Chlorinated Ethenes Research Articles

Integrating Enzyme-Based Kinetics in Reactive Transport Models to Simulate Spatiotemporal Dynamics of Biomarkers during Chlorinated Ethene Degradation.

Biomarkers such as functional gene mRNA (transcripts) and proteins (enzymes) provide direct proof of metabolic regulation during the reductive dechlorination (RD) of chlorinated ethenes (CEs). Yet, current models to simulate their spatiotemporal variability are not flexible enough to mimic the homologous behavior of RDase functional genes. To this end, we developed new enzyme-based kinetics to model the concentrations of CEs together with the transcript and enzyme levels during RD. First, the model was calibrated to existing microcosm data on RD of cis-DCE. The model mirrored the tceA and vcrA gene expression and the production of their enzymes in Dehalococcoides spp. Considering tceA and vcrA as homologous instead of nonhomologous improved fitting of the mRNA time series. Second, CEs and biomarker patterns were explored as a proof of concept under groundwater flow conditions, considering degraders occurring in immobile and mobile states. Under both microcosm and flow conditions, biomarker-rate relationships were nonlinear hysteretic because tceA and vcrA acted as homologous genes. The mobile biomarkers additionally undergo advective-dispersive transport, which increases the nonlinearity and makes the observed patterns even more challenging to interpret. The model offers a thorough mechanistic description of RD while also allowing simulation of spatiotemporal dynamic patterns of various key biomarkers in aquifers.

Use of isotopic (C, Cl) and molecular biology tools to assess biodegradation in a source area of chlorinated ethenes after biostimulation with Emulsified Vegetable Oil (EVO)

Enhanced In Situ Bioremediation (EISB) using Emulsified Vegetable Oil (EVO) as a long-term electron donor has gained prominence for the treatment of groundwater contaminated with chlorinated ethenes (CEs). This study explores the potential of isotopic and molecular biology tools (MBT) to investigate the CEs (PCE, TCE and cis-DCE) bioremediation using EVO in a contaminated site. A multiple approach using C and Cl-CSIA, quantification of Dehalococcoides (Dhc) and specific reductive dechlorination (RD) gene population, and hydrochemical data in microcosm experiments and field samples was applied. Despite the high partitioning of CEs into the EVO phase, the carbon isotopic values of the remaining CEs fraction in the aqueous phase did not exhibit significant changes caused by phase partitioning in laboratory experiments. Both microcosm experiments and field data revealed a rapid RD of PCE and TCE, resulting in the transient accumulation of cis-DCE, which was slowly degraded to vinyl chloride (VC). These results agreed with the presence of Dhc populations and a shift to stronger reducing conditions in the field: i) RD functional genes (tceA, vcrA and bvcA) exhibited a trend to higher values and ii) a substantial increase in Dhc populations (up to 30% of the total bacterial populations) was observed over time. The dual-element isotope slope ΛC-Cl for RD of cis-DCE obtained from field data (ΛC – Cl = 5 ± 3) was similar to the one determined from the microcosm experiments under controlled anoxic conditions (ΛC – Cl = 4.9 ± 0.8). However, ΛC-Cl values differ from those reported so far for laboratory studies with Dhc strains and mixed cultures containing Dhc, i.e., between 8.3 and 17.8. This observation underscores the potential variety of reductive dehalogenases involved during cis-DCE RD and the importance of determining site-specific Λ and ɛ values in order to improve the identification and quantification of transformation processes in the field.

Enhanced reductive degradation of trichloroethylene by ball milled nitridation of bimetallic Ni-ZVI: Combination effect of electron transfer and catalytic hydrogenation

