3.3. Applications of electrooxidation

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

<sup>O</sup><sup>2</sup> <sup>þ</sup> S OH ½ �• ! <sup>S</sup> <sup>þ</sup> <sup>O</sup><sup>3</sup> <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> S ClOH ½ �• <sup>þ</sup> Cl� ! Cl<sup>2</sup> <sup>þ</sup> <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>4</sup><sup>e</sup>

<sup>H</sup>2<sup>O</sup> <sup>þ</sup> S ClOH ½ �• <sup>þ</sup> Cl<sup>2</sup> ! <sup>S</sup> <sup>þ</sup> ClO<sup>2</sup> <sup>þ</sup> <sup>3</sup>H<sup>þ</sup> <sup>þ</sup> <sup>2</sup>Cl� <sup>þ</sup> <sup>e</sup>

Indirect oxidation of pollutants occurs according to the reactions given in Eqs. (25)–(29) [26]. 2Cl� ! Cl<sup>2</sup>ð Þ<sup>g</sup> þ 2e

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

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

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

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

6ClO� þ 3H2O ! 2ClO�

ClO� þ H2O þ 2e

2H3O<sup>þ</sup> þ 2e

2H2O þ 2e

significantly increases the operating costs [25].

3.2. Operating parameters

and indirect oxidation of pollutants in the bulk solution.

operating costs due to the increase in energy consumption.

are formed as shown in Eqs. (30)–(32).

78 Wastewater and Water Quality

• <sup>þ</sup> <sup>H</sup>2<sup>O</sup> (21)

� (25)

Cl<sup>2</sup> þ H2O ⇄ HClO þ H<sup>þ</sup> þ Cl� (26)

3 2

� ! H<sup>2</sup> þ 2H2O at acidic conditions ð Þ (30)

� ! H<sup>2</sup> þ 2OH�ð Þ at alkaline conditions (31)

� ! Cl� þ 2OH� (32)

H2O þ R þ ClO� þ H<sup>þ</sup> ! RO þ H3O<sup>þ</sup> þ Cl� (28)

<sup>3</sup> þ 4Cl� þ 6H<sup>þ</sup> þ

HClO ⇄ H<sup>þ</sup> þ ClO� (27)

O<sup>2</sup> þ 6e

� (22)

� (23)

� (24)

� (29)

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



Ref. Wastewater Electrode material Parameters Research

Current density (A/m2

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

Sacrificial electrode

Interelectrode distance

material

(cm)

Electrolysis time (min) 0–5 2 Initial pH 4–11 6.6 NaCl concentration (g/L) 0–2 1.5

Applications of Combined Electrocoagulation and Electrooxidation Treatment to Industrial Wastewaters

Current intensity (A) 1–3 3 Detention time (min) 10–60 60

NaCl concentration (g/L) 0.5–3 1 H2O2 concentration (mg/L) 0–2000 1000 Detention time (min) 5–30 30

Electrolysis time (min) 20–90 90 Current intensity (A) 0.2–1 0.6 pH 5–10 6

Stirring speed (rpm) 250–750 500

[48] Dairy

[49] Anaerobic

reactor effluent (real)

[50] Pharmaceutical (synthetic)

[20] Olive mill (real)

[19] Textile (real)

(synthetic)

Anode: Al Cathode: Ti/Pt

Anode: MP RuO2/Ti Cathode: MP stainless steel Sacrificial electrode: BP Al

Anode: MP stainless steel Cathode: MP stainless steel

steel Sacrificial electrodes: BP Al

Anode: MP Ti/IrO2 Cathode: MP Ti Sacrificial electrode: BP Al or Fe

Anode: RuO2/Ti Cathode: Stainless ranges for parameters

pH 5–10 7 Ammonia removal:

pH 4.5–10.9 10.9 Diclofenac sodium

pH 3–9 4 COD removal:

) 5–40 40

0.5–1.5 1

Optimum conditions

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

) 0–75 50 COD removal:

Results at optimum conditions 81

54.16% TP removal: 42.9%

37%

98% TP removal: 98% COD removal: 72%

removal: 84% COD removal: 80%

96%

88.7% TSS removal: 97% O&G removal: 97.1% BOD5/COD: 0.46

93.5% TSS removal: 97% Color removal: 97.5% BOD5 removal:

90%

96%

99% TP removal: 97%

Turbidity removal:

Phenol removal:

Operating cost: 1.69 US \$/m3

Al-Fe Al COD removal:

BOD5 removal: 93.6% Phenol removal: 94.4% Color removal: 91.4%

Turbidity removal:

Turbidity removal:

Table 2. EOx treatment results of various wastewaters for selected examples in the literature.

treatment. By evaluating the previously published studies, it has been concluded that EOx is a remarkable treatment method in terms of organic matter, turbidity, color, phenol, total Kjeldahl nitrogen (TKN), and O&G removal efficiencies.
