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

PFAS transport under lower water-saturation conditions characterized with instrumented-column systems

The transport of PFOS and PFOA in well-characterized sand was investigated for relatively low water saturations. An instrumented column was used for some experiments to provide real-time in-situ monitoring of water saturation and matric potential. The results showed that water saturations and matric potentials varied minimally during the experiments. Flow rates were monitored continuously and were essentially constant. These results demonstrate that surfactant-induced flow and other nonideal hydraulic processes did not materially impact PFAS transport for the experiment conditions. Air-water interfacial adsorption was demonstrated to provide the great majority of retention for PFOS and PFOA. Retention was significantly greater at the lower water saturations (0.35-0.45) compared to the higher saturations (∼0.66) for both PFAS, due to the larger extant air-water interfacial areas. Retardation factors were 5 and 3-times greater at the lower water saturations for PFOS and PFOA, respectively. Early breakthrough was observed for the PFAS but not for the non-reactive tracers at the lower water saturations, indicating the possibility that air-water interfacial adsorption was rate-limited to some degree. Independently determined retention parameters were used to predict retardation factors for PFOS and PFOA, which were similar to the measured values in all cases. The consistency between the predicted and measured values indicates that PFAS retention was accurately represented. In addition, air-water interfacial adsorption coefficients measured from the transport experiments were consistent with independently measured equilibrium-based values. Based on these results, it appears that the air-water interfacial adsorption processes mediating the magnitude of PFOS and PFOA retention under lower water-saturation conditions are consistent with those for higher water saturations. This provides some confidence that our understanding of PFAS retention obtained from work conducted at higher water saturations is applicable to lower water saturations.

Air-water interfacial collapse and rate-limited solid desorption control Perfluoroalkyl acid leaching from the vadose zone

Some Per- and polyfluoroalkyl substances (PFAS) are strongly retained in the vadose zone due to their sorption to both soils and air-water interfaces. While significant research has been dedicated to understanding equilibrium behavior for these multi-phase retention processes, leaching and desorption from aqueous film-forming foam (AFFF) impacted soils under field relevant conditions can exhibit significant deviations from equilibrium. Herein, laboratory column studies using field collected AFFF-impacted soils were employed to examine the leaching of perfluoroalkyl acids (PFAAs) under simulated rainfall conditions. The HYDRUS 1-D model was calibrated to estimate the unsaturated hydraulic properties of the soil in a layered system using multiple boundary condtions. Forward simulations of equilibrium PFAS partitioning using the HYDRUS model and simplified mass balance calculations showed good agreement with the net PFAS mass flux out of the column. However, neither were able to predict the PFAS concentrations in the leached porewater. To better understand the mechanisms controlling the leaching behavior, the HYDRUS 1-D two-site leaching model incorporating solid phase rate limitation and equilibrium air-water interfacial partitioning was employed. Three variations of the novel model incorporating different forms of equilibrium air-water interfacial partitioning were considered using built-in numerical inversion. Results of numerical inversion show that a combination of air-water interfacial collapse and rate-limited desorption from soils can better predict the unique leaching behavior exhibited by PFAAs in AFFF-impacted soils. A sensitivity analysis of the initial conditions and rate-limited desorption terms was conducted to assess the agreement of the model with measured data. The models demonstrated herein show that, under some circumstances, laboratory equilibrium partitioning data can provide a reasonable estimation of total mass leaching, but fail to account for the significant rate-limited, non-Fickian transport which affect PFAA leaching to groundwater in unsaturated soils.

Repeated Aqueous Film-Forming Foams Applications: Impacts on Polyfluoroalkyl Substances Retention in Saturated Soil

Historical practices at firefighter-training areas involved repeated aqueous film-forming foams (AFFFs) applications, resulting in source zones characterized by high concentrations of perfluoroalkyl and polyfluoroalkyl substances (PFAS). Repeated applications of AFFF composed of 14 anionic and 23 zwitterionic perfluoroalkyl substances (PFAS) were conducted on a single one-dimensional saturated soil column to quantify PFAS retention. An electrofluorination-based (3M) Milspec AFFF, which was above the mixture's critical micelle concentration (CMC), was at application strength (3%, v/v). Retention and retardation of PFAS mass increased with each successive AFFF addition, although the PFAS concentration profiles for subsequent applications differed from the initial. Greater degree of mass retention and retardation correlated with longer PFAS carbon-fluorine chain length and charged-headgroup type and as a function of AFFF application number. Anionic PFAS were increasingly retained with each subsequent AFFF application, while zwitterionic PFAS exhibited an alternating pattern of sorption and desorption. Surfactant-surfactant adsorption and competition during repeat AFFF applications that are at concentrations above the CMC resulted in adsorbed PFAS from the first application, changing the nature of the soil surface with preferential sorption of anionic PFAS and release of zwitterionic PFAS due to competitive elution. Applying a polyparameter quantitative structure-property relationship developed to describe sorption of AFFF-derived PFAS to uncontaminated, saturated soil was attempted for our experimental conditions. The model had been derived for data where AFFF is below the apparent CMC and our experimental conditions that included the presence of mixed micelles (aggregates consisting of different kinds of surfactants that exhibit characteristics properties different from micelles composed of a single surfactant) resulted in overall PFAS mass retained by an average of 27.3% ± 2.7% (standard error) above the predicted values. The correlation was significantly improved by adding a "micelle parameter" to account for cases where the applied AFFF was above the apparent CMC. Our results highlight the importance of interactions between the AFFF components that can only be investigated by employing complex PFAS mixtures at concentrations present in actual AFFF at application strength, which are above their apparent CMC. In firefighter-training areas (AFFF source zones), competitive desorption of PFAS may result in downgradient PFAS retention when desorbed PFAS become resorbed to uncontaminated soil.

