Novel dual-layer hollow fiber membranes applied for forward osmosis process

Journal of Membrane Science 421–422 (2012) 238–246

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Novel dual-layer hollow fiber membranes applied for forward osmosis process Laurentia Setiawan a,b, Rong Wang a,b,n, Lei Shi b, Kang Li c, Anthony G. Fane a,b a

School of Civil and Environmental Engineering, Nanyang Technological University, 639798 Singapore, Singapore Singapore Membrane Technology Centre, Nanyang Technological University, 639798 Singapore, Singapore c Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 March 2012 Received in revised form 3 May 2012 Accepted 21 July 2012 Available online 28 July 2012

A novel dual-layer forward osmosis (FO) hollow fiber membrane has been designed and successfully fabricated by using a triple orifice spinneret. The fiber consists of two layers made from polyamideimide (PAI) polymer for the outer layer and polyethersulfone (PES) polymer for the porous inner layer. Specifically, after obtaining asymmetric microporous PAI/PES dual-layer hollow fibers via non-solvent induced phase inversion, polyethyleneimine (PEI) polyelectrolyte modification on the outer PAI layer was applied to produce a nanofiltration (NF)-like thin layer, while the PES porous inner layer remained intact as PES is inert to PEI. The membrane morphology, structure and surface property were carefully tailored by adjusting polymer dope composition and spinning conditions. These membranes were subsequently characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were utilized in FO process. It was found that the resultant dual-layer NF hollow fiber membrane can achieve pure water permeability of 15.9 l m  2 h  1 bar  1 and a high rejection to divalent cations up to 89%. In FO process, the dual-layer hollow fiber exhibited a water flux of 27.5 l m  2 h  1 in the orientation of active layer facing feed water by using 0.5 M MgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. The newly developed dual layer hollow fibers outperform all the single layer and duallayer NF hollow fibers reported in the literature for FO applications. & 2012 Elsevier B.V. All rights reserved.

Keywords: Forward osmosis Membrane fabrication Dual-layer hollow fiber membrane Positively-charged nanofiltration Chemical cross-linking

1. Introduction As an important membrane fabrication technology, the development of dual-layer hollow fiber membranes made by co-extrusion method has attracted increasing attention in the past two decades since it was invented by Yanagimoto [1]. Compared with conventional single-layer hollow fiber membranes, the dual-layer hollow fibers offer several advantages due to the flexibility of using two different polymer solutions, which are extruded simultaneously to develop two layers from a triple orifice spinneret [2]. Each material can impart desirable characteristics to the membrane, and the integration of the two layers makes it possible to avoid certain weakness of individual materials. Yang et al. [3] employed a brittle but high performance polymer of polybenzimidazole (PBI) as the selective layer supported by PES which has excellent mechanical properties. Matrimid has an impressive gas separation performance

n Corresponding author at: School of Civil and Environmental Engineering, Nanyang Technological University, 639798 Singapore, Singapore. Tel.: þ65 6790 5327; fax: þ 65 6791 0676. E-mail address: [email protected] (R. Wang).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.07.020

but it is expensive. The material cost can be reduced by co-extruding this material with polysulfone which is much cheaper [4]. A series of studies have been reported on the use of polymeric dual-layer hollow fiber membranes for specific applications, including gas separation [4,5], pervaporation [6], membrane distillation [7,8], protein separation [9], forward osmosis (FO) [3,10], and nanofiltration (NF) [2,11]. However, it is very challenging to make high performance dual-layer hollow fibers, as it involves a sophisticated spinning process in which two different phase inversion pathways occur simultaneously. The major problem in a dual-layer composite structure is the integrity and the adhesion of the two layers, and the compositions of the two polymer dope solutions play a critical role. Increasing the concentration of the outer layer polymer solution improves the adhesion considerably due to the high viscosity of the solution that decreases water (non-solvent) diffusion. Therefore, a good interpenetration of outer and inner polymer dopes can be achieved [2,12]. To date, most of dual-layer hollow fiber membranes were made using a high polymer concentration as the outer layer for gas separation, pervaporation, nanofiltration, and FO applications. For example, Yang et al. developed dual-layer hollow fiber NF membranes applied for FO process [3,10]. A polybenzimidazole

