Microbial Reductive Dechlorination Research Articles

Evaluation of trichloroethene recovery processes in heterogeneous aquifer cells flushed with biodegradable surfactants

The ability of two biodegradable surfactants, polyoxyethylene (20) sorbitan monooleate (Tween® 80) and sodium dihexyl sulfosuccinate (Aerosol® MA), to recover a representative dense non-aqueous-phase liquid (DNAPL), trichloroethene (TCE), from heterogeneous porous media was evaluated through a combination of batch and aquifer cell experiments. An aqueous solution containing 3.3% Aerosol MA, 8% 2-propanol and 6 g/l CaCl 2 yielded a weight solubilization ratio (WSR) of 1.21 g TCE/g surfactant, with a corresponding liquid–liquid interfacial tension (IFT) of 0.19 dyn/cm. Flushing of aquifer cells containing a TCE-DNAPL source zone with approximately two pore volumes of the AMA formulation resulted in substantial (> 30%) mobilization of TCE-DNAPL. However, a TCE mass recovery of 81% was achieved when the aqueous-phase flow rate was sufficient to displace the mobile TCE-DNAPL toward the effluent well. Aqueous solutions of Tween 80 exhibited a greater capacity to solubilize TCE (WSR = 1.74 g TCE/g surfactant) and exerted markedly less reduction in IFT (10.4 dyn/cm). These data contradict an accepted empirical correlation used to estimate IFT values from solubilization capacity, and indicate a unique capacity of T80 to form concentrated TCE emulsions. Flushing of aquifer cells with less than 2.5 pore volumes of a 4% T80 solution achieved TCE mass recoveries ranging from 66 to 85%, with only slight TCE-DNAPL mobilization (< 5%) occurring when the total trapping number exceeded 2 × 10 − 5 . These findings demonstrate the ability of Tween 80 and Aerosol MA solutions to efficiently recover TCE from a heterogeneous DNAPL source zone, and the utility of the total trapping number as a design parameter for a priori prediction of DNAPL mobilization and bank angle formation when flushing with low-IFT solutions. Given their potential to stimulate microbial reductive dechlorination at low concentrations, these surfactants are well-suited for remedial action plans that couple aggressive mass removal followed by enhanced bioremediation to treat chlorinated solvent source zones.

Field Study of In Situ Anaerobic Bioremediation of a Chlorinated Solvent Source Zone

A field-scale in situ bioremediation test was conducted at a chlorinated solvent (mainly 1,1,2,2-tetrachloroethane) contaminated aquifer to evaluate the feasibility of source-zone treatment through anaerobic reductive dechlorination. Contaminated groundwater was extracted at the bottom of the testing zone, amended/mixed with an electron donor (lactate) solution, and reinjected at the head of the testing zone. Both forced groundwater recirculation and lactate injection resulted in significant mobilization of chlorinated contaminants from the source zone, probably through enhanced dissolution of DNAPL (dense nonaqueous phase liquid) pools. The addition of lactate stimulated the establishment of anaerobic conditions in the aquifer and the onset of microbial reductive dechlorination processes. Dechlorination of 1,1,2,2-tetrachloroethane led to the formation of predominantly 1,1,2-trichloroethane and cis-dichloroethene. Noticeably, dechlorination occurred at aqueous 1,1,2,2-tetrachloroethane concentrations as ...

Electron Transfer from a Solid-State Electrode Assisted by Methyl Viologen Sustains Efficient Microbial Reductive Dechlorination of TCE

