**1.4 Solar photocatalytic oxidation**

220 Studies on Water Management Issues

Besides, the catalyst remains unchanged throughout the process and thus can be reuse; therefore no consumable chemical is required. All these result in a significant reduction in overall operating cost. In addition, this process can be carried out at extremely low concentrations because the pollutants were strongly adsorbed on the surface of the catalyst, allowing sub part-per-million condition. Summing up all these benefits and advantages, heterogeneous photocatalysis provides a cheap and effective alternative to clean water

In this study, various issues with respect to the attributes of the photocatalyst and the mechanism behind titania-based photocatalysis will be discussed. The following discussion may be relevant to environmental cleanup context, given that the process is subjected to both contaminant reduction and oxidation relying on the tendency of the former to either

The phrase *advanced oxidation processes* (AOP) refer specifically to processes in which oxidation of organic contaminants occurs primarily through reactions with hydroxyl radicals (Glaze et al., 1995). It involves two stages of oxidation: (1) the formation of strong oxidants (*e.g.*, hydroxyl radicals) and (2) the reaction of these oxidants with organic contaminants in water (Alnaizy & Akgerman, 2000). In water treatment applications, AOPs usually refer to a specific subset of processes that involve O3, H2O2, and/or UV light. However, often AOPs are also referred to a more general group of processes that also involve semiconductor catalysis, cavitation, E-beam irradiation, and Fenton's reaction (Fox & Dulay, 1993; Legrini et al., 1993). All these processes can produce hydroxyl radicals, which can react with and destroy a wide range of organic contaminants, including phenolics. Although many of the processes noted above have different mechanisms for destroying organic contaminants, in general, the effectiveness of an AOP is proportional to its ability to

The O3 system is one of the AOP for the destruction of organic compounds in wastewater. Basically, aqueous systems saturated with ozone are irradiated with UV light of 253.7 nm. The extinction coefficient of O3 at 253.7 nm is 3300 L.mol/cm, much higher than that of H2O2 (18.6 L.mol/cm). The decay rate of ozone is about a factor of 1000 higher than that of H2O2 (Guittonneau et al*.*, 1991). The AOP with UV radiation and ozone is initiated by the photolysis of ozone. The photodecomposition of ozone leads to two hydroxyl radicals, which do not act as they recombine producing hydrogen peroxide, as shown in the

> • <sup>3</sup> H O + O 2OH + O 2 2 <sup>2</sup> ⎯⎯*<sup>h</sup>*ν

Implosion of cavity bubbles in sonicated water containing dissolved gases results in formation of hydrogen and hydroxyl radicals by fragmentation of water molecules. These

→ (1)

2 2 2OH H O • <sup>→</sup> (2)

accept or give up electrons respectively (Rajeshwar & Ibanez, 1995).

generate hydroxyl radicals (Fox & Dulay, 1993; Legrini et al., 1993).

following Eqns. (1) and (2) (Peyton & Glaze, 1988):

production and environmental remediation.

**1.1 Advanced oxidation processes** 

**1.2 Ozonation / UV** 

**1.3 Ultrasonication** 

In the past years, there have been a number of studies and reviews about this process (Bahnemann., 2004; Fox & Dulay, 1993; Herrmann et al*.*, 2007; Hoffmann et al*.*, 1995; Legrini et al*.*, 1993). Photocatalytic oxidation is based on the use of UV light and a semiconductor. Many catalysts have been tested, although titanium dioxide (TiO2) in the anatase form seems to possess the most interesting features, such as high stability, good performance and low cost (Bahnemann, 2004; Fox & Dulay, 1993; Hoffmann et al., 1995; Legrini et al., 1993).

Matthews (1990) reported that more than 90% of nitro benzene (NB) mineralization was achieved with TiO2 and sunlight. Minero et al. (1994) studied the photocatalytic degradation of NB on TiO2 and ZnO and reported complete mineralization with TiO2. Titanium dioxide has become the most studied and used photocatalyst, because it is easily available, chemically robust and durable. It can be used to degrade, *via* photocatalysis, a wide range of organic compounds (Herrmann et al*.*, 2007; Hincapié et al*.*, 2005; Leyva et al*.*, 1998; Robert & Malato, 2002). Photocatalytic degradation of phenolic compounds by employing Degussa P-25® in presence of sunlight has been successfully studied by many researchers (Curcó et al., 1996; Minero et al., 1994).
