ARTICLE pubs.acs.org/est
Atomistic Simulations of Uranium Incorporation into Iron (Hydr)Oxides Sebastien Kerisit,* Andrew R. Felmy, and Eugene S. Ilton Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
bS Supporting Information ABSTRACT: Atomistic simulations were carried out to characterize the coordination environments of U incorporated in three Fe-(hydr)oxide minerals: goethite, magnetite, and hematite. The simulations provided information on UO and UFe distances, coordination numbers, and lattice distortion for U incorporated in different sites (e.g., unoccupied versus occupied sites, octahedral versus tetrahedral) as a function of the oxidation state of U and charge compensation mechanisms (i.e., deprotonation, vacancy formation, or reduction of Fe(III) to Fe(II)). For goethite, deprotonation of first shell hydroxyls enables substitution of U for Fe(III) with a minimal amount of lattice distortion, whereas substitution in unoccupied octahedral sites induced appreciable distortion to 7-fold coordination regardless of U oxidation states and charge compensation mechanisms. Importantly, UFe distances of ∼3.6 Å were associated with structural incorporation of U and cannot be considered diagnostic of simple adsorption to goethite surfaces. For magnetite, the octahedral site accommodates U(V) or U(VI) with little lattice distortion. U substituted for Fe(III) in hematite maintained octahedral coordination in most cases. In general, comparison of the simulations with available experimental data provides further evidence for the structural incorporation of U in iron (hydr)oxide minerals.
’ INTRODUCTION Uranium is the most common and ubiquitous radionuclide contaminant in subsurface systems associated with the U.S. Department of Energy sites where nuclear materials were processed and stored.1 The mobility of uranium in the environment is largely determined by its oxidation state, where U(VI) is relatively soluble and U(IV) is only sparingly soluble under common environmental conditions. Under oxidizing conditions where U(VI) predominates, transport of U in groundwater or the vadose zone can be attenuated by precipitation of uranyl-bearing phases or by adsorption to minerals. Consequently, there has been much research on characterizing both the solubility of U(VI) phases, e. g., see review by Gorman-Lewis et al.,2 and the effect of variable conditions (e.g., pH, ionic strength, cations, and anions) on the sorption of U(VI) by minerals that are common in soil systems, including various iron (hydr)oxides. In most cases, U(VI) adsorption has been conceptualized and modeled strictly as a surface process. However, even in simple model iron oxyhydroxide systems, adsorption is not necessarily reversible and depends on both the extraction process and sorption time.3,4 Among possible explanations, the potential for incorporation into the mineral structure is supported by the extended X-ray absorption finestructure (EXAFS) study of Duff et al.5 on the coprecipitation of uranyl and hematite under oxidizing conditions. The incorporation of U into goethite and possibly magnetite during reductive dissolution and recrystallization of ferrihydrite6,7 has also been proposed based on EXAFS measurements. In each case, EXAFS indicates that uranium lost the short trans-dioxo bonds and was incorporated into the iron (hydr)oxide structures in uranate coordination. r 2011 American Chemical Society
However, the evidence for incorporation typically rests on both the extractability of U using different agents as well as UO and UFe distances that do not match distances claimed for simple adsorption in previous EXAFS and theoretical studies (e. g., refs 8 and 9). Moreover, questions remain, including the mechanism(s) for charge balance, whether U is incorporated in a regular lattice or interstitial site, and the oxidation state of incorporated U under reducing conditions.6 Therefore, independent data from additional experimental techniques as well as from theory are needed to confirm the structural incorporation hypothesis and to provide finer grain information on U incorporation mechanisms. In this contribution, we used classical atomistic modeling to evaluate the coordination of U(IV), U(V), and U(VI) incorporated in goethite (R-FeOOH), magnetite (Fe3O4), and hematite (R-Fe2O3). Several classical atomistic models have been developed to simulate uranyl in aqueous solutions and have shown good agreement with both experimental data and ab initio molecular dynamics simulations of uranyl hydration (e.g., ref 10). Similarly, classical models for simulating UO2 are reported in the literature and give a good representation of the structural, elastic, and dielectric properties of the pure phase (e.g., ref 11). The availability of classical models for other uranium oxide phases is limited12 and there is also a lack of studies on trace incorporation of U in minerals. Received: November 8, 2010 Accepted: February 28, 2011 Revised: February 22, 2011 Published: March 10, 2011 2770
