Mechanism of the solid-liquid interfacial reaction between dissolved Cd species and the CaCO₃ surface

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^-.