Sorption of Polyfluoroalkyl Surfactants on Surface Soils: Effect of Molecular Structures, Soil Properties, and Solution Chemistry

Multimedia distribution and release characteristics of emerging PFAS in wastewater treatment plants in Tianjin, China

Legacy and emerging PFAS in the air, wastewater, and sludge from two wastewater treatment plants (WWTPs) in Tianjin were investigated in this study. The semi-quantified nontarget PFAS accounted for up to 99 % of ƩPFAS in the gas phase, and aqueous film-forming foam (AFFF)-related PFAS were predominant in wastewater (up to 2250 ng/L, 79 % of ƩPFAS) and sludge (up to 4690 ng/g, 95 % of ƩPFAS). Furthermore, field-derived air particle-gas, air-wastewater, and wastewater particle-wastewater distribution coefficients of emerging PFAS are characterized, which have rarely been reported. The emerging substitute p-perfluorous nonenoxybenzenesulfonate (OBS) and AFFF-related cationic and zwitterionic PFAS show a stronger tendency to partition into particle phase in air and wastewater than perfluorooctane sulfonic acid (PFOS). The estimated total PFAS emissions from the effluent and sludge of WWTP A were 202 kg/y and 351 kg/y, respectively. While the target PFAS only accounted for 20-33 % of the total emissions, suggesting a significant underestimation of environmental releases of the nontarget PFAS and unknown perfluoroalkyl acid precursors through the wastewater and sludge disposal. Overall, this study highlights the importance of comprehensive monitoring and understanding the behavior of legacy and emerging PFAS in wastewater systems, and fills a critical gap in our understanding of PFAS exposure.

Intense Turbulent Bursts Promote the Release of Perfluoroalkyl Acids from Sediments at High Flow Velocity

Despite frequent detection of high levels of perfluoroalkyl acids (PFAAs) in sediments, research on the environmental fate of PFAAs in sediments, particularly under hydrodynamic conditions, is rather limited, challenging effective management of PFAA loadings. Therefore, this study investigated the release and transport of 15 PFAAs in sediments under environmentally relevant flow velocities using recirculating flumes and revealed the underlying release mechanisms by identifying related momentum transfer. An increased velocity enhanced the release magnitude of total PFAAs by a factor of 3.09. The release capacity of short-chain PFAAs was notably higher than that of long-chain PFAAs, and this pattern was further amplified by flow velocity. Pore-water drainage was the major pathway for PFAA release, with the release amount predominantly determined by flow velocity-induced release intensity and depth, as well as affected by the perfluorocarbon chain length and sediment size. The weak anion exchanger-diffusion gradients in the thin-film technique confirmed that the release depth of PFAAs increased with flow velocity. Quadrant analysis revealed that the rise in the frequency and intensity of turbulent bursts driven by sweeps and ejections at high flow velocity was the underlying cause of the increased release magnitude and depth of PFAAs.

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.

Assessing the suitability of leachability-based screening levels for per- and polyfluoroalkyl substances (PFAS) risk assessment

In recent years, soil screening levels have been adopted by regulatory agencies for certain per- and polyfluoroalkyl substances (PFAS) to assess the risk of groundwater contamination through leaching. These soil screening levels, determined using an established equilibrium-based partitioning equation, have high variability among regulatory groups largely attributed to the diverse reported partitioning coefficients in the literature. This variability between reported partitioning coefficients, and subsequently soil screening levels, is due to the complex leaching behavior of PFAS not being predicted well by the standard equilibrium-based model. This has led one regulatory group to require batch leaching to assess risk rather than setting default soil screening levels based on partitioning equations. In this work, we conducted leaching experiments on five field-sampled soils impacted by aqueous film-forming foams (AFFF), following Leaching Environmental Assessment Framework (LEAF) Method 1316 and compared the results to expected leaching utilizing an equilibrium-based partitioning equation commonly employed by regulatory agencies to establish soil screening levels. Our analysis found among the six PFAS detected in the soils, which have regulatory leaching thresholds established, the partitioning values assumed by the U.S. EPA exhibited the highest accuracy in predicting leachate concentrations. These partitioning values predicted actual leaching within a ± 20 % margin of error for approximately 50 % of sample points, highlighting limitations in relying solely on equilibrium-based partitioning values as predictors of leaching behavior. This discrepancy between predicted and actual leaching has implications for site managers and regulatory entities overseeing PFAS-contaminated sites, suggesting that soil screening level determinations for PFAS might need to be revised to account for the unique transport characteristics of PFAS.

Influence of soil composition and environmental factors on the adsorption of per- and polyfluoroalkyl substances: A review

Per- and polyfluoroalkyl substances (PFASs) have garnered considerable scientific and regulatory scrutiny due to their widespread distribution across environments and their potential toxicological impacts on human health. The pedosphere serves as a vital reservoir for these chemicals, significantly determining their environmental trajectory and chemical transformations. This study offers a comprehensive synthesis of the current understanding regarding the adsorption mechanics of PFASs in soil matrices. Due to their unique molecular structure, PFASs predominantly engage in hydrophobic and electrostatic interactions during soil adsorption. This work thoroughly evaluates the influence of various factors on adsorption efficiency, including soil properties, molecular characteristics of PFASs, and the prevailing environmental conditions. The complex nature of soil environments complicates isolating individual impacts on PFAS behavior, necessitating an integrated approach to understanding their environmental destinies better. Through this exploration, we seek to clarify the complex interplay of factors that modulate the adsorption of PFASs in soils, highlighting the urgent need for future research to disentangle the intricate and combined effects that control the environmental behavior of PFAS compounds.

Adsorption behavior and the potential risk of As(V) in soils: exploring the effects of representative surfactants

Arsenic contamination in soils poses a critical global challenge, yet the influence of surfactants on arsenic adsorption behavior is often underestimated. This study aims to investigate the effects of three representative surfactants, namely cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyethylene glycol anhydrous sugar alcohol monooleate (Tween 80), on arsenic adsorption behavior in soils. The adsorption isotherm shifts from a single Temkin model without surfactants to both the Langmuir and Temkin models in the presence of surfactants, indicating the simultaneous occurrence of monolayer and multilayer adsorption for arsenic in soils. Moreover, the surfactants can inhibit the adsorption and hasten the attainment of adsorption equilibrium. SDS displayed the most inhibitory effect on arsenic adsorption, followed by Tween 80 and CTAB, due to the competitive adsorption, electrostatic interaction, and hydrophobic interaction. Variations in zeta potential with different surfactants further elucidate this inhibitory phenomenon. Through orthogonal experiment analyses, pH emerges as a primary factor influencing arsenic adsorption in soils, with surfactant concentration and type identified as secondary factors. Temperature notably affects CTAB, with the adsorption inhibition rate plummeting to a mere 0.88% at 50 °C. Sequential extraction analysis revealed that surfactants enhanced the bioavailability of arsenic. The FTIR, XRD, SEM, and CA analyses further support the mechanism underlying the effect of surfactants on arsenic adsorption in soil. These analyses indicate that surfactants modify the composition and abundance of functional groups, hinder the formation of arsenic-containing substances, and improve soil compactness, smoothness, and hydrophilicity. This study provides valuable insights into the effect of surfactants in arsenic-contaminated soils, which is often ignored in previous work.

