Polypiperazine-amide Nanofiltration Membrane ... - ACS Publications

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Polypiperazine-amide Nanofiltration Membrane Modified by Different Functionalized Multiwalled Carbon Nanotubes (MWCNTs) Shuang-Mei Xue, Zhen-Liang Xu,* Yong-Jian Tang, and Chen-Hao Ji State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ABSTRACT: In this work, three modified multiwalled carbon nanotubes (MWCNTs) with carboxyl (MWCNT-COOH), hydroxyl (MWCNT-OH) and amino groups (MWCNT-NH), respectively, were added into the aqueous phase containing piperazine (PIP) to fabricate the nanocomposite nanofiltration (NF) membranes via interfacial polymerization. The influences of functional groups of MWCNTs on the performance of modified NF membrane were investigated. The MWCNTs were characterized by TEM, FT-IR and TGA; meanwhile, the properties of the membranes were evaluated by XPS, TEM, AFM and contact angle. The XPS results proved the successful incorporation of MWCNT in the active layer of modified NF membrane. When the MWCNT concentration is 0.01% (w/v), all the nanocomposite membranes possessed the optimal separation properties, among which the membrane incorporated with MWCNT-OH demonstrated the highest water flux of 41.4 L·m−2·h−1 and the Na2SO4 rejection of 97.6% whereas the one with MWCNT-COOH had the relative lowest rejection of 96.6%. Furthermore, the increased hydrophilicity of functional groups in modified MWCNTs resulted in different nodular surface morphologies, thicknesses and hydrophilicities of the nanocomposite membranes. All the membranes possessed a molecular weight cutoff (MWCO) within 300 Da and good operation stability. KEYWORDS: nanofiltration membrane, multiwalled carbon nanotube (MWCNT), interfacial polymerization, modification, functional group

1. INTRODUCTION Clean water scarcity is gaining worldwide attention because of the aggravating environmental pollution and the growing population. Because of the advantages in less energy consumption, moderate operation conditions and environmental-friendly property,1 membrane technology has become the most common approach for water purification. Nanofiltration (NF) is a burgeoning pressure-driven membrane process with excellent separating capability of divalent ion and low-molecular-weight organics.2,3 The ultrathin top layer on the supporting base, commonly composed of aromatic polyamide, plays a vital role in the performance of NF membrane. To improve further the separating properties as well as satisfy the extra needs for chemical durability, extensive efforts have been made to explore the potential of NF membrane. The emergence of nanomaterials such as silica, 4 TiO 2 , 5 MWCNT,6 graphene oxide,7 serving as nanosized fillers give new solutions for the modification of NF membrane in both the substrate and the separating layers. Nanoparticles are postulated to provide nanosized pores as well as tiny interspace between the polymer matrixes enhancing the water channels and obtaining high selectivity for membranes. Multiwalled carbon nanotubes (MWCNTs) are allotropes of carbon with a cylindrical nanostructure and well-known for their unique pore structure, large specific surface area, and excellent thermal and mechanical properties.8 With the development of mass-produced processing method of catalytic © XXXX American Chemical Society

chemical vapor deposition, MWCNTs receive the most extensive commercial applications among all the carbon nanomaterials in various fields, such as sensing, biomedicine and electrochemistry.9,10 However, MWCNTs’ general tendency to form entangled agglomerates and its weak interfacial interactions with the polymer matrix make it difficult to prepare homogeneous MWCNT dispersion in many conventional solvents.11 Consequently, attaching hydrophilic functional group to the surface of MWCNT, aiming at enhancing the chemical compatibility with other polymers, is invented to tackle this problem. Chemical oxidation of MWCNT with mixed acids is generally employed as the preliminary modification because it is capable to graft carboxyl and hydroxyl groups to the surface of MWCNTs with a simple procedure, which subsequently enables further functionalization with other chemical reactions such as esterification, amination, amidation and silanization.12,13 The carboxyl and hydroxyl groups also serve as the precursors for the linkage with active long chain chemicals,14 hydrophobic groups,15 metal or its oxide such as Ag and TiO2.10,16 Except for the improvement in water flux and rejection for the membrane, the incorporation of MWCNT is also proved to strengthen efficiently the chlorine Received: May 9, 2016 Accepted: July 7, 2016

A

DOI: 10.1021/acsami.6b05545 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration for the preparation of different nanocomposite NF membranes via interfacial polymerization.

