Article pubs.acs.org/est
Uranium Immobilization and Nanofilm Formation on MagnesiumRich Minerals Arjen van Veelen,*,†,⊥ John R. Bargar,§ Gareth T. W. Law,‡ Gordon E. Brown, Jr.,§,∥ and Roy A. Wogelius*,†,⊥ †
Williamson Research Centre, School of Earth, Atmospheric and Environmental Sciences, and ‡Centre for Radiochemistry Research, School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. § Department of Photon Science and Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States ∥ Surface and Aqueous Geochemistry Group, Department of Geological Sciences, School of Earth, Energy, and Environmental Sciences, Stanford University, Stanford, California 94305-2115, United States S Supporting Information *
ABSTRACT: 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). Xray 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 OaxialU(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.
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INTRODUCTION Uranium (U) contamination, as a result of nuclear power production and other activities, is of worldwide concern. More specifically, huge quantities of legacy nuclear waste currently resides globally in ponds, silos, and tanks, all of which are nearing their designed lifetime. In the UK and in other countries where a similar type of reactor was used, one major challenge is the cleanup of intermediate-level waste resulting from first-generation Magnox (magnesium nonoxidizing) power plants. Over 1000 m3 of waste has accumulated in cooling ponds over a period of 60 years.1,2 Only limited information is available concerning pond inventories and the resulting chemical compositions of derived sludges. It is known, however, that brucite [Mg(OH)2] and magnesite [MgCO3] mineral phases of the corroded cladding dominate the sludge and show affinities with uranium.2−5 Information that illuminates the poorly understood interfacial chemistry of this waste is vital in order to develop a fundamental explanation of uranium-mineral surface interactions based on knowledge of © XXXX American Chemical Society
molecular structures of the adsorbate and its bonding configuration relative to the underlying lattice. In oxidized aqueous systems, uranium exists mainly as hydrated uranyl (UO22+) cations and uranyl−carbonate aqueous complexes and may also form complexes with several naturally occurring organic ligands.6 These oxidized uranium species are relatively soluble and mobile.7,8 As a consequence, in order to realistically model mass transfer and its associated risk, a comprehensive understanding of all uranyl species present, particularly interfacial species is necessary.9−11 Changes in solution composition will also affect uranyl adsorption on the solid phase and this may in turn also strongly affect mobility.9 In alkaline solutions, such as the cooling ponds which were maintained at high pH to prevent Received: December 9, 2015 Revised: February 16, 2016 Accepted: March 4, 2016
A
DOI: 10.1021/acs.est.5b06041 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology dissolution of Magnox fuel elements,12 UO2(CO3)34− will dominate uranyl solute speciation (as will typically be the case for aqueous solutions with pH > 9).13−16 Therefore, carbonate complexation of uranyl cations needs to be explicitly studied to fully understand uranium mobility. Little site-specific information on uranium on magnesite and/or brucite exists. However, with respect to solid phase incorporation, the uptake of uranium into calcite has been well studied.17,18 The size and shape of the linear transdioxo uranyl cation (OUO)2+ is considerably different than that of Ca2+ or Mg2+, which hinders substitution. Additionally, the uptake of uranium is dependent on the crystal structure of the carbonate mineral in such that hexavalent uranium is more suitable for incorporation in metastable aragonite [CaCO3] than calcite [CaCO3].17−19 Our recent study has shown that uptake of U(VI) into magnesite may be similar to, or even higher, than that for calcite.20 Information about surface specific active sites on mineral surfaces is important in order to make predictions of how uranyl may coordinate and subsequently be incorporated into the growing crystal structure.21 Moreover, identifying the important adsorption reactions which control mobility, will increase our predictive power and improve existing adsorption (e.g., molecular dynamics; MD) and reactive transport models. To that end, polarization-dependent grazing-incidence X-ray absorption fine structure (GIXAFS) in combination with X-ray reflectivity and diffuse scattering has been used in this study to constrain the adsorbate configuration of uranyl onto single crystal (hydroxylated) periclase and magnesite. Due to the rigid linear structure of U(VI) with the two axial oxygens and sensitivity of multiple scattering on polarization angle, the uranyl cation is well suited for such measurements. By