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

Unveiling the effects of an environmental abundant anion "sulfate" on the formation of calcium carbonate (CaCO₃) is essential to understand the formation mechanisms of biominerals like corals and brachiopod shells, as well as the scale formation in desalination systems. However, it was experimentally challenging to elucidate the sulfate-CaCO₃ interactions at the explicit first step of CaCO₃ formation: nucleation. In addition, there is limited quantitative information on the precise control of nucleation kinetics. Here, heterogeneous CaCO₃ nucleation is monitored in real time as a function of sulfate concentrations (0-10 mM Na₂SO₄) using synchrotron-based grazing incidence X-ray scattering techniques. The results showed that sulfate can be incorporated in the nuclei, resulting in a nearly 90% decrease in the CaCO₃ nucleation rate, causing a 120% increase in the CaCO₃ nucleus size, and inhibiting the vaterite-to-calcite phase transformation. Moreover, this work quantitatively relates sulfate concentrations to the effective interfacial energies of CaCO₃ and finds a non-linear trend, suggesting that CaCO₃ heterogeneous nucleation is more sensitive at a low sulfate concentration. This study can be readily extended to study other additives and obtain quantitative relationships between additive concentrations and CaCO₃ interfacial energies, a key step toward achieving natural and engineered controls on CaCO₃ nucleation.