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Studies on theExtraction—SimultaneousBack-Extraction of Alkali MetalPicrates Using 1,3 Bis-Benzo-Crown-6-Calix[4] Arene inHollow-Fiber ContactorsZ. Albaraka a , Suman Kumar Singh b , S. Ghnimi c ,Z. Asfari c & D. Trébouet ca Atomic Energy Commission of Syria, Damascus,Syriab Fuel Reprocessing Division, Bhabha AtomicResearch Centre, Mumbai, Indiac Université de Strasbourg, IPHC, CNRS-UMR 7178,Strasbourg, France

Available online: 18 Jul 2011

To cite this article: Z. Albaraka, Suman Kumar Singh, S. Ghnimi, Z. Asfari & D.Trébouet (2011): Studies on the Extraction—Simultaneous Back-Extraction of AlkaliMetal Picrates Using 1,3 Bis-Benzo-Crown-6-Calix[4] Arene in Hollow-Fiber Contactors,Solvent Extraction and Ion Exchange, 29:4, 534-550

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Solvent Extraction and Ion Exchange, 29: 534–550, 2011Copyright © Taylor & Francis Group, LLCISSN 0736-6299 print / 1532-2262 onlineDOI: 10.1080/07366299.2011.556930

Studies on the Extraction—SimultaneousBack-Extraction of Alkali Metal Picrates Using 1,3

Bis-Benzo-Crown-6-Calix[4] Arenein Hollow-Fiber Contactors

Z. Albaraka1, Suman Kumar Singh2, S. Ghnimi3, Z. Asfari3,and D. Trébouet3

1Atomic Energy Commission of Syria, Damascus, Syria2Fuel Reprocessing Division, Bhabha Atomic Research Centre, Mumbai, India

3Université de Strasbourg, IPHC, CNRS-UMR 7178, Strasbourg, France

Abstract: An extraction and simultaneous back-extraction process with two hol-low-fiber membrane contactors is evaluated for the continuous recovery of alkalimetals and 1,3 bis-benzo-crown-6-calix[4]arene. The results show that this con-figuration operates as a “liquid membrane” (permeable only with alkali picrate)and order of the recovery yields is in agreement with the Donnan equilibrium.It is higher for the rubidium (75.8%) than for the cesium (68.1%) while it is only11% for the potassium. This is a case where the high affinity of cesium picratefor the 1,3 bis-benzo-crown-6-calix[4]arene cannot be neglected and, as a conse-quence, its release in the back-extraction aqueous phase is more difficult. The effectof the alkali metal and the initial concentration on the system behavior showsthat the concentration gradient controls the mass transfer. The kinetics are simi-lar for the cesium and the rubidium whereas the mass-transfer resistance increasesfor the potassium. A high mass transfer in the first module can slow down theglobal-transport kinetics.

Keywords: Extraction, back-extraction, hollow-fiber contactor, Donnan equilib-rium, macrocyclic carrier

INTRODUCTION

The growing demand for the separation and recovery of nuclear, valu-able, and toxic materials from various types of lean sources poses new

Address correspondence to D. Trébouet, IPHC - DSA, Laboratoire des Procedesde Separation, ECPM, 25 rue Becquerel 67087 Strasbourg cedex 2, France. E-mail:Dominique.Trebouet@unistra.fr

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Extraction and Simultaneous Back-Extraction Process 535

challenges for conventional separation technologies. The high-partitioncoefficient for liquid-liquid extraction and a large required volume oforganic solvents are the major problems for the treatment of industrialeffluents. More significantly, at the back end of the nuclear-fuel cycle, avery selective and efficient method is required for the separation of long-lived radionuclides.[1] Macrocyclic host compounds, for example, crownether and calixarene, have been claimed as efficient extractants of metalions.[2–4] These molecules that can recognize a target metal ion by theircavity size are promising extractants for innovative solvent-extraction pro-cesses and received growing attention for processing radioactive wastes.Calix-crown molecules are good complexing agents of cesium ions showinga high cesium recovery from the acidic high-level radioactive waste[5–8] andvery high selectivity over sodium in liquid-liquid extraction.[9]

Liquid membranes-based separation techniques are promising for theseparation and enrichment of valuable metal ions.[10] Liquid membrane,as supported liquid membrane (SLM) containing a mobile calixareneextractant, is known to be one of the most promising of separationtechniques.[1,10–14] However, the main drawback to the application of SLMfor industrial use lies in the relative instability of these systems due to theprogressive losses of the extractant into the aqueous phase during suc-cessive runs.[15,16] Non-dispersive solvent extraction using a hollow-fiberextractor may be an interesting alternative for the industrial use of metalas well as for macrocyclic-carrier recovery.

