Calcite growth rates as a function of aqueous calcium-to-carbonate ratio, saturation index, and inhibitor concentration: Insight into the mechanism of reaction and poisoning by strontium

Using in situ atomic force microscopy, the growth rates of the obtuse and acute step orientations on the {101̄4} calcite surface were measured at two saturation indices as a function of the aqueous calcium-to-carbonate ratio and aqueous strontium concentration. The amount of strontium required to inhibit growth was found to correlate with the aqueous calcium concentration, but did not correlate with carbonate, suggesting that strontium inhibits attachment of calcium ions to reactive sites on the calcite surface. Strontium/calcium cation exchange selectivity coefficients, K _ex, are estimated at 1.09 ± 0.09 and 1.44 ± 0.19 for reactive sites on the obtuse and acute step orientations, respectively. The implication of this work is that, to avoid poisoning calcite growth, the concentration of calcium should be higher than the quotient of the strontium concentration and K _ex, regardless of the saturation index. Previous analytical models of nucleation of kink sites on steps are expanded to include growth rates at multiple saturation indices and the effect of strontium. The rate constants for calcium attachment are found to be similar for the two step orientations, but those of carbonate vary significantly. This work will have implications for natural or engineered calcite growth, such as to sequester subsurface strontium contamination.