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

Factors influencing residual air saturation during consecutive imbibition processes in an air-water two-phase fine sandy medium - A laboratory-scale experimental study

The residual air saturation plays a crucial role in modeling hydrological processes of groundwater and the migration and distribution of contaminants in subsurface environments. However, the influence of factors such as media properties, displacement history, and hydrodynamic conditions on the residual air saturation is not consistent across different displacement scenarios. We conducted consecutive drainage-imbibition cycles in sand-packed columns under hydraulic conditions resembling natural subsurface environments, to investigate the impact of wetting flow rate, initial fluid state, and number of imbibition rounds (NIR) on residual air saturation. The results indicate that residual air saturation changes throughout the imbibition process, with variations separated into three distinct stages, namely, unstable residual air saturation (Sgr-u), momentary residual air saturation (Sgr-m), and stable residual air saturation (Sgr). The results also suggest that the transition from Sgr-u to Sgr is driven by changes in hydraulic pressure and gradient; the calculated values followed the following trend: Sgr > Sgr-u > Sgr-m. An increase in capillary number, which ranged from 1.46 × 10-7 to 3.07 × 10-6, increased Sgr-u and Sgr-m in some columns. The increase in Sgr ranged from 0.034 to 0.117 across all the experimental columns; this consistent increase can be explained by water film expansion at the primary wetting front along with a strengthening of the hydraulic gradient during water injection. Both the pre-covered water film on the sand grain surface and a pore-to-throat aspect ratio of up to 4.42 were identified as important factors for the increased residual air saturation observed during the imbibition process. Initial air saturation (Sai) positively influenced all three types of residual air saturation, while initial capillary pressure (Pci) exhibited a more pronounced inhibitory effect on residual air saturation, as it can partly characterized the initial connectivity of the air phase generated under different drying flow rates. Under identical wetting flow rate conditions, Sgr was higher during the second imbibition than during the first imbibition due to variations in initial fluid state, involving both fluid distribution and the concentration of dissolved air in the pore water. In contrast, NIR did not have an obvious effect on the three types of residual air saturation. This work aims to provide empirical evidences and offer further insights into the capture of non-wetting phases in groundwater environments, as well as to put forward some potential suggestion for future investigations on the retention and migration of contaminants that involves multiphase interface interactions in subsurface environments.

Per- and polyfluoroalkyl substances (PFAS) as contaminants in groundwater resources - A comprehensive review of subsurface transport processes

Per- and polyfluorinated alkyl substances (PFAS) are persistent contaminants in the environment. An increased awareness of adverse health effects related to PFAS has further led to stricter regulations for several of these substances in e.g. drinking water in many countries. Groundwater constitutes an important source of raw water for drinking water production. A thorough understanding of PFAS subsurface fate and transport mechanisms leading to contamination of groundwater resources is therefore essential for management of raw water resources. A review of scientific literature on the subject of processes affecting subsurface PFAS fate and transport was carried out. This article compiles the current knowledge of such processes, mainly focusing on perfluoroalkyl acids (PFAA), in soil- and groundwater systems. Further, a compilation of data on transport parameters such as solubility and distribution coefficients, as well as, insight gained and conclusions drawn from the reviewed material are presented. As the use of certain fire-fighting foams has been identified as the major source of groundwater contamination in many countries, research related to this type of pollution source has been given extra focus. Uptake of PFAS in biota is outside the scope of this review. The review showed a large spread in the magnitude of distribution coefficients and solubility for individual PFAS. Also, it is clear that the influence of multiple factors makes site-specific evaluation of distribution coefficients valuable. This article aims at giving the reader a comprehensive overview of the subject, and providing a base for further work.

