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Electronic absorption spectra of U (III) ion in a LiCl–KCl eutectic melt at 450 °C
Microchemical Journal 96 (2010) 344–347
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Electronic absorption spectra of U (III) ion in a LiCl–KCl eutectic melt at 450 °C Young Hwan Cho ⁎, Tack-Jin Kim, Sang Eun Bae, Yong Joon Park, Hong Joo Ahn, Kyuseok Song Korea Atomic Energy Research Institute, Yuseong P.O Box 105, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 24 May 2010 Accepted 2 June 2010 Available online 15 June 2010 Keyword: Uranium Actinide f–d transition Molten salt Electronic absorption spectroscopy
a b s t r a c t We have measured the electronic absorption spectra of the U(III) ion in LiCl–KCl eutectic melt at 450 °C to understand its chemical behavior in the context of pyrochemical process of spent nuclear fuel. The UV–VIS spectra of the U(III) ion consist of two main peaks in the range of 400–600 nm which are attributable to the 5f 3–5f 26d1 transitions. With the aid of UV–VIS spectroscopic tool, in-situ measurement of chemical reactions of the U(III) with oxide ion as well as neodymium oxide was successfully achieved. The U(III) ion forms insoluble uranium oxide phases by reacting with oxide ion and lanthanide oxides. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, ionic melts have become attractive reaction media in many fields [1,2]. Molten salt based electrochemical processes, so called pyroprocessing, have been proposed as a new option for the advanced spent nuclear fuel cycle [3,4]. One of the important steps in the pyroprocessing of the spent nuclear fuel is the electrorefining of uranium in molten LiCl based media. During the course of these electrochemical processes, information on the chemical behavior of uranium ions is of great importance. The knowledge on the basic chemical properties of several f-block elements in molten salt media is essential for developing pyrochemical processes. In this respect, tools for real time measurement of lanthanide and actinide ion species in high temperature molten salt media are necessary. An electronic absorption spectroscopic method may be eligible for this purpose. However, few studies have been reported until now on the spectrochemical behavior of uranium ion in high temperature molten salt media [5,6]. Although several studies have been reported recently for the uranium ions in LiCl–KCl melt, in-depth information on electronic nature and chemical behavior is very limited. In our previous paper, we have reported how the electronic absorption measurement can be effectively applied to investigate the redox behavior of lanthanide ions in LiCl–KCl eutectic melt at 450 °C [7]. The present study is focused on elucidating the chemical nature of uranium(III) ion in an effort to obtain a better understanding of its chemical behavior in LiCl–KCl eutectic melt in the context of pyroprocessing of spent nuclear fuel. Here, we report the in-situ measurement of electronic spectra of the U(III) ion in a high temperature molten salt medium
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Y.H. Cho). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.06.001
and its application to monitoring of several chemical reactions of interests in the pyrochemical process of spent nuclear fuel.
2. Experimental Although a systematic understanding of actinide chemical speciation and related redox reactions is essential for the implementation of pyrochemical processes, the severe physical and chemical reaction environment makes in-situ measurements difficult and challenging. To accomplish this goal we designed and built a glove box interfaced with electric furnace as shown in Fig. 1. All the experiments were carried out in this glove box system. The inert atmosphere was maintained by purging with purified Ar gas to avoid exposure to oxygen and water. The O2 and H2O level were maintained to be less than 1 ppm. The LiCl–KCl eutectic (mole ratio of lithium to potassium = 59/41) mixture (melting point 723 K) was prepared from A.R. grade reagent. The U(III) ion was prepared by reacting metallic uranium with CdCl2 in LiCl–KCl eutectic mixture at 450 °C. The spectrochemical measurement system is shown schematically in Fig. 2. A rectangular quartz cell attached to a long quartz tube is placed at the center of the electric furnace. The light source beam generated by deuterium–halogen lamp (Ocean Optics Inc.) was guided into and out of the sample chamber by using an optical fiber cable. Suitable quartz lens and iris were used to collimate the beam path and adjust the intensity. In a quartz cell containing 4.5 g of LiCl–KCl eutectic mixture and suitable amount of metallic uranium was placed into electric furnace and heated to ∼450 °C. When the salt is completely melted, a stoichiometric amount of CdCl2 was added to the cell successively. The reaction of U metal with CdCl2 in a LiCl–KCl eutectic melt at 450 °C
Y.H. Cho et al. / Microchemical Journal 96 (2010) 344–347
345
Fig. 1. Glove box interfaced with high temperature reaction system and spectroscopic measurement system.
