Gallium(III) sorption onto hematite was investigated as a function of pH using batch sorption experiments, precipitation controls, zeta potential measurements, extended X-ray absorption fine structure spectroscopy, and thermodynamic surface complexation modeling. Sorption experiments at initial Ga(III) concentrations of 40 and 100 µM showed a steep sorption edge at low pH and nearly complete gallium retention over a broad pH range, from pH 3.5–8.0. Parallel precipitation experiments and thermodynamic calculations demonstrated that bulk precipitation alone cannot account for the observed retention behavior, particularly at low pH and under alkaline conditions where hematite strongly immobilizes gallium. Zeta potential measurements revealed a systematic increase in surface charge in the presence of Ga(III), indicating strong specific interaction with the hematite surface. EXAFS spectroscopy showed a highly consistent local coordination environment over the entire investigated pH range, characterized by octahedral Ga–O coordination and distinct Ga–Fe backscattering paths. The spectroscopic results indicate a structurally robust retention mechanism and are consistent with either strongly bound inner-sphere surface complexes or partial incorporation of Ga(III) into near-surface Fe(III) sites of the hematite structure. Surface complexation modeling reproduced the sorption data for both initial concentrations using a single hydrolyzed Ga surface species and lead to a logK° value of −1.23 ± 0.04 for the surface complexation formation. The combined results demonstrate that Ga(III) is strongly retained by hematite over a broad pH range, leading to substantial stabilization of gallium surface species relative to dissolved species and bulk hydroxide precipitation.

Recent synthesis and characterization of tetravalent U and Np complexes [AnIVX((S)-PEBA)3] and [AnIVX(iPr2BA)3] (X = F, Cl) bearing the (S,S)-N,N'-bis(1-phenylethyl)benzamidinate ((S)-PEBA) and the N,N'-bis(isopropyl)benzamidinate (iPr2BA) ligands, respectively, have revealed a characteristic inversion of paramagnetic 1H NMR shifts for both [UF(L)3] representatives compared to the [UCl(L)3] compounds. The same effect is not observed for [NpF(L)3] and [NpCl(L)3], with currently no satisfactory explanation for these observations. Using straightforward CASSCF-SOC-NEVPT2 calculations and point-group-optimized geometries, in combination with the Kuprov model approach [G. T. P. Charnock and I. Kuprov, Phys. Chem. Chem. Phys., 2014, 16] for the calculation of pseudo-contact shifts (PCS), the experimentally derived 1H paramagnetic shifts could be reproduced qualitatively and in most cases also quantitatively. The good agreement for most signals allowed for a first evaluation of the contribution of Fermi-contact shifts (FCS) for more strongly deviating data points, which could be supported by unrestricted DFT spin densities and spin populations. To finally gain a deeper understanding as to where the differing magnetic anisotropy for the [UF(L)3] compounds stems from, the magnetic sublevels of the ground atomic multiplet |J,mJ〉 were investigated for all complexes. These revealed that only for the [UF(L)3] compounds a high |mJ〉 (pseudo) doublet showing sufficient isolation is stabilized in the ground state, whose high axiality in the end enables the realization of a prolate χ tensor responsible for the inverse PCS field.

Abstract Two series of actinide tetrachloride complexes with the monodentate carboxylic amide ligands phthalimidine (H-phthal) and 2-pyridone (HPyO) of the general formula [AnIVCl4(L)2] (An = U, Np, Pu) were synthesized and characterized in the solid state and in solution. SC-XRD analysis revealed an octahedral coordination environment with trans orientation of either H-phthal or HPyO in all cases, with and without THF molecules hydrogen-bonded to the NH group of the ligands. In the case of [NpCl4(H-phthal)2], the absence of THF leads to a significant distortion of the coordination geometry around the metal, including the shortest Np–Cl bonds observed so far (2.4394(8) Å). A quantum chemical bonding analysis (QTAIM) showed that the main bonding interaction is found between the actinide and chloride ligands, with the amides rather loosely bound. Despite their unusually low coordination number, the complexes were found to be stable, even in coordinating solvents. This allowed for an in-depth analysis of their 1H and 13C NMR spectroscopy. Due to their pseudoD4h symmetry in the first coordination sphere, these complexes are excellent model systems to analyze their electronic structure and magnetic properties. We applied SQUID magnetometry to extract magnetic properties (μeff) and crystal field parameters ( Bqk, Wybourne) in combination with quantum chemical calculations, which allows for the comprehensive description of the electronic ground state from an experimental and theoretical point of view.

