A Self-standing, Support-Free Membrane for Forward Osmosis with No

25 concentration polarization (ICP). 14. ICP is greatly influenced by the membrane support layer. 26 structure (thickness, porosity, and tortuousity),...
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A Self-standing, Support-Free Membrane for Forward Osmosis with No Internal Concentration Polarization Meng Li, Vasiliki Karanikola, Xuan Zhang, Lianjun Wang, and Menachem Elimelech Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00117 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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Environmental Science & Technology Letters

A Self-standing, Support-Free Membrane for Forward Osmosis with No Internal Concentration Polarization

Meng Li,1 Vasiliki Karanikola,2,3 Xuan Zhang,1,2* Lianjun Wang,1 and Menachem Elimelech 2*

1) Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing 210094, China 2) Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, USA 3) Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA

*Corresponding Author: Xuan Zhang: [email protected]; Menachem Elimelech: [email protected].

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ABSTRACT 1

Conventional asymmetric or thin-film composite forward osmosis (FO) membranes suffer

2

from severe internal concentration polarization, which significantly hinders process

3

performance and practical applications. Here we report the synthesis of COOH-derived

4

polyoxadiazole copolymer for the fabrication of a self-standing selective thin film without a

5

support layer. The thickness of the membrane was controlled at merely a few micrometers

6

to achieve high rejection of the Na2SO4 draw solution, while maintaining acceptable water

7

permeability. Due to the symmetric architecture, the membrane exhibited excellent and

8

identical FO performance at both of its sides. The structural parameter of the fabricated

9

membranes was zero due to the absence of internal concentration polarization in the

10

symmetric FO membranes. Our results highlight the potential of support-free membranes

11

for the further development of FO technology.

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INTRODUCTION 13

Over the past decade, forward osmosis (FO) has attracted heightened attention as an

14

emerging technology for various separation/desalination applications.1,2 The spontaneous

15

process driven by osmotic pressure and the inherently low fouling propensity of the process

16

render FO an attractive platform for a wide range of applications, particularly those that

17

cannot be performed by reverse osmosis (RO).1,3-5 Nevertheless, the relatively low

18

module-averaged water flux6 and high energy consumption associated with the effective

19

regeneration and reuse of draw solutes1 remain as major challenges for successful

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implementation of FO.

21

Membranes for FO are commonly derived from asymmetric cellulose triacetate6-8 or

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polyamide thin-film composite (TFC) architectures9-14 and generally exhibit good water

23

permeability and rejection of draw solutes. However, as water permeates and dilutes the

24

draw solution at the interface between the porous substrate and active layer, the driving

25

force for water permeation, and hence water flux, are significantly reduced due to internal

26

concentration polarization (ICP).14 ICP is greatly influenced by the membrane support layer

27

structure (thickness, porosity, and tortuousity), which is characterized by the so-called

28

structural parameter,1,15 as well as the hydrophilicity of the support layer.16-18 Consequently,

29

extensive efforts have been devoted in recent years to fabricate membranes with reduced

30

ICP by designing membranes with thin and porous support layers as well as by enhancing

31

the hydrophilicity of the support layer.19-23 However, significant reduction of the structural

32

parameter of FO membranes is challenging because membranes with thin and highly porous

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support layers lack the necessary mechanical strength for FO applications.

34

To address this problem, the idea of support-layer-free thin films with a symmetric and

35

homogenous structure merits attention. Although the concept of an FO membrane with no

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ICP was proposed in a recent modeling study of a monolayer graphene24, the exploration of

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mechanically robust polymeric materials for scalable fabrication of symmetric FO

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membranes has yet to be systematically studied.

39

In this work, we fabricated, for the first time, a self-standing, support-free membrane for

40

forward osmosis. The polymeric membrane was synthesized by an initial polycondensation 3

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reaction between 4,4’-oxybis (benzoic acid) and hydrazine sulfate salt and subsequent

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grafting of carboxyl acid group to the side chains. The physicochemical and mechanical

43

properties of the membrane were thoroughly characterized and the water flux and reverse

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draw solute flux of membranes of various thickness were determined as a function of draw

45

solution concentration. Our study provides a new platform for the fabrication of symmetric,

46

support-free FO membranes with no internal concentration polarization.

