Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Highly Efficient and Selective Dissolution Separation of Fission Products by an Ionic Liquid [Hbet][Tf2N]: A New Approach to Spent Nuclear Fuel Recycling Fang-Li Fan,† Zhi Qin,*,† Shi-Wei Cao,† Cun-Min Tan,† Qing-Gang Huang,† De-Sheng Chen,† Jie-Ru Wang,† Xiao-Jie Yin,† Chao Xu,‡ and Xiao-Gui Feng‡ †
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China Nuclear Chemistry and Chemical Engineering Division, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
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‡
S Supporting Information *
ABSTRACT: Here, we propose the use of carboxyl-functionalized ionic liquid, [Hbet][Tf2N], to separate the fission products from spent nuclear fuels. This innovative method allows the selective dissolution of neutron poisons, lanthanides oxide, as well as some fission products with high yield, leaving most of the UO2 matrix and minor actinides behind in the spent nuclear fuel and accomplishing the actinides recovery as a group. Water-saturated [Hbet][Tf2N] can dissolve lanthanides oxide from simulated spent nuclear fuel with a dissolution ratio of 100% at 40 °C. However, the dissolution of uranium is almost negligible (99%) was purchased from Lanzhou Green ILs, LICP, CAS (Lanzhou, China). Hydrochloric acid solutions were prepared from concentrated HCl (37%, Acros Organics, Geel, Belgium) and water. Water was always of ultrapure quality, deionized to a resistivity of >18.2 MΩ cm with a Milli-Q Academic ultrapure water system. Caution! Natural uranium and radioactive species 237NpO2 were used in the experiments. All radioactive materials were handled in radioactively controlled facilities that are equipped with personal safety equipment. Instrumentation and Methods. 1H NMR spectra were recorded on a Bruker 400 Avance III HD NMR spectrometer operated at 400 MHz. The samples were prepared by dissolving a small amount of product in d6-DMSO. Metal concentrations were determined using an inductively coupled plasma optical emission spectrometer (ICPOES; Agilent, type 5100SVDV) with an axial plasma configuration. The concentration of 237Np was measured by liquid scintillation counting (LSC, PerkinElmer, Quantulus 1220). X-ray Powder Diffraction (XRD) patterns were obtained using a PANalytical X’Pert3 Powder diffractometer. Synthesis of Carboxyl-Functionalized Ionic Liquid [Hbet][Tf2N]. The ionic liquid betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N] (see Scheme 1) was synthesized according to a onestep literature method based on the reaction between HbetCl and LiTf2N.15a HbetCl (0.174 mol, 26.7 g) and LiTf2N (0.174 mol, 50 g) were dissolved in water (50 mL) and stirred for 2 h at room
Scheme 1. Structure of the Ionic Liquid Betainium Bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N]
B
DOI: 10.1021/acs.inorgchem.8b02783 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry temperature. The mixture was then allowed to phase-separate, and the water phase containing LiCl was removed. The ionic liquid phase was then washed several times with cool water to remove chloride impurities until the AgNO3 test was negative in the water phase after the washing step. The dried [Hbet][Tf2N] is a white solid at room temperature, whereas it forms a viscous liquid with water. Many papers have reported that the water content in this functionalized ionic liquid affects the selectivity and kinetics of the dissolution process. Water accelerates the dissolution of metal oxides because it lowers the viscosity, facilitates the exchange of protons from the betaine groups, and enhances the solvation of ions in the ionic liquid. The viscosity of this ionic liquid with saturated water at 30 °C is 22 cP, and it will be even lower at high temperature.17a Considering the future convenience application, the water-saturated [Hbet][Tf2N] was used directly, which was a homogeneous phase. The watersaturated [Hbet][Tf2N] was obtained after washing. The content of water is about 13%, which was measured by 1HNMR (see Figure S1). Preparation of Simulated Spent Nuclear Fuel. The simulated spent nuclear fuel was prepared by mixing an amount of UO2 with Nd2O3 powder. Subsequently, in order to ensure uranium form as UO2, the mixture was reduced at a fixed temperature through 4% H2Ar for 4 h using a high-temperature furnace. The samples with different reduction temperatures 700−1200 °C were prepared to observe the difference. An XRD pattern of the simulated spent nuclear fuel is given in the Supporting Information (see Figure S2). In order to examine the dissolution selectivity of water-saturated [Hbet][Tf2N], the different samples with different Nd content (0.61%, 1.01%, 2.00%, 2.97%, and 4.98%) were prepared. Dissolution and Separation. Small glass vials (5 mL) were filled with 2 mL of water-saturated [Hbet][Tf2N] and 100 mg of sample, resulting in a solid/liquid ratio of 50 mg/mL. A magnetic stirring bar was then added to each of the vials after which were closed using a plastic screw cap. The dissolution experiments were carried out on a heating plate with an integrated magnetic stirrer and a temperature sensor. A stainless steel block was placed on top as the vial container. The vials were placed in the stainless steel block at the appropriate temperature and stirred at 1000 rpm for a certain period of time. Another dissolution method is that the vials were placed in the ultrasonic cleaner and the temperature was set at 25 °C for a certain period of time. Separation of the solution phases and the rest of the solid particles was assisted by centrifugation for 5 min at 7000 rpm. The metal contents dissolved in solution phase were determined using ICP-OES. The percentage dissolution (%) is defined as the amount of metal dissolved to the water-saturated ionic liquid over the initial amount.
Figure 1. Effect of time on the dissolution of Nd2O3, UO2, and U3O8 at 40 °C.
Figure 2. Dissolution separation of Nd2O3 from simulated nuclear fuel. The solid:liquid ratio is 50 mg/mL. The dissolution time is 60 min. The dissolution separation was performed in the ultrasonic cleaner.
temperature furnace. As anticipated, the good selective leaching for the rare earth Nd2O3 was observed. Although the dissolution of Nd2O3 will decrease to 88% when the reduction temperature was 1200 °C, it could be solved by increasing the reaction time. It can be deduced that the high dissolution of Nd2O3 is caused by easy dissociation of Nd3+ from the Nd2O3 lattice, and promoted by the formation of complexes between Nd and O atoms on the carboxylate groups of the betaine in [Hbet][Tf2N].15a More impressively, the water-saturated [Hbet][Tf2N] shows much less affinity toward UO2 and very little dissolution of uranium was observed (10 000 kJ/ mol) are insoluble, while those with smaller U/x values (