The ability to transfer electrons, via an extracellular path, to solid surfaces is typically exploited by microorganisms which use insoluble electron acceptors, such as iron-or manganese-oxides or inert electrodes in microbial fuel cells. The reverse process, i.e., the use of solid surfaces or electrodes as electron donors in microbial respirations, although largely unexplored, could potentially have important environmental applications, particularly for the removal of oxidized pollutants from contaminated groundwater or waste streams. Here we show, for the first time, that an electrochemical cell with a solid-state electrode polarized at -500 mV (vs standard hydrogen electrode), in combination with a low-potential redox mediator (methyl viologen), can efficiently transfer electrochemical reducing equivalents to microorganisms which respire using chlorinated solvents. By this approach, the reductive transformation of trichloroethene, a toxic yet common groundwater contaminant, to harmless end-products such as ethene and ethane could be performed. Furthermore, using a methyl-viologen-modified electrode we could even demonstrate that dechlorinating bacteria were able to accept reducing equivalents directly from the modified electrode surface. The innovative concept, based on the stimulation of dechlorination reactions through the use of solid-state electrodes (we propose for this process the acronym BEARD: Bio-Electrochemically Assisted Reductive Dechlorination), holds promise for in situ bioremediation of chlorinated-solvent-contaminated groundwater, and has several potential advantages over traditional approaches based on the subsurface injection of organic compounds. The results of this study raise the possibility that immobilization of selected redox mediators may be a general strategy for stimulating and controlling a range of microbial reactions using insoluble electrodes as electron donors.

Enantiomeric composition of chiral polychlorinated biphenyl atropisomers in dated sediment cores

Enantiomer fractions (EFs) of seven chiral polychlorinated biphenyls (PCBs) were measured in dated sediment cores of Lake Hartwell (SC, USA) and Lake Ontario (USA) to detect, quantify, and gain insight regarding microbial reductive dechlorination of PCBs in lake sediments with high and low concentrations, respectively. Lake Hartwell sediments had high total PCBs (5-60 microg/g), with significantly nonracemic EFs that generally were consistent with those from previous laboratory microcosm reductive dechlorination experiments using sediments from these sites. Thus, stereoselective reductive dechlorination had occurred in situ, including at total PCB concentrations of less than the threshold of approximately 30 to 80 microg/g suggested as being necessary for reductive dechlorination. Enantiomer fractions of PCBs 91, 95, 132, and 136 in Lake Hartwell cores were significantly correlated both with concentrations of those individual congeners and with total PCB concentration for some sites. This result indicates that enantioselective microbial dechlorination activity increases with higher concentrations within sediments for these congeners. Enantiomer composition reversed with depth for PCBs 91, 132, and 176, suggesting that multiple microbial populations may be present within the same core that are enantioselectively dechlorinating PCBs. Such observations indicate that concentration and time are not the only factors affecting biotransformation, complicating prediction of enantioselectivity. Comparison of EFs with dates suggested biotransformation half-lives of approximately 30 years, which is on the same time scale as sequestration by burial. In contrast, Lake Ontario sediments (maximum total PCBs, 400 ng/g) had racemic or near-racemic amounts of most congeners throughout the core profile, which is consistent with achiral indicators suggesting no microbial biotransformation within Lake Ontario sediments. Thresholds for reductive dechlorination may exist, but they would be at concentrations of less than 30 to 80 microg/g.

Effects of the Nonionic Surfactant Tween 80 on Microbial Reductive Dechlorination of Chlorinated Ethenes

Recent field studies have indicated synergistic effects of coupling microbial reductive dechlorination with physicochemical remediation (e.g., surfactant flushing) of dense nonaqueous phase liquid (DNAPL) source zones. This study explored chlorinated ethene (e.g., tetrachloroethene [PCE]) dechlorination in the presence of 50-5000 mg/L Tween 80, a nonionic surfactant employed in source zone remediation. Tween 80 did not inhibit dechlorination by four pure PCE-to-cis-1,2-dichloroethene (cis-DCE) or PCE-to-trichloroethene (TCE) dechlorinating cultures. In contrast, cis-DCE-dechlorinating Dehalococcoides isolates (strain BAV1 and strain FL2) failed to dechlorinate in the presence of Tween 80. Bio-Dechlor INOCULUM (BDI), a PCE-to-ethene dechlorinating consortium, produced cis-DCE in the presence of Tween 80, further suggesting that Tween 80 inhibits dechlorination by Dehalococcoides organisms. Quantitative real-time PCR analysis applied to BDI revealed that the number of Dehalococcoides cells decayed exponentially (R(2) = 0.85) according to the Chick-Watson disinfection model (pseudo first-order decay rate of 0.13+/-0.02 day(-1)) from an initial value of 6.6 +/-1.5 x 10(8) to 1.3+/-0.8 x 10(5) per mL of culture after 58 days of exposure to 250 mg/L Tween 80. Although Tween 80 exposure prevented ethene formation and reduced Dehalococcoides cell numbers, Dehalococcoides organisms remained viable, and dechlorination activity pist cis-DCE was recovered following the removal of Tween 80. These findings suggest that sequential Tween 80 flushing followed by microbial reductive dechlorination is a promising strategy for remediation of chlorinated ethene-impacted source zones.