predominantly produces the deep wine red colored U(III) species as shown in Fig. 3. In order to identify the reactions of U(III) with oxide ion and lanthanide oxides, we used spectrophotometric monitoring method. In a LiCl–KCl melt containing known amounts of U(III) ions, stoichiometric amounts of Li2O and Nd2O3 are added successively, while recording the UV–VIS spectra.
3. Results and discussion 3.1. Electronic absorption spectra of U(III) ion Fig. 3 shows the absorption spectrum of U(III) obtained from the reaction of U metal with CdCl2 in LiCl–KCl at 450 °C.
The UV–VIS spectrum of U(III) consists of two main peaks in the range of 400–600 nm. Although similar UV–VIS spectra were reported before, the peak assignment has not been made for the trivalent uranium ion in the molten salt media. However, detailed results of spectral analysis were reported recently for U(III) ion diluted in several crystal lattices. Systematic studies of U(III) ions diluted into solid phase have been made to explain the 5f–6d transitions [8–11]. Our observation of UV–VIS spectra of U(III) can be interpreted on the basis of these reported results. The absorption lines in Fig. 3 are attributed to the 5f 3–5f 26d1 transitions of U(III) ion with 5f3 electronic configuration. The energy levels 6d states are split into two by the octahedral chloride ligand field effect. In general, two types of electronic transitions are expected in f-block ions in vacuum UV (VUV) to near infra red energy range. The first is intra-configurational transitions occurring within f–f levels. They are
Fig. 2. Schematic diagram of reaction system interfaced with spectroscopic measurement unit.
346
Y.H. Cho et al. / Microchemical Journal 96 (2010) 344–347
Fig. 3. Result of in-situ monitoring of U(III) formation at 450 °C.
forbidden to first order by the parity conservation rule but this selection rule is partly relaxed by the admixture of opposite parity configuration states into f q state functions. The mixing is due to the perturbing crystal field (CF) potential. These transitions are called forced electric dipole transitions and they happen to appear on the spectra rather weak and narrow [12]. Absorption peaks of U(III) ions due to 5f–5f transition lines were observed in the range of 700–900 nm with very low intensity compared with the main peaks in 400–600 nm range. The second is inter-configurational transitions: For ions with 5f q 3 (5f for U3+) ground configuration, the inter-configuration transitions promote one f-electron into unoccupied orbitals of higher-lying configurations. In the energy range considered in optical spectroscopy, the transitions generally occur between the 5nf q and 5nf q − 16d configurations of opposite parity. They are called 5f–6d transitions and are orbitally allowed. Consequently, they are much more intense (with molar absorption coefficient N103) than 5f–5f transitions and because of a strong coupling to the lattice vibrations due to the extended nature of the 6d wave function. These electronic transitions give rise to broad bands mainly vibronic in nature. This phenomenon accounts for the observed spectra in Fig. 3. Until now much less are known about the inter-configurational 5f– 6d transitions in actinide ions. To the best of our knowledge, spectra presented in Fig. 3 are the first observed 5f–6d transitions of actinide ions in high temperature molten salt. Increasing the temperature of the melt had no effect on the changes of spectral pattern.
