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Synthesis and performance characterization of PS-PPEES nanoporous membranes with nonwoven porous support
*Corresponding authors. Address: Membrane Technology Division, Department of Chemistry, National Institute of Technology, Surathkal, Mangalore 575 025, India. Tel.: +91 824 2474000x3206; fax: +91 824 2474033 isloor@yahoo.com (Arun M. Isloor)
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer-review under responsibility of King Saud University.
Available online 30 June 2011
Abstract
The present work describes about the synthesis and characterization of Polysulfone blend nanoporous membrane with nonwoven support. This Nonwoven support provides mechanical strength to membrane while filtration process and minimizes membrane fouling. Hence it helps in better membrane performance in terms of salt rejection, improved flux, thermal stability and fairly increases in proton conductivity. In this work we have used K.C.270 nonwoven material consisting of fine polyester fibers and has a thickness of below 110 μm.
Keywords
PS-PPEES
Nonwoven support
Flux
Salt rejection
Proton conductivity
1 Introduction
In the recent past, membrane processes gradually have found their way into industrial applications and serve as viable alternatives for more traditional processes like distillation, evaporation or extraction. Different membranes are categorized and they are used for different purpose of separation process, depending on their physical and chemical properties (Lhassani et al., 2001). Among membranes, Nanofiltration (NF) membrane is characterized by its surface charge and Nano pore size (Szymczyk et al., 2003). NF membrane shows high efficiency with a reasonable water flux at relatively low pressures, therefore Nanofiltration (NF) membranes are used in a wide range of drinking water, wastewater, (Al-Sofi, 2001) and industrial applications (Van der Bruggen and Vandecasteele, 2003). Nanofiltration (NF) membranes can retain multivalent ions (e.g., calcium, magnesium, aluminum, sulfates) (Bowen et al., 1997) and non-ionized organic compounds with a molar mass exceeding about 300 g/mol. Hence it shows potential applications such as selective demineralization (for water softening) (Schaep et al., 1998) and concentration of organic compounds with low molar mass. As a consequence, scientists and industrialists nowadays feel more confident about what can be expected from a NF membrane, and more and more applications proved to be successful including water treatment, wastewater and desalination of dyestuffs, acid and caustic recovery and color removal (Zhijuan et al., 2006). In this study polysulfone based novel NF membranes were synthesized and their characterizations were made based on Tg, proton conductivity and flux rejection study. Above mentioned performance study gave a number of conclusions which are very helpful in this further research.
It is a well known fact that membrane filtration technology was greatly limited by membrane fouling. Membrane fouling phenomenon increases operation and maintenance costs by deteriorating membrane performances (flux decline vs. time, zeta potential changing during time, etc.) and eventually shortening membrane life (Ahmed and Robert, 2007). Hence it is sensible to use nonwoven porous support which generally has the superior dirt holding capacity and does not have a much wider pore size distribution. Such quality along with reinforcement of mechanical strength to the membranes gives better performance with decreased fouling (Violleau et al., 2005).
2 Experimental
2.1 Materials and instruments
Polysulfone having molecular weight of 35,000 PS, poly(1,4-phenylene ether ether-sulfone), PPEES were obtained from Sigma Aldrich in the form of semitransparent beads. Reagent grade N-methyl pyrrolidone, NMP was obtained from Merck-India and was used without any further purification. Non woven support, was from K.C.270 Cranemat KC & SC made up of polypropylene. All analytical grade chemical reagents used in the experiment were purchased from Merck India Ltd. Glucose, sucrose and polyethylene glycols (PEGs 600, 800, 1000 Da) for the MWCO test were measured using CL 157 Colorimeter. IR spectra were recorded using Nicolet Avatar 5700 FTIR (Thermo Corporation) spectrometer. Impedance study was done using ACM Instruments, England. SEM images of the cross section of the newly prepared membranes were recorded on Jeol JSM-84. DSC study was carried out on a Shimadzu DSC 60 instrument, Japan. The permeation experiments were performed by a self fabricated salinity checking apparatus with membrane disk which has an effective area of 6.5 cm2. The deionized water used for preparation of the salt solutions was obtained through demineralization using ion exchange followed by reverse osmosis. Surface roughness was determined using Atomic Force Microscopy (Nanosurf, EasyScan2).
