The Adsorption Of Some Key Radionuclides On Bentonite Composite And Beishan Granite

Beishan (in Gansu province) is a preliminarily selected site for the repository of high-level radioactive waste in China, and Gaomiaozi (GMZ) bentonite has been selected as buffer/backfilling material. Thereby, it is of significance to study the adsorption of key radionuclides on GMZ bentonite and Beishan granite. In this dissertation, the adsorption of Am(III), Eu(Ⅲ), Np(V) and U(Ⅵ) on graphene oxides (GO)-GMZ bentonite (Bent) composite (GO-Bent), and the adsorption of Se(Ⅳ) and Se(Ⅵ) on Beishan granite (borehole No.10) were investigated.GO-Bent was prepared, and characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetry analysis, and potentiometric titration. The adsorption of Am(Ⅲ), Eu(Ⅲ), Np(V) and U(Ⅵ) on GO-Bent was studied as a function of GO content, pH value and ionic strength using a batch technique. It was found that the adsorption of the four elements obviously increased with GO content, due to oxygen-containing functional groups on GO. The adsorption of Am(Ⅲ) and Eu(Ⅲ) on GO-Bent decreased with ionic strength in the low pH range, indicating that the adsorption was dominated by outer-sphere surface complexation or ion exchange reactions. In comparison, no obvious effect of ionic strength on Am(Ⅲ) and Eu(Ⅲ) adsorption on GO-bent was observed in the high pH range, suggesting that inner-sphere surface complexation was the main adsorption mechanism. There was no apparent effect of ionic strength on the adsorption of Np(Ⅴ) and U(Ⅵ) on GO-Bent. The adsorption isotherms of Am(Ⅲ) and Eu(Ⅲ) on GO-Bent can be described by Langmuir model, whereas the adsorption isotherms of Np(V) and U(Ⅵ) can be described by Freundlich model.75Se(Ⅳ) and 75Se(VI) radiotracers were prepared by the oxidation of 75Se(0) with nitric acid and hydrogen peroxide, respectively. The adsorption of Se(Ⅳ) on Beishan granite was studied as a function of contact time, solid-to-liquid ratio, pH, ionic strength, Se concentration and temperature with 75Se(IV) radiotracer. The results indicated that Se(Ⅳ) adsorption kinetics on Beishan granite can be described by a pseudo-second- order rate equation. Se(IV) adsorption equilibrium was reached within 48 h. The adsorption of Se(IV) decreased with increasing pH value, and was independent of ionic strength, suggesting an inner-sphere surface complexation mechanism. No apparent temperature effect on Se(IV) adsorption on Beishan granite was observed in the temperature range of 25-80℃. In addition, the adsorption of Se(Ⅵ) on Beishan granite was relatively weak, with distribution coefficients less than 1 in all experimental conditions of this study.