PFAS remediation in soil: An evaluation of carbon-based materials for contaminant sequestration

The presence of per- and poly-fluoroalkyl substances (PFAS) in soils is a global concern as these emerging contaminants are highly resistant to degradation and cause adverse effects on human and environmental health at very low concentrations. Sequestering PFAS in soils using carbon-based materials is a low-cost and effective strategy to minimize pollutant bioavailability and exposure, and may offer potential long-term remediation of PFAS in the environment. This paper provides a comprehensive evaluation of current insights on sequestration of PFAS in soil using carbon-based sorbents. Hydrophobic effects originating from fluorinated carbon (C-F) backbone "tail" and electrostatic interactions deriving from functional groups on the molecules' "head" are the two driving forces governing PFAS sorption. Consequently, varying C-F chain lengths and polar functional groups significantly alter PFAS availability and leachability. Furthermore, matrix parameters such as soil organic matter, inorganic minerals, and pH significantly impact PFAS sequestration by sorbent amendments. Materials such as activated carbon, biochar, carbon nanotubes, and their composites are the primary C-based materials used for PFAS adsorption. Importantly, modifying the carbon structural and surface chemistry is essential for increasing the active sorption sites and for strengthening interactions with PFAS. This review evaluates current literature, identifies knowledge gaps in current remediation technologies and addresses future strategies on the sequestration of PFAS in contaminated soil using sustainable novel C-based sorbents.

Perfluoroalkyl acid precursors in agricultural soil-plant systems: Occurrence, uptake, and biotransformation

Perfluoroalkyl acid (PFAA) precursors have been used in various consumer and industrial products due to their hydrophobic and oleophobic properties. In recent years, PFAA precursors in agricultural soil-plant systems have received increasing attention as they are susceptible to biotransformation into metabolites with high biotoxicity risks to human health. In this review, we systematically assessed the occurrence of PFAA precursors in agricultural soils, taking into account their sources and biodegradation pathways. In addition, we summarized the findings of the relevant literature on the uptake and biotransformation of PFAA precursors by agricultural plants. The applications of biosolids/composts and pesticides are the main sources of PFAA precursors in agricultural soils. The physicochemical properties of PFAA precursors, soil organic carbon (SOC) contents, and plant species are the key factors influencing plant root uptakes of PFAA precursors from soils. This review revealed, through toxicity assessment, the potential of PFAA precursors to generate metabolites with higher toxicity than the parent precursors. The results of this paper provide a reference for future research on PFAA precursors and their metabolites in soil-plant systems.

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.

Enhanced aggregation and interfacial adsorption of an aqueous film forming foam (AFFF) in high salinity matrices

Per- and polyfluoroalkyl substances (PFAS) exist in contaminated groundwater, surface water, soil, and sediments from use of aqueous film forming foams (AFFFs). Under these conditions PFAS exhibit unusual behavior due to their surfactant properties, namely, aggregation and surface activity. Environmental factors such as salinity can affect these properties, and complicate efforts to monitor PFAS. The effect of high salinity matrices on the critical micelle concentration (CMC) of a AFFF formulation manufactured by 3M and the surface accumulation of PFAS was assessed with surface tension isotherm measurements and bench-scale experiments quantifying PFAS at the air-water interface. Conditions typical of brackish and saline waters substantially depressed the CMC of the AFFF by over 50% and increased the interfacial mass accumulation of PFAS in the AFFF mixture by up to a factor of 3, relative to values measured in ultrapure water. These results indicate that high salinity matrices increase the aggregation and surface activity of PFAS in mixtures, which are key properties affecting their transport.

Generation Mechanism of Perfluorohexanesulfonic Acid from Polyfluoroalkyl Sulfonamide Derivatives During Chloramination in Drinking Water

Per- and polyfluoroalkyl substances (PFASs), including perfluorohexanesulfonic acid (PFHxS), as emerging persistent organic pollutants widely detected in drinking water, have drawn increasing concern. The PFHxS contamination of drinking water always results from direct and indirect sources, especially the secondary generations through environmental transformations of precursors. However, the mechanism of the transformation of precursors to PFHXS during the drinking water treatment processes remains unclear. Herein, the potential precursors and formation mechanisms of PFHxS were explored during drinking water disinfection. Simultaneously, the factors affecting PFHxS generation were also examined. This study found PFHxS could be generated from polyfluoroalkyl sulfonamide derivatives during chlorination and chloramination. The fate and yield of PFHxS varied from different precursors and disinfection processes. In particular, monochloramine more favorably formed PFHxS. Several perfluoroalkyl oxidation products and decarboxylation intermediates were detected and identified in the chloraminated samples using Fourier-transform ion cyclotron resonance mass spectrometry. Combined with density functional theory calculations, the results indicated that the indirect oxidation via the attack of the nitrogen atom in sulfonamide groups might be the dominant pathway for generating PFHxS during chloramination, and the process could be highly affected by the monochloramine dose, pH, and temperature. This study provides important evidence of the secondary formation of PFHxS during drinking water disinfection and scientific support for chemical management of PFHxS and PFHxS-related compounds.

Phytoextraction of per- and polyfluoroalkyl substances (PFAS) by weeds: Effect of PFAS physicochemical properties and plant physiological traits

Phytoextraction is a promising technology that uses plants to remediate contaminated soil. However, its feasibility for per- and polyfluoroalkyl substances (PFAS) and the impact of PFAS properties and plant traits on phytoextraction efficacy remains unknown. In this study, we conducted greenhouse experiment and evaluated the potential of weeds for phytoextraction of PFAS from soil and assessed the effects of PFAS properties and plant traits on PFAS uptake via systematic correlation analyses and electron probe microanalyzer with energy dispersive spectroscopy (FE-EPMA-EDS) imaging. The results showed that 1) phytoextraction can remove 0.04%- 41.4%wt of PFAS from soil, with extracted PFAS primarily stored in plant shoots; 2) Weeds preferentially extracted short-chain PFAS over long-chain homologues from soil. 3) PFAS molecular size and hydrophilicity determined plant uptake behavior, while plant morphological traits, particularly root protein and lipid content, influenced PFAS accumulation and translocation. Although plants with thin roots and small leaf areas exhibited greater PFAS uptake and storage ability, the impact of PFAS physicochemical properties was more significant. 4) Finally, short-chain PFAS were transported quickly upwards in the plant, while uptake of long-chain PFOS was restricted.

Enhanced electrokinetic remediation by magnetic induction for the treatment of co-contaminated soil

The electroplating industry site is an important reservoir of per- and poly-fluoroalkyl substances (PFASs) and heavy metals. In this work, a novel electrokinetic in-situ chemical oxidation system was established to restore an actual soil co-contaminated with high concentrations of heavy metals (Cr, Cu, Zn and Ni) and PFASs. Potassium persulfate (PS, K₂S₂O₈) and industrial waste steel slag were used as the oxidant and activator, respectively. The steel slag was evenly added in the soil, while PS was dosed in the cathode chamber. Citric acid fermentation broth produced by Aspergillus niger was added in the anode chamber to act as the metal chelator. A periodic alternating magnetic field was employed to enhance the catalytic performance of steel slag for PS. After 15-day treatment, 86.7% of PFASs and 87.2% of heavy metals were removed without PFASs accumulation in the electrolyte, with a defluorination percentage of 79.2%. The remediated soil had no phytotoxicity for wheat seed growth based on 7-day cultivation results. The quality of remediated soil could reach the national Class II criteria for residential use. Electron paramagnetic resonance spectroscopy analysis demonstrated that SO₄^•- and ^•OH were the major oxidative radicals responsible for PFASs degradation. Adding steel slag in the soil performed better than that in the cathode chamber based on pollutant removal and alleviating soil acidification. Magnetic induction could enhance PS activation by promote the corrosion of steel slag and thermal activation, thus increasing electrical current and electroosmotic flow, enhancing the transport of citric acid and PS, significantly improving the removal efficiency of heavy metals and PFASs.

