Design of a novel interfacial enhanced GO-PA/APVC nanofiltration membrane with stripe-like structure
Graphical abstract
Introduction
Nowadays, on account of the rapid growth of the world population and the increase in the industrialization, the pollution of water resources and the scarcity of pure drinking water are increasing [1,2]. Therefore, it is crucial to find alternative sources of clean water to meet industrial and domestic water needs. As a modern and efficient separation technology, membrane technology has received widespread attention for solving such problems, which has been widely used in water treatment, pharmaceutical, biochemical, medical, food and other fields [3]. As we all know, different applications have different requirements on the structure and properties of membrane materials. Among various types of separation membranes, nanofiltration membrane is considered to be more successful, especially in seawater desalination and purification [4,5].
Nanofiltration technology is a pressure-driven membrane separation process between ultrafiltration and reverse osmosis [6,7]. Generally, nanofiltration membranes have a pore diameter of 0.5–2 nm, which endow it with superior rejection compared with ultrafiltration membranes and excellent water permeability compared with reverse osmosis membranes [[8], [9], [10]]. Among the variety of nanofiltration membrane preparation methods, the interfacial polymerization is one of the most widely used methods, and the prepared membrane is called the thin film composite nanofiltration (TFC NF) membrane. Interfacial polymerization occurs at the interface of water/organic solution and forms a thin active layer on the porous support layer surface [11]. The advantage of interfacial polymerization is that the support layer and active layer can be optimized separately for optimal overall separation performance. However, the TFC NF membrane also has some disadvantages, such as the weak interfacial bonding, the poor water flux and antifouling performance.
For the sake of improving the interfacial bonding of the TFC NF membrane, the researchers have done a lot of works. Peng et al. [12] selected PAN as a substrate membrane, which was first hydrolyzed to produce a carboxyl group on the surface, and then activated by 1-ethyl-(3-3-dimethylaminopropyl) carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) to enhance reactivity with the amine group. Finally, interfacial polymerization was carried out on the modified PAN support layer to fabricate the composite nanofiltration membrane. This is to utilize a functional group to optimize the substrate membrane to form a covalent bond between the support layer and active layer, thereby improving the interfacial bonding. Li et al. [13] modified the support layer by depositing polydopamine on the PES membrane, then the interfacial polymerization was carried out on the modified PES membrane to form the polyamide (PA) active layer. This is to apply bio-adhesive to the surface of the substrate to enhance the physical interaction between the two layers. Kim et al. [14] utilized hydrophilic materials for plasma treatment to modify the PSF and PP support layer, then the modified support layer could be sufficiently and uniformly infiltrated by the aqueous solution during interfacial polymerization. Thereby the interfacial polymerization could be fully carried out to form the active layer. This is by improving the hydrophilicity of the support layer to enhance the physical interaction between the two layers of the TFC NF membrane.
There are many researches have been reported on improving the water flux and antifouling performance of the TFC NF membranes. One of the best ways to adjust the properties of the membrane is to modify the surface with hydrophilic and low-fouling materials. Lately, various nanomaterials (e.g., silver nanoparticles, carbon nanotubes, and graphene oxide (GO)) were utilized to modify the PA active layer [[15], [16], [17], [18]]. Liu et al. [19] exploited a simple method for covalently immobilizing Ag nanoparticles onto the active layer of the TFC NF membrane by layer-by-layer interfacial polymerization, which effectively mitigated biofouling. This study provided a feasible way to directly incorporate multifarious antifouling and surface customized nanomaterials on the TFC NF membrane surface to achieve antifouling performance. Bano et al. [18] introduced different contents of GO into the aqueous solution in the process of interfacial polymerization. The addition of GO not only improved the water flux but also enhanced antifouling performance of the TFC NF membrane. Choi et al. [20] coated the GO multilayer on the surface of TFC NF membrane by depositing GO nanosheets with opposite charges layer by layer. As a result, the conformal GO coating layer could improve the hydrophilicity of the surface, thereby observably improving the antifouling property. However, among the numerous researches, not only modified the support layer to enhance interfacial bonding through the action of covalent bonds, but also modified the active layer to improve the antifouling property of the TFC NF membrane are rare.
