1

Chapter 1

INTRODUCTION

Synthetic organic compounds such as dyes, phenols, pesticides, fertilizers,

detergents, herbicides, surfactants and other chemical products are excessively used

in today‘s life; disposed of directly into the environment, without being treated,

controlled or uncontrolled and without an effective treatment strategy particularly

in developing countries. The toxicity, stability to natural decomposition and

persistence of organic compounds in the environment has been the cause of much

concern to societies and regulation authorities around the world. Therefore it

becomes imperative to develop treatment processes for complete degradation of

these organic compounds before their discharge into the environment.

1.1 Synthetic Dyes; Environmental Concern

Synthetic dyes are extensively used in many fields of upto-date technology, e.g., in

various branches of the textile industry [Sokolowska et al., 1996; Gupta et al., 1992;

Shukla and Gupta, 1992], of the leather tanning industry [Kabadasil et al., 1999;

Tunay et al., 1999], in paper production [Ivanov et al., 1996], in food technology

[Slampova et al., 2001; Bhat and Mathur, 1998], in agricultural research [Cook and

Linden, 1997; Kross et al., 1996], in light-harvesting arrays [Wagner and Lindsey,

1996], in photoelectrochemical cells [Wrobel et al., 2001], and in hair colorings

[Scarpi et al., 1998]. Moreover, synthetic dyes have been employed for the control of

the efficacy of sewage [Sagastume et al., 1997] and wastewater treatment [Orhon et

al., 1999; Hsu and Chiang, 1997], for the determination of specific surface area of

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activated sludge [Sorensen and Wakeman, 1996], for ground water tracing [Field et

al., 1995] etc.

Effluents of a large variety of industries usually contain considerable quantities of

synthetic organic dyes. The exact amount of synthetic organic dyes produced in the

world is unknown, although financial reports estimate their continuous increase in

the worldwide market up to about US$ 11 billion in 2008 with a production of

dyestuffs over 7×105 tons [Guivarch et al., 2003].Due to their large-scale production

and extensive application, synthetic dyes can cause considerable nonaesthetic

pollution and serious health-risk factors. Although the growing impact of

environmental protection on industrial development promotes eco-friendly

technologies [Desphande, 2001], reduced consumption of water and lower output of

wastewater [Kumar and Joseph, 2006; Montero et al., 2000]. The release of synthetic

dyes to the environment and to meet strict legislatory requirements is serious

challenge to environmental scientists [Forgacs et al., 2004; Robinson et al., 2001].

1.1.1 Dyes and their Classification

All aromatic compounds absorb electromagnetic energy but only those that absorb

light with wavelengths in the visible range (~350-700 nm) are colored. Dyes contain

chromophores; delocalized electron systems with conjugated double bonds, and

auxochromes; electron-withdrawing or electron-donating substituents that cause or

intensify the colour of the chromophore by altering the overall energy of the

electron system. Usual chromophores are -C=C-, -C=N-, -C=O, -N=N-, -NO2, quinoid

rings, and the auxochromes are -NH3, -COOH, -SO3H and –OH groups.

The historical development of the synthetic dyestuffs dates back to 1856, when

eighteen year old, W.H. Perkin discovered the synthesis of Mauveine, a basic dye, by

3

accident, while he was engaged in the study of the action of potassium dichromate

on aniline sulphate. The process was successfully converted in laboratory to a large-

scale production, and the application of the dye on silk was demonstrated. These

was followed by the synthesis of numerous dye stuffs and further the development

of organic compounds like β-naphthol H-acid J-acid, Primuline base, various

anthraquinone derivatives etc., lead to discovery of the novel dyes.

Dyes are classified based on two criteria, both used by the Colour Index (Colour

Index International, 2002), which lists all dyes and pigments commercially used for

large scale colouration purposes: - According to their chemical structure or according

to their method of application over the substrate. The Colour Index (C.I.) is published

since 1924 (and revised every three months) by the Society of Dyers and Colourists

and the American Association of Textile Chemists and Colorists. Each different dye is

given a C.I. generic name determined by its application characteristics and its colour.

1.1.1.1 Classification of Dyes According to their Chemical Structure

The chemical structure of dyes are so varied that it is difficult to classify them into

distinct groups. The color index classifies dyes; in some cases a particular dye could

be placed in one or other group. Based on chemical structure or chromophore, 20-30

different groups of dyes can be discerned (Table 1.1).

