Unveiling the anaerobic bioremediation of chlorinated solvents
Chlorinated compounds have been 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.
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 and often resistant to natural degradation. However, under favorable conditions they transform and degrade through microbially mediated processes.
There therefore is great interest in understanding the transformations of these compounds in situ and in manipulating these systems to achieve more complete remediation.
Removing the Mystery Surrounding Chlorinated Compound Degradation
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. Their work has identified some of the conditions that can promote the biodegradation of chlorinated aliphatic contaminants.
The researchers, for example, have shown that the presence of hydrogen, which 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. In particular, the researchers showed that the rate of PCP (pentachlorophenol, a wood preservative) dechlorination is related to the type of electron donor provided and the loading of external electron supply.
Applying Lab Data to Field Conditions
These investigations, and others, have provided the basis for the 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 (perchloroethylene) 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 (trichloroethylene).
There is abundant evidence from contaminated sites that the complete transformation of chlorinated aliphatics usually doesn't occur. 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:
- supporting growth of methanogenic or sulfate-reducing microbial communities that produce strongly reducing conditions, and
- providing hydrogen or other direct electron donor for use by the dehalogenating bacteria.
Pointing to the Future
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, the research at the University of Washington* 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.
Contact: The University of Washington, John F. Ferguson, PI, Box 352700, Seattle, WA 98195-2700, Tel: 206-543-5176, Fax: 206-685-9185, or e-mail jferg@u.washington.edu.
* The National Institute of Environmental Health Sciences (NIEHS) Superfund Basic Research Program supports this research.
The previous case study was excerpted and adapted from "Anaerobic transformations and bioremediation of chlorinated solvents," a version of SBRP "Research Brief—Number 69," edited and posted by Managing Editor Kate Goff to Solid Waste Online, a companion site to Pollution Online in the VerticalNet e-commerce family.