High-Flux Positively Charged Nanocomposite ... - ACS Publications

Research Article www.acsami.org

High-Flux Positively Charged Nanocomposite Nanofiltration Membranes Filled with Poly(dopamine) Modified Multiwall Carbon Nanotubes Feng-Yang Zhao,† Yan-Li Ji,† Xiao-Dan Weng,‡ Yi-Fang Mi,† Chun-Chun Ye,† Quan-Fu An,*,† and Cong-Jie Gao‡,§ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China. ‡ College of Chemical Engineering and Bioengineering, Zhejiang University, Hangzhou 310027, China § The Development Center of Water Treatment Technology, Hangzhou 310012, China S Supporting Information *

ABSTRACT: The poor dispensability of pristine carbon nanotubes in water impedes their implications in thin-film nanocomposite membranes for crucial utilities such as water purification. In this work, high-flux positively charged nanocomposite nanofiltration membranes were exploited by uniformly embedding poly(dopamine) modified multiwall carbon nanotubes (PDA-MWCNTs) in polyamide thin-film composite membranes. With poly(dopamine) modification, fine dispersion of MWCNTs in polyethyleneimine (PEI) aqueous solutions was achieved, which was interracially polymerized with trimesoyl chloride (TMC) n-hexane solutions to prepare nanocomposite membranes. The compatibility and interactions between modified MWCNTs and polyamide matrix were enhanced, attributed to the poly(dopamine) coatings on MWCNT surfaces, leading to significantly improved water permeability. At optimized conditions, pure water permeability of the PEI/PDAMWCNTs/TMC nanofiltration membrane (M-4) was 15.32 L m−2 h−1 bar−1, which was ∼1.6 times increased compared with that of pristine PEI/TMC membranes. Salt rejection of M-4 to different multivalent cations decreased in the sequence ZnCl2 (93.0%) > MgCl2 (91.5%) > CuCl2 (90.5%) ≈ CaCl2, which is well-suited for water softening and heavy metal ion removal. KEYWORDS: poly(dopamine), multiwall carbon nanotubes, polyethyleneimine, nanocomposite, nanofiltration membrane

1. INTRODUCTION

Carbon nanotubes (CNTs) have been widely used in fabricating polymer matrix nanocomposite NF membrane,10−13 whereas the above-mentioned problems exist as a result of the Van der Waals forces between hydrophobic pristine CNTs.14,15 Great efforts have been devoted to these problems, either through chemical modifications (strong covalent interactions) or physical modifications (weak noncovalent interactions) of CNTs.16−22 Despite the improved water permeability of polymer−matrix nanocomposite NF membranes by incorporating acid-treated CNTs or its derivatives, the interactions between CNTs and polymer matrices were only weak interactions mediated by electrostatic and/or hydrophobic interactions, etc.23 The weak noncovalent interactions between CNTs and polymer matrix will create nonselective defects or large gaps that decrease salt rejection24 and reduce performance stability.13,25 Thus, it remains elusive to simultaneously address the two challenges for thin-film composite NF membranes.

Polymer−matrix nanocomposite membranes are emerging materials for advanced applications such as water purification, desalination, and gas separation.1−3 The incorporation of nanomaterials (nanoparticles, nanotubes, and nanosheets, etc.) in polymer matrix may construct interfacial regions between embedded nanomaterials and polymer matrix, which fosters low-resistance channels for enhancing water permeability.4,5 In the context of nanofiltration (NF), considerable interest is being directed to this concept for engineering membranes with high permeability and salt rejection.6 Two challenges should be addressed to fabricate polymer−matrix nanocomposite NF membranes. Agglomeration is the first and foremost problem preventing nanomaterials from being uniformly dispersed in polymer matrices, leading nonselective defects or large gaps that reduce membrane selectivity.7 Compatibility and interactions of nanomaterials with polymer matrices are other problems which determine both the stability of nanomaterials inside the polymer matrix and membrane performances.8,9 © 2016 American Chemical Society

Received: January 12, 2016 Accepted: February 22, 2016 Published: February 22, 2016 6693

DOI: 10.1021/acsami.6b00394 ACS Appl. Mater. Interfaces 2016, 8, 6693−6700

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Preparation of PEI/PDA-MWCNTs/TMC NF Membranes

