Preparation and characterization of novel nanoporous SBA-16-COOH embedded polysulfone ultrafiltration membrane for protein separation

Journal Pre-proof Preparation and characterization of novel nanoporous SBA-16-COOH embedded polysulfone ultrafiltration membrane for protein separation Vahid Vatanpour, Hesamoddin Rabiee, Mohammad Hossein Davood Abadi Farahani, Majid Masteri-Farahani, Mahsa Niakan

PII:

S0263-8762(20)30042-3

DOI:

https://doi.org/10.1016/j.cherd.2020.01.029

Reference:

CHERD 3980

To appear in:

Chemical Engineering Research and Design

Received Date:

3 November 2019

Revised Date:

4 January 2020

Accepted Date:

22 January 2020

Please cite this article as: Vatanpour V, Rabiee H, Davood Abadi Farahani MH, Masteri-Farahani M, Niakan M, Preparation and characterization of novel nanoporous SBA-16-COOH embedded polysulfone ultrafiltration membrane for protein separation, Chemical Engineering Research and Design (2020), doi: https://doi.org/10.1016/j.cherd.2020.01.029

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Preparation and characterization of novel nanoporous SBA-16-COOH embedded polysulfone ultrafiltration membrane for protein separation

Vahid Vatanpour

a, *

, Hesamoddin Rabiee

a, b, 1

, Mohammad Hossein Davood Abadi

Farahani a, c, 1, Majid Masteri-Farahani d, Mahsa Niakan d

a

Department of Applied Chemistry, Faculty of Chemistry, Kharazmi University, 15719-14911

b

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Tehran, Iran Advanced Water Management Centre, The University of Queensland, St. Lucia, QLD 4072,

Australia

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4

Engineering Drive 4, Singapore 117585, Singapore

Department of Inorganic Chemistry, Faculty of Chemistry, Kharazmi University, 15719-

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d

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14911 Tehran, Iran

These authors contributed equally to this work.

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1

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c

* Corresponding author:

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Tel/Fax: +98 26 34551023

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[email protected], [email protected] Graphical abstract

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Highlights •

Carboxylated nanoporous SBA-16 was used to modification of polysulfone UF

membrane. •

Flux of SBA-16-COOH mixed membranes improved due to hydrophilicity

improvement. SBA-16-COOH addition elongated finger-like pores in membranes structure.

•

Embedding of the SBA-16-COOH enhanced fouling resistance and BSA rejection.

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•

Abstract

Polysulfone ultrafiltration membrane was modified by novel nanoporous SBA-16-COOH

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during the membrane preparation via the phase inversion. The pure water flux and bovine serum albumin (BSA, as the foulant) flux were measured at 2 bar and the membranes’

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antifouling behavior were analyzed. The membranes showed higher water flux after SBA-16-

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COOH addition up to 2 wt% and after that the flux slightly decreased which is attributed to the aggregation of SBA-16-COOH particles at the higher concentrations. SBA-16-COOH addition improved the surface hydrophilicity and led to elongated finger-like pores within the

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membranes cross section structure. The water flux after BSA flux was still higher than the one before BSA, thereby SBA-16-COOH addition resulted in better antifouling properties. In terms

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of BSA rejection, the nanocomposite SBA-16-COOH-based membranes outperform the

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pristine PSf membrane with rejection values up to 98.9%. The water contact angle confirmed the enhanced hydrophilicity of the membranes’ surface due to -COOH functional groups of the nanomaterials which led to a higher permeability and an enhanced fouling resistance.

Keywords: Ultrafiltration (UF); Nanoporous SBA-16; Nanocomposite; Antifouling; Membrane separation

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1. Introduction Membrane processes has received noteworthy attention as a sustainable process in separation technology due to its several advantages like high efficiency, simplicity, compact design, low ecological footprint and low cost [1, 2]. In addition to the stated advantages, a key limitation of the membranes applied in the separation processes is flux decline because of membrane fouling, resulted from unwanted adsorption and deposit of foulants into the membrane pores or

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onto surface, which reduce the lifespan of the membranes and reduce their efficiency [3]. Fouling is major issue in ultrafiltration (UF) membranes because of direct filtration solutions with high fouling chance during various processes such as protein separation and fractionation,

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oil/water filtration, and fruit juice clarification [4, 5].

