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Polyphenol-assisted in-situ assembly for antifouling thin-film composite nanofiltration membranes
Author’s Accepted Manuscript Polyphenol-assisted in-situ assembly for antifouling thin-film composite nanofiltration membranes Runnan Zhang, Mingrui He, Daihong Gao, Yanan Liu, Mengyuan Wu, Zhiwei Jiao, Yanlei Su, Zhongyi Jiang www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(18)31431-5 https://doi.org/10.1016/j.memsci.2018.09.010 MEMSCI16455
To appear in: Journal of Membrane Science Received date: 23 May 2018 Revised date: 28 August 2018 Accepted date: 2 September 2018 Cite this article as: Runnan Zhang, Mingrui He, Daihong Gao, Yanan Liu, Mengyuan Wu, Zhiwei Jiao, Yanlei Su and Zhongyi Jiang, Polyphenol-assisted in-situ assembly for antifouling thin-film composite nanofiltration membranes, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.09.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyphenol-assisted in-situ assembly for antifouling thin-film composite nanofiltration membranes Runnan Zhang†,a,b, Mingrui He†,a,b, Daihong Gao a,b, Yanan Liua,b, Mengyuan Wua,b, Zhiwei Jiaoa,b, Yanlei Sua,b, Zhongyi Jianga,b* a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.
*
Corresponding author: Tel: +86-22-27406646; fax: +86-22-23500086. E-mail address: [email protected] † These authors contributed equally to this work. 1
Abstract Construction of a selective layer with high hydrophilicity is a promising way to prepare thin-film composite (TFC) nanofiltration (NF) membranes with high separation and antifouling performances. Nowadays, a common strategy is to incorporate hydrophilic additives through interfacial polymerization process, which is effective but complex. In this study, a facial polyphenol-assisted in-situ assembly approach during non-solvent induced phase separation (NIPS) was developed to prepare antifouling TFC NF membranes. Polyvinylpyrrolidone (PVP), a common pore-forming agent in the polyethersulfone (PES) casting solution, was in-situ assembled with tannic acid (TA) in the coagulation solution through hydrogen bonding interactions during solvent exchange in NIPS. A hydrophilic TA-PVP complex was generated as the selective layer on membrane surface. The chemical compositions and hydrophilicity of the membrane surface can be tuned by varying the TA content in the coagulation solution. The optimized TA-PVP/PES TFC membrane exhibited a high pure water flux of 152.9 ± 3.2 Lm-2h-1 and satisfying rejections to small organic molecules (methyl orange of 73.6%, organic GII of 90.0%, congo red, methyl blue and alcian blue of nearly 100%). Moreover, the TA-PVP/PES TFC membrane showed superior antifouling performance against bovine serum albumin (BSA) during ternary-cycle filtration test, as well as satisfying long-term operational pH stability and chlorine-resistant property. Key words: Polyphenol; In-situ assembly; Hydrogen bond; Thin-film composite nanofiltration membrane; Antifouling
2
1. Introduction Nanofiltration (NF) has been widely applied in sustainable water purification, such as water softening, wastewater treatment, etc. Nevertheless, the existence of complex contaminants in water environments may lead to severe membrane fouling, which has become a ubiquitous problem [1]. Therefore, preparation of NF membranes with high separation performances (permeability and selectivity) as well as antifouling property is essential for maintaining high separation efficiency of the NF process. Nowadays, the majority of commercial NF membranes are thin-film composite (TFC) NF membranes, which possess a selective skin layer on porous substrates. Various methods have been developed to prepare TFC NF membranes, including interfacial polymerization [2, 3], bioinspired deposition [4-6] and layer-by-layer assembly [7-9], etc. Among those approaches, interfacial polymerization is most commonly used. Basically, the interfacial polymerization process involves a quick cross-linking reaction between an aqueous phase monomer, usually piperazine (PIP), and an organic phase monomer, usually trimesoyl chloride (TMC), at the water/organic interface [10]. However, conventional TFC membranes prepared by interfacial polymerization, such as polyamide TFC membranes, usually suffer from fouling because of the multiple interactions between foulants and membrane surfaces [1, 11, 12]. To deal with this problem, a common strategy is to construct TFC membranes with a hydrophilic selective layer. Following this strategy, numerous hydrophilic organic materials (e.g., poly(ethylene glycol) (PEG), and zwitterions) and inorganic nanomaterials (e.g., TiO2 and GO) have been incorporated in the interfacial polymerization procedure to improve the hydrophilicity of the selective layer [11, 13-18]. Although extensively developed, the interfacial polymerization approach needs pre-preparation of porous substrates and twice immersion steps, which are usually time consuming and involve toxic organic solvents. Therefore, it is still necessary to develop a more facile and environmental-benign approach to 3
prepare TFC NF membranes with a hydrophilic selective layer. Besides interfacial polymerization, non-solvent induced phase separation (NIPS) is sometimes utilized to prepare asymmetric NF membranes. Research interest was mainly focused on modifying membrane by incorporating functional additives in the membrane casting solution and/or the coagulation bath [19-25]. For example, Zhang Yatao and coworkers incorporated various kinds of pre-modified inorganic nanofillers (e.g., SiO2 nanoparticle (NPs) [19, 20], halloysite nanotubes (HNTs) [21] and montmorillonite (MMT) and graphene oxide (GO) nanosheets [22, 24]) into the PES casting solution and fabricated positively/negatively-charged loose nanocomposite PES membranes by NIPS. The resultant membranes exhibited high rejections to dyes (e.g., above 90% rejection for reactive black 5). Liu et al. used 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) to activate carboxylated cardo poly(arylene ether ketone)s (PAEK-COOH) in the membrane casting solution. Then the activated polymer film was immersed in a polyethylenimine (PEI) aqueous solution, in which in-situ amidation crosslinking took place during NIPS, generating positively charged asymmetric NF membranes [25]. Nevertheless, most approaches involved complex synthesis of additives or specific membrane materials. Moreover, no literature has been reported to prepare TFC NF membranes by NIPS. Interestingly, NIPS involves a solvent exchange process at the initial water/solvent interface, during which the pore-forming agent (usually a hydrophilic polymer like PEG and polyvinylpyrrolidone (PVP)) may penetrate into the coagulation bath (usually water) along with the solvent. This process is somewhat similar to the monomer diffusion at the water/organic interface during the interfacial polymerization process. Tannic acid (TA), a polyphenol, can assemble with PVP through hydrogen bonds and form cross-linked polymer hydrogels [26]. Inspired by this, we used TA aqueous solution as a novel coagulation bath, PVP as the pore-forming agent and polyethersulfone (PES) as the bulk membrane material. In-situ 4
assembly between TA and PVP took place during the solvent exchange at the water/solvent interface, generating a hydrophilic TA-PVP selective layer on the membrane surface. The influence of TA concentration on the membrane structure, separation and antifouling performances was systematically investigated. Under the optimized condition, the resultant TA-PVP/PES TFC membrane showed high flux, satisfying rejections to a variety of dyes and excellent antifouling property. To our best knowledge, this is the first time to in-situ prepare TFC NF membranes by NIPS in such a facile way.
