Citrate effects on amorphous calcium carbonate (ACC) structure, stability, and crystallization

Understanding the role of citrate in the crystallization kinetics of amorphous calcium carbonate (ACC) is essential to explain the formation mechanisms, stabilities, surface properties, and morphologies of CaCO₃ biominerals. It also contributes to deeper insight into fluid-mineral interactions, both in nature and for industrial processes. In this study, ACC formation and its crystallization are monitored in real time as a function of citrate (CIT) concentration in solution. Additionally, synchrotron radiation pair distribution function analyses combined with solid-state, spectroscopic, and microscopic techniques are used to determine the effect of CIT on ACC structure, composition, and size. Results show an increase in ACC lifetime coupled with an increase in CIT uptake by ACC and slight changes in ACC atomic structure with an increase in CIT concentration. ACC does not form at concentrations ≥ 75% CIT/Ca and vaterite is absent in all cases where CIT is present. These findings can be explained by CIT binding with Ca ions, thereby forming Ca-CIT complexes in solution and decreasing ACC and calcite saturation levels. The formation of CIT-bearing ACC with calcitic structure and the absence of vaterite formation suggest that these solution complexes form a calcite-type atomic arrangement while CIT probably also acts as a growth inhibitor.