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Microbial U(VI) reduction influences the uranium mobility in contaminated subsurface environments and can affect the disposal of high-level radioactive waste by transform-ing the water-soluble U(VI) to less mobile U(IV). The reduction of U(VI) by the sulfate-reducing bacterium Desulfosporosinus hippei DSM 8344T, a close phylogenetic relative to naturally occurring microorganism present in clay rock and bentonite, was investigat-ed. D. hippei DSM 8344T showed a relatively fast removal of uranium from the superna-tants in artificial Opalinus Clay pore water. Combined speciation calculations and lumi-nescence spectroscopic investigations showed the dependence of U(VI) reduction on the initial U(VI) species. Scanning transmission electron microscopy coupled with ener-gy-dispersive X-ray spectroscopy showed uranium-containing aggregates on the cell surface and the formation of membrane vesicles. By combining different spectroscopic techniques, including UV/Vis spectroscopy, as well as uranium M4-edge X-ray absorp-tion near-edge structure (XANES) recorded in high-energy-resolution fluorescence-detection (HERFD) mode and extended X-ray absorption fine structure (EXAFS) analy-sis, the partial reduction of U(VI) could be verified, whereby the formed U(IV) product has an unknown structure. Furthermore, the U M4 HERFD-XANES showed the presence of U(V) during the process, suggesting a single-electron transfer mechanism for the microbial U(VI) reduction by sulfate reducers. These findings offer new insights into the U(VI) reduction by sulfate-reducing bacteria and contribute to a comprehensive safety concept for a repository for high-level radioactive waste.

Structural investigations of three actinide(IV) 4-phosphoryl 1H-pyrazol-5-olate complexes (An = Th(IV), U(IV), Np(IV)) and their cerium(IV) analogue display the same metal coordination in the solid state. The mononuclear complexes show the metal centre in a square antiprismatic coordination geometry composed by the two O-donor atoms of four deprotonated ligands. Detailed solid state analysis of the U(IV) complex shows that dependent on the solvent used altered arrangements are observable, resulting in a change in the coordination polyhedron of the U(IV) metal centre to bi-capped trigonal prismatic. Further, single crystal analyses of the La(III) and Ce(III) complexes show that the ligand can also act as a neutral ligand by protonation of the pyrazolyl moiety. All complexes were comprehensively characterized by NMR, IR and Raman spectroscopy. A single resonance in each of the 31P NMR spectra for the La(III), Ce(III), Ce(IV), Th(IV) and Np(IV) complex indicates the formation of highly symmetric complex species in solution. Extended X-ray absorption fine structure (EXAFS) investigations provide evidence for the same local structure of the U(IV) and Np(IV) complex in toluene solution, confirming the observations made in the solid state.

The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (Kd) of 1.23 × 104 L kg−1 for uranium and 3.07 × 105 L kg−1 for neptunium. The spectroscopic approach by X-ray absorption spectroscopy (XAS) and ATR FT-IR spectroscopy, which was applied on BIOS samples, showed the formation of inner-sphere complexes. The structural data obtained at the uranium LIII-edge and the neptunium LIII-edge indicate the formation of bidentate edge-sharing surface complexes, which are known as the main sorption species on abiotic ferrihydrite. Since the rate of iron precipitation in G. ferruginea-dominated systems is 60 times faster than in abiotic systems, more ferrihydrite will be available for immobilization processes of heavy metals and radionuclides in contaminated environments and even in the far-field of high-level nuclear waste repositories.

When assessing the safety of a nuclear waste repository, the interactions of dissolved long-lived radionuclides, such as the actinide neptunium, with corrosion products in the near-field of the repository are crucial processes that have to be taken into account. Zirconia (ZrO₂), the main corrosion product of the zircaloy cladding material of nuclear fuel rods, constitutes a first barrier against the release of radionuclides into the environment. A multimethod approach was pursued to gain a thorough understanding of the Np(V) sorption processes on the water−zirconia interface. For the macroscopic description of the Np(V)−ZrO₂ system, pH-dependent batch sorption experiments (varying ionic strength, Np(V) concentration, and the solid-to-liquid ratio) as well as a sorption isotherm experiment at pH 6 were conducted. The uptake of Np(V) was pH-dependent, with an increased sorption starting from pH 3 and being at a maximum at pH 6 and above. The Np(V) sorption was independent of ionic strength, hinting to the presence of Np(V) inner-sphere complexes on the zirconia surface. This was supported by zeta potential measurements in the presence of neptunium, where a shift to higher pH values of the isoelectric point of the neat ZrO₂ was observed. The Np(V) sorption edge was shifted towards lower pH values with increasing solid-to-liquid ratio, indicating the presence of different kinds of sorption sites, which was also deduced from the shape of the sorption isotherm. Molecular information of the surface species were obtained by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and in situ Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) revealing the predominant formation of inner-sphere Np(V) surface complexes. A short Np−Zr distance derived from EXAFS spectra suggested the presence of Np(V) bidentate complexes at the zirconia surface. Reliable information about the number and denticity of surface species obtained by macroscopic and molecular spectroscopic investigations potentially facilitate modeling approaches such as surface complexation modeling (SCM) that in turn will contribute to a more profound prediction of the environmental behavior of neptunium.

