**3. Applications**

In this section, several applications of homogeneous and heterogeneous AOP are discussed, focusing on the degradation and mineralization of organic pollutants such as pesticides, pharmaceutical formulations and dyes.

#### **3.1. Homogeneous AOP applied to degradation of the herbicide chlorimurom-ethyl and the antineoplastic agent mitoxantrone**

The use of the thermal Fenton and the photo-Fenton reactions for the treatment of the pesticide chlorimurom-ethyl (CE) and the antineoplastic agent mitoxantrone (MTX) is described here, along with the optimization of the parameters involved in these processes, including the sources of iron (free or complexed) and irradiation (lamp or possibility of using sunlight) and the concentrations of iron and hydrogen peroxide, etc. Ozone and ozone combined with UV and H2O2 were also used as alternative treatments of these pesticides.

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

out heterogeneous photochemical reactions, much work remains to be done before TiO2

Figure 5 shows the evolution of publications related to heterogeneous photocatalysis by TiO2,

1988 1992 1996 2000 2004 2008 2012

semiconductor

Year

**Figure 5.** Number of publications per year indexed in the Science Finder Scholar database retrieved using the key‐

In this section, several applications of homogeneous and heterogeneous AOP are discussed, focusing on the degradation and mineralization of organic pollutants such as pesticides,

**3.1. Homogeneous AOP applied to degradation of the herbicide chlorimurom-ethyl and the**

The use of the thermal Fenton and the photo-Fenton reactions for the treatment of the pesticide chlorimurom-ethyl (CE) and the antineoplastic agent mitoxantrone (MTX) is described here, along with the optimization of the parameters involved in these processes, including the sources of iron (free or complexed) and irradiation (lamp or possibility of using sunlight) and the concentrations of iron and hydrogen peroxide, etc. Ozone and ozone combined with UV

and H2O2 were also used as alternative treatments of these pesticides.

reflecting the potential for application of this technology on an industrial scale.

Photocatalysis, TiO2

photocatalysis becomes a generally applicable technique.

0

20

40

60

Publications

150 Organic Pollutants - Monitoring, Risk and Treatment

words "photocatalysis" and "TiO2 semiconductor".

pharmaceutical formulations and dyes.

**antineoplastic agent mitoxantrone**

**3. Applications**

80

100

120

140

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

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

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 caused extensive degradation of the target compound. Therefore, in the photo-Fenton process, UV radiation makes a significant contribution to both mineralization and CE removal.

Normative Instruction nº 2, published on January 3, 2008, by the Brazilian Ministry of Agri‐ culture (MAPA) [34], regulates the practice in Brazil for treatment of pesticide residues in effluents generated by agricultural aviation companies. The Ministry recommends ozonation for a minimum of six hours using a system with a minimum capacity for producing one gram of ozone per hour for each charge of four hundred and fifty liters of pesticide residue derived from washing and cleaning of aircraft equipment [34]. To verify the efficiency of this system for the mineralization of CE-contaminated water, the samples were treated with ozone alone and with ozone in combination with UV light and H2O2. Although oxidation of CE was very fast with all the ozonation methods studied, the use of ozone alone proved to be of limited utility with regard to the mineralization of the organic content of CE-contaminated waters. The combination of O3/UV/H2O2 did, however, achieve a high extent of mineralization (80%), indicating that the mineralization of the organic content is mediated by the HO• radical.

When compared to the other systems studied, the photo-Fenton system showed the best results, with mineralization exceeding 85%, making it the preferred technique for the treatment of wastewater containing this pesticide.

Figure 7. Comparison of the efficiencies of mineralization of a commercial formulation of CE in aqueous solution by the different AOP. [CE]0 = 0.060 mmol L–1; [TOC]0 = 65 mg L-1; when present, [Fe2+]=0.2 mmol L-1, [H2O2] = 68.4 mmol L-1 and [O3]= 25 mg mL-1. **3.1.2. Degradation of Mitoxantrone (MTX) Figure 7.** Comparison of the efficiencies of mineralization of a commercial formulation of CE in aqueous solution by the different AOP. [CE]0 = 0.060 mmol L–1; [TOC]0 = 65 mg L-1; when present, [Fe2+]=0.2 mmol L-1, [H2O2] = 68.4 mmol L-1 and [O3]= 25 mg mL-1.

#### Antineoplastic agents (drugs employed in cancer chemotherapy) are pollutants due their mutagenic, carcinogenic, and genotoxic *3.1.2. Degradation of Mitoxantrone (MTX)*

were the photo-Fenton (with Fe2+, Fe3+, and potassium ferrioxalate - K3(FeOx) - as iron sources), solar photo-Fenton, Fenton and UV/H2O2 reactions. The MTX degradation experiments were carried out using an annular glass photochemical reactor (working volume, 1 L) and a quartz tube for introduction of the radiation source (a 125 W mercury vapor lamp). Antineoplastic agents (drugs employed in cancer chemotherapy) are pollutants due their mutagenic, carcinogenic, and genotoxic potential, even at trace levels [35]. The AOP selected for degradation of the antineoplastic drug mitoxantrone (MTX), Figure 8 [36], were the photo-Fenton (with Fe2+, Fe3+, and potassium ferrioxalate - K3(FeOx) - as iron sources), solar photo-Fenton, Fenton and UV/H2O2 reactions. The MTX degradation experiments were carried out

constant (*K*) of 1.47 104 M–2 , indicative of a high affinity of MTX for Fe3+.

Figure 8. Molecular structure of Mitoxantrone.

irradiation led to a TOC removal of 59%.

potential, even at trace levels [35]. The AOP selected for degradation of the antineoplastic drug mitoxantrone (MTX), Figure 8 [36],

using an annular glass photochemical reactor (working volume, 1 L) and a quartz tube for

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

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Degradation of MTX by the photo-Fenton process was investigated with several different concentrations of Fe(II) (0.54, 0.27, and 0.13 mmol L–1) and H2O2 (4.0, 9.4, and 18.8 mmol L–1). The results showed a low removal of TOC, with a mineralization of only 14-35%. One explan‐ ation for this low efficiency is that MTX has nitrogen and oxygen atoms that might serve as complexation sites for iron(III), making it unavailable for participation in the Fenton reaction. The possibility of complexation between MTX and iron(III) was investigated by spectropho‐ tometric measurements. Indeed, addition of Fe(NO3)3 to solutions of MTX caused significant spectral changes, including a shift and a decrease in the absorbance of the long-wavelength absorption band (608-658 nm) of the drug. Spectrophotometric titrations suggested that the complex has a 2:1 Fe3+:MTX stoichiometric ratio with a complexation constant (*K*) of 1.47 ×