using the polarization vector of the incoming synchrotron X-ray beam ε, which is highly polarized in the plane of the storage ring, we can constrain the orientation and bonding conformation of uranyl on mineral surfaces. A polarized EXAFS study of uranium sorption on Mg-bearing layered silicates (vermiculite and hydro-biotite) showed that under conditions greatly favored by ion-exchange, uranyl dominantly formed outer-sphere complexes with a preferred orientation of the uranyl axis parallel to the mineral layers.21 However, upon dehydration in that study the ion-exchange complexes adopted a less symmetric structure, consistent with an inner-sphere complex, and a less pronounced orientation of uranium was therefore observed. In addition, another study using polarized EXAFS of uranyl adsorption onto montmorillonite suggested outer-sphere complexation to dominate but could not resolve orientation information at different angles relative to the polarization vector (ε).22 However, the results did suggest that the uranyl unit was tilted at an intermediate angle between 0 and 90°. With the help of a simulation, this average angle was deduced as 45°. Results from previous experiments with natural calcite have shown that there is possibly step selective adsorption of uranyl on the (101̅4) surface via equatorial oxygens.23 Furthermore, the data of Denecke et al.23 showed that uranyl was oriented perpendicular (possibly in a constant tilt) to the surface of the calcite (101̅4) cleavage plane. In this study we provide a detailed picture of uranyl bonding conformation on brucite-like basal-plane and magnesite cleavage surfaces. These solids represent two important classes of surfaces expected in storage and disposal scenarios. We hypothesized that the uranyl cation will be adsorbed: (1) in an classic outer-sphere fashion on brucite(0001), with the uranyl-
axis parallel to the mineral layer (between the hydroxide layers) via its equatorial oxygen atoms, and (2) as an inner-sphere complex with the axial oxygens perpendicular to the magnesite surface (similar to that of calcite). Here we combine polarization dependent GIXAFS with X-ray reflectivity (XRR), diffuse scatter (XDS), and grazing incidence XRD (GI-XRD) to characterize formation of thin-films of uranyl adsorption complexes on the mineral surfaces. These surfacespecific results will be useful in making accurate predictions about the adsorption and incorporation of uranyl in the solids derived from Magnox sludges. These results will also allow us to predict uranyl behavior with other high-Mg phases such as backfill materials or mafic phases within a host lithology for a geological nuclear waste disposal facility due to the relative importance of brucite, magnesium carbonates, and brucite-type layers within phyllosilicates.
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EXPERIMENTAL SECTION Batch experiments on single crystals of periclase [MgO(111)] and magnesite [MgCO3(101̅4)] were performed to determine the adsorption of U(VI) on these surfaces. Full details of the experimental method, sampling and analysis are given in the Supporting Information (section Methodology). In brief, high quality single crystals of periclase and magnesite were bought and cleaved (surface roughness of 7−12 and 3.1 Å respectively). Upon submersion in solution, both the periclase and magnesite surfaces became hydroxylated (Figure S7 and Table S3) similar to that described in the works of Newberg et al.,24,25 and Stipp et al.,26 and therefore, periclase is henceforth referred to as brucite (0001). Uranium adsorption experiments (48 h, ±2h) were performed at U(VI) concentrations above (500, 50 ppm) and below (5 ppm) solubility limits of schoepte [(UO2)8O2(OH)12·12H20], which was thermodynamically modeled (PHREEQC,27 LLNL database) (Table 1) and at ambient (PCO2 10−3.5) or reduced partial pressure of CO2 (PCO2 10−4.5). In addition, a coprecipitation experiment was performed to investigate the effects of uranyl coprecipitating with Mg2+. In all experiments slightly elevated concentrations of Mg were observed after reaction (Table 1), but no visible Table 1. Experimental Conditions for Polarization Dependent GIXAFSa reduced PCO2d
ambient PCO2 500
50
5
500
Magnesite (1014̅ ) pHb 8.60 8.30 8.36 8.42 adsorbedc 19.69 0.43 0.05 7902f Mge 0.9 2.7 Brucite (0001) pH 8.31 8.35 8.27 8.42 adsorbed 0.93 0.25 0.16 5700f Mg 53.4 80.7 Brucite (0001) Coprecipitation with 0.1 M MgCl2 pH 8.32 8.25 8.31 adsorbed 36.22 0.57 0.18
5 8.54 93.45f
8.54 33.9f 46.8
a Substrates reacted in 30 mL solution for 48 h (±2h) under either ambient or reduced CO2 levels. bProbe error ±0.05 pH units. cOnto ∼2 cm2 surface area; determined by ICP-AES after desorption with HNO3. dPCO2 estimated from PO2 decrease measured by an oxygen sensor. eDissolved Mg2+ after 48 h reaction. fDetermined by ICP-AES from the difference of U(VI) concentration after 48 h reaction.
