Pollution and Pollution Control - Environmental Biotechnology: Theory and Application - page 69

the particular, are often more important concerns than the more theoretically
applicable general considerations.

Practical Toxicity Issues

The general factors which influence toxicity have already been set out earlier in
this discussion, but before moving on to consider wider practical issues it is help-
ful to mention briefly the manner in which the toxic action of pollutants arises.
There are two main mechanisms, often labelled 'direct' and 'indirect'. In the
former, the effect arises by the contaminant combining with cellular constituents
or enzymes and thus preventing their proper function. In the latter, the damage
is done by secondary action resulting from their presence, typified by histamine
reactions in allergic responses.
The significance of natural cycles to the practical applications of environmental
biotechnology is a point that has already been made. In many respects the func-
tional toxicity of a pollution event is often no more than the obverse aspect of
this same coin, in that it is frequently an overburdening of existing innate systems
which constitutes the problem. Thus the difficulty lies in an inability to deal with
the contaminant by normal routes, rather than the simple presence of the substance
itself. The case of metals is a good example. Under normal circumstances, pro-
cesses of weathering, erosion and volcanic activity lead to their continuous release
into the environment and corresponding natural mechanisms exist to remove them
from circulation, at a broadly equivalent rate. However, human activities, partic-
ularly after the advent of industrialisation, have seriously disrupted these cycles
in respect of certain metals, perhaps most notably cadmium, lead, mercury and
silver. While the human contribution is, clearly, considerable, it is also important
to be aware that there are additional potential avenues of pollution and that other
metals, even though natural fluxes remain their dominant global source, may also
give rise to severe localised contamination at times.
The toxicity of metals is related to their place in the periodic table, as shown in
Table 4.1 and reflects their affinity for amino and sulphydral groups (associated
with active sites on enzymes).
In broad terms, type-A metals are less toxic than type-B, but this is only a gen-
eralisation and a number of other factors exert an influence in real-life situations.
Passive uptake by plants is a two-stage process, beginning with an initial
binding onto the cell wall followed by diffusion into the cell itself, along a

Table4.1 Metalperiodicityandtoxicity
Metal group Relative toxicity

Group IA Na < K < Rb and Cs

Group IB Cu < Ag < Au

Group IIA Mg < Ca < Sr < Ba

Group IIB Zn < Cd < Hg

Group IIIA Al < Ga < In < Tl