Kinetic mechanisms of cementation of cadmium ions by zinc powder from sulphate solutions

The cementation of cadmium ions by zinc powder was studied in a batch reactor at low and high concentrations at pH 5.2-5.4 and it is shown that the reaction is first-order. XRD and SEM analysis confirm that the deposited layer is metallic with no evidence of basic zinc sulphate or re-dissolution of cadmium. Dependence of the reaction constant on initial cadmium concentration proves the reaction mechanism. Different possible kinetic controlling models of cadmium ion (Cd2+) cementation from aqueous solution by zinc powder were studied with respect to initial cadmium concentration, temperature, zinc powder size and stoichiometric ratio. The experiments demonstrate that at initial cadmium concentrations > 1000 ppm, the ash diffusion control model prevails, while at concentrations < 500 ppm, the data has good agreement with the film diffusion model. For concentrations between 500 ppm and 1000 ppm, a combination of ash diffusion and film diffusion models controls the reaction rate. Based on numerical analysis, the calculated activation energies at high and low concentrations are 9.6 and 7.2 kJ/mol, respectively. Statistical data analysis was performed and different reaction rate constants were estimated from the equations for high and low initial cadmium concentrations. (C) 2006 Elsevier B.V. All rights reserved.