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Oriented Clay Nanotube Membrane Assembled on Microporous Polymeric Substrates Lijuan Qin, Yafei Zhao, Jin-dun Liu, Jingwei Hou, Yatao Zhang, Jing Wang, Junyong Zhu, Bing Zhang, Yuri M. Lvov, and Bart Van der Bruggen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12858 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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ACS Applied Materials & Interfaces

Oriented Clay Nanotube Membrane Assembled on Microporous Polymeric Substrates Lijuan Qin, a ‡ Yafei Zhao, a ‡ Jindun Liu, a Jingwei Hou, b Yatao Zhang, a* Jing Wang, a, c Junyong Zhu, c Bing Zhang, a Yuri Lvov d** and Bart Van der Bruggen c a School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China b UNESCO Centre for Membrane Science and Technology, University of New South Wales, Sydney, Australia c Department of Chemical Engineering, KU Leuven, Heverlee, Belgium d Institute for Micromanufacturing, Louisiana Tech University, LA 71272, USA and I. Gubkin Russian State University of Oil and Gas, Moscow 119991, Russia ‡

These authors are contributed equally. *Corresponding authors: E-mail: [email protected] (Yatao Zhang); [email protected] (Yuri Lvov)

Abstract: Organized arrays of halloysite clay nanotubes have great potential in molecular separation, absorption, and biomedical applications. A highly oriented layer of halloysite on polyacrylonitrile porous membrane was prepared via a facile evaporation-induced method. Scanning electronic microscopy, surface attenuated total reflection Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy mapping indicated formation of the nanoarchitecture-controlled membrane. The well-ordered nanotube coating allowed for the excellent dye rejection (97.7 % for Reactive Black 5) with high salt permeation (86.5 % for aqueous NaCl), and thus these membranes were suitable for dye purification or concentration. These well-aligned nanotubes’ composite membranes also showed very good fouling resistance against dye accumulation and bovine serum albumin adsorption as compared to the pristine polyacrylonitrile or membrane coated with disordered halloysite layer. Keywords: Oriented halloysite nanotubes; composite membranes; self-assembly; controlled thickness; antifouling

1.

Introduction

Dyes have been extensively applied in various fields, such as textile industry, printing industry, and food technology.1 More than 700,000 tons of commercial dyes are produced annually worldwide and most of them are prepared via chemical synthesis.2 Their by-products, including inorganic salts and residual compounds, need to be removed to meet quality demands for the market dyes.3-4 Membrane technology, as a facile, effective and low energy consumption method, has been applied to tackle the dye/salt separation problem.5 Functional nanomaterials, e. g. carbon nanotubes (CNTs), titanium dioxides (TiO2) nanotubes and boron nitride (BN) nanotubes have been explored to fabricate organic-inorganic hybrid membranes owing to their hollow structure.6-8 Natural clay halloysite nanotubes (HNTs) with tubular structure have also drawn many researchers’ attention. Compared with other nanotubes, HNTs offer an abundant and 1

