PVDF nanocomposite membrane

Accepted Manuscript Effect of different additives on the physicochemical properties and performance of NLDH/PVDF nanocomposite membrane Samira Arefi Oskoui, Vahid Vatanpour, Alireza Khataee PII: DOI: Reference:

S1383-5866(18)31850-1 https://doi.org/10.1016/j.seppur.2018.09.039 SEPPUR 14937

To appear in:

Separation and Purification Technology

Received Date: Accepted Date:

29 May 2018 12 September 2018

Please cite this article as: S. Arefi Oskoui, V. Vatanpour, A. Khataee, Effect of different additives on the physicochemical properties and performance of NLDH/PVDF nanocomposite membrane, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.09.039

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.

Effect of different additives on the physicochemical properties and performance of NLDH/PVDF nanocomposite membrane

Samira Arefi Oskoui, a Vahid Vatanpour,b Alireza Khataee a,c*

a

Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of

Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran b

Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran

c Department of Materials Science and Nanotechnology, Faculty of Engineering, Near East University, 99138 Nicosia, North Cyprus, Mersin 10, Turkey

*Corresponding author: Tel.: +98 41 33393165; Fax: +98 41 33340191 E–mail address: [email protected]

1

Abstract The main objective of this research was to introduce an appropriate additive for improving the properties and performance of the NLDH/PVDF nanocomposite ultrafiltration membrane. For this purpose, the effects of the different additives including hydrophilic polymers e.g. PVP (with molecular weight of 10000 and 29000) and PEG (with molecular weight of 1500 and 6000) and amphiphilic copolymer e.g. pluronic F-127 were investigated on the properties and performance of NLDH/PVDF nanocomposite membrane. The properties of the fabricated membranes were studied using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), atomic force microscopy (AFM) and water contact angle techniques. Moreover, the pure water flux, water flux of BSA solution and flux recovery ratio (FRR) were determined for fabricated membranes in order to investigate their permeability and antifouling property. The results indicated that there was an interaction between the NLDH nanolayers and additive molecules, and so the effect of both NLDH and additive should be simultaneously considered. The results obtained from analysis techniques indicated that the surface porosity, average surface pore size, surface hydrophilicity and cross-sectional morphology of the NLDH/PVDF nanocomposite membrane were efficiently improved by introducing 1 wt.% of PVP29000 to the matrix of the nanocomposite membrane. Furthermore, in the presence of PVP29000 as optimum additive, the NLDH/PVDF nanocomposite represented high pure water flux (702.2 L/m2 h), high water flux of BSA solution (119.3 L/m2 h) and good antifouling property (FRR of 73.41%).

Keywords: Ultrafiltration; Nanocomposite; Mg-Al nanolayered double hydroxide; Polyvinyl pyrrolidone; Polyethylene glycol.

2

1. Introduction Nowadays, membrane-based separation processes have received a great deal of attention owning to their remarkable advantages such as low energy consumption, cost-infectivity, easily scale up, and operation at ambient temperature [1, 2]. Among all type of membrane separation methods, ultrafiltration (UF) process which is classified in the intermediate range of nanofiltration and microfiltration has received growing attention in water and wastewater treatment, medical industry, food industry and biotechnology [3, 4]. Solutes with diameter of 1-100 nm can be retained by ultrafiltration membranes [3]. In the field of ultrafiltration technology, both inorganic materials e.g. ceramics and organic materials e.g. polymers can be used for fabrication of UF membranes. Ceramic UF membranes are of high cost and brittle, so a large part of commercial UF membranes is made of polymers. From different polymers which are suitable for UF membranes fabrication, polyvinylidene fluoride (PVDF) has been widely used for this purpose, due to its remarkable characteristics and properties including chemical resistance, mechanical strength and high thermal stability [5-7]. However, PVDF is a hydrophobic polymer with poor wettability which limit its further development and application in the field of membrane. Indeed, membranes with high hydrophobicity and poor wettability are susceptible to fouling, inducing high operation cost and restricting their wide application in industry [8]. Therefore, hydrophilicity improvement of PVDF membranes is of great importance in order to produce fouling-resistance membranes. Two main methods including surface modification, and blending modification can be used for hydrophilic enhancement in polymeric membranes. Blending modification technique can cause significant changes in the structure of the membrane, resulting in producing membranes with different properties. In the field of blending modification,

3

incorporation of inorganic nanomaterials in the matrix of polymeric membranes has voted considerable attention owning to simplicity and high efficiency [9-13]. Nanolayered double hydroxides (NLDHs) are a class of anionic clay with two dimensional structure, which can be generally represented by chemical formula of [M2+1-xM3+x(OH)2]x+[An-x/n. mH2O]x- [14]. In the positive charged layers of these materials, divalent (M 2+) and trivalent (M3+) cations are coordinated octahedrally by hydroxyl groups. The positive charge of the layers can be counter-balanced with the anions (An-) present in the interlayer gallery of the NLDHs. It should be noted that, water molecules are also present in the interlayer region along with the anions. The molar ratio of M3+/M2++M3+ determine the value of x in the general formula of NLDH. There is a relative weak interaction between the layers, inducing excellent expanding property to the NLDH. In recent years, NLDHs have received much interest due to their practical application in a wide variety of fields including catalysis, pharmaceuticals, adsorbent, electrochemical electrodes and ion exchangers [15, 16]. Moreover, high surface area, low toxicity, high hydrophilicity and good thermal stability of these materials make the NLDH an appropriate candidate for improving the performance of the polymeric membranes. In the field of blending modification, in addition to the inorganic nanoparticles, hydrophilic polymers and amphiphilic copolymers have been known as desirable additives for improving the performance and properties of polymeric membranes [16-18]. Our research review demonstrated that the effects of different hydrophilic polymers and amphiphilic copolymers on the properties and the performance of the PVDF membranes have been investigated. Nonetheless, to the best of our knowledge, there is not any report on investigating the effects of different hydrophilic polymers and amphiphilic copolymers on the performance and properties of PVDF-based nanocomposite membranes.

4

In this research, MgAl-CO32- was synthesized using co-precipitation method, followed by a hydrothermal treatment. The as-synthesized NLDH was then characterized using XRD and SEM analysis. In the next step, the prepared NLDH was incorporated to the matrix of PVDF membrane as inorganic nanofiller to improve the performance of this polymeric membrane, resulting in fabrication of NLDH/PVDF nanocomposite membrane. The main objective of this research was to investigate the effect of the different hydrophilic polymers: PVP 10000, PVP 29000, PEG 1500, PEG 6000 and amphiphilic copolymer (pluronic acid F127) on the characteristic and performance of NLDH/PVDF nanocomposite membranes. For this, the characteristics of the fabricated membranes were studied using SEM, AFM, EDX and contact angle measurement techniques. Moreover, the permeability, hydrophilicity and antifouling performance of the membranes were determined to investigate the effect of the mentioned hydrophilic polymers and amphiphilic copolymer on the performance of the NLDH/PVDF nanocomposite membrane. The effect of the NLDH concentration and also influence of solvent type on the performance and structure of the NLDH/PVDF nanocomposite membrane have been investigated by our research group before [19, 20]. In the continuous of our research, in this paper the effects of the different additives e.g. PVP, PEG and pluronic F-127 on the performance and structure of NLDH/PVDF nanocomposite ultrafiltration membrane have been investigated. The main objective of this research was to investigate the effect of different additives e.g. PEG and PVP with different molecular weights (as hydrophilic polymers) and Pluronic acid F-127 (as amphiphilic copolymer) on the characteristics and performance of NLDH/PVDF nanocomposite membranes. For this purpose, the characteristics (e.g. surface and cross-sectional morphology, surface roughness and hydrophilicity) and performance (e.g. pure water flux, water flux of BSA solution and FRR%) of nanocomposite membranes containing 0.5 wt.% of NLDH and 1 wt.% of

5

different additive were compared with nanocomposite membranes containing only 0.5 wt.% of NLDH and with bare PVDF membranes without NLDH and additive. The obtained results are discussed in the following section.