2. Experimental 2.1. Materials PES (Ultrason 6020 P, Mw = 29,000) was purchased from BASF Co. (Germany) and dried at 110 oC for 12 h before use. PVP (Mw = 10,000 Da), TA, NaH2PO4 and Na2HPO4 were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). PEGs (Mw = 200, 300, 400, 600, 800, 1000, 1500, 2000, 4000, 6000, 8000 Da) and N, N-dimethyl formamide (DMF) were purchased from Kewei Chemicals Co. (Tianjin, China). Bovine serum albumin (BSA) as the model foulant was obtained from Institute of Hematology, Chinese Academy of Medical Science (Tianjin, China). The water used throughout this study was deionized using a three-stage Milli-Q Plus system (Millipore, Billerica, USA) at pH 6.0. All the chemical reagents were of analytical grade and used without further purification. 2.2. Preparation of the TA-PVP/PES membranes The TA-PVP/PES TFC nanofiltration membranes were prepared via a polyphenol-assisted in-situ assembly method during NIPS. In details, the casting solution containing 16 wt% PES, 16 wt% PVP and 68 wt% DMF was stirred for 4 h at 80 oC and then left for 4 h to release the bubbles completely. Then the solution was cooled to room temperature and cast on glass with a steel knife. A liquid membrane with the 5
thickness of 240 μm was formed and immersed in a water coagulation bath containing TA (concentration varied from 0 to 3.0 wt%) at 25 oC. After 5 min, the pristine membrane was taken out and rinsed with deionized water to remove the unreacted PVP and TA on the membrane surface. At last, the resultant membrane, designated as PVP/PES membrane and TAn-PVP/PES membrane (n is TA concentration), was immersed in deionized water for at least 12 h to remove the residual solvent before use. In addition, the TA/PES membrane was prepared by immersing the casting solution without PVP in the TA aqueous solution (TA concentration of 2.0 wt%). 2.3. Characterization of the TA-PVP/PES membranes Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). ATR-FTIR (Vector 22 FTIR spectrometer, Bruker Optics) was applied to analyze the surface functional groups of the membranes. The spectra were collected in a reflection mode over a scan range of 400 to 4000 cm-1 with the resolution of 4 cm-1. Baseline correction was applied and the scan number was fixed at 32. X-ray photoelectron spectroscopy (XPS). Surface chemical compositions of the resultant membranes were quantitatively characterized using an X-ray photoelectron spectrometer (PHI-1600, USA) with Mg Ka (1254.0 eV) as radiation source. Survey spectra of the membranes were recorded in the region of 0-1100 eV and the photoelectron take-off angle was set at 90o. The near-surface atomic coverage ratios of the membrane components (Фcomponent) were calculated based on the near-surface elemental contents, since PES and PVP were the only sources of sulfur and nitrogen, respectively. ФPES =
AS N PES (C ,O , S ) N PES ( S )
100%
AN N PVP (C ,O , N )
(1)
100%
(2)
ФTA = 1 ФPES ФPVP 100%
(3)
ФPVP =
N PVP ( N )
6
where AS and AN (at%) are the near-surface element content of S and N, respectively. NPES(C, O, S) and NPVP(C, O, N)
are the sum of atom numbers in each unit of PES and PVP molecule, respectively. NPES(S) and NPVP(N) is
the number of S atom in each unit of PES molecule and N atom in each unit of PVP molecule, respectively. Field emission scanning electron microscopy (FESEM). Morphologies of the surface and cross-section of the resultant membranes were observed using a FESEM (Nanosem 430). The operating voltage was set at 10 kV. Before SEM examination, all membrane samples were frozen in liquid nitrogen, broken and gold-sputtered. Atomic force microscopy (AFM). To further measure the membrane surface morphologies and analyze membrane roughness quantitatively, AFM (Multimode 3, Bruker Co.) equipped with Si3N4 cantilevers was employed with the resonance frequency of 1.0 Hz and scan size of 5 μm × 5 μm. Differential scanning calorimetry (DSC). DSC (200F3, NETZSCH Co.) was applied to measure the thermal properties of the prepared membranes at a heating and cooling rate of 10 oC/min under a nitrogen (N2) atmosphere. Static water contact angle measurements. A contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) was applied to measure the static water contact angles of the prepared membranes at room temperature. In details, a water droplet (~ 5 mL) was dropped onto the membrane surface with a microsyringe and a photograph was taken after 5 s to calculate the contact angle. In order to acquire a reliable value, contact angles of at least six water droplets at different locations on each membrane surface were measured and averaged. Zeta potential measurements. The surface charge features of the membranes were analyzed using a SurPASS zeta potential analyzer (Anton Paar GmbH, Austria). The measurement was carried out under a circulation of a 1.0 mmolL-1 KCl solution (pH = 6.0 ± 0.2) through the measuring cell at room temperature. 7
The zeta potential values were obtained from the software. Every sample was tested for at least 4 times to obtain a reliable value. 2.4. Evaluation of the separation and antifouling performance of the TA-PVP/PES membranes Membrane separation performances were evaluated using a dead-end filtration system, as described in our previous study [4, 27]. In details, a membrane sample with an effective area of 28.7 cm2 was placed in a filtration cell (model 8200, Millipore Co.), which had an inner diameter of 62 mm and volume capacity of 200 mL, and equipped with a solution reservoir. The operation pressure was maintained by nitrogen gas from a nitrogen gas cylinder. Each membrane was initially compacted by water under 1.5 bar for 30 min to maintain a stable separation performance. Then the operation pressure was set down to 1.0 bar, and was measured by collecting the permeate water through the membrane using an electronic balance and the pure water flux Jw (Lm-2h-1) was calculated using the following equation:
Jw
V At
(4)
where V (L) is the volume of permeated water, A (m2) is the effective area of the tested membrane, Δt (h) was the operation time. In order to evaluate the rejection performance of the prepared membranes, various kinds of aqueous organic dyes (orange GII, congo red, methyl blue and alcian blue) solutions (0.01 wt%, pH = 6.0 ± 0.2) were used as model feeding solutions. The rejections (R) to dyes were measured under 1.0 bar with a magnetic stirring speed of 400 rpm and calculated by equation (4). Dye concentrations were collected using a UV-vis spectrophotometer (Hitachi UV-2800, Hitachi Co., Japan) under the testing wavelength at which the maximum adsorption of each dye solution was obtained. The antifouling feature of the TA-PVP/PES membranes was tested by a ternary-cycle filtration experiment using BSA as the model foulant. In details, the initial pure water flux (Jw1) of the membrane 8
was measured first. Then the water in the filtration cell was replaced by BSA aqueous solution (0.1 wt%). The fouling test was carried out for 30 min under the operation pressure of 1.0 bar and the permeation flux (Jp) was collected every 1 min. Subsequently, the filtration cell was emptied and the fouled membrane was rinsed by pure water at 400 rpm for 30 min and the pure water flux (Jw2) of the cleaned membrane was recorded again. Then the filtration cell was refilled with the BSA solution before carrying out the next testing cycle. Four indexes, including total flux decline ratio (DRt), irreversible flux decline ratio (DRir), reversible flux decline ratio (DRr) and flux recovery ratio (FRR) were introduced to evaluate the antifouling features of the membranes during each cycle and were calculated with the equations bellow [27, 28].
