Electrokinetic behaviour of calcite—the relationship with other calcite properties

Abstract

Six carbonate samples of different origin were investigated for their properties: composition, grain size (Mz), specific surface area (SSA) and electrokinetic ( ζ) potential. Four of the samples were of natural origin—one sample was sea sediment, one lake sediment and two limestone rocks. The remaining two samples were synthetic calcites. The results of characterisation have shown that the samples consisted mostly of calcite. Traces of impurities (Si, Al, Fe, Mg) and organic matter were found in natural samples. The grain size of samples ranged between 2.0 and 10.3 μm, and SSA values were between 17.0 and 1.0 m 2 g −1 accordingly. The electrokinetic mobility measurements were performed in the pH range from 6 to 10. The results have shown two different types of behaviour: synthetic calcites had positive ζ-potential at the point of equilibrium (pH eq=8.4±0.1), which decreased with either the increase or the decrease of the pH. Natural calcites, in contrast, had negative ζ-potential throughout the entire investigated pH range. Electrokinetic behaviour of synthetic samples was the result of the exchange between the constituent ions in the solution and the available surface sites. The pH decrease caused the dissolution of the particles, and consequently the diminution of the grain size and surface area. Lower values of the ζ-potential were then the result of the specific adsorption of constituent anions on such reduced surface area. Going towards higher pH values, the decrease in the ζ-potential (and finally the charge reversal) was caused by the increasing concentration of the CO 3 2−. The isoelectric point (pH IEP) was reached at about pH≈9.5. The electrokinetic behaviour of natural samples on the other hand was not the result of the exchange of constituent ions between the surface and the solution. Their ζ-potential was governed by the organic matter attached to the surface or incorporated in the structure of calcite.