Spin-assisted interfacial polymerization strategy for graphene oxide-polyamide composite nanofiltration membrane with high performance

Journal Pre-proofs Full Length Article Spin-assisted interfacial polymerization strategy for graphene oxide-polyamide composite nanofiltration membrane with high performance Xu Kang, Xin Liu, Jinghua Liu, Yan Wen, Jinyao Qi, Xin Li PII: DOI: Reference:

S0169-4332(19)34015-2 https://doi.org/10.1016/j.apsusc.2019.145198 APSUSC 145198

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Applied Surface Science

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4 August 2019 10 December 2019 25 December 2019

Please cite this article as: X. Kang, X. Liu, J. Liu, Y. Wen, J. Qi, X. Li, Spin-assisted interfacial polymerization strategy for graphene oxide-polyamide composite nanofiltration membrane with high performance, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145198

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Spin-assisted interfacial polymerization strategy for graphene oxide-polyamide composite nanofiltration membrane with high performance Xu Kang1, Xin Liu1, Jinghua Liu2, Yan Wen1, Jinyao Qi1*, Xin Li2* 1

School of Environment, Harbin Institute of Technology, Harbin 150090, China

2

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage,

School of Chemistry and Chemical Engineering, State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150001, China *Corresponding authors: Jinyao Qi; Xin Li E-mail addresses: [email protected] (J.Y. Qi); [email protected] (X. Li)

ABSTRACT: Graphene oxide (GO) has been addressed to have greatly potential in nanofiltration membranes. The state-of-the-art GO-based nanofiltration is still limited by the low permeability and instability for the large-scale industrially application. Herein, we developed a novel interfacial polymerization technology for high permeability and advanced stability GO-polyamide membrane with the help of spin. The high-rough microfiltration membrane was chosen as the substrate for providing large effective filtration area and trapping GO nanosheets in valley structures. Aided with centrifugal force field from the spin, the excess low concentration amine monomer solution could be removed homogeneous to form ultrathin and defect-free polyamide (PA) layer. The pure water permeability of the graphene oxide-polyamide (GO-PA) composite nanofiltration membrane could be up to 35.14 LMH/bar with Na2SO4 rejection of 93.56% at 4 bar, which is comparable to the top permeability of GO-based nanofiltration membranes with the similar salt rejection that has been 1

reported. The membranes also behave high Na2SO4/NaCl selectivity (15-56) due to the extremely low NaCl rejection. In addition, the GO-PA composite nanofiltration membranes exhibit good stability in harsh environment and solvents. This work promotes GO-based nanofiltration for practical applications and provides a novel route for high-performance nanofiltration membranes. Keywords: nanofiltration membrane; spin; interfacial polymerization; salt rejection; high permeability 1 Introduction Water purification has been a worldwide issue owing to the development of industry and increasing pollution of environment[1, 2]. Innovative water treatment technology should be rapidly developed for human growing demand and development for clean water[3]. Nanofiltration (NF) membrane separation technology has been proven to be a very effective way for water purification, especially for hardness, heavy metal, and dissolved organic matter removal[4]. The separation performance of NF membranes may be attributed to a combined effect of steric, Donnan, dielectric and transport [5] . In the past few decades, many studies have contributed to improving NF membrane performance. Whatever, its performance is still not high enough for affordable energy cost. Up to now, NF membranes are faced numerous challenges, like poor antifouling, high energy consumption and "trade-off" limits between permeability and selectivity[6]. Most high performance NF membranes are generally fabricated by interfacial

2

polymerization(IP) to form thin film composite (TFC) polyamide (PA) layer on a porous substrate[7]. The PA layer affects the overall performance of the membrane, such as permeability and selectivity, even greater than the influence of the substrate[8]. Extensive efforts have been invested to enhance the performance of the PA activated layer. One feasible way is to incorporate nanoparticles in the skin PA layer for optimizing the PA layer structure. Hoek and co-workers developed a strategy for membrane fabrication to embed nanoparticles within the skin thin layer and used the conception of thin film nanocomposite (TNF) membrane in early 2007[9]. The introduction to nanoparticles in the skin layer could reduce the resistance of water transport and enhance the permeation without sacrificing rejection[10]. However, the nanoparticles are easily to leak out during the IP or filtration process, due to the poor interaction between nanopatricles and PA matrix[11]. Limited by the inherent property of the PA material itself, it's hard to significantly improve the physicochemical properties of the PA layer for achieving high separation performance[12]. Another practical approach to optimize the PA layer for reducing water transport resistance is to prepare thinner PA layer. However, it's difficult to form a thin and defect-free PA layer due to the rapid reaction of IP. Introducing an interlayer to control the diffusion rate of amine monomer and acid chloride may be a feasible strategy for obtaining a thin PA active layer[13, 14]. A hydrophilic interlayer could make the substrate more uniform to help amine monomers diffusing homogeneously, which enabled efficient spread of the monomer solution and hold a dilute aqueous layer on

