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1.1 TREATMENT OF INDUSTRIAL EFFLUENTS
Contamination of water by industrial effluents is a serious problem experienced by nations throughout the developed and developing world. Recently, rapid industrial expansion especially petrochemical, pharmaceutical, textile, agricultural, food and chemical industries all produce waste effluent contaminated with organic compounds such as aromatics, haloaromatics and dyes has contributed to the contamination of fresh water in the ecosystem (Robertson et al., 2005). The released of untreated organic pollutants are of high priority concern since they are harmful to the environment and even their contamination in water at a few mg/L levels are highly carcinogenic to human and animals. In Malaysia, the number of water pollution sources was reported to be increased by 26 % from 13992 sources in 2000 to 18956 sources in 2006 (WHO, 2005; DOE, 2006). In this regard, a stricter water quality control standards and regulations such as Environmental Quality Act have been implemented in Malaysia in an effort to achieve a goal in environment protection management policy. Therefore, the enforcement of the existing environmental laws is essential to ensure the capability of the industrial sector in destructing the potentially harmful compounds from the effluent before safe disposal into the natural waters.
A variety of conventional biological, chemical and physical methods are presently available to treat the harmful compounds in the effluents. However, these conventional wastewater treatments have limitations of their own in order to reach the degree of purity required for final use. Biological treatment (aerobic or anaerobic digestion) usually is not effective in the wastewater treatment due to some of the toxic compounds present in the industrial effluent are found not readily biodegradable and may kill the active microbes (Sanromán et al., 2004). Chemical treatment (chlorination and ozonation) gave particular problems where chlorinated organic compounds as by-product after the chlorination treatment can be generated (Moonsiri et al., 2004). Due to the instability and hazardous nature, the use of ozone may be more harmful to the environment (Bizani et al., 2006). Finally, physical treatment (charcoal adsorption, reverse osmosis and ultrafiltration) is non-destructive and usually comprises a simple transfer of organic pollutants from a dispersed phase to a concentrated phase (Kabir et al., 2006), thus causing secondary pollution.
In this way, new and more efficient treatment technologies to degrade the complex refractory molecules into simpler molecules must be considered to reduce the deteriorating water quality.
1.2 PHOTOCATALYSIS IN WASTEWATER TREATMENT
In recent years, heterogeneous photocatalysis is one of the advanced oxidation processes (AOP) that has been accepted as a promising new alternative method in the area of wastewater treatment (Chen and Ray, 1999; Bekkouche et al., 2004; Cao et al., 2005; Liu et al., 2007; Merabet et al., 2009a). Compared with conventional wastewater treatments, heterogeneous photocatalysis has such advantages as: (1) pollutants are not merely transferred from one phase to another, but they are chemically transformed and completely mineralized to environmentally harmless compounds (2) this process is immune to organic toxicity to make it attractive for the degradation of toxic organic compounds and (3) this process has the potential to utilize sunlight or visible light for irradiation, thereby will give advantage in economic saving especially for large-scale operations (Chang et al., 2005; Yu et al., 2007a).
Generally, three basic components must present in heterogeneous photocatalysis in order for the reaction to take place: an emitted photon (in the appropriate wavelength), a catalyst surface (usually TiO2) and oxygen (Lasa et al., 2006). Photocatalytic process is occurred when the catalyst is activated by UV light and followed by the excitation of an electron from the valence band to conduction band, leaving a positive hole behind in the valence band. These positively charged holes will react with water molecules leading to the formation of the hydroxyl radicals (•OH), which are the strong oxidants to degrade the organic molecules (Zhang et al., 2005a).