In order to minimize the effects of the complexation of Fe(III) by MTX, the use of more stable, but photoactive iron complexes as the source of iron in the degradation process was examined. One such complex is potassium ferrioxalate K3(FeOx). This complex is often employed because of its high quantum efficiency of photodecomposition and strong absorption in the UV-visible region (up to 500 nm), compatible with the use of solar irradiation in a K3(FeOx) - mediated

Figure 9 compares the efficiencies of several different AOP for the degradation of MTX. The photo-Fenton process employing K3(FeOx) and the UV/H2O2 process were the most efficient for mineralizing MTX, with 82% and 90% total organic carbon removal, respectively. Total degradation of MTX was observed in the thermal Fenton process, but only 65% degradation of MTX occurred under UV irradiation alone; However, TOC data show that there was no appreciable mineralization of MTX under direct photolysis and in the thermal Fenton reaction,

introduction of the radiation source (a 125 W mercury vapor lamp).

**Figure 8.** Molecular structure of Mitoxantrone.

M–2, indicative of a high affinity of MTX for Fe3+.

104

photo-Fenton process [37].

Degradation of MTX by the photo-Fenton process was investigated with several different concentrations of Fe(II) (0.54, 0.27, and 0.13 mmol L–1) and H2O2 (4.0, 9.4, and 18.8 mmol L–1). The results showed a low removal of TOC, with a mineralization of only 14- 35%. One explanation for this low efficiency is that MTX has nitrogen and oxygen atoms that might serve as complexation sites for iron(III), making it unavailable for participation in the Fenton reaction. The possibility of complexation between MTX and iron(III) was investigated by spectrophotometric measurements. Indeed, addition of Fe(NO3)3 to solutions of MTX caused significant spectral changes, including a shift and a decrease in the absorbance of the long-wavelength absorption band (608-658 nm) of the drug. Spectrophotometric titrations suggested that the complex has a 2:1 Fe3+:MTX stoichiometric ratio with a complexation

In order to minimize the effects of the complexation of Fe(III) by MTX, the use of more stable, but photoactive iron complexes as the source of iron in the degradation process was examined. One such complex is potassium ferrioxalate K3(FeOx). This complex is often employed because of its high quantum efficiency of photodecomposition and strong absorption in the UV-visible region (up

Figure 9 compares the efficiencies of several different AOP for the degradation of MTX. The photo-Fenton process employing K3(FeOx) and the UV/H2O2 process were the most efficient for mineralizing MTX, with 82% and 90% total organic carbon removal, respectively. Total degradation of MTX was observed in the thermal Fenton process, but only 65% degradation of MTX occurred under UV irradiation alone; However, TOC data show that there was no appreciable mineralization of MTX under direct photolysis and in the thermal Fenton reaction, even after long treatment periods, whereas the photo-Fenton reaction using solar

Although the UV/H2O2 process is usually slower than the photo-Fenton process, due to the complexation of MTX with Fe(III) in the latter, the UV/H2O2 process proved to be more efficient in this case. To corroborate this, the amount of photogenerated Fe(II) was

to 500 nm), compatible with the use of solar irradiation in a K3(FeOx) - mediated photo-Fenton process [37].

using an annular glass photochemical reactor (working volume, 1 L) and a quartz tube for introduction of the radiation source (a 125 W mercury vapor lamp).

**Figure 8.** Molecular structure of Mitoxantrone.

caused extensive degradation of the target compound. Therefore, in the photo-Fenton process, UV radiation makes a significant contribution to both mineralization and CE removal.

Normative Instruction nº 2, published on January 3, 2008, by the Brazilian Ministry of Agri‐ culture (MAPA) [34], regulates the practice in Brazil for treatment of pesticide residues in effluents generated by agricultural aviation companies. The Ministry recommends ozonation for a minimum of six hours using a system with a minimum capacity for producing one gram of ozone per hour for each charge of four hundred and fifty liters of pesticide residue derived from washing and cleaning of aircraft equipment [34]. To verify the efficiency of this system for the mineralization of CE-contaminated water, the samples were treated with ozone alone and with ozone in combination with UV light and H2O2. Although oxidation of CE was very fast with all the ozonation methods studied, the use of ozone alone proved to be of limited utility with regard to the mineralization of the organic content of CE-contaminated waters. The combination of O3/UV/H2O2 did, however, achieve a high extent of mineralization (80%), indicating that the mineralization of the organic content is mediated by the HO• radical.

When compared to the other systems studied, the photo-Fenton system showed the best results, with mineralization exceeding 85%, making it the preferred technique for the treatment

**3.1.2. Degradation of Mitoxantrone (MTX)** 

**Figure 7.** Comparison of the efficiencies of mineralization of a commercial formulation of CE in aqueous solution by the different AOP. [CE]0 = 0.060 mmol L–1; [TOC]0 = 65 mg L-1; when present, [Fe2+]=0.2 mmol L-1, [H2O2] = 68.4 mmol L-1

Antineoplastic agents (drugs employed in cancer chemotherapy) are pollutants due their mutagenic, carcinogenic, and genotoxic potential, even at trace levels [35]. The AOP selected for degradation of the antineoplastic drug mitoxantrone (MTX), Figure 8 [36], were the photo-Fenton (with Fe2+, Fe3+, and potassium ferrioxalate - K3(FeOx) - as iron sources), solar photo-Fenton, Fenton and UV/H2O2 reactions. The MTX degradation experiments were carried out

0 15 30 45 60 75 90

Reaction time/min

Figure 8. Molecular structure of Mitoxantrone.

irradiation led to a TOC removal of 59%.

constant (*K*) of 1.47 104 M–2 , indicative of a high affinity of MTX for Fe3+.

0.060 mmol L–1; [TOC]0 = 65 mg L-1; when present, [Fe2+]=0.2 mmol L-1, [H2O2] = 68.4 mmol L-1 and [O3]= 25 mg mL-1.

volume, 1 L) and a quartz tube for introduction of the radiation source (a 125 W mercury vapor lamp).

