Incorporation of neptunium(VI) into a uranyl selenite

Article abstract of DOI:10.1021/ic301682b

The incorporation of neptunium(VI) into the layered uranyl selenite Cs[(UO₂)(HSeO₃)(SeO₃)] has yielded the highest level of neptunium uptake in a uranyl compound to date with an average of 12(±3)% substitution of Np^VI for U^VI. Furthermore, this is the first case in nearly 2 decades of dedicated incorporation studies in which the oxidation state of neptunium has been determined spectroscopically in a doped uranyl compound and also the first time in which neptunium incorporation has resulted in a structural transformation.

Full text of DOI:10.1021/ic301682b

Communication
pubs.acs.org/IC
Incorporation of Neptunium(VI) into a Uranyl Selenite
Nathan A. Meredith, Matthew J. Polinski, Jian Lin, Antonio Simonetti, and Thomas E. Albrecht-Schmitt*
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States
Department of Chemistry and Biochemistry and Department of Civil Engineering and Geological Sciences, University of Notre
Dame, Notre Dame, Indiana 46556, United States
S
* Supporting Information
Table 1. Crystallographic Information for CsUSeO and
CsUSeO+NpVI
ABSTRACT: The incorporation of neptunium(VI) into
the layered uranyl selenite Cs[(UO2)(HSeO3)(SeO3)] has
sample
descriptions
space group
a (Å)
CsUSeO
CsUSeO+NpVI
yellow prism
P21/n (No. 14)
8.4966(2)
10.3910(3)
10.2087(3)
93.693(1)
899.44(4)
4
yielded the highest level of neptunium uptake in a uranyl
compound to date with an average of 12( 3)%
substitution of NpVI for UVI. Furthermore, this is the
first case in nearly 2 decades of dedicated incorporation
studies in which the oxidation state of neptunium has been
determined spectroscopically in a doped uranyl compound
and also the first time in which neptunium incorporation
has resulted in a structural transformation.
yellow plate
P21/c (No. 14)
13.8169(5)
10.5942(4)
12.5411(5)
100.845(2)
1799.1(1)
8
b (Å)
c (Å)
β (deg)
V (Å3)
Z
This is the first observation of a structural transformation
following neptunium incorporation because, while the anionic
sheet topology remains the same, the compound crystallizes in
a different space group when neptunium(VI) is incorporated. It
is interesting to note that, in the original structure, there are
two unique uranium sites, whereas in the doped structure, only
one site is observed. Furthermore, it was not possible to solve
the doped structure in P21/c with two unique sites.
A primary concern for the disposal of nuclear waste is the
presence of high activity and/or long-lived radionuclides in
used nuclear fuel. Among the radionuclides of greatest concern
are 237Np (t1/2 = 2.14 × 106 years), 129I (t1/2 = 1.57 × 107
years), 135Cs (t1/2 = 2.30 × 106 years), and 79Se (t1/2 = 6.50 ×
105 years).1 The potential release of these radionuclides during
corrosion of the waste form has led to investigations into a
means of limiting their environmental mobility.2
UV−vis−near-IR (NIR) spectra of several crystals were
obtained using a microspectrophotometer. An example
spectrum is shown in Figure 1. The peak occurring at 1200
nm is characteristic of neptunium(VI), indicating that the initial
neptunium(V) in solution was oxidized by iodate (NpO22+ + e−
Burns et al. proposed that uranium compounds that form
during oxidation and corrosion of used nuclear fuel may be able
to host transuranium elements and other radionuclides by
incorporation into the structure via appropriate charge-
balancing substitutions.3 Several studies have shown that
neptunium(V) can substitute for uranium(VI) in a variety of
compounds. However, the oxidation state of neptunium has not
been identified in these studies.4 Additional studies have shown
the successful incorporation of iodine and selenium oxoanions
and cesium by exchange for anions or cations, respectively,
within the structure.5 Most notably, Wu et al. showed that
cosubstitution of neptunium(V) and iodate increases the
uptake of both species by providing an intrinsic charge
balance.6
The uranyl selenite Cs[(UO2)(HSeO3)(SeO3)] (CsUSeO)
forms anionic layers that consist exclusively of vertex-sharing
2+
−
2−
UO2 pentagonal bipyramids and HSeO3 and SeO3
polyhedra.7 Neptunium(V) and iodate cosubstitution into this
compound was studied following the procedure for CsUSeO by
adding NpO2+ and IO3− at 10% of the UO3 and SeO2 amounts,
respectively.
The reaction yielded yellow prismatic crystals. Crystallo-
graphic data for the doped and undoped crystals are given in
Table 1. Additional information can be found in the Supporting
Information. Black crystals of molecular iodine were also
observed, but these immediately dissolved upon rinsing of the
product with methanol.
Figure 1. UV−vis−NIR spectra for CsUSeO and CsUSeO+NpVI,
which shows the characteristic peak for neptunium(VI) at 1200 nm.
Received: July 31, 2012
Published: October 2, 2012
© 2012 American Chemical Society
10480
dx.doi.org/10.1021/ic301682b | Inorg. Chem. 2012, 51, 10480−10482

