The influence of anionic ligands on the sorption behaviour of Eu(III) on natural hematite

The solid-water interface reaction is investigated for the sorption of the Eu(III) ion, as a trivalent actinide homologue, onto well characterized nat ural hematite at pH less than or equal to 6 in 0.1 M NaClO4 in the presence of various ligands: oxalate, humic and fulvic acid (HA/FA). In each case a pH dependence of the Eu(III) sorption is observed that differs from the so rption of Eu(III) in the absence of ligands. At higher pH the sorption is d ecreased by competition of dissolved ligands with hematite surface sites. A t lower pH uptake of Eu(III) increases due to its interaction with surface sorbed ligands. An attempt is made to describe the reactions by a surface c omplexation model taking into account the surface complexation constants fo r Eu(III) onto hematite, the complexation constants for the respective liga nds and the experimentally determined solid/liquid distribution of each lig and. In the case of oxalate the model describes the distribution of Eu(III) between solid and liquid phases reasonably well. However, the same approac h is found not suitable to describe the influence of humic and fulvic acid on the Eu(III) sorption. The divergence of experimental and modelling resul ts is ascribed to blocking of the hematite surface hydroxyl groups by sorbe d humic or fulvic acid and formation of strong ternary surface complexes of the type =S-O-Eu=(HA/FA).