Selective separation of americium from europium using 2,9-bis(triazine)-1,10-phenanthrolines in ionic liquids: a new twist on an old story

Article abstract of DOI:10.1039/C6CC09823A

Bis-triazine phenanthrolines have shown great promise for f-block metal separations, attributable to their highly preorganized structure, nitrogen donors, and more enhanced covalent bonding with actinides over lanthanides. However, their limited solubility in traditional solvents remains a technological bottleneck. Herein we report our recent work using a simple 2,9-bis(triazine)-1,10-phenanthroline (Me-BTPhen) dissolved in an ionic liquid (IL), demonstrating the efficacy of IL extraction systems for the selective separation of americium from europium, achieving separation factors in excess of 7500 and selectively removing up to 99% of the americium. Characterization of the coordination environment was performed using a combination of X-ray absorption fine structure spectroscopy (XAFS) and density functional theory (DFT) calculations.

Full text of DOI:10.1039/C6CC09823A

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Reactivity differences of Sc
first methanofullerene derivatives of Sc
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DOI: 10.1039/C6CC09823A
Journal Name
COMMUNICATION
Selective Separation of Americium from Europium Using 2,9-
Bis(triazine)-1,10-phenanthrolines in Ionic Liquids: A New Twist
on an Old Story
Received 00th January 20xx,
Accepted 00th January 20xx
ab†
a†
b
c*
b*
Neil J. Williams, Jérémy Dehaudt, Vyacheslav S. Bryantsev, Huimin Luo, Carter W. Abney,
and Sheng Dai
DOI: 10.1039/x0xx00000x
ab*
www.rsc.org/
Bis-triazine phenanthrolines have shown great promise for plants has nevertheless afforded dramatic environmental and
f-block metal separations, attributable to their highly
preorganized structure, nitrogen donors, and more enhanced
covalent bonding with actinides over lanthanides. However, their
limited solubility in traditional solvents remains a technological
bottleneck. Herein we report our recent work using a simple 2,9-
bis(triazine)-1,10-phenanthroline (Me-BTPhen) dissolved in an
public health benefits; over the past three decades, nuclear power
has prevented the generation of 64 Gt CO -equivalent
2
greenhouse gases and more than 1.84 million air pollution-
related deaths.4, 5
One major criticism of nuclear power is the generation of
ionic liquid (IL), demonstrating the efficacy of IL extraction spent nuclear fuel, for which few (if any) long-term disposal
systems for the selective separation of americium from europium,
achieving separation factors in excess of 7500 and selectively
removing up to 99% of the americium. Characterization of the
economics. Efficient separation of fission products and other
coordination environment was performed using a combination of
X-ray absorption fine structure spectroscopy (XAFS) and density
solutions are available, and the volume of which should be
minimized for the sake of safety, proliferation resistance, and
spent fuel constituents is necessary to enable various disposal or
recycle options. The minor actinides (An(III)), such as
functional theory (DFT) calculations.
growing global population combined with rapid americium (Am(III)), are an example of such spent nuclear fuel
A
6
development of emerging economies and a universal desire for constituents. A long-lifetime alpha emitter, Am(III) contributes
improved standards of living drives an increasing demand for significantly to the total radiation of spent nuclear fuel, but is also
clean, renewable, and affordable sources of energy.1, 2 Although a potential fuel source for next generation fuel cycles.7
great effort has been devoted toward the development and Unfortunately, the selective recovery of Am(III) is challenging
deployment of renewable energy sources, e.g. solar and wind due to possessing similar chemical reactivity and physical
power, such technologies are inherently intermittent, requiring properties as lanthanides (Ln(III)). Due to their large neutron
either extensive over-building to account for day-to-day cross sectional areas, contamination with Ln(III) would prohibit
variabilities, or installation of infrastructure to store power for the recycle of Am(III) for future nuclear power generation.
increased delivery during times of high demand.3 In contrast,
The discovery and comparatively recent development of
nuclear energy remains the only mature, carbon neutral bis(5,6-dimethyltriazine)-pyridines (Me-BTPs),8-11 shown in
technology capable of sustained base-load power generation. Figure 1, dramatically improved the selectivity for minor
Although comprising only 15% of the global electrical power actinides over other fission products. It was determined12 that
4
production portfolio, their use in place of coal-fired power highly preorganized ligands possessing electron-donating
substitutents on a rigid 1,10-phenathroline backbone greatly
increased recovery of Am(III) from acidic waste media.13-17
a.
Department of Chemistry, University of Tennessee, Knoxville, TN 37916, USA
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN
Unfortunately,
the
2,9-bis(5,6-dimethyltriazine)-1,10-
b.
