Nucleation of Aragonite (CaCO3) from Monohydrocalcite (CaCO3.H2O) Slurries: Kinetics and Activation Energy from Raman Spectroscopy

Abstract

The ability to control the polymorph phase is one of the ultimate challenges in crystallization, where a key goal is to produce specific high-purity phases as required. Biomineralization processes are excellent examples, where organisms can select for specific polymorphs with perfect fidelity. In the present study, we report new insights into aragonite nucleation from monohydrocalcite slurry as a function of temperature (from room T (≈23 °C) to 90 °C) and as a function of initial ionic concentration at the same Mg/Ca molar ratio. Herein, competing monohydrocalcite and nesquehonite nucleation and aragonite nucleation from monohydrocalcite slurry were monitored in real time by using Raman spectroscopy. On the other hand, at higher concentrations, monohydrocalcite was purified from a monohydrocalcite and nesquehonite mixture by simple 2–3 successive washings using tap water. Here, nesquehonite is dissolved, leading to high-purity monohydrocalcite synthesis (more stable in tap water for this Ca–Mg system). As expected, the transformation kinetics from monohydrocalcite into aragonite is strongly dependent on the temperature and also on the residual Mg onto/into monohydrocalcite crystals. Herein, kinetic rate constants as a function of temperature were determined from the fitting of an empirical sigmoidal model. Then, kinetic rate constants were used to calculate the related activation energy by using the Arrhenius equation. This promising study then reports a simple method to synthesize massively high-purity monohydrocalcite, an emergent material to remove pollutants from water and as an additive in concrete materials (CO2 sequestration and improving mechanical properties). In addition, we demonstrated that selected aragonite can be also produced abiotically at room temperature, systematically observed and attributed exclusively to the life of several sea shell animals.