**2. Advanced Oxidation Processes (AOP)**

Advanced Oxidation Processes (or Advanced Oxidative Technologies) stand out among the new technologies potentially useful for the minimization of environmental impacts to biota (Ismail et al., 2009), and, among these technologies, are the photocatalytic degradation of contaminants in the environment, especially using solar radiation (Martin et al., 1995; Ziolli & Jardim, 1998; Machado et al., 2003a; Duarte et al.,2005; Augugliaro et al., 2007; Machado et al., 2008). They are characterized by being able to degrade a wide range of organic contaminants into carbon dioxide, water and inorganic anions through reactions involving oxidizing species, particularly hydroxyl radicals which have a high oxidizing power (Eo=2.8 V) (Nogueira & Jardim, 1998; Machado et al., 2003a; Machado et al., 2008; Kumar & Devi, 2011).

Among the AOP can be cited processes involving the use of ozone, hydrogen peroxide, catalytic decomposition of hydrogen peroxide in acid medium (Fenton or/and photo-Fenton reactions), and semiconductors such as titanium dioxide (heterogeneous photocatalysis) (Nogueira & Jardim, 1998; Kumar & Devi, 2011). The heterogeneous photocatalysis is considered one of the most promising advanced oxidation technologies. In heterogeneous photocatalytic processes, highly oxidizing reactive oxygen species (ie hydroxyl radicals, superoxide radical-ions, etc.) are generated from interaction between the semiconductor electronically excited, oxygenated species and other substrates (Andreozzi et al., 1999; Fujishima et al., 2007; Machado et al., 2008; Kumar & Devi, 2011).

The solar photocatalysis deserves special attention, since the sun is a virtually inexhaustible source of energy at no cost (Machado et al., 2008; Amat et al., 2011).

#### **2.1 Heterogeneous photocatalysis**

The great potential of heterogeneous photocatalysis has been demonstrated mainly in the treatment of industrial effluents and wastewater through the degradation of contaminants (Malato et al., 1997; Andreozzi et al., 1999; Malato et al., 2002; Sattler et al., 2004a, 2004b; Duarte et al., 2005; Pons et al., 2007; Palmisano et al., 2007a; Machado et al., 2008). A significant number of these studies have focused on the photocatalytic properties of TiO2, suggesting a promising use of this material in heterogeneous photocatalysis (Mills & Hunte, 1997; Malato et al., 2002; Mills et al., 2002; Machado et al., 2003a; Machado et al., 2003b; Sattler et al., 2004a, 2004b; Duarte et al., 2005; Palmisano et al., 2007a; Pons et al.,2007; Machado et al., 2008; Oliveira et al., 2012).

The potential of heterogeneous photocatalysis has been demonstrated in studies originally reported by Fujishima and Honda (Fujishima & Honda, 1971, 1972). The photoactivation of a semiconductor is based on its electronic excitation by photons with energy greater than the band gap energy. This tends to generate vacancies in the valence band – VB (holes, h+) and regions with high electron density (e-) in the conduction band – CB (Hoffmann et al., 1995; Nogueira & Jardim, 1998; Kumar & Devi, 2011). These holes have pH dependent and strongly positive electrochemical potentials, in the range between +2.0 and +3.5 V, measured against a saturated calomel electrode (Khataee et al., 2011). This potential is sufficiently positive to generate hydroxyl radicals (**HO.** ) from water molecules adsorbed on the surface of the

production of hydrogen for subsequent generation of energy (Jing et al., 2010; Kim & Choi,

Advanced Oxidation Processes (or Advanced Oxidative Technologies) stand out among the new technologies potentially useful for the minimization of environmental impacts to biota (Ismail et al., 2009), and, among these technologies, are the photocatalytic degradation of contaminants in the environment, especially using solar radiation (Martin et al., 1995; Ziolli & Jardim, 1998; Machado et al., 2003a; Duarte et al.,2005; Augugliaro et al., 2007; Machado et al., 2008). They are characterized by being able to degrade a wide range of organic contaminants into carbon dioxide, water and inorganic anions through reactions involving oxidizing species, particularly hydroxyl radicals which have a high oxidizing power (Eo=2.8 V) (Nogueira &

Among the AOP can be cited processes involving the use of ozone, hydrogen peroxide, catalytic decomposition of hydrogen peroxide in acid medium (Fenton or/and photo-Fenton reactions), and semiconductors such as titanium dioxide (heterogeneous photocatalysis) (Nogueira & Jardim, 1998; Kumar & Devi, 2011). The heterogeneous photocatalysis is considered one of the most promising advanced oxidation technologies. In heterogeneous photocatalytic processes, highly oxidizing reactive oxygen species (ie hydroxyl radicals, superoxide radical-ions, etc.) are generated from interaction between the semiconductor electronically excited, oxygenated species and other substrates (Andreozzi et

The solar photocatalysis deserves special attention, since the sun is a virtually inexhaustible

The great potential of heterogeneous photocatalysis has been demonstrated mainly in the treatment of industrial effluents and wastewater through the degradation of contaminants (Malato et al., 1997; Andreozzi et al., 1999; Malato et al., 2002; Sattler et al., 2004a, 2004b; Duarte et al., 2005; Pons et al., 2007; Palmisano et al., 2007a; Machado et al., 2008). A significant number of these studies have focused on the photocatalytic properties of TiO2, suggesting a promising use of this material in heterogeneous photocatalysis (Mills & Hunte, 1997; Malato et al., 2002; Mills et al., 2002; Machado et al., 2003a; Machado et al., 2003b; Sattler et al., 2004a, 2004b; Duarte et al., 2005; Palmisano et al., 2007a; Pons et al.,2007;