Fig. 4. On-line monitoring of the reaction of U(III) with Li2O in a LiCl–KCl eutectic melt at 450 °C. (a) before Li2O addition (b,c) successive Li2O addition (d) excess Li2O addition.
no peaks are observed in the spectra. As a result of the reaction, uranium is precipitated mostly to UO2 phase. This illustrates that 5f– 6d absorption lines of the U(III) may be used as an indicator for the monitoring the reaction where the U(III) ion is involved. 3.3. In situ monitoring of the U(III) reaction with Nd2O3 The reactivity of lanthanide oxide along with oxide ion is of great concern in the pyrochemical process of spent nuclear fuel. Because several lanthanide oxides (or Li–lanthanide mixed oxide) may exist in the electro refining process, where the U(III) ion exists as a predominant species, the effect of the presence of lanthanide oxides on efficiency of the electro refining of uranium [14]. In order to clarify this side reaction, we applied in-situ electronic absorption spectroscopy to monitor the reaction of uranium (III) and lanthanide oxide in LiCl–KCl eutectic melt at 450 °C. Amongst many lanthanide oxides, Nd2O3 was chosen as a suitable lanthanide oxide phase in two reasons, (1) its high contents compared with other lanthanide elements produced in the spent nuclear fuel, (2) its intense electronic absorption lines suitable for spectrochemical measurement [15]. The electronic absorption spectra shown in Fig. 5 were obtained by in-situ monitoring of the reaction of
3.2. In-situ monitoring of U(III) reaction with oxide ion In the pyrochemical process of spent nuclear fuel, the U(III) ion is the dominant uranium species in electrorefining and electrowinning steps. Therefore, its reaction with potential reactant materials (both ion and oxide material) are of great interests. Oxide ion, O2−, is regarded as one of the key parameters that may affect the pyrochemical process because of its high chemical reactivity with the metal ions. Actually, the molten salt based pyrochemical process of spent nuclear fuel involves t lithium oxide (Li2O) which dissociates into oxide. According to our solubility measurements, Li2O dissolves in molten LiCl at 700 °C by up to a ca. 8 wt.% [13]. Dissolved Li2O dissociates completely to produce an oxide ion (O2−) when the solubility is less than a ca. 4 wt.%. Thermodynamic calculations show that lithium oxide may react with actinide ions in high temperature molten salt media. The reaction of lithium oxide with the U(III) is of great concern in this respect. So, we monitored the reaction of the U(III) with lithium oxide by measuring the electronic absorption spectra in in-situ manner as shown in Fig. 4. The decrease of the U(III) absorption intensity act as an indicator of the reaction progress. When the reaction is completed,
Fig. 5. On-line monitoring of the UV–VIS spectra for the reaction of the U(III) with Nd2O3 in LiCl–KCl melt at 450 °C.
Y.H. Cho et al. / Microchemical Journal 96 (2010) 344–347
the U(III) ion with the Nd2O3 added. It shows the changes in the U(III) and Nd(III) ions simultaneously. The intensity of the U(III) peak decreased as the reaction with the Nd2O3 proceeded, and consequently the intensity of the Nd(III) peaks increased. In general, molar extinction coefficient of lanthanide ions is much lower than the U(III) ion. The Nd (III) spectra showed the same features as reported in the literature [14]. The characteristic peak patterns of the Nd(III), which are attributed to the 4G5/2, 2G7/2 ← 4I9/2 hypersensitive transitions, are clearly seen. The reactions of the U(III) with other lanthanide oxides (and Li mixed oxides) exhibited similar results. The electronic absorption lines of the other trivalent lanthanide ions are so weak in intensity that the increase of that is difficult to measure. However, the decrease of the U(III) ion peaks is the direct evidence of the reaction with all other lanthanide oxides. By analyzing the UV–VIS spectra and XRD data of uranium precipitates, the reaction of the U(III) with Nd2O3 can be expresses as follows: 2UCl3 þ Nd2 O3 →UO2 ↓ þ UO þ 2NdCl3 As was shown in this study, in situ UV–VIS measurement provided detailed information regarding the oxidation state and reactivity of the U(III) and some lanthanide ions in high temperature molten salt. Further studies are ongoing to apply the electronic absorption spectroscopy to various actinide ions in high temperature molten salt in combination with electrochemical method.
