Homogeneous polyethersulfone hybrid membranes prepared with in-suit synthesized magnesium hydroxide nanoparticles by phase inversion method

Author’s Accepted Manuscript Homogeneous polyethersulfone hybrid membranes prepared with in-suit synthesized magnesium hydroxide nanoparticles by phase inversion method Senjian Han, Lili Mao, Tao Wu, Haizeng Wang www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)30456-2 http://dx.doi.org/10.1016/j.memsci.2016.05.040 MEMSCI14515

To appear in: Journal of Membrane Science Received date: 18 March 2016 Revised date: 24 May 2016 Accepted date: 24 May 2016 Cite this article as: Senjian Han, Lili Mao, Tao Wu and Haizeng Wang, Homogeneous polyethersulfone hybrid membranes prepared with in-suit synthesized magnesium hydroxide nanoparticles by phase inversion method, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.05.040 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 galley proof before it is published in its final citable 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.

Homogeneous polyethersulfone hybrid membranes prepared with in-suit synthesized magnesium hydroxide nanoparticles by phase inversion method Senjian Han1, Lili Mao1, Tao Wu, Haizeng Wang* Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, P. R. China [email protected] [email protected] * Corresponding author. Tel: +86 532 66782503; fax: +86 532 66782481.

Abstract Mg(OH)2/PES (magnesium hydroxide/polyethersulfone) hybrid membranes were prepared by in-suit synthesized phase inversion method. The highlight of this method is that it affords uniform distribution of Mg2+ on an atomic level in the casting solution, reacted with OH- in the coagulation bath during the formation of membrane via phase inversion process, and nano-sized Mg(OH)2 was in-suit formed and distributed uniformly in PES matrix. SEM, XPS and BET analysis were performed to characterize the membranes, and ultrafiltration properties were investigated and compared. The results indicated that all the tested performance of the PES membranes was generally improved by the existence of Mg(OH)2. Additionally, with the increase of MgCl2∙6H2O, the surface hydrophilicity, porosity, and permeation flux improved proportionally, while EA retention rate and antifouling property increased firstly and then decreased. It is suggested that this research will provide a new line for the preparation of homogeneous organic-inorganic membranes. Keywords Mg(OH)2 nanoparticles; In-suit formation; Hybrid membrane; Uniform 1. Introduction As an important membrane technology, ultrafiltration (UF) has been established in several industrial applications, such as wastewater treatment, food processing, bioseparation and pharmaceutical manufacturing [1-3]. Serving as high performance standards, high flux, high selectivity and low fouling of UF membrane have attracted considerable attention [4]. However, the use of pure polymeric membrane or pure inorganic membranes cannot meet the increasing demand to improve the performance of UF membranes [5]. Combining polymer and inorganic substance in a hybrid membrane results in favorable selectivity and permeability, as a consequence, it gains more and more importance [6]. Using inorganic nanoparticles (NPs) as additives in polymeric membranes aimed 1

These authors have contributed equally to this study.