dx.doi.org/10.1021/es1037639 | Environ. Sci. Technol. 2011, 45, 2770–2776
Environmental Science & Technology This approach is computationally relatively inexpensive, which facilitated simulating many combinations of host minerals, uranium oxidation states, incorporation sites, and charge compensating schemes as well as many configurations for each combination. In addition, it enabled the use of large simulation cells to more closely model the infinite dilution limit. Given thoroughly benchmarked potential parameters, the strength of classical atomistic modeling lies in determining geometries, which are the only experimental data on the systems of interest currently available for comparison. The specific objectives of this contribution are to simulate UO and UFe distances, coordination numbers, and a measure of the induced local lattice distortion for U incorporated in occupied and unoccupied sites as a function of different charge compensation schemes (CCS). As mentioned, the strong point of this approach is deriving geometries, not necessarily energies. Indeed, the electronic structure of the model systems is not explicitly included, which potentially decreases the accuracy, particularly for energetics, relative to quantum mechanical methods. Accordingly, we have used the available experimental/EXAFS data to make the following assumptions: (1) U forms a limited solid solution in Fe(III)-(hydr)oxides and (2) U is incorporated in uranate, not uranyl coordination. Further, we did not compute which oxidation state and incorporation scheme is the most stable. Nonetheless, given the scope of this study, the classical atomistic modeling approach is well suited to meet our objectives and the results can be used as a starting point for more restricted, higher-level-of-theory studies of U incorporation into Fe-(hydr)oxides. It should also be noted that it is not yet known whether the experimentally derived U coordination environments in these systems correspond to thermodynamic equilibrium or are influenced by kinetic effects. The long-term goal is to provide an independent means to help interpret complicated spectroscopic data, such as often provided by EXAFS, on the association of U with Fe-(hydr)oxides.
’ COMPUTATIONAL METHODS Potential Model and Parameters. In the simulations, atoms are represented as point-charge particles that interact via longrange Coulombic forces and short-range interactions. The latter are described by parameterized functions and represent the repulsion between electron-charge clouds, van der Waals attractive forces, and, where applicable, covalent effects. The polarizability of oxygen and uranium ions was simulated by a shell model,13 where the ion is composed of a core and shell which share the ion’s charge and are linked by a harmonic spring. In the context of this study, the use of a shell model is important for taking into account the polarization of the lattice due to the presence of defects. The shell model performs better than the rigid-ion model with, for example, improved defect14 and reorganization energies.15 We note, however, that the ionic charges remain fixed in the simulations and that, therefore, the potential model does not account for any possible partial charge transfer. The ionic charges and potential parameters used for describing uranium, iron, oxygen, and hydroxyl were taken from Ball,12 Lewis and Catlow,16 Catlow,17 and Baram and Parker18 and are reported in Supporting Information (SI). Model Validation. A detailed comparison between experimental and calculated lattice parameters for all test phases is provided in SI. The potential model was evaluated, in part, by its ability to reproduce the lattice parameters of ten iron (hydr)oxide phases, containing iron in both ferrous and ferric oxidation states,
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and seven binary and ternary uranium oxide phases, containing uranium in oxidation states IV, V, and VI. The calculations were performed with METADISE.19 For the iron (hydr)oxide series, 70% of the lattice parameters were reproduced within 3% and all but one within 5%, namely, the c lattice parameter of Fe(OH)2. For the uranium oxide series, with one exception, all calculated lattice parameters fell within 2% of the corresponding experimental value. The ability of the potential model to describe coordination environments containing both iron and uranium was evaluated by calculating the structures of three well-characterized uraniumiron oxides: UFeO4,20 Ba2FeUO6,21 and Sr2FeUO6,22 where Fe and U are known to