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Transport of silver ions through a flat-sheet supported liquid membrane
Hydrometallurgy 103 (2010) 144–149
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Transport of silver ions through a flat-sheet supported liquid membrane Sureyya Altin ⁎, Yilmaz Yildirim 1, Ahmet Altin 2 Zonguldak Karaelmas University, Department of Environmental Engineering, 67100, Turkey
a r t i c l e
i n f o
Article history: Received 19 August 2009 Received in revised form 12 March 2010 Accepted 16 March 2010 Available online 25 March 2010 Keywords: Supported liquid membrane Silver Toluene DC18C6 PTFE membrane
a b s t r a c t Toxic metals from industrial wastewaters are an important environmental issue. The use of supported liquid membrane processes has gained momentum in recent years, as it allows the reuse of water and toxic metals. The aim of this study is to investigate the active transport of silver ion through a supported liquid membrane (micro-porous Fluoropore PTFE) of DC18C6 (Dicyclohexano18crown6) in toluene under various experimental conditions. For this purpose, the effects of various parameters including binary carriers, carrier concentration, feed phase concentration, the nature and concentration of stripping agents in the stripping phase and flow rates of feed and stripping phases on transport efficiency were also investigated. The maximum transport efficiency was observed at the following conditions: flow rates of 50 mL/min in both phases, 0.05 M DC18C6 in toluene as carrier solution, 50 ppm Ag+ dissolved in 0.015 M HNO3 as feed solution and 0.08 M Na2S2O3 as stripping solution Optimum operation time was determined as 240 min. Under these conditions, 94% of the silver ions were transported from the feed phase to the membrane phase. However, the transport rate from the membrane phase to the stripping phase remained at approximately 81%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The presence of silver in the natural water environment is of great concern because of its toxicity to aquatic plants and animals, especially when it is in the free ion form (Ag+) (Petering and McClain, 1991; Ratte, 1999; Irwin et al., 1998; Wang et al., 2003). The major sources of silver in the environment are wastes from the medical and photographic-imaging industry, and wastes from the manufacturing of electronics, silverware, and jewelry (Purcell and Peters, 1998; Rounaghi et al., 2008). An estimated 150,000 kg of silver enters the aquatic environment every year from the photography industry, mine tailings, and electroplaters (Irwin et al., 1998). Therefore, separation and recovery of silver from industrial wastes is of great importance. Recently, liquid membrane technologies have commonly been used in order to separate and recover heavy metals from diluted solutions. Liquid membrane systems in which solvents containing carriers are placed in a porous support material are called supported liquid membrane (SLM) (Boyadzhiev and Lazarova, 1995). Compared to other liquid membrane methods, the major advantage of the SLM method is that the SLM requires less membrane liquid for the formation of the support matrix. Moreover, its low operating costs, minimum product contamination, no phase separation problem,
⁎ Corresponding author. Tel.: + 90 372 2574010 1565; fax: +90 372 2574023. E-mail addresses: [email protected] (S. Altin), [email protected] (Y. Yildirim), [email protected] (A. Altin). 1 Tel.: + 90 372 2574010 1540; fax: +90 372 2574023. 2 Tel.: + 90 372 2574010 1291; fax: +90 372 2574023. 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.03.015
selectiveness and flexibility are the other advantages. However, the instability of the liquid membrane in the pores of the inert membrane support (carrier lost) is the main disadvantage (Izatt et al., 1986; Boyadzhiev and Lazarova, 1995; Calzado et al., 2001). In supported liquid membranes, the support material which contains membrane liquid can be flat-sheet or hollow fiber. Both of these support materials are generally produced from polymers which can be moisturized by oleophilic and membrane liquid. Although there are some studies in which hollow fiber was used as SLM (Breembroek et al., 1998a; Marchese and Campderros, 2004), flatsheet membranes produced from PVDF or PTFE were mainly used in many SLM studies (Breembroek et al., 1998b; Zhang and Gozzelino, 2003; Bansal et al., 2005). There are many previous studies to separate silver ions from aquatic environment using bulk or emulsion liquid membranes. In a study by Rounaghi et al. (2008), silver ion transport in bulk membrane was analyzed using different acyclic and cyclic ligands as carriers and nitrobenzene as a membrane solvent. El-Bachiri et al. (1996) analyzed silver nitrate transport in a dichloromethane membrane containing DC18C6 as a carrier in bulk liquid membrane. Vajda et al. (2000) used Cyanex471X as a carrier in a bulk liquid membrane system. They observed that silver transport increased as the HNO3 concentration in the aqueous phase and the carrier concentration in the solvent increased. Safavi and Shams (1999) separated silver from aqueous phase in Ag (CN)2− ions using a Victoria Blue carrier. A study by Izatt et al. (1985) investigated silver transport as AgBr2− in emulsion liquid membranes. The organic membrane was formed from DC18C6 dissolved in toluene and Li2S2O3 was used as the stripping phase.
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The number of studies about silver ion transport through SLM is limited. Chaudry et al. (2008) analyzed the silver ion transport in acidic solutions using trietanolamine (TEA) as a carrier in SLMs. In addition, the effects on transport efficiency of HNO3 and Ag+ concentrations in the feeding phase and KCN concentration in the stripping phase were also analyzed. There are also some previous studies on the transport of Ag+ ions in SLMs with different carriers, membrane solvents and feeding–stripping phases, by Boyahzhiev and Dimitrov (1994), Zolgharnein et al. (2003), Alexandrova et al. (2001), Shamsipur et al. (2006) and Zaghbani et al. (2008). As mentioned above, crown ethers are usually employed in metal transportation studies. Complexation between crown ethers and cations depends on two possibilities. The first possibility is the cavity of ligand and dimension of cation. If the cavity and dimension are approximately close to each other, the stable complexes form. Since dimension of silver cation is 2.52 A° and the cavity of 18C6 is 2.6– 3.02 A°, cation can be accepted into the cavity. In general, stability of complexes depends on properties of donor atoms. The second possibility is wrapping around the metal. If there is an interaction between functional groups and the metal, crown ether is then wrapped around the metal. In this study, the complexation was not studied but regarding on some studies that stability constants (equilibrium, K in L/mol) for the complex formation of DC18C6-Ag+ were given as 1.59 in water,5.63 in acetone and 7.98 in propylene carbonate (Izatt et al., 1985; Buschmann et al., 2001). In previous studies about the separation of silver from aqueous solutions through SLMs, the silver ions were transported after being turned into various complexes. In the Ag+ ions transport through liquid membrane systems, CN− and Br−, which generally form a complex with Ag+ easily and which are quite toxic, were preferred in many previous studies. But, in this study, transport of Ag+ ion in HNO3 solution through SLM using DC18C6 in toluene without any toxic complexation agent (CN− or Br−) were experimentally investigated. Another important aspect of the present study is that the effects of stripping solution and carriers with different characteristics on the silver ion transport were studied. All the data obtained in the study were applied to Fick's first law of diffusion and their permeability values were estimated. 2. Materials and methods 2.1. Reagents Dicyclohexano18crown6 (DC18C6), tri-n-octylamine (TOA), tributyl phosphate (TBP) and tri-n-octylphosphine oxide (TOPO) (Aldrich Chemical Company) were used as carriers. Reagent grade toluene (Merck) was used as an organic membrane solvent. All other chemicals (AgNO3, HNO3, Na2S2O3, NaOH, HCl and NH3) used in this study were of the highest a supporting purity available (Merck Chemical Company). A hydrophobic PTFE membrane (Fluoropore trade mark) with pore size 0.22 µm, a thickness of 175 µm and a porosity of 70% was selected as medium to hold the membrane solution containing carrier. 2.2. Experimental apparatus The flat-sheet SLM apparatus was used in the study. The membrane module was produced from Teflon material as two different cells. The membrane was placed between two cell compartments which were tightly clamped together. The total volume of the feeding and stripping solutions was 200 mL for each. The flow of the solution in the membrane module was enabled by two peristaltic pumps (Heidolph PD5206) and the flow rate was controlled by two flow meters (Raczek KFR-4256NS). Digital magnetic stirrers (Heidolph MR 3004S) were used to homogenize feeding and stripping solutions.