L. Setiawan et al. / Journal of Membrane Science 421–422 (2012) 238–246

(PBI) dope solution with a PBI concentration of 22.6 wt% was employed as the outer selective layer while polyethersulfone (PES) was used to form the inner support layer. The membranes showed a water flux of 33.8 l m  2 h  1 using 5 M MgCl2 as the draw solution facing the outer active layer and de-ionized (DI) water as the feed facing the inner support layer. Considering that the draw solution concentration used was quite high, obviously, the water flux needs to be further improved. In the FO process, the low permeate flux is mainly due to internal concentration polarization (ICP) [13,14]. The ICP can be mitigated by fabricating membrane with a porous substrate [15–17]. Recently, single-layer hollow fiber membrane with a NF-like selective layer was successfully fabricated by utilizing asymmetric microporous hollow fibers made of Torlons polyamide-imide (PAI) material as the porous substrate followed by polyelectrolyte posttreatment using polyethyleneimine (PEI), which imparted positive charges to the membrane due to amine groups in PEI and densified the outer skin of the membrane [18]. However, despite the fact that the membrane exhibited a high water flux and high salt rejection in the FO process, the chemical cross-linking still resulted in a denser substrate which adversely affected the water flux. To overcome the drawback of cross-linking modification on entire PAI membrane, a dual-layer hollow fiber with PAI polymer as the outer layer, supported by an inner layer made of other material inert to PEI polyelectrolyte, may be an option due to the selective crosslinking of the PAI layer. The main objective of present study is to develop composite PAIPES dual-layer hollow fiber membrane applied for FO process. Microporous PAI-PES dual-layer hollow fibers were fabricated by one-step co-extrusion, followed by PEI polyelectrolyte post-treatment, resulting in a positively charged NF-like thin selective layer supported by a porous PES inner layer. To the best of our knowledge, the cross-linking modification on microporous dual-layer hollow fiber membranes has not been explored previously as an alternative strategy to fabricate NF membranes applied for FO application.

2. Materials and methods 2.1. Materials Torlons 4000T (copolymer of amide and imide) (PAI, Solvay Advanced Polymers, Alpharetta GA) and GafoneTM polyethersulfone (PES, Solvay Advanced Polymers, Gujarat) were used as selective layer and support layer materials, respectively. N-methyl-2-pyrrolidone (NMP, 499.5%, CAS#872-50-4, Merck Chemicals, Singapore) was used as solvent. Lithium chloride (LiCl, anhydrous, CAS#744741-8, Merck) and polyethylene glycol (PEG, MW 400, Merck) and were used as additives/pore formers. Purified water by a Milli-Q system (18 M O cm) was used as the internal coagulant and solvent for the preparation of aqueous solutions. Dextrans with different molecular weights (from 6000 to 500,000 Da, CAS#9004-54-0, Sigma) were used to characterize the molecular weight cut-off (MWCO) of hollow fiber membranes. Polyethyleneimine (PEI) ethylenediamine end-capped (molecular weight (Mw) 600, Sigma Aldrich) and PEI branched (Mw 50–100 kDa, Polysciences) were used to perform chemical post-treatment of the microporous hollow fibers. For filtration experiments, magnesium chloride (MgCl2, hexahydrate), purchased from Merck, was employed. All the reagents were used as received. 2.2. Preparation of polymer dope solutions and dope viscosity measurement PAI and PES were dried in a 50 1C vacuum oven for at least 1 day to remove the moisture prior to the preparation of polymer

239

Table 1 Composition of polymer solutions. Code Compositiona

A B C D E a

Outer dope

Inner dope

Flow rate ratio outer:inner (wt/wt)

PAI/NMP 14/86 PAI/LiCl/NMP 14/3.8/82.2 PAI/LiCl/NMP 14/3.8/82.2 PAI/LiCl/NMP 14/3.8/82.2 PAI/LiCl/NMP 14/3.8/82.2

PES/LiCl/NMP 16/6/78 PES/NMP 16/84 PES/PEG400/NMP 16/10/74 PES/PEG400/NMP 16/10/74 PES/LiCl/NMP 16/6/78

01:02.6 01:03.0 01:03.2 01:02.4 01:02.4

Composition is based on wt%.

