Effects of Mass Transfer Limitations on PFAS Breakthrough in Colloidal Activated Carbon Groundwater Barriers

Per- and polyfluoroalkyl substances (PFAS) persist in groundwater and are difficult to remove. Colloidal activated carbon (CAC) is increasingly used in porous barriers to slow PFAS migration in groundwater, but it is unclear when grain-scale mass transfer limitations reduce the barrier performance. To address this knowledge gap, breakthrough and elution profiles for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) were measured in columns packed with CAC-amended sand. Freundlich isotherms for the two compounds were measured and incorporated into a two-site kinetic transport model. The model reproduced measured breakthrough and elution profiles for PFOA and PFOS, but only when mass transfer limitations were considered. The calibrated model was applied at the field scale to determine whether grain-scale mass transfer limitations are important at larger scales. In general, model results indicate that these mass transfer limitations for PFOA and PFOS are not important at field sites under natural head gradients but can become important under active pumping conditions when groundwater residence times in CAC barriers become small. The Damköhler number is proposed as a framework to compare advective and grain-scale mass transfer time scales and better understand when equilibrium conditions can be used to predict PFAS breakthrough times in CAC barriers.