Efficient masking of corrosion and fission products such as Ni(II) and Pd(II) in the presence of the minor actinide Am(III) using hydrophilic anionic or cationic bis-triazines

Article abstract of DOI:10.1039/c5cc02336j

Water soluble anionic and cationic bis-triazine ligands are able to suppress (mask) the extraction of corrosion and fission products such as Ni(II) and Pd(II) that are found in PUREX raffinates. Thus it is possible to separate these elements from the minor actinide Am(III). Although some masking agents have previously been developed that retard the extraction of Pd(II), this is the first time a masking agent has been developed for Ni(II).

Full text of DOI:10.1039/c5cc02336j

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Efficient masking of corrosion and fission
products such as Ni(II) and Pd(II) in the presence
of the minor actinide Am(III) using hydrophilic
anionic or cationic bis-triazines†
Cite this:DOI: 10.1039/c5cc02336j
Received 19th March 2015,
Accepted 1st May 2015
Frank W. Lewis,*ab Laurence M. Harwood,*a Michael J. Hudson,a Udo Mullichc and
DOI: 10.1039/c5cc02336j
¨
Andreas Geistc
www.rsc.org/chemcomm
Water soluble anionic and cationic bis-triazine ligands are able to
suppress (mask) the extraction of corrosion and fission products
such as Ni(II) and Pd(II) that are found in PUREX raffinates. Thus it is
possible to separate these elements from the minor actinide Am(III).
Although some masking agents have previously been developed
that retard the extraction of Pd(II), this is the first time a masking
agent has been developed for Ni(II).
After the removal of plutonium and uranium in the PUREX
process, the dominant part of the long-term radiotoxicity and
heat load of spent nuclear fuels arises from minor actinides
such as americium. Separation of the minor actinides, which
include Am, Cm and Np, from fission products such as the
lanthanides by solvent extraction has been achieved using soft
N-donor molecules containing the 1,2,4-triazine moiety.1–7 Mole-
cules such as the quadridentate ligands CyMe4-BTBP 18,9 and
CyMe4-BTPhen 210–12 (Fig. 1) are able to extract and separate the
minor actinides [Am(III), Cm(III)] from the lanthanides with very
Fig. 1 Structures of CyMe4-BTBP 1, CyMe4-BTPhen 2 and 3,30-bis(1,2,4-
triazine) ligands 3 and 4.
high selectivities. Indeed, CyMe4-BTBP 1 is the current European the minor actinides in these processes but certain corrosion and
reference extraction reagent for further development of the fission products such as Ni(II), Pd(II), Ag(I) and Cd(II) and their
SANEX (Selective ActiNide Extraction) process.13 In this process presence in the waste stream can interfere with the extraction protocol.
the radionuclides and non-fissile fission products are in an The solid state structures of some of these [M(CyMe4-BTBP)n]m+
aqueous nitric acid phase with the extraction reagent in an complexes have been recently reported.19
organic solvent. Furthermore, CyMe4-BTBP 1 is also being studied
Although much progress has been made concerning the
both for the direct selective extraction of Am(III) and Cm(III),14–17 partitioning of the minor actinides from the lanthanides,1–7 far
and for the co-extraction of all trans-uranic elements directly from less progress has been made on the separation of the minor
PUREX raffinate.18 Unfortunately, CyMe4-BTBP 1 not only co-extracts actinides from corrosion and fission products such as Ni(II),
Pd(II), Ag(I) and Cd(II). One way to separate these elements from
the minor actinides could be to hold the fission products in the
aqueous phase during the selective extraction of the actinides
a Department of Chemistry, The University of Reading, Whiteknights,
Reading RG6 6AD, UK. E-mail: l.m.harwood@reading.ac.uk
b Department of Applied Sciences, Faculty of Health and Life Sciences,
by CyMe4-BTBP 1 into the organic phase by using a hydrophilic
complexing agent. This new reagent in the aqueous phase
would need to be able to complex the corrosion and fission
products selectively without complexing either the minor acti-
nides or lanthanides. Such reagents could also be useful in
conventional hydrometallurgy as selective complexing agents
for certain precious metals. In addition, the hydrophilic reagent
needs to be able to form water soluble complexes. L-Cysteine14–17
Northumbria University, Newcastle Upon Tyne NE1 8ST, UK.
E-mail: frank.lewis@northumbria.ac.uk
c
¨
¨
Karlsruher Institut fur Technologie (KIT-INE), Institut fur Nukleare Entsorgung,
Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany.
E-mail: andreas.geist@kit.edu
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data and spectra for all compounds. Procedures for solvent
extraction measurements. See DOI: 10.1039/c5cc02336j
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and a chelating agent derived from methionine20 have previously
been used as selective complexing or ‘masking’ agents for Pd(II), while
trans-1,2-diaminocyclohexane-N,N,N0,N0-tetraacetic acid (CDTA) can
prevent the extraction of Pd(II) and Zr(IV) by hydrophobic O-donor
ligands.21,22 However, no reagents have been reported to date
that can mask a range of corrosion and fission products.
