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 beha vior. The adsorption isotherms were obtained as a function of clay min eral structures, pH and ionic strength by sensitive polyelectrolyte ti tration and radiotracer methods. The nature and the location of bindin g sites for PAA-PMLA copolymer on aluminol sites at the edge surface c an be stated. In the case of the adsorption on kaolinite, the pi-I-dep endent interaction in relatively low ionic strength is discussed not o nly in terms of electrostatic contributions, but evidences for a ligan d exchange mechanism are also presented. Increasing the ionic strength also enhances the adsorption by a screening of the charges in the ads orbed layer especially when the charges of the copolymer and the surfa ce are of the same sign. The adsorbed PAA-PMLA copolymer can be displa ced by phosphate compounds such as monophosphate and sodium tripolypho sphate (STP). The observed desorption process results from a competiti on between PAA-PMLA copolymer and phosphate compounds for the same bin ding sites on the kaolinite surface. Adsorption equilibrium constants can thus be derived from single and mixture adsorption isotherms. Effe cts due to the heterogeneity of the kaolinite surface and to the confo rmation of the adsorbed PAA-PMLA copolymer are also discussed. Strongl y bound PAA-PMLA copolymer traces support the Ligand exchange mechanis m at the aluminol sites of the edge surface. Clay minerals may thus ac t as natural barriers in the soil transport of the polycarboxylates us ed in phosphate-free detergents. (C) 1997 Academic Press.