Synthesis, characterization and evaluation of phosphoramides for actinide extraction

Article abstract of DOI:10.1016/j.ica.2019.05.034

A new class of extractants called hexaalkylphosphoramides has been chosen for actinide separation applications in the present study. Hexaalkylphosphoramides [(NR₂)₃P[dbnd]O] are compounds which possess three nitrogen sub-units along with a strong donor oxygen atom. The studies on them provide an opportunity to synthesize a large variety of structurally diverse analogs. Thus, the desired properties of the extractant can be tuned for a specific application. We discuss the synthesis, purification, and characterization of two such hexaalkylphosphoramides (Propyl and Butyl) in this paper. Their physico-chemical properties were measured and the extraction behavior was evaluated with U(VI), Th(IV) and Pu(IV). The results were compared with trialkyl phosphates such as tri-n-butyl phosphate (TBP) and tri-n-amyl phosphate (TAP). The results revealed that the extraction behaviors of phosphoramides are comparable to trialkyl phosphates.

Full text of DOI:10.1016/j.ica.2019.05.034

Accepted Manuscript
Research paper
Synthesis, characterization and evaluation of phosphoramides for actinide ex-
traction
G. Jegan, A. Suresh, C.V.S. Brahmmananda Rao, B. Sreenivasalu, N. Sivaraman
PII:
S0020-1693(19)30362-7
https://doi.org/10.1016/j.ica.2019.05.034
ICA 18935
DOI:
Reference:
Inorganica Chimica Acta
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18 May 2019
Please cite this article as: G. Jegan, A. Suresh, C.V.S. Brahmmananda Rao, B. Sreenivasalu, N. Sivaraman,
Synthesis, characterization and evaluation of phosphoramides for actinide extraction, Inorganica Chimica Acta
(2019), doi: https://doi.org/10.1016/j.ica.2019.05.034
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Synthesis, characterization and evaluation of phosphoramides for
actinide extraction
G. Jegan, A. Suresh*, C.V.S. Brahmmananda Rao, B. Sreenivasalu, N. Sivaraman
Homi Bhabha National institute, MC&MFCG, IGCAR, Kalpakkam-603102
Abstract
A new class of extractants called hexaalkylphosphoramides has been chosen for actinide
separation applications in the present study. Hexaalkylphosphoramides [(NR2)3P=O] are
compounds which possess three nitrogen sub-units along with a strong donor oxygen atom. The
studies on them provide an opportunity to synthesize a large variety of structurally diverse
analogs. Thus, the desired properties of the extractant can be tuned for a specific application. We
discuss the synthesis, purification, and characterization of two such hexaalkylphosphoramides
(Propyl and Butyl) in this paper. Their physico-chemical properties were measured and the
extraction behavior was evaluated with U(VI), Th(IV) and Pu(IV). The results were compared
with trialkyl phosphates such as tri-n-butyl phosphate (TBP) and tri-n-amyl phosphate (TAP).
The results revealed that the extraction behaviors of phosphoramides are comparable to trialkyl
phosphates.
Keywords: Trialkyl phosphates, alternate extractants, phosphoramides, physical properties,
actinides separation.
1 Introduction
In recent years, there has been increasing interest in the reprocessing of spent nuclear fuels for
the effective utilization of the fissile and fertile materials of the irradiated fuel. PUREX [1] and
THOREX [2] processes have been widely employed for the processing of uranium and thorium
based irradiated fuels, respectively. In PUREX process, 1.1 M solution of TBP in n-dodecane has

