Incorporating attapulgite nanorods into graphene oxide nanofiltration membranes for efficient dyes wastewater treatment

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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Incorporating attapulgite nanorods into graphene oxide nanofiltration membranes for efficient dyes wastewater treatment Cai-Yun Wang, Wen-Juan Zeng, Ting-Ting Jiang, Xi Chen, Xiao-Liang Zhang

⁎

Institute of Advanced Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene oxide Attapulgite nanorods Dyes wastewater Nanofiltration

Graphene oxide/attapulgite (GO/APT) composite membranes were successfully fabricated by the vacuum-assisted filtration for efficient dyes wastewater treatment. By characterization of FTIR, XPS, Raman, XRD and FESEM, APT nanorods were confirmed to be incorporated into GO laminar layers via grafting modification, which would influence GO interlayer distance (d-spacing), membrane surface microstructure (laminate morphology, structure, and hydrophilicity) and even water separation performance. Comparison of those of pristine GO membrane, the calculated d-spacing of GO/APT membranes gradually increased from 0.90 nm to 1.07 nm, while water contact angles decreased from 71.0° to 43.3° with the increasing APT/GO ratios. Moreover, GO/APT membranes exhibited rough hierarchical microstructure and higher surface hydrophilicity, which was in conjunction with larger interlayer spacing to synergistically improve separation performance. The water permeated flux increased from 3.4 of pristine GO membrane to 13.3 L m−2 h−1 of GO/APT membrane with preserving high rejection nearly to 100% for 7.5 mg L−1 Rh B wastewater under optimized conditions. Similarly, membrane thickness, dye concentrations and separating species in feed solutions were also found to affect membrane separation performance. Dye molecules were efficiently rejected through GO/APT nanofiltration membranes by the synergistic separation mechanism: size exclusion effect because of the unimpeded water channels formed into 3D network laminate structure, and electrostatic interactions between the oxygen-containing functional groups on membrane surface and charged molecules. Such these GO/APT membranes demonstrated efficiently separation properties and thus provided new insight into the potential applications in water purification and dyes wastewater treatment.

1. Introduction

three distinct mechanisms, i.e. molecular sieving, electrostatic interactions and dielectric exclusion [4,8–11]. NF membrane exhibits a high water permeated flux and reasonably high rejection ratio even under relative lower operating pressures, which is lower than that of reverse osmosis process [3]. Therefore, NF membrane separation technology is presently being widely exercised in various wastewater treatment applications for the removal of neutral or charged organic molecule and others [4–11]. As a typical 2D nanomaterial with a variety of oxygen-containing functional groups, graphene oxide (GO) provides a great prospect for fabricating separation membrane [4,8–12]. The 2D laminar nanochannels of GO membrane can be tuned by the adjacent interlayer distance (d-spacing) of GO sheets, which is an effective path for water molecules and blocks other species larger than the space [8,9]. Water transported through the nanochannels unexpectedly flows rapidly, which is substantially impermeable to other liquids and helium gas because of high capillary pressure and low frictional surface of the nonoxidized GO [8,9,13,14]. Therefore, GO membrane demonstrates

The rapid development of industry over the past twenty years has caused increasing wastewater pollution, which poses the serious threatening to human health and aquatic ecosystems [1,2]. Significant industry effluents such as dyes wastewater are discharged into rivers and lakes, which can be strongly accumulated along water and become one of the most severe environmental challenges [2–4]. Adsorption, filtration, biological treatment, chemical degradation and membrane separation technology (nanofiltration, reverse osmosis, electrodialysis, etc) are the most currently applicable methods for treatment of dyes wastewater [2–6]. Compared with others, membrane separation technology has been considered as more effective, lower cost, easier operation for wastewater treatment in the past decades, which is widely used in industry [6,7]. Nanofiltration (NF) is a pressure-driven separation process, which are rejected particles and dissolved molecules smaller than about 2 nm (molecular weight cutoff of about 200–1000 Da). The separation process of NF is usually governed by ⁎

Corresponding author. E-mail address: [email protected] (X.-L. Zhang).

https://doi.org/10.1016/j.seppur.2018.04.079 Received 30 December 2017; Received in revised form 8 April 2018; Accepted 28 April 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Wang, C.-Y., Separation and Purification Technology (2018), https://doi.org/10.1016/j.seppur.2018.04.079

