Assessment of Ecotoxic Metal Leaching Behaviour of Coal Gangue Using a Sequential Extraction Procedure

Abstract The environmental leaching characteristics of the ecotoxic metals Cr, Co, Ni, Cu, Zn, As, Se, Ag, Cd, Tl, and Pb within six coal gangue samples from mines located in Poland were assessed using the BCR-2 sequential extraction procedure. The samples’ bulk mineralogical and chemical compositions were determined by petrographic examination, x-ray fluorescence spectroscopy, x-ray diffraction, and inductively-coupled mass spectrometry (following acid digestion). Of the metals studied, Cd (predominantly oxidizable but often exchangeable) and Pb (predominantly reducible) likely pose the greatest hazard to the environment. Leachable concentrations of these metals were as high as 1 µg/g and 60 µg/g, respectively, implying that water/rock mass ratios as high as 25,000 and 80,000 might be required during weathering for them to comply with EU freshwater quality standards. Ni (predominantly oxidizable), Cr (predominantly reducible), Zn (predominantly exchangeable), and Cu (predominantly exchangeable) were also abundant, exhibiting leachable concentrations as high as 31 µg/g, 70 µg/g, 1,560 µg/g, and 72 µg/g, respectively. Leaching of all metals is likely to be accelerated by the gangue samples’ acid-forming propensity, which arises from their considerable (albeit variable) inventories of framboidal pyrite. This is especially the case for the gangues from the Bogdanka and Piast mines, since these possess an order of magnitude more sulfidic material than the other wastes (up to 1.9 wt%). Management of repositories containing these materials will require specific measures to prevent acid generation and ecotoxic metal release into local watercourses. Specifically, storage beneath covers that limit access by oxygen, as well as encapsulation within acid-neutralizing (e.g. calcite-bearing) materials, is essential for the wastes from the Bogdanka and Piast mines, but is also recommended for those from the Jankowice, Marcel, Staszic, and Chwalowice mines.