Facilitated Cd(II) transport across CTA polymer inclusion membrane using anion (Aliquat 336) and cation (D2EHPA) metal carriers

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Journal of Membrane Science 310 (2008) 438–445

Facilitated Cd(II) transport across CTA polymer inclusion membrane using anion (Aliquat 336) and cation (D2EHPA) metal carriers Ounissa Kebiche-Senhadji a , Lynda Mansouri a , Sophie Tingry b , Patrick Seta b , Mohamed Benamor a,∗ a b

Laboratoire des Mat´eriaux Organiques, Universit´e de B´ejaia, DZ-06000 B´ejaia, Algeria Institut Europ´een des Membranes, UMR CNRS 5635, 2 Place Eug`ene Bataillon CC 047, 34095 Montpellier Cedex 5, France

Received 31 July 2007; received in revised form 8 November 2007; accepted 12 November 2007 Available online 19 November 2007

Abstract PIMs have been involved as affinity membranes for recovery of metals (Cd, Pb, Zn) by facilitated transport from aqueous solutions under different speciation forms, either anionic or cationic. The motivation of this work is to compare the efficiency of the recovery process in the case of Cd(II) using extractants such as D2EHPA and Aliquat 336 that can form complexes with the cation Cd2+ or the anions CdCl3 − and CdCl4 2− , respectively. The maximal Cd(II) recovery factors obtained in 8 h are 97.5% and 91.8% with D2EHPA and Aliquat 336, respectively. Although the transport fluxes with both carriers are not strongly different (ca. 2 ␮mol m−2 s−1 ), the recovery process in case of mixture of metals is better achieved with Aliquat 336. PIMs have shown a very good stability and a constancy of the transmembrane transport flux over 12 replicate measurements, each one lasting for 8 h repeated every 24 h. © 2007 Elsevier B.V. All rights reserved. Keywords: Polymer inclusion membrane (PIM); Facilitated membrane transport; Metal ions separation and recovery; Aliquat 336; D2EHPA

1. Introduction The need of more specific practical systems for recovery of dilute metal species in liquid media, from both ecological and economic aspects, has led to the development of new separation techniques [1]. Liquid membranes (LMs) have been shown to have advantages over traditional solvent extraction techniques, mainly because the separation process is achieved in a single step (unitary process) and much less perturbed by interfacial emulsion formation and solvent evaporation [2]. In the last three decades liquid membranes, supported liquid membranes (SLMs) have been studied extensively [3–9]. The SLMs are formed by physical immobilisation (wetting process) of an organic phase containing a mobile carrier into the pores of a hydrophobic microporous polymer as membrane support

∗

Corresponding author. Tel.: +213 34 20 51 94; fax: +213 34 20 51 94. E-mail address: [email protected] (M. Benamor).

0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.11.015

[10]. Even though SLMs have many promising assignments, they found until now very few industrial applications. The major problem encountered is failure due to leaching of the liquid membrane components into the aqueous phases [11]. Several attempts to overcome these instability problems of SLMs have been subject to assays [12–14] without leading to real very significant improvements. Thus different routes for immobilization of carriers were tested and an alternative approach proposed by Sugiura et al. [15], which encountered a real success, was to synthesize membranes with carriers incorporated in a plasticized thermoplastic polymer. Hence a new class of membranes, called polymer inclusion membranes (PIMs), is increasingly retaining attention because of its long term stability, which let us think that industrial applications are not as hypothetical as they were considered until now [16]. The physical structure of PIMs is subject to investigations using different techniques of chemistry as well as physics, to point out the manner by which the carrier is immobilised into a plasticized polymer matrix and interacts with it. Mostly, polyvinylchloride (PVC) and cellulose triacetate (CTA) are used

