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Novel Cellulose Esters for Forward Osmosis Membranes Rui Chin Ong,† Tai-Shung Chung,*,† Bradley J. Helmer,‡ and Jos. S. de Wit‡ †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 Eastman Chemical Company, P.O. Box 1972 Kingsport, Tennessee 37662, United States

‡

S Supporting Information *

ABSTRACT: Syntheses and evaluation of novel cellulose esters consisting of a wide range of chemical compositions targeted for forward osmosis membrane fabrication have been carried out. Preliminary studies on the effects of the degree of substitution of hydroxyl, acetyl, and propionyl or butyryl functional groups on the membrane formation and permeation characteristics were conducted. Experiments results show that cellulose esters with high content of hydrophobic functional groups content exhibit great salt rejection, because of low water solubility and small hydrated free volume of hydrophobic cellulose esters. However, as the content of bulky hydrophobic functional groups increases, there is an accompanying increase in free volume due to poor chain packing, which causes a decrease in salt rejection. A high OH content in cellulose esters results in membranes with high salt and water permeation. Among the evaluated cellulose esters, those which have the highest hydrophobicity are unable to form membranes with defect-free selective layers via normal phase inversion casting conditions. gMH). However, their water fluxes usually did not exceed 20 LMH when a 2 M NaCl draw solution was used.22,27 Aiming to improve the water flux performance, many researchers shifted their asymmetric membranes made of the conventional phase inversion process to thin film composite (TFC) membranes fabricated via interfacial polymerization (IP). The pioneering TFC membranes for FO were synthesized by forming a thin polyamide (PA) selective layer on polysulfone and polyethersulfone (PES) porous substrates.28,29 Flat sheet TFC membranes were then fabricated by various research groups using different porous substrates such as polysulfone (Psf),30,31 electrospun polyethersulfone (PES)/Psf and PES nanofibers,32,33 polydopamine (PDA)-modified commercial Psf RO membranes with PET fabrics removed,34 fully sponge-like macrovoid-free sulfonated polymer,35 PES/ sulfonated Psf (SPsf),36 cellulose acetate propionate substrates37 and also zeolite embedded PA thin film on Psf substrates.38 Compared to CA and CTA membranes, TFC membranes generally have much higher water fluxes, because of the extremely thin but highly selective PA layer, which poses less resistance toward water flow. TFC membranes also offer the flexibility of tailoring support substrate and selective layer separately, which is impossible for conventional phase inversion asymmetric membranes. However, the challenges of TFC membranes lie in (1) the higher fouling propensity, (2) weak chlorine resistance of the selective PA layer and (3) possibly weak adhesion between the PA layer and the membrane substrate. Apart from the conventional asymmetric membranes and TFC membranes, layer-by-layer (LbL) FO membranes were also developed by assembling polyelectrolytes of opposite charges.39−41 The LbL membranes showed a high FO water

1. INTRODUCTION There has been a tremendous growth of interest in osmotic based membrane processes such as forward osmosis (FO) and pressure retarded osmosis (PRO) recently due to the demand for sustainable production of clean water and energy.1−9 In fact, the concept of membrane-based osmotic processes for water production and power generation has long been demonstrated and studied for decades.10−17 However, the lack of highperformance membranes has been the major obstacle rendering FO not feasible for large-scale applications. It is only until recently that considerable improvements and breakthroughs on FO membranes have been achieved. Earlier research works focused on the use of nanofiltration (NF) and reverse osmosis (RO) membranes for FO. Polybenzimidazole (PBI) NF hollow fiber membranes showed high rejections toward divalent ions but exhibited low fluxes for FO processes.17,18 The use of commercial RO membranes with their support fabric removed was also evaluated for FO applications.19 However, these earlier publications did not evaluate the reverse diffusion of draw solute across the membrane which is a detrimental phenomenon affecting the long-term performance in FO processes. Influenced by Hydration Technology Innovations’ (HTI) FO membranes,20 the use of cellulose acetate (CA) and cellulose triacetate (CTA) for FO membranes also received attention. A unique double-skinned CA FO membrane was invented which was capable of mitigating the effect of internal concentration polarization (ICP) within the membrane and reducing fouling propensity by the introduction of a second “skin” to the membrane.21,22 Subsequently, the transport phenomenon of double-skinned membranes was studied and it was mathematically proven to be indeed useful for ICP mitigation.23 Double-skinned hollow fibers were then developed accordingly for FO processes.24 Hollow fiber membranes made of CA and CTA have also been fabricated for FO processes.25,26 Some of these cellulose ester based membranes showed excellent salt rejections with reverse fluxes of draw solute as low as less than 5 g m2 h−1 (hereafter referenced as 5 © 2012 American Chemical Society

