Comparing the effect of incorporation of various nanoparticulate on the performance and antifouling properties of polyethersulfone nanocomposite membranes

Journal of Water Process Engineering 27 (2019) 47–57

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Comparing the effect of incorporation of various nanoparticulate on the performance and antifouling properties of polyethersulfone nanocomposite membranes Mohammad Hossein Davood Abadi Farahania, Hesamoddin Rabieeb, Vahid Vatanpoura, a b

T

⁎

Department of Applied Chemistry, Faculty of Chemistry, Kharazmi University, P.O. Box 15719-14911, Tehran, Iran Advanced Water Management Centre, The University of Queensland, St. Lucia, QLD 4072, Australia

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanocomposite membranes Ultrafiltration Polyethersulfone Antifouling Nanoparticulate

We have prepared polyethersulfone (PES) nanocomposite membranes comprising of cloisite 30B clay, SiO2, TiO2, hydroxyl-functionalized multi-walled carbon nanotubes (MWCNTs-OH), and carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH). The nanoparticulate at five different concentrations were added into the polymeric dope solutions including 15 or 18 wt% of PES, N,N-dimethylacetamide (DMAc) or 1methyl-2-pyrrolidone (NMP) as solvents, and 1 wt% of pore forming agent. These nanoparticulate may improve porosity, hydrophilicity, and open the channels up for improved water transport. Although many nanoparticulate with various properties and functionalities have been used for the same aims, the efforts have been made to choose the best nanomaterial and its loading among these five famous nanofillers for modifying PES ultrafiltration membrane. The nanocomposite membranes showed better antifouling characteristics and water permeability as compared with the pristine PES membrane because of the porosity and hydrophilicity improvements. However, further increasing nanoparticulate loading led to a possible agglomeration and diminished the water flux and fouling resistance. The effect of polymer concentration and solvent showed that membranes fabricated using 15 wt% of PES concentration possessed greater fouling resistance and water flux compared to those of fabricated using 18 wt% of PES concentration. Also, membranes fabricated using DMAc exhibited a more porous structure with considerably greater water flux as compared with those of fabricated using NMP as the solvent. Based on the results, 1 wt% of TiO2 is recommended as the best nanoparticulate for the PES ultrafiltration membrane modification since it exhibited the superlative performance with a 320% water flux enhancement, nearly 98% BSA rejection, and 130% FRR improvement.

1. Introduction Recently, more attentions have been paid to membrane-based separation technologies owing to their remarkable benefits in industrial processes. The membrane separations’ key advantages include relatively low energy consumption, high separation efficiency, ease of scale-up and operation, and no (low) phase changes or chemical additives. Additionally, separation, concentration, and purification are industrially capable using this technology [1]. Therefore, membranes have been widely applied for quite a few processes, for example, water and wastewater treatment [2–5], gas purification [6], food process [7], pharmaceutical industry [8], and environmental-associated problems [9]. Membranes determine both practical application and efficiency of a membrane-based separation process. Now, almost all industrially ⁎

available membranes are fabricated by means of inorganic or/and organic (polymers) materials, and the latter leads the existing membrane marketplace thanks to the ease of preparation and modification as well as comparatively low price. Instances of organic polymers include polyamide, polyimide, polyacrylonitrile, polytetrafluoroethylene, polysulfone (PSf), polyethersulfone (PES), and polyvinylidene fluoride (PVDF). The characteristic properties of PES, a commercially available polymer, comprise excellent mechanical, thermal, and chemical strength, environmental toleration, simple process, and relative resistance to heat aging; these are, actually, the key motives for the extensive application of PES [10,11]. As yet, several reports have been published on diverse implementation of PES in membrane-based separation processes, including biofuel recovery [12], membrane distillation [13], gas separation [14], ultrafiltration (UF) [15,16],

Corresponding author. E-mail addresses: [email protected], [email protected] (V. Vatanpour).

https://doi.org/10.1016/j.jwpe.2018.11.012 Received 14 August 2018; Received in revised form 6 November 2018; Accepted 21 November 2018 2214-7144/ © 2018 Published by Elsevier Ltd.