Impacts of Perfluoroalkyl Substances on Aqueous and Nonaqueous Phase Liquid Dechlorination by Sulfidized Nanoscale Zerovalent Iron

Per- and poly fluoroalkyl substances (PFASs) are often encountered with nonaqueous phase liquid (NAPL) in the groundwater at fire-fighting and military training sites. However, it is unclear how PFASs affect the dechlorination performance of sulfidized nanoscale zerovalent iron (S-nFe0), which is an emerging promising NAPL remediation agent. Here, S-nFe0 synthesized with controllable S speciation (FeS or FeS2) were characterized to assess their interactions with PFASs and their dechlorination performance for trichloroethylene NAPL (TCE-NAPL). Surface-adsorbed PFASs blocked materials' reactive sites and inhibited aqueous TCE dechlorination. In contrast, PFASs-adsorbed particles with improved hydrophobicity tended to enrich at the NAPL-water interface, and the reactive sites were re-exposed after the PFASs accumulation into the NAPL phase to accelerate dechlorination. This PFASs-induced phenomenon allowed the materials to present a higher reactivity (up to 1.8-fold) with a high electron efficiency (up to 99%) for TCE-NAPL dechlorination. Moreover, nFe0-FeS2 with a higher hydrophobicity was more readily enriched at the NAPL-water interface and more reactive and selective than nFe0-FeS, regardless of coexisting PFASs. These results unveil that a small amount of yet previously overlooked coexisting PFASs can favor selective reductions of TCE-NAPL by S-nFe0, highlighting the importance of materials hydrophobicity and transportation induced by S and PFASs for NAPL remediation.

The influence of NAPL distribution on the transport of PFOS in Co-contaminated media

The objective of this research was to examine the influence of nonuniform NAPL distribution and hydraulic-conductivity heterogeneity on the retention and transport of PFAS. Two-dimensional flow cell experiments were conducted to investigate the impact of NAPL configuration on PFOS retention. Two simplified NAPL configurations were used, one with decane present as residual and the other with greater-than-residual (pool) NAPL saturation. The measured and predicted retardation factors were 1.7 and 1.6 for the residual-NAPL flow cell. The fractions of retention associated with solid-phase sorption, NAPL-water interfacial adsorption, and NAPL absorption were 0.56, 0.43, and 0.1, respectively. The measured retardation factor was 1.4 for the pool-NAPL flow cell. Notably, the predicted retardation factor assuming no contribution from NAPL-water interfacial adsorption was 1.5. Comparison of the two values indicates a minimal contribution of NAPL-water interfacial adsorption to transport, which is attributed to the impact of by-pass flow effects that limited contact of PFOS with the NAPL present within the pool. Mathematical-model simulations were conducted to examine the impact of heterogeneity on PFOS transport in model NAPL-contaminated heterogeneous source zones. The results demonstrated that the impact of NAPL-water interface adsorption on PFAS transport in NAPL-contaminated source zones can be influenced by the magnitude of hydraulic-conductivity heterogeneity.

Transport behavior difference and transport model of long- and short-chain per- and polyfluoroalkyl substances in underground environmental media: A review

Perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonates (PFSAs), which are the most commonly regulated and most widely concerned per- and polyfluoroalkyl substances (PFAS) have received increasing attention on a global scale due to their amphiphilicity, stability, and long-range transport. Thus, understanding the typical PFAS transport behavior and using models to predict the evolution of PFAS contamination plumes is important for evaluating the potential risks. In this study, the effects of organic matter (OM), minerals, water saturation, and solution chemistry on the transport and retention of PFAS were investigated, and the interaction mechanism between long-chain/short-chain PFAS and the surrounding environment was analyzed. The results revealed that high content of OM/minerals, low saturation, low pH, and divalent cation had a great retardation effect on long-chain PFAS transport. The retention caused by hydrophobic interaction was the prominent mechanism for long-chain PFAS, whereas, the retention caused by electrostatic interaction was more relevant for short-chain PFAS. Additional adsorption at the air-water and nonaqueous-phase liquids (NAPL)-water interface was another potential interaction for retarding PFAS transport in the unsaturated media, which preferred to retard long-chain PFAS. Furthermore, the developing models for describing PFAS transport were investigated and summarized in detail, including the convection-dispersion equation, two-site model (TSM), continuous-distribution multi-rate model, modified-TSM, multi-process mass-transfer (MPMT) model, MPMT-1D model, MPMT-3D model, tempered one-sided stable density transport model, and a comprehensive compartment model. The research revealed PFAS transport mechanisms and provided the model tools, which supported the theoretical basis for the practical prediction of the evolution of PFAS contamination plumes.

