Controlling CaCO₃Particle Size with {Ca²⁺}:{CO₃^2–} Ratios in Aqueous Environments

Abstract

The effect of stoichiometry on the new formation and subsequent growth of CaCO 3 was investigated over a large range of solution stoichiometries (10 –4 < r aq < 10 4 , where r aq = {Ca 2+ }:{CO 3 2– }) at various, initially constant degrees of supersaturation (30 < Ω cal < 200, where Ω cal = {Ca 2+ }{CO 3 2– }/ K sp ), pH of 10.5 ± 0.27, and ambient temperature and pressure. At r aq = 1 and Ω cal < 150, dynamic light scattering (DLS) showed that ion adsorption onto nuclei (1–10 nm) was the dominant mechanism. At higher supersaturation levels, no continuum of particle sizes is observed with time, suggesting aggregation of prenucleation clusters into larger particles as the dominant growth mechanism. At r aq ≠ 1 (Ω cal = 100), prenucleation particles remained smaller than 10 nm for up to 15 h. Cross-polarized light in optical light microscopy was used to measure the time needed for new particle formation and growth to at least 20 μm. This precipitation time depends strongly and asymmetrically on r aq . Complementary molecular dynamics (MD) simulations confirm that r aq affects CaCO 3 nanoparticle formation substantially. At r aq = 1 and Ω cal ≫ 1000, the largest nanoparticle in the system had a 21–68% larger gyration radius after 20 ns of simulation time than in nonstoichiometric systems. Our results imply that, besides Ω cal , stoichiometry affects particle size, persistence, growth time, and ripening time toward micrometer-sized crystals. Our results may help us to improve the understanding, prediction, and formation of CaCO 3 in geological, industrial, and geo-engineering settings.

Asymmetrical Dependence of {Ba2+}:{SO42–} on BaSO4Crystal Nucleation and Growth in Aqueous Solutions: A Dynamic Light Scattering Study

“…Barite new formation was observed here to be faster, resulting in larger (prenucleation) clusters being observed earlier at ideal and barium-limited conditions (Figures and ). A similar nonsymmetrical dependence of crystal new formation on solution stoichiometry was previously reported by Seepma et al for CaCO 3 . For both BaSO 4 and CaCO 3 , the combined process of nucleation and growth is greatly hindered at nonstoichiometric conditions compared to pure crystal growth of these minerals under ideal 1:1 conditions, ,,,− but in our case more by the lack of anions than cations.…”

“…On top of that, crystals nucleated and grew under constantly changing conditions within one experiment because Ω barite dropped, to a lesser extent I dropped and, at extreme r aq , r aq shifted toward more extreme values by several magnitudes as well. This was previously discussed for similar experiments with CaCO 3 by Seepma et al These changing conditions likely have caused more heterogeneity with regard to crystal formation rates and morphology over time during batch DLS experiments and were also not accounted for.…”

“…Moreover, the new formation of barite with varying r aq has not been monitored extensively using dynamic light scattering (DLS). Seepma et al showed, with the use of DLS, that the {Ca 2+ }:{CO 3 2– } affects the size and timing of new formation of CaCO 3 crystals. The aim of this work is to determine if r aq also affects the new formation of barite crystals.…”

The Effect of pH, Ionic Strength and the Presence of PbII on the Formation of Calcium Carbonate from Homogenous Alkaline Solutions at Room Temperature

“…In particular, the different concentrations employed in other publications lead to a different Ca 2+ /CO 3 2− activity ratio at the nucleation stage. This effect has been pointed out as a crucial variable during calcite formation [27][28][29]. The values calculated from these different experiments show remarkable consistency, assuming an experimental error of about 0.1 units that derive from the intrinsic criticisms presented in the Results.…”

Counterintuitive Crystallization: Rate Effects in Calcium Phosphate Nucleation at Near-Physiological pH

“…The presence of different precursors means that the system is taking a different pathway when precipitating the final solid phase; the various experimental conditions lead to unique nucleation pathways. For both the calcium phosphates and carbonates, it has been shown that the initial solution stoichiometry is critical in determining the nucleation and growth processes, even when controlling other parameters. , The ratio of precursor ions in solution, at least in part, controls which nucleation mechanism is available. Such a result further underlines the complex nature of nucleation.…”