3

the substrate interface[15]. Graphene and its derivatives are one of the most promising candidate materials of interlayer in the filtration application for their superior water permeation and thickness of an atom. Compared with graphene, graphene oxide (GO) contains oxygen-containing functional groups on both the plane and the edge, which is more hydrophilic[16]. In recent years, GO has been proved to possess great potential for the NF membranes application[17-19]. Due to the advantages of easy preparation and robust physicochemical property, GO-based membranes are expected to be industrial-scale production. A series of different techniques, such as vacuum filtration[20,

21],

printing[22],

spin-coating[23],

spray-evaporation[24,

25],

layer-by-layer assembly[26, 27], and shear alignment[28] have been applied to prepare GO-based membranes. By the poor interaction force between GO and substrate, the GO interlayer is easily exfoliation during filtration[29, 30]. Applications of GO-based membranes are inhibited by the insufficient stability in aqueous environment[31-33]. To address this issue, many studies have attempted to develop high steady GO-based membrane[34]. In addition to the above aspects which can improve the performance of the membrane, the polymerization conditions can also greatly affect the PA layer. Numerous studies have been focused on the polymerization conditions including: support, monomers, polymeric reagents, additives, post treatments and polymerization timeto develop novel TFC nanofiltration membrane[35-42]. Among these factors, the monomers almost have the greatest influence on the polymerization. The porosity of

4

the thin PA layer is dependent on the amount of water that diffuses with the aqueous monomer into the formed PA layer[43]. And the thickness of PA layer could be controlled by the amine monomer concentration[44]. Therefore, during the IP process, the concentration and amount of the amine monomer remaining in the substrate will affect the structure of the PA layer. It is reasonable to believe that the removal of the monomer solution by different methods affects the distribution of the monomers and the amount of monomers actually involved in the reaction, resulting in different PA structures. Typical methods for removing excess monomer solution include rubber roller, air knife and gravity removing. These removal ways are performed manually, which introduces unwanted defects and is poorly repeatable. What’s more, polymerization could also take place inside the pores of the substrate as a result of penetration of the monomer through the PA layer[45]. The normal removal methods only remove monomer on the surface of the substrate without affecting the internal monomer. So it’s necessary to develop a novel method for uniform and controlled removal of monomer throughout the substrate. To address these challenges for further promoting the performance of NF membranes, herein we propose a spin-assisted IP strategy on the high-rough microfiltration porous substrate for forming GO-PA composite membrane. Spin coating is a process of coating a film on bottom plate by centrifugal force. Unlike other monomers removal methods that only work on the surface of the substrate, centrifugal force acts on the entire substrate during the spin coating. Since the

5

centrifugal force is present in all directions, the monomer solution is mixed and redistributed in the substrate to make the monomer distribution uniform. The amount of excess monomer removal can be achieved by controlling the spin speed. Therefore, spin coating can be a useful technique for uniformly removing monomers by a design procedure. The Shematic porcess diagram of GO-PA composite membrane fabrication via spin-assisted IP technique is shown in Fig.1. At first, a high-rough microfiltration membrane was chosen as the substrate, which could trap GO nanosheets in the valley morphology locus and provide high effective filtration area. Trace GO was then used as interlayer to coat part of the microfiltration substrate. The trace GO can maintain more piperazine (PIP) with increasing almost negligible thickness of the substrate. With the help of centrifugal force that formed by spin, the redundant low concentration PIP (0.05 w/v%) was removed homogeneously without physical damage. Finally, a thin and defect-free PA activate layer was achieved. By this strategy, a GO-PA composite nanofiltration membrane with high nanofiltration performance and good stability is achieved.

Fig. 1 Shematic porcess diagram of GO-PA composite membrane fabrication via spin-assisted IP 6

technique.

