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Jul 17, 2007 - A plasticized cellulose triacetate (CTA) membrane consisting of N-6-(t-dodecylamido)-2-pyridine carboxylic acid (t-DAPA) as a new carri...
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Ind. Eng. Chem. Res. 2007, 46, 5715-5722

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Extraction Equilibrium and Membrane Transport of Copper(II) with New N-6-(t-Dodecylamido)-2-Pyridinecarboxylic Acid in Polymer Inclusion Membrane Tsutomu Tasaki, Tatsuya Oshima, and Yoshinari Baba* Department of Applied Chemistry, Faculty of Engineering, UniVersity of Miyazaki, 1-1 Gakuen-Kibanadai Nishi, Miyazaki, 889-2192, Japan

A plasticized cellulose triacetate (CTA) membrane consisting of N-6-(t-dodecylamido)-2-pyridine carboxylic acid (t-DAPA) as a new carrier to facilitate membrane transport of copper(II) has been prepared to develop the selective recovery system for copper(II) from other divalent metal ions. First of all, the solid (CTA)liquid extraction equilibrium of copper(II) was examined to obtain information concerning Cu(II)-t-DAPA complex stoichiometry and its extraction constant in the CTA membrane. Membrane transport studies were performed in a two-compartment cell. The CTA-t-DAPA membrane exhibited uphill transport of copper(II) against the concentration gradient. The influences of the aqueous and membrane components on the permeability of copper(II) were studied to elucidate its transport mechanism. Their results suggest that the transport mechanism consists of a diffusion process through an aqueous diffusion film, a fast interfacial chemical reaction, and diffusion through the membrane itself. The mass-transfer coefficients in the aqueous film phase (ka) and membrane phase (km) were determined based on the diffusion model, using the stoichiometric relationship of the extracted species of copper(II) and the extraction equilibrium constant (Kex), determined independently by solid-liquid extraction. Their mass-transfer coefficients obtained were ka ) 7.8 × 10-6 m/s and km ) 2.0 × 10-9 m/s. Introduction There is general concern to minimize the discharge of hazardous metal ions in liquid effluent streams. Aqueous streams contaminated with heavy-metal ions are frequently encountered in various industrial processes, such as mining, further hydrometallurgical operation, and metal plating.1 Solvent extraction is widely used in the recovery and separation of metal species from effluent streams. However, solvent extraction suffers some disadvantages, such as problems of emulsification and high solvent losses, and a large amount of organic solvent and expensive extractant is required.2 To reduce the amount of extractants and toxic solvent, and also reduce the environmental and economic problems resulting from solvent extraction processes, several new separation techniques (such as membranebased separation) have been proposed over the past decade. Among these, the liquid membrane (LM) process has been widely investigated to develop the potential alternative separation techniques.3-5 This is a highly selective, low-cost, energysaving separation process. The types of liquid membrane systems that have been extensively studied so far are the supported liquid membrane (SLM) and emulsion liquid membrane (ELM) systems. Although LM processes have many advantages over conventional solvent extraction, SLMs show an inherent lack of stability caused by leaching extractants into the aqueous phases, whereas emulsion breakage is the main problem associated with ELMs. The mentioned factors hinder the practical use of liquid membranes for many large-scale applications.6-8 More recently, in an attempt to stabilize the LM process and increase membrane lifetime, a new type of membrane systems commonly called the polymer inclusion membrane (PIM) systemshas been developed.9-12 PIMs are formed by casting * To whom correspondence should be addressed. Tel.: +81-98558-7307. Fax: +81-985-58-7323. E-mail address: [email protected].

