Sorption of Th(IV) onto Iron Corrosion Products: EXAFS Study

Mar 17, 2009 - Corresponding author phone: +0034 93 4017388; fax: +0034 93 ... of Th(IV) onto 2-line-ferrihydrite (FeOOH·H2O) and magnetite (Fe3O4), ...
0 downloads 0 Views 348KB Size
Download PDF
Environ. Sci. Technol. 2009, 43, 2825–2830

Sorption of Th(IV) onto Iron Corrosion Products: EXAFS Study FERRAN SECO,† CHRISTOPH HENNIG,‡ J O A N D E P A B L O , †,§ M I Q U E L R O V I R A , † I S A B E L R O J O , † V I C E N S M A R T ´I , † , § ´ N E Z , * ,§ L A R A D U R O , | JAVIER GIME ´ ,| AND JORDI BRUNO| MIREIA GRIVE Environmental Technology Area, CTM-Centre Tecnolo`gic, Av. Bases de Manresa 1, 08240 Manresa, Spain, Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry, P.O. Box 510119, 01314 Dresden, Germany, Chemical Engineering Department, Universitat Polite`cnica de Catalunya, 08028 barcelona, Spain, and AMPHOS 21 Consulting S.L. Passeig de Rubı´, 29-31, 08197 Valldoreix, Spain

Received December 19, 2008. Revised manuscript received February 23, 2009. Accepted February 27, 2009.

Long-termperformanceassessmentofnuclearwasterepositories is affected by the ability of the outer barrier systems to retain radionuclides after possible corrosive leakage of waste containers. The mobility of the radionuclides released from the spent fuel depends strongly on the processes that take place in the backfill material. The interaction of steel corrosion products and radionuclides is part of such a scenario. In this work, the sorption of Th(IV) onto 2-line-ferrihydrite (FeOOH · H2O) and magnetite (Fe3O4), used as models for steel corrosion products, has been studiedusingEXAFSspectroscopy.Sorptionsampleswereprepared in 0.1 M NaClO4 solutions at acidic pH (initial pH values in the range 3.0-4.2) either from undersaturation and supersaturation conditions with respect to amorphous ThO2. Two oxygen subshells, one at 2.37 Å and another at 2.54 Å, were observed in the first hydration sphere of Th in the case of the ferrihydrite samples. Th-Fe distances for the different ferrihydrite samples are ∼3.60Å. These results indicate a corner sharing surface complex of Th(IV) ion onto the ferrihydrite surface where the Th atom shares one O atom with each of two coordinated octahedra. The longer Th-O distance accounts for coordinated water molecules. No significant changes in the structural environment of Th in terms of coordination numbers and distances were detected as a function of Th(IV) concentration. Magnetite samples sorbing Th(IV) also showed also a strong distortion of the O shell, but in contrast to ferrihydrite, two types of nearest Fe atoms were detected at 3.50 Å and 3.70 Å. These results indicatethatTh(IV)ionsorbsontothemagnetitesurfaceasbidentatecorner sharing arrangements to [FeO6] octahedra and [FeO4] tetrahedra.

Introduction Iron oxides and oxyhydroxides are of particular importance due to their ubiquitous presence in nature and also as corrosion products of the steel canister in a nuclear waste * Corresponding author phone: +0034 93 4017388; fax: +0034 93 4015814; e-mail: [email protected]. † CTM-Centre Tecnolo`gic. ‡ Institute of Radiochemistry. § Universitat Polite`cnica de Catalunya. | AMPHOS 21 Consulting. 10.1021/es803608a CCC: $40.75

Published on Web 03/17/2009

 2009 American Chemical Society

repository. They have a high capacity to sorb radionuclides resulting in an important influence on their mobility in the environment. In a high-level nuclear waste (HLNW) repository, the first physical barrier to the migration of radionuclides to the geosphere is the container. If the container is made of steel (the material chosen by different agencies) it might be corroded by the groundwater under anoxic conditions (1, 2). Under these conditions, Fe(II) is formed in a first step by the reaction of Fe(s) with the hydrogen ion: Fe(S) + 2 H+ ) Fe2+ + H2(g)

(1)

or with water: Fe(s) + 2 H2O ) Fe(OH)2(s) + H2(g)

(2)

