Testing Considerations for Quantifying PFAS Adsorption by a Soil-Bentonite Matrix Amended with Granular Activated Carbon

PFAS pose a significant environmental and health threat due to their toxicity and persistence. When conventional in situ remediation strategies are inadequate for treating PFAS contamination in groundwater, engineered vertical barriers with enhanced adsorption capacity offer a potentially viable long-term containment solution. As part of a larger study on the effectiveness of soil-bentonite (SB) cutoff walls enhanced with granular activated carbon (GAC) for long-term containment of four PFAS compounds (PFOS, PFOA, PFHxS, and PFBS), important considerations for quantifying PFAS adsorption were investigated. These considerations included: (1) impacts of solid/liquid separation methods (syringe cartridge filters, centrifugation, ultracentrifugation), (2) performance of different syringe cartridge filter materials (nylon, PES, PTFE, PVDF), and (3) the test duration required to achieve equilibrium conditions in batch equilibrium adsorption tests (BEATs). To protect analytical instrumentation from sample solids, centrifugation followed by filtration with PES cartridge filters proved most effective for supernatant clarification without significant loss of most PFAS compounds tested. Kinetic experiments suggested that a duration of 7 days was important to allow adsorption tests to approach equilibrium. Employing these experimental considerations in BEATs with PFBS showed that there was minimal contaminant adsorption to unamended SB, while SB amended with 1% (wt/wt) GAC exhibited significantly enhanced adsorption that was well described by the Freundlich model. Transport model simulations predict that 1% GAC in an SB barrier could delay PFBS breakthrough by 1,500 times compared to unamended SB.