Novel Halloysite Nanotubes Intercalated Graphene Oxide Based

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Novel halloysite nanotubes intercalated graphene oxide-based composite membranes for multifunctional applications: oil/water separation and dyes removal Guangyong Zeng, Yi He, Zhongbin Ye, Xi Yang, Xi Chen, Jing Ma, and Fei Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02723 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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Industrial & Engineering Chemistry Research

Novel halloysite nanotubes intercalated graphene oxide-based composite membranes for multifunctional applications: oil/water separation and dyes removal Guangyong Zenga, b, c

Yi Hea, b, c*

Zhongbin Yea, b, c*

Jing Maa, b

Xi Yanga, b

Xi Chena, b

Fei Lia, b

(a. State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, P R of China; b. College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, P R of China ; c. Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu, Sichuan 610500, P R of China ) *Address correspondence to this author. Email: [email protected]; [email protected]. Phone and Fax: +86 02883037315

Abstract: In recent years, owing to the unique properties such as high surface area and flexibility, graphene oxide (GO) has been extensively studied in the field of membrane preparation for wastewater treatment. However, GO-based membrane usually has a low water flux because of the narrow layer spacing between adjacent GO sheets. In this study, dopamine-modified halloysite nanotubes (D-HNTs) were used to increase the layer spacing of GO sheets, and then a series of novel D-HNTs/GO composite membranes were fabricated by vacuum filtration. The incorporation of D-HNTs could not only enhance the water flux of GO membrane significantly (from 9.6 to 218 L·m-2·h-1), but also improve the surface morphology and structure, which presented an underwater superoleophobic property. In addition, the wettability of membrane was further increased with the introduction of ethylenediamine in D-HNTs/GO membrane. More importantly, novel membranes were successfully applied for the simultaneous removal of oils and dyes from wastewater, which exhibited high water flux, good rejection ratio and even superior anti-oil-fouling property. In summary, the novel GO-based composite membranes could have a bright application prospect for wastewater treatment.

Keywords: Halloysite nanotubes; GO-based membrane; Oil/water separation; Superoleophobic; Dye removal

1. Introduction Water pollution has become a matter of concern to humankind. Owing to polluted water often contains insoluble oils and soluble dyes organics, discharge arbitrarily would threaten the ecosystem and human health1, 2. Therefore, several novel approaches (e.g., adsorption, burn, membrane separation and photocatalytic degradation) have been efficiently applied for wastewater purification3, 4. Among them, membrane separation has been demonstrated to be one of the most promising candidates for effectively treatment of wastewater5. Traditional membranes like polymeric and ceramic membranes have been successfully applied in the practical application of wastewater treatment. However, polymeric membranes usually can’t stand with harsh environments like strong acids and alkaline6. Besides, the applied pressure may compress the membrane and lead to the decrease of pore size, and thus causes the decline of membrane

