DaltonTransactions

PAPER

Cite this: Dalton Trans., 2013, 42, 8413

Received 27th November 2012,Accepted 20th March 2013

DOI: 10.1039/c3dt32832e

www.rsc.org/dalton

Fe-containing ionic liquids as effective and recoverableoxidants for dissolution of UO2 in the presence ofimidazolium chlorides†

Aining Yao and Taiwei Chu*

Imidazolium-based Fe-containing ionic liquids (ILs) can directly dissolve UO2 in the presence of their cor-

responding imidazolium chlorides without additional oxidants. The dissolution process follows pseudo

first-order kinetics initially. Raman spectroscopic studies indicate that FeCl42− is the predominant

reduction product after UO2 dissolution, and attenuated total reflection-Fourier transform infrared spec-

troscopy indicates that the UO22+ complex is the principal product in the ILs. The dissolved uranyl species

can be successfully separated from the Fe-containing ILs via a combination of centrifugation and solvent

extraction, and also, the Fe-containing ILs can be recovered easily. In conclusion, imidazolium-based

Fe-containing ionic liquids in the presence of imidazolium chlorides could be used as effective and re-

coverable oxidants for the dissolution of UO2.

Introduction

The nuclear fuel cycle is a complex process, and it is veryimportant for the continued development of nuclear energy.The potential of ionic liquids (ILs) in the nuclear industry hasalso been explored in recent years, and it has been shown thationic liquids have advantageous properties, making them par-ticularly suitable for use in nuclear fuel separations.1–12 Imida-zolium-based ILs show a relatively high radiation resistancewhich is attributed to the presence of aromatic rings whichcan absorb radiation energy and relax nondissociatively.13

They are more thermally stable than alkyl ammonium ionicliquids because they are resistant to ring fission duringthermal rearrangements, and in addition, their thermal stabi-lity can be enhanced by a methyl substitution at the C2-position.14 Therefore, imidazolium-based ILs have been mostwidely used among a series of ionic liquids.6

In spent nuclear fuel reprocessing, uranium dioxide isthe main chemical form. To our knowledge, only a fewpublications have presented the results of the dissolutionof uranium oxides in ILs.15–20 Billard et al. reported the

dissolution of UO2 in a 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide [Bmim][Tf2N] IL using concen-trated nitric acid.19 Meanwhile, Wai et al. presented a methodof dissolving uranium oxide in [Bmim][Tf2N] with a TBP-(HNO3)1.8(H2O)0.6 complex.18 Protonated betaine bis(trifluoro-methylsulfonyl)imide, [Hbet][Tf2N] can also dissolve largequantities of metal oxides, for example, uranium(VI) oxide.20 Inaddition to HNO3, Fe(III) compounds can also be used as oxi-dants. Ram et al. reported that UO2 can be dissolved by ferricions (Fe3+) from a ferric sulphate solution in 0.15 M sulphuricacid and the dissolution of uranium dioxide was found tomost closely follow first order kinetics.21 Due to the additionof HNO3 or other acids, the dissolution system is no longertotally dry and thus it will not be suitable for the separationof some lanthanide oxides and UO2 by way of selectivedissolution.

Recently, Fe-containing ionic liquids containing the FeCl4−

anion have been reported,22–25 and the incorporation of theFeCl4

− ion may change the physicochemical properties of theoriginal IL, e.g., hydrophilic 1-butyl-3-methylimidazoliumchloride [Bmim]Cl26 turns into hydrophobic [Bmim]FeCl4.

27,28

Moreover, FeCl4− brings a high oxidation ability, and nano-

sized conducting polymers can be prepared without additionaloxidants.29–31 In this study, imidazolium-based Fe-containingionic liquids are used as a medium to dissolve UO2.

It is known that the bulk of the spectroscopic studies onuranium complexes in ILs have been completed using UV-visible spectroscopy.32–34 Fe-containing ILs show an intenseUV-Vis absorbance which comes from the intra-configurationald-transition Fe3+ in a tetrahedral ligand field,35 and as a

†Electronic supplementary information (ESI) available: the maximum dissolu-tion amount of UO2 in Fe-containing ionic liquids at 160 °C (Table S1). Infraredspectra of UO2 dissolution in ILs [Bmim]FeCl4/[Bmim]Cl (Fig. S1) and [Bdmim]-FeCl4/[Bdmim]Cl (Fig. S2). See DOI: 10.1039/c3dt32832e

Beijing National Laboratory for Molecular Sciences, Radiochemistry and Radiation

Chemistry Key Laboratory of Fundamental Science, College of Chemistry and

Molecular Engineering, Peking University, Beijing 100871, China.

