EFFECT OF CALCIUM-IONS ON THE ADSORPTION OF POLYACRYLIC-ACID ONTO ALUMINA

The adsorption isotherms of polyacrylic acid on alumina were determined in aqueous solutions, and the influence of pH and calcium ions investigated. In alkaline media, PAA adsorption strongly depends on the calcium concentration, and isotherms exhibit a particular shape; in acidic media only a slight influence of calcium ions was found. We observed that the adsorption reaches a maximum when PAA is fully complexed in the solution (0.35 Ca2+ per monomer unit). Due to electrostatic effects, the complexation ratio is slightly higher in the adsorption layer than in the bulk of the solution. The decrease in the adsorption isotherm as PAA increases, at constant calcium concentration, is due to a decrease in the complexation ratio. This explanation also holds for the dependence of the adsorption isotherms on the solid content. When using electrochemical equilibria for the interactions between the surface, the polymer, and calcium ions, calculated values of the adsorbed amounts parallel the experimental results. © 1993 by Academic Press, Inc.