PHOTOCATALYTIC REMOVAL OF PHENOL AND BASIC BLUE 3 (BB3) USING ZnO/C
3N
4UNDER
OUTDOOR LIGHT IRRADIATION
NOOR IZZATI BT MD. ROSLI
UNIVERSITI SAINS MALAYSIA
2016
PHOTOCATALYTIC REMOVAL OF PHENOL AND BASIC BLUE 3 (BB3) USING ZnO/C3N4 UNDER OUTDOOR LIGHT IRRADIATION
by
NOOR IZZATI BT. MD. ROSLI
Thesis submitted in fulfillment of the requirements for the degree of
Master of Science
January 2016
ii
ACKNOWLEDGEMENT
This master thesis would not have been possible and completed without the guidance and help of several individuals who in way or another contributed and spending their valuable time in completion of this thesis.
Foremost, my sincere gratitude goes to my supervisor, Prof. Abdul Rahman Bin Mohamed for being an outstanding adviser and excellent professor. His valuable suggestions, kind support, constant encouragement, and caring throughout my study made my master program and thesis successful.
In addition, I sincerely thank all the staff and technicians of School of Chemical Engineering for their help given directly or indirectly. My special thanks go to Pn. Nur Ain Natasha, En. Ismail, and En. Mohd Faiza for their support and assistance throughout my study. Their invaluable help and assistance was always critical for the successful completion of my master program.
I sincerely to thanks my friends Lam Sze Mun, Sin Jin Chung, and Noorul Aisyah Md Suhaimi for helping me get through the difficult times. Thanks for their kindness, friendliness and open-arms to share the knowledge throughout my study. Not to forget my friends Yanna Syamsudin, Norhaslinda, Shahreen and Nurul Alia for giving me support and valuable ideas for me to complete this program.
I am gratefully acknowledging My Brain 15 through Malaysia government and USM Research University grant (Grant no. 814176) for providing me financial support throughout my Master study.
Last but not least, this work is dedicated to my parents Md Rosli Hj Osman and Mek Limah Bt Saaud, whose love, encouragement and moral support have been critical in completing this master program. Also, thanks to my brother and my sisters for giving me their advice and support throughout my study
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TABLE OF CONTENTS
Acknowledgement………... ii
Table of Contents………..………... iii
List of Tables………..………... viii
List of Figures………..……….... x
List of Plates………...………... xiii
List of Abbreviations………..………... xiv
List of Symbols………..……... xv
Abstrak………..………... xvi
Abstract………..……... xviii
CHAPTER 1 - INTRODUCTION………..…... 1
1.1 Industrial Water Pollution………..……… 1
1.2 Advanced Oxidation Process………...………... 2
1.3 Semiconductor Coupling………..………... 3
1.4 Problem Statements………..………... 4
1.5 Research Objectives………..…………... 6
1.6 Scope of Study………... 7
1.7 Thesis Organization………..…... 7
CHAPTER 2 – LITERATURE REVIEW………... 9
2.1 Phenol………..………... 9
2.1.1 Properties of Phenol………... 9
2.1.1 (a) Phenol in environment………...………... 10
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2.2 Dye………..….………... 12
2.2.1 Properties of Basic blue 3 (BB3) ………... 12
2.2.1 (a) Basic blue 3 (BB3) in environment……... 13
2.3 Advanced Oxidation Process………..…... 14
2.3.1 Heterogeneous catalysis…..………..……... 16
2.3.2 Mechanism of heterogeneous catalysis.………….……... 17
2.4 Semiconductor Catalyst………..……...……… 19
2.4.1 ZnO catalyst………...…... 20
2.5 Modifications of ZnO Catalyst………..………… 22
2.5.1 Modification by C3N4………..…...……… 23
2.6 Catalyst Preparation………...…... 26
2.7 Process Variables Studies………..……… 31
2.7.1 Catalyst loading………... 31
2.7.2 Initial substrate concentration………... 32
2.7.3 Solution pH………...…... 34
