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Facilitated Transport of Cd(II) Through a Supported Liquid Membrane with Aliquat 336 as a Carrier a

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Süreyya Altin , Sonay Alemdar , Ahmet Altin & Yılmaz Yildirim

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Department of Environmental Engineering , Zonguldak Karaelmas University , Zonguldak, Turkey Published online: 26 Mar 2011.

To cite this article: Süreyya Altin , Sonay Alemdar , Ahmet Altin & Yılmaz Yildirim (2011) Facilitated Transport of Cd(II) Through a Supported Liquid Membrane with Aliquat 336 as a Carrier, Separation Science and Technology, 46:5, 754-764, DOI: 10.1080/01496395.2010.537726 To link to this article: http://dx.doi.org/10.1080/01496395.2010.537726

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Separation Science and Technology, 46: 754–764, 2011 Copyright # Taylor & Francis Group, LLC ISSN: 0149-6395 print=1520-5754 online DOI: 10.1080/01496395.2010.537726

Facilitated Transport of Cd(II) Through a Supported Liquid Membrane with Aliquat 336 as a Carrier Su¨reyya Altin, Sonay Alemdar, Ahmet Altin, and Yılmaz Yildirim

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Department of Environmental Engineering, Zonguldak Karaelmas University, Zonguldak, Turkey

Selective removal of cadmium from wastewaters is very important, because cadmium is toxic for the environment and for human health. This work is a comprehensive study on the selective removal of Cd(II) from aqueous solutions by using a co-current flow flat sheet supported liquid membrane system. 4.4
104 M Cd(II) concentration was used as a feed solution in the experiments. Toluene containing Aliquat 336 was used as the membrane liquid in the membrane system. Parameters such as the properties of feed and stripping solutions, carrier concentration, and flow rate, which have roles in transport of Cd(II) ions, were optimized. The efficiency of the system is expressed in terms of permeability and flux values, and transport efficiency. The optimum process conditions for the Cd(II) transport are experimentally found as follows: The feed solution as 2 M HCl, the carrier concentration as 0.1 M Aliquat 336, the stripping solution as 0.06 M EDTA, and the flow rates for the feed and stripping solutions as 50 mL/min and 80 mL/min, respectively. Under these conditions, the Cd(II) transport efficiency is found to be 82%. Keywords Aliquat 336; Cd(II) removal; facilitated transport; metal separation; supported liquid membrane

INTRODUCTION Cadmium is a toxic heavy metal for organisms in aquatic environments and has been used in many industries such as paint, electronic, coating, and ceramics. Cadmium enters aquatic ecosystems through the discharge of industrial wastewater. Cadmium recovery or removal from industrial discharges is of great importance, both scientifically and technologically (1). Cadmium can cause serious adverse effects on human health (2–4). Wastewaters containing cadmium can be treated by several advanced treatment methods. In recent years, some studies have been carried out on the removal or recovery of cadmium from wastewaters by using liquid membrane systems, which have quite a few advantages over the conventional treatment methods (1–8). Recently, some preliminary research has been conducted for scaling-up the supported liquid membrane systems (SLM). Received 18 May 2010; accepted 2 November 2010. Address correspondence to Su¨reyya Altin, Department of Environmental Engineering, Zonguldak Karaelmas University, Zonguldak 67100, Turkey. E-mail: [email protected]

A few studies have been reported in literature about facilitated transport of Cd(II) by using Aliquat 336 as a carrier in membrane systems. These studies may be broadly classified into two groups: supported liquid membranes (11,12) and polymer inclusion membranes (PIMs) (7,9,10). In previous studies different organic solvent-carrier combinations were used to investigate the transport of Cd(II) in liquid membrane. These solvent-carrier combinations include amine-kerosene (6), amine-carbon tetrachloride (4), D2EHPA–chloroform (9), D2EHPAkerosene (8,12), D2EHPA-tetradecane (13), TBPcyclohexane (3), TBP-kerosene (14), and TOA (trioctylamine)-kerosene (15). The importance of this study is the consideration of all of the physical and chemical phenomenon affecting transport efficiency in the SLM’s. The obtained data is important for new studies in this field. The novelty of this study may also be indicated as follows: Initially, selective complexation between carrier and metal to form a suitable complex is essential. In order to find suitable carrier for Cd(II) transportation, both literature survey and extraction experiments were employed in this study. After the literature survey, pre-extraction experiments were carried out between Cd(II) and the carriers having different chemical properties like; Aliquat 336 (Quaternary ammonium salt), TOA, Alamine 336 (amine), TBP, TOPO (phosphine oxide), EDTA (metal complexing), D2EHPA (phosphoric acid). Therefore, Aliquat 336 as a suitable carrier was experimentally selected for Cd(II) transport. Secondly, an appropriate solvent for Cd(II) transport was experimentally determined. The activity of the carrier and the complex in organic solvent depend on their interaction with the organic solvent. Therefore, a few different solvent-carrier combinations such as kerosene-aliquat 336, toluene-aliquat 336, and chloroform-aliquat 336 were experimentally investigated in this study. Consequently, toluene-Aliquat 336 as membrane liquid was defined. This method is the novelty of this study and is not available in the literature. The properties of feed and strip solution are important for the occurrence of the desired interface

