Evaluation of the Behavior of Uranium Peroxocarbonate Complexesin Na−U(VI)−CO3−OH−H2O2 Solutions by Raman SpectroscopyKwang-Wook Kim,* Euo-Chang Jung, Keun-Young Lee, Hye-Ryun Cho, Eil-Hee Lee,and Dong-Yong Chung

Korea Atomic Energy Research Institute, Daedeok-daro 989-111, Yuseong-gu, Daejeon, 305-353, Republic of Korea

ABSTRACT: In this work, the formation of uranium species and theirstabilities in Na−U(VI)−CO3−OH−H2O2 solutions at different pHs arestudied by Raman spectroscopy. In this solution, the UO2(O2)(CO3)2

4− specieswas formed together with three other uranium species of UO2(O2)2

2−,UO2(CO3)3

4−, and a speculated uranium species of the uranyl carbonatehydroxide complex, UO2(CO3)x(OH)y

2−2x−y, which showed remarkable Ramanpeaks at approximately 769, 848, 811, and 727 cm−1, respectively. TheUO2(O2)(CO3)2

4− species disappeared at pH conditions where bicarbonatedominated, and its Raman peak could be clearly observed in only a narrow pHrange from approximately 9 to 12. When the pH of the solution increasedfurther, the UO2(O2)(CO3)2

4− species changed to UO2(CO3)34− and the

UO2(CO3)x(OH)y2−2x−y species. Moreover, the UO2(O2)(CO3)2

4− speciescontinuously decomposed into uranyl tricarbonate in the carbonate solutionat an elevated temperature because of the instability of the peroxide ion, O2

2−, in alkaline conditions.

1. INTRODUCTION

When nuclear spent fuel is disposed of in a geologicalrepository, uranium peroxide material of studtite, UO2(O2)-(H2O)4, with a very low solubility is found on the surface ofnuclear spent fuel contacting with water.1−4 Peroxide ion, O2

2−,which is generated as a radiolysis product of water, stronglycombines with uranium in the form of the uranium peroxidecompound, which is an important alternation of nuclear waste.This could have a significant impact on the dissolution andmigration behavior of actinides in geological repositories fornuclear waste disposal.1,2 Additionally, uranium is selectivelydissolved to form uranyl peroxocarbonate species with a highsolubility, UO2(O2)x(CO3)y

2−2x−2y, in a carbonate solutioncontaining hydrogen peroxide, H2O2. In contrast to the uranylperoxocarbonate complex, most transition-metal ions have verylow solubilities in such an alkaline carbonate solution and donot form metal peroxo complex ions.5−15 Uranium in the mixedsolution of hydrogen peroxide and sodium carbonate can beprecipitated as uranium peroxide when the solution is acidifiedto a pH of 2−3.10−12 On the basis of these chemicalcharacteristics of uranium, several carbonate-based processeshave been studied and suggested to treat uranium-bearingwastes, uranium scraps generated during uranium fuelfabrication, uranium sludge, and spent nuclear fuel.8,10−17 Forthese reasons, actinide peroxide chemistry has recently attractedsignificant attention.Hydrogen peroxide is easily decomposed in heated alkaline

solutions.2,18,19 Additionally, the peroxide ion is not stable, andthe uranium peroxocarbonate complex can self-decompose intouranyl tricarbonate complex, UO2(CO3)3

4−, that is stable in acarbonate solution.20 The chemical speciation of uranyl

carbonate, uranyl hydroxide, and uranyl peroxide species atdifferent pHs has been intensively studied in near-neutral mediaby many workers.2,21−34 However, there are only a limitednumbers of studies on the stability and the speciation of uranylperoxocarbonate and uranyl tricarbonate in carbonate solutionsat high pH values.2,20 The uranyl peroxide carbonate system isinherently complex because of the formation of mixed ligands,mono- and polynuclear uranyl complexes and the catalyzeddecomposition of peroxide in alkaline media. When the uranylperoxocarbonate complex is changed into other species, thedetermination of the chemical species by means of conventionalUV−visible spectrophotometry is very difficult because the veryintense and wide absorption band of the uranyl peroxocar-bonate complex may strongly interfere with the neighboringabsorption bands of other species, such as uranyl tricarbonate.The molar absorptivity of uranyl peroxocarbonate complexes ismuch higher than that of uranyl tricarbonate complexes.2

Raman spectroscopy has been proven to be one of the mostuseful tools for the investigation of molecular structure andsolution speciation.1−3,27,31−34 However, most Raman spectro-scopic data on uranium to date have been obtained for uranylcarbonate, uranyl hydroxide, and uranyl peroxide species inneutral pH conditions rather than for uranyl peroxocarbonatein high pH conditions. In this study, the characteristic behaviorof uranium peroxocarbonate complexes in a carbonate solutionat different pH values and the stability of the complexes wereinvestigated by Raman spectroscopy.

