PVA composite membrane for nanofiltration

Separation and Purification Technology 137 (2014) 21–27

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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Chlorine resistant binary complexed NaAlg/PVA composite membrane for nanofiltration Saira Bano, Asif Mahmood, Seong Joong Kim, Kew-Ho Lee ⇑ Laboratory for Functional Membranes, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea University of Science & Technology, 176 Gajung-dong, Yuseong-gu, Daejeon 305-350, Republic of Korea

a r t i c l e

i n f o

Article history: Received 14 June 2014 Received in revised form 9 September 2014 Accepted 10 September 2014 Available online 30 September 2014 Keywords: Sodium alginate Polyvinyl alcohol Nanofiltration Chlorine resistant

a b s t r a c t A composite nanofiltration (NF) membrane was prepared by coating a thin layer of sodium alginate (NaAlg)-polyvinyl alcohol (PVA) blend on the polysulfone support, then cross linked in two steps with calcium chloride and glutaraldehyde respectively. Structural and morphological characteristics of the NF composite membranes were determined by FT-IR and SEM. The performance of the membrane was evaluated in nanofiltration studies, and the effect on performance of experimental parameters including the NaAlg/PVA blend ratio, cross linking reaction time, membrane thickness and operating time was investigated. The permeance properties of the membrane were determined using 2000 ppm solutions of NaCl and MgSO4. A water flux of 80 L/m2.h with 46% salt rejection was observed for the monovalent salt solution. However, a decrease in flux to 68 L/m2 h with an increase in salt rejection to 80% was noticed for the divalent ions. The chlorine tolerance of the composite membrane was tested by exposing it to the NaOCl solution for a different time span. A slight change in water flux and salt rejection was observed, revealing a good stability of the membrane in the chlorine retaining environment. Hence, the prepared binary cross-linked NaAlg/PVA composite membrane can successfully be utilized for the purpose of nanofiltration and/or desalination. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Nanofiltration (NF) is a rapidly emerging membrane technology and an effective tool for the production of potable water from saline water without remineralization. It is characterized by high water flux and good salt rejection for multivalent ions at a low pressure. NF membrane stands at the interface of ultrafiltration (UF)/reverse osmosis (RO) membranes [1]. Most of the commercial NF membranes are thin film composite (TFC) polyamide (PA) membranes with a polysulfone support. However, these membranes are unstable in an environment containing chlorine, due to the presence of amide linkage [2]. The chemical instability renders its water purification cost much higher due to the additional chemical processing steps. To counteract the chlorine sensitivity of polyamides, several other materials has recently been employed for NF membranes e.g. cellulose acetate, sodium alginate, chitosan, polyvinyl alcohol, etc. [3–13]. Among them, polysaccharides (NaAlg, chitosan etc.) are of special interest because of their high hydrophilicity. A high water flux and selectivity have been ⇑ Corresponding author at: Laboratory for Functional Membranes, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea. http://dx.doi.org/10.1016/j.seppur.2014.09.024 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.

reported for dehydration of organic solvents by pervaporation and/or vapor permeation processes using polysaccharide-based membranes [14–18]. The sodium salt of alginic acid (a typical marine bio-polymer) has been reported as an efficient material for desalination [19] and also been widely used as a fouling model in desalination studies [20–22]. The inherent high hydrophilicity of alginate along with its good stability in chlorine makes it a perfect material for the development of NF membranes. In contrast, the high hydrophilicity of alginate leads to a significant swelling of the membrane in an aqueous medium and consequently, decreases the performance of the membrane. Different methods such as blending, cross-linking, and grafting have been reported to overcome the swelling problem and tune the permeation properties of the NaAlg membrane [14–18,23–24,38–40,46–48]. Polyvinyl alcohol (PVA) is another attractive membrane material because of its higher hydrophilic and film forming capacity [25–29]. It also has good characteristics in the NF process because of the presence of a large number of hydroxyl-group, which make the membrane surface too polar. Moreover, like NaAlg, the permeation properties of PVA can also be tuned by cross-linking with different multi-functional compounds [30–37]. Keeping in mind the good permeance property of PVA and rejection property of alginate; it is

