In organic chemistry, oxidation-reduction (redox) refers to the transfer of atoms rather than direct electrons as is the case of inorganic chemistry. Oxidation of an organic compound frequently involves a gain in oxygen and a loss in hydrogen atoms, and the reduction involves a gain in hydrogen and a loss in oxygen content. Oxidation-reduction reactions greatly affect contaminant transport and are usually closely related to the microbial activity and the type of substrates available to the organisms. Organic contaminants provide the reducing equivalents for the microbes. After the oxygen in the subsurface environment is depleted, the most easily reduced materials begin to react and, along with the reduced product, dictate the dominant potential.

The occurrence of oxidation in the subsurface is a function of the electrical potential in the reacting system (Dragun and Helling 1985). For oxidation to occur, the potential of the soil system must be greater than that of the organic chemical. Soil reduction potentials can be generally classified as follows:

Aerated soils:

Moderately reduced soils: +0.4 to +0.1 volts Reduced soils: +0.1 to -0.1 volts

Highly reduced soils:

A number of organic chemicals can hydrolyze, oxidize, and reduce quickly and sometimes violently upon contact with groundwater. Table 9.12.2 lists several classes of organic chemicals that react rapidly and violently with groundwater.

The hydrolysis, oxidation, or reduction of one organic chemical usually results in the synthesis of one or more new organic chemicals. Organic chemistry textbooks identify the basic reaction products. Soil minerals can significantly influence the chemical structure of reaction products. In addition, certain organic chemicals can form significant amounts of residues that bind to the soil. Examples of such chemicals include anilines, phenols, tri-azines, urea herbicides, carbamates, organophosphates, and cyclodiene insecticides (Sax 1984).

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