UNIVERSITI PUTRA MALAYSIA

DEGRADATION OF DYES USING ZINC OXIDE AS THE PHOTOCATALYST

LEE KIAN MUN

FS 2008 8

DEGRADATION OF DYES USING ZINC OXIDE AS THE PHOTOCATALYST

LEE KIAN MUN

MASTER OF SCIENCE UNIVERSITI PUTRA MALAYSIA

2008

i

DEGRADATION OF DYES USING ZINC OXIDE AS THE PHOTOCATALYST

By

LEE KIAN MUN

Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia, in Fulfilment of the Requirements for the Degree of Master of Science

AUGUST 2008

ii

Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of the requirement for the degree of Master of Science

DEGRADATION OF DYES USING ZINC OXIDE AS THE PHOTOCATALYST

By

LEE KIAN MUN

AUGUST 2008

Chair : Associate Professor Abdul Halim bin Abdullah, PhD

Faculty : Science

In this study, ZnO was synthesized via precipitation method. The resulting ZnO

catalyst was characterised by X-ray Diffraction (XRD), Fourier Transform Infrared

(FT-IR), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy

(SEM), Particle Size Analysis (PSA) and surface area measurement (BET method).

XRD analysis showed that hydrozincite (Zn5(CO3)2(OH)6) was formed during

precipitation process and the decomposition of hydrozincite was completed at

temperature ~ 400 °C after 2 hours calcination in air. The ZnO produced was

spherical in shape (morphology), has a surface area of 25.8 m2g-1and particle size of

255 ± 2 nm with hexagonal crystal structure.

The ZnO produced was tested for photodegradation of Methyl Orange (MO),

Methylene Blue (MB), and Reactive Orange 16 (RO 16) under the illumination of

ultraviolet (UV, λmax = 365 nm) light. Various parameters affecting the degradation

performance such as catalyst loading, initial dye concentrations, initial pH, light

intensity, different light sources and addition of oxidants was examined. The removal

percentage of dyes increased with increasing mass of ZnO up to an optimum mass

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but decreased with increasing initial concentrations of the dye. Enhanced colour

removal for MO, MB and RO 16 was observed when the UV lamp used was changed

from 6 to 100 watts. In addition, the highest removal was achieved at pH 11 and

addition of H2O2 and K2S2O8 led to an enhancement in the removal of all the three

dyes. The photocatalytic degradation of mixed dyes solution (consists of a mixture of

MO, MB and RO 16) was conducted and 64.90 % removal was observed. The

photodegradation of dyes followed first-order kinetics.

iv

Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai memenuhi keperluan untuk ijazah Master Sains

DEGRADATION OF DYES USING ZINC OXIDE AS THE PHOTOCATALYST

Oleh

LEE KIAN MUN

OGOS 2008

Pengerusi : Profesor Madya Abdul Halim bin Abdullah, PhD

Fakulti : Sains

Dalam kajian ini, zink oksida (ZnO) disintesis melalui kaedah pemendakan. ZnO

yang dihasilkan dikaji dengan Pembelauan Sinar-X (XRD), Inframerah (IR), Analisis

Termogravimetrik (TGA), Mikroskopi Pengimbasan Elektron (SEM), Analisis Saiz

Zarah (PSA) dan Pengukuran Luas Permukaan (kaedah BET). Daripada analisis

XRD, hydrozincite (Zn5(CO3)2(OH)6) telah dihasilkan semasa proses pemendakan

dan penguraian hydrozincite berlaku sepenuhnya di suhu sekitar 400 °C selepas 2

jam pengkalsinan dalam udara. ZnO yang dihasilkan berbentuk sfera, mempunyai

luas permukaan sebanyak 25.8 m2g-1 dan saiz zarah 255 ± 2 nm dengan struktur

kristal heksagon.

