Facilitated transport of copper(II) across supported liquid membrane and polymeric plasticized membrane containing 3-phenyl-4-benzoylisoxazol-5-one as carrier

Journal of Membrane Science 325 (2008) 605–611

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Facilitated transport of copper(II) across supported liquid membrane and polymeric plasticized membrane containing 3-phenyl-4-benzoylisoxazol-5-one as carrier L. Mitiche a , S. Tingry b , P. Seta b , A. Sahmoune a,∗ a b

Laboratoire de Recherche sur l’Eau, Université Mouloud Mammeri, BP 17, 15000 Tizi-Ouzou, Algeria Institut Européen des Membranes, UMR-CNRS 5635, 2 place Eugène Bataillon, CC 047, 34095 Montpellier Cedex 5, France

a r t i c l e

i n f o

Article history: Received 1 May 2008 Received in revised form 15 July 2008 Accepted 12 August 2008 Available online 20 August 2008 Keywords: Copper Supported liquid membrane Plasticized polymeric membrane Facilitated transport 3-Phenyl-4-benzoylisoxazol-5-one (HPBI)

a b s t r a c t The potential of 3-phenyl-4-benzoylisoxazol-5-one (HPBI) as metal extractant has been evaluated for the first time for Cu(II) transport from aqueous nitrate solutions by supported liquid membrane (SLM) in the solvents chloroform, 2-nitro phenyl octyl ether (NPOE) and dodecyl nitro phenyl ether (DNPE). The efficiency of the membrane transport was optimized as a function of pH, temperature, aqueous phases and membrane composition. It follows the sequence CHCl3 > DNPE > NPOE. The results suggested that the transport mechanism was mainly controlled by the diffusion of the Cu(PBI)2 complex in the membrane core. A comparative investigation of Cu(II) transport ions has been made between SLM and polymeric plasticized membrane (PPM), containing HPBI with NPOE and DNPE as organic solvents or plasticizers in order to evaluate the feasibility of PPM with HPBI. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Copper is one of the most important metals as it plays an essential role in various industrial applications [1]. The majority of effluents and waste waters of hydrometallurgy industry, rinse water of electroplating process, waste electrolytes of tannery, contain copper along with other metals such as zinc, nickel, chrome, cadmium, manganese in low concentrations [2,3]. To extract these heavy metals from the effluents, the use of liquid membrane processes that combine extraction, diffusion across membrane core and stripping operations is well adapted as compared to conventional methods for concentrating and recovering metal ions from their diluted mixture in solution. This considerable and increasing attention for recovery and separation of heavy metals from aqueous media by liquid membrane techniques [4–6] is due to the development of highly selective extractants. Many works deal with facilitated transport of copper through various liquid membrane systems (bulk liquid membrane (BLM), emulsion liquid membrane (ELM), supported liquid membrane (SLM)) using various types of carriers such as: lauric acid [7], LIX860 [8],

∗ Corresponding author. Tel.: +213 26 21 15 27; fax: +213 26 21 15 27. E-mail addresses: [email protected] (L. Mitiche), [email protected] (A. Sahmoune). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.08.021

LIX 984N [9], D2EHPA [10,11], Crown ether DB18C6 [12–14] and oxime derivate [15]. As an important drawback of SLM is their poor long term stability [9,16], polymer plasticized membranes (PPM) have been designed and applied for Cu(II) transport metal in presence of commercially plasticizers like Aliquat 336 and LIX 84-I [17]. The ␤-diketone compounds composed only of carbon, hydrogen and nitrogen atoms are also interesting extractants of divalent metals and are widely used in copper hydrometallurgy investigation. Among them, the pyrazolone 1-phenyl-3-methyl-4benzoyl-5-pyrazole (HPMBP) and the acyl-isoxazole 3-phenyl-4benzoylisoxazol-5-one (HPBI) highly acidic extractants have been studied for liquid extraction of divalent, trivalent metals and actinides [18–25]. These extractants are particularly interesting because they allow metal extractions from acidic aqueous media. The potential of HPBI as metal extractant has been evaluated for the first time by Jyothi and Roa [21] for the extraction of divalent cations Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd (II). HPBI and HPMBP have been also used for the synergistic extraction of Cd, Cu, Mn and Zn in presence of solvating extractants like trioctylphosphine oxide [19]. Up to know, there are no studies that describe copper(II) or another divalent metal ions transport through membrane using HPBI as carrier. Only one work has demonstrated the feasibility of 3-phenyl-4-acyl-5-isoxazolones derivates as extractants in SLM for Fe(III) extraction [26].

