Spectroscopic studies of vanadyl-calcite-water-oxygen systems and characterization of oxo-vanadium species deposited on CaCO3

The reactivity of vanadyl ions towards calcite has been studied in deoxygen ated and oxygenated ultra-pure water at room temperature using several tech niques: electron paramagnetic resonance (EPR), infrared (IR), laser Raman s pectroscopy (LRS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and liquid-phase and solid-state V-51 NMR. Our investig ations reveal that the surface chemistry of calcite depends strongly on the concentrations of VO2+ solutions applied in the process. Indeed, for low V O2+ concentrations (less than or equal to 5x10(-5) mol dm(-3)) in interacti on with calcite (4x10(-2) mol dm(-3)), it was found that vanadium(iv) is we ll dispersed on CaCO3 surface in the form of solid solutions, (VO)(x)Ca1-xC O3, and the kinetics of its oxygenation on a monolayer type structure is re latively rapid (half-life time: 9-10 min). However , for higher VO2+ concen trations (greater than or equal to 10(-4) mol dm(-3)), metallic multilayers (and/or clusters) grow in the medium, and a three or four components solid solution of CaCO3-VOCO3-VO(OH)(2)-(H2O) appears as a new phase. Such VO(ii ) complexes (that can be written as follows: (OH)(z)(H2O)(y)(VO)(x)Ca1+(z/2 )-xCO3) in contact with oxygen lead slowly to the generation of polyoxovana date species at the calcite surface that contain both V(iv) and V(v) atoms. The combined use of EPR, LRS, IR, XPS and V-51 NMR techniques has allowed the successful monitoring of these calcite surface phenomena, proving the e xistence of these layers, and even identifying the chemical composition of such coatings.