Preparation and modification of thin film PA membranes with improved

Journal of Membrane Science 430 (2013) 158–166

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Preparation and modification of thin film PA membranes with improved antifouling property using acrylic acid and UV irradiation Y. Mansourpanah n, E. Momeni Habili Department of Applied Chemistry, Lorestan University, Khorramabad, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 June 2012 Received in revised form 23 November 2012 Accepted 26 November 2012 Available online 12 December 2012

In this study interfacial polymerization technique was employed by applying trimesoyl chloride (TMC) and piperazine (PIP) as reagents for preparation of polyamide (PA) skin layer on a polyethersulfone (PES) support. Acrylic acid (AA) as a hydrophilic monomer and UV irradiation as a physical procedure were utilized to modify the obtained thin layers. The effect of UV-irradiation and AA concentration on the performance and morphology of modified TFC membranes were investigated. The different concentrations of AA (1%, 5% and 10 wt%) and UV-irradiation times (30, 60 and 120 s) were chosen to modification process. Two sets of modified membranes were prepared: (i) membranes which were modified using UV-irradiation and AA during formation of PA thin film, (ii) membranes which were modified using UV-irradiation and AA after formation of PA thin film. The membranes were characterized by contact angle, SEM, AFM, FTIR, cross-flow filtration set-up and antifouling measurements. First category represented the higher flux and rejection compared to the unmodified membrane. Second category also showed higher rejection. Moreover, the antifouling properties and flux recovery of membranes improved by UV photo-grafting of hydrophilic monomers. Obtained results indicate that first category has significant properties due to the increasing of both flux and rejection. These membranes showed significant anti-fouling. & 2012 Elsevier B.V. All rights reserved.

Keywords: Interfacial polymerization UV irradiation Acrylic acid Thin film composite Surface modification

1. Introduction The membrane process is an attractive separation technology due to the fast and energy efficient process without any phase change [1]. Nanofiltration (NF) membranes are gaining interest worldwide due to the advantages such as low-operation pressure, high-permeate flux, and high retention of multivalent ion salts [2–4]. Several processes have been reported for the preparation of composite NF membranes that one major approach is interfacial polymerization [5–7] which is one of the most commonly used procedures. Morgan put the concept of interfacial polymerization forward in 1965 [8]. The NF process has been used in many applications such as wastewater reclamation, industrial water production, water softening, pharmaceutical, chemical, paper, semiconductor, textile and in the separation of compounds having different molecular weights. Most NF membranes developed to date have a thin-film composite (TFC) structure due to key advantages compared to asymmetric membranes [9]. In a TFC membrane, the support layer provides the appropriate mechanical strength with low resistance to permeate flow, and each layer

n

Corresponding author. Tel.: þ98 916 3611750; fax: þ98 661 2202782. E-mail addresses: [email protected], [email protected] (Y. Mansourpanah). 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.065

can be optimized for the desired combination of permeate flux and solute rejection. A composite membrane is obtained by forming an ultra-thin dense layer on a porous support. Controlling the structure and surface properties of ultra-thin layer are vital for membrane performance. The composition and morphology of composite membranes prepared by interfacial polymerization are dependent on various parameters such as thin layer preparation conditions and post-treatment [10]. The most prepared active layers of TFC membranes are PA [11,12] which employ amine and acid chloride compounds as monomers in interfacial polymerization. The separation performances of the TFC membranes are highly dependent on the monomers used in interfacial polymerization [13,14]. A large number of TFC membranes have been successfully developed from different polymers such as polyurea [15], polyamides [16–19], polyurea-amides [20,21], polyimide [22], polyester [23], polysulfonamide [24], etc. For preparation of developed TFC membranes, nowadays, studies have been focused on choosing or synthesizing of new monomers with special functional groups to fabricate the TFC membrane. For instance, a novel nanofiltration membrane was prepared with poly (amidoamine) (PAMAM) dendrimer and trimesoyl chloride (TMC) by in situ interfacial polymerization on ultrafiltration membrane [25]. Also, several techniques have been employed in order to increase surface hydrophilicity of thin film membranes [26–29]. Among the graft polymerization methods, UV-initiated grafting is the