E-mail: twchu@pku.edu.cn; Fax: +86 10 62754319; Tel: +86 10 62754319

This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 8413–8419 | 8413

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article OnlineView Journal | View Issue

consequence, the UV-Vis absorbance of Fe will cover the uranylabsorption band. Vibrational spectroscopy can also provideimportant information regarding uranyl(VI) complexation insolution and has been extensively used for the characterizationof uranyl compounds.18,36 Hence, Raman spectroscopy andFourier transform infrared spectroscopy (FTIR) were used tocharacterize the uranium species dissolved in Fe-containingILs in the present study. The recovery of the dissolved uraniumspecies and Fe-containing ILs was also investigated.

ExperimentalChemicals and reagents

The ILs 1-ethyl-3-methylimidazolium chloride (EmimCl;>99%), 1-butyl-3-methylimidazolium chloride (BmimCl;>99%), and 1-butyl-2,3-dimethylimidazolium chloride(BdmimCl; >99%) were purchased from the Lanzhou Instituteof Chemical Physics, Chinese Academy of Sciences. All the ILswere used without any further purification. Uranium dioxide(natural uranium: 99.2745% of 238U, 0.7200% of 235U and0.0054% of 234U) and a uranium standard solution (100 μg mL−1)were purchased from the Beijing Research Institute of Chemi-cal Engineering and Metallurgy (Beijing, China). FeCl3·6H2Oand anhydrous iron(III) chloride, FeCl3, were purchased fromXilong Chemical Co. Ltd (Guangdong, China) and used asreceived. Imidazolium-based Fe-containing ionic liquids wereprepared following an established procedure.24 Negative-ionESI-MS spectrum: m/z 197.8 [FeCl4], 100%.

Instruments

An ESI-MS study was performed using an APEX IV FourierTransform Ion Cyclotron Resonance Mass Spectrometer(Bruker, USA) with methanol as the solvent. A Prodigy HighDispersion inductively coupled plasma atomic emissionspectrometer (Teledyne Leeman Labs, USA) was used for theanalysis of uranium. Elemental analyses were obtained using aVario EL (Selb, Germany). Infrared spectra were measuredusing an IRAffinity-1 Fourier Transform Infrared Spectropho-tometer (Shimadzu, Japan) with a zinc selenide horizontalAttenuated Transmitted Reflectance (ATR) crystal. A Horiba-Jobin Yvon Labram HR800 instrument with a 785 nm laser(Horiba, France) was used to obtain Raman spectra for theuranyl species dissolved in the IL phase.

Analysis of uranium by ICP-AES

The analysis of the uranium in solutions containing ionicliquids was performed using inductively coupled plasmaatomic emission spectrometry (ICP-AES). The wavelength was393.202 nm. The sample uptake delay used was 30 s, theinstrument stabilization delay was 15 s and the RF power was1.2 kW. For internal consistency, all samples were analyzed24 h after preparation. Each sample (the reaction solution andthe uranium standard solution) was tested three times, andthe uranium concentration ([U]) determined was the average of

the replicates; the errors of all testing results were not morethan 5%.

Aliquots of the reaction solution were taken periodicallyand recorded by weight rather than by volume in view of theinfluence of the ILs’ viscosity. The samples were diluted withacetone and centrifuged at a speed of 4000 rpm for 30 min toremove the undissolved UO2 from the suspension. Acetone wasremoved from the upper liquid phase using rotary evaporation.After that, the acquired IL containing dissolved uranium wasdiluted with a HNO3 solution (0.01 mol L−1) to obtain a solu-tion that contained a suitable amount of uranium for ICP-AESanalysis.

The uranium standard solution (100 μg mL−1) was added tofive 50 mL volumetric flasks to obtain final uranium concen-trations of 0, 5, 10, 15 and 25 μg mL−1. In accordance with theionic liquid amount of analyzed reaction solution, a HNO3

solution (0.01 mol L−1) containing 0.72% w/v Fe-containingILs was prepared and 25 mL of the solution was then pipettedinto the flasks. The flasks were then made up to volume by theHNO3 solution (0.01 mol L−1).

Dissolution of UO2

The dissolution of UO2 in ionic liquids was performed using a10 mL glass vial placed on a magnet stirrer at a constant stir-ring speed. Before the dissolution experiments began, the Fe-containing ILs and their corresponding imidazolium chlorideILs were placed overnight in a vacuum oven at 70 °C. The Fe-containing IL (1 mL) and its corresponding imidazolium chlor-ide IL at the molar ratio of 1 : 1 were added to a 10 mL round-bottom flask and then 25.3 mg of UO2 (0.094 mmol) wasadded to the solution. The flask was immersed in an oil bathat a specific temperature ranging from 40 to 160 °C for therequired time at 820 ± 5 rpm. To investigate the effect of thedifferent Fe-containing ILs and the temperature on the UO2

dissolution rate, the reaction sample was taken out at set inter-vals to evaluate the amount of dissolved uranium by ICP-AESanalysis.

