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DEPARTMKNT OF CHEMISTRY ALIGARH MUSLIM UNIVERSITY,

ALIGARH-202002

Dated: .s^bfc^. ^1

CERTIFICATE

This is to certify that the present work entitled "TRANSPORT PHENOMENA OF

INORGANIC PRECIPITATE MEMBRANE" has been done by Mr. Mohd Arsalan

under my supervision in the Department of Chemistry, Aligarh Muslim University

Aligarh, for the partial fulfillment of degree of Master in philosophy in chemistry.

Dr. Rafluddin (Associate Professor)

Department of Chemistry Aligarh Muslim University, Aligarh

A CKNO WLEDGEMENT

I wish deepest sense of gratitude to respected Dr. Rajluddin who guided to carry out

this research work

I also wish deep sense of gratitude to respected chairman Prof. Zafar A.

Siddiqui Chairman Department of Chemistry for providing facility to carry out thesis

work.

I am also thankful to my senior Dr. Mohammad Mujahid Ali Khan who

delicately helped me to carry out successful thesis work for the M.Phil. I am also

thankful to my Beloved brother Mohd Zeeshan who constantly encouraged me to

carrying out my work. Thanks are exceed to Ahmad Asim, Nigar Fatima, Ahmad

Faraz, Ahmad Sheeraz and Ahmad Maaz

I am also thankful to Fahad Alam, (Research Scholar IIT Kanpur)

Mohammad Ehtisham Khan, Akbar Mohammad, Mohd, Luquman and Y. Naseem,

Thanks are extend to my Lab mate Dr. Zoya Zaheer, Dr. Saima Sultana, Urfi

Ishrat, Rami Khan and Zafar Iqbal for providing a good environment for carrying

out to finalize the report.

Above all I am thankful to ALLAH, the ultimate source of knowledge; with

blessings the author got the success in completing the work.

(MOHi

CONTENTS

Chapter -I

• Introduction

> Definition of Membrane

> Types of Membrane

> Characterization of Membrane

Chapter - II

> Nomenclature

> Theory

> Experimental

> Results and Discussion

> Conclusion

Chapter -III

> Introduction

> Experimental

> Results and Discussion

> Future perception

> Conclusion

References

CHAPTER -1

INTRODUCTION

Definition of membrane

Membrane is a thin sheet of natural or synthetic material that is permeable to substances in

solution, or it is a physical barrier that allows cer1;ain compounds to pass through it, which

depends upon the size of the molecule, electronic charge etc.

The composite membranes are nowadays used in many separation processes and a great

number of ion-exchange membranes have been extensively studied due to their multipurpose

applications in food industry, drug and chemical industries and waste water management also.

Furthermore Ion-exchange membranes have alio been used in electrodialysis for brackish water

desalination from several decades [1], In the electrodialysis the seawater produces edible salts

and the separation of ionic materials from non-ionic materials takes place and by diffiision

dialysis the recovery of acid and alkali can be done from waste acid and alkali solution [2].

Cation-exchange membranes (CEMs) have been used for various industrial purposes such as

separation of environmental polluting metal ions from hard water, chloro-alkali electrolysis, fuel

cells, and desalination of electrolyte solutions also [3]. Those membranes which are having

reverse osmosis (RO) property are utilized in a wide range of applications such as drinking water

treatment, waste water recovery and making of ultra pure water. Composite and asymmetric

membranes are used in desalting processes under pressure gradient such as nanofiltration and

reverse osmosis due to their high salt rejection, volume flux and mechanical stability. These

membranes can be considered as multilayer systems coupled in series by infinitely thin layers of

aqueous solutions in local equilibrium with both adjacent layers [4]. From the last 2-3 decades

significant improvements in the performance of polymeric membranes for gas separation have

been made and perceptive of the relationships among the structure, selectivity and permeability

of polymeric membranes has been greatly advanced, therefore polymeric membrane materials

such as PVC based .Polystyrene based polyamides (PI) and cross-linked polyethylene glycol

(PEG) have been constantly developed [5]. Organic-inorganic composite membranes/materials

have been widely studied for a long time, based on the size these may be micro composites with

the size range of 200-400 mesh or nanocomposites which employ nanosized inorganic phases.

Organic-inorganic nanocomposite membranes are generally organic polymer composites with

inorganic nanoscale building blocks. They combme the advantages of the inorganic material

(e.g., rigidity, thermal stability) and the organic polymer (e.g., flexibility, dielectric, ductility, and

processability). Moreover, they usually also contain special properties of nanofillers, leading to

materials with improved properties [6]. Organic-iiiorganic nanocomposite membranes have

attracted wide interest because the addition of inorganic particles to polymers can enhance

conductivity, mechanical toughness, optical activity and catalytic activity. Therefore such type of

composite membranes may also be usefiil for molecular separations, bimolecular purification,

seawater desalination, environmental remediation, petroleum chemicals and fuel production.

These separations are classically proficient with well-known technologies such as distillation,

absorption and adsorption which are often extremely energy and capital-intensive or recently by

selective permeation through membranes. Membranes are attractive because they are a low-cost

and energy efficient green technology. The extensive use of composite membrane is in gas

separations which have been limited by the difficulty of preparing membranes with the desirable

combination of high selectivity which gives high purity product yields and low operating costs,

and high permeability which reduces membrane area and capital cost [7]. Presently, polymers are

still the main materials in membrane technology with the advantages of good membrane-forming

ability, flexibility and low cost. However, limited chemical, mechanical and thermal resistance

restricts the application of polymer materials [8]. The inorganic materials in the membranes have

higher thermal and chemical resistance as well as a longer lifetime, but they are still expensive

and brittle with poor membrane-forming ability. Composite materials could combine the basic

properties of organic and inorganic materials and offer specific advantages for the preparation of

artificial membranes with excellent separation performances, good thermal 'and chemical

resistance, showing adaptability to insensitive environments and with comparable membrane

forming ability. Thus, organic-inorganic composite materials as new membrane materials have

attracted more and more attention [9].

Membrane-based chemical separations constitute an emerging research area and industrial

teclinology. The objective is to develop membranes that selectively transport a particular target

molecule which are having definite sizes and discard (or transport at much lower rates) other

molecules that might be present in the feed solution or gas. By the help of Potential applications

industrial gas, pharmaceutical and petrochemical separations also take place. Membrane- based

technologies are potentially less energy demanding than other types of challenging separation

technologies and as such, it can be viewed as an example of "green chemistry." However,

materials with higher chemical selectivity for the desired target molecule can be incorporated

into geometries that require high flux of this particular molecule [10].

As previously studied, the fuel cells should be used from solid polymeric electrolytes which have

the ability of proton transferring. These ionic polymer electrolytes include the gel-like polymeric

structures that are harshly swollen and carry fixed positive and negative charges. These polymers

don't have the ability of proton transferring in dry conditions and only have this property in the

moist state [11]. The main applications of poly (vinyl alcohol) (PVA), polystyrene,

polyvinylchloride (PVC) based inorganic composite membranes are in pervaporation but the

swelling of membranes in aqueous media is a serious problem. The polymeric inorganic

composites attracted our interest because such hybrids may show excellent dimensional, thermal,

and mechanical stability, convenient physical properties, as well as good hydrophilic and

electrochemical properties which are very essential for an ion exchange membrane [12].

There is also widespread interest in morphological features of membranes synthesized from

composites of hard inorganic particles and soft polymeric materials. The polymers enhance the

membrane separation processes because they contribute to low cost, energy efficient green

technology where inorganic materials enhance the selectivity and permeability in membranes.

[13]

From the latest years, many polymers and their composites with their functionality for example

polyarylene ether sulfone and sulfonated poly (ether, ether ketone) (SPEEK) had been

extensively studied and showed superb stabilities and electrochemical properties [14]. Where as

the precipitated membranes are of promising interest in analytical chemistry for two reasons:

they may be useful as direct measurement of ion concentration or activity and for titrations

involving precipitation reactions, and they may provide information about the sufface electrical

characteristics of the precipitates, thus it gives fundamental knowledge concerning precipitation

processes. Several other types of membranes had been studied and used as indicating electrodes.

In subsequent work, it was shown that selection of suitable membrane characteristics for specific

processes will allow higher efficiency and improvements in the process.

Currently the membrane processes are being studied for several applications of practical

importance. In such processes, electrolyte ions interact with the membrane, with water and with

each other in a complex fashion. Ion exchange charged membranes, which are now widely

utilized in industries, have been given considerable attention due to their extraordinary properties

and practical demands and consequently a large number of researchers have concentrated on

these investigations for many years [15,16].

The membrane separation techniques such as nanofiltration, ultrafiltration, microfiltration, gas

separation, reverse osmosis, pervaporation, and liquid membranes, etc. have been studied and

industrially applicable. The ion-exchange membrane is one of the most sophisticated separation

membranes among these separation membranes. The important property of the cation-exchange

membrane and anion-exchange membrane is to selectively permeate cations or anions through

the membrane [17].

