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Graphene oxide as effective barrier on a porous nanofibrous membrane for water treatment Jianqiang Wang, Pan Zhang, Bin Liang, Yuxuan Liu, Tao Xu, Lifang Wang, Bing Cao, and Kai Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12723 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016

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ACS Applied Materials & Interfaces

Graphene oxide as effective barrier on a porous nanofibrous membrane for water treatment Jianqiang Wang†ab, Pan Zhang†a, Bin Lianga, Yuxuan Liua, Tao Xua, Lifang Wanga, Bing Caoa, and Kai Pan*a a

Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical

Technology, Beijing, 100029, P. R. China. b

Department of Civil Engineering, The University of Hong Kong, Hong Kong, 999077, P. R.

China. KEY WORDS: graphene oxide, electrospun, nanofibrous membrane, nanofiltration, water treatment ABSTRACT: A novel graphene oxide (GO) based nanofiltration membrane on a highly porous polyacrylonitrile nanofibrous mat (GO@PAN) is prepared for water treatment application. GO of large lateral size (more than 200 µm) is firstly synthesized through an improved Hummers method, and then assembled on a highly porous nanofibrous mat by vacuum suction method. The prepared GO@PAN membrane is characterized by scanning electron microscopy, transmission electron microscopy, Raman spectrum and X-ray diffraction et al. The results show that graphene oxide can form a barrier on the top of PAN nanofibrous mat with controllable thickness. The obtained graphene oxide layer exhibits “ideal” pathways (hydrophobic nanochannel) for water molecule between the well stacked GO nanosheets. Water flux under an extremely low

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external pressure (1.0 bar) significantly increased due to the unique structure of GO layer and nanofibrous support. Also the GO@PAN membrane shows high rejection performance (nearly 100% rejection of Congo red, and 56.7% for Na2SO4). A hydrophilic-hydrophobic “gate”nanochannel model is presented for explaining the water diffusion mechanism through GO layer. This method for fabrication of GO membrane on a highly porous support may give many new opportunities for high performance nanofiltration applications.

1. Introduction Global scarcity of energy, natural resources, and water are grand challenges faced by humanity today. Novel technologies are highly required especially for water treatment.1-3 Recent years, membrane separation technologies have attracted growing interests because of their advantages of high-efficiency, energy-saving and environment-friendly.4-6 Nanofiltration (NF), a separation technique between ultrafiltration and reverse osmosis, has aroused great research interests due to its unique advantages.7-8 Nanofiltration has the separation capability of molecular weights (MW) between 200 and 1000 g mol-1 and has been extensively applied in desalination, waste water treatments, and industrial substances separations.9-11 More and more attentions are paid to nanofiltration membranes, as the core factor of the nanofiltration system. Generally, composite membrane technology is an effective method for preparing nanofiltration membranes. A composite membrane is composed of a thin skin layer and a porous support layer.12-13 Traditionally used porous support layers are mainly ultrafiltration membranes produced by non-solvent induced phase inversion method. However, the low porosity and relatively dense skin layer of the ultrafiltration membranes limited the separation performances. Therefore, many studies have been focused on


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structural tailoring of nanofiltration membranes to improve separation performances. To overcome the disadvantages of traditional support layer, electrospun nanofibrous mat, with highly porous structure, is advantageous for membranes processing because of its low mass transfer resistance.

14-15

Up to now, various nanofibrous mats supported

composite membranes have been fabricated with improved performances.16-20 At the same time, many works have been performed to obtain skin layers with better separation performance and they can be summarized to two directions. One is to employ monomers with different functionality in the process of interfacial polymerization to provide new skin layers with expected properties.8, nanoparticles (e.g. SiO2 and TiO2, nano-NaX zeolite

24

21-23

The other is to introduce functional

multi-walled carbon nanotubes (MWNTs),25-26 and

27

) into the skin layers to improve their permeability, stability or

separation efficiency. More recently, graphene and graphene oxide (GO) has been considered as a potential candidate for using as barrier layer of the separation membranes due to its ultra-thin 2D structure and controllable surface chemistry.28-32 Because of its unique physical and chemical properties, GO nanosheets have attracted intensive interests in the ﬁeld of membrane applications. Many efforts have been made to fabricate highly permeable GObased membranes by taking advantage of the nano-channels between GO sheets.33-39 When dried, GO nanosheets can be tightly packed, and the spacing between GO nanosheets can be as low as 0.3 nm.29 Such a small channel can just let water vapour permeate through.40 The GO barrier layer on ultrafiltration membranes showed promising potential applications for water treatment. However, the directly preparing a GO layer on


