Sulfonated graphene oxide incorporated thin film nanocomposite nanofiltration membrane to enhance permeation and antifouling properties

Desalination 470 (2019) 114125

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Sulfonated graphene oxide incorporated thin film nanocomposite nanofiltration membrane to enhance permeation and antifouling properties

T

Yesol Kanga, M. Obaida,b, Jaewon Janga, In S. Kima,

⁎

a

Global Desalination Research Center (GDRC), School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST),123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, South Korea b Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, 61111, Egypt

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Nanofiltration Polyamide Thin film nanocomposite (TFN) Sulfonated graphene oxide Hydrophilicity

Thin film nanocomposite (TFN) membranes embedded with sulfonated graphene oxide (SGO) were fabricated to improve the hydrophilicity, salt rejection and antifouling of nanofiltration (NF) membrane. In this study, the surface properties and structure of polyamide (PA) layer were improved by incorporating SGO into the PA layer. It was prepared by interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on the polysulfone substrate surface. The PA layer thickness of TFN membranes decreased and the hydrophilicity and zeta potential of them were improved after incorporating the SGO. According to NF filtration tests, TFN membrane including 0.3 wt% of SGO (TFN 0.3) gave the highest water flux, which was 87.3% higher than that of TFC membrane. Interestingly, despite the significant increase in water permeability, the salt rejection of both membranes was similar due to the improved cross-linking and low molecular weight cut-off (MWCO) of the TFN 0.3 membrane. Furthermore, it showed high antifouling property in long-term NF test, compared to the pristine TFC membrane. The current study may lead to a facile strategy by using SGO nanosheets to improve the hydrophilicity and antifouling properties of NF membrane.

⁎

Corresponding author. E-mail address: [email protected] (I.S. Kim).

https://doi.org/10.1016/j.desal.2019.114125 Received 15 May 2019; Received in revised form 23 August 2019; Accepted 25 August 2019 Available online 31 August 2019 0011-9164/ © 2019 Published by Elsevier B.V.

Desalination 470 (2019) 114125

Y. Kang, et al.

1. Introduction

graphene supermarket, USA), sulfuric acid (H2SO4; 98%, OCI Materials Co., Ltd. Korea) and methanol (OCI) were used for synthesis sulfonated graphene oxide (SGO). Piperazine (PIP, 99%, Sigma-Aldrich) and trimesoyl chloride (TMC, 98%, Tokyo Chemical Industry Co.) were used as monomers to prepare polyamide active layer on the surface of PSf substrate. Deionized water (DI) obtained from a Milli-Q ultrapure water purification system and n-hexane (OCI) were utilized to dissolve PIP and TMC monomers, respectively. For salt rejection test, sodium sulfate (Na2SO4, Daejung), sodium chloride (NaCl, OCI), and magnesium sulfate (MgSO4, OCI) were used for evaluating membrane performance. For antifouling test, bovine serum albumin (BSA; 67 kDa, SigmaAldrich), and humic acid (HA, Sigma-Aldrich) were used as foulants to study the behavior of fouled membrane. The commercial NF membranes (NF270, NF90) were obtained from Dow-Film Tec to compare them with fabricated membranes.

Nanofiltration (NF) process has been widely used in water purification system, such as heavy metal ions removal, water softening, metal recovery in wastewater, virus removal and industry fields to produce demineralized water at low operating pressure [1–3]. The separation characteristic of NF membrane is located between reverse osmosis (RO) and ultrafiltration (UF) processes [4,5]. Owing to its looser structure than RO membranes, it exhibits higher water flux and lower operating pressure than RO [5]. Although NF has a high rejection of small neutral matter (molecular weight: 100–1000 Da) and charged multivalent ions, water molecules and monovalent ions can easily pass through. The permeance of monovalent ions could be attributed to its looser rejection (active) layer with nanosized pores diameter of 0.5–2 nm [6]. Among several NF fabrication methods, the interfacial polymerization (IP) process is commonly used in preparation of commercial thin film composite (TFC) NF membrane [7]. The diamine (piperazine in aqueous solution) and acyl chloride (trimesoyl chloride in organic solution) were allowed to react on the porous substrate forming the thin film active layer [8]. Performance of prepared TFC-NF membrane can be improved by optimizing individual active layer, substrate, or both together. For example, optimization of the active layer via controlling monomer ratio, interaction time and curing time have been investigated [9]. On the other hand, except for above development method, NF membrane could be improved by adding nanoparticles to the support or polyamide (active) layers [10–12]. It could be one of the innovative ways to solve the technical obstacles due to the unique properties of nanomaterials. Hydrophilic nanomaterials, such as cerium oxide [13], nanotubes [14], and zeolite [15] have been embedded in polyamide (PA) layer to improve membrane performance. In recent years, the number of studies applying graphene oxide (GO) to NF membranes is increasing more and more [16–18]. The abundant oxygen functional groups (carboxyl, hydroxyl, and epoxy) are placed in two-dimensional (2D) graphene sheet and their derivatives are promising candidates to enhance the membrane hydrophilicity as well as the overall performance [7,19]. Interestingly, the addition of GO to PA layer improved both the water flux and rejection of fabricated membranes [20]. However, one of the main problems with GO nanosheets is the difficulty of dispersing them homogeneously in the membrane matrix. Agglomeration occurs easily in a high concentration of nanomaterials, and membrane performance is eventually different from the expected results such as decreased water flux and hydrophilicity [21]. Therefore, in order to obtain high-performance membrane, it is important to reduce the agglomeration of nanoparticles by attaching different functional groups in its surface by various chemical reactions [22]. In order to improve its negative charge, anti-agglomeration, as well as enhance the hydrophilicity, GO was modified with sulfonic acid to synthesize sulfonated graphene oxide (SGO) [23,24]. In this research, SGO was incorporated in the PA layer during the IP process to enhance the cross-linking, hydrophilicity, zeta potential, pure water flux, salt rejection and antifouling properties of the NF membranes. We examined the effect of various concentrations of SGO nanosheets on NF membrane properties and performance.

2.2. Preparation of SGO The SGO was synthesized with GO nanosheets by using our previously reported method [24]. In brief, 1 g of GO powders were dispersed into 15 mL 0.5 M H2SO4 and 20 mL of methanol solution and sonicated for 1 h to get a homogeneous solution. Thereafter, the mixture solution was reacted in an oven (Han Baek Scientific CO., Korea) at 100 °C for 24 h to obtain solid-state SGO nanosheets, successfully. The synthesized SGO was stored in a desiccator. 2.3. Preparation of PSf substrate PSf substrate was fabricated via the phase inversion method as follows. An appropriate amount of PSf (15 wt%) and PVP (1 wt%) were dissolved in NMP solvent with continuous stirring at 60 °C for 24 h. Then, the prepared casting solution was kept at room temperature for 24 h to remove the gas bubble. A PET fabric was placed on a clean glass plate and it was wetted with NMP to enhance the adhesion with polymer solutions. After that, the polymer solution was cast on the PET fabric using an automatic membrane casting coater (MTI Corp., MSKAFA-I, USA). Finally, the cast membrane film was immediately immersed in a pure DI water bath and then stored in water at lab temperature for 48 h to remove residual NMP solvent in the PSf membrane. The fabricated membranes were stored in a laboratory refrigerator at 4 °C before use. 2.4. Preparation of TFC/TFN membranes The polyamide (PA) active layer was synthesized via interfacial polymerization (IP) reaction of PIP and TMC on the top of PSf substrate. In details, a specific amount of SGO nanoparticles (0, 0.05, 0.1, 0.3, to 0.5 wt%) was dispersed in an aqueous solution containing 2 wt% PIP and sonicated for 1.5 h to obtain a dispersed well solution. Then, the PIP aqueous solution was poured onto the PSf substrate for 2 min. Excess aqueous solution was removed using a soft rubber roller, afterward, TMC/hexane (0.2 g/100 mL) organic solution was poured onto the substrate layer for 60 s to generate PA layer. Subsequently, the fabricated TFC/TFN membranes were cured at 70 °C for 5 min for further polymerization. After all the reactions, they were thoroughly cleaned with DI water and kept in a refrigerator prior to use. The fabricated NF membranes were labeled according to the SGO concentrations as TFC (No SGO; control), TFN 0.05, TFN 0.1, TFN 0.3, and TFN 0.5.

