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+] = [CO32–] = 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.