**3. Advanced oxidation processes**

When selecting the most suitable wastewater treatment method for the specific effluent, both the feasibility of the treatment as well as the economics of the process need to be considered. There are multiplicities of different kinds of techniques available such as physical, chemical and biological wastewater treatments and their combinations.

Advanced oxidation processes (AOPs) belong to the chemical treatment category and are used to oxidise organic compounds found in wastewater which are difficult to handle biologically into simpler end products. Advanced oxidation processes involve the generation of free hydroxyl radical (HO), a powerful, non-selective chemical oxidant (Table 3) (Munter, 2001).


Table 3. Relative oxidation activity of some oxidising agents (Munter, 2001).

wastewater are listed in Table 2. All these compounds can be found or may be developing in

NH2(CH2)5NH2

CH3(CH2)3SH Skunk

Dead body

**Odorous compound Chemical formula Odour quality**  Amines CH3NH2, (CH3)3NH Fishy Ammonia NH3 Ammonia

Hydrogen sulphide H2S Rotten eggs Mercaptans (1-2 carbon) CH3SH, CH3(CH2)SH Decayed cabbage

Organic sulphides (CH3)2S, (C6H5)2S Rotten cabbage Skatole C9H9N Faecal matter Table 2. Malodorous compounds in untreated wastewater (Metcalf & Eddy, 2003).

In the complete characterisation of odour, four independent factors can be classified: intensity, character, hedonics and detectability. Odours can be measured by sensory methods and a specific odorant concentration can be measure by instrumental methods, such as GC-MS analysis (Metcalf & Eddy, 2003). In the sensory method, a panel of human subjects is exposed to odour-free air diluted odours and the minimum detectable threshold odour concentration (MDTOC) is noted. This procedure can be performed according to the

When selecting the most suitable wastewater treatment method for the specific effluent, both the feasibility of the treatment as well as the economics of the process need to be considered. There are multiplicities of different kinds of techniques available such as

Advanced oxidation processes (AOPs) belong to the chemical treatment category and are used to oxidise organic compounds found in wastewater which are difficult to handle biologically into simpler end products. Advanced oxidation processes involve the generation of free hydroxyl radical (HO), a powerful, non-selective chemical oxidant (Table

**Oxidising agent Relative oxidation activity** 

physical, chemical and biological wastewater treatments and their combinations.

Positively charged hole on titanium dioxide, TiO2+ 2.35 Hydroxyl radical 2.05 Atomic oxygen 1.78 Ozone 1.52 Hydrogen peroxide 1.31 Permanganate 1.24 Hypochlorous acid 1.10 Chlorine 1.00

Table 3. Relative oxidation activity of some oxidising agents (Munter, 2001).

wastewaters, depending on ambient conditions (Metcalf & Eddy, 2003).

Diamines NH2(CH2)4NH2,

Mercaptans (over 2 carbon) (CH3)3CSH,

Standard Method 2150B Threshold Odour Test (APHA, 1998).

**3. Advanced oxidation processes** 

3) (Munter, 2001).


Table 4. Reaction rate constants for ozone and hydroxyl radical for organic compounds (Munter, 2001).

Hydroxyl radical is one of the most active oxidising agents known. It acts very rapidly with most organic molecules with rate constants in the order of 108 – 1011 M-1 s-1 (Table 4) (Munter, 2001). Depending upon the nature of the organic species, generated hydroxyl radical can attack organic radicals by radical addition, hydrogen abstraction, electron transfer and radical combination.

*Radical addition.* Reaction of the hydroxyl radical and unsaturated or aliphatic organic compound produces organic radical which can further oxidise by oxygen or ferrous iron to form stable oxidised end products.

$$\text{R} + \text{HO}^{\cdot} \rightarrow \text{ROH} \tag{2}$$

*Hydrogen abstraction.* Generated hydroxyl radical can be used to remove hydrogen from an organic compound forming an organic radical and initiating a chain reaction where the organic radical reacts with oxygen. This produces a peroxyl radical, which can react with another organic compound, and so on.

$$\rm R + HO^{\cdot} \rightarrow R + H\_2O \tag{3}$$

*Electron transfer.* Electron transfer results in the formation of ions with a higher valence. Oxidation of a monoatomic negative ion will result in the formation of an atom or a free radical.

$$\rm R^{n} + HO^{\cdot} \rightarrow R^{n-1} + HO^{\cdot} \tag{4}$$

*Radical combination.* Two radicals form a stable product.

$$\rm HO^{\cdot} + HO^{\cdot} \rightarrow H\_2O\_2 \tag{5}$$

Generally, the reaction of hydroxyl radicals and organic compounds will produce water, carbon dioxide and salts (SES, 1994). However, the attack of the HO radical, in the presence of oxygen, generates a complex series of oxidation reactions in which the exact routes of these reactions to complete mineralisation of the organics are still not quite clear. Chlorine containing organic compounds, for example, are oxidised first to intermediates, such as aldehydes and carboxylic acids, and finally to carbon dioxide and water, and to chlorine ions (Munter, 2001).