3
7831,USA. E-mail:; abneycw@ornl.gov; dais@ornl.gov
phenanthrolines (BTPhens) are insoluble in the traditional
organic solvents utilized in separations processes, requiring the
addition of alcohol modifiers or use of expensive fluorinated
c.
Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak
Ridge, TN 37831, USA, E-mail: luoh@ornl.gov
solvents.12, 13 While more synthetically complex BTPhens have
been made in an attempt to improve solubility,12, 18 only modest
improvements in performance have been achieved in common
organic solvents due to the disfavored energetics of forming
†
‡
These authors contributed equally on this manuscript
Electronic Supplementary Information (ESI) available: materials and synthetic
methods; determination of distribution ratios, slope analysis, nitrate concentration;
computational methods; X-ray absorption fine structure spectroscopy data
collection and processing, principal component analysis EXAFS fitting models;
Cartesian coordinates for calculated structure models. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Journal Name
When [C
found to be nearly complete at
the amount of Eu removed was
4 2
mim][NTf ] was used, the recovery of Am(III) was
View Article Online
≥
99.0 % re
D
m
O
o
I:
v
1
a
0
l
.1
a
0
t
3
p
9/
H
C6
=
C
1
C
,
0
w
98
hile
2
3
A
≤
1.19 %. This affords a
separation factor greater than 7500, which exceeds the next best
separation factor reported with triazine ligand by 2.5×.26 When
the concentration of the nitric acid increased, separation factors
decreased drastically due to marked suppression of the amount
of Am(III) extracted, while the uptake of Eu(III) increased
Figure 1. Structures of bis-triazine heterocycles.
highly charged metal-ligand complexes in nonpolar solvents. slightly. This result is most likely due to the increasing ionic
Preservation of charge balance either requires the undesirable co- strength of the aqueous phase which retards the cation-exchange
extraction of counterions or addition of an organic-soluble cation mechanism and thus prevents charge balance in the organic
exchanger. While BTPhen ligands display vast potential for solution.28, 29 The effective distribution ratio (D) of Am(III) and
Am(III)/Ln(III) separations, there remains tremendous need for Eu(III), corresponding separation factors (SF), and percent Am
fundamental breakthroughs in how they can be practically and and Eu extracted are listed in Table 1.
efficiently deployed.
One way to alleviate the problems discussed above is to
substitute ionic liquids (ILs) in place of traditional molecular
solvents. ILs have a proven track record in metal ion
separations19-21 and are capable of dissolving otherwise insoluble
compounds,22, 23 can readily accommodate highly charged metal-
[
HNO
3
]
D
Am
D
Eu
SFAm/Eu
%Am
%Eu
0
0
1
.1
.5
.0
94.9
3.25
1.07
0.0120 7857.1
0.132
0.152
99.0
1.19
24.6
7.0
76.47
51.57
11.67
13.21
Table 1. Distribution ratios (D), separation factors (SF), and percent
ligand complexes, and can achieve charge balance through recovery of Am and Eu by 4 mM Me-BTPhen for various nitric acid
concentrations.
exchange of cationic imidazolium moieties into the aqueous
phase.24, 25 Earlier studies have reported BTPs dissolved in ILs
Additional experiments were performed to investigate the
structure of the ligand-metal complexes formed upon extraction
into [C mim][NTf ], in an effort to rationalize the remarkably
4 2
can achieve remarkable Am(III)/Ln(III) separation factors
(
SFAm/Eu > 3000),26, 27suggesting that if ILs could serve as a
solubilizing system for BTPhens, significant improvements in
separation performance could be achieved. Herein, we report the
efficacy of Me-BTPhen dissolved in ILs for the separation of
Am(III) from Eu(III) in nitric acid media, as well as an
investigation of the resulting metal-ligand complex through
application of X-ray absorption fine structure (XAFS)
spectroscopy and density functional theory (DFT) calculations.
high efficiency for Am removal. Slope analysis was applied to
determine the stoichiometry of ligand-metal complex. The
ligation of the metal species can be determined from the slope of
the line in the plots, revealing the Me-BTPhen forms a 2-to-1
complex (Figure S3), as reported previously in the literature.27, 30
While clearly demonstrating the number of ligands bound to each
metal, there remained uncertainty as to whether the IL solvent
could also be participating in the extraction through direct
interaction with the metal cation in the inner coordination sphere.
In an effort to determine the complete structure of the complex
in solution, we applied high level DFT calculations
complemented by XAFS spectroscopy.
