Reductive Dechlorination Processes Research Articles

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.

Potential and Limitations of Microbial Reductive Dechlorination for Bioremediation Applications

Current knowledge and recent advances in the area of microbial reductive dechlorination of polychlorinated organic compounds are summarized. Factors which may limit the efficacy of the dechlorination process for the in situ bioremediation of contaminated soil and sediment systems are identified. Results of recent studies on the anaerobic biotransformation of soil-sorbed chlorinated ethenes and sediment-sorbed chlorinated benzenes are provided to illustrate how low contaminant bioavailability may control the rate and extent of dechlorination in subsurface systems, especially those with long-term contamination. Use of nonionic, polysorbate surfactants as the sole electron donors of a mixed, methanogenic culture supported the microbial sequential reductive dechlorination of either free or sediment-bound hexachlorobenzene (HCB) to primarily 1,3-dichlorobenzene, but did not enhance the bioavailability of sediment-bound HCB as compared to microcosms, which used glucose. Because current knowledge on the interactions of dechlorinating populations with other microbial populations in the presence of alternative terminal electron acceptors (e.g., nitrate, Fe3+ , Mn4+) is limited, such interactions and their effect on the dechlorination process in subsurface systems need to be further explored to improve our understanding of the reductive dechlorination process in complex environmental systems and lead to the development of more efficient in situ bioremediation technologies and strategies.

Acetate versus hydrogen as direct electron donors to stimulate the microbial reductive dechlorination process at chloroethene-contaminated sites.

A study to evaluate the dechlorination end points and the most promising electron donors to stimulate the reductive dechlorination process at the chloroethene-contaminated Bachman Road site in Oscoda, MI, was conducted. Aquifer materials were collected from inside the plume and used to establish microcosms under a variety of electron donor conditions using chlorinated ethenes as electron acceptors. All microcosms that received an electron donor showed dechlorination activity, but the end points depended on the sampling location, indicating a heterogeneous distribution of the dechlorinating populations in the aquifer. Interestingly, several microcosms that received acetate as the only electron donor completely dechlorinated PCE to ethene. All acetate-amended microcosms rapidly converted PCE to cis-DCE, whereas PCE dechlorination in H2-fed microcosms only occurred after a pronounced lag time and after acetate had accumulated by H2/CO2 acetogenic activity. The microcosm experiments were corroborated by defined co-culture experiments, which demonstrated that H2 sustained PCE to cis-DCE dechlorination by acetotrophic populations in the presence of H2/CO2 acetogens. In sediment-free nonmethanogenic enrichment cultures derived from ethene-producing microcosms, acetate alone supported complete reductive dechlorination of chloroethenes to ethene, although the addition of H2 resulted in higher cis-DCE and VC dechlorination rates. Measurements of H2 production and consumption suggested that syntrophic acetate-oxidizing population(s) were active in the enrichment cultures. These findings demonstrated that either acetate or H2 alone can be sufficient to promote complete

Sequential anaerobic/aerobic biodegradation of chlorinated hydrocarbons in activated carbon barriers

The aim of this study is to develop a long lasting, sequential anaerobic/aerobic biological activated carbon barrier. In the biobarrier, pollutant adsorption on granular activated carbon (GAC) and biodegradation occur simultaneously. Trichloroethene (TCE), chlorobenzene (CB), and benzene were used as model pollutants. In the first barrier, that was operated under anaerobic conditions with sucrose and ethanol as auxiliary substrates, TCE was completely converted to lower chlorinated metabolites, predominantly cis-dichloroethene (cis-DCE). The reductive dechlorination process was stable for about 300 d, although the concomitant sulphate-reducing and methanogenic processes varied considerably. In the second barrier, that was operated with addition of hydrogen peroxide and nitrate, dechlorination was limited by a lack of oxygen and restricted mainly to CB biodegradation. Additional aerobic batch tests revealed that the metabolites of anaerobic TCE dechlorination, i.e. cis-DCE and vinyl chloride, were oxidatively dechlorinated in the presence of suitable auxiliary substrates such as ethene, CB, benzene, or sucrose and ethanol. During periods of low biological activity, elimination of TCE and CB occurred by adsorption in the GAC barriers. The pre-sorbed pollutants were available for subsequent biodegradation resulting in a bioregeneration of the activated carbon barriers.