2.8 Mineralization Studies………..………. 35
2.9 Summary………...………... 36
CHAPTER 3 – MATERIAL AND METHOD………... 37
3.1 Materials and Chemicals………..……… 37
3.2 Equipment………..………... 39
3.2.1 Photocatalytic batch reactor………... 39
3.2.2 High Performance Liquid Chromatography (HPLC)………. 41
3.2.3 UV-vis spectrophotometer………...………... 41
3.3 Preparation of ZnO/C3N4 Catalyst………... 41
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3.4 Characterization of ZnO/C3N4 Catalyst………... 43
3.4.1 Scanning Electron Microscopy (SEM) ………..……... 43
3.4.2 X-ray Diffraction (XRD) …………..………... 43
3.4.3 High-Resolution Transmission Electron Microscope (HRTEM) ………. 44 3.4.4 UV-vis Diffuse Reflectance Spectra (UV-vis DRS)…... 44
3.5 Photocatalytic Test………... 44
3.6 Control Studies For Phenol and BB3 Degradation………….…... 45
3.7 Catalyst Preparation Study………..………. 45
3.8 Process Parameters Study………... 46
3.8.1 Phenol………. 46
3.8.1 (a) Effect of catalyst loading………..………… 46
3.8.1 (b) Effect of initial phenol concentration..……... 46
3.8.1 (c) Effect of solution pH……….…..…... 46
3.8.2 Basic blue 3 (BB3)………..……… 47
3.8.2 (a) Effect of catalyst loading………..……… 47
3.8.2 (b) Effect of initial BB3 concentration.………... 47
3.8.2 (c) Effect of solution pH……….………... 47
3.9 Mineralization Studies………..………… 48
3.10 Kinetic Studies………..……… 48
CHAPTER 4 – RESULTS AND DISCUSSION………. 49
4.1 Effect of C3N4 Loadings on Catalyst Preparation…..…………... 50
4.1.1 Catalyst characterization………...…………... 50
4.1.1 (a) X-ray Diffraction (XRD)……..…………... 50
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4.1.1 (b) Scanning Electron Microscope (SEM)……... 52
4.1.1 (c) UV-vis Diffuse Reflectance Spectra (UV-vis DRS)…... 54 4.1.1 (d) High-Resolution Transmission Electron Microscopy (HRTEM)…... 56 4.1.2 Photocatalytic degradation of phenol at different C3N4 loadings………... 58 4.2 Photocatalytic Performance of 3.5wt% ZnO/C3N4 Catalyst……... 60
4.2.1 Control Studies………..………….. 60
4.3 Process Variables Studies………... 63
4.3.1 Effect of process parameters to degradation of phenol…... 63
4.3.1 (a) Effect of catalyst loading………..………… 63
4.3.1 (b) Effect of initial phenol concentration... 65
4.3.1 (c) Effect of solution pH………... 67
4.3.2 Effect of process parameters to degradation of basic blue 3 (BB3)………... 69 4.3.2 (a) Effect of catalyst loading………..……… 69
4.3.2 (b) Effect of initial BB3 concentration...……… 71
4.3.2 (c) Effect of solution pH………..………... 72
4.4 Total Organic Carbon (TOC)………..………. 74
4.5 Kinetic Studies………..………... 76
4.5.1 Apparent pseudo-first order rate constant………..……… 77
4.5.2 Langmuir-Hinshelwood (L-H) kinetic model…………... 80
4.5.3 Initial reaction rate………...……….. 82
4.6 Photocatalytic Mechanism………... 85
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4.7 Reusability of the Catalyst………...………... 88
4.8 Summary………..………. 90
CHAPTER 5 - CONCLUSIONS AND RECOMMENDATION….……... 92
5.1 Conclusions………..……… 92
5.2 Recommendations………... 93
REFERENCES………...………... 94
APPENDICES
LIST OF PUBLICATION
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LIST OF TABLES
Page Table 2.1 Basic physical and chemical properties for phenol (Busca et al.,
2008).
10
Table 2.2 Basic physical and chemical properties for BB3 (Keshmirizadeh and Farajikhajehghiasi, 2014).
13
Table 2.3 Comparative oxidizing power of different oxidants (Huling and Pivetz, 2006).
15
Table 2.4 Examples of advanced oxidation processes (AOP). 15 Table 2.5 Organic contaminants mineralized by heterogeneous catalysis. 17 Table 2.6 Works reported on ZnO/C3N4 catalyst. 26 Table 2.7 Studies of photocatalytic degradation by different preparation
methods.