Received: July 17, 2012Revised: November 25, 2012Published: November 26, 2012

Article

pubs.acs.org/JPCA

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2. EXPERIMENTAL SECTION

All reagents used in this work were of reagent grade, and theywere dissolved in demineralized water prepared by doubledistillation followed by ion exchange (Milli-Q plus; 18.2 MΩ).A solution containing uranyl peroxocarbonate complexes,UO2(O2)x(CO3)y

2−2x−2y, with a uranium concentration of 42mM was prepared by dissolving UO2 powder in a mixedsolution of 0.5 M Na2CO3 and 1 M H2O2 at room temperature.The UO2 powder with a particle diameter of approximately 9μm was completely dissolved within approximately 10 min. The1 M concentration of H2O2 used for the dissolution of UO2powder in a 0.5 M Na2CO3 solution represents only the initialconcentration used for the preparation of the uranylperoxocarbonate complex solution. The H2O2 plays two rolesas an oxidant during the dissolution of UO2 in carbonatesolution and the source of a ligand to form uranylperoxocarbonate complex ions. The free H2O2, which doesnot contribute to form the uranium peroxocarbonate complex,is known to be almost decomposed within a few hours.20

Therefore, the peroxide component, O22−, in the solution a

sufficient time later after the preparation of uranyl peroxocar-bonate complex solution exists only as a ligand of the uranylperoxocarbonate complex where UO2

2+ and O22− combine in

the stoichiometric molar ratio of 1:1.2,20,26 The pHs of prepareduranium solutions were controlled by adding HNO3 or NaOHsolutions and measuring them by using a pH meter (Orion,Ross).A solution containing only UO2(CO3)3

4− was prepared bythe dissolution of uranium dinitrate hexahydrate (UNH,UO2(NO3)26H2O) in 0.5 M Na2CO3 with a uraniumconcentration of 42 mM instead of UO2 powder because thedissolution rate of UO2 powder in Na2CO3 solution withoutH2O2 was too slow. The uranyl peroxocarbonate and uranyltricarbonate tinged the solutions orange−red and bright yellow,respectively, and the chemical speciation of these species in theprepared solutions was first confirmed by using a UV−visiblespectrophotometer (Hewlett-Packard 8453). Figure 1 showsthe representative absorption spectra of UO2(CO3)3

4− andUO2(O2)(CO3)2

4− species in a 0.5 M carbonate solution. Theuranyl peroxocarbonate complex with a uranium concentrationof 0.42 mM showed a strong peak at approximately 340 nmwith a molar absorptivity of 1063 M−1 cm−1, which was similarto the reported value2 of 1023 ± 19 M−1 cm−1. The uranyl

tricarbonate complex with a uranium concentration of 12 mMshowed the well-known peaks at 423, 435, 448, and 462 nmwith a molar absorptivity of 26.7 M−1 cm−1 at 448 nm.2,13,20,34

To investigate the stability of uranyl peroxocarbonatecomplex in the carbonate solution, Raman spectra of thesolutions at room temperature and 60 °C were regularlymeasured for a period of several days. Micro-Raman spectros-copy with an optical microscope (Olympus, BX43) and aspectrometer based on the Czerny−Turner optical design(Andor, SR-500) was adopted in this work. The Ar+ laser(Coherent, Innova 70) beam at the wavelength of 514.5 nmwas used as the light source. The laser beam was focused usingan objective lens with a magnifying power of 10. The samplesolution of 0.2 mL was put into a capillary quartz tube (innerdiameter 1.6 mm, outer diameter 3.3 mm, and length 100 mm),and the backscattered light from the sample was collected intoan optical fiber with a core diameter of 0.4 mm, which wasconnected to an inlet slit of the spectrometer. The Ramansignal was recorded with a CCD detector (Andor, DV401A-BV) that was placed at the outlet slit of the spectrometer. Toreduce the background noise, the detector was cooled to 40 °Cbelow zero. Each pixel received the signal for 10 s, and its 50times accumulated data were stored to record a Ramanspectrum. The wavenumber of the Raman spectra measured inthis work was calibrated by using well-known Raman peaks of acyclohexane (C6H12) solution.

