Mechanism of Magnesium's Influence on Calcium Carbonate Crystallization: Kinetically Controlled Multistep Crystallization

Crystallization of calcium carbonate under high concentration of magnesium (Mg/Ca > 5) follows a multistep pathway that is controlled by kinetics. There is a high concentration of calcium ions (supersaturated) that stably coexisted with the amorphous calcium carbonate and monohydrocalcite in our experiments. The authors propose that most of the calcium ions in the solution existed as the prenucleation cluster that had been stabilized by the hydrated magnesium ions. The dehydration free energy barrier of magnesium is higher than that of removing hydrated magnesium ions from the prenucleation cluster. Thereby, the prenucleation cluster slowly evolves into a more ordered structure by removing the hydrated magnesium ions, which is faster than the dehydration of magnesium. In addition, the crystallization via the agglomeration of the prenucleation clusters is one of the major pathways. However, before transforming into more ordered structure with less hydrated magnesium ions, prenucleation cluster was inactive in agglomeration. Thus, the oscillation of calcium concentration is observed in the continuous feeding crystallization reactor, due to the accumulation of active prenucleation cluster before rapid crystallization happened in the solution.