Biodegradation Mechanism in Biodegradable Polymers/plastics

Biodegradation is a process in which the enzymes produced by microorganisms breakdown or assimilate the organic substances. Such living organisms use these organic substances as a carbon and food/energy source. Polyhydroxyalkanoates, the common class of bacterial polyesters, is an ideal carbon-energy storage material for such microorganisms. Biodegradation is the disintegration or degradation by the action of microorganisms such as bacteria, fungi or algae and may result in loss in some useful physical or mechanical properties of polymers. A variety of pathways are involved through which polymer degradation occurs and it is the nature of the polymer that decides the mechanism through which it will degrade.

Looking from the chemical point of view, biological degradation of polymers occurs either in the presence of oxygen (aerobic biodegradation) or in the absence of oxygen (anaerobic biodegradation) in which carbon dioxide, water and biomass are the ultimate end products in both the processes while methane, an additional byproduct, is also formed in the anaerobic biodegradation. However, the above process of byproducts formation that is also referred to as mineralization may not reach to 100 % as a smaller percentage of the mass of the polymer gets converted into the microbial biomass, humus and other residual products. Biological degradation of polymers is mainly activated by microorganisms that grow on the polymer surface and act enzymatically to degrade the polymers.  The amounts and types of available microorganisms and their microbial activities greatly affect biodegradation.

Generally, the mechanism occurs in two steps in which during the first step, chain cleavage occurs and polymers are converted into their corresponding oligomers and monomers which is also called as depolymerization. This is followed by mineralization in which monomers and oligomers formed are of sufficiently smaller in size and are transported to the cytoplasmic cells of the microorganisms and get completely mineralized. This is only the process in which various byproducts such as Carbon dioxide, water, methane and other inorganic substances are formed depending on whether the process is aerobic or anaerobic.

The initial step of depolymerization becomes essential as the polymer macromolecules are generally insoluble in water and are larger in size so as to make it difficult for the microorganisms to transport such materials directly into their cells where most biochemical processes take place. The extracellular enzymes are first excreted by the microorganisms and such enzymes depolymerize the polymers outside the cells so that the smaller size oligomers and monomers formed can be absorbed and biodegraded within the microbial cells. Due to the difficulty in penetration of these large extracellular enzymes into the polymeric material they initiate their attack from the surface and thus biodegradation process is generally also referred to as surface erosion process.

There is a variety of factors that govern the biodegradability and the rate of biodegradation of polymers. It is not only the chemical composition of the polymers that is directly related to the degradation mechanism but also the complex biological environment and several other environmental factors play a very important role in controlling the degradation kinetics of polymers. Along with the polymer degradation, they also greatly affect the microbial population as well as the activity of various microorganisms themselves. Factors that are related to polymer properties affecting biodegradability are chemical structure of polymers, length of backbone chain, molecular weight, crystallinity, polymer blends, copolymers and additives, hydrophillicity and molecular size.

On the other hand, factors that are related to the environment are availability of oxygen in that particular environment, temperature, moisture, pH, light and the presence of trace metals and salts, nutrients and the co-metabolites. Microorganisms belong to various classes of bacteria, algae, fungi; protozoa etc. and they are collectively responsible for the biodegradation of polymers. The growth and development of such microorganisms is dependent on all the above mentioned environmental factors.