The performance optimization of zerovalent iron (ZVI) based materials for reductive dechlorination is critical but remains challenging. Combining the advantages of bimetallic and heteroatomic modification may lead to a novel strategy for reductive dechlorination of chlorinated ethenes (CEs) pollution. In this study, three different types of dual-modified micron ZVI (mZVI) composite particles, namely, pre-bimetallic modified mZVI (Ni/mZVI-N), pre-nitrogen modified mZVI (N/mZVI-Ni), and simultaneously modified mZVI (Ni/N-mZVI), were prepared by adjusting the order of ball milling modification of nickel and melamine, in which ball milling mechanical-chemical interactions contributed to the generation of enriched M-N structures. In terms of combined reactivity and selectivity, Ni/N-mZVI exhibited the best performance, with trichloroethylene (TCE) dechlorination rate (kobs,TCE) and electron efficiency (εe) of 0.14 h−1 and 8.9 %, respectively, which were 10.7 and 7.5 times higher than those of mZVI. The distribution and percentage of degradation products further illustrated the reductive dechlorination of nickel-nitrogen dual-modified mZVI via both electron transfer and atomic hydrogen (H*) pathways. A series of characterization and mechanistic analyses showed that the enhancement of TCE reductive dechlorination performance could be attributed to the combined effect of Fe−N to accelerated electron transfer and Ni-N to enhance catalytic hydrogenation. In addition to decreased H2 accumulation, Ni/N-mZVI reduced leaching ions thus mitigating secondary contamination. This double-modification strategy employed to enhance the optimization of mZVI for groundwater remediation establishes a foundation for ongoing progress in the synthesis of reactive materials.

Strategies and mechanisms for improving groundwater remediation efficiency of chlorinated ethenes by controlling the particle size of polyhydroxyalkanoate

The efficacy of in-situ bioremediation of chlorinated ethenes (CEs)-contaminated groundwater is heavily contingent upon the provision of exogenous electron donors to facilitate reductive dechlorination by dechlorinating bacteria. Nonetheless, methanogenic archaea have been reported to engender competition for the limited electron donor supply, thereby diminishing dechlorination efficiency. This study proposes a novel strategy to enhance reductive dechlorination by reducing the particle size of polyhydroxyalkanoate (PHA) to the micrometer scale, thereby inhibiting methanogenic competition for electron donors. The results demonstrate that trichloroethylene (TCE) biodegradation efficiency peaked at a PHA particle size of 500 μm, significantly surpassing the efficiency observed in groups with particle sizes of 50 and 2000 μm. Furthermore, the electron utilization efficiency of dechlorinating bacteria (represented by Geobacter, Dehalobacter, Anaerolinea, etc in this study) was as high as 94.82 % over methanogenic archaea (represented by Methanobacterium) in the 500 μm group. Mechanistic analyses revealed that the 500 μm PHA particles activated dark oxygen, leading to the generation of reactive oxygen species that decreased the abundance of the methanogenic functional gene McrA and increased the abundance of dechlorinating functional gene TceA relative to the total microbial genome gene (in terms of 16S rRNA). In contrast, PHA particles of 50 μm exhibited lower TCE biodegradation efficiency due to particle aggregation, while particles of 2000 μm exhibited reduced efficiency due to lower specific surface area. This study moves a step for the in-situ microbial remediation of CEs-contaminated groundwater.

An approach for estimating microbial growth and chlorinated ethenes biotransformation in groundwater based on discrete observations

The bioremediation of chlorinated ethenes (CEs) contaminated groundwater is attracting increasingly attention in practical remediation projects. However, modelling of microbial metabolic processes under the constraints of substrate and environmental factors is inadequate. This study developed a new kinetic model, which incorporated the logistic model and Dual-Monod kinetic to represent the interaction between the controlled microbial growth and the bioavailable substrates in CE-contaminated groundwater. The proposed model was based on discrete observations to simulate microbial growth under the constraints of substrate and environmental conditions, reducing the amount of observational data required for the model. Meanwhile, the proposed model introduced two new kinetic parameters, the effective specific growth rate μeff and the real self-limiting coefficient of microbial growth keff,sl, to simplified the number of independent parameters. A parameter estimation method based on the quasi-Newton's algorithm for the proposed model was also developed. The model was validated based on the hypothetical data, experimental results, and a published dataset, demonstrated the successful simulation of microbial growth and the sequential biodegradation of PCE in groundwater systems (*E < 0.3). The monitoring duration and the sampling schedule have significant impacts on estimating the biological parameters, and large errors would be induced when the data from the periods of extremely low substrate concentration or microbial growth decline were involved in parameter estimation. The research suggested that the proposed kinetic model provided a new insight to express the limitation of microbial population growth due to the available substrates and environmental factors, and is hoping to be applied in actual CE-contaminated sites.

