Extraction of actinides by Tri-n-butyl phosphate derivatives: Effect of substituents

Article abstract of DOI:10.1016/j.ica.2017.07.048

Tri-n-butyl phosphate (TBP) is most extensively used extractant in the nuclear fuel cycle. The present work investigates the effect of substituents and their role in the basicity of organophosphorus extractant on the uptake of actinides. In this connection, we have synthesized six different analogues of TBP by altering one of its butoxy group. The synthesized TBP derivatives were well characterized and evaluated for their solvent extraction behavior towards U(VI), Th(IV), Pu(IV) and Am(III), as well as acid uptake as a function of nitric acid ranging from 0.01 to 6 M and the data provides the comparison of their extraction behavior with that of 30% (1.1 M) tri-n-butyl phosphate under identical conditions. It was observed that distribution coefficient values strongly depend on the nature and size of the substituents. The presence of electron donating groups enhances the uptake of the actinides and the distribution coefficient values were significantly larger as compared to that of TBP. In addition, the effect of sodium nitrate on the extraction of uranium and enthalpy of extraction were also studied and revealed that the extraction process was exothermic.

Full text of DOI:10.1016/j.ica.2017.07.048

Accepted Manuscript
Research paper
Extraction of Actinides by Tri-n-butyl Phosphate Derivatives: Effect of Sub-
stituents
Suresh Annam, Gopinadhanpillai Gopakumar, C.V.S. Brahmmananda Rao, N.
Sivaraman, Akella Sivaramakrishna, Kari Vijayakrishna
PII:
S0020-1693(17)30981-7
http://dx.doi.org/10.1016/j.ica.2017.07.048
ICA 17771
DOI:
Reference:
Inorganica Chimica Acta
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Accepted Date:
19 July 2017
24 July 2017
Please cite this article as: S. Annam, G. Gopakumar, C.V.S. Brahmmananda Rao, N. Sivaraman, A. Sivaramakrishna,
K. Vijayakrishna, Extraction of Actinides by Tri-n-butyl Phosphate Derivatives: Effect of Substituents, Inorganica
Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.07.048
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Extraction of Actinides by Tri-n-butyl Phosphate
Derivatives: Effect of Substituents
Suresh Annam1, Gopinadhanpillai Gopakumar2, C.V.S. Brahmmananda Rao2, N. Sivaraman2,
Akella Sivaramakrishna1, Kari Vijayakrishna1,*
1Department of Chemistry, School of Advanced Sciences, VIT University, Vellore-632014, Tamil Nadu, India.
2Material Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research (IGCAR),
Kalpakkam-603102, Tamil Nadu, India.
*Corresponding Author:
E-mail: kari@vit.ac.in, vijayakrishnakari@gmail.com
Kari Vijayakrishna,
Department of Chemistry, School of Advanced Sciences,
VIT University, Vellore 632014, Tamil Nadu, India
Telephone: +91 416 224 2334
Fax: +91 416 224 3092
.

Abstract
Tri-n-butyl phosphate (TBP) is most extensively used extractant in the nuclear fuel cycle.
The present work investigates the effect of substituents and their role in the basicity of
organophosphorus extractant on the uptake of actinides. In this connection, we have synthesized
six different analogues of TBP by altering one of its butoxy group. The synthesized TBP
derivatives were well characterized and evaluated for their solvent extraction behavior towards
U(VI), Th(IV), Pu(IV) and Am(III), as well as acid uptake as a function of nitric acid ranging
from 0.01–6 M and the data provides the comparison of their extraction behavior with that of
30% (1.1 M) tri-n-butyl phosphate under identical conditions. It was observed that distribution
coefficient values strongly depend on the nature and size of the substituents. The presence of
electron donating groups enhances the uptake of the actinides and the distribution coefficient
values were significantly larger as compared to that of TBP. In addition, the effect of sodium
nitrate on the extraction of uranium and enthalpy of extraction were also studied and revealed
that the extraction process was exothermic.
Keywords: Tri-n-butyl Phosphate; Phosphonates; Solvent extraction; Actinides; Density
functional theory.
1. Introduction
Tri-n-butyl phosphate (TBP) is one of the commonly used extractant in PUREX process
since last six decades and has been the workhorse for the recovery and purification of uranium
and plutonium from the irradiated nuclear fuels [1]. The process employs 30% (1.1M) solution of
TBP diluted with suitable diluents [2]. TBP has been the unanimous choice for the reprocessing
industry due to its optimum physical properties and its selectivity for uranium and plutonium
over the fission products. TBP has also been employed in various separation processes such as
separation of 233U from irradiated thorium by THOREX/Interim-23 process [3]. However, TBP
has some limitations such as undesirable aqueous solubility, radiation degradation and
susceptibility to third-phase formation under high loading of tetravalent actinides [4]. To
overcome these properties various higher homologous of TBP such as tri-n-amyl phosphate, tri-
n-hexyl phosphate (THP) [5], tri-iso-amyl phosphate (TAP) [6] etc., were studied and found to
have a lesser tendency towards third phase formation coupled with lower aqueous solubility.
Dialkyl amides are another class of extractants which have been developed as an alternative to

