From Ground Water Currents, September 1996, Issue No. 16

Chlorinated Solvents Biodegradation

By John Wilson, National Risk Management Research Laboratory, Ada, OK

The environmental chemistry of a site in part determines the rate of biodegradation of chlorinated solvents at that site. The initial metabolism of chlorinated solvents such as tetrachloroethylene, trichloroethylene and carbon tetrachloride in ground water usually involves a biochemical process described as sequential reductive dechlorination. The occurrence of different types and concentrations of electron donors such as native organic matter, and electron acceptors such as oxygen and chlorinated solvents, determines to a large degree the extent to which reductive dechlorination occurs during the natural attenuation of a site. For instance, reductive dechlorination only occurs in the absence of oxygen; and, the chlorinated solvent actually substitutes for oxygen in the physiology of the microorganisms carrying out the process. As a result of the use of the chlorinated solvent during this physiological process it is at least in part dechlorinated.

The chemical term "reduction" was originally derived from the chemistry of smelting metal ores. Ores are chemical compounds of metal atoms coupled with other materials. As the ores are smelted to the pure element, the weight of the pure metal are reduced compared with the weight of the ore. Chemically, the positively charged metal ions receive electrons to become the electrically neutral pure metal. Chemists generalized the term "reduction" to any chemical reaction that added electrons to an element. In a similar manner, the chemical reaction of pure metals with oxygen results in the removal of electrons from the neutral metal to produce an oxide. Chemists have generalized the term "oxidation" to refer to any chemical reaction that removes electrons from a material. For a material to be reduced, some other material must be oxidized.

The electrons required for microbial reduction of chlorinated solvents in ground water are extracted from native organic matter, from other contaminants such as the benzene, toluene, ethylene and xylene compounds released from fuel spills, from volatile fatty acids in landfill leachate or from hydrogen produced by the fermentation of these materials. The electrons pass through a complex series of biochemical reactions that support the growth and function of the microorganisms that carry out the process.

To function, the microorganisms must pass the electrons used in their metabolism to some electron acceptor. This ultimate electron acceptor can be dissolved oxygen, dissolved nitrate, oxidized minerals in the aquifer, dissolved sulfate, a dissolved chlorinated solvent or carbon dioxide. Important oxidized minerals used as electron acceptors include iron and manganese. Oxygen is reduced to water, nitrate to nitrogen gas or ammonia, iron (III) or ferric iron to iron (II) or ferrous iron, manganese (IV) to manganese (II), sulfate to sulfide ion, chlorinated solvents to a compound with one less chlorine atom and carbon dioxide to methane. These processes are referred to as aerobic respiration, nitrate reduction, iron and manganese reduction, sulfate reduction, reductive dechlorination and methanogenesis, respectively.

The energy gained by the microorganisms follows the sequence listed above: oxygen and nitrate reduction provide a good deal of energy, iron and manganese reduction somewhat less energy, sulfate reduction and dechlorination a good deal less energy and methanogenesis a marginal amount of energy. The organisms carrying out the more energetic reactions have a competitive advantage; as a result, they proliferate and exhaust the ultimate electron acceptors in a sequence. Oxygen and then nitrate are removed first. When their supply is exhausted, other organisms are able to proliferate, and manganese and iron reduction begins. If electron donor supply is adequate, then sulfate reduction begins, usually with concomitant iron reduction, followed ultimately by methanogenesis. Ground water where oxygen and nitrate are being consumed is usually referred to as an oxidized environment. Water where sulfate is being consumed and methane is being produced is generally referred to as a reduced environment.

Reductive dechlorination usually occurs under sulfate-reducing and methanogenic conditions. Two electrons are transferred to the chlorinated compound being reduced. A chlorine atom bonded with a carbon receives one of the electrons to become a negatively charged chloride ion. The second electron combines with a proton (hydrogen ion) to become a hydrogen atom that replaces the chlorine atom in the daughter compound. One chlorine at a time is replaced with hydrogen; as a result, each transfer occurs in sequence. As an example, tetrachloroethylene is reduced to trichloroethylene, then any of the three dichloroethylenes, then to monochloroethylene (commonly called vinyl chloride), then to the chlorine-free carbon skeleton ethylene, then finally to ethane.

This article is, for the most part, excerpted from Environmental Chemistry and the Kinetics of Biotransformation of Chlorinated Organic Compounds in Ground Water, John T. Wilson, Donald H. Kampbell and James W. Weaver, Symposium on Natural Attenuation of Chlorinated Organics in Ground Water, pp. 124-127.

To obtain a copy of the full Symposium on Natural Attenuation of Chlorinated Organics in Ground Water (EPA Document No. EPA/540/R-96/590) call Kay Cooper (405-436-8651) at the R. S. Kerr Environmental Research Center or call the Center for Environmental Research Information (CERI) at 513-569-7562. The document will also be available on the EPA's Office of Research and Development (ORD) home page, Bio Site, at: http://www.epa.gov/docs/ord/ in November 1996.