André Rossberg

Complexation of Uranium by Cells and S-Layer Sheets of Bacillus sphaericus JG-A12

ABSTRACT Bacillus sphaericus JG-A12 is a natural isolate recovered from a uranium mining waste pile near the town of Johanngeorgenstadt in Saxony, Germany. The cells of this strain are enveloped by a highly ordered crystalline proteinaceous surface layer (S-layer) possessing an ability to bind uranium and other heavy metals. Purified and recrystallized S-layer proteins were shown to be phosphorylated by phosphoprotein-specific staining, inductive coupled plasma mass spectrometry analysis, and a colorimetric method. We used extended X-ray absorption fine-structure (EXAFS) spectroscopy to determine the structural parameters of the uranium complexes formed by purified and recrystallized S-layer sheets of B. sphaericus JG-A12. In addition, we investigated the complexation of uranium by the vegetative bacterial cells. The EXAFS analysis demonstrated that in all samples studied, the U(VI) is coordinated to carboxyl groups in a bidentate fashion with an average distance between the U atom and the C atom of 2.88 ± 0.02 Å and to phosphate groups in a monodentate fashion with an average distance between the U atom and the P atom of 3.62 ± 0.02 Å. Transmission electron microscopy showed that the uranium accumulated by the cells of this strain is located in dense deposits at the cell surface.

EXAFS investigation of uranium(VI) complexes formed at Bacillus cereus and Bacillus sphaericus surfaces

Uranium(VI) complex formation at vegetative cells and spores of Bacillus cereus and Bacillus sphaericus was 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 Bacillus strains with those of the uranyl structure family indicates that the uranium is predominantly bound as uranyl complexes with phosphoryl residues.

Characterization of U(VI)-Acidithiobacillus ferrooxidans complexes using EXAFS, transmission electron microscopy, and energy-dispersive X-ray analysis

Summary We used a combination of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Transmission Electron Microscopy (TEM) and Energy-Dispersive X-ray (EDX) analysis to conduct molecular scale studies on U(VI) interaction with three recently described eco-types of Acidithiobacillus ferrooxidans. On the basis of the information obtained by using these methods, we concluded that uranyl phosphate complexes were formed by the cells of the three eco-types studied. The uranium accumulated by A. ferrooxidans cells was located mainly within the extracellular polysaccharides, and on the cell wall. Smaller amounts were also observed in the cytoplasm.

Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-L(III)-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous (242)Pu(III) and (242)Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO(2) phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO(2) for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

The Rossendorf beam line ROBL - A dedicated experimental station for XAFS measurements of actinides and other radionuclides

X-ray absorption fine structure (XAFS) spectroscopy is a powerful tool for obtaining basic molecular-level information, which is required for a better understanding of the mechanisms responsible for radionuclide transport in the environment. A unique experimental station dedicated to the study of actinides and other radionuclides by XAFS spectroscopy has become operational at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The main characteristics of the Rossendorf Beamline, ROBL, and its radiochemistry end station and selected results obtained on Tc and Np solutions are presented.

Spectroscopic and Microscopic Characterization of Uranium Biomineralization in Myxococcus xanthus

In this work, synchrotron-based X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) studies were carried out to elucidate at molecular scale the interaction mechanisms of Myxococcus xanthus with uranium at different pH values. Extended X-ray absorption fine structure (EXAFS) spectroscopic measurements showed that there are significant differences in the structural parameters of the U complexes formed by this bacterium at pH 2 and 4.5. At very low acidic pH of 2, the cells accumulated U(VI) as organic phosphate-metal complexes. At pH 4.5, however, the cells of this bacterium precipitated U(VI) as meta-autunite-like phase. TEM analyses demonstrated that at pH 2 the uranium accumulates were located mainly at the cell surface, whereas at pH 4.5 a uranium precipitation occurred on the cell wall and within the extracellular polysaccharides (EPS) characteristic of this bacterium. Dead/live staining studies showed that 30% and 50% of the uranium treated cell populations were alive at pH 2 and 4.5, respectively. The precipitation of U(VI) as mineral meta-autunite-like phase is possibly due to the bacterial acidic phosphatase activity. The precipitation of uranium as mineral phases may lead to more stable U(VI) sequestration that may be suitable for remediation purposes. These observations, combined with the very high uranium accumulation capability of the studied bacterial cells indicate that M. xanthus may significantly influence the fate of uranium in soil environments where these bacterial species are mainly found.