1.1.1.2 Classification of Dyes According to Application

The method of application of dye depends on the nature of the fiber and dye. Fibers

are of two types: synthetic and natural fibers. Natural fibers are further of two types:

cellulosic vegetable fibers e.g., cotton, linen, flax, hemp and jute; proteinaceous

animal fibers, e.g., wool, silk, leather and fur, which bound to the dye molecule in

different ways (Table 1.2).

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Table 1.1: Classes of Synthetic Dyes According to Chemical Structure [Wesenberg et

al., 2003]

Code Chemical Class code Chemical Class Code Chemical Class

10000 Nitroso 42000 Triarylmethane 53000 Sulphur

10300 Nitro 45000 Xanthene 55000 Lactone

11000 Monoazo 46000 Acridine 56000 Aminoketone

20000 Diazo 47000 Quinoline 57000 Hydroxyketone

30000 Triazo 48000 Methine 58000 Anthraquinone

35000 Polyazo 49000 Thiazole 73000 Indigoid

37000 Azoic 49400 Indamine/Indophenols 74000 Phthalocyanine

40000 Stilbene 50000 Azine 75000 Natural

40800 Carotenoid 51000 Oxazine 76000 Oxidation Base

41000 Diphenylmethane 52000 Thiazine 77000 Inorganic

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Table 1.2: Classification of Dyes According to Application

Dye class

Characteristics Fiber Chemical Class Attachment mechanism

Acid Anionic, highly water soluble, poor wet fastness

Polyamide, wool, silk and modified acryl

azo, anthraquinone or triarylmethane, azine, xanthene, nitro and nitroso compounds

Ionic bond

Basic Cationic, highly soluble

Synthetic fiber (modified polyacryl)

diarymethane, triarylmethane, anthraquinone or azo compounds

Ionic bond

Direct Anionic, highly water soluble, poor wet fastness

Synthetic fibers (acryl, polyester, poly-amide, cellulose acetate)

Diazo, triazo dyes or phthalocyanine, stilbene or oxazine

Vander Waal forces

Disperse

Colloidal dispersion very low water solubility, good wet fastness

Polyester, nylon, acrylic cellulose acetate

small azo/nitro compds (yellow to red), anthraquinones (blue and green) metal complex azo compds (all colors)

Colloidal impregnation and adsorption

Metal-complex (Cr, Co, Cu)

Anionic, low water solubility, good wet fastness

Wool, nylon azo Ionic bond

Reactive

Anionic, highly water soluble, good wet fastness

Cotton, viscose, wool

Azo, metal complex azo, anthraquinone, phthalocyanine

Covalent bond

Sulphur Colloidal after reaction in fiber, insoluble

Cellulose (cotton, viscose)

Polymeric aromatics with heterocyclic S-containing rings

Dye precipitated in situ in fiber

Vat Colloidal after reaction in fiber, insoluble

Cellulose (cotton, viscose)

Anthraquinones or indigoids

Reduced dye impregnate the fabric followed by oxidation

Azoic/ingrain

Colloidal in fiber, insoluble

Cellulose (cotton, viscose)

Naphthol dyes in situ in fiber

The chromophores in ionic and nonionic dyes are mostly azo or anthraquinone

types. The reductive cleavage of azo linkages results in the formation of toxic

amines. Sulphonated azo dyes with their high water solubility make their removal

difficult. Anthraquinone-based dyes are most resistant to degradation due to their

fused aromatic structures and effluent containing such structures retains their colour

for longer time. Basic dyes have a high brilliance and intensity and are highly visible

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at low concentrations. The metal-complexed dyes are mostly based on carcinogenic

chromium [D'Souza, 2008].

1.1.2 Discharge Statistics of Dyes

Synthetic dyes exhibit considerable structural diversity as discussed earlier. Table 1.3

contains a representative listing of dyes grouped according to their chemical

structure.

The chemical classes of dyes employed more frequently on industrial scale are the

azo, anthraquinone, sulphur, indigoid, triphenylmethyl (trityl), and phthalocyanine

derivatives. Azo dyes represent the largest class of organic colorants listed in the

Colour Index (60-70% of the total) and their relative share among reactive, acid and

direct dyes is even higher, it can be expected that they constitute the vast majority

of the dyes discharged by textile-processing industries. Anthraquinone dyes are the

second largest class (~15%), followed by triarylmethanes (~3%) and phthalocyanines

(~2%) of the entries in the Colour Index.