Degradation of MTX by the photo-Fenton process was investigated with several different concentrations of Fe(II) (0.54, 0.27, and 0.13 mmol L–1) and H2O2 (4.0, 9.4, and 18.8 mmol L–1). The results showed a low removal of TOC, with a mineralization of only 14- 35%. One explanation for this low efficiency is that MTX has nitrogen and oxygen atoms that might serve as complexation sites for iron(III), making it unavailable for participation in the Fenton reaction. The possibility of complexation between MTX and iron(III) was investigated by spectrophotometric measurements. Indeed, addition of Fe(NO3)3 to solutions of MTX caused significant spectral changes, including a shift and a decrease in the absorbance of the long-wavelength absorption band (608-658 nm) of the drug. Spectrophotometric titrations suggested that the complex has a 2:1 Fe3+:MTX stoichiometric ratio with a complexation

In order to minimize the effects of the complexation of Fe(III) by MTX, the use of more stable, but photoactive iron complexes as the source of iron in the degradation process was examined. One such complex is potassium ferrioxalate K3(FeOx). This complex is often employed because of its high quantum efficiency of photodecomposition and strong absorption in the UV-visible region (up

Figure 9 compares the efficiencies of several different AOP for the degradation of MTX. The photo-Fenton process employing K3(FeOx) and the UV/H2O2 process were the most efficient for mineralizing MTX, with 82% and 90% total organic carbon removal, respectively. Total degradation of MTX was observed in the thermal Fenton process, but only 65% degradation of MTX occurred under UV irradiation alone; However, TOC data show that there was no appreciable mineralization of MTX under direct photolysis and in the thermal Fenton reaction, even after long treatment periods, whereas the photo-Fenton reaction using solar

Although the UV/H2O2 process is usually slower than the photo-Fenton process, due to the complexation of MTX with Fe(III) in the latter, the UV/H2O2 process proved to be more efficient in this case. To corroborate this, the amount of photogenerated Fe(II) was

to 500 nm), compatible with the use of solar irradiation in a K3(FeOx) - mediated photo-Fenton process [37].

of wastewater containing this pesticide.

152 Organic Pollutants - Monitoring, Risk and Treatment

0.0

*3.1.2. Degradation of Mitoxantrone (MTX)*

and [O3]= 25 mg mL-1.

0.2

0.4

TOC/TOC0

 UV Fenton O3 UV/O3 UV/H2 O2 /UV photo-Fenton

0.6

0.8

1.0

Degradation of MTX by the photo-Fenton process was investigated with several different concentrations of Fe(II) (0.54, 0.27, and 0.13 mmol L–1) and H2O2 (4.0, 9.4, and 18.8 mmol L–1). The results showed a low removal of TOC, with a mineralization of only 14-35%. One explan‐ ation for this low efficiency is that MTX has nitrogen and oxygen atoms that might serve as complexation sites for iron(III), making it unavailable for participation in the Fenton reaction. The possibility of complexation between MTX and iron(III) was investigated by spectropho‐ tometric measurements. Indeed, addition of Fe(NO3)3 to solutions of MTX caused significant spectral changes, including a shift and a decrease in the absorbance of the long-wavelength absorption band (608-658 nm) of the drug. Spectrophotometric titrations suggested that the complex has a 2:1 Fe3+:MTX stoichiometric ratio with a complexation constant (*K*) of 1.47 × 104 M–2, indicative of a high affinity of MTX for Fe3+.

Figure 7. Comparison of the efficiencies of mineralization of a commercial formulation of CE in aqueous solution by the different AOP. [CE]0 = Antineoplastic agents (drugs employed in cancer chemotherapy) are pollutants due their mutagenic, carcinogenic, and genotoxic In order to minimize the effects of the complexation of Fe(III) by MTX, the use of more stable, but photoactive iron complexes as the source of iron in the degradation process was examined. One such complex is potassium ferrioxalate K3(FeOx). This complex is often employed because of its high quantum efficiency of photodecomposition and strong absorption in the UV-visible region (up to 500 nm), compatible with the use of solar irradiation in a K3(FeOx) - mediated photo-Fenton process [37].

potential, even at trace levels [35]. The AOP selected for degradation of the antineoplastic drug mitoxantrone (MTX), Figure 8 [36], were the photo-Fenton (with Fe2+, Fe3+, and potassium ferrioxalate - K3(FeOx) - as iron sources), solar photo-Fenton, Fenton and UV/H2O2 reactions. The MTX degradation experiments were carried out using an annular glass photochemical reactor (working Figure 9 compares the efficiencies of several different AOP for the degradation of MTX. The photo-Fenton process employing K3(FeOx) and the UV/H2O2 process were the most efficient for mineralizing MTX, with 82% and 90% total organic carbon removal, respectively. Total degradation of MTX was observed in the thermal Fenton process, but only 65% degradation of MTX occurred under UV irradiation alone; However, TOC data show that there was no appreciable mineralization of MTX under direct photolysis and in the thermal Fenton reaction, even after long treatment periods, whereas the photo-Fenton reaction using solar irradiation led to a TOC removal of 59%.

**3.2. Preparation of TiO2 semiconductors and their application in the heterogeneous**

spectroscopy, etc. and were applied for the degradation of a herbicide and a dye.

TiO2 is an important, widely studied photocatalytic material [38]. Several samples of TiO2 are commercially available, but Evonik (Degussa) P-25 (70% anatase and 30% rutile) is the most popular and, in most cases, gives the best results. However, different methods such as sol-gel process [39-41], electrochemical anodization [42], and molten-salt synthesis [43] can be used to prepare TiO2 in the form of powders, nanoparticles, thin film, nanotubes, etc. This section considers heterogeneous photocatalysis employing TiO2 in the forms of nanotubes obtained by electrochemical anodization, of nanoparticles prepared by sol-gel or molten salt techniques, and of Ag-doped TiO2 nanoparticles. These catalysts were characterized by a series of techni‐ ques, including scanning electron microscopy, elemental analysis, energy dispersive x-ray

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

TiO2 prepared by the sol-gel process (acid hydrolysis of titanium(IV) isopropoxide) was used for the photocatalytic degradation of the herbicide methyl viologen (MV2+, Figure 10), which is widely employed in over 130 countries on crops of rice, coffee, sugar cane, beans, and soybeans, among others [44], despite a high power of intoxication. The performance under irradiation of nanoparticles of TiO2 prepared by the sol-gel technique (TiO2 SG) was compared to TiO2 SG doped with Ag (0.5%-4.0%), and to undoped and doped TiO2 P25. The materials were characterized by thermogravimetric analysis, X-ray diffraction, surface area, infrared spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. X-Ray diffraction analysis showed that TiO2 synthesized by the sol-gel method is similar to TiO2 P25 with both anatase and rutile peaks, but with a lower crystallinity and an increase in the surface area compared to P25. The surface area of TiO2 SG, determined experimentally by BET, was

values of the band gap energy (Egap), determined by diffuse reflectance spectroscopy. Higher percentages of Ag resulted in a decrease the Egap value, shifting the light absorption to the visible region. Additionally, energy dispersive spectroscopic analysis confirmed the presence of Ag in the doped materials. Scanning electron microscopic (SEM) analysis (Figure 11) indicated that silver changed the oxide morphology, depending on the amount. In materials with 0.5% (Figure 11 A) and 1.0% of Ag (Figure 11B), the agglomerates were larger, while in samples with 2.0% (Figure 11C) and 4.0% (Figure 11D) the particles were smaller and more well-defined. This indicates that the presence of larger quantities of silver in the sol-gel oxide

g-1). The doping with Ag influenced the

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155

**photocatalysis of methyl viologen, methylene blue and xylidine**

g-1, 1.5 times larger than TiO2 P25 (46.18 m2

modified the material surface, making it more uniform.

