Going deeper into plutonium sorption affected by redox.

Sorption of Pu(VI) onto synthesized goethite under oxidizing and normal conditions was investigated, which revealed its pH dependence on different solid/liquid ratios. Pu speciation upon sorption on the solid phase was characterized via extended X-ray absorption fine structure (EXAFS) spectroscopy, while that in solution was assessed using ultraviolet-visible (UV-Vis) spectroscopy and liquid-liquid extraction. The obtained results demonstrate differences in plutonium behavior in the studied systems. Pu(VI) remains hexavalent on the goethite surface and in solution under oxidizing conditions. While Pu(IV) is stabilized on the mineral and Pu(V) is found in solution under normal conditions. This study provides the thermodynamic descriptions of these reactions.

Formation of Neptunium(V) Carbonates: Examining the Forceful Influence of Alkali and Alkaline Earth Cations.

Herein, neptunium(V) carbonates containing sodium or potassium cations were synthesized via chemical precipitation. Various techniques such as scanning electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetry combined with differential scanning calorimetry, X-ray diffraction, and X-ray absorption spectroscopy were used to analyze the microstructures and elemental compositions of these samples. The crystal structures of hydrated NaNpO2CO3·3H2O (P1, a = 4.3420(2) Å, b = 4.8962(2) Å, c = 10.0933(11) Å, α = 91.014(7)°, ß = 77.834(11)°, and γ = 90.004(10)°) and KNpO2CO3 (P63/mmc, a = b = 5.0994(2) Å, c = 10.2210(15) Å) were determined for the first time using the Rietveld method. The synthesized carbonates exhibited distinct structural features and decomposition behaviors, as demonstrated through thermogravimetry analysis, which revealed the presence of crystalline hydrate water in sodium neptunium(V) carbonate. Furthermore, calcium-containing neptunium(V) carbonates were synthesized and characterized. Samples with the general composition Ca0.5NpO2CO3 were obtained using the ion exchange method and chemical precipitation from solutions containing competing cations (Ca2+, Na+, K+, and Mg2+). The synthesis conditions notably affected the diffraction patterns of the obtained calcium neptunium(V) carbonates. This investigation enhances our understanding of the structural properties and thermodynamic stability of neptunium(V) carbonates in the presence of diverse cations commonly found under radioactive waste disposal conditions.

Laboratory-based X-ray spectrometer for actinide science.

X-ray absorption and emission spectroscopies nowadays are advanced characterization methods for fundamental and applied actinide research. One of the advantages of these methods is to reveal slight changes in the structural and electronic properties of radionuclides. The experiments are generally carried out at synchrotrons. However, considerable progress has been made to construct laboratory-based X-ray spectrometers for X-ray absorption and emission spectroscopies. Laboratory spectrometers are reliable, effective and accessible alternatives to synchrotrons, especially for actinide research, which allow dispensing with high costs of the radioactive sample transport and synchrotron time. Moreover, data from laboratory spectrometers, obtained within a reasonable time, are comparable with synchrotron results. Thereby, laboratory spectrometers can complement synchrotrons or can be used for preliminary experiments to find perspective samples for synchrotron experiments with better resolution. Here, the construction and implementation of an X-ray spectrometer (LomonosovXAS) in Johann-geometry at a radiochemistry laboratory is reported. Examples are given of the application of LomonosovXAS to actinide systems relevant to the chemistry of f-elements, the physical chemistry of nuclear power engineering and the long-term disposal of spent nuclear fuel.

The features of Cs sorption onto peaty-podzolic-gleyic soil.

This article studies sorption regularities and estimates the strength of 137Cs fixation by various soil horizons in peaty-podzolic-gleyic soils under a model experiment. The interaction time varied from 1 week to 3 months while Cs concentration varied from trace to micromolar concentrations. To understand the interaction mechanisms of cesium with individual soil components, we used the method of sequential removal of organic matter and non-silicate iron compounds from the soil before the sorption experiment. Illite and vermiculite were found to be the main soil components for radiocesium sorption in the mineral soil horizons. The highly selective positions (FES - frayed edge sites) of the abovementioned minerals fixed radiocesium so strongly that even strong acid solutions could not extract it from soil. Organic matter significantly contributed to the fixation of radiocesium in the soil only in the ELih horizon which contained 4.71% Corg. In the ELg horizon, a small amount of organic matter was able to inhibit sorption by blocking highly selective to radiocesium FES. The Tessier sequential extraction method of radiocesium revealed that all the studied soil samples could strongly fix the radionuclide. Increasing the interaction period up to three months under periodic wetting and drying contributed to the increased proportion of strongly bound 137Cs. The results of sorption experiments carried out before and after the removal of non-silicate iron compounds from the soil indicate that 137Cs has little or no sorption on the surface of iron hydroxides.