dope solutions. The detailed of the polymer dope compositions are shown in Table 1. A desired amount of particular polymer and non-solvent additive was dissolved in NMP in a jacket flask equipped with a mechanical stirrer at a controlled temperature of 70 1C. Inorganic salt of LiCl and organic macromolecules of PEG have been chosen as the non-solvent additive. Both LiCl and PEG are highly soluble in water and NMP, and able to promote pore formation in membrane fabrication [19,20]. Once a homogeneous solution was formed, the solutions were cooled down to room temperature, filtered by using a 15 mm stainless steel filter and subsequently degassed under vacuum at ambient temperature over night prior to spinning. The viscosity of polymer dope solutions was measured by a Physica MCR 101 rheometer (Anton Paar). The measurements were performed using a 25 mm cone plate (CP25-1) in the range of 0.01 to 100 s  1 shear rate at 25 1C. The viscosity of the dope solutions was taken at the shear rate of 10 s  1. Measurements were repeated four times. 2.3. Fabrication of ultrafiltration PAI-PES dual-layer hollow fiber substrates PAI-PES UF dual-layer hollow fibers were fabricated by a dry–jet wet spinning technique. The inner polymer solution, connected to a high pressure nitrogen gas cylinder, was extruded through the spinneret at a specific flow rate using a Zenith gear pump, while the outer polymer solution and the bore fluid were extruded using two syringe pumps, respectively. The nascent fibers went through an air gap before immersing into an external coagulation bath at a controlled temperature, and then collected by a roller at a free fall take-up speed. Different air gap distances ranging from 1 to 10 cm and different ratio of inner dope flow rate to internal bore fluid flow rate were applied to investigate the optimum spinning conditions of desirable dual layer hollow fiber membranes. The codes of resultant membranes are designated as composition (A–E, see Table 1)—ratio of inner dope flow rate to bore fluid flow rate by weight (1:1 or 2:3) – air gap (1, 5, or 10 cm) – external coagulant bath temperature (25 1C as default or 40 1C). For instance, the membrane substrate B-11-1 refers to the membrane made of composition B with 1:1 (wt/wt) ratio of inner dope flow rate to bore fluid flow rate and an air gap of 1 cm at 25 1C; the C-23-5-T40 means that the membrane was made of composition C with 2:3 (wt/wt) ratio of inner dope flow rate to bore fluid flow rate and an air gap of 5 cm at 40 1C. Tap water and Milli-Q water were used as external and internal coagulants, respectively, at ambient temperature. The details of the spinning conditions used for experiments are shown in Table 1 and Table 2. The resultant hollow fiber membranes were stored in a water bath for approximately two day at room temperature to ensure that the residual solvent has been removed completely. A post-treatment was performed in order to minimize the membrane shrinkage during drying process for storage purpose.

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represents the pressure difference between the feed side and the permeation side of the membrane (bar).

Table 2 Spinning parameters. Substrate code

Composition

Flow rate ratio inner dope:bore fluid (wt/wt)

Water bath temp, (1C)

Air gap, (cm)

A-23-1 A-23-5 B-11-1 B-11-5 B-11-10 B-23-1 B-23-5 B-23-10 C-23-1 C-23-5 C-23-5-T40 D-23-5-T40 E-23-1 E-23-5

A A B B B B B B C C C D E E

2:3 2:3 1:1 1:1 1:1 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3

25 25 25 25 25 25 25 25 25 25 40 40 25 25

1 5 1 5 10 1 5 10 1 5 5 5 1 5

2.5.3. Molecular weight cut off (MWCO) and pore size distribution MWCO and pore size distribution of dual layer hollow fiber substrates were assessed by using a 2000 ppm dextran aqueous solution containing a mixture of several different molecular weights from 6000 to 500,000 Da. The dextran solution was circulated through the shell side of the hollow fiber module and the permeate was collected for analysis by gel permeation chromatography (GPC) on a Polymer Laboratories-GPC 50 plus system (double PL aquagel-OHMixed-M 8m columns). The details of the pore size distribution calculation can be found elsewhere [22]. The molecular weight of dextran that gave 90% rejection was recorded as the MWCO.

The membranes were immersed in a glycerol/water mixture (1:1 by volume) for 24 h. This process allowed the glycerol to stay inside the membrane pores as pore supporter to alleviate the pore collapse during membrane drying process. Removing the glycerol can be done by immersing the membranes in DI water for 24 h prior to use.

