Construction of highly water-stable metal-organic framework UiO-66 thin-film composite membrane for dyes and antibiotics separation

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Construction of highly water-stable metal-organic framework UiO-66 thinfilm composite membrane for dyes and antibiotics separation Si-Yuan Fanga, Peng Zhanga, Ji-Lai Gonga, , Lin Tanga, , Guang-Ming Zenga, , Biao Songa, Wei-Cheng Caoa, Juan Lia, Jun Yeb ⁎

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a

Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China b Hunan Qing Zhi Yuan Environmental Protection Technology Co., Ltd, Changsha 410082, PR China

HIGHLIGHTS

GRAPHICAL ABSTRACT

water-stable and negatively • Highly charged UiO-66 TFC membrane was constructed.

TFC membrane showed high per• The meability and selectivity for dye and antibiotic.

property of UiO-66 TFC • Antifouling membrane presented FRR values higher than 92%.

exclusion and electrostatic inter• Size action of UiO-66 TFC membrane were elaborated.

ARTICLE INFO

ABSTRACT

Keywords: Metal-organic frameworks UiO-66 Thin-film composite membrane Nanofiltration Dye and antibiotic

Metal-organic frameworks (MOFs) materials show great potential in wastewater separation applications. In this study, zirconium-based MOFs (UiO-66) were prepared and constructed universal, high-performance, and flexible nanofiltration (NF) membrane for various dyes and antibiotics separation. The substrate was constructed by twosteps, a) doping graphene oxide (GO) sheets into the membrane casting solution containing polyacrylonitrile (PAN) and forming 2D-3D connecting pores by phase immersion conversion method (defined as GO@PAN), b) immersing the GO@PAN substrate into dopamine solution to self-polymerize into chain macromolecules, which obtaining good compatible and flexible substrate (defined as PGP). Moreover, the UiO-66 was synthesized via solvothermal method and subsequently coated onto the PGP substrate by vacuum-assisted filtration. Benefiting from the water stability, porous structure, and negative charge of UiO-66, the outstanding filtration performance of the UiO-66/PGP thin-film composite (TFC) membrane was achieved. The pure water permeability of the UiO66/PGP TFC membrane was 31.33 ± 0.75 L m−2h−1 bar−1, and the rejection rates for dye (Congo red, Methyl orange, Rhodamine B, and Methylene blue) and antibiotic (Tetracycline hydrochloride, Oxytetracycline, and Ciprofloxacin) were above 94%. Therefore, we further explored the retention mechanism of the UiO-66/PGP TFC membrane for dye and antibiotic separation (size exclusion and electrostatic interaction). More importantly, antifouling properties and long-time stability of the UiO-66/PGP TFC membrane were also evaluated. These results indicated that the prepared UiO-66/PGP TFC membrane is promising for wastewater treatment, separation, and purification in many industrial and pharmaceutical fields.

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Corresponding authors. E-mail addresses: [email protected] (J.-L. Gong), [email protected] (L. Tang), [email protected] (G.-M. Zeng).

https://doi.org/10.1016/j.cej.2019.123400 Received 21 August 2019; Received in revised form 31 October 2019; Accepted 4 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Si-Yuan Fang, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123400

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1. Introduction

UiO-66 had superior hydrothermal and water-stable stability [35]. Herein, highly water-stable UiO-66 selective layer was vacuum-assisted filtrated on a polydopamine (PDA) modified graphene oxides (GO)-polyacrylonitrile (PAN) flexible porous support layer for dye and antibiotic separation. As mentioned in the previous researches [36], GO was a kind nanofiller to enhance polymeric substrate hydrophilicity due to abundant oxygen functional groups on their basal planes and edges. And the well-constructed lamellar structure of GO could achieve controllable pore channels and robust stability [37]. Meanwhile, PDA was used to further strengthen the combination and stability between polymeric support layer and UiO-66 selective layer. This is because that PDA can be easily deposited on many polymeric membranes owing to its strong adhesion ability [38,39]. Under this configuration, water permeability and separation experiments of the UiO-66/PGP membrane were conducted. Both antifouling and stability performance of the prepared TFC membrane were evaluated. It was also attempted to explore the separation mechanism of the UiO-66/PGP membrane for dye and antibiotic.

Over the past few decades, emission of organic contaminants into the environment was seriously excessive owing to the accelerated urbanization and overgrew population [1–3]. Environmental remediation, especially water purification, attracted much attention in the whole world [4,5]. Among many organic contaminants, synthetic dyes and antibiotics are the main pollutants, possessing intrinsic bio-toxicity and recalcitrance, that may lead to detrimental effect on the aquatic environment and human health [6,7]. Compared to conventional liquid purification methods such as distillation, evaporation, adsorption, and ion exchange, membrane-based separation processes are known as promising and advanced technologies for water purification. This is due to their low energy consumption, convenient compact operation, and environmentally friendly [8–10]. Nanofiltration (NF), as the most attractive pressure-driven membrane separation technology, has been gained great concentration on organic contaminants separation [11,12]. However, commercially available NF membranes are generally adopted polymers as the dominant materials, including polyimide (PI), polyamide (PA), poly(ethyleneimine) (PEI), and polydimethylsiloxane (PDMS) [13]. And these materials show some inevitable shortcomings, especially the barrier between high permeability and good selectivity, which limits NF membrane’s large-scale application [10,13]. Therefore, tremendous efforts have been devoted to developing advanced membranes to improve the NF separation performance [14–16]. For example, Bai et al. integrated cellulose nanocrystals into the polyamide layer to achieve high rejection efficiency for divalent and monovalent ions [8]. Shao and his group fabricated composite membranes using interfacial polymerization technique, which exhibited over 90% retention ratios for safranin o and aniline blue [14]. Moreover, Chen and co-workers successfully constructed ultrathin polyamide membranes with additional passageways to recycle and remove organic solutes [11]. Based on these researches, thin-film composite (TFC) membrane comprised of an ultrathin selective layer is the most popular and energy efficient membrane in the field of NF [17,18]. This is because that their selective layer and support layer can be independently designed and optimized to meet the requirements for target separation [17,19]. Although water permeability and solute rejection of conventional TFC membrane have gradually improved, their low hydrophilicity, fouling-prone, as well as high cost hampered their large-scale applications [20]. Hence, the development of advanced TFC membrane with effective anti-fouling and high stability is highly desirable for direct treatment synthetic dyes and antibiotics in wastewater. Metal-organic frameworks (MOFs), as a new generation porous crystalline material consisted of organic linkers and inorganic metal or clusters, have been driven intensive interest on environmental remediation, including adsorption [21,22], degradation [23], catalysis [24], and membrane separation [25–27]. Since their intrinsic well-defined porous structure, distinct chemical versatility, and tailored functionality, MOFs are considered as an ideal candidate for advanced TFC membrane preparation [10,13,25,27]. For instance, Liu et al. constructed ultrathin MOFs nanosheets as gutter layer in high efficiency, flexible thin-film composite membranes (TFCMs) for CO2 separation [28]. Zhang and co-workers prepared ultrathin reduced graphene oxide/MOF composite membrane for dye and heavy metal ions separation [29]. And precious metal recovery also realized through fibrous MOFs membranes synthesized by Liu’s group [30]. Among these MOFs, UiO-66 or its derivatives have been verified with superior performance serving as the molecular sieve for selective permeation [31,32]. It stemmed from the fact that UiO-66 has tailored nanometer pore sizes that their aperture size (~6.0 Å) much bigger than water molecules (~2.8 Å), accelerating water transportation efficiency [33]. Besides, high pore volume, surface areas, and functional groups of UiO66 increased defects and active sites during the synthetic process, promoting organic contaminants adsorption [34]. Last, the existence of the highest coordination between zirconium atom and carboxyl linkers,

