**4. Results and discussion**

### **4.1 Scanning electron microscopy (SEM)**

The surface quality and morphology of the modified and unmodified ceramic microfiltration layer were examined by scanning electron microscopy. **Figure 2** shows pictures of the surfaces of grafted and ungrafted microfiltration layers. The obtained photo shows that the modified surface is homogeneous without defects and was completely covered with fluoroalkylsilane (**Figure 2b**). We note the same that grafting

*Preparation and Evaluation of Hydrophobic Grafted Ceramic Membrane: For Application… DOI: http://dx.doi.org/10.5772/intechopen.104899*

**Figure 2.** *SEM photo of surface of ungrafted (a) and grafted (b) membrane.*

perfluorodecyltriethoxysilane on the surface of the membrane (Sand/Zirconia) led to a sharp decrease in pore size (**Figure 3a** and **b**).

### **4.2 Determination of pore size**

The method based on N2 adsorption and desorption was used for the determination of the pore size of the modified zirconia membrane. **Figure 3** showed a type IV isotherm with hysteresis behavior associated with capillary condensation of the adsorbate in mesopores. This is done because the smaller pores became completely filled with liquid nitrogen by reducing the saturation vapor pressure, according to the Kelvin Eq. [37]. The pore diameters measured are in order of 10 nm. The decrease in the pore size of 0.22 μm before grafting to values of 10 nm after grafting clearly confirms the densification of the membrane surface shown by SEM (**Figure 3**).

### **4.3 Contact angle measurement**

The hydrophobic character of the ceramic membrane was determined by measuring the contact angle of the water drop. The low contact angle of the membrane, which is approximately 18° (**Figure 4**), is attributed to the very hydrophilic character of the membrane due to the high density of the hydroxyl group on the surface of the membrane. After grafting, the value of contact angle increased exceeding 170°, which confirmed that the grafted membrane acquired a very hydrophobic character (**Figure 4**). After grafting the membrane surface, the measured contact angle increased by more than 170°. This result confirms that the grafted membrane has become very hydrophobic (**Figure 5**). In addition, the C8 used in this research work has a long alkyl chain of eight carbon atoms, leading to a very high increase in surface hydrophobicity.

### **4.4 Cross-flow filtration experiments**

Water permeability measurements of the membrane were determined to assess and demonstrate the hydrophobic character of the membrane after grafting. For this, a test with grafted and ungrafted membranes was achieved. The water permeability determination of the non-grafted membrane is of the order of 720 L.h−1.m−2.bar−1.

**Figure 3.** *Nitrogen adsorption-desorption of modified zirconia membrane.*

**Figure 4.** *Time dependence of contact angles for the modified and unmodified membranes.*

After grafting, there was a very large reduction in permeability, indeed for the microfiltration membrane grafted only 7 L.h−1.m−2.bar−1 was obtained (**Figure 6**). So, we can say that the grafted molecules were responsible for reducing the size of the pores causing the decrease in membrane permeability, which reflected the efficiency of the graft (C8) on the zirconium oxide membrane.

### **4.5 Membrane distillation process of saline water**

Air interval membrane distillation experiments were conducted in saline water through the prepared hydrophobic membrane. The evolution of permeate flow and *Preparation and Evaluation of Hydrophobic Grafted Ceramic Membrane: For Application… DOI: http://dx.doi.org/10.5772/intechopen.104899*

**Figure 5.** *Schematic representation of grafting process.*

### **Figure 6.**

*Variation of the flux, with the bulk feed temperature in desalination of a 1 M NaCl solution by AGMD using modified zirconia MF membrane.*

discharge rates with temperature was then determined. The feed side temperature varied from 75 to 95°C, while keeping the cooling system temperature.