Zink oksida yang dihasilkan telah diuji untuk fotopemangkinan Metil Jingga (MO),

Metilena Biru (MB) dan Reaktif Jingga 16 (RO 16) di bawah penyinaran cahaya

ultralembayung (UV, λmaks = 365 nm). Pelbagai parameter yang mempengaruhi

prestasi fotopemangkinan seperti amaun ZnO, kepekatan pewarna, pH larutan,

keamatan lampu, sumber cahaya yang berbeza dan penambahan agen pengoksidaan

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telah dikaji. Peratusan penyingkiran pewarna meningkat dengan peningkatan amaun

ZnO sehingga ke tahap optimum tetapi menurun dengan peningkatan kepekatan

perwarna. Peningkatan penyingkiran pewarna juga diperhatikan bagi MO, MB dan

RO 16 masing-masing apabila lampu UV yang digunakan ditukar daripada 6 ke 100

watt. Tambahan pula, penyingkiran tertinggi dicapai pada pH 11 dan penambahan

H2O2 dan K2S2O8 membawa kepada peningkatan dalam penyingkiran ketiga-tiga

pewarna. Selain itu, fotopemangkinan juga dikaji ke atas campuran pewarna yang

terdiri daripada MO, MB dan RO 16 dan sebanyak 64.90 % penyingkiran telah

diperhatikan. Fotopemangkinan ketiga-tiga pewarna didapati mengikut model kinetik

tertib pertama.

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ACKNOWLEDGEMENTS

First and foremost, my deepest appreciation goes to my supervisor, Assoc. Prof. Dr.

Abdul Halim bin Abdullah, for his effort and support in guiding me throughout my

MSc research. I would also like to express heartfelt thanks to my co-supervisors,

Prof. Dr. Zulkarnain bin Zainal and Assoc. Prof. Dr. Azmi bin Zakaria for giving me

guidance and opinions to improve my research.

Sincere thanks are also extended to Madam Rusnani Amirudin (FT-IR Perkin-Elmer

Model 1725X) and Mr. Kamal Margonal (TGA Mettler Toledo model) from

chemistry department of UPM. Beside that, I also would like to express my thank

you to Madam Faridah for helping me operates scanning electron microscopy (SEM)

analysis at Institute Bioscience. I also would like to express my special thank you to

Miss Siti Murni Zawawi who helps me to operate surface area measurement (BET

method). My appreciation also goes to all my lab mates Norsalinda, Umar Gaya,

Nurulhuda bin Abdullah for their support and encouragement.

Special appreciation also goes to Siew Cheng, Sook Keng, Wenn Jiunn, Kok Leong,

Yee Ching and Yuen who gave me a lot of support and assistance whenever I need it.

Last but not least, I would like to thank all those who had contributed to the success

of my work in one way or another including my father, mother and brother for their

support, encouragement and love that gave me the strength to complete my research

and thesis.

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I certify an Examination Committee met on date of viva to conduct the final examination of Lee Kian Mun on his Master of Science thesis entitled “Photocatalytic Degradation of Dyes using ZnO as Photocatalyst” in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree) regulations 1981. The Committee recommends that the candidate be awarded the relevant degree. Members of the Examination Committee are as follows: Chairman, PhD Professor Name of faculty/ institute Universiti Putra Malaysia (Chairman) Examiner 1, PhD Professor Name of faculty/ institute Universiti Putra Malaysia (Internal Examiner) Examiner 2, PhD Professor Name of faculty/ institute Universiti Putra Malaysia (Internal Examiner) External Examiner, PhD Professor Name of faculty/ institute Universiti Putra Malaysia (External Examiner) HASANAH MOHD GHAZALI, PhD Professor/ Deputy Dean School of Graduate Studies Universiti Putra Malaysia Date:

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This thesis was submitted to the Senate of Universiti Putra Malaysia and has been accepted as fulfilment of the requirement for the degree of Master of Science. The members of the Supervisory Committee were as follows: Abdul Halim bin Abdullah, PhD Associate Professor Faculty of Science Universiti Putra Malaysia (Chairman) Zulkarnain Zainal, PhD Professor Faculty of Science Universiti Putra Malaysia (Member) Azmi Zakaria, PhD Associate Professor Faculty of Science Universiti Putra Malaysia (Member) AINI IDERIS, PhD Professor and Dean School of Graduate Studies Universiti Putra Malaysia Date: 16 October 2008