Natural attenuation of chlorinated solvents at Area 6, Dover Air Force Base: characterization of microbial community structure

A polyphasic approach based on cultivation and direct recovery of 16S rRNA gene sequences was utilized for microbial characterization of an aquifer contaminated with chlorinated ethenes. This work was conducted in order to support the evaluation of natural attenuation of chlorinated ethenes in groundwater at Area 6 at Dover Air Force Base (Dover, DE). Results from these studies demonstrated the aquifer contained relatively low biomass (e.g. direct microscopic counts of <10 7 bacteria/g of sediment) comprised of a physiologically diverse group of microorganisms including iron reducers, acetogens, sulfate reducers, denitrifiers, aerobic and anaerobic heterotrophs. Laboratory microcosms prepared with authentic sediment and groundwater provided direct microbiological evidence that the mineralization of vinyl chloride and cis-dichloroethene as well as each step in the complete reductive dechlorination of tetracloroethene to ethene can occur in the Area 6 aquifer. Enrichment cultures capable of the oxidative degradation of cis-1,2-dichloroethene ( cis-DCE) and vinyl chloride (VC) were obtained from groundwater across the aquifer demonstrating the possible importance of direct, non-cometabolic oxidation of cis-DCE and VC in natural attenuation. Culture-independent analyses based upon recovery of 16S rRNA gene sequences revealed the presence of anaerobic organisms distributed primarily between two major bacterial divisions: the delta subdivision of the Proteobacteria and low-G+C gram positive. Recovery of sequences affiliated with phylogenetic groups containing known anaerobic-halorespiring organisms such as Desulfitobacterium, Dehalobacter, and certain groups of iron reducers provided qualitative support for a role of reductive dechlorination processes in the aquifer. This molecular data is suggestive of a functional linkage between the microbiology of the site and the apparent natural attenuation process. The presence and distribution of microorganisms were found to be consistent with a microbially driven attenuation of chlorinated ethenes within the aquifer and in accord with a conceptual model of aquifer geochemistry which suggest that both reductive and oxidative mechanisms are involved in heterogeneous, spatially distributed processes across the aquifer.

Evaluation of TCDD biodegradability under different redox conditions

Polychlorinated dibenzo- p-dioxins have been generated as unwanted by-products in many industrial processes. Although their widespread distribution in different environmental compartments has been recognized, little is known about their fate in the ultimate environment sinks. The highly stable dioxin isomer 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) has been called the most toxic compound known to man. In this laboratory microcosm study, TCDD bioavailability was evaluated under five reduction/oxidation (redox) conditions including aerobic biodegradation, aerobic cometabolism, methanogenesis, iron reduction, and reductive dechlorination. Activated sludge and aquifer sediments from a TCDD and a pentachlorophenol (PCP) contaminated site were used as the inocula. Acetate, sludge cake, and cane molasses were used as the primary substrates (carbon sources) in cometabolism and reductive dechlorination microcosms. After a 90-day incubation period, microcosms constructed under reductive dechlorination conditions were the only treatment showing promising remediation results. The highest TCDD degradation rate [up to 86% of TCDD removal (with an initial concentration of 96 μg/kg of soil)] was observed in the microcosms with anaerobic activated sludge as the microbial inocula and sludge cakes as the primary substrates. Except for reductive dechlorination microcosms, no significant TCDD removal was observed in the microcosms prepared under other conditions. Thus, application of an effective primary substrate to enhance the reductive dechlorination process is a feasible method for TCDD bioremediation. Bioremediation expense can be significantly reduced by the supplement of some less expensive alternative substrates (e.g., sludge cakes, cane molasses). Results would be useful in designing a scale-up in situ or on-site bioremediation system such as bioslurry reactor for field application.