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performance7, 8. Compared with polymeric membranes, porous ceramic membranes have good chemical resistance, as well as high flux and rejection ratios. But the inorganic ceramic membrane precursors are usually much more expensive than traditional polymeric membrane materials. This also causes the operating cost of membrane modules for industrial applications9, 10. Therefore, it’s important to find more superior materials for membrane filtration. Graphene oxide (GO), a single sheet prepared by oxidative exfoliation of graphite material, has moved into the spotlight in recent years for its polar oxygenated functional groups, including carboxyl, epoxy and hydroxyl groups, as a consequence of the attributes in able building blocks and accessible interface designs11-13. Recently, GO nanosheets with unique transport properties have attracted much attention in the field of water purification membrane14-17. Xu et al.18 synthesized some novel hybrid polyvinylidene fluoride (PVDF) ultrafiltration membranes by blending organosilane-functionalized GO into the membrane matrix, which improved the anti-fouling and mechanical properties of membrane significantly. Meanwhile, GO is highly stackable with self-assembly characteristics. Therefore, GO based membranes can be self-assembled via vacuum/pressure assisted filtration at the liquid-air interface19-21. Zhang et al.22 prepared GO framework composite membranes by layer-by-layer self-assembly and found both the rejection performances for heavy metal and water permeability were improved. However, the channels of GO-based membrane are usually too narrow to reach high permeation flux23. Recently, many reports24, 25 indicated that coupled nano-sized materials with the GO-based membrane could not only synergistically tune the channel structures for water transport, but also interconnect with the adjacent GO sheets so as to reinforce the membrane stability, which were both beneficial for efficient separation. Zhao et al.26 intercalated palygorskite nanorods into adjacent GO nanosheets to synergistically control the channel and structures of GO-based membranes, which brought high permeation flux and outstanding oil/water separation performance. Halloysite nanotubes (HNTs) are a kind of natural mineral with hollow tubular structure and have excellent physical and chemical properties, thereby providing opportunity for advanced applications in the field of membrane separation27-29. Compared with other nanomaterials, like carbon nanotubes, HNTs are more easily modified and possess a relatively low price. Zhang and his colleagues30-32 have investigated a series of novel membranes by blending functionalized HNTs into polyethersulfone (PES) membrane matrix to improve the hydrophilicity and anti-fouling ability of membranes. Hence, owing to the above environmental friendly properties, HNTs may also provide a potential to explore the novel separation by preparing GO-based HNTs composites membranes. In this work, we prepared HNTs modified GO-based composite membranes through a vacuum-assisted filtration self-assembly process on a cellulose acetate membrane support layer. In order to obtain more homogeneous composite membranes, HNTs were firstly modified by dopamine (D-HNTs), which is a mussel-inspired catechol-amine and can strongly adhere on virtually any types of solid surface33. Due to the existences of catechol and primary amine functional groups, dopamine can not only improve the dispersity of HNTs, but also provides more adsorption sites for the reaction between membrane and dye molecule. Then, ethylenediamine (EDA) was introduced to the composite membrane to further enhance the stability of the composite membrane. More importantly, the effect of HNTs on the wettability of membrane was discussed in detail. Novel membranes exhibited excellent performances of removal both insoluble oils and soluble dyes in water. In summary, no similar work that investigated homogeneous


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D-HNTs/GO membrane has been reported to the best of our knowledge, and this study has a strong theoretical and practical significance for wastewater treatment.

2. Experimental 2.1. Materials Natural graphite powders were obtained from Chengdu Organic Chemistry Co. Ltd. (Chengdu, China). Dopamine hydrochloride (DPA) (98%) and tris(hydroxymethyl) aminomethane (99%) were supplied by Aladdin (China). Halloysite nanotubes (HNTs, length 0.5-1.5µm, outer diameter = 30-70 nm, 95%) were purchased from Sigma-Aldrich. Cellulose acetate membranes (CAM) were provided by Solarbio science & Technology Co. Ltd, Beijing, China. Ethanediamine (EDA), methylene blue, hydrochloric acid (HCl) and sodium hydroxide (NaOH) (99%) were purchased from Kelong Chemical Co. Ltd, Chengdu, China. All reagents were all analytical grade without further purification. 2.2 Synthesis of GO and D-HNTs. GO was prepared from graphite powders according to the typical Hummers method. Besides, a typical synthesis process was represented for the preparation of D-HNTs as follows34: DPA (200 mg) and HNTs (100 mg) were dissolved in tris buffer solution (10mM, pH=8.5, 20ml) by ultrasonication and magnetically stirred for 12 h. Afterward, the precipitates were washed by deionized (DI) water repeatedly until the color of solution changed from dark to clear. After being centrifuged and dried, the D-HNTs were obtained eventually. 2.3 Fabrication of modified membrane. The preparation of D-HNTs/GO/EDA cellulose acetate membranes are depicted in Fig.1. GO (5 mg) was dissolved in DI water (100 mL) and D-HNTs (20 mg) were dissolved in DI water (200 mL) by ultrasonic cleaner. Afterward, GO dispersion solution (20 mL) was mixed with D-HNTs dispersion solution (5mL, 10mL and 15mL, respectively). Then, 2 mL of 1wt.% EDA water solution was fully dissolved in as-prepared solution by dripping slowly. The above solution was filtered on a CAM (with diameter of 4 cm and average pore sizes of 0.22 µm) under vacuum filtration (0.09 MPa). Table 1 summarized the composition of different solutions, and corresponding membranes were marked as M-0, M-1, M-2, M-3 and M-4, respectively.