2.5.4. Salt rejection measurement of modified dual layer hollow fibers The salt rejection of chemically modified PAI-PES dual layer hollow fibers was conducted in a bench scale cross-flow filtration unit. The hydraulic pressure of 1 bar was applied on the shell side of the hollow fiber membrane module. The salt rejection experiment was carried out using a 500 ppm MgCl2 solution based on conductivity measurement (Ultrameter II, Myron L Company, Carlsbad, CA) of permeate and feed solutions. 2.6. Performance in FO process

2.4. Chemical cross-linking by using PEI polyelectrolyte The chemical post-treatment was conducted by immersing the hollow fiber substrate into a 500 ml of 1% (wt/wt) PEI aqueous solution at the temperature of 70 1C for 75 min. Next, the membranes were rinsed using purified water and stored for further characterization. Details procedure can be found elsewhere [18]. 2.5. Characterization and analysis 2.5.1. Membrane morphology observation and mechanical property measurement The dimension of dual-layer hollow fiber membranes was measured by a Keyence VHX 500F Digital Microscope. Four different fibers were taken and a mean value was calculated for each sample measurement. The structure and morphology of resultant membranes were examined by a Zeiss EVO 50 scanning electron microscope (SEM). Dry membrane samples were frozen in liquid nitrogen and subsequently cracked in order to obtain the cross sections. The samples were then carefully mounted on the SEM stubs and dried overnight in a 50 1C vacuum oven. An Emitech SC7620 gold sputter coater was used to deposit a layer of gold on the samples under argon environment. 2.5.2. Pure water permeability (PWP) of dual-layer hollow fibers membranes before and after cross-linking modification Four pieces of dual-layer hollow fibers were potted into a module and sealed to prepare a lab-scale module with an effective length of 21 cm. PWP experiments were performed by using two to three modules from the same batch of the membrane spinning process. Milli-Q ultra pure water was circulated through the shell side of the membrane module under a pressure of 1 bar for 90 min to compact the membrane prior to PWP measurement [21]. The PWP of the membranes (l m  2 h  1 bar  1) was calculated by: PWP ¼

V tADP

ð1Þ

where V is the volume of permeate taken (l) per determined time, t (h); A is the filtration area of the dual-layer membrane (m2); and DP

The schematic diagram of a lab-scale cross-flow FO unit used in this study can be found elsewhere [23]. The same modules used for PWP and salt rejection measurements were used for FO experiment. One variable-speed gear pump was used to supply the draw solution and two variable speed peristaltic pumps were used to supply the feed and dosing solution, respectively. The volumetric flow rates of the shell side and lumen side were 1250 and 500 ml min  1, respectively, to ensure a similar Reynolds number (around 2600) of the liquid flowing both in the module shell and fiber lumen. FO experiments were performed in two configurations: (1) draw solution flowed in shell side or active layer facing draw solution (AL-DS), or known as PRO mode, and (2) draw solution flowed in lumen side or active layer facing feed water (AL-FW), or known as FO mode. The volumetric water flux, Jv, was determined at a certain time interval by measuring the weight changes of the feed tank with a digital mass balance connected to a data logging system.

3. Results and discussions 3.1. Fabrication of dual-layer hollow fibers substrates 3.1.1. Effect of the air gap and the ratio of bore fluid to inner dope flow rate on the morphology of dual layer hollow fiber membranes The effect of the air gap and ratio of bore fluid to inner dope flow rate were observed by simultaneous spinning of PAI/LiCl/ NMP (14/3.8/82.2 wt%) as the outer polymer dope solution and PES/NMP (16/84 wt%) as the inner polymer dope solution. Fig. 1 shows the cross-section morphology of dual layer hollow fiber samples and illustrates how the air gap affects the inner contour morphology at 1:1 (wt/wt) ratio of inner dope to bore fluid flow rate. It is clearly observed from Fig. 1a that the fiber spun at 1 cm air gap has irregular inner contour resulting in uneven wall thickness. There are two possible reasons associated with this phenomenon. At a lower air gap, the polymer molecules have a shorter time to relax and release the stress after extruding out of the spinneret. The instantaneous demixing and solidification at

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Fig. 1. Cross-section morphology of dual layer hollow fiber membrane spun by using dope formula B (PAI/LiCl/NMP 14/3.8/82.2 wt% and PES/NMP 16/84 wt%) at different air gaps: (a) 1 cm; (b) 5 cm; (c) 10 cm with inner dope flow rate to bore fluid flow rate ratio of 1:1 (wt/wt).

