**2.6.1 Degradation of organics**

228 Studies on Water Management Issues

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

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

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)

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

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

sunlight, which is a natural resource for reaction activation (Narayana et al., 2011).

than degradation using Degussa P25 which is 85.9% (Han et al*.*, 2009).

compared to doping of one ion (Wang et al., 2010).

photocatalytic activity and separability (Wang et al., 2010).

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

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