Impacts of Groundwater Constituents and Colloidal Activated Carbon (CAC) Surface Chemistry on the Adsorption of Perfluoroalkyl Acids (PFAA) in Aqueous Film-Forming Foam (AFFF)-Impacted Groundwater

Abstract

In situ sequestration of per- and polyfluorinated alkyl substances (PFAS) using colloidal activated carbon (CAC) is a growing technology at aqueous film-forming foam (AFFF)-impacted sites, but its long-term effectiveness remains uncertain due to complex groundwater chemistry. Perfluoroalkyl acid (PFAA) adsorption was quantified in four groundwaters collected from AFFF-impacted sites using two CAC materials. PFAA adsorption was inhibited in each groundwater, with short-chain PFAA (<7 perfluorinated carbons) being more impacted than long-chain PFAA (>7 perfluorinated carbons). Groundwater with high concentrations of total organic carbon (TOC) and diesel-range organics (DRO) caused the greatest decrease in adsorption compared to that of the control system (1 mM NaHCO₃, pH = 7.5). Correlation analysis confirmed TOC and DRO were most strongly associated with decreased adsorption performance. Addition of individual groundwater solutes to unimpacted groundwater showed TOC has the strongest impact on long-chain PFAA adsorption while an additive effect was observed for short-chain PFAA. CAC with a high point of zero charge (pH_PZC, 9.5) performed better than CAC with a low pH_PZC (6.7) in every groundwater due to favorable electrostatic interactions, but this advantage was minimized in groundwater with elevated ionic strength. Scientists and practitioners will benefit from the results presented, which will inform future CAC barrier design and implementation.

Diesel-range organics and dissolved organic carbon in AFFF-impacted groundwater have the greatest impact on colloidal activated carbon performance for PFAS adsorption.