Y. Mansourpanah, E. Momeni Habili / Journal of Membrane Science 430 (2013) 158–166

most used technique for the membrane surface modification [30,31]. For example, some researchers [30] modified PES membrane using UV-photografting and found that the UV-modified membrane has a lower tendency of natural organic matter (NOM) fouling than the unmodified membrane. It is worth quoting that very few studies have been reported for the modification of nanofiltration (NF) membranes [32–34]. Yamagishi et al. [26] studied UV-initiated grafting of poly(aryl sulfone) membrane. It was reported that UV-method can be applied to poly(aryl sulfone) membranes without requiring any photo initiator or photo sensitizer to generate free radical and active sites. The reaction mechanism started with initial cleavage of the virgin membrane backbone followed by attachment of the vinyl grafts at the free radical and active sites generated by UV. The main objective of this paper is to study the UV-photo grafting modified membranes with improved NF anti-fouling properties using different concentrations of AA (as a hydrophilic monomer) and alterations in irradiation time. For investigation of the performance and property of the obtained membranes we used SEM, AFM, FTIR–ATR, hydrophilicity measurements as well as membrane set-up.

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occurred. After 10 min reaction time the excess organic solution was poured off from the surface and the frame with membrane was held in a hot air dryer for a certain period of time so that a skin layer was formed on the support membrane. To investigate the effect of different conditions on the thin layer properties, we prepared two categories of thin layers: (i) At first category an aqueous phase containing PIP (0.15 wt%) and TEA (0.4 wt%) and different concentration (1, 5 and 10 wt%) of AA were poured on top of the support and allowed to wet for 1 min in the ambient temperature. As mentioned above and after pouring the organic phase into the frame, support and organic phase were exposed to UV irradiation at different times of UV (30, 60 and 120 s) and 160 W powers. The frame with membrane was held for a certain period of time (10 min). Then the excess organic solution was poured off from the surface and the frame with membrane was held in a hot air dryer for a certain period so that a skin layer was formed on the support membrane. (ii) At second category after formation a thin layer, modified membranes were prepared via immersion in aqueous phase containing of AA monomers and then modified by UV irradiation. These membranes were prepared by dipping the unmodified membrane in different concentrations (1, 5 and 10 wt%) of AA for various times (30, 60 and 120 s) of UV-irradiation.

2. Experimental 2.1. Materials Polyethersulfone (PES, MW¼58,000 g/mol) in powder form was purchased from BASF (Germany) for the formation of porous supports. N,N-dimethylformamide (DMF); the solvent for membrane fabrication was purchased from Sigma Aldrich (Milwaukee, USA), Poly(vinyl pyrrolidinone), (PVP) with 25,000 g/mol molecular weight, acrylic acid (AA), trimesoyl chloride (TMC), piperazine (PIP) and triethylamine (TEA) from Merck were used. NaCl and Na2SO4 salts (Merck) were utilized for investigation of ion rejections. Distilled water was used throughout the study. 2.2. Preparation of PES support The polyethersulfone support was prepared by dissolving 18 wt% PES in dimethylformamide (DMF) with 10 wt% polyvinylpyrrolidone (PVP) by stirring for 4 h at 50 1C. The stirring was carried out at 300 rpm. After formation of a homogeneous solution, the dope solution was hold at the ambient temperature for around 4 h to remove the air bubbles. Afterwards, the dope solution was cast on the non-woven polyester (with 150 mm thickness) at 150 mm height using a film applicator at the room temperature without evaporation. After coating, the support was immersed into a distilled water bath for at least 24 h for removing most of the solvent and water-soluble polymer.

2.4. Membrane performance evaluation The performance of prepared membranes was analyzed using a cross flow system. The membrane surface area in the filtration cell was 22 cm2. The flux and rejection of each membrane was determined at 10 min intervals under the 8 MPa pressure. The experiments were carried out at 25 1C. The cross flow velocity was approximately 0.6 m/s for all tests. The permeation rate and salt rejection were determined for all membranes using the Na2SO4 and NaCl solutions in the 1000 ppm concentration. The rejection was obtained by: lp R% ¼ 1 100 ð1Þ

lf

where lp and lf are the ion conductivity in the permeate and feed, respectively. The ion rejection was investigated by measuring the permeate conductivity using a conductivity meter (Hanna 8733 Model, Italy). 2.5. Membrane characterizations SEM and AFM microscopes were employed for investigation of membrane morphology, surface property, and surface pore size. SEM apparatus (Philips, USA) was employed to obtain images of the membrane surface. Membranes were snapped in liquid

