Removal and recovery of Ag(CN)2- from synthetic electroplating baths by polymer inclusion membrane containing Aliquat 336 as a carrier

Chemical Engineering Journal 295 (2016) 207–217

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Removal and recovery of AgðCNÞ 2 from synthetic electroplating baths by polymer inclusion membrane containing Aliquat 336 as a carrier Imen Iben Nasser a, Fathia Ibn El Haj Amor a, Laura Donato b, Catia Algieri b, Angelo Garofalo b, Enrico Drioli b, Chedly Ahmed a,⇑ a b

Laboratoire des Interfaces et Matériaux Avancés (LIMA), Université de Monastir, Faculté des Sciences de Monastir, Bd. de l’Environnement, 5019 Monastir, Tunisia Research Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Via P. Bucci, Cubo 17/C, 87030 Rende (CS), Italy

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Phase inversion technique for

synthesis of PIMs based on Aliquat 336 was used.  Morphological and chemical characterizations of PIMs were investigated.  Application of PIMs for the removal and recovery of silver cyanide complex.  Investigation of the membrane selectivity towards AgðCNÞ 2 and ZnðCNÞ2 4 anions.  Lifetime and durability of PIM have been performed.

a r t i c l e

i n f o

Article history: Received 25 December 2015 Received in revised form 19 February 2016 Accepted 9 March 2016 Available online 14 March 2016 Keywords: Silver cyanide complex Aqueous solutions Polymer inclusion membrane Aliquat 336 Anionic exchange

a b s t r a c t The current research deals with the first application of a polymer inclusion membrane (PIM) for the removal and recovery of silver cyanide complex from an aqueous solution. The effective PIM containing Aliquat 336 as the carrier, cellulose triacetate (CTA) as the base polymer, and Bis (2-ethylhexyl) sebacate (BEHS) as a plasticizer were prepared by phase inversion technique via solvent evaporation and characterized by different techniques. The effects of several factors on the transport efficiency including the nature and the concentration of the carrier, the type of base polymer and the plasticizer, initial concentration of the feed phase and the stripping phase nature have been studied. This study shows that the membrane composition affects considerably the AgðCNÞ 2 transport. The transport mechanism is implemented by an anionic exchange between the fixed site of the Aliquat 336 and the anionic species in the solution followed by their diffusion across the membrane. We revealed that the transport of AgðCNÞ 2 found to be enhanced by the increasing of initial complex concentration and the best silver cyanide removal was achieved when HCO 3 is used as the stripping agent. Moreover, we found that PIM exhibited an important 2 selectivity towards AgðCNÞ 2 than ZnðCNÞ4 . These promising results lead us to apply this technology for the removal of silver cyanide complex from real wastewater. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction ⇑ Corresponding author. Tel.: +216 73 500 280; fax: +216 73 500 278. E-mail addresses: [email protected] (I. Iben Nasser), hajamorfethia@ yahoo.fr (F. Ibn El Haj Amor), [email protected] (L. Donato), [email protected] (C. Algieri), [email protected] (A. Garofalo), [email protected] (E. Drioli), [email protected] (C. Ahmed). http://dx.doi.org/10.1016/j.cej.2016.03.034 1385-8947/Ó 2016 Elsevier B.V. All rights reserved.

With significant industrial development, which is spread all over the world, the industry is generating large amounts of wastes that are often very hazardous to environment. Metal ions such as

