Characterization and modeling of mercury speciation in industrially polluted areas due to energy production and mineral processing in south africa

[摘要] Coal combustion is recognized as the primary source of anthropogenic mercury emissionin South Africa followed by gold mining. Coal is also known to contain traceconcentrations of mercury which is released to the environment during coal mining,beneficiation or combustion. Therefore, determining the mercury speciation in coal is ofimportance 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) forgold recoveries until 1915 and is still used in illegal artisanal mining. Consequences ofthese activities are the release of mercury to the environment. Nowadays, gold (anduranium) is also recovered through the reprocessing of old waste dumps increasing theconcern related to mercury pollution.While much effort has been put in the northern hemisphere to understand and controlproblems related to anthropogenic mercury release and its fate to the ecosystem, riskassessment of mercury pollution in South Africa was based, until very recently, on totalelement concentrations only or on non systematic fragmental studies. It is necessary toevaluate mercury speciation under the country’s semi arid conditions, which are differentto environmental conditions that exist in the northern hemisphere, and characterizepotential sources, pathways, receptors and sinks in order to implement mitigationstrategies and minimize risk.In this study, analytical methods and procedures have been developed and/or optimizedfor the determination of total mercury and the speciation of inorganic and organic formsof 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 figuresof merit and TGM concentrations obtained when using Au/TiO2, as a mercury trap, weresimilar to those obtained with traditional sorbents.The combination of isotope dilution with the hyphenated gas chromatography-inductivelycoupled plasma mass spectrometry (ID GC-ICP-MS) was also achieved and usedsuccessfully for the speciation analysis of mercury in solid, liquid and biological samples. The developed, or optimized, methodologies were used to estimate the average mercurycontent and characterize the speciation of mercury in South African coals, and also tostudy the speciation of mercury in selected South African environmental compartmentsimpacted by gold mining activities.The obtained average mercury content in coals collected from the Highveld andWaterberg coalfields (0.20 ± 0.03 mg kg-1) was close to the reported United StatesGeological Survey (USGS) average for South African coals. Speciated isotope dilutionanalyses and sequential extraction procedures revealed the occurrence of elementalmercury, inorganic and organo-mercury species, and also the association of mercurymainly 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 geophysicalinformation obtained through satellite images. This study showed a relatively importantpollution in three of the four sites, namely the Vaal River west site near Klerksdorp, theWest Wits site near Carletonville (both in the North-West Province) and the Randfonteinsite in the West Rand (Gauteng Province). Only one site, the closed Rietfontein landfillsite in the East Rand (Gauteng Province) was found to be not impacted by mercurypollution.The methylation of mercury was characterized in all sites and factors governing themercury methylation process at the different study sites were also investigated.Geochemical models were also used to explain the distribution, transport and fate ofmercury in the study systems.

[发布机构] University of the Witwatersrand