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wastewater, especially with different charged dyes and neutral molecules, to better understand the influence of APT nanorods incorporating into GO membranes on the GO interlayer distance, membrane surface structure, and even water permeability [30–32]. Moreover, the effects between GO laminates/APT nanorods and dye molecules remain unclear for GO nanofiltration membranes. Herein, we propose a facile controllable fabrication of GO/APT composite membranes by APT nanorods intercalated into GO laminar layers to enhance water separation efficiency for dyes wastewater treatment. The effects of GO/APT loading ratios on d-spacing, surface microstructure (laminate morphology, structure, and hydrophilicity), and even water permselective performance for these membranes were investigated in details. Also, the separation mechanism through GO/ APT membranes was systematically discussed with different positively/ negatively charged dyes and neutral molecules.

high permselective performance for precise ionic and molecular sieving in various applications including gas separation, pervaporation separation and nanofiltration [15–20]. However, pristine GO membranes show fairly lower water flux during the nanofiltration process for wastewater treatment [9,14,21–26]. The d-spacing of GO laminates in air (dry state) is typical small about 0.9 nm. Recently, carbon nanodots [21], graphene oxide quantum dot [22], and carbon nanotubes [23–26] has been conveniently incorporated into GO layers to expand GO layer spacing thus improving water permeability. Although these carbonaceous materials display excellent compatibility with GO and good properties, they are more complex or expensive for preparation, and are not conducive to large-scale preparation of GO-based membranes. If inexpensive larger sized nanoparticles or nanofibers as spacers are intercalated into GO laminate layers with good compatibility, an enlarged more than 1-nm GO spacing may be achieved thus profitably enhancing water permeated flux [14]. Therefore, it is of great urgency to fabricate such a low-cost GO-based nanofiltration membrane with high water permeability for efficient dyes wastewater treatment application. Attapulgite (APT, also called palygorskite) is a naturally available 1D natural hydrophilic clay, which display the hydrated magnesium aluminum silicate structure with theoretical composition of Mg (Al)5Si8O20(OH)2(OH2)4·4H2O [27–30]. As shown in Fig. 1, it exhibits the typical nanorod-like structure with a diameter of ∼30 nm and a length range of 0.5–1 μm. And it contains plentiful hydrophilic eOH groups and composes of the tetrahedral and octahedral sandwich layered structures to form zeolite-like channels with a size of about 0.38 nm × 0.63 nm [28,29]. Such these channels could serve as a diffusion path for water transport and facilitate to construct straight-selective channels in APT-based hybrid membranes [30,31]. APT nanorods were introduced into alginate or polyvinylidene fluoride matrix to fabricate hybrid membranes, which presenting higher hydrophilicity, thermal stability and enhancing permeation selectivity. Recently, Zhao et al. [32] developed a free-standing GO-APT nanohybrid membrane for oil/water separation. APT nanorods intercalated into adjacent GO nanosheets could create 3-fold synergistical effects, i.e. rendering enlarged mass transfer channels, elevating hydration capacity, and creating hierarchical membrane nanostructures [32]. Compared with GO membrane, this nanohybrid membrane showed 7-fold permeated fluxes improvement, high separation efficiency and superior anti-oilfouling properties in a variety of oil-in-water emulsion. It exhibited the great prospects of GO-APT-based membranes for application in water purification and wastewater treatment. Unfortunately, there are still few reports on the fabrication and characterization of GO-APT-based membranes for water treatment, especially for dyes wastewater treatment. Nevertheless, it is still necessary to further investigate water permselective penetration properties and separation mechanism in dyes

2. Experimental 2.1. Materials Graphene oxide (JCGO-99-1-2, diameter of 1–5 µm, thickness of 0.8–1.2 nm, purity > 99%) was purchased from Nanjing JCNANO Technology Co., Ltd and attapulgite powder (JC-TW03, average size of 7 μm) was provided by Jiangsu Jiuchuan Nanomaterial Technology Co., China. Sodium hexametaphosphate (Na6P6O18), sucrose (C12H22O11), D-(+)-glucose (C6H12O6) and rhodamine B (C28H31ClN2O3, RhB) were obtained from Aladdin Industrial Corporation (Shanghai, China). Evans blue (C34H24N6Na4O14S4, EB), methylene blue (C16H18ClN3S, MEB), methyl blue (C37H27N3Na2O9S3, MB), concentrated hydrochloric acid (HCl), and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co. China, Ltd. All of the chemical reagents were of analytical grade without further purification. Deionized water was home-made from a Millipore system through the experiment. Polycarbonate filters (PC, diameter of 25 mm, average pore size of 0.22 µm, Isopore® GTTP02500, Millipore) were used as porous supports for preparation of GO/APT composite membranes. 2.2. Preparation of GO/APT composite membranes As shown in Fig. 2, attapulgite powder pretreated with acid [33] was dispersed in 3 wt% Na6P6O18 aqueous solution, followed by ultrasonication for 5 min to get a 0.4 g L−1 uniform APT dispersion. Simultaneously, GO nanosheets were also dispersed in water and treated ultrasonically to exfoliate a 0.4 g L−1 homogeneous brown GO suspension. Then, GO suspension and APT dispersions were mixed with a certain ratios (the mass ratios of GO/APT varied from 1/1 to 1/6) and vigorously stirred for 3 h at room temperature to ensure GO sheets