2. Experimental 2.1. Materials

2.5. Characterization of GO & SGO

Polysulfone (PSf; 60,000 MW, BASF), N-methyl-2-pyrrolidone (NMP, Daejung, Korea), polyvinylpyrrolidone (10 kDa, PVP, SigmaAldrich) were used for microporous PSf substrate fabrication and polyethylene terephthalate (PET) nonwoven fabric was used as support layer (AMFOR Inc., USA). Commercially available graphene oxide (GO,

The morphologies of GO and SGO were characterized through field emission scanning electron microscope (FESEM, S-4700, Hitachi, Japan). The crystal structure and structural transformations were studied by powder X-ray diffractometer (XRD, Empyrean, Malvern 2

Desalination 470 (2019) 114125

Y. Kang, et al.

Panalytical Ltd., Malvern, UK) and Raman spectroscopy (LabRAM HR Evolution, Horiba Jovin-Yvon, France). The chemical structure and functional groups were observed with Fourier transform infrared spectroscopy (FTIR, Spectrum 400, PerkinElmer, USA). Their surface chemical compositions were detected by an X-ray photoelectron spectroscopy (XPS, K-ALPHA+, Thermo Scientific, UK).

and HA were individually dissolved in 25 ppm Na2SO4 solution to prepare a single-foulant solution of 250 ppm, which used as a feed to test the antifouling properties of the control TFC and TFN membranes. The long-term experiments have been conducted, where the permeate flux was recorded at 25 °C and 0.5 MPa for 800 min. 2.9. Acid-base chemical cleaning test

2.6. Characterization of membranes

Chemical cleaning solutions were prepared by hydrochloric acid (HCl, 35%, OCI) and sodium hydroxide (NaOH, OCI). The concentration of acid cleaning solution was 0.2% and the pH value of base cleaning solution was 12. To compare the morphology and performance of the membrane before and after cleaning, SEM, water flux, and Na2SO4 salt rejection were studied. First, the water flux and salt rejection of Na2SO4 of pure membrane were measured. After that, they were soaked into 0.2% HCl solution for 1 h at 40 °C. It was thoroughly washed with DI water and then soaked into NaOH (pH = 12) solution at 40 °C for 1 h. Finally, the salt rejection and water flux were measured again by using cleaned membranes.

The top surface and cross-sectional morphologies of TFC/TFN membranes were observed with FESEM (S-4700, Hitachi, Japan). The surface roughness and morphology were also analyzed by atomic force microscope (AFM, XE-100, Park systems, Korea) with the sample scan size of 5 μm × 5 μm. The functional groups and surface chemical composition of fabricated membranes were characterized by FTIR (Spectrum 400, PerkinElmer, USA) and XPS (K-ALPHA+, Thermo Scientific, UK). The atomic percentage of each element obtained using XPS, where the degree of cross-linking value of the PA layer can be calculated using the following Eq. (1) [25,26]:

Degree of crosslinking (%) =

m × 100 m+n

3. Results and discussion

(1)

where m and n are the cross-linking parts and linear-liking parts of the PA layer, respectively. The values of m and n were calculated based on oxygen/nitrogen (O/N) ratio that can be obtained from the XPS result using Eq. (2) and (3):

m+n=1

(2)

O 3m + 4n = N 3m + 2n

(3)

3.1. Characterization of GO & SGO nanoparticles FESEM images of GO and SGO are shown in Fig. 2(a) and (b). GO nanosheets exhibited highly wrinkled structure. On the other hand, SGO morphology is a little bit changed due to the removal of oxygen functional groups during the partial reduction. However, overall structures were similar between GO and SGO. It is, therefore, concluded that sulfonated reaction did not change the original GO nanosheet structure and morphology. The XRD patterns of GO and SGO were shown in Fig. 2(c). The characteristic GO diffraction peak (2θ) was sharply observed at 10.7°, which suggested that the interlayer spacing (d-spacing) of GO is 0.822 nm. Interestingly, SGO exhibited a weak and broad peak at around 23.7°, corresponding the d-spacing is roughly 0.359 nm. This decrease in the d-spacing, compared to GO, is attributed to the effective reduction and removing of the oxygen functional groups, resulting in a partial restacking of the nanosheets [27]. The Raman spectra of the GO and SGO nanoparticles (Fig. 2(d)) shows the intensities of D and G bands. In the case of GO, D band represents the structural defect on the carbon base planes that is affected by epoxide and hydroxyl groups, corresponding to the vibration of sp3 carbon atom appeared at 1349 cm−1. Meanwhile, the G band related to the vibration of sp2 carbon atoms, which was occurred in-plane displacement of carbon atoms in the hexagonal carbon sheet, is displayed at 1599 cm−1 [28,29]. For SGO, the Raman bands were slightly shifted to 1348 and 1597 cm−1 for D and G band, and the intensity of D band (ID) increased but the intensity of G band (IG) decreased. However, the Raman spectra of GO and SGO seem to be highly similar, indicating that SGO still has the same main structure as GO. Also, the integrated intensity ratio (ID/IG) increased from 0.983 (GO) to 1.079 (SGO), indicating that the structure of SGO has changed during the sulfonation process, and hexagonal carbon sheet was functionalized with –SO3H groups. The sp2 bonded carbon atoms decreased, while the sp3 carbon atoms increase conversely at the same time [30]. FTIR was used to compare the presence of functional groups in GO and SGO nanosheets. As can be seen in Fig. 3(a), several major characteristic peaks were found in the GO spectrum at 1712, 1578, 1367, and 1210 cm−1 corresponded to the C]O stretching vibration from carbonyl and carboxylic groups, C]C skeletal vibration, the vibration of C–OH and C–O–C stretching vibration, respectively [31–33]. It showed that GO nanosheets contain various hydrophilic oxygen-containing functional groups. Compared to GO nanosheets, significant new peaks are appeared for SGO, such as asymmetric stretching of O=S=O at 1270 cm−1, symmetric stretching vibrations of S]O at 1141 cm−1,

