Mercury speciation in South African coal

Abstract

Total mercury (HgT) and mercury speciation in coal samples from power stations was analysed using the automatic mercury analyser (AMA-254) and capillary electrophoresis (CE) coupled with photodiode array (PDA) detector. For mercury (Hg) speciation analysis, the samples were first extracted by acid leaching, followed by complexation with cysteine before analysis.Total Hg analysis of reference materials show good agreement with the respective certified values provided for reference materials. However, total Hg analysis in acid extracts showed that 80% of Hg was extracted with the respective certified values provided for reference materials. The same trend was observed in coal samples from various power stations. This is due to the fact that Hg in South African coal is associated with pyrite, followed by organic fraction and carbonate. Pyrite is linked to the content of sulphur in the coal. Pyrite is soluble in HCl and HNO3 hence 80% recovery of Hg was obtained. The 20% not recovered can be associated with insoluble forms, either organic matter and silicates which are insoluble in HCl and HNO3.Mercury speciation using CE in acid extracts showed that only Hg2+ species was detected from the extracts and that 96% of total Hg in acid extracts is in the Hg2+ species form. From results and discussions on acid extraction in coal, it can be assumed that the 4% which was not detected by the CE is organic mercury and that its concentration is below the detection limit (1ppm for methylHg+ and ethylHg+). Based on results obtained in this study, it can be concluded that although the coal samples contain mainly inorganic mercury, small amounts of organic mercury is also present.