Interaction of humic substances with silica and alumina colloids: adsorption and stability - Electro-optical study

The adsorption of a humic substance - Na-humate - on aluminium oxide (gamma-Al2O3) and aerosil (SiO2) particles was investigated using electric light scattering and electrophoresis. The electro-optic effect in gamma-Al2O3 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. (C) 1999 Elsevier Science B.V. All rights reserved.