Th (IV) and U(VI) removal by TODGA in ionic liquids: Extraction behavior and mechanism, and radiation effect

Th (IV) and U(VI) removal by TODGA in ionic liquids: Extraction behavior and mechanism, and radiation effect 1 2 first-author 1 2 1 1 2 corresp Long Zhao ryuuchou@hotmail.com Long Zhao ryuuchou@hotmail.com ryuuchou@hotmail.com 3 3 3 Non-power Nuclear Technology Collaborative Innovation Center, School of Nuclear Technology and Chemistry & Biology, Hubei University of Science and Technology, Xianning 437100, China Non-power Nuclear Technology Collaborative Innovation Center, School of Nuclear Technology and Chemistry & Biology, Hubei University of Science and Technology, Xianning 437100, China School of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China School of Nuclear Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Division of Chemistry and Ionizing Radiation Measurement Technology, Shanghai Institute of Measurement and Testing Technology, Shanghai 201203, China Division of Chemistry and Ionizing Radiation Measurement Technology, Shanghai Institute of Measurement and Testing Technology, Shanghai 201203, China Extraction behavior of thorium(IV) and uranium(VI) from nitric acid (HNO 3 ) was studied using N,N,N’,N’-Tetraoctyldiglycolamide (TODGA) as extractant in different ionic liquids, and isooctane as comparison. Slope analyses with varying HNO 3 concentrations and diluents revealed the extraction mechanism. With increasing length of alkyl chain and HNO 3 concentration, the extraction mechanism of TODGA/IL system changed from cation exchange to neutral complex or/and anion exchange, and the molar ratio between TODGA and metal ions varied gradually from 2:1 to 1:1 for Th(IV) and 3:1 to 1:1 for U(VI). The kinetics and thermodynamic studies of Th(IV) and U(VI) by the best TODGA/[C 2 mim][NTf 2 ] system showed that the exaction reached equilibrium within 2 h and extraction reactions were endothermic. Compared to TODGA/isooctane system, TODGA/[C 2 mim][NTf 2 ] system presented higher radiation stability under γ-irradiation. Therefore, it would have a promising application in spent fuel reprocessing. Extraction TODGA Ionic liquids Thorium(IV) and Uranium(VI) Radiation effect 1. Introduction 1 Nuclear energy is playing an important role as energy demands increase rapidly. Uranium, as the major commercial nuclear fuel, is a crucial nuclide in spent fuel reprocessing [ 1 1 ] . Thorium (Th) is three times more abundant than U and can be transformed to 233 U by neutron irradiation. Thorium-based molten salt reactor is one of the six potential candidates for Generation-IV nuclear energy system [ 2 2 , 3 3 ] . Thus, it is important for the sustainable development of nuclear energy to recycle U and Th from spent fuel reprocessing. Currently, solvent extraction is widely applied in separation and recovery of U and Th. Different extractants such as malonamide, tri- n -butyl phosphate (TBP), octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phoshine oxide (CMPO) [ 4 4 ] and diglycolamide (DGA) have been employed for partitioning of actinide ions. However, malonamide-based TBP are easy to form third-phase during extraction [ 5 5 , 6 6 ] , while CMPO produces large amount of secondary waste [ 7 7 , 8 8 ] . Comparatively, N,N,N’,N’-tetraoctyldiglycolamide (TODGA), as representative of DGA, is a promising candidate extractant for recovery of actinide ions [ 9 9 , 10 10 ] , in terms of its good solubility in solvents, good radiolytic stability [ 11 11 , 12 12 ] and extractability towards actinides from highly acidic solutions [ 13 13 ] . In addition, TODGA composes of C, H, O, and N atoms (the CHON principle), which makes it fully combustible to gaseous products, minimizing the generation of secondary waste [ 14 14 ] . However, TODGA is usually used in organic diluents, which resulting in health hazard and inflammability risk [ 15 15 ] . Therefore, it is essential to develop alternative diluents to overcome the drawbacks. Ionic liquids (ILs) has received increasing attention as alternative diluents in nuclear fuel cycle, because of their low vapor press and no flammability, high thermal, chemical and irradiation stability, and high solubility [ 16 16 , 17 