Trends in small angle neutron scattering of actinide-trialkyl phosphate complexes: A molecular insight into third phase formation

Article abstract of DOI:10.1039/c6ra20175j

The "third phase" formation phenomenon in solvent extraction is due to the aggregation of extracted species and formation of reverse micelles. As Small Angle Neutron Scattering (SANS) is a powerful tool to probe colloidal particles, it is considered as an important technique to study the aggregation behaviour of actinide complexes in solvent extraction systems. The actinide specific trialkyl phosphate (TalP) based extractants, namely, tri-n-butyl phosphate (TBP), tri-iso-butyl phosphate (TiBP), tri-sec-butyl phosphate (TsBP) and tri-sec-amyl phosphate (TsAP) have been examined for the first time using the SANS technique to investigate third phase formation phenomena with some of the actinides. SANS was employed to get insight into third phase formation in the extraction of Th(iv) and U(vi) from 1 M HNO₃ by 1.1 M solutions of TalP in deuterated dodecane (n-C₁₂D₂₆, 98 atom% D). Deuterated diluent was used in order to provide contrast during the neutron scattering measurements. Potential energy and the stickiness parameter of reverse micelles formed in the above solvent systems have been quantified as a function of organic metal loading. The data are fitted using Baxter's sticky-sphere model. The stickiness parameter, (τ^-1) a measure of the attractive interaction between the micelles, as well as the attractive potential energy (U₀) was quantified. A clear correlation has been established between the stickiness parameter and the tendency for third phase formation with TalP systems. As U(vi) does not form a third phase with these extractants at 1 M HNO₃, comparative studies were carried out with U(vi)-TalP complexes. These studies established lower stickiness and attraction between the micelles with the U(vi) system. The correlation between SANS parameters and third phase formation tendency was extended to a temperature dependence study and these studies established higher third phase limits when the temperature was enhanced, corroborating well with our experimental results. Our studies also revealed the "prediction of third phase formation" before its occurrence for a range of actinide-extractant systems.

Full text of DOI:10.1039/c6ra20175j

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Trends in small angle neutron scattering of
actinide–trialkyl phosphate complexes:
Cite this: RSC Adv., 2016, 6, 92905
a molecular insight into third phase formation
a
a
a
Aditi Chandrasekar, A. Suresh, N. Sivaraman and V. K. Aswalb
*
The “third phase” formation phenomenon in solvent extraction is due to the aggregation of extracted
species and formation of reverse micelles. As Small Angle Neutron Scattering (SANS) is a powerful tool to
probe colloidal particles, it is considered as an important technique to study the aggregation behaviour
of actinide complexes in solvent extraction systems. The actinide specific trialkyl phosphate (TalP) based
extractants, namely, tri-n-butyl phosphate (TBP), tri-iso-butyl phosphate (TiBP), tri-sec-butyl phosphate
(TsBP) and tri-sec-amyl phosphate (TsAP) have been examined for the first time using the SANS
technique to investigate third phase formation phenomena with some of the actinides. SANS was
employed to get insight into third phase formation in the extraction of Th(IV) and U(VI) from 1 M HNO3 by
1.1 M solutions of TalP in deuterated dodecane (n-C12D26, 98 atom% D). Deuterated diluent was used in
order to provide contrast during the neutron scattering measurements. Potential energy and the
stickiness parameter of reverse micelles formed in the above solvent systems have been quantified as
a function of organic metal loading. The data are fitted using Baxter's sticky-sphere model. The stickiness
parameter, (sꢀ1) a measure of the attractive interaction between the micelles, as well as the attractive
potential energy (U0) was quantified. A clear correlation has been established between the stickiness
parameter and the tendency for third phase formation with TalP systems. As U(VI) does not form a third
phase with these extractants at 1 M HNO3, comparative studies were carried out with U(VI)–TalP
complexes. These studies established lower stickiness and attraction between the micelles with the U(VI)
system. The correlation between SANS parameters and third phase formation tendency was extended to
a temperature dependence study and these studies established higher third phase limits when the
temperature was enhanced, corroborating well with our experimental results. Our studies also revealed
the “prediction of third phase formation” before its occurrence for a range of actinide–extractant systems.
Received 10th August 2016
Accepted 22nd September 2016
DOI: 10.1039/c6ra20175j
www.rsc.org/advances
rich third phase and a diluent rich phase.6,7 Due to its high
viscosity, third phase formation can disturb solvent extraction
processes, especially during large scale processing of fuels.8,9
1. Introduction
Thorium is an important actinide for the long term sustain-
ability of the nuclear programme in India, due its abundant
reserves.1,2 Thorium occurs along with uranium in its major ore,
monazite and has to be separated from uranium to obtain it in
the pure form required for the nuclear fuel cycle.3 The separa-
tion of uranium and thorium is achieved by solvent extraction
in which tri-n-butyl phosphate (TBP) is used as an extractant.