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DECLARATION

I declare that the thesis is my original work except for the quotations and citations which have been duly acknowledged. I also declare that it has not been previously, and is not concurrently, submitted for any other degree at Universiti Putra Malaysia or at any other institution. LEE KIAN MUN Date:

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TABLE OF CONTENTS Page ABSTRACT iii ABSTRAK v ACKNOWLEDGEMENTS vii APPROVAL viii DECLARATION x LIST OF TABLES xiii LIST OF FIGURES xiv LIST OF ABBREVIATIONS xviii CHAPTER

1 INTRODUCTION 1 1.1 Dye 3 1.2 Methods for Treating Dye Effluents 4 1.3 Semiconductor Photocatalysis 9

1.3.1 Semiconductor 9 1.3.2 Theory of Semiconductor 9 1.3.3 Photocatalysis 10 1.3.4 Photocatalysts 11 1.3.5 Semiconductor Photocatalysis Mechanism 12

1.4 Objectives 14

2 LITERATURE REVIEW 15 2.1 Preparation of Zinc Oxide Catalyst 15 2.2 Removal of Organic Contaminants by ZnO 19

3 MATERIALS AND METHODOLOGY 26 3.1 Materials 26 3.2 Methods 28

3.2.1 Preparation of Zinc Oxide Catalyst 28 3.3 Characterisation of Zinc Oxide Catalyst 28 3.3.1 X-ray Diffractometry (XRD) Analysis 28 3.3.2 Fourier Transform-Infra Red (FT-IR) Analysis 29 3.3.3 Thermogravimetric Analysis (TGA) 29 3.3.4 Scanning Electron Microscopy (SEM) Analysis 29 3.3.5 Surface Area and Porosity Measurement (BET Method) 30 3.3.6 Particle Size Analysis (PSA) 30 3.4 Photocatalytic Degradation of Various Types of Dyes 30 3.4.1 Preparation of Dyes Solutions 30 3.4.2 Determination of Wavelength at Maximum Absorption 31 (λmax) and Construction of Standard Calibration Curve for Dyes

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3.4.3 General Photocatalytic Degradation Procedure 31 3.4.4 Effect of ZnO Loadings 32 3.4.5 Effect of Initial Dye Concentrations 33 3.4.6 Effect of Initial pH 33 3.4.7 Effect of Light Intensity 33 3.4.8 Effect of Different Light Sources 34 3.4.9 Effect of Oxidants 34 3.4.10 Photodegradation of Mixed Dyes 34

4 RESULTS AND DISCUSSION 36 4.1 Characterisation of ZnO Catalyst 36 4.1.1 X-ray Diffractometry (XRD) Analysis 36 4.1.2 Fourier Transform-Infra Red (FT-IR) Analysis 38 4.1.3 Thermogravimetric Analysis (TGA) 39 4.1.4 Scanning Electron Microscopy (SEM) Analysis 39 4.1.5 Surface Area and Porosity Measurement (BET Method) 40 and Particle Size Analysis (PSA) 4.2 Photocatalytic Degradation Studies 41 4.2.1 Preliminary Tests 41 4.2.2 Effect of ZnO Loadings 43 4.2.3 Effect of Initial Dye Concentrations 48 4.2.4 Effect of Initial pH 64 4.2.5 Effect of Light Intensity 70 4.2.6 Effect of Different Light Sources 72 4.2.7 Effect of Oxidants 78 4.2.8 Photodegradation of Mixed Dyes 91

5 CONCLUSIONS AND RECOMMENDATIONS 94 REFERENCES 97 APPENDICES 108 BIODATA OF STUDENT 118

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LIST OF TABLES

Table Page

1.1 Advantages and disadvantages of the current methods of dye removal from industrial effluents.

6

3.1 Properties of Methyl Orange, Methylene Blue and Reactive Orange 16.

26

4.1 2θ, full width half maximum (FWHM), d-spacing (d) and crystallite size (t) of ZnO.