4. Conclusions We have measured the electronic absorption spectra of the U(III) ion in LiCl–KCl eutectic melt at 450 °C. The UV–VIS spectra of the U(III) ion consist of two main peaks in the range of 400–600 nm which are attributable to the 5f 3–5f 26d1 transitions. With the aid of UV–VIS spectroscopic tool, in-situ measurement of chemical reactions of the U (III) with oxide ion, lanthanide oxides was successfully achieved.
347
Acknowledgement This study was supported by the Atomic Energy R&D Fund of the Korea Ministry of Education, Science and Technology. References [1] K.R. Seddon, Ionic liquid for clean technology, J. Chem. Technol. Biotechnol. 68 (1997) 351. [2] D.J. Fray, Emerging molten salt technologies for metal production, J. Metall. (2001) 26. [3] T. Usami, M. Kurata, T. Inoue, H.E. Sims, S.A. Beetham, J.A. Jenkins, Pyrochemical reduction of uranium dioxide and plutonium dioxide by lithium metal, J. Nucl. Mater. 300 (2002) 15. [4] Christian Le. Brun, Molten salts and nuclear energy production, J. Nucl. Mater. 360 (2007) 1. [5] D.M. Gruen, R.L. McBeth, Oxidation states and complex ions of uranium in fused chloride and nitrates, J. Inorg. Nucl. Chem. 9 (1959) 290. [6] H. Yamana, T. Fujii, O. Shirai, Proceedings of International Symposium on Ionic Liquids in Honour of Marcelle Gaune-Escard, Carry le Rouet, France, June 27–28, 2003. [7] Y.H. CHO, T.J. Kim, Y.J. Park, H.J. Im, K.S. Song, Electronic absorption spectra of Sm (II) and Yb(II) ions in a LiCl–KCl eutectic melt at 450 °C, J. Lumin. 130 (2010) 280. [8] M. Karbowiak, J. Drozdzynski, 5fN–5fN-16d1 transitions of U3+ and U4+ ions in high-symmetry sites, J. Phys. Chem. A 108 (2004) 6397. [9] M. Krabowiak, A. Mech, J. Drozdzynski, W. Ryba-Romanowski, M.F. Reid, Electronic structure of U3+ in Cs3Lu2Cl9 and Cs3Y2I9 single crystals, J. Phys. Chem. B 109 (2005) 155. [10] C.-K. Duan, M.F. Reid, G. Ruan, A simple model for f–d transition of actinide and heavy lanthanide ions in single crystals, Curr. Appl. Phys. 6 (2006) 359. [11] I.B. Polovov, V. Volkovich, J.M. Charnock, B. Kraij, R.G. Lewin, H. Kinoshita, I. May, C.A. Sharrad, In situ spectroscopy and spectroelectrochemistry of uranium in high-temperature alkali chloride molten salts, Inorg. Chem. 47 (2008) 7474. [12] T. Fujii, T. Nagai, A. Uehara, H. Yamana, Electronic absorption spectra of lanthanides in a molten chloride III. Absorption characteristics of trivalent samarium, dysprosium, holmium, and erbium in various molten chlorides, J. Alloy. Comp. 441 (2007) L10–L13. [13] Young-Hwan Cho, Jong-Seon Jeon, Jei-Won Yeon, In-kyu Choi, Won-Ho Kim, Inline monitoring of an oxide ion in LiCl molten salt using a YSZ based oxide ion selective electrode, J. Korean Nucl. Soc. 36 (2004) 415. [14] Y.H. Kang, S.C. Hwang, H.S. Lee, E.H. Kim, S.W. Park, J.H. Lee, Effect of neodymium oxide on an electrorefining process of uranium, J. Mater. Process. Technol. 209 (2009) 5008. [15] T. Fujii, H. Moriyama, H. Yamana, Electronic absorption spectra of lanthanides in a molten chloride: I. Molar absorptivity measurement of neodymium(III) in molten eutectic mixture of LiCl–KCl, J. Alloy. Comp. 351 (2003) L6–L9.
