Polyurethane TFC nanofiltration membranes based on interfacial polymerization of poly(bis-MPA) and MDI on the polyethersulfone support

Separation and Purification Technology 162 (2016) 37–44

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Polyurethane TFC nanofiltration membranes based on interfacial polymerization of poly(bis-MPA) and MDI on the polyethersulfone support Hossein Mahdavi ⇑, Fariba Razmi, Taieb Shahalizade School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6455, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 19 July 2015 Received in revised form 5 February 2016 Accepted 8 February 2016 Available online 8 February 2016 Keywords: Polyurethane TFC membrane Nanofiltration Interfacial polymerization Polyethersulfone

a b s t r a c t A new type of thin film composite (TFC) nanofiltration membranes was prepared based on a polyethersulfone (PES) support using interfacial polymerization of poly(bis-MPA) (BoltronÒ) and methylene diphenyl diisosyanate (MDI). The prepared TFC membranes were fully characterized using several techniques, including ATR-FTIR, SEM, AFM and water contact angle measurements. Nanofiltration performance of the prepared membranes was studied using determination of the water flux, and dye, salt, and transition metal rejections. The results clearly were indicated that solute rejection (dye, salt and transition metal) improved but water flux decreased with an increase of generation and/or concentration of the hyperbranched polyester. Finally, anti-fouling properties of the membranes were also investigated by static protein adsorption studies. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction During recent decades, rapid population growth and industrial development without a consistent increase in the watertreatment facility have resulted in worldwide deterioration in the water quality. Therefore, the most pressing challenges today include the recovery of clean drinking water from salty or sea water, by far the most abundant global water resource, and the treatment and recycle of wastewater and removal of hazardous components produced from industries such as textile, leather, tanning, and paint [1]. Especially, due to complex aromatic structure, textile effluents are resistant to degradation by conventional treatment method such as bleaching, ozonation, electrochemical techniques and active slug [2,3]. Membrane technology have gradually been developed as a valid replacement for common treatment processes. This technology is comparable to the conventional methods because of its industrially attractive advantages such as reduced energy consumption, higher selectivity, and thermally insensitive compounds [4–12]. Nanofiltration is a relatively recent kind of pressure-driven membrane process with separation characteristics in between ultrafiltration and reverse osmosis, and with molecular weight cut-off (MWCO) ranging from 200 to 1000 Da (pore size of approx-

⇑ Corresponding author. E-mail address: [email protected] (H. Mahdavi). http://dx.doi.org/10.1016/j.seppur.2016.02.018 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.

imately 0.5–2 nm) [13,14]. Nowadays, this process isof particular interest from the numerous fields such as, biotechnology, pharmaceutical, food industry and especially water purification and treatment industries [15,16]. Generally, two methods have been used for preparation of nanofiltration membranes: phase inversion technique for asymmetric membranes and interfacial polymerization for TFC membranes. However, most of the commercial nanofiltration membranes are prepared based on interfacial polymerization [17,18]. The prominent advantage of TFC membranes is that, both layers of TFC membranes (top thin selective layer and bottom porous substrate) can be independently chosen and optimized to gain desired water flux and solute rejection with excellent strength and compression resistance [19]. Up till now, polyamide thin film composites are the most studied TFC membranes prepared by IP technique [20–22], while thin film polyester and other polymers are less explored. However, major limitation of thin film polyamide membranes is the tendency to organic fouling. Thus, development of TFC membranes with a new active layer is a desirable issue of research. Hilal et al. [23], were attempted to synthesis of thin film polyester membranes by interfacial polymerization of bisphenol A and trimesoyl chloride to improve antifouling character and indicated that the polyester thin film, which has relatively high negative charge, exhibited lower irreversible fouling by humic acid component. Hyperbranched polymers with tree-dimensional architecture have been known as a subclass of dendritic polymers. These

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polymers have gained considerable attention because of their unique structure and properties [24]. Simple synthesis, highly branched structure and large number of functional groups make hyperbranched polymers an appropriate choice to use as the key component in the thin film composite membranes preparation. Despite their unique properties, few studies have been reported about using hyperbranched polymers in preparation of TFC nanofiltration membranes so far. Wei and Chiang were studied preparation of nanofiltration membranes using hyperbranched polyester and hyperbranched polyethyleneimine, respectively [25,26]. The aim of the current study is to prepare polyurethane TFC nanofiltration membrane by interfacial polymerization of methylene diphenyldiisocyanate (MDI) and different generations of well-known hyperbranched polyesters (poly(bis-MPA) or BoltronÒ).

