Chemical retention. i.e.. partition of the element between aqueous Solution and mineral surface, is a key phenomenon for assessing the safety of possible nuclear waste disposal. For this purpose, the sorption of Eu(III) onto a model mineral-a-alumina-is Studied here, including the effects of groundwater chemistry: pH and concentrations of small organic and inorganic ligands (acetate, oxalate, and carbonate anions). This work presents some experimental evidence for a synergic mechanism of sorption of europium-ligand complexes onto the alumina. Only cationic complexes were necessary to consider to model experimental results. Using the ion-exchange theory (IET) and a corresponding restricted set of parameters-exchange capacities and thermodynamic equilibrium constants-the whole set of sorption experiments of Eu(III) cationic species onto the alpha-alumina was modeled Under various chemical conditions. (c) 2006 Elsevier Inc. All rights reserved.