Effect of Molecular Configuration of Additives on the Membrane

Subscriber access provided by STEPHEN F AUSTIN STATE UNIV

Article

Effect of molecular configuration of additives on the membrane structure and water transport performance for forward osmosis Jia Xu, Panpan Li, Mengzhen Jiao, Baotian Shan, and Congjie Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b01039 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Effect of molecular configuration of additives on the membrane structure and water transport performance for forward osmosis

Jia Xu*, Panpan Li, Mengzhen Jiao, Baotian Shan, Congjie Gao

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, China *

Corresponding author: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In this work, we chose cis-trans isomers (Trans-2-butenedioic acid, TBA and cis-2butenedioic acid, CBA) as pore-forming additives to fabricate cellulose acetate (CA) forward osmosis (FO) membranes and explored the underlying correlations between additive molecular structure and membrane properties. Most significant fact to note is that molecular configuration of additive indeed leads to a considerable influence on membrane performance, although these additive molecules have the same constitution, conformation, molecular formula and functional groups. CBA tends to form a more porous structure in the macro-void middle region (noticeable finger-like and loose sponge-like structure) and two perfectly dense top and bottom surfaces with higher hydrophilicity, which facilitates the water transport with no sacrifice of salt rejection. On the basis of our observations, it confirms that there are some residual CBA molecules in the prepared membranes while TBA totally transfers from the casting film to the coagulation bath by TGA, which contributes to the more hydrophilic surface and the instantaneous demixing. Moreover, additive transfer rate during the phase inversion depends on the superiority of two interactions between additive-CA and additive-water from the point of hydrogen bonds. This work provides insights into the additive selection and membrane design for water reuse and desalination.

Keywords: forward osmosis; membrane; water transport; cellulose acetate; cis-trans isomer

INTRODUCTION

2 Environment ACS Paragon Plus

Page 2 of 31

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The shortage of fresh water resources has become a major challenge due to the ever increasing population and deteriorating environment [1]. Various technologies for clean water production and wastewater reclamation have been developed to address this issue [2]. However, some conventional techniques are found to be unsuitable or unsustainable in terms of environmental friendliness and impact on the eco-system health [3]. Recently, forward osmosis (FO) has attracted attentions as an emerging technology to conventional pressure-driven membrane processes for wastewater reclamation and seawater desalination [4-6]. FO employs the osmotic pressure gradient across a membrane as the driving force for water transport from a dilute feed solution (FS) to a concentrated draw solution (DS) [4,7,8]. FO with a low fouling propensity normally consumes less energy and produces less brine discharge than a pressure-driven membrane process [9]. Moreover, the separation mechanisms of FO membrane are extensively explored [10,11]. However, the lack of available high-performance FO membranes is one of the bottlenecks which affect industrial implementation of the FO technology [12]. FO membranes are typically fabricated using the non-solvent induced phase inversion [13,14], interfacial polymerization [15-18] and layer-by-layer assembly [19,20]. Currently, the commercially available FO membranes appear to be the cellulose triacetate membranes and polyamide thin-film composite membranes from Hydration Technology Innovations (HTI), LLC [21]. One of the potential issues with FO membranes is severe internal concentration polarization in the membrane substrate, and thus FO membranes often suffer from a lower water-permeability, especially for the conventional asymmetric membranes prepared by the phase inversion method (i.e., cellulose acetate-based membranes) [9]. However, compared to the thin-film-composite (TFC) membranes made



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

by interfacial polymerization, asymmetric membranes have some special advantages such as excellent chlorine tolerance and higher solute rejection, which are considered to be suitable for extensive oxidizing wastewater reclamation [22,23]. Cellulose acetate (CA), derived from the most common natural organic compounds on the earth, has considered to be a popular, green and renewable material of asymmetric membrane because of its advantages on the high hydrophilicity, vast availability, low cost and good mechanical strength [2,24,25]. Generally, using pore-forming additives is an effective strategy to control and optimize the CA membrane structure such as pore tortuosity, membrane porosity and thickness so as to improve the separation performance [26,27]. Idris and Yet [26] fabricated an asymmetric CA dialysis membrane using polyethylene glycol (PEG) with various molecular weights as pore-forming additives for the urea clearance, and the results indicated that PEG-200 as additive could enhance the separation performance. Sairam et al. [27] investigated the effect of various additives on the membrane performance and the results showed that lactic acid, maleic acid and zinc chloride were suitable additives to improve both water flux and salt rejection. In spite of plenty of pore-forming additives reported in literature, the current studies on the poreforming additives were usually focused on the additive type and concentration, which cannot provide information on the underlying interaction between the molecular structure of additives and the separation performance of the resulting membranes. Consequently, from the point of view of the membrane design, the investigation on the effect of additive structure on the membrane morphology and separation performance is urgently required. In this work, we chose cis-trans isomers as pore-forming additives to fabricate CA membranes and explored the underlying correlations between the molecular structure


Page 4 of 31

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(difference in configuration) and the membrane properties in terms of morphology, hydrophilicity and separation performance. FO process, considered as a better way to reflect the intrinsic transport behavior of water molecules and salt ions due to the absence of external hydraulic pressure compared to other filtration processes, was applied in this study. Trans-2-butenedioic acid (TBA) and cis-2-butenedioic acid (CBA) were chosen as the cis-trans isomers to understand the relationship between such additives with different configurations and the membrane morphology and FO performance of the resulting CA membranes. CBA and TBA, with the same chemical formula of C4H4O4, have distinct physical property and chemical reactivity [28] because of their cis- and transconfigurations of the double bond. For example, CBA has a higher solubility in water due to its higher polarity and a lower melting point than TBA. This work may provide insights into the additive selection and membrane designs for water reuse and desalination.

