Precipitation Characteristics of Uranyl Ions at Different pHs Depending

Mar 4, 2009 - This work studied the dissolution of uranium dioxide and precipitation characteristics of uranyl ions in alkaline and acidic solutions d...
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Environ. Sci. Technol. 2009, 43, 2355–2361

This work studied the dissolution of uranium dioxide and precipitation characteristics of uranyl ions in alkaline and acidic solutions depending on the presence of carbonate ions and H2O2 in the solutions at different pHs controlled by adding HNO3 or NaOH in the solution. The chemical structures of the precipitates generated in different conditions were evaluated and compared by using XRD, SEM, TG-DT, and IR analyses together. The sizes and forms of the precipitates in the solutions were evaluated, as well. The uranyl ions were precipitated in the various forms, depending on the solution pH and the presences of hydrogen peroxide and carbonate ions in the solution. In a 0.5 M Na2CO3 solution with H2O2, where the uranyl ions formed mixed uranyl peroxy-carbonato complexes, the uranyl ions were precipitated as a uranium peroxide of UO4(H2O)4 at pH 3-4, and precipitated as a clarkeite of Na2U2Ox(OH)y(H2O)z above pH 13. In the same carbonate solution without H2O2, where the uranyl ions formed uranyl tris-carbonato complex, the uranyl ions were observed to be precipitated as a different form of clarkeite above pH 13. The precipitate of uranyl ions in a nitrate solution without carbonate ions and H2O2 at a high pH were studied together to compare the precipitate forms in the carbonate solutions.

oxide in carbonate media and its precipitation can be used for a recovery of uranium from the dirty scrap generated during nuclear fuel fabrication as well as for the separation of uranium from spent nuclear fuel and uranium ore to enhance the environmental friendliness in those processes because the carbonate solute used in such processes can be easily recycled without the generation of a secondary waste (13, 14). The precipitation or solubility characteristics of uranium or TRU elements in carbonate media have usually been studied for understanding their geochemical behaviors at neutral aqueous conditions for the case of a direct disposal of spent nuclear fuel (15-20). Most published papers on the solubilities of metal ions in carbonate solutions have been carried out in very low carbonate concentrations of underground water, and the studies on the precipitation of uranium in high concentration carbonate solutions are very scarce. Therefore, in this work, the dissolution of uranium dioxide and precipitation characteristics of uranyl ions in alkaline solutions were studied depending on the presence of carbonate ions and hydrogen peroxide in the solutions and their solution pHs, and then the chemical forms of the uranium precipitates generated in those solutions were evaluated. Materials and Methods. The uranium used for the dissolution and the precipitation in the carbonate solutions in this work was uranium dioxide (UO2) or UNH (UO2 (NO3)26H2O). The uranium dioxide was prepared by pulverizing unirradiated UO2 pellets to a size of about 9.5 µm. The uranium oxide was dissolved in a carbonate solution of 0.5 M Na2CO3 with or without 1.0 M H2O2 to make a uranium solution of 50 g/L, while the solution pH was monitored. The UNH was dissolved in demineralized water. After the uranium solution was prepared, the solution pH was adjusted up to 1 or up to 14 by adding HNO3 or NaOH into the solution to obtain the uranium precipitates. The uranium concentration of the supernatant after the precipitation was analyzed by a colorimetric method using Arsenazo III to evaluate the precipitation yield. The particle size distribution of the precipitate was measured by a particle size analyzer (Microtrac UPA-150). The precipitated uranium was filtered and dried at about 40 °C for 3 h. The finally prepared uranium precipitates were analyzed at several temperatures by SEM, X-ray diffractometer (SIMENS D5000), infrared spectroscopy (SENSIR Travel IR), a thermal analyzer (TA Instrument DSC2920-TGA 2950), etc. to evaluate the chemical compositions and the structures of the precipitates. All the experiments were carried out at a room temperature of 25 ( 1 °C.

