**2. Theory of advanced oxidation processes (AOPs)**

The need for an efficient treatment of phenolic compounds in water is very important. When conventional treatment methods such as biological processes fail due to the recalcitrant nature of the contaminants, physical and chemical methods are a good solution. Therefore, oxidation processes are preferred to degrade such organics present. For example, the chemical processes are commonly used to degrade recalcitrant substances. High degradation efficiencies are possible with direct oxidation methods. However, pollution load, process limitation and operation condition are the more important factors considered during the selection for most oxidation processes [15]. The main treatment methods of industrial effluents with biological processes are aerobic, anaerobic and enzymatic. The physical processes are decantation, filtration and adsorption. The chemical methods are incineration, electrochemical and advanced oxidation processes(AOP), for example, photocatalysis, ozonation, Fenton/photo-Fenton and direct contact thermal treatment (DiCTT) [4].

The AOPs have been a viable alternative method for the wastewater treatment containing toxic and refractory organic pollutants, being studied in various combinations and are mainly based on intermediate reactions of hydroxyl radicals (•OH), an unstable and very reactive species, resulting in the degradation of toxic organic contaminants due to its high oxidizing potential of 2.8 V under acidic conditions and these processes have the major advantage of being a destructive treatment. Depending on the species to be degraded the hydroxyl radicals of reactive species, which attack the main part of organic molecules with a rate constant frequently in the order of 106 –109 L mol−1s−1, and reacts 106 –1012 times faster than ozone [16]. According to Tisa et al. [10] apud Bach et al. [17] and Garrido-Ramírez et al. [18], the principles of •OH generation is based on various combinations of strong oxidants, such as oxygen, ozone, hydrogen peroxide (H2 O2 ), ultra-violet (UV) and electron beam.

The AOPs are classified according to the reactive phase (homogeneous and heterogeneous). The homogeneous including: ozone (O<sup>3</sup> ), ozone/ultraviolet (O3 /UV), ozone/hydrogen peroxide (O3 / H2 O2 ), (O<sup>3</sup> /H2 O2 /UV), and (H2 O2 /UV) processes, as well as Fenton (Fe2+/H2 O2 ), Fenton-like (Fe3+/ mn+/H2 O2 ), photo-Fenton (UV/Fe2+/H2 O2 ), sono-Fenton (US/Fe2+/H2 O2 ), electro-Fenton, sonoelectro-Fenton, photo-electro-Fenton, sono-photo-Fenton. The heterogeneous includes: (TiO2 / ZnO/CdS+UV), (TiO2 /H2 O2 ), (H2 O2 +Fe2+/Fe3+/mn+-solid), (H2 O2 +Fe0 /Fe-nano-zero valente iron) and (H2 O2 +immobilized nano-zero valente iron). The non-conventional AOPs include ionizing radiation, microwaves and pulsed plasma techniques. Depending on the matrix and on the pollutant, degradation kinetics of AOPs can be zero order, first order and second order. Firstorder kinetics constant is achieved for pollutant degradation due to concentration of hydroxyl radicals within 1–10−4 s−1 [15, 4]. The mechanisms of different AOPs are presented in **Table 2**.

In the AOPs, oxygen and their reactive species (O*<sup>x</sup>* , HO*<sup>x</sup>* , *x* = 1, 2, 3, 4) act as main precursors during the oxidation that occurs in step of degradation of the organic component [23].

Devlin and Harris [24] proposed in their experimental trials that levels of O2 decrease quickly in accordance with the rates of degradation of aromatic compounds, due to the high concentrations of phenol, in the temperature range from 420 to 498 K, near or above the stoichiometric conditions. These results led to demonstrate that the concentration of radicals (•O) is the dominant mechanism for this temperature range, in which the intermediate rings resulting from the oxidation of phenol are degraded.

Section 3 indicates that the direct contact thermal treatment technology is recent, with a limited investigation and was initially developed in Canada by Benali et al. [25, 26], being the unique experimental results available in the literature, until this moment. The DiCTT process provides a promising novel means to induce degradation and mineralization of organic pollutants in water being an AOP treatment method with respective advantages and limitations. Since combining AOP treatment process with reactors can be more promising in industrial applications, this research area needs to be explored further. Section 3 describes the DiCTT technique with applications in degradation and mineralization of the organic pollutant, being elaborated to highlight the important factors and their effects, as also a detailed investigation of the effective design and operating parameters are summarized.