The non-dispersive solvent extraction is a configuration of the conven-tional solvent-extraction process where a microporous membrane separatesboth the immiscible phases, one of which impregnates the membrane, thusbringing the liquid-liquid interface to one side of the membrane. A reviewof membrane-based non-dispersive solvent extraction has been publishedby Pabby and Sastre.[17] The main benefit of the hollow-fiber contactors isthe value of the interfacial area per unit volume that can be much higherthan in conventional liquid-liquid extraction processes. The interfacial areais easily calculated, and the scale up of the process is straightforward.[18]

No density difference between both fluids is required and emulsion forma-tion is avoided because of the absence of fluid/fluid dispersion. Kathios etal.[19] and Geist et al.[20] investigated the feasibility of hollow-fiber mem-brane contactors for processing radioactive wastes. Moreover, membraneextractors may be connected in series reducing the used organic-phase vol-ume and recovering the macrocyclic compound in the organic side forpossible further processing. Hence, very selective and expensive extractantscan be used such as calixarene. The detailed literature survey reveals thatthough various studies are being carried out using hollow-fiber contac-tors for the separation and enrichment of nuclear metals from their leansources, on the other hand there are very few works which have reported onthe extraction and simultaneous back-extraction kinetics of alkaline ionsusing macrocyclic extractants in a hollow-fiber contactor.

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536 Z. Albaraka et al.

The proposed study concerns the extraction and the simultaneousback-extraction of alkali metal picrates with the 1,3 bis-benzo-crown-6-calix [4] arene in toluene (95%)-decanol (5%) in closed system. Alkalipicrates (MPic) have frequently been employed to estimate the efficienciesand selectivities of new macrocycle ionophores in metals separations. Asoft, polarizable, weakly-hydrated picrate is a convenient counter-ion thatprovides greater distribution ratio/membrane-transport rates for metalions than do inorganic anions such as nitrates.[21] A high concentration ofalkali chlorides (0.1 M) is added to the feed solution in order to increase thedistribution coefficient.[22,23] The performance of this process can only beestimated with an efficient extraction system, the high ionic strength, andthe hydrophilic character of chlorides in which the aqueous phase led to anincrease of the alkali picrates transfer (lipophilic character). It is importantto note that no alkali chloride is extracted and the chloride concentrationin the feed phase is constant as analyzed by ion chromatography. Lastly,the back-extraction with water enables the regeneration of the extractantand avoids the formation of unwanted compounds.

Calixarene was mainly studied with bulk-liquid membrane (BLM) andsupported liquid membrane (flat sheet and hollow fiber), but there are fewerapplications. Their use with non-dispersive solvent extraction could be apromising alternative. The objective of this study is a preliminary evalua-tion of a non-dispersive solvent-extraction process with two microporoushollow-fiber membrane modules in order to check that this configura-tion could be worked as a “pseudo-liquid membrane” with simultaneousrecovery of the alkali metals and the calixarene carrier. The effects of thealkali metal picrates and the initial alkali metal-picrate concentration areassessed in the system behavior.

MATERIALS AND METHODS

Materials

The 1,3 bis-benzo-crown-6-calix[4]arene and alkali picrates were synthe-sized at the Laboratoire de Chimie Analytique et Minérale from Universityof Strasbourg.[24] The toluene and the decanol are Fluka products with apurity of 99%, and are mixed in a ratio of 95%–5% (v/v) before use. Thecesium chloride was a Sigma-Aldrich product with a purity of 95%. Allother reagents were of AR grade.

