Declining flux and narrowing nanochannels under wrinkles of compacted graphene oxide nanofiltration membranes

Accepted Manuscript Declining flux and narrowing nanochannels under wrinkles of compacted graphene oxide nanofiltration membranes Yi Wei, Yushan Zhang, Xueli Gao, Yiqing Yuan, Baowei Su, Congjie Gao PII:

S0008-6223(16)30629-7

DOI:

10.1016/j.carbon.2016.07.056

Reference:

CARBON 11183

To appear in:

Carbon

Received Date: 2 March 2016 Revised Date:

25 July 2016

Accepted Date: 26 July 2016

Please cite this article as: Y. Wei, Y. Zhang, X. Gao, Y. Yuan, B. Su, C. Gao, Declining flux and narrowing nanochannels under wrinkles of compacted graphene oxide nanofiltration membranes, Carbon (2016), doi: 10.1016/j.carbon.2016.07.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Declining Flux and Narrowing Nanochannels Under Wrinkles of Compacted Graphene Oxide Nanofiltration Membranes Yi Wei a, Yushan Zhang b, *, Xueli Gao a, *, Yiqing Yuan a, Baowei Su a, Congjie Gao a

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of

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Education, Ocean University of China, Qingdao 266100, China

The Institute of Seawater Desalination and Multipurpose Utilization, Tianjin 300112, China

Professor Yushan Zhang

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Corresponding Author:

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Affiliation: The Institute of Seawater Desalination and Multipurpose Utilization,

Detailed permanent address: No.1, Keyan East Road, Tianjin 300192, China

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Email address: [email protected] Tel: +86 13902147480

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Professor Xueli Gao

Affiliation: Key Laboratory of Marine Chemistry Theory and Technology,

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Ministry of Education, Ocean University of China

Detailed permanent address: No.238, Songling Road, Qingdao 266100, China

Email address: [email protected]

Tel: +86 0532-66782017

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Declining Flux and Narrowing Nanochannels

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Under Wrinkles of Compacted Graphene Oxide

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Nanofiltration Membranes

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Yi Wei a, Yushan Zhang b, *, Xueli Gao a, *, Yiqing Yuan a, Baowei Su a,

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Congjie Gao a

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Key Laboratory of Marine Chemistry Theory and Technology, Ministry of

Education, Ocean University of China, Qingdao 266100, People’s Republic of China b

The Institute of Seawater Desalination and Multipurpose Utilization, Tianjin

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300112, People’s Republic of China

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ABSTRACT

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Graphene oxide (GO), prepared using Hummers method, is used to fabricate

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nanofiltration membranes by pressure-assisted self-assembly method. The reasons for the

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formation of wrinkles on the surface of GO membranes are analyzed in this paper. GO

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membranes have serious flux attenuation and obvious changes in surface morphology due

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to hydraulic pressure. At 1.0 MPa, the water flux of GO membranes decreases about 75%,

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while at 1.5 MPa, sodium sulfate rejection increases from 21.32% to 85.84%. GO

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membranes were compacted and wrinkles became narrower under the influence of

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hydraulic pressure. By comparing flux decline between support membranes and GO

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*Corresponding author. E-mail: [email protected] , Tel: +86 13902147480 (Yushan Zhang), E-mail: [email protected] , Tel: +86 0532-66782017 (Xueli Gao)

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membranes and analyzing the structure of GO laminate, we concluded that flux decline is

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due to the changes in the support membrane and GO laminate synergistically. This study

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reveals a defect of GO membranes and provides a profound analysis of water permeation

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through GO membranes.

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1. Introduction

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Graphene oxide (GO) containing oxygenated functional groups [1] is a two-

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dimensional nanomaterial. Similar to other nanomaterials, GO has attracted extensive

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attention in the field of water treatment and is used to improve performance of various

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separating membranes [2–12]. Additionally, laminated GO can function as two-

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dimensional water channels because of its planar construction, good dispersity [13], and

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hydrophilicity [14]. These nanochannels endow GO membranes with the ability of

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filtrating ions and molecules. GO membranes, prepared by pressure-assisted self-

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assembly (PASA) [15–22] or layer-by-layer self-assembly [23–27], are particularly

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attractive. PASA method, in which individual GO sheets are stacked into a layered

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structure at the filter–suspension interface, is an economic technique for producing GO

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membranes [15]. The microstructure of GO membranes is different from that of

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interfacial polymeric nanofiltration (NF) membranes. Water permeation through GO

