Study of the diffusion of metals and metals-EDTA complexes in water by capillary method

EDTA can complex with radionuclides (RNs) to form negatively charged complexes, making it difficult for clay minerals to retard the diffusion of RNs waste. The diffusion coefficient of RNs in water (Dw ) is an important parameter for the safety assessment of the repository. In this study, the effectsof EDTA on the diffusion of metal ions (Cu2+ , Sm3+ , Nd3+ , Lu3+ and Zn2+ ) were investigated by a capillary method. The experimental results showed that [Cu-EDTA]2- , [Sm-EDTA]- and [La-EDTA]- have higher Dw thanthe Mn+. Whereas, [Nd-EDTA]- and [Zn-EDTA]2- have lower Dw than Nd3+ and Zn2+ cations. The Dw is consistent with the literatures, indicating the validity of the capillary method to determine the diffusion coefficients. According to Stokes-Einstein relation, the ionic radius and ionic potential of the ion are in disproportional to the Dw value. Cu-, Sm-and La-EDTA complexes have smaller molecular size than the uncomplexed metal ions, indicating that the Mn+ ions might be associated with many water molecules to form hydrated ions with larger ionic radius. Whereas the [Nd-EDTA]- and [Zn-EDTA]2- have larger molecular size than Nd3+ and Zn2+ cations.