Cometabolism Of Trichloroethylene Research Articles

Acetylene Tunes Microbial Growth During Aerobic Cometabolism of Trichloroethene.

Microbial aerobic cometabolism is a possible treatment approach for large, dilute trichloroethene (TCE) plumes at groundwater contaminated sites. Rapid microbial growth and bioclogging pose a persistent problem in bioremediation schemes. Bioclogging reduces soil porosity and permeability, which negatively affects substrate distribution and contaminant treatment efficacy while also increasing the operation and maintenance costs of bioremediation. In this study, we evaluated the ability of acetylene, an oxygenase enzyme-specific inhibitor, to decrease biomass production while maintaining aerobic TCE cometabolism capacity upon removal of acetylene. We first exposed propane-metabolizing cultures (pure and mixed) to 5% acetylene (v v-1) for 1, 2, 4, and 8 d and we then verified TCE aerobic cometabolic activity. Exposure to acetylene overall decreased biomass production and TCE degradation rates while retaining the TCE degradation capacity. In the mixed culture, exposure to acetylene for 1-8 d showed minimal effects on the composition and relative abundance of TCE cometabolizing bacterial taxa. TCE aerobic cometabolism and incubation conditions exerted more notable effects on microbial ecology than did acetylene. Acetylene appears to be a viable approach to control biomass production that may lessen the likelihood of bioclogging during TCE cometabolism. The findings from this study may lead to advancements in aerobic cometabolism remediation technologies for dilute plumes.

Recent advances and trends of trichloroethylene biodegradation: A critical review.

Trichloroethylene (TCE) is a ubiquitous chlorinated aliphatic hydrocarbon (CAH) in the environment, which is a Group 1 carcinogen with negative impacts on human health and ecosystems. Based on a series of recent advances, the environmental behavior and biodegradation process on TCE biodegradation need to be reviewed systematically. Four main biodegradation processes leading to TCE biodegradation by isolated bacteria and mixed cultures are anaerobic reductive dechlorination, anaerobic cometabolic reductive dichlorination, aerobic co-metabolism, and aerobic direct oxidation. More attention has been paid to the aerobic co-metabolism of TCE. Laboratory and field studies have demonstrated that bacterial isolates or mixed cultures containing Dehalococcoides or Dehalogenimonas can catalyze reductive dechlorination of TCE to ethene. The mechanisms, pathways, and enzymes of TCE biodegradation were reviewed, and the factors affecting the biodegradation process were discussed. Besides, the research progress on material-mediated enhanced biodegradation technologies of TCE through the combination of zero-valent iron (ZVI) or biochar with microorganisms was introduced. Furthermore, we reviewed the current research on TCE biodegradation in field applications, and finally provided the development prospects of TCE biodegradation based on the existing challenges. We hope that this review will provide guidance and specific recommendations for future studies on CAHs biodegradation in laboratory and field applications.

Using qPCR Assays to Predict Rates of Cometabolism of TCE in Aerobic Groundwater

AbstractMonitored natural attenuation (MNA) is commonly used as a remedy for trichloroethene (TCE) in anaerobic groundwater; however, MNA has not been applied to TCE contamination in aerobic groundwater. Under aerobic conditions, bacteria initiate the degradation of many organic substances with oxygenase enzymes. Several of these enzymes are known to degrade TCE through a fortuitous reaction known as cometabolism. There are commercially available qPCR assays that can determine the number of gene copies of these enzymes. If the qPCR assay could be used to predict the first‐order rate constant for cometabolism of TCE, the qPCR assay could be used to screen sites to determine whether MNA was a plausible remedy for TCE contamination. This study reevaluated data from water samples that were collected from 19 wells on five sites in Minnesota, New York, and Utah. Data had previously been published on the rate constant for cometabolism of TCE in the water samples as determined by a 14C‐assay and the abundance of gene copies for five enzymes that cometabolize TCE as determined using a qPCR assay. The Michaelis‐Menten (Haldane) kinetic parameters for cometabolism of TCE and the abundance of DNA for the five oxygenase enzymes were used to predict the rate constant for cometabolism of TCE. The predicted rate constants were evaluated and validated by comparing them to the rate constants derived from the 14C‐assay. For predicted rate constants greater than 0.003 per year, the predicted rate constants agreed with the measured rate constants within a factor of three. The qPCR assay serves as a convenient screening tool to determine whether MNA is a plausible remedy for an aerobic plume of TCE.

