Improved Salts Transportation of a Positively Charged Loose

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Improved salts transportation of a positively charged loose nanofiltration membrane by introduction of poly (ionic liquid) functionalized hydrotalcite nanosheets Liang Yu, Jianmian Deng, Huixian Wang, Jin-dun Liu, and Yatao Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00343 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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ACS Sustainable Chemistry & Engineering

Improved salts transportation of a positively charged loose nanofiltration membrane by introduction

of

poly

(ionic

liquid)

functionalized hydrotalcite nanosheets Liang Yu a, c, Jianmian Deng b, Huixian Wang b, Jindun Liu a, and Yatao Zhang a * a

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou

450001, China b

School of Civil Engineering and Communication, North China University of Water

Resources and Electric Power, Zhengzhou 450045, China c

Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama

Higashihiroshima, 739-8527, Japan

*Corresponding author: Tel: +86-371-67781734; Fax: +86-371-67739438 E-mail: [email protected]

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Abstract: Basically, commercialized nanofiltration membranes exhibit a salt (NaCl) rejection of >30%, which are difficult to accomplish the separation of low-molecular-weight organics from their salts-containing wastewater. To solve this problem, in this study, a facile and novel loose nanofiltration membrane was developed by the embedment of modified hydrotalcite (mHT) in polyethersulfone (PES) membrane matrix upon a phase inversion method. Membrane performance was characterized by scanning electron microscopy (SEM), water contact angle, transmission electron microscope (TEM), atomic force microscope (AFM), water uptake, tensile strength and percentage elongation, thermal stability. Nanofiltration tests were performed using a series of salts (MgCl2, MgSO4, NaCl, and Na2SO4, 0.5 g/L) and dyes (reactive black 5 and reactive red 49, 1g/L) aqueous solutions to evaluate membrane permeation properties. The resulted membrane showed higher surface hydrophilicity, enhanced mechanical and thermal stability, as well as higher dyes retention (above 95% for reactive black 5 and around 90% for reactive red 49) and near-zero salts rejection properties. Moreover, the short-term operation test demonstrated the stability of flux and rejection of mHT mixed PES membrane for dyes desalination. Therefore, this loose nanofiltration membrane may have potential applications in separation of dyes from salts-containing wastewater. Keywords: loose nanofiltration membrane; positively charged; hydrotalcite nanosheets; poly (ionic liquid) brushes; dyes desalination

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INTRODUCTION Polymer membranes, which can be made in flat sheet or hollow fiber configurations and are easy to scale up, have been extensively used in various industrial processes ranging from microfiltration (MF) to reverse osmosis (RO), as well as gas separation.1 Nonetheless, to some extent, their performance is unsatisfactory due to limited chemical, mechanical and thermal stabilities, a trade-off relating permeability and selectivity. Alternatively, mixed matrix membranes are supposed to combine the processability of polymers with especial separation selectivity of inorganics in a single membrane and offer specific advantages for the preparation process, adaptability to the harsh environments.1-3 Many different types of fillers, such as Al2O3,4 multi-walled carbon nanotubes (MWCN),5, frameworks (MOFs),8-10 zeolite,11,

12

6

palladium,7 metal-organic

graphene oxide (GO),13-16 nano tungsten

disulfide (nano-WS2),17, 18 have been incorporated in membrane matrix to produce improvements in permeability and selectivity of obtained membranes. In our previous studies, halloysite nanotubes (HNTs), were functionalized and then introduced to polyethersulfone (PES) matrix to fabricate mixed matrix membranes, which exhibited superior properties for different applications.19-24 In printing and dyeing industry of reactive dyes, large amount of wastewater with high salinity (usually NaCl, Na2CO3, and Na2SO4) and chroma is often generated resulting in serious environmental pollution, waste of resources as well as increase of cost.25-27 In the view of resources recycling and environmental protection, it is of great 3



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significance to develop membranes with efficient separation capacity for low-molecular-weight dyes from salts-containing wastewater. Nanofiltration (NF) is regarded as a potential technology for this objective with lower-energy requirement, facile operation condition, relatively lower investment and maintenance cost.28-30 The nominal molecular weight cut off (MWCO) of a NF membrane is in the range of 100-1000 Da representing an approximate pore size of 1 nm. This provides possibilities for fine-tuning of membrane performance with high retention of low-molecular-weight dyes and high passage of salts based on molecular sieving effect. Over the past decades, NF membranes have been extensively studied and research efforts are ongoing with increasing attention. However, most commercial or reported NF membranes exhibited higher rejection ratio for salts (> 30% for NaCl), 31 difficult to achieve efficient separation of low-molecular-weight organics and salts. Charged

mosaic

NF

membrane

shows

potentials

for

the

separation

of

low-molecular-weight organics and salts, which can induce concurrent migrations of cations and anions along the respective fixed charges (ion exchange sites).32 However, unfortunately, the practical application has not been attained except for some special use with small scale to up to date. Nowadays，commercialized NF membranes are generally neutral or negatively charged. These membranes often possess higher retention for di- or multi-valent anions and are unfavorable for desalination of reactive dyes. Therefore, it is of great theoretical and practical significance to study positively charged loose NF membranes, which have enhanced permeation performance for

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anions because of electrostatic attraction and the tailored loose structure. Typically, positively charged NF membrane can be fabricated by coating a positively charged separation layer to a supporting membrane or introducing positive charge groups to an existing membrane surface.33-36 But these methods are unable, or not so convenient, to synthesize membranes with relatively looser separation layer compared to conventional NF membranes. Mg/Al hydrotalcite, layered double hydroxides (LDHs), can produce new type ultimate exfoliated two-dimensional nanosheets with higher positive charge density and as well can be modified with organic chains effortlessly to improve the compatibility with polymers. Herein, a poly (ionic liquid) brushes modified Mg/Al hydrotalcite (mHT) mixed polyethersulfone (PES) loose NF membrane was fabricated using nonsolvent induced phase separation (NIPS) method. The purpose of the utilization of ionic liquid in this work was not only to realize the organic modification of Mg/Al hydrotalcite but also to strengthen the positively charged property. Moreover, the incorporation of mHT increased the viscosity of PES casting solution and thereby delaying the phase transformation to form a loose, thickened separation layer. As a consequence, the positively charged, loose and porous skin layer may improve the salts transportation properties without an impressive decrease in retention for reactive dyes because of the electrostatic interaction and the fine-tuning of pore structure. Scheme 1 illustrates a brief process of the fabrication of mHT/PES mixed matrix membrane.