Apparatus, Module, and Hollow Fiber

The membrane-contactors modules were assembled in our laboratoryusing Accurel PP50/280 hollow fibers (MEMBRANA society) glass tube

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Extraction and Simultaneous Back-Extraction Process 537

Table 1. Characteristics of the hollow-fibers module

Hollow fiber

Name Accurel PP 50/280Material polypropyleneInternal Diameter (Dint) 280 µmThickness 50 µmPore size 0.05 µmPorosity 50–55%ModuleMaterial glassInternal Diameter (Dint) 4 mmLength 22 cmNumber of hollow fibers 60Specific exchange area (exchange area/module volume) 0.154 cm2.cm−3

Vm (total internal volume of the fibers) 0.81 cm3

VC (volume of the organic phase inside the module) 1.27 cm3

and epoxy resin. The module contains 60 polypropylene hollow fibers of anaverage length of 22 cm. A detailed description of the membrane moduleis given in Table 1.

Analytical Methods

Two analytical approaches are used according to the presence or theabsence of alkali chlorides in the aqueous phase. In alkali picrate systemswithout chloride (back-extraction aqueous phase), the concentration ofalkali cation in the aqueous phase is directly measured by atomic absorp-tion (2380 PERKIN ELMER). For the picrate-chloride systems (in thefeed aqueous phase), the extraction kinetics are followed by the analysisof alkali picrates, which can be analyzed by UV-Vis spectrophotometry[Spectronic 501 apparatus (MILTON ROY) at 355 nm].

The alkali-cation analysis in the organic phase by atomic absorptionrequires a stripping step: 1 mL of organic phase is shaken with 5 mL ofan aqueous solution of hydrochloric acid (pH = 1) for one hour. The massbalance is checked within less than 5% in all cases.

Membrane-Extraction Procedure

In all the experiments, a closed system is used (as presented in Figs. 1 and 2)for studying the extraction and back-extraction behavior of alkali picrates

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Extraction

module

Back-extraction

module

Organic

phase

Feed and back-extraction

aqueous phase

Figure 1. Photograph of the experimental set up for simultaneous extraction andback-extraction of metal picrates (color figure available online).

Back–extraction

Toluene

1–decanol

Water

M+Pic–Cl–

Extraction

M+Cl–

M+Pic–

M+

Aqueous

phase 1

Aqueous

phase 2

Pic–

Figure 2. Mechanism of simultaneous extraction and stripping of metals usinghollow fiber contactor (color figure available online).

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Extraction and Simultaneous Back-Extraction Process 539

using hollow-fiber contactors. The aqueous phases (feed/back-extraction)are pumped from reservoirs, continuously circulating through the lumenside of the module [inside the fibers] to the reservoir. The organic phaseflows counter-currently through the shell side of the modules. Owing tothe hydrophobic nature of the fibers, a slight overpressure (0.4 bar) isapplied to the aqueous phases in order to stabilize the interface withinthe membrane pores which simultaneously avoids the displacement of theorganic phase. All the experiments are carried out at constant temperature(25◦C) using jacketed glass beakers for the aqueous- and organic-phasereservoirs. The aqueous- and organic phases-flow rates are fixed at 12mL.min−1 (Re = 20, laminar flow). In these flow conditions we con-sider that the boundary layers on both sides have reached a minimumthickness as described in a previous work.[25] For the extraction exper-iment, the feed phase contains cesium picrates and cesium chlorides(0.1 mol.L−1). The concentration (L0) of calix[4]arene is 1.0 × 10−3 Min solution of toluene–decanol. For the back-extraction experiment, thefeed phase contains the alkali metals and calixarene complex and anultrapure water solution is selected as a back-extraction reagent. The pHof the back-extraction phase is fixed around 6.5 to 7 in order to avoidthe hydrolysis of picrates leading to the formation of picric acid. Thefeed phase- and the solvent phase-reservoir volumes are equal to 100 mLwhereas the back-extraction phase reservoir is equal to 200 mL. Suitablealiquots were taken at various intervals of time during the experiments tomeasure the mass transfer of metal ions. This volume variation is takeninto account in the results and in the mass balance. The experiments arecarried out in triplicate under similar conditions and the reproducibilityis found to be within 10%.

THEORY

Extraction Equilibrium

The understanding of the overall mass-transport phenomenon requires theknowledge of equilibrium conditions from the aqueous-feed phase to theorganic phase (extraction) and then from the loaded-organic phase to theaqueous back-extraction phase (back-extraction).

