Influence of the diamine structure on the nanofiltration performance, surface morphology and surface charge of the composite polyamide membranes

Journal of Membrane Science 279 (2006) 266–275

Influence of the diamine structure on the nanofiltration performance, surface morphology and surface charge of the composite polyamide membranes S. Ver´ıssimo a , K.-V. Peinemann b,∗ , J. Bordado a a

Departamento de Engenharia Qu´ımica, Torre Sul, Instituto Superior T´ecnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal b GKSS Forschungszentrum Geesthacht, Max-Planck-Str., D-21502 Geesthacht, Germany Received 13 June 2005; received in revised form 3 December 2005; accepted 11 December 2005 Available online 31 January 2006

Abstract Most nanofiltration membranes are composite and have a polyamide thin film prepared by interfacial polymerization. Their characteristics and performance are mainly determined by the thin film and consequently by the monomers used for its preparation. In this work, different thin films were prepared with small structural differences to help understanding how the amines structure influences the membranes nanofiltration performance, surface charge and morphology. Composite membranes were prepared by interfacial polymerization of piperazine (PIP), N,N -diaminopiperazine (DAP), 1,4-bis(3-aminopropyl)piperazine (DAPP) and N-(2-aminoethyl)-piperazine (EAP) with trimesoylchloride (TMC) separately. Their nanofiltration performance was evaluated with solutions of NaCl, MgSO4 and Na2 SO4 (3 g/l and pH 6) at 10 × 105 Pa. The surface charge was investigated by zeta-potential measurements and the morphological studies by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The average water permeability of the samples was correlated with hydrophobic/hydrophilic character of the monomers by use of the octanol–water partition coefficient. At pH 6 the membranes from PIP, DAP and EAP presented the following order of rejection NaCl < MgSO4 < Na2 SO4 characteristic of negatively charged membranes. The rejection order of the DAPP–TMC membrane was Na2 SO4 < NaCl < MgSO4 since it was positively charged at pH 6. The membranes surfaces were in general flat except for the PIP–TMC membrane which had higher roughness. The DAP– and EAP–TMC presented an extremely thin film, undetectable by SEM analysis of the membranes cross-section. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanofiltration; Composite membrane; Amine structure; Polyamide; Interfacial polymerization; SEM; AFM; Zeta-potential

1. Introduction Nanofiltration membranes have performance between those of ultrafiltration and reverse osmosis membranes leading to a wide range of applications. Among other are included: water softening, removal of natural organic matter, drinking and industrial water production, food processing and wastewater treatment. Since the applications are diverse, the feed solutions have different compositions, pH values and foulant contents. Then, the membranes used should have special performances and properties. The membranes charge affects the electrostatic repulsion between the ions or charged molecules and the membrane sur-

∗

Corresponding author. Tel.: +49 41 52 872420; fax: +49 41 52 872466. E-mail address: [email protected] (K.-V. Peinemann).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.12.014

face. Also, the pH of the system affects the selectivity of the membrane because of the protonation and deprotonation of the functional groups of the thin film [1]. Most NF membranes are negatively charged, and electrolyte solutes with higher anioniccharge densities and/or with low cationic-charge densities are rejected more effectively [2]. NF membranes with positive surface charge have also been reported and their performance studied [3]. Membrane fouling is responsible for the decline in membrane performance over time and is an important problem in membrane technology. Several authors have stated that fouling is directly related to membrane surface roughness [4,5]. One of the techniques used for the preparation of nanofiltration membranes is interfacial polymerization. The resulting composite membrane is constituted by a selective thin film on top of a support membrane (usually ultrafiltration membrane). The performance of the membrane is mainly determined by the thin film, therefore by the monomers used in

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Fig. 1. SEM image of the cross-section (a) and surface (b) of the PEI support. The support membrane presents sponge structure and homogeneous distribution of pores in the surface.

