This paper describes the influence of different Ca2 +  and HCO3 concentrations on the precipitation of calcite in aqueous solutions. Mixtures of CaCl2 and NaHCO3 solutions with different concentrations were stirred, covering a wide range of supersaturation and precipitation of calcite. The resulting reduction of the Ca2 +  concentration was recorded as a function of time by measuring the electric conductivity and the pH value. The nucleation rate increased with increasing supersaturation and can be described with the classical theory of nucleation. For different solutions with similar values of supersaturation, the hydrogen carbonate/calcium ratio had no significant influence on the rate of nucleation. At a given calcium concentration the precipitation rate increased with increasing supersaturation. This effect was more pronounced at higher supersaturations. Measurements at similar values of supersaturation showed that the calcite precipitation rate increased with increasing hydrogen carbonate/calcium ratio. These results can be explained by applying a surface complexation model. The crystal surface concentrations of the two species > CaCO3 and > CO3 and the adsorption of CaCO30 ion pairs are responsible for catalysing calcite precipitation.

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