Radiation Effect of Carboxyl-Functionalized Task-Specific Ionic

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Radiation Effect of Carboxyl-Functionalized Task-Specific Ionic Liquids on UO22+ Removal: Experimental Study with DFT Validation Yinyong Ao, Jian Chen, Yue Wang, Hongbing Chen, Jiuqiang Li, and Maolin Zhai J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11395 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on February 7, 2017

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The Journal of Physical Chemistry

Radiation Effect of Carboxyl-Functionalized Task-Specific Ionic Liquids on UO22+ Removal: Experimental Study with DFT Validation Yinyong Ao,a* Jian Chen,a Yue Wang,b Hongbing Chen,a Jiuqiang Li,b and Maolin Zhai,b* a

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang

621900, P. R. China b

Beijing National Laboratory for Molecular Sciences (BNLMS), Department of Applied Chemistry,

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China * Corresponding authors: Tel/Fax: +86-0816-2482203, E-mail: [email protected]; Tel/Fax: +8610-62753794, E-mail: [email protected]

Abstract: Experimental study with DFT validation about effect of radiation on 1carboxymethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)

imide

([HOOCCH2MIM][NTf2]) as solvent and extractant in rapid homogeneous extraction of UO22+ were performed for the first time. The radiolytic products of the anions and cations of [HOOCCH2MIM][NTf2] were identified by

19

F NMR and high-resolution ESI-MS,

respectively, and they were attributed to a decrease in UO22+ partitioning. Experimental study with DFT validation for the complexing reactions between [HOOCCH2MIM][NTf2] and radiolytic products proved that F- competition was one of the main reasons for the decrease in UO22+ partitioning. However, the UO22+ partitioning in irradiated [HOOCCH2MIM][NTf2]

1 ACS Paragon Plus Environment


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can largely be recovered after thorough water-washing because of the removal of radiolytic products of [NTf2]-.


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1. INTRODUCTION In recent years, room-temperature ionic liquids (RTILs) have been considered promising solvents for the separation of radioactive isotopes in advanced nuclear fuel cycles because of their unique physicochemical properties.1-5 RTILs often have a high viscosity, which is detrimental for its practical application, and are used only as solvents during liquid-liquid extraction.6-9 Functionalized task-specific ionic liquids (TSILs), that contain functional groups on either the cation or anion of the ionic liquids, can be used as a solvent and an extractant in solvent extraction systems.10-14 The equilibrium extraction time can be reduced significantly by the formation of a homogeneous phase by incorporating functional groups into TSILs.15-17 The Binnemans group investigated the switchable phase behaviour of TSILs for metal ions separation.10,18,19 They proved that effective extraction of neodymium ions11,19, indium ions, gallium ions, and trivalent rare-earth ions20 can be achieved through homogeneous extraction without extractant addition. The separation of uranyl species was also achieved by TSILs without extractant addition.21 Rapid selective extraction of U(VI) species from rare earth metal ions is of significance in the advanced nuclear fuel cycle

12,22

.

Carboxyl-functionalized task-specific ionic liquids can remove UO22+ without the usage of additional extractants after a rapid homogeneous extraction. In nuclear applications, the radiation stability of an extraction system is important in assessing potential practical applications and it should be assessed carefully.23-25 Existing studies of radiation effects focused mainly on pure RTILs under γ-irradiation,26-31 and the water-soluble radiolytic products of NTf2-based ionic liquids were identified.28,32 Minimal work has been carried out on the radiolysis of TSILs; for example, Shkrob et al.33 used



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electron paramagnetic resonance spectroscopy to study the radiation stability of cations of TSILs. Antioxidant organic cation deprotonation from the excited state is reversible, and leads to minimal TSILs damage. There is an urgent need for further research into the radiation effect on TSILs and an analysis of radiolytic products of TSILs. The influence of their radiolytic products on UO22+ removal should be evaluated carefully before practical application. We studied the effects of radiation on UO22+ extraction by a rapid homogeneous extraction system with [HOOCCH2MIM][NTf2] as solvent and extractant. The radiolytic products of the anions and cations of [HOOCCH2MIM][NTf2] were identified by

