Cellulose acetate composite membranes tailored with exfoliated tungsten disulfide nanosheets: Permeation characteristics and antifouling ability

Accepted Manuscript Cellulose acetate composite membranes tailored with exfoliated tungsten disulfide nanosheets: Permeation characteristics and antifouling ability

S. Vetrivel, M. Sri Abirami Saraswathi, D. Rana, K. Divya, A. Nagendran PII: DOI: Reference:

S0141-8130(18)30941-3 doi:10.1016/j.ijbiomac.2018.04.091 BIOMAC 9499

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

26 February 2018 30 March 2018 17 April 2018

Please cite this article as: S. Vetrivel, M. Sri Abirami Saraswathi, D. Rana, K. Divya, A. Nagendran , Cellulose acetate composite membranes tailored with exfoliated tungsten disulfide nanosheets: Permeation characteristics and antifouling ability. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Biomac(2017), doi:10.1016/j.ijbiomac.2018.04.091

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ACCEPTED MANUSCRIPT Cellulose acetate composite membranes tailored withexfoliated tungsten disulfide nanosheets: Permeation characteristics and antifouling ability

Polymeric Materials Research Lab, PG & Research Department of Chemistry, Alagappa

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Government Arts College, Karaikudi - 630 003, India

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S. Vetrivela, M. Sri Abirami Saraswathia, D. Ranab, K. Divya, A. Nagendrana,*

Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur

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St., Ottawa, ON, K1N 6N5, Canada

*Corresponding author:

E-mail: [email protected] [email protected] (A.Nagendran); Tel.: 91-4565224283; Fax: 91-4565-227497

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ACCEPTED MANUSCRIPT Abstract An attempt has been made to demonstrate the effects of exfoliated tungsten disulphide (E-WS2) nanosheets on the fabrication, permeation and anti-fouling performance of cellulose acetate(CA) ultrafiltration membranes. The E-WS2 was prepared and characterized in terms of energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction spectroscopy (XRD).

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Pure and composite CA membranes were methodically characterized for its surface, chemical

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and morphological structure using FT-IR, XRD, SEM and water contact angle analysis.

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Filtration characteristics of membranes such as pure water flux, porosity and hydraulic resistance were also studied. The addition of E-WS2 nanosheets exhibited significant

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improvement in the surface hydrophilicity of composite membranes than the control CA membrane and are evidenced by the observed contact angle and porosity values. However at

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1 wt.% E-WS2 concentration, CA membrane showed lower water flux (92.3±0.5) due to the pore plugging effect. The flux recovery ratio (FRR), bovine serum albumin (BSA) rejection,

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reversible and irreversible fouling experimental results suggested that CA/E-WS2 (1wt.%) UF

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membranes possess better fouling resistance potential than control CA membrane as a result of enhanced hydrophilicity. This study emphasizes the strong interplay between CA and E-

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WS2 nanosheets which play a significant role in altering the permeation and antifouling characteristics of nanocomposite membranes. Cellulose acetate; Antifouling; Ultrafiltration; BSA rejection; Exfoliated

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Key words:

tungsten disulfide

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ACCEPTED MANUSCRIPT 1. Introduction Membrane technology has been raised as a noticeable technique due to its selective and efficient separation, ease in handling, stability and environmentally adaptableness in water treatment [1]. Especially ultrafiltration (UF) membranes have occupied remarkable place in water treatment since they possess superior capacity in the rejection of undesired

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particles [2,3]. It was noteworthy that the organic pollutants present in the feed solutions

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lowers the membranes separation ability through their agglomeration on the surface or inside

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the membrane pore structure. As a consequence it acts as a major limitation in sieving the feed solution during the filtration experiments and affects the efficacy of membrane

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separation performance.

Cellulose acetate (CA) is one of the chief carbohydrate polymers, well suited for

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broad-spectrumof filtration and widely used as a membrane material. Loeb and Sourirajan [4] invented the phase inversion process to fabricate CA asymmetric polymeric membranes and

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it becomes a popular material for various separation applications. The first commercial FO

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membrane was developed by Hydration Technologies Inc. (HTI), which was believed to be made of CA or a mixture of CA and cellulose triacetate (CTA) [5], showed a higher water

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flux than commercial water treatment membranes. Later on, several CA based membranes have also been developed by many researchers [6,7]. Recently, many studies have shown

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that, in order to practically utilize the cellulose-based membranes, both the membranes fouling and the internal concentration polarization issues have to be solved [8]. CA has been analyzed by several researchers because of its moderate flux, cost effectiveness, renewable source of raw material and non-toxicity. However it was easily susceptible to fouling by organics such as proteins, humic acids and micro-organisms [9]. Therefore it becomes essential to make attempt on improving the antifouling properties of CA membranes to achieve the improved permanence and separation performances.

