Reductive dechlorination

Reductive dechlorination is degradation of chlorinated organic compounds by chemical reduction with release of inorganic chloride ions by reductive dehalogenases.

Biological

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.[4][5] One particularly important example for public health[6] 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.[7]

Recently, a chloroform-degrading reductive dehalogenase enzyme has been reported in a Dehalobacter member. The chloroform reductive dehalogenase, termed TmrA, was found to be transcriptional up-regulated in response to chloroform respiration[8] and the enzyme can be obtained both in native[9] and recombinant forms.[10]

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.[11][12][13] 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.[13] 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.[12] 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.[11]

Another potent toxin that can possibly be bioremediated using reductive dechlorination is CFCs.[14] 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.[15]

Electrochemical

The electrochemical reduction of chlorinated chemicals such as chlorinated hydrocarbons and chlorofluorocarbons (CFCs) can be carried out by electrolysis in appropriate solvents, such as mixtures of water and alcohol. Some of the key components of an electrolytic cell are types of electrodes, electrolyte mediums, and use of mediators. The cathode transfers electrons to the molecule, which decomposes to produce the corresponding hydrocarbon (hydrogen atoms substitute the original chlorine atoms) and free chloride ions. For instance, the reductive dechlorination of CFCs is complete and produces several HFCs plus chloride.

Hydrodechlorination (HDC) is a type of reductive dechlorination that is useful due to its high reaction rate. It uses H2 as the reducing agent over a range of potential electrode reactors and catalysts.[16] Amongst the types of catalysts studied such as precious metals (Pt, Pd, Rh), transition metals (Ni and Mo), and metal oxides, a preference for precious metals overrides the others.As an example, palladium (Pd) often adopts a lattice formation which can easily embed hydrogen gas making it more accessible to be readily oxidized.[17] However a common issue for HDC is catalyst deactivation and regeneration. As catalysts are depleted, chlorine poisoning on surfaces can sometimes be observed, and on rare occasions, metal sintering and leaching occurs as a result.[18]

Electrochemical reduction can be performed at ambient pressure and temperature.[19] This will not disrupt microbial environments or raise extra cost for remediation. The process of dechlorination can be highly controlled to avoid toxic chlorinated intermediates and byproducts such as dioxins from incineration. Trichloroethylene (TCE) and perchloroethylene (PCE) are common targets of treatment which are directly converted to environmentally benign products. Chlorinated alkenes and alkanes are converted to hydrogen chloride (HCl) which is then neutralized with a base.[18] However, even though there are many potential benefits to adopting this method, research have mainly been conducted in a laboratory setting with a few cases of field studies making it not yet well established.

Radiation

It is known that by gamma irradiation of PCBs that they can be converted into biphenyl and inorganic chloride, this is formally a reduction of the organic compound as hydrogen is added. See the Polychlorinated biphenyl page for more details of this destruction method. This reductive destruction method works for many organochlorine compounds, for instance carbon tetrachloride when irradiated tends to form chloroform and chloride anions.

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References
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