been used as the solvent. TBP has excellent extraction properties albeit with some limitations
with respect to third phase formation in the extraction of tetravalent metal ions such as Th(IV),
Pu(IV) and Zr(IV) [3]. TBP also exhibits poor selectivity of uranium over thorium which is
crucial in U/Th separation during reprocessing of thorium based irradiated fuels. In PUREX
process, TBP undergoes hydrolyses and form dibutylphosphoric acid (HDBP) and monobutyl
phosphoric acid (H2MBP). The above hydrolyzed products formed during the processing have
the strong complexing ability towards fission products like Zr and Ru thereby reducing the
decontamination factors [4 - 6]. The third phase formation and solvent degradation are the main
concerns in fast reactor fuel reprocessing due to high plutonium content and activity of the
dissolver solutions. Therefore, the development of an alternate extractant is needed for spent fuel
reprocessing, especially, for the fast reactor fuel reprocessing. In order to identify an alternate
extractant to overcome these limitations, higher homologs of TBP such as Tri-n-amyl phosphate
(TAP), Tri-iso-amyl phosphate (TiAP), Tris-2-methylbutyl phosphate (T2MBP), etc. were
investigated in our laboratory [7]. Our studies revealed that TiAP and T2MBP are potential
extractants for fast reactor spent fuel reprocessing. These studies further revealed that with an
increase in carbon chain length of alkyl groups increases the hydrophobicity of the solvent. The
metal solvate is more compatible with the diluent, thereby reducing the tendency to form the
third phase. We have also established that TsBP is suitable for U/Th separation. The results of
the investigation indicated that branching adjacent to the P=O group of the ligand, increases the
separation factor for U and Th. Studies on N, N-dialkyl amides also revealed that increase in the
carbon number of the extractant as well as the branching around the donor group of the ligand,
significantly influences the metal ion extraction thereby yielding higher U/Th separation factor
[8 - 11].

The n-alkyl and cyclic amino phosphine oxides, which are structural analogs to TBP have also
been investigated for the selective extraction of actinide metal ions [12]. Their study also
revealed that the structural modification of phosphine oxides by its alkyl substituents
significantly changes the basicity of the compound. Investigations on phosphonates and its
derivatives also concluded that modification of the alkyl substituent around P=O group in the
molecule has its influence on the basicity of the coordinating group thereby altering the
extraction behavior [13, 14].
Hexamethylphosphoramide (HMPA) and hexaethylphosphoramide (HEPA) are phosphoramides,
which finds applications in catalysis for organic synthesis [15]. The strong basic nature of the
HMPA also makes it a good ligand and form complexes with various metal ions [16, 17].
Moreover, the phosphoramide based compounds are used as inhibitors for the enzymatic reaction
such as the urease [18], acetyl cholinesterase [19] and butyl cholinesterase [20]. In addition,
these find applications in agriculture as pesticides and insecticides [21, 22].
A large number of trialkyl phosphates and dialkyl amides have been developed over the past
three decades in order to overcome the limitations of TBP. However, phosphoramides have not
been investigated for the extraction of actinides. We ventured to investigate the extraction
behavior of actinides and fission products with phosphoramides. The results of our investigation
were compared with TBP. The feasibility of hexaalkylphosphoramides [(R2N)3PO (R=Propyl
(HPPA) and Butyl (HBPA)] for actinides separations have been examined in this paper. The

Fig. 1. Chemical structures of the extractants used for actinide extraction.
chemical structure of the phosphoramides and trialkyl phosphates are shown in Fig.1. In the
present study, hexapropylphosphoramide (HPPA) and hexabutylphosphoramide (HBPA) were
synthesized, purified, and characterized by using analytical techniques like NMR (31P, 1H& 13C),
IR spectroscopy, mass spectrometry (GC-MS).
Physicochemical properties like density, aqueous solubility, and solubility of water in
phosphoramides have been measured. The extraction behavior of U(VI), Th(IV) and Pu(IV) with
1.1M solutions of HBPA and HPPA in n-dodecane have been investigated as a function of
equilibrium aqueous phase nitric acid concentration.
2 Experimental