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flux, using only a 0.5 M MgCl2 draw solution. However, these LbL membranes have poor rejections toward monovalent ions. By using glutaraldehyde (GA) as the cross-linker and further photocross-linking under ultraviolent (UV), LbL polyelectrolyte FO membranes with good rejection toward NaCl were developed.42 Biomimetic membranes embedded with aquaporin Z have also been explored and shown to have a phenomenally high water flux of 142 LMH with a low reverse salt flux, using a 2 M NaCl draw solution.43 The biomimetic concept has been proven feasibly for FO processes, but the difficulties in large-scale production and poor mechanical strength are its major shortcomings. Since cellulose ester-based membranes have inherent advantages in low fouling propensity, good chlorine resistance, and low cost,44,45 it is worthy of investigating if we can molecularly design novel cellulose ester materials for FO applications. Therefore, the aims of this study are on (1) molecular design of novel cellulose ester materials; (2) evaluation of their potential as asymmetric FO membranes via phase inversion technique; and (3) investigation of the relationship among cellulose ester chemistry, physicochemical properties, casting conditions and FO performance. A wide range of cellulose esters with different degrees of substitution (DS) of functional groups including hydroxyl (OH), acetyl (Ac), propionyl (Pr), and butyryl (Bu) were therefore synthesized. To have a more complete coverage of various types of cellulose esters, one commercially available CAP (No. CAP14-20 from Eastman Chemical Company, USA) was also included for comparison. A conventional phase inversion method was employed to cater the needs of certain applications where the use of TFC membranes may not be suitable, especially for high fouling tendency processes. Membranes from both commercial and newly synthesized cellulose esters were fabricated and studied for their FO performance using NaCl draw solutions. In addition to identifying the optimized compositions of cellulose esters with high FO performance, this study may reveal the fundamental science to design cellulose esters with suitable degrees of substitution and functional groups as well as to process them for various FO applications.

Table 1. Degree of Substitution (DS) of Hydroxyl (OH), Acetyl (Ac), Propionyl (Pr), and Butyryl (Bu) Functional Groups of Cellulose Esters Used in This Study Degree of Substitution, DS polymer

DS(OH)

DS(Ac)

DS(Pr)

CAP_A CAP_C CAP_D CAP_I CAP_K CAP_L CAP_O CAP_P CAP_Q CAB_M CAB_N CAP141−20

0.82 0.89 1.14 0.63 0.65 0.84 1.10 0.35 0.09 0.83 0.82 0.38

1.64 1.00 0.87 1.68 0.65 0.62 1.31 1.31 1.47 1.49 0.73 1.90

0.54 1.11 0.99 0.69 1.69 1.54 0.59 1.34 1.44

DS(Bu)