Fig. 1. (a) Structure (modified from [27]) and FESEM images of APT nanorods. 2

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Fig. 2. Schematic diagram of fabrication process for GO/APT composite membrane.

2.4. Separation performance measurements

intercalated by APT nanorods and thus formed GO/APT composite mixture suspension. Finally, the GO/APT composites were obtained by repeated centrifugation (CR21GIII, Hitachi, Japan) at 12000 rpm, washed with deionized water for several times. The resulting product dried at 50 °C for 24 h, which used as samples for the preparation of membrane and following characterization. The GO/APT composite membrane was deposited on PC support by the vacuum-assisted filtration method (Fig. 2), which was described elsewhere details [34–37]. The GO/APT composites generally were mixed with deionized water by ultrasonication to form 100 mL solution (0.1 g L−1) and the thickness of membrane separation layers was controlled by the filtration volume. Then, the as-prepared GO/APT membrane was dried in vacuum oven at 40 °C for overnight. For comparison, pristine GO membrane and pure APT membrane were also prepared by the same vacuum filtration method for characterization and separation performance tests.

The GO/APT composite membranes were carried out with a selfdesigned dead-end filtration device to evaluate separation performance for charged dyes solutions and organic matters solutions. The effective filtration area of membrane was 2.83 cm2. All the tests were carried out at room temperature and the transmembrane pressure difference was kept at about 0.09 MPa by a R300-type vacuum pump. The separation performance of the membrane can be characterized by permeated water flux (J, L m−2 h−1) and rejection rate (Rej, %) as shown below:

J=

V At

Rej % =

(1)

Cf −Cp Cf

× 100%

(2)

where V is the volume (L) of permeate collected over a period time (t, h), A is the effective membrane area (m2), Cf and Cp denote the concentrations of rejection molecules in the feed side and the permeate side, respectively. The concentration of organic dye was monitored by UV–vis spectrophotometer (JASCO V-750). The concentrations of sucrose and glucose were determined by a total organic carbon analyzer (TOC, TOC-VCPH, Shimadzu) with combustion oxidation-nondispersive infrared absorption method [11].

2.3. Characterization The surface and cross-sectional morphology of membranes were observed by field emission scanning electron microscope (FESEM, SU8020, Hitachi, Japan). The chemical composition and structure of GO, APT and GO/APT composite were characterized by Fourier transform infrared spectrometer (FT-IR, Nicolet 6700, Thermo, America), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi, Al Kα source, 1486.8 eV), and Raman spectrometer (LabRAM HR, Jobin Yvon, France). The crystalline properties and d-spacing of GO/APT composites and GO/APT membranes were analyzed by X-ray diffraction (XRD) using a Rigaku Ultima IV X-ray diffractometer with a Cu Kα source (40 kV, 20 mA, λ = 0.15418 nm). Thermogravimetry (TG) and derivative thermos- gravimetry (DTG) analysis were carried out on a TG 6300 (Seiko Instruments Inc-SII, Japan) simultaneous TG-DTA apparatus with a heating rate of 10 °C min−1 under N2 flow. Static water contact angle measurements for the GO/APT composite membranes were measured with a contact angle measurement instrument (JC2000C2, Shanghai Zhongchen Digital Technical Apparatus Co., China).