EXPERIMENTAL Materials Cellulose acetate (Mw 100,000) with an averaged acetyl content of 39.8% was purchased from Acros Organics (USA). N,N’-dimethylformamide (DMF), CBA and TBA were purchased from Sigma-Aldrich (USA). Figure 1 shows the chemical structures. Sodium chloride used as feed solute and glucose used as draw solution solute were purchased from Tianjin BASF Chemical Company and Shandong Xiwang Sugar Company, respectively. Deionized water used in all the experiments was purified using a Millipore water purification system to a minimum resistivity of 18.0 MΩ cm.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

Commercial FO membranes made from cellulose triacetate (CTA) were purchased from HTI (Albany, OR) for comparison purpose. RO O

HOOC

R

O

COOH

HOOC

H

OR

R=H or

RO

O O

OR

O

*

OR

CH3

H

H

H

COOH

RO OR

n

(a)

(b)

(c)

Figure 1. Chemical structures of (a) Cellulose acetate, (b) Cis-2-butenedioic acid (CBA) and (c) Trans-2-butenedioic acid (TBA).

Membrane Preparation CA was blended with the additives to prepare FO membranes via non-solvent induced phase inversion. The casting solutions, composed of a predetermined amount of CA (polymer), DMF (solvent), and CBA or TBA (additives), as shown in Table 1, were stirred at 60 °C for 24 h and then degassed for 12 h to form homogeneous solutions. Afterwards, the polymer solution was casted onto a glass plate using a casting knife with a thickness of 150 µm. The casting film was exposed to air for 10 s to partially evaporate the solvent and then immersed into a deionized water bath at room temperature for 20 min. After peeling off from the glass, the CA membranes were annealed in deionized water at 85 °C for 15 min and finally stored in deionized water before use. Table 1 Compositions of casting solutions for the membrane fabrication. Membrane

CA (wt%)

DMF (wt%)

CBA (wt%)


TBA (wt%)

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CA20

20

80

0

0

CA22

22

78

0

0

CA24

24

76

0

0

CA22-CBA2

22

76

2

0

CA22-CBA4

22

74

4

0

CA22-TBA2

22

76

0

2

CA22-TBA4

22

74

0

4

Membrane characterization Membrane thickness was measured using a gauge (Q/DBFF2-2003, Shanghai Liuling Instrument, China). Sample surface was dried with blotting paper prior to the thickness measurements and each sample was measured at least 20 different spots and the reported data represent an average value. Water contact angle on the membrane surface was measured using a Contact Angle Goniometer (DSA100, Kruss, Germany) to evaluate the membrane hydrophilicity. Before the measurement, samples were dried at room temperature overnight, and the reported data represent an average value of more than 10 measurements. The cross-sectional and surface morphologies of the membranes were examined using a scanning electron microscopy (SEM; S-4800, Hitachi, Japan). Samples were air-dried for 24 h and then fractured in liquid nitrogen. Thermogravimetric analysis (TGA; Netzsch Instruments) was performed using argon as a carrier gas at a constant flow rate ml min-1 at a heating rate of 10 °C/min from 25 to 800 °C.

FO performance measurement



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FO performance (water flux and salt rejection) of membranes was investigated in FO mode where the selective layer was oriented towards the feed solution using a homemade lab-scale cross-flow FO filtration unit with a plate-and-frame configuration at room temperature. Effective area of the membrane was 57.4 cm2. Aqueous NaCl solution (0.1 M) and aqueous glucose solution (2 M) were used as feed and draw solutions, respectively, which flowed counter-currently along the membrane surfaces at the same velocity of 10 cm/s. Commercial HTI-CTA FO membranes were also included in the FO tests under the same testing conditions for comparison purpose. Water flux (Jv, L/m2h, abbreviated as LMH) and salt rejection R (%) were calculated from Eqn. (1) and Eqn. (2), respectively.

Jv =

∆w ρA mt

Eqn. (1)

where Δw is the mass of water collected over a given period of time t, Am is the effective membrane area, and ρ is the water density.

R = (1 -

C NaCl, ds × V ds ) × 100% C fs0 × V fs0

Eqn. (2)

where CNaCl,ds is the salt concentration in the draw solution after a given period of time, which was determined via standard curve method using a conductivity meter (DDS307a), and Vds is the volume of the draw solution. Cfs0 and Vfs0 are the initial concentration and volume of the feed solution, respectively. It should be noted that Eqn. (2) is only suitable to the nonelectrolyte draw solution to obtain accurate measurement of the amount of diffused salt. The reported values of Jv and R are the average of at least five replicates.