'polymeric-inorganic' fusion materials prepared by sol-gel process have attracted a great deal of

attentions in material science. The production of hybrid ion-exchangers with controlled

ftjnctionality and hydrophobicity could open new research for organometallic chemistry, organic

host-guest chemistry, catalysis, analytical cheraistPj/, antibiotic purification and separation of

radioactive isotopes, and find large applications in water treatment and pollution control [18].

Thus, organic-inorganic or polymeric-inorganic hybrid materials are expected to provide many

possibilities as new composite materials. Therefore such types of hybrid materials generally

show intermediate properties between those of plastics and glasses (ceramics). Consequently,

hybrid can be used to modify organic polymer materials or to modify inorganic materials. In

addition of these distinctiveness the hybrid materials can be considered as a new composite

materials that show very different properties from tJieir original components (organic polymer

and inorganic materials), especially in the case of molecular level hybrids. Therefore, the

synthesis of po]3/meric/inorganic composites Has expected a great deal of attention because it

provided new materials with special mechanical, electrochemical, chemical, and optical plus

magnetic properties [19, 20].

Nowadays membrane systems are used in many separation processes. Composite and

asymmetric membranes are used in desalting processes under pressure gradient such as

nanofiltration and reverse osmosis due to their high salt rejection, volume flux, and mechanical

stability. These membranes can be considered as multilayer systems coupled in series by

infinitely thin layers of aqueous solutions in local equilibrium with both adjacent layers. [21]

Application of extraction techniques for removal and recovery of heavy metals is'of immensely

growing importance from the viewpoint of environmental protection problems [22]. During the

second half of the last century ion-exchange membranes and their practical utilization in electro-

membrane processes have gained significant teclmical and commercial importance in water

deionization and purification as well as in electrochemical synthesis and in energy conversion

and energy storage, while other processes such as capacitive electro-deionization and electro-

dialysis with bipolar membranes or the use of ion-exchange membranes in fuel cells and energy

conversion are more recent developments which show a large number of interesting applications

[23]. Currently the inorganic-organic membranes are used in many separation processes. Ion-

exchange composite membranes find application in various processes for instance diffusion

dialysis, membrane electrolysis, electro-deionization, fuel cells, storage batteries,

electrochemical synthesis and many more. Therefore, they are valuable in pollution control,

energy saving, resource recovery, power generation and in the waste water treatment also.

A lot of ion-exchange composite membranes have been extensively studied due to their

multipurpose applications in drug synthesis, food/beverages industries, waste water treatment,

chemical industries, and in separation processes. Ion-exchange membranes have also been used

in electro dialysis of salty water desalination from several decades. For providing the intensive

applications, it is essential that ion-exchange membranes possess high conductivity and

selectivity, good mechanical and chemical stability as well as resistance to the little jerk. To

accomplish these properties, the selection of binder, adjustment of membrane using various

flmctional groups and surface layer of membranes were utilized as intentional tools. Now days,

most of the ion-exchange membrane have been focusing on polystyrene, polyvinylchloride,

sulfonated polystyrene and calcium fluoride nanoparticle based material systems [24,25]. A

composite charged membrane transforms the ions due to the differences in their concentrations,

pore sizes of the membrane and electrolyte interaction with the membrane. In addition to this, a

charged membrane can separate electrolyte ions according to the charge of the membrane.

Recently it has become necessary to analyze the ion transport phenomena specifically in

aqueous, organic, electrolyte solution systems across a charged membrane from the point of view

of industrial and medical applications. The measureraent of membrane potential is an important

phenomenon for obtaining transport property across a charged membrane. Hypothetically the

membrane potential in a charged membrane for aqueous electrolyte solution system developed

by Teorell, Meyer and Sievers (TMS) can be treated by Donnan equilibrium theory and the

Nemst Planck equation [26].

The Doiman potential depends on the membrane charge density which plays a sighificant role in

the selectivity of the charged membrane with respect to the valence of ions [27]. The diffusion

potential is totally depends on the mobility of ions in the membrane and affects the transport

property of ions in the composite membrane. The measurement of the diffusion potential and

Donnan potentials will give the important parameters that are the ionic mobility and effective

charge density [28].

The surface charge density D is the central parameter that controls the membrane phenomenon

and was calculated using the membrane potentials values for different electrolytes by using

(TMS) method [29]. Some other parameters including distribution coefficient, transport

numbers, mobility ratio and charge effectiveness etc. were also calculated by the above method.

Membrane potential studies are commonly used in the electrochemical characterization of

composite membranes [30].

For instance the study calcium fluoride nanoparticle based membranes exhibit a majority of

applications, which make them attractive to the above mentioned applications. The drawbacks

of previous membranes are: that is difficult to manage the structure of membrane due to the

copolymerization nature and cross-linking processes simultaneously; now day's such type of ion

exchange membranes have been developed to overcome these problems [31,32]. The ion

exchange capacity of the above binder is depended to the accurate ratio with the compound

therefore CF nanoparticle is one of the most well-liked water-soluble base polymers that has

been studied intensively because of its good binding nature and physical properties, high

hydrophilicity, biocompatibility, and good chemical resistance. We mentioned a new type of ion

exchange composite membranes which is based from water soluble CF nanoparticle binder and

have been examined the influence of the preparation conditions on the membrane structural and

morphological properties including water content, swelling, porosity, thickness, chemical

resistance/stability and transport number etc.[33,34]

10

Types of membrane

Filtration by membrane can be a very efficient and economical way of separating components

that are suspended or dissolved in a liquid. Types of Membrane filtration, which is based on

membrane pore sizes, used materials, size of materials and their nature are described below;

They are microfiltration membranes, Nanofiltration membranes. Ion exchange (cation, anion

exchange) membranes, Homogeneous, Heterogeneous membranes. Reverse osmosis membranes

and Organic inorganic composite membranes.

Ion exchange membrane

It is a type of membrane which can exchange the ions between two electrolytes or between an

electrolyte solution and a complex. In most cases the term is used to denote the processes of

purification, separation and decontamination of aqueous and other ion-containing solutions with

solid polymeric or mineralic ion exchangers. Typical ion exchangers are ion exchange resins

(functionalized porous or gel polymer), zeolites, montmorillonite, clay and soil humus. Ion

exchangers are either cation exchangers that exchange positively charged ions (cations) or anion

exchangers are those which can exchange negatively charged ions (anions). There are also

amphoteric exchangers that are able to exchange both cations and anions simultaneously. Ion

exchangers can be selective or have binding preferences for certain ions or classes of ions which

depends on their chemical structure. This in turn depends on the size of the ions, their charge, or

their structure.

Homogeneous and Heterogenous membranes

Ion exchange membranes are classified into tv o broad categories: homogeneous and

heterogeneous. In homogeneous membranes, the charged groups are uniformly distributed

throughout the membrane matrix were as in heterogeneous membranes, the ion exchange groups

11

are contained in small domains distributed throughout an inert support matrix, which provide the

mechanical support to the membrane.

Homogeneous Membranes

These may be cation selective and anion selective, the Cation-selective membranes are usually

made up of cross-linked polystyrene (with divinyl benzene) that has been sulfonated (with 98%

sulphuric acid) to produce sulfonate ion (—SO''") attached to the polymer. The anion-selective

membranes are usually made up of cross-linked polystyrene containing quaternary ammonium

groups ( NR^^). This can be achieved by post-treating the polystyrene with

monochloromethyl ether and aluminium chloride to introduce chloromethyl group into the

benzene ring followed by formation of quaternary amines with trimethylamine.

Homogeneous membrane electrodes are ion-selective electrodes in which the membrane is a

crystalline material prepared from either a single compound or a homogeneous mixture of

compounds (i.e. Ag2S, Agl / AgaS).

Heterogeneous Membrane

The simplest form has very fmely powdered cation or anion exchange particles uniformly

dispersed in polypropylene. Heterogeneous niembrane electrodes are formed when an active

substance or mixture of active substances is mixed with an inert matrix such as silicone rubber

or PVC or placed on hydrophobized graphite or conducting epoxy to form the heterogeneous

sensing membrane

Reverse Osmosis

Reverse osmosis (RO) membrane separates salts and small molecules from low molecular weight

solutes (typically less than 100 daltons) at relatively high pressures. RO membranes are normally

rated by their retention of sodium chloride while ultrafiltration membranes are characterized

12

according to the molecular weight of retained solutes. Millipore water purification systems

employ both reverse osmosis membranes as well as ultrafiltration membranes. Reverse osmosis

systems are primarily used to purify tap water to purities that exceed distilled water quality.

Organic inorganic composite membranes

Organic inorganic composite membranes are now one of the most important classes of

engineered materials as they offer several outstanding properties as compared to conventional

materials. The inorganic-organic composite membranes are formed by mixing organic polymers

and inorganic particles together in definite ratio. Therefore, such types of membranes show the

ideal properties of both the materials. The polymers such as PVC, Polystyrene, PVA and many

more provide the stability to the membrane and the inorganic materials provide the ion exchange

property. Therefore such composite materials have found increasingly wider applications in the

general areas of chemistry, physics, nanotechnology, material science and engineering.

