Assessment of selected properties of mine tailings at the former Kabwe mine and their potential to support plant growth
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Lead (Pb) and Zinc (Zn) were mined in Kabwe from 1904 to 1994, generating large quantities of solid mine waste. Among the solid wastes, were tailings from the leach plant which are now an environmental concern because of their high contents of Pb, Zn and other potentially toxic metals. To prevent contamination of the environment surrounding the tailings, there is need to grow plants on the tailing to stabilize them. Efforts to establish plants on the tailings in the past have proved to be difficult. A study was conducted to: (i) determine selected physical and chemical properties of the tailings (ii) to assess the suitability of the tailings for growing plants and (iii) to determine if the tailings qualify to be classified as hazardous materials due to their contents of Pb, Zn, Cu and Cd. Eighty-one (81) tailing samples from the top 0 - 60 cm depth were collected and characterized for their particle size distribution, bulk density, pH, Electrical conductivity (EC), organic matter, plant available P, exchangeable Ca, Mg, Na, and K; DPTA and Aqua regia extractable Cu, Zn, Fe, Mn, Pb and Cd. The tailings were red to dark reddish brown, with a fine earth fraction dominated by sand and silt sized particles with very low clay. They had a sandy loam texture, a low bulk density (1.00 ± 0.03 g.cm-3), and a mean total porosity of 57 %, indicating that they were fine, loose and porous materials. The tailings were very strongly acid to neutral (pH: 4.19- 6.81), and had low salinity (EC: 0.08-3.69 mS/cm).They had low organic matter (1.53 %) and plant available P (10.87 mg.kg-1), but high levels of exchangeable Ca, Mg, K and Na. They also had high to very high concentrations of DPTA and Aqua regia extractable Cu, Zn, Mn Fe, Pb and Cd. Mean concentrations of DPTA extractable metals were: 51.52 ± 3.97 mg Cu.kg-1; 1058.20 ± 96.34 mg Zn.kg-1 , 2,999.17 ± 154.65 mg Pb.kg-1 and 4.61 ± 0.39 mg Cd.kg-1. The mean Aqua regia extractable concentrations were: 272,773 ± 860 mg Fe.kg-1, 20,781 ± 455 mg Mn.kg-1, 1,462 ± 55 mgCu.kg-1, 244 ± 13 mg Zn.kg-1, 18,945 ± 550 mg Pb.kg-1, and 535 ± 74 mg Cd.kg-1. The Kabwe mine tailings were found to be unsuitable for plant growth, because of their low organic matter content, and high concentrations of bio-available Zn, Cu, Pb and Cd, which were likely to be toxic to plants. A comparison of mean total concentrations of Pb, Cu, Zn, and Cd in the tailings with the US EPA Total Threshold Limit Concentrations (TTLC) for these metals showed that the Kabwe tailings qualified to be classified as toxic materials with respect to Pb and Cd. Concentrations of plant extractable Pb and Cd were found to be inversely related to the pH of the tailings. The Kabwe mine tailings should be treated as toxic materials because of their high Pb and Cd contents. Access to the tailings should be restricted. Efforts to establish vegetation on the tailings should include means of increasing the organic matter content and measures to reduce the bioavailability of Pb, Cd, Zn and Cu. Key words: Kabwe mine, Tailings, Lead, Zinc, Copper, Cadmium, toxicity, plant growth.
The University of Zambia