carbon nanotube hollow fiber membrane with high forward osmosis performance

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

A novel reduced graphene oxide/carbon nanotube hollow fiber membrane with high forward osmosis performance Xinfei Fan, Yanming Liu, Xie Quan

⁎

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

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

A R T I C LE I N FO

A B S T R A C T

Keywords: Forward osmosis Water treatment Carbon nanotube Reduced graphene oxide Hollow fiber membrane

Forward osmosis (FO)-based water treatment and desalination processes have attracted increasing attention to address the global water crisis, but its practical application is restricted by the lack of FO membranes with high permeability and selectivity. In this work, an all nanocarbon-based FO membrane was successfully fabricated via constructing reduced graphene oxide (RGO) on a carbon nanotube (CNT) hollow fiber substrate via electrophoretic deposition coupling with chemical reduction processes. Due to the ultra-low friction and well-defined interlayer spacing, the RGO active layer provided high water permeability and ion selectivity. Meanwhile, the high porosity and good wettability ensured the CNT hollow fiber substrate with low internal concentration polarization, and thus increasing water flux. During against DI water feed using 0.5 M NaCl draw solution, the prepared RGO/CNT membrane presented an outstanding water flux of 22.6 LMH, which is 3.3 times higher than that of the commercial membrane. Meanwhile, its reverse salt flux was only 1.6 gMH in comparison to 2.2 gMH for the commercial membrane. These results indicate that the all nanocarbon-based membrane is an alternative membrane for providing clean water in the FO process.

⁎

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

https://doi.org/10.1016/j.desal.2018.07.020 Received 12 February 2018; Received in revised form 29 April 2018; Accepted 25 July 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Fan, X., Desalination (2018), https://doi.org/10.1016/j.desal.2018.07.020

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1. Introduction

tunable thickness (t) [25, 26]. Such features ensure these CNT membranes with a potentially low S (S = t·τ/σ). Meanwhile, introducing hydrated functional groups can significantly improve the hydrophilicity of the CNT membranes, which would lower the transport resistance for water and solute. Thus, the CNT membrane might be an alternative support for developing all nanocarbon-based high-performance FO membranes with a RGO-based active layer. To confirm this, an all nanocarbon-based FO membrane was designed by coating a RGO active layer onto the outer surface of CNT hollow fiber support through electrophoretic deposition and chemical reduction. The effects of voltage and deposition time on the thickness of RGO layer were studied. The FO performance of the membrane was investigated and taken in comparison to the nanocarbon-polymer FO membranes with either RGO active layer or CNT substrate.

Forward osmosis (FO) has been gaining popularity to address the worldwide water crisis by seawater desalination and wastewater treatment [1–4]. Unlike the pressure- and thermally driven membrane processes, osmotically driven FO is a spontaneously natural phenomenon with no pressure requirement and low energy consumption. However, despite advancing its practical applications by extensity efforts, developing high-performance membrane is still the primary concern for FO processes [5–7]. Recently, nanomaterials and nanotechnologies have been a term increasingly used in designing and preparing novel membranes with high permeability, selectivity and antifouling ability [8]. In particular, nanocarbons have drawn much attention in developing high-performance membranes (including FO) [9–14]. However, such FO membranes are mostly fabricated by blending nanocarbons into various polymeric matrixes. Constructing all nanocarbon-based FO membrane was rarely reported, despite it might open a new avenue for developing next-generation high-performance FO membranes, thanks to their specially topological structures and exceptional water transport properties. The conventional FO membranes possess a typically asymmetric structure with a dense active layer coated onto a highly porous support [15–17]. Such structure feature offers the possibility to optimize active layer and porous support separately for developing high-performance FO membrane. For the theoretical active layer of FO membranes, it should possess both high water transport rate and salt rejection rate [3, 7]. Reduced graphene oxide (RGO), a typical two-dimensional nanocarbon, has been a term increasingly in constructing lamellarly structured membrane for removing ions and organic molecule from water [18, 19]. The selectivity of such stacked lamellar membranes can be easily tuned by reduction degree of RGO or size of inserted crosslinkers/spacers between the 2D nanosheets [20–23]. Due to the presence of oxygen-containing groups and smoothly graphitic inner walls, the stacked laminates present water preferential adsorption and ultrafast water transport rate [24]. Thus, the stacked RGO laminates may be an alternative active layer in constructing asymmetric FO membrane. Besides, RGO membranes are expected to be economically scalable fabrication because of its easy mass production from simple chemical reduction after oxidizing and exfoliating the inexpensive graphite. Unfortunately, the conventionally polymeric support might dramatically weaken the FO performance due to potentially severe internal concentration polarization (ICP) [3, 7]. ICP is considered as the major obstacle for asymmetric FO membranes, which might result in approximately 80% flux loss especially under high draw solution. As the ICP is positively related to the structure parameter (S), a novel support layer with low S is favored for asymmetric FO membrane. As another fascinating nanocarbon, one-dimensional carbon nanotubes (CNTs) have also been used in constructing membranes over the past decades. Unlike conventional polymer membranes from phase inversion, the entangled CNT membranes present an interconnected scaffold architecture with high porosity (σ), low tortuosity (τ) and