2.3. Fabrication of thin film composite membranes Thin film composite membrane was prepared via polymerization between aqueous PIP and organic TMC solution. The procedure for the formation of PA nano composite membrane is followed: The PES support was clamped between two Teflon frames that were 0.7 cm high and 7.5 cm 20 cm inner cavity. An aqueous phase containing PIP (0.15 wt%) and TEA (0.4 wt%) were poured on the top of the support and allowed to wet for 1 min in the ambient temperature. The surface was rolled by a soft roller to eliminate any tiny bubble during the wetting procedure. After draining off the excess solution, the organic solution (n-hexane) composed of trimesoyl chloride (0.1 wt%) was poured into the frame where the conventional interfacial polymerization reaction

Fig. 1. FTIR/ATR spectra of the surface: (a) unmodified PA membrane (b) modified PA membrane with AA.

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nitrogen to break. The samples were coated with gold and viewed at 25 kV. The AFM apparatus was Dual Scope TM scanning probeoptical microscope (DME model C-21, Denmark). Small squares of the prepared membranes (1 cm2) were cut and glued on a glass substrate. The membrane surfaces were analyzed in a scan size of 1 mm 1 mm. The static contact angles were measured with a contact angle measuring instrument (G10, KRUSS, Germany). Deionizer water was used as the probe liquid in all measurements and the contact angles between water and the membrane surface were measured for the evaluation of the membrane hydrophilicity. To minimize the experimental error, the contact angle was measured at five random locations for each sample. Chemical alteration of thin film membranes was investigated using an Equniox 55 Bruker FT–IR spectrometer from Germany with an attenuated total reflection (ATR) attachment. Totally 32 scans were measured during IR study for each sample. The resolution of apparatus was 4 cm 1. 2.6. Antifouling properties and flux recovery Membrane fouling can be quantified by the resistance appearing during the filtration, and cleaning can be specified by the removal of this resistance. The resistance is due to the formation of a cake or gel layer on the membrane surface. The flux (J) through the cake and the membrane may be described by the following equation: J¼

m ADt

ð2Þ

where m is the mass of the permeated water, A the membrane area, and Dt the permeation time. After water flux measurement (Jwi), the solution reservoir was refilled with a 0.3 g/L BSA solution and the flux was obtained (JP). After 2 h of filtration, the membrane was washed with deionized water for 10 min and the water flux of cleaned membranes was measured (Jwc). In order to evaluate the fouling-resistant capability of the membrane, the flux recovery ratio (FRR) was calculated using the following expression: J FRR ¼ wc 100 ð3Þ J wi Rr and Rir, described by Eqs. (4) and (5) show reversible deposition and irreversible fouling [35,36]: Jwc J p Rr ð%Þ ¼ 100 ð4Þ J wi Scheme 1. The mechanism plan of interfacial polymerization: (a) poly condensation reaction related to unmodified PA thin layer, (b) blend of the polycondensation and chained addition reactions related to the first category membranes, (c) chained addition reaction related to the second category membranes.

Rir ð%Þ ¼

Jwi Jwc J wi

100

Rt ¼ Rr þRir

Fig. 2. SEM surface images of (a) the unmodified PA thin layer and (b) modified PA thin layer with 10 wt% of AA and UV irradiation.

ð5Þ ð6Þ


3. Results and discussion 3.1. FTIR–ATR Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR–FTIR) was used to characterize both the unmodified and the UV-modified membranes. Details of this technique may be found elsewhere [37]. Fig. 1 shows the spectra of the

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unmodified membrane (a) and the modified membranes including 1 wt% of AA (b), respectively. As it can be seen, the UV-grafting membranes exhibit different ATR–FTIR spectra. The schematic pattern of the interfacial polymerization reaction between an aryl halide (TMC) and an amine (PIP) is shown in Scheme 1a, resulting in polyamide thin layer. The mechanism of photo-modification started with the absorption of UV-light at the presence of AA is depicted in Scheme 1b. Scheme 1c illustrates the combination of

Fig. 3. AFM topographic images of: (a) unmodified PA thin layer, (b) modified PA thin layer with 1 wt% of AA and (c) modified PA thin layer with 10 wt% of AA.