The potential of heterogeneous photocatalysis has been demonstrated in studies originally reported by Fujishima and Honda (Fujishima & Honda, 1971, 1972). The photoactivation of a semiconductor is based on its electronic excitation by photons with energy greater than the band gap energy. This tends to generate vacancies in the valence band – VB (holes, h+) and regions with high electron density (e-) in the conduction band – CB (Hoffmann et al., 1995; Nogueira & Jardim, 1998; Kumar & Devi, 2011). These holes have pH dependent and strongly positive electrochemical potentials, in the range between +2.0 and +3.5 V, measured against a saturated calomel electrode (Khataee et al., 2011). This potential is sufficiently positive to

) from water molecules adsorbed on the surface of the

Jardim, 1998; Machado et al., 2003a; Machado et al., 2008; Kumar & Devi, 2011).

al., 1999; Fujishima et al., 2007; Machado et al., 2008; Kumar & Devi, 2011).

source of energy at no cost (Machado et al., 2008; Amat et al., 2011).

**2.1 Heterogeneous photocatalysis** 

Machado et al., 2008; Oliveira et al., 2012).

generate hydroxyl radicals (**HO.**

2010; Melo & Silva, 2011).

**2. Advanced Oxidation Processes (AOP)** 

semiconductor (eqs. 1-3). The photocatalytic efficiency depends on the competition between the formation of pairs of electrons and holes in semiconductor surface and the recombination of these pairs (eq.4) (Nogueira & Jardim, 1998; Ziolli & Jardim, 1998; Ni et al., 2011).

$$\rm TiO\_2 + hv \rightarrow TiO\_2 \text{ (e} \, ^\circ\_{\text{CB}} + \text{h}^\* \, \_{VB} \tag{1}$$

$$\rm{h^{\*}} + \rm{H\_{2}O\_{ads}} \rightarrow \rm{HO^{-}} + \rm{H^{\*}} \tag{2}$$

$$\text{h} \star \text{OH}^{\cdot}\_{\text{ads.}} \rightarrow \text{HO} \cdot \tag{3}$$

$$\rm{TiO\_2} \left( \rm{e}\_{\rm{CB}} + \rm{h}^\* \rm{}\_{\rm{VB}} \right) \to \rm{TiO\_2} + \Delta \tag{4}$$

The electrons transferred to the conduction band are responsible for reducing reactions, such as the formation of gaseous hydrogen and the generation of other important oxidizing species such as superoxide anion radical. In the case of TiO2, the band gap energy, Eg, is between 3.00 and 3.20 eV (Hoffmann et al., 1995; Palmisano et al., 2007a; Jin et al., 2010; Kumar & Devi, 2011). This process can be viewed schematically in **Fig. 1**.

Fig. 1. General scheme for some primary processes that occur after photoactivation of a semiconductor and for photocatalytical production of gaseous hydrogen by decomposition of water.

The production of reactive species by a TiO2 photocatalyst is influenced by a series of factors, such as surface acidity and pH of the reaction medium, control of the kinetic of recombination of charge carriers, interfacial electron-transfer rate, optical absorption of the semicondutor, phase distribution, morphology, specific surface area and porosity (Hoffmann et al., 1995; Furube et al, 2001; Diebold, 2003; Carp et al., 2004; Kumar & Devi (2011).

The reactions (1) to (4) combined with other (Hoffmann et al., 1995; Machado et al., 2008; Kumar & Devi, 2011) give an approximate view of the chain reactions that compose a heterogeneous photocatalytic process.

Different semiconductors are able to trigger the heterogeneous photocatalytic processes. Other in addition to TiO2 are: CdS, ZnO, ZnS, and Fe2O3 (Nogueira & Jardim, 1998).

TiO2 stands in front of others for its abundance, low toxicity, good chemical stability over a wide pH range, photosensitivity, photostability, insolubility in water, low cost, chemical inertness, biological and chemical inertness, and stability to corrosion and photocorrosion (Martin et al., 1995; Augugliaro et al., 2007). However, its band gap energy limits, in principle, its application in photocatalytic processes induced by solar radiation, since the radiation incident on the biosphere consists of approximately 5 % UV, 43 % visible and 52 %, harvesting infrared (Kumar & Devi, 2011).

The introduction of changes in the crystalline structure of TiO2 through the introduction of dopant ions and/or modifying ions and associations between TiO2 and other semiconductor oxidesin order to expand the use of incident radiation, is particularly important if the aim is to use solar radiation in photocatalytic processes. The synthesis of new materials based on TiO2 has resulted in substantial progress towards the improvement of the photocatalytic activity of this semiconductor (Imhof & Pine, 1997; Cavalheiro et al.,2008; Eguchi et al., 2001; Agostiano et al., 2004; Machado et al., 2008; Zaleska et al., 2010; Batista, 2010; Machado et al., 2011b).

Titanium dioxide can be found in nature in the form of three different polymorphs: Anatase, Rutile and Brookite (Hanaor & Sorrell, 2011; Khataee et al., 2011; Kumar & Devi, 2011). Among these polymorphs, the thermodynamically more stable is the rutile, which can be obtained from the conversion of anatase, which in turn is the most photoactive polymorph (Hoffmann et al., 1995; Khataee et al., 2011).

Technological applications of titanium oxide are quite large. In addition to the previously described, TiO2 has been used in filters to absorb ultraviolet radiation (sunscreens, for example), pigments, in chemical sensors for gases (Pichat et al., 2000), as constituents of ceramic materials for bone and dental implants (Chen et al., 2008), among others.
