Stabilization of the Vaterite Structure in the Presence Of Copper(II):  Thermodynamic and Spectroscopic Studies

Abstract

The crystalline transformation of vaterite in the presence of copper(II) [Cu(II)] has been investigated in ultrapure water and at room temperature using several techniques; scanning electron microscopy (SEM), infrared (IR) and micro-Raman spectroscopy. Our findings show that a slowing down of this transformation occurs and is intimately related to the generation of CuCO 3 layers on CaCO 3 surfaces. This phenomenon is also accompanied by a decrease of vaterite solubility. All these observations have led us to suggest that these Cu(II) coatings prevent the dissolution of the metastable calcium carbonate, vaterite. To prove this suggestion, experiments on solubilities of Cu 2+ -vaterite solids were performed and interpreted in terms of thermodynamic equilibrium and stoichiometric saturation concepts. Thermodynamic and spectroscopic data demonstrate that the apparent stabilization of vaterite in the presence of Cu(II) is strongly dependent on the existence of water molecules in the lattice of the solid solution Cu x Ca 1-x CO 3 as (H 2 O) y Cu x Ca 1-x CO 3 . The chemical composition of this complex has been determined by X-ray photoelectron spectroscopy and thermogravimetry. Thus, the free energy of formation of such hydrated Cu(II) complexes has been found to be slightly lower than that for calcite. However, for high Cu(II) concentrations, metallic multilayers cannot grow indefinitely, and malachite [Cu 2 (OH) 2 (CO 3 )] appears as a new phase. SEM and micro-Raman techniques have allowed successfully monitoring these morphological phenomena/transformation.

A thermodynamic solution model for calcium carbonate: Towards an understanding of multi-equilibria precipitation pathways

“…The calculation methods are based on thermodynamic equilibria of all species in the solution, including the activity correction in or-der to approach their exact composition. More often than not, solubility calculations are only applied to calculate new dissociation constants [6][7][8][9][10] or initial solution compositions [11][12][13][14][15][16][17]. Thus, solubility calculations are only applied in general to equilibrated systems.…”

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“…In parallel, from a calcination temperature of 450°C, the band intensity at 712 cm -1 characteristic of calcite, increased between 450 and 490°C to reach a plateau. The bands obtained on the new spectrum (712 and 876 cm -1 ) are characteristics of pure calcite [11][12][13][14]. When vaterite is calcined at temperatures higher than 800°C, the IR spectrum obtained is characteristic of CaO phase.…”

“…The bands obtained for the untreated solid at 745, 876 and 1089 cm -1 are characteristics of pure vaterite [11][12][13][14]. A band at 712 cm -1 with a weak intensity was present.…”

“…The origin of this radical is probably due to the decomposition of copper nitrate precursor of Cu 2+ ions after calcination. The NO 2 o signal remained stable at higher temperatures but a signal characteristic of Mn 2+ ions [11][12][13][14], present in the solid, as impurities, appeared.…”

“…The introduction in the vaterite metal ions can stabilize or delay its transformation into calcite. This phenomenon was obtained when vaterite was doped by Cu 2+ ions [11][12][13][14]. In fact, by a simple washing of vaterite with water, it transforms rapidly into calcite.…”