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.