A very important point, which has to be considered in the case of natural waters, is the presence of carbonates. Efficient trapping of HO· radicals by bicarbonate (equation 6) and

The rate constants of the hydroxyl radicals are typically 106 – 109 times higher than the corresponding reaction rate constants of molecular ozone (Table 4). The oxidation of organic compounds may also occur due to the combination of reactions with molecular ozone and

*Ozone with hydrogen peroxide.* The addition of hydrogen peroxide to the aqueous solution of ozone enhances the decomposition of O3 with the formation of hydroxyl radicals. To summarise: two ozone molecules will produce two hydroxyl radicals (equation 9) (Munter,

The action of both ozone molecules and the generated hydroxyl radicals results in a significant improvement in the rates of decomposition of pollutants in aqueous solutions. *Ozone and catalyst.* Catalytic ozonation is another opportunity to accelerate ozonation with compounds which are weakly reactive with ozone, such as atratzine. Several homogeneous catalysts, such as zinc and copper sulfates, silver nitrate, chromium trioxide (Abdo et al., 1988) and also heterogeneous catalysts, Ru/CeO2, (Delanoë et al., 2001), MnO2 (Ma & Graham, 1997), TiO2/Al2O3 (Beltrán et al., 2004) and Pt/Al2O3 (Chang et al., 2009) have been studied. According to these studies, both homogeneous and heterogeneous catalysts are able to improve the efficiency of ozone for the removal of different organic compounds in an

In a photo-oxidation reaction, UV radiation (photon) excites an electron of an organic molecule (C) from the ground state to the excited state (C\*) (equation 10). The excited organic molecule excites further molecular oxygen (equation 11) with a subsequent recombination of the radical ions or hydrolysis of the radical cation, or homolysis (equation

12) to form radicals which can react with oxygen (equation 13) (Legrini et al., 1993).

C hv

R-X hv <sup>→</sup> R·

R·

**3.2.1 UV/ozone, UV/H2O2 and UV/O3/H2O2 processes** 

C\* + O2 → C·+ O2

The rate of the photo-oxidation reaction depends on the adsorption cross section of the medium, the quantum yield of the process, the photon rate at the wavelength of excitation and the concentration of dissolved molecular oxygen (Legrini et al, 1993). However, to achieve the complete mineralisation of the treated effluent, photolysis is usually combined with oxidising compounds (hydrogen peroxide, ozone) or semiconductors (such as titanium

The combination of UV light and ozone/hydrogen peroxide or both significantly enhances the rate of generating free radicals. Ozone adsorbs UV radiation at a wavelength of 254 nm

+ 3O2 (9)

315

<sup>→</sup> C\* (10)

·- (11)

+ X· (12)

+ O2 → RO2 (13)

2O3 + H2O2 → 2HO·

reactions with hydroxyl radicals (Munter, 2001).

2001):

aqueous solution.

**3.2 Photolysis** 

dioxide).

carbonate (equation 7), radical scavengers, can significantly reduce the efficiency of the abatement of pollutants.

$$\rm{HO} \rm{+} \rm{HCO}\_{3}^{\cdot} \rightarrow \rm{H}\_{2}\rm{O} \rm{+} \rm{CO}\_{3}^{\cdot} \tag{6}$$

$$\rm{HO^{+}} + \rm{CO\_{3}^{2-}} \rightarrow \rm{HO^{+}} + \rm{CO\_{3}^{-}} \tag{7}$$

However, the generated carbonate radical anion is also an oxidant itself, but its oxidation power is less positive compared to a HO· radical (Legrini et al, 1993).

The destruction rate of contaminants is approximately proportional to a constant rate for the pollutant with a HO radical. As we can see from Table 4, chlorinated alkenes decompose fastest because the double bond is very prone to a hydroxyl attack. Saturated molecules, such as alkanes, are more difficult to oxidise because of a slower reaction rate (Table 4). The powerfulness of the hydroxyl radical gives advanced oxidation processes the ability to achieve oxidative destruction of compounds refractory to conventional hydrogen peroxide or ozone oxidation. AOPs have been used successfully for example, destroying pesticides by photochemical degradation (UV/O3 and UV/H2O2) (Andreozzi et al., 2003), photocatalysis (TiO2/UV, Fenton and photo-Fenton process) (Legrini et al., 1993; Fallman et al., 1999) and chemical oxidation processes (O3, O3/H2O2 and H2O2/Fe2+) (Masten & Davies, 1994; Benitez et al., 2002), decomposing of organics from textile wastewater, such as surfactants and dyes, by photo-Fenton and H2O2/UV-C treatment (García-Montaño et al., 2006), photocatalysis with immobilised TiO2 (Harrelkas et al., 2008) and also for the destruction of organics in different kind of effluents, such as paper mill wastewaters by photocatalysis (Pérez et al., 2001), landfill leachates by the Fenton process (Lopez et al, 2004; Gotvajn et al., 2009 ), olive mill wastewaters by wet air oxidation (Gomes et al., 2007) etc.

Advanced oxidation methods can be split into "cold" and "hot" oxidation. Cold oxidation methods work near to ambient temperature and pressure compared to hot oxidation at elevated temperatures and pressure (Verenich, 2003). Suitable applications of cold oxidation methods include effluents containing relatively small amount of COD (≤ 5.0 g L-1). Higher COD contents would require the consumption of large amounts of expensive reactants, such as O3 and H2O2 (Andreozzi et al., 1999). For wastewaters with higher COD values (≥ 5.0 g L-1), hot oxidation techniques are more convenient (Mishra et al., 1995).

