Speciation Studies of Metals in Trace Concentrations: The Mononuclear Uranyl(VI) Hydroxo Complexes

Identification and Quantification of Multiphase U(VI) Speciation on Gibbsite with pH Using TRLFS and PARAFAC of Excitation Emission Matrices

The significant abundance of uranium in radioactive waste inventories worldwide necessitates a thorough understanding of its behavior. In this work, the speciation of uranyl(VI), (UO22+) in a gibbsite system under ambient conditions has been determined as a function of pH by deconvolution and analysis of luminescence spectroscopic data. Uniquely, a combined experimental and statistical approach utilizing time-resolved luminescence spectroscopy and parallel factor analysis (PARAFAC) of excitation emission matrices has been successfully utilized to identify four separate luminescent U(VI) species in the uranyl-gibbsite system for the first time. The speciation of all luminescent U(VI) species in an environmentally relevant system over a pH range of 6-11 is discerned through the analysis of emission fingerprints at low temperature (20 K). Comparison of the deconvoluted luminescence spectra with mineral standards and geochemical models of the system allows the assignment of the luminescent chemical species as metaschoepite, Na-compreignacite, surface adsorbed ≡AlO2-UO2(OH) and ≡AlO2-UO2(CO3)24- complexes, with assignments supported by fitting of extended X-ray absorption fine structure data. The combined spectroscopic techniques in this study show that assignment and quantification of uranyl(VI) species in a sorption system over a large pH range can be accurately achieved using PARAFAC to deconvolute a three way emission spectroscopic data set.

Systematic Raman spectroscopic study of the complexation of uranyl with fluoride

A simple aqueous complexing system of UO22+ with F- is selected to systematically illustrate the application of Raman spectroscopy in exploring uranyl(VI) chemistry. Five successive complexes, UO2F+, UO2F2(aq), UO2F3-, UO2F42-, and UO2F53-, are identified, as well as the formation constants except for the 1 : 5 species UO2F53-, which was experimentally observed here for the first time. The standard relative molar Raman scattering intensity for each species is obtained by deconvolution of the spectra collected during titrations. The results of relativistic quantum chemical first-principles and ab initio calculations are presented for the complete set of [UO2(H2O)mFn]2-n complexes (n = 0-5), both for the gas phase as well as for aqueous solution modelling bulk water using the conductor-like screening model. Electronic structure calculations at the Møller-Plesset second-order perturbation theory level provide accurate geometrical parameters and in particular reveal that k water molecules in the second coordination sphere coordinating to the F- ligands in the resulting [UO2(H2O)mFn]2-n(H2O)k complexes need to be treated explicitly in order to obtain vibrational frequencies in very good agreement with experimental data. The thermodynamics and structural information obtained in this work and the developed methodology could be instructive for the future experimental and computational research on the complexation of the uranyl ion.

Ultrastrong Electron-Phonon Coupling in Uranium-Organic Frameworks Leading to Inverse Luminescence Temperature Dependence

Electron-phonon interactions, crucial in condensed matter, are rarely seen in Metal-Organic Frameworks (MOFs). Detecting these interactions typically involves analyzing luminescence in lanthanide- or actinide-based compounds. Prior studies on Ln- and Ac-based MOFs at high temperatures revealed additional peaks, but these were too faint for thorough analysis. In our research, we fabricated a high-quality, crystalline uranium-based MOF (KIT-U-1) thin film using a layer-by-layer method. Under UV light, this film showed two distinct "hot bands," indicating a strong electron-phonon interaction. At 77 K, these bands were absent, but at 300 K, a new emission band appeared with half the intensity of the main luminescence. Surprisingly, a second hot band emerged above 320 K, deviating from previous findings in rare-earth compounds. We conducted a detailed ab-initio analysis employing time-dependent density functional theory to understand this unusual behaviour and to identify the lattice vibration responsible for the strong electron-phonon coupling. The KIT-U-1 film's hot-band emission was then utilized to create a highly sensitive, single-compound optical thermometer. This underscores the potential of high-quality MOF thin films in exploiting the unique luminescence of lanthanides and actinides for advanced applications.