Characterization of membrane

SEM characterization

In basic scanning electron microscopy (SEM), a beam of highly energetic (0.1-50 keV)

electrons is focused on a sample surface. It provides the morphology and topography of the

sample, surface Defects, Inclusions etc.

XRD Analysis

This provides Phase Identification (crystalline or amorphous). Crystal Structure Phases in

thin films Stress and Texture analysis

FTIR-Analysis

Fourier Transform Infrared Spectroscopy (FTIR) identifies chemical bonds in a molecule

by producing an infrared absorption spectrum. The FTIR generates an infrared spectral scan of

13

samples that absorb infrared light. Metals do not absorb infrared light, but polymers that contain

metals can be scanned with FTIR.

Thennogravimetric analysis/Differential thermal analysis (TGA/DTA)

It measures both heat flow and weight changes (TGA) in a material as a ftinction of

temperature or time in a controlled atmosphere. Simultaneous measurement of these two material

properties not only improves productivity but also simpHfies interpretation of the results. The

complementary information obtained allows differentiation between endothermic and exothermic

events with no associated weight loss (e.g. melting and crystallization) and those that involve a

weight loss (e.g. degradation)

CHAPTER - II

15

PREPARATION, TRANSPORT PHENOMENA AND DIELECTRIC PROPERTIES OF

POLYVINYL CHLORIDE BASED MANGANOUS PHOSPHATE COMPOSITE

MEMBRANE

Nomenclature

AR Analytical reagent

C1,C2 Concentrations of electrolyte solution on either side of the membrane (mol/L)

C| Cation concentration in membrane phase 1 (mol/L)

C, Cation concentration in membrane phase 2 (moI/L)

C, i"" ion concentration of external solution (mol/L)

C i"" ion concentration in membrane phase (mol/L)

O Charge density in membrane (eq/L) F Faraday constant (C/mol) 100 MPa 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

w Mobility of cations in the membrane phase (m2/v/s)

V Mobility of anions in the membrane phase (m2/v/s)

Vk Valency of cation

Vx Valency of fixed-charge group

U W = {u - V)/i^u + V)

16

MP Manganous phosphate PVC Polyvinyl chloride

Greek symbols

r± CD

At//,,,

A^m

" '•Don

A^

Mean ionic activity coefficients Mobility ratio

Observed membrane potential (mV)

Theoretical membrane potential (mV) Donnan potential (mV)

Diffusion potential (mV)

Ion-exchange membranes exhibit a lot of applications, in the field of textile, beverage

industries, pharmacy, medicine, waste water treatment and chemical industry, etc [35]. These

applications are credited to their high thermal resistance, chemical resistance and mechanical

strength. The inorganic-organic composite membrane formed by mixing organic polymers and

inorganic particles together in specific ratio. Therefore, membranes exhibit the ideal properties of

both the constituents [36], The polymer (PVC) provides the stability of the mernbrane and the

inorganic salt (MP) provides the ion exchange property. An organic-inorganic composite

membrane has a stable potential when there is no net ion current flowing across the membrane.

A wide range of membrane separation techniques such as filtration that are

microfiltration, na no filtration, ultra filtration, gas separation and reverse osmosis, have been

studied and industrially used [37]. The ion-exchange membrane is one of the most classical

separation membranes among these separation membranes. The important property of the cation-

exchange membrane and anion-exchange membrane is to selectively permeate cations or anions

through the membrane. Ion-exchange membranes have been used in those solutions which

contain numerous components, such as electro dialytic demineralization of saline water.

17

desalination of cheese whey solutions, treatment of industrial effluents which contain poisonous

metal ions [38].

The need for drinking water with good microbiological and chemical quality is clearly

increasing around the world. Simultaneously, membrane processes have met a large expansion in

desalting of seawater for the two last decades [39].

The measurement of the membrane potential is an important method for characterizing

the ion-transport phenomena across a charged membrane [40]. As the membrane comes mto

contact with an aqueous electrolyte solution, it acquires an electric charge through several

possible mechanisms. These are dissociation of functional groups, adsorption of polyelectrolyte,

adsorption of ions from solution, charged macro molecules and also ionic surfactants. The

charging mechanism can take place on the exterior membrane surface as on interior pore surface

of the membrane [41].

Theory

Fixed charge density theory of Teorell-Meyer-Sievers

In the Teorell-Meyer-Sievers (TMS) theory there is an equilibrium development at each

interface of solution membrane which has a proper similarity with the Donnan equilibrium. The

assumptions made are (a) the mobilities of ions and concentrations of fixed charge are constant

throughout the membrane phase and are independent of the salt concentration and (b) the water

transference may be neglected. The implications of these assumptions have been discussed [42].

Further assumptions that the activity coefficient of the salt is the similar in the solution phase and

membrane phase and 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.

A^„,=59.2

(1)

l o g - ^ - ' ' 7 = — - + [/log V ' _ zrzr C, ^4C- +D-+D V4C,' +D-+DU

U = {u-v)/{U+v)

Where u~ and v~ are the ionic mobility's of cation and anion (m2/v/s), respectively, in the

membrane phase. CI and C2 are the concentrations of the membrane and D is the charge on the

membrane expressed in equivalent per litre. The graphical method of TMS determines the fixed

charge D in eq/L and the cation-to-anion mobility ratio in the membrane phase. Charged

membrane was placed at the centre of the potentials measuring cell, which had two glass

chambers on either side of the membrane [43, 44].

Experimental

Preparation of composite membrane

Manganous phosphate(MP) precipitate have been prepared by mixing 0.2 M manganous

(II) chloride (Otto Kemi, India with purity of 99.989%) with 0.2 M tri sodium phosphate (E.

Merck, India with purity of 99.90%) solutions. The precipitate has been well washed with

deionized water (distilled water) to eliminate free electrolyte and then the precipitate have been

dried and change into powdered. For the formation of membrane using appropriate ratio of

binder into the sample. The precipitate which is having ion-exchange property was mixed with

the polyvinyl chloride (PVC) (Otto Kemi, India, AR) binder which is having the size of less than

200 meshes and then mixed together with the help of pestle and mortar and then transferred into

19

cast die for applying the pressure. The sample should be dried under proper conditions of

temperature. The resulting membrane then placed into a cell for observing the electrolyte

potential from the potentiometer reading. Our effort has been to get the membrane of adequate

chemical and mechanically stability. The membrane prepared by embedding 25% PVC and 75%

of inorganic phosphate which shows mechanically most stable and gave reproducible results.

Those sample which are containing larger amount (>25%) of PVC did not give reproducible

results, while those containing lesser amount (<25%) of PVC were unstable. The membrane was

subject to microscopic and electrochemical characterization for seeing the morphology of

membrane in which cracks and homogeneity of the surface will be considered. Those membranes

which have smooth surface and generated reproducible potentials were considered by careftilly

controlling the condition of fabrication [45,46].

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

ceil, 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:

20

SCE Solution CI

Donnan 'potential Donnan

Solution C2

potential

SCE

Measurement of membrane conductance

The electrical conductance of such type of membrane was measured by the method used

[47]. The membrane was sealed between two Pyrex glasses half cells. The half cells were first

filled with electrolyte solutions of known concentration to equilibrate the membrane, and then

the latter was replaced by purified mercury without removing the adhering surface liquid.

Platinum electrodes dipping into mercury were used to established electrical contact. The

membrane conductance was monitored on a direct reading conductivity meter (Model No. L303).

The solutions in both the compartments were vigorously stirred by magnetic stirrers at constant

500 rpm to minimize the effect of boundary layers on potential. The whole cells assembly was

kept immersed in a water thermostat maintained at the required temperatures (10-50 °C).

Characterization of membrane

Water content (% Total wet weight)

The prepared membrane was first kept in water for soaking in water to difflisible salt, after that

membrane was blotted quickly with Whatmann filter paper to remove the moisture of the surface

and then immediately weighted. These membranes was further dried a constant weight in a

vacuum over P2O5 for 24 h. The water content (total wet weight) in the membrane was calculated

as:

21

W ...- W % Total {weight ,,„ ) = ^^-^ '—^x 100

Where Ww is the weight of the soaked or wet membrane and Wj the weight of the dry

membrane.

Porosity

Porosity was determined as the volume of water incorporation in the cavities per unit

membrane volume from the water content data:

W, - W , Porosity - — '

Where Wj is the weight of the dry membrane, A is the area of the membrane, L is the thickness

of the membrane and density of water is pw.

Thickness

Membrane thickness was measured by taking the average thickness of the membrane by

using screw gauze.

Swelling

Swelling is measured as the difference between the average thickness of the membrane

equilibrated with 1 M NaCl for 24 h and the dry membrane.

Chemical stability

Chemical stability was evaluated on the basis of ASTM D543-95 method. Membrane was

exposed to several media commonly utilized. Membrane was evaluated after 24, 36, and 48 h.