0.68 1.45 0.72

under the hydrophobic region. Acetone (≥99.5), formamide (≥99.7), NaCl (≥99.5), and N-methyl-2-pyrrolidone (NMP, ≥99.5%) were purchased from Merck, Singapore. The deionized (DI) water used in experiments was produced by a Milli-Q unit (Millipore, Bedford, MA, USA) at a resistivity of 18 MΩ cm. 2.2. Membrane Preparation. Polymer powder was dried in a vacuum oven at 120 °C overnight to remove moisture prior to use. Five or six polymer solutions were prepared from each type of polymer, with concentrations ranging from 5 wt % to 20 wt % in NMP. The viscosity of polymer solutions as a function of polymer concentration was measured by an ARES Rheometric Scientific Rheometer (TA Instruments, USA) in the range of 1−100 s−1, using a 25-mm cone and plate geometry at 25 ± 1 °C. NMP was chosen as the solvent for the viscosity evaluation, because it does not evaporate as quickly as acetone and, hence, gives a more accurate evaluation. The critical concentration for each polymer at NMP was then evaluated from the viscosity vs concentration curves.46 Polymer solutions for membrane casting were then prepared by mixing each polymer into a mixed-solvent system of acetone/formamide with a ratio of 3:2. Since the viscosity profiles were evaluated by using NMP as the solvent, the critical concentrations obtained can only be used as a guideline to decide the polymer compositions of casting solutions. Generally, polymer compositions for casting solutions were fixed at or about 1−9 wt % higher than the critical concentrations as suggested by previous literatures to obtain almost defect-free membranes.46 Membranes were formed by spreading a thin layer of polymer solution on a smooth and hydrophilic glass plate using a uniform casting knife and subsequently immersing into a tap water coagulation bath at room temperature for phase inversion. The as-cast membrane was then gently peeled off from the glass plate and soaked in a tap water bath overnight to remove the residual solvent. The membranes were then annealed in deionized (DI) water at 85 °C for 5 min prior to testing. 2.3. Morphological Studies. The annealed membrane samples were subjected to morphological studies, using a fieldemission scanning electron microscope (FESEM). The membranes were freeze-dried in a freeze dryer (ModulyoD, Thermo Electron Corporation, USA) prior to FESEM studies. For cross-sectional images, the freeze-dried membranes were fractured in liquid nitrogen. Samples were coated with

2. EXPERIMENTAL SECTION 2.1. Materials. Nine proprietary CAP (CAP_A, CAP_C, CAP_D, CAP_I, CAP_K, CAP_L, CAP_O, CAP_P, CAP_Q) and two proprietary cellulose acetate butyrate (CAB_M, CAB_N) with different degrees of substitution (hereafter referred to as DS) of functional groups were synthesized by Eastman Chemical Company (USA). One commercially available CAP (CAP141-20) was also supplied by Eastman Chemical Company (USA). Their DS of different functional groups is summarized in Table 1. Figure 1 elucidates the design strategy of synthesizing these novel cellulose esters with various levels of DS(OH) and ratios of DS(Pr) or DS(Bu) to the total bulky side group. Cellulose esters with a high content of DS(OH) and a low ratio of DS(Pr)/(DS(Pr) + DS(Ac)) or DS(Bu)/(DS(Bu) + DS(Ac)) tend to be highly hydrophilic, whereas those with a low level of DS(OH) and a high ratio of DS(Pr)/(DS(Pr) + DS(Ac)) or DS(Bu)/(DS(Bu) + DS(Ac)) tend to be highly hydrophobic. As shown in the figure, five different levels of OH content and three different ratios of Pr/ (Pr + Ac) content have been synthesized. The CAP_A, CAP_D, and CAP_O fall under the hydrophilic region whereas the CAP_P, CAP_Q, and the commercial CAP141-20 fall 16136

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Figure 1. Map of design strategy for novel cellulose esters as a function of hydrophilic DS(OH) vs the ratio of hydrophobic DS(Pr) for CAP or DS(Bu) for CAB to the total DS of bulky side groups.

The salt permeability (B) can be obtained by a linear fitting using the following equation:

platinum, using a JEOL Model JFC-1100E ion sputtering device prior to observation under FESEM (JEOL, Model JSM6700). 2.4. Fractional Free Volume Calculations and Density Determination. The fractional free volume (FFV) of cellulose esters was estimated by the group contribution method of Bondi, using the following equation:

FFV =

⎤ ⎡ 1−R 1 =⎢ ⎥B ⎣ (ΔP − Δπ )A ⎦ R

2.6. Forward Osmosis Tests. Forward osmosis (FO) experiments were conducted using methods as described in the previous work.21 Volumetric flow rates for both feed and draw solutions were 0.3 L/min. NaCl solutions at various concentrations were used as model draw solutions, whereas DI water and 3.5 wt % NaCl solution (model seawater) were used as the feed. The experiments were conducted under room temperature maintained at 22 ± 1 °C. The water flux (Jv, expressed in units of L m−2 h−1 (abbreviated as LMH)) was determined from the change of volume of either feed or draw solution and can be calculated from eq 6:

Vp − V0 Vp

(1)

where Vp is the polymer specific volume determined based on the dry film density and V0 is the specific occupied volume, or volume that is unavailable for molecular transport.47 The specific volume can be calculated using eq 2:

V0 = 1.3Vw

(2)

where Vw is the van der Waals volume of the molecule, the empirical values of which can be found in ref 47. The density of each cellulose ester was determined by a Mettler Toledo balance and a density kit according to the Archimedean principle by measuring the weights of a cellulose ester dense film in air (Wair) and ethanol liquid (Wliq) and using eq 3: ⎛ w ⎞ air ⎟⎟ρ0 ρ = ⎜⎜ ⎝ wair − wliq ⎠

Jv =

ΔV Aeff Δt

(6)

where ΔV (L) is the volume of water permeated across the membrane from the feed to the draw solution in a predetermined time interval Δt (h) during FO experiments and Aeff is the effective membrane surface area (m2). When DI water is used as the feed, the reverse diffusion of draw solute from the draw solution to the feed solution can be determined by measuring the conductivity of the feed using a conductivity meter (Schott, Model LAB960E, U.K.). The salt concentration of feed solution at the end of each FO test can be calculated from the conductivity measurement using a calibration curve obtained prior to FO experiments. The reverse salt flux, Js (expressed in units of g m−2 h−1, abbreviated as gMH), was calculated using the following equation:

(3)

2.5. Pure Water Permeability, Salt Rejection, and Salt Permeability Tests. Pure water permeability (A, or PWP), salt permeability (B), and salt rejection (Rs) of the membranes were tested under room temperature with the selective layer facing the feed at a transmembrane hydraulic pressure of 10.0 bar, using a dead-end filtration cell. NaCl solutions (200 ppm) were used as the feed for salt rejection measurements. The NaCl concentrations were determined by conductivity measurements. Salt rejection Rs was calculated using eq 4: ⎛ cp ⎞ R s (%) = ⎜1 − ⎟ × 100 cf ⎠ ⎝

(5)

Js =

ΔC tVt Aeff Δt

(7)

where Ct and Vt are the salt concentration and the volume of feed solution at the end of the predetermined experiment duration, respectively.

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Figure 2. Viscosity curves of cellulose esters as a function of polymer concentration in NMP solutions at a shear rate of 10 s−1.

The dilution of draw solution was negligible as the ratio of total water permeation to the volume of the draw solution was 1) show relatively poorer NaCl rejections at 80.6% and 33.5%, respectively, but with PWPs of CAP_L > CAP_K ≈ CAP_C > CAP_I, while their salt permeability follows almost the same order: CAP_A > CAP_L > CAP_K > CAP_I > CAP_C. Interestingly, their water fluxes in the PRO mode follow exactly the same order of water permeability (CAP_A > CAP_L > CAP_K ≈ CAP_C > CAP_I), while their reverse flux obeys exactly the same sequence of salt permeability: CAP_A > CAP_L > CAP_K > CAP_I > CAP_C. Clearly, CAP_I and CAP_C have the best salt rejection and almost the lowest ratios of reverse salt flux to water flux (Js/Jv). There are many factors contributing to their superior performance: (1) they have the lowest dry state FFV, ranging from 16.3% to 16.9%; (2) CAP_C has a high content of Pr, which may lower its swelling tendency because there is a greater resistance to the access of water into the free volume since they are shielded by the bulky and hydrophobic Pr side chains. (3) CAP_I has a lower solubility parameter of 12.6, compared to other cellulose esters. As a result, less swelling can be expected and the swelling-induced salt permeation is minimized. Besides, CAP_I has a low DS(OH) of only 0.63, indicating that CAP_I inherently does not have many available sites for hydrogen bonding with water as the carbonyl oxygen atoms on the bulky side chains can only partially involve in hydrogen bonding with water due to the spatial resistance.53 Therefore, possessing either a low DS(OH) or a moderate high DS(Pr) is one of the keys to achieve higher salt rejection performance. Although CAP_A has a similar FFV of 16.3 with CAP_C and a lower FFV than CAP_I (16.3% vs 16.9%), the CAP_A membrane has a much lower salt rejection performance compared to the other two. However, comparing CAP_A to CAP_C, CAP_A has a much lower Pr content (0.54 vs 1.10), which may lead to a higher swelling tendency due to the lower steric hindrance in polymer chains. Furthermore, a higher value of solubility parameter of CAP_A, compared to CAP_I (13.1 cal1/2 cm−3/2 vs 12.6 cal1/2 cm−3/2) suggests that CAP_A indeed can have a higher water sorption, which encourages a higher degree of swelling. The situation is slightly different for CAP_K and CAP_L. Both have comparably high DS(Pr) (1.69 vs 1.54) and slightly lower DS(OH) (0.65 vs 0.84). Since they have a high content of Pr, moderate water fluxes and low salt permeability were

