Isolation of molybdenum(vi) from simulated leachates of irradiated uranium-aluminum targets using diluted and undiluted sulfate ionic liquids

Article abstract of DOI:10.1039/c9gc01626k

A method is presented to separate molybdenum from other elements commonly present in oxidative alkaline leachates of irradiated uranium-aluminum targets for the production of molybdenum-99. The separation was accomplished by selective extraction of molybdate anions using triazolium and phosphonium sulfate ionic liquid extractants, either diluted in 1-octanol or undiluted. Molybdenum was then stripped from the organic phase using a sodium hydrogen carbonate solution. The extractant was regenerated by contacting the organic phase with an alkaline sulfate solution. The extraction mechanism and the influence of the diluent on the extractant performance were investigated. The reported method provides a promising alternative to state-of-the-art chromatographic processes, showing potential for limiting the production of radioactive waste.

Full text of DOI:10.1039/c9gc01626k

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DOI: 10.1039/C9GC01626K
Manuscript for: Green Chemistry
Isolation of molybdenum(VI) from simulated leachates of irradiated uranium-aluminum
targets using diluted and undiluted sulfate ionic liquids
†
†
†
Stijn Raiguel , Wim Dehaen , Koen Binnemans *
†
KU Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. box 2404, B-3001
Leuven, Belgium.
*
Corresponding author:
Email: Koen.Binnemans@kuleuven.be
1

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DOI: 10.1039/C9GC01626K
Abstract
A method is presented to separate molybdenum from other elements commonly present in
oxidative alkaline leachates of irradiated uranium-aluminum targets for the production of
molybdenum-99. The separation was accomplished by selective extraction of molybdate anions
using triazolium and phosphonium sulfate ionic liquid extractants, either diluted in 1-octanol
or undiluted. Molybdenum was then stripped from the organic phase using a sodium hydrogen
carbonate solution. The extractant was regenerated by contacting the organic phase with an
alkaline sulfate solution. The extraction mechanism and the influence of the diluent on the
extractant performance were investigated. The reported method provides a promising
alternative to state-of-the-art chromatographic processes, showing potential for limiting the
production of radioactive waste.
Keywords: green solvents; ionic liquids; hydrometallurgy; molybdenum; solvent extraction;
technetium
2

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DOI: 10.1039/C9GC01626K
Introduction
To date, 99mTc is by far the most important diagnostic radionuclide in medicine, used in over
0 million procedures yearly.1–3 It is particularly useful due to its short half-life of 6 hours, ideal
3
-ray emission energy and rapid expulsion from the body, limiting the radiation dose to which
the patient is exposed.2–4 One major drawback associated with this short half-life is the
difficulty of delivering this radionuclide to hospitals, as significant fractions of it will have
9
9
decayed to its isomeric Tc daughter nuclide before a batch even reaches its destination.
9
9
Therefore, batches are transported to hospitals as its parent nuclide, Mo, which has a much
longer half-life of 66 hours.3,4 The radiomolybdenum, present as MoO , is adsorbed to an
99
2-
4
alumina column, which serves as a portable technetium generator. 99mTcO can be eluted from
-
4
9
9
2,3
this generator with physiological saline solution as it is formed by the decay of Mo.
Currently, most radiomolybdenum is produced from irradiated, highly or low enriched uranium
1
–
aluminum alloy (UAl ) targets. As fission of uranium produces a wide array of elements as
x
9
9
fission products, a rigorous purification process is required to isolate highly pure Mo from
9
9
this mixture. Production centers involved in Mo purification each have a unique approach to
the problem, of which the details are often kept highly secret. Most commonly, the purification
occurs in acidic solutions. At Chalk River Laboratories (CRL) in Canada, formerly the largest
production site worldwide for radiomolybdenum, dissolution of the targets in nitric acid was
5
,6
followed by a chromatographic purification on an alumina column. An acid digestion is also
employed at the Institute of Physics and Power Engineering (IPPE) in Russia, where the
digestion is followed by a solvent extraction and a subsequent chromatographic purification in
order to obtain the desired purity.6–8 The Indonesian BATAN Institute uses the legacy
CINTICHEM process for molybdenum purification, which consists of a dissolution step in
H SO /HNO , followed by precipitation of iodide as silver(I) iodide and the sequestration of
2
4
3
3

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molybdenum by precipitation with α-benzoin oxime. The precipitate is then re-dissolved in
DOI: 10.1039/C9GC01626K
sodium hydroxide and purified by chromatography.7,9–11
Other centers improve the selectivity of their leaching process by leaching under alkaline rather
than acidic conditions. At IRE (Belgium), the target is dissolved in NaOH/NaNO acidified
3
,
1
31
133
and degassed to remove and recover iodine ( I) and xenon ( Xe). The process further
comprises three chromatographic steps. The process employed by Mallinckrodt/ECN in the
Netherlands entails alkaline digestion in sodium hydroxide without acidification. The digestion
is followed by no fewer than five chromatographic steps. Similar strategies are also in use at
7
INVAP (Argentina) and ANSTO (Australia). AEC in South Africa also dissolves its targets in
sodium hydroxide with the addition of a proper oxidizing agent, followed by three
chromatographic steps.6
Several other procedures are described in literature and generally follow similar principles of
leaching followed by chromatography. Some authors have reported modifications of the
CINTICHEM process.10,12–15 Other authors have explored various column materials to treat
1
6–19
acidic UAl leachates.
Another strategy that has received some attention is the exploitation
x
of differences in the volatility of oxides and chlorides of the fission products. 20–23 Obviously,
this also allows for very efficient removal of highly volatile iodine and xenon.
Chromatography is the most frequently used method for purification of acidic digests of
9
9
7
irradiated UAl targets in Mo production. Chromatography enables the production of high-
x
purity samples from complex leachates, but consumes large quantities of chemicals and
solvents. These materials are contaminated with radioactive isotopes in the process, implying
that they must be treated as radioactive waste. The reagent consumption and waste production
can be mitigated by moving towards a solvent-extraction-based procedure. In order to obtain
pharmaceutical-grade purity of the final product, a highly selective solvent extraction procedure
4

