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

AOP are specific chemical reactions characterized by the generation of chemical oxidizing agents capable of oxidizing or degrading the pollutant of interest. The efficiency of the AOP is generally maximized by the use of an appropriate catalyst and/or ultraviolet light [1-3].

In most AOP, the objective is to use systems that produce the hydroxyl radical (HO•) or another species of similar reactivity such as sulfate radical anion (SO4 •-). These radicals react with the majority of organic substances at rates often approaching the diffusion-controlled limit (unit reaction efficiency per encounter). Both of these species are thus highly reactive and only modestly selective in their capacity to degrade toxic organic compounds present in aqueous solution. The principal reaction pathways of HO• with organic compounds include hydrogen abstraction from aliphatic carbon, addition to double bonds and aromatic rings, and electron transfer [4]. These reactions generate organic radicals as transient intermediates, which then undergo further reactions, eventually resulting in final products corresponding to the net oxidative degradation of the starting molecule [5].

The AOP are of two main types: homogeneous and heterogeneous processes, both of which can be conducted with or without the use of UV radiation. Thus, for example, the homogeneous process based on the reaction of Fe2+ with H2O2, known as the thermal-Fenton reaction process typically becomes more efficient for the mineralization of organic material present in the effluent when it is photocatalysed. This latter process (Fe2+/Fe3+, H2O2, UV-Vis) is commonly referred to as the photo-Fenton reaction. Among the heterogeneous AOP, processes using some form of the semiconductor TiO2 stand out because UV irradiation of TiO2 results in the generation of hydroxyl radicals, promoting the oxidation of organic species [1,6].

#### **2.1. Advances in research on AOP**

chemical structures into substances that are less toxic and/or more readily biodegradable by employing chemical oxidizing agents in the presence of an appropriate catalyst and/or ultraviolet light to oxidize or degrade the pollutant of interest. These technologies known as advanced oxidation processes (AOP) or advanced oxidation technologies (AOT), have been widely studied for the degradation of diverse types of industrial wastewaters. These processes are particularly interesting for the treatment of effluents containing highly toxic organic compounds, for which biological processes may not be applicable unless bacteria that are adapted to live in toxic media are available. The production of powerful oxidizing agents, such as the hydroxyl radical, is the main objective of most AOP. The hydroxyl radical reacts rapidly and relatively non-selectively with organic compounds by hydrogen abstraction, by addition to unsaturated bonds and aromatic rings, or by electron transfer. In the case of persistent organic pollutants (wastes), complete decontamination may require the sequential application of several different decontamination technologies such as a pretreatment with a photochemical

This chapter discusses the influence of different AOP on the degradation and mineralization of several different classes of organic pollutants such as pesticides, pharmaceutical formula‐ tions and dyes. The use of the Fenton and photo-Fenton reactions as tools for the treatment of pesticides and antineoplastic agents is presented, as well as examples of the optimization of the important parameters involved in the process such as the source of iron ions (free or complexed), the irradiation source (including the possibility of using sunlight), and the concentrations of iron ions and hydrogen peroxide. The chapter also reports the use of TiO2 nanotubes obtained by electrochemical anodization, nanoparticles prepared by a molten salt technique, and Ag-doped TiO2 nanoparticles as heterogeneous photocatalysts, emphasizing their potential for use in environmental applications. These catalysts were characterized by a combination of techniques, including scanning electron microscopy, elemental analysis, and

AOP are specific chemical reactions characterized by the generation of chemical oxidizing agents capable of oxidizing or degrading the pollutant of interest. The efficiency of the AOP is generally maximized by the use of an appropriate catalyst and/or ultraviolet light [1-3]. In most AOP, the objective is to use systems that produce the hydroxyl radical (HO•) or another

majority of organic substances at rates often approaching the diffusion-controlled limit (unit reaction efficiency per encounter). Both of these species are thus highly reactive and only modestly selective in their capacity to degrade toxic organic compounds present in aqueous solution. The principal reaction pathways of HO• with organic compounds include hydrogen abstraction from aliphatic carbon, addition to double bonds and aromatic rings, and electron transfer [4]. These reactions generate organic radicals as transient intermediates, which then undergo further reactions, eventually resulting in final products corresponding to the net

•-). These radicals react with the

AOP followed by a biological or electrochemical treatment.

energy dispersive x-ray spectroscopy.

