# Transport of Zn (II) by TDDA-Polypropylene Supported Liquid Membranes and Recovery from Waste Discharge Liquor of Galvanizing Plant of Zn (II)

Authors: Hanif Ur Rehman; Gul Akhtar; Haroon Ur Rashid; Nauman Ali; Imtiaz Ahmad; Saeed Ur Rehman; Kamin Khan; Muhammad Arshad
Journal: Journal of Chemistry
 (2017)

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## Abstract

The facilitated passage of Zn (II) across flat sheet supported liquid membrane saturated with TDDA (tri-n-dodecylamine) in xylene membrane phase has been investigated. The effect of acid and metal ion concentration in the feed solution, the carrier concentration in membrane phase, stripping agent concentration in stripping phase, and coions on the extraction of Zn (II) was investigated. The stoichiometry of the extracted species, that is, complex, was investigated on slope analysis method and it was found that the complex (LH)2·Zn(Cl2) is responsible for transport of Zn (II). A mathematical model was developed for transport of Zn (II), and the predicted results strongly agree with experimental ones. The mechanism of transport was determined by coupled coion transport mechanism with H+ and Cl− coupled ions. The optimized SLM was effectively used for elimination of Zn (II) from waste discharge liquor of galvanizing plant of Zn (II).


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## Body

## 1. Introduction

Over the past few decades, the prompt boost in the utilization of heavy metals has increased the flux of metallic ingredients into soils and natural water resources. The presence of even small amount of heavy metals in the environment is hazardous as the majority of them are harmful and persistent [1]. The zinc concentration in the earth crust is about 70 ppm. Zinc and its compounds have many applications and are used in galvanization, alloys, catalysts, wood preservation, vulcanization for rubber, photographic paper, ceramics, fertilizers, textiles, batteries, pigments and as dietary supplements and in medicines [2, 3].About two billion people suffer from zinc deficiency in the developing countries and this thus causes various disorders in them. In minors, zinc deficiency is responsible for growth retardation, infection susceptibility, delayed sexual maturation, and diarrhea. These disorders lead to the deaths of about 800,000 children throughout the world every year.Excess of zinc in the human body has been associated with system dysfunction which in turn can affect growth and reproduction. The presence of free zinc ion in solution has been reported to be very toxic for vertebrates, invertebrates, and plants [4–6]. Due to its toxic effect, large scale applications, and increasing demand for pure zinc, it is necessary to remove and separate zinc from industrial effluents.Traditional techniques of recovery for metal ions, such as adsorption [7, 8], solvent extraction [9–12], precipitation [13], and ion exchange [14]. have normally low efficiency, require high capital and operating cost, and produce secondary pollution complications [15]. Among novel techniques suggested for transport of metal ions, supported liquid membrane is one of the promising methods. The technique comprises the applications of solvent extraction (high selectivity and distribution coefficient) while overcoming usual extraction’s shortcomings (loss of carrier due to emulsification and dispersion) [16–18].Kanungo and Mohapatra [19] investigated the transport of Zn (II) through supported liquid membrane using bis(2,4,4-trimethylpentyl)phosphinic acid as a metal ion carrier. A model for the rate of transport of binary and ternary complex species has been discussed. The transport of Zn (II) via di-(2-ethylhexyl)phosphoric acid supported liquid membrane has been studied by Ata et al. [20]. The influence of pH of feed solution, carrier, and stripping agent concentration, temperature, and flow rate of feed and stripping solution on the transport of Zn (II) has been investigated. Wódzki and Szczepański found simultaneous recovery and bifurcation of Zn (II) and Cu (II) by means of two parallel BLMs. The two selective carriers, 5-nonylsalicylaldoxime and di-(2-ethylhexyl), were employed for transport of Zn (II) and Cu (II), respectively [21]. The selective removal of Zn (II) from other transition metals and transport across polymer inclusion membrane was investigated by Ulewicz and his coworkers [22]. It has been observed that transport selectivity of Zn (II) over Ni (II), Co (II), Cd (II), and Cu (II) decreases while increasing pH of the feed solution. Furthermore, it has been studied that Zn (II) can be selectively eliminated from dilute aqueous feed solution by solvent extraction. Kozlowska et al. studied the competitive transport of Zn (II), Cd (II), and Pb (II) through polymer inclusion membranes having organophosphorous acids as an ion carrier [23]. The nature of the extractants on cation efficiency and selectivity has been elaborated. Torz et al. reported the transport of Zn (II) from hydrochloric acid feed solution through hollow fiber modules using tributyl phosphate as a carrier [24]. It has been observed that the kinetics of the mass transfer process was restricted by the diffusion of species in the membrane openings. Lee et al. investigated the separation of zinc and copper by hollow fiber supported liquid membrane containing LIX 84 and PC-88A as mobile carriers. The influence of numerous operational factors on separation and permeation rate of Zn (II) and Cu (II) has been studied [25].In our previous papers [26–29], an effort was made to design a model for transport of Ag (I), Mn (II), Pb (II), and Tl (III), through SLM using various carriers and diluents, and successfully applied to industrial waste effluents.The present work elaborates the extraction and separation of Zn (II) from aqueous acidic feed solution through flat sheet supported liquid membrane using TDDA as a mobile carrier. In SLM studies, the influence of several process parameters like acid concentration in the feed solution, Zn (II) concentration in the feed solution, the carrier concentration in membrane phase, and NaOH concentration in strip solution on the Zn (II) flux was investigated. Another aim of current work was to investigate the species formed during the transport of Zn (II) and mechanism of Zn (II) and further to establish the stoichiometry of the chemical reactions through SLM. Furthermore, this study was concentrated to ascertain the most optimum conditions for transport of Zn (II), especially for its large scale separation and recovery from industrially galvanizing waste effluents of Zn (II). No such investigations have been done so far using TDDA SLM system for transport of Zn (II).


