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Original- "Reductive dechlorination"

Biological[edit]

In a biological context chlorine behaves similarly to other atoms in the halogen chemical series, and thus reductive dechlorination can be considered to fall within a somewhat broader class of biological reactions known as reductive dehalogenation reactions, in which the removal of a halogen substituent from an organic molecule occurs with a simultaneous addition of electrons to the molecule. This can be further subdivided into two types of reaction processes, the first of which, hydrogenolysis, is the replacement of the halogen atom with a hydrogen atom. The second, vicinal reduction (sometimes called, dihaloelimination), involves the removal of two halogen atoms that are adjacent on the same alkane or alkene molecule, leading to the formation of an additional carbon-carbon bond.[1]

Biological reductive dechlorination is often catalyzed by certain species of bacteria. Sometimes the bacterial species are highly specialized for organochlorine respiration and even a particular electron donor, as in the case of Dehalococcoides and Dehalobacter. In other examples, such as Anaeromyxobacter, bacteria have been isolated that are capable of using a variety of electron donors and acceptors, with a subset of possible electron acceptors being organochlorines.[2] These reactions depend on a molecule which tends to be very aggressively sought after by some microbes, vitamin B12.[3]

In many instances, microbiological reductive dechlorination of chlorinated organic molecules is important for bioremediation of polluted groundwater. One particularly important example for public health[4] is the organochloride respiration of the dry-cleaning solvent, tetrachloroethylene (PCE), and the engine degreasing solvent trichloroethylene (TCE) by naturally occurring anaerobic bacteria, often members of the candidate genera Dehalococcoides. Bioremediation of these chloroethenes can occur when other microorganisms at the contaminated site provide H2 as a natural byproduct of various fermentation reactions. The dechlorinating bacteria use this H2 as their electron donor, ultimately replacing chlorine atoms in the chloroethenes with hydrogen atoms via hydrogenolytic reductive dechlorination. If the soil and groundwater contain enough organic electron donor and the appropriate strains of Dehalococcoides, this process can proceed until all of the chlorine atoms are removed, and TCE is dechlorinated completely via dichloroethene (DCE) and vinyl chloride (VC) to ethene, a harmless end-product.[5]

  1. ^ Mohn and Tiedje. Microbial reductive dehalogenation. Microbiol Rev (1992) vol. 56 (3) pp. 482-507 PMID 1406492
  2. ^ Smidt and de Vos. Anaerobic microbial dehalogenation. Annu Rev Microbiol (2004) vol. 58 pp. 43-73 PMID 15487929
  3. ^ https://motherboard.vice.com/en_us/article/with-help-from-bacteria-biochemists-learn-how-to-break-up-environmental-toxins
  4. ^ Kielhorn et al. Vinyl chloride: still a cause for concern. Environ Health Perspect (2000) vol. 108 (7) pp. 579-88 PMID 10905993
  5. ^ McCarty. Breathing with chlorinated solvents. Science (1997) vol. 276 (5318) pp. 1521-2 PMID 9190688


Edit- "Reductive dechlorination"

Biological[edit]

In a biological context chlorine behaves similarly to other atoms in the halogen chemical series, and thus reductive dechlorination can be considered to fall within a somewhat broader class of biological reactions known as reductive dehalogenation reactions, in which the removal of a halogen substituent from an organic molecule occurs with a simultaneous addition of electrons to the molecule. This can be further subdivided into two types of reaction processes, the first of which, hydrogenolysis, is the replacement of the halogen atom with a hydrogen atom. The second, vicinal reduction (sometimes called, dihaloelimination), involves the removal of two halogen atoms that are adjacent on the same alkane or alkene molecule, leading to the formation of an additional carbon-carbon bond.[1]

Biological reductive dechlorination is often catalyzed by certain species of bacteria. Sometimes the bacterial species are highly specialized for organochlorine respiration and even a particular electron donor, as in the case of Dehalococcoides and Dehalobacter. In other examples, such as Anaeromyxobacter, bacteria have been isolated that are capable of using a variety of electron donors and acceptors, with a subset of possible electron acceptors being organochlorines.[2] These reactions depend on a molecule which tends to be very aggressively sought after by some microbes, vitamin B12.[3]

