One-Step Transformation from Hierarchical-Structured

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One Step Transformation from Hierarchical-structured Superhydrophilic NF Membrane into Superhydrophobic OSN membrane with Improved Anti-fouling Effect Hongxia Guo, Yiwen Ma, Zhenping Qin, Zhaoxiang Gu, Su-ping Cui, and Guojun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07106 • Publication Date (Web): 18 Aug 2016 Downloaded from http://pubs.acs.org on August 25, 2016

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ACS Applied Materials & Interfaces

One-step Transformation from Hierarchicalstructured Superhydrophilic NF Membrane into Superhydrophobic OSN Membrane with Improved Antifouling Effect Hongxia Guo,a c *, Yiwen Ma,a Zhenping Qin,b c Zhaoxiang Gu,a Suping Cui,a Guojun Zhang b* a.

College of Materials Science and Engineering, Beijing University of Technology, Beijing

100124, PR China. b.

College of Environmental and Energy Engineering, Beijing University of Technology, Beijing

100124, PR China. c.

Beijing Key Laboratory for Green Catalysis and Separation, Beijing 100124, PR China.

KEYWORDS: Superhydrophilic membrane; Superhydrophobic membrane; Antifouling effect; Nanocomposite membranes; Nanoﬁltration; Organic solvent nanofiltration.

ABSTRACT The hierarchical-structured superhydrophilic poly(ethyleneimine)/polyacrylic acid (PEI/PAA)-calcium silicate hydrate (CSH) multilayered membranes (PEI/PAA-CSH)n were prepared as aqueous nanofiltration (NF) membrane, and then it was transformed into a superhydrophobic organic solvent nanofiltration (OSN) membrane by one-step modification of

1 Environment ACS Paragon Plus


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trimethylperfluorinatedsilane (PFTS). Investigation on surface structures and properties of these multilayered membranes (PEI/PAA-CSH)n indicated that the hierarchical-structured (PEI/PAACSH)n multilayered membrane produced by in-situ incorporation of CSH aggregates into PEI/PAA multilayers facilitated its one-step transformation from superhydrophilicity into superhydrophobicity. Both of the superwetting membranes showed better nanofiltration performances for retention of dyes of water and ethanol solution, respectively. Moreover, the long-term performance and antifouling behaviors, investigated by retention of methyl blue (MB), bovine serum albumin (BSA) and humic acid (HA) aqueous water solution and non-aqueous ethanol solution indicated that both of the superhydrophilic and superhydrophobic membrane showed higher stability and excellent antifouling property.

1. INTRODUCTION Nanofiltration (NF) has garnered much attention as an effective and feasible technique in aqueous fluid treatment systems and plays an important role in separation technolo-gy.1-3 Numerous large-scale applications of NF exist currently in treatment of municipal drinking water and industrial wastewater.4,5 When the organic mixture fluids are separated, the nanofiltration technique are broadened to so-called organic solvent nanofiltration (OSN),6 in which the nanofiltration membranes with higher solvent-resistance are needed in order to preserve their separation characteristics under more harsh conditions. In comparison with traditional separation techniques, such as distillation and evaporation, the non-aqueous nanofiltration is also characterized as low energy and cost, increased process safety, and decreased solvent emission. 710


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However, both of the nanofiltration processes often inevitably suffer membrane fouling problem caused by irreversible deposition or adsorption of foulants upon membrane surface, leading to the deterioration of membrane performance. So, many efforts have been made to design and prepare antifouling membrane for maintaining high flux without sacrificing retention. Generally, manipulations on the membrane surface properties, such as hydrophilicity or hydrophobicity, as well as charge or roughness, are the common strategies to improve antifouling ability of nanofiltration membranes. 11-15 As regards to the aqueous nanofiltration membrane, endowing membrane surfaces hydrophilicity is the most common strategy to enhance antifouling property. The hydrophilic functional groups, such as polyelectrolyte and zwitterionic polymers, as well as poly(ethylene oxide) segments and polyvinyl alcohol polymer have been introduced into polyamide layer of nanofiltration membranes for improvement of fouling resistance.16-18 And the antifouling effects can be also achieved by embedding or incorporating inorganic components, such as multiwalled carbon nanotube and graphene oxide/TiO2 nanocomposite, into the active layer of thin film nanocomposite nanofiltration (TFN-NF) membrane.19,20 Recently, the hierarchical Mg(OH)2 nanostructures modified by branched poly(ethyleneimine) was produced on the nanofiltration membrane with positive charges, leading to 95% of the flux recovery ratio and 5.11% total fouling ratio.21 These results indicated that the antifouling performance can be effectively altered/modified by incorporation of hierarchical inorganic nanocomponents into membrane surfaces. Most recent researches on organic solvent nanofiltration (OSN) mainly focused on improving membrane solvent resistance ability by using hydrophobic species or incorporating inorganic nanocomponents into membranes, in order to hinder the swelling or dissolving of the polymeric



