Transport and retention of perfluorooctanoic acid (PFOA) in natural soils: Importance of soil organic matter and mineral contents, and solution ionic strength

Transport of polystyrene microplastics in bare and iron oxide-coated quartz sand: Effects of ionic strength, humic acid, and co-existing graphene oxide

This research explored the effects of widely utilized nanomaterial graphene oxide (GO) and organic matter humic acid (HA) on the transport of microplastics under different ionic solution strengths in bare sand and iron oxide-coated sand. The results found transport of polystyrene microplastics (PS) did not respond to the presence of HA in sand that contains large amounts of iron oxide. Compared to bare quartz sand, ionic strength had little effect: <20 % of PS passed through Fe sand columns. There was a significant promotion of PS transport in the presence of GO, however, which can be attributed to the increased surface electronegativity of PS and steric hindrance. Moreover, GO combined with HA significantly promoted the transport of PS in the Fe sand, and transport further increased when the concentration of HA increased from 5 to 10 mg/L. Interestingly, the degree of this increase exactly corresponded to the change in the surface charge of the microplastics, demonstrating that electrostatic interaction dominated the PS transport. Further results indicated that co-existing pollutants had significant impacts on the transport of microplastics under various conditions by altering the surface characteristics of the plastic particles and the spatial steric hindrance within porous media. This research will offer insights into predicting the transport and fate of microplastics in complex environments.

Three-year field study on the temporal response of soil microbial communities and functions to PFOA exposure

Contamination of per- and polyfluoroalkyl substances (PFAS) poses a significant threat to soil ecosystem health, yet there remains a lack of understanding regarding the responses of soil microbial communities to prolonged PFAS exposure in field conditions. This study involved a three-year field investigation to track changes in microbial communities and functions in soil subjected to the contamination of a primary PFAS, perfluorooctanoic acid (PFOA). Results showed that PFOA exposure altered soil bacterial and fungal communities in terms of diversity, composition, and structure. Notably, certain bacterial communities with a delayed reaction to PFOA contamination showed the most significant response after one year of exposure. Fungal communities were sensitive to PFOA in soil, exhibiting significant responses within just four months of exposure. After two years, the impact of PFOA on both bacterial and fungal communities was lessened, likely due to the long-term adaptation of microbial communities to PFOA. Moreover, PFOA exposure notably inhibited alkaline phosphatase activity and reduced certain phosphorus cycling-related functional genes after three years of exposure, suggesting potential disruptions in soil fertility. These new insights advance our understanding of the long-term effects of PFOA on soil microbial communities and functions at a field scale.

PFOA-contaminated soil remediation: a comprehensive review

Soil and groundwater contamination has been raised as a concern due to the capability of posing a risk to human health and ecology, especially in facing highly toxic and emerging pollutants. Because of the prevalent usage of perfluorooctanoic acid (PFOA), in industrial and production processes, and subsequently the extent of sites contaminated with these pollutants, cleaning up PFOA polluted sites is paramount. This research provides a review of remediation approaches that have been used, and nine remediation techniques were reviewed under physical, chemical, and biological approaches categorization. As the pollutant specifications, environmental implications, and adverse ecological effects of remediation procedures should be considered in the analysis and evaluation of remediation approaches, unlike previous research that considered a couple of PFAS pollutants and generally dealt with technical issues, in this study, the benefits, drawbacks, and possible environmental and ecological adverse effects of PFOA-contaminated site remediation also were discussed. In the end, in addition to providing sufficient and applicable understanding by comprehensively considering all aspects and field-scale challenges and obstacles, knowledge gaps have been found and discussed.

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.

Full-chain analysis on emerging contaminants in soil: Source, migration and remediation

Emerging contaminants (ECs) are gaining attention due to their prevalence and potential negative impacts on the environment and human health. This paper provides a comprehensive review of the status and trends of soil pollution caused by ECs, focusing on their sources, migration pathways, and environmental implications. Significant ECs, including plastics, synthetic polymers, pharmaceuticals, personal care products, plasticizers, and flame retardants, are identified due to their widespread use and toxicity. Their presence in soil is attributed to agricultural activities, urban waste, and wastewater irrigation. The review explores both horizontal and vertical migration pathways, with factors such as soil type, organic matter content, and moisture levels influencing their distribution. Understanding the behavior of ECs in soil is critical to mitigating their long-term risks and developing effective soil remediation strategies. The paper also examines the advantages and disadvantages of in situ and ex situ treatment approaches for ECs, highlighting optimal physical, chemical, and biological treatment conditions. These findings provide a fundamental basis for addressing the challenges and governance of soil pollution induced by ECs.

