Improved separation and antifouling properties of thin-film composite nanofiltration membrane by the incorporation of cGO

Accepted Manuscript Title: Improved separation and antifouling properties of thin-film composite nanofiltration membrane by the incorporation of cGO Authors: Hongbin Li, Wenying Shi, Qiyun Du, Rong Zhou, Haixia Zhang, Xiaohong Qin PII: DOI: Reference:

S0169-4332(17)30577-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.204 APSUSC 35313

To appear in:

APSUSC

Received date: Revised date: Accepted date:

20-12-2016 15-2-2017 22-2-2017

Please cite this article as: Hongbin Li, Wenying Shi, Qiyun Du, Rong Zhou, Haixia Zhang, Xiaohong Qin, Improved separation and antifouling properties of thin-film composite nanofiltration membrane by the incorporation of cGO, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.204 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Revised manuscript submission to Applied surface science February 15, 2017

Improved separation and antifouling properties of thin-film composite nanofiltration membrane by the incorporation of cGO Hongbin Li a,*, Wenying Shi a, Qiyun Du b, Rong Zhou a, Haixia Zhang a, Xiaohong Qin a,c a

School of Textiles Engineering, Henan Engineering Laboratory of New Textiles

Development, Henan University of Engineering, Zhengzhou, 450007, P. R. China b

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin

Polytechnic University, Tianjin 300387, P.R. China c

School of Textiles Science, Donghua University, Shanghai 201620, PR China

Correspondence to: Hongbin Li 1 Xianghe Road, Zhengzhou, Henan Province 450007, P.R. China Tel/Fax: +86-371-67718851 E-mail: [email protected]

Graphical abstract

(a)

(b)

cGO nanosheet Aqueous solution

Drying

Drying (c)

Silicone plate PSF/NWF substrate

(d) Organic solution

Post treatment

Glass plate NF composite membrane

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Highlights 1. NF membranes were modified through the carboxylated graphene oxide (cGO). 2. The overall performance of the cGO-incorporated NF membrane was improved. 3. The growth model of cGO-incorporated polyamide thin-film was proposed. 4. Modified NF membrane showed an excellent selectivity of MgSO4 and NaCl.

Abstract: Poly(piperazine amide) composite nanofiltration (NF) membranes were modified through the incorporation of carboxylated graphene oxide (cGO) in the polyamide layer during the interfacial polymerization (IP) process on the polysulfone (PSF)/nonwoven fabric (NWF) ultrafiltration (UF) substrate membrane surface. The composition and morphology of the prepared NF membrane surface were determined by means of ATR-FTIR, SEM-EDX and AFM. The effects of cGO contents on membrane hydrophilicity, separation performance and antifouling properties were investigated through Water Contact Angle (WCA) analysis, the permeance and three-cycle fouling measurements. The growth model of cGO-incorporated polyamide thin-film was proposed. Compared to the original NF membranes, the surface hydrophilicity, water permeability, salt rejection and antifouling properties of the cGO-incorporated NF membrane had all improved. When cGO content was 100 ppm, the MgSO4 rejection of composite NF membrane reached a maximum value of 99.2 % meanwhile membrane obtained an obvious enhanced water flux (81.6 L.m-2.h-1, at 0.7 MPa) which was nearly three times compared to the virginal NF membrane. The cGO-incorporated NF membrane showed an excellent selectivity of MgSO4 and NaCl with the rejection ratio of MgSO4/ NaCl of approximately 8.0.

Keywords: composite NF membrane; carboxylated graphene oxide; hydrophilicity; antifouling; separation performance

1. Introduction

Recently, separation process based on nanofiltration (NF) technology has been extensively developed. As a type of pressure-driven membrane processes, the separation characteristics of NF membrane are between reverse osmosis and ultrafiltration (UF) processes with the molecular weight cutoff (MWCO) ranging from 200 to 1000 Da and pore size of 1-3 nm [1, 2]. Most of modern nanofiltration (NF) membranes are thin-film composite (TFC) membranes. TFC membranes comprise an ultrathin selective layer on the surface of porous flat sheet substrate

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membranes through the surface coating and interfacial polymerization (IP) process of diamine and acyl chlorides [3, 4]. The selective layer and the substrate can be designed and optimized separately. The selective layer of composite NF membrane plays the primary selective separation function. Its structure is looser than that of reverse osmosis (RO) membrane but denser than that of ultrafiltration (UF) membrane [5]. Compared with reverse osmosis (RO) membrane, NF membrane possesses lower operating pressure and pretreatment requirements as well as higher permeability and lower investment investment, operation and maintenance costs. It has been applied in many fields such as water purification, seawater desalination, food, medicine, textile dyeing and other fields [6-8]. However, membrane fouling that is mainly derived from the solute, colloidal or organic matters deteriorates membrane separation performance during the long-time operation process and greatly restricts the application of NF technology [9]. Generally, the charge density [10], surface morphology [11] and hydrophilicity [5] of NF membranes are the major factors affecting the separation behavior of NF membrane through the changes of physical or chemical interactions with membranes. In order to prolong membrane life and enlarge its application fields, the over-all performance improvements of composite NF membrane including rejection, permeability, antifouling properties and etc have attracted more and more attention [12, 10]. Currently, there are several methods to improve the performance of composite NF membranes such as developing new monomers [5, 13], surface coating [14], surface grafting [15] and surface blending [16]. Among these methods, surface blending is a simple and effective approach to prepare high-performance composite NF membrane. A few researchers have modified the composite NF membrane through the surface blending of nanoparticles like inorganic, metallic and organic nanoparticles. Li [10] reported a novel thin film nanocomposite nanofiltration (TFN NF) membrane which was fabricated through the introduction of silica nanospheres in the interfacial polymerization process of trimesoyl chloride (TMC) and piperazine (PIP). The incorporation of silica nanospheres in polyamide layer can effectively enhance membrane salt rejection and water flux. The author attributed separation performance improvement to the optimizations of microstructures and surface features of the active barrier layer of TFN NF membrane, caused by the addition of silica nanospheres. Ghaemi [17] embedded cross-linkable acrylate-functionalized alumoxane nanoparticles into the thin-film during polymerization process to improve the characteristics of hydrophobic polypyrrole (PPy) thin-film layer. Xu [18] reported a novel method of highly-efficient one-step co-deposition to prepared positively charged nanofiltration (NF) membrane and employed this method to fabricated a novel nanocomposite organic solvent nanofiltration (OSN) membrane with a coating layer of mussel-inspired catechol and octaammonium polyhedral oligomeric silsesquioxane (POSS-NH3+Cl-) onto polyimide (PI) UF support membrane [19]. The optimized nanocomposite membrane exhibited high ethanol (EtOH) permeance and rejection to Rose Bengal (RB). The novel membrane also exhibited remarkable separation

