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Query: EC:3.4.23.17 (PCE)
 1,301 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Tetrachloroethene (PCE) and other chloroethenes are major contaminants in groundwater, and PCE is particularly resistant to attack by aerobes. We have developed an anaerobic enrichment culture that carries out reductive dechlorination of chloroethenes to ethene at high rates, thereby detoxifying them. Although the electron donor added to the culture is methanol, our evidence indicates that H2 is the electron donor used directly for dechlorination. We have recently obtained a culture from 10(-6) dilution of the original methanol/PCE culture that uses H2 as an electron donor for PCE dechlorination. Because the culture can be transferred indefinitely and the rate of PCE dechlorination increases after inoculation, we suggest that dechlorinating organisms in the culture use the carbon-chlorine bonds in chloroethenes as electron acceptors for energy conservation.
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PMID:Reductive dechlorination of tetrachloroethene by a high rate anaerobic microbial consortium. 856 11

Chlorine isotope fractionation during reductive dechlorination of trichloroethene (TCE) and tetrachloroethene (PCE) to cis-1,2-dichloroethene (cDCE) by anaerobic bacteria was investigated. The changes in the 37Cl/35Cl ratio observed during the one-step reaction (TCE to cDCE) can be explained by the regioselective elimination of chlorine accompanied by the Rayleigh fractionation. The fractionation factors (alpha) of the TCE dechlorination by three kinds of anaerobic cultures were approximately 0.994-0.995 at 30 degrees C. The enrichment of 37Cl in the organic chlorine during the two-step reaction (PCE to cDCE) can be explained by the random elimination of one chlorine atom in the PCE molecule followed by the regioselective elimination of one chlorine atom in the TCE molecule. The fractionation factors for the first step of the PCE dechlorination with three kinds of anaerobic cultures were estimated to be 0.987-0.991 at 30 degrees C using a mathematical model. Isotope fractionation during the first step would be the primary factor for the chlorine isotope fractionation during the PCE dechorination to cDCE. The developed models can be utilized to evaluate the fractionation factors of regioselective and multistep reactions.
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PMID:Chlorine isotope fractionation during reductive dechlorination of chlorinated ethenes by anaerobic bacteria. 1238 13

Stable hydrogen isotopes of two chlorinated solvents, trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA), provided by five different manufacturers, were determined and compared to their carbon and chlorine isotopic signatures. The isotope ratio for delta2H of different TCEs ranged between +466.9 per thousand and +681.9 per thousand, for delta13C between -31.57 per thousand and -27.37 per thousand, and for delta37Cl between -3.19 per thousand and +3.90 per thousand. In the case of the TCAs, the isotope ratio for delta2H ranged between -23.1 per thousand and +15.1 per thousand, for delta13C between -27.39 per thousand and -25.84 per thousand, and for delta37Cl between -3.54 per thousand and +1.39 per thousand. As well, a column experiment was carried out to dechlorinate tetrachloroethylene (PCE) to TCE using iron. The dechlorination products have completely different hydrogen isotope ratios than the manufactured TCEs. Compared to the positive values of delta2H in manufactured TCEs (between +466.9 per thousand and +681.9 per thousand), the dechlorinated products had a very depleted delta2H (less than -300 per thousand). This finding has strong implications for distinguishing dechlorination products (PCE to TCE) from manufactured TCE. In addition, the results of this study show the potential of combining 2H/1H analyses with 13C/12C and 37Cl/35Cl for isotopic fingerprinting applications in organic contaminant hydrogeology.
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PMID:Stable hydrogen, carbon and chlorine isotope measurements of selected chlorinated organic solvents. 1250 60

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.
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PMID:The quest for microbial reductive dechlorination of C (2) to C (4) chloroalkanes is warranted. 1265 50