Enhanced TCDD degradation by Fenton's reagent preoxidation

The dioxin isomer 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) has been called the most toxic compound known to man. Because of its poor bioavailability and low biodegradibility, bioremediation technology cannot effectively degrade TCDD when used alone. In this study, chemical pretreatment (partial oxidation) in combination with biodegradation technique was developed to efficiently remediate TCDD-contaminated soils. An oxidizing reagent [Fenton's Reagent (FR)] was applied in a slurry reactor to transform TCDD with a concentration of 96 μg per kg of soil to compounds more amenable to biodegradation. Up to 99% TCDD was transformed after the chemical pretreatment process. The slurry reactor was then converted to a bioreactor for the following biodegradation experiment. The detected TCDD oxidation byproducts including chlorophenols (CPs) and chlorobenzenes (CBs) were transformed in this bioreactor under aerobic conditions. Two other biodegradation experiments were performed in parallel to investigate the biodegradabiliy of TCDD under aerobic and anaerobic conditions without chemical pretreatment. Approximately 53% TCDD was transformed under anaerobic conditions possibly due to the reductive dechlorination process using organic materials contained in the activated sludge as the primary substrates. No TCDD degradation was observed under aerobic conditions. Results show that FR can oxidize TCDD to less-chlorinated and less-toxic byproducts, promoting their bioavailability to microbial communities. The bench-scale results indicate that the two-stage (partial oxidation followed by biodegradation) system has the potential to be developed to remediate TCDD-contaminated soils on-site.

Using a peat biobarrier to remediate PCE/TCE contaminated aquifers

The industrial solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) are among the most ubiquitous chlorinated compounds found in groundwater contamination. The objective of this study was to evaluate the potential installation a peat biobarrier to clean up aquifers contaminated by PCE/TCE. Peat bioavailability was evaluated using a modified biochemical oxygen demand (BOD) test. Results show that biodegradable materials were released from peat and utilized by microbial consortia contained in the activated sludge. Degradation of the released organic materials caused the oxygen consumption and a slight decrease in chemical oxygen demand (COD) measurements. Based on the COD tests, 0.5 g of peat released approximately 6 mg of COD, which has the potential to convert 1.5 mg of PCE to ethylene (ETH). Microcosms inoculated with aquifer sediments or activated sludges were constructed under both aerobic and anaerobic conditions. Results indicate that peat can serve as the diffusion source of primary substrates, and enhance the PCE and TCE biodegradation under aerobic cometabolism and reductive dechlorination processes, respectively. Compared to the microcosms with aquifer sediments as the inocula, microcosms inoculated with sludges showed higher TCE/PCE degradation ratios (0.277 μM/day for TCE removal and 0.419 μM/day for PCE removal under aerobic and anaerobic conditions, respectively), and an immediate PCE/TCE decrease. These findings make the peat biobarrier containing sludge inocula more feasible and applicable from an engineering point of view. Results from the feasibility study show that the proposed peat biobarrier has the potential to become an environmentally and economically acceptable technology for the bioremediation of chlorinated-solvent contaminated groundwater.

Model-based Estimation of an Anaerobic Reductive Dechlorination Process via an Attenuated Total Reflection-Fourier Transform Infrared Sensor

Model-based software sensors were built for the estimation of the process variables: cosubstrate, mineral chloride ion, biomass and acetic acid from an anaerobic reductive dechlorination process in a fixed-bed bioreactor, without any the knowledge of the bioprocess kinetics. The estimations were accomplished based only on measurements of chlorinated compounds, obtained by gas chromatography and on-line by an attenuated total reflection-Fourier transform infrared (ATR-FTIR) sensor. In addition, the bioreaction rate of this process was estimated by an observer-based estimator, and validated indirectly by the toxic compound analytical data.

Model-based estimation of an anaerobic reductive dechlorination process via an attenuated total reflectionfourier transform infrared sensor

Model-based software sensors were built for the estimation of the process variables: cosubstrate, mineral chloride ion, biomass and acetic acid from an anaerobic reductive dechlorination process in a fixed-bed bioreactor, without any the knowledge of the bioprocess kinetics. The estimations were accomplished based only on measurements of chlorinated compounds, obtained by gas chromatography and on-line by an attenuated total reflection-Fourier transform infrared (ATR-FTIR) sensor. In addition, the bioreaction rate of this process was estimated by an observer-based estimator, and validated indirectly by the toxic compound analytical data.