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nanofiltration (NF) [17,18] and so on. Moreover, quite a few commercially available membranes are made-up of PES materials. Besides, it allows an outstanding processability to fabricate both flat sheet and hollow fiber membranes. The common method employed to fabricate asymmetric polymeric membranes is the non-solvent induced phase separation (NIPS) technique. By using this approach to prepare the PES polymeric membrane, the performance limitation of the fabricated membranes can be categorized in some major problems: high pore blockage, high fouling tendency, and low flux. These disadvantages could be attributed to the hydrophobic nature and deposition or adsorption of bulky molecules on the pores of the PES-based membrane surface. The fouling tendency will significantly reduce the membrane efficiency and diminish the performance. Consequently, it raises the operational expenses and often leads to failure. To diminish the fouling consequences, one of the effective techniques is improving the hydrophilicity of PES membranes [16–18]. These modification techniques are categorized as surface modifications, physical blending, chemical grafting, and mixed matrix membrane (MMM), in which, manufacturing MMMs has the advantages of utilizing the excellent properties of inorganic materials—functional characteristics as well as mechanical, thermal and chemical stability—and the ease of processing and low cost of organic polymeric materials [19]. MMMs comprising a polymer and nanoscale materials such as metal oxide nanoparticulate, silica, zeolite, graphene oxide, and carbon nanotubes (CNTs) have gained substantial attention for their excellent performance in gas, pervaporation, organic solvent nanofiltration, NF, and UF membranes [19–27]. The combination of nanoscale materials and polymers results in higher hydrophilicity, improved fouling-resistance, enhanced permeability and selectivity, greater porosity, and boosted mechanical properties [26,28,29]. The MMMs development including homogeneously dispersed nanoscale materials in a polymeric matrix has been initiated many years ago [30]. Among several nanoscale materials, clay, MWCNTs-OH, MWCNTs-COOH, TiO2, and SiO2 have been found to possess low price, good stability, commercially availability, and potential for water treatment applications [18,20,25,29,31–34]. Nevertheless, there is an unanswered question, that is, “which of the mentioned nanoscale materials (and how much of them) are the most effective choice for PES UF membrane modification?” To the best of our knowledge, there is no comprehensive study to compare the effects of five various nanoparticulate and their concentrations on PES UF membrane performance. Moreover, the effects of PES concentration as well as two different solvents, namely N,N-dimethylacetamide (DMAc) and 1-methyl-2-pyrrolidone (NMP), on PES UF membrane performance were investigated. This study aims to prepare asymmetric PES flat sheet MMMs with a greater hydrophilicity, enhanced antifouling properties, and high permeability features. Consequently, a neat PES membrane and MMMs containing clay, MWCNTs-OH, MWCNTs-COOH, TiO2, and SiO2 nanoparticulate were prepared by the NIPS technique. While using a fixed polyvinylpyrrolidone (PVP) pore former concentration, the effects of (1) PES concentration, (2) solvent nature, (3) five various nanoparticulate, and (4) nanoparticulate loadings on PES UF membrane, morphology, water flux, hydrophilicity, as well as antifouling characteristics were systematically investigated.