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two above mechanism. Obtained FTIR–ATR spectra show the changes in the membrane composition. The ATR–IR spectrum indicates that the interfacial polymerization successfully occurs. A strong band at 1620 cm 1 appears, which is assigned to C¼O group in amide functional group. The peak at 1570 cm 1 is assigned to C–N stretching vibration. A wide and weak peak at 3400 cm 1 is assigned to O–H groups existing in the membrane composition. In spite of the typical PA bands, the IR spectrum of the UV–AA grafted membrane contains additional peaks at 2949 cm 1 which attributed to the symmetric C–H stretching vibration of AA monomers. Also the modified membrane shows two peaks at 1658 cm 1 which is assigned to C ¼O band of amide functional group and at 1750 cm 1 which is assigned to the C¼O groups of AA monomers.

3.2. Effect of chemical modification on membrane morphology The effect of different concentrations of AA and UV-irradiation times on the morphology and surface roughness of the membranes can be observed using SEM and AFM analysis. The surface SEM images of unmodified and modified membranes are shown in Fig. 2. According to these images, the increasing of AA concentration and UV irradiation time leads to enhancing the polymerization degree and accordingly the thin layer is become denser. In comparison with the unmodified membrane (Fig. 2a), the surface of the modified membranes (Fig. 2b) was changed. As it can be seen, a denser skin layer including compressed surface is formed. The modified skin layer plays a key role in the separation property of the membranes and rejection capability. The surfaces images of unmodified and modified membranes are shown in Fig. 3. Comprehensive reviews on the membrane characterization by AFM are available in the literature [38,39]. The dark areas represent membrane surface pores. The surface roughness parameters of the membranes which are expressed in terms of the mean roughness (Sa), the root mean square of the Z data (Sq), the mean difference between the highest peaks and lowest valleys (Sz) and the mean pore sizes of the membrane surface were calculated by SPM DME software (Table 1). The surface pore size was measured with the height profile of two-dimensional AFM images using SPM software and the averages were reported. With considering the AFM pictures, the significant changes in the structure of membrane surface were observed. Fig. 3a (unmodified membrane) shows a membrane with the larger pore sizes. According to Table 1 the roughness parameters in this membrane are relatively higher. By introducing of 1 wt% of AA in the skin layer structure and at the presence of UV irradiation, the property of skin layer surface

changed. As it can be seen in Fig. 3b, a denser and compressed skin layer is formed and the mean roughness parameter (Sa) slightly decreases (4.32 compared to 5.22 for unmodified membrane). On the other hand, by increasing of AA concentration to near 10 wt%, Sa parameter decreases to 2.29 (see Table 1). The membrane including 10 wt% of AA shows a compressed and even surface. It is obvious that by modification of skin layers with AA, the mean pore size of membrane surfaces decreased from 110 nm in the unmodified membrane to near 30 nm in the membrane composed of 10 wt% of AA. Accordingly, in the presence of AA and UV irradiation, changes in the surface of skin layer are observable. 3.3. Effect of AA concentration on the performance of PA membrane at the first category The pure water flux of UF membrane was 170 L/m2 h which decreased to 46 L/m2 h after formation of PA thin layer on the PES support. The effect of different concentrations of AA on the membrane performance is depicted in Table 2 and Fig. 4. The obtained results demonstrate that the addition of AA enhances both flux and rejection. First, by increasing the AA concentration in the casting solution, the pure water flux increases and reaches to a maximum at 5 wt% of AA (78 L/m2 h) and then decreases afterwards. On the other hand, at the presence of 10 wt% of AA, the flux decreases to near 58 L/m2 h and Na2SO4 rejection increases by 86%. Accordingly, by introducing of AA the rejection capability of the membranes shows a rising alignment (changes from 50% to near 86%). Fig. 4 clearly represents the effect of different concentrations of AA on the water permeation and Na2SO4 rejection. In these obtained thin layers, in spite of polymerization reaction between PIP and TMC, the addition reaction occurs, leading to a polycondensation and a chained addition reaction. Scheme 1a, b and c schematically illustrate the occurred mechanisms on the PES membrane during the modification of thin layers. We believe that in this process a thin layer containing of polymeric chains which possess many numbers of –COOH functional group is formed, resulting in higher hydrophilicity. According to Yu et al. [40], the pure water flux is influenced by the hydrophilicity and pore size/

Table 1 The mean pore size and roughness parameters of the membrane surfaces. Membrane

Mean pore size of surface (nm) Sa

Unmodified 110 (730) Modified with 1 wt% AA 60 (720) Modified with 10 wt% AA 30 (710)

Sz

Sq

5.22 28 6.32 4.32 33.9 6.77 2.29 21.6 3.02

Fig. 4. Effect of AA concentration on the flux and rejection behavior.

