Development of surface activated granular media for effective adsorption and filtration

... being ineffective to the removal of suspended particles and dissolved substances It is therefore desirable to develop more effective granular media as both adsorption and filtration materials for. .. experimental parts of coating PPy on the surfaces of glass beads and nylon 6,6 granules and immobilizing chitosan on the surfaces of PET and nylon 6,6 granules, as well as the adsorption and filtration. .. stages) and deposition of colloid particles 2.3 Adsorption and Granular Filtration Separation processes like adsorption and filtration refer to the operations that transform a mixture of substances

Chapter 1 Introduction and Research Objectives

Water or wastewater is a complex system that often contains various kinds of dissolved

substances (organic or inorganic) and suspended/colloidal particles Organic matters

found in water or wastewater may include such diverse species as humic substances,

carbohydrates, lignin, fats, soaps, synthetic detergents, proteins and their decomposition

products, as well as various synthetic organic chemicals from the process

industries Examples of suspended matters in water and wastewater can include various

inorganic and organic particles, such as soil or clay particles, immiscible liquids, metal

hydroxides or microorganisms, etc Many of the dissolved or suspended matters may

cause aesthetical problems and/or be toxic or hazardous to our human beings and other

aquatic lives Therefore, appropriate treatment of water or wastewater is necessary in

order to make water suitable for dinking or wastewater suitable for discharging or reuse

The unit processes used in water and wastewater treatment to remove dissolved or

suspended matters include coagulation/flocculation, sedimentation, granular media

filtration, adsorption, and biological treatment, etc Among these processes, granular

media adsorption and filtration have widely been used While the two processes are both

concerned with separating certain species present in a fluid stream, the sizes of the species

to be separated by the two processes are often different The molecular sizes of the

dissolved species removed in adsorption are usually in the order of angstroms or

nanometers, whereas the sizes of particles to be separated in granular filtration are in the

range of submicrons or microns

Nevertheless, the operation of fixed-bed adsorption is quite similar to that of granular

filtration In both operations, granular media are placed in a bed and the fluid to be treated

is allowed to flow through the granular media It is also believed that the same types of

interaction forces (such as the London-van der Waals and electrostatic double-layer

forces) are active in both adsorption and filtration (Tien, 1989) Thus, many similarities

exist between adsorption and granular filtration processes, in terms of equipment

configuration, mode of operation, and the respective underlying phenomena Because of

these similarities, the words adsorption and filtration have sometimes become

interchangeable (Tien, 1989; 1994) The removal of colloidal particles from a fluid phase

to a solid phase may be described as either adsorption or filtration (Hirtzel and

Rajagopalan, 1985) In engineering practice, granular carbon columns used to remove

organic compounds in drinking water supplies are often referred to as carbon filter

For economic reasons, sand and anthracite have been widely used as granular filter media

in almost all water and wastewater filtration in municipalities and various industries To

remove dissolved hazardous species, granular activated carbon is often applied as the

adsorbent in an adsorption process (Clark et al., 1989) Sand and anthracite themselves

are, in fact, not particularly efficient to remove suspended particles, especially those fine

submicron particles, such as colloids, bacteria (pathogens) and viruses, etc., nor are they

effective to remove any dissolved organic and inorganic substances However, because of

the difficulty and high cost in regeneration, it is impractical to use activated carbon for the

purpose of suspended particle removal Moreover, activated carbon is inefficient in an

adsorption process to remove those dissolved organic matters with larger molecular mass

and wide molecular mass distribution, e.g natural organic matters (NOM) Nowadays,

water and wastewater treatment systems often have to apply sand filtration followed by

activated carbon adsorption in order to achieve the desired level of purification This

approach not only complicates the treatment process but also results in significant cost

increase

The surface properties of granular media play an important role in the removal of

dissolved substances and suspended particles The common mechanisms of particle

removal in granular filtration include interception, sedimentation and adsorption

Interception and sedimentation are significant for large particles For most of the fine

particles or colloids in water and wastewater, adsorption is the key mechanism for their

removal, which relies on the surface interactions between the media grains and the

particles to be removed A classic theory for analysis and prediction of colloid adsorption

and deposition is the Derjaguin-Landau-Verwey-Overbeek (DLVO) model, which was the

postulate of Landon-ver der Waals and electrostatic double-layer interactions In general,

the Landon-ver der Waals force is always attractive, but the double-layer force can be

either attractive or repulsive Extensive theoretical calculations and experimental

investigations have demonstrated that colloid adsorption/deposition efficiency can be

enhanced significantly under attractive double-layer interactions

In most water and wastewater treatment processes, the dissolved substances and

suspended colloids and particles to be removed have negative surface charges The

conventional filter media, such as sand, are usually negatively charged under normal

water and wastewater treatment conditions (Redman et al., 1997) Due to the force of

electrostatic repulsion, the removal of most of the fine particles, such as colloidal

particles, bacteria and viruses, has been difficult Similarly, activated carbon also carries

negative surface charges in this pH range (Wu et al., 2001), thus decreasing the adsorption

efficiency greatly Another important factor for effective adsorption or filtration is the

surface morphology of the granular media Rough surfaces with micropores provide large

specific surface area for particle deposition The heterogeneity of the surface morphology

also possibly changes surface charge distributions, with certain surface patches favorable

for particle deposition even though the overall interactions between the media grains and

the particles to be removed may be unfavorable (Tien, et al., 1979; Choo and Tien, 1995)

The rough surface will also change the hydrodynamics of colloid adsorption/deposition

The smooth surface of the conventional filter media (e.g sand) is another reason for their

being ineffective to the removal of suspended particles and dissolved substances

It is therefore desirable to develop more effective granular media as both adsorption and

filtration materials for high efficient water and wastewater treatment and for

simplification of the treatment system In the present study, the objectives are to develop

granular media with positive surface charges, which would provide attractively

electrostatic surface interactions between the granular media and the dissolved organic

substances or suspended particles to be removed in water or wastewater, thus enhance

their removal in an adsorption or filtration process Granular media with positive surface

charges are obtained through modifying the surface of inorganic and organic granules

with polypyrrole (PPy) or chitosan The modified granules are then used to study the

performance and mechanisms in removing organic pollutant (use humic acid as a model

organic compound) and inorganic colloid (use clay particle as a model colloid) Attempts

are also made to understand the role of surface interactions in the removal of dissolved

and suspended substances in a granular media adsorption or filtration system

This thesis is organized as follows Chapter 2 presents background information on

granular adsorption or filtration in water and wastewater treatment, and the typical

pollutants in water system, and an overview of PPy and chitosan that were used in the

present study The experimental parts of coating PPy on the surfaces of glass beads and

nylon 6,6 granules and immobilizing chitosan on the surfaces of PET and nylon 6,6

granules, as well as the adsorption and filtration experiments, are discussed in Chapter 3 –