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is necessary. The radiochemical purity requirements are stipulated in the European, British and
DOI: 10.1039/C9GC01626K
United States Pharmacopoeia and are expressed as activity ratios. Generally, radiochemical γ-
-5
24
impurities below 5·10 are required per specified nuclide.
The selectivity of the solvent extraction procedure can be aided by ensuring the leachate of the
irradiated targets has the simplest possible chemical composition. Therefore, it is desirable to
work with alkaline lixiviants, as uranium and many fission products (Ru, Zr, Nb, Ba,
lanthanides, Am and Pu) do not dissolve under these conditions.6,25,26 Furthermore, unreacted
235U is more easily recovered for reprocessing and recycling from alkaline solutions, as it
remains in the solid residue.
Promising results were previously obtained by Al-Janabi et al., who dissolved irradiated U O
3
8
-1
in 6 mol L sodium hydroxide, followed by acidification of the leachate with sulfuric acid and
extraction using di(2-ethylhexyl)phosphoric acid in hexane.27 Tellurium and iodine were
coextracted to significant extents, requiring a selective stripping of molybdenum using
ammonia solution. Tellurium could be quantitatively scrubbed from the organic phase using
brine, but iodine could not be removed. As such, the organic phase is not regenerated after the
extraction procedure. A further drawback of this technique is that the leachate must be acidified
prior to solvent extraction, resulting in a high consumption of acid required simply for the
neutralization of the base used for leaching.
9
9
Some solvent extraction procedures for Mo perform well but require unacceptably dangerous
reagents. Iqbal and Ejaz described a solvent extraction procedure comprising a solvent
extraction of a HCl/KSCN leachate of the irradiated uranium product using 2-hexylpyridine in
2
8
benzene, followed by stripping with 70% perchloric acid. While selectivity for molybdenum
over other metallic fission products and uranium was very high, nonmetallic fission products
were not monitored in this study and the usage of highly oxidizing, concentrated perchloric acid
5

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renders this procedure of little practical interest. A process that combines the technology of the
DOI: 10.1039/C9GC01626K
2
9
CINTICHEM process with solvent extraction was developed by Nair et al. Herein, ruthenium
and iodine were first volatilized. α-Benzoin oxime was dissolved in ethyl acetate and used to
extract molybdenum, which was then stripped by HCl/HF solution. The resulting final extract
was purified by chromatography over Dowex® 1×8 anion exchange resin. The usage of HF is
a major drawback of this procedure.
In this paper, we describe a method to isolate molybdenum from a simulated alkaline leachate
of irradiated UAl targets entirely via solvent extraction with sulfate ionic liquids as extractants.
x
Performing solvent extraction directly on an alkaline leachate is challenging. Metals are present
as anionic complexes in alkaline media. Hence, anion exchanging extractants are required.
These are generally quaternary ammonium or phosphonium salts, which are known to
decompose in contact with strong bases.30–33 Work by Lethesh et al. describes base-stable
quaternary ammonium salts with 2-ethylhexyl side chains to prevent the cation from
decomposing via the Hoffman elimination. However, the compounds were found to be very
3
2
hydrophilic, leading to losses of extractant to the aqueous phase. Some imidazolium salts also
exhibit good resistance against strong bases.34,35 We opted to investigate two extractants. The
first is a long-chain phosphonium extractant (trihexyltetradecylphosphonium sulfate), analogs
3
6
of which have been proven to be compatible with moderately strong alkalis. The second is an
extractant based on the 1,2,3-triazolium cation (4-ethyl-1,3-bis(2-ethylhexyl)-5-propyl-1,2,3-
triazolium sulfate), which can conveniently be prepared on a larger scale. A sulfate system was
chosen as the strongly hydrophilic sulfate anion can readily be exchanged for dianionic
2
-
molybdate (MoO ). We also demonstrate that the phosphonium extractant is suitable for use
4
without diluent, i.e. as an ionic liquid. The advantage of the latter approach is a mitigation of
the health and safety risks typically associated with the use of organic solvents, due to the
negligible room-temperature vapor pressure of ionic liquids.37,38
6