**Figure 10.** Molecular structure of Methyl Viologen.

71.21 m2

Although the UV/H2O2 process is usually slower than the photo-Fenton process, due to the complexation of MTX with Fe(III) in the latter, the UV/H2O2 process proved to be more efficient in this case. To corroborate this, the amount of photogenerated Fe(II) was quantified during the irradiation of ferric ions [Fe(NO3)3] and ferrioxalate in the presence of MTX. In the presence of MTX, the photoreduction of Fe(III) generated only 75 μmol L-1 of Fe(II), while irradiation of ferrioxalate generated 285 μmol L-1 of Fe(II) under the same experimental conditions. This conclusively shows that MTX inhibits the photochemical step of the photo-Fenton reaction, making the overall process substantially less efficient.

**Figure 9.** Comparison of the mineralization of aqueous MTX solutions (0.077 mmol L–1) by different AOP (0.54 mmol L–1 iron source and 18.8 mmol L–1 H2O2, when present).

Cytotoxicity evaluation of the solution during treatment by an AOP is a very important since the intermediates and by-products formed during the oxidation of the organic material can be more toxic than the initial target compound. Cytotoxicity tests were performed using NIH/3T3 mouse embryonic fibroblast cells. The concentration (IC50) for inhibition of growth by MTX was 3.29 μg mL–1, demonstrating its toxicity to NIH/3T3 cells. In contrast, 100% growth of NIH/ 3T3 cells was observed in similar tests on aliquots of solutions of MTX that had been degraded by the H2O2/UV and photo-Fenton (UV/H2O2/ K3(FeOx)) processes, indicating an absence of toxic effects. Thus, these two AOP, which degraded MTX completely and exhibited the best mineralizations of the drug, generated no toxic by-products, confirming the potential of both of these processes for the removal of MTX from aqueous solution.

#### **3.2. Preparation of TiO2 semiconductors and their application in the heterogeneous photocatalysis of methyl viologen, methylene blue and xylidine**

even after long treatment periods, whereas the photo-Fenton reaction using solar irradiation

Although the UV/H2O2 process is usually slower than the photo-Fenton process, due to the complexation of MTX with Fe(III) in the latter, the UV/H2O2 process proved to be more efficient in this case. To corroborate this, the amount of photogenerated Fe(II) was quantified during the irradiation of ferric ions [Fe(NO3)3] and ferrioxalate in the presence of MTX. In the presence of MTX, the photoreduction of Fe(III) generated only 75 μmol L-1 of Fe(II), while irradiation of ferrioxalate generated 285 μmol L-1 of Fe(II) under the same experimental conditions. This conclusively shows that MTX inhibits the photochemical step of the photo-Fenton reaction,

0 20 40 60 80 100 120 140

 UV H2 O2 /Fe(II) UV/H2 O2 /Fe(II)

 Solar/H2 O2 /FeOx

 UV/H2 O2 /FeOx

 UV/H2 O2

Reaction time/min

**Figure 9.** Comparison of the mineralization of aqueous MTX solutions (0.077 mmol L–1) by different AOP (0.54 mmol

Cytotoxicity evaluation of the solution during treatment by an AOP is a very important since the intermediates and by-products formed during the oxidation of the organic material can be more toxic than the initial target compound. Cytotoxicity tests were performed using NIH/3T3 mouse embryonic fibroblast cells. The concentration (IC50) for inhibition of growth by MTX was 3.29 μg mL–1, demonstrating its toxicity to NIH/3T3 cells. In contrast, 100% growth of NIH/ 3T3 cells was observed in similar tests on aliquots of solutions of MTX that had been degraded by the H2O2/UV and photo-Fenton (UV/H2O2/ K3(FeOx)) processes, indicating an absence of toxic effects. Thus, these two AOP, which degraded MTX completely and exhibited the best mineralizations of the drug, generated no toxic by-products, confirming the potential of both

led to a TOC removal of 59%.

154 Organic Pollutants - Monitoring, Risk and Treatment

making the overall process substantially less efficient.

0.0

L–1 iron source and 18.8 mmol L–1 H2O2, when present).

of these processes for the removal of MTX from aqueous solution.

0.2

0.4

TOC/TOC

0

0.6

0.8

1.0

TiO2 is an important, widely studied photocatalytic material [38]. Several samples of TiO2 are commercially available, but Evonik (Degussa) P-25 (70% anatase and 30% rutile) is the most popular and, in most cases, gives the best results. However, different methods such as sol-gel process [39-41], electrochemical anodization [42], and molten-salt synthesis [43] can be used to prepare TiO2 in the form of powders, nanoparticles, thin film, nanotubes, etc. This section considers heterogeneous photocatalysis employing TiO2 in the forms of nanotubes obtained by electrochemical anodization, of nanoparticles prepared by sol-gel or molten salt techniques, and of Ag-doped TiO2 nanoparticles. These catalysts were characterized by a series of techni‐ ques, including scanning electron microscopy, elemental analysis, energy dispersive x-ray spectroscopy, etc. and were applied for the degradation of a herbicide and a dye.

