Spectrophotometric Analysis of Ternary Uranyl Systems to Replace Tri-N-butyl Phosphate (TBP) in Used Fuel Reprocessing

Article abstract of DOI:10.1007/s10953-019-00942-7

In this report, the interaction of monoamide/diamide and monoamide/diglycolamide mixtures with UO22+ are investigated in pH = 1 methanolic nitric acid media. These monoamides include N,N-dimethylacetamide (DMAA), N,N-diethylacetamide (DEAA), N,N-dibutylacetamide (DBAA) and N,N-dibutylbutanamide (DBBA). N,N,N′N′-tetraethylmalonamide (TEMA) and N,N,N′,N′-tetraethyldiglycolamide (TEDGA), which were chosen as model diamides and diglycolamides, respectively. Complex stability constants for each ligand were modelled using the Stability Quotients Using Absorbance Data program using UV–visible data. Complex stoichiometry of ligand mixtures was determined using Job plots and UV–Vis spectrometry. Monoamides were confirmed to produce only disolvate complexes with UO22+ in solution. The log₁₀(K) values for monoamides were found to be independent of amine-side chain length, but were slightly dependent on the carbonyl-side chain length. TEDGA was found to produce multiple uranyl complexes in solution. Job plot data indicated that the uranyl cation strongly prefers to bond either only with the monoamide or diamide in ternary monoamide–diamide–UO₂ systems. Monoamide–diglycolamide–UO₂ systems were more complicated, with Job plot data indicating the potential for multiple ternary species being present is dependent on the monoamide structure.

Full text of DOI:10.1007/s10953-019-00942-7

Journal of Solution Chemistry
https://doi.org/10.1007/s10953-019-00942-7
Spectrophotometric Analysis of Ternary Uranyl Systems
to Replace Tri‑N‑butyl Phosphate (TBP) in Used Fuel
Reprocessing
Adam J. Canner1 · Laurence M. Harwood2 · Joseph Cowell2 · Jasraj S. Babra2 ·
Solomon F. Brown1 · Mark D. Ogden1
Received: 12 April 2019 / Accepted: 12 August 2019
© The Author(s) 2020
Abstract
In this report, the interaction of monoamide/diamide and monoamide/diglycolamide
mixtures with UO2+ are investigated in pH = 1 methanolic nitric acid media. These
2
monoamides include N,N-dimethylacetamide (DMAA), N,N-diethylacetamide (DEAA),
N,N-dibutylacetamide (DBAA) and N,N-dibutylbutanamide (DBBA). N,N,N′N′-
tetraethylmalonamide (TEMA) and N,N,N′,N′-tetraethyldiglycolamide (TEDGA), which
were chosen as model diamides and diglycolamides, respectively. Complex stability
constants for each ligand were modelled using the Stability Quotients Using Absorbance
Data program using UV–visible data. Complex stoichiometry of ligand mixtures was
determined using Job plots and UV–Vis spectrometry. Monoamides were confrmed
to produce only disolvate complexes with UO2+ in solution. The log10(K) values for
2
monoamides were found to be independent of amine-side chain length, but were slightly
dependent on the carbonyl-side chain length. TEDGA was found to produce multiple
uranyl complexes in solution. Job plot data indicated that the uranyl cation strongly prefers
to bond either only with the monoamide or diamide in ternary monoamide–diamide–UO2
systems. Monoamide–diglycolamide–UO2 systems were more complicated, with Job plot
data indicating the potential for multiple ternary species being present is dependent on the
monoamide structure.
Keywords UV–Vis · Uranium · Amides · Job plot · Speciation
1 Introduction
A signifcant limitation in the development of advanced uranium nuclear fuel cycles is the
efcient and clean treatment of irradiated nuclear fuel (INF) [1]. INF becomes unusable in
conventional nuclear power reactors due to the accumulation of neutron-absorbing nuclei
* Adam J. Canner
acanner1@shefeld.ac.uk
1
Department of Chemical and Biological Engineering, University of Shefeld, Shefeld, UK
2
Department of Chemistry, University of Reading, Whiteknights, Reading, Berkshire, UK
1 3
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Journal of Solution Chemistry
[2]. Reprocessing this waste would not only allow for MOX fuel production from recov-
ered U and Pu, but it would also substantially reduce the volume and heat load of INF
waste to be stored in a geological disposal facility (GDF) [3], as well as the time required
to store the waste. This would ease GDF site selection and simplify construction.