2. EXPERIMENTAL SECTION

tolerance and antifouling capability of the membrane as well as altering the electrical properties.17−20 Furthermore, different physicochemical characteristics of MWCNT also have influences on the membrane functioning under the same experimental condition. Shah et al.21 studied the improvement of PSF membrane with oxidized, amide and azide MWCNTs, among which the amide MWCNT/PSf membrane exerted the best heavy metal removal ability. Zhang et al.22 investigated the effect of oxidized MWCNT with different diameters (10−20, 20−40, 40−60 nm) on the performance of PVDF/PFSA hollow fiber ultrafiltration membranes, resulting the one with the diameter of 20−40 nm possessed the highest flux of 181.2 L·m−2·h−1·bar−1. Wu et al.23 prepared MWCNT/polyester NF membranes with different surfactants in the aqueous phase and the anionic (SDS) surfactant was found to be more suitable than cationic and nonionic surfactants for the IP process. The thickness of the PA active layer is far less than that of the supporting membrane in NF membrane, so the incorporation of MWCNT during the IP process should be performed under precise control. However, no detailed experiments have been conducted to investigate how the functional groups on MWCNTs alter the property of the polyamide layer in the nanocomposite NF membranes. In this study, the influences of functional groups of MWCNTs on the properties of NF membrane were investigated. MWCNTs modified with three different functional groups (−COOH, −NH, −OH) separately along with PIP were added into the aqueous phase to fabricate an ultrathin polyamide layer on the PSF UF membrane by IP. The membrane properties (separation performance, morphology, hydrophilicity, etc.) were measured to evaluate the significance of incorporating modified MWCNT in NF membrane.

Material. The pristine MWCNTs (≥97 wt %) with 10−20 nm in outer diameter were purchased from Shenzhen Nanotech Port Co. Ltd. The support membranes, polysulfone (PSF) UF membrane (the MWCO and PWF are 50 000 Da and 125 L·m−2·h−1·bar−1, respectively), were obtained from the development center for water treatment technology (Hangzhou, China). Trimesoyl chloride (TMC, ≥98 wt %) was supplied by Qingdao Benzo Chemical Company (China). Piperazine (PIP, GR) was provided by Sigma-Aldrich. Nitric acid (HNO3, 65%), sulfuric acid (H2SO4, 98%), n-hexane (AR), thionyl chloride (AR), potassium hydroxide (AR) and N,Ndimethylformamide (AR) were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). Preparation of Modified MWCNTs. Three different functionalized multiwalled carbon nanotubes are carboxylic MWCNT (MWCNT-COOH), hydroxylic MWCNT (MWCNT-OH) and amino MWCNT (MWCNT-NH). Oxidation treatment was performed in order to graft carboxyl groups on the surface of the MWCNT as well as to remove the impurities.12 Briefly, 0.5 g of the pristine MWCNT was soaked in a 200 g solution of HNO3/H2SO4 (1:3 in vol %) and ultrasonically treated for 1 h. The suspension was refluxed at 80 °C in water bath for 12 h, then diluted with DI water and filtered through a 0.22 μm membrane. The desirable MWCNT-COOH was rinsed with DI water at least 3 times to reach to neutral pH and dried in a vacuum oven overnight at 40 °C. The hydroxylation of MWCNT (MWCNT-OH) was accomplished by hydrothermal treatment.24 1 g of pristine MWCNT was dispersed in 50 mL of aqueous potassium hydroxide solution (2.0 mol/L). After ultrasonic treatment for 1 h, the suspension was transferred to the polytetrafluoroethylene liner of a stainless steel reaction autoclave and heated at 180 °C for 2 h. The purification of MWCNT-OH was performed with same method as mentioned above. The amino MWCNT (MWCNT-NH) was synthesized by converting the carboxyl group of the MWCNT-COOH into the acyl chloride group with thionyl chloride (SOCl2) and further reacting with the diamine.25 The process for preparing MWCNT-NH is as follows: 0.5 g of MWCNT-COOH was refluxed with SOCl2 at 60 °C for 12 h. B