Introduction

Results and Discussion

Carbonate-leaching of uranium has been used to extract uranium from its ore (1-3). Recently, a great deal of interest has been shown on an oxidative leaching of uranium from spent nuclear fuel in a carbonate media instead of an acid media to enhance its safety, economic competitiveness, and to minimize the generation of secondary waste streams (4-10). A process for the recovery of uranium alone without transuranium (TRU) elements from spent nuclear fuel, where an oxidative leaching and a precipitation of uranium in a high alkaline carbonate solution with hydrogen peroxide are used, has been suggested (11, 12). The dissolution of uranium

The dissolution of UO2 in a solution is known to only occur when it has the +6 valence (16, 21-23). UO2 of the +4 valence is first oxidized to intermediate species with a mixture valence of either +4 and +5 or +5 and +6, and then it is converted to UO3 (16). Finally, it is dissolved into the solution in the form of a uranyl ion of UO22+. When UO2 is oxidized to UO3 and dissolved in a carbonate solution, uranyl tris-carbonato complex ions (UO2(CO3)3-4) are formed in the solution. The overall reaction is expressed as shown in equation 1.

Precipitation Characteristics of Uranyl Ions at Different pHs Depending on the Presence of Carbonate Ions and Hydrogen Peroxide K W A N G - W O O K K I M , * ,† Y E O N - H W A K I M , † SE-YOON LEE,† JAE-WON LEE,† KIH-SOO JOE,† EIL-HEE LEE,† JONG-SEUNG KIM,‡ KYUSEOK SONG,† AND KEE-CHAN SONG† Department of Chemistry, Korea University, Seoul 136-701, Korea, and Korea Atomic Energy Research Institute, 150 Deokjin, Yuseong, Daejeon, 305-600, Korea

Received October 21, 2008. Revised manuscript received February 04, 2009. Accepted February 13, 2009.

* Corresponding author phone:+82 42 868 2044; fax: +82 42 868 2351; e-mail address: [email protected]. † Korea University. ‡ Korea Atomic Energy Research Institute. 10.1021/es802951b CCC: $40.75

Published on Web 03/04/2009

 2009 American Chemical Society

-4 UO2 + 1/2O2 + 3CO23 + H2O ) UO2(CO3)3 + 2OH (1)

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on the solution pH as shown in eq 4 (13, 14). Accordingly, the uranyl peroxo ions to be precipitated in acid like eq 3 can be obtained from the acidification of the prepared uranyl peroxy carbonato complex solution, because the carbonate species in alkali condition are converted to carbonic acid by adding an acid solution into the carbonate solution. The carbonic acid is easily changed to CO2 because of its instability in water. Therefore, the overall precipitation reaction of UO44H2O in the uranyl peroxy carbonato complex solution by its acidification can be expressed as shown in eq 5

FIGURE 1. Changes of the uranium concentrations with pH in the solutions prepared by dissolving UNH in water or uranium oxide powder in 0.5 M Na2CO3 solutions with and without 1.0 M H2 O 2 . uranyl peroxy-carbonato complexes as shown in eq 2 where x/y has three cases of 1/2, 2/1, 3/0 (7, 24, 25). Accordingly, the uranyl complex ions can exist in the forms of UO2(O2)(CO3)24-, UO2(O2)2(CO3)4-, and UO2(O2)34- depending on the solution pH. The UO2(O2)34- ions are known to be very stable at pH >13 (25-27). UO2 + yCO23 + xH2O2 + 2xOH )

