Journal of Nuclear and Radiochemical Sciences, Vol. 4, No. 2, pp. 19-22, 2003

ArticlesMechanism of Extraction of Hexavalent Uranium from Alkaline Medium by
Aliquat-336/Kerosene Solution
Y. A. El-Nadi, N. E. El-Hefny and J. A. Daoud*
Hot Laboratories Centre, Atomic Energy Authority, 13759, Cairo, Egypt
Received: May 9, 2003; In Final Form: August 21, 2003
The kinetics of extraction of uranium(VI) from alkaline carbonate medium by tricaprylmonomethylammonium
chloride (Aliquat-336) in kerosene has been investigated using a stirred Lewis cell. The effects of different parameters affecting the extraction rate, such as hydrogen ion, carbonate, hydroxide, Aliquat-336, U(VI) concentrations
as well as temperature, were separately studied and a rate equation was deduced. The extraction process was foundto be governed by chemical reaction in the bulk phase rather than at the interface.

1. Introduction
Based on the high tendency of uranium to form soluble
carbonate complexes in alkaline medium,1, 2 an approach toalkaline dissolution of irradiated uranium was introduced by
Ali et al.3 The proposed process, based on nuclear fuel recycling in a system containing carbonate solution, relies on thesolubility of UO 2 in the carbonate medium containing
oxidizing agents followed by uptake using basic Bio-Rex 5exchanger.
On the other hand, although uranium ores can be leached byseveral solutions to solubilize uranium, only sulfuric acid or
sodium carbonate has been practiced. The alkaline digestion
method is generally preferred, not only for its simplicity andeconomic reasons, but also for the fact that the resulting
sodium phosphate may be used in the industry of fertilizers. 4
With the alkaline leach, development of extraction systems
capable of extraction of uranium in the presence of high saltcontent at the elevated pH is of great interest. In this context,
the amine extraction technique is particularly important since it
functions efficiently in strong acid solution and even in alkaline medium with quaternary ammonium ion (e.g. tricaprylmonomethylammonium chloride, Aliquat-336). 5 In thisconcern, Landgren6 used Aliquat-336 to investigate the extraction behaviour of several elements, such as Sc, Fe, Zr, somelanthanides, and some actinides, and nitric acid and reported
that Aliquat-336 shows high extraction for tetravalent elements
and anions: the extraction of multivalent cations was found todecrease in the order M4+ > MO22+ > M3+. A selective method
has been developed7 for extraction chromatographic studies ofU(VI) with Aliquat-336 (liquid anion exchanger) coated on
silica gel as a stationary phase. Quantitative extraction ofU(VI) has been achieved at 1.25 – 4 M HCl; U(VI) has been
separated from the binary, ternary and quaternary mixture ofvarious metal ions.
The knowledge of the mass transfer rate and mechanism ofextraction offered by the kinetic investigations is of major
importance for the optimization and control of the extraction
process. Tianlin et al. 8 studied the extraction kinetics ofuranium (VI) with the binary extractant system of trioctylamine and tri-n-octylphosphine oxide in n-heptane fromsulphuric acid media using stationary interface stirring cell at
298 K. The reported synergistic mechanism is controlled by atwo step continuous reaction at the interface. Awwad et al.9
*Corresponding author. E-mail: jadaoud@hotmail.com. FAX:
+202-6352474.