47 MATERIALS AND METHODS 48

Materials and Chemicals. 4,4’-Oxybis (benzoic acid) (OBBA, 98%) and polyphosphoric

49

acid (PPA, >85%) were purchased from J&K Chemical Reagent Co. Ltd. (Shanghai, China).

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Hydrazine sulfate (HS, 99%) and 4-aminobenzoic acid (p-ABA, 98%) were supplied by

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Energy Chemical (Shanghai, China). Anhydrous N-methyl-2-pyrrolidone (NMP, 99.5%),

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N,N-dimethylformamide (DMF, 99.5%), and other reagents/solvents were used as received.

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Deionized (DI) water was obtained from Millipore System (Millipore-Q, Ultrapure Water

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System, resistance 18 MΩ-cm).

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Synthesis of PODH and PTAODH. Polyoxadiazole-co-hydrazide (PODH) was

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synthesized according to previous studies.25-28 In brief, a 500 mL fully-dried three-necked

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flask was charged with OBBA (4.001 g, 15.5 mmol), HS (2.419 g, 18.6 mmol), and PPA (60.7

58

g). The solution with reactants was heated to 160oC and kept for 3 h with continuous stirring.

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Next, the highly viscous solution was poured into water to isolate the fiber-like polymers.

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After thoroughly washing with water, 5% NaOH solution, and DI water, successively, PODH

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was obtained by drying under vacuum at 100oC for 24 h (yield: 92%). FTIR (υ, cm-1): 1670

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(C=O), 1605, 1468 (C=C linkage of aromatic rings), 1237 (C-O-C from OBBA), 1085, 1030

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(C-O-C from oxadiazole ring); 1H NMR (DMSO-d6, ppm): δ = 8.5 (-NH-NH-), 8.3–8.0

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(Ar-H), 7.4-7.0 (Ar-H).

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A 100 mL fully-dried three-necked flask, equipped with N2 inlet and outlet, was charged

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with PODH (1.011 g, 4.282 mmol), p-ABA (0.588 g, 4.282 mmol), PPA (0.1 g), and NMP

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(20.0 mL). Following complete dissolution of polymers, the mixture was heated for 12 h at

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195oC. After cooling to room temperature, the brownish solution was poured into DI water. 4

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The fiber-like polymer, polytriazole-co-oxadiazole-co-hydrazide (PTAODH-1.0), was then

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washed thoroughly with hot water to remove the unreacted precursor and was dried under

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vacuum at 80oC for 24 h (yield: 79%). PTAODH-1.5 was synthesized by the same

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procedure except the feed amount of p-ABA was increased to 0.881 g (6.423 mmol). FTIR

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(υ, cm-1): 1670 (C=O), 1530, (C=N from triazole ring), 1605, 1468 (C=C), 1237 (C-O-C

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from OBBA), 1085, 1030 (C-O-C from oxadiazole ring); 1H NMR (DMSO-d6, ppm): δ =

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8.5 (-NH-NH-), 8.3–7.0 (Ar-H).

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Preparation of Self-standing Symmetric FO Membranes. Self-standing

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thin-films, PODH and PTAODH, were prepared via the solvent evaporation method. In brief,

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homogeneous solutions of both copolymers were obtained by dissolving them into DMF at a

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concentration of ~1 wt%. After being filtered and degassed, the solutions were poured into

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ultraflat petri dishes, allowing them to dry at 80oC overnight. The membranes were detached

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by first immersing them in water, followed by thorough washing with DI water and finally

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storing in DI water.

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Membrane Characterization. 1H NMR spectra of polymers were characterized by a

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Bruker AVANCEIII spectrometer (500 MHz) using DMSO-d6 as the solvent. FTIR spectra

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were obtained from a Perkin Elmer Spectrum FTIR spectrometer. Morphologies of the

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self-standing thin-film samples were studied by field emission scanning electron

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microscopy (FESEM, S-4800, Hitachi, Japan). All membrane samples were air-dried,

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fractured in liquid nitrogen, and sputter coated with gold prior to the test. Membrane surface

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charge was characterized by streaming potential using an electrokinetic analyzer with a set

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of AgCl electrodes (SurPASSIII, AntonPaar, Austria). For the streaming potential

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measurements, an electrolyte solution of 0.01 M KCl was used to provide the background

