Application of Cr(VI) Transport through the Polymer Inclusion

Mar 22, 2013 - Facilitated transport of Cr(VI) ions from aqueous chromate donor phase (2.10–4 kmol/m3) through polymer inclusion membranes (PIMs) us...
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Application of Cr(VI) Transport through the Polymer Inclusion Membrane with a New Synthesized Calix[4]arene Derivative Ahmet Kaya,† Hamza Korkmaz Alpoguz,*,† and Aydan Yilmaz‡ †

Department of Chemistry, Pamukkale University, 20070 Campus, Denizli, Turkey Department of Chemistry, Selcuk University, 42031 Campus, Konya, Turkey

‡

ABSTRACT: Facilitated transport of Cr(VI) ions from aqueous chromate donor phase (2.10−4 kmol/m3) through polymer inclusion membranes (PIMs) using a new synthesized 5,17-di-tert-butyl-11-piperidinomethyl-25,26,27,28-tetrahydroxycalix[4]arene (carrier), as ion carrier, has been investigated. The Cr(VI) passed through a PIM comprised of cellulose triacetate (CTA) as a support and 2-NPOE as a plasticizer. The prepared PIM was characterized with Fourier transform infrared (FT-IR) spectroscopy and the atomic force microscopy (AFM) techniques as well as with contact angle measurements. The efficiency of Cr(VI) transport through the PIM was investigated by studying the effects of carrier concentration on the donor phase as well as by measuring the amount of plasticizer in the membrane, the pH in the acceptor phase, the effect of acid type in donor phase, and the membrane’s stability and thickness. The kinetic parameters were calculated as permeability coefficient (P), flux (J), and diffusion coefficient (D). The transport efficiency of Cr(VI) was observed to be 99.38% after 6 h under optimized conditions. These results indicated that PIMs can be used in the long term for the removal of Cr(VI) from industrial waste waters.

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zinc(II) and rare earth cations.19,20 Transport of strontium(II), lead(II),21 copper(II),22 and chromium(VI)23 has been investigated. Walkowiak et al. have studied the competitive transport of zinc(II), cadmium(II), chromium(III), and chromium(VI) across PIMs using tertiary amine-based carriers.24,25 In 2005 Kozlowski and Walkowiak reported the use of polymer inclusion membranes (PIMs) for carrier-mediated transport of Cr(VI) from an aqueous donor phase into an aqueous acceptor phase.15 The present work deals with the transport of chromium(VI) from dichromate aqueous solutions through PIMs that contain CTA as a support, o-nitrophenyl octyl ether (2-NPOE) as a plasticizer. and calix[4]arene-based polymeric resin as an ion carrier. We tested several parameters, such as the concentration of the carrier, the effect of pH in the acceptor phase, and the thickness of the membrane. We characterized the membrane with AFM, FT-IR, and contact angles to obtain information regarding its composition.

INTRODUCTION Chromium is a ubiquitous contaminant of soils, and groundwater and is derived from both natural and anthropogenic sources.1,2 The chromium compounds are essentially used in many industries such as metallurgy, electroplating, leather tanning, pigments, stainless steel production, photography, and textiles.3 In the environment, chromium is commonly found in the two most stable oxidation states as Cr(III) and Cr(VI). Although Cr(III) is less toxic than Cr(VI), its discharge is still regulated, and many environmental authorities make no distinction between Cr(III) and Cr(VI).4,5 The high toxicity and carcinogenicity of Cr(VI) make this compound one of the most alarming and urgent metals that must be controlled. Cr(VI) is one of the major toxic elements present in environmental samples. A number of technologies, i.e., liquid−liquid extraction,6,7 adsorption,8−11 and membrane separation,12,13 are available for the separation of Cr(VI).3 To address such a pervasive problem, researchers have evaluated extensively the applicability of liquid membranes for Cr(VI) removal from aqueous solutions.14 In recent years, a remarkable increase of the applications of liquid membranes in separation processes is observed. These membranes include bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs), supported liquid membranes (SLMs), and polymer inclusion membrane (PIMs).15 A novel type of liquid membrane, called the polymer inclusion membrane (PIM), has been developed to provide metal ion transport with high selectivity as well as easy set up and operation. PIMs are formed by casting a solution containing an extractant, a plasticizer (mostly o-nitrophenyl alkyl ether), and a base polymer such as cellulose triacetate (CTA) or poly(vinyl chloride) (PVC) to form a thin, flexible, and stable film.16−18 A number of researchers used CTA membranes for carrier-mediated transport of metal ions from an aqueous donor phase into an aqueous acceptor phase. Sugiura et al. have studied the transport of © 2013 American Chemical Society

