A Study Of Solvent Extraction Of Uranium Based On Ionic Liquids And The Relevant Mechanism

Ionic liquids （ILs）, i.e. room-temperature ionic liquids or room-temperaturemolten salts, are composed of cations and anions, and their melting points aregenerally below100℃. ILs has many unique properties as compared with traditionalsolvents and electrolytes, such as low melting points, negligible vapor pressure, goodthermal and chemical stability, good dissolving ability, excellent electrochemistryproperties, recyclable and designable. These unique properties make ILs attractive fortraditional liquid-liquid extraction of high level radioactive nuclides in the nuclearindustry. Considering the shortage of uranium resources and troublesome nuclearcontamination, the recovery of uranium is in high demand. In this thesis, the solventextraction process in two commonly used hydrophobic imidazolium ILs,[C₄mim][PF₆]and [C₄mim][NTf₂], where [C4mim]+is1-butyl-3-methylimidazolium,[PF₆]-and[NTf₂]-are hexafluorophosphate and bis（trifluoromethylsulfonyl）imide, respectively,were systematically studied. The results demonstrate that the ionic liquid is feasibleand applicable to be used in the partitioning of uranium.In the first chapter, the main signification of the thesis, as well as the definition,classification, properties and applications of ILs were introduced. The recentresearches on the topic of applying ILs in many fields, including utility of ILs inliquid-liquid separations of uranium, were summarized. At last, the research contentsand aim of the thesis were presented. Then some extractants such as the neutralorganophosphorous extractants and some other novel liquid/liquid extractants wereintroduced. The extraction behavior of UO₂²⁺from nitric acid medium by dipicolinamidediluted with ionic liquid was investigated firstly in the second chapter. The solventextraction of the uranyl ions (UO₂²⁺) was performed using N, N’-diethyl-N, N’-di（para）tolyl-dipicolinamide (Et_（p）TDPA) as the extractant in a commonly used ionicliquid1-butyl-3-methylimidazolium hexafluorophosphate （[C4mim][PF6]）. The effectsof vortexing time, phase ratio and the concentration dependence of Et_（p）TDPA, nitricacid and sodium nitrate on the extraction were studied in detail. The extractionmechanism was deduced based on the slope analysis and UV-VIS measurement. Theresults demonstrate that the extraction process was ultra fast and the equilibrium wasestablished at the vortexing time of about1min. The extraction process was largelyaffected by the extractant concentration and the distribution ratio for UO₂²⁺increasedobviously with increasing Et_（p）TDPA concentration. The distribution ratio for theextraction of UO₂²⁺from3mol/L nitric acid by0.3mol/L EtTDPA/C4mimPF6wasfound to be almost100. However, the extraction of UO₂²⁺by Et_（p）TDPA/[C4mim][PF6]was slightly affected with increasing HNO₃or NaNO₃concentration suggesting thatNO₃^-does not participate in the UO₂²⁺extraction. In addition, conventional log-logplot analysis of the extraction equilibrium data suggested that the ions were extractedas a complex in2:3ratio of UO₂²⁺to extractant, and the extraction most likelyproceeded via a cation-exchange mode since the concentration of C4mim+in theaqueous phase increased linearly with the extraction percent of UO₂²⁺evidenced byUV-VIS measurement.The third chapter investigated the solvent extraction behavior of uranium （VI）with trioctylphosphine oxide using a room-temperature ionic liquid. In the experiment,we investigated the liquid–liquid extraction of the uranyl ions (UO₂²⁺) from aqueoussolutions into a commonly used ionic liquid1-butyl-3-methylimidazoliumbis（trifluoromethylsulfonyl）imide （[C₄mim][NTf₂]） using trioctylphosphine oxide （TOPO） as an extractant. The effect of various parameters such as vortexing time,TOPO concentration, the acidity and the concentration of sodium nitrate on theextraction behavior of UO₂²⁺were discussed in detail. The extraction mechanism wasdeduced based on the slope analysis and UV-VIS measurement.The results suggestedthat solutions of TOPO in [C4mim][NTf2] provided an efficient extraction of UO₂²⁺from aqueous solutions in absence of nitric acid. When TOPO concentration was only20mM, the distribution ratio was found to be over785. Slope analysis confirmed thatTOPO in [C₄mim][NTf₂] formed a1:3complex with UO₂²⁺.The extraction occuredby a cation-exchange mode and the finally extracted species was [UO₂（NO₃）（TOPO）₃]⁺. It suggested that one nitrate ion was involved in the extraction complexand the leaching of the C4mim+components to the aqueous phase was largely reduced.It was a great progress for the extraction of metal ions in ILs environment.Finally, the main conclusions and innovations of the thesis were summarized,and further researches were prospected.