Chemisorption of Cd2+ on Calcite Surfaces

The adsorption and precipitation of cadmium ions in the presence of calcium carbonate was studied in an effort to understand the processes of Cd 2+ retention in calcareous soils. Measurement of cadmium removal from solution by pure CaCO 3 using a selective ion electrode indicated that the quantity of cadmium adsorbed depended upon the CaCO 3 particle size. High levels of added cadmium apparently caused nucleation of CdCO 3 on CaCO 3 as the solutions approached saturation with respect to CdCO 3 . The initial chemisorption of Cd 2+ on calcium carbonate was very rapid, while CdCO 3 precipitation at higher cadmium concentrations was slow. Chemisorption may control Cd 2+ activity in some calcareous soils, producing solubilities much lower than predicted by the solubility product of CdCO 3 . The surface area of soil carbonates is likely to be important in determining the levels of cadmium that can be immobilized by chemisorption. The affinity of calcite for metal ions appears to be a function of ionic radius, with divalent metal ions of the same size as Ca 2+ readily substituting for Ca 2+ in accessible surface sites.