Research Article www.acsami.org
Nanocomposite Membranes via the Codeposition of Polydopamine/ Polyethylenimine with Silica Nanoparticles for Enhanced Mechanical Strength and High Water Permeability Yan Lv,†,‡ Yong Du,†,‡ Wen-Ze Qiu,†,‡ and Zhi-Kang Xu*,†,‡ †
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Hangzhou 310027, China S Supporting Information *
ABSTRACT: A defect-free and stable selective layer is of critical significance for thin film composite membrane with excellent separation performance and service durability. We report a facial strategy for fabricating thin film nanocomposite (TFN) nanofltration membranes (NFMs) based on the codeposition of polydopamine, polyetheylenimine, and silica nanoparticles. Tripled water flux can be obtained from the TFN NFMs as compared with those NFMs without silica nanoparticles. This is ascribed to the improved wettability of the membrane surfaces and the enlarged pore sizes of the selective layer. The interfacial compatibility of the inorganic fillers and the polymer matrices can be enhanced by the electrostatic interactions of silica nanoparticles with polyethylenimine and the adhesive characteristics of polydopamine, resulting in a defect-free selective layer and then good rejection for both bivalent cations and neutral solutes. The rigid silica nanoparticles also improve the surface mechanical strength of the TFN NFMs effectively and lead to structural stability and compaction resistance during the long-term filtration process. KEYWORDS: nanofiltration membrane, nanocomposite, silica nanoparticles, polydopamine, mussel-inspired chemistry used to enhance water permeation flux, thermal stability, and service durability;11−14 TiO2 NPs or nanotubes were found to promote water permeation flux and antifouling property;15,16 silver NPs could enhance antibacteria performance;17 and carbon-based nanomaterials were applied to raise mechanical strength and ion selectivity of TFN NFMs.18,19 In these cases, the interfacial compatibility must be improved between the polyamide matrices and the inorganic fillers to achieve ideal structures and high performance for these TFN NFMs.20 Plenty of efforts have been devoted to this problem by modifying the inorganic fillers, developing organic nanoparticles and synthesizing molecule of frameworks containing organic ligands, which can be used to avoid defective voids and structural instability to some extent.21−24 However, there still remains a major challenge to develop facile and environmental friendly methods to enhance the interfacial stability and consequently achieve TFN NFMs with high performances. Dopamine, known as a mussel-inspired “bio-glue”, has been widely evaluated for the surface modification of various materials because it can be oxidized and self-polymerizes under alkaline condition to form polydopamine (PDA) coatings with great adhesive strength.25−27 Most recently,
1. INTRODUCTION It is well accepted that the water crisis caused by population growth and industry is threatening the survival and development of human society all over the world.1,2 How to get clean water with high quality has become a world matter of concern. Various membrane technologies have been developed to produce clean water since the 1960s. Among them, nanofiltration is drawing much attention in the fields of drinkable water production, seawater desalination and wastewater treatment due to its superiorities of high water permeability, suitable retention to multivalent ions or organic molecules (200−1000 Da), and low operation pressure.3−5 In general, most nanofiltration membranes (NFMs) have been prepared from polymer.6,7 They have a thin film composite substructure, typically consisting of, for example, a polysulfone sublayer on a nonwoven support, and a selective layer that is made of polyamide.8 In recent years, nanocomposite membranes have become a focused research area,9−19 which consist of a polymeric bulk matrix with various inorganic fillers and then provide a promising access to combine the merits of inorganic fillers with polymers and to take in the synergistic effect between the two materials. Thin film nanocomposite (TFN) membranes have been prepared by mixing inorganic nanomaterials into the selective layer to improve water permeation flux, solute rejection, and mechanical durability as well as antifouling property.10 For examples, silica nanoparticles (SiO2 NPs) were © 2016 American Chemical Society
Received: October 27, 2016 Accepted: December 21, 2016 Published: December 21, 2016 2966
DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
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ACS Applied Materials & Interfaces
