Geology Reference
In-Depth Information
sluice lining and crushed in an oil drum, where the mercury is then
introduced, followed by manual panning.
The effects of all this activity can be seen in the increased turbidity,
with clear rivers turning a muddy brown colour, and increased fish
mortality. Furthermore, releases of mercury to the aquatic environment
have resulted in elevated concentrations of mercury in sediments and
fish. Commonly eaten fish species from the Madeira River were found to
contain mean mercury levels of 0.7 mg kg
1
and up to 3.8 mg kg
1
in the
Tapajos Valley.
103
Given the bioaccumulation and biomagnification of
mercury in many fish species and the possible microbial methylation of
mercury deposited in sediments to the even more toxic lipid-soluble
methyl mercury, CH
3
Hg
1
, there is a real prospect of enhanced dietary
uptake of the most toxic form of mercury by the local populations. This
would be in addition to exposure from inhalation of mercury vapour
during the handling of mercury and from subsequent post-amalgama-
tion treatment steps, which usually involve refinement of the extracted
gold by heating to drive the more volatile mercury off. It is thought that
there is an annual discharge of about 100 tonnes of mercury into the
Amazon ecosystem and perhaps the same amount released as mercury
vapour. Thus there could be an increased risk of the neurological
damage and foetal deformities commonly associated with mercury
poisoning, as most notably exemplified in the fishing village population
of Minimata, Japan, in the 1950s, when a chemical manufacturing plant
discharged mercury salts and CH
3
Hg
1
into the bay, to be taken up by
fish and shellfish.
3.3.2.6.2 Cyanide. Another method of extracting gold that can lead
to contamination of freshwater is cyanide heap leaching, which is used
to extract tiny amounts of gold from huge volumes of low-grade
deposits of gold-bearing rock. In this case, ore is dug from open-pit
mines, crushed, and spread on asphalt or plastic pads. The heaps are
then sprayed with cyanide, which dissolves the gold.
4Au
(s)
þ
8NaCN
þ
2H
2
O
þ
O
2(g)
-
4NaOH
þ
4NaAu(CN)
2
(3.97)
The gold-bearing cyanide solutions run off on impermeable asphalt or
plastic to collection vats and are then treated, for example with zinc, to
extract gold.
2NaAu(CN)
2
þ
Zn
(s)
-
Na
2
Zn(CN)
4
þ
2Au
(s)
(3.98)
One major environmental problem with this technique is that cyanide is
highly toxic. Although attempts are made to loop the cyanide through a
closed system to try to ensure that none is lost, cyanide does commonly