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EXPERIMENTAL SECTION

Materials Allyl chloride, 2-bromoisobutyryl bromide, γ-aminopropyl triethoxysilane (APTES), 2, 2'-bipyridine (BPy), azodiisobutyronitrile (AIBN), and polyvinyl pyrrolidone (PVP, Mn=24000) were purchased from Aldrich Chemical Co. and used as received. Polyethersulfone (PES, Ultrason E6020P with Mw=58 kDa) was obtained from BASF, Germany. Reactive red 49 and reactive black 5 (see Table 1) were purchased from Sunwell Chemicals Co., Ltd., China and used without any treatment. All the other chemicals (analytical grade) were obtained from Tianjin Kermel Chemical Reagent Co., Ltd., China and were used without further purification. The used water was deionized water.

Fabrication of poly (ionic liquid) modified Mg/Al hydrotalcite (mHT) Synthesis of Mg/Al hydrotalcite (HT) and exchange of interlayer anions: Mg/Al hydrotalcite was prepared using a “urea” method detailed elsewhere.37-40 Briefly, Mg(NO3)2·6H2O (38.46 g, 0.15 mol), and Al(NO3)3·9H2O (28.13 g, 0.075 mol) were dissolved in 250 mL of deionized water. Then a definite amount of urea (molar ratio, n (urea): n (NO3-) = 2) was added to form a homogeneous mixed solution. Subsequently, the mixed solution was put into a 500 mL three-neck flask equipped with a condenser pipe, a thermometer, and a stirring device. The system was firstly warmed to 95oC for 10 h under vigorous stirring for dynamic crystallization and then maintained at 95oC 6


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for 20 h without stirring for static crystallization. Finally, the mixture was filtered, washed with deionized water thoroughly and dried under vacuum at 60oC for 24 h. After synthesis of Mg/Al hydrotalcite, the interlayer anions were usually CO32-. In order to expand layer spacing, the interlayer anions were then exchanged as NO3upon treatment of NaNO3 solution adopting a method detailed previously elsewhere.40 Typically, as-prepared CO32--intercalating Mg/Al hydrotalcite (0.5 g) was dispersed in a 500 mL aqueous solution containing 0.75 mol of NaNO3 and 0.0025 mol of HNO3. After purging with nitrogen, the reaction vessel was well-sealed and shaken in a constant temperature incubator shaker at ambient temperature for 24 h. Then the treated sample was filtered, washed with deionized water thoroughly and dried under vacuum at 60oC for 24 h. Synthesis of initiator-coated Mg/Al HT (HT-Br): The ionic liquid monomer, allyl triethylammonium chloride (ATEA-Cl) and initiator, 2-bromo-2-methyl-N-(3-(triethoxysilyl)

propyl)

propanamide

(BTPAm)

were

synthesized as detailed in previous works.41, 42 To synthesize initiator-coated Mg/Al HT (HT-Br), typically, previously dried Mg/Al hydrotalcite nanoparticles (2 g) were dispersed evenly in a 30 mmol/L solution of BTPAm in anhydrous toluene and the mixture was then heated to 120oC for 12 h under stirring. After cooling to ambient temperature, the products were collected by centrifugation and washed with toluene, ethanol for several times, followed by vacuum drying at 60oC overnight.

Synthesis of poly (ionic liquid) brushes modified Mg/Al HT (mHT): Typically, a 7



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100 mL flask with four necks was loaded with HT-Br (1.00 g), CuBr2 (0.021 g, 0.094 mmol), 2, 2'-Bipyridine (0.029 g, 0.19 mmol), ATEA-Cl (1.50 g, 8.44 mmol), and 40 mL anhydrous acetonitrile. The mixture was stirred to form a homogeneous solution and the atmosphere was then exchanged as N2. Subsequently, another mixture of 5 mL anhydrous

acetonitrile

containing

AIBN

(0.016

g,

0.071

mmol)

and

2-bromoisobutyryl bromide (0.0032 g, 0.014 mmol) was injected into the flask through a syringe. Immediately the flask was degassed by three freeze-pump-thaw cycles with N2, then the mixture was heated at 80oC to react for 24 h under stirring. Samples about 3 mL were removed with a purged syringe at desired intervals for GPC measurement. Here, 2-bromoisobutyryl bromide, acting as non-fixed initiator, was used not only to control the polymerization but also to produce unbound polymers. This approach was usually adopted to determine the molecular weight and distribution of polymers grafted on inorganic materials.43 After the polymerization, the remaining mixture was diluted with EDTA aqueous solution and cooled to ambient temperature. The final products were then collected by centrifugation, washed with EDTA aqueous solution, ethanol for several times and dried under vacuum at 60oC for 24 h.

Fabrication of mHT mixed polyethersulfone (PES) nanofiltration membranes PES membrane and mHT mixed PES membranes (PES-mHT-x) were both fabricated via a typical phase inversion method as we used previously.19-23 The composition of casting solutions for all membranes was listed in Table 2. Briefly, a certain amount of

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mHT nanoparticles were vigorously dispersed in corresponding amounts of N, N-dimethyl acetamide (DMAc) under ultrasonic treatment for 2 h to generate a uniform mHT suspension. The resulting suspension was then shaken at ambient temperature for 3-4 d under 170 rpm in a constant temperature incubator shaker for purpose of in-situ exfoliation of mHT nanoparticles. Subsequently, a certain amount of PVP and PES were added to this suspension under sustaining stirring at room temperature. After the generation of homogeneous casting solution, the pale yellow mixture was degassed under vacuum, followed by casting on a glass substrate by a casting knife with a thickness of 0.1 mm. The glass substrate was then immersed in coagulation bath (deionized water, 20oC) for primary phase separation and membrane formation. Finally, the obtained membrane was withdrawn from coagulation bath and kept in deionized water at ambient temperature to ensure complete phase separation. To prepare unfilled PES membrane, mHT suspension in DMAc was replaced with the same amount of pure DMAc during the preparation of casting solution.

