Kinetics of calcium carbonate (calcite)-seeded crystallization: Influence of solid/solution ratio on the reaction rate constant

Abstract

Calcium carbonate (calcite) crystallization follows a parabolic rate expression over a large range of solid solution ratios. Growth occurs by a uniform advance of macro steps across the major crystallographic faces. At low seed crystal concentrations surface nucleation contributes to the observed reaction rate. Over the range of solid solution ratios examined, calcite crystallization can be described by the parabolic growth rate equation dN dt = −ksN 2, where N is the amount of calcite to be precipitated from solution to reach equilibrium, i.e., the theoretical crystal yield (moles/liter); dN dt is the rate of change of the crystal yield with respect to time (moles/liter · min); s is the concentration of added seed material (mg/liter); and k is the crystallization rate constant [(moles/liter)(mg/liter)(min)] −1. The constant k is independent of the solid solution ratio over much of the range examined here, but it is higher at low seed concentrations. This phenomenon is explained by the competition between growth on the calcite seed crystal surface and surface nucleation. Our results demonstrate that the parabolic crystallization rate equation-and a surface reaction mechanism—satisfactorily describe calcite formation over the solid/ solution range encountered in many geochemical and technical situations.