2. Experimental 2.1 Materials All reagents and solvents were commercially available and used as received without further purification. The graphite was purchased from Alfa Aesar (U.S.) for preparing the graphene oxide. The nylon substrate membrane (48 mm in diameter, 0.22 μm pore size, arithmetic mean roughness Ra=259 nm) was used as substrate, obtained from Zhengxing Special Filter Equipment Manufacturing Co., Ltd. (Haining, China). NaCl, Na2SO4, MgCl2, MgSO4 were used as salt for rejection experiment, obtained from TianDa Chemical Reagent CO., Ltd. (Tianjin, China). Polyethylene glycol (PEG, molecular weight 200-1000 Da) was supplied from Guangfu Chemical Research Institute (Tianjin, China) to test the solvent rejection. Piperazine (PIP) and 1, 3,

5-benzenetricarbonyl

trichloride

(TMC)

were

purchased

from

Macklin

Biochemical Co., Ltd. (Shanghai, China). Ethanol (EtOH), methanol (MeOH), isopropanol (IPA) and N, N-dimethylformamide (DMF) were issued by Fuyu Fine chemical Co., Ltd. (Tianjin, China). 2.2 Graphene oxide synthesis GO was self-synthesized according to the modified Hummers method[46, 47] (Seen in Support Information). The resulting GO nanosheets was diluted to 500 mL H2O. After 30 min of ultrasonication, the as-prepared GO dispersions were centrifuged at 10000 rpm for 10 min to remove the large nanosheets in the precipitation. Subsequently, the supernatant was collected and the precipitation was 7

re-dispersed. The centrifugation procedure was repeated four times and all supernatants were collected to obtain thin-layer graphene oxide dispersion. 2.3 GO-PA composite membrane synthesis The IP by piperazine (PIP) and trimesoyl chloride (TMC) was used to form the selective layer. The preparation process of GO-PA composite nanofiltration membrane is schematically illustrated in Fig. 1. Specific volume of GO dispersion was diluted to 50 mL of deionized water for ultrasonication 30 min, then coated on nylon substrate by vacuum filtration. The GO loading ranges from 3-24 mg/m2. The GO coated substrate membrane was immersed into series concentration PIP aqueous solution. After 2 min, the PIP solution was removed by spin under specific rotation speed ( 600 rpm, 40s). After contacting with the surface of membrane for 2 min, The n-hexane solution containing TMC (0.5 w/v‰) was poured out. Finally the as-treated membrane was further dried in oven at 60 Ԩ for 10 min. The prepared membranes

were donated as GOx-PAy, which x refers to GO loading (0, 3, 6, 12, 18, 24 mg/m2), y refers to PIP concentration (0.5 w/v‰, 2.0 w/v‰), stored at 5 Ԩ DI water before

use. As contrasted with spin, another three ways to remove excess PIP solution were applied: using a rubber roller, vacuum filtration and gravity removing (drained membrane vertically in air until no obvious water spot can be observed on the membrane). 2.4 Characterization The surface of GO nanosheets was recorded by TEM (Tecnai G2 F20 electron

8

microscope (FEI)). SEM images were taken from a S4800 HSD electron microscopy (Hitachi) to survey the morphology of membrane surface. The functional groups of membranes were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet is50, Thermo, America) in the wavelength range of 400–4000 cm−1 at room temperature. The crystalline structures of GO were tested by X-ray diffraction (XRD, Bruker D8-Advance diffractometer with Cu-Kα radiation) and the scan range of 2θ was 10–90°. AFM images were conducted by tapping mode of a Dimension Fastscan microscopy to observe the thickness and roughness of membranes. The chemical elements of membranes were analyzed by X-ray photoelectron spectroscopy(XPS, ESCALAB 250Xi spectrometer). The wettability of the surface was measured by conducting static contact angle (CA) measurement with Goniometer instrument equipped (JC 2000C, Zhong chen, China). The zetal potential of membrane was analyzed by electrokinetic analyzer (SurPASSTM 3, Anton Paar, Austria). 2.5 The test of membrane performance The nanofiltration separation performance was tested by a homemade cross-flow filtration cell at room temperature. The effective area of membrane was 7.1 cm2. The salt rejection was performed at 1000 ppm with four different salts (NaCl, Na2SO4, MgCl2 and MgSO4). The pure water and salt solution permeability of membrane was tested by DI water and salt solutions. The permeability J (LMH/bar) and rejection R (%) were calculated according to Eq. (1) and Eq. (2) respectively: J = V/AtP

9

(1)

Where V is the volume of permeated water (L), A is the effective area of membrane (m2), and t is the permeate time (h), P is the nitrogen pressure (bar) R=1- Cp/Cf×100%

(2)

The conductivity of the feedwater (Cf) and that of the permeate (Cp) was measured by the conductivity meter (Weiye instrument factory, DDS-11A, China). The separation factor of NaCl to Na2SO4 (α) was calculated by Eq. (3):

a=

(C (C

NaCl

/ CNa 2SO4

NaCl

/ CNa 2SO4

) )

p f

=

1- RNaCl 1- RNa 2SO4

(3)