Transport behavior of microplastics in soil‒water environments and its dependence on soil components

Microplastic (MP) pollution has become a global concern, and the transport behavior of MPs in soil-water systems is vital in determining their distribution and potential risks to the subsurface environment. To reveal the role of various soil components on MP migration, the downward transport behavior of polystyrene (PS) MPs were explored in this study via column experiments with mono or multi-soil components as porous media. Compared with the selected soil mineral volcanic rock (VR) and fine river sand (RS), condensed soil organic matter (SOM) resulted in higher transport efficiencies for PS microparticles, with greater than 90% total mass recovery under the experimental conditions. The more surface charges of SOM than minerals contribute to the high migration efficiency of PS MPs, and electrostatic repulsion is assumed a significant driving mechanism in the migration of negatively charged PS particles in soils. The ionic strength of porewater influenced the PS migration behaviors by altering the electrostatic interactions between the MPs and soil grains. The uniform mixing of SOM with mineral grains significantly enhanced the transport efficiency of PS MPs in the columns. The results provide supports for the prediction and prevention of the risks of MPs to the subsurface environment.

Pore size distribution and Al oxide content significantly regulated the effects of humic acid on perfluorooctanoic acid transport in natural soils

The ubiquity of dissolved organic matter (DOM) makes it encounter the released perfluorooctanoic acid (PFOA) in subsurface environment. However, the effect of DOM (e.g., humic acid, HA) on PFOA transport in soils and the critical influencing factors and mechanisms remain obscure. Column experiments were conducted to explore PFOA transport with the presence of different concentrations of HA in three types of soils and two types of Al oxide coated sand. Results revealed soil properties significantly regulate the effects of HA on PFOA transport, for which pore size distribution, minerals content (e.g., Al oxide) and pH were critical influencing soil-properties. For soil with large mesopore volume, pore blockage caused by HA controlled the effect of HA on PFOA transport. Large mesopore volume significantly alleviated pore blockage of HA, and led to insignificant effects of HA on PFOA transport. For soil exhibited minimum mesopore volume, Al oxide content and pH dominated the effect of HA on PFOA transport. Results from Al oxide coated sand (low mesopore volume) columns further proved that higher Al oxide content and lower pH caused more significant facilitating effect of HA on PFOA transport via site competition. Results highlighted the importance of considering pore size distribution and Al oxide content when assessing PFOA mobility capacity with co-transport with DOM in soils.

In-situ thermal conductive heating (TCH) for soil remediation: A review

This paper provides a comprehensive overview of research works on in-situ thermal conductive heating (TCH), including heat transfer in soil, desorption behavior of pollutants, and mass transfer mechanism within the site. Each stage influences the effectiveness of subsequent stages. Comparison of simulation and experimental results demonstrates that heat transfer and temperature rise in soil are related to the hydrogeological conditions, wells layout and pollutants contents. Thermal desorption of pollutants from soil particles can be influenced by four aspects: energy input, pollutant properties, soil characteristics, and the binding state of pollutant in soil. The exponential decay kinetic model exhibits better applicability for fitting thermal desorption processes. After desorption, the pollutants migrate in soil driven by high temperature and extraction pressure, while hydrogeological conditions of the site determine the actual migration path and rate. Applying convection-dispersion model allows for quantitatively describing the complex migration behavior of pollutants in heterogeneous sites. Future research should focus more on the composite effects of multiple factors in TCH and develop multi-field coupling models through the combination of numerical simulation and in-situ experiments. Accurate characterization and prediction of entire TCH process can improve remediation efficiency, reduce energy costs, and achieve sustainable low-carbon remediation.

Interactions between biofilms and PFASs in aquatic ecosystems: Literature exploration

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been detected in most aquatic environments worldwide and are referred to as "forever chemicals" because of their extreme chemical and thermal stability. Biofilms, as basic aquatic bioresources, can colonize various substratum surfaces. Biofilms in the aquatic environment have to interact with the ubiquitous PFASs and have significant implications for both their behavior and destiny, which are still poorly understood. Here, we have a preliminary literature exploration of the interaction between PFASs and biofilms in the various aquatic environments and expect to provide some thoughts on further study. In this review, the biosorption properties of biofilms on PFASs and possible mechanisms are presented. The complex impact of PFASs on biofilm systems was further discussed in terms of the composition and electrical charges of extracellular polymeric substances, intracellular microbial communities, and overall contaminant purification functions. Correspondingly, the effects of biofilms on the redistribution of PFASs in the aqueous environment were analyzed. Finally, we propose that biofilm after adsorption of PFASs is a unique ecological niche that not only reflects the contamination level of PFASs in the aquatic environment but also offers a possible "microbial pool" for PFASs biodegradation. We outline existing knowledge gaps and potential future efforts for investigating how PFASs interact with biofilms in aquatic ecosystems.

