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

Using Suction Lysimeters for Determining the Potential of Per- and Polyfluoroalkyl Substances to Leach from Soil to Groundwater: A Review

In-situ porewater samples were proposed to best represent the fraction of perfluoroalkyl and polyfluoroalkyl substances (PFAS) with the potential to migrate to groundwater. While there are many techniques for collecting porewater samples, suction lysimeters are frequently being used for PFAS investigations. Suction lysimeters use vacuum to extract porewater from vadose zone soils, typically fine to medium sands, which retain and release enough porewater for analysis. Importantly, determining the rate of PFAS migration to groundwater requires an independent measure of water percolation. This review covers the installation and sampling methods for suction lysimeters and provides suggestions to improve the utility and reduce the variability of the results. Because the volume of soil represented by the porewater sample varies significantly depending on the soil-water content, which is spatially and temporally variable, many suction lysimeters may be required to accurately represent soil heterogeneity. A similar limitation applies to soil or leaching protocol samples. Suction lysimeters may not provide a representative sample for all PFAS due to interactions with lysimeter materials, air-water interfaces, and the use of vacuum. Consequently, lysimeter data are best applied in combination with soil-leaching protocols, groundwater transects, and soil analysis when making remedial decisions.

Enhanced Reaction Kinetics in Stationary Two-Phase Flow through Porous Media

Understanding the interaction between multiphase flow and reactive transport in porous media is critical for many environmental and industrial applications. When a nonwetting immiscible phase is present within the pore space, it can remain immobile, which we call unsaturated flow, or move, resulting in multiphase flow. Previous studies under unsaturated flow conditions have shown that, for a given flow rate, the product of a mixing-driven reaction increases as wetting phase saturation decreases. Conversely, the opposite effect is observed for a given Péclet number (i.e., the flow rate is adapted depending on the wetting phase saturation). However, the impact of multiphase flow dynamics on mixing-driven reactions is poorly understood due to experimental and numerical challenges. To assess the impact of multiphase flow conditions on product formation, we use an optimized chemiluminescence reaction and an experimental setup that allows the separate injection of reactants along with a stationary two-phase flow. In our experiments, the mass of the reaction product under stationary two-phase flow conditions increases faster than Fickian beyond the diffusive time. The global kinetics initially increase before experiencing a monotonic decrease with significant fluctuations caused by the displacement of the nonwetting phase. For a given flow rate of the wetting phase, product formation depends on the flow rate of the nonwetting immiscible phase.

PFAS, PCBs, PCDD/Fs, PAHs and extractable organic fluorine in bio-based fertilizers, amended soils and plants: Exposure assessment and temporal trends

Bio-based fertilizers (BBFs) produced from organic waste contribute to closed-loop nutrient cycles and circular agriculture. However, persistent organic contaminants, such as per- and poly-fluoroalkyl substances (PFAS), polychlorobiphenyls (PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), as well as polyaromatic hydrocarbons (PAHs) can be present in organic waste or be formed during valorization processes. Consequently, these hazardous substances may be introduced into agricultural soils and the food chain via BBFs. This study assessed the exposure of 84 target substances and extractable organic fluorine (EOF) in 19 BBFs produced from different types of waste, including agricultural and food industrial waste, sewage sludge, and biowaste, and through various types of valorization methods, including hygienization at low temperatures (<150 °C) as well as pyrolysis and incineration at elevated temperatures (150-900 °C). The concentrations in BBFs (ΣPFOS & PFOA: <30 μg kg-1, Σ6PCBs: <15 μg kg-1, Σ11PAHs: <3 mg kg-1, Σ17PCDD/Fs: <4 ng TEQ kg-1) were found to be below the strictest thresholds used in individual EU countries, with only one exception (pyrolyzed sewage sludge, Σ11PAHs: 5.9 mg kg-1). Five BBFs produced from sewage sludge or chicken manure contained high concentrations of EOF (>140 μg kg-1), so monitoring of more PFAS is recommended. The calculated expected concentrations in soils after one BBF application (e.g. PFOS: <0.05 μg kg-1) fell below background contamination levels (PFOS: 2.7 μg kg-1) elsewhere in the literature. This was confirmed by the analysis of BBF-amended soils from field experiments (Finland and Austria). Studies on target legacy contaminants in sewage sludge were reviewed, indicating a general decreasing trend in concentration with an apparent half-life ranging from 4 (PFOS) to 9 (PCDD/Fs) years. Modelled cumulative concentrations of the target contaminants in agricultural soils indicated low long-term risks. Concentrations estimated and analyzed in cereal grains were low, indicating that exposure by cereal consumption is well below tolerable daily intakes.