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.

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

The evaluation of PFAS immobilization performance in laboratory experiments, especially the long-term stability, is a challenge. To contribute to the development of adequate experimental procedures, the impact of experimental conditions on the leaching behavior was studied. Three experiments on different scales were compared: batch, saturated column, and variably saturated laboratory lysimeter experiments. The Infinite Sink (IS) test - a batch test with repeated sampling - was applied for PFAS for the first time. Soil from an agricultural field amended with paper-fiber biosolids polluted with various perfluoroalkyl acids (PFAAs; 655 μg/kg ∑¹⁸PFAAs) and polyfluorinated precursors (1.4 mg/kg ∑¹⁸precursors) was used as the primary material (N-1). Two types of PFAS immobilization agents were tested: treatment with activated carbon-based additives (soil mixtures: R-1 and R-2), and solidification with cement and bentonite (R-3). In all experiments, a chain-length dependent immobilization efficacy is observed. In R-3, the leaching of short-chain PFAAs was enhanced relative to N-1. In column and lysimeter experiments with R-1 and R-2, delayed breakthrough of short-chain PFAAs (C4) occurred (> 90 days; in column experiments at liquid-to-solid ratio (LS) > 30 L/kg) with similar temporal leaching rates suggesting that leaching in these cases was a kinetically controlled process. Observed differences between column and lysimeter experiments may be attributed to varying saturation conditions. In IS experiments, PFAS desorption from N-1, R-1, and R-2 is higher than in the column experiments (N-1: +44 %; R-1: +280 %; R-2: +162 %), desorption of short-chain PFAS occurred predominantly in the initial phase (< 14 days). Our findings demonstrate that sufficient operating times are essential in percolation experiments, e.g., in column experiments >100 days and LS > 30 L/kg. IS experiments may provide a faster estimate for nonpermanent immobilization. The comparison of experimental data from various experiments is beneficial to evaluate PFAS immobilization and to interpret leaching characteristics.

Suspended particulate matter-bound per- and polyfluoroalkyl substances (PFASs) in a river-coastal system: Possible correlation with transparent exopolymer particles

The transport and ultimate fate of per- and polyfluoroalkyl substances (PFASs) are generally considered to be influenced by partitioning behavior between water, suspended particulate matters (SPM), and sediments. This study examined the distribution and partitioning of the PFASs in the water, SPM, and sediments in a densely populated urban river-coastal system. The total concentrations of eight PFASs (∑₈ PFASs) in the water phase, SPM, and sediments varied from 0.59 to 7.40 ng/L, 0.54 to 9.08 ng/g, and 0.05 to 0.13 ng/g, respectively. The PFAS concentrations in the water and SPM phase decreased as the salinity increased, confirming contaminant inputs from the upstream of the river to the estuary zone. Notably, the positive correlation between SPM-bound PFASs and transparent exopolymer particles (TEPs) content, providing first evidence that TEPs may accumulate and concentrate more PFASs on the SPM. Collectively, this results offers useful information about roles of TEPs in determining environmental fate of PFASs.

Target and nontarget screening of PFAS in drinking water for a large-scale survey of urban and rural communities in Québec, Canada

Limited monitoring data are available regarding the occurrence of emerging per- and polyfluoroalkyl substances (PFAS) in drinking water. Here, we validated an analytical procedure for 42 PFAS with individual detection limits of 0.001-0.082 ng/L. We also evaluated how different sample pH conditions, dechlorinating agents, and storage holding times might affect method performance. PFAS were analyzed in tap water samples collected at a large spatial scale in Quebec, Canada, covering 376 municipalities within 17 administrative regions. Target and nontarget screening revealed the presence of 31 and 23 compounds, respectively, representing 24 homolog classes. Overall, 99.3% of the tap water samples were positive for at least one PFAS, and the ƩPFAS ranged from below detection limits to 108 ng/L (95th percentile: 13 ng/L). On average, ƩPFAS was 12 times higher in tap water produced from surface water than groundwater; however, 6 of the top 10 contaminated locations were groundwater-based. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) had high detection rates (88% and 80%, respectively). PFOS (median: 0.15 ng/L; max: 13 ng/L) and PFOA (median: 0.27 ng/L; max: 8.1 ng/L) remained much lower than current Health Canada guidelines but higher than USEPA's interim updated health advisories. Short-chain (C3-C6) perfluoroalkyl sulfonamides were also recurrent, especially the C4 homolog (FBSA: detection rate of 50%). The 6:2 fluorotelomer sulfonyl propanoamido dimethyl ethyl sulfonate (6:2 FTSO2PrAd-DiMeEtS) was locally detected at ∼15 ng/L and recurred in 8% of our samples. Multiple PFAS that are most likely to originate from aqueous film-forming foams were also reported for the first time in tap water, including X:3 and X:1:2 fluorotelomer betaines, hydroxylated X:2 fluorotelomer sulfonates, N-trimethylammoniopropyl perfluoroalkane sulfonamides (TAmPr-FHxSA and TAmPr-FOSA), and N-sulfopropyl dimethylammoniopropyl perfluoroalkane sulfonamidopropyl sulfonates (N-SPAmP-FPeSAPS and N-SPAmP-FHxSAPS).

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.

Effect of pH, surface charge and soil properties on the solid-solution partitioning of perfluoroalkyl substances (PFASs) in a wide range of temperate soils

The pH-dependent soil-water partitioning of six perfluoroalkyl substances (PFASs) of environmental concern (PFOA, PFDA, PFUnDA, PFHxS, PFOS and FOSA), was investigated for 11 temperate mineral soils and related to soil properties such as organic carbon content (0.2-3%), concentrations of Fe and Al (hydr)oxides, and texture. PFAS sorption was positively related to the perfluorocarbon chain length of the molecule, and inversely related to solution pH for all substances. The negative slope between log K_d and pH became steeper with increasing perfluorocarbon chain length of the PFAS (r² = 0.75, p ≤ 0.05). Organic carbon (OC) alone was a poor predictor of the partitioning for all PFASs, except for FOSA (r² = 0.71), and the OC-normalized PFAS partitioning, as derived from organic soil materials, underestimated PFAS sorption to the soils. Multiple linear regression suggested sorption contributions (p ≤ 0.05) from OC for perfluorooctane sulfonate (PFOS) and FOSA, and Fe/Al (hydr)oxides for PFOS, FOSA, and perfluorodecanoate (PFDA). FOSA was the only substance under study for which there was a statistically significant correlation between its binding and soil texture (silt + clay). To predict PFAS sorption, the surface net charge of the soil organic matter fraction of all soils was calculated using the Stockholm Humic Model. When calibrated against charge-dependent PFAS sorption to a peat (Oe) material, the derived model significantly underestimated the measured K_d values for 10 out of 11 soils. To conclude, additional sorbents, possibly including silicate minerals, contribute to the binding of PFASs in soil. More research is needed to develop geochemical models that can accurately predict PFAS sorption in soils.