ACS Paragon Plus Environment


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inexpensive alternative. For example, the cost of HNTs is less than one percent of that of CNTs.9 HNTs applications are often based on exploiting the lumen as a container for sustained drug delivery, dye removal, or loading functional chemicals.10-12 HNTs have been shown to have little or no toxicity for cell cultures and for soil C. elegans microwors and infusoria, and they have been employed in biomedical nanocomposites.13-15 The incorporation of HNTs into polymeric membranes has also attracted attention. Clays can improve the thermal stability, mechanical strength, hydrophilicity, and antifouling properties of such nanocomposite membranes.16, 17 HNTs-derived functional membranes have been successfully fabricated by blending and phase inversion techniques. Poor compatibility between the polymeric matrix and inorganic clays adversely affects the performance and stability of these membranes.18 Surface modification of HNTs with functional organics is utilized to mitigate this issue.19 Polyaniline coated halloysite (PANi-HNTs) was synthesized by in situ polymerization, followed by mixing with polysulfone to prepare composite membranes for gas separation.20 Improved permeability and selectivity for CO2 and N2 could be obtained by incorporating 1% of PANi-HNTs into the composite membrane. We also loaded Ag and Cu nanoparticles onto the surface of modified halloysite for constructing hybrid membranes, which demonstrated significant enhancement of desalination ability or antifouling properties.3, 17, 21 Besides, many methods including interfacial polymerization, dip coating, and electrospinning have also developed to obtain membranes with excellent performance.22 Halloysite was incorporated into polyamide (PA) layer through interfacial polymerization to construct thin-film nanocomposite membranes for forward osmosis.23 The membrane with 0.05% HNTs had the best performance, which exhibited 66% high water permeability (from 2.80 L/m-2·h to 4.56 L/m-2·h) and fouling resistance. It also maintained salt rejection at 88%. Most of the research focused on optimizing the adhesive force of HNTs to improve their dispersion ability into the polymeric matrix or onto microporous substrates in order to obtain membranes containing HNTs.3, 17, 18, 20, 21, 23, 24 The spatial arrangement of the nanotubes in these membranes was neglected, even some studies have proved that the self-assembled materials had more excellent properties. Ultra-strong and stiff layered polymer nanocomposites could be achieved by self-assembly of montmorillonite (MMT) clay via layer-by-layer method.25 It was also reported that multi-walled carbon nanotubes (MWCNTs) could align on a polyvinyldene difluoride microfiltration membrane via vacuum filtration under the electric-field.26 The results demonstrated that this coating was conductive with anisotropic electrical behavior and the aligned MWCNTs had a lower resistance compared to their non-aligned counterparts. So, fabricating self-assembled HNTs membranes is required and the impact of alignment of HNTs on membrane performance is also demanded to investigate. We report an evaporation-introduced self-assembly technique to configure HNTs to self-assemble on membrane surface. This was based on following aspects. Firstly, evaporation has been known as a facile and controlled method for nanoparticle self-assembly. Types of nanoparticles that can be assembled include spherical polystyrene and silica and rod-like tobacco mosaic virus. Evaporation has also been applied for preparing clay-containing films.27, 28 Earlier, we have demonstrated that highly charged clay nanotubes may form oriented and densely packed layers over a smaller area by the droplet-casting method.29 Secondly, HNTs could be easily exfoliated to single tube aqueous dispersions via the modification by PSS. Finally, the formation of hydrogen bonds with -OH groups between HNTs and polyvinyl alcohol (PVA) were fully utilized to construct a stable coating on substrates.30 The focuses of this work were on fabricating negatively charged self-assembled coating for dye/salt separation on 2


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polyacrylonitrile membrane with excellent antifouling properties and investigating effect of alignment of the halloysite layer on membrane performance.

2.

Experimental section

2.1 Materials: Polyvinyl alcohol (PVA) and poly (sodium-p-styrenesulfonate) (PSS) (Mw ≈ 70,000 Da) were purchased from Sigma-Aldrich (St. Louis, MO) and J&K Scientific Ltd., respectively. Polyacrylonitrile (PAN) membranes (molecular weight cut-off, MWCO = 50,000 Da) were obtained from Sepro membranes (Beijing, China). The hydrolyzed PAN membranes were prepared by the immersion of PAN into NaOH aqueous solution (1 mol L-1) at 50 oC for 1 h. Hydrophobic polysulfone (PS) membranes (MWCO = 50,000 Da) were obtained from Hangzhou Water Treatment Technology Development Center (Zhejiang province, China). Halloysite nanotubes (HNTs) were supplied by Henan Xianghu Environmental Protection Technology Co., Ltd (Henan province, China). Glutaraldehyde (50% water solution), magnesium sulfate anhydrous (MgSO4, >99.0%) and sodium sulfate anhydrous (NaCl, >99.0%) were all purchased from Kermal (Tianjin, China). Reactive Red 49 (RR 49, Mw = 576.49 Da) and Reactive Black 5 (RB 5, Mw = 991.82 Da) were purchased from Sunwell Chemicals Co. Ltd. (Hangzhou, China). All other chemicals were used without further purification. Polyethylene glycol (PEG) 200, 400, 600, 1000 and 2000 were bought Kermal (Tianjin, China) and used for the MWCO measurement. Bovine serum albumin (BSA) was purchased by Zhengzhou Yikang Biological Engineering CO., Ltd (Zhengzhou, China). Deionized (DI) water (≧ 18.2 MΩ) was generated by a reverse osmosis water purification system and was used throughout the experiments. 2.2 HNTs modification with PSS: PSS (2 g) was dispersed in water (100 mL) and stirred for 30 min to form a homogenous suspension. Then HNTs (2 g) were added to the mixture and the modification was carried out at room temperature under constant stirring for 48 h. The excess PSS was removed by centrifugation and re-dispersion in DI water for three cycles. The modified HNTs (m-HNTs) were dried in a vacuum oven for 24 h and then grounded to fine power before further use. 2.3 m-HNTs coating on membrane surface: In this work, the evaporation technique was applied to coat the PAN or PS membrane surface with m-HNTs. 2 mL of m-HNTs suspension was initially mixed with 1 mL of 0.2 wt % PVA solutions, and then the mixture was poured into a dead-end membrane cell with an effective membrane area of 28.3 cm2 (Figure S1 in supporting information). The cell was put in an oven and evaporation of the solvent (water) was carried out at 80oC. Afterwards, the membrane was immersed in glutaraldehyde aqueous solution (0.2 wt %) for 30 min at 80 oC to form the stable coating layer. 2.4 Characterization: The pristine HNTs and m-HNTs powder were characterized by Fourier transform infrared spectroscopies (FTIR), high resolution transmission electron microscopy (HRTEM) and thermogravimetric analysis (TGA). The FTIR was performed at 2 cm-1 resolution with Thermo Nicolet IR 200 spectroscope (USA) spectra in the range of 400-4000 cm-1. TEM images were obtained by HRTEM (JEOL JEM-2100, Japan). The thermogravimetric analysis (TGA) was carried out in NETZSCH TG 209 (Germany) instrument under a nitrogen atmosphere with a heating rate of 10 ˚C/min from room temperature to 650 ˚C. ζ-potential was measured on a microelectrophoretic ZetaPlus Potential Analyzer (Brookhaven Instruments) at 25 ± 0.1˚C. The surface morphologies of the prepared membranes were obtained by field emission scanning electron microscope (FESEM, JEOL JSM-7001F, and Japan) and atomic force microscopy (AFM, Bruker Dimension Fastscan, USA) at the ScanAsyst 3



