Reductive Dechlorination Of PCE Research Articles

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.

Effects of Heavy Metal Ions on Microbial Reductive Dechlorination of 1, 2-Dichloroethane and Tetrachloroethene

Microbial reductive dechlorination has been considered an effective process for the clean-up of organohalide-contaminated sites. Heavy metal ions are commonly present as co-contaminants in various organohalide-contaminated sites. To understand the impacts of heavy metal ions on the environmental fate of organohalides, we investigated the effects of Zn2+, Cu2+ and Cd2+ on reductive dechlorination of tetrachloroethene (PCE) and 1,2-dichloroethane (1,2-DCA) in sediment microcosms and transferred enrichment cultures. PCE and 1,2-DCA-dechlorinating enrichment cultures could be consecutively transferred in the presence of up to 10 mg/L Cu2+ or 10 mg/L Zn2+; by comparison, up to 50 mg/L Cd2+ had minor impacts on the microbial reductive dechlorination of PCE and 1,2-DCA. The inhibitory effects of tested heavy metal ions on microbial reductive dechlorination ranked in descending order are Zn2+, Cu2+, and Cd2+. Community profiling and principal component analysis indicate that the concentration and type of contaminants (e.g., heavy metal ions, organohalides) shaped the microbial community structure, an observation similar to a prior report. The enrichment of certain organohalide-respring bacteria (e.g., Dehalococcoides, Dehalogenimonas) during continuous transfers exposed to heavy metal ions suggests that they are capable of tolerating high concentrations of heavy metal ions. Our findings provide insights into the impacts of heavy metal ions on microbial reductive dechlorination and may be helpful for in situ bioremediation at sites contaminated with organohalides and heavy metals.

Organohalide Respiration with Chlorinated Ethenes under Low pH Conditions.

Bioremediation at chlorinated solvent sites often leads to groundwater acidification due to electron donor fermentation and enhanced dechlorination activity. The microbial reductive dechlorination process is robust at circumneutral pH, but activity declines at groundwater pH values below 6.0. Consistent with this observation, the activity of tetrachloroethene (PCE) dechlorinating cultures declined at pH 6.0 and was not sustained in pH 5.5 medium, with one notable exception. Sulfurospirillum multivorans dechlorinated PCE to cis-1,2-dichloroethene (cDCE) in pH 5.5 medium and maintained this activity upon repeated transfers. Microcosms established with soil and aquifer materials from five distinct locations dechlorinated PCE-to-ethene at pH 5.5 and pH 7.2. Dechlorination to ethene was maintained following repeated transfers at pH 7.2, but no ethene was produced at pH 5.5, and only the transfer cultures derived from the Axton Cross Superfund (ACS) microcosms sustained PCE dechlorination to cDCE as a final product. 16S rRNA gene amplicon sequencing of pH 7.2 and pH 5.5 ACS enrichments revealed distinct microbial communities, with the dominant dechlorinator being Dehalococcoides in pH 7.2 and Sulfurospirillum in pH 5.5 cultures. PCE-to-trichloroethene- (TCE-) and PCE-to-cDCE-dechlorinating isolates obtained from the ACS pH 5.5 enrichment shared 98.6%, and 98.5% 16S rRNA gene sequence similarities to Sulfurospirillum multivorans. These findings imply that sustained Dehalococcoides activity cannot be expected in low pH (i.e., ≤ 5.5) groundwater, and organohalide-respiring Sulfurospirillum spp. are key contributors to in situ PCE reductive dechlorination under low pH conditions.

Multiple dual C-Cl isotope patterns associated with reductive dechlorination of tetrachloroethene.