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Figure 2. Characterization of pristine MWCNT, MWCNT-COOH, MWCNT-OH and MWCNT-NH: (a) TEM images, (b) FT-IR spectra, (c) TGA curves. After a purification process, the resulting MWCNT was reacted with piperazine in DMF and refluxed at 80 °C for 1 day to obtain the final product. Preparation of Nanocomposite NF Membranes. The fabrication of different nanocomposite NF membranes via the IP process is presented in Figure 1. Certain amounts of different functionalized MWCNTs (MWCNT-COOH, MWCNT-OH and MWCNT-NH) were dispersed in PIP aqueous solution (1%, w/v 100 mL), respectively. After ultrasonic dispersion of MWCNT for 2 h, the aqueous phase was cast on the PSF UF membrane surface for 3 min. Subsequently, excess solution was removed and residual droplets were blown away by compressed air. Afterward, the membrane was exposed to the organic TMC solution (0.15%, w/v) for 15 s to generate an ultrathin polyamide at the water−hexane interface. After the residual hexane was removed, the composite membrane was placed in the oven at 80 °C, heat treated for 5 min. Finally, the resulting membrane was rinsed and kept in DI water before tests. According to the functional groups on modified MWCNTs, corresponding membranes were denoted as NFMWCNT-COOH, NFMWCNTOH, NFMWCNT-NH (MWCNT content of 0.01% w/v), respectively. And the membrane without MWCNT was NFMWCNT-0. Characterization. Morphologies of pristine and modified MWCNTs were directly observed by transmission electron microscopy (TEM, JEM-2100, Japan). Chemical compositions were characterized by Fourier transform infrared (FT-IR) spectroscopy. The thermostability of pristine and three modified MWCNTs was analyzed by the thermogravimetric analysis (TGA, STA 499 F3) over a temperature range from 25 to 800 °C at a heating rate of 10 °C min−1 in N2 atmosphere. The chemical compositions of top surfaces of the nanocomposite NF membranes were probed by an X-ray photoelectron spectroscopy analysis (XPS, VG-Miclab II, UK). Membrane morphologies (cross sections and top surfaces) were visualized by scanning electron

microscope (SEM, NOVA NANOSEM450) with the amplification factor of 50 000. Atomic force microscopy (AFM, Veeco, Nanoscope IIIa Multimode AFM) with 5 μm × 5 μm scanning range was used to determine the roughness of top surface, in which rms (root-meansquare roughness) and Ra (arithmetic mean roughness) were analyzed by software Nanoscope. Hydrophilicities of the substrate and the nanocomposite membranes expressed in dynamic water contact angle (θ) were evaluated by contact angle meter (JC2000A, Shanghai Zhong Cheng Digital Equipment Co., Ltd., China) at 25 °C. NF Performance Tests. The separating performance of nanocomposite membranes was evaluated by measuring the water permeation and the rejection of salt and neutral solute. The experiment was conducted in a cross-flow NF device as described in our previous work.26 The effective area of the membrane cell was 75 cm2 and the operating pressure was kept constant at 0.6 MPa. All the experiments were performed with triplicates. Pure water flux (J) was evaluated precisely using DI water and calculated based on the following formula: J=

V A×t

Where J is the permeation flux (L/(m2·h)), over a period of time t (h), V is the volume of collected permeate (L) and A is the effective membrane area (m2). The salt rejection as well as pore size and the MWCO of the nanocomposite membranes were characterized by a series of salt solution (Na2SO4, MgSO4 and NaCl, 2000 ppm) and neutral organic solution (gluceso, sucrose, raffinose, 300 ppm), respectively. The solute rejection was determined by the following formula:

⎛ Cp ⎞ R (%) = ⎜1 − ⎟ × 100 Cf ⎠ ⎝ C


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Figure 3. XPS wide-scan and C 1s core level of (a) NFMWCNT-0, (b) NFMWCNT-COOH, (c) NFMWCNT-OH, (d) NFMWCNT-NH. The modified MWCNTs content in the nanocomposite membrane was 0.01% (w/v). Where Cf and Cp are the concentrations of different solute in the feed and permeate, separately. The concentrations of salt were measured by DDS-11A conductance meter (Shanghai Neici Instrument Company, China) while the concentrations of neutral organic were measured by a TOC analyzer (Shimadzu, Model TOCVPN Japan).