19

F nuclear

magnetic resonance (NMR) and high-resolution electrospray ionisation mass spectrometry (ESI-MS), respectively. The influence of the radiolytic products of [HOOCCH2MIM][NTf2] on UO22+ extraction was evaluated experimentally and by density functional theory (DFT) calculation.. 2. EXPERIMENTAL SECTION 2.1 Materials [HOOCCH2MIM][NTf2] (with a purity > 99%, Figure 1) was obtained from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). UO2(NO3)2·6H2O was purchased from Beijer Chemapol Co. CF3SO2NH2 (Alfa Aesar Co., 96%), CF3SOONa (Tokyo Chemical Industry Co., > 95 %), NaF and Na2SO3 (Beijing chemical corp., > 99%) were obtained for analysing quantitatively the amounts of radiolytic products of [HOOCCH2MIM][NTf2] and assessing the influence of radiolytic products on the extraction of UO22+.


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Figure 1. Chemical structure of [HOOCCH2MIM][NTf2]. 2.2 Irradiation [HOOCCH2MIM][NTf2] was irradiated by 60Co source (Institute of Applied Chemistry of Peking University) with an average dose rate of ca. 210 Gy·min-1, up to a maximum dose of 500 kGy. The dose rate was traced by Fricke dosimeter which was placed at sample position. 2.3 Fast homogeneous extraction experiments 0.40 mL organic phase of [HOOCCH2MIM][NTf2] and 0.40 mL aqueous phase containing 2 mM UO22+ were added into a plastic tube. Then the tube was kept at 75 oC for 10 min, followed by a vibrating mixer for 1 min. Whereafter, the samples were cooled to 30 o

C and centrifuged for 5 min to ensure the complete separation of two phases. Then the

aqueous solution was diluted ca. 40 times by deionized water, and the concentration of UO22+ in the diluted aqueous solution was measured by Prodigy high dispersion inductively coupled plasma atomic emission spectrometer (ICP-AES) (Teledyne Leeman Labs, USA). The extraction efficiencies were calculated by EU = (ni – nf)/ni×100%, where ni and nf designate the initial and final amount of UO22+ in the aqueous solution, respectively. All obtained values were in duplicate with uncertainty within 5%. 2.4 Characterization 19

F NMR: Separation of water-soluble radiolytic products of [HOOCCH2MIM][NTf2]

were carried out based on our previous method.28,32 The separated samples were analysed by



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19

F NMR on a Bruker AV-500. The chemical shift scale was calibrated with NTf2- at –78.78

ppm.28 Ion Chromatography analysis: The water-soluble products from irradiated samples were analysed by a Dionex ICS-900 ion chromatography (IC) System (USA). IonPac AS16 column

(4

×

250

mm)

was

chosen

to

analyse

the

radiolytic

products

of

[HOOCCH2MIM][NTf2]. The DS5 conductivity detector with ASRS-300 4-mm was applied. The injection volume was set at 10 µL, and the concentration of KOH eluent was set at 20 mmoL·L-1 with a constant flow rate of 0.90 mL·min-1. The operating back pressure was 1660 – 1700 psi. High-resolution ESI-MS: Irradiated TSILs were diluted by chromatographically pure acetonitrile. Mass spectrometry measurements of irradiated TSILs were detected by a Fourier transform ion cyclotron resonance mass spectrometer equipped with electrospray ionization source in positive ionization mode (Apex IV, Bruker, Switzerland). Capillary voltage: 3000 V; spray shield voltage: –3500 V. High purity N2 was used as the nebulizing and drying gases. Drying temperature was set at 200 °C. Mass range of m/z was set from 100 to 2000. 2.6 Theoretical calculations Electron correlation effects were included by employing density functional theory (DFT) methods and were calculated by the Gaussian 09 program package using DFT at the B3LYP level.3436

Relativistic effects were considered with the quasirelativistic effective core potentials (RECPs) for

U with the adopted large-core RECPs include 60 electrons.37-42 The 6-31+G(d,p) basis set was used for all C, H, O, N, F and S atoms. Geometry optimizations and electronic calculations for all of the species were carried out firstly in the gasphase at the B3LYP/6-31+G(d,p)/RECP level. The