28
Table 2.8 Effect of catalyst loading on the photocatalytic degradation of pollutants.
32
Table 2.9 Effect of initial substrate concentration on the photocatalytic degradation of pollutants.
33
Table 2.10 Effect of pH solution on the photocatalytic degradation of pollutants.
35
Table 3.1 List of chemicals and reagents 39
Table 4.1 Wavelength of reflectance edge and band gap energy values for pure ZnO catalyst and ZnO/C3N4 catalysts.
55
Table 4.2 Operating conditions for TOC removal of phenol and BB3. 74 Table 4.3 TOC removal and percentage removal of 3.5 wt% ZnO/C3N4 75
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catalyst for phenol and BB3 degradation under outdoor light irradiation
Table 4.4 The values of the apparent rate constant (kapp) and correlation coefficient (R2) at different initial phenol concentration.
79
Table 4.5 The values of the apparent rate constant (kapp) and correlation coefficient (R2) at different initial BB3 concentration.
79
Table 4.6 Summary of the performance of ZnO/C3N4 catalyst. 90
Table 4.7 Comparison between the current work and other studies on ZnO/C3N4.
91
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LIST OF FIGURES
Page Figure 2.1 Schematic diagram of heterogeneous catalysis mechanism
(Ahmed et al., 2011).
18
Figure 2.2 Bad edge position of different semiconductors (Zhang et al., 2012).
20
Figure 2.3 Stick-and-ball representation of ZnO crystal structures: (a) cubic rocksalt; (b) cubic zinc blende; (c) hexagonal wurtzite. Shaded gray denotes as ZnO and O atoms, respectively (Morkoc and Ozgur, 2009).
21
Figure 2.4 Structure of g-C3N4 (Zai et al., 2013). 24 Figure 3.1 Flow chart for method study in this research. 38 Figure 3.2 Schematic diagram of the experimental set up. 40 Figure 3.3 Process flow chart of catalyst preparation 42 Figure 4.1 XRD data of (a) ZnO pure, (b) 0.7 wt% ZnO/C3N4 catalyst, (c)
3.5 wt% ZnO/C3N4 catalyst, (d) 4.9 wt% ZnO/C3N4 catalyst.
51
Figure 4.2 The SEM images of (a) low magnification of ZnO, (b) high magnification of ZnO, (c) low magnification of 0.7 wt% of ZnO/C3N4 catalyst, (d) high magnification of 0.7 wt% of ZnO/C3N4 catalyst, (e) low magnification of 3.5 wt% of ZnO/C3N4 catalyst, (f) high magnification of 3.5 wt% of ZnO/C3N4 catalyst, (g) low magnification of 4.9 wt% of ZnO/C3N4 catalyst, (h) high magnification of 4.9 wt% of ZnO/C3N4 catalyst.
53
Figure 4.3 UV-vis reflectance spectra of ZnO and ZnO/C3N4 catalysts. 54
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Figure 4.4 HRTEM images for (a) pure ZnO; (b) pure C3N4; (c) 3.5 wt%
ZnO/C3N4 catalyst.
57
Figure 4.5 HRTEM images of ZnO/C3N4 catalysts reported by Chen et al.
(2014a).
58
Figure 4.6 Photocatalytic activity of phenol removal by ZnO/C3N4 catalyst.
(catalyst loading = 1 g/L; [phenol] = 5 mg/L; solution pH).
59
Figure 4.7 Control studies for photocatalytic degradation of phenol. (catalyst loading = 1 g/L; [phenol] = 5 mg/L; solution pH = 5.7).
61
Figure 4.8 Control studies for photocatalytic degradation of BB3. (catalyst loading = 1 g/L; [BB3] = 5 mg/L; solution pH = 5.8).
62
Figure 4.9 Effect of catalyst loading on the degradation of phenol over 3.5 wt% ZnO/C3N4 catalyst, ([phenol] = 5 mg/L; pH solution = 5.7).
64
Figure 4.10 Effect of initial concentration on the degradation of phenol over 3.5 wt% ZnO/C3N4 catalyst, (catalyst loading = 1 g/L; pH solution
= 5.7).
