From Nd(III) and Pu(III) Oxalates to Oxides: Influence of Nitrilotris(methylenephosphonic acid) on Chemical Composition, Structure, and Morphology

Article abstract of DOI:10.1021/acs.cgd.7b00643

Neodymium oxalate structure and morphology were modified by adding a phosphonate, NTMP, during the oxalic precipitation step. Spherical agglomerates of neodymium oxalate Nd₂(C₂O₄)₃(H₂O)₆·12H₂O composed of thin hexagonal rods were precipitated instead of decahydrated oxalate Nd₂(C₂O₄)₃(H₂O)₆·4H₂O with rod-like particles usually obtained in the absence of additive. Despite some differences in the complexation with NTMP and complexity from redox, the use of NTMP was successfully transposed to the plutonium system. Spherical agglomerates of plutonium(III) oxalate were so obtained, leading to spherical agglomerates of PuO₂ after thermal treatment.

Full text of DOI:10.1021/acs.cgd.7b00643

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Article
From Nd(III) and Pu(III) Oxalates to Oxides: Influence
of NitriloTris(MethylenePhosphonic acid) on
Chemical Composition, Structure and Morphology.
Anne-Lise Vitart, blaise haidon, Benedicte Arab-Chapelet, Murielle Rivenet, Isabelle
Bisel, Patrick Pochon, Pascal Roussel, Stephane Grandjean, and Francis Abraham
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00643 • Publication Date (Web): 28 Jul 2017
Downloaded from http://pubs.acs.org on August 4, 2017
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From Nd(III) and Pu(III) Oxalates to Oxides:
Influence of NitriloTris(MethylenePhosphonic
acid) on Chemical Composition, Structure and
Morphology.
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AnneꢀLise Vitarta,b, Blaise Haidona,b, Benedicte ArabꢀChapeleta, Murielle Rivenetb, Isabelle
Bisela, Patrick. Pochonc, Pascal Rousselb, Stéphane Grandjeand, Francis Abrahamb
aDMRC/SFMA/LPCA, Bat.399, CEA Marcoule, BP 17171, 30207 Bagnols/Cèze, France
bUniv. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de
Catalyse et Chimie du Solide, F-59000 Lille, France
c DMRC/SPDS/LDPS, Bat.57, CEA Marcoule, BP 17171, 30207 Bagnols/Cèze, France
d DMRC/SPDS/DIR, Bat.399, CEA Marcoule, BP 17171, 30207 Bagnols/Cèze, France
Corresponding author e-mail : benedicte.arab-chapelet@cea.fr
ABSTRACT:
Neodymium oxalate structure and morphology were modified by adding a
phosphonate, NTMP, during the oxalic precipitation step. Spherical agglomerates of
neodymium oxalate Nd2(C2O4)3(H2O)6⋅12H2O composed of thin hexagonal rods were
precipitated instead of decahydrated oxalate Nd2(C2O4)3(H2O)6⋅4H2O with rodꢀlike particles
usually obtained in absence of additive. Despite some differences in the complexation with
NTMP and complexity from redox, the use of NTMP was successfully transposed to the
plutonium system. Spherical agglomerates of plutonium(III) oxalate were so obtained, leading
to spherical agglomerates of PuO2 plutonium oxide after thermal treatment.
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INTRODUCTION
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Oxalic conversion is a wellꢀknown process to obtain metal oxide from cations in
solution1,2,3. In nuclear field, the recovery of valuable actinides requires, first, to dissolve the
spent fuel, then to partition the elements, and finally to recover the valuable actinides into a
solid form by means of a conversion step which can be achieved by oxalic precipitation. This
process leads to the precipitation of solid actinide oxalates used as oxide precursors4. As
oxalates keep their morphology when calcined into oxides through pseudomorphic
transformations, controlling of the size, structure and morphology of the oxalate particles is a
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key factor to optimize the conception of new fuels5,6,7,8
.
In the past decades, some ways have been found to control actinide oxalate size, structure
and morphology9,10,11,12. Increasing crystallization temperature of tetravalent actinides
An(C2O4)2.6H2O (An = Pu or Np) has, for example, a significant influence on particles size
and shape13,14,15. Considering the strong correlation between the particles shape and the
crystal structure, another way to influence the final morphology of the oxalate is to target its
crystal structure by varying chemical conditions of crystallization16,17,18,19,20,21,22,23,24,25,26,27,28
.
Few studies were dedicated to the influence of additives on actinide oxalates structure and
morphology. However, influence of some additives on oxalic precipitation was studied on
other systems as calcium oxalate, primary constituent of human kidney stones, or copper
oxalate, which permits to obtain copper oxide, used for bulk and thin films applications. For
example, polymers, such as hydroxypropyl methylcellulose (HPMC), are able to act as
surface capping agents to cover the surface of crystals and lead to the formation of rods for
copper oxalate29. Moreover, complexing agents of metallic cations should also be considered.
