Copper and uranyl extraction from aqueous solutions using bis-dithiophosphinate ligands have been characterized

Article abstract of DOI:10.1016/j.poly.2012.05.026

Dithiophosphinate ligands with groups linked together by a bridging spacer to take advantage of a chelate effect for improved selectivity have been characterized. Extractions with each ligand were performed with the ligands dissolved in methylene chloride and the metal salts dissolved in aqueous phase at pH 4. H₂L1 was found to have good differentiation between UO22+ and Gd³⁺, but only resulted in 50% extraction. H₂L2 had lower selectivity; however, copper extractions approached 95% after 24 h.

Full text of DOI:10.1016/j.poly.2012.05.026

Polyhedron 42 (2012) 271–275
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Copper and uranyl extraction from aqueous solutions using bis-dithiophosphinate
ligands have been characterized
⇑
Michael A. DeVore II, Anne E.V. Gorden
179 Chemistry, Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Dithiophosphinate ligands with groups linked together by a bridging spacer to take advantage of a che-
late effect for improved selectivity have been characterized. Extractions with each ligand were performed
with the ligands dissolved in methylene chloride and the metal salts dissolved in aqueous phase at pH 4.
Received 5 April 2012
Accepted 22 May 2012
Available online 30 May 2012
H2L1 was found to have good differentiation between UO2þ and Gd3+, but only resulted in 50% extraction.
2
H2L2 had lower selectivity; however, copper extractions approached 95% after 24 h.
Keywords:
Uranyl
Ó 2012 Elsevier Ltd. All rights reserved.
Copper
Bis-dithiophosphinite
Extraction
1. Introduction
Currently, the most widely used effective process for the extrac-
tion and separation of plutonium and uranium from the lantha-
The availability of adequate energy at a reasonable price is con-
sidered a requirement of modern society [1]. Experts predict that
the maximum allowable oil production using current methods will
occur in the next 5–25 years, and consequently the need to have
alternative energy sources to eventually replace oil is growing
[1]. Currently 13–14% of the world’s electricity is produced using
nuclear sources, and nuclear power is the dominant source of elec-
trical power for most of Europe [2]. The applications of nuclear en-
ergy for the production of electricity for general civilian use,
military applications, as well as in satellite and space exploration
applications are plagued with waste management risks that must
be addressed [3]. During the past 60 years, more than 1800 metric
tonnes of Plutonium, and substantial quantities of the ‘‘minor’’
actinides, such as Neptunium, Americium, and Curium, have been
generated in nuclear reactors [4]. There are two basic strategies
concerning the disposition of these heavy elements: (1) to ‘‘burn’’
or transmute the actinides using nuclear reactors or accelerators
[5]; (2) to ‘‘sequester’’ the actinides in chemically durable, radia-
tion-resistant materials that are suitable for geologic disposal [6].
While reprocessing is not currently performed in the United States
due to proliferation concerns subsequent to the Cold War, other
nations using nuclear power technology reprocess their spent fuel
to recycle the remaining fissile fractions [1,7]. Such a process can
provide up to 96% more energy than the once-through cycle using
the same initial amount of enriched uranium fuel [1].
nides is the Plutonium Uranium Recovery by Extraction (PUREX)
process [8–10]. This process uses the extractant tributyl phosphate
(TBP) (Fig. 1) in a hydrocarbon solvent such as kerosene or dode-
cane [6,11]. While this process is well described, it still leaves room
for improvement, and a resurgence of interest in the coordination
chemistry of the actinides has been inspired by a need to address
these environmental concerns, to develop new separations and
remediation technologies, and to continue to develop our funda-
mental understanding of the chemical behavior of the actinides
[12–17].
Organophosphorous extractants have played a major role in
actinide extraction [18]. These extractants are generally stable,
inexpensive, and commercially available, and have been widely
studied in the past few decades, in particular with respect to co-
balt–nickel separation from weakly acidic sulfate media [19]. The
earliest work was performed by Ritcey et al., as well as Flett and
co-workers in which they used an alkylphosphoric acid, di-(2-eth-
ylhexyl)-phosphoric acid (D2EHPA) (Fig. 1) [19]. Subsequently, the
development of the phosphonic and phosphinic acid extractants 2-
ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) and
bis-(2,4,4,-trimethylpentyl)-phosphinic acid (Cyanex 272) have
led to further improved separation factors in the order: phospho-
ric < phosphonic < phosphinic acid [19–21].
In recent years, Cyanex 301 and Cyanex 302 (Fig. 1), have re-
ceived considerable attention both for their ability to extract soft
transition metals [22], and for their ability to differentiate between
the chemically similar trivalent lanthanides and actinides [23,24];
however, Cyanex 301 can only differentiate between the trivalent
⇑
Corresponding author. Tel.: +1 334 844 4043.
E-mail address: gordeae@auburn.edu (A.E.V. Gorden).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.05.026