The current industry standard process for reprocessing INF is the Plutonium Uranium
Reduction EXtraction (PUREX) process [4]. Generally, this process exploits the selec-
tivity of tri-N-butyl phosphate (TBP) for tetra- and hexavalent cations to separate U(VI)
and Pu(IV) from INF liquors, followed by Pu reduction to separate U from Pu. However,
the use of TBP sufers many disadvantages [5]. These include undesired cation and anion
extraction [6–9], and the formation of third phases, stabilized by TBP degradation products
that retain radionuclides [10, 11]. The incineration of the degraded solvent at end-of-life
results in the formation of an active phosphate residue [12, 13]. These disadvantages result
in the need for extra process steps to refne the U and Pu product streams, and in the forma-
tion of radioactive phosphate secondary wastes that are difcult and expensive to process.
These issues could be eliminated, or somewhat mitigated, through the exclusive use of
CHON extractants; ligands that only incorporate carbon, hydrogen, oxygen and nitrogen in
their molecular structure. Such ligands are completely incinerable, thereby any active resi-
due should be readily incorporated in the high level liquid waste rafnate from PUREX.
Linear N,N-dialkylamides have long been studied as potential TBP replacements within
PUREX chemistry since the work of Siddall [14]. Their degradation products are relatively
innocuous to the solution chemistry (mainly carboxylic acids and secondary amines [15])
which, if small enough, may even leave the reprocessing fowsheet in the aqueous rafnate
rather than remain in the organic phase to be removed. After many studies, N,N-dihexy-
loctanamide (DHOA) has been presented as one of the best potential TBP replacements
[16–19]. It boasts excellent Pu recovery; however, U recovery is less efcient than with
TBP [17]. This has largely been attributed to the low solubility of the uranyl–nitrato–mon-
oamide complex in the organic phase [20]. While researchers have focused on DHOA’s
afnity for tetravalent cations in the developing THOREX process, there has currently been
no work to attempt to understand synergies in DHOA to improve U(VI) afnity with other
CHON ligands.
Synergism is the phenomenon whereby extraction of a metal ion with two ligands is
greater than the summed extraction by the two ligands separately [21]. Classic ligand syn-
ergism is usually achieved in similar solvent extraction (SX) processes by combining neu-
tral extractants with lipophilic Brønstead acids [22]; however, these lipophilic acids are
almost always organophosphorus compounds, which negates the benefts of using CHON
ligands; for example, DMDOHEMA and HDEHP in the DIAMEX-SANEX process [23],
or TODGA/TEHDGA and HEH(EHP) in the ALSEP process [24]. The addition of the
acidic extractant in these cases is mainly to facilitate the pH dependent separation of lan-
thanides and actinides. However, as the main issue identifed with U–monoamide sys-
tems is poor solubility of the uranyl complexes in the organic diluent, it follows that if
the U–DHOA complex is made more hydrophobic through interactions with larger amide-
based species, U recovery could potentially be increased to the point whereby a CHON-
based reprocessing fowsheet becomes feasible.
The present study is a frst step towards investigating the possibility of utilising
amide-based ligand mixtures to improve monoamide recovery of uranium from spent
fuel liquors. Monoamide–diamide and monoamide–diglycolamide mixtures have been
used to see how these ligand mixtures interact with U(VI). Ultraviolet–visible (UV–Vis)
spectra have been analyzed to assess what complexes form between uranyl cations and
several amide-based ligands in pseudo-aqueous media. The pseudo-aqueous media were
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Journal of Solution Chemistry
used to aid the dissolution of some of the more hydrophobic ligands, and have also been
reported to give clearer absorbance readings [25]. The efect of increasing monoamide size
on these interactions has been investigated. The tested ligands, shown in Scheme 1, are
N,N-dimethylacetamide (DMAA), N,N-diethylacetamide (DEAA), N,N-dibutylacetamide
(DBAA) and N,N-dibutylbutanamide (DBBA) with N1,N1,N3,N3-tetraethyl malonamide
(TEMA) or N1,N1,N5,N5-tetraethyl diglycolamide (TEDGA). Small ligands were used
as the focus of this study in order to investigate the fundamental interactions between
monoamide, diamide and diglycolamide species; as such, using larger ligands such as
DHOA was deemed unnecessary. Spectrophotometric titrations of ligands were conducted
to assess complex stoichiometry and stability. Job plots were used to assess the dominant
complex in solutions of amide ligand mixtures.
