Synthesis Of Amidoxime Functionalized Nano-materials For Sorption Of Uranium

Uranium is an important nuclear fuel resource and one of the major hazardous elements in radioactive liquid wastes. Highly efficient recovery of uranium from aqueous solutions is of extreme importance from the points of view of energy security and environmental protection. Sorption is one of the most widely used uranium recovery methods because of its low cost, easy operation and wide adaptability. Search for sorbents with excellent sorption performance is the key to the real application of the sorption method. Recently, composite materials have become the reaearch focus because of the free selection of specific substrates and functional moieties according to corresponding applications. Among various functional groups studied for uranium recovery from aqueous solutions, the amidoxime group has been grafted onto the surfaces of various substrates due to its strong complexation toward uranium. As far as the substrate is concerned, more and more attention has been paid to the nano-materials. In this work, the amidoxime group has been applied to functionalize several famous nano-materials to prepare the following composite materials, corresponding sorption performance has been evaluated, and sorption mechanisms have been analyzed:1. Amidoximated magnetite/graphene oxide compositesAmidoximated magnetite/graphene oxide (AOMGO) composites were synthesized and characterized and applied to adsorb U(VI) from aqueous solutions. The U(VI)-loaded AOMGO composites could be easily separated from the aqueous phase by an external magnetic field. The kinetic process of U(VI) sorption on AOMGO composites reached equilibrium within2h. Effects of pH, ionic strength and coexisted ions on the sorption of U(VI) on AOMGO composites were investigated. The results indicated that U(VI) sorption on AOMGO composites was strongly dependent on pH and independent of ionic strength. The sorption isotherm agreed well with the Langmuir model, having a maximum sorption capacity of1.197mmol/g at pH=5.0±0.1and T=298K. Thermodynamic parameters calculated from the temperature-dependent sorption isotherms suggested that the sorption of U(VI) on AOMGO composites was an endothermic and spontaneous process. The fast and efficient sorption performance suggests that AOMGO composites are potential and suitable candidates for the preconcentration and separation of U(VI) from contaminated wastewater and seawater.2. Amidoxime-Functionalized silica coated Fe2O4Amidoxime-functionalized silica coated Fe3o4(Fe3O4@SiO2-AO) was synthesized and carefully characterized. The prepared Fe3O4@SiO2-AO was applied to adsorb U(VI) from aqueous solutions and exhibited enhanced sorption capacity for U(VI) in comparison with raw silica coated Fe3o4due to the strong chelation of amidoxime to U(VI). Effects of contact time, pH, ionic strength, U(VI) concentration, and temperature on the sorption of U(VI) on Fe3O4@SiO2-AO were investigated. The kinetic process of U(VI) sorption on Fe3O4@SiO2-AO reached equilibrium within2h. The sorption was strongly dependent on pH and independent of ionic strength, indicating that the sorption was mainly dominated by inner-sphere surface complexation. The sorption isotherm agreed well with the Langmuir model, having a maximum sorption capacity of0.441mmol/g at pH=5.00.1and T=298K. The U(VI)-loaded Fe3O4@SiO2-AO could be readily separated from aqueous solutions by an external magnetic field and efficiently regenerated by1mol/L HCL for reuse.3. Amidoxime-Functionalized silica coated Fe3O4Amidoxime-functionalized magnetic mesoporous silica (MMS-AO) microspheres were synthesized through co-condensation of tetraethyl orthosilicate (TEOS) and2-Cyanoethyltriethoxysilane (CTES) on the surface of silica-coated Fe3O4, using hexadecyltrimethylammonium bromide as the structure-directing agent, followed by chemical modification of nitrile into amidoxime. The synthesized microspheres exhibit a typical sandwich structure with an inner core of Fe3O4, a middle layer of nonporous silica and an outer layer of amidoxime-functionalized mesoporous silica. Owing to the mesoporous structure and amidoxime functionalization, sorption of U(VI) by MMS-AO reaches a maximum sorption capacity of1.165mmol/g at pH=5.0±0.1and T=298K, which is much higher than the results previously reported for other magnetic materials. The selectivity of MMS-AO for U(VI) is remarkably improved in comparison with that of magnetic mesoporous silica without amidoxime functional ization. Due to the protection of by the silica shell, the U(VI)-loaded MMS-AO can be efficiently regenerated using1mol/L HCl. These results suggest that MMS-AO has high surface area, can be conveniently separated with an external magnetic field, and has good stability in the environment, and thus shows promise as a future candidate for selective separation of U(VI) from aqueous solutions in possible real applications. 4. Amidoxime-functionalized SBA-15mesoporous silicaBy varying the molar ratio of CTES, a series of amidoxime-functionalized mesoporous silica has been prepared by the self-assembly co-condensation of TEOS and CTES using Pluronic P123as the structure-directing agent, and has been applied as sorbents to eliminate U(VI) ions from aqueous solutions. Sorption isotherms and kinetics have been investigated to discuss the influence of organic group densitis on the sorption performance. Organic functionalization has dual influence on both sorption capacities and sorption rates. The sorption capacity increases with the initial increase of organic group density until it reaches the maximum, and then decreases with further increase of organic functionalization. The optimal dosage corresponds to the balance between the structural porosities of the sorbent and the loading contents of organic functional groups. Sorption mechanisms have been examined by the X-ray photoelectron spectra and Fourier transformed infrared spectra of sorbents before and after uranium sorption, and the results indicate that the sorption is mainly attributed to the formation of surface complexes between uranium and the amidoxime groups.Based on the above results, comparisons among the sorption performances of uranium on different sorbnets have been drawn, experimental conditions for sorbents preparation have been optimized, and sorption mechanisms have been analyzed. The results in this dissertation will provide experimental and theoretical bases for the practical application of functionalized mesoporous materials in the control of uranium pollution and selective enrichment of uranium.