a solution containing an extractant, a plasticizer, and a polymer matrix, such as cellulose triacetate (CTA) or poly(vinyl chloride) (PVC). These polymers are used to form the gel-like membranes that are able to entrap carrier molecules, increasing the viscosity and inhibiting their leaching into aqueous solutions. It was reported that a transport experiment using PIMs was operated for three months and no sign of flux decline or carrier and plasticizer losses was observed.13,14 Because the effectiveness of PIMs is dependent mainly on the extraction capability of the carrier, the use of an appropriate extractant as a carrier is very important. LIX reagents represent one of the most effective extractants for copper recovery from hydrometallurgical sources. Primarily studies on the application of a PIM containing 2-hydroxy-5-nonylacetophenone (LIX84-I) as a carrier for the treatment of wastewater have been reported.15,16 Generally, LIX reagents strongly extract copper(II) and have a high selectivity for copper(II) over other metal ions, such as nickel(II) and cobalt(II). However, LIX reagents are relatively expensive and have poor physicochemical stability, because of the decomposition of oxime groups for a long operation.17 To solve these problems, we are interested in the design and synthesis of a novel extraction reagent that has potential application in the fields of hydrometallurgy and analytical chemistry.18 In our previous report, it was demonstrated that alkylated pyridinecarboxylic acid derivatives with a pyridine moiety and a carboxylic acid as chelating ligands have a high selectivity for copper(II) over iron(III) and other metal ions in the solvent extraction system.19 All metals tested in our previous study were selectively extracted at a lower pH by 2-3 units, compared with alkyl carboxylic acids such as naphthenic acid and Versatic 10. It was thus of interest in the present work to evaluate alkylated pyridinecarboxylic acid derivatives as a novel carrier to develop a PIM system for a highly selective metal ion transport. In PIM system, we have synthesized N-6-(t-dodecylamide)-2-pyridinecarboxylic acid (t-DAPA) with highly branched alkyl chains in the C12-14 range that can provide a stable membrane. In this study, we developed a new PIM consisting of CTA as a polymer

10.1021/ie061671u CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

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Scheme 1. Synthesis of N-6-(t-Dodecylamido)-2-pyridinecarboxylic Acid (t-DAPA)

matrix, 2-nitrophenyl-n-octylether (NPOE) as a plasticizer, and t-DAPA as a carrier. The extraction abilities for various metal ions and copper(II) transport mechanism with t-DAPA in PIM were reported in this paper. Experimental Section Reagents. Analytical-grade copper(II), zinc(II), nickel(II), and cadmium(II) nitrates (Wako Pure Chemical Industries, Ltd.) were used to prepare the test solutions of the respective metals. Cellulose triacetate (CTA, acetyl content ) 43.6 wt %, Mw ) 72-74 kDa, Aldrich Chemical Co.) was used as the polymer support for the preparation of membranes. 2-Nitrophenyl octylether (NPOE, >99.0% purity, Dojindo Laboratory) was used as a plasticizer. The Primene 81-R (Rohm and Haas Co., Ltd., Philadelphia, PA) was used as one of the raw material for synthesis of the extractant. Other chemicals were analytical reagent (AR) grade and used as received. Synthesis of N-6-(t-Dodecylamido)-2-pyridinecarboxylic Acid (t-DAPA). N-6-(t-Dodecylamido)-2-pyridinecarboxylic acid was synthesized in two steps: (a) the reaction of 2,6pyridinecarboxylic acid ester with a branched primary amines (RNH2) and then (b) hydrolysis, as shown in Scheme 1. In the first step, a mixture of 2,6-pyridinecarboxylic acid ester (0.02 mol), Primene 81-R (0.02 mol), and NaOMe (0.02 mol) in toluene was refluxed for 12 h at 342 K. The reaction mixture was then added to chloroform and neutralized with aqueous sodium bicarbonate solution, followed by washing with 1.0 M HCl for the removal of the unreacted amine. The organic phase was washed with water several times and then dried over anhydrous MgSO4. After decantation, the chloroform was evaporated in vacuo. The crude product was purified by chromatography on silica gel with n-hexane/EtOAc as an eluent to afford a yellow liquid. In the second step, intermediate product (0.024 mol) and 0.5 M aqueous KOH solution were refluxed in tetrahydrofuran (THF) for 3 h at 332 K. The reaction mixture was filtered and the solvent was evaporated in vacuo. The residue was dissolved in chloroform, washed with distilled water several times, and then dried over anhydrous MgSO4. After decantation, the chloroform was evaporated and then the fine product was washed with cold n-hexane and dried in vacuo to afford viscous yellow oil with 70% yield. The identification of this product was performed using Fourier tranform infrared (FT-IR) and 1H nuclear magnetic resonance (NMR) spectra.19 Membrane Preparation. The CTA-NPOE membrane containing t-DAPA was prepared according to the reported procedure.20 The CTA solution was prepared by dissolving an appropriate amount of CTA in chloroform. Another solution in chloroform, containing known amounts of NPOE as a plasticizer and t-DAPA as carrier, was prepared. A casting solution was prepared by mixing these two solutions and homogenizing for 30 min, and then the mixture was cast onto a leveled 8-cmdiameter Petri dish. Chloroform was allowed to evaporate slowly

