**1. Introduction**

218 Studies on Water Management Issues

Žegura, B.; Heath, E.; Černoša, A.; Filipič, M. (2009). Combination of in vitro bioassays for

and drinking water samples. *Chemosphere*, 75, pp. 1453–1460

the determination of cytotoxic and genotoxic potential of wastewater, surface water

Heterogeneous photocatalysis has been intensively studied since the discovery of photoactivated water splitting process using titanium dioxide (TiO2) as electrode (Fujishima & Honda, 1972) in 1972. Heterogeneous photocatalysis can be defined as a reaction in which a catalytic process is initiated by the action of light.

Fujishima and co. discovered that water can be split into hydrogen and oxygen through this process in 1972. Hence early studies were focused on the production of hydrogen using solar energy as a clean fuel from water (Kawai & Sakata, 1980; Sato & White, 1980). Further studies found that irradiated semiconductors particles could catalyze a lot of interesting and useful reduction-oxidation reactions of organic and inorganic compounds (Fox & Dulay, 1993). Some of the semiconductor particles were found to be able to completely mineralize various organic and inorganic substances which are known as environmental pollutants (Fujishima et al., 2007). Since then, many researches were carried out based on the environmental applications of heterogeneous photocatalysis (Herrmann, 1999; Hoffman et al., 1995; Rajeshwar et al., 2001; Saravanan et al., 2009).

Various studies had been carried out to search for an ideal semiconductor photocatalyst, but titanium dioxide (TiO2) remains as a benchmark among other semiconductors. CdS, SnO2, WO3, SiO2, ZrO2, ZnO, Nb2O3, Fe2O3, SrTiO3 etc. were among the semiconductor materials that were being studied but titanium dioxide (TiO2) remained an excellent photocatalyst for its high resistance to photocorrosion and desirable band-gap energy (Ye & Ohmori, 2002). It can be used to degrade a variety of organic and inorganic pollutants (Fox & Dulay, 1993; Herrmann et al., 2007). Besides, titanium dioxide (TiO2) is easily available in the market, chemically inert and durable (Saravanan et al., 2009) and non-toxic.

In contrast with other conventional methods in environmental cleanup, heterogeneous photocatalysis involved the breakdown of the pollutants from complex molecules into simple and non-hazardous substances. Hence no residue is left and no sludge is produced from the process. Furthermore, no secondary treatment is needed to process the sludge.

Heterogeneous Photocatalytic Oxidation an Effective Tool for Wastewater Treatment – A Review 221

radicals in turn combine and generate other oxidative species such as peroxy and super

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;

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.,

The presence of non-biodegradable and toxic organic compounds in wastewater is one of the major problems in wastewater treatment. Organic compounds like phenol and its derivatives are known for their toxicity and are classified as persistent organic chemicals (POC) which is a major threat to human health. Phenol in particular, which is carcinogenic, is introduced to the water bodies by various means. Industrial manufacturers, normal households, and landfill leachate contribute these organic compounds (Bahnemann, 2004) into the water bodies and makes wastewater treatment more difficult. All these pollutants need to be removed from wastewater before it can be discharged to the environment. Such contaminants may also be found in surface and subsurface water which require treatment to achieve desirable drinking water quality (Lindner et al., 1995). Conventional water treatment process like activated carbon adsorption, membrane filter, ion exchange etc. generate and produce extra waste during the purification system, which will further increase the cost and time. As a result, many studies and researches have been carried out to develop a sustainable and cost-efficient treatment process that can effectively remove or degrade these organic and inorganic chemicals in wastewater (Ahmed et al., 2010; Zeltner et al., 1996) with

photocatalysis gaining much attention in the field of contaminant mineralization.

Majority of the natural purification of aqueous systems such as aerated lagoons or ponds, rivers and streams, lakes etc. are caused by the action of sunlight. Organic molecules were breakdown by the action of sunlight to simpler molecules and finally to carbon dioxide and

ambient conditions and the operating parameters. Such •

degradation of the organic compounds (Kidak & Ince, 2006).

OH) as well as hydrogen peroxide; the quantities of each depend on the

OH radicals are used for the

oxide radicals (•

Legrini et al., 1993).

1996; Minero et al., 1994).

**2.1 Photocatalytic process**

**2. Background of photocatalysis** 

**1.4 Solar photocatalytic oxidation** 

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 production and environmental remediation.

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

#### **1.1 Advanced oxidation processes**

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

#### **1.2 Ozonation / UV**

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

$$\text{CH}\_2\text{O}\_2 + \text{O}\_3 \xrightarrow{h\nu} 2\text{OH}^\* + \text{O}\_2 \tag{1}$$

$$\text{2OH}^\* \rightarrow \text{H}\_2\text{O}\_2\tag{2}$$

#### **1.3 Ultrasonication**

Implosion of cavity bubbles in sonicated water containing dissolved gases results in formation of hydrogen and hydroxyl radicals by fragmentation of water molecules. These radicals in turn combine and generate other oxidative species such as peroxy and super oxide radicals (• OH) as well as hydrogen peroxide; the quantities of each depend on the ambient conditions and the operating parameters. Such • OH radicals are used for the degradation of the organic compounds (Kidak & Ince, 2006).