#### **3.1 Ozone water processes**

*Ozonation at elevated values of pH.* Ozone is an effective oxidising agent (Table 3) which reacts with most compounds containing multiple bonds, such as C=C, C=N, N=N, but not with species containing single bonds (C-C, C-O, O-H) at high rates (Gogate & Pandit, 2004a). At higher pH values, ozone reacts almost unselectively with all inorganic and organic compounds present in the solution (Staehelin & Hoigne, 1982). Rising the pH of the aqueous solution increases the decomposition rate of the ozone that generates the super-oxide anion radical O2- and hydroperoxyl radical HO2. For example, the ozonide anion O3- is formed by the reaction between O3 and O2- . The ozonide anion further decomposes to a HO radical, such that, three ozone molecules will produce two HO radicals (equation 8) (Munter, 2001):

$$\text{2SO}\_3 + \text{HO}^\cdot + \text{H}^+ \rightarrow 2\text{HO}^\cdot + \text{4O}\_2 \tag{8}$$

The rate constants of the hydroxyl radicals are typically 106 – 109 times higher than the corresponding reaction rate constants of molecular ozone (Table 4). The oxidation of organic compounds may also occur due to the combination of reactions with molecular ozone and reactions with hydroxyl radicals (Munter, 2001).

*Ozone with hydrogen peroxide.* The addition of hydrogen peroxide to the aqueous solution of ozone enhances the decomposition of O3 with the formation of hydroxyl radicals. To summarise: two ozone molecules will produce two hydroxyl radicals (equation 9) (Munter, 2001):

$$\text{2O}\_3 + \text{H}\_2\text{O}\_2 \to \text{2HO} + \text{3O}\_2\tag{9}$$

The action of both ozone molecules and the generated hydroxyl radicals results in a significant improvement in the rates of decomposition of pollutants in aqueous solutions. *Ozone and catalyst.* Catalytic ozonation is another opportunity to accelerate ozonation with compounds which are weakly reactive with ozone, such as atratzine. Several homogeneous catalysts, such as zinc and copper sulfates, silver nitrate, chromium trioxide (Abdo et al., 1988) and also heterogeneous catalysts, Ru/CeO2, (Delanoë et al., 2001), MnO2 (Ma & Graham, 1997), TiO2/Al2O3 (Beltrán et al., 2004) and Pt/Al2O3 (Chang et al., 2009) have been studied. According to these studies, both homogeneous and heterogeneous catalysts are able to improve the efficiency of ozone for the removal of different organic compounds in an aqueous solution.

#### **3.2 Photolysis**

320 Food Industrial Processes – Methods and Equipment

carbonate (equation 7), radical scavengers, can significantly reduce the efficiency of the

2- → HO-

However, the generated carbonate radical anion is also an oxidant itself, but its oxidation

The destruction rate of contaminants is approximately proportional to a constant rate for the pollutant with a HO radical. As we can see from Table 4, chlorinated alkenes decompose fastest because the double bond is very prone to a hydroxyl attack. Saturated molecules, such as alkanes, are more difficult to oxidise because of a slower reaction rate (Table 4). The powerfulness of the hydroxyl radical gives advanced oxidation processes the ability to achieve oxidative destruction of compounds refractory to conventional hydrogen peroxide or ozone oxidation. AOPs have been used successfully for example, destroying pesticides by photochemical degradation (UV/O3 and UV/H2O2) (Andreozzi et al., 2003), photocatalysis (TiO2/UV, Fenton and photo-Fenton process) (Legrini et al., 1993; Fallman et al., 1999) and chemical oxidation processes (O3, O3/H2O2 and H2O2/Fe2+) (Masten & Davies, 1994; Benitez et al., 2002), decomposing of organics from textile wastewater, such as surfactants and dyes, by photo-Fenton and H2O2/UV-C treatment (García-Montaño et al., 2006), photocatalysis with immobilised TiO2 (Harrelkas et al., 2008) and also for the destruction of organics in different kind of effluents, such as paper mill wastewaters by photocatalysis (Pérez et al., 2001), landfill leachates by the Fenton process (Lopez et al, 2004; Gotvajn et al., 2009 ), olive

Advanced oxidation methods can be split into "cold" and "hot" oxidation. Cold oxidation methods work near to ambient temperature and pressure compared to hot oxidation at elevated temperatures and pressure (Verenich, 2003). Suitable applications of cold oxidation methods include effluents containing relatively small amount of COD (≤ 5.0 g L-1). Higher COD contents would require the consumption of large amounts of expensive reactants, such as O3 and H2O2 (Andreozzi et al., 1999). For wastewaters with higher COD values (≥ 5.0 g L-

*Ozonation at elevated values of pH.* Ozone is an effective oxidising agent (Table 3) which reacts with most compounds containing multiple bonds, such as C=C, C=N, N=N, but not with species containing single bonds (C-C, C-O, O-H) at high rates (Gogate & Pandit, 2004a). At higher pH values, ozone reacts almost unselectively with all inorganic and organic compounds present in the solution (Staehelin & Hoigne, 1982). Rising the pH of the aqueous solution increases the decomposition rate of the ozone that generates the super-oxide anion radical O2- and hydroperoxyl radical HO2. For example, the ozonide anion O3- is formed

radical, such that, three ozone molecules will produce two HO radicals (equation 8)

+ H+ → 2HO·

. The ozonide anion further decomposes to a HO

+ 4O2 (8)


+ CO3

· - (6)

· - (7)

HO·

HO·

power is less positive compared to a HO· radical (Legrini et al, 1993).

mill wastewaters by wet air oxidation (Gomes et al., 2007) etc.

1), hot oxidation techniques are more convenient (Mishra et al., 1995).

3O3 + HO-

+ HCO3

+ CO3

abatement of pollutants.

