Dispersion-free solvent extraction of U(VI) in macro amount from nitric acid solutions using hollow fiber contactor

Journal of Membrane Science 300 (2007) 131–136

Dispersion-free solvent extraction of U(VI) in macro amount from nitric acid solutions using hollow fiber contactor S.K. Gupta, N.S. Rathore, J.V. Sonawane, A.K. Pabby ∗ , P. Janardan, R.D. Changrani, P.K. Dey Nuclear Recycle Group, Bhabha Atomic Research Centre, Tarapur, P.O.-Ghivali, Thane 401502, M.S., India Received 21 September 2006; received in revised form 10 May 2007; accepted 15 May 2007 Available online 18 May 2007

Abstract The applicability of dispersion-free solvent extraction (DFSX), through microporous hydrophobic membrane has been studied. The hollow fiber membrane contactor, with surface area of ∼381 cm2 was employed to extract U(VI) in macro concentration (35 g dm−3 ) from aqueous acidic solutions. Prior to deployment of this technique for recovery of U(VI) from oxalate supernatant waste, chemical parameters such as extractant concentration, feed acidity, concentration of U(VI) in feed were studied. The study revealed that 30% tri-n-butyl phosphate (TBP) in n-dodecane as an extractant and feed in 3 M HNO3 gave an optimum extraction of U(VI) and it was possible to strip back utilizing 0.05 M HNO3 . It was established to recover more than 90% of U(VI) from oxalate supernatant waste, which was often generated from nuclear chemical facilities. © 2007 Elsevier B.V. All rights reserved. Keywords: TBP; Dispersion-free solvent extraction; Uranium; Hollow fiber contactor

1. Introduction Hollow fiber supported liquid membrane (HFSLM) technique have been investigated for the separation/removal/ extraction of radionuclides from various aqueous streams [1]. Also the sufficient attention has been given for the actinide separation in different configuration of liquid membrane [2–4]. Ansari et al. [5] have carried out the permeation study for Am(III) using N,N,N ,N -tetraoctyl-3-oxapentane diamide (TODGA) in n-dodecane as carrier. The use of HFSLM suffers mainly with the problems like limited lifetime and consistent performance, which diminishes the potentiality of this technique. As a solution to this, Dispersion-free solvent extraction (DFSX) technique has been reported in literature [6], which overcomes all the problems encountered in HFSLM. In this configuration of the hollow fiber membrane based solvent extraction, diversified applications were reported dealing with extraction of different metal ions [7,8] and extraction of various pollutants from wastewaters [9,10], etc.

The use of hollow fiber (HF) modules presents several advantages when compared to conventional contactor [11]. Recently Sastre and Pabby [12] published a comprehensive review on DFSX highlighting different applications in various fields. In the DFSX technique, simultaneous extraction and back extraction (BEX) of metal ion can be achieved easily through hollow fiber contactor (HFC). In view of this a comprehensive program for actinides removal from the waste solution, with laboratory fabricated hollow fiber contactor was initiated. Experiments were conducted for U(VI) extraction from aqueous acidic solutions using TBP in n-dodecane as an extractant and the study includes the effect of extractant concentration, aqueous feed acidity on the extraction of U(VI) in macro concentration. Also, the developed technique was applied for the recovery of U(VI) from oxalate supernatant waste under optimum conditions. 2. Experimental 2.1. Material

∗

Corresponding author. E-mail address: [email protected] (A.K. Pabby).

