OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

and kinetic stability of the complex (Fernando, 1995). Different metal species have different bio-availabilities,
and some metal species are more toxic than others. For example, free Cu 2+ ions are much more toxic than
most forms of Cu(II) which have been complexed by organic ligands (Moffat, 1997).
Much of the metal present in a water system will be either adsorbed onto particulate matter or in suspension,
rather than in solution as free ions (Kelly, 1988). Where metals are associated with a particulate phase, they
can be transported long distances from the original source.
Where anaerobic and reducing conditions develop in a water system, metal sulphides may form, and be
precipitated out of solution - although they can be re-dissolved when conditions change (Kelly, 1988). The
superoxide radical (O 2- ) is produced from photo-oxidation of coloured dissolved organic matter, and the radical
reacts quickly with inorganic iron and copper species in water. This results in the formation of reduced species,
and influences the overall speciation of metals such as iron and copper (Zafifiou & Voelker, 1997).
Increase in salt concentrations in the water increase solubility and mobility of sediment-bound metals, by
competition for sorption sites, and formation of soluble complexes (Förstner & Salomons, 1991).
Metal speciation and concentration are also affected by changes in river flow regimes, leading to settling, resuspension,
sorption or co-precipitation. The drop in velocity that occurs when a river enters a lake results in
deposition of suspended material, and diffusion and mechanical dispersion occur. Lake-bottom sediments may
act as metal sinks. This is also affected by sediment or clay type, as dissolution, adsorption and precipitation
are partly determined by factors such as surface area (Kelly, 1988). The presence of organic material in the
sediments has been found to enhance the transfer of metals from water to sediments, taking them out (at least
temporarily) of the water system (Nriagu et al., 1981). This process essentially buffers the availability of these
metals, with high levels of carbon inhibiting mobility of some metals, such as copper (Hettler et al., 1997).
Seasonal variations in metal content (and pH) of water systems draining mining areas have been recorded. A
suggested factor influencing this is the occurrence of soluble sulphate mineral phases (e.g. copiapite,
chalcanthite) in the waste pile. During the rainy season, the increased precipitation leads to a greater amount of
these minerals being dissolved.
2.4.3 Cyanide
Cyanide, a highly toxic substance, is indispensable in the refining of gold. Large quantities of cyanide are used
to simultaneously oxidise gold and stabilize it in solution as a gold-cyanide complex (Marsden & House, 1993).
The two main methods by which this takes place are agitation leaching, the aeration of mixed cyanide solution
and ground ore, and heap leaching, where the cyanide solution slowly percolates through the finely ground ore
(Channon, 1991; Love, in preparation). The gold is later recovered, but cyanide remains in the waste slurry,
which ends up on the numerous tailings dams.
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