**1. Introduction**

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A photocatalyst is defined as a substance which is activated by adsorbing a photon and is capable of accelerating a reaction without being consumed [1]. These substances are invaria‐ bly semiconductors. Semiconducting oxide photocatalysts have been increasingly focused in recent years due to their potential applications in solar energy conversion and environmen‐ tal purification. Semiconductor heterogeneous photocatalysis has enormous potential to treat organic contaminants in water and air. This process is known as advanced oxidation process (AOP) and is suitable for the oxidation of a wide range of organic compounds. Among AOPs, heterogeneous photocatalysis have been proven to be of interest due to its efficiency in degrading recalcitrant organic compounds. Developed in the 1970s, heteroge‐ neous photocatalytic oxidation has been given considerable attention and in the past two decades numerous studies have been carried out on the application of heterogeneous photo‐ catalytic oxidation process with a view to decompose and mineralize recalcitrant organic compounds. It involves the acceleration of photoreaction in the presence of a semiconductor catalyst [2]. Several semiconductors (TiO2, ZnO, Fe2O3, CdS, ZnS) can act as photocatalysts but TiO2 has been most commonly studied due to its ability to break down organic pollu‐ tants and even achieve complete mineralization. Photocatalytic and hydrophilic properties of TiO2 makes it close to an ideal catalyst due to its high reactivity, reduced toxicity, chemi‐ cal stability and lower costs [3]. Fujishima and Honda [4] pioneered the concept of titania photocatalysis (also known as "Honda-Fujishima effect"). Their work showed the possibility of water splitting in a photoelectrochemical cell containing an inert cathode and rutile titania anode. The applications of titania photoelectrolysis has since been greatly focused in envi‐ ronmental applications including water and wastewater treatment. This chapter provides insight into the fundamentals of the TiO2 photocatalysis, discusses the effect of variables af‐

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fecting the performance of degradation of organic pollutants in water with a view to current state of knowledge and future needs.

The presence of dissolved oxygen is extremely important during photocatalytic degradation

taining the electroneutrality of the TiO2 particles [5]. In other words, it is important for effec‐ tive photocatalytic degradation of organic pollutants that the reduction process of oxygen and the oxidation of pollutants proceed simultaneously to avoid the accumulation of elec‐

**Figure 1.** Mechanism of electron-hole pair formation in a TiO2 particle in the presence of pollutant in water [7].

A number of solids can be referred to as photocatalysts and as mentioned earlier, metal ox‐ ide semiconductors are considered to be the most suitable photocatalysts due to their photo‐ corrosion resistance and wide band gap energies [10]. Table 1 provides the band gap energies at corresponding wavelength for well known semiconductors. TiO2 stands out as the most effective photocatalyst and has been extensively used in water and wastewater treatment studies because it is cost effective, thermally stable, non-toxic, chemically and bio‐ logically inert and is capable of promoting oxidation of organic compounds [11]. The photo‐ catalytic activity of TiO2 is dependent on surface and structural properties which include

**3. Types of photocatalysts and their characteristics**

tron in the conduction band and thus reduce the rate of recombination of eCB<sup>−</sup>

/hVB+

) difficult which results in main‐

http://dx.doi.org/10.5772/53699

Photocatalytic Degradation of Organic Pollutants in Water

and hVB+

[8, 9].

197

as it can make the recombination process on TiO2 (eCB<sup>−</sup>

#### **2. Mechanism and fundamentals of photocatalytic reactions**

Heterogeneous photocatalysis using UV/TiO2 is one of the most common photocatalytic process and is based on adsorption of photons with energy higher than 3.2 eV (wavelengths lower than ~390 nm) resulting in initiating excitation related to charge separation event (gap band) [5]. Generation of excited high-energy states of electron and hole pairs occurs when wide bandgap semiconductors are irradiated higher than their bandgap energy. It results in the promotion of an electron in the conductive band (eCB<sup>−</sup> ) and formation of a positive hole in the valence band (hVB+ ) [5] (Eq. 1). The hVB+ and eCB<sup>−</sup> are powerful oxidizing and reducing agents, respectively. The hVB+ reacts with organic compounds resulting in their oxidation producing CO2 and H2O as end products (Eq. 2). The hVB+ can also oxidize organic com‐ pounds by reacting with water to generate •OH (Eq. 3). Hydroxyl radical (•OH) produced by has the second highest oxidation potential (2.80 V), which is only slightly lower than the strongest oxidant – fluorine. Due to its electrophilic nature (electron preferring), the •OH can non-selectively oxidize almost all electron rich organic molecules, eventually converting them to CO2 and water (Eq. 4).

