Preparation, electrochemical characterization and antibacterial study of polystyrene-based magnesium–strontium phosphate composite membrane

Materials Science and Engineering C 32 (2012) 1210–1217

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Preparation, electrochemical characterization and antibacterial study of polystyrene-based magnesium–strontium phosphate composite membrane Mohammad Mujahid Ali Khan, Rafiuddin ⁎ Membrane Research Laboratory, Department of Chemistry, Aligarh Muslim University, Aligarh, 202002, India

a r t i c l e

i n f o

Article history: Received 13 July 2011 Received in revised form 29 December 2011 Accepted 14 March 2012 Available online 21 March 2012 Keywords: Co-precipitation method Membrane potential Fixed-charge density XRD SEM Antibacterial study

a b s t r a c t The electrochemical characterizations of polystyrene-based magnesium–strontium phosphate (MSP) composite membrane have been worked on, as a function of membrane thickness, porosity and moisture content etc. Polystyrene-based magnesium–strontium phosphate composite membrane was characterized by XRD, FTIR, SEM, and antibacterial studies. The membrane was found to be crystalline in nature with uniform arrangement of particles, no sign of visible cracks and shows excellent inhibitory results against Escherichia coli and Pseudomonas aeruginosa bacteria. The membrane potentials of inorganic membrane were measured with uni-univalent electrolytes solution using saturated calomel electrodes and followed the order LiCl > NaCl > KCl, thus, the membrane was found to be cation selective. Membrane potential data have been used to calculate transport number, mobility ratio, distribution coefficient, charge effectiveness, and also to derive the fixed-charge density which is a central parameter governing the membrane phenomena by utilizing the Teorell, Meyer, and Sievers method. The order of surface charge density for uni-univalent electrolytes solution was found to be LiCl b NaCl b KCl. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Membrane separation processes stand out as alternative to conventional processes in chemical, pharmaceutical, biotechnological and food industries. In many cases the low energy consumption, reduction in number of processing steps, greater separation efficiency and improved final product quality are the main attractions of these processes [1]. The electro chemical properties can exert profound influence on the nature and magnitude of the interaction between the membrane and liquid feed, thus affecting permeating fluxes of the solvent and solute through membrane pores [2,3]. The surface charge properties of membrane materials are among the factors that influence membrane performances and electrical characterizations of the membranes. It also plays a very important role in understanding and predicting filtration properties of the membranes [4–6]. An inorganic precipitate ion-exchanger based on organic polymeric matrix must be an interesting material, as it should possess the mechanical stability due to the presence of organic polymeric species and the basic characteristics of an inorganic ion-exchanger regarding its selectivity for some particular metal ions [7–9], indicating its useful environmental applications [10,11]. Membrane systems also played a vital role in the purification of the earliest biotechnology products [12,13] which were developed for the blood fractionation, food, dairy and distillery industries [14].

⁎ Corresponding author. Tel./fax: + 91 571 2703515. E-mail address: rafi[email protected] (Rafiuddin). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.03.010

The current study is focussed upon the synthesis and characterization of stable polystyrene-based MSP composite membrane by coprecipitation method, involving polystyrene as a binder by the application of pressure. The values for the membrane potential are of the order of positive mV and increase with a decrease in external electrolytes’ concentration. This shows that the membrane is cation selective. Polystyrene-based MSP composite membrane demonstrates a large number of applications, which makes them attractive in filtration tasks such as beverage, textile industry, medicine, pharmacy, chemical industry, waste water treatment and others. These applications are attributed to their high thermal resistance, chemical resistance and the mechanical strength.  is the most effective parameter that The surface charge density (D) controls the membrane phenomena and was calculated using the membrane potentials values for different electrolytes by using Teorell, Meyer, and Sievers (TMS) method [15–18]. Some other parameters including distribution coefficient, transport numbers, mobility ratio and charge effectiveness etc. were also calculated for polystyrene-based MSP composite membranes. Membrane potential studies are commonly used in the electrochemical characterizations of membranes [19,20]. 2. Theoretical aspects of the composite membrane 2.1. Fixed-charge density theory of Teorell–Meyer–Sievers In the Teorell–Meyer–Sievers theory there is an equilibrium process at each solution membrane interface which has a formal analogy with the Donnan equilibrium. The assumptions

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Nomenclature

litre and the cation-to-anion mobility ratio in the membrane phase.