Effect of geochemical conditions on PFAS release from AFFF-impacted saturated soil columns

Per- and polyfluoroalkyl substances (PFASs) are frequently found at high concentrations in the subsurface of aqueous film forming foam (AFFF)-impacted sites. Geochemical parameters affect the release of PFASs from source area soils into groundwater but have not been extensively studied for soils that have been historically impacted with AFFF. This study investigated the effects of pH and salt concentrations on release of anionic and zwitterionic PFASs from AFFF-impacted soils in flow-through saturated columns. High pH (10) columns with elevated sodium concentrations had higher cumulative masses eluted of several PFASs compared to pH 3 and pH 7 columns with lower sodium concentrations, likely caused by changes to soil organic matter surface charge. Four PFASs (e.g. 4:2 fluorotelomer sulfonate, perfluorobutane sulfonamido acetic acid) eluted significantly earlier in both pH 3 and pH 10/high NaCl columns compared to pH 7 columns. The results of this study suggest that shifts in pH for soils located at AFFF-impacted sites - particularly raising the pH - may mobilize sorbed PFASs, specifically longer-chain and zwitterionic compounds that are typically strongly sorbed to soil.

Distribution of legacy and novel per- and polyfluoroalkyl substances in surface and groundwater affected by irrigation in an arid region

Frequent exchange of surface water and groundwater occurs in arid/semi-arid areas due to high evaporation and intensive irrigation activities, affecting the migration and transformation of per- and polyfluoroalkyl substances (PFASs) and threatening drinking water safety. This study analyzed legacy PFASs and potential precursors in surface water, groundwater, soil, and aquifer solid samples collected from a typical arid area, the Hetao Irrigation District of Northern China, to explore PFASs distribution and transformation between surface and ground. Total PFASs (ΣPFASs) in surface water was 29-232 ng/L, higher than 2-77 ng/L in groundwater. ΣPFASs in soil were 0.29-0.59 ng/g, higher than 0.09-0.27 in the aquifer solids. Regarding horizontal distribution, the concentration of PFASs in groundwater increased in downtowns and the areas recharged with lake water. In terms of vertical distribution, ΣPFASs decreased with the increase of depth, and more PFASs adsorbed on clay particles in the aquifer. The total oxidable precursor analysis showed that 8:2 FT and 4:2 FT were the dominant precursors of PFASs, resulting in an increment of 0.1-4 ng/L PFASs. Hydrogen and oxygen stable isotope compositions suggest similar sources between surface water and groundwater in the study area, while principal component analysis and Bayesian inference also indicate that surface water is an important source of groundwater PFASs. The annual infiltration PFASs to groundwater from Ulansuhai was estimated by the water balance approach to be 9.39 kg. Results highlight the influence of agricultural irrigation activities and lake infiltration on groundwater PFASs in the arid region.

PFASs in Soil: How They Threaten Human Health through Multiple Pathways and Whether They Are Receiving Adequate Concern

Per- and polyfluoroalkyl substances (PFASs) have been mass-produced and widely applied in consumer and industrial products, resulting in their widespread presence in the environment. Features such as environmental persistence, bioaccumulation, and high toxicity even at low doses have made PFASs an increasing concern. This brief review focuses on soil PFASs, especially the effect of soil PFASs on other environmental media and their potential threats to human health through daily diet. Specifically, soil PFASs contamination caused by different pathways was first investigated. Soil pollution from application of aqueous film-forming foams (AFFFs) is generally more severe than that from fluorochemical manufacturing plants, followed by biosolid land use, landfill, and irrigation. Factors, such as carbon chain length of PFASs, wastewater treatment technology, geographical conditions, and regional development level, are related to soil PFASs' pollution. Then, the migration, bioaccumulation, and toxicity characteristics of soil PFASs were analyzed. Short-chain PFASs have higher solubility, mobility, and bioavailability, while long-chain PFASs have higher bioaccumulation potential and are more toxic to organisms. Factors such as soil texture, solution chemistry conditions, enzymes, and fertilization conditions also influence the environmental behavior of PFASs. The risk of human exposure to PFASs through agricultural and animal products is difficult to control and varies depending on living region, age, eating habits, lifestyle, ethnicity, etc. Soil PFASs threaten drinking water safety, affect soil function, and enter food webs, threatening human health. Knowledge gaps and perspectives in these research fields are also included in current work to assist future research to effectively investigate and understand the environmental risks of soil PFASs, thereby reducing human exposure.

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.

Assessment of PFAS in collocated soil and porewater samples at an AFFF-impacted source zone: Field-scale validation of suction lysimeters

Per- and polyfluoroalkyl substances (PFAS) measurable in soil porewater authoritatively represent the mobile mass fraction critical to accurate assessment of leaching from source zones. This study evaluated PFAS occurrence in lysimeter-collected porewater samples for two depth intervals at a decades-old aqueous film-forming foam (AFFF)-impacted field site quarterly for a year. Notably, site-wide Log₁₀ (∑PFAS) concentrations did not significantly differ among sampling events despite highly variable sample yields due to a heterogeneous and dynamic soil moisture regime. However, Log₁₀ (∑PFAS) concentrations were significantly higher in the shallow interval concordant with higher mean soil concentrations and higher total organic carbon (TOC) reflecting net retention, which is supported by soil-to-groundwater annual mass discharge estimates less than 0.2% of the total source mass for any given PFAS. Interestingly, PFAS-specific Log₁₀ (soil-to-porewater ratios) significantly increased with soil concentration in both depth intervals contrary to concentration dependence resulting from the saturation of sorption sites potentially implicating self-assembly as an additional operative retention mechanism. Overall, these data validate the use of suction lysimeters for short-term site characterization deployments and emphasize the importance of in situ porewater samples for interrogating PFAS transport within source zones.

Effects of mineral species and petroleum components on thermal desorption behavior of petroleum-contaminated soil

Thermal desorption (TD) behavior of high-concentration petroleum-contaminated soil (PCS) is affected by soil composition, especially inorganic minerals. In this study, the TD behavior of petroleum-contaminated quartz (original mineral) and kaoline (clay mineral) were compared with those of pure petroleum (Petro-free); their "saturate, aromatic, resin, and asphaltenes" (SARA) fractions were investigated using thermogravimetry and differential thermogravimetry (TG-DTG). The modelling of the petroleum removal kinetics was also analyzed to provide insights into the mechanism. The results revealed that the limiting factor controlling the desorption of petroleum from quartz (Petro-Qtz) and kaoline (Petro-Kln) is the minerals, which increased the effective TD temperature by 200 °C and decreased TD efficiency by 2%. Compared to Petro-Qtz, Petro-Kln showed a lower desorption efficiency of 5% and the process was accomplished at a higher temperature of 100 °C. The investigation on SARA fractions indicated that polar fractions (i.e., aromatics, resins, and asphaltenes) were strongly captured by the minerals. The increment of the TD temperature of petroleum (resins-160 °C > aromatics-20 °C > saturates-5 °C) increased with the polarity of petroleum components. These results could be validated by thermogravimetry-gas chromatography/mass spectroscopy (TG-GC/MS) through the delayed desorption of naphthalene and acenaphthene. Furthermore, the increment of the TD temperature of SARA fractions on kaoline was higher than those on quartz. This makes sense because the kaoline decreased the diffusion of hydrocarbons due to its porosity features and higher specific surface area (kaoline 5.3300 m² g^-1, quartz 0.1153 m² g^-1). In addition, the analysis of the desorption kinetic models showed that the observed hysteresis was related to the diffusion barrier caused by chemisorption (n＜1). In consequence, the Petro-Kln showed a lower desorption efficiency, a slower desorption, and as a result, a higher energy consumption (0.476 kW h) for thermal remediation than Petro-Qtz (0.238 kW h).

