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. The results indicate that the reaction is first-order, and XRD and SEM analysis confirmed that the deposited layer is metallic without evidence of basic zinc sulphate or cadmium re-dissolution. The dependence of the reaction constant on initial cadmium concentration validates the reaction mechanism. Various kinetic controlling models regarding cadmium ion (Cd 2+) cementation from aqueous solution by zinc powder were evaluated based on initial cadmium concentration, temperature, zinc powder size, and stoichiometric ratio. Experimentation demonstrated that for initial cadmium concentrations greater than 1000 ppm, the ash diffusion control model is predominant, while concentrations below 500 ppm align well with the film diffusion model. For intermediary concentrations, a combination of both models influences the reaction rate. Numerical analysis revealed activation energies of 9.6 and 7.2 kJ/mol at high and low concentrations, respectively. Additionally, statistical data analysis facilitated the estimation of various reaction rate constants for different initial cadmium concentrations.