(20–30 nm), and TiO2 (20 nm) nanoparticulate were obtained from Southern Clay (Gonzales, TX), US Nano Co., and Degussa (P25, Germany), respectively. due to more using in membrane modification. Cloisite 30B is an organomodified hydrous aluminium phyllosilicates with particle size of > 2 μm and density of 1.98 g/cm3. The N,N-dimethylacetamide (DMAc, Sigma-Aldrich) and 1-methyl-2-pyrrolidone (NMP, Merck) were utilized as the solvents for the preparation of the dope solutions. Bovine serum albumin (BSA) and polyvinylpyrrolidone (PVP, Mw = 29,000 g/mol) were acquired from Merck. 2.2. Fabrication of PES MMMs The asymmetric flat sheet membranes and MMMs were fabricated using the NIPS technique [28,29]. Precise quantities of TiO2, SiO2, MWCNTs-COOH, MWCNTs-OH, or clay as well as 1 wt% of PVP (based on polymer mass) were added into the either DMAc or NMP. To get homogeneous dispersions, the resultant solutions were subjected to the sonication for 30 min. Next, 15 or 18 wt% of PES were added into the solutions and dissolved by constant stirring at ambient temperature for 24 h. Subsequently, the resultant solutions were degassed before casting using a sonicator for 30 min. Next, the degassed solutions were sprinkled on a glass plate for casting by means of an adjustable casting knife (Elcometer 3580) where the gap is fixed at 150 μm. The as-cast polymeric films were then immersed in a deionized water (DI water) coagulation bath at 25 °C. Afterward, the membranes were transferred to a fresh DI water bath for 24 h in order to remove any residual solvent and complete the phase separation. The membranes were kept in water for further tests. Table 1 tabulates the compositions of the prepared membranes. 2.3. Membrane characterizations A scanning electron microscope (SEM, VEGA║(TESCAN, Czech Republic)) was utilized to explore the cross-sectional and surface morphologies of the prepared membranes. The SEM was employed under a high vacuum condition at 20 kV. The freeze-dried membranes were first immersed in liquid nitrogen, fractured, and then sputtercoated with a thin layer of gold to diminish the sample charge. Images of 3 μl DI water droplets on the membrane surface were taken by a contact angle measurement instrument (DSA100, Krüss, Germany) at 25 °C and a relative humidity of 50%. DI water as the probe liquid was utilized to explore the fabricated membranes’ surface hydrophilicity. More than six images at various places on the membrane surface were taken to get the average contact angles. 2.4. Pure water flux (PWF) and separation experiments Fig. 1 illustrates the scheme of permeation setup used in this study. Membranes with the effective filtration area of 19.6 cm2 were fixed at the bottom of the dead-end permeation cell. Pure water or BSA/water solution were poured in the feed tank, where a nitrogen gas cylinder was used to provide the transmembrane pressure and the liquid inside the cell was stirred. The permeate weight was continuously recorded by a digital balance connected to a computer in a given time. The membranes were pretreated and stabilized at 2 bar for 30 min before the actual measurements. Afterward, the pure water flux and BSA rejection experiments were carried out at 1 bar. The pure water flux, PWF (L/m2 h or LMH), was determined by the Eq. (1):

2. Materials and methods 2.1. Materials The polyethersulfone (PES, Mw = 58,000 g/mol) was purchased from BASF (Germany) and used after drying under vacuum overnight at 60 °C. Hydroxyl-functionalized multi-walled carbon nanotubes (MWCNTs-OH) and carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH) with lengths of 0.5–2 μm, outer diameters of 5–15 nm and the −OH and −COOH content of 2.0 wt% were obtained from US Research Nanomaterials, Inc., the USA. Cloisite 30B clay, SiO2

PWF =

V A × Δt

(1)

where V (L) is the permeate volume, A (m ) is the effective filtration area, and Δt (h) is the permeation time. A 500 ppm BSA solution was filled into the feed tank for examining the membranes fouling resistance and rejection right after the first pure water permeation test. After 90 min BSA solution filtration, three feed 2

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Table 1 Compositions of the prepared membranes. Membrane ID

Nanoparticulate

Nanoparticulate (wt%)*

PVP (wt%)*

PES (wt%)

Solvent

P15-N X-C-P15-N X-S-P15-N X-T-P15-N X-A-P15-N X-B-P15-N P15-D X-C-P15-D X-S-P15-D X-T-P15-D X-A-P15-D X-B-P15-D P18-N X-C-P18-N X-S-P18-N X-T-P18-N X-A-P18-N X-B-P18-N P18-D X-C-P18-D X-S-P18-D X-T-P18-D X-A-P18-D X-B-P18-D

– C = Clay S = SiO2 T = TiO2 A = MWCNTs-OH B = MWCNTs-COOH – C = Clay S = SiO2 T = TiO2 A = MWCNTs-OH B = MWCNTs-COOH – C = Clay S = SiO2 T = TiO2 A = MWCNTs-OH B = MWCNTs-COOH – C = Clay S = SiO2 T = TiO2 A = MWCNTs-OH B = MWCNTs-COOH

