Electrokinetic behaviour of calcite - the relationship with other calcite properties

Six carbonate samples of different origin were investigated for their prope rties: composition. grain size (Mz), specific surface area (SSA) and electr okinetic (zeta) potential. Four of the samples were of natural origin-one s ample was sea sediment, one lake sediment and two limestone rocks. The rema ining two samples were synthetic calcites, The results of characterisation have shown that the samples consisted mostl y 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 mum, 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 fro m 6 to 10. The results have shown two different types of behaviour: synthet ic calcites had positive zeta -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 zeta -potential thro ughout the entire investigated pH range. Electrokinetic behaviour of synthetic samples was the result of the exchang e between the constituent ions in the solution and the available surface si tes. The pH decrease caused the dissolution of the: particles, and conseque ntly the diminution of the grain size and surface area. Lower values of the zeta -potential were then the result of the specific adsorption of constit uent anions on such reduced surface area. Going towards higher pH values, t he decrease in the zeta -potential land finally the charge reversal) was ca used by the increasing concentration of the CO32-. The isoelectric point (p H(IEP)) was reached at about pH approximate to 9.5. The electrokinetic behaviour of natural samples on the other hand was not t he result of the exchange of constituent ions between the surface and the s olution. Their zeta -potential was governed by the organic matter attached to the surface or incorporated in the structure of calcite. (C) 2001 Elsevi er Science B.V. All rights reserved.