3. Results and discussion 3.1. Characterization of GO/APT composites and membranes Fig. 3 shows the FT-IR spectrum of GO, APT powder and GO/APT composite to verify the relevant functional groups. The broad peaks centered at about 3300 cm−1 correspond to the stretching vibrations of CeOH (OeH) and the adsorbed H2O. The GO characteristic absorption band at 1726 cm−1 is attributed to the C]O stretching vibration of carbonyl and carboxyl groups [34,38]. The absorption peaks at 1624, 1384, 1230 and 1053 cm−1 corresponds to the C]C from unoxidized sp2 CeC bonds, C]OH from carboxyl, the stretching vibration of CeOeC from epoxy, the stretching vibration of CeO from epoxy/alkoxy groups, respectively [24,39]. Such various oxygen-containing 3

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1654 1090 1032 990

-OH (O-H)

slight red shift in the peak position of CeO (alkoxy) [41]. It indicated that APT nanorods were successfully intercalated into GO nanosheets via grafting modification reaction with the oxygen-containing functional groups of GO and thus formed homogeneous GO/APT composites. XPS analysis suggested that the C/O ratio in GO/APT (1/4) composite was decreased to 0.28 from 2.06 of pristine GO (Fig. 4a). As illustrated in Fig. 4b, the C1s spectra of both samples can be fitted with four Gaussian peaks with soft of XPS Peak 4.1: 284.7 ± 0.1 eV (eCeCe/eC]Ce), 286.3 ± 0.3 eV (eCeOe/eCeOeCe), 287.4 ± 0.2 eV (eC]O), and 288.5 ± 0.3 eV (eOeC]O) [38,39]. The fractions of eC]O and eOeC]O were decreased from 7.6 and 12.8 at.% for pristine GO to 0.2 and 10.4 at.% for GO/APT, respectively (Table S1), which further confirmed that carbonyl and carboxyl groups were partially removed via grafting modification reaction. The corresponding spectrum of O1s and Si2p were also demonstrated the similar results. Compared with those of GO and APT, the formation of CeOeSi bonds appeared in the GO/APT composite (102.7 eV of Si2p and 533.0 eV of O1s, respectively), which verified that SieOH of APT assuredly reacted with the oxygen-containing groups of GO to generate GO/APT composites. Such XPS characterization further agreed with the aforementioned results of FT-IR. Furthermore, the Raman spectra of pristine GO and GO/APT composites shown in Fig. 5 can also corroborate the intercalation of APT nanorods into GO. The Raman spectrum of all samples displays the D band at ∼1338 cm−1 and G band at ∼1600 cm−1, which can attribute to carbon lattice defects/disorders and carbon domain of sp2 hybridization, respectively [12,42–44]. Generally, the integrated intensity ratio of the D- and G-bands (ID/IG) indicates the oxidation degree and the carbon lattice distortion in the graphitized structure. The values of ID/IG increased from 0.98 for GO to 1.07 for GO/APT (1/2) composite, 1.11 for GO/APT (1/4) composite, and 1.14 for GO/APT (1/6) composite, respectively. It indicated that more structural disorders were

APT

Transmittrance (a.u.)

GO/APT(1/6) GO/APT(1/5) GO/APT(1/4) GO/APT(1/3) GO/APT(1/2)

4000

3500

3000

2500

2000

1500

1053

1726 1624

GO

1384 1230

GO/APT(1/1)

1000

500

-1

Wavenumber (cm ) Fig. 3. FT-IR spectra of GO, APT and GO/APT composites.

groups of GO would provide active sites for grafting reaction between GO and APT. Meanwhile, APT spectrum presents dominant adsorption bands at 1654, 1032 and 990 cm−1, respectively, which are attributed to the non-freezable water, SieOH, and Al(Mg)eOH groups in the APT framework, respectively [30,40,41]. Compared with those peaks of pristine GO and APT, the oxygen-containing characteristic peaks of GO/ APT composites become weakened or disappear (e.g. 1726, 1624 cm−1) and the peaks at 1032 cm−1 of GO/APT composites become more intensive with increasing loading of APT nanorods, which is ascribed to new shoulder peaks appeared at about 1090 cm−1 (CeOeSi) with a C: 17.1% O: 60.1% Si: 17.6% Mg: 5.2%

Intensity (a.u.)

Mg1s

O1s

GO/APT C1s

(b)

C-C/C=C

Si2p

O: 70.3% Si: 22.6% Mg: 7.1%

C-O/C-O-C

Intensity (a.u.)

(a)

APT

O-C=O

C=O

GO/APT C-O/C-O-C C-C/C=C C=O O-C=O

C: 67.2% O: 32.8%

GO

GO 1400

1200

1000

800

600

400

200

0

292

290

Binding energy (eV)

(c)

(d)

282

280

C-O-Si

Intensity (a.u.)

Intensity (a.u.)

GO/APT Si-O-Si

APT

GO/APT O-Si-O

OH

C-O-C

C=O

534

284

O-Si-O

OH

536

286

OH

C-O-C/C-O-Si/Si-O-Si

C=O

538

288

Binding energy (eV)

532

530

APT

GO 528

108

526

106

104

102

100

98

Binding energy (eV)

Binding energy (eV)

Fig. 4. (a) XPS survey of GO, APT, GO/APT(1/4) composite, and the corresponding (b) C1s, (c) O1s, and (d) Si2p spectra. Raw data (black line), background (black dash line), the fitting envelope (red line), and deconvolved peaks (blue lines) are presented in the figures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4

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D

Therefore, GO sheets are interlaced with each other in a parallel manner producing the laminar structures and APT nanorods are intercalated into the stacked GO lamellar layers to form stable CeOeSi bonds establishing the corrugations and nanochannels (3D network laminate structure) for fast water transport (also see Fig. 12). Moreover, it should be noted that these intercalation process of APT nanorods into GO was different from the simple randomly mixed process with GO powder and APT nanorods (GO-APT mixtures, see Fig. 6b) [46]. Similarly, except for characteristic peaks of PC supports, the GO/APT composite membranes also showed a relative strong diffraction at 2θ = 8–10° (Fig. 6c), which was gradually decreased from about 10° to 8° with the increasing APT loading. According to the Bragg’s equation (2dsinθ = nλ), the corresponding calculated interlayer spacing (d-spacing) gradually increased from approximately 0.90 nm for pristine GO membrane to 0.91 nm for GO/APT (1/1) membrane, even to about 1.07 nm for GO/APT (1/4) membrane (Fig. 6d). The enlarged shifting of d-spacing also obviously confirmed the successful incorporating APT nanorods into GO laminar nanosheets [32]. Therefore, these results of FT-IR, XPS, Raman and XRD indicated that it was highly compatible for APT nanorods intercalation into GO laminate structure, and the enlarged interlayer spacing and 3D network laminate structure of GO/ APT composite membranes would significantly improve the water permeability compared with that of pristine GO membrane.

G GO/APT(1/6)

Intensity (a.u.)

(ID/IG=1.14) GO/APT(1/4) (ID/IG=1.11) GO/APT(1/2) (ID/IG=1.07)

GO (ID/IG=0.98)

1000

1200

1400

1600

1800

2000

-1

Raman shift (cm ) Fig. 5. Raman spectra of GO and GO/APT composites.

formed during the incorporating process with increasing APT intercalation for GO/APT composites [32,43]. As shown in Fig. 6a, GO showed a broad diffraction peak at 2θ = 10.34° due to the irregular stacking from π-π interactions and hydrogen bonding, and APT nanorods exhibited a dominant peak at 8.22° corresponding to its crystal plane (1 1 0) [31,32,40,45]. However, for GO/APT composites with different GO/APT ratios, GO typical diffraction peak gradually disappeared and they just displayed one dominating composite peak at about 8.30–8.50° between 8.22° (APT) and 10.34° (GO), suggesting that these GO/APT composites existed a certain interaction during the APT intercalation process. In this work, the lateral size of GO sheets (1–5 µm) is larger than the bundle diameter of APT nanorods (∼30 nm), thus GO nanosheets would totally sandwich the APT to form 3D network laminar membrane as shown in Fig. 2.

3.2. Separation performance of GO/APT composite membranes The pure water flux and separation performance (J and Rej %) in dyes solutions were used to evaluate permeation performance of GO/ APT composite membranes. A series of pristine GO and GO/APT composite membranes with approximately same loading (∼1.75 g m−2) were fabricated with different APT/GO mass ratios from 0 to 6. As shown in Fig. 7a, pure water flux of pristine GO membrane was 5.1 L m−2 h−1, while those of GO/APT composite membranes increased

(a)

300

(b)

GO/APT composite GO-APT mixture

250

Intensity (a.u.)

APT

Intensity

GO/APT(1/6) GO/APT(1/5) GO/APT(1/4) GO/APT(1/3)

200 150 100

GO/APT(1/2) GO/APT(1/1)

50

GO

5

10

15

20

o

25

30

0

35

5

10

15

2θ ( )

20

o

25

30

35

2θ ( )

(c)

1.2

d-spacing (nm)

Intensity (a.u.)

