Chlorinated Hydrocarbons

Chlorinated hydrocarbons (CHCs) are a group of organic molecules that consist of chlorine, carbon, and hydrogen atoms. While some CHCs occur naturally, many of these compounds are synthesized for industrial purposes or are formed as disinfection by-products (DBPs). As the result of accidental spills and improper disposal methods, CHCs are among the most frequently detected hazardous chemicals at abandoned waste sites, in refuse disposal areas, and in industrial and municipal wastewaters. These compounds persist in the environment, and many bioaccumulate in the food chain. Some CHCs form separate, dense nonaqueous-phase liquids (DNAPL) with very low solubilities in water. However, the solubilities of DNAPL CHCs are orders of magnitude greater than their typical drinking water limits, making DNAPL CHCs long-term sources of groundwater contamination. While the human and environmental health risks of chlorinated hydrocarbons are compound specific, exposure to most CHCs has been linked to cancer. Studies continue to develop environmental remediation techniques for individual CHCs.

There are three major types of CHCs: aliphatic, aromatic, and heterocyclic. Aliphatic CHCs are those in which chlorine atom(s) are bonded to a straight or branched carbon chain. Some of the more well-known aliphatic CHCs are trihalomethanes (THMs) such as chloroform (CHCl3; Figure 1), chlorinated methanes such as dichloromethane (DCM; CH2Cl2), and chlorinated ethenes such as trichloroethene (TCE; C2HCl3) (Figure 2) and vinyl chloride (VC; C2H3Cl).

A variety of aliphatic CHCs, including chloroform, have been isolated from marine algae, although the majority are synthetically derived for human use. Historically, chloroform and TCE were used as medical anesthetics; however, they have been subsequently replaced due to their toxicity. Currently, these compounds and other aliphatic CHCs such as DCM are used as industrial solvents for waxes, oils, and grease because they dissolve many organic and inorganic compounds. When these compounds enter the subsurface, they become long-term sources of groundwater contamination. Aliphatic CHCs, THM, and haloacetic acids such as dichloroacetic acid (DCA; CHCl2COOH) (Figure 3) and trichloroacetic acid (CCl3COOH) are formed during chlorination of drinking water that contains dissolved organic matter.

The United States Environmental Protection Agency (USEPA) set MCLs for DBPs in drinking water in the micrograms-per-liter concentration range due to the increased risk of cancer from ingestion of water contaminated with DBPs. Many aliphatic CHCs are amendable to natural or enhanced bioremediation through microbial reductive dechlorination or oxidation pathways.

Aromatic CHCs are hydrocarbons that have chlorine atom(s) bonded to benzene ring(s) made up of six carbon atoms with double bonds between alternate carbon atoms. The aromatic CHCs of current environmental concern are chlorinated benzenes, chlorinated phenols, and polychlorinated biphenyls (PCBs). Chlorinated benzenes are used as solvents and pesticides. Similar to aliphatic CHCs, chlorinated benzenes have been found in abandoned waste sites and in many waste waters and leachates. Chlorobenzene (C6H5Cl) (Figure 4), dichlorobenzene (C6H4Cl2), and trichlorobenzene (C6H3Cl3) are DNAPLs and are therefore long-term sources of groundwater contamination.

Chlorobenzenes (CBs) are amendable to bioremediation through microbial biodegradation. Chlorinated phenols have been used as wood preservatives. For example, pentachlorophenol (PCP; C6HCl5O) (Figure 5) is used as a fungicide and to pressure-treat wood.

PCP has been detected in surface waters and sediments, rainwater, drinking water, aquatic organisms, soil, and food. PCP has been found to be degraded abiotically through photo-decomposition and biologically by aerobic and anaerobic microorganisms. Polychlorinated biphenyls (PCBs) are widely used as coolants, plasticizers, solvents, and hydraulic fluids. There are 209 possible congeners of PCBs. The United States banned the manufacturing of PCBs in [Page 398]1977. However, PCBs are very stable compounds and are still found in the environment today. While some PCB congeners have been shown to be degraded by microorganisms, these compounds are generally highly resistant to biodegradation. The pesticide dichloro-diphenyl-trichloroethane (DDT; C14H9Cl5) (Figure 6) is a chlorinated aromatic compound.

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