The Coordination Chemistry of U4+ in Aqueous Solutions: Challenges and Insights from Spectroscopy and Theory
Résumé
The aqueous U4+ ion is a key species in the chemistry of uranium, with significant implications for nuclear fuel processing, environmental chemistry, and radiochemistry. Indeed, the safety assessment of a future repository, as well as the remediation of various uranium mining and milling legacies, requires a better understanding of cation interactions with potential ions (thermodynamic database, structures, etc.) and its speciation diagram.
At low pH, techniques such as EXAFS and UV-Vis spectroscopy methods are invaluable for investigating the hydrolysis properties and gain insights into the structural properties of U(IV) in solution but do not necessarily agree.
Previous EXAFS studies [1,2,3] report similar distances between 4+ and its water environment (2.41-2.42 Å) but suggest a wide range of coordination numbers, spanning from 8 up to almost 11, influenced by solvent and concentration variations. This inconsistency introduces significant uncertainty regarding the possible coordination number at low pH and in a diluted solution, which theoretical approaches have not yet fully resolved [4,5,6]. While UV-vis spectroscopy can distinguish 4+ from the other hydrolyzed species, it does not provide information on coordination numbers [7,8,9].
This study presents a comprehensive examination of the physico-chemical properties of U⁴⁺ in aqueous solution, using a combination of highly accurate relativistic and multiconfigurational quantum calculations with a statistical approach, based on the Wigner sampling method [10,11] or MD snapshots [6]. Quantum-mechanical calculations provide detailed insights into the electronic structure and bonding environment of the U⁴⁺ ion, while statistical analysis addresses the variability and uncertainty in coordination numbers and bond distances reported in experimental studies. By merging these methodologies, we aim to reconcile the discrepancies observed in EXAFS and UV-Vis spectroscopy data, enhancing our understanding of U4+ chemistry.
[1] H. Moll, M. A. Denecke, F. Jalilehvand, M. Sandström, I. Grenthe, Inorg. Chem., 38 (8), 1795–1799, (1999).
[2] C. Hennig, J. Tutschku, A. Rossberg, G. Bernhard, A. Scheinost. Inorg Chem. 44(19):6655-6661 (2005).
[3] A. Uehara, T. Fujii, H. Matsuura, N. Sato, T. Nagai, K. Minato, H. Yamana, Y. Okamoto. Proceedings in Radiochemistry, 1(1), 161-165 (2011).
[4] R. Atta-Fynn, D. F. Johnson, E. J. Bylaska,, E. S. Ilton, G.K. Schenter, W. A. De Jong. Inorg. Chem. 51 (5), 3016–3024 (2012).
[5] R.J. Frick, A. B. Pribil, T. S. Hofer, B.R. Randolf, A. Bhattacharjee, B. M. Rode. Inorg. Chem. , 48 (9), 3993–4002 (2009).
[6] E. Acher, M. Masella, V. Vallet, F. Réal. Phys. Chem. Chem. Phys. , 22 (4), 2343–2350 (2020).
[7] K. Opel, S. Weiß, S. Hübener, H. Zänker, G. Bernhard. Radiochimica Acta, 95 (3), 143–149 (2007).
[8] S. Lehmann, H. Foerstendorf, T. Zimmermann, M. Patzschke, F. Bok, V. Brendler,T. Stumpf, R. Steudtner. Dalton Trans., 48 (48), 17898–17907 (2019).
[9] W. Cha, H.-K. Kim, H. Cho, H.-R Cho, E.C. Jung, Y. Lee. RSC Adv. 10 (60), 36723–36733 (2020).
[10] M. Barbatti, A. J. A. Aquino, H. Lischka, Phys. Chem. Chem. Phys., 12 (19), 4959–4967 (2010).
[11] R. Crespo-Otero, M. Barbatti. Theor. Chem. Acc., 131, 1237 (2012).
At low pH, techniques such as EXAFS and UV-Vis spectroscopy methods are invaluable for investigating the hydrolysis properties and gain insights into the structural properties of U(IV) in solution but do not necessarily agree.
Previous EXAFS studies [1,2,3] report similar distances between 4+ and its water environment (2.41-2.42 Å) but suggest a wide range of coordination numbers, spanning from 8 up to almost 11, influenced by solvent and concentration variations. This inconsistency introduces significant uncertainty regarding the possible coordination number at low pH and in a diluted solution, which theoretical approaches have not yet fully resolved [4,5,6]. While UV-vis spectroscopy can distinguish 4+ from the other hydrolyzed species, it does not provide information on coordination numbers [7,8,9].
This study presents a comprehensive examination of the physico-chemical properties of U⁴⁺ in aqueous solution, using a combination of highly accurate relativistic and multiconfigurational quantum calculations with a statistical approach, based on the Wigner sampling method [10,11] or MD snapshots [6]. Quantum-mechanical calculations provide detailed insights into the electronic structure and bonding environment of the U⁴⁺ ion, while statistical analysis addresses the variability and uncertainty in coordination numbers and bond distances reported in experimental studies. By merging these methodologies, we aim to reconcile the discrepancies observed in EXAFS and UV-Vis spectroscopy data, enhancing our understanding of U4+ chemistry.
[1] H. Moll, M. A. Denecke, F. Jalilehvand, M. Sandström, I. Grenthe, Inorg. Chem., 38 (8), 1795–1799, (1999).
[2] C. Hennig, J. Tutschku, A. Rossberg, G. Bernhard, A. Scheinost. Inorg Chem. 44(19):6655-6661 (2005).
[3] A. Uehara, T. Fujii, H. Matsuura, N. Sato, T. Nagai, K. Minato, H. Yamana, Y. Okamoto. Proceedings in Radiochemistry, 1(1), 161-165 (2011).
[4] R. Atta-Fynn, D. F. Johnson, E. J. Bylaska,, E. S. Ilton, G.K. Schenter, W. A. De Jong. Inorg. Chem. 51 (5), 3016–3024 (2012).
[5] R.J. Frick, A. B. Pribil, T. S. Hofer, B.R. Randolf, A. Bhattacharjee, B. M. Rode. Inorg. Chem. , 48 (9), 3993–4002 (2009).
[6] E. Acher, M. Masella, V. Vallet, F. Réal. Phys. Chem. Chem. Phys. , 22 (4), 2343–2350 (2020).
[7] K. Opel, S. Weiß, S. Hübener, H. Zänker, G. Bernhard. Radiochimica Acta, 95 (3), 143–149 (2007).
[8] S. Lehmann, H. Foerstendorf, T. Zimmermann, M. Patzschke, F. Bok, V. Brendler,T. Stumpf, R. Steudtner. Dalton Trans., 48 (48), 17898–17907 (2019).
[9] W. Cha, H.-K. Kim, H. Cho, H.-R Cho, E.C. Jung, Y. Lee. RSC Adv. 10 (60), 36723–36733 (2020).
[10] M. Barbatti, A. J. A. Aquino, H. Lischka, Phys. Chem. Chem. Phys., 12 (19), 4959–4967 (2010).
[11] R. Crespo-Otero, M. Barbatti. Theor. Chem. Acc., 131, 1237 (2012).
