NH2-Fe3O4-regulated graphene oxide membranes with well-defined laminar nanochannels for desalination of dye solutions

Desalination 476 (2020) 114227

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NH2-Fe3O4-regulated graphene oxide membranes with well-defined laminar nanochannels for desalination of dye solutions Liangliang Dong, Minghui Li, Shuo Zhang, Xuejian Si, Yunxiang Bai, Chunfang Zhang

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Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, PR China

G R A P H I C A L A B S T R A C T

Intercalated NH2-Fe3O4 can simultaneously tune the GO interlayer spacing and improve the stability of GO membrane in water.

A R T I C LE I N FO

A B S T R A C T

Keywords: GO/NH2-Fe3O4 membranes Dye/salt mixture separation High flux Low NaCl rejection NH2-Fe3O4

Graphene oxide (GO)-based nanofiltration membranes, featuring well-ordered microscopic structure, well-defined 2D nanochannels and superior molecular sieving ability, have attracted sustained research interest in molecular and ionic separation. However, most of current GO laminar membrane have a poor water flux and high rejection of both dyes and salts, which is not suitable for the dye/salt mixture separation. Herein, we report a vacuum filtration strategy to fabricate GO/NH2-Fe3O4 nanofiltration membranes with high water flux and excellent separation performance for dye/salts mixture by introducing NH2-Fe3O4. The NH2-Fe3O4 is not only worked as the rigid spherical nanospacer to tune GO interlayer spacing but also as crosslinkers to improve the stability of GO membrane in water. FTIR, XRD, SEM, zeta potential and contact angle were applied to analyze the chemical composition and morphology of as-prepared membranes. The effect of intercalated NH2-Fe3O4 nanoparticles on overall performance of the GO/NH2-Fe3O4 membranes was systematically investigated. The resulted membrane with 8 wt% of NH2-Fe3O4 loading has high water flux of up to 78 Lm−2 h−1, which is 4.8 times higher than that of pure GO membrane. Moreover, such membrane also displays high congo red rejection (94%) and low NaCl rejection (~15%), rendering the membranes promising for dye/salt mixtures separation.

1. Introduction With the rapid development of printing and textile industries, the

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negative impact of generated textile wastewater on our living environment is getting more and more attention [1–3]. As one of the largest textile-manufacturing countries in the world, China annually

Corresponding author. E-mail addresses: [email protected] (L. Dong), [email protected] (C. Zhang).

https://doi.org/10.1016/j.desal.2019.114227 Received 10 October 2019; Received in revised form 4 November 2019; Accepted 13 November 2019 0011-9164/ © 2019 Published by Elsevier B.V.

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Such membrane not only had enlarged interlayer spacing, but also exhibited well stability against delamination and degradation in aqueous solution, which showed sharply increased water flux, high organic dyes rejection and dye/salt selectively. Zhang and co-workers used in-situ growth method to introduce covalent organic framework-1 (COF-1) into GO membrane [13]. The prepared GO/COF-1 membrane exhibited an excellent water flux, high water-soluble dyes rejection (> 99%) and high salt permeability. However, the research on the application of GObased membrane in dye/salt mixtures separation is still insufficient. Continued exploration of new methods to design highly permeable GObased membrane with well-defined interlayer nanochannels, high rejection of dye molecules and low retention of salt is still of great necessity for high-efficiency dye/salt separation. In this study, nano-sized Fe3O4 intercalated GO membranes were successfully fabricated on polydopamine (PDA)-coated polyvinylidene fluoride (PVDF) support membranes via vacuum filtration, in which amino-functionalized Fe3O4 (NH2-Fe3O4) acted not only as the rigid spherical nanospacer between GO nanosheets to tune the interlayer spacing but also as crosslinkers to improve the water stability of GO membrane. The physicochemical properties of the resulting GO/NH2Fe3O4 membranes were systematically characterized by various techniques. The water flux, separation efficiency of dye/salt mixture, rejection stability, and operation conditions (e.g. operation pressure) of GO/NH2-Fe3O4 membranes were investigated in detail. This work aims to put forward a strategy to modulate the interlayer spacing and improve the water stability of GO/NH2-Fe3O4 membrane, stressing the synergistic effect between NH2-Fe3O4 and GO for the dye/salt separation and also a useful reference to design NF membrane for the treatment of textile wastewater.

