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9 Biodegradation of Plastics by Microorganisms

weight, functional groups, and co-polymers present in plastic structure, as well

as extra additives and plasticizers. Hydrophilic biodegradation is faster than the

hydrophobic degradation. This hydrophobicity is facilitated by the availability of

functional groups. Again, amorphous and soft plastics with lower molecular weight

and density degrade faster than the higher ones. Biodegradation process of plastics

is also affected by the occurrence of easily breakable bonds including ester or amide

bonds.

Exposure conditions can also be characterized into abiotic and biotic factors. The

foremost chain scission from photodegradation decreases the average molecular

weight of the polymer. Microorganisms and moisture get better accessibility to the

polymer chain through the reduction in molecular weight. Abiotic factors including

moisture, pH, and temperature can influence the rate of hydrolysis during the degra-

dation process. The increase in moisture content and temperature thus increases

the levels of hydrolysis reactions and microbial growth. As microorganisms require

moisture for their growth, survival, and multiplication, the rate of degradation of

polymer is higher in the presence of moisture. Availability of moisture enhances

the rate of hydrolysis by producing further chain scission reactions [4]. A change

in the pH (acidic or basic condition) modifies the rate of hydrolysis reactions.

Degradation of plastic products alters the pH followed by the polymer degradation

rate and microbial growth. Equally, enzymatic degradability is inversely affected by

the melting point of the polymer as also the temperature of degradation [4].

Different enzymes have specific active sites and are capable of biodegrading

polymers. As for example, polyesters with straight chain, assimilated from di-acid

monomers containing 6–12 carbons, degraded rapidly by enzymes formed by

Aspergillus flavus and Aspergillus niger as compared to any other polyesters with

straight chain monomer [18]. From the biodegradability perspective, molecular

weight plays an important role in determining certain properties of polymers. The

increase in the molecular weight decreases the degradability [4]. Bio-surfactants are

enabling to biodegrade polymers because they contained certain functional groups.

Bio-surfactants are known as amphiphilic compounds, formed on living surfaces

and very active under high salinity, pH, and temperature.

Environmental factors influencing the degradation of plastics include, in partic-

ular, UV light, temperature, humidity, and the incidence of chemicals. Two dimen-

sions influence the microbial degradation of plastics on the influence of the external

environment. At the same time, the growth and metabolism of associated organ-

isms, particularly biomass and the degradation process of microbes, can be affected

and influenced by the environment. On the other hand, aging and damage to plas-

tics can occur due to the external oxidation environment, which also accelerates the

degradation and utilization of plastics by microorganisms.

9.3.5

Microorganisms Involved in the Biodegradation Process

Microorganisms have different enzymes that enable them to utilize environmental

pollutants as their energy source. Their tiny nature helps them to encounter waste

and contaminants quickly. Thus, they are ideal for the removal of contaminants.