A REVIEW: TITANIUM DIOXIDE PHOTOCATALYSIS FOR WATER TREATMENT

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1 A REVIEW: TITANIUM DIOXIDE PHOTOCATALYSIS FOR WATER TREATMENT Farid Madjene Lamine Aoudjit Sadek Igoud Hafida Lebik Belkacem Boutra Solar Equipment Development Unit, (EPST /CDER) Development Center Of Renewable Energies, Algeria Abstract: In recent years, semiconductor photocatalytic process has shown a great potential as a lowcost, environmental friendly and sustainable treatment technology to align with the zero waste scheme in the water/wastewater industry. The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic compounds and microorganisms in water. This paper presents the review of the background and principle of photocatalysis for advanced oxidation technology. In particular, semiconductor TiO 2 photocatalysts is a successful and convenient alternative to the conventional methods for the treatment of wastewater containing organic pollutants. Key Words: TiO 2 Photocatalyst, Advanced oxidation technology Introduction: The pollution of surface and ground water is a serious problem of industrial society therefore, it is very important to develop processes for cleaning up polluted aquifers, as well as to make them available for industrial facilities. Photocatalytic detoxification, employing titanium dioxide, is a promising method for this purpose. Heterogeneous photocatalysis has recently emerged as an efficient method for purifying water. It can be considered as one of the new advanced oxidation technologies for water purification treatment. Photocatalytic oxidation reactions have the potential to completely mineralize organic compounds to carbon dioxide, water vapor and inorganic substances by solar light and to lead us to a clean and green purification technology for treatment of polluted air and water. 34

2 Since the effectiveness of the technique as a method of laboratory or industrial water pollution control has been proven, the kinetic which depends on major factors on photodegradation phenomena, it the main parameter to follow and optimize, the major factors are (Amount of catalyst used in powder form, Wavelength and light gradation, Intensity of the light radiation and Geometry and reactor dimension). The application of photochemistry to environmental remediation and treatment has also been an active area of research and heterogeneous photocatalysis using UV/TiO 2 system has often been proposed to eliminate these pollutants. TiO 2 is widely used as photocatalyst because it is photochemically stable, on toxic and low cost. In the present paper, we describe the literature on improving the photocatalytic activity of TiO 2 for applications to AOT. Characteristics of Titanium Dioxide photocatalysis for advanced oxidation technology Titania is a very well-known and well-researched material due to the stability of its chemical structure, biocompatibility, physical, optical, and electrical properties. Its photocatalytic properties have been utilized in various environmental applications to remove contaminants from both water and air [1]. Titania-based photocatalytic systems are used for a variety of applications such as decomposition of unwanted and toxic organic compounds, destruction of pollutants from contaminated water and air and killing of harmful bacteria and cancer cells [2]. The unique feature of the photocatalytic process is that it breaks down the pollutants and harmful organic compounds into simple molecules such as carbon dioxide and water [3]. Due to its stability in harsh environments, titanium dioxide (TiO 2 ) is a ceramic that has the potential to be the material of choice for gas sensors that operate at temperatures above 400 C [4]. It exists in three mineral forms viz: anatase, rutile, and brookite [5]. In general, TiO 2 is preferred in anatase form because of its high photocatalytic activity, since it has a more negative conduction band edge potential, high specific area, non-toxic, photochemically stable and relatively in-expensive. Anatase- TiO 2 for its strong photo induced redox power was found to be a superior photocatalytic material for purification and disinfection of water and air, as well as remediation of hazardous waste [6]. Background of TiO 2 photocatalysis Photocatalytic research is basically related to the development of solar energy use. The use of solar energy technology can be divided into solar batteries [7], solar heat [8] and photocatalysis.the core technology among them is to convert solar energy into chemical energy. This conversion refers to the synthesis of chemical energy to induce a chemical reaction. In the early 1970s, Fujishima and Honda [9] revealed the possibility of hydrogen production through water decomposition by photocatalysis and solar energy, and explosive research 35

3 began. Afterwards, the interest in TiO 2 photocatalysis has been growing in the academic and industrial fields, and has been applied actively to hydrogen production,air cleaning, metal anti-corrosion [10] and hydrophilic,self-purifi-cation and antibacterial activity [11]. Some of these technologies have been released on the market. Fig. 1 shows the various applications of TiO 2 catalysis in a recent. Photocatalysis may be termed as a photoinduced reaction which is accelerated by the presence of a catalyst. These type of reactions are activated by absorption of a photon with sufficient energy (equals or higher than the band-gap energy of the catalyst). The absorption leads to a charge separation due to promotion of an electron (e-) from the valence band of the semiconductor catalyst to the conduction band, thus generating a hole in the valence band, the schematic diagram of the process is presented in Fig1. The recombination of the electron and the hole must be prevented as much as possible if a photocatalyzed reaction must be favored (Fig. 1). Semiconductor molecules contain a valence band (VB) occupied with stable energy electrons and empty higher energy conduction bands (CB), the band gap of the semiconductor energy with higher energy is used to emit light inside the semiconductor to induce a reaction with the absorbent material on its surface via a redox reaction. This is called the photocatalytic reaction. Photocatalytic reactions are based on solar energy absorption in the bad gap of the semiconductor and the following photo-generated electron transfer. Therefore, all semiconductor materials can be used in photocatalysis. On the other hand, there are few effective semiconductors as photocatalysts, and TiO 2 is the most widely used among them. Fig.1 Schematic diagram of photocatalytic process initiated by photon acting on the semi conductor [12]. 36

