EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution

Article abstract of DOI:10.1021/ic0619759

The local structure of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution was investigated by U-L₃ and Th-L₃ EXAFS spectroscopy for total sulfate concentrations 0.05 ≤ [SO₄ ^2-] ≤ 3 M and 1.0 ≤ pH ≤ 2.6. The sulfate coordination was derived from U-S and Th-S distances and coordination numbers. The spectroscopic results were combined with thermodynamic speciation and density functional theory (DFT) calculations. In equimolar [SO₄^2-]/[UO ₂²⁺] solution, a U-S distance of 3.57 ± 0.02 A suggests monodentate coordination, in line with UO₂-SO ₄(aq) as the dominant species. With increasing [SO₄ ^2-]/[UO₂²⁺] ratio, an additional U-S distance of 3.11 ± 0.02 A appears, suggesting bidentate coordination in line with the predominance of the UO₂(SO₄)₂^2- species. The sulfate coordination of Th(IV) and U(IV) was investigated at [SO₄^2-]/[M(IV)] ratios ≥8. The Th(IV) sulfato complex comprises both, monodentate and bidentate coordination, with Th-S distances of 3.81 ± 0.02 and 3.14 ± 0.02 A, respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with monodentate and bidentate U-S distances of 3.67 ± 0.02 and 3.08 ± 0.02 A, respectively. By increasing the pH value to 2, a U(IV) sulfate precipitates. This precipitate shows only a U-S distance of 3.67 ± 0.02 A in line with a monodentate linkage between U(IV) and sulfate. Previous controversially discussed observations of either monodentate or bidentate sulfate coordination in aqueous solutions can now be explained by differences of the [SO₄^2-]/[M] ratio. At low [SO₄ ^2-]/[M] ratios, the monodentate coordination prevails, and bidentate coordination becomes important only at higher ratios.

Full text of DOI:10.1021/ic0619759

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Inorg. Chem. 2007, 46, 5882−5892
EXAFS Investigation of U(VI), U(IV), and Th(IV) Sulfato Complexes in
Aqueous Solution
Christoph Hennig,* Katja Schmeide, Vinzenz Brendler, Henry Moll, Satoru Tsushima, and
Andreas C. Scheinost
Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry, P.O. Box 510119,
01314 Dresden, Germany
Received October 15, 2006
The local structure of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution was investigated by U-L3 and
2-
Th-L3 EXAFS spectroscopy for total sulfate concentrations 0.05
sulfate coordination was derived from U S and Th S distances and coordination numbers. The spectroscopic
results were combined with thermodynamic speciation and density functional theory (DFT) calculations. In equimolar
e [SO4 ] e 3 M and 1.0 e pH e 2.6. The
−
−
2
2
[SO4 -]/[UO2 +] solution, a U
−S distance of 3.57 ± 0.02 Å suggests monodentate coordination, in line with UO2-
2
2
SO4(aq) as the dominant species. With increasing [SO4 -]/[UO2 +] ratio, an additional U
−
S distance of 3.11
±
0.02
2
Å appears, suggesting bidentate coordination in line with the predominance of the UO2(SO4)2 - species. The sulfate
2
coordination of Th(IV) and U(IV) was investigated at [SO4 -]/[M(IV)] ratios
g
8. The Th(IV) sulfato complex
S distances of 3.81 0.02 and 3.14 0.02 Å,
respectively. A similar coordination is obtained for U(IV) sulfato complexes at pH 1 with monodentate and bidentate
S distances of 3.67 0.02 and 3.08 0.02 Å, respectively. By increasing the pH value to 2, a U(IV) sulfate
precipitates. This precipitate shows only a U S distance of 3.67 0.02 Å in line with a monodentate linkage
comprises both, monodentate and bidentate coordination, with Th
−
±
±
U
−
±
±
−
±
between U(IV) and sulfate. Previous controversially discussed observations of either monodentate or bidentate
2
sulfate coordination in aqueous solutions can now be explained by differences of the [SO4 -]/[M] ratio. At low
[SO4 -]/[M] ratios, the monodentate coordination prevails, and bidentate coordination becomes important only at
2
higher ratios.
Introduction
Safonovskoye), Bulgaria (Orlov Dol and Selishte), Czech
Republic (Stra´z pod Ralskem), and other places in the world.