One particle three targets: Phosphate anion-modified magnetic mesoporous silica with enhanced fluorescence for sensitive detection, efficient adsorption, and repeated removal of uranium (VI) ions

An ideal adsorbent material that combines the multiple capabilities of sensitive detection, efficient adsorption, and repeatable removal of uranium (U) from the environment remains a serious challenge. Herin, a general method was developed for synthesizing a series of phosphate anions (such as: PO43-, P2O74-, P3O105- and P6O186-) modified magnetic mesoporous silica nanoparticles (Fe3O4 @mSiO2-Zn2+ NPs). The mesoporous surfaces and abundant phosphate groups provide potential, powerful uranium-binding sites for capturing U(VI) ions. Especially, the optimum adsorption capacity of Fe3O4 @mSiO2-Zn2+/P3O105- NPs was as high as 885.90 mg·g-1 (298 K), which was higher than that of unmodified or other phosphate anions-modified Fe3O4 @mSiO2-Zn2+ NPs. Meanwhile, P3O105--binding sites and mesoporous surfaces also strongly restrict U(VI) ions' fluorescence vibrational inactivation, the adsorption results in rapid green fluorescence enhancement (within 180 s), and an ultra-low detection limit (4.5 nmol·L-1), which is well below the standard in drinking water of the World Health Organization (WHO). Furthermore, even after 5 cycles, the adsorbent still maintained their original adsorption capacity of 80.21% and displayed excellent selectivity for detecting and removing U(VI) from seawater. Based on these results, the Fe3O4 @mSiO2-Zn2+/P3O105- NPs seem to be a suitable multifunctional adsorbent for the detection, adsorption, and removal of U(VI) from environment.

Experimental and computational evidence of U(VI)-OH-Si(OH)4 complexes under alkaline conditions: Implications for cement systems

The complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10-6 and 5 × 10-6 mol kgw-1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV-Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)-OH-Si(OH)4 complexes at 0.05 mol kgw-1 NaNO3. The main spectroscopic features - characteristic peak positions and decay-time - are reported for the first time for the UO2(OH)2SiO(OH)3- species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22- predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3- species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10β°(UO2(OH)2SiO(OH)3-) and log10β°(UO2(OH)2SiO2(OH)22-). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal.

Equilibrium Thermodynamics of Macropa Complexes with Selected Metal Isotopes of Radiopharmaceutical Interest

To pursue the design of in vivo stable chelating systems for radiometals, a concise and straightforward method toolbox was developed combining NMR, isothermal titration calorimetry (ITC), and europium time-resolved laser-induced fluorescence spectroscopy (Eu-TRLFS). For this purpose, the macropa chelator was chosen, and Lu3+, La3+, Pb2+, Ra2+, and Ba2+ were chosen as radiopharmaceutically relevant metal ions. They differ in charge (2+ and 3+) and coordination properties (main group vs lanthanides). 1H NMR was used to determine four pKa values (±0.15; carboxylate functions, 2.40 and 3.13; amino functions, 6.80 and 7.73). Eu-TRLFS was used to validate the exclusive existence of the 1:1 Mn+/ligand complex in the chosen pH range at tracer level concentrations. ITC measurements were accomplished to determine the resulting stability constants of the desired complexes, with log K values ranging from 18.5 for the Pb-mcp complex to 7.3 for the Lu-mcp complex. Density-functional-theory-calculated structures nicely mirror the complexes' order of stabilities by bonding features. Radiolabeling with macropa using ligand concentrations from 10-3 to 10-6 M was accomplished by pointing out the complex formation and stability (212Pb > 133La > 131Ba ≈ 224Ra > 177Lu) by means of normal-phase thin-layer chromatography analyses.

