Sulfate-Controlled Heterogeneous CaCO₃ Nucleation and Its Non-linear Interfacial Energy Evolution

Area of Science:

• Geochemistry
• Materials Science
• Biomineralization

Background:

• Calcium carbonate (CaCO3) formation is vital for biomineral structures and industrial processes like desalination.
• Understanding sulfate's role in CaCO3 nucleation is challenging due to experimental difficulties.
• Quantitative data on nucleation kinetics control is limited.

Purpose of the Study:

• To investigate the real-time effects of sulfate on heterogeneous CaCO3 nucleation.
• To quantify the relationship between sulfate concentration and CaCO3 nucleation kinetics.
• To elucidate sulfate incorporation within CaCO3 nuclei.

Main Methods:

• Synchrotron-based grazing incidence X-ray scattering (GIXS) was employed for real-time monitoring.
• Experiments were conducted across a range of sulfate concentrations (0-10 mM Na2SO4).
• Nucleation rates, nucleus size, and phase transformations were analyzed.

Main Results:

• Sulfate incorporation into CaCO3 nuclei was confirmed.
• A nearly 90% decrease in CaCO3 nucleation rate was observed with increasing sulfate.
• Nucleus size increased by 120%, and vaterite-to-calcite transformation was inhibited.
• A non-linear relationship between sulfate concentration and CaCO3 interfacial energies was established.

Conclusions:

• Sulfate significantly inhibits CaCO3 nucleation, with greater sensitivity at lower concentrations.
• This research provides quantitative insights into sulfate-CaCO3 interactions at the nucleation stage.
• The findings enable better control over CaCO3 formation in natural and engineered systems.