Aerobic biotransformation of 6:2 fluorotelomer sulfonate by Dietzia aurantiaca J3 under sulfur-limiting conditions

A review of the occurrence and microbial transformation of per- and polyfluoroalkyl substances (PFAS) in aqueous film-forming foam (AFFF)-impacted environments

Aqueous film-forming foams (AFFFs) have been extensively used for extinguishing hydrocarbon-fuel fires at military sites, airports, and fire-training areas. Despite being a significant source of per- and polyfluoroalkyl substances (PFAS), our understanding of PFAS occurrence in AFFF formulations and AFFF-impacted environments is limited, as is the impact of microbial transformation on the environment fate of AFFF-derived PFAS. This literature review compiles PFAS concentrations in electrochemical fluorination (ECF)- and fluorotelomer (FT)-based AFFFs and provides an overview of PFAS occurrence in AFFF-impacted environments. Our analysis reveals that AFFF use is a predominant point source of PFAS contamination, including primary precursors (polyfluoroalkyl substances as AFFF components), secondary precursors (polyfluoroalkyl transformation products of primary precursors), and perfluoroalkyl acids (PFAAs). Moreover, there are discrepancies between PFAS concentration profiles in AFFFs and those measured in AFFF-impacted media. For example, primary precursors constitute 52.6 % and 99.5 % of PFAS mass in ECF- and FT-based AFFFs, respectively, whereas they represent only 0.7 % total mass in AFFF-impacted groundwater. Conversely, secondary precursors, which constitute <1 % of PFAS in AFFFs, represent 4.0-27.8 % of PFAS in AFFF-impacted environments. The observed differences in PFAS levels between AFFFs and environmental samples are likely due to in-situ biotransformation processes. Biotransformation rates and pathways reported for AFFF-derived primary and secondary precursors varied among different classes of precursors, consistent with the PFAS occurrence in AFFF-impacted environments. For example, readily biodegradable primary precursors, N-dimethyl ammonio propyl perfluoroalkane sulfonamide (AmPr-FASA) and n:2 fluorotelomer thioether amido sulfonate (n:2 FtTAoS), were rarely detected in AFFF-impacted environments. In contrast, key secondary precursors, perfluoroalkane sulfonamides (FASAs) and n:2 fluorotelomer sulfonate (n:2 FTS), were widely detected, which was attributed to their resistance to biotransformation. Key knowledge gaps and future research priorities are presented to better understand the occurrence, fate, and transport of AFFF-derived PFAS in the environment and to design more effective remediation strategies.

Distinctive biotransformation and biodefluorination of 6:2 versus 5:3 fluorotelomer carboxylic acids by municipal activated sludge

Fluorotelomer carboxylic acids (FTCAs) represent an important group of per- and polyfluoroalkyl substances (PFAS) given their high toxicity, bioaccumulation potential, and frequent detection in landfill leachates and PFAS-impacted sites. In this study, we assessed the biodegradability of 6:2 FTCA and 5:3 FTCA by activated sludges from four municipal wastewater treatment plants (WWTPs) in the New York Metropolitan area. Coupling with 6:2 FTCA removal, significant fluoride release (0.56∼1.83 F-/molecule) was evident in sludge treatments during 7 days of incubation. Less-fluorinated transformation products (TPs) were formed, including 6:2 fluorotelomer unsaturated carboxylic acid (6:2 FTUCA), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), and perfluorobutanoic acid (PFBA). In contrast, little fluoride (0.01∼0.09 F-/molecule) was detected in 5:3 FTCA-dosed microcosms, though 25∼68% of initially dosed 5:3 FTCA was biologically removed. This implies the dominance of "non-fluoride-releasing pathways" that may contribute to the formation of CoA adducts or other conjugates over 5:3 FTCA biotransformation. The discovery of defluorinated 5:3 FTUCA revealed the possibility of microbial attacks of the C-F bond at the γ carbon to initiate the transformation. Microbial community analysis revealed the possible involvement of 9 genera, such as Hyphomicrobium and Dechloromonas, in aerobic FTCA biotransformation. This study unraveled that biotransformation pathways of 6:2 and 5:3 FTCAs can be divergent, resulting in biodefluorination at distinctive degrees. Further research is underscored to uncover the nontarget TPs and investigate the involved biotransformation and biodefluorination mechanisms and molecular basis.