Cotransport of different electrically charged microplastics with PFOA in saturated porous media

The fate and transport behavior of microplastics (MPs), emerging colloidal contaminant ubiquitous in natural environments, would be greatly affected by other copresent pollutants. PFOA (emerging surfactant pollutant) would interact with MPs after encounter with them in natural environments, which could alter the transport behavior of both pollutants. Relevant knowledge is still lacking, affecting accurate prediction the fate and distribution of these two emerging contaminants in natural porous media. The cotransport behavior of different surface charged MPs (negatively/positively charged, CMPs/AMPs) with PFOA (three concentrations ranged from 0.1 to 10 mg/L) in porous media in both 10 and 50 mM NaCl solutions thus was investigated in the present study. We found PFOA inhibited CMPs transport in porous media, while enhanced AMPs transport. The mechanisms leading to the altered transport of CMPs/AMPs caused by PFOA were found to be different. The decreased electrostatic repulsion between CMPs-sand induced by the decreased CMPs negative zeta potentials via the adsorption of PFOA led to the inhibited transport of CMPs in CMPs-PFOA suspension. The enhanced electrostatic repulsion between AMPs-sand due to the decreased positive charge of AMPs via the adsorption of PFOA together with steric repulsion induced by suspended PFOA resulted in the increased transport of AMPs in AMPs-PFOA suspension. Meanwhile, we found that the adsorption onto MPs surfaces also impacted the transport of PFOA. Due to the lower mobility of MPs than PFOA, the presence of MPs despite their surface charge decreased the transport of PFOA of all examined concentrations in quartz sand columns. This study demonstrates that when MPs and PFOA are co-existing in environments, their interaction with each other will alter the fate and transport behavior of both pollutants in porous media and the alteration is highly correlated with the amount of PFOA adsorbed onto MPs and original surface properties of MPs.

Predicting Interfacial Tension and Adsorption at Fluid-Fluid Interfaces for Mixtures of PFAS and/or Hydrocarbon Surfactants

Many per- and polyfluoroalkyl substances (PFAS) are surface-active and adsorb at fluid-fluid interfaces. The interfacial adsorption controls PFAS transport in multiple environmental systems, including leaching through soils, accumulation in aerosols, and treatment methods such as foam fractionation. Most PFAS contamination sites comprise mixtures of PFAS as well as hydrocarbon surfactants, which complicates their adsorption behaviors. We present a mathematical model for predicting interfacial tension and adsorption at fluid-fluid interfaces for multicomponent PFAS and hydrocarbon surfactants. The model is derived from simplifying a prior advanced thermodynamic-based model and applies to nonionic and ionic mixtures of the same charge sign with swamping electrolytes. The only required model inputs are the single-component Szyszkowski parameters obtained for the individual components. We validate the model using literature interfacial tension data of air-water and NAPL (non-aqueous phase liquid)-water interfaces covering a wide range of multicomponent PFAS and hydrocarbon surfactants. Application of the model to representative porewater PFAS concentrations in the vadose zone suggests competitive adsorption can significantly reduce PFAS retention (up to 7 times) at some highly contaminated sites. The multicomponent model can be readily incorporated into transport models to simulate the migration of mixtures of PFAS and/or hydrocarbon surfactants in the environment.