The biphasic-extraction equilibrium of alkali picrates with the 1,3 bis-benzo-crown-6-calix[4]arene is described by the simple mass-action law fora 1:1 stoichiometry:

Ls + Pic−w + M+

w � MLPics (1)

Kapp = (MLPic)s

(M+)w . (Pic−)w . (L)s, (2)

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where M+ refers to the alkaline metal ion, Pic− to the picrate ion, Lto the macrocycle extractant, and MLPic to the metal-picrate complex.It is assumed here that the activity coefficient of species in reaction (1)can be incorporated into an apparent extraction constant, Kapp, definedin the molarities scale (L2.mol−2).

The equilibrium organic-phase concentrations (C∗s) as a function of

the equilibrium aqueous-phase concentrations (C∗w) are accurately shown

in a previous publication.[23] The extraction order for alkali metals using1,3 bis-benzo-crown-6-calix[4]arene is Cs+ > Rb+ > K+ which is expectedas a per cavity size selectivity concept.[5]

Mass Transfer in Membrane Contactors

The simultaneous extraction and back-extraction processes have been for-merly described Fig. 2. The mass-transfer process of each solute from thefeed phase to the receiving one is considered to take place through thefollowing steps: diffusion through the aqueous feed layer, interfacial chem-ical reaction, diffusion through the hollow-fiber membrane, and diffusionthrough the organic-phase layer. The order of steps is reversed during theback-extraction process. Chemical reactions between the metallic solutesand the organic extractant in the extraction process, and between the com-plex organic species and the ultrapure water were assumed to take placeat the inside wall of the membrane where the aqueous-organic interface islocated.

Mathematical modeling and determination of the characteristicparameters has been widely studied in order to predict the performanceof the non-dispersive solvent-extraction processes with a single hollow-fiber contactor[26–28] and much less with an extraction and simultaneousback-extraction system.[29–31] The majority of the models used so farwere developed based on the concept of the resistance-in-series model.This resistance-in-series approach provides the overall mass-transfer coef-ficient from the three and/or four individual mass-transfer coefficients:the mass-transfer resistances inside the fibers (feed phase), across thefiber wall (membrane resistance), and outside the fibers (organic phase).Some authors consider a resistance for the rate of the reaction but fur-ther experiments must be carried out to check this hypothesis in thecase of the non-dispersive solvent extraction of the alkali picrates withthe 1,3 bis-benzo-crown-6-calix[4]arene. Indeed, the selective extractionof alkali metals and alkaline earths by macrocycles as calix[4]arene hasbeen investigated by our group with, among others, the use of liquidmembrane.[32] These studies have shown that the complexation reaction ofalkali metal with crown ether, for example, is fast as compared to the dif-fusion. The mass transfer is controlled by diffusion. Levitskaia et al.[33]

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Extraction and Simultaneous Back-Extraction Process 541

studied the Competitive Cs+ ion transport by polymer-inclusion mem-branes (PIMs) with bis(dodecyloxy)calix[4]arenecrown-6 or calix[4]arene-biscrown-6 under various conditions described the cesium transport by adiffusion-limited transport model.

RESULTS AND DISCUSSION

Donnan Equilibrium

In the first module, the extraction is carried out with a high concentrationof alkali metal chloride in feed solution (0.1 M) compared to the alkalipicrate concentration (from 1 × 10−4 to 5 × 10−3 M). The concentrationof metal in the aqueous phase is also considered as constant and equalto the chloride concentration (Cl−). It was considered that only alkalimetal picrates transferred to the organic phase, (Pic−) = C∗

w. From Eq.(2) with(L)s = (L0)s − (MLPic)s, where (L0)s is the initial concentrationof the ligand in the organic phase, the modelling of the equilibrium-distribution curves can be then written according to the Langmuir-likerelationship:

C∗s = C∗

w . Kapp1 . (L0)s .(Cl−

)w

1 + C∗w . Kapp1 . (Cl−)w

. (3)

In the second module, the back-extraction is carried out without the pres-ence of alkali metal chlorides in the back-extraction aqueous phase. Whenthe aqueous phase does not contain alkali chloride, the following relationshold: (Pic−)w = (M+)w = C∗

w, (M+)s = (MLPic)s = C∗s and (L)s = (L0)s –

(MLPic)s. From Eq. (1), the relationship between the solute concentrationin the organic and the aqueous phases at equilibrium can be expressed bythe following non-linear distribution Eq. (4):

C∗s =

(C∗

w

)2. Kapp2 . (L0)s

1 + (C∗

w

)2. Kapp2

. (4)

The apparent extraction constants Kapp1 and Kapp2, which refer to the firstand to the second module respectively, are presented in Table 2. The valuesobtained here are based on data presented in a previous article.[23] Thevalues of Kapp1 and Kapp2 are different because the extraction systems in thefirst and the second module are different.

