Immobilization of per- and polyfluoroalkyl substances (PFAS): Comparison of leaching behavior by three different leaching tests

Analyzing the Release of Per- and Polyfluoroalkyl Substances from Spent Granular Activated Carbons by Standard Leaching Procedures

The recent national primary drinking water regulation for per- and polyfluoroalkyl substances (PFAS) is expected to drive a nationwide increase in granular activated carbon (GAC) usage in water treatment facilities across the United States. Proper management of PFAS-laden GAC waste streams is essential to prevent potential recontamination. This study systematically evaluates PFOA and PFOS leaching from four commercial GACs using three standard batch leaching procedures. Our findings indicate that PFOA leached 1-2 orders of magnitude more than PFOS across all GAC types and leaching procedures. In general, PFAS leaching was more notable for alkaline leaching conditions, especially for wood-based GAC. Additionally, real groundwater spiked with an 8 PFAS mixture was used to load GAC for leaching propensity demonstration, and similar conclusions were reached, where leaching was generally greater for shorter-chain and more hydrophilic PFAS. PFBA exhibited the highest leaching (10.4%), followed by GenX (0.91%) and PFBS (0.75%), while minimal desorption (<0.02%) was observed for long-chain PFOA, PFOS, PFOSA, and PFNA. The study concluded that a complex interplay of multiple interactions between the GAC surface, PFAS molecules, and constituents of leaching solutions controls leaching.

Laboratory validation of a simplified model for estimating equilibrium PFAS mass leaching from unsaturated soils

Modelling per- and polyfluoroalkyl substance (PFAS) fate and transport in the vadose zone is inherently more complex than in the saturated zone due to the highly transient nature and the wetting phase saturation dependent hydraulic flux associated with the vadose zone. The chemical complexity of PFAS impart multiple partitioning processes which complicate the evaluation of PFAS transport in the vadose zone. To date, simplified screening models describing PFAS leaching have been developed to determine PFAS soil cleanup criteria in the vadose zone. Recent work has presented evidence that while PFAS transport in the vadose zone is governed by several non-equilibrium mechanisms, it is possible to predict PFAS mass flux using equilibrium modelling over month to year timescales. We hypothesized that by quantifying important equilibrium partitioning and hydraulic processes, we could simplify vadose zone leaching models for assessing mass flux from the vadose zone to the underlying groundwater. A mass flux, cell-based model which accounts for important partitioning processes (solid and air-water interfacial partitioning) and transience in hydraulic processes (water flux and water content) was developed and validated herein. Column studies were conducted under simulated rainfall conditions to provide transient hydraulic and PFAS leaching data. A HYDRUS 1-D with PFAS module model was calibrated to the hydraulic conditions of the simulated rainfall columns. Forward simulations were carried out using HYDRUS and the mass balance approximation models. The HYDRUS and mass balance approximations performed nearly identically for all PFAS, and both models predicted PFAS mass leaching within a half order of magnitude of most measured data. These results suggest that readily applicable empirical models and simplified numerical models can reasonably estimate month to year scale mass flux from the vadose zone for sites without major heterogeneity or transport non-ideality considerations.

Comparing PFAS analysis in batch leaching and column leaching tests

Laboratory leaching tests are tools to assess the mobility of environmental contaminants released from granular materials. Comparative leaching tests were performed using four PFAS-contaminated soils whose concentration patterns of 10 selected perfluoroalkyl and polyfluoroalkyl substances (PFAS) differed due to the two types of contamination sources. This study aimed to evaluate the equivalence of two usual laboratory-scale leaching test procedures, batch and column percolation tests, at liquid-to-solid ratios (L/S) of 2 l/kg, which is the current practice within the German assessment framework, and 10 l/kg (relevant for some EU regulations such as the landfill directive). The differences between the replicates of leaching tests investigating PFAS were smaller for column percolation tests than for batch tests, probably mainly due to the greater sample size and the better representativeness of the sample portion analyzed. It was observed that batch tests overestimate the release of shorter-chain PFAS, whereby the effect was greater with carboxylic than with sulfonic acids. Currently, the limits of detection of analyses given by the DIN standard with regard to PFCA and PFSA in soils are partly not sufficient to detect very low contents, whereas the detection of selected PFCA and PFSA in eluates is more sensitive, in accordance with the available standards. This results in limitations when calculating mass balances.