Eu(III) was chosen as an Am(III) surrogate for XAFS
investigations due to possessing similar size and chemical
reactivity while not presenting a radiological concern. DFT
calculations were performed on both Eu(III) and Am(III)
complexes with Me-BTPhen using the Gaussian 09.31 The
2,9-bis(5,6-dimethyl-1,2,4-triazin-3-yl)-1,10-phenanthroline
(
Me-BTPhen) was made following previously reported synthetic
methods.12, 13 Liquid-liquid extraction studies were done using
the radioisotopes 241Am(III) and 152/154Eu(III) to track and
quantify the removal of each cation by the Me-BTPhen.
Procedures for both synthesis and extraction experiments are
provided in the Supporting Information (SI).‡
The performance of Me-BTPhen for An(III)/Ln(III)
separation was investigated in three different solvents:
chloroform (CHCl
FS-13), and
bis(trifluoromethylsulfonyl)imide (hereafter referred to as
mim][NTf ]). Although Me-BTPhen dissolved into each
solvent at 4 mM, when mixed with the nitric acid solution only
the [C mim][NTf ] system was able to achieve an effective
extraction of the radioisotopes. In CHCl , the Me-BTPhen would
3
), trifluoromethylphenyl sulfone (known as
the IL 1-butyl-3-methylimidazolium
potential Eu(III) and Am(III) complexes investigated consisted
of 2 Me-BTPhen molecules and NO -
, NTf -
, H
O, OH , or no
-
3
2
2
[
C
4
2
anion in the first coordination shell of the metal ion. A figure
displaying the geometrically optimized structures of the
corresponding complexes with Eu(III) is given in Figure 2.
The coordination environment of the Eu-Me-BTPhen
4
2
3
first complex then subsequently partition back into the aqueous
phase. In FS-13 the ligands would complex and then precipitate
at the aqueous-organic interface. The poor performance of Me-
BTPhen in these molecular solvents is most likely due inability
to achieve charge balance in the organic phase without extraction
of three nitrate anions, and the poor solubility of the resulting
highly polar metal-ligand-nitrate complex in the non-polar
solvent.
complex in [C
spectroscopy. XAFS data were collected at the Eu L(III)-edge
6977 eV) on beamline 11-2 at Stanford Synchrotron Radiation
4 2
mim][NTf ] was also investigated through XAFS
(
Lightsource, Data were processed and analyzed using open
source fitting software.32-34 Further details regarding data
collection and processing are provided in the SI.
Principal component analysis of the nine normalized
2
| J. Name., 2012, 00, 1-3
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Journal Name
COMMUNICATION
+
3+
exchange of 3 cationic [C
of a Eu(NO
be resolved with a monodentate-bound NTf -
putatively coordinating ligand are unlikely to accommodate the
short interatomic distance necessary. Importantly,
4
mim] per Eu rather than extraction
View Article Online
D
r
O
osI:
c
1
o
0
p
.1
i
0
c
39fe/C
a
6
tu
C
r
C
e
0
c
9
ould
82
3
A
3
)
3
complex. While the spect
2
, the sterics of this
a
deprotonated [Eu(Me-BTPhen)(OH)]2+ model system also failed
to provide an adequate fit of the data due to the commensurate
lengthening of the first shell bond lengths for the Me-BTPhen
ligand (Figure S9). The aforementioned discarded fits are
displayed in the SI, accompanied by their corresponding DFT-
based model (Figures S7-S10).
In contrast to complexes displaying inner sphere interactions
between Eu and an anion, the comparatively simple 1:2 [Eu(Me-
O)]3+ complex afforded a good fit of the EXAFS
2 2
BTPhen) (H
data in both first and second coordination spheres (Figure 3), as
well as with regards to reasonable fitting parameters (Table 2).
Efforts were made to improve the goodness of fit through
inclusion of more multiple scattering paths, as well as through
Figure 2. Geometry optimized Eu-Me-BTPhen complexes obtained
from DFT calculations. The name of the complex is provided below
the corresponding structure. Eu is turquoise, C grey, O red, S
yellow, and H white.
addition of a separate σ2 for the tightly coordinating H
O.