APT m GO/APT(1/6) m GO/APT(1/5) m GO/APT(1/4) m GO/APT(1/3) m GO/APT(1/2) m

(d)

1.1

1.0

0.9

GO/APT(1/1) m GO m PC support

5

10

15

20

o

25

30

0.8

35

2θ ( )

1/0

1/1

1/2

1/3

1/4

1/5

1/6

GO/APT m (wt/wt)

Fig. 6. XRD patterns of (a) GO, APT powder and GO/APT composites, (b) random GO-APT mixtures and GO/APT composite (1/2), and (c) pristine GO, pure APT and GO/APT composite membranes, and (d) the corresponding d-spacing of GO and GO/APT composite membranes. 5

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(a)

100

(b)

50

20

J (L m h )

-2

95

-2

-1

40 30

15 90 10

Rej%

-1

Pure water flux (L m h )

60

20

0

85

5

10

1/0

1/1

1/2

1/3

1/4

1/5

0

1/6

1/0

1/1

1/2

1/3

1/4

1/5

1/6

80

GO/APT m (wt/wt)

GO/APT m (wt/wt)

Fig. 7. Influence of GO/APT ratios on (a) pure water flux and (b) separation performance in Rh B dyes solution (C0 = 7.5 mg L−1) for pristine GO and GO/APT composite membranes.

80

O

Water contact angle ( )

with the increasing APT/GO mass ratios. For example, the average flux of 9.7, 31.0 and 57.8 L m−2 h−1 were achieved for GO/APT (1/1), GO/ APT (1/4) and GO/APT (1/6) composite membranes, respectively. The highest flux was over 10 times higher than that of pristine GO membrane. Similarly, as shown in Fig. 7b, there were analogous trends for separation performance of these membranes in Rh B dyes solution. Pure APT membrane demonstrated the permeated water flux as high as 361.8 L m−2 h−1 and rejection ratio of 33.4%, respectively (not shown in Fig. 7). However, the permeated flux of just 3.4 L m−2 h−1 and rejection ratio of almost 100% were obtained for pristine GO membrane. For comparison, the flux of GO/APT composite membranes increased with the increase of APT/GO ratios from 5.6 L m−2 h−1 (GO/APT, 1/1)) to 13.3 L m−2 h−1 (GO/APT, 1/4) while the rejection remained changeless over 99.9%. Further increase of APT/GO mass ratio to 6, the permeated flux was enhanced to 16.2 L m−2 h−1 and the rejection slightly decreased to 97.0%. It might be contributed to the d-spacing, surface hydrophilicity and microstructural morphology for these GObased membranes. As APT nanorods were incorporated into adjacent GO layers, the dspacing of the resulting membranes increased from 0.90 nm for pristine GO membrane even to 1.07 nm for GO/APT (1/4) membrane (Fig. 6d). Considering the graphene sheets thickness of a ≈ 0.34 nm [9], the “empty” space (δ) of width between the GO-based layers should be as δ = (d–a) = 0.56 to 0.73 nm, which is available for molecules with various sizes to separating diffuse. These values of δ are between the kinetic diameters of water (∼0.26 nm) [38] and other molecules such as Rh B dyes (> 1.40 nm). Therefore, it is full of water molecules into the GO nanocapillaries and then water selectively permeate to the other side of GO-based membranes. According to the Hagen–Poiseuille equation, higher d-spacing indicates higher permeated water flux as shown in Fig. 7. Moreover, as shown in Fig. 8, the contact angle of a water droplet in air at pristine GO membrane surface was around 71.0°. Compared with that of GO membrane, water contact angles of GO/APT membranes decreased with APT nanorods loading even to ∼43.3° (GO/ APT, 1/6), indicating the highly hydrophilic nature of GO and GO/APT composites. Lower water contact angles demonstrated stronger hydration capability, thus showing the enhanced water permeability through these membranes in Fig. 7 [32,47]. Furthermore, the increasing APT loading decreased the water contact angles and increased surface hydrophilicity for these GO/APT membranes, which could be explained by the gradual accumulation of the microscale roughness of membrane surface. Compared with the wrinkled rough surface of GO membrane (Figs. 9b and S1a), the hierarchical nanostructure with more rough, clustered even accumulated surface were emerged for GO/APT membranes in Figs. 9 and S1, thus gradually deceased water contact angles with increasing APT loading. For example, a ∼450 nm-thick rougher selective layer was uniformly deposited on PC support with plenty of macropores and plenty of GO modified APT nanorods distributed on

60

40

20

0

1/0

1/1

1/2

1/3

1/4

1/5

1/6

GO/APT m (wt/wt) Fig. 8. Water contact angles of GO/APT composite membranes.