AR C1, C2

3. Experimental

Analytical reagent Concentrations of electrolyte solution on either side of the membrane (mol/L) Cation concentration in membrane phase 1 (mol/L) C 1þ Cation concentration in membrane phase 2 (mol/L) C 2þ Ci i th ion concentration of external solution (mol/L) i th ion concentration in membrane phase (mol/L) C i  D Charge density in membrane (eq/L) F Faraday constant (C/mol) 100–160 Pressure (MPa) q Charge effectiveness of the membrane R Gas constant (J/K/mol) SCE Saturated calomel electrode SEM Scanning electron microscopy TMS Teorell, Meyer and Sievers t+ Transport number of cation t− Transport number of anion ū Mobility of cations in the membrane phase (m 2/v/s) v Mobility of anions in the membrane phase (m 2/v/s) Vk Valency of cation Vx Valency of fixed-charge group  ¼ ðu  −v Þ=ðu  þ v Þ Ū U MSP Magnesium–strontium phosphate membrane

Greek symbols γ± Mean ionic activity coefficients  ω Mobility ratio Δψm Observed membrane potential (mV)  ΔΨm Theoretical membrane potential (mV) ΔΨDon Donnan potential (mV)  diff ΔΨ Diffusion potential (mV)

made are (a) the cation and anion mobilities, fixed-charge concentration are constant throughout the membrane phase but are independent of the salt concentration (b) the transference of water may be neglected. The implications of these assumptions have been discussed [21]. Further assumption is that the activity coefficient of the salt is same in the membrane and solution phase at each interface must also be made. The introduction of activities for concentrations can only be corrected by the Donnan potential using either the integration of Planck or Henderson equation. According to TMS theory, the membrane potential (applicable to a highly idealized system) is given by the equation at 25 °C 0

 ΔΨ m

1 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 U  2  2 þ D 4C 22 þ D C 2 4C 1 þ D þ D  B C þ U log qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 59:2@ log qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A C 1 4C 2 þ D 2 2    2 þ D  4C 1 þ D þ D U 2

3.1. Preparation of membrane MSP precipitate was prepared by mixing 0.2 M magnesium (II) chloride (99.98% purity, E. Merck, Mumbai, India) and 0.2 M strontium (II) chloride (99.98% purity, E. Merck, Mumbai, India) with 0.2 M tri-sodium phosphate (99.90% purity, E. Merck, Mumbai, India) solution. The precipitate was washed well with deionized water (Water purification system, ‘Integrate, whose RO conductivity 0–200 μs/cm and UP resistivity 1–18.3 M Ωcm′) to remove free reactant and then dried and powdered. Membrane using suitable ratio of binder (1:3) was prepared [22]. The precipitate was ground into fine powder and was sieved through 200 mesh BSS standard (granule size b0.07 mm). Pure crystalline polystyrene (Otto Kemi, India, AR) was also ground and sieved through 200 mesh. The MSP along with appropriate amount of polystyrene powder was mixed thoroughly using mortar and pestle. The mixture was then kept into a cast die having a diameter of 2.45 cm and placed in an oven (Oven-Universal, Memmert Type) maintained at 200 °C for about an hour to equilibrate the reaction mixture [23,24]. The die containing the mixture was then transferred to a pressure device (SL-89, UK), and a pressure such as 100 MPa was applied during the fabrication of the membrane. Our effort has been to prepare the membrane of adequate chemical and mechanical stability. The membranes prepared by embedding 25% polystyrene were found to be mechanically most stable and gave reproducible results. Those containing larger amount (>25%) of polystyrene did not give reproducible results, while the one containing smaller amount (b25%) was found unstable [25]. Thus, the total amount of the mixture utilized for the preparation of the membrane contained 0.125 g polystyrene 200 mesh and 0.1875 g magnesium phosphate and 0.1875 g strontium phosphate (200 mesh). The membranes were subjected to microscopic and electrochemical examinations for cracks and homogeneity of the surface and only those which had smooth surface and generate reproducible potentials were considered. 3.2. Measurement of membrane potential Membrane potential was measured by using digital potentiometer (Electronics India model 118). The freshly prepared charged membrane was installed at the centre of the measuring cell, which had two glass containers, on either side of the membrane. The various salt solutions (chlorides of K+, Na+, and Li+) were prepared from B.D.H. (A.R.) grade chemicals using deionized water. Both collared glass containers had cavity for introducing the electrolyte solution and saturated calomel electrodes. The half-cell contained 25 ml of the electrolyte solution while the capacity of each of the half cells holding the membrane was about 35 ml. The electrochemical setup used for uni-ionic potential and membrane potential measurements may be depicted as:

ð1Þ

 ¼ ðu  −v Þ=ðu  þ v Þ U

where ū and v are the ionic mobilities of cation and anion (m 2/ v/s) respectively in the membrane phase, C1 and C2 are the  concentrations of the membrane and Dis the charge on the membrane expressed in equivalent per litre. The graphical  method of TMS determines the fixed-charge Din equivalents/

3.3. Characterization of membrane The pre-requisite criterion for understanding the performance of an ion-exchange membrane is its complete physico-chemical

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characterization, which involves the determination of all such parameters that affects its electrochemical properties. These parameters include membrane water content, porosity, thickness and swelling etc. These were determined in accordance with the method described by [26,27].

(UK). The sample compartment was 200 mm wide, 290 mm deep and 255 mm high. The entrance and exit beam to the sample compartment was sealed with a coated KBr window and there was a hinged cover to seal it from the environment. 3.7. Antibacterial activity of membrane

3.3.1. Water content (% total wet weight) The conditional membrane was first soaked in water, blotted quickly with Whatmann filter paper to remove surface moisture and immediately weighed. These were further dried to a constant weight in a vacuum over P2O5 (dehydrating agent) for 24 h. The water content (total wet weight) was calculated as: %Total ðweight

wet Þ

¼

W w −W d  100 Ww

Where Ww is the weight of the soaked or wet membrane and Wd the weight of the dry membrane. 3.3.2. Porosity Porosity was determined as the volume of water incorporation in the cavities per unit membrane volume from the water content data: Porosity ¼

W w −W d ALρw

Where Wd is the weight of the dry membrane, Ww is the weight of the soaked or wet membrane, A is the area of the membrane, L is the thickness of the membrane and ρw is the density of water. 3.3.3. Thickness The thickness of the membrane was measured by taking the average thickness of the membrane using screw gauze. 3.3.4. Swelling Swelling was measured in accordance with the procedure described by Arfin and Rafiuddin [27]. It was calculated as the difference between the average thickness of the membrane equilibrated with 1 M NaCl for 24 h and the dry membrane. 3.3.5. Chemical stability Chemical stability was evaluated on the basis of ASTM D543-m95 method. Membrane was exposed to several media commonly used. Membrane was evaluated after 24, 48 and 168 h, analyzing alteration in colour, texture, brightness, decomposition, splits, holes, bubbles, curving and stickiness [28]. 3.4. SEM investigation of membrane morphology Scanning electron microscopic image was taken to confirm the microstructure of fabricated porous membrane. The membrane morphology was investigated by ZEISS EVO Series Scanning Electron Microscope EVO 50 at an accelerating voltage of 20 kV. Sample was mounted on a copper stub and sputter coated with gold to minimize the charging. 3.5. X-ray diffraction studies of the composite membrane X-ray diffraction pattern of the composite ion-exchange membrane was recorded by Miniflex-II X-Ray diffractometer (Rigaku Corporation) with Cu Kα radiation. 3.6. FTIR spectra of membrane The FTIR spectrum of Polystyrene-based MSP composite membrane was done by Interspec 2020 FT-IR spectrometer, spectrolab