than others because it was the product based on further reaction of MWCNT-COOH. Moreover, the modified MWCNTs appear to have hollow cylindrical nanostructures with relatively rough surfaces. The chemical structures of the modified MWCNTs were characterized by FT-IR and their infrared spectra were compared to that of the pristine MWCNT in Figure 2b. The pristine MWCNT possessed weak sp2 CH (2920 cm−1) and sp3 CH (2851 cm−1) stretching vibrations as well as some insignificant bands at lower wavelength. The CH binding was derived from the defects on the ends and sidewalls of pristine MWCNTs, enabling further reaction sites for the graft of other functional groups. The representative stretching bands

3. RESULTS AND DISCUSSION Characterizations of Modified MWCNTs. The morphologies of three modified MWCNTs (MWCNT-COOH, MWCNT-OH, MWCNT-NH) are illustrated in Figure 2a. The lengths of these modified MWCNTs were less than 0.5 μm, which were effectively reduced by the ultrasonic process and the modification. The length of MWCNT-NH is shorter D


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ACS Applied Materials & Interfaces of OH (3427 cm−1), CO (1123 cm−1) in MWCNT-OH spectrum demonstrated the successful attachment of −OH groups. Different from MWCNT-OH, the spectrum of MWCNT-COOH contained the OH vibration in CC OH form at 1384 cm−1 as well as two strong peaks representing the stretching vibrations of CO at 1719 and 1578 cm−1, which proved the existence of −COOH group in MWCNTCOOH. In the spectrum of MWCNT-NH, the grafted amide bond was verified by the peak of CN (1227 cm−1), CO (1636 cm−1) and the terminal amino group was confirmed by the NH vibration at 3150 cm−1.27−29 The thermogravimetric analysis curves of pristine and three modified MWCNTs with carboxyl, hydroxyl and amino groups under a nitrogen atmosphere are illustrated in Figure 2c. All the TGA curves had slight weight losses within 100 °C attributed to the desorption of atmospheric moisture. The pristine MWCNT possessed the best thermal stability with an only 1.5% weight reduction within 550−800 °C. The initial decomposition of MWCNT-COOH and MWCNT-OH were observed at about 430 and 350 °C and their mass losses were 8% and 7%, respectively, due to the pyrolysis of surface functional groups. On the other hand, thermal degradation of MWCNT-NH was perceived as a two-step process. The first stage is ascribed to the decomposition of terminal PIP (weight loss of 12%) between 170 and 450 °C, whereas the second one is attributed to the pyrolysis of acyl groups (weight loss of 4%). Chemical Composition of Top Surface of NF Nanocomposite Membrane. The chemical compositions of the top surface of single polypiperazine-amide and nanocomposite membranes are displayed in Figure 3. From the wide-scan spectra, all the membranes had three peaks at binding energy of 284.6, 399.5 and 532.5 eV, representing the C 1s, N 1s and O 1s regions, which were in agreement with the literature.30 The corresponding atom content listed in Table 1 revealed that the

area signifies the content of amido bond of the membranes,31,32 which is the determinant of the cross-linking degree of PA layer. A small peak at 289.2 eV was also observed in the spectra of NFMWCNT-COOH (see Figure 3b) and NFMWCNT-OH (see Figure 3c), which represents the existence of −COO group. As for NFMWCNT-COOH, the peak at 289.2 eV could be explained by the unreacted carboxyl group on MWCNTCOOH. However, the one in NFMWCNT-OH might be ascribed to the ester bond forming between MWCNT-OH and TMC. In addition, NFMWCNT-OH possesses the smallest peak at 288.2 eV among all the nanocomposite membranes, indicating that the formation of ester bond can interfere with the cross-linking degree of PA chain during the IP process.33 Morphology of NF Nanocomposite Membrane. The morphologies of the NF membranes are observed by SEM and AFM. From the SEM images of surface and cross section (Figure 4), the nanocomposite membranes exhibited nodular structures compared with NFMWCNT-0. The steric effect of functional groups on MWCNTs that possessed a slower reaction rate with the organic monomer than that of the PIP resulted in the difference in surface morphologies.14 The

Table 1. Elemental Composition of Different Polyamide Membranes Analyzed by XPS atom percent (%) sample