2.1 Materials
Phosphoryl chloride (POCl3), triethylamine, Di-n-butylamine, Di-n-Propylamine, Silica gel (60-
120 mesh), n-dodecane, hexane, ethyl acetate, 1-(2-Arsonophenylazo)-2-naphthol-3,6-disulfonic
acid disodium salt (Arsenazo-III), 2-(3,6-Disulfo-2-hydroxy-1-naphthylazo)benzene arsonic acid
disodium salt (Thoron), 2-(5-Bromo-2-pyridylazo)-5-(diethylamino)phenol (Br-PADAD) and
dichloromethane (DCM) (M/s. Merck.) were used as received. Nuclear grade thorium nitrate
received from Indian Rare Earths Ltd., Mumbai, India was used without further treatment. UO2
(NO3)2.6H2O obtained from BDH, Poole, England was used as received. Plutonium (principal
isotope: 239Pu) is used as radioactive tracer. Plutonium was purified with 0.5M TTA/xylene for
the removal of americium. Plutonium was selectively extracted with 0.5M TTA/xylene at 1M
HNO3 leaving Am in the aqueous phase followed by scrubbing with 7M HNO3. Subsequently,
the plutonium loaded in 0.5M TTA/xylene was stripped into aqueous phase using 8M HNO3.
The stripped plutonium is used as radioactive tracer. 1.1M solutions of phosphoramides were
prepared by taking the required quantity of phosphoramides and n-dodecane in volumetric flasks.
2.2 Synthesis of extractants
Phosphoramides were synthesized by the condensation reaction between POCl3 and
corresponding dialkyl amines as shown in Fig. 2.

Fig. 2. Schematic representation for synthesis of Hexaalkylphosphoramide.
2.2.1. Synthesis of Hexabutylphosphoramide (HBPA)
To a round bottom flask, triethylamine (0.3 mol) and DCM (100 mL) were added. Then, the
solutions of di-n-butylamine (0.3 mol) in DCM (100 mL) and phosphoryl chloride (0.1 mol) in
DCM (25 mL) were added drop wise for 2 hrs. under reflux conditions (~550C). After the
addition, the solution was refluxed for 48 hrs and subsequently treated with 50 mL of 4N HCl
followed by deionized water. The crude product was obtained by evaporating DCM by using
rotary evaporator. The crude product was made into slurry with silica gel (60-120 mesh). In a
glass column, it was packed in the ratio of 1:10 with respect to slurry and silica gel. Hexane was
passed through the column to remove non-polar impurities. Phosphoramide compound was
eluted from a column with the mixture of hexane and ethyl acetate. (SiO2, 60-120 mesh,
1
hexane/EtOAc 9.5/0.5) and the yield is around 65%. H NMR (400 MHz, CDCl3); d=0.84-0.88
(t, 18H), 1.18 - 2.26 (m, 12H), 1.44 -1.52 (m, 12H), 2.94 – 3.01 (m, 12H) ppm. 31P NMR (162.01
MHz, CDCl3); d = 28.28–28.78 (m, 1JPH = 691 Hz) ppm. 13C NMR (162.01 MHz, CDCl3); 13.51,
19.91, 29.90, 45.66 ppm.
2.2.2 Synthesis of Hexapropylphosphoramide (HPPA)

HPPA was synthesized from Di-n-propyl amine and the synthetic route for HPPA is similar to
1
HBPA. The yield is around 66%. H NMR (400 MHz, CDCl3); d = 0.80 - 0.84 (t, 18H), 1.48 -
1.56 (m, 12H), 2.91 – 2.98 (m, 12H) ppm. 31PNMR (162.01 MHz, CDCl3); d = 28.15–28.64 (m,
1JPH = 691 Hz) ppm. 13C NMR (162.01 MHz, CDCl3); 10.88, 20.90, 47.65 ppm.
2.3 Instrumentation
The structures of the extractants were elucidated by using Bruker DMX-400 spectrometer for 1H,
31
1
13C, and P NMR spectra. All the H chemical shifts were reported with respect to tetramethyl
silane (TMS) (all at 298 K, CDCl3). The characteristic stretching frequencies of functional
groups of the extractants were identified by using Shimadzu Affinity 1 FT-IR Spectrometer.
SHIMADZU UV-3600 double-beam spectrophotometer was employed for determining U(VI)
and Th(IV) present in the organic and aqueous phases using the methods described below. The
concentration of Pu(IV) was analyzed by using Liquid Scintillation Counter (LSC) (300SL
TDCR liquid scintillation analyzer, Hidex, Finland).
2.4 Physical properties of extractants
2.4.1 Density measurement
The density of phosphoramides (undiluted and 1.1M solution in n-dodecane) was measured
using mass(m) to volume(v) ratio of the compound. In this method, a glass micropipette (500µL,
BOROSIL®, India) was filled with a phosphoramide and weighed by using an analytical balance
of ±0.1mg sensitivity at 299 ±1K. In this experiment, previously weighed glass micropipette was
used for weighing the sample.
2.4.2 Solubility of water in phosphoramides