By competing oxalic acid they could influence the kinetic of precipitation and the structure of
the precipitated oxalate, so that microstructural modification can be expected.
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Among the different chelating reagents, nitrilotris(methylenephosphonic acid)
N[CH2PO(OH)2]3 (hereinafter referred as NTMP), is used in numerous technical and
industrial applications such as corrosion, scale inhibitor, in waste water treatment, selective
noble or toxic metals recovery30. It is also referenced to be a kinetic inhibitor of crystal
growth for calcium oxalate31, calcium sulfate32 and calcium phosphate such as
hydroxyapatite33 and a strong complexing agent for lanthanides34 and actinides35. The present
study focuses on the impact of this phosphonate additive on plutonium oxalate crystallization.
When in contact with metal ions, phosphonates form stable and water soluble complexes as
hybrid inorganic–organic frameworks36,37. This property was used here to modify plutonium
(III) oxalate crystalline nucleation and growth, and, therefore, its morphology.
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A preliminary work was first carried out on neodymium (III) oxalate, in order to
anticipate, to a certain extent by analogy, plutonium oxalate behavior before assessing it.
Reference experiments ꢀ without NTMP ꢀ were also systematically performed for easiest
comparison.
EXPERIMENTAL SECTION
Precursors
Because of the radioactive nature of actinides, especially plutonium, the experiments
involving this element were carried out in glove boxes with very restrictive protocols.
Plutonium(IV) and plutonium(III) solutions were prepared using the following specific
procedures. The Pu(IV) solution was prepared by dissolving PuO2 oxide in concentrated
HNO3. The Pu(III) solution was synthesized by reducing the Pu(IV) solution by hydrazinium
+
ꢀ
nitrate (N2H5 , NO3 ) at 50 °C. Hydrazinium nitrate was also used as an antiꢀnitrous agent to
stabilize the trivalent oxidation state of plutonium.
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The concentration of plutonium in these solutions is 1 mol·Lꢀ1 and that of hydrazinium
cations, in the plutonium(III) solution, is 0.2 mol·Lꢀ1. The concentration, purity and oxidation
state of plutonium in these primary solutions were essentially determined by UVꢀvisible
spectroscopy.
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Neodymium nitrate salt (Aldrich, 99.9% Reagent Grade Nd(NO3)3·6H2O) was used when
appropriate to prepare Nd(III) solutions to simulate Pu(III) solutions in pseudoꢀactive
experiments, on considering the analogies between Ln(III) and An(III) ions26,38,39. NTMP
(Aldrich, >97 %) and oxalic acid (Aldrich, 98 %) were used as received, without further
purification.
Oxalate precipitation experiments
Solutions A were first prepared by adding various amounts of NTMP ([NTMP]/[Nd]=0,
0.167, 0.33, 0.5, 0.67, 1 and [NTMP]/[Pu]=0, 0.33, 0.5) to an acidic solution (1M) of
neodymium (III) or plutonium (III) (0.16 M). As NTMP tends to precipitate with neodymium
and even more with plutonium, the acidity of solutions A was adjusted with various amounts
of nitric acid, so as to prevent this precipitation. The nitric acid concentration (mol/L) in each
solution versus the ratio [NTMP]/[Nd] or [NTMP]/[Pu] is summed up in table 1. With
plutonium, the NTMPꢀPu precipitation could not be completely prevented by higher
acidification. The solutions A containing plutonium were therefore filtered off before using.
Neodymium (III) and plutonium (III) oxalates were precipitated at around 20 °C , by
adding simultaneously the considered solution A and a nitric solution of oxalic acid (solution
Bꢀ [H2C2O4] = 0.54 M – [HNO3] = 1M) into a nitric solution containing a slight excess of
oxalic acid (solution C ꢀ [H2C2O4] = 0.15 M – [HNO3] = 1M), in order to maintain a constant
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oxalic concentration in the vortex during the precipitation. Time of addition and aging were
both fixed at 10 minutes as the reference.
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For [NTMP]/[Nd]=0.5, the influence of aging time (1 min to 24 hrs) and temperature (5 to
35°C) was also investigated.
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Table 1: Nitric acid concentrations (mol/L) in solutions A versus the [NTMP]/[cation]
ratio and sum up of the structure and morphology of the precipitated solids.
