2. Electrocoagulation mechanism and operation

#### 2.1. Mechanism

In the EC process, coagulant species are produced in situ using the electrical dissolution of sacrificial anode, usually made of iron or aluminum by electric current applied between metal electrodes [1]. At the anode, metal is oxidized into cations as shown in Eqs. (1)–(3).

$$Al\_{(s)} \rightarrow Al\_{(aq)}{}^{\mathfrak{s}+} + \mathfrak{B}e^- \tag{1}$$

4Fe OH ð Þ<sup>2</sup> þ O<sup>2</sup> þ H2O ! 4Fe OH ð Þ<sup>3</sup> (12)

� ð Þ at pH <sup>&</sup>gt; <sup>9</sup> (13)

http://dx.doi.org/10.5772/intechopen.75460

75

Fe OH ð Þ<sup>3</sup> þ OH� ! Fe OH ð Þ<sup>4</sup>

charge neutralization; and colloid entrapment [1].

coagulation efficiency and also reducing electrode lifetime [1].

2.2. Operating parameters

resulting in flock formation [1].

EC [11].

and pollutants [1].

in Eq. (14).

Removal mechanisms of various pollutants involve physical removal of dissolved pollutants during precipitation, adsorption, and complexation [1]; electrooxidation [28] or electroreduction [29] of the electro-active ion or molecules on the anode or the cathode; the direct adsorption of pollutants on the electrodes [30]; the compression of the double layer of a colloidal particle [31];

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters

The parameters affecting the effectiveness of the EC process are operating conditions such as the current density, operating time, pH, conductivity, and electrode properties [22]. The bubble density affects system hydrodynamics, which determine the mass transfer between pollutants, coagulant, and gas microbubbles, and determines the collision rate of the coagulated particles

The current density determines the coagulant dosage at the anode and the formation of hydrogen gas at the cathode. Unnecessarily high current values may negatively affect the EC efficiency as coagulant overdosage can reverse the charge of the colloids and redistribute them, reducing

pH is an important factor affecting EC performance, particularly the coagulation mechanism, since it governs the hydrolyzed metal species formed in electrolyte media [32]. Adsorption and coagulation depend on pH in particular. The superficial charge of Al or Fe precipitates can be explained by the adsorption of charged soluble monomeric species on their own hydroxide precipitates [1]. For Al and Fe electrodes, the amount of insoluble aluminum hydroxide increases sharply with increasing pH from 4.5 to 7 [1]. For a pH above 10, amorphous metal hydroxide is absent. For acidic influent, effluent pH after EC treatment would increase but can decrease for alkaline influent, which is due to the buffering effect of

Lower amounts of energy are consumed with decreasing the gap between electrodes. As the distance between electrodes becomes lower, the amount of generated gas bubbles increases, thus leading to a high mass transfer as well as to a high reaction rate between the coagulants

Applied potential difference of galvanostatic EC operation depends strongly on conductivity and ionic strength of the wastewater. Applied potential difference decreases with increasing electrolytic conductivity due to the decrease of ohmic resistance of wastewater [1]. Consequently, the energy consumption per unit volume of treated wastewater is reduced as shown

SEEC kWh=m<sup>3</sup> <sup>¼</sup> IVt

VE

(14)

$$Fe\_{(s)} \longrightarrow Fe\_{(aq)}^{2+} + 2e^- \tag{2}$$

$$\text{Fe}\_{(aq)}\text{}^{2+} + \text{O}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{Fe}\_{(aq)}\text{}^{3+} + 4\text{OH}^-\tag{3}$$

In the case of high anode potential, following reactions may occur as shown in Eqs. (4) and (5).

$$2H\_2O \to O\_{2(g)} + 4H^+ + 4e^- \tag{4}$$

$$\text{\textbullet Cl}^- \rightarrow \text{Cl}\_{2(g)} + 2e^- \text{ (in the presence of Cl}^- \text{ anions)} \tag{5}$$

Chlorine formation may lead to the formation of hypochlorous acid, an oxidizing agent which may contribute to the oxidation of dissolved organic compounds as shown in Eq. (6) [1].

$$\rm Cl\_{2(g)} + H\_2O \to HClO + H^+ + Cl^- \tag{6}$$

At the cathode, hydrogen gas and hydroxyl anions are formed by the reduction of water as shown in Eq.(7).