sulfate (MgSO4), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), calcium chloride (CaCl2), copper chloride (CuCl2), zinc chloride (ZnCl2), sodium hydroxide (NaOH), and hydrochloric acid (HCl, 33.6−38.6 wt.%) were all analytical grade, obtained from Sinopharm Chemical Reagent Co., Ltd., and used as received. Deionized (DI) water (resistance of 18 MΩ cm) was used in all experiments. Polysulfone ultrafiltration (PSF-UF) membranes (MWCO ∼ 35 000 Da) were provided by the Development Center of Water Treatment Technology, Hangzhou, China. 2.2. Preparation of PDA-MWCNTs. PDA-MWCNTs were synthesized according to the previous report.33 A typical synthesis process is described as follows: MWCNT powder (100 mg) was added into Tris-HCl buffer solution (10 mM, pH 8.5, 100 mL), and dispersed by 5 min sonication (KQ100DB, 100 W, Kunshan Ultrasonic Instruments). Subsequently, DOA (50 mg) was added, followed by sonication for another 5 min in an ice bath, and stirred at 25 °C for 24 h. Afterward, the mixture was centrifuged (10 min, 4000 rpm) to eliminate bundled residues. Finally, PDA-MWCNTs powder was collected by filtration supernatant (washed with DI water and 0.01 M HCl), and dried at 40 °C under vacuum for 24 h. 2.3. Preparation of NF Membranes. The preparation of PEI/ TMC or PEI/PDA-MWCNTs/TMC NF membranes was shown in Scheme 1. A PSF-UF supporting membrane was immersed into PEI or PEI/PDA-MWCNTs aqueous solution (pH 11.5) for 120 s. Subsequently, residual PEI or PEI/PDA-MWCNTs solution was removed from the membrane surface. Afterward, a hexane solution containing TMC was poured on the PEI or PEI/PDA-MWCNTs saturated surface of PSF-UF membranes for 60 s, during which polymerization reaction between PEI and TMC occurred at the water−hexane interface. The excess hexane solution was decanted after the interfacial polymerization, and membranes were cured at 65 °C for 15 min to facilitate further polymerization. Finally, the resulting membranes were washed and stored in DI water before tests. According to the mass ratio of PDA-MWCNTs to PEI (XPDA‑MWCNTs/PEI) in aqueous solution, corresponding membranes were denoted as M-0, M-1, M-2, M-3, M-4, M-5, and M-6, respectively (Table 1). 2.4. Characterizations. Morphologies of MWCNTs and PDAMWCNTs were visualized by transmission electron microscopy

New methods are demanded to improve both the dispersion of CNTs and its interactions with polymer matrices. Dopamine (DOA) is a mussel-inspired catechol-amine, which normally self-polymerizes under alkaline conditions to form poly(dopamine) (PDA) that can strongly adhere on virtually any type of solid surface, rendering it an effective method of CNT modification.26−28 Moreover, PDA can undergo versatile reactions with molecules presenting thiol or amine groups, hence allowing for stronger interactions with polyamide polymer matrices.29 In this work, poly(dopamine) was exploited to improve the dispersion of CNTs in water, and to enhance its interfaces with polyamide matrices. Positively charged nanocomposite NF membranes were prepared by uniformly embedding PDA modified multiwall carbon nanotubes (MWCNTs) in polyamide thin-film composite membranes via interfacial polymerization between polyethyleneimine (PEI) and trimesoyl chloride (TMC). PEI, an amino-containing polymer, can react with PDA through Michael addition and/or Schiff-base formation, when pH is above 8.5 (Figure S1).30−32 Thus, the PDA modified MWCNTs (PDA-MWCNTs) not only circumvent the agglomeration of pristine MWCNTs,33,34 but also enhances compatibility and interactions of MWCNTs with polyamide matrices by strong covalent interactions between PEI and PDA-MWCNTs simultaneously.35,36 With high permeability and performance stability, as-prepared positively charged NF membranes are uniquely positioned for water softening and heavy metal removal.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,3,5-Benzenetricarboxylic chloride (TMC, TCI, 98%), poly(ethyleneimine) (PEI, Mw = 10 000 Da, Sigma-Aldrich, 99%), MWCNTs (outer diameter = 20−40 nm, length < 2 μm, Shenzhen Nanotech Port, 95%), DOA (Sigma-Aldrich, 98%), and tris(hydroxymethyl)aminomethane (Tris, Aladdin, 99.9%) were all used as received. n-Hexane, sodium chloride (NaCl), magnesium 6694


Research Article

ACS Applied Materials & Interfaces

Paar electrokinetic analyzer (GmbH, Austria) with 0.001 M KCl aqueous solution (25 °C and pH 3.0−10.0), and repeated 3 times. 2.5. NF Performance Tests. NF membrane performances were conducted in cross-flow flat apparatus with high flow velocity (0.12 m s−1) and recirculation rate (33 L h−1) (Figure S2). The effective membrane area was 22.4 cm2. The membranes were prefiltrated at 7 bar for 1 h to reach a steady state before test. NF tests were all performed at pH = 6 ± 0.3, and repeated 3 times. The pure water flux was measured at different pressure (2−6 bar) at 25 °C. The water flux (J) and salt rejection (R) were calculated on the basis of the following equations: J = V/(At), R = 1 − Cp/Cf, where V is the total volume of the penetrate flow (L), A is the membrane area (m2), t is the filtration time (h), and Cp (g L−1) and Cf (g L−1) are the salt concentration of permeation and feed solutions, respectively.37 The concentrations of salt solutions (NaCl, Na2SO4, MgSO4, MgCl2, CuCl2, CaCl2, and ZnCl2) were determined through a conductivity meter (FE30, MettlerToledo, Switzerland).38 The pure water permeability (PWP, L m−2 h−1 bar−1) was calculated by the equation PWP = J/Δp, which substantially represents the fitting line slope of the pure water flux (J, L) versus trans-membrane pressure difference (Δp, bar).39 The intrinsic membrane hydraulic resistance Rm (m−1) was calculated on the basis of the following equation: Rm = 1/(PWP × η), where η is the water dynamic viscosity (25 °C, 0.8937 × 10−3 Pa s).40 The interfacial free energy (−ΔGSL) was calculated on the basis of the modified form of the Young−Dupre equation: −ΔGSL = γL × (1 + SAD + cos θ)/(1 + SAD), where θ is the static WCA, SAD is the surface area difference (obtained from AFM images), and γL is the pure water surface tension (25 °C, 72.8 m J m−2).41