Structure and core taxa of bacterial communities involved in extracellular electron transfer in paddy soils across China

Microbial communities with extracellular electron transfer (EET) activity are capable of driving geochemical changes and cycles, but a comprehensive understanding of the key microbiota responsible for EET in complex soil matrices is still lacking. Herein, the EET activities, in terms of maximum current density (j_max) and accumulated charge output (C_out), of 41 paddy soils across China were evaluated from the exoelectrogenic properties with a conventional bioelectrochemical system (BES). The j_max with a range of 8.85 × 10^-4 to 0.41 A/m² and C_out with a range of 0.27 to 172.21C were obtained from these soil-based BESs. The bacterial community analyses revealed that the most abundant phylum, order, and genus were Firmicutes, Clostridiales, and Clostridum-sensus-stricto 10, respectively. Bacterial network analysis displayed the positive correlations between the majority of electroactive bacteria-containing genera and multiple other genera, indicating their underlying cooperation for the EET. Partial least squares regression (PLSR) model showed remarkable performance in describing the EET activity with 75 most abundant genera as input variables, identified that 32 genera were very important for governing the EET activities. Multiple linear regression (MLR) analyses further prioritized that the genera norank-c-Berkelbacteria and Fonticella were the key contributors, while the genus Paenibacillus was the key competitor against bacterial exoelectrogenesis in paddy soils. Moreover, the spearman analysis showed that the abundance of these keystone taxa was mainly influenced by the carbon content and pH. This approach provides a promising avenue to monitor the microbial activities in paddy soils as well as the links between microbial community composition and ecological function.

Occurrence, distribution, and input pathways of per- and polyfluoroalkyl substances in soils near different sources in Shanghai

Per- and polyfluoroalkyl substances (PFAS) are complex emerging pollutants that are widely distributed in soils. The compositions of PFAS vary according to the emission sources. However, the soil distributions of PFAS from different sources are still poorly understood. In this study, the concentrations and compositions of 18 PFAS in soils close to potential sources (industrial areas, airports, landfills, fire stations and agricultural areas) were investigated in Shanghai. The total PFAS concentrations varied from 0.64 to 294 μg kg^-1_d.w.. Among the sites, the highest PFAS concentration was found near the fire station (average = 57.9 μg kg^-1_d.w.), followed by the industrial area (average = 8.53 μg kg^-1_d.w.). The detection frequencies of the 18 PFAS ranged from 47.5% to 100%. Perfluorooctanoic acid (PFOA) and perfluoroheptanoic acid (PFHpA) were detected in all samples. The detection frequencies of PFAS near the fire station were higher than those near other sources. The PFAS in soils were mainly composed of short-chain perfluoroalkyl carboxylic acids (C ≤ 8). Elevated concentrations of long-chain perfluoroalkyl carboxylic acids (C > 12) were found in industrial area. Principal component analysis revealed that long-chain PFAS had different factor loadings compared to short-chain PFAS. With the exception of agricultural soils, the correlations between individual PFAS were more positive than negative. Strong positive correlations were found within three groups of perfluoroalkyl carboxylic acids (C5-C7, C9-C12, and C14-C18), suggesting their similar inputs and transportation pathways. The PFAS in soils around the fire station were likely directly emitted from a point source. In contrast, the PFAS in soils near the other sites had multiple input pathways, including both direct emission and precursor degradation.

Assessment of perfluorohexane sulfonic acid (PFHxS)-related compounds degradation potential: Computational and experimental approaches

Perfluorohexane sulfonic acid (PFHxS) and PFHxS-related compounds are listed in Annex A of the Stockholm Convention without specific exemptions. Substances that potentially degrade to PFHxS are considered as their related compounds. Unfortunately, the degradation behavior of PFHxS precursors, an important basis for the corresponding chemical regulation, remains unclear. Herein, based on the hypothesis that bond dissociation enthalpy (BDE) is the determining factor for the degradation of PFHxS precursors, the BDE of PFHxS-related precursors to produceC6F13SO2-groups was calculated. In addition, quantitative structure-activity relationship models based on partial least squares, partial least squares discrimination analysis, and support vector machine algorithms were developed to predict the BDE of 48 PFHxS precursors and distinguish the precursors with different degradation potential. Subsequent photodegradation experiments demonstrated that the order of degradation rates was consistent with that predicted by theoretical models. Importantly, perfluorohexanoic acid (PFHxA) and perfluorobutanoic acid, and not PFHxS, were detected as the degradation products of potential PFHxS precursors. Sulfonamides, phenyl unit, and other radicals in the non-nucleus part of PFHxS precursors were identified as the critical molecular segments that affect their degradation potential. Ultimately, by comparing BDE values, it was theoretically speculated that PFHxS related compounds exhibit a greater potential to generate PFHxA than PFHxS. Results in this study indicated for the first time that not all the compounds containing C6F13SO2- groups were guaranteed to degrade into PFHxS under natural conditions.

Removal of perfluoroalkyl acids and dynamic succession of biofilm microbial communities in the decomposition process of emergent macrophytes in wetlands

Perfluoroalkyl acids (PFAAs) are emerging contaminants that pose significant environmental and health concerns. Water-sediment-macrophyte residue systems were established to clarify the removal efficiency of PFAAs, explore possible removal pathways, and profile the dynamic succession of biofilm microbial communities in the decomposition process. These systems were fortified with 12 PFAAs at three concentration levels. Iris pseudacorus and Alisma orientale were selected as the decomposing emergent macrophytes. The removal rates in the treatments with residues of I. pseudacorus (IP) and A. orientale (AO) were 34.4% to 88.9% and 36.5% to 89.9%, respectively, which were higher than those in the control groups (CG) (30.3% to 86.9%), suggesting that decomposition could alter the removal of PFAAs. Sediment made the greatest contributions (preloaded 14.5% to 77.8% of PFAAs in IP, 14.3% to 78.2% in AO, and 27.4% to 71.9% in CG). PFAAs could also be removed by macrophyte residue sorption (0.0190% to 13.0% in IP and 0.016% to 15.6% in AO) and bioaccumulation of residual biofilm (the contributions of biofilm microbes and their extracellular polymeric substances were 0.0110% to 3.93% and 0.918% to 34.4%, respectively, in IP and 0.0141% to 4.65% and 1.49% to 34.1%, respectively, in AO). Significant correlations were observed between sediment/residue adsorption and bioaccumulation of biofilm microbes, and were significantly correlated with perfluoroalkyl chain length (p < 0.05). The dynamic succession of residual biofilm microbial communities was investigated. The largest difference was found at the preliminary stage. The most similar communities were found in AO on day 70 (with specific genera Macellibacteroides and WCHB1-32) and in IP on day 35 (with specific genera Aeromonas and Flavobacterium). This study is useful to understand the removal of PFAAs during the decomposition process, providing further assistance in removing PFAAs during the life cycle of macrophytes in wetlands.

Properties and Mechanisms for PFAS Adsorption to Aqueous Clay and Humic Soil Components

The proliferation of poly- and perfluorinated alkyl substances (PFASs) has resulted in global concerns over contamination and bioaccumulation. PFAS compounds tend to remain in the environment indefinitely, and research is needed to elucidate the ultimate fate of these molecules. We have investigated the model humic substance and model clay surfaces as a potential environmental sink for the adsorption and retention of three representative PFAS molecules with varying chain length and head groups. Utilizing molecular dynamics simulation, we quantify the ability of pyrophyllite and the humic substance to favorably adsorb these PFAS molecules from aqueous solution. We have observed that the hydrophobic nature of the pyrophyllite surface makes the material well suited for the sorption of medium- and long-tail PFAS moieties. Similarly, we find a preference for the formation of a monolayer on the surface for long-chain PFAS molecules at high concentration. Furthermore, we discussed trends in the adsorption mechanisms for the fate and transport of these compounds, as well as potential approaches for their environmental remediation.

