INTERACTION OF HUMIC SUBSTANCES WITH SILICA AND ALUMINA COLLOIDS: ADSORPTION AND STABILITY. ELECTRO-OPTICAL STUDY

Abstract The adsorption of a humic substance — Na-humate — on aluminium oxide (γ-Al 2 O 3 ) and aerosil (SiO 2 ) particles was investigated using electric light scattering and electrophoresis. The electro-optic effect in γ-Al 2 O 3 dispersion is changed when varying the concentration of the humate polyanion and reduces to zero together with the electrophoretic mobility. A decrease of the rotation diffusion coefficient was registered above the isoelectric point (IEP), which indicates formation of a thick adsorption layer, stabilizing the dispersed system. The electro-optic effect relaxation frequency of the stabilized dispersed particles is greatly decreased, which is evidence of decreased counterion mobility. In the aerosil dispersed system the electro-optic effect increases with the addition of Na-humate. The frequency dependence of the electro-optic effect is also influenced by the presence of negative polyions. These changes in the electro-optic behaviour of the aerosil suspension are an indication of the fact that the polyion is adsorbed on a negatively charged surface, probably because of a strong specific interaction.