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This paper presents first investigations on a novel An(III), Ln(III), Cs(I) and Sr(II) co-extraction conceptual process using a CHONsolvent composed of the calixarene crown-ether MAXCalix and the diglycolamide TODGA in a 1-octanol/kerosene diluent. The co-extraction is followed by a sequential stripping of (1) Cs(I) and Sr(II), (2) An(III) and (3) Ln(III).

(2R,2′S)-2,2′-oxybis-(N,N-didecylpropanamide) (cis-mTDDGA) and (2R,2′R)-2,2′-oxybis-(N,N-didecylpropanamide) (trans-mTDDGA) were studied using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side-band spectroscopy (VSBS) and density functional theory calculations (DFT) to find reasons for their different extraction properties. Stability constants of the respective Cm(III) and Eu(III) complexes show cis-mTDDGA to be the superior ligand which is in agreement with results from extraction experiments. cis-mTDDGA extracts Cm(III) and Eu(III) as 1:3 complexes. In case of trans-mTDDGA, 1:2 complexes of the form [M(trans-mTDDGA)2(η1-NO3)(H2O)2]2+(M = Cm, Eu) are extracted additionally to the 1:3 complexes. VSBS and DFT confirm the presence of inner-sphere nitrate in the 1:2 complex.

The complexation of Cm(III) with the recombinant human serum albumin (rHSA) (characterized by single deletion of residue Asp-1), is studied in dependence of pH and rHSA concentration using time-resolved laser fluorescence spectroscopy (TRLFS). A Cm(III) rHSA species is formed between pH 6.4 and 10.0 with the conditional stability constant being logK = 6.47 at pH = 7.4. Competition titration experiments with Cu(II) and Zn(II) confirm complexation at the N-terminal binding site (NTS) of rHSA and exclude the involvement of the Multi-Metal Binding Site (MBS). Comparison with a previous study on Cm(III) interaction with native albumin, HSA, points out, that residue Asp-1 is involved in Cm(III) binding to HSA but is not crucial for Cm(III) complexation at the NTS. The results are of major importance for a better understanding of fundamental actinide-protein interaction mechanisms which are highly required for the identification and characterization of relevant distribution pathways of incorporated radionuclides.

The solubility of Nd(III), Th(IV) and U(VI) was studied from undersaturation conditions in the presence of selected organic cement additives and model compounds: adipic acid, methyl acrylate, citric acid, melamine, ethylene glycol, phthalic acid and gluconic acid. Experiments were performed under Ar atmosphere in NaCl (2.5 and 5.0 M), MgCl2(1.0 and 3.5 M) and CaCl2(1.0 and 3.5 M) solutions with 9 ≤ pHm≤ 13 (pHm= −log[H+]). Initial concentrations of organic ligands in solution were set constant in all systems to [L]0= 0.025 M, except in specific cases (e.g.adipic acid, melamine and phthalic acid) where the ligand concentration in the matrix solutions was lower and controlled by solubility. Adipic acid, methyl acrylate, melamine, ethylene glycol and phthalic acid do not impact the solubility of Nd(III), Th(IV) and U(VI) in the investigated NaCl, MgCl2and CaCl2systems. Citrate significantly enhances the solubility of Nd(III), Th(IV) and U(VI) in NaCl systems. A similar effect was observed for Th(IV) and U(VI) in the presence of gluconate in NaCl systems. The impact of pH on the stability of the complexes is different for both ligands. Because of the larger number of alcohol groups in the gluconate molecule, this ligand is prone to form more stable complexes under hyperalkaline conditions that likely involve the deprotonation of several alcohol groups. The complexation of gluconate with U(VI) at pHm≈ 13 is however weaker than at pHm≈ 9 due to the competition with the highly hydrolysed moiety prevailing at pHm≈ 13, i.e.UO2(OH)42−. The impact of citrate and gluconate in MgCl2and CaCl2systems is generally weaker than in NaCl systems, expectedly due to the competition with binary Mg-L and Ca-L complexes. However, the possible formation of ternary complexes further enhancing the solubility is hinted for the systems Mg/Ca-Th(IV)-GLU and Ca-U(VI)-GLU. These observations reflect again the differences in the complexation properties of citrate and gluconate, the key role of the alcohol groups present in the latter ligand, and the importances of interacting matrix cations. The screening experiments conducted within this study contribute to the identification of organic cement additives and model compounds potentially impacting the solution chemistry of An(III)/Ln(III), An(IV) and An(VI) under intermediate to high ionic strength conditions (2.5 ≤ I≤ 10.5 M). This shows evident differences with respect to investigations conducted in dilute systems, and thus represents a very relevant input in the safety assessment of repositories for radioactive waste disposal where such elevated ionic strength conditions are expected.