Fig.1. Schematic depiction of the preparation of D-HNTs/GO/EDA membrane.



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Table1.The detailed composition of different membranes. Type of membrane

GO

D-HNTs

EDA

(mg)

(mg)

(mL)

M-0

1

0

0

M-1

1

0.5

0

M-2

1

1

0

M-3

1

1.5

0

M-4

1

1

2

2.4 Preparation of oil-in-water emulsions. Different types of oil-in-water emulsions were prepared as follows35: diesel oil, petroleum ether, n-hexadecane or dichloroethane was added into DI water, respectively. Then 200 mg∙L-1 SDS as an emulsifier was added into solution and the volume ratio of oil/water was 1:100. Finally the solution was stirred over 60 min at a rate of 300r/min until homogeneous emulsion was obtained. 2.5 Instruments and characterization To identify the crystallinity of D-HNTs/GO, the X-ray diffraction (XRD) (X'Pert PRO MPD, Holland) was used at 40 kV and 40 mA. To further prove the reaction between EDA and D-HNTs/GO, X-ray photoelectron spectroscopy (XPS) (KRATOS, XSAM800) was also utilized to characterize nanoparticles composites. Surface morphologies of HNTs and D-HNTs/GO/EDA were visualized by transmission electron microscopy (TEM) (JEOL, JEM-100CX) with an accelerating voltage of 120 kV. Contact angles (CA) of different membranes were detected using a contact angle measuring instrument (Beijing Hake, XED-SPJ). The surface morphology of membrane surface was observed through scanning electron microscopy (SEM, JSM-7500F, JEOL). Besides, atomic force microscopy (AFM) (NSK, SPA300HV) was utilized to test the surface roughness of membrane. Besides, elemental mapping of nanoparticles composites on membrane surface were characterized by X-ray energy dispersive spectrometry (EDS) (JSM-7500F, INCA). The concentrations of methyl blue (MB) was determined spectrophotometrically by the UV–vis-NIR on a UV-762 (Shanghai precision scientific instrument co.). The oil concentration was measured by Total organic carbon (TOC) (Shimadzu TOC-VCPH analyzer). The optical microscopy images of the emulsion and permeate sample were observed by Motic BA300Pol microscope (Nikon Digital Sight DS-F11). 2.6 Permeation and anti-fouling performances The permeation and anti-fouling performances of membranes were tested by recording liquid flow rate, concentration and flux recovery rate of each filtration cycle. Every newly prepared membrane was pre-pressured for 10 min under the pressure of 0.09 MPa. The instantaneous permeate flux (J), flux recovery rate of each cycle (FRR) and rejection (R) of the membranes were evaluated with equation (1), (2) and (3), respectively36.


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(1)

% 1 100

(2)

% . 100

(3)

.

Where V (L) is the volume of penetrate flow, A (m2) is the membrane effective area, and t (h) is the filtration time. Cp and Cf (mg/L) are the MB or oil concentrations of the feed and permeate, Jw.1 and Jw.2 (L·m-2·h-1) are the pure water flux in the initial of different cycles, respectively.

3. Results and Discussion 3.1. Characterization of nanoparticles

Fig.2. XRD patterns of GO, D-HNTs and D-HNTs/GO. Fig.2. shows the XRD patterns of GO, D-HNTs and D-HNTs/GO composites. A sharp peak around 11.5° (001) in the XRD pattern of GO was associated with the (001) inter-layer structure of GO sheets. For D-HNTs, the diffraction peaks at 11.8, 19.8, 24.8, 34.8, 54.5 and 62.5° could be indexed to the hexagonal structured Al2Si2O5(OH)437. The XRD pattern of D-HNTs/GO nanocomposites showed same peaks with GO and D-HNTs, while no (001) diffraction of GO was observed, thus implying that the peak of GO (001) might be overlapped by the peak of D-HNTs (H001). This revealed that the preparation of D-HNTs/GO composites was successful. The XPS spectra and their curve-fitting of different nanoparticles including GO, D-HNTs/GO and D-HNTs/GO/EDA are displayed in Fig.3. For pure GO, there were plenty of oxygen shown in its structure, and the C1s curves contained five peaks centered at 284.5, 285.3, 286.1, 286.9 and 288.2eV, which corresponded with C=C sp2, C-C sp3, C-O, C=O and O-C=O bands38, respectively. However, it could be observed that three additional elements (N, Si and Al) were shown in the spectra of Fig.3 (c) and Fig.3 (e). Correspondingly, new peaks emerged at 285.2 (C-N) and 286.3eV (C=N) were formed in their C1s fitting curves. The results were attributed to the incorporation of D-HNTs39. In addition, the peak intensity of C and N of D-HNTs/GO/EDA were