How tobacco (Nicotiana tabacum) BY-2 cells cope with Eu(III) - a microspectroscopic study

The extensive use of lanthanides in science, industry and high-technology products is accompanied by an anthropogenic input of rare earth elements into the environment. Knowledge of a metal's environmental fate is essential for reasonable risk assessment and remediation approaches. In the present study, Eu(III) was representatively used as a luminescent probe to study the chemical environment and to elucidate the molecular interactions of lanthanides with a suspension cell culture of Nicotiana tabacum BY-2. Biochemical methods were combined with luminescence spectroscopy, two-dimensional microspectroscopic mappings, and data deconvolution methods to resolve the bioassociation behavior and spatial distribution of Eu(III) in plant cells. BY-2 cells were found to gradually take up the metal after exposure to 100 μM Eu(III) without significant loss of viability. Time-resolved luminescence measurements were used to specify the occurrence of Eu(III) species as a function of time, revealing the transformation of an initial Eu(III) species into another after 24 h exposure. Chemical microscopy and subsequent iterative factor analysis reveal the presence of four distinct Eu(III) species located at different cellular compartments, e.g., the cell nucleus, nucleolus and cell walls, which could be assigned to intracellular binding motifs. In addition, a special type of bioaccumulation occurs through the formation of a Eu(III)-containing oxalate biomineral, which is already formed within the first 24 hours after metal exposure. Oxalate crystals were also obtained in analogous experiments with Gd and Sm. These results indicate that tobacco BY-2 cells induce the precipitation of metal oxalate biominerals for detoxification of lanthanides, although they also bind to other cellular ligands at the same time.

Time-resolved laser-induced fluorescence spectroscopy and chemometrics for fast identification of U(VI)-bearing minerals in a mining context

We evaluated the potential of time-resolved laser-induced fluorescence spectroscopy (TRLFS) combined with chemometric methods for fast identification of U(VI)-bearing minerals in a mining context. We analyzed a sample set which was representative of several environmental conditions. The set consisted of 80 uranium-bearing samples related to mining operations, including natural minerals, minerals with uranium sorbed on the surface, and synthetic phases prepared and characterized specifically for this study. The TRLF spectra were processed using the Ward algorithm and the K-nearest neighbors (KNN) method to reveal similarities between samples and to rapidly identify the uranium-bearing phase and the associated mineralogical family. The predictive models were validated on an independent dataset, and then applied to test samples mostly taken from U mill tailings. Identification results were found to be in accordance with the available characterization data from X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX). This work shows that TRLFS can be an effective decision-making tool for environmental investigations or geological prospection, considering the large diversity of uranium-bearing mineral phases and their low concentration in environmental samples.

Europium(III) Meets Etidronic Acid (HEDP): A Coordination Study Combining Spectroscopic, Spectrometric, and Quantum Chemical Methods

Etidronic acid (1-Hydroxyethylidene-1,1-diphosphonic acid, HEDP, H₄L) is a proposed decorporation agent for U(VI). This paper studied its complex formation with Eu(III), an inactive analog of trivalent actinides, over a wide pH range, at varying metal-to-ligand ratios (M:L) and total concentrations. Combining spectroscopic, spectrometric, and quantum chemical methods, five distinct Eu(III)-HEDP complexes were found, four of which were characterized. The readily soluble EuH₂L⁺ and Eu(H₂L)₂^- species with log β values of 23.7 ± 0.1 and 45.1 ± 0.9 are formed at acidic pH. At near-neutral pH, EuHL⁰_s forms with a log β of ~23.6 and, additionally, a most probably polynuclear complex. The readily dissolved EuL^- species with a log β of ~11.2 is formed at alkaline pH. A six-membered chelate ring is the key motif in all solution structures. The equilibrium between the Eu(III)-HEDP species is influenced by several parameters, i.e., pH, M:L, total Eu(III) and HEDP concentrations, and time. Overall, the present work sheds light on the very complex speciation in the HEDP-Eu(III) system and indicates that, for risk assessment of potential decorporation scenarios, side reactions of HEDP with trivalent actinides and lanthanides should also be taken into account.

Single-Hydroxide Bridged Dimers of U and Np Actinyls: A Density Functional Study on Their Existence and Structure in Aqueous Solution

