Separation of Heavy Metal Chelates by Activated Carbon: Effect of Surface and Species Charge

An extensive experimental investigation was conducted with activated carbons to establish the adsorption capacities of heavy metals (Cd2+, Ni2+, Cu2+, and Zn2+ ) in the presence of complexing agents (EDTA, triethylenetetramine (TRIEN), citrate, etc.). Adsorption equilibria are explained by species charges and carbon surface charge characteristics. In the pH range 7.5−8.0, the active sites are positively charged. The adsorption capacities of Cd2+ (and other heavy metals), free ligands, and cadmium-ligands are also a function of feed metal concentration, types of ligands, molar ratio of ligand/metal, and pH. Complete metal-ligand species distributions are calculated by computer solutions of multiple reaction equilibria. For an equilibrium cadmium concentration of 0.1 mM (pH 7.5−8.0), the sequence in adsorption capacity for cadmium chelates is Q Cd−EDTA > Q Cd−citrate > Q Cd−TRIEN > Q Cd−tartrate. A relationship between the adsorption capacity, surface charge of activated carbon, and the average species charge for various cadmium ligands systems is presented.