Pollution and Pollution Control - Environmental Biotechnology: Theory and Application - page 70

concentration gradient. As a result, those cations which readily associate with
particulates are accumulated more easily than those which do not. In addition, the
presence of chelating ligands may affect the bio-availability and thus, the resul-
tant toxicity of metals. Whereas some metal-organic complexes (Cu-EDTA, for
example) can detoxify certain metals, lipophilic organometallic complexes can
increase uptake and thereby the functional toxic effect observed.
Although we have been considering the issue of metal toxicity in relation to
the contamination of land or water, it also has relevance elsewhere and may be of
particular importance in other applications of biotechnologies to environmental
problems. For example, anaerobic digestion is an engineered microbial process
commonly employed in the water industry for sewage treatment and gaining
acceptance as a method of biowaste management. The effects of metal cations
within anaerobic bioreactors are summarised in Table 4.2, and from which it is
apparent that concentration is the key factor.
However, the situation is not entirely clear cut as the interactions between
cations under anaerobic conditions may lead to increased or decreased effective
toxicity in line with the series of synergistic/antagonistic relationships shown
in Table 4.3.
Toxicity is often dependent on the form in which the substance occurs and
substances forming analogues which closely mimic the properties of essential
chemicals are typically readily taken up and/or accumulated. Such chemicals are
often particularly toxic as the example of selenium illustrates.

Table4.2 Theeffectofmetalcationsonanaerobicdigestion
Cation

Stimulatory Moderately inhibitory Strongly inhibitory

Sodium 100- 200 3 500- 5 500 8 000

Potassium 200- 400 2 500- 4 500 12 000

Calcium 100- 200 2 500- 4 500 8 000

Magnesium 75- 150 1 000- 1 500 3 000

Concentrations in mg/l.

Table4.3 Effectivetoxicityandsynergistic/
antagonisticrelationships
Toxic cation Synergistic Antagonistic

Ammonium-N Calcium Sodium

Magnesium
Potassium

Calcium Ammonium-N Sodium

Potassium Magnesium

Magnesium Ammonium-N Sodium

Calcium Potassium

Potassium - Sodium

Sodium Ammonium-N Potassium

Calcium