be in the trivalent and pentavalent oxidation states, respectively. Modeling each phase with Fe(III)/ U(V), the known oxidation states, yielded close agreement with experiment whereas using Fe(II)/U(VI) compared poorly with experiment. Uranyl coordination was not considered as EXAFS of U thought to be incorporated in both hematite5 and goethite6 indicated that U is in uranate coordination. Therefore, the model is not completely independent of the experimental data. Future work will consider the incorporation of uranyl in Fe-(hydr)oxides. U Incorporation in Fe-(hydr)oxides. For simulations of U(IV), U(V), and U(VI) in iron (hydr)oxides, unit cells were expanded 6 3 8, 3 3 3, and 5 5 2 for goethite, magnetite, and hematite, respectively, to generate supercells with side lengths of approximately 25 Å. Two incorporation sites were considered for each mineral: occupied and unoccupied octahedral sites for goethite and hematite, and octahedral and tetrahedral sites for magnetite. Whether substituted for iron or incorporated in an unoccupied site, U(IV), U(V), and U(VI) all increase the formal charge of the system. Therefore, four charge compensation schemes (CCS) were considered: uniform background charge density (CCS-U), reduction of neighboring Fe(III) ions (CCS-R), creation of iron vacancies (CCS-V); and for goethite, deprotonation of hydroxyl groups (CCS-H). For CCS-V, it is not possible to exactly charge compensate for any combination of U(IV) or U(V) and Fe(III) vacancies. In these cases, protonation/deprotonation (CCS-VH), iron reduction (CCS-VR), and iron oxidation (CCS-VO) were used in addition to the creation of an iron vacancy to adjust the system net charge to zero. For each combination of host mineral, incorporation site, oxidation state, and charge compensation scheme, geometry optimizations were performed with the steepest-descent technique as implemented in the computer code DL_POLY23 for all symmetrically nonequivalent configurations to determine the uranium coordination environment. In each case, only the most energetically stable configuration is reported.
’ RESULTS AND DISCUSSION For each incorporation site of each mineral, a summary table is presented, which contains, for each coordination shell, the coordination number, N; the average interatomic distance, R; the root-mean-square of the interatomic distances, σ; and the percentage change of the average UO/Fe distances for the substituted lattice relative to that in the original lattice, Δ. The latter value was determined separately for either simulated or experimentally determined lattice changes in order to normalize potential systematic errors. Both σ and Δ are measures of lattice distortion upon uranium incorporation. The individual calculated 2771
dx.doi.org/10.1021/es1037639 |Environ. Sci. Technol. 2011, 45, 2770–2776
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Figure 1. Uranium incorporation into (a) goethite, (b) magnetite, and (c) hematite. Iron atoms are shown in purple, oxygen atoms are in red, and hydrogen atoms are in white. The uranium incorporation sites are shown in yellow. The three hydrogen atoms removed to accommodate uranium in a vacant octahedral site in goethite are shown in gray. The additional uraniumoxygen bonds formed upon relaxation in some of the simulations are shown with solid lines. The hydrogen bond broken following uranium incorporation at the octahedral site in goethite is represented with a dotted line.
distances to the first shell oxygen and second shell iron ions as well as optimized structures of the incorporation sites are presented in SI. Uranium Incorporation into Goethite. Goethite has an orthorhombic unit cell with a = 4.598 Å, b = 9.951 Å, and c = 3.018 Å24 (Pbnm space group). Goethite consists of hexagonal close packed arrays of O2- stacked along the [100] direction. Fe(III) occupied and unoccupied edge-sharing octahedra form alternating double chains along the [010] direction. The double chains of empty octahedra will be referred to as channels. All Fe(III) sites are symmetrically equivalent, and each Fe(III) is coordinated to three oxygens and three hydroxyls. U was either substituted for Fe(III) or incorporated in an unoccupied octahedral site (Figure 1a). Each occupied octahedron shares edges with two pairs of nearest-neighbor FeO6 octahedra and corners with four occupied nearest-neighbor octahedra. Each vacant octahedron shares faces with two occupied nearest-neighbor octahedra and edges with two nearest-neighbor FeO6 octahedra. However, as described later, uranium increases its coordination from six to seven when placed inside a channel. As a result, two additional FeO6 octahedra were found to share an edge with the UO7 polyhedron and were thus considered as nearest-neighbor FeO6 octahedra. All