[UO2(O2)x(CO3)y]2-2x-2y + 2xH2O + 2e- (2) Before evaluating the compositions of uranium precipitates generated in the solutions with and without carbonate ions and H2O2 at different pHs, the dissolution characteristics of uranium oxide in the solutions were tested and the uranyl ion species formed in each solution were indentified by a spectrophotometer. The results are available in Supporting Information Figures S1 and S2. On one hand, in a carbonate solution of 0.5 M Na2CO3 with 1.0 M H2O2, the UO2 of less than about 60 g/L was quickly and completely dissolved in the solution within about 5 min. On the other hand, in the same carbonate solution without H2O2, the dissolution rate was very slow so that the concentration of uranium in the solution reached only about 1.4 g/L after 27 days. The typical characteristic absorbance peaks of UO2(CO3)3-4 formed through the dissolution of uranium oxide in the carbonate solution without H2O2 were identified at 435, 448, and 462 nm (4). However, the spectrum of the solution of UO2(O2)x(CO3)y2-2x-2y prepared by dissolving UO2 in a carbonate solution containing H2O2 did not reveal any specific peaks. The absorbance spectrum of UO22+ prepared by dissolving UNH in water showed the typical characteristic peaks at 404, 414, and 426 nm, as well (4, 28). Figure 1 shows the changes of the uranium concentrations in the solutions according to the pH-adjustments of the uranium solutions which were prepared by dissolving UO2 powder in a carbonate solution of 0.5 M Na2CO3 with and without 1.0 M H2O2 or by dissolving UNH in water. The initial uranium solutions are marked with a circle symbol in the figure. The acidification or alkalination of the solutions were performed by adding HNO3 or NaOH solutions into the solutions little by little. Several precipitates were observed to occur in the pH regions where the uranium concentrations in the solutions were decreased. Uranyl ions in an acid condition are known to be precipitated as uranium peroxide hydrate (UO2(O2) nH2O) by adding H2O2 into the uranyl solution as shown in eq 3 (29-31). The carbonate species of H2CO3, HCO3-, and CO32- are interchangeable depending 2356

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+ UO2+ 2 + 2H2O2 + 4H2O f UO2(O2)·4H2O + 4H

(3)

-2 H2CO3(CO2 v ) T HCO3 T CO3

(4)

UO2(O2)x(CO3)y2-2x-2y + mH+ + 2H2O f UO2(O2)·4H2O + yH2CO3 (5) where m ) 4, 6, 8 at y ) 0, 1, 2, and x/y ) 1/2, 2/1, 3/0. When a uranyl peroxy carbonato complex ion solution prepared by a complete dissolution of 56.7 g/L UO2 in a carbonate solution of 0.5 M Na2CO3 with 1.0 M H2O2, of which the initial pH and uranium concentration were about 11.2 and 50 g/L, was acidified by HNO3, the uranium concentration did not change until pH 6.5 because the solubility of the uranyl ion in that pH range, where bicarbonate and carbonate coexist in the solution, is higher than the initial uranium concentration in the prepared solution. However, when the solution pH reached 5, where the carbonate concentration in the solution becomes almost 0, the uranium concentration dropped rapidly and a lot of precipitate occurred in the solution. This implies that only carbonate species are detached from the uranyl peroxy carbonato complex ions so that the remaining uranyl peroxo ions change to uranium peroxide by proton ions like eq 3 (refer to Figure 3). At pH 3-4, the uranium ions in the solution were completely precipitated so that its concentration in the solution became almost zero. On one hand, the solubility of the uranium peroxide is very low at Ksp ) 1.34 × 10-3 (30), so that the uranium in the carbonate solution can be almost completely recovered as a uranium peroxide precipitate. However, as the pH becomes less than 3, the uranium concentration began to slightly increase because the uranium peroxide is redissolved in the more acidic solution. On the other hand, when the prepared uranyl peroxy carbonato complex ion solution was alkalinized by NaOH, other precipitations occurred at pH of higher than 12. The uranium concentration in the solution reached a minimum value at pH 13 and then increased again. The precipitates at pH 13-14 were sodium diuranate compounds of a clarkeite type (refer to Figure 4). The reason for the increase of uranium concentration in the solution at pH 14 rather than at pH 13 is considered as follows. As mentioned above, the uranyl peroxy carbonato complex ion of UO2(O2)x(CO3)y2-2x-2y at x/y ) 3/0 becomes a uranyl peroxide complex ion of UO2(O2)34-, which is not a carbonate species. As mentioned above, the uranyl peroxide ions are known to be very stable at pH > 13 (25-27). This means that the uranyl species exist in different forms before and after about pH 13 with different solubilities. Another aspect to explain the reason can come from a decomposition of peroxide under alkaline conditions. Hydrogen peroxide is well-known to be self-decomposed in alkaline conditions (28). Accordingly, the decomposition of peroxides imbedded in uranyl peroxy complex ions at the higher pH of 14 is considered to shift the equilibrium condition of the solution system, which drives some part of the uranyl peroxy complex ions to change into other uranium species such as UO2(OH)3- or UO2(OH)42- ions which have higher solubilities (29). However, an elucidation of the exact