To evaluate the maximum amount of UO2 dissolution in0.5 mL of an Fe-containing IL, separate aliquots of UO2 wereadded to the IL phase every two hours until the IL phaseturned from clear to cloudy at 160 °C. Moreover, differentmolar ratios of the Fe-containing ionic liquid and its corres-ponding imidazolium chloride (1 : 0, 1 : 0.5, 1 : 1, 1 : 2, 1 : 3 and1 : 4) were prepared to investigate the influence of the amountof imidazolium chloride on the maximum amount of UO2

dissolution.

Recovery of uranium and Fe-containing ionic liquids

After the dissolution of UO2 in an Fe-containing ionic liquid, itis necessary to separate the dissolved uranium from the ionicliquid so that the ionic liquid can be recycled. Upon cooling, itis not uncommon for yellow uranium compound powders toprecipitate from the reaction medium,37 and therefore, someuranium can be separated from the IL by centrifugation. Theprecipitate was washed with a small amount of acetone severaltimes, filtered with an acetonitrile solution and then dried to

Paper Dalton Transactions

8414 | Dalton Trans., 2013, 42, 8413–8419 This journal is © The Royal Society of Chemistry 2013

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article Online

obtain a pure solid for elemental analysis (EA). Based on thesimple chemical method for recovering [Bmim]FeCl4 from ahomogenous mixture,38 CHCl3 or CCl4 and a FeCl3 hydro-chloric acid solution were used to recover the Fe-containingionic liquid and the uranium dissolved in the ionic liquid. TheFeCl3 hydrochloric acid solution was prepared by introducingFeCl3 into 6 mol L−1 hydrochloric acid to reach saturation. Amixture of CCl4 and n-heptane with the volume ratio of 4 : 1was prepared to aggregate the IL phase. A recovery experimentwas carried out as follows: after the uranium compound pre-cipitates had been removed, 300 μL of the reaction solutionwas added to 4.8 mL of the CCl4 and n-heptane mixture in a10 mL centrifuge tube. Then, 1.2 mL of the FeCl3 hydrochloricacid solution was introduced into the tube. The tube was vigor-ously agitated for five minutes, and subsequently was centri-fuged at the speed of 4000 rpm for 10 minutes. The process ofagitation and centrifugation was repeated twice. A three-layermixture formed. After removing the aqueous solution, the ILcan be recovered after the evaporation of organic solvents. Theuranium, which was extracted into the aqueous solution, wasseparated from iron by the use of a carbonate solution.39–41

The aqueous solution was diluted five-fold with ultrapurewater and then treated with pH = 10.35 guanidine carbonatesolution. The solution was then centrifuged to separate theFe(OH)3 sediment from the uranium solution. The guanidinecarbonate solution was prepared by adjusting the amounts of0.5 mol L−1 guanidine carbonate solution and 0.5 mol L−1 HClsolution.

Spectrometry measurements

Raman spectra were acquired using samples contained insealed NMR tubes. Spectral positions were calibrated by refer-ence to the spectra of Si before the analysis of the uraniumsample containing ionic liquids. A 10× objective was used tofocus the laser beam onto the NMR tube containing thesample and the exposure time was 30 s. The spectra wereobtained under the same conditions. The intensities of thepeaks were obtained by fitting them with a Lorentzian func-tion. Infrared spectra over a 4000–700 cm−1 range were col-lected using ATR-IR, and 40 scans were taken at a 2 cm−1

resolution with a Happ-Genzel apodization.

Results and discussionICP-AES analysis

Analyses of metal species in solutions containing ionic liquidsby ICP-AES have been reported by several researchers.42,43

Ionic liquids are known to possess the physical properties ofboth organic and ionic species, which is different from water.For example, they have a distinct viscosity and surface tension,which as a result, alters their nebulization efficiencies andsample transport properties.43 Therefore, a similar amount ofIL was added into the uranium standard solution to offset theimpact of the IL. The emission intensity has a linear relation-ship with the uranium concentration. To test the accuracy of

the results, the actual dissolved uranium sample underwenta recovery test. The recovery rate (RR) was calculated asfollows44:

RR ¼ C2 � C1

C0� 100%

where C0 is the expected amount of uranium due to spiking,C1 is the amount of uranium in the original sample beforespiking, and C2 is the analyzed amount of uranium in thespiked sample. The recoveries for the spiked uranium samplewhich contained ionic liquid are 95–104%, indicating that theaddition method is reliable and accurate.

