Thermal activation of serpentine for adsorption of cadmium

Thermal activated serpentine with high adsorption capacity for heavy metals was prepared. The batch experiment studies were conducted to evaluate the adsorption performance of Cd²⁺ in aqueous solution using thermal activated serpentine as adsorbent. These samples before and after adsorption were characterized by XRD, FT-IR, SEM, XPS, and N₂ adsorption-desorption at low temperature. It was found that serpentine with layered structure transformed to forsterite with amorphous structure after thermal treatment at over 700°C, while the surface area of the samples was increased with activated temperature and the serpentine activated at 700°C (S-700) presented the largest surface area. The pH of solution after adsorption was increased in different degrees due to hydrolysis of MgO in serpentine, resulting in enhancing adsorption of Cd²⁺. The S-700 exhibited the maximum equilibrium adsorption capacity (15.21mg/g), which was 2 times more than pristine serpentine. Langmuir isotherm was proved to describe the equilibrium adsorption data better than Freundlich isotherm and pseudo second order kinetics model could fit the adsorption kinetics processes well. Based on the results of characterization with XPS and XRD, the adsorption mechanisms could be explained as primarily formation of CdCO₃ and Cd(OH)₂ precipitation on the surface of serpentine.