C 1s

N 1s

O 1s

NFMWCNT-0 NFMWCNT-COOH NFMWCNT-OH NFMWCNT-NH

70.17 71.52 71.53 71.52

14.04 13.45 13.82 15.72

15.78 15.03 14.65 12.76

proportion of carbon was slightly higher than that of the single polypiperazine-amide membrane, after doping functionalized MWCNTs into the NF membranes. The element composition of NFMWCNT-NH possessed the highest nitrogen contents and the lowest oxygen contents among all the nanocomposite membranes The C1 score-level spectrum indicated the possible interaction between the functional groups of modified MWCNTs and the PA layer. The C1 score-level spectrum of the membranes contained three main peaks at 284.6, 286.2 and 288.2 eV. According to the electron-withdrawing ability of the connected elements, the peak at 284.6 eV represents the carbon−hydrogen bond (CH) and carbon−carbon bond (CC). The peak at 286.2 eV is attributed to unreacted CN species from the aqueous monomer PIP, and it may also consist of CN species from MWCNT-NH or CO species from MWCNT-OH. Meanwhile, the peak at 288.2 eV is ascribed to CO species from formed amido bond and the simulated peak

Figure 4. SEM images of (a) top surfaces and (b) cross sections of NF membranes. The MWCNTs content in the NF membranes was 0.01% (w/v). E


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Figure 5. AFM images of (a) PSF supporting membrane, (b) NFMWCNT-0, (c) NFMWCNT-COOH, (d) NFMWCNT-OH, (e) NFMWCNTNH. The MWCNT loading in the tested membrane was 0.01% (w/v).

membrane with MWCNT-OH has the smallest nodular structures among all the nanocomposite membranes. As proved by the XPS results, during the IP process of the monomers in two phases, both the amino group on PIP and the hydroxyl group on NFMWCNT-OH condensation polymerized with the acyl chloride group on TMC, leading to the generation of PA chain and ester bond. Moreover, the reaction of hydroxyl group was much slower than that of the amino group, so the smaller nodular structure of NFMWCNT-OH was caused by synergistic effect of formed ester bond and amide bond. After functionalized MWCNTs were doped, the membrane thicknesses decreased significantly (NFMWCNT-0 105 nm, NFMWCNT-COOH 84 nm, NFMWCNT-OH 77 nm and NFMWCNT-NH 71 nm) and the one with MWCNT-NH had the thinnest PA layer of 71 nm. The AFM images and the roughness parameters (Ra, rms) of PSf supporting membrane, single polypiperazine-amide membrane and nanocomposite membranes with an area of 5 μm × 5 μm are shown in Figure 5 and Table 2. It could be observed

Table 2. Roughness of the NF Membrane membrane code

Ra (nm)

rms (nm)

PSF support membrane NFMWCNT-0 NFMWCNT-COOH NFMWCNT-OH NFMWCNT-NH

4.39 38.9 17.8 15.4 11.7

5.57 29.1 12.4 11.6 8.95

that roughness followed the order: NFMWCNT-0 > NFMWCNT-COOH > NFMWCNT-OH > NFMWCNTNH. The change in roughness was attributed to the diffusion disturbances of PIP in the presence of MWCNT. For the fabrication of NFMWCNT-0, PIP with small molecular weight can easily permeate through the interface of aqueous and organic phase, causing a thicker and rougher top surface of the PA layer. As for the nanocomposite membranes, PIP faced greater resistance to diffuse across the interface of MWCNT so F


