Fate of 6:2 fluorotelomer sulfonic acid in pumpkin (Cucurbita maxima L.) based on hydroponic culture: Uptake, translocation and biotransformation

Unveiling behaviors of 8:2 fluorotelomer sulfonic acid (8:2 FTSA) in Arabidopsis thaliana: Bioaccumulation, biotransformation and molecular mechanisms of phytotoxicity

8:2 fluorotelomer sulfonic acid (8:2 FTSA) has been commonly detected in the environment, but its behaviors in plants are not sufficiently known. Here, the regular and multi-omics analyses were used to comprehensively investigate the bioaccumulation, biotransformation, and toxicity of 8:2 FTSA in Arabidopsis thaliana. Our results demonstrated that 8:2 FTSA was taken up by A. thaliana roots and translocated to leaves, stems, flowers, and seeds. 8:2 FTSA could be successfully biotransformed to several intermediates and stable perfluorocarboxylic acids (PFCAs) catalyzed by plant enzymes. The plant revealed significant growth inhibition and oxidative damage under 8:2 FTSA exposure. Metabolomics analysis showed that 8:2 FTSA affected the porphyrin and secondary metabolisms, resulting in the promotion of plant photosynthesis and antioxidant capacity. Transcriptomic analysis indicated that differentially expressed genes (DEGs) were related to transformation and transport processes. Integrative transcriptomic and metabolomic analysis revealed that DEGs and differentially expressed metabolites (DEMs) in plants were predominantly enriched in the carbohydrate metabolism, amino acid metabolism, and lipid metabolism pathways, resulting in greater energy consumption, generation of more nonenzymatic antioxidants, alteration of the cellular membrane composition, and inhibition of plant development. This study provides the first insights into the molecular mechanisms of 8:2 FTSA stress response in plants.

Uptake, accumulation and metabolism of UV-320 in vegetables and its impact on growth and quality

UV-320 is classified as a Substance of Very High Concern (SVHC) by the European Chemicals Agency and has attracted significant attention due to its presence in the environment. Understanding the uptake, translocation and metabolic patterns of UV-320 in vegetables is essential for assessing their ability to bioaccumulate and potential risks to human health. In this study, we investigated the uptake and translocation of UV-320 in lettuce and radish by hydroponic experiments. The results showed that the root concentration factors (Croot/Csolution, RCF) of lettuce and radish were in the range of 47.9 to 464 mL/g and 194 to 787 mL/g, respectively. The transfer factors (Cshoot/Croot, TF) were observed to be 0.001-0.012 for lettuce and 0.02-0.05 for radish. Additionally, non-targeted screening identified twelve phase I and one phase II metabolites of UV-320 in vegetables, which were confirmed based on their molecular formulas and structures. The metabolic pathways involving oxidation, ketonylation and deamination were proposed in vegetables. Also, we have observed that UV-320 inhibits the growth of vegetables. Meanwhile, we evaluated the health risk of UV-320 in lettuce and radish and found that the consumption of lettuce is relatively safe, while the consumption of radish has a risk of HQ >1 for both adults and children, which should be seriously considered. This study provides valuable insights into the behavior and ecological risks of UV-320 in the environment.

Crop Contamination and Human Exposure to Per- and Polyfluoroalkyl Substances around a Fluorochemical Industrial Park in China

Due to their significant environmental impact, there has been a gradual restriction of the production and utilization of legacy per- and polyfluoroalkyl substances (PFAS), leading to continuous development and adoption of novel alternatives. To effectively identify the potential environmental risks from crop consumption, the levels of 25 PFAS, including fourteen perfluoroalkyl acids (PFAAs), two precursor substances and nine novel alternatives, in agricultural soils and edible parts of various crops around a fluoride industrial park (FIP) in Changshu city, China, were measured. The concentration of ΣPFAS in the edible parts of all crops ranged from 11.64 to 299.5 ng/g, with perfluorobutanoic acid (PFBA) being the dominant compound, accounting for an average of 71% of ΣPFAS. The precursor substance, N-methylperfluoro-octanesulfonamidoacetic acid (N-MeFOSAA), was detected in all crop samples. Different types of crops showed distinguishing accumulation profiles for the PFAS. Solanaceae and leafy vegetables showed higher levels of PFAS contamination, with the highest ΣPFAS concentrations reaching 190.91 and 175.29 ng/g, respectively. The highest ΣAlternative was detected in leafy vegetables at 15.21 ng/g. The levels of human exposure to PFAS through crop consumption for various aged groups were also evaluated. The maximum exposure to PFOA for urban toddlers reached 109.8% of the standard value set by the European Food Safety Authority (EFSA). In addition, short-chained PFAAs and novel alternatives may pose potential risks to human health via crop consumption.