142 Organic Pollutants - Monitoring, Risk and Treatment

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

oxidative degradation of the starting molecule [5].

species of similar reactivity such as sulfate radical anion (SO4

AOP and their applications have attracted the attention of both the scientific community and of corporations interested in their commercialization. This can be illustrated by means of searches, in August, 2012, of the Science Finder Scholar database (version 2012). This database covers the complete text of articles/papers indexed from over 15475 international journals and 126 databases with abstracts of documents in all areas, as well as several other important sources of academic information. The results of the searches were organized as histograms to show the evolution of the number of publications (arti‐ cles or patents) related to the different kinds of AOP. Figure 1 shows the results of a search using the keywords "advanced oxidation processes", which yielded approximately 840 publications and which nicely reflects the rapid growth in interest AOP, given the unique characteristics and the versatility of application of AOP.

**Figure 1.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the key‐ words "advanced oxidation processes".

#### **2.2. Fenton reaction**

The thermal Fenton reaction is chemically very efficient for the removal of organic pollutants. The overall reaction is a simple redox reaction in which Fe(II) is oxidized to Fe(III) and H2O2 is reduced to the hydroxide ion plus the hydroxyl radical.

$$Fe^{2+} + H\_2O\_2 \rightarrow Fe^{3+} + HO^\bullet + OH^- \tag{1}$$

is 1:5 respectively [12]. One way of accelerating the Fenton reaction is via the addition of catalysts, in general from certain classes of organic molecules such as benzoquinones or dihydroxybenzene (DHB) derivatives [13,14]. It is also possible to accelerate the Fenton reaction via irradiation with ultraviolet light, a process generally known as the photo-assisted

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145

Regarding the Fenton reaction, a search of Science Finder Scholar (2012) with the keyword "Fenton" without any refinements retrieved 14821 publications from 1986 to 2012. As shown in Figure 2, there is a clear upward trend in the publications, with 902 publications related to

One of the most efficient AOP is the photo-Fenton reaction (Fe2+/Fe3+, H2O2, UV light), which successfully oxidizes a wide range of organic and inorganic compounds. The irradiation of Fenton reaction systems with UV/Vis light (250-400 nm) strongly accelerates the rate of degradation. This behavior is due principally to the photochemical reduction of Fe(III) back

Studies of the pH dependence of the photo-Fenton reaction have shown that the optimum pH range is ca. pH 3. Studies of the photochemistry of Fe(OH)2+, which is the predominant species in solution at this pH and that is formed by deprotonation of hexaaquairon(III), have shown that Fe(OH)2+ undergoes a relatively efficient photoreaction upon excitation with UV light to produce Fe(II) and the hydroxyl radical. Therefore, irradiation of Fenton reaction systems not only regenerates Fe(II), the crucial catalytic species in the Fenton reaction, but also produces an additional hydroxyl radical, the species responsible for provoking the degradation of organic material. As a consequence of these two effects, the photo-Fenton process is faster than

The efficiency of the photo-Fenton process can be further enhanced by using certain organic

absorbs light as far out as 570 nm, i.e., well into the visible region of the spectrum. This species makes the photo-Fenton reaction more efficient because it absorbs a much broader range of wavelengths of light and because, upon irradiation, it efficiently decomposes (quantum yield

The use of photo-Fenton reaction has considerable advantages in practical applications. It generally produces oxidation products of low toxic, requires only small quantities of iron salt (which can be either Fe3+ or Fe2+) and offers the possibility of using solar radiation as the source

2 2 <sup>2</sup> <sup>4</sup> 2 24 2[ ( 0 )] 2 2 ++ - *Fe C hv Fe CO C O* +® + + (4)

acids to complex Fe(III). Thus, for example, oxalic acid forms species such as [Fe(C2O4)]+

2 <sup>+</sup> ·+ <sup>+</sup> *Fe H O hv Fe HO H* + +® + + (3)

, which

Fenton or photo-Fenton reaction, which is discussed in the following section.