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membranes is attributed to channels between GO nanosheets [16]. Penetration and

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desalination mechanisms of GO membranes, which are different from those of traditional

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NF membranes, have been intensively studied. Sun et al. investigated selective ion

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penetration through GO membranes and functionalized graphene membranes [17, 28]. Ge

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Shi et al. reported that the water flux of GO membranes can be readily controlled by

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tuning the oxidation level of GO nanosheets [21]. Yi Han et al. prepared reduced GO

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membranes that had high water flux and different rejections for different inorganic salts

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[19]. The results show that the rejection for salts is mainly dominated by the size-

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exclusion effect and electrostatic interaction effect [19, 20]. Yi Han et al. also reported

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that it took almost 0.5 h for the GO membrane flux to reach a steady state [19]. Huang et

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al. found that nanochannels of GO membranes shrank with increasing pressure [29].

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Theoretically, a perfect stratified structure of the GO membrane is indispensable to form a

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high-performance separating membrane. However, Karl et al. identified a semi-ordered

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accumulation mechanism through a series of experiments [30]. A highly ordered layering,

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which is beneficial to the performance of GO membrane, cannot be obtained by the

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PASA method. Compared with thin-film composite polyamide-based membranes, both

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unstable flux and relatively low salt rejection of GO membranes are the main issues

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observed in recent studies. Stable water permeability and high water–solute selectivity,

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which are needed for desalination application [31], require a highly ordered layering

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structure. Thus, further studies on the structure and separating property of GO membranes

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are requisite.

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Here, we fabricated GO membranes by the PASA method and evaluated their

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performance. The results revealed that attenuation of pure water flux and increase in salt

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rejection were evident and the surface morphology of GO membranes changed

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significantly after compaction. The relationship between performance and structure of

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GO membranes and the mechanism of formation of surface morphology were analyzed.

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2. Experimental

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2.1. Synthesis of GO In this study, Hummers method [32] was used to prepare GO. Flake graphite was

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oxidized in a mixture of concentrated H2SO4 and KMnO4 below 5°C, followed by

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increasing the temperature of the mixture slowly up to 30–40°C and maintaining this

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temperature for 2.5 h. Then, the resulting mixture was diluted and heated up to 95°C to

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react for 15 min. A large amount of water was added to terminate the reaction. After

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natural cooling, hydrogen peroxide was added and the color of the suspension liquid

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changed from brown to golden yellow. Finally, graphite oxide was obtained by filtration

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and washing for several times until the pH value was stable. Ultrasonic exfoliation and

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centrifugation were used to prepare GO solutions.

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2.2. Preparation of GO Membranes

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A self-designed membrane cell (Fig. S1), which could contain 100 ml of GO

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solution, was used to prepare GO membranes by dead-end filtration at 0.1 MPa. To

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fabricate GO membranes with different thicknesses of GO laminates, a series of GO

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dispersion was prepared with concentrations ranging from 0.83 to 2.5 µg/ml. Then, the

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GO solution (100 ml) was filtrated at 0.1 MPa through PSf ultrafiltration (UF)

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membranes, which had a diameter of 7.5 cm. The pressure was maintained until there was

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no water on the surface of the GO membrane. Finally, the wet GO membranes were dried

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in vacuo at 40°C for at least 2 h. The loading masses of GO are 18.86, 22.64, 28.29,

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45.27, and 56.59 mg/m2 for the characterization of separation performance of GO

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membranes with different thicknesses of GO laminates. GO membranes for compaction

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have a loading mass of 45.27 mg/m2. Some thicker GO membranes, with loading mass

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ranging from 226.47 to 2264.69 mg/m2, are used for the characterization of

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microstructures, including top surface and cross-section micromorphology. Additionally,

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because of the effect of peaks of PSf on X-ray diffraction (XRD), some ultrathick GO

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membranes with loading mass of 22.65 g/m2 were fabricated.

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2.3. Characterization

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GO was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray

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photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). FTIR pattern

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(Tensor 27, Bruker, Germany) was recorded using pressed KBr flakes at room

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temperature. XPS pattern (ThermoFisher Scientific, USA) was measured on the surface

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of GO membranes with an achromatic X-ray source of 100 W and 15 kV to analyze the

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binding energy of C1s. AFM (Multimode-V microscope, VEECO, USA) images of GO

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were used in tapping mode after solvent evaporation of GO solution on mica.