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

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

Uptake, accumulation and toxicity of short chain chlorinated paraffins to wheat (Triticum aestivum L.)

Short chain chlorinated paraffins (SCCPs) are ubiquitous persistent organic pollutants. They have been widely detected in plant-based foods and might cause adverse impacts on humans. Nevertheless, uptake and accumulation mechanisms of SCCPs in plants remain unclear. In this study, the soil culture data indicated that SCCPs were strongly absorbed by roots (root concentration factor, RCF>1) yet limited translocated to shoots (translocation factor<1). The uptake mechanism was explored by hydroponic exposure, showing that hydrophobicity and molecular size influenced the root uptake and translocation of SCCPs. RCFs were significantly correlated with logKow values and molecular weights in a parabolic curve relationship. Besides, it was extremely difficult for SCCPs to translocate from shoots back to roots via phloem. An active energy-dependent process was proposed to be involved in the root uptake of SCCPs, which was supported by the uptake inhibition by the low temperature and metabolic inhibitor. Though SCCPs at environmentally relevant concentrations had no negative impacts on root morphology and chlorophyll contents, it caused obvious changes in cellular ultrastructure of root tip cells and induced a significant increase in superoxide dismutase activity. This information may be beneficial to moderate crop contamination by SCCPs, and to remedy soils polluted by SCCPs with plants.

Stability and Biotransformation of 6:2 Fluorotelomer Sulfonic Acid, Sulfonamide Amine Oxide, and Sulfonamide Alkylbetaine in Aerobic Sludge

The 6:2 fluorotelomer sulfonamide (6:2 FTSAm)-based compounds signify a prominent group of per- and polyfluoroalkyl substances (PFAS) widely used in contemporary aqueous film-forming foam (AFFF) formulations. Despite their widespread presence, the biotransformation behavior of these compounds in wastewater treatment plants remains uncertain. This study investigated the biotransformation of 6:2 FTSAm-based amine oxide (6:2 FTNO), alkylbetaine (6:2 FTAB), and 6:2 fluorotelomer sulfonic acid (6:2 FTSA) in aerobic sludge over a 100-day incubation period. The biotransformation of 6:2 fluorotelomer sulfonamide alkylamine (6:2 FTAA), a primary intermediate product of 6:2 FTNO, was indirectly assessed. Their stability was ranked based on the estimated half-lives (t1/2): 6:2 FTAB (no obvious products were detected) ≫ 6:2 FTSA (t1/2 ≈28.8 days) > 6:2 FTAA (t1/2 ≈11.5 days) > 6:2 FTNO (t1/2 ≈1.2 days). Seven transformation products of 6:2 FTSA and 15 products of 6:2 FTNO were identified through nontarget and suspect screening using high-resolution mass spectrometry. The transformation pathways of 6:2 FTNO and 6:2 FTSA in aerobic sludge were proposed. Interestingly, 6:2 FTSAm was hardly hydrolyzed to 6:2 FTSA and further biotransformed to perfluoroalkyl carboxylic acids (PFCAs). Furthermore, the novel pathways for the generation of perfluoroheptanoic acid (PFHpA) from 6:2 FTSA were revealed.

Using regular and transcriptomic analyses to investigate the biotransformation mechanism and phytotoxic effects of 6:2 fluorotelomer carboxylic acid (6:2 FTCA) in pumpkin (Cucurbita maxima L.)

Although 6:2 fluorotelomer carboxylic acid (6:2 FTCA), which is one of the most popular substitutes for perfluorooctanoic acid (PFOA), has been widely distributed in environments, little is known about its biotransformation mechanism and phytotoxic effects in plants. Here, we showed that 6:2 FTCA could be taken up by pumpkin (Cucurbita maxima L.) roots from exposure solution and acropetally translocated to shoots. Biotransformation of 6:2 FTCA to different carbon chain perfluorocarboxylic acid (PFCA) metabolites (C2-C7) via α-and β-oxidation in pumpkin was observed, and perfluorohexanoic acid (PFHxA) was the major transformation product. The results of enzyme assays, enzyme inhibition experiments and gene expression analysis indicated that cytochrome P450 (CYP450), glutathione-S-transferase (GST) and ATP-binding cassette (ABC) transporters were involved in the metabolism of 6:2 FTCA in pumpkin. Plant-associated rhizobacteria and endophyte also contributed to 6:2 FTCA degradation through β-oxidation. The chlorophyll (Chl) content and genes involved in photosynthesis were significantly improved by 6:2 FTCA. The reductions of antioxidant and metabolic enzyme activities reflected the antioxidant defense system and detoxification system of pumpkin were both damaged, which were further confirmed by the down-regulating associated genes encoding phenylpropanoid biosynthesis, endoplasmic reticulum-related proteins, ascorbate-glutathione cycle and ABC transporters. This study is helpful to understand the environmental behaviors and toxicological molecular mechanisms of 6:2 FTCA in plants.