TBP [7]. Various types of ligands bearing S, O and N-donor atoms have been developed for the
separation of actinides from lanthanides [8-12]. Organophosphorus esters are the preferred class
of compounds for the extraction and separation of actinides. The basic oxygen atom of the
phosphoryl group is responsible for the complexing ability of TBP and other organophosphorus
ligands. Thus the extraction ability of the organophosphorus extractants depends on the basicity
of the phosphoryl oxygen and nature of the substituents attached to the phosphorus atom. It is
known that a structural change in the substituent at the phosphorus atom causes a perceptible
change in their physicochemical properties and extraction abilities [13]. The extraction ability
and the electronegativity of the phosphoryl oxygen can be enhanced by replacing the C–O–P
group in the extractant by a C–P group. The size and nature of the organic group in the esters
governs the degree of extraction since the solubility of the metal complex in the diluent is an
important parameter for higher loadings. The basicity of the phosphoryl oxygen in the neutral
organophosphorus extractant series increases in the order: phosphates < phosphonates <
phosphinates < phosphine oxides [14]. Phosphine oxides and phosphinates are more basic in
nature and hence stripping of metal ion extracted in the organic phase are difficult as compared
to phosphate and phosphonates. Phosphonates are moderately basic and the back extraction of
the metal ions from the organic phase can be carried out by adjusting the acidity of the aqueous
phase.
Extensive studies were carried out employing various H-phosphonates [15, 16],
phosphinic acids [17], symmetrical and unsymmetrical dialkylalkyl phosphonates by
Brahmmananda Rao et al. [18, 19]. In continuation with these studies on phosphonates, we have
synthesized the analogues of TBP by altering one of the butoxy group in the TBP by hydrogen,
hydroxyl, alkyl, aryl and amide substituents. The purpose of the present work is to ascertain and
understand the extraction behavior of actinides by altering the substituent on TBP molecule. In
this manuscript, we describe the extraction properties of compounds I-VI (Figure 1) towards
U(VI), Th(IV), Pu(IV) and Am(III) in the nitric acid medium at 303K. The extraction behavior
of these compounds was compared with TBP (VII).

Figure 1. Chemical structures of the extractants I-VII used in this study.
2. Experimental Section
2.1. Materials and Instrumentation
Phosphorus trichloride, 1-bromohexane, cyclohexyl bromide, l-butanol, sodium,
Arsenazo-III were procured from SD-Fine chemicals, India. Phenylphosphonic dichloride,
tributyl phosphite, tributyl phosphate (TBP), dibutyl phosphate, diethylcarbamoyl chloride were
procured from Sigma Aldrich. The solvents n-dodecane and xylene were purchased from Fluka.
The actinides, 233U, 239Pu and 241Am tracers were used from laboratory stock solutions. All other
chemicals used for synthesis and other experimental work are of reagent grade and used without
further purification.
The 1H, 13C, and 31P-NMR spectra were recorded using BRUKER DMX-400
spectrometer in appropriate deuterated solvents. FT-IR spectra were recorded on SHIMADZU
Affinity-1 FT-IR spectrometer with KBr pellet. UV–Vis absorption spectra were recorded using
SHIMADZU UV-3600 double-beam spectrophotometer. GC-MS spectra were recorded on
Perkin-Elmer Clarus 680 (GC) and Clarus 600 (EI Mass). Thermogravimetric analysis (TGA)

was carried out using a TA Thermogravimetric analyzer model Q500 from 25 °C to 300 °C with
a heating rate of 10 °C min-1 under nitrogen atmosphere.
2.2. Synthesis of extractants
The extractants I, III-VI were synthesized according to the modified literature reports
[20-23] and the synthetic routes were shown in Figure 2. Extractants II and VII were obtained
from the commercial sources.
Dibutylhydrogen phosphonate (DBHP) (I): 1-Butanol (76 g, 1.02 mol) was added drop-wise to
a mixture of phosphorus trichloride (47 g, 0.343 mol) in 200 mL of dichloromethane at 0 °C
under constant stirring. The mixture was stirred for 4 h at room temperature. Subsequently, the
reaction mixture was further diluted with dichloromethane and washed with saturated solution of
sodium carbonate, water and the organic phase was concentrated under reduced pressure. 60 g
(90% yield) of DBHP was collected as a colorless liquid. 1H NMR (400 MHz, CDCl3): δ = 7.65-
5.92 (d, 1JPH = 692 Hz, 1H), 4.08–4.03 (m, 4H), 1.69–1.69 (m, 4H), 1.44–1.35 (m, 4H), 0.94–
0.90 (t, 3H), ppm. 31P NMR (162.01 MHz, CDCl3): δ= 10.04-5.56(d, 1JPH = 691 Hz) ppm.13C
NMR (100 MHz, CDCl3) δ: 65.54, 65.48, 32.43, 32.40, 18.69, 13.50. GCMS (m/z): 195.21 [M]+.
Dibutylhexyl phosphonate (DBHeP) (III): Dibutyl H-phosphonate (40 g, 0.2 mol) was added
dropwise over a period of 30 min to a suspension of metallic sodium (5.68 g, 0.247 mol) in 100
mL toluene. After the metal had dissolved completely, freshly distilled n-bromohexane (37.5 g,
0.22 mol) was added. The mixture was refluxed for 5 h and the completion of the reaction was
monitored by 31P-NMR. The byproduct sodium bromide was separated by centrifugation, and the
solution was washed with distilled water and dried over sodium sulfate. The solvent was
removed under reduced pressure, and the residue was purified by column chromatography using
10% ethyl acetate/ n-hexane as an eluent. 37.5 g (65% yield) of DBHeP was collected after
column purification. 1H NMR (400 MHz, DMSO-d6): δ = 3.92-3.81 (m, 4H), 1.72–1.64 (m, 2H),
1.57–1.44 (m, 6H), 1.37–1.24 (m, 10H), 0.90-0.84 (m, 9H) ppm. 31P NMR (161.97 MHz,
DMSO-d6): δ= 31.96 ppm.13C NMR (100 MHz, DMSO-d6) δ: 64.31, 64.24, 32.09, 32.04, 30.75,
29.48, 29.32, 22.06, 22.01, 21.87, 18.25, 13.76, 13.35. GCMS (m/z): 279.25 [M] +.
Dibutylcyclohexyl phosphonate (DBCyHeP) (IV): The procedure adopted for the synthesis of
DBCyHeP is similar to DBHeP. The compound was prepared from dibutyl H-phosphonate (50 g,
1
0.25 mol) and bromocyclohexane (42 g, 0.25mol). The yield of the product is 42 g (60%). H
NMR (400 MHz, DMSO-d6): δ = 3.94-3.86 (m, 4H), 1.73–1.64 (m, 2H), 1.59–1.52 (m, 4H),

