Double-Skinned Forward Osmosis Membranes for Reducing Internal

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Double-Skinned Forward Osmosis Membranes for Reducing Internal Concentration Polarization within the Porous Sublayer Kai Yu Wang,† Rui Chin Ong,† and Tai-Shung Chung*,†,‡ Department of Chemical and Biomolecular Engineering and Singapore-MIT Alliance, National UniVersity of Singapore, Singapore 117576

A scheme to fabricate forward osmosis membranes comprising a highly porous sublayer sandwiched between two selective skin layers via phase inversion was proposed. One severe deficiency of existing composite and asymmetric membranes used in forward osmosis is the presence of unfavorable internal concentration polarization within the porous support layer that hinders both (i) separation (salt flux) and (ii) the performance (water flux). The double skin layers of the tailored membrane may mitigate the internal concentration polarization by preventing the salt and other solutes in the draw solution from penetrating into the membrane porous support. The prototype double-skinned cellulose acetate membrane displayed a water flux of 48.2 L · m-2 · h-1 and lower reverse salt transport of 6.5 g · m-2 · h-1 using 5.0 M MgCl2 as the draw solution in a forward osmosis process performed at 22 °C. This can be attributed to the effective salt rejection by the double skin layers and the low water transport resistance within the porous support layer. The prospects of utilizing the double-selective layer membranes may have potential application in forward osmosis for desalination. This study may help pave the way to improve the membrane design for the forward osmosis process. 1. Introduction The interrelated issues of water, energy, and environment, are affecting the future of mankind because of overpopulation and scarcity of exhaustible resources over the past few decades.1 The pervasive water scarcity is perhaps the most serious constraint on sustainable development, particularly in droughtprone and environmentally polluted areas. To solve or alleviate the global water scarcity problem, tremendous efforts were made to identify novel methods of wastewater reclamation and seawater desalination at reduced energy consumption.2 Membranebased separation has the advantage of avoiding thermally imposed efficiency limitations on heat utilization compared to thermal separation techniques. The development and application of membrane technologies is one of the most significant recent advances in chemical, environmental, and biological process engineering for sustainable growth. Membrane processes are commonly distinguished based on the main driving forces that are employed to accomplish the separation. For example, water purification by reverse osmosis (RO) uses high hydraulic pressures as the driving force. Recently, the forward (direct) osmosis (FO) process represents an emerging technology that has attracted considerable attention in various fields including wastewater treatment,3,4 seawater desalination,5-8 pharmaceutical and juice concentration,9,10 power generation,11-13 and protein enrichment.14 In a FO process, the osmotic pressure gradient between the concentrated draw solution and the saline feed supplies a spontaneous driving force for water transport.15 The water molecules are drawn spontaneously from the feed of low osmotic pressure across a semipermeable membrane to the draw solution of high osmotic pressure. The driving force of osmotic pressures used in FO can be significantly higher than the hydraulic pressures used in RO, subsequently resulting in a higher theoretical water flux. In addition, the possibility of * To whom correspondence should be addressed. E-mail: chencts@ nus.edu.sg. Fax: (65) 6779-1936. † Department of Chemical and Biomolecular Engineering. ‡ Singapore-MIT Alliance.

obtaining a high water recovery up to 85% from a seawater source is another advantage for the seawater desalination by FO to reduce the discharge volume.15 Compared to traditional pressure-driven membrane processes, FO may offer the advantages of higher rejection to a wide range of contaminants and lower membrane-fouling propensities.3,16 However, a fundamental hurdle that deters the successful implementation of this revolutionary process is the lack of desirable membranes with the appropriate separation performance. Generally, the existing composite or asymmetric membranes consist of a thick, porous, nonselective support layer (around 150 µm) covered by an ultrathin selective layer (less than 1.0 µm). Currently, the only commercially available FO membrane was developed by HTI (Hydration Technologies Inc., OR) through coating cellulose triacetate on the woven or/and nonwoven mesh.17-19 This proprietary membrane achieves a superior FO performance to those using the standard RO thin film composite (TFC) membranes as FO membranes because the former has a relatively thinner membrane thickness (less than 50 µm) and does not have a fabric support layer compared to the latter.3 However, all membranes used in the FO process suffer from the adverse influence of internal concentration polarization (ICP). It results in a sharp concentration gradient formed within the porous support layer. This ICP seriously counteracts the driving force and significantly decreases the water permeation flux.20-24 This ICP phenomenon is quite different from the external concentration polarization encountered in the pressure driven membrane processes.25-27 Moreover, the most important fact is that the ICP within the membrane porous support cannot be eliminated by enhancing the crossflow velocity and turbulence along the membrane surface, as shown in Figure 1. The effect of ICP on the FO performance has been verified experimentally for different membrane orientations, that is, pressure retarded osmosis (PRO) mode and forward osmosis (FO) mode. The lower water flux obtained in the FO mode compared to the PRO mode is attributed to the existence of adverse ICP in the porous support layer.9,20-24,28,29

