Sorption of Metal Ions on a Mixed Oxide [0.5 M SiO 2 :0.5 M Fe(OH) 3 ]

Abstract

The sorption of Zn²⁺, Ni²⁺, Cd²⁺, Ca²⁺ and Mg²⁺ ions on to a mixed oxide of iron and silicon was found to increase with increasing temperature, concentration and pH of the system. The selectivity of this mixed oxide was Cd²⁺ ≥ Zn²+ ≥ Ni²⁺ > Ca²⁺ > Mg²⁺, indicating that the mixed oxide was more selective as an exchanger towards Cd²⁺ ions relative to its components, SiO₂ and Fe(OH)₃. The sorption data fitted the linear forms of the Kurbatov and Langmuir adsorption equations. The sorption of metal cations was accompanied by the release of H⁺ ions to the bulk phase. On average 1 mol H⁺ was released for every cation sorbed. The values of the binding constants were used to estimate the apparent thermodynamic parameters, ΔH and ΔS. The phenomenon of enthalpy/entropy compensation showed that the adsorption of metal ions by the mixed oxide occurred typically through a cation-exchange mechanism.

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