Super-oxidized "activated graphene" as 3D analogue of defect graphene oxide: Oxidation degree vs U(VI) sorption.

Porous carbons are not favorable for sorption of heavy metals and radionuclides due to absence of suitable binding sites. In this study we explored the limits for surface oxidation of "activated graphene" (AG), porous carbon material with the specific surface area of ∼2700 m2/g produced by activation of reduced graphene oxide (GO). Set of "Super-Oxidized Activated Graphene" (SOAG) materials with high abundance of carboxylic groups on the surface were produced using "soft" oxidation. High degree of oxidation comparable to standard GO (C/O=2.3) was achieved while keeping 3D porous structure with specific surface area of ∼700-800 m2/. The decrease in surface area is related to the oxidation-driven collapse of mesopores while micropores showed higher stability. The increase in the oxidation degree of SOAG is found to result in progressively higher sorption of U(VI), mostly related to the increase in abundance of carboxylic groups. The SOAG demonstrated extraordinarily high sorption of U(VI) with the maximal capacity up to 5400 µmol/g, that is 8.4 - fold increase compared to non-oxidized precursor AG, ∼50 -fold increase compared to standard graphene oxide and twice higher than extremely defect-rich graphene oxide. The trends revealed here show a way to further increase sorption if similar oxidation degree is achieved with smaller sacrifice of surface area.

Oxidation and Nanoparticle Formation during Ce(III) Sorption onto Minerals.

The sorption of Ce(III) on three abundant environmental minerals (goethite, anatase, and birnessite) was investigated. Batch sorption experiments using a radioactive 139Ce tracer were performed to investigate the key features of the sorption process. Differences in sorption kinetics and changes in oxidation states were found in the case of the sorption of Ce(III) on birnessite compared to that on other minerals. Speciation of cerium onto all of the studied minerals was investigated using spectral and microscopic methods: high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and X-ray absorption spectroscopy (XAS) in conjunction with theoretical calculations. It was found that during the sorption process onto birnessite, Ce(III) was oxidized to Ce(IV), while the Ce(III) on goethite and anatase surfaces remained unchanged. Oxidation of Ce(III) by sorption on birnessite was also accompanied by the formation of CeO2 nanoparticles on the mineral surface, which depended on the initial cerium concentration and pH value.

To form or not to form: PuO2 nanoparticles at acidic pH.

The aim of this study is to synthesize PuO2 nanoparticles (NPs) at low pH values and characterize the materials using laboratory and synchrotron-based methods. Properties of the PuO2 NPs formed under acidic conditions (pH 1-4) are explored here at the atomic scale. High-resolution transmission electron microscopy (HRTEM) is applied to characterize the crystallinity, morphology and size of the particles. It is found that 2 nm crystalline NPs are formed with a PuO2 crystal structure. High energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy at the Pu M4 edge has been used to identify the Pu oxidation states and recorded data are analysed using the theory based on the Anderson impurity model (AIM). The experimental data obtained on NPs show that the Pu(iv) oxidation state dominates in all NPs formed at pH 1-4. However, the suspension at pH 1 demonstrates the presence of Pu(iii) and Pu(vi) in addition to the Pu(iv), which is associated with redox dissolution of PuO2 NPs under acidic conditions. We discuss in detail the mechanism that affects the PuO2 NPs synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Hence, the results shown here, together with the first Pu M4 HERFD data on PuF3 and PuF4 compounds, are significant for the colloid facilitated transport governing the migration of plutonium in a subsurface environment.

Effective coordination numbers from EXAFS: general approaches for lanthanide and actinide dioxides.

Extended X-ray absorption fine structure (EXAFS) is a comprehensive and usable method for characterizing the structures of various materials, including radioactive and nuclear materials. Unceasing discussions about the interpretation of EXAFS results for actinide nanoparticles (NPs) or colloids were still present during the last decade. In this study, new experimental data for PuO2 and CeO2 NPs with different average sizes were compared with published data on AnO2 NPs that highlight the best fit and interpretation of the structural data. In terms of the structure, PuO2, CeO2, ThO2, and UO2 NPs exhibit similar behaviors. Only ThO2 NPs have a more disordered and even partly amorphous structure, which results in EXAFS characteristics. The proposed new core-shell model for NPs with calculated effective coordination number perfectly fits the results of the variations in a metal-metal shell with a decrease in NP size.