754

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reactions. In this study, appropriate conditions were experimentally figured out for both feed and strip solutions. The use of a second carrier in the organic solvent may increase the Cd(II) transport efficiency as was reported in literature (16,17). For this purpose, a second carrier was also tried in order to increase the transport efficiency in this study. In order to adjust physical conditions for chemical interface reactions, various flow rates of feed and strip solutions were investigated. Finally, physicochemical parameters such as the permeability, the flux, and the transport rate were calculated by using Fick’s first law as used in the literature (8,18–20). By considering all of the above parameters, the optimum combinations for the SLM system were defined. In addition, the effects of the physical and chemical conditions were also figured out with a transport model. EXPERIMENTAL STUDIES Materials Methyl-tri-caprylyl ammonium chloride (Aliquat 336), tri-n-octylamine (TOA), tri-butyl phosphate (TBP) and tri-n-octylphosphine oxide (TOPO), trioctyldecylamine (Alamine336), Dicyclohexano18crown6 (DC18C6), di2ethylhexylphosporicacid (D2EHPA), toluene, kerosene, chloroform, and decanol (Aldrich Chemical Company) were used as carriers in the experiments. Reagent grade toluene (Merck Chemical Company) was used as a membrane solvent. All other chemicals (CdCl2  5H2O, EDTA, NaOH, HCl, and NH3) used in this study were of the highest available purity (Merck Chemical Company) level. A hydrophobic PVDF (polyvinylidene difluoride) membrane (DuraporeTM) with a pore size 0.22 mm, a thickness of 120 mm, and a porosity of 60% was selected as the supporting medium to hold the membrane solution containing carrier. All Cd(II) concentrations in the aqueous solutions were analyzed with an Atomic Absorption Spectrophotometer (AAS) that is the Perkin Elmer 1100B model.

FIG. 1.

Extraction Experiments Extraction experiments were performed by using a liquid-liquid extractor. 10 mL of 8.9
104 M Cd(II) solution in 1 M HCl and 10 mL of solvent containing 1% of the carrier were mixed for a duration of 30 minutes in the extractor. After the solution separation, Cd(II) concentrations in the aqueous solutions were analyzed; Cd(II) extraction percentage was determined by the following equation (1). Cd 2þ extractionð%Þ ¼

½Cdi  ½Cdf ½Cdi

 100

ð1Þ

where [Cd]i is the initial concentration and [Cd]f is the final concentration. Experimental Apparatus The flat-sheet SLM apparatus used in the study is schematically shown in Fig. 1. A membrane module was produced from Teflon material as two separate cells. A sheet of hydrophobic PVDF membrane (40
70 mm) was placed between the two cell compartments which were tightly clamped together. The total volume of the feeding and stripping solutions was 200 mL for each. The flow rates of the feed and the stripping solutions were controlled by two peristaltic pumps (Heidolph PD5206), and measured by two flow meters (Raczek KFR-4256NS). Digital magnetic stirrers (Heidolph MR 3004S) were also used to homogenize the feed and the stripping solutions. Experimental Procedure In the experimental studies, Aliquat 336 in toluene was used as the membrane liquid. 4.4
104 M Cd(II) concentration was used as the feed solution. It was prepared by dissolving an appropriate quantity of CdCl2 5H2O with a purity of 99.9% (Aldrich) in hydrochloric acid. When preparing the SLM module, initially the PVDF membrane was

The flat-sheet supported liquid membrane system.