17 ] . Many authors demonstrated that the extraction system using ILs as diluents presented dramatically improved extraction performance for some radionuclides (eg. Sr, Cs, An and Ln) compared to organic solvent as diluents [ 14 14 , 18 18 - 21 21 18-21 ] . Therefore, application of IL to recover U and Th has been widely investigated [ 22 22 ] . Panja et al . [5] applied TODGA in [C n mim][PF 6 ] ( n =4, 6, 8) for actinide ions Am(III), Pu(IV) and U(VI), resulting in higher distribution ratios for all the actinides. Shen et al . reported the extraction of Th(IV) [ 15 15 ] and U(VI ) [ 23 23 ] from a HNO 3 solution into various [C n mim][PF 6 ] ( n =4,6,8) using DGA-based extractants, N,N,N’,N’-tetrabutyl-3-oxapentanediamide (TBDA) and N,N’-dimethyl-N,N’-dibutyl-3-oxapentanediamide (MBDA). Besides, extraction of Th(IV) and U(VI) from HNO 3 by TODGA in [C n mim] [PF 6 ] ( n =6, 8) was investigated [ 24 24 ] . Shen extracted Th(IV) using 2-thenoyltrifluoroacetone (HTTA) in [C n mim][NTf 2 ] ( n =2, 4, 6, 8) [ 25 25 ] . Although ILs are proved to be an alternative green solvent, its high cost and solubility limit its applications sometimes. The disadvantages can be solved with the further development and promotion of ILs. In addition, few investigations were conducted on related techniques for especially large-scale use of ILs. Therefore, studies are needed on extraction technology applying ILs towards radionuclides especially. To our knowledge, little attention has been given to the extraction of Th(IV) and U(VI) using TODGA in NTf 2 -based ILs. Moreover, in spent nuclear fuel reprocessing, the extraction systems including extractants and diluents are used together to separate metal ions. Therefore, the radiation stability of extraction system is essential. However, investigations on the radiation effect were mainly focused on pure ILs and extractants themselves [ 26 26 - 28 28 26-28 ] , radiation stability of extractants in ILs under irradiation was seldom reported. The possibility to extract Th(IV) and U(VI) using TODGA as extractant in ionic liquid 1-alkyl-3- methylimidazolium bis(trifluoromethane sulfonyl)imide ([C n mim][NTf 2 ]) was investigated in this work. The effect of the solvent type, contact time, HNO 3 concentration, extractant concentration and temperature were studied and the extraction mechanisms in various diluents were discussed in detail. Besides, the radiation effect of TODGA in ILs and isooctane was investigated. It is expected to offer a new extraction system for separating and recycling of Th(IV) and U(VI) in spent fuel reprocessing. 2. Experimental 1 2.1 Materials 2 N,N,N’,N’-Tetraoctyldiglycolamide (TODGA) ( > 96%) was used as received. [C n mim][NTf 2 ] ( n =2, 12) ( > 99%) were obtained from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). Their structures are shown in Fig. 1 Fig. 1 . The other organic and inorganic compounds were reagent grade commercial products and used without further purification. 2.2 Solvent extraction of U(VI) and Th(VI) 2 The extraction solution was prepared by dissolving TODGA in [C n mim][NTf 2 ] ( n =2, 12) or isooctane, and the aqueous solution was obtained by dissolving UO 2 (NO 3 ) 2 ·6H 2 O or Th(NO 3 ) 4 ·6H 2 O in HNO 3 medium. The extraction experiments were carried out with equal volume of two phases (O/A=1) oscillating in a constant temperature incubator shaker. Then the mixtures were centrifuged for 3 min to ensure phase separation. After phase separation, the concentration of U(VI) or Th(VI) in aqueous solution were measured by Prodigy high dispersion inductively coupled plasma atomic emission spectrometer (ICPS-7510, Shinadzu, Japan). The distribution ratios ( D ) and extraction efficiencies ( E ) were calculated as follows: <math display="block" id="M1"><mrow><mi>D</mi><mo>=</mo><mtext> </mtext><mrow><mo>[</mo><mrow><mrow><mo>(</mo><mrow><msub><mi>C</mi><mtext>i</mtext></msub><mo>−</mo><msub><mi>C</mi><mtext>f</mtext></msub></mrow><mo>)</mo></mrow><mo>/</mo><msub><mi>C</mi><mtext>f</mtext></msub></mrow><mo>]</mo></mrow><mrow><mo>(</mo><mrow><msub><mi>V</mi><mrow><mtext>aq</mtext></mrow></msub><mo>/</mo><msub><mi>V</mi><mtext>o</mtext></msub></mrow><mo>)</mo></mrow><mo>,</mo></mrow></math> <math display="block" id="M2"><mrow><mi>E</mi><mo>=</mo><mrow><mo>[</mo><mrow><mrow><mo>(</mo><mrow><msub><mi>C</mi><mtext>i</mtext></msub><mo>−</mo><msub><mi>C</mi><mtext>f</mtext></msub></mrow><mo>)</mo></mrow><mo>/</mo><msub><mi>C</mi><mtext>i</mtext></msub></mrow><mo>]</mo></mrow><mo>×</mo><mn>100</mn><mi>%</mi><mo>,</mo></mrow></math> where C i and C f are the initial and final concentrations of metal ions in aqueous phase, respectively; and V aq and V o are the volume of aqueous and organic phases, respectively. 