The antibacterial study of Polystyrene-based MSP composite membrane was tested in vitro against two Gram-positive bacteria Staphylococcus aureus (MSSA 22) and Bacillus subtilis (ATCC 6051) and two Gram-negative bacteria Escherichia coli (K 12) and Pseudomonas aeruginosa (MTCC 2488) strains using disc diffusion method [29,30]. The discs measuring 5 mm in diameter were prepared from Whatmann No. 1 filter paper sterilized by dry heat at 140 °C for 1 h after then discs were soaked in a 0.2 M concentration of the test complex. The screening was performed for 114.4 μg/ml concentration of test Polystyrene-based MSP composite membrane and antibiotic discs. Tetracycline (30 μg/disc, Hi- Media) was used as positive control. The discs were placed on the agar plates, which had previously been inoculated with the above testing organisms and incubated for 24 h at 37 °C. After incubation inhibition zones appearing around the discs were measured and recorded in mm. The nutrient broth which, logarithmic serially twofold diluted amount of test Polystyrene-based MSP composite membrane and controls, was inoculated within the range 10 7–10 8 cfu/ml. The cultures were incubated for 24 h at 37 °C and growth was monitored visually and spectrophotometrically like [31]. The lowest concentration (highest dilution) required to arrest the growth of bacteria is regarded as minimum inhibitory concentration. The number of colony forming units (cfu) was counted after 24 h of incubation at 37 °C. 4. Results and discussion The characterization of membrane morphology has been studied by a number of investigators using scanning electron microscopy [32,33]. The surface morphologies of the membranes show uniform arrangement of particles while cross-sectional SEM image shows no visible cracks. The composite pore structure, micro/macro porosity, homogeneity, thickness, cracks and surface texture/morphology have been studied [34,35]. The SEM, surface cross sectional images of the Polystyrene-based MSP composite membrane prepared at 100 MPa applied pressure respectively are shown in Fig. 1(a) and (b). SEM images have provided ideas regarding the preparation of well ordered crack free membranes. Membrane had random nonpreferential orientation with no visible cracks and appeared to be composed of dense and loose aggregation of small particles. The SEM image is dense in nature and formed pores probably with nonlinear channel but not fully interconnected. Particles are irregularly condensed and adopt a heterogeneous structure composed of masses of various sizes. Due to the strong interactions between the styrene particles and the membrane pore walls, the particles do not permeate through the membrane. The distributions of charge density and mobile species within the pores are assumed to be uniform. The diameters of the polystyrene particles are clearly much smaller than the pore diameters of the membrane used [36]. X-ray scattering techniques are a family of non-destructive analytical techniques which reveal information about the crystal structure, chemical composition, and physical properties of composite materials. The X-ray diffraction spectrum of the polystyrene-based MSP composite membrane is shown in Fig. 2. The X-ray diffraction pattern of this material recorded in powdered sample exhibited strong intensities in the spectrum (2θ range) which suggest crystalline nature of the composite material. XRD results show the crystalline nature of polystyrene-based MSP composite membrane.

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Table 1 Thickness, water content, porosity and swelling properties of polystyrene-based MSP composite membrane at 100 MPa pressure.

Fig. 1. SEM images of polystyrene based MSP composite membrane at 100MPa and magnification of 5.00KX (a). Surface image (b). Cross-sectional view.

Inorganic composite membranes have the ability to generate potential when two electrolyte solutions of unequal concentration are separated by a membrane and driven by different chemical potential acting across the membrane [37,38]. The migration of charged species is regulated by the electrical behaviour of the membrane and diffusion of electrolytes from higher to lower concentration takes place through the charged membrane. In fact, the mobile species penetrate into the membrane at different magnitude and various transport phenomena, including the development of potential across it, are induced into the system [39]. The results of thickness, swelling, porosity and water content capacity of Polystyrene-based MSP composite membrane are

Fig. 2. X-ray diffraction pattern of composite polystyrene-based MSP composite membrane.

Applied pressure (MPa)

Thickness of the membrane (cm)

