Impact of Citrate Ions on the Nucleation and Growth of Anhydrous CaCO₃

Citrate has been shown to inhibit CaCO₃ growth and it is also found in biogenic calcite, but full understanding about how citrate controls CaCO₃ formation, and hence CaCO₃ polymorph stability and crystal morphology, is still lacking. We investigated the effect of citrate (CIT) on CaCO₃ crystallization by monitoring crystallization rates and crystal polymorph, size, and morphology as a function of increasing mol % CIT/Ca ([Ca²⁺] = [CO₃^2–] = 4 mM). At CIT/Ca ≤ 50%, both vaterite and calcite formed, but nucleation and growth were retarded as CIT/Ca was increased, demonstrated by the increase in induction time and decrease in the reaction rate constant, fitted using the Johnson-Mehl-Avrami-Kolmogorov model. This is partly explained by increased CIT-Ca complexation with higher [CIT], thus reducing saturation state with respect to vaterite and calcite. Simultaneously, CIT sorption inhibited crystal growth, demonstrated by smaller vaterite spheres and more elongated calcite crystals as CIT/Ca increased. At CIT/Ca ≥ 75%, vaterite formation was completely inhibited, with only elongated calcite forming and growth rate decreased further as CIT/Ca was increased. Overall, citrate inhibited growth rates most at CIT/Ca ≤ 50%, while nucleation was less affected. In contrast, at CIT/Ca > 50%, citrate strongly inhibited nucleation but had less effect on the growth rate. The results illustrate the spectrum of impacts that complexing compounds can have on CaCO₃ crystallization, by forming complexes in solution, affecting CaCO₃ saturation state, and also by interacting with the crystal surfaces, thereby controlling nucleation and growth. The extent and the type of effect depend on concentration regimes.