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Electronic absorption spectra of U (III) ion in a LiCl–KCl eutectic melt at 450 °C Young Hwan Cho ⁎, Tack-Jin Kim, Sang Eun Bae, Yong Joon Park, Hong Joo Ahn, Kyuseok Song Korea Atomic Energy Research Institute, Yuseong P.O Box 105, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 24 May 2010 Accepted 2 June 2010 Available online 15 June 2010 Keyword: Uranium Actinide f–d transition Molten salt Electronic absorption spectroscopy
a b s t r a c t We have measured the electronic absorption spectra of the U(III) ion in LiCl–KCl eutectic melt at 450 °C to understand its chemical behavior in the context of pyrochemical process of spent nuclear fuel. The UV–VIS spectra of the U(III) ion consist of two main peaks in the range of 400–600 nm which are attributable to the 5f 3–5f 26d1 transitions. With the aid of UV–VIS spectroscopic tool, in-situ measurement of chemical reactions of the U(III) with oxide ion as well as neodymium oxide was successfully achieved. The U(III) ion forms insoluble uranium oxide phases by reacting with oxide ion and lanthanide oxides. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, ionic melts have become attractive reaction media in many fields [1,2]. Molten salt based electrochemical processes, so called pyroprocessing, have been proposed as a new option for the advanced spent nuclear fuel cycle [3,4]. One of the important steps in the pyroprocessing of the spent nuclear fuel is the electrorefining of uranium in molten LiCl based media. During the course of these electrochemical processes, information on the chemical behavior of uranium ions is of great importance. The knowledge on the basic chemical properties of several f-block elements in molten salt media is essential for developing pyrochemical processes. In this respect, tools for real time measurement of lanthanide and actinide ion species in high temperature molten salt media are necessary. An electronic absorption spectroscopic method may be eligible for this purpose. However, few studies have been reported until now on the spectrochemical behavior of uranium ion in high temperature molten salt media [5,6]. Although several studies have been reported recently for the uranium ions in LiCl–KCl melt, in-depth information on electronic nature and chemical behavior is very limited. In our previous paper, we have reported how the electronic absorption measurement can be effectively applied to investigate the redox behavior of lanthanide ions in LiCl–KCl eutectic melt at 450 °C [7]. The present study is focused on elucidating the chemical nature of uranium(III) ion in an effort to obtain a better understanding of its chemical behavior in LiCl–KCl eutectic melt in the context of pyroprocessing of spent nuclear fuel. Here, we report the in-situ measurement of electronic spectra of the U(III) ion in a high temperature molten salt medium
⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Y.H. Cho). 0026-265X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2010.06.001
and its application to monitoring of several chemical reactions of interests in the pyrochemical process of spent nuclear fuel.
2. Experimental Although a systematic understanding of actinide chemical speciation and related redox reactions is essential for the implementation of pyrochemical processes, the severe physical and chemical reaction environment makes in-situ measurements difficult and challenging. To accomplish this goal we designed and built a glove box interfaced with electric furnace as shown in Fig. 1. All the experiments were carried out in this glove box system. The inert atmosphere was maintained by purging with purified Ar gas to avoid exposure to oxygen and water. The O2 and H2O level were maintained to be less than 1 ppm. The LiCl–KCl eutectic (mole ratio of lithium to potassium = 59/41) mixture (melting point 723 K) was prepared from A.R. grade reagent. The U(III) ion was prepared by reacting metallic uranium with CdCl2 in LiCl–KCl eutectic mixture at 450 °C. The spectrochemical measurement system is shown schematically in Fig. 2. A rectangular quartz cell attached to a long quartz tube is placed at the center of the electric furnace. The light source beam generated by deuterium–halogen lamp (Ocean Optics Inc.) was guided into and out of the sample chamber by using an optical fiber cable. Suitable quartz lens and iris were used to collimate the beam path and adjust the intensity. In a quartz cell containing 4.5 g of LiCl–KCl eutectic mixture and suitable amount of metallic uranium was placed into electric furnace and heated to ∼450 °C. When the salt is completely melted, a stoichiometric amount of CdCl2 was added to the cell successively. The reaction of U metal with CdCl2 in a LiCl–KCl eutectic melt at 450 °C
Y.H. Cho et al. / Microchemical Journal 96 (2010) 344–347
345
Fig. 1. Glove box interfaced with high temperature reaction system and spectroscopic measurement system.
predominantly produces the deep wine red colored U(III) species as shown in Fig. 3. In order to identify the reactions of U(III) with oxide ion and lanthanide oxides, we used spectrophotometric monitoring method. In a LiCl–KCl melt containing known amounts of U(III) ions, stoichiometric amounts of Li2O and Nd2O3 are added successively, while recording the UV–VIS spectra.