2 Experimental Methods
2.1 Materials
All reagents were ACS reagent grade or higher purity. Nitric acid (70%, Merck), metha-
nol (99.8%, Merck) DMAA (99.8%, Sigma–Aldrich) and DEAA (95%, Fluorochem) were
used as received. Uranyl nitrate hexahydrate was supplied by the Immobilisation Science
Laboratory at Shefeld University, and was dissolved in pH = 1 nitric acid to generate a
concentrated mother solution of uranyl nitrate. Aliquots from this solution were used to
generate uranyl nitrate working solutions of the required concentration in methanolic nitric
media (50:50 vol% MeOH:deionized water). All tests were conducted at pH = 1 and the
0.2 mol·L−1 ionic strength was controlled by addition of NaNO3.
2.2 Ligand Synthesis
DBAA and DBBA were synthesized through the reaction of acetyl chloride (98%, Alfa
Aesar) or butyryl chloride (99%, Acros Organics), respectively, in chloroform with equi-
molar dibutylamine (99.5%, Sigma–Aldrich) in the presence of equimolar triethylamine
base (99%, Acros Organics) in a stirred ice bath. The solution was then heated to refux
O
O
O
O
O
O
O
N
N
N
N
N
N
DMAA
DBAA
TEMA
TEDGA
O
N
O
N
DEAA
DBBA
Scheme 1 Structure and abbreviation of the monoamides (left), diamide (middle) and diglycolamide (right)
used in this study
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at the boiling point of chloroform for at least 2 h. The organic product was washed with
deionized water, 10 wt% Na2CO3 solution, 1.2 mol·L−1 HCl solution and a fnal deionized
water wash. The organic layer was dried over anhydrous Na2SO4, fltered, and the solvent
was removed under reduced pressure. Both ligands were>95% pure. The DBAA yield was
1
76.5%, and the DBBA yield was 81.9%. DBAA H NMR (500 MHz, CDCl3): δ 0.68 (m,
6H), 1.07 (m, 4H), 1.27 (m, 4H), 1.82 (s, 3H), 2.98 (t, 2H), 3.05 (t, 2H). DBBA 1H NMR
(500 MHz, CDCl3): δ 0.82 (m, 9H), 1.12 (m, 4H), 1.40 (m, 4H), 1.55 (m, 2H), 2.15 (t, 2H),
3.10 (t, 2H), 3.19 (t, 2H).
TEMA and TEDGA syntheses were carried out and supplied by Reading University,
UK. TEMA was synthesized by the addition of 2 mol. equivalents of hydroxybenzotriazole
(HOBt), triethylamine, diethylamine and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide
hydrochloride (EDC·HCl) to a suspension of malonic acid in chloroform. TEDGA was
synthesized by the addition of 2 mol. equivalents of HOBt, triethylamine, diethylamine
and EDC·HCl to a suspension of 2,2′-oxydiacetic acid in chloroform. Both reactions were
stirred at room temperature for 18 h. The crude reactions were washed with 1 mol·L−1 HCl
and 1 mol·L−1 NaOH. The combined organic layers were separately dried over MgSO4,
fltered, and the solvent was removed under reduced pressure. The TEMA yield was
1
86%. The TEDGA yield was 95%. TEMA H NMR (400 MHz, CDCl3) δ 3.46–3.37 (m,
10H), 1.20 (t, J=6.8 Hz, 6H), 1.14 (t, J=7.2 Hz, 6H); 13C NMR (101 MHz, CDCl3) δ
166.4, 42.6, 40.6, 40.3, 14.1, 12.8; (FTMS+pESI) calculated: C11H22N2Na [M+Na]+:
1
237.1573 g·mol−1; observed: 237.1578 g·mol−1. TEDGA H NMR (400 MHz, CDCl3) δ
4.30 (s, 4H), 3.35 (app dq, J=27.8, 7.1 Hz, 8H), 1.16 (app dt, J=17.6, 7.1 Hz, 12H);
13C NMR (101 MHz, CDCl3) δ 167.9, 77.5, 77.4 77.2, 76.9, 69.4, 41.0, 40.0, 14.2, 12.9;
(FTMS+pESI) calculated: C12H24N2O3Na [M+Na]+: 267.1679 g·mol−1; observed:
267.1687 g·mol−1.
