Transformation capacities of chlorinated organics by mixed cultures enriched on methane, propane, toluene, or phenol

The degradation of trichloroethylene (TCE), chloroform (CF), and 1,2-dichloroethane (1,2-DCA) by four aerobic mixed cultures (methane, propane, toluene, and phenol oxidizers) grown under similar chemostat conditions was measured. Methane and propane oxidizers were capable of degrading both saturated and unsaturated chlorinated organics (TCE, CF, and 1,2-DCA). Toluene and phenol oxidizers degraded TCE but were not able to degrade CF, 1,2-DCA, or other saturated organics. None of the cultures tested were able to degrade perchloroethylene (PCE) or carbon tetrachloride (CC(4)). For the four cultures tested, degradation of each of the chlorinated organics resulted in cell inactivation due to product toxicity. In all cases, the toxic products were rapidly depleted, leaving no toxic residues in solution. Among the four tested cultures, the resting cells of methane oxidizers exhibited the highest transformation capacities (T(c)) for TCE, CF, and 1,2-DCA. The T(c) for each chlorinated organic was observed to be inversely proportional to the chlorine carbon ratio (Cl/C). The addition of low concentrations of growth substrate or some catabolic intermediates enhanced TCE transformation capacities and degradation rates, presumably due to the regeneration of reducing energy (NADH); however, addition of higher concentrations of most amendments reduced TCE transformation capacities and degradation rates. Reducing energy limitations and amendment toxicity may significantly affect T(c) measurements, causing a masking of the toxicity associated with chlorinated organic degradation. (c) 1995 John Wiley & Sons, Inc.     

Optimization of trichloroethylene oxidation by methanotrophs and the use of a colorimetric assay to detect soluble methane monooxygenase activity

Methylosinus trichosporium OB3b biosynthesizes a broad specificity soluble methane monooxygenase that rapidly oxidizes trichloroethylene (TCE). The selective expression of the soluble methane monooxygenase was followed in vivo by a rapid colorimetric assay. Naphthalene was oxidized by purified soluble methane monooxygenase or by cells grown in copper-deficient media to a mixture of 1-naphthol and 2-naphthol. The naphthols were detected by reaction with tetrazotized o-dianisidine to form purple diazo dyes with large molar absorptivities. The rate of color formation with the rapid assay correlated with the velocity of TCE oxidation that was determined by gas chromatography. Both assays were used to optimize conditions for TCE oxidation by M. trichosporium OB3b and to test several methanotrophic bacteria for the ability to oxidize TCE and naphthalene.Abbreviations A₆₀₀ absorbance due to cell density measured at 600 nm - HPLC high pressure liquid chromatography - NADH reduced nicotinamide adenine dinucleotide - SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis - sMMO soluble methane monooxygenase - TCE trichloroethylene     

Mathematical Modeling of Chemotactic Bacterial Transport through a Two-Dimensional Heterogeneous Porous Medium

The impact of bacterial chemotaxis on in situ ground-water bioremediation remains an unanswered question. Although bacteria respond to chemical gradients in aqueous environments and under no-flow conditions, it is unclear whether they can also respond in porous media with advective flow to improve overall contaminant degradation. The effect of chemotaxis is most profound in regions with sharp chemical gradients, most notably around residual nonaqueous phase liquid (NAPL) ganglia and surrounding clay lenses or aquitards with trapped contamination. The purpose of this study is to simulate bacterial transport through a two-dimensional subsurface environment, containing one region of low permeability with trapped contaminant surrounded above and below by two regions of higher permeability. Using mathematical predictions of the effect of pore size on measured bacterial transport parameters, the authors observe a 50% decrease in both motility and chemotaxis in the finer-grained, low-permeability porous medium. The authors simulate how chemotaxis affects bacterial migration to the contaminated region under various flow and initial conditions. Results indicate that bacteria traveling through a high-permeability region with advective flow can successfully migrate toward and accumulate around a contaminant diffusing from a lower permeability region.     

