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

The role of bicarbonate on the growth rate of calcite in Lhermo-, 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 m(2) g(-1). The median values of the molalities of Ca2+ and CO32- are, respectively, 6.7 +/- 24 (x 10(-4)) and 6.9 +/- 1.9 (x 10(-4)); the saturation values (Omega) are 2.1-4. In the pH-range of 7.6-8.9 the activity ratios of HCO3- to CO32- decrease from 510 to 30. The effect of increasing bicarbonate on R-lin (ms(-1)) is expressed in the apparent rate constant, k(lin), which decreases with a factor of 2.8 in the indicated pH-range. The contribution of bicarbonate to R-lin is also apparent in plots of R-lin versus the free dissolved calcium to carbonate ratios (r) for a selection of results with Omega = 2.7 +/- 0.7 in the pH-range from 7.6 to 8.9. Where others found a dependence of R-lin with a maximum at r approximate to 1, symmetrically decreasing on either side of this value, it is shown here that R-lin 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 (0)(CaCO3) and CaHCO3+ successfully describes the data with associated k-values for both species in a narrow Omega-range. Implicitly, these ion pairs represent the effect of Ca2+, CO32- and HCO3 on the growth rate. (C) 2014 Elsevier B.V. All rights reserved