3. Results and discussion 3.1. Electronic absorption spectra of U(III) ion Fig. 3 shows the absorption spectrum of U(III) obtained from the reaction of U metal with CdCl2 in LiCl–KCl at 450 °C.
The UV–VIS spectrum of U(III) consists of two main peaks in the range of 400–600 nm. Although similar UV–VIS spectra were reported before, the peak assignment has not been made for the trivalent uranium ion in the molten salt media. However, detailed results of spectral analysis were reported recently for U(III) ion diluted in several crystal lattices. Systematic studies of U(III) ions diluted into solid phase have been made to explain the 5f–6d transitions [8–11]. Our observation of UV–VIS spectra of U(III) can be interpreted on the basis of these reported results. The absorption lines in Fig. 3 are attributed to the 5f 3–5f 26d1 transitions of U(III) ion with 5f3 electronic configuration. The energy levels 6d states are split into two by the octahedral chloride ligand field effect. In general, two types of electronic transitions are expected in f-block ions in vacuum UV (VUV) to near infra red energy range. The first is intra-configurational transitions occurring within f–f levels. They are
Fig. 2. Schematic diagram of reaction system interfaced with spectroscopic measurement unit.
346
Y.H. Cho et al. / Microchemical Journal 96 (2010) 344–347
Fig. 3. Result of in-situ monitoring of U(III) formation at 450 °C.
forbidden to first order by the parity conservation rule but this selection rule is partly relaxed by the admixture of opposite parity configuration states into f q state functions. The mixing is due to the perturbing crystal field (CF) potential. These transitions are called forced electric dipole transitions and they happen to appear on the spectra rather weak and narrow [12]. Absorption peaks of U(III) ions due to 5f–5f transition lines were observed in the range of 700–900 nm with very low intensity compared with the main peaks in 400–600 nm range. The second is inter-configurational transitions: For ions with 5f q 3 (5f for U3+) ground configuration, the inter-configuration transitions promote one f-electron into unoccupied orbitals of higher-lying configurations. In the energy range considered in optical spectroscopy, the transitions generally occur between the 5nf q and 5nf q − 16d configurations of opposite parity. They are called 5f–6d transitions and are orbitally allowed. Consequently, they are much more intense (with molar absorption coefficient N103) than 5f–5f transitions and because of a strong coupling to the lattice vibrations due to the extended nature of the 6d wave function. These electronic transitions give rise to broad bands mainly vibronic in nature. This phenomenon accounts for the observed spectra in Fig. 3. Until now much less are known about the inter-configurational 5f– 6d transitions in actinide ions. To the best of our knowledge, spectra presented in Fig. 3 are the first observed 5f–6d transitions of actinide ions in high temperature molten salt. Increasing the temperature of the melt had no effect on the changes of spectral pattern.
Fig. 4. On-line monitoring of the reaction of U(III) with Li2O in a LiCl–KCl eutectic melt at 450 °C. (a) before Li2O addition (b,c) successive Li2O addition (d) excess Li2O addition.