3. RESULTS AND DISCUSSION

3.1. Evaluation of Raman Spectra for Uranyl Tricar-bonate and Uranyl Peroxocarbonate Complexes.When acarbonate-based uranium leaching process is applied to treaturanium-bearing compounds, a high-pH carbonate solutioncontaining H2O2 is used to selectively dissolve uranium fromuranium mixture materials, which results in producingUO2(O2)x(CO3)y

2−2x−2y species, as shown in eq 1.8,11−13,17 Inthe initial carbonate solution of 1.0 M H2O2−0.5 M Na2CO3used in this work, it was confirmed that the uranylperoxocarbonate species existed in the form of UO2(O2)-(CO3)2

4−, with x/y= 1/2 in eq 1, as in previous works.2,20,34

The existence of this species was confirmed by measuring thesolution absorption spectra with a varying mole ratio of uranyland hydrogen peroxide2 or by measuring concentrations ofuranyl and peroxide ion.20 In other words, the stoichiometricmolar ratio of UO2

2+ and O22− in UO2(O2)x(CO3)y

2−2x−2y is 1:1for that carbonate solution condition.

+ + +

= + +

− −

− − −

y x x

x

UO CO H O 2 OH

[UO (O ) (CO ) ] 2 H O 2ex yx y

2 32

2 2

2 2 32 2 2

2 (1)

where y = 0, 1, and 2 and x/y = 1/2, 2/1, and 3/0.It is well-known that the peroxo component of the uranyl

peroxocarbonate complex ion can easily be decomposed at highpH values and elevated temperatures.18−20 Thus, it can bechanged into other uranium species, depending on the pH ofthe solution. This effect results in the coexistence of severaldifferent uranium species in the solution. However, there arefew studies on the chemical behaviors of uranyl peroxocar-bonate species at various pHs or of its stability, especially usingRaman spectroscopy.Figure 2 shows the measured Raman spectra of basic

components in the sample solutions used in this work. Themain bands observed in the Raman spectra are from the O−O

Figure 1. UV−visible absorption spectra of UO2(CO3)34− and

UO2(O2)(CO3)24− species in a 0.5 M carbonate solution.

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stretch in H2O2 at 874 cm−1, the C−OH stretch in HCO3− at

1014 cm−1, the C−O stretch in CO32− at 1066 cm−1, the N−O

stretch in NO3− at 1049 cm−1, and the symmetric OUO

stretch in UO22+ at 870 cm−1. In the case of NO3

− in Figure2D, the other tiny Raman band was observed at 718 cm−1,which will be discussed in detail later. In Figure 2E, the NO3

−

band was detected together with the UO22+ band because a

uranium nitrate solution (UO2(NO3)26H2O) was used. TheRaman shifts of these basic components agreed well with thosereported in previous works.2,27,35−37

Figure 3A and B shows the Raman spectra measured for twodifferent solutions containing UO2(O2)(CO3)2

4− andUO2(CO3)3

4−, respectively. In Figure 3A, the sample solutionobtained just after dissolving UO2 powder in 0.5 M Na2CO3containing 1.0 M H2O2 shows the Raman bands of H2O2 andCO3

2− at 874 and 1066 cm−1, respectively (the lower spectrumin Figure 3A). In this spectrum, however, the Raman band ofUO2(O2)(CO3)2

4− was not clearly detected because of bubblesbeing evolved in the solution due to decomposition of H2O2.One day later, when the H2O2 bubble evolution disappeared,the distinctive Raman band of stabilized UO2(O2)(CO3)2

4− wasobserved at 769 cm−1 (the upper spectrum in Figure 3A),which agreed with the results reported in previous works.2,20

