Box 55 Essential and nonessential elements

Many substances that might be considered toxic are in fact essential to life, for example the heavy metals Co, Cu, Fe, Se, Zn. Thus, there is a relationship between concentration of the substance and the health response it invokes. An element is essential to life when a deficient intake of the element consistently results in impairment of a life function from optimal to suboptimal (Fig. 1a). Moreover, when physiological levels of this element, but not of others, are supplemented or restored, the impairment is cured and optimal health is restored (Fig. 1a). By contrast, non-essential elements do not produce a positive health response. An organism may tolerate low concentrations of some non-essential elements (Fig. 1b). Even dangerous poisons such as arsenic oxide (As2O3) can be tolerated to some extent if the amount taken (dose) is increased in small degrees. The so-called 'arsenic eaters' of Styria, a mountainous Austrian state, are said to have taken six times the minimum fatal dose of arsenic oxide without ill effect. These people maintained that their curious diet supplement improved their personal appearance and increased their energy for ascents of their Alpine homeland! Of course if concentration exceeds the organism's tolerance threshold, impairment is observed (Fig. 1b). In both essential and non-essential elements, excessive concentrations will result in toxicity and ultimately death (Fig. 1). The 16th century Swiss scientist Paracelsus recognized this with his comment 'Tis the dose that maketh the poison'.

Fig. 1 Health-response curves for essential and non-essential elements (X).

Fig. 1 Health-response curves for essential and non-essential elements (X).

mental concerns revolve around both the amounts of mercury used and its accidental release into the environment. On average 1.3 kg of Hg0 is used for each kilogram of gold recovered, although much of this mercury is contained and reused. However, many hundreds of tonnes of mercury have been released into the Amazon Basin during the 1980s and 1990s. This practice is widespread not only in the Amazon, but in Latin America as a whole, the Pacific Rim countries, and Africa, which together produce thousands of tonnes of gold each year.

During the amalgamation process Hg0 is easily introduced into the local soils, rivers and atmosphere. Although the effects of leaching of Hg0 from waste tips and direct spillages into water courses are reduced by the low solubility of Hg0 in water (solubility of 0.12 X 10-6moll-1), the mercury is potentially subject to a number of chemical transformations that make it harmful to organisms. During the final stages of gold processing, burning of the Au-Hg amalgam in open pans allows evaporation of Hg0 as vapour to the local atmosphere. The annual flux to the atmosphere by this process is around 80 tonnes of Hg0, approximately 67% of Brazil's total mercury emission. In the atmosphere Hg0 can be dispersed globally (atmospheric residence time around 1 year) but much is readily oxidized to Hg2+. The reaction occurs in water droplets, the oxidant is ozone (see also Section 3.9) and experiments suggest that mercuric oxide is produced, for example:

0 0

Post a comment