Dichloroethylene Research Articles

Actual decay of tetrachloroethene (PCE) and trichloroethene (TCE) in a highly contaminated shallow groundwater system

The Quaternary shallow aquifer that served as the subject of research in this work is under the influence of strong anthropopressure manifested by extreme values of contamination with both organic and inorganic substances (max. TOC 1.6 g/l and Cl− 10 g/l) from chemical production. The main attention was paid to the dehalogenation of organochlorine compounds. The research included chlorinated ethane's and chlorinated methane's. In the groundwater samples, the most common components from the group of chlorinated ethenes were determined, i.e. tetrachloroethene (PCE), trichlorethylene (TCE), dichloroethene (DCE) and vinyl chloride (VC) as well as typical components from the group of chlorinated methane's, i.e. tetrachloromethane (PCM), trichloromethane (TCM) and dichloromethane (DCM), were also made. The determinations were made by gas chromatography with electron capture detection (GC-ECD), after liquid-liquid extraction (vinyl chloride: gas chromatography with mass detection (GC-MS), after liquid-liquid extraction).PCE initially undergoes a classical decomposition as a result of dehalogenated reduction to TCE and the Cl− ion (Ai et al., 2019) under aerobic conditions around Eh +100 (+50) to 0 mV. While maintaining the double bond between the carbon atoms, TCE decays under slightly reducing conditions of around Eh -50 to -100 mV to dichloroethene (DCE) and Cl−. Therefore, in the event that the π bond between the carbon atoms is not retained and broken, the TCE breaks down into two functional groups CHCl= and CCl2=. When hydrogen and chlorine are added to the CHCl= functional group, trichloromethane (TCM) is formed, and in the case of two hydrogen atoms it is respectively dichloromethane (DCM). Additionally TCM undergoes a classical decomposition under aerobic conditions by dehalogenation to DCM and the Cl− ion. Chlorine and hydrogen atoms can analogously join the CCl2 = function group to form DCM. Commonly in Quaternary aquifers, often shallow ones, there are favorable conditions for the decomposition of these unstable substances by oxidation, both under the conditions of availability and lack of oxygen (with the presence of e.g. Fe2+ and Mn2+) and immediate, complete mineralization to CO2, H+ and Cl− ions.

Simultaneous adsorption and biodegradation of trichloroethylene occurs in a biochar packed column treating contaminated landfill leachate

Trichloroethylene (TCE) is a human carcinogen that is commonly found in landfill leachate. Contaminated leachate plumes may be intercepted prior to reaching groundwater and treated in situ using permeable reactive barriers (PRB). This study used a packed column system containing herbal pomace and spruce biochar, previously shown to have TCE adsorptive capabilities. Influent containing raw or autoclaved landfill leachate was used to investigate the potential for environmental micro-organisms to establish a TCE-dechlorinating biofilm on the biochar, in order to prolong the operational life span of the system. TCE removal ≥ 99.7 % was observed by both biochars. No dichloroethylene (DCE) isomers were present in the column effluents, but cis-1,2 DCE was adsorbed to the biochar treating raw landfill leachate, indicating that dechlorination was occurring biologically in these columns. Known microbial species that are individually capable of complete dechlorination of TCE to ethene were not detected by 16S rRNA gene sequencing, but several species capable of partial TCE dechlorination (Desulfitobacterium spp., Sulfurospirillium spp. and Desulfuromonas spp) were present in the biofilms of the columns treating raw landfill leachate. These data demonstrate that biochar from waste material may be capable of supporting a dechlorinating biofilm to promote bioremediation of TCE.

Organohalide-Respiring Bacteria in Polluted Urban Rivers Employ Novel Bifunctional Reductive Dehalogenases to Dechlorinate Polychlorinated Biphenyls and Tetrachloroethene.