Water content as % weight of wet membrane

Porosity

Swelling of % weight wet membrane

100 120 140 160

0.090 0.085 0.080 0.075

0.07 0.06 0.05 0.04

0.082 0.071 0.059 0.053

No No No No

swelling swelling swelling swelling

summarized in Table 1. The water content of a membrane depends on the vapour pressure of the surroundings. In case of most of the transport measurements, only the membrane water content at saturation is needed, and that too mostly as a function of solute concentration. Thus, low order of water content, swelling and porosity with less thickness of membrane suggests that interstices are negligible and diffusion across the membrane would occur mainly through exchange sites [40]. Membrane was tested for chemical resistance in acidic, alkaline and strongly oxidant media. In acidic (1 M H2SO4) and in alkaline media (1 M NaOH) few significant modifications were observed after 24, 48 and 168 h, demonstrating that the membrane was quite effective in such media. However, in strong oxidant media the synthesized membrane became fragile in 48 h and broken after 168 h, losing mechanical strength. In general membranes having the same chemical composition were found to absorb same amount of water, where density ionisable groups are same throughout the membrane [41]. Moreover, in membrane transport phenomenon, membrane water content as a function of solute concentration at saturation is needed. Thus, low order of water content, swelling and porosity with less thickness of this membrane suggests that interstices are negligible and diffusion across the membrane would occur mainly through exchange sites. The FTIR spectral technique has been used as a valuable tool to explain the binding sites of polystyrene-based MSP composite membrane. The FTIR spectra indicate the various peaks (Fig. 3) of functional groups present in the composite containing polystyrene moiety with the metals i.e. magnesium and strontium. The aromatic peaks are well defined at the position 698, 754 and 869 cm − 1 exhibiting the presence of an aromatic ring [42]. The C= C of the ring shows characteristics peak at 1650 and 1456 cm − 1. The C–H stretching frequencies are shown as broad band at 2924 cm − 1. The C–H bonding peak is at 1075 cm − 1 as a broad band. The peaks in the spectra of polystyrene-based MSP composite show a comparatively negative shift as compared to those of the free ligand (polystyrene), due to coordination with metal [42]. The phosphate group present in the composite material shows the peaks at 698, 869 and 1456 cm − 1 in the spectra. The biological activity of Polystyrene-based MSP composite membrane have been studied for its antibacterial activities using disc diffusion method [29,30] in vitro against two Grampositive bacteria Staphylococcus aureus (MSSA 22) and B. subtilis (ATCC 6051) and two Gram-negative bacteria E. coli (K 12) and P. aeruginosa at concentration of 114.4 μg/ml. Tetracycline was used as standard drug for the comparison of bacterial results and screening data are given in Table 2. The novel synthesized polystyrene-based MSP composite membrane has exerted noteworthy inhibitory activity against the growth of the tested bacterial strains and data reveal that polystyrene-based MSP composite membrane have good influence on the antibacterial profile of S. aureus and B. subtilis. The polystyrene-based MSP composite membrane show excellent inhibitory results against E. coli and P. aeruginosa as reported in Table 2.

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Fig. 3. IR spectra of polystyrene-based MSP composite membrane.

Table 2 Antibacterial activity of polystyrene-based MSP composite membrane.

Table 4  × 10− 3 eq/l) for various polystyreneDerived values of membrane charge density ( D based MSP composite membrane electrolyte system using TMS equation.

Diameter of zone of inhibition in mm at 100 μg/ml and DMSO as control Bacteria Staphylococcus aureus Bacillus subtilis Escherichia coli Pseudomonas aeruginosa

MSP composite membrane 30.4 (± 1.37) 29.5 (± 1.10) 28.2 (± 0.62) 26.6 (± 1.26)

Tetracycline 20 (± 0.23) 21 (± 0.55) 25 (± 0.88) 24 (± 0.63)

The values of observed membrane potentials, measured across polystyrene-based MSP composite membrane in contact with various uniunivalent electrolyte (KCl, NaCl, LiCl) solutions at different concentrations and pressures at 25± 1 °C are given in Table 3. These values were found to be concentration dependent. At low concentration, the potential was found to be high, whereas on increasing concentration of electrolytes the potential decreased. The membrane potential offered by various 1:1 electrolytes followed the order LiCl> NaCl > KCl. The values for the membrane potential are of the order of positive mV and increase with a decrease in external electrolytes’ concentration. This shows that the membrane is negatively charged (cation selective). The membrane potential was also seen to be largely dependent on the applied pressure during the membrane formation. Application of high pressure on polystyrene-based MSP composite membranes causes reduction in thickness, contraction in pore volume and consequently a progressively higher fixed-charge density which in turn lead to a higher membrane potential as shown in Table 4. Inorganic precipitate membranes were found to have the ability to generate potentials when interpose between electrolyte solutions of different concentrations due to the presence of net charge on the membranes [43,44]. These charges play crucial role in the transport of electrolytes [45]. The membrane potential data obtained with polystyrene-based MSP composite membrane using various 1:1 electrolytes are plotted as a function of −logC2 (Fig. 4).

Applied pressure (MPa)