Dual effects of PFOA or PFOS on reductive dechlorination of trichloroethylene (TCE)

PFASs and chlorinated solvents are the common co-contaminants in soil and groundwater at firefighter training areas (FTAs). Although PFASs mixtures could have adverse impacts on bioremediation of trichloroethylene (TCE) by inhibiting Dehalococcoides (Dhc), little is known about the effect and contribution of PFOA or PFOS on dechlorination of TCE by non-Dhc organohalide-respiring bacteria (OHRB). To study this, PFOA and PFOS were amended to the growth medium of a non-Dhc OHRB-containing enrichment culture to determine the impact on dechlorination. This study demonstrated that high levels of PFOA or PFOS (100 mg L^-1) inhibited TCE dechlorination in four non-Dhc OHRB-containing community including Geobacter, Desulfuromonas, Desulfitobacterium, and Dehalobacter, but low levels of PFOA or PFOS (≤10 mg L^-1) enhanced TCE dechlorination. Four non-Dhc OHRB were less inhibited by PFOA than that by PFOS, and high level of PFOS killed Desulfitobacterium and Dehalobacter and decreased the biodiversity of bacterial community. Although most fermenters were killed by the presence of 100 mg L^-1 PFOS, two important co-cultures (Desulfovibrio and Sedimentibacter) of OHRB were enriched, indicating that the syntrophic relationships between OHRB and co-cultures still remained, and PFOA or PFOS inhibited TCE dechlorination by directly repressing non-Dhc OHRB. Our results highlight that the bioattenuation of chloroethene contamination could be confounded by non-Dhc OHRB in high levels of PFOS contaminated subsurface environments at FTAs.

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.

Adsorption of PFAAs in the Vadose Zone and Implications for Long-Term Groundwater Contamination

Perfluoroalkyl acids (PFAAs) are persistent environmental contaminants that sorb to air-water and solid interfaces throughout the vadose zone. These sorption processes lead to decadal leaching of PFAS from the source zones to groundwater systems. While these processes are increasingly well understood, critical gaps exist in describing the vertically variable adsorption in the presence of vadose zone heterogeneity and methods for efficiently upscaling the laboratory observations to predict field-scale PFAA transport and retardation. In this work, we build upon fundamental theories and scalable relationships to define a semi-analytical framework for synthesizing and upscaling PFAA adsorption in heterogeneous vadose zone systems. Solid-phase and air-water interfacial adsorption are quantified mechanistically for several PFAAs and then applied to a contaminated site in Northern Wisconsin. The results highlight the dominance of air-water and organic carbon solid-phase adsorption processes in the vadose zone. Strong sorption heterogeneity─driven by depth-dependent adsorption mechanisms─produces complex spatially variable retardation profiles. We develop vadose zone retardation potentials to quantify this field-scale heterogeneity and propose vertical integration methods to upscale spatially resolved information for transport modeling. This work highlights the importance of accounting for multiscale and multiprocess heterogeneity for accurately describing and predicting the long-term fate and transport of PFAAs in the subsurface.

Calculating PFAS interfacial adsorption as a function of salt concentration using model parameters determined from chemical structure

Per- and polyfluoroalkyl substances (PFAS) are widely-detected environmental contaminants known to concentrate at surfaces and interfaces. Many of the most commonly-detected PFAS function as ionic surfactants under environmental conditions. The interfacial behaviors of ionic surfactants, including PFAS, are strongly dependent on salt concentration and composition, with interfacial affinity potentially varying by orders of magnitude for the same compound under different conditions. The work described here presents a tool for predicting the salt-dependent adsorption of PFAS compounds based entirely on chemical structure, something of great value for predicting the real-world environmental behavior of many of the large numbers of PFAS compounds for which experimental data are not available. The approach combines two different previously-developed models, one a mass-action model designed to predict the effects of salts on interfacial adsorption of ionic PFAS (the UNSW-OU salt model), and the second a group-contribution model designed to predict interfacial adsorption of PFAS in the absence of salt based on chemical structure. The challenge of combining the two models comes from the fact that both are based on different isotherms. The salt model can produce sigmoidal isotherms under salt-limited conditions (an isotherm shape that is supported by experimental evidence), while the group-contribution model can generate Langmuir parameters from calculations based on chemical structure. Equations were derived to determine salt model isotherm parameters from Langmuir parameters (either from the group-contribution model or experimental sources) by matching surface tension curves in the vicinity of the concentration of highest second derivative. Refined group-contribution model parameters were determined based on data from an additional 40 surface tension curves to allow improved structure-based predictions for important classes of PFAS that were not sufficiently well-represented in the original model. The resulting equations provide a tool allowing quantitative predictions of PFAS behavior under realistic environmental conditions for compounds for which little or no experimental data are available.