Combustion of high-calorific industrial waste in conventional brick kilns: An emerging source of PFAS emissions to agricultural soils

The brick kilns in the South Asian region are widely documented to partially combust high-calorific waste components of synthetic-industrial origin, which contain hazardous constituents, including per- and polyfluoroalkyl substances (PFAS). Correspondingly, these establishments are necessarily built on agricultural land to easily acquire clay by excavating soil horizons, thus making cultivation soils vulnerable to PFAS contaminations. In this pioneering study, the occurrence, distribution profile, traceability and human health risk exposure to forty-four legacy and novel PFAS homologues, including two ultrashort-chain (C2-C3) PFAS, were investigated in agricultural soils around thirty-two conventional brick kilns across three districts of Pakistan. ⅀44PFAS concentrations ranged from 14.3 to 465 ng/g (median: 28.2 ng/g), which were 2 to 70 folds higher than those in background soils, and slightly higher than those reported in agricultural soils in the global literature. The highest occurrence was observed for PFAS alternatives, i.e., 6:2 fluorotelomer sulfonate (6:2 FTSA) (40 %) and 6:2 chlorinated polyfluorinated ether sulfonate (6:2 Cl-PFESA) (4.5 %). A significant positive correlation (p < 0.01) was observed among the concentrations of short-chain perfluoroalkyl acids (C4-C7) and novel PFOS substitutes, implying their origin from common sources. Furthermore, ultrashort-chain and short-chain perfluorocarboxylic acids (PFCA) (89 %) and perfluorosulfonic acids (PFSA) (63 %) dominated over long-chain's PFCA (11 %) and PFSA (37 %), respectively. The estimated daily intake to children exposed in surrounding inhabited communities, at 95th percentile concentrations was found to be approaching the European tolerable daily intake limit of 0.63 ng/kg bw/day. Therefore, the brick manufacturing industry is identified as a novel source of PFAS in the adjacent environment and for residents. This suggests the need for further investigations to elucidate the origin of emerging contaminants in the waste streams of the region to safeguard ecological integrity.

Degradation of 8:2 fluorotelomer carboxylic acid (8:2 FTCA) by plants and their co-existing microorganisms

8:2 fluorotelomer carboxylic acid (8:2 FTCA), an important precursor of perfluorocarboxylic acids (PFCAs), is widely detected in environment and biotas. Hydroponic exposures were conducted to investigate the accumulation and metabolism of 8:2 FTCA in wheat (Triticum aestivum L.) and pumpkin (Cucurbita maxima L.). Endophytic and rhizospheric microorganisms co-existing with the plants were isolated to investigate their contributions to degrade 8:2 FTCA. Wheat and pumpkin roots could take up 8:2 FTCA efficiently with the root concentration factor (RCF) as 5.78 and 8.93, respectively. 8:2 FTCA could be biotransformed to 8:2 fluorotelomer unsaturated carboxylic acid (8:2 FTUCA), 7:3 fluorotelomer carboxylic acid (7:3 FTCA), and seven PFCAs with 2-8 carbon chain length in plant roots and shoots. Cytochromes P450 (CYP450) and glutathione-S-transferase (GST) activities in plants were significantly increased, while flavin-dependent monooxygenases (FMOs) activities were not changed, suggesting that CYP 450 and GST were involved in the transformation of 8:2 FTCA in plant tissues. Twelve 8:2 FTCA-degrading endophytic (8 strains) and rhizospheric (4 strains) bacterial strains were isolated from root interior, shoot interior and rhizosphere of plants, respectively. These bacteria were identified as Klebsiella sp. based on the morphology and 16S rDNA sequence, and they could biodegrade 8:2 FTCA to intermediates and stable PFCAs.

Spatio-temporal trends in livestock exposure to per- and polyfluoroalkyl substances (PFAS) inform risk assessment and management measures

The migration of per- and polyfluoroalkyl substances (PFAS) onto agricultural properties has resulted in the accumulation of PFAS in livestock. The environmental determinants of PFAS accumulation in livestock from the grazing environment are poorly understood, resulting in limited capacity to manage livestock exposure and subsequent transfer of PFAS through the food chain. Analytical- (n = 978 samples of soil, water, pasture, and serum matrices), farm management/practice- and livestock physiology data were collated and interrogated from environmental PFAS investigations across ten farms, from four agro-ecological regions of Victoria (Australia). Statistical analysis identified perfluorooctane sulfonate (PFOS) and perfluorohexane sulfonate (PFHxS) as key analytes of concern for livestock bioaccumulation. PFOS and PFHxS concentrations in livestock drinking water were positively correlated with serum concentrations while other intake pathways (pasture and soil) had weaker correlations. Seasonal trends in PFAS body burden (serum concentrations) were identified and suggested to be linked to seasonal grazing behaviours and physiological water requirements. The data showed for the first time that livestock exposure to PFAS is dynamic and with relatively short elimination half-lives, there is opportunity for exposure management. Meat from cattle, grazed on PFAS impacted sites, may exceed health-based guideline values for PFAS, especially for markets with low limits (like the European Commission Maximum Limits or EC MLs). This study found that sites with mean livestock drinking water concentrations as low as 0.003 μg PFOS/L may exceed the EC ML for PFOS in cattle meat. Risk assessment can be used to prioritise site cleanup and development of management plans to reduce PFAS body burden by considering timing of stock rotation and/or supplementation of primary exposure sources.