However, two unidentified Raman bands, which are denoted asband 1 and band 2 in Figure 3A, were observed atapproximately 848 and 727 cm−1, respectively. To the best ofour knowledge, these newly observed Raman bands have notbeen reported to date. The identification of these unknownbands will be provided in the forthcoming explanation ofFigures 5−8 in this paper. In Figure 3B, the Raman band of theUO2(CO3)3

4− in 0.5 M Na2CO3 solution was observed at 811cm−1, which was in agreement with the results reported in aprevious work.33 It should be noted that an unidentified Ramanband denoted as band 2 was observed for both sample solutionswith different compositions in Figure 3A and B. Although thelocation of band 2 is slightly different between sample solutions,approximately 727 cm−1 in the carbonate solution with H2O2 atpH 11.4 (Figure 3A) and approximately 733 cm−1 in thecarbonate solution without H2O2 at pH 10.8 (Figure 3B), thisRaman band is considered to be ascribed to the samespecies (uranyl carbonate hydroxide complex of

UO2(CO3)x(OH)y2−2x−y) because this Raman band showed a

shift of location to the short wavelength direction withincreasing pH. The pH-dependent characteristics of this bandwill be discussed in detail later in Figure 8.To investigate the behavior of the uranyl peroxocarbonate

complex ion in bicarbonate media of HCO3−, the sample

solution prepared by dissolving UO2 in 0.5 M Na2CO3 with 1.0M H2O2 at pH 10.5 was pH-adjusted to approximately 8.5 byadding HNO3. After the pH-adjustment, the solution pH wasnot controlled anymore because it did not show a significantpH change due to some spontaneous change of solution afterthat. The Raman spectra for this sample are displayed in Figure4A. All Raman bands mentioned in Figure 3, UO2(O2)-(CO3)2

4− at 769 cm−1, UO2(CO3)34− at 811 cm−1, and the

unidentified bands at 848 and 727 cm−1, appeared simulta-neously (the lower spectrum in Figure 4A). Moreover, ashoulder on the high-wavenumber side of the band for theUO2(CO3)3

4− species appeared at approximately 819 cm−1,which disappeared in the spectrum measured after 5 days (theupper spectrum in Figure 4A). The transient shoulder of theUO2(CO3)3

4− at approximately 819 cm−1 is speculated to bedue to the uranyl dicarbonate complex, UO2(CO3)2

2−, that wasformed for a short time during the change of UO2(O2)-(CO3)2

4− into UO2(CO3)34−. It is well-known that the

UO2(CO3)22− predominates over a narrow range of bicar-

bonate pHs and ligand concentrations.29,33 Maya et al.33

reported an overlap of the Raman bands between 809 cm−1 ofthe UO2(CO3)3

4− complex and 834 cm−1 of the UO2(CO3)22−

Figure 2. Raman spectra of basic components used for evaluatinguranyl peroxocarbonate complex behavior.

Figure 3. Raman spectra of uranyl peroxocarbonate complex (A) anduranyl tricarbonate complex (B).

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complex when the UO2(CO3)22− solution changed to the

UO2(CO3)34− solution by adding excess bicarbonate in the

UO2(CO3)22− solution, which appeared as a small shoulder on

the low-frequency side of Raman band for the UO2(CO3)22−

complex. Similarly, the transient Raman band at around 819cm−1 as a shoulder on the UO2(CO3)3

4− Raman band in Figure4 is considered due to temporary appearance of UO2(CO3)2

2−

when the UO2(O2)(CO3)24− solution changed to the

UO2(CO3)34− solutions with pH adjustment from 10.5 to

approximately 8.5 by adding HNO3. In the Raman spectrummeasured 5 days after the sample preparation, the band at 769cm−1 corresponding to the UO2(O2)(CO3)2

4− species almostdisappeared (the upper spectrum in Figure 4A). It is apparentthat the band for UO2(CO3)3

4− was much more distinctlydeveloped with the disappearance of the high-wavenumber sideshoulder at 819 cm−1. In addition, the unidentified band 1 wasdeveloped as well with the disappearance of the H2O2 band.The upper spectrum represented in Figure 4A maintained itsshape and the ratios of the band intensities for a long time of upto 5 months. The disappearance of the bands representing theUO2(O2)(CO3)2

4− species and H2O2 are due to the self-decomposition of the peroxo component in the uranylperoxocarbonate complex as it converted to UO2(CO3)3

