Highly crosslinked layer-by-layer polyelectrolyte FO membranes: Understanding effects of salt concentration and deposition time on FO performance

Journal of Membrane Science 427 (2013) 411–421

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Highly crosslinked layer-by-layer polyelectrolyte FO membranes: Understanding effects of salt concentration and deposition time on FO performance Phuoc H.H. Duong, Jian Zuo, Tai-Shung Chung n Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

a r t i c l e i n f o

abstract

Article history: Received 8 August 2012 Received in revised form 10 October 2012 Accepted 11 October 2012 Available online 23 October 2012

Layer-by-layer (LbL) deposition of polyelectrolytes onto a negative charge membrane has been investigated under a highly crosslinking condition for forward osmosis (FO). The influence of salt concentration and deposition time on LbL FO membranes performance has been investigated in order to optimize the deposition process, followed by the investigation of polyelectrolyte layer’s crosslinking. In the crosslinking steps, polycation layers of the three bilayer LbL membrane were firstly crosslinked using glutaraldehyde (GA) as the crosslinker. Compared to a non-crosslinked membrane, the reverse salt fluxes were reduced by 63% and 58% under the PRO (pressure retarded osmosis) testing mode using 0.5 M MgCl2 and 0.5 M NaCl as draw solutions, respectively. Subsequently, the polyanion layer of that membrane was photo-crosslinked under ultraviolent (UV) at the wavelength of 254 nm. The reverse salt flux of the resulting membrane was further reduced by 55% and 53% under the PRO testing mode using 0.5 M MgCl2 and 0.5 M NaCl as draw solutions, respectively. Specially, the LbL membrane with only one bilayer after cross-linking by both GA and 4 h-UV could achieve a water flux of about 11 LMH and a reverse flux of 8 gMH under the PRO test using 0.3 M NaCl as a draw solution. This is the first time, a LbL polyelectrolyte membrane has been successfully demonstrated as a FO membrane with good rejection toward NaCl while none of previously reported LbL polyelectrolyte membranes have achieved based on the best of our knowledge. & 2012 Elsevier B.V. All rights reserved.

Keywords: Layer-by-layer Polyelectrolytes Forward osmosis Glutaraldehyde (GA) crosslinker UV crosslinking

1. Introduction Forward osmosis (FO) has been identified as an emerging technology for both energy and clean water production due to its low pressure operation and less fouling propensity compared to the currently dominant desalination technology—reverse osmosis (RO) [1–3]. Various methods have been applied to design FO membranes such as traditional cellulosic membranes [4], thin film composite (TFC) membranes [5–7], and recently technology called layer-by-layer (LbL) membranes [8–12]. LbL is a flexible process that offers a variety of functionalized materials for a range of applications such as biosensors [13], drug delivery [14], membrane separations (e.g. pervaporation [15–18], fuel cells [19], microfiltration [20], nanofiltration [21,22], reverse osmosis [23] and forward osmosis [8–12]), biomedical devices [24], food packaging [25] and others [26]. In LbL polyelectrolyte fabrication process, polyelectrolyte multilayers are normally assembled by the sequential adsorption of polyanions and polycations manually

n

Corresponding author. Tel.: þ65 65166645; fax: þ 65 67791936. E-mail address: [email protected] (T.-S. Chung).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.10.014

or automatically via dip-coating [21,27,28], spraying [29,30] or spin-coating [31] methods through electrostatic interaction. LbL polyelectrolyte assembly has been studied widely for membrane separations because of its simple and robust process for surface modifications with easily controllable thickness and functionalization. This technique can be applied for many porous membrane substrates with different sizes and topology that can adsorb the initial polyelectrolyte layer such as poly(ether sulfone), poly(vinylamine), poly(4-methyl-1-pentene), polyamide, polyacrylonitrile (PAN), poly(vinyl pyrrolidone), anodic alumina in flat sheet, tubular [32] or hollow fiber [33] structures. Previous studies proved that the LbL formation mostly depended on the polyelectrolytes and adsorption conditions than the substrate [28]. It was also observed that the most important adsorption condition that affects the formation of LbL films made from strong polyelectrolytes is the salt concentration in the deposition solutions [34]. Due to the flexibility of the LbL polyelectrolyte technique which produces dense and thin polyelectrolyte films with charge inside the structure, LbL polyelectrolyte membranes have been recently investigated as FO membranes [8–10] besides TFC FO membranes [5–7]. However, these reported LbL FO membranes

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only have good retention against MgCl2 and none of LbL FO membranes has exhibited high rejections for monovalent ions. Therefore, investigations of LbL polyelectrolyte FO membranes with good monovalent ion rejection are necessary so that the LbL polyelectrolyte membrane technology can be applied sufficiently for water reuse and seawater desalination. The purpose of this study is to investigate high NaCl rejection LbL polyelectrolyte membranes for FO applications with the minimum number of polyelectrolyte bilayers. A weak polyelectrolyte poly(allylamine hydrochloride) (PAH, þ) and a strong polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS,  ) as shown in Fig. 1 were deposited onto a negatively charged porous support (hydrolyzed PAN, H-PAN) at a zeta potential of  30 mV under 10 mM NaCl environments [9,12]. First, the influence of salt concentration and deposition time on the FO performance of the membranes was evaluated to obtain the optimal conditions for consequent studies. Second, PAH and PSS layers were crosslinked by GA and UV light exposure, respectively. The (PAH/PSS) LbL membranes were examined in details under different crosslinking conditions.