Clay rock represents a suitable host rock for the long-term storage of high-level radioactive waste with bentonite as backfill material. In the event of a worst-case scenario, water can enter the repository. It is possible that naturally occurring microorganisms can interact with the radionuclides and thereby change the chemical speciation or induce redox reactions. Among different sulfate-reducing bacteria, Desulfosporosinus species represent important members of the microbial communities in both clay rock and bentonite.[1,2] Desulfosporosinus hippei DSM 8344T is a close phylogenetic relative to an isolated bacterium from bentonite.[3] Therefore, this strain was selected to get a more profound insight into the uranium(VI) interactions with naturally occurring microorganisms from deep geological layers. Time-dependent experiments in artificial Opalinus Clay pore water[4] (100 µM uranium(VI), pH 5,5) showed a high removal of uranium from the supernatants within a short time range. UV/Vis studies of the dissolved cell pellets provided clear proof of a partial reduction of uranium(VI) to uranium(IV) in the samples, although bands of uranium(VI) were still observable. These findings propose a combined association-reduction process as an explanation for the ongoing interaction mechanism. Uranium aggregates formed on the cell surface were visible in TEM images. Furthermore, cells released membrane vesicles as a possible defense mechanism against cell encrustation. In addition, HERFD-XANES measurements confirmed the reduction of uranium(VI). But with these measurements also the presence of uranium(V) in the cell pellets could be demonstrated. This provides first evidence of the involvement of uranium(V) in uranium(VI) reduction by sulfate-reducing microorganisms. With the help of EXAFS measurements, different cell-related uranium species were detected. This study helps to better understand the complexity of redox processes in the environment and contribute to a safety concept for a nuclear repository in clay rock. Moreover, new insights into the uranium(VI) reduction mechanisms of sulfate-reducing bacteria were presented. References: [1] Bagnoud et al. (2016) Nat. Commun 7, 1–10. [2] Matschiavelli et al. (2019) Environ. Sci. Technol. 53, 10514–10524. [3] Vatsurina et al. (2008) Int. J. Syst. Evol. Microbiol. 58, 1228–1232. [4] Wersin et al. (2011) Appl. Geochemistry 26, 931–953.

The complexation of Np(V) with malonate and succinate is studied by different spectroscopic techniques, namely, attenuated total reflection Fourier transform infrared (ATR FT-IR) and extended X-ray absorption fine-structure (EXAFS) spectroscopy, as well as by quantum chemistry to determine the speciation, thermodynamic data, and structural information of the formed complexes. For complex stoichiometries and the thermodynamic functions (log βn°(Θ), ΔrHn°, ΔrSn°), near infrared absorption spectroscopy (vis/NIR) is applied. The complexation reactions are investigated as a function of the total concentration of malonate ([Mal2-]total) and succinate ([Succ2-]total), ionic strength [Im = 0.5-4.0 mol kg-1 Na+(Cl-/ClO4-)], and temperature (Θ = 20-85 °C). Besides the solvated NpO2+ ion, the formation of two Np(V) species with the stoichiometry NpO2(L)n1-2n (n = 1, 2, L = Mal2-, Succ2-) is observed. With increasing temperature, the molar fractions of both complex species increase and the temperature-dependent conditional stability constants log βn'(Θ) at given ionic strengths are determined by the law of mass action. The log βn'(Θ) are extrapolated to IUPAC reference-state conditions (Im = 0) according to the specific ion interaction theory (SIT), revealing thermodynamic log βn°(Θ) values. For all formed complexes, [NpO2(Mal)-: log β1°(25 °C) = 3.36 ± 0.11, NpO2(Mal)23-: log β2°(25 °C) = 3.95 ± 0.19, NpO2(Succ)-: log β1°(25 °C) = 2.05 ± 0.45, NpO2(Succ)23-: log β2°(25 °C) = 0.75 ± 1.22], an increase of the stability constants with increasing temperature was observed. This confirmed an endothermic complexation reaction. The temperature dependence of the log βn°(T) values is described by the integrated Van't Hoff equation, and the standard reaction enthalpies and entropies for the complexation reactions are determined. Furthermore, the sum of the specific binary ion-ion interaction coefficients Δen°(Θ) for the complexation reactions are obtained as a function of the t from the respective SIT modeling as a function of the temperature. In addition to the thermodynamic data, the structures of the complexes and the coordination modes of malonate and succinate are investigated using EXAFS spectroscopy, ATR-FT-IR spectroscopy, and quantum chemical calculations. The results show that in the case of malonate, six-membered chelate complexes are formed, whereas for succinate, seven-membered rings form. The latter ones are energetically unfavorable due to the limited space in the equatorial plane of the Np(V) ion (as NpO2+ cation).

Environmental conditions in deep geological repositories for radioactive waste may involve high pH values due to the degradation of concrete. However, the U(VI) sorption at such (hyper)alkaline conditions is still poorly understood. In this study, batch sorption experiments with Ca-bentonite in the pH range 8–13 at different carbonate concentrations were combined with spectroscopic investigations in order to gain insight into the underlying retention mechanisms. It was found that U(VI) sorption strongly correlates with the aqueous U(VI) speciation determined by time-resolved laser-induced luminescence spectroscopy (TRLFS). Increasing retention with increasing pH was accompanied by a change in aqueous speciation from uranyl carbonates to uranyl hydroxides. The occurrence of luminescence line-narrowing and a decreased frequency of the symmetric stretch vibration, deduced from site-selective TRLFS, indicate the presence of adsorbed U(VI) surface complexes. X-ray absorption fine structure (EXAFS) spectroscopy confirms that surface precipitation does not contribute significantly to the removal of U(VI) from solution but that retention occurs through the formation of two non-equivalent U(VI)-complexes on the bentonite surface. The present study demonstrates that in alkaline environments, where often only precipitation processes are considered, adsorption can provide effective retention of U(VI), despite the anionic character of prevailing aqueous species.

UO22+ was shown to form an interstrand crosslink between two different strands of a single DNA molecule. This crosslink hardly affected the hydrogen bonds between nucleobase pairs but destabilized the π–π stacking between the two nucleobases in the vicinity of UO22+-bound phosphate. Thereby, the fragility of the DNA backbone increased upon UO22+ binding.