Chapter 6 Chapter 7 discusses the qualitative and semiquantitative analyses of

electrochemical properties of PPy-water interface and presents a site-binding model for

humic acid adsorption onto PPy surface

Chapter 2 Literature Review

2.1 Surface Charges and Electrical Double Layer

Most solid surfaces will be charged when they are brought into contact with aqueous

solutions The possible charging mechanisms include (1) ionization of the surface groups,

(2) specific adsorption of ions or surfactants from the solutions, and (3) lattice

imperfections at the solid surface or isomorphic substitution in the crystal lattice (Stumm,

1992; Koopal, 1993; Myers, 1999) Although the presence or absence of surface charge

may often be neglected in the macroscopic systems, in the microscopic systems of

colloids and interfaces, the surface charge is a critical factor and plays an important role in

many applications, such as coagulation, flocculation, adsorption and filtration

The overall arrangement of the electric charge on the solid surface, together with the

balancing charge in the bulk solution, is often referred to as the electrical double layers

(EDL), or just double layers (Bockris and Reddy, 1970) The EDL can be regarded as

consisting of two regions: an inner region which may include adsorbed ions (i.e the

adsorption layer), and a diffuse region in which ions are distributed according to the

influence of electrical forces and random thermal motion (i.e the diffusion layer)

The Gouy-Chapman theory (Figure 2.1a) is the one-dimensional analysis of the diffusive

double layer based on Poisson’s equation and Boltzmann equation, and gives (Hunter,

1991)

ze ze

kT dx

d

2sinh

ψ =−

(2.1)

where ψ is the electrical double layer potential at a distance x from the surface, k is the

Boltzman constant, T is the absolute temperature, z is the valency of the counterions, e is

the coulombic charge, and κ is the reciprocal of the electrical double layer thickness

which is defined as

kT

z c e

N A i i

ε

κ = 1000 2∑ 2

(2.2)

where N A is the Avogadro’s number, c i is the molar concentration of the counterions of

type i, z i is the valency of counterion i, ε is the solution permittivity (equal to ε 0 ε r , ε 0

being the permitivity of vacuum and ε r the relative permittivity of the liquid)

The total charge σ d, per unit area of surface, in the diffuse layer can be calculated from

Eq (2.1) as (Hunter, 1991)

kT

ze ze

d

2sinh

where ψ d is the electrical potential at the onset of the diffuse layer

To improve the Gouy-Chapman model, Stern proposed to divide the double layer into two

parts separated by a plane (the Stern plane) located at about a hydrated ion radius from the

surface (Figure 2.1b) Specific ion adsorption may take place at the Stern plane when

electrostatic and/or van der Waals forces are strongly enough to overcome the thermal

agitation Across the Stern plane, the potential drops from the surface potential ψ s to the

potential at the Stern plane, ψ d , and further decays from ψ d to zero in the diffusion layer

The potential drop (ψ s - ψ d ) is related to the capacity of Stern layer, C 1, as (Koopal, 1993)

1

C

s d s

σψ

Stern plane Surface of shear

ψ d can be estimated from electrokinetic measurements Electrokinetic behavior depends

on the potential at the surface of shear between the charged surface and the electrolyte

solution This potential is called the electrokinetic or ζ (zeta) potential The concept of the

surface of shear implies some idealization and the exact location of the shear plane is an

unknown feature of the electrical double layer (Shaw, 1970; Kohler, 1993; Kosmulski,

1995) Usually the shear plane is supposed be located at a small distance further out from

the stern plane In general, ζ potential may not be too different from ψ d (usually a little

smaller in magnitude than ψ d ), but ψ d can often be considerably smaller than ψ s It is

customary to assume identity of ψ d and ζ potential for low values of the ionic strength

(Koopal, 1993; Kosmulski, 1995)

2.2 Surface Interactions

2.2.1 Short-Range Forces and Long-Range Forces

Surface interaction forces can generally be subdivided into two types: short-range forces

and long-range forces The action distance of short-range forces is usually no more than

0.1-0.2 nm (Garbassi et al., 1998) The typical short-range force is covalent force When

two atoms bind to form a non-ionized molecule, the force involved in bond formation is

referred to as covalent force, and the resulting bond is covalent bond Covalent bond has

certain characteristic bond length and bond angle which depends on the atoms involved

Hence covalent force is directional

Other short-range forces include hydrogen-bonding interactions and Lewis acid-base

interactions Hydrogen bond can be formed between a proton covalently bonded to a

highly electronegative atom (e.g O, N, F) and another electronegative atom nearby The

hydrogen bond energy depends, in a rather complex way, on the distance between the

participating atoms and the angle between the atoms Fowkes (1985) defined the

interactions between electron acceptors (Lewis acids) and electron donors (Lewis bases)

as Lewis acid-base interactions The interaction strength of a basic or acidic site depends

not only on the ability to donate or accept electrons, but also on the polarizability Van

Oss (1994) classified hydrogen-bonding interactions and Lewis acid-base interactions as

non-covalent interactions, whilst other researchers (Drago et al., 1977) suggested that

these interactions have, at least partly, a covalent character

As far as surface and colloidal phenomena are concerned, one often only considers the

long-range forces or physical interactions which act between discrete, non-bonded atoms

or molecules over distances significantly greater than molecular bond dimensions and are

generally nondirectional The two fundamental long-range forces include two kinds:

coulombic or electrostatic interactions, and van der Waals forces

For two point charges, Q 1 and Q 2 , the free energy of electrostatic interaction, w el, may be

given by (Myers, 1999)

r

e z z r

Q Q

w el

επεε

2 2 1 0

2 1

4

where r is the distance between the two charge, and z is the valency of each ion For two

charges of the same sign, w el is positive, which means that the interaction is repulsive In

contrast, w el is attractive for two unlike charges

Forces between molecules caused by permanent and induced dipoles and other multipoles

are collectively known as van der Waals forces The fluctuating dipole-induced dipole

interactions, described by London, is called London-van der Waals (dispersion)

interactions, which often makes the most important contribution to the total van der Waals

interactions, especially in the aqueous media containing electrolytes (Chaudhury, 1984)