0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.05.018

The tri-n-butyl phosphate (TBP) and diluent n-dodecane used were of A.R. grade. Uranyl nitrate stock solution was prepared

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Table 1 Details of the hollow fiber contactor Number of lumens Effective length i.d. of lumen i.d. of glass tube Inner surface area of lumens Volume occupied by lumens

20 27 cm 1800 ␮m 1.6 cm 305.2 cm2 13.7 cm2

Length of module Wall thickness Pore size Outer surface area of lumens Total volume of glass tube Surface area to volume ratio

30 cm 450 ␮m 0.64 ␮m 381.5 cm2 54.3 cm3 22.2 cm−1

Hollow fiber microporous polypropylene membrane equivalent to Accurel-PPS6/2 .

by dissolving U3 O8 powder in nitric acid and standardized by Davies and Gray method [13]. The experimental conditions for the various studies are given with the corresponding data. Based on five observations, the value of percentage extraction of U(VI) at about 80% extraction exhibits a coefficient of variation of ±1%. 2.2. The dispersion-free membrane set-up A schematic view of the dispersion-free membrane-based extraction process of U(VI) using a HFC in recirculation mode (with peristaltic pump) is similar as described elsewhere [14] and characteristics of hollow fiber contactor are given in Table 1. Both aqueous and organic phases were contacted in a hollow fiber module in counter-current flow for extraction or stripping run under recirculating mode. In the extraction module, the feed aqueous phase flowed through the lumen of the fibers at the flow rate of 5.83 cm3 s−1 , whereas the organic phase flowed through the shell side at the flow rate of 1.53 cm3 s−1 as shown in Fig. 1a, wetting the wall of the hydrophobic fibers. These flow rates were optimised similar to the work described in reference [14]. The aqueous phase pressure was maintained higher than the pressure of organic phase by keeping high flow rate of aqueous phase. While these conditions were maintained, the appearance of the organic phase on the other side of membrane can be prevented if as immobilized phase is maintained on this side at a pressure equal to or greater than that of the organic phase. In the stripping run, as shown in Fig. 1b, loaded organic extractor {TBP-U(VI) solvated complex} flowed through the shell-side at the flow rate of 1.66 cm3 s−1 whereas 0.05 M nitric acid solution flowed through the tube side with the flow rate of 6.11 cm3 s−1 in counter-current recirculation mode. Laboratory made modules are very simple to fabricate and are used to check the viability of the technique before introducing commercial modules. Extraction can be enhanced certainly by placing baffle and making cross-flow. The organic phase was ∼200 cm3 , diluted with n-dodecane solution with varying concentration of TBP. Also, 200 cm3 of aqueous feed solution of the desired concentration of U(VI) was used, which was prepared by taking a suitable aliquot from uranyl nitrate stock solution with varying acidity of nitric acid. Nitric acid of 0.05 M solution was used as a stripping phase for the back extraction of U(VI) in HFC. The feed and organic solutions were recirculated by means of calibrated peristaltic pumps. The measurement of U(VI) was carried out using Davies and Gray method [13] by periodically sampling the feed/strip solution. The sample of other radionu-

Fig. 1. (a) Extraction mechanism of U(VI) through hollow fiber contactor. (b) Stripping mechanism of U(VI) through hollow fiber contactor.

clides were analyzed for ␣ using PLA make ZnS silver activated counter and for ␥ using NaI(Tl) scintillation counter. 3. Theoretical background 3.1. Extraction equilibrium The extraction of U(VI) in TBP is well established [15]. The solvated type of complexes predominated in the nitric acid media ranging from 1 to 5 M HNO3 . The UO2 2+ ion in nitrate medium (HNO3 ) form UO2 (NO3 )2 ·2TBP complex with the extractant,

S.K. Gupta et al. / Journal of Membrane Science 300 (2007) 131–136

expressed as: UO2 aq 2+ + 2NO3 aq − + 2TBP ⇔ UO2 (NO3 )2 ·2TBPorg

(1)

and stripping of U(VI) from loaded TBP is as below: UO2 (NO3 )2 ·2TBPorg ⇔ UO2 aq 2+ + 2NO3 aq − + 2TBPorg (2) the extraction equilibrium can be described by the following equation and extraction constants (Kex ) for uranium. From Eq. (1) Kex =

DU =

[UO2 (NO3 )2 · 2TBP]org [UO2 2+ ]aq [NO3 − ]aq [TBP]2org 2

(3)