Page 8 of 31

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Water and salt transport property Intrinsic transport capacity of water and salt was evaluated using a cross-flow RO test setup at 2.0 bar. Feed water was either DI water for pure water permeability measurements (A) or a 500 mg/L NaCl solution for salt permeability coefficient (B) and solute rejection (R) measurements. The effective area (Am) of the membrane sample was 11.3 cm2 and all tests were controlled at 25 ± 3 °C. A was determined by weighing the amount of permeate water collected within specified time duration, as shown in Eqn. (3) [29].

A=

Jw Q = ∆ P Am ⋅ ∆ t ⋅ ∆ P

Eqn. (3)

where Q is the quantity of the permeate sample collected over a period of time (∆t), and Am is the effective membrane area for permeation. R can be expressed as Eqn. (4). Based on the values of A and R, B value can be calculated by Eqn. (5). The reported values of A, B and R are the average of at least five replicates.

 C  R = 1 − p  ×100%  Cf 

Eqn. (4)

1− R B = R A(∆P − ∆π )

Eqn. (5)

where Cp and Cf are the salt concentrations in the permeate and feed solution, respectively. ∆P and ∆π are the trans-membrane pressure and the osmotic pressure difference between feed and permeate, respectively.

Structural parameter determination 9 Environment ACS Paragon Plus


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intrinsic characteristics of FO membranes can be representatively quantified by structural parameter (S), which was determined in this study for the more objective comparison of our membranes and the commercial FO membranes. Because the membrane orientation is feed solution against the dense skin layer of membrane and draw solution against the porous layer side (FO mode), S value was calculated by the following Eqn. (6) [30].

Jv =

Aπ draw + B  D ln  S  Aπ feed + Jv + B 

Eqn. (6)

where Jv is water flux of membrane tested under FO mode, D is the solute diffusion coefficient. πfeed and πdraw are the osmotic pressures of the feed solution and draw solution, respectively. S value can be finally determined from experimentally measured A, B values [31].

RESULTS AND DISCUSSION CA membranes without additives To clearly evaluate the interaction between the additives with different configurations and the membrane structure as well as the FO performance, CA membrane without any additive should be optimized first to find the better fabricating conditions for the base CA membrane. In this study, CA content was adjusted for the optimization and the results are shown in Figure 2 and Figure 3. Figure 2 shows the FO performance in terms of water flux and NaCl rejection of the CA membrane with respect to the variation in CA content. It can be seen that the water flux increased from 7.5 to 8.5 L/m2h with an increasing CA content in the casting solution from 20% to 22%. However, a further increase in CA


Page 10 of 31

Page 11 of 31

content beyond 22% cannot improve the water flux. For example, when the CA content was increased from 22% to 24%, the water flux was decreased by 6.3% while the NaCl rejection was slightly improved by about 0.4% under the same operating conditions. Although the NaCl rejections of the resulting membranes prepared with 20% and 22% CA were lower than that of membrane with 24% CA, they are still more than 98.5% and the change between them is statistically insignificant. It should be noted that the water flux decreased with the FO time, which might be attributed to the internal concentration polarization and the lowered osmotic pressure caused by the dilution of draw solution. However, higher water flux of approximately 12 L/m2h still presents stable (Figure 4), indicating that the internal concentration polarization is mainly responsible for the decreased water flux (Figure 2). b

a 10

100 99

Rejection(%)

8

2

JV(L/m h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 20% CA 22% CA 24% CA 4

0

10

20

98 97 96 20% CA 22% CA 24% CA

95 94

30

40

50

60

0

10

time (min)

20

30

40

50

60

time (min)

Figure 2. FO performance of the resulting membrane with CA content in the casting solution. (a) Water flux, (b) NaCl rejection. Operating conditions: FO mode, 0.1 M NaCl solution as feed and 2 M glucose as draw solution.

To understand the effect of CA content on the FO performance as discussed above, the morphology of the resulting CA membranes prepared with 20-24% CA was characterized



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

by SEM (Figure 3). A typical sandwich structure consisting of two smooth, defect-free and dense CA membrane surfaces (top surface, Figure 3abc and bottom surface, Figure 3def) and a macro-void middle region with finger-like pores (Figure 3ghi) was revealed by SEM, which is consistent with the previous study from Chung’s group [2]. This sandwich structure has advantages on higher water permeability [32], an excellent mechanical property due to a porous support layer [2] and a mitigated ICP due to the effective barrier to the transport of molecules or ions across the membrane. Top surface

a

b

c

e

f

h

i

k

l

1µm Bottom surface

d

1µm Cross-section

g

100µm

j

1 µm

Figure 3. Morphology of the resulting membrane with CA content in the casting solution, (a, d, g and j) 20%, (b, e, h and k) 22%, (c, f, i and l) 24%. 12 Environment ACS Paragon Plus

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Additionally, as shown in Figure 3, there is no distinct difference in surface morphology (both top and bottom surfaces) with CA content at the magnification with scale bar of 1 µm. However, CA content has a significant impact on the cross-sectional structure. Although the number of finger-like pores in the macro-void middle region drops when CA content increases from 20% to 22%, these pores of the membrane with 22% CA become bulkier and more fully developed than those with 20% CA, which contributes to the enhanced water transport capacity. A further increase in CA content has little impact on the finger-like pore structure, but continues decreasing the number of these pores, leading to a lower water flux for the membrane with 24% CA. Moreover, high magnification SEM (Figure 3 jkl) on the sponge area showed that although the change in the sponge area is insignificant, the membrane with 22% CA seems to exhibit a relatively loose sponge structure, which can lower the transport resistance of water molecules across the membrane so as to facilitate water permeability. Therefore, FO performance depends on both pore number and pore structure. In consideration of both FO performance and membrane structure, CA content of 22% seems appropriate for the membrane fabrication in subsequent studies.