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performance for dyes removal from a wide range of solvents. Metallic nanoparticles like silver nanoparticles (AGNPS) [17], zinc oxide (ZnO) [20], titanium dioxide (TiO2) [21] and Aluminum hydroxide Al(OH)3 nanoparticles [22], etc have been widely used in the modification of NF composite membranes. Graphene oxide (GO) nanosheet having excellent mechanical strength, flexibility and hydrophilicity is believed to be a promising candidate to develop high-performance membranes [23]. GO containing functional groups, including epoxy, hydroxyl and carboxyl groups, and few vacancy defects membranes exhibit the potential in improving the water permeability. Generally, water molecules pass through the pores in the active layer of the traditional NF membranes. However, the mass transfer channels of water molecules are formed through the nanopores (defects) on GO [24], the interconnected nanochannels between adjacent GO nanosheets [25, 26] and the nanocorridors between GO and polymer matrix [27]. Besides, due to its smooth surface of the GO nanosheets, water molecules can diffuse rapidly through the transfer channels and thus high water flux can be achieved [28]. GO based membranes can also improve the rejection [29], antifouling [30] and antibacterial properties [31] as well as the chlorine resistance capacities [32]. The enhancement of these properties of NF membrane can improve its comprehensive performance, expand its application areas and enhance the market competitiveness. Many studies have reported the GO composite NF membranes with enhanced separation performance [26, 33, 34], antibacterial activity [23], antifouling properties [31], heavy metal removal rate [35], etc. The GO nanosheets were incorporated via the layer-by-layer assembly [35, 36] on substrates or blending into reactive layer [29, 34]. In order to obtain composite nanofiltration membrane with a uniform distribution of GO nanosheets, most of the studies used the ultrasonic vibration to enhance the dispersion of GO in the active layer of composite NF membrane. Compared with the GO nanosheets, carboxylated graphene oxide (cGO) introduces more hydrophilic carboxyl groups on the nanosheet matrix. Its incorporation in NF membrane would be more uniform and can further improve membrane surface hydrophilicity, permeation water flux and antifouling properties. In this study, cGO nanosheets was synthesized and introduced in the poly(piperazine amide) layer during the interfacial polymerization (IP) process. The variations of surface morphology, separation performance, hydrophilicity, and antifouling properties of the GO-incorporated composite NF membrane were well investigated. ATR-FTIR, SEM-EDX, AFM and surface zeta potential were used to characterize surface chemical feature and morphology of the resultant membranes. Membrane surface hydrophilicity and antifouling properties were examined via the water contact angle (WCA) measurement. The three-cycle fouling test was carried out to evaluate membrane antifouling properties.

2. Experimental

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2.1. Materials Polysulfone (PSF) (Solvay, Udel-3500) was dried in a vacuum oven at 80 oC for 12 h before use. Dimethylacetamide (DMAc), polyethylene glycol (PEG, MW=400), sodium chloride (NaCl) and n-hexane were all analytical reagents purchased from Tianjin Kemiou reagent Co., Ltd. (China). Piperazine (PIP) was purchased from Shanghai TIANLIAN Fine Chemical Co. (China). Trimesoyl chloride (TMC) was supplied by Beijing ODYSSEY Chemicals Co. (China) without further purification. Bovine serum albumin (BSA) and graphene oxide (GO) nanosheets were supplied by Aladdin Industrial Corporation (Shanghai, China). PET spunbonded NWF with the grammage of 20 g.m-2 was supplied from the Shanghai Tianlue Advanced Textile CO. LTD.

2.2. Preparation of carboxylated graphene oxide (cGO)

GO were activated into carboxylated graphene oxide (cGO) using chloroacetic acid under strongly basic conditions. The aim was to activate the epoxide and ester groups and to convert hydroxyl groups to carboxylic acid (COOH) moieties in GO nanosheets [37]. The detailed preparation process was described as follows. Certain amout GO nanosheets (0.2 wt%) were dispersed in aqueous solution. After ultrasonic vibration for 1 h, a transparent and homogeneous GO suspension was obtained. Then, sodium hydroxide (10 wt%) and chloroacetic acid (10 wt%) were added into the GO suspension. After stirring for 1 h and ultrasonic vibration for 1 h, the solution was put into the centrifuge (7000 r /min). Afterwards, the precipitation in the lower layer of the centrifuge tube was washed with pure water several times until a neutral washing solution was obtained. After further centrifugation, the black flocculent precipitate was put into the freeze dryer to get carboxylated graphene oxide (cGO) powders.

2.3. Preparation of PSF support membrane

PSF support membrane was prepared through the immersion precipitation phase inversion method. Certain amount of PSF granules (18 wt%) and compound additives (PEG-400, 6 wt%; LiCl, 1 wt %) were successively added and fully dissolved in DMAc (75 w%) at 60 oC for about 12 h until a homogeneous solution was obtained. Then, the solution was degassed in vacuum at 30 oC for another 12 h. PET NWF was immersed in anhydrous ethanol for at least 24 h to remove the chemical additives. Afterwards, PET NWF was dried at room temperature and pasted onto a clean glass plate. The degassed solution was cast on NWF surface through the extrusion from the gap (100 μm) between the casting knife and NWF surface. The liquid membrane was immediately immersed into coagulation bath (pure water). Finally, the nascent PSF/NWF support was washed several times using pure water

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and preserved in pure water before use.

2.4. Preparation of composite NF membrane

Composite nanofiltration membranes were prepared through the immersion coating method. Poly(piperazine amide) functional layer was formed on the surface of PSF support membrane via the interfacial polymerization of 2 wt% PIP aqueous solution as the aqueous phase and 0.05 wt% TMC in n-heptane as the organic solution. The preparation procedure was similar with our previous report [22]. Certain amount of cGO nanosheets was added into PIP aqueous solution followed by ultrasonic dispersion for 1 h before use. PSF substrate membrane was pre-dried at 40 oC and 30 % of relative humidity for 6 h. The resulted PIP aqueous solution with different cGO concentration was poured onto the top surface of PSF substrates for 15 min. The wet membrane was taken out and excess aqueous solution was drained off with filter paper followed by the drying at 40 oC and 30 % of relative humidity for 0.5 h. Then, TMC organic solution was immediately poured onto PSF membrane surface for 30 s and the poly(piperazine amide) layer was instantaneously formed on the PSF support membrane surface. Afterwards, the nascent functional layer was dried at room temperature for 30 min and carried out the post-treatment at 50 oC for another 15 min. Finally, the prepared TFC nanofiltration membranes were rinsed using pure water and preserved in pure water before use. PIP concentration in aqueous solution was fixed at 2 wt% with a different cGO content raging from 0, 25, 50, 100 to 200 ppm. The resulted TFC nanofiltration membranes were designated as 0#, 1#, 2#, 3# and 4#, respectively.