TBP is also used for the processing of thorium based spent
nuclear fuels in the THOREX/Interim-23 process.4,5
A major issue that can potentially arise during solvent
extraction of actinides is third phase formation. This happens
during metal loading in the organic phase exceeding beyond
a certain limit (limiting organic concentration, LOC) for some
metal ions and the organic phase splits into a metal extractant
The parameters that affect third phase formation have been
investigated in the past and it has been found that the nature of
extractant, the metal being extracted, diluent chosen, and
temperature, all have a role to play.10 Tetravalent metal ions
such as Zr(IV), Th(IV) and Pu(IV) are more likely to form third
phase with TalPs compared to UO22+8. Extractants with longer
carbon chains have higher third phase formation limits. For,
example, TiAP has a higher limiting organic concentration than
TBP. With respect to the diluent, this trend is reversed; diluents
with longer carbon chain length are more likely to form third
phase than diluents with shorter carbon chain under the same
experimental conditions. In the extraction of Th(IV) using TBP,
tetradecane with carbon number 14 forms third phase at lower
concentrations of Th(IV) than dodecane which contains 12
carbon atom.11 Studies on the effect of temperature on third
phase formation limits have revealed that increasing tempera-
ture increases the limiting organic concentration, allowing
aChemistry Group, Indira Gandhi Centre for Atomic Research, HBNI,
Kalpakkam–603102, India. E-mail: sivaram@igcar.gov.in
bSolid State Physics Division, Bhabha Atomic Research Centre, Mumbai–400085, India
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Scheme 1 A schematic of third phase formation showing the concentration of metal complexes in the third phase.
more metal loading in the organic phase, before it splits.10
schematic of third phase formation is shown Scheme 1.
A
phosphate (TsAP), on the other hand, does not form third phase
with both U(VI) and Th(IV) up to saturated metal concentrations
Molecular level studies related to third phase formation have when extracted from 1 M nitric acid at 303 K.10 In the present
been carried out using techniques including UV–Vis absorption work, SANS studies have been carried out using solutions of TBP,
and EXAFS to understand third phase formation mechanisms.12,13 TiBP, TsBP and TsAP in n-C12D26 loaded with U(VI) as well as
SAXS and SANS measurements have been employed to probe the Th(IV). The variation in the SANS proles, stickiness parameter
aggregation behaviour of metal complexes which leads to third and attraction potential energy have been examined as a func-
phase formation.14–17 Conductometric technique has also been tion of three parameters inuencing third phase formation
used to detect the formation of third phase during the reproc- phenomena. The effects of extractant, metal ion concentration
essing of fast reactor fuels.18 NMR studies have been carried out and temperature have been systematically investigated. Our
to understand how association and protonation behaviour in studies using SANS indicate prediction of third phase formation
malonamide complexes inuences third phase formation.19
The SANS technique has been used for a variety of applica-
tions, including the study of polymers,20–28 organic molecules,29,30
gels,31 micro32–37 and nanoscale materials30,38 and biomole-
cules.39,40 A subset of these that pertain to the current work
include, probing aggregation behaviour of micelles,20,41 cloud
point studies42–45 and temperature dependent aggregation.43,46
Earlier studies carried out in our laboratory have shown that tri-
sec-butyl phosphate (TsBP) has higher U/Th separation factor
and lesser third phase formation tendency compared to its
isomers, TBP and tri-iso-butyl phosphate (TiBP).47 Tri-sec-amyl
behaviour and these results are discussed.
2. Experimental
2.1. Chemicals
The extractants, TsBP, TiBP and TsAP employed in the present
study were synthesized by condensation reaction between
POCl3 with stoichiometric equivalent of corresponding alcohol
in the presence of pyridine.10,48 Structures of trialkyl phosphates
used in the present work are shown in Fig. 1. Diethylene-
triaminepentaaceticacid (DTPA) and 2,6-pyridinedicarboxylic
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acid (PDCA) (Lancaster Synthesis Ltd, England) were used for
As described above, organic samples containing uranium
the complexometric analysis of Th(VI) and U(VI), respectively. were also prepared by equilibrating uranyl nitrate solution with
The diluent, deuterated dodecane (n-C12D26 (Lancaster 1.1 M solutions of TBP, TiBP, TsBP and TsAP in n-C12D26. As
)
synthesis Ltd, England) was used as such without further U(VI) does not form third phase with any of the four extractants
purication. Xylenol orange indicator (TCI, Tokyo, Japan) was used, in addition to the samples loaded with 20, 30 and 40
used as received. Nuclear grade thorium nitrate (Indian Rare g Lꢀ1, a sample with a uranium organic loading of 120 g Lꢀ1 was
Earths Ltd, Mumbai, India) was used as received without also prepared and no third phase was formed at this concen-
further purication. Nuclear grade uranyl nitrate UO2(NO3)2- tration. The sample with 120 g Lꢀ1 of U(VI) was prepared by
$6H2O obtained from Nuclear Fuel Complex, Hyderabad, India, equilibrating a saturated UO2(NO3)2 solution with the organic
was used as received. All other chemicals employed were of GR phase separately for each extractant system. In all the cases, the
or AR grade.
corresponding organic phase was used for SANS experiments.