The solubility of water in phosphoramides was measured using the gravimetric method.
Phosphoramide (5 mL) was equilibrated with an equal volume of deionized water for an hour at
299±1K. After equilibration and centrifugation, the organic and aqueous layers were separated.
A known amount of separated organic layer (3-4 g) was taken into a previously weighed 25 mL
round-bottomed flask. The dissolved water from the sample was removed by vacuum using a
rotary evaporator. Once the sample has attained constant weight after the complete removal of
water, the RB was weighed. The solubility of water in phosphoramides was obtained from the
weight difference of the flask containing the sample before and after the evaporation. The
aqueous phase collected from this experiment was used for measuring the aqueous solubility of
phosphoramide. Karl Fischer method was also employed to find out the water content present in
phosphoramides (undiluted as well as the 1.1M solution).
2.4.3 Solubility of phosphoramides in water
Aqueous phase collected from the above experiment was transferred into a separating funnel and
allowed to settle. The top portion of the aqueous phase in the separating funnel was discarded to
ensure the complete elimination of organics from the aqueous sample and centrifuged prior to the
analysis. The aqueous sample was analyzed for carbon by using TOC analyzer (SHIMADZU
model TOC-VCPH).
2.5 Procedure for Solvent Extraction Experiments
2.5.1 Extraction of Nitric Acid
The extraction of nitric acid by phosphoramides was investigated by equilibrating 1.1 M
phosphoramides in n-dodecane (2 mL) with an equal volume of nitric acid solutions for 2 hrs. at
303±1K. After equilibration, the phases were allowed to settle by gravity. Nitric acid in organic

and aqueous samples was estimated by titrating suitable aliquot with standardized NaOH
solution using phenolphthalein as the indicator.
2.5.2 Extraction of U(VI), Pu(IV) and Th(IV) as a function of equilibrium aqueous phase nitric
acid concentration
Stock solutions containing 500 and 600 µg/mL of Th(IV) and U(VI), respectively were prepared
in various nitric acid media (0.01 M to 6M). The extraction behavior of U(VI), Pu(IV) and
Th(IV) was investigated as a function of equilibrium aqueous phase nitric acid concentration
(0.01M to 6M). 1.1M solution of a phosphoramide in n-dodecane (2 mL) was equilibrated with
an equal volume of nitric acid solution containing the metal ion (Th(IV) or U(VI)) for one hour
by using a magnetic stirrer. The temperature was maintained at 303±1K using a constant
temperature water bath. In the case of Pu(IV), plutonium tracer (10-4 to 10-5 M) (major isotope:
239Pu) was added to the aqueous phase. After equilibration, the phases were allowed to settle by
gravity. Metal ion concentrations in aqueous and organic phases were estimated by suitable
methods by taking appropriate aliquots. From equation 1, D values were obtained for actinide
metal ions.
[푀]표푟푔
(1)
퐷푀 =
[푀]푎푞
2.5.3 Analytical Procedures
The Spectrophotometric method was employed for the estimation of U(VI) and Th(IV) in the
aqueous and organic phases. The U(VI) was estimated by using Br-PADAP as the chromogenic
agent by measuring the absorbance at λmax of 577±1 nm. Th(IV) was estimated using Thoron as
the chromogenic agent by measuring the absorbance at λmax of 541±1 nm [23-26]. The precision

