Prediction of aqueous solubility, vapor pressure and critical micelle concentration for aquatic partitioning of perfluorinated chemicals

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.

Ab Initio Prediction of Vapor Pressure for Diverse Atomic Layer Deposition Precursors

Understanding the saturated vapor pressure (Pvap) is vital for evaluating atomic layer deposition (ALD) precursors, as it directly influences the ALD temperature window and, by extension, the processability of compounds. The early estimation of vapor pressure ranges is crucial during the initial stages of novel precursor design, reducing the reliance on empirical synthesis or experimentation. However, predicting vapor pressure through computer simulations is often impeded by the scarcity of suitable empirical force fields for molecular dynamics simulations. This challenge is further compounded by the diverse chemical substances and the introduction of new elements into modern ALD processes, necessitating robust force fields that can accommodate metals, organics, and halides. In response, this study introduces a novel approach utilizing a quantum mechanically derived force field for the prediction of vapor pressure across a wide spectrum of potential ALD precursors. This approach enables the creation of system-specific force fields through parametrization based on ab initio calculations for a single molecule. We develop a comprehensive workflow to simulate both liquid and gaseous equilibrium phases, allowing the calculation of vapor pressure across a wide temperature range. Our methodology has been validated with a diverse set of ALD precursors, demonstrating its robustness in predicting Pvap at specified temperatures. The approach yields a Pearson's correlation coefficient (R2) greater than 0.9 on a logarithmic scale and a root-mean-squared deviation in self-solvation-free energies as low as 1.3 kcal mol-1. This innovative workflow, which does not require any prior experimental data, marks a significant advancement in the computer-aided design of novel ALD precursors, paving the way for accelerating developments in technology.

Aminated clay-polymer composite as soil amendment for stabilizing the short- and long-chain per- and poly-fluoroalkyl substances in contaminated soil

Soils contaminated with per- and poly- fluoroalkyl substances (PFAS) require immediate remediation to protect the surrounding environment and human health. A novel animated clay-polymer composite was developed by applying polyethyleneimine (PEI) solution onto a montmorillonite clay-chitosan polymer composite. The resulting product, PEI-modified montmorillonite chitosan beads (MMTCBs) were characterized as an adsorptive soil amendment for immobilizing PFAS contaminants. The MMTCBs exhibited good efficiency to adsorb the PFAS, showing adsorption capacities of 12.2, 16.7, 18.5, and 20.8 mg g-1 for PFBA, PFBS, PFOA, and PFOS, respectively, which were higher than those obtained by granular activated carbon (GAC) (i.e., an adsorbent used as a reference). Column leaching tests demonstrated that amending soil with 10% MMTCBs resulted in a substantial decrease in the leaching of PFOA, PFOS, PFBA, and PFBS by 90%, 100%, 64%, and 68%, respectively. These reductions were comparable to the values obtained for GAC-modified soil, particularly for long-chain PFAS. Incorporating MMTCBs into the soil not only preserved the structural integrity of the soil matrix but also enhanced its shear strength (kPa). Conversely, adding GAC to the soil resulted in a reduction of the soil's mechanical properties.

Prediction of critical micelle concentration for per- and polyfluoroalkyl substances

In this study, we focus on the development of Quantitative Structure-Property Relationship (QSPR) models to predict the critical micelle concentration (CMC) for per- and polyfluoroalkyl substances (PFASs). Experimental CMC values for both fluorinated and non-fluorinated compounds were meticulously compiled from existing literature sources. Our approach involved constructing two distinct types of models based on Support Vector Machine (SVM) algorithms applied to the dataset. Type (I) models were trained exclusively on CMC values for fluorinated compounds, while Type (II) models were developed utilizing the entire dataset, incorporating both fluorinated and non-fluorinated compounds. Comparative analyses were conducted against reference data, as well as between the two model types. Encouragingly, both types of models exhibited robust predictive capabilities and demonstrated high reliability. Subsequently, the model having the broadest applicability domain was selected to complement the existing experimental data, thereby enhancing our understanding of PFAS behaviour.

Differential scanning fluorimetry to assess PFAS binding to bovine serum albumin protein

The rapid screening of protein binding affinity for poly- and perfluoroalkyl substances (PFAS) benefits risk assessment and fate and transport modelling. PFAS are known to bioaccumulate in livestock through contaminated food and water. One excretion pathway is through milk, which may be facilitated by binding to milk proteins such as bovine serum albumin (BSA). We report a label-free differential scanning fluorimetry approach to determine PFAS-BSA binding over a broad temperature range. This method utilizes the tryptophan residue within the protein binding pocket as an intrinsic fluorophore, eliminating the need for fluorophore labels that may influence binding. BSA association constants were determined by (a) an equilibrium-based model at the melting temperature of BSA and (b) the Hill adsorption model to account for temperature dependent binding and binding cooperativity. Differences in binding between PFAS and fatty acid analogs revealed that a combination of size and hydrophobicity drives PFAS binding.

Physicochemical properties and interactions of perfluoroalkyl substances (PFAS) - Challenges and opportunities in sensing and remediation

Per- and polyfluoroalkyl substances (PFAS) is a class of persistent organic pollutants that presents health and environmental risks. PFAS are ubiquitously present in the environment, but current remediation technologies are ineffective in degrading them into innocuous chemicals, especially high energy degradation processes often generate toxic short chain intermediates. Therefore, the best remediation strategy is to first detect the source of pollution, followed by capturing and mineralising or recycling of the compounds. The main objective of this article is to summarise the unique physicochemical properties and to critically review the intermolecular and intramolecular physicochemical interactions of PFAS, and how these interactions can become obstacles; and at the same time, how they can be applied to the PFAS sensing, capturing, and recycling process. The physicochemical interactions of PFAS chemicals are being reviewed in this paper includes, (1) fluorophilic interactions, (2) hydrophobic interactions, (3) electrostatic interactions and cation bridging, (4) ionic exchange and (5) hydrogen bond. Moreover, all the different influential factors to these interactions have also been reported. Finally, properties of these interactions are compared against one another, and the recommendations for future designs of affinity materials for PFAS have been given.

Exploring Route-Specific Pharmacokinetics of PFAS in Mice by Coupling in Vivo Tests and Physiologically Based Toxicokinetic Models

BACKGROUND

Oral ingestion, inhalation, and skin contact are important exposure routes for humans to uptake per- and polyfluoroalkyl substances (PFAS). However, nasal and dermal exposure to PFAS remains unclear, and accurately predicting internal body burden of PFAS in humans via multiple exposure pathways is urgently required.

OBJECTIVES

We aimed to develop multiple physiologically based toxicokinetic (PBTK) models to unveil the route-specific pharmacokinetics and bioavailability of PFAS via respective oral, nasal, and dermal exposure pathways using a mouse model and sought to predict the internal concentrations in various tissues through multiple exposure routes and extrapolate it to humans.

METHODS

Mice were administered the mixed solution of perfluorohexane sulfonate, perfluorooctane sulfonate, and perfluorooctanoic acid through oral, nasal, and dermal exposure separately or jointly. The time-dependent concentrations of PFAS in plasma and tissues were determined to calibrate and validate the individual and combined PBTK models, which were applied in single- and repeated-dose scenarios.

RESULTS

The developed route-specific PBTK models successfully simulated the tissue concentrations of PFAS in mice following single or joint exposure routes as well as long-term repeated dose scenarios. The time to peak concentration of PFAS in plasma via dermal exposure was much longer (34.1-83.0 h) than that via nasal exposure (0.960 h). The bioavailability of PFAS via oral exposure was the highest (73.2%-98.0%), followed by nasal (33.9%-66.8%) and dermal exposure (4.59%-7.80%). This model was extrapolated to predict internal levels in human under real environment.

