Reductive dechlorination

Reductive dechlorination is a term that is used to describe certain types of degradation of chlorinated pollutants by chemical reduction with release of inorganic chloride ions.


Tetrachloroethylene (PCE) and trichloroethylene (TCE) and other chlorinated aliphatic hydrocarbons as well as aromatic hydrocarbons in groundwater can be biodegraded by naturally occurring anaerobic bacteria, most commonly of the genera dehalococcoides. This occurs when other anaerobic bacteria present in the contaminated site take electrons from organic compounds (the "electron donors") and produce H2. The dechlorinating bacteria use the electrons in the H2 to replace a chlorine atom in TCE/PCE.

If the site soil and groundwater contain organic electron donors, 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 ethylene gas, a harmless end-product. Other solvents, such as 1,1,1-TCA and carbon tetrachloride, can also be degraded by reductive dechlorination.


The electrochemical reduction of chlorinated chemicals such as chlorinated hydrocarbons and clorofluorocarbons (CFCs) can be carried out by electrolysis of these pollutants in apropriate solvents, such as mixtures of water and alcohol. The cathode transfers electrons to the polluting 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 .


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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