Effects of carboxylic acids on the crystallization of calcium carbonate

The effects of seven carboxylic acids, i.e., acrylic acid, maleic acid, tar taric acid, malic acid, succinic acid, and citric acid, on CaCO3 crystalliz ation were studied using the unseeded pH-drift method along with a light-sc attering technique. Experiments were started by mixing solutions of CaCl2 a nd NaHCO3 in the presence or absence of additives. The crystallization was studied by recording the decrease in pH resulting from the reaction Ca2+ HCO3- --> CaCO3 + H+. A given amount of carboxylic acid was added to the so lution of CaCl2 or NaHCO3 before mixing the reactants. The pH profiles obta ined in the case of the CaCl2 solution containing an additive were similar to those for the NaHCO3 solution containing one, and when an additive was a dded after the onset of crystallization, the growth of CaCO3 immediately st opped. The light-scattering observations, in all cases, indicated that CaCO 3 nucleation occurred at 10-20 s after mixing of the reactants. The results indicated that the nucleation of CaCO3 was not influenced by the presence of carboxylic acids, but CaCO3 crystal growth was reduced by their adsorpti on to the surface of the CaCO3 crystals. These phenomena were explained by assuming a stronger affinity of the carboxylic acids for CaCO3 particles th an for the free Ca2+ ions in solution. The crystallization of CaCO3 in the presence of additives was divided into three stages: nucleation, growth inc ubation, and growth periods. (C) 2001 Academic Press.