Figure 1. Schematic diagram of the preparation process and the suggested structures of TFN NFMs. mixture was dialyzed for 3 days to remove residual lysine and the obtained silica nanoparticle solution is colorless and transparent. Preparation of the TFN NFMs. A typical process is schematically presented in Figure 1 for the preparation of TFN NFMs. First, PAN ultrafiltration membranes were hydrolyzed in NaOH solution (1.5 mol/L) for 1 h at 50 °C, and immersed into HCl solution (1 mol/L) for protonization overnight at 25 °C. Then, they were washed by ultrapure water for several times and stored in ultrapure water. Dopamine hydrochloride (2 mg/mL) was dissolved in Tris−HCl buffer solution (pH = 8.5, 50 mmol/L) with a certain content of SiO2 NPs for 15 min followed by the addition of PEI (1 mg/mL). PAN membranes were prewetted by ethanol and then immersed into the freshly prepared deposition solution to deposit for certain time at 25 °C. The as-prepared membranes (PDA/PEI/SiO2-modified membranes) were washed by ultrapure water overnight and cross-linked with a GA solution as described in our previous work.30 Additionally, surface grafting by PEI solution (2 mg/mL) was adopted in order to maintain the electro-positive characteristic of the membrane surfaces with different contents of SiO2 NPs. Finally, these TFN NFMs were rinsed several times and stored in ultrapure water for further characterization and evaluation. Membrane Characterization. Chemical structures of the membrane surfaces were characterized by attenuated total reflectance Fourier transform infrared spectrometry (FT-IR/ATR, Nicolet 6700, USA) with spectra collected from 400 to 4000 cm−1 by cumulating 32 scans at a resolution of 4 cm−1. Element components were further analyzed by X-ray photoelectron spectrometry (XPS, ThermoScientific, USA) using Al Kα excitation radiation (1486.6 eV) with a survey depth of 5−10 nm. Field emission scanning electron microscopy (FESEM, Hitachi, S4800, Japan) and transmission electron microscopy (TEM, Hitachi 7700, Japan) were used to characterize the morphologies of the membrane and the distribution of SiO2 NPs in the selective layer, respectively. The membranes were dehydrated using graded ethanol solutions and fractured in liquid nitrogen to prepare sectional samples for FESEM detection. For TEM, the dried membranes were frozen and cut by cryoultramicrotome (Leica UC7/ FC7, Germany) to obtain the cross-sectional samples. The dynamic water contact angles (WCAs) were measured by a DropMeter A-200 contact angle system (MAIST VisionInspection & Measurement Co. Ltd., China) in ambient environment to evaluate the wettability of the membrane surfaces. The membrane surface potentials were detected using a streaming potential method by an electrokinetic analyzer (SurPASS Anton Paar, GmbH, Austria) with KCl (1 mmol/L) solution as electrolyte solution. pH values were adjusted by NaOH (0.1 mol/L) and HCl (0.1 mol/L) solutions during the measurement. Surface strength of the membranes was expressed by Young’s modulus measured by atomic force microscopy (AFM, MultiMode 8, USA). Deflection-displacement loading curves of different membranes
PDA-based coatings have been suggested as the selective or intermediate layer for thin film composite NFMs.28−32 It has been demonstrated in our previous work that incorporating low-molecular-weight polyethylenimine (PEI) in the dopamine solution can promote the homogeneous polymerization of dopamine and the uniform codeposition of PDA−PEI to result in smooth and electropositive coatings.33 Dopamine can also help to improve the dispersion of inorganic fillers in aqueous solution and enhance the surface interactions in an interfacial polymerization process.22 Therefore, the codeposition of PDA with PEI and nanomaterials provides a promising solution to construct defect-free and stable selective layer for TFN NFMs. In this work, novel TFN NFMs have been fabricated via the codeposition of PDA/PEI/SiO2 NPs followed by cross-linking and PEI grafting. SiO2 NPs are introduced into the selective layer of TFN NFMs to improve the nanofiltration performance, mechanical strength and structural stability due to their inherent hydrophilicity, rigidity and electronegativity. The interfacial compatibility can be facilitated between the positively charged PDA/PEI matrices and the negative charged SiO2 NPs.13,22 The embedded SiO2 NPs endows these TFN NFMs with high water permeation flux while maintaining high rejection performance for multivalent cations. Furthermore, the as-prepared TFN NFMs exhibit relatively high surface strength and excellent structural stability during a long-term separation process.