2. Materials and experiments 2.1. Materials and chemicals All reagents and chemicals were commercially available and used as received. Zirconium tetrachloride (ZrCl4), terephthalate (H2BDC), and anhydrous copper sulfate (CuSO4) were purchased from Shanghai Yien Chemical Reagent Co., Ltd. (China) Polyacrylonitrile (PAN, MW = 85000), polyethylene glycol (PEG, MW = 800), and dopamine hydrochloride (DA) were provided by Shanghai Macklin Biochemical Co., Ltd. (China) N, N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), sodium dodecyl sulfate (SDS), hydrochloric acid (HCl), methyl alcohol (MeOH), and graphite powder were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Rhodamine B (RB), methyl orange (MO), methylene blue (MB), and congo red (CR) were supplied by Kermel Chemical Reagent Co., Ltd. (Tianjin, China) Tetracycline hydrochloride (TC), oxytetracycline (OTC), ciprofloxacin (CIP), and sulfamethoxazole (SMX) were purchased from Aladdin Chemical Co., Ltd. (China) Deionized (DI, 18.2 MΩ·cm−1, Millipore) water was used throughout the whole experiment. 2.2. Synthesis of UiO-66/PGP TFC membrane The process employed to create UiO-66/PGP TFC membrane in this study was presented in Fig. 1. Synthesis of UiO-66 nanoparticles: ZrCl4 (1.250 g, 5 mmol) and H2BDC (0.831 g, 5 mmol) were dissolved in the solvent mixtures of DMF/HCl (150 mL/10 mL) in polytetrafluoroethylene reactors and heated at 120 °C for 24 h. The resultant precipitates were collected and washed three times with fresh DMF and MeOH. Then, the resultant solids were activated at 90℃ for 1 h and vacuum-dried at 60 °C for further characterization and experiments [40,41]. Synthesis of GO@PAN (GP) substrate: The GO@PAN (GP) substrate was fabricated by phase inversion method as reported with slightly changed [12]. Specifically, graphene oxide (GO) powders were firstly prepared by Hummers’ method [36] and dispersed a certain amount in NMP solvent to form GO suspension. Then, PAN (4.5 g, 18 wt%) and PEG (0.5 g, 2.0 wt%) were added into the GO suspension, mechanically stirred under 75 °C for 4 h and maintained under 50 °C to degassed for 4 h. Finally, the casting solution was allowed to cool down to room temperature and stood overnight [42].As compared, PAN substrate was fabricated with similar steps without GO sheets. Prior knife-casting, the casting solution was first poured onto a glass plate and knife-casted into a thin membrane, using a 150 μm thick wiper rod and immersed in 25 °C DI water for phase inversion to form the GP or PAN substrate. Next, the GP or PAN substrate was immersed in fresh DI water for 24 h before using (Changed the fresh DI water every 6 h to prevent bacterial 2

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Fig. 1. The process employed to create the UiO-66/PGP TFC NF membrane in this study.

growth). Synthesis of UiO-66/PGP TFC membrane: Firstly, the GP substrate was modified with polydopamine (PDA) to enhance its surface properties [43,44]. Briefly, 0.08 g CuSO4 and 0.067 g H2O2were mixed in 100 mL Tris (50 mmol, pH = 8.5) buffer solution, with added 0.2 g DA after stirring evenly. Secondly, the GP substrate which had been previously immersed in ethanol overnight was quickly immersed in the above solution for a while (the dopamine was oxidized and self-polymerized to form PDA). After the reaction had been completed, membrane surface was washed with DI water to remove excess deposits, placed in a surface culture dish, and dried under vacuum at 30 °C for 4 h (denoted as PGP substrate). Thirdly, UiO-66 (0.01 g) and SDS (0.1 g) were added to DI water (100 mL) and then stirred for 10 h, followed by 1 h centrifuged at 10000 rpm to remove undispersed UiO-66. Finally, the UiO-66/PGP TFC membrane were prepared by vacuum filtration of a certain amount of UiO-66 dispersion onto the PGP membrane (effective surface area: 12.56 cm2) and immersed in DI water below 4 °C for further using. The UiO-66 loading ratio was achieved through the synthesis conditions and calculation according to the following Eq.,

calculated from nitrogen adsorption isotherms using an HK model. Xray diffraction (XRD) was used to confirm the crystalline structure of UiO-66 and compared to standard data from CCDC database. XRD patterns were recorded using a Bruker D8 Discover with Cu Kα radiation under 40 kV operation voltages from 5° to 80°. Additionally, to identify the chemical composition and properties of the membrane, X-ray photoelectron spectrometer (XPS) was characterized by Kratos XSAM800, which was equipped with a twin crystal mono-chromated Al Kα X-ray source. Fourier transform infrared (FTIR) spectrum was obtained using a Thermo ESCALAB 250XI at room temperature. The spectrum was scanned from 400 to 4000 cm−1 with a resolution of 4 cm−1 under the KBr disk as the background. 2.4. Membrane permeability and separation All permeation and separation tests were implemented in a laboratory cross-flow filtration system, with an effective surface area of 12.56 cm2 at room temperature. First, the fabricated membrane was pre-pressurized with DI water at 0.6 MPa for 30 min to reach a steady state (Fig. S1). After compacting, the permeance and rejection tests of the membrane were tested under 0.3 MPa for 1 h. During separation experiments, the concentration of dye (MO, MB, CR, and RB) and antibiotic (TC, OTC, CIP, and SMX) was 0.01 g/L with a fixed cross-flow rate of 10 L/h. Moreover, the permeate solution was collected every 10 min and calculated using the following Eq.,

w = m/A where w is the loading ratio of UiO-66 MOFs (μg/m2), m is the addition content of UiO-66 MOFs (g), and A is the effective surface area (12.56 cm2). 2.3. Membrane characterization

Jflux =

Field emission scanning electron microscopy (FESEM) was performed to visualize the structure of membrane surface and cross section. The membrane surface topography was also evaluated using atomic force microscope (AFM). The surface roughness parameters of each sample were determined from average values of three measurements. Contact angle (CA) measuring system (G10 KRUSS, Germany) equipped with an image processing software was used to access the surface hydrophilicity of membranes. In each measurement, an approximate 2 μL DI water droplet was dispensed onto a dry, flat membrane surface and the contact angle was obtained after 30 s of stabilization (Each CA value was a mean of three measurements at different sites of the same sample). The surface potentials (Zetasizer Nano ZS90) were measured using a 1 mM KCl electrolyte solution in a pH range of 1–11 at 25 °C. Nitrogen adsorption (77 K) isotherms were tested by an accelerated surface area and porosimetry system (ASAP 2020MC, Micromeritics Co., Ltd.). Samples were activated at 120 °C under vacuum for 6 h prior to analysis. The nominal pore size distribution was