AbstractThe complexation of Cm(III) and Eu(III) with 2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine (C5-BPP) and 2-bromohexanoic acid as lipophilic anion has been investigated by time-resolved laser fluorescence spectroscopy. Upon increasing ligand concentration three different Cm(III)-C5-BPP species with emission bands at 604.1 nm, 607.9 nm and 611.4 nm, respectively, are found and attributed to complexes with one, two and three C5-BPP molecules in the inner coordination sphere. Comparison with results of TRLFS experiments without 2-bromohexanoic acid shows that the C5-BPP ligand is able to completely displace the lipophilic anion from the inner coordination sphere, forming [Cm(C5-BPP)3]3+complexes. This complex is also found in the organic phase of an extraction experiment performed with Cm(III), demonstrating that the lipophilic anion required for the extraction is not directly coordinated to the metal ion in the species formed during extraction. In case of Eu(III) the number of different species formed cannot be determined accurately. Nevertheless, the formation of the complex [Eu(C5-BPP)3]3+in the presence of 2-bromohexanoic acid is confirmed.

AbstractThe preferential solvation of Cm(III) in binary aqueous mixtures of MeOH, tBuOH, DMSO, MeCN and acetone is studied at varied solvent composition. Solvation is investigated by time-resolved laser fluorescence spectroscopy and emission spectra and fluorescence lifetime data are obtained. At high mole fractions of organic solvent, preferential solvation of Cm(III) increases in the order: Me2CO < tBuOH ˜ MeOH < MeCN < H2O < DMSO. Thermodynamic data are derived from the spectroscopic results showing small positive standard Gibbs energies for the transfer of Cm(III) into the various solvent mixtures except for DMSO mixtures where negative values are found. The spectroscopically obtained enthalpies of transfer are fitted to a solvation model using the model parameters ? ? Hº12and (an+ßN)º, rendering valuable information on the interaction of Cm(III) with the solvent molecules. Cm(III)-solvent interaction increases in the order: tBuOH < MeCN < MeOH < DMSO.

AbstractA novel approach for the optimisation of extractants for the actinide(III)/lanthanide(III) separation is presented. Two BTP ligands are synthesised carrying OMe or Cl moieties on the pyridine ring, thus altering the electronic structure of the aromatic system. The influence of functionalisation on the ligand basicity is determined both spectroscopically and by quantum chemical calculations. Furthermore, we show that both distribution ratios and separation factors for Am(III)/Eu(III) separation increase upon increase of ligand basicity.

AbstractThe formation of aqueous Cm(III) nitrate complexes is studied in the temperature range from 5 to 200 °C by time resolved laser fluorescence spectroscopy (TRLFS). The experiments are performed in a custom build high pressure and high temperature fluorescence cell. The complex formation is measured at nitrate concentrations ranging from 0.10 to 4.61 mol/kg H2O. The mono- and dinitrate complexes are quantified by peak deconvolution of the fluorescence spectra and the complexation constants are determined as a function of the temperature. The conditional equilibrium constants are extrapolated to zero ionic strength using the specific ion interaction theory (SIT) and the thermodynamic standard state data (ΔrH°m, ΔrS°m, ΔrG°m, ΔrC°p,m) are determined from the temperature dependence of the equilibrium constants at I=0. The equilibrium constants up to 75 °C are well described by the Van´t Hoff equation (ΔrH°mindependent of Tand ΔrC°p,m=0). Modelling of the data at higher temperatures requires an extended equation including a term for the heat capacity changes (ΔrC°p,m).

The solvation of the Eu-perchlorate (ClO4–) and triflate (CF3SO3–, OTf–) salts as well as of Cm(ClO4)3and Am(ClO4)3in the ionic liquid C4mimTf2N (1-butyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide) has been comparatively investigated by application of laser fluorescence spectroscopy and X-ray absorption spectroscopy. Moreover, the ClO4–/OTf–ligand exchange reaction for the two actinide cations has been analyzed by the same spectroscopic techniques. A structural model for the different complexes was determined by the interpretation of the spectroscopic data. The lanthanide and the two actinide cations show the same coordination in C4mimTf2N. Moreover, a sequence for the strength of complexing ligands could be deduced from the spectroscopic data for the lanthanide and the two actinides: ClO4–>OTf–≥Tf2N–>H2O.