CA membranes with cis-trans isomers as additives To explore the effect of additives with different configurations on the membrane morphology and water transport performance, the cis-trans isomers such as CBA and TBA with different additive contents (2% and 4% in the casting solution) were used as pore-forming additives for the membrane fabrication. Water flux and NaCl rejection



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

measured in FO mode for the resulting CA membranes with additives were shown in Figure 4. The incorporation of pore-forming additives, regardless of CBA or TBA, into the casting solution improves the water flux at a lower additive content of 2% compared to the pure CA membranes. The membrane with 2% CBA in particular exhibits a maximum water flux of ~11.5 L/m2h, which increases by 32.7% compared to that of CA membranes. However, a relatively high content of 4% in both CBA and TBA has a negative impact on the water flux which is even lower than that of the CA membranes. It should be noted that even in this case, the water flux is increased by 28.8% when CBA replaces TBA, indicating that CBA is better than TBA for the improvement in water transport capacity when used as a pore-forming additive. Additionally, the maximum water flux for the membrane prepared with 2% CBA maintains stable with FO operating time. By contrast, three curves with relatively marked drop trends can be observed in the middle region of Figure 4a, probably resulting from a more severe ICP due to the reduction in the effective osmosis pressure with FO time. For the membrane prepared with 4% TBA, the downward tendency is less pronounced due to the lowest water flux which is contributed to a relieved ICP. In Figure 4b, additives such as CBA and TBA have a significant impact on the NaCl rejection of the resulting membranes. For example, the membrane with 2% CBA yields a maximum NaCl rejection of 98.2% among the membranes with additives, similar to the pure CA membrane. NaCl rejection decreases to 89.3% with a further increase in CBA content (4%). Moreover, compared to TBA, CBA indeed leads to a higher NaCl rejection. Based on an overall consideration of water flux and NaCl rejection, the membrane prepared with 2% CBA has an optimum separation performance in this study.


Page 14 of 31

Page 15 of 31

More importantly and remarkably, additive molecules with different molecular configurations, even if they have the same constitution, conformation, molecular formula and functional groups, indeed have a significant impact on the FO performance. Molecule with cis-form seems more suitable to facilitate the water transport with no sacrifice of salt rejection. b

a

100

12

9

Rejection(%)

2 1

JV(L/m h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 CA22

3

CA22-CBA2 CA22-TBA2

0

0

10

20

CA22-CBA4

90

80

CA22

CA22-TBA4 30

40

50

60

70

0

CA22-CBA2

CA22-CBA4

CA22-TBA2

CA22-TBA4

10

time (min)

20

30

40

50

60

time (min)

Figure 4. FO performance of the resulting membrane with additives in the casting solution. (a) Water flux, (b) NaCl rejection. Operating conditions: FO mode, 0.1 M NaCl solution as feed and 2 M glucose as draw solution.

In order to understand the effect of additive molecules with different molecular configurations on the intrinsic membrane separation properties of our membranes and the more objective comparison of our membranes and the commercial FO membranes, intrinsic characteristics of FO membranes, such as A (pure water permeability), B (salt permeability coefficient) and R (rejection in RO test) values were determined using crossflow RO tests at 2 bar and S (structural parameter) values were also calculated (Table 2). It is evident that the commercial HTI-CTA membrane from HTI showed lower A value in comparison with all the CA membrane with additives, which is agreed with other studies



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

[33]. The membranes with 2% CBA and with 4% TBA yield higher A values of approximately 1.27 and 1.35 L/m2hbar, respectively. However, the membranes with 4% TBA yield lowest R and highest B among the testing membranes, demonstrating a poor salt retention property; while relatively excellent salt retention is observed for the membrane with 2% CBA, which suggests that cis-trans isomers such as CBA and TBA could provide an effective approach to adjust the water permeability and salt permeability of the CA membranes. Moreover, the greater A value of the membrane with 2% CBA contributes partially to its higher FO water flux (Jv). Compared to the pure CA membrane, the enhancement in A and Jv values of the membrane with 2% CBA are approximately 36.6% and 49.4%, respectively, which indicates that the increased A value alone cannot be totally responsible for the observed Jv enhancement. The additional gain in Jv may be attributed to the reduced ICP effect that can be characterized by the S value. As seen in Table 2, the membrane with 2% CBA yields a much lower S value of about 780 µm, which indicates that CBA additive has greatly improved the mass transfer efficiency of the membrane [33]. The current study suggests that CBA not only improves the water permeability but also reduces the structural parameter, both effects resulting in improved FO water flux, which is consistent with the previous study [33]. Table 2 Intrinsic characteristics of FO membranes. Membrane

Jv (L/m2h)

A (L/m2hbar)