3 Results and discussion 3.1 GO characterization The characterization of GO nanosheets was analyzed by TEM, XRD, Raman spectrometer, FT-IR spectrometer and AFM. The wrinkle on the edge and surface of GO could be clearly observed from TEM image (Fig. 2a), showing the good flexibility. According to the Bragg equation, the XRD (Fig. 2b) of GO shows the characteristics of diffraction peak in 11.87°, corresponding to a 0.75 nm layer spacing. The Raman spectra of GO and graphite were also employed and displayed in Fig. 2c. The D-band of graphite is almost negligible, the peak intensity radio of D-band to G-band (ID/IG) is only 0.08. Compared with graphite, the D-band of GO increases significantly and the ratio of ID/IG reaches 0.90, indicating the presence of defects in GO[48]. The FT-IR spectra were performed to study the groups of GO (Fig. 2d). The strong peak at 3412 cm-1 is attributed to -OH stretching vibration, expressing the

10

existence of hydroxyl groups. The peak at 1730 cm-1 is assigned to C=O stretching vibration of carboxyl. The peaks at 1622 and 1100 cm-1, corresponding to the C-OH bending vibration and C-O-C vibration absorption, respectively. Based on the results of FT-IR spectra, the as-prepared GO contains various negatively charged oxygen functional groups, such as carboxyl, hydroxyl and epoxy. These hydrophilic groups of the GO can prevent the aggregation in water and lead to good dispensability. The thickness of as-prepared GO layer was measured by AFM and shown in Fig. 3e. The ultrathin GO nanosheets have only a thickness of 1.51 nm.

Fig. 2 Characterization of GO nanosheets: (a) TEM image, (b) XRD spectra, (c) Raman spectra, (d) FT-IR spectra, (e) AFM image. 11

3.2 Membrane characterization SEM images of GO coated substrates were presented in Fig. 3. The surface of nylon substrate (Fig. 3a and Fig. 3b) has many macropores, exhibiting rough mountain and valley topographies. The trace GO loading (3 mg/m2) has little effect on substrate morphology and is hardly observed on the substrate surface (Fig. 3c). As shown in Fig. 3d, GO nanosheets are trapped in the valleys of substrate. These valley structures could provide physics anchorage for GO nanosheets and improve the mechanical stability of the GO coated substrates. The disordered deposition of GO nanosheets may be attributed to the extreme flexibility of GO nanosheets. GO nanosheets stack along the topography during the deposition process, which would wrap and attach to the ridge and tuck into valleys even fall into interior pores[49]. As depicted in Fig. 3e and Fig. 3f, the substrate is partially covered by GO nanosheets when the GO loading reaches 6 mg/m2. Most GO nanosheets fall into the valley and a small excess of GO nanosheets begins to fill the valley. Due to the thin GO interlayer, the skeleton of the substrate can be observed clearly. We further studied the effect of high GO loading (12-24 mg/m2) on the substrate, as shown in Fig. S1. Until the GO loading reaches 24 mg/m2, the surface of substrate are totally coated and the morphology of substrate shows the GO laminates topographic characteristics. As the GO loading increases, the pores of substrate are gradually covered and become smoother. As a result, the substrate behaves more hydrophobic as the GO loading increasing (Fig. S2).

12

The schematic view of GO nanosheets laminated on the high-rough substrate was described in Fig. S3. At first, a small amount of GO nanosheets stack messily and are trapped in the valleys or dropped into the interior pores of the substrate. Subsequently, the more GO nanosheets gradually fill up the valley and modify the surface topography of substrate. When the GO loading is sufficient, GO nanosheets form layered laminates on the top of substrate. The substrate can be completely covered by the high GO loading and exhibits low roughness. Nevertheless, the ordering of layer-stacked GO nanosheets will lead to a lower surface area and reduce mechanical stability of the membrane. Tape test was employed to evaluate the adhesion of GO to substrate (Seen in Support Information). As shown in Fig.S4, a significant difference is observed for different GO loading. Apparent detachment is observed in high GO loading membranes (12-24 mg/m2), leaving a thin GO layer. However, in low GO loading membranes (3-6 mg/m2), there is slightly visible exfoliation in the tested region. This indicates that the GO at interface is conformal to substrate, resulting in strong adhesion. And multilayer GO has poor adhesion, leading to delamination. A lower roughness PES substrate (Ra=122 nm) was also tested to compared with high roughness nylon substrate (Fig. S4e). A thin GO layer is still remained on the both nylon and PES surface after tested. But there is a more obvious visible detachment in PES, which means the weaker adhesion. These results indicate that higher roughness substrate and trace GO loading lead to higher adhesion. According to theoretical

13

calculation, adhesion energy of graphene depends on both the substrate surface roughness and the bending modulus of graphene[50].

Therefore, we focused on the trace GO loading of 3-6 mg/m2 on nylon substrate for the following analysis.