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active layer in harsh organic solvents.22-24 For example, a hydrophobic OSN membranes were prepared by interfacial polymerization containing hydrophobic monomers of F and Si, exhibiting significantly improved non-polar solvent permeability without compromising solute rejection.22 The hydrophobic polydimethylsiloxane (PDMS) has been grafted into pores of γ-alumina membrane for solvent resistant nanofiltration, which showed higher stability in non-polar solvents of hexane, toluene, and isopropanol. 23, 24 Despite recent progress, most NF or OSN mainly concerned the wettability of membranes with water contact angles in the range 30-120º, covering the range from hydrophilic to hydrophobic surfaces.25,26 It is noteworthy that the amphiphilic NF membranes constructed by introducing both hydrophilic domains and low surface energy domains has exhibited an enhanced antifouling capabilities.27 Very few studies have been reported on tuning the membrane surfaces from superhydrophilicity to superhydrophobicity through multiple scales and surface chemistry. And the present superhydrophilic and/or superhydrophobic surfaces were mostly concentrated on anti-bacterial coatings, micro-fluidic devices, oil-water separation, drug delivery, anti-reflective, and

so

on.

28,29

We

have

demonstrated

that

the

superhydrophilic

poly(ethyleneimine)/poly(sodium 4-styrenesulfonate) (PEI/PSS)-calcium silicate hydrate (CSH) multilayer nanofiltration membranes exhibited a higher antifouling and self-cleaning behavior than the hydrophilic membrane during aqueous nanofiltration of dye solutions.30 Also, the superhydrophobic pervaporation (PV) membrane has been demonstrated to show both higher selectivity and higher permeability than the hydrophobic membrane.31 To our knowledge, onestep transformation from the aqueous NF to non-aqueous OSN membrane, especially based on converting the superhydrophilic surface into superhydrophobic one, has scarcely been exploited.


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Herein, we report a superhydrophilic aqueous NF membrane was transformed into a superhydrophobic non-aqueous OSN membrane by one-step method. The superhydrophilic (PEI/PAA-CSH)n multilayer membrane was first prepared using the similar procedure as previous work on the hydrolyzed polyacrylonitrile (PAN) substrate, which can be used for aqueous nanofiltration separation of dyes wastewater.30 Then, based on the produced hierarchical-structured surface, the superhydrophilic membrane was transformed into superhydrophobic one after modified by trimethylperfluorinatesilane (PFTS). Both of the superwetting membranes showed better nanofiltration performances for separation of water and ethanol dye solution, respectively. Moreover, the long-term stability and antifouling properties were also investigated. This cost-effective and easily produced superwetting membrane with excellent antifouling properties would show considerable potential in practical utilizations, such as water and organic solvent purification as well as dye recovery. 2. EXPERIMENTAL SECTION 2.1 Materials. Flat-sheet polyacrylonitrile (PAN)-50 ultrafiltration membrane with molecular weight cutoffs of 100 kDa was purchased from Sepro. Membranes Inc. (USA). Polyethylenemine

(PEI,

Mw=750000),

polyacrylic

acid

(PAA,

Mw=4,000,000),

Perfluorooctyltrimethoxysilane (PFTS), as well as bovine serum albumin (BSA) and humic acid (HA) were purchased from Sigma-Aldrich Chemical Company (USA). Sodium silicate, calcium acetate and ethanol were provided by Beijing chemical plant (Beijing, China). Dyes, such as rhodamine B (RB), xylenol orange (XO) and methyl blue (MB) were purchased from Tianjin Fu Chen Chemical Reagent Factory (Tianjin, China). Deionized (DI) water was used in all experiments with conductivity of 18.0 MΩ cm-1. All the chemicals used were of analytical grade and without any further purification.



