Enhanced Hydrophilicity and Water Flux of Polyethersulfone

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Enhanced Hydrophilicity and Water Flux of Polyethersulfone Membranes in the Presence of Aluminum Fumarate Metal-Organic Framework Nanoparticles: Preparation and Characterization Sara Abdi, and Masoud Nasiri ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01848 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Materials & Interfaces

Enhanced Hydrophilicity and Water Flux of Polyethersulfone Membranes in the Presence of Aluminum Fumarate Metal-Organic Framework Nanoparticles: Preparation and Characterization Sara Abdi, Masoud Nasiri* Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan 35195-363, Iran

Abstract The aim of this study is to examine the effect of the addition of aluminum fumarate (AlFu) nanoparticles on the properties of polyethersulfone (PES) membranes which the AlFu nanoparticles were synthesized as the nanofillers with the metal-organic framework and their structure was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis. Subsequently, PES/AlFu mixed-matrix membranes (MMMs) were fabricated in different weight percentages of nanofiller through a phase inversion method and the membranes characterization was accomplished by FTIR, XRD, FESEM, transmission electron microscopy (TEM), atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDX) and elemental mapping analyses. The effect of the addition of nanoparticles on the membranes properties was investigated by measuring the membrane hydrophilicity, pure water flux, solute rejection and fouling resistance using a dead-end cell under constant pressure and bovine serum albumin (BSA) as a foulant. The molecular weight cut-off (MWCO) of MMMs was measured by the rejection of polyethylene glycol (PEG) in the various molecular weights and the membranes surface roughness, porosity and mean pore radius were calculated. The results showed that AlFu nanoparticles increased the hydrophilicity and the porosity of the neat PES membranes and consequently increased the water permeability such that MMM including 0.75wt% of AlFu possessed the maximum porosity (62.2%), mean pore radius (10.2 nm) and MWCO (154kDa). Furthermore, this membrane exhibits a superlative flux recovery and minimal total resistance in the antifouling properties examinations.

Keywords Metal-Organic Framework, Aluminum fumarate, Polyethersulfone, Phase inversion, Hydrophilicity.

E-mail address: [email protected] Tel:(+9823)31533884

*

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1. Introduction Groundwaters are one of the main sources of drinking water which can be contaminated with the help of various natural pollutants and industrial effluents and greatly affect the health of humans and other living beings1,2. Removal of these contaminants from groundwater sources is one of the most challenging issues in today’s world whereas several studies have been carried out in this field using different separation methods. Out of them, membrane technologies are promising processes with minimal equipment, low energy consumption and eco-friendly3-8. Most recently, polymeric membranes are widely used due to their low cost, simple synthesis and excellent performance, such that polyethersulfone (PES) with high thermal stability and chemical resistance has rendered extensive applications in the various industrial processes9-11. The high efficiency in the membrane technologies can be achieved through the excellent water flux and antifouling properties and in order to create these conditions in the polymeric membranes, the membrane surface needs to be modified. Fabrication of mixed-matrix membranes (MMMs) is a simple and inexpensive method12 that usually involves the addition of filler. In contrast to the chemical modification method, the addition of filler does not require any changes to be made on the PES structure and only combines the features of nanoparticle and polymer13-14. On the other hand, the addition of nanoparticles will mainly affect the solvent/nonsolvent exchange rate, casting solution viscosity and membrane hydrophilicity15. One of the potential obstacles in the synthesis of MMMs is the incongruence between the polymer and filler13,16-17. To overcome this problem, metal-organic frameworks (MOFs) have been widely used in MMMs synthesis processes18-20. MOFs or porous crystalline materials are materials with excellent porosity and a specific surface area that can be marvelous options for modification of the membrane surface6, 21-27. Aluminum Fumarate (AlFu) or MIL-53(Al)-FA with a hydrophilic structure [Al(OH)(O2C-CH=CH-CO2)] is 2 ACS Paragon Plus Environment

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one of these materials, which unlike other MOFs, has higher stability in water. As shown in Figure 1, MIL-53(Al)-FA is formed by connecting fumarate with Al-OH-Al28-29 and has the same structure with MIL-53(Al)-BDC or Al(OH)(BDC)·H2O (BDC =1,4-benzenedicarboxylate) which is an extremely flexible MOF. Despite the many similarities between these two frameworks, Alvarez et al. showed that MIL-53(Al)-FA possess a rigid character structure with an invariable porosity such that the obtained porosity for MIL-53(Al)-FA was a little higher than the commercial aluminum fumarate sample (BASF Basolite A520) which confirms the reduction of inorganic impurities in the synthesized samples30.