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2.3. Experimental procedure A DC18C6 in toluene was used as the membrane liquid in the experimental studies. Except for the experiments in which the acid concentration of feeding phase was investigated, 50 mg/L Ag+ concentration in the feeding phase was prepared as dissolving AgNO3 in 0.02 M HNO3 solution. When preparing the SLM module, initially the PTFE membrane was cut into the suitable dimensions (40 × 70 mm) to fit the module and soaked in the carrier-solvent solution for one hour. The membrane was then removed from the organic solution, cleaned with filter papers to take away excess liquids 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. Samples (1.0 mL) were taken at regular time intervals from the tanks of both sides, and silver ion concentrations in the feed and stripping phases were analyzed by using an atomic absorption spectrophotometer (Perkin Elmer 1100B model). 2.4. Permeation model of silver ion The transport of metal ions through the supported liquid membrane system is considered to be composed of many elementary steps. In brief, to model the transport of metal ions, it is necessary to consider diffusion of 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 diffusion is the speed restricting step in substance transport in a membrane, it is called steady state and is explained by Fick's first law (Marchese et al., 1993; Sastre et al., 1998; Alguacil et al., 2001; Ata, 2005; Molinari et al., 2006). 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 an SLM system is given by an appropriate formulation of Fick's first law of diffusion as follow: J=−
Vf dCf D = ðCfi −Csi Þ Adt L
ð1Þ
Where: Vf is the volume of the source phase solution, D is the diffusion coefficient of the complex, L is the membrane thickness and 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. (1) is simplified to: P=
J Cf
ð2Þ
where Cf is the concentration of metal ions in the feed and P is the permeability coefficient. Membrane permeabilities were determined by analyzing silver concentration by AAS in the source phase as a function of time. The permeation coefficient (P) was computed using the following equation: ln
Cf Cf0
=−
Ae Pt Vf
ð3Þ
where Cf and Cf0 are the concentration of metal ions in the aqueous feed phase at time t and; the initial concentration of metal ions (at t = 0), respectively. Ae is the effective area (Ae = A ⁎ ε), ε (85%) is the porosity of the membrane material. By plotting ln CCf against t, a linear f0 curve is obtained. The slope of this curve can be used to calculate permeability coefficient (P) of the SLM system.
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In this study, the data obtained were applied to the model given in Eq. (3) with the help of an iteration program and the permeability values were calculated. The efficiency of the SLM process for different operational conditions was compared on the basis of these permeability values. Finally, to determine k constants at the membrane interfaces, optimum experimental data were used. For the calculation of k constants, consecutive irreversible first order reaction was assumed in the carrier-mediated liquid membrane systems (Eq. (4)) (Lakshmi et al., 2004; Bansal et al., 2005; Mohapatra et al., 2006). kf
ks
D→M→A
ð4Þ
Where, F, M and S represent the feed, the membrane and the stripping phases, respectively. kf and ks were calculated using following equations (Ata, 2005). Rf = exp ð−kf t Þ Rm =
ð5Þ
kf ½expð−kf t Þ− exp ð−ks t Þ ks −kf
Rs = 1−
ð6Þ
1 ½k expð−kf t Þ−kf exp ð−ks t Þ ks −kf s
ð7Þ
And, the fluxes were calculated using following equations: Jf = −kf
k − kf f =ðk −ks Þ k ks =ðk −ks Þ f f Js = ks f ks ks
ð8Þ
3. Results 3.1. Effect of the nature and concentration of stripping agents on Ag+ transport In order to analyze the effect of stripping phase on Ag+ transport, various stripping solutions were prepared. For this purpose, seven stripping solution including Na2S2O3, HCl, NaOH and NH3 with different concentrations were tested. The permeability values calculated for all stripping solutions, based on Fick's first law of diffusion, were given in Table 1. As can be seen from Table 1, the maximum permeability values were obtained in the experiment in which 0.04 M Na2S2O3 was used as the stripping phase. • It was well-known that, through its donor groups the El-Bachiri et al. (1996), DC18C6 (symbolize by L) is able to replace the hydration molecules of silver in liquid membranes. Silver cations are then coextracted with nitrate anions under a complex ion-pair form. In the equilibrium following reaction takes place. þ
−
Ag þ NO3 þ Lorg↔AgðLÞNO3 org
• On the stripping side, this complex will break to release the L carrier and forms a new complex structure with (S2O3)2− and Ag+ ion proceeds to the stripping phase (reaction (2)). 2−
AgðLÞNO3 org þ Na2 S2 O3 þ ↔2NaNO3 þ ½AgðS2 O3Þ2
3−
It was evaluated that kf = 4.7 × 1013 for [Ag(S2O3)2]3−. In conclusion, the mechanism of silver ion transport in a supported liquid membrane has been schematically defined for Na2S2O3 as stripping solution. Fig. 1 shows the three different phases and the transport stages of silver ion in a SLM. The permeability values which were obtained from the experiments in which only NaOH and HCl solutions are used as the stripping agent were lower than those obtained using the Na2S2O3 solution. The experiment in which only NH3 was used is not shown in Fig. 1 due to lower Ag+ ion transport. In a study by Flett and Wilson (1983) with the aim of improving the transportation, NH3–Na2S2O3 solution was tested for the extraction of Ag+ ions. It was reported that under appropriate conditions, the formation of [Ag(S2O3)2]3− complex in the stripping solution (NH3–Na2S2O3) occurs readily. Similar results and important permeability values were also obtained in the present study using NH3–Na2S2O3 as the stripping agent. However, the permeability values remained lower than that obtained using only Na2S2O3 solution as stripping agent. In order to improve the efficiency of the NaOH as the stripping agent, Na2S2O3 was added to the solution, but it was observed that the positive effect of this process on Ag+ ion transport was imperfect. The results showed that the sequence of permeability efficiency for Ag+ ion transport using NaOH as stripping agent was: 0.04 M Na2S2O3 prepared from 0.01 M NaOH N 0.01 M NaOH N 0.1 M NaOH. Accordingly, it is thought that in the presence of NaOH, the following reactions may be possible. • Using NaOH as a stripping solution raised the pH in the stripping phase after the experiment was started. In the beginning of the experiment; although pH is basic, silver ions were not precipitated due to low transportation of silver ions. However for a while later, the precipitation was occurred according to following reaction equation. In reaction (3), in the beginning of the experiment AgOH is an unstable compound (Ksp = 1.5 × 10−8); by the time the reaction proceeds and in a short time later dark colored Ag2O compound is precipitated in the stripping phase. In order to find transported silver ions in the stripping phase, the precipitated silver was dissolved in a strong acid and its concentration was analyzed by AAS. AgðLÞNO3 þ 2NaOH→2AgOH þ 2NaNO3 →Ag2 O↓ þ NaNO3 þ H2O ð3Þ
ð1Þ
Table 1 Permeability values for the transport of Ag+ ion through SLM using different stripping solutions (feeding phase: 0.02 M HNO3; membrane support: PTFE; membrane liquid: 0.01 M DC18C6 in toluene; flow rate: 50 mL/min). Stripping solution
P × 106 cm3/cm2 s
0.01 M Na2S2O3 prepared from 0.001 M NH3 0.04 M Na2S2O3 prepared from 0.001 M NH3 0.10 M HCl 0.10 M NaOH 0.01 M NaOH 0.04 M Na2S2O3 0.04 M Na2S2O3 prepared from 0.010 M NaOH
101 137 155 131 148 191 154
ð2Þ
Fig. 1. Schematic display of the transportation mechanism of Ag+ ions in SLM.