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{H}\_{2(g)} + 2\text{OH}^- \text{ (at alkaline conditions)}\tag{7}$$

The amount of dissolved metal at the anode can be calculated using Faraday's law as shown in Eq. (8).

$$m = \frac{\text{ItM}}{\text{zF}}\tag{8}$$

where m is the mass of metal in g, t is the electrolysis time in s, I is the electric current in A, M is the atomic weight of the electrode material in g/mole, z is the number of electrons transferred, and F is Faraday's constant (96486 C/mole). The most common phenomenon for metal cations released from the anode is the formation of non-soluble and easily precipitable metal hydroxide sweep flocs which have large surface areas beneficial for a rapid adsorption of soluble organic compounds and trapping of colloidal particles as shown in Eqs. (9)–(13) [1].

$$\rm Al\_{(aq)}^{3+} + \rm 3H\_2O \to Al(OH)\_3 + \rm 3H^+ \tag{9}$$

$$Al(OH)\_3 + H\_2O \to Al(OH)\_4^- + H^+ \text{ (at } pH > 9\text{)}\tag{10}$$

$$\text{Fe}\_{(aq)}\text{}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}\_2\tag{11}$$

$$4Fe(OH)\_2 + O\_2 + H\_2O \to 4Fe(OH)\_3\tag{12}$$

$$\text{Fe(OH)}\_{3} + \text{OH}^{-} \rightarrow \left[ \text{Fe(OH)}\_{4} \right]^{-} \text{(at } pH > 9 \text{)}\tag{13}$$

Removal mechanisms of various pollutants involve physical removal of dissolved pollutants during precipitation, adsorption, and complexation [1]; electrooxidation [28] or electroreduction [29] of the electro-active ion or molecules on the anode or the cathode; the direct adsorption of pollutants on the electrodes [30]; the compression of the double layer of a colloidal particle [31]; charge neutralization; and colloid entrapment [1].

#### 2.2. Operating parameters

2. Electrocoagulation mechanism and operation

Feð Þ aq

2Cl� ! Cl<sup>2</sup>ð Þ<sup>g</sup> þ 2e

In the EC process, coagulant species are produced in situ using the electrical dissolution of sacrificial anode, usually made of iron or aluminum by electric current applied between metal

<sup>3</sup><sup>þ</sup> <sup>þ</sup> <sup>3</sup><sup>e</sup>

<sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>2</sup><sup>e</sup>

� (1)

� (2)

<sup>3</sup><sup>þ</sup> <sup>þ</sup> <sup>4</sup>OH� (3)

2H2O ! O<sup>2</sup>ð Þ<sup>g</sup> þ 4H<sup>þ</sup> þ 4e� (4)

Cl<sup>2</sup>ð Þ<sup>g</sup> þ H2O ! HClO þ H<sup>þ</sup> þ Cl� (6)

2H2O þ 2e� ! H<sup>2</sup>ð Þ<sup>g</sup> þ 2OH� ð Þ at alkaline conditions (7)

� in the presence of Cl� ð Þ anions (5)

zF (8)

� þ H<sup>þ</sup> ð Þ at pH > 9 (10)

<sup>3</sup><sup>þ</sup> <sup>þ</sup> <sup>3</sup>H2<sup>O</sup> ! Al OH ð Þ<sup>3</sup> <sup>þ</sup> <sup>3</sup>H<sup>þ</sup> (9)

<sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>2</sup>OH� ! Fe OH ð Þ<sup>2</sup> (11)

electrodes [1]. At the anode, metal is oxidized into cations as shown in Eqs. (1)–(3). Alð Þ<sup>s</sup> ! Alð Þ aq

Feð Þ<sup>s</sup> ! Feð Þ aq

<sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>O</sup><sup>2</sup> <sup>þ</sup> <sup>2</sup>H2<sup>O</sup> ! Feð Þ aq

In the case of high anode potential, following reactions may occur as shown in Eqs. (4) and (5).

Chlorine formation may lead to the formation of hypochlorous acid, an oxidizing agent which may contribute to the oxidation of dissolved organic compounds as shown in Eq. (6) [1].

At the cathode, hydrogen gas and hydroxyl anions are formed by the reduction of water as

The amount of dissolved metal at the anode can be calculated using Faraday's law as shown in

<sup>m</sup> <sup>¼</sup> ItM

where m is the mass of metal in g, t is the electrolysis time in s, I is the electric current in A, M is the atomic weight of the electrode material in g/mole, z is the number of electrons transferred, and F is Faraday's constant (96486 C/mole). The most common phenomenon for metal cations released from the anode is the formation of non-soluble and easily precipitable metal hydroxide sweep flocs which have large surface areas beneficial for a rapid adsorption of soluble

organic compounds and trapping of colloidal particles as shown in Eqs. (9)–(13) [1].