Jp DRt 1 100% J w1
(5)
J DRir 1 w2 100% J w1
(6)
J w2 J p DRr 100% J w1
(7)
J w2 100% J w1
(8)
FRR
The higher FRR and lower DRt values indicate the better antifouling property of the membrane. In addition, cross-flow filtration tests were also applied to better evaluate the antifouling performance of the membranes. The operation pressure was fixed at 0.1 MPa and the operation procedure was similar to that of the antifouling evaluation by dead-end filtration test. 2.5. Evaluation of the long-term operational pH stability and chlorine-resistant property of the TA-PVP/PES membranes To ensure the long-term stability of the TA-PVP layer, a 20-day water contact angle test of the 9
membrane surface was carried out. In details, the membrane sample was incubated in deionized water under stirring of 200 rpm at 25 oC. The water was refreshed every day and the membrane sample was taken out for water contact angle measurement every 5 days. Moreover, the long-term operational pH stability of the TA-PVP/PES membrane was further evaluated by 72-hour filtration tests using Orange GII solutions with different pH values (4.0, 9.0 and 10.0) as the feeding solution. The flux and rejections were measured every 1 h and the feeding solution were refreshed every 24 h. In addition, the chlorine-resistant property of the TA-PVP/PES membrane was evaluated by separation performance stability test after exposure to the sodium hypochlorite solution for 72 hours. In detail, a piece of prepared TA2-PVP/PES membrane was immersed in a sodium hypochlorite solution with the concentration of 1 g/L. The membrane was taken out every 1 hour and the separation performance was evaluated by filtration test of orange GII solution. After each filtration test, the membrane was re-immersed in the sodium hypochlorite solution. 3. Results and discussion 3.1. Mechanism of the formation of the TA-PVP/PES membranes
10
Fig. 1. Structural illustration of the TA-PVP complex.
The generation of TA-PVP layer was based on the interactions between the carbonyl oxygen (in the keto group, C=O) in PVP as hydrogen bonding acceptor and the phenolic hydroxyl group (OH) in TA as hydrogen bonding donor (Fig. 1), which have been reported in the literature [26, 29]. The hypothesized formation mechanism of the TA-PVP/PES membrane can be described as the in-situ assembly of TA and PVP molecules at the water-solvent interface during NIPS, as shown in Fig. 2. When the liquid membrane was immersed in the coagulation bath, the solvent exchange took place due to the steep gradients in water concentration near the water-solvent interface. Driven by the chemical potential, the PVP molecules diffused into the coagulation bath which contained TA. Then the PVP molecules were assembled with TA through hydrogen bonding interactions, generating a TA-PVP polymeric complex on the PES membrane surface. In order to confirm the in-situ membrane formation process, a miniature membrane formation experiment was carried out and the photographs were shown in Fig. S1 in Supplementary Information (SI). It can be seen that both PVP/DMF solution and TA aqueous solution were clear and tranparent (Fig. S1(a) and (b)). When two solutions were mixed, flocculation took place immediately and the mixture turned 11
turbid (Fig. S1(c)), indicating that PVP and TA can easily assemble in solution. However, when a glass rod covered by PES/PVP/DMF membrane casting solution was put in the TA solution, only the solidification of the membrane can be observed while the TA solution remained clear without any flocculation (Fig. S1(d)). Therefore, the assembling of PVP and TA took place within the membrane surface, rather than in solution. This was because the solidification of PES can anchor the generated TA-PVP complex on surface and form a continuous dense layer, restraining the further diffusion of PVP with a rather high molecular weight of 10000 Da.
Fig. 2. The hypothesized formation mechanism of the TA-PVP/PES membrane.
3.2. Characterizations of the TA-PVP/PES membranes
12
Fig. 3. Cross-sectional SEM images with different scales of ((a), (a1)) the PVP/PES membrane, ((b), (b1)) the TA1-PVP/PES membrane, ((c), (c1)) the TA2-PVP/PES membrane and ((d), (d1)) the TA3-PVP/PES membrane, respectively.
To observe the structures of the membranes, SEM and AFM were utilized and the results were shown in Fig. 3 below and Fig. S2 in the Supplementary Information (SI). From the SEM images of the membrane surfaces (Fig. S2(a)-(d)), it can be seen that both of the PVP/PES and TA-PVP/PES membranes presented rather smooth surfaces. AFM analysis further measured the surface morphology and topology of the membranes, presenting a little decrease of the surface mean roughness (Ra) with the increase of TA concentration (Fig. S2(a1)-(d1)). This was because the generated TA-PVP complex was rather homogeneous and flexible. For the cross-sectional morphology (Fig. 3), although all membranes presented similar finger-like pore structures (Fig. 3(a)-(d)), obvious differences can be observed under high magnification (Fig. 3(a1)-(d1)). The PVP/PES membrane showed a sponge-like structure in the near-surface area, while the TA-PVP/PES membranes presented an apparent dense layer at the top side, which was ascribed to the in-situ assembly between PVP and TA. The thickness of the TA-PVP layer was 13
gradually increased from ~450 nm to ~950 nm with the increase of TA concentration from 1.0 to 3.0 wt%.