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.

PFAS Porewater concentrations in unsaturated soil: Field and laboratory comparisons inform on PFAS accumulation at air-water interfaces

Poly- and perfluoroalkyl substance (PFAS) leaching from unsaturated soils impacted with aqueous film-forming foams (AFFFs) is an environmental challenge that remains difficult to measure and predict. Complicating measurements and predictions of this process is a lack of understanding between the PFAS concentrations measured in a collected environmental unsaturated soil sample, and the PFAS concentrations measured in the corresponding porewater using field-deployed lysimeters. The applicability of bench-scale batch testing to assess this relationship also remains uncertain. In this study, field-deployed porous cup suction lysimeters were used to measure PFAS porewater concentrations in unsaturated soils at 5 AFFF-impacted sites. Field-measured PFAS porewater concentrations were compared to those measured in porewater extracted in the laboratory from collected unsaturated soil cores, and from PFAS concentrations measured in the laboratory using batch soil slurries. Results showed that, despite several years since the last AFFF release at most of the test sites, precursors were abundant in 3 out of the 5 sites. Comparison of field lysimeter results to laboratory testing suggested that the local equilibrium assumption was valid for at least 3 of the sites and conditions of this study. Surprisingly, PFAS accumulation at the air-water interface was orders of magnitude less than expected at two of the test sites, suggesting potential gaps in the understanding of PFAS accumulation at the air-water interface at AFFF-impacted sites. Finally, results herein suggest that bench-scale testing on unsaturated soils can in some cases be used to inform on PFAS in situ porewater concentrations.

Modeling 1-D aqueous film forming foam transport through the vadose zone under realistic site and release conditions

Aqueous film forming foams (AFFFs) have been used to extinguish fires since the 1960s, leading to widespread subsurface contamination by per- and polyfluoroalkyl substances (PFAS), an essential component of AFFF. This study presents 1-D simulations of PFAS migration in the vadose zone resulting from AFFF releases. Simulation scenarios used soil profiles from three US Air Force (USAF) installations, encompassing a range of climatic conditions and hydrogeologic environments. A three-component mixture, representative of major constituents of AFFF, facilitated the exploration of competitive and synergistic effects of co-constituents on PFAS migration. To accurately capture unsaturated transport of PFAS in porous media, the model considers (1) surfactant-induced flow, (2) non-linear sorption to the solid phase, (3) competitive accumulation at the air-water interface, and (4) the moisture-dependence of the air-water interfacial area. Defined PFAS releases were consistent with fire training exercises, emergency responses, and accidental spills of record. Simulation results illustrate the importance of hydrogeologic, climatic, geochemical, and AFFF release conditions on PFAS transport and retention. Comparison of field observations and model simulations for Ellsworth AFB indicate that much of the PFOA and PFOS mass is associated with the air-water interface and the solid phase, which limits their migration potential in the vadose zone. Results also show that rates of migration in the aqueous phase are largely controlled by hydrogeologic properties, including recharge rates and hydraulic conductivity. AFFF spill scenarios varying in volume, concentration, and frequency reveal the importance of release characteristics in determining rates of PFAS migration and concentration peaks. Variability is attributed to non-linear sorption processes, where, contrary to simple linear partitioning formulations, transport is strongly affected by the concentration of PFAS species. Simulations also demonstrate the importance of modeling the AFFF as a mixture since competitive interfacial accumulation effects are shown to enhance the mobility of less surface-active PFAS compounds.

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.