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cut into suitable dimensions (40
70 mm) to fit the module and soaked into the carrier-solvent solution for 24 hours. The membrane was then removed from the organic solution, cleaned with filter paper to wipe away excess liquid, and placed between the two half cells of the module and fixed in the SLM cell. All experiments were performed at ambient temperature. The reservoirs in which the solutions were kept were stirred with digital magnetic stirrers in order to maintain the homogeneity of feeding and stripping solutions. Except for the determination of the flow rate effecting on Cd(II) transport, the flow rate is 50 mL=min in experiments. Samples (1.0 mL) were taken at regular time intervals from the tanks of both sides, and cadmium ion concentrations in the feed and stripping solutions were analyzed. The percentage of the transport was calculated by utilizing Cd(II) concentration in the feed and stripping solutions with the following equation, Cdð%Þtransported ¼

½Cds  100 ½Cdf 0

ð2Þ

where [Cd]s and [Cd]f0 are Cd(II) concentration in the stripping solution at the final time and Cd(II) concentration in the feed solution at the initial time, respectively. Fick’s Diffusion Model for the Mass Transfer in the SLM The transport of metal ions through the supported liquid membrane system is considered to be composed of many elementary steps. In brief, in order to model the transport of metal ions, it is necessary to consider diffusion of the solute through the feed boundary layers, reversible chemical reaction at the interfaces, diffusion of the metal complex species in the membrane, chemical reaction at the stripping interface, and diffusion of metal ions through the stripping side boundary layer. If the diffusion is the resistance-controlling step in substance transport in the membrane, it is called as steady state and is explained by Fick’s first law (8,18–20). Assuming that the transport of metal ions occurs at the steady state and the concentration gradients are linear, the flux (J) of a carrier-mediated transport in the SLM system is given by an appropriate formulation of Fick’s first law of the diffusion as the following, J¼

Vf dCf D ¼ ðCfi  Csi Þ L Adt

ð3Þ

where Vf is the volume of the feed solution, D is the diffusion coefficient of the complex, L is the membrane thickness. Cfi and Csi are the concentrations of metal ions at the membrane=feed interface and the stripping=membrane interface, respectively. Under efficient stripping conditions (Cfi >> Csi) and ignoring the aqueous diffusion layer

(Cf  Cfi), Eq. (3) can be written D=L ¼ P and ðCfi  Csi Þ ¼ Cf . In this case, the flux is given by Eq. (4). P¼

J Cf

ð4Þ

where Cf is the concentration of metal ions in the feed solution and P is the permeability coefficient. Membrane permeabilities were determined by analyzing Cd(II) concentration in the source solution as a function of time. The permeation coefficient (P) was computed by using the following equation  ln

Cf Cfo

 ¼

Ae Pt Vf

ð5Þ

where Cf and Cfo are the concentration of metal ions in the aqueous feed solution at time t and the initial concentration of metal ions (i.e., at t ¼ 0), respectively. Ae is the effective area (Ae ¼ A
e) and e (60%) is the porosity of the membrane material. By plotting ln Cf =Cfo versus the time t, a linear curve is obtained. The slope of this curve can be used to calculate the permeability coefficient (P) of the SLM system. The experimental data were applied to the model given in Eq. (5) with the aid of an iteration program, then the permeability values were calculated. The efficiencies of the SLM process for different operational conditions were compared on the basis of these permeability values. Transport is continuous at the feed=membrane interface during the experiment time in our study. It is indicated that the carrier is unsaturated in the interfaces. By increasing the Aliquat 336 concentration, the transport efficiency continues to increase. If the carrier was saturated, the complex activity decreased, hence the transport efficiency also decreased (2,21). Therefore, Eq. (5) is valid for this system. RESULTS AND DISCUSSIONS The Extraction of Cd(II) According to previous studies, while kerosene and toluene were used very often in SLM’s (5,7,10,13,14,20–26), chloroform, dichloromethane, and n-decanol were seen only in a few studies (9,11). It can be concluded that kerosene and toluene were selected as carrier in the experiments. The best extraction efficiency among carriers was obtained for Aliquat 336. Also, D2EHPA was often used as a carrier in previous studies (8,9,12,13). After the carriers were determined, additional extraction experiments were conducted by using the combination with other solvents of these carriers. But a sufficient extraction efficiency could not be obtained. In this study, the extraction rates of Cd(II) are obtained from the extraction experiments, and the results are given in Table 1.

