**2.1 Photocatalytic process**

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

Heterogeneous Photocatalytic Oxidation an Effective Tool for Wastewater Treatment – A Review 223

TiO (e ) O H TiO HO O H 2 cb 2ads 2 2 2

TiO (e ) HO H H O 2 cb 2 2 2

OH radical (OH•) may be formed from the cleavage of H2O2 via one of the following

<sup>2</sup> H O O OH O HO 2 2 <sup>2</sup>

H O TiO (e ) HO HO TiO 2 2 2 cb <sup>2</sup>

The OH radical from reaction 4 is the most important oxidant formed in a photocatalytic process. It is the primary oxidant in the degradation of organic compounds (Ahmed et al.,

OH Organic CO<sup>2</sup>

There are many semiconductor materials can be used as a photocatalyst. Semiconductors like TiO2, ZnO, Fe2O3, CdS, ZnS etc. are all suitable materials to initiate a photocatalytic process. Extensive studies and research concludes that an 'ideal photocatalyst' should

Attributes of an ideal photocatalyst for heterogeneous photocatalysis

• Can be react with wide range of substrate and high adaptability to

Titanium dioxide (TiO2) has been widely recognized as an excellent photocatalyst. It is known to have superb pigmentary properties, high adsorption in the ultraviolet region, and high stability which allows it to be used in various applications such as electroceramics, glass, and photocatalytic purification of chemical in air and water. Two types of reactors have been developed which are suspension/slurry and thin film/fixed in wastewater

H2O2 could also be formed from the HO2•- species by reaction 8.

2010). The degradation reaction is expressed in reaction 12.

• Stability and sustained photoactivity • Biologically and chemically inert, non toxic

• Good adsorption in solar spectrum

various environment

• Suitability towards visible or near UV light

• High conversion efficiency and high quantum yield

Table 1. Attributes of an ideal photocatalyst for heterogenous photocatalytic process

reactions 9, 10, 11.

**2.3 Photocatalytic materials** 

possess the attributes shown in Table 1.

• Low cost

(Bhatkhande et al., 2001)

**2.3.1 Photocatalysts** 

<sup>−</sup> + •− •− <sup>+</sup> + +→ + →+ (7)

− •− + + +→ (8)

H O 2OH 2 2 *hv* • + → (9)

•−• − + → ++ (10)

• + → (12)

− •− + →++ (11)

other mineral compounds. There are many natural accelerators which can be used to accelerate this natural process. The introduction of 'colloidal semiconductor' and catalyst to catalyze distinct redox reactions on semiconductors could boost this sunlight driven natural purification process (Matthews, 1993).

Wastewater treatment using photocatalysis involves the combination of heterogeneous photocatalysis with solar technologies (Zhang et al., 1994). Semiconductor photocatalysis, especially titania-based photocatalysis has been applied to various environmental problems other than water and air purification. Different studies have been carried out from fundamental to practical aspects to improve the process and the properties of the photocatalyst in recent years (Rajeshwar & Ibanez, 1997; Schiavello, 1997; Serpone & Pelizzetti, 1989). Hoffman et al. (1995) reported in that the utilizations of irradiated semiconductors for the degradation of organic pollutants were well documented and have shown positive and encouraging results for various organic pollutants. Various studies have also been carried out from fundamental to practical aspects to improve the process and the properties of the photocatalyst in recent years.

#### **2.2 Mechanisms of generating oxidizing species**

The heterogeneous photocatalysis process is very complex. The oxidizing pathway is not very clear yet. Jean-Marie Herrmann (1999) suggested in that the overall of classic heterogeneous photocatalysis process can be divided into five steps (Herrmann, 1999):-


There are two pathways where the OH radicals can be formed. The valence band hole, h+vb can either react with the adsorbed water or the surface OH- groups on the titanium dioxide (TiO2) particle (Ekabi & Serpone, 1988). Equations 3 and 4 show the two reactions.

