The use of Mössbauer spectroscopy in environmental research

Abstract

The impact that mining has on the environment is becoming an ever increasing problem all over the world. South Africa, Brazil and India are main producers of various valuable resources such as for example iron ore, platinum, gold and coal, of which coal and platinum mining will be discussed in this paper. Dumping of ash, waste and discards, result in the formation of acid mine drainage (AMD) due to the high sulphur content of the coal and the waste products. The main Fe-S-bearing minerals in the coals investigated were pyrite, jarosite and ferrous sulphate, a weathering product of pyrite. In the ash produced due to combustion or gasification of the coal, the main Fe-constituents are Fe2+,3+ glass (≈ 30%) and hematite (70%). The amorphous phase of the sample was composed mainly of SiO2 and Al2O3 with trace element inclusions of Hg, Ti, Cd and As. The soil, sediment and overburden in the coal mining areas contain pyrite as Fe-S-mineral and also ferrous sulphate as weathering product, with illite the main clay mineral. From laboratory leaching products of coal and ash, sulphur in the form of SO\(_{4}^{2-}\), was found to be one of the most leached ions with a concentration ranging between 100–1000 ppm. The amount of Fe leached out from the ash samples was between 2–5 ppm, but the Fe-leachability depends on the pH, with higher amounts leached out at pH ≤ 1.5. Magnetite losses, to the amount of about 1kg per tonne of magnetite used, occur during the dense medium separation process (DMS) used in cleaning the coal, which also reports in the waste product. South Africa is the largest producer of platinum and smelting of the ore can lead to various forms of pollution. Magnetite formation in the 2-stage furnace process is used as an indicator of the effectiveness of the reduction and the Fe2+ and Fe3+ ratio is used to monitor the process. In the flash furnace the ratio is 2–6, whilst in the electric furnace it is ≤0.02. If not monitored closely a large amount of nickel loss will occur if sent to the waste dump. Mossbauer spectroscopy was used to identify the Fe-species and the results were augmented by High Resolution-Transmission Electron microscopy (HR-TEM), Field Emission-Scanning Electron Microscopy (FE-SEM)/EDS, X-Ray Diffraction (XRD) and ICP-EOS results.