DISCUSSION

Based on these data, we predict the following: PFAS were absorbed quickly via nasal exposure, whereas a distinct hysteresis effect was observed for dermal exposure. Almost all the PFAS to which mice were exposed via gastrointestinal route were absorbed into plasma, which exhibited the highest bioavailability. Exhalation clearance greatly depressed the bioavailability of PFAS via nasal exposure, whereas the lowest bioavailability in dermal exposure was because of the interception of PFAS within the skin layers. https://doi.org/10.1289/EHP11969.

Growth Hormones in Broad Bean (Vicia faba L.) and Radish (Raphanus raphanistrum subsp. sativus L.) Are Associated with Accumulated Concentrations of Perfluoroalkyl Substances

In this study, we grew radish (Raphanus raphanistrum subsp. sativus L.) and broad beans (Vicia faba L.) in a greenhouse on soils spiked with a mixture of 15 per- and polyfluoroalkyl substances (PFASs) and investigated the association between accumulated ∑PFAS concentrations, growth, and hormone levels. Short-chained PFASs dominated aboveground tissues, whereas long-chained PFASs were most abundant in the plant roots. Our results showed that the presence or absence of exodermal Casparian strips, as well as the hydrophobicity and anion exchange capacities of PFASs, could explain the translocation of PFASs within plants. Significant associations found between accumulated PFAS concentrations and levels of gibberellins (GA1 and GA15), methionine, and indole-3-acetic acid (IAA) imply potential effects of PFASs on plant development and growth. This study provides the first evidence of associations between PFAS accumulation in plants and growth hormone levels, possibly leading to growth reduction of the apical dome and effects on the cell cycle in pericycle cells and methionine metabolism in plants.

Expanding the applicability domain of QSPRs for predicting water solubility and vapor pressure of PFAS

This work aimed to verify whether it is possible to extend the applicability domain (AD) of existing QSPR (Quantitative Structure-Property Relationship) models by employing a strategy involving additional quantum-chemical calculations. We selected two published QSPR models: for water solubility, logSW, and vapor pressure, logVP of PFAS as case studies. We aimed to enlarge set of compounds used to build the model by applying factorial planning to plan the augmentation of the set of these compounds based on their structural features (descriptors). Next, we used the COSMO-RS model to calculate the logSW and logVP for selected chemicals. This allowed filling gaps in the experimental data for further training QSPR models. We improved the published models by significantly extending number of compounds for which theoretical predictions are reliable (i.e., extending the AD). Additionally, we performed external validation that had not been carried out in original models. To test effectiveness of the AD extension, we screened 4519 PFAS from NORMAN Database. The number of compounds outside the domain was reduced comparing the original model for both properties. Our work shows that combining physics-based methods with data-driven models can significantly improve the performance of predictions of phys-chem properties relevant for the chemical risk assessment.

Elevated Levels of Ultrashort- and Short-Chain Perfluoroalkyl Acids in US Homes and People

Per- and polyfluoroalkyl substances (PFAS) make up a large group of fluorinated organic compounds extensively used in consumer products and industrial applications. Perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), the two perfluoroalkyl acids (PFAAs) with 8 carbons in their structure, have been phased out on a global scale because of their high environmental persistence and toxicity. As a result, shorter-chain PFAAs with less than 8 carbons in their structure are being used as their replacements and are now widely detected in the environment, raising concerns about their effects on human health. In this study, 47 PFAAs and their precursors were measured in paired samples of dust and drinking water collected from residential homes in Indiana, United States, and in blood and urine samples collected from the residents of these homes. Ultrashort- (with 2 or 3 carbons [C2-C3]) and short-chain (with 4-7 carbons [C4-C7]) PFAAs were the most abundant in all four matrices and constituted on average 69-100% of the total PFAA concentrations. Specifically, trifluoroacetic acid (TFA, C2) and perfluoropropanoic acid (PFPrA, C3) were the predominant PFAAs in most of the samples. Significant positive correlations (n = 81; r = 0.23-0.42; p < 0.05) were found between TFA, perfluorobutanoic acid (PFBA, C4), and perfluoroheptanoic acid (PFHpA, C7) concentrations in dust or water and those in serum, suggesting dust ingestion and/or drinking water consumption as important exposure pathways for these compounds. This study demonstrates that ultrashort- and short-chain PFAAs are now abundant in the indoor environment and in humans and warrants further research on potential adverse health effects of these exposures.

Polanyi adsorption potential theory for estimating PFAS treatment with granular activated carbon

Per- and polyfluoroalkyl substances (PFAS) are a group of chemicals that have gained interest because some PFAS have been shown to have negative health effects and prolonged environmental and biological persistence. Chemicals classified as PFAS have a wide range of chemical moieties that impart widely variable properties, leading to a range of water treatment process efficacies. The Polanyi Potential Adsorption Theory was used to estimate Freundlich isotherm parameters to predict the efficacy of granular activated carbon (GAC) treatment for 428 PFAS chemicals for which the vast majority had no previously published treatment data. This method accounts for the physical/chemical characteristics of the individual PFAS beyond molecular weight or chain length that have previously been employed. From a statistical analysis of available data and model results, many of the 428 PFAS were predicted to be effectively treatable by GAC. Although not directly applicable to full-scale design, the approach demonstrates a systematic method for predicting the effectiveness of GAC where isotherm or column data are not available. This then can be used to prioritize future research.

Accumulation and effects of perfluoroalkyl substances in Arabidopsis thaliana in a temperature-dependent manner: an in vitro study

The replacement of long-chained per- and polyfluoroalkyl substances (PFAS) with their short-chained homologues may have an impact on the accumulation in plants. The extent to which PFAS are absorbed by plants may differ among species and may depend on environmental factors, including temperature. The effect of an increased temperature on root uptake and translocation of PFAS in plants has been poorly studied. In addition, very few studies have examined toxicity of environmentally realistic PFAS concentrations to plants. Here, we investigated the bioaccumulation and tissue-distribution of fifteen PFAS in Arabidopsis thaliana L. grown in vitro at two different temperatures. Additionally, we examined the combined effects of temperature and PFAS accumulation on plant growth. Short-chained PFAS mainly accumulated in the leaves. The perfluorocarboxylic acid (PFCA) concentrations in roots and leaves, and the relative contribution of PFCAs to the ΣPFAS concentrations increased with carbon chain length regardless of temperature, with the exception of perfluorobutanoic acid (PFBA). An increased uptake of PFAS in leaves and roots at higher temperatures was observed for PFAS containing either eight or nine carbon atoms and could hence potentially result in higher risks for human intake. Leaf:root ratios of PFCAs followed a U-shaped pattern with carbon chain length, which is attributed to both hydrophobicity and anion exchange. Overall, no combined effects of realistic PFAS concentrations and temperature on the growth of A. thaliana were observed. PFAS exposure positively affected early root growth rates and root hair lengths, indicating a potential effect on factors involved in root hair morphogenesis. However, this effect on root growth rate became negligible later on in the exposure, and solely a temperature effect was observed after 6 days. Temperature also affected the leaf surface area. The underlying mechanisms on how PFAS stimulates root hair growth require further examination.

Tubing material considerably affects measurement delays of gas-phase oxygenated per- and polyfluoroalkyl substances

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental pollutants associated with negative health impacts. Assessments of tubing-related measurement bias for volatile PFAS are lacking, as gas-wall interactions with tubing can delay quantification of gas-phase analytes. We use online iodide chemical ionization mass spectrometry measurements to characterize tubing delays for three gas-phase oxygenated PFAS - 4:2 fluorotelomer alcohol (4:2 FTOH), perfluorobutanoic acid (PFBA), and hexafluoropropylene oxide dimer acid (HFPO-DA). Perfluoroalkoxy alkane and high-density polyethylene tubing yielded relatively short absorptive measurement delays, with no clear dependence on tubing temperature or sampled humidity. Sampling through stainless steel tubing led to prolonged measurement delays due to reversible adsorption of PFAS to the tubing surface, with strong dependence on tubing temperature and sample humidification. Silcosteel tubing afforded shorter measurement delays than stainless steel due to diminished surface adsorption of PFAS. Characterizing and mitigating these tubing delays is crucial for reliable quantification of airborne PFAS.Implications: Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants. Many PFAS are sufficiently volatile to exist as airborne pollutants. Measurements and quantification of airborne PFAS can be biased from material-dependent gas-wall interactions with sampling inlet tubing. Thus, characterizing these gas-wall interactions are crucial for reliably investigating emissions, environmental transport, and fates of airborne PFAS.