O. Kebiche-Senhadji et al. / Journal of Membrane Science 310 (2008) 438–445

as polymer matrix for preparing PIMs and 2-NPOE is the main compound used as plasticizer. Other plasticizers have been tried in order to understand the role of the plasticizing effect on the transport efficiency of PIMs [17,18] and more precisely on the physical state of such type of membrane [19]. The large number of reference devoted to PIMs during the last decades [20–22] reveals the great interest for these membranes. A general review concerning extraction and transport of metal ions and small organic molecules using polymer inclusion membranes (PIMs) has been recently published [16], pointing out better mechanical properties and chemical resistance than traditional SLMs. The aim of this work is to compare the transport efficiency of Cd(II) between two PIMs based on cellulose triacetate polymer incorporating two kinds of carriers, an acidic one D2EHPA and a basic one Aliquat 336. D2EHPA is a commercial carrier specially suited for metal cations recovery like Pb(II), Cd(II) and Zn(II) [23–26]. To our knowledge, only one work was devoted to the extraction of Cd(II) with PIMs composed of cellulose triacetate, D2EHPA and o-nitrophenyl pentyl ether as plasticizer [27]. It was reported that the recovery factor of Cd(II) in mixture with Pb(II), Cd(II) and Zn(II), is around 25%. Aliquat 336 is an anion carrier largely used for the recovery of different types of metal anions [28–30] and it has been essentially applied to the recovery of Cd(II) in PIMs composed of poly(vinyl chloride). The authors have proposed a mechanism for the extraction of Cd(II) involving ion-exchange reaction. They showed that in the case of 50% Aliquat 336/PVC membrane, more than 50% of Cd(II) was extracted after 20 h of contact time [30]. Only one work was devoted to the extraction of Cd(II) with PIMs composed of cellulose triacetate, Aliquat 336 and o-nitrophenyl pentyl ether as plasticizer [31]. The authors reported that the selectivity in a competitive transport of Cd(II) and Pb(II), depended on the membrane surface area. For a small surface area (0.8 cm2 ), Cd(II) was transported preferentially, however for a large area (15 cm2 ) this was reversed. In this work, the efficiency of the recovery process using D2EHPA and Aliquat 336 on the transport of Cd(II) is compared by varying the composition of the membrane and of the aqueous phase. The selectivity of the recovery process for Cd(II) mixed with Pb(II) and Zn (II) is studied. Besides, the membranes have been characterized to obtain information regarding the composition and the potential interactions between the components. 2. Experimental 2.1. Reagents Organophosphorus extractant, D2EHPA was kindly supplied by Albright and Wilson (USA) and used as received. Tricaprylmethylammonium chloride (Aliquat 336), CTA (cellulose triacetate), 2-NPOE (2-nitrophenyloctylether) were obtained from Aldrich and were used without purification. Chloroform (CHCl3 ) was acquired from Prolabo. Other inorganic chemicals: Zn(NO3 )2 , NaNO3 and NaCl (Labosi), CdCl2 , HNO3 and