2.3. Irradiation 2 Irradiation of TODGA/[C 2 mim][NTf 2 ] and TODGA/isooctane solution were conducted in air at room temperature in a 60 Co source (Institute of Applied Chemistry of Peking University), where Fricke dosimetry system is used. 3. Results and Discussion 1 3.1 Effect of diluents 2 The extraction efficiency of Th(IV) and U(VI) ions with TODGA in the ILs system as function of HNO 3 concentration are shown in Fig.2 Fig.2 , together with that in the isooctane. The extraction for Th(IV) and U(VI) ions in TODGA/[C n mim][NTf 2 ] was much more efficient than that in TODGA/isooctane system. For TODGA/ [C 2 mim][NTf 2 ] system, the extraction efficiencies of Th(IV) and U(VI) ions were always high, despite their decrease at 1 M HNO 3 ; while the extraction efficiencies of the TODGA/[C 12 mim][NTf 2 ] system and TODGA/isooctane, which were low at low HNO 3 concentrations, increased with HNO 3 concentration. A similar phenomenon was reported by Cocalia et al . [ 29 29 ] . This implied similar extraction equilibrium for both diluent systems. Therefore, isooctane and [C 12 mim][NTf 2 ] were used as diluents at 3 M HNO 3 in further experiments, due to their low D Th and D U at low HNO 3 concentrations. 3.2 Effect of concentration of TODGA 2 The extraction efficiencies for Th(IV) and U(VI) as a function of TODGA concentration in [C n mim][NTf 2 ] or isooctane are shown in Fig. 3 Fig. 3 . The amount of extracted Th(IV) and U(VI) ions increased with the TODGA concentration in different diluents. This indicates that extractant TODGA participated in the extraction process to form the extracted species [ 7 7 ] . 3.3 Extraction mechanism and stoichiometry determination for U(IV) and Th(VI) with TODGA 2 The variations trend of E for Th(IV) and U(VI) ions in TODGA/[C n mim][NTf 2 ] system of different HNO 3 concentrations is accorded with the universal chemical mode proposed by Billard [ 30 30 ] : each of the three basic extraction mechanisms, i.e. cation exchange, anion exchange and neutral complex extraction, contributes depending on the extraction conditions such as HNO 3 concentration and IL types [ 6 6 , 30 30 ] . In order to further reveal the possible extraction mechanism of the three extraction systems, the relationship of lg D against lg[TODGA] in different diluents was investigated to determinate the stoichiometry of metal ion and TODGA. For traditional TODGA/isooctane system, it has been found that neutral complex extraction mechanism dominated in organic solvent system according to Eqs. (3) and (4). The slopes in Fig. 3(D) Fig. 3(D) are both 1, which indicates that the Th(IV) and U(VI) ions are extracted as mono-solvated species with TODGA in TODGA/isooctane system. <math display="block" id="M3"><mrow><msub><mrow><mtext>UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mrow><mtext>2+</mtext></mrow></msup></mrow><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><msub><mrow><mtext>+2NO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub><mtext>+</mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(org)</mtext></mrow></msub><msub><mrow><mtext>↔[UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>2</mtext></msub><mtext>•</mtext><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow><msub><mtext>]</mtext><mrow><mtext>(org)</mtext></mrow></msub><mtext>,</mtext></mrow></math> <math display="block" id="M4"><mrow><msup><mrow><mtext>Th</mtext></mrow><mrow><mtext>4+</mtext></mrow></msup><msub><mrow/><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><msub><mrow><mtext>+4NO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub><mtext>+</mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(org)</mtext></mrow></msub><mtext>↔[Th</mtext><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>4</mtext></msub><mtext>•</mtext><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow><msub><mtext>]</mtext><mrow><mtext>(org)</mtext></mrow></msub><mo>,</mo></mrow></math> where, (aq) and (org) denote the species in the aqueous phase and organic phase separately. TODGA/[C 2 mim][NTf 2 ] system is of excellent extraction performance with HNO 3 concentrations of 0.01–3 M. The distribution ratio of Th(IV) and U(VI) ions is always high, with a decrease at 1 M HNO 3 concentration, though. According to previous investigations, the extraction of metal ions happened at 0.01 M HNO 3 by cation exchange mechanism according to Eqs. (5) and (6). In this paper, the slopes at 0.01 M HNO 3 in Fig. 3(A) Fig. 3(A) are 3.22 for U(VI) ion and 2.10 for Th(IV) ion. This implies that U(VI) ion and Th(IV) ion are in 1:3 complex and 1:2 complex with TODGA, respectively. <math display="block" id="M5"><mrow><msub><mrow><mtext>UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mrow><mtext>2+</mtext></mrow></msup></mrow><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><mtext>+3</mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+2</mtext><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><msub><mrow><mtext>↔[UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow></mrow><mtext>3</mtext></msub><mrow><mo>]</mo><mrow><msub><mrow><msup><mrow/><mrow><mtext>2+</mtext></mrow></msup></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+2</mtext></mrow><mo>[</mo></mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim]</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> <math display="block" id="M6"><mrow><msup><mrow><mtext>Th</mtext></mrow><mrow><mtext>4+</mtext></mrow></msup><msub><mrow/><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><mtext>+2</mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+4</mtext><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>↔[Th</mtext><msub><mrow><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow></mrow><mtext>2</mtext></msub><mrow><mo>]</mo><mrow><msub><mrow><msup><mrow/><mrow><mtext>2+</mtext></mrow></msup></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+4</mtext></mrow><mo>[</mo></mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim]</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> where, (IL) refers to the species in ionic liquid phase. The H + concentration increases with HNO 3 concentration, which is a competitive cation with metal ions to coordinate with TODGA, as described by Eq. (7), resulting in the decrease in D value. <math display="block" id="M7"><mrow><msup><mtext>H</mtext><mtext>+</mtext></msup><msub><mrow/><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><mtext>+ </mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>↔[H</mtext><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow><mrow><mo>]</mo><mrow><msub><mrow><msup><mrow/><mtext>+</mtext></msup></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+ </mtext></mrow><mo>[</mo></mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim]</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> As HNO 3 concentration further increases, NO 3 − participates in extraction to coordinate with metal ions, leading to an increase in D values, just like isooctane system. Therefore, the neutral complex extraction mechanism is enhanced, as described by Eqs. (8) and (9), and in Fig. 3(B) Fig. 3(B) at 3 M HNO 3 the slopes are 2.38 for U(VI) ion and 1.88 for Th(IV) ion. <math display="block" id="M8"><mrow><msub><mrow><mtext>UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mrow><mtext>2+</mtext></mrow></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub><msub><mrow><mtext>+mNO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub><msup><mrow><mtext>+nH</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub><msub><mrow><mtext>+xTODGA</mtext></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><mrow><mo>(</mo><mrow><mtext>2+n-m</mtext></mrow><mo>)</mo></mrow><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>→</mtext><msup><mrow><mrow><mo>[</mo><mrow><msub><mrow><mtext>UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>m</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow></mrow><mtext>x</mtext></msub></mrow><mo>]</mo></mrow></mrow><mrow><mrow><mo>(</mo><mrow><mtext>2-m</mtext></mrow><mo>)</mo></mrow><mtext>+</mtext></mrow></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><msup><mrow><mtext>+nH</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><mrow><mo>(</mo><mrow><mtext>2+n-m</mtext></mrow><mo>)</mo></mrow><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> <math