Alternated phenol and trichloroethylene biodegradation in an aerobic granular sludge reactor

Co-metabolic removal of a synthetic solvent, trichloroethylene (TCE) was conducted. Phenol was used as the primary substrate, and aerobic granules as the biocatalyst. An airtight reactor was designed and constructed with glass, and the configuration was verified of its applicability with volatile solvent. Phenol and TCE were fed into the reactors alternately, and both were efficiently removed during 6 weeks of operation. Compared with the control reactor receiving no TCE, biomass in the TCE-fed reactors had lower phenol-dependent specific oxygen utilization rate, but higher concentration, bigger size, and better settling ability. The morphology of the biomass in TCE and control reactors exhibited distinct characteristics. Granules in the TCE reactors slightly broke up early in the operation, but generally retained their shape and structure throughout. The biomass in the control reactor, however, lost its granular structure and totally disintegrated into flocs. Therefore, TCE co-metabolism likely improved the structural integrity of aerobic granular sludge, and co-metabolic degradation of TCE by phenol-grown aerobic granules showed long term stability.

Toxic and inhibitory effects of trichloroethylene aerobic co-metabolism on phenol-grown aerobic granules

Aerobic granule, a form of microbial aggregate, exhibits good potential in degrading toxic and recalcitrant substances. In this study, the inhibitory and toxic effects of trichloroethylene (TCE), a model compound for aerobic co-metabolism, on phenol-grown aerobic granules were systematically studied, using respiratory activities after exposure to TCE as indicators. High TCE concentration did not exert positive or negative effects on the subsequent endogenous respiration rate or phenol dependent specific oxygen utilization rate (SOUR), indicating the absence of solvent stress and induction effect on phenol-hydroxylase. Phenol-grown aerobic granules exhibited a unique response to TCE transformation product toxicity, that small amount of TCE transformation enhanced the subsequent phenol SOUR. Granules that had transformed between 1.3 and 3.7mg TCE gSS−1 showed at most 53% increase in the subsequent phenol SOUR, and only when the transformation exceeded 6.6mg TCE gSS−1 did the SOUR dropped below that of the control. This enhancing effect was found to sustain throughout several phenol dosages, and TCE transformation below the toxicity threshold also lessened the granules' sensitivity to higher phenol concentration. The unique toxic effect was possibly caused by the granule's compact structure as a protection barrier against the diffusive transformation product(s) of TCE co-metabolism.

Natural attenuation potential of tricholoroethene in wetland plant roots: Role of native ammonium-oxidizing microorganisms

Bench-scale microcosms with wetland plant roots were investigated to characterize the microbial contributions to contaminant degradation of trichloroethene (TCE) with ammonium. The batch system microcosms consisted of a known mass of wetland plant roots in aerobic growth media where the roots provided both an inoculum of root-associated ammonium-oxidizing microorganisms and a microbial habitat. Aqueous growth media, ammonium, and TCE were replaced weekly in batch microcosms while retaining roots and root-associated biomass. Molecular biology results indicated that ammonium-oxidizing bacteria (AOB) were enriched from wetland plant roots while analysis of contaminant and oxygen concentrations showed that those microorganisms can degrade TCE by aerobic cometabolism. Cometabolism of TCE, at 29 and 46μgL−1, was sustainable over the course of 9weeks, with 20–30mgL−1 ammonium-N. However, at 69μgL−1 of TCE, ammonium oxidation and TCE cometabolism were completely deactivated in two weeks. This indicated that between 46 and 69μgL−1 TCE with 30mgL−1 ammonium-N there is a threshold [TCE] below which sustainable cometabolism can be maintained with ammonium as the primary substrate. However, cometabolism-induced microbial deactivation of ammonium oxidation and TCE degradation at 69μgL−1 TCE did not result in a lower abundance of the amoA gene in the microcosms, suggesting that the capacity to recover from TCE inhibition was still intact, given time and removal of stress. Our study indicates that microorganisms associated with wetland plant roots can assist in the natural attenuation of TCE in contaminated aquatic environments, such as urban or treatment wetlands, and wetlands impacted by industrial solvents.

