Selective Separation of Silver ( I ) Ion Through a Bulk Liquid Membrane Containing 1 , 1 ’-( 1 , 3-Phenylene ) bis ( 3-allylthiourea ) as Carrier

The competitive bulk liquid membrane transport of six metal cations from an aqueous source phase (SP) containing Ag, Pb, Cu, Zn, Co and Ni through an organic membrane phase (MP) facilitated by 1,1’-(1,3-phenylene)bis(3-allylthiourea) as a carrier into an aqueous receiving phase (RP) was studied and compared. Fluxes and selectivities for competitive metal cation transport have been determined in a variety of source solution pH and membrane solvent types. The obtained results showed that the carrier is selective for Ag cation. The effect of different experimental conditions that affect the transport efficiency were studied and optimized. In the optimum condition, the transport of a 5 × 10−4 mol L−1 solution of Ag cations was observed 92.3 ± 5.3% after 4 h in the presence of equimolar concentrations of other metal cations. A possible application of this carrier system and transport process for separation and recovery of Ag cation from real samples has also been examined.


Introduction
Nowadays, rapid industrial development and the diversified character of the world economy led to handle with the large volumes of different types of wastewaters.Wastewaters are a potential source of organic and metal pollutants and discharging them into the environment without prior treatment is a serious problem of concern.Furthermore, the presence of metal ions in wastewater might inhibit biodegradation of organic pollutants which are present in the wastewater. 1 Separation of metal ions is not only a very incident event in biological systems as well as in large scale industrial applications. 2Therefore, separation and removal of metal ions from industrial effluents is of interest in recovering the metal ions as well as for abating the environmental pollution problems. 3Silver is one of the most ancient metal used in the production of jewellery, coins, tableware, alloys and arts.7][18][19] The main properties of membrane separations are expressed as simultaneous removal and recovery of pollutants and materials in a single unit, simple in concept and operation, easy to scale-up, non-equilibrium mass transfer, high selectivity, high fluxes, reusability and low energy consumption; make them ideal for industrial applications. 20mong other membrane separation techniques, liquid membrane transport includes processes incorporating liquid-liquid extraction and membrane separation in one continuously operating device.Liquid membrane system involves an organic liquid membrane which is an immiscible with the two aqueous source phase (SP) and receiving phase (RP) that serves as a semipermeable barrier between these two aqueous phases.The mechanism of the extraction is basically the same as that of the liquid-liquid extraction except that the transport process is governed by kinetic rather than equilibrium parameters, under nonequilibrium mass transfer. 21In this method, solute species dissolve in the membrane and diffuse across the membrane due to an imposed concentration gradient. 22The efficiency and selectivity of transport across the membrane not only depends on the solubility and diffusion coefficient of solute in the membrane, but also enhances by the presence of a mobile complexation agent in the membrane which reacts rapidly and reversibly with the solute to form a complex.The best carrier is a compound that formed remarkably stable and selective complexes and the rate of the transport increases with the increase of stability constant of complexes. 23,24nquestionably, design and synthesis of the reagents for the formation of remarkably stable and selective complexes with metal cations is very significant in the efficient transport of cations through the liquid membrane system in a range of industrial and analytical applications.][30] However, the development of selective techniques for the separation and recovery of transition and post transition heavy metal cations is a challenging task. 31,32n this present study, we studied the competitive bulk liquid membrane transport of six metal cations involving Ag + , Pb 2+ , Cu 2+ , Zn 2+ , Co 2+ and Ni 2+ with 1,1'-(1,3-phenylene)bis(3-allylthiourea) as a carrier using chloroform (CHCl 3 ), dichloromethane (DCM), 1,2-dichloroethane (1,2-DCE) and nitrobenzene (NB) as an membrane phase (MP).When DCM was used as an MP, a good transport selectivity has been observed for silver(I) cation over several other cations.The influence of different experimental conditions such as pH of the SP, type of the MP, carrier concentration in the MP, concentration of picric acid in the SP, type and concentration of stripping agents in the RP, stirring rate and time was investigated.Also, a possible application of this carrier-facilitated transport system to separation and recovery of Ag + cation from real and industrial samples was examined.