The transformation of Fe(II) hydroxide to magnetite occurs via the Schikorr reaction. The oxidation of Fe(II) hydroxide produces a layer that consists of an oxide Fe3-xO4 with a spinel structure varying in composition from stoichiometric magnetite (Fe3O4) in oxygen-free solutions, to Fe2.67O4 in the presence of oxygen (3). In the presence of relatively high (>10-4 mol · dm-3) concentrations of sulfate, carbonate or chloride in the contacting solution, other products such as green-rusts, which are mixed Fe(II)/Fe(III) hydroxides, can be formed. Nevertheless, magnetite is the dominant longterm corrosion product of steel (4)>. Further intrusion of oxidants into the system may lead to the formation of Fe(III) oxides and hydroxides such as ferrihydrite. Amorphous ferrihydrite is of particular interest because it typically occurs initially in natural aqueous environments prior to more crystalline material (5). Ferrihydrite has been proven to be important in sorption processes of heavy metals and radionuclides, affecting their mobility in a subsurface environment (5-7). The elucidation of the structure of the sorption species will support the understanding of their sorption mechanisms, providing a better assessment of radionuclide immobilization. Thorium has been used as an analogue for the other tetravalent actinides which are difficult to keep in a pure tetravalent oxidation state (8). Besides, thorium disposal could increase due to the use of different Th-Pu MOX fuels (9). It is also useful as a tracer when studying important environmental processes; hence several processes affecting the mobility and the solubility of thorium in natural waters have been investigated (10-15). The sorption of Th(IV) onto mineral surfaces has been sparingly studied, with silica (13, 16), clay minerals (17), hematite and goethite (18-21), alumina (22, 23), or titania (24). Because sorption data on magnetite and ferrihydrite are missing in the literature, we studied in this work the sorption of Th(IV) onto these solids as a function of Th(IV) concentration.

Materials and Methods Iron Oxides. The magnetite used in this work was supplied by Aldrich, with a purity of 98% and particle size less than 5 µm. Surface area was determined by the BET methodology and the value obtained was 1.58 ( 0.02 m2.g-1. Ferrihydrite used in the experiments was synthesized at the laboratory by precipitation with alkali according to the method described in ref 25. Ferrihydrite was characterized by powder X-ray diffraction (XRD). In the pattern of 2-line ferrihydrite (not shown) appear the two characteristic broad peaks at ∼34 and 62° 2θ as described in the literature (25). A surface area of 202 ( 3 m2.g-1 was determined by the BET method (N2). VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2825

TABLE 1. Experimental Conditions for the Preparation of the Sorbed Samples: Th Sorbed onto Magnetite (THM1, THM2) and Ferrihydrite (THF1, THF2, THF3) sample

[Th(IV)]i (mol · dm-3)

THF1 THF2 THF3 THM1 THM2

[Th(IV)]aq (mol · dm-3)

-4

[Th(IV)]sorbed (mol · g-1)

-4

8.2 × 10 3.8 × 10-4 1.7 × 10-4 3.3 × 10-4 3.8 × 10-5

6.6 × 10 3.1 × 10-4 5.1 × 10-6 2.3 × 10-4 4.3 × 10-9

-5

3.3 × 10 1.4 × 10-5 3.2 × 10-5 2.0 × 10-5 7.6 × 10-6

pHi

pHf

mass (g)

volume (ml)

time (h)

3.4 3.0 3.8 4.2 4.2

3.5 3.5 6.0 4.0 9.3

0.3 0.3 0.2 0.3 0.2

60 60 40 60 40

4 4 40 4 40

TABLE 2. EXAFS Structural Parameters for the Th(IV) Hydrated Ion (THAQ) and Th Sorbed onto Magnetite (THM1, THM2) and Ferrihydrite (THF1, THF2, THF3) sample

shell

R (Å)a

Nb

σ2 (Å2)c

F

∆E (eV)

Th(IV)(aq) (THAQ) THM1

Th-O1 Th-O2 O Fe1 Fe2 O Fe1 Fe2 O1 O2 Fe O1 O2 Fe O1 O2 Fe

2.45 3.63 2.40 3.50 3.70 2.40 3.51 3.69 2.37 2.54 3.58 2.37 2.54 3.56 2.37 2.53 3.58/9

8.9 3.5 7.8 1.1 1.1 7.8 1.4 1.5 3.6 3.1 1.2 3.6 3.0 1.2 3.6 2.8 0.7

0.007 0.008 0.015 0.003 0.003 0.015 0.003 0.003 0.003 0.003 0.009 0.003 0.003 0.009 0.003 0.003 0.009

0.24

7.6

0.22

4.3

0.20

4.9

0.21

5.9

0.25

6.6

0.27

6.5

THM2 THF1 THF2 THF3

a

Error in distance is ( 0.02 Å.

b

Errors in coordination numbers ( 20%. c Debye-Waller factor.

Preparation of Sorbed Samples. Samples of Th sorbed onto ferrihydrite and magnetite were prepared with mechanical stirring in a glovebox under N2(g) atmosphere (in order to avoid any influence of atmospheric oxygen and CO2) at room temperature and at a mass/volume ratio of 5 g · L-1. Total Th concentrations in the range 4.0 × 10-5 to 8.0 × 10-4 mol · dm-3 were used. The pH of the initial contacting solution was adjusted to the desired values with either diluted nitric acid or NaOH free from carbonate. The pH was measured in the supernatant without agitation using a pH electrode (Crison 52-21, 3 M KCl background electrolyte) calibrated with commercial buffer solutions at pH 7.00 and 9.21. Additional details on the chemical conditions for the preparation of the sorbed samples as well as the final pH values and measured aqueous thorium concentrations are shown in Table 1. Preliminary results (data not shown) indicated that equilibrium was reached in less than four hours. For the Th(IV) aquo species reference, a 4.3 × 10-3 mol · dm-3 standard thorium nitrate solution in 1% nitric acid was used. Methodology. Extended X-ray absorption fine structure (EXAFS) measurements were carried out at the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF) (26). The monochromator, equipped with a water cooled Si(111) double-crystal system, was used in channel-cut measuring mode. Higher harmonics were rejected by two Pt-coated mirrors. All measurements were performed at room temperature. The samples were measured in fluorescence mode using a four-pixel Ge detector with a sample orientation of 45° and a detector orientation of 90° to the incident beam. The energy was calibrated by assigning the first inflection point in the first derivative of the L1 edge absorption spectrum of a Pb metal foil to 15858 eV. The sorbed samples were measured as dry pastes in polyethylene holders. All holders were sealed with Kapton film. 2826

