Assessment of true laterites for their use as liners in karst-depression tailings impoundments

The true laterites mantling karst depressions acted as liners in tailings impoundments in the Gejiu mining area, southwestern China. To investigate the feasibility of true laterites as liners to retain pollutants in tailings impoundment, geotechnical, chemical, mineralogical and isothermal adsorption tests were performed on true laterites, with Kunming laterites rich in typical clay minerals as a control. The results indicated that true laterites in the depressions were dominated by Al-Fe-Mn oxide nodules and only contained trace amounts of typical clay minerals such as kaolinite. Nevertheless, true laterites were still fine-grained, and the geotechnical properties satisfied the basic requirements as a hydraulic barrier material. Additionally, true laterites exhibited a great potential to adsorb Heavy metal ions, with removal efficiencies of up to 97%, 99%, and 99% for Mn²⁺, Hg²⁺, and Cd²⁺, respectively, at an initial concentration of 20 mg/L. Compared to Kunming laterites, although Gejiu true laterites lacked typical clay minerals and had smaller specific surface area and cation exchange capacity, the adsorption capacity of true laterites for Mn²⁺, Hg²⁺ and Cd²⁺ did not weaken as expected, and even had a better adsorption capability for Hg²⁺, which can be attributed to the presence of Al-Fe-Mn concretions rich in true laterites. The complexation of metals with surface functional groups, combining with physical sorption and ion exchange were likely the primary mechanisms for Mn²⁺, Hg²⁺ and Cd²⁺ adsorption on true laterites. Considering the above, the true laterites in karst depressions could be used as the most cost-effective liner materials for tailings impoundments.