Synthetic Polymers
Eg., Plastics, polyethylene- PE, polyvinyl chloride -PVC and polystyrene - PS
- Easily molded into complex
shapes, have high chemical resistance and are more or less elastic
- Can be formed into fibres
or thin, transparent films
- Popular in the manufacture
of garments, durable and disposable goods, packaging materials
- More than 90% of plastic
material in municipal garbage contains polyethylene, polyvinyl chloride
(PVC) and polystyrene (PS)
- More than one third of
total plastic production include disposable goods and packaging material;
largest environmental impact
- These materials have molecular weights ranging from several thousand to lakhs and resist biodegradation indefinitely- Resistance to biodegradation is associated with excessive molecular size
- If molecular size of polyethylene is reduced to molecular weight under 500 daltons by pyrrolysis, the fragments are found susceptible to biodegradation
- Other reports on plastic degradation focus on the degradation of plasticizers, the additives designed to render the plastic pliable
- Plasticizers- esters of long chain
fatty acids and alcohols
- Plasticizer biodegradation renders the material brittle but does not affect the polymer structure
- Plastics are biologically
inert, so were regarded as an aesthetic nuisance but not hazard
- However, oceans dumped
with plastic wastes and floating plastic spherules being ingested by small
fish with consequent intestinal blockage- much concern
- Remains of plastic nets
and ties also harm wildlife
- Agricultural practices
involving plastics (film mulching technique)- covering the fields with
plastic films in order to control weeds and conserve moisture
- Large accumulation of
plastics in the field are undesirable-reuse or collection at the end of
growing season not practical
- Research has now focused
on bioplastics such as poly β-hydroxy alkoanates produced by Alcaligenes,
Bacillus and various Pseudomonads
- These biodegradable intracellular storage products have thermoplastic properties and can be molded- properties can be influenced by the substrate fed to the bacteria
- β-hydroxy butyric and
β-hydroxy valeric acid copolymer give the best properties
- High price (5-7 times
higher than petrochemical based polyethylene) restrict to its use
- Modified starch and other natural polymers can be useful biodegradable plastics
- Synthetic caprolactone
polyester contains hydrolyzable bonds-good candidate for biodegradable
plastics
- Polyethylene, polybutene
etc are susceptible to photochemical biodegradation- Materials that
sustain sufficient photochemical damage during a growing season are later susceptible to microbial degradation
- Manufacturers sell these as degradable or environment friendly products – they contain photosensitizers alone or in combination with 3-15% modified starch
- Prolonged sun irradiation (weeks to months) causes breaks in the polymer making it brittle and susceptible to biodegradation
Main components of anionic detergents-surface active with polar (water soluble) sulphate and nonpolar alkyl end
Aid in emulsification of fatty substances and cleaning occurs while these molecules make a monolayer around lipophilic droplets or particles.
Their molecules orient with their nonpolar end towards the lipophilic substance and the sulphonate end towards water.
Nonlinear alkylbenzene sulphonates or Branched alkylbenzene sulfonates (BAS) are recalcitrants and resistant to biodegradation and cause foaming problems in sewage treatment and in the rivers.
ABS are easier to manufacture and bear superior detergent properties but the methyl branching of alkyl chain in them interferes with biodegradation of ABS, because tertiary carbon atom blocks the normal β-oxidation.
Nonlinear alkylbenzyl sulphonates or Branched alkylbenzene sulfonates (BAS) were replaced with linear alkylbenzene sulfonates (LAS) during the 1960s due to environmental concerns
LAS (linear alkylbenzyl sulphonates) are biodegradable
It is biodegraded rapidly under aerobic conditions within weeks- oxidative degradation initiates at the alkyl chain.
Under anaerobic conditions it degrades very slowly or not at all, accumulates in sewage sludge, but once oxygenated, it will rapidly degrade
Thus, BAS to LAS conversion is a story where a synthetic molecule was redesigned to remove obstacles to biodegradation, while preserving the other useful characteristics of the compound.
Reference
Microbial Ecology: Fundamentals and
Applications- Ronald M. Atlas, Richard Bartha
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