Sewage sludge biochars as effective PFAS-sorbents

The use of sewage sludge to produce biochar-based sorbents for per- and polyfluoroalkyl substances (PFAS) removal from water and soil may be an economically and environmentally sustainable waste management option. This study compared the sorption of six perfluorinated carboxylic acids (PFCAs) by two sewage sludge biochars (SSBCs) and one wood chip biochar (WCBC), dry pyrolyzed at 700 °C. Batch sorption tests were conducted by adding individual PFCAs and a PFCA-mixture to pure biochars and mixtures of biochar and a sandy soil (1.3% TOC). PFAS-sorption to the SSBCs exhibited log-linear biochar-water distribution coefficients (log K_d), comparable to those previously reported for commercial activated carbons (e.g., 5.73 ± 0.02 for perfluorooctanoic acid at 1 µg/L). The strong sorption of PFCAs was attributed to the SSBCs relatively high pore volumes in the pore size range that can accommodate these compounds. Sorption was attenuated by the presence of soil (by factors 3-10), by the presence of a mixture of PFCAs (by factors of 6-532) and by both together (by factors of 8-6581), indicating strongly competitive sorption between PFCA-congeners, and less severe sorption attenuation by soil organic matter. These findings could enable sustainable value chains for SSBs in soil remediation and water filtration solutions.

Occurrence of perfluoroalkyl substances in the environment compartments near a mega fluorochemical industry: Implication of specific behaviors and emission estimation

With the stringent restrictions on long-chain per- and polyfluoroalkyl substances (PFASs), ether-PFASs are being widely used as alternatives. We estimated that the mega fluorochemical industrial park (FIP) in Shandong, China, had emitted a maximum of 5040 kg and 1026 kg of hexafluoropropylene oxides (HFPOs), and 7560 kg and 1890 kg of perfluorooctanoic acid (PFOA) to water and air during 2021. In the surface water, groundwater, outdoor dust, soil, tree leaf and bark collected in the vicinity of the FIP, PFOA was predominant, followed by HFPOs. The much higher percentage of HFPO dimer acid (HFPO-DA) in groundwater than in surface water verified that this compound was more mobile in porous media. The strong correlations between the main PFASs in outdoor dust and surface soil suggested that the soil PFASs were mainly derived from air deposition, particularly for HFPO trimer acid (HFPO-TA), which has a stronger binding affinity with particles than PFOA. High percentage of the hydroxylated product of 6:2 polyfluorinated ether sulfonic acid was observed in groundwater, implying reductive dechlorination might occur in groundwater. Strong correlations between PFASs in outdoor dust and those in tree leaf and bark magnified that tree could serve as a sampler to effectively monitor airborne PFASs. This study provides the first line of information about the discharge, transport, and fate of novel ether-PFASs in the multiple environmental media near a point source.

Emerging and Legacy Perfluoroalkyl Substances in Breastfed Chinese Infants: Renal Clearance, Body Burden, and Implications

BACKGROUND

Human breast milk is a primary route of exposure to perfluoroalkyl substances (PFAS) in infants. To understand the associated risks, the occurrence of PFAS in human milk and the toxicokinetics of PFAS in infants need to be addressed.

OBJECTIVES

We determined levels of emerging and legacy PFAS in human milk and urine samples from Chinese breastfed infants, estimated renal clearance, and predicted infant serum PFAS levels.

METHODS

In total, human milk samples were collected from 1,151 lactating mothers in 21 cities in China. In addition, 80 paired infant cord blood and urine samples were obtained from two cities. Nine emerging PFAS and 13 legacy PFAS were analyzed in the samples using ultra high-performance liquid chromatography tandem mass spectrometry. Renal clearance rates (CLrenals) of PFAS were estimated in the paired samples. PFAS serum concentrations in infants (<1 year of age) were predicted using a first-order pharmacokinetic model.

RESULTS

All nine emerging PFAS were detected in human milk, with the detection rates of 6:2 Cl-PFESA, PFMOAA, and PFO5DoDA all exceeding 70%. The level of 6:2 Cl-PFESA in human milk (median concentration=13.6 ng/L) ranked third after PFOA (336 ng/L) and PFOS (49.7 ng/L). The estimated daily intake (EDI) values of PFOA and PFOS exceeded the reference dose (RfD) of 20 ng/kg BW per day recommended by the U.S. Environmental Protection Agency in 78% and 17% of breastfed infant samples, respectively. 6:2 Cl-PFESA had the lowest infant CLrenal (0.009mL/kg BW per day), corresponding to the longest estimated half-life of 49 y. The average half-lives of PFMOAA, PFO2HxA, and PFO3OA were 0.221, 0.075, and 0.304 y, respectively. The CLrenals of PFOA, PFNA, and PFDA were slower in infants than in adults.

CONCLUSIONS

Our results demonstrate the widespread occurrence of emerging PFAS in human milk in China. The relatively high EDIs and half-lives of emerging PFAS suggest potential health risks of postnatal exposure in newborns. https://doi.org/10.1289/EHP11403.

Estimation of vapor pressures of perfluoroalkyl substances (PFAS) using COSMOtherm

The widespread presence of per- and polyfluoroalkyl substances (PFAS) in the environment and a recognition of their possible health effects has, over the past decade, raised public concerns and led to much new research on these materials. In this field, with so many compounds of potential interest or concern, measuring the physical properties of even a small fraction of these compounds is a formidable task. The research community has turned to use of computational methods to begin to predict many useful properties, based just upon the structure of the compound. In this work, a quantum chemistry computational method (COSMO-RS) has been applied for exploring the possibility and accuracy of PFAS compound property estimation. The vapor pressures and boiling points of eleven PFAS are calculated with COSMOtherm and compared with available experimental data and literature calculation data using other packages. In the meantime, these measured results have permitted evaluation of this popular property estimation technique, which has not yet been fully validated for this class of compounds.

A Machine Learning Model to Estimate Toxicokinetic Half-Lives of Per- and Polyfluoro-Alkyl Substances (PFAS) in Multiple Species

Per- and polyfluoroalkyl substances (PFAS) are a diverse group of man-made chemicals that are commonly found in body tissues. The toxicokinetics of most PFAS are currently uncharacterized, but long half-lives (t_½) have been observed in some cases. Knowledge of chemical-specific t_½ is necessary for exposure reconstruction and extrapolation from toxicological studies. We used an ensemble machine learning method, random forest, to model the existing in vivo measured t_½ across four species (human, monkey, rat, mouse) and eleven PFAS. Mechanistically motivated descriptors were examined, including two types of surrogates for renal transporters: (1) physiological descriptors, including kidney geometry, for renal transporter expression and (2) structural similarity of defluorinated PFAS to endogenous chemicals for transporter affinity. We developed a classification model for t_½ (Bin 1: <12 h; Bin 2: <1 week; Bin 3: <2 months; Bin 4: >2 months). The model had an accuracy of 86.1% in contrast to 32.2% for a y-randomized null model. A total of 3890 compounds were within domain of the model, and t_½ was predicted using the bin medians: 4.9 h, 2.2 days, 33 days, and 3.3 years. For human t_½, 56% of PFAS were classified in Bin 4, 7% were classified in Bin 3, and 37% were classified in Bin 2. This model synthesizes the limited available data to allow tentative extrapolation and prioritization.