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• When using the mixture of Na2S2O3 and NaOH as stripping solution, the stripping yield was slightly more than that of NaOH alone. Since the medium was basic, reaction (3) was more effective on silver ions. Due to high complex formation constant of (S2O3)2− ions with silver ions, the stripping yield was enhanced. • When using the mixture of Na2S2O3 and NH3 as stripping solution, complex formation constant of [Ag(S2O3)2]3− (kf = 4.7 × 1013) was 7 higher than that of Ag(NH3)+ 2 (kf = 1.7 × 10 ). Therefore as seen from Fig. 1, the silver transportation depends on the concentration of (S2O3)2− ions in the stripping phase. • When using Na2S2O3 alone as stripping solution, silver forms [Ag (S2O3)2]3− complex according to reaction (2) at the interface of membrane/stripping phase. 3.2. Effect of Na2S2O3 concentration on Ag+ ion transport In order to determine the effect of the concentration of Na2S2O3 solution as the stripping phase on Ag+ ion transport, 0.01 M, 0.03 M, 0.04 M, 0.08 M, 0.1 M, 0.15 M Na2S2O3 were used. Fig. 2 shows the permeability values calculated from the experimental results, based on the Fick's first law. As can be seen from Fig. 2, the maximum permeability value was obtained for 0.08 M Na2S2O3. However, the permeability values showed a tendency to decrease at lower or higher concentrations of Na2S2O3. In the studies by Nowier et al. (2000) and Rounaghi et al. (2008), it was reported that increasing stripping phase concentration facilitates achieving an optimum permeability value. The reason for this may be explained as the concentration of the solute increases, the solution reaches saturation value and therefore solubility decreases. In this case, the amount of (S2O3)2− ions which would form a complex with the silver ions in the membrane/stripping interface decreases.
Fig. 3. Permeability values, calculated according to Fick's first law, for the transport of Ag+ ion through the SLM process using different stripping solutions (stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane liquid: 0.01 M DC18C6 in toluene; flow rate: 50 mL/min).
If mass transport is performed by the diffusion process in the SLM system and carrier concentration is higher than that a particular level, this increases the viscosity of membrane liquid. In this case, since the resistance of the membrane against carrier movement increases, the transport rate of the ion decreases. Therefore, there is an optimum carrier concentration in all supported liquid membrane systems and this concentration needs to be determined experimentally. As seen from Fig. 4, if the carrier concentration was higher than 0.05 M DC18C6, it negatively affected Ag+ ion transport and reduced the permeability value significantly. Similar results were also obtained by Alonso et al. (2006).
3.3. Effect of HNO3 concentration on Ag+ ion transport
3.5. Effect of the flow rates on Ag+ transport
In this part of the study, HNO3 of 0.02, 0.20 and 2.0 M concentrations were used in order to determine the effect of acid concentration in feeding phase solution on Ag+ ion transport. The permeability values of Ag+ ion transport for each HNO3 concentration are given in Fig. 3. According to the Fig. 3, increasing HNO3 concentration caused the permeability values to decrease in the SLM process. This result shows that at low [H+] ion concentrations, silver ions form a strong complex with DC18C6 ligands (Ag(L)NO3) in the membrane surface.
Co-current flow pattern was used the SLM system of this study. In this part of the study, the flow rates of stripping and feeding solutions were varied and the effects of its changes were analyzed. The results are given in Fig. 5a and b. In the experiment, while the flow rates of a phase were being tested, the flow rate of the other phase was kept constant at 50 mL/min. As can be seen in Fig. 5a and b, the silver ion transport is slightly faster when the stripping phase flow rate is higher than the feeding phase flow rate. The maximum permeability value (238 cm3/cm2 s) was obtained when the flow rate of the feeding and the stripping phase were 50 mL/min and 80 mL/min respectively. The reason may be that the complex which is formed slowly in the feeding/membrane interface and break more rapidly
3.4. Effect of carrier concentration on Ag+ transport In order to analyze the effect of carrier concentration on Ag+ transport, DC18C6 with different concentrations (0.01 M, 0.05 M, 0.1 M ve 0.15 M) were used. The calculated permeability values are given in Fig. 4.
Fig. 2. Permeability values, calculated according to Fick's first law, for the transport of Ag+ ions through the SLM process using different stripping solutions (feeding phase: 0.02 M HNO3; membrane support: PTFE; membrane liquid: 0.01 M DC18C6 in toluene; flow rate: 50 mL/min).
Fig. 4. Permeability values calculated for the transport of Ag+ ions through the SLM process using different stripping solutions (feeding phase: 0.02 M HNO3; stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane solvent: toluene; flow rate: 50 mL/min).
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Fig. 6. Permeability values calculated for transport of Ag+ ion through the SLM process using a binary carrier (feeding phase: 0.02 M HNO3; stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane liquid: 0.05 M DC18C6 in toluene; feeding phase flow rate: 50 mL/min; concentration of the second carriers: 0.001 M).
3.7. Optimum operating conditions in Ag+ transport through SLM process
Fig. 5. Permeability values calculated for transport of Ag+ ion through the SLM process in different flow rate of feeding (a) and stripping (b) phases (feeding phase: 0.02 M HNO3; stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane liquid: 0.05 M DC18C6 in toluene).
in the membrane/stripping interface. Consequently, the ion transport that occurred with the flow rate on both surfaces of the membrane affects the complexation and decomposition of the carrier–metal complex in the feeding/membrane and membrane/stripping interfaces respectively. It was observed that keeping the flow rate of stripping phase higher than that of the feeding phase does not contribute greatly to permeability. Therefore, it is possible to consider a flow rate value of 50 mL/min as an optimum value for the both phases. 3.6. Effect of using a second carrier in membrane on Ag+ transport In order to increase mass transport efficiency with SLM process, it is possible to use two different metal complexion agents as cooperative carrier. However, it was indicated that using two different carriers may affect transport efficiency positively or negatively (Gaikwad, 2004; Bansal et al., 2005). Tri-n-octylamine (TLA), tributyl phosphoric acid/tributyl phosphate (TBP) and tri-n-octyl phosphinoxide (TOPO) complexion agents have been generally used in supported liquid membrane systems in order to transport the metal complexes in acidic solutions. In this study, TBP, TOPO and TOA having different transport mechanisms were added to DC18C6, and their permeability values were compared with DC18C6 carrier alone (Fig. 6). According to the Fig. 6, the permeability values calculated for the experiments using two carriers were lower than that of the experiment using DC18C6 carrier alone. The reason is probably that the experiments were performed under more appropriate conditions for DC18C6 (feeding and stripping phase type, feeding and stripping concentration, membrane solvent type, carrier concentration etc.). In addition, using a second carrier may have increased viscosities in the aqueous and membrane phases and hindered the metal ion transfer due to the limited loading capacity of the carriers in the membrane.