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DOI: 10.1039/C9GC01626K
Experimental
Chemicals
2
-Ethylhexylamine (99%), triethylamine (99%), 1-hexanol (98%), 1-octanol (99%), NaOD (40
wt% in D O, 99 atom% D), sodium periodate (99%), sodium aluminate (technical grade) and
2
sodium azide (99%) were purchased from Acros Organics (Geel, Belgium). Sodium dihydrogen
phosphate (anhydrous, 98%), heptane (mixture of isomers, 99%), 4-aminobenzoic acid (99%),
molybdenum standard solution (1000 ppm), aluminum standard solution (1000 ppm) and
molybdenum standard oil solution (1000 ppm) were purchased from Chem-Lab (Zedelgem,
Belgium). 4-Heptanone (99%) was purchased from Alfa Aesar (Karlsruhe, Germany).
Ammonia (25 wt% in water), sodium hydrogen carbonate (analytical reagent grade), ethyl
acetate (technical grade), toluene (reagent grade), magnesium sulfate (dried), sodium sulfate
(analytical reagent grade) and sodium hydroxide (analytical grade) were purchased from VWR
(
Oud-Heverlee, Belgium). Chloroform-d (CDCl , 99.8 atom% D), mesyl chloride (98%),
3
cesium hydroxide monohydrate (99.95%) and sodium molybdate dihydrate (99+ %) were
purchased from Sigma-Aldrich (Overijse, Belgium). Telluric acid was purchased from TCI
(
Zwijndrecht, Belgium). Cobalt standard oil solution (5000 ppm) was purchased from
PerkinElmer (Zaventem, Belgium). Lithium sulfate (high purity) was purchased from UCB
Brussels, Belgium). All reagents were used as received, without further purification.
(
Instrumentation
1H NMR spectra were recorded on a Bruker Avance 300 spectrometer operating at 300 MHz.
13C, 31
P and quantitative H NMR spectra were recorded on a Bruker Avance II 400
1
1
3
31
1
spectrometer operating at 100 MHz for C, 162 MHz for P and 400 MHz for H. Samples for
1
13
H and C NMR were diluted in CDCl and internally referenced to tetramethylsilane. Samples
3
3
1
95
for P NMR were diluted in CDCl and externally referenced to 10 wt% H PO in D O. Mo
3
3
4
2
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NMR spectra were recorded on a Bruker Avance 600 spectrometer equipped with a 10 mm
DOI: 10.1039/C9GC01626K
probe and operating at 39 MHz. Samples were measured undiluted and externally referenced to
-1
2
00 mg L Na MoO ·2H O in water. The Spinworks 4.2.8 and Mesternova 6.0.2 software
2 4 2
packages were used in the analysis of nonmetal and metal NMR spectra, respectively. A
9
5
polynomial baseline correction was applied to the Mo spectra.
FTIR spectra were recorded in ATR mode on a Bruker Vertex 70 infrared spectrometer
equipped with a platinum ATR module and a diamond sample crystal. Mass spectra were
recorded on a Finnigan LCQ Advantage electrospray ionization ion trap mass spectrometer.
Viscosities were measured on an Anton Paar Lovis 2000M/ME combined densitometer-rolling
ball viscometer. Densities of water-saturated ionic liquids were also measured using this device.
Densities of pure ionic liquids were measured by pycnometry due to their high viscosity. The
water content of organic samples was determined by Karl Fischer titrimetry, performed on a
Mettler-Toledo C30S coulometric titrator. To facilitate sample injection, a measured mass of
the organic sample was dissolved in 1 mL of dry DMSO before injection. A blank measurement
of 1 mL of dry DMSO was also recorded. The water content of the blank was then subtracted
from the measured water content of the sample solution in order to obtain the actual water
content of the organic sample. Water used for analysis was always of ultrapure quality
(resistivity 18.2 MΩ cm at 25 °C), provided by a Millipore Milli-Q Reference A+ system.
Quantitative NMR
1
The organic content of the aqueous phase was determined using quantitative H NMR. Spectra
were recorded on a Bruker Ascend 400 spectrometer operating at 400 MHz. A recycle delay
(d1) time of 3 s was applied for a total of 32 scans per sample. Samples were prepared by
contacting a 100 mg aliquot of the extractant with 1 mL of a D O sample with an appropriate
2
concentration of NaOD. Subsequently, 10 µL of methanol was added to 500 µL of the D O
2
solution as internal standard. The extractant concentration of the aqueous solution was
8

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DOI: 10.1039/C9GC01626K
determined from the relative integrals of the methanol CH signal and the upfield signals of the
3
hydrocarbon chains of the extractants. A phase and baseline correction was applied to obtain a
more accurate estimate of the organic content of dilute samples.
ICP-OES
Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was used for the analysis
of molybdenum and aluminum. Aqueous phase samples were prepared by diluting 100 µL of
the analyte solution to 10 mL with 2 vol% HNO in ultrapure water. Scandium standard solution
3
(1 ppm final concentration) was added as internal reference. Calibration curves for
molybdenum and aluminum were constructed using an external standard series. Analyses of
aqueous samples were carried out on a PerkinElmer Optima 8300 spectrometer equipped with
a GemTip Cross-Flow II nebulizer, Scott double-pass spray chamber, alumina injector and
PerkinElmer Hybrid XLT ceramic-quartz torch. The radial viewing mode was used for all
measurements. The line at 202.031 nm was used for the determination of molybdenum, while
the line at 396.152 nm was used for aluminum. These wavelengths were selected on their
performance in the measurement of samples with known concentrations. Samples were
measured in triplicate.
Samples of the organic phase were prepared by diluting a 50 µL aliquot of the organic phase to
5
mL with 1-octanol. Cobalt standard oil solution (10 ppm final concentration) was added as
internal reference. An external standard series, prepared using molybdenum standard oil
solution, was used to construct the calibration curves. Organic samples were analyzed using a
PerkinElmer Avio 500 spectrometer equipped with a GemCone Low-Flow Nebulizer, baffled
cyclonic spray chamber, alumina injector and PerkinElmer Hybrid XLT Organics torch. The
line at 202.031 nm was measured in radial viewing mode for the determination of molybdenum.
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This wavelength was selected on its performance in the measurement of samples with known
DOI: 10.1039/C9GC01626K
concentrations. Samples were prepared in duplicate, each measured in triplicate.
TXRF
Iodine cannot be reliably quantized via ICP-OES devices due to its low emission intensity,
while the speciation of cesium in ICP-OES devices is too variable to allow accurate
determination. Therefore, these elements were determined using total reflection X-ray
fluorescence (TXRF) spectroscopy on a Bruker S2 Picofox benchtop TXRF spectrometer.
Samples for TXRF were prepared by first diluting 30 µL of the solution to be analyzed to 1000
µL with a solution containing Triton® X-100 (5 vol%) and HNO (5 vol%) in ultrapure water.
3
Cerium standard solution (30 ppm Ce as final concentration) was added as internal standard.
Triton® X-100 ensures a homogeneous sample distribution after drying (vide infra) while
HNO prevents base-induced precipitation of the Ce standard.
3
A 2 µL aliquot of the sample solution was pipetted onto a quartz carrier, previously pretreated
with a silicone solution in isopropanol (Serva electrophoresis, Heidelberg, Germany) to prevent
spreading of the droplet. The carrier was then dried for 30 min at 60 °C and subsequently
analyzed with the X-ray source operating at 50 kV and 600 µA. Samples were measured for
1
200 seconds each. Samples were prepared and analyzed in triplicate, to account for possible
inhomogeneity in the sample morphology. Relative standard deviations on the calculated
concentrations were generally about 110%, depending on the salt concentration of the sample
and its concentration (higher relative standard deviations mostly at high percentages extraction,
when the raffinate is very dilute).
Solvent extraction
Appropriate dilutions of the extractants in the required diluent were made. The organic phase
was presaturated with a blank NaOH solution by shaking vigorously and subsequently
1
0