Reagents and materials
All chemicals and reagents used in this study were commercially available grade and were purchased from either Aldrich (Saint Louis, MO, USA) or Merck (Darmstadt, Germany).CHCl 3 , DCM, 1,2-DCE and NB with highest purity were used as liquid membrane.Nitrate salts of silver, lead, copper, cobalt, nickel and zinc were the highest purity and used without any further purification.The stock solutions of metal ions were prepared by direct dissolution of proper amount of nitrate salts of them in 1% nitric acid solution.The fresh standard and working solutions were obtained daily by appropriately diluting of the stock solutions with deionized water.The pH of aqueous solutions was adjusted by the buffering of aqueous SP and RP.

Instrumentation
All UV-Vis spectra were recorded on a computerized double-beam 1601pc Shimadzu spectrophotometer (Tokyo, Japan) using two matched 1 cm quartz cells.In a typical experiment, 2.5 mL of ligand solution (1.0 × 10 −5 mol L −1 ) in acetonitrile was placed in the spectrophotometer cell and the absorbance of the solution was measured.Then a known amount of the solution of Ag + ions in acetonitrile (1.0 × 10 −4 mol L −1 ) was added stepwise using a 10 μL Hamilton syringe.The electronic absorption spectra of solution were measured after each addition at 270 nm.The Ag + ion solution was continually added until the desired metal to ligand mole ratio was achieved.
The infrared (IR) spectra were recorded with Shimadzu IRPrestige-21 FTIR instrument (Tokyo, Japan) in the solid state as KBr discs.The determinations of the metal ion concentrations were performed on a GBC Sens AA flame atomic absorption spectrometer (Victoria, Australia) equipped with D 2 lamp background correction and hollow cathode lamps at respective wavelengths as radiation sources.An air-acetylene flame atomizer was used for all atomic absorption measurements under the recommended instrumental conditions for each metal ion.The pH measurements were made with a Metrohm 744 digital pH meter (Herisau, Switzerland) using a combined glass-calomel electrode.

Ligand preparation
The preparation of ligand containing nitrogen and sulfur donor atoms was performed as described in the previous work. 33In a typical procedure, 2 mmol of allyl isothiocyanate was added dropwise to a stirred ethanol solution of 1 mmol of 1,3-phenylenediamine.The reaction mixtures were heated at 50-60 °C for about 30 min and the mixture was kept at room temperature for 24 h.Then, the final product was filtered and washed with ethanol, recrystallized from ethanol:water (1:1) to give corresponding dithiourea, which was used as a suitable carrier in the transport experiments.Vol. 27, No. 1, 2016   Bulk liquid membrane transport Transport measurement was performed in a 'U' shape glass cell 34 in which the SP (5 mL) and RP (5 mL) were separated by an MP (10 mL) (Figure 1).The inner dimension of the transport apparatus was 10 mm diameter × 150 mm depth.The SP containing an equimolar mixture of metal nitrate salts (1.0 × 10 −3 mol L −1 ) and picric acid (1.25 × 10 −3 mol L −1 ) was buffered at pH 5 using CH 3 COOH/CH 3 COONa buffer solution and placed in one limb of the 'U' cell.The MP contained 6.0 × 10 −4 mol L −1 of carrier which was placed at the bottom of the 'U' shape cell.The RP containing a concentration of 5.0 × 10 −3 mol L −1 of thiourea was buffered at pH 3 using HCOOH/HCOONa buffer solution and placed in the other limb of the 'U' cell.The MP was magnetically stirred by a Teflon-coated magnetic bar.All transport experiments were carried out at ambient temperature and after each experiment, both aqueous phases were analyzed for determination of metal content by flame atomic absorption spectroscopy.A similar transport experiment was also performed in the absence of the carrier as a reference, and no leakage of metal ions from SP into the RP was observed.To check the reproducibility, all measurements were carried out in duplicate.