Using Network Analysis and Predictive Functional Analysis to Explore the Fluorotelomer Biotransformation Potential of Soil Microbial Communities

Microbial transformation of per- and polyfluoroalkyl substances (PFAS), including fluorotelomer-derived PFAS, by native microbial communities in the environment has been widely documented. However, few studies have identified the key microorganisms and their roles during the PFAS biotransformation processes. This study was undertaken to gain more insight into the structure and function of soil microbial communities that are relevant to PFAS biotransformation. We collected 16S rRNA gene sequencing data from 8:2 fluorotelomer alcohol and 6:2 fluorotelomer sulfonate biotransformation studies conducted in soil microcosms under various redox conditions. Through co-occurrence network analysis, several genera, including Variovorax, Rhodococcus, and Cupriavidus, were found to likely play important roles in the biotransformation of fluorotelomers. Additionally, a metagenomic prediction approach (PICRUSt2) identified functional genes, including 6-oxocyclohex-1-ene-carbonyl-CoA hydrolase, cyclohexa-1,5-dienecarbonyl-CoA hydratase, and a fluoride-proton antiporter gene, that may be involved in defluorination. This study pioneers the application of these bioinformatics tools in the analysis of PFAS biotransformation-related sequencing data. Our findings serve as a foundational reference for investigating enzymatic mechanisms of microbial defluorination that may facilitate the development of efficient microbial consortia and/or pure microbial strains for PFAS biotransformation.

Toward the development of a molecular toolkit for the microbial remediation of per-and polyfluoroalkyl substances

Per- and polyfluoroalkyl substances (PFAS) are highly fluorinated synthetic organic compounds that have been used extensively in various industries owing to their unique properties. The PFAS family encompasses diverse classes, with only a fraction being commercially relevant. These substances are found in the environment, including in water sources, soil, and wildlife, leading to human exposure and fueling concerns about potential human health impacts. Although PFAS degradation is challenging, biodegradation offers a promising, eco-friendly solution. Biodegradation has been effective for a variety of organic contaminants but is yet to be successful for PFAS due to a paucity of identified microbial species capable of transforming these compounds. Recent studies have investigated PFAS biotransformation and fluoride release; however, the number of specific microorganisms and enzymes with demonstrable activity with PFAS remains limited. This review discusses enzymes that could be used in PFAS metabolism, including haloacid dehalogenases, reductive dehalogenases, cytochromes P450, alkane and butane monooxygenases, peroxidases, laccases, desulfonases, and the mechanisms of microbial resistance to intracellular fluoride. Finally, we emphasize the potential of enzyme and microbial engineering to advance PFAS degradation strategies and provide insights for future research in this field.

Dynamic spatiotemporal changes of per- and polyfluoroalkyl substances (PFAS) in soil and eggs of private gardens at different distances from a fluorochemical plant