Transmittance (%)
3436 cm-1
3422 cm-1
TA TA3-PVP/PES TA2-PVP/PES
3422 cm-1 3420 cm-1
TA1-PVP/PES PVP/PES
1725 cm-1
3500
3000
2000
1040 cm-1 1500
1000
-1
Wavenumber (cm ) Fig. 4. FTIR spectra of pure TA and the PVP/PES, TA1-PVP/PES, TA2-PVP/PES, TA3-PVP/PES membranes.
Fig. 4 presents the FTIR spectra of pure TA and the prepared membranes. Compared with the spectrum of the PVP/PES membrane, additional peaks can be found at 1040, 1725 and around 3400 cm-1 in the spectra of both TA and the TA-PVP/PES membranes, corresponding to C-O-C stretching vibrations, C=O stretching vibrations of ester groups and O-H stretching vibrations of phenol groups, respectively [30, 31]. Furthermore, the intensity of these peaks was increased with the increase of TA concentrations. The FTIR results indicated that TA was incorporated into the surface of the TA-PVP/PES membrane and the TA content in membrane was elevated with the increase of TA concentration in the coagulation bath. Moreover, compared with the spectrum of pure TA, the O-H stretching vibration peak in the spectrum of TA1-PVP/PES membrane exhibited an obvious red shift from 3436 to 3420 cm-1, ascribed to the formation of strong hydrogen bonding interactions between TA and PVP [32]. With the TA concentration further increased from 1.0 to 3.0 wt%, the O-H stretching vibration peak slightly shifted to 3422 cm-1, due to the 14
increase of residual free phenol groups of TA on the membrane surface. To further verify the existence of the hydrogen bond between TA and PVP, thermal properties of the membranes were analyzed by DSC, as shown in Fig. 5. Two endothermic relaxation peaks can be observed in each DSC curve, corresponding to the glass transition temperature (Tg) of PVP and PES, which existed in the separated phases [32, 33]. From Fig. 5(a), it can be calculated that of PVP in the PVP/PES membrane was 43.05oC. When TA concentration in the coagulation solution was 1.0 wt%, the Tg of PVP in the resultant TA-PVP/PES membrane was dramatically increased to 47.90oC. The change of Tg was caused by the hydrogen bonding interactions between TA and PVP, which lower the mobility of PVP molecules. Further increase of TA concentration to 3.0 wt% only led to little change of the Tg of PVP to 48.14oC, indicating the slight enhancement of the hydrogen bonding interaction. This was due to the gradual saturation of the coordinated TA molecules for each PVP molecule [32]. In addition, the Tg of PES was also changed with the change of TA concentration in the coagulation, as shown in Fig. 5(b). With the increase of TA concentration to 1.0 wt%, no obvious change of the Tg of PES was observed. This was because most of the TA molecules were coordinated with PVP as mentioned above. Further increase of TA concentration to 3.0 wt% resulted in apparent decrease of the Tg of PES from 222.28oC to 219.77oC. The reason was that the excess TA molecules were bonded to PES through hydrogen bonding interactions between the pyrogallol and ester groups [34]. This may lower the crystallinity of PES and decrease the Tg [33].
15
Fig. 5. DSC 2nd-heating thermograms within the (a) low temperature range and (b) high temperature range of the PVP/PES, TA1-PVP/PES, TA2-PVP/PES and TA3-PVP/PES membranes, respectively.
Aiming to analyze the surface chemical compositions of the membranes quantitatively, XPS was used and wide-scan spectra were shown in Fig. 6(a). From the spectra, it can be seen that the TA/PES membrane presented atomic signals of C1s, O1s, S2s and S2p in the spectrum, while the PVP/PES membrane presented an additional N1s signal. With the addition of TA in the coagulation solution, the intensity of N1s signal was increased, while that of S signals was decreased, simultaneously. The near-surface elemental compositions and Фcomponent were calculated and listed in Table 1. The XPS results showed that ФTA of the TA/PES membrane was only 1.4%. Moreover, the deconvolution of the high-resolution C1s peak showed two signals of *C-C and *C-O for the TA/PES membrane (Fig. 6(b)) but an additional *C=O peak for the PVP/PES and TA2-PVP/PES membranes (Fig. 6(c) and (d)), which was assigned to the ester groups of TA and pyrrolidone groups of PVP. The combined results verified that TA can hardly self-condense and deposit on PES membrane surface during NIPS without the presence of PVP. Without TA in the coagulation solution, the ФPVP value of the PVP/PES membrane was only 19.2%. This 16
was because the interaction between PVP and PES was weak and the majority of PVP molecules were leached into water during NIPS. When the concentration of TA was 1.0 wt%, ФPVP and ФTA were dramatically elevated to 50.88% and 11.84%, respectively, while ФPES was decreased down to 37.28%. The results indicated that the PVP molecules were assembled with TA and generated a TA-PVP layer on the membrane surface. When TA concentration was increased to 2.0 wt%, ФPVP and ФTA were further increased along with the decrease of ФPES. This was because higher TA concentration led to higher cross-linking degree of the TA-PVP layer, which anchored more TA and PVP molecules. However, although further increase of TA concentration to 3.0 wt% resulted in higher ФTA, ФPVP was no longer elevated but decreased a little bit, indicating that the in-situ assembly process within the near-surface region was completed. Moreover, even with the addition of 3.0 wt% TA, there still existed PES with the atomic coverage ratio of 18.72 wt% within the near-surface of the membrane [27]. As the XPS detection depth (~ 10 nm) was much smaller than the observed thickness of the generated TA-PVP layer (Fig. 3), it can be concluded that the TA-PVP layer was inserted inside the PES framework, not upon the membrane surface. This was caused by the “top-down” formation process of the TA-PVP layer. During the in-situ assembly process, a thin nascent TA-PVP layer was first generated, which restricted the penetration of PVP with a rather high molecular weight. Nevertheless, TA molecules, which are relatively small, can still penetrate through the nascent TA-PVP layer. Therefore, further assembly was carried out inside the bulk membrane and the TA-PVP layer was developed inwards.
