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

Descostes, Michael, Pointeau, Ingmar, Radwan, Jean, Poonoosamy, Jenna, Lacour, Jean-Luc, Menut, Denis, Vercouter, Thomas, Dagnelie, Romain V.H.
Journal of hydrology 2017 v.544 pp. 125-132
EDTA (chelating agent), adsorption, anions, carbon, carbonates, clay, diffusivity, europium, hydrology, iron oxides, models, sediments, tracer techniques
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)=1.5–1.7·1012m2s−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)∼6–14Lkg−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.