B
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Figure 1. Polarization dependency of uranyl on magnesite and brucite (A and B) under ambient and reduced partial pressures of CO2 respectively. Note the difference in intensity of features F1 and F2 when uranyl is aligned parallel or perpendicular to the polarization vector (ε) of the incoming X-rays. (C) Thin film formation and feature formation at different loadings on the crystal surfaces. (D) Postulated relationship of polarization dependence and crystal surface roughness.
dissolution features were observed during the reaction. Changes in the surface topography and structure were measured using Xray scattering methods. Polarization dependent grazing incidence EXAFS were performed at beamline 11−2 at the Stanford Synchrotron Radiation Lightsource (SSRL) of the SLAC National Accelerator Laboratory, USA. Fluorescence yield data were collected using a 30-element solid-state Ge detector, positioned at an angle of 45° to the crystal surface. Crystal alignment was performed so that the crystal was positioned parallel to the
incident beam and cut half of the beam intensity. Then the angle of the crystal surface relative to the beam was set at ∼25 millidegrees θc, but below the critical angle for total external reflection, to create standing waves. Measurements were performed at 0° (upward reflecting crystal sample) and 90° (side-reflecting sample geometry) in the Eulerian χ axis (rotation axis parallel to the X-ray beam direction). Due to the tightly coordinated linearity of the UO22+ molecule, distinct XANES spectra as a function of χ can be distinguished, which would allow us to constrain not only the molecular bonding C
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Figure 2. Polarized U LIII edge GIXAFS (left panel) and the Fourier back transformed EXAFS (right panel) on hydrated magnesite (101̅4) [MgCO3] and brucite(0001) [Mg(OH)2] under reduced PCO2 and ambient conditions. The area in green represents the polarization effect from the carbonate groups.
The decrease in the white line only occurs when the uranyl axial bonds are systematically oriented parallel to ε so that the contributions of the axial oxygens of uranyl are augmented (hence the shoulder intensity increase).29,30 As a result, the constructive interference strongly increases the axial-oxygen shoulder at the white line.29,31 Since all spectra show multiple maximum scattering amplitude of the uranyl atom when the crystal surface is oriented perpendicular to the beam vector ε (at χ = 90°), we conclude that the central rotational axis of uranyl on all of our model substrates is orientated at high angles to the crystal surfaces. We have labeled the curves to highlight the relationship between the uranyl molecule and the polarization vector, but in all cases this shows without question that the uranyl molecule is “standing up” on the surface. However, in our coprecipitation experiment, these polarization effects seemed to be lost (see Figure S4). In both the EXAFS and Fourier back transformed spectra (Figure 2A and B) the polarization dependent features related to scattering in the axial oxygen atoms are clear. This is shown in the EXAFS by the slightly increased amplitude of the oscillation with a maximum at k ∼ 5.1 Å−1 (Figure 2A) for the ε⊥ orientation as compared to the ε∥ data. When comparing the Fourier transformed data at the two orientations, we note that the intensities of the uranyl oxygen atoms at R ∼ 1.3 (Figure 2B) in particular varies significantly when comparing ⊥ against = in the same sample. These spectral variations are comparable to those previously published by Hudson et al.29 and Denecke et al.23 of UO2Ac2 and uranyl on α-Al2O3. Additional changes in the spectra are also easily resolved. In