17
(b)
(a) TA3-PVP/PES TA2-PVP/PES TA1-PVP/PES
*C-C
PVP/PES
*C-O
TA/PES
800
700
600
500
400
300
200
100 290
288
286
284
282
280
Binding energy (eV)
Binding energy(eV)
(d)
(c)
*C-C *C-O (*C-N)
*C-C *C-O (*C-N)
*C=O
290
*C=O
288
286
284
Binding energy(eV)
282
280
290
288
286
284
282
280
Binding energy (eV)
Fig. 6. (a) Wide-scan XPS spectra of the TA/PES, PVP/PES, TA1-PVP/PES, TA2-PVP/PES and TA3-PVP/PES membranes and high-resolution C1s XPS spectra of the (b) TA/PES, (c) PVP/PES and (d) TA2-PVP/PES membranes.
18
Table 1 Near-surface elemental compositions and Фcomponent of the TA/PES, PVP/PES, TA1-PVP/PES, TA2-PVP/PES and TA3-PVP/PES membranes, respectively. TA concentration
Фcomponent (at%)
Near-surface atomic element contents (at%)
(wt%)
C
O
N
S
PES
PVP
TA
PES
69.65
24.19
-
6.16
98.6
-
1.4
0
74.35
18.2
2.4
5.05
80.8
19.2
0
1
74.8
16.51
6.36
2.33
37.28
50.88
11.84
2
82.86
9.04
6.92
1.18
18.88
55.36
25.76
3
81.84
10.61
6.38
1.17
18.72
51.04
30.24
The surface hydrophilicity and charge of the membrane were assessed by static water contact angle and zeta potential measurements and the results were shown in Fig. 7. Fig. 7(a) showed that the PVP/PES membrane exhibited a quite hydrophobic surface with the water contact angle of 62.7 ± 0.8o, ascribed to the large coverage of PES on the membrane surface. With the addition of TA in the coagulation solution, the membrane surface became hydrophilic. This could be explained by the generation of the TA-PVP layer with residual pyrogallol and pyrrolidone groups, which could strongly bind with water molecules. Moreover, the surface hydrophilicity was improved with the elevation of TA concentration from 1.0 to 2.0 wt%. This was because of the higher surface coverage of TA, leading to more bonding sites for water. With the further increase of TA concentration to 3.0 wt%, the water contact angle remained stable because of the saturation of TA on membrane surface. The zeta potential results in Fig. 7(b) showed that all membranes possessed negatively charged surfaces. With the increase of the TA concentration, the membrane surface
19
charge became more negative. This was caused by the increase of TA coverage, leading to more residual phenol groups on the membrane surface, which can generate negative charges after deprotonation. 80
TA concentration (wt%)
(a)
0
1.0
2.0
2.5
3.0
60 50
Zeta poteintial (mV)
Water contact angle (o)
70
0
40 30 20 10 0
0
1.0
1.5
2.0
2.5
-10
-20
-30
-40
3.0
(b)
TA concentration (wt%)
-50
Fig. 7. (a) Water contact angles and (b) zeta potential results of the PVP/PES, TA1.5-PVP/PES, TA2-PVP/PES, TA2.5-PVP/PES and TA3-PVP/PES membranes.
3.3. Separation and antifouling performances of the TA-PVP/PES membranes The effect of TA concentration on the separation performances of the prepared membranes was investigated systematically and the results were shown in Fig. 8(a). The PVP/PES membrane exhibited a pure water flux of 209.1 ± 5.1 Lm-2h-1 and relatively low dye rejections (34.4% for methyl orange and 60.1% for organic GII). This was because the size of the sponge-like pores was larger than those of dye molecules and could only reject some large dye aggregates. With TA concentration increased to 3.0 wt%, the dye rejections were dramatically increased to 82.5% for methyl orange and 97.4% for organic GII, while the pure water flux was first slightly elevated to 234.5 ± 6.8 Lm-2h-1 and then sharply down to 34.3 ± 1.5 Lm-2h-1. The variation of the separation performances was caused by the change of the physical (cross-linking degree and thickness) and chemical (surface hydrophilicity) structures of the TA-PVP layer. 20
The rejections were mainly affected by the cross-linking degree, while the pure water flux was influenced a “trade-off” effect. On one hand, with the increase of TA concentration, the cross-linking degree as well as the thickness of the TA-PVP layer was elevated, resulting higher resistance for permeation of both water and dye molecules [35, 36]. On the other hand, the surface hydrophilicity was enhanced simultaneously, which was beneficial for water absorption and penetration. Under a low TA concentration (1.0 wt%), the chemical structure of the TA-PVP layer dominated the water permeability, leading to a slight increase of the pure water flux. Under high TA concentrations (1.5 wt% to 3.0 wt%), the physical structure took the dominated influence, resulting in a dramatic drop of the pure water flux. After considering the separation performance comprehensively, 2.0 wt% was chosen as the optimized concentration.
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Fig. 8. (a) Separation performances of the PVP/PES, TA1-PVP/PES, TA1.5-PVP/PES, TA2-PVP/PES, TA2.5-PVP/PES and TA3-PVP/PES membranes. (b) Separation performances of the TA2-PVP/PES membrane for different dyes.
To better verify the broad applicability of the resultant membranes for dye/water separation, the TA2-PVP/PES membrane was chosen to evaluate the separation performance for various dye solutions. From the results (Fig. 8(b)), the membrane exhibited satisfying rejections to organic dyes (73.6% for 21
methyl orange, 90.0% for organic GII, 99.4% for congo red, 99.9% for methyl blue and 99.8% for alcian blue). The permeation fluxes during filtration of dye solutions were a little bit lower than the pure water flux, owing to the concentration polarization and adsorption of dye molecules on the membrane surface. In addition, the separation performances of the TA-PVP/PES membranes were compared with the reported data in literatures, as shown in Fig. S3 with detailed data listed in Table S1 in SI. It can be seen that the TA-PVP/PES membranes exhibited high pure water permeance and dye rejections under a low operation pressure (1 bar) and outperformed the majority of the NF membranes reported in literatures. 300 PVP/PES TA1-PVP/PES
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Fig. 9. (a) Time-dependent fluxes, (b) DRt (sum of DRr, DRir) and (c) FRR values of the PVP/PES, TA1-PVP/PES, TA1.5-PVP/PES, TA2-PVP/PES, TA2.5-PVP/PES and TA3-PVP/PES membranes during the ternary-cycle filtration test using BSA as the model foulant.
