9.3 Biodegradation of Plastics

131

Deterioration

Fragmentation

Change in physical

appearance

Oligomer

Dimer

Monomer

Assimilation

Plastic is consumed

as substrate

Mineralization

Aerobic

Anaerobic

Biodegradation

mechanism

Released into

environment

CO2, H2O,

Metabolic

by products

CH2, H2O,

CO2,

Other

residues

Microbial

characteristics

Enzymes

Molecular weight

Density

Strength

Shape

Size

Type of polymer

Additive i.e. dye

Hydrophobicity

Biofilm

Plastic characteristics

Figure 9.3

Mechanism for the biodegradation of plastics. Source: Jaiswal et al. [17].

polymers. The formation of microbial biofilms depends on the structure and com-

position of polymers. Upon completion of bio-deterioration, microbial degradation

is known as biofragmentation, where microbial enzymes include their catalytic

action on polymers.

Bacteria contain oxygenases (mono-oxygenases and di-oxygenases), which

have potential to break down and degrade the polymers. They also bind oxygen

molecules to a long carbon chain and produce less recalcitrant, less environmental

damaging alcohol and peroxyl products. In addition, lipases and esterases as well

as amide group endopeptidases catalyze the mechanism of carboxylic group’s

transformation. Brevibacillus borstelensis, Rhodococcus ruber, Pseudomonas chloro-

raphis, Pseudomonas putida AJ, Thermomonospora fusca, Alcaligenes faecalis,

and Clostridium sp., etc. are some microorganisms found to degrade polymers.

The ultimate results of microbial polymer degradation are mineralization and

assimilation. Plastic monomers formed by biofragmentation which is essential to

cross the microbial cell membranes. Few of these monomers which are not able

to penetrate through cell membrane generally stay outside of the membrane and

do not get assimilated. In the cells, plastic monomers are oxidized by catabolic

pathways and energy produced generates a new biomass of a cell. The assimilation

cycle requires atom incorporation into the microbial cell for complete degradation.

9.3.4

Factors Affecting Biodegradation of Plastics

Biodegradation of polymers is affected by several factors. These include the charac-

teristics and exposure conditions of plastics, such as flexibility, mobility, molecular