Removal of Aqueous Cu2+ by Amorphous Calcium Carbonate: Efficiency and Mechanism

Crystalline calcium carbonate (CaCO3, such as calcite) could scavenge aqueous metals via adsorption and coprecipitation. As a precursor to crystalline CaCO3, amorphous calcium carbonate (ACC) is poorly understood on metals removal. Herein, we synthesized silica-stabilized ACC and investigated its Cu2+ removal efficiency and mechanism. The results showed that the Cu2+ removal efficiency by ACC is controlled by the initial solution pH, initial Cu2+ concentration, contacting time, and ACC dosage. The maximum Cu2+ removal capacity was 543.4 mg/g at an ACC dosage of 1 g/L, an initial pH of 5.0, an initial Cu2+ concentration of 1000 mg/L, and an equilibrium time of 20 h. X-ray powder diffraction (XRD) and scanning electron microscope with an energy dispersive spectrometer (SEM-EDS) revealed that Cu2+ precipitated as paratacamite (Cu-2(OH)(3)Cl, space group: R (3) over bar) at an ACC dosage of 1 g/L, whereas botallackite (Cu-2(OH)(3)Cl, space group: P2(1)/m) was the Cu-bearing product for crystalline calcite using the same dosage as ACC. However, Cu2+ preferred to incorporate into calcite, which is transformed from ACC at high ACC loading (such as 4 g/L). Our results demonstrated that the crystallinity and dosage of CaCO3 could control the Cu2+ removal mechanism.