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European Commission

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.

 

Short description of the work

This project was a continuation of a previous JRP grant (TALI-C02-07) and the prior research work of Michal S. Dutkiewicz (SyntCryst4NS) on a set of reactions that represent the first use of a redox reaction between two actinide complexes to make heterobimetallic complexes. The complex [(UVIO2)(thf)(H2LEt)], in which the uranyl(VI) is supported by a calix[4]pyrrole Schiff base macrocycle, H4LEt reacts with the potential reductants AnIII An(Cp)3 (An = U, Np, Cp = C5H5-) was studied. The reaction between U(Cp)3 with the uranyl(VI) complex results in one-electron reduction to uranyl(V) and concomitant binding to one uranyl oxo group forming [(Cp)3UIVOUVO(thf)(H2LEt)]; the first selective functionalization of the uranyl oxo by another actinide ion. However, when the analogous reaction with Np(Cp)3 was carried out a novel trimetallic oxo‑neptunium(IV) compound, [(Cp)3Np(μ‑O)Np(Cp)2(μ-O)Np(Cp)3]. This unusual tri-Np complex, for which no other early An analogue exists is an interesting candidate for magnetic analyses as there should be strong coupling of the ions through the oxo bridges.

 

Short description of the work

Preliminary search for extractable mixed-ligand Am(III), Cm(III) and Eu(III) complexes in solvent extraction systems with lipophilic TODGA and hydrophilic SO3-Ph-BTP4– ligands, carried out using TRLFS (Time Resolved Laser Fluorescence Spectroscopy) studies with TEDGA – a hydrophilic homologue of TODGA – allows to detect the heteroleptic TEDGA / SO3-Ph-BTP complexes of Cm(III) and Eu(III) in dilute aqueous solutions. In the first step, the formation of heteroleptic complexes was studied in monophasic experiments. An aqueous solution containing SO3-Ph-BTP and Cm(III) [analogue of Am(III)] ions in 1 mmol/L HClO4 was titrated with N,N,N',N'-tetraethyl diglycolamide (TEDGA), a water soluble homologue of lipophilic TODGA. The following complexes were identified by their distinct emission spectra: Cm(III)(SO3-Ph-BTP), Cm(III)(TEDGA), Cm(TEDGA)2, Cm(TEDGA)3 and moreover two unknown yet heteroleptic complexes, Cm(III)(TEDGA)m(SO3-Ph-BTP)n. The heteroleptic complexes were identified as Cm(III)(TEDGA)(SO3-Ph-BTP) and Cm(III)(TEDGA)2(SO3-Ph-BTP) by slope analysis. In the second step, post extraction organic phases were examined. This was done by extracting Cm(III) or Eu(III) from an aqueous phase containing 20 mmol/L SO3-Ph-BTP in 0.5 mol/L HNO3 into an organic phase (0.2 mol/L TODGA + 5% 1-octanol in kerosene) and recording the Cm(III) or Eu(III) emission spectra of the organic phase samples. Unfortunately, only the emission spectrum of the respective M(III)(TODGA)3 complex was detected, no additional features from heteroleptic complexes were observed. Furthermore, Eu(III) excitation spectra were recorded. One sample was a blank organic phase, the other one was loaded with Eu(III) by extracting from an aqueous phase containing 30 mmol/L Eu(NO3)3 and 20 mmol/L SO3-Ph-BTP in 0.5 mol/L HNO3. Again, only the emission spectra of the Eu(III)(TODGA)3 complexes were detected regardless of the excitation wavelength. In conclusion, we could prove that heteroleptic DGA/SO3-Ph-BTP complexes do form; however, we were not able to detect them in the organic phase in the extraction experiments so far. This may be due to their being present only at relatively low concentrations as compared to the M(III)(TODGA)3 complexes.

 

Short description of the work

The work at this second visit involved collecting experimental data for the estimation of complexation constants using capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS). Traditional methods of determining complexation constants involve potentiometry, spectrophotometry, solvent extraction, and calorimetry. These methods can be time consuming, labor intensive, and require work with larger quantities of materials than CE-ICP-MS. Efforts are being made to find additional techniques for determining these values while minimizing time and reducing material usage and hazardous and/or radioactive waste generation. Doing so helps to improve safety while also minimizing costs. CE-ICP-MS can provide estimates of complexation constants very quickly and uses minimal sample volumes. CE-ICP-MS can be used to relate electrophoretic mobilities of the metal-ligand complexes with ligand concentration; the electrophoretic mobilities can then be used to estimate relevant complexation constants.
For this second visit, we investigated the complexation constants between 2-hydroxy-2-methylbutyric acid (HMBA) and acetic acid, and actinides of various oxidation states. We also investigated the impact of solvent composition on these complexation constants. The solvent media explored were composed of water and methanol. CE-ICP-MS allows for the rapid, simultaneous detection of elements in solution matrices, such as the 10% and 30% (v/v) methanol media used in these experiments. The actinides used in the experiments were curium (III), plutonium (IV), neptunium (V), and uranium (VI).
In fully aqueous media, we were able to determine that mobilities of the actinides decreased with an increasing concentration of the ligand. This behavior indicates that the speciation of the each actinide is changing as the mobility is dependent on both the size and charge of the analyte. As the ligand concentration is increased, the prevalence of higher order species is increased, which in turn decreases the charge on the overall actinide-ligand complex as well as increasing the size of the complex. This same trend was also present in the mixed solvent media.
The main goal of this work is to use the experimental data collected at INE to estimate complexation constants in this mixed aqueous-methanol system. Experiments with varying ligand concentrations were repeated in aqueous solvent systems that included 10% and 30% methanol. The impact of methanol on complexation was investigated for both acetate and HMBA. The experimental results are currently being evaluated for the estimation of the stability constants, and will be reported in peer reviewed manuscripts in the future.

 
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