*2.2.5 Electrochemical techniques*

The term electrochemical techniques refers to application of DC current from a suitable source and carrying out an entire degradation of pollutants by electro-oxidation or reduction processes to inorganic materials. The main pollutants for electrochemical methods are not only dyes, but also other pollutants like pharmaceuticals, pesticides, herbicides, herbicides, detergents and many other harmful contaminants are the subjects of investigation. Electrochemical techniques usually incorporated for wastewater abatements are electrochemical reduction, electrochemical oxidation, electrocoagulation methods, photoelectrocatalytic and photo-assisted Fenton's oxidation techniques. Electrochemical oxidation techniques are further subdivided into two types which include direct and indirect oxidation techniques [28]. Over the past 10 years, the electrochemical methods have received a remarkable attention. This is because these methods have promising water decontamination potentials, have shown great novelty due to their versatility and potential cost-effectiveness and offer the most promising, clean, safe, efficient and green technologies for the decolourization of wastewater. Wastewater abatement using electrochemical methods has a fantastic advantage of environmental compatibility because its sole reagent, i.e. the electron, is a safe, clean and green reagent produced in situ and works most efficiently. Furthermore these methods are gaining attention of environmental chemists due to their excellent efficiencies, flexibility of automation and safe applicability over a broad spectrum of organic dyes [28]. These techniques need mild conditions for their operation, no heating of the samples are required and work under ambient conditions of both parameters, i.e. temperature and pressure. Nowadays a vast variety of electrochemical techniques are in practice such as electrochemical oxidation (EO), electrocoagulation (EC) using a variety of anodes, active chlorine indirect oxidation, etc. Recently emerging techniques which utilize twin technologies of both electrochemical cells and suitable light like UV light or sono-electrochemical degradations of dyes in wastewater are gaining much attention and appreciation [29, 30]. These photo- and sono-assisted electrochemical setups have been categorized as electrochemical advanced oxidation processes (EAOPs) [21]. The use of electrochemical methods for abatement of contaminants in wastewater was pioneered by Nilsson et al. 1973 [31] by electrochemical oxidation of phenolic-based wastes; later in the early 1980s, these studies were proceeded in collaboration with Chettier and Watkinson [32].

Over the past two decades, much of the research regarding decolourization of wastewater by electrochemical oxidation has been focused on the use of different anodic materials, their relative efficiencies, exploration of various factors affecting process efficiency (like PH, temperature, nature and concentration of electrolytes, etc.), kinetics and mechanism of oxidation of a variety of pollutants in water. The electrochemical oxidation is of two types, the first type is direct oxidation also called anodic oxidation by using suitable anode material. Direct oxidation is carried out by physically adsorbed hydroxyl free radical (\* OH) or chemisorbed active oxygen in oxide lattice. The second type of electrochemical oxidation is indirect oxidation using appropriate oxidant material such as hypochlorite (OCl<sup>−</sup><sup>1</sup> ) which is formed anodically [33, 34]. The main degradation products of electrochemical oxidation of organic pollutants are CO2 and water; thus during electrochemical oxidation of dyes, neither sludge formation occurs nor further treatment of degradation product is required; moreover no harmful by-products are formed; this is one of the greatest superiorities of electrochemical oxidation over other methods used for decolourization of wastewater [35]. A flow sheet representation of various techniques utilized for dyes abatement is shown below (**Figure 1**).