Table 1. Composition of Different PEI/PDA-MWCNTs/ TMC NF Membranes. membrane

PEI (wt %)

TMC (wt %)

XPDA‑MWCNTs/PEI

M-0 M-1 M-2 M-3 M-4 M-5 M-6

0.7 0.7 0.7 0.7 0.7 0.7 0.7

0.025 0.025 0.025 0.025 0.025 0.025 0.025

0 0.005 0.01 0.025 0.05 0.075 0.10

(TEM) (HT-7700, Hitachi, Japan) with an accelerating voltage of 120 kV, and field emission scanning electron microscope (FESEM) (Hitachi S-4800, Japan). Chemical structure was characterized by Fourier transform infrared spectroscopy (Vector-22 FT-IR spectrometer) (Bruker Company, Germany). Thermogravimetric analysis (TGA) (Perkin-Elmer Pyris 1 TGA system) was used to characterize thermal stability over a temperature range 50−800 °C (heating rate 10 °C min−1 and in N2 atmosphere). Membrane samples were dried thoroughly in a vacuum oven (25 °C, 24 h) prior to characterization. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) examinations were conducted on a Nicolet Nexus 6700 spectrometer equipped (Thermo Fisher Scientific) with ZnSe internal reflection element (angle of incidence of 45°). Morphologies (surface and cross-sectional) of membranes were visualized by SEM, atomic force microscopy (AFM) (SPI-3800N, Seiko Instruments Inc., Japan), and TEM (JEM-1230, JEOL, Japan) with an accelerating voltage of 120 kV. SEM samples were sputter-coated with a thin layer of gold before test. Membrane resin embedded semithin sections were prepared for TEM samples. AFM was also used to analyze membrane surface roughness with silicon tips (NSG10, NT-MDT, ca. 330 kHz) at atmospheric conditions in tapping mode. Water contact angle (WCA) meter (OCA-20) (Data Physics Instruments GmbH, Germany) was used to measure the surface hydrophilicity of membranes at ambient temperature (25 °C). The zeta potential of NF membranes was evaluated from a streaming potential method using SurPASS Anton

3. RESULTS AND DISCUSSION 3.1. Characterization of PDA-MWCNTs. FT-IR, TGA, TEM, and SEM were performed to characterize the PDA modified MWCNTs. FT-IR spectrum of PDA-MWCNTs (Figure 1a) shows that there are three adsorption bands at 3420, 1620, and 1508 cm−1, which correspond to the OH, indole, and indoline groups of PDA, respectively.34,42 Moreover, weight loss of MWCNTs, PDA-MWCNTs, and PDA are 1.6%, 16.9%, and 46.1%, respectively, at temperature up to 800

Figure 1. Characterization of PDA-MWCNTs: (a) FTIR spectra, (b) TGA curves, (c) TEM images, (d) SEM images, and (e) dispersion of MWCNTs (1) and PDA-MWCNTs (2), after standing for 1 min (left), 30 min (middle), and 7 days (right). 6695


Research Article


Figure 2. (a) ATR-FTIR spectra of PSF, M-0, and M-4. (b) FTIR spectra of PEI, PDA-MWCNTs, and PEI-PDA-MWCNTs.

1633 cm−1 is attributed to carbonyl (CO) stretching vibration of an amide (CON(H)) group, which indicates that a polyamide selective layer has been successfully prepared on PSF-UF membranes by amidation reaction between TMC and PEI.44 Meanwhile, the adsorption band at 3351 cm−1 suggests the existence of unreacted amine groups of PEI, rendering NF membranes positively charged.45 In the spectrum of M-4, as a result of the introduction of PDA-MWCNTs inside the polyamide selective layer, the adsorption band at 3400 cm−1 was enhanced as compared with M-0 (Figure 2a). In addition, a new peak at around 1660 cm−1 has been seen in the ATR-FTIR spectrum of M-4, indicating that the introduced PDAMWCNTs formed the CN bond with PEI.46 As supporting evidence, an adsorption band at 1660 cm−1 was also seen for PEI modified PDA-MWCNTs (PEI-PDA-MWCNTs, Figure 2b). Cross-sectional morphologies of M-0 and M-4 were investigated by TEM (Figure 3). It was seen that a polyamide layer (∼145 ± 55 nm) was formed by interfacial polymerization (Figure 3a), in which MWCNTs were uniformly, but yet randomly, distributed with minimal agglomeration (Figure 3b). Individual tubes were clearly visualized, which were either perpendicular or parallel to the surface of polyamide membranes. In contrast, the pristine M-0 reveals a net morphology without MWCNTs. Moreover, pristine MWCNTs without PDA modification were incorporated into the polyamide layer in the same method, yielding macroscale agglomeration which was noticeable even by naked eyes (Figures S3 and S4). As such, the fine dispensability of MWCNTs is likely due to the PDA layer on MWCNT surfaces undergoing strong interactions with PEI polymer through Michael addition and/or Schiff-base reactions.35,36 As supporting evidence, it was found that MWCNTs were readily uniformly wrapped by PEI nanolayer (ca. 1.2 nm, Figures S5 and S6). 3.3. Effect of PDA-MWCNT Concentration on PEI/PDAMWCNT/TMC NF Performance. The PEI/TMC NF membranes were investigated, and the optimum conditions for membrane preparation were established (Figures S7−10). Under this condition, the effect of PDA-MWCNTs concentration on NF separation performance was investigated (Figure 4). In general, the incorporation of PDA-MWCNTs improved water flux, despite a threshold at XPDA‑MWCNTs/PEI = 0.05 (M-4) being observed beyond which the water flux begins to decrease (Figure 4). In detail, water flux increases from 31.2 (M-0) to 83.5 L m−2 h−1 (M-4) with increasing XPDA‑MWCNTs/PEI from 0

Figure 3. Cross-section morphologies of (a) M-0 and (b) M-4 observed by TEM.