Dual isotope slopes are increasingly used to identify transformation pathways of contaminants. We investigated if reductive dechlorination of tetrachloroethene (PCE) by consortia containing bacteria with different reductive dehalogenases (rdhA) genes can lead to variable dual C-Cl isotope slopes and if different slopes also occur in the field. Two bacterial enrichments harboring Sulfurospirillum spp. but different rdhA genes yielded two distinct δ(13)C to δ(37)Cl slopes of 2.7 ± 0.3 and 0.7 ± 0.2 despite a high similarity in gene sequences. This suggests that PCE reductive dechlorination could be catalyzed according to at least two distinct reaction mechanisms or that rate-limiting steps might vary. At two field sites, two distinct dual isotope slopes of 0.7 ± 0.3 and 3.5 ± 1.6 were obtained, each of which fits one of the laboratory slopes within the range of uncertainty. This study hence provides additional insight into multiple reaction mechanisms underlying PCE reductive dechlorination. It also demonstrates that caution is necessary if a dual isotope approach is used to differentiate between transformation pathways of chlorinated ethenes.

Effectiveness of stimulating PCE reductive dechlorination: A step-wise approach

Reductive dechlorination of tetrachloroethene (PCE) and its daughter products in aquifers is often hampered by Fe(III) reducing conditions. Rigorous treatment to adjust the redox potential and stimulate dechlorination may be costly and potentially have negative effects on other aquifer functions. A step-wise experimental strategy was applied to investigate the effectiveness of various adjustment scenarios. Batch experiments with ascorbic acid (AA) and sodium lactate (SL) showed that 75μmol electron equivalents per gram dry mass of aquifer material was required to reach a sufficiently low redox potential for the onset of PCE dechlorination. Similar effects of either AA or SL on the measured redox potential suggest electron donors are not specific. However, the relative rates of Fe(III) and sulphate reduction appeared to be specific to the electron donor applied. While redox potential stabilised around −450mV after titration and sulphate was reduced to zero in both treatments, in the AA treatment a faster production of Fe2+ was observed with a final concentration of 0.46mM compared to only 0.07mM in the SL treatment. In subsequent batch experiments with aquifer material that was pre-treated with AA or SL, PCE reductive dechlorination occurred within 30days. Further stimulation tests with extra electron donor or inoculum revealed that adding electron donor can accelerate the initiation of PCE biodegradation. However, bioaugmentation with dechlorinating bacteria is required to achieve complete reductive dechlorination to ethene. The findings from step-wise approaches are relevant for improving the cost-effectiveness of the design and operation of in-situ bioremediation at initially unfavourable environmental conditions.

Microbially enhanced dissolution and reductive dechlorination of PCE by a mixed culture: Model validation and sensitivity analysis

Reductive dechlorination catalyzed by organohalide-respiring bacteria is often considered for remediation of non-aqueous phase liquid (NAPL) source zones due to cost savings, ease of implementation, regulatory acceptance, and sustainability. Despite knowledge of the key dechlorinators, an understanding of the processes and factors that control NAPL dissolution rates and detoxification (i.e., ethene formation) is lacking. A recent column study demonstrated a 5-fold cumulative enhancement in tetrachloroethene (PCE) dissolution and ethene formation (Amos et al., 2009). Spatial and temporal monitoring of key geochemical and microbial (i.e., Geobacter lovleyi and Dehalococcoides mccartyi strains) parameters in the column generated a data set used herein as the basis for refinement and testing of a multiphase, compositional transport model. The refined model is capable of simulating the reactive transport of multiple chemical constituents produced and consumed by organohalide-respiring bacteria and accounts for substrate limitations and competitive inhibition. Parameter estimation techniques were used to optimize the values of sensitive microbial kinetic parameters, including maximum utilization rates, biomass yield coefficients, and endogenous decay rates. Comparison and calibration of model simulations with the experimental data demonstrate that the model is able to accurately reproduce measured effluent concentrations, while delineating trends in dechlorinator growth and reductive dechlorination kinetics along the column. Sensitivity analyses performed on the optimized model parameters indicate that the rates of PCE and cis-1,2-dichloroethene (cis-DCE) transformation and Dehalococcoides growth govern bioenhanced dissolution, as long as electron donor (i.e., hydrogen flux) is not limiting. Dissolution enhancements were shown to be independent of cis-DCE accumulation; however, accumulation of cis-DCE, as well as column length and flow rate (i.e., column residence time), strongly influenced the extent of reductive dechlorination. When cis-DCE inhibition was neglected, the model over-predicted ethene production ten-fold, while reductions in residence time (i.e., a two-fold decrease in column length or two-fold increase in flow rate) resulted in a more than 70% decline in ethene production. These results suggest that spatial and temporal variations in microbial community composition and activity must be understood to model, predict, and manage bioenhanced NAPL dissolution.