2. EXPERIMENTAL SECTION Materials. Polyacrylonitrile ultrafiltration membranes (PAN, MWCO ranging from 10 to 30 kDa) were purchased from Shanghai MegaVision Membrane Engineering & Technology Co. Ltd. (China). Dopamine hydrochloride and PEI (Mw = 600 Da) were procured from Sigma-Aldrich (USA) and Aladdin (China), respectively. LLysine, tetraethyl orthosilicate, tris(hydroxymethyl) aminomethane, ethanol, sodium hydroxide (NaOH), hydrochloric acid (HCl) solution (12 mol/L), glutaraldehyde (GA) solution (50 wt %), and inorganic salts were all obtained from Sinopharm Chemical Reagent Co., Ltd. and used without further treatment. Ultrapure water (18.2 MΩ) was produced from an ELGA Lab Water system (France). Synthesis of SiO2 NPs. SiO2 NPs with the diameter of about 10 nm were prepared according to an amino-acid-catalyzed method reported by Yokoi et al. (Figure S1 in Supporting Information, SI).34,35 Briefly, 0.2 g L-lysine was dissolved in 50 mL of ultrapure water first, and then 3.815 mL of tetraethyl orthosilicate was added to the solution and the mixture was vigorously stirred at 60 °C for 24 h. Finally, the 2967
DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
Research Article
ACS Applied Materials & Interfaces were obtained, and the Young’s modulus of the membrane surface was calculated by the following eq 1:36,37
z=d+
k (π /2)[E /(1 − v 2)]tan α
d (1)
where E is the Young’s modulus, d and z are the cantilever deflection and piezo displacement, respectively. k is the spring constant of the cantilever (k = 39 N/m, provided by the supplier, Nanosensors, Switzerland), ν is Poisson’s ratio (ν = 0.5), and α is the opening angle of the cone (α = 35°). Membrane Performance Evaluation. The filtration performance of the as-prepared TFC NFMs was tested using a laboratory scale cross-flow flat membrane module with an effective membrane area of 7.07 cm2. A series of polyethylene glycol (PEG) with different molecular weights (200, 400, 600, 1000, 1500, and 2000 Da) were used as electroneutral solutes to detect the molecular weight cutoff (MWCO) of the membranes.31 Various salts, including MgCl2, CaCl2, MgSO4, Na2SO4, and NaCl were used as solutes in feed solutions at a concentration of 1000 mg/L to test the separation performance of the TFN NFMs. All of the filtration experiments were operated under 0.6 MPa at 30 °C with a fixed cross-flow rate of 30 L/h. The water flux (Fw, L/m2 h) was calculated by the following eq 2:
Fw =
Q A·t
(2) 2
where Q (L), A (m ), and t (h) represent the volume of permeated water, the effective membrane area, and the permeation time, respectively. And the rejection (R, %) was calculated by the following eq 3:
⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cf ⎠ ⎝
(3)
where Cp (mg/L) and Cf (mg/L) are the solute concentrations in permeate and feed, respectively. The concentration of salt solution was detected by an electrical conductivity meter (Mettler Toledo, FE30, China). Moreover, the structural stability was evaluated according to the variation of the nanofiltration performance during a long-term operation process. In detail, the NFMs were continuously tested on the apparatus for 120 h with the water flux and salt rejection measured every 6 h. All of the experimental results presented were repeated at least three times.
Figure 2. (A) XPS spectra of the composite NFMs with (a) and without (b) SiO2 NPs, and (B) FT-IR/ATR spectra of the prepared membranes containing SiO2 NPs at different fabrication steps. (C) Typical TEM image from the cross section of the TFN NFMs with SiO2 NPs in the selective layer.