## 2. Theory

Transport of metal ion through SLM occurs by diffusion of metal-carrier complex via various basic steps. First, the metal ion diffuses across aqueous diffusion layer from aqueous bulk feed solution to membrane feed interface and the metal-carrier complex is prepared at feed membrane interface. The complex is then diffused through liquid membrane phase to strip membrane interface owing to the concentration gradient. The complex is finally dissociated at strip membrane interface, carrier returns back across liquid membrane phase, and metal ion diffuses through aqueous diffusion layer into bulk strip solution. The theoretical model of mass transfer of Zn (II) for this purpose is important for the diffusion of metal-carrier complex.


## 3. Experimental

### 3.1. Chemicals and Reagents

ZnCl2 (97% BDH) in HCl (37% Merck) was used as feed in various concentration. NaOH (99-100% Merck) in distilled water in various concentrations was used as a stripping solution and TDDA (≥95% Merck) with diluent xylene (99.5%) to get the various composition of metal ion carrier. Double distilled and deionized water was used in all experiments. All other chemicals used were of analytical or better grade.


### 3.2. Analytical Instruments

Atomic absorption spectrometer of Perkin Elmer model 400 was used to measure the concentration of zinc (II) and other metal ions in feed and strip solutions. pH was determined through pH meter of Metrohm model 827. Viscosity determination of TDDA (in xylene) was performed via viscometer/rheometer of Brookfield LVDV-III.


### 3.3. Permeation Cell

The metal ion transport study was performed in two-compartment cell made of Perspex material as reported in our previous work [26]. Each half cell of the permeation cell had the volume capacity of 300 cm3 and active membrane area was 23.79 cm2. Each half was equipped with synchronous motors, pH electrode, and sampling port. The stirring speed of 1500 rpm [26] was optimized for the similar type of permeation cell and carrier; therefore this stirring speed was used in all the permeation study. All the experiments were conducted at 25±0.5°C.


### 3.4. SLM Preparation

The microporous polypropylene thin coating of Celgard 2400 was applied as a solid support for a liquid organic carrier with an active pore size of 0.02µm, the thickness of 25 µm, and porosity of 38%. The membrane was cut into rectangular pieces of 8×6 cm and was soaked in the premeasured concentration of TDDA diluted with xylene in Petri dish for overnight. The membrane was then taken out from the organic solution. The excess amount of organic carrier and solvent were removed by allowing them to drain off the membrane for about five minutes.


### 3.5. SLM Transport Study

The membrane after preparation was placed in between the two compartments of the cell and screwed together to form a water tight seal. Both compartments of the cell were filled with 300 mL of predetermined concentration of feed and strip solutions. To avoid concentration polarization at feed and strip membrane interfaces, the two solutions in feed and strip compartments were continuously stirred. Samples were collected after a specific time interval from both feed and strip solution and tested for metal ion concentration. For most of the experiments, the carrier concentration was 0.80 mol/dm3 in xylene.The flux (J) was calculated as follows [26]:(1)Flux=concentration  of  Zn2+mol/dm3×solution  volume  in  feed  or  stripdm3effective  membrane  aream2×Δt.


## 3.1. Chemicals and Reagents

ZnCl2 (97% BDH) in HCl (37% Merck) was used as feed in various concentration. NaOH (99-100% Merck) in distilled water in various concentrations was used as a stripping solution and TDDA (≥95% Merck) with diluent xylene (99.5%) to get the various composition of metal ion carrier. Double distilled and deionized water was used in all experiments. All other chemicals used were of analytical or better grade.


## 3.2. Analytical Instruments

Atomic absorption spectrometer of Perkin Elmer model 400 was used to measure the concentration of zinc (II) and other metal ions in feed and strip solutions. pH was determined through pH meter of Metrohm model 827. Viscosity determination of TDDA (in xylene) was performed via viscometer/rheometer of Brookfield LVDV-III.