439

HClO4 (Prolabo), HCl (Cheminova), Cd(NO3 )2 ·4H2 O (Fluka), ZnCl2 and Pb(NO3 )2 (Riedel-de Ha¨en) and PbCl2 (RECTAPUR) were analytical reagent grade and were used for the preparation of the different solutions. Redistilled deionised water was used in all experiments. 2.2. Preparation of polymeric membrane The membrane was prepared using a procedure similar to that reported by Hayashita et al. [32]. CTA (200 mg) was dissolved in 20 ml of CHCl3 at room temperature. 0.3 ml of 2-NPOE in 5 ml of CHCl3 was then added. After vigorous stirring, the carrier (D2EHPA or Aliquat 336) was added and the solution was stirred for 30 min to obtain a homogenous solution. The solvent of this mixed solution was allowed to slowly evaporate in a 9.0 cm diameter Petridish, which was covered loosely, overnight evaporation. A small quantity of water was deposited on the film to help its unsticking of the glass support. The membranes thickness was measured by digital micrometer (Mitutoyo) with 0.1 ␮m standard deviation over 10 readings. Two samples were cut out from the same piece of membrane for duplicate experiments. 2.3. Membrane characterizations FTIR spectra were acquired using FTIR 710 de Nicolet spectrometer. Measures were taken in wave number range from 500 to 4000 cm−1 . Scanning electronic microscopy images of the PIMs were obtained using a HITACHI S4500 microscope that can reach a resolution of 1.5 nm. Thermogravimetric analyses were achieved using an apparatus of thermogravimetry of high resolution, TGA 2350 (MT instruments) with a temperature going from ambient until 1000 ◦ C. 2.4. Membrane transport experiments The membrane cell consisted of two plexiglass compartment (250 cm3 ). The chambers were separated by the PIMs and clamped together (the area of each side of the PIMs in contact with the solution was 10.74 cm2 ). The source phase was an aqueous cadmium(II) solution in NaNO3 0.1 M (pH 4.5) or NaCl 0.5 M (pH 7). Aqueous receiving phase was 0.1 M HNO3 or 0.1 M HClO4 . Transport experiments were conducted at room temperature (23–25 ◦ C) and both source and receiving aqueous phases were stirred at 900 rpm with two mechanic stirrers (Heidolp RZR 2101). The metal concentration was evaluated by sampling at different time intervals aliquots of 0.2 ml each from the feed and strip solutions and analysed with an atomic absorption spectrophotometer (SCHIMADZU AA6500). The kinetics of the transport across PIMs was described as a first-order reaction in metal-ion concentration:   C Ln = −kt (1) Ci

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where C is the metal-ion concentration at a given time in the feed phase, Ci the initial concentration of metal ion in the feed phase, k the rate constant (s−1 ), and t is the time transport (s). The k-values were calculated from the plots of ln(Ci /C) versus time. The relationship of ln(Ci /C) versus time used to plot the data obtained in this work was linear as expected from Eq. (1): The initial flux (Ji ) was determined as Ji =

−kV ACi

Table 1 Main identified IR absorption bands in the membrane samples Membrane

Absorption bands (cm−1 )

Chemical groups

CTA

3600–3200 1735 1210–1035 2960–2850 1370

O–H C O C–O–C C–H C–H

2-NPOE

1525 1465 2960–2850 1127 1232 1351 720 730–675

NO2 –CH3 (octyl) –CH2 – C–O–C R–O–CH2 C–N –CH2 – C–H

D2EHPA

1230 1020

P O P–O

Aliquat 336

1235

(R)3 –N+

(2)

where V is the volume of aqueous feed phase and A is the area of the effective membrane. 3. Mechanism of the complexation The overall transport process is quite similar for all types of carriers. However, because of different complexation mechanisms involved, the transport characteristics and the choice of the ionic composition of both the source and receiving phases with respect to each type of carrier are distinct [16]. As reported in several studies [33,34] the mechanism of bivalent ions complexation with D2EHPA can be written as follows: M2+ + (2 + n)HRm ↔ MR2 (HR)nm + 2Haq +

(3)

As shown in Eq. (3), the transport of Cd(II) is coupled to that of the released protons since the forward and backward reactions imply the substitution of bivalent metal ion in the feed solution for two protons and the substitution of two protons in the receiving solution for one metal ion. This imposes that the difference between the sum of the chemical potentials for the transported ions in the receiving and the feed phases has to lie below zero (μ < 0). In the case of Aliquat 336 (quaternary ammonium compound), the carrier reacts as an anion-exchanger forming an ion-pair with a metal anion complex from the aqueous solution. The mechanism proposed by Wang et al. [31] is given in Eqs. (4) and (5): AClm + [MCl3 ]a − ↔ A[MCl3 ]m + Claq −

(4)

2AClm + [MCl4 ]a 2− ↔ A2 [MCl4 ]m + 2Claq −

(5)

where M is the bivalent metal, (HR)n D2EHPA (n = 1 or 2), ACl the Aliquat 336, and the subscripts m and aq refer to the membrane and the aqueous phases, respectively. 4. Results and discussion 4.1. Membrane characterization Several techniques were investigated to characterize the PIMs such as FTIR, SEM and ATG. In FTIR spectra the main observed bands are those of the individual constituents of the membrane as shown in Table 1, as already demonstrated in literature for PIMs with other carriers [22,35].

