Theoretical studies of UO2(OH)(H2O)n+, UO2(OH)2(H2O)n, NpO2(OH)(H2O)n, and PuO2(OH)(H2O)n+ (n≤21) complexes in aqueous solution

Adsorption and diffusion of actinyls on the basal gibbsite (001) surface: a theoretical perspective

Actinides are an important component of nuclear fuel for nuclear power and affect human health, and a key process in the transport of radionuclides in the environment is adsorption on mineral surfaces. In this work, we have used density functional theory (DFT) to investigate the microscopic adsorption and diffusion mechanisms of actinyls, U(V), U(VI), Np(V), Np(VI) Pu(V), and Pu(VI), on the gibbsite (001) surface. Actinyls(VI) are attached to the gibbsite surface through two An-Os bonds, which results in a bidentate inner sphere mode, while actinyls(V) favor a monodentate inner sphere adsorption mode with the gibbsite (001) surface. The solvent effects were considered through an explicit water cluster model. All the actinyls studied can be efficiently adsorbed on the gibbsite (001) surface with binding energies ranging from -113.9 kJ mol-1 to -341.2 kJ mol-1. Electronic structure analyses indicate that the cooperation of the An-Os bonds and hydrogen bonds leads to high adsorption stability of the actinyls with the gibbsite surface. The diffusion barriers of the actinyls on the gibbsite surface were determined, and the high energy barriers indicate that this type of gas-phase diffusion process is not likely to take place.

Nature and coordination geometry of geologically relevant aqueous Uranium(VI) complexes up to 400 ºC: A review and new data

The structure of the uranyl aqua ion (UO₂²⁺) and a number of its inorganic complexes (specifically, UO₂Cl⁺, UO₂Cl₂⁰, UO₂SO₄⁰, [Formula: see text] , [Formula: see text] and UO₂OH₄^2-) have been characterised using X-Ray absorption spectroscopy/extended X-Ray absorption fine structure (XAS/EXAFS) at temperatures ranging from 25 to 326 ºC. Results of ab initio molecular dynamics (MD) calculations are also reported for uranyl in chloride and sulfate-bearing fluids from 25 to 400 ºC and 600 bar to 20 kilobar (kb). These results are reported alongside a comprehensive review of prior structural characterisation work with particular focus given to EXAFS works to provide a consistent and up-to-date view of the structure of these complexes under conditions relevant to U mobility in ore-forming systems and around high-grade nuclear waste repositories. Regarding reported EXAFS results, average equatorial coordination was found to decrease in uranyl and its sulfate and chloride complexes as temperature rose - the extent of this decrease differed between species and solution compositions but typically resulted in an equatorial coordination number of ∼3-4 at temperatures above 200 ºC. The [Formula: see text] complex was observed at temperatures from 25 to 247 ºC and exhibited no major structural changes over this temperature range. UO₂OH₄^2- exhibited only minor structural changes over a temperature range from 88 to 326 ºC and was suggested to manifest fivefold coordination with four hydroxyl molecules and one water molecule around its equator. Average coordination values derived from fits of the reported EXAFS data were compared to average coordination values calculated using the experimentally derived thermodynamic data for chloride complexes reported by Dargent et al. (2013) and Migdisov et al. (2018b), and for sulfate complexes reported by Alcorn et al. (2019) and Kalintsev et al. (2019). Sulfate EXAFS data were well described by available thermodynamic data, and chloride EXAFS data were described well by the thermodynamic data of Migdisov et al. (2018b), but not by the data of Dargent et al. (2013). The ab initio molecular dynamics calculations confirmed the trends in equatorial coordination observed with EXAFS and were also able to provide an insight into the effect of pressure in equatorial water coordination - for a given temperature, higher pressures appear to lead to a greater number of equatorially bound waters counteracting the temperature effect.