Study of complexation between 1,1'-(1,3-phenylene)bis(3allylthiourea) and Ag +
Selective transport of metal cations by carrier-mediated liquid membrane depends on the nature of the ligand used as a carrier in the membrane. 35,366][27] In the preliminary experiment, we tested the ligand ability for the competitive transport of Ag + cation among some other metal cation such as Pb 2+ , Cu 2+ , Zn 2+ , Co 2+ and Ni 2+ from SP into the RP through DCM organic solvent as a MP.It was found that the ligand has the desired ability to transport Ag + cations against its concentration gradient through bulk liquid membrane.In order to investigate the complexation process between 1,1'-(1,3-phenylene)bis(3-allylthiourea) and Ag + cations, the spectrophotometric titration was utilized and the complexation procedures were analyzed in acetonitrile at 25 °C due to the absorbance alteration which occurs with the cation to ligand ([M]/[L]) ratio conversion.The electronic absorption spectra of ligand (1.0 × 10 −5 mol L −1 ) without and in the presence of Ag + (1.0 × 10 −4 mol L −1 ) ions are exhibited in Figure 2. As can be seen, absorbance of the acetonitrile solution of ligand at wavelength of 270 nm decreased with the addition of Ag + ions to the solution.On the other hand, the observed spectral evolution involved the formation of a well-defined isobestic point at around 250 nm, indicating the presence of one absorbing complex compound between ligand and Ag + ions.The mole ratio method was also used for estimation of the stoichiometry of the Ag + -ligand complex and a typical mole-ratio plot is represented in Figure 3.The obtained results show that the slope of the curve changes at 1:1 [M]/[L], that indicate the stoichiometry of the complex formed between ligand and Ag + cation is 1:1.In addition, the Fourier transform infrared (FTIR) spectrophotometric technique was also applied to study the possible complexation reaction between Ag + and ligand.The FTIR spectra of the ligand and Ag + -ligand complex are shown in Figure 4.

Effect of pH of the SP on transport efficiency
Reported literatures in recent years is a conformation of achievable high enrichment factor for the liquid membrane transport technique with making a small pH difference between SP and the acidic RP. 37 When the transport mechanism has benefit from proton pump or pH gradient, pH difference leads to back diffusion of protons from the RP into the SP. 38On the other hand, when a legating agent has used to transport a specific metal cation, pH can alter the conditional stability constant of the resulting metal complex. 24Hence, pH is a dominant factor in the separation of metal cation using liquid membrane transport.The effect of pH on the competitive transport of six-metal cations from SP at a pH ranges of 4-6.5 into acidic RP (pH 3) was investigated and the results are given in Table 1.It was found that maximum Ag + cation transport occurs at pH 5 of the SP.It seems that at lower pH values of the SP, the efficiency of silver transport decreased due to a subtraction in the hydroxide concentration for exchanging proton with Ag + or slow partition coefficients of the Ag + at the source/membrane phases interface.In other words, at higher pH values of the SP, the efficiency of Ag + transport decreased probably due to the complex formation of Ag + ion with hydroxide ion.

Effect of solvent on transport efficiency
Choosing the right organic membrane solvent is the prime issue in establishing transport efficiency. 39The physicochemical properties of solvents play noticeable roles in solute-solvent and solute-solute interactions in solutions.The log K f value for the formation of the cationligand complex in the organic membrane, the diffusion path length and diffusion coefficients of all mobile species in the unstirred boundary layers depend on the organic membrane solvent nature. 40The thickness of boundary layers is a function of the viscosity of the solvent and the partitioning of the cations into the organic membrane should be affected by the dielectric constant of the solvent. 41,42The selected solvent should be immiscible with the water, has low viscosity and volatility and at the same time it must have a suitable polarity related to an optimal value of the association constant of the complex. 43,44The results of the competitive transport of six-metal cations from an aqueous SP through CHCl  ), the competition between solvent and the ligand for the metal cation decrease which leads to an increase in the stability of the formed complex between the Ag + and the ligand.Also, the lower dielectric constant of DCM (ε = 8.93) compared to the 1,2-DCE (ε = 10.66) and NB (ε = 34.80)possibly end up in stabilization of the ion-pair in the organic phase which results in a better transport rate in this liquid membrane.