Concentration of metal ions (Th and U) and HNO3 of samples
used for SANS experiments are shown in Table 1.
2.2. Experimental procedure
2.2.1. Preparation of trialkyl phosphate solutions in n-
2.3. Analytical procedures
C
12D26. The 1.1 M extractant solutions in n-C12D26 were
2.3.1. Determination of Th(IV), U(VI) and HNO3. Thorium
concentrations in both the phases were estimated by com-
plexometric titrations using DTPA (0.03 M and 0.01 M, respec-
tively, for the analysis of concentrated and dilute thorium
solutions) as titrant and xylenol orange as an indicator. Uranium
concentrations in both phases were estimated by com-
plexometric titrations using PDCA solution (0.01 M) using
arsenazo I as an indicator. Acid concentrations in organic
samples were analysed by acid–base titration with a standard
dilute NaOH solution using phenolphthalein as an indicator. The
U(VI) or Th(IV) metal ions were complexed with neutral saturated
potassium oxalate solution prior to “free acidity” determination.
prepared by taking an appropriate quantity of extractant (on
weight basis) in a standard ask, and making up the volume
with n-C12D26. Prior to solvent extraction experiments, the
extractant solutions were washed by equilibrating with 5 M
NaOH for two hours to remove any acidic impurities present in
the extractant. The NaOH traces were removed from the organic
phase by washing it several times with water until the wash
water was nearly neutral. The purity of the extractants was
established using gas chromatographic technique (>98%).
2.2.2. Preparation of samples for SANS experiments. A
solution of thorium nitrate in 1 M nitric acid was equilibrated
with an organic phase (1.1 M solutions of TBP, TiBP, TsBP and
TsAP in n-C12D26). Equal volumes of aqueous and organic phases
along with a magnetic stirring bar were taken in an equilibration
tube. During these experiments, temperature was maintained at
303 K by placing the ask in a water bath contained in a double
walled jacket. The two phases were equilibrated for 30 minutes
by stirring at a speed enough to mix the two phases. As some of
the acid from the aqueous phase was extracted in to the organic
phase, acidity of aqueous phase dropped below the required
concentration of 1 M. Hence, acidity of the aqueous phase was
maintained at 1 M by adding few drops of conc. nitric acid (16 M)
and equilibrating again such that the nal equilibrium aqueous
concentration of acid was 1 M. The concentrations of thorium
and nitric acid were adjusted such that the equilibrium aqueous
acidity was 1 M and the organic phase thorium loading was 20,
30 and 40 g Lꢀ1 for each extractant system. These concentrations
were chosen such that the organic phase loading does not exceed
the LOC, aer which the organic phase splits into a third phase
and a diluent rich phase.10
2.4. SANS measurements
For the SANS measurements, the organic phase containing the
metal complexes of known concentration in 1.1 M TalP/n-
C
12D26 was used. The neutron scattering experiments were
carried out using the SANS facility at Dhruva reactor, in Bhabha
Atomic Research Centre (BARC), India.49 A mean neutron
Table 1 Concentration of metal ions/acid in the samples used for
SANS experiments
Metal free
samples
Concentration of
metal ion
Conc. of HNO3 (M)
in the organic phase
Organic phase
U(VI)
Th(IV)
1.1 M TBP/n-C12D26
20 g Lꢀ1
30 g Lꢀ1
40 g Lꢀ1
120 g Lꢀ1
20 g Lꢀ1
30 g Lꢀ1
40 g Lꢀ1
120g Lꢀ1
20 g Lꢀ1
20 g Lꢀ1
30 g Lꢀ1
40 g Lꢀ1
0.23
1.1 M TiBP/n-C12D26
20 g Lꢀ1
30 g Lꢀ1
40 g Lꢀ1
0.20
1.1 M TsBP/n-C12D26
1.1 M TsAP/n-C12D26
20 g Lꢀ1
30 g Lꢀ1
40 g Lꢀ1
20 g Lꢀ1
30 g Lꢀ1
40 g Lꢀ1
0.26
0.22
40 g Lꢀ1
80 g Lꢀ1
120 g Lꢀ1
Fig. 1 Structure of trialkyl phosphates studied in the present work.
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˚
wavelength of 5.2 A with a resolution of ꢁ15% was used for the less of the metal species are extracted into the organic phase.
experiments. The scattered neutrons were measured using Moreover, at higher acidities the third phase limit is lowered
a one-dimensional 3He gas detector. The differential scattering and a wide range of metal loading in the organic phase cannot
cross-section per unit volume (dS/dU) was measured as a func- be achieved. When metal ions are extracted, both the metal
tion of scattering vector (Q), where Q is given by the equation:
complex and the H+ species are present in the organic phase.