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ACCEPTED MANUSCRIPT A good way to formulate the antifouling membrane is surface modification through blending of nanoparticles [10]. The excellent properties of nanoparticles such as hydrophilicity, high surface area, photo sensitivity, chemical stability, antibacterial and low cost, made them great membrane modifiers in the field of waste and industrial water treatment [11,12]. Hence, a variety of nanoparticles including ZrO2, SiO2, Al2O3, Ag and

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carbon nanotubes [13-17] were used as an additive in the fabrication of modified composite

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UF membranes. Evidently two dimensional layered graphene oxide nanoparticles

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incorporated membranes had improved ability to permeate more water through their capillary networks bounded by oxygen containing functional groups [18,19] suggested that MoS2

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nanosheets have all the surface atoms on both sides of the surface of the layer, which make the whole surface hydrophilic and it provides higher affinity to water through hydrogen

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bonding.

Among the various 2D nanosheets, WS2 have been highly investigated these days for

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their outstanding performance in enhancing the membrane hydrophilicity. It was reported that

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the thermal, mechanical and antibacterial properties of poly (ether ether ketone) was increased after the addition of the WS2 nanomaterials [20, 21]. Katz et al. utilized nano WS2

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as a self-lubricating coating for orthodontic wires to demonstrate the toxicological test and they found that it shows no toxicity to human and animals [22].

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Herein pure and nanocomposite CA membranes with 0.25, 0.5 and 1 wt. % WS2 (designated as CA-0.25, CA-0.5, and CA-1) were prepared by phase inversion technique. All the fabricated UF membranes were characterized for hydrophilicty, morphology, permeability and anti-fouling ability.

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ACCEPTED MANUSCRIPT 2. Experimental section 2.1 Materials CA (398-30, 39.8% acetyl content) was obtained as a gift sample from Eastman Chemical Company, Mumbai, India and re-crystallized from acetone after that dried in a vacuum oven at 70°C for 24 h prior to use. N,N-dimethyl formamide (DMF), bovine serum

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albumin (BSA) (69 kDa), and sodium lauryl sulphate (SLS) of analar grades were purchased

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from Sigma Aldrich, USA. Anhydrous sodium monobasic phosphate and sodium dibasic

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phosphateheptahydrate were also purchased from Sigma Aldrich, USA.De-ionized water was used in the preparation ofmembrane and filtration experiments.

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2.2 Preparation and characterization of E-WS2 nanosheets

Bulk WS2 powder was dissolved in exfoliation medium (DMF) and sonicated for 5 h

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at a rate of 3000 rpm at room temperature. After that the supernatant part was collected, filtered and dried at room temperature to obtain exfoliated E-WS2nanosheets [23]. To find the

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crystallography of the E-WS2sheets a P-XRD diffractometer (Bruker, ECO D8 advance) was

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employed at a scan rate of 1° per minute with the 2θ for the range of 10 to 70° using Cu Kα radiation. To identify the complete reduction of WO3, the percentage of major elements in the

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E-WS2 sheets was determined using EDXS (bruker XL30 FEG). 2.3 Preparationof CA and CA/E-WS2 nanocompositeUF membranes

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The CA polymer (17.5%) without E-WS2 nanosheets additive is added with polar DMF solvent and kept under constant stirring for 4h at 90°C. The obtained blend solution was cast on a glass plate using a doctor’s knife. Then the casted polymer dope was kept 30 s for evaporation of solvent and immersedalong with the glass plate into gelation bath containing distilled water, 2wt.% of DMF and 0.2wt. % of surfactant (SLS) for 1 h at 25°C. Then membrane was thoroughly washed with distilled water for the removal of surfactant. Finally it is stored in de-ionized water consisting of 0.1wt.% formalin solution to prevent