Influence of kinetic air-water interfacial partitioning on unsaturated transport of PFAS in sandy soils

This study investigates the impact of kinetic air-water partitioning on the transport of perfluoroalkyl substances (PFAS) within homogeneous and heterogeneous sandy vadose zones under transient unsaturated flow conditions. These experimental conditions are realistic for field behavior, where transient flow foments the continual growth and collapse of air-water interfaces (AWIs), and where layered heterogenous conditions enhance the perturbations of AWIs. Short-chain PFAS behave like conservative tracers with negligible air-water interface partitioning, whereas longer-chain PFAS demonstrate non-equilibrium retention behavior, especially in heterogeneous media. AWI partitioning kinetics were found to be important in controlling PFAS transport and mass flux, particularly during PFAS sorption to the air-water interface, which results because of the different nature and more rapid changes in AWI during drainage, wherein PFAS are moving toward the interface to achieve equilibrium, than during imbibition, where PFAS are leaving the interface to achieve equilibrium. Neglecting these kinetic AWI sorption processes can result in an underestimate of the PFAS transport velocities and mass flux reaching the water table. The presence of trapped air may also inhibit PFAS partitioning in a similar manner by causing longer diffusion paths from bulk water to a portion of the AWIs. The modified HYDRUS effectively captured the transport processes and provided an excellent match to the measured breakthrough curves. To assess relevance using realistic transient infiltration rates, simulations were conducted using precipitation data from an actual site. The results showed that accounting for kinetic AWI partitioning increases the cumulative PFOS mass flux to groundwater by a factor of 2.3 compared to equilibrium conditions, significantly impacting PFAS porewater concentrations. This difference was threefold under experimental conditions, suggesting that the importance of kinetic effects may vary significantly over the long term and under different climatic conditions or soil types, due to their strong dependence on water flux.

Fate, distribution, and transport dynamics of Per- and Polyfluoroalkyl Substances (PFASs) in the environment

Per- and Polyfluoroalkyl Substances (PFASs) are persistent organic pollutants with significant environmental and health impacts due to their widespread occurrence, bioaccumulation potential, and resistance to degradation. This paper comprehensively reviews current knowledge of PFAS fate and transport mechanisms by correlating PFAS leaching, retention, and movement to their physicochemical properties and environmental factors based on observing PFAS fate and transport in unsaturated zones, surface water, sediments, plants, and atmosphere. The complex and unique physiochemical properties of PFASs, such as their carbon-fluorine bonds and amphiphilic nature, determine their environmental behavior and persistence. Recent studies emphasize that concentration-dependent affinity coefficients predict the transport of diverse PFAS mixtures by considering the impact of the Air-Water Interface (AWI). These studies highlight the complex interactions that influence PFAS behavior in environmental systems and the need for refined modeling techniques to account for transport dynamics. Competitive adsorption at the AWI, influenced by PFAS physicochemical properties and environmental factors, is crucial. PFAS chain length profoundly affects PFAS volatility and mobility, i.e., longer chains show higher solid matrix adsorption, while shorter chains exhibit greater atmospheric deposition potential. Solution chemistry, encompassing pH and ionic strength, variably alters PFAS sorption behaviors. Mathematical models, such as the Leverett Thermodynamic Model (LTM) and Surface Roughness Multipliers (SRM), effectively predict PFAS retention, offering enhanced accuracy for surface-active solutes through empirical adjustments. Co-contaminants' presence influences the transport behavior of PFASs in the environment. Microbial activity alters PFAS retention, while microplastics, especially polyamide, contribute to their adsorption. These complex interactions govern PFAS fate and transport in the environment. The paper identifies critical gaps in current understanding, including the fate of PFASs, analytical challenges, ecological risk assessment methods, and the influence of episodic events on PFAS transport dynamics. This paper also investigates the research gap in refining current models and experimental approaches to predict PFAS transport accurately and enhance risk mitigation efforts. Addressing these gaps is crucial for advancing remediation strategies and regulatory frameworks to mitigate PFAS contamination effectively.