The surface hydrophilicity of prepared membranes was measured by the sessile drop method on a contact angle analyzer (Phoenix 300, Surface Electro Optics Co., USA). At least five samples were determined to minimize the measurement error. The zeta potential of membrane surface was measured by zeta potential instrument (ELSZ-2000, Otsuka Electronics Corp., Japan) using electrophoretic light scattering. The zeta potential values of skin layer were measured at pH 8 solution and its ionic strength was 10 mM NaCl. 2.7. Evaluation of nanofiltration performance and molecular weight cut-off (MWCO) The pure water flux and rejection of fabricated TFC/TFN membranes were conducted using a lab-scale pressure-driven dead-end filtration system (Fig. 1). All experiments were performed at 0.5 MPa using an effective membrane area of 7.06 cm2. Before measuring permeate flux, each membrane was compacted at 0.5 MPa for 1 h to reach a steady state flux. After compaction, the water flux data was collected automatically every minute with RS-Key data acquisition software. To measure the membrane rejection, three different single-salt solutions (2500 mg/L of MgSO4, Na2SO4, and NaCl) were used as a feed solution, and the membrane solute rejection (R) was calculated by using feed and permeate concentrations which were measured by a TDS meter (COND2700, Eutech Instruments, Singapore). The MWCO of the membranes were defined by using the PEG with different molecular weight (200, 600, 1000, and 2000 Da) at a concentration of 100 mg/L. The concentrations of PEG solutions were estimated using a total organic carbon analyzer (TOC analyzer, TOC-L, Shimadzu, Japan). The MWCOs of membranes were defined as a molecular size of PEG being 90% rejected. 2.8. Antifouling test The antifouling test of TFC/TFN membranes was carried out by using two different standard foulants [7]. The model foulants of BSA 3

Desalination 470 (2019) 114125

Y. Kang, et al.

Pressure gauge

Nitrogen pressure Reducing valve

Membrane cell

N2 Data logging system

Feed

Permeate

Stirrer

Balance

Fig. 1. Schematic diagram of dead-end filtration unit for nanofiltration system.

Fig. 2. SEM images of GO (a) and SGO (b). (c) denotes the XRD spectra and (d) displays the Raman spectra of GO and SGO. 4

Desalination 470 (2019) 114125

Y. Kang, et al.

Transmittance (%)

Transmittance (%)

(a)

GO SGO

4000

3500

3000

2500

2000

1500

1000

500

1712 1578 1707

GO SGO

2000

1750

(c ) Intensity (a.u.)

Intensity (a.u.)

SGO GO

S 2p

70,000

GO

60,000 50,000

1500

1000

800

600

400

1250

1000

750

500

200

0

284.8 C-C

286.9 C-O-C

40,000 30,000

288.1 C=O

20,000 10,000

1200

1023 878

Wavelength (cm-1)

O 1s

C 1s

1141

1367 1210

Wavelength (cm-1)

(b)

1271

0

Binding Energy (eV) 50,000

Intensity (a.u.)

Intensity (a.u.)

(d) 169.5 S2p3/2

170.7 S2p1/2

SGO

284.8 C-C

40,000 30,000 20,000 10,000

289.0 C=O

286.1 C-O-C

0 174

172

170

168

166

164

162

160

292

Binding Energy (eV)

290

288

286

284

282

280

Binding Energy (eV)

Fig. 3. Characterization of GO and SGO nanosheets (a) FTIR spectra (b) XPS full-scan spectra (c) high-resolution C 1 s spectra of GO (upper) and SGO (below) (d) S 2p spectrum for SGO.

stretching of S]O at 1023 cm−1, stretching of SeO at 878 cm−1. Furthermore, the peak of C]O stretching vibration (1707 cm−1) became broader [34–36]. To compare the surface compositions of GO and SGO, XPS was used. The XPS full-scan spectra were presented in Fig. 3(b), the SGO survey spectrum showed a significant peak at 170.2 eV, approximately. It confirms the presence of the oxidized sulfur groups in SGO; however, this peak was not found in case of GO. Furthermore, the oxygen atomic percentage of SGO was significantly increased from 35.1% to 51.7%. From this data, it can be seen that sulfonic acid groups carrying considerable amounts of oxygen and sulfur atoms were successfully introduced on the GO surface. In addition, the S 2p spectrum (Fig. 3(d)) shows high binding energy associated with sulfonate groups [37]. The high resolution of XPS spectrum of C 1 s is presented in Fig. 3 (c), as shown in the figure, the carbon bonds were found in three different peaks including CeC (284.8 eV), C–O–C (286.9, 286.1 eV), and O–C=O (288.1, 289.0 eV), respectively. Although SGO contains same functional groups like GO, the SGO intensity of carbon-oxygen linked groups

decreased, demonstrating that they were reduced and changed to carbon-sulfur linkage bonding [38,39]. Overall, these results indicate that the successful formation of sulfonate groups on the surface of GO. 3.2. Characterization of TFC/TFN membranes 3.2.1. The analysis of membrane surface morphology and roughness The top surface and cross-section morphology of TFC and TFN membranes were detected using FESEM. Fig. 4(a1–e1) shows that the dense PA layer was successfully fabricated on the PSf substrate and TFN 0.3 had smaller cluster sizes than other membranes. Fig. A.2 represents the particle size distribution graphs of TFC and TFN membranes. It was confirmed that TFN 0.3 showed smaller particle sizes (88.6 ± 11.8 nm) than the rest and TFN 0.5 had largest particles (443 ± 147 nm) on the surface among the membranes due to the agglomeration effect resulted from excessive SGO concentration. From the cross-sectional images of TFC and TFN membranes (Fig. 4(a2–e2)), all the membranes exhibited asymmetric structure including very dense and thin PA layers and 5

Desalination 470 (2019) 114125

Y. Kang, et al.

Fig. 4. SEM images of the top surfaces (a1-e1) and cross sections (a2-e2) of (a) TFC, (b) TFN 0.05, (c) TFN 0.1, (d) TFN 0.3, and (e) TFN 0.5 membranes.

finger-like macrovoids PSf substrates. Importantly, the PA layer thickness of TFC, TFN 0.05, TFN 0.1, TFN 0.3 and TFN 0.5 are 183.9 ± 2.6, 88.6 ± 3.6, 105 ± 3.3, 113.8 ± 2.9, and 128.4 ± 3.3 nm, respectively. Compared to the TFC membrane, the PA layer thickness of all TFN membranes was decreased by 51.8–30.2%. It is because the SGO nanoparticles helped the aqueous PIP solution to absorb much more in the support layer. Similar results have been reported in which the

thickness of active layer was decreased by adding hydrophilic additives to aqueous solutions before interfacial polymerization [40–42]. It can therefore be assumed that the SGO content affects the formation of PA active layer strongly. The surface roughness of TFC and TFN was measured by AFM and three-dimensional AFM surface images are shown in Fig. 5. The roughness average (Ra) value of fabricated membranes are 6

Desalination 470 (2019) 114125

Y. Kang, et al.

Fig. 5. Three-dimensional AFM images and Ra value of TFC, TFN 0.05, TFN 0.1, TFN 0.3, and TFN 0.5 membranes.