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ACCEPTED MANUSCRIPT growth of microorganisms. Thickness of the membranes was determined using digimatic micrometer (mitutoyo, MDC-25B, Japan) and it was kept at 0.22±0.02mm. To prepare CA/E-WS2 nanocomposite membranes initially a predetermined amount of E-WS2nanosheets mixed separately with DMF solvent and kept under ultra-sonication for 2 h to attain the complete dispersion of E-WS2 in the solvent [24]. Then the CA was mixed to

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the above obtained homogeneous mixture and it was stirred for 4 h at 90o C. Afterwards, the

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procedure was followed as it carried out for fabrication of control CA membrane. Polymer,

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additive and solvent blend composition of pure CA and CA/E-WS2 nanocomposite membranes are given in Table 1.

Membrane

Polymer (wt.%) (CA)

CA

17.5

CA – 0.25

17.25

DMF

0

82.5

0.25

82.5

17

0.5

82.5

16.5

1

82.5

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CA –1

(E-WS2)

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CA – 0.5

Additive (wt.%) Solvent (wt.%)

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Blend composition of control and nanocomposite CA membranes

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Table 1

2.4 Characterization of pure CA and CA/E-WS2 nanocomposite membranes

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2.4.1 XRDand FT-IR

The X-ray diffraction peaks of control and nanocomposite CA membranes were analyzed by XRD (Bruker, ECO D8 advance) diffractometer with Cu Kα radiation at a scan range of 10 to 80°. FT-IR analysis was performed at a range of 600-4000 cm-1using Bruker Optik GmbH, Germany-TENSOR 27) to confirm the presence of E- WS2 nanosheets in the nanocomposite membranes.

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ACCEPTED MANUSCRIPT 2.4.2 SEM and contact angle The cross-section morphologies of the pure CA, CA-0.25, CA-0.5 and CA-1 nanocomposite membranes were analyzed with a scanning electron microscope (SEM) (FEI Quanta 250, Eindhoven, Netherlands) under vacuum conditions. Prior to SEM analysis sampleswere sputtered with a thin conductive layer of gold. Surface hydrophilicity of the

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membranes was measured by contact angle study using VCA Optima surface analyzing

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system, AST Products Inc., Billerica, MA.

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2.4.3 Porosity

The effect of E-WS2 nanosheets in the porosity of the membrane structure was

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evaluated by gravimetric method. Initially membranes were dried in oven at 50° C for 24 h, after that each sample was broken into five small pieces (1cm×1cm) and weighed,

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subsequently immersed in de-ionized water at 25°C for 24 h. By measuring the average value

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of wet and dry weights, the porosity was calculated by equation (1) [25].

Where, M1 and M2 are the weight (g) of the wet and dry membranes respectively,

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is the density of water (0.998g/cm3), ‘A’ is the area of the membrane (41.8 cm2) and ‘L’ is the membrane thickness (0.02cm) in wet state. 2.4.4 Pure water flux and hydraulic resistance The control and nanocomposite membranes were cut into the effective membrane area of 38.5 cm2and fit in to the UF batch cell (Amicon8400, Millilpore Corp., Bedford, MA) with a Teflon-covered magnetic paddle. The feed water was filled in the cell and stirred well using magnetic stirrer at a rate of 300rpm. The membranes were initially compacted at a

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ACCEPTED MANUSCRIPT transmembrane pressure of 414 kPa and followed by kept at 345 kPa to find its pure water flux, measured byequation (2) [26].

membrane area (cm2). The resistance (

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Where, Q is the quantity of permeate collected (L), ∆t is the sampling time (sec) and A is the to hydraulic pressure of each membrane was

versus the transmembrane pressure

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calculated from the slope of pure water flux (

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determined at different transmembrane pressures (69, 138, 207, 276, 345 and 414 kPa) and

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difference (∆P) by equation (3) [27].

2.4.5 BSA rejection

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Protein solution was prepared by mixing 1000 mg of BSA protein per litre of

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phosphate buffer (0.2M Na2HPO4 and NaH2PO4 at pH 7.2) and used as a feed in the rejection and fouling experiments. The rejected BSA percentage was determined on the basis of

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absorbance at 280 nm using UV-Vis spectroscopy (Systronics, 2201, Ahmedabad, India)

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using equation (4) [28].

Where Cp and Cf are represented the concentration of permeate and concentration of feed.