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enthalpies (Hg), entropies (Sg), and Gibbs free energies (Gg) were calculated at the same level of theory in the gas phase (298.15 K). For obtaining the enthalpies, entropies, and Gibbs free energies of these species in solvents (water (Haq, Saq, and Gaq); [HOOCmim][NTf2] (Horg, Sorg, and Gorg)) at 298.15 K, these structures were optimized in solvents and calculated by frequency analysis at the M05-2X43/6-31G(d)/RECP level of theory based on the universal continuum solvation model of SMD44, which was known to predict energies of solvation well.45 The static dielectric constant at 66.4 determined by PCM-1A dielectric constant detector and refractive index at 1.4454 determined by Abbe refractometer were adopted for [HOOCCH2MIM][NTf2]. 3. RESULTS AND DISCUSSION 3.1 Influence of radiation effect on the removal of UO22+ by [HOOCCH2MIM][NTf2]. Carboxyl-functionalized task-specific ionic liquid shows the possibility for UO22+ removal without using additional extractants after a rapid homogeneous extraction.21 In nuclear applications, the radiation stability of the extraction system can be used to assess the possibility in practical applications, so the influence of radiation dose on the homogeneous extraction of UO22+ by [HOOCCH2MIM][NTf2] was studied as shown in Figure 2. EU decreases obviously with increasing dose, and UO22+ cannot be extracted into the organic phase when the radiation dose exceeds 100 kGy. The irradiated [HOOCCH2MIM][NTf2] was washed

four

times

with

deionized

water

before

extraction.

The

washed

[HOOCCH2MIM][NTf2] was dried at 60 oC for 15 h, and these samples were used to remove UO22+ from aqueous solution. The UO22+ partitioning of [HOOCCH2MIM][NTf2] almost recovers after a thorough wash of the irradiated [HOOCCH2MIM][NTf2] at 100 kGy. The decrease in UO22+ partitioning of the irradiated [HOOCCH2MIM][NTf2] is attributed to



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radiation-formed water-soluble radiolytic products. However, as shown in Figure 2, UO22+ partitioning in the irradiated [HOOCCH2MIM][NTf2] at 500 kGy cannot recover fully after washing the irradiated sample (Figure S1), which proves that [HOOCCH2MIM][NTf2] lose its

partial

extraction

ability

after

irradiation,

and

indicates

the

radiolysis

of

[HOOCCH2MIM]+ during irradiation. Therefore, radiolysis of [HOOCCH2MIM]+ and [NTf2]- can likely be attributed to the decrease in UO22+ partitioning in irradiated [HOOCCH2MIM][NTf2].

Figure 2. Influence of dose on the extraction of UO22+ by irradiated [HOOCCH2MIM][NTf2]. *Irradiated samples were washed for 4 times by deionized water before extraction. 3.2 Identification of radiolytic products of [HOOCCH2MIM][NTf2] and their influence on the extraction of UO22+. To identify the water-soluble radiolytic products of irradiated [HOOCCH2MIM][NTf2], 0.5 mL of irradiated sample and 0.5 mL of deuterium oxide were contacted for about 10 min with a vibrating mixer, followed by centrifuging to ensure that the two phases were contacted fully. The aqueous phase from washing irradiated [HOOCCH2MIM][NTf2] was analysed by NMR spectroscopy, and the chemical shift scale of

19

19

F

F NMR was calibrated with [NTf2]- at –


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78.78 ppm. Figure S2 shows several new peaks at –79.31, –87.03 and –164.6 ppm in the