### *2.2.6 Electrochemical oxidation and its types*

Environmental chemists took a long comfortable breath when they incorporated electrochemistry for removal of organics from wastewater. Electrochemical oxidation effectively degrades organics in wastewater. Electrochemical oxidation may be either indirect or direct oxidation processes. Indirect electro-oxidation also called mediated oxidation involves electro-generations of oxidizing species (EOS) in water at electrodes (anode) like active chlorine species, physisorbed hydroxyl free radicals or chemisorbed active oxygen atoms (in metal oxide anode lattice) which are very efficient, and oxidation mediated by these EOS gives rise to partial or complete decontamination of organics. Indirect oxidation processes can be elaborated by twin approaches:


During electrochemical transformation, the stable and refractory organic contaminants are converted into biodegradable materials (most often in to carboxylic acids) by chemisorbed active oxygen atoms. Then during electrochemical combustion which is also called electrochemical incineration, the organic contaminants are entirely mineralized to CO2, water and inorganic ions by virtue of physisorbed hydroxyl free radicals (*OH*̇ ). These hydroxyl free radicals are potentially strong oxidants with a powerful oxidation potential of (Eo = 2.80 V) vs. SHE; that is why this radical has been ranked as second most powerful oxidizing agent after fluorine in electrochemical series. The (*OH*̇ ) radicals induce excessive dehydrogenation and hydroxylation of organic contaminants which ultimately oxidizes to yield CO2. The mechanism of indirect oxidation of a dye pollutant at a metal oxide (MOx) anode can be summarized as follows [36, 37]:

**119**

*Contamination of Water Resources by Food Dyes and Its Removal Technologies*

MOx + H2O → MOx(O.

of electrode (anode) materials, i.e. active and non-active electrodes.

liberation of a proton and an electron as shown below:

MOx+1 O.

*MO*x (*O* .

This surface redox couple *M Ox*/ *MOx*+1*O*̇

First of all water is electrolyzed at metal oxide interface generating physically

Now the next step is a crucial one which entirely depends on selection of anode material; moreover this step exclusively differentiates between two limiting classes

3.Active anodes like DSA types of anodes which carry out indirect oxidation of organics through formation of strong oxidants like hydroxyl free radicals, aqueous chlorine, i.e., Cl2(aq), hypochlorous acid (HOCl) or hypochlorite

where higher oxidation states of metal electrode atoms (M) are available, the physically adsorbed hydroxyl free radicals undergoes transformation to chemically adsorbed (Chemisorbed) active oxygen with increase in oxidation state of metal atom (M) to form higher oxide (MOx + 1) accompanied by

*<sup>H</sup>*) → *MO*x+1 O.

to intermediate compounds (RO), while the metal oxide anode *(MOx)* (*M Ox*) is

As in active electrodes like DSA, the chemisorbed active oxygen atoms are chemically bonded to electrode surface; they do not have full freedom to attack dye molecules; that is why they carry out very poor oxidation of dyes in the presence of supporting electrolytes like Na2SO4 in the absence of chloride medium, but in the presence of NaCl as supporting electrolytes, the situation is entirely different. The second reason for decreased efficiency of DSA and likewise active electrodes have low oxygen evolution overpotentials (they start the unlikely Oxygen Evolution

4.At non-active anodes such as BBD and PbO2 where the possibility of higher oxidation states for anode are entirely excluded (e.g. in PbO2 electrode, the lead atom has oxidation state equal to +4, and it does not show oxidation state greater

to electrode surface through weak Van der Waals forces which breaks easily) and \*

free radicals enjoy full freedom of attack on dye contaminants, these electrodes show greater degradation of dye contaminants. The second reason is that the non-active anodes have higher oxygen evolution over potentials (do not start OER easily).

pollutant "R" non-selectively and completely mineralizes dye contaminants into organic species like CO2 and H2O, while the metal oxide anode *(MOx)* is regener-

H)+R → αM Ox + mC O2 + n H2O + x H+1 + y e−1 (4)

OH radicals are physisorbed (i.e. not chemically bonded but are attached

than +4), the physisorbed hydroxyl free radicals, i.e. MO x(O.

), selectively attack dye molecule (R) and partially oxidize it

.

H) as shown below;

, etc. Now at active electrodes like DSA

H) + H+1 + 1 e−1 (1)

+ *H*+1 + 1 e−1 (2)

H) , attack dye

OH

also called chemisorbed active oxygen

+ R → MOx+ RO (3)

*DOI: http://dx.doi.org/10.5772/intechopen.90331*

ion (OCl<sup>−</sup>), ClO2

atoms, i.e. (*MOx*+1*O*̇

regenerated as shown below:

Reaction easily at low voltages).

ated as shown below.

