MECHANISM OF KAOLINITE DISSOLUTION AT ROOM TEMPERATURE AND PRESSURE: PART 1. SURFACE SPECIATION - KINETICS OF SURFACE-CONTROLLED DISSOLUTION OF OXIDE MINERALS

Abstract

Surface speciation of Georgia kaolinite was investigated by detailed potentiometric titrations (pH 2-12) at various ionic strengths in KClO4 solutions (0.1, 0.01, 0.001 M) under N2 atmosphere, corrected for Si and Al released due to dissolution. Proton adsorption or desorption was computed according to surface complexation models (nonelectrostatic model and constant capacitance model), assuming the presence of multi-sites at the surface. The behaviour of the surface may be explained by the formation of four active sites (two >AlOH2+, >AlO-, >SiO-). The pH of zero proton charge was found to be ~5.5. Below pH 5.5, the positive charge is due to the proton adsorption on aluminium sites of the octahedral sheet. The more acidic group corresponds to the external hydroxyls of the octahedral sheet, whereas the second protonation may take place either at the inner hydroxyl groups or at the edge aluminol groups. The corresponding intrinsic surface dissociation constants for these species (pKa1=1.9 and pKa1=4.1, respectively) are substantially lower than that for aluminium oxides (pKa1=4.5-8). Above pH 5.5, the kaolinite surface undergoes two successive deprotonations, the first starts at pH around 5.5 and the second at pH approximately equal to 9. In comparison with the pK values for surface dissociation of silica and alumina, it is possible to deduce that the first deprotonation takes place at Si sites, while the second occurs at Al sites.