B (L/m2h)

R (%)

S (μm)

CA22

8.5-8.6

0.91-0.95

0.09-0.12

92.7-94.2

1132.0-1138.1

CA22-CBA2

11.6-12.7

1.23-1.31

0.12-0.14

93.7-94.3

740.5-832.3

CA22-CBA4

7.0-7.1

1.13-1.28

0.21-0.26

88.7-89.6

1700.3-1761.6

CA22-TBA2

8.8-8.9

1.21-1.22

0.20-0.32

85.9-90.6

1228.1-1253.4


Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CA22-TBA4

5.1-5.3

1.31-1.40

0.32-0.41

84.5-84.7

2714.4-2809.7

HTI-CTA

7.6-7.8

0.90-0.93

0.07-0.09

94.4-95.4

1320.8-1343.7

Presented as images in Figure 5, SEM was performed to investigate the change in membrane morphology with different additive types and contents. It can be seen that additives do not change the typical sandwich structure which is similar to the pure CA membrane, composed of the top and bottom dense layers and a porous middle region with finger-like pores. However, there are discrete pores (more precisely, holes) of 10-50 nm in diameter disorderly distributed on the bottom surface for the membranes with 4% CBA and 2%, 4% TBA, which might be largely contributed to the pore-forming function of the additives, although there is no distinct difference in the defect-free top surface compared to the pure CA membrane. It should be noted that the membrane with 2% CBA exhibits a dense and defect-free bottom surfaces, which might play an important role of the effective barrier to the glucose molecules transport from draw solution to feed so as to lower the ICP. The released ICP is one of explanations for the higher water flux of the membrane with 2% CBA (Figure 4a). Moreover, the formation of porous structure on the bottom surface might be responsible for the lower NaCl rejection (Figure 4b). It might be explained using a concept of “double skin-layer FO membranes” [34-36]. The double skin-layer could serve as an effective barrier to the glucose molecules transport from draw solution to feed and as a second barrier to salt transport from feed to draw solution.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

Top surface

Page 18 of 31

b

c

d

f

g

h

j

k

l

n

o

p

1 µm Bottom surface

e

1 µm Cross-section

i

100 µm

m

1 µm

Figure 5. Morphology of the resulting membrane with CBA and TBA additives, (a, e, i and m) 2%CBA, (b, f, j and n) 4% CBA, (c, g, k and o) 2% TBA, (d, h, l and p) 4% TBA.

From the cross-sectional images in Figure 5i, the membrane with 2% CBA yields a large number of finger-like pores in the macro-void middle region, although the fingerlike pores are not fully developed within polymer matrix. Moreover, from the higher magnification images, its sponge-like structure seems much looser and more noticeable compared to the membranes with 4% CBA and 2%, 4% TBA. As reported in literature [37], higher water flux depends on the reduction in sponge-like structure. However, such membrane structure with noticeable finger-like pores and loose sponge-like pores


Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


enlarges the water passages to improve the water transport across the membrane during FO process, which is another reason for the excellent water flux. In contrast, tight sponge structure is observed in the membrane with TBA, whatever the TBA content is. It demonstrates that the molecular configuration has significant impacts not only on the FO performance, but also on the membrane structure. Concretely speaking, additive with cisform tends to form a more porous structure in the macro-void middle region and two perfect non-porous top and bottom layers. Based on the cross-sectional morphology in Figure 5ijkl, membrane thickness was summarized in Figure 6. It can be seen that all the membranes prepared with additives are thicker than the pure CA membrane. As previous studies [38], quick solidification of the cast film in the coagulation bath (finishing the membrane formation) could cause a halt in reduction of the thickness of the prepared membranes. In this study, the increasing membrane thickness indicates that instantaneous demixing might occur during the phase inversion for the cast films in the presence of CBA or TBA additives. Moreover, instantaneous demixing favors to the formation of a more porous structure as expected [39] and it is the case (Figure 5). In addition, the higher thickness and more porous structure for the membrane with CBA demonstrates that compared with TBA, CBA further facilitates the instantaneous demixing during the phase inversion. Surface hydrophilicity of the resulting membranes is evaluated by the water contact angle measurements and the results are also shown in Figure 6. Although there is no big difference in the contact angle, the change in the effect of additives on the contact angle is observed. For the top surface, with 2% CBA as additives, the contact angle decreases from 63° of the pure CA membrane to 55°, which might be responsible for an excellent



water flux presented in Figure 4a and the reduced S value presented in Table 2. This is consistent with the previous work [39]. The membrane exhibits a higher contact angle (59°) when CBA is replaced with TBA, indicating that CBA favors to form a more hydrophilic membrane surface. It might be explained by the difference in hydrophilicity of CBA and TBA. Compared with TBA, CBA possesses a stronger polarity due to the lack of molecular symmetry, which leads to a higher hydrophilicity. As expected, a more hydrophilicity molecule is considered as a good pore-forming agent to exchange totally into coagulation bath during the phase inversion, resulting in a negligible effect on the membrane hydrophilicity. However, in this study, CBA improves the membrane hydrophilicity, indicating that CBA does not only serve as a pore-forming agent, which will be discussed later. It is also worth noting that the hydrophilicity of the bottom surface is lowered compared with that of the top surface whatever the membrane is and follows the orders of “membrane with CBA > membrane with TBA ≈ pure CA membrane” and “membrane with additive content of 2% > membrane with additive content of 4%”. All of the results demonstrate the significant impact of additives with different configurations on the surface property of the prepared membranes. top surface contact angle bottom surface contact angle thickness

100

80

80

60

60

40

40

20

20

0

0

A 22 C 4 BA -T

2 BA -T

2 BA

4 BA -C

A 22

-C

A 22 C

C

A 22 C

0


thickness (µm)

Contact angle (ο)

100

A 22 C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Water contact angle and thickness of the membranes with additives.