The potential for natural attenuation of volatile organic compounds (VOCs) in landfill covers was investigated in soil microcosms incubated with methane and air, simulating the gas composition in landfill soil covers. Soil was sampled at Skellingsted Landfill at a location emitting methane. In total, 26 VOCs were investigated, including chlorinated methanes, ethanes, ethenes, fluorinated hydrocarbons, and aromatic hydrocarbons. The soil showed a high capacity for methane oxidation resulting in very high oxidation rates of between 24 and 112 microg CH4 g(-1) h(-1). All lower chlorinated compounds were shown degradable, and the degradation occurred in parallel with the oxidation of methane. In general, the degradation rates of the chlorinated aliphatics were inversely related to the chlorine to carbon ratios. For example, in batch experiments with chlorinated ethylenes, the highest rates were observed for vinyl chloride (VC) and lowest rates for trichloroethylene (TCE), while tetrachloroethylene (PCE) was not degraded. Maximal oxidation rates for the halogenated aliphatic compounds varied between 0.03 and 1.7 microg g(-1) h(-1). Fully halogenated hydrocarbons (PCE, tetrachloromethane [TeCM], chlorofluorocarbon [CFC]-11, CFC-12, and CFC-113) were not degraded in the presence of methane and oxygen. Aromatic hydrocarbons were rapidly degraded giving high maximal oxidation rates (0.17-1.4 microg g(-1) h(-1)). The capacity for methane oxidation was related to the depth of oxygen penetration. The methane oxidizers were very active in oxidizing methane and the selected trace components down to a depth of 50 cm below the surface. Maximal oxidation activity occurred in a zone between 15 and 20 cm below the surface, as this depth allowed sufficient supply of both methane and oxygen. Mass balance calculations using the maximal oxidation rates obtained demonstrated that landfill soil covers have a significant potential for not only methane oxidation but also cometabolic degradation of selected volatile organics, thereby reducing emissions to the atmosphere.
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PMID:Attenuation of methane and volatile organic compounds in landfill soil covers. 1496 59

Abiotic reductive dechlorination of chlorinated ethylenes (tetrachloroethylene (PCE), trichloroethylene (TCE), cis-dichloroethylene (c-DCE), and vinylchloride (VC)) by iron-bearing phyllosilicates (biotite, vermiculite, and montmorillonite) was characterized to obtain better understanding of the behavior of these contaminants in systems undergoing remediation by natural attenuation and redox manipulation. Batch experiments were conducted to evaluate dechlorination kinetics and some experiments were conducted with addition of Fe(II) to simulate impact of microbial iron reduction. A modified Langmuir-Hinshelwood kinetic model adequately described reductive dechlorination kinetics of target organics by the iron-bearing phyllosilicates. The rate constants stayed between 0.08 (+/-10.4%) and 0.401 (+/-8.1%) day(-1) and the specific initial reductive capacity of iron-bearing phyllosilicates for chlorinated ethylenes stayed between 0.177 (+/-6.1%) and 1.06 (+/-7.1%) microM g(-1). The rate constants for the reductive dechlorination of TCE at reactive biotite surface increased as pH (5.5-8.5) and concentration of sorbed Fe(II) (0-0.15 mM g(-1)) increased. The appropriateness of the model is supported by the fact that the rate constants were independent of solid concentration (0.0085-0.17 g g(-1)) and initial TCE concentration (0.15-0.60 mM). Biotite had the greatest rate constant among the phyllosilicates both with and without Fe(II) addition. The rate constants were increased by a factor of 1.4-2.5 by Fe(II) addition. Between 1.8% and 36% of chlorinated ethylenes removed were partitioned to the phyllosilicates. Chloride was produced as a product of degradation and no chlorinated intermediates were observed throughout the experiment.
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PMID:Abiotic reductive dechlorination of chlorinated ethylenes by iron-bearing phyllosilicates. 1526 67

Stable isotopic characterization of chlorine in chlorinated aliphatic pollution is potentially very valuable for risk assessment and monitoring remediation or natural attenuation. The approach has been underused because of the complexity of analysis and the time it takes. We have developed a new method that eliminates sample preparation. Gas chromatography produces individually eluted sample peaks for analysis. The He carrier gas is mixed with Ar and introduced directly into the torch of a multicollector ICPMS. The MC-ICPMS is run at a high mass resolution of >/=10 000 to eliminate interference of mass 37 ArH with Cl. The standardization approach is similar to that for continuous flow stable isotope analysis in which sample and reference materials are measured successively. We have measured PCE relative to a laboratory TCE standard mixed with the sample. Solvent samples of 200 nmol to 1.3 micromol (24-165 microg of Cl) were measured. The PCE gave the same value relative to the TCE as measured by the conventional method with a precision of 0.12 per thousand (2x standard error) but poorer precision for the smaller samples.
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PMID:GC/multiple collector-ICPMS method for chlorine stable isotope analysis of chlorinated aliphatic hydrocarbons. 1680 79