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mode. A spectrometer (Nicolet iS50, Thermo Scientific, USA) equipped with an attenuated total reflectance (ATR) unit was used to obtain the ATR-FTIR spectra of the PAN and m-HNTs/PAN membranes with ten scan time. Energy Dispersive X-ray Spectroscopy (EDXS) was carried out in a JEOL JSM-7500F FESEM to determine the distribution of m-HNTs on PAN substrates. Hydrogen bonds between m-HNTs and PVA polymer were confirmed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, and USA) recorded by using monochromatized Al Kα radiation (1486.6 eV). The hydrophilicity of PAN substrates and m-HNTs/PAN membranes was analyzed by measuring dynamic water contact angles. The dynamic water contact angles were measured by a contact angle goniometer (Maist Drop Meter A-100P) equipped with a high-speed charge-coupled device (CCD) camera via the sessile drop method. 2.5 Data analysis: The length and diameters of the m-HNTs were determined via HRTEM images by using ImageJ 1.46r (National Insititutes of Health, USA). The average external and internal diameters are around 57.5 ± 14.2 and 13.5 ± 4.1 nm, respectively. The length of m-HNTs is 0.76 ± 0.53 µm (size distribution in Figure S2 in supporting information). Therefore, the major-axis/minor-axis aspect of m-HNTs is about 13.2. The XPS peak differentiation imitating analysis was conducted by the XPSPEAK 4.1 software. The ζ-potential obtained by Smoluchowski formula was -36 mV for HNTs and -64 mV for m-HNTs, respectively. Raw AFM images were further processed by utilizing the NanoScope Analysis V.1.5. software (Bruker). 2.6 MWCO measurement and calculation of mean pore size: The MWCO and mean pore size are crucial parameters that significantly affect the separation performance via sieving effect. The MWCO measurement was conducted by using different molecular weight PEG ranging from 200 to 2000 Da. As depicted in the previous studies,17, 21, 31 a cross-flow filtration setup was utilized and the test was conducted by using PEG with 500 mg/L as the feed under 0.8 MPa. The rejection (R) of PEG can be calculated from concentration of the feed (Cf) and concentration of the permeate (Cp) according to following equation:32 R (%) = (1 −

Cp Cf

) × 100

(1)

Moreover, according to the relationship between MWCO and Stokes-Einstein radii, the mean pore radius of the corresponding membrane can be obtained by following equation: 32, 33 rs = 0.0262 nm (

M g ) − 0.03 nm mol

(2)

Where M is the molecular weight of PEG which is rejected at least 90 % by the membrane. 2.7 Filtration tests: The permeability and selectivity of membranes were also measured with the same cross-flow filtration setup. For the permeability test, each membrane was initially held at a pressure of 0.8 MPa for about 30 min to allow the flux to reach a steady state before testing. Equations (3) and (4) are used to determine water flux (J) and permeability (A) of the fabricated membranes, respectively. J=

∆V Am ⋅ ∆t

(3)

A=

J ∆P

(4)