were purchased from Sigma-Aldrich (USA). N-methyl-2-pyrrolidone (NMP) and sodium chloride (NaCl) were obtained from Merck (Germany). 2.2. Preparation of PAN supports

2. Experimental

Porous PAN support membranes were fabricated using a phase inversion method as described previously [35]. The compositions of casting solution used in this study were PAN (18%), LiCl (2%) and NMP (80%). Membranes were casted on a glass plate using a casting knife with a thickness of 100 mm, followed by immediate immersion in a tap water coagulant bath at room temperature. The as-cast membranes were gently peeled off from the glass plate and kept in water bath overnight for completely phase separation and solvent removal. To increase the negative charge and hydrophilicity of PAN support membranes, PAN was hydrolyzed using the previously described method [36] with slightly modifications. Briefly, PAN membranes were immersed in 1.5 M NaOH at 45 1C for 1.5 h. The hydrolyzed PAN membranes (H-PAN) were washed with DI water to remove the remaining NaOH until the pH of the washing solution reached neutral.

2.1. Materials

2.3. LbL polyelectrolyte membrane fabrication

Polyacrylonitrile (PAN) with an average Mw of 1,000,000 was kindly provided as a gift by Prof. Hui-An Tsai from Chung Yuan Christian University (Taiwan) and was vacuum-dried overnight at 60 1C to remove moisture prior to use. Poly(allylamine hydrochloride) (PAH) with an average Mw of  58,000, poly(sodium 4-styrenesulfonate) (PSS) with an average Mw of  70,000, N-(3-Dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), 50% (w/w) glutaraldehyde (GA) solution in water, sodium hydroxide (NaOH), anhydrous lithium chloride (LiCl), magnesium chloride hexahydrate (MgCl2  6H2O)

In this work, H-PAN membranes were used as supports for LbL membranes. The support membranes were firstly placed in a frame so that only their top surfaces were exposed to the polyelectrolyte solutions as illustrated in Fig. 2. LbL films were prepared using a manual dip-coating method by alternating deposition of positive polyelectrolyte PAH( þ) and negative polyelectrolyte PSS( ) with a three-time-washing step in between to avoid contamination and stabilize weakly adsorbed polyelectrolytes [28] as shown in Fig. 3. The washing time after each adsorption step was 1 min. The deposition process was performed at room temperature (2370.5 1C). PAH and PSS were dissolved in DI water at a concentration of around 1.2 mg/mL. The influence of deposition time on membrane’s performance was studied when the NaCl concentration in deposition solutions was fixed at 1 M. The deposition time varying from 5 to 40 min was employed for the deposition of each layer. The effect of ionic strength to LbL membrane’s performance was investigated at different amounts of NaCl in the deposition solutions (0, 0.25, 0.5, 1 and 2 M). The LbL membranes with 1–3 polyelectrolyte bilayers were prepared and named as (PAH/PSS)n with n is the number of bilayers. 2.4. Internal crosslinking of LbL films

Fig. 1. Molecular structures of two polyelectrolytes: (A) Anion-PSS and (B) Cation-PAH.

(PAH/PSS)n membranes were crosslinked after the LbL depositions were completely finished. The crosslinking procedures were performed by crosslinking PAH with GA first, followed by crosslinking PSS with the aid of UV exposure. Fig. 4A illustrates the cross-linking mechanism between PAH and GA utilizing the

Plastic Silicon ring PAN support Plastic Fig. 2. Lab scale LbL dipping process.

P.H.H. Duong et al. / Journal of Membrane Science 427 (2013) 411–421

COO-Na+

-

-

413

COO-Na+ - COO-Na+

-

-

-

-

PAN

1. PAH 2. DI water NH3+

+

+

NH3

+

+

+

+ NH3+ NH3 NH3+ COOCOO-

-

-

-

-

NH3

+

+

-

+

NH3

-

-

+

+

-

-

SO3NH3+

+

NH3 COO-

-

+

3. PSS 4. DI water

PAN

SO3-

SO3

-

-

-

-

+

+

+

-

-

-

SO3

SO3SO SO3 + 3 + NH3 NH3

+ NH3+ NH3 NH3+ COOCOO-

+

NH3 COO-

-

-

+

NH3

+

-

PAN

Fig. 3. Schematic of the first two layers of the LbL process.

Fig. 4. Schematic of (A) PAH crosslinking with GA linker and (B) possible PSS crosslinking reactions under UV [36].

reaction between the aldehyde group of GA and the amine group of PAH [9]. The membranes were crosslinked by pouring a GA solution on the top layer of (PAH/PSS)n. Different concentrations of GA solution and reaction time were examined to determine the optimal conditions for subsequent studies. The resultant membranes were washed with DI water for at least 5 times in 1 h to remove completely unreacted GA. All GA crosslinked LbL membranes were represented as (PAH/PSS)nx in this study. (PAH/PSS)nx membranes were subsequently exposed to ultraviolent (UV) irradiation under vacuum to photo-crosslink PSS layers (see Fig. 4B) [37]. The samples were purging with inert gas (N2) in DI water for 30 min to remove oxygen. Then, the samples were placed onto a glass plate with the active layers facing up and covered with a quartz plate to form a sandwich in order to prevent the samples to contact with air. The samples were exposure to UV at 254 nm (Vilber Lourmat Corporation, France) for 2–4 h. The LbL membranes crosslinked by GA and UV were represented as (PAH/PSS)nxx.