display="block" id="M9"><mrow><msup><mrow><mtext>Th</mtext></mrow><mrow><mtext>4+</mtext></mrow></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub><msub><mrow><mtext>+mNO</mtext></mrow><mtext>3</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub><msup><mrow><mtext>+nH</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub><msub><mrow><mtext>+xTODGA</mtext></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><mrow><mo>(</mo><mrow><mtext>4+n-m</mtext></mrow><mo>)</mo></mrow><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>→</mtext><msup><mrow><mrow><mo>[</mo><mrow><mtext>Th</mtext><msub><mrow><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow></mrow><mtext>x</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>m</mtext></msub></mrow><mo>]</mo></mrow></mrow><mrow><mrow><mo>(</mo><mrow><mtext>4-m</mtext></mrow><mo>)</mo></mrow><mtext>+</mtext></mrow></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><msup><mrow><mtext>+nH</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><mrow><mo>(</mo><mrow><mtext> 4+n-m</mtext></mrow><mo>)</mo></mrow><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> At high HNO 3 concentration, Th(IV) and U(VI) ions exist as anion nitrate species [Th(NO 3 ) 5 ] − and [UO 2 (NO 3 ) 3 ] − , so the D U increase may be attributed to anion exchange of NTf 2 − with [Th(NO 3 ) 5 ] − and [UO 2 (NO 3 )] − [ 8 8 , 20 20 ] , shown as follows: <math display="block" id="M10"><mrow><msup><mrow><mrow><mo>[</mo><mrow><msub><mrow><mtext>UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>3</mtext></msub></mrow><mo>]</mo></mrow></mrow><mtext>-</mtext></msup><msub><mrow/><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><mtext>+</mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(IL)</mtext></mrow></msub><msub><mrow><mtext>+NTf</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(IL)</mtext></mrow></msub><msub><mrow><mtext>↔[UO</mtext></mrow><mtext>2</mtext></msub><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>3</mtext></msub><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow><msup><mtext>]</mtext><mtext>-</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><msub><mrow><mtext>+ NTf</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> <math display="block" id="M11"><mrow><msup><mrow><mrow><mo>[</mo><mrow><mtext>Th</mtext><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>5</mtext></msub></mrow><mo>]</mo></mrow></mrow><mtext>-</mtext></msup><msub><mrow/><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><mtext>+</mtext><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(IL)</mtext></mrow></msub><msub><mrow><mtext>+NTf</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>↔[Th</mtext><msub><mrow><mrow><mo>(</mo><mrow><msub><mrow><mtext>NO</mtext></mrow><mtext>3</mtext></msub></mrow><mo>)</mo></mrow></mrow><mtext>5</mtext></msub><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow><msup><mtext>]</mtext><mtext>-</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><msub><mrow><mtext>+ NTf</mtext></mrow><mtext>2</mtext></msub><msub><mrow><msup><mrow/><mtext>-</mtext></msup></mrow><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> When the ionic liquid is [C 12 mim][NTf 2 ], the alkyl chain length of 12 possesses higher hydrophobicity, which restrains cation exchange. The variation trend with the increasing HNO 3 concentration is similar to that in isooctane. The slopes in Fig. 3(C) Fig. 3(C) at 3 M HNO 3 in TODGA/[C 12 mim][NTf 2 ] system are 1.08 for U(VI) and 1.36 for Th(IV), being comparable to those in isooctane but much smaller than those in [C 2 mim][NTf 2 ]. Thus, neutral complex mechanism dominants. 