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2.2 Preparation of membranes The superhydrophilic membrane was first prepared using the similar procedure as previous work with difference of that the weak polyelectrolyte of PAA instead of PSS was used in this work.30 Specifically, the similar defect-free polyelectrolytes (PEs) cap layer (PEI/PAA)2.0 was produced by alternating deposition of homogeneous solution of PEI solution (pH=6) and PAA solution (pH=8) on the hydrolyzed PAN substrates for two bicycle depositions. Then, the membrane of (PEI/PAA)2.0 cap layers was immersed into an uniform and stable solution of 0.5 g/L PEIcalcium acetate (pH=6, containing 0.05 mol/L of calcium acetate), and then rinsed by DI water to remove redundant polyelectrolytes. Next, the membrane was introduced into solution of 0.1 g/L PAA-sodium silicate (pH=8, containing 0.05 mol/L of sodium silicate) for 20 min, followed by rinsed using DI water to remove redundant polyelectrolytes. During this process, CSH nanoparticles were in-situ generated by the reaction of Ca2+ and SiO32-. This procedure was repeated until the desired superhydrophilic (PEI/PAA-CSH)n membrane obtained. In order to convert the superhydrophilic membrane into a superhydrophobic one, the obtained (PEI/PAA-CSH)2.0 membrane was immersed into 0.75wt% PFTS of anhydrous ethanol solutions for one hour at room temperature, and then taken out and washed thoroughly before moving to the oven at 60℃ for 2 hours. Finally, the superhydrophobic membrane was obtained by one-step of PFTS modification as shown in Scheme 1.


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Scheme 1. Schematic transformation of the membrane from superhydrophilicity into superhydrophobicity 2.3 Characterizations The surface morphologies of membranes were observed by field-emission scanning electron microscope (FE-SEM, Hitachi SU-8020, Japan) operated at 15kV. Before SEM observation, the samples were coated with gold by an ion sputter (Hitachi E-1010, Japan) at vacuum degree below 10Pa for 60s. The surface roughness were analyzed using an atomic force microscopy (AFM) on a Pico ScanTM 2500 Microscope System (Agilent Technologies, USA) in the tapping mode under ambient conditions. The wettability of membrane surface was measured by using the sessile drop method on a DSA100 instrument (Kruss company, Germany) at ambient temperature. A droplet of water 3.0 µL was placed on the surface of the films using a syringe, and the images were taken with a video system. The representative contact angle is the average value of no less than 5 different locations. The functional groups of the membrane were analyzed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Vertex-70 FTIR spectrophotometer, Bruker, Germany). The characteristic bands were obtained from 4000 to 400 cm-1 with resolution of 4 cm-1 at room temperature. The thickness of the film was decided by Profilometer (Dektak XT, Bruker, Germany) at room temperature. Zeta potentials of the



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membrane were determined via streaming potential measurements by the electrokinetic analyzer (Anton Paar, Surpass, Germany) with 0.83 mM KCl solution operating at 300 mMPa at 25±1.0 ℃.

For estimating the solvent resistance of the superhydrophobic membrane during non-aqueous nanofiltration, the area swelling and solvent uptake were measured according to ref.

32

The area

swelling and solvent uptake were calculated by the change of the membrane area (A) and weight

(W) according to the equation:

area swelling ( % ) = (

Awet − 1) ×100% Adry

Solvent uptake ( % ) = (

and

wwet − 1) × 100% wdry

,

respectively, where Adry and Wdry is the area and weight of the thoroughly dried membrane, respectively, and Awet and Wwet is the area and weight of the membrane after immersed into pure ethanol solvent for 48 hours at room temperature. 2.4 Separation performance of the membranes The separation performances of the superhydrophilic and superhydrophobic membrane were evaluated by retention of dyes of MB, RB and XO from aqueous water solution and non-aqueous ethanol solution, respectively, using a home-made cross-flow NF system as our previous works.30,33,34 The effective membrane area for filtration in the cell was 22.9 cm2. Prior to measurement, the system was pressurized to 0.4 MPa for 40 min to reach stable state. The rejection rate (R) was measured by R=(1–Cp/Cf)×100%, Where Cp and Cf was the concentrations of the permeate and feed solution, respectively. And the concentration of dyes solution was determined by the absorbance according to Lamber-Beer law, which was measured on a UV-Vis spectrophotometer (UV-3200, Mapada, Co. Shanghai) at the maximal absorption wavelength of RB, XO, and MB at 554nm, 432nm, and 580nm, respectively. The permeate flux J was


