Mobility of Mn and other trace elements in Mn-rich mine tailings and adjacent creek at Kanye, southeast Botswana

Abstract

Mine tailings rich in Mn and an adjacent creek at Kanye, southeast Botswana, have been investigated by total decomposition of solids, X-ray diffraction, EMPA techniques, sequential extraction, and water leaching to understand binding and mobility of Mn and other trace elements. The investigation revealed weathering of tailings close to the surface of the tailings and higher concentrations of Fe compared to Mn. There also are high concentrations of Ba and Pb in the tailings solids. The principal minerals of Mn and Fe are cryptomelane, K(Mn4+, Mn2+)8O16, and hematite, Fe2O3, respectively. Most Mn is bound to less crystalline phases dissolved in the reducible step of sequential extraction, suggesting high potential for release of Mn and related trace elements. The situation is very different for Fe, which is found mostly in the residual (Aqua Regia) insoluble fraction. This is in a good agreement with high concentrations of mobilized Mn and with low concentrations of mobilized iron in the water leaching test. The order of water-leached trace elements is Ba>Zn > As>V > Cu > Co > Pb where trace elements bound in Mn-oxyhydroxides are preferentially mobilized. In particular, As and V may represent a potential risk for the environment during rainy periods when mine tailings are probably flooded, and tailings material is transported by water in an ephemeral creek. Currently, sediments in the creek are contaminated at least to the boundaries of the Kanye settlement and they can be a source of groundwater contamination during the rainy period. Another possible source of contamination is wind transport and settling of dust particles from mine tailings with resulting ingestion when local farming products are consumed.