This should suggest that only weak interactions between constituents of the PIMs such as van der Waals or hydrogen bonds. From SEM images, all membranes show a uniform surface and appear dense with no apparent porosity. We used thermogravimetric analyses (TGA) to link specific temperature and height of mass changes to the degradation of a specific compound or fragment of it. The thermograms were close to those of individual components. However, in our experiments with the carriers Aliquat 336 and D2EHPA, it was somewhat different and the general trend was an absence of actual separation of the degradation steps attributed to individual components. Fig. 1 shows thermograms (% weight loss vs. temperature) for: (a) CTA, (b) CTA + 2-NPOE, (c) CTA + 2NPOE + D2EHPA and (d) CTA + 2-NPOE + Aliquat 336. The degradation of the CTA (thermogram a) occurs after water loss, in only one stage at a temperature close to 325 ◦ C (approximately 81% of the initial mass), after which the remaining compounds are lost by carbonization. In case of the membrane containing 2-NPOE + CTA (thermogram b), the loss weight starts at around 190 ◦ C, which corresponds to the loss of only part of it (43,65%), the boiling temperature of 2-NPOE being 198 ◦ C. The 2-NPOE weight corresponding to 61% of the total weight of the membrane sample is not fully lost at this temperature. At 210 ◦ C a loss of 21% of the membrane weight can be attributed to the rest of 2-NPOE presumably interacting with the polymer chains. The degradation of the CTA polymer starts at around 370 ◦ C. The PIM containing CTA + 2-NPOE + Aliquat 336 (thermogram c) shows a first weight loss (3.066%), which occurs at a temperature lower than 100 ◦ C attributed to dehydration. At 190 ◦ C the loss of about 35% is due to one part of the 2-NPOE (40.6%of the initial mass sample). Then at 210 ◦ C, the recorded loss is essentially due to the volatilization of Aliquat 336 (33% of the initial mass sample), the boiling temperature of the latter being 225 ◦ C. At about 300 ◦ C, the loss of 24% of the total

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Fig. 1. Thermograms of: (a) CTA, (b) CTA + 2-NPOE, (c) CTA + 2-NPOE + Aliquat 336 and (d) CTA + 2-NPOE + D2EHPA.

mass corresponds to degradation of CTA (26% of the initial mass sample). In case of PIM with CTA + 2-NPOE + D2EHPA (thermogram d), a loss of 40% of the initial mass is registered at 180 ◦ C, which does not correspond to the entire amount of D2EHPA (53%). Thus the volatilization of part of D2EHPA or part of D2EHPA with part of 2-NPOE (28% of the initial mass sample) has to be taken into account, as the boiling temperature of D2EHPA is 155 ◦ C. At 225 ◦ C, an additional loss of 52% of the membrane weight, more or less in a single step, is released bringing the sum of weight losses to approximately 92% of the membrane sample weight. The feature of the thermogram d seems to indicate peculiar interactions between the components in the membrane. Such affinities between the components have been also pointed out and characterized for PIMs based on TAC, 2-NPOE and carboxylic carrier [19]. 4.2. Transport experiment Flux experiment using the PIM without carrier showed that in absence of carrier no transport of cadmium from the source phase to the strip phase was detected. From this result it can be established that the PIM without carrier served as an effective barrier to ion permeation.