Curium is an artificial transuranic element with atomic number 96. It is typically found in its +III oxidation state, which is stabilized by a 5f7 electron configuration. CmIII exhibits intense luminescence from its first excited to its ground state in the red part of the visual spectrum. Due to the nature of the 5f electron shell, this luminescence is sensitive to changes in the chemical environment of the CmIII probe, while being detectable in the trace concentration range. This unique combination has established CmIII luminescence spectroscopy as an ideal tool for speciation studies in complex systems, particularly those relevant to the nuclear fuel cycle. In this review, we present an overview of the developments and applications of CmIII luminescence spectroscopy in the last 20 years since the last comprehensive review was published. The discussed studies have been categorized according to their chemical environment into reactions at the water/mineral interface, studies of solids containing CmIII, aqueous complexation studies, spectroscopy in non-aqueous systems, and interaction of CmIII with biomolecules and biota. These systems correlate in large parts with areas of application in nuclear waste disposal science, separation processes within current and proposed nuclear fuel cycles, and radioecological research. We summarize the most important findings in the studies, identify emerging trends and persistent challenges in the field of CmIII luminescence spectroscopy. Finally, we offer an outlook on potential future developments and research directions in this area.

Complexes of tetravalent actinides (An: Th, U, Np, and Pu) with the bidentate (N,S)-donor ligand pyridine-2-thiolate (2-PyS, PyS-) were synthesized in 1:4 or 1:5 ratios. This includes the first structurally characterized Np complex with (N,S)-donor ligands, filling a notable gap in the An coordination chemistry. An improved synthetic approach with PyS-SiMe3 enabled efficient formation of the 1:4 complexes in THF as a coordinating solvent. The compounds were comprehensively characterized in solution and in the solid phase, supported by quantum chemical calculations. Experimental and theoretical results show matching trends in the binding behavior of AnIV. The covalent bond contributions in An-N and An-S bonding increase along the series of An from Th to Pu. The bonds to the soft sulfur donors consistently have the highest covalent contributions with a remarkably high percentage for Pu-S (IQA analysis: >34%) over the harder N donors or An-O bonds of coordinating THF. High-resolution X-ray absorption and infrared spectroscopy indicate similar electronic and structural properties across AnIV complexes, while SQUID magnetometry uncovered significant differences in magnetic behavior at low temperatures depending on the complex compositions. This work advances the understanding of An-ligand bonding, emphasizing covalency and electronic structures, and expands fundamental insights into An chemistry.

Isostructural trivalent lanthanide and actinide amidinates bearing the N,N'-bis(isopropyl)benzamidinate (iPr2BA) ligand [LnIII/AnIII(iPr2BA)3] (Ln = La, Nd, Sm, Eu, Yb, Lu; An = U, Np) have been synthesized and characterized in both solid and solution states. All compounds were examined in the solid state utilizing single crystal X-ray diffraction (SC-XRD), revealing a notable deviation in the actinide series with shortened bond lengths compared to the trend in the lanthanide series, suggesting a nonionic contribution to the actinide-ligand bonding. Quantum-chemical bonding analysis further elucidated the nature of these interactions, highlighting increased covalency within the actinide series, as evidenced by higher delocalization indices and greater 5f orbital occupation, except for Th(III) and Pa(III), which demonstrated substantial 6d orbital occupancies. An in-depth paramagnetic NMR study in solution also sheds light on the covalent character of actinide-ligand bonding, with the separation of pseudocontact (PCS) and contact shift (FCS) contributions employing the Bleaney and Reilley method. This analysis unveiled significant contact contributions in the actinide complexes, indicating enhanced covalency in actinide-ligand bonding. To corroborate these observations, an accurate PCS calculation method based on the Kuprov equation, incorporating both the distribution of electronic spin density and magnetic susceptibility obtained from CASSCF calculations, was applied and compared with experimental values.