discharges about 2.37 billion tons of textile wastewater [4]. The textile wastewater usually contains high concentration of toxic dyes and high salinity (e.g. ~5.6 wt% Na2SO4 and ~6.0 wt% NaCl) [5,6]. Direct discharge not only jeopardizes the ecological environment and human health, but also leads to serious resource waste because both dyes and inorganic salts are valuable resources in terms of sustainability. Therefore, it is of great necessity to develop an effective technology for separation of dye/salt mixture in the aqueous solution. Compared with traditional methods such as coagulation-flocculation, degradation and adsorption, membrane separation technologies possess some unique features such as minor energy consumption, costeffectiveness and high-efficiency, which make them particularly appealing in water treatment and recycling [7–9]. Among these technologies, nanofiltration (NF) has been widely applied for the treatment of textile wastewater, since it has high water flux and rejection to multivalent ions and organic molecules with a range of molecular weight cutoffs (100–1000 Da), as well as low operation pressure. However, since most commercial NF membranes are polymer composite membranes with dense selective layer, several intrinsic drawbacks such as both high rejection of dyes and multivalent salts, weak chemical resistance and poor membrane fouling limit their performance [10–12]. Recently, a growing research focuses on the design of novel membranes based on ultrathin 2D materials such as GO [13–24], MXene [25–28], MoS2 and COF nanosheets [29–35]. Among these 2D materials, GO has attracted sustained research interest due to its easy accessibility, hydrophilicity, excellent mechanical strength, high chemical and thermal stability, which makes it highly interesting and promising for desalination, purification and molecular separation [13–16,21–24,36–40]. The GO membranes can be produced by several methods, such as filtration assisted, spray coating and layer-by-layer assembly, in which the interlayer nanochannels between adjacent GO nanosheets allow water to permeate through while rejecting unwanted ions or molecules larger than the interlayer spacing. However, pure GO membrane has narrow interlayer spacing (< 0.8 nm), leading to a poor water flux and high rejection of both dyes and salts, which is not suitable for the dye/salt separation [38,41–43]. In order to address this issue, various nano-sized materials have been inserted to tune the interlayer nanochannels through non-covalent or covalent strategies, including small organic molecules [15,16,22,24,37,44–46], inorganic nanoparticles [14,47–50] and nanotubes/fibers [2,51–56]. Non-covalent strategy typically includes electrostatic, π-π, cation-π and coordination interactions. For example, Xu and co-workers selected cationic tetrakis(1-methyl-pyridinium-4-yl)porphyrin (TMPyP) as an upholder to design TMPyP/GO NF membrane through π-π and electrostatic interaction. The incorporation of TMPyP not only restricted the swelling problem of pure GO membrane in water, but also increased interlayer spacing of TMPyP/GO NF membrane [37]. Jin's group, Gao's group and Sitko's group respectively intercalated CNT between adjacent GO or rGO sheets through noncovalent interaction to realize the enhancement of GO interlayer spacing [2,51,57]. Unfortunately, since non-covalent interactions usually depend strongly on solvent environment such as pH values, the stability of GO-based membrane through such interactions sometimes is not satisfactory. Therefore, covalent strategy is a good alternative to construct stable GO-based membrane. Various amines [3,14,16,44,45], acyl chlorides [24], pyridines [22] and sulfonic acids [15] have been adopted to covalently bridge adjacent GO nanosheets. Although these methods successfully realize the modulation of interlayer nanochannels of GO membrane, there are still some limitations that restrict their further application in dye/salt mixtures separation. That is, most of GO membranes still keep high rejection of both dyes and salts or high rejection of dyes and moderate rejection of salts even after the interlayer nanochannels are enlarged, resulting in low dye/salt selectively and separation efficiency [7,58]. Recently, Jiang and co-workers reported a GO-covalent triazine framework (CTF) membranes based on the combination of π-π interaction and chemical bonding between -NH2 groups of CTF and -COOH groups of GO [59].

