Application of kaolin based ceramic membrane for liquid phase separation
3.10. Results and Discussion
3.10.2. Microfiltration of TiO 2 NPs
filtration of cellulase fermentation broth using attapulgite microfiltration membrane having a mean pore size of 0.12 µm.
Permeate sample collected at each applied pressure was subjected to turbidity measurement to estimate the recovery efficiency of TiO2 NPs, and the results are presented in Fig. 3.16b. As apparent from the figure, the recovery performance of the membrane is found to be excellent with almost 100% recovery of TiO2 NPs achieved at all the applied pressures. The possible reason for complete recovery of NPs from its suspension is that the membrane with a small pore size restricts the passage of larger sized TiO2 agglomerates through the membrane in the permeate side (Shukla et al., 2000). Similar results were observed by Qin et al. (2015), where fly ash based ceramic membranes having mean pore diameters of 1.25 and 0.3 µm displayed complete recovery of suspended particles from kiwi (fruit) juice due to small pore size of the membranes as compared to large-sized suspended particles present in the juice.
Fig. 3.16 Effect of applied pressure on (a) permeate flux and (b) recovery of TiO2 NPs (Feed concentration = 0.1 wt%; pH = 6.5; CFV = 2.41×10-3 m/s)
3.10.2.2. Effect of cross flow velocity
The membrane performance in terms of flux and recovery at different cross flow velocities (2.41×10-3, 4.02×10-3 and 5.63×10-3 m/s) for treating TiO2 NPs suspension is depicted in Fig.
3.17. It is observed from Fig. 3.17 (a and b) that a high cross flow velocity helps in enhancing
the permeate flux almost linearly. An increase in cross flow velocity enhances the shear stress on the surface of the membrane, i.e., increasing sweeping action over the membrane, resulting in a decrease in the thickness of the filtration layer on the membrane surface (Tanudjaja et al., 2017). The permeate flux is only affected by membrane resistance during the initial stages of MF operation, and as the filtration layer grows, the flux decreases. Conversely, mass boundary layers tend to decrease with increasing cross flow velocity and mass transfer coefficients tend to increase (Yang et al., 2017). As evident from Fig. 3.17c, TiO2 recovery performance of the membrane in terms of turbidity is almost the same and found to be 100% at all the investigated cross flow velocity values. This result indicates that the membrane's recovery performance is not affected by the cross flow velocity as the average size of TiO2 agglomerates in the feed is significantly greater than the membrane pore size. Hence, complete recovery of TiO2 from the suspension is attributed to the simple size exclusion mechanism. The obtained result is consistent with the results reported by Zhao et al. (2002) for the removal of TiO2 particles using α-alumina membrane with an average pore diameter of 0.2 μm, which is substantially smaller than the particle size (2.89 μm).
Fig. 3.17 Effect of cross flow velocity on (a) permeate flux (b) linear plot of permeate flux vs. cross flow velocity and (c) recovery of TiO2 NPs (Feed concentration = 0.1 wt%; pH = 6.5; Applied pressure = 276 kPa)
3.10.2.3. Effect of feed concentration
The permeate flux profile with time for different TiO2 concentrations (0.05, 0.1 and 1 wt%) at an applied pressure of 276 kPa and cross flow velocity of 2.41×10-3 m/s is shown in Fig. 3.18 (a). As expected, the permeate flux decreased with increasing TiO2 concentration (Fig. 3.18a).
Comparison of the flux profile for the three feed concentrations, it was observed that at higher TiO2 concentration (1 wt%), the flux declined drastically due to rapid formation of filtration layer on the membrane surface at the beginning of microfiltration itself, which compacted
quickly to form a hard and thick filtration towards the end of the MF process. Besides, the quantity of permeate produced was significantly less than that of the other two feed concentrations (0.05 and 0.1 wt% TiO2) due to the thick and stiff filtration layer formation on the membrane surface. Similar results were reported by Akamatsu et al. (2020) for the microfiltration of Bovine serum albumin (BSA) using polyethylene microfiltration membrane (0.06 µm); the permeate flux decreased when the BSA concentration was increased from 10 to 5000 mg/L. Fig. 3.18 (b) demonstrates 100% recovery at all three feed concentrations. These results also reveal that TiO2 NPs aggregated at all the studied feed concentrations, which helped to recover TiO2 completely by microfiltration. Fig. 3.18 (c) shows comparison of the images of feed and permeate samples, which reveals that the permeate obtained is transparent due to complete recovery of TiO2 particles by microfiltration.
Fig. 3.18 Effect of different feed concentrations (0.05 – 1.0 wt% of TiO2) on (a) permeate flux and (b) recovery, and (c) images comparing feed and permeate samples (Applied pressure =
3.10.2.4. Effect of pH
It is well reported that the pH will strongly influence zeta potential, which ultimately influence TiO2 NPs agglomerate size in the feed (Weimin et al., 2001). Hence, in order to understand the effect of feed pH on the average permeate flux and recovery rate, microfiltration of TiO2 NPs was examined at five different pH of feed (4, 5.1, 6.5, 9, 11) containing a fixed TiO2
concentration of 0.1 wt%. The results obtained are presented in Fig. 3.19 (a), which shows that the maximum flux is produced using the feed at pH 5.1. Compared with the experimental condition at pH 4 of the feed, the experiment carried out at feed pH 5.1 resulted in the highest flux. At pH 5.1(IEP), the TiO2 NPs possessed zero charge and its size (i. e., aggregates) maximum (Fig. 3.15b). Such large particles may produce filtration with high porosity, whereas at a pH above and below the IEP, fine particles offer more resistance to permeate flux. It has been well documented that the permeate flux of various nanoparticles (TiO2 and SiO2) through ceramic membrane is enhanced at pH close to its IEP, whereas at a pH above and below its IEP, the permeate flux decreases (Le et al., 2019). Vyas et al. (2000) reported that the permeate flux increased with the large particles owing to scouring of the filtration surface. A previous study also indicated that for the separation of neutral charged nanoparticles by ceramic microfiltration membrane, the filtration layer formed on the membrane surface is thin due to the back transport of particles from the membrane (Le et al., 2019). At all the investigated feed pH values, 100% recovery of TiO2 NPs was observed (Fig. 3.19 (b)).
Even though the membrane characterization results clearly demonstrated that the developed membrane is stable in the acidic environment (pH 4 and 5.1), it is also equally important to know if any leaching of materials from the membrane occurs when the membranes are used to treat acidic solutions. In this regard, after performing the microfiltration experiment with the feed suspension at pH 4, few characterizations were done to identify any changes in the membranes. FESEM images portrayed in Fig. 3.20 revealed no enlarged pores present after
treating the membrane with the feed suspensions at pH 4. Comparison of XRD results of the fresh and used membrane at feed pH 4 also elucidated that there is no change in its composition as similar peaks appeared in both cases (Fig. 3.20 (c)). Moreover, the turbidity of the permeate was zero. From these observations, it can be concluded that at this experimental condition (feed pH 4; applied pressure 276 kPa; CFV 2.41×10-3 m/s), leaching from the membrane was absent.
It is clear from the above discussion that the reason for obtaining high flux at feed pH 4 and 5.1 is attributed to the size of TiO2 NPs at these values.
Fig. 3.19 Effect of pH on (a) average permeate flux and (b) recovery of TiO2 NPs (CFV = 2.41×10-3 m/s; Applied pressure = 276 kPa; Feed concentration = 0.1 wt%)
Fig. 3.20 (a-b) FESEM image of inner surface of membrane before and after microfiltration of TiO2 NPs at pH and (c) XRD analysis