66
Figure 4.11 Effect of solution pH on the degradation of phenol over 3.5 wt%
ZnO/C3N4 catalyst, (catalyst loading = 1 g/L; [phenol] = 5 mg/L)
68
Figure 4.12 Effect of catalyst loading on the degradation of BB3 over 3.5 wt%
ZnO/C3N4 catalyst, ([BB3] = 5 mg/L; solution pH =5.8).
70
Figure 4.13 Effect of initial concentration on the degradation of BB3 over 3.5 wt% ZnO/C3N4 catalyst, (catalyst loading = 0.5 g/L; solution pH
=5.8).
71
Figure 4.14 Effect of solution pH on the degradation of BB3 over 3.5 wt%
ZnO/C3N4 catalyst, (catalyst loading = 0.5 g/L; [BB3] = 5 mg/L).
73
Figure 4.15 Degradation and TOC removal for phenol and BB3, respectively. 75
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Figure 4.16 Kinetic of phenol degradation for different concentrations of phenol. (3.5 wt% ZnO/C3N4 catalyst; catalyst loading = 1 g/L;
solution pH = 5.7).
78
Figure 4.17 Kinetic of BB3 degradation for different concentrations of BB3.
(3.5 wt% ZnO/C3N4 catalyst; catalyst loading = 1 g/L; solution pH
= 5.8).
78
Figure 4.18 Linearization of L-H model for phenol. 83
Figure 4.19 Linearization of L-H model for BB3. 83
Figure 4.20 Propose mechanism representing the charge transfer in ZnO/C3N4
catalyst (Wang et al., 2011; Chen et al., 2014).
86
Figure 4.21 Recycling of phenol over 3.5 wt% ZnO/C3N4 catalyst, 88 Figure 4.22 Recycling of BB3 over 3.5 wt% ZnO/C3N4 catalyst, 88
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LIST OF PLATES
Page
Plate 3.1 Experimental set up 40
Plate 4.1 Sedimentation of catalyst after 15 min (a) commercial TiO2; (b) 3.5 wt% ZnO/C3N4
catalyst in phenol; (c) 3.5 wt% ZnO/C3N4
catalyst in BB3.
89
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LIST OF ABBREVIATIONS
AOPs Advanced oxidation processes
BB3 Basic blue 3
CB Conduction band
C3N4 Carbon nitride
CO2 Carbon dioxide
HCl Hydrochloric acid
H2O Water
H2O2 Hydrogen peroxide
HPLC High performance liquid chromatograph
HRTEM High Resolution Transmission Electron Microscopy
L-H Langmuir-Hinshelwood
O2 Oxygen
NaOH Sodium hydroxide
NHE Normal hydrogen electrode SEM Scanning Electron Microscope
TiO2 Titanium dioxide
TOC Total organic carbon
UV Ultraviolet
UV-vis DRS Ultraviolet-visible Diffuse Reflectance Spectra
VB Valence band
XRD X-ray Diffraction
ZnO Zinc oxide
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LIST OF SYMBOLS
Unit
C Concentration at time t mg/L
C0 Initial concentration mg/L
e- Electron -
Eg Energy band gap eV
h+ Holes -
h Planck’s constant eVs
HO2● Hyperoxyl radical -
hv Photon energy -
K Adsorption equilibrium constant L/mg
kapp Apparent rate constant 1/min
kr Reaction rate constant mg/L/min
O2⦁- Superoxide radical anion -
OH- Hydroxyl ion -
●OH Hydroxyl radical -
r Reaction rate mg/L.min
R2 Correlation coefficient -
zpc Point charge zero -
𝜆 Wavelength nm
𝜃 Bragg’s angle in degree -
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PENYINGKIRAN FENOL DAN BASIC BLUE 3 (BB3) MELALUI PEMFOTOMANGKINAN MENGGUNAKAN ZnO/C3N4 DI BAWAH
PENYINARAN CAHAYA LAMPU LUARAN.
ABSTRAK
Pencemaran persekitaran telah menjadi masalah yang besar terutamanya kepada negara- negara membangun. Pencemaran air oleh pelbagai jenis bahan yang berbahaya boleh mengakibatkan impak yang negatif kepada persekitaran. Proses Pengoksidaan Lanjutan (AOP) telah diketahui mampu untuk merawat air yang telah tercemar sebelum dilepaskan.