3 2

this topic being reported in just the first half of 2012.

to Fe(II), for which the overall process can be written as:

the conventional thermal Fenton process.

of the order of unity) to Fe(II) and CO2:

**2.3. Photo-fenton reaction**

The ferric ion produced in Equation 1 can in principle be reduced back to ferrous ion by a second molecule of hydrogen peroxide:

$$Fe^{3+} + H\_2O\_2 \rightarrow Fe^{2+} + HO\_2^{\bullet} + H^+ \tag{2}$$

However, this thermal reduction (Equation 2) is much slower than the initial step (Equation 1) and the addition of relatively large, essentially stoichiometric amounts of Fe(II) may be required in order to degrade the pollutant of interest [7]. Another important limitation of the Fenton reaction is the formation of recalcitrant intermediates that can inhibit complete mineralization. Despite these potential limitations, the conventional Fenton reaction has been widely used for the treatment of effluents [6, 8-11].

**Figure 2.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the keyword "Fenton".

For the degradation of organic molecules, the optimum pH for the Fenton reaction is typically in the range of pH 3-4 and the optimum mass ratio of catalyst (as iron) to hydrogen peroxide

is 1:5 respectively [12]. One way of accelerating the Fenton reaction is via the addition of catalysts, in general from certain classes of organic molecules such as benzoquinones or dihydroxybenzene (DHB) derivatives [13,14]. It is also possible to accelerate the Fenton reaction via irradiation with ultraviolet light, a process generally known as the photo-assisted Fenton or photo-Fenton reaction, which is discussed in the following section.

Regarding the Fenton reaction, a search of Science Finder Scholar (2012) with the keyword "Fenton" without any refinements retrieved 14821 publications from 1986 to 2012. As shown in Figure 2, there is a clear upward trend in the publications, with 902 publications related to this topic being reported in just the first half of 2012.

#### **2.3. Photo-fenton reaction**

**2.2. Fenton reaction**

144 Organic Pollutants - Monitoring, Risk and Treatment

The thermal Fenton reaction is chemically very efficient for the removal of organic pollutants. The overall reaction is a simple redox reaction in which Fe(II) is oxidized to Fe(III) and H2O2

The ferric ion produced in Equation 1 can in principle be reduced back to ferrous ion by a

However, this thermal reduction (Equation 2) is much slower than the initial step (Equation 1) and the addition of relatively large, essentially stoichiometric amounts of Fe(II) may be required in order to degrade the pollutant of interest [7]. Another important limitation of the Fenton reaction is the formation of recalcitrant intermediates that can inhibit complete mineralization. Despite these potential limitations, the conventional Fenton reaction has been

1988 1992 1996 2000 2004 2008 2012

Year

**Figure 2.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the keyword

For the degradation of organic molecules, the optimum pH for the Fenton reaction is typically in the range of pH 3-4 and the optimum mass ratio of catalyst (as iron) to hydrogen peroxide

2 2 <sup>+</sup> -·+ *Fe H O Fe HO OH* + ®+ + (1)

2 2 2 <sup>+</sup> ·+ <sup>+</sup> *Fe H O Fe HO H* + ®++ (2)

is reduced to the hydroxide ion plus the hydroxyl radical.

second molecule of hydrogen peroxide:

widely used for the treatment of effluents [6, 8-11].

0

200

400

600

Publications

"Fenton".

