Coal combustion is recognized as the primary source of anthropogenic mercury emission
in South Africa followed by gold mining. Coal is also known to contain trace
concentrations of mercury which is released to the environment during coal mining,
beneficiation or combustion. Therefore, determining the mercury speciation in coal is of
importance in order to understand its behavior and fate in the environment.
Mercury was also used, at a large extent, in the Witwatersrand Basin (South Africa) for
gold recoveries until 1915 and is still used in illegal artisanal mining. Consequences of
these activities are the release of mercury to the environment. Nowadays, gold (and
uranium) is also recovered through the reprocessing of old waste dumps increasing the
concern related to mercury pollution.
While much effort has been put in the northern hemisphere to understand and control
problems related to anthropogenic mercury release and its fate to the ecosystem, risk
assessment of mercury pollution in South Africa was based, until very recently, on total
element concentrations only or on non systematic fragmental studies. It is necessary to
evaluate mercury speciation under the country’s semi arid conditions, which are different
to environmental conditions that exist in the northern hemisphere, and characterize
potential sources, pathways, receptors and sinks in order to implement mitigation
strategies and minimize risk.
In this study, analytical methods and procedures have been developed and/or optimized
for the determination of total mercury and the speciation of inorganic and organic forms
of mercury in different sample matrices such as air, coal, sediment, water and biota.
The development of an efficient and cost effective method for total gaseous mercury
(TGM) determination was achieved using nano-gold supported metal oxide (1% wt Au)
sorbents and cold vapor atomic fluorescence spectrometry (CV-AFS). Analytical figures
of merit and TGM concentrations obtained when using Au/TiO2, as a mercury trap, were
similar to those obtained with traditional sorbents.
The combination of isotope dilution with the hyphenated gas chromatography-inductively
coupled plasma mass spectrometry (ID GC-ICP-MS) was also achieved and used
successfully for the speciation analysis of mercury in solid, liquid and biological samples. The developed, or optimized, methodologies were used to estimate the average mercury
content and characterize the speciation of mercury in South African coals, and also to
study the speciation of mercury in selected South African environmental compartments
impacted by gold mining activities.
The obtained average mercury content in coals collected from the Highveld and
Waterberg coalfields (0.20 ± 0.03 mg kg-1) was close to the reported United States
Geological Survey (USGS) average for South African coals. Speciated isotope dilution
analyses and sequential extraction procedures revealed the occurrence of elemental
mercury, inorganic and organo-mercury species, and also the association of mercury
mainly to organic compounds and pyrite.
The environmental pollution assessment was conducted within the Witwatersrand Basin,
at four gold mining sites selected mainly for their mining history and from geophysical
information obtained through satellite images. This study showed a relatively important
pollution in three of the four sites, namely the Vaal River west site near Klerksdorp, the
West Wits site near Carletonville (both in the North-West Province) and the Randfontein
site in the West Rand (Gauteng Province). Only one site, the closed Rietfontein landfill
site in the East Rand (Gauteng Province) was found to be not impacted by mercury
pollution.
The methylation of mercury was characterized in all sites and factors governing the
mercury methylation process at the different study sites were also investigated.
Geochemical models were also used to explain the distribution, transport and fate of
mercury in the study systems.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/11829 |
| Date | 27 August 2012 |
| Creators | Makiese, Julien Gilles Lusilao |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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