U(IV) Aqueous Speciation from the Monomer to UO2 Nanoparticles

May 16, 2017 - X-ray scattering and UV−vis spectroscopy document aqueous speciation as a function of pH and heat. The zwitterionic glycine ligand re...
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U(IV) Aqueous Speciation from the Monomer to UO2 Nanoparticles: Two Levels of Control from Zwitterionic Glycine Ligands Clément Falaise,# Harrison A. Neal, and May Nyman* Energy Frontier Research Center, Materials Science of Actinides Department of Chemistry, Oregon State University, Gilbert Hall, Corvallis, Oregon 97331, United States S Supporting Information *

ABSTRACT: The fate of U(IV)O2 in the environment in a colloidal form and its dissolution and growth in controlled environments is influenced by organic ligation and redox processes, where both affect solubility, speciation, and transport. Here we investigate U(IV) aqueous speciation from pH 0 to 3 with the glycine (Gly) ligand, the smallest amino acid. We document evolution of the monomeric to the hexameric form from pH 0 to 3 via UV− vis spectroscopy and small-angle X-ray scattering (SAXS). Crystals of the hexamer [U6O4(OH)4(H2O)6(HGly)12]·12Cl−·12(H2O) (U6) were isolated at pH 2.15. The structure of U6 is a hexanuclear oxo/hydroxo cluster U6O4(OH)4 decorated by 12 glycine ligands and 6 water molecules. The effect of pH and temperature on U6 conversion to UO2 nanoparticles, or simply reversible aggregation, is detailed by transmission electron microscopy imaging, in addition to SAXS and UV-spectroscopy. Because of the zwitterion behavior of glycine, pH and temperature control over U(IV) speciation is complex. Unexpectedly, stability of the polynuclear cluster actually increases with increased pH. Speciation is sensitive to not only metal-oxo hydrolysis but also ligand lability and hydrophobic ligand−ligand interactions.

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INTRODUCTION Since the beginning of the Manhattan project and the development of nuclear energy, large amounts of actinides have been accidentally released in the environment. This radioactive contamination has promoted considerable interest in the behavior of actinides in the environment. Currently, the transport, solubility, and speciation of uranium in soil, groundwater, and the ocean are major concerns of stored nuclear wastes and waste forms, and potential roadblocks to broader acceptance of nuclear energy. Nuclear fuel is composed predominantly of tetravalent uranium oxide, U(IV)O2. Meanwhile, hexavalent uranium (uranyl; UO22+) can be reduced to U(IV) in anaerobic environments containing organics, microbes,1−4 or iron-containing minerals.5,6 Because of its strong Lewis acid character, U(IV) cations are susceptible to hydrolysis and condensation (through olation and oxolation), leading to the formation of insoluble uraninite or oxo-hydroxo clusters and inorganic colloids that have transient solubility.7−9 Complexation of the tetravalent uranium by carboxylic ligands that are abundant in the environment also affect the solubility of inorganic−organic colloids, leading to increasing of the U(IV) mobility.4 The formation of these soluble species (clusters and colloids) accelerates distribution of reduced uranium in the environment. Meanwhile, coordination of U(IV) with carboxylates and related ligands is a means of controlling speciation and enhancing solubility, both important factors in separations for the many stages of the nuclear fuel cycle, for not only U(IV) and U(VI), but also variable oxidation states of neptunium and plutonium. Numerous studies have been conducted to improve our understanding of the polynuclear species {Un} which could © 2017 American Chemical Society