Fig. 3 SEM and photo images of the GO coated membranes for different GO loading: 0 mg/m2 (a-c), 3 mg/m2 (d-f), 6 mg/m2 (g-i). The inserted images are their corresponding high magnification SEM images.

The GO loading and PIP concentration have significant influence on the surface 14

morphology of GO-PA composite membranes. As presented in Fig. 4, the top rough PA functional layer with many irregular micro-nano structures is formed after IP process. GO0-PA0.5 and GO0-PA2 composite membranes without GO loading are appeared to be smooth (with Ra=127 nm, 143 nm), while the GO-loaded GO-PA membranes show rougher surface due to the presence of GO. The schematic diagram of the IP process with and without GO nanosheets was depicted in Fig. 5. In the absence of GO loading, the TMC solution can penetrate deep into the substrate such that the IP reaction occurs not only on the surface but also in the vertical direction. The resulting PA active layer is embedded in the substrate, exhibiting lower roughness. On account of the effects of GO nanosheets, GO laminates can maintain more PIP to promote the IP process. At the same time, GO laminates can act as a barrier to limit the IP reaction to the surface of substrate that leads to higher roughness.

Fig. 4 SEM and AFM˄upper right˅images of GO-PA composite membrane: (a) GO0-PA0.5, (b) GO3-PA0.5, (c) GO6-PA0.5, (d) GO0-PA2, (e) GO3-PA2, (f) GO6-PA2. 15

Fig. 5 Schematic diagram of interfacial polymerization process with and without GO nanosheets.

As illustrated in Fig. S7 and Fig. S8, no matter what the PIP concentration is, the larger GO loading would reduce surface roughness. It's possible that the more GO layer provides a more uniform and hydrophobic substrate for IP to form a smoother and denser surface. The SEM images of composite membranes cross-section were surveyed to measure the thickness of functional layer. As can be seen from Fig. 6, the obtained GO6-PA0.5 membrane has extremely thin functional layer, only about 20 nm thickness. When the PIP concentration reaches 2.0 w/v‰, the thickness of functional layer rises to about 35 nm.

Fig. 6 SEM images of the cross section for the GO-PA composite membrane: (a) GO6-PA0.5 and (b) GO6-PA2. 16

Surface functional groups of the GO-PA membranes were examined by the FT-IR spectra, as shown in Fig. 7. Due to the irradiation depth (>300 nm) of FT-IR exceeding the function layer, the strong signal of the substrate has covered most information assigned to top layer of membranes. The typical peak detected at 1638cm-1 is corresponding to the amide C=O stretching vibration (amide I band). The peak at 3300cm-1 and 1542cm-1 is assigned to the amide N-H stretching vibration and bending vibration (amide II band). Due to the PA layer and substrate having the same structure of amide, it's hard to distinguish the sources of these peaks. The new weak peaks presenting at 3500cm-1 and 1730cm -1are appointed to the O-H stretching vibration and the C=O stretching vibration, which are found in GO-loaded membranes, confirming the existence of GO on the membrane surface.

Fig. 7 FT-IR spectra of the membranes.

For analyzing the surface elemental and the cross-linking degree of PA skin layer, XPS was applied to characterize the membrane surfaces. Table 1 had the results of C, O, N and S elements concentration of membrane surface. The O/C radio of 17

GO-coated substrate is slightly higher than nylon substrate, due to the oxygen-containing functional group of GO. The S element might come from the substrate. The cross-linking degree can be revealed by the O/N radio. Compared with the O/N radio of GO6-PA0.5, GO0-PA0.5 shows a lower O/N radio of 1.439, which indicates the higher cross-linking degree of PA skin layer. The O/N radio of GO6-PA0.5 is beyond than 2, expressing the relatively low cross-linking degree of PA skin layer. The presence of GO is likely to impede PIP to react with TMC at PIP concentration of 0.5 w/v‰ by damaging the integrity of PA network. However, the effect of GO is exactly the opposite at 2.0 w/v‰ PIP concentration. This difference may be due to the fact that GO stores enough PIP to help achieving a full cross-linking degree of PA layer at high PIP concentration. These results are agreed with the following analysis of water static contact angle. Table 1 Surface compositions of the Nylon substrate, GO coated substrate and GO-PA composite membranes from XPS. Sample

C (%)

O (%)

N (%)

S (%)