no peaks are observed in the spectra. As a result of the reaction, uranium is precipitated mostly to UO2 phase. This illustrates that 5f– 6d absorption lines of the U(III) may be used as an indicator for the monitoring the reaction where the U(III) ion is involved. 3.3. In situ monitoring of the U(III) reaction with Nd2O3 The reactivity of lanthanide oxide along with oxide ion is of great concern in the pyrochemical process of spent nuclear fuel. Because several lanthanide oxides (or Li–lanthanide mixed oxide) may exist in the electro refining process, where the U(III) ion exists as a predominant species, the effect of the presence of lanthanide oxides on efficiency of the electro refining of uranium [14]. In order to clarify this side reaction, we applied in-situ electronic absorption spectroscopy to monitor the reaction of uranium (III) and lanthanide oxide in LiCl–KCl eutectic melt at 450 °C. Amongst many lanthanide oxides, Nd2O3 was chosen as a suitable lanthanide oxide phase in two reasons, (1) its high contents compared with other lanthanide elements produced in the spent nuclear fuel, (2) its intense electronic absorption lines suitable for spectrochemical measurement [15]. The electronic absorption spectra shown in Fig. 5 were obtained by in-situ monitoring of the reaction of
3.2. In-situ monitoring of U(III) reaction with oxide ion In the pyrochemical process of spent nuclear fuel, the U(III) ion is the dominant uranium species in electrorefining and electrowinning steps. Therefore, its reaction with potential reactant materials (both ion and oxide material) are of great interests. Oxide ion, O2−, is regarded as one of the key parameters that may affect the pyrochemical process because of its high chemical reactivity with the metal ions. Actually, the molten salt based pyrochemical process of spent nuclear fuel involves t lithium oxide (Li2O) which dissociates into oxide. According to our solubility measurements, Li2O dissolves in molten LiCl at 700 °C by up to a ca. 8 wt.% [13]. Dissolved Li2O dissociates completely to produce an oxide ion (O2−) when the solubility is less than a ca. 4 wt.%. Thermodynamic calculations show that lithium oxide may react with actinide ions in high temperature molten salt media. The reaction of lithium oxide with the U(III) is of great concern in this respect. So, we monitored the reaction of the U(III) with lithium oxide by measuring the electronic absorption spectra in in-situ manner as shown in Fig. 4. The decrease of the U(III) absorption intensity act as an indicator of the reaction progress. When the reaction is completed,
Fig. 5. On-line monitoring of the UV–VIS spectra for the reaction of the U(III) with Nd2O3 in LiCl–KCl melt at 450 °C.
Y.H. Cho et al. / Microchemical Journal 96 (2010) 344–347
the U(III) ion with the Nd2O3 added. It shows the changes in the U(III) and Nd(III) ions simultaneously. The intensity of the U(III) peak decreased as the reaction with the Nd2O3 proceeded, and consequently the intensity of the Nd(III) peaks increased. In general, molar extinction coefficient of lanthanide ions is much lower than the U(III) ion. The Nd (III) spectra showed the same features as reported in the literature [14]. The characteristic peak patterns of the Nd(III), which are attributed to the 4G5/2, 2G7/2 ← 4I9/2 hypersensitive transitions, are clearly seen. The reactions of the U(III) with other lanthanide oxides (and Li mixed oxides) exhibited similar results. The electronic absorption lines of the other trivalent lanthanide ions are so weak in intensity that the increase of that is difficult to measure. However, the decrease of the U(III) ion peaks is the direct evidence of the reaction with all other lanthanide oxides. By analyzing the UV–VIS spectra and XRD data of uranium precipitates, the reaction of the U(III) with Nd2O3 can be expresses as follows: 2UCl3 þ Nd2 O3 →UO2 ↓ þ UO þ 2NdCl3 As was shown in this study, in situ UV–VIS measurement provided detailed information regarding the oxidation state and reactivity of the U(III) and some lanthanide ions in high temperature molten salt. Further studies are ongoing to apply the electronic absorption spectroscopy to various actinide ions in high temperature molten salt in combination with electrochemical method.
4. Conclusions We have measured the electronic absorption spectra of the U(III) ion in LiCl–KCl eutectic melt at 450 °C. The UV–VIS spectra of the U(III) ion consist of two main peaks in the range of 400–600 nm which are attributable to the 5f 3–5f 26d1 transitions. With the aid of UV–VIS spectroscopic tool, in-situ measurement of chemical reactions of the U (III) with oxide ion, lanthanide oxides was successfully achieved.
347
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