## 3.3. Permeation Cell

The metal ion transport study was performed in two-compartment cell made of Perspex material as reported in our previous work [26]. Each half cell of the permeation cell had the volume capacity of 300 cm3 and active membrane area was 23.79 cm2. Each half was equipped with synchronous motors, pH electrode, and sampling port. The stirring speed of 1500 rpm [26] was optimized for the similar type of permeation cell and carrier; therefore this stirring speed was used in all the permeation study. All the experiments were conducted at 25±0.5°C.


## 3.4. SLM Preparation

The microporous polypropylene thin coating of Celgard 2400 was applied as a solid support for a liquid organic carrier with an active pore size of 0.02µm, the thickness of 25 µm, and porosity of 38%. The membrane was cut into rectangular pieces of 8×6 cm and was soaked in the premeasured concentration of TDDA diluted with xylene in Petri dish for overnight. The membrane was then taken out from the organic solution. The excess amount of organic carrier and solvent were removed by allowing them to drain off the membrane for about five minutes.


## 3.5. SLM Transport Study

The membrane after preparation was placed in between the two compartments of the cell and screwed together to form a water tight seal. Both compartments of the cell were filled with 300 mL of predetermined concentration of feed and strip solutions. To avoid concentration polarization at feed and strip membrane interfaces, the two solutions in feed and strip compartments were continuously stirred. Samples were collected after a specific time interval from both feed and strip solution and tested for metal ion concentration. For most of the experiments, the carrier concentration was 0.80 mol/dm3 in xylene.The flux (J) was calculated as follows [26]:(1)Flux=concentration  of  Zn2+mol/dm3×solution  volume  in  feed  or  stripdm3effective  membrane  aream2×Δt.


## 4. Results and Discussion

### 4.1. Reactions at Feed Membrane Interface

If tri-n-dodecylamine (TDDA) is represented by L, the following possibilities of reactions may take place. The tri-n-dodecylamine can be protonated to LH+ in acidic medium (HCl).(2)L+H+⇌LH+It is supposed that ZnCl2 in feed solution in the presence of excess chloride ions due to HCl is converted to(3)ZnCl2aq+nCl-aq⟶ZnCl2+nn-aqThe cationic (LH+)(org) and anionic species ZnCl2+nn-(aq) then interact at feed membrane interface and form the complex as(4)nLH+org+ZnCl2+nn-aq⇌LHn·ZnCl2+norgThe subscript org represents organic phase and aq aqueous phase and n indicates the number of Cl−, H+, and L molecules associating with Zn (II) ions to form the neutral complex (LH)n·Zn(Cl2+n)(org).To express the contribution of H+ and Cl−, (3) may be represented as(5)nLorg+nH+aq+ZnCl2aq+nCl-aq⇌LHn·ZnCl2+norg


### 4.2. Reactions at Strip Membrane Interface

The neutral complex formed at feed membrane interface as per (3)/(4) is extractable in liquid organic phase and disperses from feed membrane interface to strip membrane interface. The complex at strip membrane interface is dissociated owing to NaOH in the stripping phase as follows:(6)LHn·ZnCl2+norg+nNaOH⟶ZnII+2Cl-+H2O+nNaCl+nLThe free carrier molecule (L) diffuses back through the liquid membrane phase towards feed membrane interface and again forms the complex. As per Wilke-Chang, the diffusion coefficient of the forward moving complex LHn·ZnCl2+norg should be noticeably lesser than the diffusion coefficient of the backward moving free carrier molecules [30]. Owing to this reason, the free carrier concentration at feed membrane interface will constantly be higher than the complex. Figure 1 represents the schematic transport mechanism of Zn (II) through supported liquid membrane.Figure 1
Transport mechanism of Zn (II) through supported liquid membrane.The equilibrium constantKZn of (4) for Zn (II) can be written as follows:(7)Kzn=LHn·ZnCl2+norgLnorg·H+naq·Zn2+aq·Cl-naq.The distribution coefficient of Zn2+ (λzn) for distribution between the membrane and aqueous phases can be represented as follows:(8)λzn=LHn·ZnCl2+nnorgZn2+aq,(9)Kzn=λznLnorg·H+naq·Cl-naq. And on rearranging of (8),(10)λzn=Kzn·Lnorg·H+naq·Cl-naqNow considering the extraction constant, distribution coefficient, and laws of diffusion via the same path as [29], it can be indicated as:(11)log⁡Jη=log⁡y+log⁡T+nlog⁡Lorg+nlog⁡H+aq+nlog⁡Cl-aq+log⁡Cfl,(12)log⁡Jη=constant+nlog⁡Lorg+nlog⁡H+aq+nlog⁡Cl-aq+log⁡CfEquation (11) shows that, at a constant temperature, the flux (J) of Zn (II) is directly proportional to the concentration of Cl−, H+, L, and Cf. This equation can be used to find the number of H+ related to L in the form of LH+. This can be determined by various methods: one method is to keep [Cl−], [L], and Cf constant in (11) and plotting log⁡Jη against log⁡[H+]; the slope of the curve will provide the “n” value for a number of H+ ions in the complex. Likewise by drawing log⁡Jη versus log⁡[L] and maintaining [Cl−], [H+], and Cf constant, the slope of the plot will give a number of moles (n) of TDDA contributing in complex formation of Zn (II).