Correlation between molecular configuration and membrane property Pore-forming additives, usually a kind of water-soluble small-molecule, have been confirmed to transfer from cast film to coagulation bath during the phase inversion, which affects the demixing rate to change the membrane structure and produces pores in the membrane to improve the water transport. However, the nature of the correlation between additives and membrane properties in a micro-level for the membrane design remains an open question. On the basis of our observations, we identified the interaction between additive molecules and membrane polymers from the point of molecule structure to explain the significant impact of additives with different configurations on the membrane morphology and FO performance. As shown in Figure 5efgh, a larger number of the discrete pores of 10-50 nm in diameter were disorderly distributed on the bottom layer when TBA replaced CBA, indicating that TBA favors to make pores compared with CBA. In Figure 6, a more hydrophilic membrane surface is achieved by adding CBA yet TBA, showing that CBA does not just serve as a pore-forming agent. According to the above experimental measurements, we speculate preliminary that it might be dependent on the difference in transfer rate between CBA and TBA during the phase inversion, which is related to the superiority of the two interactions between additive molecules and CA polymers or between additive molecules and water. To verify this speculation, TGA measurements were conducted to characterize the membrane composition and the results are present in Figure 7. The pure CA membrane is thermally stable in inert atmosphere, with evident



decomposition temperatures above 325 °C. As illustrated in inset in Figure 7, the membrane with TBA yields the same decomposition temperature window between 320330 °C as that of the pure CA membrane, while thermal decomposition of the membranes with CBA occurs at lower temperature, with evident loss of weight starting at 280 °C. These results demonstrate that the membrane composition has no change after adding TBA into the casing solution, while CBA varies the membrane composition. It further verifies that during the phase inversion, TBA totally transfers from the casting film to the coagulation bath, while CBA partially dissolves in coagulation bath. The residual CBA in the membrane might disturb the lattice structure of the CA polymer and the lattice irregularity leads to a weight loss at lower temperature. 1.0

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.00 0.95

0.8

0.90

0.6

0.85 240

260

280

300

CA22

0.4

CA22-CBA2 CA22-CBA4

0.2

CA22-TBA2 CA22-TBA4

0.0

0

100 200 300 400 500 600 700 800

Temperature(°C)

Figure 7. Thermogravimetric analysis of the resulting membrane with additive in the casting solution.

As shown in Figure 8, compared with TBA, CBA molecule yields a stronger repulsive force due to the two homolateral carboxy groups (relative to the double bond), resulting in much higher bond energy. Capacity of the formed hydrogen bonds between CBA and


Page 22 of 31

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CA polymer is stronger than that between TBA and CA, which weakens the interaction between CBA and water when water molecules diffuse into the casting film to exchange with casting solvent. Moreover, intramolecular hydrogen bonds are easily formed after first stage acid dissociation of CBA in the presence of water, which also leads to a weak interaction between CBA and water. Therefore, there are some partial residual CBA molecules in the prepared membranes, which is responsible for a more hydrophilic surface and the instantaneous demixing to form a more porous structure.

Figure 8. Interaction of additives with glass and CA polymer for the membrane formation.

Additionally, hydrophilic-hydrophilic interaction might be responsible for the membrane structure observed on the bottom surface. In the case of no TBA or CBA,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

when the casting solution was casted onto the glass substrate, the hydrophilic interaction between CA polymers and glass occurs, resulting that CA molecules which directly contact the glass substrate adhere to the glass surface and aggregate to form a dense bottom skin (Figure 3def) during the phase inversion [2]. Obviously porous bottom surfaces were observed after using TBA and CBA as additives in Figure 5efgh, which might be contributed to a stronger hydrophilic interaction between additive molecules and glass due to the higher hydrophilicity of these additives. In the region near the glass, additive molecules could preferentially attach to the glass and occupy a particular position to hinder the adherence of CA molecules, which leads to a porous structure of the bottom surface during the phase inversion. Moreover, the pore number observed on the bottom surface increases when TBA replaces CBA, which might also be based on the hydrogen bonds. Compared to CBA, the lower capacity of the formed hydrogen bonds between TBA and CA polymer enhances the hydrophilic interaction between TBA molecules and glass and hence a more porous structure is formed during the phase inversion.