Figure 1. The structure of AlFu nanoparticles (Plotted by using Basolite A520 CIF data30 and Mercury 3.6 software)

In recent years, numerous efforts have been devoted to the fabrication of composite membranes with MOFs3,

13, 31-36.

Composite membranes ZIF-8@GO/Nafion37 and ZIF-8@GO/Polyamide4

have been investigated for the improvement of proton conductivity and increasing the antibacterial activity of water, respectively. Yin et al. combined a MOF including amine functional groups with a ceramic ultrafiltration membrane for the first time and used it for the removal of lead ions from wastewater38. The results of the synthesized membrane of UiO-66/GO displayed that an increase 3 ACS Paragon Plus Environment


in the water flux or the membrane hydrophilicity improved the organic dye rejection14. MMMs including AlFu and cellulose acetate phthalate were prepared for the removal of fluoride and other pollutants such as iron and hardness of water29. Also, AlFu was combined with a polyacrylonitrile hollow fiber membrane1 which led to an increase in the hydrophilicity and permeability of these membranes. A new method for increasing the selectivity and permeability of MMMs was created by Xie et al. and their results exhibited excellent performance in the separation of CO2 and N2 39. Up to now, the effect of aluminum fumarate as one of the materials with the metal-organic framework on the hydrophilicity and changing the properties of polyethersulfone membranes has not been investigated. In this study, AlFu was synthesized and its structure was characterized by X-ray powder diffraction (XRD), transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Then, these nanofillers were employed in the fabrication of PES/AlFu MMMs and the membranes characterization was accomplished by FTIR, XRD, FESEM, transmission electron microscopy (TEM), atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDX) and elemental mapping analysis. To investigate the membranes hydrophilicity, the contact angle as a key parameter was measured and the membrane water permeability was studied in a dead-end ultrafiltration cell under constant pressure. The membrane performance was evaluated by the calculation of bovine serum albumin (BSA) rejection, flux recovery and membrane fouling resistances. Further, the MWCO of MMMs was assessed by the rejection of PEG in the various molecular weights. To recognize the membrane with the best performance, the surface roughness, porosity and mean pore radius of MMMs were estimated. 2. Experimental 2.1. Materials and methods 4 ACS Paragon Plus Environment

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Aluminum sulfate, fumaric acid, 1-methyl-2-pyrrolidone (NMP), polyethylene glycol (PEG; with molecular weights of 20, 35, 100 and 200 kDa) and polyvinylpyrrolidone (PVP, 24kDa) were purchased from Sigma-Aldrich and PES was supplied by BASF Co. Ltd., Germany. In all experiments deionized water (DI) was used as the non-solvent. Composite membranes were synthesized using a hand-casting knife and the ultrafiltration tests were conducted in a dead-end cell under constant pressure. 2.2. Synthesis of AlFu nanoparticles AlFu was synthesized according to other reports28,40. To do this, 18.55 g of aluminum sulfate was added to 79.5 ml of deionized water and was heated at 60°C for one hour to be completely dissolved. In another container, 6.45 g fumaric acid and 4.75 g sodium hydroxide were dissolved in 95.5 ml of DI water for 2 hours and were added to the first solution drop by drop. During this time, the solution temperature was maintained at 60°C and the final solution was centrifuged. After that, it washed multiple times with DI water until the pH reached about 7. The obtained solid after a drying step at 100°C was ground using a mortar and pestle. 2.3. Preparation of the MMMs In this study, the composite membranes were synthesized by the technique of phase inversion that NMP was employed as a solvent and PVP was used to increase the membrane water content. Initially, PES was mixed well with NMP solvent for 4 hours at 70°C, and then a completely homogeneous solution containing NMP and PVP was added to the first solution. The final composition of the casting solution was composed of 18wt% PES and 8wt% PVP in NMP solvent. The above process was repeated by adding AlFu nanoparticles with a weight percentage of 0.25, 5 ACS Paragon Plus Environment