With quantum chemical calculations at the density functional theory level, we examined the structure and the stability of diactinyl monohydroxo complexes [(AnO₂)₂(OH)]^3+/+ in aqueous solution for An = U(VI), Np(VI), and Np(V). In particular, this study contributes to understanding the hydrolysis of Np(VI) and Np(V), which is less well characterized than for U(VI). [(UO₂)₂(OH)]³⁺ is a known hydrolysis complex of U(VI) at low pH. Although not yet found in experiments, [(NpO₂)₂(OH)]³⁺ is suggested to exist due to the similarity between Np(VI) and U(VI) complexes, while [(NpO₂)₂(OH)]⁺ is a hypothetical species thus far. Our calculations suggest that the An(VI) complexes favor the parallel orientation of actinyls, whereas for the Np(V) complex a perpendicular arrangement is stabilized by hydrogen bonds between aqua ligands and the actinyl oxygen atoms. The Np(VI) complex [(NpO₂)₂(OH)]³⁺ features a structure and stability similar to its U(VI) analogue. From calculated formation constants for An(VI) diactinyl monohydroxo complexes, we find qualitative agreement with the experiment for U(VI). Both An(VI) complexes are only slightly less stable than the separate mononuclear constituents, the actinyl aqua and the monohydroxo complex. For the Np(V) species [(NpO₂)₂(OH)]⁺, we calculated a considerably lower complexation constant than for its An(VI) analogues, but it is more stable against decay into its constituents. Thus, this complex may exist at about the pH where Np(V) hydrolysis starts at not too low Np(V) concentrations.

Surface Coverage- and Excitation Laser Wavelength-Dependent Luminescence Properties of U(VI) Species Adsorbed on Amorphous SiO2

Time-resolved luminescence spectroscopy is usefully used to identify U(VI) surface species adsorbed on SiO2. However, the cause of the inconsistent luminescence lifetimes and spectral shapes reported previously remains undetermined. In this study, the U(VI) surface coverage (Γ) and excitation laser wavelength (λex) were examined as the predominant factors governing the luminescence properties of U(VI) surface species. At neutral pH, the luminescence lifetimes of U(VI) surface species increased with decreasing Γ. In the low-Γ region, where a relatively large number of adjacent surface sites are involved in the formation of multidentate surface complexes, the displacement of more number of coordinated water molecules in the equatorial plane of U(VI) results in a longer lifetime. The pH-dependent luminescence lifetimes of U(VI) surface species at the same U(VI) to SiO2 concentration ratio in the pH range of 4.5–7.5 also explain the effect of the surface binding sites on the luminescence lifetime. The time-resolved luminescence properties of the U(VI) surface species were also investigated at different excitation wavelengths. Continued irradiation of the SiO2 surface with a UV laser beam at λex = 266 nm considerably reduced the luminescence intensities of the U(VI) surface species. The higher the laser pulse energy, the greater the decrease in luminescence intensity. Laser-induced thermal desorption (LITD) of U(VI) surface species is suggested to be the origin of the decrease in luminescence intensity. LITD effects were not observed at λex = 355 and 422 nm, even at high laser pulse energies.

Microscopic and spectroscopic bioassociation study of uranium(VI) with an archaeal Halobacterium isolate

The safe disposal of high-level radioactive waste in a deep geological repository is a huge social and technical challenge. So far, one of the less considered factors needed for a long-term risk assessment, is the impact of microorganisms occurring in the different host rocks. Even under the harsh conditions of salt formations different bacterial and archaeal species were found, e. g. Halobacterium sp. GP5 1-1, which has been isolated from a German rock salt sample. The interactions of this archaeon with uranium(VI), one of the radionuclides of major concern for the long-term storage of high-level radioactive waste, were investigated. Different spectroscopic techniques, as well as microscopy, were used to examine the occurring mechanisms on a molecular level leading to a more profound process understanding. Batch experiments with different uranium(VI) concentrations showed that the interaction is not only a simple, but a more complex combination of different processes. With the help of in situ attenuated total reflection Fourier-transform infrared spectroscopy the association of uranium(VI) onto carboxylate groups was verified. In addition, time-resolved laser-induced luminescence spectroscopy revealed the formation of phosphate and carboxylate species within the cell pellets as a function of the uranium(VI) concentration and incubation time. The association behavior differs from another very closely related halophilic archaeon, especially with regard to uranium(VI) concentrations. This clearly demonstrates the importance of studying the interactions of different, at first sight very similar, microorganisms with uranium(VI). This work provides new insights into the microbe-uranium(VI) interactions at highly saline conditions relevant to the long-term storage of radioactive waste in rock salt.

