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.

Graphical abstractDownload high-res image (212KB)Download full-size image