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a highly porous nanofibrous mat is still a challenge, because of the big pore size of nanofibrous mat. In this paper, for the first time, GO nanosheets were assembled on a highly porous nanofibrous mat directly using vacuum suction method. To successfully achieve this target, GO nanosheets with large lateral size (> 200 µm) were firstly synthesized using a newly developed method in our group.41 Then, a thin GO skin layer with 2D nanochannels was obtained using a facile vacuum suction method on polyacrylonitrile (PAN) nanofibrous support layer. Structures and separation performances of these novel composite membranes were investigated in detail.

2. Experimental section 2.1. Chemicals Graphite powder was purchased from Qingdao Henglide Graphite Co., Ltd. PAN (Mn=150,000) was purchased from Sigma-Aldrich. N.N-Dimethylformamide (DMF), sodium hydroxide and hydrogen chloride were purchased from Beijing Chemical Co. Ltd. All of the water used in this work was Milli-Q deionized water (18.1 MΩ cm at 25 oC).

2.2. Preparation of GO aqueous suspensions GO aqueous suspensions were prepared using a method as reported in our previous study.41 Electrochemical intercalation method was used for the preparing of expanded graphite with particle size larger than 200 µm. Typically, graphite sheets were used as electrodes. The distance between cathode and anode was kept at 2 cm, and the H2SO4 solution (2 mol L-1) was used as electrolyte. A constant voltage of 5 V from the DC


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power was applied between the two electrodes for 5 min. After that, the obtained expanded graphite sheets were washed with deionized water and dried at 80 oC under atmospheric pressure for 8 h. Then 70 mL H2SO4 and 9.0 g KMnO4 were slowly added to the pretreated graphite in an ice bath. The mixture was reacted at 35 oC for 2 h, and distilled water was added for hydrolysis. After 2 h, 5 mL 30% H2O2 was added to the mixture. The mixture was allowed to stand for at least 12 h, after which the clear supernatant was decanted. The remaining precipitate was washed with 5% HCl solution and washed again with distilled water. The ﬁnal solution was centrifuged and ultrasonicated for 4 h. The concentration of the obtained GO was 1.23 g L-1. The resulting GO aqueous suspension was sonicated for 30 min followed by centrifugation at 10,000 rpm for 30 min to obtain a homogeneous dispersed aqueous suspension. Then the GO suspensions were diluted with concentrations in the range of 3.0 µg mL-1 to 15 µg mL-1 for fabrication of GO layer.

2.3. Preparation of PAN nanofibrous mat Electrospun procedures are similar to our previous studies

42-43

. Typically, PAN (5.0 g)

was dissolved in DMF (45.0 g) and then stirred at 60 °C for 12 h. The electrospun of PAN solution was carried out at ambient temperature by applying a voltage of 15 kV on a metal needle (inner diameter of 0.7 mm). The flow rate and the distance between spinneret and collector were set to be 1.0 mL h-1 and 10 cm, respectively. The obtained nanofibrous mat was cut into many circles with a diameter of 5.4 cm before use.

2.4. Preparation of GO@PAN nanofibrous membrane


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GO nanosheets with large lateral size were synthesized using our newly developed method

41

. The aqueous suspension of GO (100 mg L-1) was prepared by adding GO into

ultrapure water followed by sonication for 10 min. To control the thickness of GO layer on PAN nanofibrous mat, the initial GO suspension was diluted to different concentrations ranging from 3.0 µg mL-1 to 22.5 µg mL-1. Considering the high flux of nanofibrous mat, the diluted GO suspensions (200 mL) were filtrated stepwise through the nanofibrous mat (30 mL each time) with the assistant of vacuum. After filtration, samples were air-dried before use.