Figure 4. Effect of PDA-MWCNT content on the separation performance of PEI/PDA-MWCNTs/TMC NF membranes, and the optical images of M-0 and M-4 (1 g L−1 aqueous MgCl2 solution, 25 °C, and 6 bar).

°C (Figure 1b). On the basis of the mass loss, it is calculated that PDA-MWCNTs contain ∼42 wt % of PDA. Through TEM (Figure 1c) and SEM (Figure 1d), it was visualized that PDA nanolayers (ca. 7 nm) were successfully coated on the outer surfaces of MWCNTs. As a result of PDA modification, aqueous dispersion of PDA-MWCNTs was stable in a period of 7 days, in stark contrast to the dispersion of pristine MWCNTs that aggregated in 30 min (Figure 1e). We consider that the improved dispersion of PDA-MWCNTs in water is due to the successful coating of hydrophilicity PDA layer on MWCNTs.43 3.2. Characterization of NF Membranes. With its improved dispersion in water, PDA-MWCNTs were mixed with PEI aqueous solution and interracially polymerized with TMC to prepare NF membranes (Scheme 1), which were investigated by ATR-FTIR (Figure 2a). Absorption band at 6696


Research Article


Figure 5. (a) Pure water flux of M-0 and M-4 (operating pressure 2−6 bar); (b) dynamic WCAs of PSF, M-0, and M-4; and (c) AFM surface morphologies of PSF, M-0, and M-4.

Table 2. PWP, Rm, WCA, RMS, SAD, and −ΔGSL of M-0 and M-4 membrane

PWP (L m−2 h−1 bar−1)

Rm (1013 m−1)

CA (deg)

RMS (nm)

SAD

−ΔGSL (mJ m−2)

M-0 M-4

5.88 15.32

6.8 2.6

78.3 70.8

4.6 20.0

2.4% 6.5%

87.2 95.3

Table 3. Crystal Ionic Radii (rx), Stokes Radii (rs), and Hydrater Radii (rh) of Various Ions ion

crystal ionic radii (rx, Å)

H+ Na+ Ca2+ Cu2+ Mg2+ Zn2+

0.95 0.99 0.72 0.65 0.74

Stokes radii (rs, Å) hydrater radii (rh, Å) (0.28) 1.84 3.10 3.25 3.47 3.49

(2.82) 3.58 4.12 4.19 4.28 4.30

to 0.05. With further increasing XPDA‑MWCNTs/PEI from 0.050 to 0.100, the water flux of PEI/PDA-MWCNTs/TMC NF membrane decreases from 83.5 to 42.0 L m−2 h−1. First, mobility of PEI chains is confined by the incorporated PDAMWCNTs, leading to smaller cross-linking density at the interfacial region between PDA-MWCNTs and the polyamide matrix. It may construct low-resistance pathways for water molecules passing through polyamide selective layer.5 Zhang et al.24 had indicated that well-dispersed MWCNTs might offer fast transport channels for the permeation of water molecules through polyamide membranes. Second, the incorporation of PDA-MWCNTs decreases the polyamide chains packing density and, consequently, enhances its free volume.45 However, excessive PDA-MWCNT (XPDA‑MWCNTs/PEI > 0.05) aggregations, i.e., bundles, occur as a result of intertube interactions.7 In addition, a higher PDA-MWCNT content may take up more space from the polyamide selective layer, which will form a relatively thicker polyamide selective layer compared to the original membrane.19 For these reasons, the intrinsic membrane hydraulic resistance (Rm) will be increased when XPDA‑MWCNTs/PEI > 0.05, resulting in the decrease of water fluxes (Figure 4). In addition to the improved water flux, it is

Figure 6. Schematic water transport paths in (a) PEI/TMC NF membranes and (b) PEI/PDA-MWCNTs/TMC NF membranes.

Figure 7. Water flux and salts rejection of M-4 for separating different salts (1 g L−1 aqueous solution, 25 °C, and 6 bar).

6697


Research Article


Figure 8. (a) Effect of operating pressure on the separation performance of M-0 and M-4 (1 g L−1 aqueous ZnCl2 solution, 25 °C, and 2−6 bar). (b) Effect of feed concentration on the separation performance of M-0 and M-4 (0.5−3.0 g L−1 aqueous ZnCl2 solution, 25 °C, and 6 bar). (c) Effect of operating temperation on the separation performance of M-0 and M-4 (1 g L−1 aqueous ZnCl2 solution, 25−50 °C, and 6 bar). (d) Stability performance of M-0 and M-4 (1 g L−1 aqueous ZnCl2 solution, 25 °C, and 6 bar).