Di antara proses tersebut, pemfotomangkinan heterogen oleh pemangkin ZnO telah menarik minat sejak akhir-akhir ini. Walaubagaimanapun, masalah besar yang dihadapi oleh ZnO adalah kadar penyatuan semula pasangan e-/h+ yang tinggi yang boleh mengurangkan degradasi pemfotomangkinan. Jadi, gandingan ZnO dengan C3N4 yang mempunyai jurang tenaga yang rendah boleh menghalang penyatuan pasangan e-/h+ ini.
Oleh itu, dalam kajian ini pemangkin ZnO/C3N4 dengan pelbagai peratusan berat C3N4
(0.7-4.9 berat%) telah berjaya disediakan melalui kaedah pengisitepuan yang mudah.
Objektif kajian ini adalah untuk mensintesis dan mencirikan pemangkin ZnO/C3N4, untuk menilai kesan proses parameter ke atas degradasi pemfotomangkinan fenol dan basic blue 3 (BB3), untuk mengetahui pemineralan bahan pencemar di bawah keadaan terbaik, dan untuk mengesahkan proses kinetik pemfotomangkinan bahan pencemar di bawah keadaan terbaik. Bagi sampel yang terhasil; Pembelauan Sinar-X (XRD), Mikroskop Penghantaran Elektron Beresolusi Tinggi (HRTEM), dan Spectrum UV-vis Pantulan telah digunakan untuk pencirian. Keputusan XRD menunjukkan penambahan C3N4 di dalam penyediaan pemangkin tidak menunjukkan puncak sepadan dengan C3N4 mungkin disebabkan oleh
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kandungan C3N4 yang rendah. Keputusan SEM mendedahkan permukaan pemangkin kurang dipengaruhi oleh berat C3N4. Walaubagaimanapun, imej HRTEM menunjukkan bahawa hubungan yang rapat antara ZnO dan C3N4 sememangnya terbentuk dan hal ini berfaedah kepada degradasi pemfotomangkinan. Kemudian, aktiviti pemfotomangkinan untuk ZnO tulen dan pemangkin yang disediakan telah diuji untuk degradasi fenol dan BB3. Tiga pembolehubah proses telah dikaji; kesan beban pemangkin, kesan kepekatan awal bahan cemar, dan kesan bendalir pH. Keputusan menunjukkan semua pemangkin ZnO/C3N4 yang disediakan mempamerkan aktiviti yang lebih baik berbanding ZnO tulen terutamanya pemangkin dengan 3.5 berat% ZnO/C3N4 yang menunjukkan penyingkiran tertinggi peratusan. Keadaan optimum bagi beban pemangkin didapati pada 1g/L dan 0.5 g/L masing-masing untuk fenol dan BB3. Selain itu, kedua-dua bahan pencemar menunjukkan penyingkiran tertinggi peratusan pada kepekatan awal 5 mg/L dengan peratusan 99.4% dan 89.6% masing-masing untuk fenol dan BB3. Degradasi fenol paling bagus adalah pada pH 5.7 (99.4%) sementara untuk BB3 pada pH 7 (96.7%). Pada keadaan terbaik di atas, analisis TOC menunjukkan hanya 56.5% dan 63.6% penyingkiran TOC telah dicapai masing-masing untuk fenol dan BB3. Kinetik bagi degradasi fenol dan BB3 turut dikaji. Keputusan menunjukkan bahawa kinetik bagi kedua-dua bahan pencemar ini mematuhi model Langmuir-Hinsheilwood (L-H). Akhir sekali, pemangkin 3.5 berat%
ZnO/C3N4 mempunyai kebolehulangan dan keupayaan pemisah yang bagus mencadangkan potensi penggunaanya di dalam rawatan air sisa.
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PHOTOCATALYTIC REMOVAL OF PHENOL AND BASIC BLUE 3 (BB3) USING ZnO/C3N4 UNDER OUTDOOR LIGHT IRRADIATION
ABSTRACT
Environmental pollution has become a major problem especially for developing countries.
