Process-Specific Effects of Sulfate on CaCO₃ Formation in Environmentally Relevant Systems

Additives, such as ions, small molecules, and macromolecules, have been found to regulate the formation of CaCO₃ and control its morphologies and properties. However, a single additive usually affects dominantly one process in CaCO₃'s formation and is seldom found to significantly affect multiple CaCO₃ formation processes. Here, we used in situ grazing incidence X-ray techniques to observe the heterogeneous formation of CaCO₃ and found that a series of formation processes (i.e., nucleation, growth, and Ostwald ripening) were modulated by sulfate. In the nucleation process, increased interfacial free energy and bulk free energy cooperatively increased the nucleation barrier and decreased nucleation rates. In the growth process, sulfate reduced the electrostatic repulsion between CaCO₃ precursors and nuclei, promoting CaCO₃ growth. This influence on the growth counteracted the inhibition effect in the nucleation process, causing a nearly 100% increase in the volume of heterogeneously formed CaCO₃. Meanwhile, adsorbed sulfate on CaCO₃ nuclei may poison the surface of smaller CaCO₃ nuclei, inhibiting Ostwald ripening. These revealed sulfate's active roles in controlling CaCO₃ formation advance our understanding of sulfate-incorporated biomineralization and scaling phenomena in natural and engineered aquatic environments.