Polluted urban river sediments could be a sink of persistent and toxic polychlorinated biphenyls (PCBs) in urban areas and provide desired growth niches for organohalide-respiring bacteria (OHRB). In this study, microcosms were set up with surface sediments of nationwide polluted urban rivers in China, of which 164 cultures could dechlorinate tetrachloroethene (PCE) to dichloroethenes (DCEs) and to vinyl chloride and/or ethene. Further in vivo tests showed extensive PCB dechlorination with different pathways in 135 PCE pregrown cultures. Taking reductive dechlorination of PCB180 (2345-245-CB) as an example, 121 and 14 cultures preferentially removed flanked para- and meta-chlorines, respectively. Strikingly, all in vitro assays with the 135 PCE pregrown cultures showed identical PCB dechlorination pathways with their living cultures, implying the involvement of bifunctional reductive dehalogenases (RDases) to dechlorinate both PCBs and PCE. Further 16S rRNA and RDase gene-based analyses, together with enantioselective dechlorination of chiral PCBs, suggested that Dehalococcoides and Dehalogenimonas in the 135 cultures largely employed distinctively different novel bifunctional RDases to catalyze PCB/PCE dechlorination. Quantitative assessment of the community assembly process with the modified stochasticity ratio (MST) indicated three different stages in enrichment of OHRB. The second stage, as the only one controlled by stochastic processes (MST > 0.5), required extra attention in monitoring community successional patterns to minimize stochastic variance for enriching the PCB/PCE-dechlorinating OHRB.

Structure and Electron Transport at Metal Atomic Junctions Doped with Dichloroethylene.

We have investigated the structure and electron transport at dichloroethylene-doped metal atomic junctions at low temperatures (20 K) in ultra-high vacuum, using Fe, Ni, Pd, Cu, Ag, and Au. The metal atomic junctions were fabricated using the mechanically controllable break junction technique. After introducing the dichloroethylene (DCE), the conductance behavior of Fe, Ni, and Pd junctions was considerably changed, whereas little change was observed for Cu, Ag, and Au. For the Pd and Cu junctions, a clear peak was observed in their conductance histograms, showing that the single-molecule junction was selectively formed. To investigate the structure of the metal atomic junctions further, their plateau lengths were analyzed. The length analysis revealed that the Au atomic wire was elongated, and the metal atomic wires were formed for the other transition metals: those that do not normally form metal atomic wires without DCE doping, as DCE adsorption stabilized the metal atomic states. There is a strong interaction between DCE and the metals, where DCE supports the formation of the metal atomic wire for Fe, Ni, and Pd.

Pilot-Scale Evaluation of a Permeable Reactive Barrier with Compost and Brown Coal to Treat Groundwater Contaminated with Trichloroethylene

This study evaluates, under field conditions, the efficiency of a permeable reactive barrier (PRB) with compost and brown coal to remove trichloroethylene (TCE) (109 µg/L) from contaminated groundwater. Three stainless steel boxes (1.2 × 0.5 × 0.5 m) with the brown coal-compost mixture at three different mixing ratios of 1:1, 1:3, and 1:5 (by volume) were installed to simulate the PRB. Groundwater from the TCE-contaminated aquifer was pumped into the system at a flow rate of 3.6 L/h. Residence times in the boxes were of: 25, 20, 10 h, respectively. Effluent samples were analyzed for TCE and its daughter products: dichloroethylene (DCE), vinyl chloride (VC) and ethane. During the 198-day experimental period TCE concentrations in groundwater decreased below ≤1.1 µg/L, i.e., much lower than groundwater and drinking water standards in Poland. After 16 days cis-1,2-DCE was monitored indicating possible reductive dechlorination of TCE. However, complete transformation of TCE into non-toxic byproducts was not evidenced during the time of experiments, indicating that reductive dechlorination slowed down or stopped at DCE, and that the designed residence times were not long enough to allow the complete dechlorination process.