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centrifuging to disengage the two phases. The employed NaOH concentration was identical to
DOI: 10.1039/C9GC01626K
that of the aqueous phase used for solvent extraction. A 500 µL aliquot of the organic layer was
taken and combined with 1.000 mL of the aqueous feed solution. The vial containing both
phases was then shaken for 30 minutes using an Allsheng TMS-300 Thermo Shaker. The vials
were shaken in the vertical position at 30 °C and 3000 rpm. After shaking, phase disengagement
was expedited by centrifuging using a Heraeus Labofuge 200. Samples of the aqueous phase
were used for further analysis.
Using concentration data for the aqueous phase before and after extraction, the percentage
extraction was calculated using Eq. 1:
푐푎푞, 푖 ― 푐푎푞, 푓
%
퐸 =
·100%
(1)
푐푎푞, 푖
The distribution ratio was calculated using Eq. 2:
푐표푟푔, 푓
퐷 = 푐푎푞, 푓
(푐푎푞, 푖 ― 푐푎푞, 푓)·푉푎푞
=
(2)
푐푎푞, 푓·푉표푟푔
Where corg and c indicate organic and aqueous phase concentrations, respectively. Subscripts
aq
i and f respectively denote initial (before extraction) and final (after extraction) values. Vorg and
Vaq are the organic and aqueous phase volumes.
Stripping
The loaded organic phase used in stripping experiments was prepared by contacting 2.00 mL
of the extractant or an appropriate dilution thereof with 4.00 mL of a solution containing
-1
Na MoO (1000 ppm Mo) and NaOH (2.00 mol L ). The samples were shaken for 30 minutes
2
4
in centrifuge tubes using a Burrell Scientific Wrist-Action Shaker. Phase separation was
expedited by centrifuging using a Heraeus Labofuge 200.
-1
A stripping liquor solution was prepared by dissolving 1.00 mol L sodium hydrogen carbonate
in ultrapure water. A 500 µL aliquot of the loaded organic phase was then contacted with 2.00
1
1

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mL of the stripping liquor for 30 minutes at 30 °C in an Allsheng TMS-300 Thermo Shaker,
DOI: 10.1039/C9GC01626K
operating at 3000 rpm. Phase separation was expedited using a Heraeus Labofuge 200. The
organic phase before and after stripping was analyzed by ICP-OES.
The percentage stripping was calculated according to Eq. 3:
푐표푟푔, 푖 푉표푟푔,푖 ― 푐표푟푔, 푓푉표푟푔,푓
%
푆 =
·100%
(3)
푐표푟푔, 푖푉표푟푔,푖
The final phase volume was determined by measurement of the density and the water content
of the organic phase before and after stripping (see ESI page S15). When diluted extractants
were used, phase volume changes upon stripping were negligible, allowing the simplified Eq.
4
to be used.
푐표푟푔, 푖 ― 푐표푟푔, 푓
푐표푟푔, 푖
%
푆 =
·100%
(4)
The distribution ratio under stripping conditions is calculated according to Eq. 2.
Results and discussion
Synthesis and characterization of the ionic liquid extractants
Trihexyltetradecylphosphonium sulfate, [P66614]2[SO ], was prepared from commercial
4
Cyphos® IL 101 by an anion metathesis reaction with aqueous NaHSO followed by treatment
4
with aqueous NaOH. The product is a deliquescent solid at room temperature which melts into
a highly viscous liquid in the range of 2834 °C.
4
-Ethyl-1,3-bis(2-ethylhexyl)-5-propyl-1,2,3-triazolium sulfate, [EhEhT ] [SO ], was
23 2 4
3
9
synthesized using a method reported previously. This method entailed the synthesis of the
precursor, 4-ethyl-1-(2-ethylhexyl)-5-propyl-1,2,3-triazole, from 4-heptanone, 2-
ethylhexylamine and 4-azidobenzoic acid. Subsequently, this precursor was quaternized using
-ethylhexyl methanesulfonate. The methanesulfonate salt was then converted to the
2
1
2

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DOI: 10.1039/C9GC01626K
corresponding sulfate salt by an anion metathesis reaction with aqueous NaHSO , followed by
4
treatment with an aqueous NaOH solution. Full synthetic procedures and characterization data
of both extractants are provided in the Electronic Supplementary Information (page S2-S4).
The structures of both extractants are shown in Fig. 1.
P
P
SO42-
[
P6 ] [SO ]
6614 2 4
N
N
N
N
SO42-
N
N
[
EhEhT ] [SO ]
23 2 4
Fig. 1: Structures of the extractants used in this work.
The physicochemical properties of the extractants were determined and are summarized in
Table 1. Both compounds melt well below 100 °C and can thus be considered ionic liquids.37,38
[
P6 ] [SO ] melts above room temperature, in the range of 2834 °C. By contrast,
6614 2
4
[
EhEhT ] [SO ] remains liquid even after prolonged storage at room temperature. This
2
3 2
4
reduction in melting point is presumed to result from the high degree of charge delocalization
in the triazolium cation. Whilst the high charge of the sulfate dianion causes the dry ionic liquids
to be very viscous, it also induces the uptake of large amounts of water in solvent extraction
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systems, reducing their viscosity to more practical levels. When saturated with water, both
DOI: 10.1039/C9GC01626K
compounds remained liquid at room temperature, allowing them to be used in diluent-free
extraction systems. The viscosity of [EhEhT ] [SO ] is markedly lower than that of the
2
3 2
4
phosphonium ionic liquid, once again a presumed result of the charge delocalization of the
aromatic cation.
The mutual miscibility of both phases is an important factor determining the efficacy of solvent
extraction systems. [P66614]2[SO ] is poorly soluble even in pure water due to its highly
4
hydrophobic cation, with a solubility of about 0.63 wt%. [EhEhT ] [SO ] is miscible with pure
2
3 2
4
-1
water. Upon addition of 2.00 mol L of NaOH to the aqueous phase, [EhEhT ] [SO ] formed
2
3 2
4
an aqueous biphasic system. However, due to the high mutual miscibility of both phases,
EhEhT ] [SO ] should be diluted with an organic solvent for use in solvent extraction (vide
[
2
3 2
4
infra). The equilibrium concentration of water in the organic phase remains high even in contact
-1
with 2.00 mol L NaOH: nearly 27 wt% for [P
] [SO ] and 50 wt% for [EhEhT ] [SO ].
6614 2 4 23 2 4
6
While remarkable, this does not impede the use of the undiluted ionic liquids in solvent
extraction systems.
1
4

