This result is in agreement with

The effect of pH on the adsorption of Cd II by kaolinite was examined by Srivastava et al. (2005). Their work focused on two modes of R-7128 binding Cd on kaolinite namely ionic exchange sites of permanent charge, and complexation of amphoteric Si-OH groups and Al-OH on the edges of particles. These sites are dependent on the pH. When the pH of the clay suspension increases, the second mode of binding is weak which explains the low value of the adsorbed quantities.
3.5. Isotherms for exchange
The study of the isotherms has often been regarded as the best approach in the study of the theoretical aspects of all adsorption systems. These isotherms are superior vena cava graphs where the amounts adsorbed are plotted against the pressure (or concentration of the adsorbate) in the external phase at constant temperature and usually under conditions of equilibrium as in Gilles (1974). The adsorption results were analyzed in terms of Langmuir and Freundlich isotherms.
The data obtained for the experiments were fitted to the Langmuir and Freundlich isotherms (Langmuir, 1918 and Freundlich, 1906) as reported in Figure 4 and Figure 5.