form in natural and synthetic systems. At room temperature U(IV) tetramers have been isolated in nonaqueous condition with monocarboxylate ligands such as acetate,10 trifluoroacetate,11 and diethylcarbamate.12 By the controlled hydrolysis of the low-valent uranium in organic solvent, Mazzanti and coworkers have prepared larger aggregates ({U6}, {U10}, {U16}) passivated with benzoate ligand.13,14 More recently, Loiseau and co-workers have prepared through solvothermal treatment (N,N′-dimethylformamide or tetrahydrofuran) several clusters with different nuclearities ({U3}, {U4}, {U6}, {U12}, and {U38}) ligated with aromatic carboxylate ligands.15−19 The synthesis of U(IV) carboxylates in organic solvent is relatively well documented. Meanwhile characterization of the aqueous counterparts is only emerging,20−22 due to the difficulties in controlling their stability. Aqueous U(IV) chemistry is far more challenging, due to the complication of easier oxidation in water, as well as the instability toward further hydrolysis and condensation reactions that yield insoluble precipitates that defy characterization. Despite numerous efforts to synthesize new U(IV) clusters, an in-depth understanding of formation processes, stability, and the interaction of polynuclear species in water remains relatively unknown, mainly due to the difficulty in controlling solubility of U(IV) in water. From extended X-ray absorption fine structure (EXAFS) data, Takao and co-workers showed that the formation of aqueous U(IV) hexamer in diluted formic acid solution is controlled by pH.20 In addition to pH, it seems the temperature can play a key role in the oligomerization of Received: March 17, 2017 Published: May 16, 2017 6591

DOI: 10.1021/acs.inorgchem.7b00616 Inorg. Chem. 2017, 56, 6591−6598

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Single-Crystal X-ray Diffraction. An irregular-shape green-brown crystal of U6 (Figure SI1) was selected under a polarizing optical microscope. X-ray intensity data were collected on a Bruker DUO four-circle diffractometer equipped with an APEX II CCD detector using Mo-Kα radiation (λ = 0.71073 Å). Data reduction was accomplished using SAINT V7.53a.27 The substantial redundancy in data allowed a semiempirical absorption correction (SADABS V2.1028) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix least-squares on all F2 data using SHELX program.29 Hydrogen atoms of the organic molecules (glycine) were included in calculated positions and allowed to ride on their parent atoms. For the U6, the μ3-oxygen atoms of the hexanuclear core are disordered in two positions with an occupancy of 0.5 each. The final refinements include anisotropic thermal parameters of all non-hydrogen atoms. The crystal data are given in Table 1. CCDC-1530220 contains the supplementary

tetravalent uranium cations. In fact, based on the structural characterization of the solid, it was shown that the heating of a mixture of U(IV) and dicarboxylic ligand in organic media (DMF) promoted the formation of oxo/hydroxo hexamer in the range 100−130 °C and the oxo infinite chain forms above 140 °C.19 Despite these recent efforts, there is a paucity of information on the evolution of U(IV) species in water and the reaction pathways. To fill this knowledge gap, we have explored the reactivity of the glycine ligand with the tetravalent uranium in aqueous solution under inert atmosphere (argon). Glycine, NH 2 CH 2 COOH, exists in water as a zwitterion NH3+CH2COO− at neutral pH, NH2CH2COO− at high pH, and NH3+CH2COOH at low pH. It has been used to isolate hexanuclear clusters with tetravalent actinides including Th,23,24 Np,25 and Pu.26 Here we report not only the crystal structure of [U6O4(OH)4(H2O)6(HGly)12]·12Cl−·12(H2O) (U6), but also the aqueous speciation from the U(IV) monomer to UO2 colloids, and the effects of the glycine ligand and temperature on speciation. We have exploited small angle X-ray scattering (SAXS), UV−visible spectroscopy, and transmission electron microscopy (TEM) for solution characterization of speciation. These studies reveal the influence of the complex ligand behavior (protonation/deprotonation at two sites) on cluster aggregation and nanoparticle size and formation in water.

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Table 1. Crystal Data and Structure Refinement for U6, [U6O4(OH)4(H2O)6(HGly)12]·12Cl−·12(H2O) identification code CCDC number empirical formula formula weight temperature/K crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 Z ρcalc, g/cm3 μ/mm−1 F(000) crystal size/mm radiation 2Θ range for data collection/° index ranges