 × 10− 3 KCl D

 × 10− 3 NaCl D

 × 10− 3 LiCl D

100 120 140 160

9.83 10.20 10.80 11.10

7.70 6.90 7.56 8.22

5.78 5.29 6.08 7.30

The fixed-charge concept of TMS model [15–18] for charged membrane is an appropriate starting point for the investigations of the actual mechanisms of ionic or molecular processes which occur in membrane phase. The membrane potential according to TMS applicable to an idealized system is represented in Eq. (1). The charge densities of membranes were estimated from the membrane potential measurement and can also be estimated from the transport number. For evaluating this parameter for the simple case of a 1:1 electrolyte. The theoretical and observed potentials were plotted as a function of −logC2 as shown by solid and broken lines, respectively in Fig. 5. Thus, the coinciding curve for various electrolytes system gave  within the membrane phase as the value of the charge density D  of polystyrene-based shown in Table 4. The surface charge density D MSP composite membranes is found to depend on the applied pressure to which the membrane was subjected to its initial stage of preparation. The thickness of the polystyrene-based MSP composite membranes was diminished continuously from 0.090 cm to 0.075 cm with progressive increase in the applied pressure. The increase in  the values ofDwith higher applied pressure is due to the successive increase of charge per unit volume as well as the decrease in pore volume of calcium phosphate membrane and therefore, the degree of selectivity for ions is enhanced with the modification in surface  of microstructure of membrane. Surface charge density D

Table 3 Observed membrane potential in mV across the polystyrene-based MSP composite phosphate membrane in contact with various 1:1 electrolytes at different concentrations and pressures at 25 ± 1 °C. Applied pressure C2 (mol/l)

100 (MPa) KCl

Membrane potential (mV) 1 2.5 0.1 7.4 0.01 14.7 0.001 28.5 0.0001 42.3

120 (MPa)

140 (MPa)

160 (MPa)

NaCl

LiCl

KCl

NaCl

LiCl

KCl

NaCl

LiCl

KCl

NaCl

LiCl

4.2 10.6 17.5 30.4 44.8

5.8 12.6 20.3 34.7 47.5

3.2 8.0 15.4 29.1 43.5

4.8 11.3 18.6 31.2 45.7

6.2 13.4 21.6 35.5 48.0

4.1 8.8 16.2 30.4 44.6

5.3 12.6 19.7 32.0 46.5

7.5 14.2 22.4 36.7 49.3

5.6 9.3 17.0 31.5 45.2

6.0 13.5 20.4 33.6 47.8

8.0 15.3 23.7 37.5 50.4

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Charge Density (eq/l) x 10-3

11 KCl NaCl LiCl

10 9 8 7 6 5 100

110

120

130

140

150

160

Applied Pressure (MPa) Fig. 4. Plots of observed membrane potentials against logarithm of concentration for polystyrene-based MSP composite membrane using various 1:1 electrolytes at 100 MPa pressure.

membrane for 1:1 electrolytes (KCl, NaCl and LiCl) were plotted against pressures (Fig. 6) and the order of charge density for electrolytes used were found to be KCl > NaCl > LiCl. The TMS Eq. (1) can also be expressed by the sum of Donnan potential Δ Don, between the membrane surfaces and the external  diff within the membrane solutions, and the diffusion potential ΔΨ [46,47].   ΔΨ m;e ¼ ΔΨDon þ ΔΨ diff

ð2Þ

Where Δψ

Don

¼−

γ C C RT ln 2 2  1þ Vk F γ1 C 1 C 2þ

! ð3Þ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi    2   V xD γ C 2 V x D þ − q 2V k 2V k

ð4Þ

q¼

rffiffiffiffiffiffiffi γ K

Charge Density

D = 1.0

50

ð5Þ

where K± is the distribution coefficient expressed as; C i ; Ci

 C i ¼ C i −D

Where C i the i th ion concentration in the membrane is phase and Ci is the i th ion concentration of the external solution. The transport of electrolyte solutions in pressure-driven membrane has shown that the transport properties of membrane are also controlled by ion distribution coefficients. It appeared that utilizing the Eq. (6) for evaluating the distribution coefficients were found to be low at lower concentration and as the concentration of electrolytes increases the value of distribution coefficients, sharply increases and thereafter, a stable trend was observed and is presented in Table 5.

D = 0.1

C2 (mol/l)

D = 0.01

40

D = 0.003 D = 0.0025

30

D = 0.0020

100 MPa 120 MPa 140 MPa 160 MPa

20

10

D = 0.0012 D=

0.0010

0 0

1

2

3

4

ð6Þ

Table 5  andK± ,q, C þ evaluated using Eqs. (9), (6), (5) and (4) respectiveThe values of t+, Ū, ω ly, from observed membrane potentials for various electrolytes at different concentrations for polystyrene-based MSP composite membrane prepared at 100 MPa pressure.

