Biomedical Engineering Reference
In-Depth Information
Often wrongly referred to as a toxic metal, and though it has some metallic
properties, selenium is a non-metal of the sulphur group. It is an essential trace
element and naturally occurs in soils, though in excess it can be a systemic poison
with the LD
50
for certain seleniumcompounds being as low as 4
g/kg bodyweight.
In plants, sulphur is actively taken up in the form of sulphate SO
4
2-
.The
similarity of selenium to sulphur leads to the existence of similar forms in
nature, namely:
•
Selenite SeO
3
2-
;
•
Selenate SeO
4
2-
.
As a result, selenium can be taken up in place of sulphur and become incor-
porated in normally sulphur containing metabolites.
μ
Practical Applications to Pollution Control
In the next chapter contaminated land and bioremediation, which typically form
a wider area of concern for environmental biotechnology will be considered, in
some detail.
To give a practical context with which to close this section, however, a brief
discussion of air pollution and odour control follows.
Bacteria normally live in an aqueous environment which clearly presents a
problem for air remediation. Frequently the resolution is to dissolve the con-
taminant in water, which is then subjected to bioremediation by bacteria, as in
the following descriptions. However, there is scope for future development of
a complementary solution utilising the fact that many species of yeast produce
aerial hyphae which may be able to metabolise material directly from the air.
A variety of substances can be treated, including volatile organic carbon con-
taining compounds (VOCs) like alcohols, ketones or aldehydes and odorous
substances like ammonia and hydrogen sulphide (H
2
S). While biotechnology
is often thought of as something of a new science, the history of its application
to air-borne contamination is relatively long. The removal of H
2
S by biological
means was first discussed as long ago as 1920 and the first patent for a truly
biotech-based method of odour control was applied for in 1934. It was not until
the 1960s that the real modern upsurge began, with the use of mineral soil fil-
ter media and the first true biofilters were developed in the succeeding decade.
This technology, though refined, remains in current use. The latest state-of-the-art
developments have seen the advent of the utilisation of mixed microbial cultures
to degrade xenobiotics, including chlorinated hydrocarbons like dichloromethane
and chlorobenzene.
There are a number of general features which characterise the various
approaches applied to air contamination. Typically systems run at an operational
temperature within a range of 15-30
◦
C, in conditions of abundant moisture,
at a pH between 6 and 9 and with high oxygen and nutrient availability. In
addition, most of the substances which are commonly treated by these systems
are water soluble.
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