37

4.2 Removal of various dyes under different conditions in 4 hours.

42

4.3 The percentage removal of dyes for the different amount of ZnO loaded.

43

4.4 Percentage removal for dyes with different initial concentrations.

48

4.5 The apparent rate constant, k1 and half time, t1/2 and correlation factor, R2 values for the photodegradation of dyes with different initial concentrations.

58

4.6 The second-order rate constant, k2 and the correlation factor, R2 values for the photodegradation of dyes with different initial concentrations.

60

4.7 The apparent rate constant, k1, half-life time, t1/2 and percentage of removal for the photodegradation of dyes at different pH values.

69

4.8 The apparent rate constant, k1, half-life time, t1/2 and percentage of removal for the photodegradation of dyes at different light intensity.

71

4.9 The apparent rate constant, k1 and half-life time, t1/2 for the photodegradation of dyes with different light sources.

72

4.10 The percentage of removal for H2O2 without the presence of ZnO or UV light.

79

4.11 The percentage of removal, apparent rate constant, k1 and half-life time, t1/2 for the photodegradation of dyes at different H2O2 volumes.

83

4.12 The removal percentage of all the three dyes upon the addition of K2S2O8.

85

4.13 Effect of K2S2O8 on the removal of MO, MB and RO 16 dye under different conditions.

86

4.14 Percentage removal of various dyes.

91

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LIST OF FIGURES

Figure Page

1.1 Conduction bands and valence bands for conductor, semiconductor and insulator.

10

1.2 General mechanism of the photocatalysis.

13

3.1 Structural formula of Methyl Orange, Methylene Blue and Reactive Orange 16.

27

3.2 Experimental set-up for photodegradation process.

32

4.1 XRD patterns of (a) precursor and (b) synthesized ZnO.

36

4.2 IR spectrum of the (a) precursor and (b) synthesized ZnO.

38

4.3 TGA thermograms of hydrozincite.

39

4.4 SEM micrograph of synthesized zinc oxide with magnification of 10000 X.

40

4.5 Particle size distribution of synthesized ZnO.

40

4.6 Percentage of removal of dyes under different conditions.

42

4.7 Graph C/Co Vs Time, t/min for the effect of different amount of ZnO loaded (240 minutes). [Conditions: Initial concentration (10 ppm), 6 W UV lamp, 1000 mL of Methyl Orange solution, and temperature 28 ˚C].

44

4.8 Graph C/Co Vs Time, t/min for the effect of different amount of ZnO loaded (240 minutes). [Conditions: Initial concentration (10 ppm), 6 W UV lamp, 1000 mL of Methylene Blue solution, and temperature 28 ˚C].

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4.9 Graph C/Co Vs Time, t/min for the effect of different amount of ZnO loaded (240 minutes). [Conditions: Initial concentration (10 ppm), 6 W UV lamp, 1000 mL of Reactive Orange 16 solution, and temperature 28 ˚C].

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4.10 Graph C/Co Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (1.20 g), 6 W UV lamp, 1000 mL of Methyl Orange solution, and temperature 28 ˚C].

49

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4.11 Graph C/Co Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Methylene Blue solution, and temperature 28 ˚C].

50

4.12 Graph C/Co Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Reactive Orange 16 solution, and temperature 28 ˚C].

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4.13 Graph ln C/Co Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (1.20 g), 6 W UV lamp, 1000 mL of Methyl Orange solution, and temperature 28 ˚C].

55

4.14 Graph ln C/Co Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Methylene Blue solution, and temperature 28 ˚C].

56

4.15 Graph ln C/Co Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Reactive Orange 16 solution, and temperature 28 ˚C].

