The Adsorption Studies Of²³⁷Np Onto Granite

Although237Np in spent nuclear fuel is only about0.05%by mass, it has a very long half-life (2.144million years), and also is a high-toxic radionuclide, so there is a concern about its mobility in the distant future, when radioactive storage sites might be expected to degrade. In this paper, the adsorption of Np(V) and Np(IV) onto Beishan granite powders under the N2atmosphere condition was studied by the batch experiment.Firstly, the Beishan granite was pretreated (removing carbonate and converting into Na+type), then was characterized by X-Ray Diffraction, Fourier Transform Infrared Spectroscopy and X-Ray fluorimetry. Meanwhile, the surface area of granite powders was measured by N2-BET method, the cation exchange capacity was measured by the ammonium acetate method, and granite powders was titrated by potentiometric titration as well. In addition, proper measuring methods of237Np were establish, namely, liquid scintillation counter was used for determining conventional concentration of237Np solution, alpha spectrometer and inductively coupled plasma mass spectrometry were also employed. Adsorption of Np(V) onto Na-granite as a function of contact time, solid-to-liquid ratio, ionic strength, temperature, pH and concentration of Np(V) was studied. Results shows, the adsorption of Np(V) onto granite was fairly rapid. At lower pH, the slurry concentration has few effects on the adsorption, while at higher pH, the greater the slurry concentration, the more the adsorption of Np(V) onto granite, and then maintaining a small change. Ionic strength has little impact on the adsorption of Np(V) onto granite at both low and high pH value. The adsorption was largely dominated by the pH value, and the pH sorption edge move to the left with the increasing temperature and decreasing concentration of Np(V). The Dubinin-Radushkevich (D-R), Langmuir, and Freundlich isotherms models were used to simulate the adsorption of Np(V), and the D-R model turned out to be the best, indicating a chemisorption process involved.In the second place, Np(V) was reduced to Np(IV) by sodium iodide and hydrazine hydrochloride at boiling temperature. Then, the analogous adsorption experiments were carried out, and the hydrazine hydrochloride was also added to the tubes except as otherwise noted. Nevertheless, the adsorption results are somewhat different from the adsorption of Np(V):the adsorption of Np(IV) decreased with the increasing slurry concentration, and the ionic strength has some impact on the adsorption as well. The liquid-liquid extraction of neptunium (IV) of the equilibrium solutions into 0.5mol/L2-thenoyl trifluoroacetone-xylene turned out that Np(IV) was partly oxidized into Np(V), this may be ascribed to the oxidization of granite powders.