3. RESULTS AND DISCUSSION Surface Structures of the Prepared TFN NFMs. Chemical structures of the membrane surfaces were characterized by XPS and FT-IR/ATR. Our previous work indicates that composite NFMs can be facilely fabricated by the codeposition of PDA/PEI followed by cross-linking with GA.30 XPS spectra (Figure 2A) show that, compared with composite NFMs without SiO2 NPs, new peaks of Si 2s and Si 2p arise in the spectrum of TFN NFMs. The deconvoluted spectra of C 1s, O 1s, and N 1s further exhibit the binding energy of CC (CC), CN (CO), CN, CO, and SiO in detail (Figure S2 in SI). The atomic percentage is calculated as 2.92% for Si element in the membrane surface, indicating SiO2 NPs are successfully embedded into the selective layer (Table S1 in SI). In accordance with these results, FT-IR/ATR spectra of the TFN NFMs with SiO2 NPs shows a new peak at 1107 cm−1 ascribed to SiOSi stretching vibration, and the intensity of OH vibration peak at 3400 cm−1 is enhanced due to the abundant hydroxyl groups on the surfaces of SiO2 NPs (Figure S3 in SI). In detail, it can be seen that peaks arise at 1107, 1398, 1566, and 1663 cm−1 due to the SiO, CN, NH, and CN vibrations after the codeposition of PDA−PEI/SiO2
NPs. After cross-linking with GA, the peak of CN enhances slightly ascribed to the reaction between aldehyde groups and amine groups whereas other peaks change little. The peaks of CN, NH, and CN increase obviously due to the PEI grafting on to the membrane surface through Michel addition between PEI and PDA and Shiff base reaction between PEI and GA. Besides, the intensity of SiO peak decreases because of the coverage by PEI chains. Chemical composition on the membrane surfaces varies consistently with above results at different fabrication steps (Table S1 in SI). Additionally, TEM was used to observe the distribution of SiO2 NPs in the selective layer of TFN NFMs. It depicts that the SiO2 NPs are embedded in the selective layer with slight aggregation, which can act as rigid cross-linking sites to improve the mechanical strength of the membrane surface and the stability of the selective layer (Figure 2B, Figure S4a in SI). Surface Morphology and Property of the TFN NFMs. Figure 3 shows the surface morphology of the composite NFMs with and without SiO2 NPs embedded, which are of 2968
DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
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Figure 3. Surface morphologies of the composite NFMs without (a) and with (b) SiO2 NPs. (c) Dynamic water contact angles and (d) surface ζpotentials at various pH values of the composite NFMs with (tilted ◪) and without (◑) SiO2 NPs. (The deposition time is 4 h, and the concentration of SiO2 NPs is 0.5 mg/mL.)
Figure 4. (a) Deflection−displacement loading curves of composite NFMs with and without SiO2 NPs (the deposition time is 4 h, the concentration of SiO2 NPs is 0.5 mg/mL), and (b) effect of the concentration of SiO2 NPs on the Young’s modulus of the membrane surfaces of the TFN NFMs.
increases slightly with the prolonged deposition time (Figure S5 in SI). These results should be ascribed to the inherent hydrophilicity of the embedded SiO2 NPs and the resulted rough surface morphology (Figure S6 in SI). The TFN NFMs show good surface wettability and can be quickly infiltrated by water, which is beneficial for the permeation performance. On the other hand, the surface potential plays a very crucial role for NFMs in the retention of charged solutes during the nanofiltration process because of the Donnan exclusion effects to co-ion at the interphase of the membrane and the solution. Figure 3d reveals that the membrane surfaces are positively charged with pH value ranging from 4 to 8 for those composite NFMs without SiO2 NPs, which is due to the abundant amino groups of the codeposited and the grafted PEI chains. It is interesting that the membrane surfaces show a lower ζ-
great importance for rejection and permeation performance. The membrane surfaces are defect-free without visible pores. It can be seen that the TFN NFMs exhibit a lot of nodules on the membrane surfaces, while the composite NFMs without SiO2 NPs are relatively smooth. These nodules should be attributed to the aggregation of SiO2 NPs. Moreover, the selective layer increases from 55 to about 500 nm (Table S2 in SI) with deposition time, which is much thicker than that of the composite NFMs without SiO2 NPs.30 The surface wettability of the TFN NFMs is essential for water permeation performance, which was evaluated by timedependent and static water contact angle (WCA) measurements (Figure 3c, Figure S5 in SI). WCA is lower on the TFN NFMs surfaces than those of the composite NFMs without SiO2 NPs. It decreases further with increased SiO2 NPs and 2969
DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
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Figure 5. (a) MWCOs and (b) separation performances for different salts of the TFN NFMs. Compaction resistance of the composite NFMs (c) with and (d) without SiO2 NPs with operation time (MgCl2 as solute). Test conditions: all inorganic salt concentration is 1000 mg/L, 30 °C, pH = 6, 0.6 MPa, cross-flow rate = 30 L/h.