Characterization of mHT The chemical structure change of mHT was investigated by FTIR spectra. FTIR spectra were performed at 2 cm−1 resolution with a Thermo Nicolet IR 200 spectroscope (Thermo Nicolet Corporation, USA). Typically, 64 scans were signal-averaged to reduce spectral noise. The spectra were recorded in the 400-4000 cm-1 range using KBr pellets. TGA measurements were carried out using a TG-DTA, DT-40 system (Shimadzu,

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Japan). Samples (10 mg) were heated from room temperature to 800oC at a heating rate of 10oC per min under flowing nitrogen. Elemental composition was analyzed using energy dispersive X-ray fluorescence spectrometry. Measurements were performed on an Oxford ED 2000 X-ray fluorescence spectrometer with a silver cathode and helium was used as the inert gas. The relative contents of elements were evaluated using XpertEase software in General Condition (high voltage 25 keV). Number average molecular weights (Mn), and polydispersity index (PDI, Mw/Mn) of poly (ionic liquid) grafted on mHT were determined using gel permeation chromatography (GPC) in THF at 30oC with a flow rate of 1 mL min-1. The original structure of Mg/Al hydrotalcite was inspected by SEM using a JEOL model JSM-6700F scanning electron microscope (JEOL, Japan). A FEI model TECNAI G2 transmission electron microscope (FEI, America) was used to study the morphology of mHT. The samples were dispersed in solvent with the aid of ultrasound. The suspended particles were transferred to a copper grid (400 meshes) coated with a strong carbon film and dried. XRD analysis was carried out on mHT before and after exfoliation in DMAc by PAN Alytical X'Pert Pro (PANalytical, Netherlands) in the scanning range of 2θ between 5o and 90o using copper Kα as the source of radiation.

Characterization of mHT induced mixed membranes The cross-section and surface morphologies of mixed membrane were observed by a

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JEOL model JSM-6700F scanning electron microscope (SEM) (JEOL, Japan). The membrane samples were gold-sputtered before SEM analysis. TEM images of cross -section of mixed membrane were also taken to observe the distribution of mHT nanoparticles in membrane matrix. Membrane samples were embedded in epoxy resin and cross sections with a thickness of 60 nm were obtained sectioning with Leica Ultracut UTC ultramicrotome. Then the ultrathin sections were mounted on the carbon-coated TEM copper grids. The surface hydrophilicity of mixed membrane was evaluated through water contact angle surveyed on a contact angle goniometer (OCA20, Dataphysics Instruments, Germany) at 25oC and 50% relative humidity. Deionized water (1 µL) was carefully dropped on the top surface and the contact angle between the water and membrane was measured until no further change was observed. To minimize the experimental error, the contact angle was measured at five random locations for each sample and then the average was reported. Atomic force microscopy (AFM) (Nanoscope IIIA) was used to measure the roughness of the membranes and the measurement was performed in the tapping mode in air atmosphere. Water uptake (Wu) of mixed membrane was investigated adopting a traditional method. The tested membranes were dried under vacuum at 40oC to constant weight then immersed into deionized water for 36 h at ambient condition. Subsequently, the surface water of moist membranes was dried by bibulous papers and the weight was

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surveyed. For each membrane sample, 3 times measurements were carried out and the average value was reported. The water uptake (Wu) can be calculated by the following equation:

Wu =

Ww − Wd ×100% Wd

(1)

Where Ww is the weight of wet membrane after dried by bibulous paper, Wd is the weight of the membrane which was dried under vacuum at 40oC thoroughly. Tensile strength and percentage elongation were measured on testing strips using a model UTM2203 electronic universal testing machine (Jinan Huike Test Instrument Co., Ltd., China) mounted with a 100 N load cell at room temperature at a constant crosshead speed of 5 mm/min with aluminum sample holder. Rectangle shapes (40 mm length and 10 mm width) were cut from the membranes and the thickness of the membranes were obtained from the SEM results of membranes. Particular attention was given to the macroscopic homogeneity of membranes and only apparently homogeneous membranes were used for the mechanical tests. The thermal stability of mixed membrane was evaluated by thermo gravimetric analysis (TGA, Perkin-Elmer Pyris Diamond). The TGA measurements were carried out under nitrogen atmosphere at a heating rate of 10oC from room temperature to 800oC. Ionic exchange capacities (IEC) of mixed membranes were conducted using a conventional method. The tested membranes were converted to Cl- ionic form, and then back titrated with 0.1 mol/L of AgNO3 solution.

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Separation performance of mHT induced mixed membranes Reactive red 49, reactive black 5 and a series of salts (NaCl, Na2SO4, MgCl2, and MgSO4) were used for NF experiments to investigate membrane separation properties. Water fluxes of prepared membranes were measured under 0.4 MPa at ambient temperature, prior to which the membranes were pre-pressurized under 0.6 MPa for 0.5 h. The permeation flux (J) and rejection (R) of tested membranes were calculated using the following equations:

J=

V A∆t

(2)

R = (1 − Cp ) × 100% Cf

(3)

Where V is the volume of permeate pure water (L), A is the effective area of the membrane (m2), and ∆t is the permeation time (h), Cp is the permeate concentration and Cf is the feed concentration. The primal feed concentration for reactive dyes was 1 g/L (pH, 7~8) and 0.5 g/L for salts. The concentrations of monodispersed salts (MgCl2, MgSO4, NaCl, and Na2SO4) solutions were measured with an electrical conductivity meter and the concentrations of dyes were obtained by a UV-Vis spectrophotometer (Shimadzu, Japan). Moreover, in order to investigate the effect of operation pressure on pure water flux of mixed membranes, the pure water flux under a series of pressures were also measured carefully. Time course of water fluxes and rejections for organic dyes and salts were subsequently performed to evaluate the short-term stability of mixed membranes.