2.5. Characterization of PSF support membrane

The pure water flux of five membrane modules was tested in a cross-flow filtration cell with an

effective membrane area of 33.2 cm2 at 25 oC. Membranes were initially pre-compacted at 0.20 MPa to get a steady flux. Then, water flux was measured at 0.10 MPa and calculated by the following equation.

F

V At

(1)

F-water permeate flux (L.m-2.h-1); V-water permeation volume (L); A-the effective membrane area (m2); t-filtration time (h). The PEG-20, 000 concentrations in the feed and the permeate solutions were measured by a UV-vis spectrophotometer (TU-1901, Purkinje General Instrument Co. Ltd., China) at a wavelength of 510 nm. The PEG rejection (R) is calculated as Eq. (3):

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R  1

Cp Cf

(2)

where Cp and Cf were PEG concentrations in the permeate and feed solutions, respectively.

Membrane porosity (ε) was evaluated through the mass loss of wet membrane after drying. The ratio of pore volume to membrane geometrical volume was defined as Eq. (3):



(Ww  Wd ) Al 

(3)

where Ww and Wd were the weight (g) of wet membrane and dry membrane, respectively. A, L, and ρ were the sample area (cm2), average thickness (cm) and pure water density at atmosphere temperature (g.cm-3).

2.6. Characterization of structure and morphology of composite NF membranes

Chemical structure of different membranes was characterized through the Attenuated Total Reflection-Fourier transform infrared spectra (ATR-FTIR) on a Vector 22 FTIR spectrometer (BRUKER Corporation, Germany) with Zinc Selenide (ZnSe) as an internal reflection element at an incident angle of 45 o. ATR-FTIR spectra of the membranes were recorded in a wave number of 600-4000 cm-1. Membrane samples were rinsed with pure water and dried at 25 oC in a vacuum oven overnight before the FTIR analysis. Membrane surface morphology was characterized through scanning electron microscopy (SEM, FEI Quanta 250, USA). The crystal structure variations of functional layer after the cGO incorporation were examined through the X-ray diffraction (XRD, Rigaku, D/MAX-2500, Japan). The scanning range (2θ) from 10 º to 45 º was collected with a scanning speed of 2 º/min along the equatorial direction and a step width of 2θ=0.02 º. In addition, membrane surface chemical compositions were probed by Energy Dispersive X-Ray Spectroscopy (EDX) instruments during the SEM observations. Samples were freeze-dried for 24 h and sputtered with gold before SEM observation. Surface morphology of different membrane samples was also observed by Atomic Force Microscopy (AFM, Agilent AFM 5500, USA). The observation was operated in a tapping mode at room temperature in air. Roughness was obtained in terms of the average roughness (Ra), root-mean-square (Rrms) and the maximum of peak-to-valley distance (Rmax). Membrane surface hydrophilicity was evaluated using static contact angle (WCA) formed between the membrane surface and water. It was performed on a Kruss Instrument (CM3250-DS3210, Germany) at ambient temperature. One water droplet (1μL) was dropped on membrane surface with an automatic piston syringe and captured by a camera. At least five locations were chosen to measure

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the contact angle to minimize the experimental errors and their average value was collected. The surface zeta potential of different NF membranes were measured using a streaming potential measurements (SurPASS, Anton Paar) with a plated sample cell in 0.001 mol/L of KCl aqueous solution at pH=6.5 and temperature of 25±0.5 oC. The samples were conditioned in the KCl solution for at least 24h before use.

2.7. Characterization of separation performance of composite NF membranes

The permeation tests of different NF membranes were carried out in a cross-flow filtration set-up at 25 oC under 0.7 MPa. The feed was 2 g/L MgSO4 or NaCl aqueous solution. Membrane was pre-compacted at 0.8 MPa until a stable permeate volume was obtained. Water flux (F) and salt rejection (R) were also calculated through Eqs. (1) and (2). Cp and Cf here were the salt concentrations of permeate and the feed, respectively. The salt concentrations were examined via a conductivity meter (DDS-11A, Shanghai Leici Instrument Works, China). If there was no special instruction, the rejection was all referred to that of MgSO4.

2.8. Membrane antifouling experiments

The three-cycle antifouling experiments were carried out through a cross-flow filtration of BSA solution under 0.7 MPa at 25 oC. Water flux data were collected at an interval of 5 min during the whole filtration process. A filtration cycle was described as follows and the other two cycles were repeated as this. First, 10 readings of pure water flux (J0) at least were measured. Then, the feed was replaced by 1 g.L-1 BSA phosphate buffer solution (PBS, 0.01mol.L-1, pH=7.4) and water flux of BSA solution was measured. Subsequently, the fouled membranes were flushed through pure water for 30 min. Afterwards, pure water flux was collected again. The normalized flux was used to characterize the flux variation which was equal to the ratio of the water flux during the whole filtration process (Ft) and pure water flux at the beginning (F0) as listed in Eq. (4). The flux recovery ratio (RFR) was defined as the ratio of the pure water flux after flushing (Fre) and pure water flux at the beginning (F0) which was described as Eq. (5).

Fn 

Ft  100% F0

Rre 

Fre 100% F0

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(4)

(5)

3. Results and discussion

3.1. Characterizations of GO/cGO nanosheets

(Insert Fig. 1 here)

The ATR-FTIR spectra were obtained in order to characterize the oxygen-containing functional groups in GO nanosheets after the carboxylation. Fig. 1 showed the ATR-FTIR spectra of GO and cGO. It could be seen that the FTIR spectra of GO exhibited a broad band ranging from 3600 to 3250 cm-1 which was assigned to the -OH group. Four characteristic peaks of GO emerged at 1711, 1053, 1349 and 1572 cm-1 which were attributed to the stretching vibration for C=O from the carboxylic groups, the stretching vibrations of C-O from the ether group, the in-plane -OH bending mode and the unoxidized C=C aromatic ring stretching peak, respectively [38, 39]. In addition, the peaks at 1412, 1276 and 1070 cm-1 were corresponded to O-H deformation, C-O epoxy stretching, and C-O alkoxy stretching, respectively. For carboxylated GO, the C=O vibration was significantly more intense than the C=C vibrations. Besides, the peak width of -OH group ranging from 3600 to 3250 cm-1 became wider and intenser in the FTIR of cGO than that of GO. These results suggested that further oxidation appeared on the surface of GO.

(Insert Fig. 2 here) The SEM images of GO and cGO nanosheets were presented in Fig. 2. It could be seen from Fig. 2 (a) and (a') that the virginal GO nanosheets had a jeep-shaped surface which was due to the exfoliation of one sheet. Compared to GO, the general structure of cGO was changed. It was found from Fig. 2 (b) and (b') that the jeep-shaped structure disappeared in cGO nanosheets and was transformed into a wrinkled appearance of one sheet. This suggested the carboxylation was well carried out so that the GO nanosheets could be further exfoliated [38]. Additionally, the wrinkled appearance of one sheet can promise the appropriate dispersion of cGO nanosheets in the active layer of NF membrane [30].