22

analyzing alteration in colour, texture, brightness, decomposition, splits, holes, bubbles, curving

and stickiness [48,49].

SEM investigation of membrane morphology

Scanning electron microscope image was used to confirm the micro structure of fabricated

porous membrane. The membrane morphology was investigated by Leo 4352 at an accelerating

voltage of 20 kV. Sample was mounted on a copper stub and sputter coated with gold to

minimize the charging.

X-ray diffraction studies of the composite membrane

X-ray diffraction pattern of the PVC based MP composite membrane was recorded by

Miniflex-II X-Ray diffractometer (Rigaku Corporation) with Cu Ka radiation.

FTIR Spectra of membrane

The FTIR spectrum of pure PVC and PVC based MP composite membrane were done by

Interspec 2020 FTIR spectrometer, spectrolab (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.

Thermo gravimetric analysis (TGA) of the composite membrane

The degradation process and thermal stability of the PVC based MP composite membrane

was investigated using thermo gravimetric analysis (TGA) (Shimdzu DTG-60H), under nitrogen

atmosphere using a heating rate of 20 Cmin"'from25 C to 800 C.

23

Dielectric properties of the composite membrane

The dielectric and impedance measurements were made from 75 KHz up to 5MHz using

Agilent (Model; 4284A) precision LCR meter.

Results and discussion

The FTIR spectral has been used as an important tool to provide details of the binding

sites of poly vinyl chloride based (MP) composite membrane. Spectra of the membrane sample

were performed as KBr pellets. The FTIR data Figure 1 showed the presence of various

significant bands of the functional group present in the membrane sample. There is a sharp peak

present at 582.64cm"', which shows the presence of C-H group. In the mangnous phosphate there

IS a broad peak near 1010.80, 1066.30 and 117.84cm"' which indicate the presence of the (0-H)

group. The less intense peak found near the 1613.40 showed the presence of (N-H) bond in the

PVC based (MP) membrane. The next N-H peak was observed at the range of the 2319.07-

2358.73 [50].

40

20

0

20

OJ O

S

i ° s Q

t "

__ . — — ' N ' " _ ' .

CM

S)

CO

O

s

u ss

1

. \ \ 1

1 1

\

i n

o C O

• *

1 (N

I' K

!'\ ' \.~

O

S

i n

00

/

\ '

IN CO

(0

C D in

1 7,

S

I

4000 3000 2000

WaveJiuinberhlcm')

1000

Figure 1 (a) FTIR spectra of pure PVC

24

100

50

0

- > . V ^ \

\

i 2 « s

I

--—.

i J ? s

1

- - ' '

i £ S

s

1

/ ^'

« • t ^

^ s .

5 V

? s

1

^ " ' ^ ^ / •

V

J ? S .o

X - —

3 - I

^ %

I

N

\ _

1 t t I

till r1 ' . J " ^

1 ^

? s !

/ r^J

i" K

S

I

\

\ 1

? J J t^ r- s.

t T T T- ra S

s S

W.ivc numbers (cm') 2000 1000

Figure 1 (b) FTIR spectra of PVC based MP composite membrane

The sample was prepared in powdered form and was mounted in the sample holder with

the help of glass slide and used for X- ray study. By pattern was recorded using a Rigaku

Mmiflex X-ray diffractometer with the Cu K a radiation (/, = 1.54060A°) in 2d ranging from

20° to 80° to find out the structure and lattice parameters of the sample. The XRD spectrum of

Mn3 (P04)2 is depicted in Figure 2 .The XRD analysis of the particles was also analyzed.The

lattice constants of Mna (P04)2 was found to be as (a) 5.758 A° and (c) 9.443 A° ).The XRD

pattern of particles showed a most intense peak at 20 = 26.30°, which corresponds to (211) plane

of Mn3 (P04)2 [15]. Crystallite size of the sample was found to be ~ 26.8 nm calculated from the

most mtense peak .The presence of peaks in the spectra shows that the particles were present in

the crystalline form.

zoo

T50

l o o ­

se-

N 1

i^j -/ V W kH%d'/^h¥^A^hf^fk i*0 3 0 40 SO

— 1 — 60

I 70 SO

29

Figure 2 XRD spectra of the PVC based MP composite membrane

25

The SEM surface images of the PVC based MP composite membrane prepared at 100

MPa applied pressure are shown in Figure 3(a), 3(b) .It was showed that the PVC based MP

membrane is resistant to compaction and did not found any break or crack and swelling. The

information obtained from SEM images liave provided direction for the preparation of well-

ordered precipitates, their pore structure, micro or macro porosity, thickness, homogeneity,

surface texture and crack-free membranes [51,52].

26

Figure 3 SEM image of PVC based MP composite membrane at 100 MPa. (a) Surface

SEM image, (b) Cross-sectional view

With the help of Thermo gravimetric analysis (TGA), we determined the weight gain or

loss of the membrane due to gas release or absorption as a ftjnction of temperature figure 4. The

measurement of TGA analysis showed that there is a weight loss with increasing temperature. At

the same time it can be used to determine the temperatures of endothermic and exothermic

transitions at temperatures up to 800 °C.

27

3 00

SOD

4 DC

Mjd Poinl

Weignt Loss

127 41C

-0 891 mg

13 996% \

J

Mid Pont

Weight loss

604 79C

-0137nng

2 152% _

~~ \

- - "'

0.00 200.00 400.00 600 00 800.00 rcmp i'T]

Figure 4 TGA curve of PVC based MP composite membrane

The dielectric and impedance measurements were made Irom 75 KHz up to 5MHz. The

samples were pressed into circular pallets and coated with silver paste on adjacent faces, thereby

forming parallel plate capacitor geometry and then values of Z, 6 and Cp were recorded. With the

help of these recorded data various dielectric parameters were calculated as shown in figure 5.

Dielectric loss was calculated by using formula:

tanS =l/tan6

Where, tan5 is the dielectric loss tangent which is proportional to the loss of energy from

the applied field into the sample (this energy is dissipated as heat) and therefore denoted as

dielectric loss.

28

The ac conductivity of the samples was determined by using the relation:

Where, (o is the angular frequency

Real part and Imaginary part of impedance were calculated using the formula (5) and (6)

respectively

Z' = ZcosO and ^'' = Zsin3

Therefore from the graph we measure dielectric constant with respect to frequency and

we obtain frequency dependent behaviour which indicates that dielectiic constant decreases with

increasing frequency and the dielectric loss is also showing the complementary result with

respect to dielectric constant which indicates from the graph that the dielectric loss is

proportional to dielectric constant.

With the help of graph the real part of impedance shows frequency dependent behaviour

at low frequency and it can be revealed that at frequency near about 50MHz and above it shows

frequency independent behaviour and the imaginary part of impedance also showing the similar

result from the real part of impedance but it shows frequency independent behaviour than the

higher frequency from the real part of impedance.

29

'<i~t •

" • • " - • - ^

PVC/Mn{PO,iJ -. • \

* PVC/Mn{PO^)|

*\

'""•'^-

• PVCWi1{PO,)l

Figure 5 Dielectric behavior of PVC based MP membrane

Applied pressure

(MPa)

100

Thickness of

membrane

0.80

Water content as

% weight of wet

membrane

0.056

Porosity

0.065

Swelling of %

wet membrane

No swelling

Tablel The thickness, swelling, porosity and water content capability of synthesized manganous

phosphate composite membrane.

The water content property of a membrane is associated with the pressure of water vapor from

the surroundings. Information of the pore size distribution and the water structure in a membrane

might contribute for categorizing a particular type of membrane showing a solution-diffusion, a

fme-porous, or a coarse-porous membrane [53,54]. The low order of water content, swelling and

porosity with less thickness of such membrane suggests that the interstices are insignificant and

30

diffusion within the membrane would happen mostly tlirough PVC based (MP) Membrane was

tested for chemical resistance by incubating in acidic, alkaline and strongly oxidant media, in

acidic (1 M HNO3) and in alkaline media (I M NaOH). Few considerable modifications were

observed after 12, 24, 36 and 48, representing that the membrane is effective in such media.

Though, in strong oxidant media the prepared membrane became weak in 36 h and it was broken

after 48 h, losing their mechanical resistance.

Applied pressure 100 (MPa)

Membrane potential (raV)

Cone. (mol/L) KCl NaCl LiCl

1 0.1

0.01

0.001

0.0001

Table-2 Observed membrane potential in mV across the PVC based MP composite membrane in

contact with various 1:1 electrolytes at different concentrations and pressures at 25±1 °C.

The observed potential for manganous phosphate membrane ia contact with various 1:1

electrolyte solutions at 25±1 °C is given in Table 2. The values of the membrane potential

decreases with an increase in external electrolyte's concentrations showing the order of positive

mV potential This shows that the membrane is cation selective i.e. (negatively charged) and the

selectivity is the proportional to the dilution by the structural changes created in the electrical

double layer at the solution membrane boundary [55]. The membrane potential data obtained

4.2

8.2

12.2

21.4

32.5

12.5

26.5

33.0

40.0

46.0

16.6

28.5

34.6

42.3

48.4

31

with PVC based MP composite membrane using various 1:1 electrolytes are plotted as a ftinction

of-logC? Figure 6.