potential for the prepared TFN NFMs than those composite NFMs without SiO2 NPs. And the ζ-potential changes with the SiO2 NPs and the deposition time (Figure S7 in SI). This is because the synthesized SiO2 NPs are negatively charged (with ζ-potential of −30.4 mV) whereas the PEI chains are positively charged when the pH value is lower than 10.5 (isoelectric point). Therefore, our TFN NFMs are still positively charged when the nanofiltration process is typically conducted at pH 6.0. Mechanical Strength of the Membrane Surfaces. SiO2 NPs are expected to act as cross-linking sites for enhancing the mechanical strength of the membrane surface and thus increasing the compaction resistance of the selective layer as mentioned above. Young’s modulus (E) of the selective layer was calculated from the indentation curves detected by AFM (Figure 4a). E of the membrane surface is improved to 43.97 GPa for the TFN NFMs as compared with 14.29 GPa for the composite NFM without SiO2 NPs. This value gradually increases to 75 GPa with the concentration increases of SiO2 NPs from 0 to 2.0 mg/mL (Figure 4b). It can be attributed to the increased amount of rigid inorganic nanoparticles in the selective layer. It should be noticed that the error bars become bigger as the SiO2 NPs concentration increases, indicating an enlarged data dispersivity of Young’s modulus. This should be ascribed to the increased nanoparticle agglomeration, which is also consistent with the results shown in the TEM images (Figure S4b in SI). Nanofiltration Performance of the TFN NFMs. The separation performance of the composite NMFs was evaluated with a typical cross-flow nanofiltration process. We studied the
effects of the deposition time and the concentration of SiO2 NPs on the nanofiltration performance in detail as they could influence the surface topology and the thickness of the selective layer (Figure S8 in SI). 4 h is adopted as an optimized deposition time after considering both the permeation flux of water and the rejection of MgCl2. The rejection increases slightly with the deposition time because the compactness and the surface potential of the selective layer reaches a steady state when the deposition is longer than 4 h (Figure S7b, Figure S8a, and Figure S9 in SI). On the other hand, the water flux declines due to the increased thickness of the selective layer whereas the surface morphology and the wettability remain changeless in these cases (Table S2, Figure S5b, Figure S8b, Figure S9, and Figure S10 in SI). We also determined MWCOs of the TFN NFMs with different concentrations of SiO2 NPs by neutral solute filtration using PEGs. It can be seen that the MWCO increases from 200 Da to over 2000 Da with the concentration of SiO2 NPs from 0 to 2.0 mg/mL (Figure 5a). It means the selective layer becomes loose when the SiO2 NPs are incorporated into the membrane surface. This is reasonable because the SiO2 NPs are more rigid than the deposited PDA/ PEI matrix and they will aggregate at high concentration (Figure S6 in SI).6,7 Therefore, the rejection to MgCl2 decreases dramatically with the concentration of SiO2 NPs, which are resulted from the loose selective layer and the decreased surface potential (Figure S7a and Figure S8b in SI). However, the water flux of TFN NFMs increases to 70 L/m2 h due to the improved surface wettability and the loose structures (Figure S5a and Figure S8b in SI). Thus, 0.5 mg/mL is chosen 2970
DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
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ACS Applied Materials & Interfaces as the optimized concentration of SiO2 NPs for the preparation of TFN NFMs. Various salt solutions were then used to evaluate the separation performances of the TFN NMFs prepared under the optimized conditions (Figure 5b). The rejection ratio reaches as high as 90% for bivalent cations whereas it is lower than 30% for monovalent cations. The salt rejection obeys the order of CaCl2 ≅ MgCl2 > MgSO4 > Na2SO4 > NaCl, which is reasonable for positively charged NFMs and mainly determined by the Donnan and dielectric effects under typical conditions. It can be seen that the rejection for Na2SO4 is higher than that for NaCl because the hydration ion radius of SO42− is larger than that of Cl−. Additionally, it should be noticed that, the water flux of TFN NFMs reaches as high as 32 L/m2 h, which is about triple of the composite NFM without SiO2 NPs (Figure S8b in SI). This indicates that the incorporation of SiO2 NPs into the selective layer improves the water permeability of the composite NFMs effectively. Moreover, we evaluated the structural stability and compaction resistance of the selective layer with a long-term operation process of continuous filtration for 120 h. Both of the water flux and the rejection maintain stable during this process, indicating good structural stability for the TFN NFMs (Figure 5c). However, the water flux decreases by about 30% whereas the rejection increases from 87% to 95% for the composite NFMs without SiO2 NPs (Figure 5d). It can be deduced that the improvement of the compaction resistance is closely related to the addition of SiO2 NPs. The enhanced structural stability is beneficial for the stable separation performance of the composite NFMs, which is promising for practical nanofiltration operation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhi-Kang Xu: 0000-0002-2261-7162 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support is acknowledged to the National Natural Science Foundation of China (Grant no. 21534009), the Open Research Fund Program of Zhejiang Provincial Collaborative Innovation Center Program 2011 (Grant no. 2016ZD04), and Zhejiang Provincial Collaborative Innovation Center Program 2011 (Grant no. G1504126001900).
