LIGAND EFFECT ON THE ADSORPTION OF HEAVY-METALS - THE SULFATE CADMIUMGOETHITE CASE

Complexing ligands may alter the adsorption behaviour of heavy metals compared to a ligand free system, either by changes in solution specia tion, or by interactions at the adsorbing mineral surface. In this pap er the effect of sulfate on the adsorption of cadmium onto goethite ha s been studied. Adsorption experiments under defined conditions reveal ed the following effects: Cadmium adsorption from sulfate containing s olutions by goethite was higher than from pure NaNO3-solutions. Simult aneously sulfate adsorption increased, indicating synergic effects. In creasing the ionic strength of the metal-sulfate solutions caused a de crease both in heavy metal and in sulfate adsorption. Model calculatio ns on simultaneous adsorption of cadmium and sulfate lead to the follo wing conclusions: 1.) Sulfate adsorption reduces the surface potential so that the surface becomes more attractive to heavy metals. 2.) In a ddition, new, so called ternary surface complexes (SOCd+-SO42-) betwee n reactive surface sites, heavy metals and sulfate-anions must be assu med in order to get agreement between observed and calculated effects.