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In this paper, the transport of Cu(II) ions was studied through SLM containing HPBI as carrier in the solvents chloroform, 2-nitro phenyl octyl ether (NPOE) and dodecyl nitro phenyl ether (DNPE). The parameters affecting the transport efficiency were evaluated such as the aqueous components (stirring, T◦ , pH, Cu(II) concentration) and the membrane composition. Finally, a comparative investigation of Cu(II) transport ions has been made between SLM and PPM, containing HPBI with NPOE and DNPE as organic solvents or plasticizers in order to evaluate the feasibility of PPM with HPBI. 2. Experimental 2.1. Reagents and solutions The specific carrier 3-phenyl-4-benzoylisoxazol-5-one (HPBI) was prepared in the Tizi-Ouzou laboratory by benzoylation of 3-phenyl-isoxazolone (H2 PI) (Aldrich) in 1.4-dioxan (Fluka) in presence of Ca(OH)2 (Aldrich) according to the method already described [27]. The purification of the HPBI compound in PBI− form was achieved by its selective phase transfer from toluene to water in which it precipitates in a yellow form. HPBI was analyzed by 1 H NMR (CDCl3 ): 7.00–7.46 ppm (m, 9H); melting point T◦ f = 153 ◦ C and elemental analysis (%C = 72.4 (72.4), %N = 5.24 (5.28), %H = 4.10 (4.18)). Copper(II) nitrate, chloroform, cellulose triacetate (CTA), 2-nitro phenyl octyl ether (NPOE), dodecyl nitro phenyl ether (DNPE) were purchased from Fluka and were used are received. 2.2. Membrane preparation 2.2.1. Supported liquid membranes (SLM) preparation The polymeric SLM support used was Celgard 2500 polypropylene (Hoeshst celanese, chascotte NC) with the following characteristics: nominal porosity 45%; thickness 25 ± 2.5 ␮m, effective pore size 0.075 × 0.25 ␮m. The membranes were impregnated during 24 h with solutions containing various concentrations of HPBI dissolved in different organic solvents CHCl3 , NPOE and DNPE. The excess of organic solution wetting the membrane was wipped off with a tissue paper. 2.2.2. Polymeric plasticized membranes (PPM) preparation Several membranes were prepared by a procedure already described [28]. Amounts of CTA (0.05 g), plasticizer (NPOE or DNPE) (0.2 ml) and 10−4 M of HPBI carrier were dissolved in chloroform and stirred during 8 h in order to ensure the complete dissolution. The solution thus prepared was poured in 9 cm diameter covered circular glass Petri dish and the solvent was allowed to evaporate over 24 h at room temperature. Polymeric films obtained was then separated from the bottom of the glass Petri dish by immersion in cold water and then placed between the two half cells of the membrane transport cell with the side exposed to the air during evaporation facing the strip solution. The membrane thickness was 50 ␮m, measured by using a digital micrometer Mitutoyo 42.300.

2.4. Transport experiments Transport experiments were performed in a permeation cell in Teflon. The SLM or PPM were placed in the transport cell between the aqueous feed phase and strip phase each having a volume of 200 cm3 . The exposed membrane area was 3.2 cm2 . The two aqueous phases were shaken at 25 ◦ C with a mechanical stirrer at controlled speed (600 rpm). In the case of SLM, the feed solution consisted of aqueous copper(II) solution of various concentrations ranging from 10−4 to 10−2 M at pH 5.5, and the strip solution consisted of HNO3 with concentrations from 10−1 to 4 M. In the case of PPM, the copper(II) concentration in the source phase was 1.5 × 10−4 M at pH 5.5 and the strip phase was 1 M HNO3 . The pH of aqueous phases was measured with a WTW 526 pHmeter. For both SLM and PPM systems, aliquots of 0.5 ml of both feed and strip phases were withdrawn periodically and analyzed by atomic absorption with a SHIMADZU 6800 spectrophotometer at wavelength of 324.75 nm to determine copper(II) concentration. Permeability P (m s−1 ) of the copper ions transferred from the feed phase to the strip phase was calculated for both membrane types by the following equation [29]: ln

C Ci

=−

A V

The scanning electron microscopy (SEM) observations of the membrane samples were made by a 8 kV scanning electron microscope HITACHI S4500. The samples were mounted with conductive glue to metal stubs and then coated with gold by sputtering. These samples were then viewed in the SEM at around 5000× magnification.