2. Experimental 2.1. Chemicals MgCl2.6H2O, AlCl3, NaOH, Na2CO3, N-methyl-2-pyrrolidone (NMP), polyethylene glycol (PEG, MW= 1500 and 6000 g/mol) and pluronic F-127 were purchased from Merck Co., Germany. Bovine serum albumin (BSA) and polyvinyl pyrrolidone (PVP, MW = 10000 and 29000 g/mole) were purchased from Sigma-Aldrich Co. Polyvinylidene fluoride polymer (PVDF) was purchased from Alfa Aesar, Germany.

2.2. Synthesis of Mg-Al NLDH In this research, Mg-Al NLDH which was used as a nanofiller, was synthesized using facile coprecipitation and subsequent hydrothermal route [21, 22]. For this purpose, 14 mmoles of MgCl2.6H2O and 7 mmoles of AlCl3 were dissolved in 70 ml of distilled water. The obtained solution was added into a basic solution containing NaOH (0.15 mol/L) and Na 2CO3 (0.013 mol/L) under intensive stirring. The stirring was continued for another 20 min, and then the resultant slurry was centrifuged. In the next step, the collected sediment was dispersed in 280 mL of distilled water under ultrasonic irradiation. Subsequently, the obtained suspension was transferred into a stainless steel autoclave with a Teflon lining and treated hydrothermally for 16 h at 100°C. Finally the crystallites of the Mg-Al-NLDH were assembled using centrifuge and dried at 50°C.

6

2.3. Preparation of NLDH/PVDF nanocomposite membranes In this research, simple phase inversion method was applied for fabricating the NLDH/PVDF nanocomposite membranes in the form of flat sheet [10, 23]. For preparing the polymeric dope solution, firstly, 0.5 wt.% of Mg-Al NLDH was dispersed in NMP as solvent under ultrasonic irradiation. Then, 18 wt.% of PVDF and 1 wt.% of appropriate additive including PEG1500, PEG6000, PVP10000, PVP 29000 or pluronic F-127 were added into the as-prepared suspension under stirring at 50 ºC. In order to achieve a homogeneous dope solution in which the polymer and additive has been ompletely dissolved, the suspension was stirred for 24 h at 50 ºC. The dope solution was kept in oven at 50 ºC for 6 h for removing the air bubbles. The prepared degassed dope solution was casted on the polyester nonwoven fabrics using a casting knife with the thickness of 150 µm. Instantly after casting, the casted membrane was drowned in a water bath containing tape water at room temperature for immersion precipitation. All the fabricated membranes were stored in distilled water in closed container until being used. 2.4. Pure water permeation, BSA filtration, antifouling performance and rejection of the membranes In this research, dead-end filtration cells with an effective area of 19.6 cm2 were applied for determining the permeation of the fabricated membranes toward the pure water and BSA solutions. In addition, these cells were used for performing the protein rejection tests. To decrease the compaction effects and to reach a steady permeation flux, the prepared membranes were pre-compacted by passing pure water through the membranes under pressure of 0.3 MPa for 30 min. The pure water flux and water flux of BSA solution of the prepared membranes were determined according to Eq. 1 at operating pressure of 0.2 MPa:

7

J

dV Adt

(1)

where J stands for flux of permeate (L/m2 h). V, A and t stand for permeate volume (L), membrane surface area (m2) and time of the permeation (h), respectively. To determine the error, at least four membrane samples were tested and the average value was reported. The protein rejection (R) of the fabricated membranes was determined according to Eq. 2:

R %  (1 

Cp )  100 Cf

(2)

Where Cp and Cf shows the concentration of protein (g/L) in permeate solution and feed solution, respectively. The concentrations of protein in permeate and feed were determined using Bradford method [24]. In order to investigate the antifouling property of the fabricated membranes, flux recovery ratio (FRR) was calculated for fabricated membranes. For this purpose, after filtration of pure water for 90 min and determining the pure water flux (Jw,1), BSA protein solution was replaced as a model foulant with concentration of 0.5 g/L, and was filtered for 90 min at the operating pressure of 0.2 MPa. The flux of the BSA protein solution (Jp) was calculated using Eq. 1. In the next step, the membranes were removed from cells, washed with distilled water and left in distilled water for 20 min. Subsequently, the membranes were loaded on cells and second pure water was filtered for 90 min. The flux of pure water after protein filtration, named Jw,2, was calculated using Eq. 1. The parameter of FRR, representing the fouling-resistant ability of membranes was calculated using Eq. 3:

FRR%  (

J w ,2 )  100 J w ,1

(3)

8

2.5. Characterization analysis X-ray diffraction D5000 diffractometer (Germany), with monochromatic high intensity Cu Kα radiation (l = 1.5406 Å), an accelerating voltage of 40 kV, and an emission current of 40 mA was used for investigating the crystallite structure of prepared Mg-Al NLDH. For investigating the morphology of the prepared NLDH and also the cross-sectional and surface morphology of the fabricated membranes a Tescan SEM Model Vega (Czech Republic) was applied. Digimizer software was used to determine the thickness and surface porosity of the fabricated membranes. The distribution of NLDH in the matrix of fabricated nanocomposite membranes was studied using energy dispersive X-ray (EDX) analysis by applying INCA (England) instrument. The hydrophilicity of the fabricated membranes was studied using contact angle analysis. In this study, the average of at least four contact angles was reported for each membrane. In order to investigate the surface morphology and surface roughness parameters of the fabricated membranes, atomic force microscopy analysis was used. A Nanosurf Mobile S (version 1.8) scanning probe-optical microscope (Switzerland) was applied to determine the average roughness (Sa), the root mean square of the Z data (Sq) and the mean difference between the highest peaks and the lowest valleys (Sy).

3. Results and discussion 3.1. Characterization of the as-synthesized Mg-Al NLDH The crystallite structure of the as-synthesized NLDH was investigated using XRD analysis. The characteristic peaks centered at 2θ of 11.4°, 23.0°, 35.1°, 38.7°, 45.5°, 60.5° and 62.0° confirmed that the prepared Mg-Al CO32- has been synthesized in typical nanolayered double hydroxide structure (see Fig. S1 in supporting information) [25]. Based on the peak centered at 2θ of 11.4°

9

as sharpest peak and using the Debye-Sherrer’s equation, the average crystallite size of asprepared NLDH was determined to be 14.6 nm [26]. The SEM image of the as-synthesized MgAl-CO32- is represented in Fig. 1. As can be seen, the prepared NLDH consists of homogeneous nanosheets with approximately width diameter of 38 nm.

3.2. Effect of additive type on the characteristics of NLDH/PVDF nanocomposite membranes 3.2.1. Morphology of the membranes The surface and cross-sectional morphologies of the fabricated NLDH/PVDF nanocomposite membranes were studied using SEM analysis. The surface SEM images and the pore size distribution diagrams of the fabricated membranes are represented in Fig. 2 and Fig. 3. As presented in Fig. 2, an approximately dense layer with low surface porosity can be observed for bare PVDF membrane without any additives and NLDH. Comparing the surface SEM image of NLDH/PVDF nanocomposite membrane presented in Fig. 3 with that of bare PVDF shown in Fig. 2, and considering the surface porosity values reported in Table 1, revealed that by introducing 0.5 wt.% of NLDH as a hydrophilic inorganic nanofiller into the matrix of PVDF membrane, the surface porosity and the average pore diameter were enhanced. The observed enhancement can be ascribed to the hydrophilic property of NLDH, which was resulted from the presence of a large number of hydroxyl groups on the layers of the NLDH. Indeed, the presence of hydrophilic nanofiller in the dope solution increases the phase inversion rate which consequently results in increasing the surface porosity and average surface pore diameter [27, 28].