with a much higher DS(Bu) than CAB_M (1.45 vs 0.68) shows a much lower salt permeability (0.06 LMH vs 0.40 LMH) and water permeability (0.093 LMH bar−1 vs 0.51 LMH bar−1) than CAB_M. In FO experiments, the former also displays a low water flux of 7 LMH at the PRO mode with a low reverse salt flux of 7 gMH, while the latter exhibits a much higher water flux and a higher reverse salt flux. Besides, the CAP_A membrane with a low Pr content shows a low NaCl rejection and a high reverse salt flux under FO experiments. Therefore, one may conclude that the higher content in bulky side group leads to the higher salt rejection, while the higher hydrophilic side group (OH) content leads to the higher salt permeability. However, currently, it is still unclear why the CAP_D membrane exhibits a lower-than-expected water permeability of mere 0.18 LMH bar−1, despite being hydrophilic; more work will be done in the future to investigate the intrinsic properties of various cellulose esters. Since water permeability and salt transport are proportional to the volume fraction of water absorbed in the polymers,52,53 the FFV of these polymers were calculated from the Bondi group contribution method to examine the availability of free volume available in different cellulose esters for water sorption. Total solubility parameters of the cellulose esters were also calculated using method discussed in ref 54. The FFV and solubility parameters are summarized in Table 5. Three groups of cellulose esters can be classified for easy comparison, namely, (1) the hydrophilic group (CAP_D and CAP_O), (2) the CAP group with moderate hydrophilicity (CAP_A, CAP_C, CAP_I, CAP_K, and CAP_L) and (3) the CAB group (CAB_M and CAB_N). The hydrophobic CAP materials (CAP_P, CAP_Q, and CAP141-20) are exempted because they cannot form almost defect-free membranes during the phase inversion process, hence a fair comparison cannot be made. Future works will be done to evaluate the cellulose esters intrinsic transport phenomena. According to Zhang et al.,53 there are two types of free volume to be considered in the cellulose ester polymers. One is the free volume in the dry state, while the other is the hydrated free volume in the wet state. The dry state free volume can be correlated well with FFV, which has been calculated in Table 5, using the Bondi group contribution method, whereas the hydrated free volume may be the same or slightly larger than the former depending on content and the characteristics of functional groups. Since permeability is a product of water solubility and diffusivity, the amount of water molecules absorbed in the free volume and the diffusion process of penetrants across the membrane plays important roles in determining the permeability. Both Zhang et al.53 and Hodge et al.55 have reported the much larger hydrated free volume than the dry-state free volume for hydrophilic polymers such as cellulose esters and poly(vinyl alcohol). This increase in hydrated free volume in cellulose esters arises from the fact that hydrophilic cellulose esters tend to have a low free volume in the dry state, because of the hydrogen bonding within polymer chains, but they undergo a significant expansion in free volume at the hydrated state. The enlargement in free volume is attributed to the plasticization and swelling of polymer chains induced by the incorporation of nonfreezing water which involves strong hydrogen bonding with the polymer.53,55 In contrast, dry state and hydrated free volumes may not differ with each other for hydrophobic cellulose esters, because of low water sorption and tortuous diffusion paths among the hydrophilic and hydrophobic functional groups. 16141

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Figure 5. Water fluxes and reverse salt fluxes of CAB_M membranes at PRO and FO modes as a function of draw solution concentration using DI water as the feed.