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most hydrophilic carboxyl group had the strongest water affinity. Effect of MWCNT Content on the NF Nanocomposite Membrane Performance. The pure water flux and rejection of NF membranes with three kinds of MWCNTs in different contents are presented in Figure 7. In general, the incorporation of modified MWCNT significantly improved pure water flux (PWF) whereas the Na2SO4 rejections were slightly decreased but still stayed above 90%. The increase in PWF of nanocomposite membranes is ascribed to strong hydrophilicity improved by modified MWCNTs. Besides, the accessorial water channels created by the tubular structure of MWCNT and the interspace between PA chain and MWCNT further promote the water flux of the membranes. All the nanocomposite membranes had the optimal PWF and rejection at the MWCNT loading of 0.01% (w/v). When the MWCNT content increased from 0 to 0.01% (w/v), the pure water flux increased significantly with slight decline in Na2SO4 rejection. When excessive MWCNT was added into the aqueous phase, the microvoids between MWCNT and the polymer matrix in the skin layer increased due to their limited compatibility, leading to potential defects. For this reason, the improvement in pure water flux was not obvious whereas the rejection decreased greatly when the MWCNT content was over 0.01% (w/v). Among three types of nanocomposite membranes, NFMWCNT-COOH had the lowest rejection due to the low interaction of carboxyl group with the PA chain. Because the carboxyl group is the hydrolysis product of the acyl chloride group, MWCNT-COOH cannot react with TMC during the IP process. The interaction of the carboxyl group with the polymer matrix mostly relies on hydrogen bond and van der Waals force, which are lower than the chemical bond formed by MWCNTOH and MWCNT-NH with TMC. As a result, NFMWCNTNH kept a high rejection above 94.02% which could be explained by the tight adhesion of terminal amino group on MWCNT-NH to the PA chain. However, the variation in performance of NFMWCNT-OH with MWCNT loading was different from those of NFMWCNT-COOH and NFMWCNT-NH. As proved by XPS results, the ester linkage was formed by the existence of MWCNT-OH, whereas the

the formed active layer decline in the surface roughness and thickness. Hydrophilicity of NF Nanocomposite Membrane. Hydrophilicity, expressed by dynamic contact angles (θ), is generally correlated with the membrane performance and the antifouling capability. The dynamic contact angles of PSf supporting membrane and nanocomposite membranes illustrated in Figure 6 followed the order: NFMWCNT-COOH

Figure 6. Dynamic contact angles of the PSF supporting membrane and the nanocomposite membranes. The MWCNTs content in the tested membranes was 0.01% (w/v).

(12.5°) < NFMWCNT-OH (14.5°) < NFMWCNT-NH (16°) < NFMWCNT-0 (21°) < PSf (89.0°). It was shown that the doping of MWCNT into the PA active layer could effectively improve the hydrophilicity of membranes. Compared with NFMWCNT-0, nanocomposite membranes benefited from the hydrophilic groups of MWCNT which enhance the affinity of water with membrane surface. Furthermore, the hydrophilicities of the nanocomposite membranes are in accordance with those of the functional groups grafted on the MWCNTs, namely the

Figure 7. Effect of MWCNT loading on the performance of NF membranes (0.6 MPa, 20 ± 0.5 °C, 2000 ppm of Na2SO4). G


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ACS Applied Materials & Interfaces Table 3. Rejections of Organic Neutral Solutes of the Nanocomposite Membrane solute rejection (%) solute

molecular weight (Da)

Stokes radius (nm)

NFMWCNT-COOH

NFMWCNT-OH

NFMWCNT-NH

glucose sucrose raffinose

180 342 504

0.359 0.462 0.538

66.9 ± 1.7 90.7 ± 1.1 93.5 ± 1.6

70.2 ± 0.9 93.4 ± 1.7 95.7 ± 1.4

68.7 ± 1.1 92.8 ± 0.8 95.1 ± 1.3

Figure 8. Permeate flux and rejections of inorganic salts of the nanocomposite membranes.

Table 4. Various Modified MWCNTs for the Fabrication of Nanocomposite NF Membranes and Their Separation Properties salt rejection (%) −2

−1

−1

membrane material of active layer

PWF (L·m ·h ·bar )

Na2SO4

MgSO4

NaCl

MWCNT-COOH/polyamide (PIP/TMC) MWCNT-OH/polyamide (PIP/TMC) MWCNT-NH/polyamide (PIP/TMC) PMMA-MWCNT/polyamide (PIP/TMC) acid-MWCNTs/polyamide (PEI/IPD) PDA-MWCNTs/polyamide (PEI/TMC) MWCNT/PDMS rGO/acid-MWCNT