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EXPERIMENTAL SECTION Materials and Chemicals. Organic compounds, such as CTA (Mn = 72.000−74.000), 2-NPOE, and dichloromethane, were purchased from Fluka and used without further purification. Inorganic compounds, such as potassium dichromate, hydrochloric acid, sulfuric acid, nitric acid, ammonium acetate, and acetic acid, were purchased from Merck and used without further purification. Synthesis of Carrier. The starting material 4-tert-butylcalix[4]arene (a) was prepared according to the previously reported method.26 The compounds b, c, and d were reported previously,27,28 while the compound e is reported in the present Received: Revised: Accepted: Published: 5428

November 26, 2012 March 19, 2013 March 22, 2013 March 22, 2013 dx.doi.org/10.1021/ie303257w | Ind. Eng. Chem. Res. 2013, 52, 5428−5436

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Figure 1. Schematic representation showing synthesis pathway of calix[4]arene Mannich bases 5, (i) benzoyl chloride, K2CO3, MeCN; (ii) AlCl3, toluene; (iii) EtOH/H2O, NaOH; (iv) piperidine, THF/DMF, AcOH, HCHO.

128.48, 128.07, 127.62, 127.41, 125.95, 125.78, 122.27, 63.32, 54.44, 34.07, 32.14, 32.21, 31.48, 25.82, 24.29. FAB-MS m/z: 656.69 (M + Na)+. Anal. Calcd for C42H51NO4 (633.86): C, 79.58; H, 8.11; N, 2.21. Found: C, 79.83; H, 7.95; N, 2.13. Preparation of the Polymer Inclusion Membrane. We prepared a solution comprised of a support (CTA), an ion carrier (calix[4]arene-based polymeric resin), and a plasticizer (2NPOE) in dichloromethane. A portion of this solution was poured into a membrane mold comprised of a 9.08-cm glass ring adhered to a glass plate with CTA−dichloromethane glue. The organic solvent was allowed to evaporate overnight, and the resulting membrane was separated from the glass plate by immersion in cold water.29,15 Next, the prepared membrane was placed in a permeation cell. Characterization of PIM. In order to characterize the PIM, we used FT-IR and atomic force microscopy (AFM) techniques as well as contact angle measurements. The FT-IR spectra were acquired using a FT-IR spectrometer (Perkin-Elmer 1600 Series). Measurements were taken in wavenumber range from 400 to 4000 cm−1. AFM images were obtained by using a Veeco diCaliber instrument. The speed of the scan was 2 kHz. We chose to use the contact mode of AFM in air to investigate the membrane surface morphology. The sessile drop method was used to measure the contact angle of the prepared membranes. A 4 μL droplet of distilled water was placed on the membrane surface by means of a 0.10mL syringe, and the contact angle was measured with a horizontal beam comparator (KSV CAM 200).4,5,14

