Acid mine drainage from mine tailings at Selebi Phikwe, eastern Botswana, has been investigated using a combination of total decomposition, sequential extraction, X-ray diffraction, Mössbauer spectroscopy, and SEM analyses of solid phase samples, water analyses, isotopic analyses, and geochemical modeling. The principal ferric phases in the seepage stream sediments are jarosite and goethite, which incorporate Ni and Cu. The Mössbauer spectroscopy (MS) indicated exclusively 3+ oxidation state of iron with typical features of ferric hydroxides/sulfates. A fraction of dissolved sulfate is also sequestered in gypsum which precipitates further downstream. Significant portions of Fe, Ni, and Cu are transported in suspension. Values of pH decreased downstream due to H+ generated by the precipitation of jarosite. Values of δ2H and δ18O indicate evaporation of pore water in the mine tailings before seepage. Values of δ34S(SO4) are consistent with the oxidation of sulfides, but sample from the seepage face is affected by dissolution of gypsum. No minerals of Ni and Cu were detected and the principal attenuation processes seem to be adsorption and co-precipitation with jarosite. Higher contents of Cu are sequestered in solid phases compared to Ni, in spite of much higher dissolved Ni concentrations. Based on the speciation calculations, seepage water is undersaturated with respect to all Ni and Cu phases and adsorption and co-precipitation with jarosite seems to be the principal attenuation processes. Direct geochemical modeling was able to reproduce downstream pH trends, thus confirming the precipitation of jarosite as the principal pH-controlling process.

Czech Geological Survey Klárov 3 118 21 Prague Czech Republic

Department of Geology Faculty of Science Palacky University 17 listopadu 12 771 46 Olomouc Czech Republic

Department of Geology Faculty of Science University of Botswana 4775 Notwane Rd Gaborone Botswana

Department of Soil Science and Soil Protection Faculty of Agrobiology Food and Natural Resources Czech University of Life Sciences Prague Kamýcká 1296 165 00 Prague Czech Republic

Institute of Geochemistry Mineralogy and Mineral Resources Faculty of Science Charles University Albertov 6 128 43 Prague Czech Republic

Regional Centre of Advanced Technologies and Materials Palacky University Šlechtitelů 27 78371 Olomouc Czech Republic

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