Aerobic/anaerobic/aerobic sequenced biodegradation of a mixture of chlorinated ethenes, ethanes and methanes in batch bioreactors

A novel aerobic/anaerobic/aerobic treatment was implemented in batch reactors containing aquifer materials from a site contaminated by tetrachloroethylene (PCE), trichloroethylene (TCE), vinyl chloride (VC), 1,1,2-trichloroethane (1,1,2-TCA) and chloroform (CF). Consortia grown aerobically on methane, propane, n-pentane and n-hexane completely biodegraded the chlorinated solvent mixture, via aerobic cometabolism of VC, CF, TCE and 1,1,2-TCA, followed by PCE reductive dechlorination (RD) to 1,2-cis-dichlorothylene (cis-DCE) or TCE, and cis-DCE/TCE cometabolism in a further aerobic phase. n-Hexane was the best substrate. No electron donor was supplied for RD, which likely utilized cellular material produced during the aerobic phase. Chloride release was stoichiometric with chlorinated solvent biodegradation. According to the Lepidium sativum ecotoxicity test, a decreased toxicity was observed with propane, n-pentane and n-hexane, but not methane. A kinetic study of PCE RD allowed to estimate the PCE maximum specific rate (0.57±0.07mgmgprotein-1day-1) and half-saturation constant (6.7±1.5mgL-1).

Impacts of microbial community composition on isotope fractionation during reductive dechlorination of tetrachloroethylene

Isotope fractionation has been used with increasing frequency as a tool to quantify degradation of chlorinated aliphatic pollutants in the environment. The objective of this research was to determine if the electron donor present in enrichment cultures prepared from uncontaminated sediments influenced the extent of isotope fractionation of tetrachloroethylene (PCE), either directly, or through its influence on microbial community composition. Two PCE-degrading enrichment cultures were prepared from Duck Pond (DP) sediment and were incubated with formate (DPF) or H(2) (DPH) as electron donor. DPF and DPH were significantly different in both product distribution and extent of isotope fractionation. Chemical and isotope analyses indicated that electron donors did not directly affect the product distribution or the extent of isotope fractionation for PCE reductive dechlorination. Instead, restriction fragment length polymorphism (RFLP) and sequence analysis of the 16S rRNA clone libraries of DPF and DPH identified distinct microbial communities in each enrichment culture, suggesting that differences in microbial communities were responsible for distinct product distributions and isotope fractionation between the two cultures. A dominant species identified only in DPH was closely related to known dehalogenating species (Sulfurospirillum multivorans and Sulfurospirillum halorespirans) and may be responsible for PCE degradation in DPH. Our study suggests that different dechlorinators exist at the same site and can be preferentially stimulated by different electron donors, especially over the long-term (i.e., years), typical of in-situ ground water remediation.

