ADSORPTION OF CADMIUM(II) ON KAOLINITE

Three types of experiment were used to study the adsorption of Cd(II) onto two kaolinite samples at 25 degrees C. (1) Adsorption edges were characterised by a plateau around pH 5-6 separating an initial adsorpt ion stage beginning about pH 4, and a second stage in the pH range abo ut 7-9. The plateau was higher for the sample with greater face area. (2) Adsorption isotherms at constant pH could be fitted closely by a s imple Langmuir model at pH 5.50, but a two-site Langmuir model was bet ter for the data at pH 7.50. One of the model sites at pH 7.50 had a s imilar maximum adsorption as the single site at pH 5.50, but the equil ibrium constant was greater. At pH 5.50 one proton was released into t he solution for the adsorption of about five cadmium ions, but at pH 7 .50 the ratio was about 1:1. (3) Potentiometric titrations of kaolinit e suspensions in the presence and absence of Cd(II) could be modeled v ery closely by a surface complexation model assuming constant capacita nce. Parameters from this model were used in turn to predict the adsor ption edges with remarkable precision. The results from all the experi ments are consistent with the view that Cd(II) adsorbs to kaolinite by two distinct processes: ion-exchange at the permanently-charged sites on the silanol faces, and complexation to aluminol and perhaps silano l groups, which occur in particular at the crystal edges. (C) 1997 Els evier Science B.V.