16.47 ± 0.39, 19.61 ± 1.11, 21.74 ± 0.79, 23.48 ± 0.13 and 20.28 ± 1.28 nm for TFC, TFN 0.05, TFN 0.1, TFN 0.3, and TFN 0.5, respectively. To minimize the experimental error, we measured three different spots of each sample and averaged them (Table A.1). As can be observed, the surface roughness of the membrane was increased by mixing a high concentration of SGO. The further increase of SGO reduces the roughness, but at high concentration (0.5%), the aggregation of SGO makes micro-size clusters as shown in Fig. 4(e1) and Fig. 5. These micro-size clusters could affect the cantilever movement, due to the large differences between the maximum (ridge) and minimum (valley) height, as a result affecting the roughness results.

hydroxyl groups of absorbed water on the membrane surface, amine groups and hydroxyl stretching vibration, aromatic CeH bands of SGO, C]O amide I group from poly (piperazinamide), and epoxy group of CeO, respectively [18,25,43]. These results suggested that semi-aromatic poly (piperazineamide) PA active layers were successfully fabricated on the PSf substrate during the IP process [44]. It was also shown that 2936 and 1060 cm−1 peaks present detectable variation with increasing SGO amount in the PA layer. XPS analysis detected four different elements, including carbon (C), oxygen (O), nitrogen (N), and sulfur (S) to confirm the interaction between SGO nanosheets and PA layer and to calculate the cross-linking degree of PA layer. Table 1 provides the percent of C, N, O, and S elements and O/N ratio. Compared to the base TFC membrane, TFN membranes exhibited an increase in C and N atomic concentrations, while O and S atomic concentrations were decreased. The TFN membranes had a high percentage of C element since SGO nanosheets had abundant carbon elements [17]. The O/N ratio was decreased with the content of SGO increased; however, the degree of cross-linking was increased. It indicated that the cross-linking degree of TFN PA layer was higher than that of the TFC membrane. Since, when the SGO embedded TFN membranes were prepared by interfacial polymerization, the remaining SGO nanosheets can attract far more PIP monomers on the surface of support layer owing to their hydrophilicity characteristic [25,45]. As a result, the reaction between PIP and TMC monomers was improved when forming PA layers, thereby improving the degree of polymerization. However, TFN 0.5 showed a 2.2% decline in the crosslinking degree, corresponding to aggregation and the non-well dispersion of the SGO. Furthermore, there are several possible explanations for this result, for example, hydrogen bonding with PIP and SGO nanosheets was improved since the amount of SGO nanoparticles increased, resulting in a decrease of cross-linking between PIP and TMC [10]. Additionally, when PIP diffusion was performed on the TMC organic phase, SGO at high concentration might have surrounded the PIP monomers and eventually prevented PIP from reacting with TMC. Fig. 7 presents the full scan and high-resolution C 1 s and S 2p spectra of TFC and TFN 0.3 membranes to compare chemical compositions. All membranes have four different peaks with binding energy at 530.6, 399.0, 284.7, and 167.5 eV, representing O 1 s, N 1 s, C 1 s, and S 2p, respectively (Fig. 7(a)). As can be seen, compared to PSf membrane, the N and O atomic percentages of nanofiltration membranes were increased, while the concentrations of S atomic were decreased. It means

Transmittance (%)

3.2.2. The chemical composition of TFC/TFN membranes The interaction between the PA and SGO was confirmed with FTIR (Fig. 6) and XPS (Fig. 7). Fig. 6 compares the surface chemical structures of TFC and TFN membrane by FTIR analyzing over a range of 3800–1000 cm−1. Typical characteristics peaks for NF membranes detected at 3700, 3450, 2936, 1627, and 1060 cm−1 corresponding to

3700 free -OH

3450 -OH Amine TFC

3800

3600

2936 C-H

TFN 0.05 TFN 0.1 TFN 0.3 TFN 0.5

3400

3200

3000

2800

1627 C=O 1600

1060 C-O 1400

1200

1000

Wavelength (nm-1) Fig. 6. FTIR spectra of TFC, TFN 0.05, TFN 0.1, TFN 0.3 and TFN 0.5 membranes. 7

Desalination 470 (2019) 114125

Y. Kang, et al.

(a)

C 1s

O 1s N 1s

Intensity (a.u.)

TFN 0.3

S 2p

TFC

PSf 1200

1000

800

600

400

200

0

Binding Energy (eV)

(c)

Intensity (a.u.)

TFC C 1s

286.1 C-N C-O 283.8 C=C

287.5 N-C=O C=O

292

TFN 0.3 C 1s

284.8 C-C

Intensity (a.u.)

(b)

290

288

286

284

282

285.8 C-N C-O 287.5 N-C=O C=O

292

280

290

167.9 Sulfonate group

169.1 Sulfone group

172

170

168

166

164

TFN 0.3 S 2p

286

284

282

280

162

160

168.0 Sulfonate group

Intensity (a.u.)

Intensity (a.u.)

(e )

TFC S 2p

288

Binding Energy (eV)

Binding Energy (eV)

(d)

284.8 C-C

162

172

160

Binding Energy (eV)

170

168

166

164

Binding Energy (eV)

Fig. 7. (a) XPS survey spectra of TFC and TFN 0.3 nanofiltration membranes, the comparison of spectra for C 1 s between (b) TFC and (c) TFN 0.3 and for S 2p between (d) TFC and (e) TFN 0.3.

that thin PA layer was perfectly formed on the surface of PSf substrate [46]. The high-resolution C 1 s spectra of TFC and TFN 0.3 are shown in Fig. 7 (b) and (c). TFC contains 4 peaks, corresponded to N–C=O (amide bond, 287.5 eV), CeN and CeO (286.1 eV), CeC (284.8 eV), and C]C (283.8 eV). While TFN 0.3 has only 3 peaks, which are N–C=O (287.6 eV), CeN and CeO (285.8 eV), and CeC (284.8 eV), respectively [37,47]. According to the existing of CeN and N–C=O bond peaks, it is apparent that PA active layers were successfully synthesized both TFC and TFN 0.3 membranes [1]. In the case of TFN 0.3, comparing the surface areas of those peaks, the ratio of N–C=O and

CeN bonds are increased from 12% to 17% and 21% to 35%, respectively. This supports that TFN 0.3 has higher cross-linking parts than TFC membrane in PA active layer. Fig. 7(d–e) show the high resolution of S 2p spectra. Compared with the TFC control membrane, TFN 0.3 membrane only showed the sulfonate group without any sulfone groups from PSf substrate, which suggested that the high degree of crosslinking was successfully formed than TFC membrane [48,49]. 3.2.3. Molecular weight cut-off (MWCO) of membranes Fig. 8 shows the MWCO of membranes. It was studied based on PEG 8

Desalination 470 (2019) 114125

Y. Kang, et al.

layer of TFN membranes, the contact angle value decreased and reached its minimum value of 39.12° for the TFN 0.3 membrane. These results suggest that the hydrophilicity of TFN membranes is much better than that of the original TFC membrane. It can be seen that hydrophilicity of SGO embedded TFN membranes was improved due to the presence of hydrophilic functional groups in SGO nanosheets [51]. Furthermore, surface roughness is also an important factor in contact angle results. For a not flat surface (rough surface, R > 1), the contact angle value is changed by Wenzel's law ‘cosθ*=r cos θ’ (where θ* is measured angle, θ is Young angle, r is roughness factor = actual surface/flat surface). Following this law, the contact angle will be increased as increasing the roughness factor on a hydrophobic surface (θ > 90°), in contrast with a hydrophilic surface (θ < 90°), the contact angle is decreased as roughness increases [52–55]. The contact angles of fabricated membranes were affected by Wenzel's law since the TFC was hydrophilic state (θ = 50.25°) and all samples had rough surfaces (r > 1). Therefore, they were decreased as increasing the roughness of membrane from TFC to TFN 0.3. However, the contact angle of TFN 0.5 was slightly increased due to its decreased roughness by agglomeration effect even though TFN 0.5 had much more hydrophilic SGO particles in membrane. Fig. 9(b) shows the zeta potential values of TFC and TFN membranes at pH 8. All membranes showed negatively charged zeta potential at a weak base pH. As the concentration of SGO increased, the zeta potential was improved from −3.00 to −23.01 mV. It is a reasonable result since SGO had abundant carboxyl, hydroxyl, and sulfonic acid groups. These groups were located on the surface of membranes to make membrane surface charged, thereby, the negative charge of membrane surface more intensified.