Structural investigations of three actinide(iv) 4-phosphoryl 1H-pyrazol-5-olate complexes (An = Th(iv), U(iv), Np(iv)) and their cerium(iv) analogue display the same metal coordination in the solid state. The mononuclear complexes show the metal centre in a square antiprismatic coordination geometry composed by the two O-donor atoms of four deprotonated ligands. Detailed solid state analysis of the U(iv) complex shows that dependent on the solvent used altered arrangements are observable, resulting in a change in the coordination polyhedron of the U(iv) metal centre to bi-capped trigonal prismatic. Further, single crystal analyses of the La(iii) and Ce(iii) complexes show that the ligand can also act as a neutral ligand by protonation of the pyrazolyl moiety. All complexes were comprehensively characterized by NMR, IR and Raman spectroscopy. A single resonance in each of the 31P NMR spectra for the La(iii), Ce(iii), Ce(iv), Th(iv) and Np(iv) complex indicates the formation of highly symmetric complex species in solution. Extended X-ray absorption fine structure (EXAFS) investigations provide evidence for the same local structure of the U(iv) and Np(iv) complex in toluene solution, confirming the observations made in the solid state.

Technetium-99 ($^{99}$Tc) is one of the most concerning fission products due to its long half-life (2.14∙10$^{5}$ years) and the mobility of the anion pertechnetate (TcO$_{4-}$). [1] However, Tc migration decreases when Tc(VII) is reduced to Tc(IV). This scavenging step is favored by reductive material, among which Fe(II) minerals have been widely studied due to their versatility, low cost and ubiquity. [2] Green rust is a Fe(II)-Fe(III) mixed hydroxide that possesses adsorption, anion exchange and reduction capabilities. Its presence is expected in the near- and far-field of a nuclear waste repository because it is an iron corrosion product, and it is also formed in the environment when Fe$^{2+}$ interacts with Fe(III) minerals. [3] Batch contact studies have been performed under a wide range of conditions, i.e. pH (3-11), Tc concentration (nM-mM), and ionic strength (0-0.1 M). X-ray diffraction, Raman microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) provided information on Tc oxidation state and speciation as well as on secondary redox products related to the Tc interaction with green rust. In addition, re-oxidation experiments have been performed during six months. The results show that green rust removes Tc from solution with efficiencies between 80% (Kd = 8.0∙10$^{3}$ mL/g) and ≈100% (Kd = 9.9∙10$^{5}$ mL/g) for pH > 6.0, regardless on the ionic strength and the Tc concentration. In contrast, Tc removal for pH < 6.0 drops with decreasing pH, and ranges from 80% to 50% (Kd = 2.0∙10$^{3}$ mL/g), reaching a minimum at pH 3.5. XPS analysis reveals the predominance of Tc(IV) at all evaluated pH values (3.5 to 11.5), supporting that Tc reductive immobilization is the main retention mechanism. Re-oxidation experiments show that Tc is slowly solubilized when time increases. Kd[mL/g] =([Tc]$_{removed}$/[Tc]$_{solution}$)x(V/m) We thank the German Federal Ministry of Economic Affairs and Energy (BMWi) for funding the VESPA II project (02E11607B). [1] Meena, A.H.; Arai, Y. Env. Chem Lett (2017), 15, 241–263. [2] Pearce, C.I. et al. Sci. Total Environ. (2020), 716, 132849. [3] Usman, M. et al. Chem. Rev. (2018), 118, 3251–3304

The uptake of the fission product technetium (Tc) by chukanovite, an FeII hydroxy carbonate mineral formed as a carbon steel corrosion product in anoxic and carbonate-rich environments, was studied under anoxic, alkaline to hyperalkaline conditions representative for nuclear waste repositories in deep geological formations with cement-based inner linings. The retention potential of chukanovite towards TcVII is high in the pH range 7.8 to 12.6, evidenced by high solid-water distribution coefficients, log Rd ~ 6, and independent of ionic strength (0.1 or 1 M NaCl). Using Tc K-edge X-ray absorption spectroscopy (XAS) two series of samples were investigated, Tc chukanovite sorption samples and coprecipitates, prepared with varying Tc loadings, pH values and contact times. From the resulting 37 XAS spectra, spectral endmembers and their dependence on chemical parameters were derived by self-organizing (Kohonen) maps (SOM), a neural network-based approach of machine learning. X-ray absorption near-edge structure (XANES) data confirmed the complete reduction of TcVII to TcIV by chukanovite under all experimental conditions. Consistent with mineralogical phases identified by X-ray diffraction (XRD), SOM analysis of the extended X-ray absorption fine-structure (EXAFS) spectra revealed the presence of three species in the sorption samples, the speciation predominately controlled by pH: Between pH 7.8 and 11.8, TcO2-dimers form inner-sphere sorption complexes at the surface of the initial chukanovite as well as on the surface of secondary magnetite formed due to redox reaction. At pH ≥ 11.9, TcIV is incorporated in a mixed, chukanovite-like, Fe/Tc hydroxy carbonate precipitate. The same species formed when using the coprecipitation approach. Reoxidation of sorption samples resulted in a small remobilization of Tc, demonstrating that both the original chukanovite mineral and its oxidative transformation products, magnetite and goethite, contribute to the immobilization of Tc in the long term, thus strongly attenuating its environmental transport.

This study combines bulk structural and spectroscopic investigations of Eu3+- or Y3+/Eu3+ co-doped tetragonal and cubic zirconia polymorphs to gain an in-depth understanding of the solid solution formation process. Our bulk structural characterizations show that the dopant is homogenously distributed in the ZrO2 host structure resulting in an increase of the bulk symmetry with increasing dopant substitution (from 8 to 26 mol%). The local site symmetry around the Eu3+ dopant, however, determined with luminescence spectroscopy (TRLFS), remains low in all samples. Results obtained with X-ray pair distribution function and X-ray absorption spectroscopy show that the average coordination environment in the stabilized zirconia structures remains practically unchanged. Despite this very constant average dopant environment, site-selective TRLFS data show the presence of three nonequivalent Eu3+ environments in the ZrO2 solid structures. These Eu3+ environments are assumed to arise from Eu3+ incorporation at superficial sites, which increase in abundance as the size of the crystallites decrease, and incorporation on two bulk sites differing in the location of the oxygen vacancies with respect to the dopant cation.

