**2.5 Synthesis and doping method**

Many studies had been carried out to alter the characteristics of the titanium dioxide (TiO2) in order to improve the practical and commercial values of titanium dioxide (TiO2) as a photocatalyst. For most of the cases, doping was carried out to improve the photocatalytic activity, the absorption of visible region of the solar spectrum, and to impart separable property.

Ao et al. (2009) reported the degradation of a dye (Red X-3B) under sunlight using N-doped titania-coated g-Fe2O3 magnetic activated carbon (NT-MAC). The titanium dioxide (TiO2) was doped with nitrogen to improve the visible light absorption while the g-Fe2O3 magnetic activated carbon was coated to impart the magnetic properties. The preparations were carried out under low temperature and ambient pressure. It is reported that the photocatalytic of the NT-MAC was approximately three times than that of Degussa P25. The separation can be done easily using an external magnetic field. Furthermore, the prepared NT-MAC can be recycled and reused without any mass losing and the degradation of the X-3B remains higher than 85% after six cycles (Ao et al., 2009).

Heterogeneous Photocatalytic Oxidation an Effective Tool for Wastewater Treatment – A Review 229

fuel using hydrogen, heterogeneous photocatalysis has taken a new step since its breakthrough in the environmental remediation field. Today, this technique has been implemented in various applications, from water and air treatment to health applications.

The degradation of organics perhaps is the most important applications of photocatalysis. The degradations of organic compounds such as alcohols, carboxylic acids, phenolic derivatives or chlorinated aromatics into non-hazardous and harmless products or residues such as carbon dioxide, water or other minerals had been well documented (Bhatkande et al., 2001; Chen & Ray, 2001; Michael et al., 1995; Mills et al., 1993; Pirkanniemi & Sillanpaa, 2002). Joanna et al. (2000) reported that oily water can also be treated effectively by photocatalysis. Herbicides and pesticides like 2, 4, 5, trichlorophenoxyacetic acid, 2, 4, 5, trichlorophenol, s-triazine herbicides and DDT which generally considered as hazardous

Other than the organic chemical compounds mentioned above, a variety of inorganic compounds are sensitive to photochemical conversion on the catalyst surfaces. Inorganics such as chlorate and bromate (Mills et al., 1996), azide, halide ions, nitric oxide (NO), palladium (Pd) and rhodium (Rh) species, and sulphur species can be broken down (Michael et al., 1995). Metal salts like silver nitrate (AgNO3), mercury (II) chloride (HgCl) and organometallic materials can be eliminated from water (Bhatkande et al., 2001), as well as cyanide (CN), thiocyanate (SCN-), ammonia (NH3), nitrates (NO3-) and nitrites (NO2-)

Humic substances (HS) can be generally defined as a class of naturally existing biogenic heterogeneous organic substances that can be further classified as being yellow-brown and having high molecular weights (MacCarthy, 2001). HS can also be defined as the fraction of filtered water that adsorb on XAD-8 resin (a non-ionic polymeric adsorbent) at pH 2 (Obernosterer & Herndl, 2000). They are the major components of the dissolved organic carbon (DOC) pool in surface water (marine waters and fresh waters) and sub-surface or ground waters. They are often said to be a main factor that lead to yellowish-brown colour in the water bodies (Schmitt-Kopplin, 1998). The concentration of the HS differs from place to place; seawater normally contains 2-3mg/L of HS. According to Gaffney et al. (1996), their physical properties like size and their chemical properties like the structure and the number and position of the functional group differ, relying on the origin and the age of the

HS are known to have the ability to change the behaviour of certain pollutants considerably, such as trace metal speciation and toxicity, (Bekbolet & Balcioghu, 1996; Shin et al., 1996), solubilisation and adsorption of hydrophobic contaminants (Chiou et al., 1986; Tanaka et al., 1997) and aqueous photochemistry (Fukushima et al., 2000). HS can act as substrates for bacterial growth, hinder the bacterial degradation of impurities (colours), interact with heavy metals such as Fe, Mn and Pb and thus making them difficult to remove, help to

pollutants can also be completely mineralized (Olis et al., 1991).