57

4.16 Graph 1/[C]t Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (1.20 g), 6 W UV lamp, 1000 mL of Methyl Orange solution, and temperature 28 ˚C].

61

4.17 Graph 1/[C]t Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Methylene Blue solution, and temperature 28 ˚C].

62

4.18 Graph 1/[C]t Vs Time, t/min for the effect of different initial dye concentrations (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Reactive Orange 16 solution, and temperature 28 ˚C].

63

4.19 Graph C/Co Vs Time, t/min for the effect of different pH values (240 minutes). [Conditions: ZnO loading (1.20 g), 6 W UV lamp, 1000 mL of Methyl Orange solution (10 ppm), and temperature 28 ˚C].

66

4.20 Graph C/Co Vs Time, t/min for the effect of different pH values (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Methylene Blue solution (10 ppm), and temperature 28 ˚C].

67

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4.21 Graph C/Co Vs Time, t/min for the effect of different pH values (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Reactive Orange 16 solution (10 ppm), and temperature 28 ˚C].

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4.22 Effect of light intensity on the removal efficiency of dyes.

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4.23

4.24

4.25

Graph C/Co Vs Time, t/min for the effect of different light sources (240 minutes). [Conditions: ZnO loading (1.20 g), 1000 mL of Methyl Orange solution (20 ppm), and temperature 28 ˚C]. Graph C/Co Vs Time, t/min for the effect of different light sources (240 minutes). [Conditions: ZnO loading (0.60 g), 1000 mL of Methylene Blue solution (20 ppm), and temperature 28 ˚C]. Graph C/Co Vs Time, t/min for the effect of different light sources (240 minutes). [Conditions: ZnO loading (0.60 g), 1000 mL of Reactive Orange 16 solution (20 ppm), and temperature 28 ˚C].

73

74

75

4.26

4.27

4.28

4.29

Experimental set-up for fluorescent light bulb type and long type. Graph C/Co Vs Time, t/min for the effect of different H2O2 volumes (240 minutes). [Conditions: ZnO loading (1.20 g), 1000 mL of Methyl Orange solution (10 ppm), and temperature 28 ˚C]. Graph C/Co Vs Time, t/min for the effect of different H2O2 volumes (240 minutes). [Conditions: ZnO loading (0.60 g), 1000 mL of Methylene Blue solution (10 ppm), and temperature 28 ˚C]. Graph C/Co Vs Time, t/min for the effect of different H2O2 volumes (240 minutes). [Conditions: ZnO loading (0.60 g), 1000 mL of Reactive Orange 16 solution (20 ppm), and temperature 28 ˚C].

77

80

81

82

4.30

4.31

Graph C/Co Vs Time, t/min for the effect of different K2S2O8 loadings (240 minutes). [Conditions: ZnO loading (1.20 g), 6 W UV lamp, 1000 mL of Methyl Orange solution (10 ppm), and temperature 28 ˚C]. Graph C/Co Vs Time, t/min for the effect of different K2S2O8 loadings (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Methylene Blue solution (10 ppm), and temperature 28 ˚C].

88

89

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4.31 Graph C/Co Vs Time, t/min for the effect of different K2S2O8 loadings (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of Reactive Orange 16 solution (20 ppm), and temperature 28 ˚C].

90

4.32 Graph C/Co Vs Time, t/min for different type of dye (240 minutes). [Conditions: ZnO loading (0.60 g), 6 W UV lamp, 1000 mL of each dye solution (10 ppm), and temperature 28 ˚C].