O/N

Nylon substrate

73.42

10.77

14.98

0.83

0.719

GO-coated substrate

70.84

18.01

10.49

0.66

1.717

GO0-PA0.5

63.58

21.49

14.93

-

1.439

GO6-PA0.5

62.46

25.83

10.56

1.14

2.446

GO0-PA2

71.15

16.20

12.65

-

1.281

GO6-PA2

66.75

16.53

15.88

0.84

1.041

The water static contact angle (CA) was tested to measure the wettability of membrane (Fig. 8). At a low PIP concentration of 0.5 w/v‰, although the addition of GO reduces the roughness of membrane, there is a negative correlation between GO 18

loading and CA. Since hydrophilic functional groups of GO having bigger effect on wettability, it can promote a decrease in contact angle. According to the analysis of the XPS, we know that GO could block the cross-linking of PA at 0.5 w/v‰ PIP concentration and it's opposite at 2.0 w/v‰ PIP concentration. On account of full cross-linking degree of PA skin layer, the addition of GO can form a denser and smoother PA layer of GO-PA2 membranes. Therefore, GO-PA2 membranes show different wetting behaviors. The GO loading is beneficial for improving the hydrophobicity of GO-PA2 membranes.

Fig. 8 Water contact angles of GO-PA composite membranes.

The electrostatic interaction and physical sieving are the two main mechanisms of nanofiltration membranes, which could be evaluated by zeta potential and PEG rejection. Owing to the functional groups of GO nanosheets providing negative charges, the negative zeta potential of membrane is promoted. As shown in Fig. 9a, the membranes at different GO loading are negatively charged under neutral and basic conditions. Within the pH range of 5-10, GO6-PA0.5 has the most negatively charges and a zeta potential of -30.25 mV at pH=7, which has the strongest repulsive force to the negative ions under 0.5 w/v‰ PIP concentration. However, at 19

2.0 w/v‰ PIP concentration, there is no significant correlation between GO loading and Zeta potential. That may due to the high cross-linking PA at high PIP concentration, which masks the effect of trace GO loading. According to the results of PEG rejection (Fig. 9bˈFig. S9 and Table S1), the GO-PA composite membranes exhibit narrower pore size distribution (PSD) with the increase of GO loading. It should be noted that at the PIP concentration of 0.5 w/v‰, although GO can weaken the cross-linking degree of PA, the final effect of GO on the membrane is still to increase the densification of membrane pores. The possible reason is that the coverage modification effect of GO on the substrate is greater than the negative effect on the cross-linking degree of PA. Therefore, despite of GO having two different effects on the cross-linking of PA at different PIP concentrations, its comprehensive effects on the GO-PA composite membrane are the same, which can facilitate the membranes to form a narrower PSD and smaller average pore size. These results are well consistent with the following nanofiltration test.

Fig. 9 (a) Zeta potential of the GO-PA composite membranes surface as a function of solution pH and (b) pores size distributions of the GO-PA composite membranes.

20

3.3 The effect of excess PIP removal Different spin speed for 40 s was designed to determine the best procedure for removing the excess PIP solution. The significantly effect of the speed on separation performance was observed in Fig. 10a. Under low spin speed (˘600 rpm), the centrifugal force is insufficient to remove PIP solution adequately. This will allow more PIP monomers to react with TMC to form a denser PA layer and result in lower permeability. At 600 rpm speed, salt rejection of membrane decreases slightly from 97.50% to 96.72%, while the permeability increases from13.11LMH/bar to 26.68 LMH/bar. When the spin speed rises above 600 rpm, the PIP solution is removed too much, resulting in insufficient PIP to react with TMC and lower salt rejection. So only an appropriate spin speed can achieve a good balance between permeability and rejection. The following tested membranes were prepared by spin at 600rpm, 40s. We also tested the separation performance of membranes by four different PIP removal methods, as depicted in Fig. 10b. By vacuum filtration removal method, the membrane exhibits the highest permeability and lowest salt rejection, indicating that vacuum filtration has the most powerful to remove the PIP solution. By the other three removal methods, the membranes show similar permeability, while the membrane by spin removal method has the highest salt rejection. It demonstrates that the spin removal method could remove PIP solution more uniformly and fewer defects.

21

Fig. 10 Separation performance of GO6-PA0.5comopsite membrane:(a) under different spin speed and (b) different removal methods.

3.4 Performance of membrane separation The pure water permeability (PWP) performance of GO-PA composite membranes was evaluated at 4 bar. The tendency of PWP could be clearly observed that falling from 51.59 LMH/bar to 26.63 LMH/bar (Fig. 11). With 3 mg/m2 GO loading of GO3-PA0.5 membrane, the PWP decreased by over 20.1% that of the GO0-PA0.5 membrane, indicating the profound effect of GO on PWP. On account of the thickness of GO6-PA2 active layer being thicker than that of GO6-PA0.5, the PWP of GO6-PA2 further declined from 35.14 LMH/bar to 26.63 LMH/bar. Since the PA active layer formed by the IP process is only 20-35 nm, this allows water to penetrate the membrane with a short penetration path. At the same time, the higher roughness of PA top layer facilitates a larger surface area for water transport. Overall, the high PWP of GO-PA membrane is due to the high roughness surface and the ultra-thin selectivity layer.

