Ethene chlorinated ethenes

Fogel MM, Taddeo AR, Fogel S. 1986. Biodegradation of chlorinated ethenes by a methane-utilizing mixed culture. Appl Environ Microbiol 51 720-724. 

There has been considerable interest in the abiotic dechlorination of chlorinated ethenes at contaminated sites. Reductive dehalogenation has therefore been examined using a range of reductants, many of them involving reduced complexes of porphyrins or corrins. 

Attention is drawn to the dechlorination by anaerobic bacteria of both chlorinated ethenes and chlorophenolic compounds that serve as electron acceptors with electron donors including formate, pyruvate, and acetate. This is termed dehalorespiration and is important in the degradation of a range of halogenated compounds under anaerobic conditions, and is discussed further in Chapter 3, Part 2 and Chapter 7, Part 3. 

Maym6-Gatell X, T Anguish, SH Zinder (1999) Reductive dechlorination of chlorinated ethenes and 1,2-dichlooroethane by Dehalococcoides ethenogenes" 195. Appl Environ Microbiol 65 3108-3113. 

Numata M, N Nakamura, H Koshikawa, Y Terashima (2002) Chlorine isotope fractionation during reductive dechloirination of chlorinated ethenes by anaerobic bacteria. Environ Sci Technol 36 4389-4394. 

Three anaerobic dechlorinating consortia were used to examine fractionation during dechlorination of tetrachloroethene and TCE to cii-dichloroethene (Numata et al. 2002). Fractionation factors (a) for the first reaction ranged from 0.987 to 0.991 for the three consortia, and for the second reaction were 0.9944 for all consortia. Some important limitations were pointed out (a) the chlorinated ethenes were not separated so that the isotopic... 

Slater GF, BS Lollar, BE Sleep, EA Edwards (2001) Variability in carbon isotope fractionation during biodegradation of chlorinated ethenes implications for field applications. Environ Sci Technol 35 901-907. 

Tetrachoroethylene (perchloroethylene, PCE) is the only chlorinated ethene that resists aerobic biodegradation. This compound can be dechlorinated to less- or nonchlorinated ethenes only under anaerobic conditions. This process, known as reductive dehalogenation, was initially thought to be a co-metabolic activity. Recently, however, it was shown that some bacteria species can use PCE as terminal electron acceptor in their basic metabolism i.e., they couple their growth with the reductive dechlorination of PCE.35 Reductive dehalogenation is a promising method for the remediation of PCE-contaminated sites, provided that the process is well controlled to prevent the buildup of even more toxic intermediates, such as the vinyl chloride, a proven carcinogen. 

Hollinger, C., The anaerobic microbiology and biotreatment of chlorinated ethens, Curr. Biol., 6, 347-351, 1995. 

Cometabolism refers to the degradation of the chlorinated solvent as a by-product of the degradation of other substrates by microorganisms, and does not benefit the microorganism. As the degree of dechlorination decreases, the cometabolism rates increase. Thus, less oxidized or chlorinated solvents such as chlorinated ethenes (excluding PCE) biodegrade more favorably under aerobic conditions. 

Vancheeswaran S, Semprini L, Williamson KJ, Ingle JD Jr (1998) Final Report to the Lawrence Livermore National Laboratory Site-300. Project Intrinsic Transformation of Alkoxysilanes and Chlorinated Ethenes at Site-300... 

Newton R, Aranovich L (1996) Simple granulite melting in concentrated NaCl-KCl solutions at deep crustal conditions. Geol Soc Am Annu Meet Abstracts with Programs 158 Numata M, Nakamura N, Koshikawa H, Terashima Y (2002) Chlorine isotope fractionation during reductive dechlorination of chlorinated ethenes by anaerobic bacteria. Env Sci Tech 36(20) 4389-4394 Numata M, Nakamura N, Gamo T (2001) Precise measurement of chlorine stable isotopic ratios by thermal ionization mass spectrometry. Geochem J 35(2) 89-100 Owen HR, Schaeffer OA (1995) The isotope abundances of chlorine from various sources. J Am Chem Soc 77 898-899... 

Yang, Y., Pesaro, M., Sigler, W., and Zeyer, J., 2005, Identification of microorganisms involved in reductive dehalogenation of chlorinated ethenes in an anaerobic microbial community. Water Res. 39 3954-3966. 

CH3-CH2CI CH2CI-CH2CI Cl2C = CHCI Chlorinated ethanes Chlorinated ethenes... 