Partitioning of uranium in contaminated bottom sediments: The meaning of fractionation.

Sequential extraction tests were used to study partitioning of U in the bottom sediments of two reservoirs that have been used for the temporary storage of nuclear waste at the "Mining and Chemical Combine" (Zheleznogorsk, Krasnoyarsk region, Russia). Various sequential extraction protocols were applied to the bottom sediment samples and the results compared with those obtained for laboratory-prepared simulated samples with different speciation and partitioning, e.g., U(VI) sorbed onto various inorganic minerals and organic matter, as well as uranium oxides. The distributions of uranium in fractions extracted from simulated and actual contaminated samples were compared to shed light on the speciation of U in the bottom sediments. X-ray absorption spectroscopy, X-ray diffraction, and scanning electron microscopy were also used to analyze the partitioning of U in contaminated sediments. We also compared the results obtained using the spectroscopic and microscopic techniques, as well as sequential extraction.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3 nm. By combining high-energy X-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis, we have characterised for the first time both the short- and medium-range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

Invited for the cover of this issue is Lucia Amidani and co-workers from the The European Synchrotron, Helmholtz Zentrum Dresden-Rossendorf, Lomonosov Moscow State University, Kurchatov Institute, and the Université Grenoble Alpes. The image depicts the atomic structure of the sample being viewed through "atomic googles", which represent the X-ray techniques used in this work. Read the full text of the article at 10.1002/chem.202003360.

Speciation of Uranium and Plutonium From Nuclear Legacy Sites to the Environment: A Mini Review.

The row of 15 chemical elements from Ac to Lr with atomic numbers from 89 to 103 are known as the actinides, which are all radioactive. Among them, uranium and plutonium are the most important as they are used in the nuclear fuel cycle and nuclear weapon production. Since the beginning of national nuclear programs and nuclear tests, many radioactively contaminated nuclear legacy sites, have been formed. This mini review covers the latest experimental, modeling, and case studies of plutonium and uranium migration in the environment, including the speciation of these elements and the chemical reactions that control their migration pathways.

Enhanced Sorption of Radionuclides by Defect-Rich Graphene Oxide.

Extremely defect graphene oxide (dGO) is proposed as an advanced sorbent for treatment of radioactive waste and contaminated natural waters. dGO prepared using a modified Hummers oxidation procedure, starting from reduced graphene oxide (rGO) as a precursor, shows significantly higher sorption of U(VI), Am(III), and Eu(III) than standard graphene oxides (GOs). Earlier studies revealed the mechanism of radionuclide sorption related to defects in GO sheets. Therefore, explosive thermal exfoliation of graphite oxide was used to prepare rGO with a large number of defects and holes. Defects and holes are additionally introduced by Hummers oxidation of rGO, thus providing an extremely defect-rich material. Analysis of characterization by XPS, TGA, and FTIR shows that dGO oxygen functionalization is predominantly related to defects, such as flake edges and edge atoms of holes, whereas standard GO exhibits oxygen functional groups mostly on the planar surface. The high abundance of defects in dGO results in a 15-fold increase in sorption capacity of U(VI) compared to that in standard Hummers GO. The improved sorption capacity of dGO is related to abundant carboxylic group attached hole edge atoms of GO flakes as revealed by synchrotron-based extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy.

The missing pieces of the PuO2 nanoparticle puzzle.

The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 nanoparticles (NPs) at the atomic level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L3 edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L3 and M4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidation states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analyzed with state-of-the-art theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of ∼2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(iv) oxidation state in all NPs, reported here for the first time, indicate that the structure of PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.

A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles.

Here we provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alkaline conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4 PuO2 CO3 , which is stable over a period of several months. For the first time, state-of-the-art experiments at Pu M4 and at L3 absorption edges combined with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase.

Towards the surface hydroxyl species in CeO2 nanoparticles.

