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Mode and sites of incorporation of divalent cations in vaterite

Electron paramagnetic resonance (EPR) spectroscopy, along with other analytical methods and techniques (X-ray diffraction, atomic absorption spectroscopy, etc.)have been used to study the mode of incorporation ofdivalent cations (Mg^2+, Sr^2+, Ba^2+, Cu^2+, Cd^2+ and Pb^2+) in vaterite. Vaterite, the least stable of the calcium carbonate polymorphs, was prepared by spontaneous precipitation from aqueous solutions and the doping of vaterite was performed by adding a particular divalent cation into calcium chloride solution before mixing it with a sodium (or potasium) carbonate solution. All vaterite samples, pure or doped with divalent cations, were subjected to gamma-irradiation. By studying EPR spectra of the pure vaterite powder the following free radicals were determined: CO3^- holes freely rotating at the surface of the spherullites (g=2.0113) ; CO2^- frozen in the matrix (g = 2.0031 and g = 1.9998), and freely rotating CO2^- (g = 2.0006) probably located between the carbonate layers. It was found that the alkaline-earth metal cations (Mg^2+, Sr^2+, Ba^2+) did not affect the formation and stability of the above free radicals. Cu^2+ and Cd^2+ doped to the vaterite phase trapped electrons by forming Cu^+ and Cd^+, respectively, which led, as a consequence, to increased formation of freely rotating CO3^- holes in the near neighbourhood. Pb^2+ released electrons readily by forming Pb^3+ thus causing a decrease in formation of CO3^- holes. In vaterite doped with Cd^2+ two additional paramagnetic centers (g = 1.9886 and g = 1.9939), imbedded in two different sites in the vaterite structure were detected.

calcium carbonate; calcite; vaterite; divalent cations; EPR spectra

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