**3.5.4 External loop airlift reactor as a continuous reactor**

68 Electrochemical Cells – New Advances in Fundamental Researches and Applications

An excess amount of fluoride anions in drinking water has been known to cause adverse effects on human health. To prevent these harmful consequences, especially problems resulting from fluorosis, the World Health Organization (WHO) fixed the maximum acceptable concentration of fluoride anions in drinking water to 1.5 mg/L (Essadki et al.,

The defluoridation of drinking water by EC is studied in the same reactor. Current density values j between 2.8 and 17 mA/cm2 are investigated, which corresponded to current (I=jS) in the range 0.5–3 A. Samples are filtered and the concentrations of the remaining fluoride content are determined in the solution by means of a combined selective fluoride electrode ISEC301F and a PhM240 ion-meter (Radiometer Analytical, France), using the addition of a

adding sodium fluoride NaF (Carlo Erba Réactifs, France). The efficiency of fluoride

Figure 20 shows the effect of the current intensity on the evolution of the fluoride concentration. For *I* = 0.5A corresponding to a current density of 2.85mA/cm2, fluoride concentration reaches only 4.5 mg/L for an electrolysis time of 30 min. Conversely, for exceeding 1A, i.e. for a current density higher than 5.7mA/cm2, one converges towards a concentration of 1mg/L and more rapidly as current density is increased. This confirms that defluoridation can be achieved at low current density. The relatively low efficiency observed at 0.5A can be attributed to the weak charge loading produced in this case, 0.47 F/m3. As

(%) 100x

expected, the efficiency of EC depends on the amount of coagulant produced in situ.

Fig. 20. Evolution of fluoride concentration during EC in the STR: influence of current

*Y*

0

 

*F F*

*F*

0

]0 between 10-20 mg/L by

(19)

TISAB II buffer solution to prevent interference from other ions. Experiments are carried using an initial fluoride concentration [F-

**3.5.3.3 Defluoridation of drinking water** 

removal can be calculated as follows:

]0 : the initial fluoride concentration .

intensity (initial pH 7.4 and *C*0 = 15mg/L).

] : the remaining concentration of fluoride .

2009).

[F-

[F-

The reactor is operated in a continuous flow of liquid. The inlet volumetric liquid flow-rate QL varied between 0.1 and 2 L/min.

A study of the residence time distribution (RTD) analysis of liquid phase has been performed. The liquid RTD is determined by means of the tracer response technique. An approximated δ-Dirac pulse of tracer solution (NaCl) is injected into the reactors at a certain time (t = 0) and the outlet signal is detected by conductivity probe and recorded by the acquisition system. The tracer is injected as quickly as possible to obtain as closely as practical a -pulse of tracer at the inlet.

The syringe (S1) was introduced in the drain that is open to the flow and representing the inlet of the whole reactor. The probe conductivity is placed at the exit position of the reactor (Fig.12). Examples of RTD measurements are shown in in figure 21. Two kinds of signal are observed in this figure. One showed two peaks for the case of QL = 0.36 L/min, I = 6 A, H1 = 7cm and the other showed one peak for the case of QL = 0.73 L/min, I = 1 A, H1 = 47 cm.

The main flow (QL) is divided into two flows: one exit directly the rector by crossing the junction and the other crosses the riser, the separator zone and the downcomer to exit. The percentage of flow that quit the reactor without reacting increased when the main flow increased and the current intensity decreased. The experiments confirm also that the liquid crosses the reactor without achieving loops in the case of the continuous flow.

So to support the reaction during the electrocoagulation, it is necessary to amplify the current intensity and to decrease the inlet flow.

Interesting results are also obtained:


Fig. 21. E-curve as a global RTD in External-loop airlift reactor for (QL = 0.36 L/min, I = 6 A, H1 = 7cm: 2 peaks) and (QL = 0.73 L/min , H1 = 7 cm, I = 1 A, 47 cm: 1 peak).

**4** 

**Electrochemical Cells with the Liquid** 

**Metallic and Oxide Systems** 

*1Chemistry Department, Lomonosov University, Moscow,* 

Valery Vassiliev1 and Weiping Gong2

*2Institute of Huizhou,* 

*1Russia 2China* 

**Electrolyte in the Study of Semiconductor,** 

The first publications devoted to a study of the thermodynamic properties of metallic alloys, using electrochemical cells (EMF method) was known since 1936 year (Strikler & Seltz,

Co-workers from Moscow State University (Geyderih et al., 1969) have considered some

A new attempt to generalize the electrochemical methods on thermodynamic studies of metallic systems was made again in the book (Moratchevsky, 1987). The general aspects of the thermodynamics of nonstoichiometric compounds were presented there and the methods for experimental studies of the thermodynamic properties of molten metal and salt

The different types of electrochemical cells with solid and liquid electrolytes and dynamic EMF methods were examined in the recent book (Moratchevsky et al., 2003). A separate chapter of this book deals with methods of treatment and presentation of experimental data. In recent decades the important step of qualitative development of EMF method had been

In the present chapter we focus on those experimental techniques that help to increase

The knowledge of thermodynamic properties and phase diagrams of binary, ternary and multi component systems is necessary for solving materials science problems and for designing new products and technologies fitted to actual needs. A rational study of equilibria among phases and of the given system thermodynamic properties not only leads to the discovery of unknown phases but also to the determination of phase thermodynamic stability, to homogeneity domain boundaries, and finally to the elaboration of analytical description of the system by using thermodynamic models which are based on the dependence of phase Gibbs energies on such parameters as temperature, concentration and

1936). This was the groundwork for all the next studies.

questions about this experimental method.

made and it was not considered in this book.

significantly the experimental result precision.

**1. Introduction** 

systems were discussed.

pressure.

#### **4. Conclusion**

External-loop airlift reactors have been shown to be versatile tools to carry out EC with complete flotation, using only electrochemically generated H2 bubbles to achieve an overall liquid circulation and good mixing conditions. Consequently, the use of mechanical agitation, pumping or compressed air was not necessary. This could not be achieved in other kinds of conventional gas-liquid contacting devices than airlift reactors. External-loop devices are particularly adapted because they offer specific designs for the disengagement section that allow large distance between riser and downcomer. This improves flotation by minimizing the recirculation of aluminum or iron particles in the downcomer. These results were obtained by the adequate selection of the axial position of the electrodes (*H1*) and the liquid height in the separator section (*h*) in order to avoid floc break-up in the riser and floc erosion at the free surface. A limiting value of the liquid velocity in the downcomer was defined, while *ULd* was correlated to dispersion height *hD* and current density *j* (Equation 17). These can be used at constant *j* and *Ar*/*Ad* ratio for scale-up purpose.

To increase the efficiency of EC in a continuous reactor, the mean residence time should be increased. The experiments showed that this effect is reached in the case of a relatively high value of current density and weak value of the inlet flow-rate. This study highlighted the hydrodynamic aspect of the flow in the external airlift reactor functioning as a batch and continuous reactor. The design of this kind of reactor should be improved to allow the reactant to follow the compartment in which the reaction takes place (riser).

### **5. References**

Chen G., (2004), Sep. Purif. Technol. 38, 11–41.