10

Similar results were obtained by adding PVP10000 and PVP29000 as hydrophilic polymers to the matrix of PVDF membranes. As reported in Table 1, the surface porosity of PVDF membrane was increased from 1.30% to 2.32% and 2.98% by adding 1 wt.% of PVP10000 and PVP29000 to the matrix of the PVDF membrane, respectively. In addition, the results of surface pore diameter of PVP10000/PVDF and PVP29000/PVDF membranes presented in Fig. 2, demonstrated that the average pore size diameter was increased by adding PVP10000 and PVP29000. Moreover, the results showed that the average pore diameter and surface porosity were increased by increasing the molecular weight of PVP from 10000 to 29000. Certainly, the mobility of the PVP chains is limited by increasing the molecular weight due to the enlargement of the molecular chains. In addition, the higher molecular weight is, the less solubility of it is resulted. Therefore, more of PVP will be trapped in the dope solution by increasing the molecular weight. Considering that the PVP is a hydrophilic polymer, presence of a large amount of it in the dope solution can increase the phase inversion rate, resulting in enhancing the surface porosity [29, 30]. On the other hand, PVP is highly soluble in water and can act as a pore former in fabrication of polymeric membranes. Consequently, some of the entrapped molecules of PVP leave the solid polymer and are dissolved in water, which results in forming pores on the surface of polymeric membranes [30]. According to the discussed reasons, PVP with higher molecular weight and larger molecular size form larger pores. Comparing the surface SEM images of PVP10000/NLDH/PVDF and PVP29000/NLDH/PVDF with SEM image of NLDH/PVDF in Fig. 3 revealed that by adding PVP10000 and PVP29000 to the matrix of the NLDH/PVDF nanocomposite membrane both surface porosity and average pore diameter were increased. These results demonstrated that the simultaneous presence of PVP10000 and PVP29000 along

11

with NLDH could enhance the surface porosity of the membrane, with no negative interaction between the PVP molecules and layers of NLDH. Comparing the surface SEM images of the PEG1500/PVDF and PEG6000/PVDF membranes (Fig. 2) with surface SEM image of bare PVDF (Fig. 2) showed that by adding PEG as hydrophilic polymer to the matrix of the PVDF, the surface porosity and average pore diameter were increased. The results reported in Table 1 showed that by adding 1 wt.% of PEG1500 and 1 wt.% of PEG6000 to the matrix of the PVDF membrane, the surface porosity was enhanced from 1.30% to 1.72% and 2.13%, respectively. Indeed, the presence of PEG molecules as a hydrophilic polymer in the dope solution increases the phase inversion rate, which consequently results in enhancing the surface porosity of the membrane. Moreover, the addition of PEG1500 to the matrix of the NLDH/PVDF nanocomposite membrane resulted in enhancing the surface porosity and average pore diameter (see Fig. 3 and Table 1). The results showed that despite adding of PEG1500 to the matrix of NLDH/PVDF nanocomposite membrane (which enhanced the surface porosity and average pore size), adding of PEG6000 to the matrix of the NLDH/PVDF nanocomposite membrane resulted in a significant reduction in surface porosity and average pore diameter. The observed decline in surface porosity (Table 1) and average pore diameter (Fig. 3) can be ascribed to the rheological domination. Indeed, by adding hydrophilic polymers to the dope solution as additives, the phase inversion process can be thermodynamically and rheologically affected. The hydrophilic polymers can affect the phase inversion process thermodynamically via increasing the rate of phase inversion process, resulting in enhancing the surface porosity [8, 31]. On the other hand, if the viscosity of the dope solution increased significantly by adding hydrophilic polymer, the phase inversion process could be affected by rheological feature [32-34]. The enhancement in

12

rheologic feature leads to the fabrication of a polymeric membrane with dense structure, low surface porosity and low average pore diameter. Furthermore, by adding 1 wt.% of PEG6000 as a high molecular weight additive to the dope solution containing NLDH and PVDF, the viscosity of the dope solution increases significantly, resulting in fabrication of membranes with low surface porosity and low average pore diameter. In other words, by adding 1 wt.% of PEG6000 to the NLDH/PVDF dope solution, the rheological feature become dominant, which results in a membrane with dense structure (see surface SEM image of the PEG6000/NLDH/PVDF in Fig. 3). According to the surface porosity values reported in Table 1, by adding PVP10000, PVP29000 and PEG 1500 into the matrix of the NLDH/PVDF nanocomposite the surface porosity was increased 48.59%, 165.42% and 17.28%, respectively. The results demonstrated that by adding 1 wt.% of Pluronic F-127 as an amphiphilic copolymer to the matrix of PVDF membrane, the surface porosity and average pore diameter were not changed significantly and only negligible improvement was observed (compare surface SEM image of pluronic F-127/PVDF with that of PVDF in Fig. 2). Pluronic F-127 is a triblock amphiphilic copolymer consisting of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO) parts in the form of PEO-PPO-PEO [18, 35]. By introducing the Pluronic F-127 as the additive to the matrix of the polymeric membrane, the hydrophobic parts (PPO) mained connected to the matrix of the polymer, whereas the hydrophilic PEO part extended toward the water at the interface of membrane water owning to its hydrophilicity. The literature review [36, 37] demonstrated that Pluronic F-127 could not act as a pore former in some polymeric membranes and has been used as a surface modifier in order to improve the antifouling property of the polymeric membranes. As aforementioned, the Pluronic F-127 was not able to act as a pore former in PVDF membrane which is in good agreement with the results of

13

other researches [36, 37]. However, comparing the surface SEM image of Pluronic F127/NLDH/PVDF membrane with that of NLDH/PVDF (Fig. 2) revealed that by adding Pluronic F-127 into the matrix of nanocomposite membrane, the surface porosity and the average pore diameter were increased. Therefore, unlike the PVDF membranes, the Pluronic F-127 could act as a pore former in the NLDH/PVDF nanocomposite membrane. Simultaneous presence of hydrophilic NLDH and hydrophilic parts of the Pluronic F-127 in the matrix of the PVDF membrane can significantly increase the rate of the phase inversion and results in observed surface porosity and average pore diameter enhancement [38]. According to the surface porosity values reported in Table 1, by adding PVP10000, PVP29000, PEG 1500 and Pluronic F-127 into the matrix of NLDH/PVDF nanocomposite, the surface porosity was increased 48.59%, 165.42%, 17.28% and 15.88%, respectively. Therefore, it seems that among the investigated additives, PVP 29000 is the most suitable additive for improving the surface porosity of NLDH/PVDF nanocomposite membranes compared with PVP10000, PEGs and Pluronic F-127. The cross-sectional SEM images of the PVDF and NLDH/PVDF nanocomposite membranes containing different additives are represented in Fig. 4 and Fig. 5. As can be seen in Fig. 4, sponge-like pores were formed in the cross-section of bare PVDF membrane. Comparing the cross-sectional SEM images of NLDH/PVDF nanocomposite membrane in Fig. 5 with that of bare PVDF membrane in Fig. 4 disclosed that the sublayer morphology of the PVDF membrane was significantly affected by adding NLDH as a hydrophilic nanofiller. The change of spongelike pores to the macro finger-like pores by adding 0.5 wt.% of NLDH can be attributed to the enhancement in the phase inversion rate in the presence of hydrophilic NLDH in the dope solution [8]. However, as can be observed in Fig. 5, the formed finger-like pores were not well

14

developed in the cross section of NLDH/PVDF membrane from top layer to bottom, and some sponge-like pores can be observed in the bottom part. The results indicated that by adding PVP10000 and PVP29000 as hydrophilic polymers to the PVDF dope solution, the sponge-like pores were transformed into the macro finger-like pores developed in cross section of membrane (see Fig. 4). In addition, the cross-sectional SEM images represented in Fig. 5 demonstrated that the presence of PVP 10000 and PVP29000 in the matrix of NLDH/PVDF nanocomposite membrane resulted in the developing of finger-like pores in the cross section of nanocomposite membranes from top to bottom. Therefore, the cross-sectional porosity of the NLDH/PVDF nanocomposite membranes was improved in the presence of hydrophilic PVP polymers in both 10000 and 29000 molecular weights. Similar to the PVP polymers, the sponge-like pores of PVDF were altered to macro finger-like pores in the presence of PEG1500 and PEG 6000 as hydrophilic polymers (see Fig. 4). Moreover, cross-sectional SEM image in Fig. 5 demonstrated that the addition of PEG1500 and PEG6000 to the NLDH/PVDF nanocomposite dope solution resulted in the formation of welldeveloped macro finger-like pores in the cross section of these membranes. The cross-sectional SEM images of Pluronic acid/PVDF and Pluronic acid/NLDH/PVDF membranes presented in Fig. 4 and Fig. 5, respectively, demonstrated that the presence of Pluronic F-127 as an amphiphilic copolymer in dope solution lead to the formation of welldeveloped macro finger-like pores in cross section of the mentioned membranes. The dispersion of the NLDH in the matrix of the NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives was investigated using EDX-mapping technique. As can be observed in Fig. 5, NLDH was homogeneously dispersed in the matrix of the NLDH/PVDF nanocomposite membrane containing different additives e.g. PVP, PEG and Pluronic F-127.

15

The thickness of the fabricated membranes was determined using cross-sectional SEM images and Digimizer software, and the results are reported in Table 1. Although a casting knife with the thickness of 150 µm was used for casting the polymeric dope solution on the nonwoven, the results demonstrated that the thickness of fabricated membranes varied in the range of 20 µm115 µm. Based on the membrane formation kinetics, the casted membrane was immediately shrinked when immersed in a nonsolvent bath and contacted with molecules of nonsolvent [39]. The rapid outflow of solvent from the casted membrane was the main reason for the membrane shrinkage [39]. In the next step, when the phase separation was occurred, the macrovoid structure and finger-like pores growth in sublayer, resulted in the bulking the casted membrane. By solidification of the polymer, the bulking of the casted membrane stopped. Hence, the fast rate of phase separation results in the formation of macro pores in cross section of membranes which leads to the increased the thickness of the membranes [39]. The thickness of bare PVDF membrane without any additive and nanofiller was found to be 20.9 µm. As can be seen in Table 1, this membrane with sponge-like morphology showed the highest shrinkage and lowest membrane thickness. This can be attributed to the hydrophobicity of the PVDF polymer which resulted in the low phase separation rate and consequently, induced sponge-like morphology with low thickness. From Table 1, the thickness of the membrane was significantly increased from 20.9 µm to 65.2 µm by embedding the NLDH as the hydrophilic nanofiller. Similar results was obtained by embedding PVP, PEG and Pluronic F-127 as an additives into the dope solution of PVDF. Indeed, in the presence of hydrophilic additives, the molecules of the water as the nonsolvent diffused the dope solution of casted membrane very fast, resulting in a rapid phase separation. As aforementioned, based on the kinetics, the fast diffusion of the non-solvent and consequently rapid phase separation favors the formation of more pores with larger sizes before

16

solidification [39]. Thickness enhancement in the PVDF membrane in the presence of NLDH and different additives e.g. PVP, PEG and pluronic acid F-127 showed the formation of macro pores in the cross section and porosity enhancement of membrane in sublayer. The results demonstrated that among the different additives used in this research, the most thickness enhancement and consequently, the highest sublayer porosity were accomplished by adding PVP into the PVDF dope solution, which can be ascribed to the its high hydrophilicity compared with the PEG and Pluronic F-127 (the hydrophilicity of the used additive is compared in section 3.2.2).

Comparing the

membrane thickness

of PVP10000/PVDF

(100.2

µm)

and

PVP29000/PVDF (105 µm) demonstrated that by increasing the molecular weight of PVP, the size of the finger-like pores were increased, resulting in enhancing the membrane thickness. In general, the solubility of the polymers decreased by increasing the polymer molecular weight, resulting in trapping more polymer molecules in dope solution when casted membrane immersed in a non-solvent bath and contacted with water molecules. The presence of high amount of hydrophilic polymer molecules in the dope solution can increase the phase separation rate by increasing the rate of water molecules diffusion and favors the formation of macro finger-like pores with large size. Therefore, the high thickness of membrane in the presence of PVP29000 compared with PVP10000 can be ascribed to the given explanations. In addition, the results presented in Table 1 demonstrate that the addition of both PVP10000 and PVP29000 to the NLDH/PVDF nanocomposite dope solution resulted in increasing the membrane thickness and consequently sublayer porosity. This demonstrates that the simultaneous presence of NLDH as the hydrophilic nanofiller and PVP as the hydrophilic polymer in dope solution can significantly increase the phase separation rate, resulting in improving the sublayer porosity. The results show

17

that the membrane thickness was increased from 20.9 µm to 46.9 µm and 84.3 µm by adding PEG1500 and PEG6000 to the dope solution of PVDF, respectively. As can be seen, the membrane thickness and consequently sublayer porosity showed further enhancement by adding PEG with high molecular weight. This phenomenon was similar to that mentioned for PVP and can be interpreted as explained for PVP. The results showed that adding PEG1500 to the NLDH/PVDF dope solution increased the membrane thickness and the sublayer porosity. However, by adding the PEG with a higher molecular weight (PEG6000) into the NLDH/PVDF nanocomposite dope solution, the membrane thickness was significantly decreased from 65.2 µm to 45.3 µm. This can be attributed to the significant enhancement in nanocomposite dope solution viscosity by adding the high molecular weight of PEG, which results in forming dense structure in sublayer. As reported in Table 1, the membrane thickness and consequently sublayer porosity of the NLDH/PVDF nanocomposite enhanced from 65.2 µm to 108.7 nm by adding 1 wt.% of Pluronic F-127 as an amphiphilic copolymer.

3.2.2. Hydrophilicity of the membranes Hydrophilicity of the polymeric membranes is one of the most important parameters which can basically influence the permeation and antifouling properties of the membranes. This property can be evaluated by determining the contact angle of water drop. Moreover, the smaller contact angle of water drop equals to higher hydrophilicity of the membrane. The contact angles of PVDF membranes and NLDH/PVDF nanocomposite membranes prepared using different additives e.g. PVP, PEG and Pluronic F-127 are reported in Table 1. As can be seen, the contact angle of bare PVDF as a hydrophobic membrane was found to be 77.4° which significantly decreased to 73.8° by embedding 0.5 wt.% of NLDH as the hydrophilic nanofiller to the matrix

18

of membrane. The contact angle decrement of NLDH/PVDF nanocomposite membrane demonstrated the hydrophilicity and wettability improvement of PVDF membrane in the presence of NLDH. The high hydrophilic property of NLDH can be ascribed to the presence of numerous hydroxyl groups on the surface of NLDH layers, which can easily form hydrogen bond with water molecules [40, 41]. The contact angle results reported in Table 1 demonstrated that the contact angle was decreased by embedding the hydrophilic polymers (PVP and PEG) and amphiphilic copolymer (Pluronic F-127) to the matrix of the PVDF membrane, confirming the hydrophilicity improvement of polymeric membrane in the presence of mentioned additives. Based on the reported contact angles, the hydrophilicity order of the PVDF membranes containing different additives was in the order of PVP29000/PVDF> PVP10000/PVDF> PEG6000/PVDF> PEG1500/PVDF> pluronic F-127/PVDF. As a result, hydrophilic polymer of PVP could improve the hydrophilicity of the PVDF membrane more than other additives. Moreover, the contact angle of the NLDH/PVDF nanocomposite membrane was decreased from 73.8° to 69.8°, 67.3°, 70.1°, 68.5° and 71.5° by adding PVP10000, PVP29000, PEG1500, PEG6000 and Pluronic F-127 into the matrix of the nanocomposite membrane, respectively. These results show that hydrophilicity and wettability of the PVDF membrane can be significantly improved in the simultaneous incidence of the NLDH and additive e.g. PVP, PEG and Pluronic F-127. Furthermore, the results showed that the contact angle values were decreased by increasing the molecular weight of PVP and PEG. Generally, the mobility of the polymeric molecules in the dope solution is limited by increasing the molecular weight, due to polymeric chain enlargement. Therefore, by increasing the molecular weight of hydrophilic additives, more polymeric molecules are expected to be trapped in the matrix of the membrane during the phase inversion process, inducing more hydrophilicity to the fabricated membranes.