Unveiling complete natural reductive dechlorination mechanisms of chlorinated ethenes in groundwater: Insights from functional gene analysis

Monitored natural attenuation (MNA) of chlorinated ethenes (CEs) has proven to be a cost-effective and environment-friendly approach for groundwater remediation. In this study, the complete dechlorination of CEs with formation of ethene under natural conditions, were observed at two CE-contaminated sites, including a pesticide manufacturing facility (PMF) and a fluorochemical plant (FCP), particularly in the deeply weathered bedrock aquifer at the FCP site. Additionally, a higher abundance of CE-degrading bacteria was identified with heightened dechlorination activities at the PMF site, compared to the FCP site. The reductive dehalogenase genes and Dhc 16 S rRNA gene were prevalent at both sites, even in groundwater where no CE dechlorination was observed. vcrA and bvcA was responsible for the complete dechlorination at the PMF and FCP site, respectively, indicating the distinct contributions of functional microbial species at each site. The correlation analyses suggested that Sediminibacterium has the potential to achieve the complete dechlorination at the FCP site. Moreover, the profiles of CE-degrading bacteria suggested that dechlorination occurred under Fe3+/sulfate-reducing and nitrate-reducing conditions at the PMF and FCP site, respectively. Overall these findings provided multi-lines of evidence on the diverse mechanisms of CE-dechlorination under natural conditions, which can provide valuable guidance for MNA strategies implementation.

Aerobic biodegradation of chlorinated ethenes by Pseudonocardia sp. D17: Biodegradation ability without auxiliary substrates and concurrent biodegradation with 1,4-dioxane

Bioremediation is a promising approach for mitigating commingled contaminations of chlorinated ethenes (CEs) and 1,4-dioxane (DX). However, aerobic bioremediation to simultaneously remove CEs and DX remains challenging. This study aimed to explore the ability of Pseudonocardia sp. D17 (D17) to aerobically degrade CEs and its applicability for concurrent removal of CEs and DX. Aerobic degradation experiments of individual CEs revealed that D17 could degrade trichloroethene (TCE), three isomers of dichloroethene (DCE), and vinyl chloride (VC), and the trend of its degradation ability was cis-1,2-DCE (cDCE) > VC > TCE >trans-1,2-DCE > 1,1-DCE. Notably, the CE-degrading activity of D17 was expressed even without any auxiliary substrates. Further, when TCE, cDCE, or VC was co-present with DX (each at 1 mg/L), D17 could degrade both compounds without any significant inhibition (for TCE and cDCE) or with only a transient and reversible suspension of its DX degradation ability (for VC). These findings indicated that D17 is a promising agent for the aerobic bioremediation of CEs and DX co-contamination and provide novel insights into the future development of efficient aerobic bioremediation strategies.

Reductive Dechlorination of Chlorinated Ethenes at the Sulfidated Zero-Valent Iron Surface: A Mechanistic DFT Study.

Sulfidated nano- and microscale zero-valent iron (S-(n)ZVI) has shown enhanced selectivity and reactive lifetime in the degradation of chlorinated ethenes (CEs) compared to pristine (n)ZVI. However, varying effects of sulfidation on the dechlorination rates of structurally similar CEs have been reported, with the underlying mechanisms remaining poorly understood. In this study, we investigated the β-dichloroelimination reactions of tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), and trans-1,2-dichloroethene (trans-DCE) at the S and Fe sites of several S-(n)ZVI surface models by using density functional theory. Dechlorination reactions were both kinetically and thermodynamically more favorable at Fe sites compared to S sites, indicating that maintaining the accessibility of reactive Fe sites is crucial for achieving high S-(n)ZVI reactivity with contaminants. At Fe sites adjacent to S atoms, the reactivity for CE dechlorination followed the order trans-DCE ≈ TCE > cis-DCE > PCE. PCE degradation was hindered at these sites due to the steric effects of S atoms. At the S sites, the energy barriers correlated with the CEs' energy of the lowest unoccupied molecular orbital in the order PCE < TCE < DCE isomers. Our findings reveal that the experimentally observed selectivity of S-(n)ZVI materials for individual CEs can be explained by an interplay of the varying reactivities of Fe and S sites in CE dechlorination reactions.