study, via Mannich reaction from d in THF, DMF, acetic acid with piperidine as secondary amine, and formaldehyde. The target compound (e) was synthesized as shown in Figure 1. Characterization of compound e was performed by a combination of 1H NMR, 13C NMR, FABMS, and elemental analysis. The 1H NMR spectrum of compound e showed one triplet (1H) at 6.72 ppm and a multiplet (8H) at range 6.95−7.12 ppm for aromatic protons. The conformation of calix[4]arenes can be interpreted via the signal of the ArCH2Ar methylene protons in 1H NMR spectrum. The ArCH2Ar methylene protons showed two doublets at 3.52 and 4.26 (J = 13.1 Hz). According to the results, compound e is in cone conformation. 5,17-Di-tert-butyl-11-piperidinomethyl-25,26,27,28tetrahydroxycalix[4]arene (e). Added to a solution of d (4.6 mmol) in a mixture of 70 mL of THF/DMF (5:2) were 1.33 mL of acetic acid, piperidine (5.5 mmol), and aqueous formaldehyde (0.16 mL, 37%), and the reaction mixture was stirred for 24 h at room temperature. The solvent was removed in vacuo, and the residue was dissolved in 40 mL of deionized water. After extraction with diethyl ether, the precipitate that formed was filtered, dried in vacuo, and recrystallized from chloroform/ methanol to give pure compound (e). Yield: 62%. Mp: 147 °C. 1 H NMR (CDCl3): δ 1.28 (s, 18H, But), 1.42 (brs, 2H, CH2CH2CH2), 1.57 (p, J = 5.4 Hz, 4H, CH2CH2N), 2.33 (bs, 4H, CH2NCH2), 3.27 (s, 2H, ArCH2N), 3.52 (d, J = 13.1 Hz, 4H, ArCH2Ar), 4.26 (t, J = 13.1 Hz, 4H, ArCH2Ar), 6.72 (t, J = 7.4 Hz, 1H, ArH), 6.95−7.12 (m, 8H, ArH), 8.53(bs, 4H, OH). 13C NMR (CDCl3): δ 148.83, 146.71, 144.52, 129.79, 128.93, 5429

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Transport Studies. As can be seen from Figure 2, transport experiments were carried out in a permeation cell that consisted

Figure 2. Polymer inclusion membrane apparatus for transport of Cr(VI).

Figure 3. ln(C/Ci) vs time for Cr(VI) transport through PIMs with carrier in different concentrations. (Donor phase, 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl; membrane composition, 1.75 mL 2NPOE/1 g CTA, 0.025, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 kmol/m3 carrier; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5).

of two identical cylindrical compartments (half-cell volume 45 mL). An aqueous solution of 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl was used as a donor phase. Then, an acceptor phase was buffered at pH 5 by using a 1 kmol/m3 acetic acid/ ammonium acetate buffer. Equal volumes (45 mL) of the donor and acceptor phase were placed in the respective compartments of the cell that were separated by the prepared PIM. The resulting membrane contained 32.49 wt % CTA, 59.19 wt % 2-NPOE, and 8.32 wt % calix[4]arene derivative. In each experiment, the stirring rates of both phases were equal and kept at a constant rate of 500 rpm throughout the experiment. All experiments were performed at room temperature. Samples were taken periodically with a micropipet from both cells. Cr(VI) concentrations were analyzed with an UV−vis Spectrometer Shimadzu 160A in the donor and the acceptor phases. Cr(VI) concentration was determined in an acidic medium at a wavelength of 540 nm based on the violet complex formation between 1,5-diphenyl-carbazide (DPC) and Cr(VI).30 The permeability coefficient (P) is given in eq 1 according to the mass transfer model described by Danesi.31 dC A = − Pt dt V

RF =

C AP =− t Ci V

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(1)

RESULTS AND DISCUSSION

Effect of Carrier Concentration. The effect of the carrier concentration on Cr(VI) transport was investigated under the following experimental conditions: the donor phase (2 × 10−4 kmol/m3 Cr(VI) solution in 0.75 kmol/m3 HCl) and the acceptor phase (acetic acid/ammonium acetate buffer at pH 5). Cr(VI) transport was investigated under the seven concentrations of 0.025, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 kmol/m3. The results are given in Table 1. The results indicate that the transport of Cr(VI) increases as the carrier concentration increases. However, this effect stops at a concentration of 0.2 kmol/m3. When the carrier concentration increased from 0.2 kmol/m3 to 0.5 kmol/m3, we noted that the transport of Cr(VI) decreased. Therefore, the carrier concentration was maintained at 0.2 kmol/m3 for further experiments. Table 1 shows that the best results for flux (J), permeability (P), and recovery factor (RF) were obtained with a 0.2 kmol/m3 carrier concentration. As one can deduce from Figure 4, the best RF (%) value (99.38%) was obtained for the 0.2 kmol/m3 carrier concentration (Table 1). The best flux value was 2.252 × 10−6 mol/(m2 s) for 0.2 kmol/m3. All other flux values were lower for

