Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 149

Biodegradation (transformation of a chemical by microorganisms) is the decay or breakdown of chemicals that occurs when microorganisms use an organic substance as a source of carbon and energy. For example, sewage flows to the wastewater treatment plant where many of the organic compounds are broken down; some compounds are simply biotransformed (changed), others are completely mineralized. These biodegradation processes are essential to recycle wastes so that the elements in them can be used again. Recalcitrant materials, which are hard to break down, may enter the environment as contaminants.

Another term, biotransformation, refers to the conversion of a substance through metabolization, thereby causing an alteration to the substance by biochemical processes in an organism. Metabolism is divided into the two general categories of catabolism, which is the breaking down of more complex molecules, and anabolism, which is the building up of life molecules from simpler chemicals. The substances subjected to biotransformation may be naturally occurring or anthropogenic (made by human activities) which may consist of xenobiotic molecules that are foreign to living systems.

Biodegradation is a microbial process that occurs when all of the nutrients and physical conditions involved are suitable for growth. Temperature is an important variable; keeping a substance frozen can prevent biodegradation. Most biodegradation occurs at temperatures between 10 and 35°C (50 and 95°F), and water is essential for the biodegradation process. Bacteria and fungi, including yeasts and molds, are the microorganisms responsible for biodegradation. The biodegradation of organic matter in the aquatic and terrestrial environments is a crucial environmental process. Some organic pollutants are biocidal; for example, effective fungicides must be antimicrobial in action. Therefore, in addition to killing harmful fungi, fungicides frequently harm beneficial saprophytic fungi (fungi that decompose dead organic matter) and bacteria. Herbicides are designed for plant control, and insecticides are used to control insects.

The biodegradation process can be divided into three stages: (i) biodeterioration, (ii) biofragmentation, and (iii) assimilation. The first stage (biodeterioration) is often described as a surface-level degradation that modifies the chemical, physical, and mechanical properties of the contaminant and occurs when the material is exposed to abiotic factors in the environment and allows for further degradation by weakening the structure of the contaminant. Some abiotic factors that influence these initial changes are compression (mechanical), light, temperature, and chemicals in the environment. While biodeterioration typically occurs as the first stage of biodegradation, it can in some cases occur in be parallel (simultaneously) to biofragmentation which is the conversion of the spilled chemical to lower molecular weight fragments that are more amenable to removal from the environment (or ecosystem). Assimilation occurs when the fragment (or fragments) are assimilated into the environment (or ecosystem) without any deleterious effect to the system.

The resulting products from biofragmentation can be assimilated into microbial cells; this is the assimilation stage. Some of the products from fragmentation are easily transported within the cell by membrane carriers. However, other products of the biofragmentation stage still have to undergo biotransformation reactions to yield products that can then be transported inside the cell. Once inside the cell, the products enter catabolic pathways that either lead to the production of adenosine triphosphate (ATP) or elements of the structure of the cell.

The biodegradation process is, in general, an important process for the removal of chemical compounds (especially organic chemicals) from the environment. The versatility and activity of microbial enzymes as catalysts mean that biodegradation is much more significant than purely chemical reactions such as hydrolyses and redox reactions. Enzymatically catalyzed transformation also occurs in higher organisms, but this process is quantitatively less important than the contribution from microorganisms. Some of the most important microorganism-mediated chemical reactions in aquatic and soil environments are those involving nitrogen compounds and the cycle of such compounds throughout the Earth system. Among the biochemical transformations in the nitrogen cycle are (i) nitrogen fixation, whereby molecular nitrogen is fixed as organic nitrogen, (ii) nitrification, the process of oxidizing ammonia to nitrate, (iii) nitrate reduction in which nitrogen in nitrate ions is reduced to nitrogen in a lower oxidation state, and (iv) denitrification, the reduction of nitrate and nitrite to ammonia.

Physical-chemical and biological treatment processes are employed as for wastewater treatment. In addition, chemicals are introduced for precipitation of nutrients, followed by coagulation and filtration for removing solids remaining after biological treatment. In some cases, granular activated carbon or membrane filtration or a combination of membrane-assisted solvent extraction is used for additional purification of the groundwater streams and waste streams. This higher level of treatment is advisable because of the damage that any visual traces of chemical waste can do to the appearance of the waters. In addition, the treatment may combat the potential eutrophic effect that the nutrients phosphorus and nitrogen can have on a water system.

For the most part, anthropogenic compounds resist biodegradation much more strongly than do naturally occurring compounds. This is generally due to the absence of enzymes that can cause an initial attack on the compound. A number of physical and chemical characteristics of a compound are involved in its amenability to biodegradation. Such characteristics include hydrophobicity, solubility, volatility, and affinity for lipids. Some organic structural groups impart particular resistance to biodegradation. These structural groups include branched carbon chains, ether linkages, meta-substituted benzene rings, chlorine, amines, methoxy groups, sulfonates, and nitro groups.

Usually the products of biodegradation are molecular forms that tend to occur in nature and that are in greater thermodynamic equilibrium with their surroundings than the starting materials. Detoxification refers to the biological conversion of a toxic chemical to a less toxic chemical species.

The biodegradability of a compound is influenced by its physical characteristics, such as solubility in water and vapor pressure, and by its chemical properties, including molecular mass, molecular structure, and presence of various kinds of functional groups, some of which provide a “biochemical handle” for the initiation of biodegradation. With the appropriate organisms and under the right conditions, even substances such as phenol that are considered to be biocidal to most microorganisms can undergo biodegradation.

See also: Biodegradation In Situ, Biodegradation Processes, Biodegradation – Slurry Phase, Biodegradation – Solid Phase.