Metals, their Biological Functions and Harmful Impacts - Mining and the Environment: From Ore to Metal - page 480

and sulphides. These minerals, when exposed to atmospheric processes, slowly decompose
which may liberate some metals. In the case of sulphide minerals, as discussed elsewhere in
this text, weathering may proceed rapidly with the liberation of metals in a soluble form.
Once they are dissolved in surface water or groundwater, metals are likely to be bio-available.
The distribution of elements into different forms is termed 'speciation' of the element.
Accordingly, an element such as iron may occur in a variety of ionic forms, the simplest ones
being Fe 3 and Fe 2 . Environmental conditions such as pH, redox potential, alkalinity and
the occurrence of organic and inorganic compounds play an important role in speciation.
Some metalloid elements such as arsenic may occur as cations (As 3 and As 5 ) and as anions
(HAsO 4 2 - arsenate); arsenic may also occur in solution in various organic complexes such
as methyl arsenate and di-methyl arsenate. Speciation is particularly important for the trans-
port and the bio-availability of the metals (Parametrix 1995; ICCM 2007). Also, the toxicity
of various species varies widely from highly toxic to non-toxic among different species.
The mineral cycle as depicted in Chapter One illustrates how mining and mineral
processing contribute to the release of metals into the environment. Some metals will end
up unavoidably in mine wastes, particularly in tailings. Those of environmental concern
are associated with mined metals; lead, mercury, cadmium and chromium in mine or min-
eral processing wastes that originate from the mining of other commercial metals such as
zinc, copper, nickel or gold. In fact, as noted in Chapter Six, cadmium is entirely a byprod-
uct of smelting, and there is no mining operation dedicated to its extraction.
The most important mechanisms for the release of metal from mine wastes are through
leaching into ground and surface water, fugitive dust emissions and from tailings solutions.
Leaching of metals depends on metal solubility. Since metals exist in a wide variety of physical
and chemical states, solubility characteristics vary widely. In addition, solubility depends on
many physical and chemical parameters occurring in complex patterns: e.g. pH, redox poten-
tial, presence of electron donors and acceptors, occurrence of organic and inorganic complex-
ing agents, etc. However a decrease in the pH of water (as experienced in acid rock drainage)
will cause increased leaching of most metals (e.g. Campbell and Stokes 1985; Cusimano
et al. 1986; Evans et al. 1988; Kimball and Wetherbee 1989; Pagenkopf 1983; Schubauer
et al. 1993). An increase in the acidity results in an increase in the free metal ion concentration
in solution. In sum, the solubility of metals is largely pH-dependent and for most metals the
solubility increases with a decrease in pH value ( Figure 11.3 ) . Metal hydroxides are generally
more soluble than metal sulphides, and they are amphoteric, as shown in Figure 11.3 : a metal
hydroxide has one pH at which its solubility is at a minimum.
As discussed in Chapter Six air emissions in primary smelting also constitute a major
source of metal release. Depending on the properties of the metal, such problems as vola-
tile species, fugitive dust and stack emissions may be encountered. A small portion of met-
als will end up in waste or by-products from subsequent steps in metal processing, mostly
in the form of slag, waste-water sludge, or dust.
Since only a small fraction of metals contained in i nal manufactured products are lost
during use through corrosion and wear, most metals will still be present in discarded
products. There are increased efforts to recycle metals, but metal containing products are
often disposed of, either uncontrolled or controlled, in municipal solid waste incinerators
or landi lls. As a consequence, only a small fraction of total metals released to the environ-
ment originate directly from mining and mineral processing. Other than natural sources,
metal usage, including the use of leaded gasoline and the i nal disposal of discarded metal
containing products, are eventually the main sources of metal release into environmental
media such as air, water, or soil.
There is ample literature on the potentially harmful effects of metals in the environ-
ment (e.g. IPCS monographs by WHO: WHO 1988; 1989a; 1989b; 1999; 1991; 1992; 1995;

The distribution of elements
into different forms is termed
'speciation' of the element.

The solubility of metals is largely
pH-dependent and for most
metals the solubility increases
with a decrease in pH value.

Air emissions in primary smelting
also constitute a major source of
metal release.