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DOI: 10.1039/C9GC01626K
Table 1: Physicochemical properties of the extractants.
[
P6 ] [SO ]
[EhEhT ] [SO ]
23 2 4
6614 2
4
Melting point range (° C)
28-34 °C
0.907
RTILa
0.967
b
-3
Density (g cm )
c
Dynamic viscosity, dry (cP)
7705 at 50 °C
798 at 50 °C
135 at 30 °C
4
Dynamic viscosity, water saturatedd
90
14
(cP)
Solubility in water (wt%)
0.63
244
miscible
154
e
Decomposition temperature (°C)
a
b
c
d
e
Room temperature ionic liquid
At 30 °C
Residual water content: 0.29 wt% for [P6 ] [SO ], 0.83 wt% for [EhEhT ] [SO ]
At 30 °C, in equilibrium with 2.00 mol L NaOH
Onset point at a ramp rate of 3 °C min
6614 2
-1
4
23 2
4
-1
1
-Octanol was chosen as diluent for solvent extraction studies due to its relatively high flash
4
0
point, high hydrophobicity, good biodegradability and miscibility with the extractants. The
losses of extractant to the aqueous phase under realistic extraction conditions were determined
1
-1
by quantitative H NMR. Undiluted [P
] [SO ] displayed a solubility in 2.00 mol L NaOH
4
6
6614 2
-1
of 390 ppm. Losses of [P66614]2[SO ] from a 40 vol% solution in 1-octanol to 2.00 mol L
4
NaOH in 1:1 phase volume ratio result in an aqueous [P66614]2[SO ] concentration of 206 ppm.
4
In a similar system with [EhEhT ] [SO ], an extractant concentration of 268 ppm was found.
2
3 2
4
-1
This value increases to 738 ppm when the aqueous phase contains 1 mol L of NaOH, a
concentration more suitable for extraction using this extractant (vide infra).
The stability of the extractants under extraction conditions was evaluated over extended periods
of time. Samples were prepared by contacting an aliquot (approx. 50 µL) of the ionic liquid
-1
with 1 mL of 2.00 mol L NaOH at the specified conditions. Subsequently, organic material
was extracted into 1 mL of chloroform-d. The degree of decomposition of the triazolium
1
extractant [EhEhT ] [SO ] was determined using H NMR. Our earlier research has showed
2
3 2
4
1
5

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3
9View Article Online
that 1,2,3-triazolium ionic liquids decompose to 1,2,3-triazoles in alkaline solutions. The
DOI: 10.1039/C9GC01626K
degree of decomposition could thus be determined by integration of the ionic liquid and triazole
signals. [EhEhT ] [SO ] showed excellent resistance to base-induced decomposition at 2.00
2
3 2
4
-1
-1
mol L NaOH. After 10 days of continuous exposure to 2.00 mol L NaOH at 60 °C, only 2
mol% of the extractant was found to have decomposed.
3
1
The stability of the phosphonium extractant [P66614]2[SO ] was determined using P NMR.
4
Phosphonium salts are known to decompose to phosphine oxides in alkaline solutions.30,31,33 In
-1
2
.00 mol L NaOH at 60 °C, 84 mol% of the [P
] [SO ] had decomposed to phosphine
6614 2 4
6
oxides after 10 days. At 30 °C, the percentage of decomposition drops to 10 mol% over 10 days
-1
of contact with 2.00 mol L NaOH. Evidently, the recyclability of the triazolium extractant is
far superior to that of the phosphonium-based extractant, and prolonged usage of the latter will
require precautions against radiation-induced heating during contact with the alkaline feed
solution. The NMR spectra of the extractants after contact with the alkaline solutions can be
found in the ESI (Fig. S4, S9 and S10). Note that the triazolium extractant dissolved into the
aqueous phase under the conditions used for this experiment.
Mechanistic studies
95Mo NMR and infrared spectroscopy were used to determine the speciation of molybdenum in
9
5
-1
the organic phase. Mo NMR spectra were externally referenced to a 200 g L solution of
Na MoO ·2H O in water. Spectra were recorded of pure [P ] [SO ], 40 vol% [P ] [SO ]
2
4
2
66614 2
4
66614 2
4
in 1-octanol, 20 vol% [P66614]2[SO ] in 1-octanol and pure [EhEhT ] [SO ]. All samples were
4
23 2
4
-1
-1
presaturated with a blank 2.00 mol L NaOH solution and then loaded from a 200 g L solution
-1
of Na MoO ·2H O in water containing 2.00 mol L NaOH (organic to aqueous phase volume
2
4
2
9
5
ratio: 2:5). In spite of the very large chemical shift range of Mo, the sample signal of the
organic phase species was never shifted further than 10 ppm downfield from the reference
signal. These small downfield shifts can simply be attributed to solvent and dilution effects.
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6

