Nano precipitates formed during the dissolution of calcite incorporated with Cu and Mn

Calcite, because of its strong chemical activity, is often doped with impurity ions that will affect the dissolution behavior of calcite-water interface. Solid solutions of (Ca, Cu)CO₃ and (Ca, Mn)CO₃ were synthesized by co-precipitation to study its dissolution in acidic solution (pH=5) at 25℃. This experiment aims to study the dissolution pattern of impurity ions in calcite, and reveals the effect of impurity ions in calcite on its dissolution process. ICP-OES results shown that the aqueous concentrations of Cu²⁺ and Mn²⁺ increased first and then decreased with the extension of dissolution time. It indicated that some of the aqueous Cu²⁺ and Mn²⁺ had been removed by re-adsorption or precipitation from the solution in the secondary stage. SEM and TEM observation revealed that new precipitates with needle or prism morphology formed on the surface of calcite. The size of the precipitates were 1~2μm in length and 10~20 nm in width for (Ca, Mn)CO3 and 50 ~ 200 nm for (Ca, Cu)CO3. The EDS data shown that the new formed precipitates were rich in Mn and Cu, respectively, comparing to their parents minerals. HRTEM the new formed Mn-rich precipitate was close to MnCO₃. It suggested that the dissolution and precipitates of carbonates might be changed due to the presence of impurity ions. Our results provided new insights on the behaviour of carbonate minerals and heavy metals such as Mn and Cu in the environment.