1.47–1.29 (m, 8H), 0.90-0.84 (m, 9H) ppm. 31P NMR (161.97 MHz, DMSO-d6): δ= 31.99
ppm.13C NMR (100 MHz, DMSO-d6) δ: 64.35, 64.29, 39.10, 38.89, 32.11, 32.05, 24.24, 24.19,
23.0, 22.84, 18.27, 13.40. GCMS (m/z): 277.30 [M] +.
Dibutylphenyl phosphonate (DBPP) (V): To a solution of n-butanol (24 g, 0.323 mol) and
triethylamine (32.69 g, 0.323 mol) in 100 mL of dichloromethane was added
dichlorophenylphosphine (30 g, 0.152 mol) in 30 mL of dichloromethane at 0 °C under nitrogen
atmosphere. The reaction mixture was subsequently stirred at room temperature for 24 h. The
resultant triethylamine hydrochloride in the reaction mixture was quenched with ice cold water.
The organic layer was separated and washed with saturated sodium bicarbonate, dried over
sodium sulfate and concentrated under reduced pressure to yield a colorless liquid. The yield of
the product is 33 g (80%). 1H NMR (400 MHz, DMSO-d6): δ = 7.73-7.68 (m, 2H), 7.65–7.54 (m,
3H), 3.94-3.94 (m, 4H), 1.60–1.53 (m, 4H), 1.36-1.37 (m, 4H), 0.87-0.83 (t, 3H) ppm. 31P NMR
(161.97 MHz, DMSO-d6): δ= 17.93 ppm. 13C NMR (100 MHz, DMSO-d6) δ: 132.44, 131.25,
131.15, 129.23, 128.74, 128.60, 127.47, 65.17, 65.11, 31.89, 31.83, 18.20, 13.31. GCMS (m/z):
271.18 [M] +.
Dibutyldiethylcarbamoyl phosphonate (DBDECP) (VI): Diethylcarbamoyl chloride (17.87 g,
0.131 mol) was taken in a Schlenk tube with a magnetic stirring bar. To this tributyl phosphite
(30 g, 0.119 mol) was added drop-wise at 0 °C and refluxed at 100 °C for 12 h. The reaction
mixture was concentrated under vacuum and the crude product was purified by column
chromatography using 50% ethyl acetate in n-hexane as eluent resulted in a brown color liquid.
The yield of the product is 25g (71%). 1H NMR (400 MHz, CDCl3): δ = 4.23-4.11 (m, 4H),
3.75–3.5 (q, 2H), 3.72-3.42 (q, 2H), 1.73–1.66 (m, 6H), 1.44-1.38 (m, 4H), 1.25-1.21 (t, 3H),
1.44-1.38 (m, 4H) 1.16-1.13(t 3H), 0.91-0.95(t, 3H) ppm. 31P NMR (161.97 MHz, CDCl3): δ=
0.39 ppm. 13C NMR (100 MHz, CDCl3) δ: 166.48, 164.22, 67.50, 67.42, 42.04, 39.8, 39.75,
32.40, 32.34, 18.77, 18.65, 14.24, 13.53, 12.45. GCMS (m/z): 294.27 [M] +

Figure 2. Schematic illustration of the synthesis of extractants I, III-VI.
2.3. Physicochemical properties
2.3.1. Density measurements
The undiluted, as well as 1.1 M solutions of extractants I-VII in corresponding diluents
(2 mL) were equilibrated at 303K with an equal volume of the aqueous phase of the appropriate
nitric acid concentration ranging from 0.01 to 6 M for 1 h. The density of the organic phase was
measured in triplicate by weighing 500 µL of the sample in a glass micropipette using an
analytical balance of ± 0.1 mg sensitivity.
2.3.2. Viscosity measurements
The viscosity of extractants I-VII was measured by Oswald viscometer [24]. A known
volume (~ 15 mL) of liquid was taken in a viscometer and the time taken by the liquid to flow
through the marked region of the Oswald viscometer was measured as a function of temperature.

