The economy of Botswana is largely driven by the mining sector. Aside from gold, copper, nickel, and coal, Botswana is the world's largest diamond producer by value. In general, mining activities especially the waste generated such as the mine tailings often lead to a serious and widespread occurrence of environmental pollution to both soil and water bodies by heavy metal(loid)s. Increased heavy metal(loid)s pollution in the environment beyond the tolerable limit poses a threat to life forms and ecological balance. This problem is prevalent in the mining areas of Botswana since the Environmental Impact Assessment (EIA) Act of 2011 and environmental management has not been fully enforced. In some countries, the rehabilitation and ecological restoration of mine areas or the environment is mandatory. One technique that can be applied to rehabilitate mining areas is phytoremediation. Phytoremediation is a technique that uses plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. It is considered a practical and more cost-effective remediation technology for a contaminated environment. Phytoremediation could offer an effective, environment-friendly, and economically feasible procedure to rehabilitate these mining areas. Currently, no rehabilitation efforts are being made in these mining areas, especially using phytoremediation technology. The overall aim of this study was to develop a phytoremediation technology suited to Botswana’s mine tailings and the specific objectives were (i) to assess the impact of heavy metal(loid)s contamination on soil and plant communities in mine areas; (ii) to evaluate the influence of different soil amendments on heavy metal(loid)s dynamics, survival and phytoremediation efficiency of selected plants; and (iii) to evaluate the potential of different tree species for phytoremediation of mine tailings. Five tree species were evaluated on their effectiveness in Abstract v phytoremediation of mine tailing from two mining areas. Additionally, several soil amendments like compost, fly ash, sewage sludge and mycorrhiza were also evaluated for their contribution to the survival and enhancement of the phytoremediation potential of these plants on these mine tailings. Tree species used for this phytoremediation study were Eucalyptus globulus, Colophospermum mopane, Vachellia tortilis, Vachellia luederitzii and Faidherbia albida. The heavy metal(loid)s pollution load in the tailings and in the vicinity of Monarch Gold mine and Bamangwato Concessions Limited (BCL) copper and nickel mine were assessed by quantifying the heavy metal(loid)s contents in soils and plants growing in the area. From this data, estimates of its possible impact to grazing cattle, goat, and sheep were performed through the daily intake of heavy metal(loid)s from plants and soils. Simultaneously, pot experiments were conducted under the screenhouse by growing some of the plants to assess their phytoremediation potential. This was done by applying different soil amendments following standard experimental designs in heavy metal(loid)s - contaminated soils or mine tailings. Results indicated that soils from the two mining areas had a pollution load index (PLI) of about 8.31 to 10.79 in BCL mine sites and between 1.84 to 2.87 in Monarch gold mine areas, indicating that there is severe pollution in the mine tailings and areas in the vicinity of the mine tailings. The plants growing in these areas had high concentrations of heavy metal(loid)s (As, Cu, Ni, Pb, Mn, Cd, Cr, and Zn) which were above the background level, indicating pollution. Different plants species were identified in mine tailings that are potential for phytoremediation agents include, Cenchrus biflorus, Bolboschoenus nobilis, Oxycaryum cubense, Portulaca oleracea Gossypium herbaceum, Withania somnifera, Fadogia homblei, Lantana dinteri, Schinus mole, Colosphospermum mopane, Boscia albitrunca, Senegalia mellifera Vachellia leuderitzii among others. Estimation Abstract vi of the daily intake of heavy metal(loid)s by cattle, goat, and sheep grazing on plants near the BCL mine and Monarch gold mine in Selibe-Phikwe and Francistown respectively, revealed that Cu, Ni, Pb, As, Cr were above the maximum allowable limits indicating the possibility of food chain contamination. Screenhouse trials of different trees grown in mine tailings without amendments had low survival rate (0%, 50%) compared to plants grown in mine tailings with amendments (90-100% survival rate). Plants grown in tailings without amendments also had low dry matter yield compared to plants grown in tailings with amendments. Different amendments had varying effects on mine tailings as some amendments like fly ash and compost increased the pH of the mine tailings, and reduced the availability of Cu, Ni, Cr, Pb, Mn and Zn to plants. Results obtained from various experiments and analysis indicated that Vachellia luederitzii was the best candidate for phytoremediation of mine tailings due to its high survival rate (70-100%) and high dry matter yield. V. luederitzii species also accumulated high concentrations of heavy metal(loid)s compared to other species indicating that it is a hyperaccumulator. Other tree species were ranked according to the decreasing preference based on survival rate, dry matter yield and heavy metal(loid)s accumulation. The rank was as follows: V. luederitzii F. albida>V. tortilis>C. mopane>E. globulus. In conclusion, there is contamination of heavy metal(loid)s in abandoned sites including the vicinity. This poses a risk to human health through direct exposure, bioaccumulation, and magnification through the food chain. Mine tailings in BCL and Monarch Gold mine and the vicinity can be phytoremediated with these identified tree species and the phytoremediation potential could be improved by the addition of amendments like fly ash, compost, sewage sludge, mycorrhiza, and their combination.

Thesis (PhD in Environmental Science)--Botswana International University of Science and Technology, 2024