Formation of complexes on the surface of mixed oxides. A comparative sorption study

The present work deals with the complex formation during the sorption of cadmium on mixed oxides. The various mixed oxides of iron and silicon like M0.25Si/0.75Fe, M0.50Si/0.50Fe and M0.75Si/0.25Fe were prepared by sol–gel method. The selectivity of adsorbents towards Cd2+ ion was found to be in the order M0.75Si/0.25Fe > M0.50Si/0.50Fe > M0.25Si/0.75Fe. Dissolution of silica from the surface of mixed oxides was found in a similar trend as the selectivity order for Cd2+ sorption, which points towards the formation of ternary complexes on the surface of the solids. Further, the order of the silica dissolution from the mixed oxides was same as determined by both EDX analysis and graphite furnace atomic absorption (GFAA) spectrophotometer. The complex formation on the surface of the solids has also been confirmed by FTIR spectroscopy. The thermodynamic parameters for the sorption of Cd2+ ions on all the three mixed oxides were also evaluated which show that the metal uptake process is endothermic and spontaneous in nature.

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► Cd2+ uptake and silica release increases with the Si contents in mixed oxides.
► The dissolved silicate results inner sphere complexation.
► Presence of Cd2+ in the mesopores hinders the phase change from Fe(OH)3 to Fe2O3.