The driving pressure p at all stages of the flow of a liquid is given by hρg, where h is the
difference in the heights of the liquid in upper and lower bulb, ρ density of the liquid and g
acceleration due to gravity. The viscosity of the fluid can be determined using the following
expression.
ηf = ηw (ρf tf /ρw tw)
(1)
Where ηf, ρw and tf are viscosity, density and time taken for the fluid to pass through a
marked region of the viscometer. The ηw, ρw and tw are viscosity, density and time for the water
system under identical conditions.
2.3.3. Nitric acid extraction
Equal volumes (each 2 mL) of organic solution (1.1 M solutions) and the required nitric
acid solution taken in a glass stoppered test tube were equilibrated at 303 K. After equilibrating
the solutions for an hour, the phases were allowed to settle by gravity. Suitable aliquots were
withdrawn from both the phases and analyzed for their acid concentration. The acid in the
organic phase was stripped by contacting three times with equal volume of deionized water and
quantitatively analyzed using standardized NaOH solution as titrant using phenolphthalein
indicator.
2.3.4. Distribution measurements
All the extractants of 1.1M solutions were prepared in corresponding diluent (n-
dodecane/xylene). Compounds I-IV and VII were diluted in n-dodecane and compounds V and
VI were diluted in xylene. The solubility of extractants V and VI was observed to be poor in n-
dodecane and the third phase was observed when pre-equilibrated with nitric acid because of the
limited solubility of the hydrogen complexed ligand in non-polar dodecane. All the diluted
extractants were washed with 5N sodium hydroxide to remove undesired acidic impurities. All
the experiments were carried out in triplicate. Radiometric assay of 233U and 239Pu was carried by
liquid scintillation counting technique employing an LSC system (Hidex, Finland) coupled to a
multi-channel analyzer using a dioxane-based scintillator cocktail [25]. 241Am was assayed by a
well-type Na(Tl) detector coupled to a multi-channel analyzer (ECIL, India).

(a) U(VI), Pu(IV) and Am(III) extraction
The extractants were pre-equilibrated with appropriate nitric acid concentration. A 2 mL
of the extractant and 2 mL of nitric acid (containing a requisite quantity of 233U, 239Pu and 241Am
tracer) were equilibrated in a constant-temperature bath at 303 K for 1 h. After attainment of
equilibrium, suitable aliquots from the both the phases (e.g.100µL) were taken for radiometric
assay of the actinides. The distribution ratio (DM) was calculated as the ratio of concentration
(radioactivity per unit volume per unit time) of metal ions in the organic phase to that in the
aqueous phase.
(2)
Where [M]org and [M]aq are the metal ion concentration in the organic and aqueous phases,
respectively.
(b) Th(IV) extraction
The experimental procedure used for the extraction of Th(IV) by 1.1 M extractants was
similar to that of uranium extraction. The thorium concentrations in the initial aqueous phase, as
well as at equilibrium were determined spectrophotometrically using Arsenazo-III as the
chromogenic agent [26]. The concentration of thorium in the organic samples was estimated by
subtracting the equilibrium aqueous concentration from the initial feed concentration. The
distribution ratio can then be defined as
(3)
Where [M]i,aq. and [M]f,aq.are the initial and final concentration of metal ion in the aqueous
phase.
2.4. Computational methods
Density functional theory (DFT) was applied to study the electronic structure and
geometries of the ligands DBHP (I), DBP (II), DBHeP (III), DBCyHeP (IV), DBPP (V),
DBDECP (VI) and TBP (VII). All geometry optimizations were performed at density functional
theory (DFT) level employing B3LYP functional [27, 28] in conjunction with triple-ζdef2-TZVP

basis sets [29, 30]. The stationary points were characterized as minima by performing vibrational
frequency calculations at the same level. The resolution-of-identity (RI) approximation was
applied in conjunction with the appropriate auxiliary basis sets to speed up the calculations.
Empirical Grimme-type dispersion corrections were incorporated during this step using the latest
atom-pairwise dispersion correction with Becke-Johnson damping (D3BJ) [31, 32]. Increased
integration grids (Grid6 in ORCA convention) and tight SCF convergence criteria were used
throughout the calculations. All quantum chemical calculations were performed with ORCA
version 3.0.3 program package [33]. In order to derive AIM charges, the corresponding
wavefunction files were generated at B3LYP/def2-TZVP level combining ORCA program
package and Molden2AIM utility [34] and analyzed using AIMAll program package [35]. The
natural charges were derived by performing natural population analysis using NBO program
package version 6.0 [36].
3. Results and Discussion
3.1. Characterization
We have synthesized six analogues of TBP by altering one of the butoxy group in the
TBP (VII) by hydrogen (I), hydroxyl (II), alkyl (III, IV), aryl (V) and amide (VI) substituents
respectively. The structures of the synthesized products I, III-VI were confirmed by IR,
multinuclear NMR and mass spectral analysis. Figure 3 shows the overlay of 31P NMR of
compounds I-VII. The phosphorus in the phosphonate is more deshielded than phosphates
because of the higher electron density at the phosphoryl oxygen. The 31P chemical shift values
(ppm) for the compounds I-VII follows the order VII > VI > II > I > V > III IV are shown in
Table 1.