αM Ox(O.

As these \*

adsorbed (physisorbed) hydroxyl free radicals (O

−1 , ClO3 −1 , ClO4 −1

**Figure 1.** *Different methods used to remove dyes from wastewater.*

*Contamination of Water Resources by Food Dyes and Its Removal Technologies DOI: http://dx.doi.org/10.5772/intechopen.90331*

*Water Chemistry*

out by physically adsorbed hydroxyl free radical (\*

*2.2.6 Electrochemical oxidation and its types*

elaborated by twin approaches:

1.Electrochemical transformation

virtue of physisorbed hydroxyl free radicals (*OH*̇

*Different methods used to remove dyes from wastewater.*

agent after fluorine in electrochemical series. The (*OH*̇

metal oxide (MOx) anode can be summarized as follows [36, 37]:

2.Electrochemical combustion

oxygen in oxide lattice. The second type of electrochemical oxidation is indirect oxidation using appropriate oxidant material such as hypochlorite (OCl<sup>−</sup><sup>1</sup>

is formed anodically [33, 34]. The main degradation products of electrochemical oxidation of organic pollutants are CO2 and water; thus during electrochemical oxidation of dyes, neither sludge formation occurs nor further treatment of degradation product is required; moreover no harmful by-products are formed; this is one of the greatest superiorities of electrochemical oxidation over other methods used for decolourization of wastewater [35]. A flow sheet representation of various

Environmental chemists took a long comfortable breath when they incorporated

electrochemistry for removal of organics from wastewater. Electrochemical oxidation effectively degrades organics in wastewater. Electrochemical oxidation may be either indirect or direct oxidation processes. Indirect electro-oxidation also called mediated oxidation involves electro-generations of oxidizing species (EOS) in water at electrodes (anode) like active chlorine species, physisorbed hydroxyl free radicals or chemisorbed active oxygen atoms (in metal oxide anode lattice) which are very efficient, and oxidation mediated by these EOS gives rise to partial or complete decontamination of organics. Indirect oxidation processes can be

During electrochemical transformation, the stable and refractory organic

electrochemical combustion which is also called electrochemical incineration, the organic contaminants are entirely mineralized to CO2, water and inorganic ions by

potentially strong oxidants with a powerful oxidation potential of (Eo = 2.80 V) vs. SHE; that is why this radical has been ranked as second most powerful oxidizing

dehydrogenation and hydroxylation of organic contaminants which ultimately oxidizes to yield CO2. The mechanism of indirect oxidation of a dye pollutant at a

contaminants are converted into biodegradable materials (most often in to carboxylic acids) by chemisorbed active oxygen atoms. Then during

techniques utilized for dyes abatement is shown below (**Figure 1**).

OH) or chemisorbed active

). These hydroxyl free radicals are

) radicals induce excessive

) which

**118**

**Figure 1.**

First of all water is electrolyzed at metal oxide interface generating physically adsorbed (physisorbed) hydroxyl free radicals (O . H) as shown below;

$$\text{MO}\_{\text{x}} + \text{H}\_{2}\text{O} \rightarrow \text{MO}\_{\text{x}}\text{(O}^{\cdot}\text{H)} + \text{H}^{+1} + \text{1}e^{-1} \tag{1}$$

Now the next step is a crucial one which entirely depends on selection of anode material; moreover this step exclusively differentiates between two limiting classes of electrode (anode) materials, i.e. active and non-active electrodes.