Table 2 Performance of unmodified and modified PA membranes with different concentrations of AA and the same time of UV. Membrane

Jwi (L/m2 h)

Jp (L/m2 h)

Jwc (L/m2 h)

Na2SO4 Rejection (%)

NaCl Rejection (%)

FRR (%)

Rir (%)

Rr (%)

Unmodified Modified with 1 wt% AAþ 60 UV Modified with 5 wt% AAþ 60 UV Modified with 10 wt% AAþ 60 UV

46.6 58.6 78.3 60.3

22.3 38.9 66 58.1

28 41.4 68 59

50 60 65 85.8

18 30 35 42

60 70.8 86.8 97.8

40 29.2 13.2 2.2

12 4 2 1


porosity. We believe that by increasing of AA concentration, the hydrophilicity of modified membranes increases but at the presence of 10 wt% of AA, the densification of obtained thin layer overcomes on the increasing of hydrophilicity, leading to lower flux and higher rejection. As the concentration of hydrophilic monomer increases, a higher density of AA-grafted layer could be formed on the membrane surface which leads to an increasing in the hydrophilicity parameter of the membrane and improves the water flux. On the other hand, at the presence of 10 wt% of AA and due to the grafting of polyacrylic acid chains into the membrane surface, a denser thin layer including small pores was formed (see Table 1).

3.4. Effect of UV irradiation on the performance of PA membrane at the first category The effect of irradiation time (30, 60 and 120 s) on the membrane performance was investigated and the results were shown in Table 3. By considering the data in Table 3 and Fig. 5 we can see that the effect of the increasing of UV irradiation is thoroughly similar to the increasing of AA concentration. The results show that by increasing of the UV irradiation time, the flux reaches gradually to near 58 L/m2 h for 60 s irradiation and then reduces to 42 L/m2 h in 120 s irradiation. On the other hand, by increasing of irradiation time the rejection capability of the obtained membranes increases and follows a gaining track. The rejection capability of the unmodified membrane for Na2SO4 salt is 50%. By increasing the UV irradiation the rejection increased to 55, 60 and 90% for 30, 60 and 120 s irradiation times, respectively. The same rejection trace for NaCl salt was observed. The NaCl rejection for unmodified membrane is about 18% whereas this parameter is approximately 24, 28.5 and 40% for the membranes which were exposed to 30, 60 and 120 s irradiations, respectively. The rejection capability of the membranes for Na2SO4 is higher compared to NaCl rejection. The performance of obtained membranes may be explained on the basis of charge, diffusion coefficient, and size of the ions [41]. Probably in the 120 s UV irradiation

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time a denser and compressed thin layer was formed and this property can overcome on the increasing of hydrophilicity. The contact angle measurement data for the membranes are depicted in Table 4. The contact angle of the 30 s irradiated thin layer is about 571 while for 60 and 120 s irradiated thin layers is 45 and 431, respectively. The obtained results demonstrate that the hydrophilicity of thin layers was increased by increasing of UV irradiation time. This is due to the presence and grafting of AA into the structure of modified thin layer membranes. It is supposed that two types of polymerization reaction occur: Polycondensation reaction and chain addition (CA) reaction that are two parallel competitive processes [42]: (a) Polycondensation reaction that is a reaction between PIP and TMC monomers (see Scheme 1a). In this reaction the rate of polymerization decreases with development of the reaction due to the reduction in the monomer concentrations and resistance to transferring of monomers due to the formation of a thin layer. So, the degree of polymerization becomes lower. (b) CA reaction (see Scheme 1c), which is a radical reaction, occurs under UV irradiation among AA monomers. In this reaction the degree of polymerization is higher than the polycondensation reaction [43,44]. Thus in the presence of AA additive along with UV irradiation, a blend of the reactions including polycondensation and CA reaction occurs (see Scheme 1b). In low concentration of AA, polycondensation reaction is more dominant compared to CA reaction, leading to create a structure with low density (see Fig. 2). Accordingly, the rejection is low and the permeation is relatively high. By increasing of AA concentration the radical polymerization is become remarkable i.e., the degree of polymerization increases. So, a thin layer with high density and compressed structure is formed, resulting in a development in rejection capability (see Table 2). This can be explained by the formation of smaller pores due to the grafting of AA chains into the membrane thin layer structure (see Fig. 3). In these conditions a thin layer with higher porosity (including too many pores with small sizes) and more hydrophilicity is formed, leading to more water flux (compared to the unmodified membrane). Also, the data of Table 4 show that the hydrophilicity of modified membranes increases by increasing the UV irradiation time. The above statement proves that the grafting process is carried out and is successful.