Metal free organic phases of 1.1 M TalP/n-C12D26 were also
prepared by equilibrating with 1 M HNO3 and the scattering
pattern was recorded. It is seen from Fig. 2 that the rise of
scattering intensity in the low Q region is highest for TBP,
marginally lower for TiBP, and reduces further for TsBP and
TsAP. The scattering intensity shows correlation with the
tendency of each extractant to form third phase with the metal
ions. The higher scattering intensity in the low Q region is ex-
pected to arise from increase in size and/or increase in attractive
interaction of the aggregates. SANS data are best tted by
considering the increase in the attractive interaction between
the aggregates. The extractants, TBP and TiBP have a higher
tendency to form third phase with Th(IV) compared to TsBP;
TsAP does not form third phase with Th(IV).10
4psin q
Q ¼
l
In the above equation q is the half-scattering angle and l is the
mean wavelength of the incident neutrons used. The samples
were scanned in the Q range from 0.017 to 0.35 Aꢀ1. A quartz cell
˚
with a thickness of 2 mm was used for containing the samples
during the measurements. Theoretically, the scattering intensity
from a collection of particles depends on the form factor P(Q),
structure factor S(Q), number density of particles (NP), square of
volume of particles (VP), and scattering contrast (rP ꢀ rS)2. The net
intensity, expressed as a product of these, is given by the equation:
dS
2
2
ðQÞ ¼ NPVP ðrP ꢀ rSÞ PðQÞSðQÞ
dU
3.2. Studies on 1.1 M TalP/n-C12D26–Th(NO3)4/1 M HNO3
systems
For particles experiencing an attractive force between each
other, the value of S(Q) can be calculated using Baxter's sticky
hard-sphere model.50 The stickiness parameter (sꢀ1) is calcu-
lated from s, which is obtained by tting the data using the
Baxter's sticky hard-sphere model. Further, the potential energy
of the interaction U0 can be obtained from the expression:
3.2.1. Effect of the structure of trialkyl phosphate. The
scattering intensity for Th(IV) loaded sample changes with
variation in structure of the extractant, when all other factors
are kept constant. Fig. 3 shows the scattering proles obtained
at 303 K for various TalP/n-C12D26 systems, all loaded with same
amount of metal ion (30 g Th(IV) per L). The highest scattering
intensity in the low Q region is shown by TBP followed by TiBP,
TsBP and TsAP respectively. The stickiness parameter (sꢀ1) and
the attractive potential energy U0 calculated for the four systems
are shown (Table 2a). Highest stickiness parameter was
observed with TBP, which has a higher third phase formation
tendency with Th(IV). The interaction potential energy is the
most negative indicating higher attraction between the
micelles. The stickiness parameter decreases with branching in
ꢀ
ꢁ
s þ D
12D
U0
s ¼
exp
kBT
The diameter of the micelle (s), is calculated mathematically
assuming the particles to be spherical and the width of the
˚
square-well potential (D), is taken as 3 A for the potential energy
calculations.51 In order to calculate the aggregate diameter, the
volume of a single extractant molecule was rst computed from
its density and molecular mass. The aggregation number was
taken as 3 for Th(IV) and 2 for U(VI).52,53 The expression for the
volume of a sphere was used to obtain the size of aggregates. Data
are tted with the two independent tting parameters as sticki-
ness (1/s) and size of the aggregate. The stickiness is mostly
sensitive to low Q region, whereas size is determined from the
high Q region. Throughout the data analysis, corrections were
made for instrumental smearing by the appropriate resolution
function. The parameters in the analysis were optimised by
means of non-linear least-squares tting program. As third phase
formation is inherently an aggregation phenomenon, sticky hard
sphere model was adopted during the tting of the SANS data.
3. Results and discussion
3.1. Study on metal free 1.1 M TalP/n-C12D26–1 M HNO3
system
Extraction of metals was carried out at 1 M acidity to ensure
higher loading of metal complexes in to the organic phase. At
Fig. 2 SANS data of metal free 1.1 M TalP/n-C12D26–1 M HNO3
higher acidities, the H+ ion competes for the free extractant and systems at 303 K.
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aggregation number was taken as 3 for Th(IV) complexes.47 An
˚
aggregate diameter of 13.8 A was obtained for TBP, TiBP and
˚
TsBP and a diameter of 14.6 A for TsAP, based on the
assumption that the micelles are spherical as discussed above.
For a given system, the size of the aggregates does not change,
though the potential varies depending on the conditions. This
has been previously studied in the work of Verma et. al.52,53
which showed that aggregate number changed only in the
particle growth model applied to higher metal loading, e.g. third
phase regions. Our studies, however, are restricted to lower
metal loading where radii remain constant as consistent with
earlier study.52,53
3.2.2. Effect of metal loading. For a given extractant
system, the scattering intensity, stickiness parameter and
attraction potential energy change when the metal concentra-
tion is varied. At higher concentration of Th(IV) in the organic
phase (40 g Lꢀ1), the scattering intensity in the low Q region is
highest for all extractant systems at a xed temperature of 303
K, (Fig. 4A–D). Similar to the trend observed in Fig. 3, TBP
system shows highest scattering intensity for a given metal
loading, and least for TsAP system. The difference in intensity
between metal loading of 20 g Lꢀ1 and 40 g Lꢀ1 is most
pronounced in the case of TBP (Fig. 4C), which has the highest
tendency to form third phase and the least difference was
observed for TsAP system, which does not form third phase with
Th(IV)/1 M HNO3 at all Th concentrations. From the patterns, it
can be seen that for extractants such as TBP and TiBP which
Fig. 3 SANS data obtained for the 1.1 M TalP/n-C12D26–Th(NO3)4/1 M
HNO3 systems loaded with 30 g Th per L at 303 K.
