate installed CAC barriers to extend their performance lifetime. Ultimately, these findings will provide information about the required properties for effective and regenerable CAC materials, and about PS activators that can be injected along with PS to provide in situ PFAS mass destruction. This should ultimately lower the costs of managing and closing PFAS-impacted sites, thereby prioritizing the rapid development of affordable, scalable solutions to safeguard personnel, bolstering our national security posture. (Anticipated Project Completion - 2026)