4


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Where Am, ∆V, ∆t and ∆P refer to effective membrane area (12.57 cm2), permeate volume (m3), filtration time (h), and transmembrane pressure difference (MPa), respectively. Single salt (NaCl or MgSO4, 1 g L-1) rejection was measured under 0.8 MPa. The salt rejection (Rs) of each membrane is also calculated using the equation (1). Cf and Cp are the salt concentrations in the feed and permeate solution, which could be determined by conductivity measurement. The rejection test of a single dye solution was measured under the same conditions with either Reactive Black 5 (0.5 g L-1) or Reactive Red 49 (0.5 g L-1) as the feed. The calculation of dye rejection (Rd) is similar to that of salt according to the equation (1). Different with the determination of salt concentration, the dye concentration was detected by UV-Vis spectrophotometry. The detailed information about dyes, UV spectra and standard curve is presented in Table S1. The dye/salt selectivity (α) can be calculated as the following equation (5):4

α=

1-Rs 1-Rd

(5)

2.8 Antifouling experiment: The dynamic filtration fouling test was conducted by utilizing BSA as the model protein pollutant. The procedures of the fouling experiment were conducted by following previous report.34 Firstly, the membrane was hold with DI water under 0.8 MPa at least for 30 min until the water flux stabilized and then water flux (Jw) was measured at 15 min intervals. Secondly, 0.1 g L-1 BSA (pH 7.0 in phosphate buffer solution) was used to test permeation flux (Jp) at the same time intervals. Thirdly, the water flux (Jwf) was measured again with the DI water after the membrane was fully washed by DI water. The water flux recovery ratio (FRR) and decline of flux caused by irreversible fouling (Rir) and reversible fouling (Rr) were measured to estimate the antifouling property of the membrane. They can be calculated by the following equations.32

FRR (%) =

J wf Jw

× 100

J  Rir (%) =  1 − wf Jw 

(6)

  × 100 

(7)

 J −Jp   × 100 Rr (%) =  wf  Jw 

(8)

The antifouling properties of the membrane for dyes (RB 5 and RR 49) were also tested through filtrating the dyes solution for a certain time.

3.

Results and Discussion

The fabrication process of halloysite nanotube membranes (m-HNTs/PAN) is exhibited in Figure 1a. Firstly, the HNTs were modified with PSS to enhance the dispersion in DI water (Figure 1b). The well dispersed modified HNTs (m-HNTs) allowed for preparing a homogeneous suspension of m-HNTs and PVA aqueous solution (Figure S3). Then, the m-HNTs/PAN membrane could be fabricated through coating the suspension onto the surface of PAN substrate via gradual evaporation, followed by 5



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cross-linking with glutaraldehyde (GA) at 80 °C. This process resulted in a well-aligned and dense nanotube layer coating on the PAN membrane surface.

Figure 1. Fabrication process of m-HNTs/PAN composite membrane (a), dispersion of HNTs and m-HNTs in water (b), schematic diagram of HNTs (c) and TEM images of HNTs and m-HNTs (d). HNTs are formed by rolled aluminosilicate sheets with tetrahedral silica oxide on outside surface and octahedral aluminum inside (Figure 1c). The hollow tubule structure and different inside / outside chemistry are beneficial for selective functionalization (Figure 1d).12 FTIR, TG and TEM were employed to verify the modification of HNTs with PSS. Figure 2 shows FTIR spectra of the original HNTs and m-HNTs. The characteristic peaks at 3693 and 3624 cm-1 belonged to the stretching vibration of hydroxyl groups of HNTs, and a strong adsorption at 1033 cm-1 originated from the asymmetric flexible vibration of O-Si bond due to the plenty of O-Si-O groups on the outer surface of HNTs.31 Compared with the original HNTs, a new peak at 1230 cm-1 and increased strength at 1200 cm-1 were ascribed to the asymmetric and symmetric vibration of O=S groups of -SO3Na and partial peaks could not be discriminated due to the strong and broad peak of HNTs.21, 35 The result suggested that PSS was attached onto HNTs surface through electrostatic interaction. The existence of PSS in the m-HNTs can also be confirmed according to the TG curves of HNTs and m-HNTs as shown in Figure S4 and the loading amount of PSS in the m-HNTs is 1.09%. Figure 1d shows the TEM images of the pristine HNTs and the m-HNTs. After the modification of HNTs with PSS, the m-HNTs kept the hollow structure and no distinct difference was found. This was due to the fact that the amount of the adsorbed PSS was very small and this agreed with the result of TGA. These results confirmed that HNTs were successfully modified with PSS. Furthermore, compared with pristine HNTs, PSS modified HNTs suspensions exhibited no aggregation after 24 h storage (Figure 1b). It is well known that the nanoparticles with larger zeta potential absolute value possess high colloid stability.36 In this work, the ζ-potential has changed from -36 mV (for HNTs) to -64mV (for m-HNTs), which greatly elevated the dispersion of HNTs in DI water. Besides, steric hindrance of the PSS polymer can also facilitate the colloid stability via the enhanced and effective repulsion between nanoparticles.37