the outer side of the fiber caused by the strong external coagulant tended to freeze the polymer molecules, forcing the stress held by the macromolecules to release toward lumen radial direction [24]. The second is that, when the pressure inside the fiber lumen is low due to insufficient supply of bore fluid, there will be a compression of the inner wall because of a rapid formation of the lumen skin when a strong bore fluid is used [25]. Two approaches were carried out to confirm the above hypotheses. (1) Fig. 1b and c show the SEM pictures of fiber cross section morphology spun at 5 and 10 cm air gap, respectively. It can be seen that the irregularity of the inner contour of the fibers became less severe with an increase in the air gap. When the air gap was increased, the stress accumulated within the polymer chains in the spinneret can be reduced gradually, as the outer part of the nascent fiber was still soft allowing the polymer chains to re-arrange and relax in the period of passing through the air gap [24,26]. (2) Fig. 2a shows the cross section morphology of hollow fiber membrane spun at 1 cm air gap and inner dope to bore fluid ratio of 2:3 (wt/wt). By increasing the bore fluid flow rate, the irregularity of the inner contour can be prevented. Santoso et al. [25] also reported that increasing the flow rate of bore fluid can maintain a uniform inner layer structure. However, it seems that increasing the bore fluid flow rate in combination with an increase in air gap can lead to the formation of big macrovoids (Fig. 2b) and cracks on the fiber cross-section (Fig. 2c) though it can avoid the irregularity of the inner contour. The formation mechanism of macrovoids in the phase separation has been studied extensively in the literature [27–29]. Since water was also used as the bore fluid that was in contact with the inner polymer dope first, resulting in an earlier phase inversion than the outer polymer dope, the macrovoids tended to develop from the inner side toward the outer layer. Big macrovoids are not generally favorable because it can reduce the mechanical strength of the hollow fiber membrane.

3.1.2. Effects of coagulation bath temperature and dope flow rate Coagulation bath temperature has an effect on the structure and morphology of the resultant hollow fiber membranes. Thus the membrane structure can be tailored simply by varying the system temperature [30]. Fig. 3 shows the morphologies of dual layer hollow fiber membranes spun at 25 and 40 1C coagulation bath temperatures. A significant change of the cross-section morphology has been observed when the coagulation bath temperature increased from 25 to 40 1C. The twisted finger-like macrovoids formed at 25 1C spun (Fig. 3a) disappeared and became straight at 40 1C (Fig. 3b). The increase in coagulation bath temperature enhanced the thermodynamic stability of the polymer solutions, leading to a delayed demixing that seems to allow the polymer molecules to re-arrange [19]. However, the increase in water bath temperature made the boundary between the outer layer and inner layer visible as pointed by the arrow on Fig. 3b. The major reason might be due to the different solidification rates between the outer and the inner polymer dope solutions, as the bore fluid temperature was at ambient temperature (25 1C). The inner layer experienced instantaneous demixing due to the thermodynamic instability while the delayed demixing occurred on the outer layer. The fibers that spun based on the ratio of outer to inner dope flow rate of 1:3.2 (wt/wt) has an outer layer with a thickness of around 25 mm. As the ratio changes to 1:2.4 (wt/wt), the outer layer became thicker which is around 40 mm (Fig. 3c).

3.1.3. Effect of non-solvent additive on the morphology of the duallayer hollow fiber membranes The effect of the addition of an additive in the outer dope of PAI/ NMP system is shown in Figs. 4 and 5. Fig. 4 shows the cross section morphology of the dual layer hollow fiber membrane obtained by simultaneous spinning of PAI/NMP (14/86 in wt%) as the outer layer

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Fig. 2. Cross-section morphology of dual layer hollow fiber membrane spun by using dope formula B (PAI/LiCl/NMP 14/3.8/82.2 wt% and PES/NMP 16/84 wt%) at different air gaps: (a) 1 cm; (b) 5 cm; (c) 10 cm with inner dope flow rate to bore fluid flow rate ratio of 2:3 (wt/wt).

Fig. 3. Cross-section morphology of dual layer hollow fiber membrane spun at coagulation bath temperature of 25 1C (a) and 40 1C (b) and (c) at air gap of 5 cm with PEG 400 as the additive of inner polymer dope. Ratio of outer layer to inner layer thickness: (b) 1/4 and (c) 1/3.

and PES/LiCl/NMP (16/6/78 in wt%) as the inner layer (composition A). A separation of two layers formed by each polymer dope solution can be clearly observed, suggesting the interpenetration delay of the inner and outer dopes. Fig. 5 shows the cross section morphology of the dual layer hollow fiber membrane obtained by simultaneous spinning of PAI/LiCl/NMP (14/3.8/82.2 in wt%) as the outer layer and PES/LiCl/NMP (16/6/78 in wt%) as the inner layer (composition E). No delamination can be observed as the outer dope viscosity increased

significantly to 8.2 Pa s when 3.8% LiCl was added. The high dope viscosity delays the diffusion of the non-solvent to the interface. Similar result has been reported elsewhere [2]. Further investigation is needed to achieve a better understanding for this delamination issue. In order to fabricate a dual layer membrane with a porous inner layer, two types of additives, PEG400 and LiCl, were added into the PES solution as the inner dope individually. As shown in