4.2.1. Influence of the membrane composition 4.2.1.1. Effect of the carrier concentration. The ability of the Aliquat 336 and D2EHPA/TAC polymer membrane to transport Cd(II) from a feed to a receiving phase is demonstrated in Fig. 2. In the case of D2EHPA, the feed solution is an aqueous solution of 12 mg/L Cd(NO3 )2 in NaNO3 (0.1 M) pH 4.5 and the receiving solution is HNO3 (0.1 M). In the case of Aliquat 336, the feed solution is an aqueous solution of CdCl2 in NaCl (0.5 M) pH 7.0 and the receiving solution is HClO4 (0.1 M). The effect of the membrane composition on the Cd(II) extraction is shown by the increase of Cd(II) with the carrier concentration. First, it must be noticed that fluxes using the two carriers are comparable as the standard deviation of these measurements is of the order of ±5% of the values. These values reach at the maximum 2.25 ␮mol m−2 s−1 with D2EHPA and 2.58 ␮mol m−2 s−1 with Aliquat 336. In the concentration region (10–20%, w/w) in case of Aliquat 336, the flux tends to reach a kind of plateau. However, this effect cannot be attributed to accumulation of the carrier in the membrane as Jfeed = Jstrip (see Fig. 3). Font`as et al. have discussed the incidence of the concentration ratio between components of the PIM on the efficiency of the metal cation transport [19]. They supposed the formation of

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Fig. 2. Cd(II) fluxes as function of carrier concentration membrane consisted of CTA (200 mg), 2-NPOE (312 mg) and various content of D2EHPA () or Aliquat 336 (䊉). Feed solution, Cd(II): 12 mg/L in NaNO3 0.1 M () or in NaCl 0.5 M (䊉); receiving solution, HNO3 0.1 M () or HClO4 0.1 M (䊉). The fluxes obtained from permeation experiments are expressed as the mean value of transport process for 8 h.

liquid state domains in PIM, similar to liquid inner core of SLM, according to a mechanism of phase transition. The variation of the flux should depend on the occurrence of such transition, itself depending also on other parameters such as the external phase composition, the nature of the metal cations, etc. Second, there is an optimum concentration of carrier (50%, w/w) for D2EHPA and slightly less for Aliquat 336 (34%, w/w) at which a maximum rate of transport is achieved. Above this concentration, the permeate flux decreases for the membrane with D2EHPA. This is probably due to high viscosity in the membrane which limits the diffusivity of the ion-carrier complex in the membrane [36]. It is suggested that the difference in carrier concentration to reach the same flux should be related to differences in extraction ability of the carriers. In the following experiments, D2EHPA and Aliquat 336 contents in PIMs were maintained constant to 50% and 34% (w/w), respectively.

Fig. 4. Cd(II) transport flux vs. pH for D2EHPA/CTA () and Aliquat 336/CTA (䊉) PIMs. Feed solution, Cd(II): 12 mg/L in NaNO3 0.1 M + HNO3 () or in NaCl 0.5 M + HCl (䊉); receiving solution, HNO3 0.1 M () or HClO4 0.1 M (䊉).

4.2.1.2. Concentration profiles versus time. The concentration profiles is shown for Cd(II) concentration of 30 mg/L in the feed solution. The maximal recovery factors of Cd(II) are 97.5% and 91.8% D2EHPA and Aliquat 336, respectively. When D2EHPA is used as carrier, a little amount of cadmium stays in the membrane as shown in Fig. 3(a). The metal amount inside the membrane rises at the beginning to attain a maximum after 2 h of transport, and decreases slowly corresponding to the progress of the metal ions discharge into the receiving phase. This result is comparable with that obtained by Resina et al. [37] who observed a maximum retention (16%) after 1 h of transport and a total discharge after at 6 h, for Cd(II), Cu(II) and Zn(II) transport with D2EHPA as carrier in CTA-polysiloxanes hybrid membrane. In contrast no retention was noticed in transport by Aliquat 336/CTA based membrane (Fig. 3(b)). This result is not in accordance with that obtained by Wang and Shen [28] and who observed a retention of Cd(II) in the membrane experiments of transport using Aliquat 336 as carrier in PVC membrane.

Fig. 3. Concentration profile vs. time: (a) DE2HPA and (b) Aliquat 336. Feed solution, Cd(II): 30 mg/L in NaNO3 0.1 M (a) or in NaCl 0.5 M (b); receiving solution, HNO3 0.1 M (a) or HClO4 0.1 M (b).

