**3.2 Reactions at electrodes and different modes of electrodes connection**

In the case of aluminum electrodes, the reactions taking place at the electrodes are as follow:

At the anode, takes place oxidation:

$$\text{Al} \twoheadrightarrow \text{Al}^{3+} + \text{3e}^{\cdot}$$

For higher current density:

54 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Fig. 7. Schematic diagram of two-electrode electrocoagulation (EC) cell.

i. Formation of coagulants by electrolytic oxidation of sacrificial anode

iii. Aggregation of the destabilized phases to form flocs.



ii. Destabilization of the pollutants, particulate suspension and breaking of emulsions

It is impossible to use the decantation as a technique to eliminate the maximum of particles. This remark is especially valid for colloids. Thus, colloids are organic or mineral particles in which the size is between some nanometers and approximately 1 responsible for color and




In the case of aluminum electrodes, the reactions taking place at the electrodes are as follow:

The destabilization mechanism of the pollutants can be summarized as follows:

**3.2 Reactions at electrodes and different modes of electrodes connection** 

interactions of ions generated by oxidation of the sacrificial anode.

Three successive stages occur during EC:

The pollutants can be in form of:


turbidity.

process.

$$\text{4 OH}\\
\text{:}\\
\text{ + 2 H}\\
\text{H}\\
\text{2O}\\
\text{+ O}\\
\text{+ 2 e}\\
\text{e}$$

At the cathode, takes place reduction:

$$\begin{array}{rcl} \text{2 н} \text{ 2C } + \text{ 2 е } \rightarrow \text{ 2 H } \text{ 2 } + \text{ 2 OH} \cdot \text{ } \end{array}$$

Although the sacrificial anodes deliver Al cation, their dissolution produces hydroxides, oxyhydroxides or polymeric hydroxides as a function of pH (Chen et al., 2004). These can adsorb or precipitate.

Al3+ ions may generate Al(H2O)63+, Al(H2O)5OH2+, Al(H2O)4(OH)2+. Many monomeric and polymeric species will be formed by hydrolysis such as, Al(OH)2+, Al(OH)2+, Al2(OH)24+, Al(OH)4-, Al6(OH)153+, Al7(OH)174+, Al8(OH)204+, Al13O4(OH)247+, Al13(OH)345+ over a wide pH range.

In the case of iron electrodes, Fe(OH)n with n = 2 or 3 is formed at the anode. The production of Fe(OH)n follow two mechanisms.


Polymeric hydroxy complexes are also generated namely : Fe(H2O)63+, Fe(H2O)5(OH)2+, Fe2(H2O)8(OH)24+ and Fe2(H2O)6(OH)4 4+, depending on the pH of aqueous medium.

Because of the workable rate of metal dissolution, a two-electrode EC is not always suitable. That's why large surface area is needed for a good performance. The electrodes configuration can be divided into three modes:


As observed in figure 8, the parallel arrangement consists of pairs of conductive metals plates placed between two parallel electrodes and DC power source. All cathodes are connected to each other and to negative pole of DC; in the same manner, all sacrificial anodes are connected to each other and to positive pole of DC. The electric current is divided between all the electrodes in relation to the resistance of the individual cells.

Electrochemical Probe for Frictional Force and Bubble Measurements

Fig. 10. Bipolar electrodes in series connections.

**3.3.1 Current density or charge loading** 

chloride anions and can be summarized as follows:

The specific electrode consumption per kg pollutant (

avoided.

expression:

AlCl3 +3H2O → Al(OH)3 +3HCl

**3.3 Parameters affecting electrocoagulation/electroflotation** 

electroflotation. The most important parameters are described bellow.

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 57

Several parameters can affect the efficiency of removal by electrocoagulation/

The current density (j) is expected to exhibit a strong effect on EC: higher the current, shorter the treatment. This is ascribed to the fact that at high current density, the extent of anodic dissolution of aluminum (iron) increases, resulting in a greater amount of precipitate for the removal of pollutants. Moreover, bubble generation rate increases and the bubble size decreases with increasing current density. These effects are both beneficial for high pollutant removal by H2 flotation. As a first approximation, the amount of Al released is proportional to the product φAlIt. The values of the Faradic yield *φ*Al are between 100% and 160%; they decrease with increasing time in the first minutes of the run, but also with higher current density. These trends have already been reported in the literature (Essadki et al., 2009). This mass overconsumption of aluminum electrodes may be due to the chemical hydrolysis of the cathode but it also often explained by the "corrosion pitting" phenomenon which causes holes on the electrode surface. The mechanism suggested for "corrosion pitting" involves

2Al + 6HCl → 2AlCl3 +3H2

This mechanism can therefore produce more aluminum hydroxide flocs and H2 bubbles than the equivalent current supplied should. Conversely, high current density allows the passivation of the cathode to be reduced but an increase in energy consumption that induces heating by Joule effect. As a result, too high current densities have generally to be

*Al*) is determined by the following

Fig. 8. Monopolar electrodes in parallel connections.


Each pair of sacrificial electrodes is internally connected with each other, and has no interconnections with the outer electrodes (figure 9). The same current would flow through all the electrodes.

Fig. 9. Monopolar electrodes in series connections.


The cells are in series. The sacrificial electrodes are placed between the two parallel electrodes without any electrical connection. Only the two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial electrodes.

Fig. 10. Bipolar electrodes in series connections.

56 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Each pair of sacrificial electrodes is internally connected with each other, and has no interconnections with the outer electrodes (figure 9). The same current would flow through all

The cells are in series. The sacrificial electrodes are placed between the two parallel electrodes without any electrical connection. Only the two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial

Fig. 8. Monopolar electrodes in parallel connections.


Fig. 9. Monopolar electrodes in series connections.


electrodes.

the electrodes.