EXPERIMENTAL SECTION

Synthesis. Caution! Uranium metal is a radioactive, pyrophoric, and chemically toxic reactant, so precautions with suitable care and protection for handling such substances have been followed. All experiments were carried out under argon atmosphere using the following chemical reactants: uranium turning (U0; Alfa-Aesar, 99.7%), glycine (C7H6O2, HGly, Sigma-Aldrich, 98.5%), and hydrochloric acid. The starting chemical reactants have been used without any further purification. Preparation of [U6O4(OH)4(H2O)6(HGly)12]·12Cl·12(H2O) (U6). The UIV/Cl solution was prepared by the dissolution of uranium turnings (U0) in hydrochloric acid. A mixture of 1 mL of UIV/Cl solution ([U] ≈ 150 mM and Cl/U = 40) and 1.8 mL of glycine (2 M) was prepared in a glovebox in a 20 mL glass vial. The pH of this dark green solution is 2.15. After a slow evaporation, dark green crystals of U6 appear. These crystals are readily soluble in water and insoluble in alcohol (methanol, ethanol, and isopropanol) or other organic solvents (DMF, THF, and DMSO). U6 could be obtained only with excess of glycine (U4+/glycine: 1/24). During the evaporation process, glycine (colorless) cocrystallizes along with U6. In attempt to obtain phase pure U6, we tried removing glycine crystals by washing the solid product by various common organic solvents (DMF, THF, ethanol). Unfortunately glycine is not soluble in these. Because of these difficulties associated with separation of U6 from glycine crystals, we cannot provide a product yield. pH Studies. The aqueous speciation of U(IV) was studied in the range of pH 0.5−3. A solution of U(IV) and glycine ([U] ≈ 70 mM; [HGly] ≈ 0.28 M and a ratio Cl/U = 10) was prepared. Then the pH of this solution was modified by adding HCl (4 M) or NaOH (1 M). After 1 day of aging, the resulting solutions were analyzed by SAXS and UV−visible spectroscopy. For the pH higher than 3, a precipitation was observed. Temperature Studies. The influence of temperature was investigated on two different solutions (sol-U1 and sol-U6). The solU1 (U(IV) monomers) has a glycine/uranium ratio of 4, and a uranium concentration of ∼70 mM, and a pH of 1.1. The sol-U6 has a glycine/uranium ratio of 40, and a uranium concentration of ∼70 mM, and a pH of 2.1. Exactly 1 mL of each solution was pipetted into a 4 mL glass vial and then placed into an oven at 80 and 110 °C during 20 h. The resulting solution was analyzed by SAXS and UV−visible immediately following thermal treatment.

reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff. peak/hole/e·Å−3

U6 1530220 C12H30Cl6N6O25U3 2332.67 296.15 trigonal R3̅ 22.3240(9) 22.3240(9) 15.0936(7) 90 90 120 6514.3(6) 6 3.568 22.44 6025 0.18 × 0.14 × 0.12 MoKα (λ = 0.71073) 3.424 to 78.746 −39 ≤ h ≤ 39 −34 ≤ k ≤ 39 −26 ≤ l ≤ 25 40665 8610 [Rint = 0.0934] 8610/0/169 1.006 R1 = 0.0425 wR2 = 0.0960 R1 = 0.0765 wR2 = 0.1054 2.52/−3.37

crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https:// www.ccdc.cam.ac.uk/structures-beta/. Small Angle X-ray Scattering. SAXS data were collected on an Anton Paar SAXSess instrument utilizing Cu-Kα radiation and line collimation. Data were recorded on an image plate in the range of 0.08−2.5 Å−1. The sample to image plate distance is 26.1 cm. Solutions were measured in 1.5 mm glass capillaries. Pure water was used for the background, and scattering was typically measured for 30 min. SAXSQUANT software was used for data collection, treatment, and preliminary analysis (normalization, primary beam removal, 6592