2.5. Performance of GO@PAN nanofibrous membrane A dead-end filtration device with an effective area of 14.51 cm2 was employed for the measurements of water flux and membrane rejection. The conditions for nanofiltration experiments were as follows: applied pressure 1.0 bar; temperature 30 °C; volume of feed solution 300 mL. The water flux was recorded every five minutes until it went steadily and the stable value was recorded as the permeate flux. Congo red and different salts were used for the study of membrane rejection performance.

2.6. Characterizations Raman spectrum of GO powder was obtained on a Renishaw Micro-Raman Spectroscopy System (Renishaw in Via) using a 633 nm as excitation laser. XRD pattern was acquired with a single crystal XRD diffractometer (Bruker AXS D8 ADVANCE, Germany) using Cu Kα (1.5406 Å) radiation. XPS result was obtained using X-ray photoelectron spectroscopy (Thermo Electron Corporation ESCALAB250) with an Al K α X-ray source


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(1486.6 eV). ATR-FTIR spectra were acquired using a Perkin-Elmer spectrum RXI with a resolution of 4 cm-1. TEM images were gathered with JEM-1230 transmission electron microscopy (JEOL Japan). SEM images of nanofiber and GO membrane were collected using a Hitachi S-4700 SEM. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer Pyris 6 TGA instrument under nitrogen at a heating rate of 10oC min-1. Contact angle measurements were carried out on a JC 2000 (MAIST Vision Inspection & Measurement Ltd. Co.) with an approximate 5 µL droplet. UV-visible spectra were gathered on a Hitachi U-3900 spectrophotometer.

3. Results and discussions 3.1. Characterization of GO structures Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to characterize the nanoscale morphologies of GO samples prepared using modified Hummer’s method. From SEM image (Figure 1a), exfoliated GO thin flakes with large lateral size (at least 200 µm) can be found. Such a large lateral size of GO will facilitate the fabrication of GO membrane on a porous support with large pore size. Figure 1b shows TEM image of GO samples. The GO sheets have a shape similar to large crumpled thin flakes, in consistent with the SEM results. In addition, a huge and transparent layer, featuring folded basal plane and rolled edges which is intrinsic to grapheme for gaining thermodynamic stability can be observed.44 The as-prepared GO powder was further characterized by ATR-FTIR, TGA, XRD and Raman Spectroscopy. The ATR-FTIR spectrum of GO in Figure 1c indicated the presence of oxygen-containing functional groups on GO laminates. In detail, the broad peak between 3050 cm-1 and 3800


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cm-1 is corresponding to hydroxyl groups on GO sheet. The peaks at 1725 cm-1 and 1629 cm-1 belong to the carboxyl groups and un-oxidized sp2 C=C bonds in the carbon lattice.45-46 Typical TGA curve of GO is shown in Figure 1d. The mass loss before 250 oC is about 42.2 %, which were mainly due to the decomposition of oxygen-containing groups (e.g. alcoholic hydroxyl, epoxy and carboxyl et al). The mass loss after 400 oC is corresponding to the pyrolysis of carbon skeleton.47 A peak at 2θ=11.1° is observed in the XRD pattern of GO powder (Figure 1e), corresponding to interlayer spacing of 7.96 Å. The number of GO layers was test using HR-TEM and reported in our previous publication.41 The results showed that approximately 36% of GO flakes were monolayer, nearly 54% exhibited two to five layers, and nearly 10% had six to ten layers. Raman spectrum (Figure 1f) is another useful technique to characterize the graphene materials. The G band at 1588 cm-1 corresponds to the sp2 hybridization of the graphitized structure and the D band at 1354 cm-1 indicates the local lattice defect.48 The ID/IG ratio is 0.85 reveals less defects in the graphitized carbon structure.