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 8, 2009

The ab initio curved-wave multiple-scattering program FEFF8.2 (27) was used to calculate phase and amplitude functions of the neighboring atoms in order to gain structural information from the raw data. Thorianite (ThO2) was used as structure model (28). The EXAFS spectra were analyzed according to standard procedures using the computer program EXAFSPAK (29). Due to the increasing noise level at higher k-range, data analysis was restricted to the k-range 2.9-9.6 Å-1. The theoretical distance resolution, ∆Rres ) π/2∆k, was 0.23 Å. The amplitude reduction factor, S02, was defined as 1 in the FEFF calculations and fixed to that value in the data fits. The fitted energy shift parameter, ∆E0, was linked for all shells in each sample. The scattering behavior of the electron wave in the atomic potentials creates a phase shift Φij. Then, the peaks in Fourier transform (FT) appear at smaller values R + ∆ relative to the true near-neighbor distances, R. This shift of around ∆ ) -0.2 to -0.5 Å was considered in the shell fitting procedure by introducing appropriate Φij functions from FEFF calculations. The high correlation between the coordination number, N, and the Debye-Waller factor, σ2, makes the comparison of the EXAFS measurements for the ferrihydrite and the magnetite samples difficult. For this reason, the strategy was to fit each sample separately and to choose a Debye-Waller factor value for each shell as a result of this previous fitting. Subsequently, the Debye-Waller factors were fixed to the same value for both ferrihydrite and magnetite samples in order to compare them.

Results and Discussion Aqueous Sample Th Hydrate Complex. Table 2 shows the structural parameters obtained in the fit, the spectrum obtained is very similar to the one obtained by Hennig et al. (30) for the same aqueous complex. The first peak in the FT corresponds to a Th-O interaction at a distance of 2.45 Å

and agrees well with the values given in the literature for the Th(IV) hydrate. A freely fitted coordination number indicated ∼9 oxygen atoms. This coordination number is slightly larger than that of eight previously determined by large angle X-ray scattering (LAXS) in (31) and lower than in previous EXAFS studies on the structural environment of Th in acidic solutions where coordination numbers ranged between 10 and 12 oxygen atoms (32-34). For U(IV) hydrate, a coordination number of 8.7 was obtained in (35). This value is lower than that of 10 previously determined by EXAFS (34) but slightly larger than the coordination number of 8 determined by LAXS (36, 37). Finally, a coordination number of 9 for U(IV) is concluded in ref 38 by means of the density functional theory. Taking into account that Th(IV) is isostructural with U(IV) (with respect to ionic radius and ionic charge), a coordination number of 9 for the Th(IV) aqueous ion might be expected. In fact, the bond length difference between U-O (R ) 2.42 Å, reference (35)) and Th-O (R ) 2.45 Å, this work) is 0.03 Å which is close to the difference of the effective ionic radii of U4+ and Th4+, 0.04 Å (39). This suggests that the coordination number for both aqua ions should be the same. If the hydration numbers would be different, a deviation in the differences of the corresponding M(IV)-O bond distances and the ionic radii would be expected. This is in good agreement with the ab initio calculation that indicates for both Th(IV) and U(IV) a hydration number of 9 (38, 40). An additional peak at longer distance was present. This peak has been also observed in previous studies of aqueous Th(IV) (32, 34). The best fitting result has been obtained using a single shell of about 3-4 oxygen atoms at 3.63Å with a Debye-Waller factor of σ2 ) 0.008 Å2. This backscattering signal has been also observed in colloidal ThO2 · xH2O(s) particles (32). It suggests the presence of a small amount of colloids in the aqueous sample. Solubility studies of thorium oxide indicate the formation of colloidal thorium particles at pH lower than 4 (41, 42). For example, in ref 42, coulometric pH titrations combined with laser-induced breakdown detection (LIBD) were performed to detect initial colloid formation as a function of H+ and Th(IV) concentration. The titrations were performed in the pH range 1.5-2.5 and led to the formation of ThO2(s) colloids which subsequently agglomerate to a microcrystalline precipitate. The solubility product for the freshly formed ThO2(s) colloids was determined to be log Ks0 ) -52.8 ( 0.3. At these low pH values, the solid particles are formed in solutions where Th4+ is the predominant aqueous species and the structure of these freshly formed colloidal thorium particles is believed to have a crystalline structure. The same titration-LIBD method was applied in ref 41 in the pH range of 3-5. They observed that hydrolysis and polynucleation led to the formation of amorphous thorium hydroxide colloids and they determined a log Ks0 ) -47.8 ( 0.3 for the Th(OH)4(am). The theoretical solubility curves of amorphous Th(IV) hydroxide and crystalline ThO2(s) at ionic strength I ) 0.1 mol · dm-3 and T ) 25 °C have been calculated using the hydrolysis constants and solubility products taken from refs 12, 41, and 43 (data not shown). The calculations indicate that the aqueous reference sample, THAQ, is undersaturated with respect to amorphous Th(OH)4(am) but supersaturated with respect to the crystalline form and therefore, the presence of microcrystalline ThO2 · xH2O(s) colloids can not be ruled out. Magnetite Sorbed Samples. From the EXAFS data obtained in this work, the most probable geometries of the thorium sorption onto magnetite will be discussed. Figure 1 shows the EXAFS and Fourier transformed (FT) spectra of Th(IV) sorbed onto magnetite. Table 2 shows the EXAFS fit parameters obtained from the EXAFS data analysis of Th(IV) sorbed onto magnetite. The fit to the EXAFS data

