Cadmium and Zinc Adsorption Kinetics onto Activated Carbon in Single and Binary Systems

In this paper, we are presenting a study aimed to understand the adsorption kinetics of a binary system with zinc and cadmium. Firstly, the zinc adsorption kinetic study as single compound was carried out in terms of initial concentration and carbon particle size. Then the adsorption kinetics of the Zn/Cd binary system was accomplished in different experimental configurations. For a thorough comprehension of the multicomponent adsorption, a specific preloaded test was also carried out by adding a zinc solution to a single-compound cadmium-carbon suspension at equilibrium. The experiments showed that the zinc adsorption rate decreases with the increase of its initial concentration, while remaining almost constant with the carbon particle size. The preloaded test revealed that the cadmium desorbed once the zinc was added and the system reached suddenly the equilibrium condition of the binary system. The experimental data were interpreted by several kinetic models and it turned out that the Elovich model described adequately most of the adsorption data. In conclusion, the activated carbon presents a higher affinity toward zinc with respect to cadmium, and the binary tests showed a competition toward the same active sites between the two compounds, being zinc ions always preferred.