at preparing organic-inorganic hybrid membranes is extensively reported, such as, carbon nanotube, Fe3O4, TiO2, ZnO, Al2O3 and SiO2, among these, SiO2 is the most convenient and widely used [7-12]. Mg(OH)2, as another good example of inorganic substances, are traditionally well known with such structurally OH groups, attracting significant attention [13]. Based on its hydrophilicity, antibacterial and environmental favored properties, Mg(OH)2 can be used in the preparation of hybrid membranes [14]. To the best of our knowledge, so far there has been only one report on Mg(OH)2 hybrid membrane by adding Mg(OH)2 NPs solely into casting solution to enhance the antifouling performance of PVDF membrane [15]. However, the reported research was performed by blending Mg(OH)2 NPs with casting solution conventionally. The presence of NPs helps improve several performance of the membranes, including hydrophilicity, permeability and antifouling property [16]. But, the difficulty of uniform dispersion NPs in casting solution leads to defects and further loose initial enhancement purposes, which was attributed to the high viscosity of the casting solution and the prone agglomeration of NPs [17, 18]. Organic-inorganic hybrid membranes can be fabricated most commonly by solution blending method, in-situ polymerization and sol-gel method [6]. Much effort including method evolution and additives change has been done to promote the dispersion of the NPs as well as prevent agglomeration in polymers [19-21]. With the same consideration, this research is aimed at solving the problem radically by bringing soluble substance as precursor into the casting solution, which can realize a real sense of uniform distribution. It is worth to mention to the readers at this point that there is no published work that has prepared hybrid membranes by dissolving inorganic substance and then in-suit forming other substances in the membrane matrix via a simple method. In this paper, in-suit synthesized phase inversion method preparing uniform Mg(OH)2/PES (magnesium hydroxide/polyethersulfone) hybrid membranes was presented. MgCl2∙6H2O (magnesium chloride hexahydrate) particles could be dissolved into PES/DMAc (N, N-dimethylacetamide) directly forming a homogeneous casting solution, and NaOH solution (1 mol/L) was used as coagulation bath. In the formation of the hybrid membranes, Mg2+ in the casting solution reacted with OH- in-suit, and Mg(OH)2 NPs were obtained efficiently, distributed uniformly in PES network. Severed as references, the other pure PES ultrafiltration membranes were prepared with deionized (DI) water as coagulation bath and MgCl2∙6H2O just as porogen. While during the formation of pure PES membranes, MgCl2 in the casting solution dissolved into the water, the space they once occupied transformed into pores in the resulted ultrafiltration membranes. The effect of Mg(OH)2 NPs on the properties of PES UF membrane was evaluated using of XPS, SEM, BET, pure water permeability and hydrophilicity studies. Further the membranes were subjected to antifouling studies using egg albumen (EA) as the model protein for rejection.

2. Experimental 2.1 Materials Polyethersulfone (PES) with molecular weight of 58,000 Da was purchased from BASF (Qingdao, China). N, N-dimethylacetamide (DMAc, regent grade) was purchased from Guangchen Chemical Reagent Co. Ltd. (Tianjin). Both sodium hydroxide (NaOH, regent grade) and magnesium chloride hexahydrate (MgCl2∙6H2O, regent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purification. Deionized water used throughout this study was purified using a Millipore water purified system to a minimum resistivity of 18.2 MΩ cm. 2.2 Membrane preparation To obtain a homogeneous casting solution, PES and MgCl2∙6H2O was well dissolved in DMAc, with full stirring at 60°C overnight in an airtight container. Until no bubbles were observed, the cooled transparent solution was poured onto a clean glass plate with a casting thickness of 100 μm at room temperature. After 10-60 s exposure to air, the cast film was immediately immersed in DI water and NaOH (1 mol/L) bath, respectively. After complete coagulation, both of the two membranes were washed with DI water until the solution was neutral as well as the residual solvent and MgCl2 was removed, and then the membranes were dried in the air at room temperature. With different dosage of MgCl2∙6H2O added into the casting solution, a series of membranes differ in composition were obtained. The formulations of all membrane casting solutions were described in Table 1. The membranes prepared with DI water were marked with M1, M2, M3, M4, M5, M6, and the membranes prepared with NaOH solution were marked with M1′, M2′, M3′, M4′, M5′, M6′, respectively. Table 1 The ratio of MgCl2∙6H2O and PES in the casting solution Casting solution compositions (wt.%)

Membranes M1, M1′ M2, M2′ M3, M3′ M4, M4′ M5, M5′ M6, M6′

PES

MgCl2∙6H2O

DMAc

17.5 18 17.5 17 17.5 17

1.5 2 2.5 3 3.5 4

81 80 80 80 79 79

2.3. Membrane characterization 2.3.1. X-ray photoelectron spectroscopy analysis The chemical composition of the membrane surface was carried out by X-ray

photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi, USA), with an Al Kradiation source (1486.6 eV). High resolution scans were obtained by averaging 15 scans for Mg1s peak with pass energy at 50.0 eV and energy step size of 0.05 eV. For data analysis, XPS Peak software package was used, and the spectra were subtracted by Shirley-type background. 2.3.2. Scanning electron microscopy (SEM) The surface morphology and the cross-sectional structure of the prepared membranes were analyzed by SEM (S-4800, Hitachi, Japan). Before analyzing, the membrane samples were dried overnight, cut into appropriate size, mounted on brass plates, and then sputter coated with gold. Additionally, the membrane samples were fractured in liquid nitrogen prior to cross-sectional analysis. 2.3.3. BET nitrogen adsorption The membrane samples were characterized by specific surface area analyzer (NOVA 2200e, Quantachrome, USA) at liquid nitrogen temperature. Based on the Brunauer, Emmett, and Teller (BET) and Joyner and Halenda (BJH) models, parameters of specific surface area, pore distribution were identified according to their volume and diameter [22]. 2.3.4. Hydrophilicity measurements The hydrophilicity of the prepared membranes was characterized qualitatively by the contact angle, which can be measured by a contact angle measuring instrument (DSA100, Kruss, Germany). The membrane samples were stored in a vacuum oven before measurement, and the measurement was carried out at room temperature. 5 μL of DI water was pumped out from the micro syringe, dropped onto the membrane surface, and the instant image was recorded to measure the contact angles. For each membrane sample, the contact angle was determined by more than 10 different locations, and the average value was taken as the reported data. 2.3.5. Porosity The porosity of the prepared membranes was represented by the volume occupied by water, which can be measured by the gravimetric method. The membrane maintained in DI water was wiped the surface water with cleaning paper, followed by weighting immediately (Ww). Then, the wet membrane already weighted was dried in a vacuum oven at 60°C for 24 h and weighted (Wd) again in dry state. The porosity of membrane (P) was calculated as follows: 𝑃(%) =

𝑊𝑤 −𝑊𝑑 𝜌𝑤

𝐴 ×𝛿

× 100

(1)

where Ww and Wd are the weights of wet and dry membranes (g), respectively. ρw is the density of pure water (g/cm3), A is the area of membrane in wet state (cm2), and δ is the thickness of membrane in wet state (cm).

2.3.6. Pure water permeability The pure water permeability measurements were carried out using a self-made apparatus with an effective filtration area of 19.63 cm2. The membranes mounted in the membrane cell were pre-compacted with DI water at 0.2 MPa for 1 h until no further variation in permeation was observed. Under steady state, the permeated flux was recorded over a period of time at different operation pressures from 0.08 to 0.16 MPa. The pure water permeability (Pw) was calculated with the following equation: 𝑄

𝑃𝑤 = 𝐴∆𝑡Δ𝑃

(2)

where Q is the volume of permeate water (L), Δt is the collection time (h), A is the effective membrane area for filtration (m2), and ΔP is the trans-membrane pressure (MPa). All the Pw (L/m2hMPa) is calculated at different pressures, and the reported data represents an average value. 2.3.7. Separation and antifouling performance The prepared membranes were subject to a series of filtration experiments to evaluate the separation performance of membranes. Before the filtration experiments were conducted, the membranes were also pre-compacted with DI water at 0.2 MPa for 1 h until steady permeation was reached. Then an aqueous egg albumin (EA, 45 kDa) solution with concentration of 3 g/L at pH 10~11 was prepared and filtered through the membrane under a pressure of 0.1 MPa. Both the concentrations in feed and permeate solution were measured by a UV-vis spectrophotometer (UV-2450, Shimadzu, Japan) at a wavelength of 280 nm. The EA rejection rate (R) was calculated by the following equation: 𝐶𝑝

𝑅(%) = (1 − 𝐶 ) × 100 f

(3)

where Cp and Cf are the EA concentrations in permeate and feed, respectively. To investigate the long-term antifouling property of membranes, dead end filtration experiments were carried out. After the above mentioned pre-compaction, DI water was performed at 0.1 MPa for 30 min to measure the pure water flux (Pw1). Then, the membranes were subjected to EA solution for 1.5 h, after that, simple physical cleaning was chosen to deal with membrane fouling. The membrane system was changed to cross-flow filtration, and tangential flow rate of DI water was maintained at 30 L/h for 30 min. The water flux of the cleaned membranes (Pp1) and the EA permeation flux were measured (Pw2), respectively. The whole experiment includes four times run of pure water and three times run of EA solution. According to the permeation fluxes before and after cleaning, the relative flux reduction (RFR) and flux recovery ratio (FRR) were obtained and calculated using the following expression: 𝑃𝑝