Understanding the complex chemistry of functional nanomaterials is of fundamental importance. Controlled synthesis and characterization at the atomic level is essential to gain deeper insight into the unique chemical reactivity exhibited by many nanomaterials. Cerium oxide nanoparticles have many industrial and commercial applications, resulting from very strong catalytic, pro- and anti-oxidant activity. However, the identity of the active species and the chemical mechanisms imparted by nanoceria remain elusive, impeding the further development of new applications. Here, we explore the behavior of cerium oxide nanoparticles of different sizes at different temperatures and trace the electronic structure changes by state-of-the-art soft and hard X-ray experiments combined with computational methods. We confirm the absence of the Ce(iii) oxidation state at the surface of CeO2 nanoparticles, even for particles as small as 2 nm. Synchrotron X-ray absorption experiments at Ce L3 and M5 edges, combined with X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS) and theoretical calculations demonstrate that in addition to the nanoceria charge stability, the formation of hydroxyl groups at the surface profoundly affects the chemical performance of these nanomaterials.

Understanding the size effects on the electronic structure of ThO2 nanoparticles.

Developing characterization techniques and analysis methods adapted to the investigation of nanoparticles (NPs) is of fundamental importance considering the role of these materials in many fields of research. The study of actinide based NPs, despite their environmental relevance, is still underdeveloped compared to that of NPs based on stable and lighter elements. We present here an investigation of ThO2 NPs performed with High-Energy Resolution Fluorescence Detected (HERFD) X-ray Absorption Near-Edge Structure (XANES) and with ab initio XANES simulations. The first post-edge feature of Th L3 edge HERFD XANES disappears in small NPs and simulations considering non-relaxed structural models reproduce the trends observed in experimental data. Inspection of the simulations of Th atoms in the core and on the surface of the NP indeed demonstrates that the first post-edge feature is very sensitive to the lowering of the number of coordinating atoms and therefore to the more exposed Th atoms at the surface of the NP. The sensitivity of the L3 edge HERFD XANES to low coordinated atoms at the surface stems from the hybridization of the d-Density of States (DOS) of Th with both O and Th neighboring atoms. This may be a common feature to other oxide systems that can be exploited to investigate surface interactions.

Redox-mediated formation of plutonium oxide nanoparticles.

Precipitates formed by the neutralisation of Pu(iii), Pu(iv), Pu(v), and Pu(vi) solutions were characterised by HRTEM, SAXS, and XRD in the suspensions. PuO2 nanoparticles uniform in size (typical diameter around 2.5 nm) and phase composition were observed in all cases under equilibrium conditions. For Pu(vi), the precipitation reactions proceed via an intermediate product.

The role of colloid particles in the albumin-lanthanides interaction: The study of aggregation mechanisms.

We studied the interaction between bovine serum albumin (BSA) and lanthanide ions in aqueous solution in the 4.0÷9.5pH range. A strong increase of the solution turbidity was observed at pH values exceeding 6, which corresponds to the formation of Ln(OH)3 nanoparticles, while no changes were observed near the isoelectric point of BSA (pH 4.7). The results of the dynamic light scattering and protein adsorption measurements clearly demonstrated that the observed turbidity enhancement was caused by albumin sorption on the surface of Ln(OH)3 and colloid particles bridging via adsorbed protein molecules. Upon pH increase from 4.5 to 6.5, albumin adsorption on lanthanide colloids was observed, while the following increase of pH from 6.5 to 9.5 led to protein desorption. The predominant role of the electrostatic interactions in the adsorption and desorption processes were revealed in the zeta-potential measurements. No reversibility was observed upon decreasing pH from 9.5 to 4.5 that was suggested to be due to the other interaction mechanisms present in the system. It was shown that while for all lanthanide ions the interaction mechanism with BSA was similar, its manifestation in the optical properties of the system was significantly different. This was interpreted as a consequence of the differences in lanthanides hydrolysis constants.

Graphene oxide for effective radionuclide removal.

Here we show the efficacy of graphene oxide (GO) for rapid removal of some of the most toxic and radioactive long-lived human-made radionuclides from contaminated water, even from acidic solutions (pH < 2). The interaction of GO with actinides including Am(III), Th(IV), Pu(IV), Np(V), U(VI) and typical fission products Sr(II), Eu(III) and Tc(VII) were studied, along with their sorption kinetics. Cation/GO coagulation occurs with the formation of nanoparticle aggregates of GO sheets, facilitating their removal. GO is far more effective in removal of transuranium elements from simulated nuclear waste solutions than other routinely used sorbents such as bentonite clays and activated carbon. These results point toward a simple methodology to mollify the severity of nuclear waste contamination, thereby leading to effective measures for environmental remediation.