Q A× t

where Jflux (L m−2h−1 bar−1) represents the flux of permeate water or solution, Q (L) is the volume of permeate water or solution, A (m2) is the effective area of tested membrane (12.56 cm2) and t (h) is the permeation time. The concentration of dye or antibiotic in feed and permeate solution was measured using an ultraviolet spectrophotometer (UV-2550, Shimadzu, Japan). The maximum absorption wavelengths of different organic contaminants were 462 nm (MO), 498 nm (CR), 552 nm (RB), 660 nm (MB), 257 nm (SMX), 264 nm (OTC), 268 nm (CIP), and 357 nm (TC). Rejection ratio (R) was calculated using the following equation:

R(%) = 1

Cp Cf

× 100

where Cp (mg/mL) is the permeate concentration, and Cf (mg/mL) is the feed concentration. All results were conducted at least three 3

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Fig. 2. The FESEM images for materials and membranes. (a) GO sheets, (b) PDA nanoparticles, (c) UiO-66 MOFs, (d) the size distribution histogram for the synthesized UiO-66 MOFs. A series of planar graphs of (e) PAN substrate, (f) GP substrate, (g) PGP substrate, and (h) the UiO-66/PGP TFC membrane. The third row was the cross-sectional FESEM image of (i) PAN substrate, (j) GP substrate, (k) PGP substrate, and (l) the UiO-66/PGP TFC membrane.

independently experiments.

Rr (%) =

2.5. Membrane antifouling property

Rir (%) =

To investigate the antifouling property of the UiO-66/PGP TFC membrane, a cross-flow nanofiltration system with an effective surface area of 12.56 cm2 was employed. The tested membrane was initially pre-pressurized with DI water at 0.6 MPa for 30 min and then operated under 0.3 MPa during the whole procedure. Since macromolecular organic and biological substance were tended to be absorbed and formed a cake layer on the membrane surface, HA (humic acid) and BSA (bovine serum albumin) were selected to simulate organic and biological foulants to evaluate the membrane backwashing capacity and antifouling efficiency [8,45]. Moreover, in order to quickly test backwashing capacity and antifouling performance of the UiO-66/PGP TFC membrane within an effective time frame, the concentration of HA and BSA was 1.0 g/L in the antifouling experiment [12,16]. First, the membrane was subjected to DI water permeation for 1 h and collected permeate pure water every 10 min to calculate the pure water flux (Jw1). Then, HA or BSA solution with a concentration of 1.0 g/L was selected as feed solution and filtered another 1 h to obtain the flux of HA or BSA solution (Jf). After finished filtration, the fouled membrane had been cleaned with DI water for 30 min before measuring the water flux of cleaned membrane (Jw2). The flux recovery ratio (FRR) was calculated using the following Eq.:

FRR (%) =

Jw2

Jf

Jw1 Jw1

Rt (%) = 1

× 100

Jw2 Jw1 Jf Jw1

× 100

× 100 = Rr + Rir

2.6. Membrane stability performance One of the major issues that TFC membrane was applied in practical application is leaching out the incorporated nanoparticles from the membrane due to low adhesion and compatibility between selective layer and support layer. Therefore, stability of UiO-66/PGP TFC membrane was investigated by continuously dynamical cross-flow filtration system. For long-time stability experiment, the UiO-66/PGP TFC membrane was pre-pressurized with DI water at 0.6 MPa for 30 min, and then the membrane was born to DI water permeation at 0.3 MPa in uninterruptedly operating during 1000 min. In addition, we also tested the capability of adapting the UiO-66/PGP TFC membrane to separate MO and TC after 1000 min operation. The feed solution contained 0.01 g/L MO or TC solution and the experiment was conducted in the same conditions. The permeability and rejection ratios for MO and TC were calculated using the above equations. All data were obtained from three independent experiments. 3. Results and discussion

Jw2 × 100 Jw1

3.1. Characterization

To analyze the antifouling resistance performance in detail, several parameters including reversible fouling ratio (Rr), irreversible fouling ratio (Rir), and total fouling ratio (Rt) were calculated using the following equations:

All the characterizations were performed on partial materials (GO, PDA, and UiO-66), PAN substrate, GP substrate, PGP substrate, and the UiO-66/PGP TFC membrane. 4

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3.1.1. FESEM and BET The morphologies of materials and membranes were studied by FESEM in Fig. 2. According to FESEM images, the synthesized GO sheets demonstrated folding lamellar structure, and the self-polymerized PDA represented irregular agglomerating particles with microsize. The as-synthesized UiO-66 MOFs appeared tetrahedron crystals with particle size around in 200 nm, corroborating good crystallized solid mode by solvothermal method (Fig. 2a–c). Meanwhile, particle size distribution of UiO-66 MOFs was shown in Fig. 2d and indicated UiO-66 MOFs particle sizes ranged from 190 to 280 nm. The average size of UiO-66 MOFs is 205 nm, which was consistent with the FESEM analysis. On the other hand, the morphological structures of the UiO66/PGP TFC membrane were also investigated. Fig. 2e–h shows the top surfaces FESEM images for PAN, GP, PGP substrates and the UiO-66/ PGP TFC membrane. Compared with pristine PAN substrate with few voids, GP substrate presented stripe folds and large pores. The reason was that GO was a two-dimensional sheet-like material. When folded or continuous topography of GO had been synthesized in individual sheets, the kinked and wrinkled areas would appear at the edges [7]. Besides, when PDA deposited on GP substrate, a few particles randomly scattered and the support layer surface also maintained smooth. This was due to the CuSO4/H2O2 oxidants can accelerate polymerization efficiency of dopamine [43,44]. After UiO-66 MOFs had been filtered on PGP substrate, more small nodular structures emerged, leading to rougher surfaces than those of other substrates. In order to further confirmed the successful preparation of the UiO-66/PGP TFC membrane directly, we explored a series of cross-sectional FESEM images for substrates and the TFC membranes. Fig. 2i–k demonstrate that the morphology of substrates presented intuitive changes with addition of GO sheets and formation of PDA. In the GP substrate, there were many sheets in the original holes, resulting in a reduction of membrane channels. The reason was that the insertion of GO sheets with 2D planar structure. However, the PDA nanoparticles formed by self-polymerization in GP substrate did not affect the pores of the membrane. The crosssection of the UiO-66/PGP TFC membrane reveals a typical composite structure. The UiO-66 selective layer constructed a thickness of 1.56 μm and uniformly stacked in preferred orientation (Fig. 2l), which could maintain membrane’s integrity and stability under relatively high pressure [28]. The porosity and specific surface areas of all membranes were determined by nitrogen adsorption-desorption isotherms and summarized in Table S1. In general, the UiO-66/PGP TFC membrane had smaller pore sizes and larger surface areas than that of those substrates. It was because UiO-66 MOFs possessed a cubic rigid three-dimensional pores system with regular and orderly pore structure [46]. The BET surface area of the UiO-66/PGP TFC membrane was 823.17 m2/g, and the mean pore size was 2.88 nm. Thus, the prepared UiO-66/PGP TFC membrane had an ideal porous structure, whose nanopores or nanochannels might be played a significant role in exclusion of organic molecules.

Fig. 3. The XRD pattern of the UiO-66/PGP TFC membrane.