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silver, zinc, chromium, lead and copper are some of the most harmful species found in these wastes [1]. Pollutant rejected by the surface treatment industry represents 30–40% from all industries. It is considered the main source of pollution by heavy metals. Among the toxic metals, silver is recognized to be both a toxic and precious metal with a high market value thus, its separation and recovery from industrial effluents is of great interest [2]. Silver finds numerous applications in industry (photography, galvanoplasty, jewellery, alloys against corrosion, electroplating etc.). As a consequence, these activities produces wastewaters that usually contain large amounts of metal cyanide complexes having the general formula [M(CN)n]x, where M represents the metal cation, n is the number of coordinated cyanide ions, and x is the total anionic charge of the complex [3]. Silver and zinc cyanide complexes are the ‘‘most known” metal cyanide compounds generated by electroplating industry. Silver cyanide complex is a stable complex (log b2 = 21.1) with the formula AgðCNÞ 2 while zinc cyanide complex ZnðCNÞ2 is less stable (log b4 = 16.7) [3]. Theses toxic com4 plexes are a serious threat to human health. Thus, considering the ultimate environmental fate of metallic cyanide complexes and mainly the economic loss of the silver metal, it is important to treat effluents containing these cyano-complexes before their discharge into the environment by removing and recovery of these pollutants. In this context, many conventional techniques have been applied to solve this environmental problem. Gupta et al. [4] explored the ion exchange method for treating wastewaters containing zinc cyanide complex by using a strong base anion resins. Lindstedt and Doyle [5] used Rohm and Haas IRA-400 anions exchange resin for the recovery of AgðCNÞ 2 from electroplating rinse water baths. Silver cyanide complex is extracted by adsorption on activated carbon known as carbon-in-pulp (CIP) [6]. Marugan et al. [7] have examined the feasibility of removal and recovery of dicyanoargentate complexes by heterogeneous photocatalysis with titanium dioxide. The use of membrane process for the removal of silver cyanide complex has been investigated. In fact, Akretche et al. [8] has examined the removal of silver in cyanide medium as a monovalent form through various anion exchange membranes by using Donnan dialysis as a membrane technique. Authors found that this process is beneficial for the possibility of recycling the dicyanoargentate complex and other cyanide complexes with a good impact on the environment. Supported liquid membranes (SLMs) have received an increasing attention as alternatives to liquid–liquid extraction over the past two decades. Silver cyanide complex was removed through a supported liquid membrane (SLM) containing K+-crown ether as a carrier [3]. The results obtained showed that the SLM system studied could be potentially used for removal, purification and separation of silver and copper cyanide complexes from their mixture. The use of Supported liquid membranes (SLMs) seems to be an advantageous alternative for removal of cyanide complexes due to its high selectivity, operational simplicity, low energy consumption and zero effluent discharge [9]. Nevertheless, the main drawback of supported liquid membranes is their relatively short lifetime due to leaching of the organic extracting solution into the adjacent aqueous phases that result in limited sampler’s stability [9]. To overcome these instability problems of SLMs, considerable attention is concentrated upon a new kind of membranes, named polymer inclusion membranes (PIMs). PIMs are characterized by the efficient immobilization of the carrier. Sugiura et al. [10], who succeeds to synthesize this type of membrane by incorporating the carrier in a plasticized thermoplastic polymer, discovered this method. These membranes have been known by their simple preparation, their long-term stability, versability, good chemical resistance and better mechanical prop-

erties when compared to supported liquid membranes [9]. They contain a base polymer providing the mechanical strength, a carrier molecule that effectively binds and transports the ions through the membrane, and a plasticizer that provides elasticity and acts as the solvent, in which the carrier molecule can diffuse [11]. PIMs are made by casting a solution containing a carrier, a plasticizer and a base polymer, such as cellulose triacetate (CTA) or poly (vinyl chloride) (PVC), to form a thin, flexible and stable film. The concept of PIMs and the role of its components have been reviewed by Nghiem et al. [11]. Among several carriers used for PIMs preparation, Aliquat 336 is one of the most extractants used in numerous applications. It is a commercial anion exchanger carrier widely used for the recovery of diverse types of metal anions [12]. In our knowledge, up to date no work was devoted to the removal of AgðCNÞ 2 by PIMs containing Aliquat 336 as a carrier. In this present work, we describe the novelty of our original results concerning the removal and recovery of silver in its anionic form AgðCNÞ 2 by diffusion across polymer inclusion membrane. Several factors have been varied in order to accomplish the optimum conditions for an effective silver cyanide complex removal by studying the composition of the membrane and the aqueous phases. The selectivity of the recovery process for AgðCNÞ 2 mixed with ZnðCNÞ2 4 is studied. The CTA-membranes have been characterized using chemical techniques as well as Fourier Transform Infra-Red (FTIR), Thermal Analysis (TGA) and morphological behavior. 2. Experimental part 2.1. Chemicals The chemical reagents used for the preparation of the PIM are: trioctylmethylammonium chloride (Aliquat 336), the tetrabutyl ammonium bromide and the Polyvinyl-chloride (PVC) (Mw = 72,000–74,000) were purchased from Aldrich. Cellulose triacetate (CTA), Bis (2-ethylhexyl) sebacate (BEHS), 2-nitrophenyl octyl ether (2-NPOE) and dioctyl phthalate (DOP) were purchased from Fluka and dichloromethane from Merck (Germany). NaHCO3, NaNO3, NaClO4, NaCH3COO, Zn (NO3)24H2O, K[Ag(CN)2] were also purchased from Merck (Germany). Laboratory grade ultrapure water (Millipore, Australia) was used for the preparation of all solutions. 2.2. Preparation of polymeric membrane Flat sheet membrane was prepared via dry phase inversion technique (solvent evaporation) as reported by Kumar [13] with slight modifications. In fact, a mixture of 200 mg of CTA, 100 mg of a plasticizer and the anion carrier Aliquat 336 (R3 MeN+ Cl) in various quantities was dissolved in 15 ml of dichloromethane at room temperature. After vigorous stirring, the obtained solution was homogeneous. The solvent of this mixed solution was slowly evaporated in a 9.0 cm diameter Petri dish, which was covered loosely, overnight. A small quantity of water was deposited on the obtained film to peel it out from the dish and placed in the transport cell. Blank membrane were prepared by the same procedure but without carrier. The obtained thin PIMs were completely clear, homogeneous, and flexible. PIMs exhibited a good mechanical strength, indicating that the CTA serves as a good supporting matrix for the solvent and the carrier. The membrane thickness was determined by measuring different locations of the membrane using a micrometer and calculating the average thickness. PIMs have a thickness about 70 lm.