k-space, the spectra for ambient brucite (0001) showed remarkable differences between the two orientations in the k-range of 6−9 Å−1, where a single broad positive antinode split into two peaks in the second EXAFS oscillation when the crystal surface was parallel to ε (see the 5 ppm experiment, bottom curve, in particular). This phenomenon can be attributed to the augmentation of scattering in this orientation by the equatorial oxygen and carbon atoms. When ε∥, then both the C and O atoms from the carbonate group are aligned in such a way as to augment backscattering. This feature has been observed in previous
conformation of the uranyl on the crystal surfaces, but also its orientation relative to the underlying lattice. Background subtraction, data normalization, and fitting of the EXAFS spectra were performed in the Demeter software package, using Athena 0.9.18 and Artemis 0.9.18.28 To take into account the polarization effect on uranyl, especially, the effective coordination number (ECN)23,29 was used, described as Nie =
1 Ni(1 + 3cos2 θi) 2
(1)
where i identifies a specific shell with its own Ri and θi. θi in turn defines the angle between the polarization vector ε of the beam and the absorbing atom of the scatterer. This equation, given that the axial oxygen and equatorial oxygen atoms are at approximately 90°, provides a weighting function for numbers in a shell as a function of angle. By using this equation and knowing the first-shell configuration of uranyl, we may determine the angle of uranyl in the adsorbate complex with respect to the mineral surfaces and thus extract the molecular bonding configuration. Further detailed theoretical information on polarized EXAFS can be found in the Supporting Information (section Methodology) and in the work of Hudson et al.29 Lastly, the crystal surfaces and film-formation were fully characterized using X-ray reflectivity (roughness, density and film-thickness), diffuse scatter (in-plane features), and grazing incidence XRD (long-range order).
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RESULTS Polarization-Dependent GIXAFS. All XANES spectra from all experimental conditions show clear polarization dependence (Figures 1A and B). When changing the angle of the crystal surface (χ) relative to the beam vector ε it is apparent that the average uranyl bond angle relative to ε is also changing. The red curve in Figure 1A was measured at χ = 90° and the black curve was measured at χ = 0°. As previously reported,29,30 the absorption at the white line will be decreased (Figure 1A, B, and F1) and the first oscillation contribution of the axial oxygen atom multiple scattering (the shoulder region, F2) will increase when the uranyl molecular axis is parallel to ε. D
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Environmental Science & Technology Table 2. Shell-by-Shell Polarized GIXAFS Fit Results of Five Representative Surfaces at ∼pH 8.5 after 48 h orientation
shell
CNa
5 ppm U(VI) ε∥ (0001)b
U-Oax U-Oeq U−C U-Oax U-Oeq
1.6 5.9 1.1 2.9 3.5
U-Oax U-Oeq U−C U-Oax U-Oeq U−C
1.6 4.1 4.3 3.8 3.9 3.4
U-Oax U-Oeq U-Oeq2 U−C U-Oax U-Oeq U-Oeq2 U-Oax U-Oeq U−C U−Mg U-Oax U-Oeq U−C U-Oax U-Oeq U−C U-Oax U-Oeq U-Oeq2 U−C
1.6 5.4 2.4 3.8 3.2 2.9 4.4 1.8 6.3 6.6 1.5 3.2 6.1 2.4 1.4 8.4 5.3 3.4 2.4 7.4 2.5
ε⊥ (0001)
500 ppm U(VI) ε∥ (1014̅ ) ε⊥ (101̅4)
500 ppm U(VI) ε∥ (0001)
ε⊥ (0001)
50 ppm U(VI) ε∥ (0001)
ε⊥ (0001)
5 ppm U(VI) ε∥ (0001) ε⊥ (0001)
a
σ2
R (Å)
