Chemistry of actinides in aqueous solutions

Summary:

The scientific tasks within this area of research are related to the fundamental laws of behavior of eigen and pseudo-colloidal particles containing actinides in aqueous solutions. Understanding the processes of actinide dioxide formation in natural and technological systems is of great fundamental and practical importance. It is particularly important for the safety assessment of newly designed radioactive waste disposal facilities and for development of remediation technologies for territories contaminated with radionuclides.

Plutonium is the most radiotoxic actinide which is characterized by complicated chemical properties and is of the greatest interest among 5f elements. The study of PuO2 nanoparticles is carried out together with its structural and chemical analogues – thorium and cerium dioxides. ThO2 is the simplest analogue, while CeO2 has a non-stoichiometric composition and is suggested as a Pu analogue under reducing conditions, where Pu(III) presence is possible.

This research interests also include model experiments to the study radionuclide sorption and colloidal formation under strictly controlled lab conditions at the mineral/water interface and determination of changes in radionuclide speciation during the processes. The obtained data provide the basis for thermodynamic modeling of radionuclide reactions with environmental media through the use of contemporary computer codes.


Main scientific results:

  • It has been established that during precipitation from Pu(III), Pu(IV), Pu(V), and Pu(VI) containing solutions hydrated PuO2 nanoparticles are formed of equal size (no more than 2.5 nm) and crystallinity. Earlier, the problem of plutonium dioxide non-stoichiometry has been widely discussed in scientific papers, however, the researchers have reliably shown that plutonium, similarly to cerium, only exists in oxidation rate +4 in PuO2 nanoparticles. A unique combination of modern spectroscopic methods including synchrotron radiation allowed us to demonstrate that the process of PuO2 precipitation from Pu(VI) solution occurs slower, through the stage of formation of a metastable phase NH4PuO2CO3.
  • A significant increase in ThO2 lattice parameters (up to 1.1%) has been demonstrated and confirmed at the transition to nanoscale state. The effects of changing the local environment of thorium atoms during reducing of ThO2 particles to ultra-small sizes have been discovered and studied for the first time using modern synchrotron methods and theoretical modeling.
  • Experimental data on solubility of CeO2 nanoparticles in a wide pH range were obtained (including the radioactive tracer method), results were described using the reductive dissolution model, and the solubility product was calculated, including the values for nanoparticles of various sizes.
  • It was found that sorption of Pu(IV,V,VI) on α-Fe2O3 and TiO2 is accompanied by redox reactions resulting in Pu(IV) stabilization on surface of solid particles. It was shown that the driving force for the reduction of Pu(V,VI) to Pu(IV) is the thermodynamic stability of sorbed Pu(IV). Equilibrium constants of sorption reactions of actinide ions on the surface of α-FeOOH, α-Fe2O3 and TiO2 were determined. Linear free energy ratios between sorption and hydrolysis reactions of cations were formulated.

Selected publications:

1. L. Amidani, G.B.M. Vaughan, T.V. Plakhova, A.Yu. Romanchuk, E. Gerber, R. Svetogorov, S. Weiss, Y. Joly, S.N. Kalmykov, and K.O. Kvashnina The application of HEXS and HERFD XANES for accurate structural characterization of actinide nanomaterials: application to ThO2 // Chemistry – A European Journal, 2020, DOI: 10.1002/chem.202003360 (IF: 4,857)
2. E. Gerber, A.Yu. Romanchuk, I. Pidchenko, L. Amidani, A. Rossberg, C. Hennig, G. B. M. Vaughan, A. Trigub, T. Egorova, S. Bauters, T. Plakhova, M.O.J.Y. Hunault, S. Weiss, S.M. Butorin, A.C. Scheinost, S.N. Kalmykov, and K.O. Kvashnina. The missing pieces of the PuO2 nanoparticle puzzle // Nanoscale, 2020, DOI: 10.1039/d0nr03767b (IF: 6,970)
3. K.O. Kvashnina, A.Yu. Romanchuk, I. Pidchenko, L. Amidani, E. Gerber, A. Trigub, A. Rossberg, S. Weiss, K. Popa, O. Walter, R. Caciuffo, A. Scheinost, S. Butorin, and S. Kalmykov. A novel metastable pentavalent plutonium solid phase on the pathway from aqueous Pu(VI) to PuO2 nanoparticles // Angewandte Chemie International Edition, 2019, DOI: 10.1002/ange.201911637 (IF: 12,257)
4. L. Amidani, T.V. Plakhova, A.Yu. Romanchuk, E. Gerber, S. Weiss, A. Efimenko, C.J. Sahle, S.M. Butorin, S.N. Kalmykov, and K.O. Kvashnina. Understanding the size effects on the electronic structure of ThO2 nanoparticles // Physical Chemistry Chemical Physics, 2019, DOI: 10.1039/C9CP01283D (IF: 3,906)
5. T.V. Plakhova, A.Yu. Romanchuk, S.M. Butorin, A.D. Konyukhova, A.V. Egorov, A.A. Shiryaev, A.E. Baranchikov, P.V. Dorovatovskii, T. Huthwelker, E. Gerber, S. Bauters, M.M. Sozarukova, A.C. Scheinost, V.K. Ivanov, S.N. Kalmykov, and K.O. Kvashnina. Towards the surface hydroxyl species in CeO2 nanoparticles // Nanoscale, 2019, DOI: 10.1039/C9NR06032D (IF: 6,970)
6. T.V. Plakhova, A.Yu. Romanchuk, D.V. Likhosherstova, A.E. Baranchikov, P.V. Dorovatovskii, R.D. Svetogorov, T.B. Shatalova, T.B. Egorova, A.L. Trigub, K.O. Kvashnina, V.K. Ivanov, and S.N. Kalmykov. Size effects in nanocrystalline thoria // Journal of Physical Chemistry C, 2019, DOI: 10.1021/acs.jpcc.9b04379 (IF: 4,309)
7. A.Yu. Romanchuk, T.V. Plakhova, A.V. Egorov, T.B. Egorova, P.V. Dorovatovskii, Y.V. Zubavichus, A.A. Shiryaev, and S.N. Kalmykov. Redox-mediated formation of plutonium oxide nanoparticles // Dalton Transactions, 2018, DOI: 10.1039/C8DT02396D (IF: 4,099)
8. T.V. Plakhova, A.Yu. Romanchuk, S.N. Yakunin, T. Dumas, S. Demir, S. Wang, S.G. Minasian, D.K. Shuh, T. Tyliszczak, A.A. Shiryaev, A.V. Egorov, V.K. Ivanov, and S.N. Kalmykov. Solubility of nanocrystalline cerium dioxide: Experimental data and thermodynamic modeling // Journal of Physical Chemistry C, 2016, DOI: 10.1021/acs.jpcc.6b05650 (IF: 4,309)
9. A.Y. Romanchuk, S.N. Kalmykov, A.V. Egorov, Y.V. Zubavichus, A.A. Shiryaev, O.N. Batuk, S.D. Conradson, D.A. Pankratov, I.A. Presnyakov. Formation of crystalline PuO2+x•nH2O nanoparticles upon sorption of Pu(V,VI) onto hematite // Geochimica at Cosmochimica Acta, 2013, DOI: 10.1016/j.gca.2013.07.016 (IF: 3,884)