(1)

where Ci is the initial copper concentration (mol l−1 ) in feed phase and C is the copper concentration at a given time, A is the effective area of membrane (m2 ) and V is the volume of aqueous feed phase (m3 ). The membrane flux Ji was determined as: Ji = PCi

(2)

The experiments have been repeated three times. The experimental accuracy of the measurements was ±5% of the mean value. 3. Results and discussion Several factors in both the aqueous solutions and the membrane phase were investigated in order to determine the efficiency of Cu(II) transport across SLM or PPM systems by the carrier HPBI. 3.1. Preliminary liquid–liquid extraction experiments In order to evaluate the adequate conditions for the Cu(II) transport through both SLM and PPM systems, the global equilibrium dissociation constant KA of HPBI carrier between aqueous phase [1 M (Na, H)NO3 ] and chloroform organic phase, and the dissociation acidic constant Ka were determined by liquid–liquid distribution procedure. HPBI ↔ HPBIorg HPBI ↔ H+ + PBI−

2.3. Membrane characterization

P×t

Kd =

[HPBI]org [HPBI]

Ka =

[H+ ][PBI− ] [HPBI]

(3) (4)

A value for log Kd = 2.76 has been determined experimentally with UV-spectrophotometry by measuring HPBI absorbance at 322 nm. The HPBI pKA (KA = [H+ ][PBI− ]/[HPBI]org = Ka /Kd ) value determined experimentally from the intercept of the curve log ([HPBI]org /[PBI− ]) vs pH at pH 0 was 4.05, similar to the value 4.25 obtained by Torkestani et al. [30] in chloroform (1 M NaNO3 ). The pKa calculated value was 1.29. Other pKa values of HPBI have been

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Table 1 Variation of copper permeability flux vs temperature using SLM impregnated by 10−2 M HPBI in CHCl3 Temperature (◦ C)

Flux (10−5 mol m−2 s−1 )

25 30 40 50 60

6.98 6.32 5.87 5.45 4.43

Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: 10−1 M HNO3 .

viscosity due to a decreased volume of solvent in the pores of the polypropylene membrane [33].

3.2. Influence of some parameters of the aqueous components

3.2.2. Influence of the aqueous phase composition The facilitated transport of divalent metal ions through membrane systems containing acidic organic compounds such as HPBI is generally ensured by a counter-transport of protons. The carrier exchanges one metal ion at the feed solution–membrane interface releasing two protons. The complex formed between the carrier and the metal diffuses through the membrane and one metal ion is released at the membrane–strip phase interface by substitution of two protons. It is clear that the pH difference between feed and strip aqueous solutions may play the role of driving force for the transfer of metal from the feed to the strip aqueous solutions. Fig. 2 shows a typical plot of variation of Cu(II) concentration vs time in the feed phase and strip phase obtained with SLM prepared with HPBI dissolved in chloroform. The extracted Cu(II) concentration in feed phase decreases with time accompanied by a related increase of Cu(II) concentration in the strip phase. After 420 min, 36% of Cu(II) are extracted from the feed phase whereas 20% of Cu(II) are recovered in the strip phase. This result and the lower permeability value of exit flux (0.39 × 10−5 mol m−2 s−1 ) than those of entrance flux (6.98 × 10−5 mol m−2 s−1 ) in membrane may be explained by a low diffusion rate of the Cu(PBI)2 complex in the membrane core. A maximum of 60% of Cu(II) was extracted by the membrane in 1 day. In order to improve the dissociation of the complex in the strip phase, the variation of the extracted Cu(II) amount was studied as a function of the pH of the strip phase by varying the concentration of HNO3 from 10−1 to 4 M. The pH of the feed phase has