Variations in expression of carbon isotope fractionation of chlorinated ethenes during biologically enhanced PCE dissolution close to a source zone

The stable carbon isotope values of tetrachloroethene (PCE) and its degradation products were monitored during studies of biologically enhanced dissolution of PCE dense nonaqueous phase liquid (DNAPL) to determine the effect of PCE dissolution on observed isotope values. The degradation of PCE was monitored in a 2-dimensional model aquifer and in a pilot test cell (PTC) at Dover Air Force Base, both with emplaced PCE DNAPL sources. Within the plume down gradient from the source, the isotopic fractionation of dissolved PCE and its degradation products were consistent with those observed in biodegradation laboratory studies. However, close to the source zone significant shifts in the isotope values of dissolved PCE were not observed in either the model aquifer or PTC due to the constant input of newly dissolved, non fractionated PCE, and the small isotopic fractionation associated with PCE reductive dechlorination by the mixed microbial culture used. Therefore the identification of reductive dechlorination in the presence of PCE DNAPL was based upon the appearance of daughter products and the isotope values of those daughter products. An isotope model was developed to simulate isotope values of PCE during the dissolution and degradation of PCE adjacent to a DNAPL source zone. With the exception of very high degradation rate constants (>1/day) stable carbon isotope values of PCE estimated by the model remained within error of the isotope value of the PCE DNAPL, consistent with measured isotope values in the model aquifer and in the PTC.

Integrated approach to PCE‐impacted site characterization, site management, and enhanced bioremediation

AbstractTetrachloroethene (PCE)‐ and trichloroethene (TCE)‐impacted sites pose significant challenges even when site characterization activities indicate that biodegradation has occurred naturally. Although site‐specific, regulatory, and economic factors play roles in the remedy‐selection process, the application of molecular biological tools to the bioremediation field has streamlined the assessment of remedial alternatives and allowed for detailed evaluation of the chosen remedial technology. The case study described here was performed at a PCE‐impacted site at which reductive dechlorination of PCE and TCE had led to accumulation of cis‐dichlorethene (cis‐DCE) with concentrations ranging from approximately 10 to 100 mg/L. Bio‐Trap® samplers and quantitative polymerase chain reaction (qPCR) enumeration of Dehalococcoides spp. were used to evaluate three remedial options: monitored natural attenuation, biostimulation with HRC®, and biostimulation with HRC‐S®. Dehalococcoides populations in HRC‐S‐amended Bio‐Traps deployed in impacted wells were on the order of 103 to 104 cells/bead but were below detection limits in most unamended and HRC‐amended Bio‐Traps. Thus the in situ Bio‐Trap study identified biostimulation with HRC‐S as the recommended approach, which was further evaluated with a pilot study. After the pilot HRC‐S injection, Dehalococcoides populations increased to 106 to 107 cells/bead, and concentrations of cis‐DCE and vinyl chloride decreased with concurrent ethene production. Based on these results, a full‐scale HRC‐S injection was designed and implemented at the site. As with the pilot study, full‐scale HRC‐S injection promoted growth of Dehalococcoides spp. and stimulated reductive dechlorination of the daughter products cis‐DCE and vinyl chloride. © 2008 Wiley Periodicals, Inc.

Electrolytic Methanogenic−Methanotrophic Coupling for Tetrachloroethylene Bioremediation: Proof of Concept

Coupling of methanogenic and methanotrophic catabolisms was performed in a single-stage technology equipped with a water electrolysis cell placed in the effluent recirculation loop. The electrolysis-generated hydrogen served as an electron donor for both bicarbonate reduction into CH4 and reductive dechlorination, while the O2 and CH4, supported the cometabolic oxidation of chlorinated intermediates left over by the tetrachloroethylene (PCE) transformation. The electrolytical methanogenic/methanotrophic coupled (eMaMoC) process was tested in a laboratory-scale setup at PCE loads ranging from 5 to 50 micromol/L(rx) x d (inlet concentrations from 4 to 11 mg/L), and at various hydraulic residence times (HRT). Degradation followed essentially a reductive dechlorination pathway from PCE to cis-1,2-dichloroethene (DCE), and an oxidative pathway from DCE to CO2. PCE reductive dechlorination to DCE was consistently over 98% while a maximum oxidative DCE mineralization of 89% was obtained at a load of 4.3 micromol PCE/ L(rx) x d and an HRT of 6 days. Controlling dissolved oxygen concentrations within a relatively low range (2-3 mg/L) seemed instrumental to sustain the overall degradation capacity. Degradation kinetics were further evaluated: the apparent half-saturation constant (K(s)) had to be set relatively high (29 microM) for the simulated data to best fit the experimental ones. In spite of such kinetic limitations, the eMaMoC system, while fueled by water electrolysis, was effective in building and sustaining a functional methanogenic/methanotrophic consortium capable of significant PCE mineralization in a single-stage process. Hence, degradation standards are within reach so long as the methanotrophic DCE-oxidizing potential, including substrate affinity, are optimized and HRT accordingly adjusted.