Effect of Temperature and Cell Viability on Uranium Biomineralization by the Uranium Mine Isolate Penicillium simplicissimum

The remediation of heavy-metal-contaminated sites represents a serious environmental problem worldwide. Currently, cost- and time-intensive chemical treatments are usually performed. Bioremediation by heavy-metal-tolerant microorganisms is considered a more eco-friendly and comparatively cheap alternative. The fungus Penicillium simplicissimum KS1, isolated from the flooding water of a former uranium (U) mine in Germany, shows promising U bioremediation potential mainly through biomineralization. The adaption of P. simplicissimum KS1 to heavy-metal-contaminated sites is indicated by an increased U removal capacity of up to 550 mg U per g dry biomass, compared to the non-heavy-metal-exposed P. simplicissimum reference strain DSM 62867 (200 mg U per g dry biomass). In addition, the effect of temperature and cell viability of P. simplicissimum KS1 on U biomineralization was investigated. While viable cells at 30°C removed U mainly extracellularly via metabolism-dependent biomineralization, a decrease in temperature to 4°C or use of dead-autoclaved cells at 30°C revealed increased occurrence of passive biosorption and bioaccumulation, as confirmed by scanning transmission electron microscopy. The precipitated U species were assigned to uranyl phosphates with a structure similar to that of autunite, via cryo-time-resolved laser fluorescence spectroscopy. The major involvement of phosphates in U precipitation by P. simplicissimum KS1 was additionally supported by the observation of increased phosphatase activity for viable cells at 30°C. Furthermore, viable cells actively secreted small molecules, most likely phosphorylated amino acids, which interacted with U in the supernatant and were not detected in experiments with dead-autoclaved cells. Our study provides new insights into the influence of temperature and cell viability on U phosphate biomineralization by fungi, and furthermore highlight the potential use of P. simplicissimum KS1 particularly for U bioremediation purposes.

Graphical Abstract

High-Gradient Magnetic Separation of Compact Fluorescent Lamp Phosphors: Elucidation of the Removal Dynamics in a Rotary Permanent Magnet Separator

In an ongoing effort towards a more sustainable rare-earth element market, there is a high potential for an efficient recycling of rare-earth elements from end-of-life compact fluorescent lamps by physical separation of the individual phosphors. In this study, we investigate the separation of five fluorescent lamp particles by high-gradient magnetic separation in a rotary permanent magnet separator. We thoroughly characterize the phosphors by ICP-MS, laser diffraction analysis, gas displacement pycnometry, surface area analysis, SQUID-VSM, and Time-Resolved Laser-Induced Fluorescence Spectroscopy. We present a fast and reliable quantification method for mixtures of the investigated phosphors, based on a combination of Time-Resolved Laser-Induced Fluorescence Spectroscopy and parallel factor analysis. With this method, we were able to monitor each phosphors’ removal dynamics in the high-gradient magnetic separator and we estimate that the particles’ removal efficiencies are proportional to (d2·χ)1/3. Finally, we have found that the removed phosphors can readily be recovered easily from the separation cell by backwashing with an intermittent air–water flow. This work should contribute to a better understanding of the phosphors’ separability by high-gradient magnetic separation and can simultaneously be considered to be an important preparation for an upscalable separation process with (bio)functionalized superparamagnetic carriers.

Polymer Ligand-Sensitized Lanthanide Metal-Organic Frameworks for an On-Site Analysis of a Radionuclide