investigated the kinetics of extraction of U(VI) and Th(IV)from nitric acid solution by tributylphosphate and Cyanex-921
in kerosene using single drop technique. The extraction rateequations for both elements have been deduced, compared anddiscussed in terms of metal ion, nitric acid, and extractantconcentrations.
The present work is directed to investigate the kinetics ofuranium extraction from alkaline carbonate medium by
Aliquat-336 in kerosene in order to get information on thefactors affecting the extraction rate of U(VI) in this system andto propose a mechanism describing the extraction process.
2. Experimental
Chemicals and Reagents. Uranyl nitrate of analyticalreagent grade (AR) was a Fluka product, while sodium
carbonate and hydrogen peroxide were obtained from Winlab.
Aliquat-336 and sodium hydroxide were Merck products.
Kerosene (non aromatic) was supplied by Misr Petrol Ltd.,
Egypt, while 1-octanol was obtained from BDH.
Procedure. Aqueous solution was prepared by addition ofoxalic acid to the solution of uranyl nitrate in order to precipitate uranyl oxalate. The precipitate was separated, dried, andheated at 850˚C for eight hours to convert the uranyl oxalate
into uranium oxide (U3 O 8 ). An exact weight of U3 O 8 was
dissolved in alkaline carbonate solution composed of 12% (1M) Na2CO3, 15% (1 M) NaOH and 10% (30%) H2O2 to form
0.005 M uranium solution. This mixture was heated up to60˚C with stirring to complete the dissolution process. The
concentration of uranium in the aqueous alkaline solution was
spectrophotometrically determined by measuring its maximum
absorbance at 360 nm using a Shimadzu double beamrecording spectrophotometer model 160 A.
Organic solution was composed of Aliquat-336 (5.0 vol%)dissolved in kerosene containing 5.0 vol% of 1-octanol as
modifier in order to prevent the formation of a third phase. 2
This produced 0.1 M solution with respect to Aliquat-336.
In the kinetic experiments, the concentrations of Aliquat336, U(VI), sodium carbonate, and sodium hydroxide were
fixed at 0.1 M, 0.005 M, 0.12 M, and 0.15 M, respectively
unless their concentrations were otherwise stated. The temperature was fixed at 25 ± 0.1˚C by using a Julabo (Germany)
circulator connected to a stirred Lewis cell. The cell description and general procedures were previously described.10
A stirring rate of 250 rpm was chosen for the kinetic investigations performed in this work. The plots of ln(Ceq-C) versus
time (t) always yielded straight lines indicating a first-order© 2003 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/06/2003

20El-Nadi

J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003

reaction, where Ceq and C represent the concentration of U(VI)in the organic phase at equilibrium and that at time t, respectively. The observed rate constants (kobs) were calculated fromthe respective slopes of these straight lines.
The surface tension at the Aliquat-336-kerosene/aqueous
alkaline carbonate interface was measured by Du Nouy ring
method using a CsC Du Nouy Tensiometer model 70545.
3. Results and Discussion
In stirred Lewis cell technique, the controlling process isdetermined from the effect of the stirring speed as well as theeffect of temperature. When the extraction is controlled bydiffusion, the rate of extraction increases with the increase in
the stirring rate, while there is no effect on the extraction rate
when it is governed by chemical reaction.11 The effect of stirring rate on the rate of extraction of U(VI) by Aliquat-336 inkerosene from alkaline medium was studied in the stirring
range 100 – 400 rpm and a plateau region is obtained in therange 200 – 300 rpm. As the stirring rate used in the present
work is fixed at 250 rpm which lies in this plateau region, theextraction process is assumed to be mainly controlled bychemical reactions.
The effect of temperature on the rate of extraction ofsystems governed by diffusion is less pronounced than thatcontrolled by chemical reactions since the activation energy inthe former case does not usually exceed 20.9 kJ mol−1.12 In thepresent work, the effect of temperature on the extraction rate ofU(VI) by Aliquat-336 was investigated in the range 10 – 45˚C.
The plot of the calculated values of ln kobs against the corresponding values of 1/T, in Figure 1, shows a linear decrease inln kobs reciprocally with the increase in temperature, which
obeys the Arrhenius equation kobs = Ae−E/RT (T is the absolutetemperature and R is the universal gas constant and A is aconstant). The activation energy (E) calculated from the slopeof this line was found to be 32.26 kJ mol−1, which verifies thatthe extraction rate of U(VI) in the investigated system iscontrolled by chemical reaction taking place either in the bulk
phase or at the interface.
The enthalpy of activation (∆H*) and the entropy of activation (∆S*) of the investigated system were calculated to be
29.78 kJ mol−1 and -57.2 kJ mol−1 K−1, respectively.
The effect of the interfacial area on the extraction rate wasstudied to identify whether the system is controlled by chemical reaction taking place in the bulk phase or at the interface.
The rate of extraction increases linearly with the increase in
the specific interfacial area ( a) (wherea = interfacial area (Q) /volume of the phase (V)) in systems when it is controlled bychemical reaction at the interface, while there is no effect ofawhen the controlling reaction takes place in the bulk phase.13
In this concern, the effect of varying the specific interfacial
area on the rate of extraction of U(VI) was studied in the range
6.6 – 15.9 cm2 by using different cells at V = 30 mL for eachphase, keeping all other experimental conditions constant. The
plot of kobs against the corresponding specific interfacial area
( a ) gave constant values of the reaction rate constant withincreasinga, indicating that the extraction rate of U(VI) is
independent of the surface area between the aqueous andorganic phases (Figure 2). This means that the rate controlling
reaction takes place in the bulk phase rather than at the interface.
The interfacial tension measurements are particularly important because they can provide useful information on the interfacial properties of the extractant. As octanol was always
added to Aliquat-336-kerosene solution, interfacial tension wasmeasured in the presence of this modifier. The interfacial
tension measurements were carried out at different Aliquat-336
concentrations in the range 0.02 – 0.14 M in kerosene
containing a certain amount of octanol (5.0 vol.%). In spite oflow distribution of Aliquat-336 in aqueous phase,2 the resultsof the interfacial measurements show that, under the experimental conditions in the present study, the organic/aqueousinterfacial tension remains constant independent of the extractant concentration (Figure 3). This indicates that Aliquat-336
is not adsorbed at the interface but lies mainly in the organic
phase and verifies that the rate of U(VI) extraction iscontrolled by chemical reaction in the bulk phase rather thanat the interface.
The effect of Aliquat-336 concentration on the extraction
rate of U(VI) in the system was studied in the range 0.06 –0.14 M. The metal extraction rate was found to increaselinearly with the increase in Aliquat-336 concentration (slope= 1). The increase in U(VI) concentration from 0.002 to 0.01
M had almost no effect on the extraction rate. The effects ofcarbonate and hydroxide concentrations in the respective
0.006