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calculated by equation: J=V/(A·t·∆P), Where V is the total volume of the permeate collected under transmembrane pressure ∆P (MPa) on a time scale t (h), and A is the effective area of the membrane (m2). 2.5 Long-term stability and antifouling properties of membranes. The long-term performances of the superhydrophilic and superhydrophobic membrane were investigated for nanofiltration of 10.0 ppm MB aqueous water solution and non-aqueous ethanol solution, respectively. The antifouling properties of the membrane was evaluated by typical foulants of BSA and HA. Prior to the fouling experiments, the pure water or ethanol flux (Jw1) was measured at 0.4 MPa for 2 hours. Then, the feed liquids were changed into 100 mg/L of BSA or HA of water and non-aqueous ethanol solution, respectively. The corresponding flux (Jp) was recorded during the performing time. The flux attenuation trend can be obtained from the normalized flux ratio (Jp/Jw1) during the continuous testing process at constant pressure.

35, 36

Following the fouling experiments, membrane cleaning was conducted to determine the fluxes recovery. The membranes after fouling were rinsed for 2 hours at the pressure of 0.4 MPa using deionized water or ethanol, the pure water or ethanol flux of the cleaned membrane (Jw2) was measured again. The antifouling property of the membrane were manifested and calculated by: the flux recovery ratio (FRR, FRR=(Jw2/Jw1)×100%), total flux decline ratio (DRt, J  DR t =  1 − p J w1 

  × 100% 

), reversible flux decline ratio (DRr,

flux decline ratio (DRir,

 J DRir =  1 − w 2 J w1 

  × 100% 

 J w2 − J p  D Rr =   × 100% J w1  

). 37

3. RESULTS AND DISCUSSION


), and irreversible


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3.1 The transformation from superhydrophilic membrane into superhydrophobic membrane The superhydrophilic membrane was prepared in the first step, using the similar procedure as our previous work with difference of that the weak polyelectrolyte PAA was used.

30

The surface

morphology and wettability revealed by SEM and water contact angle (CA) are shown in Figure 1. As expected, the surface of hydrolyzed PAN substrate displayed a large number of pores (Fig. 1(a)) with water CA of 34.7° (right bottom of Fig.1(a)). After deposition of the (PEI/PAA)2.0 cap layer, the pores and defects of the substrates had been covered; and the membrane surface showed relatively smooth and compact (Fig.1(b)), with water CA increase to 70.2° (right bottom of Fig.1(b)), illustrating that the interfacial interactions between polycationic PEI and polyanionic PAA (exhibited by FTIR spectra in Fig. S1) induced a less hydrophilic surface. After in-situ generated CSH nanoparticles, the surface of (PEI/PAA-CSH)2.0 multilayer in Figure 1(c) became more rough and bumpy with about 20 nm diameters of CSH nanoparticles distributed uniformly on the surface. From the magnified image in upper left inset of Figure 1(c), the CSH nanoparticles aggregated into cluster with about 280 nm. Such rugged and rough surface can also be seen from the cross-section SEM images of the membrane (shown in Fig. S2(a)), on which an uneven skin multilayer was covered. The water CA of 3.8° (bottom inset of Fig. 1(c)) indicated a superhydrophilic surface, much similar to that of our (PEI/PSS-CSH)2.0 membrane.

30

This

indicated that the superhydrophilic surface can be easily prepared using our in-situ precipitation of layer-by-layer (LbL) assembly method. Next step, we modified the superhydrophilic membrane with PFTS layer. SEM image in Figure 1(d) showed that the hierarchical structured surface covered by CSH aggregates exhibiting almost no change, while the existence of fluorine element in EDX spectrum (upper left inset of Fig. 1(d)) clearly manifested the PFTS coating