3.4 Effect of contact time 2 Equilibrium time, a key parameter in extraction experiment, is related to the extraction speed. The extraction kinetics of Th(IV) and U(VI) ions in TODGA/ [C 2 mim][NTf 2 ] system at 25°C were investigated in 0.01 M HNO 3 aqueous. As shown in Fig. 4 Fig. 4 , both the processes were rapid, over 95% of the metal ions were extracted within 1 h. The Th(IV) and U(VI) ion extractions reached equilibrium within 2 h. At equilibrium, the Th(IV) ions were almost completely extracted into IL phase with E Th =99.8%, while for U(VI) ions, the E U =96%. The results indicate that the TODGA/[C 2 mim][NTf 2 ] system performs better in extraction of Th(IV) ions. Therefore, to ensure to reach equilibrium, two hours of contact time was chosen for extraction of all samples. 3.5 Effect of extraction temperature 2 The dependency of extraction temperature on D by TODGA/[C 2 mim][NTf 2 ] system in 0.01 M HNO 3 was investigated, as shown in Fig. 5 Fig. 5 . D values of both Th(IV) and U(VI) ions increase with the temperature, indicating that the extraction process in TODGA/[C 2 mim][NTf 2 ] system is endothermic. The enthalpy change (∆ H ) and entropy change (∆ S ) for the extraction can be obtained by the Van’t Hoff equation: <math display="block" id="M12"><mrow><msub><mrow><mo stretchy="false">[</mo><mfrac><mrow><mo>∂</mo><mi>lg</mi><mi>D</mi></mrow><mrow><mo>∂</mo><mo stretchy="false">(</mo><mfrac><mn>1</mn><mi>T</mi></mfrac><mo stretchy="false">)</mo></mrow></mfrac><mo stretchy="false">]</mo></mrow><mi>P</mi></msub><mo>=</mo><mfrac><mrow><mo>−</mo><mi>Δ</mi><mi>H</mi></mrow><mrow><mn>2.303</mn><mi>R</mi></mrow></mfrac><mo>.</mo></mrow></math> The plot of lg D vs 1/ T is linear with slope of –∆ H /(2.303R). The Gibb’s free energy (∆ G ) can be expressed as: <math display="block" id="M13"><mrow><mi>Δ</mi><mi>G</mi><mo>=</mo><mo>−</mo><mi>R</mi><mi>T</mi><mi mathvariant="normal">lnK</mi><mo>'</mo><mo>,</mo></mrow></math> where K' is the conditional extraction constant. At 0.01 M HNO 3, the extraction of Th(IV) and U(VI) ions with TODGA can be represented as: <math display="block" id="M14"><mrow><msup><mtext>M</mtext><mi>n</mi></msup><msub><mrow><msup><mrow/><mtext>+</mtext></msup></mrow><mrow><mrow><mo>(</mo><mrow><mtext>aq</mtext></mrow><mo>)</mo></mrow></mrow></msub><mtext>+</mtext><mi>m</mi><msub><mrow><mrow><mo>[</mo><mrow><mtext>TODGA</mtext></mrow><mo>]</mo></mrow></mrow><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><mi>n</mi><msup><mrow><mrow><mo>[</mo><mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><mtext>mim</mtext></mrow><mo>]</mo></mrow></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>↔[M</mtext><msub><mrow><mrow><mo>(</mo><mrow><mtext>TODGA</mtext></mrow><mo>)</mo></mrow></mrow><mi>m</mi></msub><mrow><mo>]</mo><mrow><msup><mrow><msup><mrow/><mi>n</mi></msup></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(IL)</mtext></mrow></msub><mtext>+</mtext><mi>n</mi></mrow><mo>[</mo></mrow><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim]</mtext></mrow><mtext>+</mtext></msup><msub><mrow/><mrow><mtext>(aq)</mtext></mrow></msub></mrow></math> <math display="block" id="M15"><mrow><mi>D</mi><mo>=</mo><mfrac><mrow><msub><mrow><mo stretchy="false">[</mo><msub><mrow><mtext>M(TODGA)</mtext></mrow><mtext>m</mtext></msub><msup><mrow/><mrow><mi>n</mi><mo>+</mo></mrow></msup><mo stretchy="false">]</mo></mrow><mrow><msub><mrow/><mrow><mtext>IL</mtext></mrow></msub></mrow></msub></mrow><mrow><msub><mrow><mo stretchy="false">[</mo><msup><mtext>M</mtext><mi>n</mi></msup><msup><mrow/><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>aq</mtext></mrow></msub></mrow></mfrac></mrow></math> Equilibrium constant K for extraction reaction can be written as <math display="block" id="M16"><mrow><mi>K</mi><mo>=</mo><mfrac><mrow><msub><mrow><mo stretchy="false">[</mo><msub><mrow><mtext>M(TODGA)</mtext></mrow><mtext>m</mtext></msub><msup><mrow/><mrow><mi>n</mi><mo>+</mo></mrow></msup><mo stretchy="false">]</mo></mrow><mrow><msub><mrow/><mrow><mtext>IL</mtext></mrow></msub></mrow></msub><msub><mrow><mo stretchy="false">[</mo><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim</mtext></mrow><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>aq</mtext></mrow></msub><msup><mrow/><mi>n</mi></msup></mrow><mrow><msub><mrow><mo