2. Experimental 2.1. Materials Polyethersulfone (PES, Mw = 60,000 g/mol) was supplied from BASF. Polyethylene glycols (PEG), 2,2-bis(hydroxymethyl) propionic acid (bis-MPA), p-toluene sulfonic acid (p-TSA) and trimethylol propane (TMP), methylene diphenyldiisosyanate (MDI), sodium chloride (NaCl), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), magnesium sulfate (MgSO4), nickel(II) chloride (NiCl2) and copper(II) sulfate (CuSO4
5H2O) were purchased from Merck company. Direct yellow RL, Maxilon red BL, Rosbengal T, Brown HT and Bovine serum albumin (BSA) were purchased from Aldrich Company.

2.2. Synthesis of hyperbranched polyesters Different generations of hyperbranched polyester polyols were synthesized by melt polycondensation of stoichiometric amounts of bis-MPA with TMP using p-TSA as the catalyst [27]. In this work, the hyperbranched polyesters were synthesized in three (2nd, 3rd and 4th) generations and coded as H20, H30 and H40. Preparation of H20 was carried out according to the following method: bisMPA (50.0 mmol, 6.71 g), TMP (5.55 mmol, 0.745 g) and p-TSA (33.6 mg) were mixed in a three-necked round bottom flask equipped with inert gas inlet, dry tube, and mechanical stirrer. The reaction mixture was placed into an oil bath, which previously heated up to 140 °C and mixed under a stream of nitrogen for 2 h to remove water formed as the byproduct. After this, the stream of argon was stopped and flask was connected to vacuum pump for 1 h to remove excess water. For synthesis of H30, bis-MPA (66.7 mmol, 8.94 g) and p-TSA (44.8 mg) were added to the previous flask and again was connected to nitrogen and for 2 h to complete reaction at 140 °C and after that, the flask was vacuumed for another 1 h. Finally, in order to synthesis of H40, the first step was repeated but with stepwise addition of the calculated stoichiometric amount of the monomer. Different characteristics of the hyperbrached polyesters are listed in Table 1.

Table 1 Different characteristics of the hyperbrached polyesters. Entry

Generation

TMP/bis-MPA

Mw (g/mol)

Mw/Mn

Tg (°C)

1 2 3

2 (H20) 3 (H30) 4 (H40)

1/9 1/21 1/45

1903 2616 4472

1.36 1.39 1.47

34 37 41

2.3. Preparation of polyethersulfone ultrafiltration support PES support was prepared by dissolving PES (17%) and PEG-600 (10%) in DMF as solvent. All components were mixed for 12 h by a magnetic stirrer at 40 °C. After removing air bubbles under air tight condition, homogenous solution was cast with an approximate thickness of 200 lm by casting blade at the ambient temperature. The film was immersed in the distilled water as coagulation bath at room temperature for instant phase inversion. In order to guarantee complete phase separation, the prepared membrane was kept in the coagulation bath for at least 2 h, and then washed for several times and kept in distilled water for further analyses. 2.4. Preparation of TFC membrane The polyurethane top layer was prepared by an interfacial polymerization process. Different amount of HBPE was dissolved in water-ethanol mixture (2:3) as aqueous phase whereas the organic phase was prepared by dissolving MDI in the toluene. The PES support firstly was immersed into the aqueous phase for 10 min and then it was taken out and was drained off to remove excess solvent. Subsequently, the support was immersed in organic phase for 1 min, and then was dried in the air before was placed in the oven for 20 min at 80 °C in order to complete polycondensation. 2.5. Characterization of the polyurethane TFC nanofiltration membranes Separation performances of the membranes were characterized by different solutions: (1) NaCl, MgCl2, MgSO4 and Na2SO4 solutions at concentration of 2000 ppm; (2) Dye solutions (Table 2) at a concentration of 1000 ppm and (3) Heavy metal solutions (NiCl2 and CuSO4
5H2O in deionized water). Flux and rejection tests were carried out using a dead-end filtration system. Firstly, all membrane samples were submersed in water for 24 h and afterward, pre-pressurized at 0.3 MPa for 10 min in a dead-end batchtype cell to reach to the stable state. For salt rejection, electrical conductance of feed and permeate was measured by pen-type conductance meter (az-8361, AZ instrument); while concentration of dye in feed and permeate were analyzed by UV spectrophotometer (UV-Shimadzu 2100) and concentration of heavy metal in feed and permeate was measured by ICP (ICP, Varain VISTA-MPX, Brucker). All permeation tests were performed at the operation pressure of 0.5 MPa and ambient temperature. Pure water flux was calculated by following equation:

J w ¼ Q =A  Dt

ð1Þ 2

where Jw is pure water flux (L/m h), Q is the total volume of permeate (L), A is the effective membrane area (m2), and Dt is the time (h). The rejection was calculated by following equation:

R ¼ 1  C p =C f

ð2Þ

where Cp and Cf are the concentration of permeation and feed solution, respectively. Table 2 Different characteristics of applied dyes. Entry

Dye

Molar mass (g/mol)

Charge

Absorption (nm)

Molecular formula

1 2

Brown HT Direct Yellow RL Maxilon Red BL Rose Bengal

652 475.54

Anionic Anionic

462 404

C27H18N4NaO9 C21H14N3NaO3S3

245.303

Cationic

531

C12H17N6

973.6

Neutral

550

C20H4Cl4I4O5

3 4

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39

Fig. 1. Schematic structure of polyurethane top-layer.

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Equinox 55, Bruker) was used to investigate the surface functionality of the TFC membranes. Scanning electron microscope (SEM, AIS2100, Seron technology) and atomic force microscopy (AFM, DME model C-21, Denmark) was utilized to characterize morphology of the PES support and TFC membranes. Surface roughness values of the membranes were calculated from the height profile of AFM images using SPM software. Water

contact angle (CA, OCA 15 plus, Dataphysics) was employed to characterize hydrophilicity of the surface of the TFC membranes and the average values reported.

Table 3 HBPE and MDI weight percent used in preparation of the TFC membranes. Entry

Membrane

HBPE (w %)

MDI (w %)

1 2 3 4 5 6 7 8 9 10

N0 N1 N2 N3 N4 N5 N6 N7 N8 N9

1 1 1 2 2 2 2 2 3 4

0.07 0.1 0.15 0.2 0.25 0.3 0.4 0.5 0.3 0.3

Fig. 2. ATR-FTIR spectra of PES support and HBPE-based (N9) composite membranes.

Fig. 3. Surface (a), cross-section (b) and high magnified cross-sectional (c) FE-SEM images of the PES support.

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The PES support and prepared TFC membranes were first ultrasonically treated in phosphate buffer solution (PBS, pH 7.0, 0.1 M) for 5 min, and then immersed in BSA phosphate buffer solution (1.0 mg/ml) at 25 °C. for 4 h. After adsorption, these membranes were rinsed three times using PBS, and then undertaken ultrasonic treatment in PBS at room temperature for 2 min to drive off the protein from the surface. The amounts of adsorbed proteins on the membranes were determined by measuring the protein concentration in the PBS using Coomassie brilliant blue method [28]. 3. Results and discussion In this study, a new type of TFC nanofiltration membranes was developed by interfacial polymerization of HBPE (in three generations: H20, H30 and H40) and MDI on PES-UF membrane as support. The descriptive structure of the prepared polyurethane top layer was shown in Fig. 1. In order to obtain optimized polyurethane based top-layer, this step was carried out with different ratios of HBPE and MDI molecules as shown in Table 3. 3.1. ATR-FTIR study Surface characterization of the prepared TFC membranes was carried out by ATR-FTIR technique. The FTIR spectra of the PES support, HBPE and prepared TFC membranes are shown in Fig. 2.

Comparison of these spectra clearly showed that some new characteristic bands appeared at 1728 cm1 (related to carbonyl groups of ester and urethane linkage) and 2900–3000 cm1 (related to ACH2 and ACH3 group of HBPE). The broad absorption band at 3300–3400 cm1 was attributed to unreacted hydroxyl groups (AOH) of HBPE overlapped with newly formed urethane linkage (NAH stretching) [29,30]. As shown in Fig. 2 none of polyurethane TFC samples exhibited an apparent band at 2250–2270 cm1, which means that all ANCO groups of MDI were reacted with AOH groups of the HBPE [31–33]. Other peak observed for HBPE (Fig. 2b) and H30-based sample (Fig. 2d) can be related to C@O group hydrogen bonded (1596.13 cm1 stretching) [31].

3.2. Morphological studies In order to investigate surface morphology of the PES support and prepared TFC membranes, FE-SEM imaging was used. Fig. 3 shows FE-SEM images of surface and cross-section of the PES support. The PES support exhibited a smooth surface without porous structure. On the other hand, cross-section image of the PES showed a micro-porous structure with regular finger-like pores with sponge-like walls. Fig. 4 shows FE-SEM images of surface and cross-section of the TFC nanofiltration membranes (N9 for each generation). Compared to the PES support, the prepared TFC nanofiltration membranes clearly showed a rougher and

Fig. 4. SEM image of HBPE-based composite membranes (a) H40-based (b) H30-based (c) H20-based, 1: the surface, 2: the cross-section, 3: the upper part of cross-section with higher magnification.