2. Experimental 2.1. Materials N,N-dimethylformamide (DMF, anhydrous, 99.8%), sodium chloride (NaCl) and polyvinyl butyral (PVB) were purchased from Sinopharm Chemical Reagent Co., Ltd. Multi-walled carbon nanotubes (MWNTs, diameter: 60–100 nm; length: 5–15 μm) were supplied by Shenzhen Nanotech Port Co. Ltd. Unless otherwise specified, all chemicals with analytical grade were used as received without further purification. A commercially asymmetric cellulose triacetate (CTA) FO membrane (thickness in 50 μm) was purchased from Hydration Technologies Inc., because it receives popular recognition as the benchmark membrane in developing new FO membranes. 2.2. Fabrication of CNT hollow fiber substrate The CNT hollow fiber was prepared through a wet-spinning method coupling with a pyrolysis process. Briefly, 1.0 g oxidized MWNTs from chemical oxidation were dispersed homogeneously in PVB/DMAc (0.5 g/8.5 g) solution to yield a uniform dope suspension. After degasification for 24 h, the suspension was dispensed through a spinneret and directly immersed into a water coagulation bath by using an injection pump at room temperature. DMAc/water (v/v, 40/60) was used as the bore-fluid. Finally, the final CNT hollow fiber was obtained after PVB pyrolysis at 800 °C for 1 h in Ar atmosphere. 2.3. Preparation of RGO active layer Graphene oxide (GO) was firstly fabricated from a modified Hummers method [27], and then exfoliated in DI water (appr. 1.0 mg/ mL) under ultrasonication for 4 h. A homogeneous GO suspension was obtained after a centrifugation process at 8000 rpm for 5 min. The GO/ CNT hollow fiber membrane was obtained via an electrophoretic deposition method (Fig. 1). The electrophoretic deposition was performed in a titanium tube (2.0 cm in diameter and 12 cm in length) with one

Fig. 1. Schematic diagram for fabricating RGO/CNT hollow fiber membrane. 2

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(a)

(c)

(b)

128 μm

915 μm 250 μm

(e)

(d)

50 μm (f)

2.50 μm

2.50 μm

2.50 μm

Fig. 2. Photograph (a) and SEM images of CNT hollow fiber substrate: cross-section (b, c and d), outer surface (e) and inner surface (f).

end sealed with Taflon. A direct current (DC) voltage of 3.5 V was applied between a titanium (Ti) cathode and CNT hollow fiber anode. After deposition for 30 s, the sample was chemically reduced by introducing HI vapor into its lumen side for 5 min.

salt diffusion coefficient, osmotic pressure of draw and feed solution, respectively.