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RESULTS AND DISCUSSION Chemical structure and morphology of mHT As described previously elsewhere,

37-39, 44

Mg-Al hydrotalcite can be chemically

modified straightforwardly by covalent attachment of functional organics because of the presence of rich surface hydroxyl and layered structure. Fig. 1a shows the FTIR spectra of pristine Mg-Al HT, HT-Br, and mHT. A new band assigned to stretching vibration of C-H was seen in the range of 2800-3000 cm-1 for HT-Br as well as mHT. The bending vibration of C-H can be clearly observed around 1570 cm-1 in the spectrum of mHT. In addition, a significant absorption centered at 1090 cm-1 was generally considered as the stretching vibration of C-N+ in quaternary ammonium groups. In contrast with pristine Mg-Al HT, FTIR results revealed the success of the polymerization of ionic liquid on the layered structure of Mg-Al HT. The degrees of modification of pristine Mg-Al HT were measured by three techniques in this work including TGA measurement, X-ray fluorescence, and GPC measurement. Fig. 1b shows TGA curves of pristine Mg-Al HT, HT-Br, and mHT as a function of temperature under N2 atmosphere. There were no distinct differences for three samples below 250 oC and the residual weight showed the same appreciable decrease of approximately 15%, probably due to desorption of physical adsorbed water. From 250 to 600oC, clear differences among three samples were observed. Mg-Al HT showed a slight and then an arresting decrease in residual weight in the ranges of 250-380oC and 380-550oC, respectively. These two weight losses may be due to the 14


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dehydroxylation of structural Al-OH groups (250-380oC) and dehydroxylation of structural Mg-OH, decomposition of exchangeable anions in the interlayer (380-550 o

C). HT-Br as well as mHT revealed significant weight losses approximately between

250 and 480oC attributing to the decomposition of organic structures grafted on Mg-Al HT. A little decrease in residual weight of either HT-Br or mHT was observed from approximately 480oC to the end of heating process, most likely corresponding to the ongoing decomposition of organic structures and dehydroxylation of residual structural Mg-OH. Considering the differences of weight losses among three samples in the whole heating process, approximate grafting degrees can be grossly calculated from the TGA curves. Accordingly, the grafting degrees of initiator on Mg-Al HT and poly (ionic liquid) (PIL) on HT-Br were calculated as 0.210 g BTPAm/g Mg-Al HT, 0.253 g PIL/g HT-Br, respectively. Fig. 1c shows the changes of element composition for Mg-Al HT before and after modification. An obvious increase in silicon and the presence of bromine were observed in HT-Br nanoparticles in comparison with pristine Mg-Al HT, which indicated the loading of initiator. And furthermore, a distinct increase in chlorine and however a reduction in bromine of mHT was found when compared with HT-Br. Such results showed that poly (ionic liquid) brushes containing chlorine ions were grafted on HT-Br nanoparticles via RATRP method and most bromine atoms were exchanged as chlorine during the polymerization process. The number-average molecular mass (Mn) of poly (ionic liquid) grafted on HT-Br was determined through an equivalent method and the results are shown in Fig. 1d. It

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can be seen clearly that Mn of PIL increased approximately linearly with polymerization time and when the time was 24 h, Mn of PIL reached 4200 g/mol. Besides, the values of polydispersity index (PDI) were just between 1.0 and 1.1, which exhibited rather narrow molecular weight distribution, indicating the “control/living” characteristic of RATRP. Fig. 2 shows the morphologies of Mg-Al HT and exfoliated mHT. SEM micrographs of pristine Mg-Al HT are shown in Fig. 2a-b, where pristine Mg-Al HT nanoparticles were observed in forms of sheets and took on the layers accumulatively. Most of Mg-Al HT sheets were agglomerated as spheral forms with an approximate diameter from 0.5 to 2 µm and the nanosheets were clearly seen in the magnified image (Fig. 2b). After the exfoliation of mHT in DMAc, the as-prepared colloidal solution was observed by TEM. As shown in Fig. 2c-d, exfoliated mHT exhibited small lamellae structure with a small size of approximately 20-100 nm. XRD patterns of pristine Mg-Al HT, intercalated HT and exfoliated mHT are recorded in Fig. 2e. Pristine Mg-Al HT exhibited highly crystalline nature, layered geometry and a typical strong diffraction peak at 11.7o corresponding to an interlayer spacing of 0.760 nm. After the intercalation of nitrate ions, the reflections of intercalated HT shifted to lower 2θ angle, that is, from 11.7o to 9.9o. This indicated an increase in interlayer distance from 0.760 to 0.897 nm because of the intercalation of nitrate ions. However, after in-situ exfoliation of mHT in DMA, all the feature diffraction peaks disappeared leaving a very small bump around 17o because of the diffraction peak of the glass substrate.

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Thus, such results suggested that mHT nanosheets with layered structure were in-situ exfoliated completely in DMAc.

Characterization of mHT induced mixed membranes. Fig. 3 shows the SEM micrographs of cross-section of unfilled PES membrane and mHT mixed PES membranes with various contents of mHT nanoparticles. Each membrane consisted of a support layer with finger-like structure and a dense skin layer serving as a separation layer, with a total thickness of approximately from 70 to 90 µm. Reportedly, cross-section morphologies of membranes prepared by nonsolvent induced phase separation (NIPS) depended on the rate of polymer precipitation from coagulation

bath.45-47

Slow

precipitation

rates

produced

membranes

with

“sponge-like” structure and fast precipitation rates produced membranes with large “finger-like” macrovoids in substructure. Generally, in most cases, membranes with “finger-like” structure had the advantages of high water fluxes because of the ultrathin top layer and the macrovoids-rich sublayer. Mixed PES membranes exhibited thickened skin layer with an increased thickness from 0.68 µm of unfilled PES membrane to 3.98 µm of PES-mHT-1.0% membrane due to the enhanced viscosity of casting solution in consequence of the addition of nanofillers. Table 3 summarizes viscosities of casting solutions with various loading amount of nanofillers. Viscosity obviously increased with loading amount of nanofillers from 1530 mPa·s of unfilled PES membrane to 1850 mPa·s of PES-mHT-1.5% membrane. The increase in viscosity was also observed in many other works