over 24 h to obtain a polymer membrane with a smooth surface. After the evaporation of chloroform, the obtained membrane was carefully peeled off the dish. The resulted membranes were completely clear and exhibited a good mechanical strength, although they contained a high amount of NPOE and t-DAPA, indicating that the CTA serves as a good supporting matrix for the plasticizer and carrier. The membrane containing NPOE and t-DAPA was identified via FT-IR analysis (model FT-IR 300, Perkin-Elmer, Japan). The membrane thickness was measured via optical microscopy (model LH50A, Olympus, Japan) with a calibrated lens (Carton Optical Industries, Ltd., Japan). Solid-Liquid Extraction with the CTA-NPOE-t-DAPA Membrane. Solid-liquid extraction experiments were performed to obtain additional information concerning the complexformation stoichiometry and its extraction equilibrium constant in the membrane matrix. A 0.3 mM aqueous metal solution containing 0.1 M 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) buffer solution with pH adjusted with HNO3 or NaOH was prepared. 0.2 M NaNO3 was added to the solution to keep a constant ionic strength. Membrane mass used in this experiment was 20 mg. The CTA-NPOE-t-DAPA membrane pieces with the same thickness were immersed into the solution (volume of 15 cm3), and the latter was shaken mechanically by the shaker thermostated at 303 ( 0.1 K in a water bath. After equilibrium, the pH of aqueous solution was measured with pH meter (HM-30S, DKK-TOA Co., Ltd., Japan). Initial and after-equilibrium concentrations of metal ions were determined via atomic absorption spectrometry (AAS, model AAnalyst 100, Perkin-Elmer). The percentage extraction and the amount of adsorption (q, given in units of mol/kg) were calculated according to eqs 1 and 2, respectively.

percent extraction ) q)

[M]0 - [M]eq [M]0

[M]0 - [M]eq ×V w

× 100

(1)

(2)

where [M]0 and [M]eq represent the concentrations of metal ions in the aqueous phase before and after equilibrium, respectively. V is the volume of aqueous solution, and w is the amount of membrane used in the experiments. Transport of Copper(II) with CTA-NPOE-t-DAPA Membrane. Membrane transport studies were conducted in a two-compartment cell that was thermostated at 303 ( 0.1 K in a water bath. Each compartment has a volume capacity of 100 cm3. The effective membrane surface area was 1.25 × 10-3 m2. The membrane was sandwiched between the two compartment cells. The stirring rates in both the feed and receiving solutions were measured with digital tachometer (model HT4100, Ono Sokki Co., Ltd., Japan), and each solution was stirred at a constant rate (120 rpm) throughout each experiment. The initial conditions used were a feed solution of ∼0.1-0.3 mM copper(II) in 0.1 M HEPES buffer with pH adjusted with concentrated HNO3 solution and a receiving solution of 1.0 M HCl. The transport of copper(II) was monitored by taking 1.0 cm3 aliquots from each compartment at preselected time intervals and metal concentration in each solution was determined via AAS. Transport Mechanism of Copper(II) with CTA-NPOEt-DAPA Membrane. Several factors in both the aqueous solution and the membrane solution were investigated to elucidate the mechanism of transport of copper(II) across the PIM using t-DAPA as the carrier. The copper(II) permeability

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Figure 1. Effect of pH on the percentage extraction of metal ions with a PIM containing 25 wt % t-DAPA and 50 wt % NPOE. Initial concentration of metal ions ) 0.3 mM, weight of adsorbent ) 20 mg, [HR] ) 1.5 M.