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believed that a blend of NaAlg/PVA may serve as a potential material for the NF membranes. We have previously reported a NaAlg/PVA (5/ 95) blend membrane crosslinked with glutaraldehyde (GA) for NF studies, but the water flux was much lower [42–44]. In this study, we have prepared a thin-film composite membrane by coating a blend of NaAlg/PVA on the polysulfone support. Here we have employed a novel multistage cross-linking process to tune the permeation properties of the membrane. The prepared thin film composite membrane was cross-linked in two steps: first with calcium chloride, then glutaraldehyde. After that, the membrane performance for NF experiment and its chlorine tolerance were examined. The cross-linking of NaAlg/PVA membrane with calcium ions has mostly been studied for the pervaporation separation of a water–alcohol mixture [14–16,18–21] but not previously applied to nanofiltration. The prepared membrane was also characterized by Fourier Transform Infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM).

de-ionized water separately. Both solutions were then mixed in various ratios of NaAlg/PVA (90/10, 80/20, 50/50, 20/80) and stirred well. After removing air bubbles, the resultant solutions were over-coated on the surface of a dried polysulfone UF membrane by dip coating and dried at room temperature. The aqueous medium stability of the NaAlg/PVA layer was achieved by cross-linking with two cross-linking agents in a gradient manner. In the first step, the composite membrane was dipped in a solution of 5 wt.% calcium chloride (CaCl2) for different times (10–220 min), washed with deionized water and then immersed in a 5 wt.% aqueous solution of glutaraldehyde (GA) for 20 min (the optimum cross-linking time described previously [44]). The cross-linked membrane was thoroughly washed a second time with deionized water to remove the excessive reagent. The prepared membranes were then stored in DI water before testing.

2. Experiments

NF performance test of prepared membrane with an effective area 19.5 cm2 were performed using cross-flow membrane equipment as shown in Fig. 1. Initially membranes were compacted at 225psi, for 30 min using 2000 ppm aqueous solution of mono and divalent salts separately [41–44]. The permeation flux of the membrane was determined by measuring the volume of the permeated water through the membrane over a certain period of time. It was calculated by Eq. (1):

2.1. Materials Sodium alginate (NaAlg) was purchased from SHOWA Chemical Incorporation. Polyvinyl alcohol (PVA) with an average molecular weight (Mw: 85,000–124,000 g/mole, hydrolyzed 99+%) and anhydrous calcium chloride were supplied by the Aldrich Chemical Company. Glutaraldehyde (GA) 25 wt.% solution was purchased from Junsei Chemical Co. Ltd. Korea. The polysulfone UF membrane was supplied by WoongJin Chemical Company, Korea. All the reagents and chemicals were of analytical grade and used without further purification. De-ionized water prepared in the laboratory was used for preparing the membrane and in the NF experiments. 2.2. Membrane characterization SEM images were taken using a scanning electron microscopy model XL30S-FEG (Philips), fitted with a Bruker EDS detector. All the samples were coated with platinum, and the analysis was done in the N2 medium. Infrared (IR) spectra were measured with a Bruker Alpha-P FTIR spectrometer in attenuated total reflectance (ATR) mode. The salt concentrations were determined with a LaMotte conductivity meter model CON 6 Plus. 2.3. Preparation of composite nanofiltration membranes The casting solution was prepared by dissolving 0.1–0.5 wt.% each of sodium alginate (NaAlg) and polyvinyl alcohol (PVA) in

2.4. Permeation experiments

F¼

V At

ð1Þ

where ‘F’ is the permeate flux in L/m2 h, ‘A’ is the effective area of the membrane in meter squared (m2), ‘t’ is the time for permeation in hours (h) and ‘V’ is the volume of the permeated fluid passing through the membrane in liters (L). The salt rejection was calculated by Eq. (2):

