Environmental Engineering Reference
In-Depth Information
Microorganisms play a major role in the marine biodegradation process. They produce
both exoenzymes (degrading chains from the terminal groups) and endoenzymes
(degrading randomly along the chain). A large MW loss and high degradation rate
is shown by the exoenzymes and a comparatively small MW loss is shown with
endoenzymes. Low MW polyoleins are utilised faster by some organisms when
compared to high MW ones [12]. Most of the plastics including PE, PP and polystyrene
allow slow microbial growth, however, low MW hydrocarbons (hexadecane and so
on) can be degraded easily by the microorganisms. The hydrocarbons are taken in by
the microbial cells and converted to cellular metabolites within them. This may not
occur if the MW of the sample is too high. If the molecule is too large, the conversion
of high MW compounds to low MW ones occurs outside the microbial cells by the
extracellular enzymes produced by the organisms. The MW of plastic may decrease
by the action of photo or chemical degradation, which helps the microbial attack.
For all the alkane derived materials, uptake and intercellular degradation do not
occur at the same threshold MW. When the length of the polymer chain exceeds
24−30 carbon atoms (parafins, polyethylene glycols, and linear alkyl benzene
sulfonates) very slow degradation is observed [13]. The average MW of LDPE is
150,000 Da, which means that it contains 11,000 carbon atoms. Reducing its size
to a biologically acceptable value requires destruction of the PE matrix. This can be
achieved by the incorporation of another synthetic polymer with PE or biodegradable
natural polymers, including starch and cellulose, or by adding additives. Action of
arthropods, millipedes, snails and macrofouling organisms in the marine environment
also destroys the polymer chain. Insertion of carbon monoxide into the chain permits
chainscissionbyaNorrishtypereaction[14].
The MWD has an effect on the rate of degradation. This effect has been demonstrated
for a number of polymers, where a critical lower limit must be present before the
process will start. This effect is attributed to a range of causes including changes in
enzyme accessibility, chain lexibility which will help the polymer chain to it into the
active site of the enzyme, crystallinity or other morphological reasons.
3.2.4 Hydrophobic and Hydrophilic Characteristics
Surface properties of the polymer play a crucial role in polymer biodegradation in
the marine environment. The organisations of the microorganisms on a surface are
generally material and microorganism speciic. The surface properties may include
roughness, hydrophobicity, charges, lexibility and so on. Synthetic polymers that
are soluble in water [polyvinyl chloride, PVA, polyacrylic acid and polyether] are
more biodegradable than water insoluble polymers of a similar MW and structural
complexity. Increasing the hydrophilicity of the polymeric surface by chemical