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higher than that of D-HNTs/GO, which could be further affirmed by the analysis of surface element compositions in Table2. D-HNTs were existed in the inner and outer structure of GO and GO was further coated by EDA, which caused the decline of Si and Al contents eventually. The above results could provide a strong evidence for the synthesis of composites formed between D-HNTs and GO.

(a)

(b)

GO

O1s

SP2 C=O

C-O

C1s

O-C=O C-C

1000

800

600

400

200

0

280

Binding Energy(eV)

(c)

282

284

286

288

290

292

Binding Energy(eV) GO-DHNTs

(d)

O1s

C-O

sp2 C-N

C=O

C1s

C=N

O-C=O N1s

Si2p Al2p

1200

1000

800

600

400

200

0

280

282

284

Binding Energy(eV)

(e)

286

288

290

292

Binding Energy(eV)

(f)

GO-DHNTs/EDA

O1s

sp2

C1s

C=N

C-N

C-O

N1s

C=O

O-C=O

Si2p Al2p

1200

1000

800

600

400

200

0

280

282

284

286

288

290

292

Binding Energy(eV)

Binding Energy(eV)

Fig.3. XPS spectra and their curve-fitting of C1s: (a), (b) for GO, (c), (d) for D-HNTs/GO, and (e), (f) for D-HNTs/GO/EDA.

Table2. Detailed element content of D-HNTs/GO and D-HNTs/GO/EDA. Samples D-HNTs/GO D-HNTs/GO/EDA

element content（at %） C

N

O

Si

Al

55.41 63.21

3.72 12.07

34.60 24.05

3.25 0.66

3.02 -


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Fig.4. TEM images of (a) HNTs, (b) D-HNTs, (c) D-HNTs/GO and (d) D-HNTs/GO/EDA. The surface morphologies of different nanoparticles are observed by TEM and the images are shown in Fig.4. Nascent HNTs had a hollow tubular structure with a smooth surface. The internal diameters of HNTs ranged from 20 to 30 nm, with the external diameters about 30 to 70 nm. After the treatment of HNTs with dopamine, the outer surfaces of D-HNTs became much rougher with some irregular substance and the wall thickness was increased slightly in Fig.4 (b)34. In addition, it could be observed that some multi-layered and wrinkled structures were displayed in Fig.4 (c) and Fig.4 (d), which were consistent with GO typical morphology. HNTs were mainly anchored onto the surface or intercalated between the GO nanosheets40. Importantly and interestingly, it was clear that D-HNTs were relatively more evenly distributed in the GO matrix after EDA treatment. It was due to the fact that the abundant hydrophilic amino groups in EDA grafted on D-HNTs, which further improved the dispersion of D-HNTs in GO sheet structure.

3.2 Morphology of GO-based membranes



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Fig.5. Surface SEM images of different membranes: (a) M-0, (b) M-1, (c) M-2, (d) M-3 and (e) M-4. The surface morphologies of membranes are shown in Fig.5. The as-prepared ultrathin pristine GO membrane (M-0) exhibited a relatively smooth surface with several wrinkled corrugations and without visible defects. Such wrinkles of M-0 arose from the hydrogen bonding between the hydrophilic functional groups on the basal planes and edges of the GO sheets. This hydrogen bonding affected how the convoluted GO sheets were assembled and thus contributing to the wrinkles. As for the D-HNTs modified membranes, there were some tubular structure appeared on the surface. In addition, the intercalation of D-HNTs within the interlayers of GO nanosheets did not damage the morphology of graphene layer of the membrane due to the good flexibility of GO and the excellent compatibility between graphene and D-HNTs. When EDA was added into membrane, the surfaces of GO nanosheets were evenly distributed with 1D interpenetrating D-HNTs shown in Fig.5 (e). As described above, not only because the EDA could enhance the dispersion of D-HNTs, but also provide abundant amino groups, which were beneficial for achieving well-distributed surfaces. For all of these vacuum filtrated composite membranes, no visible defects appeared on their surface. More importantly, the composite membrane could be bent randomly without any damage, which was presented in Fig.1.