Kinetics of the Microbial Reductive Dechlorination of Pentachloroaniline

The microbial reductive dechlorination kinetics of pentachloroaniline (PCA) and less chlorinated anilines (CAs) were investigated with a mixed, fermentative/ methanogenic culture. Batch dechlorination assays were performed with all available CAs at an initial concentration of 3 microM, and an incubation temperature of 22 degrees C. Dechlorination of PCA, two tetrachloroanilines (2,3,4,5- and 2,3,5,6-TeCA), five trichloroanilines (2,3,4-, 2,3,5-, 2,4,5-, 2,4,6-, and 3,4,5-TrCA), and one dichloroaniline (3,5-DCA; low extent) was observed but none of the five remaining dichloroanilines and three monochloroanilines were dechlorinated by the enrichment culture during batch assays. The dechlorination rates (k') and half-saturation coefficients (Kc) were measured using nonlinear regression based on the integrated Michaelis-Menten equation under conditions of electron donor saturation and assuming constant biomass concentration over the relatively short batch incubation period. At an initial concentration of CAs of about 3 microM, the values of k' and Kc ranged from 0.25 to 1.19 microM/day and from 0.11 to 1.72 microM, respectively, corresponding to half-lives in the range of 1.5-8.5 days. Model simulations of the sequential dechlorination reactions based on a branched-chain Michaelis-Menten model and using independently measured k' and Kc values matched the experimental data very well.

Microbial Reductive Dechlorination of Weathered and Exogenous Co-planar Polychlorinated Biphenyls (PCBs) in an Anaerobic Sediment of Venice Lagoon

The occurrence of reductive dechlorination processes towards pre-existing PCBs and five exogenous coplanar PCBs were investigated in a contaminated sediment of Porto Marghera (Venice Lagoon, Italy) suspended, under strictly anaerobic conditions, in water collected from the same site. PCB dechlorination started after five months of incubation, when sulfate initially occurring in the microcosms was completely depleted and methanogenesis was in progress. It was ascribed to sulfate-reducing bacteria. Several pre-existing hexa-, penta- and tetra-chlorinated biphenyls were slowly bioconverted into tri- and di-, ortho-substituted PCBs from the 5th to the 16th month of experiment. Spiked coplanar PCBs, i.e., 3,3',4,4'-tetrachlorobiphenyl, 3,3',4,4',5- and 2,3',4,4',5-pentachlorobiphenyls, 3,3',4,4',5,5'- and 2,3,3',4,4',5-hexachlorobiphenyls, were extensively transformed (by about 90%) into lower chlorinated congeners, such as 3,3',5,5'-/2,3',4,4'-tetrachlorobiphenyl, 3,3',5-, 2,4,4'-, 2,3',4- and 2,3',5-trichlorobiphenyl, 3,4-/3,4'- and 3,3'-dichlorobiphenyl and 2-chlorobiphenyl. The reductive dechlorination of spiked PCBs did not influence significantly the biotransformation rate and extent of pre-existing PCBs.

Biostimulation and bioaugmentation enhances aerobic biodegradation of dichloroethenes

The accumulation of dichloroethenes (DCEs) as dominant products of microbial reductive dechlorination activity in soil and water represent a significant obstacle to the application of bioremediation as a remedial option for chloroethenes in many contaminated systems. In this study, the effects of biostimulation and/or bioaugmentation on the biodegradation of cis- and trans-DCE in soil and water samples collected from contaminated sites in South Africa were evaluated in order to determine the possible bioremediation option for these compounds in the contaminated sites. Results from this study indicate that cis- and trans-DCE were readily degraded to varying degrees by natural microbial populations in all the soil and water samples tested, with up to 44% of cis-DCE and 41% of trans-DCE degraded in the untreated soil and water samples in two weeks. The degradation rate constants ranged significantly ( P < 0.05) between 0.0938 and 0.560 wk −1 and 0.182 and 0.401 wk −1, for cis- and trans-DCE, respectively, for the various treatments employed. A combination of biostimulation and bioaugmentation significantly increased the biodegradation of both compounds within two weeks; 14% for cis-DCE and 18% for trans-DCE degradation, above those observed in untreated soil and water samples. These findings support the use of a combination of biostimulation and bioaugmentation for the efficient biodegradation of these compounds in contaminated soil and water. In addition, the results clearly demonstrate that while naturally occurring microorganisms are capable of aerobic biodegradation of cis- and trans-DCE, biotransformation may be affected by several factors, including isomer structure, soil type, and the amount of nutrients available in the water and soil.

Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?

Guest Editorial: Contaminant Source Zones: Remediation or Perpetual Stewardship?

Anaerobic biodegradability of Tween surfactants used as a carbon source for the microbial reductive dechlorination of hexachlorobenzene

Three structurally-related, nonionic, polysorbate surfactants (Tween 60, 61, and 65) were used as the sole carbon source to sustain the microbial, sequential reductive dechlorination of hexachlorobenzene (HCB) in a mixed, methanogenic culture derived from a contaminated estuarine sediment. The surfactants were partially degraded and fermented to methane with no measurable accumulation of volatile fatty acids, indicating that methanogenesis was rapid relative to the rates of hydrolysis and acidogenesis. Addition of the methanogenesis inhibitor 2-bromoethanesulfonic acid resulted in acetate accumulation without impact on the sequential dechlorination of HCB. An anaerobic biodegradability assay was performed and the following data were obtained for the Tween 60, 61, and 65, respectively: 53, 62, and 62% COD destruction; 35, 57, and 48% COD to methane conversion; and 38, 38, and 45% COD to acetate conversion. These data suggest that the hydrophobic moiety (stearate) of the surfactants was preferentially degraded, most likely through beta-oxidation, to acetate and ultimately to methane and carbon dioxide. Between 38 and 47% of the initial surfactant COD remained after 46 d incubation, which most likely corresponds to the hydrophilic polyoxyethylene moiety. An anaerobic biodegradation pathway of the Tween surfactants is proposed.

Reductive Dechlorination of Tetrachloroethene Following Abiotic Versus Biotic Reduction of Hexavalent Chromium

ABSTRACT A feasibility evaluation identified chemical reduction and biostimulation as a potential remedy for a plume containing hexavalent chromium (Cr(VI)) and tetrachloroethene (PCE) at an industrial site in southern California. The objectives of this laboratory study were to determine the stoichiometry of calcium polysulfide (CaSx) reaction with Cr(VI) in the presence of sediment, the effect of CaSx on the potential for in situ biological reductive dechlorination of PCE, and the potential to reduce Cr(VI) and PCE by addition of only an electron donor. Approximately 1 L of CaSx solution (containing 50 g S2−/L) was required per 1000 L of groundwater containing 45 mg/L of Cr(VI) (i.e., 1.8 mol S2− per mol Cr(VI)). The sediment also exerted a sulfide demand (≥0.38 g S2 − per kg sediment), but at a slower rate than the Cr(VI). In microcosms prepared with lactate, corn syrup, soybean oil, or methanol, but no CaSx, the Cr(VI) was biologically reduced in the treatments with lactate and corn syrup, but much more slowly than with CaSx. Even after 20 months of incubation, no significant reductive dechlorination of PCE occurred in any of the microcosms, including those in which the Cr(VI) was removed with CaSx. Bioaugmentation was tested with the microcosms that received lactate and corn syrup (following 20 months of incubation), using an enrichment culture that actively dechlorinates trichloroethene. PCE dechlorination began within 1 month in the lactate-only treatment; in the corn syrup–amended treatment, PCE dechlorination occurred in only one of the three bottles. However, no PCE dechlorination occurred following bioaugmentation of the lactate and corn syrup microcosms that were initially treated with CaSx, indicating that CaSx (and/or its reaction products) exerted a negative impact on the chlororespiring microbes. This outcome highlights the need to evaluate sites on a case-by-case basis when in situ chemical treatment is applied prior to microbial reductive dechlorination.

