Effect of contact time, pH, and ionic strength on Cd(II) adsorption from aqueous solution onto bentonite from Gaomiaozi, China

Bentonite has been studied extensively because of its strong adsorption capacity. The Gaomiaozi (GMZ) bentonite is selected as the first choice of Chinese buffer materials for the high-level radioactive waste repository. The adsorption of Cd(II) on the GMZ bentonite as a function of contact time, pH, ionic strength, bentonite content, and Cd(II) concentration was studied by using batch technique. The adsorption process achieved equilibrium within 50 min, and the adsorption capacity of Cd(II) by the raw GMZ bentonite was about 3.16 mg/g under the given experimental conditions. The adsorption ability of the GMZ bentonite increases with increasing pH at 2–12 but decreases with increasing ionic strength from 0.01 to 0.1 M KNO₃. The uptake of Cd(II) on this bentonite increases in response to an increase of the bentonite content. Chemisorption was considered to be a key of process and adsorption mechanism could be concluded to be complexation, ion exchange, and electrostatic interaction. The equilibrium adsorption data were fitted to the second-order kinetic equation. Furthermore, the Langmuir adsorption isotherm model was used for the description of the adsorption process very well.