3.2. Microstructure of GO@PAN nanofibrous membrane PAN nanofibrous mat was employed as the support for GO layer in our experiments due to its easy electrospun procedure. Compared to conventional support layer, the nanofibrous mat has high porosity and interconnected pore structure, which can significantly reduce membrane mass transfer resistance and thus enhance water flux.14, 17, 49

SEM images of PAN nanofibrous supporting layer are shown in Figure 2. As shown in

Figure 2a, a clear edge of GO layer can be seen on PAN nanofibrous mat. And no fragments of GO can be found in the inner of nanofibrous membrane, which may due to


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the help of large lateral size of GO. From Figure 2b, we can see that PAN nanofibers are quite uniform with diameter around 300 nm (Figure 2b, inset). The pore size of PAN nanofibrous mat is in the range of micron-scale. The inter-connected porous structure of nanofibrous mat guarantees direct paths for the diffusion of water, making it an excellent supporting layer for filtration applications. Successful deposition a thin layer of GO on nanofibrous substrates should meet two key requirements, one is the size distribution of GO monolayer should be at least tens of microns or more that can be intercept by the nanofiber networks. The other one is the concentration of GO suspension should be low to ensure tightly formation of GO laminates. Thus, extremely low concentration of GO dispersion (3.0 to 22.5 µg mL-1) was applied in this work. Though the total volume of GO suspension was just 200 mL, an intact GO layer was quickly formed within 2.0 h. The optical image of GO@PAN membrane is shown in Figure 2e. After drying in vacuum, the obtained GO@PAN membrane showed excellent flexibility with a diameter of 4.3 cm and water contact angle of 61 ± 3° (Figure 2f inset). Cross-section of GO@PAN membrane is shown in Figure 2f. Both GO laminates and nanofibrous substrate can be clearly observed, revealing successful formation of GO layer on top of PAN nanofibrous mat. The SEM images in supporting Figure S1 show the typical surface morphology of GO laminates. For composite nanofiltration membrane, thickness of skin layer determines the separation performance directly.50-51 To get a better understanding of the filtration performance, GO@PAN membranes with different thickness of GO layer were fabricated by manipulation GO loading amount ranging from 41.35 to 310 µg cm-2. The thickness of GO layer was measured based on SEM images. Figure 3 shows the SEM images of


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GO@PAN membranes with different thickness of GO layer. The thickness of GO layer increased with GO loading amount, and followed a linear growth trend as shown in Figure 3g. The results indicated that thickness of GO layer can be well controlled by a simple concentration manipulation.

3.3. Nanofiltration performance The results of water fluxes versus filtration time of GO@PAN membranes with different GO thickness were investigated and are shown in supporting information Figure S2. The results indicated that water flux decreased sharply in the beginning and then went steady after 30 min. The same trend of flux decline was also reported by others.34 Thus in the current study, all fluxes were recorded after 30 min. As we know, the thickness of skin layer has a great effect on membrane filtration performance, therefore water fluxes of GO@PAN membranes with different GO thickness were studied (as shown in Figure 4a). As can be seen from Figure 4a, the water flux decreased in a style of exponentially trend. The sharp decrease of water flux is mainly due to the increase of mass transfer resistance when GO thickness increased. The highest water flux (8.2 L m-2 h-1 bar-1) was obtained when GO layer thickness is 34 nm (as shown in Figure 4a). This water flux is more than 60% higher than the data reported by a previous study (GO layer thickness: 33 nm; pure water flux 5.0 L m-2 h-1 bar-1).34 Such a high water flux can be explained by HagenPoiseuille theory. In this case, the distance between two adjacent graphene oxide sheets can be seen as 2D channel between carbon walls.52-54 Furthermore, the hydrophobic nature of carbon wall in graphene oxide is also helpful for water transport through the slip flow theory. The water firstly goes to the hydrophilic “gate” (space between edges of two


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adjacent GO nanosheets or defects of GO) for aggregation, and then slipps through into hydrophobic 2D nanochannels (a conceptual illustration is shown in Figure 5). Because of the arrayed hydrophilic points (the edge of GO nanosheets and the defects) and hydrophobic points (the carbon wall of GO nanosheets), the water can be easily aggregated at the hydrophilic places. The structure of assembled GO layer is actually very similar to the back structure of Stenocara beetles, which survived in Namib Desert due to its superb skill for water collection.55 After the aggregation, water gets into the hydrophobic 2D nanochannels and slips through it. The hydrophilic edges and defects of GO nanosheets work as a “gate” for water molecular. However, the edges and the defects showed different properties for water passing through, the edge is much more energetically favorable for water to continue to slide along the carbon sheet than to pass through the hole (defect of GO) within GO nanosheets according to the computational results reported by D W Boukhvalov et al.56-57 That means the defects of GO may only work as aggregation points for water. After passing through the “gate”, water slide into carbon nanochannel at a high speed. Similar phenomena has been reported in the case of carbon nanotubes,52, 58 a three-four log of increase can be found when water flow through the hydrophobic carbon nanotubes. The main reason is because of the low friction (also known as drag-reducing effect