The environmental fate of radiotoxic actinides may be controlled by their interactions with feldspars. Here, the sorption of trivalent minor actinides (Am, Cm) and their rare earth analog Eu onto synthetic pure Ca-feldspar (anorthite) and natural plagioclases of different Ca contents is investigated, covering ranges of [M3+] (52 nM-10 mu M), solid-liquid ratios (1-3 g/L), pH (3-9), and ionic strengths (0.01-0.1 M NaCl) under both ambient and CO2-free conditions. As a first step to understand the uptake behavior of the heavy metals, the hitherto unknown surface charge and (de-)protonation reaction of Ca-feldspars is characterized. The zeta potential shows an unusual increase and charge reversal between pH 4 and 7 , which becomes more pronounced with increasing amounts of Ca in the crystal lattice and is likely connected to adsorption and/or surface precipitation of dissolved Al3+. Streaming potential measurements yield (de)protonation constants for anorthite surface sites of log K- = -6.94 +/- 0.38 and log K+ = +6.84 +/- 0.38. Batch sorption data shows strong immobilization of M3+ by plagioclases at mildly acidic and basic pH. Time -resolved laser fluorescence spectroscopy using Cm indicates the formation of an inner -sphere complex and its two hydrolyzed forms. The complex reactivity of dissolved Al3+ at the plagioclase -water interface severely complicated the development of a surface complexation model, emphasizing the need for additional research in this area. Our study highlights the importance of molecular -level studies to understand surface reactions and uncover unknown processes, which may have significant impact on the transport of (radiotoxic) contaminants in the geosphere.

A series of isostructural early actinide AnIV complexes was synthesized in order to investigate the influence of a conjugated framework in the ligand backbone on An bonding. Therefore, the AnIV complexes [An(pyrophen)2] (An = Th, U, Np, and Pu) with the pure N-donor ligand bis(2-pyrrolecarbonylaldehyde)-o-phenylenediamine referred to as pyrophen, were synthesized and characterized. Solid state analysis via single-crystal X-ray diffraction (SC-XRD) reveals two sets of ligands binding in an almost orthogonal arrangement to the actinide center. For the larger actinides Th and U, the coordination sphere allows for additional coordination by a solvent molecule. Nuclear magnetic resonance spectroscopy (NMR) studies show the presence of highly symmetrical complexes in solution in good agreement with the solvent-free solid structures. While SC-XRD suggests mainly ionic binding, an analysis of paramagnetic NMR contributions and quantum chemical bond analysis hint towards significant covalency in the U, Np, and Pu compounds. This series of An complexes allowed for a thorough structural and theoretical comparison of a conjugated system to a closely related N-donor ligand (pyren),[1] as well as to the mixed N,O Schiff base ligands salophen (conjugated) and salen (non-conjugated).