Complete reductive dechlorination of 1,2-dichloropropane by anaerobic bacteria

The transformation of 1,2-dichloropropane (1,2-D) was observed in anaerobic microcosms and enrichment cultures derived from Red Cedar Creek sediment. 1-Chloropropane (1-CP) and 2-CP were detected after an incubation period of 4 weeks. After 4 months the initial amount of 1,2-D was stoichiometrically converted to propene, which was not further transformed. Dechlorination of 1,2-D was not inhibited by 2-bromoethanesulfonate. Sequential 5% (vol/vol) transfers from active microcosms yielded a sediment-free, nonmethanogenic culture, which completely dechlorinated 1,2-D to propene at a rate of 5 nmol min(sup-1) mg of protein(sup-1). No intermediate formation of 1-CP or 2-CP was detected in the sediment-free enrichment culture. A variety of electron donors, including hydrogen, supported reductive dechlorination of 1,2-D. The highest dechlorination rates were observed between 20(deg) and 25(deg)C. In the presence of 1,2-D, the hydrogen threshold concentration was below 1 ppm by volume (ppmv). In addition to 1,2-D, the enrichment culture transformed 1,1-D, 2-bromo-1-CP, tetrachloroethene, 1,1,2,2-tetrachloroethane, and 1,2-dichloroethane to less halogenated compounds. These findings extend our knowledge of the reductive dechlorination process and show that halogenated propanes can be completely dechlorinated by anaerobic bacteria.

Dechlorination of polychlorinated biphenyl congeners by an anaerobic microbial consortium

An anaerobic methanogenic microbial consortium, developed in a granular form, exhibited extensive dechlorination of defined polychlorinated biphenyl (PCB) congeners. A 2,3,4,5,6-pentachlorobiphenyl was dechlorinated to biphenyl via 2,3,4,6-tetrachlorobiphenyl, 2,4,6-trichlorobiphenyl, 2,4-dichlorobi-phenyl and 2-chlorobiphenyl (CB). Removal of chlorine atoms from all three positions of the biphenyl ring, i.e., ortho, meta and para, was observed during this reductive dechlorination process. Biphenyl was identified as one of the end-products of the reductive dechlorination by GC-MS. After 20 weeks, the concentrations of the dechlorination products 2,4,6-CB, 2,4-CB, 2-CB and biphenyl were 8.1, 41.2, 3.0 and 47.8 μM respectively, from an initial 105 μM 2,3,4,5,6-CB. The extent and pattern of the dechlorination were further confirmed by the dechlorination of lightly chlorinated congeners including 2-CB, 3-CB, 4-CB, 2,4-CB and 2,6-CB individually. This study indicates that the dechlorination of 2,3,4,5,6-CB to biphenyl is due to ortho, meta and para dechlorination by this anaerobic microbial consortium.

Biotransformation of Organics in Soil Columns and an Infiltration Area

AbstractLaboratory column experiments were performed to evaluate the fate of a series of chlorinated and nonchlorinated organic contaminants in Rhine sediment and in sediment from the infiltration area of the Municipal Water Works of Amsterdam, near Zandvoort, The Netherlands. Columns were operated under aerobic, denitrifying, and methanogenic conditions. All nonchlorinated and few chlorinated compounds were aerobically transformed. Of the compounds tested under denitrifying conditions, only 1,2‐dichloro‐4‐nitrobenzene was partially transformed. Methanogenic conditions favored the transformation of chlorinated substances by reductive dechlorination. Toluene was the only nonhalogenated compound that was transformed under methanogenic conditions. Steady‐state effluent concentrations after biotransformation were at least 10 times lower than the drinking water limit of 1 μg/l except in the case of 1,2,4‐trichlorobenzene which had a steady‐state effluent concentration of 2.6 μg/l. Steady‐state effluent concentrations did not depend on the influent concentration applied. Most transformations proceeded at the same steady‐state rates at a temperature of 4° C, although the process of reductive dechlorination was slower at 4° C than at 20° C. Hydrological calculations revealed that the combined action of hydrology and sorption to organic matter in the infiltration system can reduce the concentrations of 2 week pulses of polar and nonpolar contaminants by at least 80 and 95%, respectively. There was a good qualitative agreement between removals observed in column experiments and in the dune infiltration area.

