U(VI) adsorption on hematite nanocrystals: Insights into the reactivity of {001} and {012} facets

Predicting the environmental behavior of U(VI) relies on identification of its local coordination structure on mineral surfaces, which is also an indication of the intrinsic reactivity of the facet. We investigated the adsorption of U(VI) on two facets ({001} and {012}) of hematite (α-Fe₂O₃) by coupling experimental, spectroscopic and theoretical studies. Batch experiments results indicate higher removal capacity of the hematite {012} facet for U(VI) with respect to the {001} facet, due to the existence of extra singly and triply coordinated oxygen atoms with higher reactivity on the {012} facet while only doubly coordinated oxygen atoms exist on the {001} facet. The formation of surface complexes containing U(VI) is responsible for the appearance of a new sextuplet by Mössbauer spectra. The local structures of an inner-sphere edge-sharing bidentate complex on the hematite {001} and a corner-sharing complex on the {012} facet was deciphered by extended X-ray absorption fine structure spectroscopy. The chemical plausibility of the proposed structures was further verified by density functional theory calculation. This finding reveals the important influence of surficial hydroxyl groups reactivity on ions adsorption, which is helpful to better understand the interfacial interactions and to improve the prediction accuracy of U(VI) fate in aquatic environments.

Uranium Immobilization and Nanofilm Formation on Magnesium-Rich Minerals

Polarization-dependent grazing incidence X-ray absorption spectroscopy (XAS) measurements were completed on oriented single crystals of magnesite [MgCO3] and brucite [Mg(OH)2] reacted with aqueous uranyl chloride above and below the solubility boundaries of schoepite (500, 50, and 5 ppm) at pH 8.3 and at ambient (PCO2 = 10(-3.5)) or reduced partial pressures of carbon dioxide (PCO2 = 10(-4.5)). X-ray absorption near edge structure (XANES) spectra show a striking polarization dependence (χ = 0° and 90° relative to the polarization plane of the incident beam) and consistently demonstrated that the uranyl molecule was preferentially oriented with its Oaxial═U(VI)═Oaxial linkage at high angles (60-80°) to both magnesite (101̅4) and brucite (0001). Extended X-ray absorption fine structure (EXAFS) analysis shows that the "effective" number of U(VI) axial oxygens is the most strongly affected fitting parameter as a function of polarization. Furthermore, axial tilt in the surface thin films (thickness ∼ 21 Å) is correlated with surface roughness [σ]. Our results show that hydrated uranyl(-carbonate) complexes polymerize on all of our experimental surfaces and that this process is controlled by surface hydroxylation. These results provide new insights into the bonding configuration expected for uranyl complexes on the environmentally significant carbonate and hydroxide mineral surfaces.

Behind adhesion of uranyl onto montmorillonite surface: a molecular dynamics study

We have performed molecular dynamics simulations to investigate the adsorption of radionuclide elements species onto substituted Montmorillonite (001) surface in the presence of different counterions. The structure and the dynamics of uranyl ion as well as its aquo, chloride ion, and carbonate complexes are analyzed. In addition, we have studied the surface energy between layered Montmorillonite sheets and the work of adhesion between radionuclide and charged Montmorillonite. The clay model used here is a Wyoming-type Montmorillonite with 0.75e negative charge per unit cell resulting from substitutions in Octahedral and Tetrahedral sheets. The system model was constructed based on CLAYFF force field potential model. To evaluate the thermodynamic work of adhesion, each surface and clay layer regions are converted to a thin film model. One and two species of radionuclide elements (UO2(H2O)5,UO2CO3(H2O)5, and UO2Cl2(H2O)5) were deposited near the clay surface in a pseudo-two-dimensional periodic cell. Analysis shows that the uranyl ion structure is preserved with two axial oxygen atoms detected at 1.8Å. Radial distribution functions results indicate that average UOw distances are 2.45-2.61Å, and 2.29-2.40Å for UOc distance. Average UCl distances are 2.78-3.08Å, which is relatively larger than that of Uranium atom-Oxygen atom because of electrostatic factors.

Polarization-dependent total reflection fluorescence extended X-ray absorption fine structure and its application to supported catalysis

Polarization-dependent total reflection fluorescence-extended X-ray absorption fine structure (PTRF-EXAFS) is a powerful tool to investigate the structures of highly dispersed metal clusters on oxide surfaces that provide a model system for supported metal catalysts. PTRF-EXAFS provides three-dimensional structural information of the dispersed metal clusters, in addition to the metal-support interface structure in the presence of a gas phase. Results from PTRF-EXAFS have revealed that the metal species interacts strongly with surface anions. Finally the future of PTRF-EXAFS is discussed in combination with the next generation light sources, such as X-ray free electron laser (XFEL) and energy recovery linac (ERL).

Actinide uptake by transferrin and ferritin metalloproteins

In order to better understand the mechanisms of actinide uptake by specific biomolecules, it is essential to explore the intramolecular interactions between the cation and the protein binding site. Although this has long been done for widely investigated transition metals, very few studies have been devoted to complexation mechanisms of actinides by active chelation sites of metalloproteins. In this field, X-ray absorption spectroscopy has been extensively used as a structural and electronic metal cation probe. The two examples that are presented here are related to two metalloproteins in charge of iron transport and storage in eukaryote cells: transferrin and ferritin. U(VI)O2 2+, Np(IV) and Pu(IV) have been selected because of their possible role as contaminant from the geosphere.