**2.6.1 Degradation of organics** 

**2.6.2 Elimination of inorganics** 

substance (Gaffney et al., 1996).

**2.6.3 Elimination of natural organic matter** 

(Blake, 2001).

Han et al. (2009) studied the degradation of organic dyes using various modified titanium dioxide (TiO2) photocatalysts. The modifications include doping with metals (noble metals, transition metals, lanthanide metals, alkaline and alkaline earth metals, cadmium sulphide etc.) and non-metals (nitrogen, fluorine, sulphur, carbon etc.). The purposes of these modifications and doping were to improve photocatalytic efficiency, complete degradation of organic dyes, improve visible light absorption, improve stability and reproducibility, and to improve recycle and reuse abilities of titanium dioxide (TiO2). The modified titanium dioxide (TiO2) showed considerably improved photocatalytic activity. For example, a complete degradation of Rhodamine (RB) in 105 minutes was observed using silver doped indium (III) oxide-coated TiO2 (Ag/In2O3-TiO2) as photocatalyst in 2008. It is more efficient than degradation using Degussa P25 which is 85.9% (Han et al*.*, 2009).

Narayana et al. (2011) studied the photocatalytic decolourization of basic green dye using pure and ferum (Fe) and cobalt (Co) doped titanium dioxide (TiO2) under sunlight irradiation. The purpose of doping was to improve the visible light absorption of the photocatalyst. The doped titanium dioxide (TiO2) was prepared using sol-gel method. The Fe-doped titanium dioxide (TiO2) showed the highest photoactivity among the other two with a 98% degradation of dye under sunlight illumination. Hence, doped titanium dioxide (TiO2) can have very high commercial value in wastewater treatment since it utilizes only sunlight, which is a natural resource for reaction activation (Narayana et al., 2011).

Wang et al. (2010) doped titanium dioxide (TiO2) with tin (Sn) and nitrogen (N) intended to improve the visible light absorption of titanium dioxide (TiO2) photocatalyst. The doping was successfully carried out via simple sol-gel method. Pure TiO2, N-doped TiO2, Sn-doped TiO2, and co-doped N/Sn-TiO2 were tested separately to compare their characteristics. N/Sn-TiO2 recorded the highest absorption in the visible region of solar spectrum. Besides, N/Sn-TiO2 also recorded the highest visible-light activity among the other three by using 4 chlorophenol (4-CP) in water under visible light illumination. Surprisingly, N/Sn-TiO2 also had the highest photoactivity under UV irradiation. This implies that the co-doping of two foreign ions is more efficient in improving photoactivity of titanium dioxide (TiO2) compared to doping of one ion (Wang et al., 2010).

A simple sol-gel method to prepare titania-coated magnetic porous silica (TMS) photocatalyst was reported by Wang et al. (2010). The TMS was then employed in the degradation of red X-3B dye under UV and visible light irradiation to determine its photocatalytic activity. The same was done using commercialized titanium dioxide (TiO2), Degussa P25 for comparison purpose. They recorded that the TMS had considerably higher photoactivity compared to that of Degussa P25, under either UV or visible light illumination. The TMS can be separated by applying external magnetic and thus can be reused without any mass loss. Hence, TMS can be a suitable photocatalyst for practical water purification system due to its high photocatalytic activity and separability (Wang et al., 2010).

#### **2.6 Applications of photocatalysis on water and wastewater treatment**

Since the discovery of water splitting phenomenon via photocatalysis by Fujishima and Honda in 1972, the research and development of the heterogeneous photocatalytic process has never been stop and has been growing rapidly (Linsebigler et al., 1995). Though early studies and researches were focused on the energy production i.e. the production of clean fuel using hydrogen, heterogeneous photocatalysis has taken a new step since its breakthrough in the environmental remediation field. Today, this technique has been implemented in various applications, from water and air treatment to health applications.