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ACCEPTED MANUSCRIPT 2.4.6 FRR To find the antifouling properties of control and CA nanocomposite membranes, the flux recovery ratio (FRR) was also computed with the help of the following procedure and

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expression:

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All the membranes are washed cleanly using de-ionized water for 30 min after BSA for each

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rejection test [29]. Then water flux was measured as done in the measurement of washed membrane. The measured water flux is designated as

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2.4.7 Reversible and irreversible fouling

During the filtration experiments the BSA might be diffused into the pores of the

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membranes, are usually irretrievable by back washing. Therefore such type of fouling

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occurred internally within the membrane pores was desired to assess the antifouling

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properties by the equation (6) [30].

The BSA might also be retained on the membrane’s surface known as reversible fouling. It can be minimized by easy surface cleaning and calculated by the equation (7) [31].

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ACCEPTED MANUSCRIPT 3 Result and discussion 3.1 XRD and EDXS study of E-WS2sheets The XRD pattern of the prepared E-WS2 shown in Fig 1.From the results it was observed that the E-WS2 sheets have inorganic fullerene like structure and the peak at 2θ angle of 25.4º is corresponding to (111) plane [32]. The peak observed at this angle was

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found to be identical with the standard pattern of an orthorhombic WO3.H2O [33]. Thus the

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prepared E-WS2 sheets have also contains the WO3.H2O were indicated that all theWO3 nano

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particles were not completely reduced to WS2. To establish the incomplete reduction of WO3 nanoparticles the EDXS analysis was also conducted. The average value of composition of

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the E-WS2 is given in Table 2. From the table it could be inferred that the E-WS2 sheets contains lower weight percentage of 21.34±0.3 wt.% for sulphur element than the theoretical

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weight percentage of 25.8. Hence EDXS result was strongly supported that the presence of

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WO3 nanoparticles in the synthesized E-WS2.

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Table 2 Composition of the main elements in E-WS2

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Main elements in E-WS2

Composition (Wt.%)

W

73.64± 0.9

S

21.34 ± 0.3

O

5.02 ±1.2

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Fig. 1 XRD pattern and EDXS of E-WS2 sheets.

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3.2 Characterization of control and nanocomposite CA membranes 3.2.1 XRD and FT-IR

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The XRD pattern of pure CA, CA-0.25, CA-0.5 and CA-1 is shown in Fig 2. All the

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prepared membranes showed a peak at 2θ angle of 14º corresponds to (002) plane of WS2 except pure CA. Thus it was established that the incorporated E-WS2 nanosheets were present

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in the CA membrane matrix.

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Fig. 2 XRD pattern of CA, CA-0.25 CA-0.5 and CA-1 membranes.

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Fig. 3 FT-IR spectrum of CA, CA-0.25, CA-0.5 and CA-1 membranes.

FT-IR spectrum of control and nanocomposite CA membranes is shown in Fig 3. Pure CA membrane exhibited characteristic peaks at 1737, 1369, and 1222 cm-1 for C=O, C-CH3

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and C-O-C stretching frequencies respectively [34]. All the IR absorptions frequency of CA

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is almost identical to those of nanocomposite CA membranes. The peaks corresponding to sulphur containing groups are not observed in the IR absorption bands of CA nanocomposite membranes. It was due to the complete uniform distribution of E-WS2 nanosheets over CA matrix [35]. 3.2.2 Scanning electron microscopy The cross section images of pure and nanocomposite CA membranes are shown in Fig. 4.Nanocomposite membranes displayed a typical macrovoid structure [36, 37]. The EWS2 nanosheets present in the CA matrix decreased the thermodynamic stability of the 12

ACCEPTED MANUSCRIPT casting solution when it is immersed in waterand results in the rapid phase de-mixing and a consequence of this macrovoids are formed [38]. It was noted that there is a significant differences in the size and number of macrovoids formation in the CA-0.25, CA-0.5 and CA1 nanocomposite membranes. At 0.25 wt. % and 0.5 wt. % of E-WS2 nanosheets, greater the size and numbers of macrovoids are formed. At this low concentration of E-WS2 in the

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polymer matrix caused a hindrance in lowering the interaction between solvent and polymer

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[39], as a result of this, a large number of macrovoids are formed in the nanocomposite

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membranes. However the macrovoids formation at 1 wt.% concentration of E-WS2 nanosheetsis lowered, because at this stage the casting solution were reached to the highly

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viscous state, which acted as an obstacle for the water molecules diffusion through the

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casting polymer solution [40,41].