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TABLE 1 Cd2þ Extraction (%) efficiencies used different solvent and carrier combination Solvent

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Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Chloroform Chloroform Decanol

Carrier

Ext. %

Solvent

Carrier

Ext. %

Aliquat 336 DC18C6 TOA Alamine 336 TBP TOPO EDTA D2EHPA D2EHPA Aliquat 336 D2EHPA

70 10 26 53 25 11 6 8 13 24 7

Kerosene Kerosene Kerosene Kerosene Kerosene Kerosene Kerosene Kerosene Dichloromethane Dichloromethane Decanol

Aliquat 336 DC18C6 TOA Alamine 336 TBP TOPO EDTA D2EHPA D2EHPA Aliquat 336 Aliquat 336

50 10 51 15 40 11 9 7 12 48 6

As indicated in Table 1, Toluene-Aliquat 336 and Toluene-Alamine 336 solvent-carrier pairs were found to be most effective for the Cd(II) extraction. Based on these results, the reactions which influence the extraction can be explained as follows. Initially, Cd(II) ions are ionized in aqueous solution as CdCl2 , Cd 2þ þ 2Cl 

Reac: ð1Þ

Next, ionized CdCl2 forms CdCl3 and CdCl42 complexes in the acid solution, according to the following reactions: H þ þ Cd 2þ þ 3Cl  , HCdCl3 , H þ þ CdCl3 Reac: ð2Þ 2H þ þ Cd 2þ þ 4Cl  , H2 CdCl4 , 2H þ þ CdCl42 Reac: ð3Þ The species of CdCl2 can form in high concentration HCI solutions. Within the acid concentration range of 1–2 M, CdCl 3 complex was the highest one, and also there was a small amount of CdCl2 4 complex (27,28). Apart from these two complexes, the species did not have a structure to react with Alamine 336 (R3N, tertiary amine) and Aliquat 336 (R4NCl, quaternary amine). The tertiary amines and the quaternary amines give the following reactions with 2 CdCl 3 and CdCl4 complexes (4);

(Reac. (5)), and CdCl2 (Reac. (6)). Therefore, Aliquat 4 336 is more effective in Cd(II) extraction. According to the results obtained from the extraction experiments, the transport mechanism occuring within a SLM system in the presence of Aliquat 336 carriers is summarized in Fig. 2. The Effect of Acid Concentration in Feed Solution HCl solutions of 0.1, 0.5, 1, and 2 M were used to determine the effect of the acid concentration in the feed solution on Cd(II) transport within a supported liquid membrane system. Figure 3 indicates the changes of Cd(II) ion at the feed and stripping solutions for different HCI concentrations. Table 2 indicates Cd(II) transport rate (as %), permeability, and the fluxes values. As seen in Table 2, the transport efficiencies of Cd(II) ions increases with the increment of the HCI concentrations in the feed solution. It can be suggested that if 2 M HCl is used as the feed solution, reaction (3) will result in the most effective Cd(II) transport. When 0.1 M HCl is used for the feed solution,

þ þ CdCl42 þ 2R3 Norg , ðR3 NHÞ2 CdCl4org Reac: ð4Þ 2Haq

CdCl3 þ R4 NClorg , ðNR4 ÞCdCl3org þ Cl 

Reac: ð5Þ

CdCl42 þ 2R4 NClorg , ðNR4 Þ2 CdCl4org þ 2Cl  Reac: ð6Þ In comparison with Alamine 336, Aliquat 336 is more efficient in the extraction of Cd(II) ions. According to reaction (4), Alamine 336 only reacts with CdCl2 4 . On the other hand, Aliquat 336 reacts with both CdCl 3

FIG. 2. Transport mechanism of cadmium at the supported liquid membrane.

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FIG. 3. The variation of Cd2þ concentration in the feed and stripping solutions versus time for different HCl concentrations (Stripping solution: 0.1 M HCl; carrier; 0.01 M Aliquat 336; the feed flow rate: 50 mL=min, the stripping flow rate: 50 mL=min).