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Fig. 5. AFM images of the PES support and HBPE-based composite membranes.

un-uniform surface. Morphology of the polyurethane active layers according to the different generation of HBPE molecules are clearly observed in Fig. 4. Surface of H40-based TFC membranes (Fig. 4a1) showed a uniform film which totally covered surface of support. However, H30 and H20-based TFC membranes exhibited a non-uniform film on the surface of support (Fig. 4b2 and c2). H-bond interactions between the polar hydroxyl end-group, lead to the formation of stable clusters, even in polar organic solvents. Even different dissolution procedure or the thermal prehistory of HBPE could cause a discrepancy in the observed results [34]. These complexities limit the understanding the behavior of the poly(bis-MPA) during dissolution at the molecular level. But, we can conclude HBPEs with lower molecular weight and smaller hydrodynamic volume could diffuse better in organic phase during interfacial reaction. However, smaller generations could not form a widespread layer on top and mostly diffused into pores or formed agglomerated and globular structures. To obtain more information about surface morphology and roughness of the PES support and TFC nanofiltration membranes (N9 sample for each generation), AFM analyses were used. Fig. 5 shows AFM images of both PES support and three TFC nanofiltration membranes based on different generations of HBPE that revealed different extents and occurrences of surface roughness. The average surface roughness (RMS) of PES support and H20, H30 and H40-based TFC membranes were 18.8, 89, 35 and 27.2 nm, respectively. Clearly, it can be seen that the roughness of prepared TFC membranes increased compared to the PES support. In addition, the surface of TFC membranes possess ‘‘ridge-and-valley” morphology that gives a rougher surface, which observed in previous works, too [35–39]. Surface roughness of H40-based TFC membranes (27.2 nm) is higher than TMC/HPE TFC nanofiltration membrane (12.5 nm) [25]. This high roughness may related to

the rigid molecular structure of MDI, due to existence of the benzene ring. Furthermore, the roughness order of the TFC membranes (H20-based > H30-based > H40-based) were consistent with SEM analyses. 3.3. Hydrophilicity and static protein adsorption studies Water contact angle data of the PES support and prepared TFC membranes are listed in Table 4. To perform a reliable evaluation, the contact angles should be measured from different locations of Table 4 Water contact angle and protein adsorption of the composite membranes. Entry

Composite membranes

Water contact angle

Roughness (nm)

1 2 3 4

PES-blank H20-based membrane (N9) H30-based membrane (N9) H40-based membrane (N9)

79.4 ± 2 60.2 ± 5.2 53.7 ± 3.1 45.6 ± 6.4

18.8 89 35 27.2

Fig. 6. Protein adsorption of the TFC membranes.

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Fig. 7. Water flux of the HBPE-based TFC membranes.

membrane surface and surface roughness should also be taken into consideration [40], Roughness can enhance surface hydrophobicity due to the trapped air. Hence, the smaller membrane surface roughness, the smaller the contact angle [41,42]. As expected, contact angle values of HBPE-based TFC membranes are significantly lower than the PES support, which it could be contributed to the numerous unreacted terminal hydroxyl groups of HBPE. According to Table 4, among the different generation of HBPE, H40-based TFC membrane exhibited the highest hydrophilicity. Considering the similar hydroxyl content of H20, H30 and H40, we could conclude that the H40-based N9 sample contains more unreacted hydroxyl groups, due to higher steric hindrance. Moreover, according to the AFM images, and roughness values, H40-based sample surface is smoother than the other two. As a result, much more decrease in roughness, numerous unreacted polar groups in HBPE and

increased chemical hydrophilicity could overcome the smaller mean pore size, which is stronger factor affecting contact angle value [43]. It is well known that higher membrane hydrophilicity can effectively improve membrane fouling resistance and reduce protein adsorption [39,44,45]. Generally, the amount of proteins adsorbed on a membrane surface is considered as one of the most important evidences to evaluating the fouling resistance of the membranes [28,46]. The amounts of surface protein adsorption of PES support and prepared TFC membranes (N9 samples) were measured (Fig. 6). As it could be seen, all resulting TFC nanofiltration membranes have lower BSA adsorption than the PES support. In addition, the adsorption of BSA decreased with increasing of HBPE generation and H40based TFC membrane exhibited lowest BSA adsorption as a result of its high hydrophilicity. This phenomenon could be tentatively explained as follows: A hydrated layer on the surface of membrane which formed by water molecules attached to the unreacted hydroxyl groups of HBPE by hydrogen bonding, introduce a significant steric repulsion effect and inhibited protein adsorption. These observation were in agreement with surface roughness values and it could be concluded that the smoother surface in case of H40based N9 sample, trapped lowest amount of protein. 3.4. Membrane performance Pure water fluxes of the TFC membranes were measured at the ambient temperature, and the results are shown in Fig. 7. Obviously, water flux has a linear correlation with concentration of MDI and HBPE and was decreased with the increase of their concentration. The reason of this fact is that with increase of the

Fig. 8. Dye rejection of H20-based TFC membranes.

