Bentonite colloids immobilization and release in quartz column and its influence on selenite migration.

Immobilization and release of colloids are important for colloids-facilitated migrations, and in the safety assessment of geological disposal for high-level radioactive waste, the association between the immobilization and release process of the bentonite colloids with selenite migration has not been well revealed. In this work, the migration of bentonite colloids under different conditions is evaluated, and the effects of colloids immobilization and release on selenite migration are studied. In addition, the cases of in-migration (colloids are immobilized in the quartz sand, and then selenite migrates through the quartz sand with immobilized colloids) and co-migration (colloids bearing selenite are immobilized in the quartz sand) are investigated. The results show that in the systems containing 3.0 mM Mg2+, the mobility of the colloids is highly hindered and the colloids are immobilized in the quartz sand mainly by straining effect. The immobilization of bentonite colloids affects selenite migration differently according to the immobilization process (in-migration or co-migration). A more significant retardation effect is observed in the co-migration process than in-migration due to the additional inner-sphere complexed selenite in the co-migration. The immobilized colloids can be more easily released by alkaline DI-water (pH 11.0) than acidic one (pH 6.0) as a result of the more negative surface charges of the immobilized bentonite colloids. The average size of the released colloids is larger than the initial colloids at the same pH. Selenite is found to be released ahead of colloids in either in- or co-migration process, and part of selenite is discovered migrating with released colloids in co-migration process. Since colloids immobilization and release would influence radionuclides migration, further research about colloids immobilization and release with broad range of pH and ionic strength in the host rock and its influence on the migration of other radionuclides are needed.

Multiple investigations of aqueous Eu(III)-oxalate complexes: the reduction in coordination number and validation of spectral linear correlation.

Detailed information on the An(iii)/Ln(iii) complexation properties in solution is essential for separation chemistry and the prediction of their potential for radionuclide migration from nuclear waste repositories into natural aquifers. In the present study, to better reveal and confirm the structural information of [Eu(Ox)x (H2O)h-2x]3-2x (h = 8, 9; x = 0-3) aqueous species, especially the variable coordination number (CN), and explore the validity of the spectral linear correlation between the luminescence lifetime and the residual hydration number in the first coordination sphere of Eu(iii) compounds in solution, a comparison between the spectral results and the theoretical calculations in a wide parametric space in terms of the pH value and oxalate concentration was carried out by combining time-resolved luminescence spectroscopy (TRLS) with speciation modelling and density functional theory (DFT) calculations. We have found direct and clear evidence for the 9-fold to 8-fold coordination number reduction of Eu(iii) atoms upon coordination with more than one oxalate in an aqueous medium, and as well systematically validated the applicability of the spectral linear correlation in an aqueous system (otherwise solid state) involving multiple species with the support of relatively reliable and clear speciation modelling.

Macroscopic and spectroscopic characterization of U(VI) sorption on biotite.

Knowledge of the geochemical behavior of uranium is critical for the safe disposal of radioactive wastes. Biotite, a Fe(II)-rich phyllosilicate, is a common rock-forming mineral and a major component of granite or granodiorite. This work comprehensively studied the sorption of U(VI) on biotite surface with batch experiments and analyzed the uranium speciation with various spectroscopic techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and time-resolved fluorescence spectra (TRFS). Our results indicated that uranyl ions could penetrate into the interlayer of biotite, this ion-exchange process was pH-dependent and only favorable under acidic condition. Instead of precipitation or reduction to uraninite, the TRFS results strongly suggests U(VI) forms surface complexes under the neutral and alkaline condition, though the number and structure of surface species could not be identified accurately. Besides, the oxidation of biotite with peroxide hydrogen showed that structural Fe(II) would have a very low redox reactivity. With leaching experiments, zeta potential analysis and thermodynamics calculation, we discussed the possible reasons for inhibition of U(VI) reduction at the biotite-water interface. Our results may provide insight on interaction mechanism of uranium at mineral-water interface and help us understand the migration behavior of uranium in natural environments.

Sorption of U(VI) onto natural soils and different mineral compositions: The batch method and spectroscopy analysis.

This research studied the sorption behavior of uranium(VI) onto two different kinds of soils: surface soil and undersurface soil that taken from the depth of 30 m undersurface. The soil samples were collected from a low and medium-level radioactive waste disposal site in the southwest of China. The effects of pH, solid-liquid ratio and contact time on the adsorption behavior were studied by batch adsorption method. The experiment results show that the mineral composition of soil and the speciation of U in natural groundwater are two main influencing factors. Muscovite and clinochlore, two of the main minerals of soil samples, dominate the sorption behavior of uranium onto natural soils at weak acidic and near neutral pH range. Under neutral and weak alkaline conditions, the thermodynamic calculation results show that Ca2+ and CO32- have significant influence on the species of uranium in aqueous solution. The U sorption reduced sharply due to the formation of the CaUO2(CO3)32- (aq) complex. This work provides a better insight of the sorption behavior of uranium onto natural soils, and gives an in-depth understanding about the influence of aqueous and surface speciation.

Influence of Fe(II) on the Se(IV) sorption under oxic/anoxic conditions using bentonite.

79Se, one of the key radionuclides for nuclear waste disposal, threatens the quality of the environment, as well as human health. Therefore, it needs to be permanently isolated from the biosphere. The aim of the study was to investigate the effects of Fe(II)/Fe(III) on the removal of 79Se using bentonite in the pH range of 2.0-10.0 under oxic/anoxic conditions. Under oxic conditions, Se(IV) prefers to form inner-sphere complexes with Fe(III)-oxyhydroxide, derived from the oxidization of Fe(II) using oxygen. Interestingly, Se(IV) will interact with Fe(III) and form a poorly soluble ferric selenite at pH ∼4 under oxic conditions. Under anoxic conditions, however, the concentration of Fe(II) is closely related to the sorption process of Se(IV) on bentonite. When the concentration of Fe(II) was less than 1%, Fe(II) combined with the hydroxyl, forming Fe(OH)2, which generated a disproportionation at pH ∼8 and formed a new sorbent, Fe3O4. However, when the concentration of Fe(II) was increased to 5%, reduction precipitation was the primary way to remove Se(IV) in aqueous solution. XANES (X-ray Absorption Near Edge Structure) spectra showed that higher pH values are beneficial for the formation of the final thermodynamic reduction product, Fe selenide. These results suggested that Fe(II) significantly affect the Se(IV) sorption. Overall, this study confirmed the significant role of Fe(II) on the retardation of 79Se and on remediation for Se(IV) contamination in the hydrosphere.