The influence of molecular structure on PFAS adsorption at air-water interfaces in electrolyte solutions.

Fluid-fluid interfacial adsorption has been demonstrated to be an important retention process for per and polyfluoroalkyl substances (PFAS) in porous media with air or non-aqueous phase liquids (NAPLs) present. The objective of this study was to characterize the influence of PFAS molecular structure on air-water interfacial adsorption in electrolyte solutions. Measured and literature-reported surface-tension data sets were aggregated to generate the largest compilation of interfacial adsorption coefficients measured in aqueous solutions comprising environmentally representative ionic strengths. The surface activities and interfacial adsorption coefficients (Ki) exhibited chain length trends, with greater surface activities and larger Ki values corresponding to longer chain length. The impact of multiple-component PFAS solutions on the surface activity of a select PFAS was a function of the respective surface activities and concentrations. Quantitative structure-property relationship analysis (QSPR) employing a single molecular descriptor (molar volume) was used successfully to characterize the impact of PFAS molecular structure on air-water interfacial adsorption. A previously reported QSPR model based on PFAS data generated for deionized-water solutions was updated to include more than 60 different PFAS, comprising all head-group types and a wide variety of tail structures. The QSPR model developed for PFAS in electrolyte solution compared favorably to the model developed for deionized water. Additionally, the magnitude of ionic strength for non-zero ionic strength systems was determined to have relatively minimal impact on interfacial adsorption coefficients. The new QSPR model is therefore anticipated to be representative for a wide variety of PFAS and for a wide range of ionic compositions.

Contribution of Nonaqueous-Phase Liquids to the Retention and Transport of Per and Polyfluoroalkyl Substances (PFAS) in Porous Media.

Per and polyfluoroalkyl substances (PFAS) cocontamination with nonaqueous-phase organic liquids (NAPLs) has been observed or suspected at various sites, particularly at fire-training areas at which aqueous film-forming foams (AFFFs) were applied. The objectives of this study are to (1) delineate the relative significance of specific PFAS-NAPL processes on PFAS retention, including partitioning into the bulk NAPL phase and adsorption to the NAPL-water interface; (2) investigate the influence of NAPL properties, saturation, and mass-transfer constraints on PFAS retention; and (3) determine whether PFAS may impact NAPL distribution through mobilization or dissolution. Perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are used as representative PFAS, and trichloroethene (TCE) and decane are used as representative NAPLs. NAPL-water interfacial adsorption was quantified with NAPL-water interfacial-tension measurements; partitioning into NAPL was quantified with batch experiments, and retardation factors (R) in the absence and presence of residual NAPL were determined with miscible-displacement transport experiments. R values increased in the presence of residual NAPL, with adsorption to the NAPL-water interface accounting for as much as ∼77% of retention and solid-phase adsorption also significantly contributing to retention. Additionally, this study provides the first QSPR analysis focused on NAPL-water interfacial adsorption coefficients, with results consistent with those from previous air-water studies. Lastly, this initial investigation into PFAS impacts on NAPL behavior determined that PFOS/PFOA are unlikely to enhance solubilization or mobilization of NAPL under the conditions present at many AFFF legacy sites.

Column versus batch methods for measuring PFOS and PFOA sorption to geomedia.

The objective of this study is to compare the consistency between column and batch experiment methods for measuring solid-phase sorption coefficients and isotherms for per and polyfluoroalkyl substances (PFAS). Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are used as representative PFAS, and experiments are conducted with three natural porous media with differing geochemical properties. Column-derived sorption isotherms are generated by conducting multiple experiments with different input concentrations (multi-C0 method) or employing elution-front integration wherein the entire isotherm is determined from a single breakthrough curve (BTC) elution front. The isotherms generated with the multi-C0 column method compared remarkably well to the batch isotherms over an aqueous concentration range of 3-4 orders of magnitude. Specifically, the 95% confidence intervals for the individual isotherm variables overlapped, producing statistically identical regressions. The elution-front integration isotherms generally agreed with the batch isotherms, but exhibited noise and systematic deviation at lower concentrations in some cases. All data sets were well described by the Freundlich isotherm model. Freundlich N values ranged from 0.75 to 0.81 for PFOS and was 0.87 for PFOA and are consistent with values reported in the literature for different geomedia. The results of this study indicate that column and batch experiments can measure consistent sorption isotherms and sorption coefficients for PFOS and PFOA when robust experimental setup and data analysis are implemented.

Nonideal Transport and Extended Elution Tailing of PFOS in Soil.

The objective of this research was to examine the influence of nonideal sorption/desorption on the transport of polyfluorinated alkyl substances (PFASs) in soil, with a specific focus on characterizing and quantifying potential extended, mass-transfer-limited elution behavior. Perfluorooctane sulfonic acid (PFOS) was used as a representative PFAS, and miscible-displacement experiments were conducted with two soils comprising contrasting geochemical properties. The influence of nonlinear, rate-limited, hysteretic, and irreversible sorption/desorption on transport was investigated through experiments and model simulations. The breakthrough curves measured for PFOS transport in the two soils were asymmetrical and exhibited extensive elution tailing, indicating that sorption/desorption was significantly nonideal. The widely used two-domain sorption kinetics model could not fully simulate the observed transport behavior, whereas a multirate model employing a continuous distribution of sorption domains was successful. The overall results indicated that sorption/desorption was significantly rate-limited and that nonlinear, hysteretic, and irreversible sorption/desorption had minimal impact on PFOS transport. Comparison of PFOS transport data to data reported for two hydrophobic organic contaminants (HOCs) showed that the HOCs exhibited much more extensive elution tailing, likely reflecting differences in sorption/desorption mechanisms. The projected influence of rate-limited sorption/desorption on PFOS transport at the field scale was investigated through simulation. The results of the study suggest that rate-limited sorption/desorption may affect the field-scale transport of PFOS and other PFAS for systems influenced by transient or short-residence-time conditions and in some cases could possibly increase the amount of flushing required to reduce PFOS concentrations to levels below those associated with human-health concerns.

The influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces.

The objective of this research is to examine the influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces. Surface tensions were measured for select PFAS, as well as a representative hydrocarbon surfactant. These data are supplemented with data sets collected from the literature. The influence of surfactant headgroup charge, specifically for zwitterionic PFAS, was investigated. The impacts of surfactant counterion for ionic PFAS and the influence of headgroup size for nonionic PFAS were also investigated. In addition, the influence of solution ion composition, ionic strength, and pH was examined. The impact of co-solutes, specifically ethanol, humic acid, and trichloroethene, was also examined, as well as the behavior of PFAS mixtures and fluorocarbon-hydrocarbon surfactant mixtures. The data were interpreted within the framework of a QSPR model recently developed to predict fluid-fluid interfacial adsorption coefficients (Ki) of PFAS. The results demonstrate that all of the factors investigated have some degree of impact on Ki values. Thus, the composition of soil-pore water and groundwater is likely to affect the magnitude of PFAS adsorption at air-water and organic liquid-water interfaces. However, the influence on Ki of most of the factors investigated is small for lower PFAS concentrations (less than ∼1-10 mg/L). Hence, their impacts on fluid-fluid interfacial adsorption are likely to be relatively minor at the low PFAS concentrations representative of many environmental systems, especially compared to the impact of other factors such as fluid saturations, porous-medium properties, and PFAS molecular structure. The results of this study indicate that the revised QSPR model provides reasonable first-order predictions of Ki for a wide range of PFAS in environmental systems.

Comprehensive retention model for PFAS transport in subsurface systems.

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.