An assessment of toxic metals in soda mine tailings and a native grass: A case study of an abandoned Nyala Magnesite mine, Limpopo, South Africa

Abstract

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. Thus A. congesta is recommended to rehabilitate the mine tailings and provide a cover against wind and water erosion. © 201 WIT Press.