Removal of perfluorooctanoic acid (PFOA) in the liquid culture of Phanerochaete chrysosporium

Perfluorooctanoic acid (PFOA) is becoming a concern due to its persistence, bioaccumulation, and potential harmful effects on humans and the environment. In this study, the fungus Phanerochaete chrysosporium (P. chrysosporium) was used to remove the PFOA in liquid culture system. The results showed that the average activities of laccase (Lac), lignin peroxidase (LiP), and manganese peroxidase (MnP) enzymes secreted by P. chrysosporium were 0.0003 U/mL, 0.013 U/mL, and 0.0059 U/mL, respectively, during the incubation times of 0-75 days. The pH of 3 and incubation time of 45-55 days were the optimum parameters for the three enzymes activities. The enzyme activities in P. chrysosporium incubation system were firstly inhibited by adding PFOA and then they were enhanced after 14 days. The maximum removal efficiency of PFOA (69.23%) was achieved after 35 days in P. chrysosporium incubation system with an initial PFOA concentration of 0.002 mM and no veratryl alcohol (VA). Adsorption was not a main pathway for PFOA removal and the PFOA adsorbed in fungi mycelial mat accounted for merely 1.91%. The possible products of PFOA contained partially fluorinated aldehyde, alcohol, and aromatic ring. These partially fluorinated compounds might result from PFOA degradation via a combination of cross-coupling and rearrangement of free radicals.

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

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

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

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

In Situ Insight into the Availability and Desorption Kinetics of Per- and Polyfluoroalkyl Substances in Soils with Diffusive Gradients in Thin Films

The physicochemical exchange dynamics between the solid and solution phases of per- and polyfluoroalkyl substances (PFAS) in soils needs to be better understood. This study employed an in situ tool, diffusive gradients in thin films (DGT), to understand the distribution and exchange kinetics of five typical PFAS in four soils. Results show a nonlinear relationship between the PFAS masses in DGT and time, implying that PFAS were partially supplied by the solid phase in all of the soils. A dynamic model DGT-induced fluxes in soils/sediments (DIFS) was used to interpret the results and derive the distribution coefficients for the labile fraction (K_dl), response time (t_c), and adsorption/desorption rates (k₁ and k_-1). The larger labile pool size (indicated by K_dl) for the longer chain PFAS implies their higher potential availability. The shorter chain PFAS tend to have a larger t_c and relatively smaller k_-1, implying that the release of these PFAS in soils might be kinetically limited but not for more hydrophobic compounds, such as perfluorooctanesulfonic acid (PFOS), although soil properties might play an important role. K_dl ultimately controls the PFAS availability in soils, while the PFAS release from soils might be kinetically constrained (which may also hold for biota uptake), particularly for more hydrophilic PFAS.

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

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

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

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

Using colloidal AFM probe technique and XDLVO theory to predict the transport of nanoplastics in porous media

The plastic concentration in terrestrial systems is orders of magnitude higher than that found in marine ecosystems, which has raised global concerns about their potential risk to agricultural sustainability. Previous research on the transport of nanoplastics in soil relied heavily on the qualitative prediction of the mean-field extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO), but direct and quantitative measurements of the interfacial forces between single nanoplastics and porous media are lacking. In this study, we conducted multiscale investigations ranging from column transport experiments to single particle measurements. The maximum effluent concentration (C/C0) of amino-modified nanoplastics (PS-NH₂) was 0.94, whereas that of the carboxyl-modified nanoplastics (PS-COOH) was only 0.33, indicating PS-NH₂ were more mobile than PS-COOH at different ionic strengths (1-50 mM) and pH values (5-9). This phenomenon was mainly attributed to the homogeneous aggregation of PS-COOH. In addition, the transport of PS-NH₂ in the quartz sand column was inhibited with the increase of ionic strength and pH, and pH was the major factor governing their mobility. The transport of PS-COOH was inhibited with increasing ionic strength and decreasing pH. Hydrophilicity/hydrophobicity-mediated interactions and particle heterogeneity strongly interfered with interfacial forces, leading to the qualitative prediction of XDLVO, contrary to experimental observations. Through the combination of XDLVO and colloidal atomic force microscopy, accurate and quantitative interfacial forces can provide compelling insight into the fate of nanoparticles in the soil environment.

Effect of solution chemistry on the transport of short-chain and long-chain perfluoroalkyl carboxylic acids (PFCAs) in saturated porous media

Perfluorocarboxylic acids (PFCAs) are one of the most widely detected classes of PFAS in the global environment after decades of intensive use. This study investigated the impact of perfluorinated carbon chain length on the transport behavior of PFCAs by testing and modeling two short-chain (PFPeA and PFHxA) and two long-chain PFCAs (PFOA and PFDA) in laboratory water-saturated columns. Moreover, their transport behavior was examined under different solution chemistry conditions, including pH, ionic strength, and cationic type. The experimental and simulation results indicated that the chain length had a limited impact on transport behaviors of PFPeA, PFHxA, and PFOA under various pH and ionic strengths, evidenced by their tracer-like breakthrough curves. In contrast, the mobility of PFDA was significantly affected by pH and ionic strengths. Additionally, the transport of all four PFCAs was inhabited in the presence of the divalent cation Ca²⁺. This study could help predict migration behavior and assess the potential risk of PFCAs in the subsurface system.