2. Materials and methods 2.1. Materials Flat-sheet PVDF ultrafiltration (UF) membrane with pore size of 50 nm was prepared by the traditional phase separation method, which was used as a substrate. GO (500 mm average particle diameter) was purchased from XFNANO Co. Ltd. (Nanjing, China). (3-Aminopropyl) triethoxysilane (APTES) was purchased from Energy Chemical Reagent Co. Ltd. (Shanghai, China). Fe3O4 nanoparticle (20 mm average particle diameter), ethanol and dopamine hydrochloride (DOPA) were obtained from Aladdin Reagent Co. Ltd. (Shanghai, China). Sodium sulfate (Na2SO4), magnesium chloride (MgCl2), calcium chloride (CaCl2), sodium chloride (NaCl), congo red, methylene blue and methyl orange were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were used as received without further purification. 2.2. Preparation of amino-functionalized Fe3O4 (NH2-Fe3O4) NH2-Fe3O4 used in this study was synthesized as the literature procedure [60,61] and the scheme of the synthesis process is shown in Scheme 1. Briefly, 0.25 g Fe3O4 nanoparticles were firstly added into 80 mL of ethanol-water mixed solvent (Vethanol:Vwater = 1:1) to form stable disperse solution under ultrasonication for 1 h. Then 2 mL of APTES was added to the above solution. The temperature was maintained at 50 °C and the reaction was carried out for 24 h. After that, the obtained crude product was washed by fresh ethanol and water and further centrifuged several times. The final product was dried at 60 °C under reduced pressure. 2.3. Preparation of GO/NH2-Fe3O4 membrane The fabrication process of GO/NH2-Fe3O4 membranes is shown in Scheme 2. Firstly, we prepared a porous PVDF membrane via the traditional phase separation method, and then dip-coated it in dopamine 2

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Scheme 1. Schematic of NH2-Fe3O4 nanoparticles.

2.5. Characterization of GO, NH2-Fe3O4 and GO/NH2-Fe3O4 membrane

hydrochloride solution containing 2 g/L dopamine hydrochloride and 10 mM Tris buffer at pH 8.5. Dopamine can polymerize to form PDA on the PVDF support membrane to enhance the interaction between GO layer and support membrane, which can finally improve the stability of GO/NH2-Fe3O4 membrane. After that, the stable GO/NH2-Fe3O4 suspensions were prepared by mixing different mass ratios of NH2-Fe3O4 to GO (2:1, 4:1, 6:1, 8:1 and 10:1) into DI water under ultrasonication for 1 h. Then, the mixtures were filtered on a PDA coated PVDF membrane under vacuum filtration of 80 kPa. After filtration, the obtained membranes designated as GO/NH2-Fe3O4-X were air-dried before use, where X is the mass ratios of NH2-Fe3O4 to GO.

The physicochemical properties of GO powder and NH2-Fe3O4 nanoparticle were characterized by a FTLA 2000 type Fourier transform infrared (FT-IR) spectrometer, X-ray diffraction (XRD) using Cu-Kα radiation (λ = 0.15406 nm), zeta-potential measurement using Zetasizer Nano ZS and JEM-2100 transmission electron microscopy (TEM). The chemical structures of GO/NH2-Fe3O4 membrane were characterized by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and XRD. The morphologies of membranes were investigated by Hitachi S4800 scanning electron microscope (SEM) and Multimode 8 atomic force microscopy (AFM). Static contact angles of the membranes were measured by contact angle goniometer OCAH200 using sessile-drop method. Surface zeta-potentials of the membranes were performed by the SurPASS electrokinetic analyzer with 0.01 M potassium chloride (KCl) as an electrolyte solution.

