Calcite growth inhibition by copper(II): I. Effect of supersaturation

Abstract

The effectiveness of the inhibition of calcite growth by copper(II) ions depends not only on copper concentration but also on calcite supersaturation and solution composition (i.e., pH, carbonate concentration, etc.). This paper presents an investigation of the effect of supersaturation; a second paper will address the effect of solution composition. Calcite growth inhibition was investigated over the supersaturation range 2.5–10, at four solution compositions. Growth rates in the absence and presence of copper were measured using a constant composition seeded growth technique. Calcium ion activity, copper ion activity, and pH were measured and controlled in situ using ion selective electrodes. With this technique, rate data at a known inhibitor concentration were obtained. In the absence of copper, the rate-determining step for calcite growth changed from a first-order process at low pH and low carbonate concentration to a second-order process at high pH and high carbonate concentration. First- and second-order growth rates are consistent with the layer growth mechanism which has been observed by other investigators using atomic force microscopy. In the presence of copper, the order of the growth rate was significantly larger than its absence. At supersaturations above about S=7, the order of the growth rate was larger than 2 (i.e., between 2.7 and 4.9 depending on solution composition); values which strongly suggest that under these conditions surface nucleation is the controlling growth mechanism. At supersaturations below S=7, the order of the growth rate was slightly larger in the presence of copper but it was not larger than 2. This suggests that the rate-determining step is shifted from a first-order process to a second-order process due to the presence of copper. Until now, the effects of an inhibitor on the rate determining step or on the growth mechanism have not been accounted for in existing rate laws for growth inhibition. This study clearly shows the need to include these effects in experimental studies and in the development of rate equations for crystal growth in the presence of inhibitors.