Abstract
Complexing ligands may alter the adsorption behaviour of heavy metals compared to a ligand free system, either by changes in solution speciation, or by interactions at the adsorbing mineral surface. In this paper the effect of sulfate on the adsorption of cadmium onto goethite has been studied. Adsorption experiments under defined conditions revealed the following effects: Cadmium adsorption from sulfate containing solutions by goethite was higher than from pure NaNO3-solutions. Simultaneously sulfate adsorption increased, indicating synergic effects. Increasing the ionic strength of the metal-sulfate solutions caused a decrease both in heavy metal and in sulfate adsorption. Model calculations on simultaneous adsorption of cadmium and sulfate lead to the following conclusions: 1.) Sulfate adsorption reduces the surface potential so that the surface becomes more attractive to heavy metals. 2.) In addition, new, so called ternary surface complexes (SOCd+-SO4 2−) between reactive surface sites, heavy metals and sulfate-anions must be assumed in order to get agreement between observed and calculated effects.
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