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Table 2 Transport recovery (%) and fluxes (Ji ) of M(II) (Cd(II) or Pb(II) or Zn(II)) transported as individual metal species with D2EHPA (50% (w/w)) or Aliquat 336 (34% (w/w)) contents in CTA/PIM from a source solution of [M(II)] = 12 mg/L

Cd(II) Pb(II) Zn(II)

Transport recovery (%)

Ji (␮mol m2 s−1 )

D2EHPA

Aliquat 336

D2EHPA

Aliquat 336

97.65 96.88 98.01

95.84 92.79 81.96

1.45 1.08 0.94

1.54 1.10 0.55

Source phase: NaNO3 0.1 M (D2EHPA) or in NaCl 0.5 M (Aliquat 336). Receiving phase: HNO3 0.1 M (D2EHPA) or HClO4 0.1 M (Aliquat 336).

Fig. 5. Variation of the transported Cd flux vs. the Cd concentration in the feed for D2EHPA/CTA () and Aliquat 336/CTA (䊉) PIMs. Feed solution, Cd(II) in NaNO3 0.1 M () or in NaCl 0.5 M (䊉); receiving solution, HNO3 0.1 M () or HClO4 0.1 M (䊉). The fluxes have been normalized to mole number of carriers and thickness d of the membrane (PIM with Aliquat 336, d = 84 ␮m and PIM with D2EHPA d = 120 ␮m).

4.2.2. Influence of the aqueous phase 4.2.2.1. Effect of the pH of the source phase. The pH of the receiving phase being maintained constant at pH 1, the pH of the source phase has been varied from 3 to 7.5. As is shown in Fig. 4, the flux is highest at pH 4.5 and 7.5 for D2EHPA and Aliquat 336/PIM transport, respectively. For D2EHPA, when the pH of the source phase is increased the transport flux tends to decrease. This result is unexpected; as classically when the driving force is increased the flux should be increased too. This phenomenon was attributed to the formation of hydroxide forms of the metal less extractable with D2EHPA [38]. Salazar-Alvarez et al. proposed another explanation based on the formation of emulsion, which depends both on pH, and concentration of the carrier [39]. To our point of view an explanation could rely on the proposed association mechanism between the metal cation and the D2EHPA (see Eq. (3)), this mechanism involves the implication of a monomeric (LH) and a dimeric ((LH)2 ) forms of D2EHPA to form the complex ((ML2 (LH)2 ). The existence of ((LH)2 is more probable in a range of pHs lower than the pKa of D2EHPA, while the formation of the ML2 (needing HL form) species predominates in the opposite range. Thus an existence of an overlap between both species should lie in between these two pH domains, where the associated form (ML2 )(LH)2 predominates. From the data shown in Fig. 4, the pH of max-

imum formation of these latter species should be around pH 4.5. For the Aliquat 336/PIM based transport, the highest fluxes are obtained at neutral pH. This may be explained by the speciation of the metal in the presence of chloride ions whose chloride forms such as [MCl3 ]a − are more stable at neutral and higher pHs favoring the complexation with the cationic Aliquat 336. No degradation of the CTA was observed when membranes were in contact with oxidizing acids HNO3 and HClO4 , commonly used in transport experiments. 4.2.2.2. Effect of Cd(II) concentration. Fig. 5 shows the effect of the initial cadmium in the feed solution on the permeate flux, the carrier concentrations in the PIMs being 53% (w/w) for D2EHPA and 34% for the Aliquat 336, respectively. The fluxes were normalized to the mole number of carriers and the thickness of the membrane. For both carriers, the permeate flux increases with increasing cadmium concentrations. PIMs with Aliquat 336 are more efficient as the permeate flux of Cd(II) is higher. 4.3. Selectivity study (efficiency of metal recovery) 4.3.1. Case of single metal cations We studied the individual transport of two other metal ions Pb(II) and Zn(II) by D2EHPA and Aliquat 336 under the optimum conditions for transport of Cd(II). The results of transport recovery and fluxes are assembled in Table 2. For both carriers, lower fluxes are obtained for Pb(II) and Zn(II) recovery compared to Cd(II) recovery. 4.3.2. Case of competitive transport (cations mixture) Selectivity is an important issue in the implementation of PIMs for many reasons. For instance, in environmental appli-

