Mineral speciation and fate of some hazardous contaminants in coal waste pile from anthracite mining in Portugal

The coal (anthracite A) in Douro Coalfield (NW of Portugal) has been exploited for many years and has been mainly used as fuel supply by a thermal power plant. The mining activities inevitably impacted the environment, which includes a large number of coal waste piles emplaced over the old mine sites and adjacent areas of the Douro Coalfield. The disposal of coal mining residues represents significant environmental concerns due to their potential influence on soils and sediments, as well as on the surface and groundwater of the surrounding areas. In the present study, the development of sequential extraction combined with various advanced analytical techniques was performed to provide an improved understanding of the complex processes related with sulfide-rich coal waste oxidation, sequences of mineral formation, and the transport mechanisms of hazardous elements by specific neoformed soluble minerals. The results showed the presence of amorphous iron (oxy-) hydroxides and goethite with various degrees of crystallinity, containing hazardous elements, such as As, Cr, Hg, Mo, Se, Pb, U, and others. Some of the neoformed minerals found in the coal waste material are the same as those commonly associated with coal acid drainage, in which oxidation of sulfides plays an important role. The precipitated neoformed minerals include pickeringite, blödite, and a mixture of epsomite, pickeringite, and hexahydrite. As these sulfates may dissolve after the first rain, they may release above-mentioned elements into surrounding water bodies.

► Oxidation is the major process for mineral transformation in coal waste.
► The electron bean methodology has been applied to investigate neoformed minerals.
► Coal waste geochemisty can provide increase information in coal-ming areas.