Bioremediation using reductive dechlorination

In many instances, microbiological reductive dechlorination of chlorinated organic molecules is important for bioremediation of polluted groundwater. One particularly important example for public health[4] is the organochloride respiration of the dry-cleaning solvent, tetrachloroethylene (PCE), and the engine degreasing solvent trichloroethylene (TCE) by naturally occurring anaerobic bacteria, often members of the candidate genera Dehalococcoides. Bioremediation of these chloroethenes can occur when other microorganisms at the contaminated site provide H2 as a natural byproduct of various fermentation reactions. The dechlorinating bacteria use this H2 as their electron donor, ultimately replacing chlorine atoms in the chloroethenes with hydrogen atoms via hydrogenolytic reductive dechlorination. If the soil and groundwater contain enough organic electron donor and the appropriate strains of Dehalococcoides, this process can proceed until all of the chlorine atoms are removed, and TCE is dechlorinated completely via dichloroethene (DCE) and vinyl chloride (VC) to ethene, a harmless end-product.[5]

Additionally, reductive dechlorination can be further used in bioremediation of other toxins such as PCBs and CFCs. The reductive dechlorination of PCBs is performed by anaerobic microorganisms that utilize the PCB as an electron sink. The result of this is the reduction of the "meta" site, followed by the "para" site, and finally the "ortho" site, leading to a dechlorinated product.[6][7][8] Under experimental conditions, microorganisms undergoing reductive dechlorination in the Hudson River have shown to remove 53% of the total chlorine levels following 16 weeks. This is accompanied by a 9 fold increase in the proportion of monochlorobiphenyls and dichlorobiphenyls which are less toxic and more easily degradable by aerobic organisms compared to their chlorinated counterparts.[8] The prominent drawback that has prevented the widespread use of reductive dechlorination for PCB detoxification and has decreased its feasibility is the issue of the slower than desired dechlorination rates.[7] However, recently, it has been suggested that bioaugmentation with DF-1 can lead to enhanced reductive dechlorination rates of PCBs through stimulation of dechlorination. Additionally, high inorganic carbon levels do not affect dechlorination rates in low PCB concentration environments.[6]

Another potent toxin that can possibly be bioremediated using reductive dechlorination is CFCs.[9] Reductive dechlorination of CFCs including CFC-11, CFC-113, chlorotrifluoroethene, CFC-12, HCFC-141b, and tetrachloroethene occur through hydrogenolysis. Reduction rates of CFC mirror theoretical rates calculated based on the Marcus theory of electron transfer rate.[10] 

- Milad5858 (talk) 06:45, 9 October 2017 (UTC)

  1. ^ Cite error: The named reference :0 was invoked but never defined (see the help page).
  2. ^ Cite error: The named reference :1 was invoked but never defined (see the help page).
  3. ^ Cite error: The named reference :2 was invoked but never defined (see the help page).
  4. ^ Cite error: The named reference :3 was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference :4 was invoked but never defined (see the help page).
  6. ^ a b Payne, Rayford B.; May, Harold D.; Sowers, Kevin R. (2011-10-15). "Enhanced Reductive Dechlorination of Polychlorinated Biphenyl Impacted Sediment by Bioaugmentation with a Dehalorespiring Bacterium". Environmental Science & Technology. 45 (20): 8772–8779. doi:10.1021/es201553c. ISSN 0013-936X.
  7. ^ a b Tiedje, James M.; Quensen, John F.; Chee-Sanford, Joann; Schimel, Joshua P.; Boyd, Stephen A. (1994). "Microbial reductive dechlorination of PCBs". Biodegradation. 4 (4): 231–240. doi:10.1007/BF00695971.
  8. ^ a b QUENSEN, J. F.; TIEDJE, J. M.; BOYD, S. A. (4 November 1988). "Reductive Dechlorination of Polychlorinated Biphenyls by Anaerobic Microorganisms from Sediments". Science. 242 (4879): 752–754. doi:10.1126/science.242.4879.752.
  9. ^ Lovley, Derek R.; Woodward, Joan C. (1992-05-01). "Consumption of Freons CFC-11 and CFC-12 by anaerobic sediments and soils". Environmental Science & Technology. 26 (5): 925–929. doi:10.1021/es00029a009. ISSN 0013-936X.
  10. ^ Balsiger, Christian; Holliger, Christof; Höhener, Patrick. "Reductive dechlorination of chlorofluorocarbons and hydrochlorofluorocarbons in sewage sludge and aquifer sediment microcosms". Chemosphere. 61 (3): 361–373. doi:10.1016/j.chemosphere.2005.02.087.
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