10.1021/ie901592d  2010 American Chemical Society Published on Web 04/20/2010

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Figure 1. Transport profile in the FO process using RO and commercial FO membranes under different membrane orientations.

Figure 2. Structural comparison of composite RO membranes with single selective layer (left) and specially designed FO membranes with double skin layers (right).

For water production using FO, a membrane is utilized as a semipermeable barrier between the draw and feed solutions which allows the transport of water while rejecting dissolved solutes. Another separation and reconcentration process is required to produce high quality water and regenerate the draw solution. The development of improved semipermeable membranes is critical for the advancement of the FO process. The requirements for membranes used in RO and FO applications are significantly different. One major consideration in the fabrication of RO membranes is the mechanical robustness of the porous support layer to withstand high pressures. However, the asymmetric/composite membrane structure with a thicker support layer designed for RO may not be applicable for the FO application.17-19 The ultimate goals in a FO process are high water flux and high solute rejection. These objectives may be attainable via a membrane that comprises two thin selective skin layers and a highly porous sublayer with minimal resistance to water transport. The two thin selective skin layers may have different degrees of selectivity for ions and solutes. The prototype of the desirable FO membrane structure is shown in Figure 2. Besides, sufficient membrane mechanical strength is also essential for sustaining low pressure operations. In this work, a novel membrane structure design for FO applications is proposed to reduce the adverse influence of ICP within the porous support layer. The membrane structure comprises double selective skins with a porous support layer in-between. The effect of membrane structure on the water and salt transports is investigated together with its potential application in drawing water from a salted feed. 2. Experimental Section 2.1. Materials and Methods. Cellulose acetate (CA-39830, acetyl content, 39.8%) was supplied by Eastman, USA. Acetone (>99.5%), and N-methyl-2-pyrrolidone (NMP, >99.5%) used as solvents, magnesium chloride (MgCl2, 98%) and sodium

chloride (NaCl, 99.5%) was purchased from Merck, Germany. The deionized (DI) water used in experiments was produced by a Milli-Q unit (Millipore, USA) with the resistivity of 18 MΩ cm. 2.2. Fabrication and Characterization of Cellulose Acetate Double-Skinned Layer Membrane. Cellulose acetate is the first material that has been used for the fabrication of early commercial RO membranes.30-36 In this work, a homogeneous polymer solution was prepared by dissolving the cellulose acetate (22.5 wt %) in a mixture of acetone (5.0 wt %) and NMP (72.5 wt %). To fabricate the FO membranes, the polymer solution was poured on a horizontal glass plate (21 cm ×30 cm), and a casting knife with a gap of 50 µm was used to spread the polymer solution on the glass surface. The glass plate with a uniform polymer layer was immersed into a coagulant bath containing tap water for phase inversion at 25 °C. There was a very short evaporation period of about 30 s before plunging the cast membrane into the coagulant bath of water. After being peeled off from the glass plate, the membrane was rinsed with tap water for 6 h to remove the residual solvent. The selective top skin layer was formed during the phase inversion. The freshly as-cast CA film was water-swollen, and the membrane structure was therefore “tightened” via hot water annealing which would improve its salt rejection performance. The cast CA flat sheet film was annealed in hot water at 85 °C for 15 min and then stored in tap water prior to use. To study the membrane morphology, the wet CA membranes were dried by a freeze-dryer (ModulyoD, Thermo Electron Corporation, USA) and fractured in the liquid nitrogen followed by platinum sputtering using a Jeol JFC-1100E ion sputtering device. The membrane morphologies were observed by a field emission scanning electronic microscope (FESEM, JEOL JSM-6700). 2.3. Measurements of Water Permeability and Salt Permeability of CA Membrane. The permeability and salt rejection of the FO membrane was determined by testing the membrane in the RO mode in a pressurized cross-flow filtration cell. The feed flow rate was kept at 0.3 m/s. The water permeability coefficient (A) was obtained from pure water flux within the applied transmembrane pressure of 0-1.0 bar. The salt rejection (Rs) was determined using a feedwater containing 100 ppm NaCl or MgCl2, based on conductivity measurements of the permeate and feedwater. The salt permeability coefficient (B), which is the intrinsic property of a membrane to retain salt, was determined on the basis of the solution-diffusion theory:11,32,37