FIGURE 1. Left: Th LIII-edge k3-weighted EXAFS spectra of the sorbed samples: Th sorbed onto magnetite (THM1, THM2) and ferrihydrite (THF1, THF2, THF3). Right: the corresponding Fourier transforms. Theoretical curves are depicted as dots and experimental ones as continuous lines. Fe1 and Fe2 are the two iron subshells used in the fitting. yielded a Th-O distance of 2.40Å for both magnetite samples with a large Debye-Waller factor of 0.015Å2, that indicates a superposition of several scattering contributions. Due to the high Debye-Waller factor values obtained for the oxygen shell, different set of the parameters to fit the EXAFS data considering a splitting into two subshells were tested. This strategy did not improve the fits in all cases, and eventually, a single oxygen shell in the first coordination environment of thorium was considered in the final fit procedure. The shorter Th-O distance with respect to the Th(IV) ion suggests the formation of an inner-sphere Th(IV) complex and corresponds to an average distance composed of the surface interaction Th-O and the contribution of the hydration sphere Th-OH2. The Th-Fe shell shows two backscattering contributions at 3.51 and 3.69 ( 0.02Å. Although this shell is split, the distance difference of 0.20Å between Th-O bond lengths is close to the theoretical distance resolution 0.23 Å. Different attempts to fit the EXAFS data with a single Fe shell were tested but the best fits were obtained considering two Fe shells: a Th-sFe1 at ∼3.50Å and a Th-Fe2 at ∼3.70 Å for both magnetite sorbed samples. The Debye-Waller factor values were linked in the fit procedure for the two Fe shells and we obtained σ2 ) 0.003 Å2 for both magnetite samples and show coordination numbers of N ) 1.1 for THM1 and around N ) 1.5 for THM2. A Th-Th backscattering signal was not observed in the FT’s above the experimental noise level. Magnetite is an iron oxide with inverse spinel structure containing both Fe2+ and Fe3+ ions in cavities of cubic closed packed O2- ion lattice. The site distribution of ferrous and ferric ions is [Fe3+]tet[Fe2+Fe3+]octO4, with the Fe2+ ions occupying half the octahedral sites and that the Fe3+ ions distributed over remaining octahedral and tetrahedral sites (44). Figure 2 shows a schematic representation of possible sorption positions of Th(IV) to the [FeO6] octahedron of the magnetite surface. These configurations are monodentate, bidentate edge-sharing and bidentate corner-sharing arrangements, respectively. The expected Th-Fe distances for these different coordinations are also indicated in Figure 2. They are based on crystal structural data from (45) (dFe-O ) 2.14 Å, dFe-Fe ) 2.97 Å and dO-O ) 3.08 Å; O-Fe-O angle ) 92°) and the Th-O bond length from our EXAFS data analysis. The results obtained from the fits suggest that a monodentate VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2827

FIGURE 2. Schematic illustration of the possible coordinations of Th(IV) to the [FeO6] octahedron of the magnetite surface; (a) monodentate corner-sharing, (b) bidentate corner-sharing, (c) bidentate edge-sharing. The expected Th-Fe distances are indicated. Blue: Th; Green: Fe; Red: O.