𝑅 𝑅(%) = (1 − 𝑃 ) × 100 𝑤

𝑃

𝑅𝑅(%) = 𝑃𝑤 × 100 𝑤

(4) (5)

Where Pp3 is the EA permeation flux after three cycles, Pw1 is the initial pure water

flux, and Pw4 is the forth pure water flux during the whole process [23]. 3. Results and discussion 3.1. X-ray photoelectron spectroscopy analysis In order to detect the residue amount of MgCl2 in the pure PES membranes and the loss of Mg(OH)2 in the hybrid membranes after a long-run filtration, membrane M4 and M4′ (before and after 48h filtration of DI water) were further analyzed as representative by XPS analysis. As shown in Fig. 1, membrane M4 has no significant difference with raw material in XPS spectra, and no emission peak of Cl or Mg appears in M4, indicating that more than 93% of MgCl2 was released into water, so the M series membranes were called pure PES membranes. For hybrid membrane M4′, a peak at 198 eV ascribing to the binding energy of Cl 2p vanished, while a peak at 1303 eV ascribing to the binding energy of Mg1s just weakened after 48h filtration of DI water, which manifests that residual MgCl2 was washed away but about 78% of Mg(OH)2 nanoparticles were retained in membrane matrix. Furthermore, the fitted high resolution spectra for Mg1s with related chemical bonding is displayed in Fig. 2. The peak at 1303.2 eV assigned to Mg-OH accounts for 76%, while the peak at 1304.2 eV assigned to Mg-Cl accounts for 24% [24]. From the percentage of Mg1s in Mg(OH)2, it can be concluded that 76% of MgCl2∙6H2O is converted into Mg(OH)2, which we call “efficiently formed”. Clearly, all the results are well corresponding to the previous theoretical assumptions.

C1s

O1s Mg1s

S2p Cl2p

M4' before filtration

M4' after filtration

M4

Raw material PES

0

500

1000

1500

Binding energy (eV) Fig. 1 XPS spectra of raw material PES, membrane M4 and membrane M4′ (before and after 48h filtration).

Survey curve Fitting curve Basline Mg-OH (76.0%)

Counts /s

Mg-Cl (24.0%)

1296

1298

1300

1302

1304

1306

1308

1310

Binding energy (eV) Fig. 2 High resolution XPS scans for Mg1s peak in membrane M4′ before filtration.

3.2. Morphological study SEM analysis was carried out to determine the changes on the morphology of the prepared membranes. All the SEM images show that Mg(OH)2 NPs are equally distributed in the hybrid membranes both on the surface and matrix (M1′-M6′), which is along the lines of previous estimates on the subject. The cross-section morphology of the hybrid membrane (M6′) matrix is shown in Fig. 3. It can be seen that the generated Mg(OH)2 particle size distribution was narrow and the mean particle size was nano-sized, which indicated precursor MgCl2∙6H2O particles were well-dissolved in the polymer casting solution and converted into Mg(OH)2 NPs efficiently.

Fig. 3 SEM image of the generated Mg(OH)2 in the hybrid membrane (M6′).

The SEM images of the prepared membranes surface are shown in Fig. 4. It can be seen that the number of pores both on the two membranes surface increased with the addition of MgCl2∙6H2O, and large amount of Mg(OH)2 formed in the hybrid membranes surface. Meanwhile, the formation of Mg(OH)2 resulted to macro-pores and more pores compared to the corresponding PES membrane, due to the formed Mg(OH)2 NPs was hydrophilic, which provided more sites for water penetration and further accelerate water penetration rate.

Fig.4 SEM surface images of the prepared membrane. (a) M1, (b) M3, (c) M5,(d) M1′, (e) M3′ and (f) M5′.