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(

Rs ) 1 +

B A(∆P - ∆π)

-1

)

(1)

where ∆P and ∆π are the pressure difference and osmotic pressure difference across the membrane, respectively. 2.4. Water Flux through Forward Osmosis Tests. FO experiments were conducted on a lab-scale cross-flow filtration unit, as shown in Figure 3. The crossflow permeation cell was a plate-and-frame design with a rectangular channel (8.0 cm in length, 2.0 cm in width, and 0.25 cm in height) on each side of the membrane. There is no spacer in the flowing channel. The flow velocities of solutions during the FO testing were kept at 0.2 L/min (corresponding to a crossflow velocity of 8.3 cm/s) for both the feed and draw solutions which cocurrently flowed through the cell channels. The temperatures of the feed and draw solutions were maintained at 22 ( 0.5 °C. The pressures at the two channel inlets were kept at 6 kPa. MgCl2 and NaCl solutions with different concentrations were prepared and used as the draw solutions. DI water and 3.5 wt % NaCl solution was used as the feed solution. During the FO testing, the dilution of the draw solution was ignored since the ratio of water permeation flux to the volume of the draw solution was less than 2%. The membranes were tested in two different operation modes. In the top mode, the selective top layer was on the draw solution side while in the bottom mode, the draw solution was in contact with the selective bottom layer. Under the same operating conditions (draw solution, flowing velocities) the external osmotic driving forces should be equal for both the top and bottom modes. Each experiment was conducted for 20 min and repeated three times. The water permeation flux (Jw, L · m-2 · h-1, abbreviated as LMH) is calculated from the volume change of the feed or draw solution. Jw )

∆V S∆t

(2)

where ∆V (L) is the volume of water that has permeated across the membrane in a predetermined time ∆t (h) during the FO process and S is the effective membrane surface area (m2). When using DI water as the feed, the reverse salt flux (salt reverse-diffusion) from the draw solution to the feed was calculated by measuring the salt concentration in the feed solution at the end of each experiment. The salt concentration in the feed was determined by the conductivity measurement based on a calibration curve for the single salt solution. Then, the reverse salt flux, Js in g · m-2 · h-1 (abbreviated as gMH), can be obtained by considering the increase in the feed salt content: Jw )

∆(CtVt) S∆t

(3)

where Ct and Vt are the salt concentration and the volume of the feed at the end of FO tests, respectively. 3. Results and Discussion 3.1. Morphology of the CA Double-Skinned Membrane. The newly developed double-skinned CA membrane has a thickness of 36 ( 3 µm. Figures 4 and 5 show the membrane cross-section morphology that consists of two skin layers with different thicknesses. The selective top layer is about 0.15 µm; whereas the selective bottom layer is about 0.05 µm. There is almost little visible difference between untreated and heat-annealing CA membranes. One highly porous reticulated sublayer is

Figure 3. Schematic diagram of the laboratory-scale FO setup (cocurrent crossflow of feed and draw solutions).