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Fig.6. 3D AFM images of different membranes: (a) M-0, (b) M-1, (c) M-2, (d) M-3 and (e) M-4.

Fig.7. EDS elemental mapping of D-HNTs/GO/EDA membrane surface (M-4). The AFM image also indicated the uniform distribution of HNTs nanotubes among the surface of kinds of membranes shown in Fig.6. Obviously, almost all the membranes containing D-HNTs showed larger roughness compared with M-0. The average roughness (Ra) of M-3 reached 56.1nm, whereas the Ra of M-0 was just 27.3nm. The result of the enhancement of membrane roughness could improve the capacity of underwater superoleophobicity of interfaces. Some water molecules were adsorbed on membrane surface and thus formed a protective layer between oil and GO membrane, which prevented the oil drop contacting on the GO membrane surface eventually. The result was also consistent with the conclusion of hydrophilicity test discussed in a later section. The EDS elemental mapping of M-4 is presented in Fig.7, which revealed uniformly dispersion of C, N, O, Al and Si elements on the top surface of D-HNTs/GO/EDA membrane. The results confirmed the formation of D-HNTs/GO/EDA on the CAM substrates. The presences of C



and O elements were derived from GO, whereas Al and Si elements were ascribed to D-HNTs. In addition, the element N was derived from EDA. Overall, the above results demonstrated that the self-assembly process of D-HNTs modified GO-based composite membrane was orderly and uniform.

3.3 The hydrophilicity of membranes

(a) 80

(b) 180

Underwater OCA(°)

160

70

CA (°) 60

140

120

100

80

60

50

M-0

M-1

M-2

M-3

dichlorom

M-4

diesel oil

corn oil

petroleum 1,2-dichlorom

(C) 250 200

-2

-1

Flux (L. m ⋅ h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150

100

50

0

M-0

M-1

M-2

M-3

M-4

Fig.8. The hydrophilic property of membranes: (a) water contact angle; (b) underwater oil contact angle and (c) pure water flux. The hydrophilicity of the membranes were evaluated by water contact angle (CA), underwater oil contact angle (OCA) and pure water flux (PWF), which are shown in Fig.8, respectively. It could be noticed that the M-0 membrane showed a CA about 73.5° in air. In contrast, the CA of membrane was gradually decreased with the increasing of D-HNTs content and decreased to as low as 67.6° after adding 1.5 mg of D-HNTs. However, when EDA was added into the D-HNTs/GO composite, the as-prepared (M-4) exhibited the best water wettability with a CA of 59.4°. In addition, dichloromethane was chosen to further test the OCA of M-4 in underwater environment. As is shown in Fig.8 (b), M-4 exhibited a large OCA of about 156.5°± 2.5°and there was invisible deformation of oil drop during the contact test. Also, all the underwater OCAs of M-4 were larger than 150°. It indicated that the D-HNTs/GO/EDA membrane possessed an extremely low oil adhesion and excellent underwater superoleophobic property regardless of the types of oil. The pure water fluxes of various membranes are displayed in Fig.8 (c). For M-0, the pure


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water flux was just 9.6 L·m-2·h-1, which was extremely low at a relatively low pre-compacting pressure (0.09 MPa). By contrast, all these D-HNTs/GO membranes (M-1, M-2, M-3 and M-4) showed much higher fluxes than M-0. This was due to that the introduction of D-HNTs could enlarge the inter-lamellar spacing of GO nanosheets. However, the thickness of GO membrane was also increased after further adding D-HNTs, which extended the time for water molecule permeating through the membrane. Hence, the water flux of M-3 was lower than that of M-2. Generally speaking, M-4 showed the highest water permeation flux reached 218 L·m-2·h-1. The result was further attributed to the generation of abundant hydrophilic groups (such as –NH2) within the interlayers of GO nanosheets after being introduced by EDA. In addition, the surface hydrophilicity and low-oil adhesion property were also significant factors in determining the antifouling performance of membranes.