Hamaker (1937) calculated the London-van der Waals force between individual atoms of

spherical particles His treatment was rather coarse, but the concept of a Hamaker

constant has still been used A more accurate calculation was developed by Lifshitz and

co-workers (Dzyaloshinskii et al., 1961), who described the van der Waals force as

originating from spontaneous electromagnetic fluctuations at interfaces

The London-van der Waals forces are characterized as being universal and almost always

attractive over relatively long distances The simplest situation in analyzing London-van

der Waals force is of two hard, flat, and infinite surfaces separated by a distance, d, in a

vacuum The free energy of attraction per unit area, w vdw, in such a case is given by

where A is the Hamaker constant, which depends on the dielectric properties of the two

interacting plates and the intervening medium and typically amounts to about 10-20 J

Although short-range forces are regarded as inferior factors to long-range physical forces

in the systems of colloids and surfaces, short-range forces may have indirect effects on

interactions such as hydrogen bonds or chemical bonds between the surface and the

solvent molecules, and the solvation layer influences the long-range forces between two

approaching surfaces greatly Moreover, adsorbed macromolecule layer strongly affects

the interaction between surfaces (Fleer et al., 1993a; Claesson, 1998)

2.2.2 DLVO Theory

The combined action of van der Waals forces and electrostatic or electrical double layer

forces in aqueous systems is described by the DLVO theory, which was developed by two

groups of researchers, Derjaguin and Landau, and Venwey and Overbeek, independently

in the late 1940s (Shaw, 1970) For two symmetrically charged plates in an electrolyte

solution, the total free energy of interactions in the DLVO theory is expressed by the sum

of the van der Waals and electrostatic contributions (Kohler, 1993; Garbassi et al., 1998):

2 2

0

12

)]

4[tanh(

64

d

A e

kT

e kT

c w

where c 0 is the bulk concentration of the electrolyte solution, and ψ s is the surface

potential

In Eq (2.8), the term of van der Waals forces tends to infinity as separation distance, d,

approaches to zero, while the electrostatic term remains finite Therefore, the van der

Waals attractive interactions always prevail at small distances On the other hand, the van

der Waals’ term is insensitive to the change of ionic concentration or solution pH, which

greatly affects the electrostatic term For a given colloid suspension system, van der

Waals attraction outweighs electrostatic repulsion at small separations, while at

prevents the colloid surfaces from contacting each other As the ionic concentration

increases, the thickness of the double layers, κ -1, decreases (see Eq (2.2)), and thus the

repulsion between the colloid surfaces are reduced (decrease of e -κd outweighs the

increase of c 0 /κ) Hence, increasing ionic concentration favors flocculation and

coagulation of colloids

DLVO theory represents the pillar of colloid science Ninham (1999) asserted that DLVO

stands at the same level of importance for colloid science as does Darwin’s theory of the

origin of species in biology However, DLVO theory failed at short distance from the

surface (Sposito, 1984) Non-DLVO forces, or so-called “extra-DLVO” forces (Pashley,

1981; Israelachvili, 1992), such as solvation, hydration, hydrophobic, oscillator, capillary

and water structure forces may become operative at separations below 5 nm Moreover,

the agreement between DLVO theory and experimental measurement is less satisfactory

for biological regime or for any system where the ionic concentration is in the order of 0.1

M or higher Boström et al (2001a, 2001b) pointed out that this might be, in part, due to

the reason that the dispersion forces of specific ion effects are ignored

Despite these limitations, the classical DLVO theory has appeared to work reasonably

well at separations of intermediate distance and for low ionic concentrations (<5×10-2 M)

A recent review on the application of DLVO theory for colloid adsorption and deposition

at solid/liquid interfaces was given by Adamczyk and Weroński (1999) It was

demonstrated in this review that the electrostatic interactions played the most important

role in both adsorption (especially at the initial adsorption stages) and deposition of

colloid particles

2.3 Adsorption and Granular Filtration

Separation processes like adsorption and filtration refer to the operations that transform a

mixture of substances into two or more products (King, 1980)

2.3.1 Adsorption

2.3.1.1 Definition and Applications

Adsorption is a term to describe the existence of a solute concentration (dissolved

substance) at the interface between a fluid and a solid higher than that present in the fluid

(Masschelein, 1992) Generally, adsorption is classified as physical adsorption (i.e

physisorption) in which the van der Waals interactions are involved and chemical

adsorption (i.e chemisorption) in which the adsorbed molecules are attached by chemical

bonding Adsorption is of great technological importance in separation process, industrial

catalysis and pollution control Adsorption phenomena also play a vital role in many

solid-state reactions and biological mechanisms (Rouquerol et al., 1999) For colloid and

surface science, adsorption affects the surface charge of suspended particles and colloids,

and so influences their aggregation and transport, which is important in water and soil

science (Stumm, 1992)

As a separation process, most of the adsorption-based processes have a single fluid feed

stream (liquid or gas) containing one or more species to be removed, or so-called

adsorbates The separating agent is the adsorbent pellets or powder, i.e adsorbents, used

in the process The products included the fluid steam with the adsorbate species removed

or depleted and the adsorbents saturated with the adsorbates initially present in the feed

steam (King, 1980; Tien, 1994)

One of the earliest adsorption applications is purification, such as the removal of H2S and

obnoxious fumes from air and the removal of organic compounds from liquid water (Al

Duri, 1996) Adsorption has also been applied to dyestuffs removal from textile industry,

removal of odor and color from edible oils, decolorization in the sugar industry, and

removal of unwanted hydrocarbons in oil refining (Pollard, 1993; Nogueira, 1996;

Ahmedna et al., 2000) Some biological materials and precious metals, e.g gold, are also

recovered by adsorption (Nakajima and Sakaguchi, 1993)

Adsorption has been demonstrated to be an especially efficient and economically feasible

unit process in water and wastewater treatment Adsorption by granular activated carbon

(GAC) is a widely used water and wastewater treatment operation to remove both natural

and man-made micro-pollutants such as natural organic matters (NOM), pesticides,

industrial chemicals, tastes and odors and algal toxins (Newcombe, 1999; Shawwa et al.,

2001) The aluminum hydroxide and ferric hydroxide solids formed during coagulation

process may adsorb organic compounds (Julien et al., 1994) or remove microorganisms

(Bitton, 1994) Adsorption processes for high-level treatment of metal-bearing tap water

or wastewater using low-coast adsorbents such as bark (Martin-Dupont et al., 2002),

lignin (Lalvani et al., 1997), fungus (Kapoor and Viraraghavan, 1997), algae (Hamdy,

2000), and chitosan (Guibal et al., 1998; Inoue et al., 1999) are becoming increasingly

attractive in recent years

2.3.1.2 Adsorption Isotherms

Adsorption is often described in terms of isotherms which show the relationship between

the bulk concentration of adsorbate and the adsorbed amount at constant temperature