3.Active anodes like DSA types of anodes which carry out indirect oxidation of organics through formation of strong oxidants like hydroxyl free radicals, aqueous chlorine, i.e., Cl2(aq), hypochlorous acid (HOCl) or hypochlorite ion (OCl<sup>−</sup>), ClO2 −1 , ClO3 −1 , ClO4 −1 , etc. Now at active electrodes like DSA where higher oxidation states of metal electrode atoms (M) are available, the physically adsorbed hydroxyl free radicals undergoes transformation to chemically adsorbed (Chemisorbed) active oxygen with increase in oxidation state of metal atom (M) to form higher oxide (MOx + 1) accompanied by liberation of a proton and an electron as shown below:

$$\text{MO}\_{\text{x}}\text{(}\stackrel{\cdot}{\text{O}}\text{H)}\rightarrow\text{MO}\_{\text{x}+1}\text{O}^{\cdot}\text{ + }\text{H}^{\cdot 1}\text{ + }\text{1e}^{-1}\tag{2}$$

This surface redox couple *M Ox*/ *MOx*+1*O*̇ also called chemisorbed active oxygen atoms, i.e. (*MOx*+1*O*̇ ), selectively attack dye molecule (R) and partially oxidize it to intermediate compounds (RO), while the metal oxide anode *(MOx)* (*M Ox*) is regenerated as shown below:

$$\text{MO}\_{\text{x}\ast 1}\text{O}^{\cdot}\text{ + }\text{R}\rightarrow\text{MO}\_{\text{x}\ast 2}\text{ RO}\tag{3}$$

As in active electrodes like DSA, the chemisorbed active oxygen atoms are chemically bonded to electrode surface; they do not have full freedom to attack dye molecules; that is why they carry out very poor oxidation of dyes in the presence of supporting electrolytes like Na2SO4 in the absence of chloride medium, but in the presence of NaCl as supporting electrolytes, the situation is entirely different. The second reason for decreased efficiency of DSA and likewise active electrodes have low oxygen evolution overpotentials (they start the unlikely Oxygen Evolution Reaction easily at low voltages).

4.At non-active anodes such as BBD and PbO2 where the possibility of higher oxidation states for anode are entirely excluded (e.g. in PbO2 electrode, the lead atom has oxidation state equal to +4, and it does not show oxidation state greater than +4), the physisorbed hydroxyl free radicals, i.e. MO x(O. H) , attack dye pollutant "R" non-selectively and completely mineralizes dye contaminants into organic species like CO2 and H2O, while the metal oxide anode *(MOx)* is regenerated as shown below.

$$\text{a}\text{MO}\_{\text{x}}\text{(O'H)} + \text{R} \rightarrow \text{a}\text{MO}\_{\text{x}} + \text{mCO}\_{2} + \text{n}\text{H}\_{2}\text{O} + \text{x}\text{H}^{\*1} + \text{y}\text{e}^{-1}\tag{4}$$

As these \* OH radicals are physisorbed (i.e. not chemically bonded but are attached to electrode surface through weak Van der Waals forces which breaks easily) and \* OH free radicals enjoy full freedom of attack on dye contaminants, these electrodes show greater degradation of dye contaminants. The second reason is that the non-active anodes have higher oxygen evolution over potentials (do not start OER easily).

Both physisorbed and chemisorbed active oxygen atoms undergo a simultaneous undesirable and unlikely competitive side reaction which is called oxygen evolution reaction (OER). This reaction substantially reduces the efficiency of electrodes for electrochemical oxidation of organics. Now as a general rule, the anode materials like active electrodes (DSA and others) have low oxygen evolution over potentials (i.e. they are excellent electro-catalysts for undesirable process of oxygen evolution reaction, i.e. OER; that is why they are active anodes) and hence carryout partial oxidation of organics. On the contrary the non-active anodes such as BDD and PbO2 anodes have high oxygen evolution over potentials (i.e. they are poor electrocatalysts for OER; that is why they are termed as non-active anodes), and they carry out complete mineralization of organics to inorganic species like CO2 and H2O. The oxygen evolution potential of DSA (RuO2) is 1.47 volts (Vs SHE in 0.5 M H2SO4), while that of BDD is 2.3 volts (Vs SHE in 0.5 M H2SO4). That is why non-active electrodes are most widely used anodes for elimination of organic contaminants during wastewater treatment in the absence of chloride medium [38–40].