**3.1 Ozone water processes** 

by the reaction between O3 and O2-

(Munter, 2001):

In a photo-oxidation reaction, UV radiation (photon) excites an electron of an organic molecule (C) from the ground state to the excited state (C\*) (equation 10). The excited organic molecule excites further molecular oxygen (equation 11) with a subsequent recombination of the radical ions or hydrolysis of the radical cation, or homolysis (equation 12) to form radicals which can react with oxygen (equation 13) (Legrini et al., 1993).

$$\mathbf{C} \xrightarrow{\text{hv}} \mathbf{C}^\* \tag{10}$$

$$\text{C}^\* + \text{O}\_2 \rightarrow \text{C}^\* \text{O}\_2^- \tag{11}$$

$$\mathbf{R}\cdot\mathbf{X} \xrightarrow{\text{hv}} \mathbf{R}' + \mathbf{X}' \tag{12}$$

$$\rm{R} + \rm{O}\_{2} \rightarrow \rm{RO}\_{2} \tag{13}$$

The rate of the photo-oxidation reaction depends on the adsorption cross section of the medium, the quantum yield of the process, the photon rate at the wavelength of excitation and the concentration of dissolved molecular oxygen (Legrini et al, 1993). However, to achieve the complete mineralisation of the treated effluent, photolysis is usually combined with oxidising compounds (hydrogen peroxide, ozone) or semiconductors (such as titanium dioxide).

#### **3.2.1 UV/ozone, UV/H2O2 and UV/O3/H2O2 processes**

The combination of UV light and ozone/hydrogen peroxide or both significantly enhances the rate of generating free radicals. Ozone adsorbs UV radiation at a wavelength of 254 nm

its anatase form has an energy bandgap of 3.2 eV and can be activated by UV radiation with a wavelength up to 387.5 nm. Therefore, many researchers have focused on examining the use of sunlight in photocatalytic processes. Unfortunately, only a few percent of solar energy reaches the surface of the earth that could in principle be utilised as a direct exciter to

Degussa P-25 TiO2 catalyst is probably the most active catalyst in photocatalytic reactions however its optimum effective will always be strongly dependent on the type and concentration of the treated pollutant (Gogate & Pandit, 2004a). In several studies, the doping of TiO2 with metals, such as, platinum (Hufschmidt et al, 2002), silver, zirconium and iron (Kment et al, 2010) as well as, sulphur, carbon and nitrogen (Menendez-Flores et al,

Fenton´s reagent consists of H2O2 and ferrous iron, which generates hydroxyl radicals

+ HO· (24)

317

+ Fe2+ (26)

Fe3++ H2O2 <sup>→</sup> H+ + Fe-OOH2+ (25)

Fe3++ H2O → Fe�OH�2++ H+ (27)

<sup>→</sup> Fe2+ + HO· (28)

Fe2++ H2O2 → Fe3++ HO-

Fe-OOH2+ → HO2

The generated ferric ion decomposes H2O2 forming hydroxyl radicals (equations 25, 26):

·

After the reaction (26), the formed ferrous iron again decomposes H2O2 (24) etc. The hydrogen peroxide decomposition is an iron salt catalyzed reaction and in reactions (25, 26)

The most important operating parameter in the Fenton process is the pH of the solution. According to the majority of researchers, the optimum operating pH to be observed is 3. However, the Fenton process effectively generates hydroxyl radicals, it consumes one molecule of Fe2+ for each HO· radical produced, which results in a high concentration of

In photoassisted Fenton process, Fe3+ ions are added to the H2O2/UV process. In acidic pH,

The photolysis of Fe3+ complexes allows Fe2+ regeneration and formation of hydroxyl

The combination of H2O2/UV and iron salt produces more hydroxyl radicals compared with a conventional Fenton process or photolysis, thus the technique enhances the degradation of

Fe�OH�2+ hv

2011; Wang et al, 2011) has been proven to enhance the activity of the catalyst.

TiO2 (Munter, 2001).

**3.3 Fenton processes** 

(equation 24) (Munter, 2001):

iron is regenerated again to iron(II).

**3.3.1 Photoassisted Fenton processes** 

radicals (equation 28) (Munter, 2001):

treated pollutants (Gogate and Pandit, 2004b).

a Fe(OH)2+ complex is formed (equation 27) (Munter, 2001):

Fe(II) (Munter, 2001).

(equation 14) producing hydrogen peroxide as an intermediate, which decomposes further to hydrogen peroxide radicals (equation 15) (Munter, 2001):

$$\text{O}\_3 + \text{hv} \rightarrow \text{O}\_2 + \text{O}(^{1}\text{D})\tag{14}$$

$$\text{H}\_{\text{(1D)}} + \text{H}\_{2}\text{O} \rightarrow \text{H}\_{2}\text{O}\_{2} \rightarrow 2\text{HO}^{\cdot}\tag{15}$$

The mechanism for the photolysis of hydrogen peroxide is the cleavage of the molecule into two hydroxyl radicals (equation 16) (Munter, 2001):

$$\mathrm{H\_2O\_2} \xrightarrow{\mathrm{hV}} \mathrm{2HO}^\cdot \tag{16}$$

Depending on the pH value of the aqueous H2O2 solution, HO2- also absorbs UV radiation at 254 nm to form a hydroxyl radical (equations 17, 18):

$$\text{H}\_2\text{O}\_2 \leftrightarrow \text{HO}\_2^\cdot + \text{H}^+\tag{17}$$

$$\rm{HO}\_{2}^{\cdot} \xrightarrow{\rm{hv}} \rm{HO^{\cdot}} + \rm{HO^{\cdot}} + \rm{O^{\cdot}} \tag{18}$$

The combination of UV photolysis and ozone/hydrogen peroxide will be beneficial only for contaminants which require a relatively higher level of oxidation conditions (higher activation energy) (Gogate & Pandit, 2004b).