There are a few equations or models that are available to describe the adsorption

isotherms Yu and Neretnieks (1990) thoroughly reviewed model isotherms for

single-component adsorption Only two of the more common equations, the Langmuir and

Freundlich equations, are presented here because of their simplicity and wide utility

Adsorption of molecules can be represented as a chemical reaction:

A

where A represents the adsorbate, B the adsorbent, and A·B the adsorbed compounds

Adsorbates are attached to the surface of the adsorbent by various types of chemical

forces such as hydrogen bonds, dipole-dipole interactions, as well as van der Waals

forces Based on the assumptions that (1) adsorption is restricted to monolayer coverage,

(2) adsorption is localized, and (3) the heat of adsorption is independent of the amount of

adsorbate adsorbed, Langmuir isotherm equation can be obtained by applying the

equilibrium equation and mass law to Eq (2.9) as:

C K

C K q

q

a

a m

+

=

where q is the amount of adsorbate adsorbed per unit weight of adsorbent at equilibrium

concentration, C is the equilibrium or final concentration of the adsorbate in the solution,

q m is the maximum adsorption at monolayer coverage, and K a is the adsorption

equilibrium constant The values of q m and K a can be determined from a plot of 1/q versus

1/C in accordance with a linearized form of Eq (2.10):

C q K q

11

Like many classic approaches such as DLVO theory, Langmuir equation has its

fundamental weaknesses The assumptions of Langmuir equation mentioned above are

almost never met in practice However, Langmuir equation has been found to be widely

applicable in many systems when such conformity doses not imply the requirement of the

Langmuir assumptions Langmuir model accurately describes a significant amount of

adsorption data in a mathematically simple method, and this makes it invaluable as a basis

for adsorption studies (Mayers, 1999)

The Freundlich equation is another classic adsorption isotherm that is very useful at

describing adsorption This equation is an empirical equation which has the form

n

f C K

intensity When 1/n < 1, the adsorption is said to be "favorable", and 1/n > 1 is called

"unfavorable" Although the Freundlich equation was developed empirically, it can also

be derived theoretically using a model in which it is assumed that the heat of adsorption is

not constant but varies exponentially with the extent of surface coverage (Halsey and

Taylor, 1947; Halsey, 1948), which is more reasonable than the Langmuir assumption in

most cases

The Freundlich equation applies very well for solids with heterogeneous surface

properties However, this equation cannot be applied to all values of C As C increases, q

increases (according to Eq (2.12)), only until the adsorbents reach saturation At

saturation, q is a constant and is independent of further increase in C, thus the Freundlich

equation no longer applies In addition, there is no assurance that adsorption data will be

in a good agreement with the Freundlich equation over all concentrations less than

saturation

2.3.2 Granular Filtration

2.3.2.1 Definition and Applications

Granular filtration is a fluid-solid separation process commonly applied to remove minute

quantities of small particles (liquid or gas suspensions) from various kinds of fluid by

passing through granular media (Aitken, 1969; Tien, 1989) As the fluid or suspension is

forced through the voids or pores of the granular media, the solid particles are retained on

the medium’s surface or, in some cases, on the walls of the pores, while the fluid (i.e

filtrate) passes through (Cheremisinoff, 1998)

Both Sanskrit medical lore and Egyptian inscriptions give clear evidence that granular

filtration was used for water treatment as early as 200 B.C The versatility of granular

filtration is evident from its scope of application as well as from the manner in which it is

carried out Besides water or air, systems that may be treated by granular filtration include

such diverse substances as flue gas, molten metal, petrochemical feedstocks, polymers,

and alcoholic or nonalcoholic beverages Although granular filtration is frequently carried

out in the fixed-bed mode, it may also be conducted in a moving-bed or fluidized-bed

mode so that the operation is continuous

The most commonly known application of granular filtration is for water and wastewater

treatment to remove solids, including bacteria present in surface waters, suspended clay

particles, precipitated hardness from lime-softened waters, and precipitated iron and

manganese (Uchrin, 1983) It has been reported that various forms of sand filtration have

been used to purify water for centuries (Montiel et al., 1988) Slow sand filtration was

first developed to purify surface water for drinking purposes Since the mid-19th century,

slow sand filtration has been widely employed in treating community water supplies in

many countries against water-borne disease A significant improvement to water

treatment in the 1880s and 1890s was the development of rapid sand filters, which could

handle considerably larger volumes of water

At present, granular filtration of water and wastewater is inevitably applied in conjunction

with sedimentation and/or coagulation Granular filtration is widely used to remove

residual biological floc in settled effluents from secondary treatment by trickling filters or

activated sludge process, to remove residual chemical-biological floc after alum, iron, or

lime precipitation of phosphates in secondary settling tanks of biological treatment

processes, and to remove solids remaining after the chemical coagulation of wastewaters

in tertiary or independent physical-chemical waste treatment (Cheremisinoff, 1995)

2.3.2.2 Filtration Theory

Although granular filtration is one of the most widely used processes in a variety of

applications, the design and operation of filters is still carried out on an almost entirely

empirical basis The reason for this is that a filter itself may be a relatively simple device,

but the process of filtration is quite dynamic and extremely complex In general, filtration

process involves two sequential steps, i.e transportation and attachment (O’Melia, 1985)

Particles in the suspension to be filtered are first transported from the bulk of the fluid to

the vicinity of the stationary surface of the filter media by physical forces Then

attachment of the particles to the collectors (filter media grains) occurs through various

physical and chemical interactions (Amirtharajah and Westein, 1980) The major surface

interaction forces between the collector grains and the suspended or colloidal particles

include the London-van der Waals force and the double layer force The former is

attractive and remains essentially constant during particle deposition The double layer

force can be either attractive or repulsive and may change during particle deposition (Bai

and Tien, 1997; 2000a) While much progress has been made in the area of filtration

modeling, no generally applicable models for the process have been developed yet

Until about 1970, most of the modeling work in filtration had followed what is known as

the phenomenological approach In its simplest form, the process is described by two

equations (Iwasaki, 1937)

c Z

∂

∂

Z

c u

t

σ

where c is the concentration of particles, Z the depth of filter bed, λ the filter coefficient, σ

the (absolute) specific deposit and u the approach velocity Eq (2.14) basically describes

the rate of removal and Eq (2.15) is a mass balance equation The solution of Eqs (2.14)

and (2.15) will depend on the filter coefficient which is usually taken to be a function of σ

Various attempts have been made to find a correlation between the initial filter coefficient

