Biological Degradation of Polymers

Biological degradation of polymer may take place either on the surface 

Although the addition of biocide is the current practice in much of the oil industry for polymer flooding and many other water injection and near well treatments, some care should still be taken. For example, the biocide may interfere with other additives in the process in the case of polymer flooding, it was discussed earlier in this chapter how the biocide may interact in a detrimental way with the action of chemical stabiliser packages. Thus, the task is to find a suitable biocide that is compatible with other fluid additives. For further details, the reader is referred to the papers quoted in this section and the references therein. 

On the other hand there are polymers that are capable of being degraded, generally in a controlled way, to yield low molar mass molecules that can be safely metabolised by living organisms. Polymers of this type need to have relatively polar substituents, preferably ester and hydroxyl groups. One such 

In nature, there are several sources of enzymes that are capable of catalysing the hydrolysis of PHB. The polymer itself is produced by bacteria and occurs in cells as discrete inclusion bodies. These bodies contain the necessary enzymes for degrading the polymer, preventing its build-up in the cell. As well as this, there are numerous bacteria and fungi, many of which are found in the soil, that are capable of secreting the necessary enzymes outside their cell walls, and thus of iiufiating degradation of PHB. 

Biodegradable polymers are likely to be increasingly important materials in the future, finding use in applications as diverse as medicine, agriculture, and pharmacy. For applications such as packaging, they remain expensive. However, with changing public attitudes towards enviromnental pollution, it is likely that objections based purely on cost will dimiiush, and that such applications will also grow in the years ahead. 

Biodegradation is an enzyme-catalysed process, and typically occurs in two stages. In the first of them, the enzyme binds to the 

Two key steps occur in the microbial polymer degradation process first, a depolymerisation or chain cleavage step, and second, mineralisation. The first step normally occurs outside the organism due to the size of the polymer chain and the insoluble nature of many of the polymers. Extracellular enzymes are responsible for this step, acting in either an endo (random cleavage on the internal linkages of the polymer chains) or exo (sequential cleavage on the terminal monomer units in the main chain) manner. 

Enzymes are the biological catalysts, which can induce enormous (10 -10 fold) increases in reaction rates in an environment otherwise unfavourable for chemical reactions. All enzymes are proteins, i.e., polypeptides with a complex three-dimensional structure. 

Glycosidic bonds, as well as peptide bonds and most ester bonds (e.g., in proteins, nucleic acids, polysaccharides, and polyhydroxyalkanoic acids), are cleaved by hydrolysis. A number of different enzymes are involved, depending of the type of bond to be hydrolysed proteases, esterases, and glucoside hydrolases. 

Proteolytic enzymes (proteases) catalyse the hydrolysis of peptide (amide) bonds and sometimes the related hydrolysis of ester linkages. Proteases are divided into four groups on the basis of their mechanism of action  

The names indicate one of the key catalytic groups in the active site. They have been reviewed in detail by Whitaker [17]. 

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Microorganisms generate enzymes that will promote biodeterioration of different classes of polymers. Figure 4.33 summarizes the possible pathways for biological degradation of polymers. 

A large wealth of literature is available for biological degradation of polymers in soil or compost, whereas little information is available on the degradation processes in aqueous environments. Since the applications of polymers in contact with aqueous environments are now gaining popularity, biodegradation under aqueous conditions becomes a matter of concern. The durability and lifetime of polymers certainly depends on the water body composition wherein they are used (seawater, freshwater, etc.) and also on the availability of methods that can detect the level of degradation in such conditions. 

The biological degradation of polymers can be characterised by several laboratory degradation tests (Grima, 2002, Domenek et al, 2004). These tests are generally standardised (Decriaud et al. 1998) but tests in real conditions can also be performed and consist of burying specimens in agricultural soils. 

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