The Role of Interfaces in Controlling Pb2+ Removal by Calcium Carbonate Minerals

The possibility to develop a process for lead removal from wastewater, based on calcium carbonate minerals, depends on the overall efficiency of the uptake process. Aqueous Pb tends to form cerussite (CER) via a dissolution-precipitation reaction when interacting with aragonite (ARG) and calcite (CAL). From a thermodynamic perspective, the two processes have a similar driving force, because the solubility of CAL and ARG is approximatively the same (K-s approximate to 1 x 10(-8)). Experimentally, the macroscopic yield of reaction was found to be very different. Using ex situ electron microscopy, diffraction, and in situ atomic force microscopy, we demonstrate that the Pb uptake mechanism by the two most abundant CaCO3 polymorphs is controlled by the kinetics of processes at the solid-solid and solid-liquid interfaces. Aragonite is isostructural with the product phase (CER) that easily precipitates taking advantage of the template effect offered by the surfaces of the substrate. The reaction proceeds through an interface-coupled dissolution-precipitation that leads to a mineral replacement. Because of a crystallographic mismatch, the reaction between CAL and CER mainly occurs as a simple solvent-mediated transformation. Our study unveiled the mechanistic reasons behind the different reaction yields shown by CAL and ARG toward Pb uptake. Similar conclusions can be extended to other contaminant((aq))-CaCO3(s) systems, thus increasing the predictability of limestone efficiency toward the uptake of heavy metals.