**3.3 Parameters affecting electrocoagulation/electroflotation**

Several parameters can affect the efficiency of removal by electrocoagulation/ electroflotation. The most important parameters are described bellow.

### **3.3.1 Current density or charge loading**

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 chloride anions and can be summarized as follows:

$$\text{2Al} + 6\text{HCl} \rightarrow 2\text{AlCl}\_3 + 3\text{H}\_2$$

$$\text{AlCl}\_3 + 3\text{H}\_2\text{O} \rightarrow \text{Al(OH)}\_3 + 3\text{HCl}$$

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 avoided.

The specific electrode consumption per kg pollutant (*Al*) is determined by the following expression:

$$\mu\_{Al} \text{(kgAl / kgpollutant)} = \frac{3600. M\_{Al}.It.\rho\_{Al}}{3F} \frac{1}{V \left(\mathbb{C}\_{0}Y\right)}\tag{11}$$

using initial pollutant concentration *C0* (kg/m3), current intensity *I* (A), cell voltage *U* (V), electrolysis time *t* (h), liquid volume *V* (m3), molar weight of aluminum *MAl* = 0.02698 constant *F* (96,487 C/mol*e*–) and the faradic yield *Al* of Al dissolution, Y is the removal efficiency. *Al*is estimated as the ratio of the weight loss of the aluminum electrodes during the experiments *mexp* and the amount of aluminum consumed theoretically at the anode *mth*:

$$
\phi\_{Al} = \frac{\Im F}{3600. M\_{Al^\*} I.t} \Delta m\_{\exp} \tag{12}
$$

Electrochemical Probe for Frictional Force and Bubble Measurements

at high pH values:

**3.3.4 Power supply** 

**3.3.5 Temperature** 

aluminum oxide film on the electrode surface.

(2004) is often recommended for non-passivated electrodes:

**3.4 Design of electrocoagulation cell** 

Al(OH)4<sup>−</sup>

as follows:

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 59

At low pH, such as 2–3, cationic monomeric species Al3+ and Al(OH)2+ predominate. When pH is between 4–9, the Al3+ and OH− ions generated by the electrodes react to form various monomeric species such as Al(OH)2+, Al2(OH)22+, and polymeric species such as Al6(OH)153+, Al7(OH)174+, Al13(OH)345+ that finally transform into insoluble amorphous Al(OH)3(s) through complex polymerization/precipitation kinetics. When pH is higher than 10, the monomeric Al(OH)4− anion concentration increases at the expense of Al(OH)3(s). In addition, the cathode may be chemically attacked by OH− ions generated together with H2

2Al + 6H2O + 2OH−→ 2Al(OH)4− +3H2 (10) Two main mechanisms are generally considered: *precipitation* for pH lower than 4 and adsorption for higher pH. Adsorption may proceed on Al(OH)3 or on the monomeric

From an energetic point of view, energy consumption during EC is known to vary as the product *UIt*. Energy requirements per kg of pollutant removed (*E*pollutant) to achieve a certain

The specific electrical energy consumption per kg pollutant removed (*E*pollutant) is calculated

... ( / pollutant) 1000. .

Few papers were investigated to show the effect of temperature on EC efficiency. The current efficiency of aluminum was found to be increased with temperature until about 60°C (Chen, 2004) where a maximum was found. Further increase in temperature results in a decrease of EC efficiency. The increase of temperature allows to a destruction of the

The position of the electrodes in the reactor can be optimized as a function of hydrodynamic parameters and current density (j). Complementary rules should include the influences of electrode gap (e) and operating conditions on voltage U (and consequently on energy consumption). The measured potential is the sum of three contributions, namely the kinetic overpotential, the mass transfer overpotential and the overpotential caused by solution ohmic resistance. Kinetic and mass transfer overpotentials increase with current density, but mass transfer is mainly related to mixing conditions: if mixing is rapid enough, mass transfer overpotential should be negligible. In this case, the model described by Chen et al.,

0.76 0.20 ln ( ) *<sup>e</sup> U jj <sup>k</sup>*

<sup>0</sup>

(14)

*UIt <sup>E</sup> kWh kg V Y <sup>C</sup>* (13)

percentage of efficiency (Y) shows a continuous increase of *E*pollutant with *j*.

This parameter depends upon the pH and the amount of other species present in solution, for example co-existing anions.

#### **3.3.2 Conductivity**

The increase of the conductivity () by the addition of sodium chloride is known to reduce the cell voltage U at constant current density due to the decrease of the ohmic resistance of wastewater. Energy consumption, which is proportional to *U.I*, will therefore decrease. Chloride ions could significantly reduce the adverse effects of other anions, such as HCO3 – and SO4 <sup>2</sup>−, for instance by avoiding the precipitation of calcium carbonate in hard water that could form an insulating layer on the surface of the electrodes and increase the ohmic resistance of the electrochemical cell (Chen et al., 2004). Chloride anions can also be oxidized and give active chlorine forms, such as hypochlorite anions, that can oxidize pollutants. The main mechanism is as follows:

$$\rm Cl\_2 + 2e^- \to 2Cl^-$$

$$\rm Cl\_2 + H\_2O \to Cl^- + ClO^- + 2H^+$$

However, an excessive amount of NaCl (higher than 3 g/L) induces overconsumption of the aluminum electrodes due to "corrosion pitting" described above; Al dissolution may become irregular.

#### **3.3.3 pH effect**

pH is known to play a key role on the performance of EC. An optimum has to be found for the initial pH, in order to optimize the EC process. However, the pH changed during batch EC. Its evolution depended on the initial pH. EC process exhibits some buffering capacity because of the balance between the production and the consumption of OH- (Chen et al., 2004), which prevents high change in pH. The buffering pH seems just above 7: when the initial pH is above this value, pH decreases during EC; otherwise, the opposite behavior is observed.

The effect of pH can be explained as follows. The main reactions during EC are:

Anode: Al0(s)→ Al3+ + 3e− Cathode: 2H2O + 2e- → H2(g) +2OH- At low pH, such as 2–3, cationic monomeric species Al3+ and Al(OH)2+ predominate. When pH is between 4–9, the Al3+ and OH− ions generated by the electrodes react to form various monomeric species such as Al(OH)2+, Al2(OH)2 2+, and polymeric species such as Al6(OH)153+, Al7(OH)174+, Al13(OH)345+ that finally transform into insoluble amorphous Al(OH)3(s) through complex polymerization/precipitation kinetics. When pH is higher than 10, the monomeric Al(OH)4− anion concentration increases at the expense of Al(OH)3(s). In addition, the cathode may be chemically attacked by OH− ions generated together with H2 at high pH values:

$$\text{2Al} + 6\text{H}\_2\text{O} + 2\text{OH}^- \rightarrow 2\text{Al(OH)}^{4-} + 3\text{H}\_2 \text{(10)}$$

Two main mechanisms are generally considered: *precipitation* for pH lower than 4 and adsorption for higher pH. Adsorption may proceed on Al(OH)3 or on the monomeric Al(OH)4<sup>−</sup>