dried samples were freeze-fractured in liquid nitrogen to maintain their original structure. The samples were coated with platinum particles by a JFC 1300 auto fine coater (JEOL, Japan) before analyses. The images were obtained with a FESEM JSM 6700F (JEOL, Japan) at an accelerating voltage of 5 kV. The functional groups formed on the LbL membranes after different modifications were analyzed under the attenuated total reflectance (ATR) mode on a Fourier transform infrared spectroscopy (FTIR) FTS 135 (Bio-Rad, USA) over the range of 800– 4000 cm  1. The number of scans for each sample was 16. The changes in surface chemistry of LbL membranes after surface modifications were further characterized by an X-ray photoelectron Spectrometry (XPS) Kratos AXIS UltraDLD (Kratos Analytical, UK) using mono Al Ka (hn ¼1486.71 eV, 5 mA, 15 kV) as the X-ray source over the scan range of 1100 to 5 eV.

2.5. Characterization of membranes

In order to further characterize the micro-morphology of noncrosslinked and crosslinked LbL membranes, freeze-dried (PAH/ PSS)3, (PAH/PSS)3xx—UV 2 h and (PAH/PSS)3xx—UV 4 h membranes were characterized using PAS. Doppler broadening energy

The morphologies of LbL membranes were observed under a field-emission scanning electron microscope (FESEM). The freeze-

2.6. Positron annihilation spectroscopy (PAS)

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spectroscopy (DBES) of PAS was used to detect the changes in free volume and its variation as a function of depth profile. The DBES spectra which measured the annihilation gamma photon with a center line at 511 keV were recorded using an HP Ge detector and presented as a function of positron incident energy ranging from 0 keV to 30 keV [38]. The S parameter, one of the characteristic parameters of DBES spectra, was employed to calculate the free volume variation with the depth profile in polymeric materials. To quantitatively analyze the results, the S parameter was fitted by VEPFIT program by using a three-layer mode [39,40]. 2.7. Reverse osmosis tests The water permeability (A, Lm  2h  1/bar, referred to as LMHbar  1), salt rejection (Rs, %) and salt permeability (B, Lm  2h  1, referred to as LMH) of the LbL membranes were determined by testing the membranes using a lab-scale dead-end filtration set-up under the RO mode. The tests were carried out at room temperature with an effective membrane area of approximately 10 cm2. The A values were obtained from the test using DI water as the feed under an applied pressure of 1 bar. The Rs values were determined by carrying out the tests using a 200 ppm NaCl solution as the feed under a trans-membrane pressure of 1 bar. The B values of LbL membranes were determined from the solution-diffusion theory 1Rs B ¼ AðDPDpÞ Rs where DP is the applied pressure and Dp is the osmotic pressure different across the membrane. 2.8. Forward osmosis tests FO experiments were conducted on a lab-scale crossflow filtration unit as illustrated earlier [5]. All experiments were carried out at room temperature (2375 1C) with a linear velocity of 3.3 cm/s in the flow channels without any spacer [4]. The system was operated with a draw solution and a feed solution flowing countercurrently on each side of the membrane. LbL membranes were tested in two different operation modes: PRO and FO. In the PRO mode, the LbL polyelectrolyte layer faces against the draw solution, while, in the FO mode, the PAN substrate of the LbL polyelectrolyte layer was in contact with the feed solution. Each experiment was repeated at least three times. Performance of LbL membranes was evaluated by water flux (Jv, Lm  2h  1, abbreviated as LMH) and reverse salt flux (Js, gm  2h  1, abbreviated as gMH), which were calculated using the following equations

3. Results and discussion 3.1. Evaluation of the deposition time on the FO performance of LbL membranes During LbL membrane preparation via dipping method, polyelectrolyte layers were formed on the charged porous supports via a two-step process: a fast deposition of polyelectrolytes onto the supports and then a slow reconfiguration of polyelectrolyte layers [41]. In other words, polyelectrolytes firstly diffused from the dilute bulk solution to the support’s surface and were subsequently adsorbed onto the oppositely charged substrate via electrostatic interaction. Secondly, the conformation of newly deposited polyelectrolytes was changed slowly to lower the overall energy and maximize the interaction with the support. In order to facilitate the bilayer formation in the shortest deposition time, the FO performance of (PAH/PSS)3 membranes was investigated by varying each layer’s deposition time from 5 to 40 min while keeping 1 M NaCl in the deposition solution. Fig. 5 shows the FO performance of (PAH/PSS)3 membranes versus the deposition time. As the deposition time increases, the membranes show a better selectivity as represented by a lower reverse salt flux and reach a plateau at the value of around 7.5 gMH after 20 min deposition. This indicates that 20 min deposition could be sufficient for the adsorption and relaxation of compact PAH/PSS layers under these experimental conditions. The water flux results also demonstrate a similar trend, no improvement in water flux is observed after 20 min deposition time. However, (PAH/PSS)3 membranes prepared under shorter deposition times (i.e., 5 and 10 min) have lower water fluxes under the FO testing mode. A similar phenomenon has been also observed under the RO testing mode in which the water permeability values of (PAH/ PSS)3 membranes prepared from 5, 10 and 20 min deposition times are 3.93, 4.82 and 4.96 LMH bar  1, respectively. This phenomenon could be explained by the naturalization of H-PAN in an acidic environment [42] due to the acidic characteristics of PAH and PSS deposition solutions at pH around 4.75. As a result, H-PAN is slowly naturalized during the LbL coating process which increases the pore size of H-PAN support [42]. Hence, a longer LbL deposition time leads to the resultant support membrane with a higher water flux until the system reaches equilibrium. Since this LbL membrane system could achieve the maximum FO performance with the minimum deposition time of 20 min, a deposition time of 20 min was adopted for subsequent experiments.