$$\text{TiO}\_2 + \text{hv} \left( \text{<387 nm} \right) \rightarrow \text{e}\_{\text{CB}} \text{"+h}\_{\text{VB}} \text{"} \tag{1}$$

$$\text{H}\_{\text{VB}}\text{}^{\text{+}}\text{R}\rightarrow\text{intermediate}\rightarrow\text{CO}\_{2}\text{+H}\_{2}\text{O}\tag{2}$$

$$\text{CH}\_2\text{O} + \text{H}\_{\text{VB}} \text{ }^\text{+} \rightarrow \text{"OH} + \text{H}^\* \tag{3}$$

$$\text{H}^+\text{OH} + \text{R} \rightarrow \text{intermediate} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} \tag{4}$$

where R represents the organic compound.

The conductive band can react with O2 forming an anion radical superoxide as shown in Eq. 5. Further reaction can lead to the formation of hydrogen peroxide which lead to the forma‐ tion of •OH [6]. The mechanism of the electron hole-pair formation when the TiO2 is irradia‐ tied is given in Figure 1 [7].

$$\text{Fe}\_{\text{CB}}\text{"+O}\_{2} \rightarrow \text{O}\_{2}\text{"}\tag{5}$$

The presence of dissolved oxygen is extremely important during photocatalytic degradation as it can make the recombination process on TiO2 (eCB<sup>−</sup> /hVB+ ) difficult which results in main‐ taining the electroneutrality of the TiO2 particles [5]. In other words, it is important for effec‐ tive photocatalytic degradation of organic pollutants that the reduction process of oxygen and the oxidation of pollutants proceed simultaneously to avoid the accumulation of elec‐ tron in the conduction band and thus reduce the rate of recombination of eCB<sup>−</sup> and hVB+ [8, 9].

fecting the performance of degradation of organic pollutants in water with a view to current

Heterogeneous photocatalysis using UV/TiO2 is one of the most common photocatalytic process and is based on adsorption of photons with energy higher than 3.2 eV (wavelengths lower than ~390 nm) resulting in initiating excitation related to charge separation event (gap band) [5]. Generation of excited high-energy states of electron and hole pairs occurs when wide bandgap semiconductors are irradiated higher than their bandgap energy. It results in

and eCB<sup>−</sup>

pounds by reacting with water to generate •OH (Eq. 3). Hydroxyl radical (•OH) produced by has the second highest oxidation potential (2.80 V), which is only slightly lower than the strongest oxidant – fluorine. Due to its electrophilic nature (electron preferring), the •OH can non-selectively oxidize almost all electron rich organic molecules, eventually converting

( ) - +

The conductive band can react with O2 forming an anion radical superoxide as shown in Eq. 5. Further reaction can lead to the formation of hydrogen peroxide which lead to the forma‐ tion of •OH [6]. The mechanism of the electron hole-pair formation when the TiO2 is irradia‐


reacts with organic compounds resulting in their oxidation

<sup>2</sup> CB VB TiO + hv <387 nm e + h ® (1)

VB 2 2 h + R intermediates CO + H O ® ® (2)

•OH + R intermediates CO + H O ® ® 2 2 (4)

+ • <sup>+</sup> H O + h OH + H 2 VB ® (3)

CB 2 2 e + O O ® (5)

) and formation of a positive hole

can also oxidize organic com‐

are powerful oxidizing and reducing

**2. Mechanism and fundamentals of photocatalytic reactions**

the promotion of an electron in the conductive band (eCB<sup>−</sup>

producing CO2 and H2O as end products (Eq. 2). The hVB+

+

where R represents the organic compound.

tied is given in Figure 1 [7].

) [5] (Eq. 1). The hVB+

state of knowledge and future needs.

196 Organic Pollutants - Monitoring, Risk and Treatment

in the valence band (hVB+

agents, respectively. The hVB+

them to CO2 and water (Eq. 4).