0.5, 0.75 and 1.0 to a solution containing polymers (The calculations were based on without solvent). After that, a thin layer of the polymer solution was cast onto a glass plate with a 150micron casting knife. After casting, the glass plate was immediately immersed in a 25°C water bath for as long as the phase separation occurred and until the membrane separated from the glass. The membranes were kept in fresh water for 24 hours until the separation process was completed and eventually were stored between two filter papers for several days. 2.4. Characterization of AlFu nanoparticles and MMMs To identify the structure of AlFu nanoparticles and MMMs, the Thermo Nicolet Avatar 360 FTIR spectrometer was used in the range 500-4000 cm-1 and XRD analysis was performed using D4 Bruker Cu-Kα (λ= 0.15406 nm) in the 2θ range of 5-50º and a step size of 0.05º to obtain the morphology of the samples. The cross-sectional images and surface morphologies were also recorded by FESEM analysis (MIRA III TESCAN-XMU, The Czech Republic). In this method, at first the membranes were immersed in the liquid nitrogen and then fractured easily. To get the proper resolution, the fracture surface of the membranes was coated with gold metal. To better demonstrate the presence of AlFu nanoparticles in the synthesized membranes, the additional characterizations such as EDX spot, line, elemental mapping (or EDX mapping) and TEM analyses were conducted for the membrane with 1wt% of AlFu by FESEM instrument at 15 kV and TEM Philips EM208S-100kV microscope, respectively. For investigation of the membrane hydrophilicity, the contact angle was measured at ambient temperature and pressure using a contact angle apparatus by the sessile drop method from Fars EOR Technology. Due to the high sensitivity of the device in identifying completely uniform


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surfaces, the membrane samples were cut in small dimensions and pasted to the glass slides to create a uniform and thick surface. The surface roughness of the membranes was estimated by AFM (VEECO, USA) analysis in a scan size of 5µm×5µm. In this test, the device with considering the force between tip needle and the sample surface is located at a very close distance from the surface and sweeps it for image processing. The calculated surface roughness parameters including Ra (average roughness), Rq (root mean square roughness) and R3Z (the distance between the third highest peak and the third lowest valley) were employed to assess the surface roughness of the membranes. 2.5. Membrane performance To investigate the pure water flux of the synthesized membranes, a dead-end ultrafiltration system with an effective surface area of 33.1 cm2 was employed. The schematic picture of this setup is shown in Figure 2. In this system, to achieve a permanent state in the membranes and compact them, the pressure was adjusted to 4 bar and water was passed through the membrane for 30 minutes. After that, the water flux of membranes containing 0, 0.25, 0.5, 0.75 and 1.0wt% of nanoparticles was measured for 60 minutes under the pressure of 2 bar. The membrane water flux was determined using equation (1): 𝑄

(1)

𝐽𝑤1 = 𝐴𝛥𝑡

where Jw1 is the pure water flux (m/s), Q is the volume of the collected water in the permeation flow (m3), A is the effective membrane surface area (m2), and Δt is the sampling time (s).



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Figure 2. The schematic picture of the dead-end ultrafiltration system

Since proteins have extremely inclined to be absorbed on the membrane surface and cause the membrane pores clogging, BSA with a molecular weight of 66.5 kDa was used to evaluate the membrane rejection. The same procedure was described by Ma et al.14. For this purpose, initially, the pure water flux of each composite membrane is measured for 60 minutes (Jw1). Then, a 1000 mg/L BSA solution is passed through a dead-end ultrafiltration cell for 60 minutes under the constant pressure of 2 bar (JP). Subsequently, UV analysis (the UV-Visible spectrophotometer, S2100S model) was utilized at a 280-nm wavelength to measure the BSA concentration in the permeate stream. The rejection rate was determined by the equation (2).

%R  1 

Cp

(2)

CF


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where Cp and CF are the permeate and feed concentration, respectively. Finally, the fouled membrane with BSA was washed with distilled water for 10 minutes and then the flux of DI water was measured through this cleaned membrane (Jw2). To assess the efficiency of the membranes, the flux recovery (FR) and the fouling membrane resistances were calculated by the following equations13.

FR(%) 

J w2  100 J w1

(3)

Rt  Rm  Rr  Rir

(4)

Rm 

TMP   J w1

(5)

Rir 

TMP  Rm   J w2

(6)

Rr 

TMP  Rm  Rir   JP

(7)

where Rt, Rm, Rr and Rir are the total resistance (1/m), membrane resistance (1/m), reversible resistance (1/m) and irreversible resistance (1/m), respectively. In the above equations, TMP is the transmembrane pressure between the feed and permeate stream, which is considered 2 bar in this study and μ is the viscosity of the permeate stream. The MMMs porosity was also measured using BoneJ software To characterize a membrane, MWCO is a useful parameter that can describe the membrane pores size, such that the smaller MWCO demonstrates narrower pores in the membrane. MWCO is the minimum molecular weight of the polymer which 90% of the solute particles are rejected by the 9 ACS Paragon Plus Environment


