Controlling CaCO₃ Particle Size with {Ca²⁺}:{CO₃ ^2-} Ratios in Aqueous Environments

Area of Science:

• Geochemistry and Materials Science
• Focuses on the chemical and physical processes governing mineral formation.

Background:

• Calcium carbonate (CaCO3) precipitation is crucial in geological, industrial, and geo-engineering applications.
• Understanding CaCO3 formation mechanisms is vital for controlling crystal size and properties.

Purpose of the Study:

• To investigate the influence of solution stoichiometry on CaCO3 nucleation and growth.
• To determine how varying Ca2+:CO32- ratios affect CaCO3 particle characteristics and precipitation kinetics.

Main Methods:

• Experimental investigation of CaCO3 formation across a wide range of solution stoichiometries (r_aq) and supersaturation levels (Ω_cal).
• Utilized dynamic light scattering (DLS) to analyze nanoparticle size and formation mechanisms.
• Employed cross-polarized light microscopy to measure precipitation times to micrometer scale.
• Validated findings with molecular dynamics (MD) simulations of CaCO3 nanoparticle behavior.

Main Results:

• At stoichiometric conditions (r_aq = 1) and moderate supersaturation, ion adsorption dominates nanoparticle formation.
• At higher supersaturation, aggregation of prenucleation clusters becomes the primary growth mechanism.
• Non-stoichiometric conditions (r_aq ≠ 1) result in smaller nanoparticles (<10 nm) persisting for extended periods (up to 15 h).
• Precipitation time to 20 μm depends strongly and asymmetrically on solution stoichiometry (r_aq).
• MD simulations confirm stoichiometry's substantial effect on CaCO3 nanoparticle formation and size.

Conclusions:

• Solution stoichiometry, in addition to supersaturation, critically influences CaCO3 particle size, persistence, growth, and ripening.
• Stoichiometry dictates the dominant nucleation and growth mechanisms of CaCO3.
• Findings provide insights for controlling CaCO3 formation in diverse environmental and industrial contexts.