#### **3.2.2 Photocatalysis**

In the photocatalytic process, semiconductor material (often TiO2) is excited by electromagnetic radiation possessing energy of sufficient magnitude, to produce conduction band electrons and valence band holes (equation 19) (Andreozzi et al, 1999):

$$\text{TiO}\_2 \xrightarrow{\text{hv}} \text{e}^\text{e} + \text{h}^\text{\textasciicircum} \tag{19}$$

Formed electrons can reduce some metals and dissolved oxygen to produce a superoxide radical ion O2·- (equations 20, 21):

$$\mathbf{M} \mathsf{\tau} \mathsf{\mathsf{e}}^{\circ} \to \mathsf{O}\_{2}^{\circ} \tag{20}$$

$$\rm O\_2 + e^\cdot \to \rm O\_2^\cdot \tag{21}$$

Remaining holes then oxidise and adsorbed H2O or HO- to reactive hydroxyl radicals (equations 22, 23):

$$\text{TiO}\_2\text{(h}^+\text{)} + \text{H}\_2\text{O}\_{\text{ad}} \rightarrow \text{TiO}\_2 + \text{HCO}\_{\text{ad}}^\cdot + \text{H}^+\tag{22}$$

$$\text{TiO}\_2\text{(h}^+\text{)} + \text{HO}^\cdot\_{\text{ad}} \rightarrow \text{TiO}\_2 + \text{HO}^\cdot\_{\text{ad}} \tag{23}$$

Formed hydroxyl radicals may also react with organic compounds in water as described in the equations (2)-(5).

Several catalytic materials have been studied in photocatalysis although TiO2 in the anatase form seems to possess the best photocatalytic performance (Andreozzi et al, 1999). TiO2 in its anatase form has an energy bandgap of 3.2 eV and can be activated by UV radiation with a wavelength up to 387.5 nm. Therefore, many researchers have focused on examining the use of sunlight in photocatalytic processes. Unfortunately, only a few percent of solar energy reaches the surface of the earth that could in principle be utilised as a direct exciter to TiO2 (Munter, 2001).

Degussa P-25 TiO2 catalyst is probably the most active catalyst in photocatalytic reactions however its optimum effective will always be strongly dependent on the type and concentration of the treated pollutant (Gogate & Pandit, 2004a). In several studies, the doping of TiO2 with metals, such as, platinum (Hufschmidt et al, 2002), silver, zirconium and iron (Kment et al, 2010) as well as, sulphur, carbon and nitrogen (Menendez-Flores et al, 2011; Wang et al, 2011) has been proven to enhance the activity of the catalyst.

#### **3.3 Fenton processes**

322 Food Industrial Processes – Methods and Equipment

(equation 14) producing hydrogen peroxide as an intermediate, which decomposes further

The mechanism for the photolysis of hydrogen peroxide is the cleavage of the molecule into

hv

Depending on the pH value of the aqueous H2O2 solution, HO2- also absorbs UV radiation at

The combination of UV photolysis and ozone/hydrogen peroxide will be beneficial only for contaminants which require a relatively higher level of oxidation conditions (higher

In the photocatalytic process, semiconductor material (often TiO2) is excited by electromagnetic radiation possessing energy of sufficient magnitude, to produce conduction

> hv → e-

Formed electrons can reduce some metals and dissolved oxygen to produce a superoxide

M+ e- → O2

O2+ e- → O2

Remaining holes then oxidise and adsorbed H2O or HO- to reactive hydroxyl radicals

�+ H2Oad → TiO2+ HOad

Formed hydroxyl radicals may also react with organic compounds in water as described in

Several catalytic materials have been studied in photocatalysis although TiO2 in the anatase form seems to possess the best photocatalytic performance (Andreozzi et al, 1999). TiO2 in


�+ HOad


H2O2

H2O2 ↔ HO2

HO2 - hv <sup>→</sup> + HO·

band electrons and valence band holes (equation 19) (Andreozzi et al, 1999):

TiO2�h+

TiO2�h+

TiO2

O3 + hv → O2 + O(1D) (14)

<sup>→</sup> 2HO· (16)

+ H<sup>+</sup> (17)

+ O·- (18)

+ h+ (19)

·- (20)

·- (21)

· + H+ (22)

· (23)

O(1D) + H2O → H2O2 → 2HO· (15)

to hydrogen peroxide radicals (equation 15) (Munter, 2001):

two hydroxyl radicals (equation 16) (Munter, 2001):

254 nm to form a hydroxyl radical (equations 17, 18):

activation energy) (Gogate & Pandit, 2004b).