Carbon isotope effects in the sorption of chlorinated ethenes on biochar and activated carbon

As an alternative to activated carbon, biochar is a promising, environmentally friendly sorbent that can be used to remove organic groundwater pollutants, such as chlorinated ethenes (CEs). Stable isotope fractionation in biofilters is used to quantify pollutant degradation and to distinguish degradation from pollutant sorption on e.g. biochar. However, the sorption of CEs on biochar, and the potential abiotic fractionation processes remain to be tested. The sorption process of CEs and ethene on activated carbon and biochar was investigated with regard to the isotope effects for the differentiation from microbial degradation processes. Results from physical and chemical characterization of biochar indicated that biochar feedstock and pyrolysis conditions determined sorption performance depending on the surface chemistry and the pore size distribution of the coarse sorbent particles. The sorption capacity of the activated carbon was significantly higher with highly chlorinated ethenes, but similar to the biochars with low chlorination. Apparent carbon isotope fractionation factors (ε) of +0.1 to −4.4 ‰ were found above measurement uncertainties of GC/IRMS. The extent of isotope enrichment of the 13C bearing isotopologues in the residual aqueous phase (ε < 0) was characteristic for individual pairs of pollutant and sorbent material and could be related to pore-filling processes limited by the micropore size distribution of sorbent materials and the chemical properties of sorbed pollutants. Especially the large isotope fractionation during the sorption of ethene led to the assumption that diffusion processes within the pore matrix of the sorbent particles contributed to the observed isotope effects, but should still be considered a property of sorption. Concluding on the results indicated that sorption processes can have a significant contribution to carbon isotope fractionation in CEs and ethene. These should not be neglected in the evaluation of biofilters for groundwater purification, in which CEs are simultaneously degraded by microbes.

Sequential Anaerobic/Aerobic Microbial Transformation of Chlorinated Ethenes: Use of Sustainable Approaches for Aquifer Decontamination

Chlorinated ethene contamination is a worldwide relevant health issue. In anaerobic aquifers, highly chlorinated ethenes are transformed by microbially-mediated organohalide respiration metabolism. For this reason, in the last few years, bioremediation interventions have been developed and employed in situ for aquifer decontamination. Biostimulation has been demonstrated to be efficient in enhancing organohalide respiration activity. The use of agrifood wastes that replace engineered substrates as biostimulants permits the low carbon impact of bioremediation treatment as part of a circular economy approach. The present work depicts the effects of available bio-based substrates and discusses their efficiency and impact on microbial communities when applied to contaminated aquifers. As a drawback of anaerobic organohalide respiration, there is the accumulation of more toxic lower-chlorinated ethenes. However, compounds such as dichloroethene (DCE) and vinyl chloride (VC) can be mineralized by metabolic and co-metabolic pathways in aerobic conditions. For this reason, sequential anaerobic/aerobic treatments proposed to stimulate the natural biotransformation activity can achieve complete degradation of chlorinated ethenes. The aim of this work is to provide an up-to-date revision of anaerobic/aerobic microbial transformation pathways towards chlorinated ethenes and to discuss their application in real scenarios and futurable microbial bioelectrochemical systems to remediate contaminated aquifers.

UAV-based sampling systems to analyse greenhouse gases and volatile organic compounds encompassing compound-specific stable isotope analysis