Figure 3. 31P NMR overlay of extractants I-VII.
Figure 4. FT-IR overlay of extractants I-VII.
The IR spectra of organophosphorus compounds contain characteristic bands attributed to
P=O and P–O–C vibrations. The position of the P–H and P=O absorption bands depend largely
on the type of substituents. The stretching frequency of P=O was employed as an index of
electron density at phosphoryl oxygen. The electron density at the phosphoryl oxygen is
attributed to the mesomeric and inductive effect of -OR groups. The replacement of -OR by -R
eliminates the mesomeric effect and the inductive nature changes from -I to +I effect, which
increases the electron density and the basicity of phosphoryl oxygen atom. This increase in the
basicity makes the P=O bond behaves more like a single bond and hence stretching frequency of
P=O in phosphonate is lower than that of phosphates (Figure 4). The P=O stretching frequency
of the compounds I-VII follows the same trend as 31P NMR (see Table 1).
Table 1. Spectral characteristics of the extractants I-VII.
Extractant
31P
P=O
Calculated P=O
Shift
(ppm)
Stretching stretching frequencies
Frequency (cm-1) at B3LYP/def2-
(cm-1)
TZVP level
DBHP (I)
DBP (II)
6,10
0.49
31.95
1251
1220
1230
1242
1246
1287
1291
1276
1277
1286
DBHeP (III)
DBCyHeP (IV) 31.99
DBPP (V) 17.93

DBDECP (VI) 0.35
TBP (VII) -0.4
1247
1276
1283
1290
The thermal stability of extractants I-VII was studied by thermogravimetric (TG) analysis
and the results are summarized in Table 2. Among seven extractants, DBP (II) showed highest
thermal stability (started decomposing after 200 °C), whereas DBHP (I) showed least thermal
stability as can be seen in Figure 5.
Figure 5. Thermogravimetric analytic curves of extractants I-VII.
Table 2. Thermal degradation data of extractants I-VII derived from TGA.
Extractant
Td 5%a (°C) Td 10%b (°C)
Td max%c (°C)
DBHP (I)
DBP (II)
51.00
181.54
69.38
194.59
185.00
239.34
DBHeP (III)
108.53
131.62
108.54
147.17
156.23
132.52
192.89
171.41
219.10
222.03
188.73
DBCyHeP (IV) 83.77
DBPP (V)
122.13
137.85
118.93
DBDECP (VI)
TBP (VII)
a Temperature at weight loss value of 5% for non-isothermal degradation.
b Temperature at weight loss value of 10% for non-isothermal degradation.
c Temperature at which a maximum decomposition rate is reached for non-isothermal degradation.
3.2. Density
The density difference between the organic phase and the aqueous phase is an important
parameter which decides the phase separation time (PST) in liquid–liquid extraction. Figure 6
shows the variation in density of nitric acid loaded organic phase with the equilibrium aqueous
nitric acid at 303 K. The density of organic phase increases with an increase of nitric acid

concentration in the organic phase. In our case, a marginal increase in the density of the organic
phase was noticed as the aqueous phase acidity increases from 0.01 M to 6 M nitric acid
concentration. In the case of DBP, the density decreases with increasing acid concentration due
to the increase in the aqueous solubility of DBP in nitric acid medium. A similar trend was
observed for DBHP up to 2M followed by a rise in the density above 2M nitric acid
concentration. The uncertainty in the estimation of density in these studies is ±0.2%.
Figure 6. Density variations of extractants I-VII with an initial concentration of nitric acid at
303 K.
3.3. Viscosity
The variation in the viscosity of extractants I-VII as a function of temperature is shown
in Figure S30 (ESI). The relative standard deviation for the measurement of viscosity by Oswald
viscometer is 1-2%. The deviations can be maximum at higher temperatures due to an error in
the assumption of invariance in density as a function of temperature. In case of water, density at
303K and 323 K is 0.997 and 0.988 g/mL respectively. This corresponds to an uncertainty of 1%
in the calculation of viscosity by this method. Among all the extractants, DBP (II) exhibits high
viscosity due to the presence intramolecular hydrogen bonding between the -OH groups. The
effect of temperature on the viscosity can be fitted with the Arrhenius type of relationship as
shown in Eq.4
/
?? = ??0?????? ????
(4)

Where ??0 is dynamic viscosity (Pa⋅s); ?? is the pre-exponential factor (Pa⋅s); ???? is the
exponential constant which is known as activation energy (J/mol); ?? is the gas constant
(J/mol.K) and ?? is the absolute temperature (K). The plot of ln?? against 1/?? for phosphonates
is shown in Figure 7. At 303K, the activation energy ???? (calculated from the slope of the
straight lines) values for I-VII are 12.13, 23.99, 17.96, 12.83, 17.15, 25.19, 14.63 kJ/mol
respectively.
Figure 7. Arrhenius plot for the variation in viscosity for the extractants I-VII with temperature.
3.4. Nitric acid extraction
The basicity of neutral extractants is an important parameter that decides the efficiency
towards metal ion extraction. Ligand basicity can be determined to a certain extent on the basis
of nitric acid extraction. Figure 8 shows the extraction of nitric acid by 1.1 M solutions of I-VI in
n-dodecane/xylene at 303 K in comparison with extraction of nitric acid by 1.1 M TBP/n-
dodecane at 303 K [37]. Extraction of nitric acid by DBHeP (III), DBCyHeP (IV) and DBPP (V)
is almost same throughout the acidities. The presence of carbonyl group (C=O) in DBDECP (VI)
strongly decreases the electron density at phosphoryl oxygen, resulting in lower nitric acid
extraction, which is nearly similar to TBP (VII). The presence of free -OH group in DBP (II)
and tri-coordinated phosphite form [38] of DBHP (I) contribute to the acidity to the molecule and
hence are not reported here.