60

50

DV Jv ¼ Aef f Dt DðC t V t Þ Aef f Dt

where DV (L) is the volume of water that has permeated across the membrane in a predetermined time Dt (h) during the test. Aeff is the effective membrane surface area (m2). Ct and Vt are the salt concentration (g/L) and the volume of the feed (L) at the end of FO tests, respectively. The salt concentration was measured by a conductivity meter 856 conductivity module (Metrohm, Switzerland). The dilution of the draw solution was ignored, because the ratio of water permeation flux to the volume of the draw solution was less than 1%.

Water flux (LMH )

Js ¼

15 40

30

10

20

Reverse salt flux (gMH )

and

20

5 10

0

0

0

10

20

30

40

Deposition time (min) Fig. 5. Influence of deposition time on FO performance of (PAH/PSS)3 membranes in PRO tests using 0.5 M MgCl2 as the draw solution and DI water as the feed under 1 M NaCl condition.

P.H.H. Duong et al. / Journal of Membrane Science 427 (2013) 411–421

3.3. Effect of crosslinking to the LbL membrane’s performance

3.2. Study the effect of Ionic strength to LbL membrane’s performance

140

50

120 100

40

80 30 60 20

40

10

Reverse salt flux (gMH)

Water flux (LMH)

The ionic strength of dipping solutions has been proven as one of the most important factors that influence the polyelectrolyte layer formation [28,34,43–46]. Increasing ionic strength of the deposition solution results in a larger amount of adsorbed polyelectrolytes [46] and an increase in polyelectrolyte’s coil structure [47]. Hence, the thickness of polyelectrolyte films is proportional to the ionic strength of the deposition solution [34,45]. In this study, the FO performance of (PAH/PSS)3 membranes with different ionic strengths was investigated by the addition of various NaCl concentrations from 0 to 2 M in polyelectrolyte solutions. As illustrated in Fig. 6, the FO performance of LbL membranes depends on NaCl concentration. LbL membranes fabricated from a higher NaCl concentration show a higher water flux and a lower reverse salt flux. The higher water flux could be explained by the higher amount of trapped salt and less compact structure as more coil structure of polyelectrolytes is formed [47]. The presence of salt in LbL layers may provide additional osmotic pressure which not only increases the water flux but also reduces the reverse salt flux. A larger amount of polyelectrolytes is adsorbed onto the oppositely charged membrane with an increase in NaCl concentration in deposition solutions [46], which could enhance the surface coverage of polyelectrolytes on the support; therefore, a lower reverse salt flux was observed. A membrane with the highest water flux and the lowest reverse salt flux of 53 LMH and 7 gMH respectively using 0.5 M MgCl2 as a draw solution is obtained under the preparation condition with 1 M NaCl in the deposition solutions. However, (PAH/PSS)3_2 M membrane shows a lower water flux and a slightly higher reverse salt flux than (PAH/PSS)3_1 M membrane. It could be explained by the thicker polyelectrolyte layers [34,45] and more globular polyelectrolyte conformations [47] formed at a higher ionic strength. Thicker polyelectrolyte layers may increase tortuosity and cause higher transport resistance which reduce the efficiency of osmotic driving force. As mentioned above, the amount of salt inside LbL films may increase with increasing NaCl concentration in the dipping solution which can enhance the osmotic driving force. However, LbL films could only adsorb a certain amount of salt. Thus, the effect of salt in LbL layers may not be dominant compared to the thickness increment when increasing NaCl concentration from 1 to 2 M. Hence, the water flux of the (PAH/PSS)3_2 M membrane is lower than that of the (PAH/PSS)3_1 M membrane. The higher salt reverse flux is observed in FO tests because of the higher degree of coil chains of polyelectrolytes which may cause a less compact LbL film with more defects. 60

20

0

0 0

0.5

1

1.5

415

2

NaCl concentration in deposition solutions (M) Fig. 6. Effect of NaCl concentration in deposition solutions on FO performance of (PAH/PSS)3 membranes in PRO tests using 0.5 M MgCl2 as the draw solution and DI water as the feed.