When the three phases (feed, organics, and back-extraction phases) arein balance (connected modules), the organic phase, which is the commonphase and is circulated between the two aqueous systems, can be written as:

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Table 2. Apparent extraction[23] and Donnan constants

Metal Kapp1 (L2.mol−2) Kapp2 (L2.mol−2) Kapp1/Kapp2

Cs 1,50E+06 3,75E+06 0, 40Rb 1,80E+05 3,95E+05 0, 46K 4,00E+03 3,85E+04 0, 10

C∗s =

(C∗

w2

)2. Kapp2 . (L0)s

1 + (C∗

w2

)2. Kapp2

= C∗w1 . Kapp1 . (L0)s .

(Cl−

)w

1 + C∗w1 . Kapp1 . (Cl−)w

(5)

then,

C∗w1 . Kapp1 .

(Cl−

)w(

C∗w2

)2. Kapp2

= 1 (6)

and

Kapp1

Kapp2=

(C∗

w2

)2

(0.1) × C∗w1

. (7)

This relation describes the behavior of the extraction and simultane-ous back-extraction process in a closed system and shows the conditionswhich make it possible “to empty” the feed phase using Donnan equilib-rium conditions (equilibrium of a semi-permeable membrane). The alkalipicrates from the aqueous feed are continuously extracted in the medi-ating solution (organic phase) which is used simultaneously as the feedsolution for the back-extraction aqueous phase. The solute concentrationin the back-extraction phase is a function of the solute concentration inthe feed phase. Therefore, this equation provides the theoretical concentra-tion limit of alkali picrate that can be transferred from the feed phase tothe back-extraction phase via the organic phase. The extraction constantsratio (Kapp1/Kapp2) are given in Table 2. These results show three differ-ent behaviors of the semi-permeable system—for a higher ratio the metalrecovery increases. In this case, the organic phase provides a higher capac-ity to concentrate the rubidium picrate in the back-extraction phase thanthe other alkali picrates. Alternatively, a lower ratio (when both Kapp1 andKapp2 are high) leads to a lower recovery. Lastly, the potassium is consideredas another case in which the back-extraction is low because the extractionis low.

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Extraction and Simultaneous Back-Extraction Process 543

Influence of Alkali Picrate on the Extraction and SimultaneousBack-Extraction Kinetics

The experimental results on the kinetics of extraction and simultaneousback-extraction of alkali metal picrates (CsPic, RbPic, and KPic) with 1,3bis-benzo-crown-6-calix[4] arene in the feed, organic, and back-extractionphases are presented in Figs. 3, 4, and 5 respectively. The initial feed con-centration of alkali picrates are 4.2 × 10−4 mol.L−1, 4.37 × 10−4 mol.L−1

and 6 × 10−4 mol.L−1 for the cesium, rubidium, and potassium picrates,respectively.

It is observed that the trends of extraction kinetics are similar in thecase of cesium and rubidium picrates. The alkali-metals concentration inthe feed reservoir decreases quickly in the early step of the extraction pro-cess indicating fast extraction of metal picrates while the concentration ofalkali metal starts increasing in the back-extraction phase only after a fewminutes. The concentration of metals in the organic phase increases quicklyand reaches a maximum and then decreases slowly with time. Accordingto Figs. 3 and 4, a qualitative comparison of the reservoir concentrationvariation with time (δC/δt) in the early step of the process shows that theextraction kinetic is slightly faster for the cesium than it is for the rubid-ium. A fast-system response and a high concentration of solute in theorganic phase show a high-mass transfer from the feed-aqueous phase tothe organic phase indicating a high extraction of metal picrates. This highextraction of metal in the first module is due to the presence of alkali chlo-ride in feed solution and the high affinity of calixarene for alkali metals.