Coupling Aggressive Mass Removal with Microbial Reductive Dechlorination for Remediation of DNAPL Source Zones: A Review and Assessment

The infiltration of dense non-aqueous-phase liquids (DNAPLs) into the saturated subsurface typically produces a highly contaminated zone that serves as a long-term source of dissolved-phase groundwater contamination. Applications of aggressive physical–chemical technologies to such source zones may remove > 90% of the contaminant mass under favorable conditions. The remaining contaminant mass, however, can create a rebounding of aqueous-phase concentrations within the treated zone. Stimulation of microbial reductive dechlorination within the source zone after aggressive mass removal has recently been proposed as a promising staged-treatment remediation technology for transforming the remaining contaminant mass. This article reviews available laboratory and field evidence that supports the development of a treatment strategy that combines aggressive source-zone removal technologies with subsequent promotion of sustained microbial reductive dechlorination. Physical–chemical source-zone treatment technologies compatible with posttreatment stimulation of microbial activity are identified, and studies examining the requirements and controls (i.e., limits) of reductive dechlorination of chlorinated ethenes are investigated. Illustrative calculations are presented to explore the potential effects of source-zone management alternatives. Results suggest that, for the favorable conditions assumed in these calculations (i.e., statistical homogeneity of aquifer properties, known source-zone DNAPL distribution, and successful bioenhancement in the source zone), source longevity may be reduced by as much as an order of magnitude when physical–chemical source-zone treatment is coupled with reductive dechlorination.

Stimulated Microbial Reductive Dechlorination following Surfactant Treatment at the Bachman Road Site

A pilot-scale demonstration of surfactant-enhanced aquifer remediation (SEAR) was conducted in July 2000 at the Bachman Road site located in Oscoda, MI. The Bachman aquifer is a shallow, relatively homogeneous, unconfined aquifer formation composed primarily of sandy glacial outwash with relatively low organic carbon content (0.02 wt %). A 6 wt % aqueous solution of Tween 80 (a nonionic, food-grade surfactant) was flushed through a localized dense nonaqueous phase liquid (DNAPL) source zone to recover approximately 19 L of tetrachloroethene (PCE). Post-treatment monitoring revealed PCE concentrations were reduced by up to 2 orders of magnitude within the source zone, and there was no evidence of concentration rebound after more than 450 d. Concentrations of PCE dechlorination products (trichloroethene, cis-1,2-dichloroethene) 450 d after SEAR operations ceased were more than 2 orders of magnitude greater than pretreatment values, suggesting stimulation of native dechlorination activity. Post-treatment monitoring detected increased concentrations of volatile fatty acids generated from the fermentation of residual-level Tween 80 surfactant. These field data suggest that Tween 80 not only induced and maintained anaerobiosis but also provided reducing equivalents to reductively dechlorinating populations present in the oligotrophic Bachman aquifer. Experience from this site supports application of staged treatment strategies that couple SEAR and microbial reductive dechlorination to enhance mass removal and reduce contaminant mass flux emanating from treated source zones.

Microbial reductive dechlorination of organochloride pollutants in the marine environment

Anaerobic reductive dechlorination of chlorinated aromatic or aliphatic compounds is generally perceived as a rare and highly specialized activity in the environment. However, recent studies have revealed a phylogenetic cluster of marine and estuarine dehalorespiring bacteria within the green nonsulfur group that are capable of attacking a wide range of chlorinated pollutants and are widespread in the marine environment. Each microorganism described so far from this phylogenetic cluster appears to have reductive dechlorination activity. Furthermore, this group can dechlorinate a wide range of chlorinated compounds, including chlorinated biphenyls, benzenes, and ethenes, and multiple congeners within each type of organochloride. In this study we discuss the ability of polychlorinated biphenyl dechlorinators 0-17 and DF-1 to dechlorinate chloroethene and chlorobenzenes and we present the phylogenetic status of this group. Ongoing studies focus on the distribution and diversity of these microorganisms and their potential role in the global cycling of organochlorides in the marine environment.