19

The research review [42-44] indicates that the contact angle of the porous surface can be affected by the mean pore size of the surface in addition to hydrophilicity/hydrophobicity of the surface. The results of these researches demonstrate that by increasing the average pore diameter of the porous surface, the apparent contact angle decreases. Therefore, the contact angle decrement of the PVDF membrane and NLDH/PVDF nanocomposite membrane resulted by adding 1 wt.% of different additives e.g. PVP, PEG and Pluronic F-127 (Table 1) can be attributed to average pore size enhancement (see section 3.2.1 ).

3.2.3. Surface roughness of membranes The surface morphology of the PVDF membranes and NLDH/PVDF nanocomposite membranes containing different additives e.g. PVP, PEG and Pluronic F-127 was investigated using AFM analysis. The three-dimensional AFM images of the fabricated membranes in the scale of 5 µm×5 µm are represented in Fig. 6. In these images, bright regions represent the highest point, while dark regions represent the valleys or pores of the membranes surface. The roughness parameters of the fabricated membranes were determined using the AFM images and the results are reported in Table 2. From Table 2, the surface roughness was increased by embedding 0.5 wt.% of NLDH into the matrix of the PVDF membrane, which can be attributed to the surface porosity and pore size enhancement resulted by embedding NLDH (compare the surface porosity of PVDF and NLDH/PVDF in Table 1). Similar results were obtained by adding 1 wt.% of PVP10000, PVP29000, PEG1500, PEG6000 and Pluronic F-127 into the matrix of the PVDF membrane. As shown in Table 1, surface porosity of the PVDF membrane was increased by adding mentioned additives, which resulted in enhancing the surface roughness. The order of the surface roughness increment for additive/PVDF membranes was found to be as follows:

20

PVP29000/PVDF> PVP10000/PVDF> PEG6000/PVDF> PEG1500/PVDF> Pluronic F127/PVDF. This order was in good agreement with the surface porosity values reported in Table 1. Also, surface roughness of the NLDH/PVDF nanocomposite membrane was increased by adding PVP10000, PVP29000, PEG1500 and Pluronic F-127. However, adding PEG6000 into NLDH/PVDF nanocomposite dope solution resulted in the formation of smooth surface with low surface roughness. As above-mentioned, the rheological feature of the NLDH/PVDF dope solution was dominated by adding 1 wt.% of PEG6000, resulting in formation of dense surface layer with low porosity which consequently resulted in decreasing the surface roughness. Research review [45-48] demonstrates that in addition to hydrophilicity/hydrophobicity of membrane, contact angle of membranes can be affected by surface roughness of membranes. The effect of the surface roughness on the contact angle can be represented by Wenzel model [46]: Cos R  RCos E

(4)

Where θE is an angle between a flat solid surface and a tangent line from a contact point to an air-liquid interface, θR is apparent contact angle on the rough surface (which is obtained as an experimental value), and R is the ratio of the actual area to projected area (the value of R can be determined using AFM analysis). This model indicates that θR would be lower than θ E (θR< θE) when the θE is less than 90°. Considering that the value of R is always bigger than unity for rough surface, the contact angle of surface is expected to decrease by increasing the surface roughness. As reported in Table 2, by adding different additives e.g. PVP, PEG and Pluronic F127 into the matrix of the bare PVDF membrane and NLDH/PVDF nanocomposite membrane, the surface roughness increased, which can affect the contact angles values. As reported in Table 1, the contact angle value of bare PVDF membrane and NLDH/PVDF nanocomposite membrane is 77.4° and 73.8°, respectively, which are lower than 90°. Therefore, according to the Wenzel

21

equation represented in Eq. 4, the surface roughness enhancement of PVDF and NLDH/PVDF nanocomposite membrane can result in decreasing the contact angle. Therefore, the contact angle decrement of PVDF membrane and NLDH/PVDF nanocomposite membrane in the presence of 1 wt.% of different additives e.g. PVP, PEG and Pluronic F-127 (reported in Table 2) can be attributed to the surface roughness enhancement in addition to hydrophilic nature of these additives. 3.3. Effect of the additive type on the performance of NLDH/PVDF nanocomposite membranes 3.3.1. Permeation flux of NLDH/PVDF nanocomposite membranes In order to study the influence of the additive types on the permeation of PVDF membrane and NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives e.g. PVP, PEG and Pluronic F-127, pure water flux was assessed for fabricated membranes, and the results are represented in Fig. 7. From the literature, the pure water flux of membrane is mainly influenced by different parameters such as surface and cross-sectional morphology, surface porosity, membrane thickness, hydrophilicity and surface roughness of membranes. From Fig. 7, pure water flux of the bare PVDF membrane was 291.2 L/m2 h, which was increased to 366.9 L/m2 h by embedding 0.5 wt.% of NLDH as the nanofiller. The observed permeability improvement of PVDF membrane in the presence of NLDH can be attributed to the surface porosity enhancement, hydrophilicity and surface roughness increment. Typically, more efficient surface area is provided for permeation of water molecules by increasing the surface roughness, resulting in increased pure water flux. In addition, the cross-sectional morphology change of PVDF membrane from sponge-like to finger-like in the presence of NLDH can be considered as the main reason for observed pure water flux improvement. Moreover, pure water flux

22

improvement of NLDH/PVDF nanocomposite membrane compared with bare PVDF can be attributed to the hydrophilicity improvement of the membrane in the presence of hydrophilic NLDH (see Table 1), resulting in enhancing the wettability and consequently, the pure water flux improvement. As can be seen in Fig. 7, the order of the pure water flux of PVDF membranes containing different

additives

was

PVP29000/PVDF>

PVP10000/PVDF>

PEG6000/PVDF>

PEG1500/PVDF> Pluronic F-127/PVDF, which was in good agreement with surface porosity values (reported in Table 1), surface hydrophilicity (reported in Table 1) and surface roughness parameters (reported in Table 2). Moreover, the results represented in Fig. 7 demonstrate that the order of the pure water flux of nanocomposite membranes containing different additives was as follow:

PVP29000/NLDH/PVDF>

PVP10000/NLDH/PVDF>

PEG1500/NLDH/PVDF>

Pluronic F-127/NLDH/PVDF, which is well corresponded to the results of surface porosity, surface hydrophilicity and surface roughness. However, the results showed that the pure water flux of the NLDH/PVDF nanocomposite membrane was decreased from 366.9 L/m2 h to 302.8 L/m2 h by adding 1 wt.% of PEG6000. The observed reduction in pure water flux of NLDH/PVDF nanocomposite membrane in the presence of PEG6000 can be attributed to the surface porosity, surface roughness and also sublayer porosity decrement resulted by rheological feature domination of NLDH/PVDF dope solution in the presence of PEG6000.

3.3.2. Protein rejection ability of NLDH/PVDF nanocomposite membranes The rejection ability of the fabricated membranes was investigated using BSA protein molecules as model molecules. The results indicated that the protein rejection ability of the membranes was

23

not decreased by adding different additives e.g. PVP, PEG and Pluronic F-127 into the matrix of the PVDF membrane and NLDH/PVDF nanocomposite membrane, and all fabricated membranes were able to reject the BSA molecules more than 98%. High protein rejection of the nanocomposite membranes containing different additives confirmed that embedding of PVP, PEG and Pluronic F-127 for improving the performance of the PVDF-based membrane did not affect the structure of the membrane negatively.