The key affecting factors in the fouling of a membrane are the membrane’s surface properties

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such as hydrophilicity, pore size and roughness. An intensification in the membrane

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hydrophilicity and a reduce in the roughness could enhance membrane’s fouling resistance [6, 7]. There are some methods to improve polymeric membranes antifouling properties such as grafting a hydrophilic polymer to membrane surface [8], blending a hydrophilic polymer with

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the base polymer matrix [9, 10], addition of hydrophilic agents like surfactants to the membrane structure [11, 12], incorporating in situ hydrophilic surface modifying macromolecules [13],

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coating an antifouling material [14], mixing with nanoparticles [15-17], and more. Specifically,

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the strategy of blending nanomaterials with the polymer matrix to fabricate nanocomposite membranes is promising, due to the simplicity of its dope preparation and fabrication, small use of nanomaterials, and its effectiveness [18-20]. The nanoparticles can efficiently improve hydrophilicity, antifouling and anti-compaction characteristics of membranes. For example, Liao et al. [21] prepared SiO2 dispersed polyvinylidene fluoride (PVDF) nanocomposite membrane and showed that the hydrophilicity and mechanical stability of the resulted

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membranes have been improved to some extent. Zhang et al. [22] used porous ZrO2 solid super acid shell/void/TiO2 core nanoparticles to prepare PVDF nanocomposite membrane for enhancing the capabilities of antifouling, anti-compaction and hydrophilicity of PVDF membranes. PVC-based UF membranes were modified with addition of ZnO and TiO2 nanoparticles by our team and the results showed remarkable enhancement in antifouling properties of the membranes, in addition to better water flux [20, 23]. Zeolites are one of the most used nanoparticles in the preparation of the nanocomposite

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membranes. The studies have presented that zeolites could be suitable in enhancing the permeability and antifouling properties of polymeric matrices when the proper characteristics of zeolite material are selected [24, 25]. These characteristics include zeolite hydrophilicity, size,

crystal

size,

and

morphology.

Among

these

materials,

ordered

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pore

mesoporous/nanoporous molecular sieves have received widespread interest [26]. Due to their

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high surface areas, possibility of functionalization, adjustable and larger pore size they have

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established to be promising nanofillers in membranes especially in gas separation membranes [27, 28]. Also, SiO2 has been widely considered for modification of membranes because of its good chemical stability and high hydrophilicity [29, 30]. However, most of the earlier

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researches have used non-ordered mesoporous silica particles. Compared with the SiO2 nanoparticles, nanoporous silica such as SBA-16 (SBA = Santa Barbara Amorphous) has high

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porosity, large surface area and great density of silanol groups (SiOH) distributed over the

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silica surfaces [31]. These significant features make the hydrophilicity of SBA-16 superior than that of routine SiO2. The SBA-16 silica shows attractive textural properties, like as large specific surface areas (above 1000 m2·g−1), thick framework walls, uniform-sized pores (in range 4–30 nm), complementary textural porosity and small crystallite size of primary particles [32]. Also, these materials are easily surface functionalized.

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Using ordered mesoporous/nanoporous silica in UF membranes is relatively rare. Zhao et al. reported application of mesoporous titanium dioxide in preparation of PVDF ultrafiltration membranes to improve the mechanical properties, thermal stability, hydrophilicity, flux, and antifouling performance [33]. Wu et al. prepared the polyamide (PA) thin film nanocomposite membranes with blending amino functionalized mesoporous silica nanoparticles in piperazine (PIP) aqueous solution [34]. By adding a suitable amount of mesoporous silica, the pure water flux of the nanocomposite membrane was 1.5 times of the unfilled TFC membrane, while the

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rejection of Na2SO4 stayed at a relatively high level. Martín et al. used mesostructured functionalized silica particles for modification of polyethersulfone membranes [26]. The results showed a great effect of mesostructured silica incorporation on the membrane flux and

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antifouling properties. Liao et al. used unfunctionalized SBA-15 in the fabrication of PVDF hybrid membrane [35]. The obtained modified membrane showed no specific effect on the