Statistical Mapping of PFOA and PFOS in Groundwater throughout the Contiguous United States

Per-and polyfluoroalkyl substances (PFAS) are synthetic chemicals that are increasingly being detected in groundwater. The negative health consequences associated with human exposure to PFAS make it essential to quantify the distribution of PFAS in groundwater systems. Mapping PFAS distributions is particularly challenging because a national patchwork of testing and reporting requirements has resulted in sparse and spatially biased data. In this analysis, an inhomogeneous Poisson process (IPP) modeling approach is adopted from ecological statistics to continuously map PFAS distributions in groundwater across the contiguous United States. The model is trained on a unique data set of 8910 PFAS groundwater measurements, using combined concentrations of two PFAS analytes. The IPP model predictions are compared with results from random forest models to highlight the robustness of this statistical modeling approach on sparse data sets. This analysis provides a new approach to not only map PFAS contamination in groundwater but also prioritize future sampling efforts.

Transport and competitive interfacial adsorption of PFOA and PFOS in unsaturated porous media: Experiments and modeling

Among emerging contaminants, per- and polyfluoroalkyl substances (PFAS) have captured public attention based upon their environmental ubiquity and potential risks to human health. Due to their typical surface release conditions and amphiphilic properties, PFAS tend to sorb to soil and accumulate at the air-water interface within the vadose zone. These processes can result in substantial plume attenuation. Although there is a growing body of literature on vadose zone transport, few studies have explored PFAS mixture transport, particularly under conditions where nonlinear sorption processes are important. The present study aims to advance our understanding of PFAS transport in variably saturated porous media through integration of experiments and mathematical modeling. Experiments include batch studies to quantify sorption to the solid phase, interfacial tension (IFT) measurements to estimate adsorption at the air-water interface (AWI), and column studies with F-70 Ottawa sand at 100 % and ca. 50 % water saturation to explore transport mechanisms. Employed PFAS solutions encompass individual solutes and binary mixtures of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) at concentration levels spanning four orders of magnitude to assess competitive and nonlinear sorption at the AWI. Observations demonstrate that concentration levels and competitive effects substantially influence PFAS transport in unsaturated systems. In the presence of PFOS, PFOA experienced less retention than would be anticipated based on single-solute behavior, and effluent breakthrough curves exhibited chromatographic peaking. The presented mathematical model for simultaneous flow and transport of PFAS was able to capture experimental observations with a consistent set of parameters and minimal curve fitting. These results demonstrate the robustness of the model formulation that included rate-limited interfacial mass transfer, an extended Langmuir-Szyszkowski model for adsorption at the AWI, and a scaled Leverett thermodynamic model to predict the AWI specific area. Overall, the results of this work underscore the importance of the AWI in PFAS transport and highlight the relevance of competition effects in adsorption formulations.

The effect of heavy precipitation on the leaching of heavy metals from tropical coastal legacy tailings

The continued growth in demand for mineral resources has led to a large amount of mining wastes, which is a major challenge in the context of carbon neutrality and climate change. In this study, runoff migration, batch leaching, and column experiments were used to investigate the short-, medium-, and long-term leaching of heavy metals from legacy tailings, respectively; the cumulative metal release kinetic equations were established, and the long-term effects of tailings leaching were verified by HYDRUS-1D. In runoff migration experiments, surface dissolution of tailings and the co-migration of adsorbed soil particles by erosion were the main carriers in the early stages of leachate formation (Mn ∼ 65 mg/L and SO42- up to 2697.2 mg/L). Batch leaching tests showed that the concentration of heavy metals in soil leached by acid rain were 0.1 ∼ 22.0 μg/L for Cr, 0.7 ∼ 26.0 μg/L for Cu, 4.8 ∼ 5646.0 μg/L for Mn, 0.3 ∼ 232.4 μg/L for Ni, and 1.3 ∼ 448.0 μg/L for Zn. The results of column experiments indicated that some soluble components and metals with high mobility showed a significant decreasing trend at cumulative L/S ≤ 2. Additionally, the metals have higher leaching rates under TCLP conditions, as shown by Mn > Co > Zn > Cd > Ni > Cu > Pb > Cr. The fitting results of Langmuir equation were closer to the cumulative release of metals in the real case, and the release amounts of Mn, Zn, Co, and Ni were higher with 55, 5.84, 2.66, and 2.51 mg/kg, respectively. The water flow within tailings affects the spatial distribution of metals, which mainly exist in relatively stable chemical fractions (F3 + F4 + F5 > 90 %) after leaching. Numerical simulation verified that Mn in leachate has reached 8 mg/L at a scale of up to 100 years. The research results are expected to provide technical basis for realizing the resource utilization of tailings in the future.