생물벽체를 이용한 PCE로 오염된 지하수의 환원성 탐염소화: 칼럼 실험

The applicability of in situ biobarrier or microbial filter technology for the remediation of groundwater contaminated with chlorinated solvent was investigated through column study. In this study, the effect of packing materials on the reductive dechlorination of PCE was investigated using Canadian peat, Pahokee peat, peat moss and vermicompost (or worm casting) as a biobarrier medium. Optimal conditions previously determined from a batch microcosm study was applied in this column study. Lactate/benzoate was amended as electron donors to stimulate reductive dechlorination of PCE. Hydraulic conductivity was approximately and no difference was found among the packing materials. The transport and dispersion coefficients determined from the curve-fitting of the breakthrough curves of using CXTFIT 2.1 showed no difference between single-region and two-region models. The reductive dechlorination of PCE was efficiently occurred in all columns. Among the columns, especially the column packed with vermicompost exhibited the highest reductive dechlorination efficiency. The results of this study showed the promising potential of in situ biobarrier technology using peat and vermicompost for the remediation of groundwater contaminated with chlorinated solvents.

Ultrastructure of a bio-electrolytic methanogenic/methanotrophic granular biofilm for the complete degradation of tetrachloroethylene in contaminated groundwater

The electrolytical methanogenic/methanotrophic coupling (eMaMoC) process was tested in a laboratory-scale single-stage reactor for the treatment of tetrachloroethene (PCE)-contaminated waters. A water electrolysis cell was placed directly in the effluent recirculation loop for the supply of both O2 and H2 to the system: H2 serving as the electron donor for both carbonate reduction into CH4 and reductive dechlorination. The concurrent presence of O2 and CH4 could be used by the methanotrophs for co-metabolically oxidising the chlorinated intermediates left over by the anaerobic transformation of PCE. At a PCE inlet of 33-52 microM and a hydraulic residence time (HRT) of 5.6 days, PCE reductive dechlorination to dichloroethene (DCE) was over 95% with a maximum DCE mineralisation of 83%. Fluorescence in situ hybridisation with 16S rRNA probes related to type I and type II methanotrophic bacteria were utilised to localise the methanotrophic communities in the anaerobic/aerobic granules. It evidenced that with operational time, along with increasing oxygenation rate, methanotrophic communities were specifically colonising onto the outermost layer of the anaerobic/aerobic granule.

Kinetic and phylogenetic characterization of an anaerobic dechlorinating microbial community.