[NTMP]/[cation]
0
[HNO3]A
1
Structure
Morphology
Nd2(C2O4)3(H2O)6
·
4H2O
Rodꢀlike particles
0.167
0.33
0.5
Nd2(C2O4)3(H2O)6
+
·4H2O
Spherical
agglomerates
Nd
3.8
(Nd2(C2O4)3(H2O)6·12H2O
0.67
Small particles
1
0
3.8
1
amorphous
without defined shape
Rodꢀlike particles
Sphereꢀlike
0.33
0.5
3.8
2
Pu
agglomerates
Sphereꢀlike
agglomerates
After 10 minutes of precipitation by mixing solutions A, B and C, followed up by 10 min
of maturation, fine pale purple powder of neodymium compounds and blue to green/blue
powder of plutonium compounds are obtained (Figure 1). Those powders are filtered off on a
Buchner funnel under vacuum and then dried during 24 hours at room temperature.
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(a)
(b)
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Figure 1: Neodymium oxalate (a) and plutonium oxalate (b) powders obtained without additive
Thermal conversion
In some cases, a heat treatment of the asꢀobtained oxalates was carried out in a tubular
furnace. The samples were calcined at 1098 K under flowing air, at a heating rate of
10°.minꢀ1.
UV-visible analysis
The acidified solutions A were analyzed by UVꢀvis spectroscopy to study the
complexation of neodymium or plutonium by NTMP, using a spectrophotometer Cintra 10e
CBG. For neodymium, the analysis is direct (PMMA UVꢀcuvette, 1 cm optical path), whereas
for plutonium, the optical signal is transmitted through optical fibers (10 m length, Hellma)
from the disposable UVꢀcuvette (PMMA, 0.2 cm optical path) inside the gloveꢀbox to the
spectrophotometer placed outside. Solution is scanned from 350 to 900 nm with a speed of
270 nm/min, a measuring range of 0.479 nm and a gap width of 1.5 nm. Additional ratios of
[NTMP]/[Nd] and [NTMP]/[Pu] compared to ratios chosen for the precipitation experiments
were studied, in order to get a sufficient number of data to simulate the spectra by
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chemiometry, following an algorithm developed by Pochon et al.40. This method gives
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information about the number of species in solution and complex constants.
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XRD acquisition
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After precipitation, compounds were characterized by Xꢀray powder diffraction (XRD).
XRD data were obtained with a BRUKER D8 Advanced diffractometer (LynxEye detector) in
Bragg Brentano θ–θ mode using CuꢀK
α radiation (λ=1.5418 Å). Each powder pattern was
recorded within an angular range of 5°ꢀ60° in 2θ, with steps of 0.02° and a counting time of
0.5 s per step.
The stripping of the Kα2 radiation according the Rachinger method as well as the baseline
and angle shift corrections were done using empirical calculations and DIFFRACplus EVA
software41. Plutonium oxalates were mixed with an epoxy resin to prevent contamination
spreading. Silicon was added to the samples as an internal standard to calibrate the angular
positions of the observed XRD lines.
In order to study the broadening of some peaks, powder patterns of some neodymiumꢀ
containing oxalates were recorded with a RIGAKUꢀSMARTLAB Xꢀray diffractometer in θ–θ
mode using Cu–Kα radiation within the angular range 5°ꢀ50° in 2θ, with steps of 0.01° and a
counting time of 0.3 s per step. Refinement was realized using the powder option of JANA
200642.
SEM analysis
The SEM analyses of neodymium compounds were performed on a HitachiꢀS3400N
microscope, equipped with a tungsten filament (acceleration voltage = 15 kV, secondary
electron mode, working distance = 5 to 10 mm). Plutonium compounds were studied on a
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Zeiss Supra 55/55VP fieldꢀemission scanning electron microscope (acceleration voltage = 5
kV, secondary electron mode, working distance 5 to 10 mm, diaphragm= 30 µm). When EDS
analyses were performed, the following conditions were chosen: acceleration voltage = 15
kV, working distance 8.6 mm, diaphragm= 240 µm. Both with neodymium and plutonium
based samples, powders were coated with gold prior to the analysis.
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Infrared spectroscopy
The neodymium solids were analyzed by IR spectroscopy using a Perkin Elmer Spectrum
Two, equipped with a single reflection attenuated total reflectance, between 4000 and 400
cmꢀ1 with a 1 cmꢀ1 resolution.
Thermogravimetric analysis
For neodymium oxalates, the thermogravimetric analyses (TGA) were carried out on a
thermoanalyser 92 SETARAM TGA up to 700 °C, with a platinum crucible, under air
conditions and with a heating rate of 5°C.minꢀ1. Thermogravimetric analyses of plutonium
precipitates were performed on a NETZSCH STA 409C thermal analysis system with an
alumina crucible up to 950°C, under argon flow and with a heating rate of 20°C minꢀ1. The
normalized weight loss is calculated considering the mass/final oxide mass ratio (normalized
weight loss = 1 for final oxide).
Specific Surface area (BET)
The specific surface area was measured at 77 K by the BET method with
a
MICROMERITICS ASAP 2020 (surface area and porosity analyser).