Bioaccumulation of per- and polyfluoroalkyl substances by freshwater benthic macroinvertebrates: Impact of species and sediment organic carbon content

Per- and polyfluoroalkyl substances (PFAS) in aquatic environments have caused global concern due to their persistence, toxicity, and potential bioaccumulation of some compounds. As an important compartment of the aquatic ecosystem, sediment properties impact PFAS partitioning between aqueous and solid phases, but little is known about the influence of sediment organic carbon content on PFAS bioaccumulation in benthic organisms. In this study, three freshwater benthic macroinvertebrates - worms (Lumbriculus variegatus), mussels (Elliptio complanata) and snails (Physella acuta) - were exposed for 28 days to PFAS spiked synthetic sediment equilibrated with a synthetic surface water. Using microcosms, sediment organic carbon content - 2%, 5% and 8% - was manipulated to assess its impact on PFAS bioaccumulation. Worms were found to have substantially greater PFAS bioaccumulation compared to mussels and snails. The bioaccumulation factors (BAFs) and biota sediment accumulation factors (BSAFs) in worms were both one to two magnitudes higher than in mussels and snails, likely due to different habitat-specific uptake pathways and elimination capacities among species. In these experiments, increasing sediment organic carbon content decreased the bioaccumulation of PFAS to benthic macroinvertebrates. In worms, sediment organic carbon content was hypothesized to impact PFAS bioaccumulation by affecting PFAS partitioning and sediment ingestion rate. Notably, the BSAF values of 8:2 fluorotelomer sulfonic acid (FTS) were the largest among 14 PFAS for all species, suggesting that the benthic macroinvertebrates probably have different metabolic mechanisms for fluorotelomer sulfonic acids compared to fish evaluated in published literature. Understanding the impact of species and sediment organic carbon on PFAS bioaccumulation is key to developing environmental quality guidelines and evaluating potential ecological risks to higher trophic level species.

Uptake and translocation of both legacy and emerging per- and polyfluorinated alkyl substances in hydroponic vegetables

The extensive use of per- and polyfluorinated alkyl substances (PFASs) and their substitutes has resulted in their frequent detections in environmental matrices. However, limited information is known about their uptake into vegetables and health risk through diet, particularly for those emerging alternatives. In this study, a total of 17 PFASs (namely 12 legacy PFASs and 5 of their alternatives) were compared for their accumulation into four staple vegetables (lettuce, Chinese cabbage, chrysanthemum coronarium, and cucumber) in hydroponic system with single PFAS concentration being 10 μg/L, except for 8:2 chlorinated polyfluoroalkyl ether sulfonate (Cl-PFESA) as 0.5 μg/L. The sum concentrations of 17 PFASs in edible parts were in the order of Chinese cabbage leaf (13,456 ng/g) > lettuce leaf (5996 ng/g) > cucumber fruit (4115 ng/g) >chrysanthemum coronarium stem (3999 ng/g). For perfluorooctanoate acid (PFOA) and its alternatives, hexafluoropropylene oxide trimer acid (HFPO-TA) preferentially accumulated in roots than PFOA with root concentration factors being 35.7-99.9. Translocation to edible parts was more remarkable for hexafluoropropylene oxide dimer acid (HFPO-DA) compared with PFOA in lettuce and cucumber. For perfluorooctanesulfonate acid (PFOS) and its alternatives, roots of all the four vegetables were found to more readily accumulate 8:2 Cl-PFESA than PFOS, but 8:2 Cl-PFESA was hardly translocated to the aerial parts. Significantly (p < 0.05) higher edible concentrations of 8:2 and 6:2 fluorotelomer sulfonic acids (FTSA) than that of PFOS were observed for cucumber.