Inorganic Chemistry
Communication
→ NpO2 , E° = 1.16 V; 2IO3− + 12H+ + 10e− → I2 + 2H2O, E°
The high level of neptunium uptake in this compound may
+
= 1.195 V).9,10 The given potentials are for reactions at 25 °C
and 1 atm and should only be considered as a guide because the
reaction was conducted at 180 °C. This is the first study since
Burns et al. first proposed neptunium incorporation in spent
fuel corrosion products in which the oxidation state of
neptunium doped into a uranyl compound was able to be
determined spectroscopically.
be attributed to the presence of neptunium(VI), as the NpO2
2+
cation, which may substitute directly for UO22+ in the structure
without the need for a cosubstituent to maintain charge
2+
2+
neutrality (Figure 3). NpO2 is slightly smaller than UO2
,
Three crystals were selected for laser-ablation inductively
coupled plasma mass spectrometry (LA-ICP-MS) analysis.
Because of the size of the crystals, only one ablation was
conducted per sample. In a typical measurement, a 60 s
background ion signal was obtained with the laser shutter in
place followed by a 60 s ablation conducted at a single spot on
the crystal using a 15 μm spot size. As seen in Figure 2, the
neptunium signal is consistent with the signals for uranium,
cesium, and selenium, indicating that it is uniformly
incorporated within the structure.
2+
Figure 3. Illustration of the potential substitution of NpO22+ for UO2
in CsUSeO.
which may account for the observed unit-cell transformation.8
In fact, the doped phase adopts a unit cell similar to those of
M[(UO2)(HSeO3)(SeO3)] (M = K, Rb, Tl, NH4), which are
isotypic with CsUSeO.7 The axes of the doped phase are all
slightly longer than those for the structures containing smaller
cations, while the β angle is smaller, as would be expected with
the larger Cs+ cation.
Several possible mechanisms exist for the incorporation of
iodine. The first could be intercalation of iodic acid into the
interlayer, as has previously been observed in uranyl iodate.9
Two other possibilities are based on the substitution of iodate
for selenite. The substitution, IO3 ↔ HSeO3 , maintains a
charge balance and results in iodate with one terminal oxygen
atom. Alternatively, IO3− may exchange with SeO32− with three
bridging oxygen atoms. This substitution results in an excess of
positive charge, which may be relieved by the stoichiometric
removal of a Cs+ cation or cosubstitution with some remaining
NpV for UVI. Further exploration is needed to evaluate the
likelihood of each of these proposals.
Neptunium incorporation in CsUSeO has revealed several
new observations that have not been observed in previous
incorporation studies. In addition to the highest level of
neptunium uptake to date, this study is the first in which the
oxidation state of neptunium was determined by UV−vis−NIR
spectroscopy and also the first observation of a significant
change in the unit cell parameters of the crystal structure in a
doped compound. This work reinforces the critical role of
Figure 2. Typical LA-ICP-MS spectrum for CsUSeO+NpVI showing
the initial 60 s background with the laser shutter in place followed by
the 60 s ablation measurement. The relatively flat profile of the
ablation data indicates that the constituent elements are present
uniformly in the crystal.
−
−
The signal intensities obtained from the LA-ICP-MS
measurements were used to calculate ratios of the elements
present in the crystals (Table 2). According to these results,
between 10.3( 0.8)% and 16.4( 0.6)% of the uranium in
CsUSeO+NpVI has been replaced by neptunium. This is the
highest level of neptunium uptake by a uranyl phase to date,
with the previous maximum value being 7.93( 0.83)%
substitution.6 A small presence of iodine [0.33( 0.05)% to
1.5( 0.2)%] was also observed.
Table 2. LA-ICP-MS Results for Neptunium-Doped CsUSeO
sample
Se/(U + Np)
Cs/(U + Np)
Np/(U + Np)
I/(Se + I)
a
CsUSeO
CsUSeO+NpVI
0.38 0.10
0.372 0.009
0.38 0.01
0.34 0.02
0.72 0.26
0.71 0.06
0.71 0.04
0.69 0.06
0.164 0.006
0.107 0.005
0.103 0.008
0.0056 0.0007
0.0033 0.0005
0.015 0.002
a
Ratio averaged from six measurements (three ablations × two crystals). Three crystals were originally analyzed; however, one fractured upon
ablation, giving anomalous measurements (Se/U ≈ 0.20, for example), and was therefore excluded.
10481
dx.doi.org/10.1021/ic301682b | Inorg. Chem. 2012, 51, 10480−10482