IR absorption spectra for chlorinated ethenes in water, calculated using density functional theory

Calculations are presented of vibration absorption spectra for tetrachloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE) and vinyl chloride (VC) molecules in water, using density function theory (DFT). DFT can provide interpretation of absorption spectra with respect to molecular structure for excitation by electromagnetic waves at frequencies within the IR range. The DFT-calculated absorption spectrum corresponding to vibration excitation states of these molecules in continuous water background provides quantitative estimates that can be correlated with additional information obtained from laboratory measurements. This case study provides proof of concept for using DFT-calculated spectra to construct templates, which are for spectral-feature comparison and, thus, detection of spectral-signature features associated with target materials.

Reductive dechlorination of high concentrations of chloroethenes by a Dehalococcoides mccartyi strain 11G.

Chloroethenes are common groundwater and soil contaminants due to extensive historic utilization and inappropriate discharge. The tendency for chloroethenes to become sequestered as dense non-aqueous phase liquids (DNAPL)-a point source to groundwater contamination and causing high concentrations of chloroethenes in proximal aquifers poses a great challenge for remediation of chloroethene contaminated sites. In this study, we report isolation and characterization of a Dehalococcoides mccartyi strain 11G which couples growth with reductive dechlorination of trichloroethenes (TCE), dichloroethene (DCE) isomers and vinyl chloride (VC) to ethene at a growth yield ranging from 2.47±0.23 × 108 to 5.64±0.43 × 108 cells/µmoles Cl- released and co-metabolically dechlorinates tetrachloroethene (PCE) in the presence of TCE. Compared with previous D. mccartyi strains showing dechlorination of TCE at up to 2.0 mM, strain 11G is distinguished by its capacity to dechlorinate chloroethenes at initial concentrations of DCE isomers as high as 4 mM and TCE as high as 3.5 mM to ethene. Bioaugmentation of a contaminated microcosm with strain 11G resulted in complete detoxification of a mixture of 5 mM chloroethenes (2.5 mM of each TCE and cis-DCE) after 40 days. Strain 11G is a promising candidate for in situ bioremediation of high-concentration-chloroethene contaminated sites.

Enhanced reductive de-chlorination of a solvent contaminated aquifer through addition and apparent fermentation of cyclodextrin

In a field study, aqueous cyclodextrin (CD) was investigated for its ability to extract chlorinated volatile organic compounds (cVOC), such as trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), and dichloroethene (DCE) through in-situ flushing of a sandy aquifer. After cessation of aquifer flushing, a plume of CD was left. Changes in CD, cVOC, and inorganic terminal electron acceptors (TEAs) (DO, nitrate, sulfate, iron) were monitored in four rounds of wellwater sampling (20, 210, 342, and 425days after cessation of active pumping). Post-CD flushing VOC levels rebounded (850% for TCE, 190% for TCA, and 53% for DCE) between the first two sampling rounds, apparently due to rate-limited desorption from aquifer media and dissolution from remaining NAPL. However, substantial reduction in the mass of TCE (6.3 to 0.11mol: 98%) and TCA (2.8 to 0.73mol: 74%) in groundwater was observed between 210 and 425days. DCE should primarily be produced from the degradation of TCE and is expected to subsequently degrade to chloroethene. Since DCE levels decreased only slightly (0.23 to 0.17mol: 26%), its degradation rate should be similar to that produced from the decaying TCE. Cyclodextrin was monitored starting from day 210. The mass of residual CD (as measured by Total Organic Carbon) decreased from 150mol (day 210) to 66 (day 425) (56% decrease). The naturally anaerobic zone within the aquifer where residual CD mass decreased coincided with a loss of other major potential TEAs: nitrate (97% loss), sulfate (31%) and iron (31%). In other studies, TCE and 1,1,1-TCA have been found to be more energetically favorable TEAs than sulfate and iron and their degradation via reductive dechlorination has been found to be enhanced by the fermentation of carbohydrates. Such processes can explain these observations, but more investigation is needed to evaluate whether residual levels of CD can facilitate the anaerobic degradation of chlorinated VOCs.