4−

and due to decomposition of H2O2 in the alkaline solution,respectively.18−20

In Figure 4A, it should be noted that Raman peaks at 1049cm−1 for NO3

− show the intensity difference with the lapse oftime in the same sample. The reason for this phenomenon isinterpreted as a kind of inner filter effect. As shown in Figure 1,the absorption of the laser beam at the wavelength of 514.5 nmis stronger for the solution containing UO2(O2)(CO3)2

4− thanthat for the solution containing UO2(CO3)3

4−. This effectresults in the weaker laser beam intensity at the focal point,where the Raman signal is generated, for the UO2(O2)-(CO3)2

4− solution than that for the UO2(CO3)34− solution.

Thus, the Raman intensity for NO3− increased with the lapse of

time accordingly as the UO2(O2)(CO3)24− species changed to

the UO2(CO3)34− species .

To confirm whether the uranyl peroxocarbonate complexdirectly forms in bicarbonate media or not, UO2 powder wasdissolved in 0.5 M NaHCO3 with 1.0 M H2O2. The final pH ofthis solution was approximately 9. The Raman spectra of thissolution are shown in Figure 4B. Just after the samplepreparation, the bands of UO2(CO3)3

4− species with thehigh-wavenumber side shoulders and H2O2 predominated, asshown in the lower spectrum in Figure 4B. In the spectrummeasured 5 days after the sample preparation, only the bandscorresponding to the UO2(CO3)3

4− species and the twounidentified species designated as bands 1 and 2 remained, asshown in the upper spectrum in Figure 4B. It is noteworthythat the band of the UO2(O2)(CO3)2

4− species is indistinctivefor both Raman spectra in Figure 4A and B. These resultssuggest that the uranyl peroxocarbonate complex cannot beformed in a solution in which bicarbonate ion dominates in thesolution, even though sufficient H2O2 exists in the solution.This will be discussed more in detail in Figure 8.

3.2. Identification of Uranyl Peroxide and UranylCarbonate Hydroxide Species in the UO2

2+−O22−−OH−−

CO32− System. To identify the species related to bands 1 and

2 observed in Figures 3 and 4 in which carbonate species areincluded in the solutions, Raman measurements of uraniumspecies in NaOH solutions at various pHs adjusted by theaddition of HNO3 without and with H2O2 were carried out, andtheir results are shown in Figure 5A and B, respectively. InFigure 5A, no Raman bands associated with uranium species inNaOH solution at different pHs from 8 to 13 were observed. Inthe inset of Figure 5A, which shows an enlargement of the smallbands from 650 to 800 cm−1, a Raman band of NO3

− wasdetected at 717 cm−1 without a peak shift in the pH range from8 to 13.35−37 The uranyl nitrate solution in Figure 5A wasprepared by adding UNH of 2 g/L in a NaOH solution at pH13, and then, the prepared solution pH was adjusted to lowerpH by adding HNO3. Accordingly, the uranyl nitrate solutionsat lower pHs showed a more distinct Raman band of NO3

−

because the solutions at lower pHs contained more NO3− in

the solution. In Figure 5B, no Raman band associated withuranium species was detected in the UO2

2+−OH− solution atpH 14, as depicted by the dotted line. While H2O2 was addedto the solution, the Raman band at 848 cm−1 denoted as band 1in Figures 3 and 4 appeared abruptly, as shown in the topspectrum in Figure 5B. Similar phenomena were observed forthe UO2

2+−OH− solutions with H2O2 at different pHs of 11.8and 12.5, as shown in the bottom and middle spectra,respectively. On the basis of the results of Figures 3−5, theuranium species corresponding to band 1 at 848 cm−1 isthought to be related to UO2

2+ and O22− species, that is, uranyl

Figure 4. (A) Changes in the Raman spectra of the uranylperoxocarbonate complex after pH adjustment (pH of 8.5) withtime. (B) Change in Raman spectra of the uranyl carbonate speciesdissolved in NaHCO3 solution with H2O2 at pH 9 with time.

