Schematic Diagram of Sorptive Barrier

Per-and polyfluoroalkyl substances (PFAS) have been identified by federal and state regulatory agencies as emerging chemicals of concern. As of March 2018, PFAS concentrations exceeding the U.S. Environmental Protection Agency lifetime health advisory were detected in 60% of the 2,668 groundwater wells sampled at U.S. Department of Defense (DoD) installations. The anticipated future liability for DoD to remediate PFAS will likely be significant, thus highlighting the importance of reliable remediation techniques. The overarching objective of this project is to assess the long-term effectiveness of colloidal activated carbon (CAC) used for in situ remediation of coastal groundwater sites impacted by PFAS. The specific technical objectives of the project are to:

1. Assess the impact of groundwater solutes, co-occurring chemicals, carbon surface chemistry, and PFAS chain length and functional group on PFAS adsorption on and desorption from fresh CAC adsorbents;
2. Evaluate the adsorption of PFAS on and desorption from CAC particles aged through physical, chemical, and microbiological processes;
3. Examine the remobilization of fresh and aged CAC particles from aquifer material matrix upon perturbation of groundwater flow and solution chemistry; and
4. Establish models for predicting the longevity of CAC barrier for treating PFAS-impacted coastal groundwater.

In Task 1, the team will provide an objective assessment of the long-term performance of CAC for PFAS containment. This includes determining the effect of surface properties of activated carbon (AC), groundwater solution chemistry, co-occurring chemicals, and PFAS chain-length and end group on their equilibrium adsorption characteristics and the rate of desorption from ACs. The team anticipates that dissolved organic matter and the ionic strength of the groundwater will affect PFAS desorption behavior, with the magnitude of the effects depending on the chain length and functional groups of PFAS, and the anion exchange capacity and hydrophobicity of ACs.

In Task 2, the team will characterize CACs aged via physical, chemical, and biological processes and determine the correlation between different aging processes and the adsorption/desorption of PFAS on CACs. The impact of biofilms formed on the surface of the CACs on PFAS sorption will also be assessed. The team anticipates that PFAS adsorption capacity and desorption rate will be altered after the CAC particles are aged.

In Task 3, the team will characterize the surface charge and colloidal stability of fresh and aged CAC particles, assess the extent and rate of CAC remobilization from model and real aquifer materials under perturbation of solution chemistry and flow rate, elucidate the remobilization mechanisms, and explore ways to suppress the remobilization of CAC particles. The team will incorporate the PFAS adsorption and desorption parameters as well as CAC remobilization parameters into reactive-transport models to predict the long-term effectiveness of CAC barriers for treating PFAS-impacted coastal groundwater. Overall, this research will allow for quantitative performance assessment of this technology under a variety of hydraulic and geochemical conditions, and will provide a basis for remedy selection using CAC products for large-scale cleanup of DoD contaminated coastal sites.

This project will address the critical question of whether the CAC sorptive barrier will retain its effectiveness as an in situ remediation approach for PFAS-impacted coastal groundwater under perturbation of groundwater conditions and aging of CAC particles. It will also provide appropriate parameters that can be integrated into models for predicting the longevity of CAC sorptive barriers in coastal sites, and offer strategies that allow for significant improvement of the longevity of CAC sorptive barrier via engineering carbon properties, controlling aging processes, and suppressing CAC particle remobilization. This project will improve the fundamental and practical understanding of CAC-based remediation technology, help practitioners select appropriate CAC products for PFAS remediation, and accelerate technology acceptance. (Anticipated Project Completion - 2025)