2
absorption datasets resulted in identification of only one
mathematical component at the > 99.9% confidence level
Figure S4). This statistical approach suggests there is only one
However, application of the F-test revealed the apparent gains
were not statistically significant.35-37 Expansion of the data range
and inclusion of distant scatterers was also attempted in an effort
to fit the feature at 4 Å, but was ultimately unsuccessful. The
possibility of a dimeric complex was also considered by
investigating a model generated from a crystal structure
(
solution component contributing to the spectral response. DFT-
optimized complexes (Figure 2) were used to prepare structure
models for fitting to the EXAFS data. Reasonable preliminary
3
+
2
fits were obtained for [Eu(Me-BTPhen) ] , [Eu(Me-
2 2 2
containing Eu Phen (OH) , but the Eu-Eu scattering path did not
3
+
2+
BTPhen)
monodentate [Eu(Me-BTPhen)
spectra collected on [Eu(CyMeBTPhen)
2
(H
2
O)] ,
[Eu(Me-BTPhen)
2
(NO
)] . Similar to EXAFS
(H O)](NO crystals
3
)] ,
and
align well with the feature at 4 Å and had a deleterious effect. It
is expected multiple scattering paths and scattering from distant
atoms on Me-BTPhen ligands are convoluted with those from
anions in the second coordination sphere, making definitive
refinement of these features particularly challenging.
2
+
2
(NTf
2
2
2
3 3
)
or the solvated compound in cyclohexanone,27 a shoulder is
apparent at 1.5 Å in the Fourier transformed data, albeit more
pronounced for the IL spectrum, and best resolved by inclusion
of a tightly coordinating water molecule. Similar spectral
contributions cannot be reasonably achieved with a chelating
2
-3
2
Path
Coord. No. Bond
Length (Å)
σ (×10 Å )
NO -
due to bond length considerations (Figure S8). This is
3
Eu→OH
2
1
4
4
4
4
8
1.93 ± 0.02 4.9 ± 0.09
2.53 ± 0.02 4.9 ± 0.09
2.55 ± 0.01 4.9 ± 0.09
3.39 ± 0.02 2 ± 0.1
3.43 ± 0.01 2 ± 0.1
3.44 ± 0.01 2 ± 0.1
-
further supported by a constant NO
3
concentration post-
Eu→Nphen
Eu→Nazine(1)
Eu→Cphen
extraction, also indicating charge balance is achieved by
Eu→Nazine(2)
Eu→Cazine
Eu→Nphen→Cphen 24
3.64 ± 0.02 7 ± 0.1
2
E
0
= 9.5 ± 0.8
R = 0.96%
χ
ν
= 32.7
3
+
Table 2. Data for EXAFS fit with [Eu(Me-BTPhen)
Model.
2 2
(H O)] Structure
Based on the structural information obtained from DFT and
EXAFS, we can speculate on the possible origin of enhanced
Am(III) over Eu(III) selectivity in the IL solvent compared to
traditional organic solvents. As nitrate anions are extracted
together with the trivalent metal ions into the organic phase,
present DFT calculations and crystallographic evidence27
suggest that one nitrate can enter the inner sphere of the complex
and adopt a chelate coordination mode. Substitution of bidentate
Figure 3 Fourier transform of the Eu LIII-edge EXAFS spectrum in
2 4 2
nitrate in organic solvent by H O in [C mim][NTf ] leads to a
R-space (black line), with accompanying fit afforded by the
3
+
[
Eu(BTPhen)
2
(H
2
O)] model. The imaginary component (grey
shorter average metal ion-ligand bond distance and a stronger
ligand binding (Table S1), which could in turn lead to a higher
selectivity for Am(III) over Eu(III).
line) and fit are offset beneath. Grey dashed lines denote the fit
window.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
8
.
Y. Wei, K. N. Sabharwal, M. Kumagai, TV.ieAw sAartkiculeraO,nliGne.
Conclusions
Uchiyama and S. Fujine, Journal of Nuclear Science and
DOI: 10.1039/C6CC09823A
In conclusion, we report the remarkable capability of a
Technology, 2000, 37, 1108-1110.
simple Me-BTPhen extractant to achieve highly efficient 9.
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M. Hudson, M. Foreman, C. Hill, N. Huet and C. Madic,
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complex upon extraction into the IL phase, while fitting of the
EXAFS spectrum confirmed a coordination number of 9 and
supports the non-interaction of the anion in the first coordination
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sphere.
Implementation of this technology in the processing
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5.
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1
7.
term storage.
2
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insightful review of this manuscript. This research was supported
financially by the Division of Chemical Sciences, Geosciences, and
Biosciences, Office of Basic Energy Sciences, U.S. Department of
Energy. This research used resources of the National Energy Research
Scientific Computing Center, a DOE Office of Science User Facility
supported by the Office of Science of the U.S. Department of Energy
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under Contract No. DE-AC05-00OR22725 with the U.S. Department of
Energy. The United States Government retains and the publisher, by
accepting the article for publication, acknowledges that the United States
6
Government retains a non-exclusive, paid-up, irrevocable, worldwide 25.
license to publish or reproduce the published form of this manuscript, or
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29, 149-151.
4
| J. Name., 2012, 00, 1-3
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