The ubiquitous β-Proteobacterium Gallionella ferruginea is known as stalk-forming, microaerophilic iron(II) oxidizer, which rapidly produces iron oxyhydroxide precipitates. Uranium and neptunium sorption on the resulting intermixes of G. ferruginea cells, stalks, extracellular exudates, and precipitated iron oxyhydroxides (BIOS) was compared to sorption to abiotically formed iron oxides and oxyhydroxides. The results show a high sorption capacity of BIOS towards radionuclides at circumneutral pH values with an apparent bulk distribution coefficient (K

Plutonium plays an important role within nuclear waste materials because of its long half-life and high radiotoxicity. The aim of this study was to investigate with high spatial resolution the reactivity of the more oxidized forms of Pu(V,VI) within Opalinus Clay (OPA) rock, a heterogeneous, natural argillaceous rock considered as a potential repository host. A combination of synchrotron based X-ray microprobe and bulk techniques was used to study the spatial distribution and molecular speciation of Pu within OPA after diffusion and sorption processes. Microscopic chemical images revealed a pronounced impact of geochemical heterogeneities concerning the reactivity of the natural barrier material. Spatially resolved X-ray absorption spectroscopy documented a reduction of the highly soluble Pu(V,VI) to the less mobile Pu(IV) within the argillaceous rock material, while bulk investigations showed second-shell scattering contributions, indicating an inner-sphere sorption of Pu on OPA components. Microdiffraction imaging identified the clay mineral kaolinite to play a key role in the immobilization of the reduced Pu. The findings provide strong evidence that reduction and immobilization do not occur as linked processes on a single reactive phase but as decoupled, subsequent, and spatially separated reactions involving different phases of the OPA.

The complexation of Am(III) with formate in aqueous solution is studied as a function of the pH value using a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy, iterative transformation factor analysis (ITFA), and quantum chemical calculations. The Am LIII-edge EXAFS spectra are analyzed to determine the molecular structure (coordination numbers; Am–O and Am–C distances) of the formed Am(III)–formate species and to track the shift of the Am(III) speciation with increasing pH. The experimental data are compared to predictions from density functional calculations. The results indicate that formate binds to Am(III) in a monodentate fashion, in agreement with crystal structures of lanthanide formates. Furthermore, the investigations are complemented by thermodynamic speciation calculations to verify further the results obtained.

The use of uranium and to a minor extent plutonium as fuel for nuclear energy production or as components in military applications is under increasing public pressure. Uranium is weakly radioactive in its natural isotopy but its chemical toxicity, combined with its large scale industrial utilization, makes it a source of concern in terms of health impact for workers and possibly the general population. Plutonium is an artificial element that exhibits both chemical and radiological toxicities. So far, uranium (under its form uranyl, U(vi)) or plutonium (as Pu(iv)) decorporation or protecting strategies based on molecular design have been of limited efficiency to remove the actinide once incorporated after human exposure. In all cases, after human exposure, plutonium and uranium are retained in main target organs (liver, kidneys) as well as skeleton although they exhibit differences in their biodistribution. Polymers could represent an alternative strategy as their tropism for specific target organs has been reported. We recently reported on the complexation properties of methylcarboxylated polyethyleneimine (PEI-MC) with uranyl. In this report we extend our work to methylphosphonated polyethyleneimine (PEI-MP) and to the comparison between actinide oxidation states +IV (thorium) and +VI (uranyl). As a first step, thorium (Th(iv)) was used as a chemical surrogate of plutonium because of the difficulty in handling the latter in the laboratory. For both cations, U(vi) and Th(iv), the uptake curve of PEI-MP was recorded. The functionalized PEI-MP exhibits a maximum loading capacity comprised of between 0.56 and 0.80 mg of uranium (elemental) and 0.15-0.20 mg of thorium (elemental) per milligram of PEI-MP. Complexation sites of U(vi) and Th(iv) under model conditions close to physiological pH were then characterized with a combination of Fourier transform Infra Red (FT-IR) and Extended X-Ray Absorption Fine Structure (EXAFS). Although both cations exhibit different coordination modes, similar structural parameters with phosphonate functions were obtained. For example, the coordination sites are composed of fully monodentate phosphonate functions of the polymer chains. These physical chemical data represent a necessary basic chemistry approach before envisioning further biological evaluations of PEI-MP polymers towards U(vi) and Pu/Th(iv) contamination.

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO

Cellulosic materials present as tissue, paper, wood, or filter materials in low and intermediate level waste will degrade under alkaline conditions if water ingresses in a cementitious backfilled repository. The main degradation product is isosaccharinic acid. Complex formation with isosaccharinic acid may adversely affect the retention of radionuclides by the sorption or formation of solid phases. Hence, this compound is of particular concern in the context of nuclear waste disposal. Structural information of complexes is limited to spherical metal centers and little is known about the interaction of uranyl (UVIO22+) with isosaccharinic acid. Therefore, the interaction of UO22+ with α-isosaccharinate (ISA) was studied under acidic conditions focusing particularly on the structural characterization of the formed complexes. Attenuated total reflection Fourier-transform infrared (ATR-FTIR), nuclear magnetic resonance (NMR), UV-Vis, extended X-ray absorption fine structure (EXAFS) spectroscopy and electrospray-ionization mass spectrometry (ESI-MS) were combined with theoretical calculations to obtain a process understanding on the molecular level. The dominant binding motifs in the formed complexes are 5- and 6-membered rings involving the carboxylic group as well as the α- or β-hydroxy group of ISA. Two concentration dependent complex formation mechanisms were identified involving either mono- ([UO2(ISA)(H2O)3]+) or binuclear ([(UO2)2(ISA)(H2O)6]3+) species. Furthermore, this study unveils the interaction of UO22+ with the protonated α-isosaccharinic acid (HISA) promoting its transformation to the corresponding α-isosaccharinate-1,4-lactone (ISL) and inhibiting the formation of polynuclear UO22+-ISA species. Future studies on related systems will benefit from the comprehensive knowledge concerning the behavior of ISA as a complexing agent gained in the present study.