Abstract. The study herein reports on the development and testing of sampling systems (and subsequent analytical setups) that were deployed on an unmanned aerial vehicle (UAV) for the purpose of analysing greenhouse gases (GHGs) and volatile organic compounds (VOCs) in the lower atmospheric boundary layer. Two sampling devices, both of which can be mounted to an UAV with a payload capability greater than 1 kg, were tested for respective sampling and analysis of specific GHGs (carbon dioxide, CO2, and methane, CH4) and VOCs (chlorinated ethenes, CEs). The gas analyses included measurements of the molar amounts and the respective stable carbon isotope ratios. In addition to compound calibration in the laboratory, the functionality of the samplers and the UAV-based sampling was tested in the field. Atmospheric air was either flushed through sorbent tubes for VOC sampling or collected and sampled in glass vials for GHG analysis. The measurement setup for the sorbent tubes achieved analyte mass recovery rates of 63 %–100 % (more favourable for lower chlorinated ethenes), when prepared from gaseous or liquid calibration standards, and reached a precision (2σ) better than 0.7 ‰ for δ13C values in the range of 0.35–4.45 nmol. The UAV-equipped samplers were tested over two field sampling campaigns designed to (1) compare manual and UAV-collected samples taken up a vertical profile at a forest site and (2) identify potential emissions of CO2, CH4 or VOC from a former domestic waste dump. The precision of CO2 measurements from whole air samples was ≤7.3 µmol mol−1 and ≤0.3 ‰ for δ13C values and ≤0.03 µmol mol−1 and ≤0.2 ‰ for CH4 working gas standards. The results of the whole air sample analyses for CO2 and CH4 were sufficiently accurate to detect and localise potential landfill gas emissions from a secured former domestic waste dump using level flight. Vertical CO2 profiles from a forest location showed a causally comprehensive pattern in the molar ratios and stable carbon isotope ratios but also the potential falsification of the positional accuracy of a UAV-assisted air sample due to the influence of the rotor downwash. The results demonstrate that the UAV sampling systems presented here represent a viable tool for atmospheric background monitoring, as well as for evaluating and identifying emission sources. By expanding the part of the lower atmosphere that can be practicably sampled over horizontal and vertical axes, the presented UAV-capable sampling systems, which also allow for compound-specific stable isotope analysis (CSIA), may facilitate an improved understanding of surface–atmosphere fluxes of trace gas.

Reductive dechlorination of chlorinated ethenes by ball milled and mechanochemically sulfidated microscale zero valent iron: A comparative study

Ball milling is an effective technique to not only activate and reduce the size of commercial microscale zero valent iron (mZVI) but also to mechanochemically sulfidate mZVI. Yet, little is known about the difference between how chlorinated ethenes (CEs) interact with ball milled mZVI (mZVIbm) and mechanochemically sulfidated mZVI (S-mZVIbm). We show that simple ball milling exposed the active Fe0 sites, while mechanochemical sulfidation diminished Fe0 sites and meanwhile increased S2- sites. Mechanochemical sulfidation with [S/Fe]dosed increased from 0 to 0.20 promoted the particle reactivity most for TCE dechlorination (∼14-fold), followed by PCE and 1,1-DCE while it diminished the reactivity for trans-DCE (∼0.4-fold), cis-DCE (∼0.02-fold) and VC (∼0.002-fold) compared to simple ball milling. Sulfidation also improved the electron efficiency of CE dechlorination, except for cis-DCE and VC. The kSA of cis-DCE, VC and trans-DCE dechlorination positively correlated with surface Fe0 content, suggesting their dechlorination was mainly mediated by Fe0 site or reactive atomic hydrogen. The kSA of TCE dechlorination positively correlated with surface S2- content and the dechlorination mainly occurred on S2- sites via direct electron transfer. Increased sulfidation favored direct electron transfer mechanism. The kSA of PCE and 1,1-DCE was not dependent on either parameter and their dechlorination was equally achieved through either mechanism.

Iron nitride nanoparticles for rapid dechlorination of mixed chlorinated ethene contamination.

Sulfidation and, more recently, nitriding have been recognized as promising modifications to enhance the selectivity of nanoscale zero-valent iron (nZVI) particles for trichloroethene (TCE). Herein, we investigated the performance of iron nitride (FexN) nanoparticles in the removal of a broader range of chlorinated ethenes (CEs), including tetrachloroethene (PCE), cis-1,2-dichloroethene (cis-DCE), and their mixture with TCE, and compared it to the performance of sulfidated nZVI (S-nZVI) prepared from the same precursor nZVI. Two distinct types of iron nitride (FexN) nanoparticles, containing γ'-Fe4N and ε-Fe2-3N phases, exhibited substantially higher PCE and cis-DCE dechlorination rates compared to S-nZVI. A similar effect was observed with a CE mixture, which was completely dechlorinated by both types of FexN nanoparticles within 10 days, whereas S-nZVI was able to remove only about half of the amount, most of which being TCE. Density functional theory calculations further revealed that the cleavage of the first C-Cl bond was the rate-limiting step for all CEs dechlorinated on the γ'-Fe4N(001) surface, with the reaction barriers of PCE and cis-DCE being 29.9, and 40.8kJmol-1, respectively. FexN nanoparticles proved to be highly effective in the remediation of PCE, cis-DCE, and mixed CE contamination.