6.2 6.9 5.3 7.0 4.1 15.3 6.0 11.3

96.6 97.6 96.8 99.0

93.7 97.1 95.0 96.0

34.0 35.3 35.1 44.1

2000 2000 2000 2000

45.2 81.0 83.5

76.1 63.0 44.2

33.8 22.0 51.4

1000 ppm 200 ppm 0.01 mol/L

reaction rate was slower than that of the amide bond formed by PIP with TMC. This competition for the organic monomer caused a synergistic effect and interfered with the cross-linking degree of PA chain.34 When MWCNT-OH content was lower than 0.01% (w/v), the hydroxyl groups makes the PA layer more compact and leads to higher rejection. NFMWCNT-OH has the highest rejection of 97.55% and a relative high flux of 41.44 L·m−2·h−1·bar−1 at the MWCNT loading of 0.01% (w/v). However, excessive MWCNT-OH disturbed the reaction of PIP with TMC. Because of the low reaction rate of the hydroxyl group on MWCNT-OH, the polymerization degree was lower than that of the single polypiperazine-amide chain within the same reaction time. As a result, the synergistic effect had a negative effect on the rejection of the NFMWCNT-OH whereas it improved the pure water flux when MWCNT-OH content continued to increase. Separation Property of NF Nanocomposite Membrane. Three electrically neutral organics (raffinose, sucrose and glucose) with a solution concentration of 300 ppm were used to test the pore size of NF membranes under 0.6 MPa. The organic solute rejection rate of the nanocomposite membranes was tabulated in Table 3. Because of the electric neutrality, the rejection of organic solute depends largely on the sieve effect of nanosized pores on the separating layer. All the

salt concentration ppm ppm ppm ppm

operation pressure (bar)

ref.

6 6 6 10 3.45 6 4 5

this work this work this work 15 36 20 37 38

nanocomposite membranes are capable of excluding over 90.7% sucrose, corresponding to a pore size within 300 Da, among which NFMWCNT-OH has the relatively smallest pore size whereas NFMWCNT-COOH has the biggest one. Three inorganic salts (NaCl, MgSO4, Na2SO4) with a solution concentration of 2000 ppm were used to evaluate the salt separation properties of three nanocomposite membranes. The comparison in flux and different salt rejections of NFMWCNT-COOH, NFMWCNT-OH and NFMWCNTNH was illustrated in Figure 8. Similar to the separation properties tested by pure water, the permeate flux of the same salt solution followed the order: NFMWCNT-OH > N F M W C N T - C O O H > NF M W C N T - N H , w h e r e as NFMWCNT-OH had the highest salt rejection and NFMWCNT-COOH had the lowest one. In addition, the performance of NFMWCNT-OH is expected to be better at higher pH because the ionization of hydroxyl group occurs at high pH.35 These different rejection rates of the same salt can be attributed to their corresponding pore sizes proved in the separation of organic solutes. Because of the nanoscale aperture, the sieve effect plays the dominant role in holding back of SO42−, thus the rejection of NaCl was far less than that of the Na2SO4 and MgSO4. On the basis of the negative charge of PA separating layer, when it comes to different separation H


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Figure 9. Long-term test of the nanocomposite membranes during 10 days (0.6 MPa, 20 ± 0.5 °C, 2000 ppm of Na2SO4).

properties of Na2SO4 and MgSO4, the divalent ion Mg2+ has more negative influence on the anionic electric field and subdued Donnan effect, leading to lower rejection rate of MgSO4 compared to Na2SO4. The nanocomposite NF membranes fabricated in this work are compared with several NF membranes with different modified MWCNTs in the available literature (Table 4). The performance of the three nanocomposite membranes possessed superior rejections with moderate water fluxes. Stability of NF Nanocomposite Membrane in LongTime Running. In the practical industrial application of the membranes, the operational stability is an essential parameter for the nanocomposite membranes. In Figure 9, all the nanocomposite membranes showed good stability, the permeate flux and salt rejection underwent slight variation in the 10-day duration. NFMWCNT-NH had the least significant fluctuation in rejection and flux, which could further support the phenomenon that MWCNT-NH had the tightest adhesion to the PA matrix among all the functionalized MWCNTs.

ACKNOWLEDGMENTS

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REFERENCES

The authors are thankful for the financial support received from the National Science and Technology Support Project of China (2014BAB07B01 and 2015BAB09B01), Project of National Energy Administration of China (2011-1635 and 2013-117) and the Key Technology R&D Program of Jiangsu Committee of Science and Technology in China (BE2013031).