Enhanced uptake and modified distribution of mercury(II) by fulvic acid on the muscovite (001) surface

Evidence is increasing for the mobility and bioavailability of aqueous mercury(II) species being related to the interactions of mercury with dissolved organic matter (DOM). Here, we assess the relative roles of the mineral surface and DOM in controlling mercury(II) uptake at the muscovite (001)-solution interface using interface-specific X-ray reflectivity combined with element-specific resonant anomalous X-ray reflectivity. Experiments were performed with single crystals of muscovite and solutions of 100 mg/kg Elliott Soil Fulvic Acid II and 0.5-1 x 10(-3) mol/kg Hg(NO3)2 at pH 2-12 Mercury(II) adsorbed from a 1 x 10(-3) mol/kg Hg(II) solution at pH 2 without fulvic acid (FA) as inner- and outer-sphere complexes that compensated 55(4)% of the permanent negative charge of the muscovite surface. The remaining charge presumably was compensated by hydronium. The enhanced uptake of Hg(II) (compensating 128% of the muscovite surface charge) and FA (43% more adsorbed compared to the amount from a similar solution without Hg), along with a broader distribution of Hg(II) at the interface, occurred by adsorption from a premixed solution of 1 x 10(-3) mol/kg Hg(NO3)2 and 100 mg/kg FA at pH 2. Adsorption of Hg(II) and FA, likely as complexes, decreased significantly as pH increased from 3.7 to 12 in solutions of 0.5 x 10(-3) mol/kg Hg(NO3)2 and 100 mg/kg FA. Preadsorbed FA molecules provide different binding environments and stability for Hg(II) than dissolved FA, which may be attributed to conformational differences, fractionation, or kinetic effects in the presence of the mineral surface, at least at these relatively high concentrations of aqueous Hg(II).

Water structure and aqueous uranyl(VI) adsorption equilibria onto external surfaces of beidellite, montmorillonite, and pyrophyllite: results from molecular simulations

Molecular dynamics simulations were performed to provide a systematic study of aqueous uranyl adsorption onto the external surface of 2:1 dioctahedral clays. Our understanding of this key process is critical in predicting the fate of radioactive contaminants in natural groundwaters. These simulations provide atomistic detail to help explain experimental trends in uranyl adsorption onto natural media containing smectite clays. Aqueous uranyl concentrations ranged from 0.027 to 0.162 M. Sodium ions and carbonate ions (0.027-0.243 M) were also present in the aqueous regions to more faithfully model a stream of uranyl-containing groundwater contacting a mineral system comprised of Na-smectite. No adsorption occurred near the pyrophyllite surface, and there was little difference in uranyl adsorption onto the beidellite and montmorillonite, despite the difference in location of clay layer charge between the two. At low uranyl concentration, the pentaaquouranyl complex dominates in solution and readily adsorbs to the clay basal plane. At higher uranyl (and carbonate) concentrations, the mono(carbonato) complex forms in solution, and uranyl adsorption decreases. Sodium adsorption onto beidellite occurred both as inner- and outer-sphere surface complexes, again with little effect on uranyl adsorption. Uranyl surface complexes consisted primarily of the pentaaquo cation (85%) and to a lesser extent the mono(carbonato) species (15%). Speciation diagrams of the aqueous region indicate that the mono(carbonato)uranyl complex is abundant at high ionic strength. Oligomeric uranyl complexes are observed at high ionic strength, particularly near the pyrophyllite and montmorillonite surfaces. Atomic density profiles of water oxygen and hydrogen atoms are nearly identical near the beidellite and montmorillonite surfaces. Water structure therefore appears to be governed by the presence of adsorbed ions and not by the location of layer charge associated with the substrate. The water oxygen density near the pyrophyllite surface is similar to the other cases, but the hydrogen density profile indicates reduced hydrogen bonding between adsorbed water molecules and the surface.

Molecular dynamics simulation of uranyl(vi) adsorption equilibria onto an external montmorillonite surface

We used molecular dynamics simulations to study the adsorption of aqueous uranyl species (UO(2)(2+)) onto clay mineral surfaces in the presence of sodium counterions and carbonato ligands. The large system size (10,000 atoms) and long simulation times (10 ns) allowed us to investigate the thermodynamics of ion adsorption, and the atomistic detail provided clues for the observed adsorption behavior. The model system consisted of the basal surface of a low-charge Na-montmorillonite clay in contact with aqueous uranyl carbonate solutions with concentrations of 0.027 M, 0.081 M, and 0.162 M. Periodic boundary conditions were used in the simulations to better represent an aqueous solution interacting with an external clay surface. Uranyl adsorption tendency was found to decrease as the aqueous uranyl carbonate concentration was increased, while sodium adsorption remained constant. The observed behavior is explained by physical and chemical effects. As the ionic strength of the aqueous solution was increased, electrostatic factors prevented further uranyl adsorption once the surface charge had been neutralized. Additionally, the formation of aqueous uranyl carbonate complexes, including uranyl carbonato oligomers, contributed to the decreased uranyl adsorption tendency.