Aerobic degradation of trichloroethylene by co-metabolism using phenol and gasoline as growth substrates.

Trichloroethylene (TCE) is a common groundwater contaminant of toxic and carcinogenic concern. Aerobic co-metabolic processes are the predominant pathways for TCE complete degradation. In this study, Pseudomonas fluorescens was studied as the active microorganism to degrade TCE under aerobic condition by co-metabolic degradation using phenol and gasoline as growth substrates. Operating conditions influencing TCE degradation efficiency were optimized. TCE co-metabolic degradation rate reached the maximum of 80% under the optimized conditions of degradation time of 3 days, initial OD600 of microorganism culture of 0.14 (1.26 × 107 cell/mL), initial phenol concentration of 100 mg/L, initial TCE concentration of 0.1 mg/L, pH of 6.0, and salinity of 0.1%. The modified transformation capacity and transformation yield were 20 μg (TCE)/mg (biomass) and 5.1 μg (TCE)/mg (phenol), respectively. Addition of nutrient broth promoted TCE degradation with phenol as growth substrate. It was revealed that catechol 1,2-dioxygenase played an important role in TCE co-metabolism. The dechlorination of TCE was complete, and less chlorinated products were not detected at the end of the experiment. TCE could also be co-metabolized in the presence of gasoline; however, the degradation rate was not high (28%). When phenol was introduced into the system of TCE and gasoline, TCE and gasoline could be removed at substantial rates (up to 59% and 69%, respectively). This study provides a promising approach for the removal of combined pollution of TCE and gasoline.

Enhancing trichloroethylene degradation using non-aromatic compounds as growth substrates

The effect of non-aromatic compounds on the trichloroethylene (TCE) degradation of toluene-oxidizing bacteria were evaluated using Burkholderia cepacia G4 that expresses toluene 2-monooxygenase and Pseudomonas putida that expresses toluene dioxygenase. TCE degradation rates for B. cepacia G4 and P. putida with toluene alone as growth substrate were 0.144 and 0.123μg-TCE/mg-proteinh, respectively. When glucose, acetate and ethanol were fed as additional growth substrates, those values increased up to 0.196, 0.418 and 0.530μg-TCE/mg-proteinh, respectively for B. cepacia G4 and 0.319, 0.219 and 0.373μg-TCE/mg-proteinh, respectively for P. putida. In particular, the addition of ethanol resulted in a high TCE degradation rate regardless of the initial concentration. The use of a non-aromatic compound as an additional substrate probably enhanced the TCE degradation because of the additional supply of NADH that is consumed in co-metabolic degradation of TCE. Also, it is expected that the addition of a non-aromatic substrate can reduce the necessary dose of toluene and, subsequently, minimize the potential competitive inhibition upon TCE co-metabolism by toluene.

Cometabolism of CAHs while Growing on BTEX in Soil Slurry

An indigenous Pseudomonas sp., isolated from the heavily petroleum-contaminated soil and identified as Pseudomonas plecoglossicida, was evaluated for its aerobic cometabolic removal of mixture of two representative chlorinated aliphatic hydrocarbons (CAHs), cis-1,2-dichloroethylene (cis-DCE) and trichloroethylene (TCE), with BTEX/toluene provided as substrate in a laboratory-scale soil slurry. The aerobic simultaneous bioremoval of the cis-DCE/TCE/BTEX mixture was studied under different conditions. Results showed that the increased BTEX concentration from 400 to 600 mg/kg prolonged the bioremoval of BTEX. The cometabolism of cis-DCE and TCE was significantly greater when toluene was provided as growth substrate compared to the BTEX mixture as substrates. Additionally, the bioremoval of toluene in the treatment with toluene as sole growth substrate was higher compared to the treatment with BTEX mixture as substrate. Results would enhance the applicability of bioremediation technology to the mixed wastes-contaminated sites.