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Table 1.3: Structure of Representative Organic Dyes

Azo Reactive Orange

16 C.I. 17757

S

O

O

N N

OH

NH

O

CH3

SO3Na

NaO3SOH2CH2C

Xanthene Basic Violet 10 C.I. 45170

Thiazine Methylene Blue C.I. 52015

N

S N+ CH3

CH3

N

CH3

CH3 Cl-

Anthraquinone Reactive Blue 4 C.I.61205

O

O NH2

NH NH

N

N

NSO3

Cl

Cl

Triphenylmethane Basic Violet 4 C.I. 42.600

Phthalocyanine Reactive Blue 15 C.I. 74459

Indigo Indigo Carmine C.I. 73015 N

H

NH

O

O

SO 3Na

NaO 3S

Quinoline D & C Yellow 10 C.I. Acid Yellow

3

NH O

O

SO3NaNaO3S

Phenanthrene D & C Green 8 C.I. Acid Green 9

O H

SO 3 NaNaO 3 S

NaO 3 S

O

COOH

N(C 2H 5 )(H 5 C 2 )N

Cl-

+

N(C2H5)2

N(C2H5)2(H5C2)2N

Cl-

+

8

On a global scale, over O.7 million tons of organic synthetic dyes are manufactured

each year mainly for use in textile, leather goods, industrial painting, food, plastics,

cosmetics and consumer electronics sector. Unfortunately, exact data on the

quantity of dyes discharged in the environment are not available. It is assumed that a

loss of 1–2% in production and 1–10% loss in use are a fair estimate. Due to large-

scale production and extensive application and their direct discharge into aquatic

streams, synthetic dyes can cause considerable environmental damage and related

health problems.

Reactive dyes constitute approximately 12% of the worldwide production of the

commercialized synthetic dyes and are extensively used in the textile industry

[Scharmm et al., 1988]. Reactive dyes contain a reactive anchor (e.g. vinylsulfone,

chlorotriazine) that bonds covalently with the fiber during the dyeing process [Pierce

et al., 1994]. Some hydrolyzed reactive dyes, which undergo side-reactions of

nucleophilic addition with water, have little affinity for the fabric in the dyeing

process [Weber and Stickney, 1993] and approximately 10-50% of reactive dyes are

lost to effluents. Table 1.4 presents the percentage of dye lost to the effluent, for

each class of dye.

The level of unexhausted reactive dyes typically remain at 0.06 g/L, but possibly even

as high as 0.6–0.8 g/L in dyehouse effluents [O’Neill et al., 1999] can lead to severe

organic and colour pollution in the water environment.Strict environmental

regulations, water scarcity and sustainable approach have forced the industrial

sector to adopt the practice of recycling and reuse of treated wastewater. Therefore

dye recovery is not an option with reactive dyes but treatment process must lead to

final destruction or disposal of these contaminants.

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Table 1.4: Percentage of Dye Lost to Effluent

Dye class Loss to effluent (%)

Acid 5-20

Basic 0-5

Direct 5-30

Disperse 0-10

Metal-complex (Cr, Co, Cu) 2-10

Reactive 10-50

Sulphur 10-40

Vat 5-20

Azoic/Ingrain 2-3

1.1.3 Dye Intermediates

Apart from the dye classes mentioned above, dye intermediates are aromatic

compounds with low biodegradability which are formed as a result of destruction of

the chromophoric system of the dyes during degradation process. These

intermediates or byproducts formed may be toxic/nontoxic in nature, when

introduced into the aquatic system cause various health hazards thereby increasing

the environmental risks. Moreover the requirement for recycling the decolourized

water are i) it should have lower toxicity than untreated waste water ii) no

intermediate/byproduct formed during degradation process should be present that

negatively affect the dyeing process. Various dyes intermediates found in the

effluents are chloro or bromo derivatives of naphthalene or sulphonated, hydroxy,

amino or nitro aromatic compounds.