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RESULTS
Characterization of solutions A (neodymium/plutonium + NTMP in nitric acid
medium)
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Neodymium -NTMP interactions
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UVꢀvis analyses performed on Nd(III)ꢀNTMP nitric acid solutions (solutions A) revealed
the complexation of neodymium by NTMP (Figure 2). Indeed, the Nd characteristic spectrum
evolves as NTMP concentration increases, evidencing modifications of the cation
coordination. For example, absorption bands at 740 and 795 nm are shifted toward higher
wavelengths, which are characteristic of 4I9/2ꢀ4F5/2 and 4I9/2ꢀ4F3/2 transitions, respectively43.
The UVꢀvisible spectra were analyzed and interpreted by modeling factor analysis (MFA)
techniques coupled with principal components analysis (PCA). PCA evidenced the formation
of a single anionic complex in the form M(H6ꢀnL)nꢀ, where M stands for neodymium and L for
NTMP. In the pH range studied, H5Lꢀ and H4L2ꢀ forms of NTMP coexist44 and the
coordination mode of NTMP in these neodymium complexes could be either monodentate or
bidentate34.
Figure 2: UVꢀvis study of neodymium complexation by NTMP
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Plutonium -NTMP interactions
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In Pu(III)ꢀNTMP nitric acid mixtures (solutions A), high acidification up to nearly 4M
and NTMP presence did not affect Pu(III) stability. A slight complexation of plutonium(III)
by NTMP was shown by UVꢀvis spectroscopy but the chemiometric study could not give
further information on the nature of the complex, due to the fact that even slight amounts of
plutonium precipitate with NTMP into the disposable UVꢀcuvette during the analysis (Figure
3).
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Figure 3: UVꢀvis study of plutonium(III) complexation by NTMP, after removal of the PuꢀNTMP
precipitate
Oxalic precipitation experiments
Influence of the NTMP content
Without NTMP, in reference precipitation conditions, the expected rodꢀlike
neodymium oxalate Nd2(C2O4)3(H2O)6⋅4H2O45, crystallizing in the monoclinic P21/c space
group, was obtained as a pure phase (Figure 4a and Figure 5) whereas in presence of NTMP
with [NTMP]/[Nd] ratios equal to 0.167, 0.33, 0.5 and 0.67 the additional neodymium oxalate
46
ꢀ
Nd2(C2O4)3(H2O)6
·
12H2O , crystallizing in the rhombohedral R3 space group, was observed
(Figure 5). Increasing the [NTMP]/[Nd] ratio decreases the Nd2(C2O4)3(H2O)6⋅4H2O content
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to the benefit of Nd2(C2O4)3(H2O)6·12H2O until reaching [NTMP]/[Nd]=1 for which a XꢀRay
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amorphous compound was obtained (Figure 5). The oxalic precipitation from complexed
neodymium, with [NTMP]/[Nd] ratios equal to 0.167, 0.33, 0.5 and 0.67, led to spherical
agglomerates of about 8 µm in diameter made of the radial agglomeration of thin hexagonal
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rods assumed to be the neodymium oxalate Nd2(C2O4)3(H2O)6
·
12H2O46 (Figure 4bꢀe). Some
XRD peaks of Nd2(C2O4)3(H2O)6 12H2O, such as those located at 2θ = 13.9 (2ꢀ21) and 15°
·
(211), are very broad compared to the others, indicating a strong anisotropy of either
crystallite size or microstrains (or both) within the observed sample. For the ratio
[NTMP]/[Nd]=1, the pattern is characteristic of an amorphous phase, and related to small
particles without defined shape on SEM images (Figure 4f).
Figure 4: SEM analyses of neodymium oxalates precipitated with various amounts of NTMP:
[NTMP]/[Nd] = a) 0, b) 0.167, c) 0.33, d) 0.5, e) 0.67 and f) 1
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Figure 5: Powder XꢀRay Diffraction analyses of neodymium samples precipitated with various amounts of NTMP
([NTMP]/[Nd] = 0, 0.167, 0.33, 0.5, 0.67, 1). Peaks under dashed lines correspond to Nd2(C2O4)3(H2O)6⋅12H2O46..
Other peaks belong to the Nd2(C2O4)3(H2O)6⋅4H2O45.
The anisotropic line broadening of Xꢀray peaks was thoroughly investigated for the
[NTMP]/[Nd]=0.5 sample. Full powder pattern matching refinement was done considering
the two phases with Le Bail method47. The peak profile of Nd2(C2O4)3(H2O)6⋅4H2O was
modeled using a conventional PseudoꢀVoigt function, whereas the peak profile of
Nd2(C2O4)3(H2O)6⋅12H2O was refined using a [001] broadened PseudoꢀVoigt function. After
deconvolution of the broadening due to the instrument (estimated using a LaB6 NIST
standard), WilliamsonꢀHall analysis was performed and lead to platelets with sizes along a
and b around 140 nm to compare to 6 nm in the perpendicular direction, i.e. a very anisotropic
size, as expected. Refined cell parameters for this phase are a=31.204(4) and c=7.317(13) Å
ꢀ
(space group R3), and a=11.197(6) b=9.640(4), c=10.272(6) Å and β=114.40(4) (space group
P21/c), for Nd2(C2O4)3(H2O)6⋅4H2O. The Xꢀray pattern, as well as a scheme of the obtained
morphology is reported Figure 6.