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Figure 1. View of the hexanuclear building unit U6O4(OH)4 decorated by 12 glycine ligands in U6. Green spheres represent uranium atoms, orange spheres represent μ3-oxo groups, and yellow spheres represent μ3-hydroxo groups (oxo and hydroxo are each 50% occupied). Hydrogen atoms are omitted for ease of viewing. background subtraction, desmearing, and smoothing). The spherical fits of the scattering data were carried out utilizing size distribution and PDDF in the IRENA30 macros within IGOR Pro. Simulated scattering curve of the unit [U6O4(OH)4(H2O)6(HGly)12] was generated using SolX,31 utilizing structural files (xyz) containing the selected portion of the structure U6 with no symmetry elements. UV−Visible Spectroscopy. UV−visible spectra of the solutions have been collected by using Thermo Scientific Evolution 220 spectrophotometer in the range 350−1100 nm. The estimation of the species distributions (U(IV) monomer and hexamer) was determined by the ratio of the absorption intensity at two wavelengths (650 and 664 nm). We cannot directly determine the concentration of each uranium species by measuring the absorbance values of the maxima, because there are overlapping bands. In fact, for each uranium species, there electronic transitions (3H4 → 3P0, 3H4 → 1G4, and 3H4 → 1D2) can be observed in the range 600−700 nm. To tackle the problem associated with overlapping bands, we used the ratio of the absorption intensity at two wavelengths (650 and 664 nm). For pure phase of U6 (U6 crystals dissolved in DI water), the ratio of I650 nm/I664 nm = 0.915; this value is used as reference for 100% U6 and 0% U-monomer. For pure U-monomer, the ratio I650 nm/I664 nm = 2.615; this value is used as reference for 0% U6 and 100% U-monomer. The evolution of the ratio I650 nm/I664 nm is linear with species distribution (z% of U6 and (100 − z)% of U-monomer). The equation of this linear curve is y = −58.8x + 153.8 where y corresponds to the % of U6 and x corresponds to the value of I650 nm/I664 nm. By this approximation, we obtain a good estimation of the U based species distribution. Infrared Spectroscopy. Infrared spectra was recorded in attenuated reflectance mode (ATR) using a Thermo Scientific Nicolet iS10 FT-IR spectrometer. Transmission Electron Microscopy. U6 samples were prepared by dispensing 1 μL of diluted nanoparticles on gold 300 mesh sample grids with carbon backing and allowed to dry. The grids were then placed in a single tilt holder and analyzed in a Titan G2 80−200 keV TEM operating at 80 keV. Standard TEM alignments were done on the sample for imaging. The TEM settings were as follows: condenser aperture 5, spot 8.

longer trans distance is 5.43 Å and the U−U distance between two adjacent uranium centers is 3.84 Å. This octahedral building block has been reported for tetravalent uranium with monocarboxylate (formate,20 benzoate14) and polycarboxylate ligands16 (fumarate, terephthalate, 2,6-naphthalenedicarboxylate, and 4,4′-biphenyldicarboxylate). Each uranium atom is 9fold coordinated by four carboxyl oxygen atoms, four tricoordinated oxygen atoms (μ3-oxygen), and one water molecule, which define a monocapped square antiprismatic geometry (Figure SI2). The μ3-oxygen atoms are disordered on two positions and differed by two set of U−O bond distances. The shorter bond distances, in the range 2.233(7)−2.250(2) Å (i.e., U−O−U bond angles: ∼117.5(5)°), correspond to the oxo group. The longer U−O distance 2.441(7)−2.454(7) Å (i.e., U−O−U bond angles: ∼103.5(2)°), are attributed to the hydroxo group. Bond valence sum (BVS) calculation confirms both the attribution of hydroxo and oxo groups (Table SI1). The disorder observed on the μ3-oxygen atoms of the An6O4(OH)4 (An = Th(IV), U(IV), Pu(IV)) unit is very common when this octahedral unit crystallizes in high symmetry systems including orthorhombic,18,32 tetragonal,33 trigonal,26,32 or cubic.16,34 The second square plane of the antiprism is composed four oxygen atoms from four distinct carboxylic arms of the glycine ligand (U−Oglycine = 2.395(4)− 2.501(4) Å). Each glycine ligand bridges two U(IV) metalcenters. The last oxygen atom of the uranium coordination sphere, corresponding to a water molecule, is located above the square face defined by the four carboxyl oxygen atoms. This aquo ligand has a long bond to U(IV) (2.645(4) Å). The [U6O4(OH)4]12+ core is decorated by 12 glycine ligands which act as syn-syn bidentate bridging linkers. The glycine ligand can exist as the neutral glycine (HGly = NH3CH2COO) or the negatively charge form (Gly− = NH2CH2COO−). The infrared spectrum (Figure SI3 and Table SI2) shows a large absorption bands at 1594, 1502, and 1122 cm−1, corresponding to the presence of NH3 groups of the glycine ligand. These results suggest that glycine ligands coordinated to the hexanuclear core are neutral involving the {U6}-glycine complex is positively charge (+12). The analysis of the crystallographic data reveals 12 chloride ions (Cl−) per hexanuclear complex are intercalated between the clusters; unambiguously provide charge-balance