800

1000

1200

1400

2 3

3 2

Fenton

One of the most efficient AOP is the photo-Fenton reaction (Fe2+/Fe3+, H2O2, UV light), which successfully oxidizes a wide range of organic and inorganic compounds. The irradiation of Fenton reaction systems with UV/Vis light (250-400 nm) strongly accelerates the rate of degradation. This behavior is due principally to the photochemical reduction of Fe(III) back to Fe(II), for which the overall process can be written as:

$$Fe^{3+} + H\_2O + h\nu \rightarrow Fe^{2+} + HO^\* + H^+ \tag{3}$$

Studies of the pH dependence of the photo-Fenton reaction have shown that the optimum pH range is ca. pH 3. Studies of the photochemistry of Fe(OH)2+, which is the predominant species in solution at this pH and that is formed by deprotonation of hexaaquairon(III), have shown that Fe(OH)2+ undergoes a relatively efficient photoreaction upon excitation with UV light to produce Fe(II) and the hydroxyl radical. Therefore, irradiation of Fenton reaction systems not only regenerates Fe(II), the crucial catalytic species in the Fenton reaction, but also produces an additional hydroxyl radical, the species responsible for provoking the degradation of organic material. As a consequence of these two effects, the photo-Fenton process is faster than the conventional thermal Fenton process.

The efficiency of the photo-Fenton process can be further enhanced by using certain organic acids to complex Fe(III). Thus, for example, oxalic acid forms species such as [Fe(C2O4)]+ , which absorbs light as far out as 570 nm, i.e., well into the visible region of the spectrum. This species makes the photo-Fenton reaction more efficient because it absorbs a much broader range of wavelengths of light and because, upon irradiation, it efficiently decomposes (quantum yield of the order of unity) to Fe(II) and CO2:

$$2[Fe(C\_2\,0\_4)]^+ + h\nu \to 2Fe^{2+} + 2CO\_2 + C\_2O\_4^{2-} \tag{4}$$

The use of photo-Fenton reaction has considerable advantages in practical applications. It generally produces oxidation products of low toxic, requires only small quantities of iron salt (which can be either Fe3+ or Fe2+) and offers the possibility of using solar radiation as the source of light in the reaction process sunlight constitutes an inexpensive, environmentally friendly, renewable source of ultraviolet photons for use in photochemical processes.

**2.4. Ozone**

(OH-

decomposition of ozone.

H2O2 (Equation 7):

two hydroxyl radicals:

Ozone is a powerful oxidizing agent with a high reduction potential (2.07V) that can react with many organic substrates [18,19]. Using ozone, the oxidation of the organic matrix can occur via either direct or indirect routes [20,21]. In the direct oxidation route, ozone molecules can react directly with other organic or inorganic molecules via electrophilic addition. The electrophilic attack of ozone occurs on atoms with a negative charge (N, P, O, or nucleophilic carbons) or on carbon-carbon, carbon-nitrogen and nitrogen-nitrogen pi-bonds [22,23]. Indirectly, ozone can react via radical pathways (mainly involving HO•) initiated by the

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A process that employs ozone is only characterized as an AOP when the ozone decomposes to generate hydroxyl radicals (Equation 5), a reaction that is catalyzed by hydroxide ions

The efficiency of ozone in degrading organic compounds is improved when combined with H2O2, UV radiation or ultrasound. The initial step in the UV photolysis of ozone is dissociation to molecular oxygen and an oxygen atom (Equation 6), which then reacts with water to produce

In a second photochemical step (Equation 8), H2O2 photodissociates into the active species,

The O3/UV process has been employed commercially to treat ground water contaminated with chlorinated hydrocarbons, but cannot compete economically with the H2O2/UV process. A major problem with the use of ozone for water treatment is bromine formation in waters containing bromide ion. Strategies such as addition of H2O2 (O3/H2O2) can reduce bromine formation and assure the suitability of ozone for treating drinking and wastewater [26].