6


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Transmittance (%)

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1230 Stretching vibration of -SO3Na

3693 3624 Stretching vibration of Al-OH or Si-OH

1200

1033 Flexible vibration of Al-OH or Si-OH

HNTs m-HNTs 4000

3500

3000

2500

2000

1500

-1

1000

500

Wavenumber (cm ) Figure 2. FTIR spectra of the HNTs and the m-HNTs. 10, 20, 30 and 40 mg m-HNTs were applied for 28.3 cm2 area membrane coating to obtain coherent and flat composite membranes (resulting membranes were denoted as m-HNTs-10, m-HNTs-20, m-HNTs-30 and m-HNTs-40). Further increasing of m-HNTs concentration to 50 mg would result in uneven coating with cracks (Figure S5), and the m-HNTs-50 membrane was not used. SEM, ATR-FTIR and EDXS are employed to verify the formation of m-HNTs/PAN membranes. Figure 3 shows the SEM images of the m-HNTs/PAN membranes and the correlation between the amount of m-HNTs and coating layer thickness. As shown in Figure 3 (a-h), nanotubes in the coating layer had well alignment and were tightly packed thus diminishing gaps in the layer. No visible macro-voids or defects were observed on the cross-sectional images with the increased amount of the m-HNTs. With higher halloysite concentration, the coating thickness linearly increased and did not change the well-aligned tubes’ arrangement (Figure 3i). Figure 4 displays the ATR-FTIR spectra of several membranes. The PAN substrate had characteristic peaks at 2922 and 2862 cm-1 which were ascribed to the stretching vibration of -CH2- or -CH3 of the backbone,38 and a peak at 2243 cm-1 corresponded to the stretching vibration of -C≡N.39 Coating with m-HNTs layer on the PAN substrate, main characteristic peaks of m-HNTs at 3689 and 3620 cm-1 emerged, indicating that this substrate was totally covered by m-HNTs nanomaterial. Many studies have demonstrated that GA was an effective cross-linker for PVA.40, 41 Herein, GA was utilized to keep the stability of the m-HNTs coating (seeing mechanism of cross-linking in Figure S6). In ATR-FTIR, after the m-HNTs coating treated by GA, the formation of -C-O-C- bonds via the cross-linking of GA and PVA lead to a broader peak at 1100 ~ 950 cm-1.40 EDXS was carried out on the cross-section of composite membrane to analyze the distribution of m-HNTs by detecting the relevant chemical elements. Figure 5 shows the distribution of elements (Al, Si and O) which originated from m-HNTs, and thus demonstrated that the m-HNTs were mainly on the top-layer. The results of EDXS were in great agreement with the SEM and ATR-FTIR, further confirming that the in situ evaporation method was successfully applied to construct the m-HNTs coating on the PAN membrane surface.21 7



Figure 3. SEM images of membrane surface and cross-section for m-HNTs-10 (a and e), m-HNT-20 (b and f), m-HNTs-30 (c and g), m-HNTs-40 (d and h) and correlation of the m-HNTs amount and coating layer thickness (i).

Reflectivity (%)

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2862 2922 -CH2- or -CH3

2243 -C≡N

3689 3620 Al-OH or Si-OH

PAN Untreated m-HNTs-10 m-HNTs-10 4000

3500

3000

2500

2000

1100~950

1500

1000

500

-1

Wavenumber (cm ) Figure 4. ATR-FTIR spectra of PAN substrate and m-HNTs-10 membrane treated by glutaraldehyde or not.

8


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Figure 5. EDXS mapping of the cross-section of m-HNTs-10 membrane. Scale bar: 1µm. PVA can be used to stabilize m-HNTs coating layer via hydrogen bonds.25, 30 The m-HNTs layer was not stable without PVA and the m-HNTs could re-disperse in water after soaking. The XPS full-scan spectra of m-HNTs membranes (containing PVA) and m-HNTs membranes without PVA are shown in Figure 6 (for the atomic concentration seeing Table S2). In the position of C1s, the bonding energy of C-O appeared at 286.02 eV, indicating the existence of PVA.42 The characteristic peak of Si2p originated from Si-O-Si / Si-O-Al, and the main peak of Al2p was attributed to the Al-OH and Al-O-Si. After the addition of PVA, no new peaks appeared in that positions, while the binding energies of Al2p and Si2p have shifted from 74.54 eV and 102.86 eV to 74.30 eV and 102.65 eV, respectively. This was mainly attributed to the formation of strong hydrogen bonds between hydroxyl groups of m-HNTs and PVA, which would facilitate the formation of dense m-HNTs layer on the PAN substrate.43 PVA assisted the m-HNTs attachment onto the membrane surface, and it further avoided the detachment of m-HNTs during the next cross-linking process. Moreover, the strong attachment of m-HNTs on PAN support also benefited the formation of defect-free membranes, which would be demonstrated by comparing the m-HNTs coating layer on the hydrophilic or hydrophobic substrate in the following section. Previously, our study has proved that the modification of HNT was selective through the unchanged surface wettability and diffusion coefficient of HNTs.29 Here, a new peak (1230 cm-1) emerged in the FTIR spectra of m-HNTs while the S element was not detected on m-HNTs/PAN membrane surface by XPS. This result also confirms the selective PSS modification of the inner surface of the nanotubes, and this is in line with our previous work.