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ionic strength and was automatically titrated with 0.05 M HCl and 0.05 M NaOH to

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investigate the effect of pH on the zeta potential. Membrane hydrophilicity was assessed

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using a contact angle and drop shape analyzer (KRÜSS, DSA30, Germany). At least three

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measurements were made at different locations for each membrane surface and their

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averages were recorded. The number- and weight-average molecular weights (Mn and Mw)

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of PODH and PTAODH polymers were determined by gel permeation chromatography 5

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(GPC) on a Waters 1515 HPLC system with a fixed 1.0 mL min-1 flow rate;

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N,N-dimethylformamide (DMF) containing 0.05 M LiBr was used as eluate. Mechanical

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properties of membranes were measured by a universal material tester (AGS-100NX,

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Shimadzu) at room temperature (20oC) and 60% relative humidity. Membrane samples were

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cut into rectangle shapes (2 cm × 0.5 cm) and fixed onto the cantilever, with the extension

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speed set at 2 mm s-1.

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Membrane Performance Testing. To evaluate the performance of the fabricated

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membranes, a lab-scale cross-flow FO module was utilized, with an effective membrane

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area of 10 cm2 as described elsewhere.5,21 DI water and Na2SO4 at different concentrations

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(0.5, 0.75, 1, 1.25, and 1.5 M) were used for feed solution (FS) and draw solution (DS),

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respectively. The flow rate for both solutions was fixed at 10.36 cm/s. The variation in

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Na2SO4 concentration of the FS was calculated by measuring the electric conductivity

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(Conductivity Meter DDS-307, Shanghai, China) and the rate of weight change of the FS

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was recorded by a digital weight balance. All experiments were carried out at room

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temperature (25oC).

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The stability of the PTAODH-1.0 membrane (thickness of 8 µm) was evaluated by a

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continuous 16 h test at 25oC. DI water and Na2SO4 solution were used as FS and DS,

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respectively. The concentration of Na2SO4 solution was monitored every ~2 h, and

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additional Na2SO4 was added to the draw solution to keep the salt concentration constant at

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1.5 M. To ensure the symmetric character of the membrane, water fluxes (Jw) were

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measured for both the top and bottom sides of the membrane.

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Modeling. Water flux, Jw, and salt flux, Js, in FO can be determined by6

 π exp −  − π exp Jw   Fb  kF   Db Jw =A πDm − πFm  = A   1+ B exp Jw  − exp −    kF  Jw

(1)

 c exp −  − c exp Jw   Fb  kF   Db Js =B (cD − cFm ) = B   m 1+ B exp Jw  − exp −    kF  Jw

(2)

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where A and B are the membrane water and solute permeability coefficients, respectively;

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πDm and πFm are the osmotic pressures of the draw and feed solutions at the

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membrane/solution interface, respectively; cDm and cFm are the solute concentrations of the

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draw and feed solutions at the membrane/solution interface, respectively; kF and kD are the

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mass transfer coefficients of the solute at the feed boundary layer and of the feed at the draw

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boundary layers; πDb and πFb are the osmotic pressures of the draw and feed bulk solutions,

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respectively; and cDb and cFb are the draw and feed bulk solution concentrations,

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respectively. Calculation of the mass transfer coefficient kF is described in Supporting

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Information. Internal concentration polarization is neglected in this study as the dense

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membranes synthesized are support-free and the structural parameter, S, is zero due to the

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lack of a support layer.

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The sum of squared errors method was used to fit the permeability coefficients A and B

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by calculating the absolute difference between the predicted (eq 1 and 2) and the

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experimental water and salt fluxes. With the aid of the Solver function in Excel, the total

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sum of errors was then minimized by fitting the permeability coefficients A and B in the

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water and salt flux equations. The new obtained values for A and B were then compared to

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the obtained experimental water and solute permeability coefficients (the latter is described

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in Supporting Information). The difference between the value fitted by the experimental

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data and the value fitted based on the sum of squared error is always less than 10%.