Table 1 Elemental composition percentage, O/N ratio and degree of cross-linking of TFC and TFN membranes. Membrane

TFC TFN TFN TFN TFN

Atomic concentration (%)

0.05 0.1 0.3 0.5

C

N

O

S

67.17 69.36 70.77 72.22 73.44

12.15 12.81 12.87 13.33 12.53

19.29 17.07 15.89 14.28 13.62

1.4 0.76 0.47 0.17 0.41

O/N ratio

Degree of crosslinking (%)

1.59 1.33 1.23 1.07 1.09

31.9 57.2 68.5 89.7 87.5

94

PEG rejection (%)

92 90 88

TFC TFN 0.05 TFN 0.1 TFN 0.3 TFN 0.5

86 84 82 80 400

800

1200

1600

2000

Molecular weight of PEG (Da) Fig. 8. PEG rejection of TFC and TFN membranes depending on the molecular weight of PEG.

3.3. Nanofiltration performance of TFC/TFN membranes

rejection using different molecular weight of PEGs from 200 to 2000 Da. It was reported that the MWCO values of TFC, TFN 0.05, TFN 0.1, TFN 0.3, and TFN 0.5 were 826, 705, 640, 469, and 546 Da, respectively. As it can be seen, the MWCO of SGO embedded membranes tended to decrease, which demonstrated that SGO nanosheets can affect the structures of active layers [50]. The results also related to the degree of cross-linking percentage.

The pure water flux and salt rejections of TFN and TFC membranes were evaluated and the experiment results are shown in Fig. 10(a). From the data in Fig. 10(a), it can be seen that the water flux increases with increasing SGO content (up to TFN 0.3), but the flux decreases slightly at TFN 0.5. The trend of contact angle, zeta potential (Fig. 9(b)) and water flux were well-matched each other. In other words, the hydrophilicity and negative charge of membrane surface improved by the oxygen-rich functional groups of SGO, eventually it showed increased water flux from 6.3 LMH (TFC) to 11.8 LMH (TFN 0.3). For these reasons, TFN membranes can attract water molecules and allow them to pass through the membrane matrix quickly [56]. However, even the cross-linking of TFN 0.5 slightly reduced, the water flux of TFN 0.5 was decreased owing to an agglomeration effect of SGO nanosheets. The

3.2.4. Contact angle and zeta potential of membranes The water contact angle and zeta potential of TFC and SGO composited TFN membranes were shown in Fig. 9(a). As can be seen from the figure, all the SGO incorporated TFN membranes showed lower contact angles than TFC membrane. The contact angle of TFC membranes was 50.25°. However, after adding SGO nanosheets in the PA

(a)

(b) 50.25

50

46.60

TFC

44.24 43.08

z-potential (mV)

Contact angle (º)

Membrane

10

60

39.12

40 30 20

TFN 0.05

TFN 0.1

TFN 0.3

TFN 0.5

0

-10

-20

10

-30

0 TFC

TFN 0.05

TFN 0.1

TFN 0.3

TFN 0.5

Membrane Fig. 9. The results of contact angle (a) and zeta potential (b) of TFC, TFN 0.05, TFN 0.1, TFN 0.3, and TFN 0.5 membranes. 9

Desalination 470 (2019) 114125

Y. Kang, et al.

(a)

(b) 100

14

100

11.86

10

80

8.97

8.40

60 8

6.35

6

40

90

4

Salt rejection (%)

10.88

Rejection (%)

Water flux (LMH)

12

TFC TFN 0.3

80 70 60 50

20 2

40

0

30

0 TFC

TFN 0.05

TFN 0.1

TFN 0.3

TFN 0.5

Na2SO4

MgSO4

Membrane

NaCl

Salt solution

Fig. 10. (a) Pure water flux and salt rejection (Na2SO4 2500 mg/L) of the composite membranes, and (b) the rejection of different kinds of salts of TFC and TFN 0.3 nanofiltration membranes.

hand, low-valent anions (Cl−) demonstrated relatively low rejection due to the Donnan electrostatic exclusion mechanism and the molecular sieving mechanism.

Table 2 Comparison of the performance characteristics of prepared membranes with other NF membranes. Membrane

Additive

Flux (LMH/ bar)

Rejection (%)

Salt

Ref

TFC TFN 0.3 TFC IP-UT M-5h-0.5 TFN-1g/m2 HPEC-NFM 0.2MWCNT NF270 NF90

– SGO SDS PXDC SNW-1 MWCNTs/SA MWCNT – –

1.27 2.37 7.2 2.75 19.25 4.5 2.3 11.70 5.33

96.62 96.45 38 92.8 83.5 83.5 85 98.42 98.23

Na2SO4 Na2SO4 NaCl Na2SO4 Na2SO4 MgCl2 Na2SO4 Na2SO4 Na2SO4

This work

3.4. Long-term antifouling test of TFC and TFN 0.3 membranes Two model foulants were prepared to investigate the antifouling properties of prepared NF composite membranes, using 250 ppm BSA or HA in 25 ppm Na2SO4 solution. The normalized flux (current flux divided by initial flux) of TFC and TFN 0.3 membranes were compared according to time (Fig. 11). The normalized flux of the TFN 0.3 membrane slightly decreased to 14% for BSA and 17% for HA in the early stage, while the TFC membrane showed a high decrease tendency of 31% for BSA and 46% for HA. In the case of BSA antifouling test, the flux of TFN 0.3 was maintained higher than that of the TFC membrane. Especially, the TFC normalized flux of BSA test decreased rapidly after 550 min. The HA test showed that the flux of TFN 0.3 was maintained high until 200 min, thereafter it decreased to the same value as TFC. Throughout these results, it can be seen that BSA antifouling property was improved by incorporating SGO into PA layer since the hydrophilic functional groups of SGO increased the hydrophilicity of NF membrane. The surface hydrophilicity can reduce the adsorption of hydrophobic foulants on the membrane surface, therefore high hydrophilicity surface can generally have better antifouling performance [61]. However, HA fouling performance showed similar results eventually after

[42] [58] [59] [2] [60] Measured

non-uniformly dispersed and excessive concentration of SGO eventually decreased the water uptake and flux [7,17]. In case of Na2SO4 rejection of TFC and TFN membranes showed similar rejection values (> 95.0%) even if TFN membranes presented higher water flux than pristine TFC membrane without significant decline by nanosheet embedding. This finding can be explained by the enhanced roughness, cross-linking degree (MWCO decline), hydrophilicity, and negative charge on the surface of membranes. A high degree of PA cross-linking and a low MWCO value are generally associated with a smaller pore radius, which indicates that salt removal capacity is improved [25]. The high negatively charged membrane also correlate with the salt rejection enhancement through electrostatic interaction with the negatively charged ions [57]. From the performance test results, TFN 0.3 membrane is the best optimum membrane by taking its water flux value (87.3% higher than pristine one) and salt rejection value into consideration. Table 2 compares the water flux and salt rejection of membranes prepared in this study, commercial membranes, and recently reported membranes which were improved by incorporating different additives (e.g. SDS, PXDC, SNW-1 or MWCNTs) in the substrates or active layers. The water fluxes of fabricated membranes in this study were lower than other membranes, but the salt rejections were higher than other studies. To compare the additional salt rejections of TFC and TFN 0.3 membranes, three different salt solutions (2500 mg/L of Na2SO4, MgSO4, and NaCl) were prepared. As can be seen in Fig. 10(b), the rejections of all the prepared membranes are between 95.2% to 98.1% except NaCl solution test. Their rejection against NaCl solution shows the lowest values, indicating 77.6% (TFC 0.3) and 75.3% (TFN). The negatively charged polyamide layer can reject high-valent anions (SO42−), resulting in high rejection of MgSO4 and Na2SO4. On the other