0.00 X = 0.2, 0.5, 1, 2, 4 X = 0.2, 0.5, 1, 2, 4 X = 0.2, 0.5, 1, 2, 4 X = 0.05, 0.1, 0.2, 0.3, X = 0.05, 0.1, 0.2, 0.3, 0.00 X= 0.5 X= 0.5 X= 1 X = 0.05 X = 0.05 0.00 X = 0.2, 0.5, 1, 2, 4 X = 0.2, 0.5, 1, 2, 4 X = 0.2, 0.5, 1, 2, 4 X = 0.05, 0.1, 0.2, 0.3, X = 0.05, 0.1, 0.2, 0.3, 0.00 X= 0.5 X= 1 X= 0.5 X = 0.2 X= 0.1

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0 18.0

NMP NMP NMP NMP NMP NMP DMAc DMAc DMAc DMAc DMAc DMAc NMP NMP NMP NMP NMP NMP DMAc DMAc DMAc DMAc DMAc DMAc

0.5 0.5

0.5 0.5

* Based on polymer mass.

where Cp and Cf are the BSA concentrations in permeate and feed, respectively. 2.5. Porosity The gravimetric method based on the water sorption was used to measure the overall porosity (ε; %) of the prepared membranes, as written in the following equation [35]:

ε (%) =

w1 − w 2 × 100 A × l × dw

(4)

where w1 and w2 are the weights of membranes in wet and dry conditions (kg), respectively; A is the dry membrane area (m2), l is the dry membrane thickness (m), and dw is the water density (998 kg/m3). 3. Results and discussion 3.1. Morphological studies The surface, cross-sectional, and enlarged cross-sectional morphology of the pristine PES membranes and PES MMMs (clay/PES, SiO2/PES, TiO2/PES, MWCNTs-OH/PES, and MWCNTs-COOH/PES) fabricated using two different concentrations of PES, namely 15 and 18 wt%, were observed by SEM, as illustrated in Figs. 2 and 3, respectively. The cross-sectional images for both pristine PES membranes and MMMs fabricated in this work presented a typical porous and asymmetric structure, which owns a very thin semi-dense top layer as well as a porous and rich in macrovoids sub-layer. Based on the crosssectional SEM images, both pristine PES membranes and PES MMMs possess a relatively alike macrovoid structure that is due to the significant affinity between solvents (NMP or DMAc) and non-solvent (water) as well as immediate demixing during phase inversion. However, fabricated membranes using PES concentration of 15 wt% (Fig. 2) apparently demonstrate higher porosity (Fig. 4) and more macrovoids in their structure compared to those fabricated membranes using PES concentration of 18 wt% (Fig. 3). Generally, the incorporation of hydrophilic nanoparticulate influences both kinetics and thermodynamic of the phase inversion process, which leads to the observation of dissimilar morphologies [36,37]. Unlike pristine PES dope solution,

Fig. 1. Scheme of the in-house dead-end permeation setup.

samples and permeates were obtained every 10 min to make sure the rejection is stable and then the BSA rejection was measured. Next, the membranes were brought out of the cells in order to putting them in fresh water for 30 min. Meanwhile, the feed tank was washed using DI water. To calculate the flux recovery ratio (FRR, %) of the membranes, the second pure water flux experiment was then performed. The following equation was used to determine FRR:

FRR (%) =

PWFwater ,2 × 100 PWFwater ,1

(2)

where PWFwater,2 and PWFwater,1 are the pure water flux after and before BSA solution permeation test, respectively. To evaluate the separation ability of the membranes, the BSA rejection (R, %) was determined using the following equation:

Cp ⎞ R (%) = ⎜⎛1 − ⎟ × 100 Cf ⎠ ⎝

(3) 49

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Fig. 2. Surface, cross-sectional, and enlarged cross-sectional SEM images of the a) pristine PES membrane, b) clay/PES, c) SiO2/PES, d) TiO2/PES, e) MWCNTs-OH/ PES, and f) MWCNTs-COOH/PES MMMs fabricated using 15 wt% of PES in NMP and DMAc.