the interfacial polymerization [3,6–11]. Even small changes in the monomers structure can strongly influence the performance of the membranes [12,13]. Piperazine (PIP) is a wellknown monomer for interfacial polymerization, which reacted with trimesoyl chloride, leads to a nanofiltration membrane [7]. We decided to investigate the influence on membrane performance of the use of different piperazine derivatives. The derivates used were 1,4-bis(3-aminopropyl)-piperazine (DAPP), N,N -diaminopiperazine (DAP) [14] and N-(2-aminoethyl)piperazine (EAP). Besides different performance, different films usually have distinct surface morphology and charge. The surfaces of the four TFC membranes were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM) and zeta-potential (ζ) measurements. 2. Experimental 2.1. Membrane preparation procedure A polyetherimide (PEI) flat sheet membrane was used as support and was prepared by casting a PEI mixture on a non-woven. The mixture was constituted by 17 wt.% PEI and 30 wt.% ␥butyrolactone in N,N-dimethylacetamide. The membrane, after precipitation in water, was washed in hot water and dried at 70 ◦ C. Before composite membrane preparation, it was rewetted with 40 vol.% methanol in water for 10 s and then left in deionized water overnight. The water permeability was 135 × 10−5 l (m−2 h−1 Pa−1 ) and SEM images of the support (Fig. 1) reveal a spongy structure and homogeneously distributed pores in its surface. The composite membranes were prepared by interfacial polymerization of the different amines and trimesoylchloride (TMC). The chemical structure of the monomers and suppliers is shown in Table 1 and the composition of the different amine solutions in Table 2. The wet PEI support was immersed for 30 s in an aqueous solution of the amine and excess solution was removed from the surface by gently pressing an absorbing surface to the support membrane. Subsequently, the impregnated membrane was dipped into an organic solution of TMC in cyclohexane (0.13 wt.%) for 10 s. After that, the organic solvent was allowed

to evaporate at room temperature and the membrane stored in water overnight. 2.2. Measurement of the NF performance The performance of the membranes was evaluated at 10 × 105 Pa in a stirred cell (Berghof) with a membrane area of 38.5 cm2 . The concentration of the solutions of NaCl, MgSO4 and Na2 SO4 was 3 g/l and pH was 6. The water flux was calculated by Eq. (1): JW = A(P − πP + πN )

(1)

where Jw is the water flux, A the water permeability coefficient, P the applied pressure, πP the osmotic pressure of the permeate and πN is the average osmotic pressure between the feed and retentate. The salt rejection of the membrane is defined as the following ratio: R = (CF − CP )/CN × 100%

(2)

where R is the salt rejection, CF the feed concentration, CP the permeate concentration and CN is the average concentration between the feed and retentate. 2.3. Surface studies Morphological observations of the membrane surfaces were conducted by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM studies were done with a LEO 1550 VP Gemini from ZEISS. The AFM analysis was conducted with a NanoScope IIIa from Digital Instruments, in constant force mode, at room temperature. The very sharp SFM tip (NanoProbe ESPC-CONT, radius rtip ≈ 15 nm, made from silicon) glides over the surface (scan size 100 ␮m2 ) and tracks the profile line by line. Each line is a convolution between the tip and the local roughness profile of the sample. The images consist of 512 lines with 512 pixel per line. The samples were in wet state. The surface roughness was evaluated by determination of the average plane roughness (Ra ), root mean square (Rms ), surface area (As ) and maximum

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Table 1 Chemical structure of the monomers and suppliers Amine

Molecular structure

Supplier

Piperazine (PIP)

Merck

N,N -Diaminopiperazine (DAP)

Synthetised at GKSS according to [14]

1,4-Bis(3-aminopropyl)-piperazine (DAPP)

Aldrich

N-(2-Aminoethyl)-piperazine (EAP)

Aldrich

Trimesoylchloride (TMC)

Merck

difference of peak and valley (Rmax ). The definition of these quantities is indicated in [15,16]. Tangential streaming potential measurements were performed using the Electro Kinetic Analyzer, EKA (Anton Paar KG, Austria), equipped with a flat plate measuring cell similar to that first described by van Wagenen and Andrade [17] Measurements were carried out at 25 ± 0.5 ◦ C with aqueous KCl solution (ionic strength 5 × 10−3 mol/l), adjusted to the desired pH with equimolar KOH solutions. A silicon spacer 0.34 mm thick in between the both sample surfaces was used in this work, forming an electrolyte channel of an effective area of 2 × (74 × 15) mm2 , the effective channel height was in the order of 0.3 mm. Prior to use, the samples were conditioned for at least 24 h in 5 × 10−3 mol/l KCl solution. Zeta-potential was calculated according to Hunter [18] using the average slope of Table 2 Composition of the amine solutions Amine