Self-cleaning liner for halogenated hydrocarbon control in landfill leachate

Sorptive landfill liners can prevent the migration of the leachate pollutants. However, their sorption ability will decrease over time. A method should be developed to maintain the sorption ability of landfill liners. In this study, we combined cetyltrimethylammonium bromide-bentonite (CTMAB-bentonite) and zero-valent iron (ZVI) to develop a self-cleaning liner that can retain its sorption ability for a long period. Batch experiments and calculation simulations were employed to analyse the sorption ability of this liner material and the ecological risk of halogenated hydrocarbons. The results showed that CTMAB-bentonite could sorb halogenated hydrocarbons well, with saturated sorption capacities (Qm) of 10.2, 14.5, 6.69, 18.5, 29.4, and 49.7 mg·g-1 for dichloroethane (DCA), trichloroethane (TCA), dichloroethene (DCE), trichloroethylene (TCE), tetrachloroethylene (PCE), and 1,3- dichloropropene (1,3-DCP), respectively. Using the mixture of 0.5 g iron and 0.5 g CTMAB-bentonite could dramatically increase the removal efficiency of DCE, TCE, and PCE. The reaction with ZVI did not change the structure of CTMAB-bentonite and its sorption ability remained consistent. Calculation results suggested that the self-cleaning landfill liner would dramatically decrease the hazard index (HI) of the eluate. However, the humic acid and salt in leachate would cause a reduction in the removal of halogenated hydrocarbons.

In situ remediation of tetrachloroethylene and its intermediates in groundwater using an anaerobic/aerobic permeable reactive barrier.

Tetrachloroethylene (PCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. Its chlorinated degradation by-products remain highly toxic. In this study, an anaerobic/aerobic permeable reactive barrier system consisting of four different functional layers was designed to remediate PCE-contaminated groundwater. The first (oxygen capture) layer maintained the dissolved oxygen (DO) concentration at <1.35mg/L in influent supplied to the second (anaerobic) layer. The third (oxygen-releasing) layer maintained DO concentration at >11.3mg/L within influent supplied to the fourth (aerobic) layer. The results show that 99% of PCE was removed, mostly within the second layer (anaerobic). Furthermore, the toxic by-products trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride (VC) were further degraded by 98, 90, and 92%, respectively, in layer 4 (aerobic). Thus, the designed anaerobic/aerobic permeable reactive barrier system could control both PCE and its degradation by-products, showing great potential as an efficient remediation alternative for the in situ treatment of PCE-contaminated groundwater.

Cometabolic degradation of dichloroethenes by Comamonas testosteroni RF2

An environmental isolate Comamonas testosteroni strain RF2, which has been found to cometabolize trichloroethene (TCE) in the presence of phenol and sodium lactate as growth substrates, was tested to investigate its capacity for degrading 1,2-cis-dichloroethene (cDCE), 1,2-trans-dichlorothene (tDCE), and 1,1-dichloroethene (1,1DCE). Degradation assays were performed for single DCEs, as well as for a mixture of DCEs with TCE, which resembled contaminated plume in groundwater. RF2 was capable of efficiently removing all three dichloroethenes (DCEs) at the initial aqueous concentrations of 6.01 mg L−1 for cDCE, 3.80 mg L−1 for tDCE and 0.65 mg L−1 for 1,1DCE, with a removal efficiency of 100% for cDCE, 65.8% for tDCE, and 46.8% for 1,1DCE. Furthermore, complete removal of TCE, cDCE and 1,1DCE (122.5 μg L−1, 84.3 μg L−1 and 51.4 μg L−1, respectively) was observed in a mixture sample that also contained 72.33 μg L−1 of tDCE, which was removed to the amount of 72.3%. Moreover, degradation of cDCE (6.01 mg L−1) led to a 93.8% release of inorganic chloride, and 2,2-dichloroacetaldehyde was determined as the first intermediate of cDCE transformation. The findings of this study suggest that the strain RF2 exhibits the potential to remediate groundwater contaminated with less chlorinated ethenes.