**3.2.2 Photocatalysis** 

(equations 22, 23):

the equations (2)-(5).

radical ion O2·- (equations 20, 21):

Fenton´s reagent consists of H2O2 and ferrous iron, which generates hydroxyl radicals (equation 24) (Munter, 2001):

$$\text{Fe}^{2+} + \text{H}\_2\text{O}\_2 \rightarrow \text{Fe}^{3+} + \text{HO}^\cdot + \text{HO}^\cdot \tag{24}$$

The generated ferric ion decomposes H2O2 forming hydroxyl radicals (equations 25, 26):

$$\text{Fe}^{3+} + \text{H}\_2\text{O}\_2 \rightarrow \text{H}^+ + \text{Fe-COOH}^{2+} \tag{25}$$

$$\text{Fe-OOH}^{2+} \rightarrow \text{HO}\_2^{\cdot} + \text{Fe}^{2+} \tag{26}$$

After the reaction (26), the formed ferrous iron again decomposes H2O2 (24) etc. The hydrogen peroxide decomposition is an iron salt catalyzed reaction and in reactions (25, 26) iron is regenerated again to iron(II).

The most important operating parameter in the Fenton process is the pH of the solution. According to the majority of researchers, the optimum operating pH to be observed is 3. However, the Fenton process effectively generates hydroxyl radicals, it consumes one molecule of Fe2+ for each HO· radical produced, which results in a high concentration of Fe(II) (Munter, 2001).

#### **3.3.1 Photoassisted Fenton processes**

In photoassisted Fenton process, Fe3+ ions are added to the H2O2/UV process. In acidic pH, a Fe(OH)2+ complex is formed (equation 27) (Munter, 2001):

$$\text{Fe}^{3+} + \text{H}\_2\text{O} \rightarrow \text{Fe(OH)}^{2+} + \text{H}^+ \tag{27}$$

The photolysis of Fe3+ complexes allows Fe2+ regeneration and formation of hydroxyl radicals (equation 28) (Munter, 2001):

$$\text{Fe(OH)}^{2+} \xrightarrow{\text{hv}} \text{Fe}^{2+} + \text{HO}^{\cdot} \tag{28}$$

The combination of H2O2/UV and iron salt produces more hydroxyl radicals compared with a conventional Fenton process or photolysis, thus the technique enhances the degradation of treated pollutants (Gogate and Pandit, 2004b).

molecular oxygen) to intermediates (short-chain organic molecules), CO2 and water. The degree of oxidation is dependent on temperature, oxygen partial pressure, operating time and, of course, the oxidisability of the compounds under consideration. To summarise, it can be said that, the higher the operating temperature the higher is the extent of oxidation

According to Lixiong et al. (1991) the WAO reaction starts with the reaction of oxygen and the weakest C-H bonds of the oxidised organic compound (R denotes the organic functional

At the high operating temperature of WAO, H2O2 decomposes rapidly in the homogeneous

The chain reaction continues with the oxidation of organic compounds by hydroxyl radicals

Compared with conventional WAO, catalytic wet air oxidation (CWAO) has lower energy requirements. Due to the presence of homogeneous or heterogeneous catalysts, lower operating conditions (air/oxygen pressure and temperature) can be used to achieve much higher oxidation rates. Various heterogeneous catalysts have been synthesised and tested in CWAO reactions, based either on metal oxides or supported noble metals (Levec & Pintar, 2007). Mixtures of metal oxides such as Cu, Zn, Co and Al, are reported to exhibit good activity, but leaching of these metals has been detected (Mantzavinos et al., 1996a). Supported noble metal catalysts, such as Pt, Pd, Rh, Re, Ru on Al2O3 (Mantzavinos et al., 1996b) and ceria including doped-ceria supported Pt and Ru (Keav et al., 2010) are less prone to leaching and are also more effective for oxidising organic compounds than metal oxide catalysts. CWAO processes have also been commercialised. In Japan, several companies have developed catalytic wet air oxidation technologies relying on heterogeneous supported noble metal catalysts (Harada et al., 1987; Ishii et al., 1994). In Europe, homogeneous CWAO processes such as Ciba-Geigy, LOPROX, ORCAN and

In a supercritical water oxidation process the reaction temperature is above 374 °C and the oxygen containing gas is at a pressure of 221 bar, which is the critical point of water. Above this critical point, water is an excellent solvent for both organic compounds and gases; therefore oxidisable compounds and oxygen can be mixed in a single homogeneous phase. This is the great advantage of supercritical water oxidation and in general, the destruction efficiencies of pollutants are in the order of 99-99.9% at 400-500 °C for a residence time

H2O2 + M → 2HO·

+ HO2

·

· (32)

319

and organic molecule:

+ H2O2 (33)

+ M (34)

RH+ O2 → R·

RH+ HO2 · → R·

following a hydrogen abstraction mechanism described earlier (equation 3).

ATHOS have already been developed in the 1990s (Luck, 1999).

More organic radicals are formed with the reaction of HO2

or heterogeneous species (term M) to hydroxyl radicals:

achieved (Mishra et al. 1995).

group) forming free radicals:

**3.4.3 Catalytic wet air oxidation** 

**3.4.4 Supercritical wet air oxidation** 

interval of 1-5 minutes (Mishra et al., 1995).

#### **3.3.2 Electro-Fenton processes**

In the Electro-Fenton process, Fe2+ and H2O2 are generated electrochemically, either separately or concurrently. Hydrogen peroxide can be electrogenated by the reduction of dissolved oxygen (equation 29), and ferrous iron by the reduction of ferric iron (equation 30) or by oxidation of a sacrificial Fe anode (equation 31) (Szpyrkowicz et al., 2001):

$$\text{O}\_2 + 2\text{H}^+ + 2\text{e}^\cdot \rightarrow \text{H}\_2\text{O}\_2\tag{29}$$

$$\text{Fe}^{3+} + \text{e}^{\cdot} \rightarrow \text{Fe}^{2+} \tag{30}$$

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^{\cdot} \tag{31}$$

The reaction between H2O2 and Fe2+ produces hydroxyl radicals (equation 24).

