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Short description of the work

The overall goal of the proposed study is to unravel the effects of solute and surface speciation of redox sensitive radionuclide and of the nature of Fe associated to clay minerals on the RN reduction kinetics and efficiency. Structural iron affects the physicochemical properties of clay minerals and, depending on its redox state, may oxidize or reduce radionuclides at their surface thus altering their mobility and/or (bio) availability. The study focuses on a natural montmorillonite (SWa), but also on a nontronite (a Fe rich smectite) interacting with two different radionuclides: Tc and the actinide U. By performing Tc(VII)/U(VI) and Tc(IV) adsorption/reduction experiment on clay samples with increasing levels of reduction, and by combining Tc/U EXAFS analysis and 57Fe Mössbauer analysis to wet chemical analyses (especially Tc/U retention and solid Fe speciation), it is possible to discriminate the contributions of the different mechanisms leading to the overall Tc(VII)/U(VI) adsorption and reduction kinetics. Tc experiments will be a good test to judge if predictive adsorption/reduction model could be established based on the changes of the clay surface properties as a function of pH and redox while U experiments will help to understand the effects of surface and solute speciation changes on redox interactions.

 

Short description of the work

Using the TALISMAN pooled facilities at KIT-INE, we were able to study the extraction capabilities and coordination chemistry of a family of newly designed ligands based on DTPA and EDTA appended with amino acids using nuclear magnetic resonance (NMR) and time resolved laser induced fluorescence spectroscopy (TRLIFS) spectroscopies. The precursors are readily available and affordable, allowing effective separation of trivalent lanthanides and actinides in the reprocessing of spent nuclear fuel. The process we have developed is a simplified approach to lanthanide/actinide separations based on a modified TALSPEAK process. In our hands, the polyaminocarboxylates DTPA and EDTA are appended with a series of amino acids that act to self-buffer the solution and lower the pH of the extraction processes to that of the pKa of the amino acids themselves (ca. 1.5). Our preliminary results show that the ligands show high selectivity for An(III) over Ln(III) with separation factors over 100, importantly without the need for an additional buffer agents or modifiers. The pooled facilities enabled us to examine the separation behavior of four ligands containing representative amino acids; DTPA-bis-alanine, DTPA-bis-histidine, EDTA-bis-alanine and EDTA-bis-histidine. In addition to the extraction experiments over the pH range 1-3 with all ligands and americium(III) and lanthanide(III), we were able to study the coordination environment of all ligands with americium(III) by NMR spectroscopy and that of the curium(III) derivatives by TRLIFS. Both these spectroscopic techniques showed that all the ligands possess fast complexation kinetics and verified the 1:1 binding motif of the chelates in solution, additionally giving information on the relative stabilities of the curium(III) complexes.

 

Short description of the work

The hexavalent state is the prevalent oxidation state for uranium in aqueous solution under oxic conditions, where it occurs as a linear, dioxo uranyl cation, UO2(2+). Under strongly oxidizing conditions also neptunium and plutonium occur as linear NpO2(2+) and PuO2(2+) cations. The multiple bonds in these cations are strong and normally unreactive, making the oxo groups to weak Lewis bases. However, the reactivity and the Lewis basicity of the oxo groups depend strongly on the ligands in the equatorial plane and their binding to the metal. To deepen the knowledge about the actinyl-ligand bonds and effect of the ligands onto the oxo bonds, various experimental and theoretical methods are possible. Direct determination of bond energies in actinyl complexes is difficult. Therefore, other methods should be utilized, such as different spectroscopic methods, among them vibrational spectroscopy accompanied with force field calculations is regarded as the most prominent one. Theoretical quantum chemical methods at different levels of theory may also be used to gain information about the structure and the relative stability of a complex, although it should be emphasized that the results may depend strongly on the molecular cluster size, boundary conditions, and solvent effects, the latter being particularly crucial for aqueous systems due to the problems with correct description of hydrogen bond network. As a result, data obtained by theoretical methods typically require comparison with the experimental information.

 

Short description of the work

The experiments carried out in HZDR-IRC were focused on determination of mechanisms of interaction of neptunium(III,IV and VI) ions with ionic species containing technetium with various oxidation states in presence of nitric acid.
The spectrophotometric titration clearly shows that neptunium bulk solutions initially contain Np(IV). These neptunium species can be electrochemically reduced in 4M H2SO4 to relatively stable Np(III) ions. Both technetium and neptunium species were electrogenerated separately in electrochemical cells equipped with gold/RVC or platinum electrodes. Such generated Tc and Np species were used for preparation of the mixed solution for Tc and Np interaction studies. UV-Vis-NIR was employed to study the interactions between both elements. The obtained results show that the interaction of technetium(III,IV) ionic species with neptunyl(VI) ions leads to formation of intermediate species characterized spectroscopically by a band with maxima at 460 nm and 760 nm also in the presence of nitric acid. Both neptunium(VI) and nitrate ions act as technetium oxidazing agents and both species are able to oxidize technetium(IV) ions in acidic media to pertechnetates. Noteworthy is the fact that in nitric acid solutions the Tc(V) intermediate species are detectable even after few minutes from its formation.

 
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