TiO2 prepared by the sol-gel process (acid hydrolysis of titanium(IV) isopropoxide) was used for the photocatalytic degradation of the herbicide methyl viologen (MV2+, Figure 10), which is widely employed in over 130 countries on crops of rice, coffee, sugar cane, beans, and soybeans, among others [44], despite a high power of intoxication. The performance under irradiation of nanoparticles of TiO2 prepared by the sol-gel technique (TiO2 SG) was compared to TiO2 SG doped with Ag (0.5%-4.0%), and to undoped and doped TiO2 P25. The materials were characterized by thermogravimetric analysis, X-ray diffraction, surface area, infrared spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. X-Ray diffraction analysis showed that TiO2 synthesized by the sol-gel method is similar to TiO2 P25 with both anatase and rutile peaks, but with a lower crystallinity and an increase in the surface area compared to P25. The surface area of TiO2 SG, determined experimentally by BET, was 71.21 m2 g-1, 1.5 times larger than TiO2 P25 (46.18 m2 g-1). The doping with Ag influenced the values of the band gap energy (Egap), determined by diffuse reflectance spectroscopy. Higher percentages of Ag resulted in a decrease the Egap value, shifting the light absorption to the visible region. Additionally, energy dispersive spectroscopic analysis confirmed the presence of Ag in the doped materials. Scanning electron microscopic (SEM) analysis (Figure 11) indicated that silver changed the oxide morphology, depending on the amount. In materials with 0.5% (Figure 11 A) and 1.0% of Ag (Figure 11B), the agglomerates were larger, while in samples with 2.0% (Figure 11C) and 4.0% (Figure 11D) the particles were smaller and more well-defined. This indicates that the presence of larger quantities of silver in the sol-gel oxide modified the material surface, making it more uniform.

**Figure 10.** Molecular structure of Methyl Viologen.

To test the photocatalytic activity of the oxides, MV2+ photodegradation experiments were performed. The amount of herbicide solution treated was 500 mL and herbicide concentration was determined by spectrophotometric analysis at 250 nm. Titanium dioxide synthesized by the sol-gel method (Figure 12B) had a lower rate of degradation than TiO2 P25 (Figure 12A). This difference can be attributed to several factors, including the preparation method, crystal structure, surface area, size distribution and porosity. Although the sol-gel oxide had a higher surface area, it contained non-uniform particles of different sizes and therefore had a lower porosity than TiO2 P25. The oxides synthesized with 2.0% silver showed improved photoca‐ talytic activity for degradation of MV. However, in the presence of oxide doped with 4.0% silver, there was an inhibition of the photocatalytic process, probably due to the excessive

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

amount of silver, which occupied most of the active sites of the catalyst.

0 15 30 45 60 75 90

0 15 30 45 60 75 90

Reaction time/min

**B**

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0.5 0.6 0.7 0.8 0.9 1.0

 Photolysis SG 0,0% Ag SG 2,0% Ag SG 4,0% Ag

[MV2+]/[MV2+]

**Figure 12.** Results of MV2+ (0.2 mmol L-1) degradation by heterogeneous photocatalysis (0.5 g of photocatalyst) with

An alternative method for preparing TiO2 is via molten-salt synthesis. This approach employs an eutectic mixture of salts, for example NaCl/KCl or NaNO3/KNO3 in the desired proportion, together with other reagents (oxalates or metals oxides). Molten-salt synthesis [43] was used to prepare TiO2 using TiOSO4.xH2O.xH2SO4 as the Ti precursor with a melt phase of either

where the symbol [A] indicates the alkali metal cation used in the molten salt ([Na] or [K]).

The oxides synthesized in this manner were characterized by X-ray diffraction and diffuse reflectance spectroscopy. The X-ray diffraction diffractograms (Figure 13) show only the presence of the anatase phase for both oxides synthesized by the molten-salt method. The

4 3 2 24 2 2 [ ] <sup>1</sup> 2 2 2 *TiOSO ANO TiO A A SO NO O* + ® + ++ (14)

0

Reaction time/min

NaNO3 or KNO3. The synthesis reaction occurs according to Equation 14.

1.0 **<sup>A</sup>**

0.2

0.4

 Photolysis P25 0,0% Ag P25 2,0% Ag P25 4,0% Ag

[MV2+]/[MV2+]

(A) TiO2 P25 and (B) TiO2 SG.

0.6

0.8

0

**Figure 11.** Scanning electron micrographs of: (A) TiO2 SG 0.5% Ag; (B) TiO2 SG 1.0% Ag; (C) TiO2 SG 2.0% Ag; (D) TiO2 SG 4.0% Ag.

Laser flash photolysis is a technique for producing and investigating excited states and transient reaction intermediates and the kinetics of photochemical reactions. The photocata‐ lytic reduction of MV by TiO2 or by Ag-doped TiO2 (2%) in the presence and absence of sodium formate was investigated via the formation of MV•<sup>+</sup> at different initial concentrations of MV2+ (0.05, 0.07, 0.1, 0.15 and 0.2 mmol L-1), monitoring the transient absorption of MV•<sup>+</sup> at 605 nm [45]. As reported by Tachikawa et al. [45], the transient absorption decays by first order kinetics. The bimolecular electron transfer rate constants in the absence and presence of sodium formate, listed in Table 1, were obtained from linear plots of the observed first-order rate constants versus the concentrations of MV2+. In the presence of sodium formate there is an increase in electron transfer constant for all photocatalysts analyzed; according Tachikawa et al [45], this occurs because the initial oxidation of organic additives, such as sodium formate, generates the CO2• radical, which has strong reducing power and can easily reduce other substrates. There is an increase in the electron transfer rate constants in the presence of sodium formate and in the presence of silver, demonstrating the improved efficiency of the oxidation/ reduction in the presence of the metal.


**Table 1.** Electron transfer rate constants for MV2+ in the presence of the different TiO2 photocatalysts.

To test the photocatalytic activity of the oxides, MV2+ photodegradation experiments were performed. The amount of herbicide solution treated was 500 mL and herbicide concentration was determined by spectrophotometric analysis at 250 nm. Titanium dioxide synthesized by the sol-gel method (Figure 12B) had a lower rate of degradation than TiO2 P25 (Figure 12A). This difference can be attributed to several factors, including the preparation method, crystal structure, surface area, size distribution and porosity. Although the sol-gel oxide had a higher surface area, it contained non-uniform particles of different sizes and therefore had a lower porosity than TiO2 P25. The oxides synthesized with 2.0% silver showed improved photoca‐ talytic activity for degradation of MV. However, in the presence of oxide doped with 4.0% silver, there was an inhibition of the photocatalytic process, probably due to the excessive amount of silver, which occupied most of the active sites of the catalyst.