Abstract. The transport of radionuclides in the environment is a major problem for the safety assessment of radioactive waste repositories. Storage in deep geological repositories is considered a safe disposal strategy because of their ability to isolate hazardous components from the biosphere for hundreds of thousands of years. Minor actinides (i.e. Am, Cm and Np) dominate the radiotoxicity of spent nuclear fuel over geological timescales. In underground repositories, reducing conditions are expected, and therefore the trivalent oxidation state is dominant for Am and Cm as well as possibly for Pu. For investigations of the mobility of the trivalent actinides Am3+ and Cm3+, the less toxic trivalent rare earth elements, in particular Eu3+, are commonly used. Besides clay and salt, crystalline rock is considered a possible host rock for deep geological repositories. Crystalline rock (e.g. granite) consists mainly of quartz, mica and feldspar. The latter forms common aluminosilicates making up ∼60 vol. % of the earth's crust, but their sorption behaviour is not well understood, especially for the Ca-bearing members of the group. Here, we study the sorption of trivalent actinides and their rare earth element homologues on plagioclase which are Ca-bearing feldspars, quantitatively and mechanistically. Zeta potentials of various Ca feldspars show an unexpected increase at pH 4–7, which becomes more pronounced as the amount of Ca in the crystal lattice increases. This can be interpreted by assuming uptake of Al3+ and/or the precipitation of an Al phase, where Al originates from feldspar dissolution at different pH values. Nevertheless, only minor differences were found in the retention and surface speciation of Cm3+ on Ca and K feldspars (Neumann et al., 2021). Ca feldspar has a slightly higher potential to retain trivalent metal ions compared to K feldspar. An inner-sphere (IS) complex and its two hydrolysis forms have been identified on both minerals, but the hydrolysis of the IS complex is stronger in the Ca-rich mineral. A surface complexation model for Ca feldspar was developed by combining the batch sorption data and the spectroscopically identified surface complexes to describe the experimental data. These data will be the basis for the improvement of transport simulations for a reliable safety assessment of potential radioactive waste repositories in crystalline rock.

We have previously reported complex effects of cytokine-containing T cell supernatants on the interleukin (IL)4 plus phorbol 12-myristate 13-acetate (PMA)-induced proliferative response of murine thymocytes. Here we show that recombinant murine IL-2, IL-6, and IFN-γ each differentially regulate the IL-4/PMA-driven growth of thymocyte subpopulations. Thymocytes fractionated into four subpopulations on the basis of CD4 and CD8 expression were stimulated to proliferate by IL-4/PMA. Interferon-γ (IFN-γ) caused almost complete inhibition of the CD4+/CD8− response but had no measurable effect on the growth of CD4−/CD8+ or CD4−/ CD8− populations. This inhibitory effect was also observed on splenic CD4+/CD8− T cells. In contrast, IL-6 strongly enhanced the proliferative response of CD4+/CD8− thymocytes, but showed no effect on peripheral CD4+/CD8− T cells, suggesting that IL-6 may be an important regulator of growth in the thymus. IL-2 also enhanced the proliferation of both CD4−/CD8+ and CD4−/CD8− thymocytes to IL-4 and PMA. To test whether the IL-4/PMA stimulus provided all the signals required to initiate growth in each subpopulation, we titrated cell number and examined the relationship between cell dose and cell response. Growth of CD8+/CD4− cells was cell density independent, indicating that IL-4/PMA is sufficient stimulus to induce growth of these cells. In contrast, growth of CD4−/CD8− and CD4+/CD8− cells is cell density dependent, suggesting a requirement for another signal provided by the cells themselves. These observations suggest that more signals remain to be identified in this thymocyte growth system.

Isonicotinic acid (INA), as a prototypical N,O-donor bifunctional ligand, has demonstrated its ability to differentiate Th4+ from representative ions for products in spent nuclear fuels (Cs+,Ba2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Pd2+, ReO4-,La3+, Ce3+, Ce4+, UO22+), yielding an actinide metal-organic framework, Th-INA-1, by selective crystallization. This unprecedented motif with the highest ligand-binding number (i.e., 16) shows promise as a primary waste form due to its structural integrity, especially exposed to beta-or gamma-irradiation to achieve doses of 6 MGy.

To assess a reliable safety case for future deep underground repositories for highly active nuclear waste the retention of radionuclides by the surrounding host rock must be understood comprehensively. Retention is influenced by several parameters such as mineral heterogeneity and surface roughness, as well as pore water chemistry (e.g., pH). However, the interplay between those parameters is not yet well understood. Therefore, we present a correlative spectromicroscopic approach to investigate sorption of the actinide Cm(III) on: 1) bulk K-feldspar crystals to determine the effect of surface roughness and pH (5.5 and 6.9) and 2) a large feldspar grain as part of a complex crystalline rock system to observe how sorption is influenced by the surrounding heterogeneous mineralogy. Our findings show that rougher K-feldspar surfaces exhibit increased Cm(III) uptake and stronger complexation. Similarly, increasing pH leads to higher surface loading and stronger Cm(III) binding to the surface. Within a heterogeneous mineralogical system sorption is further affected by neighboring mineral dissolution and competitive sorption between mineral phases such as mica and feldspar. The obtained results express a need for investigating relevant processes on multiple scales of dimension and complexity to better understand trivalent radionuclide retention by a potential repository host rock.

