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8 µm thick PET films were purchased from DuPont Co., Ltd, US. Dopamine hydrochloride, PEI (MW = 600 Da), Zirconium sulfate tetrahydrate, tris(hydroxymethyl) aminomethane were purchased from Aladdin Chemistry Co., Ltd, China. Other reagents, including ethanol, sodium chloride (NaCl), magnesium chloride (MgCl2), sodium sulfate (Na2SO4), magnesium sulfate (MgSO4), sodium hydroxide (NaOH) and concentrated hydrochloric acid (HCl) were obtained from Tianjin Damao Chemical Reagent Co., Ltd, China.
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The fabrication process for PET ITM is shown in Fig. 1. First, the PET membrane was irradiated by
$ ^{129}{\rm{Xe}} $ ions (19.5 MeV/u) with the beam normal to the membrane surface at the Heavy Ion Research Facility in Lanzhou (HIRFL, China), and the ion latent tracks highly parallel to each other were formed. Herein, the ion fluence of$ 1\times 10\ {{\rm{ions}}/{\rm{cm}}^2} $ was adopted. Subsequently, both sides of the irradiated PET membranes were exposed to the ultra-violet (UV) light (MUA-165, MEJIRO GCNOSSEN, Japan) with 365 nm peak wavelength for 1 h. The radiation intensity of the UV light was adjusted to$ 30\ {{\rm{mW}}/{\rm{cm}}^{2}} $ . Then, the sensitized PET membrane was immersed in the solution of 2 M 50 °C NaOH for 4 min to carry out chemical etching. After the etching, the ITM was washed in deionized water for three times to remove the chemical residuals completely. -
The fabrication process of composite nanofiltration membrane was performed according to the reports from Lv
$et~al$ .[21], and is shown in Fig. 2. The PET ITM was firstly immersed in hydrochloric acid solution (2 mol/L) for 10 min at room temperature, and then transferred into ethanol and treated for 10 mins. PDA and PEI were dissolved in Tris-HCl buffer solution (pH = 8.5, 50 mmol/L). The mass ratio of PDA and PEI was fixed at 1:1 with a total concentration of 2 mg/mL. After that, the ITM pre-wetted by ethanol were moved into the freshly as-prepared PDA/PEI solution and gently stirred at room temperature for 1 h. The resulting composite membrane was washed with deionized water several times and dried naturally. 0.7 g Zirconium sulfate tetrahydrate was dissolved in 500 mL hydrochloric solution with the concentration of 40 mmol/L. Subsequently, the PDA-PEI coating ITM was immersed in the solution at room temperature for 10 h. Finally, the resulting organic-inorganic composite nanofiltration membrane was washed in deionized water several times and dried in oven at 40 °C for 5 h to further evaluate its performance and characterization. The thin ZrO2-layer obtained from the hydrolysis process of zirconium sulfate tetrahydrate (Zr(SO4)2) under equilibrium reactions as follows:Figure 2. Schematic diagram of the preparation process for the organic-inorganic composite nanofiltration membranes with ultrathin ZrO2 film as the selective layer on the PDA-PEI coating ITM. (color online)
$$ {2[{\rm{Zr}}({\rm{SO_4^{}}})_2^{}]} + 3{{\rm{H_2^{}O}}}\to {[{\rm{Zr_2^{}}}({\rm{OH}})_3^{}({\rm{SO_4^{}}})_4^{}]^{3-}} + 3{{\rm{H}}^+}, $$ (1) $$ {[{\rm{Zr_2^{}}}({\rm{OH}})_3^{}({\rm{SO_4^{}}})_4^{}]^{3-}} + 3{{\rm{OH}}^-} \to {{\rm{Zr_2^{}}}({\rm{OH}}){\rm{_6^{}SO_4^{}}}} + 3{{\rm{SO_4}}^{2-}}, $$ (2) $$ {{\rm{Zr_2}}^{}({\rm{OH}})_6^{}{\rm{SO_4}}^{}} + 2{{\rm{OH}}^-} \to {\rm{ZrO_2}}^{} + {{\rm{SO_4^{}}}^{2-}}+4{{\rm{H_2}}^{}{\rm{O}}}. $$ (3) -
The chemical structures and detailed components of membrane surfaces were analyzed by attenuated total reflectance Fourier transform infrared spectrometer (FT-IR/ATR, vertex 70, Bruker, Germany). Scanning electron microscopy (SEM, Nano-SEM 450, FEI, US) and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the morphologies and elemental distribution of the membrane. The morphology and roughness of the membranes were observed by atom force microscopy (AFM, Cypher S, Asylum Research, UK). The water contact angles before and after composite were measured by a contact angle system (SZ-CAMB, Sunzern, China) at room temperature.
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Nanofiltration performance of composite membrane was evaluated by measurement of pure water flux and salt rejection using a lab-scale cross-flow filtration apparatus (effective membrane area is
$ 7.1\ {{\rm{cm}}^2} $ ). Membranes were pre-compacted under 0.65 MPa for over 30 min before evaluation and tested under 0.6 MPa at room temperature. Various salts including NaCl, MgCl2, Na2SO4, MgSO4 were dissolved in deionized water and used as feed solutions with a concentration of 1 g/L. The permeate flux was calculated by the following equation:$$ F = \frac{V}{A\times t}, $$ where
$ V $ is the volume of permeated solution;$ A $ is the effective membrane area;$ t $ is the filtration time. The salt rejection was calculated as follow:$$ R = \left( { 1-\frac{c_{\rm{p}}^{}}{c_{\rm{f}}^{}} } \right)\times 100\text{%}, $$ where
$ c_{\rm{p }}^{} $ and$ c_{\rm{f}} ^{}$ are the concentration of the permeated and feed solution, respectively. The concentration of the salt solution was determined by the solution conductivity monitored using the electrical conductivity meter (METTLER TOLEDO S230, Switzerland).
Fabrication of Organic-inorganic Composite Nanofiltration Membrane Based on PET Ion Track Membrane
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摘要: 纳滤膜在海水淡化及饮用水净化领域起着越来越重要的作用,核孔膜作为一种重要的分离材料,具有均匀的孔径、可调的孔密度及无缺陷的表面,在水处理及精确分离应用方面有着潜在的优势,高孔密度的超薄核孔膜在制备高性能纳滤膜上具有很大的潜力。基于此,利用氧化锆作选择层,核孔膜作为支撑层制备出了三层结构的有机无机复合纳滤膜,聚多巴胺(PDA)与聚乙烯亚胺(PEI)中间层促进无机纳米粒子在膜表面形成无机层。制备出的复合膜可实现对不同盐的部分截留,表现出的截留顺序为Na2SO4>MgSO4>MgCl2>NaCl。Abstract: Nanofiltration membranes are playing more and more important roles in the fields of sea water desalination and drinking water purification. The ion track membrane (ITM), as a kind of important separation membrane with uniform pore size, regulable pore density, and free-defect surface, has potential permission in water treatments and precise separation. The thin ITMs with high pore density as substrates have great potential in the preparation of high-performance nanofiltration membranes. Herein, we report a triple-layered organic-inorganic composite membrane with zirconia (ZrO2) as a selective layer and polyethylene terephthalate ion track membrane (PET ITM) as support. The polydopamine (PDA)-polyethyleneimine (PEI) intermediate coating facilitates inorganic nanoparticles to form an inorganic layer on the membrane surfaces. The composite membrane can achieve partial rejection for various kinds of salts and exhibit a salt retention order of Na2SO4>MgSO4>MgCl2>NaCl.
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Key words:
- nanofiltration /
- composite membrane /
- ion track membrane
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