**Figure 11.** Scanning electron micrographs of: (A) TiO2 SG 0.5% Ag; (B) TiO2 SG 1.0% Ag; (C) TiO2 SG 2.0% Ag; (D) TiO2

Laser flash photolysis is a technique for producing and investigating excited states and transient reaction intermediates and the kinetics of photochemical reactions. The photocata‐ lytic reduction of MV by TiO2 or by Ag-doped TiO2 (2%) in the presence and absence of sodium

MV2+ (0.05, 0.07, 0.1, 0.15 and 0.2 mmol L-1), monitoring the transient absorption of MV•<sup>+</sup> at 605 nm [45]. As reported by Tachikawa et al. [45], the transient absorption decays by first order kinetics. The bimolecular electron transfer rate constants in the absence and presence of sodium formate, listed in Table 1, were obtained from linear plots of the observed first-order rate constants versus the concentrations of MV2+. In the presence of sodium formate there is an increase in electron transfer constant for all photocatalysts analyzed; according Tachikawa et al [45], this occurs because the initial oxidation of organic additives, such as sodium formate,

substrates. There is an increase in the electron transfer rate constants in the presence of sodium formate and in the presence of silver, demonstrating the improved efficiency of the oxidation/

TiO2 P25 5.4 x 109 M-1s-1 6.0 x 109 M-1s-1 TiO2 P25 2.0% Ag 6.5 x 109 M-1s-1 8.0 x 109 M-1s-1 TiO2 SG 3.2 x 109 M-1s-1 3.6 x 109 M-1s-1 TiO2 SG 2.0% Ag 4.5 x 109 M-1s-1 6.6 x 109 M-1s-1

**Table 1.** Electron transfer rate constants for MV2+ in the presence of the different TiO2 photocatalysts.

radical, which has strong reducing power and can easily reduce other

**Absence of NaHCO2 Presence of NaHCO2**

at different initial concentrations of

formate was investigated via the formation of MV•<sup>+</sup>

SG 4.0% Ag.

generates the CO2•-

reduction in the presence of the metal.

156 Organic Pollutants - Monitoring, Risk and Treatment

**Figure 12.** Results of MV2+ (0.2 mmol L-1) degradation by heterogeneous photocatalysis (0.5 g of photocatalyst) with (A) TiO2 P25 and (B) TiO2 SG.

An alternative method for preparing TiO2 is via molten-salt synthesis. This approach employs an eutectic mixture of salts, for example NaCl/KCl or NaNO3/KNO3 in the desired proportion, together with other reagents (oxalates or metals oxides). Molten-salt synthesis [43] was used to prepare TiO2 using TiOSO4.xH2O.xH2SO4 as the Ti precursor with a melt phase of either NaNO3 or KNO3. The synthesis reaction occurs according to Equation 14.

$$TiOSO\_4 + 2AlO\_3 \to TiO\_2[A] + A\_2SO\_4 + 2NO\_2 + \bigvee\_2 O\_2 \tag{14}$$

where the symbol [A] indicates the alkali metal cation used in the molten salt ([Na] or [K]).

The oxides synthesized in this manner were characterized by X-ray diffraction and diffuse reflectance spectroscopy. The X-ray diffraction diffractograms (Figure 13) show only the presence of the anatase phase for both oxides synthesized by the molten-salt method. The Egap values for TiO2[K] (3.13 eV) and TiO2[Na] (3.15 eV) were similar to that of P25 (3.13 eV), as expected.

**Figure 15.** Molecular structure of Methylene Blue.

0.0

0.2

0.4

[MB]/[MB]0

11 synthesized by the molten salt method.

22 diameter of the nanotubes was 66 nm.

18 soluble complex fluoride.

the molten salt method.

0.6

0.8

1.0

of organic dyes like MB.

1

9

Figure 16 compares the degradation of MB using the two catalysts synthesized by the moltensalt method. TiO2[K] produced a net degradation efficiency of 99%. In constrast, TiO2[Na] degraded only 61% of MB. The lower rate of degradation of MB by TiO2[Na] and the lower overall efficiency may be related to the differences in aggregation observed in the SEM images of the two oxides (Figure 14). On the basis of these results, TiO2[K] obtained by the molten salt method would appear to be a promising alternative material for the catalytic photodegradation

0 15 30 45 60 75

Figure 16. Degradation of MB (0.40 mmol L-1) by heterogeneous photocatalysis using TiO2 (0.5g L-1 10 )

**Figure 16.** Degradation of MB (0.40 mmol L-1) by heterogeneous photocatalysis using TiO2 (0.5g L-1) synthesized by

TiO2 nanotubes has been subject of several recent studies due to their unique electronic transport properties and their mechanical strength, large surface area and well-defined geometry, which improve their performance in many applications compared to other forms of titanium dioxide. Several studies have reported that highly ordered and uniform TiO2 nanotubes can be easily obtained using anodization in titanium fluoride [42,47]. The formation of TiO2 nanotubes by electrochemical anodization is based on a competition between the

TiO2 12 nanotubes has been subject of several recent studies due to their unique electronic transport properties and their mechanical strength, large surface area and well-defined geometry, which improve their performance in many applications compared to other forms of titanium dioxide. Several studies have reported that highly ordered and uniform TiO2 nanotubes can be easily obtained using anodization in titanium fluoride [42,47]. The formation of TiO2 16 nanotubes by electrochemical anodization is based on a competition between the anodic oxide formation and its dissolution as a

Ti/TiO2 19 electrodes (4 x 2.8 cm) were prepared by anodization of Ti foil using an applied voltage of 20 V in 0.15 mol L-1 NH4F in glycerol (10% H2O). Self-assembly of nanotubular TiO2 20 arrays can be 21 seen on films of Ti obtained under these anodization conditions (Figure 17). The average internal

anodic oxide formation and its dissolution as a soluble complex fluoride.

Reaction time/min

 photolysis TiO2 [K] TiO2 [Na]

3 Figure 16 compares the degradation of MB using the two catalysts synthesized by the molten-salt method. TiO2[K] produced a net degradation efficiency of 99%. In constrast, TiO2 4 [Na] degraded only 61% of MB. The lower rate of degradation of MB by TiO2 5 [Na] and the lower overall efficiency 6 may be related to the differences in aggregation observed in the SEM images of the two oxides (Figure 14). On the basis of these results, TiO2 7 [K] obtained by the molten salt method would appear 8 to be a promising alternative material for the catalytic photodegradation of organic dyes like MB.

2 Figure 15. Molecular structure of Methylene Blue.

18 Book Title

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

http://dx.doi.org/10.5772/53188

159

**Figure 13.** X-ray diffractograms of TiO2 synthesized by the molten salt method: (A) in NaNO3 and (B) in KNO3.

The morphologies of the oxides, observed by SEM (Figure 14), exhibited different forms of agglomeration, presumably due to an influence of the alkaline metal nitrate molten salt used in the synthesis.