Fig. 9. Dye rejection of H30-based TFC membranes.

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Fig. 10. Dye rejection of H40-based TFC membranes.

Table 5 Salts and heavy metal rejection of the prepared TFC membranes. Entry

1 2 3

Membrane type

H20-based (N9) H30-based (N9) H40-based (N9)

Rejection % (Average |%error|) MgSO4

MgCl2

Na2SO4

NaCl

Ni2+

Cu2+

25 (1.8) 31 (0.77) 37 (2.1)

13 (0.88) 17 (1.23) 23 (0.59)

20 (2.3) 23 (1.65) 30 (0.75)

7 (1.14) 12 (2.5) 20 (0.96)

23 (2.97) 42 (1.55) 66 (1.2)

37 (1.19) 53 (2) 84 (1.9)

MDI and HBPE concentration, a thicker and higher compact layer was formed, and consequently, water flux dramatically decreased. It seems intermolecular and intramolecular reactions of MDI and HBPE were created a well-developed three-dimensional polyurethane network over the PES support. As it could be observed the order of water flux is as follows: PES support > H20-based > H30-based > H40-based. This fact may be due to the bigger size of H40 molecules, which are less likely to diffuse into the pores of support and therefore, mostly remained on the surface and a formed a more uniform thin film layer. In addition, generally increase of surface roughness, leads to higher permeability through the membrane. Then H20-based TFC membrane with greatest roughness shows the highest water flux and H40-based with smoothest surface have lowest water flux. Compared to the TFC membrane prepared from TMC/HPE, H40based TFC membrane showed higher water flux [25]. It is maybe resulted because of lower functionality of the MDI molecule as cross-linker, which formed less compact thin film layer than the TMC with three functional groups. In order to have more information about pore size and surface electrostatic charge of the prepared TFC membranes, rejection of different dyes were studied and data were shown in Figs. 8–10. As seen, Rose Bengal, which has the largest size and neutral charge was completely rejected by all the prepared TFC membranes. However, there is a significant difference in the case of Brown HT because of smaller molecular size. Interestingly, rejection percent of the Direct yellow RL (Anionic, Mw = 475 g/mol) was lower than the Maxilon red BL (Cationic, Mw = 245 g/mol) for most of the membrane samples, which is in contradiction with molecular weights of dyes. This could be related to the positive surface charge of the membranes and indicates that the Donnan exclusion is the main rejection mechanism in case of dye solutions [47].

and multivalent transition metal ions). The results of salt rejection are given in Table 5. Generally following order was observed: MgSO4 > Na2SO4 > MgCl2 > NaCl. From these results it could be considered that the Donnan exclusion and sieving effect are playing main roles in filtration process. This fact that the MgSO4 and Na2SO4 have been rejected more than the chloride salts, may be addressed to more powerful sieving effect in case of salt solutions, and larger effective solvation radius of sulfate. Also, N9 sample from H40-based series with uniform active layer showed highest rejection in both cases of dyes and salts solutions.

4. Conclusions This work described preparation of efficient TFC nanofiltration membranes by interfacial polymerization of HBPE and MDI on a PES ultrafiltration support. The results indicated that a dense, relatively rough, and hydrophilic active layer is formed in case of TFC prepared using 4th generation HBPE trough this technique. Obviously, the hydrophilicity of HBPE-based TFC membranes was higher than its PES support. The dye rejection studies showed following order: Rose Bengal > Brown HT > Maxilon red BL > Direct yellow RL. These results confirmed that the prepared TFC membranes are positively charged on surface. Furthermore, it was found that surface hydrophilicity and consequently, overall performance of the membranes improved with increasing of HBPE generation. These novel hyperbranched polyester based polyurethane TFC membranes could be promising for dye removing applications, due to ease of preparation process, suitable hydrophilicity and water flux.

References 3.5. Salt rejection study In order to further evaluation of the nanofiltration performance of prepared TFC membranes, sample N9 was selected and tested via a standard salt rejection study (MgSO4, MgCl2, Na2SO4, NaCl

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