Ruthenium Nitrosyl Structure in Solvent Extraction Systems: A

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Ruthenium nitrosyl structure in solvent extraction systems: A comparison of tributyl phosphate (TBP), tetrabutyl urea (TBU), N-methyl, N-octyl ethylhexanamide (MOEHA) and N, N, N’, N’ tetraoctyl diglycolamide (TODGA) Thomas Dirks, Thomas Dumas, Pier Lorenzo Solari, and Marie-Christine Charbonnel Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02555 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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Industrial & Engineering Chemistry Research

Ruthenium nitrosyl structure in solvent extraction systems: A comparison of tributyl phosphate (TBP), tetrabutyl urea (TBU), N-methyl, N-octyl ethylhexanamide (MOEHA) and N, N, N’, N’ tetraoctyl diglycolamide (TODGA) Thomas Dirks†, Thomas Dumas* ,†, Pier Lorenzo Solari‡, Marie-Christine Charbonnel†† †CEA,

DEN, MAR, DMRC, SPDS, LILA, F-30207 Bagnols-sur-Cèze Cedex, France

‡Synchrotron SOLEIL, L’Orme des Merisiers, BP 48, St Aubin, 91192 Gif sur Yvette, France ††CEA,

DEN, MAR, DMRC, SPDS, F-30207 Bagnols-sur-Cèze Cedex, France

Ruthenium, PUREX, solvent extraction, FTIR, Raman, EXAFS, monoamide, TODGA, carbamide, TBP, N,N dialkyamides

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ABSTRACT

In the course of nuclear fuel treatment, solvent extraction cycles separate uranium and plutonium from fission products and minor actinides. During these steps, small amounts of the fission product ruthenium may follow the uranium and plutonium products. Herein, we show by Raman spectroscopy how ruthenium complexes in the aqueous phase differ as a function of the nitric acid concentration. Furthermore, we compare ruthenium extraction by the industrially used solvent trin-butylphosphate (TBP) to other extractants such as tetrabutyl urea (TBU), N-methyl, N-octyl ethylhexanamide (MOEHA) and N, N, N’, N’-tetraoctyl diglycolamide (TODGA). Analysis by ICP-AES demonstrates that all of these solvents have different ruthenium distribution ratios. In contrast, similar ruthenium complexes were identified by FTIR and EXAFS spectroscopy in all extractions from 4 M nitric acid. Hence, different distribution ratios of ruthenium are not due to different ruthenium complexes. By comparing ruthenium extraction with nitric acid and water extraction, we show a linear correlation between ruthenium and water concentration in the organic phase. This suggests an interaction between the solvent and water ligands during ruthenium extraction. In order to limit the ruthenium co-extraction in nuclear fuel treatments, we suggest further investigation of solvents with low water co-extraction.

INTRODUCTION Since nuclear reprocessing has been industrialized in France, spent fuels from different countries have been treated in order to reduce the toxicity and volume of high-level wastes, and reduce the reliance on natural uranium resources. Current commercial nuclear fuel reprocessing applies the plutonium and uranium refining by extraction (PUREX). [1] The process uses two immiscible liquids: a solvent composed of the extractant tri-n-butyl phosphate (TBP) in hydrogenated


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tetrapropylene (TPH) dilution, and nitric acid solutions at different concentrations. Initially, a ~3-4 M nitric acid solution of dissolved spent nuclear fuel is mixed with the TBP solvent for uranium (VI) and plutonium (IV) extraction. In the extraction equilibrium, the extractant TBP behaves as a neutral ligand L that coordinates cations Ax+ of uranium (VI) and plutonium (IV). The coordinated cations are finally extracted into the organic phase in form of neutral nitrate complexes, following the general extraction equilibrium: [2, 3, 4] 𝐴𝑥 + + 𝑥𝑁𝑂3― + 𝑛𝐿 ⇌ [𝐴(𝑁𝑂3)𝑥𝐿𝑛] (1) After the extraction steps, the loaded solvent passes a number of purification cycles–called scrubbing–to remove extracted impurities, like the fission product ruthenium. In scrubbing cycles, the loaded solvent is contacted with fresh nitric acid, which reaches an equilibrium concentration of about 2 M in the first scrubbing cycle, and 5 M in the second scrubbing cycle. Under such conditions, most of the uranium and plutonium remain in the organic phase, while impurities are scrubbed into the aqueous phase. After the plutonium back-extraction step realized by Pu(IV) reduction to Pu(III), the solvent loaded with uranium is contacted with a fresh nitric acid solution 2 M, despite its low molecular concentration compared to the other solvents. MOEHA and TBU have a significantly lower DRu than TBP and TODGA while extracting from 3 M. Only TODGA significantly extracts ruthenium over the entire studied nitric acid concentration range. The plot (c) shows nearly linear proportional relations between ruthenium and water extraction. Generally, more the applied solvents extract water, more they extract ruthenium. Specifically, this linear correlation has a higher slope for extractions with the bifunctional extractant TODGA than is observed for the monofunctional extractants TBP, TBU and MOEHA. The higher slope observed for TODGA may arise from a different number of extractants coordinating in the second coordination sphere of ruthenium complexes, or from the presence of n-octanol, which affects the water extraction. However, the relation between the ruthenium distribution ratio and water extraction confirms the former hypothesis of TBP-ruthenium interactions via water ligands, [18] and generalizes this interaction for different solvent extraction systems. TBP extracts more ruthenium than TBU and MOEHA, since it better interacts with water molecules, and, consequently, water ligands of ruthenium complexes. TBU follows the trend until 3 M nitric acid, but the ruthenium extraction becomes higher than it would have been estimated according to the linear correlation between water and ruthenium extraction. This increased ruthenium extraction of TBU argues for a secondary interaction between the protonated TBU and ruthenium, since the ruthenium extraction does not decline for higher nitric acid concentrations. The speciation analysis by Raman, FTIR and EXAFS is consistent with this correlation, since it is consistent with an outer-sphere interaction of all extractants with ruthenium complexes. The interaction between extractant and ruthenium occurs via the ligands sphere, and water ligands are the main connecting elements.