[UO2 (NO3 )2 · 2TBP]org [UO2 2+ ]aq

or Kex =

DU

In DFSX systems, overall mass transfer coefficients are a weighted average of the individual mass transfer coefficients in the aqueous feed phase, across the membrane, and in the organic phase. The reciprocal of the overall mass transfer coefficient (the total resistance to mass transfer) can be described as the sum of mass transfer resistances inside the fiber (feed phase), across the fiber wall (membrane resistance) and outside the fiber (organic phase). Recent work assumes that the main resistance to the solute transport lies in the membrane [7,11]. Among those who deal with ionic species in the extraction step, it is also widely assumed that chemical reactions are fast enough to be considered to be instantaneous; then the reacting species are present in equilibrium concentration at the interface everywhere. The overall mass transfer coefficient of extraction and stripping for the module, which was used throughout the present study was calculated by slope analysis technique using Eq. (5). 4. Results and discussion

[NO3 − ]aq [TBP]2org 2

The values of Kex for U(VI) with TBP in dodecane were calculated from the DU values. The partition coefficient could be presented as log DU = log Kex + 2 log[NO3 − ] + 2 log [TBP]org .

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(4)

3.2. Mass transfer coefficient The mechanism of mass transfer in HFC can be explained as the diffusion of metal ions through the pores of the fibers from one phase to another without dispersion of the two contacting phases in one another. More detailed technical steps involved in DFSX are described elsewhere [16]. The rate at which a component is transferred between two different phases depends on the mass transfer coefficient, the interfacial area and the degree of departure of the component from its partition equilibrium. Evaluation of mass transfer coefficients is of importance because these determine the rate at which equilibrium is approached and control the time required for a given separation, and therefore the size and the cost of equipment to be used as derived by D’Elia E or K S et al. [16–18], the key equation for the calculation of KU U for counter-current flow is,   0 /H − C 0 ) (Ce/s f ln 0 /H − C 0 ) + (V /HV )(C 0 − C ) (Ce/s e/s f f f f   E 1− exp(−4KU /d(1/Qf −1/Qe/s H))][1/Vf +1/Ve/s H =t E V /d)(1/Q − 1/Q H)] 1/Qf − 1/Qe/s exp[(−4KU m e/s f (5) where Qf and Qe/s are the feed and extract/strip flows; Vf and 0 are the Ve/s are the feed and extract/strip volumes; Cf0 and Ce/s concentrations of the solute in the feed and in the extract/strip solutions at time zero; Cf is the concentration of the solute in the feed at time t; Vm is the volume of all the hollow fibers; H is the distribution ratio of the metal; and d is the diameter of one fiber.

4.1. Effect of extractant concentration To evaluate the effect of extractant concentration on extraction of U(VI), different concentration of TBP in n-dodecane (v/v) were employed. Fig. 2 shows that % extraction of U(VI) was also increased when the TBP concentration is increased from 5 to 50%. For economical plant scale operation 30% TBP was selected as optimum concentration to evaluate furE values were calculated using Eq. (5) as ther parameters. The KU a representative case for the effect of the concentration of TBP on the mass-transfer and depicted in Fig. 3. The mass transfer E ) of uranium was increased from 1.5 × 10−5 to coefficient (KU 1.9 × 10−5 cm s−1 for TBP (5%) to TBP (50%, v/v). At lower concentration of TBP, mass transfer control is in the membrane. Further, it remained constant for higher concentration of TBP. This behavior could be due to the following reasons: (1) At higher concentrations of TBP, mass transfer control is shifted to the aqueous phase, so increase in TBP concentration does not influence the mass transfer significantly. (2) The higher concentration of TBP is resulting in higher viscosity of orgnaic solution leads to lower diffiusion coefficient of the TBP-uranium complex [13]. D=

kT 6πrη

(6)

where k is the Boltzmann constant, T the absolute temperature, r the molecular radius of the uranium complex and η is the viscosity of the organic phase equilibrated with the aqueous phase. This will finally result in lower uranium percentage extraction as indicated in Fig. 2. (3) Since, uranium concentration is low (∼0.1 M), higher concentration of TBP will allow only a small fraction of TBP to be complexed with uranium (maximum capacity ∼120 g U/0.5 M TBP), hence more TBP is available for making complex with HNO3 (nitric acid forms 1:1 complex with TBP at 8 M HNO3 ). Therefore acidity of feed phase comes