Dissolution of uranium dioxide in Fe-containing ILs

To investigate the effects that the different Fe-containing ILsand the temperature have on the UO2 dissolution rate, an Fe-containing ionic liquid (1 mL) and its corresponding imidazo-lium chloride IL at a molar ratio of 1 : 1 were used to dissolveUO2. The dissolution process was measured using ICP-AES toobtain the dissolved amount of uranium at different times asshown in Fig. 1, and % U dissolved represents the percentageof dissolved uranium species compared with the initialamount of uranium. When media of [Bmim]FeCl4/[Bmim]Cland [Emim]FeCl4/[Emim]Cl in a 140 °C oil bath were used,25.3 mg of UO2 (0.094 mmol) could almost fully dissolve inthe ILs during the first half an hour; however, an hour and ahalf was needed when a medium of [Bdmim]FeCl4/[Bdmim]Clwas used.

Parallel experiments were set up at the same stirring speed,and therefore, the diffusion effect became comparablebetween experiments. In the present dissolution system, thestrong oxidant was in a large excess (the molar ratio of Fe andU was more than 40 : 1) and was miscible with its correspond-ing imidazolium chloride IL. At the beginning of the dissolu-tion process, the dissolved uranium percentage increasesexponentially, which is an experimental indication of first-order kinetics. A plot of ln[(C∞ − C)/C∞] versus time is shown

Fig. 1 % U dissolved in the Fe-containing IL and its corresponding chloride ILat a molar ratio of 1 : 1 at 140 °C as a function of time.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 8413–8419 | 8415

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article Online

in Fig. 2, where C is the concentration of the dissolveduranium at time t and C∞ is taken as the concentration at900 min. There is a linear relationship between ln[(C∞ − C)/C∞] and t, indicating that the initial dissolution appears tofollow pseudo first-order kinetics. The slope of the line is0.037 min−1, which may be regarded as the rate constant ofthe initial pseudo first-order dissolution process of UO2 in[Bdmim]FeCl4 containing an equimolar amount of [Bdmim]Clat 140 °C.

Moreover, the apparent activation energy was calculated byapplying the Arrhenius equation (Fig. 3). The dissolutionprocess is expected to involve a number of steps including thediffusion of FeCl4

− to the UO2 surface, the oxidation of UO2 touranyl on the UO2 surface, and the diffusion of uranyl speciesfrom the solid surface to the IL phase. It can be seen fromFig. 3 that the apparent activation energy obtained from UO2

dissolved in [Bdmim]FeCl4/[Bdmim]Cl is larger than the appa-rent activation energy obtained from the other mixed Fe-

containing IL systems. It has been reported that the ionicliquid’s viscosity is enhanced by a methyl substitution at theC2-position.45 As a result of the ionic liquid’s viscosity, thediffusion of the uranyl species from the UO2 surface to the ILphase of [Bdmim]FeCl4/[Bdmim]Cl was much harder. There-fore, the IL’s viscosity affects the diffusion of the uranylspecies from the UO2 surface to the IL phase and may furtherinfluence the dissolution rate of UO2.

To evaluate the maximum amount of UO2 dissolution inthe Fe-containing ILs, different amounts of UO2 were added tothe IL phase repeatedly every two hours. After the addition of alarge excess of UO2, a black slurry started to appear in the ILphase, suggesting that the system had reached its saturationpoint under the experimental conditions. We have found thatthe dissolution rate of UO2 increases with an increase in thetemperature. Therefore, we only studied the maximum dissolu-tion of UO2 at 160 °C. Without the imidazolium chloride ILin the UO2 reaction system, the maximum amount of UO2 dis-solution in 0.5 mL of [Bmim]FeCl4 at 160 °C was about 3.6 mg.When the imidazolium chloride IL was added to the reactionsystem, the maximum amount of UO2 dissolution was about232.1 mg (0.86 mmol). This was achieved in 0.5 mL of a solu-tion of [Bmim]FeCl4 (2.0 mmol) that contained a molaramount of [Bmim]Cl (4.0 mmol) which was twice that of[Bmim]FeCl4 . When the amount of the Fe-containing IL isequal to that of its imidazolium chloride IL, as shown inFig. 4, the maximum amount of UO2 dissolved in the Fe-containing IL system increases with an increase in the molaramount of the imidazolium chloride IL over a certain range(when the molar ratio of Cl− : FeCl4

− is between 0 and 2). Fordetailed information on the maximum dissolution amount ofUO2 in these ILs, see Table S1 in ESI.†

The direct dissolution of UO2 in ionic liquids using concen-trated nitric acid or a TBP(HNO3)1.8(H2O)0.6 complex has beenreported by several researchers.17–19 The maximum amount ofUO2 dissolution is about 320 mg in 3.6 mL of [Bmim][Tf2N]containing 16.7% by volume of the TBP(HNO3)1.8(H2O)0.6complex. Therefore, the Fe-containing ionic liquids can be

Fig. 2 A plot of ln[(C∞ − C)/C∞] versus time for the dissolution of UO2 in[Bdmim]FeCl4 containing an equimolar amount of [Bdmim]Cl at 140 °C.