Al OH ð Þ<sup>3</sup> þ H2O ! Al OH ð Þ<sup>4</sup>

Feð Þ aq

Alð Þ aq

2.1. Mechanism

74 Wastewater and Water Quality

shown in Eq.(7).

Eq. (8).

The parameters affecting the effectiveness of the EC process are operating conditions such as the current density, operating time, pH, conductivity, and electrode properties [22]. The bubble density affects system hydrodynamics, which determine the mass transfer between pollutants, coagulant, and gas microbubbles, and determines the collision rate of the coagulated particles resulting in flock formation [1].

The current density determines the coagulant dosage at the anode and the formation of hydrogen gas at the cathode. Unnecessarily high current values may negatively affect the EC efficiency as coagulant overdosage can reverse the charge of the colloids and redistribute them, reducing coagulation efficiency and also reducing electrode lifetime [1].

pH is an important factor affecting EC performance, particularly the coagulation mechanism, since it governs the hydrolyzed metal species formed in electrolyte media [32]. Adsorption and coagulation depend on pH in particular. The superficial charge of Al or Fe precipitates can be explained by the adsorption of charged soluble monomeric species on their own hydroxide precipitates [1]. For Al and Fe electrodes, the amount of insoluble aluminum hydroxide increases sharply with increasing pH from 4.5 to 7 [1]. For a pH above 10, amorphous metal hydroxide is absent. For acidic influent, effluent pH after EC treatment would increase but can decrease for alkaline influent, which is due to the buffering effect of EC [11].

Lower amounts of energy are consumed with decreasing the gap between electrodes. As the distance between electrodes becomes lower, the amount of generated gas bubbles increases, thus leading to a high mass transfer as well as to a high reaction rate between the coagulants and pollutants [1].

Applied potential difference of galvanostatic EC operation depends strongly on conductivity and ionic strength of the wastewater. Applied potential difference decreases with increasing electrolytic conductivity due to the decrease of ohmic resistance of wastewater [1]. Consequently, the energy consumption per unit volume of treated wastewater is reduced as shown in Eq. (14).

$$\text{SEECC} \left( \text{kWh/}m^3 \right) = \frac{\text{IVt}}{V\_E} \tag{14}$$

where SEEC is the specific electrical energy consumption, I is the electric current in A, V is the applied potential difference in V, t is the electrolysis time in h, and VE is the volume of treated wastewater in L.

#### 2.3. Applications of electrocoagulation

In this section, recent studies on the application of EC method in the treatment of different wastewaters and pollutants such as chemical oxygen demand (COD), total organic carbon (TOC), total phosphate (TP), phenol, oil and grease (O&G), biochemical oxygen demand (BOD5), and total suspended solids (TSS) are presented. Ranges and optimum values of operating parameters, pollutant removal efficiencies, energy consumption, and operating cost values of previously published work are given in Table 1. It has been reported that EC


treatment is an effective method in removing organic matter, turbidity, color, phenol, phos-

Parameters Research ranges

pH 2–10 8

Operating time (min) 5–50 30

Na2SO4 concentration (mM) 1–7 3 Initial pH 3.12–8.83 6

Current density (mA/cm<sup>2</sup>

for parameters

) 0.1–5 3

Al or Fe Electrode material Al-Fe Fe COD removal:

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters

Optimum conditions

) 10–50 30 COD removal:

Results at optimum conditions

77

Operating cost: 0.6 \$/m3 Sludge: 2.1 kg/ m<sup>3</sup>

SEEC: 42.44 kWh/m<sup>3</sup> Energy cost: 1.91 €/m3 Sludge: 1.91 g/g COD

88% O&G removal: 90% Cl� removal: 50%

http://dx.doi.org/10.5772/intechopen.75460

62.5%

EOx may occur either by direct oxidation of pollutants using hydroxyl or hydroperoxyl radicals produced on anode surface or by an indirect process where oxidants like chlorine, hypochlorous acid, and hypochlorite or hydrogen peroxide/ozone are formed at electrodes. Direct anodic oxidation of pollutants and formation of indirect oxidizing agents occur according to the reac-

<sup>H</sup>2<sup>O</sup> <sup>þ</sup> <sup>S</sup> ! S OH ½ �• <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