Solution composition

PIP DAP DAPP EAP

1 wt.% PIP 0.2 wt.% NaOH 2 wt.% DAP 1 wt.% DAPP 1 wt.% EAP

six different streaming potential curves ␦U/␦p over a range of pressure difference ␦p from 20 to 70 mbar along the channel. Surface conductivity was taken into account according to Fairbrother and Mastin [19]. 3. Results and discussion Composite membranes were prepared by interfacial polymerization of PIP, DAP, DAPP and EAP with TMC separately. Their nanofiltration performance was evaluated as well as the surface morphology and charge. 3.1. PIP–TMC composite membrane PIP is an already known monomer for the preparation of composite membranes for nanofiltration. It was initially introduced by Cadotte [6] and since then has been subject to a lot of interest [10,16,20]. Fig. 2 shows the water permeabilities and salt rejection of the PIP–TMC membranes. The average water permeability of the PIP–TMC membrane is 6.6 × 10−5 l (m−2 h−1 Pa−1 ) and the average rejection to the divalent salts MgSO4 (93%) and Na2 SO4 (95%) is higher than to the monovalent salt NaCl (40%).

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Fig. 2. Water permeability and rejection of NaCl, MgSO4 and Na2 SO4 of the PIP–TMC composite membranes (transmembrane pressure difference: 10 × 105 Pa; salt concentrations: 3 g/l; pH 6).

Concerning the zeta-potential measurements (Fig. 3), one can conclude that the PIP–TMC membrane has its isoelectric point situated at pH 3.8 and in the range of pH between 3.8 and 9 the membrane is negatively charged. The curves shape is typical of surfaces with slight acidic behavior. The plateau at pH > 7 is due to the presence of dissociated acid groups ( COOH → COO− ) from the TMC. The isoelectric point at pH < 5 is too high when only acidic groups are present at the surface. It indicates the presence of some groups containing N from the piperazine monomer (mostly unreacted NH groups). For the pH of 6 used in the experiments, the membranes surface is then negatively charged. The order of rejections for the different salts obtained are then in full agreement with what is expected for a negatively charged membrane surface. The electrolyte solutes with higher anioniccharged densities and/or with low cationic-charged densities are rejected more effectively.

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Fig. 4. AFM image of the membrane surface for the PIP–TMC membrane.

In Fig. 4 is represented the AFM surface image of the PIP–TMC membrane. The surface contains many thin picks, which are responsible for the roughness (29 nm) and surface area (111 ␮m2 ) presented in Table 3. The cross-section and surface of the PIP–TMC membrane analyzed by SEM (Fig. 5) presents a smooth surface where the film does not totally cover the surface and pores are still visible. 3.2. DAP–TMC composite membrane DAP has two primary amines bound to the nitrogen atoms of the piperazine structure. The composite membranes prepared by reaction of DAP and TMC have a superior average water permeability compared to the other membranes prepared. Fig. 6 shows the water permeabilities and salt rejections of the DAP–TMC membranes. The average water permeabilities are 7.9 × 10−5 , 8.7 × 10−5 and 9.9 × 10−5 l (m−2 h−1 Pa−1 ) for the NaCl, MgSO4 and Na2 SO4 solutions, respectively. The same sequence is present for the salt rejections. The rejection to NaCl is 21%, for MgSO4 is 72% and for Na2 SO4 is 89%. The lower rejection of the monovalent salt, NaCl, is an advantage because the nanofiltration membranes are often used as pre-treatment before RO. In the RO step, the monovalent salts are totally eliminated. Its partial removal, already in the nanofiltration step, has as consequence the need of higher transmembrane pressures, which reflect negatively on the operation costs. Table 3 Average plane roughness (Ra ), square mean plane roughness (Rms ), surface area (As ) and maximum difference of peak and valley (Rmax ) for the different membranes determined by AFM analysis

Fig. 3. Zeta-potential vs. pH for the PIP–TMC membrane.

