Calcium carbonate as sorbent for lead removal from wastewaters

Low-cost and largely available industrial by-products such as calcite (CaCO3) have been considered as sorbents to remediate wastewaters from toxic elements, such as lead, in compliance with the European circular economy strategy. To date few articles are reporting results on lead sorption at the calcite-water solution interface by X-ray photoelectron spectroscopy (XPS) and this investigation aims to clarifying the mechanism of the interaction of Pb2+ model solutions over a wide concentration range, from 0.1 μM to 80 mM, with commercial calcite. X-ray powder diffraction (XRPD), scanning electron microscopy (SEM, EDX) and XPS analysis indicate that when CaCO3 particles are soaked in Pb2+ 0.1 mM and 1 mM solutions, hexagonal platelets of hydrocerussite [(PbCO3)2 Pb(OH)2] precipitate on its surface. When the concentration of Pb2+ is equal or higher than 40 mM, prismatic acicula of cerussite [PbCO3] precipitate. Solution analysis by atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) indicate that Pb2+ removal efficiency is nearly 100%; when the initial Pb2+ concentration was equal to 0.1 μM it was below the limit of detection (LOD) and the efficiency could not be determined. The sorption capacity (qe) increases linearly with increasing initial Pb2+ concentration up to a value of 1680 (20) mg/g when the initial Pb2+concentration is 80 mM. These findings suggest that heterogeneous nucleation and surface co-precipitation occur and calcite can be well considered a very promising sorbent for Pb2+ removal from wastewaters within a wide initial concentration range.