[en] Full text of publication follows: The adsorption of heavy metals on clay minerals such as kaolinite is an important process that affects the migration and retardation of neptunium and other actinides in the geosphere. The sorption of Np(V) onto the reference clay mineral kaolinite KGa-1b was investigated both by batch experiments and EXAFS measurements. The aim of our study was to combine macroscopic studies (batch experiments) with microscopic techniques (EXAFS) to study the Np(V) speciation at the kaolinite surface. The batch experiments were done under relevant environmental conditions with Np(V) concentrations of 10^-11 and 10^-12 mol/L. Sorption samples were prepared in 0.1 mol/L NaClO₄, 4 g/L kaolinite, pH 6.0 to 10.5, presence and absence of ambient CO₂, and 60-h equilibration. The sorption curves for 10^-11 and 10^-12 mol/L Np(V) obtained in the presence and absence of CO₂, respectively, show that the adsorption edge occurs at pH 8.5. The uptake of Np(V) by kaolinite strongly increased above pH 7.0 and reached its sorption maximum (70 %) at pH 9.0. Above pH 9.0, the amount of Np(V) sorbed onto kaolinite decreased and reached ca. 30 % at pH 10.5 due to the formation of Np(V) carbonato species in the aqueous solution. In the CO₂-free system, the sorption of Np(V) increased continuously with pH until the sorption maximum of 100 % was reached at pH 10.5. The same sorption behavior was found in batch experiments in the CO₂ equilibrated system with Np concentrations ranging from 1 μmol/L to 10 μmol/L. EXAFS experiments on some of these batch samples indicated the formation of Np(V) carbonato species at the kaolinite surface at pH 9.0 where the uptake of Np(V) by kaolinite reaches its maximum [1]. [1] T. Reich, S. Amayri, Ta. Reich, J. Drebert, A. Jermolajev, P. Thoerle, N. Trautmann, C. Hennig, S. Sachs, Feasibility of EXAFS experiments at the Np L-edge to investigate neptunium sorption on kaolinite, Institut fuer Kernchemie, Universitaet Mainz, Annual Report 2004 (in press). (authors)

[en] Full text of publication follows: We investigated the surface sorption process of U(VI) and Np(V) on kaolinite by extended X-ray absorption fine structure (EXAFS) spectroscopy in the 10 μM concentration range. Batch experiments with kaolinite in CO₂-equilibrated systems showed that the adsorption edge of U(VI) occurs at pH 5.5, i.e., near the pH_PZC of kaolinite. The adsorption edge of Np(V) occurs well above the pH_PZC value at pH 8.5. This may indicate that the bonds between Np(V) and the surface functional groups of kaolinite are not as strong as in the case of U(VI). U(VI) and Np(V) have in common that the amount which is adsorbed decreases when the pH is increased beyond the absorption maximum. This behavior can be attributed to the formation of U(VI) and Np(V) carbonato complexes in the aqueous solutions. The aim of this comparative EXAFS study was to investigate the reason for the different affinities of U(VI) and Np(V) for kaolinite by measuring their local environments at the clay surface. Samples were prepared from 4 g/L kaolinite, 0.1 M NaClO₄, pH 3.0 - 10.5, presence and absence of ambient CO₂. The U L₃- and Np L₂-edge EXAFS spectra of the wet paste samples were measured at room temperature in fluorescence mode at the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility. The measured U-O and U-Al/Si distances indicate inner-sphere sorption of U(VI) on kaolinite. There was no evidence of uranium neighbors in the EXAFS spectra, suggesting that the adsorbed U(VI) complexes were predominantly monomeric. The average distance between uranium and its equatorial oxygen atoms, O_eq, increased from 2.32 to 2.38 Angstrom in the presence of atmospheric CO₂ when the pH was increased from 5.0 to 8.5. In the CO₂-free system, the U-O_eq distance was independent from pH and equal to 2.32 Angstrom. The lengthening of the average U-O_eq distance in the presence of carbonate (or bicarbonate) suggests the formation of ternary U(VI)-carbonato complexes with kaolinite. In contrast to U(VI), Np(V) samples prepared at pH 9.0 and ambient CO₂ did not show any indication of an inner-sphere sorption. The measured distances of 1.85, 2.55, 4.24 (all Np-O), 2.94 (Np-C), and 4.86 Angstrom (Np-Np) are consistent with the precipitation of a Np(V) carbonato species at the kaolinite surface. (authors)

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