5 0 -

^ 4 5 -> ^ 4 0 -

• £ 3 5 -<u

Q. 30-H

ro 25-H X I

^ 20-H

E •D 1 5 -

i • 0) 10 -(/)

0 5 -

n

V

" .^^

1—'

' 1 ' 1 '

. 1 ^

^

- • - KCI - • - N a C I

- LiCI

1 1 ' 2 3

-logC,(mol/l)

Figure 6 Plots of observed membrane potentials against logarithm of concentration for PVC

based MP composite membrane using various 1:1 electrolytes at 100 MPa pressure.

The variation in the selectivity nature of the membrane is due to the adsorption of

particular ions leading to a condition in which the membrane surface making cation selective

nature due to the presence of net negative charge on it. The inorganic precipitate has the

capability to create potentials due to the interphase among electrolyte solutions having different

concentrations [56].

32

The charge on the membrane plays a vital role in the adsorption and transport of simple

electrolytes. In synthetic as well as ordinary membranes there ^re some significant

electrochemical properties, the most important are being the differences in the permeability of

co-ions, counter ions as well as neutral molecules. The number of charge needed to generate the

potential particularly using dilute solution is small. This is dependent on the porosity of the

membrane [57]. [f the membrane pores are broad resulting any sum of charge does to generate

good potentials on the membrane. On the other hand, if the pores are narrow, a little quantity of

charge can give rise to appropriate potential values. Therefore, it shows the transport mechanism

of simple electrolytes within the charged membrane appear to be incomplete without the

evaluation of the thermodynamically effective fixed charge density of the membrane [58,59].The

membrane carrying various charge densities which can be written as, D < 1. The theoretical

Potential showed by solid line and observed potential showed by broken line were plotted as a

function of-log C2 Figure 7. Thus, the coinciding curves for various electrolytes system gave the

value of the charge density D within the membrane phase Table 3.

Applied pressure KCl NaCl LiCl

(MPa)

Too 9^617 9^78 9 66

Table-3 Derived values of membrane charge density (Dx 10''' eq/1) for various PVC based MP

composite membrane electrolyte system using TMS equation

The surface charge density D of PVC based MP composite membrane is found to depend

on the initial stage of preparation. Thus, order of charge density for electrolytes used which were

found to be KCl > NaCl >LiCl. The charge density is higher m the case of KCl than in the NaCl

case due to the size factor (the smaller size, the larger ionic atmosphere).

33

1 2 3 -logC, (mol/l)

Figure 7 Plots of membrane potential (theoretical and observed) (mV) versus - log Q (mol/l) at

different concentrations of KCl electrolyte solution for PVC based MP composite membrane

prepared at pressure of 100 Mpa.

The TMS Eq. (1) can also be expressed by the sum of Donnan potential, A^oon among

the membrane surfaces and the external solutions, and the diffusion potential A Poiff within the

membrane [60,61]

^H^m.e^^WDon-^^M^d.ff (2)

V C C (3)

The R, T and F have their usual significance;

34

Yuand Y2i a]e the mean ionic activity coefficients; Ci± and C2±are the cation concentration in the

membrane phase first and second, respectively. The cation concentration is given by the

equation.

c "^-^ \ :

1 I ' • " ( 4 )

^K

Where Vk and Vx denotes the valency of cation and fixed-charge group on the membrane matrix,

where q is the charge effectiveness of the membrane and is defined by the following equation.

Where K± is the distribution coefficient expressed as;

K, = ^,C.=C,-D (6)

Where Ci is the ith ion concentration in the membrane phase and Ci is tne itn ion concentration

of the external solution. The transport of electrolyte solutions in membrane has shown that the

transport properties of membrane are also controlled by ion distribution coefficients. It appeared

that the Eq. (6) for evaluating the distribution coefficients were found to be low down at lower

concentration and as the concentration of electrolytes increases the value of distribution

coefficients, significantly increases and thereafter, a stable trend was observed.

35

Table-4 The values of ;^, U, co anAK^ ,<7, Q evaluated using Eq. (9), (6), (5)

and Eqs. (4) respectively, irom observed membrane potentials across various

electrolytes at different concentrations for polyvinyl chloride (PVC) based

MP composite membrane.

KCl (Electrolyte)

C2(moiyi)

1.000

.1000

.0100

.0010

.0001

1.000

.1000

.0100

.0010

.0001

1.000

.1000

.0100

.0010

.0001

K

0.54

0.57

0.61

0.69

0.78

0.61

0.73

0.78

0.84

0.89

0.65

0.75

0.80

0.86

0.90

U

0.08

0.14

0.22

0.38

0.56

0.22

0.46

0.56

0.68

0.78

0.30

0.50

0.60

0.72

0.80

CO

1.17

1.32

1.56

2.22

3.54

NaCl

1.56

2.70

3.54

5.20

8.09

LiCl

1.85

3.00

4.00

6.14

9.00

^ ±

0.9910

0.9010

0.4050

-8.600

-95.00

0.9906

0.9050

0.5300

-8.400

-93.00

0.9900

0.9000

0.0940

-8.000

-89.60

«?

1.010

1.110

2.500

0.100

0.010

1.010

1.104

1.886

0.119

0.010

1.010

1.111

10.638

0.125

0.011

C

0.99105

0.09121

0.00345

0.00045

0.00005

0.99135

0.09146

0.00290

0.00030

0.00002

0.99165

0.91832

0.00327

0.00030

0.00004

36

The diffusion potential A Poiff is expressed as.

RTo)-\ , Aw ,„ = X In (7)

Where co = u/ v is the mobility ratio of the cation towards the anion in membrane phase.

Therefore total membrane potential A*Pme was obtained by following.

RT , Ai// = In

V / i +

RTco-] V,Fco + \

( i -

xln (fi^+l)C + (F,,/F,)Z)

(8)

I

-logC2(mol/l)

Figure 8 The plot for mobility ratio of PVC based MP membrane for 1:1 electrolyte against

concentration.

For the applicability of these theoretical equations for the system the Donnan potential and

37

diffusion potential were separately calculated fi-om the membrane parameters which were

obtained from membrane potential measurements by using a distinctive membrane which were

prepared at a pressure of 100 MPa [62,63]. The transport properties of the membrane in

electrolyte solutions are important parameters to investigate for further study represented in the

following equation

F C, (9)

Where t ^ _ u

t v"

0.90-

tl- 0.80 •

1 ' 2 3

logC,(mol/l)

Figure 9 plots showing the transport number of cation for the PVC based MP membrane for

electrolytes (KCI, NaCI, LiCI) Vs concentration

38

To get the value of transport number, t+ Eq. (9) will be used and t" from experimental

membrane potential data and consequently, the mobility ratio co = u /v were calculated. The

mobility of the electrolyte in the membrane phase were found to be high and follow the order of

LiCl > NaCl > KCl as showed in Figure 8. The high mobility is due to the higher transport

number of comparatively free cations of electrolytes. The similar trend of mobility is also seen

in least concentrated solution.

The transport number of cation of the various electrolytes (KCl, NaCl and LiCl) increases

with decreasing the concentration of electrolytes and follows the increasing order KCl < NaCl

<LiCl Figure 9.

Donnan and diffusion potential at various electrolyte concentrations can be calculated from

the parameters y^^, X:±. C^^, C^ , co, V^, V^ and the experimentally derived values of

charge density D by using Equations (3) and (7). The values of the parameters iC ., q and Q

also derived for the system are also shown in Table 4. The values of Y± were the usual charted

values for electrolyte (KCl, NaCl and LiCl).

Conclusion

The polyvinyl chloride (PVC) based manganous phosphate (MP) composite membrane

was prepared by sol-gel process and it was found quite stable. The fixed-charge density is the

central parameter governing transport phenomena in membranes, and depends upon the feed

composition and applied pressure due to the preferential adsorption of ions and accounted for

altering the charge density and, in turn, performance of membrane. The membrane potential of

PVC based MP composite membrane for different 1:1 electrolytes was found to follow an

increasing order KCl < NaCl < LiCl and surface charge density follow reverse order.

39

CHAPTER - III

40

CHARACTERIZATION, ELECTROCHEMICAL NATURE AND TRANSPORT

PHENOMENA OF POLYSTYRENE BASED MANGANOUS TUNGSTATE

COMPOSITE MEMBRANE

Introduction

A great number of ion-exchange membranes have been widely studied due to their

versatile applications in chemical industries, food, and drugs, waste water treatment, and in many

separation processes also [64]. Furthermore ion-exchange composite membranes find

applications in various processes such as electrodialysis, desealination, diffusion, electro-

deionization, membrane electrolysis, electrochemical synthesis, foel cells, storage batteries, and

others. Therefore, they are useful in pollution control, energy saving, power generation, and

resource recovery etc [65]

Such composite membranes have also been used in electro dialysis for briny water

desalination from several times. For doing that ion-exchange membranes should possess high

selectivity and conductivity. Separation of solutes by a neutral membrane takes place by the

differences of solute sizes and their interaction with the membrane. But charged membrane can

separate the charged solutes according to their nature. Now days it is very necessary to

investigate the ion transport phenomena specifically in aqueous, organic, electrolyte solution

systems across a charged membrane from the perspective of medical and industrial applications.

