Retention of PFOS and PFOA Mixtures by Trapped Gas Bubbles in Porous Media

Abstract

The transport of per-and polyfluoroalkyl substances (PFAS) in soil and groundwater is important for site investigation, risk characterization, and remediation planning. The adsorption of PFAS at air−water interfaces has been shown to significantly contribute to PFAS retention, with subsequent effects on concentrations and the time scales of transport. In this study, column experiments were conducted to investigate the transport of perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and 6:2 fluorotelomer sulfonate (6:2 FTS) individually and in binary mixtures in the presence of a trapped gas phase, using clean sands to isolate adsorption to air−water interfaces. Consistent with previous studies, the transport of PFOS, PFOA, and 6:2 FTS was retarded by adsorption at the air−water interface, with greater retention of PFOS due to its higher affinity for the air−water interface. Chromatographic separation occurred in the experiments using binary mixtures of PFOS and PFOA, with greater retention at lower influent concentrations. The mixture experiments also showed enhanced breakthrough of PFOA in the presence of PFOS, where effluent concentrations of PFOA were temporarily greater than the influent concentration prior to the breakthrough of PFOS. This enhanced breakthrough was attributed to competition between PFOS and PFOA for adsorption to the air−water interface.

Predicting Concentration- and Ionic-Strength-Dependent Air–Water Interfacial Partitioning Parameters of PFASs Using Quantitative Structure–Property Relationships (QSPRs)

“…Some components in the mixture (i.e., PFOS, PFHpS, Nafion ByProduct 2, PFO3OA) demonstrate 10- to 20-fold greater equilibrium K ia values in the batch studies than measured in the low flow rate column experiments. This discrepancy is consistent with the discrepancy between Langmuir vs Freundlich models of K ia , and the discrepancy observed between CD method data , vs several other methods. ,,− , The presence of rate-limited air–water interfacial sorption under nonideal, quasi-saturated conditions suggests that rate-limited partitioning could develop under CD method conditions as well, thereby compromising K ia values determined using the CD method.…”

“…Both equilibrium K ia values from pan film experiments and equilibrium K ia values measured under quasi-saturated conditions follow the same trend as predicted K ia values from Schaefer et al 2019 (blue squaresFigure ). Predicted values from Schaefer et al 2019 are likely greater because they are predicted from a Freundlich isotherm model using IFT data which is prone to a higher degree of nonlinearity than what has been empirically observed in pan film, quasi-saturated column, , and static column data . Therefore, IFT data may not be the most suitable to determine PFAS isotherm behavior below the CRC.…”

“…At the time of Le et al 2021a publication, assumption of the Langmuir isotherm for air–water interfacial partitioning was generally considered valid. However, recent studies ,, and a lack of fit to empirical data (i.e., 10–100× deviations from predictions in Figure of Le et al 2021) demonstrate that this assumption is not valid. Equation 9 of Le et al 2021a can be empirically modified to incorporate the Freundlich isotherm for PFASs at concentrations present below the CRC.…”

“…residual air saturation) column method to PFASs to minimize the impact of non-Fickian transport in multiphase systems. The quasi-saturated condition is significantly different from drainage or imbibition conditions because the only nonwetting phase (air in this case) remaining in the system is fully entrapped. − Previous quasi-saturated studies utilized highly ideal sands and suggested that equilibrium partitioning of PFASs could be measured using this method, even at relatively high linear pore velocities (0.25 < v < 0.45 cm/min). , …”

“…38−41 Previous quasi-saturated studies utilized highly ideal sands and suggested that equilibrium partitioning of PFASs could be measured using this method, even at relatively high linear pore velocities (0.25 < v < 0.45 cm/ min). 30,37 PFASs are generally present as mixtures 8,12,42,43 in the aqueous environment and also coexist with inorganic ions in solution. 3,44−47 Competition for air−water interfacial sorption sites has been demonstrated to reduce the K ia values of PFASs and nonfluorinated surfactants when concentrations of any single surfactant are at or above the CRC.…”