FIGURE 3. Schematic illustration of (a) edge-sharing coordination of Th(IV) ion to the Fe tetrahedra of magnetite. The calculated distance considering a Fe-Th-O angle between 100 and 110° is also indicated. (b) corner-sharing coordination where Th(IV) ion shares double corners with the Fe tetrahedra of the magnetite surface. The expected Th-Fe distance for a minimum Fe-O-Th angle of β) 114° is also shown. coordination of Th(IV) to the magnetite surface can be ruled out due to the Th-Fe distance that is too long in comparison with the observed distance. Based on the crystal structural data from ref 45 a bidentate edge-sharing coordination is expected to have Th-Fe distances around 3.30-3.40 Å depending on the Fe-O-Th angle (β ) 93-97°). A bidentate corner-sharing arrangement would give Th-Fe distances longer than in the case of an edge-sharing configuration due to the wider Fe-O-Th angle expected. Geometric calculations indicate that for a Fe-O-Th angle between β ) 101-109°, a Th-Fe distance of ∼3.50-3.70 Å would be expected. Figure 3 shows a schematic representation of possible sorption positions of Th(IV) to the [FeO4] tetrahedra. The Fe-O distance in the tetrahedral sites of the magnetite structure is 1.75 Å (45). An edge-sharing coordination to Fe tetrahedra is expected to give a Th-Fe distance shorter than in an edge-sharing arrangement to the octahedra. If we consider a Th-O-Fe angle in the range 100-110°, the expected Th-Fe distance would range between 3.20 and 3.42 Å. This type of coordination has been observed in the sorption of Th onto silica (13) and onto huttonite (ThSiO4) (46), where Th(IV) is partly bidentate coordinated to SiO4 tetrahedra with 2828

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 8, 2009

a Th-Si bond length of 3.20 Å. The difference in ionic radii of Si4+ and Fe3+ is 0.38Å which is larger than the difference in Th-Si and Th-Fe bond lengths (0.30 Å) and therefore suggests a different coordination of Th(IV) onto the magnetite surface. On the other hand, it can be considered that Th(IV) bonds to the magnetite surface by sharing double corners with Fe tetrahedra (see Figure 3). This coordination has been observed in the study on the Th uptake on montmorillonite, (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2 · nH2O (33), where Th(IV) bonds by sharing double corners with SiO4 tetrahedra with a Th-Si distance of ∼3.87 Å. In this case, the difference in Th-Si and Th-Fe bond length is 0.37 Å which is very close to the difference in their ionic radii (0.38 Å) and suggests a similar coordination of Th(IV) ion in magnetite. The expected Th-Fe distance for this coordination considering a minimum Fe-O-Th angle of β ) 114° would be equal or greater than 3.50Å. Taking into account the geometric constraints discussed above and the results obtained from the fits, Th(IV) is present in at least two different sorption sites. The most probable geometries are bidentate-corner arrangements both onto the [FeO4] tetrahedra and onto the [FeO6] octahedra. This interpretation corresponds well with the observed Th-O

bond length in several Th(IV) crystal structures. For example, the monodentate and bidentate coordination mode of the SO42- tetrahedron in K4Th(SO4)4(H2O)2 lead to two different Th-O distances (47). Monodentate coordinated [SO42-] has a short Th-O distance of 2.33Å and bidentate [SO42-] has a longer Th-O distance of 2.53 Å. Its short Th-O distance of 2.40 Å is in agreement with the proposed coordination mode. A Th-Th backscattering signal was not observed in the FT’s above the experimental noise level. However, the lack of a Th-Th interaction does only exclude the presence of a fraction of a surface Th precipitate or sorbed polynuclear species. Ferrihydrite Sorbed Samples. Figure 1 shows the EXAFS and Fourier transformed (FT) spectra of Th(IV) sorbed onto ferrihydrite under the different experimental conditions described above (Table 2, THF1, THF2, and THF3). All three samples exhibit a FT peak at R + ∆ ∼ 2.40 Å which is attributed to an average Th-O distances at 2.54 and 2.37 Å from both water molecules and oxygen atoms binding to the ferrihydrite surface, respectively. A further peak at R + ∆ ∼ 3.20 Å was attributed to a Th-Fe interaction while a Th-Th backscattering contribution could not be clearly observed. An initial fit was obtained with a single oxygen shell at a distance of 2.41 Å with a variation of (0.02 Å between the three samples. Coordination numbers of ∼7.0 and DebyeWaller factors in the range σ2 ) 0.012- 0.015 Å2 are obtained for the three samples. Th-Fe distances of R ) 3.55-3.56 Å were obtained from the fits for the three cases with coordination numbers around 1.0. The Debye-Waller factors were also relatively high (σ2 ) 0.011-0.015 Å2) indicating a broad distribution of both Th-O and Th-Fe distances. As in the case of the magnetite samples, different data analysis strategies were tested due to the high Debye-Waller factor values obtained for the oxygen shell. In this case, the separation into two subshells improved the fit, indicated by a 10-15% lower F value. Therefore, the presence of two oxygen subshells in the first coordination environment of thorium was considered in the final fit procedure although the distance difference of ∼0.18 Å2 is below the distance resolution in the k-range of 2.9-9.6 Å-1. The presence of two oxygen subshells in the first coordination environment of thorium agrees with previous EXAFS studies on the uptake of thorium onto amorphous silica (13) and onto montmorillonite (33). Table 2 shows the EXAFS fit parameters obtained from the EXAFS data analysis of Th(IV) sorbed onto ferrihydrite. For the three sorbed samples, the Debye-Waller factors were fixed to 0.003 and 0.009 Å2 for the oxygen and iron shells, respectively. These σ2 values were chosen taking in account the convergence of most of the previous fits. The fit to the EXAFS data yielded a Th-O distance of 2.37 Å for the first oxygen shell with a coordination number of N ) 3.6 for all three samples. This shift to a shorter Th-O distance together with the decrease of the coordination number with respect to the aqueous Th(IV) ion suggests the formation of an innersphere Th(IV) bidentate complex to the ferrihydrite surface. A Th-O distance of 2.54 ( 0.02 Å for the second oxygen shell was obtained for THF1, THF2, and THF3. The coordination number obtained for this second shell was ∼3 for all three samples. This Th-O distance seems too large to be the result of coordinated O atoms from the ferrihydrite octahedra and therefore, it might be the result of coordinated water molecules. In contrast to the magnetite, no splitting of the Fe shell is observed for the ferrihydrite samples. A single Fe shell with a Th-Fe distance of 3.56-3.59 Å is obtained for the three samples with coordination numbers ∼1.0 but slightly shorter for the THF3 sample (N ) 0.7). The fundamental structure unit within ferrihydrite is the [FeO6] octahedron. In freshly prepared ferrihydrite only short uncoupled chains joined sharing edges are present and

