Adsorption of PFOA at the Air–Water Interface during Transport in Unsaturated Porous Media

Abstract

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.

The influence of molecular structure on the adsorption of PFAS to fluid-fluid interfaces: Using QSPR to predict interfacial adsorption coefficients

“…This is illustrated in Figure S1, which presents the full range of K i values determined for three selected PFAS. These values were determined using the best-fit of the Szyszkowski equation (see below) to the measured surface-tension data sets, with K i values calculated using local slopes for each discrete C. It is relevant to note that the results of recent studies have demonstrated that K i values determined from surface-tension data in this manner are consistent with values determined from miscible-displacement transport experiments for PFOA (Lyu et al, 2018) and PFOS (Brusseau et al, 2019).…”

“…Measured surface-tension data for the homologous Na-perfluorocarboxylate series. Data reproduced from Lunkenheimer et al (2015) and Tamaki et al (1989), except for C8-UAZ, which is from Lyu et al (2018). QSPR model for log K i versus molar volume for all data.…”

“…Extensive research has demonstrated that a wide range of organic compounds undergo adsorption at air-water interfaces (e.g., Karger et al, 1971; (QSPR) analysis methods can be used to provide empirical-based estimates. This approach is widely used for the estimation of many phase-transfer (partition) coefficients, and it has been applied to PFAS (e.g., Gabriel et al, 1996;Bhhatarai and Gramatica, 2011;Kim et al, 2015;Lyu et al, 2018). For example, Bhhatarai and Gramatica conducted an extensive QSPR analysis for PFAS, and showed that aqueous solubility, vapor pressure, and critical micelle concentration (CMC) were all well described by simple QSPR models.…”

“…Similarly, Kim et al (2015) demonstrated the effectiveness of simple QSPR models for estimating vapor pressures, aqueous solubilities, octanol/water partition coefficients, air/water partition coefficients, and octanol/air partition coefficients of PFAS. Lyu et al (2018) successfully applied QSPR analysis to air-water interfacial adsorption coefficients for a homologous series of perfluoroalkyl carboxylates.…”

Sources, Fate, and Plant Uptake in Agricultural Systems of Per- and Polyfluoroalkyl Substances

“…Given the importance of phosphorus as a nutrient in agricultural systems, further studies on the effect of phosphorus speciation and concentration on sorption of additional PFAS are warranted. Lastly, other sorption mechanisms unique to PFAS as surfactants include the potential for enhanced retention at the air-water interface in the vadose zone [73,74] (the unsaturated zone of soil above the water table including the root zone).…”