Zeolite-Ceramic Composite Membrane
5.3 Results and discussion
5.3.2 Ultrafiltration of Cr (VI)
5.3.2.1 Influence of applied pressure on the performance of zeolite composite membranes
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for all zeolite composite membranes. Fig. 5.3(e) displays pure water flux as a function of applied pressure for all zeolite composite membranes. It is noticed that with the deposition of MCM-41, MCM-48 and FAU zeolite on the ceramic support, the pure water flux is drastically reduced for all zeolite composite membranes. It indicates that the pore size and water permeability of zeolite membranes are decreased, which justifies the pore size obtained from Hagen-Poiseuille equation.
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membrane. Fig. 5.4(d) illustrates the permeate flux of zeolite membranes (MCM-41, MCM- 48 and FAU) as a function of applied pressure. MCM-41 membrane displays marginally higher flux value, whereas MCM-48 offers less flux compared to other two membranes. This may be due to a slightly bigger pore size of MCM-41 when compared to other membranes.
Among these membranes, MCM-48 composite membrane has the lowest pore size. The deposition of FAU zeolite on the ceramic support acquires promising permeate flux during the separation of Cr (VI). This may be due to better compatibility of liquid phase separation and hydrophilicity nature of FAU zeolite membrane. Even though FAU zeolite membrane has slightly smaller pore size than that of MCM-41 composite membrane, it displays almost similar permeate flux of MCM-41 composite membrane at a fixed flow rate.
Fig. 5.4 Variation of permeate flux with time at different applied pressures for (a) MCM- 41,(b) MCM-48, (c) FAU and (d) permeate flux as a function of applied pressure (feed
concentration = 1000 ppm, natural pH~ 2.35)
0 10 20 30 40 50 60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Permeate flux× 105 (m3 m-2 s-1 )
Time (min)
69 kPa 138 kPa 207 kPa 276 kPa 345 kPa (C) FAU Membrane
0 10 20 30 40 50 60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Permeate flux× 105 (m3 m-2 s-1 )
(b) MCM-48 Membrane
Time (min)
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa
50 100 150 200 250 300 350 400
0.0 0.5 1.0 1.5 2.0 2.5
Permeate flux× 105 (m3 m-2 s-1 )
Applied pressure (kPa) MCM-41 Membrane
MCM-48 Membrane FAU Membrane (d)
0 10 20 30 40 50 60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Permeate flux× 105 (m3 m-2 s-1 )
Time (min)
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa (a) MCM-41 Membrane
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Fig. 5.5 Variation of rejection of Cr (VI) with time at different applied pressures for (a) MCM-41, (b) MCM-48, (c) FAU and (d)rejection as a function of applied pressure (feed
concentration = 1000 ppm, natural pH~ 2.35)
The variation of percentage rejection of MCM-41, MCM-48 and FAU zeolite composite membranes with time at various applied pressures for a fixed cross flow rate of 1.667 × 10-7 m3s-1 is presented in Fig. 5.5(a-c). For all the zeolite membranes, the rejection of Cr (VI) marginally increases with the duration of the process for all the studied pressure. This is
10 20 30 40 50 60
0 20 40 60 80 100
Rejection (%)
Time (min)
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa (a) MCM-41 Membrane
10 20 30 40 50 60
0 20 40 60 80 100
Rejection (%)
Time (min)
69 kPa 138 kPa 207 kPa 276 kPa 345 kPa (b) MCM-48 Membrane
0 10 20 30 40 50 60
0 20 40 60 80 100
Rejection (%)
Time (min)
69 kPa 138 kPa 207 kPa 276 kPa 345 kPa (c) FAU Membrane
50 100 150 200 250 300 350
0 20 40 60 80 100
Rejection (%)
Applied pressure (kPa)
MCM-41 Membrane MCM-48 Membrane FAU Membrane
(d)
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at the membrane surface (Danis and Keskinler 2009). The rejection also augments with increasing applied pressure for all the composite membranes. At the initial stage of process as well as at lower applied pressure, the formation of concentration polarization effect doesn’t take place. However, with enhancing time, it starts forming due to retention of ions within the pore of the membrane. When the applied pressure increases, the convective transport becomes more important than the diffusive transport and hence the retention will increase.
Therefore, Cr (VI) rejection increases with enhancing pressure due to the dilution effect, as the higher transport solvent flux would result in a dilution of permeate. As a result, higher rejection is observed at an elevated pressure (Gherasim and Mikulasek 2014).
In order to get comprehensive information on the influence of applied pressure on the removal of Cr (VI), the rejection against applied pressure is plotted. Fig. 5.5(d) illustrates the rejection of Cr (VI) as a function of applied pressure for zeolite composite membranes. It represents increasing rejection of Cr (VI) with an increase in the applied pressure (Mehiguene et al., 1999). A maximum rejection of 75, 77 and 82%, is achieved using MCM-41, MCM-48 and FAU zeolite composite membrane, respectively at a higher applied pressure (345 kPa).
The surface charge is an important factor for determining the separation efficiency of the membrane with ionic solution and also it varies with the pH of the solution. The isoelectric point (IEP) of MCM-41, MCM-48 and FAU zeolite membrane is 3.9, 3.2 and 3.8, which are determined using zeta potential measurement. MCM-41, MCM-48 and FAU zeolite membrane have zeta potential value of +14.6, +2.486 and +2.95 mV, respectively, at natural pH (2.35) of chromium solution (1000 ppm). In addition to charge density, the removal of Cr (VI) is also influenced by the amount of zeolite materials deposited on the ceramic support.
The quantity of FAU zeolite deposition (1.26 g) on ceramic support is more than that of MCM-41 (0.89 g), whereas zeta potential value of MCM-41 is higher than the FAU zeolite, and both membranes have almost similar IEP value. However, the rejection of FAU zeolite
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composite membrane is found to be higher when compared to MCM-41 membrane and this may be due to more hydrophilic nature of FAU zeolite membrane. The removal of Cr (VI) for MCM-48 membrane is slightly higher compared to MCM-41 membrane due to a larger quantity of MCM-48 depsoition (1.28 g) on the ceramic support. Among the three zeolite membranes, the rejection of FAU membrane is higher as it has almost equal amount of deposition with MCM-48 and its hydrophilicity is also greater than that of other two composite membranes.