Fig. 3 The dissolution rate as a function of 1/T in different Fe-containing ILswith their corresponding chloride ILs at a molar ratio of 1 : 1; ▲ [Emim]FeCl4, ●

[Bmim]FeCl4, ■ [Bdmim]FeCl4. From the slopes of these curves, apparent acti-vation energies (Eapp) are obtained by applying the Arrhenius equationln κ2

κ1¼ � Ea

R1T2� 1

T1

� �.

Fig. 4 The maximum dissolution amount of UO2 in 0.5 mL [Bmim]FeCl4 withdifferent molar amounts of [Bmim]Cl at 160 °C.

Paper Dalton Transactions

8416 | Dalton Trans., 2013, 42, 8413–8419 This journal is © The Royal Society of Chemistry 2013

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article Online

used as effective oxidants for the dissolution of UO2 in thepresence of imidazolium chlorides without an additionaloxidant, and they can be used as not only solvents but also asoxidants.

An investigation into the dissolution mechanism by aspectroscopic study

The infrared-active vas(UO2) and Raman-active vs(UO2) modesare sensitive to changes in the uranyl(VI) coordination environ-ment, shifting to a lower wavenumber when the coordinationof uranyl(VI) by a ligand weakens the OvUvO bands. Recently,it has also been reported that ATR-IR was used to characterizeuranyl(VI) nitrate complexes in ILs,36 and accordingly, weadopted ATR-IR to characterize the uranyl complexes in the Fe-containing ILs. Fig. 5 shows the infrared spectra of [Emim]-FeCl4 with and without dissolved uranyl species. The peak at914 cm−1 appearing in the absorption spectra for the UO2 dis-solution system is a very common value for the antisymmetri-cal OvUvO stretching vibrations in uranyl compounds, andis roughly the same as the literature value.46,47 Moreover,characteristic vibrations of the imidazole cation were detected.The vibrational bands at 829 and 741 cm−1 in the infraredspectra for [Emim]FeCl4 should only be assigned to the in-plane and out-of-plane flexural vibration mode of the imidazo-lium ring, respectively.48 After UO2 was oxidized in the [Emim]-FeCl4 and [Emim]Cl ILs, it is possible that the joint factorsfrom the new solvent environment and the more complicatedinteraction of ions led to changes in the flexural vibrationbands of the imidazolium ring.49–51 The infrared spectra forthe UO2 dissolution in the other two Fe-containing ionicliquids are given in the ESI† (Fig. S1 and S2).

In addition, Raman scattering was used to detect thecoordination of the uranyl complexes and most importantly toanalyze the structures of the iron ions after the dissolution ofUO2 in the Fe-containing ionic liquids. Fig. 6 shows Ramanspectra for the Fe-containing ionic liquids. The feature at333 cm−1 in these ionic liquids corresponds very well with

literature value for FeCl4− from the [Bmim]FeCl4 ionic liquid.

22

Fig. 6b shows an obvious uranyl peak at 832 cm−1, whichmatches very well with the FT Raman spectrum for [UO2Cl4]

2−

in a basic aluminum chloride-1-ethyl-3-methylimidazoliumchloride ionic liquid.52 When the intensity of the Ramanspectra (Fig. 6a) is magnified by approximately 9.5 times,

Fig. 5 IR spectra for (a) [Emim]FeCl4 only, (b) 150 mg UO2 dissolved in themixture of [Emim]FeCl4 and [Emim]Cl (molar ratio: 1 : 0.5).