2.2 Membrane synthesis
Solutions containing different wt.% of PS and PPEES (Table 1) in 4.5 ml of 1-methyl-2-pyrrolidone (NMP) were prepared by mild stirring for one day at constant temperature of 75 °C. So obtained viscous solution was casted over non woven support (K.C.270) using K-Control coater. Casted membrane was again heat-treated for 45 s at 220 °C, then washed thoroughly with deionized water and immersed in de-ionized water for 24 h (Chitrakara et al., 2011).
Membrane code
Wt.% composition (PS)
Wt.% composition (PPEES)
M1
90
10
M2
80
20
M3
70
30
M4
60
40
2.3 Water up take
The swelling characteristics were determined by water uptake measurements. The membrane samples were first immersed in deionized water until there was no weight difference in the membrane. Further wet membrane then blotted to dry to remove surface droplets was quickly weighed. The wet membranes were vacuum dried at 80–100 °C and weighted again. The water uptake of the membranes was calculated by weight gain of absorbed water with reference to the dry membrane and reported as weight percent water absorption. The water uptake can be calculated as follows (Chitrakar et al., 2009).
where mwet is the weight of wet membrane and mdry is the weight of dry membrane. The result of the same has been presented in Table 2. These observed water uptakes are much more than our earlier work (Chitrakar et al., 2009).
Membrane code
Water up-take (%)
M1
45
M2
57
M3
68
M4
72
2.4 Permeation experiment
Salts with different valence distribution are used for NF membrane experiments to investigate membrane properties. The permeability of pure water through this NF membrane was also measured. Flux, F (l/m2 h), was calculated as Eq. (1):
Schematic representation of the self made salinity checking unit.
2.5 Membrane hydraulic resistance (Rh)
To determine the Membrane hydraulic resistance (Rh), the pure water flux of the membrane was measured at different transmembrane pressure (ΔP). The variation of pure water flux was plotted as a function of transmembrane pressure for all the prepared membranes. Membrane hydraulic resistance (Rm) was determined from the inverse of slopes, using following equation (Abdoul et al., 2007).
2.6 Molecular weight cut-off (MWCO)
Molecular weight cut-off or MWCO refers to the lowest molecular weight solute (in Daltons) in which 90% of the solute is retained by the membrane or the molecular weight of the molecule (e.g., globular protein) that is 90% retained by the membrane. To determine the MWCO for the resultant membrane, solutes with molecular weight range of 180–1000 Da were chosen, namely glucose, sucrose and polyethylene glycols (PEGs 600, 800, 1000 Da) at a concentration of 1000 mg/L. The rejection was obtained according to Eq. (2) (Ruihua et al., 2009).
2.7 Structural characterization
Surface analysis can be done with different tools, each one with its own specificity with regard to the conditions of use and to the information it provides. In our study, we used scanning electron microscope (SEM) and an atomic force microscope (AFM). The membrane was cryogenically fractured in liquid nitrogen and then sputtered with gold. SEM provides information on surface porosity and layer thickness. To prepare samples for AFM, the membranes were dried in vacuum at room temperature. The membranes were taken out of water and air-dried and they were ready for AFM observation. AFM Imaging was performed in tapping mode with resonant frequency of 200–400 kHz, nominal tip radius of 5–10 nm. Atomic force microscopy (AFM) provides surface RMS roughness (Chitrakar et al., 2009, 2011).
2.8 DSC analysis
Differential-scanning calorimetry is a thermodynamic technique widely used for studying thermal characteristics of the membrane. The ability to monitor phase transitions in polymeric membrane has not only provided data on thermodynamic stability for these important molecules, but also made it possible to examine the details of unfolding processes and to analyze the characteristics of intermediate states involved in the melting of membrane polymers. It is well known that PS has a high melting point and hydrophobic. A DSC-60 Shimadzu calorimeter was used to analyze the thermal behavior of differently processed membranes, with the heating rate of 15 °C /min up to 300 °C. DSC curve of the resultant membranes were studied with increase in temperature at the rate of 10 °C/min. Each sample was subjected to several heating/cooling cycles to obtain reproducible Tg values. The initial onset of the change midpoint of slope in the DSC curve is taken to be the Tg (Helen et al., 2007; Sturtevant, 1996).