According to the results of the study, the optimum conditions for transportation of Ag+ ions through flat-sheet SLM are as follows: feeding phase; 50 mg/L Ag+ dissolved in 0.015 M HNO3, stripping phase; 0.08 M Na2S2O3, membrane liquid; 0.05 M DC18C6 in toluene, flow rate of feeding and stripping phases; 50 mL/min and operation time was 240 min. Under the optimum conditions, variations of mass balances of Ag+ ions in three phases (feed, stripping and membrane) in the SLM system as a function of time are given in Fig. 7. The amount of Ag+ ions in the membrane phase was determined using a mass balance calculation and the other phases were determined experimentally. As can be seen in Fig. 7, Ag+ ions can be transported from feed phase to membrane phase at the rate of 94% under optimum conditions. However, the transport efficiency from the membrane phase to the stripping phase remained at approximately 81%. In accordance with the results of the mass balance, it is thought that approximately 12–13% of the Ag+ ions accumulated in the membrane phase. Accumulation in the membrane phase reaches a maximum value (17%) at 120 min and then starts to decrease. This may have arisen from the fact that the formation of metal– carrier complex in the feeding/membrane interface is higher than the decomposition of the complex in the membrane/stripping interface, or the
Fig. 7. Mass balance of Ag+ ions as a function of time in the feed, stripping and membrane phases for the supported liquid membrane system operating under optimum conditions (stripping phase: 0.08 M Na2S2O3; feed phase: 0.02 M HNO3; membrane support: PTFE; membrane liquid: toluene-DC18C6 (0.05 M); flow rate: 50 mL/min).
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composed complex may be precipitated in the membrane liquid. The reaction rate constants and fluxes were obtained at the optimum conditions as follows: kf =1.3×10−2 min−1, ks =2.9×10−2 min−1 and Jf =−2.6×10−2, Js =1.5×10−2. 4. Conclusions As results of this study, it was determined that Ag+ ions can effectively be separated from aqueous solutions through a flat-sheet SLM process containing the membrane liquid prepared from a mixture of DC18C6 and toluene. Transportation in the process is performed by the formation of a carrier–metal complex in the stripping/membrane interface and the break down of this complex in the membrane/stripping interface. If the stripping solution is prepared from Na2S2O3, transport efficiency increases because Ag+ ions and (S2O3)2− ions can easily form a complex. However, when the stripping solution was prepared by dissolving Na2S2O3 in NaOH and NH3, transport efficiency decreased. A similar effect was observed in the experiment using the membrane liquids contained two different carriers such as TBP, TOPO and TOA. Therefore, in order to improve metal transport efficiency, instead of using a second carrier, it may be more useful to add a hydrophobic substance to reduce the viscosity of the membrane liquid. In the SLM, since there is a hydrophobic organic phase between two liquid phases, the interaction between the two liquid phases is not possible. Therefore, it is concluded that ionic strength of the liquid phases doesn't affect the material transportation. A study by Pei et al. (2009) also presented that the ionic strength could be neglected due to a similar reason. In addition, the feeding phase acid concentration should be kept as low as to limit the transport of H+ ions and should be at a high enough level to give support to the transport of silver ion as a primary transport mechanism by forming a carrier–metal complex. In this study, toluene (0.47 g/L at 20 °C), DC18C6 (very low) which are only slightly soluble in water were used. Therefore, they last for a longer time in hydrophobic membrane support and they increase the membrane stability. Organic interferences are not observed in the metal analyses performed at the aqueous phases. Therefore, it is accepted that the membrane is stabile during the time of the in our work. For a longer process time, the membrane stability can be affected. The transport of Ag+ ions from the membrane phase to the stripping phase is not as rapid and efficient as the transportation from the feeding phase to the membrane phase. The reason is that the reactions taking place in the feeding/membrane and the membrane/ stripping interfaces occur at different reaction speeds or the composed complex may be precipitated in the membrane liquid causing some Ag+ ions accumulation in the membrane itself. Under optimum conditions, 94% of Ag+ ions are carried from the feeding phase to the membrane phase. Acknowledgement This study was supported by The Scientific and Technological Research Council of Turkey (TÜBİTAK) as a research project (project no.: 106Y085). References Alexandrova, S., Dimitrov, K., Saboni, A., Boyadzhiev, L., 2001. Selective recovery of silver from dilute polymetal solutions by rotating film pertraction. Sep. Pur. Tech. 22–23, 567–570. Alguacil, F.J., Alonso, M., Sastre, A.M., 2001. Modelling of mass transfer in facilitated supported liquid membrane transport of copper(II) using MOC–55 TD in iberfluid. J. Membr. Sci. 184, 117–122. Alonso, M., Lopez-Delgado, A., Sastre, A.M., Alguacil, F.J., 2006. Kinetic modelling of the facilitated transport of cadmium(ii) using cyanex 923 as ionophore. Chem. Eng. J. 118, 213–219.