The development of the nuclear industry has raised multiple questions about its impact on the biotope and humans. Proteins are key biomolecules in cell machinery and essential in deciphering toxicological processes. Phosvitin was chosen as a relevant model for phosphorylated proteins because of its important role as an iron, calcium, and magnesium storage protein in egg yolk. A multitechnique spectroscopic investigation was performed to reveal the coordination geometry of two oxocations of the actinide family (actinyl UVI , NpV ) in speciation with phosvitin. IR spectroscopy revealed phosphoryl groups as the main functional groups interacting with UVI . This was confirmed through laser luminescence spectroscopy (U) and UV/Vis absorption spectroscopy (Np). For UVI , X-ray absorption spectroscopy at the LIII edge revealed a small contribution of bidentate binding present, along with predominantly monodentate binding of phosphoryl groups; for NpV , uniquely bidentate binding was revealed. As a perspective to this work, X-ray absorption spectroscopy speciation of UVI and NpV in the extracted yolk of living eggs of the dogfish Scyliorhinus canicula was determined; this corroborated the binding of phosphorous together with a reduction of the actinyl moiety. Such data are essential to pinpoint the mechanisms of heavy metals (actinyls) accumulation and toxicity in oviparous organisms, and therefore, contribute to a shift from descriptive approaches to predictive toxicology.

A comprehensive molecular analysis of a simple aqueous complexing system. U(VI) acetate. selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

International audience; Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO2 and ThO2 nanoparticles displayed as dispersed colloids, nanopowders or nanostructured oxide powders, allow to establish a clear relationship between the size of the nanocrystals composing these oxides and their corresponding An(IV) local structure investigated by EXAFS spectroscopy. Particularly, the probed An(IV) first oxygen shell evidences an analogous behaviour for both Pu and Th oxides. This observation suggests that the often observed and controversial splitting of the Pu-O shell on the Fourier transformed EXAFS signal of PuO2 samples is attributed to a local structural disorder driven by a nanoparticle surface effect rather than to the presence of PuO2+x species.

The complexation of the trivalent lanthanides Nd(III) and Eu(III) and of the actinide Am(III) with malate was studied using a multi−method approach. The combination of structural and thermodynamic studies was required for the interpretation of the stoichiometry and thermodynamic data (logβ0, ΔrH0m,2, ΔrS0m, ΔrG0m) of the lanthanide/actinide malate complexes leading to a profound molecular understanding of the system. The structure-sensitive methods vibrational spectroscopy and extended X–ray absorption fine structure spectroscopy complemented with quantum-mechanical ab–initio molecular dynamics calculations revealed a tridentate ring structure of the respective metal complexes. The metal is coordinated by two carboxylate groups and a hydroxyl group. UV–Vis, laser fluorescence and calorimetric studies consistently yielded two complex species having a 1:1 and a 1:2 (metal:malate) stoichiometry. Parallel factor analysis and iterative transformation factor analysis were applied to decompose experimental spectra into their single components and to determine stability constants. The 1:1 and 1:2 Nd(III) malate complexation constants determined by isothermal titration calorimetry were extrapolated to zero ionic strength using the specific ion interaction theory, yielding logβ10 and logβ20 of about 6 and 9, respectively. The respective complexation enthalpies ΔrH0m,1 and ΔrH0m,2 showed average values of 5 kJ·mol−1 which are typical for small organic molecules. The comparison of Nd(III) and Am(III) malate complexes showed that the malate binding motif, the speciation and the thermodynamics can be transferred from lanthanides(III) to actinides(III) supporting the 4f–/ 5f–element homology.

Layered double hydroxides (LDH) play a decisive role in regulating the mobility of contaminants in natural and engineered environments. In this work, the retention of an Fe(II)-Al(III)-Cl LDH towards pertechnetate (TcO4–), which is the most stable and highly mobile form of Tc under aerobic conditions, is investigated comprehensively as a function of pH, Tc concentration and ionic strength. For a technetium initial concentration of 5 µM, its retention yield is higher than 80% from pH 3.5 to pH 10.5, especially at NaCl concentration below 0.1 M. A combination of vibrational and X-ray absorption spectroscopy provides structural information on the retention mechanism on a molecular scale. X-ray absorption near edge spectroscopy (XANES) confirms that most of the Tc uptake is due to an initial Tc(VII) reduction to Tc(IV), and consecutive Tc(IV) interaction with the solid. The analysis of the extended X-ray absorption fine structure (EXAFS) reveals two different mechanisms of Tc(IV) interaction with hematite (sub-product of the LDH oxidation and confirmed by Raman microscopy). At low pH, sorption of Tc(IV) dimers via inner-sphere monodentate complexation on hematite dominates. In contrast, under alkaline conditions, Tc(IV) is incorporated into the structure of hematite. Additionally, in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR) evidences a small contribution of the total uptake corresponding to Tc(VII) anion exchange. The derived molecular structures increase confidence in predictive modelling of Tc migration patterns in subsurface environments, e.g. in the vicinity of a radioactive waste repository and treatment sites or in polluted areas due to other anthropogenic Tc sources.

Up until now, molecular chelating agents, such as diethylenetriamine pentaacetic acid (DTPA), have been the standard method for actinide human decorporation. Mainly active in blood serum, their distribution within the body is thus limited. To treat a wider range of organs affected by plutonium contamination, a potential new class of macromolecular decorporation agents is being studied. Polyethyleneimine methylenecarboxylate (PEI-MC) is one such example. It is being considered here because of its capacity for targeting the liver and bones.