Invariant chlorine isotopic signatures during microbial PCB reductive dechlorination

In order to develop more robust insight into the natural attenuation of polychlorinated biphenyls (PCBs), the chlorine isotopic composition of residual 2,3,4,5-tetrachlorobiphenyl (2,3,4,5-CB) was monitored as it underwent microbial reductive dechlorination to 2,3,5-trichlorobiphenyl (2,3,5-CB) in laboratory cultures. Reverse-phase high performance liquid chromatography (HPLC) was employed to isolate the former compound from the experimental matrix for δ 37Cl measurement. No detectable isotopic fractionation was observed over the 90 day incubation with sterile control, standard, and inoculated samples all exhibiting δ 37Cl values with a range of ∼0.5‰. These results show that this type of biological activity can be discriminated from other transformations by the absence of a measurable isotope effect during microbial reductive dechlorination. The utility of HPLC isolation for compound-specific δ 37Cl analyses of environmentally relevant species is also demonstrated.

Carbon Isotope Fractionation of PCE and TCE During Dechlorination by Vitamin B12

AbstractReductive dechlorination of perchloroethylene (PCE) and trichloroethylene (TCE) by vitamin B12 is an analogue of the microbial reductive dechlorination reaction and is presently being applied as a remediation technique. Stable carbon isotopic analysis, an effective and powerful tool for the investigation and monitoring of contaminant remediation, was used to characterize the isotopic effects of reductive dechlorination of PCE and TCE by vitamin B12 in laboratory microcosms. In laboratory experiments, 10 mg/L vitamin B12 degraded &gt;90% of the initial 20 mg/L PCE with TCE, the primary product of PCE degradation, accounting for between 64% and 72% of the PCE degraded. In experiments with TCE, 147 mg/L vitamin B12 degraded &gt;90% of the initial 20 mg/L TCE with cis‐dichloroethene (cDCE), the primary product of degradation accounting for between 30% and 35% of the TCE degraded. Degradation of both PCE and TCE exhibited first‐order kinetics. Strong isotopic fractionation of the reactant PCE and of the reactant TCE was observed over the course of degradation. This fractionation could be described with a Rayleigh model using enrichment factors of −16.5%o and −15.8%o for PCE, and −17.2%o and −16.6%o for TCE. Fractionation was similar in all experiments, with a mean enrichment factor of −16.5%o ± 0.6%o. The occurrence of such large enrichment factors indicates that isotopic analysis can be used to monitor the dechlorination of PCE and TCE by vitamin B12 and remediation of ground water plumes. Evidence indicates that isotopic fractionation is taking place during complexation of the chlorinated ethenes to vitamin B12, as has been suggested for reductive dechlorination by zero valent iron. The differences between e values for this reaction and those observed for anaerobic biodegradation of the chlorinated ethenes suggest that there may be differences in the rate‐determining step for these two processes.

Microbial reductive dechlorination of pre-existing PCBs and spiked 2,3,4,5,6-pentachlorobiphenyl in anaerobic slurries of a contaminated sediment of Venice Lagoon (Italy)

Reductive dechlorination of polychlorinated biphenyls (PCBs) pre-existing (at ≈1 mg kg −1) in a marine sediment of Porto Marghera (Venice Lagoon, Italy) was investigated in anaerobic slurries developed in water of the same contaminated site. Some microcosms were pasteurized whereas others were amended with 2-bromoethanesulfonic acid, molybdate or eubacteria-inhibiting antibiotics (without and in the presence of exogenous carbon sources) to preliminarily characterize the microbial populations involved in the process. Bioconversion of highly chlorinated PCBs into tri- and di-chlorinated, ortho-substituted biphenyls was detected from the 11th week of incubation both in the non-amended and in the pasteurized microcosms, where a significant consumption of sulfate and no methane production were observed. Conversely, no significant PCB transformation was detected in the microcosms with molybdate, where no sulfate consumption and a significant methane evolution occurred. Neither was PCB transformation observed in the microcosms supplemented with antibiotics and exogenous carbon sources, where a strong methane evolution and no sulfate consumption were recorded until the 11th week. The addition of exogenous 2,3,4,5,6-pentachlorobiphenyl showed preferential dechlorination at the meta and para positions, and did not significantly influence the onset of pre-existing PCB dechlorination. These results indicate that endogenous PCBs pre-existing in the marine sediment underwent reductive dechlorination. They also suggest that the process was not ‘primed’ upon 2,3,4,5,6-pentachlorobiphenyl addition, and was likely to be mediated by sulfate-reducing, spore-forming bacteria.