Critical review on phytoremediation of polyfluoroalkyl substances from environmental matrices: Need for global concern

Poly- and perfluoroalkyl substances (PFAS) are a class of emerging organic contaminants that are impervious to standard physicochemical treatments. The widespread use of PFAS poses serious environmental issues. PFAS pollution of soils and water has become a significant issue due to the harmful effects of these chemicals both on the environment and public health. Owing to their complex chemical structures and interaction with soil and water, PFAS are difficult to remove from the environment. Traditional soil remediation procedures have not been successful in reducing or removing them from the environment. Therefore, this review focuses on new phytoremediation techniques for PFAS contamination of soils and water. The bioaccumulation and dispersion of PFAS inside plant compartments has shown great potential for phytoremediation, which is a promising and unique technology that is realistic, cost-effective, and may be employed as a wide scale in situ remediation strategy.

Chronic toxicity of PFAS-free AFFF alternatives in terrestrial plant Brassica rapa

Fluorine (F)-free firefighting foams will be replacing per- and polyfluoroalkyl substances (PFAS)-containing aqueous film-forming foams (AFFFs) at U.S. military installations imminently, yet the environmental impacts of F-free foams are largely unknown. Ecotoxicity assessment of F-free foams is urgently needed to avoid replacement regret. In this study, we comparatively assessed phytotoxicity of six F-free formulations and one current short-chain fluorinated AFFF in terrestrial plant Brassica rapa. Five of six F-free formulations exerted higher toxicity than the short-chain AFFF to the growth and reproduction of B. rapa, with 8-51 times and > 10 times lower EC₅₀ values, respectively. Nontargeted analysis indicated the occurrence of transformation products of the test formulations in the above-ground plant tissues. In agreement with their phytotoxicity, the five highly toxic F-free formulations generated more transformation products with higher peak intensities in plant tissues than the two weakly toxic formulations. The most abundant transformation products detected in plant extracts were suspect transformation products derived from diethylene glycol monobutyl ether, a common ingredient of the five toxic formulations. This study provides ecotoxicological data that, combined with data from all related ongoing research, should be used in decision making regarding recommendations for manufacturing and use of candidate F-free foams.

Behaviors of 6:2 fluorotelomer sulfonamide alkylbetaine (6:2 FTAB) in wheat seedlings: Bioaccumulation, biotransformation and ecotoxicity

As a new alternative to perfluorooctane sulfonate (PFOS), 6:2 fluorotelomer sulfonamide alkylbetaine (6:2 FTAB) has been currently used in industrial and consumer applications, which has been frequently detected in environment media. However, the behaviors of 6:2 FTAB in plants are still unclear. This study investigated the bioaccumulation, biotransformation and ecotoxicity of 6:2 FTAB in wheat (Triticum aestivum L.) by hydroponic exposure. 6:2 FTAB was easily taken up by roots with the root concentration factor (RCF) as high as 94.8, but difficult to be acropetally translocated in the shoots with the translocation factor (TF) as low as 0.058. Two intermediates and six terminal perfluorocarboxylic acid (PFCA) metabolites were detected in roots and shoots. The detected metabolites included 6:2 fluorotelomer sulfonic acid (6:2 FTSA), 6:2 fluorotelomer carboxylic acid (6:2 FTCA), perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), perfluorobutyric acid (PFBA), pentafluoropropionic acid (PFPrA) and trifluoroacetic acid (TFA), and 6:2 FTSA was the main metabolite. 6:2 FTAB significantly reduced the biomass of plant and prevented chlorophyll (Chl) accumulation, while caused no significant change in malondialdehyde (MDA) content. Significant reduction in glutathione (GSH) contents, excess production of reactive oxygen species (ROS), and obvious inhibition of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and glutathione-s-transferase (GST) activities were observed, suggesting damage of antioxidant defense systems and failure to detoxication of 6:2 FTAB in wheat. These findings provide important knowledge for the fate of 6:2 FTAB in plants.

Insights into the impacts of dissolved organic matter of different origins on bioaccumulation and translocation of per- and polyfluoroalkyl substances (PFASs) in wheat

Per- and polyfluoroalkyl substances (PFASs) have been found to be widely present in soil. Dissolved organic matter (DOM) in soil are supposed to greatly affect the bioavailability of PFASs in soil. Herein, hydroponic experiments were conducted to understand the impacts of two kinds of typical DOM, bovine serum albumin (BSA) and humic acid (HA), on the uptake and translocation of legacy PFASs and their emerging alternatives, perfluorooctane sulfonic acid (PFOS), perfluorooctane acid (PFOA), perfluorohexane sulfonic (PFHxS) and 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA) in wheat (Triticum aestivum L.). The results indicated that both HA and BSA significantly inhibited the bioaccumulation and translocation of PFASs in the roots and shoots of wheat, and the impacts of BSA were greater than HA. This difference was explained by the greater binding affinities of the four PFASs with BSA than with HA, as evidenced by the equilibrium dialysis and isothermal titration calorimetry (ITC) analyses. It was noting that inhibition impacts of the BSA-HA mixture (1:1) were lower than BSA alone. The results of Fourier transform infrared (FT-IR) spectroscopy and excitation-emission matrix (EEM) fluorescence spectroscopy suggested that HA could bind with the fluorescent tryptophan residues in BSA greatly, competing the binding sites with PFASs and forming a cover on the surface of BSA. As a result, the binding of PFASs with BSA-HA complex was much lower than that with BSA, but close to HA. The results of this study shed light on the impacts of DOM in soil on the bioaccumulation and translocation of PFASs in plants.