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layer. The water CA in Figure 1(d) (right bottom) obviously increased to about 155.6°, displaying a superhydrophobic surface. This indicated that our superhydrophilic membrane have been successfully transformed into a superhydrophobic one, after modified with PFTS of lower surface energy. As well-documented in the literatures,38 the superwetting surface is governed simultaneously by surface geometrical structures and surface free energy. We assumed that such superwetting transformation was attributed to the surface roughness, together with the coated PFTS of low surface energy. Herein, the roughness of the membrane surface was investigated by three-dimensional (3D) images of AFM. As can be seen from Figure 2(a), that the superhydrophilic (PEI/PAA-CSH)2.0 membrane showed a rough and bumpy surface with 251 nm of roughness and Z-axis height of 2.17µm (Table S1), which were much higher than that of PAN substrate and the (PEI/PAA)2.0 cap layer (supporting information of Fig. S2 and Table S1). We attributed such high roughness to the in-situ produced CSH hierarchical structure composing of micro- and nano-scaled aggregates. The cross-section thickness trace in AFM (Fig. 2(c)) indicated that the vertical distance of hill-to-valley surface of aggregated CSH clusters is 814 nm. And the exponentially increased thickness in Figure 3 also manifested such roughness. The thickness of (PEI/PAA)2.0 cap layer was 36.5 nm, and the thickness of (PEI/PAA-CSH)1.0 multilayer increased markedly to 183.66 nm due to in-situ produced CSH nanoaggregates. Then the thickness of (PEI/PAACSH)2.0 multilayer showed exponential increase to 318.73 nm. Thus, it can be seen that the prominent CSH nanoparticles with micro- and nano-scaled hierarchical structures, lead to the superhydrophilic surface (Fig. 1(c)). After modified by PFTS layer, the 3D AFM image (Fig. 2(b)) and the cross-section SEM picture (Figure S2(b) of the superhydrophobic (PEI/PAACSH)2.0/PFTS membrane exhibited likewise a relatively coarse surface, in which the Rq



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roughness and Z-axis height showed a little decrease to 138 nm and 1.35µm, respectively. While the thickness (Fig. 3) showed a slight increase to 354.73 nm, we attributed the decreased roughness to the PFTS coating layer filling some surface concavities. To further demonstrate the role of such hierarchical structure in the superwetting transformation, a (PEI/PAA)4.0 multilayer membrane was prepared and then modified by PFTS layer. As shown in Figure 4(a), the (PEI/PAA)4.0 membrane without CSH aggregates was much flat with roughness about 29.4 nm. The water CA of ~70.8° indicated its hydrophilicity. After modified with PFTS, the surface roughness (Fig. 4b) showed little change, and the water CA was increased to 133.3°, not achieving superhydrophobicity. According to Herminghuas, 39 the self-affine profiles of surface roughness can render the surface with microscopic contact angle non-wet. Thus, it was fully confirmed that the coarse surface with relatively higher roughness (Fig. 2) together with the water-repellent PFTS layer really contributed to the transformation of superhydrophilic (PEI/PAA-CSH)2.0 membrane (Fig. 1 c ) into a superhydrophobic surface (Fig. 1 d).


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Figure 1 SEM images and water contact angle of (a) the hydrolyzed PAN substrate, (b) (PEI/PAA)2.0 cap layer, (c) the superhydrophilic (PEI/PAA-CSH)2.0 multilayer, and (d) the superhydrophobic (PEI/PAA-CSH)2.0/PFTS membrane. The bottom inset images are corresponding water CAs, and the upper inset of (c) is the magnified corresponding images; the upper inset of (d) is the corresponding EDX spectrum.



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Figure 2 AFM morphologies of (a) the superhydrophilic (PEI/PAA-CSH)2.0 membrane, (b) the superhydrophobic (PEI/PAA-CSH)2.0/ PFTS multilayer membrane, and (c) an AFM crosssection of the line shown in image (a).

Figure 3 Thickness alterations along with the number of deposition bilayers


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Figure 4 AFM images and Water contact angles of (a) (PEI/PAA)4.0 multilayer and (b) PFTS modified (PEI/PAA)4.0 membrane 3.2 The aqueous NF and non-aqueous OSNF performance. NF and OSNF have been regarded as the ideal process for textile dye wastewater treatment and organic solvent recovery in industry.40,41 Herein, based on the prepared superhydrophilic (PEI/PAA-CSH)2.0 membrane, 1.0 ppm of aqueous solutions of MB, XO and RB dyes in water were used as feed solution to evaluate the aqueous nanofiltration, respectively. Figure 5(a) showed the rejections of 99.68%, 98.51% and 74.98% for MB, XO and RB dyes, along with higher flux of 202.5, 210.0, and 212.5 L/m2·h·MPa, respectively. Such rejection difference was mainly attributed to that the molecular weight of MB (799.81 g/mol) was larger than that of XO (760.60 g/mol) and RB (479.02 g/mol). In addition, the reduced Donnan exclusion effect between the cationic RB dye and the negative membrane surface (Fig. S3) led to the relatively



