Constant composition kinetics studies of the simultaneous crystal growth of alkaline earth carbonates; the calcium/strontium system

Abstract

The kinetics of crystal growth of strontium carbonate have been investigated over a range of supersaturation using the constant composition method. The rate is second order in relative supersaturation suggesting a surface controlled spiral growth mechanism and the reduction in rate in the presence of calcium ions can be interpreted in terms of a Langmuir adsorption isotherm with an affinity coefficient of (1.4±0.35)X10 4 L mol -1. By using ion specific hydrogen and calcium ion electrodes to control titrants for strontium carbonate and calcium carbonate crystallization, respectively, a dual constant composition method has been developed for studying the simultaneous crystal growth of these phases in solutions supersaturated with respect to both salts. When strontium carbonate seed crystals alone are introduced into the mixed supersaturated solutions, calcium carbonate nucleates at the surface and both alkaline earth carbonates undergo crystal growth. Conversely, strontium carbonate crystal growth is induced at calcite surfaces immediately upon adding these seed crystals to the mixed supersaturated solutions. X-ray data suggest that the presence of strontium ion induces the formation of aragonite having some incorporated strontium ions in the crystal lattice. However, when strontium carbonate is induced at the surface of calcium carbonate seed crystals in the mixed supersaturated solutions, the only calcium carbonate X-ray peaks observed are those characteristic of calcite. The nucleation and subsequent growth reactions are also confirmed by Fourier transformation infrared (FTIR) spectroscopy and energy dispersive X-ray (EDX) analysis.