Thin-Film Nanocomposite (TFN) Membranes Incorporated with Super

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Thin-film nanocomposite (TFN) membranes incorporated with super-hydrophilic metal-organic framework (MOF) UiO-66: Towards enhancement of water flux and salt rejection Dangchen Ma, Shing Bo Peh, Gang Han, and Shing Bor Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14223 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017

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ACS Applied Materials & Interfaces

Thin-Film Nanocomposite (TFN) Membranes Incorporated with Super-Hydrophilic MetalOrganic Framework (MOF) UiO-66: Towards Enhancement of Water Flux and Salt Rejection Dangchen Ma, Shing Bo Peh, Gang Han,* Shing Bor Chen* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore KEYWORDS: Seawater desalination; forward osmosis (FO); thin-film nanocomposite (TFN); metal-organic framework (MOF); internal concentration polarization (ICP); UiO-66

ABSTRACT

Zirconiumv

(IV)-carboxylate

metal-organic

framework

(MOF)

UiO-66

nanoparticles were successfully synthesized and incorporated in the polyamide (PA) selective layer to fabricate novel thin-film nanocomposite (TFN) membranes. Compared to unmodified pure polyamide thin-film composite (TFC) membranes, the incorporation of UiO-66 nanoparticles significantly changes the membrane morphology and chemistry, leading to an improvement of intrinsic separation properties due to the molecular sieving and super-

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hydrophilic nature of UiO-66 particles. The best performing TFN-U2 (0.1 wt% particle loading) membrane not only shows a 52% increase of water permeability but also maintains salt rejection levels (~95%) similar to the benchmark. The effects of UiO-66 loading on the forward osmosis (FO) performance were also investigated. Incorporation of 0.1 wt.% UiO-66 produced a maximum water flux increase of 40% and 25% over the TFC control under PRO and FO modes, when 1 M NaCl was used as the draw solution against deionized water feed. Meanwhile, solute reverse flux was maintained at a relatively low level. In addition, TFN-U2 membrane displayed a relatively linear increase in FO water flux with increasing NaCl concentration up to 2.0 M, suggesting a slightly reduced internal concentration polarization effect. To our best knowledge, the current study is the first to consider implementation of Zr-MOFs (UiO-66) onto TFN-FO membranes.

1. INTRODUCTION Global water scarcity has turned into a critical issue mainly because of the severe industrial contamination and rapid population growth. Therefore, development of feasible water treatment technologies with low operational cost and high efficiency is urgently desired to mitigate this challenge.1-5 Forward osmosis (FO) has shown significant promise as separation technique for water treatment, such as wastewater treatment,6-10 osmosis membrane bioreactor (OMBR),11-13 seawater desalination,14-19 power generation,20,21 and food processing.22,23 In the FO process, the hydraulic pressure in RO is replaced by an osmotic pressure difference, which arises from a difference in chemical potential, to move water from a feed to a draw solution.24 The low pressure operating conditions of FO offers numerous advantages including low energy


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consumption and equipment cost, superior anti-fouling propensities, and great rejection to various contaminants.1,4,25-29 Thin-film composite (TFC) membranes, experiencing considerable success in RO, has been tailored for FO applications.30-36 An interfacial polymerization reaction is employed to produce an ultra-thin polyamide (PA) rejection layer above a porous support layer. Compared to commercially available cellulosic membranes directly made by phase inversion, TFC membranes generally

offer

superior

permeability

characteristics

and

greater

resistance

against

biodegradation.2,4,33 However, the intrinsic internal concentration polarization (ICP), fouling and solute reverse diffusion have restricted the development of TFC FO membranes.2,4,32,65 As a result, a careful design of membrane structure and chemistry is necessary to maximize the FO separation performance. Incorporating nanomaterials into the PA layer provides additional degrees of freedom to tune the material properties and enhance the separation performance. Jeong et al. introduced the concept of fabricating and using nanocomposite membranes for RO separation.37 Later, Ma et al. explored the potential of thin-film nanocomposite (TFN) membranes in FO using NaY zeolite nanoparticles in the active layer.16 Other nanomaterials such as functionalized carbon nanotubes and TiO2 have also been incorporated to optimize the membrane FO performance.38-40 The incorporation of inorganic nanoparticles allows for enhanced surface hydrophilicity and hence water flux. However, the non-selective voids induced by the incorporated nanoparticles may lead to deterioration in salt rejection. The resulting severe solute reverse flux decreases the net trans-membrane osmosis driving force. Furthermore, poor affinity of inorganic nanoparticles with the polymer matrix may lead to their dissociation from the PA layer and a significant loss in water flux over the duration of operation.


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Metal-organic frameworks (MOFs) are a new type of hybrid material consisting of metal clusters or ions as inorganic building bricks coordinated by organic ligands as linkers.41-47 The highly porous structure, mild synthesis conditions and tunable chemical functionality of MOFs make them promising candidate materials for water treatment.48-51 The presence of organic ligands in MOFs can enhance their compatibility with the polyamide layer matrix when compared to conventional inorganic particles. Consequently, one can more easily blend MOFs and monomers to reduce the free-volume voids in the heterogeneous system. For example, Lee et al. employed various MOFs (HKUST-1, MIL-53, Fe-BTC) as removable templates to enhance the porosity of FO mixed matrix membranes upon decomposition.48 The membranes exhibited improved flux when 0.5 M MgCl2 was used as draw solution against deionized water feed solution. However, water-unstable fillers are not suitable for incorporation within the PA rejection layer because loss of MOF structure upon contact with water may induce defects and deteriorates the membrane’s salt rejection performance.51 On the other hand, MOFs with good water stability such as UiO-66, ZIF-8, and MIL-101 (Cr)58 may impart beneficial characteristics for aqueous phase separations. For FO desalination, the mesoporous structure of MIL-101 (Cr) with cage diameter of 29 Å and 34 Å could lead to severe salt leakage,68 while the hydrophobicity of ZIF-8 nanoparticle may result in a reduced water flux.51 Hence, UiO-66 was selected as the filler material to fabric TFN FO membranes. UiO-66 belongs to the family of zirconium (IV)-carboxylate MOFs (Zr-MOFs) with excellent chemical and thermal stabilities.5256

Its outstanding stability in water allows for sustained performance during operation in the

aqueous phase as compared to other MOFs. In addition, the well-defined sub-nanometer pores of UiO-66 may also behave like water channels to facilitate water permeation while blocking


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hydrated cations, e.g., Na+, K+, Ca2+, Mg2+, etc.52,57 The hydrophilicity and favorable stability characteristics of UiO-66 make it suitable for incorporation in the PA layer. The present study aims to fabricate TFN membranes by incorporating UiO-66 MOF nanoparticles into the PA layer, and to characterize their separation properties and FO performances. The effects of UiO-66 incorporation on transforming the PA layer morphology, the membrane surface hydrophilicity and chemistry, as well as the membrane separation performance will be investigated systematically. To our best knowledge, the present study is the first to characterize Zr-MOFs incorporated TFN-FO membranes. Successful fabrication of highperformance TFN membranes is anticipated to advance the application of nanotechnology in membrane development.

2. MATERIALS AND METHODS 2.1 Materials. All chemicals were used as received without further purification and were of analytical grade unless specified. Udel polysulfone (PSf, UDELP-3500) was used as the membrane substrate polymer with N-methyl-2-pyrrolidone (NMP, Merck) as solvent for the casting solution. Lithium chloride (LiCl, Merck) and polyvinylpyrrolidone (PVP, average Mw ~ 1,300,000 g/mol, Sigma-Aldrich) were used as additives. m-phenylenediamine (MPD, > 99%, Sigma-Aldrich) and trimesoyl chloride (TMC, 98%, Sigma-Aldrich) were employed as the building block units for the PA selective layer. Sodium dodecyl sulfate (SDS, >99%) from Sigma-Aldrich was dissolved in MPD aqueous solution prior to interfacial polymerization (IP), while n-hexane (> 99%, Merck) was used as the solvent for TMC monomer. Zirconium chloride (ZrCl4, > 99.5%, Sigma-Aldrich), benzene-1,4-dicarboxylic acid (BDC, 98%, Sigma-Aldrich),


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acetic acid (>99%, Merck), and N,N-dimethylformamide (DMF, >99.8%, Merck) were used for the synthesis of UiO-66. For FO desalination, sodium chloride (NaCl, Merck) with various amounts were dissolved in deionized (DI) water and used as draw solution. A Milli-Q unit (Millipore, USA) was used to supply the DI water with a resistivity of 15 MΩ cm. 2.2 UiO-66 nanoparticles synthesis. UiO-66 nanoparticles were prepared via solvothermal reaction following a procedure reported elsewhere.58,59 BDC (114 mg or 0.686 mmol) was dissolved in 20 mL of DMF/acetic acid mixed solvent (9:1). Stoichiometric amount of ZrCl4 (160 mg) was then added. Subsequently, the reaction mixture was sonicated for 30 min and maintained at solvothermal conditions (120 °C for 40 h). The white solids were recovered by centrifuge (5 min, 5000 rpm) and washed with DMF for three times. In order to completely remove any ligands that were unreacted and trapped inside the framework, the solids were soaked in DMF and heated at 80℃ for 12 h. Then, the samples underwent a 3-day solvent exchange process using methanol at ambient temperature, followed by drying at 120℃ in vacuum oven overnight to afford the final products. 2.3 PSU membrane substrates preparation. The fabrication of PSU substrates followed a reported method with some minor changes.60 Briefly, dried PSU (15.5 wt%), LiCl (3 wt%), and PVP (0.5 wt%) were dissolved in NMP (81 wt%) and stirred at 70 ℃ for 24 h. After complete dissolution, the solution was degassed over 24 h. Subsequently, the membrane was cast over a clean glass plate using a blade with 150 µm in height, followed immediately by immersion into a water bath for phase inversion. The resulting substrates remained in water for over 48 h for thorough removal of residual solvents. Prior to further application, all membranes were stored in room-temperature deionized water.


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2.4 TFC and TFN membranes preparation. The polyamide active layer of the TFC membranes (benchmark) was formed via IP reaction of TMC with MPD. To position the solvent interface directly on top of the substrate layer, the membrane substrates were first thoroughly wetted from 2-min immersion in aqueous MPD (2 wt%) solution. After gentle removal of excess surface droplets using filter paper, the 0.15 wt% TMC/n-hexane solution was introduced via careful pouring and then drained after 1-min reaction. In the whole process, the membrane substrates were fixed in frames so that interfacial polymerization reaction happened only at their top surfaces. Fabrication of UiO-66/polyamide TFN membranes was similar to that of TFC membrane, except that UiO-66 was added in the 0.15 wt% TMC/n-hexane solution before interfacial polymerization. UiO-66 with various concentrations from 0.05 wt% to 0.2 wt% was dispersed in TMC/n-hexane solution via ultrasonication at room temperature for 30 min. The MPD-soaked membrane substrates were immediately placed in contact with the resultant UiO-66 TMC/nhexane solution for formation of TFN membranes. The synthesized TFN membranes were denoted by TFN-U1, TFN-U2, TFN-U3, and TFN-U4, corresponding to a particle loading of 0.05 wt%, 0.1 wt%, 0.15 wt%, and 0.2 wt%, respectively. Prior to further characterizations, the membranes were all kept in DI water. Table 1 summarizes the interfacial polymerization conditions for TFN and TFC membranes.


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Table 1. Interfacial polymerization conditions of TFC and TFN membranes. Membrane ID

Chemical concentration in solvent (wt%) MPD/water

SDS/water

TMC/n-hexane

MOF UiO-66/n-hexane

TFC

2

2

0.15

-

TFN-U1

2

2

0.15

0.05

TFN-U2

2

2

0.15

0.1

TFN-U3

2

2

0.15

0.15

TFN-U4

2

2

0.15

0.2

2.5 UiO-66 particle characterizations. 2.5.1 Transmission-Fourier Transform Infrared spectroscopy. Fourier transform infrared spectrum (Bio-Rad TFS-3500 FTIR in transmission mode) of UiO-66 over the wavenumber range 400-4000 cm-1 was measured at room temperature. The as-synthesized nanoparticles were freeze-dried overnight (ModulyoD, Thermo Electron Corporation, USA) and blended with KBr. The samples were then compressed into pellets for direct FTIR measurements. 2.5.2 X-ray diffraction spectroscopy. The X-ray diffraction (XRD) patterns of UiO-66 nanoparticles were obtained via a Bruker D8 Advance X-ray Power Diffractometer using Cu K excitation radiation (λ = 0.154 nm at 30 mA and 40 kV). 2.5.3 Field-emission scanning electron microscopy. The morphologies and sizes of UiO66 nanoparticles were determined by field-emission scanning electron microscope (FESEM, JEOL JSM-6700F). The as-synthesized nanoparticles were first dispersed in hexane solution with ultra-low concentration. Then, approximately 10 µL of resultant solution was transferred onto silicon wafer and dried. A thin platinum layer was applied with a sputter coater to ensure conductivity (JEOL LFC-1300).


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2.6 Membrane characterizations. 2.6.1 Membrane morphology, contact angle, and porosity. The membrane morphologies were observed with a field-emission scanning electron microscope. Prior to characterization, the samples were freeze dried in vacuum for 24 h. Subsequently, the dried samples were frozen and fractured in liquid nitrogen before applying sputter-coating of a thin platinum layer. Contact Angle Goniometer (Rame Hart, USA) was used to characterize the surface hydrophilicities of PSU substrate, TFN and TFC membranes via the sessile drop technique. Using Milli-Q DI water as the drop-forming liquid, the measurement was immediately conducted at room temperature as a small droplet touched the membrane surface. In order to reduce the experimental error, 10 random spots of each membrane sample were tested, and the averaged value was recorded. The overall porosity (%) of the membrane was determined by the following expression:32,33 = (

( )⁄

(1)

)⁄ ⁄

The wetted membrane weight (m1, g) was recorded after overnight immersion of the membrane sample in DI water and careful blotting of excess droplets by tissue paper from the membrane surface. Next, the sample was freeze dried overnight to obtain the dry weight (m2, g). The porosity calculation was carried out with water (ρw) and PSU (ρp1) densities taken to be 1.00 and 1.24 g cm-3 respectively. 2.6.2 Mean pore size and pore size distribution of PSU substrate. Pure water permeability (PWP) (in L m-2 h-1 bar-1) of the PSU substrate was first measured via a dead-end permeation cell at 1 bar with DI water as feed. This setup was utilized also for solute rejection


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experiments to estimate the pore-size distribution and mean effective pore size of the PSU membrane substrate. The cell was fed with 200 ppm polyethylene oxide (PEO) or polyethylene glycol (PEG) solutions of various molecular weights. A total organic carbon analyzer (TOC ASI5000A, Shimadzu, Japan) was employed to determine the neutral solute concentrations of the permeate (Cp) and the feed (Cf). The effective solute rejection coefficient R (%) was determined by32

= 1 − × 100%

(2)

The dependence of the Stokes radius rs (nm) on the molecular weight Mw (g mol-1) is empirically given by32 for PEG: = 16.73 × 10 " × #$%.&&'

(3)

for PEO: = 10.44 × 10 " × #$%.&)'

(4)

The solute rejection R is plotted against its diameter dp in a log-normal probability diagram. Assuming negligible effects from the hydrodynamic and steric interaction between PEG/PEO and membrane pores, we can approximate the geometric standard deviation σp and the mean effective pore size µp to be σs (the ratio of rs at R = 84.13% to that at R = 50%) and µs (the mean solute radius at R = 50%). The membrane pore size distribution can then be calculated from the probability density function based on the value of µp and σp:32 *+(* ) *(* )

=*

,

-./ √12

345 6−

(-.* -.7 ) 1(-./ )

8

(5)

2.6.3 Characterization of membrane separation properties. The separation properties of TFN and TFC membranes were characterized in a dead-end permeation cell in the RO mode.


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The effective membrane area for the permeation cell was 9.5 cm2. The water permeability coefficient A (L m-2 h-1 bar-1) was then obtained by measuring pure water flux across the membrane over low transmembrane pressure of 2 bars to avoid membrane damage. The value of A was then determined by following equation: 9=

:

∆
=(

,%%% +?

− 1) × @

(8)

2.6.4 FO performance measurement. The lab-scale FO filtration unit32,61 was used to evaluate the FO performances of the synthesized membranes. The plate and frame system consists of rectangular channels (1.5 cm in height, 1.0 cm in width, and 2.0 cm in length) separated by the membrane without use of a spacer. The feed and draw solutions flowed along the channels in counter-current configuration at flow rate of 0.1 L min-1. NaCl with various concentrations (0.5 M, 1 M, 1.5 M, and 2 M) and deionized were used as draw and feed solutions, respectively. Two modes were tested for TFC and TFN membranes: (1) pressure retarded osmosis (PRO) mode, where the active layer of membrane faces the draw solution (AL-


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DS orientation); (2) forward osmosis (FO) mode with the opposite facing compared to PRO (ALFS orientation). In addition, the water influx into the draw solution leads to negligible dilution as total amount of water transported across the membrane during the experiment is less than 1% of the total volume of draw solution. The FO water flux @A (L m-2 h-1, abbreviated as LMH) was then determined by weight difference of feed solution over a time interval: @A =

∆BBC ⁄ BBC

(9)

∆D×EF

where GHII* is the density of feed solution (1.00 g cm-3 for DI water); ∆JHII* is the weight difference of feed solution; ∆K is the time interval; 9 is the area of permeation cell. The solute reverse flux @ (g m-2 h-1, or gMH) was evaluated by calculating the change in salt concentration of the feed over a time interval: @ =

L M LM

(10)

∆D×EF

where NH and NO are the final and initial salt concentration of the feed water solution, respectively; PH and PO are the corresponding feed volumes. For every nominal composition, experiment was carried out for at least five membrane samples, which were randomly selected from independently fabricated batches. The results were then averaged and reported. 2.6.5 Estimation of membrane structure parameter S. Structure parameter Q=

-R

(11)

S

with l, T and being the thickness, tortuosity and porosity of membrane substrate is a key parameter of FO membranes to understand the membrane boundary layer phenomena:62-64


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Generally a large S value indicates considerable internal concentration polarization, which leads to a reduced net osmotic pressure driving force. S can be calculated using experimental data along with a formula first proposed by Loeb et al. as follows:62 U

Concentrative ICP (PRO mode): @$ = V ln U

E2Y,F : [ E2\,] [

E2Y,] [

Dilutive ICP (FO mode): @$ = V ln E2

\,F : [

(12)

(13)

where ^ (10-9 m2 s-1) is the solute diffusion coefficient; _`, and _U, refer to the osmotic pressures at surfaces of the membrane in the feed and draw solutions; _`,a and _U,a are the osmotic pressures of bulk feed solution and draw solution, respectively. 2.6.6 Other characterizations. Attenuated total reflectance Fourier Transform Infrared Spectroscopy (TFS-3500 FTIR Bio-Rad in ATR mode) measurements were performed in the range of wavenumber 1000-1800 cm-1 to validate the successful incorporation of UiO-66 nanoparticles in polyamide layer of TFN membranes. XRD patterns of TFN with various loadings and TFC membranes were obtained with a Bruker D8 Advance X-ray Power Diffractometer with a Cu K excitation radiation at wavelength λ = 0.154 nm.

3. RESULTS AND DISCUSSION 3.1 UiO-66 nanoparticles characterization. As Figure 1(a) shows, the featured two peaks at 7.4° and 8.5° in the XRD patterns of as-synthesized MOF particles are attributed to (111) and (200) crystal planes, respectively, which are consistent with UiO-66’s simulated standard patterns.52 The MOF structure of the UiO-66 nanoparticles is also validated by FTIR


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spectroscopy. As seen from Figure 1(b), the characteristic peaks at 744, 671, 552, 482 cm-1 are ascribed to a mixture of C-H vibrational modes in aromatic compounds and Zr-O modes.52 In addition, the carboxylate group of ligand BDC has two strongly coupled C-O bonds, which give rise to two sharp peaks: a symmetric C-O stretching band at 1396 cm-1 and an asymmetric C-O stretching band at 1585 cm-1.52 The peak at 1662 cm-1 occurs due to the presence of DMF in the sample.52

Figure 1. Characteristics of the as-synthesized MOF UiO-66 nanoparticles: (a) XRD patterns from 5° to 40°, and (b) FTIR spectra in a range of 400 to 4000 cm-1.

The SEM images of UiO-66 nanoparticles are presented in Figure 2. By measuring the diameter of each nanoparticle (Figure 2(a)), the averaged particle size is around 512 nm with a standard deviation of 100 nm. This is consistent with value reported in literature.52,59 In addition, all synthesized UiO-66 nanoparticles show octahedral crystal structure, where the edges of the octahedron are connected by the carboxylate groups of BDC ligands. The Zr-clusters embedded at the center of crystal have a coordination number of 12, which has been the largest among all


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MOFs reported thus far.52 The high degree of connectivity is considered to be the reason of its extraordinary chemical resistance and stability. In addition, the aperture size of UiO-66 nanoparticles is estimated to be ~6.0 Å,52 which is larger than H2O (~2.8 Å),57 but smaller than other hydrated ions, e.g. Na+ (7.16 Å) and Cl- (6.64 Å).57 Therefore, the UiO-66 nanoparticles are expected to exhibit a high selectivity of water over salts.

Figure 2. FESEM morphologies of the as-synthesized MOF UiO-66 nanoparticles: (a) morphology with X10,000 magnification, (b) a zoom-in observation of UiO-66 morphology with X50,000 magnification.

3.2 Characterization and performances of PSU substrates, TFN and TFC membranes. 3.2.1 Characteristics of PSU membrane substrate. Figure 3 illustrates the cross-section, top, and bottom micrographs of PSU substrate membrane. From SEM images, the thickness of PSU substrate is measured to be ~62 µm. The PSU substrate possesses a finger-like cross-section structure formed with high degree of sub-micrometer pores. This morphology is favored by FO membranes as it tends to reduce the structure parameter (S).32,60,62 Gravimetric tests confirmed that the PSU membrane substrate has a high porosity of 78.5±1.5% mainly because of the addition of LiCl, a well-known pore former. Based on FO water flux measurements, the structure


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parameter (S) of the PSU substrate is estimated to be 532 µm from Eqn (12) and (13). The small S value is attributed to the low tortuosity, high degree of porosity, and thin structure of the PSU substrate. As displayed in Figure 3(b) and (c), a thin sponge-like layer can be observed above the finger-like structure. This layer functions as a cushion to provide sufficient mechanical strength for the top polyamide selective skin. The bottom of the membrane substrate shows well-defined pores with size smaller than 1 micrometer (Figure 3(d)). The porous structure is likely produced by the intrusion of non-solvent during phase inversion.

Figure 3. FESEM micrographs of PSU substrate membrane: (a) cross-section, (b) zoom-in crosssection, (c) top surface, and (d) bottom surface.


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The results of pore size and pure water permeability (PWP) for the PSU substrate are shown in Table 2 and Figure 4. The high values of the molecular weight cut-off (MWCO) (238 kDa) and PWP (267±12 LMH bar-1) for the as-casted membrane are mainly because of its high surface porosity and relatively large mean effective pore size µp (10.1 nm). In addition, the narrowed pore-size distribution with small geometric standard deviation σp (1.36) indicates that the PSU substrate possesses a large portion of submicron pores. Table 2. Properties of the PSU membrane substrate. Membrane ID µp (nm) σp PSU

10.1

1.36

PWP (LMH bar-1)

MWCO (kDa)

267±12

238

Figure 4. The pore size distribution of PSU substrate.

3.2.2 Membrane morphologies with various UiO-66 nanoparticle concentrations. Figure 5 have illustrates the surface morphologies of TFN with different UiO-66 particle loadings and TFC membranes. The cross-section microstructures resemble the PSU substrate except that on


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top of the membranes an additional polyamide (PA) thin layer is developed through interfacial polymerization. Detailed observations of the TFN membrane cross-sections confirm the successful incorporation of UiO-66 nanoparticles in the PA layers. Compared to the surface morphology of the as-synthesized UiO-66 nanoparticles illustrated in Figure 2(b), most of UiO66 nanoparticles in the TFN membranes are covered by PA layers. The top surface morphologies show that although TFN and TFC membranes display similar ridge-and-valley structures due to reactions between TMC and MPD monomers, they are slightly different in terms of the size of the ridge structure. The difference is even more pronounced as the particle loading increases, indicating that UiO-66 nanoparticles may affect interfacial polymerization process. In addition, when the concentration of UiO-66 is high (0.2 wt%), more nanoparticles are exposed at the top surfaces of membranes surrounded by bare polyamide layers without apparent particle aggregations as labeled in red. Since the average particle size of UiO-66 is larger than the thickness of a typical PA layer (~200 nm), partial embedment of UiO-66 in the PA selective layer can take place with the particle bottom part tucked into the PA matrix and the remainder exposed.


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Figure 5. FESEM micrographs of cross-section and top surface for FC and TFN membranes with different UiO-66 loadings: TFC (0 wt%); TFN-U1 (0.05 wt%); TFN-U2 (0.1 wt%); TFNU3 (0.15 wt%); TFN-U4 (0.2 wt%).

Table 3 presents the average thickness and contact angle of the rejection layer for TFN and TFC membranes. The thickness is found to vary with the loading of UiO-66 nanoparticles and follows the order: TFN-U4 (0.2 wt.% loading) > TFN-U3 (0.15 wt.% loading) > TFN-U2 (0.1 wt.% loading) > TFN-U1 (0.05 wt.% loading) > TFC (0 wt.% loading). The contact angle results show that the layer hydrophilicity increases by raising UiO-66 nanoparticle content in the range of 0-0.2 wt%; it drops from 47.8° for pure TFC membrane to 29.6° for TFN-U2 (0.1 wt%), and further decreases to 23.9° when the particle loading goes up to 0.2 wt%. The improved hydrophilicity of TFN membranes may be attributed to the exposed UiO-66 nanoparticles on the top surface of the membranes. The super-hydrophilic nature of UiO-66 is also one of the factors


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resulting in the increased layer thickness with increasing particle loadings. Since TMC monomers are less soluble in water, the interfacial polymerization mainly takes place in the organic phase. In order to react with TMC, diffusion of MPD monomers from the aqueous phase to the organic phase is required. For TFN membranes, such diffusion is facilitated due to the presence of UiO-66 nanoparticles in the organic phase. Consequently, a thicker rejection layer is formed as the UiO-66 concentration is increased. A hydrophilic thin-film layer generally enhances water permeation flux, fouling resistance, and salt rejection in membrane filtration.2,4 Table 3. Summary of the averaged polyamide (PA) layer thickness and contact angle for TFN with various UiO-66 concentrations and TFC membranes. a

The averaged values of PA layer thickness were calculated based on 10 random measurements. Membrane ID

PA layer thickness (nm)a

Contact angle (degree)b

TFC 230±63 47.8±3.6 TFN-U1 450±67 36.7±6.1 TFN-U2 603±188 29.6±5.1 TFN-U3 760±267 24.1±4.0 TFN-U4 1058±192 23.9±5.9 b The averaged values of surface contact angle were calculated based on 10 random measurements.

3.2.3 ATR-FTIR and XRD spectra of membranes. The ATR-FTIR measurements for PSU support, TFN-U2, and TFC membranes are shown in Figure 6(a). The characteristic bands of polyamide at ~1500 and ~1580 cm-1, which correspond to the amide N-H bending and aromatic amide C=O stretching respectively, were observed in both TFN-U2 and TFC membranes, indicating the successful formation of a thin rejection layer by interfacial polymerization.32 An intensified sharp peak at 1397 cm-1 was observed for TFN-U2 membrane, which is assigned to the symmetric stretching of C-O bond in UiO-66 nanoparticles.52


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Discrepancies in the spectra of TFN membranes with different loading over the range of 1500 cm-1 to 1700 cm-1 (Figure 6(b)), together with the differences in membrane top surface morphology (Figure 5), suggest that the incorporation of UiO-66 may affect interfacial polymerization and hence slightly changes the microstructure of PA layer.

Figure 6. ATR-FTIR of (a) the PSU support, TFN-U2, and TFC membranes, and (b) TFN membranes with various UiO-66 concentrations (0.05 – 0.2 wt%).

The XRD measurements were conducted for TFN membranes with different UiO-66 loadings in order to validate the crystal structure of UiO-66 nanoparticles in the PA layer of the membranes. As shown in Figure 7, the presence of featured peaks at 7.4° and 8.5° confirms the successful incorporation of UiO-66 without losing the crystallinity.52 The broad peak from 10° to 24° is attributed to the amorphous PA layer. With increased particle loading, the intensity of characteristic bands of UiO-66 magnified while the broad peak weakens. In summary, both ATR-FTIR and XRD data have confirmed the successful incorporation of UiO-66 in the PA rejection layer of the TFN membranes.


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Figure 7. XRD patterns of TFN membranes with different UiO-66 loadings.

3.3 Effects of UiO-66 loading on the membrane separation properties. Table 4 presents the water permeability coefficient A, salt rejection Rs, and salt permeability coefficient B of TFC and TFN membranes with 0-0.2 wt% UiO-66 nanoparticle loadings. A non-monotonic behavior of A is observed when the UiO-66 concentrations increased. The benchmark TFC membrane shows a water permeability of 2.19b0.30 LMH/bar. After incorporation of UiO-66 in the PA rejection layer, the water permeability coefficient of TFN membrane increases significantly, and reaches a maximum of 3.33b0.48 LMH/bar for TFN-U2 membrane (0.1 wt% loading). This is attributable to the improved hydrophilicity of TFN membranes. However, further increase in the content of UiO-66 leads to a decrease of water permeability coefficient, for instance, 2.63b0.17 LMH/bar for TFN-U4 (0.2 wt% loading), probably because of the development of a thicker PA rejection layer, resulting in a longer residence time for water passage.


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Compared to water permeability coefficient, the salt rejection Rs shows an opposite trend. It decreases very slightly from 95.5% for TFC membrane to 95.3% for the TFN-U2 membrane. Further addition of UiO-66 nanoparticles improves salt rejection marginally. A possible explanation for the persistently high rejection is the size exclusion of hydrated ions like Na+ and Cl- for the transport through the sub-nanometer pores of the UiO-66 nanoparticles.52,57 Moreover, the B/A ratio is a critical parameter to evaluate the membrane selectivity; a smaller B/A is indicative of a greater selectivity and hence preferred. Table 4 reveals that the TFN and TFC membranes have similar B/A ratios, which are 740% smaller than the commercial CTA-HW membranes reported elsewhere,10,60 implying that the UiO-66/Polyamide TFN membranes may achieve good performance in desalination. Overall, incorporation of UiO-66 allows a significant increase in membrane water permeability with minimal compromise in salt rejection. Table 4. Summary of the intrinsic transport properties of TFC and TFN membranes. A Membrane Water permeability -1 (LMH bar ) ID TFC 2.19±0.30

a

b

Salt rejection, Rs (%) 95.5±1.6

Salt permeability B (LMH) 0.20±0.08

B/A (kPa) 9.4±3.6

TFN-U1

2.62±0.30

95.4±1.0

0.26±0.07

9.7±2.2

TFN-U2

3.33±0.48

95.3±0.2

0.33±0.05

10±0.5

TFN-U3

2.88±0.15

95.3±0.8

0.29±0.06

9.9±1.8

TFN-U4 2.63±0.17 95.4±1.4 0.26±0.10 a Evaluated using a RO setup by applying 2 bar against deionized water. b Evaluated using a RO setup by applying 2 bar with 1000 ppm NaCl as feed.

9.7±3.1

3.4 FO performance. Figure 8 illustrates the FO performance of membranes with different UiO-66 loadings under PRO and FO modes using DI water and 1 M NaCl as the feed and draw solutions, respectively. In the PRO mode the unmodified TFC membrane displays a water flux of 26.2 LMH. As the content of UiO-66 increases, the water flux is enhanced and reaches the


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highest value of 36.7 LMH for the TFN-U2 membrane (0.1 wt%). Further increase in UiO-66 loading causes a reduction in water flux down to 31.7 LMH for the TFN-U4 membrane (0.2 wt%). This trend is consistent with that derived from the RO experiments in the prior subsection. A similar tendency is observed in the FO mode with a maximum water flux of 20.7 LMH obtained by the TFN-U2 membrane (0.1 wt%). In addition, all the TFN membranes exhibit great improvement in water flux compared to the unmodified TFC membrane, where a flux increase of ~40% in the PRO mode and ~25% in the FO mode is achieved by TFN-U2 membrane (0.1 wt%). Figure 8(a) and (b) show the solute reverse flux under PRO and FO modes for TFC and TFN membranes, respectively. Solute reverse flux increases to a maximum at 0.1 wt% UiO-66 loading, and then decreases with further increase in the UiO-66 content. The trend for solute reverse flux is congruent with the behavior for salt rejection, i.e., a higher salt rejection shows a lower solute reverse flux. Interestingly, although the increments of solute reverse flux of TFN membranes are moderate when compared with the benchmark in current study, the solute reverse flux of TFN membranes are lower than TFN and TFC membranes reported in the literature (see Table 6), implying that an almost defect-free polyamide layer is produced after incorporating UiO-66 nanoparticles.


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Figure 8. The FO performance of TFC and TFN membranes: (a) under the PRO mode, and (b) under the FO mode. Feed: DI water, draw solution: 1M NaCl.

The solute reverse flux/water flux (Js/Jw) ratio of the resultant TFC and TFN membranes are summarized in Table 5. A small Js/Jw usually indicates a higher FO selectivity. In general, the error bar of Js/Jw is comparatively large owing to sample variation. However, a trend of selectivity for both PRO and FO modes can still be observed. The selectivity deteriorates slightly when UiO-66 is incorporated, with the lowest value obtained for TFN-U2. The small reduction of selectivity for TFN-U2 may be due to the non-selective voids formed at the interface of the PA layer, where solute can permeate with lower resistance. Nevertheless, Js/Jw of TFN-U2 membrane is still much smaller than most TFN membranes incorporated with other fillers, such as zeolite,16 TiO2,65 functionalized carbon nanotubes (CNT),66 etc.

Table 5. Js/Jw ratio for different membranes under PRO and FO mode. Feed: DI water; draw solution: 1M NaCl. Membrane ID Js/Jw (g/L) PRO mode 0.16±0.04 0.15±0.03 0.20±0.04 0.18±0.05 0.18±0.09

TFC TFN-U1 TFN-U2 TFN-U3 TFN-U4

FO mode 0.16±0.03 0.14±0.02 0.21±0.06 0.20±0.04 0.15±0.04

In summary, the high water flux, low solute reverse flux and small Js/Jw attained using TFN membranes suggests that the incorporation of UiO-66 nanoparticles is a useful approach to optimize the membrane FO performance.


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The FO performances of benchmark and the best performing membrane (TFN-U2) are further investigated under PRO and FO modes by varying NaCl concentration (0.5-2.0 M) in the draw solution. Figure 9(a) has shown that under PRO mode the water fluxes of TFN-U2 membrane increase with NaCl concentration up to 2 M, which is the highest concentration commonly investigated in the literature. Moreover, the average water flux increment per 0.5 M increase in the concentration of draw solution is approximately 7.3 LMH. At 2 M draw solution concentration, water flux of the TFN-U2 membrane is 40% higher than benchmark, indicating a slightly reduced ICP effect by incorporation of UiO-66. It is hypothesized that the small pore size of UiO-66 efficiently limits the solute reverse flux even at high draw solution concentration, thereby mitigating the internal concentration polarization and afford higher net osmotic driving force. As shown in Figure 9(b), the water fluxes of TFN-U2 under FO mode reveal similar trends. The mean water flux increment per 0.5 M increase in draw solution is around 2.8 LMH due to more severe ICP phenomenon.

Figure 9. Water flux of the TFC and TFN-U2 membrane against different draw solution concentrations (0.5-2 M): (a) under PRO mode, and (b) under FO mode. Feed solution is DI water.


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To provide useful comparison, we show in Table 6 the FO performances of the TFN-U2 membrane, the TFC-control in current study, together with other membranes investigated in the literature. Compared to TFC (control) membrane, TFN-U2 membrane has exhibited superior water flux while maintaining relatively high selectivity. Although there is a selectivity decline with increased DS concentration, the observed Js/Jw ratio out-performs TFN membranes incorporated with other filler materials at similar osmotic pressure differences. The current study demonstrates that UiO-66 presents significant advantages over conventional fillers in the polyamide layer in controlling the solute reverse diffusion.

Table 6. Comparison of FO performance for TFN-U2 with various TFC and TFN membranes in the literature. Polyamide Filler Materiala 0.1 wt% UiO-66 0.1 wt% UiO-66 TFC (control) TFC (control) 0.05 wt% CNT 0.1 wt% NaY 0.05 wt% silanized TiO2 TFC HTI TFCb a b

FO performance Jw Js (LMH) (gMH) 36.7 7.1 20.7 4.3 51.3 12.3 27.0 6.1 26.2 4.4 16.5 2.6 36.7 8.9 22.8 5.7 33.6 9.0 24.0 6.0 38.7 15 17.5 7.5 37.0 10.3 20.0 8.8 38.0 6.0 23.0 4.0 33.3 18.3

Js/Jw (g/L) 0.19 0.21 0.24 0.23 0.16 0.16 0.24 0.25 0.27 0.25 0.39 0.43 0.28 0.44 0.16 0.17 0.55

Testing conditions membrane Feed Draw orientation solution solution PRO Deionized 1 M NaCl FO Deionized 1 M NaCl PRO Deionized 2 M NaCl FO Deionized 2 M NaCl PRO Deionized 1 M NaCl FO Deionized 1 M NaCl PRO Deionized 2 M NaCl FO Deionized 2 M NaCl PRO 10 mM NaCl 2 M NaCl FO 10 mM NaCl 2 M NaCl PRO Deionized 1 M NaCl FO Deionized 1 M NaCl PRO 10 mM NaCl 2 M NaCl FO 10 mM NaCl 2 M NaCl PRO Deionized 2 M NaCl FO Deionized 2 M NaCl PRO Deionized 1 M NaCl

References

Present work Present work Present work Present work Present work Present work Present work Present work (65) (65) (16) (16) (66) (66) (32) (32) (67)

All membranes except HTI TFC are fabricated on polysulfone substrate. HTI TFC membrane is fabricated by Hydration Technology Inc (HTI) on proprietary materials.


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4. CONCLUSIONS In the current study, thin-film nanocomposite (TFN) membranes with superior permeability, rejection and FO performance have been successfully fabricated and characterized. Incorporating super-hydrophilic Zr-MOF UiO-66 MOF nanoparticles affects the hydrophilicity, surface morphology, and chemistry of the top PA selective layer, resulting in a significantly improved water permeability and negligible reduction in rejection. With 0.1 wt% UiO-66 loading, the TFN-U2 membrane exhibits the highest water permeability, which is approximately 50% higher than unmodified TFC membrane with a comparable salt rejection. It also shows the highest water flux of 51.3 LMH in the PRO mode and 27 LMH in the FO mode, when DI water was used as the feed and 2 M NaCl as the draw solution. More importantly, the TFN-U2 membrane shows a relatively linear increase in water flux and a limited solute reverse diffusion with increasing draw solution concentration up to 2 M NaCl, suggesting that the internal concentration polarization effect is slightly reduced. The present study has provided a promising avenue for fabrication of high-performance membranes with incorporation of Zr-MOFs, and will stimulate further development in near future.

AUTHOR INFORMATION Corresponding Author *Tel: (65) 65164349; Fax: (65) 67791936; Email: [email protected] (G.H.)., *Tel: (65) 65165237; Fax: (65) 67791936; Email: [email protected] (S.-B.C.)


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the financial support from the Singapore National Research Foundation under its Environment & Water Research Programme administered by PUB, Singapore’s national water agency, for the project titled “Membrane development for osmotic power generation, part 1. Materials development and membrane fabrication” (1102-IRIS-11-01) and the NUS grant no. R-279-000-381-279.

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TOC. For Table of Contents Only.


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Thin-Film Nanocomposite (TFN) Membranes Incorporated with Super

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