A search of the Science Finder Scholar database retrieved using only the keyword "ozone" retrieved, as expected, an enormous number of publications, nearly 130,000. Refinement with the additional keyword "degradation" reduced this to 3057 publications, which is a significant

number when compared with other AOPs, especially in recent years (Figure 4).

3 2 <sup>2</sup> <sup>2</sup> 2 2 2 2 · · *O H O HO O HO* + ® ++ (5)

3 2 · *O hv O O* +® + (6)

<sup>2</sup> 2 2 · *HO O HO* + ® (7)

2 2 2 · *H O hv HO* + ® (8)

) in alkaline medium or by transition metal cations [18,24,25].

The disadvantages of the photo-Fenton process include the low pH values required and the need for removal of the iron catalyst after the reaction has terminated. If necessary, however, the residual Fe(III) can usually be precipitated as iron hydroxide by increasing the pH. Any residual hydrogen peroxide that is not consumed in the process will spontaneously decompose into water and molecular oxygen, being thus a "clean" reagent itself. These features make homogeneous photo-Fenton based AOPs the leading candidate for cost-efficient, environ‐ mental friendly treatment of industrial effluents on a small to moderate scale [6, 15-17]. Currently much research activity is focused on attempts to develop new catalysts that function at neutral pH that do not require acidification of the effluent in order to react and that also do not require removal of the catalyst at the end of the reaction.

A search of Science Finder Scholar (2012) with the keyword "photo-Fenton" (Figure 3) showed a modest increase during the 1990s followed by a much more robust upward trend since ca. 2000.

**Figure 3.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the keyword "photo-Fenton".

#### **2.4. Ozone**

of light in the reaction process sunlight constitutes an inexpensive, environmentally friendly,

The disadvantages of the photo-Fenton process include the low pH values required and the need for removal of the iron catalyst after the reaction has terminated. If necessary, however, the residual Fe(III) can usually be precipitated as iron hydroxide by increasing the pH. Any residual hydrogen peroxide that is not consumed in the process will spontaneously decompose into water and molecular oxygen, being thus a "clean" reagent itself. These features make homogeneous photo-Fenton based AOPs the leading candidate for cost-efficient, environ‐ mental friendly treatment of industrial effluents on a small to moderate scale [6, 15-17]. Currently much research activity is focused on attempts to develop new catalysts that function at neutral pH that do not require acidification of the effluent in order to react and that also do

A search of Science Finder Scholar (2012) with the keyword "photo-Fenton" (Figure 3) showed a modest increase during the 1990s followed by a much more robust upward

photo-Fenton

1988 1992 1996 2000 2004 2008 2012

Year

**Figure 3.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the keyword

renewable source of ultraviolet photons for use in photochemical processes.

not require removal of the catalyst at the end of the reaction.

Publications

"photo-Fenton".

trend since ca. 2000.

146 Organic Pollutants - Monitoring, Risk and Treatment

Ozone is a powerful oxidizing agent with a high reduction potential (2.07V) that can react with many organic substrates [18,19]. Using ozone, the oxidation of the organic matrix can occur via either direct or indirect routes [20,21]. In the direct oxidation route, ozone molecules can react directly with other organic or inorganic molecules via electrophilic addition. The electrophilic attack of ozone occurs on atoms with a negative charge (N, P, O, or nucleophilic carbons) or on carbon-carbon, carbon-nitrogen and nitrogen-nitrogen pi-bonds [22,23]. Indirectly, ozone can react via radical pathways (mainly involving HO•) initiated by the decomposition of ozone.