$$\text{TiO}\_2\text{(h}\_{\text{vb}}^{+}\text{)} + \text{H}\_2\text{O}\_{\text{ads}} \rightarrow \text{TiO}\_2 + \text{OH}\_{\text{ads}}^{\bullet} + \text{H}^{+} \tag{3}$$

$$\text{TiO}\_2(\text{h}\_{\text{vb}}^{+}) + \text{OH}^-\_{\text{ads}} \rightarrow \text{TiO}\_2 + \text{OH}^\bullet\_{\text{ads}} \tag{4}$$

Generally, an acceptor molecules (A) such as O2 will be adsorbed and react with an electron in the conduction band while a donor molecules (D) such as H2O will be adsorbed as well and react with a hole in the valence band. The above reactions are presented in reactions 5 and 6.

$$\text{TiO}\_2(\text{e}\_{\text{cb}}^-) + \text{A}\_{\text{asd}} \rightarrow \text{TiO}\_2 + \text{A}\_{\text{asd}}^- \tag{5}$$

$$\text{TiO}\_2\text{(h}^+\_{\text{vb}}) + \text{D}\_{\text{asd}} \rightarrow \text{TiO}\_2 + \text{D}^+\_{\text{asd}} \tag{6}$$

It is widely accepted that O2 plays an important role in these reactions. Oxygen can trap conduction band electrons to form superoxide ions (O2•-) according to reaction 7. These O2• can then react with hydrogen ions (H+) from the water splitting process to form HO2•-.

$$\text{TiO}\_2(\text{e}\_{\text{cb}}^-) + \text{O}\_{2\text{ads}} + \text{H}^+ \rightarrow \text{TiO}\_2 + \text{HO}\_2^{\bullet-} \rightarrow \text{O}\_2^{\bullet-} + \text{H}^+ \tag{7}$$

H2O2 could also be formed from the HO2•- species by reaction 8.

$$\text{TiO}\_2(\text{e}\_{\text{cb}}^-) + \text{HCO}\_2^{\bullet-} + \text{H}^+ \rightarrow \text{H}\_2\text{O}\_2 \tag{8}$$

OH radical (OH•) may be formed from the cleavage of H2O2 via one of the following reactions 9, 10, 11.

$$\text{CH}\_2\text{O}\_2 + hv \rightarrow 2\text{OH}^\bullet\tag{9}$$

$$\text{CH}\_2\text{O}\_2 + \text{O}^{\bullet-}\_{2} \rightarrow \text{OH}^{\bullet} + \text{O}\_2 + \text{HO}^- \tag{10}$$

$$\text{TiO}\_2\text{O}\_2 + \text{TiO}\_2(\text{e}\_{\text{cb}}^-) \rightarrow \text{HCO}^\bullet + \text{HCO}^- + \text{TiO}\_2\tag{11}$$

The OH radical from reaction 4 is the most important oxidant formed in a photocatalytic process. It is the primary oxidant in the degradation of organic compounds (Ahmed et al., 2010). The degradation reaction is expressed in reaction 12.

$$\text{CH}^\bullet + \text{Organic} \to \text{CO}\_2 \tag{12}$$

#### **2.3 Photocatalytic materials**

#### **2.3.1 Photocatalysts**

222 Studies on Water Management Issues

other mineral compounds. There are many natural accelerators which can be used to accelerate this natural process. The introduction of 'colloidal semiconductor' and catalyst to catalyze distinct redox reactions on semiconductors could boost this sunlight driven natural

Wastewater treatment using photocatalysis involves the combination of heterogeneous photocatalysis with solar technologies (Zhang et al., 1994). Semiconductor photocatalysis, especially titania-based photocatalysis has been applied to various environmental problems other than water and air purification. Different studies have been carried out from fundamental to practical aspects to improve the process and the properties of the photocatalyst in recent years (Rajeshwar & Ibanez, 1997; Schiavello, 1997; Serpone & Pelizzetti, 1989). Hoffman et al. (1995) reported in that the utilizations of irradiated semiconductors for the degradation of organic pollutants were well documented and have shown positive and encouraging results for various organic pollutants. Various studies have also been carried out from fundamental to practical aspects to improve the process and the