A systematic investigation of single solute, binary and ternary PFAS transport in water-saturated soil using batch and 1-dimensional column studies: Focus on mixture effects

This study aimed to investigate the transport and release of per- and polyfluoroalkyl substances (PFAS), as single solutes and binary and ternary mixtures, and associated competitive sorption effects in water-saturated soil. Batch sorption isotherm and desorption, and one-dimensional miscible displacement studies were conducted. For the batch study, the mixtures exhibited extensive sorption isotherm nonlinearity at aqueous concentrations exceeding 20 µg/L. At and above this threshold, competitive effects significantly decreased PFAS sorption, mostly affecting perfluorooctanoic acid (PFOA) and perfluorohexane sulfonate (PFHxS). Importantly, mixture effects exacerbated isotherm nonlinearity and may increase the leaching of PFAS in subsurface soil and groundwater. Further, up to 100% desorption occurred for single solutes and mixtures, indicating that the studied PFAS were weakly sorbed. For the column study, at influent concentrations (21 - 27 µg/L, depending on PFAS) near the threshold, PFOA and PFHxS breakthrough curves (BTC) generally exhibited equilibrium (nonlinear) transport, whereas perfluorooctane sulfonate (PFOS) exhibited nonequilibrium transport, with minimal or no mixture effects. Nonequilibrium transport of PFOS was driven by rate-limited sorption, especially as flow interruption tests confirmed the absence of physical nonequilibrium. The sorption distribution coefficients (Kd) from moment and frontal analyses, and 2-site modelling of the BTC, were consistent with the batch-derived Kd, although comparatively smaller. Such discrepancies may limit the applicability of batch-derived Kd values for predictive transport modelling purposes. Overall, understanding mixture impacts may aid effective predictive modelling of PFAS transport and leaching, especially in aqueous film forming foam (AFFF)-source zone areas associated with elevated PFAS concentrations. At low or environmental PFAS concentrations, mixture effects can be expected to be play a minor role in influencing PFAS transport.

Investigating rate-limited sorption, sorption to air-water interfaces, and colloid-facilitated transport during PFAS leaching

Various sorption processes affect leaching of per- and polyfluoroalkyl substances (PFAS) such as PFOA and PFOS. The objectives of this study are to (1) compare rate-limited leaching in column and lysimeter experiments, (2) investigate the relevance of sorption to air-water interfaces (AWI), and (3) examine colloid-facilitated transport as a process explaining early experimental breakthrough. A continuum model (CM) with two-domain sorption is used to simulate equilibrium and rate-limited sorption. A random walk particle tracking (PT) model was developed and applied to analyze complex leaching characteristics. Results show that sorption parameters derived from column experiments underestimate long-term PFOA leaching in lysimeter experiments due to early depletion, suggesting that transformation of precursors contributes to the observed long-term leaching in the lysimeters (approximately 0.003 µg/kg/d PFOA). Both models demonstrate that sorption to AWI is the dominant retention mechanism for PFOS in lysimeter experiments, with retardation due to AWI being 3 (CM) to 3.7 (PT) times higher than retardation due to solid phase sorption. Notably, despite a simplified conception of AWI sorption, the PT results are closer to the observations. The PT simulations demonstrate possible colloid-facilitated transport at early time; however, results using substance-specific varying transport parameters align better with the observations, which should be equal if colloid-facilitated transport without additional kinetics is the sole mechanism affecting early breakthrough. Possibly, rate-limited sorption to AWI is relevant during the early stages of the lysimeter experiment. Our findings demonstrate that rate-limited sorption is less relevant for long-term leaching under field conditions compared to transformation of precursors and that sorption to AWI can be the dominant retention mechanism on contaminated sites. Moreover, they highlight the potential of random walk particle tracking as a practical alternative to continuum models for estimating the relative contributions of various retention mechanisms.

Potential impact of bacteria on the transport of PFAS in porous media

The transport and fate of per- and poly-fluoroalkyl substances (PFAS) in soil and groundwater is a topic of critical concern. A number of factors and processes may influence the transport and fate of PFAS in porous media. One factor that has received minimal attention to date is the impact of bacteria on the retention and transport of PFAS, which is the focus of this current study. The first part of this work comprised a critical review of prior studies to delineate observed PFAS-bacteria interactions and to summarize the mechanisms of PFAS sorption and retention by bacteria. Retention of PFAS by bacteria can occur through sorption onto cell surfaces and/or by incorporation into the cell interior. Factors such as the molecular structure of PFAS, solution chemistry, and bacterial species can affect the magnitude of PFAS sorption. The influence of bacteria on the retention and transport of PFAS was investigated in the second part of the study with a series of batch and miscible-displacement experiments. Batch experiments were conducted using Gram-negative Pseudomonas aeruginosa and Gram-positive Bacillus subtilis to quantify the sorption of perfluorooctane sulfonic acid (PFOS). The results indicated that both bacteria showed strong adsorption of PFOS, with no significant difference in adsorption capacity. Miscible-displacement experiments were then conducted to examine the retention and transport of PFOS in both untreated sand and sand inoculated with Pseudomonas aeruginosa or Bacillus subtilis for 1 and 3 days. The transport of PFOS exhibited greater retardation for the experiments with inoculated sand. Furthermore, the enhanced sorption was greater for the 3-day inoculation compared to the 1-day, indicating that biomass is an important factor affecting PFOS transport. A mathematical model representing transport with nonlinear and rate-limited sorption successfully simulated the observed PFOS transport. This study highlights the need for future studies to evaluate the effect of bacteria on the transport of PFAS in soil and groundwater.

