Miscible-displacement experiments are conducted with perfluorooctanoic acid (PFOA) to determine the contribution of adsorption at the air-water interface to retention during transport in water-unsaturated porous media. Column experiments were conducted with two sands of different diameter at different PFOA input concentrations, water saturations, and pore-water velocities to evaluate the impact of system variables on retardation. The breakthrough curves for unsaturated conditions exhibited greater retardation than those obtained for saturated conditions, demonstrating the significant impact of air-water interfacial adsorption on PFOA retention. Retardation was greater for lower water saturations and smaller grain diameter, consistent with the impact of system conditions on the magnitude of air-water interfacial area in porous media. Retardation was greater for lower input concentrations of PFOA for a given water saturation, consistent with the nonlinear nature of surfactant fluid-fluid interfacial adsorption. Retardation factors predicted using independently determined parameter values compared very well to the measured values. The results showed that adsorption at the air-water interface is a significant source of retention for PFOA, contributing approximately 50-75% of total retention, for the test systems. The significant magnitude of air-water interfacial adsorption measured in this work has ramifications for accurate determination of PFAS migration potential in vadose zones.

A comprehensive compartment model is presented for PFAS retention that incorporates all potential processes relevant for transport in source zones. Miscible-displacement experiments were conducted to investigate separately the impact of adsorption at the air-water and decane- water interfaces on PFAS retention and transport. Two porous media were used, a quartz sand and a soil, and perfluorooctanesulfonic acid (PFOS) was used as the model PFAS. The breakthrough curves for transport under water-unsaturated conditions were shifted noticeably rightward (delayed arrival) compared to the breakthrough curves for saturated conditions, indicating greater retardation due to adsorption at the air-water or decane-water interface. The retardation factor was 7 for PFOS transport in the sand for the air-water system, compared to 1.8 for saturated conditions. PFOS retardation factors for transport in the soil were 7.3 and 3.6 for unsaturated (air-water) vs saturated conditions. Air-water interfacial adsorption is a significant source of retention for PFOS in these two systems, contributing more than 80% of total retention for the sand and 32% for the soil. For the experiments conducted with decane residual emplaced within the sand, adsorption at the decane-water interface contributed more than 70% to total retention for PFOS transport. Methods to determine or estimate key distribution variables are presented for parameterization of the model. Predicted retardation factors were similar to the measured values, indicating that the conceptual model provided adequate representation of the relevant retention processes and that the parameter estimation methods produced reasonable values. The results of this work indicate that adsorption by fluid-fluid interfaces in variably saturated porous media can be a significant retention process for PFAS that should be considered when characterizing their transport and fate behavior in source zones.

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