R-002

Morgan Milner

morgan.milner@sjc.ox.ac.uk

Hugo A. Saint, Chunhong Lei, Jake M. Yang and Richard G. Compton

University of Oxford, United Kingdom

Dissolution of Vaterite and Vaterite/Hydroxyapatite Core-Shell Microspheres: Mechanism and Kinetics

Calcium carbonate plays a vital role in various biological and geological systems, with growing interest in its functions fuelled by ecological and environmental concerns.(1) Understanding the dissolution mechanisms and kinetics of calcium carbonate-based materials provides valuable insights into their roles in natural processes. Fundamental research on the role of calcium carbonate in geochemistry—specifically its formation and dissolution in the ocean and its involvement in the carbon cycle— has parallels to Grotthuss’ work on hydrogen sulfide formation in Smardone spring waters, which he related to the dissolution of gypsum deposits and sulfide reduction (Grotthuss, 1816).

In the presented work, the dissolution of porous spherulitic vaterite particles, vaterite being one of three calcium carbonate polymorphs, in aqueous solution is investigated through microscopic monitoring of particle size over time.(2,3) This approach reveals a clear distinction between kinetically controlled dissolution, where rate is determined by interfacial kinetics, and dissolution governed by thermodynamic control, where local ion concentrations are pinned by the solubility product of vaterite, followed by diffusion into the bulk solution. Results confirm that vaterite dissolves under thermodynamic control, allowing estimation of its solubility product, in the light of accounting for known solution-phase equilibria involving ion pairs such as CaCO₃, CaOH⁺, and CaHCO₃⁺.

The synthesis and dissolution study of novel vaterite/hydroxyapatite core-shell microspheres are also presented.(4) Size–time profiles revealed two distinct dissolution phases: an initial phase corresponding to the hydroxyapatite shell, followed by the dissolution of the more soluble porous vaterite core. Quantitative analysis confirmed that both the core and shell dissolved under thermodynamic control.

References:

(1) Batchelor‐McAuley, C.; Yang, M.; Rickaby, R. E. M.; Compton, R. G. Calcium Carbonate Dissolution from the Laboratory to the Ocean: Kinetics and Mechanism. Chemistry – A European Journal 2022, 28 (68). https://doi.org/10.1002/chem.202202290.

(2) Milner, M. P.; Yang, M.; Compton, R. G. Vaterite Dissolution: Mechanism and Kinetics. The Journal of Physical Chemistry C 2024, 128 (25), 10388–10396. https://doi.org/10.1021/acs.jpcc.4c02074.

(3) Milner, M. P.; Yang, M.; Compton, R. G. Correction to Vaterite Dissolution: Mechanism and Kinetics. The Journal of Physical Chemistry C 2024, 128 (40), 17196–17196. https://doi.org/10.1021/acs.jpcc.4c06304.

(4) Milner, M. P.; Saint, H. A.; Lei, C.; Yang, J. M.; Compton, R. G. Vaterite/Hydroxyapatite Core–Shell Microspheres: Dissolution Kinetics and Mechanism. The Journal of Physical Chemistry C 2025. https://doi.org/10.1021/acs.jpcc.4c07444.