272
M.A. DeVore II, A.E.V. Gorden / Polyhedron 42 (2012) 271–275
Fig. 1. Current and previously used extractants.
lanthanides and trivalent actinides in solutions of a pH lower than
3 [25]. Several methods of separations of spent nuclear fuel (SNF)
have focused on extraction agents containing a single dithiophos-
phinic acid group like Cyanex 301. Here, we report the preparation
and simple extractions with a class of compounds – the bisdithio-
phosphinites [26,27]. The purpose of this design was to incorporate
two dithiophosphinic acid groups connected by a linker to take
advantage of a potential chelate effect to increase selectivity for
actinides.
H2L1 and H2L2, quantitatively soluble in DCM, were used for
extraction studies. Fresh solutions of CuCl2ꢁ2H2O, UO2(NO3)2
6H2O, or GdCl3 were prepared in DI water, and the pH was adjusted
with HNO3 and KOH ( 0.05). Simple extractions were tested at pH
4 with a ratio of 1:1 metal to ligand for compounds H2L1 and H2L2.
The phases were agitated by stirring for time periods indicated, al-
lowed 24 h to equilibrate, and the organic layer drawn off into a
separate vial.
3. Results and discussion
2. Experimental
The ligands H2L1 and H2L2 each possess 4 potential donor
atoms to complex with metal atoms and take advantage of the che-
late effect to enhance extractions. While there are few metal com-
plexes with similar ligands, it has been shown through mass
spectrometry that these complexes can be monomer or dimers,
and mononuclear or dinuclear [27]. Ligands without the carbon
chain bridge can form many crystallographic species. Verani
et al. reported crystal structures with Pt2+, Pd2+, and Ni2+ that have
a 1–2 metal to ligand structure [28,29], while Karakus et al. have
described a 2–4 metal to ligand crystal structure with cadmium
in which two ligands have both donors coordinating to cadmium
metal, and the other two ligands have their donors split between
the two metal centers [30]. While crystals suitable for X-ray dif-
fraction were not able to be grown, based on above papers and
what was observed for extraction we would expect coordination
as shown in Fig. 2.
2.1. General procedure
Lawesson’s reagent, uranyl nitrate, cuprous chloride, gadolin-
ium chloride, 1,3-propanediol, and 1,5-pentanediol, were pur-
chased from Acros and used without any purification. The pH
was recorded on a Fischer Scientific AR15 pH meter. UV–Vis data
was collected on a Cary 50 UV–Vis spectrophotometer with a
xenon lamp in the range of 200–1200 nm. The 1H, 13C, and 31P
NMR was recorded on a Bruker AV 250 spectrophotometer with
d4-MEOD, d1-CDCl3, or d6-DMSO as the solvent using tetramethyl-
silane as the reference. 100 ppm standards were purchased and
diluted to ꢀ2 ppm and used for calibration of the ICP-OES.
2.2. Synthesis of ligands
Ligands were prepared according to literature procedures
[26,27]. To a solution of diol (10 mmol) in toluene (30 mL), 4.0 g
(10 mmol) of Lawesson’s Reagent (1) was added. The mixture
was stirred at 70 °C until all the solids had dissolved and left over-
night. The solvent was removed on a roto-vap until the remaining
volume was ꢀ10 mL. Hexane (30 mL) was added to precipitate the
product as a green oil (Scheme 1).
3.1. Hydrolysis and extraction studies
Stock solutions of H2L1 and H2L2 were prepared by dissolving
the respective compound in methylene chloride (100 mL each).
An equivalent amount of an aqueous solution at pH 1–14 ( 0.05)
(adjusted with HNO3 and KOH) was added to separate vials con-
taining H2L1 or H2L2 in organic solvent and shaken for 60 s. The
solution was left undisturbed overnight, and the organic layer iso-
lated for hydrolysis studies employing UV–Vis. The extent of
hydrolysis was interpreted relative to the spectra of the ligand at
neutral pH.
2.3. Extraction and hydrolysis studies
Two-phase extraction studies in methylene chloride/water
(DCM/H2O) were performed to determine the extraction capability
for the removal of Cu2+ ions from aqueous solution. The ligand
Scheme 1. Synthesis of ligands.