stretchy="false">[</mo><msup><mtext>M</mtext><mi>n</mi></msup><msup><mrow/><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>aq</mtext></mrow></msub><msub><mrow><mo stretchy="false">[</mo><mtext>TODGA</mtext><mo stretchy="false">]</mo></mrow><mrow><mtext>IL</mtext></mrow></msub><msup><mrow/><mi>m</mi></msup><msub><mrow><mo stretchy="false">[</mo><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim</mtext></mrow><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>IL</mtext></mrow></msub><msup><mrow/><mi>n</mi></msup></mrow></mfrac><mo>=</mo><mfrac><mi>D</mi><mrow><msub><mrow><mo stretchy="false">[</mo><mtext>TODGA</mtext><mo stretchy="false">]</mo></mrow><mrow><mtext>IL</mtext></mrow></msub><msup><mrow/><mi>m</mi></msup></mrow></mfrac><mo>×</mo><mfrac><mrow><msub><mrow><mo stretchy="false">[</mo><msub><mi>C</mi><mn>2</mn></msub><msup><mrow><mtext>mim</mtext></mrow><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>aq</mtext></mrow></msub><msup><mrow/><mi>n</mi></msup></mrow><mrow><msub><mrow><mo stretchy="false">[</mo><msub><mi>C</mi><mn>2</mn></msub><msup><mrow><mtext>mim</mtext></mrow><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>IL</mtext></mrow></msub><msup><mrow/><mi>n</mi></msup></mrow></mfrac><mo>,</mo></mrow></math> <math display="block" id="M17"><mrow><mi>lg</mi><mi>D</mi><mo>=</mo><mi>lg</mi><mi>K</mi><mo>+</mo><mi>log</mi><mo stretchy="false">(</mo><mfrac><mrow><msub><mrow><mo stretchy="false">[</mo><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim</mtext></mrow><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>IL</mtext></mrow></msub><msup><mrow/><mi>n</mi></msup></mrow><mrow><msub><mrow><mo stretchy="false">[</mo><msub><mtext>C</mtext><mtext>2</mtext></msub><msup><mrow><mtext>mim</mtext></mrow><mo>+</mo></msup><mo stretchy="false">]</mo></mrow><mrow><mtext>aq</mtext></mrow></msub><msup><mrow/><mi>n</mi></msup></mrow></mfrac><mo stretchy="false">)</mo><mo>+</mo><mi>n</mi><mi>lg</mi><mo stretchy="false">[</mo><mtext>TODGA</mtext><mo stretchy="false">]</mo><mo>,</mo></mrow></math> where [C 2 mim + ] IL /[C 2 mim + ] aq is regarded as constant. Therefore, lg D can be calculated by Eq. (18) <math display="block" id="M18"><mrow><mi>lg</mi><mi>D</mi><mo>=</mo><mi>lg</mi><mi>K</mi><mo>'</mo><mo>+</mo><mi>n</mi><mi>lg</mi><mo stretchy="false">[</mo><mtext>TODGA</mtext><mo stretchy="false">]</mo></mrow></math> The K' value can be calculated from the intercept of a plot of lg D vs. lg[TODGA] at 0.01 M HNO 3 [ 7 7 , 23 23 ] . The ∆ S at certain temperature can be calculated by Eq. (19) <math display="block" id="M19"><mrow><mi>Δ</mi><mi>G</mi><mo>=</mo><mi>Δ</mi><mi>H</mi><mo>−</mo><mi>T</mi><mi>Δ</mi><mi>S</mi></mrow></math> Thermodynamics parameters of Th(IV) and U(VI) extraction from TODGA/[C 2 mim][NTf 2 ] at 0.01 M HNO 3 at 25°C are given in Table 1 Table 1 . The negative value of ∆ G indicates the spontaneous extraction reactions. The extraction of U(VI) ions (∆ G = −25.8 kJ·mol −1 ) by TODGA/[C 2 mim][NTf 2 ] system is energetically more favourable than extraction of Th(IV) ions (∆ G = −5.5 kJ·mol −1 ). The extraction of Th(IV) and U(VI) ions, being endothermic process with a positive entropy change, might be attributed to the loss of rotational entropy of TODGA during complexation [ 7 7 ] . 1 1 Ions 1 1 ∆ H (kJ·mol −1 ) 1 1 ∆ S (J·K −1 ·mol −1 ) 1 1 ∆ G (kJ·mol −1 ) 1 1 Th(IV) 1 1 104.5 1 1 367 1 1 −5.50 1 1 U(VI) 1 1 68.7 1 1 315 1 1 −25.8 3.6 Radiation effect 2 In order to evaluate the applicability of TODGA in radiation conditions, it is essential to study the radiation stability of TODGA in different diluents [ 31 31 ] . In Fig. 6 Fig. 6 , the γ-irradiation effect on TODGA/[C 2 mim][NTf 2 ] system for extracting Th(IV) and U(VI) ions is compared with that on TODGA/isooctane system. Obviously, the TODGA/[C 2 mim][NTf 2 ] system is more radiation-resistant than the TODGA/isooctane system. Comparing the TODGA/isooctane systems before and after irradiation, one sees that the radiation affects greatly the extraction of Th(IV) and U(VI), and the E U and E Th decreased dramatically with increasing doses. Under irradiation, isooctane might decompose and generate active radicals, which could induce serious radiolysis of TODGA [ 6 6 ] . Sugo et al . demonstrated