the alkyl chain as in the case of TiBP and is further lower for
TsBP, which has lower third phase formation tendency with
Th(IV). For TsAP, the value of stickiness parameter drops much
lower and it is also observed from solvent extraction experi-
ments that TsAP does not form third phase with Th(IV), when
extracted from 1 M HNO3 at all metal concentrations. Based on
the experimentally derived metal–ligand ratio in the complex,
Table 2 The fitted parameters of SANS data for (a) 1.1 M TalP/n-C12
D
26–Th(NO3)4/1 M HNO3 systems, loaded with 30 g Lꢀ1 of Th(IV) at 303 K. (b)
20 g Lꢀ1 and 40 g Lꢀ1 Th(IV) loaded TalP/n-C12D26 systems at 303 K. (c) 1.1 M TalP/n-C12
D26–Th(NO3)4/1 M HNO3 systems, loaded with 40 g Th
per L as a function of temperature
(a) Conditions: TalP/n-C12D26/1 M HNO3 systems, loaded with 30 g Lꢀ1 of Th(IV) at 303 K
System
Aggregation number (N)
Aggregate diameter (D)
Stickiness parameter (1/s)
Potential energy [U0 (kBT)]
˚
1.1 M TBP/n-C12D26
1.1 M TiBP/n-C12D26
1.1 M TsBP/n-C12D26
1.1 M TsAP/n-C12D26
3
3
3
3
13.8 A
9.3
8.5
7.7
4.7
ꢀ1.47
ꢀ1.38
ꢀ1.28
ꢀ0.84
˚
13.8 A
˚
13.8 A
˚
14.6 A
(b) Conditions: 20 g Lꢀ1 and 40 g Lꢀ1 Th(IV) loaded TalP/n-C12D26 systems at 303 K
˚
˚
˚
˚
TBP (N ¼ 3, D ¼ 13.8 A)
TiBP (N ¼ 3, D ¼ 13.8 A)
TsBP (N ¼ 3, D ¼ 13.8 A)
TsAP (N ¼ 3, D ¼ 14.6 A)
Metal conc.
1/s
U0 (kBT)
1/s
U0 (kBT)
1/s
U0 (kBT)
1/s
U0 (kBT)
20g Lꢀ1
40g Lꢀ1
7.4
11.3
ꢀ1.24
ꢀ1.66
7.3
9.2
ꢀ1.23
ꢀ1.46
6.9
8.9
ꢀ1.17
ꢀ1.42
4.6
4.9
ꢀ0.81
ꢀ0.87
(c) Conditions: 1.1 M TalP/n-C12
D26–Th(NO3)4/1 M HNO3 systems, loaded with 40 g Th per L
˚
˚
˚
TBP (N ¼ 3, D ¼ 13.8 A)
TiBP (N ¼ 3, D ¼ 13.8 A)
TsBP (N ¼ 3, D ¼ 13.8 A)
Temp (ꢂC)
1/s
U0 (kBT)
1/s
U0 (kBT)
1/s
U0 (kBT)
40
50
60
70
9.8
8.8
7.8
7.4
ꢀ1.52
ꢀ1.41
ꢀ1.29
ꢀ1.24
8.4
7.6
7.2
6.7
ꢀ1.37
ꢀ1.27
ꢀ1.21
ꢀ1.14
8.0
7.5
6.9
6.4
ꢀ1.32
ꢀ1.25
ꢀ1.17
ꢀ1.10
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have higher third phase formation tendency, the scattering attraction potential is larger at higher metal concentrations,
intensity is more sensitive to variation in organic metal indicating more attraction between the micelles. Among the
concentration.