3.3.3. Flux recovery ratio study of NLDH/PVDF nanocomposite membranes The effects of different additives such as PVP, PEG and Pluronic F-127 was investigated on the antifouling performance of the NLDH/PVDF nanocomposite membranes. For this purpose, the flux of the BSA solution with the concentration of 500 mg/L and the FRR parameter was determined for fabricated membranes using Eq. 1 and Eq. 3, respectively. Fig. 8 represents the flux of BSA after 90 min of filtration. The water flux of BSA solution of the PVDF membrane was increased by embedding 0.5 wt.% of NLDH as hydrophilic nanofiller into the matrix of the membrane. The observed enhancement in water flux of BSA solution in the presence of NLDH can be mainly attributed to the hydrophilicity improvement of the PVDF membrane in the presence of hydrophilic layers of NLDH. Mainly, by increasing the hydrophilicity of the membrane, a thin layer of the water molecules is formed on the surface of the membrane which inhibits the adsorption of BSA molecules on the surface of the membrane [8]. As can be seen in Fig. 8, the order of the water flux of BSA solution for fabricated membrane is similar to the pure water flux. Therefore the explanations mentioned for order of pure water flux can be considered for interpreting the order of the water flux of BSA solution. The results represented in Fig. 8 showed that by embedding 1 wt.% of different additives e.g. PVP, PEG and Pluronic F-127 into

24

the matrix of PVDF membrane, the FRR parameter was improved in the following order: Pluronic

F-127/PVDF>

PVP29000/PVDF>

PVP10000/PVDF>

PEG6000/PVDF>

PEG1500/PVDF. The literature [49, 50] indicated that the fouling potential of the membranes with a rough surface is more than membranes with smooth surface. This can be attributed to the accumulation of foulant molecules in the valleys of the membrane with a rough surface. The results showed that despite the surface roughness enhancement of PVDF membrane in the presence of different additives, the antifouling performance of these membranes was improved. The observed antifouling improvement can be attributed to the hydrophilicity enhancement of PVDF membrane in the presence of applied additives. As can be seen, the highest FRR and also water flux of BSA solution were resulted from addition of Pluronic F-127 into the matrix of the PVDF membrane. Considering that the hydrophilicity improvement of the PVDF membrane by adding Pluronic F-127 was low compared with other additives, the significant improvement in antifouling performance of PVDF membrane in the presence of Pluronic F-127 cannot be attributed to the hydrophilicity enhancement. Pluronic F-127 is an amphiphilic copolymer consisting of both hydrophilic and hydrophobic parts. As mentioned earlier, by introducing the Pluronic F-127 as an amphiphilic additive to the matrix of the PVDF membrane, the hydrophilic PEO part of it was extended toward the water at the interface of membrane and water. However, the hydrophobic parts remained in the matrix of the membrane. Therefore, the PEO parts exhibiting on the surface of Pluronic F-127/PVDF membrane act as a brush layer and prevented the adsorption of the BSA molecules on the surface of the membrane, resulting in improving the antifouling performance. Therefore, the high antifouling performance improvement of the Pluronic F-127/PVDF membrane can be mainly attributed to the presence of PEO parts of the Pluronic F-127 on the surface of the membrane. However, the antifouling performance

25

improvement of other additive/PVDF membrane including PVP and PEG with different molecular weight was well correspond to the hydrophilicity parameter (see Table 1). Moreover, the results presented in Fig. 9 indicated that the FRR parameter of NLDH/PVDF nanocomposite membrane was enhanced by adding different polymeric hydrophilic additives into the matrix of the nanocomposite membrane. The order of the FRR parameter for NLDH/PVDF nanocomposite membranes was as follows: PVP29000/NLDH/PVDF> Pluronic F-127/NLDH/PVDF>

PEG6000/NLDH/PVDF>

PVP10000/NLDH/PVDF>

PEG1500/NLDH/PVDF. As can be seen, the NLDH/PVDF nanocomposite membrane showed the best antifouling performance by adding 1 wt.% of PVP29000, which can be attributed to the high surface hydrophilicity of this membrane. It should be noted that the order of the antifouling performance improvement of additive/NLDH/PVDF membranes was different from the order reported for additive/PVDF membranes, indicating that NLDH and the molecules of the additive simultaneously affect the antifouling performance of the membranes. Therefore, the influence of both of them should be simultaneously considered. The FRR of the PVDF membrane was increased 59.32% by adding Pluronic F-127, whereas only 3.44% enhancement was resulted for NLDH/PVDF nanocomposite by adding Pluronic F-127. The low FRR improvement of the nanocomposite membrane can be ascribed to the high surface roughness of the membrane in the presence of NLDH. Considering that the Pluronic F-127, as an amphiphilic copolymer, was not able to improve the surface hydrophilicity of the membrane (see Table 1), the high surface roughness of the NLDH/PVDF nanocomposite is the dominant parameter affecting the antifouling performance of the nanocomposite membrane, resulting in low improvement in antifouling performance.

26

In addition, the results showed that the FRR parameter of the NLDH/PVDF nanocomposite membrane was increased 13.79% by adding 1 wt.% of PEG6000, which is high compared with FRR enhancement of PVDF membrane in the presence of PEG6000 (8.33%). As aforementioned, the surface roughness of the NLDH/PVDF nanocomposite was decreased by adding 1 wt.% of PEG6000 into matrix of nanocomposite membranes. Therefore, surface roughness decrement resulted by adding PEG6000 into the matrix of the nanocomposite membrane induced high antifouling performance to the NLDH/PVDF nanocomposite membrane.

4. Conclusion In this research, the effect of the different additives including PVP (with molecular weight of 10000 and 29000) and PEG (with molecular weight of 1500 and 6000) as hydrophilic polymers and Pluronic F-127 as an amphiphilic copolymer was investigated on the structure and performance of the NLDH/PVDF nanocomposite membranes. Facile phase inversion method was applied for fabrication of NLDH/PVDF nanocomposite membranes. The surface and crosssectional morphology, NLDH distribution in matrix of nanocomposite membrane, surface roughness and hydrophilicity of the fabricated membranes were studied using SEM, EDX, AFM and contact angle analysis, respectively. The permeability and the antifouling properties of the as-prepared membranes were investigated via determination of pure water flux, water flux of BSA solution and flux recovery ratio (FRR), respectively. The results revealed that there was an interaction between the layers of the NLDH and molecules of the additive. The order of the pure water flux of the NLDH/PVDF nanocomposite membranes containing different additives was found to be as: PVP29000/NLDH/PVDF> PVP10000/NLDH/PVDF> PEG1500/NLDH/PVDF> Pluronic F-127/NLDH/PVDF, which is well corresponded to the results of surface porosity,

27

surface hydrophilicity and surface roughness. However, the results showed that the pure water flux of the NLDH/PVDF nanocomposite membrane decreased from 366.9 L/m2 h to 302.8 L/m2 h by adding 1 wt.% of PEG6000, which can be attributed to the surface porosity, surface roughness and also sublayer porosity decrement resulted by rheological feature domination of NLDH/PVDF dope solution in the presence of PEG6000. The order of the water flux of BSA solution and FRR parameter for NLDH/PVDF nanocomposite membranes was found to be as follow: PVP29000/NLDH/PVDF> Pluronic F-127/NLDH/PVDF> PEG6000/NLDH/PVDF> PVP10000/NLDH/PVDF> PEG1500/NLDH/PVDF.

Among the

studied additives,

the

PVP29000 was able to efficiently improve the surface porosity, surface hydrophilicity and crosssectional morphology of the NLDH/PVDF nanocomposite membrane. In addition, the highest enhancement in pure water flux (702.2 L/m2 h), water flux of BSA solution (119.3 L/m2 h) and FRR (73.41%) was obtained by introducing 1 wt.% of PVP29000 to the matrix of the nanocomposite membrane. Accordingly, the PVP29000 was introduced as an optimum additive for fabrication of NLDH/PVDF nanocomposite membrane.

Acknowledgments We sincerely thank University of Tabriz and Kharazmi University for all the support. We also acknowledge the support of Iran Science Elites Federation.