The cross-section morphologies of the prepared membranes are shown in Fig. 5, it can be seen that all the membranes present typical asymmetric cross-section structure. An increase in the dosage of MgCl2∙6H2O results in continuously

attenuation in sponge-like bottom layer, accompanied with the increase of finger-like pores. Furthermore, an increase in the dosage of Mg(OH)2 NPs results in more formation of finger-like pores, and the voids become increasingly large.

Fig.5 SEM cross-section images of the prepared membrane. (a) M2, (b) M4, (c) M6, (d) M2′, (e) M4′ and (f) M6′.

3.3. BET nitrogen adsorption To further characterize the nano-scale pore structure of the membrane, membrane M4 and M4′ were analyzed as representative by BET analysis. Fig. 6a and b show nitrogen adsorption/desorption isotherm and the corresponding pores size distribution curve (inset) from adsorption branch using BJH method of the two membranes respectively. From the isothermal curves, it can be found that the two membranes have the same isotherm type, but M4′ has higher adsorbed quantity. At P/P0> 0.90, a large increase in volume (the take-off on the curve) is observed, which suggests that there is a large amount of macro-pores in the samples [25]. Furthermore, M4′ has relatively larger average pore width and exhibits higher BET surface area of 20.352 m2 than M4 found to be 15.938 m2. The BET results are all consistent with those obtained from the SEM images.

35 Adsorption Pore volume (cm3/nm/g) ×10-3

Adsorbed volume (cm3/g STP)

30 25 20 15

a

Desorption

1.6

1.2

0.8

0.4

0

5

10 15 20 25 30

85

90

95

Pore diameter (nm)

10 5 0 0.0

0.2

0.4

0.6

Relative pressure (P/P0)

0.8

1.0

45 Adsorption Pore volume (cm3/nm/g)×10-3

Adsorbed volume (cm3/g STP)

40 35 30 25 20

2.4

b

Desorption

1.6

0.8

0.0 0

20

100

Pore volume (nm)

15 10 5 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Fig. 6 Nitrogen adsorption–desorption isotherm of the prepared membrane. (a) M4, (b) M4′. Inset: Corresponding pore size distribution curve determined from the N2-adsorption isotherm.

3.4. Hydrophilicity of membranes The surface hydrophilicity is always a considering factor to improve the membranes performance, including permeability and antifouling property. In general, the contact angle is used as a qualitative measurement of surface hydrophilicity, the increase in which is a clear indication of increasing hydrophilicity of the membranes, and the observed values are shown in Fig. 7. It can be seen that he hybrid membranes have relatively lower water contact angle values, which could be caused by the OH group from Mg(OH)2 NPs in the matrix, except for M6′. Beyond this, for the hybrid membranes, contact angle values decrease at first and then increase with the increase of MgCl2∙6H2O. When MgCl2∙6H2O content is 1.5-3.5 wt.%, the pore size of the hybrid membranes surface does not change obviously (Fig. 4), so the decreased contact angle reveals an improved hydrophilicity. However, when MgCl2∙6H2O content is up to 4 wt.%, there are some larger pores on the membrane surface than the less ones, through which the water droplets possibly partly penetrate into the bulk, affecting the measurement of contact angle, and thus the increased water contact angle may not reflect the real surface hydrophobicity [26]. Considering this case, the increased contact angle can be ignored. From a view of tendency, the surface hydrophilicity is increased with the increase of MgCl2∙6H2O in hybrid membranes.

M1-M6 M1'-M6'

80

Water contact angle (degree)

70 60 50 40 30 20 10 0 M1

M1' M2

M2' M3

M3' M4

M4' M5

M5' M6

M6'

Membrane type Fig. 7 Water contact angle measurements of the prepared membranes.