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peroxide species, not uranyl species combined with CO32− or

OH− ions, because the band was detected in the UO22+−O2

2−−OH− and UO2

2+−O22−−OH−−CO3

2− systems among thesolutions containing UO2

2+−OH−, UO22+−O2

2−−OH−,UO2

2+−OH−−CO32−, and UO2

2+−O22−−OH−−CO3

2− inFigures 3−5. The uranyl peroxide is so far known to exist inthree forms, UO2(O2), UO2(O2)2

2−, and UO2(O2)34−. The

UO2(O2) is solid, and the UO2(O2)34− ion is known to be very

stable at pH values higher than 14. In contrast, the UO2(O2)22−

predominates in the pH range from 6 to 12.6,7 Accordingly, theRaman band at 848 cm−1 observed in the pH range from 9 to14, which is denoted as band 1 in this paper, represents theexistence of UO2(O2)2

2− species. A recent paper by Meca34

mentioned that the UO2(O2)2(OH)24− complex predominates

at H2O2 concentrations higher than 10−4 M and at a totalCO3

2− concentration lower than 5 × 10−4 M. However, theconcentration of CO3

2− used in this work is 1000 times higherthan this value. Thus, the existence of UO2(O2)2(OH)2

4−

species was not considered in this paper. Because the solutionchemistry of the uranyl peroxide system is not well-known atthis time, further study on the identification and thermody-namic characterization of U(VI)−H2O2 complexes in solutionshould be done.33 Another possibility of the Raman bandobserved at 848 cm−1 could be considered to be the

polynuclear uranyl peroxide species with big molecular weights,large cage clusters of uranium peroxide, which have beenrecently reported.39−41 However, these big polynuclear uranylperoxide species are formed only in specific solutions thatcontain high concentrations of uranyl nitrate (2 to 3 M) andhydrogen peroxide (∼10 M) in LiOH (∼4 M), accompanied byother agents in the solution, while the solution is evaporated fora long time at elevated temperature. Such solutions are quitedifferent from the solution system used in this work. Therefore,the possibility of existence of a polynuclear U(VI) complex inthis solution system was ruled out in this work.In the solutions of UO2

2+−OH− and UO22+−O2

2−−OH− inFigure 5, the Raman band denoted as band 2, which wasobserved in the solution systems of UO2

2+−O22−−OH−−

CO32− and UO2

2+−OH−−CO32− in Figures 3 and 4, was not

found. To identify this unknown band, Raman measurementsfor uranium nitrate samples in 0.5 M Na2CO3 without H2O2 atthree different pHs were carried out, and the results aredisplayed in Figure 6. The solutions were prepared by

dissolving uranium nitrate in 0.5 M Na2CO3 solution at theinitial pH of 11.5, followed by a pH adjustment to 9 and 7 byadding HNO3. All three solutions at pHs 7, 9, and 11.5 showdistinctive Raman bands of uranyl tricarbonate complex at 811cm−1, as shown in Figure 6. In the inset of Figure 6, whichshows an enlargement of the small bands from 650 to 800cm−1, in the initial solution at pH 11.5, the Raman band atapproximately 727 cm−1, denoted as band 2 in Figures 3 and 4,was observed with a very weak Raman band of NO3

− at 717cm−1. However, in the solutions with the pH adjusted to 7 and9 by adding HNO3, the band of NO3

− at 717 cm−1 was moredeveloped, and its tail reached the Raman band atapproximately 727 cm−1, resulting in the two bands apparentlyoverlapping each other. From these results, the Raman banddenoted as band 2 in Figures 3 and 4 is thought to beUO2(CO3)x(OH)y

2−2x−y or UO2(CO3)x2−2x because the band

was commonly found in solutions of UO22+−O2

2−−OH−−CO3

2− and UO22+−OH−−CO3

2− but not in solutions ofUO2

2+−OH− and UO22+−O2

2−−OH−. The uranyl carbonatecomplexes have, to date, been known to exist in the form ofUO2(CO3), UO2(CO3)2

2−, and UO2(CO3)34−. However,

UO2(CO3) is precipitated in the solution. In this work, onlysupernatants without any precipitate were used for the Raman

Figure 5. Raman spectra of uranium species in NaOH solutions atdifferent pHs without H2O2 (A) and with H2O2 (B) in the solutions.

Figure 6. Changes in the Raman spectra of the uranyl carbonatecomplex at different pHs.