Role of Mineral-Organic Interactions in PFAS Retention by AFFF-Impacted Soil

A comprehensive, generalized approach to predict the retention of per- and polyfluoroalkyl substances (PFAS) from aqueous film-forming foam (AFFF) by a soil matrix as a function of PFAS molecular and soil physiochemical properties was developed. An AFFF with 34 major PFAS (12 anions and 22 zwitterions) was added to uncontaminated soil in one-dimensional saturated column experiments and PFAS mass retained was measured. PFAS mass retention was described using an exhaustive statistical approach to generate a poly-parameter quantitative structure-property relationship (ppQSPR). The relevant predictive properties were PFAS molar mass, mass fluorine, number of nitrogens in the PFAS molecule, poorly crystalline Fe oxides, organic carbon, and specific (BET-N2) surface area. The retention of anionic PFAS was nearly independent of soil properties and largely a function of molecular hydrophobicity, with the size of the fluorinated side chain as the main predictor. Retention of nitrogen-containing zwitterionic PFAS was related to poorly crystalline metal oxides and organic carbon content. Knowledge of the extent to which a suite of PFAS may respond to variations in soil matrix properties, as developed here, paves the way for the development of reactive transport algorithms with the ability to capture PFAS dynamics in source zones over extended time frames.

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

Air-water interfacial retention of poly- and perfluoroalkyl substances (PFASs) is increasingly recognized as an important environmental process. Herein, column transport experiments were used to measure air-water interfacial partitioning values for several perfluoroalkyl ethers and for PFASs derived from aqueous film-forming foam, while batch experiments were used to determine equilibrium K_ia data for compounds exhibiting evidence of rate-limited partitioning. Experimental results suggest a Freundlich isotherm best describes PFAS air-water partitioning at environmentally relevant concentrations (10¹-10⁶ ng/L). A multiparameter regression analysis for K_ia prediction was performed for the 15 PFASs for which equilibrium K_ia values were determined, assessing 246 possible combinations of 8 physicochemical and system properties. Quantitative structure-property relationships (QSPRs) based on three to four parameters provided predictions of high accuracy without model overparameterization. Two QSPRs (R² values of 0.92 and 0.83) were developed using an assumed average Freundlich n value of 0.65 and validated across a range of relevant concentrations for perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), and hexafluoropropylene oxide-dimer acid (i.e., GenX). A mass action model was further modified to account for the changing ionic strength on PFAS air-water interfacial sorption. The final result was two distinct QSPRs for estimating PFAS air-water interfacial partitioning across a range of aqueous concentrations and ionic strengths.

A review of cardiovascular effects and underlying mechanisms of legacy and emerging per- and polyfluoroalkyl substances (PFAS)

Cardiovascular disease (CVD) poses the leading threats to human health and life, and their occurrence and severity are associated with exposure to environmental pollutants. Per- and polyfluoroalkyl substances (PFAS), a group of widely used industrial chemicals, are characterized by persistence, long-distance migration, bioaccumulation, and toxicity. Some PFAS, particularly perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexanesulfonic acid (PFHxS), have been banned, leaving only legacy exposure to the environment and human body, while a number of novel PFAS alternatives have emerged and raised concerns, such as polyfluoroalkyl ether sulfonic and carboxylic acid (PFESA and PFECA) and sodium p-perfluorous nonenoxybenzene sulfonate (OBS). Overall, this review systematically elucidated the adverse cardiovascular (CV) effects of legacy and emerging PFAS, emphasized the dose/concentration-dependent, time-dependent, carbon chain length-dependent, sex-specific, and coexposure effects, and discussed the underlying mechanisms and possible prevention and treatment. Extensive epidemiological and laboratory evidence suggests that accumulated serum levels of legacy PFAS possibly contribute to an increased risk of CVD and its subclinical course, such as cardiac toxicity, vascular disorder, hypertension, and dyslipidemia. The underlying biological mechanisms may include oxidative stress, signaling pathway disturbance, lipid metabolism disturbance, and so on. Various emerging alternatives to PFAS also play increasingly prominent toxic roles in CV outcomes that are milder, similar to, or more severe than legacy PFAS. Future research is recommended to conduct more in-depth CV toxicity assessments of legacy and emerging PFAS and explore more effective surveillance, prevention, and treatment strategies, accordingly.

Effects of biofilms on the retention and transport of PFOA in saturated porous media

Understanding the fate and transport of perfluorooctanoic acid (PFOA) in soil and groundwater is essential to reliable assessments of its risks. This study investigated the impacts of Gram-positive Bacillus subtilis (BS), Gram-negative Pseudomonas aeruginosa (PA) and wild microbiota (WM) biofilm on the transport of PFOA in saturated sand columns at two ionic strengths (i.e., 1.0 and 20.0 mM NaCl). The retention of PFOA in biofilm-coated sand columns was higher than that in uncoated sand columns, due to biofilm-induced reinforced hydrophobic interactions and surface roughness, and decreased zeta potential. However, the retention effects varied among biofilm bacterial species with PFOA retardation factors in PA, WM and BS columns of 1.29-1.38, 1.21-1.29 and 1.11-1.15, respectively. Notably, PA biofilm had the most pronounced effect on PFOA retention. While increasing ionic strength promoted the retention of PFOA in BS biofilm-coated sand, it had no significant impact on PFOA transport in PA and WM biofilm-coated sand. This could be attributed to the differences in biofilm composition, deviating the ionic strengths effects on electrostatic double layer compression. The advection dispersion equation coupled with two-site kinetic retention model well described the transport of PFOA in all saturated columns. Our findings reveal that biofilm plays important roles in PFOA transport in porous media, instructive for risk assessment and remediation of PFOA contamination.