It has been shown, however, that bidentate molecules contain-
ing the 1,2,4-triazine moiety do not interact significantly with
Am(III) or with the trivalent lanthanides [Ln(III)].23 We considered,
therefore, that hydrophilic bidentate 3,30-bis(1,2,4-triazine) ligands
could be capable of masking several fission and corrosion products
based on a series of observations. Hydrophobic 3,30-bis(1,2,4-
triazine) ligands such as 3 and 4 (Fig. 1) form strong complexes
with several late transition metals such as Fe(II), Co(II), Ni(II), Cu(II),
Ru(II) and Pt(II).24–29 Furthermore, it has been observed that hydro-
phobic 3,30-bis(1,2,4-triazine) ligands do not extract or separate
Am(III) or Eu(III).23 Indeed, ligand 4 has been used as a phase-
transfer agent in the separation of Am(III) from Eu(III) by the BTBPs
but 4 does not itself extract either element.30 We thus proposed
that hydrophilic 3,30-bis(1,2,4-triazine) molecules could be able to
complex several corrosion and fission products and prevent their
extraction by CyMe4-BTBP 1, without complexing either Am(III) or
Ln(III). In this communication, we investigate the ability of two
new hydrophilic 3,30-bis(1,2,4-triazine) molecules to mask some
problematic corrosion and fission products that are co-extracted
from the PUREX raffinate by CyMe4-BTBP 1 using Ni(II), Pd(II),
Ag(I) and Cd(II) as exemplars for a wider range of elements.
The hydrophilic 3,30-bis(1,2,4-triazines) chosen for considera-
tion were molecules 10 and 13. The former is a sulfonic acid
derivative and the latter has quaternary ammonium groups.
The novel tetrasulfonated 3,30-bis(1,2,4-triazine) molecule 10
was synthesized as shown in Schemes 1 and 2. Intermediate 8
was synthesized from dithiooxamide 5 in two steps according to
the literature (Scheme 1).23,27
Scheme 2 Synthesis of tetrasulfonated masking agent 10.
Scheme 3 Synthesis of quaternary ammonium salt masking agent 13.
Initial extraction experiments were performed in order to
judge whether the molecules 10 and 13 had any potential for
Sulfonation of 8 with chlorosulfonic acid afforded the tetra- masking the above fission and corrosion products that would
sulfonyl chloride 9 and hydrolysis of 9 with sodium hydroxide in otherwise be extracted by CyMe4-BTBP 1 into 1-octanol. Neither
methanol gave the desired tetrasodium sulfonate 3,30-bis(1,2,4- masking agent had any significant influence on the extraction
triazine) ligand 10 (Scheme 2).
or separation of Am(III) and Eu(III) by CyMe4-BTBP 1. Ni(II) extraction
The tetra quaternary ammonium salt 3,30-bis(1,2,4-triazine) by 1 was suppressed both by 10 and 13. Pd(II) extraction by 1 was
ligand 13 was synthesized as shown in Scheme 3. Intermediate suppressed by 13. Ag(I) extraction by 1 seems to be suppressed by
12 was synthesized from 6 and 11 according to the literature both 10 and 13 but the results were inconclusive. No results could
procedure31 and subsequently reacted with triethylamine in be obtained for Cd(II) which was below the detection limit in all
methanol to afford the desired hydrophilic ligand 13 (Scheme 3). aqueous samples. Therefore, Cd(II) extraction by CyMe4-BTBP 1 was
not suppressed by either masking agent. This is not surprising, as
the BTBPs are quadridentate ligands and are thus stronger ligands
toward Cd(II) than the bidentate ligands 10 and 13.
More detailed experiments were then performed by varying the
contact-time in the extraction experiments and adding a second
extracting agent, N,N,N0,N0-tetraoctyldiglycolamide (TODGA), which
is used to accelerate the rate of actinide extraction by CyMe4-BTBP 1.17
As shown in Fig. 2 left, CyMe4-BTBP 1 extracts Ni(II) slowly but with
high distribution ratios (D) in the absence of a masking agent.
For contacting times of several minutes, DNi(II) E 1, meaning that
approximately 50% is extracted. However, once extracted, Ni(II) is
difficult to remove from the organic phase since its distribution
Scheme 1 Synthesis of intermediate 8.
ratio is greater than 1000 at equilibrium.
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Fig. 2 Influence of masking agents 10 and 13 on the extraction of Ni(II)
into CyMe4-BTBP 1. Organic phase: 10 mM CyMe4-BTBP 1 + 5 mM
TODGA in 1-octanol. Aqueous phase: Ni(II), Pd(II), Ag(I), Cd(II) (1 mM each)
with or without 20 mM masking agent 10 or 13 (left), or with varied concentra-
tions of 13 (right) in 2 M HNO3. T = 20 1C, A/O = 1 ((CH2NEt3X)2-BT = ligand 13,
(PhSO3Na)2-BT = ligand 10).