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measurement, and the Raman bands of UO2(CO3)22− and

UO2(CO3)34− are positioned at approximately 810−830 cm−1,

as reported in the literature.33,38 Therefore, these uranylcarbonate complexes can be ruled out as a material related tothe unknown band 2 species. At this time, it can be consideredthat the Raman band observed at approximately 727 cm−1 isattributed to the UO2(CO3)x(OH)y

2−2x−2y complex species,even though its chemical form cannot be exactly assigned inthis work. The solution chemistry of UO2

2+−O22−−OH−−

CO32− and UO2

2+−OH−−CO32− is inherently complex, and

there can be various uranyl carbonate hydroxide complexspecies at high pH conditions.29,30,33,42−45 Thus, the exactidentification of the Raman band at 727 cm−1 was not furthercarried out in this work.The Raman data set illustrated as the lower spectrum in

Figure 4A, in which the Raman bands of all the uranium speciesconsidered in this work appeared together, is deconvoluted intoeach component in Figure 7. The solid and dotted lines

represent the measured spectrum and the calculated results,respectively. The identification of corresponding uraniumspecies and their positions are listed in Table 1.3.3. Behaviors of Raman Spectra for Uranium

Complex Species with pH and Time. The stability ofuranyl peroxocarbonate is examined as a function of pH andtime. Figure 8 shows the change of Raman spectra of theuranium samples in a mixed solution of 0.5 M Na2CO3 and 1.0M H2O2 with a change of pH from 9 to 14, in which the pHs ofthe solutions were controlled by NaOH or HNO3. The uranylperoxocarbonate solution at pH 10.6 prepared by dissolvingUO2 in 0.5 M Na2CO3 with 1.0 M H2O2 contained a dominanturanyl peroxocarbonate complex species, UO2(O2)(CO3)2

4−,together with UO2(O2)2

2− and UO2(CO3)x(OH)y2−2x−y spe-

cies, but the UO2(CO3)34− species barely existed. As the pH

increased, the UO2(O2)(CO3)24− changed into UO2(CO3)3

4−

because of decomposition of the peroxide ion, O22−, in the

highly alkaline solution, as reported in the literature.18−20

Above pH 12.4, the Raman band of the UO2(O2)(CO3)24− at

769 cm−1 almost disappeared, while the Raman band ofUO2(CO3)3

4− was clearly developed instead. At this condition,uranium was experimentally observed to begin to precipitate inthe solution. The uranium precipitate was known to beNa2U2Ox(OH)y(H2O)z with poor crystallinity in previousworks.20,28,41,46 The reason that the Raman band of theUO2(O2)2

2− species at 848 cm−1 can be observed even over pH14 while the Raman band of UO2(O2)(CO3)2

4− at 769 cm−1

disappeared to decompose into UO2(CO3)34− is considered to

be attributed to the difference in the stability constants of thosespecies.6,28 The stability constant (log K) of 72 for UO2(O2)2

2−

is much higher than that of 4 for UO2(O2)(CO3)24−. In Figure

8, the intensities of Raman bands of the uranium species greatlydecreased over pH 14 because of the lack of uranium species inthe solution due to the uranium precipitation. When the pHdecreased from 10.6 to 9.3, the pH at which the bicarbonatespecies predominates, the Raman band of UO2(O2)(CO3)2

4−

almost disappeared with the development of the Raman bandof UO2(CO3)3

4−, as shown in the bottom spectrum in Figure 8.These results suggest that the uranyl peroxocarbonate complexexists in a narrow pH range between 9 and 12. The reason thatthe uranyl peroxocarbonate complex does not exist in thebicarbonate pH range is considered to be because bicarbonate,HCO3

−, cannot bind with uranyl ion, UO22+, or peroxide ion,

O22−, as a ligand to form UO2(O2)(CO3)2

4− or UO2(CO3)34−.

No uranyl bicarbonate species has been known yet.44

As observed in Figure 8, the Raman bands of UO2(CO3)34−

and UO2(O2)22− at approximately 811 and 848 cm−1 were

observed over the whole pH range without a change in theirposition, while the Raman band at approximately 727 cm−1,which is thought to be UO2(CO3)x(OH)y

2−2x−y, was shifted upto 710 cm−1 in the direction toward shorter wavenumbers asthe pH increased. This phenomenon is similar to the shift ofthe Raman bands of uranyl hydrolysis species of

Figure 7. Deconvolution of the Raman spectrum obtained from theuranyl carbonate complex solution pH-adjusted to 8.5 in Figure 4A.