A process that employs ozone is only characterized as an AOP when the ozone decomposes to generate hydroxyl radicals (Equation 5), a reaction that is catalyzed by hydroxide ions (OH- ) in alkaline medium or by transition metal cations [18,24,25].

$$2O\_3 + 2H\_2O \to 2HO^\* + O\_2 + 2HO\_2^\* \tag{5}$$

The efficiency of ozone in degrading organic compounds is improved when combined with H2O2, UV radiation or ultrasound. The initial step in the UV photolysis of ozone is dissociation to molecular oxygen and an oxygen atom (Equation 6), which then reacts with water to produce H2O2 (Equation 7):

$$O\_3 + h\nu \to O\_2 + O^\* \tag{6}$$

$$H\_2O + O^\* \to H\_2O\_2 \tag{7}$$

In a second photochemical step (Equation 8), H2O2 photodissociates into the active species, two hydroxyl radicals:

$$H\_2O\_2 + h\nu \rightarrow 2HO^\bullet \tag{8}$$

The O3/UV process has been employed commercially to treat ground water contaminated with chlorinated hydrocarbons, but cannot compete economically with the H2O2/UV process. A major problem with the use of ozone for water treatment is bromine formation in waters containing bromide ion. Strategies such as addition of H2O2 (O3/H2O2) can reduce bromine formation and assure the suitability of ozone for treating drinking and wastewater [26].

A search of the Science Finder Scholar database retrieved using only the keyword "ozone" retrieved, as expected, an enormous number of publications, nearly 130,000. Refinement with the additional keyword "degradation" reduced this to 3057 publications, which is a significant number when compared with other AOPs, especially in recent years (Figure 4).

The electron (e-

radicals [30,31].

according to Equation 13.

) and hole (h+

electron donors to the holes (h+

by Equations 10 and 11.

) pair produced by absorption of UV light can migrate to the

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Application of Different Advanced Oxidation Processes for the Degradation of Organic Pollutants

) of the catalyst [27,29], generating hydroxyl radicals, as shown

2 <sup>+</sup> +· *h H O HO H* +®+ (10)

+- · *h HO HO* + ® (11)

2 2 - - *Oe O* + ® (12)

2 2 -·- *H O e HO OH* +® + (13)

[32],

surface of the anatase particle, where they react with adsorbed oxygen, water, hydroxide ion or organic species via electron transfer reactions. Both water and hydroxide ion can act as

When dissolved molecular oxygen is present or is deliberately added to the medium, it can act as an acceptor of the electron in the conduction band, generating the superoxide radical (Equation 12) and triggering a series of reactions that can lead to the formation of hydroxyl

Alternatively, one can increase the oxidative efficiency of TiO2 photocatalysis by adding H2O2. The electrons in the conduction band then reduce the added H2O2 to HO• and HO-

The use of TiO2 also makes it possible to degrade organic molecules that are resistant to oxidation, since they can potentially be reduced by the electrons in the conduction band.

TiO2 photocatalysis has a number of important advantages in relation to other AOP and, in some aspects, even some biological treatments. In particular, unlike other AOP, the TiO2/UV system can be employed to treat pollutants in the gas phase, as well as in solution. In addition, TiO2 has a relatively low cost, is essentially insoluble in water and biologically and chemically inert. Moreover, it can be used to treat effluents containing a wide range of concentrations of pollutants, in particular very low concentrations. Solar radiation can be used to activate the catalyst; and the excellent mineralization efficiency is observed for organochlorine com‐ pounds, chlorophenols, nitrogen-containing pesticides, aromatic hydrocarbons, dioxins, carboxylic acids, etc. The principle limitations of TiO2 photocatalysis in practical applications are the low quantum efficiency of the process and the limited depth of penetration of the incident radiation into suspensions of TiO2, due to the strong scattering of light by the opaque white catalyst particles. Incrustation of the reactor walls with catalyst can also reduce the amount of incident light. Batch reactors also require additional unit operations in order to physically separate the catalyst from the solution at the end of the irradiation for recycling. Although substantial progress has been made in developing larger-scale reactors for carrying

**Figure 4.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the key‐ words "ozone" and "degradation".

#### **2.5. Heterogeneous AOP**

Another important class of AOP is based on the use of solid semiconductors as heterogeneous catalysts for the mineralization of organic compounds. In this type of photocatalysis, an electron in the valence band of the semiconductor (CdS, TiO2, ZnO, WO3, etc.) is promoted into the conduction band upon excitation. The electron in the conduction band typically reacts with O2, while the hole in the valence band can react with an adsorbed pollutant or oxidize water to produce a surface-bound HO• radical [2].