3 Results and discussion
3.1 Pure water permeability
At different pressure (bar), flux for pure water is shown in Fig. 2. The plot depicts a linear relationship between the pure water flux and transmembrane pressure. It is seen that there is a slow and steady increase of pure water flux with respect to decrease in PS wt.%. This is due to the fact that PPEES leads predominantly to swelling rather than leaching out from the membrane-forming system. Consequently, the flow path in the membrane was reduced and hence the increase in the flux was not steep. Flux values of M1 and M2 further reinforces above explanation (Ruihua et al., 2009).
Pure water flux at different pressure (bar).
3.2 Membrane hydraulic resistance (Rm)
Membrane hydraulic resistance was calculated from the inverse of slopes of the corresponding water flux versus pressure lines and is shown in Table 3. It is evident from these values that as the concentration of PS increases in the blend system, the Rm decreases. This can be explained by the fact that PS is relatively more hydrophobic in nature whereas increase in PPEES leads to the formation of pores, which in turn increases the flux thereby decrease in membrane resistance (Sivakumar et al., 2000). The values of the same have been represented in Table 3.
Membrane code
Hydraulic membrane resistance (Rm), m2 h/L
M1
1.45
M2
1.36
M3
1.27
M4
1.19
3.3 Effect of operating pressure on membrane performance
To illustrate the effect of operating pressure on newly synthesized membrane, the feed concentration (NaCl) was fixed at 1000 mg/L. Water flux across the membrane increases in direct relationship with increase in feed water pressure but the rejection of salts by the NF membrane is more complicated as it is depends on both molecular size and Donnan exclusion effects (Szymczyk et al., 2003), it is well understood that solute (NaCl) is hydrophilic in nature where as membrane as a whole is hydrophobic. This hydrophobic interaction makes minimum passage for salt, thus as shown in Figs. 3–6 increased feed water pressure increases salt rejection but, to a lower extent than water flux. Because NF membranes are imperfect barriers to dissolved salts in feed water, there is always some salt passage through the membrane. As feed water pressure is increased, this salt passage is increasingly overcome as water is pushed through the membrane at a faster rate than salt can be transported. However, there is an upper limit to the amount of salt that can be excluded via increasing feed water pressure. As the plateau in the salt rejection curve (Figs. 3–6) indicates, above a certain pressure level, salt rejection no longer increases and some salt flow remains coupled with water flowing through the membrane.
Performance of membrane M4 with operating pressure.
Performance of membrane M3 with operating pressure.
Performance of membrane M2 with operating pressure.
Performance of membrane M1 with operating pressure.
3.4 Effect of feed concentration on membrane performance
To describe the effect of feed concentration on membranes, MgCl2 and NaCl solutions with different concentration were used. The results are shown in Tables 4 and 5 respectively. Both the rejection and flux decrease with an increase in feed concentration, especially the NaCl solution. As the feed concentration increases, the repulsion effect of the charged membrane on the ions in feed decreases, leading to a lower rejection; meanwhile the double electrical layer near to membrane surface became thicker, thus the flux decreased (Petersen, 1993).