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Membr. Sci. 147, 195–206. Buschmann, H.J., Wego, A., Schollmeyer, E., 2001. Silver complexes with large crown ethers in propilen carbonate. Inorg. Chem. Commun. 4, 9–11. Calzado, J.A., Palet, C., Valiente, M., 2001. Metal affinity liquid membrane. Part III: characterization of transport selectivity. J. Sep. Sci. 24, 533–543. Chaudry, M.A., Bukhari, N., Mazhar, M., 2008. Coupled transport of Ag(I) ions through triethanolamine–cyclohexanone-based supported liquid membranes. J. Membr. Sci. 320, 93–100. El-Bachiri, A., Hagege, A., Burgard, M., 1996. Recovery of silver nitrate by transport across a liquid membrane containing dicyclohexano 18 crown 6 as a carrier. J. Membr. Sci. 121, 159–168. Flett, D.S., Wilson, J.C., 1983. Thiosulfate leaching of gold and silver ores. Trans. Inst. Min. Metall. 92, 216–222. Gaikwad, A.G., 2004. Synergetic transport of europium through a contained supported liquid membrane using trioctylamine and tributyl phosphate as carriers. Talanta 63, 917–926. Irwin, R.J., Van Mouwerik, M., Stevens, L., Seese, M.D., Basham, W., 1998. Environmental Contaminants Encyclopedia. National Park Service, Water Resources Division, Fort Collins. Izatt, R.M., Clark, G.A., Christensen, J.J., 1985. Transport of AgBr2 − in an emulsion liquid membrane using Mn+-DC18C6 carriers (Mn+ = Li+, Na+, K+ or Mg2+). J. Membr. Sci. 24, 1–5. Izatt, R.M., Clark, G.A., Bradshaw, J.D., Lamb, J.D., Christensen, J.J., 1986. Macrocycle facilitated transport of ion in liquid membrane system. J. Sep. Pur. Rev. 15, 21–72. Lakshmi, D.S., Mohapatra, P.K., Mohan, D., Machanda, V.K., 2004. Uranium transport using a PTFE flat-sheet membrane containing alamine 336 in toluene as the carrier. Desalination 163, 13–18. Marchese, J., Campderros, M., 2004. Mass transfer of cadmium ions in a hollow-fiber module by pertraction. Desalination 164, 141–149. Marchese, J., Campderros, M.E., Acosta, A., 1993. Mechanistic study of cobalt, nickel and copper transfer across a supported liquid membrane. J. Chem. Technol. Biotechnol. 57, 37–42. Mohapatra, P.K., Lakshmi, D.S., Manchanda, V.K., 2006. Diluent effect on Sr(II) extraction using di-tert-butyl cyclohexano 18 crown 6 as the extractant and its correlation with transport data obtained from supported liquid membrane studies. Desalination 198, 166–172. Molinari, R., Poreio, T., Argurio, P., 2006. Selective removal of Cu2+ versus Ni2+, Zn2+ and Mn2+ by using a new carrier in a supported liquid membrane. J. Membr. Sci. 280, 470–477. Nowier, H.G., El-Said, N., Aly, H.F., 2000. Carrier-mediated transport of toxic elements through liquid membranes transport of Cd(II) from high salinity chloride medium through supported liquid membrane containing TBP/Cyclohexane. J. Membr. Sci. 177, 41–47. Pei, L., Yao, B., Zhang, C., 2009. Transport of Tm(III) through dispersion supported liquid membrane containing PC-88A in kerosene as the carrier. Sep. Pur. 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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Transport of silver ions through a flat-sheet supported liquid membrane Sureyya Altin ⁎, Yilmaz Yildirim 1, Ahmet Altin 2 Zonguldak Karaelmas University, Department of Environmental Engineering, 67100, Turkey
a r t i c l e
i n f o
Article history: Received 19 August 2009 Received in revised form 12 March 2010 Accepted 16 March 2010 Available online 25 March 2010 Keywords: Supported liquid membrane Silver Toluene DC18C6 PTFE membrane
a b s t r a c t Toxic metals from industrial wastewaters are an important environmental issue. The use of supported liquid membrane processes has gained momentum in recent years, as it allows the reuse of water and toxic metals. The aim of this study is to investigate the active transport of silver ion through a supported liquid membrane (micro-porous Fluoropore PTFE) of DC18C6 (Dicyclohexano18crown6) in toluene under various experimental conditions. For this purpose, the effects of various parameters including binary carriers, carrier concentration, feed phase concentration, the nature and concentration of stripping agents in the stripping phase and flow rates of feed and stripping phases on transport efficiency were also investigated. The maximum transport efficiency was observed at the following conditions: flow rates of 50 mL/min in both phases, 0.05 M DC18C6 in toluene as carrier solution, 50 ppm Ag+ dissolved in 0.015 M HNO3 as feed solution and 0.08 M Na2S2O3 as stripping solution Optimum operation time was determined as 240 min. Under these conditions, 94% of the silver ions were transported from the feed phase to the membrane phase. However, the transport rate from the membrane phase to the stripping phase remained at approximately 81%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The presence of silver in the natural water environment is of great concern because of its toxicity to aquatic plants and animals, especially when it is in the free ion form (Ag+) (Petering and McClain, 1991; Ratte, 1999; Irwin et al., 1998; Wang et al., 2003). The major sources of silver in the environment are wastes from the medical and photographic-imaging industry, and wastes from the manufacturing of electronics, silverware, and jewelry (Purcell and Peters, 1998; Rounaghi et al., 2008). An estimated 150,000 kg of silver enters the aquatic environment every year from the photography industry, mine tailings, and electroplaters (Irwin et al., 1998). Therefore, separation and recovery of silver from industrial wastes is of great importance. Recently, liquid membrane technologies have commonly been used in order to separate and recover heavy metals from diluted solutions. Liquid membrane systems in which solvents containing carriers are placed in a porous support material are called supported liquid membrane (SLM) (Boyadzhiev and Lazarova, 1995). Compared to other liquid membrane methods, the major advantage of the SLM method is that the SLM requires less membrane liquid for the formation of the support matrix. Moreover, its low operating costs, minimum product contamination, no phase separation problem,
⁎ Corresponding author. Tel.: + 90 372 2574010 1565; fax: +90 372 2574023. E-mail addresses: [email protected] (S. Altin), [email protected] (Y. Yildirim), [email protected] (A. Altin). 1 Tel.: + 90 372 2574010 1540; fax: +90 372 2574023. 2 Tel.: + 90 372 2574010 1291; fax: +90 372 2574023. 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.03.015
selectiveness and flexibility are the other advantages. However, the instability of the liquid membrane in the pores of the inert membrane support (carrier lost) is the main disadvantage (Izatt et al., 1986; Boyadzhiev and Lazarova, 1995; Calzado et al., 2001). In supported liquid membranes, the support material which contains membrane liquid can be flat-sheet or hollow fiber. Both of these support materials are generally produced from polymers which can be moisturized by oleophilic and membrane liquid. Although there are some studies in which hollow fiber was used as SLM (Breembroek et al., 1998a; Marchese and Campderros, 2004), flatsheet membranes produced from PVDF or PTFE were mainly used in many SLM studies (Breembroek et al., 1998b; Zhang and Gozzelino, 2003; Bansal et al., 2005). There are many previous studies to separate silver ions from aquatic environment using bulk or emulsion liquid membranes. In a study by Rounaghi et al. (2008), silver ion transport in bulk membrane was analyzed using different acyclic and cyclic ligands as carriers and nitrobenzene as a membrane solvent. El-Bachiri et al. (1996) analyzed silver nitrate transport in a dichloromethane membrane containing DC18C6 as a carrier in bulk liquid membrane. Vajda et al. (2000) used Cyanex471X as a carrier in a bulk liquid membrane system. They observed that silver transport increased as the HNO3 concentration in the aqueous phase and the carrier concentration in the solvent increased. Safavi and Shams (1999) separated silver from aqueous phase in Ag (CN)2− ions using a Victoria Blue carrier. A study by Izatt et al. (1985) investigated silver transport as AgBr2− in emulsion liquid membranes. The organic membrane was formed from DC18C6 dissolved in toluene and Li2S2O3 was used as the stripping phase.