Recovery of per- and polyfluoroalkyl substances after solvent evaporation

Per- and polyfluoroalkyl substances (PFAS) are persistent, bioaccumulative chemicals that can be toxic at very low levels. Many of these compounds have unusual chemical properties that can have a large impact on analytical methods intended to quantitate them. When analyzing environmental samples, concentrating extraction eluents can greatly increase the sensitivity of PFAS extraction and analysis workflows. However, data on PFAS stability when evaporated under vacuum drying conditions are lacking. In this study two common sample preparation methods were replicated (methanol or methanolic ammonium hydroxide) to determine if PFAS material would undergo any observable loss during vacuum evaporation. Standards containing 49 different analytes from 7 different PFAS classes were evaporated to dryness under vacuum either with or without heat and reconstituted using one of two methods. It was found that recovery of some classes (e.g. PFSA, PFESA, FTS) was not greatly impacted by evaporation conditions or reconstitution method. Some analytes such as the very long chain PFCAs were not affected by evaporation conditions but saw drastic differences in recovery depending on the reconstitution method. Others analytes, for example PFSAms, experienced significant loss during evaporation that could not be mitigated by the chosen reconstitution method. This difference could be due to the number of fluorines present on the compound which correlated with a compound's hydrophobicity. Due to these findings, it is recommend that researchers consider PFAS class, chain length, and fluorine number when designing concentration and reconstitution protocols for PFAS to ensure conditions are optimal for the specific analytes of interest.

A novel procedure for predicting chronic toxicities and ecological risks of perfluorinated compounds in aquatic environment

Perfluorinated compounds (PFCs) can pose adverse effect on aquatic species and community structure. However, little is known about how the characteristics of molecules of PFCs affect their chronic toxic potencies to aquatic species, and the species sensitivity distributions (SSDs) and ecological risk assessments of PFCs are hampered by limited available data of chronic toxicity. In the present study, a novel procedure is proposed to obtain the ecological risk of PFCs using existing exposure concentrations of PFCs and SSDs integrated with the chronic toxicity prediction through robust QSAR models. The results showed that the energy of the lowest unoccupied molecular orbital (E_LUMO) exhibited the strongest correlation with the chronic toxicities of 15 PFCs (R² > 0.844, F > 16.206, p < 0.05). SSDs of 15 PFCs on eight species were first constructed, and the SSD fitting parameters were significantly correlated with E_LUMO (R² > 0.610, F > 19.471, p < 0.05). The QSAR-SSDs support the evaluation of hazardous criteria of PFCs for which data are lacking. Given environmental exposure distributions (EEDs) of the national presence of PFCs in aquatic systems in China, the QSAR-SSDs models allow the development of the ecological risk assessment for PFCs. This way, it was concluded that negligible environmental risk (defined as 5% of the species being potentially exposed to concentrations able to cause effects in < 5% of the case) could be expected from exposure to PFCs in surface waters in China. This method may be helpful for providing an evidence-based approach to guide the risk management for PFCs in aquatic environment.

Molecular Screening and Toxicity Estimation of 260,000 Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) through Machine Learning

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are a class of chemicals widely used in industrial applications due to their exceptional properties and stability. However, they do not readily degrade in the environment and are linked to contamination and adverse health effects in humans and wildlife. To find alternatives for the most commonly used PFAS molecules that maintain their desirable chemical properties but are not adverse to biological lifeforms, a novel approach based upon machine learning is utilized. The machine learning model is trained on an existing set of PFAS molecules to generate over 260,000 novel PFAS molecules, which we dub PFAS-AI-Gen. Using molecular descriptors with known relationships to toxicity and industrial suitability followed by molecular docking and molecular dynamics simulations, this set of molecules is screened. In this manner, increasingly complex calculations are performed only for candidate molecules that are most likely to yield the desired properties of low binding affinity toward two selected protein receptors, the human pregnane x receptor (hPXR) and peroxisome proliferator-activated receptor γ (PPAR-γ), and high industrial suitability, defined by critical micelle concentration (CMC). The selection criteria of low binding affinity and high industrial suitability are relative to the popular PFAS alternative GenX. hPXR and PPAR-γ are selected as they are PFAS targets and facilitate a variety of functions, such as drug metabolism and glucose regulation, respectively. Through this approach, 22 promising new PFAS substitutes that may warrant experimental investigation are identified. This integrated approach of molecular screening and toxicity estimation may be applicable to other chemical classes.

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.

Volatilization Potential of Per- and Poly-fluoroalkyl Substances from Airfield Pavements and during Recycling of Asphalt

Per- and poly-fluoroalkyl substances (PFAS) in water are typically present in their ionic (nonvolatile) forms; however, these can transition to their nonionic (volatile) forms when in contact with organic solvents and organic matrices. In particular, when PFAS are dissolved in organic solvents such as residues left from firefighting foams, fuels, and bitumen present in asphalt, the equilibrium between ionic and nonionic forms can trend toward more volatile nonionic forms of PFAS. We assessed the volatility of common PFAS based on calculated and available experimental data across ambient temperature ranges experienced by airfield pavements and at elevated temperatures associated with reworking asphalts for reuse. Volatilities are shown to be comparable to hydrocarbons in the semivolatile range, suggesting that volatilization is a viable loss mechanism for some PFAS that are nonvolatile in water. The present study points to future investigative needs for this unexplored mass loss mechanism and potential exposure pathway. Environ Toxicol Chem 2022;41:2202-2208. © 2022 Commonwealth of Australia. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Precise heteroatom doping determines aqueous solubility and self-assembly behaviors for polycyclic aromatic skeletons

Developing effective strategies to improve the hydrophilicity or aqueous solubility of hydrophobic molecular scaffolds is meaningful for both academic research and industrial applications. Herein, we demonstrate that stepwise and precise N/O heteroatoms doping on a polycyclic aromatic skeleton can gradually alter these structures from hydrophobic to hydrophilic, even resulting in excellent aqueous solubility. The Hansen solubility parameters (HSP) method shows that the three partial solubility parameters are closely related to N/O doping species, numbers and positions on the molecular panel. The hydrogen bonding solubility parameter indicates that the hydrogen bonding interactions between N/O doped molecules and water play a key role in enhancing hydrophilicity. Moreover, three optimized water-soluble molecules underwent a self-assembly process to form stable nanoparticles in water, thus facilitating better hydrogen bonding interactions disclosed by HSP calculations, NMR and single crystal X-ray analysis. These ensembles even show quasi-solid properties in water from NMR and luminescence perspectives.

Is the use of paper food contact materials treated with per- and polyfluorinated alkyl substances safe for high-temperature applications? - Migration study in real food and food simulants

Migration of per- and polyfluorinated alkyl substances (PFAS) from paper food contact materials (FCMs) can pose a consumer risk. However, risk assessment procedures typically do not consider PFAS contribution from FCMs. Moreover, migration studies are often limited to one subclass of PFAS or simplified by using food simulants (FS). To assess the risk comprehensively, migration of three PFAS subclasses (perfluorinated carboxylic acids/ sulfonic acids (PFCAs/PFSAs), polyfluoroalkyl phosphate esters (PAPs), and fluorotelomer alcohols (FTOHs)) from six FCMs were investigated to FS (50% and 20% ethanol) and food (oatmeal porridge, muffins, and tomato soup) under high-temperature conditions. Migration of PFCAs and FTOHs to all food samples was observed. Migration of PFCAs and FTOHs to 50% ethanol was significantly higher than migration to real food whilst FTOHs did not migrate into 20% ethanol. Estimated dietary PFAS exposure for children (1.06 - 5.67 ng/kgbw/day) exceeded EFSA's proposed safety threshold (0.63 ng/kgbw/day), risking consumer health.