Prediction of Redox Potentials and Isomer Distribution Yields for Reductive Dechlorination of Chlorobenzenes

Abstract Gibbs free energies of reductive dechlorination processes of chlorobenzenes are calculated from thermodynamic data. The results are utilized to predict redox potentials and isomer distribution yields. The model predicts a standard redox potential of 0.680 V for the reduction of hexachlorobenzene to pentachlorobenzene, tapering off to 0.535 V for the reduction of monochlorobenzene to benzene. It is shown that under anaerobic conditions, reductive dechlorination is more likely to occur, while aerobic conditions favour the formation of chlorophenols. An isomer distribution yield is predicted for each of the reductive dechlorination processes of chlorobenzenes. Predicted yields correspond to experimental values within 10%. The model includes a possibility to establish a temperature dependence of the relative isomer yields.

Effect of chlorinated alkenes on the reductive dechlorination and methane production processes

Mixed, acetate-fed methanogenic cultures were used to assess the effect of increasing tetrachloroethylene (PCE), trichloroethylene (TCE) and cis-1,2-dichloroethylene (cDCE) concentrations on the reductive dechlorination and primary metabolism (methane production) processes. In addition, the effect of TCE on the rate and extent of the reductive dechlorination of PCE was investigated. All cultures were developed in serum bottles and incubation was carried out at 35°C in the dark. Soil samples from two sites contaminated with chlorinated solvents served as the inoculum for this study. All chlorinated alkenes used were supplied as saturated solutions in the culture media. The rate of PCE dechlorination increased with increasing PCE concentrations. Both TCE and cDCE showed an increase in the dechlorination rate up to a solvent threshold concentration followed by a decrease. In all cases, the methane production rate decreased as the chlorinated alkene concentrations increased. However, the effect of chlorinated alkenes was more pronounced on methane production rather than on the extent of dechlorination. The relative toxicity of the chlorinated alkenes tested in this study formed the following ascending series: PCE &amp;lt; TCE &amp;lt; &amp;lt; cDCE.

Mineralization of the herbicide 2,3,6-trichlorobenzoic acid by a co-culture of anaerobic and aerobic bacteria

Bacteria from an anaerobic enrichment reductively removed chlorine from the ortho- position of 2,3,6-trichlorobenzoic acid (2,3,6-TBA) producing 2,5-dichlorobenzoate (2,5-DBA). The strictly aerobic bacterium Pseudomonas aeruginosa JB2 subsequently used 2,5-DBA as a growth substrate in the presence of oxygen. The anaerobic dechlorinating microbial population was grown with P. aeruginosa JB2 in continuous culture. Inside the liquid culture, a nylon netting, on a stainless-steel support, contained vermiculite particles to provide a strictly anaerobic environment within the aerated culture. Complete mineralization of 2,3,6-TBA depended on the extent of oxygen input into the reactor. Under strictly anaerobic conditions 2,5-DBA and Cl− were produced stoichiometrically through the reductive dechlorination of 2,3,6-TBA. This process of reductive dechlorination was not inhibited by (moderate) aeration resulting in an O2-concentration of 0.3–0.5 μM in the culture liquid.