Figure 2. Effect of pH on the distribution ratio of Cu(II) with the CTANPOE-t-DAPA membrane.

coefficient (P) can be defined by eq 3; it is related to changes in copper(II) ion concentration with time in the feed solution.21

J)ln

V dC ) P[Cu2+]f A dt

[Cu2+]f [Cu2+]0f

A ) - (Pt) V

(3) (4)

Here, V is the volume of the transport cell in each solution, A the effective membrane area, t the time, and J the copper(II) flux. [Cu2+] and [Cu2+]0 indicate the transient and initial concentrations of copper(II) in the feed solution, respectively. P can be used to characterize the PIM transport efficiency. Results and Discussion Extraction Selectivity with the CTA-NPOE-t-DAPA Membrane. Figure 1 shows the effect of pH on the percentage extraction of the metal ions with the CTA-NPOE-t-DAPA membrane. As seen from Figure 1, the CTA-NPOE-t-DAPA membrane selectively adsorbed copper(II) over zinc(II), nickel(II), and cadmium(II) at low pH, indicating its very high complexation ability for copper(II). The order of the selectivity with the CTA-NPOE-t-DAPA membrane was

Figure 3. Effect of dimer concentration of the extractant on the distribution ratio of Cu(II) with the CTA-NPOE-t-DAPA membrane.

organic phase.19 That is, assuming that the solubility of the extractant and the metal complex in the aqueous phase are negligible and all extractant molecules are present as dimers in the organic phase. In the solid-liquid experiments, a distribution coefficient (D) was defined as

D)

[Cu(II)]T,m [Cu(II)]T,a

Cu(II) . Zn(II) ≈ Ni(II) > Cd(II) The extraction selectivity of metal ions with the CTA-NPOEt-DAPA membrane in solid-liquid extraction was the same as that of t-DAPA in liquid-liquid extraction in our previous report,19 showing that the membrane extraction is dependent on the extraction abilities of the carrier with metal ions. The selectivity of t-DAPA can be attributed to the high affinity of carboxylic group and pyridine moiety with divalent metal ions, such as copper(II), zinc(II), and nickel(II). Moreover, the hypothesized selectivity of the t-DAPA toward copper(II) can be explained by the formation of a neutral complex in the membrane phase, where metal should be present in a tetracoordinated geometry. It is known that copper(II) can form these complexes, whereas other metal ions, such as zinc(II) and nickel(II), preferably form hexacoordinated complexes.22,23 Extraction Equilibrium of Copper(II) with the CTANPOE-t-DAPA Membrane. In work on the stoichiometric relation for the extraction of copper(II) complex with t-DAPA, we previously proposed an extraction equation between the metal complex in the aqueous phase and the extractant in the

where the subscripts “T”, “m”, and “a” denote the total concentration, membrane, and aqueous phase, respectively. As for t-DAPA molar total concentration expression in the membrane phase, the weighted quantity of t-DAPA was considered to be dissolved in the mixture of volumes of t-DAPA and NPOE. From the dependences of the distribution ratio on pH and the concentration of dimer extractant in Figures 2 and 3, the extraction equilibrium of copper(II) with t-DAPA (HR) in the membrane can be expressed as follows:

Cu2+ + 2(HR)2 a CuR22HR + 2H+ Kex )

[CuR22HR][H+]2 [(HR)2]2[Cu2+]

(5) (6)

where the overbar denotes a species in the membrane phase and Kex is the extraction equilibrium constant. The mass balance equation for (HR)2 is given by the following expression:

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[(HR)2]T ) [(HR)2] + 2[CuR22HR]

(7)

D can be expressed, according to the equation previously defined, as

D)

[CuR22HR]T,m [Cu]T,a

(8)

Hence, by combining eqs 6 and 8, the following equation was obtained in its logarithmic form:

log D ) 2 log[(HR)2] + 2pH + log Kex

(9)

To determine Kex, the experimental results were plotted in Figure 4, according to eq 9. The Kex value was obtained from the intercept of the straight line with the ordinate in Figure 4. The value of Kex was determined to be 2.90. The theoretical line based on eq 9 was fitted in Figure 4 and in good agreement with the experimental results. The complex stoichiometry of membrane extraction, CuR22HR is the same as that of classical liquid-liquid extraction in our previous study.19 The Kex value of the CTA-NPOE-t-DAPA membrane in this study was very high, compared to the extraction constant value obtained in the liquid-liquid extraction (Kex ) 0.31). The differences is probably due to the fact that a different medium was used for t-DAPA in membrane extraction and solvent extraction.24 Copper(II) Adsorption Capacity of the CTA-NPOE-tDAPA Membrane. The adsorption isotherm of copper(II) with CTA-NPOE-t-DAPA membrane is shown in Figure 5. The adsorption amount of copper(II) increases with increasing copper(II) concentration and reaches a constant value that is the maximum adsorption capacity (qmax) of the CTA-NPOEt-DAPA membrane: 0.39 mol/kg. The value is comparable to that previously evaluated in the extraction study of lead(II) using commercial resin impregnated calix[4]arene tetracarboxylic acid.25 The maximum adsorption capacity of PIM seems to be insufficient for industrial use. However, the fast adsorption rate, negligible leakage of extractant, and effective separation as a membrane are noteworthy in the fields of analytical chemistry and hydrometallurgy. Active Transport of Copper(II) with CTA-NPOE-t-DAPA Membrane. The permeation of copper(II) through the CTA membrane was measured using a two-compartment glass cell.26,27 Figure 6 shows one example of the transport results for copper(II) with the CTA-NPOE-t-DAPA membrane. It can be observed that the complete transfer of copper(II) from the feed to the receiving solution occurs within 24 h. The rapid transport of copper(II) is a promising result. Note that the driving force for the active transport is the pH gradient between the feed and receiving solutions, as we expected from eq 5. Effect of pH in the Feed Solution on the Transport of Copper(II). To calculate P values, the plot of ln[Cu2+]f/ [Cu2+]0f versus time was performed according to eq 4 and shown in Figure 7. Figure 8 shows the effect of pH in the feed solution on the permeability coefficients of copper(II), calculated from the slopes of the linear parts of the plot in Figure 7. As shown in Figure 8, the permeability coefficient increased with an increase in pH region from 1.0 to 2.0, although at higher pH, it remained unaffected. This indicates that the diffusion of a metal complex through the membrane is the rate-determining step at low pH, whereas the diffusion of a metal cation across the aqueous boundary layer is the rate-determining step at high pH values. The finding of this study is similar to that previously observed with a PIM membrane study of lead(II) from nitrate

Figure 4. Plot of log([(HR)2]/[H+]) versus the distribution ratio of Cu(II) with the CTA-NPOE-t-DAPA membrane. Solid line is the theoretical line based on eq 9.

Figure 5. Adsorption isotherm on the CTA-NPOE-t-DAPA membrane. Weight of adsorbent ) 20 mg, [HR] ) 1.5 M, initial pHeq ) 1.75.