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low rejection to RB. Moreover, the (PEI/PAA-CSH)2.0 superhydrophilic membrane in this work showed a higher permeable flux (more than 200.0 L/m2·h·MPa) than our previous (PEI/PSSCSH)2.0 membrane (191.5 and 183.5 L/m2·h·MPa for XO and RB aqueous solutions), due to the higher roughness of the (PEI/PAA-CSH)2.0 membrane (Table S1) leading to the relatively larger transferring area of membrane.42 Also, the long-term performance of the membrane shown in Figure 6(a) indicated that both of the rejection and flux of the membrane exhibited little fluctuation during 60 hours of testing period, within which the rejection of MB showed a slight increase from 91.86% to 95.47%. And the flux decreased slightly from 162.5 to 155.0 L/m2·h·MPa. Similar to our previous (PEI/PSSCSH)2.0 membrane, the superhydrophilic (PEI/PAA-CSH)2.0 membrane in this work also showed a stable operation performance. The SEM morphology of top surface (Fig. 6(b)) and crosssection (Fig. 6(c)) of the membrane indicated that the dye foulants were just attached onto the membrane surface in colloidal state and did not permeate into the inner channel during the running time. This was mainly attributed to the superhydrophilicity together with the electrostatic extrusion effect between anionic MB molecules and the negatively charged membrane surface (supporting information Fig. S3), alleviating the dyes adsorption. In addition, the slightly decreased flux can be recovered to 160.7 L/m2·h·MPa after cleaning with pure water for 2 hours at the pressure of 0.4 MPa, showing a flux recovery ratio of 98.9%. Therefore, it was concluded that the water layer easily formed onto the superhydrophilic membrane surface, together with the unblocked inner channel, was responsible for the enhanced permeation flux and long-term stability.


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Figure 5 Separation performance of (a) the superhydrophilic (PEI/PAA-CSH)2.0 membrane for aqueous nanofiltration, and (b) the superhydrophobic (PEI/PAA-CSH)2.0/PFTS membrane for non-aqueous nanofiltration. (Separation conditions: 1.0 ppm of MB water/or ethanol solution, 0.4MPa, 25℃)

Figure 6 (a) Long-term performance of the (PEI/PAA-CSH)2.0 superhydrophilic membrane, and SEM morphologies of (b) top surface and (c) cross-section of the membrane after performing for 60 hours. (Separation conditions: 10.0 ppm of MB aqueous solution, 0.4MPa, 25℃) Furthermore, the non-aqueous nanofiltration was performed by as-prepared superhydrophobic (PEI/PAA-CSH)2.0/PFTS membrane for rejection of the same three dyes of ethanol solution, respectively. Figure 5(b) presented the similar rejections of 98.91%, 94.86% and 70.38% for the



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same dyes of MB, XO and RB, along with the permeable flux of 60.45, 65.08, and 76.28 L/m2·h·MPa, respectively. Moreover, long-term performance of the superhydrophobic membrane shown in Figure 7(a) indicated that the flux decreased gradually from 53.7 to 21.98 L/m2·h·MPa, meanwhile, the rejection increased from 97.1% to 100% within 40 hours of operation time. In order to verify whether the decreased flux was resulted from swelling effect, the prepared superhydrophobic membrane was immersed into absolute ethanol for 48h, and the area swelling and solvent uptake were measured, respectively. As a result, the membrane displayed lower area swelling of 3.12% and relatively high solvent uptake of 18.01%, exhibiting a good solvent resistance. Also, after cleaning with absolute ethanol for 2 hours, the flux can be recovered to 45.2 L/m2·h·MPa along with rejection of 98.1%. Thus, it was concluded that the gradually decreased flux was mainly ascribed to the accumulation of MB dye molecules forming a gel layer on the surface with extended operation time. And this gel layer can be easily cleaned by pure solvent.

This can be further manifested from Figure 7(b) and (c)), in which the dye

foulant was just existent onto the membrane surface, and did not permeate into the inner channel of the membrane. In addition, the comparison of the OSN performance between this work and other OSN membranes reported in literatures was shown in Table 1. It can be noted that our superhydrophobic membrane showed much higher permeable flux for the polar solvent ethanol and a comparable rejection of dyes.