3.2. The properties of PSF support membrane

Generally, support membranes with an appropriate pore size and high permeation flux would be in favor of the preparation of high-performance composite membrane [40]. The properties of PSF support membrane including the PEG rejection, pure water flux and porosity were listed in Table 1. It could be seen that the resultant PSF support membrane had a high rejection of PEG-20, 000. In

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addition, membrane exhibited a relatively high pure water flux and porosity which was beneficial to obtain high permeation water flux for composite NF membrane.

(Insert Table 1 here)

3.3. Chemical compositions of membrane surface

(Insert Fig. 3 here)

The ATR-FTIR spectra of different NF membranes were shown in Fig. 3. It could be seen that compared with PSF support membrane, new characteristic peaks emerged at around 1625, 1377, 3254

and 3435 cm-1 for all NF membranes which were ascribed to the stretching vibration of C=O in amide group, the mixture of C-N stretching and in plane NH deformation amide III, OH stretching and N-H stretching vibration [41-43]. These results confirmed the successful formation of poly(piperazine amide) layer during the IP process on PSF support membrane surface. By comparing the spectra of cGO NF membranes (1-4#) and the virginal NF membrane (0#), there was no obvious difference among them. This was because the amount of cGO is so low that it is difficult to detect the variation of light intensity via the FTIR detection.

3.4. Surface morphology of different membranes

(Insert Fig. 4 here)

Fig. 4 showed the surface SEM images and EDS curves of the prepared membranes. It could be observed from the EDS curves that 7.04 At% S element emergend in the EDS curve of PSF support membrane (Fig. 4 (PSF')) which was assigned to the O=S=O groups in PSF molecules. It should be pointed here that "At%" is an atomic fraction (at is an abbreviation for atom), used to describe the atomic content (percentage) of various elements in a substance. Followed by the interfacial polymerization, nodular structure with a few dish-like bulges could be observed on 0# NF membrane surface as shown in Fig. 4(0#) which was the typical characteristic of a polyamide layer surface obtained from the polycondensation of TMC and PIP [12, 43]. However, after embedding cGO nanosheets, the dish-like bulges nearly disappeard and the nodules became small which could be seen from Fig. 4(1#). With the further increasing of cGO concentration, the nodule number was gradually decreased and its size was slowly diminished as seen from Fig. 4(2, 3 and 4#). The EDS data of 0# and 3# composite NF membranes suggested that a few oxygen-containing groups probably -OH

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connected to the cGO nanosheets and the polyamide molecules inferred from FTIR analysis emerged on membrane surface. This was beneficial to the enhancement of membrane water flux. It could also be seen from SEM images that the bare NF membrane surface was roughness. After the introduction of cGO nanosheets, the NF membrane gradually became relatively smooth. These results could also be confirmed from the roughness data of different membranes as listed in Table 2 in the AFM observations. It could be well seen from Table 2 that the surface roughness of PSF support membrane and the bare NF membrane were the smallest and the biggest of all membranes which indicated that the surface of these two membranes was the smoothest and the coarsest, respectively. With the incorporation and increasing of cGO nanosheets, the membrane surface roughness gradually decreased which suggested the membrane surface was gradually became relatively smooth. The decline of surface roughness of different membranes could be due to the hydrogen bonding between the carboxyl groups and the polyamide layer. The similar results were also obtained from the previous studies which modified the polyamide composite membranes through the embedding of hydrophilic additives [44-46].

(Insert Fig. 5 here) To better investigate the surface morphology of the prepared membranes, the membrane surface was further characterized through the AFM analysis. Fig. 5 showed the two and three-dimensional AFM scans of the bare and the modified NF membranes with a scan area of 5 μm×5 μm. Table 2 summarizes the roughness parameters of the prepared membranes, including average roughness (Ra), root-mean-square (Rrms) and the maximum of peak-to-valley distance (Rmax) (i. e. the distance between the brightest point and the darkest point). It could be seen that all membrane surface showed the typical ridge-valley morphology. PSF support membrane surface was relatively flat with small dimension of valleys and peaks (Fig. 5(PSF and PSF')). With the completion of the interfacial polymerization reaction on PSF substrates, more bright and big peaks (Fig. 5(0#)) emerged on the original NF membrane surface which indicated the surface roughness would be obviously increased as listed in Table 2. With the incorporation of cGO nansheets, the nodule size gradually diminished as shown in Fig. 5(1-4#). The variations of membrane surface morphology could be clearly seen from the three-dimensional AFM surface images in Fig. 5. The peak intensity and its height in Fig. 5(0#') gradually weakened and decreased after the introduction of cGO nansheets as shown in Fig. 5(1-4#'). Table 2 further confirmed that the surface roughness was greatly affected by embedding cGO in the polyamide matrix. With the cGO addition, the surface roughness gradually decreased. As mentioned above and in the previous reports, the roughness decline could be concerned in the hydrogen bonding between the carboxyl groups in cGO and the polyamide layer [44, 47]. Surface roughness would strongly affect membrane fouling characteristics by balancing the

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adsorption/desorption of foulants on membrane surface. A smooth membrane surface favors for the antifouling potential because the foulant trapping and adhesion on a flat surface are more difficult [48, 49]. This signified that the smoother surface of cGO NF membranes would contribute to the improvement of antifouling properties which will be further discussed in the next text.

(Insert Table 2 here) The formation and morphological evolution of the cGO-incorporated thin-film layer during the IP process can be illustrated according to a proposed growth model as shown in Fig. 6. There are three main stages during the IP process. According to the previous studies [50, 51], during the ultrasonication of the cGO/PIP aqueous solution, due to the π-π interactions and electrostatic attractions, the amine groups (-NH) of PIP may interact with the carboxyl groups (-COOH) of cGO to form new amide bonds and hydrogen bonds could emerge between the -NH and -CO connected with –COOH of cGO. The TMC contacted and instantly reacted with PIP molecules that bonded on the PSF surface (Fig. 6(a)) where acyl chloride groups (-COCl) of TMC would not only react with PIP, but also with cGO to form anhydride (-COOCO-) and ester groups (-OCO-). The cGO which interacted with PIP and TMC could bonded with the poly(piperazine amide) via the ester groups (-OCO-) and the hydrogen bonds as illustrated in Fig. 7.

(Insert Fig. 6 here) Instantaneously, the thin-film layer was preliminarily formed on PSF substrate surface. Meanwhile, PIP molecules that saturated in the support membrane micropores continually diffused to the interface adjacent to the organic phase and reacted instantaneously with TMC (Fig. 6(b)). This induced the formation of a grain-like morphology with a high roughness. With the proceeding of the IP process, PIP molecules were inclined to diffuse and permeate from the lateral of the “grains” upon micropores. Consequently, the “grain” accumulated into a belt-like structure (Fig. 6(c)). The surface morphology as shown in SEM (Fig. 4(0#)) and AFM (Fig 5(0#)) exhibited the typical grain-like structure with visible “grain” accumulation which was consistent with the model in Fig. 6.