Fig. 6 Raman spectra for the ionic liquids, from top to bottom (a, b and c):[Emim]Cl only, [Emim]FeCl4 only, 150 mg UO2 dissolved in the mixture of [Emim]-FeCl4 and [Emim]Cl (molar ratio: 1 : 2) in a 160 °C oil bath. The intensity axis onplot b is magnified by almost 4.5 times. The intensity axis on plot c is magnifiedby about 9.5 times.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 8413–8419 | 8417

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article Online

a new peak at approximately 263 cm−1 is visible, as shown inFig. 6c, which is most likely due to the A1 vibration of FeCl4

2−.This assignment is consistent with that found in the literature,where the A1 feature has been reported at 265 cm−1 in a[Bmim]2FeCl4 ionic liquid.22 Also, upon the addition of asmall amount of the reaction sample to a K3Fe(CN)6 solution,a dark-blue deposit formed from the solution which proves thepresence of Fe(II). In UO2 reaction systems that did not containthe imidazolium chloride IL, there was a lower amount of thedark-blue deposit. This indicates that the imidazolium chlor-ide IL contributes to the effective dissolution of UO2 in the Fe-containing ILs. According to the Raman spectra, FeCl4

2− is thepredominant reduction Fe-containing product after the dis-solution of UO2 in the mixed ILs. The addition of imidazoliumchloride favored the dissolution of UO2, and therefore it canbe postulated that the existence of imidazolium chloride pro-moted the formation of FeCl4

2−. Consequently, the dissolutionprocess of UO2 in an Fe-containing IL may go through the fol-lowing process, represented by eqn (1):

UO2 þ 2FeCl4� þ 4Cl� ! UO2Cl42� þ 2FeCl42� ð1Þ

The separation of the Fe-containing ILs and uranium

Uranyl(VI) species have been reported to precipitate from aroom temperature ionic liquid medium upon cooling, and theamount of precipitate varies even under similar reaction con-ditions.37 This phenomenon was also present in our study. Atthe very most, nearly 87% dissolved uranium can precipitatein the reaction system. The uranium precipitate can be sepa-rated from the ILs by centrifugation. The precipitate which wasin the ILs was analyzed by Raman scattering in NMR tubes,and the uranyl peak was found at 829 cm−1. After purification,the pure precipitate was characterized by EA and is consideredto be [imidazolium]2[UO2Cl4]. Elemental analysis (%) calcd forthe precipitate [Bmim]2[UO2Cl4]: N 8.12, C 27.83, H 4.35;found: N 8.15, C 27.93, H 4.38.

According to Wang et al.’s report, inorganic salts andorganic solvents were successfully used to recover [Bmim]FeCl4from a homogeneous mixture of [Bmim]FeCl4 and H2O.

38 Theorganic solvents of CHCl3 or CCl4 and inorganic salts includ-ing anion Cl− synergistically promoted the phase division. Theaddition of Cl− anions favored the backward direction of theequilibrium in eqn (2).

½Bmim�FeCl4 Ð Bmimþ þ FeCl4�

Ð Bmimþ þ Fe3þ þ 4Cl� ð2ÞIn view of the simple chemical recovery method, CHCl3 or

CCl4 and a FeCl3 hydrochloric acid solution were used torecover the Fe-containing ionic liquid and the uranium dis-solved in the ionic liquid. The uranium dissolved in the IL canbe fully extracted into an aqueous phase unless the volumeratio of the FeCl3 hydrochloric acid solution to the reactionmixture is greater than 4, and accordingly, the additionvolume of the organic solvents should be four times greaterthan the aqueous volume so as to aggregate the ILs well.

During the recovery process, a three-layer mixture formed, inthe order of aqueous solution followed by organic solvent fol-lowed by IL from top to bottom, which is consistent with theorder of their densities. After the recovery process, the additionof a small amount of the Fe-containing ionic liquid to the pot-assium hexacyanoferrate(III) solution, did not produce a dark-blue deposit, indicating that the Fe(II) had been oxidized toFe(III) by air during the recovery process. The collected Fe-containing IL was analyzed by ESI-MS and Raman scattering.The feature at 333 cm−1 in the Raman spectra correspondsvery well with the literature value for FeCl4

− 22 and the ESI-MSspectra were the same as that for the unreacted Fe-containingionic liquids. This shows that the recovery of the Fe-containingILs was successful, and moreover, almost 95% Fe-containingILs can be recovered.

The uranium extracted into the aqueous solution was sepa-rated from Fe by the use of carbonation.39–41 Within themedium of carbonate, the amount of uranium absorbed byFe(OH)3 fluctuated according to pH.41 At a pH of almost 10.35,uranium was separated from iron to the greatest degree in ourwork; almost 98.7% uranium was parted from the Fe(OH)3sediment in the form of a uranyl carbonate complex.

