Heavy metals in soil, plants, and associated risk on grazing ruminants in the vicinity of Cu-Ni mine in Selebi-Phikwe, Botswana.

The impact of BCL Cu-Ni mines on the surrounding environment has indicated high levels of heavy metal contamination in soil and some plant species. A comprehensive assessment of heavy metal concentration in plants, heavy metal concentration and availability in soils, and the estimates of risk associated with grazing animals in the area are presented. Exhaustive quantification of heavy metal contents in 82 plant samples revealed that some plants such as Boscia albitrunca and Boscia foetida are suitable for multi-metal phytoextraction, and others can accumulate one or two of the metals in soils or tolerate high levels of contamination. Current levels of soil contamination were manifested by acidification and high electrical conductivity, high contamination factor, and a pollution index between 8.31 and 10.79. The amount of exchangeable fractions of metals was higher than ordinary soils which is attributed to the high solubility of deposited materials on the soil surfaces. Daily intake estimates showed a possible risk associated with Pb and Cu contamination among grazing animals in the study area. Overall, the information has identified potential plants or combination of plants that could be utilized for the rehabilitation of the study area through phytoremediation. In addition, the estimates of the daily intake of the minerals due to consumption of plants in the vicinity of the BCL mines warrant for evaluation of the actual levels of heavy metals in grazing animals near the study area and in other mining areas in Botswana.

Influence of mycorrhiza and fly ash on the survival, growth and heavy metal accumulation in three Acacia species grown in Cu-Ni mine soil.

Acacia albida, Acacia luederitzii, and Acacia tortilis are dominant acacia species in Botswana and have the potential to rehabilitate the heavy metal degraded environment. To establish this claim, experiments to assess the influence of mycorrhizal inoculation and fly ash amendments on the survival, growth and heavy metal accumulation of these species in mine tailings were conducted. A two-factor (AM inoculation × fly ash) in CRD was done on each of the three Acacia species consisting of four treatments: control (no mycorrhizal, no fly ash coded as - AM/- FA), with mycorrhizal but no fly ash (+ AM/- FA), no mycorrhizal but with fly ash (- AM/+ FA), and with mycorrhizal and with fly ash (+ AM/+ FA). After 24 weeks, results showed that the survival and dry matter yield of all Acacia species were enhanced by 10% with fly ash amendments. However, mycorrhiza inoculation alone improved the survival of A. albida and A. luederitzii but reduced that of the A. tortilis in mine tailings. Fly ash amendments increased the pH of the mine tailings, reduced the availability of Cu, Ni, Pb, Mn and Zn and consequently reduced the concentration of these metals in shoots. On the other hand, it increased the availability of As in the mine tailings. In addition, mycorrhizal inoculation reduced the concentration of these metals in shoots regardless of fly ash amendments. Overall, combined mycorrhizal inoculation and fly ash amendment enhanced the establishment of A. luederitzii in heavy metal-contaminated soils by reducing the heavy metal availability and metal uptake, thus increasing the survival and dry matter yield of plants.