59-60

)

between water and hydrophobic surface. As for the higher water flux of the prepared GO@PAN membrane compared to others, it is maybe mainly due to the following two reasons: (1) nanofibrous mat used in this study has a relatively high porosity than traditional microfiltration membranes, which render a low transport resistance;61 (2) the large lateral size of GO nanosheets used in our experiments make a relatively longer


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hydrophobic 2D nanochannel, therefore water gets a higher speed at the end of these channels. For GO@PAN membrane with a GO layer thickness of 128 nm, the water flux increased linearly with pressure at the range of 1.0 to 4.0 bar (as shown in Figure 4b). The results also indicated that the GO@PAN membrane could stand higher pressure if the thickness of GO layer increasing. In fact the thin separation layer supported on nanofibrous mat usually cannot afford high pressure,62 but GO@PAN composite membrane showed favorable pressure endurance. This is mainly attributed to the excellent mechanical properties of graphene oxide. Another reason may be due to the large lateral size of GO, the interlaced area increased as the increase of lateral size. Therefore, the pressure endurance performance can be improved. Rejection performance of GO@PAN membrane was studied using organic dye (Congo red) as model pollutant. Rejection of Congo red was determined by the ratio of concentration in permeates and feed solutions. The applied pressure was in the range of 1.0 to 3.0 bar, depending on the thickness of GO layer. The results of rejection for Congo red are shown in Figure 6a, and indicated that rejection increased with the increase of thickness of GO layer. When thickness of GO layer reached to 128 nm, the rejection of Congo red maintained at 99.9%. Figure S3 and the inset showed the UV spectra and digital graph of Congo red feed solution and permeate. As can be seen from Figure S3, the permeate solution is transparent and no UV-vis absorption peak can be achieved. The high rejection performance of GO nanofiltration membrane for Congo red is mainly attributed to two mechanisms: (1) physical sieving by 2D nanochannels formed by the space between GO nanosheets, and (2) electrostatic repulsion between negatively charged


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Congo red and GO nanosheets. As previous study has reported that the average size of the 2D nanochannels is smaller than the size of organic dye (such as methyl blue and direct red). 34 Therefore, the dominant rejection mechanism is considered as the physical sieving. Meanwhile, electrostatic repulsion also contributes for the rejection of Congo red dye due to the large amount of carboxylic groups at the edge of GO nanosheets. Adsorption of dyes on the GO nanosheets has been proved limited contribution for the high rejection performance.34 Salts rejection is another important factor for nanofiltration membranes. Therefore, the rejection of Na2SO4 and NaCl of GO@PAN membrane (with 128 nm GO thickness) was tested. The results are shown in Figure 6b. The rejection for Na2SO4 and NaCl were 56.7% and 9.8% respectively. The rejection mechanism for salts is maybe mainly due to physical sieving and Donnan exclusion. Considering the hydrated radiuses of SO42- (3.79 Å), Cl(3.32 Å) and Na+ (3.58 Å) are slightly smaller than the radius (3.98 Å, as calculated from the XRD result shown in Figure 1e) of carbon nanochannel (formed by the two adjacent GO nanosheets) 30 physical sieving maybe has limited effect for salt rejection in this study. The Donnan exclusion mechanism is usually used for the explaining of rejection performance of charged nanofiltration membranes.63-64 According to Donnan exclusion, the ionic concentrations at membrane surface are not equal to those in the bulk solution. The counter-ion (ions with opposite charge of the membrane) concentration is higher at the membrane surface compared to bulk solution. Co-ion (ions with same charge of the membrane) concentration is lower at the membrane surface. When an external pressure is applied on the membrane, water can pass through the membrane, while co-ions are rejected due to the Donnan potential. At the same time, counter-ions are also rejected


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because of the requirements of electroneutrality. Donnan theory provides the following equation for the rejection (R) of different salts: R=1−

| |

= 1 − ( |

) / |

(1)

where zi and zj are the valence of co-ions and counter-ions, ci and cim are the concentrations of co-ions in the bulk solution and membrane respectively, cxm is the membrane charge concentration, and subscripts i and j indicate co-ions and counter-ions respectively. Based on equation (1), rejection for a 1:2 salt (such as Na2SO4) should be higher than a 1:1 salt (such as NaCl). Our results are identical with the theory calculation (as shown in Figure 6b), which suggests that salts are rejected mainly because of Donnan effect.

