|Other Abstract||The distribution, migration and transformation of radionuclides in the environment directly affect their environmental risks and ecotoxicity. In geological environment, the adsorption-desorption, precipitation-dissolution, oxidation-reduction and other reactions of radionucliedes will occur, which make their geochemical behavior more complex, and bring great challenges to accurately describe and predict the environmental behavior of radionuclides. However, the compositions of geological medium are complex, and the ability to absorb and block radionuclides is significantly different for each component. Typical media, such as titanium rich phase, calcite and mica minerals, which have strong retardation ability to radionuclides, play an important role in controlling the environmental behaviors such as radionuclide retardation, migration and transformation. However, the interaction mechanism between these typical media and radionuclides is still unclear. On the one hand, radionuclides can interact with the media in other ways besides adsorptionon the other hand, the interaction between radionuclides and environmental media will affect the structure and properties of environmental media. Therefore, in-depth study on the interaction and micro-mechanism between typical environmental media and radionuclides can provide the most direct theoretical support for the construction of a model that can accurately describe and predict the environmental behavior of radionuclides, which is of great guiding significance for in-depth discussion of the geochemical behavior and environmental risk of radionuclides, as well as for the safety assessment of geological disposal of radioactive wastes.
This thesis mainly focused the adsorption behavior and micro-mechanism of radioactive uranium on typical environmental media (TiO2, calcite, biotite), discussed the adsorption law of U(VI) under a wide range of environmental conditions (acidity, salinity, temperature, coexisting ions and coexisting ligands, etc.), and investigated the adsorption/coprecipitation behavior and micro mechanism of U(VI) on the surface at a molecular scale by means of spectral technology. By observing the mechanism, the adsorption law, occurrence state, microstructure of uranium, and the structural change of medium itself was revealed. The corresponding results provided comprehensive and in-depth understanding on the environmental behavior, provided a basis for the construction of uranium geochemical model, and provided solutions for radioactive pollution control, safety disposal as well as safety evaluation.
In chapter 2, the adsorption behavior of U(VI) on TiO2 was studied in detail by batch method and spectroscopic technique. The effects of equilibrium time, pH, solid-liquid ratio, coexisting ions, humic acid (HA) and temperature on the adsorption behavior of U(VI) on TiO2 were investigated. The experimental results showed that the adsorption of U(VI) on TiO2 conformed to the quasi second-order kinetics, and was significantly affected by the ionic strength and pH value. HA promoted the adsorption of U(VI) on TiO2 at low pH value, and showed a significant inhibition on the adsorption of U(VI) at high pH value. With the greater HA concentration, more obvious promotion and inhibition was observed. High temperature was more favorable for the adsorption of U(VI) on TiO2, and the thermodynamic fitting results showed that the adsorption of U(VI) on TiO2 was a spontaneous endothermic process. High background electrolyte concentration promoted the adsorption of U(VI) on TiO2, which was due to the aggregation of TiO2 through (001) surface under high ionic strength. More (101) surface was exposed, and the exposed (101) surface showed a certain reduction effect on uranium immoblization. The XPS spectrum analysis confirmed that U(VI) was reduced to U(IV) at high ionic strength, so at high ionic strength, part of U(VI) was reduced on (101) surface, thus promoting the removal of U(VI).
Calcite has a stronger thermodynamic stability, which exists widely in the near surface environments. In chapter 3, the adsorption and incorporation behavior of U(VI) on calcite was concerned, the effect of U(VI) incorporation on the formation of CaCO3 were studied by batch experiments and advanced spectroscopy. The effects of U(VI) concentration and aging time on the recrystallization of CaCO3 from aragonite to calcite were evaluated. The adsorption isotherms showed that when pH increased from 6.5 to 10.0, the adsorption capacity of U(VI) on calcite increased, the uranyl carbonate species was formed at high pH. At the same time, with the increase of adsorption capacity of U(VI), the concentration of Ca2+ in the aqueous phase also increased linearly, which indicated that the adsorption mechanism of U(VI) changed significantly at different pH, which involved ion exchange and surface complexation. The U(VI)-carbonate species may enter calcite lattice to replace Ca atom. The coprecipitation of U(VI)-CaCO3 showed that the removal rate of U(VI) decreased with the increase of aging time in the first 200 hours, and increased significantly when aging time was more than 200 hours, which was consistent with the change trend of Ca(II) concentration in coprecipitation process. The results of scanning electron microscope (SEM), X-rays diffraction (XRD), X-ray absorption near-edge structures (XANES) and fourier transform attenuated total reflection infrared spectrum (ATR-FTIR) analysis showed that three stages was passed after adding U(VI) into CaCO3: vaterite, vaterite/calcite and calcite. In addition, with the increase of U(VI) concentration, the transition from aragonite to calcite took longer time, which indicated that the existence of U(VI) could stabilize the aragonite phase and increased the stable existence time. extended X-ray absorption fine structure (EXAFS) confirmed that in the process of U(VI) incorporation into CaCO3, the local structure of uranyl changed. The species of U(VI) was similar to uranyl carbonate, but different from the main aqueous species of U(VI) in the high pH range of calcite.
Biotite plays an important role in the enrichment and retardation of radionuclides in granite system. In chapter 4, the adsorption of U(VI) on biotite was studied. The influence of environmental factors such as acidity, ionic strength, temperature and organic matter on the adsorption of U(VI) was discussed. Sorption mechanism was discussed in depth with the aid of X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT-IR) and extended X-ray absorption fine structure (EXAFS). The results showed that the adsorption of U(VI) on the biotite surface was significantly affected by pH, the ionic strength showed little effect on the adsorption, the inner sphere complex formed on the edge sites of biotite, the interlayer site showed little contribution to the adsorption of U(VI). The presence of phosphate and humic acid (HA) significantly promoted the adsorption of U(VI) by the formation of ternary surface complex on the surface. HA was easy to be adsorbed on the surfacee, and the adsorbed HA provided a large number of functional groups and active sites for the further adsorption of U(VI) by forming ternary complex. High temperature was favorable for the adsorption of U(VI) on biotite. The spectral results showed that part of U(VI) was reduced by Fe(II) in the structure of biotite, forming U(V)/U(IV) mineralized products or nano clusters on the surface of biotite.|