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2.3. AgðCNÞ 2 analysis The anionic complex concentration was determined by means of potentiometric titration using the specific silver electrode 6.0430.100 Ag Titrode with Ag2S coating, the titrant was the AgNO3 according to the following chemical reaction:

AgðCNÞ2 þ Agþ $ 2AgCN ðsÞ 2.4. Membrane characterizations Membranes were characterized using chemical techniques as well as Fourier transform infra-red (FTIR), mechanical properties, thermogravimetric analyses (TGA), scanning electron microscopy (SEM) and contact angle. All the membranes were characterized by FTIR analysis using a Perkin–Elmer Spectrum One FT-IR spectrometer (Thermo Scientific Nicolet IS50ATR) according to the Attenuated Total Reflectance (ATR) technique. Elastic characterization of CTA blank, PIM with CTA and PIM with PVC based membranes was carried out with samples of 1 cm width and 3 cm length using a force digital gauge (Mark-T, LLOYD LS5 model) with a maximum tension of 100 N and length accuracy of ±0.01 mm connected to a computer. Measurements were performed at a strength rate of 10 mm/s. The morphology of the prepared membranes was investigated by scanning electron microscopy, (SEM) using a FEI QUANTA 200 F microscope at 20 kV. Membrane samples were freezed in liquid nitrogen for producing a clear brittle rupture. All samples were sputter-coated with gold before microscopic analysis. Thermogravimetric analysis (TGA) was carried out under a nitrogen atmosphere at a heating rate of 10 °C min1, using a TA Instruments Q50. Contact angle measurements were done with the contact angle measurement instrument GBX Scientifique Instrument (RomanFrance). The sessile drop method was used and ultrapure water was the wetting liquid. 2.5. Membrane transport experiments Transport experiments were carried out using a permeation cell in which the membrane film was tightly clamped between two cell compartments. Its effective area is 19.6 cm2. Magnetic stirrer bars stirred both feed and receiving aqueous phases (180 mL each) and all experiments were carried out in quasi-stationary regime at 25 °C. During the process, we measured the concentration of AgðCNÞ 2 in the feed and in the receiver phase. The kinetics of the transport across PIMs was described as a first-order reaction in anion complex concentration:

ln

Ct ¼ k t Ci

where Ct is the anion complex concentration at a given time in the feed phase, Ci the initial concentration of anion complex in the feed phase, k the rate constant (s1), and t is the time transport (s). The constant k values were calculated from the plots of ln (Ct/Ci) versus time. It is derived from the dependence ln (Ct/Ci) = f (t), and confirmed by the determination factor r2. The permeability coefficient (P) is then calculated using the following equation [14]:

V P ¼ ð Þ  k A where, V is the volume of the source solution (m3) and A represents the effective area of the membrane (m2). For the calculation of the initial fluxes (Ji) the following expression was used [14]:

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Ji ¼ P  C i In order to express the efficiency of the anion complex from the feed phase, we access to the recovery factor (RF%) by using the following expression [14]:

RFð%Þ ¼ ½ðC i  C f Þ=C i   100 where Ci is the initial concentration of AgðCNÞ 2 in the source solution and Cf represents its concentration in the source compartment at a final time (mol L1).

