Effect of inorganic phosphate ions on the spontaneous precipitation of vaterite and on the transformation of vaterite to calcite

The effect of inorganic orthophosphate ions on both the precipitation of vaterite and the transformation of vaterite to the thermodynamically stable calcite was investigated during the precipitation process of calcium carbonate under conditions of constant supersaturation at 25 degrees C and at pH 9.0 and 10.0, The calcite content of the solid precipitates was determined quantitatively by powder X-ray diffraction (XRD) using the appropriate standards mixed at known amounts. The presence of very small amounts of inorganic phosphate ions was found to reduce the rate of vaterite crystallization. Application of the kinetic data to a Langmuir-type model suggested that adsorption of phosphate at the active growth sites was responsible for the reduction in the crystal growth rates. At relatively high supersaturation ratios (1.5-2.1), where the transformation rate decreases with increasing supersaturation ratio, it was found that the transformation of the initially formed vaterite to calcite depends only on the supersaturation ratio and not on the pH of the supersaturated solutions in which precipitation takes place. It was suggested on the basis of the kinetic results, that the transformation takes place through dissolution of vaterite followed by crystallization of calcite. At relatively high supersaturation ratios (1.5-2.1) the transformation is controlled by the dissolution of vaterite, whereas at lower supersaturation ratios (1.1-1.5) the rate of vaterite dissolution is comparable with the respective rate of calcite crystallization. Finally, it was found that the presence of phosphate ions in the precipitating solution stabilized the initially formed vaterite, by decreasing markedly the rate of its transformation to calcite. The stability of vaterite in the presence of the phosphate ions was ascribed to the retardation of both the dissolution of vaterite and of the crystallization of calcite, caused by blocking of the active sites for the dissolution of vaterite and for the crystallization of calcite, by the adsorbed phosphates. (C) 1999 Elsevier Science B.V. All rights reserved.