22

Fig. 11 Pure water permeability of GO-PA composite membranes.

Four salt solutions of Na2SO4, MgSO4, MgCl2 and NaCl were used as typical divalent and monovalent salts to be filtrated for testing the separation performance of membranes. As Fig. 12 presented, the salt rejection of all prepared membranes is consistent with the trend of Na2SO4˚MgSO4˚MgCl2˚NaCl. The high performance of membranes is achieved by the increase of GO loading and PIP concentration. With a trace GO loading of 3 mg/m2, GO3-PA0.5 has the high divalent salt rejection over 93.56% for Na2SO4 and 75.20% for MgSO4. Compared with GO0-PA0.5, rejection of Na2SO4 by GO3-PA0.5 increases 23.20% and the permeability declines from 39.80 LMH/bar to 30.30 LMH/bar, indicating that even trace GO loading also has a great effect on the salt rejection and permeability. The GO6-PA2 membrane exhibits the highest salt rejection, which is 98.51% of Na2SO4 and 95.88% of MgSO4. At the same time, GO6-PA2 still keeps high permeation permeability of 20.93 and 24.68 LMH/bar, respectively. The main separation mechanisms of nanofiltration membrane are the combination effects of sieving exclusion, dissolution diffusion and charge repulsion[5]. The 23

MWCO of GO-PA composite membranes ranges from 0.58 nm to 0.77 nm (Table S1), which is much larger than the hydrated radius of SO42- (0.30 nm)[51]. So, the sieving exclusion may not play a major role for salt rejection. The intensive negative charges of membranes make a larger contribution to salt rejection. According to the C1s XPS analysis results (Fig. S10, Table S2), the membrane surface is rich in carboxyl, hydroxyl, epoxy and other negatively charged oxygen functional groups. Based on the Donnan effects, these groups could provide a strong repulsion to co-ions. In order to maintain the electrical neutrality on both sides of the membrane, it will prevent the passage of positive ions at the same time. The strong electrostatic repulsive force leads to outstanding rejection of divalent salt. However, the prepared membranes have a poor retention rate against monovalent, which is less than 15% for NaCl. That maybe caused by the relatively low charge of NaCl and the MWCO of GO-PA composite membranes being much larger than the hydrated radius of NaCl. The separation factor of Na2SO4/NaCl ranges from 15 to 56, demonstrating the excellent selectivity for divalent and monovalent ions. The high ion selectivity of membranes has potential applications in bivalent/monovalent ion separation of brines.

24

Fig. 12 Separation performance of GO-PA composite membranes:(a) Na2SO4, (b) MgSO4, (c) MgCl2, (d) NaCl.

To approach the eơects of GO on the permeability properties of GO-PA composite membranes, full range of GO loading (0-24 mg/m2) was further evaluated by testing their rejection against single salt solution at a PIP concentration of 0.5 v/w‰. The relationship between the salts rejection performance and the GO loading was shown in Fig. 13. For all prepared membranes, GO loading significantly improves the performance of membranes. It's found that when the GO loading reaches to 24 mg/m2, the rejection of Na2SO4 can be as high as 98.98% and the permeability declines to 13.24 LMH/bar. The rejection of MgSO4 is up to 88.26% and permeability is 16.77 LMH/bar. With the increase of GO loading, the permeability decreases rapidly and the rejection increases, the cause of which can be simply explained as an increase of mass transfer resistance. Combined with the previous analysis, the substrate could be 25

completely covered by the high GO loading, which is inevitable to greatly increase the water penetration resistance. Meanwhile, the GO can maintain more PIP and limit the IP reaction to the surface to form a defect-free selective layer, that is beneficial to increase the salt rejection.

FIg. 13 Salts rejection of the GO-PA0.5 composite membranes :(a) Na2SO4, (b) MgSO4, (c) MgCl2, (d) NaCl.

The divalent salts separation performance of our membranes has been compared with literature reported GO-based membranes (Table 2). Our membranes can operate under low pressure for achieving the superior water permeability and salts rejection.

26

Table 2 Comparison of divalent salts rejection and water permeability abilities of different GO-based nanofiltration membranes.

membrane material

GO

solution

rejection

Pure water

Salt solution

Applied

permeability

permeability

pressure

(LMH/bar)

(LMH/bar)

(bar)

4.08

15

[52]

4.45

8

[14]

6.06

10

[53]

12.23

4

[54]

12.4

5

[55]

14.3

6

[56]

15.63

4

[57]

Na2SO4

95.6%

Na2SO4

95.8%

MgSO4

97.7%

GO/TiO2/polyamide

Na2SO4

93.57%

GO/PVA/ glutaraldehyde

Na2SO4

96.83%

MgSO4

94.18%

Na2SO4

98.2%

MgSO4

97.8%

Na2SO4

98.0%

Na2SO4

96.56%

MgSO4

90.5%

GO/polyamide

Na2SO4

93.56%

35.14

30.30

GO/polyamide

Na2SO4

98.50%

26.63

20.93

GO/polyamide

MgSO4

95.88%

26.63

24.68

GO/polyamide

GO/PEI GO/polyamide GO/polyamide

Ref.