### **4.6 Effects of temperature**

The effects of temperature on permeate flow and release rates in the distillation of the air spacer membrane for aqueous solutions of NaCl were determined using the modified zirconium membrane microfiltration membrane at a feed rate of 2.6 m.s−1. The temperature on the feed side varied from 75 to 95°C; keeping the cooling system temperature constant at 5°C. **Figure 6** shows the flux variations of a 1 mol.L−1 permeate of NaCl solution examined at different temperatures. Increasing the temperature of the source solution from 75 to 95°C led to an increase of permeate flux from 76 to

155 L.day−1.m−2 for modified zirconia membrane, in what the vapor pressure differences increase with the increase of the temperature. The effect of variable temperature on the permeates of aqueous solutions with different concentrations of NaCl was investigated using the modified zirconia membrane. The results of all these experiments are presented in **Figure 7**. As observed, for each temperature, the permeate flow increases with the decrease in the concentration of the NaCl solution.

### **4.7 Effect of concentration**

In order to study the effects of feed concentration on permeate flow, several series of experiments were carried out. The experimental conditions were cooling system was maintained at 5°C, a feed velocity of 2.6 m.s−1, and a feed temperature of 95°C. We can observe from **Figure 8** that the permeate flux decreases when the feed NaCl

### **Figure 7.**

*Evolution of permeate flux as a function of the feed solution temperature for modified zirconia MF membrane at different NaCl concentrations.*

### **Figure 8.**

*Variation of the flux, with the NaCl concentration in desalination by AGMD using modified zirconia MF membrane.*

*Preparation and Evaluation of Hydrophobic Grafted Ceramic Membrane: For Application… DOI: http://dx.doi.org/10.5772/intechopen.104899*

### **Figure 9.**

*Variation of the permeate flux as a function of the temperature. The values reported on the graph correspond to the rejection rates calculated for modified zirconia MF membrane after seawater filtration.*

concentration increased from 0.5 to 3 mol.L−1. Thus, increasing the salt concentration from 0.5 to 3 mol.L−1 led to a decrease of permeate flux from 173 to 120 L.day−1.m−2 for modified MF membrane. Raoult's Law can be used to explain these results. From **Figure 8**, we observed that the air-gap membrane distillation efficiency decreased over 30% when the NaCl concentration increased from 0.5 to 3 mol.L−1. According to Raoult's Law, the water vapor pressure over salt solutions is P = P0\*(1 − Xsalt). It is concluded that the reduction of permeate flow to a percentage of 5 to 7% can be interpreted by this law. It should be noted that in all the results, the salt rejection was always greater than 99%. The rejection rate for MF-modified membrane is not modified when NaCl concentration varies.

### **4.8 Desalination of seawater**

Seawater desalination aims to obtain fresh water for drinking. In this work, the treated seawater is taken from the sea of the region of SIDI MANSOUR Sfax (Tunisia). The measurements of the rejection rates and permeate flow were carried out by the filtered pilot used later. The feed side temperature was thus varied from 75 to 95°C, while keeping the cooling system temperature constant at 5°C. As it is shown in **Figure 9**, the rejection rate of NaCl is about 100% for microfiltration modified sand/zirconia membrane. These results proved that in the air-gap membrane distillation with aqueous solutions containing nonvolatile compounds such as NaCl, only water vapor is transported through the membrane.

### **5. Conclusion**

Membrane distillation is a new technology used for desalination. This technique differs from other membrane technologies in that the driving force responsible for desalination is the difference in water vapor pressure across the membrane. In this research work, very encouraging results have been found for distillation experiments with a microfiltration modified sand/zirconia membrane. An important

influence of the feed temperature and NaCl concentration on the permeate flux was observed. At the same time, very high salt rejection rates have been found in this research with grafted sand/zirconia ceramic membranes, the rejection rate of NaCl is about 100%. The membranes for membrane distillation are hydrophobic, which allows water vapor to pass. The vapor pressure gradient is created by heating the source water. It is expected that the total costs for drinking water with membrane distillation depend on the source of the thermal energy required for the evaporation of water through the membrane. Solar energy could very much help this process in our countries, which are very sunny resulting in a reduction of energy costs. Thus, membrane distillation could become competitive relative to other processes.