S2a) [7]. Moreover, the XRD patterns of GP substrate and PGP substrate only showed an identical peak at 2θ = 9.66° with a broad peak appeared around 2θ = ~17.0° (Fig. S2c), which was stemmed from the combination of amorphous PAN [48]. In addition, the TGA curves of UiO-66 MOFs in a nitrogen atmosphere also revealed the thermal stability and composition of UiO-66 MOFs, which yielded two distinct mass-loss steps of degradation behavior (Fig. S2d). Firstly, a progressive structured weight loss up to 300 °C, which was attributed to the absorbed water and DMF guest molecules. Secondly, in the temperature range of 450 ~ 700 °C, there is a sharp decline occurred (weight loss 44.1%) and the sample is left with the 42.1% of the starting weight. The reason was that the organic linkers of UiO-66 MOFs was decomposed, which is in accordance with previous reported articles [33,47,49]. Therefore, the synthesized UiO-66 MOFs had completely crystalline structure and thermal stability. The utilization of UiO-66 MOFs as selective layer was beneficial to the integrity and stability of the UiO-66/ PGP TFC membrane. 3.1.3. FTIR The FTIR spectra of materials (PAN, GO, PDA, and UiO-66 MOFs) and membranes (PAN, GP, PGP, and the UiO-66/PGP TFC membrane) were measured and shown in Fig. 4. These patterns present good agreement with that previously reported [29,33,42,50]. From the spectrum of PAN (Fig. 4a), it could be observed some major adsorption peaks at 1072, 1454, 1635, and 2244 cm−1, respectively, assigning to the functional groups of NeN, CeH, C]N, and C^N in PAN. The substrong peaks appear at 835 and 2940 cm−1 were corresponded to the wagging and stretching vibration of hydrogen bond, and the peak at 3242 cm-1is associated with NeH functional groups [42]. For the GO sheets (Fig. 4c), the broadband at wavelengths 3180, the typical double peaks at 1701 and 1618 cm−1 were associated with characteristic vibrations of OeH, C]O and C]C bonds, respectively [29,47]. The two adsorption peaks appeared at 1238 and 1051 cm−1 were assigned to the functional groups of CeO, and OeCeO in carboxyl, respectively [7,29]. As shown in Fig. 4b, a broad peak at 3242 cm−1 was related to eNHe and eOH groups in PDA, and a significant peak at 1480 cm−1 was attributed to the C]C in aromatic ring [51]. Since dopamine produced a series of by-products during the self-polymerization process such as dopaminequinone, leukodopaminechrome, and 5,6-dihydroxyindole, the two peaks in the wavelength of 573 and 822 cm−1 were attributed to eNHe deformation vibration and ]CH2 wagging vibration [52]. Besides, the characteristic peaks of UiO-66 were mainly presented in wavelengths ranging from 700 to 1600 cm−1. Considering carboxylterminated terephthalic acids as organic ligand of UiO-66, there were different peaks derived from carboxylic and aromatic groups in the

3.1.2. XRD and TGA To further prove the integrity and crystalline of the UiO-66/PGP TFC membrane, the XRD results were exhibited in Figs. 3 and S2. As can be seen in Fig. 3, three obvious characteristic peaks were exhibited at 2θ = 7.42°, 8.53° and 9.64°, with relative two weak peaks at 12.5° and 26.1°. Which were consistent with the XRD patterns of UiO-66 and GO (Fig. S2a-b). The two peaks at 7.42° and 8.53° represented the crystal planes (1 1 1) and (2 0 0) of the face-centered cubic crystal of UiO-66 [2,47], and they were retained in the UiO-66/PGP TFC membrane. It demonstrated that the vacuum-assisted filtration method would not disrupt the crystallinity of UiO-66 MOFs. A sharply single peak at 9.64° was the diffraction peak of GO, representing the (0 0 2) crystal plane and an interlayer spacing of 9.1 Å. It was ascribed to large amounts of oxygen containing functional groups formed on the graphitic planes of GO [3,37]. Whereas an unobvious peak at 19.42° represented the partially oxidized graphite oxide sheets during exfoliation process (Fig. 5

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Fig. 4. FTIR spectra of materials (a) PAN, (b) PDA, (c) GO, and (d) UiO-66 MOFs. The FTIR results of membranes also showed in (e) PAN, GP, PGP substrates and (f) the UiO-66/PGP TFC membrane (the insert figure represents the partial enlargement of wavenumbers between 700 and 1800 cm−1).

FTIR spectrum: 1398 and 1578 cm−1 for symmetric and asymmetric stretching vibration of CeO in BDC ligand, respectively [12]. As organic ligand possessed primary aromatic groups connecting two symmetrical carboxyl groups, the peaks at wavelengths of 1508 cm−1 was assigned to the aromatic ring stretching vibration [16]. Additionally, the characteristic peaks at 1016, 1157, and 3400 cm−1 were corresponded to CeO, CeOeC, and OeH [47]. The peaks at 746 cm−1 were represented the ZreO bond (Fig. 4d) [41]. Moreover, compared FTIR results of materials with PAN, GP, PGP substrates and the UiO-66/PGP TFC membrane, the functional groups NeN, CeN, C]N and C^N derived from PAN substrate were completely retained in the PGP substrate. And newly emerging peaks at 1051, 1238, and 1736 cm−1 were appeared, which were attributed to the GO containing OeCeO, CeO, and C]O bonds. Compared with the FTIR spectrum of GO, the C]O bond in PGP substrate appeared red shift from 1701 to 1736 cm−1, it was probably ascribed to the dopamine converted phenolic hydroxyl groups into carbonyl groups during self-polymerization and produced conjugation effect, resulting in electron cloud density averaging (Fig. 4e). The vibration frequency was reduced and the wavelength was increased [52]. Besides, the FTIR absorbance of the UiO-66/PGP TFC membrane was mainly identified a number of contributing peaks between 700 and 1800 cm−1 (Fig. 4f). Intense double peaks at 1578 and 1398 cm−1 were assigned to the asymmetric and symmetric stretching vibration of carboxylate groups in UiO-66. The sub-major adsorption peaks at wavelengths of 1508, 1157, 1016, and 746 cm−1 were assigned to the function groups of C]C (benzene ring), CeOeC, CeO, and ZreO in UiO-66, respectively [33,41].

286.37 eV ascribed to CeO or CeN bonds, 3) the peak at 288.64 eV was attributed to OeC]O and NeC]O bonds, and 4) the peak at 292.17 eV corresponded to π-bond in benzene ring [4,39]. The N 1s core-level spectra can also be curve-fitted with four peak components. The peak at 398.96 eV might be assigned to CeN group in eCOeNHe, while the peak at 399.68 eV was assigned to NeC]C group. The peak at 400.86 eV was likely corresponded to NeC]O group and the last peak at 401.67 eV was assigned to the amino groups in the ammonium form (NH3+) [53]. Otherwise, the 3d3/2 and 3d5/2 component peaks for Zr (185.25 and 182.86 eV) emerged after the intercalation of UiO-66 as the selective layer in the UiO-66/PGP TFC membrane [39,41]. Additionally, to better understand and identify the GO and PDA incorporation, we also carried out the peak fitting maps of all substrates (PAN, GP, and PGP) in Fig. S3. As shown in Fig. S3(a, c, e), C 1s XPS spectrum exhibited intensive peaks of CeC/CeH (284.06 eV), CeN (285.11 eV), and C^N (287.31 eV), which was resulting from the nitrile group existed. After GO and PDA incorporation, three new characteristic peaks appeared at 283.98 eV (CeOeC), 285.37 eV (CeC]O), and 287.21 eV (OeC]O). And the deconvolution spectra of N 1s also verified the presence of PDA in PGP substrate again. This result was consistent with the XPS analysis of the UiO-66/PGP TFC membrane and testified the GO and PDA had been successfully incorporated into the original PAN substrate. 3.1.5. AFM The AFM topographic images of three substrates, the UiO-66/PGP TFC membrane and their calculated surface roughness parameters including the average roughness (Ra) and the root mean square of the z date (Rq) were shown in Fig. 6 and Table 1. The height information of the membrane surface is inflected by the variation of bright spots in these figures (scan area of 5 μm × 5 μm), and the brighter region represents the higher membrane surface. It was observed that the large peaks and valleys in the PAN substrate surface were substituted by many small ones in the GP substrates, leading to an obvious decrease in the surface roughness from 11.9 to 10.1 nm. When the substrates had been immersed in DA solution to form PDA, the surface with smaller pores become dense, and the roughness parameter was further reduced