<sup>R</sup> <sup>þ</sup> S OH ½ �• ! <sup>S</sup> <sup>þ</sup> RO <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

<sup>H</sup>2<sup>O</sup> <sup>þ</sup> <sup>S</sup> <sup>þ</sup> Cl� ! S ClOH ½ �• <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>2</sup><sup>e</sup>

<sup>H</sup>2<sup>O</sup> <sup>þ</sup> S OH ½ �• ! <sup>S</sup> <sup>þ</sup> <sup>O</sup><sup>2</sup> <sup>þ</sup> <sup>3</sup>H<sup>þ</sup> <sup>þ</sup> <sup>3</sup><sup>e</sup>

<sup>H</sup>2<sup>O</sup> <sup>þ</sup> S OH ½ �• ! <sup>S</sup> <sup>þ</sup> <sup>H</sup>2O<sup>2</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

<sup>R</sup> <sup>þ</sup> S ClOH ½ �• ! <sup>S</sup> <sup>þ</sup> RO <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> Cl� (18)

� (15)

� (16)

� (17)

� (19)

� (20)

phate, heavy metals, pharmaceuticals, O&G from wastewater.

Table 1. EC treatment results of various wastewaters for selected examples in the literature.

Fe Current density (mA/cm<sup>2</sup>

3. Electrooxidation mechanism and operation

tions given in Eqs. (15)–(24) [26, 38, 39].

3.1. Mechanism

Ref. Wastewater Electrode

[36] Car wash (real)

[37] Industrial estate (real)

material

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters http://dx.doi.org/10.5772/intechopen.75460 77


Table 1. EC treatment results of various wastewaters for selected examples in the literature.

treatment is an effective method in removing organic matter, turbidity, color, phenol, phosphate, heavy metals, pharmaceuticals, O&G from wastewater.

### 3. Electrooxidation mechanism and operation

#### 3.1. Mechanism

where SEEC is the specific electrical energy consumption, I is the electric current in A, V is the applied potential difference in V, t is the electrolysis time in h, and VE is the volume of treated

In this section, recent studies on the application of EC method in the treatment of different wastewaters and pollutants such as chemical oxygen demand (COD), total organic carbon (TOC), total phosphate (TP), phenol, oil and grease (O&G), biochemical oxygen demand (BOD5), and total suspended solids (TSS) are presented. Ranges and optimum values of operating parameters, pollutant removal efficiencies, energy consumption, and operating cost values of previously published work are given in Table 1. It has been reported that EC

Parameters Research ranges

NaCl concentration (g/L) 0.5–3 2 Initial pH 2–10 4.2

Inlet flow rate (L/h) 15–60 15 Initial concentration (mg/L) 50–300 < 300 Initial pH 2.3–8.8 7.74 Residence time (min) 5–55 35

Initial pH 2–12 7

NaCl concentration (g/L) 0–2 1

pH 3–12 5.82 Temperature (C) 10–50 22.62 Electrolysis time (min) 5–45 10.83 Current intensity (A) 0.5–1.8 1.22

Current density (mA/cm<sup>2</sup>

Electrical conductivity

(mS/cm)

Al Electrolysis time (min) 0–60 30 COD removal:

Al or Fe Electrode material Al-Fe Al COD removal:

) 2.5–35 14

2.3–4.2 2.72

Current density (A/m<sup>2</sup>

Al Current density (A/m<sup>2</sup>

for parameters

) 10–4000 250

Al Electrolysis time (min) 3–20 15 Color removal:

Optimum conditions

) 100–400 300 Turbidity removal:

Results at optimum conditions

Phenol removal:

2.63 kWh/kg CODr Operating cost: 0.27 €/kg CODr

19.5 kWh/kg dye removed

Turbidity removal:

Turbidity removal:

TSS removal: 97.06%

91% COD removal: 83%

87% SEEC:

90% Color removal: 97% SEEC:

74.56% BOD5 removal: 96.28%

98.91% SEEC: 3.36 kWh/m<sup>3</sup>

65.33% Color removal: 100%

98.61%

SEEC: 8.82 kWh/kg COD

wastewater in L.