### 4.3. Effect of Carrier Concentration

The carrier in the liquid membrane phase of the supported liquid membrane has a critical role in the extraction of metal ions by SLM [26]. Various concentrations of carrier ranging from 0.157 mol/dm3 to 1.103 mol/dm3 were used in solvent xylene to observe the effect of carrier concentration on the transport of Zn (II) through SLM. During this study, the Zn (II) concentration in feed solution was kept at 7.385×10-4 mol/dm3 in 2.0 mol/dm3 of HCl and the NaOH concentration in strip solution was fixed at 2.0 mol/dm3.Figure2 shows that with the increase of carrier concentration from 0.157 to 0.80 mol/dm3 in xylene in the membrane phase has a significant effect on flux of Zn (II). It follows (11) where flux (J) is directly proportional to carrier concentration [L]. However, the flux is insignificant as the concentration of carrier increases beyond 0.80 mol/dm3. This reduction in transport of Zn (II) might be the result of enhanced friction of liquid membrane phase owing to high viscosity, as with the increase in carrier concentration, the viscosity of liquid membrane phase increases [29]. Hence 0.80 mol/dm3 of tri-n-dodecylamine was considered the optimum carrier concentration and more investigations were performed with this concentration of carrier.Figure 2
Effect of carrier concentration on transport of Zn (II) in the stripping phase. [HCl] in feed solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.157 mol/dm3 to 1.103 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, [Zn (II)] = 7.385×10-4 mol/dm3, time = 5.0 h.Finding the number (n) of tri-n-dodecylamine (L) taking part in complex LHn·Zn(Cl2+n) was determined by plotting log⁡[TDDA] versus log⁡Jη as shown in Figure 3. The slope of the plot is 2.008, and this indicates that two molecules of TDDA take part in the complex formation.Figure 3
Plot oflog⁡[TDDA] versus log⁡Jη (same operating conditions as given in Figure 2).In our previous study for the stability of membrane for such type of carrier, it has been observed that supported liquid membrane is quite stable for a period of 120 h with very minute flux variation in the metal ion flux and no indication of structural deformation of polypropylene membrane was investigated. Furthermore, the membrane was reused several times without leakage and metal ion flux decline [28].


### 4.4. Role of HCl Concentration in Feed Phase

The HCl performs an important part in the transportation of Zn (II) because it provides H+ and Cl− for the formation of complex LHn·Zn(Cl2+n) as per (4). The effect of HCl concentration on extraction Zn (II) was observed by varying the concentration of HCl in the range of 0.5 mol/dm3 to 3.0 mol/dm3, while keeping the concentration of TDDA in the liquid membrane phase at 0.80 mol/dm3 and concentration of NaOH on strip side at 2.0 mol/dm3. Figure 4 shows that as the concentration of HCl increases from 0.5 mol/dm3 to 2.0 mol/dm3 and the flux of Zn (II) increases. Thus, it can be concluded that with an increase of HCl concentration, the concentration of H+ and Cl− ions in feed solution also increases. This, in turn, increases the formation of the complex as per (4), which ultimately enhances the transport of Zn (II). Although, by further increasing the concentration of HCl beyond 2.0 mol/dm3, the transport of Zn (II) decreases and this may be due to the formation of Hn·ZnCl2n type species due to the large quantity of H+ and Cl− in feed phase and reaction (4) is hindered in forward direction. This study shows that 2.0 mol/dm3 of HCl is the most favorable concentration for transport of Zn (II), and further studies were carried out with this concentration of HCl to evaluate various parameters for subsequent studies.Figure 4
Effect of HCl concentration in feed solution on flux (J) of Zn (II). [HCl] in feed solution = 0.50 mol/dm3 to 3.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, time = 5.0 h.To investigate that how much hydrogen is taking part in complexation of Zn (II),log⁡Jη was plotted versus log⁡[HCl] (Figure 5). The slope of this plot was approximately 2.0, indicating that two hydrogens take part in a complex of Zn (II) transport. As the transport study concluded that two molecules of TDDA and H+ are involved in the complex formation of Zn (II), the complex formed during this extraction study may be LHn·Zn(Cl2+n).Figure 5
Plot oflog⁡[HCl] versus log⁡Jη (same operating conditions as given in Figure 4).