Haloarchaea represent a predominant part of the microbial community in rock salt, which can serve as host rock for the disposal of high level radioactive waste. However, knowledge is missing about how Haloarchaea interact with radionuclides. Here, we used a combination of spectroscopic and microscopic methods to study the interactions of an extremely halophilic archaeon with uranium, one of the major radionuclides in high level radioactive waste, on a molecular level. The obtained results show that Halobacterium noricense DSM 15987T influences uranium speciation as a function of uranium concentration and incubation time. X-ray absorption spectroscopy reveals the formation of U(VI) phosphate minerals, such as meta-autunite, as the major species at a lower uranium concentration of 30 μM, while U(VI) is mostly associated with carboxylate groups of the cell wall and extracellular polymeric substances at a higher uranium concentration of 85 μM. For the first time, we identified uranium biomineralization in the presence of Halobacterium noricense DSM 15987T cells. These findings highlight the potential importance of Archaea in geochemical cycling of uranium and their role in biomineralization in hypersaline environments, offering new insights into the microbe-actinide interactions in highly saline conditions relevant to the disposal of high-level radioactive waste as well as bioremediation.

In peatlands, arsenite was reported to be effectively sequestered by sulfhydryl groups of natural organic matter. To which extent porewater arsenite can react with reduced sulfur to form thioarsenates and how this affects arsenic sequestration in peatlands is unknown. Here, we show that, in the naturally arsenic-enriched peatland Gola di Lago, Switzerland, up to 93% of all arsenic species in surface and porewaters were thioarsenates. The dominant species, monothioarsenate, likely formed from arsenite and zerovalent sulfur-containing species. Laboratory incubations with sulfide-reacted, purified model peat showed increasing total arsenic sorption with decreasing pH from 8.5 to 4.5 for both, monothioarsenate and arsenite. However, X-ray absorption spectroscopy revealed no binding of monothioarsenate via sulfhydryl groups. The sorption observed at pH 4.5 was acid-catalyzed dissociation of monothioarsenate, forming arsenite. The lower the pH and the more sulfhydryl sites, the more arsenite sorbed which in turn shifted equilibrium toward further dissociation of monothioarsenate. At pH 8.5, monothioarsenate was stable over 41 days. In conclusion, arsenic can be effectively sequestered by sulfhydryl groups in anoxic, slightly acidic environments where arsenite is the only arsenic species. At neutral to slightly alkaline pH, monothioarsenate can form and its slow transformation into arsenite and low affinity to sulfhydryl groups suggest that this species is mobile in such environments.

Since the past few years, Pu(V) has gained much attention due to its potential contribution to the environmental migration of actinides. However, the preparation of concentrated (up to mM) and pure Pu(V) solutions is quite difficult and often hindered by its great instability towards disproportionation, thus limiting the accessibility to physical and chemical property data. This work describes the rapid and facile sonochemical preparation of relatively stable Pu(V) solutions in the millimolar range free from the admixtures of the other oxidation states of plutonium. The mechanism deals with the sonochemical reduction of Pu(VI) in weakly acidic perchloric solutions by using the in situ generated H2O2, where the kinetics can be dramatically enhanced under high frequency ultrasound and an Ar/O2 atmosphere. The quasi-exclusive presence of the Pu(V) aqua ion in solution was evidenced by UV-vis absorption spectroscopy. The prepared solutions were found to be stable for more than one month which allowed the accurate XAFS and NMR investigations of Pu(V). EXAFS spectra revealed the presence of two trans dioxo Pu[double bond, length as m-dash]O bonds at 1.81 Å and 4–6 equatorial Pu–Oeq interactions at 2.47 Å characteristic of coordinated water molecules. The exact number of water molecules (N[Oeq(H2O)] = 4) was determined by simulating the EXAFS spectra of the PuO2+ aqua complexes using DFT calculations (geometry and the Debye–Waller factor) and comparing them with experimental signals. For the first time, the magnetic susceptibility of the pentavalent state of plutonium in aqueous solutions was also determined (χM = 16.3 × 10−9 m3 mol−1 at 25 °C) and the related Curie constant was estimated (C = 6.896 × 10−6 m3 K mol−1).

The contamination of the environment by U may affect plant life and consequently may have an impact on animal and human health. The present work describes U(VI) sequestration by Acidovorax facilis using a multidisciplinary approach combining wet chemistry, transmission electron microscopy, and spectroscopy methods (e.g. cryo-time resolved laser-induced fluorescence spectroscopy, extended X-ray absorption fine structure spectroscopy, and in-situ attenuated total reflection Fourier transform infrared spectroscopy). This bacterial strain is widely distributed in nature including U-contaminated sites. In kinetic batch experiments cells of A. facilis were contacted for 5 min to 48 h with 0.1 mM U(VI). The results show that the local coordination of U species associated with the cells depends upon time contact. U is bound mainly to phosphate groups of lipopolysaccharide (LPS) at the outer membrane within the first hour. And, that both, phosphoryl and carboxyl functionality groups of LPS and peptidoglycan of A. facilis cells may effectuate the removal of high U amounts from solution at 24–48 h of incubation. It is clearly demonstrated that A. facilis may play an important role in predicting the transport behaviour of U in the environment and that the results will contribute to the improvement of bioremediation methods of U-contaminated sites.

