Dehalogenation Of PCE Research Articles

Biodegradation of tetrachloroethylene in upflow anaerobic sludge blanket reactor

The laboratory study using upflow anaerobic sludge blanket (UASB) reactor in high strength wastewater containing tetrachloroethylene (PCE) was carried out to develop granular sludge in the presence of PCE (2 mg/l) and to assess the potential of UASB reactor in treating PCE containing wastewater. The granules of 0.25–4 mm size were observed after 82 days having mostly Methanothrix and Methanosarcina bacteria. Influent PCE concentration of 5–50 mg/l decreased to less than 0.23 mg/l (98.5 ± 1% removal) in most cases. The trichloroethylene (TCE), cis-1,2-dichloroethylene (cis-DCE), vinyl chloride (VC) and ethylene were formed on dehalogenation of PCE. Under steady state operation conditions the COD removal of 94 ± 2% and biogas production of 0.559–0.508 m 3/kg COD rem with methane content of 64 ± 2% was achieved. The maximum PCE dechlorination rate ( q max), was 14.28 mg PCE/g VSS d and the half velocity coefficient, ( K s ), was 0.417 mg PCE/l under steady state conditions. The UASB reactor may prove to be a potential wastewater treatment process for PCE containing wastewater.

Interaction between methanogenic and sulfate-reducing microorganisms during dechlorination of a high concentration of tetrachloroethylene.

A methanogenic and sulfate-reducing consortium, which was enriched on medium containing tetrachloroethylene (PCE), had the ability to dechlorinate high concentrations of PCE. Dehalogenation was due to the direct activity of methanogens. However, interactions between methanogenic and sulfate-reducing bacteria involved modification of the dechlorination process according to culture conditions. In the absence of sulfate, the relative percentage of electrons used in PCE dehalogenation increased after an addition of lactate in batch conditions. The sulfate reducers would produce further reductant from lactate catabolism. This reductant might be used by methanogenic bacteria in PCE dechlorination. A mutualistic interaction was observed in the absence of sulfate. However in the presence of sulfate, methanogenesis and dechlorination decreased because of interspecific competition, probably between the H(2)-oxydizing methanogenic and sulfate-reducing bacteria in batch conditions. In the semicontinuous fixed-bed reactor, the presence of sulfate did not affect dechlorination and methanogenesis. The sulfate-reducing bacteria may not be competitors of H(2)-consuming methanogens in the reactor because of the existence of microbial biofilm. The presence of the fixed film may be an advantage for bioremediation and industrial treatment of effluent charged in sulfate and PCE. This is the first report on the microbial ecology of a methanogenic and sulfate-reducing PCE-enrichment consortium.

In vitro studies on reductive vinyl chloride dehalogenation by an anaerobic mixed culture

Reductive dehalogenation of vinyl chloride (VC) was studied in an anaerobic mixed bacterial culture. In growth experiments, ethene formation from VC increased exponentially at a rate of about 0.019 h(sup-1). Reductive VC dehalogenation was measured in vitro by using cell extracts of the mixed culture. The apparent K(infm) for VC was determined to be about 76 (mu)M; the V(infmax) was about 28 nmol (middot) min(sup-1) (middot) mg of protein(sup-1). The VC-dehalogenating activity was membrane associated. Propyl iodide had an inhibitory effect on the VC-dehalogenating activity in the in vitro assay. However, this inhibition could not be reversed by illumination. Cell extracts also catalyzed the reductive dehalogenation of cis-1,2-dichloroethene (cis-DCE) and, at a lower rate, of trichloroethene (TCE). Tetrachloroethene (PCE) was not transformed. The results indicate that the reductive dehalogenation of VC and cis-DCE described here is different from previously reported reductive dehalogenation of PCE and TCE.

Cobalamin-Mediated Reduction of cis- and trans-Dichloroethene, 1,1-Dichloroethene, and Vinyl Chloride in Homogeneous Aqueous Solution: Reaction Kinetics and Mechanistic Considerations

Since cobalamin is involved in the enzymatic reduction of halogenated ethenes by a variety of anaerobic bacteria and since cobalamin has been suggested as electron transfer mediator for the treatment of halogenated solvents, its reactions with such compounds are presently of great interest. In this paper, it is shown that, in homogeneous aqueous solution containing titanium(III) citrate as the bulk electron donor, superreduced cobalamin reductively dechlorinated cis- and trans-dichloroethene (cis-DCE and trans-DCE), 1,1-dichloroethene (1,1-DCE), and vinyl chloride (VC) in pH-dependent reactions to ethene and ethane. Evidence is given that the initial step was the addition of cob(I)alamin to the chlorinated ethenes (CEs) with simultaneous protonation. Only for 1,1-DCE at high pH, a dissociative electron transfer mechanism as suggested for tetrachloroethene (PCE) and trichloroethene (TCE) in earlier work was important. 1,1-DCE reacted about 30 times faster than VC, 600 times faster than trans-DCE, and 3000 times faster than cis-DCE. Acetylene and ethene were found to react at similar rates as 1,1-DCE and VC, respectively. However, at more positive redox potentials, the reductive cleavage of the addition products, particularly of the adducts of acetylene, ethene, and VC with cob(I)alamin, may become very slow, thus preventing the regeneration of cob(I)alamin. The results of this study demonstrate that, at more negative potentials and at low pH, cobalamin is a potent electron transfer mediator for the complete dehalogenation of PCE and TCE without significant accumulation of VC.

