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.
Keywords: CaCO3; calcite; heterogeneous nucleation; interfacial energy; sulfate; vaterite.