9



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Figure 6. The survey XPS (a) of m-HNTs-40 membrane with (gray) or without (black) PVA, the C1s core-level signal of m-HNTs-40 membrane with PVA (b) and the high resolution details of m-HNTs-40 membrane with (gray) or without (black) PVA for Si2p (c) and Al2p (d). Note: this two membranes were not cross-linked by GA for more accurately estimating hydrogen bonds. Surface roughness of the m-HNTs/PAN membranes is also investigated through the AFM analysis. Figure 7 presents the three-dimensional AFM images of PAN, m-HNTs-10, m-HNTs-20, m-HNTs-30 and m-HNTs-40 membrane. The lighter color represents a higher position on the membrane surface and the darker color refers to valleys or membrane pores. After coating by m-HNTs, the mean square roughness (Rq) of the prepared membrane decreased, suggesting the formation of a relatively smooth surface.44 Figure S7 also shows the AFM peak force images for PAN and m-HNTs/PAN membrane. In great accordance with SEM images, the m-HNTs were well aligned according to the AFM images. For a two dimensional (2D) system, orientational order parameter S is represented the orientation degree. The larger absolute value of S, the higher orientation degree. The equation S=2cos2φ-1, in which φ is the average angle between the main tube axis and most optimal aligned direction, can be utilized to calculate S. Herein, SEM and AFM peak force images are employed to calculate S and the values for PAN membrane and m-HNTs/PAN membrane are shown in Table S3. The surface porosity of the m-HNTs/PAN membrane is also calculated by the analysis of SEM using ImageJ.16 The surface porosities of all the m-HNTs/PAN membranes are around 1.1% (Table S3). Stack of m-HNTs on the PAN substrate almost not affected by the amount of m-HNTs and this was in great accordance to the 10


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results of SEM images. Therefore, evaporation of solvent can give the self-assembled m-HNTs/PAN membrane and the amount of m-HNTs only slightly affects the structure of the coating layer.

Figure 7. AFM images of PAN substrate (a), m-HNTs-10 (b), m-HNTs-20 (c), m-HNTs-30 (d) and m-HNTs-40 membrane (e). Scale bar: 20 µm. Temperature, pH, and salts can influence the orientation of m-HNTs in a droplet on hydrophilic silicon wafer and the influences of these factors have been discussed in our previous study.29 Here, m-HNTs were applied to coat on a hydrophobic PS substrate. Interestingly, the coating of halloysite nanotubes on hydrophobic membrane surface resulted in randomly orientated nanotubes. During evaporation process, the liquid layer of m-HNTs dispersion on substrate surface gradually becomes thinner. However, considering that the PS membrane doesn’t have a relatively hydrophilic surface, the PS membrane surface is readily to be dewetted. The dewetting of relatively thick fluid halloysite layer can result in the randomly orientated tubes on the PS substrate surface (Figure 8). We speculated that this is why the m-HNTs can assemble on a hydrophilic surface rather than a hydrophobic one. The orientation parameter S for HNTs/PS membrane with loading 10 mg m-HNTs (-0.55) is also calculated and it is smaller compared to the m-HNTs-10 membrane (0.77).