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.

Sorption of microcystin-RR onto surface soils: Characteristics and influencing factors

Microcystins are frequently detected in cyanobacterial bloom-impacted sites; however, their mobility potential in soils is poorly understood. This study aimed to elucidate the sorption behaviors of microcystin-RR (MC-RR) in heterogeneous soils and evaluate critical affecting factors. MC-RR sorption followed the pseudo-second-order kinetics and Freundlich model. All isotherms (n = 0.83-1.03) had no or minor deviations from linearity. The linear distribution coefficients (K_d) varied from 2.64 to 15.2 across soils, depending mainly on OM and CEC. Stepwise regression analysis indicated that the K_d was predictable by the fitting formula of: K_d = 2.56 + 0.15OM + 0.28CEC (R² = 0.45). The sorption was an endothermic physisorption process, involving electrostatic forces, cation exchange and bridging, H-bonding, ligand exchange, and van der Waals forces. The sorption of MC-RR (dominantly behaved as electroneutral zwitterions) at pH > 5 was insensitive to pH change, while more MC-RR (anionic species) was adsorbed at lower pH and in the presence of Ca²⁺. The study provides insights into the sorption of MC-RR across a range of soil properties and water chemistry for the first time, which is of importance for a better understanding of the mobility potential of microcystins in the terrestrial systems.

Binding of per- and polyfluoroalkyl substances (PFASs) by organic soil materials with different structural composition - Charge- and concentration-dependent sorption behavior

The charge- and concentration-dependent sorption behavior of a range of per- and polyfluoroalkyl substances (PFASs) was studied for three organic soil samples with different organic matter quality, one Spodosol Oe horizon (Mor Oe) and two Sphagnum peats with different degrees of decomposition (Peat Oi and Peat Oe). Sorption to the two peat materials was, on average, four times stronger compared to that onto the Mor Oe material. In particular, longer-chained PFASs were more strongly bound by the two peats as compared to the Mor Oe sample. The combined results of batch sorption experiments and ¹³C NMR spectroscopy suggested sorption to be positively related to the content of carbohydrates (i.e., O-alkyl carbon). Sorption of all PFAS subclasses was inversely related to the pH value in all soils, with the largest pH effects being observed for perfluoroalkyl carboxylates (PFCAs) with C₁₀ and C₁₁ perfluorocarbon chain lengths. Experimentally determined sorption isotherms onto the poorly humified Peat Oi did not deviate significantly from linearity for most substances, while for the Mor Oe horizon, sorption nonlinearity was generally more pronounced. This work should prove useful in assessing PFAS sorption and leaching in organic soil horizons within environmental risk assessment.

Developing innovative treatment technologies for PFAS-containing wastes

The release of persistent per- and polyfluoroalkyl substances (PFAS) into the environment is a major concern for the United States Environmental Protection Agency (U.S. EPA). To complement its ongoing research efforts addressing PFAS contamination, the U.S. EPA's Office of Research and Development (ORD) commissioned the PFAS Innovative Treatment Team (PITT) to provide new perspectives on treatment and disposal of high priority PFAS-containing wastes. During its six-month tenure, the team was charged with identifying and developing promising solutions to destroy PFAS. The PITT examined emerging technologies for PFAS waste treatment and selected four technologies for further investigation. These technologies included mechanochemical treatment, electrochemical oxidation, gasification and pyrolysis, and supercritical water oxidation. This paper highlights these four technologies and discusses their prospects and the development needed before potentially becoming available solutions to address PFAS-contaminated waste.Implications: This paper examines four novel, non-combustion technologies or applications for the treatment of persistent per- and polyfluoroalkyl substances (PFAS) wastes. These technologies are introduced to the reader along with their current state of development and areas for further development. This information will be useful for developers, policy makers, and facility managers that are facing increasing issues with disposal of PFAS wastes.

Insights into the effects of salinity on the sorption and desorption of legacy and emerging per-and polyfluoroalkyl substances (PFASs) on marine sediments

Per-and polyfluoroalkyl substances (PFASs) have attracted extensive attention since this century due to their wide distribution, persistence, bioaccumulation/biomagnification potential, and (eco)toxicity. In the present study, we investigated the sorption kinetics, sorption isotherms and desorption behaviors of legacy and emerging PFASs with different chain lengths and functional end groups onto marine sediments at four different salinities (0, 10, 20, and 30 practical salinity units (psu)). Results revealed that the sorption of PFASs onto sediment can be well described by the pseudo-second-order kinetic model. PFASs sorption was influenced by both compound-specific and solution-specific parameters. The distribution coefficient (K_d) for PFASs were increased with the increase of perfluorocarbon chain length and salinity, suggesting that hydrophobic and electrostatic interactions were involved in the adsorption process. 6:2 FTSA showed the lowest adsorption among PFASs with eight carbon atoms (6:2 FTSA, PFOA and PFOS). The increase of perfluorocarbon chain length of PFASs and salinity would result in the decrease of desorption rate of PFASs from sediment. In addition, PFCAs were desorbed more easily from the sediment than the PFSAs with the same perfluorocarbon chain length at all salinity groups. The present study demonstrated that salinity can apparently influence the fate of PFASs in aquatic environment and provided valuable data for modeling the fate of PFASs in real environment.

Increasing ionic strength and valency of cations enhance sorption through hydrophobic interactions of PFAS with soil surfaces

The effect of soluble cations on sorption in soils of a range of anionic PFAS is not well studied. We investigated the role of three common cations (Na⁺, Ca²⁺, and Mg²⁺) at varying solution concentrations on the sorption coefficients (K_d) of 18 anionic PFAS in two contrasting soils. The effective charge of the soil suspension (Zeta potential) became less negative as the concentration of these cations increased in the soil solutions. Perfluorinated compounds showed greater sorption than polyfluorinated compounds, with sulfonates of comparable chain lengths showing higher sorption than the carboxylates. We observed that the K_d values of several PFAS in the two soils were positively correlated with the concentration of cations in solution, especially in the presence of polyvalent cations (Ca²⁺and Mg²⁺). The changes in sorption with cation concentration were more prominent for long-chain PFAS, with C > 10 PFAS being completely removed from solution at higher cation concentrations. The emerging PFAS (replacement compounds GenX and ADONA) showed negligible or little sorption (K_d < 0.6 L/kg). While several mechanisms contribute towards sorption of PFAS in the presence of cations, we conclude that the primary effect of cations is through screening of negative charges on head groups of PFAS and reorientation of molecules at the interface between organic matter surfaces and soil solution as well as charge neutralisation at soil solid surface. Screening of negative charges allows for greater hydrophobic interaction between hydrophobic tails of PFAS and soil surfaces resulting in greater sorption. Increasing cation concentrations in soil solutions could thus reduce mobility of PFAS through a soil profile.