93

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xviii

LIST OF ABBREVIATIONS BET Brunauer-Emmet-Teller surface area measurements

Eg Band gap energy

FT-IR Fourier Transform-Infra Red

FWHM Full width at half maximum

JCPDS Joint Committee of Powder Diffraction Standards

MO Methyl Orange

MB Methylene Blue

PSA Particle Size Analysis

RO 16 Reactive Orange 16

SEM Scanning Electron Microscopy

TGA Thermogravimetric analysis

UV Ultraviolet

XRD X-ray Diffraction

ZnO Zinc Oxide

CHAPTER 1

INTRODUCTION

There is no possible living world without water. In general, water accounts almost 70

to 90 % of the weight of living organisms. One cannot imagine life without clean

water. No doubt, there is no raw material in the world that more than water. Thus, the

quality of this valuable resource will directly influence the normal life of human

being. In general, water pollution refers to the degradation of water quality. Although

natural phenomena may change the water quality and also the ecological status of

water, however it cannot be considered as pollution unless it is not suitable to be

used in particular application.

The growth of the world’s population and industry has increased the demand for

water supply. The domestic use and industrial activity especially in developed

countries produce large amount of wastewater which then disposed into natural

channels that may lead to high pollution risk. In 2007, dos Santos et al. commented

that colour in water can be resulted either from natural phenomenon like the presence

of humic substances or from artificial phenomenon such as the disposal of dyes

effluents. Light played a major role for proper development of aquatic life. Thus, the

presence of large quantities of coloured wastewater blocked the penetration of light

into the water and subsequently causes an imbalance in their ecosystem.

According to Allègre et al. (2006), textile industries are one of the industries that

consumed large volumes of water in the processing operations including pre-

treatment, dyeing, printing and finishing. In dyeing process, water vapour is used as

heating agent for the dye baths, while liquid water ensures the transmission of the

dyes onto the fibre (Hessel et al., 2007). The release of the treatment baths which are

highly toxic and heavily coloured after the dyeing process contributes to water

pollution. One cannot estimate the total amount of pollutants being generated each

time the dyestuff is discharged into the water stream. Therefore, coloured water

required costly treatment operations before being consumed as drinking water. The

critical state of the water bodies has raised public concern and awareness as well as

stringent legislations being imposed by the government. This has encouraged

researchers in developing techniques such as electro-flocculation, reverse osmosis,

chlorination, adsorption and ozonation to minimise water pollution and restoring the

natural state of the water bodies.

Nowadays, semiconductor photocatalysis attracts increased interest since it has a

great potential to contribute to such environmental problems. The usage of ZnO,

TiO2 and SnO2 has been widely applied in the removal of organic contaminants

(Sakthivel et al. (2003), Akyol et al. (2004) and Irmak et al. (2004)). Moreover,

some studies have confirmed that ZnO exhibits a better efficiency than TiO2 in

photocatalytic degradation of some dyes, even in aqueous solution (Gouvêa et al.

(2000) and Dindar and Icli (2001)).

As a well-known photocatalyst, ZnO has received much attention in the degradation

and complete mineralisation of environmental pollutants (Richard et al. (1997),

Driessen et al. (1998) and Yeber et al. (2000)). Since ZnO has almost the same band-

gap energy (3.2 eV) as TiO2, its photocatalytic capability is anticipated to be similar

to that of TiO2. Moreover, ZnO is relatively cheaper compared to TiO2 whereby the

2

usage of TiO2 and platinum catalyst are uneconomic for large scale water treatment

operations (Daneshvar et al., 2004b). In addition, ZnO can be a suitable alternative

photocatalyst other than TiO2 since its photodegradation mechanism has been proven

to be similar to that of TiO2 (Dindar and Icli (2001) and Pirkanniemi and Sillanpää

(2002)). In some cases, ZnO shows higher photocatalytic tendency towards

photodegradation and mineralisation of environmental organic or inorganic

pollutants (Richard et al. (1997), Gouvêa et al. (2000) and Dindar and Icli (2001)).

1.1 Dye

A dye is a coloured substance that has specific affinity to a given substrate. Dyes are

applied to various substrates (textiles, leather, paper, hair etc.) from liquid in which

they are completely, or at least partially soluble (Zollinger, 2003).