The reductive dechlorination (RD) of tetrachloroethene (PCE) to vinyl chloride (VC) and, to a lesser extent, to ethene (ETH) by an anaerobic microbial community has been investigated by studying the processes and kinetics of the main physiological components of the consortium. Molecular hydrogen, produced by methanol-utilizing acetogens, was the electron donor for the PCE RD to VC and ETH without forming any appreciable amount of other chlorinated intermediates and in the near absence of methanogenic activity. The microbial community structure of the consortium was investigated by preparing a 16S rDNA clone library and by fluorescence in situ hybridization (FISH). The PCR primers used in the clone library allowed the harvest of 16S rDNA from both bacterial and archaeal members in the community. A total of 616 clones were screened by RFLP analysis of the clone inserts followed by the sequencing of RFLP group representatives and phylogenetic analysis. The clone library contained sequences mostly from hitherto undescribed bacteria. No sequences similar to those of the known RD bacteria like 'Dehalococcoides ethenogenes' or Dehalobacter restrictus were found in the clone library, and none of these bacteria was present in the RD consortium according to FISH. Almost all clones fell into six previously described phyla of the bacterial domain, with the majority (56.6 %) being deep-branching members of the Spirochaetes phylum. Other clones were in the Firmicutes phylum (18.5 %), the Chloroflexi phylum (16.4 %), the Bacteroidetes phylum (6.3 %), the Synergistes genus (1.1 %) and a lineage that could not be affiliated with existing phyla (1.1 %). No archaeal clones were found in the clone library. Owing to the phylogenetic novelty of the microbial community with regard to previously cultured micro-organisms, no specific microbial component(s) could be hypothetically affiliated with the RD phenotype. The predominance of Spirochaetes in the microbial consortium, the main group revealed by clone library analysis, was confirmed by FISH using a purposely developed probe.

Reductive Dechlorination of PCE and TCE by Vitamin B12 and ZVMs

The reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) catalyzed by vitamin B12 was examined when zero valent metals (ZVMs) were used as bulk electron donors in batch reactors. UV-visible spectra showed that zinc reduces vitamin B12 Co(III) to vitamin B12 Co(I) through B12 Co(II) and iron reduces vitamin B12 Co(III) to vitamin B12 Co(II). Thus iron, the most popular ZVM reductant, does not have enough reduction potential to reduce vitamin B12 to the super-reduced vitamin B12(I), which has been shown to be an active species in reductive dechlorination. Dechlorination of PCE and TCE by iron and zinc in the presence of vitamin B12 showed that the zinc and vitamin B12 combination greatly enhances the reaction rates for both PCE and TCE, but iron and vitamin B12 result in an increase in reactivity only for PCE degradation, not for TCE degradation in comparon with metals only. This result indicates vitamin B12(I) is active towards both PCE and TCE degradation while vitamin B12(II) is active only towards PCE. Calculated activation energies for the dechlorination of PCE in the presence of Vitamin B12 showed that vitamin B12 lowered the activation energy about 40-60 kJ mol−1 for both metals.

Dechlorination of PCE by Mixed Methanogenic Cultures Using Hollow-Fiber Membranes

The study investigated the use of hollow-fiber membranes for hydrogen (H2) delivery to support the biological reductive dechlorination of tetrachloroethene (PCE) Two experiments were performed in which H2 was supplied through membranes placed in stirred batch reactors containing a mixed methanogenic/dechlorinating culture and PCE (≤10 µM. Reductive dechlorination of PCE to cis-dichloroethene was sustained in the reactors receiving H2 (1% H2 and 50% H2), while negligible dechlorination was observed in control reactors (100% N2). The 1%-H2-fed reactor outperformed the 50%-H2-fed reactor in the first experiment. However, the dechlorinating performance in the two reactors was similar in the second experiment. Despite relatively high H2 concentrations (4.6 to 178 µM) that led to H2 consumption (and CH4 production) by methanogens, dechlorination was effectively maintained for the duration of the experiments (35 to 62 days). The results of this study are significant in that dechlorination was sustained in a minimal medium by membrane-delivered H2. Dechlorination was also maintained at aqueous H2 concentrations that exceeded the thermodynamic thresholds for not only dechlorination (<0.1 to 2 nM, but also methanogenesis (∼10 nM) and homoacetogenesis (94 to 400 nM. The results of these experiments suggest that membranes are a promising H2 delivery technology for stimulating the bioremediation of chlorinated ethene-contaminated aquifers.