International audience; Since the 1940s, great amounts of Plutonium (Pu) have been produced for both military and civil purposes. Until now, the standard therapy for decorporation following inhalation has been the intravenous injection of diethylenetriaminepentaacetic acid ligand (Ca-DTPA form). This method offers a strong complexing constant for Pu(IV) but has poor chemical specificity, therefore its efficacy is limited to actinides present in the blood. Consequently, there is no decorporation treatment currently available which efficiently removes the intracellular Pu(IV) trapped in the pulmonary macrophages. Our research shows that a nanoparticle approach could be of particular interest due to large contact area and ability to target the retention compartments of the lungs. In this study, we have focused on the inhalation process involving forms of Pu(IV) with poor solubility. We explored the design of biocompatible nanoparticles able to target the macrophages in the lung alveoli and to chelate the forms of Pu(IV) with poor solubility. Nanoparticle formation was achieved through an ionic cross-linking concept using a polycationic polymer and an anionic chelate linker. We chose N-trimethyl chitosan, for its biocompatibility, as the polycationic polymer base of the nanoparticle and the phosphonic analogue of DTPA, diethylenetriamine- pentamethylenephosphonic acid (DTPMP) as the anionic chelating linker in forming NPs TMC-DTPMP. The synthesis and physico-chemical characterization of these NPs are presented. Secondly, the complexation mechanisms of TMC-DTPMP NPs with Thorium (Th(IV)) are discussed in terms of efficiency and structure. The Extended X-Ray Absorption Fine Structure (EXAFS) of the TMC-DTPMP complex with Th(IV) as well as Pu(IV) are defined and completed with DFT calculations to further delineate the plutonium coordination sphere after complexation. Finally, preliminary cytotoxicity tests onto macrophages were assayed.

Hematite plays a decisive role in regulating the mobility of contaminants in rocks and soils. The Np(V) reactions at the hematite-water interface were comprehensively investigated by a combined approach of in situ vibrational spectroscopy, X-ray absorption spectroscopy and surface complexation modeling. A variety of sorption parameters such as Np(V) concentration, pH, ionic strength, and the presence of bicarbonate was considered. Time-resolved IR spectroscopic sorption experiments at the iron oxide-water interface evidenced the formation of a single monomer Np(V) inner-sphere sorption complex. EXAFS provided complementary information on bidentate edge-sharing coordination. In the presence of atmospherically derived bicarbonate the formation of the bis-carbonato inner-sphere complex was confirmed supporting previous EXAFS findings.1 The obtained molecular structure allows more reliable surface complexation modeling of recent and future macroscopic data. Such confident modeling is mandatory for evaluating water contamination and for predicting the fate and migration of radioactive contaminants in the subsurface environment as it might occur in the vicinity of a radioactive waste repository or a reprocessing plant.

The complexation of acetate with Am(III) is studied as a function of the pH (1–6) by extended X-ray absorption fine-structure (EXAFS) spectroscopy. The molecular structure of the Am(III)–acetate complexes (coordination numbers, oxygen and carbon distances) is determined from the rawk3-weighted AmLIII-edge EXAFS spectra. The results show a continuous shift of Am(III) speciation with increasing pH value towards the complexed species. Furthermore, it is verified that acetate coordinates in a bidentate coordination mode to Am(III) (Am—C distance: 2.82 ± 0.03 Å). The EXAFS data are analyzed by iterative transformation factor analysis to further verify the chemical speciation, which is calculated on the basis of thermodynamic constants, and the used structural model. The experimental results are in very good agreement with the thermodynamic modelling.

The complexation of U(vi) at the proteinaceous surface layer (S-layer) of the archaeal strain Sulfolobus acidocaldarius was investigated over a pH range from pH 1.5 to 6 at the molecular scale using time-resolved laser-induced fluorescence spectroscopy (TRLFS) and U L(III)-edge extended X-ray absorption fine structure (EXAFS). The S-layer, which represents the interface between the cell and its environment, is very stable against high temperatures, proteases, and detergents. This allowed the isolation and purification of S-layer ghosts (= empty cells) that maintain the size and shape of the cells. In contrast to many other microbial cell envelope compounds the studied S-layer is not phosphorylated, enabling the investigation of uranyl carboxylate complexes formed at microbial surfaces. The latter are usually masked by preferentially formed uranyl phosphate complexes. We demonstrated that at highly acidic conditions (pH 1.5 to 3) no uranium was bound by the S-layer. In contrast to that, at moderate acidic pH conditions (pH 4.5 and 6) a complexation of U(vi) at the S-layer via deprotonated carboxylic groups was stimulated. Titration studies revealed dissociation constants for the carboxylic groups of glutamic and aspartic acid residues of pK(a) = 4.78 and 6.31. The uranyl carboxylate complexes formed at the S-layer did not show luminescence properties at room temperature, but only under cryogenic conditions. The obtained luminescence maxima are similar to those of uranyl acetate. EXAFS spectroscopy demonstrated that U(vi) in these complexes is mainly coordinated to carboxylate groups in a bidentate binding mode. The elucidation of the molecular structure of these complexes was facilitated by the absence of phosphate groups in the studied S-layer protein.

A fundamental understanding of actinide incorporation processes in envisioned nuclear waste forms, such as monazite ceramics, is required for a reliable prediction of the long-term stability of such ceramic materials for safe nuclear disposal. The present study provides structural insights into the formation of monazite solid solutions by incorporation of PuIII and verifies previous results on surrogate materials, where Eu and Gd served as inactive analogues for trivalent actinides. A solid state method was used to synthesize La1-xPuxPO4 (x = 0.01, 0.05, 0.10, 0.15, 0.5) solid solutions with monazite structure. XRD measurements of the compounds with x = 0.50 revealed the formation of two phases: (La,Pu)PO4-monazite and a cubic phase (PuO2). Pure-phase La1-xPuxPO4-monazite solid solutions were obtained for materials with x = 0.00-0.15 and confirmed by a linear dependence of the lattice parameters on composition according to Vegard’s law. X-ray absorption spectroscopy (XAS) analysis at the Pu-LIII and La-LIII edges verified the +III valence state of plutonium in the monazite solid solutions. The local environment of Pu is similar as in PuPO4-like along the solid solution series, except for the longest fitted cation-cation distance, which may be an indication of cluster formation consisting of a few Pu-atoms in the La-Pu-monazite lattice.

