Adsorption-desorption behavior of acrylic-maleic acid copolymer at clay minerals

The interactions between acrylic-maleic acid (PAA-PMLA) copolymer with clay minerals are investigated in terms of adsorption/desorption behavior. The adsorption isotherms were obtained as a function of clay mineral structures, pH and ionic strength by sensitive polyelectrolyte titration and radiotracer methods. The nature and the location of binding sites for PAA-PMLA copolymer on aluminol sites at the edge surface can be stated. In the case of the adsorption on kaolinite, the pi-I-dependent interaction in relatively low ionic strength is discussed not only in terms of electrostatic contributions, but evidences for a ligand exchange mechanism are also presented. Increasing the ionic strength also enhances the adsorption by a screening of the charges in the adsorbed layer especially when the charges of the copolymer and the surface are of the same sign. The adsorbed PAA-PMLA copolymer can be displaced by phosphate compounds such as monophosphate and sodium tripolyphosphate (STP). The observed desorption process results from a competition between PAA-PMLA copolymer and phosphate compounds for the same binding sites on the kaolinite surface. Adsorption equilibrium constants can thus be derived from single and mixture adsorption isotherms. Effects due to the heterogeneity of the kaolinite surface and to the conformation of the adsorbed PAA-PMLA copolymer are also discussed. Strongly bound PAA-PMLA copolymer traces support the Ligand exchange mechanism at the aluminol sites of the edge surface. Clay minerals may thus act as natural barriers in the soil transport of the polycarboxylates used in phosphate-free detergents. (C) 1997 Academic Press.