Rð%Þ ¼

ðC f  C p Þ  100 Cf

ð2Þ

where ‘Cp’ is the permeate concentration and ‘Cf’ is the feed concentration. 2.5. Chlorine resistance For the measurement of the chlorine resistance, the membrane was immersed in an aqueous solution of sodium hypochlorite (300 ppm) at pH = 7 at room temperature for different time period. The membranes were rinsed with water and permeation experiment was conducted using 2000 ppm NaCl as feed solution.

Permeate

Pressure Guage

NF-Cell

Pressure Guage Coolant Water

Coolant Water Rotameter

Feed Tank Feed Pump

Fig. 1. Schematic diagram of NF setup, (15 °C, 225 psi, 3 L/min).

S. Bano et al. / Separation and Purification Technology 137 (2014) 21–27

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and 1104 cm1 suggest the presence of CAOAC and CAOAH groups respectively [41]. In short, the FT-IR spectra shown in Fig. 2 confirm the complete miscibility of the two components as well as the successful two-step cross-linking process of the NaAlg and PVA.

NaAlg NaAlg/PVA

PVA

3.2. SEM images of membrane

4000

3500

3000

2500

2000

1500

1000

500

-1

Wave number (cm ) Fig. 2. FT-IR Spectra of the NaAlg, PVA and binary complexed NaAlg/PVA composite membrane (80:20, Conc.0.3%, x-linking time, 1st step 120 min, 2nd step 20 min).

Surface and cross sectional SEM images of the composite NF membrane are shown in Fig. 3a and b. These images depict a very thin active layer with an average thickness of 1 lm and a compact surface morphology that is present on a finger-like morphology of the supporting layer of the polysulfone UF membrane. The role of thin functional layer is to control the flux and rejection properties of the composite NF membrane [41,45]. As shown in Fig. 3a and b, a continuous interconnected network is well formed in the thin film with a homogeneous phase surface morphology. The cross-sectional image of the composite NF membrane (Fig. 2b) reveals a good attachment of the active layer to the UF support. 3.3. Effect of PVA contents

Chlorine tolerance of the membrane is determined by the variation of flux and rejection after chlorine exposure.

3. Results and discussion 3.1. Infrared spectra of membrane A multi-component system can be efficiently characterized by FT-IR spectroscopy, which provides information about both the blend composition and the polymer–polymer interaction. The IR spectrum of the NaAlg, PVA and NaAlg/PVA blend membrane was measured in the range of 500–4000 cm1 and is shown in Fig. 2. Pure PVA membrane shows a broad band at 3100– 3500 cm1 that is attributed to AOH stretching vibrations. For the blend NaAlg/PVA membranes a strong absorption appeared at 3376 cm1, which is a characteristic of the hydrogen-bonded hydroxyl group. The absence of a sharp free peak at 3600– 3500 cm1 implies the absence of the non-hydrogen bonded (free) OH group [15]. An additional medium peak at 2900– 3000 cm1 is appeared for PVA; whereas, it diminishes in blended membrane’s spectra, expressing the hydrogen bonded OH groups and CH stretching respectively. A very weak absorption observed for blend membrane at 2968 cm1 corresponds to the sp3-hybridized CAH stretching. As expected, two peaks were observed at 1640 cm1 and 1585 cm1, corresponding to the presence of two different types of AC@O groups in the complex composite In case of NaAlg the strong absorption at 1590 cm1 corresponds to the C@O stretching of alginate and is the characteristic peak of the ionic bonded carboxylic group of alginate. Similar peak is observed for the blend membranes as well but with low intensity. membrane. The peak at 1640 cm1 is due to the AC@O groups of ester or acetal [17,39–40]. It confirms that the prepared membrane is successfully cross-linked with calcium ions as well as with glutaraldehyde. In fact, the carboxylic groups of the NaAlg are cross-linked with divalent calcium ions, while the AOH groups of NaAlg and PVA are cross-linked with glutaraldehyde to form an acetal [44]. Moreover, the weak absorption of AC@O at 1640 cm1 is due to the low proportion of PVA in the membrane. The peak at 1486 cm1 can be assigned to the bending vibration of the CAH bond. As most of the part of the membrane containing NaAlg has a cyclic ether linkage, a strong sharp band is observed at 1237 cm1. Two different CAO stretching peaks at 1149 cm1

