Reductive Dehalogenation of Chlorinated Organics in Anaerobic SedimentsEPA Grant Number: U915194
Title: Reductive Dehalogenation of Chlorinated Organics in Anaerobic Sediments
Investigators: Ridgway, Robin M.
Institution: Purdue University
EPA Project Officer: Carleton, James N
Project Period: January 1, 1997 through January 1, 2001
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1997) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Engineering and Environmental Chemistry , Fellowship - Civil/Environmental Engineering
The objective of this research project is to offer another possible explanation as to why anaerobic cultures have shown the ability to reductively dechlorinate organic pollutants. Bioremediation has the potential to offer the field of environmental restoration an effective and cost-efficient method for reducing the environmental and human health hazards of chlorinated organic pollutants such as polychlorinated biphenyls, chlorinated ethenes, or chlorinated ethanes. The bulk of anaerobic microbial degradation of chlorinated compounds is through sequential dehalogenation, or a step-wise removal of chlorines from the compound with subsequent mineralization of the remaining organic portion. The primary route of anaerobic dechlorination is via reductive dehalogenation. The carbon in chlorinated organic compounds is usually highly oxidized, particularly in compounds with many chlorines. This makes highly chlorinated compounds thermodynamically more amenable to reductive degradation processes than to oxidative processes. Many mixed cultures have been shown to have dechlorinating capabilities, but the reason why anaerobic microorganisms reductively dechlorinate has yet to be determined.
Many environments contaminated with chlorinated and nonchlorinated organic pollutants are anaerobic and extremely reducing (due to high biochemical oxygen demand loading), such as river sediments and some aquifers. These environments also tend to have normal biological electron acceptors in short supply. In these environments, fermenting organisms predominate, and this is one reason that fermenting organisms may be favorable candidates for examination for their dechlorinating abilities. In addition, fermenters could potentially benefit from dechlorination reactions by utilizing oxidized chlorinated compounds (as an electron sink) as an alternative to hydrogen production, for example. The electron sink would function to redistribute the microorganism's fermentation balance so that the balance becomes more energetically favorable than fermentation without an external electron sink (Schink, 1991). This approach views reductive dechlorination as almost a cometabolic process in which the microorganism benefits energetically from the reaction, but the chlorinated (halogenated) compounds would not be required for growth. This scheme would explain the past difficulties in culturing anaerobes capable of mediating reductive dechlorination, because halogenated compounds cannot be used in selective culturing if they are not a required growth substrate.