Co-metabolic degradation activities of trichloroethylene by phenol-grown aerobic granules

Aerobic granule, a novel form of microbial aggregation, was explored for its potential in a co-metabolic degradation process using trichloroethylene (TCE) as the model compound and phenol as the primary substrate. Granulation process was conducted with phenol as the sole carbon and energy source, and phenol-acclimated sludge as seed sludge. TCE and phenol degradation activity of the biomass was monitored during granulation, together with other sludge characteristics. The biomass’ settling ability improved significantly with granulation, and the mean size increased from 100 to 600μm. The biomass showed efficient and robust TCE co-metabolism activity, but the phenol-dependent specific oxygen utilization rate (SOUR), TCE instant transformation rate (Tr) and ultimate transformation capacity (TC) all exhibited decreasing trends during granulation. Compared to the seed sludge, stable granules retained most of the SOUR and Tr, but lost half of the original TC. Phenol and TCE degradation kinetic parameters of the granules were determined to be Vmax 1.59mggSS−1min−1, KS 37.2mgL−1, KI 461.8mgL−1 for phenol; and Vmax 0.0142mggSS−1min−1, KS 1.02mgL−1 for TCE. 2.5molCl− was released per mol TCE transformed, and the TC was approximately 0.011gTCEggranule−1.

Cometabolism of trichloroethylene: concepts, limitations and available strategies for sustained biodegradation

Due to its toxicity and persistence in the environment, trichloroethylene (TCE) has become a major soil and groundwater contaminant in many countries. A group of aliphatic- and aromatic-degrading bacteria expressing nonspecific oxygenases have been reported to transform TCE through aerobic cometabolism in the presence of primary substrate such as methane, ammonia, propane, phenol, toluene or cumene. This paper reviews the fundamentals and results of TCE cometabolism from laboratory and field studies. The limitations associated with TCE cometabolism including the causes and effects of substrate and/or inducer utilization rate and depletion, enzyme inhibition and inactivation, and cytotoxicity during TCE oxidation among various TCE-degrading bacteria and enzymes are discussed. In addition, the potential strategies e.g. addition of primary substrate/inducer or external energy substrate, use of a two-stage reactor and application of cell immobilization for sustained TCE degradation are highlighted. The review summarizes important information on TCE cometabolism, which is necessary for developing efficient TCE bioremediation approaches.

Application of enhanced bioremediation for TCE-contaminated groundwater: a pilot-scale study

Abstract The industrial solvent trichloroethylene (TCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. The objective of this pilot-scale study was to apply the combined biosparging and enhanced in situ bioremediation technology to remediate TCE-contaminated groundwater at a TCE-spill site. A biosparging well was installed inside the TCE plume for oxygen supplement. Primary substrate (cane molasses) was injected into the TCE plume through the biosparging well to enhance the rate of TCE co-metabolism. Three monitoring wells were installed in series downgradient of the biosparging well along the groundwater flow path. Results of polymerase chain reaction and nucleotide sequence analysis revealed that no appropriate TCE-degradation enzymes were observed in site groundwater. Thus, aerobic-activated sludge containing TCE-degraders collected from an industrial wastewater treatment plant were injected into the biosparging and three monitoring wells as an inoculum to provide...