Table 4 Contact angle measurements with different irradiation times (constant AA concentration).

Fig. 5. Effect of UV time on the flux and rejection behavior.

Membrane

Contact angle (o)

Unmodified Modified with 1 wt% AA þ 30 UV Modified with 1 wt% AA þ 60 UV Modified with 1 wt% AA þ 120 UV

– 57.9 (7 1.36) 45.5 (7 0.42) 43.5 (70.8)

Table 3 Performance of unmodified and modified PA membranes with different times of UV irradiation and the same concentrations of the AA. Membrane

Jwi (L/m2 h)

Jp (L/m2 h)

Jwc (L/m2 h)


NaCl Rejection (%)

FRR (%)

Rir (%)

Rr (%)


46.6 47.3 58.6 42.3

22.3 29 38.9 29.3

28 31.4 41.4 30.3

50 55 60 90

18 23.8 28.5 40

60 66.4 70.8 71.6

40 33.6 29.2 28.4

12 5 4 2

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3.5. Fouling behavior of obtained membranes To investigate the influence of photochemical grafting of AA monomer on fouling parameters, the reversible resistances (Rr), irreversible resistances (Rir), and flux recovery ratios (FRR) of unmodified membranes and modified membranes obtained and represented in Tables 2 and 3. The flux behavior of membranes during 2 h of filtration by the BSA solution was investigated. The flux measurements show that the flux of BSA solution decreases during filtration. The results illustrate that the resistance factor in the modified membranes is lower while the flux recovery ratio of this membrane is higher. The irreversible resistance (Rir) of the membranes considerably reduced from 40% for the unmodified membrane to 2.1% for the photochemical grafted membrane composed of 10 wt% of AA. These obtained results present that the modified membranes show remarkable anti-fouling properties. On the other hand, the reversible resistance (Rr) decreased from 12% (for the unmodified membrane) to near 1% for the membrane composed of 10 wt% AA. This may be

attributed to the presence of hydrophilic –COOH functional groups in the membrane structure which prevent from settlement of BSA molecules on the membrane surface. Other factor can affect on the increasing of resistance is densification of the surface which prevents from the penetration of BSA molecules into the thin film structure. Also, the flux recovery ratio changed and significantly enhanced from 60% in the unmodified membrane to about 98% in the modified membrane. Actually, the amount of adsorbed molecules onto the membrane surface is reduced due to the grafting of hydrophilic (AA) monomers onto the membrane surface. This is due to the existence of –COOH groups in AA chains which leads to an enhancement in hydrophilicity property of modified membranes. Accordingly, water flux also increased. On the other hand, the higher flux recovery suggests that the anti-fouling property of modified membranes is very high due to the modification of thin layer surface. In summary, the recovery ratios (FRR), reversible resistances (Rr), and irreversible resistances (Rir) of modified membranes were improved i.e., the surface properties of membrane were modified by photochemical grafting of AA onto the membrane surface.