19

F

NMR spectrum of the irradiated [HOOCCH2MIM][NTf2]. According to our previous investigations28,32, these water-soluble radiolytic products are CF3SOONH2, CF3SOOH, and HF.46 SO32- was also one of the radiolytic products of [NTf2]-.28 The amount of these watersoluble radiolytic products was analysed quantitatively by ion chromatography and the results are given in Figure S3. Compared with unirradiated sample, the amounts of the main radiolytic products increased with increasing dose, and the radiation yields of these watersoluble radiolytic products were: G(F-) = 0.19±0.01 µmol·J-1 > G(CF3SO2N2-) = 0.070±0.003 µmol·J-1> C(SO32-) = 0.050±0.001 µmol·J-1 > C(CF3SOO-) = 0.016±0.001 µmol·J-1. The influence of water-soluble radiolytic products on UO22+ extraction was studied by adding standard compounds separately in unirradiated samples based on the above quantitative analysis, such as HNO3, CF3SOONH2, CF3SOONa, NaF, and Na2SO3. As shown in Figure 3, CF3SOONa, CF3SOONH2, and Na2SO3 influence the UO22+ extraction slightly. However, EU decreases from 83.2% to 63.1 % and 83.2% to 23.2% when HNO3 and NaF are added, respectively. Deprotonation of the carboxyl groups is inhibited by H+, which is detrimental to the co-ordination with UO22+. It is proposed that F- has a stronger co-ordination ability to UO22+ than the carboxyl groups of [OOCCH2MIM], which results in a decrease of UO22+ partitioning. Therefore, the effect of H+ and F- was confirmed for the main change of UO22+ partitioning. The influence of water-soluble radiolytic products can be eliminated through the water-washing process before extraction.



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Figure 3. Influence of radiolytic products H+, CF3SO2NH-, SO32-, CF2SOO-, and F- on the extraction of UO22+ by unirradiated [HOOCCH2MIM][NTf2] in combination with standard compounds.([CF3SO2NH2]: 10 mM; [Na2SO3]: 10 mM; [CF2SOONa]: 10 mM; [NaF]: 30 mM; [HNO3]: 70 mM). UO22+ partitioning in the irradiated [HOOCCH2MIM][NTf2] at 500 kGy did not recover fully. Thus, the irradiated [HOOCCH2MIM][NTf2] at 500 kGy was analysed by highresolution ESI-MS with positive ionization mode. Figure 4 shows the high-resolution ESIMS spectra of [HOOCCH2MIM][NTf2] before and after irradiation at 500 kGy. Serial peaks with a relative high intensity at 281.126, 562.048, 702.105, 842.162, 1123.084, 1263.141 and 1404.009 are visible for the unirradiated sample and are attributed to the association of cations with [HOOCCH2MIM][NTf2] in clusters (Table S1). As illustrated in Figure 4(b), several new signals at 446.037, 504.044, and 516.043 are observed in the high-resolution ESI-MS spectra of irradiated [HOOCCH2MIM][NTf2]. These products at m/z = 446.037, 504.044, and 516.043 were designated to radiolytic products of [HOOCCH2MIM]+ (Table 1), which were identified as [[C4H7N2][NTf2]+C4H7N2]+ (P1, theoretical exact m/z is 446.039), [[HOOCCH2MIM][NTf2]+C4H7N2]+

(P2,

theoretical

exact


m/z

is

504.044),

and

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[[HOOCCH2MIM][NTf2]+C5H7N2]+ (P3, theoretical exact m/z is 516.044). The formation of C4H7N2+ and C5H7N2+ indicated that the carboxyl groups of [HOOCCH2MIM]+ were fragmented during irradiation. Nockemann et al.12 found that three carboxyl groups were coordinated bidentately to the uranyl in the crystal structure of [(UO2)( [OOCCH2MIM])3]2+ complexes formed between [HOOCCH2MIM][NTf2] and UO22+. Therefore, radiolysis of the cation of [HOOCCH2MIM][NTf2] were responsible for the inhibition in extraction.

Figure 4. High resolution ESI-MS spectra of [HOOCCH2MIM][NTf2] before (a) and after irradiation at 500 kGy (b). Table 1. Designation of the radiolytic products of cation of [HOOCCH2MIM][NTf2]. Nu

Theoretica Experimental m/z

Designation

m

P1

l m/z

446.037

446.039



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P2

504.044

504.044

P3

516.043

516.044

3.3 DFT calculations about the influence of radiolytic products on the extraction of UO22+.

Figure

5.