### 4.5. Influence of Feed Concentration

To study the capability of this SLM for metal ion transport, various concentrations of Zn (II) from3.692×10-4 mol/dm3 to 18.463×10-4 mol/dm3 were used in the feed solution. During this particular study, the concentrations of the carrier, stripping phase, and HCl in feed solution were kept at 0.80 mol/dm3, 2.0 mol/dm3, and 2.0 mol/dm3, respectively.Figure6 shows that as Zn (II) concentration in feed solution rises, the transport of Zn (II) also increases; this is as per (11), where flux (J) is directly proportional to feed concentration (Cf). Such type behavior has already been studied in our previous study [27, 28]. This study shows that up to 18.463×10-4 mol/dm3 no saturation of carrier with metal ion takes place.Figure 6
Effect of Zn (II) concentration in feed solution on its flux. [Zn(II)] in feed =3.692×10-4 mol/dm3 to 18.463×10-4 mol/dm3, [HCl] in feed = 2.0 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, time = 5.0 h.


### 4.6. Influence of Stripping Phase Concentration

The stripping agent dissociates the complexLHn·Zn(Cl2+n) at strip membrane interface and releases Zn (II) in strip solution. To investigate the effect of NaOH on the transport of Zn (II), numerous concentrations of NaOH in the range of 0.05 mol/dm3 to 2.5 mol/dm3 were used, while keeping the TDDA concentration at 0.80 mol/dm3 and HCl concentration in feed solution at 2.0 mol/dm3.Figure7 shows that as the concentration of NaOH rises from 0.05 mol/dm3 to 0.1 mol/dm3, the flux of Zn (II) decreases and, beyond this concentration of NaOH, the flux of Zn (II) increases. The decrease in transport of Zn (II) at a lower concentration of NaOH can be explained that it forms a precipitate of Zn(OH)2 which is insoluble and blocks the pores of polypropylene membrane and transport of Zn (II) is restricted [31]. The SEM (Figure 8(b)) indicates the blocked SLM at a lower concentration of NaOH. More increase in the concentration of NaOH increases the transportation of Zn (II), as the precipitate of Zn (OH)2 is soluble in excess of NaOH and more OH− are available that enhance the decomposition of the complex at strip membrane interface as per (5).Figure 7
Effect of NaOH concentration in strip solution on the flux of Zn (II). [NaOH] in stripping solution 0.05 mol/dm3 to 2.5 mol/dm3, [HCl] in feed solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, time = 5.0 h.Figure 8
(a)
SEM of the membrane before Zn (II) transport
(b)
SEM of membrane at 0.1 mol/dm3 of NaOH ([HCl] in feed solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, [Zn (II)] = 7.385×10-4 mol/dm3).


## 4.1. Reactions at Feed Membrane Interface

If tri-n-dodecylamine (TDDA) is represented by L, the following possibilities of reactions may take place. The tri-n-dodecylamine can be protonated to LH+ in acidic medium (HCl).(2)L+H+⇌LH+It is supposed that ZnCl2 in feed solution in the presence of excess chloride ions due to HCl is converted to(3)ZnCl2aq+nCl-aq⟶ZnCl2+nn-aqThe cationic (LH+)(org) and anionic species ZnCl2+nn-(aq) then interact at feed membrane interface and form the complex as(4)nLH+org+ZnCl2+nn-aq⇌LHn·ZnCl2+norgThe subscript org represents organic phase and aq aqueous phase and n indicates the number of Cl−, H+, and L molecules associating with Zn (II) ions to form the neutral complex (LH)n·Zn(Cl2+n)(org).To express the contribution of H+ and Cl−, (3) may be represented as(5)nLorg+nH+aq+ZnCl2aq+nCl-aq⇌LHn·ZnCl2+norg