76 Wastewater and Water Quality

Ref. Wastewater Electrode

[33] Olive mill (real)

[34] Textile (synthetic)

[35] Dairy (real)

[22] Pulp and paper (real)

2.3. Applications of electrocoagulation

material

EOx may occur either by direct oxidation of pollutants using hydroxyl or hydroperoxyl radicals produced on anode surface or by an indirect process where oxidants like chlorine, hypochlorous acid, and hypochlorite or hydrogen peroxide/ozone are formed at electrodes. Direct anodic oxidation of pollutants and formation of indirect oxidizing agents occur according to the reactions given in Eqs. (15)–(24) [26, 38, 39].

$$H\_2O + S \to S[OH]^\bullet + H^+ + e^- \tag{15}$$

$$\text{R} + \text{S}[\text{OH}]^\bullet \rightarrow \text{S} + \text{RO} + \text{H}^+ + e^- \tag{16}$$

$$\text{H}\_2\text{O} + \text{S} + \text{Cl}^- \rightarrow \text{S[ClOH]}^\bullet + \text{H}^+ + 2\text{e}^- \tag{17}$$

$$\text{R} + \text{S}[\text{ClOH}]^\bullet \rightarrow \text{S} + \text{RO} + \text{H}^+ + \text{Cl}^- \tag{18}$$

$$\text{H}\_2\text{O} + \text{S}[\text{OH}]^\bullet \rightarrow \text{S} + \text{O}\_2 + \text{3H}^+ + \text{3e}^- \tag{19}$$

<sup>H</sup>2<sup>O</sup> <sup>þ</sup> S OH ½ �• ! <sup>S</sup> <sup>þ</sup> <sup>H</sup>2O<sup>2</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup> � (20)

$$\text{S[OH]}^\bullet + H\_2O\_2 \rightarrow \text{S} + HO\_2^\bullet + H\_2O \tag{21}$$

It should be noted that although there is no agreement on the effect of the conductivity on the overall oxidation efficiency, the cell voltage is reduced for a given current density as the conductivity of the electrolyte increases [12]. For this reason, the electrochemical oxidation process is more cost-effective if the treated wastewater already has a high conductivity [12].

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters

When reactions are carried out in chloride-containing media, the pH value may affect the rate of oxidation because it determines the primary active chloro species present in the effluent [12]. During indirect oxidation, chlorine evolution occurs at the anode (Eq. (25)). At pH values lower than 3.3, the primary active chloro species is Cl2, while at higher pH values, it forms HClO at pH <7.5 (Eq. (26)) and ClO at pH >7.5 (Eq. (27)). Operation at strongly acidic conditions is favorable since chlorine is the strongest oxidant followed by HClO and ClO, respectively [12]. Depending on the electrolytic cell design, desorption of chlorine from reaction medium may prevent its action as an oxidizing agent. Thus, neutral or mildly alkali medium may be preferred since HClO and ClO species remain almost unaffected by desorption of gases and they can act as oxidizing reagents in the whole volume of wastewater [40].

This section briefly describes the treatment of different wastewater types by EOx method in the last few years. Table 2 shows a general insight on minimum, maximum, and optimum values of operating parameters, pollutant reduction, energy consumption, and operating cost of EOx

Current density (A/dm<sup>2</sup>

Current density (A/m<sup>2</sup>

Type of support electrolyte

Concentration of support electrolyte (mM)

Current density (mA/cm<sup>2</sup>

NaCl concentration (g/L) 1–5 3

Graphite Initial pH 4–10 4 COD removal:

NaCl concentration (g/L) 0–2 2 Electrolysis time (min) 10–130 110

Mixing rate (rpm) 0–600 0 Dilution ratio 1/5–5/5 1/5 pH 2–8 4.5–4.7

Temperature (C) 10–50 10

ranges for parameters

) 27.78–138.89 27.78

Electrolysis time (h) 1–24 5 COD removal:

Na2SO4, KCl, NaCl

0.25–1.25 0.25

) 2.5–15 7.69

Optimum conditions

http://dx.doi.org/10.5772/intechopen.75460

79

) 2.5–5 2.5 COD removal:

NaCl

Results at optimum conditions

67.99% SEEC: 37.67 kWh/kg COD

90.78% Color removal: 96.27% SEEC: 23.58 kWh/kg COD

100% TOC removal: 78% Phenol removal: 100% SEEC: 451.25 kWh/m3 Energy cost: 6.02 \$/kg COD

Ref. Wastewater Electrode material Parameters Research

3.3. Applications of electrooxidation

Anode: Catalytic oxide coated titanium Cathode: Stainless steel

Anode: Ti/Pt mesh Cathode: Ti mesh

[41] Pulp and paper industry (real)

[42] Textile (real)