four charge compensation schemes were considered for goethite. The nearest-neighbor FeO6 octahedra described above served as sites for iron reduction, CCS-R, or vacancy formation, CCS-V. For CCS-VH (i.e., U(IV) or U(V)), oxygen coordinated to the vacancy were protonated as necessary to adjust the system net charge to zero. For CCS-H, the uranium first shell hydroxyls were deprotonated as needed. Additionally, for U incorporation in a vacant octahedron, the three nearest hydroxyl groups were deprotonated to create room for uranium prior to charge compensation. When U substitutes for Fe(III), the average simulated UO distances are 620% longer than the average simulated FeO distance and the first shell coordination numbers, N, range from 6 to 8 (Table 1). Higher N is associated with higher U valence and CCS-R, CCS-V, and CCS-VH. CCS-R, CCS-V, and CCSVH promote high N because oxygen is underbonded to Fe(II) and vacancies. Consequently, U relaxes toward the second shell oxygens (Figure 1a) and forms 12 additional UO bonds. The first additional oxygen bridges the two corner-sharing FeO6
octahedra (Figures 1a and S1a). The relaxation of this oxygen toward U causes its hydrogen bond with a neighboring hydroxyl group to break (Figures 1a and S1a). The second additional oxygen is the closest second shell hydroxyl (Figures 1a and S1b). The exception is for U(VI) and CCS-V, where U is coordinated by two additional oxygens associated with a vacancy created by removing Fe(III) from an edge-sharing FeO6 octahedron. Importantly, deprotonation of first shell hydroxyl groups leads to shorter uraniumoxygen bonds and exclusively 6-fold coordination. Further, CCS-H produces more symmetrical uranium coordination environments and less local distortion of the lattice for U(V) and U(VI) compared to other CCS, as indicated by low values of σ and Δ (Table 1 and Figure S1c). U incorporated in an unoccupied octahedral site relaxes toward the center of the channel and binds to one more oxygen, regardless of U valence and CCS (Figures 1a and S1d). As a result, two additional FeO6 octahedra shared an edge with the UO7 polyhedron. The face-sharing octahedra are pushed out and, due to the displacement of the uranium ion toward the center of the channel, the distances to the face-sharing octahedra and to the first set of edge-sharing octahedra increase whereas the distances to the second set of edge-sharing octahedra decrease (Table 2). Uranium is in 7-fold coordination in all cases when in this site. Although the model validation set consists of solid phases with uranium in a range of coordination environments, none contain U in 7-fold coordination. Consequently, one should be cautious about predicted coordination environments in the unoccupied octahedral site. Uranium Incorporation into Magnetite. Magnetite adopts the inverse spinel structure with a = 8.394 Å25 and consists of cubic close-packed arrays of oxygen ions stacked along the [111] direction. Octahedral layers alternate with mixed tetrahedral/octahedral layers along the [111] direction. The tetrahedral sites are occupied by Fe(III) whereas the octahedral sites are occupied by Fe(II) and Fe(III). The bulk of the octahedral Fe sites were assigned an ionic charge of þ2.5 to represent the rapid electron transfer between octahedral Fe(II) and Fe(III) ions at room temperature.26 Tetrahedral and octahedral sites (Figure 1b) were considered for U incorporation. The tetrahedral site has twelve corner-sharing nearest-neighbor FeO6 octahedra whereas the octahedral site shares edges with six nearest-neighbor FeO6 octahedra. All CCS were considered except for CCS-H, as we assumed that magnetite was 2772
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Table 1. Uranium Incorporation into Goethite: Occupied Octahedral Sitea oxygen 1st shell ox. 6
N
CCS
σ (Å) Δ (%) 5
N
R (Å)
1.0
3.08(3)
σ (Å)
iron 2nd shell (eso#2) Δ (%)
N
R (Å)
σ (Å)
-
2
1.0
3.2(1)
-
iron 2nd shell (cso) Δ (%)
N
R (Å)
σ (Å)
Δ (%)
3
2.0
3.58(4)
-
4
3.9(4)
2.12(2)
-
U(IV) U
6
2.26
0.14
13
2
3.33
0.00
6
2
3.59
0.00
7
4
3.44
0.12
3
U(IV) R
6
2.24
0.12
12
2
3.30
0.03
5
2
3.55
0.01
6
4
3.48
0.05
4
U(IV) VH
8
2.40
0.20
20
2
3.45
0.03
10
2
3.81
0.03
14
3
3.46
0.25
4
U(IV) H
6
2.26
0.17
13
2
3.32
0.00
5
2
3.43
0.00
2
4
3.48
0.12
5
U(V) U(V)
U R
7 7
2.20 2.20
0.13 0.13
10 10
2 2
3.38 3.38
0.00 0.02
7 7
2 2
3.64 3.70
0.00 0.06
9 10
4 4
3.43 3.41
0.19 0.20
3 2
U(V)
VH
8
2.28
0.22
14
2
3.42
0.07
9
2
3.80
0.03
14
3
3.46
0.26
4
U(V)
H
6
2.11
0.06
6
2
3.22
0.00
2
2
3.39
0.00
1
4
3.51
0.01
6
U(VI) U
8
2.18
0.12
9
2
3.41
0.00
9
2
3.71
0.00
11
4
3.45
0.20
4
U(VI) R
8
2.18
0.12
9
2
3.44
0.01
9
2
3.74
0.00
12
4
3.42
0.21
3
U(VI) V
8
2.20
0.13
10
1
3.64
0.00
16
2
3.56
0.19
6
4
3.58
0.18
7
U(VI) H
6
2.07
0.01
4
2
3.29
0.00
4
2
3.30
0.00
1
4
3.58
0.03
7
exp.