Brucite (0001) Reduced PCO2 1.78 (06) 0.001 (05) 2.30 (5) 0.016 (1) 2.93 (4) 0.010 (2) 1.79 (02) 0.005 (03) 2.26 (04) 0.008 (06) Magnesite (101̅4) Reduced PCO2 1.79 (05) 0.001 (09) 2.34 (08) 0.011 (2) 2.99 (1) 0.004 (2) 1.79 (03) 0.006 (06) 2.25 (07) 0.009 (1) 2.95 (2) 0.004 (3) Brucite (0001) Ambient Conditions 1.75 (03) 0.003 (04) 2.25 (04) 0.016 (1) 2.39 (1) 0.018 (3) 2.85 (07) 0.003 (1) 1.80 (02) 0.004 (04) 2.19 (06) 0.004 (09) 2.40 (04) 0.003 (06) 1.76 (05) 0.001 (07) 2.40 (06) 0.009 (1) 2.94 (2) 0.012 (3) 3.75 (5) 0.004 (4) 1.81 (03) 0.004 (05) 2.39 (05) 0.012 (1) 2.90 (2) 0.003 (2) 1.75 (2) 0.002 (1) 2.34 (2) 0.013 (1) 2.89 (1) 0.002 (1) 1.81 (05) 0.001 (06) 2.21 (1) 0.009 (3) 2.41 (08) 0.007 (1) 2.91 (2) 0.007 (5)
ΔE0 (eV)
χv2
R
4.53 ± 0.36
410.38
0.012
−0.37 ± 0.39
87.72
0.008
4.41 ± 0.37
74.56
0.012
−1.35 ± 0.41
79.97
0.010
−3.37 ± 0.30
83.83
0.013
5.73 ± 0.41
68.75
0.017
5.69 ± 0.43
184.64
0.032
8.14 ± 0.36
41.95
0.020
−0.42 ± 0.42
89.33
0.018
7.17 ± 0.40
39.92
0.013
Error of CN ±20%. bRefers to angle between ε and mineral orientation
When the crystal surface was rotated to be perpendicular to ε, EXAFS fits determined 2.9−3.8 axial oxygen atoms at ∼1.80 Å, which indicates that the central uranyl rotational axis is more or less aligned with ε at this position. In agreement with our XANES spectra, these results indicate that, because uranyl has two axial oxygen atoms, the uranyl axis is positioned at an angle of 60−85° relative to the crystal surface in all of our systems. The polarization effect on the equatorial oxygen atoms is less pronounced and is only notable on the magnesite surfaces. In the dilute systems (5 and 50 ppm) there are 6−8.5 oxygen atoms in the equatorial plane at a distance of 2.30−2.40 Å, which means an actual CN of 5−6 equatorial oxygen atoms. In the supersaturated solution (500 ppm), eight oxygen atoms could be fitted, equal to 5−6 oxygen atoms, in a split planar configuration. In addition, carbon atoms could be fitted at R ∼ 2.85−2.97 Å. To improve the models, three multiple scattering paths for UO of the uranyl atom at 3.5−3.6 Å were tested. Contributions from these paths were high when the uranyl axis was parallel to ε and must be accounted for in that geometry; however, they did not improve the fit significantly when the uranyl axis was perpendicular to ε. To conclude, XANES and EXAFS spectral differences and fit results indicate that regardless of the PCO2 of our systems, uranyl axis is attached approximately perpendicular to the
studies and is unequivocally assigned to equatorial carbonate.32−34 The observed polarization dependence from carbonate decreases for brucite (0001) with increasing concentration of uranium in solution, and disappears completely in the precipitate experiment. In precipitates on brucite, a “split” equatorial oxygen coordination occurs (see Figure S5 and Table S1) which is consistent with a schoepitetype structure. In addition, these precipitates apparently contain less equatorial C, which in turn contributes to the disappearance of the antinode at k 7.3−7.4 Å−1. This is also suggestive of the precipitation of a noncarbonate bearing phase. Consistent with these observations at ambient PCO2, under reduced partial pressure of CO2 the k-space spectra show only minor differences in both orientations. Our EXAFS results show an affinity for surface adsorbed uranyl−carbonate complexes under ambient conditions35 and uranyl−hydroxylate36 complexes under reduced PCO2 or high U concentrations. As previously stated, changing the angle of uranyl relative to ε has a significant effect on the effective coordination number (ECN) of axial oxygens when uranyl is aligned parallel (ECN ∼ 4) or aligned perpendicular (ECN ∼ 1) to the polarization vector ε. EXAFS fits, with the mineral surface parallel to ε, resolved 1.4 to 1.8 oxygen atoms at 1.75−1.79 Å (Table 2). E
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the diffraction peaks also suggests a less well-organized adsorbate thin-film on brucite (0001). In the 500 ppm coprecipitate system, XRR and XDS values for σ, ξ, and H were beyond the resolution of the instrumentation.