3.2.1. Effect of stirring speed and temperature on the transport of copper In order to establish the optimal stirring speed in feed and receiving phases, the variation of the flux of copper across the SLM was studied in the speed range of 200–1000 rpm at 25 ◦ C. The experiments were conducted with a SLM prepared from 10−2 M HPBI in CHCl3 . The flux values were calculated from Eq. (2). As observed in Fig. 1, the permeability flux of copper increases with increasing stirring speeds until a maximum value of 6.98 × 10−5 mol m−2 s−1 at 600 rpm. It diminishes at higher stirring speed values probably due to the leaching of the CHCl3 from the membrane into the aqueous phases. In the following experiments, the agitation of both aqueous phases was thus fixed at the stirring speed 600 rpm corresponding to the maximum flux. The copper permeability flux through SLM was also measured as function of the temperature (25 ◦ C up to 60 ◦ C). The results, reported in Table 1, show the decrease of the flux with the increase of temperature. This result could be explained by the fact that at high temperature, the CHCl3 solvent evaporates easily, thus diminishing the number of filled pores where the free diffusion of the carrier is active, and as a consequence reducing the transport flux of copper. Another explanation could be linked to the carrier loss in the aqueous phases by the increase of its solubility and/or to the increase of

Fig. 2. Variation of copper concentration in the feed phase () and strip phase (䊉) vs time. SLM impregnated by 10−2 M HPBI in CHCl3 . Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: 10−1 M HNO3 .

Fig. 1. Variation of copper permeability flux vs stirring speed of both aqueous phases using SLM impregnated by 10−2 M HPBI in CHCl3 . Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: 10−1 M HNO3 .

determined in 0.2 M NaCl (0.87) [31] and in 0.1 M NaClO4 (1.23) [32]. The extraction of Cu(II) from 1 M NaNO3 aqueous phase in chloroform was also studied. The plots of the distribution curves log D vs pH and vs log [HPBI] were straight lines with slopes of 2 that agree with the extraction of the 1:2 metal to ligand Cu(PBI)2 complex. As a conclusion, the extraction process at the aqueous phase/membrane interface can be represented by the following reaction: Cu2+ + 2HPBI(org) ↔ Cu(PBI)2 (org) + 2H+

(5)

The apparent constant value of the extraction was log Kex = 0.90. In chloroform, no aggregation of HPBI was observed. Thus, both properties of good lipophilicity and great ability to complex the metal ions are precisely required for using the extractant HPBI as carrier in liquid membrane systems.

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Fig. 3. Effect of the pH of the strip phase on copper transport across SLM impregnated by 10−2 M HPBI in CHCl3 . Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: (䊉: 4 M HNO3 ); (: 1 M HNO3 ); (: 10−1 M HNO3 ).

been fixed at 5.5, taking into account the maximum efficiency of experimental biphasic extraction obtained in this pH range. From the results shown in Fig. 3, when a larger difference in pH between the feed and strip phases is applied, the extracted Cu(II) concentration increases from 14.5 to 22%. The permeate flux increases from 3.9 × 10−6 to 12.7 × 10−6 mol m−2 s−1 . One can suggest that the increase of the pH difference between the feed and the strip phases enhances the release of the metal ion in the strip phase by a facilitated protonation of the carrier with two protons as expected in Eq. (5). Besides, the variation of the extracted Cu(II) amount was studied as a function of thiourea in the strip phase. Thiourea is known to form cationic species with Cu(II) in acidic medium in the pH range 0–1 [33]. It was chosen as a complexing reagent in the strip phase as it forms at the concentration of 0.1 M more stable copper complexes (global stability constant log ˇ = 15.4 [33]) than HPBI (log ˇCu(PBI)2 = 6.3). As observed in Fig. 4, the presence of thiourea in the strip phase improves slightly the dissociation of the Cu(HPBI)2 complex accumulated at the membrane interface. This result corroborates the previous assumption that the transport mechanism of Cu(II) is mainly controlled by the diffusion of Cu(PBI)2 complex in the core of the membrane. 3.2.3. Effect of initial concentration of copper(II) The effect of initial copper concentration range between 10−4 and 10−1 M in feed phase on the permeation flux is presented in Table 2. The permeate flux increases with increasing copper concentrations from 10−4 to 10−2 M and tends to stabilize for higher concentrations. At Cu(II) = 10−1 M and after 400 min of transport time, the occurrence of a milky layer was observed on the membrane surface. This observation was likely one of the reasons of the flux limit. However, other reasons could be related to the kinetics of the metal uptake, which could have reached its maximum value, or to complexes of different stoichiometries formed at highest metal ion concentration less soluble in the membrane core. SEM images of the surface of the SLM membrane exposed to the feed phase corrobotates this observation (see Fig. 5). The film formed on the interface could presumably be due to aggregation of metal–carrier species at the entry of membrane pores thus dimin-