(2)

where C is the chromium(VI) concentration (kmol/m3) in the donor phase at a given time, Ci is the initial Cr(VI) concentration in the donor phase, t is the transport time (s), V is the volume of the aqueous in the donor phase, and A is the area of the membrane. To calculate the P values, we prepared a plot of ln(C/Ci) versus time. Examples of such plots are given in Figure 3. The relationship of ln(C/Ci) versus time was linear, which was confirmed by the high values of the determination coefficient (r2) 0.9538−0.9996. The initial flux (Ji) was determined as equal to eq 3. Ji = PCi

(4)

Liquid−Liquid Extraction. The chromate extraction experiments by the calix[4]arene derivative were performed following Pedersen’s procedure.32 An aqueous solution of potassium salt of anion (10 mL of a 2.0 × 10−4 kmol/m3; in 0.75 kmol/m3 HCl) and calix[4]arene ligand (10 mL of 1.0 × 10−4 kmol/m3) in dichloromethane was shaken vigorously in a stoppered glass tube with a mechanical shaker for 2 min and then magnetically stirred in a thermostatted water bath at 25 °C for 1 h, and finally left standing for an additional 30 min.33 The concentration of Cr(VI) in liquid−liquid extraction was measured at 540 nm based on the reaction of diphenylcarbazide and Cr(VI) using a UV−vis spectrophotometer.30 Blank experiments showed that no dichromate anions extraction occurred in the absence of calix[4]arene derivatives.

This mass balance can be derived by integration of eq 1. In

Ci − C × 100% Ci

(3)

To describe the efficiency of Cr(VI) removal from the donor phase, we calculated the recovery factor (RF) with eq 4. 5430

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Table 1. Effect of Carrier Concentrations on the Transport of Cr(VI)a carrier concentration (kmol/m3)

P × 10 (m/s)

J × 10 (mol/(m2·s))

D × 10 (m2/s)

RF (%)

0.025 0.05 0.1 0.2 0.3 0.4 0.5

5.714 7.384 7.875 11.260 7.211 4.381 2.418

1.143 1.477 1.575 2.252 1.442 0.876 0.484

2.607 3.233 3.413 4.759 3.178 2.123 1.506

92.07 95.92 96.92 99.38 95.31 86.69 67.46

6

6

10

Kex =

[((LHn +)n , A nn −)]org n [An −]naq [LHn +]org

(5)

Equation 6 is obtained by arranging eq 5 as below. log D = log Kex + n log[LHn +]org

(6)

where D is described as the ratio of the concentration of the anion An− in both phases:

D = [A]org /[A]aq

(7)

As shown in Figure 5, the plot of log D versus log [L] gave a straight line with the slope of 1.06 for Cr(VI) where [L] is the

Donor phase, 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5; membrane composition, 1.75 mL 2-NPOE/1 g CTA, different carrier concentrations. a

Figure 5. log D versus log [L] for the extraction Cr(VI) by calix[4]arene derivative from an aqueous phase into dichloromethane at 25 °C. Figure 4. Effect of carrier concentration on the transport of Cr(VI) through PIMs. Conditions same as in Figure 3.

concentration of the calix[4]arene. It was shown that the stoichiometry of the calix[4]arene derivative with Cr(VI) is 1:1, so the extraction equation of Cr(VI) using calix[4]arene derivative can be represented by the reaction below.