The heterogeneous photocatalysis process is very complex. The oxidizing pathway is not very clear yet. Jean-Marie Herrmann (1999) suggested in that the overall of classic heterogeneous photocatalysis process can be divided into five steps (Herrmann, 1999):-

There are two pathways where the OH radicals can be formed. The valence band hole, h+vb

TiO (h ) H O TiO OH H 2 vb 2 ads 2 ads

TiO (h ) OH TiO OH 2 vb ads 2 ads

Generally, an acceptor molecules (A) such as O2 will be adsorbed and react with an electron in the conduction band while a donor molecules (D) such as H2O will be adsorbed as well and react with a hole in the valence band. The above reactions are presented in reactions 5

TiO (e ) A TiO A 2 cb asd 2 asd

TiO (h ) D TiO D 2 vb asd 2 asd

It is widely accepted that O2 plays an important role in these reactions. Oxygen can trap conduction band electrons to form superoxide ions (O2•-) according to reaction 7. These O2• can then react with hydrogen ions (H+) from the water splitting process to form HO2•-.

(TiO2) particle (Ekabi & Serpone, 1988). Equations 3 and 4 show the two reactions.

groups on the titanium dioxide

<sup>+</sup> • + + →+ + (3)

<sup>+</sup> − • + →+ (4)

<sup>−</sup> <sup>−</sup> +→ + (5)

<sup>+</sup> <sup>+</sup> +→ + (6)

purification process (Matthews, 1993).

properties of the photocatalyst in recent years.

1. Transfer of reactants to the surface 2. Adsorption of one of the reactants

4. Desorption of the product(s)

and 6.

**2.2 Mechanisms of generating oxidizing species** 

3. Reactions of the reactants in the adsorbed phase

can either react with the adsorbed water or the surface OH-

5. Diffusion of the product(s) from the surface

There are many semiconductor materials can be used as a photocatalyst. Semiconductors like TiO2, ZnO, Fe2O3, CdS, ZnS etc. are all suitable materials to initiate a photocatalytic process. Extensive studies and research concludes that an 'ideal photocatalyst' should possess the attributes shown in Table 1.


• Good adsorption in solar spectrum

Table 1. Attributes of an ideal photocatalyst for heterogenous photocatalytic process (Bhatkhande et al., 2001)

Titanium dioxide (TiO2) has been widely recognized as an excellent photocatalyst. It is known to have superb pigmentary properties, high adsorption in the ultraviolet region, and high stability which allows it to be used in various applications such as electroceramics, glass, and photocatalytic purification of chemical in air and water. Two types of reactors have been developed which are suspension/slurry and thin film/fixed in wastewater

Heterogeneous Photocatalytic Oxidation an Effective Tool for Wastewater Treatment – A Review 225

wastewater treatment process. Different principles and technologies were adopted including sunlight concentrating system. Four most frequent used photoreactors will be presented in

A parabolic through reactor adopted the principle of parabolic through solar concentrating system' to concentrate the sunlight on the focal point using Dewar tube. The schematic presentation is given in Fig. 1. The PTR concentrates the parallel (direct) rays of the photocatalytically active ultra-violet part of the solar spectrum and can be characterized as a typical plug flow reactor. Borosilicate glass tube which positioned along the focal line was filled with contaminant with titanium dioxide (TiO2) in suspension with a flowrate ranges between 250–3500Lh-1. This type of reactor had been selected as the first solar detoxification loops in Albuquerque and California in USA and Almeria in Spain (Bahnemann, 2004). Several research groups from the European continent have tested the PTR which installed at the Plataforma Solar de Almeria (PSA) in Spain for solar wastewater purification in the early

Sunlight

Focal point

Fig. 1. A schematic view of a parabolic through solar concentrator

peristaltic pump and ranges from 1–6.5Lh-1 (Bockelmann et al., 1995).