PFOS Mass Flux Reduction/Mass Removal: Impacts of a Lower-Permeability Sand Lens within Otherwise Homogeneous Systems

Perfluorooctane sulfonic acid (PFOS) is one of the most common per- and polyfluoroalkyl substances (PFAS) and is a significant risk driver for these emerging contaminants of concern. A series of two-dimensional flow cell experiments was conducted to investigate the impact of flow field heterogeneity on the transport, attenuation, and mass removal of PFOS. A simplified model heterogeneous system was employed consisting of a lower-permeability fine sand lens placed within a higher-permeability coarse sand matrix. Three nonreactive tracers with different aqueous diffusion coefficients, sodium chloride, pentafluorobenzoic acid, and β-cyclodextrin, were used to characterize the influence of diffusive mass transfer on transport and for comparison to PFOS results. The results confirm that the attenuation and subsequent mass removal of the nonreactive tracers and PFOS were influenced by mass transfer between the hydraulically less accessible zone and the coarser matrix (i.e., back diffusion). A mathematical model was used to simulate flow and transport, with the values for all input parameters determined independently. The model predictions provided good matches to the measured breakthrough curves, as well as to plots of reductions in mass flux as a function of mass removed. These results reveal the importance of molecular diffusion and pore water velocity variability even for systems with relatively minor hydraulic conductivity heterogeneity. The impacts of the diffusive mass transfer limitation were quantified using an empirical function relating reductions in contaminant mass flux (MFR) to mass removal (MR). Multi-step regression was used to quantify the nonlinear, multi-stage MFR/MR behavior observed for the heterogeneous experiments. The MFR/MR function adequately reproduced the measured data, which suggests that the MFR/MR approach can be used to evaluate PFOS removal from heterogeneous media.

Leaching and Transformation of Perfluoroalkyl Acids and Polyfluoroalkyl Phosphate Diesters in Unsaturated Soil Column Studies

Per- and polyfluoroalkyl substances (PFAS) are environmentally ubiquitous, anthropogenic substances with adverse effects on organisms, which shows the need to study their environmental fate and leaching behavior. In the present soil columns study, the leaching behavior and fate of nontransformable and transformable (precursors) were investigated. Ten nontransformable PFAS in two different soils, two precursors and two field soils, which were already contaminated with a mixture of PFAS, and two uncontaminated controls, were set up for a time span of 2 years. At the end of the study, the molecular balance could not be closed for nontransformable PFAS. This effect was positively correlated to the fluorinated carbon chain length. The precursors, which were both polyfluoroalkyl phosphate diesters (diPAP), had different transformation products and transformation rates, with a higher rate for 6:2 diPAP than 8:2 diPAP. After 2 years, amounts of diPAP were still found in the soil with no significant vertical movement, showing high adsorption to soils. Transformation products were estimated to be simultaneously formed. They were predominantly found in the percolation water; the amounts left in soil were negligible. Up to half of the initial precursor amounts could not be balanced and were considered missing amounts. The results of contaminated field soil experiments showed the challenge to estimate PFAS leaching without knowing all occurring precursors and complex transformation dynamics. For this purpose, it was shown that a broad examination of contaminated soil with different analytical methods can help with qualitative estimations of leaching risks. For a better quantitative estimation, analytical determination of more PFAS and a quantification of the missing amounts are needed. Environ Toxicol Chem 2022;41:2065-2077. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Per- and polyfluoroalkyl substance (PFAS) retention by colloidal activated carbon (CAC) using dynamic column experiments

Developing effective remediation methods for per- and polyfluoroalkyl substance (PFAS)-contaminated soils is a substantial step towards counteracting their widespread occurrence and protecting our ecosystems and drinking water sources. Stabilisation of PFAS in the subsurface using colloidal activated carbon (CAC) is an innovative, yet promising technique, requiring better understanding. In this study, dynamic soil column tests were used to assess the retardation of 10 classical perfluoroalkyl acids (PFAAs) (C₅-C₁₁ perfluoroalkyl carboxylic acids (PFCAs) and C₄, C₆, C₈ perfluoroalkane sulfonates (PFSAs)) as well as two alternative PFAS (6:2 and 8:2 fluorotelomer sulfonates) using CAC at 0.03% w/w, to investigate the fate and transport of PFAS under CAC treatment applications. Results showed high retardation rates for long-chain PFAS and eight times higher retardation for the CAC-treated soil compared to the non-treated reference soil for the ∑PFAS. Replacement of shorter chain perfluorocarboxylic acids (PFCAs), such as perfluoropentanoic acid (PFPeA), by longer chained PFAS was observed, indicating competition effects. Partitioning coefficients (K_d values) were calculated for the CAC fraction at ∼10³-10⁵ L kg^-1 for individual PFAS, while there was a significant positive correlation (p < 0.05) between perfluorocarbon chain length and K_d. Mass balance calculations showed 37% retention of ∑PFAS in treated soil columns after completion of the experiments and 99.7% higher retention rates than the reference soil. Redistribution and elution of CAC were noticed and quantified through organic carbon analysis, which showed a 23% loss of carbon during the experiments. These findings are a step towards better understanding the extent of CAC's potential for remediation of PFAS-contaminated soil and groundwater and the limitations of its applications.

PFAS concentrations in soil versus soil porewater: Mass distributions and the impact of adsorption at air-water interfaces

Determining the risk posed by PFAS leaching from soil to groundwater requires quantification of the magnitude and temporal/spatial variability of PFAS mass discharge from the vadose zone, which is governed in part by the concentrations of PFAS in soil porewater. Porewater concentrations are impacted and mediated by the properties of the PFAS and soil, multiple transport and fate processes, and site conditions. The objective of this research was to delineate the relationship between soil porewater concentrations and soil concentrations, based on a comprehensive model of PFAS mass distribution within a soil sample volume. Measured parameters representing solid-phase sorption and air-water interfacial adsorption are used to illustrate the impact of soil and PFAS properties on the distribution of representative PFAS between soil and soil porewater. Literature data reported for soil and soil porewater concentrations of several PFAS obtained from outdoor lysimeter experiments are used to test the distribution model. Soil-to-porewater concentration ratios predicted with the model compared very well to the measured concentration ratios. The nondimensional distribution coefficient that describes the distribution of PFAS mass amongst all domains within a soil sample was observed to be a function of PFAS molecular size. Numerical simulations conducted for a model fire-training source area were used to illustrate the ranges in magnitude of soil versus porewater concentrations for representative field conditions. The results of the measured and simulated data sets demonstrated the importance of air-water interfacial adsorption for the distribution of the longer-chain PFAS within soil samples. PFAS soil porewater concentrations are anticipated to range from ng/L to mg/L depending upon soil concentrations, which in turn depend upon the nature of the site.