The risk of in situ leaching technology is that the leaching
liquid migrates out of the mining zone and contaminates
drinking water reservoirs. For example, in situ leaching of
uranium from sandstone with sulfuric acid, applied in the
Ko¨nigstein uranium mine (Saxony, Germany) between 1984
and 1990,1 resulted in contamination risk of an aquifer used
for the drinking water supply of the Dresden area. Such
problems are the motivation to gain better insight in the
general complexation behavior of U(VI), U(IV), and Th-
(IV) sulfato complexes in aqueous solution. The knowledge
of interactions between sulfate and these radionuclides is
necessary to predict their migration capability into aquifers.
This includes a description of the chemical behavior of
Sulfate is able to form strong complexes with uranium
and thorium, thereby affecting speciation and migration of
these radionuclides in the subsurface environment. Technical
processing of uranium ores with sulfuric acid drastically
increases the sulfate concentrations in affected groundwaters,
producing high amounts of uranium sulfate near tailings.
Another artificial source of uranium sulfate is in situ leaching
of uranium ore, so-called solution mining, where a leaching
liquid like sulfuric acid or ammonium carbonate is injected
into the ore deposit and the uranium bearing liquid is
pumped back. Such in situ leaching is, or has been, applied
in the U.S. (Wyoming, Nebraska, and Texas), Australia
(Beverley and Honeymoon), China (Tengchong and Yining),
Russia (Dalmatovkoye), Ukraine (Devladove, Bratske, and
(1) Zeissler, K. O.; Nindel, K.; Hertwig, T. In Uranium Mining and
Hydrogeology IV; Merkel, B. J., Hasche-Berger, A., Eds.; Technische
Universita¨t Bergakademie Freiberg: Freiberg, Germany, 2005;
p 713.
* To whom correspondence should be addressed. E-mail: hennig@
esrf.fr.
5882 Inorganic Chemistry, Vol. 46, No. 15, 2007
10.1021/ic0619759 CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/20/2007

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EXAFS InWestigation of Sulfato Complexes
Scheme 1
Å. The angle R is 138-147° for monodentate and 96-105°
for bidentate sulfate arrangement. Typical bond lengths of
U(VI), U(IV), and Th(IV) are given in the Supporting
Information (Tables S3 and S4).
Thermodynamic speciation provides the complex stoichi-
ometry but does not reveal the coordination mode of the
sulfate ligands. The structure of uranium and thorium sulfato
species in aqueous solution is still under debate. Evidence
for bidentate U(VI) sulfate coordination at high sulfate
concentrations was found up to pH 5 by EXAFS.7 A similar
result, indicating a bidentate coordination mode of sulfate,
was obtained with Raman spectroscopy for a high [SO42-]/
[UO22+] ratio at pH 2.8 In this study, the existence of UO2-
uranium sulfate under oxic and anoxic conditions which can
be modeled by variation of the redox potential.
2-
SO4(aq) (860 cm-1), UO2(SO4)2 (852 cm-1), and UO2-
(SO4)3 (843 cm-1) was assumed from the frequency shift.
In contrast, monodentate sulfate coordination to uranyl units
has been suggested by infrared spectroscopy.9 This observa-
tion was confirmed by high-energy X-ray scattering (HEXS)
for equimolar [SO42-]/[UO22+] solutions.10 Recent quantum
chemical calculations on uranyl sulfate assume bidentate
sulfate coordination.11 The comparison of these references
gives a contradicting impression on the sulfate coordination
mode of UO22+. Structural investigations of Th(IV) and
U(IV) aquo sulfato species are missing in the literature up
to now.
The migration of uranium in the environment is influenced
by pH, the redox potential of the solutions, and the
availability of potential complexation agents. In its oxidized
state, U(VI) is highly soluble under acidic and alkaline
conditions. Under anoxic conditions, U(IV) precipitates easily
except for strongly acidic conditions. At neutral pH and under
reduced conditions, uranium is almost immobile. In situ
leaching with sulfuric acid creates a low pH in the concerned
area keeping uranium sulfate soluble even under reduced
conditions.
Sulfate is a moderately strong base and is thus partly
protonated at low pH. The uranyl ion, UO22+, forms binary
sulfato complexes in slightly acidic solution. Thermodynamic
data are available for three sulfato species, UO2SO4(aq),
UO2(SO4)22-, and UO2(SO4)34- 2,3 At pH values >3, hydro-
.
lysis of uranyl causes formation of various oligomeric species
Extended X-ray absorption fine structure (EXAFS) spec-
troscopy is an established technique for the determination
of the coordination in aqueous solution.12 Due to its element
selectivity, the analysis of coordination allows the determi-
nation of solution species in complex matrices. Our study
intends to clarify the discrepancies of reported results on the
structure of the U(VI) sulfato complexes and to elucidate
the structural features for Th(IV) and U(IV) sulfato species.
