From: Public Services and Procurement Canada
Reductive dechlorination is a biologically based in situ remediation technology used for the treatment of soil and groundwater contaminated with chlorinated organic compounds. This technology consists of anaerobic biodegradation of chlorinated compounds. The implementation of this technology requires the injection of an organic substrate (electron donor) into the contaminated media to stimulate microbial growth and generate hydrogen through fermentation reactions. Nutrients sometimes must also be injected in order to meet nitrogen and phosphorus requirements of the microorganisms involved.
During reductive dechlorination, the chlorine(s) on the contaminant is (are) replaced by a hydrogen molecule. The hydrogen molecule is generated by the fermentation of a naturally occurring or of an imported electron donor, including lactate, acetate, vegetable oil, methanol and molasses. Hydrogen can also be supplied directly.
The reaction may be performed using chlorinated organic compounds (the contaminant) as either a primary substrate (dehalorespiration) or by cometabolism. When the chlorinated organic compound is the primary substrate, it is used as an electron acceptor to generate energy and act as a carbon source.
Reductive dechlorination by cometabolism is the reduction of a chlorinated organic compound by an enzyme or co-factor produced during the metabolism or another substrate. Since the contaminant reduction reaction doesn’t directly benefit microorganisms, a primary substrate must be present in the environment as a carbon and energy source. As contaminant is not the stimulator of microbial activity, cometabolism allows the remediation of low contaminant concentrations, thus achieving undetectable concentrations.
The transformation of chlorinated organic compounds by cometabolism is more common and is more effective in the transformation of highly chlorinated organic compounds, which explains why the resulting less-chlorinated compounds tend to accumulate in the contaminated matrix. Dehalorespiration is more effective in the biodegradation of less-chlorinated organic compounds. In both cases, reductive dechlorination produce intermediate metabolites (less chlorinated compounds) and a localized increase in chloride concentrations.
Projects implementation may include:
Injected materials vary widely according to contaminant, general groundwater composition and practitioner. Common generic compounds include urea (as a nitrogen source), ammonium nitrate (as a nitrogen source), lactate, acetate, methanol, ethanol, vegetable oils, etc.
Notes:
Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.
Reductive dechlorination, using campaign-based injection is potentially applicable to remote northern sites where impediments to material transport and injection equipment mobilization can be overcome. Extreme cold can hamper biodegradation and volatilization in shallow material, but deep soil temperatures (below permafrost) are relatively constant over the course of the year.
Follow-up monitoring may be required to verify that the remediation objectives as well as applicable regulations are met once the groundwater system normalizes; after stimulation is withdrawn and excess biomass dies off.
Reductive dechlorination of certain chlorinated aliphatic hydrocarbons could produce metabolites of greater concern than the parent compound. The formation of chloroethane or vinyl chloride may warrant the use of an aerobic biostimulation step. Bench scale and/or pilot testing, as well as strict quality control for injected materials, are typically required.
Bioaugmentation may be required if there is an insufficient number of dehalogenators present. Fracturing injection methods or soil mixing could enhance substrate delivery in the subsurface.
Because by-products and metabolites produced during reductive dechlorination are in general more easily biodegraded under aerobic conditions (e.g. chloroethane or vinyl chloride), aerobic biostimulation sometimes follows reductive dechlorination.
Application examples are available at these addresses:
The treatment time for site remediation using reductive dechlorination varies according to the contaminant types and properties, the indigenous microbial population and the physical and chemical characteristics of the site.
A large variety of different substrate can be used including proprietary compounds. Performance can vary significantly with the type of substrate used.
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Composed by : Karine Drouin, M.Sc., National Research Council
Updated by : Karine Drouin, M.Sc., National Research Council
Updated Date : April 1, 2008
Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée
Updated Date : March 31, 2018