Ligand solutions were prepared by diluting the required mass of ligand with pre-pre-
pared pH = 1 methanolic nitric acid at 0.2 mol·L−1 ionic strength.
2.3 Spectrophotometric Titrations and Stability Constants
Incremental additions of 0.125 mol·L−1 ligand solution were made via a burette into a
stirred 0.025 mol·L−1 uranyl nitrate solution. After each addition, a small aliquot was taken
and its absorption spectrum recorded in a 1 cm path length quartz cuvette. The aliquot
was then reintroduced into the experiment. Volume losses were minimal. Spectra were
recorded on a VWR UV-6300PC Double Beam Spectrophotometer calibrated between 190
and 1100 nm. Absorption across this entire range was recorded; however extreme absorp-
tion from the nitrate anions prevented reliable readings below~330 nm. Concentrations
of the initial uranium solution were confrmed against a calibration curve. It was assumed
that the pH and ionic strength remained constant throughout the experiments. For each
test, 24 samples were taken as this was the upper limit of sample inputs for the MODS-
QUAD version of the SQUAD (Stability Quotients Using Absorbance Data) program. It
was found that above a ligand molar excess of~4.6, dilution efects dominate the spectral
changes which reduces the accuracy of SQUAD. To that end, the ligand was added in 0.2
molar steps to uranyl nitrate. All tests were carried out in triplicate at room temperature
(19 2 °C).
Stability constants were determined using the MODSQUAD version of the SQUAD
program considering metal dilution [26]. SQUAD utilizes a nonlinear least-squares
approach to calculate the best values for complex stability constants in a given model by
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Journal of Solution Chemistry
reducing the standard deviation of the absorbance data, refned stability constant and of
each spectrum through an iterative process. SQUAD convergence confrmed the validity of
each tested model, but this validity was also checked through comparison of the outputted
molar absorptivities and concentrations of each model component with the experimentally
determined spectra. All stability constants reported in this work were calculated over the
range 350–499 nm. The Gibbs energy of complex formation ΔGꢀ was calculated from the
determined stability constants using Eq. 1:
ΔGꢀ = −2.303RT log10(Kxyz
)
(1)
where R is the gas constant (8.314 J·mol−1·K−1), T is the absolute temperature, and Kxyz
is the stability constant; x, y and z denote the number of metal ions, protons and ligands
involved in the complex, respectively.
2.4 Job Plots
Job’s method is an established method for the determination of metal complex stoichiom-
etry [27]. By varying the concentration of metal to ligand, or ligand to ligand, the stoi-
chiometry of the dominant species can be determined by monitoring the absorbance of
the solution. After accounting for the absorbance of the individual solution components,
the remaining absorbance is due to the complexed species. When plotting this absorbance
against mole fraction of ligand, a peak in this absorbance (and hence species concentra-
tion) describes the stoichiometry of the dominant species. For example, a peak at 0.5 mol
fraction ligand indicates a 1:1 species; a peak at 0.67 indicates a 2:1 species, etc.
For single-ligand Job plots, separate stock solutions of uranyl nitrate, DMAA, DEAA,
DBAA, DBBA, TEMA and TEDGA were generated at equal concentration. Varying vol-
umes of uranyl nitrate and ligand solutions were mixed in separate vials to result in the
required mole fraction of metal and ligand. Spectra were recorded as above. All tests were
carried out in triplicate at room temperature (19 2 °C).
For mixed ligand Job plots, U–ligand stock solutions were prepared such that
the ligand was in 4.6 fold molar excess to uranium, mimicking the end point of the
spectrophotometric titrations. Varying volumes of U–monoamide/U–diamide or
U–monoamide/U–diglycolamide solutions were mixed in separate vials to result in the
required mole fraction of diamide or diglycolamide, respectively. Uranium concentration
was assumed to be constant throughout the experiment. Spectra were recorded as above.
All tests were carried out in triplicate at room temperature (19 °C 2 °C).
3 Results
3.1 Uranyl Speciation in Pseudo‑Aqueous Media
Figure 1 shows the molar absorptivity of uranyl nitrate in pH = 1 nitric acid in both aque-
ous and pseudo-aqueous media at 0.2 mol·L−1 total ionic strength. This test was to con-
frm that no uranyl hydrolysis has taken place when introduced into the methanolic nitric
acid media. The retention of the distinct spectral “fngerprint” in the ligand-to-metal charge
transfer band (LMCT,~350–500 nm) indicates there is no hydrolysis of the uranyl species.