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) in groundwater: current understandings and challenges to overcome

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been frequently detected in groundwater globally. With the phase-out of perfluorooctane sulfonate (PFOS) and perfluorooctanate (PFOA) due to their risk to the ecosystem and human population, various novel PFASs have been used as replacements and detected in groundwater. In order to summarize the current understanding and knowledge gaps on PFASs in groundwater, we reviewed the studies about environmental occurrence, transport, and risk of legacy and novel PFASs in groundwater published from 1999 to 2021. Our review suggests that PFOS and PFOA could still be detected in groundwater due to the long residence time and the retention in the soil-groundwater system. Firefighting training sites, industrial parks, and landfills were commonly hotspots of PFASs in groundwater. More novel PFASs have been detected via nontarget analysis using high-resolution mass spectrometry. Some novel PFASs had concentrations comparable to that of PFOS and PFOA. Both legacy and novel PFASs can pose a risk to human population who rely on contaminated groundwater as drinking water. Transport of PFASs to groundwater is influenced by various factors, i.e., the compound structure, the hydrochemical condition, and terrain. The exchange of PFASs between groundwater and surface water needs to be better characterized. Field monitoring, isotope tracing, nontarget screening, and modeling are useful approaches and should be integrated to get a comprehensive understanding of PFASs sources and behaviors in groundwater.

Estimation of Transport Parameters of Perfluoroalkyl Acids (PFAAs) in Unsaturated Porous Media: Critical Experimental and Modeling Improvements

Predicting the transport of perfluoroalkyl acids (PFAAs) in the vadose zone is critically important for PFAA site cleanup and risk mitigation. PFAAs exhibit several unusual and poorly understood transport behaviors, including partitioning to the air-water interface, which is currently the subject of debate. This study develops a novel use of quasi-saturated (residual air saturation) column experiments to estimate chemical partitioning parameters of both linear and branched perfluorooctane sulfonate (PFOS) in unsaturated soils. The ratio of linear-to-branched air-water interfacial partitioning constants for all six experiments was 1.62 ± 0.24, indicating significantly greater partitioning of linear PFOS isomers at the air-water interface. Standard breakthrough curve analysis and numerical inversion of HYDRUS models support the application of a Freundlich isotherm for PFOS air-water interfacial partitioning below a critical reference concentration (CRC). Data from this study and previously reported unsaturated column data on perfluorooctanoate (PFOA) were reevaluated to examine unsaturated systems for transport nonidealities. This reanalysis suggests both transport nonidealities and Freundlich isotherm behavior for PFOA below the CRC using drainage-based column methods, contrary to the assertions of the original authors. Finally, a combined Freundlich-Langmuir isotherm was proposed to describe PFAA air-water interfacial partitioning across the full range of relevant PFAA concentrations.

Occurrence and Risk Assessment of Perfluorooctanoate (PFOA) and Perfluorooctane Sulfonate (PFOS) in Surface Water, Groundwater and Sediments of the Jin River Basin, Southeastern China

Perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) levels were determined in surface water, groundwater and sediments of the Jin River Basin, southeastern China. PFOA was detected in most of the samples, and its concentrations ranged from 0.53 to 8.77 ng/L, 0.26 to 15.1 ng/L and not detected (ND) to 23.9 ng/g in surface water, groundwater and sediments, respectively. Unlike PFOA, the detection frequency of PFOS was lower than 32%, and its concentrations ranged from ND to 2.56 ng/L, ND to 7.01 ng/L, ND to 11.1 ng/g in surface water, groundwater and sediments, respectively. The environmental risk assessment showed that PFOA could pose a high risk to surface water and groundwater, and both PFOA and PFOS posed a high risk to sediments. Moreover, the adults living in the Jin River Basin were at insignificant health risk to exposure to PFOA and PFOS through water consumption.

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

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

Transport and deposition behaviors of microplastics in porous media: Co-impacts of N fertilizers and humic acid

Due to the interaction of fertilizers with microplastics (MPs) and porous media, fertilization process would influence MPs transport and distributions in soil. The co-impacts of N fertilizers (both inorganic and organic N fertilizers) and humic substance on MPs transport/retention behaviors in porous media were examined in 10 mM KCl solutions at pH 6. NH₄Cl and CO(NH₂)₂ were employed as inorganic and organic N fertilizers, respectively, while humic acid (HA) was used as model humic substance. We found that for all three sized MPs (0.2, 1 and 2 µm) without HA, both types of N fertilizers decreased their transport/increased their retention in porous media (both quartz sand and soil). N fertilizers adsorbed onto surfaces of MPs and sand/soil, lowering the electrostatic repulsion between MPs and porous media, thus contributed to the enhanced MPs deposition. MPs with N fertilizers in solutions more tightly attached onto porous media and thus were more difficult to be re-mobilized by low ionic strength solution elution. Via steric repulsion and increasing electrostatic repulsion between MPs and porous media due to adsorption onto their surfaces, HA could increase MPs transport with N fertilizers in solutions.

