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Homogeneous precipitation of calcium carbonate polymorphs by catalyzed decomposition of urea

Precipitation processes of carbonate particles of different size and shape have been intensely investigated, due to their importance in geo- and biosciences, as well as in numerous industrial applications. The aim of this presentation is to show that uniform calcium carbonate polymorphs of different size and shape can be obtained by homogeneous precipitation in solutions of calcium salts in the presence of urea at elevated temperature. It was found that the formation of different polymorphs of calcium carbonate was affected mostly by agitation and the concentration of the reactants, as well as by the mixing procedure. Thus, under otherwise the same experimental conditions, uniform needles of aragonite were formed without stirring (Fig. 1a), rhombohedral calcite appeared when aging experiments were carried out under the influence of magnetic stirring (Fig. 1b), while mixing the preheated reactant solutions resulted in unique 'flower-type' vaterite particles (Fig. 1c). In addition, catalytic decomposition of urea by the enzyme, urease, rapidly precipitates calcium carbonate particles at room temperature. It is apparent that urease macromolecules are important in the development of the calcite faces, and that these particles show the characteristic features of intercellular crystals that appear in living organisms (Fig. 1d). The latter experimental system may simulate processes that take place in nature, where certain bacteria excrete the enzyme, urease, which then interacts with the urea formed by biodegradation processes of organic materials in the presence of calcium salts. For additional information on the homogeneous precipitation of uniform carbonate particles by thermal and enzymatic decomposition of urea the readers are referred to papers published elsewhere.

calcium carbonate; polymorphism; catalyzed decomposition of urea; calcite; aragonite; vaterite

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