The effect of the presence of zinc on the precipitation of CaCO3 from supersaturated solutions

Precipitation and dissolution processes of calcium carbonate in natural aquatic systems, are responsible not only for the regulation of pH but also for the transport of pollutants to and from sediments. Of interest in these processes, is the role of the presence of heavy metals in the aquatic environment. In the present work the mechanism of calcium carbonate formation from supersaturated solutions in the absence and presence of zinc (II) was investigated at 25°C, pH 8.50, 0.1M NaCl, at conditions of constant supersaturation. Kinetics analysis suggested that the precipitation of calcium carbonate both in the absence and in the presence of zinc over a concentration range 10-30 M, is controlled by surface diffusion. The presence of zinc in the supersaturated solutions reduced the rate of crystal growth of the calcite seed crystals and the rate reduction was smaller the higher the solution supersaturation. The inhibition was probably due to the adsorption of zinc species to the active sites of crystal growth of the seed crystals. Provided that the adsorption could be described according to the Langmuir model, the assumption was justified both by fitting kinetics data of calcium carbonate crystal growth and by independent equilibrium adsorption studies.