Survival of Vinyl Chloride Respiring Dehalococcoides mccartyi under Long-Term Electron Donor Limitation.

In anoxic groundwater aquifers, the long-term survival of Dehalococcoides mccartyi populations expressing the gene vcrA (or bvcA) encoding reductive vinyl chloride dehalogenases are important to achieve complete dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) to nonchlorinated ethene. The absence or inactivity of vcrA-containing Dehalococcoides results in the accumulation of the harmful chlorinated intermediates dichloroethene (DCE) and vinyl chloride (VC). Although vcrA-containing Dehalococcoides subpopulations depend on synergistic interaction with other organohalide-respiring populations generating their metabolic electron acceptors (DCE and VC), their survival requires successful competition for electron donor within the entire organohalide-respiring microbial community. To understand this dualism of synergy and competition under growth conditions relevant in contaminated aquifers, we investigated Dehalococcoides-level population structure when subjected to a change in the ratio of electron donor to chlorinated electron acceptor in continuously stirred tank reactors (CSTRs) operated over 7 years. When the electron donor formate was supplied in stoichiometric excess to TCE, both tceA-containing and vcrA-containing Dehalococcoides populations persisted, and near-complete dechlorination to ethene was stably maintained. When the electron donor formate was supplied at substoichiometric concentrations, the interactions between tceA-containing and vcrA-containing populations shifted toward direct competition for the same limiting catabolic electron donor substrate with subsequent niche exclusion of the vcrA-containing population. After more than 2000 days of operation under electron donor limitation, increasing the electron donor to TCE ratio facilitated a recovery of the vcrA-containing Dehalococoides population to its original frequency. We demonstrate that electron donor scarcity alone, in the absence of competing metabolic processes or inhibitory dechlorination intermediate products, is sufficient to alter the Dehalococcoides population structure. These results underscore the importance of electron donor and chloroethene stoichiometry in maintaining balanced functional performance within consortia composed of multiple D. mccartyi subpopulations, even when other competing electron acceptor processes are absent.

Effects of tidal operation on pilot-scale horizontal subsurface flow constructed wetland treating sulfate rich wastewater contaminated by chlorinated hydrocarbons.

Three different flow regimes were carried out in a pilot-scale horizontal subsurface flow constructed wetland-treating sulfate rich wastewater contaminated with monochlorobenzene (MCB) and perchloroethene (PCE). The three regimes were continuous flow, 7-day cycle discontinuous flow, and 2.5-day cycle discontinuous flow. The results show that intensifying the tidal regime (2.5-day cycle) significantly enhanced MCB removal before 2m from the inlet and increasing PCE removal efficiency at 0.5m. The PCE dechlorination process was promoted with tidal operation, especially under the 2.5-day cycle regime, with significant increases of cis-1,2- dichloroethenes (DCEs), vinyl chloride (VC), and ethene, but trans-1,2-DCE was significantly decreased after tidal operation. Due to the high sulfate concentration in the influent, sulfide was observed in pore water up to 20 and 23mgL-1 under continuous flow and 7-day cycle regime, respectively. However, sulfide concentrations decreased to less than 4mgL-1 under intensified tidal operation (2.5-day cycle). The increase of oxygen concentration in pore water through intensified tidal operation resulted in better MCB removal performance and the successful inhibition of sulfate reduction. In conclusion, intensifying tidal operation is an effective approach for the treatment of chlorinated hydrocarbons and inhibiting sulfide accumulation in horizontal subsurface flow constructed wetland.

