Selective sorption of cadmium by mixed oxides of iron and silicon

The present work deals with the synthesis, characterization and cadmium adsorption studies on SiO2, Fe(OH)3 and their mixed oxides. Mixed oxides of iron and silicon of composition [0.5 M SiO2:0.5 M Fe(OH)3] were prepared by physical mixing (M1) and sequential precipitation methods (M2). The selectivity of adsorbents towards Cd2+ ion was found to be in the order M2 > M1 = Fe(OH)3 > silica. Dissolution of silica, mixed oxides M1 and M2 was also investigated in the pH range 3–10. The greater dissolution of mixed oxide M2, along with the higher selectivity towards Cd2+ ions showed the formation of the ternary complexes on the surface of M2. The complex formation was also confirmed by IR studies and by the increase in Cd2+ sorption with the addition of the silicates anions in the medium. The thermodynamic parameters showed the Cd2+ sorption to be endothermic and spontaneous in nature.