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Much larger changes in chemical shift are expected should speciation changes occur upon
DOI: 10.1039/C9GC01626K
extraction, for instance the oligomerization of molybdate anions. The obtained chemical shift
2
-
values thus demonstrate that molybdenum is extracted as MoO , the same species that exists
4
in the aqueous phase. Furthermore, the narrow linewidth observed in all spectra is in agreement
2
-
with the high T symmetry of the MoO anion. All sample spectra are shown in Fig. 2.
d
4
Fig. 2: 95Mo NMR spectra of (A) loaded, undiluted [EhEhT23
[SO ] diluted in 1-octanol and (D) loaded 20 vol% [P66614
MoO ·2H O in water, set to 0 ppm.
]
2
[SO
], (B) loaded, undiluted [P66614
[SO ] diluted in 1-octanol. All spectra are referenced to
4
2 4
] [SO ], (C) loaded 40 vol%
[
2
P
66614
]
2
4
]
2
4
-
1
00 g L Na
2
4
2
The above findings were corroborated by infrared spectroscopy of the loaded organic phases.
Sulfate and molybdate anions, being structurally analogous and isoelectronic, both display
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7

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strong absorption bands in their infrared spectra attributed to their asymmetric stretching
DOI: 10.1039/C9GC01626K
modes.41–43 The molybdate band appears at lower wave numbers (approximately 800820 cm-
1)
than the sulfate band (approximately 1060-1080 cm ) as a result of the greater atomic mass
-1
of molybdenum and the comparatively weaker Mo=O bond. These bands can be used to detect
presence of either anion and can clearly be seen in the spectra of pure [P66614]2[SO ] and
4
Na MoO ·2H O. [P
] [SO ] that was maximally loaded with molybdenum displays a strong
66614 2 4
2
4
2
molybdate asymmetric stretching band, but lacks the corresponding sulfate signal. Hence,
sulfate is exchanged for molybdate in the organic phase during extraction. Moreover, this
implies that the fully loaded extractant can be viewed as the ionic liquid [P66614]2[MoO ]. The
4
spectra before and after loading are shown and compared in Fig. 3.
1
8

Page 19 of 41
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DOI: 10.1039/C9GC01626K
Fig. 3: Comparison of the infrared spectra of loaded and unloaded [P66614
]
2
[SO
4
] and solid Na
2
MoO
4
·2H
2
O. The sulfate and
S
molybdate stretching bands are highlighted and respectively labeled as ν and νMo. Loading was accomplished by contacting
-1
-1
2 4 2
the organic phase with a 200 g L solution of Na MoO ·2H O containing 2.00 mol L NaOH in 2:5 volume ratio.
1
9

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DOI: 10.1039/C9GC01626K
Similarly, when [EhEhT ] [SO ] was loaded with molybdenum, the molybdate band appeared
2
3 2
4
in the spectrum. Due to the high water content of the loaded organic phase, background
absorption prevents clear evaluation of the intensity of the sulfate absorption band. Regardless,
the infrared spectra of loaded and unloaded 50 wt% [EhEhT ] [SO ] in 1-octanol do show a
2
3 2
4
striking attenuation of the sulfate absorption band upon loading. The spectra before and after
loading are shown and compared in Fig. 4.
Fig. 4: Comparison of the infrared spectra of loaded and unloaded [EhEhT23
Na MoO ·2H
was accomplished by contacting the organic phase with a 200 g L solution of Na
in 2:5 volume ratio.
]
2
[SO
4
] (50 vol% diluted in 1-octanol) and solid
and νMo. Loading
2
4
2
O. The sulfate and molybdate stretching bands are highlighted and respectively labeled as ν
S
-1
-1
2
4 2
MoO ·2H O containing 2.00 mol L NaOH
9
5
Based upon the aforementioned Mo NMR and infrared spectra, an extraction mechanism can
be inferred in which sulfate anions in the organic phase is replaced by molybdate anions from
2
0

Page 21 of 41
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the aqueous phase. The proposed extraction mechanism is thus an anion-exchange process,
DOI: 10.1039/C9GC01626K
which is expected to be favorable due to the larger ionic radius of the molybdate anion with
respect to that of the sulfate anion. This difference in radius leads to a lower charge density and
thus comparatively weaker hydration of the molybdate anion. As a result, the transfer of sulfate
anions to the aqueous phase in exchange for molybdate anions is favorable. Such phenomena
4
4
were studied and discussed in detail by Dupont et al. The process can be represented by the
following reaction equation:
2
―
+ SO 24 ―
[Q]2
[
SO4
]
+ MoO ⇌ [Q]2[MoO4
]
(4)
4
In which Q denotes the organic cation of the extractant and an overbar denotes species in the
organic phase.
Diluent and cation effects
The infrared spectra of undiluted [EhEhT ] [SO ] and [P
] [SO ] show marked differences
66614 2 4
2
3 2
4
in their sulfate ν-regions. The sulfate (ν ) band is shifted to higher wave numbers in
S
-1
[
EhEhT ] [SO ] with respect to [P
] [SO ] (1109 vs. 1079 cm ), as can be seen in Fig. 5.
2
3 2
4
66614 2 4
The δ -band is located at approximately the same wave number in both compounds. One can
S
thus conclude that significant differences exist in the coordination of the sulfate anion to cations
in the pure extractants.
The use of a diluent also affects the solvation of the sulfate anion. When [P66614]2[SO ] is diluted
4
-1
in 1-octanol, the sulfate asymmetric stretch (ν ) band is shifted from 1079 to 1059 cm .
S
-1
Furthermore, the bending mode (δ ) band, which is prominently present at 607 cm in undiluted
S
-1
[
P6 ] [SO ], is shifted to 619 cm and nearly completely absent from the diluted sample (Fig.
6614 2 4
5
). Both are evidence of solvation of the sulfate anion in diluted [P66614]2[SO ], which is not
4
present in the pure extractant.42,45,46 The lack of sulfate solvation in the latter can be attributed
to the tetrahedral structure of the cation, in which polarized protons and the positively charged
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1