(Insert Fig. 7 here)

As reported in previous literature [52], the amine compounds such as pyrrole can be absorbed onto the GO surface due to the interaction and electrostatic attractions between GO and the pyrrole molecules. Compared with GO, cGO nanosheets having more -COOH groups can absorb more amine compounds like PIP used in this study. Therefore, the IP process could occur around cGO nanosheets.

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From the previous literature, when substrate membrane is taken out from the aqueous solution, GO nanosheets tend to orient along the membrane surface horizontally because of the Langmuir–Blodgett film deposition [32, 53]. cGO is a micron scale particles, even if it has good hydrophilicity, the micron dimension will hinder the PIP molecules in the micropores to reach the interface of organic phase. Therefore, the diffusion rate of PIP molecules from aqueous phase to the interface between the organic solution and the nascent layer will slow down. This restrained the formation of large grains and the so-called “belts”. As a result, with the increasing of cGO nanoflakes more and more smaller particles gradually occurred on the cGO-incorporated polyamide layer as shown in in SEM (Fig. 4(1-4#)) and AFM (Fig. 5(1-4#)). Previous references have interpreted this and obtained the similar morphology variation with this study. Shen [51] incorporated GO into the polyamide (PA) selective layer via the interfacial polymerization of m-phenylenediamine (MPD) and 1, 3, 5- trimesoyl chloride (TMC). It could be found from the SEM photos and AFM images that with the increase of GO content in polyamide layer, composite membrane surface gradually turned smooth. The author attributed this to the hinderance from the horizontal-oriented GO nanosheets along membrane surface which would retard the diffusion of MPD into the organic phase, leading to a smoother surface. Chae [54] prepared the GO-incorporated polyamide layer on the GO-incorporated polysulfone (PSf) support membrane surface. The similar variation trends of membrane surface morphology with the change of GO content were obtained. The authors also attributed these phenomena to the fact that the horizontally oriented GO retarded the diffusion rate of MPD and caused the reduction in surface roughness.

3.5. Crystal structure analysis of the polyamide layer

(Insert Fig. 8 here)

XRD analysis was used to examine the effect of the incorporation of cGO flakes on the crystal structure of functional layer of NF membranes. Fig. 8 showed the X-ray diffractograms of GO/cGO and different membranes. The XRD curve of GO showed a sharp peak at 2θ=9.64 o which corresponded to the (001) reflection of GO [55]. There was no obvious peak at about 26° in the XRD patterns of GO and cGO which was ascribed to the characteristic peak of unoxidized graphite (002) [51, 56], suggesting that the GO and as-synthesized cGO was completely oxidized. No clear diffraction peak emerged in the curves of 0# which was due to the non-crystallization probability of poly(piperazine amide). An intensive characteristic peak occurred at 13.35 o in the curve of cGO nanosheets which was assigned to the further carboxylation of GO. Compared to the characteristic peak of GO, the increasing of 2θ in the XRD pattern of cGO indicated that the interlay space

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decreased according to Bragg's formula [38, 57]. This suggested the carboxylation further exfoliated the GO nanosheets. After the introduction of cGO flakes into polyamide layer, one obvious diffraction peak emerged at 2θ=16.77 o in the X-ray diffractogram of 1# NF membrane as shown in Fig. 8. The previous studies pointed that the appropriate sonication could decrease the size of the GO nanosheets and improve the GO hydrophilicity [58, 59]. Therefore, the GO after further carboxylation could also acquire dimension reduction and hydrophilicity improvement as that of GO nanosheets. Consequently, a small quantity of cGO nanosheets could homogeneously disperse in the polyamide matrix after the proper sonication. And this result reflected the shift of 2θ from 13.35 o to 16.77 o in the XRD diffractograms of cGO and 1# membrane, respectively. With the increasing of cGO content, this characteristic peak gradually declined from 16.77 o, 15.91 o, 15.22 o to 14.96 o which corresponded to 1#, 2#, 3# and 4# NF membranes, successively. This result simplied the gradual increasing of interlay space which was probably derived from the aggregation when the cGO nanosheet content gradually increased. Previous literatures pointed that the amphiphilicity of GO which had the same compositions was mainly size-dependent and the appropriate sonication could decrease the size of the GO nanosheets resulting in the improvement of GO hydrophilicity [58, 59]. However, the carboxylated GO prepared in this study had more hydrophilic groups (-COOH) than that of GO which facilitated the uniformly dispersion and stabilization of cGO in aqueous solution.

3.6. Hydrophilicity of membrane surface

(Insert Fig. 9 here)

Fig. 9 presented the surface water contact angle (WCA) of different NF membranes prepared in this work. It could be seen that the WCA value greatly decreased from 48.8 o of 0# NF membrane to 37.2 o of 1# NF membrane and with the increasing of cGO content the WCA value continuously decreased to 22.3 o of 4# NF membrane. All cGO nanocomposite NF membranes displayed lower WCA value in the range of 22-37°. The decline of WCA value suggested that the hydrophilicity of NF membrane surface was enhanced after the incorporation of cGO into polyamide layer. There are two major factors affecting the WCA value including the membrane surface roughness and intrinsic wettability of the material itself [60]. Membrane surface with higher surface roughness and more wettable groups would have a smaller WCA value. It could be obtained from the AFM and EDS observations that with the increasing of cGO nanosheet content the surface roughness of modified NF membrane gradually declined and more and more cGO nanosheets emerged on NF membrane surface. Since the roughness decline was not obvious (in the range of about tens of nm as listed in Table 2), the effect of surface roughness on the WCA value could be negligible compared with that of the surface

14

wettability improvement of the material itself. The hydrophilicity improvement of polyamide layer after the cGO incorporation determined the decline of WCA value and the increasing of membrane permeate water flux.

3.7. Zeta potential analysis

(Insert Fig. 10 here) The surface properties of the functional layer were mainly related to the negatively charged nature. Previous studies have well confirmed the enhanced negatively charged property of GO-embedded polyamide layer [61, 62]. It can be expected that the cGO-embedded polyamide layer would show more negative charges than that of GO-embedded polyamide layer. More COOH groups emerged on the surface of the cGO-embedded polyamide layer and the deprotonation of the carboxyl groups (-COOH→COO-) produced enhanced negative charge as shown in Fig. 10. It could be clearly seen from Fig. 10 that with the incorporation of cGO flakes, the zeta potential of membrane surface had an obvious decrease which was mainly derived from the introduction of more COOH groups in cGO flakes. Besides, the introduction of cGO flakes could hinder the diffusion of PIP molecules into the organic phase during the IP process. This resulted in the decline of cross-linking degree of poly(piperazine-amide) layer and the formation of a relatively loose and roughness network (as shown in AFM images) with some possible defects in the selective layer of membranes. Consequently, more carboxyl groups appeared on membrane surface and the dissociation of COOH groups favored the enhancement of the negatively charged density [44, 10].

