Lead and cadmium immobilization on calcitic limestone materials

Calcite minerals are used as an alternative low-cost retention material for metal removal in aquatic systems. The pollutant retention process occurring on the surface of these minerals has not been studied in sufficient detail particularly as regards the influence of surface characteristics of calcite-containing soil. To tackle the above, batch equilibrium sorption tests of three sampled limestone solids with different calcite percentages were carried out at different lead and cadmium concentrations in solution. The sorption of the heavy metal ions was found to follow the order Pb(II) > Cd(II). Limestone solid with 41.0% of calcite was found to be most effective in removing Pb(II) with a maximum lead removal capacity of 40 mg/g at pH 5–7, while maximum cadmium removal capacity was barely 1.3 mg/g. Soil characteristics were analyzed by X-ray diffraction and N2 adsorption at 77 K, prior and subsequent to the sorption test. As an approximation to the mechanism on the surface, in this study we considered the environment on the boundary layer and the species produced on this layer as adsorbates to be fixed onto the surface. In this way, equilibrium retention data were fitted to empirical isotherm models such as Langmuir, Freundlich and Temkin.