Air-water interfacial adsorption of C4-C10 perfluorocarboxylic acids during transport in unsaturated porous media

The impact of chain length on air-water interfacial adsorption of perfluorocarboxylic acids (PFCAs) during transport in unsaturated quartz sand was investigated. Short-chain (C4-C7: PFBA, PFPeA, PFHxA, PFHpA) and long chain (C8-C10: PFOA, PFNA, PFDA) PFCAs were selected as a representative homologous series. Surface tensions were measured to characterize surface activities of the selected PFCAs. Miscible-displacement column experiments were conducted for each of the PFCAs to characterize the magnitudes of air-water interfacial adsorption under transport conditions. The transport of the long-chain PFCAs exhibited greater retardation than the short-chain PFCAs. Air-water interfacial adsorption (AWIA) was the predominant source of retention (≥63%) for the long-chain PFCAs. Conversely, AWIA contributed less to retention than did solid-phase sorption for the short-chain PFCAs, with the former contributions ranging from 4% to 40%. Direct examination of the breakthrough-curve profiles as well as mathematical-modeling results demonstrated that transport of the two longest-chain PFCAs was influenced by nonlinear AWIA, whereas that of the shorter-chain PFCAs was not. This disparate behavior is consistent with the input concentration used for the transport experiments in comparison to the respective surface activities and critical reference concentrations of the different PFCAs. Quantitative-structure/property-relationship (QSPR) analysis was applied to characterize the influence of molecular size on air-water interfacial adsorption. The logs of the air-water interfacial adsorption coefficients (K_ia) determined from the transport experiments are a monotonic function of molar volume, consistent with prior QSPR analyses of surface-tension measured values. The K_ia values determined from the transport experiments are very similar to those measured from surface-tension data, indicating that the transport experiments produced robust measurements of AWIA.

Assessment of Mobilization Potential of Per- and Polyfluoroalkyl Substances for Soil Remediation

This study investigated the mobilization of a wide range of per- and polyfluoroalkyl substances (PFASs) present in aqueous film-forming foams (AFFFs) in water-saturated soils through one-dimensional (1-D) column experiments with a view to assessing the feasibility of their remediation by soil desorption and washing. Results indicated that sorption/desorption of most of the shorter-carbon-chain PFASs (C ≤ 6) in soil reached greater than 99% rapidly─after approximately two pore volumes (PVs) and were well predicted by an equilibrium transport model, indicating that they will be readily removed by soil washing technologies. In contrast, the equilibrium model failed to predict the mobilization of longer-chain PFASs (C ≥ 7), indicating the presence of nonequilibrium sorption/desorption (confirmed by a flow interruption experiment). The actual time taken to attain 99% sorption/desorption was up to 5 times longer than predicted by the equilibrium model (e.g., ∼62 PVs versus ∼12 PVs predicted for perfluorooctane sulfonate (PFOS) in loamy sand). The increasing contribution of hydrophobic interactions over the electrostatic interactions is suggested as the main driving factor of the nonequilibrium processes. The inverse linear relationship (R² = 0.6, p < 0.0001) between the nonequilibrium mass transfer rate coefficient and the Freundlich sorption coefficient could potentially be a useful means for preliminary evaluation of potential nonequilibrium sorption/desorption of PFASs in soils.

Influence of Residual Nonaqueous-Phase Liquids (NAPLs) on the Transport and Retention of Perfluoroalkyl Substances

Per- and polyfluoralkyl substances (PFAS) are known to accumulate at interfaces, and the presence of nonaqueous-phase liquids (NAPLs) could influence the PFAS fate in the subsurface. Experimental and mathematical modeling studies were conducted to investigate the effect of a representative NAPL, tetrachloroethene (PCE), on the transport behavior of PFAS in a quartz sand. Perfluorooctanesulfonate (PFOS), perfluorononanoic acid (PFNA), a 1:1 mixture of PFOS and PFNA, and a mixture of six PFAS (PFOS, PFNA, perfluorooctanoic acid (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanesulfonate (PFHxS), and perfluorobutanesulfonate (PFBS)) were used to assess PFAS interactions with PCE-NAPL. Batch studies indicated that PFAS partitioning into PCE-NAPL (K_nw < 0.1) and adsorption on 60-80 mesh Ottawa sand (K_d < 6 × 10^-5 L/g) were minimal. Column studies demonstrated that the presence of residual PCE-NAPL (∼16% saturation) delayed the breakthrough of PFOS and PFNA, with minimal effects on the mobility of PFBS, PFHpA, PFHxS, and PFOA. Breakthrough curves (BTCs) obtained for PFNA and PFOS alone and in mixtures were nearly identical, indicating the absence of competitive adsorption effects. A mathematical model that accounts for NAPL-water interfacial sorption accurately reproduced PFAS BTCs, providing a tool to predict PFAS fate and transport in co-contaminated subsurface environments.