interchain linkage is absent or severely disordered (48). With aging, however, dioctahedral chains become more abundant and these chains link to others by sharing corners to form a cross-linked structure. The possible binding sites of Th(IV) ion onto an [FeO6] octahedra located on the ferrihydrite surface are a monodentate species and two possible bidentate species (bidentate edge-sharing and bidentate cornersharing). Based on atomic coordinates of Fe and O atoms determined from hematite (49), a Th-Fe distance approximately in the range of 3.31-3.48 Å (depending on the Th-O-Fe bond angle) would be expected for a bidentate edge-sharing coordination of Th(IV) ion to the ferrihydrite octahedra, whereas a longer distance would be expected for a bidentate corner-sharing configuration. The observed Th-Fe distance of 3.56-3.59 Å suggests that Th(IV) sorbs by forming a bidentate corner-sharing surface complex. These distances match very well with the expected bond distances of this kind of configuration (3.53-3.60 Å for a Fe-O-Th binding angle in the range β ) 104-107°). The results also indicate that a monodentate coordination of Th(IV) to the ferrihydrite surface can be ruled out due to the longer Th-Fe distance expected for this type of arrangement. While no significant differences in either the structural parameters or in the EXAFS spectra were observed for THF1 and THF2 samples, local surface precipitation or sorption of polynuclear species might take place in THF3 sample as inferred by the small feature that appears at R + ∆ ∼ 4.0 Å in the corresponding FT, which can be attributed to a Th-Th interaction. Although the intensity of this peak is close to the background noise, the introduction of a Th shell in the fit procedure of the THF3 sample was tested, and it was found that the F value was 5% reduced. The coordination number was fixed to 1.0, and a Th-Th distance of 3.99Å was obtained. This distance is relatively close to that for the amorphous thorium oxide reported in ref 26 and indicates the presence of amorphous Th(IV) oxide or sorbed polynuclear thorium species. Actually, both thorium concentration and pH of the THF3 sample were inside the predominance region of polynuclear and colloidal Th(IV) species according to Rothe et al. (32). The mechanism for Th(IV) sorption onto magnetite and ferrihydrite deduced from this work has been recently used to model experiments on thorium incorporation to magnetite and ferrihydrite as a function of pH (50).

Acknowledgments Financial support for this research has been provided by ENRESA (Spanish Radioactive Waste Management Co.) in the framework of the European Comission FUNMIG project (Fundamental processes of radionuclide migration, 516514FI6W-2004) and the Spanish “Ministerio de Educacio´n y Ciencia” by means of the “Ramon y Cajal” and “Torres Quevedo” programs, the AMAME project (Contract number: CTM2005-07037-C03-01/TECNO) and the MOMIES project (Contract number: CTM2008-06662-C02-01).

Literature Cited (1) Smailos, E.; Schwarzkopf, W.; Kienzler, B.; Ko¨ster, R. Corrosion of carbon-steel container for heat-generating nuclear waste in brine environment relevant for a rock-salt repository. Mater. Res. Soc. Symp. Proc. 1992, 257, 399–406. (2) Rovira, M.; de Pablo, J.; El Aamrani, S.; Duro, L.; Grive´, M.; Bruno, J. Study of the Role of Magnetite in the Immobilisation of U(VI) by Reduction to U(IV) under the Presence of H2(g) in Hydrogen Carbonate Medium, Technical Report TR-03-04; SKB: Stockholm, Sweden, 2003. (3) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; Wiley Interscience:New York, 1996. (4) Rovira, M.; El Aamrani, S.; Duro, L.; Gime´nez, J.; de Pablo, J.; Bruno, J. Interaction of uranium with in situ generated magnetite on steel. J. Hazard. Mater. 2007, 147, 726–731. VOL. 43, NO. 8, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

