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9 Biodegradation of Plastics by Microorganisms

Polyvinyl Alcohol (PVOH) It is a biodegradable vinyl polymer. The degrading microor-

ganisms require selective enrichment to successfully mineralize polyvinyl alcohol

(PVOH) or PVOH blends. Water solubility depends on the hydrolysis ratio. It is com-

monly used as coatings (e.g. carbon dioxide barrier of PET), adhesives parts, and

additive in the production of paper and board.

9.3

Biodegradation of Plastics

9.3.1

General Outline

Biodegradation is the process where the constituent polymer gets converted into

several compounds by the action of the enzyme secreted by microorganisms.

Commonly used enzymes are lipases, proteinase k and dehydrogenases [12].

Biodegradable plastics typically decompose in the natural environment. Both

synthetic and natural plastic materials can be biologically degraded by bacteria,

fungi, and actinomycetes. These microorganisms turn the polymeric materials into

their metabolic products by chemical degradation (e.g. H2O, CO2, CH4, biomass,

etc.). The process of biodegradation proceeds dynamically under conditions such as

soil and its properties. Soil pH, oxygen, moisture, temperature, and light are factors

that affect the optimal growth of microorganisms. The degradation characteristics

and the rate are strongly dependent on soil pH, oxygen, moisture, temperature, and

light. They consume various substances as a source of food in order to eliminate its

original form [6]. Plastics with high molecular weight are usually hard to degrade.

Different characteristics of plastic materials such as morphology, mobility, presence

of functional group, molecular weight, additives, and cross-linking usually control

the degradation process [12]. Amorphous plastics are easily biodegradable than

crystalline polymers. Moreover, plastics having high melting point also make them

less biodegradable. Chemical and physical properties are important as they play a

significant role in the biodegradation of plastics. As for example, plastics having

side chain are less biologically degradable than those without side chains.

However, the biodegradation of plastic is a steady process. Primarily, it begins by

environmental factors, like temperature, pH, and UV rays. Biodegradation of plastics

involves following steps:

(a) Attachment of microorganisms on to the polymeric surface area.

(b) Growth and development of microorganisms (by using the polymer as a carbon

source), and

(c) Final degradation of plastic material.

Microorganisms can be attached to the surface of plastic until it is hydrophilic.

Once attached to the surface, microorganisms proliferate by using plastic as a carbon

source. Initially, enzymes (extracellular) secreted by microorganisms cause the main

chain to cleave. This leads to the generation of low-molecular-weight fragments,

i.e. monomers, dimes, or oligomers. Once transformed into their monomers, they

begin to transform into a mineralized form. In the case of large polymers, it creates