FIGURE 2. SEM photographs of the precipitates generated in UO2 uranyl peroxy carbonato complex solutions at pH 3 (A), pH 13 (B),and pH 14 (C), and in a uranyl carbonato solution at pH 14 (D). reason of the increased solubility of uranium in the H2O2carbonate solution at pH 14 needs furthermore studies. In Figure 1, the initial pH and uranium concentration of the uranyl tris-carbonato complex ion solution in the carbonate solution without H2O2 were about 11 and 2.8 g/L, respectively. When the prepared solution was acidified by HNO3, the uranium concentration in the solution did not change from pH 11 to pH 1 except for around pH 4 where a slight precipitation was observed. The solubilities of the uranyl ions in the solution at pH of more than 5, where bicarbonate or carbonate ions exist, and at pH of less than 3, where the solution became carbonate-free (13), were higher than the uranium concentration of the initial carbonate solution, so that there is no change of the uranium concentration in those pH ranges. The slight precipitation at pH 4 is because the uranyl ions in the carbonate-free solution are hydrolyzed. When the uranyl tris-carbonato complex ion solution was alkalinized at a pH of more than 12 by NaOH, the uranium concentration decreased to about 0.6 g/L at pH 14 and a precipitation occurred. The precipitate was a sodium diuranate compound of a clarkeite type (refer to Figure 5). In Figure 1, the uranyl solution of UO22+ prepared by dissolving UNH in water had the initial uranium concentration of 50 g/L and pH about 2. The uranyl ions are known to be hydrolyzed to form complicated uranyl hydroxide complex ions of UO2(OH)x(2-x)+ as shown in eq 6 (22, 34). The uranium concentration in the solution began to decrease from pH of higher than 3 and reached almost zero over pH 5. The precipitate at pH 14 is a sodium diuranate compound of a clarkeite type (33) (Refer to Figure 6).

(2-x)+ + xH+ UO2+ 2 + xH2O f UO2(OH)x

(6)

Figure 2 shows the SEM photographs after drying the precipitates of Figure 1 generated in a UO2(O2)x(CO3)y2-2-2y solution at pH 3 (A), pH 13 (B), and at pH 14 (C), and in a UO2(CO3)34- solution at pH 14 (D). The particle size of the precipitate generated at pH 3 by adding HNO3 into the UO2(O2)x(CO3)y2-2-2y solution was less than 0.1 µm. Those generated at pH 13 and pH 14 by adding NaOH into the UO2(O2)x(CO3)y2-2x-2y solution were about 0.2 and 0.3 µm, respectively. The particle size of the precipitate generated at pH 14 by adding NaOH into the UO2(CO3)34- solution was about 0.5 µm. The particles generated in the carbonate solution without H2O2 are much larger and more clearly shaped than those generated in the carbonate solution with H2O2. In the case of the precipitate generated by adding NaOH into the uranyl solution, distinct solid particles were not formed, so the SEM photograph of it was not presented in this paper. The actual particle size distributions in the solutions of the precipitates of Figure 1 were measured and their data are available in Figure S3 of the Supporting Information. The average sizes of the particles precipitated in each solution are larger than the individual particles shown in Figure 2. It means that the individual particles grown through the first nucleation are agglomerated so that the particles exist in the form of a cluster which is much larger than the individual particle. XRD, thermal-gravimetry, and infrared analyses were carried out to elucidate the chemical composition of the precipitates in Figure 1. Figure 3 shows the XRD spectra (A) and TG (thermal gravity) DT (differential temperature) spectra (B) of the precipitates generated in a UO2(O2)x(CO3)y2-2-2y solution at pH 3 before and after sintering at 800 °C. The VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. XRD and TGA spectra of the precipitate generated in a uranyl peroxy carbonato complex solution at pH 3 controlled by HNO3. XRD result showed that the initial precipitate was UO4 4H2O. After sintering the precipitate at 800 °C, the precipitate changed to U3O8 (35), the TG results shows that the UO4 4H2O changes to UO4 2H2O at about 90 °C, and UO4 at about 200 °C, which are considered to be attributed to the losses of water from the structure, i.e., a loss of water of crystallization, and finally to U3O8 at 550 °C. The change of the TG weight loss shows exactly the same change of weight due to the change of UO4 to U3O8. Figure 4 shows the XRD (A), TG-DT (B), and infrared (C) spectra of the precipitates generated in the UO2(O2)x(CO3)y2-2x-2y solution at pH 14 before and after sintering at several temperatures. The initial precipitate has no XRD peaks because it has a poor crystallinity, but it begins to show XRD peaks of Na2U2O7 material after sintering above 250 °C, and these peaks become more distinct as the sintering temperature increases until 1300 °C. But some peaks of U3O8 were observed at 1450 °C among the main Na2U2O7 peaks. This is considered to be because Na atoms are released from the Na2U2O7 structure by their evaporation. The TG-DT spectra shows only one phase change until about 150 °C due to the loss of water and another weak peak due to the phase transition to Na2U2O7 at around 550 °C. In order to evaluate the composition of the initial precipitate, infrared spectra of the precipitates before and after sintering at 100, 200, 800 °C were measured. The infrared spectra show that the initial precipitate and the precipitate sintered at 200 °C where the water of crystallization imbedded within the structure disappears have the bands occurring at around 3600 cm-1 and about 1630 cm-1 which are attributed to OsH stretching and HsOsH stretching, respectively. The band at around 850 cm-1 is ascribed to UsO stretching (36-38). Although the structure of initial precipitate generated in the 2358