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Conclusion This article has demonstrated the dependence of ruthenium speciation in aqueous solution as a function of nitric acid concentration using Raman spectroscopy. Extractions of ruthenium from 4M nitric acid–similar to PUREX condition– were performed with TBP, TBU, MOEHA and TODGA in TPH diluent. Extracted ruthenium complexes have been analyzed and compared by use of FTIR and EXAFS spectroscopy. The FTIR results have been compared with the Raman spectrum of the initial RuNO(NO3)3 solution in 4 M nitric acid. For all studied extractants, the DRu of extractions from 1-5 M nitric acid have been determined by ICP-AES. The co-extraction of water and nitric acid has been measured after solvent pre-conditioning. Ruthenium distribution ratios for different acidities were plotted as a function of nitric acid and water coextraction, in order to determine the relationship between water, nitric acid and ruthenium extraction. The results have shown that none of the studied extractants interacts with ruthenium directly, after extractions from 4 M nitric acid. The absence of extractant-ruthenium coordination was proven by the absence of a carbonyl/phosphoryl and nitrosyl shift in the FTIR spectra, and by EXAFS spectroscopy. Furthermore, all spectra indicate a similar inner sphere coordination of ruthenium in 4 M nitric acid solution and in all corresponding organic solvents after extractions. Differences in the ruthenium distribution ratios for extractions from 4 M nitric acid are driven by outer-sphere interactions in all analyzed solvents. TBU partly interacts with nitrate ligands of ruthenium complexes, particularly for extraction from > 3 M nitric acid solution. TBP, MOEHA and TODGA predominantly bond to water ligands of ruthenium nitrosyl complexes during extraction, as shown by the linear relation between ruthenium and water extraction.


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Overall, the predominant interaction of the extractants with ruthenium nitrosyl complexes via water ligands enables to postulate the purification effort for future solvent extraction systems. We demonstrate the correlation between ruthenium and water extraction. Regarding the design of advanced solvent extraction processes, we forecast that solvents with high water extraction capacities must anticipate higher ruthenium co-extraction.


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Acknowledgement Thanks for the partial financial support from Orano, EDF and the GENIORS project (H2020 Euratom Research and Innovation Programme under grant agreement n°755171). Thanks to Vanessa Holfeltz for scientific advises and language corrections.

Abbreviations MOEHA, N-methyl, N-octyl ethylhexanamide; TBP, tributyl phosphate; TBU, tetrabutyl urea; TPH, hydrogenated tetrapropylene; TODGA, N, N, N’, N’ tetraoctyl diglycolamide.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Thomas Dumas: 0000-00016426-6484 Notes The author declares no competing financial interest. Author Contributions The manuscript was written by Thomas Dirks under scientific supervision from Thomas Dumas and Marie-Christine Charbonnel. All sample preparations, titrations and spectroscopic


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measurements were done by Thomas Dirks, exempt from the EXAFS measurement and data exploitation, which was predominantly done by Thomas Dumas and Pier Lorenzo Solari. All authors have given approval to the final version of the manuscript.

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FOR TABLE OF CONTENT ONLY

TBP

TBU

MOEHA

TODGA


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Ruthenium Nitrosyl Structure in Solvent Extraction Systems: A

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