3. Results and discussion 3.1. Physical and chemical characteristics of cellulose triacetate membranes Various techniques were investigated to characterize the PIMs such as FTIR, mechanical properties, contact angle, SEM and TGA. In order to investigate the absorption bands of the constituents of the membranes containing CTA + BEHS + Aliquat 336, FTIR was performed. As it can be observed from FTIR spectra (Fig. 1), the presence of the bands between 2853 and 2963 cm1 attributed to the stretching vibration of –C–H and –CH2–. The absorption at 1735 cm1 is assigned to the stretching vibration of C@O in CTA. An absorption band between 1030 and 1220 cm1 are assigned to the C–O–C group and the peak at 1123 cm1 are attributed to C–N stretching vibrations of Aliquat 336. The mechanical properties of the prepared membranes are evaluated by measuring the Young’s modulus, the elongation at break and the tensile stress at break. The characterization is done to provide information on membrane strength. In the tensile test, the specimen is loaded by tensile force vertically that continuously increases and the observation was simultaneously done on the extension of specimen. The results reported in Table 1 and displayed in Fig. 2 shown that CTA blank and CTA-PIM membranes have good mechanical properties, compared by PIM based PVC, in spite the difference between both CTA membranes. In fact, the tensile strength of the blank CTA membrane was higher than that of CTA- PIM. This fact can be explained by the inclusion of the Aliquat 336 to the matrix, which lead to decrease the tensile strength of the membrane and enlarge the elastic behavior of CTA membrane. We observe also from Table 1 that the incorporation of Aliquat 336 in the matrix causes a decrease in the Young modulus. This behavior was in agreement with that found by Vázquez et al. [15]. The hydrophilic character of the membranes surfaces was determined by contact angle (/) measurements. Fig. 3 shows a comparison of the contact angle for both materials; blank membrane and CTA membrane containing Aliquat 336. As it can be observed, both membranes possess an hydrophilic surface but with different hydrophilicity level. Indeed, the inclusion of Aliquat 336 in the CTA matrix modifies the membrane behavior passing from a character slightly hydrophilic (h/i 65.5° for CTA blank membrane) to another more hydrophilic (h/i 34.7° for PIM containing CTA + 25% Aliquat 336). The reason of the different hydrophilicity level between blank membrane and PIM is explained by the presence of Aliquat 336 in the CTA matrix. In fact, a quaternary ammoniums group forms Aliquat 336 and it is known that this group presents the hydrophilic property induced by quaternary amine positively charged. As well as polarity of this functional group, exposed on the surface, makes the membrane more hydrophilic. This result is in agreement with results found by Vázquez et al. [15]. The morphology of the membranes was observed by the SEM images displayed in Fig. 4. It appears from the observation of the

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Fig. 1. FTIR spectrum of CTA membrane containing BEHS and Aliquat 336.

Table 1 Mechanical properties of CTA blank membrane and PIMs. Membrane

Tensile strength (MPa)

Elongation at break (%)

Young’s modulus (MPa)

CTA blank CTA–PIM PVC–PIM

331.45 30 6.5

12.11 25.66 57.43

9943.6 546.10 174.37

As illustrated in diagram (b) of the membrane containing BEHS + CTA + Aliquat 336, the weight loss begins at about 233 °C, which corresponds to the loss of 41.32% of both Aliquat 336 and BEHS in a single step. Essentially, this loss is may be due to the volatilization of a part of Aliquat 336, (the boiling temperature of the latter being 225 °C) and a part of the plasticizer due to its interaction with the polymer chains (its boiling temperature being 256 °C). The degradation of the CTA polymer starts at around 368 °C with the loss of 40.45% of its total mass in the membrane. It appears that the total of weight losses is about 82% of the membrane sample weight. 3.2. Transport experiment

Fig. 2. Stress strain-curve of CTA blank and PIM based CTA, and PIM based PVC.

cross section of the two membranes that they have a uniform and dense surface appeared without apparent porosity. Thermogravimetric analyses (TGA) were performed in order to link specific temperature and height of mass changes to the degradation of a specific compound of PMs. The thermograms of the CTA blank membrane (a) and PIM containing Aliquat 336 (b) were displayed in Fig. 5. We can notify from thermogram (a) that the degradation of CTA occurs after water loss, in only one stage at a temperature close to 351 °C (approximately 82% of the initial mass), after which the remaining compounds are lost by carbonization.

Based on transport experiment using the PIM without carrier, it has been noted that in absence of carrier, no transport of AgðCNÞ 2 from the feed phase to the receiver phase was perceived. In order to confirm this result, FT-IR spectrum of the blank membrane and PIM before and after silver cyanide complex transport have been performed as illustrated in Fig. 6. Concerning the CTA blank membrane, we do not observe any modification of surface chemical bonds, this may conclude that the CTA polymer is not the responsible for the diffusion of the complex. While, In the case of PIM membrane containing Aliquat 336, we observe the appearance of a new vibration band centered at 2142 cm1 attributed to the (C„N) bond related to the AgðCNÞ 2 complex. This observation confirm the retention of the complex by the fixed site of the membrane without any modification of the principal chemical bonds of the membrane. From these results, it can be proven that the PIM without carrier served as an effective barrier to ion permeation. This is in agreement with the results found in the literature [14]. 3.2.1. Effect of base polymer Given that, there are a vast number of polymers currently used for many engineering purposes [16], PVC and CTA are still the most commonly polymer matrices used in PIMs preparation which provide a good mechanical strength for the membrane and which are compatible with most carriers. In order to select the best base polymer for the transport and recovery of AgðCNÞ 2 , transport of the anion complex at 25 °C

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Fig. 3. Water contact angle measurement of the (a) CTA blank membrane and (b) CTA plasticized membrane containing 25% Aliquat 336.

Fig. 4. SEM images of the cross section of CTA blank membrane (a) and the CTA plasticized membrane containing the carrier Aliquat 336 (b).