Changes in Enantiomeric Fractions during Microbial Reductive Dechlorination of PCB132, PCB149, and Aroclor 1254 in Lake Hartwell Sediment Microcosms

The enantioselectivity of microbial reductive dechlorination of chiral PCBs in sediments from Lake Hartwell, SC, was determined by microcosm studies and enantiomer-specific GC analysis. Sediments from two locations in the vicinity of the highest levels of PCB contamination were used as inocula. Dechlorination activity was monitored by concentration decreases in the spiked chiral PCBs and formation of dechlorination products using both achiral and chiral chromatography. Live microcosms spiked with PCB132 (234-236) exhibited dechlorination of PCB132 to PCB91 (236-24) and PCB51 (24-26). Meta dechlorination was the dominant mechanism. Microcosms spiked with PCB149 (245-236) exhibited preferential para dechlorination of PCB149 to PCB95 (236-25), followed by meta dechlorination to PCB53 (25-26) and subsequently PCB19 (26-2). Dechlorination of chiral PCB132 and PCB149 was not enantioselective. In Aroclor 1254-spiked microcosms, reductive dechlorination of PCB149 also was nonenantioselective. These results suggest that dechlorinating enzymes responsible for the dehalogenation of the chiral PCB132 and PCB149 congeners bind the two enantiomers equally. Reductive dechlorination of PCB91 and PCB95, however, occurred in an enantioselective manner, indicating that the dechlorinating enzymes for these PCBs are enantiomer-specific. The chlorine substitution pattern on the biphenyl ring appears to influence whether reductive dechlorination of chiral PCB congeners is enantioselective. Enantioselective PCB dechlorination by the microbial population of Lake Hartwell sediments occurs for select chiral PCBs; thus, certain chiral PCBs might be useful as markers for in situ reductive dechlorination.

The quest for microbial reductive dechlorination of C (2) to C (4) chloroalkanes is warranted.

C (2) to C (4) chloroalkanes have been used for a wide range of industrial applications. Consequently, numerous leaks to the environment have occurred. It is generally observed that the lower chlorinated members of the group, containing 1-3 chlorine atoms, accumulate in environments where reductive conditions prevail. Their half-lives under these conditions often exceed several decades. To date, successes in rapid and complete in situ reductive dechlorination have only been obtained with tetrachloroethene (PCE) and trichloroethene (TCE), but not with chloroalkanes. Since the key-player PCE- and TCE-dechlorinating bacteria involved have been studied, these organisms could be used as very efficient tools for low-cost in situ bioremediation. Except for one 1,2-dichloroethane-dehalorespiring bacterium with limited application possibilities and a recent isolate which partly dechlorinates some polychloroethanes, all bacterial reductive conversions of C (2) to C (4) chloroalkanes are based on slow, mostly incomplete and poorly controllable cometabolic dechlorinations. Furthermore, metals such as Fe(0) cannot dechlorinate most lower-chlorinated C (2) to C (4) alkanes. Hence, pump and treat, or aerobic degradation are the applied technologies, although they are expensive and time-intensive. However, energetic consideration of chloroalkane dechlorination suggests that metabolizing anaerobes may exist. Isolation and characterization of these organisms is warranted in order to develop cost-efficient, controlled, fast and complete in situ remediation technologies.

Acetate versus Hydrogen as Direct Electron Donors To Stimulate the Microbial Reductive Dechlorination Process at Chloroethene-Contaminated Sites

Vorticity, which represents the rotation of a fluid element, is an important characteristic of turbulence. Various methods have been used to measure vorticity. A hot-wire/hot-film anemometer (HWA) was used here to measure the vorticity in turbulent flows. The velocity components and their partial derivatives were simultaneously measured with a new 6-sensor hot-wire (HW) probe assuming ideal yaw and pitch factors with Jorgensen's expression and Taylor's hypothesis to analyze the data. The accurate 6-sensor hot-wire probe results for the velocity field were used to determine the velocity gradients and, therefore, the vorticity vector field. The data was measured in an isothermal model of a tangentially fired furnace. The experimental results in the tangentially fired furnace agree with numerical results.