Effects of changes in adsorbed polyacrylic acid conformation on alumina flocculation

Conformational changes of adsorbed polyacrylic acid induced by changes in such properties as pH can cause flocculation or stabilization of colloidal suspensions. In this study, conformation of adsorbed polyacrylic acid on alumina fines was monitored along with electrokinetic and flocculation responses at different polyacrylic acid concentration. Fluorescence spectroscopy was used to measure the polyacrylic acid coiling/stretching on the particles. Upon raising the pH of an alumina/polyacrylic acid system, originally at pH 4, at the low polymer dosage flocculation increased, while at the higher dosage flocculation increased up to pH 6 and then decreased markedly leading to stabilization above pH 10. These trends are explained here on the basis of changes in conformation of adsorbed polyacrylic acid and zeta potential of the particles. At the low polymer concentration (i.e., low surface coverage), increase in pH caused coiled polyacrylic acid on the surface to stretch out resulting in better bridging of the particles by the polymer and hence enhanced flocculation, whereas at the higher concentration (i.e., at high surface coverage) adsorbed polyacrylic acid exhibited less stretching with opposite flocculation effects. Zeta potential of the particles played a role in promoting flocculation at least up to pH 10 at the low polymer dosage, and between pH 4 and 6 at high polymer dosage.