FIG. 4. The variation of Cd2þ concentration in the feed and stripping phase for different stripping solutions (the feed solution: 2.0 M HCl; the carrier: 0.01 M Aliquat 336; the feed flow rate: 50 mL=min, the stripping flow rate: 50 mL=min).

there are not enough species to form a complex with Aliquat 336 (27,28). These interpretations were also supported by the transport rate (%), permeability, and flux values given in Table 2.

complexing ability of EDTA with Cd(II) (complexformation constant, kf ¼ 2.9
1016 (29)) facilitates the transition of Cd(II) ions to the stripping solution. Therefore, the removal of Cd(II) from the feed solution and the release of Cd(II) to the stripping solution significantly increased. When NaOH was used as the stripping solution, the transport efficiency could not be determined. Back reactions of Reac. 5 and Reac. 6 must be occurred at the membrane= strip interface to provide satisfactory stripping. The back reactions depend on properties of the stripping solution. By using NaOH as stripping solution, the back reactions and transport of Cd(II) wasn’t taken place. The reason for this may be a high pH of the stripping solution.

The Effect of Stripping Solution In order to determine the effect of stripping solution on Cd(II) transport, different stripping solutions were tested. In previous studies deionized water, H2SO4, HCl, H3PO4, NaOH (6), and EDTA (3) were used as stripping solutions in our experiments. The variation of Cd(II) in the feed and stripping solutions is given in Fig. 4, and the corresponding Cd(II) transport rate, permeability, and flux values are given in Table 3. The transport efficiency was found in the order of deionized water < H3PO4 < H2SO4 < HCl < EDTA. The highest transport rate was obtained for EDTA. The TABLE 2 Cd2þ transport rate, permeability and flux values for different HCl concentration in the feed solution (Stripping solution: 0.1 M HCl; carrier : 0.01 M Aliquat 336; flow rate: 50 mL=min) 6

10

HCl (M)

P
10 (cm=s)

J
10 (mol=cm2  s)

0.1 0.5 1 2

10.30 32.04 36.33 51.97

45.9 142.8 162.0 231.7

2þ

Cd

transport rate (%) 9.10 33.95 36.86 48.71

The Effect of the Concentration of Stripping Solution The use of HCl and EDTA solutions in the stripping solution gave the most efficient result in Cd(II) transport. TABLE 3 Cd2þ transport rate, permeability and flux values for different stripping solutions (the feed solution: 2.0 M HCl; the carrier: 0.01 M Aliquat 336; the flow rate: 50 mL=min) Stripping solution Deionized water H2SO4 H3PO4 HCl EDTA

P
106 (cm=s)

J
1010 (mol=cm2  s)

Cd2þ transport rate (%)

37.11 45.47 43.95 51.97 59.64

165.50 202.70 196.00 231.70 265.90

36.95 48.98 40.30 48.71 55.86

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SELECTIVE REMOVAL OF CADMIUM FROM WASTEWATERS

FIG. 5. The variation of Cd2þ concentration in the feed and stripping phases for different concentrations of HCl as stripping solution (the feed solution: 2.0 M HCl; the carrier: 0.01 M Aliquat 336; the feed flow rate: 50 mL=min, the stripping flow rate: 50 mL=min).

Therefore, different concentrations of HCl and EDTA solutions were used to analyze the effect of the concentration of stripping solution on the transport. Transport graphs obtained from experimental data are given in Figs. 5 and 6 for HCl and EDTA as stripping solutions, respectively. Permeability, flux, and transport rates are also presented in Table 4. When the stripping solution was prepared in HCI, the highest Cd(II) transport was obtained at 0.5 M HCl. The

FIG. 6. The variation of Cd2þ concentration in the feed and stripping phases for different concentrations of EDTA as stripping solution (the feed solution: 2.0 M HCl; the carrier: 0.01 M Aliquat 336; the feed flow rate: 50 mL=min, the stripping flow rate: 50 mL=min).