that charge transfer from radical cation of dodecane to TODGA caused the sensitization effect on radiolysis of TODGA [ 31 31 ] . Consequently, the extractability of irradiated TODGA/isooctane system remarkably decreased. ILs have high radiation stability, with little radiolysis after 400 kGy irradiation [ 32 32 ] . For the TODGA/[C 2 mim][NTf 2 ] system, the E Th did not change virtually up to 500 kGy. It might be that the generated radiolysis products had extraction ability to Th(IV) ion. However, the irradiation affected greatly the E U , being reduced significantly with increasing doses. Sun et al . [ 27 27 ] found that the radiolytic products of [C 4 mim][NTf 2 ] were F − and SO 4 2− , which led to a sharp reduction in the extractability of U(VI) in TBP/irradiated-[C 4 mim][NTf 2 ]. Yuan et al . [ 33 33 ] reported that the extractability declined using irradiated [C 4 mim][NTf 2 ] as diluents. Irradiating [C 4 mim][NTf 2 ] generates H + , which competes with metal ions to interact with extractant. SO 3 2− was observed in irradiated [C 2 mim][NTf 2 ] by Zhou et al . [ 6 6 ] and Yuan et al . [ 34 34 ] . In order to confirm whether the water-soluble radiolytic products of [C 2 mim][NTf 2 ] led to the decreased extractability of irradiated TODGA/[C 2 mim][NTf 2 ] system, the irradiated TODGA/[C 2 mim][NTf 2 ] were washed with water for 3 times before extraction. We found that the extraction efficiency of irradiated TODGA/[C 2 mim][NTf 2 ] for both Th(IV) and U(VI) were recovered completely, as shown in Fig.6 Fig.6 from histograms marked with (c). It can be concluded that the main reason for decease in extractability in TODGA/[C 2 mim][NTf 2 ] system is the presence of water-soluble radiolytic products (F − , SO 4 2− , SO 3 2− , H + ) from radiolysis of [C 2 mim][NTf 2 ]. In one circle of the reprocessing of spent nuclear fuel, the adsorbed dose of extraction system is less than 10 kGy, which has litter impact on the extractability of TODGA/[C 2 mim][NTf 2 ] [ 27 27 ] . Therefore, TODGA/[C 2 mim][NTf 2 ] have a potential application in spent fuel reprocessing. 4. Conclusion 1 Th(IV) and U(VI) ions could be efficiently extracted from HNO 3 medium using TODGA as extratant in different diluents including [C 2 mim][NTf 2 ], [C 12 mim][NTf 2 ] and isooctane as comparison. The dominating extraction mechanism shifted from cation exchange to neutral complex/anion exchange with increasing HNO 3 concentration and alkyl chain length of [C n mim] + . The TODGA/[C 2 mim][NTf 2 ] presented the best extraction performance to both Th(IV) and U(VI) ions with E U > 75% and E Th > 80% in HNO 3 medium ranged from 0.01 M to 6 M. The extraction of TODGA/[C 2 mim][NTf 2 ] system against Th(IV) and U(VI) ions was endothermic reaction and the extraction equilibrium can reach within 2 h. TODGA/ [C 2 mim][NTf 2 ] system showed higher radiation stability than TODGA/isooctane, and the decrease in extractability of irradiated TODGA/[C 2 mim][NTf 2 ] system was mainly due to the presence of water-soluble radiolytic products, which could be easily eliminated by washing irradiated TODGA/[C 2 mim][NTf 2 ] with water. Therefore, TODGA/[C 2 mim][NTf 2 ] system would have a promising application in separation of Th(IV) and U(VI) ions in spent fuel reprocessing. Reference S.H. Ha , R.N. Menchavez , Y.M. Koo , Reprocessing of spent nuclear waste using ionic liquids . Korean J Chem Eng , 2010 , 27 : 1360 - 1365 . 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DOI: 10.1039/C4RA08308C http://doi.org/10.1039/C4RA08308C Influence of radiation effect on extractability of an isobutyl-BTP/ionic liquid system: quantitative analysis and identification of radiolytic products 10.1007/s41365-017-0214-y This work is financially supported by the National Natural Science Foundation of China (No. 11475112), the Ph.D. Programs Foundation of Ministry of Education of China (No. 20130073120051), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. 2016-06-01 2016-10-15 2016-11-03 2017-03-28 2017-05-01 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2017 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-017-0214-y http://dx.doi.org/10.1007/s41365-017-0214-y . 2017 Nuclear Science and Techniques 05 28