60

Membrane Potential(mV)

where Vk and Vx refer to the valency of cation and fixed-charge group on the membrane matrix, q is the charge effectiveness of the membrane and is defined by the equation

K ¼

The R, T and F have their usual significance; γ1 ± and γ2 ± are the mean ionic activity coefficients; C 1þ and C 2þ are the cation concentration in the membrane phase first and second, respectively. The cation concentration is given by the equation C þ ¼

 of polystyrene-based MSP composite memFig. 6. The plot of surface charge density D brane for 1:1 electrolytes (KCl, NaCl and LiCl) versus pressures.

5

-logC2(mol/l) Fig. 5. Plots of membranes potential (mV) versus − log C2(mol/l) at different concentrations of KCl electrolyte solution for polystyrene-based MSP composite membranes prepared at different pressures of 100–160 MPa.

t+

KCl (electrolyte) 1.000 0.53 0.1000 0.57 0.0100 0.63 0.0010 0.75 0.0001 0.86 NaCl 1.000 0.54 0.1000 0.59 0.0100 0.65 0.0010 0.76 0.0001 0.88 LiCl 1.000 0.55 0.1000 0.61 0.0100 0.68 0.0010 0.80 0.0001 0.91

Ū

 ω

K±

q

C þ

0.06 0.14 0.26 0.50 0.72

1.12 1.32 1.70 3.00 6.14

0.9902 0.9020 0.0200 − 8.8000 − 97.000

1.0049 1.0529 7.0710 0.3370 0.1015

0.9895 0.0899 0.00364 0.00002 0.000007

0.08 0.18 0.30 0.52 0.76

1.17 1.43 1.85 3.16 7.33

0.9923 0.9230 0.2300 − 6.7000 − 76.000

1.0038 1.0408 2.0851 0.3863 0.1144

0.9916 0.0918 0.0042 0.00003 0.000009

0.10 0.22 0.36 0.60 0.82

1.22 1.56 2.12 4.00 10.11

0.9943 0.9430 0.4300 − 4.7000 − 56.000

1.0028 1.0297 1.5249 0.4612 0.1336

0.9936 0.0936 0.0051 0.00004 0.00001

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The high mobility is attributed to higher transport number of comparatively free cations of electrolyte which was also found to have the similar trend as the mobility in least concentrated solution. The transport number of cation of the various electrolytes (KCl, NaCl, LiCl) increases with decreasing the concentration of electrolytes and follows the increasing order KCl b NaCl b LiCl shown in Fig. 8. Donnan and diffusion potential at various electrolyte concentrations  V x; V k can be calculated from the parameters γ 1  ; γ 2  ; C 1þ ; C 2þ ; ω;  by using and the experimentally derived values of charge density D Eqs. (3) and (7). The values of the parametersK±, q and C þ derived for the system are also shown in Table 5. The values of γ± were the usual charted values for electrolyte (KCl, NaCl and LiCl).

Fig. 7. The plot of mobility ratio of polystyrene-based MSP composite membrane for 1:1 electrolytes (KCl, NaCl and LiCl) versus concentrations.

Fig. 8. The plot of transport number of cation of polystyrene-based MSP composite membrane for 1:1 electrolytes (KCl, NaCl and LiCl) versus concentrations.

 diff , is expressed in the form The diffusion potentialΔΨ   þ 1ÞC 2þ þ ðV x =V k ÞD  ðω R T ω−1   ln ΔΨ diff ¼ −   þ1  þ 1ÞC 1þ þ ðV x =V k ÞD Vk F ω ðω

! ð7Þ

 ¼u  v is the mobility ratio of the cation to the anion in the where ω  , was thus membrane phase. The total membrane potential ΔΨ m;e obtained by simple addition of Eqs. (3) and (7). !  γ C C RT RT ω−1 ln 2 2  1þ −  þ1 Vk F Vk F ω γ 1 C 1 C 2þ !   þ 1ÞC 2þ þ ðV x =V k ÞD ðω  ln   þ 1ÞC 1þ þ ðV x =V k ÞD ðω

 Δψ m;e ¼ −

ð8Þ

In order to test the applicability of these theoretical equations for the system under investigation, the Donnan potential and diffusion potential were separately calculated from the membrane parameters obtained from membrane potential measurements using a typical membrane prepared at a pressure of 100 MPa. The transport properties of the membrane in various electrolyte solutions are important parameters to further investigate the membrane phenomena as shown in Eq. (9)  m¼ Δψ

 C RT t þ −t − ln 2 F C1

tþ  v ¼u t−

ð9Þ

Eq. (9) was first used to get the values of transport numbers t+ and t− from experimental membrane potential data and consequent ¼u  of  v and Ū were calculated. The mobility ω ly, the mobility ratio ω the electrolyte in the membrane phase were found to be high and the order is LiCl > NaCl > KCl and shown in Fig. 7.