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Journal of Solution Chemistry
Fig. 1 Molar absorptivity of
uranyl nitrate in both pH = 1
nitric acid and pH = 1 metha-
nolic nitric acid (50:50 vol%
methanol:deionized water) at
0.2 mol·L−1 total ionic strength
Fig. 2 Spectrophotometric titrations with incremental additions of 0.125 mol·L−1 ligand into UO2+
2
in pH = 1 methanolic nitric acid (50:50 vol% MeOH:deionized water) at 0.2 mol·L−1 ionic strength.
The black solid and dashed lines denote the start and end points, respectively. Results for a DMAA into
0.0251 mol·L−1 UO2+ up to [M]:[L]=1:4.6, b DEAA into 0.0250 mol·L−1 UO2+ up to [M]:[L]=1:4.6,
2
2
c DBAA into 0.0247 mol·L−1 UO2+ up to [M]:[L]=1:4.7, d DBBA into 0.0243 mol·L−1 UO2+ up to
2
2
[M]:[L]=1:4.7, e TEMA into 0.0249 mol·L−1 UO2+ up to [M]:[L]=1:4.6, f TEDGA into 0.0254 mol·L−1
2
UO2+ up to [M]:[L]=1:4.5
2
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Journal of Solution Chemistry
Table 1 U(VI) stability
constants 2 SD at 19 °C in
pH = 1 methanolic nitric acid
(50:50 vol% MeOH:deionized
water) at 0.2 mol·L−1 ionic
strength
Ligand
log10K101
log10K102
log10K104
DMAA
DEAA
DBAA
DBBA
TEMA
TEDGA
–
–
–
–
3.74 0.02
3.86 0.02
3.72 0.02
4.03 0.03
4.28 0.02
4.85 0.04
–
–
–
–
–
–
2.44 0.03
9.0 0.1
“–”indicates that no successful SQUAD model incorporated this com-
plex. For Kxyz and ΔGxyz; x, y and z denote the number of metal ions,
protons and ligands involved in the complex, respectively
Table 2 G 2 SD (kJ·mol−1
)
Ligand
ΔG101
ΔG102
ΔG104
at 19 °C in pH = 1
methanolic nitric (50:50 vol%
MeOH:deionized water) at
0.2 mol·L−1 ionic strength
DMAA
DEAA
DBAA
DBBA
TEMA
TEDGA
–
–
–
–
−20.9 0.1
−21.6 0.1
−20.8 0.1
−22.5 0.2
−23.1 0.1
−27.1 0.2
–
–
–
–
–
–
− 13.6 0.2
− 50.9 0.6
“–”indicates that no successful SQUAD model incorporated this com-
plex. For Kxyz and ΔGxyz; x, y and z denote the number of metal ions,
protons and ligands involved in the complex, respectively
3.2 Single‑Ligand Complex Speciation and Stability
Figure 2 shows the results of the spectrophotometric titrations within the range
350–500 nm. This spectral region is dominated by a LMCT band [28], and as such, it is
sensitive to the ligand environment in the uranyl equatorial plane. As this band is not the
result of f–f electronic transitions, it is not subject to selection rules. Therefore, “silent”
complexes with linear actinyl cations identifed by Tian et al. [29] and Rao and Tian [30]
need not be considered in the modelling process. The initial and fnal solutions are denoted
by the thick and dashed black lines, respectively. The grey lines denote 0.4 molar steps of
ligand to uranium.
The stability constants for the U(VI) complexes with the tested ligands calculated by
SQUAD are shown in Table 1. The Gibbs energies for these complexes at 19 °C are given
in Table 2. The molar absorptivities of the SQUAD-proposed complexes are shown in
Fig. 3.
Speciation of the dominant single ligand complex with uranyl nitrate and each of the
tested ligands is shown in the Job plots in Fig. 4. These were conducted to confrm the
validity of reported species from MODSQUAD. It is difcult to infer any kind of trend for
the monoamide systems shown in Fig. 4a–d; due to the relatively small amount of measur-
able interaction, even small amounts of error dominate any perceivable trend in the data.