Electrochemical and Complexation Behavior of Neptunium in Aqueous Perchlorate and Nitrate Solutions

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.

Uranium speciation in plants

Several spectroscopic methods specially the time-resolved laser-induced fluorescence spectroscopy and the X-ray-absorption spectroscopy were used for the determination of the uranium speciation in plants. Differences between the uranium speciation in the initial solution and inside the plants could be detected. The chemical speciation of uranium is identical in the roots, shoot axis and leaf. It is independent from the uranium speciation in the initial solution and the type of the plant. The results indicate that the uranium is predominantly bound as uranium phosphate (phosphoryl) groups in the plants.

Solution coordination chemistry of uranium in the binary UO22+-SO42- and the ternary UO22+-SO42--OH- system

The structure and reaction dynamics in the systems UO22+-SO42- and UO22+-SO42--OH- were investigated using EXAFS and 17O-NMR spectroscopy. Uranium LIII edge EXAFS indicated a bidentate coordination mode of sulfate to uranyl. In solution, this is characterized by an U-S distance of 3.11 Å. Approximately 5 oxygen atoms were observed in the equatorial plane at 2.39-2.43 Å. The kinetics in the binary uranyl sulfate system can be described by four dominant exchange reactions: (1) UO22+ + SO42- ⇔ UO2SO4 (k1), (2) U*O22+ + UO2SO4 ⇔ U*O2SO4 + UO22+ (k2), (3) UO22+ + UO2(SO4)22- ⇔ 2 UO2SO4 (k3), and (4) UO2SO4 + SO42- ⇔ UO2(SO4)22- (k4). These reactions have rate constants indicating that the exchange is not of the simple Eigen-Wilkins type. Ternary uranyl sulfate hydroxide species were characterized by their 17O chemical shift and by potentiometry. There are no separate signals for the possible isomers of the ternary species indicating that they are in fast exchange with each other.

Aqueous uranium(VI) complexes with acetic and succinic acid: speciation and structure revisited.

We employed density functional theory (DFT) calculations, and ultraviolet-visible (UV-vis), extended X-ray absorption fine-structure (EXAFS), and attenuated total reflection Fourier-transform infrared (IR) spectroscopy analyzed with iterative transformation factor analysis (ITFA) to determine the structures and the pH-speciation of aqueous acetate (ac) and succinate (suc) U(VI) complexes. In the acetate system, all spectroscopies confirm the thermodynamically predicted pH-speciation by Ahrland (1951), with the hydrated uranyl ion and a 1:1, a 1:2 and a 1:3 U(VI)-ac complex. In the succinate system, we identified a new 1:3 U(VI)-suc complex, in addition to the previously known 1:1 and 1:2 U(VI)-suc complexes, and determined the pH-speciation for all complexes. The IR spectra show absorption bands of the antisymmetric stretching mode of the uranyl mojety (υ3(UO2)) at 949, 939, 924 cm(-1) and at 950, 938, 925 cm(-1) for the 1:1, 1:2 and 1:3 U(VI)-ac and U(VI)-suc complexes, respectively. IR absorption bands at 1535 and 1534 cm(-1) and at 1465 and 1462 cm(-1) are assigned to the antisymmetric υ3,as(COO) and symmetric υ3,s(COO) stretching mode of bidentately coordinated carboxylic groups in the U(VI)-ac and U(VI)-suc complexes. The assignment of the three IR bands (υ3(UO2), υ3,as(COO), υ3,s(COO)) and the stoichiometry of the complexes is supported by DFT calculations. The UV-vis spectra of the equivalent U(VI)-ac and U(VI)-suc complexes are similar suggesting common structural features. Consistent with IR spectroscopy and DFT calculations, EXAFS showed a bidentate coordination of the carboxylic groups to the equatorial plane of the uranyl moiety for all uranyl ligand complexes except for the newly detected 1:3 U(VI)-suc complex, where two carboxylic groups coordinate bidentately and one carboxylic group coordinates monodentately. All 1:1 and 1:2 complexes have a U-Owater distance of ∼2.36 Å, which is shorter than the U-Owater distance of ∼2.40 Å of the hydrated uranyl ion. For all complexes the U-Ocarboxyl distance of the bidentately coordinated carboxylic group is ∼2.47 Å, while the monodentately coordinated carboxylic group of the 1:3 U(VI)-suc complex has a U-Ocarboxyl distance of ∼2.36 Å, that is, similar to the short U-Owater distance in the 1:1 and 1:2 complexes.