Membrane code
MgCl2 feed concentration, mg/L
Flux, L/m2/h
%R
M1
1000
46
68
2000
37
51
3000
30
44
M2
1000
50
65
2000
35
58
3000
29
51
M3
1000
42
62
2000
39
59
3000
28
48
M4
1000
40
60
2000
37
52
3000
29
47
Membrane code
NaCl feed concentration, mg/L
Flux, L/m2/h
%R
M1
1000
47
67
2000
33
57
3000
29
41
M2
1000
46
60.2
2000
32
56
3000
27
35
M3
1000
38
57
2000
37
54
3000
25
46
M4
1000
35
55
2000
40
50
3000
28
43
3.5 Effect of the salt solution on membrane performance
The average rejection and flux of different synthesized membranes (M1, M2, M3 and M4) for salt solutions are listed in Table 6. The rejection order is CaCl2 > MgCl2 > MgSO4 > NaCl > KCl > Na2SO4 > K2SO4. For CaCl2 > MgCl2 > NaCl > KCl and MgSO4 > Na2SO4 > K2SO4, corresponding to the increasing order of the cation charge densities, because the active layer has quaternary ammonium groups contribution and has a stronger repulsion to Mg2+ and Ca2+ than Na+ and K+, Mg2+ and Ca2+ are rejected easily. However, for MgSO4 > NaCl, it may be speculated that the repulsion force on cation is much stronger than the attraction one on anion.
Salt conc., 2000 mg/L
Flux, L/m2/h
%R
MgCl2
35
55
MgSO4
43
43
Na2SO4
39
40
K2SO4
42
33
KCl
39
38.5
CaCl2
43
60
NaCl
31
45
3.6 MWCO of membrane
The curve for the rejection for the model organic solutions against molecular weight is shown in Fig. 7. The MWCO was the molecular weight of organic substance with a rejection of 90%. The MWCO of the all resultant membrane lies approximately between 900 Da to 910 Da.
MWCO curve for newly synthesized NF membrane.
3.7 Morphology of membrane
Tapping mode AFM images give information related to surface features of the membranes. Figs. 8 and 9 show the surface profile of membrane M1 and membrane M4 respectively. The membrane containing 90 wt.% of PS shows almost smooth and less intense crater and valley (Fig. 8) with reduced surface roughness rather than more surface roughness as like in M4 (Fig 9). This reduction in roughness of membrane clearly affirmed that PPEES incorporated well with PS matrix. Such a membrane matrix may give smooth electrical connectivity leading to better ionic conductivity. This explanation matches with our proton conductivity data (Pramod et al., 2010). Figs. 10 and 11 show the surface SEM pictures of membranes M4 and M1 respectively. The cross-section image of the composite membrane in Figs. 12 and 13 clearly shows that there is a thin selective layer on a finger-like ‘micro voids’ support layer suggesting the composite structure of this membrane. Meanwhile, the surface picture (Figs. 10 and 11) gives an overall view of pore size and pore distribution from SEM image, it is certain that the spongy layer provides the sustained structure that could support high pressure. The porous finger-like layer allows transporting the permeated solvent (Blanco et al., 2006).
AFM surface roughness image of membrane M1.
AFM surface roughness image of membrane M4.
SEM surface image of the membrane M4.
SEM surface image of the membrane M1.
SEM cross section image of the membrane M4.
SEM cross section image of the membrane M1.
3.8 DSC analysis
Fig. 14 shows the thermograms of DSC measurement of the membranes. The Tg values of the M1, M2, M3, M4 membranes are 223, 213, 203, 200 °C, respectively. The relationship between Tg and the composition of blended membranes depends upon percentage of poly sulfone, lower PS composite membrane shows Tg range from 200 °C to 203 °C whereas higher PS composition show Tg range from 213 °C to 223 °C. Table 8 shows the Tg values of the synthesized membranes. Results were summarized in Table 7.
DSC curve of the membranes.
Membrane code
Tg (°C)
M1
223
M2
213
M3
203
M4
200
4 Conclusion
The result shows newly synthesized NF membrane has better salt rejection performance capacity than our previous work on NF membrane without nonwoven porous support. Increased performance of membrane confirms that supporting material (K.C.270) helps in minimizing membrane fouling. Present membrane shows increased rejection percentage for divalent cation than monovalent cation. Noticeable increase in the water uptake was observed, which play vital role in proton hoping to exhibit proton conductivity. Interestingly it was observed that, increase in polysulfone concentration increases Tg value whereas increase in PPEES content increases number of pores.
Acknowledgments
The Authors extend their appreciation to The Deanship of Scientific Research at King Saud University for the funding the work through the research group project No. RGP-VPP-207.
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