Perfluoroalkyl and Polyfluoroalkyl Substances in Groundwater Used as a Source of Drinking Water in the Eastern United States

In 2019, 254 samples were collected from five aquifer systems to evaluate perfluoroalkyl and polyfluoroalkyl substance (PFAS) occurrence in groundwater used as a source of drinking water in the eastern United States. The samples were analyzed for 24 PFAS, major ions, nutrients, trace elements, dissolved organic carbon (DOC), volatile organic compounds (VOCs), pharmaceuticals, and tritium. Fourteen of the 24 PFAS were detected in groundwater, with 60 and 20% of public-supply and domestic wells, respectively, containing at least one PFAS detection. Concentrations of tritium, chloride, sulfate, DOC, and manganese + iron; percent urban land use within 500 m of the wells; and VOC and pharmaceutical detection frequencies were significantly higher in samples containing PFAS detections than in samples with no detections. Boosted regression tree models that consider 57 chemical and land-use variables show that tritium concentration, distance to the nearest fire-training area, percentage of urban land use, and DOC and VOC concentrations are the top five predictors of PFAS detections, consistent with the hydrologic position, geochemistry, and land use being important controls on PFAS occurrence in groundwater. Model results indicate that it may be possible to predict PFAS detections in groundwater using existing data sources.

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

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

Comprehensive adsorption behavior and mechanism of PFOA and PFCs in various subsurface systems in China

The adsorption-desorption performance of perfluorooctanoic acid (PFOA), one of the environmentally persistent pollutants which is refractory to degrade in soil, was investigated and reported. The adsorption-desorption process of PFOA was firstly conducted using different fractions (sand, coarse silt and fine silt) of soil collected from Shanghai, China. More than 50% of PFOA (2.0 mg/L) could be adsorbed by soils while only less than 10% of which could be desorbed once contamination occurs. The kinetics and particle diffusion rates of PFOA in different fractions of soil were calculated and analyzed in detail. Apart from this, the retention of short-chained PFCs, which can be generated as degradation products of PFOA, were also measured. In single solute systems, the adsorption of pollutants in soils dramatically increased as the chain length of PFCs grew longer. Similarly, in mixed solutions, preferential adsorption of longer-chained PFCs over shorter chains in soils were sited, attributable to the stronger hydrophobicity of the pollutants. However, the desorption of them performed in reverse, where the desorption rates of longer-chained PFCs were far lower than those of shorter ones. Furthermore, influencing factors including pH, temperature and co-existing matters were studied during the adsorption process. After comprehending the adsorption behavior of PFOA in soil fractions, the situation of the adsorption of PFOA in various soils chosen from nine provinces in China was investigated and compared. There was an obvious discrepancy, whether it be from the rate or the amount of adsorption of PFOA (approximately 10%), in the nine different soils. Finally, a multiple linear regressive equation was employed to sort influencing parameters which are prone to affect the adsorption of PFOA in soils, the contribution of these are provided in order of relevance. These results demonstrate the adsorption performance and behavior of PFOA and PFCs in different soils, which can be utilized as a scientific reference for maximizing remediation of PFOA polluted sites in the future.

Adsorption and Reductive Defluorination of Perfluorooctanoic Acid over Palladium Nanoparticles

Per- and polyfluoroalkyl substances (PFASs) comprise a group of widespread and recalcitrant contaminants that are attracting increasing concern due to their persistence and adverse health effects. This study evaluated removal of one of the most prevalent PFAS, perfluorooctanoic acid (PFOA), in H₂-based membrane catalyst-film reactors (H₂-MCfRs) coated with palladium nanoparticles (Pd⁰NPs). Batch tests documented that Pd⁰NPs catalyzed hydrodefluorination of PFOA to partially fluorinated and nonfluorinated octanoic acids; the first-order rate constant for PFOA removal was 0.030 h^-1, and a maximum defluorination rate was 16 μM/h in our bench-scale MCfR. Continuous-flow tests achieved stable long-term depletion of PFOA to below the EPA health advisory level (70 ng/L) for up to 70 days without catalyst loss or deactivation. Two distinct mechanisms for Pd⁰-based PFOA removal were identified based on insights from experimental results and density functional theory (DFT) calculations: (1) nonreactive chemisorption of PFOA in a perpendicular orientation on empty metallic surface sites and (2) reactive defluorination promoted by physiosorption of PFOA in a parallel orientation above surface sites populated with activated hydrogen atoms (H_ads*). Pd⁰-based catalytic reduction chemistry and continuous-flow treatment may be broadly applicable to the ambient-temperature destruction of other PFAS compounds.

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.

Chemical and photo-initiated aging enhances transport risk of microplastics in saturated soils: Key factors, mechanisms, and modeling

Microplastics (MPs) inevitably undergo aging transformation and transport process in environmental compartments. In this study, the polystyrene MPs were aged via three different oxidation methods including persulfate oxidation (PS), UV irradiation (UV), and UV irradiated persulfate oxidation (UVPS). All three treatments induced the great transformation of MPs, with the significant increase in surface roughness and in oxygen-containing functional groups, i.e., COOH or COOC. The UVPS aging showed synergetic effect due to the strengthened photo-initiated chemical oxidation, compared to UV and PS alone. All aged MPs exhibited the enhanced transport (34.9%-89.2%) in sandy and clay loam soils than pristine MPs (30.5%), and the synergetic effect was also observed in the transport behaviors of the UVPS MPs. Higher transport of MPs and aged MPs occurred in sandy soil than that in clay loam soil since the latter one contained high Fe minerals that tend to retain MPs, which was confirmed by the model quartz sand column experiment. Modeling on the migration of MPs retained in soil under a rainstorm scenario showed that the aged MPs had the stronger remobility and greater proportion of cumulative flux than pristine ones in the soil profile. These findings provided new insights on the fate and transport of MPs in natural soil and their potential risk to groundwater contamination.