3.3 The separation performances of membranes 3.3.1 oil/water separation 100

130

240 220

-1

97 110

200

Flux (L⋅m-2⋅h-2)

120 98

-2

Rejection (%)

99

Flux (L⋅m ⋅h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


180 160 140 120

96

100 95

100

Dichloromethane

Diesel oil

Corn oil

80 0

5

10

15

20

25

Time (min)

Fig.9. Oil/water separation performance of M-4: (a) oil rejection and flux, (b) reusability behavior, (c) emulsion solution before and after filtration. To test the separation capability of the as-prepared membrane, the oil/water separation process was performed under a trans-membrane pressure of 0.09 MPa. A series of water-in-oil emulsions, which were named dichloromethane, diesel oil and corn oil, respectively, were prepared to evaluate the separation property of M-4. The microscope image showed that there were uniformly oil droplets with size of 1–10 µm in feed solution shown in Fig.9 (c). After being filtrated by novel D-HNTs/GO/EDA membrane, it was clearly observed that the collected filtrated solution barely contained any oil droplets. The results also showed that M-4 exhibited efficient separation ability with oil rejection ratio over 99% and relatively high fluxes for different emulsions displayed in Fig.9 (a). In addition, the antifouling capacity and reusability of the super-wetting membrane were tested using diesel oil-in-water emulsion as typical pollutants and the membrane was washed by DI water after each cycle. As is shown in Fig. 9 (b), the flux of



novel membrane was decreased gradually along with the filtration of emulsion. The result was attributed the accumulations of pollutants in membrane surface, which formed oil cake near the membrane surface and even blocked the membrane pores, and thus would increase filtration resistances especially in a dead-end experimental device. However, it was found that the flux of membrane had a good recovery after being cleaned by water for every cycle. After three times of fouling-washing tests, the flux recovery ratio of M-4 could still reach about 90%. The surface roughness of membrane was decreased after being modified by hydrophilic EDA. In summary, the above results further indicated that D-HNTs/GO/EDA membrane presented an excellent anti-fouling property.

3.3.2 Dye removal experiments

(a)

100

225 80

200

50 50

Critical Volume(mL)

Rejection Ratio (%)

(b)

250

100

Flux (L⋅m-2⋅h-1)

25

60

40

20

0

0 M-0

M-2

0

M-4

(c)

M-1

2.0

M-2

M-3

M-4

MB CAM M-0 M-2 M-4

1.5

Absorbance

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1.0

0.5

0.0 350

400

450

500

550

600

650

700

750

800

Wavelength(nm)

Fig.10. Dye removal of different membranes: (a) MB rejection and flux, (b) critical solution volume, and (c) absorption experiments. The performance of these membranes for dye removal from wastewater is shown in Fig10. As an example, methylene blue (MB) was used as model contaminant to test different types of membranes. The MB solutions became colorless after being filtered by the membranes. The rejection of membrane was above 99% for the MB solutions with an initial concentration of 10 ppm, which is shown in Fig10 (a). The reasons for this could be listed as follows41. For one thing, there were plenty of carboxyl groups among every layer of GO sheet, which made GO present negative charge. Hence, the positive charge MB molecules were adsorbed during the permeation of dye wastewater. For another thing, it have been demonstrated that HNTs showed strong negative charge at different ranges of pH value (3 to 11). Besides, functional groups such as catechol groups, aromatic moieties and amine groups on the surface of D-HNTs could further have an adsorption reaction with cationic dyes. However, the dye fluxes of D-HNTs intercalated