M.A. DeVore II, A.E.V. Gorden / Polyhedron 42 (2012) 271–275
273
Fig. 2. Proposed coordination of uranyl and ligand.
The two-phase hydrolysis study of H2L1 indicates that the li-
gand hydrolyzes in extreme pH conditions (pH 1, 2 and 12–14)
while H2L2 also hydrolyzes in at these pH conditions (pH 1, 2
and 11–14). The hydrolysis profile of compounds H2L1 and H2L2
are shown in Fig. 3.
The two-phase extraction studies described here are at pH 4.0
only since H2L1 and H2L2 hydrolyze in the acidic region. H2L1
and H2L2 were dissolved in methylene chloride (10
the metal salts were dissolved in the aqueous phase (10
two phases were agitated by stirring on a magnetic stir plate for
the indicated time. At higher concentrations- above 10 M, a third
l
M) while
l
M). The
l
layer can clearly be seen, and hence, using UV–Vis spectroscopy to
track the uranyl peak was undesirable.
The H2L1 ligand was found to extract about 48% of the copper
ion after 8 h and 57% after 12 h (Fig. 4); however, after 24 h, the
concentration of the metal ion increased in the aqueous phase to
about 50% of the original solution, again indicative of a possible
third layer formation. The third layer formation was observed at
higher concentrations of the metal and ligand when initial studies
were to employ the use of UV–Vis spectroscopy. Since the layers
would often be cloudy because of this third layer, it was impossible
to use that technique. The extraction of uranyl ion was modest at
about 41% after 8 and 12 h, but as in the case with the copper
ion after 24 h, there is an increase in the uranyl ion concentration
after 24 h. With the four sulfur donors being ‘‘softer’’ donors, better
able to overlap and bond with the 5f orbital of the uranyl, it was
expected that the extraction of gadolinium would not be very good
and for H2L1 it was not with only 17% being the highest extraction
after 8 h. With the smaller binding pocket associated with H2L1,
the gadolinium metal ion would be small enough to fit but the
bond distances from the donors could be far enough away as to
not form stable complexes. The concentration in the aqueous phase
does increases as it does with the other metal ions.
½Mꢂorg
The distribution ratio (defined as D ¼
, Table 1) of copper
½Mꢂaq
with H2L1 was found to be ꢀ1 while it is much less for the other
metal ions. The separation factor of uranyl over gadolinium is 3–
5, which while low, indicates a preference of uranyl of gadolinium
and use in the separation of trivalent actinides from trivalent lan-
thanides is perhaps possible with additional suitable modifications
to the ligand.
H2L2 with the five carbon chain has a bigger binding pocket for
which a metal ion or metal ions to bind and be extracted into the
organic phase (Fig. 5). After 8 h, 80% of the copper in the aqueous
phase had been extracted with 95% after 24 h. This was interesting,
so a shorter time scale was used to see how quickly the copper
could be extracted (Fig. 6). After only 5 min of stirring, 87% of the
copper ions had been extracted out of the aqueous phase. The per-
Fig. 3. Absorption spectra showing hydrolysis of H2L1 and H2L2 between pH 1–14.
The bands indicated by ꢃ correspond to spectra at neutral pH. The bottom graph is a
close up expansion of the previous graph from 290 to 380 nm.