other carrier concentrations, which can be explained by the effect of increasing the Cr(VI)−carrier complex. Increasing this complex results in its extraction into the membrane phase. The transport of the Cr(VI)−carrier complex increases its concentration gradient within the membrane through the membrane thickness. The membrane viscosity also increases with the increase of the carrier concentration. The high viscosity in the membrane thus limits the diffusion of the Cr(VI)−carrier complex into the membrane phase.34 In addition, we performed a blank experiment with no carrier present in the membrane. We found no detectable movement of the Cr(VI) ions in the blank experiment through the PIM, which might suggest that the transport of Cr(VI) ions was fulfilled by the carrier through the PIM. Extraction Mechanism of Cr(VI) with Calix[4]arene Derivative. Liquid−liquid extraction study was carried out to examine the extraction mechanism between Cr(VI) and calix[4]arene derivative. For researching the extraction mechanism by calix[4]arene derivative in dichloromethane, the conventional slope analysis method was selected. The extraction equilibrium of an anion (A) with the anion receptor (L) can be expressed as below.

[LH+]org + [HCrO4 −]aq ⇌ [LH+, HCrO4 −]org

Effect of the Amount of Plasticizer in the Membrane. To investigate the amount of the plasticizer’s effect on transport efficiency, we prepared membranes with a constant carrier, CTA concentrations, and six different amounts of 2-NPOE/1 g CTA (from 1.0 to 2.25 mL). The relations between P, J, and D and the amount of 2-NPOE are given in Figure 6. The transport permeability, fluxes, and diffusion coefficient of carrier were observed to increase from 1.0 to 1.75 mL (2-NPOE/ 1 g CTA) until the limited value was reached. At that point, the transport permeability, fluxes, and diffusion coefficient of carrier decreased. Increasing the amount of 2-NPOE, which is a high dielectric constant, increases the membrane thickness and viscosity, so it begins to decrease after the maximum value. Effect of Acceptor Phase pH. The acceptor phase pH plays an important role in the transport of Cr(VI), because a pH gradient between the donor and acceptor phases is the driving force for the transport of Cr(VI) through the membrane phase. While Cr(VI) is in any medium with a pH from 1 to 6, it takes the form of Cr2O72− ions. At pH > 6, Cr(VI) takes the form of a chromate ion.35 The pH of this study’s donor phase Cr2O72− solution was 1, so the transport of Cr(VI) was investigated at five different pH levels in the acceptor phase. The experimental results indicate that the transport flux increased from pH 4 to pH

n− n(LHn +)org + n A aq ⇌ ((LHn +)n , A nn −)org

The extraction constant Kex is described by eq 5 as below. 5431

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Figure 6. Change of the transport permeability, fluxes, and diffusion coefficient with the amount of 2-NPOE (donor phase, 2 × 10−4 kmol/ m3 K2Cr2O7 in 0.75 kmol/m3 HCl; membrane composition, 1.00−2.25 mL 2-NPOE/1 g CTA, 0.2 kmol/m3 carrier; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5).

Figure 7. Change of the transport permeability, fluxes, and the diffusion coefficient with the type of acid: A1, HNO3; A2, H2SO4; A3, HCl (donor phase, 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 different acid type; membrane composition, 1.75 mL 2-NPOE/1 g CTA, 0.2 kmol/m3 carrier; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5).

5 (Table 2). Then, at pH 5.5 and 6, the transport flux decreased. These results are in full agreement with those of previous Table 2. Effect of Acceptor Phase pH on the Transport of Cr(VI)a pH of acceptor phase

P × 106 (m/s)

J × 106 (mol/(m2·s))

D × 1010 (m2/s)

RF (%)

4 4.5 5 5.5 6

0.956 8.936 11.260 7.751 2.374

0.191 1.787 2.252 1.550 0.475

1.079 3.824 4.759 3.375 1.519

37.23 98.15 99.38 96.46 65.62

When nitrate (NO3−) was present in the medium, Cr(VI) did not form a similar complex like Cr(VI)−H2SO4 and Cr(VI)− HCl. In our study we observed the best flux value in HCl; therefore HCl, was used as the donor phase solution in the following experiments. Stability of PIM. To investigate the transport behavior of the PIMs, we conducted 10 experiments under the same conditions without changing the membrane where the donor and acceptor phases were renewed. The results indicate that the transport efficiency of the PIM is reproducible. The RF values were over 90.00% in the first four cycles of the PIM (each cycle, 6 h; Figure 8). After the 4th cycle, the RF values decreased; at the 13th cycle,