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RESULTS AND DISCUSSION Structure Description. The compound U6 crystallizes in the space group R3̅ (148) and consists of a hexanuclear unit U6O4(OH)4 decorated by 12 glycine ligands (Figure 1). The crystal structure is built from six uranium cations lying in one crystallographically independent site. The uranium centers are located on the corners of an ideal octahedral core, where the 6593

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eliminated by addition of electrolytes.44 In fact, this feature is not present in reaction solutions that contain excess HCl and glycine, as described below. All analyses of sol-U6-1 by SAXS indicate that U6 is stable and intact in neat water. The UV−visible spectrum of sol-U6-1 (Figure 3) shows the typical absorption bands of tetravalent uranium.45−49 The

for the 12+-charged polycation. In addition to the intercalated chloride ions, 12 free water molecules per hexanuclear unit were located in the electron density map. The arrangement of the clusters on the ab-plane, as viewed down the c-axis is shown in Figure SI4. The structure of compound U6 is analogous to the glycine complexes [M6O4(OH)4(H2O)x(HGly)y(Gly)z](12−z)+ isolated with Zr(IV) (x = 8; y = 8; z = 0),35 Th(IV) (x = 6; y = 6; z = 6 and x = 6; y = 12; z = 0),23,24 and Pu(IV) (x = 6; y = 12; z = 0).26 Recently, Ce(IV)6O4(OH)4 was also observed with mixture of glycine, nitrate, and water surrounding the hexanuclear core.36 This suggests the hexanuclear unit M6O4(OH)4 dominates for aqueous acidic solutions of M(IV) metal cations that favor coordination numbers greater than 6. With similar synthesis conditions for M6O4(OH)4 (M= Zr, Th, Pu and U), the actinide based hexamers reported are decorated by 12 ligands, while the zirconium based hexamer is surrounded by only eight glycine molecules. On the basis of this observation, we surmise that the tetravalent cationic radius influences the number of decorated glycine ligands. Hexanuclear Core U6O4(OH)4 in Water. Crystals of U6 were dissolved in water. This solution (noted sol-U6-1) was analyzed by SAXS and UV−visible spectroscopy. SAXS is a powerful technique to characterize cluster speciation and interactions in solution. Quantitative analysis of the scattering data gives information on cluster size, shape, and structuring of ions.37−43 The scattering curve observed for sol-U6-1 is similar to the simulated scattering curve of the hexanuclear UIV-glycine complex for q > 0.4 Å−1 (Figure 2), including the portion of the

Figure 3. (Top) Evolution of the UV−visible spectra of the U/HGly solution as a function of pH. (Middle) Species distribution in U/Gly solution. (Bottom) Visual observation of color change with species evolution.

comparison of the spectra of the sol-U6-1 and U4+/Cl solution (putative U4+ monomers in water, sol-U1) reveals that the electronic transitions of the hexamer are shifted to the longer wavelength (Figure SI5). The signals around 433, 500, and 554 nm could be attributed to the transitions 3H4 → 3P2, 3H4 → 1I6, and 3H4 → 3P1, respectively. Between 610 and 690 nm, an intense absorption band with two maximum peaks located at 653 and 664 nm corresponds to the three electronic transitions 3 H4 → 3P0, 3H4 → 1G4, and 3H4 → 1D2. The flat band in the range 790−940 nm is assigned to the transition 3H4 → 3H6. The last band situated at 1090 nm is attributed to the two transitions 3H4 → 3F3 and 3H4 → 3F4. Despite efforts, we were unable to crystallize a U(IV) monomer. SAXS data do not provide information about the coordination sphere. Thereby we can only hypothesize the ligation of U(IV) monomeric species. Possibilities include monomeric aqua species U(H2O)x(OH)y or glycine complexes U(glycine)x(H2O)y(OH)z. This hypothesis is based on previous observations reported in the literature. Using EXAFS data, Takao et al. reported the presence of monomeric species (aqua