3.1.4. XPS A more detailed study of the membrane chemical bonds and elemental compositions was obtained via conducting XPS measurement. Fig. 5 depicts the XPS survey spectra of substrates and the UiO-66/PGP TFC membrane. Fig. 5(b–d) are peak fitting maps of C (285 eV), N (399 eV), and Zr (184 eV) in the UiO-66/PGP TFC membrane in sequence. For the C 1s case, four peaks were obtained including: 1) the peak at 284.66 eV corresponding to CeC or CeH bonds, 2) the peak at 6

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Fig. 5. Comparison of XPS survey of (a) PAN, GP, PGP substrates and the UiO-66/PGP TFC membrane. XPS peak fitting images of the UiO-66/PGP TFC membrane in (b) C 1s, (c) N 1s, and (d) Zr 4d.

Fig. 6. Surface AFM figures of different substrates: (a) PAN substrate, (b) GP substrate, (c) PGP substrate, and (d) the UiO-66/PGP TFC membrane. 7

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reason might be that GO had outstanding mechanical properties and abundant interlaced areas at the edges, which could stand higher pressure [56]. However, the simple GP support layer had a low interception effect to MO and TC, and the weak binding force with UiO-66 promoted further modification to improve compatibility and integrity of UiO-66/ PGP TFC membrane. Dopamine is a class of protein-inspired molecules with excellent biocompatibility and adhesion. It can be spontaneously oxidized and self-polymerized in alkalescent environment due to the existence of amine groups and catechol groups [6,57,58]. The asformed polydopamine (PDA) coatings could be conglutinated on a number of substrates and their remaining catechol groups could be further modified by Michael addition or Schiff base reactions [52]. Hence, we further modified PDA coatings on GP substrate to improve membrane surface hydrophilicity and enhance the binding force among GP substrate and UiO-66 selective layer. The time for dopamine to polymerize is known to be key to the formation of PDA coatings. As shown in Fig. S4c, the unmodified GP substrate showed a water flux of 78.51 L m−2h−1 bar−1 and MO or TC rejection less than 50% (Fig. S5c), suggesting its larger pore sizes. It was noted that the water flux had been increased slightly during the first 10 min. This was because the rapid polymerization process of PDA presumably consumed a large number of active groups on the GP substrate, changing the conformation of surface voids [9]. Moreover, a sharply margin of water flux was presented down to 12.53 L m−2h−1 bar−1 after 10 min. The reason was that CuSO4/H2O2 produced many reactive oxygen species, which initiated dopamine polymerization and accelerated deposition rate, resulting in a highly uniform and dense PDA layer [43,44,54]. However, due to the H2O2 as an intermediate product of dopamine polymerization, too higher concentration of H2O2 could reverse the catechol groups in PDA, which were deprotonated and oxidized to quinone, leading to a loose surface layer that possessed lower mass transfer resistance [44]. Therefore, the water flux gradually increased to 41.65 L m−2h−1 bar−1 along with deposition time prolonged. In addition, we also explored rejection rates of PGP substrate for MO and TC under different deposition time. As shown in Fig. 5c and d, the permeate flux presented the same with a little floating before deposition time reached 40 min. The possible reason was that the shorter selfpolymerization time was not enough to form a dense PDA layer [57]. While the rejection rates for MO and TC presented a continuing upward trend and reached the highest value at 40 min. It was proposed that the PDA layer generated many active sites cross-linked with MO and TC due to abundant catecholamine in PDA [6]. Moreover, with the self-polymerization time prolonged, the rejection rate of TC decreased dramatically as the permeate flux increased gradually from 22 L m−2h−1 bar−1 to 42 L m−2h−1 bar−1. The results could be rationalized as due to smaller amounts of PDA aggregated and affected the resultant PDA layer structure became looser. Thus, more TC molecules passed through the membrane [57]. In contrast, the retention rate of MO was only decreased slightly (85.22%) and maintained 95.40% with that of 40 min. It was related to the reason that intrinsic cross-linked and stacked PDA structure could provide a sieve effect for dyes separation to some extent and gave rise to an increased MO rejection [52]. Thus, deposition time of 40 min was chose for further experiment. On the other hand, the strong interfacial binding of UiO-66 could be directly translated to excellent mechanical properties of a corresponding composite thin film. And the thickness of selective layer played a crucial role in the retention of UiO-66/PGP TFC membrane. Therefore, we further evaluated the effects of UiO-66 loading ratio on the membrane permeation and retention efficiency (Fig. S4d). It was a remarkable fact that the water flux declined almost linearly with the UiO-66 loading ratio up to 0.08 μg/m2, since MOF films took homogeneous shape and closed the path for the water molecules [32]. Subsequently, the loading increment from 0.08 to 0.12 μg/m2 led to only a slight decrease in water flux due to the formation of compact film reached steady and had little effect on the transport of water molecules.

Table 1 Surface Roughness Parameters of membranes. membranes

PAN GP PGP UiO-66/PGP

Roughness parameters Ra (nm)

Rq (nm)

Rz (nm)