The well-known \* OH free radical is a powerful oxidant (E<sup>o</sup> = 2.80 V vs. SHE), but it is unstable and has much shorter life than HClO (Eo = 1.49 V vs. SHE); therefore in the dye decontamination from wastewater which is heavily loaded with inorganic salts like chlorides, the indirect electrochemical oxidation mediated by active chlorine species in situ provides an excellent tool for such wastewater abatement. The non-active electrodes like BDD, PbO2 and SnO2 are more active with respect to generation of physisorbed \* OH free radicals for wastewater abatement; however they cannot be utilized for generation of active chlorine species because they preferably generate \* OH free radical along with other mild oxidants like *peroxo-diphosphate*, *peroxodicarbonate*, *peroxodisulphate*, etc. On the contrary the active anode materials like platinum and DSA have remarkably high efficiency for oxidation of chloride ion, thereby generating active chlorine oxidants rather than generating physisorbed \* OH free radicals at their surfaces [41].

When NaCl is used as supporting electrolyte with active electrodes like Ti-based DSA (which has been selected as model anode in this study), then chloride ions (Cl<sup>−</sup>) undergo oxidation at anodes to produce a very strong dye killing oxidants in situ, i.e. aqueous chlorine (i.e. Cl2 (aq)), HOCl, OCl<sup>−</sup>, ClO2 −1 , ClO3 −1 and ClO4 −1 [30, 42, 43]. These species mediates the oxidation of organics as shown below.

Reaction at anode:

$$2\text{ Cl}^{-1} \rightarrow \text{Cl}\_2(aq) + 2e^{-1} \tag{5}$$

Reaction at cathode:

$$2H\_2O + 2e^- \to H\_2 + 2OH^{-1} \tag{6}$$

Reaction at bulk:

$$\text{Cl}\_2(aq) \, + \, H\_2O \, \rightarrow \text{HOCl} \, \star H^{\ast 1} \, \star \text{Cl}^{-1} \tag{7}$$

$$\text{HOCl} \rightarrow \text{OCl}^{-1} + H^{\*1} \quad \text{(} pK\_a = 7.54\text{)}\tag{8}$$

**121**

**Figure 2.**

*Contamination of Water Resources by Food Dyes and Its Removal Technologies*

MOx + O.

reaction (disproportionation reaction) and forms HOCl, while at PH greater than 7.5, hypochlorite ions (OCl<sup>−</sup>) exist in solution which is a much weaker oxidizing

**Figure 2** shows that in the first step, the hydroxide from solution adsorbs on

(see reaction 10); this adsorbed hypochlorous radical attacks organic pollutant (R) and degrades it to CO2 and H2O, and chloride ions are regenerated (see reaction 11). This degradation reaction proceeds simultaneously with chlorine evolution reaction (CER) to produce Cl2, some of which Cl2 gas evolves (if applied current density is very high), while some chlorine gas dissolve in water in the form of aqueous chlorine Cl2 (aq). This aqueous chlorine reacts with hydroxide ions to generate HOCl, OCl<sup>−</sup><sup>1</sup>

) to yield adsorbed hypochlorous acid radical, i.e. *MOX*(*HO*.

−1, etc. (depending on pH of solution), which are active chlorine species

<sup>H</sup> → MOx(O.

H) + Cl−1 → MOx (O.