140 RESULTS AND DISCUSSION 141

Synthesis and Physicochemical Properties of Polymer Membranes. Scheme S1

142

shows the synthesis route of PDOH and PTAODH. The final polymer was synthesized by

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an initial polycondensation reaction between OBBA and HS, followed by side chain

144

grafting to form a COOH-functionalized pendant. In contrast to a typical condensation

145

reaction, which generally requires an equivalent feed ratio of the reactants, the molar ratio

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used here for OBBA and HS was 1:1.2. This ratio is consistent with a previous report

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optimized by statistical experimental design and kinetic modeling.27 The subsequent step 7

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was the nucleophilic attack of the amino group on the electron-deficient oxadiazole ring.29,30

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It should be noted that although the conditions of the reaction were similar to the synthesis

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of OH-modified polyoxadiazoles,26,31 the conversion ratios in our case were always found in

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the low range of 10-20% (calculated from 1H NMR spectra). One possible explanation is

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the existence of a strong electro-withdrawing group (-COOH) which may result in a

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decrease in nucleophilicity of –NH2 group, thus hindering the substitution reaction.

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The chemical structures of both copolymers were studied by 1H NMR and FTIR spectra.

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In Figure 1(A)-(a), the sharp peaks around 8.5 ppm and 8.3-7.0 ppm are assigned to the

156

hydrazide and phenyl protons in PODH, respectively. However, the spectra turn into the

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scrambled states for both PTAODH-1.0 and PTAODH-1.5 with the occurrence of several

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new peaks which overlapped (Figure 1(A)-(b) and (c)), indicating the complexity of the

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grafting reaction. It has been reported that PODH was chemically unstable in an acidic

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medium, undergoing a ring-opening reaction of the oxadiazole ring and hydrolysis of

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hydrazide group.27,32 However, since a dehydrating agent is critically needed for the

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subsequent nucleophilic substitution reaction, PPA is typically used as described

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elsewhere.25,27 Such a predicament leads to the unavoidable degradation of PODH during

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the synthesis. As proposed in Figure S1, degradation occurs either by the scission of

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oxadiazole or of hydrazide units. Such scission produces more terminated carboxylic acid

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groups at the end of the polymer chain; these groups could further react with p-ABA in the

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presence of acid at high temperature. Yet, for the PTAODH polymers, a clear absorption

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band at 1530 cm-1 is observed, corresponding to the stretching vibration of C=N from the

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triazole ring,26 (Figure 1(B)), confirming the successful formation of the triazole ring.

170 171

Figure 1

172 173

The surface zeta potential was determined for the three fabricated membranes as a

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function of pH (Figure S2). PODH, without -COOH groups, exhibited an electroneutral

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character with a constant zeta potential of near zero over the pH range investigated. On the

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other hand, highly negative zeta potentials were observed for the PTAODH-1.0 and 8

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PTAODH-1.5 membranes, with the latter exhibiting the highest negative charge among the

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three membranes. We attribute this observation to the more degraded polymeric skeletons in

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the

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COOH-terminated groups, as we previously hypothesized. These findings are in accordance

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with the water contact angles shown in Figure S3. Both PTAODH membranes showed

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much lower contact angles after the introduction of polar carboxyl acid groups. Specifically,

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the contact angle changed from the original 78.6±1.3 degree of PODH to 63.7±1.1 and

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59.4±1.0 degrees for the PTAODH-1.0 and PTAODH-1.5 membranes, respectively,

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suggesting improved surface hydrophilicity. We note that no significant difference in

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contact angles existed between the top and bottom surface for any of the three membranes,

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which further demonstrates their symmetric nature.

case

of

PTAODH-1.5

rather

than

PTAODH-1.0,

which

generated

more

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Morphological Properties of Fabricated Membranes. The surface and

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cross-section morphologies of all membranes were observed by FE-SEM. As shown in

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Figure 2, the surface morphologies for all membranes are almost identical, and no visible

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pores were identified in all the images (both top and bottom), suggesting their rather dense

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character. In addition, uniform structures were also found in the cross-section images of all

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membranes, which again indicates their symmetric properties.