From the preliminary results of the above studies, it can be concluded that the optimal deposition time and ionic concentration for the fabrication of (PAH/PSS)3 membranes in this FO application should be 20 min and 1 M NaCl, respectively. Hence, all the subsequent experiments were carried out under these deposition conditions. As presented in Tables 1 and 2, the (PAH/ PSS)3 membrane exhibits an extremely poor salt rejection toward NaCl with 18.78% rejection under the RO mode; high salt reverse fluxes of 222 and 145 gMH for PRO and FO modes, respectively, using a 0.5 M NaCl draw solution. In order to further reduce reverse salt flux, PAH and PSS layers of LbL membranes were crosslinked as shown in Fig. 4. Due to the negative charge of HPAN, the layer-by-layer deposition of PAH(þ)/PSS( ) pairs onto the HPAN substrates leads to the formation of LbL membranes with PSS as the top layer. The crosslinking yield of PAH layers is affected by the diffusion of GA molecules from the bulk solution into the LbL layers and react with PAH molecules. Therefore, the PAH crosslinking reaction is carried out before the PSS top layer is crosslinked by UV to minimize the diffusion resistance for PAH. In order to optimize the PAH crosslinking conditions, the reactions were examined under different GA concentrations and reaction times as shown in Fig. 7. First, GA with concentrations ranging from 0–0.5 wt% were used to crosslink PAH under 2 h reaction time. The FO performance of 3 bilayer LbL membranes after being crosslinked with different GA concentrations shows that 0.1 wt% GA is the optimize concentration with the lowest salt reverse flux and reasonable water flux as shown in Fig. 7A. Subsequently, the GA crosslinking time was optimized under 0.1 wt% GA concentration (Fig. 7B). It can be seen that the salt reverse flux reduces with an increase in reaction time from 0 to 2 h. However, a further increase in reaction time from 2 h to 3 h does not help to reduce the reverse flux but the water flux decreases. Therefore, 0.1 wt% GA and 2 h reaction time should be the optimal condition for this study. Three membranes with different degrees of crosslinking: (1) (PAH/PSS)3—noncrosslinked membrane, (2) (PAH/PSS)3x—2 h GA crosslinked membrane and (3) (PAH/PSS)3xx—2 h GA þ2 h UV crosslinked membrane were studied. Tables 1 and 2 summarize their transport properties and FO performance. Clearly, the decrease in water permeability coefficient (A) and increase in salt rejection (Rs) have been observed with a higher degree of crosslinking which is due to the denser structure of the LbL film with a higher degree of crosslinking. The increase in membrane’s salt rejection comes with a trade-off in water flux because of the decrease in water diffusivity across the denser LbL film. A clear trend is observed, an increase in the degree of crosslinking from the non-crosslinked membrane to both PAH and PSS crosslinked membranes results in a substantial reduction in reverse salt flux, Table 1 Effect of crosslinking condintions on the transport properties of LbL membranes. Membrane

Water permeabilitya, A (LMH bar-1)

Salt rejectionb, Rs(%)

Salt permeabilityb, B (LMH)

B/A (kPa)

(PAH/PSS)3 (PAH/PSS)3x (PAH/PSS)3xx UV—2 h (PAH/PSS)3xx UV—3 h (PAH/PSS)3xx UV—4 h

4.967 0.28 1.767 0.03 0.697 0.05

18.78 70.79 44.07 70.72 72.39 72.81

19.85 7 1.18 1.947 0.06 0.237 0.03

400.2 110.2 33.3

0.577 0.06

84.98 72.85

0.0887 0.019

15.4

0.577 0.02

87.83 71.13

0.0677 0.007

11.8

a b

Tested at 1 bar using DI water as the feed. Tested at 1 bar using 200 ppm NaCl as the feed.

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Table 2 Effect of the crolinking conditions on the FO performance of LBL membranes. Membrane

(PAH/PSS)3 (PAH/PSS)3x (PAH/PSS)3xx (PAH/PSS)3 (PAH/PSS)3x (PAH/PSS)3xx (PAH/PSS)3xx (PAH/PSS)3xx

Feed

UV—2 h

UV—2 h UV—3 h UV—4 h

DI DI DI DI DI DI DI DI

Draw

water water water water water water water water

0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M

PRO mode

MgCl2 MgCl2 MgCl2 NaCl NaCl NaCl NaCl NaCl

FO mode

Water flux (LMH)

Reverse salt flux (gMH)

Js/Jv (g/L)

Water flux (LMH)

Reverse salt flux (gMH)

Js/Jv (g/L)

53.53 7 0.42 31.407 2.51 21.007 1.00 19.53 7 1.50 17.73 7 2.42 13.707 0.26 12.037 0.42 11.807 0.74

7.497 1.29 2.777 0.87 1.257 0.27 222.60 7 27.62 93.26 7 19.51 43.52 7 6.55 21.93 7 1.64 12.93 7 0.92

0.1407 0.023 0.0877 0.023 0.0607 0.014 11.5117 2.324 5.349 7 1.454 3.175 7 0.456 1.821 7 0.079 1.096 7 0.022

23.33 7 1.30 15.807 0.72 14.53 7 2.34 18.007 0.80 15.87 7 0.81 12.807 0.80 11.68 7 0.77 11.55 7 0.46

8.26 71.02 1.83 70.25 1.07 70.21 144.82 78.11 43.55 72.80 26.27 75.23 13.02 71.19 8.06 70.28

0.3557 0.050 0.1177 0.021 0.0767 0.023 8.0607 0.650 2.752 7 0.270 2.0717 0.500 1.114 7 0.068 0.6987 0.007

60

20

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FO 5 PRO

10

Water flux (LMH)

PRO

30

15 40 PRO 10

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PRO

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FO 0

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Salt reverse flux (gMH)

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Salt reverse flux (gMH)

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Water flux (LMH)

20

60

0.2

0.3

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0

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0

GA concentration (wt %)

1

2

3

GA crosslinking time (h)

Fig. 7. Effect of (A) GA concentration under 2 h reaction time and (B) GA crosslinking time using 0.1 wt% GA concentration on the performance of (PAH/PSS)3x membranes.