mineral surfaces. Under ambient conditions, uranium tends to adsorb as a uranyl−triscarbonato species and under reduced PCO2 as a uranyl-hydroxyl species. Both types of surface complexes tend to show signs of an “organized” adsorbate, hence the strong polarization dependence. X-ray Reflectivity, Diffuse Scatter, and GI-XRD. All analyzed surfaces showed indications of film formation at the surface. Prior to reaction with the uranium containing solutions, both magnesite and periclase showed a thin-film of water on each surface of ∼2 and 2.5 nm, respectively. Due to the comparability of the flat and stable surface of magnesite to calcite, a thin, low density layer 2.8 Å thick was included in the fit as previously described for calcite.37 After reaction, the roughness of the brucite (0001) surfaces increased significantly over the length of the reaction time, and only XDS was useful to acquire information about the brucite (0001)/aqueous solution interface. For the X-ray reflectivity (XRR) fitting results, diffuse scatter (DS), and GI-XRD, we refer to the Supporting Information, Tables S2A and S2B and Figure S6. Magnesite. The increase in amplitude of the “Kiessig fringes” on the magnesite surfaces indicates an increase in the densities (ρ) of the surface film layers relative to the water film on the unreacted crystal by a factor of 2 or more (see Figure 1C and Supporting Information, Table S2; XRR). XRR fits on magnesite with 500 or 50 ppm U(VI) improved when a concentration gradient was included as a function of distance from the crystal surface, with higher densities at the crystal surface and a slightly decreased density of the topmost-layer. Under ambient conditions with 500 and 50 ppm U(VI), these films are overall approximately 21 Å thick. The density of these films is consistent with a mixed phase, composed of bayleyite [Mg2(UO2)(CO3)3·18H2O] plus Mg uranyl hydrate [Mg(UO2)3(OH)2·4H2O] as determined via GI-XRD on the surfaces reacted at high U concentrations. Precipitate formation is also suggested by the much higher amounts of uranium concentrated at the surfaces of these crystals as shown in Table 1 relative to the 5 ppm experiments. GI-XRD of the 500 ppm experiment suggests that sheets of uranyl-hydrate (a schoepitelike phase) are apparently registered on the crystal surface with d-spacing perpendicular to the surface plane of approximately 7 Å, including domains of bayleyite-like phases. XDS fits in addition show that the crystal surface topography is not strongly altered after reaction because even though the in plane correlation distance (ξ) did decrease, it remained relatively large over the duration of the reaction period (Table S2B). Therefore, higher energy steps on the crystal surface may be minimal. There was however a sign of surface strain relaxation, due to the appearance of a “double-peak” in the diffuse scatter indicating a mosaic surface had developed. Brucite (0001). XRR on the product surfaces was not possible in this case because the surfaces became too rough after reaction. However, even though the roughness increased to well over 50 Å, XDS could still be measured (Figure 1C). XDS shows that the surfaces after reaction are jagged with many hill and valley features, with ξ and H parameters significantly changed after reaction (Supporting Information, Table S2B). These jagged features, possibly formed through coupled hydroxylation of the periclase (111) surface to form the brucite (0001) surface and dissolution, would have an effect on the organization and uptake of the adsorbed uranyltriscarbonato complexes. GI-XRD resolved similar precipitates in the 500 ppm periclase experiments, comparable to the results with magnesite. However, the significantly lower intensity of
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DISCUSSION XANES and EXAFS results show, in contrast with our expectations, that without question uranyl is adsorbed with the uranyl axis at high angles to the mineral surfaces examined here. In addition, XRR and XDS results suggest that these uranyl films are hydrated. At high loadings on both surfaces, diffraction analysis reveals the presence of poorly crystalline hydrated and hydrous uranium bearing phases. Therefore, based on surface chemistry and structure, it is most likely that uranyl is adsorbed with its axial oxygen atom not directly attached to the mineral surfaces, but via a hydrated monolayer on the surface. The polarization dependence of uranyl X-ray absorption spectroscopy was strongest on the magnesite surfaces. The magnesite crystal’s surface starting roughness (3.1 Å) was lower than the brucite (0001) surfaces (11.8 Å, determined by XRR) and remained more stable during reaction as would be expected.38,39 XRR and XDS (Figure 1C) confirmed that the crystal surface of magnesite is much more stable than the brucite-like surfaces. After 48 h, the brucite (0001) surfaces became too rough to be accurately