Aerobic biodegradation of trichloroethylene using a consortium of five bacterial strains

Degradation with an aerobic consortium was used to evaluate the bioremediation trichloroethylene (TCE) as a model substrate. After one week, 228-1186 mg TCE l(-1) was degraded at rates of 20-50 microg TCE l(-1) h(-1). The introduction of 10 mg toluene l(-1) enhanced the degradation rates for TCE when greater than 600 mg l(-1). Using isolated enzymes, a TCE degradation intermediate(s) appears inhibitory to the oxygenase enzymes thereby diminishing the overall degradation.     

Trichloroethylene Degradation by Toluene-Oxidizing Bacteria Grown on Non-aromatic Substrates

The potential of trichloroethylene (TCE) to induce and non-aromatic growth substrates to support TCE degradation in five strains (Pseudomonas mendocina KR1, Ralstonia pickettii PKO1, Pseudomonas putida F1, Burkholderia cepacia G4, B. cepacia PR1) of toluene-oxidizing bacteria was examined. LB broth and acetate did not support TCE degradation in any of the wild-type strains. In contrast, fructose supported the highest specific levels of TCE oxidation observed in each of the strains tested, except B. cepacia G4. We discuss the potential mechanisms and implications of this observation. In particular, cells of P. mendocina KR1 degraded significant amounts of TCE during cell growth on non-aromatic substrates. Apparently, TCE degradation was not completely constrained by any given factor in this microorganism, as was observed with P. putida F1 (TCE was an extremely poor substrate) or B. cepacia G4 (lack of oxygenase induction by TCE). Our results indicate that multiple physiological traits are required to enable useful TCE degradation by toluene-oxidizing bacteria in the absence of aromatic cosubstrates. These traits include oxygenase induction, effective TCE turnover, and some level of resistance to TCE mediated toxicity.     

Demonstration of efficient trichloroethylene biodegradation in a hollow‐fiber membrane bioreactor

Rapid cometabolism of trichloroethylene (TCE) by pure cultures of Methylosinus trichosporium OB3b PP358 was demonstrated in a two‐stage hollow‐fiber membrane bioreactor over the course of 3 weeks. PP358 was grown in a continuous‐flow chemostat and circulated through the shell of a hollow‐fiber membrane module (HFMM), while TCE contaminated water (160 to 1450 μg/L) was pumped through the fiber lumen (fiber interior). In parallel‐flow HFMM biological experiments, 82% to 89% of the influent TCE was removed from the lumen (5.1‐min residence time) with 99% of the transferred TCE undergoing biodegradation. Biological experiments in a larger capacity baffled radial‐flow HFMM resulted in 66% to 99% TCE transferred and 93% to 96% TCE biodegradation at lumen residence times of between 1.5 and 3.7 min. Biodegradation was maintained throughout the experiments at pseudo‐first‐order biodegradation rate constants of 0.41 to 2.8 L/mg TSS/day. Best‐fit computer modeling of the baffled radial‐flow biological process estimated mass transfer coefficients as large as 2.7 × 10⁻² cm/min. The computer model was also shown to simulate the experimental results quite well. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 62: 681–692, 1999.     

Trichloroethylene and dichloroethylene: a critical review of teratogenicity

Trichloroethylene (TCE) and dichloroethylene (DCE) are high-volume industrial chemicals frequently found as contaminants in public drinking water supplies. The developmental toxicity of both chemicals has been evaluated in laboratory and epidemiologic studies. It has been suggested that TCE and DCE are specific cardiac teratogens and that drinking water contaminated with them increases the risk of congenital heart defects in exposed human populations. In contrast, other laboratory and epidemiologic studies do not find an increase in developmental effects, either in general or specifically affecting the heart. This laboratory and epidemiologic base was reviewed to evaluate the strengths and weaknesses of the conflicting published reports. We conclude that the weight of experimental and epidemiologic evidence does not support the hypothesis that TCE or DCE is a selective developmental toxicant in general or a cardiac teratogen specifically.     