Homegrown food serves as an important human exposure source of per- and polyfluoroalkyl substances (PFAS), yet little is known about their spatiotemporal distribution within and among private gardens. This knowledge is essential for more accurate site-specific risk assessment, identification of new sources and evaluating the effectiveness of regulations. The present study evaluated spatiotemporal changes of legacy and emerging PFAS in surface soil from vegetable gardens (N = 78) and chicken enclosures (N = 102), as well as in homegrown eggs (N = 134) of private gardens, across the Province of Antwerp (Belgium). Hereby, the potential influence of the wind orientation and distance towards a major fluorochemical plant was examined. The ∑short-chain PFAS and precursor concentrations were higher in vegetable garden soil (8.68 ng/g dry weight (dw)) compared to chicken enclosure soil (4.43 ng/g dw) and homegrown eggs (0.77 ng/g wet weight (ww)), while long-chain sulfonates and C11-14 carboxylates showed the opposite trend. Short-term (2018/2019-2022) changes were mostly absent in vegetable garden soil, while changes in chicken enclosure soils oriented S-SW nearby (<4 km) the fluorochemical plant were characterized by a local, high-concentration plume. Moreover, soil from chicken enclosures oriented SE and remotely from the plant site was characterized by a widespread, diffuse but relatively low-concentration plume. Long-term data (2010-2022) suggest that phaseout and regulatory measures have been effective, as PFOS concentrations nearby the fluorochemical plant in soil and eggs have declined from 25.8 to 2.86 ng/g dw and from 528 to 39.4 ng/g ww, respectively. However, PFOS and PFOA concentrations have remained largely stable within this timeframe in gardens remotely from the plant site, warranting further rapid regulation and remediation measures. Future monitoring efforts are needed to allow long-term comparison for multiple PFAS and better distinction from potential confounding variables, such as variable emission outputs and variability in wind patterns.

The effects of two short-chain perfluoroalkyl carboxylic acids (PFCAs) on northern leopard frog (Rana pipiens) tadpole development

Short-chain perfluoroalkyl carboxylic acids (PFCAs) have been detected in the environment globally. The presence and persistence of these compounds in the environment may lead to chronic wildlife exposure. We used northern leopard frog (Rana pipiens) tadpoles to investigate the chronic toxicity and the bioconcentration of two short-chain PFCAs, perfluorobutanoic acid (PFBA) and perfluorohexanoic acid (PFHxA). We exposed Gosner stage 25 tadpoles to PFBA and PFHxA (as individual chemicals) at nominal concentrations of 0.1, 1, 10, 100, and 1000 µg/L for 43-46 days. Tadpoles exposed to 0.1 to 100 µg/L of PFBA and PFHxA had significantly higher mean snout-to-vent lengths, mean masses, and scaled mass indexes than control tadpoles. These results indicate that exposure to short-chain PFCAs influences tadpole growth. Further investigation into the mechanism(s) causing the observed changes in tadpole growth is warranted. We observed a significantly higher proportion of males in the PFBA 1 µg/L treatment group, however further histological analyses are required to confirm visual sex identification before making concrete conclusions on the effects of PFCAs on amphibian sex ratios. PFBA concentrations in tissues were higher than PFHxA concentrations; a pattern that contrasts with previously published studies using fish, suggesting potential differences between taxa in PFBA and PFHxA bioconcentration. Bioconcentration factors were <10 L/kg wet weight, indicating low bioconcentration potential in tadpoles. Our results suggest that PFBA and PFHxA may have effects at environmentally-relevant concentrations (0.1-10 µg/L) and further investigation is required before these compounds can be deemed a "safe" alternative to their long-chain counterparts.

Assessing construction and demolition wood-derived biochar for in-situ per- and polyfluoroalkyl substance (PFAS) removal from landfill leachate

With regulations for per-and polyfluoroalkyl substances (PFAS) impending, the abundance of these chemicals of emerging concern in municipal solid waste (MSW) landfill leachate increasingly challenges landfill operators to seek on-site leachate pre-treatment options. This two-staged study explores the potential reuse of biochar derived from construction and demolition debris (CDD) wood as an in-situ PFAS sorbent for application within MSW landfill leachate collection systems. Batch leaching tests were first used to examine the feasibility of capturing PFAS from landfill leachate using two sources of CDD-wood-derived biochar. Then, columns were used to test the in-situ sorption capabilities of the same biochars under simulated landfill conditions. All leachates were characterized for pH, chemical oxygen demand, ammonia-nitrogen, and 92 PFAS. Seventeen PFAS were detected in the batch leaching experiment, and nine PFAS were detected in column leachates. In the batch leaching scenario, Biochar 1 achieved a maximum of 29% PFAS reduction compared to controls. Columns containing Biochar 1 generated leachates with PFAS concentrations 50% to 80% higher than those in control columns for the duration of the experiment. Columns containing Biochar 2 generated leachates with PFAS concentrations 44% less than controls in week 1 and similar concentrations in weeks 2, 3, and 4. In this study, PFAS removal from landfill leachate using biochar derived from CDD wood was not significant. Further research on biochar derived from CDD wood is needed before it can be recommended as an in-situ landfill leachate pre-treatment method.