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Fig. 2. Influence of organic concentration on U(VI) extraction: feed acidity: 3 M HNO3 , conc. of U(VI): ∼35 g dm−3 , conc. of extractant (TBP in n-dodecane): variable, feed flow rate: 5.83 cm3 s−1 , organic (pseudo-emulsion) flow rate: 1.53 cm3 s−1 .

down. TBP-uranium system has lower D value at low acidity. This will maintain the mass transfer coefficient either steady or lower. 4.2. Influence of the feed acidity From Fig. 4, it is seen that the % extraction increased up to 4 M and there after remained constant (maximum 80% extraction was obtained in 105 min) and decreases on further increase E values were calculated from Eq. (5). The in feed acidity. The KU E KU value was increased from 1.4 × 10−5 to 1.9 × 10−5 cm s−1 when feed HNO3 concentration increased upto 3 M HNO3 , further it remained constant for higher concentration of HNO3 . Eq. (4) clearly indicates that DU value depends on square of E value NO3 − ion concentration. Therefore, initial increase in KU is due to the increase in NO3 − concentration. This effect is more prominent when concentration of HNO3 is upto 3 M HNO3 . On further increase in concentration of HNO3 acid, it starts competing with uranium and TBP could be loaded with nitric acid as TBP forms TBP·HNO3 type of complex. 4.3. Effect of U(VI) concentration on extraction In order to recover maximum uranium from the feed solution, experiments were conducted in three consecutive run to

Fig. 3. Mass transfer coefficients for extraction: feed acidity: 3 M HNO3 , conc. of U(VI): ∼35 g dm−3 , conc. of extractant: 30% TBP in n-dodecane, feed and organic flow rates same as in Fig. 2.

Fig. 4. Effect of nitric acid molarity on U(VI) extraction: conc. of U(VI): ∼35 g dm−3 , conc. of extractant: 30% TBP in n-dodecane, feed acidity (HNO3 ): variable, feed and organic flow rates same as in Fig. 2.

remove complete U(VI) from the effluent. Fig. 5 clearly indicated that % recovery of U(VI) in three successive steps were 60%, 27%, 11%, respectively. Thus, total recovery more than 98% was achieved in three consecutive run with fresh extractant without adjusting the feed acidity. To study the effect of macro concentration of uranium in DFSX, the concentration of uranium was varied from 16 to 116 g dm−3 and the results from Fig. 6 revealed that 81% extraction within 135 min was achieved in case of U = 16 g dm−3 which reduced to 51% when the concentration of U(VI) was increased to 116 g dm−3 . The decrease in the extraction percentage could be further enhanced either by increasing the surface area of the module or running the experiment for longer time. 4.4. Stripping of U(VI) from loaded organic For the quantitative recovery of metal ion, it is essential to get back extracted uranium from the loaded organic. With this aim, experiments were conducted with 0.05 M HNO3 . From Fig. 7 it is clearly indicated that in the two consecutive runs of 375 min each with fresh strippant, more than 90% back extraction was achieved. This suggests that two HF contactor in series or hydrophilic membrane could be better option for efficient E value for stripping was also stripping in single batch. The KU

Fig. 5. Extraction of U(VI) in three consecutive runs: feed acidity: 3 M HNO3 , conc. of U(VI): ∼80 g dm−3 , conc. of extractant: 30% TBP in n-dodecane (fresh extractant used in each run), feed and organic flow rates same as in Fig. 2.

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135

Table 2 Composition of the oxalate supernatant waste Uranium (U) Ruthenium (106 Ru) Nitric acid

5 g dm−3 0.0032 mCi dm−3

Plutonium (Pu) Ceasium (137 Cs)

3 mol dm−3

H2 C2 O4

25 mg dm−3 0.003 mCi dm−3 0.1 mol dm−3

Fig. 6. Effect of U(VI) concentration on extraction: feed acidity: 3 M HNO3 , conc. of extractant: 30% TBP in n-dodecane, conc. of U(VI): variable. Feed and organic flow rates same as in Fig. 2.