Two modes of TiO2 as photocatalyst: (1) suspended TiO2 powder and immobilized TiO2 are typically used in the photocatalytic degradation processes. Both types of TiO2 offered various advantages and disadvantages. Suspended TiO2 powder has been the most commonly used because of its simplicity and offers high surface area for reaction with almost no mass transfer limitation. Nevertheless, additional separation processes are required to recover the TiO2 powder at the end of the treatment, either by filtration or centrifugation which is expensive in term of time and cost. Another concern is suspended TiO2 powder tends to agglomerate into larger particles at high concentration, which reduces the catalytic activity. Thus, in terms of large scale application, immobilized TiO2 is preferable. However, there is another problem that activity of immobilized TiO2 system may lower than slurry system due to the surface area reduction and mass transfer limitation (Li et al., 2005; Damodar et al., 2008; Song et al., 2008).
1.3 PROBLEM STATEMENT
In recent years, increasing use of immobilized photocatalyst in the heterogeneous photocatalysis has witnessed its significant application in the wastewater treatment (Kang, 2002; Zhang et al., 2006; Zhu and Zou, 2009). Even though immobilized TiO2 allows the easily continuous use of the photocatalyst by eliminating the need of additional separation processes in a slurry system, there are still technical challenges that must be further investigated and overcame. It is well established that the photocatalytic performance of TiO2 are strongly influenced by the physiochemical properties such as crystallinity, crystal size and surface area, which are governed by the preparation method (Jang et al., 2001; Senthilkumaar et al., 2006; Tian et al., 2009). Synthesis of immobilized nanosized TiO2 is important to compensate for the reduced performances associated with the immobilization process due to its large surface area and consistent with a high volume fraction of active sites available on the surface for substrate adsorption. Hence, knowledge especially in the synthesis of immobilized nanosized TiO2 still requires better understanding.
As most commonly known, sol-gel, chemical vapour deposition (CVD) and hydrothermal are prominent methods for the synthesis of TiO2. Sol-gel and CVD usually generate a relatively homogeneous TiO2 coating but high calcination temperature above 450˚C is usually required to induce crystallization. This is not economical and can cause crystal growth (Shang et al., 2003; Sayilkan et al., 2007). To avoid these defects, hydrothermal has been considered as an alternative method for the preparation of immobilized TiO2 in a nanocrystalline state, where low reaction temperature is available, and physiochemical properties such as crystal size, morphology and crystalline phase of the prepared photocatalyst can be controlled (Kolen'ko et al., 2003; Yu et al., 2005; Zhao et al., 2007).
Besides, the selection of a proper substrate as a support for immobilized TiO2 is essential to increase the photocatalytic degradation activities. Early works mainly focused on coating TiO2 on non-adsorbent supports such as glass, quartz sand and stainless steel substrate (Shang et al., 2003; Sonawane et al., 2004; Pozzo et al., 2006). The photocatalyst separation problem is somewhat can be solved, but no improvement in the photoefficiency is observed due to the diffusion limitation of pollutants to the surface of TiO2. To avoid this problem, much attention is given to support TiO2 on adsorptive materials such as zeolite, activated carbon (AC) and silica gel (Zhang et al., 2006; Mahalakshmi et al., 2009; Sun et al., 2009). Among these supports, AC is used in this study owing to its superiority of adsorption capacity, high surface area and lower cost (Sun et al., 2009)
In addition, an effective reactor design is considered important in the photocatalytic degradation reaction where intimate contact can be achieved between UV light, photocatalyst and reactants. In this sense, fluidized bed reactor is believed can increase the photocatalytic efficiency owing to its excellent reactant contact, high photocatalyst loading and efficient UV light exposure (Nam et al., 2002; Nelson et al., 2007). However, technical development of fluidized bed reactor is still not widely studied in heterogeneous photocatalysis technology for wastewater treatment. Thus, it is imperative to conduct a thorough study on the effect of operating parameters to investigate the photocatalytic performance of the prepared photocatalyst in a fluidized bed reactor. The importance of the present work is to exploit the wide and ever-growing application of TiO2 photocatalysis to be more practical in the wastewater treatment by studying the criteria in the synthesis of immobilized TiO2 with its photocatalytic performance in a fluidized bed reactor.