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Surface morphology of GO membranes was characterized by AFM and scanning

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electron microscope (SEM). After compaction or soaking in water, samples for AFM

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were adhered on iron sheets used in tapping mode when water on the sample surface was

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evaporated. Samples for SEM were fractured in liquid nitrogen to prepare the cross-

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section sample. Gold sputtering was performed on the sample before acquiring SEM

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images. We also investigated GO membranes by XRD (D8 ADVANCE, Bruker,

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Germany). Dry samples for XRD were directly measured with a Cu Kα radiation source.

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Wet GO membranes for XRD were obtained after soaking in water or compaction

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experiments.

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For GO membranes with different thicknesses of GO laminates, pure water flux and

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salt rejection were performed on a cross-flow filtration device (Fig. S2) by maintaining a

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flow rate of 5 m/s at 25°C. All water samples were collected after prepressing for 3 h at

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1.5 MPa. Compaction experiments in pure water were conducted on a dead-end filtration

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device (Fig. S3) driven by pressurized air at room temperature. After compaction in pure

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water for 5 h, GO membranes and PSf membranes were evaluated in the cross-flow

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filtration device to obtain the data of salt rejection at 1.5 MPa. Decolorization ability of

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GO membranes was evaluated by a cross-flow instrument, which could furnish 0.4 MPa

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at the most.

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3. Result and discussion

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Before evaluating GO membranes, we analyzed GO nanosheets by several

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characterization methods. In the AFM image (Fig. S4), the thickness of GO nanosheet

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was 0.736 nm. The FTIR spectrum of GO in Figure S5 suggested the presence of

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carboxyl groups (C=O stretching at 1727 cm−1), unoxidized C=C bonds (stretching at

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1623 cm−1), C–OH (stretching at 1396 cm−1), epoxy groups (C–O–C stretching at 1224

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cm−1), and alkoxy groups (C–O stretching at 1054 cm−1), which corresponded with

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previous studies [33–35]. The C1s XPS spectrum of GO in Figure S6 showed five peaks

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at binding energies of 284.2, 284.8, 286.6, 286.9, and 288.6 eV, attributed to C–C, C–OH,

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C–O–C, C=O, and COOH, respectively[28]. The XPS peak area ratio is 100(C–C):97(C–

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OH):137(C–O–C):100(C–O–C):22(COOH).

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As seen in the SEM image (Fig. 1a), the fracture edge of GO membranes fabricated

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by the PASA method reveals well-packed layers. However, on the surface (Fig. 1b), there

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are many wrinkles with different heights. As seen in Figure S7, the GO membranes

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showed good stability in the evaluation process due to the presence of some multivalent

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metal cations such as Mn2+, a by-product of GO synthesis [36].

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Figure 1. SEM images of cross-sectional (a, c) and surface (b) morphology of GO membrane. The GO loading masses of samples in (a), (b), and (c) are 0.97, 0.23, and 1.52 g/m2 respectively. The fracture in Figure 1c reveals that the surface wrinkle is composed of layers of

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wrinkles. With the deposition of GO nanosheets, initial wrinkles gradually grow (Fig. 2)

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and form surface wrinkles with different altitudes on the GO membrane [37, 38]. The

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AFM image (Fig. 3) shows wrinkles with different heights (1.008 and 0.498 nm). The

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different heights of initial wrinkles affect the altitudes of surface wrinkles. Besides, to

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some extent, the altitude of surface wrinkle depends on the initial position of the

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nethermost wrinkle. In other words, to some extent, the thickness of surface wrinkle

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determines its altitude. Figure 4 shows that different thicknesses of GO laminates lead to

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obvious differences among SEM images of surface morphologies. Figure S8 illustrates

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the AFM images showing more tiny surface wrinkles on GO membranes of different

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thicknesses.

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Figure 2. Schematic diagram of the formation of surface wrinkle from initial wrinkle (a), folding (b) and stacking (c) of GO nanosheets and the deformation of wrinkle after compaction (d).

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Figure 3. AFM image and schematic diagrams of initial wrinkle, folding, and stacking of GO nanosheets and their height profiles.

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Figure 4. SEM images of surface morphology of GO laminates with different thicknesses

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(2.26 g/m2 (a), 1.13 g/m2 (b), 0.57 g/m2 (c), and 0.23 g/m2 (d)).