S. Altin et al. / Hydrometallurgy 103 (2010) 144–149
The number of studies about silver ion transport through SLM is limited. Chaudry et al. (2008) analyzed the silver ion transport in acidic solutions using trietanolamine (TEA) as a carrier in SLMs. In addition, the effects on transport efficiency of HNO3 and Ag+ concentrations in the feeding phase and KCN concentration in the stripping phase were also analyzed. There are also some previous studies on the transport of Ag+ ions in SLMs with different carriers, membrane solvents and feeding–stripping phases, by Boyahzhiev and Dimitrov (1994), Zolgharnein et al. (2003), Alexandrova et al. (2001), Shamsipur et al. (2006) and Zaghbani et al. (2008). As mentioned above, crown ethers are usually employed in metal transportation studies. Complexation between crown ethers and cations depends on two possibilities. The first possibility is the cavity of ligand and dimension of cation. If the cavity and dimension are approximately close to each other, the stable complexes form. Since dimension of silver cation is 2.52 A° and the cavity of 18C6 is 2.6– 3.02 A°, cation can be accepted into the cavity. In general, stability of complexes depends on properties of donor atoms. The second possibility is wrapping around the metal. If there is an interaction between functional groups and the metal, crown ether is then wrapped around the metal. In this study, the complexation was not studied but regarding on some studies that stability constants (equilibrium, K in L/mol) for the complex formation of DC18C6-Ag+ were given as 1.59 in water,5.63 in acetone and 7.98 in propylene carbonate (Izatt et al., 1985; Buschmann et al., 2001). In previous studies about the separation of silver from aqueous solutions through SLMs, the silver ions were transported after being turned into various complexes. In the Ag+ ions transport through liquid membrane systems, CN− and Br−, which generally form a complex with Ag+ easily and which are quite toxic, were preferred in many previous studies. But, in this study, transport of Ag+ ion in HNO3 solution through SLM using DC18C6 in toluene without any toxic complexation agent (CN− or Br−) were experimentally investigated. Another important aspect of the present study is that the effects of stripping solution and carriers with different characteristics on the silver ion transport were studied. All the data obtained in the study were applied to Fick's first law of diffusion and their permeability values were estimated. 2. Materials and methods 2.1. Reagents Dicyclohexano18crown6 (DC18C6), tri-n-octylamine (TOA), tributyl phosphate (TBP) and tri-n-octylphosphine oxide (TOPO) (Aldrich Chemical Company) were used as carriers. Reagent grade toluene (Merck) was used as an organic membrane solvent. All other chemicals (AgNO3, HNO3, Na2S2O3, NaOH, HCl and NH3) used in this study were of the highest a supporting purity available (Merck Chemical Company). A hydrophobic PTFE membrane (Fluoropore trade mark) with pore size 0.22 µm, a thickness of 175 µm and a porosity of 70% was selected as medium to hold the membrane solution containing carrier. 2.2. Experimental apparatus The flat-sheet SLM apparatus was used in the study. The membrane module was produced from Teflon material as two different cells. The membrane was placed between two cell compartments which were tightly clamped together. The total volume of the feeding and stripping solutions was 200 mL for each. The flow of the solution in the membrane module was enabled by two peristaltic pumps (Heidolph PD5206) and the flow rate was controlled by two flow meters (Raczek KFR-4256NS). Digital magnetic stirrers (Heidolph MR 3004S) were used to homogenize feeding and stripping solutions.
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2.3. Experimental procedure A DC18C6 in toluene was used as the membrane liquid in the experimental studies. Except for the experiments in which the acid concentration of feeding phase was investigated, 50 mg/L Ag+ concentration in the feeding phase was prepared as dissolving AgNO3 in 0.02 M HNO3 solution. When preparing the SLM module, initially the PTFE membrane was cut into the suitable dimensions (40 × 70 mm) to fit the module and soaked in the carrier-solvent solution for one hour. The membrane was then removed from the organic solution, cleaned with filter papers to take away excess liquids 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. Samples (1.0 mL) were taken at regular time intervals from the tanks of both sides, and silver ion concentrations in the feed and stripping phases were analyzed by using an atomic absorption spectrophotometer (Perkin Elmer 1100B model). 2.4. Permeation model of silver ion The transport of metal ions through the supported liquid membrane system is considered to be composed of many elementary steps. In brief, to model the transport of metal ions, it is necessary to consider diffusion of 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 diffusion is the speed restricting step in substance transport in a membrane, it is called steady state and is explained by Fick's first law (Marchese et al., 1993; Sastre et al., 1998; Alguacil et al., 2001; Ata, 2005; Molinari et al., 2006). 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 an SLM system is given by an appropriate formulation of Fick's first law of diffusion as follow: J=−
Vf dCf D = ðCfi −Csi Þ Adt L
ð1Þ
Where: Vf is the volume of the source phase solution, D is the diffusion coefficient of the complex, L is the membrane thickness and 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. (1) is simplified to: P=
J Cf
ð2Þ
where Cf is the concentration of metal ions in the feed and P is the permeability coefficient. Membrane permeabilities were determined by analyzing silver concentration by AAS in the source phase as a function of time. The permeation coefficient (P) was computed using the following equation: ln
Cf Cf0
=−
Ae Pt Vf
ð3Þ
where Cf and Cf0 are the concentration of metal ions in the aqueous feed phase at time t and; the initial concentration of metal ions (at t = 0), respectively. Ae is the effective area (Ae = A ⁎ ε), ε (85%) is the porosity of the membrane material. By plotting ln CCf against t, a linear f0 curve is obtained. The slope of this curve can be used to calculate permeability coefficient (P) of the SLM system.
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In this study, the data obtained were applied to the model given in Eq. (3) with the help of an iteration program and the permeability values were calculated. The efficiency of the SLM process for different operational conditions was compared on the basis of these permeability values. Finally, to determine k constants at the membrane interfaces, optimum experimental data were used. For the calculation of k constants, consecutive irreversible first order reaction was assumed in the carrier-mediated liquid membrane systems (Eq. (4)) (Lakshmi et al., 2004; Bansal et al., 2005; Mohapatra et al., 2006). kf
ks
D→M→A
ð4Þ
Where, F, M and S represent the feed, the membrane and the stripping phases, respectively. kf and ks were calculated using following equations (Ata, 2005). Rf = exp ð−kf t Þ Rm =
ð5Þ
kf ½expð−kf t Þ− exp ð−ks t Þ ks −kf
Rs = 1−
ð6Þ
1 ½k expð−kf t Þ−kf exp ð−ks t Þ ks −kf s
ð7Þ
And, the fluxes were calculated using following equations: Jf = −kf
k − kf f =ðk −ks Þ k ks =ðk −ks Þ f f Js = ks f ks ks
ð8Þ
3. Results 3.1. Effect of the nature and concentration of stripping agents on Ag+ transport In order to analyze the effect of stripping phase on Ag+ transport, various stripping solutions were prepared. For this purpose, seven stripping solution including Na2S2O3, HCl, NaOH and NH3 with different concentrations were tested. The permeability values calculated for all stripping solutions, based on Fick's first law of diffusion, were given in Table 1. As can be seen from Table 1, the maximum permeability values were obtained in the experiment in which 0.04 M Na2S2O3 was used as the stripping phase. • It was well-known that, through its donor groups the El-Bachiri et al. (1996), DC18C6 (symbolize by L) is able to replace the hydration molecules of silver in liquid membranes. Silver cations are then coextracted with nitrate anions under a complex ion-pair form. In the equilibrium following reaction takes place. þ