and of solvents and PES [43,44] used were tabulated in Table 2. PES is considered as a polar polymer and usually more entanglements will form on a molecular chain when a solvent with greater solubility parameter and polarity is utilized. This is because of the more powerful interactions between solvent and polymer molecules [43]. As shown in Table 2, the polarity (δp) and solubility parameter (δt) of NMP are greater than DMAc. Consequently, further stress will be fixed between entanglements in the membranes fabricated using NMP as the solvent. Consequently, more shrinkage will be appeared in the drying process. Furthermore, comparing δt and δp parameters (Table 2), as well as porosity measurements (Fig. 5), suggested that a greater polarity and solubility parameter of solvent lead to a lower porosity (or higher shrinkage) of the membrane fabricated from the casting solution with the solvent. Based on the top surface SEM images, the pristine PES membranes and PES MMMs all display a relatively smooth top surface with no significant difference. Comparing of trend of porosity change with different nanoparticulates show different porosity trends for the membranes casted from 15 and 18 wt% PES solutions. For the membranes casted from 15 wt% PES solution the membrane porosity decrease in the row: PES-clay > PES-

adding a hydrophilic nanoscale material in the dope induces a much faster solvent/non-solvent exchange rate (a faster water diffusion into the as-cast polymeric film), resulting in macrovoid development and overall porosity improvement (Fig. 4) [20,25,26,38]. Interestingly, at higher loadings of nanoparticulate, the dope solution viscosity increases, which can be seen visually. Consequently, higher viscosity affects the phase inversion kinetic, which diminishes the solvent/nonsolvent exchange rate during the membrane formation. The phase inversion rate and solvent/non-solvent immediate demixing are the main dissuasive for any structural changes, and accordingly, the membrane’s top layer morphology becomes denser [15,20]. With increasing nanoparticulate loading, regardless of the membrane structural transformation, the cross-sectional morphology did not change to sponge-like. This observation has been seen previously for polymers like PVDF and PSf [20,39]. These results are also in a good agreement with literature findings on nanocomposite membranes [16,20,40–42]. Membranes fabricated using DMAc tend to have higher porosity based on both visual observation (Figs. 2 and 3) and porosity measurements (Fig. 5). This change may arise from their dissimilar precipitation paths and phase inversion rates. The solubility parameter (δt) 50

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Fig. 3. Surface, cross-sectional, and enlarged cross-sectional SEM images of the a) pristine PES membrane, b) clay/PES, c) SiO2/PES, d) TiO2/PES, e) MWCNTs-OH/ PES, and f) MWCNTs-COOH/PES MMMs fabricated using 18 wt% of PES in NMP and DMAc.

the gravitational force or interface energy [45,46]. A reduction in water contact angle usually results in a greater surface hydrophilicity for the MMMs, which is more advantageous to the water permeability and antifouling characteristics. The results show that the contact angle of membranes with DMAc as a solvent is higher than the membranes prepared by NMP in 15 w% PES, but it has different results in 18 wt% PES membranes. It should be mentioned that the contact angle is related to membrane surface porosity, pore size and roughness in addition to its chemistry [47]. By increasing of polymer concentration, the porosity and pore size of the membranes decrease and probably cause to reduce of surface roughness. The membranes with lower pore size have lower roughness [48]. Also, it is reported that the membrane with higher surface roughness has higher contact angle [49]. Based on these knowledges, by increasing of PES concentration in NMP solvent, the porosity of the 18 wt % PES/NMP membranes extremely reduced related to PES/DMAc (see Fig. 5), causing to increase of contact angle of PES/NMP membranes.