Figure 8. Variation in nitric acid concentration of extractants I-VII in organic phase as a
function of initial aqueous nitric acid concentration.
3.5. Density Functional Theory Studies
We have applied DFT calculations to probe the effect of structural change on the electronic
structure of the ligands. The ligand geometries were systematically optimized considering ten
starting geometrical conformers resulting from C-C single bond rotation. Our calculations
suggested that the energy difference between these conformers is relatively small (~5 kcal/mol)
and suggests the conformational flexibility of the system under room temperature. This is in
accordance with our previous studies reported on di-n-alkyl phosphine oxides [39] and tri-n-
butyl phosphate ligands [40, 41]. Therefore we have considered the lowest energy conformer in
each set as the representative structure for each ligand. The optimized geometries of the ligands
DBHP (I), DBP (II), DBHeP (III), DBCyHeP (IV), DBPP (V), DBDECP (VI) and TBP (VII)
are illustrated in Figure 9. The lowest energy conformer for TBP (VII) is already reported in the
literature [41] and therefore considered from the corresponding reference. The calculated gas
phase P=O stretching frequencies for all ligands were in agreement with the corresponding
values derived experimentally (Table 1). There is also a general trend in computed P=O
stretching frequencies in neutral compounds, as we move from DBHeP to TBP, i.e. from 1276
cm-1 to 1290 cm-1.

I
II
III
IV
V
VI
VII
Figure 9. Optimized geometries of ligands I-VII considered in the present study at B3LYP/def2-
TZVP level. Color code: Orange ball is phosphorus, red balls are oxygen, blue ball is nitrogen,
grey balls are carbon, and white balls are hydrogen atoms
It is conventionally understood that replacement of –OR group by –R increases basicity of
the phosphoryl oxygen atom. In order to verify this hypothesis, we have performed electron
population analysis on computationally derived representative structures. To check the
sensitivity of the computed charges, we have compared the Mulliken electron population and that
derived using advanced methodologies, such as natural bond orbital (NBO) and AIM. Upon
structural modification, the phosphoryl oxygen atom shows no considerable change in electron
population. This can be reconfirmed by comparing trends in computed electronic charges at

phosphoryl oxygen using three different methodologies, viz. Mulliken, NBO and AIM (Table S1,
ESI). On the other hand, the absolute values of computed charges at phosphorus atom show
small numerical differences for different ligands. A general trend such as a large positive charge
on phosphorous for TBP (VII) and a small value for phosphorous on DBHeP (III) was observed
and this trend is in agreement with the trend in 31P chemical shift values. In summary, the
difference in extraction behavior of ligands considered in the present study cannot be attributed
alone to the difference in electron density at phosphoryl oxygen arising from the structural
modification. Factors other than basicity of P=O group, such as geometry of the corresponding
complexes, nitrate ion concentration, etc. will also contribute to the D value.
3.6. Distribution studies of U(VI), Th(IV), Pu(IV) and Am(III)
The extracting power of neutral organophosphorus extractants can be enhanced by an
increase in the electron availability on the donor atom. The introduction of longer alkyl chain,
branching in the alkyl groups near the donor atom are likely to make a marginal increase in the
basicity of the extractant. However, extraction is also decided by the stability of the metal
complex and on the structure of the extractant.
The data on the variation in D for the trace level extraction of U(VI) by 1.1M solutions of
extractants I-VII with equilibrium aqueous nitric acid concentrations from 0.01 to 6M at 303 K
are shown in Figure 10 and compared with 1.1 M TBP/n-dodecane. The variation in distribution
ratio for U(VI) by 1.1M dibutyl phosphate (DBP) as a function of nitric acid concentrations was
investigated and it was observed that at lower acidity DU(VI) value for DBP (II) is considerably
large and it decreases gradually with the increase of acid concentration. A high distribution value
of DBP (II) at lower acidity is attributed to cation exchange mechanism [39] and involves the
following reaction.
(5)
The equilibrium constant Keq, for the reaction is given by
(6)
Rearranging eq. (6),
(7)

DBP (II) has two chelating sites, viz. P=O and P-OH groups, at lower acidity ion exchange
mechanism is the predominant mode of extraction of actinides, at higher acidity, the higher
concentration of H+ drives the reaction to the reactant side with no metal complex formation. The
phosphoryl group is thus solely responsible for the extraction at higher acidity. The DU(VI) values
of DBHP (I) are high at low acidities as compared to those of TBP under similar conditions. At
lower acidities, the DBHP (I) behave like acidic extractant via cation exchange mechanism,
while at higher acidity the extraction occurs through solvation as that of DBP (II). The graph
also shows the comparative data for the extraction of U(VI) for DBHeP (III), DBCyHeP (IV),
and DBPP (V). Among the three hexyl derivatives, DBPP (V) shows lower distribution values
compared to DBHeP (III), due to the presence of phenyl group, which is moderately activating
group compared to the n-hexyl group. The decrease in electron density on phosphoryl oxygen
leading to the formation of the weaker complex bond, thereby it resulting low distribution values
compared to DBHeP (III) and DBCyHeP (IV). DU(VI) values for DBDECP (VI) were lower and
closer to TBP (VII) due to electron withdrawing carbonyl group (C=O), which is directly
attached to the central phosphorus atom and it decreases the electron density on phosphoryl
oxygen.
Figure 10. Variation in the ??U(VI) of extractants I-VII in organic phase with equilibrium aqueous
phase nitric acid at 303 K.
Figure 11 shows the variation in ??Th(IV) as a function of nitric acid concentration. The
trend in thorium is similar to U(VI). The D values of Th(IV) for all four phosphonates are in the