**Figure 1.** Mechanism of electron-hole pair formation in a TiO2 particle in the presence of pollutant in water [7].

### **3. Types of photocatalysts and their characteristics**

A number of solids can be referred to as photocatalysts and as mentioned earlier, metal ox‐ ide semiconductors are considered to be the most suitable photocatalysts due to their photo‐ corrosion resistance and wide band gap energies [10]. Table 1 provides the band gap energies at corresponding wavelength for well known semiconductors. TiO2 stands out as the most effective photocatalyst and has been extensively used in water and wastewater treatment studies because it is cost effective, thermally stable, non-toxic, chemically and bio‐ logically inert and is capable of promoting oxidation of organic compounds [11]. The photo‐ catalytic activity of TiO2 is dependent on surface and structural properties which include crystal composition, surface area, particle size distribution, porosity and band gap energy [12]. TiO2 is also known as titania, titanic oxide, titanium white, titanic anhydride, or titanic acid anhydride. It is prepared using ilmenite and rutile in crystalline forms called anatase and rutile. The anatase form is achieved by processing of titanium sulphate, which is ach‐ ieved when ilmenite is treated with sulphuric acid. Rutile crystalline form is obtained when raw rutile is chlorinated and the resulting titanium tetrachloride is subjected to vapor phase oxidation [13]. When photon energy (hv) of higher than or equal to the bandgap energy of TiO2 is illuminated onto its surface, typically 3.2 eV (anatase) or 3.0 eV (rutile), the lone elec‐ tron is photoexcited to the empty conduction band in femtoseconds [7]. Degussa P25 which is the most widely used form of TiO2 is composed of 75% anatase and 25% rutile and has a specific BET surface area of 50 m2 /g. The high effectiveness of D25 is related to the inhibition of recombination process on TiO2 (eCB<sup>−</sup> /hVB+ ) due to the smaller band gap of rutile that ab‐ sorbs photons and generates electron-hole pairs and the electron transfer from the rutile conductive band to the electron traps occurs in the anatase phase [14].

**5. Photocatalytic reactors**

degradation of organic pollutants in water.

**5.1. Slurry reactors**

**5.2. Immobilized TiO2 reactors**

Photocatalytic reactors can be classified based on the deployed state of the photocatalyst, i.e., suspended or attached. Photocatalytic reactors can use either UV or solar radiation. So‐ lar photocatalytic reactors have been of great interest for the photoxodation of organic con‐ taminants in water. Such kind of reactors can be divided into concentrating or nonconcentrating reactors [16]. Both the reactor types extend certain advantages and disadvantages. For example, non-concentrating reactors have negligible optical losses and therefore can use direct and diffuse sun irradiation but are larger in size compared with the concentrating reactors and have high frictional pressure losses [16]. However, the use of so‐ lar radiated photoreactors is limited due to the intrinsic nature of the TiO2 particles. Follow‐ ing section provides details on the type of reactors used in various studies for the

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199

Until recently, TiO2 slurry reactors are most commonly type used in water treatment. These show largest photocatalytic activity compared with the immobilized photocatalyst and pro‐ vide a high total surface area of photocatalyst per unit volume which is one of the most impor‐ tant factor configuring a photocatalytic reactor [7]. However, these reactors require separation of the sub-micron TiO2 particles from the treated water which complicates the treatment proc‐ ess. Several techniques were proposed to achieve post-treatment separation such as the use of settling tanks (overnight particle settling) or external cross-flow filtration system [7]. However

Photocatalytic reactors with immobilized TiO2 are those in which catalyst is fixed to support via physical surface forces or chemical bonds. These reactors extend the benefit of not re‐ quiring catalyst recovery and permit the continuous use of the photocatalyst [16]. Hybrid photocatalytic membrane reactors have been developed to achieve the purpose of down‐ stream separation of photocatalyst. The photocatalytic membrane reactors can be general‐ ized in two categories (1) irradiation of the membrane module and (2) irradiation of feed tank containing photocatalyst in suspension [17]. Various membranes such as microfiltra‐ tion, ultrafiltration, and nanofiltration membranes may be used for this purpose depending on the requirements of the treated water quality [7]. Photocatalytic membrane reactors have been successfully used for the degradation of tricholoroethylene and 4-nitrophenol [18, 19]. However, these reactors possess drawbacks such as low surface area to volume ratios, cata‐ lyst fouling and significant pressure drop [16]. Another problem associated with the mem‐ brane photocatalytic reactors is the diffusion of organic compounds to the catalyst surface which is slow particularly when the organic compounds concentration is low [20]. One pos‐ sible solution to the slow diffusion is using pores of nano size to enable photocatalyst to per‐

the use of filtrations systems increases the cost of the treatment process.

form selective permeation and to produce an oxidized permeate stream [21].


**Table 1.** Band gap energies of various semiconductors at relevant wavelengths [15]

#### **4. Radiation sources for photocatalysis**

Both artificial UV lamps and sunlight can be used as the radiation source for photocatalytic process. Artificial UV lamps containing mercury are the most commonly used source of UV irradiation. These can be divided into low pressure mercury lamp, medium pressure mercu‐ ry lamp and high pressure mercury lamp. Sunlight has also been used in the photocatalytic process as nearly 4-5% of the sunlight that reaches the earth's surface is in the 300-400 nm near UV light range. Furthermore solar energy has limitations due to the graphical varia‐ tions when compared with the artificial UV lamps. However ongoing interests and develop‐ ments in harnessing solar energy are expected to increase its use in photocatalytic degradation applications.