Water contamination by various kinds of hazardous substances might give negative impacts to the environment. Advanced Oxidation Processes (AOP) are known to have the ability to treat the contaminated water before being discharged. Among them, heterogeneous photocatalysis by ZnO catalyst have attracted recent years. However, the major problem suffers by ZnO is a high recombination rate of e-/h+ pairs which can decrease the photocatalytic degradation. Thus, coupling of ZnO with small band gap value of C3N4 can prevent these recombination of e-/h+ pairs. Therefore, in this study ZnO/C3N4
catalyst with various C3N4 weight percentage (0.7-4.9 wt%) were successfully prepared by simple impregnation method. The objectives of this study are to synthesize and characterize the ZnO/C3N4 catalyst, to evaluate the effect of process parameters on photocatalytic degradation phenol and basic blue 3 (BB3), to determine the mineralization of pollutants under the best condition, and to validate the kinetic process of photodegradation of pollutants under the best condition. For the prepared samples; X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), High Resolution Transmission Electron Microscopy (HRTEM), and UV-vis Diffuse Reflectance Spectra (UV-vis DRS) have been used for characterization. XRD result showed that the addition of C3N4 in catalyst preparation gave no peak corresponding to C3N4 possibly because of the amount C3N4 was very low. SEM result revealed that the catalyst surface was less affected by C3N4
loading. However, HRTEM images showed that the intimate contact between ZnO and
xix
C3N4 was indeed formed and it is advantageous for the photocatalytic degradation. Then, the photocatalytic activity of pure ZnO and as-prepared catalyst were evaluated for the degradation of phenol and BB3. Three process variables were studied; effect of catalyst loading, effect of catalyst loading and effect of solution pH for both phenol and BB3. The result showed that all the ZnO/C3N4 catalysts prepared catalyst exhibited better activity compared to pure ZnO especially with 3.5 wt% ZnO/C3N4 catalyst which showed highest removal percentages. The optimum catalyst loading was found at 1 g/L and 0.5 g/L for phenol and BB3, respectively. Besides, both pollutants showed the highest removal percentage at the initial concentration of 5 mg/L with the percentages of 99.4% and 89.6%
for phenol and BB3, respectively. Degradation of phenol was found favor at pH 5.7 (99.4%) while for BB3 at pH 7(96.7%). At the best condition above, the TOC analysis revealed that only 56.5% and 63.6% of TOC removal for phenol and BB3, respectively.
Kinetic degradation of phenol and BB3 also has been studied. The result showed that the reaction kinetic for both pollutants obeyed Langmuir-Hinshelwood (L-H) kinetic model.
Finally, 3.5 wt% ZnO/C3N4 catalyst also has good repeatability and good separation ability suggested its potential application in wastewater treatment.
1
CHAPTER 1 INTRODUCTION
Water covers about ¾ of the earth's surface and its consumption contribute approximately 60 % - 70% of livings worldwide. Most of the human activities are dependable to the water as their main source. The growth of the human population and industries causes the increase of demand for fresh water while the sources of water supply remain constant. The quality of fresh water supply has been affected mainly due to the increased of industrial activities and human populations. In particular, developing countries have high potential to discharge more polluted water through domestic use and industrial activities. On the global level, the question of the fresh water supply has been a concern.
1.1 Industrial Water Pollution
Nowadays, water pollution becomes the main threat and challenge that a human must face. Water pollution occurs because of emission of pollutants (particles, chemicals or substances that cause water to be contaminated) are discharged directly or indirectly into water system without proper treatment. Pollutants enter into the water system mainly by human causes or factors. Daily human activities introduce pollutant and wastes into the river and streams, lakes, groundwater aquifers, and oceans. These pollutants eventually will affect the groundwater system and become dangerous for human consumption. These contaminants mainly contributed from the textile industry, haloalkanes, aromatic compounds, alcohol, detergents and surfactants, agriculture wastes like insecticides, herbicides and pesticides (Gordon and Jules, 2009), inorganic compounds like heavy metals like mercury, nickel, and lead (Fenglian et al., 2011; Ming et al., 2012); and pathogens like bacteria and fungi (Vinu and Madras, 2012). Although these contaminants
2
may exist at trace levels, they can change the quality of drinking water and can cause adverse effects to the environment and human health.
