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

Abstract

The reactivity of vanadyl ions towards calcite has been studied in deoxygenated and oxygenated ultra-pure water at room temperature using several techniques: electron paramagnetic resonance (EPR), infrared (IR), laser Raman spectroscopy (LRS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and liquid-phase and solid-state ⁵¹V NMR. Our investigations reveal that the surface chemistry of calcite depends strongly on the concentrations of VO²⁺ solutions applied in the process. Indeed, for low VO²⁺ concentrations (⩽5×10^-5 mol dm^-3) in interaction with calcite (4×10^-2 mol dm^-3), it was found that vanadium(IV) is well dispersed on CaCO₃ surface in the form of solid solutions, (VO)_xCa_1-xCO₃, and the kinetics of its oxygenation on a monolayer type structure is relatively rapid (half-life time: 9–10 min). However , for higher VO²⁺ concentrations (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)_xCa_1+(z/2)-xCO₃) in contact with oxygen lead slowly to the generation of polyoxovanadate species at the calcite surface that contain both V(IV) and V(V) atoms. The combined use of EPR, LRS, IR, XPS and ⁵¹V NMR techniques has allowed the successful monitoring of these calcite surface phenomena, proving the existence of these layers, and even identifying the chemical composition of such coatings.