located between two selective skin layers. These two tailored thin skin layers may ensure the rejection of ions, sugar, and other dissolved solutes, while allowing water to be transported freely; the middle porous sublayer supports two thin selective layers. As a result, depending on pore size and solute size, the solutes in the feed and draw solutions may be prevented from penetrating into the membrane support layer. This type of trilayer membrane structure prevents the direct contact of the porous support layer with the feed and draw solutions, thus retarding the build-up of an internal concentration gradient in the porous sublayer. Therefore, the salt concentration gradient within the middle porous layer may be mitigated. However, the external concentration polarization (ECP) still exists during the FO process, but this has less effect on the water permeation flux and can be reduced by enhancing the crossflow velocity and turbulence. The proposed water transport profiles for this prototype CA FO membrane are demonstrated in Figure 6. 3.2. Water Transport and Separation Properties of CA Double-Skin Membranes. The membrane separation properties, that is, the water permeability coefficient A and the salt permeability coefficient B (100 ppm NaCl, MgCl2 as the feed), are determined by testing the CA double-skin FO membranes in a cross-flow RO mode at 1.0 bar. External concentration polarization is minimized by performing the filtration tests at a relatively low pressure and under a cross-flow velocity of 0.3 m/s. Table 1 shows that the pure water permeability is 0.78 L · m-2 · h-1 · bar-1, and the salt permeability for MgCl2 and NaCl are 0.25 L · m-2 · h-1 (7.5 × 10-8 m/s) and 0.46 L · m-2 · h-1 (1.73 × 10-7 m/s), respectively, which are comparable to those of the commercial membranes.23 The membrane exhibits a higher NaCl permeability than MgCl2 which may affect the salt reverse flux in the forward osmosis. 3.3. Water Flux through the Double-Skinned CA Membrane. For the double-skinned CA membrane after thermal annealing, in both top and bottom operation modes, the water flux increases with an increase in draw solution concentration because of greater effective osmotic pressures which provide larger driving forces as depicted in Figure 7. However, the water flux vs osmotic pressure departs from linearity, especially at high osmotic pressures. This is due to the dilutive external concentration polarization within the boundary layer because of the low crossflow velocity applied. Another reason is the

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Figure 4. Morphology of the as-cast CA double-skinned FO membrane.

Figure 5. Morphology of the CA double-skinned FO membrane after thermal annealing at 85 °C for 15 min.

Figure 6. Transport profile in the FO process using the CA double-skinned FO membrane under different membrane orientations.

salt leakage from the draw solution, although it is little, which decreases the effective driving force. Using 5.0 M MgCl2 and DI water as the draw and feed solutions, respectively, a water flux of 48.2 LMH is achieved in the bottom mode and this is higher than the 27.4 LMH obtainable by the top mode. The reverse salt fluxes under bottom and top modes are reduced to

as low as 6.5 and 3.9 gMH, respectively. The water flux is almost 4 orders of magnitude higher than the reverse salt flux for the corresponding draw solution used in the FO process. The low reverse salt flux under both membrane orientations also demonstrates that the fabricated double-skinned layer FO membrane is a kind of semipermeable membrane. In addition,

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Table 1. Water Transport and Separation Properties of CA Double-Skinned Membranes water permeability, salt permeability, salt rejection, A, L · m-2 · h-1 · bar-1 B, L · m-2 · h-1 (m/s) Rs, 1.0 bar

feed

0.78 ( 0.11 0.75 ( 0.06 0.72 ( 0.05

DI water 100 ppm MgCl2 100 ppm NaCl

0.25 (7.5 × 10-8) 0.46 (1.7 × 10-7)

79% 58%

the reverse salt diffusion rate did not change much with respect to the variation in the draw solution concentration. This may be due to the high MgCl2 rejection by the membrane. Generally, a higher water flux is obtained in the PRO mode as compared to the FO mode when commercially available RO and FO membranes are used in forward osmosis. This is attributed to the presence of serious ICP in the porous support layer for asymmetric/composite membranes.9,16,20-24,28,29 The newly developed double-skinned membranes display the opposite results because a thinner bottom selective layer produces a higher water flux by the bottom mode. The water flux in the top mode is less than that in the bottom mode under the same draw solution concentration. This implies that there is internal concentration polarization within the top layer. It may be because the top skin has pores or water channels and is not perfectly semipermeable compared to the bottom skin although the thickness of the top skin layer is higher than that of the bottom skin layer. From the SEM images in Figure 5,

the thickness of the selective top layer is larger than that of the selective bottom layer. Assuming the negligible water transport resistance within the boundary layers, the water transport resistance through the double-skinned layer membrane may be represented as 1 1 1 1 ) + + km ktop ksub kbottom