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Figure 6 : Final LeBail plot of the sample precipitated for [NTMP]/[Nd]= 0.5 and platelets scheme. Observed
data points are indicated as circles; the bestꢀfit profile and the difference are drawn as upper trace and lower
blue trace, respectively. Vertical bars indicate the angular positions of the allowed Bragg reflections for
Nd2(C2O4)3(H2O)6⋅4H2O (lower bars) and Nd2(C2O4)3(H2O)6·12H2O (upper bars) phases.
The FTIR spectra of the different precipitates are quite similar and characteristic of
oxalate compounds (Figure 7). Indeed, the broad absorption band around 3240 cmꢀ1
corresponds to stretching vibration of OꢀH bonds of coordinated water molecules, whereas the
intense peak at 1605 cmꢀ1 is due to the combined effects of distortion vibrations in the water
molecules plane and of asymmetric stretching vibration of C=O bonds of oxalate groups48.
The 1360 and 1316 cmꢀ1 peaks correspond to symmetric OꢀCꢀO stretching vibration. Finally,
the metalꢀoxygen bonds are confirmed by the presence of peaks at 800 and 490 cmꢀ1 49. The
main differences between these spectra are observed in the 1250ꢀ800 cmꢀ1 range and for a
[NTMP]/[Nd] ratio of 1, large peaks at 1169, 1038 and 885 cmꢀ1 are observed whereas for
other ratios these absorption bands are weak. These bands don’t match with spectra of
neodymium oxalates but with the stretching mode of phosphorusꢀoxygen bonds50 and indicate
the presence of phosphorous in the precipitated solid, coming the most probably from NTMP
or NTMP degradation products. The amount of phosphorous is higher in the precipitate
obtained for [NTMP]/[Nd] ratio of 1.
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Figure 7: IR analyses of neodymium powders precipitated with various amounts of
NTMP ([NTMP]/[Nd] = 0, 0.167, 0.33, 0.5, 0.67, 1). Peaks in dashed boxes correspond
to the phosphorusꢀoxygen bonds.
Influence of the temperature
For the ratio ([NTMP]/[Nd]=0.5), for which spherical agglomerates are obtained, the
influence of temperature was further investigated by precipitating powders at 5, 20 and 35 °C.
At the lowest temperature, the Nd2(C2O4)3(H2O)6⋅12H2O phase is obtained as a major phase
containing small amounts of Nd2(C2O4)3(H2O)6⋅4H2O (Figure 8). The powder XꢀRay patterns
exhibit the as previously described broadening of the peaks located at 13.9° and 15.1°. As the
temperature increases, the amount of Nd2(C2O4)3(H2O)6⋅4H2O increases and the relative
intensities of the powder XRD pattern of the Nd2(C2O4)3(H2O)6⋅12H2O phase tend to be
similar to that expected from the calculated diagram. The structural changes are accompanied
by microstructural modifications in that the smooth spheres observed at 5°C disappear to give
rise to thin hexagonal rods at 35°C.
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Figure 8: XRD analyses of neodymium powders precipitated with an amount of [NTMP]/[Nd] = 0.5 for
various precipitation temperatures (5, 20, 35 °C). Peaks under dashed lines correspond to
Nd2(C2O4)3(H2O)6⋅12H2O45. Other peaks belong to Nd2(C2O4)3(H2O)6⋅4H2O46.
Influence of the aging time
Same effects are observed by varying the aging time. At the first step, a Xꢀray
amorphous compound forms. When precipitation is carried out within a few minutes to 5
hours, smooth spheres of Nd2(C2O4)3(H2O)6⋅12H2O are precipitated. A longer aging time
increases the crystallinity of Nd2(C2O4)3(H2O)6⋅12H2O and replaces the spheres by isolated
thin rods corresponding, at 24 hours, to a mixing of Nd2(C2O4)3(H2O)6⋅4H2O and
Nd2(C2O4)3(H2O)6⋅12H2O (Figures 9 and 10). XꢀRay fluorescence combined with XRD and
SEM analyses show evidence that a high phosphorus content is maintained as long as the Xꢀ
ray amorphous compound and the spheres of Nd2(C2O4)3(H2O)6⋅12H2O are found in the solid
phase. XꢀRay fluorescence analyses support that the amount of NTMP decreases as
maturation proceeds (Figure I, SI).