Isolation and Characterization of a Facultatively Aerobic Bacterium That Reductively Dehalogenates Tetrachloroethene to cis-1,2-Dichloroethene

A rapidly-growing facultatively aerobic bacterium that transforms tetrachloroethene (PCE) via trichloroethene (TCE) to cis-1,2-dichloroethene (cis-1,2-DCE) at high rates in a defined medium was isolated from a contaminated site. Metabolic characterization, cellular fatty acid analysis, and partial sequence analysis of 16S rRNA showed that the new isolate, strain MS-1, has characteristics matching those of the members of the family Enterobacteriaceae. Strain MS-1 can oxidize about 58 substrates including many carbohydrates, short-chain fatty acids, amino acids, purines, and pyrimidines. It can transform up to 1 mM PCE (aqueous) at a rate of about 0.5 (mu)mol of PCE(middot) h(sup-1)(middot)mg (dry weight) of cell(sup-1). PCE transformation occurs following growth on or with the addition of single carbon sources such as glucose, pyruvate, formate, lactate, or acetate or with complex nutrient sources such as yeast extract or a mixture of amino acids. PCE dehalogenation requires the absence of oxygen, nitrate, and high concentrations of fermentable compounds such as glucose. Enterobacter agglomerans biogroup 5 (ATCC 27993), a known facultative bacterium that is closely related to strain MS-1, also reductively dehalogenated PCE to cis-1,2-DCE. To our knowledge, this is the first report on isolation of a facultative bacterium that can reductively transform PCE to cis-1,2-DCE under defined physiological conditions. Also, this is the first report of the ability of E. agglomerans to dehalogenate PCE.

Tetrachloroethene metabolism of Dehalospirillum multivorans

Dehalospirillum multivorans is a strictly anaerobic bacterium that is able to dechlorinate tetrachloroethene (perchloroethylene; PCE) via trichloroethene (TCE) to cis-1,2-dichloroethene (DCE) as part of its energy metabolism. The present communication describes some features of the dechlorination reaction in growing cultures, cell suspensions, and cell extracts of D. multivorans. Cell suspensions catalyzed the reductive dechlorination of PCE with pyruvate as electron donor at specific rates of up to 150 nmol (chloride released) min-1 (mg cell protein)-1 (300 microM PCE initially, pH 7.5, 25 degrees C). The rate of dechlorination depended on the PCE concentration; concentrations higher than 300 microM inhibited dehalogenation. The temperature optimum was between 25 and 30 degrees C; the pH optimum at about 7.5. Dehalogenation was sensitive to potential alternative electron acceptors such as fumarate or sulfur; nitrate or sulfate had no significant effect on PCE reduction. Propyl iodide (50 microM) almost completely inhibited the dehalogenation of PCE in cell suspensions. Cell extracts mediated the dehalogenation of PCE and of TCE with reduced methyl viologen as the electron donor at specific rates of up to 0.5 mumol (chloride released) min-1 (mg protein).-1 An abiotic reductive dehalogenation could be excluded since cell extracts heated for 10 min at 95 degrees C were inactive. The PCE dehalogenase was recovered in the soluble cell fraction after ultracentrifugation. The enzyme was not inactivated by oxygen.

Effects of three concentrations of mixed fatty acids on dechlorination of tetrachloroethene in aquifer microcosms

AbstractChloroethenes are among the most common organic contaminants of ground water. The biotransformation of these compounds by reductive dechlorination is a promising technology for in situ treatment. The effects of three concentrations of a fatty acids mixture on the reductive dehalogenation of tetrachloroethene (PCE) were studied in methanogenic microcosms. These microcosms were constructed with slurries of aquifer solids collected from an area impacted both by aviation gasoline and chlorinated ethenes at Traverse City, Michigan. The microcosms were amended with approximately 30 μM PCE and one of three concentrations (0.1, 1, or 10 mM total acids) of a mixture of low‐molecular‐weight organic acids, or were part of a control set that received no amendment. The observed lag or adaptation times before the onset of PCE dehalogenation were 51 d (10 mM acids), 65 d (1 mM acids), 86 d (0.1 mM acids), and >233 d (no acids). After 233 d of incubation, no PCE was detectable in any of the fatty‐acid‐supplemented microcosms, but 23 to 27 μM of combined tri‐ and dichloroethenes (TCEs, DCEs) was detected. PCE was not dechlonnated in microcosms without a fatty acid supplement. Although there were observed differences in the length of the lag time, the amount of ultimately dechlorinated PCE was similar. A zero‐order rate constant of 0.3 μM d−1 was calculated for PCE dechlorination and for TCE dechlorination for the two series of microcosms receiving 0.1 and 1.0 mM fatty acid supplements. Of the fatty acids tested in the mixture, butyrate oxidation appeared to be the most probable link to PCE dechlorination.