2829

(5) Dodge, C. J.; Francis, A. J.; Gillow, J. B.; Halada, G. P.; Eng, C.; Clayton, C. R. Association of uranium with iron oxides typically formed on corroding steel surfaces. Environ. Sci. Technol. 2002, 36, 3504–3511. (6) Novikov, A. P.; Kalmykov, S. N.; Utsunomiya, S.; Ewing, R. C.; Horreard, F.; Clark, S. B.; Tkachev, V. V.; Myasoedov, B. F. Colloid transport of plutonium in the far-field of the Mayak Production Association, Russia. Science 2006, 314, 638–641. (7) Dardenne, K. Scha¨fer. T.; Denecke, M. A.; Rothe, J.; Kim, J. I. Identification and characterization of sorbed lutetium species on 2-line ferrihydrite by sorption data modeling, TRLFS and EXAFS. Radiochim. Acta 2001, 89, 469–478. (8) Choppin, G. R. Utility of oxidation state analogs in the study of plutonium behavior. Radiochim. Acta 1999, 85, 89–96. (9) David, S.; Huffer, E.; Nifenecker, H. Revisiting the thoriumuranium nuclear fuel cycle. Europhysics News 2007, 38, 24–27. (10) Reiller, P.; Casanova, F.; Moulin, V. Influence of addition order and contact time on Th(IV) retention by hematite in the presence of humic acids. Environ. Sci. Technol. 2005, 39, 1641–1648. (11) Reiller, P.; Moulin, V.; Casanova, F.; Dautel, C. Retention behaviour of humic substances onto mineral surfaces and consequences upon Th(IV) mobility: case of iron oxides. Appl. Geochem. 2002, 17, 1551–1562. (12) Fangha¨nel, Th.; Neck, V. Aquatic chemistry and solubility phenomena of actinide oxides/hydroxides. Pure Appl. Chem. 2002, 74 (10), 1895–1907. ¨ sthols, E.; Manceau, A.; Farges, F.; Charlet, L. Adsorption of (13) O thorium on amorphous silica: An EXAFS study. J. Colloids Interface Sci. 1997, 194, 10–21. (14) Quigley, M. S.; Honeyman, B. D.; Santschi, P. H. Th sorption in the marine environment: Equilibrium partitioning in the hematite/water interface, sorption/desorption kinetics and particle tracing. Aquat. Geochemi. 1996, 1, 277–301. (15) Landa, E. R.; Le, A. H.; Luck, R. L.; Yeich, P. J. Sorption and coprecipitation of trace concentrations of Th with various minerals under conditions simulating an acid uranium mill efluent environment. Inorg. Chim. Acta 1995, 229, 247. ¨ sthols, E. Thorium sorption on amorphous silica. Geochim. (16) O Cosmochim. Acta 1995, 59, 1249. (17) Akc¸ay, H.; Kilinc¸, S. Sorption and desorption of thorium from aqueous solution by montmorillonite. J. Radioanal. Nucl. Chem. 1996, 212, 173–185. (18) Laflamme, B. D.; Murray, J. W. Solid/solution interaction: the effect of carbonate alkalinity on adsorbed thorium. Geochim. Cosmochim. Acta 1987, 51, 243–250. (19) Hunter, K. A.; Hawke, D. J.; Kwee Choo, L. Equilibrium adsorption of thorium by metal oxides in marine electrolytes. Geochim. Cosmochim. Acta 1988, 52, 627–636. (20) Cromie`res, L.; Moulin, V.; Fourest, B.; Guillaumont, R.; Giffaut, E. Sorption of thorium onto hematite colloids. Radiochim. Acta. 1998, 82, 249–256. (21) Murphy, R. J.; Lenhart, J. J.; Honeyman, B. D. The sorption of thorium (IV) and uranium (VI) to hematite in the presence of natural organic matter. J. Colloid Surf., A 1999, 157, 47–62. (22) Righetto, L.; Bidoglio, G.; Marcandalli, B.; Bellobono, I. R. Surface interactions of actinides with alumina colloids. Radiochimica Acta. 1988, 44/45, 73–75. (23) Righetto, L.; Bidoglio, G.; Azimonti, G.; Bellobono, I. R. Competitive actinide interactions in colloidal humic acidmineral oxide systems. Environ. Sci. Technol. 1991, 25, 1913–1919. (24) Jakobsson, A. M. Measurement and Modeling of Th Sorption onto TiO2. J. Colloid Interface Sci. 1999, 220, 367–373. (25) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory: Preparation and Characterization; VCH: Weinheim, 1991. (26) Matz, W.; Schell, N.; Bernhard, G.; Prokert, F.; Reich, T.; Claussner, J.; Oehme, W.; Schlenk, R.; Dienel, S.; Funke, H.; Eichhorn, F.; Betzl, M.; Pro¨hl, D.; Strauch, U.; Hu ¨ ttig, G.; Krug, H.; Neumann, W.; Brendler, V.; Reichel, P.; Denecke, M. A.; Nitsche, H. ROBL - a CRG beamline for radiochemistry and materials research at the ESRF. J. Synchrotron Radiat. 1999, 6, 1076–1085. (27) Ankudinov, A. L.; Rehr, J. Theory of solid-state contributions to the x-ray elastic scattering amplitude. J. Phys. Rev. B. 2000, 62, 2437. (28) Whitfield, H. J.; Roman, D.; Palmer, A. R. X-ray study of the system ThO2 - CeO2 - Ce2O3. J. Inorg. Nucl. Chem. 1966, 28, 2817–2825.