of spectrophotometric determination was better than ±3% and nitric acid estimation was better
than ±1%. The amount of Pu(IV) in aqueous and organic phases was measured by liquid
scintillation counting (LSC) technique by taking suitable aliquots and precision of the method is
±1%.
3 Results and discussion
3.1 Density of hexaalkylphosphoramides
Density is an important physical property of the extractant for separation processes. An optimum
difference in densities of the aqueous and organic phases results in faster separation of the phases
after extraction.
Table 1. Density of phosphoramides and trialkyl phosphates (dry, water-saturated (WS) and 1.1
M solutions in n-dodecane) at 299±1K
Density, ρ (g/mL)
Undiluted
Extractants
HPPA
HBPA
TBP
Dry
WS
1.1 M solutions in n-DD
0.971
0.924
0.976
0.954
0.981
0.954
0.980
0.960
0.826
0.834
0.812
0.814
TAP
*precision of the density measurement is ±0.2%
The trends in the variation of density as a function of the number of carbon atoms present in the
molecule for various systems (n-alkyl phosphoramides, n-alkanes, n-alkyl phosphates) are shown
in (Fig.3.)

Fig. 3. Density variation of the extractants (phosphoramides and phosphates) and hydrocarbons
with an increase in the carbon atoms.
The density of an organic compound depends on its mass, shape and intermolecular interactions.
In the case of linear hydrocarbons, the density gradually increases with an increase in the number
of carbon atoms [27]. This can be attributed to “Van der Waals” interaction between the
molecules. Therefore, the strength of Van der Waals force is directly related to its size of the
molecule. Hence, there is an increase in density with an increase in the carbon number in the
case of hydrocarbons. For trialkyl phosphates, the density of the compound decreases with
increase in molecular mass. It is reported that there is a decrease in density of trialkyl phosphates
with an increase in the number of carbon atoms [7]. Unlike hydrocarbons, trialkyl phosphates

have dipole-dipole interactions operate between the molecules besides van der Waals interaction.
If the size of the molecule is small, intermolecular dipole-dipole interaction between the P=O
groups of molecules is predominant and leads to close packing of molecules. The dipole-dipole
interaction diminishes and the Van der Waals force of attraction becomes dominant with the
increase in the number of carbon atoms. In such cases, the net effect is a reduction in density due
to the close packing of molecules. Therefore, phosphoramides having non-polar alkyl groups and
polar P=O entity also likely to exhibit similar trends. This can be illustrated by comparing
different phosphoramides with a large difference in carbon number. For example, hexamethyl
phosphoramide (HMPA) with six carbon atoms has a density of around 1.030 g/mL, whereas,
hexahexyl phosphoramide (HHPA) with thirty-six carbon atoms has a density of around 0.812
g/mL [31]. The present study also indicates that the crowding near P=O group of the extractant
as in the case of phosphoramides due to the presence of more number of alkyl groups diminishes
the dipole-dipole interaction and lowers the density of the molecule. This effect can be seen from
Table 1 which provides a comparison of densities between trialkyl phosphates and hexaalkyl
phosphoramides containing three and six alkyl groups, respectively. It is interesting to note that
despite the higher molecular weight, both the phosphoramides (HPPA and HBPA) exhibit lower
density of 0.971 g/mL and 0.921 g/mL, respectively, as compared to TBP (0.976 g/mL).
However, the densities of 1.1M solutions of phosphoramides in n-DD are comparable to that of
1.1M TBP/n-DD due to the presence of higher volume fraction of the extractant.
3.2 Solubility of Water in phosphoramides
Table 2. Data on solubility of water in Phosphoramides and Trialkyl phosphates at 299±1K
Extractants
Solubility of H2O in extractant
1.1 M solutions in
n-DD
mg/mL mg/mL mg/mL mol/L
mg/mL
mol/Lit