3.5. Porosity The result of porosity measurement is shown in Fig. 8, from which it can be seen that the porosity is in proportion to the dosage of MgCl2∙6H2O, and the hybrid membranes have a higher porosity than the pure PES membranes. As revealed by SEM images, the increase of MgCl2∙6H2O caused an increase number of the pores, and the existence of Mg(OH)2 NPs caused an enlarged aperture of the prepared membranes, compared to the inexistence ones, both of which lead to higher porosity. That is to say, M′ > M, and the porosity of membranes both in the two membranes decreases in the order of M6(′)> M5(′)> M4(′)> M3(′)> M2(′)> M1(′).

100

M1-M6 M1'-M6'

Porosity (%)

90

80

70

60

50 M1

M1' M2

M2' M3

M3' M4

M4' M5

M5' M6

M6'

Membrane type Fig. 8 Porosity of the prepared membranes.

3.6. Pure water permeability The permeability property of the prepared membranes was studied by collecting the pure water through a certain time at different certain pressures ranging from 0.08 to 0.16 MPa. The obtained average values are displayed in Fig. 9, consist with the trends observed in the SEM images. With the increase of MgCl2∙6H2O, both kinds of membranes have the same increasing trend on the water permeability, but a higher level was observed in the case of hybrid membranes. An increase in the dosage of MgCl2∙6H2O leading to more porous structure and the existence of Mg(OH)2 leading to enlarged pores, which consequently facilitate the permeation of pure water, results in an increase of water permeability. It is worth mentioning that not only morphology but other factor such as hydrophilicity has effect on the permeability property of membranes [27]. As mentioned above, both macro-pores and high hydrophilicity are of importance to the higher water permeability of the hybrid membranes.

1200

Pure water permeability (L/m2hMPa)

M1-M6 M1'-M6' 1000

800

600

400

200

0 M1

M1' M2

M2' M3

M3' M4

M4' M5

M5' M6

M6'

Membrane type Fig. 9 Pure water permeability of the prepared membranes

3.7. Separation and antifouling performance It is well known that both the retention rate and flux contribute to the membrane performance. In this study, 3 g/L egg albumin (EA) solution was used as model protein to evaluate the separation performance, and DI water was used as cleaning agent to investigate the antifouling property of the prepared membranes. According to the Fig. 10, when the content of MgCl2∙6H2O is 1.5-3.5 wt.%, the rejection rate of hybrid membranes is higher than the pure PES membranes, but it is opposite when the content of MgCl2∙6H2O is up to 4 wt.%. Additionally, for the hybrid membranes, a slightly increase in rejection rate is observed with MgCl2∙6H2O increasing from 1.5 to 2.5 wt.%, and then it begins to decrease more and more drastically. While for the pure PES membranes, the rejection rate remains unchanged at first and then gradually decreases with the addition of MgCl2∙6H2O. According to Fig. 4 and BET analysis, the size of pores became larger due to the composition change, as a result, the number of EA passed through the pores was increased, which may lead to decreased retention rate. However, the hydrophilicity of the hybrid membrane was improved by Mg(OH)2 NPs on the surface, and the numbers of adsorbed EA molecules were reduced, which contributed to the antifouling property of the membranes, leading to increased retention rate [28]. In brief, the final retention rate depends on the dominant effect. For the hybrid membranes, when the content of MgCl2∙6H2O is from 1.5 to 2.5 wt.%, the hydrophilicity plays the leading role in the hybrid membranes, leading to the increase of retention rate. However, with the continuing increase of MgCl2∙6H2O, more macrovoids generated leads to more passed

EA molecules, which cannot be offset by the increased hydrophilicity, so the retention rate decreased instead. While for the pure PES membranes, MgCl2∙6H2O used as pore-forming agent, can only increase the number of pores without changing the aperture, and the retention rate basically remains unchanged (95%±1%) with lower content of MgCl2∙6H2O. But when the content of MgCl2∙6H2O is continued to increase, maybe the connection of pores causes enlarged aperture, which leading to the decrease of retention rate.

M1-M6

100

M1'-M6'

Rejection (%)

80

60

40

20

0 M1

M1' M2

M2' M3

M3' M4

M4' M5

M5' M6

Membrane type Fig. 10 EA rejection of the prepared membranes.