For the U(VI) sulfato complexes, DFT calculations were
applied to compare the total bonding energy of different
structural isomers. The species with the stable oxidation
states, U(VI) and Th(IV), will be discussed first, followed
by U(IV).
comprising bridging hydroxide/oxide and sulfate ligands.
The major ternary complexes found so far are (UO2)2(OH)2-
2-2n
(SO4)22-, (UO2)3(OH)4(SO4)34-, and (UO2)5(OH)8(SO4)n
,
with n ) 5 or 6.4,5 However, the most recent NEA database
for uranium3 does not consider them to be proven yet.
For U(IV), the actual NEA thermodynamic database
2+
reports only two sulfate species reported for U(IV), USO4
,
and U(SO4)2(aq).3 For Th(IV) there are no commonly agreed
thermodynamic data available for Th(IV) (a NEA review
Experimental Section
volume is in preparation). Important sulfate species in the
2-
acidic pH range include ThSO42+, Th(SO4)2(aq), Th(SO4)3
,
Sample Preparation. A summary of the samples investigated
by EXAFS spectroscopy is given in Table 1. Details of the
preparation are given below.
4- 6
and Th(SO4)4
.
Sulfate is able to coordinate either in monodentate or
bidentate arrangements (Scheme 1). X-ray diffraction studies
of U(VI) sulfates indicate, that monodentate sulfate ligands
typically show U-Smon distances in the range of 3.57-3.68
Å and short U-Oeq distance of 2.30-2.37 Å. Sulfate in
bidentate arrangement shows U-Sbid distances in the range
of 3.09-3.10 Å and longer U-Oeq distances of 2.42-2.48
U(VI) and U(IV) Hydrate. UO3 was obtained by heating UO2-
(NO3)2‚nH2O (Lachema) at 300 °C for 2 days. A U(VI) stock
solution (0.1 M) was prepared by dissolving UO3 in 0.3 M HClO4.
The solution of U(VI) hydrate (sample A) was prepared by adding
(7) Moll, H.; Reich, T.; Hennig, C.; Rossberg, A.; Szabo, Z.; Grenthe, I.
Radiochim. Acta 2000, 88, 559-566.
(8) Nguyen-Trung, C.; Begun, G. M.; Palmer, D. A. Inorg. Chem. 1992,
31, 5280-5287.
(2) Geipel, G.; Brachmann, A.; Brendler, V.; Bernhard, G.; Nitsche, H.
Radiochim. Acta 1996, 75, 199-204.
(3) Guillaumont, R.; Fangha¨nel, T.; Fuger, J.; Grenthe, I.; Neck, V.;
Palmer, D. A.; Rand, M. H. Update on the chemical thermodynamics
of uranium, neptunium, plutonium, americium and technetium; Elsevier
Science Publishers: Amsterdam, 2003.
(4) Peterson, A. Acta Chem. Scand. 1961, 15, 101.
(5) Grenthe, I.; Lagerman, B. Radiochim. Acta 1993, 61, 169-176.
(6) Langmuir, D.; Herman, J. S. Geochim. Cosmochim. Acta 1980, 44,
1733-1766.
(9) Ga´l, M.; Goggin, P. L.; Mink, J. Spectrochim. Acta A 1992, 48, 121-
132.
(10) Neuefeind, J.; Skanthakumar, S.; Soderholm, L. Inorg. Chem. 2004,
43, 2422-2426.
(11) Craw, J. S.; Vincent, M. A.; Hillier, I. H.; Wallwork, A. L. J. Phys.
Chem. A 1995, 99, 10181-10185.
(12) Antonio, M. R.; Soderholm, L. In The Chemistry of the Actinides and
Transactinides; Springer: Dordrecht, The Netherlands, 2006; Vol. 5,
pp 3086-3189.
Inorganic Chemistry, Vol. 46, No. 15, 2007 5883

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Hennig et al.
Table 1. List of Samplesa
volume was increased to 10 mL by adding water (sample I). The
pH value was adjusted with NaOH.
Th(IV) Sulfate. Samples J and K were prepared according to
the U(VI) sulfate solutions by adding aliquots of stock solutions
of Th(NO3)4, H2SO4, and solid (NH4)2SO4.