Figure 2. Comparison of the log(q)-log(I(q)) scattering curves of simulated U6 (simulated) and sol-U6-1 (experimental). Sol-U6-1 was obtained by dissolution of U6 crystals in distilled water.

first oscillation (beginning at q ≈ 1.1 Å−1) that is observable within the q-range of the instrument. The Rg (radius of gyration, the root-mean-square distance of all the electrons from the center of gravity of the cluster) is directly correlated to the diameter of the cluster (Rg2 = 3/5(Ø/2)2; Ø = diameter for a spherical cluster). The pair-density distribution functions (PDDF) analysis is one way to extract an Rg, and it gives a value of 3.63 Å. The theoretical Rg from simulated31 scattering data is 3.9 Å. Below q = 0.4 Å−1, there is a broad Coulombic peak (from q = 0.06 to 0.4 Å−1), and this indicates structuring in solution that results from cluster−cluster repulsion. This is typical of solutions containing highly charged clusters and is 6594

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the vector. The maximum linear extent where scattering probability goes to 0 is the diameter of the scattering particle. The first Gaussian peak is consistent with the U6 cluster with a radius of ∼5 Å. Subsequent peaks at greater distance indicate mixture of clusters aggregates, in addition to U6, where the aggregates are up to ∼22 Å in diameter. Aggregation normally occurs by hydrolysis reactions, as pH increases, for example, initiated by displacement of glycine ligands (L), i.e.:

complexes or formate complexes), in acidic mixtures of U(IV) and formic acid.20 The same group proposed the formation of monocarboxylate complexes of Np(IV), Np(RCOO)(OH)2+, acidic mixtures of Np(IV), and monocarboxylic acid.25 During the investigation of Th(IV) behavior in acid condition containing glycine, Hennig et al.23 successfully crystallized a 1D coordination polymer [Th(H2O)3(HGly)3]·(ClO4)·4H2O from a solution with a pH value below the stability range of the hexanuclear complex. This compound is based on Th(IV) monomers which is coordinated by six oxygen from glycine ligands and three oxygen from water molecules. On the basis of EXAFS studies, the authors assumed that this one-dimensional chain (or fragment) does not exist in solution. On the basis of literature of tetravalent actinides An (Th, U, and Np) and monocarboxylic acid, it seems predominantly monomers coexist in solution, with no evidence for dimers in the presence of monocarboxylate. Some of them are purely aqua species (i.e., An(H2O) x(OH) y), while others are carboxylate complexes (i.e., U(COO)x(H2O)y(OH)z). U(IV) Speciation. Effect of the pH. Assuming no long-lived intermediate uranium(IV) species between the monomer and hexamer in the studied solutions, we have quantified these as a function of pH from 0.5 to 3 ([U] = 70 mM; [Gly] = 0.28 M) in Figure 3b. We observe visually and by UV−vis spectroscopy an evolution from pale green to darker green-brown with increasing pH, and finally precipitation above pH = 3 (Figure 3c). At low pH (between 0.6 and 1.4) the UV−visible spectra are similar to that of the UIV/Cl solution without addition of glycine, indicating the predominance of the monomer. At higher pH (between 1.85 and 2.8), the spectra are analogous to that obtained by dissolution of crystallized U6 in water, meaning the hexanuclear complex is the main species observed in the mixture. Solutions at intermediate pH have absorption bands of both monomer and hexamer species. X-ray scattering of pH 0.9, 1.65, 2.1, and 2.8 solutions show an evolution of the scattering with pH increase (Figure 4). No

2M6O4 (OH)4 Lx + 2OH− → 2M6O4 (OH)4 Lx − 1(OH) + 2L

(1)

2M6O4 (OH)4 Lx − 1(OH) → [M 6O4 (OH)4 Lx − 1]2 O + H 2O (2)