Figure 6. Transient concentration of Cu(II) in (b) the feed solution and (0) the receiving solution during a transport experiment across a PIM containing 25 wt % t-DAPA and 50 wt % NPOE. Initial concentration of Cu(II) ) 0.3 mM, [HR] ) 1.5 M, initial pHfeed ) 3.20.

solution using di(2-ethylhexyl) phosphoric acid (D2EHPA).28 Judging from the experimental result of Figure 1, it is considered that copper(II) can be selectively transported in the presence of other metal ions in the feed solution, under optimum pH values of 2.0. Effect of Carrier Concentration on the Transport of Copper(II). A PIM without a carrier results in no transport of copper(II) from the feed to the receiving solutions. The effect of carrier concentration on the transport of copper(II) from feed

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Figure 7. Effect of pH on the facilitated transport of Cu(II) across a PIM. Initial concentration of Cu(II) ) 0.1 mM, [HR] ) 1.5 M.

Figure 8. Cu(II) permeability as a function of pH in the feed solution. Solid line is the theoretical curve based on eq 16.

Figure 9. Cu(II) permeability as a function of the t-DAPA dimer concentration. Initial concentration of Cu(II) ) 0.1 mM, pHfeed ) 2.80. Solid line is the theoretical curve based on eq 16.

solution containing 0.1 mM copper(II) at pH 2.80 to the receiving solution of 1.0 M HCl is shown in Figure 9. The concentration of t-DAPA was changed in the range of 0.505.88 M in the membrane. It was observed that permeability coefficient increases with an increase in carrier concentration. However, further increases in carrier concentration do not enhance the transport of copper(II). This is due to the fact that, in the low carrier concentrations, the diffusion of metal complex through the membrane is the rate-determining step, whereas, at higher carrier concentrations, the diffusion of metal cations across the aqueous boundary layer is the rate-determining step.29

Figure 10. Concentration profile of the species across PIM containing t-DAPA as the carrier.

Figure 11. Graphical determination of mass-transfer coefficients in the aqueous solution and in the membrane phase.

In previous papers,30-32 it was reported that, in a LM system, metal complexes themselves diffuse across the membrane, whereas in a PIM system, metal complexes jump or hop from a molecular site to another. The PIM system has shown different behavior, compared to the conventional liquid membranes, even if the transport is ensured by the same carrier.33 Although the PIM system is generally considered to have lower fluxes than those of the SLM system, the maximum flux obtained in this study was calculated to be 7.8 × 10-7 mol m-2 s-1, which is comparable to those reported earlier for the SLM system.34,35 This suggests that the affinity between the extractant and the matrix, because of their physicochemical properties, is very important to form stable membranes, which actively transport ions and work for a long time. Modeling the Transport of Copper(II) across the PIM. From the experimental results, the permeation model was quantitatively developed based on the steady-state transport model and the following simplifying assumptions.36-39 (i) Chemical reaction of copper(II) and t-DAPA instantaneously occur at both aqueous/membrane interfaces. (ii) Mass transport within the membrane is described by Fick’s first law, and the concentration gradients for the metal species across the aqueous boundary layer are linear. (iii) Metal concentration at the feed interface is negligible, compared to the carrier concentration in the membrane, thus resulting in the carrier concentration at the feed/membrane interface that is identical to the carrier concentration within the membrane. (iv) Metal carrier complexes concentrations at the membrane/ receiving interface are negligible, because the distribution coefficient at this interface is very low.

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(v) Both the feed and receiving phases are ideally mixed. Figure 10 shows a possible transport scheme for copper(II) across the PIM. The extraction process at the feed/membrane interface can be represented by the following reaction:

Cuf,i2+ + 2(HR)2 a CuR22HRf,i + 2H+

(10)

with

Kex )

[CuR22HR]f,i[H+]2 [(HR)2]2[Cu2+]f,i

(11) D hm )

Hence, the flux of copper(II) across the membrane can be described by applying Fick’s first law to the diffusion of metal across the different interfaces. The copper(II) flux at feed/ membrane interface (Ja) is defined as

Ja ) ka([Cu2+]f - [Cu2+]f,i)

(13)

where ka and km represent the mass-transfer coefficient on the aqueous solution side and in the membrane phase, respectively. Because the copper(II) concentration at the membrane/ receiving interface is very low, compared to that at the feed/ membrane interface, Jm was simplified to

Jm ) km[CuR22HR]f,i

(14)