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The formation of initial wrinkle is shown in Figure 5. In the PASA method, water

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between soft GO nanosheets [39] and UF membrane is gradually drained. If water under

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GO nanosheet forms a wrinkle and is drained further, the GO nanosheet contacts with

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the target UF membrane and forms a slender wrinkle (Fig. 5a–d). The height of wrinkle

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depends on the mass of water under it. AFM samples were obtained by dropping

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dispersion liquid of GO on a freshly cleaved mica surface. The evaporation of solvent

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on mica is similar to the draining of water in the PASA method. The slender wrinkles

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can also be observed in the graphene transfer process [40, 41]. Additionally, because of

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the flexibility of GO in water, folding or stacking GO can also form an initial wrinkle

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(Fig. 3), which can develop into a surface wrinkle (Fig. 2(a–c)).

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Figure 5. Schematic diagrams of the formation of initial wrinkle of GO.

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The flux as a function of thickness of GO membrane is shown in Fig. 6. Exponential

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decrease in flux and increase in salt rejection were observed with increasing GO loading.

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Conventional evaluation of GO membranes has been studied [42]. Figs. S9–S11 show our

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results including more detailed fluxes and salt rejections as functions of pressure. The

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pure water flux of GO membrane increased linearly with increasing operating pressure.

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During permeation, water molecules are absorbed onto the gaps with oxygen-containing

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functional groups around the GO nanosheets [43] and move into the empty space between

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the GO layers. Cracks and pin holes of GO nanosheets can serve as the entrance of GO

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laminate for water molecules [42, 44]. Meanwhile, the spaces between the GO layers

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become larger [43, 45, 46] and allow more water molecules to permeate through the GO

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membrane for hydration. Water molecules take a curving path, which is composed of the

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empty spaces of unoxidized areas of the GO nanosheets [16, 47]. The rejection (R)

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sequence of salt solution (as seen in Fig. S12) was R (Na2SO4) > R (NaCl) > R (MgSO4) >

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R (MgCl2). This phenomenon can be explained by the Donnan exclusion theory, which is

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generally used to explain desalination mechanism for NF and reverse osmosis (RO)

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membranes [48–50].

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Figure 6. Variations in pure water flux and salt rejection as functions of GO loading

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coated on the PSf membranes.

In the evaluation process, we found that the pure water flux declined obviously and

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higher pressure resulted in a higher and faster attenuation of flux (as shown in Fig. 7).

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Although most of the NF and RO membranes were composed of UF membrane and

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functional layers, they had smaller flux decline than GO membranes. Generally, pure

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water flux of interfacial polymerization membrane declines slightly due to the

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densification and modest fouling from the measurement equipment [52]. The main

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reason for compaction is the change in support structure, which could result in 32%

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decline of pure water flux at 1724 kPa [53]. However, as shown in Fig. 2, flux

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attenuation of GO membranes is much larger than that of conventional NF or RO

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membranes. In particular, pure water flux declined about 75% at 1.00 MPa after 300

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min.

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Figure 7. Attenuation of pure water flux of GO membranes (45.27 mg/m2). To confirm the influence of support layer attributing compaction of GO membranes,

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we evaluated dry PSf membranes at 1 MPa and other conditions were the same as those

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used in the compaction experiments of GO membranes. As shown in Fig. 8, pure water

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flux of PSf membranes decreased about 36% in 300 min at 1 MPa.

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Figure 8. Pure water flux of PSf membranes in compaction experiment at 1 MPa.

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In the compaction process (0.4 MPa), the rejection of methylene blue (MB) (100

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ppm) changed from 80.93% to 97.60% in 2 h and reached 99.62% in 3 h. Table 1 shows

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that the rejection for Na2SO4 (2000 ppm, 1.5 MPa) of GO membranes increased from

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21.32% to 85.84% after compaction. Under the same conditions, the rejection of PSf

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membranes increased from 1.46% to 10.0% after compaction. For GO membranes, in the

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compaction process, enhanced retention for MB is attributed to the formation of cake

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layer. Nevertheless, the increased rejection for Na2SO4 resulted from the compaction of

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the structure of GO membranes. These increasing rejections for inorganic salt indicate

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that the size-exclusion effect enhances in the compaction process.

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Table 1. Salt rejections (R) of Na2SO4 (2000 ppm, 1.5 MPa, 25°C) of PSf and GO membranes (45.27 mg/m2) before and after compaction. R (%) before compaction

R (%) after compaction

PSf membranes

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10.00

21.32

85.84

GO membranes

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To investigate the contribution of GO laminates in compaction, we studied the

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change in surface morphology before and after compaction by AFM. To avoid the

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influence of dry process on the surface morphology of GO laminates, we acquired AFM

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images after the evaporation of water on the surface of the samples. As shown in the

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AFM images (Fig. 9), the wrinkles on the surface of GO membranes became narrower

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than those before compaction. More AFM images of GO membranes are shown in Fig.