Fig. 5. Thermograms of: (a) CTA and (b) CTA + BEHS + Aliquat 336.

through PIMs was performed. The membrane composition contain: 0.2 g CTA or 0.4 g PVC as a base polymer, 25% (w/w) Aliquat 336 as a carrier and bis (2-ethylhexyl) sebacate as a plasticizer. The strip phase composition contain 0.1 M HCO 3 as a driven anion and the feed phase is composed by 102 M AgðCNÞ 2. As shown in Table 2, membrane based on CTA exhibits the best performance for the complex removal. In fact, the percent recovery of AgðCNÞ 2 is higher with CTA membrane compared to PVC membrane. Less than 5% was transported to the strip phase across PVC membrane during 12 h of transport process.

The obtained results make evident the great influence of the nature of polymer matrix on the AgðCNÞ 2 transport efficiency. This can be attributed to the better immobilization of the Aliquat 336 in the membrane due to the ionizable groups of CTA that participate in dipole–dipole interactions. This finding is in agreement with that of Pérez Silva et al. [17] who have proven the high performance of CTA membrane as compared with that on PVC. This difference between both polymers can be also explained that CAT is a polar polymer with a crystalline structure while PVC is less polar than CTA and has a predominantly amorphous structure with a

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Fig. 6. FTIR spectrum of CTA blank membrane (a) and PIM membrane (b) before and after silver cyanide transport.

Table 2 Influence of the base polymers in anion complex transport. Base polymers

AgðCNÞ 2 removal (%)

J lmol cm2 s1

CTA PVC

47.16 4.5

1.22 0.058

small degree of crystallinity [18]. The efficiency of the CTA membrane led us to select it for the rest of this work. 3.2.2. Effect of the carrier nature Fully substituted quaternary ammonium compounds, such as the famous Aliquat 336, were considered as a basic carrier by several authors [11,19] despite the absence of a lone electron pair at the nitrogen atom. This criterion has been attributed to the resemblance in the extraction mechanism that involves amines and this type of quaternary ammonium. By using this category of quaternary ammonium, it is therefore an anion exchange of the carrier which forms an ion pair with the anionic complex from the aqueous phase. In this context, the transport has been accomplished with two carriers using a CTA as the base polymer and a same plasticizer (Bis (2-ethylhexyl) sebacate). Each carrier was evaluated in terms of the variation of the AgðCNÞ 2 concentration in strip phase as a function of time. The carriers selected for this study were: tetrabutylammonium bromide TBAB and Aliquat 336. Fig. 7 represents the evolution of the metal complex concentration in the strip phase during 12 h of the process by using an aqueous solution of HCO 3 0.1 M as a driving anion. The results show that the concentration of AgðCNÞ 2 in the strip compartment obtained with TBAB is very low compared with that of Aliquat 336. The difference between the two carriers is in their chemical structure. In fact, TBAB have a steric hindrance effect affected by the voluminous alkyl groups of the carrier molecule that limit the accessibility of AgðCNÞ 2 ions to the active sites while Aliquat 336 has less hydrocarbon chain. The other important factor is the mobile ion (chloride for the Aliquat 336 and bromide for TBAB) responsible for the anionic exchange phenomenon. Bromide ion is more voluminous than chloride ion, which means that, the diffusion of the complex is very slow across the membrane because the anion exchange between Br and AgðCNÞ 2 did not completely took place. This is probably due to the poor affinity of TBAB for the AgðCNÞ 2 anion that results in negligible exchange between the silver complex anion in solution and Br anion in the membrane. This results can be also explained based on the hydratized radius which are in the order Br > Cl [20]. Although, Cl is more

mobile than Br leading that the transport of the complex into the receiver compartment became more difficult with increasing the hydratized radius of ions. Therefore, it can be concluded that the transport mechanism may be limited by the diffusion of the more mobile anion that is in our case the chloride ion. Aliquat 336 was found to be a suitable quaternary ammonium for the removal of AgðCNÞ 2. 3.2.3. Effect of the carrier content In order to establish the optimal membrane composition for anionic complex transport, PIMs with various content of Aliquat 336 (10–40% w/w) were tested in membrane transport experiments, the achieved transport process being presented in Fig. 8. Before studying the influence of the membrane composition already reported in Section 3.2, we remind that transport of 102 M AgðCNÞ 2 across the membrane containing only CTA and the plasticizer bis (2-ethylhexyl) sebacate (without Aliquat 336) show that no AgðCNÞ 2 species in the receiver phase was perceived. This indicates that the complex transport from the feed to the stripping compartment results not only from the diffusion of AgðCNÞ 2 through the membrane but also from the anion exchange between the anionic complex and the fixed-site carrier. From the results displayed in Fig. 8, we can observe that the initial flux increases with the increasing of Aliquat 336 percent and reaches a maximum when the membrane contains 25% (w/w) of the carrier and decreases for higher content of the carrier. The increase of Aliquat 336 concentration induces the increase of the anionic complex concentration gradient within the membrane along the membrane thickness [21]. This optimum percent content of Aliquat336 is supposed to be enough to create the liquid pathways inside the membrane required to carry out the transport of the anionic complex [21]. Beyond this percent value, the flux decreases which could be explicated by the higher viscosity of the membrane containing more organic carrier. As a consequence, high viscosity in the phase liquid membrane limits the diffusivity of the anionic complex across the membrane seen that the carrier is incorporated into a polymeric material gel network [15]. This finding was also studied by Guell et al. [22] who noticed that the highest concentration Aliquat 336 is related to important parameters such as membrane conductivity and dielectric constant. In fact, they noted that when the carrier concentration increases, in parallel these parameters increase while the membrane area resistance decreases [22]. In terms of conclusion of this part, 25% (w/w) of Aliquat 336 was selected as the best percent as cited by Yildiz et al. [23]. This value will be used for the rest of our work.