Normalized flux (J/J0)

1.2 TFC with BSA TFN 0.3 with BSA TFC with HA TFN 0.3 with HA

1.0 0.8 0.6 0.4 0.2 0.0 0

100

200

300

400

500

600

700

800

Time (minute) Fig. 11. Long-term antifouling test of TFC and TFN 0.3 membranes with BSA (bovine serum albumin) and HA (humic acid). 10

Desalination 470 (2019) 114125

Y. Kang, et al.

(b) 14 Flux before cleaning Flux after cleaning Flux ratio

120

1.6

110

1.4 0.98

0.96

8

1.2 1.0 0.8

6

Salt rejection (%)

10

Flux ratio

Water flux (LMH)

12

1.8

0.6 4 0.4 2 0 TFC

1.8 Salt rejection before cleaning Salt rejection after cleaning Salt rejection ratio

1.6 1.4

100

1.2

90

1.00

0.99 1.0

80 0.8 70

0.6

60

0.2

50

0.0

40

TFN 0.3

0.4 0.2 0.0 TFC

Membrane

Salt rejection ratio

(a)

TFN 0.3

Membrane

Fig. 12. Comparison of the flux (a) and salt rejection (b) ratio of TFC and TFN 0.3 membranes after chemical cleaning.

250 min. This phenomenon was related to the molecular weight of each foulants. The molecular weights of BSA and HA are 67 kDa and 2–500 kDa, respectively. They cannot pass through the membranes (MWCO: 546–826 Da); however, small particles of HA can easily block membrane pores and eventually severe fouling caused on the membrane surfaces.

Environment (MOE) (1485016274).

3.5. Mechanical stability of membranes in a cleaning process

References

Table A.2 and Fig. 12 present the membrane surface SEM images and the comparison of before and after cleaned membrane's performances. The SEM images of before and after membrane morphologies were similar. It means that there were not available any defects on the surface. Also, to find out more about cleaning effects, the water flux and salt rejection were measured. The performance results of membranes were similar even if they were cleaned with acid-base cleaning solution for 2 h. The flux ratio of TFC and TFN 0.3 membranes were 0.96 and 0.98. It showed that the flux was decreased by 4% and 2% only. The salt rejections also showed stable value. These results indicated that the poly (piperazine) active layers were stable in acid-base cleaning agents.

[1] Y. Zhang, S. Zhang, T.S. Chung, Nanometric graphene oxide framework membranes with enhanced heavy metal removal via nanofiltration, Environ. Sci. Technol. 49 (2015) 10235–10242, https://doi.org/10.1021/acs.est.5b02086. [2] F.Y. Zhao, Q.F. An, Y.L. Ji, C.J. Gao, A novel type of polyelectrolyte complex/ MWCNT hybrid nanofiltration membranes for water softening, J. Membr. Sci. 492 (2015) 412–421, https://doi.org/10.1016/j.memsci.2015.05.041. [3] H.K. Shon, S. Phuntsho, D.S. Chaudhary, S. Vigneswaran, J. Cho, Nanofiltration for water and wastewater treatment - a mini review, Drink. Water Eng. Sci. 6 (2013) 47–53, https://doi.org/10.5194/dwes-6-47-2013. [4] X. Li, R. Wang, F. Wicaksana, C. Tang, J. Torres, A.G. Fane, Preparation of high performance nanofiltration (NF) membranes incorporated with aquaporin Z, J. Membr. Sci. 450 (2014) 181–188, https://doi.org/10.1016/j.memsci.2013.09.007. [5] H. Li, W. Shi, Q. Du, R. Zhou, H. Zhang, X. Qin, Improved separation and antifouling properties of thin-film composite nanofiltration membrane by the incorporation of cGO, Appl. Surf. Sci. 407 (2017) 260–275, https://doi.org/10.1016/j.apsusc.2017. 02.204. [6] K.Y. Wang, T.S. Chung, J.J. Qin, Polybenzimidazole (PBI) nanofiltration hollow fiber membranes applied in forward osmosis process, J. Membr. Sci. 300 (2007) 6–12, https://doi.org/10.1016/j.memsci.2007.05.035. [7] S. Bano, A. Mahmood, S.J. Kim, K.H. Lee, Graphene oxide modified polyamide nanofiltration membrane with improved flux and antifouling properties, J. Mater. Chem. A 3 (2015) 2065–2071, https://doi.org/10.1039/c4ta03607g. [8] Y. Zhang, C. Zhao, S. Zhang, L. Yu, J. Li, L. an Hou, Preparation of SGO-modified nanofiltration membrane and its application in SO42 − and Cl− separation in salt treatment, J. Environ. Sci. (China) 78 (2019) 183–192, https://doi.org/10.1016/j. jes.2018.09.019. [9] N. Misdan, W.J. Lau, C.S. Ong, A.F. Ismail, T. Matsuura, Study on the thin film composite poly(piperazine-amide) nanofiltration membranes made of different polymeric substrates: effect of operating conditions, Korean J. Chem. Eng. 32 (2015) 753–760, https://doi.org/10.1007/s11814-014-0261-6. [10] M.E.A. Ali, L. Wang, X. Wang, X. Feng, Thin film composite membranes embedded with graphene oxide for water desalination, Desalination 386 (2016) 67–76, https://doi.org/10.1016/j.desal.2016.02.034. [11] W.J. Lau, S. Gray, T. Matsuura, D. Emadzadeh, J. Paul Chen, A.F. Ismail, A review on polyamide thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches, Water Res. 80 (2015) 306–324, https://doi.org/10. 1016/j.watres.2015.04.037. [12] A.F. Ismail, M. Padaki, N. Hilal, T. Matsuura, W.J. Lau, Thin film composite membrane - recent development and future potential, Desalination 356 (2015) 140–148, https://doi.org/10.1016/j.desal.2014.10.042. [13] S.R. Lakhotia, M. Mukhopadhyay, P. Kumari, Cerium oxide nanoparticles embedded thin-film nanocomposite nanofiltration membrane for water treatment, Sci. Rep. 8 (2018) 1–10, https://doi.org/10.1038/s41598-018-23188-7. [14] M. Ghanbari, D. Emadzadeh, W.J. Lau, S.O. Lai, T. Matsuura, A.F. Ismail, Synthesis and characterization of novel thin film nanocomposite (TFN) membranes embedded with halloysite nanotubes (HNTs) for water desalination, Desalination 358 (2015) 33–41, https://doi.org/10.1016/j.desal.2014.11.035. [15] H. Dong, L. Zhao, L. Zhang, H. Chen, C. Gao, W.S. Winston Ho, High-flux reverse osmosis membranes incorporated with NaY zeolite nanoparticles for brackish water

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2019.114125.