1.4 RESEARCH OBJECTIVES
The aim of this research is to develop an immobilized photocatalyst with high photoactivity, which is capable of degrading and mineralizing phenol under UV irradiation. The objectives of this research include:
- To synthesize nanosized TiO2 immobilized on granular activated carbon (TiO2/GAC) using a hydrothermal method.
- To characterize the prepared TiO2/GAC based on its chemical and physical properties.
- To study the performance of TiO2/GAC on the photocatalytic degradation of phenol in a fluidized bed reactor.
- To study the effect of operating parameters such as TiO2 loading, inorganic anions, pH, air flow rate, H2O2 concentration and initial phenol concentration on photocatalytic degradation of phenol in a fluidized bed reactor.
- To obtain optimum operating parameters by using response surface methodology.
- To study the kinetic of photocatalytic degradation of phenol over TiO2/GAC.
1.5 SCOPE OF STUDY
This research is focused on the development of highly effective immobilized TiO2 using a hydrothermal method. The development of the photocatalyst includes studying the effect of hydrothermal temperature (120oC - 200oC) and GAC as a support on the TiO2 photocatalytic activity. The freshly prepared immobilized photocatalyst are characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and Brunauer-Emmett-Teller (BET). Their photocatalytic activities are evaluated through phenol degradation in a fluidized bed reactor.
Various operating parameters such as TiO2 loading (1 layer - 4 layers), pH (3.0 - 11.0), inorganic anions (Cl-, HCO3-, CO32- and SO42-), air flow rate (1.0 L/min - 3.0 L/min), H2O2 concentration (50 mg/L - 400 mg/L) and initial phenol concentration (20 mg/L - 110 mg/L) are studied to evaluate the photocatalytic performance of TiO2/GAC in a fluidized bed reactor. Data analysis is further studied using 23 factorial experimental design of response surface methodology (RSM) to optimize and analyze the possible interaction between the process variables on phenol degradation. Finally, kinetic study based on Langmuir-Hinshelwood kinetics model is studied to determine the rate of reaction in the phenol degradation.
1.6 ORGANIZATION OF THESIS
There are five chapters in this thesis. Chapter 1 (Introduction) provides a brief description of treatment of industrial effluent and photocatalysis in wastewater treatment. This chapter also includes the problem statement that describes the problem faced and the needs of the current research. The objectives and scopes of this study are then explained in this chapter. This is followed by the organization of the thesis.
Chapter 2 (Literature Review) provides the past research works in the photocatalysis field. A brief explanation about advanced oxidation process is in the first part and followed by the overview of photocatalysis. Subsequently, information regarding with the TiO2 as a photocatalyst, the immobilization onto the support and photocatalytic reactor are discussed in the second part. Next, the characteristic of phenol and details of phenol degradation are described. The effects of various operating parameters that affect the photocatalytic activity are included. Finally, the design of experiment (DOE) is discussed.
Chapter 3 (Materials and Methods) covers the experimental part. Details of the materials and chemical reagents with a general description about the photocatalytic reactor that are used in the present study are described in the first part. This is followed by the discussion on the detailed of the photocatalyst preparation and characterization techniques throughout this research. Lastly, process studies and experimental design are described in this chapter.
Chapter 4 (Results and Discussion) presents the experimental findings together with the discussion. It is divided into eight parts: (a) characterization of TiO2/GAC, (b) Effect of hydrothermal temperature on the photocatalytic performance, (c) determination of factors affecting the photocatalytic activity, (d) performance comparison between immobilized TiO2 and suspended TiO2, (e) extraction studies, (f) effect of operating parameters, (g) optimization studies and (h) kinetics studies.
Chapter 5 (Conclusions and Recommendations) summarizes the results reported in the chapter 4 and recommends the possible ways to improve the present studies for the future research in this field.