2.4. Characterization of hollow fiber membrane

The FO performance of the synthesized RGO/CNT hollow fiber membrane was investigated in a lab-scale cross-flow filtration module. The lab-scale module was prepared with polypropylene (PP), which has a lumen diameter of about 1.5 cm and lumen length of ~7.0 cm. The hollow fiber membrane with surface area of 2.0 cm2 was installed parallelly in the lumen center of the module (Fig. S2). The feed (DI water) and draw solution (0.5 M NaCl) were simultaneously pumped into the membrane module at flow rate of 25 cm/s and temperature of 25 °C under the FO mode (RGO active layer facing DI water and CNT substrate facing 0.5 M NaCl solution). The water flux (Jw, L/(m2·h), abbreviated LMH) was calculated by the change in feed solution weight as follows:

S=

The microstructure and morphology of CNT hollow fiber and RGO/ CNT membrane were characterized by using field emission scanning electron microscopy (FE-SEM, Hitachi S4800, 5 kV). Zeta potentials of GO at different pH were measured through a Malvern nano zetasizer equipment. The water contact angle (WCA) was measured on an optical contact angle & interface tension meter (KINO SL 200 KB) with 2 μL water droplet. Fourier transform infrared spectrophotometer (FT-IR) was used to characterize the chemical groups on the samples in the wavelength range of 500–4000 cm−1. The nanochannel information (i.e. interlayer spacing) was determined by X-ray diffractometry (XRD, Shimadzu LabX-6000) with a Cu Kα radiation at 40 kV and 30 mA over the 2θ range of 5–40° with a scanning speed of 4°/min. X-ray photoelectron spectroscopy was performed by using a VG ESCALAB 250 spectrometer with a focused monochromatized Al Kαsource (hv of 1486.6 eV). Raman spectra were recorded with a Renishaw MicroRaman system 2000 spectrometer with He-Ne laser excitation (wavelength of 623.8 nm).

Jw =

Js = The transport properties (including water permeability coefficient (A), salt rejection rate (Rs), and salt permeability coefficient (B)) and structural parameters (S) of the prepared membranes through the reported standard protocols [6]. Briefly, A was determined from pure water flux (25 °C) under trans-membrane pressure (ΔP) of 1.0 bar. Rs was studied by monitoring the conductivity differences between feed and permeate water by filtering 500 mg/L NaCl solution at 25 cm/s cross-flow rate under 1.0 bar. Both A and Rs were obtained by using a stainless steel module in a pressured cross-flow RO mode (Fig. S1). B, an intrinsic property of FO membranes, was calculated based on the average rejection value (3 replicates) at a given pressure using the solution-diffusion theory:

(

1 − Rs Rs

⎜

⎟

Δm ρ⋅M⋅Δt

where Δm (kg) denotes the weight of produced water in predetermined time Δt (h). M is the membrane surface area (m2). ρ is the water density. Change of conductivity in feed was monitored and used to calculate the salt reverse-diffusion (Js, g/(m2·h), gMH) via a NaCl calibration curve.

2.5. Membrane properties and performance evaluation

B = Jw ×

D A × πdraw + B ⎞ × ln ⎛ × JW A πfeed + B + Jw ⎠ ⎝

(Ct⋅mt − C0⋅m 0 ) ρ⋅M⋅Δt

where m0 and C0 is the initial feed weight (kg) and salt concentration (g/L), while mt and Ct denote the feed weight and salt concentration after predetermined time Δt (h). 3. Results and discussion 3.1. Microstructure of CNT hollow fiber substrate According to the SEM image in Fig. 2a, the prepared hollow fiber substrate is in black color due to the presence of CNTs. Its outer diameter is about 915 μm (Fig. 2b). Fig. 2c displays that typical finger-like voids can be found in the cross-section, and the wall thickness is about 128 μm. This typical hollow fiber structure is attributed to the instantaneous precipitation of PVB contacting with water bath, consistent with the conventional polymeric membranes from phase inversion [6,

) × exp (− )where Δπ denotes the osmotic pressure Jw k

difference across the membrane. k is the mass transfer coefficient for the crossflow channel of the RO membrane cell. The S value was determined by the following equation in which D, πdraw and πfeed are the 3

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16, 28]. More importantly, such hollow fiber structure did not collapse after the PVB pyrolysis at high temperature. Meanwhile, the high-resolution SEM images reveal fascinatingly interconnected scaffold structures by the entangled CNTs in the cross-section, outer and inner surfaces (Fig. 2d–f). For various membrane processes, both the fingerlike voids and interconnected pores can significantly increase membrane porosity, which then lowers the water transport resistance and increases water flux. By measured through the gravimetric method, the CNT hollow fiber substrate showed a porosity of 91%, which is much higher than 40% and 60–80% for the ceramic and polymeric membranes, respectively [29–31]. Meanwhile, the substrate presented a mean pore size of 194 nm, suggesting it is a microfiltration membrane. As a result, the CNT hollow fiber possessed a high water permeability near to 8500 L·m−2·h−1·bar−1, suggesting low resistance to water transport.