According to Alfano and coworkers [27], the anatase form of titanium dioxide (TiO2) is the material most indicated for use in photocatalytic water treatment, considering aspects such as toxicity, resistance to photocorrosion, availability, catalytic efficiency and cost. Using TiO2 as the semiconductor, the photocatalysis is based on the activation of anatase by light [28]. The band gap or energy difference between the valence and conduction bands of anatase is 3.2 eV. Thus, UV light of wavelength shorter than 390 nm is capable of exciting an electron (*e*- ) from the valence to the conduction band.

$$\text{TiO}\_2 + h\text{v} \rightarrow \text{e}^- + \text{h}^+ \tag{9}$$

An important feature of TiO2 photocatalysis is the very high oxidation potential of the holes left in the valence band (3.1 eV at pH 0), making it possible for photoexcited TiO2 to oxidize most organic molecules.

The electron (e- ) and hole (h+ ) pair produced by absorption of UV light can migrate to the surface of the anatase particle, where they react with adsorbed oxygen, water, hydroxide ion or organic species via electron transfer reactions. Both water and hydroxide ion can act as electron donors to the holes (h+ ) of the catalyst [27,29], generating hydroxyl radicals, as shown by Equations 10 and 11.

$$\text{H}^+ + \text{H}\_2\text{O} \rightarrow \text{HO}^\bullet + \text{H}^+ \tag{10}$$

$$h^\* + HO^- \to HO^\* \tag{11}$$

When dissolved molecular oxygen is present or is deliberately added to the medium, it can act as an acceptor of the electron in the conduction band, generating the superoxide radical (Equation 12) and triggering a series of reactions that can lead to the formation of hydroxyl radicals [30,31].

1988 1992 1996 2000 2004 2008 2012

Year

**Figure 4.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the key‐

Another important class of AOP is based on the use of solid semiconductors as heterogeneous catalysts for the mineralization of organic compounds. In this type of photocatalysis, an electron in the valence band of the semiconductor (CdS, TiO2, ZnO, WO3, etc.) is promoted into the conduction band upon excitation. The electron in the conduction band typically reacts with O2, while the hole in the valence band can react with an adsorbed pollutant or oxidize

According to Alfano and coworkers [27], the anatase form of titanium dioxide (TiO2) is the material most indicated for use in photocatalytic water treatment, considering aspects such as toxicity, resistance to photocorrosion, availability, catalytic efficiency and cost. Using TiO2 as the semiconductor, the photocatalysis is based on the activation of anatase by light [28]. The band gap or energy difference between the valence and conduction bands of anatase is 3.2 eV. Thus, UV light of wavelength shorter than 390 nm is capable of exciting an electron (*e*-

An important feature of TiO2 photocatalysis is the very high oxidation potential of the holes left in the valence band (3.1 eV at pH 0), making it possible for photoexcited TiO2 to oxidize

2 - + *TiO hv e h* +®+ (9)

) from

Ozone, degradation

0

water to produce a surface-bound HO• radical [2].

the valence to the conduction band.

most organic molecules.

40

80

120

Publications

148 Organic Pollutants - Monitoring, Risk and Treatment

words "ozone" and "degradation".

**2.5. Heterogeneous AOP**

160

200

240

Alternatively, one can increase the oxidative efficiency of TiO2 photocatalysis by adding H2O2. The electrons in the conduction band then reduce the added H2O2 to HO• and HO- [32], according to Equation 13.

$$O\_2 + e^- \to O\_2^- \tag{12}$$

$$H\_2O\_2 + e^- \rightarrow HO^\bullet + OH^- \tag{13}$$

The use of TiO2 also makes it possible to degrade organic molecules that are resistant to oxidation, since they can potentially be reduced by the electrons in the conduction band.