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FIGURE 4. XRD, TGA, and IR spectra of the precipitates generated in a uranyl peroxy carbonato complex solution at pH 14 controlled by NaOH. UO2(O2)x(CO3)y2-2x-2y solution at pH 14 was not identified by XRD, it can be estimated with the results of XRD, TG-DT, and infrared spectra. Bessonov et al. (25) studied the formation of sodium uranate by adding H2O2 and alkali material into a uranyl solution without carbonate ions. Finch et al. (32) and Krupka et al. (33) studied the existence of clarkeite, Na(UO2)1-xO1-y(OH)1+y (H2O)0 to1, of which the crystallinity was very poor (32). From a formation of Na2U2O7 at 800 °C, and from the stability of Na and U atoms within the precipitation structure until 1300 °C together with the TG-DT and infrared results, the initial precipitate can be said to be a sodium uranate compound with at least a ratio of Na:U ) 1:1 as an amorphous clarkeite form. Therefore, the general precipitation reaction by adding NaOH into a UO2(O2)x(CO3)y2-2x-2y solution can be expressed as shown in eq 7.

UO2(O2)x(CO3)y2-2x-2y + mOH+ + nNa+ + qH2O f Nan(UO2)(O2)x(OH)m(H2O)q + yCO2(7) 3 In the equation, x/y can be 1/2, 2/1, 3/0, as mentioned above. However, from the fact that the atomic ratio of U and Na is 1:1 in the precipitate, only the case of x/y ) 1/2 is available. Other cases of x/y cannot have a 1:1 ratio of U and Na. Accordingly, at x/y)1/2, eq 7 can be reexpressed, as follows: + 2U2(O2)(CO3)-4 2 + 2OH + 2Na + qH2O f