A dual grafted fluorinated hydrocarbon amine weak anion exchange resin polymer for adsorption of perfluorooctanoic acid from water

Perfluorooctanoic acid (PFOA) is a persistent and recalcitrant organic contaminant of exceptional environmental concern, and its removal from water has increasingly attracted global attention due to its wide distribution and strong bioaccumulation. Adsorption is considered an effective technique for PFOA removal and more efficient PFOA sorbents are still of interest. This study developed a dual grafted fluorinated hydrocarbon amine weak anion exchange (WAX) polymeric resin (Sepra-WAX-KelF-PEI) for PFOA removal from water. This polymer was synthesized by a two-step amine grafting reaction procedure involving first the reaction of the Sepra-WAX hydrocarbon polymer with poly(vinylidinefluoride-chlorotrifluoroethylene) (Kel-F 800) and then a second reaction with polyethyleneimine (PEI). Characterization of the synthesized polymers was performed using scanning electron microscopy and elemental analysis (F and Cl) by energy dispersive X-ray spectroscopy. The PFOA adsorption performance evaluations were conducted by packed column flow analyses with on-line detection. The results show the breakthrough of the Sepra-WAX-KelF-PEI synthesized with optimum stoichiometry was two times better than the starting anion exchange polymer Sepra-WAX, and six times better than powdered activated carbon, when using the same column size. The adsorption mechanisms of this novel adsorbent including hydrophobic interaction and electrostatic interaction were also clarified in this study. The adsorption kinetic parameters of the two optimum synthesized sorbents were determined using the Thomas model, the Yoon-Nelson model, and batch isotherm studies, and compared with those found with activated carbon and the starting WAX resin. Good agreement of the batch isotherm and column studies with respect to adsorption capacities trends between all three polymers (Sepra-WAX, Sepra-WAX-KelF, and Sepra-WAX-KelF-PEI) were noted.

Comparison of methods to estimate air-water interfacial areas for evaluating PFAS transport in the vadose zone

When performing calculations or numerical simulations for the fate and transport of PFAS and other surface-active solutes in the vadose zone, accurately representing the relationship between the area of the air-water interfaces (A_aw) as a function of water saturation (S_w), and changes in that relationship resulting from changes in soil texture, are equally important as accurately characterizing interfacial adsorption coefficients and the concentration dependence for PFAS solutes. This is true because the magnitude of the A_aw directly governs the degree of air-water interfacial adsorption, which contributes to the transport retardation of these solutes within unsaturated porous media. Herein, a well-known thermodynamic-based model for predicting the A_aw-S_w relationship is evaluated through comparisons to literature data collected using various measurement techniques for model sands and a limited number of soils using data collected from the current published literature. This predictive model, herein termed the Leverett thermodynamic model (LTM), relies on the characterization of the soil-water retention curve (SWRC) for a given soil, using the van Genuchten (VG) equation for the pressure head-vs-S_w relationship. Therefore, methods to estimate the VG equation parameters are also compared as to the A_aw-S_w relationships predicted. Comparisons suggest that the LTM provides the best estimate of the actual A_aw-S_w relationships for water containing non-surface-active solutes. Because PFAS solutes are also surface-active, A_aw measurement methods utilizing surface-active tracers are considered to provide the most accurate representation of the A_aw-S_w relationship for these solutes. Differences between A_aw-S_w relationships derived from tracer methods and the LTM are described in relation to media surface roughness effects. Based on the available literature data, a practical empirical model is proposed to adjust the LTM prediction to account for the effects of surface roughness on the magnitude of the A_aw for surface-active solutes. Finally, example retention calculations are performed to demonstrate the sensitivity of the predicted A_aw-S_w relationship on the vadose zone transport of of a representative PFAS, perfluorooctane sulfonate.

The impact of multiple-component PFAS solutions on fluid-fluid interfacial adsorption and transport of PFOS in unsaturated porous media

The objective of this research was to investigate the impact of multiple-component PFAS solutions on the retention of PFOS during transport in unsaturated porous media. Surface tensions were measured to characterize the impact of co-PFAS on the surface activity of PFOS. Miscible-displacement experiments were conducted to examine the air-water interfacial adsorption of PFOS during transport in single and multi-PFAS systems. Literature data for transport of PFOS in NAPL-water systems were also investigated for comparison. A mathematical model incorporating surfactant-induced flow, nonlinear rate-limited sorption, nonlinear rate-limited fluid-fluid interfacial adsorption, and competitive adsorption at the fluid-fluid interface was used to simulate the transport of PFOS. The results indicate that the presence of co-PFAS had no measurable impact on solid-phase sorption of PFOS during transport under the extant conditions of the experiments. Conversely, the air-water interfacial adsorption of PFOS was decreased by the presence of co-PFAS during transport under unsaturated-flow conditions for relatively high input concentrations. The multiple-component Langmuir model could not predict the competitive adsorption behavior observed during transport. Conversely, competitive interactions were not observed for transport with a lower input concentration. The results indicate that the retention and transport of individual PFAS in mixtures may in some cases be impacted by the presence of co-PFAS due to competitive fluid-fluid interfacial adsorption effects. Reduced retention due to competitive interfacial-adsorption interactions has the potential to decrease PFOS retardation during transport, thereby increasing migration rates in sources zones and enhancing groundwater-pollution risks. SYNOPSIS: The impact of PFAS mixtures on the retention and transport of PFOS in unsaturated porous media is examined with a series of experiments and mathematical modeling.

The Co-Transport of PFAS and Cr(VI) in porous media

PFAS and Cr are present at some sites as co-contaminants. The objective of this research was to investigate the co-transport behavior of per- and polyfluoroalkyl substances (PFAS) and hexavalent chromium (Cr(VI)) in porous media. Miscible-displacement experiments were conducted using two soils and an aquifer sediment with different geochemical properties. Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) were employed as model PFAS. The retardation of PFOS was decreased in the presence of Cr(VI). Conversely, the transport and retardation of PFOA was not affected by the presence of Cr(VI). The reduction of PFOS retardation caused by Cr(VI) is likely due to sorption competition for both organic-carbon and inorganic (metal-oxides and clay minerals) domains. The relative contributions of the three soil constituents to PFOS sorption and the potential for competition between PFOS and Cr(VI) is a function of the geochemical composition of the porous media (i.e., organic carbon, metal-oxides and clay minerals). The PFAS had minimal impact on the retention and transport of Cr(VI). To our knowledge, the results presented herein represent the first reported data for PFOS and Cr(VI) co-transport in porous media. The results of this study indicate that the presence of Cr(VI) has the potential to increase the migration potential of PFOS in soil and groundwater, which should be considered when characterizing electroplating facilities, leather tanning facilities, and other co-contaminated sites.