Antifouling properties have been recognized as crucial features of NF membranes in long-term practical applications, such as wastewater purification [1, 37, 38]. In order to evaluate the antifouling performances of the prepared membranes, ternary-cycle filtration tests were carried out using BSA as the model foulant and the time-dependent flux curves were shown in Fig. 9(a). In each cycle, a common tendency can be observed that the permeation flux declined after exposure to BSA solution and recovered to a certain extent after water rinsing. Four indexes (DRt, DRr, DRir and FRR) were applied to evaluate the antifouling performances precisely. As shown in Fig. 9(b), membrane fouling (corresponding to DRt) can be divided into reversible fouling (corresponding to DRr) caused by concentration polarization and formation of the cake layer, and irreversible fouling (corresponding to DRir) caused by BSA adsorption, according to the resistance-in-series model [39, 40]. With the increase of TA concentration, the antifouling performances were obviously enhanced. Taking the first cycle as an example, both of the DRt and DRir values were dramatically decreased from 66.2% and 49.0% for the PVP/PES membrane down to 7.6% and 1.7% for the 23
TA3-PVP/PES membrane, while the FRR value were increased from 51.0% for the PVP/PES membrane up to 98.3% for the TA3-PVP/PES membrane. The antifouling properties are closely related to the chemical structure of the membrane surface. The generation of TA-PVP layer introduced hydrophilic groups to the membrane surface, which can bind with water molecules. Hence a continuous hydration layer was generated over the membrane surface, suppressing BSA adsorption [1]. In addition, as shown in Fig. S4, the TA2-PVP/PES membrane also exhibited a superior antifouling performance in the cross-flow antifouling test, compared with the PVP/PES membrane.
3.4. Membrane stability The long-term stability of the TA-PVP layer was evaluated by measuring the change of the water contact angle of the membrane surface, as shown in Fig. 10. According to the structure verified by SEM and XPS results, the disassembly of the TA-PVP layer may lead to further exposure of the PES matrix and the decrease of surface hydrophilicity. During 20 days, the water contact angle of the PVP/PES membrane was gradually increased. The change of the surface hydrophilicity was caused by the leaching of the PVP molecules to water, which increased the surface coverage of hydrophobic PES matrix. However, the water contact angle of the TA2-PVP/PES membrane remained relatively unchanged. On one hand, the hydrogen bonds between TA and PVP were strong and stable in water [26, 29]. On the other hand, its highly cross-linked structure made it physically anchored to the PES matrix. In addition, the hydrogen bonds between PES and the residual TA molecules of the TA-PVP layer provided additional interactions to maintain the structural stability [34].
24
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Time (d) Fig. 10. Water contact angle variation of the PVP/PES and TA2-PVP/PES membrane during 20 days.
In addition, the long-term operational pH stability of the membrane was evaluated and the results were shown in Fig. 11(a)-(c). From Fig. 11(a) and (b), it can be seen that the separation performance of the TA2-PVP/PES membrane remained relatively stable with the pH value at both 4.0 and 9.0. When the pH value was further increased to 10.0, the pure water flux of the membrane was increased while the dye rejections were decreased, indicating that the TA-PVP layer was partially destroyed. This was because under high pH value, the phenol groups of TA were more likely to be deprotonated, decreasing the number of the hydrogen bonding donors. However, even after operation at pH of 10.0 for 72 h, the rejection to orange GII still remained as high as 70%. Therefore, the TA-PVP/PES membrane exhibited high structural stability under a wide range of pH conditions. Considering that the operation pH for NF is around 6.0 [41], the TA-PVP/PES membrane can remain stable during real applications. Chlorine-resistant property is important for NF membranes, due to the demand for chemical cleaning of membranes in real applications. The results in Fig. 11(d) showed that after exposure to the sodium hypochlorite solution (1 g/L) for 72 h, the rejection to orange GII remained stable at about 90%, while the 25
flux was slightly increased. The excellent chlorine-resistant property of the TA-PVP/PES membrane could be ascribed to the existence of radical scavenging phenol groups in the active layer [42]. 300
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Fig. 11. (a)-(c) Long-term operational stability of the TA2-PVP/PES membrane under the pH of 4.0, 9.0 and 10.0, respectively and (d) the time-dependent separation performance of the TA2-PVP/PES membrane after immersion by the sodium hypochlorite solution with a concentration of 1 g/L. 4. Conclusions In summary, a novel TA-PVP/PES TFC membrane was prepared through a facile polyphenol-assisted in-situ assembly approach during phase separation. PVP, the pore-forming agent, was in-situ cross-linked by TA in the coagulation bath through hydrogen bonds, generating a hydrophilic TA-PVP selective layer. The chemical composition and the physical structure of the TA-PVP layer can be facilely tailored by 26
altering TA concentration. Under the optimized condition, the TA-PVP/PES membrane exhibited a high pure water flux (152.9 ± 3.2 Lm-2h-1) with high rejections to various dyes. Compared with the PVP/PES membrane, the TA-PVP/PES membrane showed higher surface hydrophilicity and superior antifouling property. Moreover, the cross-linked structure and strong hydrogen bonding interactions rendered the TA-PVP/PES membrane high stability. Considering the convenience and controllability of the procedure, this approach shows great potential for large-scale commercial membrane preparations. Acknowledgements This research was financially supported by National Natural Science Foundation of China (21621004) and National Key Research and Development Program of China (No. 2016YFB0600503). Appendix. Supplementary Information Supplementary data associated with this article can be found in the online version at…
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Highlights 1.
Nanofiltration membranes were fabricated by polyphenol-assisted in situ assembly.
2.
The assembly process was mediated by hydrogen bonds between TA and PVP.
3.
Both structure and surface composition of the selective layer were manipulated.
4.
The membrane showed high fluxes, dyes rejections and antifouling performance.
5.
The membrane showed satisfying pH and long-term operational stability.
30
PII: DOI: Reference:
S0376-7388(18)31431-5 https://doi.org/10.1016/j.memsci.2018.09.010 MEMSCI16455
To appear in: Journal of Membrane Science Received date: 23 May 2018 Revised date: 28 August 2018 Accepted date: 2 September 2018 Cite this article as: Runnan Zhang, Mingrui He, Daihong Gao, Yanan Liu, Mengyuan Wu, Zhiwei Jiao, Yanlei Su and Zhongyi Jiang, Polyphenol-assisted in-situ assembly for antifouling thin-film composite nanofiltration membranes, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.09.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyphenol-assisted in-situ assembly for antifouling thin-film composite nanofiltration membranes Runnan Zhang†,a,b, Mingrui He†,a,b, Daihong Gao a,b, Yanan Liua,b, Mengyuan Wua,b, Zhiwei Jiaoa,b, Yanlei Sua,b, Zhongyi Jianga,b* a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.
