cg7b00796_si_001.pdf (1.1 MB)
Impact of Citrate Ions on the Nucleation and Growth of Anhydrous CaCO3
journal contribution
posted on 2017-08-29, 00:00 authored by G. Montanari, J. D. Rodriguez-Blanco, N. Bovet, S. L. S. Stipp, D. J. ToblerCitrate 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.