CONCLUSIONS In summary, we have prepared cellulose acetate forward osmosis membranes with cistrans isomers (CBA and TBA) as pore-forming additives. It was found that membrane with 2% CBA exhibited a maximum and highly stable water flux with an increase of 36% compared to CA membranes and a NaCl rejection of more than 98% under FO conditions of 0.1 M NaCl solution as feed and 2 M glucose as draw solution, which indicated that additive molecules with cis-form are more suitable to facilitate the water transport with


Page 24 of 31

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


no sacrifice of salt rejection. Moreover, CBA tended to form a more porous structure in the macro-void middle region and two perfect dense top and bottom layers as well as a more hydrophilic membrane surface, which might be explained by considering the more instantaneous demixing due to CBA. In addition, it was confirmed that there were some residual CBA molecules in the prepared membranes while TBA totally transferred from the casting film to the coagulation bath, which was responsible for a more hydrophilic surface and the instantaneous demixing. In addition, the additive transfer rate during the phase inversion depended on the superiority of the two interactions between additive molecule and CA polymer or between additive molecule and water from the point of hydrogen bonds. Hydrophilic-hydrophilic interaction between additive molecules and glass substrate is also responsible for the more porous structure of bottom surface.

AUTHOR INFORMATION Corresponding Author * Tel.: +86 5326678223. E-mail address: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT The authors thank financial supports from NSFC (No. 21306178), Shandong Province Key Project (2014GHY115033), Qingdao applied basic research project (No. 15-9-1-21jch) and Fundamental Research Funds for the Central Universities (No. 201564015).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES 1. Semiat, R. Energy issues in desalination processes. Environ. Sci. Technol. 2008, 42, 8193-8201. 2. Zhang, S.; Wang, K. Y.; Chung, T. S.; Chen, H. M.; Jean, Y. C.; Gary, A. Wellconstructed cellulose acetate membranes for forward osmosis: minimized internal concentration polarization with an ultra-thin selective layer. J. Membr Sci. 2010, 360, 522-535. 3. Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and technology for water purification in the coming decades. Nature. 2008, 452, 301-310. 4. Kravath, R. E.; Davis, J. A. Desalination of sea water by direct osmosis. Desalination. 1975, 16, 151-155. 5. Phuntsho, S.; Shon, H. K.; Hong, S. K.; Lee, S. Y.; 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. 6. Quintanilla, V. Y.; Li, Z. Y.; Valladares, R.; Li, Q. Y.; Amy, G. Indirect desalination of Red Sea water with forward osmosis and low pressure reverse osmosis for water reuse. Desalination. 2011, 280, 160-166. 7. Cath, T. Y.; Childress, A. E.; Elimelech, M. Forward osmosis: principles, applications, and recent developments. J. Membr. Sci. 2006, 281, 70-87. 8. Chung, T. S.; Zhang, S.; Wang, K. Y.; Su, J. C.; Ling, M. M. Forward osmosis processes: yesterday, today and tomorrow. Desalination. 2012, 287, 78-81.


Page 26 of 31

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. Ong, R. C.; Chung, T. S.; de Wit, J. S.; Helmer, B. J. Novel cellulose ester substrates for high performance flat-sheet thin-film composite (TFC) forward osmosis (FO) membranes. J. Membr. Sci. 2015, 473, 63-71. 10. Gai, J. G.; Gong, X. L. Zero internal concentration polarization FO membrane: functionalized graphene. J. Mater. Chem. A. 2014, 2, 425-429. 11. Gai, J. G.; Gong, X. L.; Wang, W. W.; Zhang, X.; Kang, W. L. An ultrafast water transport forward osmosis membrane: porous graphene. J. Mater. Chem. A. 2014, 2, 4023-4028. 12. Zhang, X.; Gai, J. G. Single-layer graphyne membranes for super-excellent brine separation in forward osmosis. RSC Adv. 2015, 5, 68109-68116. 13. Wang, K. Y.; Yang, Q.; Chung, T. S.; Rajagopalan, R. Enhanced forward osmosis from chemically modified polybenzimidazole (PBI) nanofiltration hollow fiber membranes with a thin wall. Chem. Eng. Sci. 2009, 64, 1577-1584. 14. Zhang, S.; Wang, K. Y.; Chung, T. S.; Jean, Y. C.; Chen, H, M. Molecular design of the cellulose ester-based forward osmosis membranes for desalination. Chem. Eng. Sci. 2011, 66, 2008-2018. 15. Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M. High performance thin-film composite forward osmosis membrane. Environ. Sci. Technol. 2010, 44, 3812-3818. 16. Han, G.; Zhang, S.; Li, X.; Widjojo, N.; Chung, T. S. Thin film composite forward osmosis membranes based on polydopamine modified polysulfone substrates with enhancements in both water flux and salt rejection. Chem. Eng. Sci. 2012, 80, 219-231.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