*
Corresponding author: Tel: +86-22-27406646; fax: +86-22-23500086. E-mail address: [email protected] † These authors contributed equally to this work. 1
Abstract Construction of a selective layer with high hydrophilicity is a promising way to prepare thin-film composite (TFC) nanofiltration (NF) membranes with high separation and antifouling performances. Nowadays, a common strategy is to incorporate hydrophilic additives through interfacial polymerization process, which is effective but complex. In this study, a facial polyphenol-assisted in-situ assembly approach during non-solvent induced phase separation (NIPS) was developed to prepare antifouling TFC NF membranes. Polyvinylpyrrolidone (PVP), a common pore-forming agent in the polyethersulfone (PES) casting solution, was in-situ assembled with tannic acid (TA) in the coagulation solution through hydrogen bonding interactions during solvent exchange in NIPS. A hydrophilic TA-PVP complex was generated as the selective layer on membrane surface. The chemical compositions and hydrophilicity of the membrane surface can be tuned by varying the TA content in the coagulation solution. The optimized TA-PVP/PES TFC membrane exhibited a high pure water flux of 152.9 ± 3.2 Lm-2h-1 and satisfying rejections to small organic molecules (methyl orange of 73.6%, organic GII of 90.0%, congo red, methyl blue and alcian blue of nearly 100%). Moreover, the TA-PVP/PES TFC membrane showed superior antifouling performance against bovine serum albumin (BSA) during ternary-cycle filtration test, as well as satisfying long-term operational pH stability and chlorine-resistant property. Key words: Polyphenol; In-situ assembly; Hydrogen bond; Thin-film composite nanofiltration membrane; Antifouling
2
1. Introduction Nanofiltration (NF) has been widely applied in sustainable water purification, such as water softening, wastewater treatment, etc. Nevertheless, the existence of complex contaminants in water environments may lead to severe membrane fouling, which has become a ubiquitous problem [1]. Therefore, preparation of NF membranes with high separation performances (permeability and selectivity) as well as antifouling property is essential for maintaining high separation efficiency of the NF process. Nowadays, the majority of commercial NF membranes are thin-film composite (TFC) NF membranes, which possess a selective skin layer on porous substrates. Various methods have been developed to prepare TFC NF membranes, including interfacial polymerization [2, 3], bioinspired deposition [4-6] and layer-by-layer assembly [7-9], etc. Among those approaches, interfacial polymerization is most commonly used. Basically, the interfacial polymerization process involves a quick cross-linking reaction between an aqueous phase monomer, usually piperazine (PIP), and an organic phase monomer, usually trimesoyl chloride (TMC), at the water/organic interface [10]. However, conventional TFC membranes prepared by interfacial polymerization, such as polyamide TFC membranes, usually suffer from fouling because of the multiple interactions between foulants and membrane surfaces [1, 11, 12]. To deal with this problem, a common strategy is to construct TFC membranes with a hydrophilic selective layer. Following this strategy, numerous hydrophilic organic materials (e.g., poly(ethylene glycol) (PEG), and zwitterions) and inorganic nanomaterials (e.g., TiO2 and GO) have been incorporated in the interfacial polymerization procedure to improve the hydrophilicity of the selective layer [11, 13-18]. Although extensively developed, the interfacial polymerization approach needs pre-preparation of porous substrates and twice immersion steps, which are usually time consuming and involve toxic organic solvents. Therefore, it is still necessary to develop a more facile and environmental-benign approach to 3
prepare TFC NF membranes with a hydrophilic selective layer. Besides interfacial polymerization, non-solvent induced phase separation (NIPS) is sometimes utilized to prepare asymmetric NF membranes. Research interest was mainly focused on modifying membrane by incorporating functional additives in the membrane casting solution and/or the coagulation bath [19-25]. For example, Zhang Yatao and coworkers incorporated various kinds of pre-modified inorganic nanofillers (e.g., SiO2 nanoparticle (NPs) [19, 20], halloysite nanotubes (HNTs) [21] and montmorillonite (MMT) and graphene oxide (GO) nanosheets [22, 24]) into the PES casting solution and fabricated positively/negatively-charged loose nanocomposite PES membranes by NIPS. The resultant membranes exhibited high rejections to dyes (e.g., above 90% rejection for reactive black 5). Liu et al. used 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) to activate carboxylated cardo poly(arylene ether ketone)s (PAEK-COOH) in the membrane casting solution. Then the activated polymer film was immersed in a polyethylenimine (PEI) aqueous solution, in which in-situ amidation crosslinking took place during NIPS, generating positively charged asymmetric NF membranes [25]. Nevertheless, most approaches involved complex synthesis of additives or specific membrane materials. Moreover, no literature has been reported to prepare TFC NF membranes by NIPS. Interestingly, NIPS involves a solvent exchange process at the initial water/solvent interface, during which the pore-forming agent (usually a hydrophilic polymer like PEG and polyvinylpyrrolidone (PVP)) may penetrate into the coagulation bath (usually water) along with the solvent. This process is somewhat similar to the monomer diffusion at the water/organic interface during the interfacial polymerization process. Tannic acid (TA), a polyphenol, can assemble with PVP through hydrogen bonds and form cross-linked polymer hydrogels [26]. Inspired by this, we used TA aqueous solution as a novel coagulation bath, PVP as the pore-forming agent and polyethersulfone (PES) as the bulk membrane material. In-situ 4
assembly between TA and PVP took place during the solvent exchange at the water/solvent interface, generating a hydrophilic TA-PVP selective layer on the membrane surface. The influence of TA concentration on the membrane structure, separation and antifouling performances was systematically investigated. Under the optimized condition, the resultant TA-PVP/PES TFC membrane showed high flux, satisfying rejections to a variety of dyes and excellent antifouling property. To our best knowledge, this is the first time to in-situ prepare TFC NF membranes by NIPS in such a facile way.