Conclusion

The dissolution of UO2 has been successfully achieved inimidazolium-based Fe-containing ILs with the help of theircorresponding imidazolium chloride ILs without additionaloxidants. The dissolution process follows pseudo first-orderkinetics initially. The maximum amount of UO2 dissolution isabout 232.1 mg (0.86 mmol), and was achieved in 0.5 mL of asolution of [Bmim]FeCl4 (2.0 mmol) that contained a molaramount of [Bmim]Cl (4.0 mmol) which was twice that of[Bmim]FeCl4. Raman spectra show that FeCl4

2− is the predomi-nant reductive iron-containing product after UO2 dissolution.The dissolved uranyl species can be successfully separatedfrom the Fe-containing ILs and the Fe-containing ILs can alsobe recovered. Our work may provide a route to the direct dis-solution of spent nuclear fuels in dry ILs, and further researchin this area is currently in progress.

Acknowledgements

We are very grateful to the National Science Foundation ofChina (Grant No. 91026011) for financial support.

References

1 X. Sun, H. Luo and S. Dai, Chem. Rev., 2011, 112, 2100–2128.

2 L. Berthon, S. I. Nikitenko, I. Bisel, C. Berthon, M. Faucon,B. Saucerotte, N. Zorz and P. Moisy, Dalton Trans., 2006,2526–2534.

3 J. F. Wishart, Energy Environ. Sci., 2009, 2, 956.

Paper Dalton Transactions

8418 | Dalton Trans., 2013, 42, 8413–8419 This journal is © The Royal Society of Chemistry 2013

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article Online

4 Y.-F. Hu, Z.-C. Liu, C.-M. Xu and X.-M. Zhang, Chem. Soc.Rev., 2011, 40, 3802–3823.

5 A. Ouadi, B. Gadenne, P. Hesemann, J. J. E. Moreau,I. Billard, C. Gaillard, S. Mekki and G. Moutiers, Chem.–Eur. J., 2006, 12, 3074–3081.

6 I. Billard, A. Ouadi and C. Gaillard, Anal. Bioanal. Chem.,2011, 400, 1555–1566.

7 S. Aparicio, M. Atilhan and F. Karadas, Ind. Eng. Chem.Res., 2010, 49, 9580–9595.

8 A. P. Abbott, G. Frisch, J. Hartley and K. S. Ryder, GreenChem., 2011, 13, 471–481.

9 M. Srncik, D. Kogelnig, A. Stojanovic, W. Korner,R. Krachler and G. Wallner, Appl. Radiat. Isot., 2009, 67,2146–2149.

10 N. Asanuma, Y. Takahashi and Y. Ikeda, Prog. Nucl. Energy,2011, 53, 944–947.

11 A. Rout, K. A. Venkatesan, T. G. Srinivasan and P. R.Vasudeva Rao, Sep. Purif. Technol., 2012, 97, 164–171.

12 A. Marciniak, Fluid Phase Equilib., 2010, 294, 213–233.13 K. Binnemans, Chem. Rev., 2007, 107, 2592–2614.14 W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris Jr,

T. E. Sutto, J. Callahan, P. C. Trulove, H. C. DeLong andD. M. Fox, Thermochim. Acta, 2004, 409, 3–11.

15 K. Servaes, C. Hennig, I. Billard, C. Gaillard, K. Binnemans,C. Görller-Walrand and R. Van Deun, Eur. J. Inorg. Chem.,2007, 2007, 5120–5126.

16 S. Dai, Y. S. Shin, L. M. Toth and C. E. Barnes, Inorg.Chem., 1997, 36, 4900–4902.

17 A. E. Bradley, C. Hardacre, M. Nieuwenhuyzen, W. R.Pitner, D. Sanders, K. R. Seddon and R. C. Thied, Inorg.Chem., 2004, 43, 2503–2514.

18 C. M. Wai, Y.-J. Liao, W. Liao, G. Tian, R. S. Addleman,D. Quach and S. P. Pasilis, Dalton Trans., 2011, 40,5039–5045.

19 I. Billard, C. Gaillard and C. Hennig, Dalton Trans., 2007,4214–4221.

20 P. Nockemann, B. Thijs, S. Pittois, J. Thoen, C. Glorieux,K. Van Hecke, L. Van Meervelt, B. Kirchner andK. Binnemans, J. Phys. Chem. B, 2006, 110, 20978–20992.