The interaction between the Pseudomonas fluorescens biofilm and U(VI) were studied using extended X-ray absorption fine structure spectroscopy (EXAFS), and time-resolved laser fluorescence spectroscopy (TRLFS). In EXAFS studies, the formation of a stable uranyl phosphate mineral, similar to autunite (Ca[UO2]2[PO4]2•2-6H2O) or meta-autunite (Ca[UO2]2[PO4]2•10-12H2O) was observed. This is the first time such a biomineralization process has been observed in P. fluorescens. Biomineralization occurs due to phosphate release from the cellular polyphosphate, likely as a cell's response to the added uranium. It differs significantly from the biosorption process occurring in the planktonic cells of the same strain. TRLFS studies of the uranium-contaminated nutrient medium identified aqueous Ca2UO2(CO3)3 and UO2(CO3)3 (4-) species, which in contrast to the biomineralization in the P. fluorescens biofilm, may contribute to the transport and migration of U(VI). The obtained results reveal that biofilms of P. fluorescens may play an important role in predicting the transport behavior of uranium in the environment. They will also contribute to the improvement of remediation methods in uranium-contaminated sites.

The mechanism by which oxygen bound in UO22+ exchanges with that from water under strong alkaline conditions remains a subject of controversy. Two recent NMR studies independently revealed that the key intermediate species is a binuclear uranyl(VI) hydroxide, presumably of the stoichiometry [(UO2(OH)42−)(UO2(OH)53−)]. The presence of UO2(OH)53− in highly alkaline solution was postulated in earlier experimental studies, yet the species has been little characterized. Quantum-chemical calculations (DFT and MP2) show that hydrolysis of UO2(OH)42− yields UO3(OH)33− preferentially over UO2(OH)53−. X-ray absorption spectroscopy was used to study the uranium(VI) speciation in a highly alkaline solution supporting the existence of a species with three U−O bonds, as expected for UO3(OH)33−. Therefore, we explored the oxygen exchange pathway through the binuclear adduct [(UO2(OH)42−)(UO3(OH)33−)] by quantum-chemical calculations. Assuming that the rate-dominating step is proton transfer between the oxygen atoms, the activation Gibbs energy for the intramolecular proton transfer within [(UO2(OH)42−)(UO3(OH)33−)] at the B3LYP level was estimated to be 64.7 kJ mol−1. This value is in good agreement with the activation energy for “yl”−oxygen exchange in [(UO2(OH)42−)(UO2(OH)53−)] obtained from experiment by Szabó and Grenthe (Inorg. Chem. 2010, 49, 4928−4933), which is 60.8 ± 2.4 kJ mol−1. Both the presence of UO3(OH)33− and the scenario of an “yl”−oxygen exchange through a binuclear species in strong alkaline solution are supported by the present study.

The sorption of pentavalent neptunium, Np(V), on corundum (α-Al2O3) was investigated in the absence and presence of trivalent europium or gadolinium as competing element under CO2-free conditions. The objective of this study was to investigate how a trivalent metal ion with a higher charge than that of the neptunyl ion would affect the sorption of Np(V) when allowed to adsorb on the mineral surface before the addition of Np(V). Batch sorption experiments conducted as a function of pH (pH-edges) and as a function of Np(V) concentration (isotherms) in the absence and presence of 1×10-5 M Eu(III) showed no sign of Eu being able to block Np sorption sites. Surface complexation modelling using the diffuse double layer model was employed to the batch data to obtain surface complexation constants for the formed Np(V) complexes on corundum. To account for potential changes occurring in the coordination environment of the neptunium ion in the presence of a trivalent lanthanide, X-ray absorption spectroscopic (XAS) studies of samples containing only Np(V) or Np(V) and Gd(III) were conducted. The XAS-measurements reveal the presence of a bidentate Np(V) edgesharing complex on the corundum surface in the absence of Gd(III). In the presence of Gd(III) our Np(V) EXAFS data show a contraction of the Np-Al distance together with the formation of an additional peak that is not resolved in the absence of the competing metal. These differences might point toward a change in the Np(V) surface configuration on corundum when Gd(III) is added to the sample before Np(V).

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.

In U1–xAmxO2±δ compounds with low americium content (x ≤ 20 atom %) and oxygen-to-metal (O/M) ratios close to 2.0, AmIII+ cations are charge-balanced by an equivalent amount of UV+ cations while the fluorite structure of pure UIV+O2 is maintained. Up to now, it is unknown whether this observation also holds for higher americium contents. In this study, we combined X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate a U0.5Am0.5O2±δ compound. Our results indicate that americium is again only present as AmIII+, while UV+ remains below the amount required for charge balance. Unlike lower americium contents, this leads to an overall oxygen hypostoichiometry with an average O/M ratio of 1.92(2). The cationic sublattice is only slightly affected by the coexistence of large amounts of reduced (AmIII+) and oxidized (UV+) cations, whereas significant deviations from the fluorite structure are evidenced by both extended X-ray absorption fine structure and Raman spectroscopies in the oxyge...

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-LIII-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous 242Pu(III) and 242Pu(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 PuO2 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 PuO2 for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

U(VI) accumulation by the acidothermophilic archaeon Sulfolobus acidocaldarius at a moderate acidic pH of 4.5 was investigated. This pH value is relevant for some heavy metal and uranium polluted environments where populations of S. acidocaldarius were found to persist. We demonstrate that U(VI) is rapidly complexed by the archaeal cells. A combination of X-ray absorption spectroscopy and time-resolved laser-induced fluorescence spectroscopy revealed that at pH 4.5 organic phosphate and carboxylic groups are involved in the U(VI) complexation. These results are in contrast to those published for most bacteria which at this pH precipitate U(VI) mainly in inorganic uranyl phosphate phases. As demonstrated by TEM only a limited part of the added U(VI) was biomineralized extracellularly in the case of the studied archaeon. Most of the U(VI) accumulates were localized in a form of intracellular deposits which were associated with the inner side of the cytoplasma membrane. Observed differences in U(VI) bioaccumulation between the studied archaeon and bacteria can be explained by the significant differences in their cell wall structures as well as by their different physiological characteristics.