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transport efficiency decreased significantly at the concentrations higher and lower than 0.5 M. When Cl concentration increases in the stripping solution, substitution reaction with CdCl2 complex easily occurs at the 4 stripping=membrane interface. Therefore, the rise in Clconcentration improves the transport efficiency up to a peak value. However, the gap between HCl concentration of feed and stripping solutions were decreased to a higher HCI concentration in the stripping solution. This decrement negatively affects the transport rate. Basic carriers like amines can be used to carry Hþ ions together with negatively charged ions, such as Cl in the same direction. Surfaces are shifted in opposite directions. This creates a concentration gradient of different forms of the carrier (with and without ions) in the membrane, and results in the directed ion flux through the membrane (30). In experiments where EDTA is used as a stripping solution, the increament of the EDTA concentration between 0.01 and 0.06 M rises the transport efficiency. This is because the EDTA in the stripping solution selectively removes Cd(II) from (R4N)2CdCl4 complex at the membrane=stripping interface, and irreversibly forms a stable complex. Transport would continue, depending on the activity of EDTA on the membrane=stripping interface. However, the solubility of EDTA decreases in its higher concentrations. By reaching saturation of EDTA in stripping solution, the back reaction at interface membrane= stripping complicates. In this case, the transport of Cd(II) into the stripping solution becomes slow. The Cd(II) transport efficiency can be negatively affected due to high EDTA concentrations. The Effect of Carrier Concentration Aliquat 336 was used at 0.01, 0.02, 0.05, and 0.1 M concentration values to determine the effect of the carrier concentration on the Cd(II) transport. The resulting change of Cd(II) in the feed and stripping solutions over time are shown in Fig. 7, and the corresponding Cd(II) transport rate, the permeability, and flux values are given in Table 5. Increased carrier concentration raises the transport efficiency; however, when a certain concentration level is exceeded, the transport efficiency remains constant or decreases (3). It can be suggested that the carrier concentration affects the transport efficiency through the solubility of the carrier on the membrane liquid and the viscosity of the medium (i.e., the mobility of the complex) (2,3,4,6). As Aliquat 336 concentration was increased from 0.01 M to 0.1 M, it was observed that the Cd(II) ion transport rate also increased. Since the feed=membrane interface does not reach full saturation at low carrier concentrations, the flux rises with the increment of the carrier concentration (3). This indicates that Aliquat 336 dissolves and moves well in toluene solvent.

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TABLE 4 Cd2þ transport rate, the permeability and flux values for different concentrations of HCl and EDTA as stripping solutions (the feed solution: 2.0 M HCl; the carrier: 0.01 M Aliquat 336) (HCl) concentration P
106 (EDTA) P
106 J
1010 Cd2þ transport J
1010 Cd2þ transport 2 2 (M) rate (%) concentration (M) (cm=s) (mol=cm  s) rate (%) (cm=s) (mol=cm  s)

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0.1 0.5 1

57.38 63.83 51.97

255.90 284.60 231.70

48.71 48.97 39.85

The transport efficiency increases by an increment in the carrier concentration, initially (3). But, when the carrier concentration continues to increase, the efficiency may change or decrease. In our study for 0.05 and 0.1 M of Aliquat 336 concentrations, the efficiencies are also very close to each other. Therefore, 0.1 M Aliquat 336 can be regard as the optimum carrier concentration for this study. Thus, the vast numbers of these carrier molecules in the solvent play a role actively in the transport of Cd(II) ion. Nearly, all of Cd(II) through the membrane liquid were transported into the stripping solution. Consequently, the Aliquat 336-Cd(II) complex was not able to accumulate in the membrane structure. This reveals that complexes easily decompose at the membrane=stripping interface, and the carrier released in the liquid membrane quickly takes a role in the transport.

0.01 0.04 0.06 0.08 0.1

59.64 80.95 92.26 77.14 52.74

265.90 361.00 411.40 344.00 234.60

60.84 64.64 68.96 61.42 55.86

The Effect of Flow Rates in Stripping and Feed Solutions The supported liquid membrane system used in the study was designed according to co-current flow conditions. To analyze the effect of flow rates on the transport, the experiments were performed by using different flow rates for feed and stripping solutions. In the first stage, while the flow rate of the stripping solution was kept constant at 50 mL=min, the flow rates for feed solution were varied as 30 mL=min, 50 mL=min, 80 mL=min, and 100 mL=min. On the second stage, while the flow rate of the feed solution was kept constant at 50 mL=min, the flow rates of the stripping solution (30 mL=min, 50 mL=min, 80 mL=min, and 100 mL=min) were varied. The variation of Cd(II) concentration in the feed and stripping solutions for their different flow rates are shown in Figs. 8 and 9, respectively. Table 6 indicates the changes of Cd(II) concentrations at feed and stripping solutions when various flow rates were used. According to Fick’s Diffusion Model, the metal ions reach the membrane by diffusion from the bulk layer. The thickness of the bulk layer indicates the transition distance of metal from the moving layer to the membrane surface. In other words, a resistance was applied to the metal. As known, the diffusion is reversely correlated with the distance. In this case, if the bulk layer is thin enough, the diffusion of metal ions from this layer are easier. By TABLE 5 Cd2þ transport rate, the permeability and flux values for different concentration of Aliquat336 (the feed solution; 0,06 M EDTA, the stripping solution; 2.0 M HCl; the carrier: Aliquat 336) Concentration of P
106 J
1010 Cd2þ transport 2 Aliquat 336 (M) (cm=s) (mol=cm  s) rate (%)

