The solid-liquid interfacial reactions between heavy metal ions and CaCO3 play important roles in geochemical and environmental processes. In particular, the sorption of dissolved Cd species is characterized by two reaction steps. Initially, Cd2+ is adsorbed onto the CaCO3 surface. Further, the adsorbed Cd species formed a calcium-cadmium carbonate solid solution. However, the mechanism of the solid-liquid interfacial reaction between the dissolved Cd species and the CaCO3 surface at the primary step of sorption is not clear. In this study, the adsorption behavior was investigated to elucidate the mechanism for the solid-liquid interfacial reaction, and then a detailed analysis of the chemical state of the adsorbed Cd species was performed by X-ray absorption spectroscopy. The adsorption experiments were performed for 4 h with Cd initial concentration of 1.00 μmol/dm3 and various amounts of fine CaCO3 powder while bubbling N2. The adsorption behavior was not suppressed by the presence of NaClO4. In addition, the adsorption isotherm fitted well to the Langmuir type. Therefore, it was indicated that the dissolved Cd species were adsorbed specifically onto CaCO3. The Cd species adsorbed on CaCO3 was characterized via Cd K-edge and L3-edge X-ray absorption spectroscopy. From the results, it was revealed that the dissolved Cd species was specifically adsorbed at >CaCO3H on the surface of CaCO3. Accordingly, the dissolved Cd species formed an inner-sphere complex that coordinates bidentately with two >CaCO3-.
