An appraisal of the physical environmental quality of the Selebi Phikwe Ni-Cu mine area, South-Eastern

Abstract

This research project focused on the environmental impact of mining and smelting nickel-copper (Ni-Cu) at the Selebi Phikwe area, south-eastern Botswana. Physico-chemical properties, mineralogical identification and characterisation, and heavy metals concentrations of elements for samples of tailings dump, soils, particulate air matter (PAM), Colophospermum mopane (mopane plant), and Imbrasia belina (phane caterpillar) were investigated. Physico-chemical properties studied on tailings dump and soil samples included soil texture and colour, particle size distribution (PSD), pH, electrical conductivity (EC), cation exchange capacity (CEC) and descriptive petrography. Identification and characterisation of minerals contained in tailings dump, soil, and PAM samples were performed employing X-ray powder diffraction (XRPD) techniques which included clay size and heavy minerals fractionation. Chemical analyses for heavy metals (cadmium, Cd; cobalt, Co; chromium, Cr; nickel, Ni and selenium, Se) concentrations in tail ings dump, soils, PAM, mopane leaves and phane caterpillar were measured with a graphite furnace atomic absorption spectrometer (GFAAS) whereas the flame atomic absorption spectrometer (FAAS) measured copper, Cu; iron, Fe and zinc, Zn concentration levels. The clay and silt soil components made up to 50 wt % of soil. Very acidic soils were located close to the smelter/concentrator plant, and both soil EC and CEC va lues were significantly low. Physical tests revealed albite, NaAISi30 s; cristobalite, a- Si02; chalcopyrite, CuFeS2; pyrrhotite, Fe1_xS; tremolite, Ca2MgsSis022(OHh; and pentlandite, (Fe,Ni)9Ss; to be contained in tailings dump. Soil colour varied from pale yellow, reddish yellow to dark reddish brown. The tailings dump comprised of nickelbloedite, Na2(Ni(S04h.4H20; pyrrhotite; quartz, Si02; pentlandite; malachite,Cu2C03(OHh; chalcopyrite; actinolite, Ca2(Mg,Fe)sSis022(OHh; cristobalite; tremolite; kaolinite, AI2SbOs(OH)4; mica and albite. The PAM consisted of quartz, Si02; pyrrhotite; chalcopyrite, CuFeS2; albite, and djurleite, CU31 S16. Bulk soil samples consisted of actinolite, albite, quartz, microciine, KAISi30 s; pyrrhotite, silicon sulphide, SiS; and cobalt oxide, CoO whereas the < 2 f.In fraction was made of kaolinite, smectite, Nao.3(AI,MghSi4010(OH)2.xH20; anorthite, CaAI2Si20 s; illite, KAI2(Si~1010)(OH)2 and quartz. Ojurleite polymorphs (CU31S16 and CU193S) were formed from secondary mineralisation of chalcopyrite and the S02 released from concentration/smelting processes. Ambient temperature and an acidic milieu created favourable conditions for the formation of nickelblodite and malachite from the primary ore minerals: pentlandite, chalcopyrite and pyrrhotite in tailings dump. Cobalt oxide and silicon sulphide identified in surface soils were indicative of environmental chemical alteration of mining waste deposited on surface soils. High concentrations of heavy metals recorded in different environmental media had affected the physical environmental quality at Selebi Phikwe. Heavy metals including Cd, Co, Cr, Cu, Fe, Ni, Se and Zn, which are deleterious to the environment, and pose as health hazards to humanbeings, were associated with these minerals. Contamination of waterbodies around Selebi Phikwe might have been possible by the heavy ions in solution. Consumption of stunted phane might pose as health hazard. In overcoming pollution problems at Selebi Phikwe, aspects of pollution management such as phytoremediation and phytomining, environmental desulphurisation, phytostabilisation, and biotechnology could be introduced as pollution control measures.