Investigation Of U(?)Interaction Mechanisms At Mineral/water Interfaces

Radionuclides have both radiological and chemical toxicity,and could enter the human body in a variety of ways once released into the ecosystem,thus causing harm to human health.Deep geological disposal has been accepted worldwide as the best option for isolating high-level radioactive waste.However,radioactive wastes may be subsequently released into the biosphere as time goes by.Oxide and clay mineral are most important media in the natural environment,which could retard the mobility of radionuclides.A thorough investigation of the mechanism of radionuclides reacting at the mineral/water interfaces is of great significance for reliably describing and predicting the environmental behavior of radionuclides as well as managing radioactive wastes.In this paper,U???is selected as the typical representative of radionuclide ions,?-Al₂O₃ and hematite as the representatives of oxide minerals and kaolinite as the representative of clay minerals.Combined with batch experiments and advanced extended X-ray absorption fine structure spectroscopy?EXAFS?,the macroscopic adsorption princples and microscopic reaction mechanisms of U?VI?on different minerals under different environmental conditions were investigated.The main research results are as follows:1)The effect of silicate on U???sorption to ?-Al₂O₃ was investigated.The presence of silicate enhanced the sorption of U???on the surface of ?-Al₂O₃,which is attributed to the formation of the ternary inner-sphere complex.The sorption geometries of U?VI?and silicate on ?-Al₂O₃ were studied by EXAFS.The fitting results of the sorption samples consist of two axial oxygen atoms and five equatorial oxygen atoms.The third coordination shell is the Al atom shell.The presence of silicate increased the coordination number of the Al atom shell and enhanced the sorption of U?VI?,which is ascribed to that silicate could act as a bridge and help to form ternary inner-sphere complexes between U???and ?-Al₂O₃.2)The effect of silicate on the surface property of kaolinite and on U???sorption to kaolinite was investigated.The presence of silicate desceased the Zeta potential of kaolinite and inhibited the aggregation of kaolinite particles.Batch experimental results showed that the presence of silicate promoted the sorption of U???on kaolinite at low pH,which could be explained by the formation of ternary U???-silicate-kaolinite surface complexes.While the presence of silicate inhibited the sorption of U???at high pH,which is ascribed to the competition between silicate and U???for sorption sites.The adsorption time curves can be described more accurately by pseudo-second-order kinetic model The Langmuir model fitted the adsorption isotherms more accurately.The thermodynamic fitting parameters showed that the sorption process is spontaneous and endothermic.3)The retention of U???by Fe precipitates formed from oxidation of FeC1₂ dilute solution under mild oxidation conditions was investigated.The X-ray diffraction spectra showed that the presence of U???promoted the formation of lepidocrocite phase.The higher retention e fficiency of the coprecipitation experiments for U???is attributed to the synergistic effect of incorporation and sorption processes.The energy dispersive X-ray spectroscopy and fluorescence excitation-emission matrix spectroscopy analysis demonstrated that for the precipitates after successive acid-washing process,the adsorbed U was washed out,while the incorporated U was still retained in the precipitates.X-ray photoelectron spectroscopy implied that the incorporated U is in the U???or U???valence state.4)The sorption of U???on hematite {001} and {012} was investigated.The results of batch experiments indicated that hematite {012} facet is more capable of adsorbing U???than the {001} facet,which may be closely related to the difference in the density and type of surfacial hydroxyl groups.Based on the analysis of Mossbauer spectra,the formation of U???surface complexes is responsible for the new sextuplet spectra.By means of EXAFS analysis,it was found that the bidentate edge-sharing complex of U???is formed on the {001} facet and the bidentate corner-sharing complex on the {012} facet.In this paper,the interaction processes of U???with minerals including adsorption and coprecipitation under different environmental conditions were investigated in depth.The macroscopic reaction behaviors were related to the structures and properties of minerals themselves.The intrinsic mechanisms of the interfacial interaction processes were revealed,which provides new insights into the reactivity of minerals,better co nstrains the surface complexation model,and increases the prediction accuracy of the fete of U???in the environment.