-
R (Å) b
iron 2nd shell (eso#1)
a
eso and cso stand for edge-sharing octahedra and corner-sharing octahedra, respectively. b The low experimental coordination number obtained by Nico et al.6 reflects the fact that U was partitioned between adsorbed and incorporated and some uranyl(VI) remained adsorbed despite bicarbonate extraction.
Table 2. Uranium Incorporation into Goethite: Unoccupied Octahedral Sitea oxygen 1st shell ox. exp.6
a
N
CCS -
R (Å)
3.9(4) 2.12(2)
iron 2nd shell (fso)
σ (Å) Δ (%) -
1
N
R (Å)
1.0
3.08(2)
σ (Å) -
iron 2nd shell (eso#1) Δ (%)
N
R (Å)
33
1.0
3.2(1)
σ (Å) -
iron 2nd shell (eso#2) Δ (%)
N
R (Å)
18
2.0
3.58(4)
σ (Å) -
Δ (%) 1
U(IV) U
7
2.28
0.09
7
2
3.01
0.05
33
2
3.27
0.00
24
2
3.35
0.00
5
U(IV) R
7
2.29
0.09
7
2
3.00
0.05
33
2
3.30
0.00
26
2
3.31
0.00
6
U(IV) VH
7
2.31
0.13
8
1
3.01
0.00
34
2
3.29
0.00
25
2
3.20
0.00
9
U(IV) H
7
2.28
0.16
7
2
2.98
0.04
32
2
3.26
0.00
24
2
3.37
0.00
4
U(V)
U
7
2.17
0.06
2
2
3.07
0.06
36
2
3.32
0.00
26
2
3.30
0.00
6
U(V)
R
7
2.18
0.06
2
2
3.09
0.08
37
2
3.28
0.00
25
2
3.30
0.00
7
U(V)
VH
7
2.20
0.12
3
1
3.11
0.00
38
2
3.27
0.00
25
2
3.39
0.00
4
U(V)
H
7
2.17
0.06
2
2
3.03
0.03
34
2
3.29
0.00
25
2
3.36
0.01
5
U(VI) U
7
2.14
0.06
0
2
3.49
0.39
55
2
3.32
0.00
26
2
3.34
0.00
5
U(VI) R
7
2.13
0.04
0
2
3.19
0.07
42
2
3.31
0.01
26
2
3.35
0.00
5
U(VI) V U(VI) H
7 7
2.15 2.13
0.08 0.06
1 0
1 2
3.18 3.43
0.00 0.36
41 52
2 2
3.31 3.41
0.00 0.00
26 30
2 2
3.42 3.31
0.00 0.06
3 6
fso and eso stand for face-sharing octahedra and edge-sharing octahedra, respectively.