(1)
(2)
average
HPPA
HBPA
TBP
204.8
74.8
66.3
41.4
201.2
71.6
67.7
40.0
203.0
73.2
67.0
40.6
11.27
4.06
3.37
2.26
44.6
31.1
9.1
2.477
1.727
0.506
0.433
TAP
7.8
*(1) Gravimetry method: Precision of solubility measurement is ±3%.
(2) Karl Fischer titration: Precision of solubility measurement by Karl Fischer titration is ±3%.
The solubility of water in trialkyl phosphates is attributed to hydrogen bonding between the P=O
group of the extractant and water molecules [7]. Earlier, we have investigated the effect of
structural modification of trialkyl phosphates on the solubility of water in these extractants. In
the present study, similar measurements have been made with phosphoramides. Results indicated
that the solubility of water in phosphoramides decreases with an increase in the number of
carbon atoms. This is in accordance with the hydrophobicity of the extractants, i.e. solubility of
water decreases with an increase in the hydrophobicity of the extractants. Therefore, the
structural modification of the extractant by varying the alkyl groups can significantly influence
the solubility of the water in phosphoramides. Data on the solubility of water in undiluted HPPA
and HBPA at 298±1K are given in Table 2 and found to be 203 mg/mL and 73.2 mg/mL,
respectively. The results obtained from both the methods (gravimetry as well as Karl Fischer
method) have shown excellent agreement.
3.3 Solubility of phosphoramides in water
The solubility of extractants in water is decided by dipole-dipole interactions of the P=O group
in extractant and water molecules [7]. Generally, aqueous solubility decreases with an increase in
the carbon number and also branching near the P=O group of the extractant.
Table 3. Data on the aqueous solubility of undiluted (neat) phosphoramides and trialkyl
phosphates.

Extractants
HPPA
HBPA
TBP
Solubility in mg/L
80
60
388
19
TAP
*Precision of TOC analysis: ±3%
Moreover, the hydrophobicity of the extractant also increases by increasing the alkyl chain
length of the extractant. Data on the aqueous solubility of HBPA and HPPA are given in Table 3.
The results presented in this work clearly revealed that HPPA (80 mg/mL) with eighteen carbon
atoms have higher aqueous solubility than that HBPA (60 mg/L) with twenty-six carbon atoms.
The aqueous solubility of an extractant is an important concern with respect to safety
considerations due to ‘red oil” formation. Some incidents reported related to “red oil” formation
was due to the higher solubility of TBP in water and nitric acid medium [32]. In addition, loss of
extractant is also important during reprocessing of spent nuclear fuels due to its aqueous
solubility. Therefore, it is always desirable to employ an extractant with lower aqueous
solubility. The solubility of phosphoramides is significantly lower than that of TBP (388 mg/L)
and is an advantage of phosphoramides for nuclear materials processing applications.
3.4 Extraction of Nitric acid
Table 4. Data on extraction of nitric acid by 1.1 M solutions of phosphoramides in n-DD at
303±1K.
[HNO3](mol/L)
Initial acidity
HPPA
org
HBPA
org
(mol/L)
aq
aq
0.14
0.99
2.09
3.95
5.77
0.01
0.17
0.44
0.81
1.11
0.11
0.81
1.62
3.13
4.25
0.01
0.17
0.62
0.85
1.11
0.10
0.78
1.36
2.70
4.09
*Precision of nitric acid determination is ±1%

Basicity of neutral extractants can be correlated with nitric acid extraction. Horwitz et al. have
studied the extraction of nitric acid by replacing the alkoxy groups with alkyl groups on N, N-
diethylcarbamoylmethyl phosphoryl derivatives and the results indicated that extraction of nitric
acid is enhanced by increasing the basicity of the extractant [28]. Generally, a significant change
in the nitric acid extraction is not observed within the family of extractant by increasing its alkyl
chain length. In the present work, the extraction of nitric acid by HBPA and HPPA have been
studied (Fig. 4) and the results were compared with 1.1M solutions of TBP and TAP in n-DD
(Table 4) under identical conditions [29]. Moreover, for better comparison, the distribution data
for the extraction of nitric acid by 1.1M solutions of HPPA and HBPA in n-DD at 303±1K are
presented in Table 4.