11.9 10.1 9.08 15.7

16.3 14.3 11.4 23.4

11.4 12.2 11.7 40.7

to 9.08 nm. The decreased surface roughness would reduce the contact areas between the contaminated and membrane surface, which is favorable to improve antifouling properties of the membrane. With the combination of UiO-66 on PGP substrate, the surface roughness grew gradually due to the crystal structure and accumulation of UiO-66 in the skin layer of the membrane. Noticeably, when the loading mass of UiO-66 had reached 0.08 μg/cm2, the average surface roughness was suitable for contaminants separation in this work. This is because UiO66 had strong electro-negativity, the electrostatic repulsion between contaminants and thin selective layer does not form a cake layer on the top surface of the membrane, which can be verified by contact angle and subsequent experimental data. To sum up, these results testify that the successfully prepared UiO-66/PGP TFC membrane possessed effective nanoscale pore size and abundant oxygen-containing functional groups. Besides, these characteristic structure and property of the UiO66/PGP TFC membrane might provide effective application in smaller organic molecules separation. 3.2. Optimization performance Experimental conditions were investigated including doping amount of GO, deposition time of DA, loading ratio of UiO-66 MOFs and operating pressure on the performance of the UiO-66/PGP TFC membrane. The pure water permeability experiment was conducted firstly, and then a series of separation experiments were also carried out for MO and TC retention with a concentration of 0.01 g/L. The water flux and pollutants rejection of the membrane were shown in Figs. S4 and S5. As shown in Fig. S4a, the PAN substrate possessed the highest water flux and the lowest pollutants rejection due to its ultrafiltration nature. With an increase of GO doping amount from 0.0 to 0.4 wt%, the water flux was sharply decreased from 487.4 to 87.8 L m−2h−1 bar−1 while the rejection rates increased from 21.44 to 42.46% for MO, and 25.44 to 48.95% for TC, respectively (Fig. S5(a, b)). The larger decrease of water flux was explained that the two-dimensional laminar structure of the GO increased tortuosity of flow paths for water molecules and its pleated surface could effectively reduce the pore size of the pristine PAN substrate [7,12]. In addition, the increased charge density barrier effect induced by GO may play an important role in slightly improving the contaminants retention [54]. Also, the addition of GO into the PAN substrate could increase negatively charge on the membrane surface due to the large amount of oxygen-containing functional groups at the edges of GO. That resulted in relatively high rejection of MO and TC owing to the charge repulsion and size exclusion effects [12,16,55]. Nevertheless, the water flux was kept stable at the GO doping amount increases to 0.5 wt%. It was because that GO sheets aggregated partially in the porous layer and blocked the pathways, leading to the resistance increase for water molecules passed through [51,56]. At the same time, we tested the different operation pressure on the performance of PAN and GP substrates (Fig. S4b). It can be observed that the GP substrate induced a much lower water flux in comparison with that of the pure PAN substrate. Meanwhile, compared with the continuously decrease of water flux for PAN substrate, the water flux of GP substrate tended to be stable and reached to 90 L m−2h−1 bar−1 when the pressure reaches 0.3 MPa. This result might be the higher pressure deformed finger-like structure of PAN from the vertical direction. For GP substrate, the 8

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Meanwhile, the increased loading ratio of UiO-66 enhanced membrane thickness with smaller pore sizes, thus causing lower water flux [11]. However, further lifting the loading ratio of UiO-66 to 0.20 μg/m2, a relatively thicker skin layer maybe generated and the partial agglomeration of UiO-66 on the dense membrane surface may create large defects [16]. Theoretically speaking, the configuration energy of PAN and UiO-66 was much lower (−46 kJ/mol), which suggested the much weaker π-π interactions between PAN monomer and UiO-66. Therefore, few of π–π configurations with low configuration energy were established in UiO-66/PGP TFC membrane, which generated loose combinations of PAN and UiO-66 nanoparticles, leading to the low resistance of UiO-66/PGP TFC membrane [30]. Then the water flux rose up to 31.28 L m−2h−1 bar−1. We also found the MO and TC interception results presented an initial rising and a subsequent declining with loading ratio of UiO-66 increased (Fig. S5(e, f)). It was worth noting that both the molecular weight and molecular diameter of TC were greater than that of MO, but its rejection was less than the MO rejection rate, and a significant decline after the loading ratio of UiO-66 increased from 0.08 to 0.20 μg/m2. This might be the reason of TC can be ionized as a zwitterion in aqueous solution [59], leading to their weaker electronegativity than that of MO. Moreover, π-π interaction between BDC linker of UiO-66 and aromatic structure in TC was dominated in pH values ranging from 4 to 10, which generated competitivity with electrostatic repulsion, causing the rejection rate of TC dramatically decreased [24]. In comparison under the same condition, the rejection rate of dye that used MO as model contaminant was better than that used of TC. This can be explained by two reasons. First, the molecular weight of MO (327.33 g/mol) is less than that of TC (480.9 g/mol). The compact and homogeneous membrane surface presented higher resistance to MO while blocking TC molecules. Second, the results of FTIR spectra of UiO-66/PGP TFC membrane suggested that acidic functional groups (carboxyl) had been enhanced, which could induce negatively charged membrane surface, causing the high rejection rate between negative dye (MO) and negative surface [16]. Hereafter, we choose loading ratio of UiO-66 was 0.08 μg/m2 to prepare UiO-66/PGP TFC membrane for further comprehensive investigation, as such a loading yielded the best rejection ratio with satisfactory permeability.

corresponding order of solution permeability was CR (37.39 ± 0.80) > MO (37.18 ± 0.90) > MB (34.92 ± 0.24) > RB (24.04 ± 0.73). It is of noteworthy that the solution permeability almost negatively correlated to the molecular diameter except for CR. This might be the reason that CR molecules had many hydrophobic functional groups. Its enhanced hydrophobic and electrostatic repulsion could facilitate the eCOOH gate opening [23]. Moreover, the molecular diameter of H2O (~2.8 Å) molecules was much smaller than the cages of UiO-66 (~8 Å and ~11 Å) [31]. These two interactions promoted the water molecules apace went through the pores of UiO-66, causing the highest permeability among these dye molecules. Additionally, the slightly reduced retention ratio between CR (99.56 ± 2.20%) and MO (94.84 ± 2.16%) was due to the relatively smaller molecular diameter of MO than CR, promoting its easier to pass through the membrane surface pores [16]. The similar phenomenon can be observed in the performance for antibiotics exclusion. For the antibiotics separation, the rejection rate order was CIP (98.55 ± 2.35%) > TC (95.50 ± 1.64%) > OTC (94.77 ± 2.31%) > SMX (83.05 ± 1.41%), which is inversely proportional to the order of antibiotics permeability (SMX (27.46 ± 0.61) > OTC (17.61 ± 0.57) > TC (17.25 ± 0.70) > CIP (16.09 ± 0.47)). Owing to the molecular size of SMX is about 0.827 nm, it was closed to the tetrahedral cavities of UiO-66 (d 0.8 nm) and smaller to the octahedral cavities (d 1.1 nm), leading to the SMX molecules easily entered the pore channels of UiO-66 and flowed into the percolate. The OTC (1.105 nm) and TC (1.098 nm) belong to the same tetracycline antibiotics and have similar molecular diameters. Thus, the permeability and rejection rate of OTC were basically the same as those of TC. Furthermore, although CIP molecule was only 0.996 nm in diameter, which was approximately less than the UiO-66 octahedral cages, CIP had the highest rejection rate. This result showed that there were other effects also affected the separation efficiency while the size exclusion was contributed to organic small molecules. Since symmetrical dicarboxylic groups were existed on the benzene ring of UiO-66, the result of ζ-potential of the UiO-66/PGP TFC membrane was presented negatively at pH 7.0 (25 °C, −44.1 mV) and endowed negatively charged membrane surface (Fig. S7) [63]. It indicated that the UiO-66/PGP TFC membrane had a stronger repulsive force for electronegative pollutants. CR was easily dissociated into organic anions (CR−) and tended to be an anion in aqueous solution, which inevitably caused strong electrostatic repulsion between CR and the UiO-66/PGP TFC membrane surface. So, CR owned a good rejection rate while maintaining the highest permeability. Conversely, MB was cationic in aqueous solution, which had the best retention ratio when the solution permeability was only 34.92 L m−2h−1 bar−1. As MB molecules have deprotonated amine group, there are weak electrostatic attraction between MB and active sites and spaces of the UiO-66/PGP TFC membrane, leading to surface pore channels blocked and water permeability declined [23]. Moreover, since RB possessed amide group and carboxyl group simultaneously, which exhibited electrical neutrality in solution and had less electrostatic repulsion on membrane surface than CR [63]. In addition, because the ionization state of most antibiotics was controlled by solution pH and acid dissociation constant (pKa) of the antibiotic, we further expounded the corresponding dissociation equilibrium and pKa values of TC, OTC, CIP, and SMX in various pH ranges (Fig. S8). As an amphoteric compound, OTC (or TC) is positively charged below pH 3.22 (3.32) and negatively charged above pH 7.46 (7.78). It exhibited a net charge of nearly neutral within pH range of 3.22–7.46 (3.32–7.78). Similarly, CIP is positively charged below pH 8.70 and neutral within the range of 8.70 < pH < 10.58, while SMX is negatively charged above pH 5.60 [53,59,64]. Thus, during this filtration process (pH 7.0), these antibiotics exhibited electrically neutral (OTC and TC), electropositive (CIP), and electronegative (SMX), respectively. In terms of individually electrostatic interaction, SMX had the highest rejection rate while CIP had the lowest one in theory. But the experimental results showcased opposite, probably owing to the size exclusion was far stronger than electrostatic interaction during antibiotic nanofiltration. Therefore, the results