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membrane. The MWCO was measured experimentally by the rejection of PEG in the average molecular weights of 20, 35, 100 and 200 kDa under the constant pressure of 2 bar. The total organic carbon analyzer (TOC, SHIMADZU) was used for the detection of PEG concentration in the feed and permeate solution and the rejection of PEG was calculated by the equation (2). The mean pore radius of the membranes can be determined using the below equation41: rm(nm) = 0.1[0.262(MWCO(Da))0.5 ―0.3]

(8)

To examine the stability of AlFu in the mixed-matrix membranes, the concentration of Al ions in the permeate solution was studied. For this purpose, three solutions containing distilled water and popular membrane cleaning solutions (nitric acid 10wt% and sodium hydroxide 10wt%) were prepared and for each feed solution, the membrane with 0.75wt% of AlFu was placed in the deadend ultrafiltration cell for 60 min under the constant pressure of 2 bar. After that, the concentration of Al ions released in the permeate solution was measured by the inductively coupled plasma atomic emission spectrophotometer (ICP-AES, Optima, 7300DV, USA) analysis. It was expected this concentration be low as the AlFu nanoparticles were in the membrane casting solution and they didn’t coat on the membrane surface which can be easily separated. 3. Results and discussion 3.1. AlFu nanoparticles characterization The MOF structure is investigated by FTIR analysis and the results are shown in Figure 3(a). As seen from this figure, the peaks of 500-650 cm-1, 700-1200 cm-1, 1428 cm-1 and 3400 cm-1 related to the bending vibration of Al-O, stretching vibration of Al-O, the bending vibration of O-H and the stretching vibration of O-H, respectively. The AlFu structure is also accredited with XRD analysis and the results are illustrated in Figure 3(b). AlFu nanoparticles possess sharp peaks at


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10.0, 15.6, 18.3, 21.1, 31.8 and 43.9 degrees which are in good agreement with the experimental XRD results for a BASF Basolite A520 sample as a commercial and available aluminum fumarate28,30,42 that confirm the successful synthesis of these nanoparticles. The FESEM analysis was employed to study the surface morphology of the AlFu nanoparticles. As can be observed in Figure 3(c), the nanostructure of aluminum fumarate is a rice-shaped elongated which is clearly visible at high magnification (Figure 3(d)). According to this figure, the average size of AlFu nanoparticle is obtained about 26 nm. 120

a

100

Transmittance(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0 4000

3500

3000

2500

2000

Wavenumber(cm-1)


1500

1000

500


b

experimental XRD for a (commercial) Basolite A520 sample 42

Intensity(a.u.)

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5

10

15

20

25

30

35

40

45

50

2θ(degree) c

d

Figure 3. (a) FTIR, (b) XRD, (c) FESEM image and (d) High resolution of FESEM image of AlFu nanoparticles

3.2. Characterization of MMMs To confirm the structure of composite membranes and to evaluate their morphology, FTIR, XRD, FESEM, TEM, EDX and elemental mapping analyses were applied to each membrane sheets, and the results are represented in Figures S1-S3(Supporting Information) and 4(a)- 4(d). The results of measuring the average water contact angle of PES composites are shown in Figure 5 and 3D AFM images of the composite membranes are demonstrated in Figure 6. 12 ACS Paragon Plus Environment

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Figure S1 illustrates the FTIR patterns of composite membranes containing pure PES, 0.25wt%, 0.5wt%, 0.75wt% and 1.0wt% of AlFu nanoparticles which peaks of 1097 and 1148 cm-1 represent stretching band of S=O, 1232, 1249 and 1336 cm-1 correspond to the C-O-C bond stretching and 1484 and 1569 cm-1 can be attributed to the benzene ring stretching which is in good consonance with the literature43,44. As can be seen in this figure, there is no discrepancy between neat PES membrane and other MMMs patterns and similar peaks can be observed in the range of 600-3600 cm-1. It can be concluded these nanoparticles could not create a new functional group in the mixed matrix membrane structure. The XRD patterns of composite membranes containing pure PES, 0.25, 0.5, 0.75 and 1.0wt% of AlFu nanoparticles are shown in Figure 4(a), which is compared with the XRD pattern of AlFu nanoparticles. The PES has only one wide peak at 18.5 degrees due to its amorphous structure. In the composite membranes, first, the weight percentage of the nanoparticles is low and the structure is very similar to the PES. With an increase in the weight percentage of the nanoparticles, at 0.75wt% and 1wt%, the AlFu peaks appeared at 18.3 and 31.8 degrees, which confirms the presence of nanoparticles in the PES structure. The FSEM images of the cross-section morphology of composite membranes consisting of pure PES, 0.75wt% and 1wt% of AlFu nanoparticles are shown in Figure 4(b). As shown in this figure, all composite membranes are composed of a dense layer attached to a porous substrate with a finger-like structure. By increasing the distance from the membrane outer layer and approaching to the porous layer and substrate, the structure becomes more porous. With an increase in the weight percentage of the nanoparticles compared to the pure PES, these finger-like pores are slightly more elongated and thicker, and a macrovoid structure can be observed which improves the water permeability. The cause of this phenomenon can be attributed to the rapid exchange of 13 ACS Paragon Plus Environment