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243

Fig. 4. Cross-section morphology of dual layer hollow fiber membrane spun by using dope formula A (PAI/NMP 14/86 wt% and PES/LiCl/NMP 16/6/78 wt%) at different air gaps: (a) 1 cm; (b) 5 cm; with inner dope flow rate to bore fluid flow rate ratio of 2:3 (wt/wt).

Fig. 5. Cross-section morphology of dual layer hollow fiber membrane spun by using dope formula E (PAI/LiCl/NMP 14/3.8/82.2 wt% and PES/LiCl/NMP 16/6/78 wt%) at coagulation bath temperature of 25 1C at different air gap: (a) 1 cm; (b) 5 cm.

Fig. 3, using PEG as an additive, the resultant dual layer hollow fibers exhibited large macrovoids and a big portion of sponge-like structure, which are unfavourable due to weak mechanical strength. In contrast, when a small molecular additive, LiCl, was used, the large macrovoids in the inner layer was suppressed, as shown in Fig. 5. These observations were believed to be associated with the change in the thermodynamic properties and phase inversion kinetics of the PES systems without and with the addition of an additive [20,31]. Hansen solubility parameter for each component in PES system is summarized in Table 3, from which the total solubility parameter difference can be derived (Table 4). PES systems without an additive and with 10% PEG 400 as the additive have the total solubility parameter difference (Ddt) of 1.08 and 0.98 (MPa)1/2. The addition of 6% LiCl significantly

increases Ddt to 11.9 (MPa)1/2 which enhances the thermodynamic instability [32]. However, the addition of LiCl increased the viscosity of PES solution significantly from 0.5 Pa s (polymer and solvent only) to 6.9 Pa s (with 6% LiCl), as listed in Table 4, due to the strong interaction of the additive with the solvent. A high viscosity of a spinning dope slows down the non-solvent diffusion into the polymer solution leading to a delayed demixing, and thus, suppresses the formation of big macrovoids [20]. 3.2. Substrate characteristics: pure water permeability (PWP), MWCO, and pore size distribution The PWP and MWCO of hollow fiber membranes prepared with different conditions are presented in Table 5. It can be seen

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Table 3 Hansen solubility parameter. Component

dd (MPa)1/2

dp (MPa)1/2

dh (MPa)1/2

PES PEG 400 LiCl NMP

17.6 19.2 – 18.0

10.4 3.5 –12.3

7.8 3.6 – 7.2

n

dt (MPa)1/2n

Ref.

21.9 [33] 19.8 [34] 94.2 [35] 23.0 [32] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 Total solubility parameter (dt) is calculated by dt ¼ dd þ dp þ dh [32]

after PEI cross-linking for PES. Hereafter the membrane was denoted as dual layer FO hollow fiber with a code of membrane substrate—PEI molecular weight, such as C-23-5-50K. The intrinsic properties of PAI-PES dual layer hollow fiber membranes with a positively charged NF-like selective layer such as water permeability and rejection of MgCl2 are tabulated in Table 6. It can be seen from Table 6 that when the membrane substrates of C-23-5 and E-23-5 were cross-linked with PEI 600 Da, the substrate with a higher MWCO (E-23-5) had a higher water permeability and a lower rejection of MgCl2. This can be explained as follows. The C-23-5, C-23-5-T40, and E-23-5

Table 4 Viscosity and solubility parameter of polymer dope solutions. Composition

PAI/NMP PAI/LiCl/NMP PES/NMP PES/PEG400/NMP PES/LiCl/NMP

Viscosity, (Pa s)

14/86 14/3.8/82.2 16/84 16/10/74 16/6/78

1.5 8.2 0.5 0.8 6.9

b

1.6 n

Ddt ¼9dsolvent-additive  dpolymernn9 (MPa)1/2 – – 1.08 0.98 11.9

a

Composition is based on wt%. Viscosity was measured at shear rate of 10 s  1 at 25 1C. n dsolvent-additive was calculated based on mol fractions of total solubility parameter of solvent and additive. nn dpolymer is total solubility parameter of polymer. b