Enzymatic synthesis of fluorinated compounds

Fluorinated compounds are widely used in the fields of molecular imaging, pharmaceuticals, and materials. Fluorinated natural products in nature are rare, and the introduction of fluorine atoms into organic compound molecules can give these compounds new functions and make them have better performance. Therefore, the synthesis of fluorides has attracted more and more attention from biologists and chemists. Even so, achieving selective fluorination is still a huge challenge under mild conditions. In this review, the research progress of enzymatic synthesis of fluorinated compounds is summarized since 2015, including cytochrome P450 enzymes, aldolases, fluoroacetyl coenzyme A thioesterases, lipases, transaminases, reductive aminases, purine nucleoside phosphorylases, polyketide synthases, fluoroacetate dehalogenases, tyrosine phenol-lyases, glycosidases, fluorinases, and multienzyme system. Of all enzyme-catalyzed synthesis methods, the direct formation of the C-F bond by fluorinase is the most effective and promising method. The structure and catalytic mechanism of fluorinase are introduced to understand fluorobiochemistry. Furthermore, the distribution, applications, and future development trends of fluorinated compounds are also outlined. Hopefully, this review will help researchers to understand the significance of enzymatic methods for the synthesis of fluorinated compounds and find or create excellent fluoride synthase in future research.Key points• Fluorinated compounds are distributed in plants and microorganisms, and are used in imaging, medicine, materials science.• Enzyme catalysis is essential for the synthesis of fluorinated compounds.• The loop structure of fluorinase is the key to forming the C-F bond.

Interactions between Lemna minor (common duckweed) and PFAS intermediates: Perfluorooctanesulfonamide (PFOSA) and 6:2 fluorotelomer sulfonate (6:2 FTSA)

Perfluorooctanesulfonamide (PFOSA) and 6:2 fluorotelomer sulfonate (FTSA) are widely present intermediates of per- and polyfluorinated substances (PFAS). Although detected at high concentrations in landfill leachate and groundwater, the interactions of these two compounds with plants have not been investigated much. In this work, uptake of these two PFAS intermediates at 10 and 200 μg/L by Lemna minor (common duckweed) were studied in detail. It was found that the biomass production of L. minor was not impacted negatively by PFOSA and FTSA at concentrations equal to or lower than 200 μg/L. Between these two target compounds, FTSA had much higher concentrations in L. minor when the concentrations and exposure times were the same as those for PFOSA. In addition, this compound at 200 μg/L inhibited the activities of catalase in L. minor significantly compared to the controls. This study indicates that PFOSA with low water solubility has low toxicity to L. minor, while FTSA at high concentration may accumulate in the floating plants and cause adverse effects on plant's antioxidative defense system. Longer-term studies of L. minor with these two and other PFAS are warranted given the important role of this floating plant in the ecosystem.

Extending the knowledge about PFAS bioaccumulation factors for agricultural plants - A review

A main source of perfluoroalkyl and polyfluoroalkyl substances (PFASs) residues in agricultural plants is their uptake from contaminated soil. Bioaccumulation factors (BAFs) can be an important tool to derive recommendations for cultivation or handling of crops prior consumption. This review compiles >4500 soil-to-plant BAFs for 45 PFASs from 24 studies involving 27 genera of agricultural crops. Grasses (Poaceae) provided most BAFs with the highest number of values for perfluorooctanoic acid and perfluorooctane sulfonic acid. Influencing factors on PFAS transfer like compound-specific properties (hydrophobicity, chain length, functional group, etc.), plant species, compartments, and other boundary conditions are critically discussed. Throughout the literature, BAFs were higher for vegetative plant compartments than for reproductive and storage organs. Decreasing BAFs per additional perfluorinated carbon were clearly apparent for aboveground parts (up to 1.16 in grains) but not always for roots (partly down to zero). Combining all BAFs per single perfluoroalkyl carboxylic acid (C4-C14) and sulfonic acid (C4-C10), median log BAFs decreased by -0.25(±0.029) and -0.24(±0.013) per fluorinated carbon, respectively. For the first time, the plant uptake of ultra-short-chain (≤ C3) perfluoroalkyl acids (PFAAs) was reviewed and showed a ubiquitous occurrence of trifluoroacetic acid in plants independent from the presence of other PFAAs. Based on identified knowledge gaps, it is suggested to focus on the uptake of precursors to PFAAs, PFAAs ≤C3, and additional emerging PFASs such as GenX or fluorinated ethers in future research. Studies regarding the uptake of PFASs by sugar cane, which accounts for about one fifth of the global crop production, are completely lacking and are also recommended. Furthermore, aqueous soil leachates should be tested as an alternative to the solvent extraction of soils as a base for BAF calculations.