4 is positively charged, and its zeta potential (4.1 ± 1.4 mV) is slightly lower than that of M-0 (6.0 ± 1.5 mV) at pH 6 ± 0.3. According to the electrostatic repulsion principle, Na + (monovalent) has weaker electrostatic repulsive interaction to the same charged membrane surfaces compared with Zn2+, Mg2+, Cu2+, and Ca2+ (multivalent). As such, M-4 shows lower rejection to sodium salts. Moreover, the hydrated cationic radii of Zn2+, Mg2+, Cu2+, and Ca2+ are 4.30, 4.28, 4.19, and 4.12 Å, respectively (Table 3).50 On the basis of the steric hindrance principles, rejection of M-4 to different multivalent cationic decreases in the following order: ZnCl2 (93.0%) > MgCl2 (91.5%) > CuCl2 (90.5%) ≈ CaCl2. Meanwhile, the hydrated radius of SO42− ion (3.79 Å) is larger than Cl− ion (3.32 Å) in size, which results in higher rejection of Na2SO4 (45.2%) than NaCl (33.8%). Additionally, both electrostatic repulsion and steric hindrance principles determine the selectivity of M-4. Thus, rejection of MgSO4 (76.1%) is between that of Na2SO4 and CaCl2. The above results indicate that M-4 suits well for water softening and heavy metal ions removal. 3.4. Effects of Operating Conditions on NF Performance. Heavy metal ions (e.g., Zn2+ and Cu2+, etc.) represent toxic pollutants in wastewater, whereby NF is an emerging method for removing heavy metal ions from water. Here, the effects of operation conditions on the separation of ZnCl2 (a model heavy metal ion) by M-0 and M-4 were investigated in detail (Figure 8). With increasing operating pressure from 2 to 6 bar, the salt water flux of M-0 and M-4 increases linearly, while the rejection to ZnCl2 is stable (>93.0%, Figure 8a).39 In addition, both the salt water flux and salt rejection of M-0 and M-4 decrease with increasing ZnCl2 concentration from 0.5 to 3.0 g L−1 (Figure 8b). By increasing the ZnCl2 concentration, the osmotic pressure of the feed solution is increased, while the salt water flux is decreased as a result of the reduced driving

noteworthy that selectivity retains stable; e.g., the MgCl2 rejection stays stable above 90% (within 3.5% variation) in the whole range of XPDA‑MWCNTs/PEI. This indicates that the PDA-MWCNTs is very compatible with the polyamide matrix with minimum nonselective defects or large gaps (Figure 3b). Furthermore, the PWPs of M-0 and M-4 were calculated (Figure 5).39 The PWP of M-4 is 15.32 L m−2 h−1 bar−1, which is ∼1.6 times higher than M-0 (Table 2). Both the surface hydrophilicity and microstructures of a membrane influence its separation performance; 47 thus, we use −ΔG SL as a comprehensive parameter representing the surface properties of membranes.48 The −ΔGSL of M-4 (95.3 mJ m−2) is only slightly higher than that of M-0 (87.2 mJ m−2) (Table 2), indicating that surface physicochemical properties are not the key factor affecting the PWP of M-4. Therefore, the improved PWP of M-4 is mainly attributed to low-resistance channels at the interfacial region between the PDA-MWCNTs and polyamide matrixes. Herein, the water permeation path in pristine PEI/TMC NF membranes only exists in the continuous polyamide matrix (Figure 6a). By contrast, additional water channels were created in PEI/PDAMWCNTs/TMC NF membranes. That is, water molecules can permeate through the continuous polyamide matrix, and/or the low-resistance channels at the interfacial region between the PDA-MWCNTs and polyamide matrix (Figure 6b). In a comparison with flat-sheet positively charged NF membrane data reported in the literature, M-4 shows higher water flux and an acceptable salt rejection (Figure S11, Table S1). Selectivity of NF membranes is determined by both electrostatic repulsion and steric hindrance principles.49 Figure 7 shows that salt rejection of M-4 for seven different inorganic salts decreases in the sequence ZnCl2 > MgCl2 > CuCl2 ≈ CaCl2 > MgSO4 > Na2SO4 > NaCl. As shown in Figure S12, M6698


Research Article


National Natural Science Foundation of China (No. 21306163), and Zhejiang University K. P. Chao’s High Technology Development Foundation.

force of NF membranes. Meanwhile, the ZnCl2 rejection is decreased due to the augment of the electrostatic shield effect of Cl− ions toward the surface of positively charged membranes. With increasing operating temperature from 25 to 50 °C (Figure 8c), the salt water flux of M-0 and M-4 is significantly increased, despite a slight decrease of salt rejection. This is primarily due to the decreasing viscosity of the feed solution and the higher diffusion coefficient for water.51,52 Furthermore, the performance stability of NF membranes was also tested. The ZnCl2 rejection of M-0 and M-4 shows a slight increase during the 168 h continuous filtration test, maintaining in the ranges 94.5−98.6% and 93.0−97.0%, respectively. Meanwhile, the salt water flux of M-0 and M-4 shows a slight decrease after 168 h continuous operation (Figure 8d). These results suggest that both the M-0 and M-4 have a good NF stability during the long-term operation. As such, high-flux, positively charged nanocomposite NF membranes with long-term stability were exploited by uniformly embedding PDA-MWCNTs in polyamide thin-film composite membranes.