274
M.A. DeVore II, A.E.V. Gorden / Polyhedron 42 (2012) 271–275
Fig. 4. Percent extraction of various metals by H2L1 at 8, 12, and 24 h (Cu2+ = ꢄ;
Fig. 6. % Copper extraction at shorter time lengths.
Gd3+ = - -; UO22+ = –).
Table 1
Table 2
Distribution of metal ions after extraction of H2L2 at 8, 12, and 24 h.
Distribution of metal ions after extraction by H2L1 at 8, 12, and 24 h.
Metal
8 h
12 h
24 h
Metal
8 h
12 h
24 h
Copper
Gadolinium
Uranyl
0.91
0.21
0.72
1.28
0.13
0.70
0.98
0.18
0.47
Copper
Gadolinium
Uranyl
4.17
0.85
1.00
11.69
1.30
0.81
20.68
0.76
0.95
able to employ the use of UV–Vis spectroscopy to track extraction
of uranyl ion.
Distribution (Table 2) of the copper ion increases after 8 h to a
max of 20 after 24 h while gadolinium peaks at 12 h and falls at
24 h. Uranyl does the opposite where the distribution decreases
between 8 and 12 h but then increases at 24 h.
The separation factor values of H2L2 of copper over uranyl in-
creases, as there is an increase in time. While the goal would be
to have a higher separation factor of uranyl over copper, this is still
in interesting result and confirmation of the problem of uranyl and
copper binding in the same coordination pocket of a ligand [31].
4. Conclusions
Extractions were performed on three metals (Cu2+, Gd3+, and
UO22+) using two bisdithiophophinate ligands H2L1 and H2L2. Un-
like the Cyanex 301 ligand, H2L1 and H2L2 hydrolyze under very
acidic conditions so all extractions were performed at pH 4. While
H2L1 had about 50% extraction for copper and 40% for uranyl, it
was not very good at extraction of gadolinium expected, according
to the Pearson theory of Hard and Soft Acids and Bases [32]. The
H2L2 with the bigger binding pocket was a much better ligand
for the extraction of all three metals, especially copper where there
was nearly 100% extraction. Until modifications can be made such
that the ligands are more soluble in more suitable solvents like ker-
osene, extractions could prove to be difficult with the formation of
third layer systems or metallopolymer byproducts.
Fig. 5. Percent extraction of various metals by H2L1 at 8, 12, and 24 h (Cu2+ = ꢄ;
Gd3+ = - -; UO22+ = ꢅ).
cent extracted from the aqueous layer is consistent until about 8 h
in when it drops to 80%, before more is extracted at 12 and 24 h.
While the bigger pocket does help the uranyl extractions, it also al-
lows for better extraction of gadolinium as the percent extracted
increases from ꢀ20% with H2L1 to ꢀ50% with H2L2. H2L2 also
had issues of a third layer formation at higher concentrations,
although not as severe as H2L1, still significant enough to not be
Acknowledgements
The research was sponsored in part by the Fuel Cycle Research
and Development Program, Office of Nuclear Energy, US Depart-

M.A. DeVore II, A.E.V. Gorden / Polyhedron 42 (2012) 271–275
275
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.05.026.
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