0,0

0,2

0,4

0,6

0,8

1,0

0 1000 2000 3000 4000 5000

C/C

°

Time (min)

Figure 3. Kinetics of simultaneous extraction and back-extraction of CsPic withBC6, C◦ = 4.2 × 10−4 mol.L−1, (BC6) = 1 × 10−3 mol.L−1, QW1 = QS = QW2 =12 mL m−1, (CsCl) = 0.1 mol.L−1, VW1 = Vs = 100 mL, VW2 = 200 mL, � (CsPic)in aqueous phase 1, � (CsPic) in organic phase, � (CsPic) in aqueous phase 2.

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0,0

0,2

0,4

0,6

0,8

1,0

0 1000 2000 3000 4000 5000

C/C

°

Time (min)

Figure 4. Kinetics of simultaneous extraction and back-extraction of RbPic withBC6, C◦ = 4.37 × 10−4 mol.L−1, (BC6) = 1 × 10−3 mol.L−1, QW1 = QS = QW2 =12 mL m−1, (RbCl) = 0.1 mol.L−1, VW1 = Vs = 100 mL, VW2 = 200 mL, � (RbPic)in aqueous phase 1, � (RbPic) in organic phase, �(RbPic) in aqueous phase 2.

0,0

0,2

0,4

0,6

0,8

1,0

0 1000 2000 3000 4000 5000

C/C

°

Time (min)

Figure 5. Kinetics of simultaneous extraction and back-extraction of KPic withBC6, C◦ = 6.0 × 10−4 mol.L−1, (BC6) = 1× 10−3 mol.L−1, QW1 = QS = QW2 = 12mL.m−1, (KCl) = 0.1 mol.L−1, VW1 = Vs = 100 mL, VW2 = 200 mL, � (KPic) inaqueous phase 1, � (KPic) in organic phase, �(KPic) in aqueous phase 2.

The mass transfer between the feed solution and the organic phase is fasterthan the mass transfer from the organic to the back-extraction phase. Themass transfer of metal ion is associated with its concentration in the feedphase. Initially the organic phase starts saturating with the carrier-metalpicrate complex leading to high-metal extraction and then decreases

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Extraction and Simultaneous Back-Extraction Process 545

significantly due to decrease and exhaustion in metal-picrates concentra-tion in feed solution and a very small amount of the metal ion is transferredto the back-extraction side. Once the organic phase is saturated becauseof the high-concentration gradient in the organic phase, the mass transferof alkali metals to the back-extraction phase reaches maxima. Then, therate of mass transfer decreases as the concentration of alkali metal ionsdecreases in the feed solution caused by the low-concentration gradient.

The behavior of the system is very different in the case of potas-sium. The low selectivity of 1,3 bis-benzo-crown-6-calix[4]arene towardsthe potassium increases the mass-transfer resistance. In this case, the masstransfer in the first module is so low that the concentration in the organicreservoir hardly changes during the run of experiments. On the other hand,it is interesting to compare the recovery yields for each alkali picrate usingthe same system. In the case of back-extraction, as a matter of fact, it ishigher for the rubidium (75%) than for the cesium (68%) respectively whileit is only 11% for the potassium. This order (Rb > Cs > K) is in agreementwith the Donnan equilibrium (Table 2). This is a case where the high affin-ity of cesium picrate for the 1,3 bis-benzo-crown-6-calix[4]arene cannot beneglected and, as a consequence, its release in the back-extraction aqueousphase is more difficult. This result confirms that a good extractant is notalways a good carrier.

Influence of Initial Cesium Concentration on the Extraction andSimultaneous Back-Extraction Kinetics

The effect of the initial cesium concentration on the system behavior isdescribed by Figs. 3, 6, and 7. The initial concentrations of cesium picratesare 4.2 × 10−4 mol.L−1, 1.46 × 10−4 mol.L−1, and 1.0 × 10−3 mol.L−1,respectively. The variation of the cesium concentration with time in thefeed reservoir is higher in the early steps of the process when its concentra-tion is the lowest (1.46 × 10−4 mol.L−1) and the cesium concentration inthe organic phase is maximum for this concentration. This is a case wherethe mass transfer from the feed-aqueous phase to the organic phase is fasterthan the mass transfer from the organic phase to the back-extraction phase.It is important to note that the high ionic strength, the hydrophilic char-acter of the chlorides in the feed-aqueous phase, and the solute affinity forthe organic phase always led to an increase of the cesium picrates trans-fer in the first module. In this case (CsPic, C◦ = 1.46 × 10−4 mol.L−1), theglobal-mass transfer from feed to the back-extraction phase is slow and thesteady state is not reached before 40 hours.