2.4. Nanofiltration performance of GO/NH2-Fe3O4 membrane The nanofiltration performance of GO/NH2-Fe3O4 membranes was evaluated by a cross-flow circulation system (Fig. S1) with the effective membrane area of 23.7 cm2. In order to make the membrane reach the steady state, each membrane was firstly compacted at 0.6 MPa for 30 min. Then the pressure was kept constant at 0.5 MPa with a crossflow velocity of 30 L h−1. The pure water flux (Jw, L/(m2h)) was calculated using the following Eq. (1):

Jw =

V S·t

3. Results and discussion 3.1. Physicochemical properties of GO and NH2-Fe3O4 Several characterization techniques were conducted to investigate the structures and properties of the GO and NH2-Fe3O4. The FT-IR spectrum of GO in Fig. 1(a) shows the presence of oxygen-containing functional groups on GO nanosheets. Specifically, the broad peak around ~3407 cm−1 is assigned to OeH stretching vibration. The peaks at ~1725, 1384 and 1229 cm−1 are ascribed to the CeO stretching of the C]O, C-O-C and C-OH groups. The C]C stretching vibration appears at ~1588.1 cm−1 [24]. From the TEM image (Fig. 1(b)), it can be seen that GO has a thin and typical sheet-shaped structure with the size of several hundreds of square nanometers. The surface of GO is relatively smooth without any impurities, while some wrinkles and folding appear on the edges of GO sheets. The powder XRD pattern of GO is shown in Fig. 1(c). A single intense peak appears at 2θ of 11.6°, corresponding to an interlayer spacing of 0.79 nm. The inset of Fig. 1(c) shows that GO has excellent dispersions in aqueous solution, which is beneficial for preparation of GO membrane without defects. FT-IR spectra were used to identify the surface modification of Fe3O4 nanoparticle by APTES, as shown in Fig. 2(a). Pure Fe3O4 nanoparticle has two characteristic adsorption bands of OeH groups on

(1)

where V (L) is the permeate volume, S (m2) is the effective membrane area, and t (h) is the operation time. To determine the rejection properties of the membranes, the pure saline solutions (1 g/L) including Na2SO4, MgCl2, CaCl2 and NaCl, pure dye solutions (0.1 g/L) including congo red, methylene blue and methyl orange, and mixed dye/salts solutions were used as the feed solutions. The salt concentrations were measured by electrical conductivity (DDS11A, Shanghai Leichi Instrument Co., Shanghai, China) and dye concentrations were calculated by an ultraviolet-visible spectrophotometer at the maximal absorption wavelength of the dye. The rejection ratio (R) was calculated by the following Eq. (2):

Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝

(2)

where Cp (mg/L) and Cf (mg/L) are the concentration of dyes and salts in permeate and feed solutions, respectively.

Scheme 2. Schematic representation of GO/NH2-Fe3O4 membrane fabrication. 3

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Fig. 1. Characterization of GO: (a) FT-IR spectrum; (b) TEM image; (c) powder XRD pattern.

the surface of Fe3O4 nanoparticle at ~3423 cm−1 and FeeO bonds of bulk magnetite at ~582 cm−1 [60,61]. Compared to pure Fe3O4, the NH2-Fe3O4 shows three new peaks at ~2923, ~1010 and ~833 cm−1, which correspond to the stretching vibration of the CeH moiety, Si-O-Si and SieO groups of APTES, respectively. These new peaks confirm that APTES is successfully grafted on the surface of the Fe3O4 nanoparticles. The zeta potential of NH2-Fe3O4 is 19.3 ± 1.8 mV (Fig. 2(b)), which indicates NH2-Fe3O4 contains a positive charge due to the protonation of amine groups. Fig. 2(c) exhibits the XRD patterns of Fe3O4 and NH2Fe3O4 nanoparticles. For the Fe3O4 nanoparticle, all the diffraction peaks can match well with typical crystal structure of Fe3O4, indicating that Fe3O4 has inverse cubic spinel structure. The diffraction patterns of the NH2-Fe3O4 are in accordance with that of pure Fe3O4, confirming that the presence of APTES modification doesn't destroy the crystalline structure of Fe3O4 nanoparticles. The mean particles sizes of Fe3O4 and NH2-Fe3O4 can be calculated based on the line width of the (311) plane refraction peak in XRD pattern by using Scherrer equation [60]. The resulted crystallite sizes of the Fe3O4 and NH2-Fe3O4 nanoparticles using this equation were about 17 nm and 20 nm, respectively.