48-50

indicating the occurrence of

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good interaction between nanofillers and the polymers, playing an important role in the morphology of membranes prepared upon phase inversion method. Meanwhile, from the inset of Fig. 3, it can be also observed that mixed membranes showed much looser and more porous skin layer, increased connectivity and more uniform macrovoids in support layer compared to unfilled PES membrane. Such enlarged/uniform macrovoids may result from the fusion of disappearing macrovoids because of the improved viscosity.50 Importantly, this structure of mHT mixed membranes may facilitate the permeation of water molecules and salts and meanwhile remain entrapment of low-molecular-weight organics. Fig. 4 shows the water contact angle results of mHT mixed PES membranes as a function of nanofillers loading amount. It is commonly acknowledged that the lower contact angle represents the higher surface energy, greater tendency for water to wet the membrane and the higher hydrophilicity.51 All the mixed PES membranes (Fig. 4b-d) showed hydrophilic nature as a result of the introduction of mHT nanofillers compared with unfilled PES membrane (Fig. 4a). Unfilled PES membrane exhibited the highest contact angle of 83.5o whereas this value decreased to 64o of PES-mHT-1.5% membrane with the loading amount of nanofillers. This result was consistent with that of membrane surface morphologies obtained from SEM and AFM images. As indicated in Fig. 5, surface SEM and 10 ×10 µm roughness analysis of AFM images for both unfilled PES membrane and mHT mixed PES membranes were performed. Unfilled PES membrane exhibited a rough surface with numerous

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attachments (pollutants) on it while the mixed PES membrane showed a rather smoother surface. Membrane samples were kept in deionized water and flushed with deionized water before observation, hence the attachments on the surface of unfilled PES membrane contributed part of the roughness and may be due to the growth and attachment of bacteria. However, under the same conditions, mHT mixed membrane showed a clean and smooth surface, possibly benefiting from the improvement of antifouling abilities induced by the enhanced hydrophilicity as stated hereinbefore. Furthermore, mHT nanoparticles can be clearly observed at a higher magnification on the surface mHT mixed PES membrane (Fig. 5b). The existence of mHT nanoparticles on the membrane surface may contribute for the membrane surface roughness. However, the addition of mHT nanoparticles with high hydrophilicity, positively charged characteristic and affinity with polymers may evidently reinforce the

membrane

surface

hydrophilicity

and

interfacial

interaction

of

nanoparticles-polymer. As a result, mHT mixed membrane presented a smoother, hydrophilic, antifouling and uniform surface compared with that of unfilled PES membrane. Similar results were found and verified in AFM images of unfilled PES membrane and mHT mixed membrane as shown in Fig. 5c-f. Both top-view and side-view of AFM images confirmed the smoother surface of mHT mixed PES membrane. The mean roughness (Ra) decreased from 13.804 nm of unfilled PES membrane to 11.977 nm of PES-mHT-1.0% membrane and the root-mean-square roughness (Rms) represented the same tendency. In general, membrane roughness

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facilitates the fouling phenomenon in various membrane processes especially in NF, in which it might be somewhat more complex because the interactions leading to fouling occur at nanoscale.52-54 It is accepted that surface roughness increases the membrane fouling by increasing the rate of attachments onto membrane surface.55 Therefore, membranes with a lower surface roughness as well as a higher surface hydrophilicity can be beneficial to decrease the membrane fouling, in particular, colloidal fouling in NF process. Such membrane morphologies in combination with that of water contact angle revealed that mHT mixed PES membranes provided potential antifouling properties. In order to investigate the distribution of mHT nanoparticles in PES membrane matrix, cross-sectional TEM images were introduced and are shown in Fig. 6. The fairly thin skin layer and porous support structure were clearly observed in Fig. 6a, consistent with that of SEM images as shown in Fig. 3. Under a magnifying image of the skin layer (Fig. 6b), mHT nanosheets were relatively uniformly dispersed in PES membrane matrix in different particle sizes with small agglomeration. Usually, nanoparticles mixed matrix membrane was considered as a viable candidate to break the traditional permeability-selectivity trade-off of polymeric membrane. Inorganic nanoparticles were added to polymer matrix to fabricate mixed matrix membranes and meanwhile were expected to endow resulting membranes with better properties to overcome the limitations of regular polymer/inorganic membranes. Fig. 7 shows the stress-strain curves for mHT mixed PES membranes as a function of

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loading amount of nanoparticles. Stress-strain curve of unfilled PES membrane was also plotted as a control acquiring a good mechanical stability with a tensile strength of 3.06 MPa at elongation of 33.87%. All the tested membranes represented elastic deformation in the initial stage and the mHT mixed membranes showed increased tensile strengthes ranging from 3.36 to 3.54 MPa compared to unfilled PES membrane. Table 4 summarizes the detailed information of tensile strengthes, Young’s moduli and elongations of all the tested membranes. Young’s moduli of mixed membranes were also enhanced with the loading amount of nanoparticles from 117.4 MPa of unfilled PES membrane to 144.7 MPa of PES-mHT-1.5% membrane at possible expense of elongatons. This phenomenon was probably affected by the particle-polymer adhesion force between the interface of particles and polymer. Nanoparticles reinforced the tensile strengthes of mixed membranes upon adhesion forces with polymer chains, which resulted in a dense chain packing and a suppression of chain mobility.56 Fig. 8 describes the thermostability of mHT mixed PES membranes. The discomposing temperature of a membrane is defined as a temperature with a residual weight at 97%,57 that is, a temperature at which a membrane begins to represent an obvious weight loss of 3% during the heating process. All the membranes revealed the similar weight loss stages: (I) the first stage was the evaporation of physical absorbed water from the matrix of membranes around 30-150 oC; (II) the second stage, occupying the most weight loss in the whole process, can be attributed to the degradation of PES backbones from 370-650 oC; (III) the third stage from 650 oC to