This 4 work

3.5 The stability of membranes Many studies have pointed out that GO membrane would swell and disintegrate in water, leading to instability in water[31, 58]. In addition, layer-stacked GO nanosheets have poor interaction with the substrate, which would be split off by the lateral forces during cross-flow filtration. In order to further investigate the stability of GO-PA membrane in application, we employed a long-time filtration, ultrasonic 27

treatment and organic solvents immersion for GO3-PA0.5 membrane. It should be pointed out that the high roughness surface of membrane could provide more efficient area for filtration to enhance the permeation, but at the same time it has a negative impact on the anti-pollution performance of the membrane. As shown in Fig. 14, a low permeability decline ratio of 11.5% after 300 min of filtration at 4 bar, whereas high rejection of Na2SO4 is maintained at above 94.0% throughout the filtration. These conditions may be due to compression of substrate or membrane fouling.

Fig. 14 Long time filtration of GO3-PA0.5 composite membrane.

The membrane mechanical stability in turbulent situation was estimated by ultrasonic treatment. The photo images of the ultrasound treated membrane were shown in Fig. 15a. No visible damages could be observed after 2 h ultrasonication. Due to the flexibility of GO nanosheets, the order of layered-stacking GO nanosheets are destroyed and deposited messily. GO nanosheets are trapped in the valleys or into the pores, making GO interlayer attaching to the substrate tightly. As a result, the mechanical stability of membrane was greatly improved. To evaluate the solvent stability, the GO6-PA0.5 composite membrane was 28

immersed in various organic solvents to detect the weight loss ratio. Five conventional solvents were chosen to test the solvent stability of membrane (Fig. 15b). After 60 h immersion, the weight of GO3-PA0.5 composite membrane almost has not changed. No weight loss of membrane is detected in EtOH, MeOH and IPA. The weight loss in DMF and ACN is only 0.4% and 0.6%, respectively. The solvent resistance of typical TFC membranes is often determined by the support layer, as the PA layer is highly cross-linking[39]. This high solvent stability may be attributed to the chemical stability of membrane components. The nylon substrate and PA layer of our membranes have high stability in most organic solvents.

Fig. 15 (a) Photo images of GO3-PA0.5 composite membrane after ultrasonication and (b) mass loss ratio of GO3-PA0.5 composite membrane immersed in different solvents for 60 h.

4. Conclusions In summary, we developed a facile yet effective strategy for IP process to achieve high performance nanofiltration membranes by the assistance of spin. Centrifugal force was employed to solve the challenges of uniform removal low concentration PIP solution. Defect-free and ultrathin PA layer was formed through the specific design of spin process. The minimal amount of GO was used as the modification layer of the 29

substrate to maintain more PIP solution and control the IP reaction on the substrate surface. The pure water permeation of GO3-PA0.5 is as high as 35.14 LMH/bar, with a remarkable Na2SO4 rejection of 93.56%, outperforming the separation performance of commercial nanofiltration membranes. Meanwhile, the prepared membranes show a high selectivity between divalent and monovalent ions. What's more, our membranes also exhibit outstanding stability that could resist ultrasound for 2 h without visible damages. After immersed in solvents 60 h, our membranes have almost no weight loss, indicating the good solvents resistance. This research provides us a novel approach to fabricate high-performance nanofiltration membranes.

Acknowledgements We are grateful for the financial support of this research from the National Natural Science Foundation of China (51779065 and 51579057), and State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (2019DX11).

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34

Graphical Abstract

35

Highlights: (1) A thin and defect-free PA layer is formed by spin-assistance interfacial polymerization. (2) The PWP of the membrane could be up to 35.14 L/m2/h/bar with remarkable salts rejection. (3) The membranes exhibit excellent stability by trapping the GO nanosheets.

36

Author Contribution Statement

Xu Kang : Conceptualization, Methodology, Writing- Original draft preparation. Xin Liu: Data curation, Software. Jinghua Liu: Visualization, Investigation. Yan Wen: Validation. Jinyao Qi: Project administration, Supervision. Xin Li: Writing- Reviewing and Editing.

37

Declaration of interests

‫ڦ‬The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

38