Complete remediation of PCE contaminated unsaturated soils by sequential anaerobic-aerobic bioventing

Bioventing principles have been applied to completely dechlorinate tetrachloroethylene vapors in the unsaturated zone in a sequential anaerobic-aerobic pattern. The aerobic step yields trans-DCE and VC as PCE reductive dechlorination byproducts, while TCE and cis-DCE are observed as intermediates. The aerobic step results in rapid oxidation of the VC and trans-DCE to carbon dioxide. Hydrogen was delivered in the gas phase as a reducing agent for the anaerobic step at levels of 1%, and oxygen at 4.2% was used as an electron acceptor in the aerobic step. PCE and VC half lives in the anaerobic and aerobic steps respectively, where less than 10 min.

Efficient Dechlorination of Tetrachloroethylene in Soil Slurry by Combined Use of an Anaerobic Desulfitobacterium sp. Strain Y-51 and Zero-Valent Iron.

A laboratory test was conducted to examine the combined effect of bioaugmentation of an anaerobic bacterial Desulfitobacterium sp. strain Y-51 and addition of zero-valent iron (Fe0) on the reductive dechlorination of tetrachloroethylene (PCE) in a non-sterile soil slurry. Introduction of a strain Y-51 culture in soil (3 mg vss (volatile suspended solids)/kg soil) containing PCE (at 60 micromol/kg soil) led to complete conversion of PCE to cis-1,2-dichloroethylene (cis-DCE) within 40 d. Treatments of the same soil slurry with Fe0 (0.1-1.0%) resulted in extended PCE dechlorination to ethylene (ETH) and ethane (ETA). The combined use of a strain Y-51 culture and Fe0 showed effective dechlorination of PCE than did the individual use. The cis-DCE produced from biological PCE dechlorination by strain Y-51 was totally converted to non-chlorinated end products by the following chemical reduction by Fe0. Furthermore, anaerobic corrosion of Fe0 was found to stimulate the biological reductive dechlorination of PCE by keeping proper levels of pH and oxidation-reduction potential (ORP) and by producing cathodic hydrogen, which might be used as an electron donor for respiratory PCE dechlorination. These findings suggest that the combined use of bacterial strain Y-51 and Fe0 is effective for practical treatment of PCE and other chlorinated ethylenes in contaminated sites.

Microbial Community Changes Associated with a Shift from Reductive Dechlorination of PCE to Reductive Dechlorination ofcis-DCE and VC

Subcultures that reductively dechlorinate cis-dichloroethene (cis-DCE) or vinyl chloride (VC) were derived from three independent enrichments that completely dechlorinated tetrachloroethene (PCE) to ethene in order to study the reductive dechlorination of the lesser chlorinated ethenes. These subcultures completely dechlorinated cis-DCE and VC and could be transferred indefinitely in basal salts minimal medium with H2 as the electron donor. After 10 transfers (1% V/V) the cis-DCE and VC-dechlorinating subcultures from two of the PCE enrichments failed to dechlorinate PCE, but the subcultures from the third PCE enrichment maintained the ability to dechlorinate PCE. Analysis of the 16S rRNA genes from these enrichments by terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE) demonstrated shifts in the community composition of the subcultures that had lost the PCE-dechlorinating activity but not in the subcultures that maintained the PCE-dechlorinating ...

Microbial utilization of vadose zone organic carbon for reductive dechlorination of tetrachloroethene

Abstract Aqueous extracts from a calcareous spodosol were used as the primary substrate to study the reductive dechlorination of tetrachloroethene (PCE). A comparison was made between extracts obtained using pure water and water saturated with trichloroethene (TCE). The latter solutions were intended to simulate solutions formed by rainwater percolating through TCE‐contaminated soils. The results indicated that TCE‐saturated water apparently mobilized more organic materials than did pure water. The extracts obtained from TCE‐saturated water were noticeably more turbid than those obtained using pure water, suggesting greater mobilization of colloids. After centrifugation, the extracts contained similar amounts of dissolved organic carbon (DOC), 16 and 20 mg/L, respectively. Microcosm studies indicated that both centrifuged extracts were capable of sustaining reductive dechlorination of PCE as evidenced by the production of TCE, dichloroethene (DCE), and methane. These results indicate that the inclusion of...