Legacy and Emerging Per- and Polyfluoroalkyl Substances Behave Distinctly in Spatial Distribution and Multimedia Partitioning: A Case Study in the Pearl River, China

Per- and polyfluoroalkyl substances (PFASs) have attracted worldwide attention due to their ubiquitous occurrence, bioaccumulation, and toxicological effects, yet the fate of PFASs in a lotic ecosystem is largely unknown. To elucidate spatial distribution and multimedia partitioning of legacy and emerging PFASs in a lotic river flowing into an estuary, PFASs were synchronously analyzed in water, suspended particulate matter (SPM), sediment, and biota samples collected along Guangzhou reach of the Pearl River, South China. Geographically, the concentrations of PFASs in the water phase showed a decreasing trend from the upper and middle sections (urban area) to the down section (suburban area close to estuary) of the river. While perfluorooctanoic acid predominated in water and SPM, more diverse compositions were observed in sediment and biota with the increase in contributions of long-chain PFASs. Field-derived sediment-water partitioning coefficients (K_d) and bioaccumulation factors (BAFs) of PFASs increased with the increase in perfluorinated carbons. Besides hydrophobicity, water pH and salinity significantly affected the multimedia partitioning of PFASs in a lotic ecosystem. In addition, 87 homologues (63 classes) were identified as emerging PFASs in four media using suspect analysis. Interestingly, K_d and BAF of the emerging PFASs were often higher than legacy PFASs containing the same perfluorinated carbons, raising a special concern on the environmental risk of emerging PFASs.

Reduced bioaccumulation of fluorotelomer sulfonates and perfluoroalkyl acids in earthworms (Eisenia fetida) from soils amended with modified clays

Soils contaminated by per- and polyfluoroalkyl substances (PFAS) pose long-term sources to adjacent water bodies and soil invertebrates. The study investigated the stabilization using a modified clay adsorbent (FLUORO-SORB100®) in reducing the bioaccumulation of 13 anionic PFAS by earthworms (Eisenia fetida), as compared to coal-based granular activated carbon. The target PFAS included four perfluoroalkyl sulfonates such as perfluorooctane sulfonate (PFOS), six perfluoroalkyl carboxylates (e.g., perfluorooctanoate PFOA), and three (X:2) fluorotelomer sulfonates. Laboratory-spiked surface soil and the soil collected from a site contaminated by aqueous film-forming foams were examined. Both adsorbents resulted in reduced earthworm PFAS body burdens at the end of the 28-day uptake phase. The highest adsorbent amendment concentration (4 w/w%) was most effective, achieving >95% reduction of PFAS body burden. Soil leaching tests indicated better immobilization performance by the clay adsorbent for most analytes; in comparison, the activated carbon performed better at reducing total PFAS body burdens, possibly owing to the avoidance of larger-sized particles by earthworms. Strong positive logarithm relationships were observed between leachate concentrations and earthworm body burdens for most PFAS in the spiked soil. The study demonstrated that stabilization of PFAS using modified clay adsorbents can achieve concurrent benefits of lowering leachability and reducing bioaccumulation.

Per- and Polyfluoroalkyl Substances in Contaminated Soil and Groundwater at Airports: A Canadian Case Study

The occurrence of 93 classes of per- and polyfluoroalkyl substances (PFASs) was investigated at aqueous film-forming foam (AFFF)-impacted sites of four Canadian airports. Surface/subsurface soil and groundwater samples were characterized using high-resolution mass spectrometry (HRMS) and an improved total oxidizable precursor (TOP) assay. PFAS profiles, loads, and spatial trends were highly site-specific, influenced by the AFFF use history, variations in sorption, transport, and in situ transformation potential of PFASs. All sites have been impacted by more than one AFFF chemistry, with the active firefighter training area exhibiting a greater PFAS variety and total PFAS burden than decommissioned sites. Zwitterionic and cationic compounds composed a large percentage (34.5-85.5%) of the total PFAS mass in most surface soil samples in the source zone but a relatively low percentage (<20%) in groundwater samples. Background soils surrounding the source zone contained predominantly unidentified precursors attributed to atmospheric deposition, while in AFFF-impacted soils, precursors originating from AFFFs can be largely captured by HRMS using available suspect lists. Horizontal transfer of PFASs in surface soils was limited, but vertical migration down the soil column occurred even in locations of low permeability. This study provides a critical data set to support developing new priority analyte lists and integrating TOP assay for comprehensive PFAS monitoring at AFFF-impacted sites.

Modelling the sorption behaviour of perfluoroalkyl carboxylates and perfluoroalkane sulfonates in soils

A simple parametric model was developed to predict the sorption of perfluoroalkyl substances (PFASs) in soils. Initially, sorption and desorption solid-liquid distribution coefficients (K_d and K_d,des respectively) of eight PFASs (five perfluoroalkyl carboxylates, PFCAs, and three perfluoroalkane sulfonates, PFSAs) in seven soils with organic carbon (OC) content ranging from 1.6 to 41% were quantified using batch experiments. The information obtained helped to fill the gaps in a literature-based database of K_d values of PFASs, which was lacking data on soils with high OC content. The overall dataset finally comprised 435 entries. Normalized sorption coefficients for the soil OC and mineral fraction contents (K_OC and K_MIN respectively) were deduced for each PFAS by correlating the corresponding K_d values obtained under a wide range of experimental conditions with the fraction of organic carbon (f_OC) of the soils. Furthermore, the sorption mechanisms in each phase were shown to depend mainly on PFAS chain length. The dependence of K_OC and K_MIN values on PFAS chain length defined the basic equations to construct the model for predicting PFAS sorption, applicable to both PFCAs and PFSAs with chain lengths ranging from 3 to 11 fluorinated carbons. The validation of the proposed model confirmed its ability to predict the K_d of PFASs based only on the soil OC and silt+clay contents and PFAS chain length. Therefore, it can be used in the first stages of a risk assessment process aiming at estimating the potential mobility of PFASs in soils after a contamination event. SYNOPSIS: This study develops a new parametric model to predict the sorption of perfluoroalkyl substances (PFASs) in soils.

Release of Per- and Polyfluoroalkyl Substances from Aqueous Film-Forming Foam Impacted Soils

Per- and polyfluoroalkyl substances (PFASs) are highly mobile in the saturated subsurface, yet aqueous film-forming foam (AFFF)-impacted source zones appear to be long lasting PFAS reservoirs. This study examined the release of over one hundred anionic and zwitterionic PFASs from two AFFF-impacted surface soils under saturated conditions with packed soil columns. Perfluoroalkyl acids (PFAAs) were released more rapidly than their polyfluorinated precursors, while anionic PFASs that were present in partially uncharged states were released more slowly than PFASs that were present entirely as anions, as were zwitterionic PFASs with terminal cationic functional groups when compared with analogous zwitterions with only anionic terminal groups. Nonideal transport was observed in both per- and polyfluorinated classes, as soil column effluent concentrations of slowly released PFASs increased by up to 107-fold with sustained artificial groundwater flow. A flow-interruption experiment suggested the influence of rate-limited desorption on diverse PFAS classes, including PFAAs with as few as four perfluorinated carbons. These results suggest that during infiltration the slow, rate-limited desorption of anionic and zwitterionic PFAA precursors may result in these compounds comprising an increasingly large fraction of the remaining PFASs in AFFF-impacted surface soils.

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.

Limitations of Current Approaches for Predicting Groundwater Vulnerability from PFAS Contamination in the Vadose Zone

Published literature for reported sorption coefficients (K_d) of eight anionic per- and polyfluoroalkyl substances (PFAS) in soil was reviewed. K_d values spanned three to five log units indicating that no single value would be appropriate for use in estimating PFAS impacts to groundwater using existing soil-water partition equations. Regression analysis was used to determine if the soil or solution parameters might be used to predict K_d values. None of the 15 experimental parameters collected could individually explain variability in reported K_d values. Significant associations between K_d and soil calcium and sodium content were found for many of the selected PFAS, suggesting that soil cation content may be critical to PFAS sorption, as previously noted in sources like Higgins and Luthy (2006), while organic carbon content was significant only at elevated levels (>5%). Unexplained discrepancies between the results from studies where PFAS were introduced to soil and desorbed in the laboratory and those that used material from PFAS-impacted sites suggest that laboratory experiments may be overlooking some aspects critical to PFAS sorption. Future studies would benefit from the development and use of standardized analytical methods to improve data quality and the establishment of soil parameters appropriate for collection to produce more complete data sets for predictive analysis.