5. Conclusion The polystyrene-based MSP composite membrane was prepared after preliminary investigations, the binder polystyrene was selected because its cross-linked rigid framework provides an adequate adhesion to the MSP composite, which accounts for the mechanical stability to the membrane. Particles are irregularly condensed and adopt a heterogeneous structure composed of masses of various sizes and membrane is crystalline in nature. The membrane potential of polystyrene-based MSP composite membranes for different uni-univalent electrolytes was found to follow an increasing order KClb NaClb LiCl. The result indicates that the behaviour of investigated membrane is cation selective. Fixed-charge density is the central parameter governing transport phenomena in membrane and depends upon the feed composition. The order of surface charge density for uni-univalent electrolytes solution was found to be LiClb NaCl b KCl. Acknowledgement The authors gratefully acknowledge the Chairman, Department of Chemistry, Aligarh Muslim University, Aligarh (India) for providing the necessary research facilities. We are also thankful to UGC for providing financial assistance and the Indian Institute of Technology (IIT), Delhi for providing scanning electron microscopy facility. References [1] M. Cesar de Coutinho, Ming Chih Chiu, Rodrigo Correa Basso, Ana Paula Badan Ribeiro, Lireny Aparecida Guaraldo Gonçalves, Luiz Antonio Viotto, Food Res. Int. 42 (2009) 536–550. [2] A. Kitahara, A. Watanabe, Electrical phenomena at interface: fundamentals, and application, Marcel Dekker, New York, 1984. [3] A. Szymczyk, P. Fievet, M. Mullet, J. Membr. Sci. 143 (1–2) (1998) 189. [4] H.P. Hsieh, Inorganic membrane for separation and reaction, Elsevier, Amsterdam, 1996. [5] D. Elzo, I. Huisman, E. Middelink, V. Gekas, Colloids Surf. A Physicochem. Eng. Aspects 138 (1998) 145. [6] L. Palacio, P. Pradanos, J.I. Calvo, G. Kherif, A. Larbot, A. Hernandez, Colloids Surf. A Physicochem. Eng. Aspects 138 (1998) 291. [7] A.A. Khan, M.M. Alam, React. Funct. Polym. 55 (2003) 277. [8] K.G. Varshney, N. Tayal, A.A. Khan, R. Niwas, Colloid Surf. A Physicochem. Eng. Aspects 181 (2001) 123. [9] A.P. Gupta, H. Agarwal, S. Ikram, J. Indian Chem. Soc. 80 (2003) 57. [10] A.A. Khan, Inamuddin, M.M.A. Khan, React. Funct. Polym. 63 (2005) 119–133. [11] M.M.A. Khan et. al., Journal of Industrial and Engineering Chemistry, doi:10.1016/ j.jiec.2012.01.042. [12] M.R. Ladisch, K.L. Kohlmann, Biotechnol. Progr. 8 (1992) 469. [13] H.W. Blanch, D.S. Clark, Biochemical Engineering, Marcel Dekker, NewYork, 1997. [14] F.V. Kosikowski, Membrane separations in food processing, in: W.C. McGregor (Ed.), Membrane Separations in Biotechnology, Marcel Dekker, Inc., New York, (1991). [15] T. Teorell, Proc. Soc. Exp. Biol. 33 (1935) 282. [16] T. Teorell, Proc. Natl. Acad. Sci. U. S. A. 21 (1935) 152. [17] K.H. Meyer, J.F. Sievers, Helv. Chim. Acta 19 (1936) 649, 665, and 987. [18] T.J. Chou, A. Tanioka, J. Phys. Chem. B 102 (1998) 7198. [19] K. Singh, A.K. Tiwari, J. Membr. Sci. 34 (1987) 155. [20] K. Singh, V.K. Shahi, J. Membr. Sci. 49 (1990) 223. [21] F. Jabeen, Rafiuddin, Journal of Porous Mater 16 (2009) 257–265. [22] F. Jabeen, Rafiuddin, J. Sol-Gel Sci. Technol. 44 (2007) 195. [23] M.N. Beg, F.A. Siddiqi, S.P. Singh, P. Prakash, V. Gupta, Electrochem. Acta 24 (1979) 85.

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