Use of specific gene analysis to assess the effectiveness of surfactant-enhanced trichloroethylene cometabolism

The objective of this study was to evaluate the effectiveness of in situ bioremediation of trichloroethylene (TCE)-contaminated groundwater using specific gene analyses under the following conditions: (1) pretreatment with biodegradable surfactants [Simple Green™ (SG) and soya lecithin (SL)] to enhance TCE desorption and dissolution, and (2) supplementation with SG, SL, and cane molasses as primary substrates to enhance the aerobic cometabolism of TCE. Polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE), and nucleotide sequence analysis were applied to monitor the variations in specific activity-dependent enzymes and dominant microorganisms. Results show that TCE-degrading enzymes, including toluene monooxygenase, toluene dioxygenase, and phenol monooxygenase, were identified from sediment samples collected from a TCE-spill site. Results from the microcosm study show that addition of SG, SL, or cane molasses can enhance the aerobic cometabolism of TCE. The TCE degradation rates were highest in microcosms with added SL, the second highest in microcosms containing SG, and lowest in microcosms containing cane molasses. This indicates that SG and SL can serve as TCE dissolution agents and act as primary substrates for indigenous microorganisms. Four dominant microorganisms ( Rhodobacter sp., Methyloversatilis sp., Beta proteobacterium sp., and Hydrogenophaga pseudoflava) observed in microcosms might be able to produce TCE-degrading enzymes for TCE cometabolic processes.

Aerobic cometabolism of trichloroethene and cis-dichloroethene with benzene and chlorinated benzenes as growth substrates

Using inoculum from a microcosm study that exhibited aerobic transformation of cis-1,2-dichloroethene (cDCE) and trichloroethene (TCE) commensurate with biodegradation of monoaromatic compounds, enrichment cultures were developed by providing benzene, chlorobenzene (CB), dichlorobenzene (DCB) isomers and 1,2,4-trichlorobenzene as carbon and energy sources. Isolates that grow on benzene, CB, 1,2-DCB and 1,3-DCB were identified as Rhodococcus, Ralstonia, Variovorax and Ralstonia spp., respectively. Cometabolic transformation of cDCE and TCE by resting cells was demonstrated. Transformation capacities ( T c = 0.47–1.0 μg TCE mg −1 biomass; 1.3–5.3 μg cDCE mg −1 biomass), transformation yields ( T y = 0.18-0.27 μg TCE mg −1 substrate; 0.46–2.1 μg cDCE mg −1 substrate), and pseudo-first-order cometabolic degradation rate constants (0.00081–0.0031 L mg TCE −1 d −1; 0.0012–0.030 L mg cDCE −1 d −1) for resting cells grown on benzene, CB, 1,2-DCB and 1,3-DCB were generally lower in comparison to phenol and toluene-grown isolates. Cometabolic transformation of cDCE and TCE also occurred while the cultures concurrently consumed their growth substrate ( T c ′ = 0.15–0.33 μg TCE mg −1 biomass; 4.9–11 μg cDCE mg −1 biomass; T y ′ = 0.06–0.11 μg TCE mg −1 substrate; 1.7–4.6 μg cDCE mg −1 substrate), a condition that is more likely to be encountered in situ compared to cometabolic transformations by resting cells. This study is the first to report transformation rates, capacities, and yields for cometabolism of cDCE and TCE during aerobic growth on benzene, CB, 1,2-DCB and 1,3-DCB. This type of information is needed to predict the potential for natural attenuation when these compounds occur as co-contaminants.

Proteomic and targeted qPCR analyses of subsurface microbial communities for presence of methane monooxygenase

The Test Area North (TAN) site at the Idaho National Laboratory near Idaho Falls, ID, USA, sits over a trichloroethylene (TCE) contaminant plume in the Snake River Plain fractured basalt aquifer. Past observations have provided evidence that TCE at TAN is being transformed by biological natural attenuation that may be primarily due to co-metabolism in aerobic portions of the plume by methanotrophs. TCE co-metabolism by methanotrophs is the result of the broad substrate specificity of microbial methane monooxygenase which permits non-specific oxidation of TCE in addition to the primary substrate, methane. Arrays of experimental approaches have been utilized to understand the biogeochemical processes driving intrinsic TCE co-metabolism at TAN. In this study, aerobic methanotrophs were enumerated by qPCR using primers targeting conserved regions of the genes pmoA and mmoX encoding subunits of the particulate MMO (pMMO) and soluble MMO (sMMO) enzymes, respectively, as well as the gene mxa encoding the downstream enzyme methanol dehydrogenase. Identification of proteins in planktonic and biofilm samples from TAN was determined using reverse phase ultra-performance liquid chromatography (UPLC) coupled with a quadrupole-time-of-flight (QToF) mass spectrometer to separate and sequence peptides from trypsin digests of the protein extracts. Detection of MMO in unenriched water samples from TAN provides direct evidence of intrinsic methane oxidation and TCE co-metabolic potential of the indigenous microbial population. Mass spectrometry is also well suited for distinguishing which form of MMO is expressed in situ either soluble or particulate. Using this method, pMMO proteins were found to be abundant in samples collected from wells within and adjacent to the TCE plume at TAN.