Fig. 4 SEM cross section images of control CA (a), CA-0.25 (b), CA-0.5 (c) and CA-1 (d) membranes. 3.2.3 Contact angle, porosity, pure water flux and hydraulic resistance The images of the contact angle were provided in Fig. 5 and the values are given in Table 3. The contact angle values are decreased from pure CA to CA-1. It was signified that

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ACCEPTED MANUSCRIPT surface hydrophilicity of CA membrane was enhanced with the increasing concentration of E-WS2 nanosheets. The enhancement of hydrophilicity with the increase of nanoheets could be related to the formation of clusters. The clusters formed by the hindrance aroused due to the agglomeration of nanosheets in the membrane matrix increased the size of surface pores of the membranes [42]. Hence hydrophilicity was increased with augment of E-WS2

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nanosheets concentration. The improvement of hydrophilicity in the CA membrane matrix

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could also be related to the presence of small amount of WO3.H2O in the incorporated E-WS2

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nanosheets [43, 44]. The existence of WO3.H2O particles was confirmed by XRD and EDXS measurement as it has higher affinity towards water molecules increased the hydrophilicity of

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the nanocomposite membranes.

Fig. 5 Contact angle of (a) CA, (b) CA-0.25 (c) CA-0.5 and (d) CA-1 membranes.

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ACCEPTED MANUSCRIPT Table 3 Hydrophilicity, permeation and fouling resistance characteristics ofcontrol and nanocomposite CA membranes. Contact

Porosity

PWF

Rm

Rr

Rir

FRR

SR

code CA

angle (°) 100.8 ± 2.8

(%) 10.3±0.2

(L m-2 h-1) 18.2±2.8

(kPa/l m-2h -1) 33.2±1.8

(%) 6.1±1.2

(%) 19.2±1.5

(%) 85.6±0.4

(%) 88.4±1.2

CA-0.25

94.7±2.5

13.4±0.3

67.8±1.9

21.4±0.9

15.2±0.9

12.1±0.8

91.4±1.2

90.8±0.4

CA-0.5

74.0±1.8

16.2±0.5

129.5±2.2

10.9±0.5

19.8±0.7

8.4±0.6

96.8±1.7

94.2±1.3

CA-1

65.1±1.6

14.3±0.3

107±2.3

19.3±1.2

23.4±0.2

6.3±0.5

99.2±0.8

97.2±2.8

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Membrane

It was shown from the Table 3 and Fig. 6 that highest porosity of 16.2±0.5% and

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water flux of 129.5±5.2 Lm-2h-1was obtained for CA-0.5 nanocomposite membrane. It was

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mainly noted that both the porosity and permeability of the CA membrane was decreased when the E-WS2 nanoparticles concentration beyond 0.5 wt. %. It was found that there was a

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strong relationship between the porosity and pure water flux of the membranes [45]. Upto 0.5

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wt.% of E-WS2 concentration, higher pure water flux is observed due to the enhancement of membrane hydrophilicity and the change in membrane structure. But pure water flux (PWF) of the membranes in which nanosheets concentration was beyond 0.5 wt. % was decreased

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although hydrophilicity still increases. The decrease in pure water flux was occurred as a

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consequence of changes in membrane morphology irrespective of the increase in hydrophilicity. At higher concentration of E-WS2 nanosheets, membrane pores are blocked due to the accumulation of nanocontents on the surface [46]. Hence the pure water flux of the membrane at 1.0% concentration of E-WS2 nanosheets was lowered.

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Fig. 6 Pure water flux and porosity of CA, CA-0.25, CA-0.5 and CA-1 membranes.

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It was clearly seen from Table 3 that the hydraulic resistance (Rm) was decreased with

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the increase of E-WS2 loading upto 0.5 wt.%. The decrease in Rm was due to the increase in macrovoids [46]. It was also measured that the Rm of the CA/WS2 membrane was enhanced

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after rising nanoparticles beyond 0.5%.The rise in hydraulic resistance might be happened because of the effect of pore plugging and reduction in the size of macrovoids [42].