**Figure 14.** Scanning electron micrographs of (A) TiO2[K]; (B) TiO2[Na].

In order to evaluate the photocatalytic activities of the synthesized oxides, they were used for the photodegradation of the dye methylene blue (MB, Figure 15). Although MB is not consid‐ ered to be a very toxic dye, it can cause harmful effects on living beings. After inhalation, symptoms such as difficulty in breathing, vomiting, diarrhea and nausea may occur in humans [46]. The degradation of MB was carried out in aqueous solution in a 400 mL reactor with an 80 W mercury vapor lamp as the irradiation source. The concentration of MB was determined from its absorption at 654 nm.

**Figure 15.** Molecular structure of Methylene Blue.

9

22 diameter of the nanotubes was 66 nm.

Egap values for TiO2[K] (3.13 eV) and TiO2[Na] (3.15 eV) were similar to that of P25 (3.13 eV),

0

50

100

150

Intensity (a.u.)

<sup>220107</sup> <sup>204</sup> <sup>213</sup>

**Figure 13.** X-ray diffractograms of TiO2 synthesized by the molten salt method: (A) in NaNO3 and (B) in KNO3.

The morphologies of the oxides, observed by SEM (Figure 14), exhibited different forms of agglomeration, presumably due to an influence of the alkaline metal nitrate molten salt used

In order to evaluate the photocatalytic activities of the synthesized oxides, they were used for the photodegradation of the dye methylene blue (MB, Figure 15). Although MB is not consid‐ ered to be a very toxic dye, it can cause harmful effects on living beings. After inhalation, symptoms such as difficulty in breathing, vomiting, diarrhea and nausea may occur in humans [46]. The degradation of MB was carried out in aqueous solution in a 400 mL reactor with an 80 W mercury vapor lamp as the irradiation source. The concentration of MB was determined

10 20 30 40 50 60 70 80

2 (degree)

200

TiO2 - Anatase Spatial group: I 41/am d Simetry: tetragonal

004

103

**Figure 14.** Scanning electron micrographs of (A) TiO2[K]; (B) TiO2[Na].

from its absorption at 654 nm.

101

<sup>105</sup> <sup>211</sup> <sup>116</sup> <sup>112</sup>

158 Organic Pollutants - Monitoring, Risk and Treatment

Intensity (a.u.)

in the synthesis.

200

250 **B**

101

10 20 30 40 50 60 70 80

200

TiO2 - Anatase Spatial group: I 41/am d Simetry: tetragonal

<sup>105</sup> <sup>211</sup> <sup>116</sup> <sup>103004112</sup>

220 <sup>107</sup> <sup>204</sup> <sup>213</sup>

2 (degree)

as expected.

Figure 16 compares the degradation of MB using the two catalysts synthesized by the moltensalt method. TiO2[K] produced a net degradation efficiency of 99%. In constrast, TiO2[Na] degraded only 61% of MB. The lower rate of degradation of MB by TiO2[Na] and the lower overall efficiency may be related to the differences in aggregation observed in the SEM images of the two oxides (Figure 14). On the basis of these results, TiO2[K] obtained by the molten salt method would appear to be a promising alternative material for the catalytic photodegradation of organic dyes like MB. 1 2 Figure 15. Molecular structure of Methylene Blue. 3 Figure 16 compares the degradation of MB using the two catalysts synthesized by the molten-salt method. TiO2[K] produced a net degradation efficiency of 99%. In constrast, TiO2 4 [Na] degraded only 61% of MB. The lower rate of degradation of MB by TiO2 5 [Na] and the lower overall efficiency 6 may be related to the differences in aggregation observed in the SEM images of the two oxides (Figure 14). On the basis of these results, TiO2 7 [K] obtained by the molten salt method would appear

8 to be a promising alternative material for the catalytic photodegradation of organic dyes like MB.

Figure 16. Degradation of MB (0.40 mmol L-1) by heterogeneous photocatalysis using TiO2 (0.5g L-1 10 ) 11 synthesized by the molten salt method. **Figure 16.** Degradation of MB (0.40 mmol L-1) by heterogeneous photocatalysis using TiO2 (0.5g L-1) synthesized by the molten salt method.

TiO2 12 nanotubes has been subject of several recent studies due to their unique electronic transport

13 properties and their mechanical strength, large surface area and well-defined geometry, which 14 improve their performance in many applications compared to other forms of titanium dioxide. Several 15 studies have reported that highly ordered and uniform TiO2 nanotubes can be easily obtained using anodization in titanium fluoride [42,47]. The formation of TiO2 16 nanotubes by electrochemical 17 anodization is based on a competition between the anodic oxide formation and its dissolution as a 18 soluble complex fluoride. Ti/TiO2 19 electrodes (4 x 2.8 cm) were prepared by anodization of Ti foil using an applied voltage of 20 V in 0.15 mol L-1 NH4F in glycerol (10% H2O). Self-assembly of nanotubular TiO2 20 arrays can be 21 seen on films of Ti obtained under these anodization conditions (Figure 17). The average internal TiO2 nanotubes has been subject of several recent studies due to their unique electronic transport properties and their mechanical strength, large surface area and well-defined geometry, which improve their performance in many applications compared to other forms of titanium dioxide. Several studies have reported that highly ordered and uniform TiO2 nanotubes can be easily obtained using anodization in titanium fluoride [42,47]. The formation of TiO2 nanotubes by electrochemical anodization is based on a competition between the anodic oxide formation and its dissolution as a soluble complex fluoride.

8

Ti/TiO2 electrodes (4 x 2.8 cm) were prepared by anodization of Ti foil using an applied voltage of 20 V in 0.15 mol L-1 NH4F in glycerol (10% H2O). Self-assembly of nanotubular TiO2 arrays can be seen on films of Ti obtained under these anodization conditions (Figure 17). The average internal diameter of the nanotubes was 66 nm.

19

Initial studies of photoelectrocatalytic oxidation employing these electrodes was carried out using 2,4 xylidine (Figure 19) as the model pollutant. In 0.1 mol L-1 Na2SO4 supporting

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

http://dx.doi.org/10.5772/53188

161

Although there has been a considerable increase in research activity related to advanced oxidation processes (AOP) since 2000, a number of significant challenges must still be over‐ come to make AOP generally applicable for the treatment of polluted waters and effluents. AOP involving both homogeneous and heterogeneous catalysis have shown good results for degradation of pollutants leading to efficient mineralization. The use of TiO2 nanoparticles and nanotubes as the photocatalyst have been shown to be viable alternatives for the photodegra‐ dation of methylene blue (MB) and for the photoelectrocatalytic oxidation of xylidine. These studies underline the importance of synthesizing new molecules and testing the catalytic efficiencies of novel materials. In addition, new experimental conditions and new AOP technologies need to be developed for the efficient, cost-effective oxidative mineralization of

electrolyte applying a potential of 0.6 V, a TOC removal of 62% was obtained.