−
Ag þ NO3 þ Lorg↔AgðLÞNO3 org
• On the stripping side, this complex will break to release the L carrier and forms a new complex structure with (S2O3)2− and Ag+ ion proceeds to the stripping phase (reaction (2)). 2−
AgðLÞNO3 org þ Na2 S2 O3 þ ↔2NaNO3 þ ½AgðS2 O3Þ2
3−
It was evaluated that kf = 4.7 × 1013 for [Ag(S2O3)2]3−. In conclusion, the mechanism of silver ion transport in a supported liquid membrane has been schematically defined for Na2S2O3 as stripping solution. Fig. 1 shows the three different phases and the transport stages of silver ion in a SLM. The permeability values which were obtained from the experiments in which only NaOH and HCl solutions are used as the stripping agent were lower than those obtained using the Na2S2O3 solution. The experiment in which only NH3 was used is not shown in Fig. 1 due to lower Ag+ ion transport. In a study by Flett and Wilson (1983) with the aim of improving the transportation, NH3–Na2S2O3 solution was tested for the extraction of Ag+ ions. It was reported that under appropriate conditions, the formation of [Ag(S2O3)2]3− complex in the stripping solution (NH3–Na2S2O3) occurs readily. Similar results and important permeability values were also obtained in the present study using NH3–Na2S2O3 as the stripping agent. However, the permeability values remained lower than that obtained using only Na2S2O3 solution as stripping agent. In order to improve the efficiency of the NaOH as the stripping agent, Na2S2O3 was added to the solution, but it was observed that the positive effect of this process on Ag+ ion transport was imperfect. The results showed that the sequence of permeability efficiency for Ag+ ion transport using NaOH as stripping agent was: 0.04 M Na2S2O3 prepared from 0.01 M NaOH N 0.01 M NaOH N 0.1 M NaOH. Accordingly, it is thought that in the presence of NaOH, the following reactions may be possible. • Using NaOH as a stripping solution raised the pH in the stripping phase after the experiment was started. In the beginning of the experiment; although pH is basic, silver ions were not precipitated due to low transportation of silver ions. However for a while later, the precipitation was occurred according to following reaction equation. In reaction (3), in the beginning of the experiment AgOH is an unstable compound (Ksp = 1.5 × 10−8); by the time the reaction proceeds and in a short time later dark colored Ag2O compound is precipitated in the stripping phase. In order to find transported silver ions in the stripping phase, the precipitated silver was dissolved in a strong acid and its concentration was analyzed by AAS. AgðLÞNO3 þ 2NaOH→2AgOH þ 2NaNO3 →Ag2 O↓ þ NaNO3 þ H2O ð3Þ
ð1Þ
Table 1 Permeability values for the transport of Ag+ ion through SLM using different stripping solutions (feeding phase: 0.02 M HNO3; membrane support: PTFE; membrane liquid: 0.01 M DC18C6 in toluene; flow rate: 50 mL/min). Stripping solution
P × 106 cm3/cm2 s
0.01 M Na2S2O3 prepared from 0.001 M NH3 0.04 M Na2S2O3 prepared from 0.001 M NH3 0.10 M HCl 0.10 M NaOH 0.01 M NaOH 0.04 M Na2S2O3 0.04 M Na2S2O3 prepared from 0.010 M NaOH
101 137 155 131 148 191 154
ð2Þ
Fig. 1. Schematic display of the transportation mechanism of Ag+ ions in SLM.
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• When using the mixture of Na2S2O3 and NaOH as stripping solution, the stripping yield was slightly more than that of NaOH alone. Since the medium was basic, reaction (3) was more effective on silver ions. Due to high complex formation constant of (S2O3)2− ions with silver ions, the stripping yield was enhanced. • When using the mixture of Na2S2O3 and NH3 as stripping solution, complex formation constant of [Ag(S2O3)2]3− (kf = 4.7 × 1013) was 7 higher than that of Ag(NH3)+ 2 (kf = 1.7 × 10 ). Therefore as seen from Fig. 1, the silver transportation depends on the concentration of (S2O3)2− ions in the stripping phase. • When using Na2S2O3 alone as stripping solution, silver forms [Ag (S2O3)2]3− complex according to reaction (2) at the interface of membrane/stripping phase. 3.2. Effect of Na2S2O3 concentration on Ag+ ion transport In order to determine the effect of the concentration of Na2S2O3 solution as the stripping phase on Ag+ ion transport, 0.01 M, 0.03 M, 0.04 M, 0.08 M, 0.1 M, 0.15 M Na2S2O3 were used. Fig. 2 shows the permeability values calculated from the experimental results, based on the Fick's first law. As can be seen from Fig. 2, the maximum permeability value was obtained for 0.08 M Na2S2O3. However, the permeability values showed a tendency to decrease at lower or higher concentrations of Na2S2O3. In the studies by Nowier et al. (2000) and Rounaghi et al. (2008), it was reported that increasing stripping phase concentration facilitates achieving an optimum permeability value. The reason for this may be explained as the concentration of the solute increases, the solution reaches saturation value and therefore solubility decreases. In this case, the amount of (S2O3)2− ions which would form a complex with the silver ions in the membrane/stripping interface decreases.
Fig. 3. Permeability values, calculated according to Fick's first law, for the transport of Ag+ ion through the SLM process using different stripping solutions (stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane liquid: 0.01 M DC18C6 in toluene; flow rate: 50 mL/min).
If mass transport is performed by the diffusion process in the SLM system and carrier concentration is higher than that a particular level, this increases the viscosity of membrane liquid. In this case, since the resistance of the membrane against carrier movement increases, the transport rate of the ion decreases. Therefore, there is an optimum carrier concentration in all supported liquid membrane systems and this concentration needs to be determined experimentally. As seen from Fig. 4, if the carrier concentration was higher than 0.05 M DC18C6, it negatively affected Ag+ ion transport and reduced the permeability value significantly. Similar results were also obtained by Alonso et al. (2006).
3.3. Effect of HNO3 concentration on Ag+ ion transport
3.5. Effect of the flow rates on Ag+ transport
In this part of the study, HNO3 of 0.02, 0.20 and 2.0 M concentrations were used in order to determine the effect of acid concentration in feeding phase solution on Ag+ ion transport. The permeability values of Ag+ ion transport for each HNO3 concentration are given in Fig. 3. According to the Fig. 3, increasing HNO3 concentration caused the permeability values to decrease in the SLM process. This result shows that at low [H+] ion concentrations, silver ions form a strong complex with DC18C6 ligands (Ag(L)NO3) in the membrane surface.
Co-current flow pattern was used the SLM system of this study. In this part of the study, the flow rates of stripping and feeding solutions were varied and the effects of its changes were analyzed. The results are given in Fig. 5a and b. In the experiment, while the flow rates of a phase were being tested, the flow rate of the other phase was kept constant at 50 mL/min. As can be seen in Fig. 5a and b, the silver ion transport is slightly faster when the stripping phase flow rate is higher than the feeding phase flow rate. The maximum permeability value (238 cm3/cm2 s) was obtained when the flow rate of the feeding and the stripping phase were 50 mL/min and 80 mL/min respectively. The reason may be that the complex which is formed slowly in the feeding/membrane interface and break more rapidly
3.4. Effect of carrier concentration on Ag+ transport In order to analyze the effect of carrier concentration on Ag+ transport, DC18C6 with different concentrations (0.01 M, 0.05 M, 0.1 M ve 0.15 M) were used. The calculated permeability values are given in Fig. 4.
Fig. 2. Permeability values, calculated according to Fick's first law, for the transport of Ag+ ions through the SLM process using different stripping solutions (feeding phase: 0.02 M HNO3; membrane support: PTFE; membrane liquid: 0.01 M DC18C6 in toluene; flow rate: 50 mL/min).
Fig. 4. Permeability values calculated for the transport of Ag+ ions through the SLM process using different stripping solutions (feeding phase: 0.02 M HNO3; stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane solvent: toluene; flow rate: 50 mL/min).
