ADSORPTION OF POLYACRYLATE AND CARBOXY METHYL CELLULOSE ON KAOLINITE - SALT EFFECTS AND COMPETITIVE ADSORPTION

The adsorption of sodium polyacrylate (NaPAA) and sodium carboxy methyl cellulose (NaCMC) on homoionic sodium and calcium kaolinite has been investigated. Adsorption is strongly dependent on Ca2+ /polyelectrolyte interactions in solution, while the nature of the kaolinite is of less importance. Adsorption takes place on two types of sites where the energetically more favorable sites appear to be the edges and the other sites are the basal planes. On addition of Ca2+ to a PAA solution, a maximum binding (saturation) corresponding to 0.306 Ca2+ /PAA segment is reached. An adsorption model has been developed in which it is assumed that while any PAA may be adsorbed on the more favorable sites, only PAA molecules that are saturated with Ca2+ adsorb on the other sites. Equilibrium calculations using apparent binding constants of Ca2+ to the PAA determined from potentiometric titration show that this model is able to predict the experimentally determined isotherms with good accuracy. Although detailed calculations have not been performed, the model appears to be applicable also to the adsorption of NaCMC. © 1990.