FIG. 7. Cd2þ ion concentration changes in the feed and stripping solutions for different concentration of Aliquat 336 as the carrier (the stripping solution; 0,06 M EDTA, the feed solution; 2.0 M HCl; the feed flow rate: 50 mL=min, the stripping flow rate: 50 mL=min).

0.01 0.02 0.05 0.1

92.26 101.23 113.04 124.33

411.40 451.40 504.10 554.50

68.96 73.84 77.98 78.46

SELECTIVE REMOVAL OF CADMIUM FROM WASTEWATERS

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thickness should be maintained for the interface reactions that are necessary for the transport (12). As a conclusion, an efficient flow rate should be slow enough to allow for interface reactions, and fast enough to minimize the resistance to metal ions diffusion. Therefore, in each SLM system, appropriate individual flow rates should be determined for feed and stripping solutions. In the present study, optimum flow conditions were found to be 50 mL=min for the feed solution and 80 mL=min for the stripping solution.

FIG. 8. The variation of Cd2þ concentration in the feed and stripping solutions for different flow rate of the feed solution (the feed solution: 2.0 M HCl; the stripping solution: EDTA, 0.06 M; the carrier: Aliquat 336, 0.1 M; the flow rate of stripping solution: 50 mL=min.).

increasing the flow rate, the bulk layer becomes thinner. Therefore, the metal can easily reach the membrane surface. This phenomena affects transport efficiency positively. But for an excessively thin bulk layer the required time of the metal complexing reaction for the metal may not be sufficient. Consequently, the optimum layer

FIG. 9. The variation of Cd2þ concentration in the feed and stripping solution for different flow rate of stripping solution (the feed solution: 2.0 M HCl; the stripping solution: EDTA, 0.06 M; the carrier: Aliquat 336, 0.1 M; the flow rate of the feed solution: 50 mL=min).

The Effect of Using a Second Carrier in the Membrane A second carrier to increase cation transport efficiency was employed in experiments at some previous studies (6,16,17). Similarly, in the present study, TBP, TOA, and TOPO complexes were used as a secondary carrier in addition to Aliquat 336. The use of a secondary carrier generally increases the ion transport efficiency. However, contrary to the expected result, the combined use of binary carriers resulted in a reduced transport efficiency. Different carrier combination and their effects on the Cd(II) ion transport are presented in Fig. 10. The permeability, flux values, and Cd(II) transport rate are also given in Table 7. All of the second carriers employed in this study were tertiary amines. Potential reactions of tertiary amines on feed=membrane interfaces are designed as follows: In the first step, tertiary amines react with the acid in the feed solution to form complex (Reac. (7)). Then, this complex (R3NHþCl-) reacts with the CdCl2 4 complex in the feed solution to form (R3NH)2CdCl4 complex as seen in reaction (8). These complexes are transported to the membrane=stripping interface. Both reaction (7) and reaction (8) for the metal transport by the tertiary amines are needed to occur as follows: R3 NHþ þ Cl ! R3 NHþ Cl

Reac: ð7Þ

R3 NHþ Cl þ CdCl2 4 ! ðR3 NHÞ2 CdCl4

Reac: ð8Þ

Aliquat 336 is a quaternary ammonium salt having R4NCl structural formula, and it gives the same ion exchange reaction in a single step. Therefore, it is more effective in the Cd(II) ion transport compared to tertiary amines. As indicated in the reactions, the metal transport with tertiary amines occurs as a result of the reaction between the tertiary amine and the acid in feed solution. If the feed solution has a high acid concentration, a stable H2CdCl4 is formed in the feed as given by reaction (9). Therefore, stable H2CdCl4 and R3NHþ cations do not form a complex at the feed=membrane interface. þ CdCl2 4 þ 2H $ H2 CdCl4