strictly anhydrous. Local charge balance around U was maintained by reduction or creation of vacancies at second nearest-neighbor Fe(III) sites. Pairs of second nearest-neighbor octahedral Feþ2.5/ Feþ2.5 ions were changed to explicit representation (i.e., Feþ2.0/ Feþ3.0) to allow for vacancy creation and/or iron reduction. The first shell coordination number of U substituted for Fe(III) is six (Figure S2), regardless of oxidation state and CCS, with the exception of U(VI) in the presence of an Fe(III) vacancy (Table 3). The average uraniumoxygen bond distances are 19% longer than the average simulated FeO distance. The higher repulsive
electrostatic interaction between U and Fe relative to between Fe and Fe, pushes the second shell irons out, although UFe bond distances are always within 10% of those for FeFe. In general, the values of Δ and σ are low, especially for CCS-U and CCS-R. Collectively, the calculations indicate that magnetite can incorporate uranium with minimal distortion. For U incorporated in the tetrahedral site, U(IV) retains 4-fold coordination for all CCS, U(V) distorts to 6-fold coordination only for CCS-VH, whereas U(VI) consistently bonds to two additional oxygens regardless of the CCS (Table 4 and Figure 2773
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Table 3. Uranium Incorporation into Magnetite: Octahedral Site oxygen 1st shell ox.
N
CCS
Table 4. Uranium Incorporation into Magnetite: Tetrahedral Sitea
iron 2nd shell (eso)
R (Å) σ (Å) Δ (%)
N
oxygen 1st shell
R (Å) σ (Å) Δ (%)
exp.6 -
3.9(4) 2.12(2) -
3
0.9(5) 2.89(3)
-
U(IV) U U(IV) R
6 6
2.17 2.17
0.00 0.02
6 6
6 6
3.12 3.12
0.00 0.02
U(IV) VO 6
2.23
0.11
9
5
3.19
U(V) U
6
2.08
0.00
2
6
U(V) R
6
2.08
0.01
2
6
U(V) VO 6
2.10
0.05
3
U(VI) U U(VI) R
6 6
2.06 2.06
0.00 0.00
U(VI) V
7
2.13
0.05
ox.
3
N
CCS
iron 2nd shell (cso)
R (Å) σ (Å) Δ (%)
N
R (Å) σ (Å) Δ (%) 17
exp.6 -
3.9(4) 2.12(2) -
12
5 5
U(IV) U U(IV) R
4 4
2.12 2.11
0.00 0.00
12 11
12 12
3.56 3.56
0.00 0.01
2 2
0.14
8
U(IV) VO 4
2.11
0.02
11
11
3.56
0.04
3
3.16
0.00
7
U(V) U
4
2.02
0.00
6
12
3.57
0.00
3
3.15
0.01
6
U(V) R
4
2.02
0.00
6
12
3.57
0.01
3
5
3.18
0.08
7
U(V) VO 6
2.14
0.08
13
11
3.61
0.22
4
1 1
6 6
3.21 3.22
0.00 0.00
8 9
U(VI) U U(VI) R
6 6
2.10 2.09
0.09 0.07
11 10
12 12
3.54 3.57
0.17 0.21
2 3
4
5
3.25
0.08
10
U(VI) V
6
2.08
0.04
10
11
3.65
0.22
5
a
S2). In order to obtain 6-fold coordination, U moves in the [100] direction and attracts two oxygen ions (Figures 1b and S2d), which significantly stretches those FeO bonds. Interestingly, U(V) appear to be able to enter the tetrahedral site with minimal distortion for CCS-R (low σ and Δ for first and second coordination shells, Table 4). Uranium Incorporation into Hematite. Hematite has a hexagonal unit cell with a = 5.038 Å and c = 13.772 Å.27 The structure of hematite consists of arrays of hexagonal close-packed oxygen ions stacked in the [001] direction. Two thirds of the octahedral sites are occupied by Fe(III). Therefore, uranium was either incorporated into an unoccupied octahedral site or substituted for Fe(III) (Figure 1c). Each occupied octahedron is linked to one occupied nearest-neighbor octahedron in a facesharing configuration and to three occupied nearest-neighbor octahedra through shared edges. Each vacant octahedron shares faces with two occupied nearest-neighbor octahedra and edges with six occupied nearest-neighbor octahedra. Nearest-neighbor FeO6 octahedra were used as sites for Fe(III) reduction or vacancy formation. When U substituted for Fe(III) given CCS-V, the oxygens around the vacancy were protonated as necessary to obtain a net charge of zero (CCSVH). Conversely, second nearest-neighbor Fe(III) were reduced to ensure a net charge of zero when U was placed at the unoccupied site in CCS-VR. When uranium substitutes for Fe(III), the simulated average UO distances are longer than the simulated average FeO distance by 210% (Table 5). Uranium maintains 6-fold coordination except in two cases that involve U(VI), where N increases to 9 (CCS-U) and 7 (CCS-V), causing appreciable distortion to the local hematite structure (Figure S3). U obtains 9- and 6-fold coordination when placed in a vacant octahedral site for CCS-U and CCS-R, respectively (Table 6 and Figure S3). For CCS-U, there are three short and six long bonds (Table SXII), which cause the extension of neighboring ironoxygen bonds. Uranium essentially moves out of the unoccupied octahedral site by pushing the Fe of a face-sharing octahedron along the [001] direction and then relaxes to a position close to the original Fe position. The displaced Fe, in turn, takes the position of a second Fe by pushing it along [001] direction into an unoccupied
0.9(5) 2.89(3)
-
cso stands for corner-sharing octahedra.