extractants, TBP has the highest stickiness, followed by TiBP,
When the metal loading is higher, the tendency for the TsBP and TsAP. This trend matches with the trends observed in
system to form third phase is higher, because of the stronger solvent extraction experiments, which showed a similar trend
attractive interaction between the metal solvate species while for third phase formation tendency among these extractants.10
approaching the LOC. When the number density of particles Stickiness parameter is markedly lower for TsAP than the other
increase, they are forced to pack in such a way that they come three phosphate systems with butyl carbon chain, indicating
closer, making this a factor for aggregation. In general, the that its third phase limits are higher. This has been observed by
attraction leading to third phase formation/aggregation can be earlier solvent extraction experiments carried out in our
prevented in metal–extractant systems by increasing the alkyl laboratory.11
chain length of the extractant molecules, thus enhancing the
3.2.3. Effect of temperature. Apart from investigating the
intermicellar distance making the central polar cores stay away effects of structure of the alkyl groups of the extractant and
from each other. In addition, for systems consisting of isomers organic metal loading on neutron scattering, a third parameter
of extractants with similar micellar radius, the structure of the namely, temperature was probed in the present work using the
extractant may inuence the intermicellar distance, in a way same tool, keeping all other factors similar. Fig. 5A–C shows the
this is achieved by introducing branching on the carbon chain variation in SANS patterns for TBP, TiBP and TsBP systems, as
close to the polar core. Thus the stickiness parameter increases the temperature was increased from 313 to 343 K in steps of 10
from 7.7 in TsBP to 9.3 in TBP system. It is interesting to note K. For all extractant systems, the Th(IV) concentration was kept
that even in the nanoparticle systems, these are oen capped at 40 g Lꢀ1. As temperature was raised, intensity of the scat-
with sterically bulky molecules to prevent them from coming tering monotonically reduces for all extractants. Among these,
close together, greatly reducing their tendency to aggregate.54,55 TBP system is most sensitive to rise in temperature from 313 to
For the four extractant systems (Fig. 4), stickiness parameters 323 K, as well as from 323 to 333 K.
have been computed and the results are shown in Table 2b. As
The increase in thermal energy suppresses aggregation in
expected, the stickiness parameter is higher at 40 g Lꢀ1 metal the solvent phase and hence prevents third phase formation.
loading compared to 20 g Lꢀ1 for all the extractants. The Thus, aggregation leading to third phase formation can be
Fig. 4 SANS plots of 1.1 M TalP/n-C12
D
26–Th(NO3)4/1 M HNO3 systems with organic Th(IV) loading of 20 g Lꢀ1 and 40 g Lꢀ1 at 303 K (A) TiBP (B)
TsAP (C) TBP and (D) TsBP.
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Fig. 5 Variation in the scattering pattern as a function of temperature for 1.1 M TalP/n-C12
D
26–Th(NO3)4/1 M HNO3 systems with a Th(IV) loading
of 40 g Lꢀ1 in the organic phase (A) TBP (B) TiBP (C) TsBP systems are shown in the panels.
minimised by increasing the temperature as observed by our a small change in the stickiness and aggregation energy as the
earlier solvent extraction experiments.10 The variations in metal loading is increased, indicating that aggregation
stickiness parameter and attraction potential energy as a result tendency of U(VI) loaded organic phase have a limited sensitivity
of temperature increase are shown in Table 2c. The data indi- to increasing metal loading. Aggregation number was taken as 2
cate that there is a gradual decrease in the stickiness parameter for all extractant systems loaded with U(VI). For TBP and TiBP,
˚
with increase in temperature for all the three solvents.
the micelle diameter obtained was 12.0 A, and for TsAP it was
˚
calculated to be 12.7 A.
As 10 g Lꢀ1 change in U(VI) had only a small effect on the
intensity and tted parameters obtained from the curves, 1.1 M
TsAP/n-C12D26 was loaded with 40 and 80 g Lꢀ1 of U(VI). Fig. 6C
shows the SANS results obtained from these two systems and
Table 3a shows the corresponding (sꢀ1) and U0 values. The
magnitude of (sꢀ1) is lower for TsAP compared to TBP and TiBP,
illustrating that its tendency to form third phase is even lower
than the two butyl phosphates. It has also been observed
experimentally that phosphates with a longer carbon chain are
less susceptible to third phase formation.11
3.3.2. Effect of the structure of trialkyl phosphate. At lower
concentrations of U(VI) in the organic phase, the stickiness
parameter was lower, because at the given acid concentration
these extractants do not form third phase with uranium even
with saturated aqueous concentration of the metal ion. SANS
studies was carried out with samples loaded with 120 g Lꢀ1 of
U(VI), which was the organic loading obtained with near satu-
rated (saturated loading ꢁ120–125 g Lꢀ1) aqueous concentra-
tion of the metal ion. Fig. 7 shows the neutron scattering
3.3. Studies on 1.1 M TalP/n-C12D26–UO2(NO3)2/1 M HNO3
systems
3.3.1. Effect of metal loading. The third phase formation
tendency of Th(IV) complexes showed a strong correlation with
the stickiness parameter obtained from the SANS patterns. It
has been observed from solvent extraction experiments that
U(VI) does not form third phase with the 1.1 M TalP/n-C12D26 in
the extraction from 1 M HNO3. Hence to investigate our
predictions, similar SANS experiments were carried out using
samples loaded with U(VI). Fig. 6A–C shows the tted SANS plots
obtained at 303 K from TiBP, TBP and TsAP systems, respec-
tively, as a function of increasing U(VI) concentration in the
organic phase. The TiBP and TBP systems were loaded with 20,
30 and 40 g Lꢀ1 of U(VI). From the proles it can be seen that
there is a marginal increase in the scattering intensity as metal
concentration was increased for all the three extractants.