Na2[(UO2)(O2)]2(OH)2(H2O)q + 4CO2(8) 3 As the weight loss of the initial precipitate to become the final material identified as Na2U2O7 after sintering the initial precipitate at 800 °C is 23.5%, the theoretical weight loss for the initial precipitate to become the final Na2((UO2)(O2))2(OH)2 should be 17.6%. From the measured TG spectrum, the weigh loss of the initial precipitate until 150 °C where the water of crystallization disappears is 16.9%, which is close to the theoretical value. This implies that the precipitation reaction of eq 8 is sound. Taking into consideration the weight decrease due to the loss of water until above 150 °C, the initial precipitate composition is evaluated to be Na2((UO2)(O2))2(OH)2(H2O)8.17, that is, Na2U2Ox(OH)y(H2O)z which is a type of clarkeite (32, 33). Therefore, the initial precipitate of a clarkeite type is considered to be converted to Na2U2O7 with the sintering temperature increasing, while the water of crystallization is first lost and then the OH components are continuously released as H2O until 550 °C while leaving some oxygen within the structure. Figure 5 shows the XRD (A), TG-DT (B), and infrared (C) spectra of the precipitates generated in the UO2(CO3)34solution at pH 14 before and after sintering at several temperatures. The XRD results show that the initial precipitate has a trace of Na2U2O7 structure even though its crystallinity is not good, but it shows more distinct peaks of Na2U2O7 as the sintering temperature increases. The TG-DT results shows a loss of water from the structure until about 150 °C and the infrared spectra show the OsH stretching for the initial precipitate and the sintered precipitate at 200 °C similarly to Figure 6 (C). The uranyl tris-carbonate ions are known to be precipitated in a strong NaOH solution as shown in eq 9 (39). + 2UO2(CO3)-4 3 + 2Na + 6OH f

Na2U2O7 + 6CO23 + 3H2O (9) However, eq 9 can be reexpressed like eq 10 because OH component and water molecular exists within the initial precipitate structure.

FIGURE 5. XRD, TGA, and IR spectra of the precipitates generated in a uranyl carbonato solution at pH 14 controlled by NaOH.

+ 2UO2(CO3)-4 3 + 2Na + 6OH f

Na2U2Ox(OH)14-2x(H2O)q + 6CO23 + (x - 4 - q)H2O (10) This equation has a constraint of 4 e x e 7 for meeting the dehydration reaction and the maximum number of oxygen atom to be imbedded in the structure. As the weight loss of the initial precipitate to become the final material identified as Na2U2O7 after sintering at 800 °C is 8.6%, the composition of the precipitate at 150 °C, where the specimen is free of water and until which its weight loss from the initial precipitate is 5.4%, can be evaluated to be Na2U2O5.11(OH)3.78 through taking into consideration the oxidation valences of each component consisting of the material. Finally, from the weight loss of the initial precipitate due to the loss of water until 150 °C, the initial precipitate can be evaluated to be Na2U2O5.11(OH)3.78 (H2O)1.38, which is a type of clarkeite, as well.

Figure 6 shows the XRD (A) and the TG-DT (B) spectra of the precipitates generated in the UO22+ solution at pH 14 controlled by adding NaOH before and after sintering at several temperatures. Similar to Figure 5, the XRD results show that the initial precipitate has a trace of Na2U2O7 structure but it has distinct peaks of Na2U2O7 after sintering at 800 °C. The TG-DT results shows a slight loss of water from the precipitate structure until about 100 °C and the infrared spectra, which is not present here, showed the OsH stretching at the initial precipitate and the sintered precipitate at 200 °C similar to Figure 5 (C). The general precipitation reaction by adding NaOH into the UO22+ solution can be expressed as shown in eq 11 with a constraint of 4 e x e 7. + 2UO2+ 2 + 2Na + 6OH f Na2U2Ox(OH)14-2x(H2O)q + (x - 4 - q)H2O (11)

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FIGURE 6. XRD and TGA spectra of the precipitates generated in a uranyl solution at pH 14 controlled by NaOH. Based on the water loss at about 100 °C and the total weight loss of the initial precipitate to Na2U2O7 at 800 °C, the composition of the initial precipitate can be evaluated to be Na2U2O6.53(OH)0.94 (H2O)1.26, which is a type of clarkeite, as well.

Acknowledgments This work was supported by the Ministry of Education, Science and Technology (MEST) of the Republic of Korea under the nuclear R&D Project.

Supporting Information Available Dissolution characteristics of uranium oxide in a carbonate solution with and without H2O2 (Figure S1), absorbance spectra of UO22+, UO2(CO3)34-, UO2(O2)x(CO3)y2-2x-2y (Figure S2), and particle size distributions in the solutions of the precipitates (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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