When marked decline in rate of biodegradation with time is observed, it indicates reduced availability of the substrate. A compound that is slowly degraded has more time to interact with physical and chemical parameters in soils, sediments resulting in alteration of its behavior. The changed material may become highly persistent and is called bound residues. Especially in soils, many insecticides, herbicides, fungicides, and other classes of chemicals undergo such changes, which result in the formation of bound residues. Many of the bound residues are in fact due to complexing of chemicals with humic materials in soils.
Shape and surface architecture of the material to which a toxic compound is adsorbed plays an important role in determining bioavailability. For example, a chemical may not be available for biodegradation if it is deposited or encapsulated in a micropore that is inaccessible to microorganisms. Soils and sediments are composed of particles of various sizes; between these particles are large and small pores. Therefore, the movement of the chemical from micropore to a site containing a bacterium that is able to degrade that chemical is by diffusion. However, if the environment has small particles, the movement of the chemical from the micropore to the bacterium may take much longer. The longer the path, the slower will be the biodegradation. There is some evidence that suggests that organic compounds are protected from microbial attack this way. The low bio-availability of polyaromatic aromatic hydrocarbons (PAHs) in soil has been attributed to their presence in inaccessible sites in soil matrix. The longer some chemicals remain in soil, the more resistant they become to desorption and to degradation.
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