Reaction of the N-heterocylic carbene ligand i PrIm (L1 ) and lithium bis(trimethylsilyl)amide (TMSA) as a base with UCl4 resulted in U(IV) and U(V) complexes. Uranium's +V oxidation state in (HL1 )2 [U(V)(TMSI)Cl5 ] (TMSI=trimethylsilylimido) (2) was confirmed by HERFD-XANES measurements. Solid state characterization by SC-XRD and geometry optimisation of [U(IV)(L1 )2 (TMSA)Cl3 ] (1) indicated a silylamido ligand mediated inverse trans influence (ITI). The ITI was examined regarding different metal oxidation states and was compared to transition metal analogues by theoretical calculations.

For a safe enclosure of contaminants, for instance in deep geological repositories of radioactive waste, any processes retarding metal migration are of paramount importance. This study focusses on the sorption of trivalent actinides (Am, Cm) and lanthanides (Eu) to the surface of muscovite, a mica and main component of most crystalline rocks (granites, granodiorites). Batch sorption experiments quantified the retention regarding parameters like pH (varied between 3 and 9), metal concentration (from 0.5 mu M Cm to 10 mu M Eu), or solid-toliquid ratio (0.13 and 5.25 g center dot L-1). In addition, time-resolved laser fluorescence spectroscopy (TRLFS) using the actinide Cm(III) identified two distinct inner-sphere surface species. Combining both approaches allowed the development of a robust surface complexation model and the determination of stability constants of the spectroscopically identified species of (equivalent to S-OH)(2)M3+ (logK(o) -8.89), (equivalent to S-O)(2)M+ (logK(o) -4.11), and (equivalent to S-O)(2)MOH (logK(o) -10.6), with all values extrapolated to infinite dilution. The inclusion of these stability constants into thermodynamic databases will improve the prognostic accuracy of lanthanide and actinide transport through groundwater channels in soils and crystalline rock systems.

Six mononuclear tetravalent actinide complexes (1-6) have been synthesized using a new Schiff base ligand 2-methoxy-6-(((2-methyl-1-(pyridin-2-yl)propyl)imino)methyl)phenol (HLpr). The HLpr is treated with tetravalent actinide elements in varied stoichiometries to afford mononuclear 1:1 complexes [MCl3-Lpr·nTHF] (1-3) and 2:1 complexes [MCl2-L2pr] (4-6) (M = Th4+ (1 and 4), U4+ (2 and 5), and Np4+ (3 and 6)). All complexes are characterized using different analytical techniques such as IR, NMR, and absorption spectroscopy as well as crystallography. UV-vis spectroscopy revealed more red-shifted absorption spectra for 2:1 complexes as compared to 1:1 complexes. 1H NMR of Th(IV) complexes exhibit diamagnetic spectra, whereas U(IV) and Np(IV) complexes revealed paramagnetically shifted 1H NMR. Interestingly, NMR signals are paramagnetically shifted between -70 and 40 ppm in 2 and 3 but are confined within -35 to 25 ppm in 2:1 complexes 5 and 6. Single-crystal structures for 1:1 complexes revealed an eight-coordinated Th(IV) complex (1) and seven-coordinated U(IV) (2) and Np(IV) (3) complexes. However, all 2:1 complexes 4-6 were isolated as eight-coordinated isostructural molecules. The geometry around the Th4+ center in 1 is found to be trigonal dodecahedral and capped trigonal prismatic around U(IV) and Np(IV) centers in 2 and 3, respectively. However, An4+ centers in 2:1 complexes are present in dodecahedral geometry. Importantly, 2:1 complexes exhibit increased bond distances in comparison to their 1:1 counterparts as well as interesting bond modulation with respect to ionic radii of An(IV) centers. Cyclic voltammetry displays an increased oxidation potential of the ligand by 300-500 mV, after coordination with An4+. CV studies indicate Th(IV)/Th(II) reduction beyond -2.3 V, whereas attempts were made to identify redox potentials for U(IV) and Np(IV) centers. Spectroscopic binding studies reveal that complex stability in 1:1 stoichiometry follows the order Th4+ ≈ U4+ > Np4+.