194 195

Figure 2

196 197

Forward Osmosis Membrane Performance. The performance of the novel thin

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films was examined in a typical FO system. Since the membranes were negatively charged,

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two typical inorganic salts, Na2SO4 and NaCl, were used as draw solutes to verify the

200

feasibility of the membranes in FO. As shown in Figure S4, PODH, the less-charged

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membrane, exhibited the highest reverse salt flux among the three membranes (609±42.8

202

mmol m-2 h-1 and 110±10.6 mmol m-2 h-1 for NaCl and Na2SO4, respectively), indicating its

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inappropriateness for FO applications. However, both PTAODH-1.0 and PTAODH-1.5

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membranes, with carboxyl side chain incorporated, showed much reduced reverse salt

205

fluxes. The reverse salt flux for Na2SO4 was lower than that for NaCl for both membranes, 9

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which is attributed to the higher electrostatic repulsion of the divalent sulfate ions by the

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negatively charged membranes.33,34 Both PTAODH-1.0 and PTAODH-1.5 membranes

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exhibited low reverse salt (Na2SO4) fluxes around 20-30 mmol m-2 h-1, highlighting their

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potential application in FO.

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At this point we will discuss the mechanical robustness of the membranes, since all the

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membranes in the current study were support-free and fabricated with a thickness of a few

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micrometers. Although the FO performance for both PTAODH membranes was quite

213

comparable, the PTAODH-1.5 membrane was more susceptible to fracture under long-term

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operation. The lower maximum yield stress and elongation for the PTAODH-1.5 membrane

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are indicative of the mechanical strength of this membrane compared to the other

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membranes (Table S2). This result highlights the critical need to select an appropriate

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degradation ratio for PODH to ensure that while satisfactory mechanical robustness is

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achieved, the surface charge is still negative enough to maintain high salt rejection by

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electrostatic repulsion.

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FO performance as a function of salt concentration was studied using the PTAODH-1.0

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as the optimal membrane, balancing performance and mechanical strength. The influence of

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membrane thickness on FO performance was also evaluated. As shown in Figure 3(A), for

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each FO membrane, water flux exhibits a linear relationship with draw solution

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concentration and increases substantially with increasing Na2SO4 concentration from 0.5 to

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1.5 M. It should be noted that water fluxes through the PTAODH-1.0 membranes were

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improved proportionally with the decrease of membrane thickness at all draw solution

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concentrations. For instance, water flux of the 5-µm PTAODH-1.0 membrane was

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approximately twice of that for the 8-µm membrane and three times of that of the 15-µm

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membrane at a given draw solution concentration. Results of reverse fluxes of draw solute

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of the PTAODH-1.0 membranes are shown in Figure 3(B) and, as expected, exhibit a

231

similar trend to the observed water flux.

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An important parameter characterizing FO membrane performance is the reverse flux

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selectivity, which is the ratio of water flux to reverse salt flux, Jw/Js.35 This ratio is

234

proportional to the membrane selectivity (A/B), and is given by  , where n is the 



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number of dissolved species created by the draw solute (3 for Na2SO4), R is the ideal gas

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constant, T is the absolute temperature, and A and B are the water and salt permeability

237

coefficients, respectively.35 For example, the averaged reverse flux selectivity was

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412.0±30.9 L mol-1 for the 8-µm PTAODH-1.0 membrane, which is comparable to a

239

commercial FO membrane, suggesting its high FO performance.35-37 Detailed comparison of

240

the selectivities and structural parameters of the fabricated membranes and the commercial

241

FO membrane is presented in Table S3 of Supporting Information.

242 243

Figure 3

244 245

By using the calculated pure water permeability (A) and solute permeability (B) values

246

(see detailed calculations in Supporting Information), the structural parameters (S) for the

247

three PTAODH-1.0 membranes were found to be 4.5±2.0, 3.5±5.7, and 1.0±5.5 µm for

248

membrane thicknesses of 5, 8, and 15 µm, respectively. Considering the inherent errors

249

during the experiments, the S values for all the above three membranes could be regarded as

250

zero, which is expected for support-free FO membranes with no ICP. In other words, the

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utilization of the osmotic pressure driving force is maximized in the symmetric FO system.