■

(1) Humplik, T.; Lee, J.; O’Hern, S. C.; Fellman, B. A.; Baig, M. A.; Hassan, S. F.; Atieh, M. A.; Rahman, F.; Laoui, T.; Karnik, R.; Wang, E. N. Nanostructured Materials for Water Desalination. Nanotechnology 2011, 22, 292001. (2) Salim, W.; Ho, W. S. W. Recent Developments on Nanostructured Polymer-Based Membranes. Curr. Opin. Chem. Eng. 2015, 8, 76−82. (3) Savage, N.; Diallo, M. S. Nanomaterials and Water Purification: Opportunities and Challenges. J. Nanopart. Res. 2005, 7, 331−342. (4) Wang, X.; Yeh, T. M.; Wang, Z.; Yang, R.; Wang, R.; Ma, H. Y.; Hsiao, B. S.; Chu, B. Nanofiltration Membranes Prepared by Interfacial Polymerization on Thin-Film Nanofibrous Composite Scaffold. Polymer 2014, 55, 1358−1366. (5) Ma, H. Y.; Burger, C.; Hsiao, B. S.; Chu, B. Highly Permeable Polymer Membranes Containing Directed Channels for Water Purification. ACS Macro Lett. 2012, 1, 723−726. (6) Daer, S.; Kharraz, J.; Giwa, A.; Hasan, S. W. Recent Applications of Nanomaterials in Water Desalination: A Critical Review and Future Opportunities. Desalination 2015, 367, 37−48. (7) Li, Y. F.; He, G. W.; Wang, S. F.; Yu, S. N.; Pan, F. S.; Wu, H.; Jiang, Z. Y. Recent Advances in the Fabrication of Advanced Composite Membranes. J. Mater. Chem. A 2013, 1, 10058−10077. (8) Kar, S.; Bindal, R. C.; Tewari, P. K. Carbon Nanotube Membranes for Desalination and Water Purification: Challenges and Opportunities. Nano Today 2012, 7, 385−389. (9) Ji, Y. L.; Zhao, Q.; An, Q. F.; Shao, L. L.; Lee, K. R.; Xu, Z. K.; Gao, C. J. Novel Separation Membranes Based on Zwitterionic Colloid Particles: Tunable Selectivity and Enhanced Antifouling Property. J. Mater. Chem. A 2013, 1, 12213−12220. (10) Das, R.; Ali, M. E.; Hamid, S. B. A.; Ramakrishna, S.; Chowdhury, Z. Z. Carbon Nanotube Membranes for Water Purification: A Bright Future in Water Desalination. Desalination 2014, 336, 97−109. (11) Goh, K.; Setiawan, L.; Wei, L.; Jiang, W. C.; Wang, R.; Chen, Y. Fabrication of Novel Functionalized Multi-Walled Carbon Nanotube Immobilized Hollow Fiber Membranes for Enhanced Performance in Forward Osmosis Process. J. Membr. Sci. 2013, 446, 244−254. (12) Gusev, A. A.; Guseva, O. Rapid Mass Transport in Mixed Matrix Nanotube/Polymer Membranes. Adv. Mater. 2007, 19, 2672−2676. (13) Madaeni, S. S.; Zinadini, S.; Vatanpour, V. Preparation of Superhydrophobic Nanofiltration Membrane by Embedding Multiwalled Carbon Nanotube and Polydimethylsiloxane in Pores of Microfiltration Membrane. Sep. Purif. Technol. 2013, 111, 98−107. (14) Ghosh, S.; Rao, C. N. R. Separation of Metallic and Semiconducting Single-Walled Carbon Nanotubes through Fluorous Chemistry. Nano Res. 2009, 2, 183−191. (15) Lau, W. J.; Gray, S.; Matsuura, T.; Emadzadeh, D.; Paul Chen, J.; Ismail, A. F. A Review on Polyamide Thin Film Nanocomposite (TFN) Membranes: History, Applications, Challenges and Approaches. Water Res. 2015, 80, 306−324. (16) Daraei, P.; Madaeni, S. S.; Ghaemi, N.; Monfared, H. A.; Khadivi, M. A. Fabrication of PES Nanofiltration Membrane by Simultaneous Use of Multi-Walled Carbon Nanotube and Surface Graft Polymerization Method: Comparison of MWCNT and PAA Modified MWCNT. Sep. Purif. Technol. 2013, 104, 32−44. (17) Ghaemi, N.; Madaeni, S. S.; Daraei, P.; Rajabi, H.; Shojaeimehr, T.; Rahimpour, F.; Shirvani, B. PES Mixed Matrix Nanofiltration Membrane Embedded with Polymer Wrapped MWCNT: Fabrication and Performance Optimization in Dye Removal by RSM. J. Hazard. Mater. 2015, 298, 111−121. (18) Wu, H. Q.; Tang, B. B.; Wu, P. Y. Optimization, Characterization and Nanofiltration Properties Test of MWNTs/polyester Thin Film Nanocomposite Membrane. J. Membr. Sci. 2013, 428, 425−433.