octahedral site (Figure S3c). In CCS-R, the two face-sharing FeO6 octahedra are always occupied by Fe(II). Again, the main disruption to the lattice is in the [001] direction where Fe atoms in the face-sharing octahedra are pushed out of their original sites. The resulting close approach of Fe(II) to neighboring Fe(III) in the [001] direction is possible because of the lower electrostatic interaction compared to two Fe(III). As a result, U remains in the center of the unoccupied octahedral site and is therefore 6-fold coordinated (Figure S3d). For all CCS-V, U moves to the vacancy in all cases, which results in configurations equivalent to those for U substitution for Fe(III) (Table 5). Therefore, CCS-V is not reported in Table 6. As for goethite, U incorporation at a vacant octahedral site yields more distortion than U substitution for Fe(III). Comparison with Experimental Data. In this section, we compare the simulations to available EXAFS of U that is thought to be incorporated into the structures of goethite, magnetite, and hematite. Nico et al. and Stewart et al.6,7 provide evidence for incorporation of U into goethite (and possibly magnetite) during Fe(II)-induced transformation of ferrihydrite into the more stable iron (hydr)oxides in the presence of U(VI). Duff et al.5 coprecipitated U(VI) and hematite from aqueous solution under oxidizing conditions. With respect to U incorporated into a vacant octahedral site of goethite, all cases yield larger Δ values than the EXAFS data of Nico et al.6 for the first pair of edge-sharing octahedra and significantly shorter ones for the second pair (Table 2). Substitution of U(V) and U(VI) for Fe(III) in goethite, particularly in combination with CCS-H, shows good agreement with the EXAFS data of Nico et al.6 In particular, the UO and UFe(eso#1) distances for CCS-H compare very well with experiment (UO: ΔU(V) = 6%, ΔU(VI) = 4%, ΔEXAFS = 5%; UFe(eso#1): ΔU(V) = 2%, ΔU(VI) = 4%, ΔEXAFS = 2%). Interestingly, for both UO and UFe distances, the simulations give similar results for U(V) and U(VI). In contrast, the calculated U(IV)O distances (Δ = 1220%) and U(IV)Fe(eso#1) distances (Δ = 510%) do not match the EXAFS determined distances. This is consistent with XANES measurements which indicate that U(IV), if present, was only a minor 2774
dx.doi.org/10.1021/es1037639 |Environ. Sci. Technol. 2011, 45, 2770–2776
Environmental Science & Technology
ARTICLE
Table 5. Uranium Incorporation into Hematite: Occupied Octahedral Site oxygen 1st shell ox. 5
CCS
N
R (Å)
iron 2nd shell (fso) σ (Å)
Δ (%)
N
R (Å)
iron 2nd shell (eso)
σ (Å)
Δ (%)
N
R (Å)
σ (Å)
Δ (%)
10
1.1(3)
3.19(2)
-
7 6
exp.