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structures of the membrane, however it effectively enhanced membrane hydrophilicity and

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antifouling performance. Hollow mesoporous silica nanoparticles enhanced water flux of thin film composite membranes by 40%, while NaCl rejection did not change considerably [36], however the membranes strength and durability increased which can be due to positive

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interactions between the silica nanoparticles and the polymer. To the best of our knowledge, study of the fabrication of nanoporous SBA-16 hybrid

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polysulfone (PSf) ultrafiltration membrane is not reported yet. In this study, SBA-16

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nanoporous material was synthesized and then, carboxylated to fabricate a hydrophilic SBA16-COOH. PSf hybrid nanocomposite membranes were prepared using different concentrations of SBA-16-COOH by the immersion precipitation induced phase inversion technique. The objective of this study was to enhance the performance (flux and antifouling) of PSf membrane by the addition of low amounts of SBA-16-COOH nanoporous material. The influences of SBA-16-COOH on the morphology, hydrophilicity, membrane flux, BSA

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rejection and antifouling behavior of the resulted nanocomposite membranes were investigated. The membranes presented higher permeability and improved antifouling properties compared to the pristine PSf membrane.

2. Experimental 2.1. Materials Polysulfone (S 6010) was purchased from BASF, Germany. N–methyl pyrrolidone (NMP),

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hydrochloric acid (HCl, 36-38%), tetraethyl orthosilicate (TEOS) and polyvinyl pyrrolidone (PVP, Mw=29000 g/mole) were all obtained from Merck company. Pluronic F127 and cetyltrimethylammonium bromide (CTAB) were purchased from Aldrich. Bovine serum

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2.2. Synthesis of SBA-16 and SBA-16-COOH

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albumin (BSA) with Mw = 67,000 g/mole was purchased from Merck.

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First, 1.26 g of pluronic F127, 0.073 g of cetyltrimethylammonium bromide (CTAB) and 71.28 g of 2 M hydrochloric acid were mixed in a 250 mL flask and next, 2.83 g tetraethyl orthosilicate (TEOS) was added to the solution. After that, the solution was stirred at 40 ºC for

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6 h. After 20 min, some solids were precipitated. In the next step, the temperature of the reaction was increased to 80 ºC for 6 h. After termination of the reaction and its cooling up to

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room temperature, the resulted precipitates were washed several times with distilled water and

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filtered. The precipitate was firstly dried in room temperature and next, placed in an oven for 6 h in 100 ºC. For removal of the surfactants, the resulted solid was calcined for 6 h in 550 ºC. The prepared SBA-16 was modified with trichlorocyanopropyl silane according to our earlier report [37]. For carboxylation of SBA-16, 1 g of the synthesized SBA-16 containing nitrile group was added to 80 mL of a mixture having concentrated sulfuric acid and distilled water (1:1) and refluxed for 3 h [38]. The resulted suspension was stirred for 2 days in ambient

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temperature. The obtained solids were separated by filtration and rinsed with distilled water. Next, the precipitates were dried in an oven at 90 ºC overnight.

2.3. Preparation of nanocomposite polysulfone membrane The precise amounts of the SBA-16-COOH particles (0.1, 0.5, 1, 2 and 4 wt% related to polymer) were added to the NMP solvent and ultrasonicated for 10 min for good dispersion. After that, first 1 wt% PVP was dissolved in this suspension and next, 18 wt% PSf was added

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and stirred for 24 h to complete dissolution. The membranes were cast on a glass plate with an applicator with thickness of about 150 µm. The membranes were quickly coagulated by immersing them in a water bath at 25 °C for 24 h to finish the phase inversion and leaching of

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solvent and PVP out of the membranes. Subsequently, the membranes were reserved in water

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before use.

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2.4. Characterization

Fourier transform infrared spectroscopy (FTIR) (Bruker, TENSOR 27) and Transmission electron microscope (TEM, Zeiss, EM10C, 80 kV) were applied to characterize the synthesized

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SBA-16 nanoparticles.

To study the surface and cross-sectional morphologies of the fabricated UF membranes, a

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scanning electron microscope (SEM, VEGA║(TESCAN, Czech Republic)) was used in a high

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vacuum condition at 20 kV. For cross-sectional images, the membranes were frozen by immersing in liquid nitrogen, fractured, and next gold sputter-coated to reduce the sample charge.