**Figure 19.** Molecular structure of 2,4-Xylidine.

organic materials in polluted waters.

AOP Advanced Oxidation Processes AOT Advanced Oxidation Technologies

HO• Hydroxyl Radical SO4 •- Sulfate Radical Anion DHB Dihydroxybenzene CE Chlorimurom-Ethyl MTX Mitoxantrone K3(FeOx) Potassium Ferrioxalate

**Abbreviations list**

**4. Conclusion**

**Figure 17.** Scanning electron micrographs of TiO2 nanotubes prepared on a Ti film in 0.15 mol L-1 NH4F in glycerol (10% H2O) at a potential of 20 V. (A) TiO2 nanotubes, (B) diameters of the nanotubes. 1

Figure 17. Scanning electron micrographs of TiO2 nanotubes prepared on a Ti film in 0.15 mol L-1 2

The photoeletrocatalytic activity of the Ti/TiO2 electrodes was evaluated by linear voltammet‐ ric scans in the potential range of -0.4 to 0.7 V under UV irradiation. The photoanodic current flow arises from the photooxidation of adsorbed water molecules or hydroxyl groups on the titania surface (Figure 18). 3 NH4F in glycerol (10% H2O) at a potential of 20 V. (A) TiO2 nanotubes, (B) diameters of the 4 nanotubes. The photoeletrocatalytic activity of the Ti/TiO2 5 electrodes was evaluated by linear voltammetric scans 6 in the potential range of -0.4 to 0.7 V under UV irradiation. The photoanodic current flow arises from 7 the photooxidation of adsorbed water molecules or hydroxyl groups on the titania surface (Figure 18).

Figure 18. Linear-sweep photovoltammograms for TiO2 nanotubes on a Ti film in 0.1 mol L-1 10 Na2SO4 (curve A) under UV illumination and in the dark (curve B). Scan rate: 5 mV s-1 11 . **Figure 18.** Linear-sweep photovoltammograms for TiO2 nanotubes on a Ti film in 0.1 mol L-1 Na2SO4 (curve A) under UV illumination and in the dark (curve B). Scan rate: 5 mV s-1.

12 Initial studies of photoelectrocatalytic oxidation employing these electrodes was carried out using 2,4 xylidine (Figure 19) as the model pollutant. In 0.1 mol L-1 Na2SO4 13 supporting electrolyte applying a

14 potential of 0.6 V, a TOC removal of 62% was obtained.

Initial studies of photoelectrocatalytic oxidation employing these electrodes was carried out using 2,4 xylidine (Figure 19) as the model pollutant. In 0.1 mol L-1 Na2SO4 supporting electrolyte applying a potential of 0.6 V, a TOC removal of 62% was obtained.

**Figure 19.** Molecular structure of 2,4-Xylidine.

### **4. Conclusion**

Ti/TiO2 electrodes (4 x 2.8 cm) were prepared by anodization of Ti foil using an applied voltage of 20 V in 0.15 mol L-1 NH4F in glycerol (10% H2O). Self-assembly of nanotubular TiO2 arrays can be seen on films of Ti obtained under these anodization conditions (Figure 17). The average

19

**Figure 17.** Scanning electron micrographs of TiO2 nanotubes prepared on a Ti film in 0.15 mol L-1 NH4F in glycerol

The photoeletrocatalytic activity of the Ti/TiO2 electrodes was evaluated by linear voltammet‐ ric scans in the potential range of -0.4 to 0.7 V under UV irradiation. The photoanodic current flow arises from the photooxidation of adsorbed water molecules or hydroxyl groups on the


Figure 18. Linear-sweep photovoltammograms for TiO2 nanotubes on a Ti film in 0.1 mol L-1 10 Na2SO4

**Figure 18.** Linear-sweep photovoltammograms for TiO2 nanotubes on a Ti film in 0.1 mol L-1 Na2SO4 (curve A) under

12 Initial studies of photoelectrocatalytic oxidation employing these electrodes was carried out using 2,4 xylidine (Figure 19) as the model pollutant. In 0.1 mol L-1 Na2SO4 13 supporting electrolyte applying a

curve B

curve A

Figure 17. Scanning electron micrographs of TiO2 nanotubes prepared on a Ti film in 0.15 mol L-1 2 3 NH4F in glycerol (10% H2O) at a potential of 20 V. (A) TiO2 nanotubes, (B) diameters of the

The photoeletrocatalytic activity of the Ti/TiO2 5 electrodes was evaluated by linear voltammetric scans 6 in the potential range of -0.4 to 0.7 V under UV irradiation. The photoanodic current flow arises from 7 the photooxidation of adsorbed water molecules or hydroxyl groups on the titania surface (Figure 18).

(10% H2O) at a potential of 20 V. (A) TiO2 nanotubes, (B) diameters of the nanotubes.

internal diameter of the nanotubes was 66 nm.

160 Organic Pollutants - Monitoring, Risk and Treatment

titania surface (Figure 18).

0

<sup>9</sup>E/mV

14 potential of 0.6 V, a TOC removal of 62% was obtained.

UV illumination and in the dark (curve B). Scan rate: 5 mV s-1.

(curve A) under UV illumination and in the dark (curve B). Scan rate: 5 mV s-1 11 .

5

10

I/mA

15

20

4 nanotubes.

1

8

Running Title

Although there has been a considerable increase in research activity related to advanced oxidation processes (AOP) since 2000, a number of significant challenges must still be over‐ come to make AOP generally applicable for the treatment of polluted waters and effluents. AOP involving both homogeneous and heterogeneous catalysis have shown good results for degradation of pollutants leading to efficient mineralization. The use of TiO2 nanoparticles and nanotubes as the photocatalyst have been shown to be viable alternatives for the photodegra‐ dation of methylene blue (MB) and for the photoelectrocatalytic oxidation of xylidine. These studies underline the importance of synthesizing new molecules and testing the catalytic efficiencies of novel materials. In addition, new experimental conditions and new AOP technologies need to be developed for the efficient, cost-effective oxidative mineralization of organic materials in polluted waters.