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Therefore, the structure, composition and microstructure of neodymium oxalate
synthesized with NTMP evolve with the aging time as if NTMP was initially inhibiting the
crystal growth of Nd2(C2O4)3(H2O)6⋅4H2O to the benefit of the metastable
Nd2(C2O4)3(H2O)6⋅12H2O neodymium oxalate, characterized by sphere shape particles, which
progressively transforms into rodꢀlike particles of Nd2(C2O4)3(H2O)6⋅4H2O neodymium
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oxalate. Under ambient conditions, the metastable Nd2(C2O4)3(H2O)6⋅12H2O neodymium
oxalate evolves towards Nd2(C2O4)3(H2O)6⋅4H2O neodymium oxalate.
Figure 9: XRD analyses of neodymium powders precipitated with an amount of [NTMP]/[Nd] =
0.5 for various aging times (1 min, 4 min, 8 min, 30 min, 5h and 24 h). Peaks under dashed lines
correspond to the Nd2(C2O4)3(H2O)6⋅12H2O phase46. Other peaks marked by squares belong to
Nd2(C2O4)3(H2O)6⋅4H2O phase45.
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Figure 10: SEM analyses of neodymium powders precipitated with an amount of
[NTMP]/[Nd] = 0.5 in function of aging time (1 min, 8 min, 30 min, 5h and 24 h)
Influence of the trivalent element
As for neodymium(III), the oxalic precipitation of plutonium(III) in an aqueous
solution acidified by HNO3 leads to the formation of Pu2(C2O4)3(H2O)6⋅4H2O exhibiting a
rodꢀlike morphology (Figures 11a and 12).
By oxalic precipitation of NTMPꢀ plutonium(III), sphereꢀlike agglomerates of about
20 µm diameter were obtained (Figure 11 bꢀc). However, in this case, they are heterogeneous
in terms of morphology and composition.
Regardless of the [NTMP]/[Pu] ratio, two morphologies are visible within the sphereꢀlike
agglomerates. They are mainly composed of thin needles (Figure 11b) and of some small
particles (Figure 11c). According to XRD analysis, the spherical agglomerates mainly consist
of Pu2(C2O4)3(H2O)6⋅4H2O, but deviation of the baseline reveals the presence of an
amorphous compound (small particles). For [NTMP]/[Pu]=0.5, a third morphology is also
visible in the powder, based on tabular particles (Figure 11b) and corresponding to a third
phase, which does not match to any phase referenced in the literature (Figure 12). The
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corresponding XRD peaks were indexed in a monoclinic symmetry by the McMaille
software51, and its unit cell parameters were refined with a=17.658(3) Å, b=9.862(2) Å,
c=8.623(1) Å and β=112.48(1) °, corresponding to a cell volume of 1387.5(3) Å3. The lack of
information about this phase does not permit to solve the structure from the XRD powder
pattern.
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Figure 11: SEM analyses of plutonium(III) powders precipitated in various conditions:
a) [NTMP]/[Pu] = 0, [HNO3]A = 1 mol/L ; b) [NTMP]/[Pu] = 0.5, [HNO3]A = 2 mol/L ;
c) [NTMP]/[Pu] =0.33, [HNO3]A = 3.8 mol/L.
Numbers correspond to the three identified morphologies: 1=needles, 2=small particles, 3= tabular particles
Figure 12: XRD analyses of plutonium powders precipitated in various conditions:
a) [NTMP]/[Pu] = 0, [HNO3]A = 1 mol/L ; b) [NTMP]/[Pu] = 0.5, [HNO3]A = 2 mol/L ;
c) [NTMP]/[Pu] = 0.33, [HNO3]A = 3.8 mol/L.
Peaks under dashed lines topped by squares correspond to Pu2(C2O4)3(H2O)6⋅4H2O, peaks under dashed
lines topped by triangles correspond to the unknown phase.
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Calcination of oxalates into oxides
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Spherical agglomerates of neodymium oxalate synthesized for [NTMP]/[Nd]=0.5
were converted into oxides by heating at 800 °C in air. During this thermal treatment,
spherical morphology of the oxalate precursor was kept (Figure 13). At the same time, the
specific area of the neodymium oxide increased from 2.5 m2/g for the rodꢀlike particles
(obtained from oxalate synthesized without NTMP) to 12 m2/g for the spherical particles.
In parallel, TG analyses were performed on this sample (Figure 13). Its thermal
decomposition exhibits one main difference with the referenceꢀsample (without NTMP), i.e. a
higher initial dehydration. This can be explained by the presence of the dodecahydrate phase
Nd2(C2O4)3(H2O)6⋅12H2O which contains a higher amount of nonꢀcoordinated water
molecules than Nd2(C2O4)3(H2O)6⋅4H2O. The second step of decomposition is similar to the
one observed for the reference compound and corresponds to the departure of the oxalate
groups at around 400 °C, accompanied by the release of CO and CO2. In agreement with the
XRD analyses, Nd2O3 is accompanied with small amounts of Nd3PO7 which indicates that
phosphorous remains in the compound despite the thermal treatment (Figure II SI).