Optimized

structures

of

[UO2([OOCCH2MIM])3]2+,

UO2(CF3SO2NH)2,

UO2(CF3SOO)2, UO2SO3, UO2F2, [UO2F4]2-, and [UO2F5]3-. Green, white, red, blue, pink, yellow, and cyan spheres represent C, H, O, N, F, S, and U, respectively. Based on previous research12, deprotonation of the carboxyl groups of [HOOCCH2MIM] is necessary before it coordinates with UO22+. The coordination abilities of [OOCCH2MIM] and water-soluble radiolytic products were investigated by using DFT calculations, and the 12 ACS Paragon Plus Environment

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coordination of H2O with UO22+ was also considered according to previous literature47. Figure 5 shows the optimized structures of complexes between radiolytic products and metal ions obtained separately in the gas phase and water. Table 2 lists the changes in enthalpy, entropy, and binding energies for the complexation reactions between UO22+ and the radiolytic products in the gas phase by the B3LYP/6-31+G(d,p)/RECP level. As presented in Table 2 (Table S2), the gas-phase reaction enthalpies are relatively large negative gas-phase binding energies and are much more negative than the T∆Sg. Radiolytic products (CF3SO2NH-, SO32-, CF2SOO-, and F-) show a high coordination ability for UO22+, especially for F- ([UO2F4]2-, ∆Gg=–2641.3 kJ/mol), which is much more negative than that of [OOCCH2MIM] ([UO2([OOCCH2MIM])3]2+, ∆Gg=–1476.8 kJ/mol) in the gas phase. During the practical extraction system, these water-soluble radiolytic products were transferred from the organic to the aqueous phase, so the solvent effects in water were considered for geometry optimizations and electronic calculations. The solvation structure was optimized in water and was calculated by frequency analysis at the M05-2X/6-31G(d)/RECP level of theory based on the universal continuum solvation model of SMD. Table 3 (Table S3) shows that these complexing binding energies (∆Gaq) are much lower than the corresponding gasphase binding energies. However, these results confirm those in the gas phase, which indicates that radiolytic products (SO32-, CF2SOO-, and F-) have a higher coordination ability over [OOCCH2MIM] or H2O for UO22+. The difference in Gibbs free energy for the complexing reactions in water proves that F- is one of the main reasons for the decrease in UO22+ partitioning, which is remarkably consistent with the above experimental results. The complexing reactions between F- and UO22+ were studied in [HOOCCH2MIM][NTf2] and water by using DFT calculations. Figure 5 (Table 3 and Table S3) shows that the calculated



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Gibbs free energy of UO2F2, [UO2F4]2-, and [UO2F5]3- formation in water is –634.0, –1146.2, –1282.6 kJ/mol, respectively, which indicates that [UO2F5]3- is the main complexing compound. Similarly, the main complexing compound between F- and UO22+ in [HOOCCH2MIM][NTf2] is calculated as [UO2F5]3-. The change in Gibbs free energy of [UO2Fn]2-n (n = 2, 4, 5) from the organic to the aqueous phase is negative (Scheme 1 and Table S4), which suggests that [UO2Fn]2-n prefers to exist in an aqueous phase and leads to a decrease in UO22+ partitioning in [HOOCCH2MIM][NTf2]. Table 2. The changes in enthalpy (∆Hg), entropy (∆Sg), and binding energies (∆Gg, 298.15 K, kJ/mol) for the complexes between radiolytic prodcuts and metal ions obtained in gas phase by the B3LYP/6-31+G(d,p)/RECP level. Complexation

∆Hg

T∆Sg

∆Gg

UO22++3[OOCCH2MIM]→[UO2([OOCCH2MIM])3]2+

-1620.0

-143.2

-1476.8

UO22++ SO32-→UO2SO3

-2505.1

-47.3

-2457.8

UO22++ CF3SOO-→[UO2CF3SOO]+

-1353.7

-49.5

-1304.2

UO22++ CF3SO2NH-→[UO2CF3SO2NH]+

-1335.8

-46.3

-1289.5

UO22++ 2CF3SOO-→UO2(CF3SOO)2

-1931.0

-94.7

-1836.3

UO22++ 2CF3SO2NH -→UO2(CF3SO2NH)2

-1871.9

-95.2

-1776.7

UO22++ 2F-→UO2F2

-2422.8

-8.4

-2414.4

UO22++ 4F-→[UO2F4]2-

-2772.9

-131.6

-2641.3

UO22++ 5F-→[UO2F5]3-

-2164.0

-163.3

-2000.7

UO22++ 5H2O→[UO2(H2O)5]2+

-998.8

-199.5

-799.3


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Table 3. The changes in enthalpy (∆Haq), entropy (∆Saq), and binding energies (∆Gaq, 298.15 K, kJ/mol) for the complexes between radiolytic products and metal ions obtained in water by the M05-2X/6-31G(d)/RECP level. Complexation