Membrane

Ra (nm)

Rms (nm)

Rmax (nm)

As (␮m2 )

PIP–TMC DAP–TMC DAPP–TMC EAP–TMC

28.962 7.404 11.843 12.264

38.289 11.703 15.224 15.406

366.78 143.41 132.85 117.74

111.46 101.83 100.39 100.55

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Fig. 5. SEM images of the cross-section (a) and surface (b) of the PIP–TMC composite membrane. The thin film covers partially the membrane surface since some pores are still visible.

The influence of the pH on the DAP–TMC membrane surface charge is represented in Fig. 7. For pH inferior to 4.6, the membrane is positively charged and for superior pHs, the surface charge is negative. The curve presents a strong slope that reflects a strong influence of the pH on the protonation and deprotonation of the film. As for the PIP–TMC film, the DAP–TMC film has also some acid behavior. The main difference between the two films is that the DAP–TMC film has more N groups. This is a consequence of the number of N groups per monomer which is four for the DAP against two for the PIP. For the pH of 6 used in the experiments, the membranes surface is then negatively charged. The order of rejections for the different salts obtained are then in full agreement with what is expected for a negatively charged membrane surface. The AFM surface study of the DAP–TMC film, Fig. 8, shows a very flat surface with a small amount of low picks. The surface roughness is the smallest of all the membranes tested, only

Fig. 6. Water permeability and rejection of NaCl, MgSO4 and Na2 SO4 of the DAP–TMC composite membranes (transmembrane pressure difference: 10 × 105 Pa; salt concentrations: 3 g/l; pH 6).

Fig. 7. Zeta-potential vs. pH for the DAP–TMC membrane.

Fig. 8. AFM image of the membrane surface for the DAP–TMC membrane.

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Fig. 9. SEM images of the cross-section (a) and surface (b) of the DAP–TMC composite membrane. The thin film is not visible in the cross-section image. Only the observation of the membrane surface and its comparison with the surface of the support membrane confirms its existence.

7 nm (Table 3) and the surface area (102 ␮m2 ) is similar to the geometric area. The flat surface characteristic of the DAP–TMC membranes is a clear advantage in terms of fouling. The results of the SEM analyses confirm the results of the AFM studies. In the cross-section image of the membrane (Fig. 9), it is not possible to distinguish a film on top of the support membrane. On the other hand, the surface is distinct from the support membrane (Fig. 1) because the pores, if existent, are smaller than the ones of the support. These observations lead us to conclude that the DAP–TMC film is very thin. This surprisingly thin film is then responsible for the membranes high water permeability. The spheres, present in Fig. 9, are believed to be artefacts. 3.3. DAPP–TMC composite membrane DAPP has a similar molecular structure to piperazine except for the presence of two aminopropyl groups bounded to the N atoms of the ring. The water permeabilities and salt rejection of the DAPP–TMC membranes are presented in Fig. 10.

Fig. 10. Water permeability and rejection of NaCl, MgSO4 and Na2 SO4 of the DAPP–TMC composite membranes (transmembrane pressure difference: 10 × 105 Pa; salt concentrations: 3 g/l; pH 6).

The DAPP–TMC membranes show relatively similar values of water permeabilities for the three salt solutions and their average water permeability is 3.2 × 10−5 l (m−2 h−1 Pa−1 ). The membranes PIP–TMC and DAP–TMC showed the following increasing order of rejection: NaCl < MgSO4 < Na2 SO4 . On the other hand, the DAPP–TMC membranes have a lower rejection to Na2 SO4 (35%) as to NaCl (57%) and MgSO4 (75%). This behavior can be explained based on the membranes surface charge. Unlike the membranes tested earlier in this work, the membrane presents positive charge at pH 6 (Fig. 11). Its isoelectric point is situated at pH 6.4 and in the range of pH between 4.4 and 6.4 the membrane is positively charged. Therefore, the electrolyte solutes with higher cationic-charged densities and/or with low anionic charge densities are rejected more effectively. The surface charge curve has the typical shape of amphoteric surfaces, i.e., surfaces with acidic and basic groups. It has a plateau at pH > 7 which results from the presence of dissociated acid groups. The existence of a plateau at acidic conditions, together with the isoelectric point at pH > 6, indicates the presence of a large quantity of basic groups (mostly NH2 and N). The AFM analysis of the DAPP–TMC membranes surface, Fig. 12 and Table 3, reveals an irregular surface with low roughness

Fig. 11. Zeta-potential vs. pH for the DAPP–TMC membrane.