[66]

In these days, ion-exchange membrane processes, as electrodialysis, have been suggested

as capable possibilities to the exclusion and the resurgence of heavy metals and other inorganic

toxic substances which were generated in electroplating processes. For the industrial practicality

41

of an electrodialysis process it is helpfiil to know the electrochemical performance of the

ionexchange composite membranes, their conductivity, selectivity, stability and their ion

transport number [67]. The inorganic-organic composite membrane formed by mixing organic

polymers and inorganic particles together in a specific ratio. Therefore, membranes exhibit the

ideal properties of both the constituents [68] By the study it is clear that polystyrene is a good

binder for showing a compact and rigid framework therefore in this study I have used this

organic polymer with the inorganic tungstate which give a stable membrane by the economical

and industrial point of view. The measurement of membrane potential is an important method

for characterizing transport phenomena across a charged ion exchange membrane [69.]

Theoretically the membrane potential in a charged membrane in aqueous electrolyte solution

system developed by Teorell, Meyer and Sievers (TMS) which can be treated by Donnan

equilibrium theory and the Nernst - Planck equation, so it is assumed that the fixed charge

groups are homogeneously distributed in the membrane [70] and the result of mean activity

coefficient of the electrolytes in the external solution is negligible. Henderson solved the Nernst -

Planck equation at the constant ion concentration gradient. The ion transport phenomena across a

charged membrane in aqueous solution systems have been studied in many previous papers.

In this paper, the membrane potential in different aqueous-electrolyte systems are

measured and applied to the equation which is based on the Dorman equilibrium and the Nernst -

Planck equation, considering the effect of electrolytes in the external solution by the mean

activity coefficient. [71] .The diffusion potential is totally depends on the mobility of ions in the

membrane and affects the transport property of ions in the composite membrane. And the

Donnan potential of the membrane depends on the charge density which plays a significant role

in the selectivity of the charged inembrane with respect to the valence of ions .The measurement

42

of the diffusion potential and Donnan potentials will give the important parameters that are the

ionic mobility and effective charge density. [72,73]

The surface charge density D is the fundamental parameter that controls the membrane

phenomenon and was calculated using the membrane potentials values for different electrolytes

with the help of (TMS) method. Some other parameters including distribution coefficient,

transport numbers, mobility ratio and charge effectiveness etc. were also calculated by the above

method. [74]

Experimental

Chemicals used:

Pure crystalline polystyrene ((Otto Kemi, India, AR) ground and sieved through 200

meshes.) used as a binder, 0.2M sodium tungstate solution (E. Merck. India with purity of

99.90%), 0.2M manganous chloride (MnCb) solution (Otto Kemi, India) and various electrolytes

solution (KCl, NaCi and LiCl) of different concentrations were also arranged

Instruments:

SEM images were taken on scaiming microscope. Leo 4352 at an accelerating voltage of

20 kV. The phase transformation analysis of the sample was conducted by X-ray diffraction

(XRD) Miniflex-II X-Ray diffractometer (Rigaku Corporation) with Cu Ka radiation. Infrared

(IR) spectra were recorded using an Interspec 2020 FTIR spectrometer, spectrolab (UK).

Thermo gravimetric (TG) analysis and differential theimal analysis (DTA) of the samples were

carried out on (TGA) (Shimdzu DTG-60H), the dielectric property of the sample were

determined with Agilent (Model; 4284A) precision LCR meter.

Preparation of manganous tungstate material

43

Manganous tungstate was prepared by sol-gel or coprecipitation process by mixing 0.2 M

aqueous solution of sodium tungstate with 0.2 M manganous chloride solution resulting a

constant stirring for 1-2 h done and after the stirring P" must be maintained. The resulting

precipitate was well washed almost 4th to 5th times with double distilled water to remove free

electrolytes and dried the material till the 3-5 h at 100°C after that it will be powdered with the

help of pestle and mortar until the size should be less than 200 meshes. Pure crystalline

polystyrene (Otto Kemi, India, AR) was also ground and sieved through 200 mesh

Preparation of polystyrene based manganous tungstate composite membrane

The precipitates were mixed with polystyrene granules (less than 200 meshes) by pestle

and mortar and then mixture was kept into a cast die having a diameter of 2.45 cm and placed in

a digital furnace by maintained temperature at 200°C tor about an hour to equilibrate the reaction

mixture and transfer to a pressure device (SL-89, UK), and apply pressure of 100 MPa during the

fabrication of the membrane. By embedding 25% polystyrene the prepared membrane were

found to be more mechanically stable and gave reproducible results. If containing greater

amounts (>25%) of polystyrene could not be reproduced while having lesser amounts of

polystyrene (<25%) were unstable [75]. Therefore the total amount of the mixture utilized for the

preparation of the membrane which contained 0.125 g of polystyrene and 0.375 g of manganous

tungstate. Then the prepared membrane was subjected to microscopic and electrochemical

examination for seeing cracks and homogeneity of the membrane surface and applying for

potential observation which had smooth surface were confident by carefially controlling the

conditions of fabrication. [76].

44

Measurement of Thickness and swelling.

The thickness measurement of the membrane obtain by the average thickness of 4-5th

rep Heat es by using the screw gauze apparatus and Swelling is measured as the difference

between the average thickness of the membrane equilibrated with 1 M NaCl for 24 h and the dry

membrane.

Measurement of water absorption and water flux

The water absorption, in the membranes was calculated by the following equation

W - W % Total {weight ,„ , ) = —^ ^ x 100

^ "

Where Ww is the equilibrium weight of the swollen membrane which is obtained by soaking in

water for 5 h and Wj the weight of the dry membrane.

The pure water flux, F, can be measured under a pressure difference of 0.2 MPa at ambient

temperature. And it can be calculated by the following equation [77]

F = V/ At

Where V is the total volume of the water permeated during the experiment, t is the operation

time, and A is the membrane area.

Water content (% Total wet weight)

Membrane was first kept in water for soaking in water to diffusible salt, after that

membrane was blotted quickly with Whatmann filter paper to remove the moisture of the surface

and then immediately weighted. These membranes was fiarther dried a constant weight in a

45

vacuum over P2O5 for 24 h. The water content (total wet weight) in the membrane was calculated

as:

w - w % Total {weight ,.„ ) = - ^ ^ x 100

Ww = weight of the soaked

Wd = weight of the dry membrane

Porosity

Porosity is the volume of water incorporation in the cavities per unit membrane

volume from the water content data:

rorosity -

Wd= weight of dry membrane, A = area of membrane, L = thickness the membrane,

pvv = density of water

Chemical stability

Chemical stability was determined by of ASTM D543-95 procedure. For analyzing the

morphological changes, alteration in color, texture, brightness, decomposition and splits, the

membrane was exposed to different acidic and basic media. And then was evaluated after 24, 36,

and 48 h.

46

Observation of Membrane Potential

With the help of digital potentiometer (Electronics India model 118) membranous

potential can be obtain as the membrane was placed in the centre of the measuring cell which is

having two compartment of definite size [78,79]. Mainly KCl, NaCl, LiCl electrolyte solutions

used and were prepared from B.D.H. (A.R.) grade chemicals and deionized water. The collared

shaped measuring cell used for injecting the electrolyte solutions and it was stirred by a magnetic

stirrer. The membrane potentials were determined with the help of potentiometer of reference

electrodes (SCE) kept dipped in to the both chamber of different concentrated solutions. All the

observation of ionic potential was made at room temperature. The electrochemical setup which

was used for membrane potential measurements may be depicted as

SCE Solution Membrane

CI Diffusion Potential

Solution SCE

Donnan Potential Donnan Potential

Membrane conductance of ion exchange composite membrane

The electrical conductance of such type of composite membrane was measured by the

method used [80]. With the help of conductivity meter (Model No. L303).

Morphological characterization of membrane

SEM investigation

With the help of SEM image the morphology of membrane was investigated. It was done

by Leo 4352 at an accelerating voltage of 20 kV. The sample was mounted on a copper stub and

47

sputter coated with gold to minimize the charging and was used to verify the micro structure of

fabricated porous membrane.

X-ray diffraction analysis

X-ray diffraction pattern of the polystyrene based MT composite membrane was recorded

by Miniflex-II X-Ray diffractometer (Rigaku Corporation) with Cu Ka radiation

FTIR Spectra of composite membrane

The FTIR spectrum of polystyrene based MT composite membrane were done by

Interspec 2020 FTIR spectrometer, spectrolab (UK).The sample compartment was 200 mm wide,

290 mm deep and 255 mm high. The entrance and exit beam to the sample chamber was sealed

with a coated KBr window and there was a hinged cover to seal it from the environment.