non-solvent and solvent during the process of membrane fabrication. As can be seen in this figure, the MMM containing 1wt% of AlFu nanoparticles shows the aggregation of hydrophilic groups near the membrane surface. The FESEM images correspond to the weight percentages of 0.25 and 0.5 of nanoparticles are shown in Figure S2. Moreover, to get the direct evidence of the nanoparticles, the top view FESEM and TEM images of the MMM containing 1wt% of AlFu nanoparticles are presented in Figures S3 (a) and (b), respectively. This figure signifies that the fabricated MMM is a pinhole free membrane and almost spherical nanoparticles can be accumulated in this loading of AlFu nanoparticles.

Figure 4(a). Comparison of XRD patterns of MMMs and AlFu nanoparticles


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a1

b1

a2

b2

a3

b3

Figure 4(b). FESEM images of the cross-section morphology of composite membranes with different AlFu loadings; (a1, b1): pure PES, (a2, b2): 0.75wt%, (a3, b3):1wt%.

The EDX spot analysis of MMM with 1wt% of AlFu nanoparticles is illustrated in Figure 4(c)-(I) which detected signals in the 1-2 keV confirm the presence of aluminum ions as the main element



of AlFu nanoparticles in the mixed-matrix membrane. In comparison with stronger signals such as C and S which accord with the PES elements, the weaker signals of Al indicate the small amount of AlFu nanoparticles in the mixed-matrix membrane. The average weight and atomic percentages of these elements are tabulated in Figure 4(c)-(I). Figure 4(c)-(II) also exhibit the EDS line analysis of this MMM which red, green and black profiles are related to the Al, S and O elements, respectively. The presented element distribution profile depicts an evenly dispensation of Al ions along the matrix membrane and also their accumulation near the membrane surface, which was in good agreement with the FESEM image of this sample (Figure 4(b)-b3). To better elucidate the presence of AlFu nanoparticles, the elemental mapping analysis is also performed for this MMM and the results are represented in Figure 4(d). EDX mapping is a worthwhile analysis which can illustrate the elemental distribution and each colored map exhibits the spreading of a specific component. It is evident that Al mapping (red points) corresponds to the AlFu nanoparticles and blue, green and purple colors identify Carbon, Oxygen and Sulfur elements in the PES, respectively.

1200

C K

Element Wt% At% C 63.59 80.66 O 3.99 3.80 Al 1.50 0.85 S 30.92 14.69

(I)

1100 1000 900 800 700

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

S K 500 400 300

O K

200 100

AlK S K

keV

0 0

5

10


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W%

35

(II)

S Al O

30

25

20

15

10

5

0

µm 0

97

Figure 4(c). EDX analysis of mixed-matrix membrane with 1wt% of AlFu nanoparticles; (I): EDX spot analysis, (II): EDX line analysis.



Figure 4(d). Elemental mapping analysis of mixed-matrix membrane with 1wt% of AlFu nanoparticles

In general, a surface with a contact angle less than 90º is known as a hydrophilic surface. The smaller contact angle is related to the greater wettability45. As shown in Figure 5, the pure PES demonstrated the maximum average water contact angle and by blending with 0.25wt% to 1wt% of AlFu nanoparticles, these values reduced in the range of 65-73º. Composite membranes containing 0.75wt% of AlFu have the lowest average water contact angle about 65º and are more willing to pass water. In other words, the membrane becomes more hydrophilic in this weight percentage and is likely play an important role in improving the water flux and antifouling properties. In amounts greater than 0.75wt%, the accumulation of hydrophilic groups on the membrane surface may block the membrane pores and consequently decrease hydrophilicity. For description of this phenomenon, it can be said that by adding more hydrophilic nanoparticles in the casting solution, the accumulation does not allow to these nanoparticles to fully cover the surface and this decrease in the surface coverage leads to an increase in the contact angle46,47.


