Magnesium Adsorption on Vaterite: Influence on Dissolution Kinetics and Inhibition of the Rate of Conversion to Calcite

We report first the influence of magnesium(II) adsorption on the dissolution kinetics of spherulitic vaterite particles. Using an inverted optical microscopy setup, the reduction in particle area in varying magnesium concentrations was tracked over time to reveal a “thermodynamically” controlled rate of dissolution. The rate was therefore controlled by both the solubility product of vaterite and the rate of diffusion of the dissolved ions away from the particle-solution surface. The variation in dissolution rate with magnesium concentration at constant ionic strength was investigated. At high magnesium concentrations, vaterite dissolution was only partially inhibited, suggesting that dissolution proceeds through at least two parallel pathways. The rate transition observed at low magnesium concentrations was fitted using a model based on Langmuirian adsorption of Mg(II). The influence of magnesium on the rate of transformation of vaterite into calcite was then investigated by aging synthesized vaterite particles in either water or a magnesium-containing solution for varying durations in the range 0 to 7 days. Scanning electron microscopy images, supported by X-ray diffraction (XRD) diffractograms, showed that the vaterite-calcite transformation was effectively inhibited by magnesium(II) concentrations of 100 mM. This suggests that seawater concentrations of magnesium suppressed calcite growth rather than reducing vaterite dissolution since the latter was measured to proceed at a significant rate in the presence of the equivalent amount of Mg(II).