Characterization and modeling of mercury speciation in industrially polluted areas due to energy production and mineral processing in south africa
Makiese, Julien Gilles Lusilao
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.