∆Haq

T∆Saq

∆Gaq

UO22++3[OOCCH2MIM]→[UO2([OOCCH2MIM])3]2+

-374.6

-167.1

-207.5

UO22++ SO32-→UO2SO3

-313.5

-49.0

-264.5

UO22++ CF3SOO-→[UO2CF3SOO]+

-108.2

-48.5

-59.7

UO22++ CF3SO2NH-→[UO2CF3SO2NH]+

-65.7

-47.4

-18.3

UO22++ 2CF3SOO-→UO2(CF3SOO)2

-210.9

-97.0

-113.9

UO22++ 2CF3SO2NH -→UO2(CF3SO2NH)2

-124.9

-97.4

-27.5

UO22++ 2F-→UO2F2

-697.1

-63.1

-634.0

UO22++ 4F-→[UO2F4]2-

-1275.1

-128.9

-1146.2

UO22++ 5F-→[UO2F5]3-

-1454.4

-171.8

-1282.6

UO22++ 5H2O→[UO2(H2O)5]2+

-317.0

-214.0

-103.0



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Scheme 1. The change of Gibbs free energy of [UO2Fn]2-n (n = 2, 4, 5) from organic phase to aqueous phase. 4. CONCLUSIONS In

conclusion,

the

influence

of

radiation

effects

on

UO22+

extraction

by

[HOOCCH2MIM][NTf2] as solvent and extractant was studied. Radiolytic products of [HOOCCH2MIM]+ and [NTf2]- were identified, and their influence on UO22+ removal was investigated experimentally with DFT validation. Water-soluble radiolytic products of [NTf2]-

are

the

main

reason

for

the

decrease

in

UO22+

partitioning,

whereas

[HOOCCH2MIM]+ radiolysis is responsible for the inhibition in extraction. The experimental results and difference in Gibbs free energy for the complexing reactions in water prove that Fcompetition is the main reason for the decrease in UO22+ partitioning. The negative change in


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Gibbs free energy of [UO2Fn]2-n (n = 2, 4, 5) from organic to aqueous phase indicates that [UO2Fn]2-n prefers to exist in an aqueous phase, and leads to a decrease in UO22+ partitioning in [HOOCCH2MIM][NTf2]. The UO22+ partitioning of irradiated [HOOCCH2MIM][NTf2] can be largely recovered after thorough water-washing because of the removal of radiolytic products of [NTf2]-. Therefore, the [HOOCCH2MIM][NTf2]-H2O extraction system without extractant can be considered in the nuclear fuel cycle.

ASSOCIATED CONTENT Supporting Information 19

F NMR spectra of [HOOCCH2MIM][NTf2] before and after irradiation; The relationship

between the concentration of water-soluble radiolytic products and radiation doses; Influence of dose on the extraction of UO22+ by water-washed [HOOCCH2MIM][NTf2]; Designation of signals of [HOOCCH2MIM][NTf2] before and after γ-irradiation; The enthalpy, entropy, and binding energies (298.15 K) for radiolytic products, metal ions and complexes obtained in the gas phase by DFT method at B3LYP/6-31+G(d,p)/RECP level; The enthalpy, entropy, and binding energies (298.15 K) for radiolytic products, metal ions and complexes obtained in the water by DFT method at M05-2X/6-31G(d)/RECP level; The enthalpy, entropy, and binding energies (298.15 K) for [UO2Fn]2-n (n = 2, 4, 5) in the organic and the aqueous phase.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The Innovation Foundation of Institute of Nuclear Physics and Chemistry (Grant No. 2015CX04) and National Science Foundation of China (NSFC, Project No. 91126014, 11079007, and 21073008) are acknowledged for supporting this research.

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TOC graphic

Radiation effect of carboxyl-functionalized task-specific ionic liquids on UO22+ removal.


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Radiation Effect of Carboxyl-Functionalized Task-Specific Ionic

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