Stickiness parameter and attraction potential energy have been
calculated for all metal concentrations (Fig. 6A–C), and are
shown in Table 3a. For all the extractant systems, there is
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Fig. 6 Variation of SANS curves as a function of metal loading obtained from 1.1 M TalP/n-C12
D26–UO2(NO3)2/1 M HNO3 systems at 303 K (A)
TiBP (B) TBP and (C) TsAP.
pattern obtained at 303 K from TBP, TiBP and TsAP systems increase in temperature as enhancement in thermal energy
loaded with 120 g Lꢀ1 of U(VI). The data in Table 3b show the reduces inter-particle attraction. The attractive energy becomes
calculated values of (sꢀ1) and U0 obtained aer tting the data. less with temperature rise.
The scattering intensity is equally high for TBP and TiBP, and
these have a stickiness parameter of 7.9 and attraction potential
of ꢀ1.2, which are higher in magnitude than the corresponding
3.4. Comparison of 1.1 M TalP/n-C12D26 solvents containing
Th(IV) and U(VI)
numbers shown in Fig. 6, where the same extractants were
loaded with a maximum of 40 g Lꢀ1 of U(VI). The scattering
intensity as well as the stickiness parameter is lower for TsAP
compared to the two butyl phosphates (Fig. 7) at the same metal
loading of 120 g Lꢀ1. This implies that TsAP has lower tendency
to aggregate than butyl phosphates do.10
As described in the previous sections, TBP and TiBP have high
tendency to form third phase with Th(IV), whereas they do not
form third phase with U(VI) under normal extraction conditions.
The SANS patterns obtained in the present study are in line with
the above behaviour and show a clear difference between
organic phases loaded with Th(IV) and U(VI) for these two
extractant systems. Fig. 9A and B shows the SANS plot obtained
at 308 K for TBP and TiBP systems, respectively. Each organic
phase was separately loaded with 40 g Lꢀ1 of Th(IV) and U(VI). It
can be seen that for both the extractant systems, the scattering
intensity is much higher for Th(IV) loaded samples compared to
samples loaded with U(VI). The data in Table 3d shows the
corresponding stickiness parameter and attraction potential
energy obtained for the above systems. The stickiness param-
eter is much higher for the Th(IV) loaded samples indicating
a higher third phase formation tendency, whereas for the U(VI)
loaded samples, the stickiness is much lower, in agreement
with the observation that the two extractant systems do not
form third phase with U(VI). From the SANS studies it is seen
3.3.3. Effect of temperature on U(VI) system. Increase in
temperature produces lower scattering intensity and lowers the
stickiness in the Th(IV) systems, thereby reducing the third
phase formation tendency. The effect of raising the temperature
has a similar effect on U(VI) loaded organic phases containing
1.1 M TalP/n-C12D26. Fig. 8A and B, respectively, shows the
scattering from TBP and TiBP extractant systems loaded with 40
g Lꢀ1 of U(VI) at different temperatures ranging from 313 to 333
K. Fig. 8C shows 1.1 M TsBP/n-C12D26 containing 20 g Lꢀ1 of
U(VI) at temperatures between 313 and 343 K. For all the
extractants, for a xed metal loading, the scattering intensity
decreases with increase in temperature. Table 3c shows the
variation in stickiness parameter and attraction potential
energy for systems at different temperatures shown in Fig. 8. For
all three phosphates, the stickiness parameter decreases with
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Table 3 The fitted parameters of SANS data for (a) 1.1 M TalP/n-C12D26–UO2(NO3)4/1 M HNO3 systems at 303 K. (b) 1.1 M TalP/n-C12D26–
UO2(NO3)2/1 M HNO3 systems loaded with 120 g U per L at 303 K. (c) 1.1 M TalP/n-C12D26/1 M HNO3 systems with U(VI): 40 g Lꢀ1 in TBP and TiBP
and 20 g Lꢀ1 in TsBP as a function of temperature. (d) 1.1 M TBP and TiBP in C12D26 loaded with 40 g Lꢀ1 Th(IV) and U(VI)/1 M HNO3 at 308 K
(a) Conditions: 1.1 M TalP/n-C12
D26–UO2(NO3)4/1 M HNO3 at 303 K
˚
˚
˚
TBP (N ¼ 2, D ¼ 12.0 A)
TiBP (N ¼ 2, D ¼ 12.0 A)
TsAP (N ¼ 2, D ¼ 12.7 A)
U(VI)
conc. (g Lꢀ1
)
1/s
U0 (kBT)
1/s U0 (kBT)
1/s
U0 (kBT)
20
30
40
80
6.6
6.7
6.9
ꢀ1.02
ꢀ1.03
ꢀ1.06
—
6.6
6.8
7.2
ꢀ1.01
ꢀ1.04
ꢀ1.10
—
—
—
4.7
4.9
—
—
ꢀ0.72
—
—
ꢀ0.76
(b) Conditions: 1.1 M TalP/n-C12D26/1 M HNO3 systems loaded with 120 g U per L at 303 K
System
Aggregation number (N)
Aggregate diameter (D)
Stickiness parameter (1/s)
Potential energy [U0 (kBT)]
1.1 M TBP/n-C12D26
1.1 M TiBP/n-C12D26
1.1 M TsAP/n-C12D26
2
2
2
12.0
12.0
12.7
7.9
7.9
5.2
ꢀ1.2
ꢀ1.2
ꢀ0.81
(c) Conditions: 1.1 M TalP/n-C12D26/1 M HNO3 systems with U(VI): 40 g Lꢀ1 in TBP and TiBP and 20 g Lꢀ1 in TsBP
TBP
TiBP
TsBP
1/s
Temp (ꢂC)
1/s
U0 (kBT)
1/s
U0 (kBT)
U0 (kBT)
40
50
60
70
5.5
5.1
5.0
—
ꢀ0.84
ꢀ0.76
ꢀ0.74
—
5.4
4.9
4.8
—
ꢀ0.82
ꢀ0.71
ꢀ0.70
—
5.3
4.8
—
ꢀ0.79
ꢀ0.69
—
ꢀ0.60
4.4
(d) Conditions: 1.1 M TBP and TiBP in C12D26 loaded with 40 g Lꢀ1 Th(IV) and U(VI)/1 M HNO3 at 308 K
1.1 M TBP/n-C12D26
1.1 M TiBP/n-C12D26
1/s
Metal ion in the organic phase
1/s
U0 (kBT)
U0 (kBT)
40 g Lꢀ1 Th(IV)
40 g Lꢀ1 U(VI)
10.9
5.8
ꢀ1.63
ꢀ0.89
8.7
5.5
ꢀ1.40
ꢀ0.83
that tetravalent metals ions with higher ionic potential e.g.
