Geochemistry and mineralogy of vanadium in mine tailings at Berg Aukas, northeastern Namibia O. Sracek a,⇑ , M. Mihaljevic ˇ b , B. Kr ˇíbek c , V. Majer c , J. Filip d , A. Vane ˇk e , V. Peníz ˇek e , V. Ettler b , B. Mapani f a Department of Geology, Faculty of Science, Palacky ´ University, Olomouc, Czech Republic b Institute of Geochemistry, Mineralogy and Natural Resources, Faculty of Science, Charles University, Praha, Czech Republic c Czech Geological Survey, Praha, Czech Republic d Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacky ´ University, Olomouc, Czech Republic e Department of Soil Science and Soil Protection, Czech University of Life Sciences Prague, Praha, Czech Republic f University of Namibia, Windhoek, Namibia article info Article history: Received 12 December 2013 Received in revised form 3 April 2014 Accepted 8 April 2014 Available online 21 April 2014 Keywords: Vanadium Mine tailings Namibia Weathering Adsorption abstract Vanadium-rich mine tailings at Berg Aukas, site with a world-known vanadium mineralization in north- eastern Namibia, were investigated using a combination of solid phase and mineralogical analyses, leach- ing tests and speciation modeling. Principal objective of the study was to determine, if vanadium can be released into the environment. In spite of >30 y of weathering and oxidation of tailings material to a maximum sampling depth of 2.4 m, a large portion of V still remains in the primary mineral descloizite, (Pb, Zn) 2 (OH)VO 4 . A part of V was mobilized and adsorbed/co-precipitated with ferric oxyhydroxides. Based on sequential extraction and 57 Fe Mössbauer spectroscopy, a large amount of ferric iron is present in insoluble hematite and goethite, where V is effectively bound. Other potential contaminants are Zn, present mostly in the primary mineral willemite, Zn 2 SiO 4 , descloizite and also in secondary smithsonite, ZnCO 3 ; and Pb, which was transferred from completely dissolved galena to cerussite, PbCO 3 and is also partly present in pri- mary descloizite. Conditions in the mine tailings are alkaline (pH generally >8.2) and oxidizing during dry period, but mobility of V is low due to low solubility of descloizite and secondary crystalline ferric phases such as hematite. In contrast, Zn and especially Pb in secondary carbonates, probably represent more serious environmental and health risks. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Vanadium is an abundant element in the earth’s crust, with an average occurrence of 120 ppm (Reimann and Caritat, 1998). More than 80 minerals of V have been described to date (Clark, 1990). It may also substitute for Fe and for this reason it is more common in mafic rocks compared to felsic counterparts. Vanadium is also very abundant in shale, especially in that of marine origin with high organic matter content, where V contents may reach up to 16,000 ppm (McKelvey et al., 1986). An important source of V-pollution is from the ferrous metallur- gical industry including smelting and foundry operations. Other anthropogenic sources include chemical and polymer industries, the mining industry, and combustion of V-rich fossil fuels (Nriagu, 1998). The geochemical behavior of V in aqueous environment is com- plex because V occurs in several oxidation states including V(III), V(IV), and V(V). Oxyanion-forming V(V) is most common under oxi- dizing conditions and its concentration is often controlled by adsorption on or co-precipitation with ferric oxyhydroxides. In some environments, such as the Pampean loess aquifers in Argentina with alkaline groundwater, V concentrations may reach up to several mg/ l and compete with arsenic for adsorption sites (Smedley et al., 2005; Bhattacharya et al., 2006). Wright and Belitz (2010) collected a large dataset from California and they also found the highest dissolved V concentrations in oxic and alkaline groundwater. Mine tailings may result in formation of acid mine drainage caus- ing serious environmental problems (Blowes et al., 2003). However, when the neutralization capacity of the solid phase is high, condi- tions are neutral and dissolved ferric iron precipitates on the surface of sulfides and these surface rims limit further oxidation (Nicholson et al., 1990; Hossner and Doolittle, 2003). Neutral or even alkaline conditions are not favorable for attenuation of oxyanionic contam- inants such as vanadium, which are desorbed from the negatively charged surface of oxyhydroxides (Cornelis et al., 2008). In arid conditions evaporation dominates water regime in unsaturated zone of mine tailings (Dold and Fontbote, 2001), contaminants http://dx.doi.org/10.1016/j.jafrearsci.2014.04.003 1464-343X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +420 585634537; fax: +420 585225737. E-mail address: srondra@yahoo.com (O. Sracek). Journal of African Earth Sciences 96 (2014) 180–189 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci