PES mixed matrix membrane; synthesis, characterization and antifouling evaluation

Accepted Manuscript Title: A novel Ce-MOF/PES mixed matrix membrane; synthesis, characterization and antifouling evaluation Authors: Farrokh Mohammadnezhad, Mostafa Feyzi, Sirus Zinadini PII: DOI: Reference:

S1226-086X(18)30751-2 https://doi.org/10.1016/j.jiec.2018.09.032 JIEC 4178

To appear in: Received date: Revised date: Accepted date:

5 June 2018 18 September 2018 18 September 2018

Please cite this article as: Farrokh Mohammadnezhad, Mostafa Feyzi, Sirus Zinadini, A novel Ce-MOF/PES mixed matrix membrane; synthesis, characterization and antifouling evaluation, Journal of Industrial and Engineering Chemistry https://doi.org/10.1016/j.jiec.2018.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A novel Ce-MOF/PES Mixed Matrix Membrane; Synthesis, Characterization and Antifouling Evaluation Farrokh Mohammadnezhad2, Mostafa Feyzi1,2*, Sirus Zinadini3 1

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Department of Physical Chemistry, Faculty of Chemistry, Razi University, P. O. Box: 6714967346, Kermanshah, Iran. Tel/fax: +98 833 4274559 2 Department of Nano Chemistry, Faculty of Chemistry, Razi University, P. O. Box: 6714967346, Kermanshah, Iran 3 Environmental Research Center (ERC), Department of Applied Chemistry, Razi University, P.O Box: 67149, Kermanshah, Iran

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*Corresponding Author Emails: [email protected], [email protected]

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Graphical abstract

Highlights

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Nanocrystals of Ce(III) metal-organic framework, [Ce(tp)(NMP)2(CH3COO)]n,1, were prepared via sonochemical irradiation. Novel hydrophilic PES ultrafiltration membranes were produced by the embedding of MOF NPs using the phase inversion method.

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Modified PES membranes showed an increase in the pure water flux relative to the bare membrane because of enhancement of the membrane hydrophilicity.

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The surface hydrophilicity of the modified membranes was improved due to the tendency of water to membrane surface.

Abstract

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A new polyethersulfone (PES) nanofiltration membrane, modified with nanocrystalline Ce(III) metal-organic framework (MOF), was produced via the phase inversion method and characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), water contact angle and porosity measurements. The morphology and performance of these membranes were investigated in terms of pure water flux, water contact angle, fouling parameters and dye removal. Modified PES membranes showed an increase in the pure water flux relative to the bare membrane. The changes in sublayer and skin layer of modified membranes and also increased pore size and porosity is obvious from the SEM images of PES membranes porosity measurements. Moreover, the surface hydrophilicity of the MOF embedded membranes was improved due to the tendency of water to the membrane surface. The antifouling properties of the membranes were evaluated by powder milk solution and measuring the flux recovery ratio (FRR). The results revealed the modified membrane with 0.5 wt. % of MOF nanoparticle (NPs) had the best antifouling property and also the highest porosity and water flux. Nanofiltration performance of membranes was appraised by probing of the retention of Direct Red 16. The result showed that all the modified membranes have a higher dye rejection capacity than the bare PES membrane. Keywords:

1. Introduction

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Nanofiltration; Mixed matrix membranes; Metal-organic frameworks; Nanoparticles; Sonochemical irradiation; Wastewater treatment; Antifouling

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Clean water and its scarcity are one of the essential human concerns in the current century and demand for potable drinking water is growing rapidly. This problem is a very important global concern and many countries have become involved especially those facing water crises [1]. Wastewater treatment via membrane separations such as driving force (trans-membrane pressure (TMP)), microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) all are confirmed procedures for providing drinkable water. The advantages of these methods can be pointed to environmentally friendly properties, low energy utilization, easy operation in a continuous flow and a technology of significant, and also their use for biochemical, food, textile and other industries [2]. One of the major challenges in membrane filtration processes is membrane fouling by foulants in the feed stream which causes an effective decrease in the permeation flux and consequently results in the need for more energy, membrane cleaning, high maintenance costs and membrane replacement [3, 4].

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Polyethersulfone (PES) membranes have been widely used in ultrafiltration processes due to their distinguished properties such as thermal stability, compressive strength and chemical inertness across all pH range. Despite these advantages, their use as membrane has been restricted because of their inherent hydrophobic nature. PES membranes have low wettability and higher resistance to water permeate flow due to the lack of hydrogen bonding interactions in the boundary layer between the membrane jointing and water [5].

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One of the most important challenges in the membrane separation process is membrane fouling. This process strongly affects the performance of membrane filtration in terms of fluid separation and usage lifetime. Several membrane modification methods have been investigated to improve the performance of the membrane by combining hydrophilic and hydrophobic properties. The main cause of developing antifouling membranes is to diminish the interactions between foulants and membrane surface by increasing the surface hydrophilicity. Membrane hydrophilicity and surface roughness of the membrane are two parameters that can affect membrane antifouling performance. The more smooth and hydrophilic surface improves membrane antifouling performance. The increase of the NPs in an optimum manner reduces the roughness of the surface, and also the functional groups of these compounds increase the hydrophilicity [6, 7]. Various methods such as hydrophilic polymer blending [8, 9], grafting with hydrophilic monomers [10, 11], grafting with short-chain molecules [12], coating [13, 14], embedding hydrophilic NPs [15, 16] were proposed to mitigate membrane fouling. Clay, Carbon nanotube, goethite, and graphene oxide, Ag, TiO2, ZnFe2 O4 /SiO2 , Chitosan/Fe3 O4 and MOFs are nanomaterials that were found to improve the anti-fouling performance of membranes. MOFs as nanoporous materials have received great attention since the introduction of MOF-5 [17]. MOFs are hybrid inorganic-organic solid compounds and have remarkable characteristics such as high surface area, tunable pore size, the availability of inner surface modification and potential applications in gas separation, energy transformation, drug delivery, sensing, and heterogeneous catalysis. These solids have regular and highly harmonic pore structures and they play a very vital role in increasing the hydrophilic property of the membrane. Framework structure of MOFs can be adapted according to the guest molecules and high selectivity for adsorption can be obtained. Thus, the blending of MOF crystals with a polymer to provide a mixed-matrix membrane (MMM) for gas and liquid separation has been widely investigated [18-21]. In the recent studies, Andrew Livingston et al (2016) reported that Hybrid Polymer/MOF membranes prepared by in-situ growth of HKUST-1 MOF within the pores of polyimide membranes improve the selectivity of membranes and allow the separation of closely related solutes [22]. Lianjun Wang et al. (2016) introduced a novel single-step approach, named phase transformation interfacial growth (PTIG), for the fabrication of a MOF membrane on a polymeric substrate [23]. Seth M. Cohen et al (2015) demonstrated a versatile approach to preparing stable functional MMMs using different MOFs in order to produce free-standing MMMs that are mechanically stable and pliable [24]. S. Zinadini et al. (2017) have shown that PES membranes which modified with ZnFe2O4/SiO2 nanofiller have a slightly higher flux and rejection rates than PES membranes and also are more hydrophilic [4]. 3

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The porosity of MOFs is important in their applications. In addition, the remarkable features of this type of porous material provide the opportunity to regulate their behavior. Fine-tuning behavior in Lanthanides MOFs is accomplished by trapping molecules in the framework pores to influence the lanthanides ions emission and thereby prepares a proper media for sensing by luminescence variations. Another advantage to using Ln-MOFs as a modifier in the membrane is their high surface area, permanent microporosity, and tunable pore sizes which lead to a better performance of membranes. Also, unsaturated metal centers in LaMOF Lanthanides ions can create coordinately unsaturated metal centers, makes their promising candidates for heterogeneous catalysis application [25, 26]. According to the mentioned considerations and since no report has yet been published on the modification of PES membranes with La-MOFs, Ce-MOF NPs with the formula [Ce(tp)(NMP)2 (CH3 COO)]n (1), as a La-MOF, was used to produce a novel modified PES membrane via wet phase inversion method. Nanocrystals of Ce-MOFs were synthesized through ultrasonic irradiation method. The prepared membrane was characterized using water contact angle, AFM, SEM, and overall porosity measurements. Then the hydrophilicity, permeation flux, morphology and antifouling performance of provided membrane were investigated.

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2. Materials and methods 2.1. Materials

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All chemicals for the synthesis and analysis used in this study were of analytical grade and used as received. Polyethersulfone (PES) (Ultrason E 6020 p, MW = 58,000 gmol−1 and glass transition temperature Tg =225 ℃) and dimethylacetamide (DMAc) as the solvent were supplied from BASF Company. Polyvinylpyrrolidone (PVP) with a molecular weight of 25,000 gmol−1 , Ce(NO3 )3 . 6H2 O, 1, 4-benzene dicarboxylic acid (H2tp) and n − methyl − 2 − pyrrolidone (NMP) were purchased from Sigma-Aldrich Company. FT-IR spectra were provided using a Bruker Equinox 55 spectrometer, equipped with a single reflection diamond ATR system, in the 600–4000 cm−1 . The AFM observations were done by Nanosurf MobileS scanning probe-optical microscope (Switzerland). Scanning electron microscopy was done using Philips-XL30 (The Netherland) with an accelerating voltage of 20 kV. The powder Xray diffraction (PXRD) pattern was obtained using a Philips X’pert diffractometer with monochromatic Cu-Kα radiation (λ= 1.5418 Å). The ultrasonic processor UP100H (100W, 30 kHz) was used for the ultrasonic irradiation. The specific surface area and pore size distribution were performed by NOVA surface area analyzer. 2.2. Preparation of nanocrystalline [𝐂𝐞(𝐭𝐩)(𝐍𝐌𝐏)𝟐 (𝐂𝐇𝟑 𝐂𝐎𝐎)]𝐧 To prepare nanocrystalline [Ce(tp)(NMP)2 (CH3 COO)]n by the sonochemical method, 1, 4BDC (2mmol) was solved in 5 ml NMP and drop wisely added into to a 5 ml solution of Ce(NO3 )3 . 6H2 O (1mmol) in NMP under the ultrasonic irradiation. The mixture was irradiated by ultrasound with a power of 60 W for 30 min at room temperature. Then, the

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colorless powders were collected using filtration, washed with methanol, and dried in air. (Yield 78.1%) NPs crystalline size diameter (𝐷) has been calculated by Debye–Scherrer equation (1):

(1) 𝐷 =

0.9𝜆 𝛽 cos 𝜃

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In this equation, 𝜃 is the position of maximum diffraction peak 𝜆 is the X-ray wavelength (1.5406 Å for Cu Kα) and 𝛽-FWHM (full-width at half-maximum) is in radians.

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2.3. Preparation of mixed matrix MOF/PES

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Nascent asymmetric PES and MOF modified PES membranes were produced using immersion precipitation phase inversion method. For this, casting solutions including PES 18 wt. %, PVP 1 wt. % and MOF (1) 0.1, 0.5 and 1 wt. % NPs in DMAc as the solvent were prepared. Table 1 shows the compositions of casting solutions for all membranes. At first, a specified amount of the Ce-MOF was mixed into DMAc and dispersed completely using sonication for 30 minutes to ameliorate homogenous solutions. After that, PES and PVP were dissolved in the primary solution and then was stirred for 24 hours uninterruptedly. In the end, the prepared homogeneous polymer solution was sonicated for another 20 minutes to remove air bubbles and efficient scattering of the Ce-MOF NPs in the polymer matrix. Afterward, the solutions were cast using a self-made casting knife with 200 µm thicknesses on a glass plate and instantly moved to an antisolvent bath (distilled water) for immersion precipitation at ambient temperature without any evaporation. After membrane formation and primarily phase separation, the membranes were placed in fresh distilled water for 24 hours to assurance the full phase separation. This step allows the water-soluble ingredients in the membrane to be washed out. As the last step, the membranes were sandwiched between two sheets of filter papers for 24 hours at room temperature and completely were dried.

Membrane Type M1 M2 M3 M4

PES (wt. %) 18.0 18.0 18.0 18.0

PVP (wt. %) 1.0 1.0 1.0 1.0

MOF (wt. %) 0.0 0.1 0.5 1.0

DMAc (wt. %) 81 80.9 80.5 80.0

Table 1 The composition of casting solutions for providing the bare membrane (M1) and modified ones with the MOF NPs (M1, M2, M3)

2.4. Membrane Characterization

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Surface and cross-sectional morphology and also roughness of the prepared membranes were investigated via SEM and AFM. For quantitative analyzing of the images, surface roughness parameters of the membranes such as mean roughness (Sa), the root mean square of the Z data (Sq) and the mean difference between the highest peaks and lowest valleys (Sz) were obtained by Nanosurf® MobileS software. Water contact angle measurement technique was used to quantify the surface hydrophilicity of the membranes. Accordingly, a digital microscope (G10, KRUSS, Germany) was used to record the status of injected water droplets on the membrane surface and to determine the hydrophilicity of the membranes prepared. All contact angle determinations were done using 2 µl of deionized water. The mean value of the contact angle on each polymer membrane was calculated using at least five random locations on each membrane to minimize the experimental errors.

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2.5. Porosity measurements

The total porosity (𝜀) of the membranes was calculated via gravimetric method, using equation 2: 𝜔1 − 𝜔2 𝐴 × 𝑙 × 𝑑𝑤

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(2) 𝜀 =

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In this equation 𝜔1 is the weight of the membrane 𝜔2 is the weight of the dry membrane, 𝐴 (m2 ) is the membrane effective area, 𝑑𝑤 (0.998 gcm−3) is the water density and 𝑙 (m) is the membrane thickness. Furthermore, Guerout–Elford–Ferry equation (3), based on the data of pure water flux and porosity, was employed in order to determine the membrane mean pore radius( r𝑚 ).

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(2.9 − 1.75𝜀) × 8ηlQ (3) r𝑚 = √ 𝜀 × 𝐴 × ∆𝑃

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Where η (8.9 × 10-4 Pa.s) is the water viscosity, Q (m3 s −1) is the volume of the permeated pure water per unit time, and ΔP (5 bars) is the operation pressure.

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2.6. Membrane performance 2.6.1. Pure water flux and antifouling experiments

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The permeate flux, separation and fouling tests of the prepared ultrafiltration membranes were accomplished in a dead-end cell with 150 ml volume and a membrane surface area of 12.56 cm2 . The cell was equipped with a barometer (Fig. 1). High-pressure nitrogen gas was applied to force the liquid flow through the membrane. To reduce the concentration polarization of the membrane, steady stimulation at a rate of 400 rpm was used. The transmembrane pressure was applied at 4.5 bars for 30 minutes to obtain a fixed permeate flow and following that, the pressure was decreased to the operating pressure of 3 bars. The pure water flux (PWF) was obtained using equation 4. 𝑀 (4) 𝐽𝑊,1 = 𝐴∆𝑡 6

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In this equation 𝐽𝑊,1 (kgm−2 h−1 ) is the pure water flux, 𝑀 (kg) is the weight of permeated water, 𝐴 (m2 ) is the membrane area and ∆𝑡 (h) is the permeation time.

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Fig. 1 Schematic of the setup for pure water flux tests using bare and modified membranes

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2.6.2 Analysis of membrane fouling

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After the PWF test, a milk powder solution (8000 mgL-1) as a suitable fouling agent [27] was swiftly replaced the stirred cell after PWF tests. The milk powder solution flux (𝐽𝑝 ) was calculated based on the water amount permeated through the membranes at 3 bars for 90 minutes. Then the fouled membranes were washed with distilled water for 15 minutes and consequently, the water flux of these washed membranes(𝐽𝑤,2 ) was measured afresh. The flux recovery ratio (𝐹𝑅𝑅) equation (5) is: 𝐽𝑊,2 (5) 𝐹𝑅𝑅 = ( ) × 100 𝐽𝑊,1 Higher FRR represents the better antifouling property of the ultrafiltration membranes. Moreover, in order to in detail analyzing of fouling process, total fouling ratio(𝑅𝑡 ), reversible fouling ratio(𝑅𝑟 ) and irreversible fouling ratio(𝑅𝑖𝑟 ) were calculated by equations 6-8. 𝐽𝑝 (6) 𝑅𝑡 (%) = (1 − ) × 100 𝐽𝑊,1 𝐽𝑤,2 − 𝐽𝑝 (7) 𝑅𝑟 (%) = ( ) × 100 𝐽𝑊,1 𝐽𝑤,1 − 𝐽𝑤,2 (8) 𝑅𝑖𝑟 (%) = ( ) × 100 = 𝑅𝑡 − 𝑅𝑡 𝐽𝑊,1

Results and Discussion 3.1. Characterization of NanoCrystalline MOF

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The single crystals of the Ce-MOF were previously synthesized and characterized by Abbasi et al. [28]. The solvothermal reaction between Ce(NO3 )3 . 6H2 O as the metal salt and H2 tp in NMP as the solvent, leads to the formation of colorless block crystals with the general formula of [Ce(tp)(NMP)2 (CH3 COO)]n . The title MOF is an air-stable compound and can maintain its own crystallinity at the environmental condition. The X-ray single crystal structure determination represents a neutral 2D Ce (III) coordination polymer based on the (Ce2 (COO)4 ) clusters as secondary building units (SBUs), which crystallizes in the monoclinic system, space group of 𝑃21 /𝑐. The resulting two-dimensional networks of CeMOF, which was obtained by the Mercury software and utilizing the CIF file of this MOF, contain one-dimensional channels decorated with NMP and acetic acid molecules, are shown in Fig. 2.

Fig. 2 The two-dimensional network of the bulk Ce-MOF contains one-dimensional channels decorated with NMP and acetic acid molecules (Hydrogens are omitted for clarity)

In this study, the Ce-MOF NPs were provided by using ultrasonic processor UP100H. Fig. 3 exhibits a comparison between PXRD patterns of the bulk MOF based on single crystal data and experimental pattern of the nanocrystalline MOF. It confirms that the phase purities of Bulk MOF agree well with those of the nanocrystalline MOF. The difference in peak intensity in these patterns can be attributed to the different orientation of the crystals in the 8

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powder samples. By using the Debye–Scherer equation, the sizes of crystalline particles have been found to be about 60 nm.

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Fig. 3 X-ray diffraction patterns comparison of bulk (A) and NPs (B) of the Ce-MOF

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Fig. 4 shows the IR spectra of single crystal and nanoscale Ce-MOF. Presence of the aromatic rings in the structure confirmed by the peaks in the 1400 cm-1 to 1600 cm−1 and 500–800 cm−1 regions. The asymmetric and symmetric stretching vibrations of carboxylate groups are detected at 1549 and 1387 cm−1 respectively. 𝛥(𝜈𝑎𝑠 − 𝜈𝑠𝑦𝑚 ) Indicate the being of a bridging mode of the carboxylate anions. Complete deprotonating of the carboxylic acid group of H2tp confirmed by the absence of characteristic bands at 1680 cm−1 to 1715 cm−1 . However, the carboxylate stretching frequencies are the brightest feature in the spectra which well affected by the binding modes of the carboxylate groups.

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Fig. 4 The FT-IR spectra for bulk (top) and NPs (down) of the Ce-MOF

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The size, volume, and shape of the pores of the MOF NPs are directly related to the performance of these compounds in a particular application. The size distribution and porosity studies of the NPs prepared by the sonochemical method were characterized by particle size and BET analyzes. The average particle size for the nanocrystalline Ce-MOF sample is around 80 nm. Fig. 5 shows the particle size distribution histogram obtained using a particle size analyzer instrument. This result is in agreement with other studies on the synthesis of NPs with ultrasound irradiation [29].

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Fig. 5 Size distribution histogram of the Ce-MOF NPs prepared by ultrasonic method shows the average particle size of 80 nm.

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Porosity studies using traditional methods such as isothermal sorption of gases confirm the Ce-MOF as an eternal porous material by the reversible flow of guests into and out of the void volume. As shown in Fig. 6 the activated sample, represented type I sorption isotherm at 77 K and 1 bar, suggesting the character of a microporous compound. The Brunauer-EmmettTeller (BET) and Langmuir surface areas of the Ce-MOF measured based on the nitrogen adsorption isotherm were 218.6 and 287.4 m2 g −1 , respectively. The Horvath Kawazoe model indicated a pore diameter of 3.7 Å and a maximum pore volume of 0.81 cm3 g −1for the Ce-MOF.

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Fig. 6 Sorption isotherm of N2 at 77 K and 1 bar for the Ce-MOF NPs, suggests the character of a microporous compound.

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3.2. Membrane Hydrophilicity and pure water flux

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Hydrophilicity is one of the most important features of membranes and it is because of the effect on the flux and antifouling ability of the membranes. To determine the Hydrophilicity of the membranes water contact angle tests were conducted using the Contact Anglemeter XCA-50. The volume of the drops of water was 4 µl and photos were recorded 10 seconds after drip. For each sample, the test was repeated 3 times and the average was reported. The images and static contact angles of the prepared nanocomposite membranes are shown in Fig. 7 and Fig. 8 respectively. The hydrophilicity of the mixed matrix membranes was progressed by adding of MOF NPs to the casting solutions. The mitigated contact angle enucleates the enhancement in hydrophilicity, which comes from the inherent high hydrophilicity of the NPs. MOF NPs showed the lowest water contact angle of 49.1. Unfilled PES membrane represented the highest water contact angle of 63.2. Addition of 0.1 0.5 and 1 wt. % the MOF NPs reduced the water contact angle to 57.2, 55.1 and 53.5 respectively. Accordingly, the contact angle of M4 (1 wt. % of the MOF NPs) membrane was low; pointing that the quantity of the MOF (1 wt. %) consumed has a significant impact on augmentation of the hydrophilicity. During membrane organization, migration of the hydrophilic MOF towards the top surface of the membrane as the top layer, bedights the functional groups of MOF on the membrane top surface and modify the membrane hydrophilicity. The augmented 12

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hydrophilicity is attributed to the entry of hydrophilic hydroxyl (-OH) and acid (-COOH) functional groups through the attachment of MOF to the PES membrane matrix.

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Fig. 7 Water contact angle images of the prepared bare and MOF/PES membranes. Unfilled PES membrane (M1) represented the highest water contact angle and the addition of 0.1 0.5 and 1 wt. % the MOF NPs reduced the water contact angle to 57.2, 55.1 and 53.5 respectively. 70

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Fig. 8 Water contact angle of the prepared bare, MOF/PES membranes, and MOF NPs.

The outcomes of the pure water flux of the nanocomposite membranes were shown in Fig. 9. Water permeation was increased up to 0.1 wt. % of MOF and decreased by the main increase of MOF concentration in the polymer matrix. Adding of the MOF NPs in the PES casting solution may have two significant effects on membrane formation: (I) an increment in the hydrophilicity of PES membranes and (II) a remodel in the pore size and structure of the membrane. The obtained results for contact angle (Fig. 8) illustrate that the hydrophilicity of 13

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all membranes collected from different weight percentages of the MOF NPs is higher than the hydrophilicity of the main PES membrane. So, the water flux for membrane M2 containing MOF NPs must be high compared to the membrane prepared unfilled MOF. Contrary to the increase in hydrophilicity in M3 and M4 membranes which contain 0.5 and 1 wt. % of MOF NPs, pure water flux has not increased for them. This flux mitigation can be attributed to pore blocking with a higher amount of MOF NPs, decreased porosity and dense skin-layer of this membrane as represented in Fig. 10. In addition, it should be insinuated that the rejection of powder milk proteins was more than 98% for all the modified membranes, stating that the augmentation in the flux was not dependent to defects or cracks in the membranes.

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Fig. 9 Pure water flux of the bare membrane (M1) and MOF/PES membranes (M2, M3, M4) at 3 bars after 60 minutes

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Fig. 10 The overall porosity of the bare (M1) and MOF/PES mixed matrix membranes (M1, M2, M3). Average and standard deviation of three replicates is reported.

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3.3 Morphology analysis

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Fig. 11 shows cross-sectional and surface morphology images of the bare PES membrane and the nanocomposite membranes. PVP as a pore maker, due to its hydrophilic nature, come out of the body of the matrix and creates cavities [30]. The changes induced in the skin-layer and sub-layer of the membranes is visible and appraisable. The membranes have a finger-like and porous bulk structure with an asymmetric morphology of a thin skin-layer. The PES membranes modified with the MOF NPs and synthesized by the phase inversion method exhibit a porous and finger-like structure through embedment between the solvent (DMAc) and non-solvent (water). The membrane with 0.1 wt. % of the MOF NPs (Fig. 11 M2) has the highest porosity and its finger-like structure was changed to spherical macro-voids. These desired changes can be justified by the impact of two factors: 1. the fast exchange of solvent and non-solvent in the phase inversion process because of the hydrophilic MOF NPs, 2. available interactions between ingredients in the casting solution. It is clear in the SEM images that the imported MOF NPs 0.1 wt. % was not only agglomerated or formed clusters but also scattered well in the polymer matrix (Fig. 11 M2). By increasing the amount of the MOF NPs in the casting solution, gradually agglomeration was appeared in the MOF/PES 0.5 and 1 wt. % membranes (Fig. 11 M3, M4). This may be related to the enhancement in viscosity of these solutions which leads to retardation of the exchange of solvent and nonsolvent and subsequently slows down the precipitation of the membranes. Chemical analysis of the bare and MOF blended membranes by EDAX technique shows the presence of cerium in the surface of the modified membrane sample which represents immigration of the MOF NPs to the upper surface of this membrane (Fig. 12). 15

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Fig. 11 Cross-sectional and surface morphology images of membranes with different mix compositions (M1) Unfilled PES, (M2) 0.1% MOF/PES, (M3) 0.5% MOF/PES and (M4) 1% MOF/PES.

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Fig. 12 Chemical composition (EDAX) of bare and modified membranes with the Ce-MOF NPs

3.4. Nanofiltration performance Investigation on Direct Red 16 retention in provided membranes was performed to evaluate the nanofiltration performance of membranes. The nanofiltration membrane dye rejection was tested under operating pressure of 4 bars, pH=6 and dye concentration of 30 mgL−1 and in a dead-end permeation cell. Fig 13 shows the retention results after 60 min filtration of the 17

dye solution. The rejection capability of the prepared Ce-MOF/PES membranes was higher than that of the unfilled PES membrane. Acidic functional groups of MOFs can induce surface negative charge throughout the entire pH range [31, 32] on the surface of the prepared membranes, causing high retention between the negative dye and negative surface. There is a decrease in retention of dye in 0.5 wt. % MOF/PES membranes which can be because of an increase in the membrane porosity (Fig. 10). 99

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Fig. 13 Dye retention performance of the prepared bare (M1) and MOF/PES ultrafiltration

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membrane (M1, M2, M3) at the pressure of 4 bars, pH=6, 30 mgL−1 Direct Red 16, after 60 minutes filtration.

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3.5 Fouling behavior of the prepared membranes

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The antifouling property of the MOF/PES ultrafiltration membranes was characterized by means of flux recovery ratio (FRR), for which the higher FRR value means the better antifouling property for the membrane. For this aim, water flux measurement was made after fouling by powder milk solution. The permeate fluxes are shown in Fig. 14 which it comprises three parts: pure water flux, the foulant flux, and water flux after fouling with the foulant. In this study, 8000 ppm of powder milk solution (for simulating protein solution) was used for fouling experiments. Fig. 14 shows that the flux of the membranes remarkably decreased when pure water was changed by the powder milk solution in the filtration cell, indicating fouling of the membranes. After washing the membranes, results showed that the pure water flux of the bare PES membrane was intensely reduced. The distinguished antifouling performance of the membranes may be attributed to the improved hydrophilicity because of the MOF NPs addition. This behavior is consistent with membrane hydrophilicity change as revealed in Fig. 8. The membrane with more hydrophilic surface illustrates lower fouling and massive flux recovery ratio. 18

40 Unfilled PES MOF 0.1 wt.%

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MOF 0.5 wt.% MOF 1 wt.%

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Fig. 14 Flux versus time of the membranes at 3 bars during three steps: water flux (60 min), 8000 ppm powder milk solution flux (90 min), and water flux (60 min) after 15 min washing with distilled.

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Flux recovery ratio depicted in Fig. 15 can apparently present the appropriate recycling properties of the modified membranes. The FRR for the bare PES membrane (62.27%) was lower than the FRR for the membranes modified with MOF NPs (more than 91%). It means that the filtration efficiency of the prepared UF membranes was increased when they were subjected to the protein solution. In the best condition, for the MOF NPs 0.1 wt.% membrane, the flux recovery percentage of the membrane was 91.55%. (better performance than similar ones [4]) Assessment of flux recovery of the MOF modified membranes proposes that this sequence of change in flux recovery are paronomasia with the membrane surface roughness displayed by AFM images (Fig. 16).

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91.55

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69.51 62.27

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M2 Membrane type M3

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Fig. 15 Water flux recovery ratio (FRR) of the unfilled (M1) and MOF/PES membranes (M1, M2, M3) in the filtration of powder milk solution (Average of three replicates was reported.)

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Studies show that membrane with lower roughness and surface energy has formidable antifouling potencies, and membrane fouling is raised by an increase in the surface roughness. In addition, foulants are likely to be absorbed in the cavities and grooves of the membrane with eccentric surfaces resulting in blocking of the cavities and grooves [33]. The AFM method was used to determine the surface roughness of the membrane. The roughness parameters and AFM images are presented in Table 2 and Fig. 16 respectively. The average roughness (Sa ) of unembedded PES membrane decreased from 43.86 to 9.23 nm for modified membranes. Generally, there is little electrostatic interaction between the MOF NPs and they are symmetrically arranged in the membrane, so the membrane surface is smooth. But by increasing the concentration of MOF NPs and consequently an increase in pore size and also an agglomeration of them, the roughness of membrane surface increases [34].

Membrane

Sa (nm)

Sq (nm)

Sz (nm)

M1 M2 M3 M4

43.86 9.23 11.59 13.52

53.23 11.87 16.08 19.58

300.95 111.69 123.92 280.93

Table 2 Surface roughness parameters of unfilled (M1) and MOF modified PES membranes (M1, M2, M3) resulted from analyzing three randomly chosen AFM images.

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M3 M4 Fig. 16 AFM images of the unfilled and modified PES membranes with different concentrations of the MOF NPs. (M2 0.1%, M3 0.5 % and M4 1 %)

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Antifouling properties of the membranes were characterized using three parameters. Reversible fouling (R r ) is the part of membrane fouling could be omitted through hydraulic cleaning. Irreversible fouling (R ir ) is the other part of fouling could not be removed only with hydraulic cleaning. Total fouling is the last parameter which shows the degree of total flux. Hydraulically reversible fouling ratio (R r ), hydraulically irreversible fouling ratio (R ir ) and total fouling ratio (R t ) values were shown in Fig. 17. These values were calculated from water flux before powder milk fouling and after hydraulic cleaning to evaluate the antifouling properties. The lower Rt value represents a better antifouling property of the membranes while the higher FRR value indicates a better antifouling property of the membrane. Unfilled PES membrane due to less hydrophilicity and surface charge had a great irreversible fouling ratio (37.72%, almost half of 76.91% of total fouling). Among the modified membranes, MOF embedded membrane with 0.5 wt. % depicted the highest irreversible fouling, which can be attributed to the higher roughness of this membrane. The 0.1 wt. % MOF/PES membrane had the highest FRR value of 91.55% and the lowest Rir value of 8.44%. As a result, Surface roughness parameters, reversible resistances (R r ), and irreversible resistances (R ir ) and the recovery ratios (FRR) of MOF modified membranes were improved.

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8.53

70 8.44 37.72 30.48

50 40 30 20

68.62

60.08 39.19

36.78

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M3

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Fig. 17 Fouling resistance ratio of bare (M1) and MOF/PES ultrafiltration membranes (M1, M2, M3).

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To determine the reproducibility of the membrane efficiency and the stability of antifouling property, five cycles of protein fouling tests were carried out for the modified membrane with 0.1 wt. % MOF NPs. Flux recovery values for five cycles were depicted in Fig. 18. Thus, reversible fouling was the dominant phenomenon in total fouling as hydraulic cleaning maintained high efficiency after five cycles.

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Fig. 18 Recycling properties of the M2 membrane during five powder milk filtration

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4. Conclusion

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In summary, nanocrystals of the Ce (III) metal-organic framework [Ce(tp)(NMP)2 (CH3 COO)]n were synthesized using sonochemical irradiation and characterized via powder X-ray diffraction, FT-IR spectroscopy, BET, SEM, and particle size analyzer. The porosity analyses of these NPs shows a nanoporous material with an average pore size of 3.7 Å, a pore volume of 0.81 cm3 g −1 and a specific surface area of 287.4 m2 g −1. The MOF NPs embedded polyethersulfone (PES) membranes were prepared through solution casting by phase inversion procedure. The addition of the NPs into the PES matrix caused an increase in the pure water flux relative to the bare membrane due to the enhancement of the membrane hydrophilicity. It was appointed that the antifouling feature of the modified membranes was ameliorated and the sensibility of the membranes to fouling diminishes with a decrease in the roughness of their surfaces. According to the results, ultrafiltration membrane produced by 0.1 wt. % MOF NPs modified PES had high antifouling property and water flux and can be used as a suitable membrane for wastewater treatment. Nanofiltration performance of the membranes, appraised by probing of the retention of Direct Red 16, showed that all the modified membranes have a higher dye rejection capacity than the bare one.

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