[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)