Environmental Engineering Reference
In-Depth Information
6
DEGRADATION OF PLASTICS IN THE
ENVIRONMENT
Though the plastics used in high volume are evidently recalcitrant materials,
being organic compounds, they invariably do break down or degrade on
exposure to the environment. Except when exposed outdoors to solar
ultraviolet radiation (UVR) where measurable rates of loss in mechanical
integrity are obtained, the process is painstakingly slow in most
environments. Under solar irradiation, plastics gradually loses strength and
mechanical properties, finally weakening to a point where surface cracks
appear and brittle fracture fragments the material. The small fragments have
high specific surface area, and if they are in a biotic medium, they will
undergo biologically mediated degradation, further weakening and
ultimately converting the material into inorganic molecules such as CH
4
,
NH
3
, and CO
2
. This last step where the plastic material is converted into
inorganicmoleculesis“mineralization.”However,becausethisentireprocess
cantakedecadesorcenturies,plasticsareoftenperceivedasbeing“forever.”
1
The biodegradation rates of plastics in most environments are far too slow
to play any meaningful role in removing plastic litter from the environment.
As all plastics being invariably biodegradable, the widely used terms
“biodegradable plastics” and “degradable plastics” are strictly speaking,
misnomers. These terms merely mean that these particular plastics degrade
or break down at a much faster, generally at an observable or measurable
rate, compared to the conventional common plastics such as polyethylenes
(PE).
Two requirements must be met to obtain biologically-mediated
mineralization of a polymer: it must be exposed to a biotic environment
conducive to degradation and its chemical structure must be amenable to
breakdown by the available set of microbial enzymes in that environment.
Thus, even the inherently biodegradable starch in grains (barley and wheat)
and dark environment they were stored in.