Fig. 4. Effect of the thiourea on copper transport in strip phase vs time. SLM impregnated by 10−3 M HPBI in CHCl3 . Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: 10−1 M thiourea in 10−1 M HNO3 .

ishing the effective membrane area. As suggested by Sastre and co-workers [15], the saturation of the membrane pores and the build-up of a carrier layer on the membrane interface enhanced the retention of the separation constituent on the entry side and thus causing the permeability flux to be constant. 3.3. Influence of membrane characteristics on the transport 3.3.1. Effect of HPBI carrier concentration Fig. 6 displays the variation of the entrance flux in the SLM membrane as a function of the carrier concentration HPBI. When the HPBI concentration increases from 10−4 to 10−2 M, JCu increases by a factor two as higher amounts of carrier are available at the feed–membrane interface favouring the formation of Cu(PBI)2 complex in the membrane. At higher HBPI concentration, the organic membrane becomes saturated in Cu(PBI)2 complex and Jcu remains constant. The main reason of the enhancement of the flux is the increase of the kinetics of metal–carrier association at the feed membrane interface as the concentration of the metal ions is also increased. 3.3.2. Effect of chemical nature of the solvent in SLM The Cu(II) concentration in the strip phase was measured for SLM s obtained from 10−2 M of HPBI in different solvents chloroform, NPOE and DNPE (Fig. 7). The influence of the nature of the solvent on the transport efficiency of SLM depends on both its viscosity and its dielectric constant. The latter parameter influences the balance between the efficiency of association and dissociation steps for the uptake of the metal ion by the carrier and its Table 2 Permeability flux (J) vs initial copper concentration in the feed phase. SLM impregnated by 10−2 M HPBI in CHCl3 [Cu]i (mol l−1 ) −4

10 10−3 10−2 10−1

Flux (10−5 mol m−2 s−1 ) 0.35 1.13 6.98 7.05

Feed phase: Cu(II) at pH 5.5, strip phase: 10−1 M HNO3 .

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Fig. 5. Scanning electronic microscopy of SLM prepared from 10−2 M HPBI in CHCl3 . (a) Before transport of Cu(II) and (b) after transport of Cu(II), phase I: Cu(II) 10−1 M at pH 5.5; phase II: 10−1 M HNO3.

release from the complex at the receiving interface. From Fig. 7, the amount of extracted Cu(II) is higher for SLM in CH3 Cl that possesses the lower dielectric constant (ε = 4.8). The lowest efficiency of the transport is observed for NPOE, which is characterized by a high dielectric constant (ε = 23) and a high viscosity (13.8 cP). The facilitated transport of Cu(II) by HPBI in SLM appears thus to be more limited by the diffusion of the complex Cu(PBI)2(org) in the membrane (NPOE being more viscous than CH3 Cl) than by the dissociation of the complex at the strip interface favoured by a higher dielectric constant solvent.

Fig. 6. Influence of carrier concentration on entrance flux (J) for SLM with CHCl3 . Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: 10−1 M HNO3 .

Fig. 7. Variation of copper concentration in the strip phase vs time as a function of the solvent of the SLM. SLM impregnated for 24 h with 10−2 M HPBI in (: CHCl3 ), (䊉: NPOE) and (: DNPE). Feed phase: 10−2 M Cu(II) at pH 5.5, strip phase: 10−1 M HNO3 .