#### **3.4 Wet oxidation processes**

The main differences in wet oxidation processes compared with "cold" oxidation techniques described earlier are the operating temperature and pressure. Typically, wet oxidation processes are operating at temperatures from 90 C (wet peroxide oxidation) to even over 600 C (supercritical wet air oxidation). The operating conditions of different wet oxidation techniques are described in Table 5.


Table 5. Operating conditions of different wet oxidation techniques (Debellefontaine et al., 1996; Mishra et al., 1995).

The wet oxidation processes are suitable for wastewaters and sludges which are both too diluted to incinerate and too concentrated for biological treatment. The COD level of the wastes appropriate to be treated by WAO techniques is typically between 5 and 200 g L-1 (Kolaczkowski et al., 1999).

#### **3.4.1 Wet peroxide oxidation**

WPO process is adapted from the Fenton process (Section 3.3) but it operates at temperatures above 100 C. The oxidation mechanism is the same as for the Fenton´s reaction (equations 24-26), but as a consequence of a higher operating temperature more efficient TOC removal can be obtained (Debellefontaine et al., 1996). A typical catalyst in the WPO is iron but other catalytic materials have also been used successfully in the process such as copper (Caudo et al, 2008), activated carbon (Gomes et al., 2010), ruthenium (Rokhina et al., 2010) etc.

#### **3.4.2 Wet air oxidation**

In the WAO process organic and oxidisable inorganic compounds of the liquid phase are oxidised at elevated temperatures and pressures (Table 5) using oxygen containing gas (air, molecular oxygen) to intermediates (short-chain organic molecules), CO2 and water. The degree of oxidation is dependent on temperature, oxygen partial pressure, operating time and, of course, the oxidisability of the compounds under consideration. To summarise, it can be said that, the higher the operating temperature the higher is the extent of oxidation achieved (Mishra et al. 1995).

According to Lixiong et al. (1991) the WAO reaction starts with the reaction of oxygen and the weakest C-H bonds of the oxidised organic compound (R denotes the organic functional group) forming free radicals:

$$\text{RH} + \text{O}\_2 \rightarrow \text{R}' + \text{HO}\_2^\cdot \tag{32}$$

More organic radicals are formed with the reaction of HO2· and organic molecule:

$$\text{RH} + \text{HO}\_2 \rightarrow \text{R}' + \text{H}\_2\text{O}\_2\tag{33}$$

At the high operating temperature of WAO, H2O2 decomposes rapidly in the homogeneous or heterogeneous species (term M) to hydroxyl radicals:

$$\rm H\_2O\_2 + M \to 2HO^\cdot + M\tag{34}$$

The chain reaction continues with the oxidation of organic compounds by hydroxyl radicals following a hydrogen abstraction mechanism described earlier (equation 3).

#### **3.4.3 Catalytic wet air oxidation**

324 Food Industrial Processes – Methods and Equipment

In the Electro-Fenton process, Fe2+ and H2O2 are generated electrochemically, either separately or concurrently. Hydrogen peroxide can be electrogenated by the reduction of dissolved oxygen (equation 29), and ferrous iron by the reduction of ferric iron (equation 30)

The main differences in wet oxidation processes compared with "cold" oxidation techniques described earlier are the operating temperature and pressure. Typically, wet oxidation processes are operating at temperatures from 90 C (wet peroxide oxidation) to even over 600 C (supercritical wet air oxidation). The operating conditions of different wet oxidation

> **Catalytic wet air oxidation (CWAO)**

The wet oxidation processes are suitable for wastewaters and sludges which are both too diluted to incinerate and too concentrated for biological treatment. The COD level of the wastes appropriate to be treated by WAO techniques is typically between 5 and 200 g L-1

WPO process is adapted from the Fenton process (Section 3.3) but it operates at temperatures above 100 C. The oxidation mechanism is the same as for the Fenton´s reaction (equations 24-26), but as a consequence of a higher operating temperature more efficient TOC removal can be obtained (Debellefontaine et al., 1996). A typical catalyst in the WPO is iron but other catalytic materials have also been used successfully in the process such as copper (Caudo et al, 2008), activated carbon (Gomes et al., 2010), ruthenium

In the WAO process organic and oxidisable inorganic compounds of the liquid phase are oxidised at elevated temperatures and pressures (Table 5) using oxygen containing gas (air,

Temperature [C] 100-140 130-250 125-320 > 374 Pressure [bar] 3-5 20-50 5-200 > 221 Table 5. Operating conditions of different wet oxidation techniques (Debellefontaine et al.,

O2+ 2H++ 2e- <sup>→</sup> H2O2 (29)

Fe3++ e- → Fe2+ (30)

Fe → Fe2++ 2e- (31)

**Wet air oxidation (WAO)** 

**Supercritical wet air oxidation (SCWO)** 

or by oxidation of a sacrificial Fe anode (equation 31) (Szpyrkowicz et al., 2001):

The reaction between H2O2 and Fe2+ produces hydroxyl radicals (equation 24).

**3.3.2 Electro-Fenton processes** 

**3.4 Wet oxidation processes** 

techniques are described in Table 5.

**Wet peroxide oxidation (WPO)** 

**Operating parameter** 

1996; Mishra et al., 1995).

(Kolaczkowski et al., 1999).