The complexation of 248Cm(III) with aqueous silicic acid is investigated in the pH range of 1.5–9.0 in 0.03 M NaCl with varying the silicate concentration under and over saturation in reference to the solubility of amorphous silica in order to characterize the reaction with monosilicic and polysilicic acid. Speciation is made by time-resolved laser fluorescence spectroscopy (TRLFS) in combination with radiometric quantification. Three different complexation products are observed: Cm-silicate(I), Cm-silicate(II) and Cm-silicate(III). The first and second species are formed in under-saturation of silicic acid, whereas all three species are produced in over-saturation of silicic acid. The formation of Cm-silicate(I) following the reaction of Cm3+with H3SiO4−appears in both under and over-saturation of silicic acid only as a minor fraction at pH=4–7. Cm-silicate(II) and Cm-silicate(III) are found to be colloidal. Cm-silicate(II) shows its spectroscopic characteristics varying with the experimental condition, whereas Cm-silicate(III) formed exclusively with polysilicic acid remains consistent and stable. The experimental results indicate that a species conversion takes place as Cm-silicate(I) → Cm-silicate(II) → Cm-silicate(III) with increasing pH and silicon concentration. Only the reaction leading to the formation of Cm-silicate(I) can be quantified for its stability constant as logβ°=7.74±0.08.

Sorption of Cm(III) onto γ-alumina coated with humic acid (HA) is studied by the Time Resolved Laser Fluorescence Spectroscopy (TRLFS). The experiments are performed at 0.1 M NaClO4, 0.44 g/L γ-Al2O3, 10 mg/L HA and at a metal ion concentration of 2×10-7mol/L. At the investigated pH range (4 to 10) HA is completely sorbed to γ-Al2O3. The excitation spectrum of Cm(III) bound to HA/γ-Al2O3in the wavelength range 370-400 nm exhibits broad flat bands very different from those obtained for the Cm(III) aquo ion and the Cm(III)-γ-Al2O3surface complex, respectively. The spectrum, lacking distinctive structure due to intramolecular energy transfer processes, points to the predominant binding of the Cm(III) to surface-bound HA. TRLFS experiments performed at two different excitation wavelengths (λex=355 and 396.6 nm) allow for a differentiation of humic-bound and non-humic-bound Cm(III). Differences in fluorescence spectra obtained at the different excitation wavelengths are found at pH<6.9. They are due to the presence of the non-complexed Cm(III) aquo ion which is not detected in the indirect excitation mode (λex=355 nm). At pH≥7, the fluorescence spectra obtained by indirect and direct excitation become congruent and again point to the existence of only humic-bound Cm(III) species. Comparison of peak maxima and fluorescence lifetimes for Cm(III)-HA and Cm(III)-HA/γ-Al2O3, however, reveal differences. The results clearly indicate a contribution of the γ-Al2O3surface to the Cm(III) binding and, thus, suggest the formation of ternary complexes such as >Al-O-Cm(III)(HA).

We studied the interaction of U(VI) with vegetative cells, heat killed cells, spores, and decomposed cells of Bacillus sphaericus. The characterization of the formed complexes was performed by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure spectroscopy (EXAFS). We observed no significant differences in the sorption behavior of vegetative and heat killed cells, whereas the spores showed a higher sorption of U(VI) (related to their dry weight). Regardless of the higher relative sorption of the spores of B. sphaericus, the fluorescence and EXAFS spectra of the vegetative cells, heat killed cells and spores were almost identical. Analysis of the data proved that U(VI) forms inner sphere complexes with organic bound phosphate groups on the cell surface. We observed no significant differences in the coordination numbers and the distances of the oxygen and phosphorus atoms in the inner coordination sphere.

Knowledge of the plutonium complexes formed with bacterial cells is critical for predicting the influence of microbial interactions on the migration behavior of actinides in the environment. This investigation describes the interaction of plutonium(VI) with cells of the aerobic soil bacteria, Bacillus sphaericus. The studies include the quantification of carboxylate and phosphate functional groups on the cell walls by potentiometric titration and the determination of the plutonium speciation by X-ray absorption fine structure (XAFS). Extended-XAFS (EXAFS) was used to determine the identity of the Pu(VI) interfacial complex with the bacteria, and the Pu(VI) was found primarily bound to phosphate groups on the cell surface. No carboxylate complexation was detected.

Uranium(VI) complex formation at vegetative cells and spores of Bacillus cereusand Bacillus sphaericuswas studied using uranium LII-edge and LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. A comparison of the measured equatorial U-O distances and other EXAFS structural parameters of uranyl species formed at the Bacillusstrains with those of the uranyl structure family indicates that the uranium is predominantly bound as uranyl complexes with phosphoryl residues.