λ 0 and the various system variables such as media grain size, approach velocity, etc., and

to determine a relationship between λ and σ From about 1970 onwards, the trajectory

analysis approach has been used extensively for the study of λ 0 and received much success

(Yao et al., 1971; Tien et al., 1979) Predicting the development of removal (λ against σ)

is however a much harder task O’Melia and Ali (1978) recognized that deposited

particles can act as additional collectors, and thus enhance particle removal in a filter

(increase in removal efficiency or so-called ripening) Chang and Tien (1985) developed a

dendrite model to quantify the increase in removal efficiency due to the presence of

deposited particles As deposition progresses, the process becomes increasingly complex

Some pores in the filter may be blocked off, and therefore become unavailable for

deposition (Tien et al., 1979) The increase of interstitial velocity in the filter pores can

also result in detachment of particles that deposited previously (Bai and Tien, 1999) In

addition, Choo and Tien (1995) have considered that the deposit may be porous for the

flow, whereas most others have assumed that the deposit layer is impermeable

More recently, Bai and Tien (2000b) have proposed the rate expressions that distinguish

different types of deposition in filtration They assume that the media grain surface may

be divided into two parts: one covered with deposited particles, and the other without

particles The nature of surface interactions between the particles and either of those two

parts of grain surfaces are therefore characterized as particle-particle and collector-particle

types The overall filter coefficient is given as

where (λ)p-p is the filter coefficient between grains covered with deposited particles and

particles to be collected, (λ)c-p the filter coefficient between clean grains and particles to

be collected, and f is the fraction of the grain surface covered with deposited particles In

many cases, (λ)p-p and (λ)c-p can be expected not to vary with time since the condition

determining the surface interactions remains the same Bai and Tien (2000b) also divide

the deposition process into two types: deposition on grains and deposition on previously

deposited particles The first type is named as the monolayer deposition while the latter as

multilayer deposition They demonstrated that Eq (2.16), although simple, together with

Eq (2.14) and (2.15), is quite versatile to describe the various filtration behaviors

2.4 Granular Media for Adsorption and Filtration

2.4.1 Conventional Granular Adsorbents and Filter Media

Adsorbents and filter media represent the hearts of adsorption and filtration devices All

practical adsorbents have large specific surface areas and are therefore highly porous or

composed of fine particles The old types of industrial adsorbents (e.g activated carbons

and silica gels) are generally non-crystalline and their surface and pore structure therefore

tend to be ill-defined and difficult to characterize Many new adsorbents possessing

intracrystalline pore structures have been developed over the past 20 years, including

carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model

mesoporous solids (Rouquerol et al., 1999) In addition, various modern spectroscopic

and microscopic techniques can now be employed for studying the surface state and the

microstructure of the adsorbents (Dobiás et al., 1999) Although several materials may be

applied in the adsorption process for water and wastewater treatment, including alumina,

silica gel, Fuller’s earth and diatomaceous earth, granular activated carbon (GAC) has

been by far the most widely used adsorbent which provides tertiary treatment for water

contaminated with organic matters

By the structure of materials, filter media may be classified into two types: flexible and

rigid media (Cheremisinoff, 1995) Rigid filter media are commonly used for granular

filtration Ceramic filter media for example are widely used in gas filtration, and in

separation of dust and liquid droplets from gases (Loff, 1981) The relatively uniform

particle size of diatomaceous media achieves high efficiency of filtration in retaining solid

particles of sizes less than 1 µm, as well as certain types of bacteria Plastic granules (e.g

polyvinyl chloride and nylon) have gained growing attention as filter media in recent

years because of their relatively low cost (Driscoll, 1977; Loff, 1981) In water and

wastewater treatment, the most commonly used filter media include silica sand (specific

gravity =2.65), garnet sand (specific gravity = 4.0-4.4) and anthracite coal (specific

gravity = 1.35-1.75)

The effectiveness of granular adsorption and filtration processes largely relies on the

surface interactions between the substances to be removed and the granular media As

discussed early, DLVO forces (London-van der Waals force and electrostatic double layer

force) play the most important role in colloid adsorption and deposition processes In

general, the London-van der Waals force is always attractive, but the double layer force

can be either attractive or repulsive Hence, the surface charges of impurities or pollutants

in water systems and those on the granular media have an impact on the efficiency of

water and wastewater treatment In face, most impurities or pollutants in waters carry

negative surface charges in the pH range of natural waters (Ives, 1990; Chen et al., 1998)

On the other hand, the activated carbon, sand and anthracite that are used to remove these

impurities or pollutants also carry negative surface charges in that pH range Therefore,

the conventional filter media or adsorbents are not particularly efficient to remove

suspended particles, nor are they effective to remove dissolved organic and inorganic

substances This problem may be solved by surface modification of the traditional

granular media to obtain the desired surface properties, i.e., positive surface charges

2.4.2 Surface Modification of Granular Media

Several studies have examined the possibility of modifying granular media to improve

their ability to remove dissolved matters and suspended particles These modifications

included impregnation of coal with metallic hydroxides (Chaudhuri and Sattar, 1986;

Lukasik et al., 1999), addition of positive charges to silica using organosilane derivative

(Zerda et al., 1985), incorporation of metallic hydroxides onto the surfaces of sand using

in situ precipitation of metallic hydroxides (Farrah and Preston, 1985; Lukasik et al.,

1996), adsorption of metallic flocs onto the surfaces of sand (Edwards and Benjamin,

1989), and modification of diatomaceous earth by precipitation of metallic peroxides

(Farrah et al, 1988)

Among these developments, coating sand with ferric and aluminum oxide or hydroxide

has received much attention Because the surface of granules became electrically positive,

it facilitated the attachment of the negatively charged particles or other pollutants found in

water and wastewater (Farrah and Preston, 1985) The coated sand was also found to

effectively adsorb microorganisms (Lukasik et al., 1996; Mansoor and Chaudhuri, 1996)

The experimental and theoretical studies by Truesdail et al (1998) demonstrated the

benefit of granule surfaces coated with metal oxides, or hydroxide rich oxide/hydroxide

mixtures in increasing the efficiencies of commercial filtration systems The

electropositive surface coatings from aluminum or mixed (hydr)oxides, had similar

average kinetic rate constants and were five times greater than the rate constants for the

uncoated sand In particular, the adsorption of natural organic matters (NOM) onto iron

oxide coated sand has been actively studied recently A new granular adsorbent based on

β-FeOOH was developed by Teermann and Jekel (1999), and was shown to have high

adsorption capacities for both smaller and higher molecular weight humic substances It

was reported that coating iron oxide particles with cationic polymer significantly

increased adsorption of humic acid at high pH, but had less effect at low pH (Kim and

