Adsorption of polyacrylamides on the different faces of kaolinites

Abstract

The adsorption densities of nonionic and anionic high molecular weight polyacrylamides, respectively on the basal and lateral faces of kaolinite, are derived from measurements on four Na-kaolinite samples with different proportions of lateral-to-total surface area. For nonionic polyacrylamide, adsorption is independent of NaCl content and pH (between pH 3 and 9) and is shown to take place on both basal and lateral faces. The adsorption density on lateral faces is high; i.e.,Γ= 3500 μg/m 2. Adsorption on basal surfaces takes place most likely on the aluminum hydroxide face with a low adsorption density, i.e.,Γ= 500 μg/m 2. Adsorption of the anionic polyacrylamide on lateral faces increases with NaCl concentration C s in two steps, due to charge screening up to C s < 1 to 2 mol/liter and then to formation of ion pairs between cations and negative sites on the surface. As pH increases from 4 to 6, adsorption drops markedly and then remains practically constant between 6 to 10, as expected from the decrease in electrostatic repulsions. Under very alkaline conditions (pH > 10), adsorption drops again markedly down to negligible values. The adsorption on basal surfaces increases with salinity and decreases as pH increases as expected from the contribution of electrostatic repulsions between the polymer and mineral surface.