Effect of the ligand concentration on transport efficiency
Carrier-facilitated transport system is a coupled transport process that combines a chemical coupling reaction with a diffusion process.In this system, the solute has first reacted with the carrier to form a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side.Carrier is a water-immiscible complexation agent which dissolved in the MP and reacts selectively, rapidly and reversibly with the desired solute to form a complex.Hence, the efficiency and selectivity of transport across the MP might be markedly enhanced by the presence of a carrier in the MP.In this study, the effect of the concentration of ligand as a suitable carrier in the MP on the transport efficiency of Ag + cations was investigated in the concentration range (5.0 × 10 −5 to 1.0 × 10 −3 mol L −1 ) of ligand and the results are presented in Figure 5.It is seen that the percentage of Ag + cation transport increases with an increase in ligand concentration in the MP and 6.0 × 10 −4 mol L −1 of ligand is much more effective in the efficient transport of Ag + ions.The ligand concentration over this amount had no considerable effect on the transport efficiency.Therefore, the ligand concentration was fixed in 6.0 × 10 −4 mol L −1 at the subsequent experiments.

Effect of picric acid in the SP on transport efficiency
To maintain electroneutrality and solute uphill pumping, many carrier-facilitated membrane systems require a coupling ion to be counter-or co-transported along with the solute ion.Since the coupling ion must also enter and cross the organic phase, it is bound to influence transport efficiency.Transport efficiency affected by several parameters such as anion hydration free energy anion lipophilicity, and anion interactions with the solvents.It seems that anion hydration free energy is the major determinant. 20Christensen et al. 45 shows that transport rate of K + with dibenzo-18-crown-6 as a carrier, decreased in the following order: picrate > PF 6 . This order is almost identical to that for increasing anion hydration free energy and demonstrates the strong correlation between anion hydration and transport efficiency: larger anions are more easily dehydrated and thus more readily enter the membrane to facilitate transport.
The results of the competitive transport of six metal cations from SP through DCM containing the ligand showed that the nitrate ion is not a suitable counter anion to accompany the ligand-Ag + complex into the MP.Therefore, we used picric acid as counter ion in the SP to increase the Ag + cation transport into the MP.The role of the picric acid may be to aid the transport process by providing a soft, polarizable and weakly hydrated counter ion in the membrane on proton loss to the aqueous phase, giving rise to charge neutralization of the Ag + ion being transport through ion pair or adduct formation. 32,46The influence of the concentration of picric acid on the transport efficiency of Ag + cation was investigated and the results are presented  in Figure 6.As can be seen, the efficiency of the Ag + cation transport increases sharply with increasing picric acid concentration up to (1.25 × 10 −3 mol L −1 ), and further increase in the concentration of picric acid caused a decrease in the percentage transport of the Ag + ion.This behavior maybe caused due to the competition of picric acid with Ag + ion for transport through the liquid membrane system.
Effect of striping agent type and its concentration in the RP on transport efficiency In the preliminary experiments, it was found that the permeability of the membrane system for Ag + cation is extremely dependent on the nature and composition of the stripping agent used in the RP.As it is observed in Table 3, among the different stripping agents used in these experiments, thiourea, with the increased complexing ability towards Ag + cation, acts as the most suitable receiver for the release of Ag + cation from the MP into the RP.
The influence of the concentration of thiourea in the RP on the transport efficiency of Ag + was also evaluated and the results are shown in Figure 7.According to the obtained results, the optimum concentration of thiourea in the RP was found to be 5.0 × 10 −3 mol L −1 .It should be noted that in the absence of the stripping agent on the RP, the Ag + cation transports only 39% into the RP.Thus, 5.0 × 10 −3 mol L −1 thiourea concentration was used for further experiments.

Effect of time on transport efficiency
The time required for maximum transport of Ag + cation through the bulk liquid membrane was inspected under the experimental conditions.The results demonstrated the acceptable transport of Ag + cation from the aqueous SP into the RP and maximum transport of Ag + cation was achieved in about 4 h.Therefore, further experiments were carried out for a period of 4 h.

Effect of stirring rate on transport efficiency
In order to explore the effect of stirring speed on the Ag + transport, several transport experiments were performed with different stirring speeds (100-600 rpm) and the results are presented in Figure 8.As can be seen, the transport efficiency of Ag + ions through the MP augments with acceleration of stirring speed which reveals that diffusion was the rate limiting step in the transport of Ag + from the SP to RP.The transport efficiency increases at lower stirring speeds (100-400 rpm) and then it is nearly constant at higher stirring speeds (500-600 rpm).