3.5 V voltage. Fig. 5a displays that a short deposition time of 30 s results in a thin RGO layer with thickness of 51 nm. When the deposition time increased to 60 s, a 79 nm RGO layer was constructed. Further increasing the time to 120 s achieved a RGO layer in 135 nm thickness. Therefore, it can be summarized that the thickness of the RGO layer can be easily controlled by adjusting the deposition time or applied voltage. The surface wettability of the CNT hollow fiber substrate, GO and RGO membranes was studied by the water contact angle (WCA). Fig. 6 shows that the CNT hollow fiber is hydrophilic with WCA of 37.8 ± 0.8°, a desirable property to lower ICP for developing high-flux FO membranes [35–37]. The pristine GO layer displays a static WCA of 33.4 ± 0.4°. This is attributed to the presence of highly hydrophilic oxygen-containing groups (e.g. eCOOH, eOH) on the GO nanosheets. After chemical reduction, as comparison, the RGO layer yields a high WCA, 85.9 ± 1.4°. This value is much lower than that of the reported graphene-based materials [38], illustrating that the RGO layer is still somewhat hydrophilic. So that, the water molecules can access the bulk of such RGO layer. To confirm the oxygen-containing groups were reduced by chemical reduction, FTIR was carried out and taken in comparison between GO and RGO. FTIR spectrum in Fig. 7 reveals that the GO sample clearly represents some typically hydrophilic functional groups. The broad band centered at 3430 cm−1 and peak at 1640 cm−1 are assigned to the eOH stretching vibration from the absorbed H2O. The peaks at 2930 and 2850 cm−1 are attributed to the eCH2e groups. The adsorption peak at 1750, 1380, 1250, and 1100 cm−1 refer to the C]O stretching vibration of carbonyl group, C-OH stretching of carboxyl group, CeOeC stretching of epoxy group and C]O vibration of alkoxy group, respectively. In contrast, the intensity of carboxyl groups at 1380 cm−1 nearly disappeared with respect to RGO sample. Meanwhile, the bands of epoxy and alkoxy groups became weaker. These results suggest that carboxyl and epoxy groups on GO was preferentially reduced during the chemical reduction process. However, the existence of strong C]O groups in the RGO nanosheets implies that the pristine GO was partially reduced. The variation of elemental atomic contents as well as functional groups of the GO and RGO laminates was determined by XPS analysis. It can be found that the C/O ratio in RGO was increased to 2.8 from 2.0 of pristine GO. This might be attributed to the loss of oxygen-containing groups during chemical reduction process. To confirm this hypothesis, peak-fit processing was performed on the C1s spectra of both samples. According to the results in Fig. 8, the C1s was resolved into five Gaussian peaks. The deconvoluted peak located at 284.7 eV is attributed to the eC]Ce and eCeCe bonds. The deconvoluted peaks centered at 285.2, 286.7, 287.1, and 288.7 eV are assigned to the eCeOH, eCeOeCe, eC]O, and eO]CeOH bonds, respectively. It is noteworthy that the oxygen-containing carbonaceous bond ratios of the eOeC]O and eCeOeC bonds for RGO decrease comparing to the corresponding bonds on the GO sample. This result further verifies that epoxy and carboxyl groups partially lost after chemical reduction, consistence with the result in FT-IR spectrum. The remained oxygen-