Perfluoroalkyl acids on suspended particles: Significant transport pathways in surface runoff, surface waters, and subsurface soils

Eroded particles from the source zone could transport a high concentration of perfluoroalkyl acids (PFAAs) to sediments and water bodies. Yet, the contribution of suspended particles has not been systematically reviewed. Analyzing reported studies, we quantitatively demonstrate that suspended particles in surface water can contain significantly higher concentrations of PFAAs than the sediment below, indicating the source of suspended particles are not the sediment but particles eroded and carried from the source zone upstream. The affinity of PFAAs to particles depends on the particle composition, including organic carbon fraction and iron or aluminum oxide content. In soils, most PFAAs are retained within the top 5 m below the ground surface. The distribution of PFAAs in the subsurface varies based on site properties and local weather conditions. The depth corresponding to the maximum concentration of PFAA in soil decreases with an increase in soil organic carbon or rainfall amount received in the catchment areas. We attribute a greater accumulation of PFAAs near the upper layer of the subsurface to an increase in the accumulation of particles eroded from source zones upstream receiving heavy rainfall. Precursor transformation in the aerobic zone is significantly higher than in the anaerobic zone, thereby making the aerobic subsurface zone serve as a long-term source of groundwater pollution. Collectively, these results suggest that suspended particles, often an overlooked vector for PFAAs, can be a dominant pathway for the transport of PFAAs in environments.

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.

The role of soils in the disposition, sequestration and decontamination of environmental contaminants

Soil serves as both a 'source' and 'sink' for contaminants. As a source, contaminants are derived from both 'geogenic' and 'anthropogenic' origins. Typically, while some of the inorganic contaminants including potentially toxic elements are derived from geogenic origin (e.g. arsenic and selenium) through weathering of parent materials, the majority of organic (e.g. pesticides and microplastics) as well as inorganic (e.g. lead, cadmium) contaminants are derived from anthropogenic origin. As a sink, soil plays a critical role in the transformation of these contaminants and their subsequent transfer to environmental compartments, including groundwater (e.g. pesticides), surface water (phosphate and nitrate), ocean (e.g. microplastics) and atmosphere (e.g. nitrous oxide emission). A complex transformation process of contaminants in soil involving adsorption, precipitation, redox reactions and biodegradation control the mobility, bioavailability and environmental toxicity of these contaminants. Soil also plays a major role in the decontamination of contaminants, and the 'cleaning' action of soil is controlled primarily by the physico-chemical interactions of contaminants with various soil components, and the biochemical transformations facilitated by soil microorganisms. In this article, we examine the geogenic and anthropogenic sources of contaminants reaching the soil, and discuss the role of soil in the sequestration and decontamination of contaminants in relation to various physico-chemical and microbial transformation reactions of contaminants with various soil components. Finally, we propose future actions that would help to maintain the role of soils in protecting the environment from contaminants and delivering sustainable development goals. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

Enhanced Recovery of Per- and Polyfluoroalkyl Substances (PFASs) from Impacted Soils Using Heat Activated Persulfate

Varying transport potential of cationic, zwitterionic, and anionic per- and polyfluoroalkyl substances (PFASs) may pose challenges for remediation of aqueous film forming foam (AFFF) impacted sites, particularly during groundwater extraction. Slow desorption of stronger sorbing, zwitterionic, and cationic PFASs may cause extended remediation times and rebound in aqueous PFAS concentrations. Persulfate oxidation has the potential to convert a complex mixture of PFASs into a simpler and more recoverable mixture of perfluoroalkyl acids (PFAAs). AFFF-impacted soils were treated with heat-activated persulfate in batch reactors and subjected to 7-day leaching experiments. Soil and water were analyzed using a combination of targeted and high resolution liquid chromatography mass spectrometry techniques as well as the total oxidizable precursors assay. Following oxidation, total PFAS composition showed the expected shift to a higher fraction of PFAAs, and this led to higher total PFAS leaching in pretreated reactors (108-110%) vs control reactors (62-90%). In both pretreated and control soils, precursors that remained following leaching experiments were 61-100% cationic and zwitterionic. Results suggest that persulfate pretreatment of soils has promise as an enhanced recovery technique for remediation of total PFASs in impacted soils. They also demonstrate that PFAS distribution may have been altered at sites where in situ chemical oxidation was applied to treat co-occurring contaminants of concern.

Occurrence, source apportionment, plant bioaccumulation and human exposure of legacy and emerging per- and polyfluoroalkyl substances in soil and plant leaves near a landfill in China

In this study, 17 legacy and emerging PFASs were investigated in soil and plant leaves near a valley-type landfill, which has been in operation for over 20 years. ΣPFASs concentrations ranged from 5.31 to 108 ng/g dw and 11.9 to 115 ng/g dw in the soil and leaf samples, respectively, and perfluorobutanoic acid (PFBA) was dominant in both soil and leaves. The concentrations of hexafluoropropylene oxide dimer acid (HFPO-DA), 6:2 chlorinated polyfluorinated ether sulfonic acid (F-53B) and 6:2 fluorotelomer sulfonic acid (6:2 FTS) were significantly higher than those of legacy PFOA and PFOS, indicating emerging alternatives were widely applied in the region. The integrated approach of PCA analysis, field investigation of relevant industrial activities in the study area, along with the Unmix model analysis quantitatively revealed that factories producing consumer products and the landfill were the major sources of PFASs in soil, accounting for 57% of total PFASs detected. Bioaccumulation factors (BAFs) of ΣPFASs in leaves varied from 0.37 to 8.59, and higher BAFs were found in camphor leaves. The log₁₀BAFs in all plant leaves showed a linear decrease with increasing carbon chain lengths for individual PFCAs (C4-C8). The BAF values of HFPO-DA, F-53B and 6:2 FTS were 0.01-3.39, 0.04-6.15 and 0.01-6.33, respectively. The human health risk assessment of EDIs showed a decreasing trend with the increasing carbon chain lengths of PFCAs (C4-C9), and the PFASs EDI indicated further study on the human health risk via vegetable consumption be warranted.

Fate and transport of per- and polyfluoroalkyl substances (PFASs) in the vadose zone

Per- and polyfluoroalkyl substances (PFASs) are a heterogeneous group of persistent organic pollutants that have been detected in various environmental compartments around the globe. Emerging research has revealed the preferential accumulation of PFASs in shallow soil horizons, particularly at sites impacted by firefighting activities, agricultural applications, and atmospheric deposition. Once in the vadose zone, PFASs can sorb to soil, accumulate at interfaces, become volatilized, be taken up in biota, or leach to the underlying aquifer. At the same time, polyfluorinated precursor species may transform into highly recalcitrant perfluoroalkyl acids, changing their chemical identity and thus transport behavior along the way. In this review, we critically discuss the current state of the knowledge and aim to interconnect the complex processes that control the fate and transport of PFASs in the vadose zone. Furthermore, we identify key challenges and future research needs. Consequently, this review may serve as an interdisciplinary guide for the risk assessment and management of PFAS-contaminated sites.