At pH 1.65, the mean diameter obtained from the size distribution modeling is close to 5.85 Å. We assume that the low diameter value for the solution with pH = 1.65 is due to the mixture of hexamer and monomer species in solution, in agreement with UV−vis spectrum (Figure 3). At pH 2.8, the scattering curve indicates aggregation, based on the slope rather than plateau at low q-values, and the analysis becomes complicated. This scattering curve can be fit with a fractal aggregate model with a radius of the primary particle size of 5 Å, a fractal dimension of 1.4, and an average aggregate size of ∼100 Å (Figure SI7). Surprisingly, this aggregation is reversible with the addition of acid. This indicates that hydrolysis of the U6(O,OH)8 core as depicted in eqs 1 and 2 is not responsible for the aggregation behavior in solution, since this is rarely reversible in a controlled way. Instead, we interpret this behavior as partial deprotonation of the Glycine amine moiety, leading to hydrophobic cluster− cluster interactions. Because the cluster charge becomes less positive with neutralization of ligands, they are attracted rather than repelled. Effect of the Temperature. While pH controls the evolution from a monomer to a hexamer, temperature promotes the formation of UO2 nanoparticles, regardless of the pH. However, these studies yielded unexpected results that we again attribute to ligand behavior. Sol-U1 initially containing mostly monomers and sol-U6 initially dominated by hexamers (see Experimental Section) were heated at 80 and 110 °C. With heating, sol-U1 darkens at 80 °C and becomes black at 110 °C, while sol-U6 color remains unchanged at 80 °C and becomes black at 110 °C (Figure SI8). The UV−visible analysis of heated sol-U1 indicates no characteristic absorption bands of the U(IV) hexamer (Figure SI9). The black color of the 110 °C solution has no distinct absorption bands, and it is colloidal UO2 as determined by TEM (discussed below). UV−vis indicates the hexamer persists in sol-U6 at 80 °C and evolves to UO2 at 110 °C (Figure SI9). These results indicate that at lower pH (1.1), aqueous UIV-glycine solutions are more readily converted to colloidal UO2 than at higher pH (2.1). This was initially surprising because, as described in eqs 1 and 2, increasing pH promotes the hydrolysis reactions that lead to UO2 formation, and current experiments suggest otherwise. X-ray scattering of sol-U1 and sol-U6 are consistent with UV−vis analyses (Figure 5). Sol-U6 at 80 °C is similar to crystallized U6 redissolved in neat water but with neither the Coulombic peak indicating repulsion (Figure 3), nor any indication of aggregation. A size distribution analysis of all four scattering curves (Figure SI10) shows the primary particle size has a radius of ∼5 Å, consistent with the size of the hexamer.

Figure 4. Evolution of the experimental log(q)−log(I(q)) scattering curves of U/HGly solution as a function of the pH.

significant scattering is observed at pH-0.9, while from pH 1.65 to 2.8, we observe an increase in scattering intensity and the formation of a distinct Guinier region at q ≈ 0.4 Å−1. PDDF modeling of the pH-2.1 solution shows three overlapping Gaussian peaks with a maximum linear extent of 21.5 Å (Figure SI6). The PDDF analysis is a histogram of scattering vectors through the species, where distance (r, Å) is the magnitude of 6595

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hydrolysis reactions. Temperature also clearly has an effect on hydrolysis, likely increasing the lability of the ligand and its rate of replacement with water, promoting hydrolysis reactions.