TiO2 photocatalysis has a number of important advantages in relation to other AOP and, in some aspects, even some biological treatments. In particular, unlike other AOP, the TiO2/UV system can be employed to treat pollutants in the gas phase, as well as in solution. In addition, TiO2 has a relatively low cost, is essentially insoluble in water and biologically and chemically inert. Moreover, it can be used to treat effluents containing a wide range of concentrations of pollutants, in particular very low concentrations. Solar radiation can be used to activate the catalyst; and the excellent mineralization efficiency is observed for organochlorine com‐ pounds, chlorophenols, nitrogen-containing pesticides, aromatic hydrocarbons, dioxins, carboxylic acids, etc. The principle limitations of TiO2 photocatalysis in practical applications are the low quantum efficiency of the process and the limited depth of penetration of the incident radiation into suspensions of TiO2, due to the strong scattering of light by the opaque white catalyst particles. Incrustation of the reactor walls with catalyst can also reduce the amount of incident light. Batch reactors also require additional unit operations in order to physically separate the catalyst from the solution at the end of the irradiation for recycling. Although substantial progress has been made in developing larger-scale reactors for carrying out heterogeneous photochemical reactions, much work remains to be done before TiO2 photocatalysis becomes a generally applicable technique.

*3.1.1. Degradation of Chlorimurom-Ethyl (CE)*

**Figure 6.** Molecular structure of Chlorimurom-ethyl (CE).

The thermal Fenton, photo-Fenton and ozonation processes were applied for the degradation of a commercial preparation of chlorimurom-ethyl (CE, Figure 6), a compound belonging to the class of sulfonylurea herbicides. This herbicide, widely used in the cultivation of soybeans,

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Experiments were performed in a photochemical reactor (1.0 L) equipped with a high pressure mercury lamp (125 W) coupled to a reservoir (2.0 L) via a recirculation pump. The photo-Fenton degradation was influenced by the initial concentrations of H2O2 and Fe2+. Experiments were performed with different H2O2 concentrations, ranging from 17 to 103 mmol L-1, main‐ taining the Fe2+ concentration constant at 0.33 mmol L-1. Subsequently, the H2O2 concentration was fixed at 68.4 mmol L-1, the value that gave the best mineralization, and the Fe2+ concen‐ trations were varied from 0.20 to 1.0 mmol L-1. The extent of mineralization of the organic material, expressed as the percentage of removal of the total organic carbon (TOC), ranged from 84% to 95%. Since the quantity of Fe2+ had only a small effect on CE removal, a concen‐ tration of Fe2+ of 0.20 mmol L-1 was used in subsequent experiments. In all cases, the extent of mineralization was higher than the percentage of degradation of CE (82-87%) determined by HPLC. This particularity reflects the fact that a commercial formulation of CE was employed in the experiments. Thus, a solution of this formulation in water that contained 30 mg L-1 of CE contained 65 mg L-1 of total organic carbon. Therefore, it can be concluded that the other organic compounds present in the composition react somewhat better with HO• than CE.

The effect of UV radiation on this optimized reaction system was used to compare the efficiencies of the thermal Fenton and photo-Fenton reactions for the mineralization of CE (Figure 7) with each other and with those of several other homogeneous AOP. Under direct photolysis there was no significant mineralization. Less than 20% TOC removal was obtained at the end of the thermal Fenton treatment. However, a considerable increase in mineralization was observed when the Fenton system was irradiated with UV light. Monitoring CE removal rather than TOC showed that both the thermal Fenton reaction and the photo-Fenton reactions

may persist in the environment and has residual phytotoxicity [33].

Figure 5 shows the evolution of publications related to heterogeneous photocatalysis by TiO2, reflecting the potential for application of this technology on an industrial scale.

**Figure 5.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the key‐ words "photocatalysis" and "TiO2 semiconductor".