membrane surface without obvious defects, exhibiting a consecutive layered nanostructure for the GO/APT (1/4) composite membrane (Figs. 9c, d and S1c). Also, these excessive stacking of APT nanorods into GO/APT (1/6) composite membrane would appear some pinholes on membrane surface (Fig. S1d), which result in slightly decline of Rh B rejection as shown Fig. 7b. Incorporating APT nanorods into GO would decrease hydrogen bonds between GO laminar sheets, thereby allowing water molecule preferentially permeate through the nanochannels formed at the edges of the GO nanosheets [48]. It was also suggested that the more rough hierarchical microstructure (3D network laminate structure) and higher surface hydrophilicity of membrane surfaces in conjunction with larger interlayer spacing synergistically improved water permeated flux for these GO-based membranes. Among these membranes, the GO/APT (1/4) composite membrane showed relatively higher flux and rejection nearly to 100%, which was also selected for further investigation as following. Fig. 10 showed the effect of GO/APT composites loading amount and Rh B feed concentration on separating performance of membrane. The thicknesses of these membranes were easily controlled by varying the volume of filtrate (i.e. GO/APT composite loading) during the vacuum filtration process [19,49,50]. As seen in Fig. 10a, both pure water and permeated water flux rapidly decreased with the increasing loading and then trended to be essentially steady when the loading exceeded 1.75 g m−2. On the contrary, the Rh B rejection in 7.5 mg L−1 feed solutions gradually increased with the loading amounts and then displayed a stable tendency over 99.0%. The influence of Rh B feed concentration on the separation performance of GO/APT (1/4) composite 6

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Fig. 9. FESEM images of (a) PC support, (b) pristine GO membrane, and (c, d) GO/APT (1/4) membrane.

membrane with optimized loading (1.75 g m−2) was also performed. As shown in Fig. 10b, both permeation flux and rejection firstly declined and then trended invariable as function of Rh B feed concentration with ranges of 0–60 mg L−1. For example, the permeated flux and rejection were achieved to 13.3 L m−2 h−1 and over 99.9% in the 7.5 mg L−1 Rh B solution, respectively. Moreover, this membrane could easily maintain separation performance in the relatively long-term runs over 10 h without loss of permeated flux and rejection. Even toward 60 mg L−1 Rh B solution, the permeated flux was as high as ∼9.9 L m−2 h−1 and the rejection was about 80%. It was attributed to unpermeated RhB molecules accumulation or fouling on the membrane surface to result in concentration polarization and even blocking transport pathways of water molecules through the laminar membranes. Even so, the membrane still showed relatively high separation performance due to electrostatic interactions as discussed in Section 3.3 [10]. After the tests

As for separation mechanism of nanofiltration membranes, size exclusion (molecular sieving) and electrostatic interaction are two main effects. The separation performance of optimized GO/APT (1/4) composite membrane for removing organic molecules were performed to

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3.3. Separation mechanism of GO/APT composite membranes

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with 60 mg L−1 Rh B solution as feed, the membrane was soaked in HCl solution (HCl:H2O = 1:1, wt%) for 1–3 h to remove the dyes on the membrane surface. It was found that the membranes exhibited good acid resistance and could restore high rejection rate nearly to 100%. Moreover, these GO/APT (1/4) composite membranes shows higher thermostability than pristine GO membranes over 200 °C as shown in Fig. S2, which is sufficient for the practical application in wastewater treatment.

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Fig. 10. Influence of (a) GO/APT composites loading amount, and (b) Rh B feed concentration on the separation performance for GO/APT (1/4) membranes. 7

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hydrophilic eOH group and Al(Mg) cations can react with the oxygen functional groups of GO nanosheets to induce CeOeSi bonds (alkoxide), and then to form a 3D network laminate structure (see Figs. 2 and 12). These oxidative groups on the membrane surface will produce strong electrostatic interactions and hydrogen bonding of water molecules, and it will also exist to a strong repulsive barrier to hinder organic molecules penetration at water/membrane interfaces [11]. Thus, such chemical and electrostatic interactions between these functional groups and organic molecules are responsible for the selective penetration properties of GO/APT composite membranes [4,10,11,32,55]. As illustrated in Fig. 12, water molecules will rapidly pass through the unimpeded “empty” interlayer spacing of GO/APT membrane (∼0.7 nm) or along the open channels of incorporating APT nanorods (∼0.38 nm) [30], while these charged dye or neutral molecules are rejected through the membrane. As an experimental result, the electronegative GO/APT membrane exhibits much higher permeated flux and rejection rate for electropositive dyes than those for electronegative dye molecules. Moreover, dye molecules with relatively larger size (molecular weight) and/or higher charge will be more efficiently rejected by these negatively charged GO/APT composite membranes. It further indicates that organic molecules are efficiently excluded through GO/APT composite membranes by the synergistic effects of molecular sieving and electrostatic interactions in this work. Such GO/ APT composite membranes with well-defined 3D network laminate nanostructure and high water permeable properties can be employed for applications in water purification and dyes wastewater treatment.

