The reactions of cadmium with calcium carbonate surfaces

The reaction of cadmium with calcite was studied with 10⁻⁶–10⁻²M Cd²⁺ using 4g CaCO₃ and 50 ml solution. The calcite-water suspension was equilibrated for 2 d before adding Cd(NO₃)₂. The CaCO₃ surface has a high affinity for Cd and at < 1 μmol g⁻¹ a linear adsorption isotherm (C-type) describes the reaction, whereas precipitation of CdCO₃ on calcite predominates at higher Cd additions. In the latter case pH decreased and solutions were undersaturated with respect to CaCO₃ indicating that CdCO₃ prevented the CaCO₃ surface from reacting with the acidity generated from CdCO₃ precipitation. The data for low Cd additions suggests that an ideal surface solid solution is formed between CdCO₃ and CaCO₃ and the Thorstenson and Plummer equation expressing the pIAP values of surface solid solutions is valid. Equilibrium was attained for low Cd additions in several minutes, whereas precipitation was a slow process; at the changeover point distinction between the two processes is difficult. The reacting surface area calculated from the maximum adsorption of Cd is 0.5-1.0 times that measured by the BET method.
The Cd-reacting surface areas of four natural calcareous minerals were much less than those found by the BET method which includes the surfaces of other minerals present. The Thorstenson and Plummer equation may be a useful way to relate solution composition and carbonate-adsorbed Cd in contaminated soils.