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S13.

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Figure 9. AFM images of wet GO membranes (0.57 g/m2) before (a) and after (b) compaction and their height profiles.

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The water channel networks in GO laminates composed of the intersheet spacing and the nanochannels in wrinkles could be responsible for water permeating and blocking

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small molecules [29, 54]. Water channels under wrinkles are larger than those under the

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flat area and reduce much resistance to water permeation (Fig. 2). Water molecules prefer

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to flow in the wide channels. Huang et al. reported that larger nanochannels could result

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in ultrafast water permeation, which is 10 times higher than that of pristine GO

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membranes [55]. However, nanochannels in wrinkles would collapse under pressure,

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leading to increase in permeation resistance of freestanding GO membranes [55]. In the

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compaction process, the number of water molecules in the channels continued to decrease

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and the spacing in wrinkles became smaller. As negatively charged GO sheets moved

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much closer to each other, the electrostatic repulsion force rapidly increased with

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compaction [29]. GO membrane has a relatively gentle and small flux attenuation under

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low transmembrane pressure. Water molecules have a higher moving rate, leading to

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larger flux under higher differential pressure. Meanwhile, the increased pressure led to

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faster and higher flux attenuation (Fig. 7). With the establishment of balance between

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electrostatic repulsion force and pressure force, the size of nanochannels and the

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penetration resistance were finally stabilized.

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As shown in Fig. 10, the thickness of GO membrane has no obvious change after

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compaction. Because samples for SEM are dry, the data of thickness cannot be used to

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represent real thickness in water. To study the structural change in the process of

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hydration and compaction, XRD measurements were conducted over the range of 3–30°.

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To obtain stronger feature diffraction peaks of GO laminates, ultrathick GO laminates

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(22.65 g/m2) were used in XRD measurement. Increasing the thickness of GO laminates

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can lead to a prolonged hydration time. As shown in Fig. 11 dry GO laminates show a

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diffraction peak at 10.9° corresponding to the interlayer spacing d≈0.811 nm. Three peaks

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at 17.8°, 22.7°, and 26.0° are feature diffraction peaks of PSf. XRD pattern of pure PSf

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membrane is shown in Fig. S14. With the increase in wetting time, the d-spacing

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increases from 0.811 (without wetting) to 1.354 nm (wetting for 8 h), which agrees well

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with that reported in the literature [44, 56, 57]. A constant level of hydration was

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achieved after 8-h water adsorption. No further increase in the d-spacing of 1.354 nm was

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observed after 8 h.

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Figure 10. SEM images of cross-sectional morphology of GO membranes (0.57 g/m2)

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before and after compaction (1 MPa, 5 h).

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Figure 11. XRD patterns of GO membranes before and after compaction.

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For wet GO laminates, it is interesting that the peak is shifted to d≈1.571 nm.

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Increased d-spacing indicates higher hydration level. Infiltration caused by pressure helps

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water to enter more unhydrated space, resulting in higher hydration level. From XRD

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patterns, we cannot draw a conclusion that the d-spacing does not diminish, because the

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channels in GO laminate are mechanically elastic and recoverable by releasing pressure

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[29, 55].

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4. Conclusion

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Compacted GO membranes have higher flux attenuation than conventional NF or

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RO membranes. By conducting a series of experiments and characterizations, we confirm

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that the structures of support membrane and GO laminate change in the compaction

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process. Wrinkles on the surface of GO laminates became narrower under pressure. This

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phenomenon indicates that water channels in wrinkles, which are part of water channels

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in GO laminates, have smaller cross-sectional area and higher permeation resistance. The

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change in support membrane and GO laminate result in flux decline synergistically. All

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these offer deeper understanding of the structure of GO membrane and its change under

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pressure. Change in other parts of water channels needs further study, in which

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compaction and characterization can proceed simultaneously. The defect of GO laminate

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needs further study to evaluate the effect of different types of water channels on flux as

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well.

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Acknowledgments

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Funding for this study was provided by the National NSF of China (21576250), the

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National Basic Research Program of China (973 Program) (2015CB655303), and the

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Nation Key Technology R&D Program (2014BAK13B02).

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References:

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