TiO2 > PES-SiO2 (Fig. 4a), while for 18 wt% PES solution the membrane porosity decrease in the row: PES-SiO2 > PES-TiO2 > PES-clay (Fig. 4b). These trends could be explained by considering polymer solution viscosity and nanoparticulates structure and size. By increasing PES concentration, the viscosity of casting solution increases and probably movement of bigger sheet form nanoparticulates like as clay is restricted and its influence is reduces causing to diminish of the clay embedded membrane porosity. 3.2. Static water contact angle The surface hydrophilicity of the fabricated PES membranes and PES MMMs are evaluated by means of the static water contact angle and the data are shown in Fig. 6. The contact angle data for the PES MMMs are slightly lower than the pristine PES membrane where 15 and 18 wt % of PES and NMP or DMAc are used to prepare the membranes. As shown in Fig. 6a & b, adding a small quantity of different nanoparticulates, bring about minor variations in membrane hydrophilicity. These variations could be related to the migration of the nanoparticulates to the membrane surface. This phenomenon is due to a decrease in 51

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Fig. 4. The porosity of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, MWCNTs-OH/PES, and MWCNTs-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP.

and then, at the higher loadings of the nanoparticulate the water flux diminishes. The incorporation of nanoparticulate into the PES matrix could affect water permeation by both flux enhancement due to the hydrophilicity and porosity increment as well as flux reduction owing to pore blockage for higher nanoparticulate loadings [29]. Embedding the nanoparticulate results in an improved connectivity

3.3. Filtration performance The pure water fluxes of all the fabricated pristine PES membranes and PES MMMs using 15 and 18 wt% of PES concentrations were examined at the operating pressure of 1 bar for 120 min. As shown in Fig. 7, as the nanoparticulate loading increases the water flux enhances

Fig. 5. The porosity of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, CNT-OH/PES, and CNT-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP and DMAc. 52

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[4]. Indeed, the addition of nanoparticulate generally boosts the porosity where a faster exchange of solvent/non-solvent and precipitation rate are anticipated [18]. Also, larger pores are likely to form because the pore size might be affected by the hydrophilic nanoparticulate and their induced faster demixing. As it has been discussed previously, the porosity decreases at higher nanoparticulate loadings owing to the minor agglomeration of nanoparticulate and higher dope solution viscosity [20,28,29]. It has been comprehensively discussed that PES/DMAc membranes be subject to possess greater porosity based on both visual observation (Figs. 2 and 3) and porosity measurements (Fig. 5) compared to that of PES/NMP membranes due to the dissimilar precipitation paths and phase inversion rates which arise from different polarity and solubility parameter of DMAc and NMP. Based on Fig. 8, it is evidence that PES/ DMAc membranes have shown a considerably better water flux compared to the PES/NMP membranes possibly due to the mentioned reasons. The BSA rejections towards the pristine PES membrane and PES MMMs are also studied. Surprisingly, the BSA rejections for all the fabricated membranes using PES concentration of 15 wt% are greater than 98% while the BSA rejections for those of using PES concentration of 18 wt% are nearly 100%.

Table 2 Solubility and polarity parameters of NMP, DMAc, and PES. Solvent and PES

δp (MPa0.5)

δt (MPa−1/2)

NMP DMAc PES

12.3 11.5 10.4

22.9 22.7 21.9

between the top layer and support layer of the membrane as well as a highly porous structure (Figs. 2 and 3), which could possibly diminish the membrane’s resistance to water permeation. Furthermore, the incorporation of nanoparticulate leads to an improved hydrophilicity (Fig. 6) and subsequently a higher water permeability. Hence, the MMMs have a tendency to absorb more water molecules and exhibit improved water penetration compared to that of the pristine membranes, because of the greater hydrophilicity and lower structural-resistance [25,26,50]. However, a higher nanoparticulate loading results in a lower water flux possibly owing to the agglomeration of the nanoparticulate and the pore blockage induced by nanoparticulate [26]. This phenomenon may diminish the advantageous effect of hydrophilicity, porosity, and structural-resistance on water permeation. Moreover, as mentioned earlier, a higher loading of nanoparticulate brings about a greater dope viscosity and it subsequently causes the formation of a thicker skin layer, which rises the membrane resistance and decreases its permeability [42,50]. Although the sonication of the nanoparticulate/solvent solution was done to obtain a well-dispersed solution prior to the addition of PES, at the higher loadings of nanoparticulate, the agglomeration of the nanoparticulate is unavoidable [26]. This observation reveals that further increasing nanoparticulate loading to a loading greater than a nanoparticulate’s optimize loading would bring about severe pore blockage and permeation decrease. The literature is also in good agreement with the findings [16,18,25,26,28,29]. Fig. 4 displays the porosity of the fabricated membranes. All the prepared membranes enjoy a porosity ranging between 52% to 75% for those prepared using PES concentration of 15 wt%, thanks to the low polymer concentration and presence of 1 wt% of pore former additive (PVP) in the dope solution [51]. However, the membrane porosities are ranged from 27% to 64% for those prepared using PES concentration of 18 wt% in the dope solution. Higher concentration of PES results in a denser membrane with much lower porosity and water flux, as expected