Thin film fixed bed reactor (TFFBR) is one of the very first solar reactor which does not utilize a solar concentrating system. It implies that the TFFBR can utilize the diffuse as well as the direct portion of the solar UV-A illumination for the photocatalytic process. A TFFBR installed at PSA is depicted in Fig. 2 (Bockelmann et al., 1995; Goslich et al., 1997; Hilgendorff et al., 1993). The most important aspect in the TFFBR is the slopping plate coated with photocatalyst like Degussa P25 (Bockelmann, 1993) and rinsed with contaminated water in a very thin film (~100µm). The flowrate was controlled by a cassette

**2.4.2 Thin film fixed bed reactor (TFFBR)** 

Dewar tube

Focal line

the following text.

1990s (Bahnemann, 2004).

**2.4.1 Parabolic through reactor (PTR)** 

treatment (Chang et al., 2000; Huang et al., 1999; Matthews et al., 1990). The details of these two reactors will be discussed later.

Titanium dioxide (TiO2) exist as many crystalline forms. The most common forms of crystalline structures are anatase and rutile. Brookite is the most uncommon form due to its instability in terms of the enthalpy of formation. Anatase is the most stable among all the different crystalline forms with 8-12kJmol-1 (Cotton et al., 1999). It can be converted to rutile when it is heated to approximately 700°C (Bickley et al., 1991). Anatase is less dense compared to rutile, has a density of 3900kg/m3 while rutile has a density of 4260kg/m3. In the application of photocatalysis, anatase is a more efficient photocatalyst compared to rutile due to its open crystalline structure.

#### **2.3.2 Titanium dioxide (Degussa P25)**

It was been used extensively in many studies regarding photocatalytic degradation. Photocatalytic degradation studies utilizing Degussa P25 have been well documented due to its chemical stability, readily availability, reproductive ability, and activity as a catalyst for oxidation process (Bekbolet et al., 1998; Bekbolet & Balcioghu, 1996; Saravanan et al., 2009). Vigorous activities and researches are in process to further develop the existing Degussa P25 or synthesizing new materials, which can initiate photocatalysis using solar energy and hence reduce the cost and shortening the total time needed for the degradation. These developments include increasing the effective surface area, increasing the photoactivity, increasing the active sites, enhancing the absorption of photon energy and reducing the band-gap energy.

#### **2.4 Photocatalytic reactors**

There are many types of reactors have been developed and can be used in photocatalytic studies. These reactors were developed based on the different needs of applications. The selection of these reactors was according to the experiment conditions and applications. Generally the reactors can be briefly categorized into two groups, a suspension/slurry type and a thin film type. A slurry type reactor uses the catalyst in a suspension form whereas a thin film type reactor uses a thin film catalyst. Both types of reactors can be designed to be an immersion well reactor or flat wall reactor. Immersion well reactors were generally used in laboratory scale works for evaluation purposes. It can be run on either batch or continuous mode. The flow of oxidant and the temperature can be easily controlled and monitored. The source of the light can either be single or multiple with or without any reflectors. A suspension form is preferred because it is normally more efficient compared to the thin film reactor. It is because in a suspension type reactor, the catalyst has a higher effective surface area and hence larger surface area in contact with the substrate. This allows a larger amount of photon to hit the surface and results in large adsorption capacity.

Other types of reactors are flat wall and tubular photoreactors. These types of reactors are simple and easy to design. Air can be use as an oxidant option for these reactors. Besides, solar energy can be utilized by these types of reactors. Moreover, reflectors are used in a tubular reactor to concentrate sunlight so that it can enhance the photoreaction.

For the past 20 years, several photocatalytic water treatment reactors have been developed and tested. Different rectors were developed to find the best way of conducting solar wastewater treatment process. Different principles and technologies were adopted including sunlight concentrating system. Four most frequent used photoreactors will be presented in the following text.