4. Conclusion In this study, a novel NF membrane was prepared by embedding hydrophilic SGO nanosheets into the PA layer, followed by IP process. Owing to the abundant oxygen functional groups, SGO can successfully enhance the wettability, negative charge, surface roughness, and crosslinking degree (low MWCO), also it reduced the thickness of PA layer. At an identical SGO content of 0.3 wt%, the TFN 0.3 membrane augmented the water flux reaching 11.86 LMH (87.3% improved) with an excellent MgSO4 salt rejection of > 95.0%, indicating that it maintained its salt rejection ability. In monovalent and divalent ions rejection test, it showed rejection values of over 95% for divalent ions and at the same time presented moderate rejection for the monovalent ions. TFN 0.3 membrane also exhibited low fouling tendency compared to the pristine TFC membrane. In conclusion, the addition of an appropriate amount of SGO to the membrane improved the hydrophilicity, water flux, salt rejection, and antifouling properties. However, our TFC membranes had less flux than other commercial and research membranes so we have to improve the performance further by changing thickness, concentrations of monomers, or flow system in the future. Acknowledgements This work was supported by Korea Environment Industry & Technology Institute (KEITI) through Industrial Facilities & Infrastructure Research Program, funded by Korea Ministry of 11

Desalination 470 (2019) 114125

Y. Kang, et al.

[16]

[17] [18] [19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29] [30]

[31]

[32]

[33]

[34] [35]

[36] [37] [38]

desalination, J. Membr. Sci. 476 (2015) 373–383, https://doi.org/10.1016/j. memsci.2014.11.054. G.S. Lai, W.J. Lau, P.S. Goh, A.F. Ismail, N. Yusof, Y.H. Tan, Graphene oxide incorporated thin film nanocomposite nanofiltration membrane for enhanced salt removal performance, Desalination 387 (2016) 14–24, https://doi.org/10.1016/j. desal.2016.03.007. J. Wang, C. Zhao, T. Wang, Z. Wu, X. Li, J. Li, Graphene oxide polypiperazine-amide nanofiltration membrane for improving flux and anti-fouling in water purification, RSC Adv. 6 (2016) 82174–82185, https://doi.org/10.1039/c6ra17284a. W. Zhao, H. Liu, N. Meng, M. Jian, H. Wang, X. Zhang, Graphene oxide incorporated thin fi lm nanocomposite membrane at low concentration monomers, J. Membr. Sci. 565 (2018) 380–389, https://doi.org/10.1016/j.memsci.2018.08.047. W. Choi, J. Choi, J. Bang, J.H. Lee, Layer-by-layer assembly of graphene oxide nanosheets on polyamide membranes for durable reverse-osmosis applications, ACS Appl. Mater. Interfaces 5 (2013) 12510–12519, https://doi.org/10.1021/ am403790s. J. Yin, G. Zhu, B. Deng, Graphene oxide (GO) enhanced polyamide (PA) thin-film nanocomposite (TFN) membrane for water purification, Desalination 379 (2016) 93–101, https://doi.org/10.1016/j.desal.2015.11.001. E. Mahmoudi, L.Y. Ng, M.M. Ba-Abbad, A.W. Mohammad, Novel nanohybrid polysulfone membrane embedded with silver nanoparticles on graphene oxide nanoplates, Chem. Eng. J. 277 (2015) 1–10, https://doi.org/10.1016/j.cej.2015.04. 107. G.F. Liu, L.J. Huang, Y.X. Wang, J.G. Tang, Y. Wang, M.M. Cheng, Y. Zhang, M.J. Kipper, L.A. Belfiore, W.S. Ranil, Preparation of a graphene/silver hybrid membrane as a new nanofiltration membrane, RSC Adv. 7 (2017) 49159–49165, https://doi.org/10.1039/c7ra07904d. S. Ayyaru, Y.H. Ahn, Application of sulfonic acid group functionalized graphene oxide to improve hydrophilicity, permeability, and antifouling of PVDF nanocomposite ultrafiltration membranes, J. Membr. Sci. 525 (2017) 210–219, https:// doi.org/10.1016/j.memsci.2016.10.048. Y. Kang, M. Obaid, J. Jang, M.-H. Ham, I.S. Kim, Novel sulfonated graphene oxide incorporated polysulfone nanocomposite membranes for enhanced-performance in ultrafiltration process, Chemosphere 207 (2018) 581–589, https://doi.org/10. 1016/j.chemosphere.2018.05.141. G.S. Lai, W.J. Lau, P.S. Goh, A.F. Ismail, Y.H. Tan, C.Y. Chong, R. Krause-Rehberg, S. Awad, Tailor-made thin film nanocomposite membrane incorporated with graphene oxide using novel interfacial polymerization technique for enhanced water separation, Chem. Eng. J. 344 (2018) 524–534, https://doi.org/10.1016/j.cej. 2018.03.116. O. Akin, F. Temelli, Probing the hydrophobicity of commercial reverse osmosis membranes produced by interfacial polymerization using contact angle, XPS, FTIR, FE-SEM and AFM, Desalination 278 (2011) 387–396, https://doi.org/10.1016/j. desal.2011.05.053. M. Vinothkannan, A.R. Kim, G. Gnana Kumar, D.J. Yoo, Sulfonated graphene oxide/ Nafion composite membranes for high temperature and low humidity proton exchange membrane fuel cells, RSC Adv. 8 (2018) 7494–7508, https://doi.org/10. 1039/c7ra12768e. N. Meng, R.C.E. Priestley, Y. Zhang, H. Wang, X. Zhang, The effect of reduction degree of GO nanosheets on microstructure and performance of PVDF/GO hybrid membranes, J. Membr. Sci. 501 (2016) 169–178, https://doi.org/10.1016/j. memsci.2015.12.004. L. Kou, C. Gao, Making silica nanoparticle-covered graphene oxide nanohybrids as general building blocks for large-area superhydrophilic coatings, Nanoscale 3 (2011) 519–528, https://doi.org/10.1039/c0nr00609b. V.G. Sreeja, A. Cheruvalathu, R. Reshmi, E.I. Anila, S. Thomas, M.K. Jayaraj, Effect of thickness on nonlinear absorption properties of graphite oxide thin films, Opt. Mater. (Amst). 60 (2016) 450–455, https://doi.org/10.1016/j.optmat.2016.08. 025. W. Peng, H. Li, Y. Liu, S. Song, Effect of oxidation degree of graphene oxide on the electrochemical performance of CoAl-layered double hydroxide/graphene composites, Appl. Mater. Today 7 (2017) 201–211, https://doi.org/10.1016/j.apmt.2017. 03.005. M. Yang, C. Zhao, S. Zhang, P. Li, D. Hou, Preparation of graphene oxide modified poly(m-phenylene isophthalamide) nanofiltration membrane with improved water flux and antifouling property, Appl. Surf. Sci. 394 (2017) 149–159, https://doi.org/ 10.1016/j.apsusc.2016.10.069. M. Obaid, Y. Kang, S. Wang, M.H. Yoon, C.M. Kim, J.H. Song, I.S. Kim, Fabrication of highly permeable thin-film nanocomposite forward osmosis membranes: via the design of novel freestanding robust nanofiber substrates, J. Mater. Chem. A 6 (2018) 11700–11713, https://doi.org/10.1039/c7ta11320j. C.C. Li, Y.P. Zheng, T.H. Wang, Sulfated mesoporous Au/TiO 2 spheres as a highly active and stable solid acid catalyst, J. Mater. Chem. 22 (2012) 13216–13222, https://doi.org/10.1039/c2jm16921e. H. Li, J. Fan, Z. Shi, M. Lian, M. Tian, J. Yin, Preparation and characterization of sulfonated graphene-enhanced poly (vinyl alcohol) composite hydrogel and its application as dye absorbent, Polymer (Guildf) 60 (2015) 96–106, https://doi.org/ 10.1016/j.polymer.2014.12.069. R. Kumar, M. Mamlouk, K. Scott, Sulfonated polyether ether ketone-sulfonated graphene oxide composite membranes for polymer electrolyte fuel cells, RSC Adv. 4 (2014) 617–623, https://doi.org/10.1039/c3ra42390e. S. Scalese, I. Nicotera, D.D. Angelo, S. Filice, Cationic and anionic azo-dye removal from water by sulfonated graphene oxide nanosheets in Nafion membranes, New J. Chem. 40 (2016) 3654–3663, https://doi.org/10.1039/C5NJ03096J. X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, F. Zhang, Deoxygenation of