Formation of perfluorocarboxylic acids (PFCAs) during the exposure of earthworms to 6:2 fluorotelomer sulfonic acid (6:2 FTSA)

6:2 fluorotelomer sulfonic acid (6:2 FTSA) is a novel perfluorooctane sulfonate (PFOS) alternative used globally in aqueous film forming foams (AFFFs). Although 6:2 FTSA has been recently detected in the environment, its fate in terrestrial invertebrates remains unclear. The uptake, elimination and biotransformation of 6:2 FTSA in earthworms (Eisenia fetida) were investigated after in vivo and in vitro exposure. 6:2 FTSA could be biodegraded by microorganisms in soil to trifluoroacetic acid (TFA), perfluoropropionic acid (PFPrA), perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA) and perfluorohexanoic acid (PFHxA). The uptake rate constant (k_u) and biota-to-soil accumulation factor (BSAF) of 6:2 FTSA in earthworms were 0.185 g_oc/g_ww/d and 0.685 g_oc/g_ww, respectively, indicating high bioaccumulative ability in earthworms. Five terminal perfluorocarboxylic acids (PFCAs) metabolites, including TFA, PFPrA, PFBA, PFPeA and PFHxA were observed in both in vivo and in vitro exposure tests, with TFA as the predominant metabolite. However, no perfluoroheptanoic acid (PFHpA) was observed in the present study. The elimination rate constants (k_e) increased in the order: 6:2 FTSA (0.057/d) < TFA (0.058/d) < PFPrA (0.071/d) < PFBA (0.084/d) < PFHxA (0.182/d) < PFPeA (0.193/d). Biodegradation of 6:2 FTSA in the earthworm homogenates, cytolchrome P450 (CYP450) enzyme solutions and glutathione-s-transferase (GST) enzyme solutions fitted well with the first order kinetics. The biotransformation rate constants (k) were in the following order: homogenates (0.012/h) > CYP450 (0.009/h) > GST (0.007/h), implying that CYP450 and GST were involved in biotransformation of 6:2 FTSA in earthworms. This study provides important theoretical evidence for the fate of 6:2 FTSA in earthworms.

Biotransformation of perfluoroalkyl acid precursors from various environmental systems: advances and perspectives

Perfluoroalkyl acids (PFAAs) are widely used in industrial production and daily life because of their unique physicochemical properties, such as their hydrophobicity, oleophobicity, surface activity, and thermal stability. Perfluorosulfonic acids (PFSAs) and perfluorocarboxylic acids (PFCAs) are the most studied PFAAs due to their global occurrence. PFAAs are environmentally persistent, toxic, and the long-chain homologs are also bioaccumulative. Exposure to PFAAs may arise directly from emission or indirectly via the environmental release and degradation of PFAA precursors. Precursors themselves or their conversion intermediates can present deleterious effects, including hepatotoxicity, reproductive toxicity, developmental toxicity, and genetic toxicity. Therefore, exposure to PFAA precursors constitutes a potential hazard for environmental contamination. In order to comprehensively evaluate the environmental fate and effects of PFAA precursors and their connection with PFSAs and PFCAs, we review environmental biodegradability studies carried out with microbial strains, activated sludge, plants, and earthworms over the past decade. In particular, we review perfluorooctyl-sulfonamide-based precursors, including perfluroooctane sulfonamide (FOSA) and its N-ethyl derivative (EtFOSA), N-ethyl perfluorooctane sulfonamido ethanol (EtFOSE), and EtFOSE-based phosphate diester (DiSAmPAP). Fluorotelomerization-based precursors are also reviewed, including fluorotelomer alcohols (FTOH), fluorotelomer sulfonates (FTSA), and a suite of their transformation products. Though limited information is currently available on zwitterionic PFAS precursors, a preliminary review of data available for 6:2 fluorotelomer sulfonamide betaine (FTAB) was also conducted. Furthermore, we update and refine the recent knowledge on biotransformation strategies with a focus on metabolic pathways and mechanisms involved in the biotransformation of PFAA precursors. The biotransformation of PFAA precursors mainly involves the cleavage of carbon-fluorine (C-F) bonds and the degradation of non-fluorinated functional groups via oxidation, dealkylation, and defluorination to form shorter-chained PFAAs. Based on the existing research, the current problems and future research directions on the biotransformation of PFAA precursors are proposed.

