The electrokinetic and shear yield stress properties of kaolinite in the presence of aluminium ions

The electroacoustic zeta potential and shear yield stress (tau(y)) properti es of kaolinite suspensions have been examined over a wide range of pH cond itions and Al(III) concentrations. At zero or low Al(III) concentrations, t he zeta potential data are dominated by the silica-like kaolinite face, whi le the corresponding tau(y) data show the characteristics of pH-dependent f ace and edge interactions. As the Al(III) concentration is raised, all zeta potential data become increasingly more positive, the electrokinetic isoel ectric point progressively shifts to higher pH, and the z,versus pH data ch ange systematically from kaolinite-like to alumina-like behaviour. These fi ndings are indicative of the adsorption of Al(III)-based hydrolysis product s, as is predicted by the adsorption model of James and Healy, J. Colloid I nterface Sci., 40 (1972) 65. A comparison of the properties of partially Al (III)-coated kaolinite particles in the presence of Cl- and SO42- shows tha t the zeta potential results are more negative and the shear yield stress d ata are lower for the kaolinite-Al(III)-SO42- system. These results can be rationalised in terms of the extent of surface coverage by Al(III) hydrolys is products and anion adsorption on the surface-adsorbed Al(III)-based prec ipitates. (C) 1999 Elsevier Science B.V. All rights reserved.