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3.2.4 BSA rejection

The solute rejection is also one of the most important parameters for evaluating the antifouling ability of the membranes. As shown in Table 3 and Fig. 7 the BSA foulant rejection of E-WS2 incorporated membranes is better than that of pure CA membrane. The improvement in the rejection of BSA foulant could be achieved by the enhanced hydrophilicity. On the account of higher hydrophilicity the nanosheet added membranes

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ACCEPTED MANUSCRIPT opposed the interactions between their surface and BSA, thereby the undesired fouling was reduced [47-48]. 3.3 Antifouling properties

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3.3.1 Flux recovery ratio

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Fig. 7 BSA rejection (%SR), %FRR, reversible and irreversible fouling of CA, CA0.25, CA-0.5 and CA-1 membranes.

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In order to analyze the extent of flux recovery, after BSA fouling flux recovery ratio

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(FRR) was measured. It was shown from Fig. 7 and Table 3 nanocomposite CA membranes had the higher FRR compared to the pure CA membrane. For instance, CA-1 membrane displayed the highest FRR of 99.2±0.8%.The rise in the FRR on the addition of E-WS2 nanosheets was stimulated on the account of increasing hydrophilicity [49].

3.3.2 Reversible and irreversible fouling When the E-WS2 nanosheets concentration kept at 1%, it was found that the CA-1 membrane had a lower irreversible fouling of 6.3±0.5% and higher reversible fouling of 17

ACCEPTED MANUSCRIPT 23.4±0.2% asshown in Fig. 7 and Table 3. It is implied that addition of E-WS2 nanosheets increased the hydrophilicity of the polymeric casting solution. Consequently a more hydrophilic membrane surface with more fouling resistance ability was obtained [50]. 4. Conclusions In this work, initially E-WS2 nanosheets was prepared and characterized by XRD and

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EDXS. Then purer CA and CA/E-WS2 UF membranes were fabricated and displayed the

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improved surface morphology, permeation and antifouling performance. SEM results

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indicated that upto CA-0.5 membrane (low E-WS2) showed larger macrovoids due to the hindrance effect of lowering the interaction between solvent and polymer whereas the

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suppression of macrovoids of the CA-1 membrane (high E-WS2) because at higher concentration of E-WS2, viscous state of the casting solution acted as a barrier for the

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macrovoids formation. With the successive addition of E-WS2 nanosheets from 0.25 to 1 wt.%, the hydrophilicity of the modified membranes were increased. The change in structure

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of the membranes and the presence of small amount of WO3.H2O were become responsible

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for the enhanced hydrophilicity of the modified membranes. Upto 0.5 wt.% of E-WS2 nanosheets in the polymer casting solution, hydrophilicity played an important role in

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increasing the water flux of the CA/E-WS2 membranes whereas at 1 wt.% of E-WS2 nanosheets the modified membrane caused flux decline due to the pore plugging effect

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irrespective of the hydrophilicity. Overall, due to the enhanced hydrophilicity, FRR, BSA rejection and lower irreversible fouling, the CA-1 nanocomposite UF membrane demonstrated excellent antifouling properties and promising for use in water treatment.

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ACCEPTED MANUSCRIPT 35. S. K. Kim, J. J. Wie, Q. Mahmood, H. S. Park, Nanoscale, 2 (2014) 7430. 36. A. Rahimpour, S. S. Madaeni, A. H. Taheri, Y. Mansourpanah, J.Membr. Sci. 313 (2008) 158–169. 37. A. Sotto, A.Boromand, R. Zhang, P. Luis, J. M. Arsuaga, J. Kim, B. Van der Bruggen, J. Colloid Interface Sci. 363 (2011) 540–550.

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ACCEPTED MANUSCRIPT 49. S. Vetrivel, M. Sri Abirami Saraswathi, D. Rana, A. Nagendran, Int. J. Biol. Macromol. 107 (2018) 1607-1612. 50. M. Sri Abirami Saraswathi, D. Rana, A. Subbiah, A.Nagendran, New J. Chem.41

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(2017) 14315-14324.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights 

Novel CA/E-WS2 nanocomposite UF membranes were fabricated effectively.

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CA/E-WS2 membranes displayed superior hydrophilicity and enhanced BSA rejection. Antifouling CA/E-WS2 membranes showed higher FRR and lower irreversible

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fouling.

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CA/E-WS2 membranes are promising for potential use in the water treatment.

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