## 4.2. Reactions at Strip Membrane Interface

The neutral complex formed at feed membrane interface as per (3)/(4) is extractable in liquid organic phase and disperses from feed membrane interface to strip membrane interface. The complex at strip membrane interface is dissociated owing to NaOH in the stripping phase as follows:(6)LHn·ZnCl2+norg+nNaOH⟶ZnII+2Cl-+H2O+nNaCl+nLThe free carrier molecule (L) diffuses back through the liquid membrane phase towards feed membrane interface and again forms the complex. As per Wilke-Chang, the diffusion coefficient of the forward moving complex LHn·ZnCl2+norg should be noticeably lesser than the diffusion coefficient of the backward moving free carrier molecules [30]. Owing to this reason, the free carrier concentration at feed membrane interface will constantly be higher than the complex. Figure 1 represents the schematic transport mechanism of Zn (II) through supported liquid membrane.Figure 1
Transport mechanism of Zn (II) through supported liquid membrane.The equilibrium constantKZn of (4) for Zn (II) can be written as follows:(7)Kzn=LHn·ZnCl2+norgLnorg·H+naq·Zn2+aq·Cl-naq.The distribution coefficient of Zn2+ (λzn) for distribution between the membrane and aqueous phases can be represented as follows:(8)λzn=LHn·ZnCl2+nnorgZn2+aq,(9)Kzn=λznLnorg·H+naq·Cl-naq. And on rearranging of (8),(10)λzn=Kzn·Lnorg·H+naq·Cl-naqNow considering the extraction constant, distribution coefficient, and laws of diffusion via the same path as [29], it can be indicated as:(11)log⁡Jη=log⁡y+log⁡T+nlog⁡Lorg+nlog⁡H+aq+nlog⁡Cl-aq+log⁡Cfl,(12)log⁡Jη=constant+nlog⁡Lorg+nlog⁡H+aq+nlog⁡Cl-aq+log⁡CfEquation (11) shows that, at a constant temperature, the flux (J) of Zn (II) is directly proportional to the concentration of Cl−, H+, L, and Cf. This equation can be used to find the number of H+ related to L in the form of LH+. This can be determined by various methods: one method is to keep [Cl−], [L], and Cf constant in (11) and plotting log⁡Jη against log⁡[H+]; the slope of the curve will provide the “n” value for a number of H+ ions in the complex. Likewise by drawing log⁡Jη versus log⁡[L] and maintaining [Cl−], [H+], and Cf constant, the slope of the plot will give a number of moles (n) of TDDA contributing in complex formation of Zn (II).


## 4.3. Effect of Carrier Concentration

The carrier in the liquid membrane phase of the supported liquid membrane has a critical role in the extraction of metal ions by SLM [26]. Various concentrations of carrier ranging from 0.157 mol/dm3 to 1.103 mol/dm3 were used in solvent xylene to observe the effect of carrier concentration on the transport of Zn (II) through SLM. During this study, the Zn (II) concentration in feed solution was kept at 7.385×10-4 mol/dm3 in 2.0 mol/dm3 of HCl and the NaOH concentration in strip solution was fixed at 2.0 mol/dm3.Figure2 shows that with the increase of carrier concentration from 0.157 to 0.80 mol/dm3 in xylene in the membrane phase has a significant effect on flux of Zn (II). It follows (11) where flux (J) is directly proportional to carrier concentration [L]. However, the flux is insignificant as the concentration of carrier increases beyond 0.80 mol/dm3. This reduction in transport of Zn (II) might be the result of enhanced friction of liquid membrane phase owing to high viscosity, as with the increase in carrier concentration, the viscosity of liquid membrane phase increases [29]. Hence 0.80 mol/dm3 of tri-n-dodecylamine was considered the optimum carrier concentration and more investigations were performed with this concentration of carrier.Figure 2
Effect of carrier concentration on transport of Zn (II) in the stripping phase. [HCl] in feed solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.157 mol/dm3 to 1.103 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, [Zn (II)] = 7.385×10-4 mol/dm3, time = 5.0 h.Finding the number (n) of tri-n-dodecylamine (L) taking part in complex LHn·Zn(Cl2+n) was determined by plotting log⁡[TDDA] versus log⁡Jη as shown in Figure 3. The slope of the plot is 2.008, and this indicates that two molecules of TDDA take part in the complex formation.Figure 3
Plot oflog⁡[TDDA] versus log⁡Jη (same operating conditions as given in Figure 2).In our previous study for the stability of membrane for such type of carrier, it has been observed that supported liquid membrane is quite stable for a period of 120 h with very minute flux variation in the metal ion flux and no indication of structural deformation of polypropylene membrane was investigated. Furthermore, the membrane was reused several times without leakage and metal ion flux decline [28].


## 4.4. Role of HCl Concentration in Feed Phase

The HCl performs an important part in the transportation of Zn (II) because it provides H+ and Cl− for the formation of complex LHn·Zn(Cl2+n) as per (4). The effect of HCl concentration on extraction Zn (II) was observed by varying the concentration of HCl in the range of 0.5 mol/dm3 to 3.0 mol/dm3, while keeping the concentration of TDDA in the liquid membrane phase at 0.80 mol/dm3 and concentration of NaOH on strip side at 2.0 mol/dm3. Figure 4 shows that as the concentration of HCl increases from 0.5 mol/dm3 to 2.0 mol/dm3 and the flux of Zn (II) increases. Thus, it can be concluded that with an increase of HCl concentration, the concentration of H+ and Cl− ions in feed solution also increases. This, in turn, increases the formation of the complex as per (4), which ultimately enhances the transport of Zn (II). Although, by further increasing the concentration of HCl beyond 2.0 mol/dm3, the transport of Zn (II) decreases and this may be due to the formation of Hn·ZnCl2n type species due to the large quantity of H+ and Cl− in feed phase and reaction (4) is hindered in forward direction. This study shows that 2.0 mol/dm3 of HCl is the most favorable concentration for transport of Zn (II), and further studies were carried out with this concentration of HCl to evaluate various parameters for subsequent studies.Figure 4
Effect of HCl concentration in feed solution on flux (J) of Zn (II). [HCl] in feed solution = 0.50 mol/dm3 to 3.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, time = 5.0 h.To investigate that how much hydrogen is taking part in complexation of Zn (II),log⁡Jη was plotted versus log⁡[HCl] (Figure 5). The slope of this plot was approximately 2.0, indicating that two hydrogens take part in a complex of Zn (II) transport. As the transport study concluded that two molecules of TDDA and H+ are involved in the complex formation of Zn (II), the complex formed during this extraction study may be LHn·Zn(Cl2+n).Figure 5
Plot oflog⁡[HCl] versus log⁡Jη (same operating conditions as given in Figure 4).