High-Resolution Depth-Discrete Analysis of PFAS Distribution and Leaching for a Vadose-Zone Source at an AFFF-Impacted Site

The long-term leaching of polyfluoroalkyl substances (PFAS) within the vadose zone of an AFFF application site for which the depth to groundwater is approximately 100 m was investigated by characterizing the vertical distribution of PFAS in a high spatial resolution. The great majority (99%) of PFAS mass resides in the upper 3 m of the vadose zone. The depths to which each PFAS migrated, quantified by moment analysis, is an inverse function of molar volume, demonstrating chromatographic separation. The PFAS were operationally categorized into three chain-length groups based on the three general patterns of retention observed. The longest-chain (>∼335 cm3/mol molar volume) PFAS remained within the uppermost section of the core, exhibiting minimal leaching. Conversely, the shortest-chain (<∼220 cm3/mol) PFAS accumulated at the bottom of the interval, which coincides with the onset of a calcic horizon. PFAS with intermediate-chain lengths were distributed along the length of the core, exhibiting differential magnitudes of leaching. The minimal or differential leaching observed for the longest- and intermediate-chain-length PFAS, respectively, demonstrates that retention processes significantly impacted migration. The accumulation of shorter-chain PFAS at the bottom of the core is hypothesized to result from limited deep infiltration and potential-enhanced retention associated with the calcic horizon.

An integrated analytical modeling framework for determining site-specific soil screening levels for PFAS

Soils at many contaminated sites have accumulated a significant amount of per- and polyfluoroalkyl substances (PFAS) and may require remediation to mitigate leaching to groundwater. USEPA's current approaches for determining soil screening levels (SSLs) were developed for non-PFAS contaminants. Because many PFAS are interfacially-active with unique leaching behaviors in soils, the current non-PFAS-specific soil screening models may not be applicable. Following USEPA's general methodology, we develop a new modeling framework representing PFAS-specific transport processes for determining site-specific SSLs for PFAS-contaminated sites. We couple a process-based analytical model for PFAS leaching in the vadose zone and a dilution factor model for groundwater in an integrated framework. We apply the new modeling framework to two typical types of contaminated sites. Comparisons with the standard USEPA SSL approach suggest that accounting for the PFAS-specific transport processes may significantly increase the SSL for some PFAS. For the range of soil properties and groundwater recharge rates examined, while SSLs determined with the new model are less than a factor of 2 different from the standard-model values for less interfacially-active shorter-chain PFAS, they are up to two orders of magnitudes greater for more interfacially-active longer-chain PFAS. The new analytical modeling framework provides an effective tool for deriving more accurate site-specific SSLs and improving site characterization and remedial efforts at PFAS-contaminated sites.

Determining air-water interfacial areas for the retention and transport of PFAS and other interfacially active solutes in unsaturated porous media

The objective of this work was to determine the methods that produce the most representative measurements and estimations of air-water interfacial area specifically for the retention and transport of PFAS and other interfacially active solutes in unsaturated porous media. Published data sets of air-water interfacial areas obtained with multiple measurement and prediction methods were compared for paired sets of porous media comprising similar median grain diameters, but one with solid-surface roughness (sand) and one without roughness (glass beads). All interfacial areas produced with the multiple diverse methods were coincident for the glass beads, providing validation of the aqueous interfacial tracer-test methods. The results of this and other benchmarking analyses demonstrated that the differences in interfacial areas measured for sands and soil by different methods are not due to errors or artifacts in the methods but rather the result of method-dependent differential contributions of solid-surface roughness. The contributions of roughness to interfacial areas measured by interfacial tracer-test methods were quantified and shown to be consistent with prior theoretical and experiment-based investigations of air-water interface configurations on rough solid surfaces. Three new methods for estimating air-water interfacial areas were developed, one based on the scaling of thermodynamic-determined values and the other two comprising empirical correlations incorporating grain diameter or NBET solid surface area. All three were developed based on measured aqueous interfacial tracer-test data. The three new and three existing estimation methods was tested using independent data sets of PFAS retention and transport. The results showed that the method based on treating air-water interfaces as smooth surfaces as well as the standard thermodynamic method produced inaccurate air-water interfacial areas that failed to reproduce the multiple measured PFAS retention and transport data sets. In contrast, the new estimation methods produced interfacial areas that accurately represented air-water interfacial adsorption of PFAS and associated retention and transport. The measurement and estimation of air-water interfacial areas for field-scale applications is discussed in light of these results.

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.