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

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

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

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

New insights into the different adsorption kinetics of gallic acid and tannic acid on minerals via 1H NMR relaxation of bound water

The slower adsorption of lower molecular weight organic molecules remains poorly understood. This study investigated the adsorption kinetics of gallic acid (GA) and tannic acid (TA) on kaolinite (Kao), montmorillonite (Mon) and hematite (Hem), with an emphasis on the role of the bound water on the minerals. The lower adsorption of TA and GA on Kao than on Mon attributed to the lower specific surface area of Kao. Because of the electrostatic attraction, the adsorption of TA and GA on Hem was higher than that on Mon, even the specific surface area of the former was much lower than that of the later. The adsorption rates of TA on the three minerals were generally two orders of magnitude higher than those of GA. The adsorption kinetics of GA was strongly diffusion dependent; however, the diffusion process had limited influence on TA adsorption kinetics. The decreased c values of the intraparticle diffusion model of GA with increasing ionic strength provided additional direct evidence for the diffusion-dependent adsorption and the reduced hindrance by bound water via hydration layer compression. However, hydration layer compression had no effect on TA adsorption kinetics. The reduced ¹H NMR relaxation rate of bound water indicated that the bound water quantity on minerals decreased with increasing ionic strength, which proved the occurrence of hydration layer compression. This study highlighted the importance of bound water and the relative sizes of organic molecules in the adsorption kinetics of organic compounds on minerals, which should be carefully considered for their environmental fate studies.

A critical review of modeling Poly- and Perfluoroalkyl Substances (PFAS) in the soil-water environment

Due to their health effects and the recalcitrant nature of their CF bonds, Poly- and Perfluoroalkyl Substances (PFAS) are widely investigated for their distribution, remediation, and toxicology in ecosystems. However, very few studies have focused on modeling PFAS in the soil-water environment. In this review, we summarized the recent development in PFAS modeling for various chemical, physical, and biological processes, including sorption, volatilization, degradation, bioaccumulation, and transport. PFAS sorption is kinetic in nature with sorption equilibrium commonly quantified by either a linear, the Freundlich, or the Langmuir isotherms. Volatilization of PFAS depends on carbon chain length and ionization status and has been simulated by a two-layer diffusion process across the air water interface. First-order kinetics is commonly used for physical, chemical, and biological degradation processes. Uptake by plants and other biota can be passive and/or active. As surfactants, PFAS have a tendency to be sorbed or concentrated on air-water or non-aqueous phase liquid (NAPL)-water interfaces, where the same three isotherms for soil sorption are adopted. PFAS transport in the soil-water environment is simulated by solving the convection-dispersion equation (CDE) that is coupled to PFAS sorption, phase transfer, as well as physical, chemical, and biological transformations. As the physicochemical properties and concentration vary greatly among the potentially thousands of PFAS species in the environment, systematic efforts are needed to identify models and model parameters to simulate their fate, transport, and response to remediation techniques. Since many process formulations are empirical in nature, mechanistic approaches are needed to further the understanding of PFAS-soil-water-plant interactions so that the model parameters are less site dependent and more predictive in simulating PFAS remediation efficiency.

Effects of ionic strength and cation type on the transport of perfluorooctanoic acid (PFOA) in unsaturated sand porous media

Current understanding of perfluorooctanoic acid (PFOA) transport in unsaturated porous media is still limited with significant variability in solution chemistry. Column experiments were conducted to systematically evaluate the impacts of ionic strength (1.5-30 mM) and cation type (Na⁺ and Ca²⁺) on PFOA transport in unsaturated quartz sand. The results showed that an increase in ionic strength (1.5-30 mM) led to greater PFOA retardation in unsaturated columns. Meanwhile, Ca²⁺ caused more PFOA retardation than Na⁺ at the same unsaturated conditions. These findings were supported by bubble column experiments, which indicated greater PFOA adsorption at the air-water interface with increasing ionic strength or in the presence of Ca²⁺ in comparison to Na⁺. Furthermore, the air-water interfacial (AWI) adsorption coefficients calculated from surface tension isotherms also increased with increasing ionic strength or in the presence of Ca²⁺ in comparison to Na⁺. These results clearly confirm that higher ionic strength or cation valence significantly promoted PFOA adsorption at the air-water interface, and thus caused greater PFOA retardation during transport in unsaturated porous media. This work points out the importance of considering solution ionic strength and cation type in assessing the transport behavior of PFOA in unsaturated porous media.