Assessment of Bioactive Surfactant Levels in Selected Cereal Products

Per- and polyfluoroalkyl substances (PFASs) are bioactive surfactants that are widespread in the environment and living organisms. This study presents measurements of PFAS in selected food of plant origin that are part of the healthy eating pyramid, including bread, rolls, flour, bran, buckwheat, millet, rice, and noodles. A simple and reliable analytical method was developed for the simultaneous determination of seven perfluorocarboxylic acids (PFCAs) and three perfluoroalkane sulfonates (PFSAs) in cereal-based products. Out of the 10 PFASs, 5 PFASs were detected at levels above LOQ. The most frequently detected compound was perfluorooctanoic acid (PFOA), which was quantified in 65.2% of samples, while none of the others were present in more than 40.0% of tested products. Perfluorobutanoic acid (PFBA) was the predominant PFAS in cereal-based products, and its maximum measured concentration was 202.85 ng/g for wheat bran. The soil–root–shoot interactions in relation to PFAS transfer to the above-ground parts of plants and PFAS’s ability to interfere with proteins are most likely the sources of these compounds in commonly consumed cereal products. As PFBA contributes greatly to total PFAS concentration, this food group should be included in future dietary exposure assessments.

Thermodynamic Properties: Enthalpy, Entropy, Heat Capacity, and Bond Energies of Fluorinated Carboxylic Acids

Fluorinated carboxylic acids and their radicals are becoming more prevalent in environmental waters and soils as they have been produced and used for numerous commercial applications. Understanding the thermochemical properties of fluorinated carboxylic acids will provide insights into the stability and reaction paths of these molecules in the environment, in body fluids, and in biological and biochemical processes. Structures and thermodynamic properties for over 50 species related to fluorinated carboxylic acids with two and three carbons are determined with density functional computational calculations B3LYP, M06-2X, and MN15 and higher ab initio levels CBS-QB3, CBS-APNO, and G4 of theory. The lowest energy structures, moments of inertia, vibrational frequencies, and internal rotor potentials of each target species are determined. Standard enthalpies of formation, Δ_fH₂₉₈^°, from CBS-APNO calculations show the smallest standard deviation among methods used in this work. Δ_fH₂₉₈^° values are determined via several series of isodesmic and/or isogyric reactions. Enthalpies of formation are determined for fluorinated acetic and propionic acids and their respective radicals corresponding to the loss of hydrogen and fluorine atoms. Heat capacities as a function of temperature, C_p(T), and entropy at 298 K, S₂₉₈^°, are determined. Thermochemical properties for the fluorinated carbon groups used in group additivity are also developed. Bond dissociation energies (BDEs) for the carbon-hydrogen, carbon-fluorine, and oxygen-hydrogen (C-H, C-F, and O-H BDEs) in the acids are reported. The C-H, C-F, and O-H bond energies of the fluorinated carboxylic acids are in the range of 89-104, 101-125, and 109-113 kcal mol^-1, respectively. General trends show that the O-H bond energies on the acid group increase with the increase in the fluorine substitution. The strong carbon fluorine bonds in a fluorinated acid support the higher stability of the perfluorinated acids in the environment.

Field-Scale Demonstration of PFAS Leachability Following In Situ Soil Stabilization

A field-scale validation is summarized comparing the efficacy of commercially available stabilization amendments with the objective of mitigating per- and polyfluoroalkyl substance (PFAS) leaching from aqueous film-forming foam (AFFF)-impacted source zones. The scope of this work included bench-scale testing to evaluate multiple amendments and application concentrations to mitigate PFAS leachability and the execution of field-scale soil mixing in an AFFF-impacted fire-training area with nearly 2.5 years of post-soil mixing monitoring to validate reductions in PFAS leachability. At the bench scale, several amendments were evaluated and the selection of two amendments for field-scale evaluation was informed: FLUORO-SORB Adsorbent (FS) and RemBind (RB). Five ∼28 m³ test pits (approximately 3 m wide by 3 m long by 3 m deep) were mixed at a site using conventional construction equipment. One control test pit (Test Pit 1) included Portland cement (PC) only (5% dry weight basis). The other four test pits (Test Pits 2 through 5) compared 5 and 10% ratios (dry weight basis) of FS and RB (also with PC). Five separate monitoring events included two to three sample cores collected from each test pit for United States Environmental Protection Agency (USEPA) Method 1315 leaching assessment. After 1 year, a mass balance for each test pit was attempted comparing the total PFAS soil mass before, during, and after leach testing. Bench-scale and field-scale data were in good agreement and demonstrated >99% decrease in total PFAS leachability (mass basis; >98% mole basis) as confirmed by the total oxidizable precursor assay, strongly supporting the chemical stabilization of PFAS.

Underlying mechanisms for the impacts of molecular structures and water chemistry on the enrichment of poly/perfluoroalkyl substances in aqueous aerosol

Enrichment of ionic poly/perfluoroalkyl substances (PFASs) in aqueous aerosol (AA) is an important pathway for them to enter atmosphere. In this study, the enrichment behaviors of 12 legacy and emerging PFASs in AA in both single solute and mixed solutions were investigated. The enrichment factors (EF) displayed a general increasing trend with the fluorinated carbon chain length. For the first time, a robust Quantitative Structure-Property Relationship (QSPR) model coupled with partial least-square method was established with fifteen quantum chemical descriptors. Four molecular descriptors, including dipole moment (μ), molecular weight (MW), the maximal value of the molecular surface potential (V_{s, max}) and molecular volume (V) were identified as the key structural variables affecting the PFASs enrichment. Inorganic salts and humic acid (HA) which are common in seawater, facilitated the PFASs enrichment as a result of enhanced hydrophobicity and the bridging effect caused by divalent cations. The typical cationic and anionic surfactants, cetyltrimethylammonium bromide and sodium dodecyl sulfate, both inhibited the enrichment due to the competition between PFASs and surfactants. It is interesting that 6:2 chlorinated polyfluorinated ether sulfonate (F53B) had the highest EF among the 12 PFASs, implying its strong potential of atmosphere transport.

Combined effects of mixed per- and polyfluoroalkyl substances on the Nrf2-ARE pathway in ARE reporter-HepG2 cells

Although the Nrf2-ARE pathway plays a critical role in cellular protection against toxicity and oxidative stress from environmental chemical stressors, the association between exposure to per- and polyfluoroalkyl substances (PFAS) mixtures and the changes of Nrf2-ARE pathway remains largely unexplored. This study evaluated the potential of PFAS to induce the Nrf2-ARE pathway as individual compounds and as binary, ternary, and multicomponent mixtures in the ARE reporter-HepG2 cells and compared the mixture toxicity data to the predictions by concentration addition (CA) model. The toxicological interactions between PFAS mixture components were also determined by the model deviation ratio (MDR) between the CA predicted and mixture toxicity values. The induction of the Nrf2-ARE pathway was quantified using the luciferase system, and the endpoint assessed was the concentration that induced an induction ratio (IR) of 1.5 (EC_IR1.5). The results showed that exposures to both individual and mixed PFAS induced the Nrf2-ARE pathway in ARE reporter-HepG2 cells. Based on the MDRs, the combinations with PFOS showed synergistic interactive effects, while the combinations with PFOA showed additive effects. These results indicate that the CA model underestimated the mixture toxicity of PFAS with PFOS co-exposures and may have health risk assessment implications.