The actual work evaluated the liquid phase flow rate (*QL* ) of 100 and 170 L h−1 and the effect of initial phenol concentration (*C*Ph0) of 500, 2000 and 3000 mg L−1. The experiments studies were performed using a molar stoichiometric ratio of phenol/hydrogen peroxide (*R*P/H) of 50%, an air excess (*E*) of 40%, a recycle rate of gaseous thermal wastes (*Q*RG) of 50%, and, a natural gas


species, resulting in the degradation of toxic organic contaminants due to its high oxidizing potential of 2.8 V under acidic conditions and these processes have the major advantage of being a destructive treatment. Depending on the species to be degraded the hydroxyl radi

cals of reactive species, which attack the main part of organic molecules with a rate constant

According to Tisa et al. [10] apud Bach et al. [17] and Garrido-Ramírez et al. [18], the princi

ples of •OH generation is based on various combinations of strong oxidants, such as oxygen,

The AOPs are classified according to the reactive phase (homogeneous and heterogeneous). The

electro-Fenton, photo-electro-Fenton, sono-photo-Fenton. The heterogeneous includes: (TiO2/

+Fe2+/Fe3+/mn+-solid), (H

radiation, microwaves and pulsed plasma techniques. Depending on the matrix and on the pollutant, degradation kinetics of AOPs can be zero order, first order and second order. Firstorder kinetics constant is achieved for pollutant degradation due to concentration of hydroxyl radicals within 1–10−4 s−1 [15, 4]. The mechanisms of different AOPs are presented in **Table 2**

during the oxidation that occurs in step of degradation of the organic component [23].

in accordance with the rates of degradation of aromatic compounds, due to the high concen

trations of phenol, in the temperature range from 420 to 498 K, near or above the stoichiomet

ric conditions. These results led to demonstrate that the concentration of radicals (•O) is the dominant mechanism for this temperature range, in which the intermediate rings resulting

Section 3 indicates that the direct contact thermal treatment technology is recent, with a limited investigation and was initially developed in Canada by Benali et al. [25, 26], being the unique experimental results available in the literature, until this moment. The DiCTT process provides a promising novel means to induce degradation and mineralization of organic pollutants in water being an AOP treatment method with respective advantages and limitations. Since com

bining AOP treatment process with reactors can be more promising in industrial applications, this research area needs to be explored further. Section 3 describes the DiCTT technique with applications in degradation and mineralization of the organic pollutant, being elaborated to highlight the important factors and their effects, as also a detailed investigation of the effective

performed using a molar stoichiometric ratio of phenol/hydrogen peroxide (

*E*) of 40%, a recycle rate of gaseous thermal wastes (

*Q L*

*C*Ph0) of 500, 2000 and 3000 mg L−1. The experiments studies were

Devlin and Harris [24] proposed in their experimental trials that levels of O

), ozone/ultraviolet (O

s−1, and reacts 10

), ultra-violet (UV) and electron beam.

/UV) processes, as well as Fenton (Fe2+/H

+immobilized nano-zero valente iron). The non-conventional AOPs include ionizing

*x* , HO *x* ,

), sono-Fenton (US/Fe2+/H

6

3

2 O 2 +Fe 0

frequently in the order of 10

ozone, hydrogen peroxide (H

H 2 O 2 ), (O 3 /H 2 O 2

mn+/H 2 O 2

and (H 2 O 2

ZnO/CdS+UV), (TiO

homogeneous including: ozone (O

/UV), and (H

2 /H 2 O 2 ), (H 2 O 2

), photo-Fenton (UV/Fe2+/H

In the AOPs, oxygen and their reactive species (O

from the oxidation of phenol are degraded.

design and operating parameters are summarized.

initial phenol concentration (

air excess (

The actual work evaluated the liquid phase flow rate (

6 –10 9 L mol−1

326 Phenolic Compounds - Natural Sources, Importance and Applications

2 O 2

2 O 2 3

2 O 2 -


3 /

.




decrease quickly

/

), Fenton-like (Fe3+

), electro-Fenton, sono-

/Fe-nano-zero valente iron)

–1012 times faster than ozone [16].

/UV), ozone/hydrogen peroxide (O

*x* = 1, 2, 3, 4) act as main precursors

2

) of 100 and 170 L h−1 and the effect of

*Q*RG) of 50%, and, a natural gas

*R*P/H) of 50%, an

2 O 2

2 O 2


**Table 2.** Mechanisms of different advanced oxidation processes. flow (*Q*GN) of 4 m3 h−1, on the oxidation of phenolic effluents by DiCTT process. The phenol concentration and mineralization content were obtained by high-performance liquid chromatography (HPLC) and mineralization Total Organic Carbon (TOC), respectively. A new data bank was compiled in this work by optimizing the operation conditions for the degradation/ mineralization of phenol by the DiCTT process. (Phenol was used as a model compound for liquid organic wastes.)