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 Fig. 7. Evolution of the concentration of silver vs. time. ½AgðCNÞ 2 t¼0 ¼ 0:01 M, ½HCO3  ¼ 0:1 M, membrane composed by 0.2 g CTA, 0.1 g of each carrier and 0.1 g of bis (2ethylhexyl) sebacate.

 2 Fig. 8. AgðCNÞ M, strip: ½HCO 2 initial flux vs. Aliquat 336 content in the PIM. Feed: ½AgðCNÞ2 0 ¼ 10 3  ¼ 0:1 M. Membrane composition: 0.2 g CTA, 0.1 g Bis (2-ethylhexyl) sebacate.

3.2.4. Effect of the plasticizer type Among the important components in PIM, the plasticizer possesses a dual role. Either that it reacts as a solvent in which the carrier diffuse or it makes the film more soft and flexible in order to improve the mechanical stability of the membrane [24]. The action of the plasticizer is attributed to its ability to reduce the intermolecular attraction forces between the chains in the polymer systems [11]. In fact, it penetrates between the polymer molecules and ‘‘neutralize” their polar groups with its own polar groups or simply to increase the distance between the polymer chains and thus reduce the strength of intermolecular forces [11]. In order to select the suitable plasticizer, the CTA PIMs were prepared by using different plasticizers having different viscosity and dielectric constants. The weight fractions of CTA, plasticizer and Aliquat 336 were 0.2 g, 0.1 g and 0.1 g respectively. The chosen plasticizers in the preparation of PIM were: 2-nitrophenyl octyl ether (NPOE), dioctylphthalate (DOP) and bis (2-ethylhexyl) sebacate (BEHS). The diffusion experiment was carried out by using 102 M of AgðCNÞ 2 solution in the feed compartment and 0.1 M of HCO 3 solution in the strip phase. The variation of concentration

of AgðCNÞ 2 in the strip phase during 12 h of dialysis as a function of nature of plasticizers is displayed in Fig. 9. As can be seen from Fig. 9, the best transport efficiency of AgðCNÞ 2 was obtained with the BEHS, while the DOP was less effective for AgðCNÞ 2 transport because it has a high viscosity (g = 40.4 cP) and low dielectric constant (er = 5.1) [13]. As it was known, the most effective plasticizer is one which possesses high polarity (high dielectric constant). This behavior is usually attributed to NPOE due to its quite high polarity (er = 24) [13]. Whereas, in our case the BEHS produced better results by comparing with the two other plasticizers despite it possesses the low value of the dielectric constant (er = 3.9) [25]. The performance of this plasticizer is may be due to its high lipophilic character (log P = 11) [25]. Consequently, this phenomenon seems to be a bit particular and was not investigated in the literature.

3.2.5. Effect of initial concentration of silver The influence of the initial concentration of the anionic complex was investigated under the optimum conditions. The experiments

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 Fig. 9. Evolution of the concentration of silver complex vs. time. ½AgðCNÞ 2 t¼0 ¼ 0:01 M, ½HCO3  ¼ 0:1 M, membrane composed by 0.2 g CTA, 0.1 g of Aliquat 336 and 0.1 g of each plasticizer.