Removal of uranium from contaminated groundwater using monorhamnolipids and ion flotation

Mining of uranium for defense-related purposes has left a substantial legacy of pollution that threatens human and environmental health. Contaminated waters in the arid southwest are of particular concern, as water resource demand and water scarcity issues become more pronounced. The development of remediation strategies to treat uranium impacted waters will become increasingly vital to meet future water needs. Ion flotation is one technology with the potential to address legacy uranium contamination. The green biosurfactant rhamnolipid has been shown to bind uranium and act as an effective collector in ion flotation. In this study, uranium contaminated groundwater (∼440 μg L-1 U) from the Monument Valley processing site in northeast Arizona was used as a model solution to test the uranium removal efficacy of ion flotation with biosynthetic (bio-mRL) and three synthetic monorhamnolipids with varying hydrophobic chain lengths: Rha-C10-C10, Rha-C12-C12, and Rha-C14-C14. At the groundwater's native pH 8, and at an adjusted pH 7, no uranium was removed from solution by any collector. However, at pH 6.5 bio-mRL and Rha-C10-C10 removed 239.2 μg L-1 and 242.4 μg L-1 of uranium, respectively. By further decreasing the pH to 5.5, bio-mRL was able to reduce the uranium concentration to near or below the Environmental Protection Agency maximum contaminant level of 30 μg L-1. For the Rha-C12-C12 and Rha-C14-C14 collector ligands, decreasing the pH to 7 or below reduced the foam stability and quantity, such that these collectors were not suitable for treating this groundwater. To contextualize the results, a geochemical analysis of the groundwater was conducted, and a consideration of uranium speciation is described. Based on this study, the efficacy of monorhamnolipid-based ion flotation in real world groundwater has been demonstrated with suitable solution conditions and collectors identified.

Multiple investigations of aqueous Eu(III)-oxalate complexes: the reduction in coordination number and validation of spectral linear correlation

Detailed information on the An(iii)/Ln(iii) complexation properties in solution is essential for separation chemistry and the prediction of their potential for radionuclide migration from nuclear waste repositories into natural aquifers. In the present study, to better reveal and confirm the structural information of [Eu(Ox)x (H2O)h-2x]3-2x (h = 8, 9; x = 0-3) aqueous species, especially the variable coordination number (CN), and explore the validity of the spectral linear correlation between the luminescence lifetime and the residual hydration number in the first coordination sphere of Eu(iii) compounds in solution, a comparison between the spectral results and the theoretical calculations in a wide parametric space in terms of the pH value and oxalate concentration was carried out by combining time-resolved luminescence spectroscopy (TRLS) with speciation modelling and density functional theory (DFT) calculations. We have found direct and clear evidence for the 9-fold to 8-fold coordination number reduction of Eu(iii) atoms upon coordination with more than one oxalate in an aqueous medium, and as well systematically validated the applicability of the spectral linear correlation in an aqueous system (otherwise solid state) involving multiple species with the support of relatively reliable and clear speciation modelling.

Ab initio molecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

Ab initio molecular dynamics (AIMD) simulations of the Mg2+, Ca2+, Sr2+ and UO22+ ions in either a pure aqueous environment or an environment containing two hydroxide ions have been carried out at the density functional level of theory, employing the generalised gradient approximation via the PBE exchange-correlation functional. Calculated mean M-O bond lengths in the first solvation shell of the aquo systems compared very well to existing experimental and computational literature, with bond lengths well within values measured previously and coordination numbers in line with previously calculated values. When applied to systems containing additional hydroxide ions, the methodology revealed increased bond lengths in all systems. Proton transfer events (PTEs) were recorded and were found to be most prevalent in the strontium hydroxide systems, likely due to the low charge density of the ion and the consequent lack of hydroxide coordination. For all alkaline earths, intrashell PTEs which occurred outside of the first solvation shell were most prevalent. Only three PTEs were identified in the entire simulation data of the uranium dihydroxide system, indicating the clear impact of the increased charge density of the hexavalent uranium ion on the strength of metal-oxygen bonds in aqueous solution. Broadly, systems containing more charge dense ions were found to exhibit fewer PTEs than those containing ions of lower charge density.

Organic Compounds of Actinyls: Systematic Computational Assessment of Structural and Topological Properties in [AnO2(C2O4) n](2 n-2)- (An = U, Np, Pu, Am; n = 1-3) Complexes

Exploring the bonding features between organics and actinide elements is a fundamental topic in nuclear waste separation. In this work, [AnO₂(C₂O₄) _n]^{(2 n-2)-} (An = U, Np, Pu, and Am; n = 1-3) complexes have been characterized by density functional theory. The actinyl oxalate complexes are found to exhibit the typical An-O_yl, An-O_eq bonds and O_yl-An-O_yl angles. Interatomic interaction analyzed by electron density difference, charge decomposition analysis, charges population, bond order, electron localization function, and quantum theory of atom in molecules indicates that An-O_eq bonds are ionic (closed-shell) bonding interaction with a small degree of covalent character. The similarities and differences between isomers have been discussed in the actinide coordination chemistry, and the orbital interactions also have been investigated through total, partial, and overlap population density of state diagrams. Besides, the electrostatic potential was used to predict the adsorption sites on the molecular vdW surface.