148
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Fig. 6. Permeability values calculated for transport of Ag+ ion through the SLM process using a binary carrier (feeding phase: 0.02 M HNO3; stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane liquid: 0.05 M DC18C6 in toluene; feeding phase flow rate: 50 mL/min; concentration of the second carriers: 0.001 M).
3.7. Optimum operating conditions in Ag+ transport through SLM process
Fig. 5. Permeability values calculated for transport of Ag+ ion through the SLM process in different flow rate of feeding (a) and stripping (b) phases (feeding phase: 0.02 M HNO3; stripping phase: 0.08 M Na2S2O3; membrane support: PTFE; membrane liquid: 0.05 M DC18C6 in toluene).
in the membrane/stripping interface. Consequently, the ion transport that occurred with the flow rate on both surfaces of the membrane affects the complexation and decomposition of the carrier–metal complex in the feeding/membrane and membrane/stripping interfaces respectively. It was observed that keeping the flow rate of stripping phase higher than that of the feeding phase does not contribute greatly to permeability. Therefore, it is possible to consider a flow rate value of 50 mL/min as an optimum value for the both phases. 3.6. Effect of using a second carrier in membrane on Ag+ transport In order to increase mass transport efficiency with SLM process, it is possible to use two different metal complexion agents as cooperative carrier. However, it was indicated that using two different carriers may affect transport efficiency positively or negatively (Gaikwad, 2004; Bansal et al., 2005). Tri-n-octylamine (TLA), tributyl phosphoric acid/tributyl phosphate (TBP) and tri-n-octyl phosphinoxide (TOPO) complexion agents have been generally used in supported liquid membrane systems in order to transport the metal complexes in acidic solutions. In this study, TBP, TOPO and TOA having different transport mechanisms were added to DC18C6, and their permeability values were compared with DC18C6 carrier alone (Fig. 6). According to the Fig. 6, the permeability values calculated for the experiments using two carriers were lower than that of the experiment using DC18C6 carrier alone. The reason is probably that the experiments were performed under more appropriate conditions for DC18C6 (feeding and stripping phase type, feeding and stripping concentration, membrane solvent type, carrier concentration etc.). In addition, using a second carrier may have increased viscosities in the aqueous and membrane phases and hindered the metal ion transfer due to the limited loading capacity of the carriers in the membrane.
According to the results of the study, the optimum conditions for transportation of Ag+ ions through flat-sheet SLM are as follows: feeding phase; 50 mg/L Ag+ dissolved in 0.015 M HNO3, stripping phase; 0.08 M Na2S2O3, membrane liquid; 0.05 M DC18C6 in toluene, flow rate of feeding and stripping phases; 50 mL/min and operation time was 240 min. Under the optimum conditions, variations of mass balances of Ag+ ions in three phases (feed, stripping and membrane) in the SLM system as a function of time are given in Fig. 7. The amount of Ag+ ions in the membrane phase was determined using a mass balance calculation and the other phases were determined experimentally. As can be seen in Fig. 7, Ag+ ions can be transported from feed phase to membrane phase at the rate of 94% under optimum conditions. However, the transport efficiency from the membrane phase to the stripping phase remained at approximately 81%. In accordance with the results of the mass balance, it is thought that approximately 12–13% of the Ag+ ions accumulated in the membrane phase. Accumulation in the membrane phase reaches a maximum value (17%) at 120 min and then starts to decrease. This may have arisen from the fact that the formation of metal– carrier complex in the feeding/membrane interface is higher than the decomposition of the complex in the membrane/stripping interface, or the
Fig. 7. Mass balance of Ag+ ions as a function of time in the feed, stripping and membrane phases for the supported liquid membrane system operating under optimum conditions (stripping phase: 0.08 M Na2S2O3; feed phase: 0.02 M HNO3; membrane support: PTFE; membrane liquid: toluene-DC18C6 (0.05 M); flow rate: 50 mL/min).
S. Altin et al. / Hydrometallurgy 103 (2010) 144–149
composed complex may be precipitated in the membrane liquid. The reaction rate constants and fluxes were obtained at the optimum conditions as follows: kf =1.3×10−2 min−1, ks =2.9×10−2 min−1 and Jf =−2.6×10−2, Js =1.5×10−2. 4. Conclusions As results of this study, it was determined that Ag+ ions can effectively be separated from aqueous solutions through a flat-sheet SLM process containing the membrane liquid prepared from a mixture of DC18C6 and toluene. Transportation in the process is performed by the formation of a carrier–metal complex in the stripping/membrane interface and the break down of this complex in the membrane/stripping interface. If the stripping solution is prepared from Na2S2O3, transport efficiency increases because Ag+ ions and (S2O3)2− ions can easily form a complex. However, when the stripping solution was prepared by dissolving Na2S2O3 in NaOH and NH3, transport efficiency decreased. A similar effect was observed in the experiment using the membrane liquids contained two different carriers such as TBP, TOPO and TOA. Therefore, in order to improve metal transport efficiency, instead of using a second carrier, it may be more useful to add a hydrophobic substance to reduce the viscosity of the membrane liquid. In the SLM, since there is a hydrophobic organic phase between two liquid phases, the interaction between the two liquid phases is not possible. Therefore, it is concluded that ionic strength of the liquid phases doesn't affect the material transportation. A study by Pei et al. (2009) also presented that the ionic strength could be neglected due to a similar reason. In addition, the feeding phase acid concentration should be kept as low as to limit the transport of H+ ions and should be at a high enough level to give support to the transport of silver ion as a primary transport mechanism by forming a carrier–metal complex. In this study, toluene (0.47 g/L at 20 °C), DC18C6 (very low) which are only slightly soluble in water were used. Therefore, they last for a longer time in hydrophobic membrane support and they increase the membrane stability. Organic interferences are not observed in the metal analyses performed at the aqueous phases. Therefore, it is accepted that the membrane is stabile during the time of the in our work. For a longer process time, the membrane stability can be affected. The transport of Ag+ ions from the membrane phase to the stripping phase is not as rapid and efficient as the transportation from the feeding phase to the membrane phase. The reason is that the reactions taking place in the feeding/membrane and the membrane/ stripping interfaces occur at different reaction speeds or the composed complex may be precipitated in the membrane liquid causing some Ag+ ions accumulation in the membrane itself. Under optimum conditions, 94% of Ag+ ions are carried from the feeding phase to the membrane phase. Acknowledgement This study was supported by The Scientific and Technological Research Council of Turkey (TÜBİTAK) as a research project (project no.: 106Y085). References Alexandrova, S., Dimitrov, K., Saboni, A., Boyadzhiev, L., 2001. Selective recovery of silver from dilute polymetal solutions by rotating film pertraction. Sep. Pur. Tech. 22–23, 567–570. Alguacil, F.J., Alonso, M., Sastre, A.M., 2001. Modelling of mass transfer in facilitated supported liquid membrane transport of copper(II) using MOC–55 TD in iberfluid. J. Membr. Sci. 184, 117–122. Alonso, M., Lopez-Delgado, A., Sastre, A.M., Alguacil, F.J., 2006. Kinetic modelling of the facilitated transport of cadmium(ii) using cyanex 923 as ionophore. Chem. Eng. J. 118, 213–219.
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