Biodegradation chemical transformations

Ribbons DW, RW Eaton (1982) Chemical transformations of aromatic hydrocarbons that support the growth of microorganisms. In Biodegradation and detoxification of environmental pollutants (Ed AM Chakrabarty), pp. 59-84. CRC Press, Boca Raton. 

Degradation is often the result of the combined effect of chemical transformation and biodegradation. For example, the oxidation/reduction of complex hydrocarbons can produce simple compounds such as peroxides, primary alcohols, and monocarbocylic acids. These compounds can then be further degraded by bacteria, leading to the formation of carbon dioxide, water, and new bacterial biomass.19-35... 

Environmental fate Chemicals released in the environment are suscephble to several degradahon pathways, including chemical (i.e., hydrolysis, oxidation, reduction, dealkylahon, dealkoxylation, decarboxylahon, methylation, isomerization, and conjugation), photolysis or photooxidahon and biodegradation. Compounds transformed by one or more of these processes may result in the formation of more toxic or less toxic substances. In addihon, the transformed product(s) will behave differently from the parent compound due to changes in their physicochemical properties. Many researchers focus their attention on transformahon rates rather than the transformahon products. Consequently, only limited data exist on the transitional and resultant end products. Where available, compounds that are transformed into identified products as weh as environmental fate rate constants and/or half-lives are listed. 

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

Chemical and biological reactions, such as hydrolysis, biodegradation, photolysis, and other chemical transformations. The relevant expressions are given by Eqs. 24-8 and 24-23. 

The primary mechanisms of degradation of chemicals in soil, water, sediment, air, and biota environments are classified as biotic (biodegradation, phytodegradation, and respiration) or abiotic (hydrolysis, photolysis, and oxidation/reduction), as shown in Figure 6.7. Biodegradation, the transformation of chemicals by microorganisms, has potential to occur in any environmental compartment that... 

The Deeper Unsaturated Soil The deeper unsaturated soil includes the soil layers below the root zone and above the saturated zone, where all pore spaces are filled with water. This compartment can encompass both the B and the C soil horizons. The soil in this layer typically has a lower organic carbon content and lower porosity than the root-zone soil. Contaminants in this layer move downward to the groundwater zone primarily by capillary motion of water and leaching. Chemical transformation in this layer is primarily by biodegradation. 

Degradation includes both chemical biodegradation and transformation. It can lead to a diversified pollution that is not always easily perceptible. Indeed, there are relatively little available toxicological and ecotoxicological data on metabohtes. 

Transformation/degradation processes biodegradation, chemical hydrolysis, oxidation-reduction reactions and photolysis, the last only at the surface of the soil. Biological transformations comprise the main degradation pathway in the soil layer, where there is an active bacterial community, possibly up to some tens of centimetres deep. 

Cellulose, the most abundant renewable and biodegradable polymer, is the promising feedstock for the production of chemicals for their appUcatimis in various industries. Annual production of cellulose in nature is estimated to be lO"—10 t in two forms, partially in a pure form, for example seed hairs of the cotton plant, but mostly as hemicelluloses in cell wall of woody plants (Klenun et al. 1998). The versatility of cellulose has been reevaluated as a useful structural and functional material. The environmental benefits of ceUulosics have become even more apparent (Hon 1996a). Cellulose is revered as a constmction material, mainly in the form of intact wood but also in the form of natural textile fibers like cotton or flax, or in the form of paper and board. The value of cellulose is also recognized as a versatile starting material for subsequent chemical transformation in production of artificial ceUulose-based threads and films as well as of a variety of cellulose derivatives for their utilization in several industries such as food, printing, cosmetic, oil well drilling, textile, pharmaceutical, etc. and domestic life. 

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