Table 3 Transport recovery (%), fluxes (Ji ) of cations and selectivity coefficient in competitive metal transport with D2EHPA (50% (w/w)) or Aliquat 336 (34% (w/w)) contents in CTA/PIM from a source solution of [M(II)] = 12 mg/L Ji (␮mol m2 s−1 )

Transport recovery (%)

Cd(II) Pb(II) Zn(II)

S = Ji (Cd)/Ji (M)

D2EHPA

Aliquat 336

D2EHPA

Aliquat 336

86.56 84.77 91.02

85.99 89.4 56.34

1.26 1.34 1.27

1.31 1.19 0.58

D2EHPA 0.94 0.99

Source phase: NaNO3 0.1 M (D2EHPA) or in NaCl 0.5 M (Aliquat 336). Receiving phase: HNO3 0.1 M (D2EHPA) or HClO4 0.1 M (Aliquat 336).

Aliquat 336 1.10 1.72

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stituents of the membrane, which will be the subject of further investigations. The comparison of the transport properties of Cd(II) by two carriers in PIMs based on different association mechanisms, has shown differences in the transport efficiency. Aliquat 336 that coordinates with anionic metal type is shown to be the most efficient. The selectivity of the transport of Cd(II) over Pb(II) and Zn(II) in mixture of these species is also in favor of Aliquat 336 as compared to D2EHPA. Accumulation in PIMs of Cd(II), Pb(II) and Zn(II) was observed in the membranes containing D2EHPA as carrier, in the opposite no accumulation had been noted in the membranes containing Aliquat 336. Acknowledgement Fig. 6. Cd(II) initial flux vs. number of replicate measurements, each one lasting for 8 h, repeated 24 h working cycles for PIM containing 50%, w/w of D2EHPA.

cations, the concentration of the target metal ions can be quite low and reasonably high selectivity is essential for an effective treatment. The selectivity of the PIMs was investigated by determining the selectivity coefficient, S, defined as the ratio between the initial flux of (Cd(II)) and the initial flux of competitive ions (Pb(II) or Zn(II)) in the solution mixture (Table 3). The results show that D2EHPA is able to extract all the studied cations without any marked selectivity, while Aliquat 336 shows an improved selectivity for Cd(II) and Pb(II) as compared to Zn(II). Kozlowska et al. [40] have obtained S(Ji (Cd)/Ji (Zn)) = 0.063 and S(Ji (Cd)/Ji (Pb)) = 0.11, with D2EPA/CTA PIM, the difference with our results is presumably due to differences in the membrane constituents. 4.4. Stability of the Cd transport flux The most important advantage of PIMs other SLMs is their durability. The reproducibility of Cd(II) transport was investigated with the PIMs with Aliquat 336 and D2EHPA. In a set of transport experiments, the same membrane was involved in 12 replicate measurements, each one lasting for 8 h repeated every 24 h. At each cycle the feed and the stripping phases were renewed. The flux varies slightly and no signs of structural weakening were observed for both PIMs. As an example, the resulting stability of the PIM with D2EHPA is shown in Fig. 6. This result corroborates the high stability of PIMs [20,41,42]. 5. Conclusion PIM systems containing 2-NPOE as plasticizer, and D2EHPA cation or Aliquat 336 anion as carriers, were studied for Cd(II), Zn(II) and Pb(II) transport from nitrate and chloride media. The maximal Cd(II) recovery factors obtained in 8 h are 97.5% and 91.8% with D2EHPA and Aliquat 336, respectively. The characterization of membranes presumes, particularly looking at the thermogram of D2EHPA/CTA based membrane, the existence of peculiar interactions between the different con-

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