Fig. 4. Variation of distribution ratio for nitric acid extraction by 1.1 M solutions of
phosphoramides and TBP in n-dodecane as a function of aqueous phase nitric acid concentration
at 303±1K.
From the data shown in Fig. 4, it can be concluded that the phosphoramides are relatively more
basic than trialkyl phosphates. Therefore, the extraction of nitric acid by phosphoramides is
marginally higher than that of trialkyl phosphates. The nitric acid extraction isotherms by HPPA
and HBPA are also shown in Fig. 5.

Fig. 5. Variation in the organic phase nitric acid concentration for 1.1 M solutions of
phosphoramides in n-dodecane as a function of aqueous phase nitric acid concentration at
303±1K.
3. 5 Extraction of U (VI)
Table 5. Data on the extraction of U (VI) by 1.1M solutions of phosphoramides and trialkyl
phosphates in n-DD from nitric acid media at 303±1K ($ - This work)
DU(VI)
Acidity
HPPA
HBPA
TBP
TAP
0.01M
0.1 M
1 M
2 M
4 M
0.35
1.81
3.21
4.8
13.3
28.00
$
0.45
0.62
2.46
3.13
16.88
29.98
$
0.01
0.34
7.30
16.10
30.90
35.70
[7]
0.01
0.40
8.76
19.6
35.2
40.3
[7]
6 M
Ref

Precision of DU(VI) measurements: ±4%.
The extraction of U(VI) by 1.1M solutions of HBPA and HPPA in n-dodecane has been
investigated as a function of equilibrium aqueous phase nitric acid concentration at 303±1K
(Fig.6). The extraction data are shown in Table 5 and Fig. 6 provide the U(VI) extraction data of
trialkyl phosphate systems for comparison.
Fig. 6. Variation in the DU(VI) for 1.1 M phosphoramides and trialkylphosphates as a function of
aqueous phase nitric acid concentration at 303±1K..
The trends in the variation of DU(VI) as a function of nitric acid concentration for phosphoramides
and trialkyl phosphates are similar. In both the cases, the DU(VI) increases with nitric acid

concentration and this is a typical trend shown by neutral extractants. However, DU(VI) values for
phosphoramide systems are marginally lower than that of phosphates. It can be seen that DU(VI)
reaches a maximum at around 6M HNO3 for trialkyl phosphates. Moreover, the DU(VI) of
phosphoramide systems gradually increases between 4-6 M HNO3. The initial increase in DU(VI)
is attributed to salting out effect and the further decrease is due to decrease in the free extractant
concentration and also the competition between the U(VI) and HNO3 for free extractant
molecules. The gradual increase after 4M HNO3 might be due to the preferential compound
formation between the protonated phosphoramides and metal nitrate. Furthermore, the formation
of protonated extractant is favored by increasing the nitric acid concentration. Thus, there is a
gradual increase in the DU(VI) with increasing nitric acid concentration in 4-6 M region. Further
studies are in progress to understand the mechanism of extraction for phosphoramides. The
extraction of U(VI) by phosphoramides as well as trialkyl phosphates increases with increase in
number of carbon atoms [7]. For example, the extraction of U(VI) increases by 14 % from TBP
to TAP but it is 26% from HPPA to HBPA at 4M HNO3.
3.6 Extraction of Pu(IV)

Fig. 7. Variation in the DPu(IV) for 1.1M phosphoramides and trialkylphosphates as a function of
aqueous phase nitric acid concentration at 303±1K..
The extraction of Pu(IV) by 1.1M HPPA and HBPA in n-DD has been investigated as a function
of equilibrium aqueous phase nitric acid concentration. The extraction data are shown in Fig. 7
and Table 6. Our earlier data on the extraction of Pu(IV) by TBP and TAP are also included to
compare phosphate and phosphoramide systems.
Table 6. DPu(IV) Values for the extraction by 1.1 M phosphoramides and trialkyl phosphates in
n-dodecane at 303±1K ($ - This work)
Extractants
HPPA
HBPA
TBP
0.5 M
0.7
0.4
0.81
0.95
1M
2.83
5.07
2.91
3.36
2M
4.0
9.15
8.42
9.43
4M
18.2
28.71
24.12
29.33
6M
34.9
40.65
38.33
42.51
Ref
$
$
[30]
[30]
TAP
Precision of DPu(IV) measurements: ±1%.