Excavated vs novel in situ soil washing as a remediation strategy for sandy soils impacted with per- and polyfluoroalkyl substances from aqueous film forming foams

In situ soil washing at the field scale has not yet been investigated as a remediation strategy for soils impacted by per- and polyfluoroalkyl substances (PFAS). This remediation strategy is a promising low-cost alternative to other costlier remediation options like excavating, transporting and landfilling large amounts of PFAS contaminated soil. However, it is unclear if it is effective at the field scale, where large areas of heterogenous soil can be challenging to saturate with infiltration water and then pump to a treatment facility. To address this for the first time, herein we established three different trials involving in situ washing of an undisturbed, 3 m deep, sandy vadose zone soil contaminated with aqueous film forming foam (AFFF). The trials were performed at a site with an established pump and treat system for treating PFAS contaminated groundwater. In situ soil washing was compared to the more conventional practice of washing excavated soil on top of an impermeable bottom lining where the PFAS contaminated water was collected and monitored in a drainage system before treatment. The measured amount of perfluorooctane sulfonate (PFOS) removed was compared with expectations based on a non-calibrated, 1-D first order rate saturated soil model using only the local soil-to-water distribution coefficient as well as the volume and irrigation rate of wash water as input. This model predicted results within a factor of 2. The suspected reasons for small discrepancies between model predictions and excavated vs in situ washing was a combination of the heterogeneity of PFOS distribution in the soil as well as preferential flow paths during soil washing that prevented full saturation. This analysis showed that in situ soil washing was more efficient and less costly than washing excavated sandy soil, particularly if a pump-and-treat system is already in place.

Release of soil colloids during flow interruption increases the pore-water PFAS concentration in saturated soil

Groundwater flow through aquifer soils or packed bed systems can fluctuate for various reasons, which could affect the concentration of natural colloids and per- and polyfluoroalkyl substances (PFAS) in the pore water. In such cases, PFAS concentration could either decrease due to matrix diffusion of PFAS or increase by the detachment of colloids carrying PFAS. Yet, the effect of flow fluctuation on PFAS transport or release in porous media has not been examined. To examine the relative importance of either process, we interrupted the flow during an injection of groundwater spiked with perfluorobutanoic acid (PFBA), perfluorooctanoic acid (PFOA), and bromide as conservative tracer through clay-rich soil, so that diffusive transport would be prominent during flow interruption. After flow interruption, the PFAS concentration did not decrease indicating an insignificant contribution of matrix diffusion. The concentration increased, potentially due to enhanced release of colloid-associated PFAS. Analysis of samples before and after flow interruption by particle size analysis and SEM confirmed an increase in soil colloid concentration after the flow interruption. XRD analysis of soil and the colloids proved that PFAS were associated with specific sites of the colloids. Due to a higher affinity of PFOA to soil colloids, the total PFOA concentration in the effluent samples increased more than PFBA after the flow interruption process. The results indicate that colloids may have a disproportionally higher role in the transport of PFAS in conditions that release colloids from porous media. Thus, fluctuations in groundwater flow can increase this colloid facilitated mobility of PFAS.

The Mass Transfer Index (MTI): A semi-empirical approach for quantifying transport of solutes in variably saturated porous media

The processes impacting solute transport through unsaturated porous media have been receiving renewed attention due to their relevance to the transport of emerging contaminants. A set of well-monitored and highly controlled experiments in sand columns were conducted to determine the effect of partial saturation on conservative solute breakthrough in porous media. The results suggest traditional transport parameter estimation methods inadequately account for the pore-scale processes of mass transfer to the immobile zones and the effects of partial saturation on advective transport, even for conservative tracers. Accurate estimation of these basic transport parameters is critical to evaluate the multi-phase partitioning of nonconservative solutes, as any errors in these parameters would bias the estimates of multi-phase partitioning parameters. Herein, we introduced the Mass Transfer Index (MTI), a semi-empirical approach for quantifying the impact of non-Fickian elements of pore-scale unsaturated solute transport (i.e. immobile water, tortuous flow paths, and non-uniform solute distribution), which become increasingly important as the wetting fluid saturation decreases. Importantly, this MTI was determined independently of chemically driven phase partitioning and is supported by experimental data. Based on this conceptualization, the 1-D equilibrium advection dispersion equation was modified to incorporate the MTI as a lumped parameter which quantifies resistance to (MTI > 1) or promotion of (MTI < 1) of advective solute flux. Analytical solutions to the modified advection-dispersion-reaction equation for pulse and step inputs were developed. Conservative tracer experiments were conducted in variably saturated sand columns to validate both the MTI conceptualization and the inversion method used to estimate the MTI. These experiments involved the use of X-ray absorption spectroscopy integrated with sensor-based measurements of soil moisture, temperature, and electrical conductivity for tracer breakthrough. The mathematical model developed herein adapts traditional macroscopic models of solute transport to account for the non-Fickian pore-scale transport behaviors observed in unsaturated porous media with significant advective flux.