Table 1 The separation performance of optimized GO/APT composite membranes for removing different organic molecules. Molecule

Molecular weight (Da)

Molecular size (nm)

Charge

J (L m−2 h−1)

Rej %

RhB MEB EB MB Sucrose Glucose

479 320 961 800 342 180

1.8 × 1.4 1.4 × 0.6 1.2 × 3.1 2.0 1.0 0.7

positive positive negative negative neutral neutral

13.3 11.9 9.4 8.2 5.4 6.6

> 99.9 99.7 79.2 69.9 93.3 83.7

Note: feed concentration, 7.5 mg L−1 for dye solutions, 10 mmol L−1 for sucrose and glucose solutions.

investigate separation mechanism with three serial organic species, namely positively charged dyes of RhB and MEB, negatively charged dyes of EB and MB, and neutral molecules of sucrose and glucose. As seen in Table 1, there is no apparent correlation between the separation performance and molecular weight for these charged dyes and neutral molecules. For example, glucose, sucrose, RhB and EB have molecular weights of 180, 342, 479, and 961 Da, respectively, but their permeated flux and rejection are 6.6, 5.4, 13.3, 9.4 L m−2 h−1, and 83.7%, 93.3%, 99.9%, 79.2%, respectively. For neutral species, the diameter of sucrose (1.0 nm) is larger than that of glucose (0.7 nm) [11] and thus sucrose has relatively lower flux and higher rejection rate, which is in agreement with previous Mi’s work [11]. It also indicates that size exclusion plays an important role in the nanofiltration process. However, the removal of charged dyes species through the membrane cannot be fully explained by molecular size. Unlike neutral molecule, RhB and EB are ionic-type species, which can dissociate chloride or sodium ion. The rejection performance of dye molecules maybe is related to three-dimensional size of molecules and charge effects [51] from the comparison photos in Fig. S3. Actually, even though negatively charged dye molecules of EB (1.2 × 3.1 nm) and MB (2.0 nm) have larger size than those of RhB (1.8 × 1.4 nm) and MEB (1.4 × 0.6 nm) [52–54], they still show much lower permeated flux and rejection (< 80%, Table1). Like pristine GO membrane, the GO/APT composite membrane was negatively charged in aqueous solution with a wide pH range of 3–12 and its surface charge density significantly trended to decay with pH (Fig. 11). Therefore, electropositive dye molecules such as RhB and MEB could be preferential effectively captured by GO/APT composite due to π-π stacking and electrostatic interactions, which resulted in the tight binding between dyes molecules and GO-based nanosheets [4]. As previously discussed in Section 3.1, APT nanorods containing more

4. Conclusions The high rejection of organic dye molecules and water purification properties of GO/APT composite nanofiltration membranes prepared via a simple vacuum filtration method were demonstrated through the controlled assembly of APT nanorods incorporating into the interlayers of GO nanosheets. APT nanorods were intercalated into the stacked GO lamellar layers to form stable CeOeSi bonds establishing 3D network laminate structure via grafting modification reaction, which would significantly improve the water permeability. Compared with pristine GO membrane, GO/APT membranes exhibited a 4-fold water flux improvement with maintaining high rejection nearly to 100% for Rh B wastewater under optimized conditions. It might be contributed to the d-spacing, surface hydrophilicity and 3D network laminate microstructural morphology for these membranes. Moreover, the separation mechanism through GO/APT membranes was systematically investigated with different positively/negatively charged dyes and neutral molecules. The size exclusion effect and electrostatic interactions were synergistically illustrated to be responsible for the efficiently permselective penetration properties of these GO/APT membranes. It indicates that such GO/APT composite nanofiltration membranes are a promising candidate for water purification and dyes wastewater treatment in future applications.

0

Zeta potential (mv)

GO GO/APT (1/4) composite -10

-20

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21766011, 21566012), Jiangxi Provincial Department of Science and Technology (Grant Nos. 20151BDH80012, 20162BCB23025, 20171BAB203020), and in part by the National Undergraduate Training Programs for Innovation and Entrepreneurship (Grant No. 201710414009).

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Appendix A. Supplementary material

12

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.seppur.2018.04.079.

Fig. 11. Zeta potentials of GO and GO/APT (1/4) composite membranes. 8

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Fig. 12. Schematic representation of water selective permeation through GO/APT composite membrane.

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