Direct biodegradation

Electron donors—In direct biodegradation pathways, the contaminant acts as the electron donor or substrate. However, during cometabolic degradation, a different electron donor is metabolized, resulting in the consequential oxidation of the contaminant. In some contaminated plumes, other electron donors, such as other constituents of gasoline, may also be present. In cases where they are not, and cometabolic degradation pathways are desired, electron donors may be added. 

The highly selective biocatalytic reactions afford a substantial reduction in waste. The overall isolated yield is greater than 90%, and the product is more than 98% chemically pure with an enantiomeric excess of >99.9%. All three evolved enzymes are highly active and are used at such low loadings that counter-current extraction can be used to minimize solvent volumes. Moreover, the butyl acetate solvent is recycled with an efficiency of 85%.The E factor (kgs waste per kg product) for the overall process is 5.8 if process water is excluded (2.3 for the reduction and 3.5 for the cyanation) [47]. If process water is included, the E factor for the whole process is 18 (6.6 for the reduction and 11.4 for the cyanation). The main contributors to the E factor are solvent losses which accounted for 51% of the waste, sodium gluconate (25%), NaCl and Na2SO4 (combined circa. 22%). The three enzymes and the NADP cofactor account for <1% of the waste. The main waste streams are aqueous and directly biodegradable. 

Deeb R. A., Scow K. M., and Alvarez-Cohen L. (2000) Aerobic MTBE biodegradation an examination of past studies, current challenges and future research directions. Biodegradation 11, 171-186. 

Apart from saturated fatty acids, Simoneit and Mazurek (1982) observed low concentrations of unsaturated fatty acids (range C,4-Ci7) a-hydroxy fatty acids (range C 0-C24) that are known components of grass wax dicarboxylic acids (range C 0-C24) that probably arise from the direct biodegradation of hydroxy fatty acids and diterpenoidal acids occurring as diagenetic products of diterpenoids from coniferous resins. 

Under these geometric conditions, short oligomers can be assumed to only diffuse along X direction. Biodegradation on this plate can be reduced to a one-dimension problem. Two data sets are used in this case to predict different biodegradation behaviours. The values of these parameters are provided in Table 15.3. 

HD is highly insoluble in water [8] and toxic to microorganisms because of its reactivity with enzymes and other proteins [3]. For these reasons HD is a poor candidate for direct biodegradation. However, HD does react with water to form hydrolysis products that are non-chlorinated, water-soluble and significantly detoxified. 

Direct biodegradation of 1 is not promising due to the high content of As in a reaction solution. 

An area of great interest in the polymer chemistry field is structure-activity relationships. In the simplest form, these can be qualitative descriptions, such as the observation that branched polymers are more biodegradable than straight-chain polymers. Computational simulations are more often directed toward the quantitative prediction of properties, such as the tensile strength of the bulk material. 

Rayon is unique among the mass produced man-made fibers because it is the only one to use a natural polymer (cellulose) directly. Polyesters, nylons, polyolefins, and acryflcs all come indirectly from vegetation they come from the polymerization of monomers obtained from reserves of fossil fuels, which in turn were formed by the incomplete biodegradation of vegetation that grew millions of years ago. The extraction of these nonrenewable reserves and the resulting return to the atmosphere of the carbon dioxide from which they were made is one of the most important environmental issues of current times. CeUulosic fibers therefore have much to recommend them provided that the processes used to make them have minimal environmental impact. 

There have been numerous communications on the subject of biodegradation test methods, including aerobic compost (30), anaerobic bioreactor (31), general methodology and future directions (32—34), and a fine review article (24). ASTM (22) and MITI (35) have also set forth standard testing protocols for plastics, as shown in Table 2, whereas OECD test methods (29) are more suited to water-soluble polymers. 

Sorbitol is the most important higher polyol used in direct esterification of fatty acids. Esters of sorbitans and sorbitans modified with ethylene oxide are extensively used as surface-active agents. Interesteritication of fatty acid methyl esters with sucrose yields biodegradable detergents, and with starch yields thermoplastic polymers (36). 

EEC Directive on methods of testing the biodegradability of anionic surfactants... 

Monochloroparaffins in this range may be dehydrochlorinated to the corresponding monoolefms and used as alkylating agents for the production of biodegradable detergents. Alternatively, the monochloroparaffins are used directly to alkylate benzene in presence of a Lewis acid catalyst to produce alkylates for the detergent production. These reactions could be illustrated as follows ... 

Alcohol and alcohol ether sulfates are commonly considered as extremely rapid in primary biodegradation. The ester linkage in the molecule of these substances, prone to chemical hydrolysis in acid media, was considered the main reason for the rapid degradation. The hydrolysis of linear primary alcohol sulfates by bacterial enzymes is very easy and has been demonstrated in vitro. Since the direct consequence of this hydrolysis is the loss of surfactant properties, the primary biodegradation, determined by the methylene blue active substance analysis (MBAS), appears to be very rapid. However, the biodegradation of alcohol sulfates cannot be explained by this theory alone as it was proven by Hammerton in 1955 that other alcohol sulfates were highly resistant [386,387]. 

Owing to the above reasons, some biopolymers have been used directly or after modification, to replace the conventional fillers leading to partial biodegradation. A number of studies have been carried out with an aim to maximize the proportion of renewable resources used while retaining acceptable material properties. 

Heller, J., and Himmelstein, K. J., Biodegradable poly (ortho esters) as drug delivery forms, in Directed Drug Delivery (R. 

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