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the end was in part owing to the ongoing pyrolysis of PES molecules and in part owing to the dehydroxylation of mHT nanoparticles for mixed PES membranes. Moreover, for mHT mixed PES membranes, the degradation of poly (ionic liquid) brushes on the surface of mHT nanoparticles occurred in the second and third stages and the mHT nanoparticles possibly remained eventually in the ashes as oxide form considering the lower weight losses of mixed membranes than unfilled PES membrane. The discomposing temperature of unfilled PES membrane emerged at 396 o

C with a total weight loss of 65%. By comparison, mHT mixed PES membranes

showed distinct enhancements in discomposing temperature with a highest value of 439 oC of PES-mHT-1.5% membrane. Collectively, these data indicated distinct improvements in thermal and mechanical behaviors, surface and cross-section morphologies of mixed PES membranes by the introduction of mHT nanoparticles. The charged properties of mixed membranes were also investigated by the measurement IEC values, which reflected the amount of ion-exchangeable groups existing in the matrix of membranes. Fig. 9 shows the IEC values of mHT mixed PES membranes as a function of nanoparticles loading amount. The IEC value increased dramatically once the charged mHT nanoparticles were added and the highest value of mHT mixed membranes could achieve to be 1.23 meq/g of PES-mHT-1.5% membrane. As shown in Fig. 9, the IEC value did not increase linearly with the loading amount of charged mHT nanoparticles as expected, which was most likely due to the limitation of test method and the part loss of charged materials during the

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membrane formation process.

Separation performance of mHT induced mixed membranes For inorganic/organic mixed membranes, interface voids at the polymer-particle interface may generate during the membrane formation process and such voids were likely to provide additional passage channels for water molecules or salts ions, particularly for di- or multi-valent anions to pass through. Fig. 10a shows water uptakes and water fluxes of mHT mixed PES membranes as a function of nanoparticles loading amount. Water uptakes of mHT mixed membranes increased significantly with nanofillers loading amount with a highest value of 183.5% for PES-mHT-1.5% compared to 120.5% of unfilled PES membrane. This result indicated the enhancement in hydrophilicity of mixed PES membranes, which had been confirmed by water contact angle results. Normally, hydrophilicity-enhanced membranes may offer higher water fluxes during water filtration process. However, mHT induced membranes presented a small decrease in water flux before increasing with mHT particles loading amount. Meanwhile, as shown in Fig. 10b, water flux of all the tested membranes shot up linearly with operation pressure and water permeability was calculated adopting to Spiegler-Kedem Model,30 which was utilized to describe this linear behavior of separation membranes. Accordingly, mHT induced membrane (PES-mHT-1.5%) exhibited a little lower value of 63.25 L/(m2·h·MPa) in comparison to 64.41 L/(m2·h·MPa) of unfilled PES membrane. Though concerned that mixed membranes may offer more pathways for water molecules to pass through, such phenomenon was not surprising. Because of the waterproof nature of nanofillers, 23



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overmuch loading amount reasonably might reduce the effective area of membranes when in use. On other hand, the increase in toplayer thickness of mHT mixed membrane might also lead to some increase of permeation resistant. In our previous study, the embedment of graphene oxide (GO) nanosheets in PES membranes reduced the pure water flux at a small level despite a significant increase in surface hydrophilicity.16 Therefore it was not uncommon to notice the slight decrease in water permeability of mHT induced membrane. However, the subsequent increase of water flux with mHT loading amount was attributed to the predominant-effects of the increase in surface hydrophilicity and interface voids around mHT particles. Fig. 11 shows the rejections for dyes and salts of mHT induced PES membranes as a function of nanoparticles loading amount. Obviously, mHT induced PES membranes showed stable rejections for either reactive black 5 (around 95%) or reactive red 49 (around 90%), except for slight fluctuations. As shown in Fig. 12, after filtration with mHT induced membranes, reactive dyes aqueous solution were almost clear with non-obivious chroma. The relatively higher rejections for reactive dyes indicated PES membranes prepared in our work had an approximate pore size range to realize the retention of reactive dyes and the introduction of mHT particles was unable to decrease the separation performance for reactive dyes. However, importantly, compared to unfilled PES membrane, mHT induced PES membranes presented apparent decrease in rejections for all the tested salts. The rejections for salts of mixed membranes were decreased to no more than half of unfilled PES membrane. The

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lowest rejection for NaCl of mixed membranes was only 8% and the value decreased to 6% for Na2SO4. The possible reason for significant decrease in rejection for salts was part due to the looser and porous skin layer and part due to electrostatic interaction. Moreover, on the other hand, the thickened skin layer may compromise on such effect in a certain degree. Furthermore, it was worth to note that mHT mixed membranes showed the same rejections order for the tested salts, i.e., MgCl2 > MgSO4 > NaCl > Na2SO4. However, a typical rejections order of positively charged NF as reported was MgCl2> CaCl2 > NaCl = KCl > MgSO4 >Na2SO4 >K2SO4, which can be explained by electrostatic and steric effects.58 In this work, for MgSO4 > NaCl, it may be partly ascribed to a stronger repulsion force on cation than the attraction one on anion and partly ascribed to a possible sieving effect induced by the small changes of separation layer of mixed membranes. Operation stability is another important factor to evaluate the use performance of membranes. A filtration test of 48 h for each mixed system (reactive black 5 + NaCl + water and reactive red 49 + NaCl + water) was also performed to evaluate the membrane stability. Each mixed system showed a similar situation that the rejections for reactive dyes presented very small increases at the very early stage and then almost maintained stable until the end of filtration test. These small increases in rejections may be attributed to the adsorption of reactive dye molecules on the membrane surface or pores surface at first stage because of the electrostatic interaction between the positively charged membrane surface and always negatively

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charged dye molecules (pH~6.5).59, 60 The adsorbed dyes on the membrane surface may generate a thin gel layer acting as an enhanced separation layer. However, because of the excellent water solubility of reactive dyes, this adsorption process may be dynamic and then the filtration process can keep stable once equilibrium was established. Nevertheless, rejections for NaCl in each system almost remained stable with some tiny fluctuations during the whole filtration process. Maybe the major impact factor on the rejections for NaCl was the charging effect and ion exchange effect than sieving effect and therefore the adsorption of reactive dyes had little effect on the rejections for NaCl in mixed systems. Recently, there are several reported works about loose nanofiltration membranes paying special attention on the separation of low-weight-molecular organics and salts as summarized in Table 5. These membranes showed relatively higher water permeability and rather lower or near-zero salts rejections compared to conventional nanofiltration membranes. Therefore, it is possible to realize the separation of salts and dyes from their mixed aqueous solutions with these membranes, which are of great significance for the reuse or recycling of textile wastewater.