[43] Olive mill (real)

$$\text{O}\_2 + \text{S}[\text{OH}]^\bullet \rightarrow \text{S} + \text{O}\_3 + \text{H}^+ + \text{e}^- \tag{22}$$

$$\text{C}\_2\text{O} + \text{S}[\text{ClOH}]^\bullet + \text{Cl}^- \rightarrow \text{Cl}\_2 + \text{S} + \text{O}\_2 + 3\text{H}^+ + 4\text{e}^- \tag{23}$$

$$\text{C} \begin{aligned} \text{H}\_2\text{O} + \text{S}[\text{ClOH}]^\bullet + \text{Cl}\_2 &\rightarrow \text{S} + \text{ClO}\_2 + \text{3H}^+ + 2\text{Cl}^- + e^- \end{aligned} \tag{24}$$

Indirect oxidation of pollutants occurs according to the reactions given in Eqs. (25)–(29) [26].

$$\text{2Cl}^- \rightarrow \text{Cl}\_{2(g)} + 2e^- \tag{25}$$

$$\text{Cl}\_2 + \text{H}\_2\text{O} \rightleftharpoons \text{HClO} + \text{H}^+ + \text{Cl}^-\tag{26}$$

$$\rm{HClO} \rightleftharpoons \rm{H}^+ + \rm{ClO}^- \tag{27}$$

$$\text{H}\_2\text{O} + \text{R} + \text{ClO}^- + \text{H}^+ \rightarrow \text{RO} + \text{H}\_3\text{O}^+ + \text{Cl}^- \tag{28}$$

$$2\text{ }6\text{ClO}^- + 3\text{H}\_2\text{O} \to 2\text{ClO}\_3^- + 4\text{Cl}^- + 6\text{H}^+ + \frac{3}{2}\text{O}\_2 + 6\text{e}^- \tag{29}$$

Chlorate is usually an unwanted product in the effluent, and its formation could also prevent the use of EOx in various applications [25]. At the cathode, hydrogen gas and chloride anions are formed as shown in Eqs. (30)–(32).

$$2\text{H}\_3\text{O}^+ + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{H}\_2\text{O} \text{(at acidic conditions)}\tag{30}$$

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{OH}^- \text{(at alkaline conditions)}\tag{31}$$

$$2\text{ ClO}^{-} + H\_{2}\text{O} + 2\text{e}^{-} \rightarrow \text{Cl}^{-} + 2\text{OH}^{-} \tag{32}$$

Here S symbolizes the active sites of the anode surface, and R represents the organic matter. To evaluate the selectivity of an anode material, competition between the oxidation of organic materials and the oxygen formation (the side reaction) at the anode (Eq. (4)) must be taken into account [12]. Oxygen formation is typically considered to be an undesirable side reaction in the electrochemical wastewater treatment because it affects the efficiency of the process and significantly increases the operating costs [25].

Removal mechanisms of various pollutants involve diffusion of pollutants from the bulk solution to the anode surface and direct oxidation at the anode surface either partially or completely [12] and generation of a strong oxidizing agent (i.e., chlorine) at the anode surface and indirect oxidation of pollutants in the bulk solution.

#### 3.2. Operating parameters

Among the variables that are usually studied in EOx treatment, the current density is one of the most frequently referenced terms since it affects the rate of reactions [12]. It should be noted that an increase in current density will not necessarily result in an increase in oxidation efficiency or oxidation rate [12]. The use of higher current densities usually leads to higher operating costs due to the increase in energy consumption.

It should be noted that although there is no agreement on the effect of the conductivity on the overall oxidation efficiency, the cell voltage is reduced for a given current density as the conductivity of the electrolyte increases [12]. For this reason, the electrochemical oxidation process is more cost-effective if the treated wastewater already has a high conductivity [12].

When reactions are carried out in chloride-containing media, the pH value may affect the rate of oxidation because it determines the primary active chloro species present in the effluent [12]. During indirect oxidation, chlorine evolution occurs at the anode (Eq. (25)). At pH values lower than 3.3, the primary active chloro species is Cl2, while at higher pH values, it forms HClO at pH <7.5 (Eq. (26)) and ClO at pH >7.5 (Eq. (27)). Operation at strongly acidic conditions is favorable since chlorine is the strongest oxidant followed by HClO and ClO, respectively [12]. Depending on the electrolytic cell design, desorption of chlorine from reaction medium may prevent its action as an oxidizing agent. Thus, neutral or mildly alkali medium may be preferred since HClO and ClO species remain almost unaffected by desorption of gases and they can act as oxidizing reagents in the whole volume of wastewater [40].