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Modeling results for water flux and reverse mass flux of draw solution are in excellent

253

agreement with the experimental data (Figure 3). The fitted intrinsic membrane

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permeabilities (i.e., material properties independent of membrane thickness) for water and

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salt are 8.37 × 10-7 L m-1 h-1 bar-1 and 1.93 × 10-7 L m-1 h-1, respectively. Using these values,

256

the water and salt permeability coefficients (i.e., A and B) for the membranes with various

257

thicknesses can be calculated. For example, for the 5-µm PTAODH-1.0 membrane, the

258

calculated A and B are 0.16 L m-2 h-1 bar-1 and 0.039 L m-1 h-1, respectively, which are in

259

close agreement with the values determined from RO experiments (Table S3). Through this

260

validated model, we can predict the performance of the support-free FO membrane for any

261

thickness, operational variables, and draw solution concentration.

262 ASSOCIATED CONTENT 11

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Supporting Information Available: Membrane performance testing methodology for

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evaluation of structural parameter (S) and mass transfer coefficients. Supporting Scheme

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includes the synthetic route of PODH and PTAODH. Supporting figures include any

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possible degradation mechanisms during the synthesis of PTAODH (Figure S1), zeta

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potential of the PODH, PTAODH-1.0, and PTAODH-1.5 FO membranes (Figure S2), static

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water contact angles (Figure S3), FO performance of PODH, PTAODH-1.0 and

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PTAODH-1.5 membranes as a function of different type of draw solutes (Figure S4), and

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stability testing of water flux for representative PTAODH-1.0 membrane (Figure S5).

271

Supporting tables include GPC data of all polymers (Table S1), mechanical properties

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(Table S2), and RO/FO performance of PTAODH-1.0 membrane with different thicknesses

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(Table S3). This material is available free of charge via the Internet at https://pubs.acs.org/. AUTHOR INFORMATION

274

Corresponding Author

275

* Xuan Zhang. E-mail: [email protected], Tel./fax: +86-25-84315916.

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* Menachem Elimelech: [email protected]. Notes

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The authors declare no competing financial interest. ACKNOWLEDGMENTS

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This work was supported by the National Natural Science Foundation of China (21774058,

279

51778292), Priority Academic Program Development of Jiangsu Higher Education

280

Institutions (PAPD), State Key Laboratory of Separation Membranes and Membrane

281

Processes (Tianjin Polytechnic University, M2-201604), and the Agnese Nelms Haury

282

Program in Environment and Justice at the University of Arizona.

283 REFERENCES 284 285

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(13) Wang, Y.Q.; Li, X. Y.; Cheng, C. L.; He, Y. B.; Xu, T. W. Second Interfacial Polymerization on Polyamide Surface using Aliphatic Diamine with Improved Performance of TFC FO Membranes. J. Membr. Sci. 2016, 498, 30-38.

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(15) Ghanbari, M.; Emadzadeh, D.; Lau, W. J.; Riazi, H.; Almasi, D.; Ismail, A.F. Minimizing Structural Parameter of Thin Film Composite Forward Osmosis Membranes using Polysulfone/halloysite Nanotubes as Membrane Substrates, Desalination 2016,377, 152-162.

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Figure 1. (A) 1H NMR spectra of (a) PODH, (b) PTAODH-1.0, and (c) PTAODH-1.5. In (b) and (c), the signals circled by the dashed ovals were attributed to the degradation of the

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polymer chains. (B) FTIR spectra of PODH, PTAODH-1.0, and PTAODH-1.5.

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Figure 2. FESEM images of all three types of membranes (PODH, PTAODH-1.0, and

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PTAODH-1.5 membrane, respectively): (a) (d) (g) top surface; (b) (e) (h) bottom surface; (c) (f) (i) cross-section.

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(A) 20 Experimental Simulated

5 µm

12

-2

-1

Jw (L m h )

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8 µm

8 4 0 0.0

15 µm

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1.2

1.6

-1

Na2SO4 Concentration (mol L )

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(B)10

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5 µm

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6 40

4

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8 µm 15 µm

0 0.0

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-1

20

2

JS (mmol m h )

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JS (g m h )

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Experimental Simulated

0

-1

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Na2SO4 Concentration (mol L )

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Figure 3. (A) Water flux (Jw) and (B) reverse draw salt flux (Js) for the PTAODH-1.0 membranes with different thickness. Experiments were carried out in an FO test cell with an

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effective membrane area of 10 cm2 and a crossflow velocity of 10.36 cm/s using different concentrations of Na2SO4 as draw solution and DI water as feed solution. At least two

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parallel tests were conducted at 25oC for 2 h, and the average values and error bars are presented.

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