1.2 Removal of Synthetic Dyes from Wastewater

A wide range of methods have been developed over last two decades to remove

colour from dye contaminated wastewaters in order to meet environmental

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regulations as well as economic constraints. The conventional technologies currently

used to degrade the colour of the dye-contaminated water include microbiological

decomposition (aerobic and anaerobic), enzymatic decomposition, chemical

precipitation, adsorption on organic supports (biogas waste slurry, orange peel,

waste bananapith etc.); inorganic supports (carbon, coal, flyash, china clay, silica,

alumina, red mud etc.), flocculation, physiochemical methods (adsorptive bubble

separation, electrocoagulation, coagulation etc.) and chemical processes

(chlorination, ozonization). However the complete degradation of these recalcitrant

organic compounds present in wastewater is not possible by these well established

techniques. Some of the advantages and disadvantages of these techniques are

listed in Table 1.5.Some of them could transfer pollutants from the aqueous phase to

a second one, but they do not destroy the pollutant. Others may be selective but

slow to moderate in destruction rate, or rapid but not selective, thus generating

appreciable reaction or energy costs. Other conversion processes can be limited by

economic reasons, effluent characteristics, or tendency to form harmful by-products.

The incapability of conventional wastewater treatment methods to effectively

remove many bio-recalcitrant pollutants leads to explore the new efficient

treatment systems which are effective for the complete degradation and

mineralization of these pollutants. The advanced oxidation processes (AOP’s) have

been emerged as one of the promising technologies for the degradation of the

pollutants. The numbers of recalcitrant molecules degrade into biodegradable

compounds via the formation of highly reactive chemical species.

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Table 1.5: Advantages and Disadvantages of Current Methods of Dye Removal

from Industrial Effluents [Robinson et al., 2001]

Physical/Chemical

methods Advantages Disadvantages

Fentons reagent Effective decolouration of

soluble and insoluble dyes Sludge generation

Ozonation Applied in gaseous state,

no alteration of volume Short half-life (20 min)

Sodium hypochlorite Initiates and accelerates

azo-bond cleavage

Release of aromatic

amines

Cucurbturil Good sorption capacity for

various dyes Expensive

Electrochemical

destruction

Break-down products are

non-hazardous

High electricity

consumption

Activated carbon Good removal of a wide

variety of dyes Very expensive

Peat Good adsorbent due to

cellular structure

Specific surface areas for

adsorption are lower than

activated carbon

Wood chips Good sorption capacity for

acid dyes

Requires long retention

times

Silica gel Effective for basic dye

removal

Side reactions prevent

commercial application

Membrane filtration Removal all dye types Concentrated sludge

production

Ion exchange No adsorbent loss due to

regeneration Not effective for all dyes

Irradiation Effective oxidation at

laboratory scale

Requires high

concentrations of

dissolved oxygen

Electrokinetic coagulation Economically feasible High sludge production

1.3 Advance Oxidation Processes

The water purification research has been growing extensively for the last 25 years.

Special attention paid to the environment by social, political and legislative

international authorities,has resulted in an intensive search for new and more

efficient wastewater treatment technologies. The AOPs are attractive alternative for

the treatment of contaminated ground, surface wastewater containing hardly-

biodegradable anthropogenic substances as well as for the purification and

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disinfection of drinking waters [Andreozzi et al., 1999; Ullmann, 1998; Casero et al.,

1997; Benites et al., 1995; Masten and Davies , 1994; Legrini et al., 1993; Carey,

1990; Miller et al., 1988; Glaze et al., 1987; Hoigne and Bader, 1983 (a),(b); Peyton et

al., 1982]. U.S. Environmental Protection Agency (EPA) has approved the inclusion of

advance oxidation technologies (AOTs) as a best available technology (BAT) to meet

the standard and specification that provide safe and sufficient pollution control of

industrial processes and remediation of contaminated sites. More than 800 organic

molecules including dyes can be degraded by this process [Robert et al., 2002].

Highly reactive hydroxyl radicals (HO˙) are traditionally thought to be the active

species responsible for the destruction of pollutants [Haag and Yao, 1992; Glaze and

Kang, 1989; Glaze et al., 1987; Peyton et al., 1982]. It has a high standard redox

potential of 2.8 V vs. NHE (Normal hydrogen electrode) in acidic media (Table 1.6)

[Titus et al., 2004; Weichgrebe, 1992; Feuerstein, 1982].