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Fig. 12. AFM image of the membrane surface for the DAPP–TMC membrane.

(12 nm) and surface area of 100 ␮m2 . The SEM images of the membrane, Fig. 13, show a thin, flat film that covers totally the surface of the support membrane except for some sporadic pores. As the DAP–TMC membrane, also the DAPP–TMC membranes surface presents a flat surface which is beneficial against fouling.

Fig. 14. Water permeability and rejection of NaCl, MgSO4 and Na2 SO4 of the EAP–TMC composite membranes (transmembrane pressure difference: 10 × 105 Pa; salt concentrations: 3 g/l; pH 6).

3.4. EAP–TMC composite membrane The molecular structure of the EAP differs from the molecular structure of piperazine in the presence of an aminoethyl group bounded to one of the nitrogen atoms of the ring. The water permeabilities and salt rejection of the EAP–TMC membranes are presented in Fig. 14. The average water permeability of the membranes is 3.2 × 10−5 l (m−2 h−1 Pa−1 ). The membranes show a higher rejection to divalent anions then to monovalent anions since the average rejection of NaCl is 31%, of MgSO4 is 90% and Na2 SO4 is 92%. The zeta-potential curve of the EAP–TMC membrane (Fig. 15) corresponds also to an amphoteric surface. The curve presents a behavior analogous to the DAPP–TMC membrane and their interpretations are similar. The surface presents acidic

Fig. 15. Zeta-potential vs. pH for the EAP–TMC membrane.

Fig. 13. SEM images of the cross-section (a) and surface (b) of the DAPP–TMC composite membrane. The smooth DAPP–TMC thin film covers totally the surface of the membrane.

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groups ( COOH) and basic groups ( NH2 and NH). For pH < 6.1 the membranes surface is positively charged and above is negatively charged. The pH at which the experiments were conducted is situated close to the boundary between positive charge and negative charge taking into attention the experimental error. Since the membranes performance is analogous to the behavior of negatively charged membranes, the rejection to Na2 SO4 is still high, we conclude that the membrane had slightly negative charge. The smooth surface of membrane EAP–TMC has a roughness of 12 nm and its area is 101 ␮m2 (Fig. 16 and Table 3). These features are confirmed by the SEM study (Fig. 17). In the crosssection image, the thin film is not perceptible and the surface image reveals a flat surface only disturbed by rare protuberances. 3.5. Influence of hydrophobic/hydrophilic character of the amines on the water permeability Fig. 16. AFM image of the membrane surface for the EAP–TMC membrane.

The hydrophobic/hydrophilic character of the polymer obtained by interfacial polymerization is naturally influenced by the hydrophobic/hydrophilic character of the monomers. Since all the membranes prepared used the same acid chloride, TMC, only the hydrophobic/hydrophilic character of the amines should influence the differences of hydrophobic/hydrophilic character of the resulting polymer. The octanol–water partition coefficient (Poct) can be used to quantify the hydrophobic/hydrophilic character of a compound. In [21] the experimental partition coefficients for PIP, DAPP and the estimated partition coefficients for DAP and EAP (Table 4) are provided. When the average water permeabilities for the three salt solutions are plot-

ted versus −log(Poct) of the amines (Fig. 18), a clear relation between these two parameters is visible. The higher the value of −log(Poct), the higher the water permeability obtained. So, the water permeability of the membranes can be correlated with the hydrophobic/hydrophilic character of the monomers. For the PIP–TMC membrane, the water permeability was higher than what could be expected from the octanol–water partition coefficient which can be explained by the higher surface area. When selecting monomers for composite membrane preparation, calculation of the Poct may save some superfluous work.

Fig. 17. SEM images of the cross-section (a) and surface (b) of the EAP–TMC composite membrane. The EAP–TMC film is very thin and only detectable by comparison of the membrane surface with the surface of the support membrane. The thin film is flat with some rare protuberances.

