Research papersAdsorption and retarded diffusion of EuIII-EDTA− through hard clay rock

- EDTA diffusion in COx clay rock was studied using [EuIII-EDTA]−.
- Adsorption of Eu was driven by anions adsorption at high EDTA concentration.
- Anion exclusion in COx clay rock induced low diffusivities of EDTA complexes.
- Diffusion was less retarded than predicted from adsorption on crushed clay.
- This early break-through was contrary to the data obtained on oxides containing rocks.

Adsorption and diffusion experiments of EuIII were performed in Callovo-Oxfordian (COx) clay rock in the presence of EDTA. The predictive model based on binary system parameters (Eu/COx and EDTA/COx) was in good agreement with the results for the Eu/EDTA/COx ternary system. At low EDTA concentrations, the behaviour of EuIII was mainly driven by Eu3+adsorption and complexation by carbonates and EDTA. At higher EDTA concentrations, the behaviour of EuIII was driven by the adsorption of [EuIII-EDTA]− anions. Europium was then used as a probe to estimate the transport of EDTA. Three through-diffusion experiments of EDTA were compared with 14C, Eu and 152Eu tracers. EuIII-EDTA was not quantitatively dissociated by diffusion through the rock. The effective diffusion coefficients quantified De(EuIII-EDTA) = 1.5-1.7 · 1012 m2 s−1 were an order of magnitude lower than that of water, evidencing the anionic exclusion of [EuIII-EDTA]− within the clay rock. Break-through curves and diffusion profiles confirmed retardation due to significant adsorption on the clay rock (Rd(EuIII-EDTA) ∼ 6-14 L kg−1) in comparison with inorganic anions. However, the model based on batch adsorption measurements failed to predict the diffusion results. All experiments displayed an early break-through of EDTA complexes. This behaviour contrasted with results on iron oxides rich sediments, which usually led to higher retardation than expected from the batch studies.

212