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80 75

Contact angle(ₒ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 65 60 55 50 45 0

0.25

0.5

0.75

1

Loading(wt%)

Figure 5. The average water contact angle of PES composite membranes containing different weight percentages of AlFu nanoparticles

The 3D AFM images of the MMMs and their calculated surface roughness parameters are presented in Figure 6 and Table 1, respectively. In this figure, the bright spots indicate areas with higher heights. The results showed that the addition of nanoparticle reduces the surface roughness from 10.4 nm to 7.4 nm as 0.75wt% of AlFu nanoparticles is added and it was reported the reduced surface roughness would decrease the interaction between foulants and membrane surface if it can be assumed that foulants are adsorbed on the surface valleys13-15. So that it can be said the lower surface roughness has a considerable effect on the anti-fouling properties of PES composite membranes. In the membrane with the loading of 1wt% of AlFu nanoparticles, the surface roughness is increased due to the accumulation of hydrophilic nanoparticles such that it has a maximum average roughness of 9.8 nm which is less than the average roughness of pure PES. As shown in Table 1, the membrane with 0.75wt% of the nanoparticle possesses the lowest average roughness (7.4 nm) and as expected the trapping of pollutants in this membrane is diminished not



only because of the smoother surface but also due to the presence of hydrophilic groups in AlFu nanoparticles15.

Figure 6. The 3D AFM images of composite membranes ((a) pure PES, (b) 0.25wt%, (c) 0.5wt%, (d) 0.75wt%, (e)

1wt% of AlFu nanoparticles)


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Table 1. Surface roughness parameters of MMMs

Roughness parameters Membranes Ra(nm)

Rq(nm)

R3Z(nm)

pure PES

10.4 ± 0.4

15.6 ± 3.8

89.2 ± 0.1

PES/0.25wt% of AlFu

8.7 ± 0.4

11.9 ± 3.1

68.4 ± 2.2

PES/0.50wt% of AlFu

7.9 ± 0.5

9.8 ± 5.2

44.4 ± 0.4

PES/0.75wt% of AlFu

7.4 ± 0.5

9.5 ± 2.3

43.2 ± 0.1

PES/1wt%

9.8 ± 0.6

15.1 ± 0.9

72.0 ± 0.1

of AlFu

3.3. Evaluation of the water flux The average water flux of the composite membranes at 200 kPa is depicted in Figure 7. With an increase in the loading of AlFu nanoparticles from 0 to 0.75wt%, the water flux is enhanced from 1.4 LMH to 7.7 LMH and with a further increase the water flux is decreased. This reduction can be attributed to the membrane pores blocking which confirms the obtained results from the hydrophilicity of composite membranes in various percentages of nanoparticles. According to the following figure, the water flux of membranes containing nanoparticles is higher than the pure PES, which indicates a good performance of the AlFu nanoparticles in the creation of more hydrophilic PES membranes.



10 9

Water flux(LMH)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8 7 6 5 4 3 2 1 0 0

0.25

0.5

0.75

1

Nanoparticle loading (wt%)

Figure 7. Average water flux of composite membranes containing various weight percentages of AlFu nanoparticles at 200 kPa after 1 hour (The error bars in each loading of nanoparticle are calculated from the average of obtained performance results in the several different membranes)

3.4. Evaluation of the BSA rejection and membrane fouling resistance As can be seen in Figure 8, the BSA rejection is varied from 79.2 % to 99.7% whereas the rejection of pure PES initially was 90.1% and after adding nanoparticles is reached to 99.7%. The protein rejection 79.2% is related to the nanoparticle loading of 0.75wt%, and this reduction can be attributed to an increase in the size of the membrane pores and its permeability. Indeed, the high membrane hydrophilicity in this weight percent creates a weakness bond between the BSA and the polymer and causes the protein molecules to remove easily from the membrane surface and enter the permeate flow.