## 4.5. Influence of Feed Concentration

To study the capability of this SLM for metal ion transport, various concentrations of Zn (II) from3.692×10-4 mol/dm3 to 18.463×10-4 mol/dm3 were used in the feed solution. During this particular study, the concentrations of the carrier, stripping phase, and HCl in feed solution were kept at 0.80 mol/dm3, 2.0 mol/dm3, and 2.0 mol/dm3, respectively.Figure6 shows that as Zn (II) concentration in feed solution rises, the transport of Zn (II) also increases; this is as per (11), where flux (J) is directly proportional to feed concentration (Cf). Such type behavior has already been studied in our previous study [27, 28]. This study shows that up to 18.463×10-4 mol/dm3 no saturation of carrier with metal ion takes place.Figure 6
Effect of Zn (II) concentration in feed solution on its flux. [Zn(II)] in feed =3.692×10-4 mol/dm3 to 18.463×10-4 mol/dm3, [HCl] in feed = 2.0 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, time = 5.0 h.


## 4.6. Influence of Stripping Phase Concentration

The stripping agent dissociates the complexLHn·Zn(Cl2+n) at strip membrane interface and releases Zn (II) in strip solution. To investigate the effect of NaOH on the transport of Zn (II), numerous concentrations of NaOH in the range of 0.05 mol/dm3 to 2.5 mol/dm3 were used, while keeping the TDDA concentration at 0.80 mol/dm3 and HCl concentration in feed solution at 2.0 mol/dm3.Figure7 shows that as the concentration of NaOH rises from 0.05 mol/dm3 to 0.1 mol/dm3, the flux of Zn (II) decreases and, beyond this concentration of NaOH, the flux of Zn (II) increases. The decrease in transport of Zn (II) at a lower concentration of NaOH can be explained that it forms a precipitate of Zn(OH)2 which is insoluble and blocks the pores of polypropylene membrane and transport of Zn (II) is restricted [31]. The SEM (Figure 8(b)) indicates the blocked SLM at a lower concentration of NaOH. More increase in the concentration of NaOH increases the transportation of Zn (II), as the precipitate of Zn (OH)2 is soluble in excess of NaOH and more OH− are available that enhance the decomposition of the complex at strip membrane interface as per (5).Figure 7
Effect of NaOH concentration in strip solution on the flux of Zn (II). [NaOH] in stripping solution 0.05 mol/dm3 to 2.5 mol/dm3, [HCl] in feed solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, time = 5.0 h.Figure 8
(a)
SEM of the membrane before Zn (II) transport
(b)
SEM of membrane at 0.1 mol/dm3 of NaOH ([HCl] in feed solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3, [Zn (II)] = 7.385×10-4 mol/dm3).


## 5. Recovery of Zinc (II) from Waste Discharge/Effluent of Galvanizing Plant

The objective of this work was to design and develop SLM for recovery of Zn (II) from industrial waste effluents. The polypropylene as supported liquid membrane, TDDA as a carrier, was utilized for extraction of Zn (II) from waste discharge liquor of galvanizing plant. To check and assess the efficiency of SLM system, samples of galvanized industrial waste were collected from the different locations of the drain carrying the galvanizing industrial effluent. The effluent samples were analyzed using the aforementioned method as described in Experimental and the percent extraction and recovery of the Zn(II) metal ions were determined using atomic absorption spectrophotometry method. The data obtained is provided in Figure9 which indicates approximately the complete extraction and recovery of Zn (II) after 210 minutes. This shows the suitability and effectiveness of this SLM for recovery of Zn (II).Figure 9
Variation in Zn (II) concentration in feed and strip solution against time (waste discharge liquor of galvanizing plant). Initial Zn (II) conc. in feed =5.16×10-4 mol/dm3, [HCl] in feed = 2.0 mol/dm3, [NaOH] in stripping solution = 2.0 mol/dm3, [TDDA] in membrane phase = 0.80 mol/dm3.