Field application of glycerol to enhance reductive dechlorination of chlorinated ethenes and its impact on microbial community

Chlorinated ethenes (CEs) are common and persistent contaminants of soil and groundwater. Their degradation is mostly driven by a process of bacterial reductive dechlorination (also called organohalide respiration) in anaerobic conditions. This study summarizes the outcomes of the long-term in-situ application of glycerol for the enhanced reductive dechlorination of CEs on a highly contaminated site. Glycerol injection resulted in an almost immediate increase in the abundance of fermentative Firmicutes, which produce essential sources of carbon (acetate) and electrons (H2) for organohalide-respiring bacteria (OHRB) and change groundwater conditions to be suitable for OHRB growth. The decreased redox potential of groundwater promoted also the proliferation of sulfate-reducing bacteria, which compete for electron donors with OHRB but at the same time support their growth by producing essential corrinoids and acetate. A considerable increase in the abundance of OHRB Dehalococcoides, concurrently with vinyl chloride (VC) reductase gene levels, was revealed by real time polymerase chain reaction (qPCR) method. Consistent with the shifts in bacterial populations, the concentrations of pollutants tetrachloroethylene and trichloroethylene decreased during the monitoring period, with rising levels of cis-1,2-dichloroethylene, VC, and most importantly, the final CE degradation products: ethene and ethane. Our study implies the importance of syntrophic bacterial interactions for successful and complete CE degradation and evaluates glycerol as convenient substrate to enhance reductive dechlorination and as an effective source of electrons for OHRB.

Detecting vinyl chloride by phytoscreening in the shallow critical zone at sites with potential human exposure

Chlorinated ethene (CE) contaminants are widespread in groundwater, and the occurrence of vinyl chloride (VC), among others, is a well-known issue due to its mobility, persistence, and carcinogenicity. Human exposure to VC may occur through inhalation after soil vapor intrusion into buildings at sites with shallow underground contamination. Soil vapor intrusion risk is traditionally assessed through indoor air and sub-slab sampling (direct evidence) or soil gas and groundwater surveys (indirect evidence). Phytoscreening (sampling and analysis of tree trunk matrices) was proven as a cost-effective alternative technique to indirectly detect shallow underground contamination by higher chlorinated ethenes and subsequent vapor intrusion risk. However, the technique has appeared barely capable to screen for the lower chlorinated VC, likely due to its fugacity and aerobic bio-degradability, with only one literature record to date showing successful detection in trees. We applied phytoscreening at two sites with severe CE contamination nearby residential buildings caused by illegal dumping of chlorinated pitches from petrochemical productions. The two sites show variable amounts of VC in the shallow groundwater (1e2 to 1e4 μg/L), posing potential sanitary risk issues. Former soil gas surveys did not detect VC in the vadose zone. At both sites, we sampled trunk micro-cores and trunk gas from poplar trees close to contaminated piezometers in different seasons. VC was detected in several instances, disproving the shared literature assumption of the inefficacy of phytoscreening towards this compound. Factors influencing the detectability of VC and other CEs in trees were analyzed through linear regressions. Two different conceptual models were proposed to explain the effective uptake of VC by trees at the two sites, i.e., direct uptake of contaminated groundwater at the first site and uptake of VC from an anoxic vadose zone at the second site. In planta reductive dechlorination of CEs is not expected based on current literature knowledge. Thus, the detection of VC in trunks would indicate its occurrence in the shallow underground, suggesting higher screening effectiveness of phytoscreening compared to soil gas; this has implications for indirect vapor intrusion risk assessment.

Dehalogenation of Chlorinated Ethenes to Ethene by a Novel Isolate, "Candidatus Dehalogenimonas etheniformans".