The composition of membrane plays an important role in its separation characteristics. In this study, NaAlg/PVA blend membranes were prepared with different PVA contents ranging from 10% to 80%. The results obtained are shown in Fig. 4. It is clear from Fig. 4 that an addition of PVA to NaAlg results in an increase in the water flux and a decrease in salt rejection. The increase in flux can be attributed to the high hydrophilicity of the PVA, while the gradual decline in salt rejection is due to a decrease in the ionic character of the membrane with a lower NaAlg content. Because the sodium alginate has ionic properties, higher alginate contents in the membrane exhibit higher repulsion for multivalent ions and hence a higher rejection ratio [42–44]. A similar trend has been reported in literature [47]. A comparison of the present study with the reported data is shown in Table 1. Generally, NaAlg-based membranes exhibited lower water flux but a higher salt rejection than PVA and/or a blend of PVA/NaAlg [44]. However, some contradictory results are shown in Table 1 that might be due to a difference in the membrane thickness, cross-linking agents and the operating conditions. It is generally believed that the flux decreased with the increased feed concentration [49], in present study a higher feed concentration was employed as compared to other in table [6,41–44] yet the flux value is high indicating superiority of the membrane. As shown in Fig. 4a binary cross-linked NaAlg/PVA membrane with 20 wt.% PVA exhibits an optimum flux with a good salt rejection. Therefore, this ratio was kept constant for all further experiments. 3.4. Effect of active layer thickness In composite membranes, generally the thin active layer plays a dominant role in controlling the performance of the membrane. It is believed that the membranes with different active layer thicknesses of show distinctive permeation properties. It is necessary to adjust the concentration or viscosity of the casting solution to obtain the desired thickness. To determine the effect of active layer thickness, polysulfone supports were coated with different concentrations (0.1–0.5 wt.%) of NaAlg/PVA (80/20) blend solutions and the approximate thickness of the active layer was measured by SEM. An increase in the salt rejection with a decrease in water flux was observed when increasing the thickness (solution concentration) of the active layer (Fig. 5). These results are in accordance with the results reported by Chen et al. [46].

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Fig. 3. SEM images of the polysulfone (psf) surface and cross section (a and c); binary complexed NaAlg/PVA (0.3 wt.%) composite membrane surface and cross-section (b and d).

60

100

40 2

60 30 40 20

20

Salt Rejection (%)

50

80

Flux (L/m .h)

3.5. Effect of cross-linking reaction time

10

Flux Salt Rejection 0

0 0

20

40

60

80

100

PVA Contents (wt. %) Fig. 4. Effect of PVA contents on flux and salt rejection (15 °C, 225 psi, 2000 ppm NaCl solution).

A low solution concentration (0.1 wt.%) deposits an active layer with a very small thickness of 0.85 lm that could weaken easily and lead to a higher flux with low salt rejection. Increasing the solution concentration from 0.1 to 0.3 wt.% gave a thickness of 1 lm that ultimately results in a decrease in flux with an increase in salt rejection. An additional rise in the polymer solution concentration to 0.5 wt.% dramatically affected the flux and rejection properties of membrane because of an excessive increase in the thickness (3.45 lm) of the membrane. Therefore, a 0.3 wt.% solution concentration was selected for further experiments, as it exhibits both good flux and rejection.