3.2. Morphology and properties of RGO/CNT hollow fiber membrane The final RGO/CNT hollow fiber FO membrane was prepared by HI reduction after GO deposition onto the outer surface of the CNT hollow fiber substrate through an electrophoretic deposition process. Prior to the electrophoretic deposition, zeta potentials of GO were firstly measured and presented in Fig. 3a. It reveals that GO is negatively charged due to the deprotonation of the carboxyl group at the edges of GO flakes. Meanwhile, the electronegativity is gradually enhanced in the range of pH 3–11. Here, GO was deposited onto the anodic CNT hollow fiber by electric field force at pH 7 during the electrophoretic process, followed by HI reduction. As displayed in the SEM image (Fig. 3b), the final RGO/CNT membrane surface displays a dense coverage with undulating and wrinkled morphology. No visible defects on the membrane surface suggest the CNT hollow fiber substrate was completely covered. The typical wrinkles were caused by the stacked RGO boundaries, which can act as not only entrances to enhance water permeability but also buffering spaces for improving mechanical strength. It is well known that the membrane thickness is an important factor that influences the membrane performance, particularly for RGO membranes. When the thickness was thicker than 500 nm, the RGO layer is reported to be impermeable to water. However, the water molecule can across the RGO layer with thickness thinner than 100 nm [32]. The possible reason could be the presence of the structural defects on the RGO nanosheets that act as the water transport channels. Hence, the RGO layer was firstly formed under different voltages with deposition time of 30 s. As presented in Fig. 4, the thicknesses of the RGO layer are 51, 64 and 95 nm at voltages of 3.5, 4.0 and 4.5 V, respectively. The increased thickness is attributed to that the enhanced electric field force under higher applied voltage drives more GO nanosheets toward to the substrate. Moreover, it is noteworthy that the RGO flakes are tightly stacked in the in-plane direction to membrane surface, consistent with the stacked RGO laminar membranes in reported works [33, 34]. Effect of deposition time on RGO thickness was also investigated at 40

zeta potential (mV)

(a)

(b) 20

0

1

3

5

7

9

pH

11

-20

5 μm -40 Fig. 3. Zeta potentials of GO (a) and SEM image of RGO/CNT membrane (b). 4

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(a)

(b)

(c) 64 nm

51 nm 250 nm

95 nm

250 nm

250 nm

Fig. 4. Thickness of RGO layer under different applied voltages of 3.5 V (a), 4.0 V (b) and 4.5 V (c) for deposition time of 30 s.

(a)

(b)

(c) 135 nm

79 nm

51 nm 250 nm

250 nm

250 nm

Fig. 5. Thickness of RGO layer under different deposition time for 30 s (a), 60 s (b) and 120 s (c) at applied voltage of 3.5 V. Raw Internsity Peak Sum C-C/C=C C-OH C-O-C C=O O=C-OH

90 Intensity (a.u.)

Water contact angle (degree)

100

80 45

RGO

GO

30 15 0

294

292

290

288

286

284

282

280

Binding Energy (eV)

CNT substrate

GO/CNT

RGO/CNT

Fig. 8. XPS spectra of GO and RGO.

Sample D

RGO

Intensity (a.u.)

G

Intensity (a.u.)

Fig. 6. Water contact angles of CNT substrate, GO/CNT and RGO/CNT membranes.

RGO

GO 2850 2963

GO

1750 1380

500

1640 1250

1500

2000

2500

3000

3500

-1

1100

3430

1000

Raman shift (cm ) Fig. 9. Raman spectra of GO and RGO.

4000 3600 3200 2800 2400 2000 1600 1200 800 400 -1

Wavelength (cm )

containing groups ensure the RGO layer somewhat hydrophilic as displayed in WCA test. Raman spectroscopy was also used to characterize the structural regularity of GO and RGO, in which two characteristic peaks, including D-band at 1346 cm−1 and G-band at 1593 cm−1, appear in the final

Fig. 7. FT-IR spectra of GO and RGO.

5

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The reverse salt flux is about 1.2 gMH for 95 nm RGO layer, while it is 1.6 gMH for 51 nm RGO layer. Specific salt flux (Js/Jw), the lost amount of draw solute per water unit, is an important parameter that is generally used to evaluate the performance of FO membrane. Fig. 11c reveals that all the RGO/CNT hollow fiber membranes display a relatively low Js/Jw value in the range of 0.06–0.07 g/L. On the other hand, both RGO/polyether sulfone (PES) and polyamide (PA)/CNT hollow fiber membrane were prepared and taken in comparison. As presented in Fig. 12a, the RGO/CNT hollow fiber membrane presents a higher water flux in comparison to RGO/PES hollow fiber membrane. As the two membranes possess same RGO active layer (Fig. 12b), the difference in water flux is mainly attributed to their different substrates. As presented in Table 1, the RGO/CNT membrane presents a relative low S value of 202 μm, comparable to 186 μm for the PA/CNT membrane. This is attributed to the both membranes were constructed on the same CNT sublayer which possesses a scaffold architecture with entangled CNTs. Such interconnected mesh structure ensures high porosity and low tortuosity, which is favorable to lowering S value. In comparison, the S value of RGO/PES membrane is 377 μm, which is more than two times of that of RGO/CNT membrane. As S value is an important parameter relating to ICP, the low S value suggests a relatively weak ICP in RGO/CNT membrane. Moreover, the CNT hollow fiber presents higher porosity (93%) and better wettability (WCA of 37°) than the PES hollow fiber (porosity of 72% and WCA of 62°). These fascinating properties are favorable to developing high-flux FO membrane via lowering ICP. As the FO selectivity is mainly dominated by the active layer, the salt reverse flux for RGO/CNT membrane is comparable to that for RGO/PES membrane. For the PA/CNT hollow fiber membrane, although it shows a slightly higher flux than the RGO/CNT hollow fiber membrane, its reverse salt is much more than those of the membranes with RGO active layer. This suggests that the RGO layer possess higher selectivity than the conventional PA layer. Fig. 12c reveals the Js/Jw values of the three FO membranes. A lowest draw solute loss was achieved on the RGO/ CNT hollow fiber membrane. In addition, the commercial CTA membrane presented Jw of only 6.9 LMH and Js much than 2.2 gMH, and Js/ Jw higher than 0.32 g/L. Therefore, the RGO/CNT hollow fiber is an alternative high-performance membrane for FO application. Moreover, as presented in Fig. 13, the FO performance of the prepared RGO/CNT membrane is comparable to and even higher than those in the reported works [6, 40–48]. The chemical and thermal stability of the RGO/CNT membrane was also investigated in acid, alkaline and high-temperature conditions. After immersing the RGO/CNT membrane into various pH for 2 h, there is no obvious flux change in permeate water and reverse salt. Meanwhile, the membrane shows a similar water and reverse salt flux (Fig. S3). These results suggest that the RGO/CNT membrane possesses excellent chemical and thermal stability, which ensures it can be applied in some special conditions such as drilling-water treatment and

Intensity (a.u.)

RGO

GO

5

10

15

20

25

30

35

40

2 Theta (Degree) Fig. 10. XRD spectrum of GO and RGO.

spectra (Fig. 9). According to the reported works [39], the D-band is generally derived from the vibrations of sp3 hybridized carbon atoms in plane terminations of structural defects during the graphite oxidation process, while the G-band is attributed to the first-order scattering of sp2 carbon atoms in a recovered 2D hexagonal network of carbon atoms with defects. Meanwhile, the intensity ratio of D- to G- bands (ID/IG) provides a sensitive measure of the disorder level of the basally graphitic plane and crystallite size on the carbon backbone. For the RGO sample, the ID/IG value for RGO sample is higher than that for the pristine GO sample. The higher ID/IG value indicates more defects possibly forming in the carbon backbone during the chemical reduction by losing oxygen-functional groups. These defects would act as the channels for water transport in the stacked RGO laminates. XRD analysis was used to examine the d-spacing to determine the nanochannels between the laminated GO or RGO nanosheets. As shown in Fig. 10, the characteristic peak (001) of the pristine GO appears at 2θ = 9.98° (d-spacing: ~0.89 nm), consistent with the reported data of GO [39]. For the RGO sample, its (001) peak was slightly up-shifted to 11.29°, implying a smaller d-value of 0.78 nm. The successfully narrowed 2D channel arises from the partially reduced oxygen-containing groups in RGO. In addition, the new peak at 22.55° (d-spacing: 0.39 nm) after chemical reduction suggests a wide range of interlayer spacing among the GO nanosheets. 3.3. Performance of RGO/CNT hollow fiber membrane Fig. 11 reveals the FO performance of RGO/CNT hollow fiber membrane with different RGO thickness. The water flux (Jw) is about 22.6 LMH for RGO layer in 51 nm, which gradually decreased to16.5 LMH with thickened RGO layer in 95 nm. It is because the increased thickness can aggravate the resistance for water transport. However, the thicker RGO layer results in lower reverse salt flux (Js) in Fig. 11b.