17. Widjojo, N.; Chung, T. S.; Weber, M.; Maletzko, C.; Warzelhan, V. The role of sulphonated polymer and macrovoid-free structure in the support layer for thin-film composite (TFC) forward osmosis (FO) membranes. J. Membr. Sci. 2011, 383, 214-223. 18. Gai, J. G.; Gong, X. L.; Kang, W. L.; Zhang, X.; Wang, W. W. Key factors influencing water diffusion in aromatic PA membrane: Hydrates, nanochannels and functional groups. Desalination. 2014, 333, 52-58. 19. Qiu, C.; Qi, S.; Tang, C. Y. Synthesis of high flux forward osmosis membranes by chemically crosslinked layer-by-layer polyelectrolytes. J. Membr. Sci. 2011, 381, 74-80. 20. Cui, Y.; Wang, H.; Chung, T. S. Micro-morphology and formation of layer-by-layer membranes and their performance in osmotically driven processes. Chem. Eng. Sci. 2013, 101, 13-26. 21. Luo, W. H.; Xie, M.; Hai, F. I.; Price, W. E.; Nghiem, L. D. Biodegradation of cellulose triacetate and polyamide forward osmosis membranes in an activated sludge bioreactor: Observations and implications. J. Membr. Sci. 2016, 510, 284-292. 22. Zhang, X. W.; Ning, Z. Y.; Wang, D. K.; Diniz da Costa, J. C. D. Processing municipal wastewaters by forward osmosis using CTA membrane. J Membr Sci. 2014, 468, 269-275. 23. Su. J. C.; Yang, Q.; Teo, J. F.; Chung, T. S. Cellulose acetate nanofiltration hollow fiber membranes for forward osmosis processes. J. Membr. Sci. 2010, 355, 36-44. 24. Loeb, S.; Sourirajan, S. Sea water demineralization by means of an osmotic membrane. Adv. Chem. Ser. 1963, 38, 117.


Page 28 of 31

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25. Su. J. C.; Chung, T. S. Sublayer structure and reflection coefficient and their effects on concentration polarization and membrane performance in FO processes. J. Membr. Sci. 2011, 376, 214-224. 26. Idris, A.; Yet, L. K. The effect of different molecular weight PEG additives on cellulose acetate asymmetric dialysis membrane performance. J. Membr. Sci. 2006, 280, 920-927. 27. Sairam, M.; Sereewatthanawut, E.; Li, K.; Bismarck, A.; Livingston, A. G. Method for the preparation of cellulose acetate flat sheet composite membranes for forward osmosis—desalination using MgSO4 draw solution. Desalination. 2011, 273, 299-307. 28. Dugave, C.; Demange, L. Cis-trans isomerization of organic molecules and biomolecules: implications and applications. Chem. Rev. 2003, 103, 2475-2532. 29. Kwan, S. E.; Bar-Zeev, E.; Elimelech, M. Biofouling in forward osmosis and reverse osmosis: Measurements and mechanisms. J. Membr. Sci. 2015, 493, 703-708. 30. Duong, P. H. H.; Chisca, S.; Hong, P. Y.; Cheng, H.; Nunes, S. P.; Chung, T. S. Hydroxyl functionalized polytriazole-co-polyoxadiazole as substrates for forward osmosis membranes. ACS Appl. Mater. Interfaces. 2015, 7, 3960-3973. 31. Song, X. X.; Liu, Z. Y.; Sun, D. D. Nano gives the answer: Breaking the bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate. Adv. Mater. 2011, 23, 3256-3260. 32. Wang, K. Y.; Ong, R. C.; Chung, T. S. Double-skinned forward osmosis membranes for reducing internal concentration polarization within the porous sublayer. Ind. Eng. Chem. Res. 2010, 49, 4824-4831.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

33. Ma, N.; Wei, J.; Qi, S.; Zhao, Y; Gao, Y. B.; Tang, C. Y. Y. Nanocomposite substrates for controlling internal concentration polarization in forward osmosis membranes. J. Membr. Sci. 2013, 441, 54-62. 34. Yang, Q.; Wang, K. Y.; Chung, T. S. A novel dual-layer forward osmosis membrane for protein enrichment and concentration. Sep. Purif. Technol. 2009, 69, 269-274. 35. Wei, J.; Qiu, C. Q.; Tang, C. Y. Y.; Wang, R.; Fane, A. G. Synthesis and characterization of flat-sheet thin film composite forward osmosis membrane. J. Membr. Sci. 2011, 372, 292-302. 36. Qi, S.; Qiu, C. Q.; Zhao, Y.; Tang, C. Y. Y. Double-skinned forward osmosis membranes based on layer-by-layer assemble——FO performance and fouling behavior. J. Membr. Sci. 2012, 405-406, 20-29. 37. Xu, J.; Tang, Y. Y.; Gao, C. J. Research Progress on Optimizing the Structure of Support Layers in Forward Osmosis Membrane. Prog. Chem. 2015, 27, 1025-1032. 38. Xu, J.; Tang, Y. Y.; Wang, Y. H.; Shan, B. T.; Yu, L. M.; Gao, C. J. Effect of coagulation bath conditions on the morphology and performance of PSf membrane blended with a capsaicin-mimic copolymer. J Membr. Sci. 2014, 455, 121-130. 39. McCutcheon, J. R.; Elimelech, M. Influence of membrane support layer hydrophobicity on water flux in osmotically driven membrane processes. J. Membr. Sci. 2008, 318, 458-466.


Page 30 of 31

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Effect of molecular configuration of additives on the membrane structure and water transport performance for forward osmosis

Jia Xu*, Panpan Li, Mengzhen Jiao, Baotian Shan, Congjie Gao

For Table of Contents Use Only

Membrane preparation procedure and the effect of molecular configuration of additives on the membrane structure and performance


Effect of Molecular Configuration of Additives on the Membrane

Recommend Documents