Development and mathematical modeling of a two-stage reactor system for trichloroethylene degradation using <Emphasis Type="Italic">Methylosinus trichosporium</Emphasis> OB3b

A two-stage reactor system was developed for the continuous degradation of gas-phase trichloroethylene (TCE). Methylosinus trichosporium OB3b was immobilized on activated carbon in a TCE degradation reactor, trickling biofilter (TBF). The TBF was coupled with a continuous stirred tank reactor (CSTR) to allow recirculation of microbial cells from/to the TBF for the reactivation of inactivated cells during TCE degradation. The mass transfer aspect of the TBF was analyzed, and mass transfer coefficient of 3.9 h⁻¹ was estimated. The loss of soluble methane monooxygenase (sMMO) activity was modeled based on a material balance on the CSTR and TBF, and transformation capacity (T _c) was determined to be 20.2 mol mg⁻¹. Maximum TCE degradation rate of 525 mg 1⁻¹ d⁻¹ was obtained and reactor has been stably operated for more than 270 days.     

USE OF NATIVE PLANTS FOR REMEDIATION OF TRICHLOROETHYLENE: II. CONIFEROUS TREES

The phytoremediation of trichloroethylene (TCE) from contaminated groundwater has been extensively studied using the hybrid poplar tree (Populus spp.). Several metabolites of TCE have been identified in the tissue of poplar including trichloroethanol (TCEOH) and dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA). In addition to the use of hybrid poplar for the phytoremediation of TCE, it is important to screen native tree species that could be successful candidates for field use. This study involves a greenhouse-based comparison of four different native southeastern conifers to a hybrid poplar species for their potential to phytoremediate TCE through the analysis of various plant tissues for TCE and major TCE metabolites, as well as several growth parameters that are desirable for phytoremediation. Longleaf pine (Pinus palustris), Leyland cypress (X Cupressocyparis leylandii), two varieties of Loblolly pine (Pinus taeda), and hybrid poplar species H11-11 (Populus trichocarpa x deltoides) were examined for the concentration of TCE and its metabolites in their tissue following treatment with either a low (50 mg L^?1) or high dose of TCE (150 mg L^?1) for 2 mo. The amount of water taken up, change in height of the tree, TCE transpiration, and total fresh weight of various tissue types were also measured. All trees contained detectable levels of TCE in their root and stem tissue. TCEOH was found only in the tissue of longleaf pine, suggesting that TCE metabolism was occurring in this tree. TCAA was only detected in the leaves of hybrid poplar and piedmont loblolly pine. Conifers took up less water over the 2-mo treatment period than hybrid poplar and grew at a slower rate. However, phytoremediation field sites may benefit from the evergreen's ability to transpire water throughout the winter months.     

Trichloroethylene and Human Cancer

Evidence to suggest that trichloroethylene may be a human carcinogen comes mainly from two small epidemiological studies with supporting evidence from human toxicity and genotoxicity studies and from rodent cancer bioassays. Careful analysis of these data reveal marked inconsistencies between the data, differences in the conclusions drawn by various authorities reviewing the same data and, for certain key human studies, a complete absence of exposure data. Much of the rodent cancer data may be dismissed as not indicative of a human hazard because the tumors result from either peroxisome proliferation, or as a consequence of exceptionally high metabolic rates that are found uniquely in mouse tissues, or because the tumors only occur in the presence of overt toxicity. A common mechanism invoked to account for the development of renal tumors in both rats and humans is unproven, there is contrary evidence to suggest that this mechanism does not result in renal cancer, and alternative mechanisms have been proposed. Overall, the uncertainty and lack of consistency throughout the trichloroethylene studies, whether they are in humans, in animals, or in tests in vitro, lead to the conclusion that it would be wholly inappropriate to classify trichloroethylene as a human carcinogen.