20 FO

10

10 5

25

250

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PRO

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FO 10

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Reverse salt flux (gMH)

15

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Water flux (LMH)

30

PRO FO

0 0

1

0 2

3

4

Number of bilayers, n

FO

0 0

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0 3

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Fig. 8. Effect of the number of polyelectrolyte bilayers on the performance of (PAH/PSS)nxx UV—3 h membranes.

an increase in salt rejection (i.e., a lower Js/Jv) and a decrease in water flux. It is also observed that a higher water flux is obtained when the LbL film is faced toward the draw solution (i.e., PRO mode) than toward the PAN porous substrate (i.e., FO mode). This is in good agreement with the FESEM images (Fig. 11) where a dense LbL film is adhered onto a dense top layer of a PAN support and hence the top layer of the resultant LbL membrane has a higher salt rejection than the bottom layer. Therefore, the water flux is higher when the top layer of the LbL film is oriented to the draw solution (i.e., PRO mode) due to the less severe ICP than the FO mode. Fig. 8 presents the FO performance of (PAH/PSS)nxx—UV 3 h membranes with the number of bilayers (n) from 1 to 3 using 0.5 M NaCl and 0.5 M MgCl2 as draw solutions. Obviously, the

water flux decreases significantly when the number of bilayers increases. Because the higher number of bilayers leads to the thicker active layer of the LbL membrane which could reduce the permeation rate due to the higher resistance. Interestingly, the membrane’s reverse salt flux is not affected when changing the bilayer number. This implies that an increase in the number of bilayers does not necessarily enhance membrane’s performance in this process because the first bilayer could form an almost defect-free film onto the support. The (PAH/PSS)3xx membranes were further investigated under a longer UV exposure time. It can be seen clearly from Tables 1 and 2 and Fig. 9 that an increase in UV irradiating time significantly improves the membrane’s salt rejection toward NaCl with a decrease in water flux. This phenomenon is similar to the

P.H.H. Duong et al. / Journal of Membrane Science 427 (2013) 411–421

20

Water flux (LMH)

15

10

20 FO

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20 150 15

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Water flux (LMH)

40

417

1

2

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0 0

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UV crosslinking time (h)

1

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UV crosslinking time (h)

Fig. 9. Effect of the UV crosslinking time on the performance of (PAH/PSS)3xx membranes. 20

15

(PAH/PSS)3xx - PRO (PAH/PSS)3xx - FO 13

(PAH/PSS)1xx - PRO (PAH/PSS)1xx - FO

Reverse salt flux (gMH)

Water flux (LMH)

15 11

9

7

10

5 5

(PAH/PSS)3xx - PRO (PAH/PSS)3xx - FO (PAH/PSS)1xx - PRO (PAH/PSS)1xx - FO

3 0

0.1

0.2

0.3

0.4

0 0.5

Draw concentration, NaCl (M)

0

0.1

0.2

0.3

0.4

0.5

Draw concentration, NaCl (M)

Fig. 10. The water fluxes and salt leakages of 4 h UV-crosslinked (PAH/PSS)1xx and (PAH/PSS)3xx membranes under the FO and PRO testing modes with various NaCl draw concentrations and DI water as the feed.

effect of increasing the degree of crosslinking in the previous session. The results indicate that a higher degree of PSS crosslinking is achieved with a longer UV irradiating time which is in good agreement with the literature [37]. An increase in UV irradiating time from 2 to 4 h results in a substantial decrease in reverse salt flux while only a slightly decrease is observed in water flux when using 0.5 M NaCl as a draw solution. Hence, the Js/Jv ratio has improved with the increase in UV irradiating time. The (PAH/PSS)3xx membrane after 4 h UV irradiation has the smallest Js/Jv ratio among the other (PAH/PSS)3xx membranes at around 1 g/L under the PRO mode using 0.5 M NaCl as a draw solution. Interestingly, there is no change in the membrane’s salt rejection toward MgCl2 when increasing the UV irradiating time from 2 to 4 h. This suggests that 2 h UV irradiation could be enough to obtain a (PAH/PSS)3xx membrane with a very good salt rejection (around 1 gMH) toward MgCl2 at a reasonable water flux of 21 LMH using 0.5 M MgCl2 as a draw solution under the PRO mode. The enhancement of water flux due to the decrease in the number of bilayers is not observed at low draw solution concentrations but more significant under high draw solution concentrations as shown in Fig. 10. This phenomenon is most likely attributed by the presence of NaCl in LbL layers as

aforementioned in Section 3.2. The residual NaCl provides additional osmotic pressure and may outperform the low concentration of draw solutions in determining the water flux. Interestingly, after UV irradiation for 4 h, a (PAH/PSS)1xx membrane could achieve water fluxes upto 11 and 10.5 LMH with the reverse salt fluxes of 8 and 5 gMH using 0.3 M NaCl as a draw solution under PRO and FO modes, respectively. Therefore, it can be concluded that one bilayer LbL FO membrane with a very good rejection to MgCl2 and an acceptable rejection to NaCl can be fabricated using this method. In comparison with previously reported LbL methods [9,22,23] which require many bilayers to get the almost defect-free membranes, the newly developed method requires the lowest number of bilayers while can achieve reasonable membrane’s rejection to NaCl in FO application which none of the previously reported LbL membranes could achieve. 3.4. Morphologies by FESEM PAN was used as a model support for the deposition of polyelectrolyte layers because of its highly negative charge surface with a relative small pore size of the selective layer. All PAN membranes used in this study were firstly hydrolyzed with NaOH to enhance the surface charge [10] and hydrophilicity [36]. The

