Study of the adhesion mechanism of CaCO3 using a combined bulk chemistry/QCM technique

Calcareous scaling of calcium carbonate was investigated by combining a quartz crystal microbalance (QCM) technique with ion concentration measurements in which adhesion of crystals of CaCO3 on the solid surface and its bulk precipitation were traced simultaneously. The mechanisms of adhesion (scale formation) on the solid surface (gold surface of QCM sensor) during the precipitation and transformation of calcium carbonate from highly supersaturated solution were obtained at 25.0degreesC. When the QCM sensor has been inserted in the reaction solution prior to the formation of the precipitate, orthorhombic calcite was nucleated and grown on the surface of the sensor after several minutes of induction period. These processes synchronized to the precipitation processes in the bulk solution, i.e., transformation of amorphous CaCO3 (ACC) to crystalline CaCO3 (calcite and vaterite) and the transformation of vaterite to calcite. When the QCM sensor was inserted in the suspension after the complete transformation of ACC, a thin disc of leaf-like vaterite was grown on the surface, whereas the vaterite of the precipitate in solution was spherical. These adhesion processes are mainly controlled by the lattice ion activity product of the solution. In addition to these direct adhesions, the indirect adhesion of the crystalline CaCO3 already precipitated in the solution was observed, although the contribution of this adhesion to the scaling is very small. The morphology and polymorphs of calcium carbonate crystals on the solid surface and of the precipitate in the bulk solution were detected using several techniques. The influences of stirring rate, inclination angle of the QCM sensor and the calcium-carbonate concentration ratio on the adhesion process were studied. (C) 2002 Elsevier Science B.V. All rights reserved.