Descriptions of the Major Types of Biological Transformation Processes

Process

Description

Mineralization

The complete conversion of an organic compound to inorganic constituents (water, carbon dioxide). Generally results in complete detoxification unless one of the products is of environmental concern, such as nitrates and sulfides under certain conditions.

Conversion of an organic compound to another organic compound without the microorganism using the compound as a nutrient. Resulting compounds may be as toxic (DDT to DDE or DDD) or less toxic (xylenes to toluic acid).

Conversion of a toxic organic compound to a nontoxic organic compound. The pesticide 2,4-D can be detoxified microbially to 2,4-dichlorophenol.

Conversion of a toxic compound to another toxic compound with similar, increased, or reduced toxicity.

Conversion of a nontoxic molecule to one that is toxic, or a molecule with low potency to one that is more potent. Examples include the formation of the phenoxy herbicide 2,4-D from the corresponding butyrate, formation of nitrosamines, and methylation of arsenicals to trimethylarsine.

Conversion of a compound capable of becoming hazardous to another nonhazardous compound by circumventing the hazardous intermediate. This has been observed in the laboratory, but not identified in the environment. An example is the direct formation of 2,4-dichlorophenol from the corresponding butyrate of 2,4-D.

Cometabolism

Detoxification

Transtoxification Activation

Defusing

Source: U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/ 6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.

molecule or inorganic species. For example, conjugation occurs when microbial processes transform inorganic mercury into dimethyl mercury.

At least 26 oxidative, 7 reductive, and 14 hydrolytic transformations of pesticides had been identified. Detailed identification and discussion of specific reactions can be found in the works of Alexander56 and Scow.57

20.3.4 Transport Processes

Many factors and processes must be considered when evaluating the movement of deep-well-injected hazardous wastes. Four factors are relevant to geochemical characteristics:

1. Hydrodynamic dispersion

2. Osmotic potential

3. Particle migration

4. Density and viscosity

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