A Study on the Effect of Polymer Concentration &
Surfactant on Performance and Molecular
Orientation of Nanofiltration-Surfactant Membranes
S. H. Zainal; A. R. Hassan*; M. H. M. Isa
Industrial Membrane Technology Laboratory (IMTL), Department of Industrial Chemical Technology Faculty of Science and Technology, Islamic Science University of Malaysia (USIM, Malaysia
*Corresponding Author E-mail:[email protected]
Abstract:
In this study, an asymmetric nanofiltration-surfactant (NFS) membrane was prepared via simple dry/wet phase inversion technique. A newly dope formulations consisting of different surfactant (SDS, CTAB) and polymer concentration (17wt% - 21wt %) were developed. The effect of these parameters on membrane performance and molecular orientation were examined in terms of pure water flux (PWF) and salt rejection. Experimental data showed that, at different ranges of polymer concentration, NFS membranes results high PWF ranging from 9.02 to 127.36 L/m2h at 5 bar operating pressure. Significantly, the addition of SDS and CTAB surfactant were found to promote the increasing of PWF up to 900 L/m2h. In addition, the NFS membranes also demonstrated of good salt rejection from 47% to 92%. Meanwhile, FTIR analysis revealed that the addition of hydrophilic surfactants into polymeric membranes creating of good molecular orientation and broader spectrum which thus produced a better membranes performance and characteristics.
Keywords: Nanofiltration; surfactant; water flux; salt rejection; molecular orientation
1. Introduction
Nanofiltration (NF) membrane is one of promising technologies for the separation of neutral and charged solutes in aqueous solutions. It has been largely developed and commercialized over the past decades.
Having a molecular weight cut-offs (MWCO) between reverse osmosis (RO) membrane and ultrafiltration (UF) membranes, the MWCO of NF membrane has ranges from 200 to 2000 Da. Another thing that makes NF membrane used widely is the separation of electrolytes due to the materials containing charged groups [1]. In this study, membranes are prepared via phase inversion process where it has been introduced by Loeb and Sourirajan. In this process, polymer solutions are cast on a substrate/glass and immersed and precipitated in a water bath. Through the process, phase separation occurs in the film where the solvent in the casting solution is exchanged with a non-solvent result on the formation of dense top layer and porous sub-layer of asymmetric membrane. Preparation of asymmetric membrane would depend on kinetic parameters, as well as on thermodynamic parameters. Thus, the materials selection
such as polymer, solvent and non-solvents is the most important for the fabrication of asymmetric membranes for
their applications [2]. Addition of surfactant as additive is the new material used in the formulation of NF membranes. Surfactant has been used in soaps, laundry detergents, dishwashing liquids and shampoos are organic chemicals that reduce surface tension in water and other liquids. Surfactants are classified by their own behavior and have their own properties. Surfactant also can be considered as amphiphilic due to the two parts which are hydrophilic part and hydrophobic part [3]. Therefore, the effect of surfactant on membrane performance has been studied by several researchers. According to Mansourpanah, addition of CTAB in the polymer solution will enhance the water flux as well as permeation of salt solutions [4]. Rahimpour reported that addition of small amount of SDS can influence macrovoids formation [5].
Thus, the aim of this study is to investigate the effect of polymer concentration and surfactant types on performance and molecular orientation in polymeric phase inversion asymmetric nanofiltration surfactant (NFS) membranes.
2. Materials and Method
2.1 Materials
Polyethersulfone (RADEL A-300) supplied by SOLVAY was used as polymer. 1- Methyl-2- Pyrrolidone (NMP) with analytical purity of 99.5% was purchased from Merck and distilled water was used as solvent and non- solvent agents respectively. Ethanol and n-hexane both
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are from Merck was used as pre-treatment solutions in membrane fabrication. Polyethylene glycol (PEG) with average molecular weight 600g/mol was purchased from Merck and used as additive. Cetyltrimethylammonium bromide (CTAB) with molecular weight 364.5g/mol from EMD Chemicals and Sodium dodecyl sulfate (SDS) with molecular weight of 288.37g/mol from Merck were used as surfactant addition in the polymer solution.
2.2 Membrane Preparation and Fabrication
Polyethersulfone was dissolved at about 50°C. PEG 600 was added when the entire polymer dissolved. The solution must be stirred for 1 hour to get a homogenous mixture. Then, surfactant was added 3 hours before the polymer solution was completed. Asymmetric nanofiltration-surfactant (NFS) membranes were fabricated according to dry/wet phase separation process.
The casting process was conducted at room temperature (30±2°C). Small amount of polymer solution was poured onto glass plate with casting knife setting at 150μm. After the membrane has been casted, the glass plate support together with the membrane was then immersed into the coagulation bath. When coagulation was completed, the membrane was immersed in water bath for 24 hours.
