The purpose of the study was to assess the level of toxic metals (Cd, Co, Cu, Cr, Mn, Pb, Ni and Zn) in an abandoned Nyala mine tailings dams and in Aristida congesta, a native grass. The mean value of toxic metals in the mine tailings soils 1 were: Mn (498.78 ± 2.42 mg/kg); Ni (114.62 ± 2.64 mg/kg); Cr (81.30 ± 3.03 mg/kg); Co (77.10 ± 0.46 mg/kg); Zn (58.31 ± 0.90 mg/kg); Cu (26.32 ± 0.12 mg/kg); Pb (14.13 ± 0.70 mg/kg) and Cd (10.00 ± 0.00 mg/kg) respectfully. The mean value of toxic metals in the mine tailings soils 2 were: Ni (959.31 ± 9.72 mg/kg); Mn (424.24 ± 0.76 mg/kg); Co (63.44 ± 0.83 mg/kg); Cr (56.52 ± 0.69 mg/kg); Zn (46.26 ± 0.92 mg/kg); Cu (22.03 ± 0.64 mg/kg); Pb (17.57 ± 0.87 mg/kg) and Cd (1.13 ± 0.12 mg/kg). The pH of mine tailings soils 1 was 7.97 ± 0.06 and electrical conductivity was 4.92 ± 0.06 mS/cm. The pH of mine tailings soils 2 was 8.94 ± 0.06 and electrical conductivity was 1.88 ± 0.10 mS/cm. A. congesta, was growing in abundance in mine tailings soils 2 and none in mine tailings soils 1. The reason may be due to cumulative toxic metals and Ni which was eight times more in mine tailings soils 1. A. congesta accumulated the toxic metals (sum of roots, stem and leaves) as follows: Cr (184.55 mg/kg); Mn (104.60 mg/kg); Ni (95.99 mg/kg); Co (18.66 mg/kg); Zn (10.13 mg/kg); Cu (8.93 mg/kg); Pb (4.40 mg/kg) and Cd (0.93 mg/kg). The bioaccumulation coefficient was 2.27 which mean that A. congesta is a hyperaccumulator for Cr. Water Pollution XI 415Thus A. congesta is recommended to rehabilitate the mine tailings and provide a cover against wind and water erosion.
Mine waste, including tailings is generally outlined as one of the largest environmental concern which faces defunct mines in South Africa and New Union Gold Mine is no exception. These tailing contain heavy metal such as chromium (Cr) and uranium (U) which poses enormous threat to the environment even at small quantity. The study focuses mainly on bioaccumulation of Cr and U in soil by Cynodon dactylon, an indigenous grass. The grass and soil sample were collected at New Union Gold Mine and Ka-Madonsi Village at Malamulele, Limpopo Province, South Africa. The concentration of Cr and U were determined with a Thermofischer ICP MS. The research findings indicate that the range in the levels of Cr and U at mine tailings dam A were 152.60 to 196.12 mg/kg and 0.51 to 0.92 µg/gm, respectively. The ranges in the levels of Cr and U at mine tailings dam B were 151.34 to 229.67 mg/kg and 0.85 to 1.06 µg/g, respectively. The levels of Cr and U at the control site were 81.31 mg/kg and 0.73 µg/g. The pH of mine tailing dam A was 3.23 to 3.34 and for tailing dam B were, 3.25 to 3.29 making both tailing acidic while for the control site, it was slightly alkaline at 7.56. The bioconcentration and translation factors of C. dactylon were variable but were dependent on pH conditions. Thus, C. dactylon was able to bioaccumulate toxic metals Cr and U from the mine tailings making them potential phytoremediation agent for the rehabilitation of exposed mine tailings. This is important in covering the mine tailings since any exposed part of mine tailings is liable to water and wind erosion. Thus, Cr and U may be exported to external environment such as aquatic ecosystem and neighboring rural communities with negative impacts.
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