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core are sterically shielded by the alkyl substituents. Further evidence of hydrogen bonding
DOI: 10.1039/C9GC01626K
between 1-octanol and the sulfate anion in [P66614]2[SO ] is presented in the electronic
4
1
supplementary information (Fig. S5), where H NMR spectra show the disappearance of the
hydroxyl proton signal upon addition of [P66614]2[SO ], which again indicates the presence of
4
rapidly exchanging hydrogen bonds.
Fig. 5: Comparison of the infrared spectra of [P66614
]
2
[SO
4
], [EhEhT23
]
2
[SO
4
] and 40 vol% [P66614
]
2
[SO
4
] diluted in 1-octanol.
The sulfate asymmetric stretching and bending bands are highlighted and respectively labeled as ν
S
and δ .
S
The effect of the diluent on the solvation of the extractant is clearly reflected in the distribution
ratios observed for molybdenum. Fig. 6 shows the distribution ratio for molybdenum (D ) as
Mo
a function of the extractant concentration for [P66614]2[SO ] dissolved in 1-octanol and toluene,
4
2
2

Page 23 of 41
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DOI: 10.1039/C9GC01626K
and for [EhEhT ] [SO ]. The indicated volume fractions denote the ratio of the extractant to
2
3 2
4
the total dry volume of the organic phase. Upon saturation with water, the volume fraction of
the extractant thus decreases further. The results shown in Fig. 6 reveal a very strong
dependence of DMo on the extractant concentration. Ideally, the logarithms of both DMo and the
extractant concentration are expected to correlate linearly with a slope of 1 as a result of the
equilibrium of Equation (4) being shifted to completion. A negative deviation from this trend
can be expected at higher extractant concentrations due to the uptake of large amounts of water,
which diminishes the difference in solvation experienced by solutes upon phase transfer.
However, the dependence of DMo on the concentration of [P66614]2[SO ] was far stronger than
4
predicted for an ideal system, particularly at low extractant concentrations. This can be
correlated to the aforementioned infrared and NMR studies, which show increased solvation of
the sulfate anion in 1-octanol with respect to the pure extractant. Thus, increasing the extractant
concentration not only increases the amount of sulfate available to drive Eq. 4 to higher values
of D , but also decreases the solvation of the sulfate anions, thus further increasing their
Mo
activity. No such effect was observed for [P66614]2[SO ] in toluene, where no strong hydrogen
4
bonding can take place. The observed values for DMo in this system are more or less constant,
regardless of the extractant concentration, which may result from the competing effects of the
increased concentrations of extractant and water in the organic phase.
[
EhEhT ] [SO ] exhibits much less affinity for molybdenum than [P
] [SO ]. The lower
66614 2 4
2
3 2
4
observed values of DMo are likely related to the higher hydrophilicity of [EhEhT ] [SO ],
23 2
4
leading to a much higher uptake of water. Furthermore, differences in solvation of the sulfate
anion could also be involved, as the strong dependence of DMo on the extractant concentration
in 1-octanol was not observed in for the triazolium extractant. This can again be correlated to
the abovementioned infrared spectra, which indicate significant differences in solvation of the
sulfate anions in both undiluted extractants.
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3

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DOI: 10.1039/C9GC01626K
Fig. 6: Variation of the distribution ratio of molybdenum as a function of the extractant concentration. The aqueous feed was
-
1
a 1000 ppm solution of Na
in toluene (b) and [EhEhT23
2
MoO
4
in 1.00 mol L NaOH. The organic phases were [P66614
4
2 4 2 4
] [SO ] in 1-octanol (a) [P66614] [SO ]
-1
]
2
[SO
] in 1-octanol (c). The organic phases were presatured with 1.00 mol L NaOH. A phase
ratio of 2:1 (aqueous to organic) was used at 30 °C.
Separation studies
To investigate the selectivity of the extractants, a solution was made containing base-soluble
elements reported to occur in leachates obtained from irradiaded UAl targets in concentrations
x
of approximately 1000 ppm.10,47,48 These were molybdenum, aluminium, cesium, iodine and
tellurium, respectively added as Na MoO , NaAl(OH) , CsOH, NaIO and Te(OH) . The
2
4
4
4
6
alkalinity of the feed solution was varied and the extraction of each element was studied. Data
on the exact composition and concentration of industrial leach solutions of irradiated UAlx
targets are scarce, as these details are generally not disclosed by producers. There is some
2
4

Page 25 of 41
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ambiguity in the literature about the presence of Ru and Zr in such leachates. A handful of
DOI: 10.1039/C9GC01626K
1
0
26
authors report the partial leaching of Ru and Zr or Ru in such solutions. Other authors
explicitly report these elements to be insoluble in the alkaline leachates.6,48 Their solubility thus
appears to depend very strongly on the exact leaching conditions used. Cs and Te are generally
reported to be present in acidic leachates, but due to the high solubility of these elements in
alkaline solutions, their presence cannot be excluded from alkaline leachates. The compounds
as which the elements were added contain each element in its highest stable oxidation state,
reflecting their speciation after alkaline oxidative leaching or alkaline leaching followed by
oxidation. The concentration of each element in UAl target leachates is again highly dependent
x
on the exact leaching process used by the manufacturer, which is often tailored to the ensuing
purification process. Therefore, concentrations of 1000 ppm were chosen as this allows accurate
quantification while being low enough to prevent saturation of the extractant. Studies reported
on by the International Atomic Energy Agency in 1989 reported a realistic total molybdenum
concentration after leaching to be roughly 20 000 ppm (32 mg dissolved in 1.575 mL of leach
4
9
solution). For iodine, this value was 4000 ppm (6.7 mg dissolved in 1.575 mL of leach
solution).
The first system to be investigated was undiluted, presaturated [P66614]2[SO ]. The results of
4
this assay are shown in Fig. 7. The percentages extraction (%E) are shown rather than the
distribution ratios, to more clearly illustrate the degree of separation obtained. As the percentage
extraction is a function of the phase ratio, it should be noted that a 2:1 aqueous to organic ratio
was used in this study. Initial pH-values are shown in Fig. 7, although the pH does not change
significantly during the extraction process as the mechanism does not involve proton or
hydroxide exchange.
Clearly, neat [P66614]2[SO ] exhibits selectivity for molybdenum over the other evaluated
4
-1
elements, especially at higher alkalinity. At 2.00 mol L NaOH, 91% of molybdenum is
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5