Phenol and basic blue 3 (BB3) are among of the hazardous pollutants released to the environment. They are contributed from many industrial activities especially from agriculture industries. Phenol and its derivatives have been known for their toxicity and carcinogenicity and they are highly resistant to many degradation processes including conventional biological and chemical treatment (Tassalit et al., 2008). Severe illness like leukaemia and some serious organs malfunction may arise if exposes in high concentration of phenol. Dyes can cause serious environmental problem since they have low biodegradability in a water system that causes a high potential threat to the environment (Fatimah et al., 2011). The release of colored waste even in trace quantities is highly desirable as it reduces the penetration of light into the water, thereby decreasing the efficiency of photosynthesis in aquatic plants. This in turn causes the ecosystem of streams to be seriously affected.
1.2 Advanced Oxidation Process
Recently, AOPs have received considerable attention for the complete destruction of contaminants. AOPs become a promising technology for wastewater treatment containing non-easily removable organic compounds. AOPs include photocatalysis system like the combination of semiconductor and light, and semiconductor and oxidants (Kansal et al., 2009). The research regarding AOP in wastewater treatment also has been reported by several authors. The AOP have been developed include heterogeneous catalysis, ozonation, Fenton processes, and oxidation by H2O2 (Vilhunen et al., 2010; Andronic et al., 2011; Rosal et al., 2011; Wu et al., 2011). Heterogeneous catalysis, among the advanced oxidation processes (AOP) is a process successfully used to oxidize organic
3
pollutants present in the aqueous systems. Heterogeneous photocatalysis offers a number of advantages as the following:
a) Destroying contaminants by decomposing into non-toxic substances with the aid of light irradiation.
b) Environmentally friendly materials can be employed as a semiconductor catalyst.
c) Photocatalytic activity can be conducted under mild condition.
d) Complete mineralization of organic pollutants without producing secondary pollutants.
Despite having some advantages in destroying organic pollutants, heterogeneous photocatalysis also have some disadvantages which are less active and selective compared to homogeneous catalysis. On the other hand, heterogeneous catalysis also suffers from the leaching of the catalyst during its use and recycle, leading to deactivation of catalyst (Erica et al., 2005).
1.3 Semiconductor Coupling
Due to the fact that the catalyst makes use of light to enhance the degradation of organic pollutants in the presence of semiconductors, several attempts have been made in this field. Semiconductors like TiO2, ZnO, and Fe2O3 can be used to act as sensitizers for light-induced redox-processes. As stated by Hofmann et al. (1995), this process occurs because of the electronic structure of the metal atoms in chemical combination, which is characterized by filled valence band (VB) and empty conduction band (CB). Upon irradiation with the energy equal or more than the band gap energy, this semiconductor molecules absorb the photon light energy, hence the electrons in the valence band excited to the conduction band. In order for the semiconductor to catalyse the reaction, the recombination of e-- h+ must be prevented as much as possible.
4
After the discovery of water splitting by Fujishima and Honda (Fujishima and Honda, 1972), TiO2 have been extensively investigated as the potential catalyst for degradation of organic contaminants by many researchers. However, ZnO has been found to be a potential catalyst in the photocatalysis research area because of the comparable band gap energy of ZnO with TiO2 (3.2 eV). ZnO offers several advantages over TiO2 in photocatalytic research area which are:
a) Can absorb a large fraction of the solar spectrum (Yulong et al., 2013).
b) A good electron donor with high optical activity and stability (Kanika et al., 2013).
1.4 Problem Statement
Removal of environmental contaminants by light-driven photocatalytic activity has received considerable attention in recent years since the demand of the fresh water supply are critical (Song et al., 2014). In the past researches regarding semiconductors have been investigated like ZnO, WO3, ZnS, Fe2O3, CdS and SrTiO3 as well as coupled semiconductors. However, producing a good catalyst may require extensive research regarding their separation efficiency. One way to improve the photoactivity of catalyst is by increasing the separation efficiency and form a composite powder between two semiconductors. A good catalyst must possess high activity, ability to utilize visible and/or near-UV light, photostable (durable) and reusable, chemically and biologically inert and cheap (Chen et al., 2008).
Among the available catalysts, ZnO is a popular catalyst used in the photodegradation of organic and inorganic compounds because of its high activity, low cost and non-toxicity. ZnO also have been reported to have higher photocatalytic activities both in air and aqueous media (Kansal et al., 2010; Wahab et al., 2013). However, the