2-
ID
chemical composition
0.01 M U(VI),
speciation
pH [SO4
0.89
]
2+
A
100% UO2
b
0.1 M HClO4
2+
B
C
0.05 M UO2SO4,
0.15 M H2SO4,
0.2 M (NH4)2SO4
15.7% UO2
26.7% UO2SO4(aq)
0.96
1.00
0.99
0.4
U(IV) Sulfate Precipitate. The electrochemical reduction of
uranyl sulfate solutions at pH (1.5 is accompanied by a precipita-
tion. This precipitation prevented a complete reduction of U(VI)
by passivating the electrodes. In order to obtain a solid U(IV)
compound, a uranyl sulfate solution was initially reduced at pH 1
(starting solution was sample B). Subsequently, the pH of the U(IV)
sulfate solution was increased to pH 2 by adding NaOH. The
precipitate (sample P) was separated from the solution by centrifu-
gation and dried under inert gas atmosphere. The elemental analysis
of this precipitate, determined by ICP-MS after wet digestion, is
[mg/g] U, 458.0; Na, 59.7; SO42-, 64.0. The precipitate can be
regarded as amorphous because no peaks were observed with X-ray
diffraction.
Zippeite. This reference sample (sample H), K(UO2)2SO4(OH)3‚
H2O, was provided by R. Vochten (University of Antwerp). Its
preparation and crystal structure are already described.13
Electrochemical Reduction. To convert uranium to its tetrava-
lent oxidation state, we used an electrochemical cell as described
previously.14 A Pt gauze was used as working electrode. The
reference potential is the Ag+/AgCl couple. The reduction of U(VI)
to U(IV) involves an electron transfer and a chemical reaction
transforming the trans-dioxo cation to the spherical coordinated U4+
cation. The reaction at the working electrode is
c
2-
53.8% UO2(SO4)2
4-
3.1% UO2(SO4)3
2+
0.05 M UO2SO4,
0.3 M H2SO4,
3.1% UO2
1.3
19.4% UO2SO4(aq)
75.1% UO2(SO4)2
c
2-
0.95 M (NH4)2SO4
4-
2.2% UO2(SO4)3
2+
D
E
F
0.05 M UO2SO4,
0.38 M H2SO4,
1.57 M (NH4)2SO4,c
0.05 M UO2SO4,
0.6 M H2SO4,
2.35 M (NH4)2SO4
0.5 M U(VI),
0.5 M H2SO4
1.7% UO2
2.0
3.0
0.5
21.2% UO2SO4(aq)
76.2% UO2(SO4)2
25.6% UO2SO4(aq) 0.99
73.1% UO2(SO4)2
2-
2-
c
2+
37.5% UO2
31.0% UO2SO4(aq)
1.96
2.55
2-
30.0% UO2(SO4)2
2+
G
0.05 M U(VI),
0.05 M H2SO4
42.9% UO2
0.05
40.3% UO2SO4(aq)
15.8% UO2(SO4)2
2-
H
I
zippeite,
K(UO2)2SO4(OH)3H2O
0.05 M Th(IV),
0.5 M HClO4
100% Th4+
1.00
1.00
2+
J
0.05 M Th(IV),
0.17 M H2SO4,
0.23 M (NH4)2SO4
0.05 M Th(IV),
0.39 M H2SO4,
1.61 M (NH4)2SO4
0.01 M U(IV),
2.4% ThSO4
0.4
2.0
70.8% Th(SO4)2(aq)
26.7% Th(SO4)3
55.1% Th(SO4)2(aq) 1.02
44.7% Th(SO4)3
2-
K
2-
UO22+ + 2e- + 4H+ H U4+ + 2H2O
(1)
L
100% U4+
prepared from Ab
0.05 M U(IV),
For security reasons, the release of any gas had to be prevented
at the beam line; hence, an electrode of second kind was used as
counter electrode, consisting of metallic Ag (rod of 1 mm diameter)
and 0.1 M [Cl-] ions in the solution. During the uranium reduction
at the cathode, twice the molar amount of Ag oxidizes to Ag+ and
reacts with Cl- from the solution to AgCl
2+
M
N
O
P
3.7% USO4
0.4
1.3
2.0
prepared from Bc
0.05 M U(IV),
96.3% U(SO4)2(aq)
2+
0.7% USO4
prepared from Cc
0.05 M U(IV),
99.1% U(SO4)2(aq)
2+
0.5% USO4
prepared from Dc
precipitate of U(IV) sulfate
99.5% U(SO4)2(aq)
2Ag + 2Cl- H 2AgClV + 2e-
(2)
a Electrolytically prepared U(IV) samples and their starting U(VI)
solutions contain Cl- ions in order to enforce the anode reaction: b 0.02 M
LiCl, c 0.1 M NaCl.