Donor phase, 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl; membrane composition, 1.75 mL 2-NPOE/1 g CTA, 0.2 kmol/m3 carrier; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 4−6. a

studies.35,36 This study thus confirmed that Cr(VI) transport is influenced by the pH of the acceptor phase. Therefore, the pH of the acceptor phase should be higher than the pH of the donor phase for efficient transport of Cr(VI). We used pH 5 in subsequent experiments within the present study. Effect of Acid Type in Donor Phase. To investigate the acid type effect in the donor phase we used three different acids, HCl, HNO3, and H2SO4, under the same conditions (each acid concentration 0.75 kmol/m3). The results are given in Figure 7. As shown in Figure 7, the transport of Cr(VI) ions through the PIM containing HCl in the donor phase is much higher than that of HNO3 and H2SO4, because Cr(VI) ions form different complexes depending on the type of acid in the aqueous phase. The reactions of Cr(VI)−H2SO4 and Cr(VI)−HCl may form mononuclear complexes in high concentrations of bisulfate and chloride.37 Figure 8. Cr(VI) RF vs number of replicate measurements (donor phase, 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl; membrane composition, 1.75 mL 2-NPOE/1 g CTA, 0.2 kmol/m3 carrier; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5).

HCrO4 − + H 2SO4 ⇌ CrO3SO4 −2 + H 2O

HCrO4 − + H+ + Cl− ⇌ CrO3Cl− + H 2O 5432

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obtained relation 1/J vs d, the authors estimated the apparent diffusion coefficient of the Cr(VI)-containing species in the membrane phase (D; Table 3).3,38−41 Surface Characterization. To understand the surface morphology of prepared PIM, we had used FT-IR and AFM techniques and contact angle measurements. Figure 10a−c all show measurements for the FT-IR spectrum of the carrier, CTA + 2-NPOE, and CTA + 2-NPOE + carrier, respectively. The main features of the spectrum in Figure 10a are the absorption bands located around 3171 cm−1, which were attributed to stretching vibrations of O−H groups. Absorption bands were also located around 2952−2866 cm−1, which was attributed to stretching vibrations of aliphatic C−H groups, as well as around 1462−1362 cm−1, which was attributed to stretching vibrations of the C−N group. Figure 10c shows where the absorption band of the O−H groups was deformed. This location indicated that the carrier was banded to CTA + 2-NPOE via O−H groups. Figure 10c also shows that the bands of 1751 cm−1 corresponded to the stretching modes of CO groups. The vibrations indicate the inclusion of CTA + 2-NPOE. A sharp and distinct stretching signal of aliphatic C−H groups (2932 cm−1) was also observed (Figure 10c), which indicates that there was a good connection between carrier and CTA + 2-NPOE. Absorption bands were also located around 3166−3058 cm−1, which was attributed to stretching vibrations of C−H aromatic groups. The vibrations indicate the inclusion of carrier and CTA + 2-NPOE. The AFM pictures of PIM formed with CTA + NPOE (Figure 11a) and CTA + NPOE + carrier (Figure 11b) were taken in a three-dimensional format of 100 μm × 100 μm. The shade density shows the vertical profile of the sample with the lightest regions being the highest points and the darkest regions being the pores (organic inclusion in CTA support). The pores are clearly visible as small, well-defined dark areas. As seen in Figure 11a, the blank membrane is nonporous and only imperceptibly wrinkled due to the varying speed of the solvent vaporization.4,14,15 The inclusion of carrier in CTA support can be distinguished clearly by noting the difference between parts a and b of Figure 11. The mean roughnesses (Ra) of the projected membrane areas for the blank membrane and PIM were 2.722 and 104.301 nm, respectively. This difference can be attributed to the differences between the surface of the blank membrane and PIM due to the inclusion of carrier. Researchers prefer to use contact angle measurements to investigate the hydrophilicity of material surfaces42,43 In this study, contact angle measurements were taken at room temperature for blank membranes and PIM containing 0.2 kmol/m3 of carrier. The angles were found to be 65.41 ± 1° and 83.44 ± 2°, respectively. When a solid surface is hydrophilic, the contact angle will be lower than 90°.43 This phenomenon can be the result of changes in surface morphology and the hydrophilic quality of carrier in the membrane. Another reason that the blank membrane has a lower contact angle than PIM is the inclusion of carrier in CTA support.