Table 1. Uranium Species and Their Raman Frequencies Considered in This Work

uranium species UO22+ UO2(CO3)3

4− UO2(CO3)22− a UO2(O2)(CO3)2

4− UO2(O2)22− UO2(CO3)x(OH)y

2−2x−y a

Raman frequency (cm−1) 870 811 819 769 848 727

aSpeculated species in this work.

Figure 8. Changes in the Raman spectra of the uranyl peroxocar-bonate complex at different pHs.

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(UO2)x(OH)y2x−y for increasing pH and the number of

coordinated ligands.2,3,26,33,47,48 The Raman bands of uranylhydroxo species with ligand numbers of 1, 2, 3, and 4 appearedat approximately 851, 833, 810, and 786 cm−1, respectively,with a shift in the direction toward shorter wavenumbers fromthe Raman band of UO2

2+ at 870 cm−1. On the basis of theseconsiderations, we speculate that the initial uranyl carbonatehydroxide species of UO2(CO3)x(OH)y

2−2x−y in the solution ofUO2

2+−O22−−OH−−CO3

2− at pH 10.6 may change to othercomplicated forms of UO2(CO3)x(OH)y

2−2x−y with a change inlocation of the Raman band in the direction of shorterwavenumbers as the pH increases up to 14. This effect isconsidered due to a slight change of ligands of OH− or CO3

2−

to UO22+ according to solution pH.

Figure 9 shows the stability of uranyl peroxocarbonatecomplexes at a pH of 10.6 and at 60 °C with a function of time.

The decomposition kinetics of the uranyl peroxocarbonatecomplex into the uranyl tricarbonate complex was studied inour previous paper,20 where its decomposition rates weremeasured at several temperatures. To accelerate the change ofthe uranyl peroxocarbonate complex, the solution was kept inan oven at 60 °C because the decomposition of the uranylperoxocarbonate complex at room temperature is relatively toolow.20 The relative ratio of the Raman band intensities ofUO2(O2)(CO3)2

4− at 769 cm−1 to UO2(CO3)34− at 811 cm−1

decreased with time. However, the intensities of the Ramanbands at 848 and 727 cm−1, which are thought to beUO2(O2)2

2− and UO2(CO3)x(OH)y2−2x−y, were almost not

changed without a significant shift of Raman bands. Theseresults indicate that only the uranyl peroxocarbonate complexcontinuously changes into the uranyl tricarbonate complexbecause of the instability of the peroxide ion component, O2

2−,in the uranyl peroxocarbonate complex species in alkalineconditions, but the peroxo component in uranyl peroxidespecies is known to be stable in alkaline conditions. Similarresults on the stability of the uranyl peroxide species in alkalineconditions were reported in the literature.6,7,26,34,49,50

4. CONCLUSIONSThe uranyl complex in the carbonate solution with hydrogenperoxide existed in various forms at different pH values. When a

uranyl peroxocarbonate complex was prepared in the CO32−−

H2O2 solution, the UO2(O2)(CO3)24− complex existed

together with three other species identified as UO2(O2)22− at

848 cm−1, UO2(CO3)34− at 811 cm−1, and a speculated

uranium species of UO2(CO3)x(OH)y2−2x−y in the wavenumber

range of 710−733 cm−1. It is confirmed that the uranylperoxocarbonate complex was not formed in the bicarbonatecondition, even though sufficient hydrogen peroxide existed inthe solution. The uranyl peroxocarbonate complex was changedto UO2(CO3)3

4− and UO2(CO3)x(OH)y2−2x−y species as the

pH of the solution increased. The uranyl peroxocarbonatecomplex could exist in only a narrow pH range fromapproximately 9 to 12, and it continuously decomposed intoa uranyl tricarbonate complex in the carbonate solution becauseof the instability of the peroxide ion in the uranylperoxocarbonate complex species in alkaline conditions.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +82 42 868 2044. Fax: +82 42 868 2351. E-mail:nkwkim@kaeri.re.kr.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Ministry of Education, Scienceand Technology (MEST) of the Republic of Korea under thenuclear R&D Project.

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