4. CONCLUSION In summary, MWCNTs modified with PDA formed a fine dispersion in water, and thus facilitated the preparation of mixed matrix NF membranes with improved separation performances. Positively charged PEI/PDA-MWCNTs/TMC NF membranes were prepared by interfacial polymerization, and characterized by ATR-FTIR, SEM, TEM, AFM, CA, zeta potential, and NF tests, respectively. As-prepared membranes feature stronger compatibility and interactions between PDAMWCNTs and polyamide matrix. Meanwhile, the lowresistance pathways constructed by PDA-MWCNTs inside polyamide matrix led to significantly improved water permeability and performance stability. At the optimized condition (XPDA‑MWCNTs/PEI = 0.05), M-4 possessed a PWP of 15.32 L m−2 h−1 bar−1, and rejection to different salts decreased in the sequence ZnCl2 > MgCl2 > CuCl2 ≈ CaCl2 > MgSO4 > Na2SO4 > NaCl at pH 6 ± 0.3. Moreover, the rejections to ZnCl2, MgCl2, CuCl2, and CaCl2 were all above 90%. Collectively, the PEI/PDA-MWCNTs/TMC NF membranes were well-suited for water softening (e. g., Mg2+ and Ca2+), heavy metal removal (e. g., Zn2+ and Cu2+), and for recovery of valuable cationic molecules (e.g., cationic dyes and antibiotics).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00394. Experimental procedures and material characterizations (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+86)-571-87953780. Fax: (+86)-571-87953780. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was financially supported by Zhejiang Province Natural Science Foundation (No. LR15B060001), National Basic Research Program of China (No. 2015CB655303), 6699


Research Article

ACS Applied Materials & Interfaces (19) Shen, J. N.; Yu, C. C.; Ruan, H. M.; Gao, C. J.; Van der Bruggen, B. Preparation and Characterization of Thin-Film Nanocomposite Membranes Embedded with Poly(Methyl Methacrylate) Hydrophobic Modified Multiwalled Carbon Nanotubes by Interfacial Polymerization. J. Membr. Sci. 2013, 442, 18−26. (20) Chan, W. F.; Chen, H. Y. T.; Surapathi, A.; Taylor, M. G.; Shao, X. H.; Marand, E.; Johnson, J. K. Zwitterion Functionalized Carbon Nanotube/Polyamide Nanocomposite Membranes for Water Desalination. ACS Nano 2013, 7, 5308−5319. (21) Tang, C. Y.; Zhang, Q.; Wang, K.; Fu, Q.; Zhang, C. L. Water Transport Behavior of Chitosan Porous Membranes Containing MultiWalled Carbon Nanotubes (MWNTs). J. Membr. Sci. 2009, 337, 240− 247. (22) Wu, H. Q.; Tang, B. B.; Wu, P. Y. MWNTs/Polyester Thin Film Nanocomposite Membrane: An Approach to Overcome the Trade-Off Effect between Permeability and Selectivity. J. Phys. Chem. C 2010, 114, 16395−16400. (23) Roy, S.; Ntim, S. A.; Mitra, S.; Sirkar, K. K. Facile Fabrication of Superior Nanofiltration Membranes From Interfacially Polymerized CNT-polymer Composites. J. Membr. Sci. 2011, 375, 81−87. (24) Zhao, H. Y.; Qiu, S.; Wu, L. G.; Zhang, L.; Chen, H. L.; Gao, C. J. Improving the Performance of Polyamide Reverse Osmosis Membrane by Incorporation of Modified Multi-Walled Carbon Nanotubes. J. Membr. Sci. 2014, 450, 249−256. (25) Vatanpour, V.; Esmaeili, M.; Farahani, M. H. D. A. Fouling Reduction and Retention Increment of Polyethersulfone Nanofiltration Membranes Embedded by Amine-Functionalized MultiWalled Carbon Nanotubes. J. Membr. Sci. 2014, 466, 70−81. (26) Ye, Q.; Zhou, F.; Liu, W. M. Bioinspired Catecholic Chemistry for Surface Modification. Chem. Soc. Rev. 2011, 40, 4244−4258. (27) Ryu, S. W.; Lee, Y. H.; Hwang, J. W.; Hong, S. K.; Kim, C. S.; Park, T. G.; Lee, H. S.; Hong, S. H. High-Strength Carbon Nanotube Fibers Fabricated by Infiltration and Curing of Mussel-Inspired Catecholamine Polymer. Adv. Mater. 2011, 23, 1971−1975. (28) Hong, S. K.; Na, Y. S.; Choi, S. H.; Song, I. T.; Kim, W. Y.; Lee, H. S. Non-Covalent Self-Assembly and Covalent Polymerization CoContribute to Polydopamine Formation. Adv. Funct. Mater. 2012, 22, 4711−4717. (29) Lee, H. S.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426−430. (30) Zhang, X. Y.; Huang, Q.; Liu, M. Y.; Tian, J. W.; Zeng, G. J.; Li, Z.; Wang, K.; Zhang, Q. S.; Wan, Q.; Deng, F. J.; Wei, Y. Preparation of Amine Functionalized Carbon Nanotubes Via a Bioinspired Strategy and their Application in Cu2+ Removal. Appl. Surf. Sci. 2015, 343, 19−27. (31) Burzio, L. A.; Waite, J. H. Cross-Linking in Adhesive Quinoproteins: Studies with Model Decapeptides. Biochemistry 2000, 39, 11147−11153. (32) Yu, M. E.; Deming, T. J. Synthetic Polypeptide Mimics of Marine Adhesives. Macromolecules 1998, 31, 4739−4745. (33) Wang, J. L.; Ren, K. F.; Chang, H.; Zhang, S. M.; Jin, L. J.; Ji, J. Facile Fabrication of Robust Superhydrophobic Multilayered Film Based on Bioinspired Poly(Dopamine)-Modified Carbon Nanotubes. Phys. Chem. Chem. Phys. 2014, 16, 2936−2943. (34) Lee, H. D.; Kim, H. W.; Cho, Y. H.; Park, H. B. Experimental Evidence of Rapid Water Transport through Carbon Nanotubes Embedded in Polymeric Desalination Membranes. Small 2014, 10, 2653−2660. (35) Liu, M. Y.; Ji, J. Z.; Zhang, X. Y.; Zhang, X. Q.; Yang, B.; Deng, F. J.; Li, Z.; Wang, K.; Yang, Y.; Wei, Y. Self-Polymerization of Dopamine and Polyethyleneimine: Novel Fluorescent Organic Nanoprobes for Biological Imaging Applications. J. Mater. Chem. B 2015, 3, 3476−3482. (36) Li, M. M.; Xu, J.; Chang, C. Y.; Feng, C. C.; Zhang, L. L.; Tang, Y. Y.; Gao, C. J. Bioinspired Fabrication of Composite Nanofiltration Membrane Based on the Formation of DA/PEI Layer Followed by Cross-Linking. J. Membr. Sci. 2014, 459, 62−71.