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Figure 8. Schematic diagrams of the HNTs coating on hydrophilic (a) and hydrophobic (b) polymeric substrate. Insert: SEM images of the corresponding membrane. Scale bar: 1 µm. In the case of coating on a hydrophilic surface, the orientation of m-HNTs can be explained according to the following aspects. On one hand, the high axis ratio of m-HNTs provides the basis for the uniform distribution of m-HNTs on polymeric substrate. According to the research of Yunker et al.,45 spherical particles (major-axis/minor-axis aspect α=1) tended to aggregate along with the drop edge in a droplet (Figure 9a). The circular edge is also known as the ‘coffee-ring’. Particles with high aspect ratio (α=3.5) would suppress the ‘coffee-ring’ effect to form a uniform distribution through the shape-dependent capillary interaction (Figure 9b). Here, the m-HNTs with very high axis ratio (α=13.2, calculated from HRTEM), could effectively produce a relatively uniform distribution (Figure 9c). Besides, a capillary flow would be developed during the evaporation process. The modification of HNTs by PSS didn’t affect the hollow structure according to the results of TEM images. The orientation of m-HNTs could also benefit from the capillary flow from the center of a drop to the edge during the evaporation (Figure 9d). On the other hand, the final thermodynamical equilibrium of a gradually evaporating system is determined by its minimum energy F=H-TS, where H is the enthalpy, T is the temperature and S is the entropy. The change of the internal enthalpy (H) during the evaporation process is negligible when compared with the TS value, thus the equilibrium state is achieved with the maximum entropy. This process is also known as entropy-driven phase transitions. In this work, the entropy loss of the ordered orientation is compensated by the increased translational entropy and rotational entropy: compared with the randomly orientated nanotubes, the halloysite with well-aligned orientation have larger free volume, which ensures their higher translational and rotational movement freedom and higher entropy.46, 47

Figure 9. Deposition of spheres (a), ellipsoids (b) and nanotubes (c) after evaporation process and schematic diagram of evaporation process (d). The dye/salt solution permeability and selectivity of PAN, m-HNTs/PS with loading of 10 mg m-HNTs and m-HNTs/PAN membranes are exhibited in Figure 10. The PAN substrate as control had the 12


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permeability about 81.25 L m-2 h-1 bar-1 and selectivity about 0.79 (for RB 5/NaCl) and 0.41 (for RB 5/MgSO4). Compared with the original PAN membrane, both the m-HNTs/PAN and m-HNTs/PS composite exhibited much higher selectivity for RB 5. This suggested that the PSS modification made the HNTs strongly negatively charged providing higher repulsion against the anionic dyes. Besides, the well-aligned nanotube structure had significant effects on the membrane filtration performance. Although the m-HNTs/PS membrane with randomly orientated nanotube structure had higher permeability (15.5 L m-2 h-1 bar-1, 4.07 times for m-HNTs-10 membrane), the dye selectivity of m-HNTs/PS membrane (4.00 for RB 5/NaCl and 3.33 for RB 5/MgSO4) was lower than that of the m-HNTs/PAN membrane (16.63 for RB 5/NaCl and 8.72 for RB 5/MgSO4 for m-HNTs-10 membrane), indicating that the irregular structure was less efficient in solute selectivity. As shown in Figure 10, for the m-HNTs/PAN membranes, the water permeability didn’t have significant changes with increase of the halloysite loading amount. This was due to the compensation of the increased membrane resistance of the thicker halloysite coating, surface hydrophilicity and the nanoscale roughness of the nanotube (Figure 3, S7 and S8). This evaporation-introduced method for fabricating self-assembled membrane allowed good performance towards dye/salt separation and membranes prepared by other methods for this purpose were concluded in Table S4 in supporting information.

Figure 10. Permeability (a) and the dye/salt selectivity (b) of PAN, m-HNTs/PS (loading 10 mg m-HNTs), m-HNTs-10, m-HNTs-20, m-HNTs-30 and m-HNTs-40 membrane. 13



Considering that negatively charged membranes were applied to handle the separation of dye/salts solution, the antifouling properties of the membranes including PAN, m-HNTs/PAN and m-HNTs/PS with loading m-HNTs of 10 mg were firstly explored by employing the organic dyes (RB 5 or RR 49) as the foulants (Figure S9-11). The organic dye fouling was obviously discovered on the original PAN membrane after only 15 min filtration of 0.5 g L-1 RB 5 (Figure S9). For the m-HNTs/PAN and m-HNTs/PS membrane, the organic foulant barely appeared on the membranes surface even after 4 h of filtration process (Figure S10). The improved antifouling performance could be attributed to the nature of negatively charged nanotubes with PSS modification. However, disordered m-HNTs/PS membrane was more prone to be fouled by the organic dye owing to the large interface voids that formed in the coating layer (see the insert in Figure 9). The MWCO of m-HNTs/PS is also greater than that of m-HNTs/PAN membrane (Figure S12). The m-HNTs/PAN membrane was also utilized to separate RR 49 with smaller molecular weight (Figure S11). It was found that the membrane was either self-cleaning (for RB 5) or the attached dye could be easily removed by immersing in alkaline aqueous rapidly (for RR 49). The stability of m-HNTs-40 membrane was further tested under 0.8 MPa by employing the RB 5 as the feed solution with the m-HNTs-40 coating (Figure 11). As time passed by, the permeability of the m-HNTs/PAN membrane was almost constant and the rejection of RB 5 could be maintained above 95.0% throughout the experiment. Therefore, the composite membrane exhibited good operational stability and thus it has a great potential for dyes purification in the neutral environment. Besides, the rejection and permeability of RB 5 in acidic or alkaline environment were also measured as shown in Figure S13. The rejection was slightly declined and the permeability increased due to the slow etch of HNTs by acid or alkali.48

