Divalent Cd and Pb uptake on calcite {101̄4} cleavage faces: An XPS and AFM study

The interaction of divalent Cd and Pb with the {101̄4} cleavage faces of calcite has been investigated with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Analysis of the {101̄4} cleavage planes of calcite was carried out with X-ray photoelectron spectroscopy (XPS) after exposure to divalent metal-bearing solutions in the 0.1-100 μM concentration range for times ranging from 1 to 24 h. The uptake of Cd²⁺ by calcite was determined to be greater than that of Pb²⁺ under similar experimental conditions (1 μM, pH 8.2, 24 h exposure time). In both cases, the majority of the divalent metal was postulated to exist in a surface precipitate. AFM results showed that the exposure of calcite to a 1 μM Pb²⁺ solution resulted in ellipsoidal surface growths that were attributed to the nucleation of a PbCO₃ bulk phase. In the Cd circumstance, AFM showed comparatively flat growth features forming on the calcite surface even at concentrations down to 0.1 μM, where the solution would be expected to be undersaturated with respect to Cd bulk phases. These features were attributed to a (Ca,Cd)CO₃ solid solution. The individual exposure of these Cd/CaCO₃ and Pb/CaCO₃ samples to water pre-equilibrated with calcite (metal free) for 1 h led to the removal of no more than 20% of the divalent metal, suggesting that if there was an adsorbed Pb or Cd complex initially on the calcite surface, it was an minority species compared to the precipitate phase. Exposure of calcite to 100 μM Cd and Pb resulted in the accumulation of precipitate on the calcite surface presumably due to the divalent metal initial solution concentrations exceeding the solubility products of CdCO₃ and PbCO₃, respectively.