Fig. 9. Percentage recovery of U(VI) from oxalate supernatant waste: conc. of organic 30% TBP in n-dodecane, strippant: 0.05 M nitric acid (composition of the oxalate supernatant waste is given in Table 2), feed and organic flow rates same as in Fig. 2.

Fig. 7. Stripping of U(VI) from loaded organic: conc. of organic 30%TBP in n-dodecane, strippant: 0.05 M nitric acid (fresh strippant used in each run), feed and organic flow rates same as in Fig. 2.

calculated from Eq. (5). The representative plot of Y versus time (t) is shown in Fig. 8. 4.5. Application of DFSX technique for U(VI) recovery from oxalate supernatant waste After optimizing all the chemical parameters, this technique was applied for the recovery of U(VI) from oxalate supernatant

waste, which is normally generated from nuclear facilities. The composition of the waste is given in Table 2. Experiments were performed with treated (oxalate ion destroyed by KMnO4 and H2 O2 ) and untreated (without oxalate decomposition) waste separately. As obvious from Fig. 9, the recovery of uranium was well above 90% in case of untreated waste while in case of treated waste it was more than 99%. Both the experiments were run in counter-current recirculation mode with A/O ratio at 1. The decrease in uranium recovery in case of untreated waste was due to the neutral soluble oxalate complex of uranium having lower DU value. Also the presence of plutonium in feed affected the overall recovery of uranium due to competitive extraction. The presence of important fission products such as 137 Cs and 106 Ru did not affect the recovery of uranium. This clearly indicate that present method could be successfully applied for the recovery of U(VI) from oxalate supernatant waste. It has been observed that physical integrity of fibers at radiation dose of 1 M Rad was not affected as no topographical and physical changes occurs at this radiation level [4]. Radiation stability of polypropylene flat sheets are also examined and discussed by Kumar et al. [1]. In present work, we estimated that total dose absorbed by fibers would be far lesser than level of doses required injuring the propylene fibers while examining the viability of this technique. 5. Conclusions

Fig. 8. Mass transfer coefficients for stripping: conc. of organic 30%TBP in n-dodecane, strippant: 0.05 M nitric acid, feed and organic flow rates same as in Fig. 2.

From the extraction data of U(VI) by TBP, it is concluded that the 30% TBP and aqueous feed in 3 M nitric acid is quite suitable for DFSX by HFC. This system can be successfully utilized for separation of both micro and macro concentration of U(VI) from aqueous acidic solutions. Due to high surface area

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to volume ratio of this technique, the feasibility of separation of uranium from nuclear waste in presence of oxalate ions using 30% TBP/n-dodecane as an extractant and 0.05 M HNO3 solution as strippant was clearly demonstrated. The extraction and stripping could be achieved simultaneously using two HFC and mass-transfer for stripping in this system could be enhanced by using the hydrophilic hollow fiber membrane. Presently, in conventional methods, uranium is recovered from treated oxalate supernatant by solvent extraction using Purex process. Hence, this will be an additional burden on recovery cycle. This step can be avoided if uranium is recovered on the spot (at the place of generation) by DFSX technique. Further, radiation stability of such polymer (polypropylene fiber) is well proven for 1 M Rad which is far lesser than the limit required to damage the fibers.

[2]

[3]

[4]

[5]

[6]

Nomenclature A/O D or H Eq. Kex E KU S KU

t

aqueous/organic ratio distribution ratio equation extraction constant overall membrane mass transfer coefficient for extraction of uranium (cm/s) overall membrane mass transfer coefficient for stripping of uranium (cm/s) time

[7]

[8]

[9]

Subscripts aq aqueous e extract f and s refer to feed and stripping solution, respectively org organic s strip U uranium

[10]

Superscript 0 refer to at time zero

[13]

[11] [12]

[14]

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