| Extensive interests have been focused on the separation and purification of uranium,the actual fuel to the production of nuclear power.Solvent extraction is the most practically mature technology for such purpose.However,the drawbacks of this traditional method are back extraction for concentrating metal ions and treatment of organic wastes,especially radioactive wastes are required.Therefore,the simplification of separation process has attracted significant attention.The self-assembly of metal ions and ligands in organic phases is found in many extraction systems,but these assemblies stop aggregation at microscopic or mesoscopic scales.If they can further aggregate to macroscopic scale,an easy-to-separate target could be obtained.The realization of self-assembly at all scale,a process with numerous interactive groups associating through non-covalent interactions in a multivalent manner,must meet two important requirements:one being intermolecular interactions,and the other being driving forces that can propel the building blocks to move towards each other and reach nanoscale distance for sufficient interaction.Several kinds of intermolecular interactions such as hydrogen bonding,host-guest molecular recognition,electrostatic interaction,metal coordination,magnetic interactions,DNA hybridization have been developed to realize self-assembly,most of which are commonly found in extraction systems.Meanwhile,it was found that the combination of Marangoni-driven self-propulsion and molecular interaction could successfully induce collisions between and the macroscopic supramolecular assembly.Nonetheless,the Marangoni effect due to surface tension gradient usually lasts for a relatively short time,which leads to a poor assembly efficiency.Very recently,the hydroxyl-functionalized IL,e.g.1-(2-Hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([HEMIm][NTf2])attracted our attention because of its surfactant-like behaviors and different surface tension as compared with similar ILs without hydroxyl groups.We intuitively believe that the introduction of hydroxyl-functionalized IL in the extraction system may induce Marangoni interactions at the aqueous-IL interface,by which self-assembly processes may be driven.Octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide(CMPO)is selected as ligand to bind uranyl ions in the designed IL-based extraction strategy.Herein,we developed a novel extraction strategy for uranium based on self-assembly with hydroxyl-functionalized IL,where one-step separation and solidification of uranyl from aqueous solution are realized without back extraction and free of radioactive waste(the uranium concentration in aqueous phase is below 0.05%of the initial,and the IL could be reused).To obtain the morphological and structural information of the uranium-rich self-assembly sample(SA sample),various characterization techniques were utilized.The EDS mapping showed that ligand and IL should both participate in the formation of the SA sample.More evidences came from the mass spectrometry and the NMR spectra results.The related infrared spectra indicating the absence of NO3-in the SA sample.The TG curve together with the pyrolysis FTIR spectra revealed that no structural water was contained.Furthermore,elemental analysis results demonstrated the composition of the SA sample was in close accordance with that of[UO2(CMPO)3]2+:[HEMIm]+:[NTf2]-=3:1:7 in molar ratio.The WAXS patterns indicate that an ordered structure is formed in the SA sample,matching with the theoretically calculated radius of UO2(CMPO)32+complex.To test whether the coordination environment of uranyl in the SA sample changes in regards to UO2(CMPO)3(NTf2)2,solid state NMR spectra of 31P and 19 F for both samples were recorded for comparison.On the basis of evidences given,the pellet was deduced to be the macroscopic assembly of {[UO2(CMPO)3]3[HEMIm]}[NTf2]7.Furthermore,a potential mechanism of the formation of macroscopic uranium-rich pellet was speculated The UO2(CMPO)32+is formed in the aqueous phase,and then extracted in the IL-phase by cation exchange mechanism.The complex and the cation/anion moieties of IL started to assemble into intermediates,and further aggregate to form the uranium-rich pellet at the aqueous-IL interface by self-assembly.[HEMIm][NTf2]plays a key role in the self-assembly process of pellet-formation.In this quite delicate process,the hydroxyl-functionalized IL in the interfacial region is consumed continually,and the surface tension gradient is established between the interfacial region and the bulk of IL,which in turn drives cation/anion moieties of IL together with the uranyl-CMPO complex to be pulled to the aqueous-IL interface by Marangoni effect.Self-assembly can then proceed as the intermediates further gather through electrostatic force,?-stacking,H-bonding and solvophobic interaction,forming a uranium-rich macroscopic pellet spontaneously,till the end of the transformation of uranyl ions into a solid pellet.In summary,our designed extraction system based on self-assembly is of efficiency,cleanliness,reusability and has overcome the drawbacks of conventional solvent extraction systems.The ion exchange between the hydroxyl-functionalized IL and uranyl ions keeps the interfacial tension gradient and thus the driven function of Marangoni effect.Meanwhile,the hydroxyl-functionalized IL participates in the formation of self-assembly sample because of the supramolecular interaction.The above process does not stop until the extraction equilibrium of uranyl ions occurs.Thus,a novel extraction strategy for uranium can be established on the basis of the formation of self-assembly.Moreover,this separation strategy can be extended to the recovery of many other metals such as iron,copper,zinc,gold,manganese,lead,vanadium and aluminum,making it a highly promising method in spent nuclear fuel reprocessing,mining and treatment of heavy metal-containing waste water.Additionally,this process enables metal complex to assemble from microscopic scale to mesoscopic scale and then to macroscopic scale by Marangoni effect in a simple system continuously and spontaneously,overcoming some long-lasting difficulties for driving forces,hence may promote development of supramolecular assembly both in mechanism and application. |
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