Table 5 Performance of unmodified and modified PA membranes with AA at presence of UV irradiation. Membrane

Jwi (L/m2 h)

Jp (L/m2 h)

Jwc (L/m2 h)


NaCl Rejection (%)

FRR (%)

Rir (%)

Rr (%)


46.6 22.5 11 7.9

22.3 11.4 9.1 6.8

28 13.9 9.3 6.9

50 55 58.3 59.5

18 27 35 40.7

60 62 84.5 87

40 38 15.5 13

12 11 1.8 1

Table 6 Performance of unmodified and modified PA membranes with UV irradiation at presence of AA. Membrane

Jwi (L/m2 h)

Jp (L/m2 h)

Jwc (L/m2 h)


NaCl Rejection (%)

FRR (%)

Rir (%)

Rr (%)


46.6 12.2 11.7 11

22.3 6.3 6.9 9.1

28 7.6 7.9 9.3

50 51 53.4 58.3

18 18.5 24.6 35

60 62.3 67.5 84.5

40 37.7 32.5 15.5

12 10 8 1.8

Table 7 The mean pore size and the roughness parameters of the membrane surfaces (second category). Membrane

Mean pore size of surface (nm)

Sa

Sz

Sq

Unmodified Modified with 10 wt% AA

110 (7 30) 50 (7 10)

5.22 2.17

28 14.9

6.32 2.67

Fig. 6. AFM topographic images of PA thin layer modified with 10 wt% of AA on the surface (second category).


3.6. Membrane performance at the second category The modified membranes in the second category were prepared via immersion technique and then modified by UV irradiation. In this category of modification, first, a PA thin layer was created. After the formation of PA thin layer, modified membranes were prepared via immersion in aqueous phase containing AA monomers and then modified by UV irradiation. As a result, a new layer composed of AA monomers is created on the surface of the PA thin layer (see Scheme 1c). The pure water flux and rejection capability of the membranes are shown in Tables 5 and 6. The results show that by increasing of AA concentration as well as the UV irradiation time, the pure water flux decreased. According to these Tables, the pure water flux decreased from 46 L/m2 h (unmodified membrane) to 8 L/m2 h in the 120 s-irradiated membrane including 10 wt% of AA. It can be explained that just the upper surface of thin layer modified by AA monomers. According to the literature, changing the hydrophilicity and porosity affects on the pure water flux and rejection [40]. Probably by increasing of AA concentration, a higher density of AA-grafted layer could be formed on the membrane surface. Furthermore, a dense and compressed thin layer was formed due to the penetration of monomers into the thin layer bulk. By grafting of AA onto the surface, further functional groups are grafted on the thin layer surface, resulting in a decrease in pore sizes (see Table 7). Fig. 6 illustrates the surface AFM image of thin layer modified with 10 wt% of AA on the surface of the membrane. Table 8 Contact angle measurements at the presence of different concentrations of AA (constant irradiation time).

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Under these cases, the size of surface pores was changed and decreased to 50 nm. The surface roughness parameters of the obtained membranes present that the surface of modified thin layer become flat and smoother. On the other hand, the Na2SO4 rejection ability developed to 59%. The antifouling properties i.e., the flux recovery ratios (FRR), reversible resistances (Rr), and irreversible resistances (Rir) of unmodified membranes and modified membranes were summarized in Tables 5 and 6. The flux recovery ratio (FRR) increases from 60% (unmodified membrane) to about 87% in the membrane modified with 10 wt% of AA. On the other hand, the resistance parameters changed and decreased to low amounts. We believe that this phenomenon is due to the increasing of surface hydrophilicity. The contact angles measurements are presented in Table 8. The contact angle amounts were decreased with photo grafting of AA. The thin layer modified with 10 wt% of AA exhibits the minimum contact angle (391), while the modified PA thin layer with 1 wt% and 5 wt% of AA show the contact angle about 441 and 41.61, respectively. In summary, the hydrophilicity property increases when the AA concentration increases. Accordingly, monomers were significantly polymerized onto the membrane surface and PA membrane was effectively hydrophilized via grafting of hydrophilic monomers onto the membrane surface. The surface SEM images of unmodified and modified membranes in this category are shown in Fig. 7. SEM images clearly show that a denser and compressed skin layer forms on the thin layer surface by increasing of AA concentration and UV irradiation time. The data in Table 7 prove the above statements.

4. Conclusion

o

Membrane

Contact angle( )


44 (7 0.14) 41.6 (7 0.14) 39.2 (7 0.16)

According to this study, the prepared thin film polyamide nanofiltration membranes were modified by acrylic acid (AA) and UV irradiation. The effect of UV-irradiation time and addition of acrylic acid (AA) on the performance and morphology of TFC nanofiltration membranes were investigated. The modification of

Fig. 7. SEM surface images of the modified PA thin layer with (a) 1 wt% of AA, (b) 5 wt% of AA and (c) 10 wt% of AA (second category).

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