Validation of an Integrative Methodology to Assess and Monitor Reductive Dechlorination of Chlorinated Ethenes in Contaminated Aquifers

Bioremediation of tetra-and trichloroethene-contaminated aquifers is frequently hampered due to incomplete dechlorination to the more toxic dichloroethene (DCE) and vinyl chloride (VC), indicating insufficient knowledge about the biological mechanisms related to aquifer functioning. A methodology based on the joint analysis of geochemical and microbiological datasets was developed to assess the presence of the biochemical potential for complete reductive dechlorination to harmless ethene and to explain the reasons for which degradation often stalls at the more toxic intermediates. This methodology is composed of three successive steps, with i) the acquisition of geochemical data including chlorinated ethenes, ii) the detailed analysis of the bacterial community structures as well as the biochemical potential for complete dechlorination using microcosms and molecular detection of organohalide-respiring bacteria and key reductive dehalogenases, and iii) a statistical Multiple Factor Analysis combining the above mentioned abiotic and biotic variables in a functional modelling of the contaminated aquifer. The methodology was validated by analyzing two chlorinated ethenes-contaminated sites. Results from the first site showed that the full biochemical potential for ethene production was present in situ. However, redox potential was overall too high and locally manganese reduction out-competed chlorinated ethenes reduction, explaining the reasons for the local accumulation of DCE and VC to a lesser extent. The second contaminated aquifer was under bioremediation by successive cheese whey injections. Analysis demonstrated that cheese whey additions led to increasingly reduced redox conditions and that hampered reductive dechlorination was not due to competition with other anaerobic respiration processes. Complete reductive dechlorination to ethene was preferentially occurring under methanogenic conditions. DCE and VC accumulation was probably induced first by low pH resulting from whey fermentation and at a later stage by phosphate limitation. In conclusions, the proposed methodology successfully allowed the identification of biogeochemical processes limiting or supporting complete dechlorination in both aquifers. The integrative approach provided fundamental information about the functional heterogeneity of the contaminated aquifers in time and space, and can be used as a reliable tool to support corrective decision-making in the development of remediation strategies based on natural attenuation of chlorinated ethenes.

Evaluation of contaminated drinking water and male breast cancer at Marine Corps Base Camp Lejeune, North Carolina: a case control study.

BackgroundSolvents contaminated drinking water supplies at Marine Corps Base Camp Lejeune during 1950s-1985.MethodsWe conducted a case–control study among Marines to evaluate associations between residential exposure to contaminated drinking water at Camp Lejeune and male breast cancer risk. The study included 71 male breast cancer cases and 373 controls identified from the Department of Veteran’s Affairs (VA) cancer registry whose military personnel records were available. Controls were selected from cancers not known to be associated with solvent exposure and included 270 skin cancers, 71 mesotheliomas, and 32 bone cancers. Base assignment and risk factor information came from military personnel and VA records. Groundwater contaminant fate/transport and distribution system models provided monthly estimated residential contaminant levels. We conducted exact logistic regression using the 50th percentile level among exposed controls to create low and high exposure categories. We calculated 95 % confidence intervals (CIs) to indicate precision of effect estimates. Exploratory analyses used proportional hazards methods to evaluate associations between exposures and age at diagnosis.ResultsAfter adjusting for age at diagnosis, race, and service in Vietnam, the odds ratio (OR) for ever stationed at Camp Lejeune was 1.14 (95 % CI: 0.65, 1.97). Adjusted ORs for high residential cumulative exposures to tetrachloroethylene (PCE), t-1,2 dichloroethylene (DCE), and vinyl chloride were 1.20 [95 % CI: 0.16-5.89], 1.50 [95 % CI: 0.30-6.11], 1.19 [95 % CI: 0.16-5.89], respectively, with a monotonic exposure response relationship for PCE only. However these results were based on two or three cases in the high cumulative exposure categories. Ever stationed at Camp Lejeune and high cumulative exposures to trichloroethylene (TCE), PCE, DCE and vinyl chloride were associated with earlier age at onset for male breast cancer; hazard ratios ranged from 1.4-2.7 with wide confidence intervals for cumulative exposure variables.ConclusionFindings suggested possible associations between male breast cancer and being stationed at Camp Lejeune and cumulative exposure to PCE, DCE, and vinyl chloride. TCE, PCE, DCE and vinyl chloride cumulative exposures showed possible associations with earlier age at onset of male breast cancer. However, this study was limited by small numbers of cases in high exposure categories.Electronic supplementary materialThe online version of this article (doi:10.1186/s12940-015-0061-4) contains supplementary material, which is available to authorized users.