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REFERENCES
(1) Gin, D. L.; Noble, R. D. Designing the Next Generation of Chemical Separation Membranes. Science 2011, 332, 674−676. (2) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Science and Technology for Water Purification in the Coming Decades. Nature 2008, 452, 301−310. (3) Hilal, N.; Al-Zoubi, H.; Darwish, N. A.; Mohamma, A. W.; Arabi, M. A. A Comprehensive Review of Nanofiltration Membranes: Treatment, Pretreatment, Modelling, and Atomic Force Microscopy. Desalination 2004, 170, 281−308. (4) Luo, J.; Wan, Y. Effects of pH and Salt on Nanofiltration - A Critical Review. J. Membr. Sci. 2013, 438, 18−28. (5) Vander Bruggen, B.; Mänttäri, M.; Nyström, M. Drawbacks of Applying Nanofiltration and How to Avoid Them: A Review. Sep. Purif. Technol. 2008, 63, 251−263. (6) Souza, V. C.; Quadri, M. G. N. Organic-Inorganic Hybrid Membranes in Separation Process: A 10-Year Review. Braz. J. Chem. Eng. 2013, 30, 683−700. (7) Li, Y.; He, G.; Wang, S.; Yu, S.; Pan, F.; Wu, H.; Jiang, Z. Recent Advances in the Fabrication of Advanced Composite Membranes. J. Mater. Chem. A 2013, 1, 10058−10077. (8) Yin, J.; Kim, E. S.; Yang, J.; Deng, B. Fabrication of A Novel ThinFilm Nanocomposite (TFN) Membrane Containing MCM-41 Silica Nanoparticles (NPs) for Water Purification. J. Membr. Sci. 2012, 423− 424, 238−246. (9) 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. (10) Yin, J.; Deng, B. Polymer-Matrix Nanocomposite Membranes for Water Treatment. J. Membr. Sci. 2015, 479, 256−275. (11) Jadav, G. L.; Singh, P. S. Synthesis of Novel Silica-Polyamide Nanocomposite Membrane with Enhanced Properties. J. Membr. Sci. 2009, 328, 257−267. (12) Hu, D.; Xu, Z. L.; Chen, C. Polypiperazine-Amide Nanofiltration Membrane Containing Silica Nanoparticles Prepared by Interfacial Polymerization. Desalination 2012, 301, 75−81. (13) Niksefat, N.; Jahanshahi, M.; Rahimpour, A. The Effect of SiO2 Nanoparticles on Morphology and Performance of Thin Film Composite Membranes for Forward Osmosis Application. Desalination 2014, 343, 140−146. (14) Li, Q.; Wang, Y.; Song, J.; Guan, Y.; Yu, H.; Pan, X.; Wu, F.; Zhang, M. Influence of Silica Nanospheres on the Separation Performance of Thin Film Composite Poly(piperazine-amide) Nanofiltration Membranes. Appl. Surf. Sci. 2015, 324, 757−764.
4. CONCLUSION In summary, novel TFN NFMs are fabricated via the codeposition of PDA, PEI, and SiO2 NPs followed by GA cross-linking and PEI grafting. The dense, hydrophilic, and positively charged selective layers endow these TFN NFMs with high rejection for bivalent cations and neutral molecules accompanied by tripled water flux compared with those composite NFMs without SiO2 NPs. The structural stability and mechanical strength of the selective layer are also enhanced due to the rigidity of SiO2 NPs and their good compatibility with the PDA/PEI matrices, which is beneficial for the compaction resistance during long-term filtration process. In brief, we propose here a promising strategy to construct TFN NFMs with defect-free, stable, and compaction resistant selective layers.
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different deposition times and concentrations of SiO2 NPs, effects of deposition time and concentration of SiO2 NPs on the nanofiltration performances of the TFN NFMs (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13761. TEM image and size distribution of the synthesized SiO2 NPs, high-resolution XPS spectra of the TFN NFM, FTIR/ATR spectra of the composite NFMs, TEM images of the selective layer from the cross-section of the TFN NFMs, SEM images from the membrane surfaces of the TFN NFMs with different deposition times and concentrations of SiO2 NPs, SEM images the membrane cross sections of the TFN NFMs and thickness of the selective layer with different codeposition times, water contact angles and ζ-potentials of the TFN NFMs with 2971
DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
Research Article
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DOI: 10.1021/acsami.6b13761 ACS Appl. Mater. Interfaces 2017, 9, 2966−2972
