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^-, CO₃^2-) ion exchanges at the water/CaCO₃-grain interface. The inclusion of Mn^II into vaterite grains occurs as adsorption followed by surface precipitation and formation of solid solutions, Mn_xCa_1-xCO₃. In this context the formation of these ‘Mn^II 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 Mn^II 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 Mn^II complexes: (H₂O)_y–Mn_xCa_1-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 Mn_xCa_1-xCO₃, and (ii) thereby explains the apparent stabilization of Mn^II-doped vaterite observed experimentally.