Polyvinyl chloride (PVC) is a widely used thermoplastic synthetic resin, which has various characteristics, such as low cost, good mechanical strength, excellent acid and alkali resistance, and has become a common material for preparing the separation membranes [21,22]. In particular, previous reports indicated that some highly reactive C–Cl bonds were present in the PVC molecular chain, providing potential functionalization sites for the PVC. Numerous studies about modifying PVC indicated that the existence of unstable C–Cl bonds and the nucleophilic substitution were the most studied reactions up to now [23]. Among which, amino-PVC prepared by the amination of PVC with polyamines has always been a linkage for further functionalization [24,25]. GO is a new type of carbon material with abundant oxygen-containing functional groups (carboxyl, hydroxyl, epoxy groups, and so on) [26]. In recent years, it has aroused the attention of researchers involved in water treatment membranes due to its distinct water transport and antifouling characters. Implanting GO into PA active layer during interfacial polymerization opens up a new way for preparing GO-based nanofiltration membrane with the superior performance [27]. However, there are few studies focus on improving the interfacial bonding, permeability and selectivity and antifouling performances of the PVC composite nanofiltration membrane. Therefore, inspired by the above researches and material properties, combined with the current problems of the TFC NF membranes, we have done the following research.
Herein, we prepared a novel TFC NF membrane with interfacial stability, good permeability and antifouling properties, which involved the preparation of aminated PVC to provide the active reaction sites for interfacial polymerization, and the interfacial polymerization of the GO-PIP aqueous solution and TMC organic solution on the aminated PVC membrane to fabricate a new-style nanofiltration membrane. The chemical structures, morphologies, surface properties, separation performances, interfacial stability and antifouling properties of the prepared membranes were investigated.
Section snippets
Materials
Polyvinyl chloride (PVC, fiber grade, DG-1000k) resin was offered by Tianjin Dagu Chemical Factory (Tianjin, China), whose average degree of polymerization is about 1030. Poly (ethylene glycol) (PEG) with average molecular weight (Mw) 6000, N, N-dimethylacetamide (DMAc), Na2SO4, MgSO4, CaCl2, MgCl2 and NaCl were all got from Tianjin Kermel Chemical Reagents Co., Ltd (Tianjin, China). Graphene oxide (GO), a material with a single layer ratio of more than 98%, a thickness of about 0.6–1.0 nm and
Chemical performances analysis
FTIR spectra of PVC and APVC membranes were detected to prove that the nucleophilic substitution reaction had occurred between TETA and PVC membrane. As depicted in Fig. 2a, a new extremely broad peak was detected at the spectrum of APVC membrane around 3307 cm-1 compared with PVC substrate, which was a typical stretching vibration of N–H. The peaks at 1574 cm-1 and 1050 cm-1 corresponded to the presence of N–H (bending vibration) and C–N (stretching vibration), respectively [5,28]. In
Conclusions
To summarize, a new-style TFC NF membrane with the stripe-like surface structure, the expected interfacial bonding, permeability and antifouling performances was fabricated by a facile method. It involved the amination of PVC membrane to form APVC membrane, and the interfacial polymerization of the TMC organic solution and the GO-PIP aqueous solution on the APVC substrate to prepare the nanofiltration membrane. The FTIR, XPS spectra, FESEM and AFM morphologies analyses proved that the APVC
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the research funding provided by the National Natural Science Foundation of China (51603146, 51673149), the Natural Science Foundation of Tianjin (18JCQNJC72200), the Science&Technology Development Fund of Tianjin Education Commission for Higher Education (2018KJ198), the industrial chain collaborative innovation major projects of the State Oceanic Administration (BHSF2017-01), the Science and Technology Plans of Tianjin (18PTSYJC00170).
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