resolved by XRR, and XDS in addition revealed a positive trend between increasing roughness and increasing U(VI) concentrations. As confirmed by previously performed experiments on dissolution rates of these substrates, the dissolution rate of magnesite (6.4−1.02 × 10−12 mol cm−2 s−1)40 is 4 orders of magnitude slower than that of hydrated periclase (1 × 10−8 mol cm−2 s−1),41 so that roughening would happen much faster on hydrated periclase. Therefore, the stability of the mineral surfaces controls the observed polarization effect (Figure 1D). This finding suggests that when a crystal has a surface roughness higher than 50 Å, the polarized beam will most likely not be able to distinguish different bond orientations clearly. This is the case with our system where we coprecipitated U(VI) with MgCl2. The coprecipitation resulted in a very rough and unorganized surface so that the polarization dependency was low between the two χ measurement angles. The hydrated magnesite surfaces, however, remained very stable and flat with large correlation lengths, so that the interaction of precipitates becomes negligible over large length scales.42 As a result, the majority of uranyl is likely to be adsorbed via surface hydroxyl with the U(VI) pushed away so that uranyl is “standing up”. EXAFS fits are in good agreement with distances and coordination numbers of previously published uranyl−triscarbonates and uranyl−hydrates under ambient and reduced PCO2 respectively.36,43−45 In the ambient under-saturated brucite system a higher CN for the carbon atom suggests a higher relative carbonate concentration in the adsorbate. Under ambient, slightly alkaline solution conditions uranyl forms very stable carbonate complexes,11,46 so that the carbonate in solution will be directly involved with uranyl complexation and adsorption. In addition, precipitates and surface adsorbates under ambient conditions can be distinguished through EXAFS by the split in equatorial oxygen shell and by the decrease in carbon signal, due to the precipitation of noncarbonate bearing schoepite-like phases. The absence of backscatters heavier than oxygen (U and Mg) in the EXAFS fits is an indicator that both the adsorbate and F
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parallel to the mineral surface. Essentially proton deficient surfaces will replace an equatorial oxygen with surface oxygen atoms, whereas protonated surfaces will bond via surface hydrogen to the axial actinyl oxygen atom. Uranyl adsorption on brucite, in contrast with the hydrated magnesite surface, may be more stable, since uranyl is adsorbed to the hydroxyl layer on the brucite basal plane (see schematic representation eq 2). These water bonds are likely to be strong and tight and result in inner-sphere type adsorption. Because calcite and magnesite are isostructural, uranyl adsorption on the (101̅4) surfaces is expected to be similar. We also expect uranyl and other actinyls, such as plutonyl and neptunyl, to be adsorbed similarly on other alkaline earth minerals, such as portlandite [Ca(OH)2], which is a key mineral expected to be present in Geological Disposal Facilities (GDF). The results presented here will improve our understanding and are fundamental to computational models of actinyl adsorption in these and similar systems. Implications for Uranium Mobility. Constraining the mobility of actinide elements is critical for designing remediation processes and for the development of future safety cases. These results show that uranyl is preferentially outersphere adsorbed on magnesite (101̅4) and inner-sphere adsorbed on brucite (0001) as an uranyl-triscarbonato (ambient) and uranyl-hydrate (reduced PCO2) complex with its uranyl axis oriented roughly perpendicular to the crystal surfaces. On all of our surfaces, uranyl interacts directly with surface hydroxyl, which pushes the uranium ion away to make it stand up on the mineral surfaces. It is expected that changes in surface charge (positive to negative) would change the observed orientation and adsorption of uranyl on these mineral surfaces. In addition, roughening of the surface has an effect on the clarity of the polarization dependency of the uranyl atom. GI-XRD results in the supersaturated uranium (schoepite) systems, suggests that these surface precipitates are comparable to U(VI) layered minerals, with multilayers of uranyl polymerizing on the mineral surfaces. As a result, these rather organized layered adsorbates are more stable than initially expected. The implications of these findings will directly feed into the optimization of the molecular dynamics computational models used for magnesite, calcite, brucite, and portlandite and will be important for the predicted adsorption behavior of other actinyls. Furthermore, based on our findings, we suggest that any mineral exposed to a low enough pH relative to its PZC may show a similar adsorbed actinyl surface orientation.
