Heavy metal contamination

Heavy metals such as mercury (Hg) and lead (Pb) are so called because of their very high densities (Hg = 13.5 gcm-3, Pb = 11.3 gcm-3) when compared to other common metals, for example Mg = 1.7gcm-3. Heavy metals are of concern because of their toxicity to humans and other animals. However, other elements, for example the semimetal arsenic (As = 5.7 g cm-3), are also toxic. It can be argued that the term heavy metals should be more encompassing to include the toxicity of the element. Toxicity depends on an element's chemistry, the mode of contact with the host organism, the concentration of the element and the host organism's biochemistry. Indeed some substances that are toxic at high concentrations may be essential to life at low concentration (Box 5.5). Heavy metals are of concern because of their toxicity to humans and other animals, but also because they are non-biodegradable. All heavy metals occur naturally, but industrial activity can markedly increase their concentrations in natural waters. These contamination maxima tend to be localized rather than widespread. In Section 5.6.1 we highlight the specific case of mercury associated with gold mining. Mercury is particularly interesting as its toxicity is intimately related to biogeochemical processes and redox conditions. In Section 5.7.2 we discuss the case of natural rather than anthropogenic arsenic contamination of groundwater.

5.6.1 Mercury contamination from gold mining

Mercury has had a host of industrial and commercial uses, ranging from use in batteries, for the production of commercial chlorine, as a fungicide on seeds and in the mining of precious metals such as silver (Ag) and gold (Au). In the latter context the unusual property of metallic mercury (Hg0)—being liquid at 'room temperature' —is exploited to help isolate particulate gold from mined gravel slurries. When mercury is added to the slurry it readily forms an amalgam with the gold:

This amalgam or alloy has a higher density than the surrounding gravel slurry allowing easy separation. While this method of separation has long been used by miners, for example in the Californian gold rush of the late 19th century, it is currently an environmental concern in many developing countries. In the early 1980s, for example, the Amazon Basin experienced its own 'gold rush'—some claim the largest single gold rush in history—and mercury is still used extensively as an amalgam in the myriad small-scale gold mines (garimpos). The environ-

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