4 Fundamentals and mechanism of TiO 2 photocatalysis The fundamentals of photophysics and photochemistry underlying the heterogeneous photocatalysis employing the semiconductor TiO 2 catalyst have been intensively reported in many literatures. The semiconductor TiO 2 has been widely utilised as a photocatalyst for inducing a series of reductive and oxidative reactions on its surface. This is solely contributed by the distinct lone electron characteristic in its outer orbital. When photon energy of greater than or equal to the band gap energy of TiO 2 is illuminated onto its surface, usually 3.2 ev (anatase) or 3.0 ev (rutile), the lone electron will be photo excited to the empty conduction band in femtoseconds Fig. 1 depicts the mechanism of the electron hole pair formation when the TiO 2 particle is irradiated with adequate hv. The light wavelength for such photon energy usually corresponds to l < 400 nm. The photonic excitation leaves behind an empty unfilled valence band, and thus creating the electron-hole pair. The series of chain oxidative reductive reactions that occur at the photon activated surface was widely postulated Operating parameters in photocatalytic processes In photocatalytic degradation of toxic organic compounds or dyes in wastewaters, the followings are operating parameters which affect the process: ph of the solution to be degraded, and the ph of the precursor solution (catalyst s solution during preparation of catalyst); oxidizing agent, calcination temperature, dopant content, and catalyst loading. These parameters will be considered one after the other as they influenced the photocatalytic processes of the degradation of dyes or toxic organic compounds in wastewaters. Methods for synthesis Many methods have been reported for the production of TiO 2 such as chemical solution decomposition, chemical vapor decomposition, two-step wet chemical method and sol gel [13]. The most widely used TiO 2 in photocatalysis is commercial Degussa P25 produced by flame hydrolysis of TiCl 4 at temperatures greater than 1200 C in the presence of hydrogen and oxygen. Recent literature revealed that sol gel is the most commonly used method for the preparation of photocatalysts, whether only TiO 2. The advantage of these methods (wet chemical methods, which include sol gel) is that they facilitate the synthesis of nanometer sized crystallized TiO 2 powder of high purity at relatively low temperature [14]. Conclusion TiO 2 photocatalyst under either UV light or solar irradiation has become more prominent owing to its low cost, safety, high photocatalytic activity, etc., and as an advanced oxidation technology for the water treatment industry. In addition to the degradation of organic contaminants, the photocatalytic activity of TiO 2 has potential use as an additive in 37

5 foods or medicines, electrodes of solar cells, etc. On the other hand, the utilization of solar energy is currently limited by the photo-inefficiency of the TiO 2 catalyst. Therefore, the development of an innovative TiO 2 photocatalyst and its optimization are needed this photocatalyst can be used commercially in photocatalytic water treatment technology. References: Li Q, Mahendra S, Lyon DY, Brunet L, Liga MV, Li D, Alvarez PJJ (2008) Water Res 42:4591 Comparelli R, Fanizza E, Curri ML, Cozzoli PD, Mascolo G, Passino R, Agostiano A (2005) Appl Catal B 55:81 Vijay M, Selvarajan V, Sreekumar KP, Yu J, Liu S, Ananthapadmanabhan PV (2009) Sol Energy Mater Sol Cells93:1540 Seeley Z, Choi YJ, Bose S (2009) Sens Actuators B 140:98 Gianluca L, Bono A, Krishnaiah JG, Collin D (2008) J Hazard Mater 157(2 3):209 Yu H-F, Zhang Z-W, Hu F-C (2008) J Alloys Compd 465:484 A. Nottrott, J. Kleissl, B. Washom, Renewable Energy 55 (2013) 230. S. Bouadila, S. Kooli, M. Lazaar, S. Skouri, A. Farhat, Applied Energy 110 (2013) 267 A. Fujishima, K. Honda, Nature 238 (1972) 37. C.K. Lee, International Journal of Electrochemical Science 7 (2012) A. Fujishima, X. Zhang, D.A. Tryk, Surface Science Reports 63 (2008) 515. Gaya UI, Abdullah AH (2008) J Photochem Photobiol C 9:1 Zhang X, Liu Q (2008) Mater Lett 62:2589 Peng F, Cai L, Huang L, Yu H, Wang H (2008) J Phys Chem Solids 69(7):

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