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extracted, with only 6% of the aluminum coextracted. The strong pH-dependence of the
DOI: 10.1039/C9GC01626K
2
-
extraction of tellurium results from its polyprotic nature. The transition of H TeO to the more
4
6
3
-
50
strongly charged H TeO has a theoretical pK of 14.3. On the grounds of their respective
3
6
a
charge densities, only the former is expected to report significantly to the organic phase in
4
4
exchange for dianionic sulfate. Similar arguments apply to periodate. In alkaline solutions,
-
3-
metaperiodate (IO ) hydrolyzes to trianionic orthoperiodate (H IO ). The theoretical pK for
4
2
6
a
2
-
3-
the transition of H IO to H IO is 11.6, although the former dimerizes, affecting the
3
6
2
6
5
1
equilibrium. As a result of these hydrolyses, the extraction of both iodine and tellurium is
observed to decrease strongly from pH 13 to 14.3, down to negligible values.
Extraction of aluminum also decreases markedly towards higher pH values. While Moolenaar
2
-
-1
and McKeever suggested the formation of Al O(OH) above 1.5 mol L NaOH on the basis
2
6
2
7
-1
of vibrational and Al NMR spectra, this form only becomes dominant above 6 mol L NaOH.
A more plausible reason is the dehydrating effect of concentrated electrolyte solutions on the
organic phase. This could even be observed visually during presaturation of the organic phases,
as the volume increase of the organic phase was much less significant for samples equilibrated
with higher NaOH concentrations. As discussed above, reduced water content in the organic
phase increases the differences in solvation experienced by anions in both phases, driving
distribution ratios to more extreme values in either direction.
The extraction of cesium was very low at all investigated pH values. Cesium does not
significantly form anionic species and thus cannot be extracted via a mechanism similar to that
proposed in Eq. 4. Note that due to analytical errors of a few percent, some extraction
percentages for cesium appear slightly below 0%.
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6

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DOI: 10.1039/C9GC01626K
Fig. 7: Variation of the percentages extraction of Mo (solid circles), Te (solid squares), I (solid triangles), Al (unfilled circles)
and Cs (unfilled squares) as a function of the initial aqueous pH. Elements were present in 1000 ppm concentrations as their
hydroxides or highest valence sodium salts. The organic phase was undiluted [P66614] [SO ] presaturated with water containing
2 4
an appropriate concentration of NaOH. A phase ratio of 2:1 (aqueous to organic) was used at 30 °C.
Similar results are obtained for extractions to an organic phase composed of a 1:1 volume
mixture of [P66614]2[SO ] in 1-octanol, under identical conditions (Fig. 8). The co-extraction of
4
aluminum, iodine and tellurium is suppressed compared to neat [P66614]2[SO ], presumably due
4
to the reduced water content of the organic phase, enhancing differences in solvation of the
anionic species in both phases and thus resulting in higher selectivity. Note again that some
very poorly extracted metals display a %E below 0% due to analytical errors.
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DOI: 10.1039/C9GC01626K
Fig. 8: Variation of the percentages extraction of Mo (solid circles), Te (solid squares), I (solid triangles), Al (unfilled circles)
and Cs (unfilled squares) as a function of the initial aqueous pH. Elements were present in 1000 ppm concentrations as their
hydroxides or highest valence sodium salts. The organic phase was 1:1 [P66614
2 4
] [SO ] in 1-octanol, presaturated with water
containing an appropriate concentration of NaOH. A phase ratio of 2:1 (aqueous to organic) was used at 30 °c.
[
EhEhT ] [SO ] is a weaker extractant for Mo than [P
] [SO ] and is miscible with water
66614 2 4
2
3 2
4
at low NaOH concentrations. However, it also displays a very low affinity for tellurium,
aluminum and iodine when diluted with 1-octanol. Diluted [EhEhT ] [SO ] is thus suitable as
2
3 2
4
a selective extractant for molybdenum at comparatively low basicities. Based on the extraction
results for molybdenum shown in Fig. 6, an extractant concentration of 30 vol% was chosen.
The extraction results for mixed feed solutions displayed in Fig. 9 show that the extraction of
molybdenum is poor at higher alkalinity. This effect is also seen in the other systems described
above, but the impact on the percentages extraction is exacerbated in this system by the lower
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8

Page 29 of 41
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distribution ratios for Mo. The suppression of molybdenum extraction is most likely caused by
DOI: 10.1039/C9GC01626K
competitive extraction of hydroxide.52 The lowest degree of coextraction of impurities is
-1
obtained at 1.00 mol L NaOH. The slight uptick in the degree of coextraction of tellurium and
-1
iodine at 2.00 mol L NaOH may be a result of a salting-out effect induced by NaOH.
Extraction results for 50 vol% [EhEhT ] [SO ] were generally poorer in terms of both the
2
3 2
4
selectivity and distribution ratios obtained for molybdenum. These are shown in the ESI
Fig.S13).
(
Fig. 9: Variation of the percentages extraction of Mo (solid circles), Te (solid squares), I (solid triangles), Al (unfilled circles)
and Cs (unfilled squares) as a function of the initial aqueous pH. Elements were present in 1000 ppm concentrations as their
hydroxides or highest valence sodium salts. The organic phase was 30 vol% [EhEhT23] [SO ] diluted in 1-octanol, presaturated
2 4
with water containing an appropriate concentration of NaOH. A phase ratio of 2:1 (aqueous to organic) was used at 30 °C.
2
9