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Figure 13: TG analyses under air of neodymium powder precipitated for [NTMP]/[Nd] = 0.5 at 20 °C and SEM pictures
of the oxalate precipitate and the resulting oxide after thermal treatment.
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Concerning the Pu system, the thermal conversion into oxides was performed by
heating the precipitated oxalates, in air. The oxides XRD analyses indicate that PuO2 is
accompanied with small amounts of PuPO4, which confirms that as for neodymium system,
some amount of phosphorous remains in the oxides despite the thermal treatment (Figure III,
SI). As for neodymium, the spherical morphology of the plutonium oxalate synthesized with
[NTMP]/[Pu]=0.33 and [HNO3]A=3.8 M was kept during its thermal transformation into
oxide (Figure 14).
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Figure 14 : SEM analyses of plutonium oxalate synthesized with [NTMP]/[Pu] = 0.33 and a nitric acid
concentration of 3.8 M in solution A, at 25 °C, and of the oxide resulting from its calcination at 950 °C
under air.
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SUGGESTED MECHANISM OF CRYSTAL GROWTH IN PRESENCE OF
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NTMP
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Precipitation of sphere-like agglomerates of neodymium oxalate
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In regard of the reported results and a previous work on the template free synthesis of
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zinc oxide hollow spheres52, a growth mechanism of neodymium oxalate in presence of
NTMP is proposed. This envisaged mechanism is summarized Figure 15.
Firstly, when contacting the neodymium and NTMP, a NdꢀNTMP complex is formed.
This acts the most probably as a nanoreservoir of neodymium, which slowly releases its
neodymium content, when the solution is put into contact with oxalate. This initial complex
formation strongly modifies the Ndꢀoxalate nucleation step and leads to the formation of a Xꢀ
ray amorphous submicronic compound made of neodymium, phosphonate and oxalate which
composition evolves in contact of oxalate ions in solution to the dodecahydrate oxalate
Nd2(C2O4)3(H2O)6⋅12H2O then to the decahydrate oxalate Nd2(C2O4)3(H2O)6⋅4H2O. The input
of neodymium from the inside to the surface of the first nuclei and the subsequent
crystallizations of Nd2(C2O4)3(H2O)6⋅12H2O and Nd2(C2O4)3(H2O)6⋅4H2O are accompanied
by the formation of hollow spheres successively composed of an assembly of dots, hexagonal
platelets and, finally, thin rods with hexagonal section.
At first, the slow diffusion of neodymium may limit the kinetic of reaction and favor
the formation of small particles of Nd2(C2O4)3(H2O)6⋅12H2O which agglomerate into a
spherical structure. At this stage, the crystal growth of Nd2(C2O4)3(H2O)6⋅12H2O is confined
in the [001] direction and the particles are allowed to grow in two directions, leading to thin
hexagonal plates. This twoꢀdimensional growth, which leads to the strong anisotropy visible
on the XRD patterns, can be explained by the inhibition of the (001) faces by NTMP which
precludes the crystal growth in this direction.
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Once NTMP starts to be released, the hydrated oxalate is allowed to grow in the third
direction, and hexagonal plates give rise to rods with hexagonal section.
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When growth is favored by an increase of either the precipitation temperature or the
precipitation time, the rods would finally break away from the initial amorphous support.
However, even a small disturbance of the precipitation conditions (local supersaturations,
kinetic changes…) would preferentially lead to the crystallization of the most stable phase,
that is to say the decahydrate Nd2(C2O4)3(H2O)6⋅4H2O. Even if no evidence supports strongly
this interpretation, it provides the advantage of suggesting a mechanism in quite good
agreement with the experimental results concerning the morphology of the precipitated
compounds.
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Figure 15: Proposed mechanism for the growth of hollow spheres of Nd2(C2O4)3(H2O)6·12H2O in presence of NTMP
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Precipitation of sphere-like agglomerates of plutonium oxalate
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The thorough study on the neodymium surrogate system enabled to propose a
mechanism based on the same principle for spherical plutonium oxalate precipitation. The
complexation of plutonium by NTMP in solution A, prior to the oxalic precipitation, would
enable the formation of an amorphous compound with oxalic acid and could then delay the
release of plutonium during the oxalic precipitation. However, if the dodecahydrate phase
Nd2(C2O4)3(H2O)6⋅12H2O was obtained with neodymium, for plutonium system, an unknown
monoclinic phase with parameters a=17,658(3) Å, b=9,862(2) Å, c=8,623(1) Å, β=112,48(1)°
was precipitated. The appearance of this unknown monoclinic phase could be favored by the
slow release of plutonium in presence of NTMP but an additional explanation can be
proposed since the plutonium system is more complex than the neodymium one. Indeed, the
tetravalent form of plutonium is stable in nitric acid medium whereas the stabilization of
trivalent plutonium requires the presence of hydrazinium ions. These monovalent cations
might be involved, with plutonium and oxalate, in the precipitation of the original monoclinic
compound.