Under the steady-state condition (J ) Ja ) Jm), by combining eqs 12 and 14, J could be obtained as

J)

kakmKex[(HR)2]2[Cu2+]f Kexkm[(HR )2]2 + ka[H+]2

(15)

The copper(II) permeability coefficient, P, can be expressed from eq 3 as follows:

P)

kakmKex[(HR)2]2 Kexkm[(HR )2]2 + ka[H+]2

(16)

From eq 16, two limiting cases can be derived:

kmKex[(HR)2]2[Cu2+]f

(a) at low pH:

J)

(b) at high pH:

J ) ka[Cu2+]f

[H+]2

This indicates that the flux is independent of the pH in the feed solution in the high-pH region. The mass-transfer coefficients in the aqueous solution and the membrane phase were obtained by plotting 1/P versus [H+]2/ Kex[(HR)2]2, according to eq 16. The extraction constant and complex stoichiometry were determined in the preliminary solid-liquid extraction experiments. The values

(θτ)D

m

(17)

The membrane porosity (θ) was calculated as the volume fraction of the plasticizer (θ ) 0.50). The membrane tortuosity (τ) was determined using the Wakao-Smith model and is expressed as the inverse of the membrane porosity:37,42

(12)

The flux of copper complexes in the membrane phase (Jm) is defined as

Jm ) km([CuR22HR]f,i - [CuR22HR]r,i)

of ka ) 7.8 × 10-6 m/s and km ) 2.0 × 10-9 m/s were obtained from the slope and intercept of the plots shown in Figure 11. The theoretical curves based on eq 16 have been fitted in Figures 8 and 9 and are in good agreement with the experimental data. The apparent membrane diffusion coefficient (Dm) was calculated from km ) Dm/δ, using the thickness of the membrane (δ ) 60.2 × 10-6 m). The true membrane diffusion coefficient (D h m), which considers morphological features inside the membrane, was calculated from the following relationship:40,41

τ ) 1 - ln θ

(18)

The value of D h m was calculated to be 4.08 × 10-13 m2/s. The mass-transfer coefficient and membrane diffusion coefficient determined from this study were observed to be similar to those previously reported in copper(II) transport studies that used Acorga M5640 and lauric acid as a carrier.24,29 Conclusions It was found out that N-6-(t-dodecylamide)-2-pyridinecarboxylic acid (t-DAPA) synthesized in this study as a carrier was very suitable to be incorporated into cellulose triacetate (CTA) for making a polymer inclusion membrane (PIM). To examine the extraction ability of t-DAPA for metal ions, solidliquid extraction was tested. It was observed that the extraction order for divalent metal ions from aqueous solution was Cu(II) . Zn(II) ≈ Ni(II) > Cd(II). Copper(II) was selectively extracted at low pH as complexes, according to the following reaction: Cu2+ + 2(HR)2 ) CuR22HR + 2H+ (where HR represents the extractant). The extraction equilibrium constant, Kex ) 2.90, was obtained at 303 K. The extraction selectivity order for metal ions and stoichiometric relationship in the membrane extraction were determined to be the same as those in solvent extraction. The transport experiments were performed to examine the ability of the newly fabricated membranes, which were made using t-DAPA as a carrier. It was observed that the PIM used in this study was very effective, in regard to actively transporting copper(II) from the feed solution to the receiving solution. The permeation transport of copper(II) with the t-DAPA/CTA membrane was reasonably explained by considering that the diffusion of metal complex across the liquid membrane or the diffusion of metal ion across the aqueous boundary layer is the rate-determining step. The mass-transfer coefficients in the aqueous boundary layer and the membrane phase were determined based on the transport model proposed in this study. Nomenclature A ) membrane area (m2) δ ) thickness (m) θ ) porosity (%) τ ) tortuosity J ) flux (mol m2/s)