Ideal versus Nonideal Transport of PFAS in Unsaturated Porous Media

Per- and poly-fluoroalkyl substances (PFAS) adsorb at air-water interfaces during transport in unsaturated porous media. This can cause surfactant-induced flow and enhanced retention that is a function of concentration, which complicates characterization and modeling of PFAS transport under unsaturated conditions. The influence of surfactant-induced flow and nonlinear air-water interfacial adsorption (AWIA) on PFAS transport was investigated with a series of miscible-displacement transport experiments conducted with a several-log range in input concentrations. Perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), and ammonium perfluoro 2-methyl-3-oxahexanoate (GenX) were used as model PFAS. The results were interpreted in terms of critical reference concentrations associated with PFAS surface activities and their relationship to the relevancy of transport processes such as surfactant-induced flow and nonlinear AWIA for concentration ranges of interest. Analysis of the measured transport behavior of PFAS under unsaturated-flow conditions demonstrated that AWIA was linear when the input concentration was sufficiently below the critical reference concentration. This includes the absence of significant arrival-front self-sharpening and extended elution tailing of the breakthrough curves, as well as the similarity of retardation factors measured for a wide range of input concentrations. Independently-predicted simulations produced with a comprehensive flow and transport model that accounts for transient variably-saturated flow, surfactant-induced flow, nonlinear rate-limited solid-phase sorption, and nonlinear rate-limited AWIA provided excellent predictions of the measured transport. A series of simulations was conducted with the model to test the specific impact of various processes potentially influencing PFOS transport. The simulation results showed that surfactant-induced flow was negligible and that AWIA was effectively linear when the input concentration was sufficiently below the critical reference concentration. PFAS retention associated with AWIA can be considered to be ideal in such cases, thereby supporting the use of simplified mathematical models. Conversely, apparent nonideal transport behavior was observed for experiments conducted with input concentrations similar to or greater than the critical reference concentration.

Impact of a Hydrocarbon Surfactant on the Retention and Transport of Perfluorooctanoic Acid in Saturated and Unsaturated Porous Media

The transport and retention behavior of perfluorooctanoic acid (PFOA) in the presence of a hydrocarbon surfactant under saturated and unsaturated conditions was investigated. Miscible-displacement transport experiments were conducted at different PFOA and sodium dodecyl sulfate (SDS) input ratios to determine the impact of SDS on PFOA adsorption at solid-water and air-water interfaces. A numerical flow and transport model was employed to simulate the experiments. The PFOA breakthrough curves for unsaturated conditions exhibited greater retardation compared to those for saturated conditions in all cases, owing to air-water interfacial adsorption. The retardation factor for PFOA with a low concentration of SDS (PFOA-SDS ratio of 10:1) was similar to that for PFOA without SDS under unsaturated conditions. Conversely, retardation was greater in the presence of higher levels of SDS (1:1 and 1:10) with retardation factors increasing from 2.4 to 2.9 and 3.6 under unsaturated conditions due to enhanced adsorption at the solid-water and air-water interfaces. The low concentration of SDS had no measurable impact on PFOA air-water interfacial adsorption coefficients (K_ia) determined from the transport experiments. The presence of SDS at the higher PFOA-SDS concentration ratios increased the surface activity of PFOA, with transport-determined K_ia values increased by 27 and 139%, respectively. The model provided very good independently predicted simulations of the measured breakthrough curves and showed that PFOA and SDS experienced various degrees of differential transport during the experiments. These results have implications for the characterization and modeling of poly-fluoroalkyl substances (PFAS) migration potential at sites wherein PFAS and hydrocarbon surfactants co-occur.

Transport of PFOS in aquifer sediment: Transport behavior and a distributed-sorption model

The objectives of this research were to examine the transport of perfluorooctane sulfonic acid (PFOS) in aquifer sediment comprising different geochemical properties, and to compare the behavior to that observed for PFOS transport in soil and sand. PFOS retardation was relatively low for transport in all aquifer media. The PFOS breakthrough curves were asymmetrical and exhibited extensive concentration tailing, indicating that sorption/desorption was significantly nonideal. The results of model simulations indicated that rate-limited sorption/desorption was the primary cause of the nonideal PFOS transport. Comparison of PFOS transport in aquifer media to data reported for PFOS transport in two soils and a quartz sand showed that PFOS exhibited more extensive elution tailing for the soils, likely reflecting differences in the relative contributions of various media constituents to sorption. A three-component distributed-sorption model was developed that accounted for contributions from soil organic carbon, metal oxides, and silt + clay fraction. The model produced very good predictions of K_d for the five media with lower soil organic‑carbon contents (≤0.1%). Soil organic carbon was estimated to contribute 19-42% of the total sorption for all media except the sand, to which it contributed ~100%. The contribution of silt + clay ranged from 51 to 80% for all media except the sand. The only medium for which the contribution of metal-oxides was significant is Hanford, with an estimated contribution of 15%. Overall, the results of the study indicate that sorption of PFOS by these aquifer media comprised contributions from multiple soil constituents.

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.

Examining the robustness and concentration dependency of PFAS air-water and NAPL-water interfacial adsorption coefficients

Determining robust values for the air-water or NAPL-water interfacial adsorption coefficient, KIA, is key to characterizing and modeling PFAS transport and fate in several environmental systems. Direct, high-resolution measurements of surfactant adsorption at the fluid-fluid interface were aggregated from the literature. This data set was used to examine the accuracy and applicability of Γ and KIA measurements determined for three PFAS from transport experiments and surface-tension data. The transport-measured Γ and KIA data were observed to be fully consistent with the directly-measured data. Specifically, Γ values for the two methods were entirely coincident in the region of overlapping concentrations, which spanned ~4 orders-of-magnitude. Furthermore, the two data sets adhered to an identical Γ-C profile. These results conclusively demonstrate the accuracy of the transport-measured values. Γ and KIA values determined from the application of the Gibbs adsorption equation to measured surface-tension data were fully consistent with the directly-measured and transport-measured data sets, demonstrating their applicability for representing PFAS transport in environmental systems. The directly-measured data were used to examine the concentration dependency of KIA values, absent the potential confounding effects associated with the use of surface-tension or transport-measured data. The directly-measured data clearly demonstrate that KIA attains a constant, maximum limit at lower concentrations. Two separate analyses of the transport-measured data both produced observations of constant KIA values at lower concentrations, consistent with the directly-measured data. These outcomes are discussed in terms of surface activities, relative surface coverages, and critical concentrations.