(b) 2.5

(a) 28

2.0

20

16

0.09

1.5

Js/Jw (g/L)

Js (gMH)

Jw (LMH)

24

12

(c) 0.12

1.0

0.03

0.5

51

64

Thickness (nm)

95

0.0

0.06

51

64

Thickness (nm)

95

0.00

51

64

95

Thickness (nm)

Fig. 11. Performance of RGO/CNT with different thickness: Jw (a), Js (b) and Js/Jw (c). (Conditions: 0.5 M NaCl draw solution against DI water feed at crossflow velocity of 25 cm/s and temperature of 25 °C under FO mode.) 6

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(a)

30

(b)10.0

(c)

0.4

25

15 10

Js/Jw (g/L)

Js (gMH)

Jw (LMH)

0.3

7.5

20

5.0

0.1

2.5

5 0

RGO/CNT

RGO/PES

0.0

PA/CNT

0.2

RGO/CNT

Sample

RGO/PES

PA/CNT

0.0

RGO/CNT

Sample

RGO/PES

PA/CNT

Sample

Fig. 12. Performance comparison among RGO/CNT, RGO/PES and PA/CNT hollow fiber membrane: Jw (a), Js (b) and Js/Jw (c). (Conditions: 0.5 M NaCl draw solution against DI water feed at crossflow velocity of 25 cm/s and temperature of 25 °C under FO mode, the thickness of RGO layer is 51 nm.)

successfully fabricated via chemically reducing electrodeposited GO layer on a CNT hollow fiber substrate. The RGO layer ensured a high excellent rejection rate and water permeability. Meanwhile, the CNT hollow fiber substrate, with high porosity and good wettability can weaken the ICP phenomenon, and thus ensured a high water flux. The thickness of RGO layer can be easily controlled by adjusting the deposition time and applied voltage. The optimal RGO/CNT hollow fiber FO membrane revealed balanced water flux and solute flux compared to RGO/PES and PA/CNT membranes. This work opens a new avenue for developing high-performance FO membranes by using nanocarbons, which possess considerable potential for application in water purification. Besides, further study and optimization are required in the future, e.g. enhancing wettability of RGO active layer for high flux via surface modification, and improving antifouling ability under electrical assistance by using the good electro-conductivity of nanocarbons.

Table 1 Transport properties and structural parameters of TFC-FO membranes. Sample

Aa, L/(m2·h·bar)

Rsb, %

B, L/(m2·h)

Sc, μm

RGO/CNT RGO/PES PA/CNT

2.11 2.02 2.37

96.6 95.7 92.4

0.051 0.055 0.108

202 377 183

a

A was obtained at 1 bar with DI water feed. Rs was measured by using pressure of 1.0 bar (500 mg/L NaCl feed). c S was calculated based on experiments in FO mode using 0.5 M NaCl as draw solution and deionized water as feed. All experiments were performed at 25 °C. b

30 Ref. 47

GOOD

Jw (LMH)

25

This work Ref. 44

20

Ref. 43

Ref. 46

Notes

Ref. 48

Ref. 48 Ref. 42

Ref. 48

The authors declare no competing financial interests.

Ref. 46 Ref. 44 Ref. 40 Ref. 45 Ref. 41

15

Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51708085 and 21437001) and the China Postdoctoral Science Foundation (No. 2016M601314).

Ref. 44

10

BAD

5

0

2

4

6

8

10

Appendix A. Supplementary data

12

Js (gMH)

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2018.07.020.

Fig. 13. Comparison of FO performance under AL-FS mode with 0.5 M NaCl draw solution against DI water feed.

References

emulsion enrichment. Considering the salt rejection of RGO/CNT hollow fiber membrane during FO process, a lab-scale desalination test was performed by using a 0.1 M NaCl solution (feed solution) against a 1.0 M sucrose (draw solution). The result reveals that the membrane present a water flux of 40.4 ± 3.7 LMH and salt rejection of about 94.0 ± 1.9%. It is noted that the salt permeation is slightly higher than that in the aforementioned FO tests. This might be attributed to that the salt and water concurrently across the membrane from RGO active layer to CNT substrate in desalination process. In contrast, the salt diffused from CNT substrate to RGO active layer in the FO test, while water transport in an opposite transport direction. This behavior might weaken the salt diffusion.

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