Chlorinated hydrocarbons biotransformation

Biotransformation of monoaromatic and chlorinated hydrocarbons at an aviation gasoline spill site. Geomicrobiology Journal, 8, 233-40. 

Phosgene is widely used as a chemical intermediate. It is used in metallurgy and in the production of pesticides, herbicides, and many other compounds. It is a by-product of chloroform biotransformation and can be generated from some chlorinated hydrocarbon solvents under intense heats. Phosgene has been used as a chemical warfare agent. 

Biotransformation of certain chlorinated hydrocarbon insecticides results in their conversion to metabolites which are less polar than the parent chemical. Heptachlor and aldrin are converted to the more lipophilic compounds heptachlor epoxide and dieldrin, respectively, whereas DDT is converted to DDE. The primary residue of DDT, which persists to the present day in animals and humans after exposure over a decade ago, is DDE. Following biotransformation, these compounds distribute to tissues which are higher in neutral lipid content than are the major organs of metabolism and excretion, the liver and kidney. These lipid-rich tissues are relatively, deficient in the so-called mixed-function oxidase (MFO) enzyme systems necessary for biotransformation of the halogenated hydrocarbons to more polar and thus more easily excreted compounds. As a result, these lipophilic chemicals remain unchanged in adipose tissue with only limited amounts returning to the circulation for possible metabolism and excretion. Paradoxically, aldrin and heptachlor metabolism results in an increased rather than reduced body load. This is opposite of the pattern seen for most other pesticide classes. 

The imide chloride is synthesized in the subsequent step at 0 °C with phosphorous pentachloride. Hydrolysis of this imide chloride yields 7-ACA. By replacement of this synthesis with the biotransformation, the use of heavy-metal salts (ZnCh) and chlorinated hydrocarbons as well as precautions for highly flammable compounds can be circumvented. The off-gas quantities were reduced from 7.5 to 1.0 kg. Mother liquors requiring incineration were reduced from 29 to 0.3 t. Residual zinc that was recovered as Zn(NH4)P04 is reduced from 1.8 to 0 t. The absolute costs of environmental protection are reduced by 90% per tonne of 7-ACA. Asahi Chemical and Toyo Jozo have produced 7-ACA since 1973 with a capacity of 90 t a-1 and Hoechst Marion Roussel since 1996 with a capacity of 2001 a-1. 

Reductive dehalogenation is a mechanism for the anaerobic biotransformation of chlorinated hydrocarbons such as hexachlorobenzene (HCB). In reductive dehalogenation, the halogenated compound serves as the electron acceptor rather than the donor that requires a separate carbon source. In a microbially catalyzed reaction, a halide ion is replaced by a hydrogen ion (Figure 13.7). The removal of halide ions results in compounds that are generally easier to degrade, and, in some instances, are completely mineralized. 

Chlorinated hydrocarbons are the most stable pesticides in the environment. Persistence of a compound is a measure of its resistance to degradation, but no pesticide is considered totally resistant to biological degradation. Table 6.3 lists the systems and metabolites in the biotransformation of chlorinated... 

Skubal K. L., Barcelona M. J., and Adriaens P. (2001) An assessment of natural biotransformation of petroleum hydrocarbons and chlorinated solvents at an aquifer plume transect. J. Contamin. Hydrol. 49(1-2), 151-169. 

Many halogenated hydrocarbons exert toxicity in their native form whereas others are more toxic and exert effects from the intermediates and metabolites formed in the biotransformation for excretion or other purposes. A special group of intermediates or by-products that are formed are different types of very reactive macromolecule and tissue-harming free radicals from carbon, oxygen and chlorine and organic radicals such as CHj", as well as hydrogen peroxide, epoxides and other electrophilic compounds emanating from the oxidative metabolism. 

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