Table 1.6: Standard Reduction Potentials of Some Oxidants in Acidic Media

Oxidant

Standard

Reduction

Potential

(V vs. NHE)

Oxidant

Standard

Reduction

Potential

(V vs. NHE)

Fluorine (F2) 3.03 Hypobromous acid (HBrO) 1.59

Hydroxyl radical (HO˙) 2.80 Chlorine dioxide (CIO2) 1.50

Atomic oxygen (O˙) 2.42 Hypochlorous acid (HCIO) 1.49

Ozone (O3) 2.07 Chlorine (Cl2) 1.36

Hydrogen peroxide (H2O2) 1.77 Bromine (Br2) 1.09

Perhydroxyl radical (O-OH) 1.70 Iodine (I2) 0.54

Potassium permanganate

(KMnO4) 1.67

These radicals are able to oxidize almost all organic compounds to carbon dioxide

and water. The formation of carbon dioxide is of great significance in wastewater

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treatment because it is the unequivocal evidence for the total destruction of organic

compounds in water. Advanced oxidation processes can be broadly classified into

homogeneous and heterogeneous processes.

1.3.1 Homogeneous Processes

The applications of homogeneous process (single phase system) to treat

contaminated water involve the use of an oxidant to generate radicals, which attack

the organic pollutants to initiate oxidation. The major oxidants used for the

homogeneous degradation are: Hydrogen peroxide (UV/H2O2), Ozone (UV/O3),

Hydrogen peroxide and Ozone (UV/H2O2/O3) and Photo-Fenton system (Fe3+ / H2O2).

Table 1.7: Homogeneous Processes

Method Key reaction Drawbacks

UV/O3 O3 + hν→ O2 + O(1D)

O (1D) + H2O → 2HO•

• Absorbs λ < 310nm, a lesser

component in solar radiation

UV/H2O2 H2O2 + hν→ 2HO•

• Low molar extinction

coefficient.

• Absorbs λ < 310nm, a lesser

component in solar

radiation.

UV/H2O2/O3 O3 + H2O2+ hν→ O2 + HO• + •OH2 • Absorbs λ < 310nm, a lesser

component in solar radiation

UV/H2O2/Fe

(Photo-

Fenton)

H2O2 + Fe2+→ Fe3+ +•OH + OH−

Fe3+ + H2O+ hν→ Fe2++ •OH + H+

• Process is expensive.

• Sludge disposal problem

formed during the process.

• Continuous supply of feed

chemicals is required.

Many of the AOP’s listed in Table 1.7 utilize the hydrogen peroxide, whose oxidising

strength alone is relatively weak, but the addition of UV light enhances the rate and

strength of oxidation through production of increased amount of hydroxyl radicals.

Hydrogen peroxide may also be used to enhance other AOP’s if added in low

concentration, as the molecule easily splits into two hydroxyl radicals.

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1.3.2 Heterogeneous Processes

Heterogeneous photocatalytic oxidation system lead to the complete mineralization

of organic compounds [Alhakimi et al., 2003; Noorjahan et al., 2003] and even the

non-biodegradable contaminants undergo partial decomposition to biodegradable

intermediates [Arabatizis et al., 2002]. Heterogeneous photocatalysis involves the

degradation of organic pollutants in slurry or immobilized mode using various

semiconductors as photocatalyst in the presence of ultraviolet (UV)/solar light. It has

been demonstrated that the heterogeneous photocatalytic systems are more

efficient than the homogeneous systems [Mansilla et al., 1997].

1.3.3 Basic Principle of Heterogeneous Photocatalysis

The basic principles of heterogeneous photocatalysis can be summarized as follows:

A semiconductor (SC) is characterized by an electronic band structure in which the

highest occupied energy band, called valence band (vb), and the lowest empty band,

called conduction band (cb), are separated by a band gap, i.e. a region of forbidden

energies in a perfect crystal. When a photon of energy higher or equal to the band

gap energy is absorbed by a semiconductor particle, an electron from the vb is

promoted to the cb with simultaneous generation of a hole (h+) in the vb. The e–

cband the h+vb can recombine on the surface or in the bulk of the particle in a few

nanoseconds (and the energy dissipated as heat) or can be trapped in surface states

where they can react with donor (D) or acceptor (A) species adsorbed or close to the

surface of the particle. Thereby, subsequent anodic and cathodic redox reactions can

be initiated (Figure 1.1).