3.4. Antifouling properties One of the key features influencing the efficiency of membrane separation is antifouling properties. Particularly, this issue becomes more significant for hydrophobic materials like PES, PVDF, and PVC [18,20,28]. The PES membranes usually experience dreadful fouling in processes including foulant such as whey, natural organic matter (NOM), or microorganism. The foulants cause water flux reduction as a result of the fouling phenomenon on the membrane surface. Nevertheless, a sufficiently effective membrane modification method would be employed to reduce fouling effect as much as possible and change it to reversible fouling where washing/backwashing method can recover the flux after foulant permeation. To evaluate the membranes antifouling properties, they were subjected to a BSA solution permeation test. The membranes fluxes decreased until they reached the steady fluxes. The interaction of the membrane surface with BSA molecules may probably form a thin gel layer, and hence, it enhances the membrane resistance and accordingly reduces flux. However, the presence of hydrophilic nanoparticulate on both inter pores and membrane surface

Fig. 6. The static water contact angles of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, CNT-OH/PES, and CNT-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP and DMAc. 53

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Fig. 7. Pure water flux of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, CNT-OH/PES, and CNT-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP. (Inset: the enlarged version of the same figure for the CNT-OH and CNT-COOH).

Fig. 8. Pure water flux of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, CNT-OH/PES, and CNT-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP and DMAc.

concentrations. The FRR increases from 54% for the pristine PES membrane (P15-N) to 124% for the 1-T-P15-N membrane, which shows the greatest antifouling properties among all the prepared membranes. The results reveal that protein fouling has a tendency to be more reversible with an increase in nanoparticulate loading. Nonetheless, higher nanoparticulate concentrations bring about a slightly reduced FRR, which might be associated to the agglomeration. Inordinate

will possibly weaken the interaction between membrane and BSA and reduce BSA adsorption, and thus, diminish the membrane fouling [20,40,50]. In our previous work, results shown that the adsorption of BSA reduces by addition of hydrophilic nanoparticles [52]. Figs. 9 and 10 demonstrate the flux recovery ratio (FRR, %) of the membranes. The membranes FRR improves with increasing the nanoparticulate loading and then slightly reduces at greater nanoparticulate 54

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Fig. 9. FRR of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, CNT-OH/PES, and CNT-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP. (Inset: the enlarged version of the same figure for the CNT-OH and CNT-COOH).

more than that of the pristine PES membrane, which confirms that agglomeration does not have a substantial effect on the MMMs performance. By embedding the hydrophilic nanoscale fillers into the membrane surface, a hydrated layer is formed, which will probably reduce the foulants attachment efficiently [52,53]. Therefore, it can be concluded that the addition of nanoparticulate into the PES matrix for manufacturing the UF membranes may boost the antifouling characteristics of the fabricated membranes. Fig. 10 compares the FRR of the PES/DMAc and PES/NMP

nanoparticulate loading causes a significant agglomeration on the membrane surface [26], declining the hydrophilic group's contact areas of the incorporated nanoparticulate and probably weakens repulsion between membrane surface and protein [38]. These findings perfectly agree with the other mixed matrix membranes used for water applications [18,20,28,29,42]. Although there is a critical value for the nanoparticulate loading, the pure water flux and FRR of the most MMMs fabricated in this work (even with higher nanoparticulate concentrations) are considerably

Fig. 10. FRR of the pristine PES membranes, clay/PES, SiO2/PES, TiO2/PES, CNT-OH/PES, and CNT-COOH/PES MMMs prepared using a) 15 wt% and b) 18 wt% of PES in NMP and DMAc. 55

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Table 3 A comparison between the antifouling properties (FRR) of the developed PC-A/PES MMMs and previously reported nanocomposite membranes. Nanomaterial

Nanomaterial concentration (wt%)a

Polymer composition (wt%)

Foulant

FRR (%)

Ref.