CONCLUSIONS In this work, poly (ionic liquid) brushes modified Mg-Al hydrotalcite (mHT) was fabricated via reverse atom transfer radical polymerization method and subsequently a novel positively charged loose mHT mixed PES membrane was prepared to be used in the separation of dyes from salts-containing wastewater. Various characterizations

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were performed to study membrane morphologies, physic-chemical properties, particularly separation behaviors and operation stability. The introduction of mHT particles as well as the loading amount had a significant influence on membrane properties. It was confirmed that the surface hydrophilicity, mechanical and thermal stability of mHT mixed PES membranes were enhanced compared to unfilled PES membrane. As the loading of mHT particles, the membrane skin layer became thicker, looser and positively charged. The fine-tuning of separation layer of mHT mixed PES membranes facilitated the transport of salts without perceptible decrease in dyes rejections. Therefore, this type of loose nanofiltration membrane may open opportunities for the separation of dyes from salts-containing textile wastewater, which is very lack in industrial applications.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI. Synthesis

of

allyl

triethylammonium

chloride

(ATEA-Cl);

Synthesis

of

2-bromo-2-methyl-N-(3-(triethoxysilyl) propyl) propanamide (BTPAm); 1H NMR spectra of ATEA-Cl; 1H NMR spectra of BTPAm; DSC (a) and TGA (b) curves of ATEA-Cl; Elution curves of poly (ionic liquid) (PIL) AUTHOR INFORMATION Corresponding Author *(Y.Z.) Phone: +86-371-67781734. E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS We gratefully acknowledge the support from National Natural Science Foundation of China (Nos. 21476215 and 21376225), Program for Science & Technology Innovation Talents in Universities of Henan Province (16HASTIT004) and Excellent

Youth

Development

Foundation

of

Zhengzhou

University

(No.

1421324066). REFERENCES (1) Teixeira, M.; Campo, M.; Tanaka, D. A.; Tanco, M. A.; Magen, C.; Mendes, A., Carbon–Al2O3–Ag composite molecular sieve membranes for gas separation. Chem. Eng. Res. Des. 2012, 90 (12), 28


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natural organic matter rejection and flux decline. Sep. Purif. Technol. 2007, 58 (1), 68-75. (53) Agenson, K. O.; Urase, T., Change in membrane performance due to organic fouling in nanofiltration (NF)/reverse osmosis (RO) applications. Sep. Purif. Technol. 2007, 55 (2), 147-156. (54) Al-Amoudi, A.; Lovitt, R. W., Fouling strategies and the cleaning system of NF membranes and factors affecting cleaning efficiency. J. Membr. Sci. 2007, 303 (1–2), 4-28. (55) Aroon, M. A.; Ismail, A. F.; Matsuura, T.; Montazer-Rahmati, M. M., Performance studies of mixed matrix membranes for gas separation: A review. Sep. Purif. Technol. 2010, 75 (3), 229-242. (56) Moore, T.; Mahajan, R.; Vu, D.; Koros, W., Hybrid membrane materials comprising organic polymers with rigid dispersed phases. AIChE J. 2004, 50, 311-321. (57) Murthy, Z. V. P.; Gupta, S. K., Estimation of mass transfer coefficient using a combined nonlinear membrane transport and film theory model. Desalination 1997, 109 (1), 39-49. (58) Huang, R.; Chen, G.; Sun, M.; Hu, Y.; Gao, C., Studies on nanofiltration membrane formed by diisocyanate cross-linking of quaternized chitosan on poly (acrylonitrile)(PAN) support. J. Membr. Sci. 2006, 286 (1), 237-244. (59) Zhao, W.; Tang, Y.; Xi, J.; Kong, J., Functionalized graphene sheets with poly(ionic liquid)s and high adsorption capacity of anionic dyes. Appl. Surf. Sci. 2015, 326, 276-284. (60) Mi, H.; Jiang, Z.; Kong, J., Hydrophobic Poly(ionic liquid) for Highly Effective Separation of Methyl Blue and Chromium Ions from Water. Polymers 2013, 5, 1203-1214. (61) Xing, L.; Guo, N.; Zhang, Y.; Zhang, H.; Liu, J., A negatively charged loose nanofiltration membrane by blending with poly (sodium 4-styrene sulfonate) grafted SiO2 via SI-ATRP for dye purification, Sep. Purif. Technol. 2015, 146, 50-59. (62) Zhu, J.; Tian, M.; Zhang, Y.; Zhang, H.; Liu, J., Fabrication of a novel “loose” nanofiltration membrane by facile blending with Chitosan–Montmorillonite nanosheets for dyes purification, Chem. Eng. J. 2015, 265, 184-193. (63) Zhu, J.; Zhang, Y.; Tian, M.; Liu, J., Fabrication of a Mixed Matrix Membrane with in Situ Synthesized Quaternized Polyethylenimine Nanoparticles for Dye Purification and Reuse, ACS Sustain. Chem. Eng. 2015, 3, 690-701. (64) Lin, J.; Ye, W.; Zeng, H.; Yang, H.; Shen, J.; Darvishmanesh, S.; Luis, P.; Sotto, A.; Van der Bruggen, B., Fractionation of direct dyes and salts in aqueous solution using loose nanofiltration membranes. J. Membr. Sci. 2015, 477, 183-193. (65) Puspasari, T.; Pradeep, N.; Peinemann, K.-V., Crosslinked cellulose thin film composite nanofiltration membranes with zero salt rejection. J. Membr. Sci. 2015, 491, 132-137.