A simple, quick and sensitive method for the compound-specific stable chlorine isotope analysis of chlorinated solvents by conventional quadrupole gas chromatography/mass spectrometry (GC/MS) is presented. With this method, compound-specific stable chlorine isotope ratios of typical chlorinated solvents like tetrachloroethene (PCE) and trichloroethene (TCE) can be determined quantitatively within 30 min by direct injection. The chlorine isotope ratios of target substances are calculated from the peak areas of several selected molecular ions and fragment ions of the substances, using a set of unique mathematical equations. The precision of the method was demonstrated through reproducibility tests. An internal precision of +/-0.4 per thousand to +/-1.1 per thousand was obtained when analyzing PCE and TCE in the 10-1000 pmol range. The validity of the method was further demonstrated by determining the chlorine isotopic fractionation factor during the reductive dechlorination of TCE in a batch experiment using zero-valent iron. The chlorine isotopic fractionation factor was calculated as 0.9976 +/- 0.0011 with a correlation coefficient of 0.9469 (n = 38). The high correlation coefficient indicates that compound-specific stable chlorine isotope analysis can be performed with sufficient accuracy using conventional quadrupole GC/MS when significant fractionation takes place during a reaction. For the first time, the chlorine isotope fractionation factor of TCE during an abiotic anaerobic dechlorination process was determined using quadrupole GC/MS, without offline sample preparation.
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PMID:Monitoring and evaluation of dechlorination processes using compound-specific chlorine isotope analysis. 1770 45

Compound-specific chlorine isotope analysis receives much interest to assess the fate of chlorinated hydrocarbons in contaminated environments. This paper provides a theoretical basis to calculate isotope ratios and quantify isotope fractionation from ion-current ratios of molecular- and fragment-ion multiplets. Because both (35)Cl and (37)Cl are of high abundance, polychlorinated hydrocarbons consist of molecules containing different numbers of (37)Cl denoted as isotopologues. We show that, during reactions, the changes in isotopologue ratios are proportional to changes in the isotope ratio assuming a nonselective isotope distribution in the initial compound. This proportionality extents even to fragments formed in the ion source of a mass spectrometer such as C 2Cl 2 (double dechlorinated fragment of perchloroethylene, PCE). Fractionation factors and kinetic isotope effects (KIE) may, therefore, be evaluated from isotope, isotopologue or even fragment ratios according to conventional simple equations. The proportionality is exact with symmetric molecules such as dichloroethylene (DCE) and PCE, whereas it is approximately true with molecules containing nonreactive positions such as trichloroethylene (TCE). If in the latter case isotope ratios are derived from dechlorinated fragments, e.g., C 2HCl 2, it is important that fragmentation in the ion source affect all molecular positions alike, as otherwise isotopic changes in reactive positions may be underrepresented.
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PMID:Evaluating chlorine isotope effects from isotope ratios and mass spectra of polychlorinated molecules. 1848 45

A method has been developed for the direct determination of the stable chlorine isotope composition (delta(37)Cl) of organochlorines that eliminates sample preparation, achieves precision comparable to earlier techniques while improving the sensitivity, and makes use of benchtop gas chromatography-quadrupole mass spectrometry instruments (GCqMS). The method is based on the use of multiple injections (n = 8-10) of the sample, bracketed by a molecularly identical isotopic standard with known delta(37)Cl, determined using off-line thermal ionization mass spectrometry (TIMS). Mass traces of two isotopologues differing by one chlorine isotope were used to calculate delta(37)Cl values. Optimization of mass spectrometry and peak integration parameters as well as method validation was achieved using tetrachloroethene (PCE), p,p'-dichlorodiphenyltrichloroethane (DDT), and pentachlorophenol (PCP), spanning a delta(37)Cl range of -5.5 to +3.2 per thousand vs SMOC. Injecting 1.6-1100 pmol resulted in standard deviations (1sigma) of 0.6-1.3 per thousand, and the delta(37)Cl results agreed with values independently measured with TIMS. The method was tested by determining the Rayleigh fractionation during evaporation of pure liquid PCE, resulting in a chlorine isotopic enrichment factor of epsilon(Cl) = -1.1 +/- 0.4 per thousand. Furthermore, position-specific delta(37)Cl analysis based on analysis of DDT mass fragments was evaluated. The GCqMS-delta(37)Cl method offers a simplified yet sensitive approach for compound-specific chlorine isotope analysis.
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PMID:Direct compound-specific stable chlorine isotope analysis of organic compounds with quadrupole GC/MS using standard isotope bracketing. 2000 May 86


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