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Journal of Solution Chemistry
Fig. 3 Molar absorptivities of the modelled uranyl complexes in pH = 1 methanolic nitric acid (50:50 vol%
MeOH:deionized water) at 0.2 mol·L−1 ionic strength given by SQUAD. a DMAA, b DEAA, c DBAA, d
DBBA, e TEMA, f TEDGA
3.3 Mixed‑Ligand Complex Speciation
Figure 5 shows the Job plots for the tested monoamide–TEMA systems, and Fig. 6 shows
the Job plots for the tested monoamide–TEDGA systems, both at 0.2 mol·L−1 ionic
strength. The ligand is in 4.6×molar excess to uranium, so these conditions mimic the end
point of the spectrophotometric titrations. Absorbance has been corrected through multiple
steps; (1) subtracting the efects of the uranyl cation and both ligands on absorption, (2)
zeroing at 700 nm to minimize the efects of any baseline deviation, (3) baseline corrected
to ensure absorption is zero when the mole fraction of TEMA/TEDGA is 0 and 1. Step 3
is a crude but necessary step to take account of absorption change resulting from uranyl
complexation with the single ligands (i.e. U–monoamide, U–diamide or U–diglycolamide
species).
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Journal of Solution Chemistry
Fig. 4 Job plots of tested ligands with uranyl nitrate in pH = 1 methanolic nitric acid (50:50 vol%
MeOH:deionized water) at 0.2 mol·L−1 ionic strength. a DMAA (419 nm), b DEAA (419 nm), c DBAA
(419 nm), d DBBA (419 nm), e TEMA (418 nm), f TEDGA (417 nm)
4 Discussion
4.1 Uranyl Speciation
It has previously been reported that the uranyl cation exists as a hydrolyzed (UO2)2O2+ dimer
species in methanolic media [31, 32]. In the case of this work, nitric acid prevents the forma-
tion of such species, as seen from the retention of the distinct spectral fngerprint in Fig. 1. As
such, all models tested with SQUAD consider a single UO22+ at the center of the complex.
Due to the uranyl cation existing in solution as a linear dioxo cation [33], any bond-
ing is restricted to the equatorial plane. It is unusual for bonding/coordination number of
uranium(VI) to exceed 8 (6+2 from covalently bonded O) [34]. However, it has been reported
that di- and tri-solvate uranyl nitrate species are produced with TODGA in acidic nitric media,
potentially indicating the formation of a higher coordinated uranium species [35–38]. There-
fore, assuming charge neutralization by two nitrate anions each via two oxygen donors, it
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Journal of Solution Chemistry
Fig. 5 Job plots of the tested monoamide–diamide systems in pH = 1 methanolic nitric acid (50:50
vol% MeOH:deionized water) at 0.2 mol·L−1 ionic strength, [UO2+] held constant at 0.013 mol·L−1 and
2
[M]:[total L]=4.6. a DMAA–TEMA (416 nm), b DEAA–TEMA (416 nm), c DBAA–TEMA (419 nm), d
DBBA–TEMA (419 nm)
Fig. 6 Job plots of the tested monoamide–diglycolamide systems in pH = 1 methanolic nitric (50:50 vol%
MeOH:deionized water) at 0.2 mol·L−1 ionic strength, UO2+=0.013 mol·L−1; a DMAA–TEDGA (420 nm),
2
b DEAA–TEDGA (420 nm), c DBAA–TEDGA (420 nm), d DBBA–TEDGA (420 nm)
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Journal of Solution Chemistry
follows that the formed complexes can contain no more (and probably less) than four coordi-
nation bonds from adduct-forming ligands. To this end, this was the limiting factor for models
considered within SQUAD.
4.2 Single‑Ligand Complex Speciation and Stability
4.2.1 UO2(NO3)2–Monoamide Species
Monoamide complexes (Fig. 2a–d) all produce very similar responses in the UV–Vis
absorption spectra; a slight bathochromic shifting of the peaks, apparent loss in vibrational
coupling and the production of at least one isosbestic point at longer wavelengths. As only
one set of “new” peaks appear to arise, this indicates that only one complex is formed in
solution. The molar absorptivity of these solutions increase slightly when dilution of the
metal ion is taken into account, indicating complex formation. Increasing the size of the
amine-side chains induces a larger increase in molar absorptivity. However, increasing the
carbonyl-side chain appears to reduce it.