same order and increased with nitric acid concentration. The uptake of Th(IV) reaches steady
value at 2M acidity after which there is only a marginal increase. Distribution values for the
phosphonate systems are significantly higher than those of TBP (VII) system and DBDECP (VI)
distribution is almost similar to TBP (VII). The presence of cyclic ring in the DBCyHeP (IV)
and DBPP (V) causes steric effect resulting in large variation of the DTh(IV) values compared to
the open chain in DBHeP (III). The decrease in D values of thorium is more profound in thorium
system than uranium. This may be due to the higher solvation number i.e. trisolvate nature of Th
solvate compared to the disolvate uranium complex [42], which reduces the amount of free
extractant in thorium extraction compared to uranium. Distribution studies of Th(IV) for DBP
(II) and DBHP (I) could not be carried out, because of the formation of emulsion at lower
acidities. As mentioned earlier, nature of substituent and steric effects have a predominant effect
in the extraction behavior of the metals.
Figure 11. Variation in the ??Th(IV) of extractants III-VII in organic phase with equilibrium
aqueous phase nitric acid at 303 K.
The batch studies for the extraction of Pu(IV) was carried out as a function of nitric acid
ranging from 0.01- 6M. The effect of the different structures of extractants I-VII on the
distribution ratios of plutonium is similar to that observed for uranium. Figure 12 gives the
variation of distribution ratios of Pu(IV) by 1.1M to solutions of extractants I-VI with
equilibrium aqueous nitric acid 303 K. This system is compared with 1.1M TBP/n-dodecane
under identical conditions. The DPu(IV) increases steeply for the extractants III-VI as compared to

VII. A decrease in D values of plutonium is observed as the nitric acid concentration is varied
from 0.01- 6M for the extractants I-II. The fall in the D values can be explained based on the
cation exchange mechanism operating in these systems.
Figure 12. Variation in the ??Pu(IV) of extractants I-VII in organic phase with equilibrium
aqueous phase nitric acid at 303 K.
The effect of equilibrium aqueous phase nitric acid concentration on the extraction of
Am(III) with 1.1M solutions of I-VI at 303K are compared with 1.1M TBP/n-dodecane under
identical conditions are shown in Figure 13. The extraction behavior of these ligands towards
Am(III) follows the same trend as that of U(VI). The data suggest that the extraction of Am(III)
is poor compared to that of tetravalent (Th) and hexavalent (U) actinide ions. The extraction
values of Am(III) by TBP (VII) and phosphonates are very low and hence cannot be used for
practical application in the solvent extraction mode. In absolute terms, the D values of
phosphonates are ten times higher than those of TBP (VII), suggesting the higher basicity of
phosphonates over phosphates.

Figure 13. Variation in the ??Am(III) of extractants I-VII in organic phase with equilibrium
aqueous phase nitric acid at 303 K.
3.7 Effect of NaNO3 on extraction of Uranium
The effect of ionic strength on the extraction of U(VI) was studied at varying
concentrations of NaNO3 (0.0–6 M) in 0.1M HNO3. The effect of variation of salt on extraction
by extractants III-VII showed that the distribution ratio increases with the increase in NaNO3
concentration in these systems as shown in Table 3. This is explained by the increase in the
thermodynamic activity of the metal and concomitant decreases in the activity of water. The
ionic strength of the solution increases and hence behaves like salting out agent. The efficiency
of extraction increases with the increase in the concentration of nitrate anions, because these
species are considered as weak complexing anions and thus behave as salting out agents. This
supports the nitrate complex is being extracted because of the higher the ionic strength of the
solution. On the other hand, to evaluate the effect of acid (protons) on extraction, we have
carried out the extraction of U(VI) with the extractants III-VI by varying the nitric acid to
sodium nitrate concentration where the total nitrate concentration is fixed at 2M. Distribution
ratios are greatly increased with the decrease in H+ concentration and increase in nitrate ion
concentration as shown in Table 4. This observation can be attributed to the salting out effect of
nitrate ions and decrease in competition between acid and metal ions for the extraction.

