| In India according
to future flow analysis the total virgin plastics
consumption is expected to reach 20,000 KT by
the year of 2030 and over 18, 800 KT of waste
can be generated. The consumption of thermoplastics
was 40 million tones in European countries (Plastemart.com
website, 2004 & 2006). Polyolefins like Polypropylene
(PP), Low density polyethylene (LDPE) & High
density polyethylene (HDPE) account for about
60% of the total plastics consumption in India.
Dumping of plastic in the environment at such
a large amount is causing already serious problems
to the flora and fauna. The conventional method
like incineration is a source of secondary hazardous
product. This plastic waste degrades the environmental
conditions at a very slow rate. The low rate of
biodegradation of plastics is usually due to properties
of the polymeric material like lack of water solubility
(Hydrophobicity) and size of the polymer molecules
(long chains and high molecular weight which prevents
the breakdown of the polymeric bond) that microbial
cell are unable to transport directly in their
cells.
Literature review
Biodegradation ultimately results in the consumption
of polymer by the microorganism. The growth activity
study of the microbes like fungi (Aspergillus
niger, A.flavus, A.oryzae, Chaetomium globusum,
Penicillium funiculosum, Pullularia pullulan),
bacteria (Pseudomonas aeruginosa,Pseudomonas
sp., Bacillus cereus,Bacillus sp.,
Coryneformes bacterium, Mycobacterium, Nocardia,
Corynebacterium and Candida) and Actinomycetales
(Streptomycetaceae) on the agar plate
for a definitetime period revealed the capability
of these microbes to degrade. Polyethylene (PE).
There are scare reports on PP biodegradation.
Fungal species (A. niger) and microbial
communities such as Pseudomonas and Vibrio
species have been reported to biodegrade PP. Isotactic
PP exposed to a bacterial consortium for 175 days
had 40% methylene chloride extractable compounds,
which was mixture of hydrocarbons (between C10H22
to C31H64). 30-60 % growth
of A. niger was observed on gamma irradiated
PP films in six weeks, indicating that the fungus
is able to grow taking this polymer as its sole
carbon source. The continuous chain of repetitive
methylene units makes PP resistant to degradation.
How to address
the problem?
For achieving the task of biodegradation it is
a prerequisite that the polymer surface is modified
to some extent. Pretreatment and blending PE with
natural polymer can modify the surface.
Pretreatments
Under environmental conditions natural weathering,
which includes solar radiation, UV and thermal,
is a process that affects polymeric properties
to some extent but at a slower rate. It is reported
that there is a synergistic effect between photo
oxidation and the biodegradation of polyethylene.
Treatments such as UV, thermal and chemical leading
to oxidation of the polymer surface
can be effectively used as a pretreatment strategy
before subjecting it to biodegradation. These
pretreatments lead to oxidation of the polymer
surface that decreases the hydrophobicity and
helps in the attachment of microorganism. The
attachment of organism to the polymeric surface
further enhances the biofilms formation. Microbes
utilize the functional groups like carbonyl, carboxyl
and ester produced on the polymer surface during
oxidation. Such studies are done with polyethylene
and have shown positive results in the form of
increase in the biodegradation with increase in
the irradiation time of UV.
Blends
Natural polymers like Poly lactic acid (PLA),
Poly  -caprolactone
(PCL) and Polysaccharides can be blended to some
extent with the synthetic polymer. In these blends
the natural polymer being biodegradable will help
in the formation of biofilm on the surface.
Current research
in our laboratory
We are studying the effect of various physical
and chemical pretreatment on the biodegradation
of LDPE, HDPE, starch blended PE and PP. Soil
and marine microorganisms have been isolated and
are being tested for their efficacy in carrying
out the biodegradation of these polymers. Several
different pretreatment strategies such as UV,
thermal, chemical are being tested to enhance
the process. The pretreated PP is then exposed
to mixed soil culture. The mixed soil culture
is a suspension of soil sample from a local dumping
site. The experiments are carried out in a minimal
media. From our one year experiments with thermally
pretreated PP and mixed soil culture we found
good results. As already reported pretreatment
of the polymer surface works in synergy with microbial
attachment to enhance biodegradation. After one
year microorganism isolated from the mixed culture
are found to be Bacillus and Pseudomonas
sp.
The polymer samples are monitored by techniques
like Baclight staining, Fourier transform infrared
(FTIR) spectroscopy, Differential scanning calorimetry
(DSC), Scanning electron microscopy (SEM), Contact
angle and Tensile strength etc. Baclight staining
helps us to observe live and dead microorganisms
on the polymer surface. SEM and Contact angle
measurements helps in studying surface changes
on the polymer whereas FTIR and DSC techniques
analyse the chemical and structural changes in
the polymer.
1.The SEM analysis of
the PP surface after 12 months
2.The Baclight staining
of PP surface after 12 months (Live organism-
green and dead organism- red in colour)
Further
reading:
Arutchelvi, J., Sudhakar, M., Ambika
Arkatkar, Mukesh Doble, Sumit Bhaduri and Parasu
Veera Uppara (2008). Biodegradation of polyethylene
and polypropylene. Indian Journal of Biotechnology.
7, 9-22.
Trishul Artham and Mukesh Doble.
(2007). Biodegradation of Aliphatic and Aromatic
Polycarbonates. Macromolecular Bioscience.
doi 10.1002/mabi.200700106 (Press).
Sudhakar, M., Mukesh Doble, Sriyutha
Murthy, P., and Venkatesan, R. (2007). Marine
Microbe Mediated Biodegradation of Low and High
Density Polyethylene . International Biodegardat
ion and Biodeterioration. doi:10.1016/j.ibiod.2007.07.011
(in press).
Sudhakar, M., Trishul, A., Mukesh
Doble, Suresh Kumar, K., Syed Jahan, S., Inbakandan,
Viduthalai, R., Umadevi, P., Sriyutha Murthy,
P. and Venkatesan, R. (2007). Biofouling and biodegradation
of polyolefins in ocean waters. Polymer Degradation
and Stability. 92, 1743-1752.
Sudhakar, M., Priyadarshini,
Mukesh Doble, Sriyutha Murthy, P., and Venkatesan,
R. (2007). Marine Bacteria Mediated Degradation
of nylon 66 and 6. International Biodeterioration
and Biodegradation. 60,
144-151.
For
more details contact:
Dr. Mukesh Doble, Ph.
D.
Lab : Bioengineering & Drug Design
Professor, Department of Biotechnology
IIT Madras, Chennai - 600036, INDIA
(Tel:044-2257 4107; Fax:044-2257 4102)
Email: mukeshd@iitm.ac.in,
mukeshd@biotech.iitm.ac.in
Website: http://www.biotech.iitm.ac.in/faculty/md.htm
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