Factors Influencing Biodgradation

The primary factor determining the ability of microorganisms to degrade an XOC, as well as the kinetics of that biodégradation, is its molecular structure. The closer that structure is to the structure of a biogenic compound, the easier the XOC will be to biodegrade because the more readily it will fit into common metabolic pathways. Xenophores are substituents on organic molecules that are physiologically uncommon or entirely nonphysiological,1 and their presence is one factor that can make a compound xenobiotic. Because they are alien to most organisms, they hinder the functioning of many enzymes and it is only the nonspecificity of those enzymes that allows them to function in the presence of xenophores. The nature, number, and position of xenophores all influence the biodegradability of an XOC. Halogens, nitro groups, and cyanide groups are typical xenophores and all reduce biodegradability in comparison to the unsubstituted compound. Furthermore, the greater the number of xenophores, the less susceptible the XOC is to biodégradation. However, it is difficult to generalize about the effect of the position of a xenophore. A given xe-nophore in one position may have little impact, whereas in another its effect may be large. Similarly, one xenophore in a given position may have a strong effect, whereas another xenophore in the same position may have none. Because of these widely diverse effects, there is strong interest in the development of structure-biodegrada-bility relationships that can be used to deduce biodegradability from a compound's molecular structure.1 It must be recognized, however, that the knowledge base for such relationships is still limited, thereby restricting their utility at this time. Prediction of biodegradability is a new science and much is still to be learned.

With a few exceptions, microorganisms are thought to degrade only organic compounds that are dissolved in the aqueous phase.1 Furthermore, as seen in Chapter 3, the rate at which bacteria grow on a substrate is a function of its liquid phase concentration. This means that solubility has a profound effect on the biodegradability of any organic compound, whether xenobiotic or biogenic. Many xenophores reduce aqueous solubility, thereby reducing bioavailability and the rate of biodégradation. Furthermore, if a compound has very low solubility, it may be difficult to induce the enzymes required for its biodégradation.1

Finally, the environment has a strong effect on biodegradability. In addition to the usual effects of pH, temperature, and the availability of nutrients and electron acceptor, the presence or absence of molecular oxygen can have a strong effect. For example, some enzymatic steps, such as those carried out by oxygenase enzymes, require the presence of molecular oxygen, whereas others, such as reductive dehal-ogenation, require its absence. The engineer must be knowledgeable about the nature of the potential pathways for biodégradation of a given XOC so that the appropriate environment can be provided. Failure to do so will result in an inadequate system that cannot meet effluent goals.

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