Reac: ð9Þ

762

S. ALTIN ET AL.

TABLE 6 Cd2þ transport rate (%), permeability and flux values and for different flow rates of feed and stripping solutions (the feed solution: 2.0 M HCl; the stripping solution: EDTA, 0.06 M; the carrier: Aliquat 336, 0.1 M) Flow rate of feed and Flow rate of Cd2þ 6 10 2þ 6 10 stripping solutions P
10 transport J
10 Cd transport stripping and feed P
10 J
10 (mL=min) rate (%) solutions (mL=min) (cm=s) (mol=cm2  s) rate (%) (cm=s) (mol=cm2  s)

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30–50 50–50 80–50 100–50

92.52 124.33 65.26 56.88

412.63 554.51 291.05 253.68

67.92 78.46 56.26 53.98

If R3NHþ and CdCl2 4 ionic species are not formed, reaction (8) will not be completed, hence Cd(II) transport may decrease. Therefore, the acid concentration of the feed solution should not be very high (12). Since R3NHþCl are stable in the strong acid solutions, the transport cannot take place. In the previous studies (2,4,9) about the transport of Cd(II) in SLM’s, it is also reported that the increment of acid concentration in feed solution causes the decrement of the transport efficiency. Furthermore, working conditions of the membrane system with Aliquat 336 were optimized in the previous experimental steps. Optimal operation conditions for each of the carriers are different. For this reason, the optimal operation conditions for Aliquat 336 used in the experiments may not be appropriate for binary carrier cases. Therefore, transport efficiencies have been decreased with the use of a second carrier.

FIG. 10. The variation of Cd2þ concentration in the feed and stripping solutions by using a second carrier in the membrane (the feed solution: 2.0 M HCl; the stripping solution: 0.06 M EDTA; the carrier: 0.1 M Aliquat 336 and 0.001 M of the second carrier; the feed flow rate: 50 mL=min; the stripping flow rate : 80 mL=min).

30–50 50–50 80–50 100–50

50.95 124.33 133.21 86.38

227.23 554.51 594.11 385.25

49.18 78.46 82.27 66.24

TABLE 7 Permeability, flux values and transport rate by using a second carrier in the membrane (the feed solution: 2.0 M HCl; the stripping solution: 0.06 M EDTA; the carrier: 0.1 M Aliquat 336 and 0.001 M of the second carrier; the feed flow rate: 50 mL=min; the stripping flow rate: 80 mL=min) Carriers Aliquat Aliquat Aliquat Aliquat

P
106 J
1010 Cd2þ transport 2 rate (%) (cm=s) (mol=cm  s) 336 133.21 336-TOPO 94.50 336-TOA 80.42 336-TBP 66.66

594.11 421.47 358.67 297.30

82.27 63.90 59.76 54.44

FIG. 11. The variation in Cd2þ concentration in the feed, stripping and membrane phases at the optimum conditions of the SLM (the feed solution: 2.0 M HCl; the stripping solution: EDTA, 0.06 M; the carrier: 0.1 M Aliquat 336 and 0.001 M secondary carriers; the flow rate of feed: 50 mL=min; the flow rate of stripping:80 mL=min).

SELECTIVE REMOVAL OF CADMIUM FROM WASTEWATERS

Figure 11 shows experimental results for the change of Cd(II) concentration over time under optimal transport conditions. Cd(II) concentration in the membrane was calculated from mass balance in the system as following. Cf þ Cs þ Cm ¼ Cf 0

ð6Þ

Cm ¼ Cf 0  ðCf þ Cs Þ

ð7Þ

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By considering Fig. 11, agglomeration of Cd(II) in the membrane did not show up. This case indicated that the conditions of the liquid membrane system are proper for interface reactions and complex action, hence Cd(II) transport takes place efficiently. CONCLUSION According to the experimental results, it is found that interface reactions are affected by feed and stripping solutions, and the carrier properties. It is also found that the rate of diffusion is influenced by the concentration of the carrier and the properties of the solvent. In this study, toluene (0,47 g=L at 20 C), kerosene (none), Aliquat 336 (0,12 g=L at 30 C), and D2EHPA (