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P ) permeability coefficient (m/s) ka ) mass-transfer coefficient for the aqueous feed boundary layer (m/s) km ) mass-transfer coefficient for the membrane phase (m/s) Dm ) membrane diffusion coefficient (m2/s) Kex ) extraction equilibrium constant D ) copper distribution coefficient q ) amount of adsorption (mol/kg) w ) amount of membrane (kg) V ) volume (m3) t ) time (s) [x] ) concentration of species x (mol dm-3) HR ) extractant M ) metal Subscripts a ) aqueous phase f ) feed phase r ) receiving phase m ) membrane phase T ) total concentration i ) interface Superscripts 0 ) refers to the concentration at time zero e ) equilibrium Literature Cited (1) Sengupta, B.; Sengupta, R.; Subrahmanyam, N. Copper extraction into emulsion liquid membrane using LIX984-C. Hydrometallurgy 2006, 81, 67. (2) Wang, L.; Paimin, R.; Cattrall, R. W.; Shen, W.; Kolev, S. D. The extraction of cadmium(II) and copper(II) from hydrochloric acid solutions using as Aliquat 336/PVC membrane. J. Membr. Sci. 2000, 176, 105. (3) Izatt, R. M.; Lamb, J. D.; Bruening, R. L. Comparison of bulk, emulsion, thin supported liquid, and hollow fiber supported liquid membranes in macrocycle-mediated cation separations. Sep. Sci. Technol. 1988, 23, 1645. (4) Draxler, J.; Marr, R. Emulsion liquid membrane part I: phenomenon and industrial application. Chem. Eng. Process. 1986, 20, 319. (5) Parthasarathy, N.; Buffle, J. Capabilities of supported liquid membranes for the metal speciation in neutral water: application to copper speciation. Anal. Chim. Acta 1994, 284, 649. (6) Dreher, T. M.; Steven, G. W. Instability mechanisms of supported liquid membranes. Sep. Sci. Technol. 1998, 33, 835. (7) Gyves, J. D.; Miguel, E. R. D. S. Metal ion separations by supported liquid membranes. Ind. Eng. Chem. Res. 1999, 38, 2182. (8) Kulkarni, P. S.; Tiwari, K. K.; Mahajani, V. V. Membrane stability and enrichment of nickel in the liquid emulsion membrane process. J. Chem. Technol. Biotechnol. 2000, 75, 553. (9) Lamb, J. D.; Nazarenko, A. Y.; Uenshi, J. C. Silver(I) ion-selective transport across polymer inclusion membranes containing new pyridineand bipyridino-podands. Anal. Chim. Acta 1998, 373, 167. (10) Kusumocahyo, S. P.; Kanamori, T.; Sumaru, K.; Aomatsu, S.; Matsuyama, H.; Teramoto, M.; Shinbo, T. Development of polymer inclusion membranes based on cellulose triacetate: carrier-mediated transport of cerium(III). J. Membr. Sci. 2004, 244, 251. (11) Kolev, S. D.; Argiropoulos, G.; Cattrall, R. W.; Hamilton, I. C.; Paimin, R. Mathmatical modeling of membrane extraction of gold(III) from hydrochloric acid solutions. J. Membr. Sci. 1997, 137, 261. (12) Baba, Y.; Hoaki, K.; Perera, J. M.; Cattrall, R. W.; Kolev, S. D. Separation of palladium(II) from copper(II) acid solutions using PVC membranes containing D2EHPA. Dokl. Bulk Akad. Nauk. 2001, 54, 69. (13) Schow, A. J.; Peterson, R. T.; Lamb, J. D. Polymer inclusion membranes containing macrocyclic carriers for use in cation separations. J. Membr. Sci. 1996, 111, 291. (14) Tayer, R.; Fontas, C.; Dhahbi, M.; Tingry, S.; Seta, P. Cd(II) transport across supported liquid membranes (SLM) and polymer plasticized membranes (PPM) mediated by Lasalocid A. Sep. Purif. Technol. 2005, 42 (2), 189.

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ReceiVed for reView December 25, 2006 ReVised manuscript receiVed April 2, 2007 Accepted June 6, 2007 IE061671U