Herein, a new strategy to increase the sensitivity of a lanthanide metal-organic framework (Ln-MOF) to UO₂²⁺ was proposed by using polymeric ligands. By utilizing [Tb(1,3,5-benzenetrisbenzoate)]n (Tb-TBT) MOF as the host, preloaded 2-vinyl terephthalic acid (VTP) was polymerized in situ, which produced a novel fluorescent composite denoted as PVTP⊂Tb-TBT. Benefiting from the coordination of PVTP to the Tb nodes, the polymeric chains performed both as molecular scaffolds that improved the water stability of the framework and as additional antennae that sensitized the photoluminescence of the Tb nodes. More importantly, the detection sensitivity and selectivity of PVTP⊂Tb-TBT to UO₂²⁺ were much improved compared to those of Tb-TBT. Detailed characterizations indicated that the incorporation of PVTP efficiently enriched UO₂²⁺ in the probe, which promoted the energy dissipation to UO₂²⁺. Besides, UO₂²⁺ was also supposed to release PVTP from PVTP⊂Tb-TBT and, thus, exposed the open metal sites to water molecules, which interrupted the sensitization effect of PVTP and induced a nonradiative energy dissipation. A limit of detection (LOD) as low as 0.75 nm was recorded by suspending the PVTP⊂Tb-TBT probe in a water sample, far below the limit in drinking water set by the United States Environmental Protection Agency (130 nm). Furthermore, a remotely controlled sampling and an on-site analysis of real water samples were realized by facilely loading PVTP⊂Tb-TBT on thin films (TFs). The LOD for UO₂²⁺ was 2.5 nm by using the TFs. This study reports a new strategy for boosting the sensitivity and selectivity of Ln-MOF to monitor UO₂²⁺ and expands the application of the strategy to an on-site analysis.

Quenching Mechanism of Uranyl(VI) by Chloride and Bromide in Aqueous and Non-Aqueous Solutions

A major hindrance in utilizing uranyl(VI) luminescence as a standard analytical tool, for example, in environmental monitoring or nuclear industries, is quenching by other ions such as halide ions, which are present in many relevant matrices of uranyl(VI) speciation. Here, we demonstrate through a combination of time-resolved laser-induced fluorescence spectroscopy, transient absorption spectroscopy, and quantum chemistry that coordinating solvent molecules play a crucial role in U(VI) halide luminescence quenching. We show that our previously suggested quenching mechanism based on an internal redox reaction of the 1:2-uranyl-halide-complex holds also true for bromide-induced quenching of uranyl(VI). By adopting specific organic solvents, we were able to suppress the separation of the oxidized halide ligand X₂^·- and the formed uranyl(V) into fully solvated ions, thereby "reigniting" U(VI) luminescence. Time-dependent density functional theory calculations show that quenching occurs through the outer-sphere complex of U(VI) and halide in water, while the ligand-to-metal charge transfer is strongly reduced in acetonitrile.

Uranium(VI) Complexes of Glutathione Disulfide Forming in Aqueous Solution

The interactions between glutathione disulfide, GSSG, the redox partner and dimer of the intracellular detoxification agent glutathione, GSH, and hexavalent uranium, U(VI), were extensively studied by solution NMR (in D₂O), complemented by time-resolved laser-induced fluorescence and IR spectroscopies. As expected for the hard Lewis acid U(VI), coordination facilitates by the ligands' O-donor carboxyl groups. However, owing to the adjacent cationic α-amino group, the glutamyl-COO reveal monodentate binding, while the COO of the glycyl residues show bidentate coordination. The log K value for the reaction UO₂²⁺ + H₃GSSG^- → UO₂(H₃GSSG)⁺ (pH 3, 0.1 M NaClO₄) was determined for the first time, being 4.81 ± 0.08; extrapolation to infinite dilution gave log K^⊖ = 5.24 ± 0.08. U(VI) and GSSG form precipitates in the whole pD range studied (2-8), showing least solubility for 4 < pD < 6.5. Thus, particularly GSSG, hereby representing also other peptides and small proteins, affects the mobility of U(VI), strongly depending on the speciation of either component.

Temperature-dependent luminescence spectroscopic and mass spectrometric investigations of U(VI) complexation with aqueous silicates in the acidic pH-range

In this study the complexation of U(VI) with orthosilicic acid (H₄SiO₄) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5 · 10^-3M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO₂OSi(OH)₃⁺ species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10^-6M, the silicon concentration was varied between 1.3 · 10^-4-1.3 · 10^-3M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO₄. The results confirmed the formation of the aqueous UO₂OSi(OH)₃⁺ complex. The conditional complexation constant at 25 °C, log *β = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *β⁰ = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction Δ_rH_m⁰ = 45.8 ± 22.5 kJ·mol^-1 and molal entropy of reaction Δ_rS_m⁰ = 152.5 ± 78.8 J·K^-1·mol^-1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.