The thermo stability of membranes

The degradation process and thermal stability of polystyrene based MT composite

membrane was investigated using thermo gravimetric analysis (TGA) (Shimadzu DTG-60H),

under nitrogen atmosphere using a heating rate of 20 C min"' from 25 C to 800 C. At the

temperature of 446.78 C the weight loss was 2.27 mg that should be the 33.07%.

Dielectric properties of the composite membrane

The dielectric and impedance measurements were made from 75 KHz up to 5MHz using Agilent

(Model; 4284A) precision LCR meter.

Result and discussion

The surface of the membrane was investigated by SEM characterization [8,82] which is

showing in the Figure 1 and was prepared at lOOMPa .The distribution of materials such as

48

Polystyrene and MP was uniformly mixed in the medium; therefore the prepared membrane was

having homogeneous nature and it's applicable for good result.

USt*

Figure 1 SEM images of polystyrene based MT composite membrane (a) Surface image (b)

Cross sectional view

For the XRD analysis sample should be in powdered form and was mounted in the sample holder

with the help of glass slide and further studied for X- ray analysis. By pattern was recorded using

a Rigaku Miniflex X-ray diffractometer with the Cu K a radiation (k = 1.54060A°) in 26

ranging from 20° to 80° to find out the structure and lattice parameters of the sample. The XRD

spectrum of polystyrene based MT is depicted in Figure 2. BY the analysis of the sample the

lattice constants of the sample was found to be as (a) 5.758 A° and (c) 9.443 A°).The XRD

pattern of particles showed two most intense peak at 29 = 28.30°and 51.50°, which corresponds

to (211) plane of polystyrene based MT [83]. The crystallite size ofthe sample was found to be ~

26.8 nm calculated from the most intense peak .The presence of peaks in the spectra shows that

the particles were present in the crystalline form.

49

£••4

,— ^ r 2

1 ,-

V.

# X%¥r\fL ,1 Is^.f

V V^\.JlA\-^ ...A/' 3 0 SO

2 0 6 0 7 0 8 0

Figure 2 X-ray diffraction curve of polystyrene based MT composite membrane

FTIR spectrum was measured to analyze the structural properties and bonds of the sample.

Figure 3 shows the FTIR spectrum of polystyrene based MT membrane. The spectrum shows

two strong IR absorption bands at ~ 676.23 and 814.30 cm-1. These are the characteristic of H-

0-H bending of the H20 molecules. This shows the presence of hydroxyl groups in the prepared

sample. The fundamental frequency at ~ 504.51-546.23 cm"' arises due to hindered rotations of

the hydroxyl ions. The band shows at 3404 cm-1 is due to KBr pallets used during record of

FTIR spectrum.

50

too y

•^M> S O

s

1 -

m S.

f I f 4 «.! » S i I i l

St ! ill

I III if i f f I nil 111

4000 3000 SODD

Wave number (cm^) tooo

Figure 3 FTIR Spectra of polystyrene based manganous tungstate composite membrane

The TGA and DTA curves of membranes are shown in Figure 4 As can be seen from the TG

curves of the polystyrene based MT membrane, there was 2.27 mg in weight loss takes place at

the ~ 600°C temperature that should be 33.076 % which represent in the graph. However, most

of the composite membrane did not have any weight loss. The polystyrene based MT membrane

had a high hydrophilicity nature and could absorb moisture from the surrounding air.

TGA

MUPWnl 44e7«C

100.00 2t}0.00 300.00 400X0 Tamp [C|

S00.0O 600.O

Fgure 4 TGA curve of polystyrene-manganous tungstate membrane

51

By Figure 5.1-5.6 the dielectric properties of the sample according to the graph can be seen

below.

1 8x10-

1 6x10'

14x10'-

1 2x10'

1 0x10'

8 0x10'-

6 0x10*

4 0x10'

2 0x10'

00

. /•

2x10

T

3x10 4x10 5X10

Figure 5.1 shows the complex impedance plot of sample. Generally, the grains are effective in

high frequency region while the grain boundaries are effective in low frequency region. Thus the

circular arc appearing in the high frequency region corresponds to grain contribution while low

frequency region corresponds to grain boundary contribution. It is evident from the plot itself

that figure show single circular arc behavior, which suggests the predominance of grain

boundary resistance over the grain resistances in this sample. Due to this fact, the single circular

arc is being observed in Cole-Cole plots. Moreover, it can be seen from above graph.

52

1

0 30 -

0 2 5 -

0 20-

015 -

0 10 -

0 05-

0 00-

-0 05-

-010-

• • •

1 - • - land

•

1 1 1 . 1

50 55 60

Uogf(Hz)

Figure 5.2 shows the variation in dielectric loss factor with frequency at room

temperature. U is observed that tan5 decreases v ith the increase in frequency for the

characterized sample which may be due to the space charge polarization. It is considered to

be caused by domain wall resonance. At higher frequencies the losses are found to be low

since domain wall motion is inhibited and magnetization is forced to change' rotation.

5x10'-

4x10'-

3x10'-

2x10'-

1x10'-

0-

•

\ •

\ • \ 1 \ •

•

• •

V ^ ^

| - B - Z |

--1 T — , , ,

5 5 8 0

Logf(H2)

53

Figure.5.3 shows the variation of the resistive part of impedance (Z') with frequency. It

has been clearly viewed from the pattern itself that Z' decreases with the increase in

frequency for characterized sample. The decrement in the real part of impedance (Z' )

with the rise in frequency may be due to the increase in ac conductivity with rise in

frequency. It is observed that Z' has higher values at lower frequencies and decreases

monotonically with the rise in frequency and attains a constant value at higher frequency

part.

1 8x10^-

1 6x10'-

1 4x10*-

1 2x10^-

"b 1 Oxio''-

jgSOxlo ' -

6 0x10'-

4 0x10*-

2 0x10'-

0 0 -

20x10*-

• \ • \ • \

•

m

•

1—

—, , 1 ,

- z |

55 60

Logf(Hz)

Figure.5.4 shows the variation of reactive part of impedance (Z") with frequency, which

indicates the same behavior as that of Z'. It is seen that Z" decreases with increasing

frequency

54

-Ac[ 0 020n

0 015-

0 0 1 0 -

0 O 0 5 -

0 0 0 0 -

0 0 0 5 -

-0010-

-0 0 1 5 -

-0 0 2 0 -

-0 025 -4 5

Loof(Hz)

Figure.5.5 shows the variation in ac conductivity with frequency for different

compositions at room temperature. The ac conductivity gradually increases for all

compositions with the increase in frequency. It has been reported that the ac conductivity

gradually increases with the increase in frequency of applied ac field because the increase

in frequency enhances the migration of electron.

4 0 0 -

3 0 0 -

2 0 0 -

100-

0 -

100-

•

• •

t^Z^

1

55 60

Logf (Hz)

Figure5.6 Ft is clear from that dielectric dispersion as e" value of the sample show a

decrease with increase in frequency. The decrease in dielectric constant is rapid at low

bs-44n 55

frequency and becomes slow at higher frequencies, approaching to frequency independent

behavior.

As a result of the electrochemical performance it is clear that prepared membranes having

a definite ratio of binder and if it containing less than 25% was mechanically unstable although

having more than above percentage showing unstability binding. [84, 85] It is due to the

membrane carries some charges and with the help of binder separation of exchanger particles

takesplace. However those electrolyte solutions separated by a membrane (which is fixed in the

cell) having unequal concentration and it is driven by different chemical potential performing

across the membrane by the concentration of the solution which was sign by the nature of the

fixed charge on the membranes. [86]

Chemical stability of composite membranes was determined by incubating in acidic, alkaline and

strongly oxidant media, as- (IM ffN03, IM NaCl and 1 M NaOH) solutions observed by putting

it into the above solutions for the 12, 24, 36 and 4S, after 48 h, membrane losing their

mechanical resistance which represent that the membrane is effective' in such media. The

thickness, porosity and swelling of the membrane is showing in the Table!. The high thermal

stability, chemical stability and the specificity for cations are the exclusive features of this

composite membrane.

Applied pressure

(MPa)

100

Thickness of membrane

0.80

Water content as % weight of wet

membrane

0.056

Porosity

0.065

Swelling of % wet membrane

No swelling

Table-1 Thickness, water content, porosity and swelling properties of PVC based MP composite

membnee

56

Membrane potentials had been regarded for a long time as a measure of the membrane

selectivity by different electrolyte solutions such as KCl, NaCl, LiCl. The measurement of ion

activity by means of membrane potential is more successful in the concentration range over

which the membrane behaves as an ideally selective one and hence obeys the Nernst equation.

By the observation it is clear that the membrane potential decreases with an increase in external

electrolyte's concentrations resulting membrane is ideally cation selective i.e. (negatively

charged)

The membrane under consideration seems to be truly selective and specific for K , Na" , Li and

were investigated by observing the membrane potentials in contact with solutions of KCl, NaCl

and LiCl (Table 2) and it was observed that the nature and charge of anion present did not

substantially influence the potentiometric respond of the membrane. The potential data obtained

for composite membrane using various 1:1 electrolytes are plotted as a function of-logC2

(Figure 4).