3.3. Permeability and rejection efficiency As is known to all, the NF membrane performance for small organic molecules was mainly contributed to the mechanism of size exclusion and electrostatic interaction. The size exclusion was primarily related to the size of solutes and membrane pores, while electrostatic interaction was related to the charge distribution between molecules and membrane surface (Scheme 1). In this work, dye molecules of different charges and antibiotics with different sizes were used to demonstrate the separation performance of UiO-66/PGP TFC membrane, and all results were shown in Fig. 7. As a whole, the UiO-66/PGP TFC membrane exhibited higher permeability for dyes than that of antibiotics. This result is corresponded with the that of optimization experiments. It might be attributed to the following two aspects: the molecular weights of antibiotics are smaller than that of dye molecules, and the hydrolysis effect of antibiotics in water is controlled by acid dissociation constant. Therefore, we combined these experimental data to analyze the trapping mechanism of the two kinds of substances in detail. On the one hand, the UiO-66 was built up from Zr6O6(CO2)12 units interlinked via terephthalate linkers, forming a cubic rigid 3D pores system consisted of octahedral cavities of diameter 1.1 nm and tetrahedral cavities of diameter 0.8 nm, respectively [35,46,60]. Moreover, the accessibility to the cavities was ensured through microporous triangular windows of free diameter within the 0.5–0.7 nm range [34,61,62]. In addition, we calculated the molecular diameters of dyes and antibiotics using Chem3D software employing MM2 force-field parameters (Fig. S6). For the dye separation, the molecular diameter was CR (2.313 nm) > RB (1.386 nm) > MB (1.226 nm) > MO (1.184 nm) in turn. The 9

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Scheme 1. Mechanism illustration of the UiO-66/PGP TFC membrane for dye and antibiotic separation. (a) The structure of UiO-66 MOFs (Zr6 polyhedral, blue; O, red; C, gray). The yellow sphere and purple red sphere represent the types of pores inside the octahedral and tetrahedral cages, respectively. H atoms on the organic ligands were omitted for clarity. The dominating separation mechanism (b) Size exclusion and (c) Electrostatic interaction of the UiO-66/PGP TFC membrane in nanofiltration process. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Chemical structures and separation performances of the dyes and the antibiotics used in this study. (a) Dyes and (b) antibiotics permeability and rejection rates of UiO-66/PGP TFC membrane under 0.3 MPa with solution concentration of 0.01 g/L. 10

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Fig. 8. Antifouling and stability performance of UiO-66/PGP TFC membrane under 0.3 MPa. (a) Flux was plotted versus time for three periods: pure water flux for 1 h, 1.0 g/L HA/BSA solution flux for 1 h, and pure water flux after hydraulic washing for 30 min. (b) Fouling resistance ratios for HA (light gray) and BSA (dark gray). (c) Long-time permeance in pure water, MO, and TC solution. (d) The rejection rates of MO and TC under continuous 1000 min filtration (concentration: 0.01 g/L).

membrane possesses a better antifouling characteristic. Additionally, other antifouling parameters, total fouling (Rt), reversible fouling (Rr) and irreversible fouling (Rir) also exhibited in detail (Fig. 8b). From comparison of these parameters, the UiO-66/PGP TFC membrane maintained higher FRR value and lower resistance factors. These results can be verified by the water contact angle and surface roughness parameters (Fig. S9 and Table 1). The water contact angle was decreased from 62.9° to 30.7° with the modification of UiO-66 MOFs, and the surface roughness parameter was lower. Therefore, the prepared UiO-66/PGP TFC membrane has superior antifouling property that can be effectively resisted the erosion from organic and biological foulants.

suggested that size exclusion and electrostatic interaction had a synergistic effect on the membrane surface, which were affected the separation efficiency of the UiO-66/PGP TFC membrane for dyes and antibiotics. 3.4. Antifouling property Membrane fouling is one of the biggest obstacles for the large-scale application of NF membrane separation process. It generates blocking or plugging membrane pores and ultimately shortening the membrane lifespans [7,19]. A good quality membrane should possess low fouling prone over a prolonged period. Thus, exploring the antifouling property of the UiO-66/PGP TFC membrane is critical. The antifouling performance of the UiO-66/PGP TFC membrane was analyzed by measuring the pure water flux recovery ratio after the membrane was fouled with 1.0 g/L HA or BSA solution. Fig. 8 shows the pure water flux as well as the HA and BSA solution flux as a function of fouling time. As can be seen in Fig. 8a, the permeability of the UiO-66/ PGP TFC membrane went through a sharp decline when pure water was substituted with the HA or BSA solution in the filtration system. This result might be stemmed from a fouling cake layer formation. And the concentration polarization or adsorption occurred on the membrane surface or the pore walls also can result in water flux reduction [7,45]. The flux recovery ratio (FRR) and other antifouling parameters of the prepared membrane were also shown in Fig. 8b. The FRR values for the UiO-66/PGP TFC membrane were reached 95.0% and 92.2% for HA and BSA, respectively. This was indicated that the UiO-66/PGP TFC

3.5. Long-time stability The stability is a critical factor for TFC membrane during the NF application. Fig. 8c presented the evolution of permeate flux as a function of testing duration including water, MO, and TC. It could be concluded that under continuous filtration for 1000 min, the UiO-66/ PGP TFC membrane only exhibited a slight decline of permeation flux. The pure water flux dropped to 80.3% of the original value, and the permeation flux for MO and TC also showed insignificant reduction. Results indicated that the prepared UiO-66/PGP TFC membrane had good stability. In addition, the rejection efficiency of the UiO-66/PGP TFC membrane was also investigated, with MO and TC as model contaminants under continuous filtration for 1000 min. As shown in Fig. 8d. the final rejection rates of MO and TC still retained 96.0% and 94.6% of the initial values. These results can be deduced that the UiO11

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Table 2 Comparison of separation performance of our membranes with the state-of-the-art NF membranes. Membrane

Operation pressure (MPa)

Water flux (L m-2h-1 bar−1)

Pollutant

Rejection (%)

Reference

PDA/PI PAN/GO PA/PEI PA-MOF/PPSU-GO PDA/PEG UiO-66/PA/PAN UiO-66@GO/PES UiO-66-(COOH)2/prGO UiO-66/PGP

0.1 0.6 0.6 0.8 0.5 0.5 0.25 0.1 0.3

0.91 33.0 18.8 59.9 46 15.4 15.7 20.0 31.3

RB MB/AR SO/AB KHI TOB/CP/CA RB/AH MO/DR 80 CR/MB MO/MB/RB/CR TC/OTC/CIP

99.0 100/99.8 90.1/98.2 96.0 99/94/93 100/97.6 88.6/99.3 98.2/92.55 94.8/100/95.5/99.6 95.5/94.8/98.6

[52] [12] [14] [17] [65] [11] [16] [29] This work This work

Polymers: PA-polyamide, PI-polyimide, PEI-polyetherimide, PES-polythersulfone, PEG-polyethylene glycol, RC-regenerated cellulose, PPSU-polyphenylsulfone. Pollutants: AR-acid red, SO-safranin o, AB-aniline blue, TOB-tobramycin, CP-clindamycin phosphate, CA-cephalexin, AH-azithromycin, DR-direct red.

