Abstract:
The colloidal-borne facilitated transport of actinides is an important aspect of radionuclide migration investigation in high-level waste geological disposal. In the present study, colloidal behavior of Np in the presence of humic acids was investigated by using micro- and ultra-filtration methods under simulated disposal conditions. All experiments were performed in argon atmosphere glove-box and at ambient temperature. No significantly effect of acidity, inorganic irons and filter wall and membrane adsorption on the detection of Np was found in these research systems. It is indicated that the counting efficiency for
237Np should be close to 100%. The humic acids would be the apparent influence factor for the
237Np counts ratios due to its quenching efficiency, however, the sample quenching induced by humic acids can be depressed by increasing the volume of liquid scintillation cocktail to 16 mL under the condition that the humic acids concentration is lower than 66.6 mg/L. The formation of neptunium colloids was observed, and the influences of oxidation states of Np, initial concentrations of Np and the present of humic acids on Np colloidal behaviors were investigated. The intrinsic colloids of Np were found in Np(Ⅳ) solutions at pH=8 under anaerobic conditions, while the formation of the colloidal species was less observed in Np(Ⅴ) solutions under the same conditions. The initial concentration of Np(Ⅳ) would influence the Np(Ⅳ) colloidal behavior, and the amount of Np(Ⅳ) colloids would be very lower under the higher Np(Ⅳ) initial concentration. The radioactive pseudocolloids of Np(Ⅳ) were formed in the presence of humic acids in aqueous solution. It is indicated that the Np(Ⅳ)-humates pseudocolloids are generated through adsorption and complexation of Np(Ⅳ) by humic acids. In this work, we found that humic acids will enhance the Np(Ⅳ) concentrations in solutions, the results demonstrate that some Np(Ⅳ) can be bound to low molecular weight humic acids and thus the 10 kDa membrane can not retard these Np(Ⅳ)-humate compounds. It can be concluded that the present of humic acids will facilitate the migration of Np(Ⅳ) even under anaerobic conditions.