Applied pressure 100 (MPa)

Membrane potential (mV)

Cone. (mol/L) KCl NaCl LiCl

0.1

0.01

0.001

0.0001

Table-2 Observed membrane potential in mV across the PVC based MP composite membrane in

contact with various 1:1 electrolytes at different concentrations and pressures at 25±1 °C.

9.0

10.3

18.5

30.6

38.5

12.5

16.3

22.7

32,8

40,6

16.6

22.5

27.6

33.7

42.8

57

The activities of concentrations can be corrected by the Donnan potential through the

integration of Planck or Henderson equation. According to TMS theory, the membrane potential

is given by the equation at 25 C.

Av7,=59.2 log---^'^ _ _+^log-^^^— z z=r C, ^AC{+D' + D ^AC; + D' + DU \

C/ = {u-v)/{tT + v)

(1)

Where u" and v" are the ionic mobility's of cation and anion (m2/v/s), respectively, in the

membrane phase. CI and C2 are the concentrations of the membrane and D is the charge on the

membrane expressed in equivalent per liter. The graphical method of TMS determines the fixed

charge D~ in eq/L and the cation-to-anion mobility ratio in the membrane phase. Charged

membrane was placed at the center of the potentials measuring cell, which had two glass

chambers on either side of the membrane The disparity in the selectivity nature of the membrane

is due to the adsorption of particular ions which leads a condition in which the membrane surface

making cation selective nature due to the presence of net negative charge on the membrane. The

inorganic precipitate has the capacity to create potentials due to the interphase among electrolyte

solutions having different concentrations [87,88].

The charge on the membrane having major role in the adsorption as well as transport of

electrolytes in synthetic as well as standard membranes. The most important electrochemical

properties are permeability differences in the co-ions, counter ions as well as neutral molecules.

The potential of diluted electrolyte solution is dependent on the porosity of the membrane [89].

Therefore without the evaluation of the thermodynamically effective fixed charge density the

58

transport mechanism of electrolytes solution within the charged membrane appear to be

incomplete

If the membrane pores are broad resulting any sum of charge does to generate good potentials on

the membrane. On the other hand, if the pores are narrow, a little quantity of charge can give rise

to appropriate potential values. Therefore, it shows the transport mechanism of simple

electrolytes within the charged membrane appear to be incomplete without the evaluation of the

thermodynamically effective fixed charge density of the membrane [90, 91], The membrane

canying various charge densities which can be written as, D < 1. The theoretical and observed

potential showed by solid and broken line respectively and were plotted as a fiinction of-log C2

Figure 5. Thus, the coinciding curves for various electrolytes system gave the value of the charge

density U within the membrane phase Table 3.

Applied pressure KCl NaCl LiCl

(MPa)

Too 0.0612 00403 0.0301

Table-3 Derived values of membrane charge density (D ,eq/l) for various PVC based MP

composite membrane electrolyte system using TMS equation

The surface charge density D of membrane is found to depend on the initial stage of

preparation. Thus, order of charge density were found to be KCl > NaCl >LiCl. The charge

density is higher in the case of KCl than in the NaCl case due to the size factor. The TMS Eq.

(1) can also be expressed by the sum of Donnan potential, A 'oon among the membrane surfaces

and the external solutions, and the diffusion potential A^ oiit within the membrane [92,93].

59

-logC2 (mol/l)

Figure 4 Plots of observed membrane potentials against logarithm of concentration for

polystyrene based BMP composite membrane using various 1:1 electrolytes at 100 MPa

pressure.

^¥m,e=^¥Don+^¥diff (2)

Ai//n = In y Don y p.

^y CC ^ (3)

The R. T and F have their usual significance;

Yi± and Y2± are the mean ionic activity coefficients; C~ii and C 2±are the cation concentration in

the membrane phase first and second, respectively. The cation concentration is given by the

equation.

60

6 0 -

5"50-E '—' "TO

^ 4 0 -B B 1 Q .

g 30-ro

0 2 0 -5

' 1 0 -

Q

/ ^ / f

1 ' y

' 1

/ ^ ^ / / / /

/ /' // / / //

' / // / //

/ /' //

' /< 11 1 / II

1 1

J"^"^"^'

' 1 '

o-o i ; — 1 D- '

1 / ) -0(101 1

£) = 0(t<)25

1 D - 0 (H)20

O - 0 0iil2

O-OOIHI)

" ,

1 2 3

-logC2 (mol/l)

Figure 5 Plots of membrane potential (theoretical and observed) (mV) versus -log C2 (mol/L) at

different concentrations of KCi electrolyte solution for polystyrene based BMP composite

membrane prepared at pressure of 100 MPa.

C =, I 2^ J

2

I <? J 2F, (4)

Where VK and Vx denotes the valency of cation and fixed-charge group on the membrane matrix,

where q is the charge effectiveness of the membrane and is defined by the following equation.

^ = , (5)

Where K± is the distribution coefficient expressed as;

K,=^,C,-C,-D (6)

61

C2(mol/l)

1.000

.1000

.0100

.0010

.0001

1.000

.1000

.0100

.0010

.0001

1.000

.1000

.0100

.0010

.0001

t. 0.58

0.59

0.66

0.76

0.83

0.61

0.64

0,70

0.78

0,85

0.65

0.69

0.74

0.79

0.87

U

0.16

0.18

0.32

0.52

0.66

0.22

0.28

0.40

0.56

0.70

0.30

0.38

0.48

0.58

0.74

KCl (Electrolyte)

a

1.38

1.43

1.76

3.16

4.88

NaCI

1.56

1.77

2.33

3.54

5.66

LiCl

1.85

2.22

2.84

3.76

6.69

^ ±

0.9388

0.388

-5.120

-60.20

-611.00

0.9597

0.5970

-3.03

-39.30

-402.00

0.9699

0.699

-2.01

-29.10

-300.0

q

1.065

2.577

0.195

0.0166

0.0016

1.014

1.675

0.330

0.025

0.002

1.013

1.430

0.497

0.034

0.0003

Q 0.99105

0.09121

0.00345

0.00045

0.00005

0.99135

0.09146

0.00290

0.00030

0.00002

0.99165

0.91832

0.00327

0.00030

0.00004

Table-4The values of/^, U, co andA'+ ,g, Q evaluated using Eq. (9), (6), (5)

and Eqs. (4) respectively, from observed membrane potentials across various

electrolytes at different concentrations for polyvinyl chloride (PVC) based

MP composite membrane

Where Ci is the ith ion concentration in the membrane phase and Ci is the ith ion concentration

of the external solution. The diffusion potential, AH diff was expressed in the form

62

^ ^ d > W RTco-\ / / -

xln f;:\ {6i + \)C,+{V,/V,)D

(7)

Where co = u/ v is the mobility ratio of the cation towards the anion in membrane phase.

Therefore total membrane potential A*Fme was obtained by following.

A(// = in ^y CC ^

y^^L^L-2+ J

RToJ-l

V,Fco + \ xln

(^ + \)C,+{V,/V,)D

(oj + \)q+(V,/V,)D (8)

2 3

-logC2 (mol/l)

Figure 6 The plot of mobility ratio of polystyrene based MT composite membrane for I:

electrolytes (KCI, NaCl and LiCl) versus concentrations

(9)

63

Where u V

(10)

0 90

2 3

-logC2 (mol/l)

Figure 7 The plot of transport number of cation of polystyrene based BMP composite membrane

for 1:1 electrolytes (KCl, NaCI and LiCl) versus concentrations.

For the applicability of these theoretical equations for the system the Diffiision potential

and Donnan potential was separately calculated from membrane potential measurements by

using a distinctive membrane at a pressure of 100 MPa [94, 95]. Esq. (9) and (10) were used to

get the values of transport numbers t+ and t- from the experimental membrane potential data and

consequently the mobility ratio (n"=u/v is within the membrane phase.

Future perception

By the above discussion it is clear that the polystyrene based MT membrane can be use

into the diffusion process, osmosis, water filteration, desealination, membrane electrolysis, fuel

cells, storage batteries, and many more and it is mainly useful in control of water pollution.

64

Conclusion

The polystyrene based manganous tungstate composite membrane was prepared by

applying the theoretical approach mentioned above and it was found quite stable, several

conclusions are drawn as: The characterization stmctural details of the membrane showed great

properties in terms of, effective charge density, mobility ratio, membrane thickness and

membrane porosity. The fixed-charge density is the central parameter governing transport

phenomena in membranes, and depends upon the feed composition and applied pressure. The

increase of electrolytes concentration tends to exhibit smaller membrane potential. The

membrane potential for different 1:1 electrolytes was found to follow increasing order KCl <

NaCl < LiCl and surface charge density follow reverse order. For upcoming works, this

membrane model can be enhanced fiirther which can be acceptable and withstands in the range

of the commercial membranes available in the markets.

65

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