Superfund Research Program
Anaerobic Transformations and Bioremediation of Chlorinated Solvents
Chlorinated aliphatic compounds, particularly chlorinated solvents, have been widely used and inadvertently or casually released into the environment for many years. In fact, the chlorinated solvents are among the most common groundwater contaminants at Superfund sites. These compounds are used, for example, as industrial solvents, degreasing agents and chemical process intermediates. They include such compounds as carbon tetrachloride (CT), tetrachloroethane (TeCA), perchloroethylene (PCE), trichloroethylene (TCE), and trichloroethane (TCA). Pentachlorophenol (PCP), a chlorinated aromatic compound, has been widely used as a wood preservative. When released into the environment, these compounds can physically and chemically interact, form non-aqueous phases, flow downward into the subsurface, sorb strongly to soil organics and minerals, and dissolve into groundwater.
The chlorinated compounds are toxic. Many are known or suspected human carcinogens. Accordingly, clean-up standards for drinking water are very stringent, especially in the U.S. While these compounds are often resistant to natural degradation, under favorable conditions they can be transformed and degraded through microbially-mediated processes. There is great interest in understanding the transformations of these compounds in situ and in manipulating these systems to achieve more complete remediation.
Researchers at the University of Washington are studying several aspects of the aerobic and anaerobic biodegradation of chlorinated aliphatics and are developing models that will help develop in situ treatment technologies. Particular compounds of study include those listed above, and many others including certain common degradation products which result from the incomplete transformation processes often seen in the natural environment. Researchers have identified some of the conditions which can promote the biodegradation of chlorinated aliphatic contaminants.
This research brief describes the work with an important, interesting and novel category of transformations, namely those that occur in anaerobic microbial communities. Project investigators have shown that the presence of hydrogen, that can serve as an electron donor, is a key determinant in the level of activity of the anaerobic dehalogenating bacteria. In laboratory experiments, degradation reactions and transformations proceed very slowly unless hydrogen is introduced either directly or through fermentation of some component within the reaction vessel. For example, the rate of PCP dechlorination was shown to be related to the type of electron donor provided and the loading of external electron supply.
These investigations, and others, have provided the basis for development of applications of this technology to assist clean-up efforts at contaminated Superfund sites. For example, the investigators have developed a mixed bacterial culture that can dechlorinate PCE to ethene. They have determined the kinetics of each dechlorination step and the optimum hydrogen concentration (the principal electron donor) for the dechlorination. This kinetic model has been combined with a flow and transport model to predict the effects of treatment trenches for in situ transformation of PCE and TCE.
There is abundant evidence from contaminated sites that the complete transformation of chlorinated aliphatics is not usually seen. Investigators have concluded that complete conversion--when it is found--probably depends on the availability of an electron donor source to perform the key functions of 1) supporting growth of methanogenic or sulfate-reducing microbial communities that produce strongly reducing conditions and 2) providing hydrogen or other direct electron donor for use by the dehalogenating bacteria.
The key role of electron donor in anaerobic bioremediation points to the importance of understanding fermentation reactions, as well as dehalogenation reactions. These findings also suggest consideration of remediation strategies that include biostimulation of contaminated aquifers with electron donor supplies. Potentially, this could result in stimulating complete conversions of toxic compounds to innocuous ones, minimizing the use of costly reagents, and avoiding excessive growth of bacteria and clogging of the aquifer.
A subsequent research brief will describe related work with aerobic transformations of chlorinated compounds.
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To learn more about this research, please refer to the following sources:
- Ferguson J, Pietari JM. 2000. Anaerobic transformations and bioremediation of chlorinated solvents. Environ Pollut 107(2):209-215. PMID:15092997
- Zou S, Anders KM, Ferguson J. 2000. Biostimulation and bioaugmentation of anaerobic pentachlorophenol degradation in contaminated soils. Bioremediat J 4(1):19-25.
- Chen CY, Ballapragada BS, Puhakka JA, Strand SE, Ferguson J. 1999. Anaerobic transformation of 1,1,1-trichloroethane by municipal digester sludge. Biodegradation 10(4):297-305. PMID:10633545
- Mohn H, Puhakka JA, Ferguson J. 1999. Effects of electron donors on degradataion of pentachlorophenol in a methanogenic fluidized bed reactor. Environ Technol 20:909-920.
- Zou S, Anders KM, Ferguson J. 1999. Pentachlorophenol dechlorination in fluidized bed reactors under methanogenic conditions. Bioremediat J 3(2):93-104.
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