observed initially. However, their higher FFV values (18.2% and 17.6%) indicate that extra free volume may have been created among the interstitial space of polymeric chains, because of the high amount of Pr substitution, which increases the steric hindrance during chain packing. As a result, both polymers exhibit higher water and salt permeability than CAP_C and CAP_I. Since CAP_L has a higher DS(OH) and a lower DS(Pr) than CAP_K ((0.84 vs 0.65) and (1.54 vs 1.69), respectively), the former is more likely to better water sorption than the latter. As a consequence, CAP_L has a higher solubility parameter and a higher water and salt permeability than CAP_K. For the CAB group, both CAB_M and CAB_N have comparably high free volumes (18.2% vs 18.9%). However, the former has a high DS(Ac) of 1.49 and a low DS(Bu) of 0.68, while the latter has a low DS(Ac) of 0.73 and a high DS(Bu) of 1.45. It has been reported that an increase in Bu content with decreasing Ac content results in an increase in moisture resistance56 and polymers with a low water uptake may experience a decrease in free volume in hydrated environments due to hydrophobic interactions that bring molecular segments closer together in a denser packing.57 As a result, it is no surprise that CAB_N has an extremely low water and salt permeability, as well as unimpressive FO performance, because of the low water flux. As a comparison with other cellulose esters, CAB_M exhibits the most satisfying FO performance with a higher water flux and a moderately lower reverse salt flux. It is interesting to point out that CAP_A and CAB_M exhibit almost the same fluxes in both PRO and FO modes ((21.4 LMH vs 21.6 LMH) and (12.0 LMH vs 11.9 LMH), respectively), but the former suffers from higher reverse salt fluxes than the latter ((17.5 gMH vs 10.6 gMH) and (9.5 gMH vs 5.1 gMH) for PRO and FO modes, respectively). Since CAP_A and CAB_M have almost the same DS(OH) values (0.82 vs 0.83), the difference in their rejection toward salt diffusion arises from the difference in substituted functional groups. CAP_A has a high DS(Ac) of 1.64 and a low DS(Pr) of 0.54, while CAB_M has a high DS(Ac) of 1.49 and a low DS(Bu) of 0.68. Since the Bu functional group is bulkier and more hydrophobic, CAB_M has a greater free volume but a lower salt permeability than CAP_A. Consequently, the CAB_M membrane has a relatively higher water flux but a much lower reverse flux than the CAP_A membrane. In summary, highly hydrophilic cellulose esters have a higher tendency to yield membranes with a high salt permeability. However, it was observed that highly hydrophobic cellulose

esters may cause defects in the selective layer during the phase inversion process under normal casting conditions. The FFV and solubility parameters of the cellulose esters also play important roles in determining membrane performance in FO applications. Membranes with low FFV and solubility parameters give rise to low water and salt permeability and vice versa. Therefore, an optimal composition should be chosen in order to obtain a desirable membrane performance to fit the final application. 3.5. FO Performance of CAB_M Membranes at Different Draw Solution Concentrations. Since the CAB_M membrane exhibits the most reasonable FO performance, they were further evaluated for their FO performance under various draw solution concentrations and using model seawater (3.5 wt % NaCl) as the feed solution. Figure 5 shows the FO performance of CAB_M membranes at draw solution concentrations ranging from 0.5 M to 2M. The water flux obtained from the PRO mode increases steadily while the FO mode water flux reaches a plateau at ∼1.0 M NaCl draw solution. This is caused by the higher severity of ICP in the FO mode, resulting from the dilution of draw solution within the porous substrate at a higher draw solution concentration. Figure 6 shows the water flux as a function of draw solution concentration when model seawater was used as the feed. The membrane has a water flux of 8 LMH under the PRO mode using a 2 M NaCl as the draw solution. As a reference, a comparison of FO performance between CAB_M membranes

Figure 6. Water fluxes of CAB_M membranes at PRO and FO modes as a function of draw solution concentration using model seawater (3.5 wt % NaCl) as the feed. 16142

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Table 6. Comparison of FO Performance between the CAB_M Membrane and HTI Membranes Reported in Various Literature

a b

membrane

annealing

feed

CAB_M CAB_M HTI (1)b HTI (2)b HTI (3)b HTI (4)b HTI (5)b HTI (6)b HTI (7)b HTI (8)b HTI (9) HTI (9)

85 °C 85 °C

DI DI DI DI DI DI DI DI DI DI DI DI

draw solution 2 1 2 2 1 1 1 1 1 1 2 1

M M M M M M M M M M M M

NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl

orientationa

water flux (LMH)

reverse salt flux (gMH)

ref

PRO FO PRO FO FO FO FO FO FO FO PRO FO

21.6 9.4 43.0 17.0 9.5 16.8 12.4 6.6 9.5 5.0 36.3 15.8

10.6 3.9 46.8 27.8 7.0 21.8 9.5 1.0 2.6 0.7 68.4 31.0

this work this work 23 23 58 59 59 59 60 60 61 61

PRO orientation indicates the active layer oriented towards draw solution; FO orientation indicates active layer oriented towards feed solution. The values were estimated from graphs.