Fig. 3 Influence of masking agents 10 and 13 on the extraction of Pd(II)
into CyMe4-BTBP 1. Organic phase: 10 mM CyMe4-BTBP 1 + 5 mM
TODGA in 1-octanol. Aqueous phase: Ni(II), Pd(II), Ag(I), Cd(II) (1 mM each)
with or without 20 mM masking agent 10 or 13 (left), or with varied
concentrations of 13 (right) in 2 M HNO3. T = 20 1C, A/O = 1 ((CH2NEt3X)2-
BT = ligand 13, (PhSO3Na)2-BT = ligand 10).
Addition of reagent 10 (20 mM) efficiently suppresses Ni(II)
extraction (DNi(II) o 0.10) by 1 even for longer contacting times
of up to 20 minutes (Fig. 2 left). It is notable that, although some
masking agents have previously been developed that retard the
extraction of Pd(II),17,32 none have so far been developed that can
retard the extraction of Ni(II).
Adding 13 (20 mM) to the aqueous phase suppresses Ni(II)
extraction by 1 but not to a large extent (after 10 min, DNi(II) E 1). We
then examined the ability of 13 to mask Ni(II) at different concentra-
tions since it would be preferred over the sulfur-containing 10 from
the viewpoint of high level waste treatment as the raffinate solution
containing the corrosion and fission products and masking agent
will be ultimately incinerated and vitrified following the separation
process. Molecule 13 could be fully incinerated to innocuous gases if
the bromide ion is exchanged for nitrate, whereas 10 would generate
acidic waste on incineration owing to its sulfur content. As shown in
Fig. 2 right, 13 does suppress the extraction of Ni(II) to a useful extent
Fig. 4 Influence of masking agents 10 and 13 on the extraction of Ag(I)
into CyMe4-BTBP 1. Organic phase: 10 mM CyMe4-BTBP 1 + 5 mM
TODGA in 1-octanol. Aqueous phase: Ni(II), Pd(II), Ag(I), Cd(II) (1 mM each)
with or without 20 mM masking agent 10 or 13 (left), or with varied
concentrations of 13 (right) in 2 M HNO3. T = 20 1C, A/O = 1 ((CH2NEt3X)2-
BT = ligand 13, (PhSO3Na)2-BT = ligand 10).
(at 100 mM 13, DNi(II) E 0.1 for contacting times of 5–10 minutes). However, both masking agents also precipitate some Ag(I), as
In the absence of a masking agent, Pd(II) is extracted rapidly by seen from the formation of a precipitate upon adding the
CyMe4-BTBP 1 (equilibrium is attained within o5 minutes with masking agent to the aqueous phase and from the measured
DPd(II) E 20, Fig. 3 left). The strong attraction by CyMe4-BTBP 1 for Ag concentrations in the aqueous samples before extraction.
Pd(II) arises from the formation of a 1 : 1 square-planar complex with The D values shown in Fig. 4 are based on the fraction of Ag(I)
Pd(II).19 Adding 10 (20 mM) to the aqueous phase slightly lowers remaining in solution. In the case of 13, Ag(I) is precipitated as
Pd(II) extraction by 1 (DPd(II) E 10) without affecting the extraction AgBr in an ion exchange reaction. Fig. 4 right shows the effect
kinetics. On the other hand, addition of 13 (20 mM) to the aqueous of adding varied concentrations of 13 on the extraction of Ag(I)
phase efficiently suppresses Pd(II) extraction by CyMe4-BTBP 1 (for by CyMe4-BTBP 1.
contacting times up to 40 min, DPd(II) o 0.01). A slow increase in
In summary, we report the first examples of hydrophilic
Pd(II) extraction was observed with contact time indicating that 13 3,30-bis(1,2,4-triazine) ligands, and their application as masking
masks Pd(II) kinetically. However, for practical reasons Pd(II) extrac- agents in order to suppress the extraction of certain problematic
tion is efficiently suppressed by 13 during the contact times typically corrosion and fission products that are to be found in nitric acid
used in waste treatment. Increasing the concentration of 13 further solutions of PUREX raffinates. The bidentate tetrasulfonated
suppresses the extraction of Pd(II) by CyMe4-BTBP 1 (Fig. 3 right). ligand 10 efficiently suppresses the extraction of Ni(II) by solutions
However, owing to the low concentrations of Pd(II) in the aqueous of CyMe4-BTBP 1. Interestingly, 10 has little effect on the suppres-
phase the D values are not precise (D values below 0.1 have sion of Pd(II) extraction by 1. The reasons for the different
errors up to ꢀ20%).
complexation behavior of the ligands towards Ni(II) and Pd(II)
In the absence of a masking agent, Ag(I) is rapidly extracted are unclear at this point, and will be the subject of further
by CyMe4-BTBP 1 (DAg(II) 4 1000, Fig. 4 left). Both masking agents ongoing studies in our laboratory. Neither masking agent is able
10 and 13 suppress the extraction of Ag(I) by CyMe4-BTBP 1 to some to prevent the extraction of Cd(II), which is known to be strongly
extent (with 20 mM 10, DAg( ) o 2, with 20 mM 13, DAg( ) E 6). extracted by solutions of CyMe4-BTBP 1 and TODGA.14–17
I
I
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We acknowledge Financial support from the Commission of the
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