3.3.3. Comparison of copper transport through SLM and PPM In order to compare the efficiency of copper transport through SLM and PPM, experiments have been performed with NPOE and DNPE as solvent or plasticizers. The values of flux have been normalized vs the SLM thickness. Generally the carrier concentration in PPM is chosen higher than in SLM. However, as we noticed the occurrence of agglomerates on the membrane surface during the PPM preparation at high carrier concentrations, the HPBI content was fixed to 10−4 M like in SLM experiments. Fig. 8 shows that the efficiency of Cu(II) transport is better with SLM compared to PPM. This phenomenon was already reported for transport of metals by the two membrane systems [7] and it was allotted to some physical properties such as the dipole moment

Fig. 8. Variation of extracted Cu(II) concentration in the strip phase across SLM and PPM membranes. SLM: 10−4 M HPBI + DNPE () or + NPOE (). PPM: 0.05 g CTA + 0.2 ml DNPE () or NPOE (䊉) + 10−4 M HPBI. Feed phase: 1.5 × 10−4 M Cu(II) at pH 5.5, strip phase: 1 M HNO3 .

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Table 3 Comparison of the permeability flux (J) between SLM and PPM containing 10−4 M HPBI in NPOE or DNPE Organic solvent/plasticizer

Flux (10−8 mol m−2 s−1 ) SLM

2-Nitro phenyl octyl ether (NPOE) Dodecyl nitro phenyl ether (DNPE)

4.15 6.1

PPMa 0.49 0.87

Feed phase: 1.5 × 10−4 M Cu(II) at pH 5.5, strip phase: 1 M HNO3 . a Fluxes are normalized for identical thickness vs the SLM.

of PPM compared to SLM was pointed out and attributed to physical properties of the solvents. Future works will be devoted both to the study of the PPM stability and the development of a microfluidic system for PPM extraction with channel dimensions that affect enrichment factors and extraction efficiencies [35]. Acknowledgements We are indebted to the research team “Interfaces and bioinspired Membranes” at the Membrane European Institute, respectively for their constant support and helps about the experimental studies. We thank the DEF and the CNRS for their travel assistance in the frame an exchange program n◦ 18429. References

Fig. 9. Variation of copper concentration in the feed phase () and strip phase (䊉) vs time. Feed phase: 1.5 × 10−4 M Cu(II) at pH 5.5, strip phase: 1 M HNO3 , membrane 0.05 g CTA + 0.2 ml DNPE + 10−4 M HPBI.

and the dielectric constant of solvent. The order of the transport efficiency of copper thus follows: SLM > PPM (Table 3). Besides, the nature of the solvent (in SLM) or plasticizer (PPM) shows that the transport of copper is enhanced in the membrane containing a plasticizer with the higher polarity constant. DNPE could increase the solvation of HPBI carrier in the polymer core of the PPM thus favouring the transition to a SLM structure as proposed by Fontas et al. [34]. In the case of the PPM with the plasticizer DNPE, an additional transport experiment of copper ions shows that 50% of copper is extracted in 30 h in the strip phase (Fig. 9). Seventy percent of copper is extracted from the feed phase and about 20% of Cu(II) is accumulated in the membrane. The lower efficiency of PPM with this carrier points out the difficulty to select the right conditions for an efficient transport, although on a practical point of view PPM should be more convenient for long term membrane use in such separation applications. 4. Conclusion The feasibility of HPBI as metal extractant in SLM has been demonstrated for the first time for Cu(II) metal ions. This extractant is easy to synthesize and is efficient for the transport of divalent metal ions across membranes in similar way than that of trivalent metal ions already reported in the literature. The Cu(II) transport has been optimized for SLM with CHCl3 , which gave the best efficiency. Despite an accumulation of Cu(II) in the membrane, a maximum of 60% of the metal was extracted in 1 day. The effects of the chemical nature of the solvent and of thiourea, as complexing agent in the strip phase, suggested that the transport mechanism was mainly controlled by the diffusion of the Cu(PBI)2 complex in the membrane core. A comparative investigation of Cu(II) transport between SLM and PPM shows the feasibility of HBPI in PPM to extract 50% of the metal ions in 30 h. However, the lower efficiency

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