(1) Mohammad, A. W.; Teow, Y. H.; Ang, W. L.; Chung, Y. T.; Oatley-Radcliffe, D. L.; Hilal, N. Nanofiltration Membranes Review: Recent Advances and Future Prospects. Desalination 2015, 356, 226− 254. (2) Lau, W. J.; Gray, S.; Matsuura, T.; Emadzadeh, D.; Chen, J. P.; Ismail, A. F. A Review on Polyamide Thin Film Nanocomposite (TFN) Membranes: History, Applications, Challenges and Approaches. Water Res. 2015, 80, 306−324. (3) Tang, Y.-J.; Wang, L.-J.; Xu, Z.-L.; Wei, Y.-M.; Yang, H. Novel High-flux Thin Film Composite Nanofiltration Membranes Fabricated by the NaClO Pre-oxidation of the Mixed Diamine Monomers of PIP and BHTTM in the Aqueous Phase Solution. J. Membr. Sci. 2016, 502, 106−115. (4) Hu, D.; Xu, Z.-L.; Chen, C. Polypiperazine-amide Nanofiltration Membrane Containing Silica Nanoparticles Prepared by Interfacial Polymerization. Desalination 2012, 301, 75−81. (5) Soroko, I.; Livingston, A. Impact of TiO2 Nanoparticles on Morphology and Performance of Crosslinked Polyimide Organic Solvent Nanofiltration (OSN) Membranes. J. Membr. Sci. 2009, 343 (1−2), 189−198. (6) Irigoyen, J.; Laakso, T.; Politakos, N.; Dahne, L.; Pihlajamäki, A.; Mänttäri, M.; Moya, S. E. Design and Performance Evaluation of Hybrid Nanofiltration Membranes Based on Multiwalled Carbon Nanotubes and Polyelectrolyte Multilayers for Larger Ion Rejection and Separation. Macromol. Chem. Phys. 2016, 217 (6), 804−811. (7) Choi, W.; Choi, J.; Bang, J.; Lee, J. H. Layer-by-layer Assembly of Graphene Gxide Nanosheets on Polyamide Membranes for Durable Reverse-osmosis Applications. ACS Appl. Mater. Interfaces 2013, 5 (23), 12510−12519. (8) Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C. Carbon Nanotube−polymer Composites: Chemistry, Processing, Mechanical and Electrical Properties. Prog. Polym. Sci. 2010, 35 (3), 357−401. (9) Avilés-Barreto, S. L.; Suleiman, D. Effect of Single-walled Carbon Nanotubes on the Transport Properties of Sulfonated Poly(styrene− isobutylene−styrene) Membranes. J. Membr. Sci. 2015, 474, 92−102. (10) Sahoo, N. G.; Rana, S.; Cho, J. W.; Li, L.; Chan, S. H. Polymer Nanocomposites Based on Functionalized Carbon Nanotubes. Prog. Polym. Sci. 2010, 35 (7), 837−867. (11) Rahmat, M.; Hubert, P. Carbon Nanotube−polymer Interactions in Nanocomposites: A Review. Compos. Sci. Technol. 2011, 72 (1), 72−84. (12) Kim, S. W.; Kim, T.; Kim, Y. S.; Choi, H. S.; Lim, H. J.; Yang, S. J.; Park, C. R. Surface Modifications for the Effective Dispersion of

4. CONCLUSIONS In summary, three functionalized MWCNTs with carboxyl, hydroxyl and amino groups respectively, were prepared and incorporated into the separating layers of NF membranes via the IP process. The influences of MWCNT functional groups on the membrane properties (separation performance, morphology, hydrophilicity and so on) were investigated. The XPS result revealed that the ester group formed between the hydroxyl group on MWCNT-OH and the acyl chloride group on TMC influenced the cross-linking degree of PA layer. MWCNT with the concentration of 0.01% (w/v) results in the optimal PWF and rejection of Na2SO4 of all the nanocomposite membranes, among which NFMWCNT-OH possessed the highest PWF of 41.44 L·m−2·h−1·bar−1 and the Na2SO4 rejection of 97.6% due to the synergistic effect of MWCNTOH and amino group in PIP. Meanwhile, NFMWCNT-NH showed higher rejecting capability and stability than NFMWCNT-COOH on the basis of different adhesion strength of amino and carboxyl group with the PA matrix. Collectively, functionalized MWCNTs with little addition were well-suited for the modification of NF membranes.

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AUTHOR INFORMATION

Corresponding Author

*Z.-L. Xu. Email: [email protected]. Tel: 86-2164253670. Notes

The authors declare no competing financial interest. I


Research Article

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