A Robust Model for Prediction of U(VI) Adsorption onto Ferrihydrite Consistent with Spectroscopic Observations

A robust model that can predict the adsorption behavior of U(VI) on ferrihydrite under a wide range of environmental conditions was developed with the aid of an extended triple-layer model. X-ray absorption spectroscopic observations from previous studies showed that the predominant U(VI) surface species on ferrihydrite was commonly a bidentate inner-sphere species under ambient CO₂ conditions. Previous surface complexation models, however, could not predict U(VI) surface speciation because of the lack of sufficient macroscopic adsorption datasets with which to estimate the surface complexation reaction. In this study, we obtained U(VI) adsorption data at U(VI) concentrations of 10nM under a wide range of pH, ionic strength, and solid concentrations in NaNO₃ solutions with/without atmospheric CO₂. We determined the stoichiometries of the U(VI) adsorption reactions and the equilibrium constants with the adsorption data and the U(VI) hydroxyl constants recently estimated from direct luminescence measurements. A single set of equilibrium constants for the reactions could reproduce reasonably well the reported adsorption datasets obtained under a wide range of pH values (2-12), U(VI) concentrations (10^-8 to 10^-4 M), ionic strengths (0.004-0.5), and CO₂ partial pressures (<10^-6 to 10^-1.7 atm). The model could also predict all U(VI) surface speciation consistent with previous spectroscopic observations under a wide range of solution conditions.

U(VI) sorption on Ca-bentonite at (hyper)alkaline conditions - Spectroscopic investigations of retention mechanisms

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.

Interdisciplinary Round-Robin Test on Molecular Spectroscopy of the U(VI) Acetate System

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.

En Route Activity of Hydration Water Allied with Uranyl (UO22+) Salts Amid Complexation Reactions with an Organothio-Based (O, N, S) Donor Base

This study provides en route activity of hydration water allied with uranyl salts amid complexation reactions with a donor species L bearing O, N, and S (phenolic, -OH; imine, -HC═N-; and thio-, -S-) donor functionalities. The UO₂²⁺/L reaction encounters a series of hydrolytic steps with hydration water released from uranyl salts during the complexation processes. Primarily, the coordinated [L_{(-HC=N)(OH)(-HC=N)} → UO₂(NO₃)₂/(OAc)₂] species formed during the complexation process undergoes partial hydrolysis of the coordinated ligand resulting in the isolation of an aldehyde coordinated uranyl species [L_{(-HC=N)(OH)(-HC=O)} → UO₂(NO₃)₂/(OAc)₂]. The influence of hydration water continued as the reaction further proceeded to the next stage resulting in alteration of the aldehyde coordinated uranyl species [L_{(-HC=N)(OH)(-HC=O)} → UO₂(NO₃)₂/(OAc)₂] to an oxidized carboxy coordinated uranyl species [L_{(-HC=N) (OH){-C(═O)O}} → (NO₃)/(OAc)]₂ without the use of any external oxidizing agents. These studies are of particular significance as they allow one to realize the adventitious role of hydration water released from commonly used uranyl salts during their reaction with organic donor substrates in nonaqueous medium. These results also form an experimental basis to understand the critical behavior of UO₂²⁺ ion activity (as oxidizing, reducing, or catalytic) relevant in many chemical, biological, and environmental processes.

Cm3+/Eu3+induced structural, mechanistic and functional implications for calmodulin

Trivalent lanthanide and actinide can strongly bind to calmodulin (CaM). The global structure of Ln/An-bound CaM were found to be similar to Ca-CaM but the local environment around Ln/An is distorted giving less structural rigidity to Ln/An-CaM.

Impact of Haloarchaea on Speciation of Uranium-A Multispectroscopic Approach

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 15987^T 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 15987^T 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.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]. J Phys Chem A 2018;122:6970-6977. [PMID: 30095911 DOI: 10.1021/acs.jpca.8b05567]

For the only water coordinated "free" uranyl(VI) aquo ion in perchlorate solution we identified and assigned several different excited states and showed that the ³Δ state is the luminescent triplet state from transient absorption spectroscopy. With additional data from other spectroscopic methods (TRLFS, UV/vis) we generated a detailed Jabłoński diagram and determined rate constants for several state transitions, like the inner conversion rate constant from the ³Φ state to the ³Δ state transition to be 0.35 ps^-1. In contrast to luminescence measurements, it was possible to observe the highly quenched uranyl(VI) ion in highly concentrated chloride solution by TAS and we were able to propose a dynamic quenching mechanism, where chloride complexation is followed by the charge transfer from the excited state uranyl(VI) to chloride. This proposed quenching route is supported by TD-DFT calculations.

