Cyanide and cyanide complexes in the goldmine polluted land in the East and Central Rand Goldfields, South Africa
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Date
2009-06-30T11:40:02Z
Authors
Nsimba, Elysee Bakatula
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Abstract
The use of cyanide in gold extraction is of concern when it is not properly
managed from the extraction process to the management of wastes. The
distribution and fate of cyanide in the environment upon release from the
tailings dumps depends on its physical-chemical speciation.
This study presents results of distribution, speciation and fate of cyanide in
selected compartments, namely: tailings, sediments and water systems in gold
mine polluted land.
Sampling of tailings in a facility that is being rehabilitated was done in 2006
and 2007 to assess the impact of AMD on cyanide release over that period.
Deposition of materials in the tailings dams stopped in 2004. The results
revealed that the pH of the tailings decreased between 2006 and 2007. Elevated
concentrations of CNfree, SCN- and CNO- were observed for 2007 compared to
2006. Most cyanide species had degraded as a result, primarily, of decrease in pH due to generation of AMD, also the oxidation of CNfree and the reaction
with active sulphur species such as S2O3.
The decrease of cyanide total (CNT) with time is a consequence of natural
attenuation of cyanides in tailings which may be attributed to physicalchemical
and microbiological mechanisms.
Cyanide and its metal complexes were found to be unstable following
generation of AMD in the dump over a period of one year. The dissociation of
metal-cyanide complexes when the pH drops, releases CNfree which is either
volatilised as HCN(g) or transported in solution with the contamination plume
or converted to SCN- ,CNO- and NH4
+.
However, in most of cases high concentrations of metal-cyanide complexes
were found even at low pH values of the tailings suggesting that these
complexes are very stable. This was substantiated by the geochemical
modelling which predicted the predominance of iron-cyanide complexes in
tailings at low pH. iii
Cyanide released from cyanide complexes flows into the central pond of the
tailings facility and partly leaches into the groundwater.
Salt crusts were observed along the capillary fringe of the central pond as well
as around other water bodies considered in the study. These crusts were found
to contain elevated concentrations of heavy metals (e.g. 12940 mg kg-1 Fe and
186.1 mg kg-1 Co) and cyanide (e.g. 118.4 mg kg-1 CNT, 14.36 mg kg-1 CNWAD
and 100.2 mg kg-1 CNSAD). This obviously has implications of secondary
pollution as these crusts tend to be very soluble in water thus leading to the
release of heavy metals and cyanide into water systems during rainfall.
Characterization of cyanide was also done in drainage water from an active
slimes dam where deposition from a reprocessing plant takes place. The slimes
dam had drainage pipes and a solution trench around it that drained away
excess water. Low concentration of CNT was obtained in pipe water from the
pipe with low pH values (2 - 4) whilst this concentration was high in water
from the trench with high pH values (5 -7). Copper and iron complexes were
the most abundant. High concentrations of SCN- and CNO- were obtained as
result of conversion of CNfree as explained previously.
Salt crusts collected around the dam presented low pH (3) and high
conductivity, the evidence of high metals content. High concentrations (198.4
mg kg-1) of CNT were obtained in the crusts with predominance of CNSAD (Fe
and Co). The bluish-green colour of the crusts and the elevated concentrations
of CNSAD as well as those for iron could suggest the presence of Prussian blue.
Analysis of the wetland sediments showed the transport of cyanide from the
tailings dumps to the wetland through the streams.
An enrichment of cyanide was observed in the sediment with the enrichment
factor of 3 for CNT with predominance of strong complexes (Fe and Co). The
sediment is rich in organic matter and cyanide is known to bind strongly with
organic matter. Although other possible sources (e.g. bacterial or microbial
sources) could have contributed to the enrichment of cyanide in sediment, this
was not investigated. Cyanide can be transported from the tailings dams to natural streams and other
surface water bodies through groundwater. A natural stream within a
reprocessing area was considered as a water system and cyanide in it was
characterised. Three clusters were observed: water collected upstream with
high pH, water from downstream with low pH (4) and the groundwater with
low pH (3). Low concentrations of CNfree were obtained downstream. This
could be due by the lost of CNfree by volatilization due acidic pH conditions.
CNT was found to be lower downstream than upstream with the predominance
of CNWAD. CNT concentrations were high at the seepage point, where the
groundwater discharges to the surface. These concentrations were similar to
those obtained in the groundwater.
Copper and iron complexes were dominant in the surface and groundwater and
this was substantiated by modelling results as well.
SCN- was not detected in surface water as it is highly soluble in water and then
leaches in the groundwater. The concentrations of CNO- were the same up and downstream.
The results obtained from the study revealed that concentration of CNfree in
most water bodies exceeded stipulated limits by bodies such as WHO, USEPA
and UE. For instance, concentrations of up to 0.304 mg l-1 of CNfree were
obtained in some instances to compare with limits of 0.07 mg l-1 by WHO, 0.02
mg l-1 by DWAF/South Africa.
Additional studies should be done to find out the impact of organic matter (e.g.
humic and fulvic acids) on the fate of cyanide. Various natural attenuation
mechanisms of cyanide in tailings dams should be investigated. An assessment
of the phytoremediation program vis-à-vis cyanide cyclisation is recommended
and a monitoring of groundwater (borehole water) quality is required.