Walker, 2001) Lai et al (2001; 2002) found that copper ions, lead ions and humic acid

could be removed efficiently from water by the iron oxide coated sand, and the adsorption

of cadmium ions and humic acid onto goethite-coated sand was highly pH-dependent

Although the coatings of metallic hydroxides, oxides, or peroxides on filter media

enhanced the removal of bacterial, viruses and turbidity from water and wastewater, the

coated layer was however subject to dissolution when the coated filter media were placed

in service (Chen, et al., 1998; Ahammed and Chaudhuri, 1996) The effect of continuous

exposure of coated media to water and wastewater treatment therefore has been difficult

to assess

2.5 Impurities and Pollutants in Water and Wastewater

Water is one of the most important requirements of life Without it, neither the individual

nor the organized community can survive Absolutely pure water is never found in nature

When the natural waters contact with their surroundings, they leach and dissolve various

minerals and salts from the components of the earth and rocks Natural waters may also

serve to nurture organisms such as bacteria and viruses The source waters therefore are

always aqueous solutions and suspensions of various compositions (Meltzer, 1993)

Municipal wastewater consists of various pollutants from homes and commercial

establishments, including various suspended and dissolved organic and inorganic

substances, and harmful viruses or bacteria

The impurities and pollutants in water and wastewater may be divided into two groups,

dissolved matters and suspended solids, based on the fact that many of the water and

wastewater treatment processes are only effective against one of them (Henze et al.,

2001) Natural organic matters (NOM) and suspended particles will be discussed in the

following sections because traditional water and wastewater treatment processes are not

able to effectively remove low concentration of these matters completely

2.5.1 Humic Substances

2.5.1.1 Characterization of Humic Substances

Natural organic matters in the environment (soils, sediments and natural waters) can be

divided into two classes of compounds: non-humic materials (e.g protein,

polysaccharides, nucleic acids and small molecules such as sugars and amino acids) and

humic substances (Hayes et al., 1989; Perdue and Gjessing, 1990; Rowell, 1994;

Ziechmann et al., 2000) Humic substances are structurally complex, heterogeneous, and

have a yellow to black appearance They consist mainly of carbon, oxygen, hydrogen and,

sometimes, small amounts of nitrogen, and occasionally phosphorous and sulphur The

amount of humic substances in natural waters can range from less than 2 mg/L up to 40

mg/L, and typically be in the range of 5-15 mg/L (Aiken et al., 1985) Humic substances

result from biological decomposition of natural materials, including forest residues,

grasses, food crops, animal remains and dead microorganisms, with molecular weights

from a few hundreds for the simple ones to the thousands for the large polymeric

materials Humic substances predominantly carry negative charges in liquid solutions

because of the abundance of carboxylic and phenolic groups (Jones and Bryan, 1998)

Due to the hydration of the charged groups and electrostatic repulsion between the

charges, the dissolved humic substances can be distributed in an extended conformation

that adjusts themselves due to changes in the environmental conditions Humic substances

affect water quality adversely in several ways: undesirable color, complexation with

metals to yield high metal concentrations, and reaction with chlorine to produce

trihalomethanes (Ruohomäki et al., 1998) Hence it is desirable to minimize the

concentration of humic substances in drinking water supplies and other process waters

Humic substances may be further divided into three fractions that can be isolated from

soil: humic acids, fulvic acids and humans The main fraction of humic substances is

humic acids and their salts - humates A rich soil with a near neutral pH would contain a

high level of humates Whereas the same soil with a low pH would be replete with humic

acids (Schnitzer and Khan, 1972) The main chemical functions in humic acid include

carboxylic, phenolic, amino and quinone with aromatic nucleuses of low degree of

condensation The presence of aromatic nucleuses with mobile p-type electrons and the

various functional groups cause humic acid to have the ability of ionic exchange, complex

formation and oxidization-reduction reactions At present, the structure of humic acid is

still ill defined despite many decades of research A proposed building block for humic

acid suggested by Jansen et al (1996) is shown in Figure 2.2 The building block contains

carboxylic groups connected to various alkyl chains and aromatic rings, phenolic groups,

quinone structure, aromatic amine groups, seven chiral centers and thus 128

stereoisomers

Figure 2.2 Proposed building block of humic acids

2.5.1.2 Removal of Humic Substances

Humic substances have several characteristics that influence on how they may be

removed from water and wastewater Humic substances carry negative charges, which

makes them often be removed through coagulation Because of the negative charge

property, humic substances are sometimes removed by ion exchange Humic substances

can also be adsorbed on some types of sorbent material, such as activated carbon

(Ødegaard et al., 1999)

Coagulation/Direct Filtration Removal of humic substances by coagulation/direct

filtration process is normally carried out by addition of coagulant, such as aluminum

sulphate, ferric chloride, and subsequent floc separation through direct filtration The task

of the coagulant is to neutralize the negative charges of the humic substances to form

flocs The tiny flocs can then be transported and attached to the surfaces of filter grains

and be separated from water Although coagulation/direct filtration for the removal of

humic substances in water and wastewater treatment has gained a lot of attention in resent

years, low molecular weight humic substances are difficult to coagulate In addition, the

process incurs high chemical operation cost and generates high volume of extra sludge

(from filter washing) that is difficult to handle

Adsorption by Activated Carbon Activated carbon is the sorbent material most widely

applied in drinking water treatment Due to the composition of the raw material and the

production process, between 5% and 20% by weight of activated carbon consists of

elements other than carbon, primarily metals and surface bound oxygen (Sontheimer et

al., 1988) The latter can be present in both acidic and basic surface functional groups as

well as in metal oxides As a result, the hydrophobicity of the ‘clean’ carbon surface is

reduced and the conditions for adsorption of hydrophilic solutes are improved There are

several physical and chemical parameters that affect the adsorption of humic substances

Summers and Roberts (1986) showed that a positively charged carbon surface is favorable

for the binding of the negatively charged humic substances The ultimate carbon capacity,

however, is more strongly influenced by the effects of pore structure and size Lee et al