The cross-linking of NaAlg/PVA membrane with calcium ions has mostly been studied for the pervaporation separation of a water–alcohol mixture [14–16,18–21] but not previously applied to nanofiltration. Glutaraldehyde, however, is a familiar crosslinking agent to enhance the compactness of the membrane. Previously, we reported a GA cross-linked NaAlg/PVA composite membranes for NF, but the water flux and rejection were not so high [42–44]. To study the effect of cross-linking reaction time, a series of NaAlg/PVA composite films were coated on the polysulfone supports and immersed in a solution of 5 wt.% CaCl2 for different time intervals. Permeation properties were evaluated in terms of flux and rejection using 2000 ppm NaCl as feed solution. With the Ca+2 crosslinking instead of a high flux very low rejection was observed. So, a two-step cross-linking approach is then employed to tune the permeation properties of the NF membrane. The cross-linking time for calcium was varied from 10 min to 220 min. After that, the membranes were dipped in a 5 wt.% solution of glutaraldehyde for 20 min, which we reported as the optimum cross-linking reaction time for a GA and NaAlg/PVA membrane in our previous study [44]. Here excessive concentrations of both cross-linking agents were used in order to achieve the maximum cross-linking density. The hydrophilicity of the mono (Ca2+) cross-linked and binary (Ca2+ and GA) cross-linked membranes were investigated by measurement of the contact angle. A contact angle values of 63.32° and 55.36° were observed for the Ca2+ cross-linked and binary cross-linked NaAlg/PVA membranes respectively. A lower contact angle value indicates higher hydrophilicity of the binary cross-linked membrane. An exponential increase in the salt rejection was observed with an exponential decrease in flux for a 2000 ppm solution of NaCl as

25

S. Bano et al. / Separation and Purification Technology 137 (2014) 21–27 Table 1 Comparison of the performance of binary complexed NaAlg/PVA membrane with the reported results. Membrane a

NaAlg/psf PVA/psf PVA/NaAlg/psfa,d PVA/NaAlg/psfa PVAa,c PVA/NaAlga,c PVA/NaAlg/psfb

Polymer conc. (wt.%)

Cross-linking agent

Flux (L/mb h)

Salt rejection (%)

Ref.

2 0.1 0.1 0.1 5 5 0.3

GA SA GA GA GA GA CaCl2/GA

34 32 52 31 8 6 79

32 35 16 13 68 70 45

[41] [6] [43] [42] [44] [44] This study

GA: glutaraldehyde, SA: succinic acid. a Feed solution: 1000 ppm. b Feed Solution: 2000 ppm. c Dense membrane. d Pretreated psf support.

maximum after a certain period of time, after which no more crosslinking occurs.

120

Flux Salt Rejection

3.6. Effect of operating time 1.01 µm

80

60

A good-quality membrane must possess a stable performance over time in combination with excellent permeation characteristics. The size and charge characteristics of the ionic species present in feed solutions significantly affect the permeation properties of the membrane over the passage of time [43]. The effect of experi-

3.45 µm

2

Flux (L/m .h) / Rejection (%)

0.85 µm 100

40

20

(a)

0

100

80 0.0

0.1

0.2

0.3

0.4

0.5

0.6

2

Fig. 5. Effect of membrane thickness on flux and salt rejection (15 °C, 225 psi, 2000 ppm NaCl solution).

Flux (L/m .h)

Polymer Solution Concentraion (wt.%) 60

40

100

200

Binary X-link Flux, +2 Ca X-link Flux,

Binary X-link Rejection +2 Ca X-link Rejection

NaCl Na2SO4

20

80

MgSO4

60 100 40

Salt Rejection (%)

0

2

Flux (L/m .h)

150

0

5

10

15

20

25

Operating Time (hours)

(b)

100

50 20

0

0 0

50

100

150

200

250

Cross linking Time (minutes) Fig. 6. Effect of cross-linking reaction time on flux and salt rejection (15 °C, 225 psi, 2000 ppm NaCl solution).