1.4 Pore size distribution, f(d)

a

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

Membrane substrate code

C-23-1 C-23-5 C-23-5-T40 D-23-5-T40 E-23-1 E-23-5

Dimension OD, (mm)

ID, (mm)

1.43 1.43 1.43 1.44 1.38 1.39

1.08 1.07 1.05 1.04 1.02 1.03

PWP, (l m  2 h  1 bar  1)

Outer skin MWCO, (kDa)

111 7 26 135 7 3 189 7 27 162 7 14 119 7 34 149 7 16

9 15.7 43.3 23.6 20.5 30.8

that all fibers possessed quite large dimensions (OD/ID 1.40/ 1.05 mm/mm). A large fiber lumen is favourable for the fluid flow with less resistance. From this table, it is clear that PWP increased as the air gap increased. A similar trend is also shown for MWCO of the outer skin. This is in agreement with the change of the pore size and pore size distribution presented in Fig. 6. As the temperature of the coagulation bath increased from 25 to 40 1C, the PWP and the outer skin MWCO also increased for fibers spun at the same air gap due to the increasing of the thermodynamic stability of the system as discussed previously. Furthermore, it is depicted from Fig. 6 that the viscosity of inner dope affects the pore size distribution of the resultant fibers. Fibers spun with inner polymer dope having viscosity of 6.9 Pa s (E-23-1 and E-23-5) have a narrower pore size distribution than that having a lower viscosity.

6 8 10 Pore diameter, nm

12

14

16

Fig. 6. Pore size distributions (f(d)) of PAI-PES dual layer hollow fiber membranes ((’) C-23-1; (&) C-23-5; (m) C-23-5-T40 ; (n) D-23-5-T40; (K) E-23-1; (J) E-23-5) .

1850

1650

1450

1250

1050

850

650

Wave length (cm-1) Fig. 7. FTIR spectra of PES before (——) and after (----) PEI cross-linking.

Table 6 Intrinsic properties of dual layer FO hollow fiber membranes*. Substrate code

3.3. Intrinsic properties of dual layer FO hollow fiber membranes The cross-linking reaction was taken place at the outer layer of the dual layer substrate between PAI and PEI. The details of the cross-linking reaction and charge characteristic can be found elsewhere [18]. The PES as the inner layer remains intact as shown in Fig. 7. There is no change in FTIR spectra before and

4

% Transmission

Table 5 Dimension, PWP and MWCO of dual-layer hollow fiber substrates spun at different conditions.

2

C-23-5 C-23-5-T40 E-23-5

Polyethylenimine, MW 600 Da Polyethylenimine, MW 50–100 kDa Water permeability, (l m  2 h  1 bar  1)

Rej. MgCl2, (%)

Water permeability, (l m  2 h  1 bar  1)

Rej. MgCl2, (%)

6.5 – 22.6

83 58

14.4 22.5 15.9

80 55 89

n Condition of polyethylenimine (PEI) crosslinking: 1% PEI aqueous solution in 70 1C water bath for 75 min.

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substrates had a MWCO of 15.7, 43.3, and 30.8 kDa, respectively. When PEI 600 Da was used, the PEI molecules were able to penetrate into the membrane pores and carried out the crosslinking reaction inside the membrane matrix. However, when the MWCO of the substrate was too big, the PEI 600 Da was unable to fully seal the big pores. Therefore, the water permeability is high while the rejection is low. To further understand the effect of MWCO of membrane substrate and the molecular weight of cross-linker on the water permeability and salt rejection, PEI 50–100 kDa was used to perform chemical cross-linking for C-235, C-23-5-T40, and E-23-5 substrates which have similar or lower MWCO than the molecular weight of cross-linker. It is interesting to see that C-23-5 and E-23-5 substrate had a good water permeability and acceptable salt rejection while C-23-5-T40 substrate had quite poor rejection. This might be due to the combination of pore size and pore size distribution of each substrate. As shown in Fig. 6, C-23-5-T40 substrate has a very broad pore size distribution. Thus, the big pores cannot be fully sealed during the cross-linking reaction.