Natural Gradient Drift Tests for Assessing the Feasibility of In Situ Aerobic Cometabolism of Trichloroethylene and Evaluating the Microbial Community Change

The objective of this study is to develop a method for using the single-well natural gradient drift test (SWNGDT) in the field to assess in situ aerobic cometabolism of trichloroethylene (TCE) and to analyze microbial community changes. The SWNGDT was performed in a monitoring well installed in a TCE-contaminated aquifer in Wonju, South Korea. The natural gradient drift biostimulation test (NGDBT) and surrogate test (NGDST) were performed by injecting dissolved solutes (bromide (a tracer), toluene (a growth substrate), ethylene (a nontoxic surrogate substrate to probe for TCE transformation activity), dissolved oxygen (DO, an electron acceptor), and nitrate (nutrient)) into the aquifer. Push–pull blocking tests (PPBT) were also performed to examine whether the monooxygenase of toluene oxidizers is involved in the degradation of toluene and the transformation of ethylene. Through the NGDBT, NGDST, and PPBT, we confirmed that the addition of toluene and oxygen in these field tests stimulated indigenous toluene utilizers to cometabolize aerobically TCE, with the following results: (1) the observed simultaneous utilization of toluene and DO; (2) the transformation of ethylene to ethylene oxide and propylene to propylene oxide; and (3) the transformation of TCE. Furthermore, the results of restriction fragment length polymorphism suggested that the microbial community shifts and the microbes capable of transforming TCE are stimulated by injecting the growth substrate, toluene.

Model simulations for in situ aerobic cometabolism of trichloroethylene

AbstractBACKGROUND: While overall removal of trichloroethylene (TCE) can be estimated from in situ pilot studies of aerobic cometabolism, no quantitative information on the relative importance of the biodegradation and sorption processes is currently available. This paper presents a quantitative method to evaluate the dynamics and the individual removal efficiencies of cometabolic biodegradation and sorption processes using model simulations for in situ aerobic cometabolism.RESULTS: The first‐order rate constant of TCE cometabolism by aerobic toluene‐utilizing bacteria was around 0.2 to 0.3 L mg−1 day−1. The TCE partition coefficient of sorption was determined at 16.4 L kg−1. The sorption process diminished as the sorption capacity of the aquifer solids became saturated. The sustainable TCE removal efficiency due to cometabolic biodegradation for the in situ pilot was determined at 25.8%, 51.8%, and 75.2% for residence times of 0.14 day, 0.57 day, and 1.16 day, respectively, after establishing an indigenous toluene‐degrading consortium.CONCLUSION: The agreement between model simulations and pilot results demonstrates the validity of a model that incorporates fundamental microbial and transport processes of advection, dispersion, and sorption onto the aquifer solids. The mathematical model aids the understanding of both the dynamics and the individual removal efficiencies of sorption and biodegradation processes of in situ bioremediation. Copyright © 2010 Society of Chemical Industry

Field Evidence for Co-Metabolism of Trichloroethene Stimulated by Addition of Electron Donor to Groundwater