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.

Aerobic biotransformation of 6:2 fluorotelomer sulfonate in soils from two aqueous film-forming foam (AFFF)-impacted sites

Although 6:2 fluorotelomer sulfonate (6:2 FTS) is a common ingredient in aqueous film-forming foam (AFFF) formulations, its environmental fate at AFFF-impacted sites remains poorly understood. This study investigated the biotransformation of 6:2 FTS in microcosms prepared with soils collected from two AFFF-impacted sites; the former Loring Air Force Base (AFB) and Robins AFB. The half-life of 6:2 FTS in Loring soil was 43.3 days; while >60 mol% of initially spiked 6:2 FTS remained in Robins soil microcosms after a 224-day incubation. Differences in initial sulfate concentrations and the depletion of sulfate over the incubation likely contributed to the different 6:2 FTS biotransformation rates between the two soils. At day 224, stable transformation products, i.e., C4C7 perfluoroalkyl carboxylates, were formed with combined molar yields of 13.8 mol% and 1.2 mol% in Loring and Robins soils, respectively. Based on all detected transformation products, the biotransformation pathways of 6:2 FTS in the two soils were proposed. Microbial community analysis suggests that Desulfobacterota microorganisms may promote 6:2 FTS biotransformation via more efficient desulfonation. In addition, species from the genus Sphingomonas, which exhibited higher tolerance to elevated concentrations of 6:2 FTS and its biotransformation products, are likely to have contributed to 6:2 FTS biotransformation. This study demonstrates the potential role of biotransformation processes on the fate of 6:2 FTS at AFFF-impacted sites and highlights the need to characterize site biogeochemical properties for improved assessment of 6:2 FTS biotransformation behavior.

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.

Estimation of per- and poly-fluoroalkyl substances mass loads in the Danube River using passive sampling

As Europe's second longest river, the Danube is an important water source for drinking water and irrigation for many countries, before discharging into the Black Sea in the East. Per- and poly-fluoroalkyl substances (PFAS) have been observed over the last two decades in concentrations exceeding the European Union's drinking water guidelines for total sum of 20 select PFAS of 0.1 μg L-1. Their presence is a result of current and historical use and high environmental persistence, necessitating their monitoring for human risk assessments. The aim of this study is to use recently developed passive sampling technology to calculate time-integrated water concentrations and mass loads of 11 select PFAS at 9 sites along the Danube River. Results indicate ∑11 PFAS concentrations in the range of 9.3-29.6 ng L-1 were not in exceedance of EU drinking water guidelines, but perfluorooctanesulfonic acid (PFOS) was in exceedance of the environmental quality standard (0.65 ng L-1) at all sampling locations. The highest ∑11 PFAS mass loads were observed at Ruse (9.5 kg day-1) and Budapest (6.3 kg day-1), believed to be driven by proximity to industrial facilities and large populations (urban runoff). Finally, we estimate 4.9 kg of total PFAS (∑11 PFAS) were delivered to the Black Sea daily over Summer 2019.

