Determination and modelling of geochemical speciation of uranium in gold mine polluted land in South Africa

The East and Central Rand of the Witwatersrand has been one of the most important gold mining areas in the world. One of the environmental consequences of this mining activity has been the release and distribution of heavy metals into the ground and surface water systems as a result of acid mine drainage arising from oxidation of pyrite (FeS2) in the tailings dumps. The research is aimed at the quantitative assessment of the distribution of uranium and the modelling of its geochemical speciation in gold mine polluted land. Uranium in the form of uraninite (UO2) and brannerite (UTi2O6) is normally associated with gold-bearing ores and studies of samples collected from the study area revealed high concentrations. Analyses of dump material indicate that U is being selectively separated from its decay products in the dumps, resulting in locally high concentrations of U. It is possible that such areas of enrichment may not be detectable by conventional gamma ray spectroscopy because of separation of U from its decay products. Separation occurs as U is oxidized to the U (VI) state. Chemical modelling indicates that U occurs predominantly as uranyl-sulphate complexes in the water, which are soluble over a wide pH range. Neutralization of the acidic water in swamp environments is associated with removal of U. This is partly due to adsorption or co-precipitation on Fe and Mn oxides, and partly due to reduction of U to the U (IV) state.