Fig. 13.9 Observed transformations of chlorinated ethenes in groundwater system. (Chapelle and Bradley 1998)...

Heimann, A. C. Jakobsen, R. Experimental Evidence for a Lack of Thermodynamic Control on Hydrogen Concentrations during Anaerobic Degradation of Chlorinated Ethenes. Environ. Sci. Technol. 2006, 40, 3501-3507. 

The BAT system operates based on principles of aerobic cometabolism. In cometabohsm, enzymes that the microbes produce in the process of consuming one particular compound (e.g., phenol) have the collateral effect of transforming another compound that normally resists biodegradation (e.g., chlorinated ethenes, especially lesser chlorinated ethenes such as dichloroethene or vinyl chloride). The BAT system operates under these principles by sorbing the chlorinated compounds from a vapor stream onto powdered activated carbon (PAC) where they are cometabolically transformed into a combination of end products, including new biomass, carbon dioxide, inorganic salts, and various acids. 

Reductive biotransformation of a contaminant can occur when the contaminant serves as the terminal electron acceptor. Many contaminants that are recalcitrant to bio-oxidation will undergo reductive biotransformations. These biotransformations can lead to detoxification, mineralization, or changes in the mobility of the targeted contaminant. Hexavalent chromium and tetra-chloroethene (PCE) have been investigated as candidates for reductive biotransformation. This technology may be most applicable for in situ remediation for the following scenarios PCE contamination, low-yield aquifers, areas contaminated by both alkylbenzenes and chlorinated ethenes, and deep aquifer contamination. 

We should point out, however, that depending on the relative importance of the various reactions, kohs may not be a simple function of pH and temperature, and that product formation may strongly depend on these two variables. Furthermore, we note that many environmentally important organic compounds exhibit halogen atoms bound to a carbon-carbon double bond, be it an olefinic (e.g., chlorinated ethenes) or an aromatic (e.g., chlorinated benzenes, PCBs) system. In many cases, under environmental conditions, these carbon-halogen bonds undergo SN or E reactions at extremely slow rates, and we therefore may consider these reactions to be unimportant. 

In our second example we consider the reduction of chlorinated ethenes including the prominent solvents tetrachloroethene (perchloroethylene, PCE) and trichloro-ethene (TCE). An overview of the hypothesized reaction sequence for reduction of these compounds by zero-valent iron (Fe(0) has been constructed (Fig. 14.15 Arnold and Roberts, 2000). Identical or very similar reaction schemes have been... 

Figure 14.15 Hypothesized reaction sequence for reduction of chlorinated ethenes and related compounds by Fe(0). Adapted from Arnold and Roberts (2000). Abbreviations are PCE (tetrachlo-roethene), TCE (trichloroethene), and DCE (dichloroethene).

Figure 14.17 Reduction of chlorinated ethenes (for structures see Fig. 14.15) at a nickel electrode and by two zero-valent metals [Fe(0), Zn(0)]. Decadic logarithms of the relative overall reduction rates plotted (a) against / 0.059 V (analogous to Eq. 14-38 E H values from Arnold and Roberts, 1998), and (b) against the C-Cl bond energy (DR X) divided by 2.3 RT (Dr.x values from Perlinger et al., 2000). The absolute surface-normalized second-order rate constants for PCE are 3 x 10-3 L m 2 s I (Ni-electrode at -1.0 V Liu et al., 2000), 6 x 10-7 L-nr2 s 1 (Fe(0) average value reported by Scherer et al., 1998), and 8 x 10 5 L - nr2 s 1 (Zn(0) Arnold and Roberts, 1998).

In our second example we look at the reduction of chlorinated ethenes at a nickel electrode and at the surfaces of two zero-valent metals [Fe(0), Zn(0)]. To gain insight into the rate-limiting process(es) in these cases, we consider how the relative overall reduction rates (relative to PCE) of PCE, TCE, and the three DCE isomers (see Fig. 14.15 for structures) vary as a function of two common descriptors used in QSARs, the one-electron reduction potential (EJ Fig- 14.17a) and the bond dissociation energy (DR X Fig. 14.176). In all these systems, the reduction rates were found to be significantly slower than diffusion of the compounds to the respective surfaces. Therefore, the large differences in the relative reactivities of the compounds between the systems reflect differences in the actual reaction at the metal surface. 

Haston, Z. C., and P. L. McCarty, Chlorinated ethene half-velocity coefficient (Ks) for reductive dechlorination , Environ. Sci. Technol., 33, 223-226 (1999). 

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