The energy level at the bottom of the ‘cb’ is actually the reduction potential of

photoelectrons and the energy level at the top of the ‘vb’ determines the oxidizing

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ability of photoholes, each value reflecting the ability of the system to promote

reductions and oxidations. The flatband potential, Vfb, locates the energy of both

charge carriers at the SC-electrolyte interface and depends on the nature of the

material and the system equilibria [Serpone, 1997].

Figure 1.1: Simplified Diagram of the Heterogeneous Photocatalytic Process

Heterogeneous photocatalytic system has the major advantages like complete

oxidation of organic pollutants in a few hours even upto the ppb level without the

production of polycyclic products using low cost solar light.

1.4 Light

Light is small portion of electromagnetic waves present in space. The UV-Vis portion

of electromagnetic spectrum occupies an intermediate position having both wave

and particle properties in varying degrees (Figure 1.2).

The UV spectrum is arbitrarily divided into three bands: UV-A (315 to 400 nm), UV-B

(280 to 315nm) and UV-C (100 to 280 nm). Of these bands UV-A and UV-C are

generally used in environmental applications. UV-A radiations are referred to as long

wavelength radiations or black light and UV-C are referred to as short wave

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radiations. Of all the energy coming from that huge reactor, the sun, the earth

receives 1.7 × 1014 kW, meaning 1.5 × 1018 kWh per year. Solar ultraviolet radiation,

as explained above is, only a very small part of the solar spectrum (between 3.5% to

8%). However this ratio may be different for a given location on cloudy and clear

days. Photocatalyst can be activated by sunlight (near UV), thus reducing

significantly operating cost of photocatalytic process.

Figure 1.2: Spectra Classification and Solar Irradiance Spectra (a) At Atmosphere,

(b) At Sea Level

1.5 Photocatalyst

A variety of photocatalysts (generally semiconductor oxides or sulfides of some

metals) which produce excited high energy states of e-/h+ pairs have been employed

for decomposition of organic pollutants and dyes [Sivakumar and Shanthi, 2001;

Vinodgopal et al., 1996; Vinodgopal and Kamat, 1992] Figure 1.3 shows the band gap

of different semiconductors[Serpone, N. 1995, Gratzel, M., 2001].

Among various semiconductor materials tested under similar conditions for the

degradation of organic compounds, titanium dioxide (TiO2) has been demonstrated

17

to be the most active photocatalst. Daneshvar et al., (2004) reported that zinc oxide

(ZnO) is a suitable alternative to TiO2 since its photodegradation mechanism has

been proven to be similar to that of TiO2.

Many other semiconductor particles like cadmium sulphide (CdS) or GaP absorb

large fraction of the solar spectrum but unfortunately these photocatalysts are

degraded during the repeated catalytic cycles involved in the heterogeneous

photocatalysis.

Figure 1.3: Band Gap Positions (Top of Valence Band and Bottom of Conduction

Band) in Various Semiconductors. The Energy Scale is Indicated in Electron Volts

Using either the Vacuum Level (Left) or the Normal Hydrogen Electrode (NHE)

(Right) as a Reference

1.5.1Titanium Dioxide as Photocatalyst

Titanium dioxide’s strong resistance to chemical and photocorrosion, its safety and

low cost and biological harmlessness limit the choice of convenient alternatives

[Pelizzetti, 1995]. Moreover TiO2 is more stable than other photocatalyst in ambient

conditions and can be recycled [Sun and Simrniotis, 2003; Kiriakidou et al., 1999].

Furthermore, TiO2 has a special feature that it can use natural UV due to appropriate

18

energetic separation between its valence and conduction bands which can be

surpassed by the energy content by a solar photon. Therefore, degradation of the

organic pollutants present in wastewater using irradiated TiO2 suspension is the most

promising process. Anatase and rutile are two forms of TiO2 being frequently studied

Rutile form of TiO2 isclaimed as a catalytically inactive or much less active form.