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CONCLUSION Aqueous formation of oxohydroxyl metal polyoxocations of low-valent transition metals, lanthanides, and actinides generally ensues by increasing the pH. Ligands including carboxylates, amines, aminocarboxylates, and inorganic oxoanions serve to isolate the clusters in a molecular form. The current study departs from these predicted trends in metal hydrolysis chemistry, and this is owed to acid−base behavior of the amino-acid ligand. The ligand behavior greatly influences hydrolysis pathways of the metal cations, as well as reversible aggregation. Normally, increasing the pH of aqueous polyoxocations leads to precipitation of metal oxyhydroxides, which cannot be readily reversed. In the case of the glycineligated U(IV) hexamer, pH increase deprotonates the amine groups of glycine, decreasing cation−cation repulsion and increasing hydrophobic interactions of the organic ligands. This type of aggregation is reversible, indicating the hexamer core is protected by the ligands from further hydrolysis reactions. On the other hand, temperature promotes precipitation of UO2. We normally expect this reaction to be accelerated at higher pH, but the U(IV) monomer at pH ≈ 1 converts to more rapidly and yields larger UO2 particles than the U(IV) hexamer at pH ≈ 2. This too is attributable to the acid−base behavior of the ligand: At lower pH, the carboxylate becomes protonated, preventing ligation that would otherwise prevent formation of UO2. In summary, the three states of the glycine ligand, anionic, neutral (zwitterionic), and cationic provide an unexpected dimension of influence over U(IV) hydrolysis and aggregation of aqueous U(IV) species. Because the processes of glycine protonation−deprotonation and U(IV) hydrolysis are likely interdependent in close spatial proximity, there may be rich chemistry that is yet undiscovered in this and related systems. This could include stabilization and isolation of small oligomers intermediate between monomers and hexamers, revealing the hydrolysis pathway in the prenucleation stages of UO2. Therefore, these findings have implications on both environmental fate and transport, and aqueous processes in several stages of the nuclear fuel cycle.

Figure 5. Experimental log(q)−log(I(q)) scattering curves of the solution sol-U1 and sol-U6 heated at 80 and 110 °C.

However, larger scattering particles or aggregates are observed in sol-U1 at both 80 and 110 °C and in sol-U6 at 110 °C. The sol-U1 at 80 and 110 °C both have a distinct Guinier region below q = 0.07 Å−1, and fitting of this data shows a predominance of particles with a radius of 6.2 nm at 80 °C, and 8.5 nm at 110 °C (Figure SI10). TEM analysis corroborated the size of the nanoparticles (Figure 6).

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. TEM image of sol-U1 heated at 80 °C, showing well-formed UO2 nanoparticles, approximately 5 nm in size, in agreement with the SAXS data.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00616. Figure SI1: Photography of crystals of the compounds U6 obtained by optical microscope; Table SI1: Bond valence sums for the U6O8 core of U6; Figure SI2. View of the coordination sphere of the unique crystallographic uranium atom U1; Figure SI3. Infrared spectra of the solid U6; Table SI2. Assignment of the infrared vibration of the U6; Figure SI4. View of U6 along the c axis; Figure SI5. Comparison of the UV-visible spectra of the

Similar to the reversibility of the U6 aggregation as a function of pH, we attribute this unexpected trend (i.e., soluble polynuclear molecular forms more stable at higher pH) to the behavior of the ligand. The pKa of the carboxyl group of glycine is 2.2 (eq 3); therefore, in sol-U1 it is mostly protonated, and in sol-U6 it is mostly deprotonated:When the carboxylate of glycine becomes protonated at lower pH, it cannot so readily bind the UIV, which increases vulnerability to 6596

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Inorganic Chemistry solution; Figure SI6. PDDF analysis of UIV/HCl/NaOH/ glycine reaction solution, Figure SI7. Fractal aggregate fit of the pH-2.8 solution of UIV/glycine/HCl/NaOH; Figure SI8. Photos showing the evolution of the color of sol-U1 and sol-U6; Figure SI9. Evolution of the UVvisible spectra of the sol-U1 and sol-U6 as a function of temperature; Figure SI10: Experimental log(q)-log(I(q)) scattering curves of the solution sol-U1 and sol-U6 heated at 80 and 110°C. Volume distribution as a function of the scattering diameter; Figure SI12. Spherical model fit of large particles in sol-U1 (PDF)

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Accession Codes

CCDC 1530220 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. URL: msa-efrc.com/ nyman.chem.oregonstate.edu/. ORCID

May Nyman: 0000-0002-1787-0518 Present Address #

Université de Rennes 1, Institut des Sciences Chimiques de Rennes (ISCR), Rennes, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Materials Science of Actinides, a Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001089.

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REFERENCES

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