3.8. Separation performance of different NF membranes

(Insert Fig. 11 here)

The separation performance of the prepared NF membranes were measured through the cross-flow permeation system using 2 g.L-1 MgSO4 and NaCl aqueous solution as the feed solution. Fig. 11(a) and (b) showed the variations of salt rejection and water flux of different NF membranes, respectively. It could be seen that with the incorporation of cGO nanosheets the MgSO4 rejection initially increased and then decreased when cGO content was higher. The NaCl rejection of cGO NF membranes continuously decreased. The MgSO4 water flux of the virginal NF membrane was 28.6 L.m-2.h-1. With the introduction of cGO nanosheets in polyamide layer, the water flux data had an obvious increase to 86.9 L.m-2.h-1 which was more than three times that of the virginal NF membrane. These results suggested that the incorporation of cGO could effectively improve the permeability and

15

rejection capability of the bare poly(piperazine-amide) NF membrane which were mainly through changing the microstructure and surface charge properties of the functional barrier layer. It is well known that the rejection performance of NF membrane is controlled by both the steric size exclusion and Donnan exclusion effects, which are the two major factors determining the rejection of a charged membrane to electrolyte [63, 64]. Two opposite effects compete along with the incorporation of cGO in polyamide layer: enhancing Donnan exclusion effects through increasing membrane surface charge and weakening size exclusion effects by enlarging membrane pore size. The former is favorable to increase the electrolyte rejection and the latter is prone to decrease the electrolyte rejection. As analyzed in the Zeta potential measurement, the Zeta-potential of cGO membrane surface gradually declined with the increase of cGO content. This implied that more negative charges would be produced on the cGO-incorporated composite membrane surface which facilatied to the rejection improvement of inorganic salt. However, with the incorporation and increase of cGO content, the pore size of the cGO-incorporated membranes was gradually enlarged which could be obtained from the gradual decline of neutral solute rejection (as shown in Fig. 12).

(Insert Fig. 12 here)

The larger pore size of the cGO-incorporated membranes was due to the fact that when adding cGO nanosheets, the diffusion of PIP molecules into the organic phase was hindered to some extent during the IP process as a result of the decline of cross-linking degree of poly(piperazine-amide) layer and the formation of a relatively loose network (as shown in AFM images). These two opposite effects together determined the variations of MgSO4 and NaCl rejections as shown in Fig. 11. Therefore, the cGO membrane with a more negatively charged surface (as analyzed in the Zeta potential measurement) had a higher rejection for multivalent SO4-2 anion (93.3-99.2 %) than that of the virginal NF membrane. However, with the cGO content increased to 200 ppm, membrane MgSO4 rejection exhibited a slight decrease to 93.3 %. This was due to the positive factor of the obvious enlarged pore size. By comparison, the continuous decline of NaCl rejection was attributed to the fact that NaCl had smaller ionic radius than that of MgSO4. And thus the effects of pore size enlargement on NaCl retention rate exceeded over the the effect of negatively charge enhancement. The similar results were reported in previous literature [65, 66]. Through comprehensive comparison, the optimal separation performance was obtained by 3# NF membrane which would be further analyzed in the following measurements. By carefully comparing the rejection data, it could be found that the prepared cGO NF membranes had more excellent permselective characteristics for SO4-2 and Cl-. The rejection ratio of SO4-2/Cl- was calculated and the results were listed in Table 3. It could be seen from Table 3 that with the increasing of cGO concentration in polyamide layer, the rejection ratio of MgSO4/ NaCl had

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obvious increase. These results were mainly ascribed to the denser negatively charged surface which could reject the higher valence sulfate anion more preferably than the monovalent chloride anion [30].

(Insert Table 3 here)

The microstructure of cGO NF membrane was changed via the SEM and AFM analysis above that the surface roughness gradually increased with the introduction and increasing of cGO content. This could enhance membrane water permeability. In addition, the improved surface hydrophilicity of cGO membrane could also increase membrane water flux. When the adding content of cGO flakes was higher (4#), the formation of a relatively loose and roughness network would be facilitate the further increase of membrane water flux. In this study, the GO-incorporated NF membranes were also prepared by the same method as cGO-incorporated NF membranes as described in Section 2.4. Table 4 listed the separation performance of cGO and GO-incorporated NF membranes prepared under the optimal conditions (GO content: 100 ppm) in this study as well as the GO NF membranes reported in previous literature [11, 29, 30, 34-36]. It could be seen that the cGO-incorporated NF membrane prepared in this study showed the better separation performance than that of other GO-incorporated NF membranes.

(Insert Table 4 here)

3.9. The effect of operating pressure on NF separation performance

(Insert Fig. 13 here)

In view of the excellent separation performance of cGO-incorporated NF membrane when the amount of cGO was 100 ppm, 3# membrane was chosen and studied for further research in the following performance measurement. Fig. 13(a) and (b) showed the variations of salt rejection and water flux of different NF membranes at different operating pressure. As shown in Fig. 13(a), with the increasing of operating pressure from 0.02 to 1.0 MPa, membrane both salt rejection of 0# and 3# membranes was gradually enhanced. Through careful observation, it could be seen that the salt rejection of these two membranes increased rapidly when the operating pressure was below 0.4 MPa and afterwards the growth rate slowed down until the operating pressure was raised to 1.0 MPa. Simultaneously, both the permeate water flux of 0# and 3# membranes had an almost linear increase with the increasing of operating pressure as shown in Fig. 13(b). This suggested the prepared composite NF membrane with the poly(piperazine amide) as the selective layer was still a pressure-driven type membrane.

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By comparison of the salt rejection of 0# and 3# membranes in Fig. 13(a), it could be found that both the rejection of the virginal and modified NF membranes had a gradual increase with the increasing of operating pressure from 0.02 to 0.7 MPa. The polyamide composite membranes are negatively charged because of the hydrolyzation of the acyl chloride groups remained in the polyamide chains. Hence, the rejection is mainly determined by Donnan exclusion and has little relation with the operating pressure [67]. The water flux will increase with the increase of the operating pressure according to the Spiegler-Kedem Model [68].