were performed using a solution of AgðCNÞ 2 , having different concentrations, as the feed solution and the strip solution used was NaHCO3 0.1 M. The complex concentration is in the range of 103 M–102 M. As shown in Fig. 10, the complex transport during 12 h of dialysis is presented in function of different initial concen tration of AgðCNÞ 2 .We notice that the transport of AgðCNÞ2 increases with its initial concentration i.e. enhanced by the increasing of initial complex concentration. The transport of the anionic complex across the membrane involves the coupled diffusion-transport wherein AgðCNÞ 2 permeates from feed to receiver compartment and HCO 3 diffuses in opposite direction. During this process, an ion exchange took place between ions accessible in both phases resulting from a difference in the chemical potentials under the driving force. This force is provided by Donnan equilibrium of HCO 3 ions which increases in the ion decreases with time. Thus, the feed feed phase while AgðCNÞ 2 phase was enriched by hydrogenocarbonate ions. This behavior agrees well with our previous work concerning the diffusion of cyanide through anion exchange membranes [26]. In term of percent recovery of silver complex, the transport percentages were determined to be 47.16%, 56.41% and 57.28% for AgðCNÞ 2 initial concentrations of 102, 5.103 and 103 M, respectively. We

observe that the transport percentage increases when the initial concentration of the complex decreases. This result can be explained by the increase in the driving force that improves the removal of the complex. Similar results were found by Arslan et al. [27] who reported that increasing the concentration of Cr (VI) in the feed phase decreased the transport percentage. 3.2.6. Effect of stripping medium The permeability of AgðCNÞ 2 through the membrane is considerably dependent on the nature of stripping agent which is among the important factors which affects the efficiency of the transport phenomenon. In order to select the suitable stripping anion for the complex anion removal, transport experiments were performed under optimum conditions by using CTA PIM containing 25%(w/w) of Aliquat 336 as a carrier and 0.1 g of bis (2-ethylexyl sebacate) as the plasticizer. The initial concentration of AgðCNÞ 2 was 102 M while the strip phase was composed by 0.1 M of different driving monovalent anions. The choice of monovalent anions is based on their important mobility across the membrane contrary to bivalent anions as was examined by Miyoshi [28], who informed that in the structure of the ions through membranes. A monovalent ion makes a pair ion

Fig. 10. Evolution of the concentration of silver complex vs. time as function of different feed initial concentration.

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with a fixed site in the membrane and it is transferred step by step between neighboring fixed ions. Whereas, in the case of bivalent ions, two fixed ions were used by bivalent ions for transport. Thus, monovalent ion needs only one fixed charged ion, which corresponds of a half-set of a bivalent ion. As presented in Fig. 11, the  effective transport was obtained with NO 3 and HCO3 . Compared to the other anions, this may be due to the strong affinity of fixed site of the membrane towards nitrate and hydrogenocarbonate anions or may be explained by the hydrophilic environment in our membrane providing their transport. This result was also demonstrated by the increase in transport rate through our CTA membranes of AgðCNÞ 2 with anions that were moderately lipophilic (NO 3 ) [29]. However and surprisingly, transport with highly hydrated and less hydrophobic driving anion  (HCO 3 ) is more effective than with ClO4 despite it is the less hydrated and the highly lipophilic anion [29]. This behavior can be explained by the fact that the less and moderately lipophilic anions can be exchanged with the AgðCNÞ 2 present in the feed phase. Given that the HCO 3 is not harmful and it is an alimentary anion, it was selected as the suitable driving anion for the silver cyanide complex removal.

Table 3 2 transported as Transport recovery (%) and fluxes (Ji) of AgðCNÞ 2 and ZnðCNÞ4 individual complexes with Aliquat 336 (25% (w/w) contents in CTA/PIM from a feed  2 solution of [complex] = 10 M and strip solution HCO3 (0.1 M). Anionic complex

Percent recovery (%)

Percent removal (%)

Ji (lmol cm2 s1)

AgðCNÞ 2

51 45.45

47 54.54

1.22 1.54

ZnðCNÞ2 4

4.2. Case of competitive transport (complex mixture) The selectivity is assuredly a very significant factor for the development and application of PIMs for reasons mainly related to the environmental processes. Particularly, regarding the low concentration of pollutants that requires a high selectivity for effective treatment as well as the necessity of their purity during their recovery. The selectivity of the PIMs was investigated by determining the selectivity coefficient, S, defined as the ratio between the initial flux of a given ion AgðCNÞ 2 and the flux of another ion existing

4. Selectivity study

in the same solution (ZnðCNÞ2 4 ). The obtained results are presented in Table 4 and Fig. 12. It is clear that the lower flux and transport percent are obtained

4.1. Case of single anionic complex

for ZnðCNÞ2 4 . This result can be explained by the difference between the two anionic complexes in term of size, geometry

In order to explore the possibility and the feasibility of our CTA membrane based on Aliquat 336 for the separation of complex mixture, we studied at first the transport of each single complex under optimal conditions found throughout this work. Transport was tested with another cyanide complex which has a different geometry, valence and size with respect to the silver cyanide com plex. The selected complex is the ZnðCNÞ2 4 found with the AgðCNÞ2 in the rinsing baths of electroplating units. The initial concentra-

tion of ZnðCNÞ2 in the feed phase is 102 M against the driving 4 (0.1 M) in the receiver compartment. ion HCO 3 Transport recovery and fluxes results are assembled in Table 3. It is clear that both complexes diffuse in a similar manner and Aliquat 336 is effective for the transport of both complexes. The slight difference between the transport of both complexes was related to the affinity of the quaternary ammonium site of the membrane towards the bivalent anions.

