Abstract
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.
Highlights
Ethylenediaminetetraacetate (EDTA) is often used as a chelate for agricultural and industrial applications, resulting in an increase of EDTA concentrations in various water sources
The diffusion coefficient (Dw) for a metal ion in NaCl solution was obtained by fitting the concentration of the metal ion in vials as a function of time by the following equation 13:
The ionic potential (Ip) is defined as: Ip z /r where z is the charge of ion and r is the ionic radius (Å)
Summary
Ethylenediaminetetraacetate (EDTA) is often used as a chelate for agricultural and industrial applications, resulting in an increase of EDTA concentrations in various water sources It may complex with free heavy metal ions to form solublecomplexation, enhancing the transportation of M-EDTA(Z-4)+ complexes in water and clays barrier systems [1,2,3]. EDTA can decrease the adsorption of metal ions on minerals and enhance their transportation due to the formation of negatively charge complexes[7, 8], and due to the reducing of the clay specific surface area 9. Descostes et al (2017) reported that EDTA increase the diffusion of RNs in clays due to the formation of negatively charged complexes. We wish to have more knowledge of the diffusion properties of MEDTA(Z-4)+ complexes before the diffusion experiment in clay starts
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