28

References [1] S. Guan, S. Zhang, P. Liu, G. Zhang, X. Jian, Effect of additives on the performance and morphology of sulfonated copoly (phthalazinone biphenyl ether sulfone) composite nanofiltration membranes, Appl. Surf. Sci. 295 (2014) 130-136. [2] Y. Liu, S. Zhang, Z. Zhou, J. Ren, Z. Geng, J. Luan, G. Wang, Novel sulfonated thin-film composite nanofiltration membranes with improved water flux for treatment of dye solutions, J. Membr. Sci. 394 (2012) 218-229. [3] X. Shi, G. Tal, N.P. Hankins, V. Gitis, Fouling and cleaning of ultrafiltration membranes: a review, J. Water Process Eng. 1 (2014) 121-138. [4] A. Shockravi, V. Vatanpour, Z. Najjar, S. Bahadori, A. Javadi, A new high performance polyamide as an effective additive for modification of antifouling properties and morphology of asymmetric PES blend ultrafiltration membranes, Microporous Mesoporous Mater. 246 (2017) 24-36. [5] X. Tan, S. Tan, W. Teo, K. Li, Polyvinylidene fluoride (PVDF) hollow fibre membranes for ammonia removal from water, J. Membr. Sci. 271 (2006) 59-68. [6] A. Bottino, G. Capannelli, A. Comite, Novel porous poly (vinylidene fluoride) membranes for membrane distillation, Desalination 183 (2005) 375-382. [7] B. Wu, K. Li, W. Teo, Preparation and characterization of poly (vinylidene fluoride) hollow fiber membranes for vacuum membrane distillation, J. Appl. Polym. Sci. 106 (2007) 1482-1495. [8] F. Liu, N.A. Hashim, Y. Liu, M.M. Abed, K. Li, Progress in the production and modification of PVDF membranes, J. Membr. Sci. 375 (2011) 1-27.

29

[9] V. Vatanpour, A. Karami, M. Sheydaei, Central composite design optimization of Rhodamine B degradation using TiO2 nanoparticles/UV/PVDF process in continuous submerged membrane photoreactor, Chem. Eng. Process. Process Intensif. 116 (2017) 68-75. [10] M. Safarpour, V. Vatanpour, A.R. Khataee, Preparation and characterization of graphene oxide/TiO2 blended PES nanofiltration membrane with improved antifouling and separation performance, Desalination 393 (2016) 65-78. [11] S. Zhao, W. Yan, M. Shi, Z. Wang, J. Wang, S. Wang, Improving permeability and antifouling performance of polyethersulfone ultrafiltration membrane by incorporation of ZnODMF dispersion containing nano-ZnO and polyvinylpyrrolidone, J. Membr. Sci. 478 (2015) 105116. [12] F. Liu, M.R.M. Abed, K. Li, Preparation and characterization of poly(vinylidene fluoride) (PVDF) based ultrafiltration membranes using nano γ-Al2O3, J. Membr. Sci. 366 (2011) 97-103. [13] K. De Sitter, C. Dotremont, I. Genné, L. Stoops, The use of nanoparticles as alternative pore former for the production of more sustainable polyethersulfone ultrafiltration membranes, J. Membr. Sci. 471 (2014) 168-178. [14] H. Abdolmohammad-Zadeh, Z. Rezvani, G. Sadeghi, E. Zorufi, Layered double hydroxides: a novel nano-sorbent for solid-phase extraction, Anal. Chim. Acta 685 (2011) 212-219. [15] N. Baig, M. Sajid, Applications of layered double hydroxides based electrochemical sensors for determination of environmental pollutants: A review, Trends Environ. Anal. Chem. 16 (2017) 1-15. [16] J. Qu, Q. Zhang, X. Li, X. He, S. Song, Mechanochemical approaches to synthesize layered double hydroxides: a review, Appl. Clay Sci. 119 (2016) 185-192.

30

[17] J.K. Koh, Y.W. Kim, S.H. Ahn, B.R. Min, J.H. Kim, Antifouling poly (vinylidene fluoride) ultrafiltration membranes containing amphiphilic comb polymer additive, J. Polym. Sci., Part B: Polym. Phys. 48 (2010) 183-189. [18] P. Kanagaraj, S. Neelakandan, A. Nagendran, D. Rana, T. Matsuura, M. Shalini, Removal of BSA and HA contaminants from aqueous solution using amphiphilic triblock copolymer modified poly (ether imide) UF membrane and their fouling behaviors, Ind. Eng. Chem. Res. 54 (2015) 11628-11634. [19] S. Arefi-Oskoui, A. Khataee, V. Vatanpour, Effect of solvent type on the physicochemical properties and performance of NLDH/PVDF nanocomposite ultrafiltration membranes, Sep. Purif. Technol. 184 (2017) 97-118. [20] S. Arefi-Oskoui, V. Vatanpour, A. Khataee, Development of a novel high-flux PVDF-based ultrafiltration membrane by embedding Mg-Al nanolayered double hydroxide, J. Ind. Eng. Chem. 41 (2016) 23-32. [21] Z.P. Xu, G.S. Stevenson, C.-Q. Lu, G.Q. Lu, P.F. Bartlett, P.P. Gray, Stable suspension of layered double hydroxide nanoparticles in aqueous solution, Journal of the American Chemical Society 128 (2006) 36-37. [22] Z.P. Xu, G. Stevenson, C.-Q. Lu, G.Q. Lu, Dispersion and size control of layered double hydroxide nanoparticles in aqueous solutions, J. Phys. Chem. B 110 (2006) 16923-16929. [23] D. Zhang, A. Karkooti, L. Liu, M. Sadrzadeh, T. Thundat, Y. Liu, R. Narain, Fabrication of antifouling

and

antibacterial

polyethersulfone

(PES)/cellulose

nanocrystals

(CNC)

nanocomposite membranes, J. Membr. Sci. 549 (2018) 350-356. [24] M.M. Bradford, A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding, Anal. Biochem. 72 (1976) 248-254.

31

[25] M.N. Sepehr, K. Yetilmezsoy, S. Marofi, M. Zarrabi, H.R. Ghaffari, M. Fingas, M. Foroughi, Synthesis of nanosheet layered double hydroxides at lower pH: Optimization of hardness and sulfate removal from drinking water samples, J. Taiwan Inst. Chem. Eng. 45 (2014) 2786-2800. [26] A. Patterson, The Scherrer formula for X-ray particle size determination, Physical Review 56 (1939) 978. [27] M.R. Mehrnia, Y.M. Mojtahedi, M. Homayoonfal, What is the concentration threshold of nanoparticles within the membrane structure? A case study of Al2O3/PSf nanocomposite membrane, Desalination 372 (2015) 75-88. [28] L.Y. Yu, Z.L. Xu, H.M. Shen, H. Yang, Preparation and characterization of PVDF–SiO2 composite hollow fiber UF membrane by sol–gel method, J. Membr. Sci. 337 (2009) 257-265. [29] N. Arahman, S. Mulyati, Effects of PEG molecular weights on PVDF membrane for Humic Acid-fed ultrafiltration process, in: IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2017, pp. 121-129. [30] H. Dong, L. Wu, L. Zhang, H. Chen, C. Gao, Clay nanosheets as charged filler materials for high-performance and fouling-resistant thin film nanocomposite membranes, J. Membr. Sci. 494 (2015) 92-103. [31] S. Zhao, Z. Wang, X. Wei, B. Zhao, J. Wang, S. Yang, S. Wang, Performance improvement of

polysulfone

ultrafiltration

membrane

using

well-dispersed

polyaniline–poly

(vinylpyrrolidone) nanocomposite as the additive, Ind. Eng. Chem. Res. 51 (2012) 4661-4672. [32] J.S. Kang, Y.M. Lee, Effects of molecular weight of polyvinylpyrrolidone on precipitation kinetics during the formation of asymmetric polyacrylonitrile membrane, J. Appl. Polym. Sci. 85 (2002) 57-68.