Chloroethene degradation and expression of Dehalococcoides dehalogenase genes in cultures originating from Yangtze sediments.

The anaerobic Dehalococcoides spp. is the only microorganism known to completely dechlorinate the hazardous compounds tetrachloroethene (PCE) or trichloroethene (TCE) via dichloroethene (DCE) and vinyl chloride (VC) to the terminal product, ethene. In this study, growth of Dehalococcoides spp. (DHC) and the expression of DHC dehalogenase genes were demonstrated for Yangtze enrichment cultures. Reductive dechlorination of chloroethenes occurred in Yangtze sediment without the addition of any external auxiliary substrates. All Yangtze enrichment cultures completely dechlorinated PCE and cis-DCE to ethene. To investigate expression of the dehalogenase genes pceA, tceA, vcrA, and bvcA, a protocol for messenger RNA (mRNA) extraction followed by reverse transcription and quantitative PCR analysis was established. During dechlorination, an increase in gene copy numbers of pceA, tceA, and vcrA was observed. However, temporary formation of mRNA was only measured in the case of the dehalogenase genes tceA and vcrA. Comparison of DHC dehalogenase patterns indicated that the Yangtze DHC community does not match any of the previously published enrichment cultures that were obtained from contaminated areas in the USA or Europe.

Comparison of 1,2-dichloroethane, dichloroethene and vinyl chloride carbon stable isotope fractionation during dechlorination by two Dehalococcoides strains

Carbon stable isotope fractionation during 1,2-dichloroethane (1,2-DCA), dichloroethene (DCE) and vinyl chloride (VC) dechlorination was analysed for two Dehalococcoides strains, Dehalococcoides mccartyi strain 195 (formerly Dehalococcoides ethenogenes strain 195) and D. mccartyi strain BTF08, and used to characterize the reaction. The isotope enrichment factors (εC) determined for 1,2-DCA were −30.8 ± 1.3‰ and −29.0 ± 3.0‰ for D. mccartyi strain BTF08 and D. mccartyi strain 195, respectively. Enrichment factors (εC) determined for chlorinated ethenes with strain BTF08 were −28.8 ± 1.5‰ (VC), −30.5 ± 1.5‰ (cis-DCE) and −12.4 ± 1.1‰ (1,1-DCE). Product, ethene, related enrichment factors (εC1,2-DCA-ethene) calculated for 1,2-DCA (−34.1 and −32.3‰ for strain BTF08 and strain 195, respectively) were similar to substrate based enrichment factors (εC1,2-DCA), supporting the hypothesis that ethene is the direct product of 1,2-DCA dichloroelimination but that VC was a side product as result of branching in the reaction.

Enhanced Reductive Dechlorination of Tetrachloroethene Dense Nonaqueous Phase Liquid with EVO and Mg(OH)2

