Electrokinetic properties of spontaneously precipitated calcium carbonate polymorphs: the influence of organic substances

The influence of organic matter on the surface charge of calcium carbonate polymorphs in the nascent stage was investigated in this work. Precipitation was initiated from the solution containing calcium and carbonate ionic species and selected organic solutes (fulvic, citric and propionic acids). Ionic strength of the solution was adjusted by the addition of NaCl. Electrophoretic mobilities of the resulting suspensions were measured and electrokinetic (ζ) potentials calculated. Precipitation from the blank solution resulted in the formation of positively charged vaterite. Fulvic acid was found to be the inhibitor of both nucleation and crystal growth. When present in the system from the beginning it completely inhibited nucleation, and momentarily stopped further precipitation when added to a suspension where vaterite seed was already present. ζ-Potential of such a suspension was negative, clearly indicating fulvic acid adsorption on the positively charged sites of vaterite surface, which disabled further crystal growth. Propionic acid had no significant influence either on the precipitation process or the surface charge. Citric acid caused the formation of another calcium carbonate polymorph-calcite. ζ-potential of the precipitate was positive but much lower than ζ-potential of vaterite formed from the blank solution. The difference in electrokinetic behaviour of those two precipitates was the result of citric acid adsorption on the positive sites of the newly formed nucleus, by which mechanism the crystal growth was induced to form calcite structure.