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Journal of Membrane Science 593 (2020) 117444

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Journal of Membrane Science 593 (2020) 117444 Journal of Membrane Science 593 (2020) 117444 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Nanofiltration membranes with hydrophobic microfiltration substrates for robust structure stability and high water permeation flux T ∗∗ ∗ Xi Zhanga,b, Chang Liua,b, Jing Yanga,b, , Cheng-Ye Zhua,b, Lin Zhangc, Zhi-Kang Xua,b, a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China b Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, China c Key Laboratory of Biomass Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China ARTICLE INFO ABSTRACT Keywords: Traditional nanofiltration membranes (NFMs) suffer from ultrafiltration substrates with low porosity, small pore Nanofiltration membrane size and relatively poor solvent stability. Herein, NFMs have been fabricated on a series of hydrophobic polymer Microfiltration substrate microfiltration substrates to address these issues. Polyphenol-based coatings of tannic acid/diethylenetriamine Surface wettability (TA/DETA) were co-deposited on the hydrophobic substrates to improve their surface wettability and to make Water permeation flux them appropriate for interfacial polymerization. The spreading behaviors of aqueous solutions, which are of Structure stability significant importance to the formation of defect-free polyamide layers, were directly visualized by laser con- focal microscopy. The influences of TA/DETA coatings on the interfacial polymerization were further demon- strated by both dynamic molecular simulation and nanofiltration performance evaluation. The as-prepared NFMs exhibit higher water permeation flux compared with traditional ones because of the large pore size and high porosity of the microfiltration substrates, as well as the relatively low cross-linking degree of polyamide layers. Internal stress during the nanofiltration process was calculated by the theory of thin plates and the results claim good pressure resistance for these NFMs. Therefore, the as-prepared NFMs can be steadily used under high operation pressures even up to 0.9 MPa, which are in accordance with the theoretical calculation. Furthermore, these NFMs also present good solvent resistance since the chemical stability of the no-polar hydrophobic sub- strates. 1. Introduction size and porosity of the porous substrates because of the reduced trans- membrane resistance and shortened water permeation pathway Nanofiltration membranes (NFMs) have drawn growing attention [18–20]. Some commercialized non-polar polymer membranes, such as over the past years due to their high water permeation flux, low op- polypropylene, polyethylene and poly(vinylidene fluoride) microfiltra- eration pressure and high rejection to multivalent ions [1–3]. They have tion membranes (PPMM, PEMM and PVDFMM), thereby emerge as been widely used in waste-water treatment, biological engineering, satisfactory candidates for support substrates of NFMs due to their medicine/food industry, and seawater desalination [4–7]. Currently, advantages of high solvent resistance, good mechanical strength, low these NFMs usually consist of a polyamide selective layer and a porous cost for raw materials, large pore size and high porosity [21,22]. support substrate [8–11]. Interfacial polymerization is one successful However, it is difficult to directly fabricate a polyamide selective method to fabricate NFMs with such structures [10]. Most of the layer on these microfiltration substrates by interfacial polymerization commercialized NFMs use polysulfone (PSF), polyethersulfone (PES) or because aqueous solutions cannot be well spread on them due to their polyacrylonitrile (PAN) ultrafiltration membranes as porous substrates poor surface wettability [23]. Improving the surface hydrophilicity of due to their proper surface hydrophilicity for carrying out the inter- these hydrophobic substrates is required for carrying out the interfacial facial polymerization [12–14]. However, these substrates exhibit low polymerization. There are many available methods to hydrophilize the chemical resistance to ketones, esters and alcohols, limiting the struc- surfaces of hydrophobic membranes, such as UV, plasma, or ozone-in- ture stability of the NFMs [15–17]. Moreover, It has been reported that duced grafting polymerization and chemical treatment [24–31]. For NFMs can obtain improved water permeability by increasing the pore example, PPMMs were hydrophilized by UV-induced grafting or ∗ Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China. ∗∗ Corresponding author. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Hangzhou, 310027, China. E-mail addresses: [email protected] (J. Yang), [email protected] (Z.-K. Xu). https://doi.org/10.1016/j.memsci.2019.117444 Received 2 May 2019; Received in revised form 29 July 2019; Accepted 4 September 2019 Available online 05 September 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved. X. Zhang, et al. Journal of Membrane Science 593 (2020) 117444 oxidation with chromic acid solution and then they were used as sub- Table 1 strates for NFMs [23,28,29]. PPMMs and PVDFMMs could also be hy- Average pore diameter, porosity and water contact angle for three kinds of drophilized by plasma treatment for the fabrication of NFMs [30,31]. hydrophobic microfiltration membranes used in this study. However, these methods usually have complex operation process and Substrate Average pore diameter (nm) Porosity (%) Water contact angle (°) high energy cost. The substrate structure may even be destroyed and pore blockage will occur, resulting in the increased trans-membrane PPMM 338.3 81.14 144.25 resistance and reduced water permeation flux [22,23,29,30]. Another PEMM 143.6 52.29 116.56 PVDFMM 307.8 61.81 126.65 disadvantage is the lack of university, which means one can only hy- drophilize one kind of hydrophobic substrate with a specific condition, limiting the selection of substrates for NFMs. Therefore, it is still a phenylenediamine (MPD, 99%), tannic acid (TA, Analytical Reagent) challenge to fabricate NFMs on hydrophobic microfiltration substrates and fluorescein sodium (Analytical Reagent) were bought from Aladdin with desirable nanofiltration performances. Chemistry Co. Ltd. (China). Acetone, ethanol, hexane, sodium chloride Polyphenols have strong solid-liquid interface activities and can (NaCl), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), mag- form coatings on various polymer materials [32–35]. Recently, we have nesium sulfate (MgSO4), hydrogen chloride (HCl) and sodium hydro- demonstrated a novel type of polyphenol coating by simple co-deposi- xide (NaOH) were all analytical reagents and obtained from Sinopharm tion of tannic acid (TA) and diethylenetriamine (DETA), which merits Chemical Reagent Co. Ltd. (China). All chemicals were used as received the advantage of simplicity, versatility and university [32,34,36]. It without further purification. Bicine buffer (pH = 7.8) was prepared should be noticed that such coating is particle-free and uniform, thus from bicine and NaOH as reported in our previous work [34]. Ultrapure exhibits a negligible influence on the surface structures of microfiltra- water (18.2 MΩ) was produced by an ELGA Lab Water System (France). tion substrates.34 Herein, we used the TA/DETA coatings to hydro- philize PPMM, PEMM and PVDFMM substrates according to our pre- 2.2. Dynamic molecular simulation vious work [34], making them proper substrates for directly carrying out the interfacial polymerization (schematically shown in Fig. 1). The Materials Studio 2017 R2 was used to carry out dynamic molecular interfacial polymerization was then conducted on these microfiltration simulation. Fig. 2 shows the molecular structures of PIP, MPD, PP and substrates to prepare NFMs. Laser confocal microscopy (LSCM) was TA/DETA coating. The Forcite module with a task of geometry opti- used to visualize the spreading behaviour of aqueous solution on the mization was used to optimize all the structures applied in the dynamic studied substrates. Water can well spread on these TA/DETA co-de- molecular simulation. Especially, PP molecule with 20 repeat units was posited substrates while it shows a de-wetting phenomenon on the used to simplify the simulation. Mixing energies of PIP-PP, PIP-TA/ nascent ones, corresponding to whether the interfacial polymerization DETA, MPD-PP and MPD-TA/DETA were calculated by the Blends can be carried out or not. The resulting NFMs show improved water module. permeation flux compared with traditional ones with ultrafiltration substrates while maintaining a high rejection to divalent ions (> 95% 2.3. Preparation of NFMs for Na2SO4). Internal stress on the polyamide selective layer was cal- culated to confirm the good pressure resistance of such NFMs. On the The hydrophobic substrates were co-deposited by TA/DETA coating other hand, our NFMs show stable nanofiltration performances against for hydrophilization. Briefly, TA was dissolved in bicine buffer to pre- ethanol and acetone treatment, demonstrating high solvent resistance pare a solution with a concentration of 2 g/L. Then DETA was added which is profited from the chemical stability of the hydrophobic sub- into the freshly prepared TA solution with a TA/DETA mass ratio of 1/ strates. 10. Substrates were
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