In situ treatment of dense nonaqueous phase liquids (DNAPL) by enhanced reductive dechlorination (ERD) can be limited by contaminant toxicity, low pH, and challenges in effectively delivering electron donor. Flushing emulsified vegetable oil (EVO), colloidal Mg(OH)2 buffer, and a bioaugmentation culture (BC) through a zone containing neat tetrachloroethene (PCE) was effective in reducing contaminant toxicity, limiting pH declines, and accelerating bioenhanced dissolution of the DNAPL. In the effluent of porous media columns with little fine material, PCE concentrations reached a maximum of 40-50 times PCE aqueous solubility in water, demonstrating NAPL PCE was distributed throughout the 1.5 m column length. In a column treated with only EVO+BC, reductive dechlorination was limited. However, a single injection of EVO+Mg(OH)2+BC was effective in reducing PCE to below detection for over 400 days with a large increase in Cl(-) and dichloroethene (DCE), accelerating bioenhanced DNAPL dissolution. Dechlorination rates gradually increased over time with the rate of total ethene (TE) release from the Mg(OH)2+EVO+BC column reaching 5-6 times the TE release rate from the EVO+BC column. The accelerated dechlorination was likely due to both Mg(OH)2 addition which limited pH declines from HCl, volatile fatty acids (VFAs), and inorganic carbon (IC) production, and formation of a mixed PCE-vegetable oil NAPL which provided a readily accessible electron donor, resulting in rapid PCE degradation with reduced PCE toxicity.

Modeling trichloroethene reduction in a hydrogen-based biofilm

We constructed a multispecies biofilm model for simultaneous reduction of trichloroethene (TCE) and nitrate (NO3(-)) in the biofilm of a H2-based membrane biofilm reactor (MBfR). The one-dimensional model includes dual-substrate Monod kinetics for a steady-state biofilm with multiple solid and dissolved components. The model has five solid components: autotrophic denitrifying bacteria (ADB), heterotrophic denitrifying bacteria (HDB), Dehalococcoides (DHC), inert biomass (IB), and extracellular polymeric substances (EPS). The model has eight dissolved components: NO3(-), TCE, dichloroethene (DCE), vinyl chloride (VC), ethene, hydrogen (H2), substrate-utilization-associated products (UAP), and biomass-associated products (BAP). We used this model to simulate a bench-scale experiment in a H2-based MBfR. The model simulated the trends well: almost complete removal of nitrate, incomplete reduction of TCE, and almost no accumulation of DCE and VC. To gain insight into reductive dehalogenation in a H2-based MBfR, we also simulated the concentrations of nitrate, TCE, DCE, VC, and ethene in the reactor effluent while varying the influent nitrate concentration. Simultaneous low concentrations of nitrate and the three chlorinated ethenes can occur as long as the influent ratio of NO3(-) to TCE is not too large, so that DHC are a significant fraction of the biofilm.

Isolation of two new Dehalococcoides mccartyi strains with dissimilar dechlorination functions and their characterization by comparative genomics via microarray analysis

Microbial reductive dechlorination of trichloroethene (TCE) in groundwater often results in the accumulation of dichloroethenes (DCEs). Dehalococcoides mccartyi (Dhc) are the only known bacteria capable of dechlorination beyond DCE to non-toxic ethene. In this study, two newly isolated Dhc strains (11a and 11a5) with dissimilar functional abilities are described. Strain 11a reductively dechlorinates TCE, 1,1-DCE, cis-DCE, trans-DCE, and vinyl chloride (VC) to ethene, while strain 11a5 dechlorinates TCE and all three DCE isomers only to VC. Each of these dechlorination reactions are coupled to growth by these strains. The VC dechlorination rate of strain 11a occurs at a rate of 258 nmol per min per mg of protein, about two times faster than previously reported stains. Strain 11a possesses the vcrA gene while strain 11a5 contains the tceA gene. Strains 11a and 11a5 share 100% 16S rRNA gene sequence identity with previously sequenced Dhc strains BAV1 and CBDB1, placing it within the Pinellas subgroup, and 85.4% and 89.5% of all genes present in the CBDB1 and BAV1 genomes were detected in strains 11a and 11a5, respectively, using a custom-designed microarray targeting four sequenced Dhc strains. Genes that were not detected in strains 11a and 11a5 are mostly within the high plasticity regions or integrated elements of the sequenced strains. This study reports the functional description and comparative genomics of two additional Dhc isolates and provides evidence that the observed functional incongruence between the activity and core genome phylogenies of Dhc strains is likely driven by the horizontal transfer of key reductive dehalogenase-encoding genes.