Salt Rejection (%)

80

60

40

NaCl Na2SO4

20

shown in Fig. 6. However, after 120 min, the salt rejection became almost constant with a slight decrease in flux. The sodium salt of alginic acid rapidly reacts with divalent Ca2+ (Ca2+ replaces Na+) to give a highly compact gel-like network [19], while the hydroxyl groups of PVA are expected to be cross-linked with glutaraldehyde in the following step. These results depict that degree of cross-linking increases with an increase of cross-linking time, but reaches a

MgSO4 0 0

5

10

15

20

25

Operating Time (hours) Fig. 7. Effect of operating time on flux and salt rejection for various salt solutions (15 °C, 225 psi).

2

100

100

80

80

60

60

40

40

Salt Rejection (%)

S. Bano et al. / Separation and Purification Technology 137 (2014) 21–27

F lux (L/m .h)

26

better permeation performance than others. Furthermore, the membrane exhibited good chlorine stability when exposed to a 300 ppm solution of sodium hypochlorite. A water flux of 80 L/ m2 h with 46% salt rejection and 68 L/m2 h with 80% salt rejection was observed for 2000 ppm solutions of NaCl and MgSO4, respectively. Although the prepared binary complexed membrane showed good chlorine stability and better permeation characteristics than the reported NaAlg and/or PVA composite membranes, further improvement in its performance can make it a good competitor of the commercially available NF membranes.

20

20

Flux Salt Rejection 0

References 0

0

20000

40000

60000

80000

NaOCl Exposure Time (ppm.h) Fig. 8. Effect of chlorine exposure on membrane flux and rejection (15 °C, 225 psi, 2000 ppm NaCl solution).

mental time on the membrane flux and salt rejection was studied for three different salt solutions each containing 2000 ppm of salt (Fig. 7). Both flux and salt rejection were seen to vary with time. A slight decrease in flux over time was observed for all the feed solutions. This could be due to the accumulation of some charged species on the surface of the membrane. The salt solutions followed the order as NaCl > Na2SO4 > MgSO4, which suggests the dependence of flux on the size of the hydrated ion. A lower flux for a bigger hydrated ion may be due to the blockage of some membrane pores over the passage of time [42–44]. The rejection properties show an inverse relationship: the highest rejection value is observed for the bigger hydrated ion (MgSO4), and vice versa. It possesses greater rejection response for Mg+2 ions. Based on selective salt rejection properties, this type of membrane can also be effectively used for the separation of monovalent and divalent ions [43]. 3.7. Effect of chlorine exposure time Chlorine tolerance of the commercial polyamide membrane is a significant factor in their sustainability, as these membranes are liable to chlorine attack [43]. Alternative polysulfone composite membranes are expected to be stable in a chlorine-containing (oxidative) environment. The chemical stability of the prepared polysulfone supported NaAlg/PVA composite membrane was studied by exposing it to a 300 ppm solution of sodium hypochlorite (NaOCl) for different times, then testing its permeation characteristics. Fig. 8 depicts that the prepared composite NF membrane is stable in a chlorine-rich environment, even after a long period of exposure. However, a slight increase in flux was found, which reduced the salt rejection. These results show a good chemical stability of the prepared composite membrane in a chlorine environment. 4. Conclusion Chlorine resistant NaAlg/PVA nanofiltration composite membranes were prepared and cross-linked with Ca2+ and glutaraldehyde in a novel two-steps process. Effects of experimental parameters like PVA contents, polymer solution concentration and cross-linking time was investigated. Compared to the other experimental condition, a change in cross-liking time showed an enormous effect on the permeation characteristics of the membranes. Membrane prepared with 0.3 wt.% of NaAlg/PVA (80:20), crosslinked with Ca2+ for 120 min and GA for 20 min showed a

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