Results indicated that DPu(IV) for phosphoramides increases with nitric acid concentration and
DPu(IV) for HBPA is marginally higher than that of HPPA (Fig. 7). At lower nitric acid
concentration, the D values for Pu(IV) extraction by phosphoramides are marginally lower than
that of trialkyl phosphates. For example, DPu(IV) values for HPPA and HBPA are 0.4 and 0.7,
respectively, at 0.5M HNO3, whereas for TBP and TAP, DPu(IV) values are 0.81 and 0.9. .Results
also revealed that for phosphoramides the DPu(IV) is higher than that of DU(VI) indicating that
phosphoramides extract Pu(IV) higher than U(VI) at a given nitric acid concentration, whereas in
the case of trialkyl phosphates, the DU(VI) is higher than that of DPu(IV)
.
3. 7 Extraction of Th(IV).
The extraction of Th(IV) by 1.1M solutions of phosphoramides in n-dodecane has been
investigated as a function of equilibrium aqueous phase nitric acid concentration at 303±1K.

Fig. 8. Variation in the DTh(IV) for by 1.1M phosphoramides and trialkyl phosphates as a
function of aqueous phase nitric acid concentration at 303±1K.
The thorium extraction data of phosphoramides along with our earlier data on TBP system are
shown in Fig. 8 and Table 7. Results indicated that D values are lower for phosphoramides as
compared to TBP, especially in the extraction from 2-4 M HNO3 media. Probably the presence
of the six alkyl groups around the P=O group in phosphoramides may be responsible for the
suppression of DTh(IV) value due to steric hindrance in Th(IV) extraction. However, at higher
acidity (eg: 6 M) there is no difference in DTh(IV) value between TBP and phosphoramides.
Table 7. DTh(IV) Values for the extraction by 1.1 M phosphoramides and trialkyl phosphates in
n-dodecane at 303±1K ($ - This work)
Extractants
HPPA
0.01 M 0.1M 1M
0.09 0.13 0.52
2M
0.81
4M
1.85
6M
3.16
Reference
$

HBPA
TBP
0.48
0.02
0.5
0.12 0.83
0.75
1.05
1.4
2.07
4.2
3.33
3.3
$
[13]
Precision of DTh(IV) measurements: ±4%.
Conclusion
We have synthesized and purified two hexaalkyl phosphoramides, HBPA and HPPA. These
31
phosphoramides were well characterized by spectroscopic techniques such as NMR (1H, P &
13C), IR and mass analysis. Their physicochemical properties were measured and extraction
behavior with U(VI), Th(IV) and Pu(IV) was investigated. Lesser density and lower aqueous
solubility of phosphoramides are attractive features to make these compounds as suitable for
solvent extraction applications. The overall results obtained from this study suggest that
phosphoramides are potential ligands for actinides separations.
The present study is only the beginning of an investigation to explore hexaalkyl
phosphoramides for actinide separations. Clearly, further studies on third phase formation,
fission product extraction, degradation aspects (thermal, chemical and radiolytical), etc. have to
be carried out to have a thorough understanding of these extraction systems prior to their
deployment for separation processes.
Acknowledgements
The author’s thanks to Dr. Kari Vijayakrishna and Dr. Suresh Reddy of Department of
chemistry, VIT University, Vellore, India-632 014 for their support and assistance in the
synthesis and characterization of phosphoramides.

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Highlights:
1. Two hexaalkylphosphoramides, HBPA and HPPA have been synthesized, purified and
characterized successfully by using analytical techniques (1H, 13C, 31P and Mass
spectrometry(GC-MS))
2. Extraction studies on phosphoramides with U(VI), Pu(IV) and Th(IV) were evaluated and
results compared with Tri-n-butyl phosphate (TBP)
3. The phosphoramides have lower density and lesser aqueous solubility than TBP