Then, it will immerse in ethanol for another 24 hours.
Finally, membrane will soak in n-hexane for 2-3 hours before dried at room temperature at least 24 hours.
2.3 Membrane Performance Evaluation
The permeation test was conducted by using a simple dead-end permeation cell. Prior to the testing, each membrane was subjected for the passages of the first 10ml permeate and it was collected for concentration analysis. The volume flux was calculated as follow;
(1)
The salt permeation test was carried out using 0.01M NaCl for monovalent salt and 0.01M for multivalent salts (MgSO4, MgCl2, Na2SO4) at 4 bar of operating pressure.
The volume flux was calculated as follow;
(2)
(3)
3. Result and Discussion
3.1 Pure Water Permeation (PWP)
Pure water permeation test was determined by plotting Jw (water flux) against pressure applied. From the results, normal nanofiltration membrane shows low water flux as
polymer concentration increases while water flux increases as operating pressure increases.
Fig. 1 shows PES 17% shows increasing of water flux as the pressure applied increase. At 5 bar, PES 17%
achieved 127.36 L/m2h of water flux. However, as the polymer concentration increase, the water flux becomes decrease although the pressure increases. PES 21% has the lowest water flux of about 10.60 L/m2h at the same operating pressure. This shows that, polymer concentration become the parameter that affect the membrane performance as well as operating pressure.
Addition of PEG 600 as additive shows the same linear graph on water flux. Besides, additive also shows significant difference between normal nanofiltration membranes. Different molecular weight of PEG will give different morphological structure of prepared membrane.
This will lead to increasing the performance of the membrane since PEG 600 acts as pore former in the polymer solution.
Figure 1. Pure water permeation of nanofiltration membrane with different concentration
Figure 2. Pure water permeation of PES/PEG 600 of nanofiltration membrane
Fig. 2 shows addition of PEG 600 in the polymer solution. The same concentration of additive was used for each different polymer concentration. Like normal nanofiltration membrane, PES 17% shows higher water flux among others membrane concentration. However, addition of additive shows slightly increases of about
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(a)
(b) 214.72 L/m2h at 5 bar. As polymer concentration
increase, water flux becomes decreased. At the same operating pressure, PES 21% shows the lowest water flux of about 68.19 L/m2h. This finding indicate that denser membrane were produced.
Figure 3. Pure water permeation of PES/PEG/SDS
Study has been done shows addition of small amount of SDS will increase the membrane performance. This has been that addition of SDS shows the higher water flux compared with other membranes. From Fig. 3, addition of SDS on 17 % of PES shows highest water flux about 851.85 L/m2h at 5 bar operating pressure. As we can see from Fig. 3, water flux will increase when pressure increases. However, as polymer concentration increase, the water flux becomes decrease. This is show by PES 21% has the lowest water flux of about 303.30 L/m2h at 5 bar. It has been study by Amirilargani that, addition of surfactant on membrane casting solution results in increase pure water permeability [6]. As in this study the effect of different types of surfactant on membrane performances, another surfactant has been used which is CTAB as cationic surfactant. Addition of CTAB has increased the macrovoids formations of the membrane.
Figure 4. Pure water permeation of PES/PEG/CTAB of nanofiltratiom membrane
Fig. 4 shows the water flux of nanofiltration membrane containing CTAB as surfactant. As usual, membrane with lower polymer concentration shows higher water flux. In this case, PES 17% with CTAB membrane gives water flux of about 911.63 L/m2h at higher operating pressure.
This is shows that CTAB will produced the lowest flux
among others prepared membrane. Mulijani et al., reported that the addition of CTAB in the casting solution will increased the pure water permeation and leads to slight decrease in salt rejection [7].
3.2 NaCl Rejection
For the nanofiltration process, the membrane productivity is expressed as the permeate flux through the membrane.
For the NaCl solutions, the flux rate of the solutions (Jv) containing small molecules, such as salt rejected by the membrane, is a function difference between volume permeation rate and the membrane area prior to testing, the experiment was carried out with a 0.01M NaCl solutions under pressure at about 4 bar.
Figure 5. (a) Flux of NaCl solution, (b) Percentage Rejection of NaCl
From Fig. 5, flux for NaCl solutions decreases when polymer concentration increases. This indicates that there is influence of ions (Na+, Cl-) on membrane performance.
The lowest flux of NaCl is shows by membrane with CTAB of about 5.973 L/m2h. However, membrane with higher polymer concentration and addition of CTAB shows higher rejection than membrane with SDS of about 84%. Higher rejection on NaCl shows that the surface of the membrane is active to reject NaCl ions. Membrane containing SDS shows low rejection due to the porous membrane structure. At 21% of polymer concentration, membrane containing SDS give about 61% of NaCl rejection.
