Transport dynamics of per- and polyfluoroalkyl substances (PFAS) in a saturated flow cell: Impacts of intermittent flow, capillary fringe dynamics, and biochar amendment

Per- and polyfluoroalkyl substances (PFAS) are environmental contaminants that are usually found in groundwater. This study examines PFAS movement in saturated sandy porous media under different hydrologic conditions, including intermittent and perennial flow, water level fluctuations, and biochar amendment, using a lab-scale flow cell. Theoretical retardation factors ( R theory ), based solely on adsorption to sand, consistently overestimated the effective retardation factors ( R eff ) of short- and moderate-chain PFAS (≤C8) but underestimated the retardation of long-chain PFAS (≥C9) under both flow conditions. While PFAS (≤C9) exhibited R eff of close to 1 under perennial flow conditions, substantially greater retardation ( R eff = 1.6 11.6) was observed under intermittent flow. This increased retardation is due to PFAS remobilization from the capillary fringe during periods of flow cessation that altered breakthrough dynamics. Co-transport experiments with inorganic fluoride (F⁻) revealed that short-chain PFAS closely followed the conservative tracer and could therefore be used as an indicator of recent or active contamination. Lower water table reduced PFAS retention due to elevated Darcy’s flow velocity (4.74 ×10 −6 m/s for higher water table vs 5.27 ×10 −6 m/s for lower water table), which overweighed the increased air–water interfacial adsorption in the newly formed unsaturated zone. Biochar amendment enhanced PFAS retention and delayed breakthrough, particularly for long-chain species, demonstrating its promise for in situ remediation applications. Molecular dynamics simulations showed that PFAS adsorption onto quartz surfaces is energetically unfavorable, whereas adsorption onto carbonaceous surfaces such as biochar is favorable and exothermic, corroborating experimental findings. Lastly, the effects of biochar and interfacial partitioning on the retention dynamics of PFAS were quantified using the Thomas kinetic model. Overall, this investigation offers a fresh perspective on the coupled hydrodynamic and geochemical mechanisms that control PFAS fate and transport in the subsurface environment. • Intermittent flow increased PFAS retardation via redistribution. • Equilibrium adsorption model overestimated short/moderate-chain PFAS retardation. • Lowered groundwater table levels decreased PFAS retention due to increased flow velocity. • Thomas kinetic model revealed effects of biochar and flow regime on PFAS retention.