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A: (PAH/PSS)3x Top x 20K

Cross-section x 800

Porous sublayer x 50K

Bottom x 50K

Cross-section x 800

Porous sublayer x 50K

Bottom x 50K

Cross-section x 800

Porous sublayer x 50K

Bottom x 50K

B: (PAH/PSS)3xx UV-2h Top x 20K

C: (PAH/PSS)3xx UV-2h Top x 20K

1 µm

10 µm

100 nm

100 nm

Fig. 11. Morphology of different LbL membranes.

thickness of all membranes were about 60 78 mm. A FESEM technique was used to examine the LbL membrane morphology. All FESEM studies were carried out using three bilayers LbL membranes. As observed under FESEM, the degree of crosslinking affects the morphology of the active layer of the LbL membrane. A comparison of top surface Fig. 11, the higher degree of crosslinking, the smoother LbL membranes but with a larger pattern can be obtained (Fig. 11(C) left) which may indicate that the UV exposure chemically alters the PSS layers. The bottom surfaces as well as cross-sectional morphologies of the H-PAN supports remain unchanged after UV irradiation under different exposure times. This suggests that the porosity of the H-PAN supports may not change significantly after UV irradiation. This is in agreement with the pure water permeability (PWP) of H-PAN and 4 h UV irradiated H-PAN, they are around 20 LMH bar  1 for both membranes.

(PAH/PSS) x

(PAH/PSS)

PAN-before UV

1900

1700

1500

1300

1100

900

Wave number (cm-1) Fig. 12. FTIR spectra of hydrolyzed PAN, (PAH/PSS)3 and (PAH/PSS)3x membranes.

UV – 2 h

3.5. FTIR characterization Fig. 12 proves that the LbL film is successfully deposited onto the H-PAN support as indicated by the presence of the SO3 group of the PSS polyelectrolyte at 1006, 1035 and 1126 cm  1. After exposure to the GA solution, the PAH layers are crosslinked as proven by the presence of the new peak at 1627 cm  1 which belongs to the stretching vibration of the –CHQN– group after the GA crosslinking [48]. From Fig. 13, the absorption peaks at 2245 and 1560 cm  1 indicate the stretching vibrations of the –CRN and the –COONa groups of H-PAN, correspondingly [36,49]. It can be seen that the H-PAN is also crosslinked under UV exposure as indicated by the presence of the new peak at 1616 cm  1 (Fig. 13). It could be assigned to the –CQN stretching vibration of PAN which occurs after crosslinking [50]. The existence of crosslinked PSS after UV

– SO

–C=N

Before UV

– C≡N

– C=N – COO2200

2000

1800

1600

1400

1200

1000

800

Wave number (cm-1) Fig. 13. FTIR spectra of hydrolyzed PAN supports before and after UV irradiation.

P.H.H. Duong et al. / Journal of Membrane Science 427 (2013) 411–421

irradiation can be proven by the decrease in the intensity of the –C–H stretching vibrations of the –CH2 group at 2860 and 2935 cm  1, and the –CH associated with styrene group at 2900 cm  1 (see Fig. 14); [37,51]. Moreover, the crosslinking of PSS is also proven by the decrease in the intensity of the stretching vibration peaks of the –SO3 group as shown in Fig. 14; [37]. The chemical changes of the membranes after LbL depositions of PAH/PSS and crosslinking were further characterized by XPS (Figs. 15 and 16). The depositions of PSS onto the HPAN substrate can be confirmed by the presence of the new peaks of S2s and S2p3 (Fig. 15) which belong to PSS on the (PAH/PSS)3 sample

(PAH/PSS)3xx UV 2h (PAH/PSS)3x – CH – SO3 3400

2900

2400

1900

1400

900

Wave number (cm-1) Fig. 14. FTIR spectra of (PAH/PSS)3x membranes before and after UV irradiation.

C1s O1s N1s

(PAH/PSS)3xx UV 2h

S2s

S2p3

(PAH/PSS)3x

(PAH/PSS)3

H-PAN

800

700

600

500

400

300

200

100

Binding Energy (eV) Fig. 15. XPS spectra of hydrolyzed PAN, (PAH/PSS)3, (PAH/PSS)3x and (PAH/ PSS)3xx UV—2 h membranes.

≡N

419

compared with the HPAN sample. The presence of PAH has been demonstrated by the new N1s peak at 401.4 eV corresponding to the nitrogen element of the NH3þ group of PAH (Fig. 16B). After GA crosslinking, the intensity of the N1s peak at 399.5 eV (Fig. 16C) increases due to the new –NQ group of crosslinked PAH. Moreover, the reduce in the intensity of S2s and S2p3 of PSS (Fig. 15) after UV exposure, (PAH/PSS)3xx UV 2 h, compared with (PAH/ PSS)3x proves that PSS has been crosslinked under UV.