The oxidation of organic compounds via active chlorine species had a serious concern about the formation of chlorinated organic compounds in water which are known as carcinogens, but Panizza and Bonfatti successfully demonstrated that by selecting optimal experimental conditions, the generations of toxic chlorinated compounds can be totally avoided in aqueous medium [46, 47]. For example,

OH) (see reaction 05 given below), and then it reacts with

Cl) + *R* → MOx + CO2 + H2O + Cl−1 (11)

*Cl*)

,

H) (9)

Cl)ads + 1 e−1 (10)

Bonfatti has presented an alternative approach to electrochemical oxidation of organics; the schematic sketch of electrochemical oxidation at non-active oxide electrode *(MOx)* and with associated CER is shown in **Figure 2**. Here "R" represents

*DOI: http://dx.doi.org/10.5772/intechopen.90331*

agent than HOCl [44].

a dye pollutant molecule [45].

and oxidize organic pollutants in situ.

MOx(O.

*Mechanism of electrochemical degradation of pollutants in chloride medium.*

MOx(O.

metal oxide to form MOx (٭

chloride ions (Cl<sup>−</sup><sup>1</sup>

ClO3 −1 , *ClO*<sup>3</sup>

HOCl has greater oxidation potential (1.49 V) than OCl<sup>−</sup><sup>1</sup> (0.94 V). The type of active chlorine species in water depends upon pH solution, for example, at pH less than 03, dissolved chlorine (Cl2) gas exists in water in the form of aqueous chlorine, i.e. Cl2 (aq). At pH range of 03–7.5, Cl2 undergoes self-oxidation/self-reduction

*Contamination of Water Resources by Food Dyes and Its Removal Technologies DOI: http://dx.doi.org/10.5772/intechopen.90331*

*Water Chemistry*

The well-known \*

they preferably generate \*

generating physisorbed \*

Reaction at anode:

Reaction at cathode:

Reaction at bulk:

respect to generation of physisorbed \*

Both physisorbed and chemisorbed active oxygen atoms undergo a simultaneous undesirable and unlikely competitive side reaction which is called oxygen evolution reaction (OER). This reaction substantially reduces the efficiency of electrodes for electrochemical oxidation of organics. Now as a general rule, the anode materials like active electrodes (DSA and others) have low oxygen evolution over potentials (i.e. they are excellent electro-catalysts for undesirable process of oxygen evolution reaction, i.e. OER; that is why they are active anodes) and hence carryout partial oxidation of organics. On the contrary the non-active anodes such as BDD and PbO2 anodes have high oxygen evolution over potentials (i.e. they are poor electrocatalysts for OER; that is why they are termed as non-active anodes), and they carry out complete mineralization of organics to inorganic species like CO2 and H2O. The oxygen evolution potential of DSA (RuO2) is 1.47 volts (Vs SHE in 0.5 M H2SO4), while that of BDD is 2.3 volts (Vs SHE in 0.5 M H2SO4). That is why non-active electrodes are most widely used anodes for elimination of organic contaminants during wastewater treatment in the absence of chloride medium [38–40].

OH free radical is a powerful oxidant (E<sup>o</sup>

therefore in the dye decontamination from wastewater which is heavily loaded with inorganic salts like chlorides, the indirect electrochemical oxidation mediated by active chlorine species in situ provides an excellent tool for such wastewater abatement. The non-active electrodes like BDD, PbO2 and SnO2 are more active with

however they cannot be utilized for generation of active chlorine species because

*peroxo-diphosphate*, *peroxodicarbonate*, *peroxodisulphate*, etc. On the contrary the active anode materials like platinum and DSA have remarkably high efficiency for oxidation of chloride ion, thereby generating active chlorine oxidants rather than

DSA (which has been selected as model anode in this study), then chloride ions (Cl<sup>−</sup>) undergo oxidation at anodes to produce a very strong dye killing oxidants in

[30, 42, 43]. These species mediates the oxidation of organics as shown below.