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120 100

Rejection(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 0 0

0.2

0.4

0.6

0.8

1

1.2

Nanoparticle loading(Wt%)

Figure 8. BSA rejection in the composite membranes with different weight percentages of nanoparticle

The calculated fouling resistance for each of the composite membranes is represented in Table 2. According to the results, the pure PES membrane possessed higher membrane resistance, reversible resistance, irreversible resistance and total resistance. The composite membrane with 0.75wt% of AlFu nanoparticles exhibited the lowest total resistance and the highest rate of flux recovery. Fouling resistances and flux recovery for other weight percentages of the nanoparticle are also less and more than pure PES, respectively. In other words, the addition of nanoparticles significantly reduced the membrane fouling compared to pure PES, which can be attributed to the ability of these nanoparticles in the creation of new pores and more hydrophilic PES membranes. Also, these nanofillers proved to be able to slightly enhance the membrane performance after fouling and rinsing such that the flux recovery in the loading of 0.75wt% of AlFu nanoparticles is shifted up to 96.88%.



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Table 2. Calculated fouling resistances for composite membranes with different weight percentages of AlFu nanoparticles Rm*1014

Rir*1014

Rr*1014

Rt*1014

(1/m)

(1/m)

(1/m)

(1/m)

Pure PES

5.04

0.41

1.14

6.58

92.51

0.25wt%

2.43

0.11

0.43

2.96

95.75

0.50wt%

2.24

0.07

0.10

2.41

96.86

0.75wt%

0.94

0.03

0.19

1.16

96.88

1wt%

3.38

0.20

0.02

3.60

94.47

FR (%)

3.5. Measurement of the porosity, MWCO and mean pore radius of MMMs The measured porosities of the mixed-matrix membranes are represented in Figure 9(a). As can be seen in this figure, by increasing the nanoparticle loading to 0.75wt%, the porosity has slightly increased compared to the pure PES. In the nanoparticle loading of 1wt%, the pores probably are blocked due to the accumulation of nanoparticles and the porosity is reduced to 54.2%.


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70 60

Porosity(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


58.6

60.8

62.2 54.2

52.8

50 40 30 20 10 0 0

0.25

0.5

0.75

1

Loading(wt%)

Figure 9(a). The surface porosity of MMMs in different weight percentages of AlFu nanoparticles

The MWCO results are shown in Figure 9(b) that based on this figure, the MWCO of the pristine PES is about 27 kDa and for modified membranes shifted up to 154 kDa. The MWCO of 27, 29, 31, 154 and 144 kDa can be related to the mean pore radiuses 4.3, 4.4, 4.6, 10.2 and 9.9 nm, respectively which are exhibited in Figure 9(c). The obtained results depict that by the addition of AlFu nanoparticles as the hydrophilic agents, the pores become larger and the surface porosity increases, and subsequently will lead to more flux. Moreover, the neat PES with a pore radius of 4.3 nm has the smallest pore size and the addition of nanoparticles increases this value such that the mean pore radius reaches to the maximum value of 10.2 nm at the nanoparticle loading of 0.75wt%. These results are consistent with the increase in permeability and reduction in the BSA rejection in this weight percentage. Also, according to the figure 9(c), the estimated mean pore radius for MMM with 0.75wt% of the nanoparticle is 10.2 nm that is smaller than the AlFu nanoparticle size. So that with a further increase in the weight percentage of nanoparticles, due to the accumulation of nanoparticles on the membrane surface and the membrane pore blocking, the mean pore radius reduces to 9.9 nm. 25 ACS Paragon Plus Environment


0 wt%

Rejection(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.25 wt%

0.5 wt%

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0.75 wt%

1 wt%

110 100 90 80 70 60 50 40 30 20 10 0 0

25

50

75

100

125

150

175

200

225

MW(kDa)

Figure 9(b). The molecular weight cut-off of pristine PES and PES/AlFu composite membranes.

180

154

160

144

140 120 100 Mean pore radius(nm) MWCO(kDa)

80 60

20

4.3

31

29

27

40

4.6

4.4

10.2

9.9

0

0

0.25 0.5 Loading of AlFu (wt%)

0.75

1

Figure 9(c). The measured MWCO and mean pore radius of MMMs

According to the literature review48-50, the MWCO provides only a rough approximation of the membrane retention for uncharged solutes. Moreover, retention of the BSA molecules can be affected not only by the MWCO but also other properties such as molecular width, steric 26 ACS Paragon Plus Environment

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hindrance, electrostatic repulsion, surface porosity and physicochemical interaction between BSA and membrane surface. The rejection diagram for organic compounds based on solute and membrane properties well categorized by Bellona et al.49. This unexpected trend between solute rejection and MWCO can be seen in the other works51,52. These results support that the MWCO is not an enough parameter for evaluation of the solute rejection behavior and a weak correlation between MWCO and solute rejection may be obtained since this parameter cannot consider the effect of the solute- membrane interaction and permeation. Also, the charge effects have an essential role in the prediction of solute retention especially when the pore sizes become larger53. The results of AlFu leaching from the mixed-matrix membrane with 0.75wt% of AlFu are shown in Figure 10. The concentration of Al ions in the permeate solution is very low (up to 3 µg/l) which is less than the maximum contaminated level of this metal in the drinking water (0.2 mg/l) and illustrates the high ability of Al ions in the chelation with PES polymer.