M6'

1200

M1' M2'

M3 M4 M5

M3' M4' M5'

M6

M6'

M1

1000

Flux (L/m2hMPa)

M2

800

600

400

200

0 0

100

200

300

400

500

600

700

t (min)

Fig. 11 Recycling potential of the prepared membranes.

Fig. 11 depicts the permeation fluxes of the prepared membranes, performed by dynamic filtration experiment of EA. All these membranes show a flux decline and partial recovery after a long time run. The relative flux reduction (RFR) and flux recovery ratio (FRR) after three cycles were calculated from Fig. 11, and the results are described in Table 2. The higher FRR means the longer time run without significant flux decline, indicating the better antifouling property, similarly, the lower RFR indicates the stronger fouling resistant ability [29]. The tendency of the antifouling property of the hybrid membranes is consistent with the separation performance, and M3′ gives the highest FRR (73.8%) and the lowest RFR (31.1%) values. It is well understood that improving the membrane surface hydrophilicity can debase protein adsorption, and so much effort has been done to improve antifouling property by increasing the dosage of the hydrophilic particles [30–32]. In this study, antifouling properties of the hybrid membranes should be improved by increasing the percentage of MgCl2∙6H2O. However, the antifouling property is not only depending on the hydrophilicity, but also affected by the membrane surface morphology, which had been summarized as the fouling tendency of the membrane surface increased with the enlarged pore size [33]. The increased dosage of MgCl2∙6H2O did not monotonously improve the antifouling property, but increased the number of pores and enlarged the pore size, which may contrarily increase the fouling tendency. As a consequence of the above combined effects, the antifouling property increases firstly then decreases with further increase of Mg(OH)2.

Table 2 Antifouling performance of the membranes for EA after three cycles. Membranes M1 M1′ M2 M2′ M3 M3′ M4 M4′ M5 M5′ M6 M6′

FRR% 35.5 70.3 35.7 71.3 36.1 73.8 36.3 69.7 27.5 48.2 23.3 28.4

RFR% 76.6 33.7 80.3 31.5 67.8 31.1 71.4 39 74.9 53.8 83.7 76

The comparison of FRR and RFR values between hybrid membranes and pure PES membranes can be summarized as the hybrid membranes yield higher FRR values, while lower RFR values are obtained from pure PES membranes. Accordingly, it can be concluded that hybrid Mg(OH)2/PES membranes have better antifouling property than the pure PES membranes. Taking all the above performance into consideration, 3 wt.% MgCl2∙6H2O in the casting solution used 1 mol/L NaOH solution as coagulation bath can obtain the most excellent membrane, including high permeability, high selectivity as well as almost 70% recovery flux ratio after three cycles EA filtration experiment. 4. Conclusions Mg(OH)2/PES hybrid membranes were prepared by in-suit synthesized phase inversion method. Dissolved in the casting solution, precursor MgCl2∙6H2O reacted with NaOH in the coagulation bath preparing uniform hybrid membranes, or released into the coagulation bath of DI water preparing pure PES membranes. The XPS results confirmed the effective generation of Mg(OH)2 in the hybrid membrane and the porogen role played by MgCl2∙6H2O in the pure PES membrane. Combined with BET and SEM results on pore structure, the hybrid membrane had larger pore width and specific surface area. From an overall view, all the tested performance of the membranes was improved by the existence of Mg(OH)2. Additionally, the difference in the concentration of MgCl2∙6H2O affected the morphology and properties of resulted membranes to some extent. Taking all the tested performance into consideration, when the content of MgCl2∙6H2O was 3 wt.%, the hybrid Mg(OH)2/PES membrane M4′ had the best performance: pure water flux improved from 430 to 720 L/m2hMPa, rejection rate was 94.58%, and recovery flux ratio was almost 70%. It can be envisioned that a series of homogeneous organic-inorganic hybrid membranes could become accessible via in-suit synthesized phase inversion method

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Dissolved MgCl2∙6H2O reacted with NaOH coagulation bath during the formation of

 

membrane. Mg(OH)2 nanoparticles were uniformly distributed in the polymer matrix. Hybrid membranes showed higher permeability and better antifouling property.