‐

active chlorine species in water depends upon pH solution, for example, at pH less than 03, dissolved chlorine (Cl2) gas exists in water in the form of aqueous chlorine, i.e. Cl2 (aq). At pH range of 03–7.5, Cl2 undergoes self-oxidation/self-reduction

2 *Cl*−1 → *Cl*2

2 *H*2*O* + 2 *e*

HOCl has greater oxidation potential (1.49 V) than OCl<sup>−</sup><sup>1</sup>

OH free radicals at their surfaces [41]. When NaCl is used as supporting electrolyte with active electrodes like Ti-based

but it is unstable and has much shorter life than HClO (Eo

situ, i.e. aqueous chlorine (i.e. Cl2 (aq)), HOCl, OCl<sup>−</sup>, ClO2

= 2.80 V vs. SHE),

and ClO4

(0.94 V). The type of

−1

= 1.49 V vs. SHE);

OH free radicals for wastewater abatement;

−1 , ClO3 −1

(*aq*) + 2 *e*−1 (5)

→ *H*<sup>2</sup> + 2 *OH*−1 (6)

*Cl*2(*aq*) + *H*2*O* → *HOCl* + *H*+1 + *Cl*−1 (7)

*HOCl* → *OCl*−1 + *H*+1 (*pKa* = 7.54) (8)

OH free radical along with other mild oxidants like

**120**

reaction (disproportionation reaction) and forms HOCl, while at PH greater than 7.5, hypochlorite ions (OCl<sup>−</sup>) exist in solution which is a much weaker oxidizing agent than HOCl [44].

Bonfatti has presented an alternative approach to electrochemical oxidation of organics; the schematic sketch of electrochemical oxidation at non-active oxide electrode *(MOx)* and with associated CER is shown in **Figure 2**. Here "R" represents a dye pollutant molecule [45].

**Figure 2** shows that in the first step, the hydroxide from solution adsorbs on metal oxide to form MOx (٭ OH) (see reaction 05 given below), and then it reacts with chloride ions (Cl<sup>−</sup><sup>1</sup> ) to yield adsorbed hypochlorous acid radical, i.e. *MOX*(*HO*. *Cl*) (see reaction 10); this adsorbed hypochlorous radical attacks organic pollutant (R) and degrades it to CO2 and H2O, and chloride ions are regenerated (see reaction 11). This degradation reaction proceeds simultaneously with chlorine evolution reaction (CER) to produce Cl2, some of which Cl2 gas evolves (if applied current density is very high), while some chlorine gas dissolve in water in the form of aqueous chlorine Cl2 (aq). This aqueous chlorine reacts with hydroxide ions to generate HOCl, OCl<sup>−</sup><sup>1</sup> , ClO3 −1 , *ClO*<sup>3</sup> −1, etc. (depending on pH of solution), which are active chlorine species and oxidize organic pollutants in situ.

$$\text{MO}\_{\text{x}} \text{+ O'H} \rightarrow \text{MO}\_{\text{x}} \text{(O'H)} \tag{9}$$

$$\text{MO}\_{\text{x}}\text{(O}^{\cdot}\text{H)} + \text{Cl}^{-1} \rightarrow \text{MO}\_{\text{x}}\text{(O}^{\cdot}\text{Cl)}\_{\text{ads}} + \text{1}\,\text{e}^{-1} \tag{10}$$

$$\text{MO}\_x\text{(O}^\cdot\text{Cl)}\text{ + }\text{R}\rightarrow\text{MO}\_x\text{+ }\text{CO}\_2\text{+ }\text{H}\_2\text{O}\text{+ }\text{Cl}^{-1}\tag{11}$$

The oxidation of organic compounds via active chlorine species had a serious concern about the formation of chlorinated organic compounds in water which are known as carcinogens, but Panizza and Bonfatti successfully demonstrated that by selecting optimal experimental conditions, the generations of toxic chlorinated compounds can be totally avoided in aqueous medium [46, 47]. For example,

**Figure 2.** *Mechanism of electrochemical degradation of pollutants in chloride medium.*

Panizza investigated electrochemical oxidation of 2-naphthol which was associated with the formation of few chlorinated organic compounds initially, but later as these reactions proceeded, chlorinated compounds were either mineralized to CO2 or converted to volatile CHCl3 which escaped off [48].
