Replacement of Calcium Carbonate Polymorphs by Cerussite

Abstract

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 μm thick) followed by a slower inward propagation of reaction fronts through a thin solution gap (∼0.1 μm wide) between the shell and the CaCO3 substrate. The replacement of vaterite is characterized by the formation of a thinner cerussite shell (≤1 μm 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.

Conversion of Electrochemically Deposited Aragonite Crystallites to Perovskite through Ion Exchange

“…31−33 A recent study by Kim et al provided insight into this conversion process for all three polymorphs of CaCO 3 and found that the mechanism is one of interface-mediated dissolution/recrystallization in acidic solution. 34 All three polymorphs showed the capacity to undergo the conversion but with different efficacies dictated by solubility constant, dissolution rate, crystal habit, and surface areas of the initial microstructures. Vaterite and aragonite, growing in polycrystalline aggregates, possess a higher surface area than euhedral single-crystal microstructures of calcite, causing faster dissolution on exposure to an acidic solution.…”

“…This causes the resulting PbCO 3 formed over the initial CaCO 3 microstructure to precipitate in much the same morphology as the layer beneath, effectively allowing for the retention of crystal morphology, though causing the change in surface texture and an apparent reduction in transparency (Figure 1B,E). 34 It is anticipated that this type of cerussite shell may be porous, allowing Pb 2+ ions in solution to migrate to the CaCO 3 /PbCO 3 interface to continue the process of conversion to PbCO 3 . 31,35 The pH of the Pb(NO 3 ) 2 conversion solution in the present work was measured and was found to be acidic with an average initial pH of 3.92; hence, we propose that the conversion process proceeds by the same mechanism.…”

“…A recent study by Kim et al provided insight into this conversion process for all three polymorphs of CaCO 3 and found that the mechanism is one of interface-mediated dissolution/recrystallization in acidic solution . All three polymorphs showed the capacity to undergo the conversion but with different efficacies dictated by solubility constant, dissolution rate, crystal habit, and surface areas of the initial microstructures.…”

Formation of Strontianite and Witherite Cohesive Layers on Calcite Surfaces for Building Stone Conservation

“…The molar volume increases involved in the calcite-by-strontianite (Δ V = + 2.07 cm 3 /mol; +5.6%) and calcite-by-witherite (+8.87 cm 3 /mol; +24.1%) replacement reactions determine the formation of very compact overgrowths which only contain a small volume of isolated micropores. The lack of interconnected porosity within these overgrowths makes them impermeable to the fluid, preventing significant mass transfer from the bulk fluid to the reactive interface between the Volmer–Weber replacement layer and the unaltered calcite core. ,− Slight differences in the molar volume change involved in both replacement reactions can result from differences in the amount of Ca incorporated into strontianite and witherite as this incorporation will contribute to the decrease of the molar volume of both phases. In any case, the impact of these differences on the textural features of the strontianite- and witherite-replaced layers will be very small and will not significantly affect the effectiveness of their armoring of the calcite surface.…”

Four-dimensional X-ray micro-tomography imaging of dynamic processes in geosciences

“…XMT has also revealed the complex interplay during dissolution/precipitation coupled reactions or phase transformation, which are explored using either single crystals [Kim et al, 2021] or aggregates confined in pressure vessel. For example, Fusseis et al [2012] and Bedford et al [2017] have provided insights in the dehydration reactions during heating of gypsum (CaSO 4 , 2H 2 O) and subsequent transformation into bassanite (CaSO 4 , 0.5H 2 O), whereas Zheng et al [2017] have investigated the hydration of periclase (MgO) into brucite (Mg(OH) 2 ).…”