Mineralization of the herbicide 2,3,6-trichlorobenzoic acid by a co-culture of anaerobic and aerobic bacteria

Bacteria from an anaerobic enrichment reductively removed chlorine from the ortho- position of 2,3,6-trichlorobenzoic acid (2,3,6-TBA) producing 2,5-dichlorobenzoate (2,5-DBA). The strictly aerobic bacterium Pseudomonas aeruginosa JB2 subsequently used 2,5-DBA as a growth substrate in the presence of oxygen. The anaerobic dechlorinating microbial population was grown with P. aeruginosa JB2 in continuous culture. Inside the liquid culture, a nylon netting, on a stainless-steel support, contained vermiculite particles to provide a strictly anaerobic environment within the aerated culture. Complete mineralization of 2,3,6-TBA depended on the extent of oxygen input into the reactor. Under strictly anaerobic conditions 2,5-DBA and Cl− were produced stoichiometrically through the reductive dechlorination of 2,3,6-TBA. This process of reductive dechlorination was not inhibited by (moderate) aeration resulting in an O2-concentration of 0.3–0.5 μM in the culture liquid.

Mineralization of the herbicide 2,3,6-trichlorobenzoic acid by a co-culture of anaerobic and aerobic bacteria

Bacteria from an anaerobic enrichment reductively removed chlorine from the ortho- position of 2,3,6-trichlorobenzoic acid (2,3,6-TBA) producing 2,5-dichlorobenzoate (2,5-DBA). The strictly aerobic bacterium Pseudomonas aeruginosa JB2 subsequently used 2,5-DBA as a growth substrate in the presence of oxygen. The anaerobic dechlorinating microbial population was grown with P. aeruginosa JB2 in continuous culture. Inside the liquid culture, a nylon netting, on a stainless-steel support, contained vermiculite particles to provide a strictly anaerobic environment within the aerated culture. Complete mineralization of 2,3,6-TBA depended on the extent of oxygen input into the reactor. Under strictly anaerobic conditions 2,5-DBA and Cl− were produced stoichiometrically through the reductive dechlorination of 2,3,6-TBA. This process of reductive dechlorination was not inhibited by (moderate) aeration resulting in an O2-concentration of 0.3–0.5 μM in the culture liquid.

Mineralization of the herbicide 2,3,6-trichlorobenzoic acid by a co-culture of anaerobic and aerobic bacteria

Bacteria from an anaerobic enrichment reductively removed chlorine from the ortho- position of 2,3,6-trichlorobenzoic acid (2,3,6-TBA) producing 2,5-dichlorobenzoate (2,5-DBA). The strictly aerobic bacterium Pseudomonas aeruginosa JB2 subsequently used 2,5-DBA as a growth substrate in the presence of oxygen. The anaerobic dechlorinating microbial population was grown with P. aeruginosa JB2 in continuous culture. Inside the liquid culture, a nylon netting, on a stainless-steel support, contained vermiculite particles to provide a strictly anaerobic environment within the aerated culture. Complete mineralization of 2,3,6-TBA depended on the extent of oxygen input into the reactor. Under strictly anaerobic conditions 2,5-DBA and Cl − were produced stoichiometrically through the reductive dechlorination of 2,3,6-TBA. This process of reductive dechlorination was not inhibited by (moderate) aeration resulting in an O 2-concentration of 0.3–0.5 μM in the culture liquid.

PCB movement, dechlorination, and detoxication in the acushnet estuary

AbstractCongener‐specific analyses of the PCBs in Acushnet Estuary (New Bedford, MA) sediments and waters were undertaken to identify the PCB alteration and transport processes occurring in coastal marine sediments. These analyses indicated that (a) the PCBs deposited at the sediment sites sampled had originally consisted of Aroclors 1242 and 1254, in widely varying proportions; (b) these PCBs had undergone vertical movement within the sediments, rather than remaining stratified, but not horizontal translocation between sites; (c) they had also undergone extraction into the water, albeit at declining rates, with some consequent changes in composition but (d) the major compositional change was caused by a previously unreported type of reductive dechlorination process, designated Process H. This presumably anaerobic microbial process, subsequently identified at several other locations as well, had selectively removed non‐ortho chlorines from most of the higher PCB congeners, especially those associated with acute toxic effects. It appeared to have begun near the upper end of the estuary and not yet reached its lower portions, thus providing a marker for tracing the origin of the PCBs in estuarine water samples.