The formation of the AgCl precipitate at the counter electrode
prevented the reoxidation of U(IV). Since the stability constants
of uranium chloro complexes are low, it can be excluded that the
sulfate speciation was affected by the presence of 0.1 M [Cl-].14
UV-vis spectra were obtained before and after the reduction in
order to verify the initial and final oxidation states. The measure-
ments were performed in a quartz glass cuvette with an optical
path length of 10 mm using a CARY-5G/Varian spectrometer. As
an example, the UV-vis spectra of samples D and O are shown in
Figure F1 of the Supporting Information. The spectrum of U(VI)
shows the typical fine structure which is dominated by the vibration
modes of the trans-dioxo cation, UO22+. The transitions in the
spectrum are mainly arising from electronic configurations of the
type (σu+)2 f σu+δu and (σu+)2 f σu+φu.15 The UV-vis absorption
spectrum of U(IV) sulfate shows four typical broad absorption bands
with a higher oscillator strength than that of U(VI).
aliquots of the U(VI) and HClO4 stock solutions. U(IV) hydrate
(sample L) was obtained by electrolysis of the U(VI) solution.
U(VI) and U(IV) Sulfate. A stock solution of 0.2 M uranyl
sulfate was prepared by dissolving UO3 in 0.2 M H2SO4. Samples
B, C, D, and E were prepared by adding aliquots of stock solutions
of uranyl sulfate, H2SO4, and solid (NH4)2SO4. The amounts of
H2SO4 and (NH4)2SO4 were varied according to the values given
at Table 1 to adjust the total sulfate concentration and the pH value.
Samples F and G were prepared by dissolving appropriate amounts
of UO3 in H2SO4 to achieve equimolar concentrations of U(VI)
and sulfate of 0.5 and 0.05 M, respectively. U(IV) sulfate solutions
(samples M, N, O) were prepared by electrolysis of the corre-
sponding U(VI) solutions.
Th(IV) Hydrate. A 0.1 M Th(IV) stock solution was obtained
by dissolving Th(NO3)4‚5H2O in 0.5 M HCl. The Th(IV) stock
solution (10 mL) was evaporated to dryness under stirring. The
residue was redissolved in 10 mL of 0.5 M HCl and again
evaporated to dryness. This was repeated twice. The residue was
redissolved in water, and Th was precipitated with NaOH. The Th
precipitate was isolated by centrifugation and washed with water
several times. Subsequently, 5 mL of 1 M HClO4 was added. The
Sample Conditions for EXAFS Measurements. The storage
of the samples with the stable oxidation states U(VI) and Th(IV)
(13) Vochten, R.; Vanhaverbeke, L.; Vanspringel, K.; Blaton, N.; Peeters,
O. M. Canad. Min. 1995, 33, 1091-1101.
(14) Hennig, C.; Tutschku, J.; Rossberg, A.; Bernhard, G.; Scheinost, A.
C. Inorg. Chem. 2005, 44, 6655-6661.
(15) Servaes, K.; Hennig, C.; Van, Deun, R.; Go¨rller-Walrand, C. Inorg.
Chem. 2005, 44, 7705-7707.
5884 Inorganic Chemistry, Vol. 46, No. 15, 2007

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EXAFS InWestigation of Sulfato Complexes
did not need any specific precautions. However, different U and
Th concentrations required individually adapted optical path lengths
for the sample vials to obtain similar noise levels of the EXAFS
spectra. Hence, polyethylene vials with an optical path length of 3
and 13 mm were used for the samples with 0.5 and 0.05 M U and
Th, respectively, encapsulated in 200 µm polyethylene film as
second confinement against radionuclide release. In situ EXAFS
measurements with the spectroelectrochemical cell with an optical
path length of 20 mm were applied to obtain spectra of U(IV)
hydrate as described previously.14 The U(VI) f U(IV) reduction
was performed under inert gas atmosphere in the electrochemical
cell. The EXAFS measurement of the U(IV) hydrate (sample L)
was performed in situ, whereas the U(IV) sulfate solutions were
transferred from the electrochemical cell into quartz glass cuvettes
(optical path length of 10 mm). The solid samples were measured
as powder pellet (H) and wet paste (P) in hot-sealed polyetylene
cuvettes.