66/PGP TFC membrane possessed durability and stability, which is beneficial for membranes’ lifespans, and it may be an ideal TFC membrane for practical nanofiltration process. Table 2 summarizes the comparison of dye and antibiotic separation performance between some state-of-the-art NF membranes and the UiO66/PGP TFC membrane presented in this study. Apparently, the prepared UiO-66/PGP TFC membrane exhibited pleasing permeability and comparable rejection ratios under relatively lower pressure. Which was intensified by UiO-66 selective layer’s distinctive crystalline structure. The porous structure of UiO-66 MOFs was tremendously slashed the pressure resistance for conveying water molecules. In practical application of nanofiltration membrane, the permeability, fouling resistance, membrane lifespan and operating pressure are all important factors influenced economic costs. According to previous studies [11,12,14,17], the operating pressure applied to nanofiltration was above 0.5 MPa. This would consume great energy and generated high cost. Objectively speaking, the prepared membrane operated on lower pressure will save 20%–40% charge. Considering the cost derived from materials, labor input and other consumption, the final price of UiO-66/ PGP TFC membrane was about 147 US$/m2, which was only ~60% cost of commercial NF membrane (~239 US$/m2). Therefore, the UiO-66/ PGP TFC membrane is a promising NF membrane for separation synthetic dyes and antibiotics in wastewater treatment.

influence the work reported in this paper. Acknowledgements The authors are grateful for the financial supports from National Natural Science Foundation of China (51579095, 51579096, 51521006, 51378190 and 21675043), Hunan Province university innovation platform open fund project (14K020). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123400. References [1] P. Zhang, J.-L. Gong, G.-M. Zeng, C.-H. Deng, H.-C. Yang, H.-Y. Liu, S.-Y. Huan, Cross-linking to prepare composite graphene oxide-framework membranes with high-flux for dyes and heavy metal ions removal, Chem. Eng. J. 322 (2017) 657–666. [2] D.X. Trinh, T.P.N. Tran, T. Taniike, Fabrication of new composite membrane filled with UiO-66 nanoparticles and its application to nanofiltration, Sep. Purif. Technol. 177 (2017) 249–256. [3] P. Zhang, J.-L. Gong, G.-M. Zeng, B. Song, S. Fang, M. Zhang, H.-Y. Liu, S.-Y. Huan, P. Peng, Q.-Y. Niu, D.-B. Wang, J. Ye, Enhanced permeability of rGO/S-GO layered membranes with tunable inter-structure for effective rejection of salts and dyes, Sep. Purif. Technol. 220 (2019) 309–319. [4] M.D. Firouzjaei, A.A. Shamsabadi, S.A. Aktij, S.F. Seyedpour, M. Sharifian Gh, A. Rahimpour, M.R. Esfahani, M. Ulbricht, M. Soroush, Exploiting synergetic effects of graphene oxide and a silver-based metal-organic framework to enhance antifouling and anti-biofouling properties of thin-film nanocomposite membranes, ACS Appl. Mater. Interfaces 10 (2018) 42967–42978. [5] S. Abdi, M. Nasiri, Enhanced hydrophilicity and water flux of poly(ether sulfone) membranes in the presence of aluminum fumarate metal-organic framework nanoparticles: preparation and characterization, ACS Appl. Mater. Interfaces 11 (2019) 15060–15070. [6] H. Gao, Y. Sun, J. Zhou, R. Xu, H. Duan, Mussel-inspired synthesis of polydopaminefunctionalized graphene hydrogel as reusable adsorbents for water purification, ACS Appl. Mater. Interfaces 5 (2013) 425–432. [7] T.A. Makhetha, R.M. Moutloali, Antifouling properties of Cu(tpa)@GO/PES composite membranes and selective dye rejection, J. Membr. Sci. 554 (2018) 195–210. [8] L. Bai, Y. Liu, N. Bossa, A. Ding, N. Ren, G. Li, H. Liang, M.R. Wiesner, Incorporation of cellulose nanocrystals (CNCs) into the polyamide layer of thin-film composite (TFC) nanofiltration membranes for enhanced separation performance and antifouling properties, Environ. Sci. Technol. 52 (2018) 11178–11187. [9] J. Zhu, S. Yuan, A. Uliana, J. Hou, J. Li, X. Li, M. Tian, Y. Chen, A. Volodin, B.V. der Bruggen, High-flux thin film composite membranes for nanofiltration mediated by a rapid co-deposition of polydopamine/piperazine, J. Membr. Sci. 554 (2018) 97–108. [10] W. Li, Y. Zhang, Q. Li, G. Zhang, Metal−organic framework composite membranes: synthesis and separation applications, Chem. Eng. Sci. 135 (2015) 232–257. [11] X. Cheng, X. Jiang, Y. Zhang, C.H. Lau, Z. Xie, D. Ng, S.J.D. Smith, M.R. Hill, L. Shao, Building additional passageways in polyamide membranes with hydrostable metal organic frameworks to recycle and remove organic solutes from various solvents, ACS Appl. Mater. Interfaces 9 (2017) 38877–38886. [12] Z. Qiu, X. Ji, C. He, Fabrication of a loose nanofiltration candidate from Polyacrylonitrile/Graphene oxide hybrid membrane via thermally induced phase separation, J. Hazard. Mater. 360 (2018) 122–131. [13] W. Li, Metal–organic framework membranes: production, modification, and applications, Prog. Mater Sci. 100 (2019) 21–63. [14] L. Shao, X.Q. Cheng, Y. Liu, S. Quan, J. Ma, S.Z. Zhao, K.Y. Wang, Newly developed nanofiltration (NF) composite membranes by interfacial polymerization for

4. Conclusions In this study, highly water-stable UiO-66 selective layer was vacuum-assisted filtrated on mussel-inspired porous PGP support layer for dye and antibiotic separation. The FESEM, AFM, FTIR, XRD, XPS and CA results revealed that the incorporation of GO and PDA into the PAN substrate can effectively enhance the membrane surface compatibility and hydrophilicity. Notably, the thermal stability and wastewater treatment performance of the UiO-66/PGP TFC membrane were remarkably improved after UiO-66 selective layer had been formed on flexible PGP support layer. Superior water flux (31.33 ± 0.75 L m−2h−1 bar−1) and outstanding rejection efficiency for dye and antibiotic (> 94.0%) were presented in this work. In addition, the UiO-66/PGP TFC membrane also exhibited superior antifouling property due to their enhanced hydrophilicity of membrane surface. Finally, we elaborated the separation mechanism of the UiO66/PGP TFC membrane for nanofiltration performance, including size exclusion and electrostatic interaction. These results indicated that the prepared UiO-66/PGP TFC membrane might be an advanced and energy-efficient thin-film composite membrane for synthetic dyes and antibiotics separation in wastewater treatment technology. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 12

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