However, Degussa P25, which has both anatase and rutile forms with ratio of

anatase to rutile equal to 3-4/1 is one of the best TiO2 photocatalyst and used

frequently as a benchmark in photocatalysis. Degussa P25 displays a clear UV

absorption, with reflectance spectrum oversetting sharply at 380 nm [Liu et al.,

2006].The XRD pattern of TiO2 (P-25) (Figure 1.4) shows peaks corresponding to

anatase and rutile phases. The XRD peak of crystal plane 101 for anatase appeared at

25.4 (2θ) and crystal plane 110 for rutile at 27.5 (2θ) were selected to determine the

percentage of anatase and rutile phases. TiO2 (P-25) contains on average 85%

anatase and 15% rutile phases.TiO2 (P-25) has very small average particle size (1.25

µm) with large surface area (50.46 m2/g) [Sakthivel et al., 2003]. SEM micrograph

(Figure 1.5) of commercial TiO2 from Degussa consists of agglomerates of particles in

the nanometer size, very difficult to separate from the aqueous solution [Aguado et

al., 2002].

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Figure 1.4: XRD Pattern of TiO2 Degussa P-25

Figure 1.5: SEM Micrographs of TiO2 Degussa P-25

20

It has been shown in numerous studies that there is positive interaction of anatase

and rutile TiO2 particles of Degussa P25, which enhances the electron hole

separation and increases the total photoefficiency [Sun and Smirniotis, 2003].

1.6 Photoreactor

The reactors used for photocatalytic treatment are categorized as follows:

• Fixed-bed photoreactors,

• Slurry batch photoreactors either mechanically or magnetically stirred.

According to various reports, mainly from laboratory scale investigations, slurry type

reactors seem to be more efficient than those based on immobilized catalyst [Bideau

et al., 1995; Chester et al., 1993; Murabayashi et al., 1993; Matthews and McEvoy,

1992]. However for engineering applications, there is an intrinsic drawback to the

first option; the need of a post radiation treatment of particle fluid separation, for

catalyst recycling and for the ultimate goal of obtaining a clean, powder free water.

The first engineering–scale outdoor reactor for solar detoxification was developed by

Sandia National Laboratories (USA) at the end of the eighties. Since then, many

different concepts with a wide variety of designs have been proposed and developed

all over the world, in a continuous effort to improve performance and to reduce the

cost of solar detoxification system. Basically the solar photocatalytic reactors are of

two types:

• Concentrating reactors

• Non-concentrating reactors

Concentrating reactors are based on the collection of only high energy, short wave

length photon to promote photochemical reactions and use direct solar radiations.

Concentrating system has the advantage of a much smaller reactor-tube area, which

21

control and handle the contaminated water to be treated. The use of high-quality

ultraviolet-light transmitting reactors and supported-catalyst devices also seem more

logical, both economically and from an engineering point of view. The main

disadvantages are that the collectors use only direct radiation and have low optical

and quantum efficiencies during cloudy or partly cloudy periods.

Non-concentrating solar collectors can make use of both direct and diffused UV

radiation; their efficiency can be noticeably higher. Non-concentrating reactors are

cheaper as compared to the concentrating reactors. Their maintenance is also easy

and is of low cost due to their simple structure and design. They do not concentrate

radiations, so that efficiency is not reduced by factors associated with concentration

and solar tracking. A disadvantage of the non-concentrating reactors is the

requirement of much larger reactor area. Researchers have proposed a number of

different designs of non-concentrating solar reactors which are Pressurized Flat

plate, Trickle-down flat plate, Tubular, Free-falling film and Shallow solar ponds.

Solar Ponds can be constructed on-site, especially for wastewater treatment. Since

industries already use ponds for biological treatment of waste water, shallow solar

ponds can be used for the front or back end of a combined solar/microbiological

treatment scheme.

1.7 Kinetics

Chemical kinetics describes the studies of both the mechanism and rate of a

chemical reaction [Hill, 1977; Smith, 1970]. Pseudo-first order reaction rate

constants are determined via the same method as first–order reaction rate

constants.

22

The photocatalytic oxidation of most contaminants species via TiO2 can be described

with Langmuir-Hinshelwood kinetics [Hugul et al., 2002; Chang et al., 2000; Serra et

al., 1994; Ku and Hsieh, 1992; Matthews, 1990; Ollis and Truchi, 1990; Turchi and

Ollis, 1990;]. Langmuir–Hinshelwood kinetics utilize both reaction rate constant, k,

and an adsorption equilibrium constant, K, to describe heterogeneous surface

reaction [Hugul et al., 2002].