21 R. Ram, F. Charalambous, J. Tardio and S. Bhargava, Hydro-metallurgy, 2011, 109, 125–130.

22 M. S. Sitze, E. R. Schreiter, E. V. Patterson andR. G. Freeman, Inorg. Chem., 2001, 40, 2298–2304.

23 H. Wang, R. Yan, Z. Li, X. Zhang and S. Zhang, Catal.Commun., 2010, 11, 763–767.

24 S. Hayashi and H. O. Hamaguchi, Chem. Lett., 2004, 33,1590–1591.

25 T. Bäcker, O. Breunig, M. Valldor, K. Merz, V. Vasylyeva andA. V. Mudring, Cryst. Growth Des., 2011, 11, 2564–2571.

26 S. Zhu, Y. Wu, Q. Chen, Z. Yu, C. Wang, S. Jin, Y. Ding andG. Wu, Green Chem., 2006, 8, 325–327.

27 K. Bica and P. Gaertner, Org. Lett., 2006, 8, 733–735.28 S. H. Lee, S. H. Ha, S. S. Ha, H. B. Jin, C. Y. You and

Y. M. Koo, J. Appl. Phys., 2007, 101.

29 J.-Y. Kim, J.-T. Kim, E.-A. Song, Y.-K. Min and H.-o.Hamaguchi, Macromolecules, 2008, 41, 2886–2889.

30 S. Shang, L. Li, X. Yang and L. Zheng, J. Colloid InterfaceSci., 2009, 333, 415–418.

31 L. Li, Y. Huang, G. P. Yan, F. J. Liu, Z. L. Huang andZ. B. Ma, Mater. Lett., 2009, 63, 8–10.

32 C. Gaillard, A. Chaumont, I. Billard, C. Hennig, A. Ouadi,S. Georg and G. Wipff, Inorg. Chem., 2010, 49, 6484–6494.

33 Y. Ikeda, K. Hiroe, N. Asanuma and A. Shirai, J. Nucl. Sci.Technol., 2009, 46, 158–162.

34 P. Nockemann, K. Servaes, R. Van Deun, K. Van Hecke,L. Van Meervelt, K. Binnemans and C. Görller-Walrand,Inorg. Chem., 2007, 46, 11335–11344.

35 A. V. Mudring, in Ionic Liquids, ed. B. Kirchner, 2009,vol. 290, pp. 285–310.

36 D. L. Quach, C. M. Wai and S. P. Pasilis, Inorg. Chem., 2010,49, 8568–8572.

37 A. E. Bradley, J. E. Hatter, M. Nieuwenhuyzen, W. R. Pitner,K. R. Seddon and R. C. Thied, Inorg. Chem., 2002, 41,1692–1694.

38 M. Wang, B. Li, C. J. Zhao, X. Z. Qian, Y. L. Xu andG. R. Chen, Korean J. Chem. Eng., 2010, 27, 1275–1277.

39 H. Shlewit and M. Alibrahim, J. Radioanal. Nucl. Chem.,2008, 275, 97–100.

40 R. K. Singhal, U. Narayanan, R. Karpe, A. Kumar, A. Ranadeand V. Ramachandran, Appl. Radiat. Isot., 2009, 67,501–505.

41 M. Wazne, G. P. Korfiatis and X. G. Meng, Environ. Sci.Technol., 2003, 37, 3619–3624.

42 A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed andP. Shikotra, Inorg. Chem., 2005, 44, 6497–6499.

43 J. A. Whitehead, G. A. Lawrance and A. McCluskey,Aust. J. Chem., 2004, 57, 151–155.

44 E. Prichard and V. Barwick, in Quality Assurance in Ana-lytical Chemistry, John Wiley & Sons, Ltd, 2007, pp. 51–98.

45 B. Chen, Q. Long and B. Z. Zheng, Prog. Chem., 2012, 24,225–234.

46 J. A. Danis, M. R. Lin, B. L. Scott, B. W. Eichhorn andW. H. Runde, Inorg. Chem., 2001, 40, 3389–3394.

47 C. D. Flint and P. A. Tanner, J. Chem. Soc., Faraday Trans. 2,1982, 78, 953–958.

48 S. Tait and R. A. Osteryoung, Inorg. Chem., 1984, 23,4352–4360.

49 M. O. Sornein, C. Cannes, C. Le Naour, G. Lagarde,E. Simoni and J. C. Berthet, Inorg. Chem., 2006, 45, 10419–10421.

50 M. Deetlefs, C. L. Hussey, T. J. Mohammed, K. R. Seddon,J. A. van Den Berg and J. A. Zora, Dalton Trans., 2006,2334–2341.

51 P. B. Hitchcock, T. J. Mohammed, K. R. Seddon, J. A. Zora,C. L. Hussey and E. Haynes Ward, Inorg. Chim. Acta, 1986,113, L25–L26.

52 T. A. Hopkins, J. M. Berg, D. A. Costa, W. H. Smith andH. J. Dewey, Inorg. Chem., 2001, 40, 1820–1825.

Dalton Transactions Paper

This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 8413–8419 | 8419

Publ

ishe

d on

21

Mar

ch 2

013.

Dow

nloa

ded

by L

omon

osov

Mos

cow

Sta

te U

nive

rsity

on

18/1

2/20

13 2

1:23

:00.

View Article Online