Jw  Lp(P   )

(6)

where Jw, Lp, ΔP and Δπ are the water flux, pure water permeability, transmembrane pressure and reflection coefficient, respectively. The salt flux is a function of salt concentration on both sides of membrane and had no direct relation to the operating pressure. With the increase of operating pressure, the water flux had a corresponding increase with a constant salt flux. Consequently, the salt concentration at the permeation side decreased which implied an increase of salt rejection [69]. With the further increasing of operating pressure from 0.7 to 1.0 MPa, the MgSO4 rejection of 3# NFmembrane continued to increase meanwhile the rejection of 0# had a slight decline. The rejection decline of the virginal NF membrane was derived from the inevitable concentration polarization near membrane surface [70, 71]. The hydrophilic cGO membrane surface with more negative charges could alleviate this phenomenon. It allowed more water molecules to penetrate through the active layer and retented more salt ions on membrane surface side. In addition, high operating pressure may cause membrane deformation which could also induce the deterioration of membrane separation performance. Previous literature have reported that the mechanical stress on the membrane may lead to membrane deformation as a consequence of compaction, changes in the pore size and shape, or even rupture of the skin layer under extreme high pressure. The consistent high rejection of the cGO-incorporated NF membrane was ascribed from the fact that the incorporation of GO into polyamide skin layer enhanced the anti-compression of polyamide functional layer [72, 73].

3.10. Stability of membrane separation performance

(Insert Fig. 14 here)

The stability of separation performance was tested and the results were shown in Fig. 12. It could be seen that show that the salt rejection of the cGO incorporated NF membrane could be well kept at a high data raging from 98-99.5% for 2 g.L-1 MgSO4 aqueous solution under 0.7 MPa and continuous

18

running for 60 h. Correspondingly, membrane permeate water flux had just a small fluctuation (80.7-82.1 L.m-2.h-1) during the whole measurement process. These results indicated that the cGO incorporated NF membrane possessed relatively stable separation performance which was attributed to the well-maintained microstructures of the modified barrier layer of cGO membrane in the running process.

3.11. The effects of feed temperature on the separation of NF membranes

(Insert Fig. 15 here)

The feed temperature has an important effect on the mass transfer especially for the water flux during the membrane separation process [74]. Fig. 15 showed the variations of NF membrane separation performance with feed temperature. It could be seen that with the raising of feed temperature, the two curves of salt rejection in Fig.15(a) kept almost stable followed by a decline when the temperature increased over 40 oC. However, both the water flux of the virginal NF membrane (0#) and the modified NF membrane (3#) in Fig. 15(b) had continuously increase. There are two major factors affecting the separation performance of NF membranes including the movements of polymer segments in polyamide and the dimension of the water/ion cluster. A higher temperature would enhance the movements of polymer segments and hence enlarges the water channels according to the dissolution-diffusion model [75, 76]. Correspongdingly, membrane water flux would have an obvious enhancement. Besides, the water flux enhancement of cGO NF membranes is indeed attributed to the decrease of water viscosity at higher feed temperature [77]. This also contributed to the increase of membrane water flux. Because the inorganic ions were also emerged in the form of of the cluster in polyamide layer, a higher temperature would similarly produce a smaller cluster. This would result in the decline of membrane salt rejection [78].

3.12. Antifouling properties of different NF membranes

(Insert Fig. 16 here)

The antifouling properties of the original NF membrane (0#) and the cGO NF membrane (3#) were evaluated in a cross-flow filtration using 1gL-1 BSA phosphate buffer solution (PBS,

0.01mol.L-1, pH=7.4) as the model feed solution under 0.7 MPa at 25 oC. Fig. 16 showed the variations of normalized flux with filtration time of different NF membranes. It could be seen that during the BSA filtration process, all membrane water flux in the three cycles had an obvious decline.

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This suggested all membranes suffered the inevitable fouling. After cleaning, membrane water flux obtained corresponding recovery. The flux recovery ratios of different membranes were shown in Fig. 17. By comparison, it could be seen that the flux recovery ratio (RFR) of 0# NF membrane exhibited a clear decrease from 92 % to 84 % during the three cycles. However, the flux recovery ratio of 3# NF membrane had a slightly reduction from 96 % to 92 %.

(Insert Fig. 17 here) Two major factors affect the antifouling properties of nanofiltration membrane including membrane surface hydrophilicity and roughness. The foulants arenot easily deposited on a hydrophilic and smooth membrane surface. Once the contaminants are deposited and adhered on membrane surface, they can also be stripped away from the hydrophilic and smooth membrane surface [74]. It could be obtained from the WCA and AFM observations that with the increasing of cGO content membrane surface hydrophilicity gradually enhanced and the surface roughness had a decline. These results induced the difficultly of BSA foulants absorbed on membrane surface. Besides, BSA protein molecules that were blocked in surface valley spots can be easily removed away by water flushing, and thus membrane water flux after washing could obtain a higher recovery.

4. Conclusions

The cGO-incorporated NF membrane was prepared through the introduction of carboxylated graphene oxide (cGO) in the polyamide layer during the interfacial polymerization (IP) process on the PSF/NWF membrane surface. FTIR results demonstrated the formation of polyamide functional layer and the incorporation of cGO nanosheets on the surface of PSF membrane. WCA analysis suggested membrane surface hydrophilicity had obviously improved with the increasing of cGO content. It was found that the resultant membranes showed high water flux, salt rejection as well as the improved membrane antifouling properties. The cGO-incorporated NF membrane showed an excellent selectivity of MgSO4 and NaCl. And the rejection ratio of MgSO4/ NaCl was approximately 8.0 which was mainly due to the steric size exclusion and Donnan exclusion effects. The enhanced antifouling properties of the cGO-incorporated NF membrane were ascribed to the higher hydrophilicity and lower roughness of membrane surface. Based on the above results, the modification through introducing cGO nanosheets in the functional layer is proven to be an effective way to obtain high-performance composite nanofiltration membrane.

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Acknowledgments

The authors gratefully acknowledge the funding for the Project supported by the National Natural Science Foundation of China (No. 51403052), the open fund of Henan Engineering Laboratory of New Textiles Development (No. GCSYS201603) and the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (13IRTSTHN024/15IRTSTHN011).

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27

Figure captions

Fig. 1. ATR-FTIR spectra of GO and cGO. Fig. 2. SEM images and local magnification Photos of GO (a/a') and cGO (b/b') nanosheets. Fig. 3. ATR-FTIR spectra of different membranes. Fig. 4. SEM image and EDS curves of different membrane surface. Fig. 5. The two and three-dimensional AFM surface images of different membranes. Fig. 6. The growth model of different polyamide thin-film during interfacial polymerization process: (Left) Thin-film layer derived from PIP and TMC; (Middle) Thin-film layer derived from PIP-cGO and TMC; (Right) Magnification of thin-film layer derived from PIP-cGO and TMC. Arrows stand for diffusion directions. Fig. 7. Schematic illustration of interactions between cGO and poly(piperazine amide). Fig. 8. X-ray diffractograms of GO/cGO and different membranes. Fig. 9. Static water contact angle of different NF membrane. Fig. 10. Zeta-potential curves of different NF membraneswith 0.001 mol/L KCl aqueous solution at pH=6.5. Fig. 11. The separation performance of the prepared NF membranes (a) Salt rejection; (b) Water flux. Fig. 12. Neutral solute rejections (glucose, saccharose and raffinose) of different composite NF membranes tested with 200 mg/l aqueous solution at 0.7 MPa bar and 25 °C. Fig. 13. Variations of salt rejection (a) and water flux (b) of different NF membranes at different operating pressure (2g.L-1 MgSO4 aqueous solution at 25 oC under 0.7 MPa). Fig. 14. The stability of separation performance of modified NF membrane (3#) in 2 g.L-1 MgSO4 aqueous solution under 0.7 MPa and continuous running for 60 h. Fig. 15. Variations of salt rejection (a) and water flux (b) of NF membranes with feed temperature in 2 g.L-1 MgSO4 aqueous solution under 0.7 MPa and 25 oC. Fig. 16. Variations of normalized flux with filtration time of different membranes. Fig. 17. Flux recovery ratio (RFR) values of different membranes (1, 2, 3 represent the times of cycles).

