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Neptunium(V) / Diffusion / Sorption / Montmorillonite / ModelingSummary. Diffusion and sorption of radionuclides in compacted bentonite/montmorillonite are key processes in the safe geological disposal of radioactive waste. In this study, the effects of carbonate and salinity on neptunium(V) diffusion and sorption in compacted sodium montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (D e ) and distribution coefficients (K d ) of 237 Np(V) in sodium montmorillonite compacted to a dry density of 800 kg m −3 were measured under four chemical conditions with different salinities (0.05/0.5 M NaCl) and carbonate concentrations (0/0.01 M NaHCO 3 ). D e values for carbonate-free conditions were of the order of 10 −10 -10 −11 m 2 s −1 and decreased as salinity increased, and those for carbonate conditions were of the order of 10 −11 -10 −12 m 2 s −1 and showed the opposite dependence. Diffusion-derived K d values for carbonate-free conditions were higher by one order of magnitude than those for carbonate conditions. Diffusion and sorption behaviors were interpreted based on mechanistic models by coupling thermodynamic aqueous speciation, thermodynamic sorption model (TSM) based on ion exchange, and surface complexation reactions, and a diffusion model based on electrical double layer (EDL) theory in homogeneous narrow pores. The model predicted the experimentally observed tendency of D e and K d qualitatively, as a result of the following mechanisms; 1) the dominant aqueous species are NpO 2 + and NpO 2 CO 3 − for carbonate-free and carbonate conditions, respectively, 2) the effects of cation excess and anion exclusion result in opposite tendencies of D e for salinity, 3) higher carbonate in solution inhibits sorption due to the formation of carbonate complexes.

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The ultrasound technique was applied to the synthesis of layered double hydroxides. The use of ultrasound in the synthesis of these compounds was studied in terms of its effect on their crystal qualities and surface properties. The crystal qualities of the compounds synthesized with ultrasound were compared with those of compounds synthesized without ultrasound to elucidate the effect of the ultrasound on the synthesis. The effect of crystal quality (crystallite size) on the adsorption behavior of humic substances was examined. The compounds synthesized under ultrasonic conditions showed a larger crystallite size and a larger adsorption capacity for humic substances than those synthesized without ultrasonic treatment. The degree of adsorption correlated well with crystal quality. INTRODUCTI ON Layered double hydroxides (LDHs) such as hydrot a l c i t e a n d h y d r o t a l c i t e -l i k e c o m p o u n d s M 2+x M 3+ y (OH) 2(x+y) A n¡ y/n ·mH 2 O (where M 2+ is a divalent metal ion, M 3+ is a trivalent metal ion, and A n¡ is an intercalated anion) are well known as inorganic anion exchangers. Because of their large ion-exchange capacity and structural thermal stability, which are difficult to achieve simultaneously in conventional anionexchange resins, these compounds are used in many industrial processes, e.g. for polymer synthesis, and as coatings that are halogen scavengers and neutralizers (Cavani et al., 1991).These compounds are commonly synthesized by precipitation from a parent salt solution of divalent and trivalent metal ions, followed by ageing at an elevated temperature (hydrothermal treatment) in the laboratory. The conditions of synthesis, such as solution pH, temperature and metal composition, along with physicochemical structure of the compounds have been studied extensively by many researchers (Allmann et al., 1970;