especially for the hydroxyl, carboxyl and epoxy groups. In addition, new peak appears at 1507 cm−1, which is assigned to NeH bending vibration. All these changes suggest that cross-linking reactions and electrostatic interaction between amine groups of NH2-Fe3O4 and epoxy/carboxyl groups of GO have occurred [14], which can eliminate the swelling of pure GO membrane in aqueous solution and improve the stability of GO/NH2-Fe3O4 membranes. Comparison of XRD patterns taken from GO and GO/NH2-Fe3O4 membrane is shown in Fig. 3(b). Compared with PDA coated PVDF support membrane, the characteristic diffraction peaks belonging to GO and Fe3O4 can be found in GO membrane and GO/NH2-Fe3O4 membranes, respectively. The XRD pattern of pure GO membrane exhibits a d-spacing of 7.9 Å based on Bragg's law, which was in agreement with other GO membranes reported previously [51,59]. However, for the GO/NH2-Fe3O4 membrane, with the increase of NH2-Fe3O4 loading, the diffraction peak belonging to GO shifts to leftward (from 11.6° to 10.0°), which means the increase of d-spacing (from 7.9 Å to 8.8 Å). The increased interlayer distance is helpful for facilitating water transport and finally improving water flux of GO/NH2-Fe3O4 membranes. Surface charge plays a vital role in determining the separation performance of NF membranes, especially for dye/salts separation. In this work, the surface zeta potential was conducted to study the surface charge of as-prepared NF membranes at 25 °C and pH of 7 (shown in Fig. 3(c)). The pure GO membrane has a negative zeta potential of −48.9 ± 2 mV. Compared with pure GO membrane, the surface zeta potential values of GO/NH2-Fe3O4 membranes gradually decrease and are getting closer to the isoelectric point (electrically neutral state) with increase of NH2-Fe3O4 loading. This is due to multiple effects of chemical reactions and electrostatic interaction between NH2-Fe3O4 and GO, which decrease the negative charge density of GO and endow the

3.2. Physicochemical properties of GO/NH2-Fe3O4 membranes Fig. 3(a) shows the ATR-FTIR spectra of the GO/NH2-Fe3O4 membranes. The ATR-FTIR spectrum of the pure GO membrane exhibits the characteristic peaks of hydroxyl groups (-OH stretching at 3360 cm−1), carboxyl groups (C]O stretching at 1720 cm−1 and C-OH stretching at 1380 cm−1), epoxy groups (C-O-C stretching at 1220 cm−1) and alkoxy groups (CeO stretching at 1056 cm−1), which are consistent with the previous reports [24,40,45]. However, in the GO/NH2-Fe3O4 membranes, the intensity of these characteristic peaks sharply decreases, 4

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Fig. 2. Characterization of Fe3O4 and NH2-Fe3O4 nanoparticles: (a) FT-IR spectra; (b) Zeta potential of NH2-Fe3O4 nanoparticles; (c) powder XRD patterns.

investigated by AFM (Fig. 5). All the membranes show typical ridge and valley topography. The surface roughness of membrane is represented by the mean roughness (Ra) and the root mean-square roughness (Rq). As shown in Fig. 5, the GO/NH2-Fe3O4 membranes have higher surface roughness (Rq) than pure GO membrane because of the intercalation of NH2-Fe3O4, which is in accordance with the SEM results (Fig. 4). This means that the GO/NH2-Fe3O4 membranes have higher effective filtration area than pure GO membrane, which contributes to high membrane permeations.

GO/NH2-Fe3O4 membranes with more neutral charge. This will facilitate the salt permeation and enhance the separation performance of dye/salts mixture. The surface chemistry of membrane was characterized by the static water contact angle test, as shown in Fig. 3(d). The addition of NH2-Fe3O4 does not change the water contact angle of GO membrane, suggesting that GO/NH2-Fe3O4 membranes retain the advantages of the excellent hydrophilicity of GO membrane, which is beneficial for eliminating the deposition and adsorption of hydrophobic foulants. Fig. S2 shows the pure GO and GO/NH2-Fe3O4 membranes on PDA coated PVDF support with a diameter of 6.5 cm. Membrane surface and cross section morphology were investigated by SEM, as shown in Fig. 4. The pure GO membrane has wrinkled surface and dense laminated microstructure with the thickness of 200 nm (Fig. S3), which has been observed from other reported GO membranes [46,51]. All the GO/NH2Fe3O4 membranes show the similar cross section morphology to pure GO membrane but more loose laminar structure (Fig. 4), which is due to the enlargement of GO interlayer spacing caused by the insertion of NH2-Fe3O4. With an increase of NH2-Fe3O4 loading, the membrane thickness of GO/NH2-Fe3O4 membranes display a slight ascent from 295 nm to 350 nm and then a sharp ascent from 410 nm to 510 nm. Generally, the loose structure can favor fast transport of water and salts through the membranes, enhancing the water and salts permeations. However, excess amount of NH2-Fe3O4 can cause the agglomeration and form some defects in the membrane (e.g. GO/NH2-Fe3O4-10), which inevitably weakens separation efficiency of dye/salts mixture. Moreover, compared to the pure GO membrane, after addition of NH2Fe3O4, all GO/NH2-Fe3O4 membranes exhibit rougher surface morphology. The detailed surface morphologies of all membranes were further

3.3. Separation performance of GO/NH2-Fe3O4 membranes 3.3.1. Single dye and salt rejection and permeation flux In this work, we firstly investigated the effect of NH2-Fe3O4 loading on the water flux and single dye and salt rejection of GO/NH2-Fe3O4 membranes at room temperature and 0.5 MPa, as shown in Fig. 6(a). We chose the NaCl and Na2SO4 as single model salt and congo red as single model dye for the tests. At a transmembrane pressure of 0.2 MPa, the water flux of pure GO membranes is only 13.5 Lm−2 h−1, but shows dramatical rise after adding the NH2-Fe3O4. For example, a loading at 10 wt% of NH2-Fe3O4, the water flux of GO/NH2-Fe3O4 membrane reaches up to 110 Lm−2 h−1, which is 8.1 times higher than that of the pure GO membrane. The increased water flux can be attributed to the following two reasons. On the one hand, the intercalation of NH2-Fe3O4 can enlarge the GO interlay distance (confirmed by the XRD results of Fig. 3(b)), forming enlarged nano-channels and more loose structure (confirmed by the cross-sectional morphology of membranes in Fig. 4), resulting in higher water flux according to the Hagen-Poiseuille equation [62]. On the other hand, as discussed in Figs. 4 and 5, the rougher surface of GO/NH2-Fe3O4 membrane means a larger effective filtration 5

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Fig. 3. ATR-FTIR spectra (a), XRD patterns (b), surface zeta potential (c) and static contact angle (d) of GO and GO/NH2-Fe3O4 membrane.

area, which is also contributed to the enhancement of water flux. In contrast to the change of water flux, the rejections of GO/NH2-Fe3O4 membranes for both NaCl and Na2SO4 exhibit sharp decline when the NH2-Fe3O4 loading continuously rises from 2 wt% to 8 wt%, while the congo red rejection is high and almost has no changed. Based on the mechanism of transport process of the GO/NH2-Fe3O4 membranes (Fig. 7), the reasonable explanation is that the enlarged nano-channel and more loose structure formed by addition of NH2-Fe3O4 deteriorate sieving effect of GO/NH2-Fe3O4 membranes for the salts, then leading to low salt rejection. Moreover, decreased density of surface charge (Fig. 3(c)) is another factor for low salt rejection since it can reduce the Donnan effect of GO/NH2-Fe3O4 membranes. Generally, water-soluble dyes (e.g. congo red) tend to form aggregates or clusters in aqueous solution because of intermolecular hydrogen bonds and/or the hydrophobic interactions of aromatic rings of neighbor dye molecules [63,64]. Therefore, actual size of a congo red in aqueous solution is bigger than monomeric congo red molecule, leading to a high congo red rejection. Nevertheless, when the NH2-Fe3O4 loading further rise to 10 wt%, the congo red rejection shows obvious decrease from 98.0% to 90.5%, which is due to some defects formed in GO/NH2-Fe3O4-10 membrane (discussed in Fig. 4). Based on above discussion, the GO/ NH2-Fe3O4–8 membrane has high congo red rejection but poor NaCl and Na2SO4 rejection, which is chosen as optimum membrane to investigate its dye/salt separation performance. To further evaluate practical value of GO/NH2-Fe3O4-8 membrane in dye wastewater system, the rejection performance of other different

types of dyes (methylene blue and methyl orange) and salts (MgCl2 and CaCl2) were also tested, as shown in Fig. 6(b). The dye and salt rejection of GO/NH2-Fe3O4-8 membrane is in the following sequence: methylene blue (70.0%) < methyl orange (75.0%) < congo red (98.0%) and CaCl2 (7.5%) < MgCl2 (9.8%) < NaCl (15%) < Na2SO4 (32.0%), respectively. The dye rejection can be explained in terms of their molecule sizes and charge characters. As shown in Fig. S4, among these three types of dyes, congo red has the largest molecule size and negative charge, resulting in the highest rejection according to sieving effect and electrostatic repulsion due to the negative charge of GO/NH2Fe3O4-8 membrane. Because methylene blue contains positive charge and has slight larger molecule size than methyl orange, so the electrostatic attraction between methylene blue and membrane becomes more obvious than sieving effect, which ultimately leads to lower rejection than methyl orange. The salt rejection can be explained mainly depending on the Donnan effect. That is, the negatively-charged membrane tends to form more electrostatic attraction force for bivalent counter-ion (Ca2+ and Mg2+) than monovalent ion (Na+) [1,29,38,39], so that the rejection of CaCl2 and MgCl2 is lower than that of NaCl and Na2SO4. In brief, except slightly high Na2SO4 rejection, the GO/NH2-Fe3O4-8 membrane has poor salt rejections below 15% (some even below 10%) but high water flux and congo red dye rejection over 95%, exhibiting the excellent dye/salt separation performance.

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Fig. 4. Surface (up) and cross section (down) SEM images of GO/NH2-Fe3O4 membranes.

electrostatic interaction of the charges in the membrane, making the “shielding effect” of NF membranes more significant. Thus, the hydration layer of charges on the membrane pores becomes thinner, which opens up the transport route of solutes through the membrane, allowing NaCl and congo red to pass through the membrane more easily [68–70]. As for the congo red, except the “shielding effect”, more uniform dispersion of congo red in the presence of NaCl can making it easier to go through the membrane and thus also result in a low dye rejection. However, compared with sharp decrease of NaCl rejection, congo red rejection show a little decrease, which means that the size exclusion mechanism plays a dominant role in dye retention over the “shielding effect” intensified by NaCl. The comparison with other GObased membranes reported in the literature (Table S1) illustrates that the GO/NH2-Fe3O4-8 membrane shows excellent desalination performance from dye/salt mixtures, since it has higher water flux and congo red rejection but lower NaCl rejection than previously reported GObased membranes. Effect of operating pressure on water flux of the as-prepared membranes is shown in Fig. 9. In theory, increasing the operation pressure would increase the driving force for transport of water molecule through the membrane, thus the water flux should increase accordingly. For the GO/NH2-Fe3O4-8 membrane, the water flux almost increases linearly with the increase of operating pressure ranging from 0.2 MPa to 0.8 MPa, which can match well with theoretical prediction. Nevertheless, the pure GO membrane only shows a slight increase with the increase of operating pressure. A reasonable explanation is that the

