Removal of cadmium(II) and lead(II) from aqueous solution using sulfur-functionalized silica prepared by hydrothermal-assisted grafting method

In this paper, silanol (SiOH) groups on the surface of silica were functionalized by the silylation reactions with bis[3-(triethoxysilyl)propyl]tetrasulfide molecules using a hydrothermal-assisted surface grafting method to give a tetrasulfide-functionalized silica sorbent, which was applied to the removal of Cd2+ and Pb2+ ions from aqueous solutions. The sorbent was characterized by scanning electron microscopy, fourier-transform infrared spectroscopy, N-2 adsorption-desorption isotherms, elemental analysis and thermogravimetric analysis. The effects of pH values of solution, the contact time and the initial concentrations of metal ions were investigated by conventional batch equilibrium adsorption test. The characteristics of the adsorption process were investigated by Scatchard plot analysis. The results showed that the maximum static adsorption capacity of tetrasulfide-functionalized silica sorbent by hydrothermal heating method was 1.5 times as much as by the conventional heating method. The experimental values of the saturated adsorption capacities of Cd2+ and Pb2+ could reach to 22.3 and 46.3 mg g(-1), respectively. The tetrasulfide-functionalized silica sorbent had a substantial binding capacity in the range of pH 3.7-8.5. Scatchard analysis revealed that two classes of binding sites were formed. The pseudo second-order kinetic model provided excellent kinetic data fitting (r(2) > 0.99). Equilibrium data Equilibrium data fitted to the Langmuir isotherm model. Various thermodynamic parameters such as Delta G degrees, Delta H degrees and Delta S degrees revealed that the adsorption was endothermic and spontaneous process. In addition, the adsorption/desorption cycle of tetrasulfide-functionalized silica sorbent could be up to five times. These results confirmed that tetrasulfide-functionalized silica sorbent could be a potential sorbent for the removal of Cd2+ and Pb2+ from aqueous solutions. (C) 2012 Published by Elsevier B.V.