Dyes are classified according to their chemical structure or according to the method

of application (Christie, 2001). According to Christie, dyes contain two groups which

are responsible for their colour. They consist of a group of atoms responsible for the

dye colour, called chromophores and the other group is auxochromes, which can be

defined as an electron withdrawing or donating substituents that essential for the

colour enhancement of the chromophores. The most important chromophores are the

azo (–N=N–), carbonyl (–C=O), methine (–CH=) and nitro (–NO2) groups. The most

important auxochromes are amine (–NH3), carboxyl (–COOH), sulfonate (–SO3H)

and hydroxyl (–OH) groups.

3

In term of method of application, dye molecules are carefully designed to ensure that

they have suitable properties for a particular application. For textile applications, dye

molecules are designed so that they are strongly attached to the fibre molecules to

which they are applied. Acid dyes and mordant dyes are mostly applied to protein

fibres whereas direct dyes, reactive dyes and vat dyes are used for cellulose fibres.

Disperse dyes and basic dyes are suitable for acrylic fibres.

1.2 Methods for Treating Dye Effluents

Generally, the release of coloured wastewater from textile industry into water bodies

such as lakes, oceans and rivers has created severe environmental problems. Most of

the coloured effluents are toxic and potentially carcinogenic hence considered as a

threat for human survival. These issues have caused public concern and awareness on

water pollution. Although various chemical and physical processes have been

employed in the removal of coloured effluents, however, those methods do not lead

to complete destruction of the dyes. Moreover, further treatment is needed since the

methods employed just transfer the contaminants from one phase to another. This

will indirectly enhance the operational cost. Some advantages and drawbacks of

current technologies for the removal of pollutants are listed in Table 1.1.

In last decade, advanced oxidation processes (AOPs) have been growing since they

are able to deal with the problem of dye destruction in aqueous systems

(Konstantinou and Albanis, 2004). The generation of hydroxyl radicals (•OH) that

oxidise various pollutants quickly and non-selectively have improved the degradation

of organic compounds compared to conventional methods.

4

5

Among AOPs, semiconductor photocatalysis appears as the most emerging

destructive technology. The main advantages of this process are highlighted below:

i. Inherent destructive nature;

ii. No mass transfer involved;

iii. Can be carried out under ambient conditions and using atmospheric oxygen

as the oxidant; and

iv. May lead to complete mineralisation of organic carbon into CO2.

In addition, semiconductors are chosen due to the following advantages:

i. Inexpensive;

ii. Non-toxic;

iii. Having high surface area;

iv. Having broad absorption spectra with high absorption coefficients;

v. Exhibits tunable properties which can be modified by size reduction, doping,

sensitizers, etc.;

vi. Can be afforded for multielectron transfer process;

vii. Capable of extended use without substantial loss of photocatalytic activity;

viii. Semiconductor particles recovered by filtration or centrifugation or when

immobilised retain much of their native activity after repeated catalytic cycle.

(Chatterjee and Dasgupta, 2005)

Table 1.1. Advantages and disadvantages of the current methods of dye removal from industrial effluents.

Physical/chemical methods Advantages Disadvantages References Chemical methods Fentons reagent Ozonation Sodium hypochloride (NaOCl) Cucurbituril

1. Suitable treatment for

wastewater which resistant to biological treatment or are poisonous to live biomass.

2. Effective decolourisation of both soluble and insoluble dye.

1. Capable of degrading

chlorinated hydrocarbons, phenols, pesticides and aromatic hydrocarbons.

2. Colourless effluent and low COD.

3. Can be applied in its gaseous state and doesn’t increase the volume of wastewater and sludge.

1. Initiates and accelerates azo-

bone cleavage. 1. Good sorption capacity for

various types of dyes.

1. Sludge generation. 1. Short ozonation half-life (20

min) and high cost. 1. Release of aromatic amines or

toxic molecules. 1. High cost.

(Slokar and Le Marechal, 1997)

(Pak and Chang, 1999) (Raghavacharya, 1997)

(Lin and Lin, 1993) (Xu and Lebrun, 1999)

(Banat et al., 1996)

(Buschmann, 1992)

6