and valence. In fact, ZnðCNÞ2 is more voluminous than AgðCNÞ 4 2 and has the largest hydratized radius [30]. For this reason, these anions complexes move more difficult through the membrane sites. In addition the linear geometry of the AgðCNÞ 2 anion enhance its diffusion across the membrane while the tetrahedral geometry of ZnðCNÞ2 4 may hinder its migration through the membrane when it is in the presence of the other AgðCNÞ 2 complex. Moreover, the transport efficiency increases when the charge of the complex decreases as reported by Miyoshi [28]. It was also clearly observed for both complexes that the flux and the transport percent are reduced compared to the case of every individual complex. It could be explained by the competition between both anions and may be due to the crowded environment around fixed site of the membrane. We followed the transport of the two complexes after 50 h of dialysis, it was observed that the transport of two complexes was

Fig. 11. Effect of counterions present in the receiver phase for silver complex removal.

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Table 4 2 Transport recovery (%), selectivity factor and fluxes (Ji) of AgðCNÞ 2 and ZnðCNÞ4 transported as mixture complexes with Aliquat 336 (25% (w/w) contents in CTA/PIM from a source solution of [complexes mixture] = 102 M and strip solution HCO 3 (0.1 M). Anionic complex

Percent recovery (%)

Percent removal (%)

Ji (lmol cm2 s1)

2 S = Ji(AgðCNÞ 2 ) /Ji(ZnðCNÞ4 )

AgðCNÞ 2 ZnðCNÞ2 4

24.56 9.1

65.2 18.2

1.66 0.193

8.6

Fig. 12. Concentration of both complexes in strip phase versus time of transport across a PIM in competitive anions transport.

Fig. 13. Life time of polymer inclusion membrane. Fig. 14. Stress-strain curve of CTA–PIM membrane before and after experimental run.

improved to reach 30% for the zinc complex and 73% for the silver complex. We may conclude that the time is an important factor for the recovery of both complexes anions. The obtained selectivity coefficient during 12 h of process led us to conclude that Aliquat 336 is selective to the silver cyanide complex and the both complexes were partially separated. 4.3. Durability and lifetime studies In the current study, percent removal value has been used as the parameter to estimate the lifetime of PIM systems. The transport of dicyanoargentate 102 M from a feed to the stripping phase containing 0.1 M HCO 3 through PIM, having the optimum compo-

sition and used throughout this work, was investigated. Transport experiments were accomplished where both feed and stripping phases were renewed each time while the membrane remained the same as in the first use. The obtained results are presented in Fig. 13. Silver cyanide complex transport during three months, remains practically constant. Even transport decreases slightly after six months, it provides a significant percentage of complex extraction. The percentage removal values are 51%, 42% and 41% for the first use, after three months and after six months respectively. Results showed that the PIM presented a long-term stability and can be reused up to 6 months without loss of its performance and no sign of structural weakening was observed for PIM. The

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membrane remained transparent, and the carrier does not leak out into aqueous phases during 6 months. These observations confirm that the PIM keeps a long-term stability and a good durability. As another confirmation of the durability of the membrane, mechanical properties were performed for the membrane after 50 h as shown in Fig. 14. Results showed that the tensile strength was reduced just a little bit from 30 MPa to 24 MPa and Young’ modulus remains practically constant (546.10 MPa for PIM before use and 534.5 MPa for PIM after experimental run), this may confirm that the membrane preserve its durability. These results corroborates the high stability of PIMs. 5. Conclusion Silver cyanide complex can be efficiently transported from feed aqueous solutions to a stripping phase using CTA PIMs with Aliquat 336 as an ion carrier. The transport of AgðCNÞ 2 is also affected by the membrane composition: carrier type and concentration, kind of plasticizer in addition to the stripping medium. From these results, it can be concluded that CTA PIM containing 25%(w/w) of Aliquat 336 as a carrier and bis (2-ethylexyl sebacate) as the plasticizer ensures better results in terms of AgðCNÞ 2 transport and removal. Moreover, the selectivity of the membrane system towards silver cyanide complex has been examinated when dealing with feed solutions containing ZnðCNÞ2 4 . We found that PIM 2 exhibited an important selectivity towards AgðCNÞ 2 thanZnðCNÞ4 . Percent removal of silver cyanide can reach 73% after 50 h of process. Lifetime study confirms that the PIM keeps a long-term stability and a good durability. These promising results open perspectives to apply this technology for the removal of silver cyanide complex from real wastewater.

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