but salt rejection is reduced. Membranes comprising a high DS(Bu) show lower water permeability than those containing high DS(Pr). More fundamental work will be carried out on the intrinsic transport properties of cellulose esters in the future in order to gain an in-depth understanding of cellulose esters.

and various HTI membranes is consolidated in Table 6. The HTI FO membranes are the current only commercially available FO membranes and consist of a rather complicated structure with embedded polyester woven mesh in a phaseinversed asymmetric membrane cast from CA/CTA.20 As observed from Table 6, the different performance of these HTI FO membranes was reported by various literature, possibly due to the fact that there are many types of HTI FO membranes are available in the market. Compared to the HTI FO membranes reported in other studies, the CAB_M membranes fabricated in this study via a simple phase inversion process has shown comparable performance. Using a 1 M NaCl draw solution at the FO mode, CAB_M membranes exhibit comparable performance with HTI (3) and HTI (7) membranes reported in refs 58 and 60, while the other HTI membranes reported in other literature have poorer performance where a high water flux is accompanied by a poor NaCl rejection and high NaCl rejection was accompanied by a poor water flux.

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ASSOCIATED CONTENT

S Supporting Information *

FESEM images on the membrane morphology of all membranes cast in this study are provided in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (65) 6156-6645. Fax: (65) 6779-1936. E-mail: chencts@ nus.edu.sg. Notes

The authors declare no competing financial interest.

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4. CONCLUSION Novel cellulose esters with different degrees of substitution (DS) of hydroxyl, acetyl, propionyl, and butyryl functional groups were used to fabricate asymmetric membranes via nonsolvent induced-phase inversion process. Both RO and FO experiments were conducted in order to measure properties such as water and salt permeability and salt rejection, as well as FO performance, such as water and reverse salt fluxes. The relationship of both polymer inherent properties and membrane FO performance, as a function of functional group content, were also evaluated. It has been found that the highly hydrophobic cellulose esters could not form defect-free selective layers, because of rapid phase inversion under normal casting conditions during the membrane formation process. These membranes exhibit low or no NaCl rejection. However, membranes with high hydrophilicity also suffer from high salt permeation, because of the large extent of swelling of polymer chains and expansion of free volume in hydrated environments, which favor salt transport across the membrane. Therefore, CAP and CAB with moderate hydrophilicity are preferred candidates as FO membrane materials. Generally, cellulose esters with higher content of bulky side groupsnamely, CAP_C and CAB_Ntend to have lower salt permeability but also lower water permeability. However, very high content of Pr results in poor chain packing due to the bulkiness of Pr groups and higher FFV values. As a consequence, water flux increases

ACKNOWLEDGMENTS The authors would like to thank Eastman Chemical Company, USA, for the research funding and the provision of the synthesized novel cellulose esters through the project titled “Investigation of Novel Materials for the Forward Osmosis Process” (Grant No. R-279-000-315-597), as well as the Singapore National Research Foundation (NRF) for the support through the Competitive Research Program for the project titled “New Advanced FO Membranes and Membrane Systems for Wastewater Treatment, Water Reuse and Seawater Desalination” (Grant No. R-279-000-336-281). Special thanks are due to Ms. Sui Zhang, Dr. Jincai Su, Ms. Xue Li, Dr. Kaiyu Wang, Dr. Natalia Widjojo, and Prof. Donald R. Paul for their valuable advice. Thanks are also due to Ms. Nguyen Thi Mai Thao, Ellie for her kind assistance in this work.

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REFERENCES

(1) Chung, T. S.; Li, X.; Ong, R. C.; Ge, Q. C.; Wang, H. L.; Han, G. Emerging forward osmosis (FO) technologies and challenges ahead for clean water and clean energy applications. Curr. Opin. Chem. Eng. 2012, 1, 246−257. (2) Phuntsho, S.; Shon, H. K.; Hong, S.; Lee, S.; Vigneswaran, S.. A novel low energy fertilizer driven forward osmosis desalination for direct fertigation: Evaluating the performance of fertilizer draw solutions. J. Membr. Sci. 2011, 375, 172−181.

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