Inorganic Chemistry
Communication
(9) Ling, J.; Albrecht-Schmitt, T. E. Inorg. Chem. 2007, 46, 346−347.
(10) Bray, T. H.; Ling, J.; Choi, E. S.; Brooks, J. S.; Beitz, J. V.;
Sykora, R. E.; Haire, R. G.; Stanbury, D. M.; Albrecht-Schmitt, T. E.
Inorg. Chem. 2007, 46, 3663−3668.
charge balance with respect to uptake levels of neptunium in
uranium compounds.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, X-ray diffraction collection parameters,
laser-ablation parameters, scanning electron microscopy and
optical images, energy-dispersive X-ray analysis data, and X-ray
crystallographic files in CIF format for CsUSeO and CsUSeO
+NpVI. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: talbrec1@nd.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for support provided by the U.S. Department of
Energy, Subsurface Biogeochemical Research Program, under
Grant ER64804.
REFERENCES
■
(1) Bruno, J.; Ewing, R. C. Elements 2006, 2, 343−349.
(2) (a) Finch, R. J.; Ewing, R. C. J. Nucl. Mater. 1992, 190, 133−156.
(b) Forsyth, R. S.; Werme, L. O. J. Nucl. Mater. 1992, 190, 3−19.
(c) Wronkiewicz, D. J.; Bates, J. K.; Gerding, T. J.; Veleckis, E.; Tani,
B. S. J. Nucl. Mater. 1992, 190, 107−127. (d) Wronkiewicz, D. J.;
Bates, J. K.; Wolf, S. F.; Buck, E. C. J. Nucl. Mater. 1996, 238 (1), 78−
95. (e) Shoewsmith, D. W. J. Nucl. Mater. 2000, 282, 1−31. (f) Finn,
P. A.; Hoh, J. C.; Wolf, S. F.; Slater, S. A.; Bates, J. K. Radiochim. Acta
1996, 74, 65−71.
(3) Burns, P. C.; Ewing, R. C.; Miller, M. L. J. Nucl. Mater. 1997, 245
(1), 1−9.
(4) (a) Klingensmith, A. L.; Burns, P. C. Am. Mineral. 2007, 92 (11−
12), 1946−1951. (b) Klingensmith, A. L.; Deely, K. M.; Kinman, W.
S.; Kelly, B.; Burns, P. C. Am. Mineral. 2007, 92 (4), 662−669.
(c) Burns, P. C.; Deely, K. M.; Skanthakumar, D. Radiochim. Acta
2004, 92 (3), 151−159. (d) Douglas, M.; Cark, S. B.; Friese, J. I.; Arey,
B. W.; Buck, E. C.; Hanson, B. D.; Utsunomiya, S.; Ewing, R. C.
Radiochim. Acta 2005, 93, 265−272. (e) Schuller, L. C.; Ewing, R. C.;
Becker, U. J. Nucl. Mater. 2011, DOI: 10.1016/j.jnucmat.2011.04.016.
(f) Buck, E. C.; Finch, R. J.; Finn, P. A.; Bates, J. K. Scientific Basis for
Nuclear Waste Management XXI. InMaterials Research Society
Symposium Proceedings; McKinley, I. G., McCombie, C. H., Eds.;
Materials Research Society: Warrendale, PA, 1998; pp 87−94.
(g) Buck, E. C.; Douglas, M.; McNamara, B. K.; Hanson, B. D.
Possible Incorporation of Neptunium in Uranyl(VI) Alteration Phases;
Pacific Northwest National Laboratory: Richland, WA, 2003.
(5) (a) Wu, S.; Chen, F.; Kang, M.; Yang, Y.; Dou, S. Radiochim. Acta
2009, 97 (8), 459−465. (b) Ling, J.; Wu, S.; Chen, F.; Simonetti, A.;
Shafer, J. T.; Albrecht-Schmitt, T. E. Inorg. Chem. 2009, 48 (23),
10995−11001. (c) Wu, S.; Wang, S.; Simonetti, A.; Chen, F.; Albrecht-
Schmitt, T. E. Radiochim. Acta 2011, 99, 573−579. (d) Chen, F.;
Burns, P. C.; Ewing, R. C. J. Nucl. Mater. 1999, 275 (1), 81−94.
(e) Douglas, M.; Blars, S. B.; Utsunomiya, S.; Ewing, R. C. J. Nucl. Sci.
Technol. 2002, 3, 504−507. (f) Burns, P. C. J. Nucl. Mater. 1999, 265,
218−223.
(6) Wu, S.; Chen, F.; Simonetti, A.; Albrecht-Schmitt, T. E. Environ.
Sci. Technol. 2010, 44, 3192−3196.
(7) (a) Almond, P. M.; Albrecht-Schmitt, T. E. Inorg. Chem. 2002, 41,
1177−1183. (b) Koskenlinna, M.; Valkonen, J. Acta Crystallogr. 1996,
C52, 1857−1859.
(8) Forbes, T. Z.; Wallace, C.; Burns, P. C. Can. Mineral. 2008, 46,
1623−1645.
10482
dx.doi.org/10.1021/ic301682b | Inorg. Chem. 2012, 51, 10480−10482