Advances and perspectives of using stable isotope probing (SIP)-based technologies in contaminant biodegradation

Stable isotope probing (SIP) is a powerful tool to study microbial community structure and function in both nature and engineered environments. Coupling with advanced genomics and other techniques, SIP studies have generated substantial information to allow researchers to draw a clearer picture of what is occurring in complex microbial ecosystems. This review provides an overview of the advances of SIP-based technologies over time, summarizes the status of SIP applications to contaminant biodegradation, provides critical perspectives on ecological interactions within the community, and important factors (controllable and non-controllable) to be considered in SIP experimental designs and data interpretation. Current trend and perspectives of adapting SIP techniques for environmental applications are also discussed.

Simultaneous nitrate and sulfate biotransformation driven by different substrates: comparison of carbon sources and metabolic pathways at different C/N ratios

Nitrate (NO₃^-) and sulfate (SO₄^2-) often coexist in organic wastewater. The effects of different substrates on NO₃^- and SO₄^2- biotransformation pathways at various C/N ratios were investigated in this study. This study used an activated sludge process for simultaneous desulfurization and denitrification in an integrated sequencing batch bioreactor. The results revealed that the most complete removals of NO₃^- and SO₄^2- were achieved at a C/N ratio of 5 in integrated simultaneous desulfurization and denitrification (ISDD). Reactor Rb (sodium succinate) displayed a higher SO₄^2- removal efficiency (93.79%) with lower chemical oxygen demand (COD) consumption (85.72%) than reactor Ra (sodium acetate) on account of almost 100% removal of NO₃^- in both Ra and Rb. Ra produced more S^2- (5.96 mg L^-1) and H₂S (25 mg L^-1) than Rb, which regulated the biotransformation of NO₃^- from denitrification to dissimilatory nitrate reduction to ammonium (DNRA), whereas almost no H₂S accumulated in Rb which can avoid secondary pollution. Sodium acetate-supported systems were found to favor the growth of DNRA bacteria (Desulfovibrio); although denitrifying bacteria (DNB) and sulfate-reducing bacteria (SRB) were found to co-exist in both systems, Rb has a greater keystone taxa diversity. Furthermore, the potential carbon metabolic pathways of the two carbon sources have been predicted. Both succinate and acetate could be generated in reactor Rb through the citrate cycle and the acetyl-CoA pathway. The high prevalence of four-carbon metabolism in Ra suggests that the carbon metabolism of sodium acetate is significantly improved at a C/N ratio of 5. This study has clarified the biotransformation mechanisms of NO₃^- and SO₄^2- in the presence of different substrates and the potential carbon metabolism pathway, which is expected to provide new ideas for the simultaneous removal of NO₃^- and SO₄^2- from different media.

Degradation of per- and polyfluoroalkyl substances in landfill leachate by a thin-water-film nonthermal plasma reactor

Per- and polyfluoroalkyl substances (PFAS) are present in landfill leachate, posing potential challenges to leachate disposal and treatment. This work represents the first study of a thin-water-film nonthermal plasma reactor for PFAS degradation in landfill leachate. Of the 30 PFAS measured in three raw leachates, 21 were above the detection limits. The removal percentage depended on the category of PFAS. For example, perfluorooctanoic acid PFOA (C8) had the highest removal percentage (77% as an average of the three leachates) of the perfluoroalkyl carboxylic acids (PFCAs) category. The removal percentage decreased when the carbon number increased from 8 to 11 and decreased from 8 to 4. The effects of various landfill leachate components, including sodium chloride, acetate, humic acids, pH, and surfactants, had no or minor impacts (<30%) on PFOA mineralization in synthetic samples. This might be explained by the plasma-generation and PFAS-degradation mainly occurring at the gas/liquid interface. Shorter-chain PFCAs were produced as intermediates of PFOA degradation, and shorter-chain PFCAs and perfluorosulfonic acids (PFSAs) were produced as intermediates of perfluorooctanesulfonic acid (PFOS). The concentrations of the intermediates decreased with decreasing carbon number, suggesting a stepwise removal of difluoromethylene (CF₂) in the degradation pathway. Potential PFAS species in the raw and treated leachates were identified at the molecular level through non-targeted Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The intermediates did not show accurate toxicity per Microtox bioassay.

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

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