precipitates are likely to be hydrated with possible different multiple hydrated uranyl phases.35 This possibility is another indication that uranium is unlikely to be adsorbed directly to exposed surface oxygen atoms of hydrated periclase(111) or hydrated magnesite(101̅4). On hydrated periclase attachment will be via surface hydroxyls analogous to those exposed on brucite(0001) as shown in the schematic representation below, where > denotes the termination of the “solid” surface.
This mode of uranyl adsorption on brucite makes it technically inner-sphere, but because of the O−H−O− bonds in between U and Mg, EXAFS could not resolve any distal heavy backscatterers. On magnesite the adsorption is chemically similar to that of brucite, except that the hydroxyls in this system are not part of the solid structure, which sensu stricto defines this as outer-sphere adsorption. This finding is also supported by the fact that the correlation lengths of the hydrated magnesite surface remained long, so that there are fewer higher energy steps to adsorb uranyl. As a result, uranyl adsorption on the magnesite(101̅4) surface is chemically identical to that on brucite(0001). Therefore, we propose that the uranyl axial oxygen atoms interact with the surface hydroxyl47 on both the hydrated magnesite and brucite surfaces, which in turn pushes away the uranium ion from the surface and organizes the equatorial carbonates and/or water into a plane parallel to the surface. Additionally, GI-XRD results suggest lattice d-spacing of approximately 7 Å. This is, in combination with our polarization results, consistent with sheet-like mineral analogous to bayleyite, rutherfordine, and schoepite, which also have uranyl normal to the layers and a similar d-spacing. These multilayer precipitates and/or adsorbate layers then register on top of the surface contact layer of uranyl, and with surface hydrates and surrounding water persist in keeping the uranyl standing up on the mineral surface. These findings on these minerals, especially for magnesite, are partly consistent with atomistic simulations performed on Ca−uranyl carbonate by Doudou et al.47 Their study shows that the affinity of uranyl with the hydrated magnesite (101̅4) is low and more stable with a vicinal layer of water on the surface which forms hydrogen bonds between the calcite surface and carbonate anions. However, the calculated uranyl orientation in that study shows uranyl oriented parallel to the mineral surface, which contradicts the findings in our study. Our experiments show that the preferred organization in these systems is indeed outer-sphere adsorption on magnesite, but with the uranyl axis oriented at high angles to the crystal surface, similar to preliminary results of uranium on calcite23 and calculations of uranyl adsorption onto a hydrated Ni(111) surface.11 Therefore, given our unequivocal EXAFS results and because both surfaces are hydroxylated and positively charged (point of zero charge, PZC) of brucite is pH 1148 and by comparison to calcite [PZC = pH 9.549] the PZC of magnesite is also most probably above our experimental pH of < 8.5, these surfacehydroxyls are the driving force behind the observed adsorption. Therefore, under contrasting conditions where the surface is deprotonated, as was the case on magnetite in the study by Singer et al.,43 uranyl adsorption occurs via its reactive equatorial oxygen atoms and as a result uranyl is oriented
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b06041. Information on methodology/techniques, GIXAFS, GIXRD, XRR, DS, and XPS results (PDF)
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A.v.V. is supported through the UK Nuclear Decommissioning Agency (RWMD) affiliated EPSRC grant EP/I036389/1. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. DOE Office of Science by Stanford University. The SSRL Structural Molecular Biology Program is supported by the U.S. DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program (P41RR001209). Notes
The authors declare no competing financial interest.
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