-
3.5(5)
2.30(1)
-
13
1.1(3)
3.19(2)
-
U(IV)
U
6
2.22
0.08
10
1
3.20
0.00
9
3
3.19
0.00
U(IV)
R
6
2.19
0.04
8
1
3.24
0.00
11
3
3.17
0.00
5
U(IV)
VH
6
2.21
0.09
10
1
3.13
0.00
7
2
3.23
0.00
8
U(V)
U
6
2.11
0.06
5
1
3.23
0.00
10
3
3.21
0.00
7
U(V) U(V)
R VH
6 6
2.09 2.09
0.03 0.01
3 3
1 0
3.28 -
0.00 -
12 -
3 3
3.19 3.19
0.01 0.00
6 6
U(VI)
U
9
2.17
0.06
7
1
3.35
0.00
15
3
3.29
0.01
10
U(VI)
R
6
2.06
0.02
2
1
3.37
0.00
15
3
3.25
0.01
8
U(VI)
V
7
2.14
0.07
6
1
3.28
0.00
12
2
3.33
0.04
11
Table 6. Uranium Incorporation into Hematite: Unoccupied Octahedral Site oxygen 1st shell ox.
CCS
N
R (Å)
iron 2nd shell (fso) σ (Å)
Δ (%)
N
R (Å)
iron 2nd shell (eso)
σ (Å)
Δ (%)
N
R (Å)
σ (Å)
Δ (%)
exp.5
-
3.5(5)
2.30(1)
-
10
1.1(3)
3.19(2)
-
60
1.1(3)
3.19(2)
-
U(IV)
U
9
2.41
0.18
14
2
3.09
0.08
65
9
3.31
0.08
12
U(IV)
R
6
2.15
0.00
2
2
2.64
0.00
40
6
3.15
0.01
7
U(V)
U
9
2.29
0.14
9
2
3.08
0.07
64
9
3.32
0.08
13
U(V)
R
6
2.07
0.00
2
2
2.70
0.00
44
6
3.18
0.01
8
U(VI)
U
9
2.23
0.10
6
2
3.10
0.08
65
9
3.35
0.07
14
U(VI)
R
6
2.05
0.13
3
2
2.80
0.00
49
6
3.23
0.01
10
component under the relevant experimental conditions. Further, the simulations showed that UFe distances on the order of ∼3.6 Å are possible for structurally incorporated U. This is important because such distances are commonly reported for U(VI) adsorbed to the surface of Fe(III)-(hydr)oxides.8,9 Consequently, it would be a mistake to assume that UFe on the order of 3.6 Å necessarily implies adsorbed U(VI). In sum, interpretation of the available EXAFS data6 in light of the calculations, suggests that U primarily substituted for Fe(III) in the structure of goethite as U(V) and/or U(VI) with deprotonation acting as the primary charge compensating mechanism. For U substitution in the octahedral site of magnetite, simulated U(V)O and U(VI)O bond distances (Δ = 23% and 14%, respectively) are in good agreement with the experimental data of Nico et al.6 (Δ = 3%), whereas U(IV)O distances are not (Δ = 69%). Note that R = 2.12 ( 0.02 Å obtained by Nico et al. was for both the goethite and magnetite first shell. In contrast, the UFe distance of 2.89 ( 0.03 Å reported in Nico et al., which is nearly equal to FeFe distances in pure magnetite, is significantly shorter than the simulated UFe distances (Table 3). Given arguments based on electrostatic repulsion (see Uranium Incorporation into Magnetite), we think it unlikely that 2.89 Å records UFe distances, and suggest that it might represent UC interactions.8 For U incorporation at the tetrahedral site, only U(IV) and U(V) can maintain 4-fold coordination and only U(IV)O
9
distances (Δ = 1112%) are a good match with the EXAFS (Δ = 12%). Although the presence of U(IV) cannot be unequivocally ruled out, the XANES data of Nico et al 6 suggest that, if present at the conditions of interest, it is only a minor component. Further, none of the uranium oxide phases used in the model validation set contained U in 4-fold coordination. Therefore, one should exert caution when matching experimental and calculated distances for the tetrahedral site. We also note that, in this particular case, uncertainties concerning the magnetite EXAFS data, as discussed previously, limit our ability to compare the simulations to experiment. The simulations of U in hematite are not in close agreement with the experiments of Duff et al.,5 where oxidizing conditions focus the discussion on U(VI). In particular, the average experimental UO distance (2.30 Å), determined by EXAFS, is much longer than the calculated average U(VI)O distances, regardless of the CCS. Long uraniumoxygen bond distances may stem from the presence of hydroxyl ions. Because U(VI) and hematite were coprecipitated from aqueous solutions at relatively low T (