(1981) found that the molecular weight distribution of humic substances in relation to the

pore size distribution of activated carbon is of great importance Humic substances with

larger molecular weight are excluded from the smaller pores of activated carbon, and thus

have a lower volume available for adsorption On the other hand, although humic

substance fractions with smaller molecular weight generally adsorb to a great extent, they

have larger polarity than larger molecules, thus decreasing the adsorption on the

hydrophobic activated carbon surface These two factors, as well as the high cost of

regeneration of activated carbon, limited the application of activated carbon to the

removal of humic substances from water and wastewater

Anion Exchange Resin Since a large fraction of humic substances can be characterized as

anionic polyeletrolytes, macroporous anion exchange resins in principle can be used for

humic substance removal Specifically, strong base resins have been found to be superior

to weak-base resins with respect to capacity (Brattebф et al., 1987) Moreover, a resin

skeleton which allows a high degree of swelling is more favorable for humic substances

removal than a rigid skeleton (Fu and Symons, 1990) These studies concluded that ion

exchange was the dominant mechanism for humic substance removal and surface

adsorption only contributed to the removal of humic substances with smaller molecular

weight The break-through curves of humic substances in ion exchange columns are not

practically favorable after a short time (Ødegaard et al., 1989), resulting in relatively poor

utilization of the capacity of the resin and thus requiring frequent regeneration The waste

regeneration solution has a disposal problem due to its high pH (12.5-14) and salt

concentrations (conductivity 10000-20000 mS/m) (Ødegaard et al., 1999)

2.5.2 Suspended Particles in Water and Wastewater

2.5.2.1 Sources of Suspended Particles

By convention, suspended particles are defined as the particles which have a diameter

larger than 0.4 µm (Eisma, 1993) Modern technologies allow particles as small as

0.02 µm to be observed, but these particles have often been considered as ‘dissolved’ The

upper size limit of suspended particles is difficult to define, because small but heavy

particles can rapidly undergo sedimentation, while large but low-density particles can stay

in suspension for a long time Suspended particles are present in almost all natural waters

and wastewater, even although sometimes they are in very limited amounts The universe

presence and the complex physical and chemical properties of the suspended particles

have made their removal a subject of significant scientific and engineering importance

Suspended particles present in natural waters include inorganic materials like clay, silt,

sand, silica and calcium carbonate, or organic remnants of plant and other substances like

fats, greases, microorganisms, oils, etc, entrained from the surface by rain water or thawed

snow and carried into basins, rivers and lakes (Sastry and Agamuthu, 1996)

Microorganisms and organic growths are common in natural surface and river waters,

including normal soil bacteria, iron or manganese bacteria, algae and crustacea

Suspended particles in municipal wastewater include organic materials such as dead

animal matter, plant tissue, organisms, synthetic (artificial) organic compounds, and

inorganic solids which may include a number of potentially toxic elements such as

arsenic, cadmium, chromium, copper, lead, mercury, zinc, etc Pathogenic viruses,

bacteria, protozoa and helminths may be suspended in raw municipal wastewater and will

survive in the environment for long periods The presence of them may cause human

infections and be an important public health problem (Payment et al., 1991)

The fine suspended mineral particles in the aquatic environment are mostly clays It is

difficult to remove clay particles from suspension because their negative charges repel

each another, and, as a result, clay particles do not agglomerate to sizes large enough to be

responsive to the sedimentation induced by gravitational forces (Meltzer, 1993) Neither

is sand filtration particularly efficient to remove fine clay particles because sand also

carries negative surface charges in water systems Furthermore, it is generally know that

clay particles are highly surface-active, and easily adsorb dissolved or colloidal materials

onto their surfaces (Eisma, 1993) It has also been suggested that bacterial surfaces are not

homogeneous with regard to charge distribution Bacteria therefore have electrostatic and

other interactions with clays in aquatic systems and tend to attach on the clays’ surface

(Beveridge et al., 1993) Clay-bacterial composites are common to most aquatic

environments (Beveridge, 1989)

Even though most suspended particles may not be hazardous, their presence can cause

color, odor or taste, making water aesthetically unpleasant for drinking, and unsuitable for

many industrial applications Suspended particles in the water can also interact with the

disinfection processes in water and wastewater treatment, making them less effective

2.5.2.2 Removal of Suspended Particles by Filtration

Filtration with granular media is most commonly used for suspended particle removal in

water and wastewater treatment Media commonly used in water and wastewater filtration

include sand and anthracite Slow sand filters have been effective in providing a safe,

potable water supply However, there are limitations in the use of this type of treatment

Generally, the average turbidity is limited to 10 ppm, with a maximum concentration of

30 ppm (Cheremisinoff, 1995) Within these limits, effective bacterial and turbidity

removals are realized Rapid sand filters may be used for treatment of waters with

high-suspended solids content when preceded by a preliminary solids separation device, e.g

coagulation and sedimentation Diatomaceous earth filtration units were found to be

effective during World War II in the prevention of amoebic dysentery by the removal of

Endomebeia histolytica cysts (Fulton, 2000)

The performance of filtration depends on several factors including particle size and

physicochemical properties of the particles and the filer media Larger particles may be

filtered by entrapment mechanism As the sizes of particles decrease, particle removal is

usually more difficult and the interactions between the particles and the collectors (filter

media) will play the most important role Onorato and Tien (1980) have found that

favorable interactions between particles and the collector can significantly increase

particle deposition They reported a 10-fold increase in the deposition of negatively

charged particles onto positively charged collectors, as compared to negatively charged

collectors A notable increase in colloid deposition rates was observed by Elimelech

(1991; 1994) when positively charged latex particles and negatively charged glass beads

was used, and the enhancement of deposition was attributed to the attractive double layer

interactions In membrane filtration, using a cationic surfactant to create a positively

charged surface was also found to increase the removal of negatively charged particles by

10% to 95% (Kang and Shah, 1997)

2.6 Polypyrrole and Chitosan

A conducting polymer, polypyrrole (PPy), and a natural biopolymer, chitosan, were

chosen to modify the surface of conventional granular media in this study because they

possess activated sites (positively charged sites) under the neutral or weak acidic solution

pH conditions

2.6.1 Polypyrrole (PPy)

2.6.1.1 Properties and Applications

During the past two decades, intrinsic conducting polymers (ICPs) with conjugated

double bonds have attracted a great deal of attention as advanced materials capable of

simultaneously presenting the properties of organic polymers and semiconductors In spite

of the active and extensive research spent on ICPs, the ‘applications’ usually still reside in

the laboratory rooms One of the ICPs that actually has had a number of commercial

applications is PPy, due to its good environmental stability, facile synthesis, possibility of

forming homopolymers or composites, and higher conductivity than many other

conducting polymers (Rodríguez et al., 1997; van Hutten and Hadziioannou, 1997; Wang

et al., 2001)

