Replacement of Calcium Carbonate Polymorphs by Cerussite

Calcium carbonate (CaCO3) polymorphs, calcite, aragonite, and vaterite, serve as a major sink to retain various metal ions in natural and engineered systems. Here, we visualize the systematic trends in reactivities of calcite, vaterite, and aragonite to Pb2+ dissolved in acidic aqueous solutions using in situ optical microscopy combined with ex situ scanning electron and transmission X-ray microscopies. All three polymorphs undergo pseudomorphic replacement by cerussite (PbCO3) but with distinct differences in the evolution of their morphologies. The replacement of calcite and aragonite occurs through the formation of a pseudomorphic cerussite shell (typically 5-10 mu m thick) followed by a slower inward propagation of reaction fronts through a thin solution gap (similar to 0.1 mu m wide) between the shell and the CaCO3 substrate. The replacement of vaterite is characterized by the formation of a thinner cerussite shell (<= 1 mu m thick) and a larger cavity between the shell and the host mineral. These systematic differences in cerussite morphology for different CaCO3 polymorphs are explained by the relative dissolution and precipitation rates of the reactant and product minerals, coupled with the role of ion transport through the cerussite shells. We also find that the replacement of calcite by cerussite is the slowest when all three polymorphs coexisted. Our results provide mechanistic insights into the growth mode of cerussite on dissolving calcium carbonate and demonstrate these CaCO3 polymorphs as promising substrate materials for removal and recycling of Pb from acidic polluted water and industrial effluents.