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Table Captions Table 1 Summary of molecular structures of reactive dyes used in this work Table 2 Summary of composition of casting solutions Table 3 Summary of Young’s modulus, tensile strength and elongation at break for the mHT derived membranes Table 4 Summary of casting solutions with various loading amount of nanofillers Table 5 Separation performance of loose nanofiltration membranes for separation of salts and dyes in the literature and this work Figure Captions Scheme 1 Schematic diagram of fabrication for mHT mixed PES nanofiltration membrane Fig. 1 Characterization of chemical structure of mHT. (a) FTIR spectra of Mg-Al HT, HT-Br and mHT. (b) TGA curves of Mg-Al HT, HT-Br and mHT. (c) Surface elemental composition of Mg-Al HT, HT-Br and mHT. (d) Number-average molecular mass (Mn) measurement of mHT. Fig. 2 Morphology of Mg-Al HT and exfoliated mHT. (a and b) Representative SEM micrographs of pristine Mg-Al HT. (c and d) Representative TEM micrographs of mHT exfoliated in DMAc. (e) XRD patterns of pristine Mg-Al HT, NO3- intercalating HT and exfoliated mHT. Fig. 3 SEM micrographs of cross section of (a) unfilled PES membrane, (b) PES-mHT-0.5%, (c) PES-mHT-1.0%, (d) PES-mHT-1.5% Fig. 4 Static water contact angle measurement of (a) unfilled PES membrane, (b) PES-mHT-0.5%, (c) PES-mHT-1.0%, (d) PES-mHT-1.5% Fig. 5 Surface morphologies of membranes: SEM morphologies of (a) unfilled PES membrane and (b) PES-mHT-1.0%; Top-view of AFM images of (c) unfilled PES membrane and (d) PES-mHT-1.0%.; Side-view of AFM images of (e) unfilled PES membrane and (f) PES-mHT-1.0% Fig. 6 TEM morphologies of (a) the cross-section of PES-mHT-1.0% membrane and (b) the dispersion of mHT nanoparticles in membrane matrix Fig. 7 Stress-strain curves for unfilled PES membrane and mHT mixed PES membranes with different nanoparticles loading amount Fig. 8 TGA curves for unfilled PES membrane and mHT mixed PES membranes with different nanoparticles loading amount Fig. 9 Ion exchange capacities of unfilled PES membrane and mHT mixed PES

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membranes as a function of nanoparticles loading amount Fig. 10 (a) Water uptake and water flux of mHT mixed PES membranes as a function of nanoparticles loading amount. (b) Effect of operation pressure on water flux of unfiled PES membrane and membrane PES-mHT-1.0% Fig. 11 Rejection for dyes and salts of mHT mixed PES membranes as a function of nanoparticles loading amount. Fig. 12 Photograph showing the separation effect of reactive dyes by membrane PES-mHT-1.0% Fig. 13 Short-term stability of mHT mixed PES membrane (operating conditions: PES-mHT-1.0% membrane, 0.4 MPa, 500 mg/L reactive dye aqueous solution with 1000 mg/L NaCl).

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Table 1 Molecular structure

Molecular weight

Reactive red 49

576.49

Reactive black 5

991.82

Table 2

1

Membrane type

PES (wt%)

mHT (wt%)

PVP (wt %)

DMAc (wt %)

Unfilled PES

22

0

1

77

PES-mHT-0.5%1

22

0.11

1

76.89

PES-mHT-1.0%1

22

0.22

1

76.78

PES-mHT-1.5%1

22

0.33

1

76.67

The percentage values of 0.5%, 1.0%, 1.5% refer to the weight percent of mHT

particles in PES membrane rather than casting solution. Table 3

Casting solution

Unfilled PES

PES-mHT-0.5%

PES-mHT-1.0%

PES-mHT-1.5%

Viscosity (mPa·s)

1530

1600

1700

1850

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Table 4

Membrane type

Young’s modulus (MPa)

Tensile strength (MPa)

Elongation (%)

Unfilled PES

117.4

3.05

33.8

PES-mHT-0.5%

129.8

3.36

31.3

PES-mHT-1.0%

127.5

3.55

34.2

PES-mHT-1.5%

144.7

3.54

18.8

Table 5

Membrane

Permeability 2

(L/m h MPa)

NaCl rejection (%)

HT/PES

384

~4*

SiO2-PIL/PES

375

~3

HNTs-PIL/PES

118

~6

SiO2-PSS/PES

225

~3

Cs-MMT/PES

156

~5

QPEI/PES

126

~6

Sepro NF 2A

105

6.9~33.3

Ssepro NF 6

137

2.6~17.9

Crosslinked RC

~18

~0

mHT/PES

63

~8

Organic rejection (%) 90~97%, PEG 1000 88~90%, Reactive black 5 94~96%, Reactive black 5 ~90%, Reactive black 5 90~95%, Reactive black 5 ~95%, Reactive black 5 ~99.9%, Direct red 80 ~99.9%, Direct red 80 ~80%, Sucrose 95~98%, Reactive black 5

*

Rejection for MgSO4

37


Ref. [39] [41]

[42]

[61]

[62]

[63]

[64]

[64]

[65]

This work


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Scheme 1

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Fig. 1

Fig. 2

Fig. 3

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Fig. 4

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Fig. 5

Fig. 6

Fig. 7

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Fig. 8

Fig. 9

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Fig. 10

Fig. 11 43



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Fig. 12

Fig. 13

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TABLE OF CONTENTS (TOC) GRAPHIC

Title: Improved salts transportation of a positively charged loose nanofiltration membrane by introduction of poly (ionic liquid) functionalized hydrotalcite nanosheets Authors: Liang Yu, Jianmian Deng, Huixian Wang, Jindun Liu, and Yatao Zhang Synopsis: A novel positively charged loose nanofiltration membrane was prepared by adding poly(ionic liquid) functionalized hydrotalcite nanosheets, which showed enhanced salts transportation.


Improved Salts Transportation of a Positively Charged Loose

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