PPy displays many interesting properties: redox activity (Deronzier and Moutet, 1996),

high surface free energy (Chehimi et al., 1999) and polarity (Liu et al., 1994), good ion

and proton exchange capacity (Ge et al., 1992), Lewis acid-base interactions (Chehimi et

al., 1993), electro-chromatographic stationary phase properties (Deinhammer, 1991),

exceptional gas transport properties (Liang and Martin, 1991), good optical properties (De

Paoli et al., 1990), strong adsorptive capabilities towards gases and macromolecules

(Chehimi, 1999; Azioune et al., 1999) Most of these properties are highly depending on

the nature of the counteranion, acid/base treatment, synthesis condition and procedure,

handling and history of the polymer

Due to its unique attractive properties, PPy has been used to prepare high performance

materials such as biosensors (Parthasarathy and Martin, 1994), gas sensors (Kincal et al.,

1998), microactuators (Smela, 1999), and pH sensing systems (Talaie, 1997) The

capability of PPy to be reversibly doped and dedoped by electrochemical methods makes

it a suitable candidate for the construction of rechargeable batteries (Alper, 1989; Gemeay

et al., 1995) PPy can also be used as functional membranes for liquid separation or gas

separation PPy membranes containing high concentrations of different counterions show

exceptional selectivity of transport of ionic species and gas (Liang and Martin, 1991;

Zhao et al., 1998) Furthermore, PPy membranes can be reversibly transformed between

polyionic and neutral forms, which may switch the transport of ionic species across the

membrane (Kyöstikontturi et al., 1998)

PPy shows good adhesion to both organic and inorganic substrates PPy can be stably

deposited on a metal surface to reduce the rate of degradation process in an aggressive

environment, hence to protect metal from corrosion (Idla et al., 1997; Su and Iroh, 2000)

PPy may be coated on porous membranes (Nikpour et al., 1999) or be combined with

membranes as composites (Morita, 1998) to improve their performance, and to prepare

novel functional membranes Polymer composites and blends of PPy materials have been

widely studied to overcome the relatively poor mechanical properties (Rodríguez et al.,

1997) One of the recent interests in research on PPy materials is the preparation of

nanocomposite of PPy PPy was coated on the surfaces of gold (Selvan et al., 1998) or

ceramics (Cho et al., 2001) using surfactants as templates for the potential application in

the field of rechargeable batteries PPy/PMMA coaxial nanocable with an electrical

conductivity of 1.7 S/cm was prepared using mesoporous silica as a nanoreactor (Jang et

al., 2001) A series of PPy nanocomposites with both electrical and ferromagnetic

behaviors were synthesized for applications in electrical-magnetic shields and

microwave-absorbing materials (Jarjayes et al., 1995; Gangopadhyay and De, 1999; Liu and Wan,

2000)

2.6.1.2 Charge Carriers in PPy

PPy can be easily prepared via chemical or electrochemical polymerization The former

method usually gives powdered PPy and the latter gives filmy PPy To prepare a large

quantity of PPy, the chemical polymerization is the better method because it is free from

the restriction of an electrode shape

PPyFigure 2.3 Reaction mechanism of chemical polymerization of pyrrole

Figure 2.3 schematically shows the process of chemical polymerization of pyrrole A

neutral molecule of pyrrole monomer (a) changes to its cation radical species (b) due to

oxidation, and (b) combines to form a dication of bipyrrole (c), and subsequently, a

neutral bipyrrole molecule (d), which can undergo further oxidation into (e), then

of the chemical polymerization (Malinauskas, 2001)

Coupling of pyrrole monomers during polymerization reaction occurs primarily through

the α-carbon atoms of the heterocyclic ring since these are the positions of highest

unpaired electron π-spin density and hence highest reactivity Ideal PPy therefore has

α,α’-linked ring (poly(2,5-pyrrole)), with 180o dihedral angles so as to form a planar and

linear chain However, two-dimensional microstructures of PPy through β-linkages have

also been detected (Bartl, et al., 1993; Joo et al., 2001)

PPy can have two structures of bond alternation (Kikuchi, et al., 1992; Fermín et al.,

1996), the aromatic configuration with long bonds between the rings and aromatic

structures within the rings, and the quinoid configuration with shortened bonds between

the rings and quinoid rings, as shown in Figure 2.4 The nondegenerate fundamental state

of PPy is characterized by a unique aromatic structure of lower energy than that of the

quinoid structure

H N

H N

H N

H N

H N

N

H N N

H N

H

N H

N H H

N

H N N

H N

H

N H

Figure 2.5 p-Type doping of PPy introducing polarons and bipolarons

The molecular arrangement of quinoid structure of PPy favors the formation of electronic

defects, mainly polarons and bipolarons (Heeger, et al., 1988) When the nondegenerate

ground state of PPy is oxidized, the structural motif of the chain segment between the

cation radical (polaron) and the unpaired electron is that of a quinoid configuration This

polaron structure is higher in energy and confines the charge and spin density to a single

self-localized structural deformation that is mobile along the chain When further

oxidation of a nondegenerate PPy chain takes place, a second electron can be removed

from a different segment to create a new polaron, or the unpaired electron of previously

formed polaron is removed and thus a spinless dication (bipolaron) is produced to confine

a single lattice deformation on the chain (Figure 2.5) The oxidation of PPy is a doping

process during which the electrons are transferred away from the polymer chain and the

condition Two charge transport mechanisms accounting for macroscopic conductivity in

PPy polymer have been proposed (Rodríguez et al., 2000): charge propagation along

one-dimensional conjugated chains, with tunneling across conjugation defects, and inter-chain

charge propagation between localized defects, a three dimensional electron-hopping

mechanism

Because the oxidation potential of PPy is lower than that of the monomer (pyrrole)

(Brédas and Street, 1985), the polymer is simultaneously oxidized during polymerization

and thus charge carriers are generated Hence the PPy prepared by both chemical and

electrochemical polymerization is in the oxidation state and the counteranions present in

the polymerization solution are incorporated into the growing polymer The doping

densities of PPy synthesized by chemical polymerization are usually in the range of 25 to

30% (Kim et al., 1995) Early studies suggested delocalized positively charged nitrogen

atoms or partially charged nitrogen atoms in doped PPy Kang et al (1997) revealed the

simultaneous presence of neutral and positively charged nitrogen atoms in the N1s

core-level XPS spectrum, an agreement between the proportion of positively charged nitrogen

and the counter anions They thus proposed the presence of unit positive charges on the

positively charged nitrogen atoms in doped PPy Malitesta et al (1995) suggested the

polaron and bipolaron structures in doped PPy as NH

+

.and HN

+

, respectively, on the basis of XPS findings

The electrical and structural properties of chemically and electrochemically synthesized