The isosbestic points observed in the monoamide spectra are good evidence that at
least two species are present in solution [39], including the uranyl cation. The best ftting
model indicates that only a 102 complex is produced, as shown in Table 1. It appears that
even when the ligand concentration is less than the uranyl concentration, a 102 complex is
strongly preferred over a potential 101 complex. These fndings also correlate with many
studies that use slope analysis to determine complex stoichiometry [5, 40–42], recent struc-
tural and spectroscopic studies of uranyl–monoamide complexes [43, 44], as well as previ-
ous spectrophotometric studies with hexavalent actinides [45, 46] when considering these
systems at low acidity ([HNO3]<4 mol·L−1).
There is no discernible trend in the UO2–monoamide stability constant as the amine-
side chain increases in size. However, it is observed that increasing the carbonyl-side chain
slightly increases stability. The larger carbonyl-side chain likely provides stability to the
amide bond, resulting in a larger donor strength on the carbonyl oxygen. The stability con-
stants determined by SQUAD are comparable to those determined by Prabhu et al. [42],
which lends support to the SQUAD methodology for stability constant determination.
4.2.2 UO2–Diamide Species
The spectrophotometric titration results for TEMA indicate that diamide interactions with
the uranyl cation are similar to those of monoamide interactions, but there are signifcant
diferences. More signifcant bathochromic shifting indicates the formation of more stable
species; this is inferred by the species absorbing a lower energy photon. Like the mono-
amides, the nature of the peak shift indicates that only one complex is formed in solution.
Modelling shows the most likely scenario is the formation of the 102 complex, as seen in
Table 1. This is in agreement with the similar monoamide spectra above, and also with lit-
erature results [47]. The higher stability of the diamide complex comes from the bidentate
nature of the ligands. It should be noted that the 101 models were also seen to be possible,
but less likely.
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Journal of Solution Chemistry
4.2.3 UO2–Diglycolamide Species
Initially in the spectrophotometric results for TEDGA, the “fngerprint” is maintained
with a large increase in molar absorptivity. As [M]:[L] increases, peak broadening
and signifcant bathochromic shifting is observed. While this could be a sign of ura-
nyl hydrolysis, the molar absorptivities indicate that the 102 and 104 species both have
broadened spectral peaks. This could possibly be due to Jahn–Teller distortions; this
implies that ftting of these ligands around the metal center distorts the bond length of
the double bonded oxygens, causing band splitting which ultimately results in a peak
broadening efect when the spectra are superimposed. The log10(K101) and log10(K102
)
values reported in Table 1 are roughly twice as large as those reported for the linear
NpO+ with tetramethyl diglycolamide [30]. This diference is likely due to the larger
form2al charge on the uranyl cation.
The Job plot of UO2 and TEDGA defnitively shows the dominant solution complex
is a 101 species, which contradicts the slope analysis studies in the literature, which
generally report the 102 species being dominant. As mentioned previously, 103 com-
plexes with diglycolamides are also reported by Sasaki et al. [36] and Peng et al. [38],
both determined from slope analysis. All models tested with a 103 complex resulted in
large amounts of error; the best ftting model of this kind was tested for 101, 102, 103
and 104 complexes. As such, it was decided that these results likely do not suggest the
presence of a 103 complex with TEDGA. It is possible that the gradient from the slope
analysis was due to a mixture of diglycolamide species, such as the range of species
reported in the present work, rather than a defnitive 103 complex.
4.3 Mixed‑Ligand Complex Speciation
These Job plots were conducted with the aim of determining the ratio of monoamide to
diamide/diglycolamide in uranyl complexes in these systems. It should be noted that the
diglycolamide solutions used for the Job plot tests already contain multiple uranyl species,
as is evident from Fig. 7. This immediately arises a source of error when baseline correct-
ing for the efect of the initial and fnal complexes in solution. To baseline correct this, a
straight line was drawn from TEMA or TEDGA at mole fraction=0 and 1.0, and this was
taken to be the baseline. While the total ligand concentration remained constant throughout
Fig. 7 Speciation of the solution
components and proposed U(VI)
complexes with TEDGA against
[M]:[L] calculated by SQUAD
in pH = 1 methanolic nitric acid
(50:50 vol% MeOH:deionized
water) at 0.2 mol·L−1 ionic
strength
1 3

Journal of Solution Chemistry
the test, the changing concentrations of each individual ligand may cause a change in com-
plex speciation. This may mean taking a linear baseline lends some inaccuracy; however it
will at least serve as a good approximation as to what is occurring in solution.