28

Fig. 1 (by Li et al)

C=O C=C C-OH

Transmittance (a. u.)

OH

GO

cGO

C-O-C

4000

3500

3000

2500

2000

1500 -1

Wave number (cm )

29

C-O

1000

500

Fig. 2 (by Li et al)

30

Fig. 3 (by Li et al)

PSF

1#

3#

0#

2#

4#

-1

1377cm -1

1625cm

-1

-1 3435cm 3254cm

4000

3600

3200

2800

2400

2000

1600

-1

Wave number (cm )

31

1200

800

Fig. 4 (by Li et al)

32

33

Fig. 5 (by Li et al)

34

35

Fig. 6 (by Li et al)

36

Fig. 7 (by Li et al)

37

Fig. 8 (by Li et al)

GO cGO 0# 1# 2# 3# 4#

o

14.96

o

Intensity (a. u.)

15.22

o

15.91

o

16.77 o

13.35

5

10

15

20

25

30

35

40



2 

38

45

50

Fig. 9 (by Li et al)

o

Water contact angle ( )

50

40

30

20

10

0

0#

1#

2#

3#

Membrane

39

4#

Fig. 10 (by Li et al) Membranes 0#

1#

2#

3#

Zeta potential (mV)

-20

-30

-40

-50

40

4#

Fig. 11 (by Li et al)

(a) 100 90

MgSO4

80

NaCl

Rejection (%)

70 60 50 40 30 20 10 0 0#

1#

2#

3#

4#

3#

4#

Membrane

(b) 100

MgSO4 NaCl

80 70

.

-2.

-1

Water flux (L m h )

90

60 50 40 30 20

0#

1#

2#

Membrane

41

Fig. 12 (by Li et al)

100

Solute rejection (%)

90

0# 1# 2# 3# 4#

80

70

60

50 100

200

300

400 .

500 -1

Molecular weight (g mol )

42

600

Fig. 13 (by Li et al)

(a) 0# 3#

100

Rejection (%)

90

80

70

60 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.8

0.9

1.0

Operating pressure (MPa)

(b) 100 0# 3#

90 80

60

.

-2. -1

Water flux (L m h )

70

50 40 30 20 10 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Operating pressure (MPa)

43

Fig. 14 (by Li et al)

100 100

70

85

-2. -1

90

.

Rejection (%)

80

Water flux (L m h )

95

90

60

80

50 75 40 70 30 65

20 10

60 0

10

20

30

40

50

Running time (h)

44

60

Fig. 15 (by Li et al)

(a) 0# 3#

100

Rejection (%)

90

80

70

60 10

20

30

40

50

60

50

60

o

Feed temperature ( C)

(b) 100 0# 3#

90

70 60

.

-2. -1

Water flux (L m h )

80

50 40 30 20 10 0 10

20

30

40 o

Feed temperature ( C)

45

Fig. 16 (by Li et al)

0# 3#

Flux recovery ratio (%)

1.0

0.9

0.8

0.7

0.6

0.5

water

wash 0.4

0

50

wash 100

water

water BSA

water BSA

BSA

150

wash 200

Time (min)

46

250

300

Fig. 17 (by Li et al)

Flux recovery ratio (%)

100

0# 3#

80

60

40

20

0

1

2

3

Cycle

47

Table 1 Properties of the resultant PSF support membrane Properties

PEG-20, 000 Rejection (%) 99.5

PSF substrate membrane

Pure water flux (L.m-2.h-1) 165.6

Porosity (%) 68.1

Table 2 Surface roughness of different membranes. Membrane sample

Surface roughness parameters Ra (nm)

Rrms (nm)

Rmax (nm)

PSF support membrane

10.5

14.9

160

0#

49.6

61.5

353

1#

43.1

58.2

307

2#

32.9

44.1

282

3#

24.3

32.6

256

4#

23.0

31.5

223

Table 3 Salt rejection and rejection ratio (MgSO4/NaCl) of different NF membranes Membrane

0#

1#

2#

3#

4#

MgSO4 rejection %)

89.6

95.6

98.2

99.2

93.3

NaCl rejection %)

29.7

20.3

16.2

12.6

11.6

Rejection ratio (MgSO4/ NaCl)

3.0

4.7

6.1

7.9

8.0

48

Table 4 Separation performance of different cGO and GO-incorporated NF membranes Membrane

cGO NF membrane

WCA

Rejection

Flux

(o)

(%)

(L.m-2.h-1)

26.2

MgSO4: 99.2

MgSO4: 81.6

0.7 MPa, 25 oC,

NaCl: 12.6

NaCl: 89.6

2 g.L-1 aqueous solution

MgSO4: 92.3

MgSO4: 43.5

0.7 MPa, 25 oC,

NaCl: 30.1

NaCl: 51.2

2 g.L-1 aqueous solution

MgSO4: 90.2

MgSO4: 17.04

0.8 MPa, 25 oC,

NaCl: 58.5

NaCl: 18.56

1 g.L-1 aqueous solution

Methyl blue:

Methyl blue:

0.5 MPa, 25 oC,

99

24.5

0.1 g.L-1 aqueous solution

MgSO4: 65

-

0.3 MPa, 25 oC,

in this study GO NF membrane

33.6

in this study GO NF membrane

25.8

in Ref [34] GO NF membrane

66.8

in Ref [36] GO NF membrane

-

in Ref [35] GO NF membrane

1 g.L-1 aqueous solution

NaCl: 62 -

NaSO4: 72.5

NaSO4: 30

42.1

PEG-200: 96.8

PEG-200: 15.7

in Ref [30]

1.0 MPa, 25 oC, 0.5 g.L-1 aqueous solution

in Ref [29] GO NF membrane

1.5 MPa, 25 oC, 2 g.L-1 aqueous solution

in Ref [11] GO NF membrane

Test conditions

52.9

Na2SO4: 93.57

Pure water:

1.0 MPa, 25 oC,

NaCl: 35.63

60.6

2 g.L-1 aqueous solution

49