According to the result presented in Fig. 7 it is clear that the varia-tion of the cesium concentration with time in the feed reservoir is very lowand linear without reaching a steady state while the concentration in theorganic phase increases slowly from the first moments to reach a low and

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0,0

0,2

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0 500 1000 1500 2000 2500 3000

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Figure 6. Kinetics of simultaneous extraction and back-extraction of CsPic withBC6, C◦ = 1.46 × 10−4 mol.L−1, (BC6) = 1 × 10−3 mol.L−1, QW1 = QS = QW2 =12 mL.m−1, (CsCl) = 0.1 mol.L−1, VW1 = Vs = 100 mL, VW2 = 200 mL, � (CsPic)in aqueous phase 1, � (CsPic) in organic phase, � (CsPic) in aqueous phase 2.

0,0

0,2

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0 1000 2000 3000 4000 5000 6000 7000 8000

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Figure 7. Kinetics of simultaneous extraction and back-extraction of CsPic withBC6, C◦ = 1.0 × 10−3 mol.L−1, (BC6) = 1 × 10−3 mol.L−1, QW1 = QS = QW2 = 12mL.m−1, (CsCl) = 0.1 mol.L−1, VW1 = Vs = 100 mL, VW2 = 200 mL, � (CsPic) inaqueous phase 1, � (CsPic) in organic phase, �(CsPic) in aqueous phase 2.

constant value. Therefore, the amount of complex designed with the extrac-tion is similar to the amount of complex used with the back-extraction. Onthe other hand, the increase in cesium concentration in the back-extractionaqueous phase is much lower than the variation of the cesium concentra-tion in the feed reservoir. It is interesting to note the appearance of an

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intense yellow color (visible to naked eye) in the first membrane contac-tor which indicates an accumulation of metal-organic complex in the fiber.The high initial concentration in the aqueous phase (C◦

w = 1.0 × 10−3 M)and the presence of alkali chloride increases the mass transfer through themembrane pores providing a high concentration at the interface betweenthe feed and membrane pores which obstructs the membrane pores andslows down the transport kinetics. The mass transfer from the organicphase to the back-extraction aqueous phase is significantly low and notenough to reduce the metal-complex concentration in the organic phase.Dozol et al.[34] studied the influence of the crown-ether concentration onstrontium transport though a flat sheet-supported liquid membrane, andalso observed the presence of a third phase (precipitation of a white solid),causing a decrease of strontium permeability, when the organic phase con-taining di-tert-butylcyclohexano-18-crown-6 dissolved in n-hexylbenzene(0.7 mol.L−1 isotridecanol) was concentrated.

CONCLUSION

The experimental extraction–simultaneous back-extraction kinetics ofalkali picrates with the 1,3 bis-benzo-crown-6-calix [4]arene show that thisconfiguration operates as a “liquid membrane” (permeable only with alkalipicrate) and the order of the recovery yields is in agreement with theDonnan equilibrium. It is higher for the rubidium (75.8%) than for thecesium (68.1%) respectively while it is only 11% for the potassium. Thisis a case where the high affinity of cesium picrate for the 1,3 bis-benzo-crown-6-calix[4]arene cannot be neglected and, as a consequence, its release(back-extraction) in the back-extraction aqueous phase is more difficult.The effect of the alkali metal and the initial concentration on the systembehavior shows that the concentration gradient controls the mass transfer.

It must be noted that this study is a preliminary evaluation of microp-orous hollow-fiber membrane modules for the simultaneous extraction–back-extraction process and experiments show some drawbacks. Theextraction and the back-extraction using two hollow-fiber contactorsincrease the membrane resistance. Consequently, the system reaches theequilibrium too slowly. The steady state is not reached before 40 hoursin the best case (CsPic, C◦ = 1.46 × 10−4 mol.L−1). The performance ofthe simultaneous extraction–back-extraction process (faster system) maybe improved optimizing the chemical and transport processes.

ACKNOWLEDGMENTS

Z. Albaraka gratefully acknowledges financial support from the AtomicEnergy Commission of Syria, BP 6091, Damascus.

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