4.3.1 UO2–Monoamide–Diamide Species
Due to the small absorbance changes observed, it is likely that the following discussion is
not entirely representative of the solution chemistry in these systems, but it is presented
here for qualitative purposes.
The low absorption change in the monoamide–diamide Job plots (Fig. 5) indicate there
is very little mixed interaction between the uranyl cation and both ligands. It appears that
the uranyl center strongly prefers to bond with either the monoamide or the diamide. Isos-
bestic points were observed in the Job plot spectra of DBAA–TEMA and DBBA–TEMA,
indicating the presence of multiple UO2 species, likely the 102 complex of both the mono-
and diamide. Absorption generally peaks at a TEMA mole fraction of 0.5, indicating the
formation of a 1:1 monoamide:diamide complex with uranyl nitrate. This species is cer-
tainly possible from a steric perspective, however, the lack of absorption in Fig. 5 suggests
that this species is in low concentration.
4.3.2 UO2(NO3)2–Monoamide–Diglycolamide Species
The monoamide–diglycolamide Job plots shown in Fig. 6 indicate this system is much more
complex than the monoamide–diamide system. The relatively large increases in absorp-
tion indicate the presence of mixed ligand complexes. As the monoamide size increases,
an increased absorbance change is observed which indicates an increase in complex con-
centration, inferring greater stability. Absorbance peaks at 0.5 mol fraction of TEDGA for
the DBAA–TEDGA system, indicate the formation of a 1:1 monoamide:diglycolamide
complex. However, the other three tested systems all clearly have a peak at 0.4 mol frac-
tion of TEDGA. This could have two interpretations; the predominant complex in solu-
tion has a ratio of 3:2 monoamide:diglycolamide, or there is a mixture of 1:1 and 2:1
monoamide:diglycolamide species. From a steric perspective, it can be argued that the lat-
ter scenario is more likely. However, EXAFS data would be required to underpin quite how
these ligands are arranged. It appears that the stoichiometry of the mixed ligand complex
is not based on the stability of the single ligand complexes; if it were the case, it might be
expected that the DMAA–TEDGA systems also solely produce 1:1 complexes, which are
not seen in the data. Nevertheless, it is suggested that mixed-ligand species are present in
solution, and the diference in speciation would undoubtedly result in changes in complex
hydrophobicity. A key limitation in the application of monoamides to spent fuel reprocess-
ing is the solubility of the extracted complex in the organic phase. Therefore, the next step
will be to see how these systems behave in a solvent extraction setting.
5 Conclusions
There is little information in the literature associated with interactions between monoam-
ides and amide-based ligands. This may be of use for developing more efcient, sustain-
able nuclear fuel waste treatment. In this study, the interactions between monoamides and
1 3

Journal of Solution Chemistry
2+
diamides or diglycolamides with UO2 were investigated in pseudo-aqueous media. The
efect of changing the monoamide structure on these interactions was also investigated.
As expected, monoamides were confrmed to produce 2:1 ligand:metal complexes with the
uranyl cation, and diamides followed this trend. Diglycolamides were seen to produce mul-
tiple species of uranyl complexes, up to 104, leading to more complex solution chemistry
than previously thought. While diamides have similar complex behavior to monoamides,
Job plots suggest that the uranyl cation strongly prefers to bond exclusively with only mon-
oamide or diamide species, regardless of the monoamide structure. Diglycolamides were
shown to produce stronger complexes with UO2 than monoamides. However, Job plots
suggest that multiple mixed-ligand species are produced in the UO2–monoamide–digly-
colamide systems, with no clear indication on the dominant species in solution. It is likely
that both 1:1 and 2:1 monoamide:diglycolamide species are produced in solution. Future
2+
work needs to assess monoamide–diglycolamide interactions with UO2 in more detail,
perhaps with EXAFS data to underpin complex geometry.
Acknowledgement The authors would like to acknowledge the SNUCER group at the University of Shef-
feld for support through this work. They would also like to acknowledge the Harwood group at the Univer-
sity of Reading for ligand synthesis. Finally, the authors would like to thank EPSRC for funding this work.
Funding This work was funded through an EPSRC Doctoral Training Grant through the Chemical and Bio-
logical Engineering Department at the University of Shefeld.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
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as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Com-
mons licence, and indicate if changes were made. The images or other third party material in this article
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permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly
from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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