Immobilization of Pb and Zn leached from mining residue materials in Kabwe, Zambia: Performance of calcined dolomite in column experiments

Legacy Pb-Zn mine in Kabwe, Zambia, has left massive stockpiles of mining residue materials (MRM) that contain potentially toxic elements, such as lead (Pb) and zinc (Zn), which pollute the environment. In situ remediation of the contaminated area is an attractive technique for immobilizing hazardous metals. In this study, the efficacy of modified dolomite, i.e., calcined dolomite (CD), was evaluated by mixing with the MRM at 1, 5, and 10 % ratios (CD1, CD5, and CD10, respectively). The leaching of hazardous metals, Pb and Zn, from the MRM was assessed by four-column experiments. Measurements of mineralogical and chemical constituents of the MRM, sequential extraction, and thermodynamic modeling of the leachates were carried out to determine the attenuation mechanisms. Sequential extraction results indicated that the leachable Pb and Zn in the MRM were mainly from the exchangeable and carbonate fractions. Furthermore, the results showed that the effluents from the column with MRM (CD0) exceeded the Zambian effluent regulatory values throughout 30 weeks of the experimental period. The introduction of the CD into the MRM increased effluent pH and decreased the leaching concentrations of Pb and Zn. The CD5 was enough to decrease the leaching concentrations of both hazardous metals below the Zambian effluent standard by prohibiting the release of Pb and Zn via the dissolution of carbonate minerals and retaining them in the carbonate fraction. The saturation indices of Pb and Zn suggest that the retention mechanisms by the CD accredit to precipitation, co-precipitation, and adsorption reactions. These results present an effective and inexpensive immobilization technology for hazardous wastes in abandoned mines.