(37) Ji, Y. L.; An, Q. F.; Zhao, F. Y.; Gao, C. J. Fabrication of chitosan/PDMCHEA Blend Positively Charged Membranes with Improved Mechanical Properties and High Nanofiltration Performances. Desalination 2015, 357, 8−15. (38) Tang, Y.; Tang, B. B.; Wu, P. Y. Preparation of a Positively Charged Nanofiltration Membrane Based on Hydrophilic-Hydrophobic Transformation of a Poly(Ionic Liquid). J. Mater. Chem. A 2015, 3, 12367−12376. (39) Peng, F. B.; Huang, X. F.; Jawor, A.; Hoek, E. M. V. Transport, Structural, and Interfacial Properties of Poly (Vinyl Alcohol)− Polysulfone Composite Nanofiltration Membranes. J. Membr. Sci. 2010, 353, 169−176. (40) Aydiner, C. A Novel Approach Based on Distinction of Actual and Pseudo Resistances in Membrane Fouling: “Pseudo Resistance” Concept and its Implementation in Nanofiltration of Single Solutions. J. Membr. Sci. 2010, 361, 96−112. (41) Tiraferri, A.; Vecitis, C. D.; Elimelech, M. Covalent Binding of Single-Walled Carbon Nanotubes to Polyamide Membranes for Antimicrobial Surface Properties. ACS Appl. Mater. Interfaces 2011, 3, 2869−2877. (42) Yang, H. C.; Liao, K. J.; Huang, H.; Wu, Q. Y.; Wan, L. S.; Xu, Z. K. Mussel-inspired Modification of a Polymer Membrane for Ultrahigh Water Permeability and Oil-in-water Emulsion Separation. J. Mater. Chem. A 2014, 2, 10225−10230. (43) Fei, B.; Qian, B. T.; Yang, Z. Y.; Wang, R. H.; Liu, W. C.; Mak, C. L.; Xin, J. H. Coating Carbon Nanotubes by Spontaneous Oxidative Polymerization of Dopamine. Carbon 2008, 46, 1795−1797. (44) Fang, W. X.; Shi, L.; Wang, R. Mixed Polyamide-Based Composite Nanofiltration Hollow Fiber Membranes with Improved Low-Pressure Water Softening Capability. J. Membr. Sci. 2014, 468, 52−61. (45) Zhao, F. Y.; An, Q. F.; Ji, Y. L.; Gao, C. J. A Novel Type of Polyelectrolyte complex/MWCNT Hybrid Nanofiltration Membranes for Water Softening. J. Membr. Sci. 2015, 492, 412−421. (46) Lv, Y.; Yang, H. C.; Liang, H. Q.; Wan, L. S.; Xu, Z. K. Nanofiltration Membranes via Co-Deposition of Polydopamine/ Polyethylenimine Followed by Cross-Linking. J. Membr. Sci. 2015, 476, 50−58. (47) Wong, M. C. Y.; Lin, L.; Coronell, O.; Hoek, E. M. V.; Ramon, G. Z. Impact of Liquid-filled Voids within the Active Layer on Transport through Thin-film Composite Membranes. J. Membr. Sci. 2016, 500, 124−135. (48) Ghosh, A. K.; Jeong, B. H.; Huang, X. F.; Hoek, E. M. V. Impacts of Reaction and Curing Conditions on Polyamide Composite Reverse Osmosis Membrane Properties. J. Membr. Sci. 2008, 311, 34− 45. (49) Ji, Y. L.; An, Q. F.; Zhao, Q; Chen, H. L.; Gao, C. J. Preparation of Novel Positively Charged Copolymer Membranes for Nanofiltration. J. Membr. Sci. 2011, 376, 254−265. (50) Nightingale, E. R. Phenomenological Theory of Ion Solvation. Effective Radii of Hydrated Ions. J. Phys. Chem. 1959, 63, 1381−1387. (51) Da, X. W.; Wen, J. J.; Lu, Y. W.; Qiu, M. H.; Fan, Y. Q. An Aqueous Sol−gel Process for the Fabrication of High-flux YSZ Nanofiltration Membranes as Applied to the Nanofiltration of Dye Wastewater. Sep. Purif. Technol. 2015, 152, 37−45. (52) Zhao, Q.; Ji, Y. L.; Wu, J. K.; Shao, L. L.; An, Q. F.; Gao, C. J. Polyelectrolyte Complex Nanofiltration Membranes: Performance Modulation Via Casting Solution pH. RSC Adv. 2014, 4, 52808− 52814.

6700