For more than 10 years, electron donor has been injected into the Snake River aquifer beneath the Test Area North site of the Idaho National Laboratory for the purpose of stimulating microbial reductive dechlorination of trichloroethene (TCE) in groundwater. This has resulted in significant TCE removal from the source area of the contaminant plume and elevated dissolved CH(4) in the groundwater extending 250 m from the injection well. The delta(13)C of the CH(4) increases from -56 per thousand in the source area to -13 per thousand with distance from the injection well, whereas the delta(13)C of dissolved inorganic carbon decreases from 8 per thousand to -13 per thousand, indicating a shift from methanogenesis to methane oxidation. This change in microbial activity along the plume axis is confirmed by PhyloChip microarray analyses of 16S rRNA genes obtained from groundwater microbial communities, which indicate decreasing abundances of reductive dechlorinating microorganisms (e.g., Dehalococcoides ethenogenes) and increasing CH(4)-oxidizing microorganisms capable of aerobic co-metabolism of TCE (e.g., Methylosinus trichosporium). Incubation experiments with (13)C-labeled TCE introduced into microcosms containing basalt and groundwater from the aquifer confirm that TCE co-metabolism is possible. The results of these studies indicate that electron donor amendment designed to stimulate reductive dechlorination of TCE may also stimulate co-metabolism of TCE.

Single-well, gas-sparging tests for evaluating the in situ aerobic cometabolism of cis-1,2-dichloroethene and trichloroethene

This study developed single-well, gas-sparging tests for assessing the feasibility of in situ aerobic cometabolism of trichloroethene (TCE) and cis-1,2-dichloroethene ( cis-DCE) using propane and methane as growth substrates. Tests were performed in groundwater contaminated with TCE (100–400 μg l −1) and cis-DCE (20–60 μg l −1). A series of gas-sparging tests was performed by first sparging (“bubbling”) gas mixtures in a well fitted with a “straddle” packer and then periodically sampling groundwater from the same well to develop concentration profiles and to estimate transformation rate coefficients. Evidence that gas-sparging of propane (or methane) and oxygen had stimulated organisms expressing a propane (or methane) monooxygenase enzyme system and the capability to transform TCE and cis-DCE included: (1) the transformation of sparged ethylene and propylene to their corresponding cometabolic by-products, ethylene oxide and propylene oxide, (2) the transformation of both cis-DCE and TCE in the propane-sparged well, (3) the transformation of cis-DCE in the methane-sparged well, and (4) the inhibition of ethylene and propylene transformations in the presence of acetylene, a known monooxygenase inactivator. At a well sparged with propane, first-order rate coefficients for propane utilization and ethylene and propylene transformation were similar, ranging from 0.007 to 0.010 h −1. At the well sparged with methane, the propylene first-order transformation rate coefficient was 0.028 h −1, a factor of 1.8 and 1.6 greater than methane and ethylene, respectively. The results demonstrated that gas-sparging tests are a rapid, low-cost means of assessing the potential for the in situ aerobic cometabolism of cis-DCE and TCE.

Field Evidence for Intrinsic Aerobic Chlorinated Ethene Cometabolism by Methanotrophs Expressing Soluble Methane Monooxygenase

ABSTRACT Idaho National Laboratory's Test Area North is the site of a trichloroethene (TCE) plume resulting from waste injections. Previous investigations revealed that TCE was being attenuated relative to two codisposed internal tracers, tritium and tetrachloroethene, with a half-life of 9 to 21 years. Biological attenuation mechanisms were investigated using a novel suite of assays, including enzyme activity probes designed for the soluble methane monooxygenase (sMMO) enzyme. Samples were analyzed for chlorinated solvents, tritium, redox parameters, primary substrates, degradation products, bacterial community methanotrophic potential, and bacterial DNA. The enzyme probe assays, methanotrophic enrichments and isolations, and DNA analysis documented the presence and activity of indigenous methanotrophs expressing the sMMO enzyme. Three-dimensional groundwater data showed plume-wide aerobic conditions, with low levels of methane and detections of carbon monoxide, a by-product of TCE cometabolism. The TCE half-life attributed to aerobic cometabolism is 13 years relative to tritium, based on the tracer-corrected method. Similarly, a half-life of 8 years was estimated for cis-dichloroethene (DCE). Although these rates are slower than most anaerobic degradation processes, they can be significant for large plumes. This investigation is believed to be the first documentation of intrinsic aerobic TCE and DCE cometabolism in an aquifer by indigenous methanotrophs.