5.28 80 4.62 60 3.96 40 3.30 20

pH=7

2.64

0 4

6

8

10

12

14

16

18

20

22

Permeability (L m-2 h-1 bar-1)

100

Rejection (%)

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24

Time (h) Figure 11. Stability of the m-HNTs-40 membrane under 0.8 MPa. Figure 12 shows the performance of PAN, m-HNTs/PS and m-HNTs/PAN membrane fouled by 0.1 g L-1 BSA solution. Generally, membrane with lower value Rir and higher value of FRR possesses the better antifouling ability.49 After coating m-HNTs on substrate, both m-HNTs/PS and m-HNTs/PAN 14


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membrane showed increased water flux recovery ratio and decreased irreversible fouling, which implied that the existence of m-HNTs coating layer could significantly improve the antifouling property. For m-HNTs/PAN membrane, the water flux recovery could reach as high as 100% even after three circles and the irreversible fouling was also close to zero, and this suggested the excellent antifouling property of the well-ordered membrane (for antifouling property in other reports, please see Table S5 in supporting information). Besides, the reversible fouling value for m-HNTs/PAN membrane maintained around 12.0%. Therefore, it was speculated that the decline of flux was mainly caused by reversible fouling of ordered m-HNTs/PAN membrane by BSA. This was different with the disordered m-HNTs/PS membrane, whose membrane fouling resulted from the blocking of large membrane pores by BSA molecules and reversible absorption of BSA. Moreover, it has been demonstrated that high surface hydrophilicity and low surface roughness could endow membrane excellent antifouling properties.17, 50, 51 Here, the well-aligned halloysite surface film functioned as a selective nanopore layer enhanced the membrane hydrophilicity with water contact angle from 50.1˚ for PAN substrate to 26.5˚ for m-HNTs-30 membrane and declined membrane roughness from 65.7 nm (for PAN substrate) to 40.4 nm (for m-HNTs-20 membrane)

Figure 12. Normalized flux (a), water flux recovery ratio (b), irreversible fouling ratio (c) and reversible fouling ratio (d) of PAN (black), m-HNTs/PS (red) and m-HNTs/PAN (blue) membrane with loading 10 mg m-HNTs under 0.8 MPa. 15



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4.

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Conclusion

We fabricated composite membranes (m-HNTs/PAN) via a simple and efficient evaporation-introduced method. The nanocomposite layer was built onto microporous membrane by adding modified halloysite clay in aqueous/PVA dispersions, followed by solvent evaporation and cross-linking with glutaraldehyde. The nanotubes are well-aligned on the polyacrylonitrile substrate, but on the polysulfone membrane they were disordered. The m-HNTs/PAN membrane with a well-arranged nanotube layer exhibited much better selectivity for dye/salt solution than polyacrylonitrile and m-HNTs/polysulfone membrane, and it also had good water permeability. Moreover, this hierarchical micro/nano composite organic-inorganic membrane has an excellent antifouling behavior against organic dyes and bovine serum albumin. The water flux recovery ratio of the m-HNTs-10 membrane can reach nearly 100 %. Therefore, this sustainable antifouling m-HNTs/PAN membrane is favorable in the separation of dye/salt solutions. Moreover, this elaborately organized halloysite arrays coating technique provides a simple approach for other surface nano-modifications, such as separation and absorption of materials and bio tissues. ASSOCIATED CONTENT Supporting Information. List of relevant figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected] Author Contributions Lijuan Qin, Yafei Zhao and Yatao Zhang conceived the research. Yuri Lvov provided expertise on the nanotube self-assembly. The primary experimental work was conducted by Lijuan Qin and Yafei Zhao. Jingwei Hou studied the formation mechanism of the aligned structure. All authors contributed to manuscript writing and editing. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work has been funded by the National Natural Science Foundation of China (Nos. 21376225 and 21476215), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004) and Excellent Youth Development Foundation of Zhengzhou University (No. 1421324066). YL thanks support by the Ministry of Education and Science of the Russian Federation (No. 14Z50310035). 16


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