Independent mode sorption of perfluoroalkyl acids by single and multiple adsorbents

Infinite dilution partition coefficients, K_p,0, of a series of unbranched perfluoralkylacids, PFAAs with 3 to 8 CF₂ units between water and commercially available weak anion exchange (WAX) and strong anion exchange (MAX) polymers, C₁₈-modified silica, hydrophilic-lipophilic balance polymer (HLB), and Al₂O₃ sorbents were determined with self-packed columns using an HPLC-MS/MS setup. The anionic WAX sorbent shows a much higher adsorption affinity (about 450 fold) for PFBA than was observed for the applied hydrophobic sorbent HLB. Since the incremental value for each CF₂ group is smaller when the electrostatic adsorption process is observed, the hydrophobic partition coefficient of HLB supersedes the electrostatic one of WAX at around PFTeDA. Adsorption of PFAAs to Al₂O₃ was weak and did not show a clear chain length dependency. A recently developed independent mode (IM) adsorption model is a more accurate model to combine the electrostatic and hydrophobic interaction terms. This model predicts the correct behaviour of especially short chain PFAAs in soil or sediment sorption experiments. Factors increasing sorption efficiency of well- and ill-defined single and multiple adsorbents towards PFAAs are discussed. The IM model provides a method to optimise sorption remediation strategies of PFAAs in contaminated waters and proposes a two-step strategy, a starting hydrophobic step followed by an electrostatic one to remove more efficiently the short chain PFAAs.

Distribution characteristics of per- and polyfluoroalkyl substances (PFASs) in human urines of acrylic fiber plant and chemical plant

Per- and polyfluoroalkyl substances (PFASs) are persistent and bioaccumulative substances that have many adverse effects on human bodies. This study investigated the PFASs distribution characteristics in urine samples of workers from an acrylic fiber plant and a chemical plant. It was found that perfluorobutanoic acid (PFBA) was the predominant PFASs both in urine samples from the chemical plant (detection frequency: 86.52%; median value: 39.01 ng/mL) and the acrylic fiber plant (detection frequency: 88.16%; median value: 44.36 ng/mL). Meanwhile, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) were detected with very low frequencies and low concentrations. Furthermore, the results showed that PFASs levels in urine samples of workers from different units of the plants were quite different. PFASs concentrations of urine samples in males were higher than those in females, especially for PFBA, PFHxA, and PFDoA. The age had limited effects on the PFASs distribution in urine samples in this study, as short-chain PFASs were the dominant compounds. The correlations between PFASs concentrations in urine and gender/ages of workers were finally analyzed by Pearson correlation. The overall results may indicate that short-chain PFASs (such as: PFBA and PFBS) were becoming dominant for human exposure, especially occupational workers.

Sensors for detecting per- and polyfluoroalkyl substances (PFAS): A critical review of development challenges, current sensors, and commercialization obstacles

Per- and polyfluoroalkyl substances (PFAS) are a class of compounds that have become environmental contaminants of emerging concern. They are highly persistent, toxic, bioaccumulative, and ubiquitous which makes them important to detect to ensure environmental and human health. Multiple instrument-based methods exist for sensitive and selective detection of PFAS in a variety of matrices, but these methods suffer from expensive costs and the need for a laboratory and highly trained personnel. There is a big need for fast, inexpensive, robust, and portable methods to detect PFAS in the field. This would allow environmental laboratories and other agencies to perform more frequent testing to comply with regulations. In addition, the general public would benefit from a fast method to evaluate the drinking water in their homes for PFAS contamination. A PFAS sensor would provide almost real-time data on PFAS concentrations that can also provide actionable information for water quality managers and consumers around the planet. In this review, we discuss the sensors that have been developed up to this point for PFAS detection by their molecular detection mechanism as well as the goals that should be considered during sensor development. Future research needs and commercialization challenges are also highlighted.

QSARINS-Chem standalone version: A new platform-independent software to profile chemicals for physico-chemical properties, fate, and toxicity

The new software QSARINS-Chem standalone version is a multiplatform tool, freely downloadable, for the in silico profiling of multiple properties and activities of organic chemicals. This software, which is based on the concept of the QSARINS-chem module embedded in the QSARINS software, has been fully redesigned and redeveloped in the Java™ language. In addition to a selection of models included in the old module, the new software predicts biotransformation rates and aquatic toxicities of pharmaceuticals and personal care products in multiple organisms, and offers a suite of tools for the analysis of predictions. Furthermore, a comprehensive and transparent database of molecular structures is provided. The new QSARINS-Chem standalone version is an informative and solid tool, which is useful to support the assessment of the potential hazard and risks related to organic chemicals and is dedicated to users which are interested in the application of QSARs to generate reliable predictions.

Nanoscale Solid-Phase Isobaric Labeling for Multiplexed Quantitative Phosphoproteomics

We established a workflow for highly sensitive multiplexed quantitative phosphoproteomics using a nanoscale solid-phase tandem mass tag (TMT) labeling reactor. Phosphopeptides were first enriched by titanium oxide chromatography and then labeled with isobaric TMT reagents in a StageTip packed with hydrophobic polymer-based sorbents. We found that TMT-labeled singly phosphorylated peptides tend to flow through the titanium oxide column. Therefore, TMT labeling should be performed after the enrichment step from tryptic peptides, resulting in the need for microscale reactions with small amounts of phosphopeptides. Using an optimized protocol for tens to hundreds of nanograms of phosphopeptides, we obtained a nearly 10-fold increase in sensitivity compared to the conventional solution-based TMT protocol. We demonstrate that this nanoscale phosphoproteomics protocol works for 50 μg of HeLa proteins treated with selumetinib, and we successfully quantified the selumetinib-regulated phosphorylated sites on a proteome scale. The MS raw data files have been deposited with the ProteomeXchange Consortium via the jPOST partner repository (https://jpostdb.org) with the data set identifier PXD025536.

Quantitative structure-property relationships for the calculation of the soil adsorption coefficient using machine learning algorithms with calculated chemical properties from open-source software

The soil adsorption coefficient (K_oc) is an environmental fate parameter that is essential for environmental risk assessment. However, obtaining K_oc requires a significant amount of time and enormous expenditure. Thus, it is necessary to efficiently estimate K_oc in the early stages of a chemical's development. In this study, a quantitative structure-property relationship (QSPR) model was developed using calculated physicochemical properties and molecular descriptors with the OPEn structure-activity/property Relationship App (OPERA) and Mordred software using the largest available K_oc dataset. Specifically, we compared the accuracies of the model using the light gradient boosted machine (LightGBM), a gradient boosting decision tree (GBDT) algorithm, with those of previous models. The experimental results suggested the potential to develop a QSPR model that will produce highly accurate K_oc values using molecular descriptors and physicochemical properties. Unlike previous studies, the use of a combination of LightGBM, OPERA and Mordred enables the prediction of K_oc for many chemicals with high accuracy. In this study, OPERA was used to calculate the physicochemical properties, and Mordred was used to calculate molecular descriptors. The wide range of chemicals covered by OPERA and Mordred enables the analysis of a diverse range of chemical compounds. We also report a method to tune the LightBGM program. The use of fast-processing software, such as LightGBM, enables parameter tuning of a method required to obtain best performance. Our research represents one of the few studies in the field of environmental chemistry to use LightGBM. Using physicochemical properties as well as molecular descriptors, we could develop highly accurate K_oc prediction models when compared to prior studies. In addition, our QSPR models may be useful for preliminary environmental risk assessment without incurring significant costs during the early chemical developmental stage.