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Also, the complexation of tetravalent elements by NTMP is very strong, it is therefore
possible that the presence of small amounts of Pu(IV) in solution leads to the formation of
insoluble NTMPꢀPu precipitates which would explain the small particles observed in the final
solid.
Finally, the plutonium system is more sensitive to supersaturation and temperature or
acidity variations, which could explain that Pu2(C2O4)3(H2O)6⋅4H2O precipitation is favored
and that the intermediate stage of crystallization is hard to observe.
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CONCLUSION
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In this study, we firstly managed to modify neodymium oxalate structure and
morphology by adding a phosphonate, NTMP, into the precipitation medium. Depending
upon the NTMP/Nd ratio and/or aging different crystallinity, morphology, particles size
distributions of precursor oxides were found starting from submicronic particles to spherical
agglomerates composed of dots, hexagonal platelets or thin hexagonal rods instead of the rodꢀ
like particles usually obtained in absence of additive. The formation of hollow sphere can be
explained by a twoꢀsteps process of nucleation/growth involving the formation of complex as
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an intermediate which favors the formation of
a
metastable compound,
Nd2(C2O4)3(H2O)6⋅12H2O. This approach of solution phase synthesis of oxide precursor is
quite interesting in that the morphology obtained the precipitation stage is kept during the
calcination process.
Despite some differences in the complexation with NTMP and complexity from redox,
the use of NTMP was successfully transposed to the plutonium system. Spherical
agglomerates of plutonium(III) oxalate were so obtained, leading to spherical agglomerates of
PuO2 plutonium oxide after thermal treatment. The PuO2 squared particles commonly
synthetized by oxalic route are replaced by spherical agglomerates. Then, varying the
conditions of oxalic precipitation, it is possible to obtain unusual morphologies of PuO2,
highlighting the flexibility of this conversion method.
ACKNOWLEDGEMENTS
The « Projet fédérateur NEEDS Matériaux ꢀ Nucléaire : Energie, Environnement, Déchets,
Société » is gratefully acknowledged for its funding participation.
Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche,
Région Nord – Pas de Calais and FEDER are acknowledged for supporting and funding
partially this work.
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Finally, the authors would like to thank the French ASTUTE ANR project no. ANRꢀ15ꢀ
CE08ꢀ0011ꢀ01 for financial support.
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SUPPORTING INFORMATION
The XꢀRay fluorescence analysis of the precipitate versus the aging time and XRD powder patterns of the Nd
and Pu oxides are available in the supporting information. This information is available free of charge via the
Internet at http://pubs.acs.org.
REFERENCES
(1) Dollimore D., “The thermal decomposition of oxalates. A review”,Thermochim. Acta,
1987, 117, 331ꢀ363.
(2) Sarsfield M. J., Reprocessing and Recycling of Spent Nuclear Fuel, 2015, edited by Robin
Taylor, Woodhead publishing series in nuclear energy Nr. 79, 2015, chapter 13
(3) Abraham F., ArabꢀChapelet B., Rivenet M., Tamain C., Grandjean S., “Actinide oxalates,
solid state structures and applications”, Coord.. Chem. Rev., 2014, 266-267, 28ꢀ68.
(4) Grandjean S., Bérès A., Rousselle J., Maillard C., Patent WO 2005 119699.
(5) MachuronꢀMandard X., Madic C., “Plutonium dioxide particle properties as a function of
calcination temperature”, J. Alloys Compd., 1996, 235, 216
(6) De Almeida L., Grandjean S., Vigier N., Patisson F., “Insights into the Thermal
Decomposition of Lanthanide(III) and Actinide(III) Oxalates – from Neodymium and Cerium
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From Nd(III) and Pu(III) Oxalates to Oxides:
Influence of NitriloTris(MethylenePhosphonic
acid) on Chemical Composition, Structure and
Morphology.
AnneꢀLise Vitarta,b, Blaise Haidona,b, Benedicte ArabꢀChapeleta, Murielle Rivenetb, Isabelle
Bisela, Patrick. Pochonc, Pascal Rousselb, Stéphane Grandjeand, Francis Abrahamb
SYSNOPSIS TOC
The chemical composition, structure and morphology of neodymium and plutonium oxalates
have been modified by adding a complexing agent (NTMP) in the precipitation medium. The
Nd/Pu complexation has been evidence by UVꢀvisible spectroscopy and leads to sphereꢀlike
agglomerates precipitation instead of rodꢀlike particles.
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