-4.5

0.005

-5.0

kobs /s-1

0.004

ln kobs /s-1

-5.5

0.003

-6.0

0.002

-6.5

-7.0
3.0

0.001
0.2

3.1

3.2

3.3

3.4

3.5

3.6

3.7

1/Tx103/K-1

Figure 1. Effect of temperature on the rate of extraction of U(VI) byAliquat-336 in kerosene from alkaline carbonate medium.
[U(VI)] = 0.005 M, [Aliquat-336] = 0.1 M,
Stirring rate = 250 rpm.

0.3

0.4

0.5

0.6

0.7

Specific Interfacial Area ( a )/cm-1− on the rate of extracFigure 2. Effect of specific interfacial area ( a)tion of U(VI) by Aliquat-336 in kerosene from alkaline carbonate
medium.
[U(VI)] = 0.005 M, [Aliquat-336] = 0.1 M,
Stirring rate = 250 rpm, T = 25˚C, Vaq. = Vorg. = 30 mL.

Mechanism of Extraction of Hexavalent Uranium

J. Nucl. Radiochem. Sci., Vol. 4, No. 1, 2003

ranges of 0.02 – 0.15 M and 0.025 – 0.20 M were examined
and are almost nothing until their concentrations of 0.1 M and0.12 M, respectively. However, afterwards, the extraction rate
constant drastically decreased with increasing the carbonate
and hydroxide concentrations giving straight lines with slopes= -1 for each case. The drastic decreasing effect of increasing
carbonate in the medium may be explained by the formation ofhigher uranium carbonate complexes, such as [UO2(CO3)3]4−,
which decrease the extraction of the main extracted species,[UO2(CO3)2]2−; 14 the decreasing effect caused by the increasein hydroxide concentration may be related to the increase in
pH which in turn decreases the extraction process.14
All data above-mentioned, the slopes of the log – log relations between kobs and [Aliquat-336], [U(VI)], [CO 32−], and[OH−], are given together in Figure 4. This figure indicatesthat the reaction is first order dependent on Aliquat-336
concentration, nearly zero order on U(VI) concentration, andinverse first order on carbonate and hydroxide concentrations.