Figure 10. Investigation of the released Al ions concentration from the MMM membrane containing 0.75wt% of AlFu nanoparticle by three feed solutions (Distilled water, nitric acid 10wt% and sodium hydroxide 10wt%)

4. Conclusion In this study, AlFu nanoparticles were initially synthesized as a metal-organic framework material and their structure was confirmed by XRD, FTIR and FESEM analysis. Subsequently, the composite membranes of PES/AlFu were prepared using the technique of phase inversion and the membranes characterization was accomplished by FTIR, XRD, FESEM, TEM, AFM, EDX and elemental mapping analysis. The measured contact angles in the various weight percentages 0.25 to 1.0 of AlFu nanoparticles is in the range of 65-73º which indicates an increase in the PES hydrophilicity, especially in the composition of 0.75wt% and by increasing this value to 1.0wt%, the membrane surface hydrophilicity decreases. The surface roughness experiments showed that the AlFu nanoparticles reduce the surface roughness and tendency to interact with the pollutants by increasing in the surface hydrophilicity. Also, the results of MMMs water permeability in a


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dead-end ultrafiltration system illustrated that the membrane with the loading of 0.75wt% of nanoparticles possesses the highest pure water flux under 200 kPa pressure, which was consistent with the membrane hydrophilicity results. The calculated fouling resistance of composites illustrated that the composite membrane with 0.75wt% of AlFu nanoparticles renders the lowest total resistance and the highest rate of flux recovery (96.88%). In this weight percentage, the membrane with the maximum porosity of 62.2%, mean pore radius of 10.2 nm and MWCO of 154kDa demonstrates a favorable performance. Acknowledgments The authors would like to express their gratitude to Semnan University for the financial support provided for this work and also thank Mr. Azhdari for collaboration in this study. Supporting Information Figure S1: FTIR patterns of MMMs with different loadings of AlFu nanoparticles; Figure S2: FESEM images of the cross-section morphology of composite membranes with different AlFu loadings; (a1, b1): 0.25wt%, (a2, b2):0.5wt%; Figure S3: a) The top view FESEM image and b) TEM image of the MMM containing 1wt% of AlFu nanoparticles. Nomenclature Symbols Jw1

Pure water flux (m/s)

Q

Volume of the collected water in the permeation flow (m3)

A

Effective membrane surface area (m2)

∆t

Sampling time (s) 29 ACS Paragon Plus Environment


R

Rejection rate (dimensionless)

CP

Permeate concentration (mg/L)

CF

Feed concentration (mg/L)

JP

BSA filtration flux (m/s)

Jw2

Water flux of cleaned membrane (m/s)

µ

Viscosity (kg/(m.s))

Rt

Total resistance (1/m)

Rm

Membrane resistance (1/m)

Rr

Reversible resistance (1/m)

Rir

Irreversible resistance (1/m)

rm

Mean pore radius (nm)

Ra

Average roughness (nm)

Rq

Root mean square roughness (nm)

R3z

The distance between the third highest peak and the third lowest valley (nm)

Abbreviations AlFu

Aluminum Fumarate

PES

Polyethersulfone

XRD

X-ray powder diffraction


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FTIR

Fourier transform infrared spectroscopy

FESEM

Field emission scanning electron microscopy

EDX

Energy dispersive X-ray spectroscopy

TEM

Transmission electron microscopy

MMM

Mixed-matrix membrane

AFM

Atomic force microscopy

PEG

Polyethylene glycol

BSA

Bovine serum albumin

MOF

Metal-organic frameworks

NMP

1-methyl-2-pyrrolidone

PVP

Polyvinylpyrrolidone

TMP

Transmembrane pressure (Pa)

MWCO

Molecular weight cut-off (kDa)

LMH

Unit of flux (L/(m2.h))

FR

Flux recovery



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Graphical Abstract


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Enhanced Hydrophilicity and Water Flux of Polyethersulfone

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