Uptake and accumulation of per- and polyfluoroalkyl substances in plants

Per- and polyfluoroalkyl substances (PFASs) are a class of persistent organic contaminants that are ubiquitous in the environment and have been found to be accumulated in agricultural products. Consumption of PFAS-contaminated agricultural products represents a feasible pathway for the trophic transfer of these toxic chemicals along food chains/webs, leading to risks associated with human and animal health. Recently, studies on plant uptake and accumulation of PFASs have rapidly increased; consequently, a review to summarize the current knowledge and highlight future research is needed. Analysis of the publications indicates that a large variety of plant species can take up PFASs from the environment. Vegetables and grains are the most commonly investigated crops, with perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) as the most studied PFASs. The potential sources of PFASs for plant uptake include industrial emissions, irrigation with contaminated water, land application of biosolids, leachates from landfill sites, and pesticide application. Root uptake is the predominant pathway for the accumulation of PFASs in agricultural crops, and uptake by plant aboveground portions from the ambient atmosphere could play a minor role in the overall PFAS accumulation. PFAS uptake by plants is influenced by physicochemical properties of compounds (e.g., perfluorocarbon chain length, head group functionality, water solubility, and volatility), plant physiology (e.g., transpiration rate, lipid and protein content), and abiotic factors (e.g., soil organic matters, pH, salinity, and temperature). Based on literature analysis, the current knowledge gaps are identified, and future research prospects are suggested.

Uptake and translocation of perfluoroalkyl acids (PFAAs) in hydroponically grown red chicory (Cichorium intybus L.): Growth and developmental toxicity, comparison with growth in soil and bioavailability implications

Short-chain perfluoroalkyl acids (PFAAs) have shown a high potential for plant (crop) uptake, making them possibly significant contributors to the total dietary exposure to PFAAs. The plant uptake of PFAAs is a complex process that needs better characterization, as it does not only depend on perfluoroalkyl chain length, but also on their polar terminal group, on the plant species and the exposure media. Here, a plant uptake study with nine perfluoroalkyl acids (PFAAs) was carried out under the hydroponic (soilless) exposure conditions. Red chicory was grown in a nutrient solution, spiked with PFAAs mixture at three different concentrations (i.e. 62.5, 125 and 250 μg/L), in order to extend the range of levels tested and reported in the literature so far. Bioaccumulation metrics and transpiration stream concentration factors (TSCFs) were employed for the plant uptake characterization and consequent comparison with the results of soil uptake experiment we previously performed with the same crop. The results showed that calculated root concentration factors (RCFs) increase with PFAA chain length, while the opposite chain length dependence was present for shoots. Plants from two treatments with the highest PFAAs concentrations manifested physiological changes (discoloration, inhibited roots and leaves growth), despite of the used exposure concentrations being much lower than previously published phytotoxicity thresholds. A comparison among RCFs and TSCFs derived from hydroponic and from the soil experiment has emphasized their different magnitudes and PFAAs chain length dependence patterns. They could not be ascribed only to soil sorption as a process decreasing PFAAs bioavailability for plants, but also to developmental differences between the root systems formed in soil and in nutrient solution and to the potential competitive PFAAs sorption to roots in hydroponics. The interchangeable use of bioaccumulation and translocation parameters derived in hydroponic and soil systems would lead to erroneous conclusions and plant uptake predictions.

Bioavailability and Bioaccumulation of 6:2 Fluorotelomer Sulfonate, 6:2 Chlorinated Polyfluoroalkyl Ether Sulfonates, and Perfluorophosphinates in a Soil-Plant System

As emerging alternatives of legacy perfluoroalkyl substances, 6:2 fluorotelomer sulfonate (6:2 FTS), 6:2 chlorinated polyfluoroalkyl ether sulfonates (6:2 Cl-PFESA), and perfluorophosphinates (C6/C6 and C8/C8 PFPiAs) are supposed to be partitioned to soil and highly persistent in the environment. The uptake of novel per- and polyfluoroalkyl substances (PFASs) by plants represents a potential pathway for their transfer in the food chain. In this study, the bioavailability of these four novel PFASs in soil and the bioaccumulation characteristics in greenhouse-grown wheat (Triticum aestivum L.), maize (Zea mays L.), soybean (Glycine max L. Merrill), and pumpkin (Cucurbita maxima L.) were investigated. The results indicated that these novel PFASs with higher hydrophobicity were more easily sequestrated in soil, and the fractions extracted by methanol could well describe their bioavailability, which could be stimulated by low-molecular-weight organic acids at rhizospheric concentrations. A negative relationship was found between root soil concentration factors (RSCFs) and hydrophobicity (log Kow) of the target PFASs. This correlation was also found in the translocation factors (TF) from roots to shoots. Furthermore, the uptake and transfer of the target PFASs were regulated by the protein contents in plant roots and shoots.