Table 3. Variation of DU(VI) with varying nitrate concentration at fixed 0.1M HNO3.
-
[NO3 ]
DU(VI)
[HNO3] [NaNO3] DBHeP DBCyHeP DBPP
DBDECP TBP
M
M
(III)
(IV)
(V)
(VI)
(VII)
0.1
0
6.0
2.6
0.98
0.43
0.29
0.1
0.1
0.1
0.1
0.1
0.4
0.9
1.9
3.9
5.9
18.60
38.02
54.01
105.21
128.13
7.48
18.51
25
4.67
9.5
1.76
5.3
0.91
4.23
20.6
67.8
86
14.7
45.4
58.68
12.70
39.70
52.29
94
102
Table 4. Variation of DU(VI) with varying nitrate concentration with respect to HNO3.
-
[NO3 ]
DU(VI)
[HNO3] [NaNO3] DBHeP DBCyHe DBPP
DBDECP TBP
M
M
P (IV)
(III)
(V)
(VI)
(VII)
2.0
0
74.01
57.83
41.88
12.08
7.75
1.75
1.5
1.0
0.5
0
0.25
0.5
1.0
1.5
2.0
84.12
64.34
68.3
47.70
48.14
55.75
71.12
95.26
13.44
15.69
17.46
23.12
29.63
9.89
91.15
11.75
15.81
18.5
101.03
119.36
141.02
76.91
93.51
114
19.2
3.8 Effect of temperature on extraction of uranium and thorium
Temperature plays an important role in solvent extraction studies. In order to determine
the effect of temperature on extraction of these metals, variable temperature extraction studies
were carried out with extractants III-VII at 2M nitric acid concentration with U(V1) and Th(IV).
Figure 14 and Figure 15 illustrated the variation of distribution ratio of U(VI) and Th(IV) with
the temperature. The extraction of uranium and thorium decreases monotonically with an
increase in the temperature within the range of experiment (290–333 K), which confirms that the

extraction of these metals with the extractants III-VII is exothermic in nature. The reeducation
in temperature aids the extraction of U(VI) and Th(IV). The DM data of U(VI) and Th(IV)
obtained for extractants at different temperature were plotted as log DM vs (l/T) and the plots for
2.0 M HNO3 are shown in Figure 16 and Figure 17.
Figure 14. Effect of temperature on the distribution ratio for extraction of U(VI) by 1.1 M
extractants III-VII at equilibrium 2M nitric acid.
Figure 15. Effect of temperature on the distribution ratio for extraction of Th(IV) by 1.1 M
extractants III-VII at equilibrium 2M nitric acid.

Figure 16. Variation of log DU(VI) against 1/T for the extractants III-IV at equilibrium 2M nitric
acid.
Figure 17. Variation of log DTh(IV) against 1/T for the extractants III-IV at equilibrium 2M
nitric acid.

Table 5. Enthalpy Change of Extraction of U(VI) and Th(IV) with 1.1M extractants III-VII at
equilibrium 2M nitric acid.
-∆H( kJ/mol)
Extractant
U(VI)
Th(IV)
DBHeP (III)
DBCyHeP (IV)
DBPP(IV)
9.48
7.7
15.16
13.58
12.21
7.23
7.27
4.63
3.74
DBDECP (VI)
TBP (VII)
4.04
From the slope of the straight lines, the enthalpy change ∆H for the extraction can be
calculated using the Van’t Hoff equation:
(8)
and the calculated values are summarized in Table 5. The negative ∆H value further suggests that
the extraction reaction of uranium and thorium by the extractants III-VII is exothermic in nature.
4. Conclusions
We have prepared six different analogues of TBP by altering one of its butoxy group by
hydrogen, hydroxyl, hexyl, cyclohexyl, phenyl and amide substituents. The synthesized TBP
derivatives were well characterized by spectroscopic methods and also measured their physical
properties such as density and viscosity. The 31P chemical shift values (ppm) for the compound
I-VII follows the order VII > VI > II > I > V > III IV. This trend suggests that phosphorus in
the phosphonate is more deshielded than in the case of phosphates because of the higher electron
density at the phosphoryl oxygen. IR studies revealed that the P=O stretching frequency of the
compounds I-VII follows the same trend as that of 31P NMR. The DFT calculated P=O
stretching frequencies for extractants I-VII were in agreement with experimental values.

The solvent extraction behavior of synthesized extractants towards U(VI), Th(IV), Pu(IV)
and Am(III) were studied and compared with standard TBP. It was found that the variations in
the distribution ratios of U(VI), Th(IV) and Am(III) were influenced by the basicity of the
phosphoryl oxygen (P=O), which correlated with the nature and size of the inductive effective
substituent, and distribution ratios are greatly influenced by the increase of sodium nitrate. The
extraction process of U(VI) and Th(IV) is exothermic. 31P chemical shift values, FT-IR
absorption frequency values are good agreement and evident for the electron density (basicity) of
the phosphoryl oxygen (P=O). Thus the field of organophosphorus based extractants are seems to
be wide open for exciting investigations.
Acknowledgements
Kari thanks IGCAR-Kalpakkam, [IGCAR/FChD/SCSS/Project-1/2014] for the financial
support. Suresh thanks IGCAR- Kalpakkam for the Junior research fellowship. Authors also
thank DST-VIT-FIST for NMR, VIT-SIF for GC-MS and other instrumentation facilities. We
gratefully acknowledge computing time on IVY cluster, the High-Performance Computing
Facility at IGCAR, India.
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Highlights
Six different analogues of TBP were prepared and evaluated for their solvent extraction behavior
towards U(VI), Th(IV) and Am(III).
The distribution coefficient values (for U(VI), Th(IV) and Am(III))
Are strongly depend on the nature and size of the substituents. Presence of electron donating
groups enhances the uptake of the actinides.