Langmuir–Hinshelwood mechanism assumes that rate of reaction r is proportional to

the surface coverage of reactants [Herrmann, 2010]. In a bimolecular reaction:

A + B → C + D

rate r varies as: r = kθAθB (1.1)

Each coverage θi varies as: θi = KiCi/(1 + KiCi) (1.2)

where Ki is the adsorption constant (in the dark) and Ci represents either the

concentration in the liquid phase or the partial pressure Pi in the gas

phase.Therefore reaction rate r becomes:

r = kθAθB = ���������

�������������� (1.3)

where k is the true rate constant. Besides the mass of catalyst, reaction rate constant

k exclusively depends on a single parameter, temperature according to the

Arrhenius’ law:

k = k0 e (−Ea/RT), with Ea = true activation energy (1.4)

Similarly, adsorption constants Ki only vary with temperature T according to

vantHoff’s law

Ki = (Ki)0 e (−∆Hi/RT) (1.5)

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Where ∆Hi is the enthalpy of adsorption of reactant i. Therefore, even if the true

photocatalytic rate constant k is independent of T, reaction rate r depends on

temperature because of the two temperature-dependent coverages θA and θB.

Generally, one of the two reactants (for instance B) is either in excess or maintained

as constant for pseudo first order reactions. Therefore, θB =1 or θB = constant, hence

r = kθBθA = k’θA = k’KACA/(1 + KACA) (1.6)

with k’ = kθB = pseudo-true rate constant. There are two limit cases:

(i) C = Cmax =>θA = 1 and thence r = k’ (1.7)

(ii) C << Cmax =>θA = (KA CA)/(1 + KA CA)≈KA CA and thence (1.8)

r ≈ k’·KA CA = kapp CA (1.9)

where kapp = apparent first order rate constant.

When the chemical concentration Co is millimolar solution (Co small), the equation

can be simplified to an apparent first-order equation [Konstantinou and Albanis,

2003; Houas et al., 2001; Tang and An, 1995].

ln (Co/C) = kKt = kappt or

C = Coe-kapptt (1.10)

A plot of ln Co/C versus time represents a straight line, the slope of which depends

upon linear regression, equals the apparent first–order rate constant kapp.

1.8 Response Surface Methodology

Response Surface Methodology (RSM) is a collection of mathematical and statistical

techniques that are useful for the modeling and analysis of problems in which a

response of interest is influenced by several variables and the objective is to

optimize this response [Montgomery, 1997; Cochran and Cox, 1962]. It is a

sequential experimentation strategy for empirical model building and optimization.

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By conducting experiments and applying regression analysis, a model of the

response to some independent input variables can be obtained. Based on the model

of the response, a near optimal point can then be deduced. RSM is often applied in

the characterization and optimization of processes.

1.9 Objectives

Although the strong potential of photocatalytic process for wastewater treatment is

widely recognized, technical development at industrial scale has not been met with

much success. This is due to high operating cost of photocatalytic oxidation process

relative to existing biological treatment. Heterogeneous processes using TiO2

photocatalysis involves the capturing of only 5% of the solar light, for practical

application of AOP, harnessing solar light may lead to cost and energy effective

technology. Thus the modification of the existing photocatalysts and synthesis of

nano photocatalysts are some of the promising approaches.

Most of the studies deal with the photocatalytic decolorization of specific textile dye

from different chemical categories, including a detailed examination of the so-called

primary processes under different working conditions. Little information is available

on the reaction mechanisms involved in the photocatalytic degradation of dyes and

on the identification of major transient intermediates which have been more

recently recognized as very important aspects of these processes, especially in view

of their practical applications [Stylidi et al.,2004; Tanaka et al., 2000]. Keeping all

these facts in view, the present work has been undertaken with the following

objectives:

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1. To study the photochemical decolourization and degradation of synthetic dyes

(Malachite Green, Acid Orange 7, Reactive Red 160 and Reactive Red 35)

present in industrial effluents.

2. To investigate the effect of various process parameters such as amount of

catalyst, nature of catalyst, pH, and concentration of dye on the efficacy of

photocatalytic process and to optimize the process parameters.

3. To study rate of degradation, mineralization and dye intermediates formed

during degradation process.

4. To investigate the effect of modification of photocatalyst by mixed system

approach on degradation of dyes.

5. To investigate solar photocatalytic system for degradation of dyes.

6. Applicability of the optimized photocatalytic system for treatment of real textile

effluents.