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

The objective of this study is to compare the consistency between column and batch experiment methods for measuring solid-phase sorption coefficients and isotherms for per and polyfluoroalkyl substances (PFAS). Perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are used as representative PFAS, and experiments are conducted with three natural porous media with differing geochemical properties. Column-derived sorption isotherms are generated by conducting multiple experiments with different input concentrations (multi-C₀ method) or employing elution-front integration wherein the entire isotherm is determined from a single breakthrough curve (BTC) elution front. The isotherms generated with the multi-C₀ column method compared remarkably well to the batch isotherms over an aqueous concentration range of 3-4 orders of magnitude. Specifically, the 95% confidence intervals for the individual isotherm variables overlapped, producing statistically identical regressions. The elution-front integration isotherms generally agreed with the batch isotherms, but exhibited noise and systematic deviation at lower concentrations in some cases. All data sets were well described by the Freundlich isotherm model. Freundlich N values ranged from 0.75 to 0.81 for PFOS and was 0.87 for PFOA and are consistent with values reported in the literature for different geomedia. The results of this study indicate that column and batch experiments can measure consistent sorption isotherms and sorption coefficients for PFOS and PFOA when robust experimental setup and data analysis are implemented.

Importance of surface roughness on perfluorooctanoic acid (PFOA) transport in unsaturated porous media

Understanding the subsurface transport of perfluorooctanoic acid (PFOA) is of considerable interest for evaluating its potential risks to humans and ecosystems. In this study, packed-column experiments were conducted to examine the influence of surface roughness on PFOA transport in unsaturated glass beads, quartz sand and limestone porous media. Results showed decreasing moisture content significantly increased the air-water interfacial adsorption of PFOA and led to greater retardation in all three types of porous media. Particularly, rougher surface (limestone > quartz sand > glass beads) and smaller grain size (i.e. a larger solid specific surface area, SSSA) significantly enhanced PFOA retardation under unsaturated conditions. These results were further supported by bubble column experiments and SSSA analysis of porous media, which demonstrate that except for the factors affecting PFOA transport in solid-water interface (e.g. surface charge and chemical heterogeneity), the greater retardation of PFOA during transport is attributed to the larger air-water interfacial areas associated with rougher surface and smaller grain size and hence greater interfacial adsorption of PFOA. Our results indicated the importance of surface roughness on the retention and transport of PFOA in the unsaturated zone.

Reductive Defluorination and Mechanochemical Decomposition of Per- and Polyfluoroalkyl Substances (PFASs): From Present Knowledge to Future Remediation Concepts

Over the past two decades, per- and polyfluoroalkyl substances (PFASs) have emerged as worldwide environmental contaminants, calling out for sophisticated treatment, decomposition and remediation strategies. In order to mineralize PFAS pollutants, the incineration of contaminated material is a state-of-the-art process, but more cost-effective and sustainable technologies are inevitable for the future. Within this review, various methods for the reductive defluorination of PFASs were inspected. In addition to this, the role of mechanochemistry is highlighted with regard to its major potential in reductive defluorination reactions and degradation of pollutants. In order to get a comprehensive understanding of the involved reactions, their mechanistic pathways are pointed out. Comparisons between existing PFAS decomposition reactions and reductive approaches are discussed in detail, regarding their applicability in possible remediation processes. This article provides a solid overview of the most recent research methods and offers guidelines for future research directions.

Fluorinated Surfactant Adsorption on Mineral Surfaces: Implications for PFAS Fate and Transport in the Environment

Fluorinated surfactants, which fall under the class of per- and polyfluoroalkyl substances (PFAS), are amphiphilic molecules that comprise hydrophobic fluorocarbon chains and hydrophilic head-groups. Fluorinated surfactants have been utilized in many applications, e.g., fire-fighting foams, paints, household/kitchenware items, product packaging, and fabrics. These compounds then made their way into the environment, and have been detected in soil, fresh water, and seawater. From there, they can enter human bodies. Fluorinated surfactants are persistent in water and soil environments, and their adsorption onto mineral surfaces contributes to this persistence. This review examines how fluorinated surfactants adsorb onto mineral surfaces, by analyzing the thermodynamics and kinetics of adsorption, and the underlying mechanisms. Adsorption of fluorinated surfactants onto mineral surfaces can be explained by electrostatic interactions, hydrophobic interactions, hydrogen bonding, and ligand and ion exchange. The aqueous pH, varying salt or humic acid concentrations, and the surfactant chemistry can influence the adsorption of fluorinated surfactants onto mineral surfaces. Further research is needed on fluorinated surfactant adsorbent materials to treat drinking water, and on strategies that can modulate the fate of these compounds in specific environmental locations.

Response of cucumber (Cucumis sativus) to perfluorooctanoic acid in photosynthesis and metabolomics

A mechanistic understanding of perfluorooctanoic acid (PFOA) toxicity to plants is essential for future risk assessment of PFOA in agricultural soil. In this study, soil-grown cucumber (Cucumis sativus) was exposed to 0, 0.2, and 5 mg/kg of PFOA for 60 days. At harvest, contaminant accumulation, cucumber biomass, photosynthesis profiles and metabolites were measured. Results showed that PFOA depressed cucumber biomass and accumulated highest in leaves. Photosynthesis analysis revealed that PFOA at both doses reduced the chlorophyll contents and net photosynthesis rate of cucumber leaves. Gas chromatography-mass spectrometry-based non-targeted metabolomics revealed that PFOA induced metabolic reprogramming in cucumber leaves, including up-regulation of phenols (at 0.2 and 5 mg/kg) and down-regulation of amino acids (at 5 mg/kg), indicating disrupted nitrogen and carbon metabolism. Results revealed how PFOA represses plant growth by down-regulating photosynthetic pigments and disturbing the metabolism of carbohydroxides, phenols and amino acids. These findings provide valuable information for understanding the molecular mechanisms involved in plant responses to PFOA-induced stress.