We have constructed an unprecedented MOF platform that accommodates a range of 5f-block metal ions (Th4+, U4+, Np4+, Pu4+) as the primary building block. The isoreticular actinide metal-organic frameworks (An-MOFs) exhibit periodic trends in the 12-coordinate metal environment, ligand configuration, and resulting ultramicroporosity. It holds potential in distinguishing neighboring tetravalent actinides. The metal ionic radius, carboxylate bite angle, anthracene plane twisting, interligand interactions, and countercation templating collectively determine an interplay between solvation, modulation, and complexation, resulting in a coordination saturation of the central actinide, while lanthanide counterparts are stabilized by the formation of a dimer-based motif. Quantum chemical calculations indicate that this large coordination number is only feasible in the high-symmetry environment provided by the An-MOFs. This category of MOFs not only demonstrates autoluminescence (4.16 × 104 counts per second per gram) but also portends a wide-bandgap (2.84 eV) semiconducting property with implications for a multitude of applications such as hard radiation detection.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The environmental fate of metal ions is influenced by their interactions with natural organic and inorganic ligands, which modify the ions' structure and charge and thus influence their interactions with mineral phases. We investigate the impact of ubiquitous sulfate on the retention of trivalent f-element cations (M(III) = Am, Eu, Y) by muscovite. We combine ex situ a spectrometry and in situ surface X-ray diffraction (i.e., crystal truncation rod and resonant anomalous X-ray reflectivity) to determine M(III) coverages and interfacial structures at the molecular level. M(III) cations adsorb as two distinct outer-sphere (OS) complexes (i.e., adsorbed and extended OS complexes whose coverages vary with increasing sulfate concentration, [SO42-]. When [SO42-] <= 0.4 mM, M(III) coverages increase with increasing [SO42-] and exceed the amounts needed for surface charge compensation of muscovite by a factor of similar to 3. This overcompensation is likely controlled by ion-ion correlations at the mineral/water interface rather than adsorption of MSO4+, which has a lower thermodynamic stability in the solutions and weaker electrostatic attraction to the mica surface than M3+. For higher [SO42-], MSO4+ and M(SO4)(2)(-) dominate solution speciation, leading to a strong decrease of the M(III) coverage due to their lower sorption affinity and weaker ion-ion correlations compared to M3+. These results indicate that interactions between electrolyte anions and metal ions at charged interfaces need to be explored for a more realistic prediction of contaminant transport in the environment.

Complexation by small organic ligands controls the bioavailability of contaminants and influences their mobility in the geosphere. We have studied the interactions of Cm3+, as a representative of the trivalent actinides, and Eu3+, as an inactive homologue, with glucuronic acid (GlcA) a simple sugar acid. Time-resolved laser-induced luminescence spectroscopy (TRLFS) shows that complexation at pH 5.0 occurs only at high ligand to metal ratios in the form of 1:1 complexes with standard formation constants log β0 = 1.84 ± 0.22 for Eu3+ and log β0 = 2.39 ± 0.19 for Cm3+. A combination of NMR, QMMM, and TRLFS reveals the structure of the complex to be a half-sandwich structure wherein the ligand binds through its carboxylic group, the ring oxygen, and a hydroxyl group in addition to five to six water molecules. Surprisingly, Y3+, which was used as a diamagnetic reference in NMR, prefers a different coordination geometry with bonding through at least two hydroxyl groups on the opposite side of a distorted GlcA molecule. QMMM simulations indicate that the differences in stability among Cm, Eu, and Y are related to ring strain induced by smaller cations. At higher pH a stronger complex was detected, most likely due to deprotonation of a coordinating OH group.