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Table 1. The comparison of OSN performance between the superhydrophobic membrane in this work and other OSN membranes reported in literatures Membranes

Dye/solvent

Retention

Flux

%

L/(m ·h·MPa)

Reference s

2

PDDA/PSS PEs

Rose Bengale/IPA

99

1.2–15.7

[43]

PPy/PAH

Rose Bengal/IPA

99.2

13

[44]

PTMS/PAN

RemazolBrillant Blue R

90

42.70

[45]

PI/Au NPs

Rose Bengal/IPA

about 90

10

membranes

Bromothymol blue/EA

about 70

25

Crosslinked PAI

Rose Bengal/IPA

97.3

64

Xylenol Orange/EA

94.86

65.08

Rhodamine B/EA

70.38

76.25

Methyl Blue/EA

98.91

60.45

(PEI/PAACSH)2.0/PFTS

[46]

[47] This work

Figure 7 (a)Long-term performance of the (PEI/PAA-CSH)2.0/PFTS superhydrophobic membrane and SEM morphologies of (b) top surface and (c) cross-section of the membrane



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after performing for 60 hours. (Separation conditions: 10.0 ppm of MB ethanol solutions, 0.4MPa, 25℃)

3.3 Antifouling properties of the superwetting membranes The antifouling capacity of the superwetting membranes was investigated using BSA and HA as typical foulants. To examine the antifouling effect of superhydrophilic (PEI/PAA-CSH)2.0 membrane, the flux decline of BSA and HA aqueous solutions were investigated by 3 cycles as shown in Figure 8 (a). It can be seen that the superhydrophilic membrane displayed less fouling with normalized flux of BSA and HA solution decreasing to 0.686 and 0.847, respectively, within the 1st running cycle of 12 hours. After rinsed by pure water for 2 hours, the normalized flux of BSA and HA aqueous solutions recovered to 0.750 and 0.968. Figure 8(b) showed that the total flux decline ratio (DRt) of BSA and HA aqueous solutions was averaged as 17.8% and 26.7%, respectively. The flux recovery ratio (FRR) was 96.8% and 75.0% for the 1st cycle, then still at 91.9%, 93.7% at 3rd cycle, respectively. These indicated that similar to our previous superhydrophilic (PEI/PSS-CSH)2.0 membrane, the superhydrophilic membrane in this work also showed a better antifouling effect. Moreover, the average irreversible flux decline ratio (DRir) for HA solution (4.72%) was lower than BSA solution (11.74%), indicating that HA was easily removed from the surface by water hydraulic cleaning, which was probably ascribed to a relatively weak interaction between HA foulant and membrane surface. In order to verify such interaction, the zeta potentials of HA and BSA of water solution were measured as shown in Fig.S4. It was noted that zeta potentials of HA was -28.6 mV, really higher than that of BSA 12.4 mV. Thus, we concluded that the Donnan repulsive effect between the negatively charged


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(PEI/PAA-CSH)2.0 membrane surface (Fig.S3) and HA induced less HA attached on the membrane surface, responsible for its antifouling property. Simultaneously, the antifouling effect of the superhydrophobic (PEI/PAA-CSH)2.0/PFTS membrane was also investigated as shown in Figure 9. It can be noted that DRt of HA and BSA ethanol solution was 23.8% and 29.8%, respectively. And FRR was 82.4% and 75.8% for the 1st cycle, and still at 89.6%, 82.9% after 3rd cycle, respectively. These indicated that the superhydrophobic membrane likewise showed a better fouling resistance to HA and BSA in nonaqueous ethanol solution, due to that the low surface free energy of the surface facilitated release of the attached organic foulants. The small difference of antifouling effect between HA and BSA in ethanol solution can be manifested from the little differences of zeta potentials of HA and BSA in Figure. S4, in which zeta potentials of HA and BSA were -4.27mV and -4.84 mV, respectively. In order to further corroborate the antifouling effect of the superhydrophobic membrane in ethanol solution, the same performance was done using the hydrophobic (PEI/PAA)4.0 membrane. It was noted that FRR of HA and BSA ethanol solution (Fig.S5) was tested as 75.6% and 38.3% for the 1st running cycle, and 56.5%, 28.7% after 3rd cycle, respectively. And the DRt of HA and BSA ethanol solution was averaged 30.4% and 72.2%, respectively. This further indicated that our superhydrophobic membrane displayed more effective fouling resistance than the hydrophobic one.