3.6. LbL membrane morphology characterized by PAS Three bilayer LbL membranes were used to study the difference in morphology after crosslinking due to the fact that a thicker selective layer of a three bilayer LbL membrane could give a better analysis signal compared to a one bilayer LbL membrane. Fig. 17 shows the measured S parameter data as a function of incident positron energy for the original LbL film—(PAH/PSS)3 and the ones after GA and UV crosslinking with different UV irradiating times. The S parameter increases rapidly near the surface which is a typical phenomenon because of back diffusion and scattering of positronium annihilation. At about 1 keV, there is a small plateau for all the three cases. Following that, the S parameter increases to another plateau and attain its maximum value. This variation of S parameter indicates fine structure change in the LbL films, which can be represented by a threelayer structure model: top selective layer, transition layer from top dense layer to porous PAN layer and porous PAN layer [52]. The fitted result is plotted in Fig. 17 and a good match is obtained with the original S parameter data. In addition, the first part of S parameter (0 keV–3 keV) which contains the information of top selective layer is enlarged and plotted in Fig. 18. The fitted results of S parameter; namely, S1 and first layer thickness L1, are tabulated in Table 3. It can be seen that the original LbL film has the largest S1 value which infers a larger fractional free volume. With increasing UV irradiation time, S1 value decreases, which shows a decrease in the fractional free volume. This is consistent with our analysis that a longer UV irradiation time can result in a higher degree of crosslinking. From Table 3, it can also be seen that the first layer thickness L1 of the LbL films is about 30–40 nm, which is a typical thickness for 3 bilayers LbL film prepared from PAH and PSS polyelectrolytes under 1 M NaCl deposition solutions [34,47]. It is interesting that L1 increases to a small extent after UV irradiation, which is because PAN substrate can possibly be crosslinked by UV [50] that slightly increases the thickness of the dense layer. As a consequence, the resultant membranes after UV irradiation shows a decrease in reverse salt flux.

NH3+

NH3+

-N=, ≡N ≡N

405

403

401

399

397

405

401 403 399 Binding Energy (eV)

397

405

403

401

Fig. 16. XPS N 1s spectra of the surfaces of hydrolyzed PAN, (PAH/PSS)3 and (PAH/PSS)3x membranes.

399

397

420

P.H.H. Duong et al. / Journal of Membrane Science 427 (2013) 411–421

0.475

0.470

S Parameter

0.465 Porous layer 0.460 Transition layer

(PAH/PSS)3 (PAH/PSS)3xx - UV 2h (PAH/PSS)3xx - UV 4h Fitted (PAH/PSS)3 Fitted (PAH/PSS)3xx - UV 2h Fitted (PAH/PSS)3xx - UV 4h

0.455

0.450 Top layer 0.445 0.00

5.00

10.00

15.00

20.00

25.00

Positron Incident Energy (keV)

about 11 LMH and a reverse salt flux of 8 gMH under the PRO test using 0.3 M NaCl as the draw solution. As a result, this technique may be a promising method for the fabrication of FO membranes. Besides, more studies on the support should be carried out in order to further increase water flux by changing the PAN support to a highly permeable one. The UV exposure time could be shortened by the optimization of the UV power and distance. Although LbL membrane technology has been studied for many years with good achievements in separation, LbL membranes still have not been commercialized. One of the main reasons could be the multiple steps in the deposition process. Our newly developed method with only one bilayer will reduce the large number of adsorption and rinsing steps in the previous LbL techniques. Therefore, this study may offer a new approach to advance LbL membranes.

Fig. 17. S parameters versus positron incident energy for top layers of LbL membranes fitted through VEPFIT in three-layer mode.

Acknowledgments 0.466

S Parameter

0.461

0.456

0.451

Transition layer

(PAH/PSS)3 (PAH/PSS)3xx - UV 2h (PAH/PSS)3xx - UV 4h Fitted (PAH/PSS)3 Fitted (PAH/PSS)3xx - UV 2h Fitted (PAH/PSS)3xx - UV 4h

Top layer

0.446 0.00

1.00

2.00

3.00

The authors would like to thank the Singapore National Research Foundation under its Competitive Research Program for the project entitled ‘‘Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination: Module designs and integrated systems for sustainable processes’’ (Grant number: R-279-000-339-281). Thanks are also due to Ms. J. Koh for her help on the experimental works. We specially thank Ms. R.C. Ong and Ms S. Zhang for their help and valuable suggestions. We are also appreciated Ms. Y. Cui for her valuable comments.

References

Positron Incident Energy (keV) Fig. 18. Close up view on the S parameters versus positron incident energy for top layers of LbL membranes fitted through VEPFIT in three-layer mode.

Table 3 VEPFIT results for the S parameter and thickness of the LBL layers. Membrane

a

a

(PAH/PSS)3 (PAH/PSS)3xx UV—2 h (PAH/PSS)3xx UV—4 h

0.4585 7 0.0007 0.4582 7 0.0007 0.4570 7 0.0007

34.3 725.6 37.6 722.9 37.8 728.7

S1

L1 (nm)

a S1 and L1 are the S parameter and the thickness of the first layer based on multilayer analysis from VEPFIT analysis in three-layer mode, respectively.

4. Conclusions This work has studied the possibility of using LbL polyelectrolyte technique with the aid of crosslinking methods to fabricate FO membranes with reasonable rejections toward NaCl and very good rejection toward MgCl2. It can be concluded that LbL assembly of polyelectrolytes is a flexible technique which can be used to fabricate FO membranes dependent on applications. A LbL membrane with a high water flux of 55 LMH and a reasonable MgCl2 reverse flux of 7.5 gMH using 0.5 M MgCl2 as the draw solution could be obtained by assembling 3 bilayers of PAH and PSS with the presence of 1 M NaCl in the deposition solutions onto the H-PAN support. Moreover, this membrane can be further modified by crosslinking the PAH and PSS layers to become a FO membrane with a reasonable reverse salt flux of 8 gMH and a reasonable water flux of 10.9 LMH using 0.3 M NaCl as the draw solution. Specially, by using this crosslinking method, a LbL membrane with only one bilayer could achieve a water flux of

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