Th(IV) have higher third phase formation tendency.
3.5. Prediction of third phase formation
3.5.1. Interpretation of scattering intensity and stickiness
parameter. Observations made from the overall SANS proles
can be extended to predicting the likelihood of third phase
formation phenomena. TBP loaded with as high as 120 g Lꢀ1 of
U(VI) (Fig. 7) shows scattering intensity (Q / 0) less than 3.5
indicating that in general for systems with scattering intensity
below 3.5 they are far from forming third phase. On the other
extreme, systems with scattering intensity (Q / 0) above 6 are
close to the third phase limit as seen from Fig. 4A, which shows
the scattering for TiBP loaded with 40 g Lꢀ1 of Th(IV), which is
close to its limiting organic concentration of 43.8 g Lꢀ1. Similar
threshold values can be set for the stickiness parameter (sꢀ1).
Systems with (sꢀ1) less than 8 are not susceptible to third phase
formation as seen from Table 3b, whereas a stickiness value
greater than 9 would signal near third phase conditions. This
Fig. 7 SANS data obtained for 1.1 M TalP/n-C12
HNO3 systems loaded with 120 g U per L at 303 K.
D
26–UO2(NO3)2/1 M
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Fig. 8 Temperature variants in SANS obtained from 1.1 M TalP/n-C12
D
26–UO2(NO3)2/1 M HNO3 systems with U(VI): (A) 40 g Lꢀ1 U(VI) in TBP (B)
40 g Lꢀ1 U(VI) in TiBP and (C) 20 g Lꢀ1 U(VI) in TsBP.
Fig. 9 SANS patterns obtained at 35 ꢂC from 1.1 M TalP/C12D26 (A) TBP and (B) TiBP loaded with 40 g Lꢀ1 Th(IV) and U(VI)/1 M HNO3.
can be seen from Table 2b which shows 9.2 to be the stickiness
parameter for TiBP system loaded with 40 g Lꢀ1 of Th(IV).
Stickiness parameter for TBP is much higher than the threshold
when loaded with 40 g Lꢀ1 of Th(IV) which is close to its limiting
organic concentration. The corresponding value for TsAP
loaded with 40 g Lꢀ1 Th(IV) is 4.9 which is way below the
threshold, and agrees with the observations that TsAP does not
form third phase with Th(IV) at 1 M acid up to saturated aqueous
concentration.10
3.5.2. Effect of the extracted metal ion. From the SANS
studies it is seen that tetravalent metal ions e.g. Th(IV) have
higher third phase formation tendency as seen from its higher
scattering intensities and stickiness parameter data. In this
context, it will be interesting to explore the stickiness parameter
details for the Zr(IV)–TBP system as it has been experimentally
shown in our laboratory that Zr is bound to form third phase
even at much lower metal ion concentration for e.g. above the
LOC of 8.19 g Lꢀ17. It is also interesting to compare the present
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study with a preliminary study on SANS of Zr(IV) loaded TBP/n- tendencies observed due to structural variation in the ligands
12D26 system carried out in our laboratory by Benadict et al.17 may be extended to include a wide range of materials and
C
Earlier studies on SANS of 1.1 M TBP/n-C12D26 loaded with Zr provide means of overcoming aggregation by ne tuning the
(0.057 mol Lꢀ1) before the LOC conditions indicated a scat- molecular structure of interacting surfaces.
tering intensity of around 5 (Q / 0) with a stickiness parameter
of 9.7 and potential energy of ꢀ1.4. The above values of Zr–TBP
system are higher than the corresponding values (stickiness
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