3.3 Multivalent Salts Rejection (MgCl
2, MgSO
4, Na
2SO
4)
0.01M of each multivalent salt concentration is used in
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(a)
(b)
(a)
(b)
(a)
(b) order to determine the flux and rejection of multivalent
ions by nanofiltration-surfactant (NFS) membrane. The operating pressure was fixed at 4 bar. Fig. 6 shows at 0.97 L/m2h, PES 21% with CTAB can reject almost 74% of MgCl2. For the same polymer concentration but different types of surfactant, membrane with SDS shows high flux of about 110.56 L/m2h and rejection shows slightly decreases to 65%.
Figure 6. (a) Flux of MgCl2, (b) Rejection of MgCl2
For MgSO4 solutions, membrane with SDS always shows high flux and low in rejection. This is might due to the structure of the membrane which is more porous than other membrane. Test with MgSO4 shows that at 3.64 L/m2h of permeation flux, PES 21% with CTAB gives promising result of rejection about 92% while membrane with SDS at permeation flux of 18.17L/m2h shows low rejection of 74% at the same polymer concentration as shown in Fig. 7. This can be conclude that as polymer concentration increase, the flux will decrease and lead to the increasing of rejection. This indicate that increasing in polymer concentration will make the membrane denser.
Figure 7. (a) Flux of MgSO4, (b) Rejection of MgSO4
Fig. 8 shows the experimental results of prepared membrane tested with Na2SO4 salt solutions. The same result was obtained by Na2SO4 solutions where membrane with SDS as surfactant produces low flux with high rejection. PES 17% is the least polymer concentration used resulted of 225.926 L/m2h of permeation flux and gives rejection of about 55%. However, PES 21%
containing CTAB as surfactant gives lower flux of about 9.148 L/m2h and shows high in rejection up to 72%. This is might due to the porous membrane structure compared to the CTAB surfactant.
Figure 8. (a) Flux of Na2SO4, (b) Rejection of Na2SO4
305 3.4 Molecular Orientation Study
In order to study the molecular orientation of each prepared membranes, Fourier-transform infrared (FTIR) has been used. FTIR will show the bands that indicate the functional group of the membrane. Moreover, FTIR will give the molecular interaction between the components in the polymer blend. Fig. 9 shows the molecular orientation of PES membrane without addition of additive and surfactant. S=O stretching vibrations are presence at 1179.3 cm-1 in the spectrum. As PES is used for the important material in membrane fabrications, C-C stretch (in ring) and C-H stretching of benzene ring shows peak at 1509.3 cm-1 and 3058 cm-1 respectively. However, H2O does not show any significant peak in the spectrum.
Figure 9. Molecular orientation of PES nanofiltration membrane
Fig. 10 shows morphological study of membrane with SDS shows strong peak of S=O at 1482.1 cm-1 resulting from vibration of SO4 when small amount of SDS is added. Presence of SDS in the solution makes S=O of PES shifted from 1179.3 cm-1 to 1208.5 cm-1. At 2884.3 cm-1, the peak shows the presence of functional groups from PEG. This shows PEG also contributes in the molecular interaction of the components.
Figure 10. FTIR spectroscopy of NF membrane with SDS
CTAB shows different kind of functional group presence in the prepared membranes. Presence of CTAB is indicated by scissoring vibrations of CH3-N+ moiety and
C-N+ stretching band at 1482 cm-1 and 966.5 cm-1 respectively. Moreover, Fig. 11 also shows presence of PEG 600 as additive in the polymer solution. Weak stretching vibration peak shows by S=O at 1153.7cm-1 and O-H stretch peak also from PEG 600 shows high peak at 3572.9 cm-1.
Figure 11. FTIR spectroscopy of membrane with CTAB
4. Conclusion
Additive was found to improve the membrane performance in terms of water flux, flux and salt rejection. It has been proved by the increasing of water flux when PEG 600 is added in the polymer solution from 127.36 L/m2h to 214.72 L/m2h. Moreover, in terms of salt rejection, PEG 600 will increase the rejection as polymer concentration increases. However, presence of SDS as surfactant in membrane gives high flux and low salt rejection. This is might due to the structure of the membrane which is more porous. Membrane with CTAB shows low flux but increase in salt rejection almost 92%.
As polymer concentration increase, the flux will increase and rejection will decrease for SDS and vice versa for CTAB. as conclusion, the CTAB was found to be the best surfactant and produced the finest membrane for salt rejection.
ACKNOWLEDGMENT
The authors would like to thank the Ministry Of Education of Malaysia (ERGS 50113) for the financial support.
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