Role of electrostatic interactions in the adsorption of cadmium(II) from aqueous solution onto vermiculite

The adsorption equilibrium data of Cd(II) on vermiculite was obtained in a batch adsorber. The adsorption data were interpreted with the Langmuir, Freundlich and Prausnitz-Radke (PR) isotherm models, and the PR isotherm best fitted the experimental data. The adsorption capacity of vermiculite towards Cd(II) was increased by raising the solution pH and temperature. The first effect was attributed to electrostatic interactions and competition between the protons and Cd(II) for the cationic sites. The second trend indicated that the adsorption was endothermic. The desorption of Cd(II) adsorbed on vermiculite at pH = 7, was irreversible and reversible when the pH of desorption solution was 7 and 3, respectively. The maximum uptake of Cd(II) on vermiculite was greater than the cationic exchange capacity. This result revealed that the adsorption of Cd(II) on vermiculite occurred by other adsorption mechanisms besides ion exchange. It was demonstrated that the greater the uptake of Cd(II) on vermiculite, the higher the reduction of the negative charge of the surface. Hence, the electrostatic interactions between the Cd2 + cations in solution and the negative zeta potential of the surface played an important role on the adsorption capacity of vermiculite. Hence, the adsorption of Cd(II) on vermiculite is mainly due to the mechanisms of ion exchange and electrostatic attraction.