Effect of heat treatment on the efficient adsorption of Cd²⁺ ions by nanosized SiO₂, TiO₂ and their composite

In this study nanosized SiO₂, TiO₂ and their composite were synthesized via the oil in water (o/w) microemulsion method and their thermal treatment was performed at 378, 573, 973 and 1273 K. The physicochemical properties of the samples were studied by surface area measurements, scanning electron microscopy, Fourier transform infra-red spectroscopy and x-ray diffraction analysis. The Brunauer, Emmett and Teller surface area of all the adsorbents increases from 378 to 573 K, while it decreases upon further heat treatment. The average crystallite size decreases by heating the samples from 378 to 573 K while it increases when the adsorbents were thermally heat treated at 973 and 1273 K. The intensity of a few IR bands was reduced along with the disappearance of most of the bands at higher temperatures. The appearance of the beta-cristobalite phase in SiO₂ and the rutile phase in TiO₂ was confirmed from the diffraction data. The heat treated samples were subjected to preliminary adsorption of Cd²⁺ ions from aqueous solution at 293 K. Based on the preliminary adsorption experiments, SiO₂, TiO₂ and their composite heat treated at 573 K were selected for further adsorption studies. The Langmuir model was found to be fitted to the sorption data of TiO₂ and the nanocomposite while the adsorption of Cd²⁺ ions by the SiO₂ nanoparticles was explained well based on the Freundlich model. In the present study, the maximum Cd²⁺ adsorption capacity of SiO₂, TiO₂ and their composite was found to be 79.72, 98.55 and 107.17 mg g⁻¹, respectively. The q _m and K _f values obtained in the present study were found to be far better than those reported in the literature. The negative values of Δ G confirm the feasibility of an adsorption process at higher temperatures. The positive values of Δ H and Δ S represent the endothermic and physical nature of the adsorption process with the increased randomness of Cd²⁺ ions at the solid/solution interface. (paper)