3.3.2. Dye/salt mixtures separation As mentioned in section of introduction, the dyeing wastewater usually contains an appreciable quantity of salts, therefore it is of great importance to study the effect of salt on separation performance of dye/ salt mixtures. As shown in Fig. 8, the presence of NaCl has a significant influence on membrane flux. For example, the flux of GO/NH2-Fe3O4-8 membrane continuously decreases from 78 Lm−2 h−1 to 63 Lm−2 h−1 with increase of NaCl concentration from 0.5 g/L to 3.0 g/L, which is the result of the rise in osmotic pressure, concentration polarization and dye adsorption into membrane. Specifically, based on the KedemKatchalsky equation [65], the membrane flux is directly proportional to net transmembrane pressure that is defined as the difference between feed pressure and osmotic pressure. Addition of salt ions (e.g. NaCl) make the charged dye molecules (e.g. congo red) disperse more uniformly in aqueous solution, thus the congo red/NaCl mixture solution is more concentrated than pure congo red solution, increasing osmotic pressure, reducing the net transmembrane pressure at the constant feed pressure, then leading to low membrane flux [66]. Meanwhile, uniformly dispersed congo red molecules can easily pass through membrane, which gives rise to higher dye absorption on the surface and inner of membrane. This would narrow the effective membrane pore size, which also reduce membrane flux [67]. For rejection of NaCl, increased salt content leads to higher concentration gradient near membrane surface, which could facilitate transport of NaCl through membrane and thus lower the rejection of NaCl [66]. Moreover, higher NaCl concentration can lower the 7

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Fig. 5. AFM images of GO and GO/NH2-Fe3O membranes: (a) pure GO; (b) GO/NH2-Fe3O4-2; (c) GO/NH2-Fe3O4-4; (d) GO/NH2-Fe3O4-8.

2D nanochannels formed by oxygen-containing groups in pure GO membrane is contracted under high pressure, thus resulting in a slight rise of water flux [71]. Instead, addition of NH2-Fe3O4 can effectively prevent the shrinkage of 2D nanochannels against increased operating pressure, indicating that GO/NH2-Fe3O4-8 membrane has outstanding mechanical stability. To examine the stability of GO/NH2-Fe3O4 membranes in aqueous

solution, pure GO and GO/NH2-Fe3O4 membranes were immersed in DI water for a certain time, as shown in Fig. 10. After 30 days, the pure GO membrane becomes broken, where some parts of GO nanosheets in GO membrane have been detached from the support membrane whereas the GO/NH2-Fe3O4 membranes keep well integrity for all time. This is because pure GO membrane often suffers from serious swelling problem in aqueous solution due to electrostatic repulsion, making the

Fig. 6. Water flux and single dye and salt rejection of GO/NH2-Fe3O4 membranes. 8

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Fig. 7. Mechanisms of transport process of GO and GO/NH2-Fe3O4 membranes.

Fig. 8. Effect of NaCl concentration on flux and rejection for GO/NH2-Fe3O4-8 membrane (Feed congo red concentration: 0.1 g/L).

Fig. 9. Effect of operating pressure on water flux of GO and GO/NH2-Fe3O4-8 membranes.

membrane unstable and causing peeling. But for the GO/NH2-Fe3O4 membranes, intercalated NH2-Fe3O4 links adjacent GO nanosheet via chemical bonds between amino groups of NH2-Fe3O4 and oxygen groups of GO, restricting the swelling and thus improving stabilization of membrane in aqueous media.

Fe3O4 nanoparticles working as new intercalated material to prepare the GO-based membranes with high separation performance and energy-efficiency for the treatment of dye wastewater.

Declaration of competing interest 4. Conclusion The 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.

In summary, we reported a simple but effective strategy to fabricate GO/NH2-Fe3O4 membranes with high water flux and excellent separation performance for dye/salts mixture. Here, the NH2-Fe3O4 nanoparticles are not only regarded as nanospacer to tune the d-spacing of GO/NH2-Fe3O4 membranes but also as crosslinkers to improve the stability of membranes in aqueous solution. The resulted membranes display high water flux of up to 78 Lm−2 h−1, which is 4.8 times higher than that of pure GO membrane. In addition, the GO/NH2-Fe3O4-8 membrane also shows high congo red rejection (94%) and low NaCl rejection (~15%). The core of this work is to emphasize that the NH2-

Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 21576114), the Fundamental Research Funds for the Central Universities (JUSRP11933, JUSRP21936) and the Natural Science Foundation of Jiangsu Province (BK20190603). 9

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Fig. 10. Digital photos of the stability of pure GO and GO/NH2-Fe3O4-8 membrane in aqueous solution.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2019.114227.

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