Table 4 Summary of the average results obtained for the four different composite membranes Amine

−Log(Poct)

A × 105 (l (m−2 h−1 Pa−1 ))

RNaCl (%)

RMgSO4 (%)

RNa2 SO4 (%)

ζ At pH 6 (mV)

Surface roughness

PIP DAP DAPP EAP

1.50 2.84 1.43 1.57

6.6 8.8 3.2 3.1

40 21 57 31

93 72 75 90

95 89 35 92

≈−70 ≈−50 ≈+15 ≈0

Slightly Flat Flat Flat

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Nomenclature A AFM As CF CN

Fig. 18. Average water permeability of the NaCl, MgSO4 and Na2 SO4 solutions for the composite membranes prepared and values of −log(Poct) for the four different monomers (transmembrane pressure difference: 10 × 105 Pa; salt concentrations: 3 g/l; pH 6).

4. Conclusions Composite membranes were prepared by interfacial polymerization of PIP, DAP, DAPP and EAP with TMC separately. The influence of the amine structure on the membranes nanofiltration performance, surface charge and morphology was studied. Their nanofiltration performance was evaluated with solutions of NaCl, MgSO4 and Na2 SO4 (3 g/l and pH 6) at 10 × 105 Pa. The surface charge was investigated by zeta-potential measurements and the morphological studies by AFM and SEM. A summary of the results is presented in Table 4. The PIP–TMC membranes presented an average water permeability of 6.6 × 10−5 l (m−2 h−1 Pa−1 ) and the average rejection to the divalent salts MgSO4 (93%) and Na2 SO4 (95%) was higher than to the monovalent salt NaCl (40%). The membranes have slight acidic behavior and have a negatively charged surface at the pH 6. The morphological studies revealed a somewhat rough surface. The DAP–TMC membranes gave the highest average water permeability, around 8.8 × 10−5 l (m−2 h−1 Pa−1 ). The average salt rejections for NaCl, MgSO4 and Na2 SO4 were 21, 72 and 89%, respectively. The membranes also showed some acidic behavior and were negatively charged at pH 6. The membranes surface was very flat and had a very thin film. The DAPP–TMC membranes have an average water permeability of 3.2 × 10−5 l (m−2 h−1 Pa−1 ) and different average salt rejections to NaCl (57%), MgSO4 (75%) and Na2 SO4 (35%). The membranes surface is amphoteric and at pH 6 is positively charged. A smooth surface characterizes the membranes. The EAP–TMC membranes presented an average water permeability of 3.1 × 10−5 l (m−2 h−1 Pa−1 ) and the average rejection to the divalent salts MgSO4 (90%) and Na2 SO4 (92%) was higher than to the monovalent salt NaCl (31%). The membranes have amphoteric surface and are negatively charged at pH 6. The membranes surface was flat and had a very thin film.

CP DAP DAPP EAP Jw NF P PEI PIP R Ra Rmax Rms SEM TMC ζ πN πP

permeability coefficient atomic force microscopy surface area feed concentration average concentration between the feed and retentate permeate concentration N,N -diaminopiperazine 1,4-bis(3-aminopropyl)-piperazine N-(2-aminoethyl)-piperazine water flux nanofiltration applied pressure polyetherimide piperazine salt rejection average plane roughness maximum difference of peak and valley root mean square scanning electron microscopy trimesoylchloride zeta-potential average osmotic pressure between the feed and retentate osmotic pressure of the permeate

It was also observed that the water permeability of the samples can be correlated with the hydrophobic/hydrophilic character of the monomers by use of the octanol–water partition coefficient. For the PIP–TMC membrane, the water permeability was higher than what could be expected from the octanol–water partition coefficient which can be explained by the higher surface area. Acknowledgements The authors greatly appreciate M. Schossig and M. Aderhold at GKSS for performing the SEM experiments and for their valuable comments. We are grateful to K. Kratz, H. Kamusewitz and M. Keller at GKSS for performing the AFM experiments and for their help with the interpretation of the results. The authors also appreciate K. Richau at GKSS for the zeta-potential measurements and help with the interpretation of the results. Gratefully acknowledged is also the financial support of the Portuguese Foundation for Science and Technology (PhD scholarship SFRH/BD/6226/2001). References [1] A.E. Childress, M. Elimelech, Relating nanofiltration membrane performance to membrane charge (electrokinetic) characteristics, Environ. Sci. Technol. 34 (2000) 3710. [2] A. Hamza, G. Chowdhury, T. Matsuura, S. Sourirajan, Study of reverse osmosis separation and permeation rate for sulfonated poly(2,6-dimethyl-

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