Dehalococcoides mccartyi strains harboring vinyl chloride (VC) reductive dehalogenase (RDase) genes are keystone bacteria for VC detoxification in groundwater aquifers, and bioremediation monitoring regimens focus on D. mccartyi biomarkers. We isolated a novel anaerobic bacterium, "Candidatus Dehalogenimonas etheniformans" strain GP, capable of respiratory dechlorination of VC to ethene. This bacterium couples formate and hydrogen (H2) oxidation to the reduction of trichloro-ethene (TCE), all dichloroethene (DCE) isomers, and VC with acetate as the carbon source. Cultures that received formate and H2 consumed the two electron donors concomitantly at similar rates. A 16S rRNA gene-targeted quantitative PCR (qPCR) assay measured growth yields of (1.2 ± 0.2) × 108 and (1.9 ± 0.2) × 108 cells per μmol of VC dechlorinated in cultures with H2 or formate as electron donor, respectively. About 1.5-fold higher cell numbers were measured with qPCR targeting cerA, a single-copy gene encoding a putative VC RDase. A VC dechlorination rate of 215 ± 40 μmol L-1 day-1 was measured at 30°C, with about 25% of this activity occurring at 15°C. Increasing NaCl concentrations progressively impacted VC dechlorination rates, and dechlorination ceased at 15 g NaCl L-1. During growth with TCE, all DCE isomers were intermediates. Tetrachloroethene was not dechlorinated and inhibited dechlorination of other chlorinated ethenes. Carbon monoxide formed and accumulated as a metabolic by-product in dechlorinating cultures and impacted reductive dechlorination activity. The isolation of a new Dehalogenimonas species able to effectively dechlorinate toxic chlorinated ethenes to benign ethene expands our understanding of the reductive dechlorination process, with implications for bioremediation and environmental monitoring. IMPORTANCE Chlorinated ethenes are risk drivers at many contaminated sites, and current bioremediation efforts focus on organohalide-respiring Dehalococcoides mccartyi strains to achieve detoxification. We isolated and characterized the first non-Dehalococcoides bacterium, "Candidatus Dehalogenimonas etheniformans" strain GP, capable of metabolic reductive dechlorination of TCE, all DCE isomers, and VC to environmentally benign ethene. In addition to hydrogen, the new isolate utilizes formate as electron donor for reductive dechlorination, providing opportunities for more effective electron donor delivery to the contaminated subsurface. The discovery that a broader microbial diversity can achieve detoxification of toxic chlorinated ethenes in anoxic aquifers illustrates the potential of naturally occurring microbes for biotechnological applications.

Genomic analysis of Acinetobacter pittii CEP14 reveals its extensive biodegradation capabilities, including cometabolic degradation of cis-1,2-dichloroethene.

Halogenated organic compounds are naturally occurring in subsurface environments; however, accumulation of the degradative intermediate cis-1,2-dichloroethene (cDCE) at soil and groundwater sites contaminated with xenobiotic chlorinated ethenes is a global environmental and public health issue. Identifying microorganisms capable of cDCE degradation in these environments is of interest because of their potential application to bioremediation techniques. In this study, we sequenced, assembled, and analyzed the complete genome of Acinetobacter pittii CEP14, a strain isolated from chloroethene-contaminated groundwater, that has demonstrated the ability for aerobic cometabolic degradation of cDCE in the presence of n-hexane, phenol, and toluene. The A. pittii CEP14 genome consists of a 3.93Mbp-long chromosome (GenBank accession no. CP084921) with a GC content of 38.9% and three plasmids (GenBank accession no. CP084922, CP084923, and CP084924). Gene function was assigned to 83.4% of the 3,930 coding DNA sequences. Functional annotation of the genome revealed that the CEP14 strain possessed all genetic elements to mediate the degradation of a range of aliphatic and aromatic compounds, including n-hexane and phenol. In addition, it harbors gene clusters involved in cytosol detoxification and oxidative stress resistance, which could play a role in the mitigation of toxic chemical intermediates that can arise during the degradation of cDCE. Gene clusters for heavy metal and antibiotic resistance were also identified in the genome of CEP14. These results suggest that CEP14 may be a versatile degrader of xenobiotic compounds and well-adapted to polluted environments, where a combination of heavy metal and organic compound pollution is often found.

Molecular Structure and Sulfur Content Affect Reductive Dechlorination of Chlorinated Ethenes by Sulfidized Nanoscale Zerovalent Iron.