Titania (Degussa P25) Titania (PC500) Oxidized-multiwalled carbon nanotubes Aminated-multiwalled carbon nanotubes Boehmite (synthesised,15–30 nm) ZnO (synthesised,180–550 nm) Polycitrate-Alumoxane ZrO2 1-T-P15-N (TiO2, Degussa P25)

16 4 0.2 0.045 2 0.5 0.5 1 1

21% 21% 18% 21% 21% 15% 15% 16% 15%

Whey Whey BSA BSA Whey BSA Activated sludge BSA BSA

90.8 88.3 87.7 100.2 90.1 86.5 95.2 95 124

[40] [40] [54] [18] [53] [55] [56] [57] This study

a

PES, PES, PES, PES, PES, PES, PES, PES, PES,

1% PVP 1% PVP 1% PVP 1% PVP 1% PVP 1.7% PVP 1% PVP 2% PVP 1% PVP

related to polymer.

4 Based on our findings, TiO2/PES (1-T-P15-N) is recommended as the best nanoparticulate for the PES UF membrane modification since it exhibited the superlative performance with a 320% water flux enhancement, nearly 98% BSA rejection, and 130% FRR improvement.

membranes. The PES/NMP membranes tend to exhibit a superior FRR compared to that of PES/DMAc membranes. This could possibly arise from the greater porosity and probably bigger pore sizes of the latter as compared with that of former because a bigger pore size is more favorable to foulant entrapments. The best performance in case of flux improvement (320%) and FRR enhancement (130%) was achieved for the 1-T-P15-N, where the membrane showed a great BSA rejection of 98%. This is probably due to the membrane’s great hydrophilicity and high porosity (66.5%) with controlled MWCO.

Acknowledgments This work has been partially supported by the National Elites Foundation and the Science & Technology Vice President of Iran. The authors gratefully acknowledge the financial support of Kharazmi University (Grant Number: D/2063).

3.5. Benchmark References Table 3 presents a comparison between the antifouling properties of the developed PES-based MMMs and previously reported PES MMMs. The FRR of the 1-T-P15-N nanocomposite membrane is found to be much higher than that of a wide variety of PES-based MMMs including commercially prepared or custom-synthesized nanomaterials.

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4. Conclusions Five various nanoparticulate comprising of clay, SiO2, TiO2, MWCNTs-OH, and MWCNTs-COOH were chosen to ameliorate the morphology, antifouling characteristics, and separation performance of the PES UF membranes. Five different concentrations of each nanoparticulate were added to the dope solutions including 15 or 18 wt% of PES, 1 wt% of PVP as a pore former, and DMAc or NMP as the solvent. The NIPS was utilized to prepare the UF membranes. The following conclusions can be drawn from this study. 1 The cross-sectional and surface SEM images revealed that there is no significant morphological change for the MMMs compared to the pristine PES membrane. However, the membranes fabricated using DMAc as solvent (or PES concentration of 15 wt%) was found to be more porous than those of fabricated using NMP (or PES concentration of 18 wt%). The MMMs’ porosity trend was found to be up and down as the nanoparticulate loading was increased in the dope. 2 The water contact angle measurements confirmed that the MMMs generally possess an improved hydrophilicity. Enhanced hydrophilicity and porosity led to a notable increase in water flux. Clay/ PES MMM (1-C-P15-N) exhibited the best water flux with an 850% higher water flux compared to the pristine PES membrane. Nevertheless, greater loadings of nanoparticulate resulted in lower water fluxes owing to a denser structure and the agglomeration of the nanoparticulate. 3 The nanoparticulate incorporation led to a significant improvement in flux recovery ratio, and thus, the PES MMMs exhibited excellent fouling resistance as compared with the pristine PES membrane. This perfection might be associated to the hydrophilicity improvement induces by the presence of the nanoparticulate.

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