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Figure 8 Antifouling test of the superhydrophilic (PEI/PAA-CSH)2.0 membrane under 0.4 MPa. (a) time-dependent flux for BSA and HA aqueous water solution; (b) antifouling indexes of the membrane

Figure 9 Antifouling test of the superhydrophobic (PEI/PAA-CSH)2.0/PFTS membrane under 0.4 MPa. (a) time-dependent normalized flux for BSA and HA non-aqueous ethanol solution; (b) antifouling indexes of the membrane. CONCLUSION The hierarchical-structured superhydrophilic (PEI/PAA-CSH)n multilayered membranes were prepared as aqueous nanofiltration (NF) membrane, and then it was transformed into a


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superhydrophobic organic solvent nanofiltration (OSN) membrane by one-step modification of trimethylperfluorinatedsilane (PFTS). The separation performances evaluated by rejection of dyes, such as MB, XO and RB, showed that the superhydrophilic (PEI/PAA-CSH)2.0 membrane displayed rejections of 99.68%, 98.51% and 74.98% for MB, XO and RB dyes in aqueous NF separation, along with the higher flux of 202.5, 210.0, and 212.5 L/m2·h·MPa, respectively. And the superhydrophobic (PEI/PAA-CSH)2.0/PFTS membrane showed rejections of 98.91%, 94.86% and 70.38% for the same dyes of MB, XO and RB in non-aqueous OSN process, along with the permeable flux of 60.45, 65.08, and 76.28 L/m2·h·MPa, respectively. Also, both of the superwetting membranes showed better long-term performances, together with excellent antifouling property for BSA and HA during NF and OSNF processes. These potential advantages are beneficial to produce either superhydrophilic or superhydrophobic membrane in facile and cost-effective strategy, with considerable potential utilization for water and organic solvent purification as well as dye recovery. ASSOCIATED CONTENT Supporting Information ATR-FTIR spectra, the cross-section SEM images of the membranes, as well as the surface zeta potential of the foulant and membranes, antifouling test of the hydrophobic membrane were supplied. This materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] (H. Guo)



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* [email protected] (G. Zhang) Tel./Fax: +86-10-67396110 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The National Natural Science Foundation of China (21476005, 21176005) Beijing Municipal Selected Excellent Overseas Scholars Project. Notes No additional relevant notes should be placed here. ACKNOWLEDGMENT The supports from the National Natural Science Foundation of China (21476005, 21176005), and Beijing Municipal Selected Excellent Overseas Scholars Project are acknowledged. The authors also thank Dr. Naixin Wang and Dr. Hongwei Fan with their helpful discussion. REFERENCES (1) Mohammad, A.W.; Teow, Y. H.; Ang W. L.; Chung Y. T.; Oatley-Radcliffe D. L.; Hilal, N. Nanofiltration Membranes Review: Recent Advance and Future Prospects. Desalination 2015, 356, 226–254. (2) Kaur, S.; Barhate, R.; Sundarrajan, S.; Matsuura, T.; Ramakrishna, S. Hot Pressing Of Electrospun Membrane Composite and its Influence on Separation Performance on Thin Film Composite Nanofiltration Membrane. Desalination 2011, 279, 201–209. (3) Amini, M.; Arami, M.; Mahmoodi, N. M.; Akbari, A. Dye Removal from Colored Textile Wastewater Using Acrylic Grafted Nanomembrane. Desalination 2011, 267, 107–113.


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One-step Transformation from Hierarchical-structured Superhydrophilic NF Membrane into Superhydrophobic OSN Membrane with Improved Antifouling Effect Hongxia Guo,a c *, Yiwen Ma,a Zhenping Qin,b c Zhaoxiang Gu,a Suping Cui,a Guojun Zhang b* a.

College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, PR China. b. College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China. c. Beijing Key Laboratory for Green Catalysis and Separation, Beijing 100124, PR China. ABSTRACT: The hierarchical-structured superhydrophilic poly(ethyleneimine)/polyacrylic acid (PEI/PAA)-calcium silicate hydrate (CSH) multilayered membranes (PEI/PAA-CSH)n were prepared as aqueous nanofiltration (NF) membrane, and then it was transformed into a superhydrophobic organic solvent nanofiltration (OSN) membrane by one-step modification of trimethylperfluorinatedsilane (PFTS). Investigation on surface structures and properties of these multilayered membranes (PEI/PAASS-CSH)n indicated that the hierarchical-structured (PEI/PAACSH)n multilayered membrane produced by in-situ

incorporation of CSH aggregates into PEI/PAA multilayers facilitated its one-step transformation from superhydrophilicity into superhydrophobicity. Both of the superwetting membranes showed better nanofiltration performances for retention of dyes of water and ethanol solution, respectively. Moreover, the long-term performance and antifouling behaviors, investigated by retention of methyl blue (MB), bovine serum albumin (BSA) and humic acid (HA) aqueous water solution and non-aqueous ethanol solution indicated that both of the superhydrophilic and superhydrophobic membrane showed higher stability and excellent antifouling property.

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