A group-contribution model for predicting the physicochemical behavior of PFAS components for understanding environmental fate

The factors controlling per- and polyfluoroalkyl substances (PFAS) environmental fate remains the subject of considerable debate and study. As surfactants, PFAS readily partition to interfaces, a property that controls their transport and fate. A group contribution model is developed to predict the extent to which PFAS partitions to the air-water interface. Langmuir adsorption and Szyszkowski equation parameters were fitted to literature air-water surface tension data for a range of PFAS and conventional hydrocarbon surfactants. This approach enabled the prediction of the impact of the hydrophilic head group, and other molecular components, on PFAS interfacial partitioning in instances when PFAS data are unavailable but analogous hydrocarbon surfactant data are available. The model was extended to predict a range of parameters (i.e., solubility, critical micelle concentration (CMC), K_D, K_oc and K_ow) that are used to predict PFAS environmental fate, including long-range PFAS transport and in multimedia models. Model predictions were consistent with laboratory and field derived parameters reported in the literature. Additionally, the proposed model can predict the impact of pH and speciation on the extent of PFAS interfacial partitioning, a potentially important feature for understanding the behaviors of some ionizable PFAS, such as fluorinated carboxylic acids. The proposed model provides a conceptually straightforward method to predict a wide range of environmental fate parameters for a wide range of PFAS. As such, the model is a powerful tool that can be used to determine parameters needed to predict PFAS environmental fate.

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.

Direct contact membrane distillation for effective concentration of perfluoroalkyl substances - Impact of surface fouling and material stability

Polyfluoroalkyl and perfluoroalkyl substances (PFAS) are ecotoxic amphiphilic compounds containing alkyl-fluorinated chains terminated with weak acid moieties, and hence difficult to be degraded or removed from water sources. Direct contact membrane distillation (DCMD) was used for concentrating and removing of perfluoropentanoic acid (PFPeA) compounds from model contaminated water using commercially available poly (tetrafluoroethylene) (PTFE) membranes. The membranes were characterised for surface morphology, roughness, contact angle and pore size distribution before and after the DCMD test to investigate and evaluate membrane fouling. During the DCMD test performed for 6 h using 10 ppm PFPeA solution, the membrane exhibited progressive increased flux (from 17 to 43 kg m^-2 h^-1) and decreased PFPeA rejection (from 85 to 58%), as the feed temperature was increased from 50 to 70 °C. Further, the feed/retentate side showed a 1.8, 2.1 and 2.8-fold increase in PFPeA concentration tested at feed temperatures 50, 60, and 70 °C, respectively. The permeate side contained less than 1 ppm of PFPeA revealing that the PFPeA moved across the PTFE membrane during DCMD, which is attributed to progressive surface diffusion over time. This study opens a new route to concentrate and remove amphiphilic molecules, such as PFAS, from source points, relevant to landfill leachates or surface waters. The study also points at gaps in materials science and surface engineering to be tackled to deal with PFAS compounds efficiently.

Versatile modelling of polyoxymethylene-water partition coefficients for hydrophobic organic contaminants using linear and nonlinear approaches

Environmental fate or transport of hydrophobic organic contaminants (HOCs) depends on the partitioning properties of compounds within various environmental phases. Due to the wide application of polyoxymethylene (POM) in the passive sampling technique, several in silico models were developed to predict POM-water partition coefficients (K_POM-w) in accordance with the guidelines of the Organization for Economic Cooperation and Development (OECD). It is an attempt to combine conventional linear method (multiple linear regression, MLR) and popular nonlinear algorithm (artificial neural network, ANN) for estimating partition coefficients of HOCs. All models were performed on a dataset of 210 chemicals from 13 different classes. The polyparameter linear free energy relationship (pp-LFER) model included 5 molecular descriptors, namely, E, S, A, B and V, and predicted log K_POM-w with R²_adj of 0.825. The values of statistical parameters including R²_adj, Q²_ext, RMSE_tra and RMSE_ext for quantitative structure-property relationship (QSPR)-MLR and QSPR-ANN models with four descriptors (ALOGP, MeanDD, E1m and Mor24s) were: (0.928, 0.877, 0.498 and 0.649) and (0.943, 0.905, 0.443 and 0.571), with high similarity for both models, which confirmed the robustness, significance, and remarkable prediction accuracy of the QSPR models. Moreover, the mechanism interpretation revealed that the molecular volume and hydrophobicity had a major impact on distribution procedure of HOCs. The models developed herein, with the broad applicability domain (AD), provide suitable tools to fill the experimental data gap for untested chemicals and help researchers better understand the mechanistic basis of adsorption behavior of POM.

Principles of QSAR Modeling

At the end of her academic career, the author summarizes the main aspects of QSAR modeling, giving comments and suggestions according to her 23 years' experience in QSAR research on environmental topics. The focus is mainly on Multiple Linear Regression, particularly Ordinary Least Squares, using a Genetic Algorithm for variable selection from various theoretical molecular descriptors, but the comments can be useful also for other QSAR methods. The need for rigorous validation, also external, and for applicability domain check to guarantee predictivity and reliability of QSAR models is particularly highlighted. The commented approach is the “predictive” one, based on chemometrics, and is usefully applied to the prioritization of environmental pollutants. All the discussed points and the author's ideas are implemented in the software QSARINS, as a legacy to the QSAR community.

Serum Half-Lives for Short- and Long-Chain Perfluoroalkyl Acids after Ceasing Exposure from Drinking Water Contaminated by Firefighting Foam

BACKGROUND

Firefighting foam-contaminated ground water, which contains high levels of perfluoroalkyl substances (PFAS), is frequently found around airports. In 2018 it was detected that employees at a municipal airport in northern Sweden had been exposed to high levels of short-chain PFAS along with legacy PFAS (i.e., PFOA, PFHxS, and PFOS) through drinking water.

OBJECTIVES

In this study, we aimed to describe the PFAS profile in drinking water and biological samples (paired serum and urine) and to estimate serum half-lives of the short-chain PFAS together with legacy PFAS.

METHODS

Within 2 weeks after provision of clean water, blood sampling was performed in all 26 airport employees. Seventeen of them were then followed up monthly for 5 months. PFHxA, PFHpA, PFBS, PFPeS, and PFHpS together with legacy PFAS in water and biological samples were quantified using LC/MS/MS. Half-lives were estimated by assuming one compartment, first-order elimination kinetics.

RESULTS

The proportions of PFHxA, PFHpA, and PFBS were higher in drinking water than in serum. The opposite was found for PFHxS and PFOS. The legacy PFAS accounted for about 50% of total PFAS in drinking water and 90% in serum. Urinary PFAS levels were very low compared with serum. PFBS showed the shortest half-life {average 44 d [95% confidence interval (CI): 37, 55 d]}, followed by PFHpA [62 d (95% CI: 51, 80 d)]. PFPeS and PFHpS showed average half-lives as 0.63 and 1.46 y, respectively. Branched PFOS isomers had average half-lives ranging from 1.05 to 1.26 y for different isomers. PFOA, PFHxS, and linear PFOS isomers showed average half-lives of 1.77, 2.87, and 2.93 y, respectively.

DISCUSSION

A general pattern of increasing half-lives with increasing chain length was observed. Branched PFOS isomers had shorter half-lives than linear PFOS isomers. https://doi.org/10.1289/EHP6785.

Property Estimation of Per- and Polyfluoroalkyl Substances: A Comparative Assessment of Estimation Methods

To accurately predict the environmental fate of per- and polyfluoroalkyl substances (PFAS), high-quality physicochemical property data are required. Because such data are often not available from experiments, assessment of the accuracy of existing property estimation models is essential. The quality of predicted physicochemical property data for a set of 25 PFAS was examined using COSMOtherm, EPI Suite, the estimation models accessible through the US Environmental Protection Agency's CompTox Chemicals Dashboard, and Linear Solvation Energy Relationships (LSERs) available through the UFZ-LSER Database. The results showed that COSMOtherm made the most accurate acid dissociation constant and air-water partition ratio estimates compared with literature data. The OPEn structure-activity/property Relationship App (OPERA; developed through the CompTox Chemicals Dashboard) estimates of vapor pressure and dry octanol-air partition ratios were the most accurate compared with other models of interest. Wet octanol-water partition ratios were comparably predicted by OPERA and EPI Suite, and the organic carbon soil coefficient and solubility were well predicted by OPERA and COSMOtherm. Acid dissociation of the perfluoroalkyl acids has a significant impact on their physicochemical properties, and corrections for ionization were included where applicable. Environ Toxicol Chem 2020;39:775-786. © 2020 SETAC.