(4)

where km (m s-1) is the total water transport coefficient, ktop, ksub, and kbottom (m s-1) are the water transport coefficients of the top, sublayer, and bottom layers, respectively. The water transport resistance through the reticulated porous sublayer may be ignored owing to its highly porous structure and larger network interstice. The water transport resistance across the thicker selective top layer may be higher than that across the thinner selective bottom layer. The thickness difference between two skin layers will produce different water transport resistances and thus will induce a water flux difference when the draw solution contacts with membrane surfaces. This also results in the water flux differences between two membrane orientations. Researchers have proposed several mechanisms which are not fully satisfactory and still inconsistent to explain the osmosis phenomenon.38-41 One hypothesis is derived from Van’t Hoff who believed that water forces its way, penetrating through the

Figure 7. Water permeation flux and reverse salt flux against osmotic pressure and MgCl2 concentration in a draw solution. Experimental conditions: crossflow velocity and temperature of both feed and draw solutions of 8.3 cm/s and 22 ( 0.5 °C, respectively. Table 2. Overview of Recent Researches on the FO Process with Different Membranes operation temp (°C)

draw solution

feed

draw solution flow rate

membrane orientation

0.2 L/min 0.2 L/min 1.5 L/min 1.5 L/min 1.5 L/min 5.5 L/min 0.3 m/s 0.21 m/s

bottom top FO FO FO PRO PRO PRO

22 ( 0.5 22 ( 0.5 23 ( 1 23 ( 1 23 ( 1 20 ( 2 22.5 ( 1.5 20.0

DI DI DI DI DI DI DI DI

20.0

DI water

1.5 M NaCl

0.21 m/s

PRO

23 ( 0.5

DI water

5.0 M MgCl2

0.73 m/s

FO

23 ( 0.5

DI water

5.0 M MgCl2

0.067 m/s

PRO

22.5 20 ( 2 20 ( 1 20 ( 1 50

DI water DI water NaCl NaCl NaCl

5.0 M MgCl2 1.5 M MgSO4 4.0 M NaCl 4.0 M NaCl 6 M NH4HCO3

0.56 m/s 5.5 L/min 0.085 m/s 0.085 m/s 0.3 m/s

PRO FO PRO FO FO

water water water water water water water water

5.0 5.0 1.0 1.0 1.0 0.5 0.5 1.5

M M M M M M M M

MgCl2 MgCl2 NaCl NaCl NaCl NaCl NaCl NaCl

membranes CA double-selective layer membrane CA double-selective layer membrane FO flat sheet membrane A, HTI. FO flat sheet membrane B, HTI. FO flat sheet membrane C, HTI. FO flat sheet membrane, HTI. FO flat sheet membrane, HTI. cellulosic RO membrane with the fabric layer removed, GE Osmonics polyamide RO membrane with the fabric layer removed, Dow Filmtec dual-layer (PBI-PES/PVP) nanofiltration hollow fiber membrane dual-layer (PBI-PES/PVP) nanofiltration hollow fiber membrane hollow fiber membrane TS80 NF TFC membrane, TriSep FO flat sheet membrane, HTI. FO flat sheet membrane, HTI. FO flat sheet membrane, HTI.

water flux (LMH)

reverse salt flux (gMH)

48.2 27.4 16.8 12.4 6.6 8.5 18.6 36.0

6.5 3.9 21.8 9.5 1.0 7.4

8.1

ref this work this work 43 43 43 4 23 17 17

24.2

0.45

29

33.8

0.55

29

11.2 1.1 37.8 27.0 36.0

0.04

28 4 16 16 22

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Figure 8. Water permeation flux and reverse salt flux against NaCl concentration in draw solution (22 ( 0.5 °C).

membrane from the feed to the draw solution.42 More likely the membrane allows water flux due to the cohesive forces in the water itself. Residual water is pulled out of the membrane midlayer by osmosis and water from the feed solution moves into that space across the selective layer due to a concentration gradient. Therefore, the thinner the semipermeable layer, the higher the water flux. Since the draw solution in the bottom mode is against the thin selective bottom layer, it has a higher flux than that of the top mode where the draw solution is against the thick selective top layer. Nevertheless, the inherent mechanism of water transport through the double-skinned layer membrane according to the solution-diffusion principle seems difficult to understand at this moment and will be studied in the future. The reverse salt flux becomes higher when using NaCl as the draw solution which may be attributable to the lower membrane rejection of NaCl, as shown in Figure 8. The water permeation flux becomes lower at the same molar concentration because NaCl solutions produce lower osmotic pressures as the driving force compared to MgCl2 solutions. Clearly, the

