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 adsorption stage beginning about pH 4, and a second stage in the pH range about 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 simple Langmuir model at pH 5.50, but a two-site Langmuir model was better for the data at pH 7.50. One of the model sites at pH 7.50 had a similar maximum adsorption as the single site at pH 5.50, but the equilibrium constant was greater. At pH 5.50 one proton was released into the solution for the adsorption of about five cadmium ions, but at pH 7.50 the ratio was about 1:1. (3) Potentiometric titrations of kaolinite suspensions in the presence and absence of Cd(II) could be modeled very closely by a surface complexation model assuming constant capacitance. Parameters from this model were used in turn to predict the adsorption edges with remarkable precision. The results from all the experiments 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 silanol groups, which occur in particular at the crystal edges. (C) 1997 Elsevier Science B.V.