32

[33] K. Roy, T. Anjali, A. Sujith, Asymmetric membranes based on poly (vinyl chloride): effect of molecular weight of additive and solvent power on the morphology and performance, J. Mater. Sci. 52 (2017) 5708-5725. [34] K.A. Gebru, C. Das, Effects of solubility parameter differences among PEG, PVP and CA on the preparation of ultrafiltration membranes: Impacts of solvents and additives on morphology, permeability and fouling performances, Chin. J. Chem. Eng. 25 (2017) 911-923. [35] W. Zhao, Y. Su, C. Li, Q. Shi, X. Ning, Z. Jiang, Fabrication of antifouling polyethersulfone ultrafiltration membranes using Pluronic F127 as both surface modifier and pore-forming agent, J. Membr. Sci. 318 (2008) 405-412. [36] Y.q. Wang, T. Wang, Y.l. Su, F.b. Peng, H. Wu, Z.y. Jiang, Remarkable reduction of irreversible fouling and improvement of the permeation properties of poly (ether sulfone) ultrafiltration membranes by blending with pluronic F127, Langmuir 21 (2005) 11856-11862. [37] S. Li, Y. Gao, H. Bai, L. Zhang, P. Qu, L. Bai, Preparation and characteristics of polysulfone

dialysis

composite

membranes

modified

with

nanocrystalline

cellulose,

BioResources 6 (2011) 1670-1680. [38] X. Li, X. Fang, R. Pang, J. Li, X. Sun, J. Shen, W. Han, L. Wang, Self-assembly of TiO2 nanoparticles around the pores of PES ultrafiltration membrane for mitigating organic fouling, J. Membr. Sci. 467 (2014) 226-235. [39] Q. Wang, Z. Wang, Z. Wu, Effects of solvent compositions on physicochemical properties and anti-fouling ability of PVDF microfiltration membranes for wastewater treatment, Desalination 297 (2012) 79-86. [40] X. Duan, D.G. Evans, Layered double hydroxides, Springer Science & Business Media, 2006.

33

[41] G. Fan, F. Li, D.G. Evans, X. Duan, Catalytic applications of layered double hydroxides: recent advances and perspectives, Chem. Soc. Rev. 43 (2014) 7040-7066. [42] X. Li, X. Fan, S. Brandani, Difference in pore contact angle and the contact angle measured on a flat surface and in an open space, Chem. Eng. Sci. 117 (2014) 137-145. [43] V. Gekas, K.M. Persson, M. Wahlgren, B. Sivik, Contact angles of ultrafiltration membranes and their possible correlation to membrane performance, J. Membr. Sci. 72 (1992) 293-302. [44] F. Bartell, W. Purcell, C. Dodd, The measurement of the effective pore size and of the water-repellency of tightly woven textiles, Discussions of the Faraday Society 3 (1948) 257-264. [45] S. Shimpalee, V. Lilavivat, Study of water droplet removal on etched-metal surfaces for proton exchange membrane fuel cell flow channel, J. Electrochem. Energy Convers. Storage 13 (2016) 011003. [46] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. Res. 28 (1936) 988-994. [47] C. Feng, B. Shi, G. Li, Y. Wu, Preparation and properties of microporous membrane from poly (vinylidene fluoride-co-tetrafluoroethylene)(F2. 4) for membrane distillation, J. Membr. Sci. 237 (2004) 15-24. [48] C. Feng, B. Shi, G. Li, Y. Wu, Preliminary research on microporous membrane from F2. 4 for membrane distillation, Sep. Purif. Technol. 39 (2004) 221-228. [49] M. Safarpour, A.R. Khataee, V. Vatanpour, Effect of reduced graphene oxide/TiO2 nanocomposite with different molar ratios on the performance of PVDF ultrafiltration membranes, Sep. Purif. Technol. 140 (2015) 32-42.

34

[50] M. Safarpour, A.R. Khataee, V. Vatanpour, Preparation of a novel polyvinylidene fluoride (PVDF) ultrafiltration membrane modified with reduced graphene oxide/Titanium dioxide (TiO2) nanocomposite with enhanced hydrophilicity and antifouling properties, Ind. Eng. Chem. Res. 53 (2014) 13370-13382.

Figure captions: Fig. 1. SEM image of MgAl-CO32- NLDH. Fig. 2. Surface SEM images and pore size distribution of PVDF membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F127. Fig. 3. Surface SEM images and pore size distribution of PVDF/NLDH nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127. Fig. 4. Cross-sectional SEM images of PVDF membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127. Fig. 5. Cross-sectional SEM images and EDX mapping of NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127. Fig. 6. AFM images of PVDF and NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F127.

35

Fig. 7. Pure water flux of PVDF and NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127. Fig. 8. Water flux of BSA solution of PVDF and NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127. Fig. 9. Flux recovery ratios of PVDF and NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127.

36

Figures

Fig. 1

37

Fig. 2

38

Fig. 2 (continued)

39

Fig. 3

40

Fig. 3 (continued)

41

Fig. 4

42

Fig. 5

43

Fig. 6

44

Fig. 7

45

Fig. 8

46

Fig. 9

47

Table 1.

Surface porosity, contact angles and thickness of PVDF and NLDH/PVDF

nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127. Membrane Membrane PVDF PVP10000/PVDF PVP29000/PVDF PEG1500/PVDF PEG6000/PVDF Pluronic F-127/PVDF NLDH/PVDF PVP10000/NLDH/PVDF PVP29000/NLDH/PVDF PEG1500/NLDH/PVDF PEG6000/NLDH/PVDF Pluronic F-127/NLDH/PVDF

Surface porosity (%) 1.3 ± 0.18 2.32 ± 0.14 2.98 ± 0.33 1.7 ± 0.08 2.1 ± 0.11 1.5 ± 0.09 2.14 ± 0.21 3.1 ± 0.23 5.68 ± 0.14 2.5 ± 0.17 1.6 ± 0.10 2.2 ± 0.23

Contact parameters angle Roughness (deg) 77.4 ± 0.8 73.1 ± 0.5 71.3 ± 0.9 75.2 ± 1.1 74.7 ± 0.6 75.9 ± 1.3 73.8 ± 1.1 69.8 ± 0.8 67.3 ± 1 70.1 ± 1.2 68.5 ± 0.9 71.5 ± 0.5

Thickness (nm) 20.9 ± 1.9 100.2 ± 2.4 105.3 ± 1.6 47.0 ± 1.8 84.3 ± 0.9 91.4 ± 1.3 65.2 ± 2.8 106.8 ± 1.2 115.0 ± 1.5 73.9 ± 3.1 45.3 ± 2.2 108.7 ± 1.4

Table 2. Surface roughness parameters of PVDF and NLDH/PVDF nanocomposite membranes containing 1 wt.% of different additives including PVP10000, PVP 29000, PEG1500, PEG6000 and pluronic F-127.

48

PVDF PVP10000/PVDF PVP29000/PVDF PEG1500/PVDF PEG6000/PVDF Pluronic F-127/PVDF NLDH/PVDF PVP10000/NLDH/PVDF PVP29000/NLDH/PVDF PEG1500/NLDH/PVDF PEG6000/NLDH/PVDF Pluronic F-127/NLDH/PVDF

Sa (nm) 9.65 37.21 41.20 30.02 34.84 13.21 32.11 42.00 61.40 41.33 37.62 32.61

49

Sq (nm) 12.37 43.63 50.53 39.81 40.99 17.64 38.67 51.78 78.86 51.55 45.93 45.34

Sy (nm) 80.35 310.41 358.39 200.7 287.21 112.32 201.32 370.61 487.54 362.65 265.52 320.45

Research highlights:  Mg-Al nanolayered double hydroxide was synthesized with CO32- anions in interlayer.  Effect of different additives on properties of NLDH/PVDF nanocomposite membranes was assessed. 

PVP29000 was introduced as an optimum additive for NLDH/PVDF nanocomposite membrane.

50