Uranyl Arsenate Complexes in Aqueous Solution: Insights from First-Principles Molecular Dynamics Simulations

In this study, the structures and acidity constants (p K_a's) of uranyl arsenate complexes in solutions have been revealed by using the first principle molecular dynamics technique. The results show that uranyl and arsenate form stable complexes with the U/As ratios of 1:1 and 1:2, and the bidentate complexation between U and As is highly favored. Speciation-pH distributions are derived based on free energy and p K_a calculations, which indicate that for the 1:1 species, UO₂(H₂AsO₄)(H₂O)₃⁺ is the major species at pH < 7, while UO₂(HAsO₄)(H₂O)₃⁰ and UO₂(AsO₄)(H₂O)₃^- dominate in acid-to-alkaline and extreme alkaline pH ranges. For the 1:2 species, UO₂(H₂AsO₄)₂(H₂O)⁰ is dominant under acid-to-neutral pH conditions, while UO₂(HAsO₄)(H₂AsO₄)(H₂O)^-, UO₂(HAsO₄)(HAsO₄)(H₂O)^2-, and UO₂(AsO₄)(HAsO₄)(H₂O)^3- become the major forms in the pH range of 7.2-10.7, 10.7-12.1, and >12.1, respectively.

Surface decontamination in fuel manufacture plants by chelating solution of nanoparticles

A nanoparticles chelating solution was synthesized by copolymerization of acrylonitrile (AN) and methacrylic acid (MAA) by radiation induced polymerization technique using 17 kGy irradiation doses. A high copolymer yield was obtained by using 80/20% of AN/MAA and comonomer concentration of 50% (w/w) at a dose rate of 2.58 KGy/h. The resultant cyano group (–CN) of nano-poly(AN/MAA) was converted by chemical modification using hydroxylamine (NH2–OH) to an amidoxime group [–C(=NOH)NH2], which was then confirmed by Fourier transform infrared spectroscopy (FTIR). The physico-chemical properties of poly(AN/MAA) and amidoximated poly(AN/MAA) nanoparticles were studied by FTIR, transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermal gravimetric analysis (TGA). The morphological analysis by TEM and DLS showed a spherical and uniform size of the amidoximated poly(AN/MAA) nanoparticles. TGA results indicated that the thermal stability of poly(AN/MAA) increased by the amidoximation process. The surface decontamination due to uranium was also investigated by the prepared chelating nanoparticles solution. A high purity germanium detector (HPGe) was used as a surface contamination detection tool. The results showed the presence of peaks at different energies, namely, 186.2 keV for Ra-226 (U-238) and 143.76 keV, 163.35 keV and 205.31 for U-235 before the decontamination process. The disappearance of these peaks after decontamination confirmed the applicability and efficiency of the nanoparticles solution in uranium surface decontamination.

Multistage bioassociation of uranium onto an extremely halophilic archaeon revealed by a unique combination of spectroscopic and microscopic techniques

The interactions of two extremely halophilic archaea with uranium were investigated at high ionic strength as a function of time, pH and uranium concentration. Halobacterium noricense DSM-15987 and Halobacterium sp. putatively noricense, isolated from the Waste Isolation Pilot Plant repository, were used for these investigations. The kinetics of U(VI) bioassociation with both strains showed an atypical multistage behavior, meaning that after an initial phase of U(VI) sorption, an unexpected interim period of U(VI) release was observed, followed by a slow reassociation of uranium with the cells. By applying in situ attenuated total reflection Fourier-transform infrared spectroscopy, the involvement of phosphoryl and carboxylate groups in U(VI) complexation during the first biosorption phase was shown. Differences in cell morphology and uranium localization become visible at different stages of the bioassociation process, as shown with scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. Our results demonstrate for the first time that association of uranium with the extremely halophilic archaeon is a multistage process, beginning with sorption and followed by another process, probably biomineralization.