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GO-based membrane (M-2 and M-4) were much higher than pure GO membrane. This was attributed to the hydrophilic D-HNTs/GO/EDA membrane containing abundant N and O-containing groups42. Moreover, different membranes maintained separation efficiency above 99% for MB solution with a critical solution volume presented in Fig10 (b). We could found that these values rose with the increasing of D-HNTs additive contents. When the EDA solution was added, the as-prepared membrane showed an even higher critical solution volume (nearly 90 mL), whereas critical solution volume of M-0 and M-3 were 36 mL and 55 mL, respectively. It was mainly due to that EDA could provide more adsorption sites for dye absorption. In addition, to confirm whether the removal of dyes was caused by absorption or interception by the CA membrane support layer, we also carried out a series of comparison experiments. All the membranes including the CAM, M0, M2 and M4 were directly dipping in MB solutions (30mL, 10ppm), respectively. These membranes were taken out once the concentrations of the MB solutions were steady. Apparently, the results are shown in Fig10 (c) and indicated that the CA substrate membrane could barely influence the absorption performance of these composite membranes, besides that, during the real separation procedure, the contaminants in feed solution were firstly and mainly contacting with the upper GO part of the composite membrane, thus the effects of CA membrane on dyes removal could be neglected. Furthermore, it could be concluded that the pure GO had a certain degree of absorption capacity for MB, and both the addition of D-HNTs and EDA could further enhance the absorption performance of composite membranes. The above results demonstrated that novel prepared GO-based composite membranes were capable for the absorption or interception of dyes, which possessed potential applications in water purification field.

3.3.3 Mixed wastewater treatment by novel membrane

MB+oil MB oil emulsion filtrate

100

1.4

0.7

100

90

80

0.0 300

90

350

400

450

500

550

600

Wavelength(nm)

650

700

750

800

0

20

40

60

Volume (mL)

Fig.11. Mixed wastewater treatment by M-4.


80

100

70

MB rejection(%)

Oil Rejection(%)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The removal performance of M-4 for insoluble oils and soluble dyes in mixed water was evaluated in the following experiments shown in Fig.11. It could be observed that the color of feed solution changed from blue to clear after being filtrated by M-4. The corresponding result was further displayed by UV-vis spectrum in Fig.11 (b). The absorption peak of MB was emerged at 664 and 609 nm. In a sharp contrast to feed solution, there was no obvious absorption peak in permeation solution, which nearly presented as a straight line. Besides, the absorbance of oil emulsion was significantly decreased after being removed by membrane. The above studies indicated that novel membrane could remove different scales of pollutions from wastewater efficiently. In order to investigate the reuse and sustainable ability of M-4 for the removal of mixed wastewater, the filtration test was sequentially carried out for ten cycles. The volume of solution is 10 mL for every cycle and the result is shown in Fig.11 (c). It could found that novel D-HNTs/GO/EDA membrane exhibited an excellent and stable ability for oil removal, and oil rejection kept a high value (nearly 99%) after ten times of filtration. In addition, although the MB rejection was slightly reduced after dealing with 70 mL solution, the MB rejection ratio still reached 95%. The results demonstrated that D-HNTs/GO/EDA membrane provided a bright and promising potential for the application in mixed wastewater treatment.

4. Conclusion In conclusion, a novel underwater superoleophobic GO-based composite membrane was successfully fabricated through a vacuum-assisted filtration self-assembly process. Dopamine modified HNTs were intercalated between GO nanosheets layer. The additions of D-HNTs and ethanediamine not only endowed the membrane with the capacity of underwater superoleophobic, but also introduced abundant O and N-containing groups. Simultaneous and continuous removal of insoluble oils and soluble dyes from wastewater was successfully realized. More interestingly and importantly, theses membranes also showed excellent recyclability and anti-fouling performance. Hence, we believed the as-prepared novel GO-based composite membrane is promising for high-end applications in treating polluted water from most industrial fields.

Acknowledgments Financial support of this work is acknowledged to the Youth science and technology creative group fund of Southwest Petroleum University (2015CXTD03), and the majorly cultivated project of sci-tech achievements transition (15CZ0005) from the education department in Sichuan Province.

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For Table and Contents Only

Schematic depiction of the preparation of D-HNTs/GO/EDA membrane.


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Novel Halloysite Nanotubes Intercalated Graphene Oxide Based

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