3.4. FO performances The FO performance of dual layer FO hollow fiber membranes with a salt rejection over 80% was determined using a 0.5 M MgCl2 solution as the draw solution and de-ionized water as the feed water at ambient temperature of 23 1C, and the results are listed in Table 7. It was found that all of the dual layer FO hollow fiber membranes have a better performance in the AL-FW orientation than in the ALDS orientation, similar to the results reported previously [18]. The Js/Jv in the AL-FW for all membranes is much smaller compared with their counterparts in the AL-DS orientation. This might be due to the shielding effect occurred at the interface between the high concentration draw solution and the positively charged active layer in the AL-DS orientation by the counter ions [36] and consequently, the active layer of the membrane was unable to reject the co-ions. Table 7 Performance of dual layer FO hollow fiber membranes applied in FO process*. Membrane code

C-23-5-600 C-23-5-50K E-23-5-50K n

AL-facing-DS

AL-facing-FW

Jv, (l m  2 h  1)

Js/Jv, (g l  1)

Jv, (l m  2 h  1)

Js/Jv, (g l  1)

14.5 15.5

5.9 – 5.4

20.6 24.3 27.5

0.3 0.3 0.2

Draw solution: 0.5 M MgCl2 and feed water: DI water.

245

However, in the AL-FW orientation, the salt concentration at the interfaces between the outer layer and the inner support layer was lower than the bulk concentration of the draw solution due to water dilution. Therefore, the shielding effect may not be severe and the co-ions can be repelled by the positively charged outer layer of the membrane. In addition, the facilitated transport in the AL-DS orientation (salt flux and salt repulsion are in the same direction) and retarded transport in the AL-FW orientation (salt flux and salt repulsion are in the opposite direction) also played roles [18]. Furthermore, the water flux in the orientation of AL-FW is higher than that in AL-DS as shown in Table 7. Normally, AL-FW is known to show a lower flux due to more severe dilutive ICP. The lower than expected water flux in AL-DS might be due to the high salt flux. As a result, the salt concentration at the interface of the selective layer and porous support increased, leading to the decrease in the effective osmotic pressure difference across the membrane. Table 8 compares the FO performance of the dual layer hollow fiber membranes developed in current work with other single layer and dual layer membranes reported in the literature. It seems that the water flux of C-23-5-600, C-23-5-50K and E-23-550K dual layer FO hollow fiber membranes in the AL-FW orientation are much higher than other membranes with comparable Js/Jv values. This may be attributed to the porous inner layer, that can be remained intact during the chemical cross-linking treatment, to overcome the concentrative ICP within the inner layer.

4. Conclusions Microporous dual layer hollow fiber membranes using PAI as the selective layer and PES as the inner support layer have been successfully fabricated with a delamination-free structure. The compositions of the outer and inner polymer solutions, air gap and coagulation water bath temperature, etc. played key roles in determining the morphology and structure of dual layer hollow fiber membranes. Following a simple chemical cross-linking treatment on the microporous dual layer hollow fibers using a PEI solution, duallayer FO hollow fiber membranes with a positively charged NFlike selective layer can be developed. The resultant novel FO membranes present promising performance in the AL-FW orientation for FO application. A water flux of 27.5 l m  2 h  1 was achieved using 0.5 M MgCl2 as draw solution and DI-water as feed at room temperature. The newly developed dual-layer hollow fibers outperform all the single layer and dual-layer NF membranes reported in the literature for FO applications.

Table 8 Comparison of various membranes used in FO process. Membrane

C-23-5-600 C-23-5-50K E-23-5-50K DL-PBI-PES Positively charged FO single layer PAI hollow fiber Positively charged FO single layer PAI hollow fiber Positively charged FO PAI flat sheet membrane Neutrally charged FO single layer hollow fiber Double dense layer flat sheet membrane (thickness 35 mm) Cellulose acetate hollow fiber NF membrane

PWP, (l m  2 h  1 bar  1)

Rejection MgCl2, (%)

FO: AL-facing-FW

Testing conditions

Jv, (l m  2 h  1)

Js/Jv, (g l  1)

Feed solution

Draw solution

DI-water

0.5 M MgCl2

Current work

DI-water DI-water

1.0 M MgCl2 0.5 M MgCl2

[3] [18]

DI-water DI-water DI-water

0.5 M MgCl2 0.5 M Na2SO4 2.0 M MgCl2

[37] [38] [39]

DI-water

2.0 M MgCl2

[40]

6.5 14.4 15.9 1.74 2.25 2.19 7.56 4.3 0.17

83 80 89 87 93 91 87 85 99

20.6 20.6 27.5 7.5 8.4 9.7 19.2 13 10.3 70.3

0.3 0.3 0.2 0.1 0.3 0.3 0.5 0.2 0.08

5.56

15

5

–

Ref.

246

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