2830

9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 8, 2009

(29) George, G.; Pickering, I. EXAFSPAK: a Suite of Computer Programs for Analysis of X-ray Absorption Spectra; Stanford Synchrotron Radiation Laboratory: Standford, CA, 1995. (30) Hennig, Ch.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A. C. EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution. Inorg. Chem. 2007, 46, 5882–5892. (31) Johansson, G.; Magini, M.; Ohtaki, H. Coordination around thorium(IV) in aqueous perchlorate, chloride and nitrate solutions. J. Solution Chem. 1991, 20 (8), 775–792. (32) Rothe, J.; Denecke, M. A.; Neck, V.; Mu ¨ ller, R.; Kim, J. I. XAFS investigation of the structure of aqueous thorium(IV) species, colloids, and solid thorium(IV) oxide/hydroxide. Inorg. Chem. 2002, 41, 249–258. (33) Da¨hn, R.; Scheidegger, A. M.; Manceau, A.; Curti, E.; Baeyens, B.; Bradbury, M. H.; Chateigner, D. Th uptake on montmorillonite: a powder and polarized EXAFS study. J. Colloid Interface Sci. 2002, 249, 8–21. (34) Moll, H.; Denecke, M. A.; Jalilehvand, F.; Sandstro¨m, M.; Grenthe, I. Structure of the aqua ions and fluoride complexes of uranium(IV) and thorium(IV) in aqueous solution. An EXAFS study. Inorg. Chem. 1999, 38, 1795–1799. (35) Hennig, C.; Tutschku, A.; Rossberg, A.; Bernhard, G.; Scheinost, A. C. Comparative EXAFS investigation of uranium(IV) and -(IV) aquo chloro complexes in solution using a newly developed spectroelectrochemical cell. Inorg. Chem. 2005, 44, 6655–6661. (36) Johansson, G. Structures of Complexes in Solution Derived from X-ray Diffraction Measurements in Advances in Inorganic Chemistry; Academic Press: London, 1992; vol. 39. (37) Poc¸ev, S.; Johansson, G. An X-ray investigation of the coordination and hydrolysis of the uranium(IV) ion in aqueous perchlorate solutions. Acta Chem. Scand. 1973, 27, 2146–2160. (38) Tsushima, S.; Tianxiao, Y. Relativistic density functional theory study on the structure and bonding of U(IV) and Np(IV) hydrates. Chem. Phys. Lett. 2005, 401, 68–71. (39) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Found. Crystallogr. 1976, 32, 751–767. (40) Tsushima, S.; Yang, T. X.; Mochizuki, Y.; Okamoto, Y. Ab initio study on the structures of Th(IV) hydrate and its hydrolysis products in aqueous solution. Chem. Phys. Lett. 2003, 375, 204– 212. (41) Neck, V.; Mu ¨ ller, R.; Bouby, M.; Altmaier, M.; Rothe, J.; Denecke, M. A.; Kim, J. I. Solubility of amorphous Th(IV) hydroxide application of LIBD to determine the solubility product and EXAFS for aqueous speciation. Radiochim. Acta. 2002, 90, 485– 494. (42) Bundschuh, T.; Knopp, R.; Mu ¨ ller, R.; Kim, J. I.; Neck, V. Fangha¨nel, Th. Application of LIBD to the determination of the solubility product of thorium(IV) colloids. Radiochim. Acta. 2002, 88, 625. (43) Altmaier, M.; Neck, V.; Mu ¨ ller, R. Fangha¨nel, Th. Solubility of ThO2.xH2O(am) in carbonate solution and the formation of ternary Th(IV) hydroxide-carbonate complexes. Radiochim. Acta. 2005, 93 (2), 83–92. (44) Fleet, M. E. The structure of magnetite. Acta Chrystallogr. 1981, B37, 917–920. (45) Fleet, M. E. The structure of magnetite: Symmetry of cubic spinels. J. Solid State Chem. 1986, 62, 75–82. (46) Taylor, M.; Ewing, R. C. The crystal structures of the ThSiO4 polymorphs: huttonite and thorite. Acta Crystallogr., B: Struct. Sci. 1978, 34, 1074–1075. (47) Arutyunyan, E. G.; Porai-Koshits, M. A.; Molodkin, A. K. The crystal structure of K4Th(SO4)4(H2O)2. Strukt. Khimii. 1963, 4, 276–277. (48) Manceau, A.; Drits, V. A. Local structure of ferrihydrite and feroxyhite by EXAFS spectroscopy. Clay Miner. 1993, 28, 165– 184. (49) Janney, D. E.; Cowley, J. M.; Buseck, P. R. Structure of synthetic 2-line ferrihydrite by electron nanodiffraction. Am. Mineral. 2000, 85, 1180–1187. (50) Rojo, I.; Seco, F.; Rovira, M.; Gime´nez, J.; Cervantes, G.; Martı´, V.; de Pablo, J. Thorium sorption onto magnetite and ferrihydrite in acidic conditions. J. Nucl. Mater. 2009, DOI: 10.1016/ j.jnucmat.2008.12.014.

ES803608A