Sulfidized nanoscale zerovalent iron (SNZVI) with desirable properties and reactivity has recently emerged as a promising groundwater remediation agent. However, little information is available on how the molecular structure of chlorinated ethenes (CEs) affects their dechlorination by SNZVI or whether the sulfur content of SNZVI can alter their dechlorination pathway and reactivity. Here, we show that the reactivity (up to 30-fold) and selectivity (up to 70-fold) improvements of SNZVI (compared to NZVI) toward CEs depended on the chlorine number, chlorine position, and sulfur content. Low CEs (i.e., vinyl chloride and cis-1,2-dichloroethene) and high CEs (perchloroethene) tended to be dechlorinated by SNZVI primarily via atomic H and direct electron transfer, respectively, while SNZVI could efficiently and selectively dechlorinate trichloroethene and trans-1,2-dichloroethene via both pathways. Increasing the sulfidation degree of SNZVI suppressed its ability to produce atomic H but promoted electron transfer and thus altered the relative contributions of atomic H and electron transfer to the CE dechlorination, resulting in different reactivities and selectivities. These were indicated by the correlations of CE dechlorination rates and improvements with CE molecular descriptors, H2 evolution rates, and electron transfer indicators of SNZVI. These mechanistic insights indicate the importance of determining the structure-specific properties and reactivity of both SNZVI materials and their target contaminants and can lead to a more rational design of SNZVI for in situ groundwater remediation of various CEs.

Natural Biodegradation of Vinyl Chloride and cis-Dichloroethene in Aerobic and Suboxic Conditions.

Chlorinated ethene (CE) groundwater contamination is commonly treated through anaerobic biodegradation (i.e., reductive dechlorination) either as part of an engineered system or through natural attenuation. Aerobic biodegradation has also been recognized as a potentially significant pathway for the removal of the lower CEs cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). However, the role of aerobic biodegradation under low oxygen conditions typical of contaminated groundwater is unclear. Bacteria capable of aerobic VC biodegradation appear to be common in the environment, while aerobic biodegradation of cDCE is less common and little is known regarding the organisms responsible. In this study, we investigate the role of aerobic cDCE and VC biodegradation in a mixed contaminant plume (including CEs, BTEX, and ketones) at Naval Air Station North Island, Installation Restoration Site 9. Sediment and groundwater collected from the plume source area, mid-plume, and shoreline were used to prepare microcosms under fully aerobic (8mg/L dissolved oxygen (DO)) and suboxic (< 1mg/L DO) conditions. In the shoreline microcosms, VC and cDCE were rapidly degraded under suboxic conditions (100% and 77% removal in < 62days). In the suboxic VC microcosms, biodegradation was associated with a > 5 order of magnitude increase in the abundance of functional gene etnE, part of the aerobic VC utilization pathway. VC and cDCE were degraded more slowly under fully aerobic conditions (74% and 30% removal) in 110days. High-throughput 16S rRNA and etnE sequencing suggest the presence of novel VC- and cDCE-degrading bacteria. These results suggest that natural aerobic biodegradation of cDCE and VC is occurring at the site and provide new evidence that low (< 1mg/L) DO levels play a significant role in natural attenuation of cDCE and VC.

Geobacter sp. Strain IAE Dihaloeliminates 1,1,2-Trichloroethane and 1,2-Dichloroethane.

Chlorinated ethanes, including 1,2-dichloroethane (1,2-DCA) and 1,1,2-trichloroethane (1,1,2-TCA), are widespread groundwater contaminants. Enrichment cultures XRDCA and XRTCA derived from river sediment dihaloeliminated 1,2-DCA to ethene and 1,1,2-TCA to vinyl chloride (VC), respectively. The XRTCA culture subsequently converted VC to ethene via hydrogenolysis. Microbial community profiling demonstrated the enrichment of Geobacter 16S rRNA gene sequences in both the XRDCA and XRTCA cultures, and Dehalococcoides mccartyi (Dhc) sequences were only detected in the ethene-producing XRTCA culture. The presence of a novel Geobacter population, designated as Geobacter sp. strain IAE, was identified by the 16S rRNA gene-targeted polymerase chain reaction and Sanger sequencing. Time-resolved population dynamics attributed the dihaloelimination activity to strain IAE, which attained the growth yields of 0.93 ± 0.06 × 107 and 1.18 ± 0.14 × 107 cells per μmol Cl- released with 1,2-DCA and 1,1,2-TCA as electron acceptors, respectively. In contrast, Dhc growth only occurred during VC-to-ethene hydrogenolysis. Our findings discover a Geobacter sp. strain capable of respiring multiple chlorinated ethanes and demonstrate the involvement of a broader diversity of organohalide-respiring bacteria in the detoxification of 1,2-DCA and 1,1,2-TCA.