The sorption of Cd(II) and U(VI) on sepiolite: A combined experimental and modeling studies

The sorption of Cd(II) and U(VI) on sepiolite as a function of contact time, pH, ionic strength, temperature and initial concentration was investigated by batch techniques. The sorption kinetics showed that sorption of Cd(II) and U(VI) on sepiolite can be satisfactorily fitted by pseudo-second order kinetic model with high correlation coefficients of 0.999. The sorption of Cd(II) and U(VI) on sepiolite significantly decreased with increasing ionic strength at pH < 4.0, whereas the sorption of Cd(II) and U(VI) on sepiolite was independent of ionic strength at pH > 5.0, indicating that cation exchange/outer sphere surface complexation and inner-sphere surface complexation dominated the Cd(II) and U(VI) sorption on sepiolite at pH < 4.0 and pH > 5.0, respectively. According to surface complexation modeling, the sorption of Cd(II) and U(VI) on sepiolite can be satisfactorily simulated by diffuse double layer model with cation exchange sites (XNa/XH) at low pH and surface complexation sites (SOCd+/SOUO2+) at high pH conditions. The finding presented herein indicated that clay minerals can potentially be served as an efficient retardant for the immobilization of heavy metals and radionuclides in environmental cleanup.