5.2 Results and discussion
5.2.1 Potential of FAU and MFI zeolite in chromium removal
5.2.1.1 Influence of applied pressure on zeolite composite membranes
Figure 5.1 shows permeate flux profiles of FAU and MFI zeolite membranes with time for various applied pressures (69, 138, 207, 278 and 345 kPa) at a fixed cross flow rate of 1.11 × 10-6 m3/s with feed concentration of 1000 ppm. It is observed from these figures that the flux of zeolite composite membranes is augmented with an increase in the applied pressure owing to enhancement of driving force. It can also be noticed from Figure 5.1(a) that the variation of permeate flux with time is almost negligible in 1 h operation and similar patterns are also observed with MFI zeolite composite membranes (Figure 5.1(b)). The permeate flux varies almost linearly with increasing applied pressure for both the zeolite composite membranes.
This may be due to the fact that there is no significant contribution of additional transport resistance from concentration polarization and adsorption. On the other hand, the chromium flux is marginally lesser than to water flux of the membrane. This typical trend is obtained owing to the generation of osmotic pressure by the retained ions, which leads to decrease the effectual pressure on the membrane surface. Rashdi et al. (2015) also observed the improvement of permeate flux with enhancing applied pressure for the separation of heavy metal ions by cross flow nanofiltration with commercial NF270 membrane. Figure 5(c) illustrates the permeate flux of FAU and MFI zeolite membranes as a function of applied pressure. Compare to FAU membrane, the MFI membrane displays marginally higher flux value; this may be due to a bigger pore size of MFI membrane (0.272 µm) when compared to FAU membrane (0.179 µm).
The deposition of zeolites on the ceramic support acquires promising steady permeate flux with the removal of chromium. This may be due to better compatibility of liquid phase separation of both zeolite membranes.
Figure 5.2(a-b) shows percentage removal of chromium with time for FAU and MFI zeolite membranes at different applied pressures with fixed cross flow rate of 1.11 × 10-6 m3/s. For both
the zeolite membranes, the percentage removal of chromium gradually augments for all the studied pressure with the filtration time. This is possibly due to the development of concentration polarization until reach the steady state with time on the surface of the membrane. The percentage of chromium removal increases with an increment in the applied pressure. This is possible due to the formation of concentration polarization effect doesn’t occur at the initial period of operation and at low applied pressure.
Figure 5.1: Influence of applied pressure on permeate flux with time for (a) FAU zeolite membrane, (b) MFI zeolite membrane and (c) permeate flux as a function of applied pressure for zeolite membranes (feed concentration = 1000 ppm, natural pH~ 2.35)
0 500 1000 1500 2000 2500 3000 3500 4000 0.00002
0.00004 0.00006 0.00008 0.00010 0.00012 0.00014 0.00016 0.00018 0.00020
Time (s) Permeate flux (m3/m2s)
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa (b) MFI membrane
0 500 1000 1500 2000 2500 3000 3500 4000 0.00001
0.00002 0.00003 0.00004 0.00005 0.00006 0.00007 0.00008
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa
Permeate flux (m3/m2s)
Time (s) (a) FAU membrane
50 100 150 200 250 300 350 400
0.00000 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012 0.00014
Permeate flux (m3 /m2 s)
Applied pressure (kPa) FAU membrane
MFI membrane (c)
Conversely, it develops with increasing time owing to retention of ions within the membrane pores. When the applied pressure increases, the convective transport turns more significant than the diffusive transport. As a result, the percentage removal of chromium enhances with increasing pressure due to the dilution effect, as the higher transport of solvent flux would result in a dilution of permeate. Therefore, the higher applied pressure facilitates to attain higher removal of chromium (Gherasim and Mikulasek 2014).
Figure 5.2: Influence of applied pressure on removal (%) with time for (a) FAU zeolite membrane, (b) MFI zeolite membrane and (c) removal as a function of applied pressure (feed concentration = 1000 ppm, natural pH~ 2.35)
10 20 30 40 50 60
0 20 40 60 80 100
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa
Removal (%)
Time (min) (a) FAU membrane
10 20 30 40 50 60
0 20 40 60 80 100
Time (min)
Removal (%)
69 kPa 138 kPa 207 kPa 278 kPa 345 kPa (b) MFI membrane
50 100 150 200 250 300 350 400
50 60 70 80 90
Removal (%)
Applied pressure (kPa) FAU membrane
MFI membrane (c)
In order to get comprehensive information on the influence of applied pressure on the removal of chromium, the removal against applied pressure is plotted. Figure 5.2(c) illustrates the removal of chromium as a function of applied pressure for both the zeolite composite membranes. It represents increasing the percentage removal of chromium with an increase in the applied pressure (Mehiguene et al. 1999). The maximum removal of chromium is achieved as 82% for FAU membrane and 78% for MFI membrane under the applied pressure of 345 kPa.
The membrane surface charge is a major factor for the influential removal efficacy of the membrane while dealing with ionic solution and also it varies with the pH of the solution. The isoelectric point (IEP) of FAU and MFI zeolite membrane is estimated as 3.8 and 4, respectively, which are determined using zeta potential measurements (Chapter 4). It means, at pH of the solution less than IEP, membranes are positively charged and when pH of the solution is greater than IEP, membranes are negatively charged. FAU and MFI zeolite membrane have zeta potential value of +2.95 mV and +10.76, respectively, at natural pH (2.35) of chromium solution (1000 ppm). In aqueous solution, chromium is usually present in the form of HCrO-4, Cr O2 27- and Cr O2 24- based on the pH (Benhamou et al. 2013). In this study, the removal experiments were conducted at the natural pH of the chromium solution (~2.35). The chromium predominantly presents in the form of acid chromate ions (HCrO4-) along with H3O+ in this experimental condition (Benhamou et al. 2013; Basumatary et al. 2015). Since the membrane is positively charged at this pH condition, therefore, the repulsion between positively charged membrane and positive species (H3O+) will take place. As the cation and anion cannot act autonomously, thereby, HCrO-4 is also rejected to maintain electroneutrality of the system, as shown in Figure 5.3 (Benhamou et al. 2013; Kumar et al. 2015a; Basumatary et al. 2015). In addition to charge density, the removal of chromium also depends on the amount of zeolite
materials deposited on the ceramic support. The quantity of FAU zeolite deposition (1.128 g) on ceramic substrate is more than that of MFI (0.6844 g). Hence, the removal efficiency of FAU membrane is slightly higher compared to MFI membrane.
Figure 5.3: Schematic representation of chromium separation by zeolite membranes