Manganese(II)/vaterite/water systems Spectroscopic and thermodynamic study

Abstract

The surface chemistry, reactivity of vaterite, and its crystalline transformation in the presence of manganese(II) have been studied in ultra-pure water and at room temperature using several techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), specific area measurements (BET), infrared (IR) spectroscopy, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). Our investigations reveal that this transformation is strongly dependent on the metal abundance in the reaction medium and cannot be considered as a simple or direct solid phase transition, i.e. vaterite→cubic calcite. Indeed, dissolution and precipitation of CaCO3 crystals proceed via (H+, HO-, Ca2+, Mn2+, HCO3-, CO32-) ion exchanges at the water/CaCO3-grain interface. The inclusion of MnII into vaterite grains occurs as adsorption followed by surface precipitation and formation of solid solutions, MnxCa1-xCO3. In this context the formation of these ‘MnII coatings’ contributes to both a decrease of vaterite solubility and a deceleration of the vaterite-to-calcite transformation. A fundamental thermodynamic approach to an understanding of the solubilities and solution properties of these MnII coatings has been addressed. The combined use of the IR and XPS techniques has allowed us to prove that vaterite grains are coated with a three component system: ‘CaCO3–MnCO3–H2O’ or hydrated MnII complexes: (H2O)y–MnxCa1-xCO3. Thermodynamic calculations demonstrated that the presence of water molecules in the lattice of Mn-vaterite contributes: (i) to a significant decrease in the free energy of formation of solid solution MnxCa1-xCO3, and (ii) thereby explains the apparent stabilization of MnII-doped vaterite observed experimentally.