To compare the membranes’ hydrophilicity, static water contact angles were captured by a goniometer (G10, KRUSS, Germany) at 25 °C. Images of 3 μL DI water droplets on the

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membranes’ surface and contact angles were taken at more than ten random places for each sample and the average values were calculated to reduce the experimental error. Powder X-ray diffraction (XRD) patterns of the materials were achieved by Philips TW-1730 equipped with CuKα radiation (λ=1.54 Å). The overall porosity (ε) of the fabricated membranes were determined by Eq. (1), based on gravimetric method [15]. 𝜔 −𝜔

1 2 𝜀 = 𝐴×𝑙×𝑑

(1)

𝑤

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where ω1 and ω2 are correspondingly the wet and dry weights of the membrane (g), dw is the water density (0.998 g/cm3), A is the surface membrane area (cm2), and l is the thickness of membrane (cm).

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The membranes mean pore radius was also determined using the Guerout-Elford-Ferry

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equation (Eq. (2)) using porosity and pure water flux data [15]. (2.9−1.75𝜀)×8𝜂𝑙𝑄

𝑟𝑚 = √

(2)

𝜀×𝐴×∆𝑃

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where, Q is the volume of permeated water per unit of time (m3/s), η is water viscosity

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(8.9⨯10−4 Pa·s) and ΔP is the operation pressure (0.2 MPa).

2.5. Calculation of permeation, flux recovery and BSA rejection

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The membranes performance was characterized in a dead-end cell setup with 19.6 cm2 effective

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membrane area under pressure of 2 bar after they were compacted in 3 bar pressure for 30 min. The rejection and the pure water flux of BSA solution were measured every 5 min for 90 min. To obtain an average value, as a minimum 6 replicates were carried out. The permeation flux, J, is determined as follows:

J 

M A .t

(1)

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where M is the total permeated mass of the water or solution (kg); A is the membrane effective area (m2); and t is the filtration time (h). BSA rejection, R, is measured by Eq.(2):

R (%)  (1 

CP )  100 CF

(2)

In this equation, CP and CF are the BSA concentration of the permeate and the feed solution, respectively.

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Flux recovery ratio (FRR) was calculated using the first and second water flux, as follows: 𝐽

𝐹𝑅𝑅(%) = (𝐽𝑤𝑎𝑡𝑒𝑟2 ) × 100

(3)

𝑤𝑎𝑡𝑒𝑟1

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where, 𝐽𝑤𝑎𝑡𝑒𝑟2 and 𝐽𝑤𝑎𝑡𝑒𝑟1 are flux of pure water after and before BSA flux, respectively.

3.1. Characterization of SBA-16-COOH

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3. Results and discussion

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The full characterization results of the pristine SBA-16 and the SBA-16-COOH were given in Figs. 1 and 2, respectively. First, characterization of the pristine SBA-16 and SBA-16-COOH

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was done by FTIR spectroscopy. The FTIR spectra of the pristine SBA-16 and SBA-16-COOH were presented in Figs. 1a and 2a, respectively. In the FTIR spectrum of both samples, the observed bands at 1631 and 3429 cm-1 could be attributed to the bending and stretching

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vibrations of surface -OH groups, respectively. Other bands at 818, 1087 and 463 cm-1 are

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related to symmetric stretching, asymmetric stretching, and bending vibrations of Si-O-Si groups [39]. Compared with the FTIR spectrum of the pristine SBA-16, the FTIR spectrum of SBA-16-COOH showed a new band at 1722 cm-1, corresponding to stretching vibrations of carboxyl group [37]. Moreover, the absence of characteristics band of nitrile group at 2255 cm1

approved the conversion of nitrile groups to carboxylic ones. These results clearly confirmed

the successful functionalization of SBA-16 with carboxylic acid groups.

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The TEM images of the pristine SBA-16 and SBA-16-COOH were presented in Figs. 1b and 2b, respectively. A body-centered cubic mesostructure can be observed for the pristine SBA16. Similar to SBA-16, the SBA-16-COOH particles showed a highly ordered cage-like arrangement of mesopores with pore diameters of about 4 nm and particle size of about 200 nm which is similar to those reported for SBA-16 material [39]. These results confirmed that the mesoporous structure of SBA-16 have not changed during the modification steps. XRD patterns of the pristine SBA-16 and SBA-16-COOH were shown in Figs. 1c and 2c,

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respectively. As can be seen, both samples exhibited one (110) diffraction peak, which is the distinguishing peak of SBA-16 mesoporous material with the cubic Im3m structure [40, 41]. However, a decrease in the intensity of the (110) peak is observed after -COOH

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functionalization owing to contrast matching between the silica walls and organic species placed in the mesopores of SBA-16. The XRD results demonstrated that the mesoporous

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structure of SBA-16 was preserved after modification.

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Nitrogen adsorption-desorption isotherms of the pristine SBA-16 and SBA-16-COOH were presented in Figs. 1d and 2d, respectively. A typical type IV isotherm by H2 hysteresis loop (consistent with the Brunauer-Deming-Deming-Teller (BDDT) classification) that is specific

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of mesoporous materials by structure of cubic cage-type, was observed for both samples [39, 41]. It can be also concluded that mesoporous structure of the original SBA-16 is well-kept

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after -COOH functionalization in spite of the decrease in nitrogen adsorption.

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3.2. Characterization of the nanocomposite membranes Contact angle test was applied at 25 ºC to investigate the changes of surface hydrophilicity after SBA-16-COOH addition, as it can be seen in Fig. 3. The results show that the membranes become more hydrophilic with continuously increasing the concentration of in the structure. Upon SBA-16-COOH addition, the membranes become more hydrophilic and the contact angle continuously decreases which is relayed to the presence of SBA-16-COOH nanoparticles on

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the surface [20]. In higher concentrations, the contact angle is almost constant due to agglomeration of the nanoparticles [42]. Therefore, the membranes more hydrophilic and the water permeability and antifouling properties will be improved.

SEM images were used to explore changes in surface and cross-sectional morphology of the nanocomposite membranes after SBA-16-COOH blending and the results can be seen in Fig.

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4. All the SEM images show normally porous and asymmetric structure for the membranes, with a porous sub-layer and a dense top layer. Addition of SBA-16-COOH powders change both kinetic (by increasing the viscosity of the solution and delaying instantaneous demixing,

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this was seen visually as viscosity measurement was not performed) and thermodynamic of the phase inversion (by causing a less stable solution and encouraging instantaneous demixing)

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which results in changes in the morphology [43, 44].

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The neat PES membrane exhibits large finger-like pores which are due to high affinity of NMP and nonsolvent (water). When the polymer film enters into the water, NMP and water exchange instantaneously and this results in a fast nonsolvent-solvent demixing, therefore membranes

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with large pores will be formed. From the SEM images, porosity of the membranes increases with addition of SBA-16-COOH powders and the connectivity between the top and bottom

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layer improves. In addition, the SBA-16-COOH/PSF membranes have more micro-void

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volume, in particular for up to 2 wt% SBA-16-COOH, and no further improvement can be seen with more SBA-16-COOH addition. Comparing the cross-sectional SEM images of bare PSF and 2 wt% SBA-16-COOH blended membrane, obviously show that by inserting the nanoparticles, the thickness of pore walls is reduced and the size of macrovoids is increased. However, for the membrane with 4 wt% SBA-16-COOH, a dense top layer is formed due to delayed instantaneous demixing caused by increment in the viscosity of the polymeric solution

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[20]. The morphology results observed here are fairly in accordance with similar studies about nanocomposite membranes [45-48]. Moreover, even for the membrane with 4 wt% SBA-16COOH, the membrane still exhibits finger-like structure and unlike another study on PSf nanocomposite membranes, the morphology does not turn to sponge-like [49]. In addition to formation of more finger-like structure after SBA-16-COOH addition, the surface of the membranes also show more porosity with rising the SBA-16-COOH content (Fig. 5). The presence of SBA-16-COOH nanoparticles on the surface of the membranes, alter the

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surface morphology and increases the surface roughness, which affect the permeability and antifouling properties of the membranes (will be discussed later). Indeed, the surface porosity in enhanced as we observed more long-finger-like pores in the cross section, however, SBA-

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16-COOH addition above 4 wt% increases the chance of particle agglomeration and pore blockage [23] which will influence the water flux properties of the membranes, negatively [50,

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51]. Due to lower concentration of the nanoparticles in polymeric matrix and nano-metric size

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of the nanoparticles, the observation of the agglomeration of this nanoparticles by SEM images is difficult.

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For approving the presence of SBA-16-COOH in the matrix of membrane and showing good dispersion of the nanomaterial, EDX-mapping technique was applied for the 2 wt% SBA-16-

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COOH/PSf membrane. As shown in Fig. 6, the Si peak was appeared in EDX spectrum

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confirming the presence of SBA-16 in membrane body. Mapping showed that the SBA-16COOH was homogeneously dispersed in the PSf membrane matrix.

3.3. Membrane filtration performance

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The effect of SBA-16-COOH addition of the pure water and BSA flux of the membranes was examined for 90 min at 2 bar operating pressure. The pure water flux rises by blending of SBA16-COOH nanomaterials up to 2 wt%, then it declines. This increase in water flux is attributed to higher surface hydrophilicity and more porous structure of the membranes [49, 52, 53]. However, excessive SBA-16-COOH addition after 2 wt%, increases the chance of particle aggregation, therefore the membranes pores will block and water flux reduces [51]. As it can be seen in Fig. 6, water flux for the membrane containing 4 wt% SBA-16-COOH is even lower

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than then membrane with 0.5 wt%, which exhibits the high chance of agglomeration of the membrane surface which suppress the water passing. The more hydrophilic membranes after SBA-16-COOH addition, can attract water easier which leads to faster water permeability [12].

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In addition, higher porosity and interconnectivity in the membranes’ structure which was seen in SEM images, means lower resistance to the water flux. A similar trend is observed for BSA

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flux in the nanocomposite membranes, except BSA flux for the membranes containing 4 wt%

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SBA-16-COOH is almost the same with the pristine PSf membranes. However, along with the positive effects of SBA-16-COOH addition for higher hydrophilicity and a structure with interconnected pores, at high of SBA-16-COOH content, particle blockage

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becomes more serious and it can neutralize the positive effect of hydrophilicity and morphology on water permeability. Moreover, a higher dope viscosity after SBA-16-COOH

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addition, results in delayed phase inversion and formation of membranes with thicker skin layer

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(as was seen for the membrane with 4wt% SBA-16-COOH addition) and this thick skin layer acts like a resistance for water flux [42, 48]. The amount of nanoparticle addition to UF membranes with the aim to improve the water flux and antifouling properties usually has a threshold this trend has been observed for other nanocomposite membranes with decreased water flux after a certain content [46-48]. Although, the SBA-16-COOH particles are functionalized with carboxylic agents before addition to polymeric solution, to make them

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hydrophilic and improve their dispersion, at higher concentrations aggregation of particles happens inevitably. Also, based on the pore size and porosity data presented in Table 1, the membranes average porosity increases until 2 wt% SBA-16-COOH addition and after that further SBA-16-COOH addition results in reduction, although all the membranes fabricated have a porosity over 65% due to the addition of PVP pore former in the casting solution [10, 54, 55]. However, further increment in porosity after PVP addition is attributed to instantaneous demixing and faster water-NMP diffusion till 2 wt% SBA-16-COOH addition

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because after that the membranes become less porous as demixing is delayed. It was seen that pore size follows the same trend as average porosity with SBA-16-COOH addition, which is due to the effects of SBA-16-COOH on the thermodynamic and kinetic of phase inversion, as

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discussed above [23, 56].

As well as sufficient water flux, the membranes must be able to reject BSA which is commonly

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tested as a foulant model for ultrafiltration membrane performance analysis [10, 12, 57]. The

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pristine PSf membrane shows almost 97% BSA rejection and later addition of SBA-16-COOH particles results in almost 99% BSA rejection (Fig. 7). BSA is more attracted to hydrophobic materials, while it is observed that SBA-16-COOH addition improves hydrophilicity of the

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membrane surface, therefore it is expected that BSA rejection improves with SBA-16-COOH addition. It is also expected that BSA rejection reduces after the threshold which is probably

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related to blockage of smaller surface pores, thereby more large pores are active and this leads

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to decrease in rejection of BSA [20, 23, 46, 58].

3.4. Antifouling behavior of the membranes The UF membranes fabricated by hydrophobic polymer such as PVC, PSf, and PES are prone to fouling which will shorten their lifespan. Foulants tend to stick to the surface and pores of the membranes and as time goes on, they increase the chance of pore blockage and therefore

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reduction in water flux. For this reason, in many industrial membrane processes, the membranes should be washed regularly after a particular time to recover their performance [59]. The membranes should have antifouling abilities and be able to recover their permeability after several times cycles with no significant drop, because fouling cannot be always recovered via washing [56, 60]. For this purpose, the water flux through the pristine and nanocomposite membranes was measured before and after BSA flux (500 ppm) to compare the ability of the membranes to recover water flux. BSA can make a layer on the surface of the membranes

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during its permeation and this can cause more resistance for water to pass the membrane. Therefore, the memebranes’ surface must be less hydrophobic with lower roughness to reduce the chance of foulant adsorption and layer formation [23, 61].

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The three step fluxes of water and BSA can be seen in Fig. 8, and as obvious all the permeations follow the same trend. Like first water flux through the membranes, BSA flux and the second

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water have the same order of flux and the highest flux is related to PSf/2 wt% SBA-16-COOH

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membrane. The flux for all the membranes decreases and reach a fairly stable plateau. When BSA contacts the membrane surface, it results in formation of hydrophobic layer which eventually leads to lower water flux and less hydrophilic surface. The presence of hydrophilic

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SBA-16-COOH on the surface and pore walls of the will lead to less BSA adsorption and thereby the membranes can keep their hydrophilicity and affinity to water [20, 23, 42, 46].

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Therefore, the membranes are expected to be more resistant to lose the water flux after SBA-

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16-COOH addition.

After BSA solution filtration, the membranes flux recovery ability was investigated with evaluating their second pure water flux, which results were presented in Fig. 9a. The flux recovery ratio (FRR) of the bare PSf membrane was 47.8%, while this value increased to 56.9 in the 0.5 wt% SBA-16-COOH membrane, i.e. the FRR was increased by almost 10% after addition of only 0.5 wt% nanoparticle. All of the modified nanocomposite membranes had

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higher FRR in comparison to the bare PSf. This increase could be related to the hydrophilicity improvement [15, 20]. However, the FRR reduced for the membranes blended by more than 0.5 wt% nanoparticle. This behavior could be related to agglomeration of nanoparticles in higher concentrations and probably surface roughness increasing [59]. In the basis of the BSA rejection and FRR value, the 0.5 wt% SBA-16-COOH blended PSf membrane displays the best performance. To show protein fouling durability of this membrane, the repeated FRR test was done five times for the bare PSf membrane and the optimum

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membrane for comparison. Fig. 9b show these results. With repeating the filtration of BSA solution and washing, the 0.5 wt% SBA-16-COOH/PSf membrane exhibited minor flux

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decline in comparison with the bare one.

To compare the results of SBA-16-COOH/PSf membrane with other reported mixed matrix

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polysulfone membranes, Table 2 is prepared. The fabricated SBA-16-COOH/PSf membrane

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showed high flux and BSA separation ability with suitable antifouling properties. 4. Conclusion

SBA-16-COOH material was added to the structure of PSf UF membranes to modify the

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morphology and antifouling properties. The membranes were prepared via facile phase inversion technique by addition of SBA-16-COOH material at five different percentages from

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0.1 wt% to 4 wt%. Addition of SBA-16-COOH material led to considerable changes in the

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morphology of the membranes and SEM images indicated more finger-like pores with connectivity across the membranes up to 2 wt% addition. After 2 wt% the nanoparticles are prone to agglomeration despite of -COOH functional groups, which it leads to lower water flux and higher chance of pore blockage. In addition, -COOH functionalized SBA-16 material enhances the hydrophilicity properties of the surface, therefore the nanocomposite membranes

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showed better water flux recovery and foulant rejection compared to the pristine PSf membrane.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgment The authors thankfully acknowledge the financial support provided by the Kharazmi

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University, Iran.

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Fig. 1. (a) FTIR spectrum, (b) TEM image, (c) XRD pattern, and (d) nitrogen adsorption-

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desorption isotherm of pristine SBA-16.

Fig. 2. (a) FTIR spectrum, (b) TEM image, (c) XRD pattern, and (d) nitrogen adsorptiondesorption isotherm of mesoporous SBA-16-COOH.

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4 wt%

62.2

2 wt%

62.9

1 wt%

64.5 66.3

0.5 wt%

69.3

0.1 wt%

0

10

20

30 40 50 Contact angle (°)

60

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72.8

Bare PSf

70

80

Fig. 3. Static water contact angle of pristine PSf and nanocomposite SBA-16-COOH/PSf

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membranes.

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28

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-p

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Fig. 4. Cross-section SEM images of SBA-16-COOH/PSf UF membranes

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Fig. 5. Surface SEM images of SBA-16-COOH/PSf UF membranes

Fig. 6. EDX and Si Mapping of 2 wt% SBA-16-COOH embedded PSf membrane.

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100.0

90.0

98.3

98.9

0.1 wt%

0.5 wt%

96.8

98.8

80.0

-p

85.0

1 wt%

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Bare PSf

98.8

98.4

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BSA rejection (%)

95.0

2 wt%

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Fig. 7. Protein rejection of the SBA-16-COOH blended PSf membranes

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4 wt%

250

(a) 2 wt% 1 wt% 0.5 wt% 0.1 wt% 4 wt% Bare PSF 18%

150

100

First pure water

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Flux (kg/m2 h)

200

50

Second pure water

BSA Filtration

0

50

-p

0 100

150

200

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Time (min)

70

40 30 20

50.2

51.2

52.8

1

2

4

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50

56.9

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Flux recovery ratio (%)

47.8

54.6

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(a) 60

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Fig. 8. Three step flux versus time for the SBA-16-COOH/PSf membranes

10

0

Bare PSF

0.1

0.5

32

250

(b)

60

Bare PSf

0.5 wt% SBA-16-COOH

FRR (%)

50 40 30 20 10 0 2

3

4

5

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1

Repeated Test

Fig. 9. Flux recovery ratios for SBA-16-COOH/PSf membranes, (a) one round for all

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membranes and (b) five rounds of BSA solution filtration for bare and optimum membrane.

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Table 1. Mean pore size and overall porosity of the SBA-16-COOH/PSf membranes. Mean pore size (nm) 6.9 (±0.4) 7.2 (±0.4) 7.2 (±0.6) 8.2 (±0.5) 8.3 (±0.6) 6.4 (±0.4)

Average porosity (%) 64.1 (±2.5) 76.3 (±2.6) 68.1 (±2.3) 70.4 (±2.8) 72.2 (±3.1) 71.1 (±3.3)

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Membrane Bare PSf 0.1 wt% SBA-16-COOH/PSf 0.5 wt% SBA-16-COOH/PSf 1 wt% SBA-16-COOH/PSf 2 wt% SBA-16-COOH/PSf 4 wt% SBA-16-COOH/PSf

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Table 2. Studies done to develop nanocomposite PSf membranes in water treatment. FRR (%)

Contact angle Ref. (°)

BSA (98%)

82

72

[16]

--

87

[17]

--

41

[49]

Nanomaterials

UF

2-hydroxy ethyl methacrylate grafted 150 (1 bar) TiO2

UF

PVP grafted silica

35 (3 bar)

UF

TiO2

470 (2 bar)

NF

Sulfated TiO2

6 (6 bar)

UF UF

TiO2 Titanium nanotube

92 (1 bar) 170 (2 bar)

PEG 40,000 (100%) Kerosene emulsified wastewater (99%) BSA (99%) Methylene blue dye (40%) Humic acid (85%) BSA (98%)

UF

SBA-16-COOH

210 (2 bar)

BSA (99%)

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-p

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Pure water flux (L/m2 h) (pressure)

Rejection (%)

Membrane type

35

86

60

[54]

-90

45 60

57

62

[58] [59] This study