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membrane structure needs further improvement to increase water flux and decrease reverse salt transport. 3.4. Water Flux against the Model 3.5 wt % NaCl Solution. Figure 9 shows the water transport performance as a function of osmotic pressure difference and MgCl2 concentration under different membrane orientations when using 3.5 wt % NaCl as the feed solution. The water permeation flux achieved is less than the water flux obtained when DI water is used as the feed because of the reduction in the driving force and the greater ECP at the feed side. Moreover, the water flux in the bottom mode is also higher than that in the top mode under the same draw concentration. However, the water flux difference between bottom and top modes during desalination is reduced compared to that when pure water is used as the feed solution. This is due to the reduction in overall osmotic pressure difference because of the use of 3.5 wt % NaCl as the feed solution. Moreover, because the concentration polarization, whether internal or external, is a function of water permeation flux and effective mass transfer coefficient, the reduced water flux will mitigate the concentration polarization which reduces the difference between top and bottom modes. Up to now, tremendous research efforts have been devoted to the identification of suitable membranes and appropriate draw solutions in the FO process in order to achieve high water flux, high salt rejection, and low energy input. However, the lower effective osmotic pressure difference induced by the ICP in the membrane porous layer results in a great difference between theoretical and experimental fluxes.19,24 From Table 2, the permeability (water flux/bulk osmotic pressure difference) of this prototype double-skinned membrane is still less than that of commercial FO membranes. Many challenges stay ahead to develop this kind of double-skinned layer membranes for real application. It is believed that through reducing the thickness of skin layers and optimizing the structure of the middle reticulated support layer, the water transport rate can be further improved and the salt leakage can be reduced simultaneously in the FO process. In addition, the newly developed FO membrane may drive further progress on FO membrane fabrication with nontraditional configuration and structure, new fundamentals on the FO theory, and separation technology. Further research on the pore size and selective skin formation

Figure 9. Water permeation flux against the osmotic pressure difference between draw solution and the bulk feed (MgCl2 as draw solution; feed, 3.5 wt % NaCl; 22 ( 0.5 °C).

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will be conducted to investigate the effect of membrane casting conditions, casting plate, polymer properties, solvent and additive, and the coagulant bath on membrane formation, pore size distribution, FO performance, and membrane fouling. 4. Conclusions In this work, a prototype double-skinned cellulose acetate membrane has been developed through phase inversion and thermal annealing. For its use in forward osmosis, it is found that water flux in the bottom mode is higher than that in the top mode which is contrary to results from other asymmetric or composite membranes under different membrane orientations. Depending on pore size and solute size, the double-skinned FO membrane may have potential to reduce the adverse influence of ICP within the membrane porous support layer. Although the formation of a second skin layer in this prototype FO membrane may induce additional water transport resistance and decrease water permeation flux, the adverse effect of the internal concentration polarization can be significantly mitigated. The membrane structure needs further improvement to increase water flux and decrease reverse salt transport. Acknowledgment The authors would like to thank King Abdullah University of Science and Technology (KAUST), Saudi Arabia, and National University of Singapore (NUS) for funding this research project with Grant Nos. R-279-000-265-597 and R-279000-265-598. Special thanks are due to Dr. Yang Qian, Ms. Low Bee Ting, Ms. Zhang Jiyuan, and Ms. Zhang Sui for their valuable assistance. Nomenclature A ) water permeability, L · m-2 · h-1 · bar-1 B ) salt permeability, L · m-2 · h-1 Ct ) salt concentration, mol · L-1 ECP ) external concentration polarization FO ) forward osmosis ICP ) internal concentration polarization Js ) reverse salt flux, g · m-2 · h-1 Jw ) water flux, L · m-2 · h-1 k ) water transport coefficient, m · s-1 PRO ) pressure-retarded osmosis S ) effective membrane surface area, m2 ∆t ) operation time interval, h ∆V ) water permeation volume, L Vt ) volume of the feed at a time interval of ∆t, L π ) osmotic pressure, bar Subscripts b ) bulk solution D ) draw solution side F ) feed solution side i ) inside of the active layer within the porous support

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ReceiVed for reView October 12, 2009 ReVised manuscript receiVed March 22, 2010 Accepted April 9, 2010 IE901592D