(Rokhina et al., 2010) etc.

**3.4.2 Wet air oxidation** 

**3.4.1 Wet peroxide oxidation** 

Compared with conventional WAO, catalytic wet air oxidation (CWAO) has lower energy requirements. Due to the presence of homogeneous or heterogeneous catalysts, lower operating conditions (air/oxygen pressure and temperature) can be used to achieve much higher oxidation rates. Various heterogeneous catalysts have been synthesised and tested in CWAO reactions, based either on metal oxides or supported noble metals (Levec & Pintar, 2007). Mixtures of metal oxides such as Cu, Zn, Co and Al, are reported to exhibit good activity, but leaching of these metals has been detected (Mantzavinos et al., 1996a). Supported noble metal catalysts, such as Pt, Pd, Rh, Re, Ru on Al2O3 (Mantzavinos et al., 1996b) and ceria including doped-ceria supported Pt and Ru (Keav et al., 2010) are less prone to leaching and are also more effective for oxidising organic compounds than metal oxide catalysts. CWAO processes have also been commercialised. In Japan, several companies have developed catalytic wet air oxidation technologies relying on heterogeneous supported noble metal catalysts (Harada et al., 1987; Ishii et al., 1994). In Europe, homogeneous CWAO processes such as Ciba-Geigy, LOPROX, ORCAN and ATHOS have already been developed in the 1990s (Luck, 1999).

#### **3.4.4 Supercritical wet air oxidation**

In a supercritical water oxidation process the reaction temperature is above 374 °C and the oxygen containing gas is at a pressure of 221 bar, which is the critical point of water. Above this critical point, water is an excellent solvent for both organic compounds and gases; therefore oxidisable compounds and oxygen can be mixed in a single homogeneous phase. This is the great advantage of supercritical water oxidation and in general, the destruction efficiencies of pollutants are in the order of 99-99.9% at 400-500 °C for a residence time interval of 1-5 minutes (Mishra et al., 1995).

pre-treatment followed by an activated sludge step provides enhancement in the removal of substrate obtained in relation to that obtained in the single aerobic treatment without ozonation, i.e. from 28% to 39 % (Benitez et al., 1999; 2003). The integrated process (ozone pre-treatment-aerobic biological oxidation-ozone post-treatment) achieved almost 80% COD reduction in the treatment of distillery wastewater along with the decolouration of the effluent compared with 35% COD removal for non-ozonated samples (Sangave et al., 2007). UV/H2O2 (Beltrán et al., 1997a) and electro-Fenton (Yavuz, 2007) processes have also been used for the treatment of distillery wastewaters. The EF process (Yavuz, 2007) seems to be a promising technique with the COD removal over 90% compared with a UV radiation and hydrogen peroxide combination whose COD reduction is only 38% (Beltrán et al., 1997a). Belkacemi et al. (1999; 2000) investigated wet oxidation and catalytic wet oxidation for the removal of organics from distillery liquors. The initial TOC of the effluent was 22500 mg L-1 while in the AOPs described earlier the total organic carbon was 10 or even 100 times lower. In the temperature and oxygen partial ranges of 180-250 °C and 5-25 bar respectively, the highest TOC removal (around 60%) was achieved with Mn/Ce oxides and Cu(II)NaY catalysts. These catalysts were found to be very effective for short contact times, while for prolonged exposures catalysts deactivation by fouling carbonaceous deposit was shown to be the prime factor responsible for the loss of catalysts activity (Belkacemi et al., 2000). In the supercritical water oxidation of alcohol distillery wastewater (Goto et al., 1998) almost complete colour, odour and TOC removal was attained when more than stoichiometric

321

amount (over 100%) of oxidant (H2O2) was used in temperatures between 200-600 °C.

g L-1 and even 190 g L-1 for the amount of suspended solids (Oller et al., 2010).

Wastewaters from olive oil extraction plants, also called olive mill wastewaters, and wastewaters generated by table olive production, contain high concentration of phenolic compounds. In olive oil production, an oily juice is extracted from the fruit through milling or centrifugation. Table olive production requires the same treatment in order to eliminate the bitterness of the fruit, due to the presence of polyphenolic compounds (Bautista et al., 2008). Olive mill wastewater contains polysaccharides, sugars, polyphenols, polyalcohols, proteins, organic acids, oil etc. and therefore, the COD of the effluent may be as high as 220

For several years, olive mill wastewater has been the most polluting and troublesome waste produced by olive mills in all the countries surrounding the Mediterranean. Thus, the management of this liquid residue has been investigated extensively and the efficiency of AOPs for treating olive mill effluents has been studied widely (Mantzavinos & Kalogerakis, 2005). Many researchers have also investigated Fenton processes in the treatment of olive

Olive mill wastewater has also been treated by several other AOPs such as ozonation or ozone/UV (Lafi et al., 2009) which have increased the biodegradability of the effluent. Minh et al. (2008) and Gomes et al. (2007) have been successful in decreasing the TOC and phenolic content of olive mill wastewater by CWAO. At reaction conditions of 190 °C and 70 bar of air using Pt and Ru supported on titania and zirconia carriers, the toxicity and phytotoxicity of the effluent decreased to a suitable level for anaerobic treatment (Minh et al., 2008). Gomes et al. (2007) reported that with the carbon supported Pt catalyst TOC and the colour of olive mill wastewater were completely removed after 8 h of reaction at 200 °C

**4.2 Olive industry wastewater** 

mill effluents (Table 6).