Experimental and Theoretical Studies on the Adsorption Mechanisms of Uranium (VI) Ions on Chitosan

An experiment on the adsorption of uranium (VI) by chitosan was conducted to investigate the efficiency of chitosan as an adsorbent for U(VI). The adsorption potential of U(VI) by chitosan was investigated with ICP-MS by varying the experimental conditions such as the pH in order to obtain the optimum conditions. Adsorption dependence on the pH was confirmed, and the highest uptake of U(VI) was observed at pH 5. In addition, to scrutinize the experimental results, quantum chemistry calculations were performed. The results, taking into account the experimental conditions, show that the adsorption efficiency increases as the total charge of the adsorbent and adsorbate species decreases if both of them are positively charged. It was also found that a slight change in the adsorption geometric configuration controls the adsorption efficiency.

Density functional theory investigations on the binding modes of amidoximes with uranyl ions

η¹-O of tautomerized amidoximes and η¹-O/η²-N–O of anionic amidoximes are all plausible coordination modes for amidoximes in ligating uranyl ions.

Theoretical study on structures and bond properties of NpO2(m+) ions and NpO2(H2O)n(m+) (m = 1-2, n = 1-6) complexes in the gas phase and aqueous solution

The equilibrium structures, vibrational frequencies, and bond characteristics of NpO2(m+) ions and NpO2(H2O)n(m+) (m = 1-2, n = 1-6) complexes have been studied by carrying out ab initio calculations in the gas phase and aqueous solution. The geometries have been obtained at the B3LYP level with the use of the polarized continuum model (PCM). The computed structural parameters that are in reasonably good agreement with the available data show that the solvation effect leads to a red shift of the IR spectra and the weakness of interaction strengths in neptunyl ions. By comparing the structural properties and the density of states (DOS) of these aqua complexes in the gas phase and aqueous solution, it is found that the solvation effect can be simulated approximately with the calculations of these aqua complexes in the gas phase. In addition, the DOS of these aqua complexes together with the binding energies between the neptunyl ion and water molecule reveal that the penta-aqua complex is preferred for neptunyl ions in aqueous solution.

Promising density functional theory methods for predicting the structures of uranyl complexes

By examining the overall accuracy of different theoretical methods in predicting the U–X bond distances (of a series uranyl complexes), we found that both the global-hybrid meta-GGA functional of BB1K and the range-seperated LC-BLYP functional are fairly good (even better than the popular B3LYP method).

Electrospray ionization mass spectrometric studies on uranyl complex with α-hydroxyisobutyric acid in water-methanol medium

RATIONALE

Hydroxycarboxylic acids are extensively used as chelating agents in the liquid chromatographic separation of actinides and lanthanides. They are also used as model compounds to understand the binding characteristics of humic substances. A systematic study of the speciation of uranyl-α-hyydroxyisobutyric acid (HIBA) in water-methanol is essential, as it is important to understand the various mechanisms responsible for the separation of these species in liquid chromatography.

METHODS

ESI-MS studies were carried out using a tandem quadrupole-time-of-flight mass spectrometer in positive and negative ion mode. The effects of solution composition, solute concentration and supporting electrolyte concentration on the ESI-MS behavior of the uranyl species were studied. Transmission parameters such as the quadrupole ion energy and collision cell energy were optimized for acquiring the spectra of uranyl-HIBA species, ensuring that the spectra reflect the solution equilibrium conditions.

RESULTS

The solution composition and concentration of the uranyl salt were found to influence the major uncomplexed uranyl species. Although the ESI parameters did not influence the species distribution of uranyl-HIBA, the transmission parameters did have a significant effect. The overall trend in the complexation reaction between uranyl and HIBA was studied as a function of ligand-to-metal ratio. The species distribution obtained in positive ion mode was similar to that obtained in negative ion mode.

CONCLUSIONS

The study presents the optimization of the mobile phase conditions and the ESI-MS parameters for the speciation of the uranyl-HIBA system. The methodology was applied to obtaining the distribution of complexed and uncomplexed uranyl species for monitoring the trend in the complexation reaction.