4. Conclusions In summary, a novel GO nanofiltration membrane was successfully fabricated on a highly porous nanofibrous mat through a simple vacuum suction method for the first time. The large lateral size of GO is a key point for fabrication of this nanofibrous mat supported GO nanofiltration membrane. GO layer was uniformly formed and the thickness can be well controlled by manipulation the concentration of GO solution. The results showed that GO nanosheets can form a unique structure with hydrophilic and negatively charged “gate” and hydrophobic 2D nanochannel. This unique structure has a significantly improvement for water flux of GO membrane based on the capture and slip flow theory. The structure of highly porous nanofibrous support also has a contribution for high water flux due to its low mass transfer resistance. Pollutant with a relatively large size (such as Congo red) can be rejected (99.9% for Congo red) by the combination of physical sieving


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and electrostatic repulsion due to the negatively charged “gate”. This “gate” is also effective for removal of salts due to the electrostatic repulsion (56.7% for Na2SO4 and 9.8% for NaCl). This nanofibrous mat supported GO membrane with unique structure (gatechannel type) exhibits promising nanofiltration application with high performance and low energy input.

Figure 1. SEM (a), TEM (b), ATR-FTIR spectrum (c), TGA curve (d), XRD pattern (e), and Raman spectrum (f) of GO.


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Figure 2. SEM (a) image of GO@PAN membrane; surface (b) and cross-section (c) of PAN nanofibrous membrane; GO layer (d), digital photo (e) and cross-section (f) of GO@PAN membrane (inset: water contact angle).

Figure 3. Cross-section SEM images of GO@PAN membranes with different GO loading (a-f); GO layer thickness in relation to GO loading amount (g).


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Figure 4. (a) Relationship between pure water flux and GO layer thickness (operated under 1.0 bar); (b) water flux in relation to pressure applied on GO@PAN membrane with GO thickness of 160 nm.

Figure 5. Conceptual illustration of hydrophilic “gate” and hydrophobic nanochannel.


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Figure 6. (a) Rejection performance of GO@PAN membrane with different GO layer thickness; (b) The rejection performances of GO@PAN membrane for salts. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” SEM images of GO@PAN membranes with different GO loading amount, time variation of pure water flux for a GO@PAN membrane and UV spectra & digital photo of Congo red solution before and after filtration AUTHOR INFORMATION Corresponding Author *[email protected] (K. Pan) Author Contributions † Jianqiang Wang and Pan Zhang contributed equally to this work.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The project is supported by the Beijing Science and Technology Project (Z141100000914001). REFERENCES (1)

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Table of Contents Graphic and Synopsis

Graphene oxide based nanofiltration membrane is successfully prepared on a highly porous nanofibrous mat with large pore size. The prepared GO@PAN membrane shows high water flux at extremely low pressure (1.0 bar), and promising rejection for dye and salts. Hydrophilic “gate” and hydrophobic nanochannel model is presented and used for explaining the membrane performance.


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SEM, TEM, ATR-FTIR spectrum, TGA curve, XRD pattern, and Raman spectrum of GO 715x433mm (72 x 72 DPI)


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SEM of the GO/PAN nanofibrous membrane 205x103mm (150 x 150 DPI)



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Thickness of the GO membranes 467x148mm (72 x 72 DPI)


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Permeability of the GO@PAN membranes 441x192mm (72 x 72 DPI)



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Mechanistic illustration of water filtration through GO membrane 154x82mm (150 x 150 DPI)


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Rejection performance of GO/PAN membrane 453x186mm (72 x 72 DPI)



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TOC picture 49x45mm (300 x 300 DPI)


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Graphene Oxide as an Effective Barrier on a ... - ACS Publications

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