## 6. Recovery and Transport of Zn(II) from Galvanizing Plant Waste

The present SLM was designed for the transport of Zn(II) metal ions and applied to zinc industrial effluents of galvanizing plants because zinc is one of the industrial important metals because of its applications and its use for protection, passivation, and decoration of some heavy elements. The data in Table2 show that TDDA-polypropylene SLM system has significant transport efficiency for Zn(II) metal ions, that is, about 99.8%, while other metal ions like Co, Cu, Cd, Mn, Cr, and Fe show very little or more precisely negligible amount of transport that is 0.0 to 0.72% or in average 0.08% for Co, 0.23% for Cu, 0.15% for Cd, 0.36% for Mn, 0.06% for Cr, and 0.28$ for Fe. This much smaller amount of transport may be attributed to experimental error. If error factor is taken into account then TDDA-polypropylene SLM system is highly selective for transport of Zn(II) metal ions only. The data are represented in Tables 1 and 2.Table 1
Analysis of galvanizing plants waste.
Metal ions
Zinc galvanizing plant-I
Zinc galvanizing plant-II
Zinc galvanizing plant-III
Zinc galvanizing plant-IV
Original concentration of actual sample
Concentration of actual sample after transport
Original concentration of actual sample
Concentration of actual sample after transport
Original concentration of actual sample
Concentration of actual sample after transport
Original concentration of actual sample
Concentration of actual sample after transport
BT  (mg/L)
AT  (mg/L)
BT  (mg/L)
AT  (mg/L)
BT  (mg/L)
AT  (mg/L)
BT  (mg/L)
AT  (mg/L)
Zn
170
±
0.57
1.0
±
0.28
130
±
0.36
0.2
±
0.05
150
±
0.64
3.0
±
0.05
191
±
0.1
1.20
±
0.1
Co
4.66
±
0.01
4.65
±
0.011
1.364
±
0.001
—
9.04
±
0.5
9.03
±
0.005
7.05
±
0.01
7.04
±
0.001
Cu
3.86
±
0.01
3.71
±
0.015
—
—
5.86
±
0.01
5.84
±
0.005
13.3
±
0.005
13.29
±
0.32
Cd
9.990
±
0.012
9.979
±
0.006
116.4
±
0.15
116.168
±
0.03
11.5
±
0.003
11.48
±
0.01
22.0
±
0.06
21.97
±
0.01
Mn
2.850
±
0.001
2.836±  0,0005
6.26
±
0.01
6.24
±
0.005
1.56
±
0.01
1.55
±
0.002
4.033
±
0.01
4.032
±
0.0005
Cr
—
—
37.48
±
0.011
37.43
±
0.01
21.70
±
0.01
21.70
±
0.01
7.51
±
0.006
7.50
±
0.006
Fe
16.84
±
0.005
16.718
±
0.001
13.2
±
0.004
13.18
±
0.01
5.835
±
0.001
5.820
±
0.01
3.14
±
0.01
3.139
±
0.0005
BT, before transport; AT, after transport; galvanizing plants I & II, Industrial State Peshawar, Pakistan; galvanizing plants III & IV, Industrial State Rawalpindi, Pakistan.Table 2
Percent transport of metal ions from galvanizing plants waste.
Percent transport of metal ions from waste of zinc galvanizing plants
Metal ions
Zinc galvanizing plant-I(% transport)
Zinc galvanizing plant-II(% transport)
Zinc galvanizing plant-III(% transport)
Zinc galvanizing plant-IV(% transport)
Zn
98.50
99.33
98
99.37
Co
0.2
00
0.011
0.14
Cu
0.38
00
0.5
0.07
Cd
0.11
0.19
0.17
0.13
Mn
0.49
0.31
0.64
0.02
Cr
00
0.13
00
0.13
Fe
0.72
0.15
0.25
0.03


## 7. Conclusions

The flux (JZn(II)) got increased by increasing carrier concentration up to 0.80 mol/dm3, and, further increasing the carrier concentration, the transport of Zn (II) was found to decrease. The Zn(II) ions transport was increased by increasing HCl concentration in feed solution up to 2.0 mol/dm3 and then decreased using a higher concentration of HCl. The slope analysis studies of log⁡Jη of Zn (II) versus log⁡[TDDA] and log⁡[HCl] showed that 2.0 mole of each tri-n-dodecylamine and hydrogen are involved in the transport of Zn (II). The maximum transport of Zn (II) was achieved at 0.80 mol/dm3 of TDDA in membrane phase and 2.0 mol/dm3 of HCl and NaOH in feed and strip solution, respectively. It was also found to be coupled coion extraction pathway, as H+, Cl−, and Zn (II) travel in the same direction. Two moles of hydrogen and TDDA interact with one mole of Zn (II) producing a complex LHn·Zn(Cl2+n), that is responsible for transport of Zn (II). This SLM system was effectively used for removal of Zn (II) to the waste discharge liquor of galvanizing plant of zinc.



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*Source: 7569354-2017-11-02.xml*