Understanding and predicting the diffusivity of organic chemicals for diffusive gradients in thin-films using a QSPR model

The diffusion coefficient (D) is a key physicochemical parameter for the diffusive gradients in thin films technique (DGT) for environmental sampling, which has been extended to organic chemicals (so called o-DGT). D can be measured in the laboratory, although for organic chemicals this parameter might be predicted based on chemical structure. Here we developed for the first time a Quantitative Structure-Property Relationship (QSPR) model to predict the D values. Twenty quantum chemical descriptors that quantify the electronic and energy properties of 120 organic compounds were selected together with molecular mass, solubility and hydrophobicity. The best QSPR model was established by using genetic algorithm and multiple linear regression (GA-MLR). The results indicated that the model derived from the average molecular polarizability (α), the chemical potential (ξ) and the global electrophilicity index (ω) could explain the diffusion of organics in o-DGT and had good statistical performance (R² = 0.767, RMSE = 0.101). Different validation strategies confirmed that the developed model was robust and predictive. 93% of tested compounds were within the applicability domain (AD) and predicted accurately. We concluded that the proposed QSPR model can serve as an efficient predictive tool for new chemicals in the AD, would be useful to cross validate measured D values and provide a better the understanding of the diffusive behaviour of organics in o-DGT and measurements in the environment. It might also be useful in the non-target analysis with o-DGT for chemicals without measured D values.

QSPR studies on water solubility, octanol-water partition coefficient and vapour pressure of pesticides

The assessment of the environmental fate and (eco)toxicological effects of pesticide compounds is of crucial importance. The present review is focused on Quantitative Structure-Property Relationships (QSPR) applications on three environmentally relevant physicochemical properties of pesticides, which can be used for assessing their environmental partition and transport, as well as exposure potential namely water solubility, octanol-water partition coefficient and vapour pressure. This article revises various interesting QSPR applications with special emphasis on studies developed during the 2009-2019 period.

Assessing the contribution of atmospheric transport and tourism activities to the occurrence of perfluoroalkyl acids (PFAAs) in an Alpine Nature Reserve

Perfluoroalkyl acids (PFAAs) are ubiquitous in the global environment, even in remote regions. With increasing production and application of PFAAs in China, their distribution patterns have been widely reported, however with less attention to inland northwestern regions. Long-range transport and direct releases from local activities have been regarded as the main reasons for PFAAs distribution in such a remote area. To identify and quantify the contributions of different sources to PFAAs occurrences, an investigation was conducted in the Tianchi lake, nature reserve. A total of 20 water samples, 8 soil, 4 sediment and 10 fresh snow samples were collected and analyzed in 2015. The mean PFAAs concentrations were 3.38 ng L^-1 in surface water, 1.06 ng g^-1 dw in soil, 0.53 ng g^-1 dw in sediment, and 3.31 ng L^-1 in fresh snow, respectively. High levels of PFAAs were observed in surface water (15.41 ng L^-1) from Western Tianchi pond and surface snow (14.24 ng L^-1) from the site near a ski resort around Tianchi Lake indicating potential pollution by local human activities. The correlation between individual concentrations among water, soil and snow indicated the snow deposition as an important source. Although with limited sample size, principal component analysis associated with multiple linear regression (PCA-MLR) and positive matrix factorization (PMF) analyses have identified two major sources, which are characterized as tourism activities with dominance of perfluorooctanoic acid (PFOA) and long-range transport with abundant perfluorobutanoic acid (PFBA). Their contributions to total levels were 41% and 52%, respectively. These two sources contributed differently to the PFAAs presences in Tianchi and Western Tianchi Lakes. Source analysis indicates that the western Tianchi lake with a relatively small catchment was affected mainly by local activities.

Distribution of eight perfluoroalkyl acids in plant-soil-water systems and their effect on the soil microbial community

Phytoremediation of per- and polyfluoroalkyl substances (PFAS) appears to be a green remediation technique. To understand distribution of PFAS in plant-soil-water systems, eight perfluoroalkyl acids (PFAAs) at three different concentrations were spiked to Juncus effusus grown in a greenhouse for 21 days. Results from this study demonstrated that mass-based plant uptake of PFAAs correlated positively with concentrations and time. On the basis of removal percentages, the higher the initial PFAA concentrations, the less removal by plant was observed. With the low level of PFAA spike, J. effusus roots and shoots accumulated 30-40% of PFAAs (C4 to C8) except PFOS with a lower uptake of approximately 20%. Together with soil sorption, >82.8% of PFAAs were removed from the aqueous solution in 21 days. Uptake of PFAAs also depended on their carbon chain length and plant compartments (roots or shoots). This dependence resulted in different bioaccumulation factors and translocation factors for different PFAAs. Besides physical and chemical distribution, PFAAs, especially those added at the high level led to significant change of soil bacterial communities in terms of composition and structure. Potential impact to the community's functions warrants further investigations.

Leachate emissions of short- and long-chain per- and polyfluoralkyl substances (PFASs) from various Norwegian landfills

Restrictions on the use of long-chain per- and polyfluoralkyl substances (PFASs) has led to substitutions with short-chain PFASs. This study investigated the presence of four short-chain PFASs and twenty-four long-chain PFASs in leachate and sediment from ten Norwegian landfills, including one site in Svalbard, to assess whether short-chain PFASs are more dominant in leachate. PFASs were detected in all sites. Short-chain PFASs were major contributors to the total PFAS leachate concentrations in six of ten landfills, though not in Svalbard. In sediment, long-chain PFASs such as perfluorooctanesulfonate (PFOS) and PFOS-precursors were dominant. Short-chain PFAS leachate concentrations ranged from 68 to 6800 ng L^-1 (mean: 980 ± 1800; median: 360 ng L^-1), whereas long-chain concentrations ranged from 140 to 2900 ng L^-1 (mean: 530 ± 730; median: 290 ng L^-1). Sediment concentrations, which contained mainly long-chain PFASs, ranged from 8.5 to 120 μg kg^-1 (mean: 47 ± 36; median: 41 μg kg^-1). National release from Norwegian landfills to the environment was estimated to be 17 ± 29 kg per year (median: 6.3 kg per year), which is in the same range as national emissions from the US, China and Germany after normalizing the data to a per capita emission factor (3.2 ± 5.5 mg per person per year). Results from this study are compared with previous and current studies in other countries, indicating a general trend that short-chain PFASs are dominating over long-chain PFASs in landfill leachate emissions.

Sorption and Desorption Mechanisms of Cationic and Zwitterionic Per- and Polyfluoroalkyl Substances in Natural Soils: Thermodynamics and Hysteresis

Sorption linearity and reversibility are implicit in models for the fate and transport of per- and polyfluoroalkyl substances (PFAS). In this study, however, we found that the sorption of cationic and zwitterionic PFAS in natural soils was highly nonlinear. The nonlinearity was so severe that it led to a variation in the coefficient of sorption by several orders of magnitude over the experimental concentration range. This implies a considerable increase in sorption as concentration falls in the natural environment. Sorption of cationic PFAS correlated strongly with the soil organic matter (SOM) content and was reversible in all soils. Sorption of zwitterionic PFAS, on the other hand, displayed concentration-dependent hysteresis in soils with a low SOM content. The irreversibility, which was associated with neither SOM, pore deformation, nor surface complexation, was likely caused by the entrapment of molecules in porous structures within inorganic components of soil aggregates. Furthermore, electrostatic interactions with negatively charged soil constituents and the hydrophobic effect were found to be major sorption driving forces for cationic/zwitterionic PFAS at low and high concentrations, respectively. The maximum electrostatic potential of PFAS ions, computed using density functional theory, was found to be a useful predictor of the sorption of ionic PFAS species.