the RF value was 38.85%. The decrease of the stability of the membrane may be caused by the partitioning of the carrier between the membrane and the aqueous solution.14 In the first 10 cycles, RF values are over 64.07%; therefore, the prepared PIM seems to be effective for long-term separation processes. Effect of Membrane Thickness. Experiments on Cr(VI) transport across the PIM were carried out with five different thicknesses and varying amounts of CTA. The prepared membranes were 42, 48, 54, 60, and 66 μm thick. The results are given in Table 3 and Figure 9. The flux values decreased as the membrane thickness increased (Figure 9), which can be explained by Fick’s first law. J=D

ΔC ΔX

(8)

Table 3. Effect of Membrane Thickness on the Transport of Cr(VI)a membrane thickness (μm)

P × 106 (m/s)

J × 106 (mol/(m2·s))

D × 1010 (m2/s)

RF (%)

42 48 54 60 66

11.260 10.715 10.011 9.629 9.253

2.252 2.143 2.002 1.926 1.850

4.759 5.187 5.473 5.859 6.222

99.38 99.15 98.77 98.61 98.15

Donor phase, 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl; membrane composition, 2-NPOE/CTA, 0.2 kmol/m3 carrier; acceptor phase, 1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5.

a

Figure 9. Influence of membrane thickness on the transport of Cr(VI) (Donor phase: 2 × 10−4 kmol/m3 K2Cr2O7 in 0.75 kmol/m3 HCl, membrane composition: 2-NPOE/CTA, 0.2 kmol/m3 carrier, acceptor phase:1 kmol/m3 acetic acid/ammonium acetate buffer at pH 5).

Assuming that d is the length of the transport path Δx = d, one can create eq 8 according to the initial concentration of ΔC ≈ Ci as follows.

Ji = D

Ci d

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CONCLUSIONS For this study, we investigated the carrier-mediated transport of Cr(VI) through a PIM containing carrier. We synthesized the carrier and characterized the prepared PIM with FT-IR and AFM techniques and contact angle measurements, after which they carried out the transport experiments under optimized conditions. The obtained results can be interpreted as follows.

(9)

The inverse relationship between the flux of Cr(VI) and membrane thickness was found to be a straight line with a correlation coefficient of r2 = 0.9923 (Figure 9), which confirms the existence of a rate-limiting transport due to the diffusion of a metal complex across a membrane. Using the slope of the 5433

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Figure 10. FT-IR spectra of carrier (a), PIM including 2-NPOE + CTA (b), and PIM containing 0.2 kmol/m3 carrier (c).

(ii) The maximum transport of Cr(VI) was obtained in the membrane phase when the carrier concentration was 0.2 kmol/m3. (iii) Increasing the amount of 2-NPOE, which is a high dielectric constant, increases the membrane thickness and

(i) The transport efficiency of Cr(VI) through the PIM depends on numerous variables, including the amount of plasticizer in the membrane, the pH level in the acceptor phase, the effect of acid type in donor phase, and the carrier concentration in the membrane phase. 5434

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viscosity; therefore, transport efficiency begins to decrease after the optimum amount of 2-NPOE. (iv) The pH is a driving force for the transport of Cr(VI) in the acceptor phase. Thus, the pH of the acceptor phase should be higher than the pH of the donor phase for the efficient transport of Cr(VI). (v) The polymer inclusion membranes with carrier show the good stability of membranes. We used it several times without loss of these properties.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +90 258 296 36 00. Fax: +90 258 296 35 35. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Scientific Research Projects (BAP) of Pamukkale University, Denizli-Turkey (2012 FBE 064).

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REFERENCES

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