[39] [40] [41] [42]

[43] [44]

[45] [46]

[47] [48]

[49]

[50]

[51]

[52]

[53] [54] [55]

[56]

[57]

[58]

[59] [60]

[61]

12

exfoliated graphite oxide under alkaline conditions: a green route to graphene preparation, Adv. Mater. 20 (2008) 4490–4493, https://doi.org/10.1002/adma. 200801306. G. Zhao, L. Jiang, Y. He, J. Li, H. Dong, X. Wang, W. Hu, Sulfonated graphene for persistent aromatic pollutant management, Adv. Mater. 23 (2011) 3959–3963, https://doi.org/10.1002/adma.201101007. A.K. Ghosh, B.H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, J. Membr. Sci. 311 (2008) 34–45, https://doi.org/10.1016/j.memsci.2007.11.038. M.S.B. Khorshidi, T. Thundat, B.A. Fleck, Thin film composite polyamide membranes: parametric study on the influence of synthesis conditions, RSC Adv. 5 (2015) 54985–54997, https://doi.org/10.1039/C5RA08317F. M. Dalwani, N.E. Benes, G. Bargeman, D. Stamatialis, M. Wessling, Effect of pH on the performance of polyamide/polyacrylonitrile based thin film composite membranes, J. Membr. Sci. 372 (2011) 228–238, https://doi.org/10.1016/j.memsci. 2011.02.012. Y. Zhang, Z. Jing, T. Kameda, T. Yoshioka, Hydrothermal synthesis of hardened diatomite-based adsorbents with analcime formation for methylene blue adsorption, RSC Adv. 6 (2016) 26765–26774, https://doi.org/10.1039/c5ra18948a. Q. Li, Y. Wang, J. Song, Y. Guan, H. Yu, X. Pan, F. Wu, M. Zhang, Influence of silica nanospheres on the separation performance of thin film composite poly(piperazineamide) nanofiltration membranes, Appl. Surf. Sci. 324 (2015) 757–764, https://doi. org/10.1016/j.apsusc.2014.11.031. W. Gao, The chemistry of graphene oxide, Graphene Oxide Reduct. Recipes, Spectrosc. Appl. (2015) 61–95, https://doi.org/10.1007/978-3-319-15500-5_3. Q. Xie, W. Shao, S. Zhang, Z. Hong, Q. Wang, B. Zeng, Enhancing the performance of thin-film nanocomposite nanofiltration membranes using MAH-modified GO nanosheets, RSC Adv. 7 (2017) 54898–54910, https://doi.org/10.1039/ c7ra11550d. N. Yin, K. Wang, L. Wang, Z. Li, Amino-functionalized MOFs combining ceramic membrane ultrafiltration for Pb (II) removal, Chem. Eng. J. 306 (2016) 619–628, https://doi.org/10.1016/j.cej.2016.07.064. D. Koushik, W.J.H. Verhees, D. Zhang, Y. Kuang, S. Veenstra, M. Creatore, R.E.I. Schropp, Atomic layer deposition enabled perovskite/PEDOT solar cells in a regular n–i–p architectural design, Adv. Mater. Interfaces 4 (2017), https://doi.org/ 10.1002/admi.201700043. M.S. Haider, G.N. Shao, S.M. Imran, S.S. Park, N. Abbas, M.S. Tahir, M. Hussain, W. Bae, H.T. Kim, Aminated polyethersulfone-silver nanoparticles (AgNPs-APES) composite membranes with controlled silver ion release for antibacterial and water treatment applications, Mater. Sci. Eng. C. 62 (2016) 732–745, https://doi.org/10. 1016/j.msec.2016.02.025. Polyphenylsulfone-based solvent resistant nanofiltration (SRNF) membrane incorporated with copper-1,3,5-benzenetricarboxylate (cu-BTC) nanoparticles for methanol separation, RSC Adv. 5 (2015) 13000–13010, https://doi.org/10.1039/ C4RA14284E. M. Safarpour, V. Vatanpour, A. Khataee, M. Esmaeili, Development of a novel high flux and fouling-resistant thin film composite nanofiltration membrane by embedding reduced graphene oxide/TiO2, Sep. Purif. Technol. 154 (2015) 96–107, https://doi.org/10.1016/j.seppur.2015.09.039. R. Ploeger, S. Musso, O. Chiantore, D. Quéré, L. Fernández, M. Sánchez, F.J. Carmona, L. Palacio, J.I. Calvo, A. Hernández, P. Prádanos, Analysis of the grafting process of PVP on a silicon surface by AFM and contact angle, Langmuir 27 (2009) 77–83, https://doi.org/10.1021/la201683p. R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994, https://doi.org/10.1021/ie50320a024. D. Quéré, Non-sticking drops, Reports Prog. Phys. 68 (2005) 2495–2532, https:// doi.org/10.1088/0034-4885/68/11/R01. R. Ploeger, S. Musso, O. Chiantore, Contact angle measurements to determine the rate of surface oxidation of artists’ alkyd paints during accelerated photo-ageing, Prog. Org. Coatings. 65 (2009) 77–83, https://doi.org/10.1016/j.porgcoat.2008. 09.018. C. Zhao, X. Xu, J. Chen, F. Yang, Effect of graphene oxide concentration on the morphologies and antifouling properties of PVDF ultrafiltration membranes, J. Environ. Chem. Eng. 1 (2013) 349–354, https://doi.org/10.1016/j.jece.2013.05. 014. I. Wan Azelee, P.S. Goh, W.J. Lau, A.F. Ismail, M. Rezaei-DashtArzhandi, K.C. Wong, M.N. Subramaniam, Enhanced desalination of polyamide thin film nanocomposite incorporated with acid treated multiwalled carbon nanotube-titania nanotube hybrid, Desalination 409 (2017) 163–170, https://doi.org/10.1016/j. desal.2017.01.029. C. Liu, Y. Sun, Z. Chen, S. Zhang, From ultrafiltration to nanofiltration: Nanofiltration membrane fabricated by a combined process of chemical crosslinking and thermal annealing, Sep. Purif. Technol. 212 (2019) 465–473, https:// doi.org/10.1016/j.seppur.2018.11.041. R. Wang, X. Shi, A. Xiao, W. Zhou, Y. Wang, Interfacial polymerization of covalent organic frameworks (COFs) on polymeric substrates for molecular separations, J. Membr. Sci. 566 (2018) 197–204, https://doi.org/10.1016/j.memsci.2018.08.044. V. Vatanpour, S.S. Madaeni, R. Moradian, S. Zinadini, B. Astinchap, Fabrication and characterization of novel antifouling nanofiltration membrane prepared from oxidized multiwalled carbon nanotube/polyethersulfone nanocomposite, J. Membr. Sci. 375 (2011) 284–294, https://doi.org/10.1016/j.memsci.2011.03.055. R.R. Choudhury, J.M. Gohil, S. Mohanty, S.K. Nayak, Antifouling, fouling release and antimicrobial materials for surface modification of reverse osmosis and nanofiltration membranes, J. Mater. Chem. A 6 (2018) 313–333, https://doi.org/10. 1039/c7ta08627j.