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. Precipitatio n was initiated from the solution containing calcium and carbonate ionic sp ecies and selected organic solutes (fulvic, citric and propionic acids). Io nic strength of the solution was adjusted by the addition of NaCl. Electrop horetic mobilities of the resulting suspensions were measured and electroki netic (xi) potentials calculated. Precipitation from the blank solution res ulted in the formation of positively charged vaterite. Fulvic acid was foun d to be the inhibitor of both nucleation and crystal growth. When present i n the system from the beginning it completely inhibited nucleation, and mom entarily stopped further precipitation when added to a suspension where vat erite seed was already present. xi-Potential of such a suspension was negat ive, clearly indicating fulvic acid adsorption on the positively charged si tes of vaterite surface, which disabled further crystal growth. Propionic a cid had no significant influence either on the precipitation process or the surface charge. Citric acid caused the formation of another calcium carbon ate polymorph-calcite. xi-potential of the precipitate was positive but muc h lower than xi-potential of vaterite formed from the blank solution. The d ifference in electrokinetic behaviour of those two precipitates was the res ult of citric acid adsorption on the positive sites of the newly formed nuc leus, by which mechanism the crystal growth was induced to form calcite str ucture. (C) 2000 Elsevier Science B.V. All rights reserved.