noise levels at higher k values, data analysis was restricted to the
k range 3.2-16.7 Å-1 for U(VI) and to 4.1-14.2 Å-1 for U(IV)
and Th(IV). The distance resolution, ∆R ) π/2∆k, is 0.12 Å for
the U(VI) spectra and 0.16 Å for U(VI) and Th(IV). The amplitude
2
reduction factor, S0 , was defined as 1.0 in the FEFF calculation
and fixed to that value in the data fits. The threshold energy, Ek)0
,
was arbitrarily defined for U(VI) and U(IV) as 17 185 eV, and for
Th(IV) as 16 320 eV and varied as a global fit parameter resulting
in the energy shift ∆Ek)0. The same energy shift was applied for
each shell. The overall goodness of the fits, F, is given by ø2
weighted by the magnitude of data.18
Double-Electron Excitations. The excited photoelectron has a
certain probability to excite a second electron into unoccupied
orbitals (shake up) or the continuum (shake off). Since the intensity
of double-electron excitation is usually only a few percent of a
single-electron excitation, their appearance is often masked by the
single-electron EXAFS oscillation. The spectra obtained from
solutions show weak EXAFS amplitudes, especially at high k
values, and therefore, double-electron excitations become more
dominant. The EXAFS data shown here are partly affected by [2p4f]
double-electron excitations.26 The spectra of Th(IV) and U(IV) are
more affected than the spectra of U(VI) due to stronger resonance
intensity of their double-electron excitations.26 While the double-
electron excitation may bias the EXAFS amplitude and hence the
coordination numbers and Debye-Waller factors, the frequency
of the main electron excitation channel is not affected; hence, the
determined distances are not influenced. Deviations between µ0-
(E) and its spline approximation lead to artificial peaks at R e 1.2
Å, which were minimized during the data extraction. The double-
electron excitation itself acts as a high-frequency feature that may
contribute spurious peaks at large R values. Fourier filtering
procedures and data analysis using reduced k ranges indicated that
the effects are small and within the usual error limits. Therefore,
for the final data analysis, the raw data itself and not the Fourier-
filtered data were used. In Figures 4, 6 and 7, double-electron
excitations are marked with a dotted line and their k values are
given. A more detailed discussion of double-electron excitation
phenomena in L3-edge X-ray absorption spectra of actinides is given
elsewhere.26
EXAFS Data Acquisition. EXAFS measurements were carried
out at the Rossendorf Beamline16 at the European Synchrotron
Radiation Facility. The monochromator, equipped with a Si(111)
double crystal, was used in channel-cut mode. Higher harmonics
were rejected by two Pt-coated mirrors. All experiments were
performed at room temperature. The spectra were collected in
transmission mode using argon-filled ionization chambers. Across
the EXAFS region, data points were collected with equidistant k
steps of 0.05 Å-1. The monochromator energy scale was calibrated
to the K-edge of an Y metal foil (first inflection point assigned to
17038 eV).
EXAFS Data Analysis. The EXAFS oscillations were extracted
from the raw absorption spectra by standard methods including a
µ0 spline approximation for the atomic background using either
the WINXAS17 or the EXAFSPAK18 software packages. A square
window function has been applied for the Fourier transform. In
order to suppress side lobe effects, care has been taken to keep
ø(k) ≈ 0 at kmin and kmax. The EXAFS data were fit using theoretical
phase and amplitude functions calculated with the FEFF 8.2 code
of Rehr et al.19 The scattering interactions were calculated using
single scattering (SS) and multiple scattering (MS) paths of the
model compounds Na10[(UO2)(SO4)4](SO4)2‚3H2O,20 Cs2Th(SO4)3‚
3H2O,21 and U(SO4)2‚4H2O22 and the hypothetical clusters UO2-
Quantum Chemical Calculations. The quantum chemical
calculations were performed at the B3LYP level in the aqueous
phase without any symmetry constraints using the Gaussian 03
program package.27 The energy-consistent small-core effective core
potential (ECP) and the corresponding basis set suggested by Dolg
(H2O)5 , , .
2+ 23 U(H2O)94+ 24 and Th(H2O)94+ 24 It has been shown that
the multiple scattering path U-Oax in the uranyl unit is dominated
by the two-fold degenerated four-legged multiple-scattering path
U-Oax1-U-Oax2.
25 This scattering path was included in the curve
fit by constraining its Debye-Waller factor and its effective
pathlength to twice the values of the corresponding, freely fitted
U-Oax single-scattering path. Taking into account the individual
(25) Hudson, E. A.; Rehr, J. J.; Bucher, J. J. Phys. ReV. B 1995, 52, 13815-
13826.
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