Groundwater Solute-Induced Desorption of Perfluoroalkyl Substances (PFAS) from Colloidal Activated Carbon (CAC)

In situ colloidal activated carbon (CAC) barriers limit the migration of per- and polyfluorinated alkyl substances (PFAS) in groundwater. While the influence of site-specific conditions on equilibrium adsorption has been studied, the kinetics and mechanisms of desorption remain unreported. Here, we quantify the extent of PFAS desorption from CAC using preloaded flow-through columns and explore mechanisms of release. PFAS release after 1600 pore volumes under baseline conditions (1 mM NaHCO₃, pH = 7.5) was controlled by chain length and headgroup and varied from <1% for perfluorooctanoic acid (PFOA) to 83% for perfluoropentanoic acid (PFPeA). Release of perfluorobutane sulfonamide (FBSA) (30%) was lower than that of perfluorobutanesulfonic acid (PFBS) (46%), which was attributed to its near-neutral pK_a and a fraction of uncharged species with greater sorption stability. Low-molecular-weight (MW) dissolved organic matter (DOM) displaced more PFAS than high-MW DOM. Diesel range organics (DRO 1400 μg/L) led to ∼70% of the total PFOA mass recovered in effluent, which was significant. Elevated ionic strength (100 mM) did not greatly impact the total mass desorbed but did decrease the rate of PFAS mass release. Results demonstrate that previously sequestered PFAS can be released following exposure to DOM and DRO, and that nonequilibrium mass transfer rate-limited processes could impact barrier performance at sites where groundwater retention times in the barrier are below what is required for equilibrium.