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.

Effects of anionic hydrocarbon surfactant on the transport of perfluorooctanoic acid (PFOA) in natural soils

The widespread usage of per- and polyfluoroalkyl substances (PFASs) has led to their ubiquitous co-existence with hydrocarbon surfactants in the subsurface environment. In this study, column experiments were conducted to investigate the effect of an anionic hydrocarbon surfactant (sodium dodecylbenzene sulfonate, SDBS, 1 and 10 mg/L) on the transport of perfluorooctanoic acid (PFOA) in two saturated natural soils under different cation type (Na⁺ and Ca²⁺) conditions. Results showed that SDBS (10 mg/L) significantly enhanced the transport of PFOA in two soils. This was likely because SDBS had a stronger adsorption affinity to the soils than PFOA, and can outcompete PFOA for the finite adsorption sites on the soil surface. The effect of SDBS on PFOA transport varied greatly in the two soils. More negatively charged soil surface and greater soil particle size likely contributed to the more noticeable transport-enhancement of PFOA resulting from the presence of SDBS. Also, the enhancement effect of SDBS (10 mg/L) with Ca²⁺ on PFOA transport was more significantly than that with Na⁺. This was possibly due to the blocking effect of SDBS to the more positively charged soil surface induced by Ca²⁺. Findings of this study point out the importance of anionic hydrocarbon surfactants on PFOA transport when assessing its environmental risks and implementing remediation efforts.

Co-occurrence and correlations of PFASs and chlorinated volatile organic compounds (cVOCs) in subsurface in a fluorochemical industrial park: Laboratory and field investigations

Fluorochemical industrial park (FIP) represents an important source of per- and polyfluoroalkyl substances (PFASs) and chlorinated volatile organic compounds (cVOCs). Exploring the co-occurrence and correlations of PFASs and cVOCs is a key step towards the understanding their distributions in the field. In this study, perfluorooctanoic acid (PFOA) was the dominant compound in groundwater and aquifer solids, and elevated concentrations of short-chain perfluoroalkyl carboxylic acids (PFCAs) and hexafluoropropylene oxide oligomers were also detected in the field, suggesting their wide applications as substitutes for PFOA. Correlation analyses between PFASs and cVOCs suggested that cVOCs had a significant influence on the distribution and composition of PFASs in the field. In addition, the presence of cVOCs in the form of dense non-aqueous-phase organic liquids (DNAPL) affected the distribution and migration of PFASs at various depths, as evidenced by the relatively high PFASs concentrations (204 μg/L) and PFOA abundance (85.4%) in the deep aquifer, likely due to DNAPL-water interfaces sorption or partition into bulk DNAPL. The log K_d values, determined in the laboratory, were found to increase in the presence of DNAPL, especially for PFOA, with more than one time higher than those of perfluorobutanoic acid (PFBA) and hexafluoropropylene oxide dimer acid (HFPO-DA). This conclusion further demonstrated that PFOA had a higher potential to participate into DNAPL, which can migrate with DNAPL to the deep aquifer, supporting the higher abundance of PFOA in the deep aquifer mentioned above. However, the log K_d-field values of PFBA and HFPO-DA in the field were higher than that of PFOA, and no significant correlations (p > 0.05) were found between log K_d-field values and the chain-length of PFCAs at various depths, suggesting that the phenomena observed in the field are a result of composite influencing factors.

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.

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 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.

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.

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.