Adsorption of Cd(II) ion from the solution using selective adsorbent of dithizone-modified commercial bentonite

Cadmium (Cd) has potential environmental risk because of its toxicity and persistence in water; therefore, it should be removed from waters. Several methods of Cd(II) removal have been proposed, and adsorption has become the choice because of its simplicity and low-cost operation. In this study, the modification of commercial bentonite (CB) with dithizone has been carried out with the purpose to increase adsorption selectivity and capacity toward Cd(II) ion. Bentonite was first activated by HCl and then modified its surface with dithizone. The synthesized adsorbents, i.e., dithizone-modified bentonite (D-CB) and their original activated commercial bentonite (A-CB), were confirmed by Fourier transform infrared spectroscopy and X-ray diffraction and then used for the removal of Cd(II) from the solution. Several parameters influencing the adsorption of Cd(II) onto A-CB and D-CB such as solution pH, mass of adsorbent, contact time as well as initial concentration of Cd(II) were optimized. The results show that the adsorption capacity of D-CB increases almost 40% as compared to that of A-CB at their optimal adsorption conditions. Results of kinetic study reveal that the adsorption of Cd(II) on the two adsorbents fits well pseudo-second order kinetic models, and their adsorption isotherms are best described by Langmuir models. Sequential desorption study using different solvents of H2O, KNO3, HONH2HCl, and Na(2)EDTA has revealed that the interaction of Cd(II) ion with the adsorbents is dominated by chemisorption. As expected, the nature of interaction changes significantly from mainly electrostatic (ion-exchange) interaction in A-CB to hydrogen bond and chelate complexation in D-CB.