21

As the predominant uranium species in alkaline carbonate
medium is considered to be UO 2 (CO 3 ) 3 4− , 15, 16 the main
extracted uranium species by Aliquat-336 is supposed to be
UO 2 (CO 3 ) 2 .2R (R: Aliquat-336 molecule). 14 Based on thekinetic results mentioned above, obtained under the employedexperimental conditions, the mechanism of U(VI) distribution
between alkaline medium and Aliquat-336 in kerosene solution
is expected to proceed through the following steps:
1. Distribution of Aliquat-336 (R +OH−) between the organicand aqueous phases.
R+OH−KD

R+OH−

KD =

[R+OH−][R+OH−](1)

Where, bars refer to the organic phase species, KD is thedistribution constant of R+OH−.
2. Slow reaction between the predominant uranium tricarbonate complex and one R+OH− in the aqueous phase withthe release of one hydroxide and one carbonate.
slow
UO2(CO3)34− + R+OH− →UO2(CO3)2·R− + OH− + CO32− (2)5k1Interfacial tension/dynecm-1

3. Fast addition of another R +OH− molecule to give neutral
uranium species with the release of one hydroxide ion.
fast UO (CO ) ·2R + OH−
UO2(CO3)2·R− + R+OH− →
23 24(3)

4. Fast transfer of the formed neutral uranium species to theorganic phase.
3

fastUO2(CO3)2·2R →
UO2(CO3)2·2R(4)Since step (2) is assumed to be the rate-determining step,
therefore, the rate of extraction is given by the following
equation:

2Rate =1
0.01

(5)

= k1[UO2(CO3)34−] [R+OH−],

0.5

0.1

–d[UO2(CO3)34−]= kobs[UO2(CO3)34−]
dt(6)[Aliquat-336]/M

Figure 3. Effect of interfacial tension of R + OH − on the alkaline
carbonate / kerosene interface.

where k1 is the rate constant of the slow reaction given in eq
2, andkobs =0.01[Aliquat-336][U(VI)][CO32-]

k1 + −[R OH ].
KD(7)

0.006

[OH - ]

0.005

kobs /s-1

kobs /s-1

0.004

0.003

0.002

0.001

0.001
0.001

0.01

0.1
2-1
-

[Aliquat-336], [U(VI)], [CO3 ] or [OH ]/M+−

2−−

Figure 4. Effect of R OH , U(VI), CO3 or OH concentration on theextraction rate of U(VI) by Aliquat-336 in kerosene from alkaline
carbonate medium.
Stirring Rate = 250 rpm, T = 25˚C.

0.000
0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

[Aliquat-336]/M

Figure 5. Relation between the reaction rate constant (kobs) and [R+OH−]
for the extraction of uranium from alkaline carbonate medium.

22J. Nucl. Radiochem. Sci., Vol. 4, No. 2, 2003
Equation 7 is an equation of a straight line passing throughthe origin. The plot of the experimental values of k obs
obtained at different initial extractant concentrations[R+OH−], shown in Figure 3, yields a straight line passing
through the origin with slope = k1/KD, which verifies eq 7
and supports the proposed mechanism.

4. Conclusions
The extraction rate constant was found to decrease withincreasing temperature. The activation energy obtained wasfound to be 32.6 kJ mol−1 , which indicates that the rate ofuranium extraction by Aliquat-336 in kerosene is controlled bychemical reaction.
The extraction rate is independent of interfacial area.
Furthermore, the interfacial tension at the alkaline/keroseneinterface is constant at different Aliquat-336 concentrations.
These verify that the rate of extraction of U(VI) in the system
is governed by a chemical reaction taking place in the bulk
phase rather than at the interface.
The obtained kinetic results show that the extraction rate ofU(VI) by Aliquat-336 from alkaline carbonate medium isaccelerated by the increase in the extractant concentration, but
in contrast, slowed down by the increase in the concentrationsof carbonate and hydroxide in their high concentration ranges.
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