Importance of Al/Fe oxyhydroxide coating and ionic strength in perfluorooctanoic acid (PFOA) transport in saturated porous media

Understanding subsurface transport of per- and polyfluoroalkyl substances (PFASs) is of critical importance for the benign use and risk management of PFASs. As one of the most commonly found PFASs, perfluorooctanoic acid (PFOA) is used as a representative PFAS and water-saturated column experiments were conducted to investigate the effect of Al/Fe oxyhydroxide coating and ionic strength on its transport at an environmentally relevant PFOA concentration (6.8 μg L^-1). Our results showed a clear increase in PFOA retardation in Al/Fe oxyhydroxide coated sand (retardation factor: Al: 1.87-5.58, Fe: 1.28-4.05) than those in uncoated sand (1.00-1.05), due to the stronger electrostatic attraction between anionic PFOA and Al/Fe oxyhydroxide coated sand surface. Notably, Al oxyhydroxide have a more profound effect on PFOA retention and retardation than Fe oxyhydroxide. Besides, higher ionic strength significantly inhabited PFOA retention and retardation in positively charged sand, and the considerable retention of PFOA (∼90%) in deionized water than those in 1.5 mM and 30.0 mM NaCl (<10%) clearly proves the role of competitive adsorption of Cl^- on PFOA transport in positively charged sand. In contrast, higher ionic strength (0 mM-30 mM NaCl) slightly increased PFOA retardation in negatively charged sand, illustrating the dominance of electrostatic interaction. Our findings advance current knowledge to understand PFOA transport in natural media with different surface charge property under environmental PFOA concentrations.

Transport of polystyrene nanoplastics in natural soils: Effect of soil properties, ionic strength and cation type

Nanoplastics as emerging pollutants have caused growing concerns and posed potential threats to the environment. Nonetheless, only few studies investigated transport behaviors of nanoplastics in natural soils. In this study, column experiments were conducted to investigate the effect of soil properties, ionic strength and cation type on the transport of polystyrene nanoplastics (PSNPs) in a desert soil (DS), a black soil (BS) and a red soil (RS). The effluent recovery of PSNPs in three soils followed the order of DS (0%-96.8%) > BS (0%-87.5%) > RS (0%). The retention of PSNPs was positively correlated with Fe/Al oxides contents (DS: Fe-2.69%, Al-12.6%; BS: Fe-4.04%, Al-15.9%; RS: Fe-6.57%, Al-26.9%), whereas negatively correlated with soil pH (DS: 9.75; BS: 6.57; RS: 4.97). Soil minerals and pH were thus identified as the crucial soil properties determining transport of PSNPs, due to their coupled effects on surface charges to affect electrostatic interactions between soils and PSNPs. In addition, increasing solution ionic strength strongly inhibited the transport of PSNPs in the DS (0%-96.8%) and BS (0%-87.5%). Ca²⁺ (IS: 1-5 mM) was more pronounced in enhancing PSNP retention than Na⁺ (IS: 1-20 mM). Our findings highlight that the transport and fate of PSNPs in natural soils are highly sensitive to soil physicochemical properties, ionic strength and cation type, and reveal that nanoplastics have strong mobility ability in soils with high pH and low Fe/Al oxides contents, which may pose potential risks to the soil and groundwater environment.

Toxicity of perfluorinated compounds to soil microbial activity: Effect of carbon chain length, functional group and soil properties

Perfluorinated compounds (PFCs) have been detected at various concentrations in different environment compartments due to their widespread usage. Nowadays, soil environment has become a prominent sink of PFCs from surface runoff and penetration, but few researches have been conducted in the toxicity of PFCs to soil microorganisms. To address the issue, microcalorimetry was applied to investigate the toxicity of six PFCs with different carbon chain length (4, 8, and 10) and functional group (carboxylic and sulfonic) to microbial activities in three Chinese soils varying widely in soil properties. Adsorption of PFCs by soil matrix was a key factor in controlling the toxicity of PFCs to soil microorganisms. The differences of carbon chain length and functional groups of PFCs have different impacts on soil microbial activity while affecting adsorption progress. Particularly, the sulfonic PFCs expressed higher toxicity than the carboxylic. It is also identified that the longer the chain length, the greater the toxicity of PFCs. Soil pH was another relevant factor of soil adsorption, and with the increase of pH, adsorption capability increased. Soil available P, N and K were essential nutrients in soil, and suggested to improve microbial activity under PFCs stress.