Calcite growth: Rate dependence on saturation, on ratios of dissolved calcium and (bi)carbonate and on their complexes

• The crystallization rate increases at high concentrations of bicarbonate and its complexes at constant supersaturation.
• A kinetic model based on solution ion pairs fits the experimental data of two independent studies.
• The maximum crystallization rate at constant supersaturation does not necessarily occur at a calcium/carbonate ratio of 1.

The role of bicarbonate on the growth rate of calcite in thermo-, chemo- and pH-stated experiments was observed in a foregoing paper by Van der Weijden et al. (1997) [31]. The data are used here to explore especially the effect of bicarbonate on the calcite growth rate in more detail. The experimental range of pH is from 7.6 to 8.9, the initial seed concentrations are 165±12 mg/L with an average surface area of 0.29 m2 g–1. The median values of the molalities of Ca2+ and CO32−CO32− are, respectively, 6.7±2.4 (×10–4) and 6.9±1.9 (×10–4); the saturation values (Ω  ) are 2.1–4. In the pH-range of 7.6–8.9 the activity ratios of HCO3−HCO3− to CO32−CO32− decrease from 510 to 30. The effect of increasing bicarbonate on Rlin (m s–1) is expressed in the apparent rate constant, klin, which decreases with a factor of 2.8 in the indicated pH-range. The contribution of bicarbonate to Rlin is also apparent in plots of Rlin versus the free dissolved calcium to carbonate ratios (r) for a selection of results with Ω=2.7±0.7 in the pH-range from 7.6 to 8.9. Where others found a dependence of Rlin with a maximum at r≈1, symmetrically decreasing on either side of this value, it is shown here that Rlin increases at r>1 for higher values of free dissolved bicarbonate, in this case at pH 7.6. In the present paper a kinetic model relating the growth rate (R  ) to the activities of CaCO30CaCO30 and CaHCO3+CaHCO3+ successfully describes the data with associated k-values for both species in a narrow Ω-range. Implicitly, these ion pairs represent the effect of Ca2+, CO32−CO32− and HCO3−HCO3− on the growth rate.