Recommended mechanism
The Ag + ion is transported from the SP into the RP via a DCM membrane with the simultaneous counter-transport of proton.Movement of the charge species through the  [Thiourea] / (10 mol L ) hydrophobic organic membrane is accomplished by the presence of the host ligand, ion paired with picrate ion as a suitable counter ion.The picrate ion not only neutralizes the charged Ag + -ligand complex, but also induces a more lipophilic character to the Ag + complex so that it can be readily extracted into the MP.During the complexation process of Ag + ion with the ligand on the source side of the membrane, the ligand splits off protons into the SP and the formed complex diffuses across the membrane.On the other side of the membrane, the release of the Ag + ion into the RP occurs via the formation of complex between Ag + ion and thiourea as a stripping agent.At this moment, the ligand associates with proton from the RP and the free ligand diffuse back across the membrane and cycle start again.A schematic diagram of this mechanism is shown in Scheme 2.

Limit of detection
The blank determination was performed using bulk liquid membrane system at optimum condition of experiments.The blank value was 0.12 ± 0.18 mg L −1 of Ag + for 5 replicates experiment and the limit of detection (calculated as 3 times of the standard diviation) was 0.54 mg L −1 of Ag + ion.

Real water analysis
Three real environmental water samples, including ground, river and tap water spiked with an equimolar mixture of six metal cations as nitrate salts (5.0 × 10 −4 mol L −1 ) separated using this membrane system under the optimal conditions of the experiments.Tap water and ground water were collected from Payame Noor University of Ardabil (Iran), and river water was collected from the Baliqly Chay River in Ardabil (Iran).The results are given in Table 4.It is obvious that Ag + is successfully transported and purified using this membrane system.In addition, to show practical utilization of this bulk liquid membrane system, it was used for recovery of Ag + ions from silver plating and photographic waste solutions.The composition of each waste solution is given in Table 5.The obtained results indicate that only silver is transported, which shows the selectivity and efficiency of this bulk liquid membrane system for silver recovery.ligand in various organic membranes showed that the rates of the cation transport are strongly influenced by the nature of the organic solvent.The efficiency of the method depends on effective parameters such as pH of the SP, carrier concentration, concentration of picric acid as counter anion, type and concentration of stripping agent, stirring rate and transport time.At the optimum conditions of transport experiments, the proposed method was triumphantly used for the separation of (5.0 × 10 −4 mol L −1 ) Ag + from real water samples in the presence of equimolar concentration of some other metal cations and found that the recoveries for tap, river and ground water after 4 h were 92.3 ± 5.3, 89.2 ± 8.5 and 83.2 ± 4.8%, respectively.Also, this bulk liquid membrane system was successfully utilized for recovery of Ag + ions from silver plating and photographic waste solutions.

Figure 1 .Figure 2 .
Figure 1.Representation of the bulk type liquid membrane cell.

Figure 5 .
Figure 5.Effect of ligand concentration in the MP on the transport of Ag + ions.SP: 5 mL solution containing 1.0 × 10 −3 mol L −1 of metal cations at pH 5; MP: 10 mL of ligand in DCM at different concentrations; RP: 5 mL solution containing thiourea (5.0 × 10 −3 mol L −1 ) at pH 3; stirred for 4 h at 400 rpm.

Scheme 2 .
Scheme 2. Schematic diagram of liquid membrane system for transport of Ag + ions.

Table 1 .
Effect of pH of the SP on the competitive transport of the six metal cations across DCM-MP using 1,1'-(1,3-phenylene)bis(3-allylthiourea) as a carrier a Percent of total metal cations in the RP after 24 h; b percent of total metal cations in the MP after 24 h; c all J (flux rate) values are × 10 −8 ; d the hyphenated symbols means that the values are about zero or they are with high uncertainties.

Table 2 .
Data for six metal cations competitive transport across organic solvents as MP with ligand a Percent of total metal cations in the RP after 24 h; b percent of total metal cations in the MP after 24 h; c all J (flux rate) values are × 10 −8 ; d the hyphenated symbols means that the values are about zero or they are with high uncertainties.

Table 3 .
Effect of type of stripping agent on the Ag + transport

Table 5 .
Application of proposed methodology for recovery of Ag + from silver plating and photographic waste solutions [M] in SP: metal concentration in source phase.