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672 A. Malekpour, A. Samadi-Maybodi and M. R. Sadati A mixture of distilled water and 2-propanol were used as the feed solution in a 10/90 ratio; the amounts of 2-propanol were estimated by gas chromatography (GC) (Back Science Model 910 with FID detector). For desalination experiments, aqueous solutions containing 1.0×10–3 mol dm-3 of the ionic species were used. Flame atomic absorption (FAA) (Varian Model Plus 20) was used for elemental analysis. The concentration of I- was measured with an ion selective electrode (Orion Model 53-94). RESULTS AND DISCUSSION Comparison of the X-ray peaks and relative intensities with the standard NaA zeolite pattern shows that pure zeolite with the LTA-type structure is the only crystalline phase present in this sample, in agreement with our previous work (Malekpour et al., 2008). Fig. 2 displays the SEM images of the NaA zeolite membranes from the top and a side view. After 3 h of hydrothermal treatment, the surface of the seeded support was completely covered with a continuous crystalline layer. The film thickness, obtained from the SEM cross-section, is about 15 μm with 1-3 μm diameter particles and was almost the same at different areas. No defects and no holes between the NaA crystals were observed. Fig. 3 shows the X-ray diffraction pattern of the ZSM-5 zeolite membranes synthesized under hydrothermal heating compared with standard ZSM-5 zeolite crystals. It can be seen that, after 40 h, the typical diffraction peaks of ZSM-5 crystals appeared, suggesting the formation of pure ZSM-5 crystals on the surface of the substrate. SEM images of the synthesized membranes from the top and side view are shown in Fig. 4. Based on these images, after 40 h of hydrothermal treatment, the support was completely covered with 2–3 μm diameter spherical ZSM-5 crystals to a thickness of about 10μm, the crystals being well compacted together on the surface of the support. Initial experiments were performed to evaluate the quality of the membranes. For this propose, water removal from 2-propanol was achieved using the PV method. Pervaporation studies with a NaA membrane have been previously reported for water removal from highly concentrated 2-propanol solutions. Related results showed that NaA and ZSM-5 membranes achieved a very high separation factor and good permeation flux for removal of water (Abdollahi et al., 2007; Malekpour et al., 2008). Table 1 shows the separation factors and fluxes obtained in the pervaporation of 90 wt% 2-propanol– water solutions by as-synthesized membranes. Based on these data, NaA membranes show higher fluxes than ZSM-5 at similar feed concentrations and temperature. This is because the ZSM-5 zeolite membrane has a higher Si/Al ratio and is more hydrophobic than the NaA zeolite membrane. As the Si/Al ratio increases, the membrane becomes more hydrophobic and, consequently, the water adsorption on the membrane surface decreases. Although ZSM- 5 pore diameters are larger than those of zeolite A, the separation factors for NaA membranes are higher than those for ZSM-5 membranes, probably because of pore diameters (about 4 Å) that are more efficient and more specific for water molecules. The results obtained show that the as-synthesised membranes are defect free and of high quality. (a) Figure 2: SEM images of NaA zeolite membrane after a three stage synthesis: (a) top; (b) cross section Brazilian Journal of Chemical Engineering (b)PDF Image | DESALINATION OF AQUEOUS SOLUTIONS ZEOLITE MEMBRANES
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