9.3 Biodegradation of Plastics

129

Polymer

Oligomers

dimers

monomers

Anaerobic

Aerobic

Microbial biomass

Microbial biomass

CH4/H2S

CO2

CO2

H2O

H2O

Figure 9.2

Reaction pathways of polymer biodegradation. Source: Gu [14].

problem to pass through the cellular membrane. As a result, depolymerization takes

place to create smaller monomers and later, the microorganisms absorb it [12].

These low-molecular-weight compounds are further used by microbes as a source

of energy and carbon. Small oligomers can also spread to the microorganism which

adapted in the internal environment [13]. The reaction pathways are demonstrated

in Figure 9.2.

The following two distinct methods affect the working mechanisms of microor-

ganisms during degradation of different plastics. (i) Direct action method: in this

process, metabolites formed during the degradation of plastics act as a nutrient for

the growth and development of microorganisms. (ii) Indirect action: where the

metabolic substances of the microbes cause further deterioration.

9.3.2

Biodegradation Phases and End Products

Abiotic and biotic are the main two classes of biodegradation. Abiotic biodegradation

includes hydrolysis and photolysis. Instead, biotic biodegradation is the microbial

degradation. Furthermore, based on organic material, biotic degradation may be fur-

ther classified into aerobic and anaerobic degradation [15].

9.3.2.1

Aerobic Biodegradation

Aerobic biodegradation is also known as aerobic respiration. It is a significant con-

stituent of the natural reduction of pollutants in the so many hazardous wastes.

Usually, this biodegradation reaction occurs in the presence of oxygen due to the

microbial breakdown of organic contaminants. The final products of this process

are water, minerals, biomass, carbon dioxide, and salt [15]. The presence of oxygen,

the causative organisms, surrounding environment, and the chemistry of the system

are categorized by oxidative conditions.

In the cellular respiration process, aerobic bacteria use oxygen to obtain energy

by the oxidation of sugars and fats. There is no production of pungent gases in the