Calcite growth inhibition by copper(II): II. Effect of solution composition

Abstract

The effectiveness of calcite growth inhibition 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 solution composition; an earlier paper (Parsiegla and Katz, J. Crystal Growth 200 (1–2) (1999) 213) addressed the effect of supersaturation. Calcite growth inhibition by copper(II) ions was investigated over the following range of solution compositions: pH 7.00–9.25, [CO 3 2−] =10 −3.0–10 −5.0 M, and [HCO 3 −] =0.1–0.001 M; all at T =25°C , a supersaturation of S=7.8, and an ionic strength of I =0.1 M . 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 known inhibitor concentrations were obtained. This study shows that calcite growth inhibition by copper(II) ions is not a simple function of the total dissolved copper concentration, [Cu] T, or the free copper ion concentration, [Cu 2+]. Instead, the observed dependencies of reduction in growth rate on [Cu] T strongly suggest that copper complexes (e.g., Cu(CO 3) 2 2− ) are involved in growth inhibition. In addition, the observed dependencies of the reduction in growth rate on pH and [CO 3 2−] are adequately fit by assuming that copper hydroxide and copper carbonate complexes, respectively, are the actual growth inhibitors. Measured reductions in growth rate were related to inhibitor concentrations in solution using the Langmuir adsorption isotherm, with the Langmuir constants as fitting parameters. The dependence of reduction in growth rate on solution composition (i.e., on pH, [CO 3 2−] , and [Cu] T) could quantitatively be described (to within a factor of 2.4) by postulating that three copper complexes, i.e., CuCO 3 0 ,Cu(CO 3 ) 2 2− , and Cu 4(CO 3) 3(OH) 3 −, were the growth inhibitors.