2. Experimental 2.1. Materials PES (Ultrason 6020 P, Mw = 29,000) was purchased from BASF Co. (Germany) and dried at 110 oC for 12 h before use. PVP (Mw = 10,000 Da), TA, NaH2PO4 and Na2HPO4 were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). PEGs (Mw = 200, 300, 400, 600, 800, 1000, 1500, 2000, 4000, 6000, 8000 Da) and N, N-dimethyl formamide (DMF) were purchased from Kewei Chemicals Co. (Tianjin, China). Bovine serum albumin (BSA) as the model foulant was obtained from Institute of Hematology, Chinese Academy of Medical Science (Tianjin, China). The water used throughout this study was deionized using a three-stage Milli-Q Plus system (Millipore, Billerica, USA) at pH 6.0. All the chemical reagents were of analytical grade and used without further purification. 2.2. Preparation of the TA-PVP/PES membranes The TA-PVP/PES TFC nanofiltration membranes were prepared via a polyphenol-assisted in-situ assembly method during NIPS. In details, the casting solution containing 16 wt% PES, 16 wt% PVP and 68 wt% DMF was stirred for 4 h at 80 oC and then left for 4 h to release the bubbles completely. Then the solution was cooled to room temperature and cast on glass with a steel knife. A liquid membrane with the 5
thickness of 240 μm was formed and immersed in a water coagulation bath containing TA (concentration varied from 0 to 3.0 wt%) at 25 oC. After 5 min, the pristine membrane was taken out and rinsed with deionized water to remove the unreacted PVP and TA on the membrane surface. At last, the resultant membrane, designated as PVP/PES membrane and TAn-PVP/PES membrane (n is TA concentration), was immersed in deionized water for at least 12 h to remove the residual solvent before use. In addition, the TA/PES membrane was prepared by immersing the casting solution without PVP in the TA aqueous solution (TA concentration of 2.0 wt%). 2.3. Characterization of the TA-PVP/PES membranes Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). ATR-FTIR (Vector 22 FTIR spectrometer, Bruker Optics) was applied to analyze the surface functional groups of the membranes. The spectra were collected in a reflection mode over a scan range of 400 to 4000 cm-1 with the resolution of 4 cm-1. Baseline correction was applied and the scan number was fixed at 32. X-ray photoelectron spectroscopy (XPS). Surface chemical compositions of the resultant membranes were quantitatively characterized using an X-ray photoelectron spectrometer (PHI-1600, USA) with Mg Ka (1254.0 eV) as radiation source. Survey spectra of the membranes were recorded in the region of 0-1100 eV and the photoelectron take-off angle was set at 90o. The near-surface atomic coverage ratios of the membrane components (Фcomponent) were calculated based on the near-surface elemental contents, since PES and PVP were the only sources of sulfur and nitrogen, respectively. ФPES =
AS N PES (C ,O , S ) N PES ( S )
100%
AN N PVP (C ,O , N )
(1)
100%
(2)
ФTA = 1 ФPES ФPVP 100%
(3)
ФPVP =
N PVP ( N )
6
where AS and AN (at%) are the near-surface element content of S and N, respectively. NPES(C, O, S) and NPVP(C, O, N)
are the sum of atom numbers in each unit of PES and PVP molecule, respectively. NPES(S) and NPVP(N) is
the number of S atom in each unit of PES molecule and N atom in each unit of PVP molecule, respectively. Field emission scanning electron microscopy (FESEM). Morphologies of the surface and cross-section of the resultant membranes were observed using a FESEM (Nanosem 430). The operating voltage was set at 10 kV. Before SEM examination, all membrane samples were frozen in liquid nitrogen, broken and gold-sputtered. Atomic force microscopy (AFM). To further measure the membrane surface morphologies and analyze membrane roughness quantitatively, AFM (Multimode 3, Bruker Co.) equipped with Si3N4 cantilevers was employed with the resonance frequency of 1.0 Hz and scan size of 5 μm × 5 μm. Differential scanning calorimetry (DSC). DSC (200F3, NETZSCH Co.) was applied to measure the thermal properties of the prepared membranes at a heating and cooling rate of 10 oC/min under a nitrogen (N2) atmosphere. Static water contact angle measurements. A contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) was applied to measure the static water contact angles of the prepared membranes at room temperature. In details, a water droplet (~ 5 mL) was dropped onto the membrane surface with a microsyringe and a photograph was taken after 5 s to calculate the contact angle. In order to acquire a reliable value, contact angles of at least six water droplets at different locations on each membrane surface were measured and averaged. Zeta potential measurements. The surface charge features of the membranes were analyzed using a SurPASS zeta potential analyzer (Anton Paar GmbH, Austria). The measurement was carried out under a circulation of a 1.0 mmolL-1 KCl solution (pH = 6.0 ± 0.2) through the measuring cell at room temperature. 7
The zeta potential values were obtained from the software. Every sample was tested for at least 4 times to obtain a reliable value. 2.4. Evaluation of the separation and antifouling performance of the TA-PVP/PES membranes Membrane separation performances were evaluated using a dead-end filtration system, as described in our previous study [4, 27]. In details, a membrane sample with an effective area of 28.7 cm2 was placed in a filtration cell (model 8200, Millipore Co.), which had an inner diameter of 62 mm and volume capacity of 200 mL, and equipped with a solution reservoir. The operation pressure was maintained by nitrogen gas from a nitrogen gas cylinder. Each membrane was initially compacted by water under 1.5 bar for 30 min to maintain a stable separation performance. Then the operation pressure was set down to 1.0 bar, and was measured by collecting the permeate water through the membrane using an electronic balance and the pure water flux Jw (Lm-2h-1) was calculated using the following equation:
Jw
V At
(4)
where V (L) is the volume of permeated water, A (m2) is the effective area of the tested membrane, Δt (h) was the operation time. In order to evaluate the rejection performance of the prepared membranes, various kinds of aqueous organic dyes (orange GII, congo red, methyl blue and alcian blue) solutions (0.01 wt%, pH = 6.0 ± 0.2) were used as model feeding solutions. The rejections (R) to dyes were measured under 1.0 bar with a magnetic stirring speed of 400 rpm and calculated by equation (4). Dye concentrations were collected using a UV-vis spectrophotometer (Hitachi UV-2800, Hitachi Co., Japan) under the testing wavelength at which the maximum adsorption of each dye solution was obtained. The antifouling feature of the TA-PVP/PES membranes was tested by a ternary-cycle filtration experiment using BSA as the model foulant. In details, the initial pure water flux (Jw1) of the membrane 8
was measured first. Then the water in the filtration cell was replaced by BSA aqueous solution (0.1 wt%). The fouling test was carried out for 30 min under the operation pressure of 1.0 bar and the permeation flux (Jp) was collected every 1 min. Subsequently, the filtration cell was emptied and the fouled membrane was rinsed by pure water at 400 rpm for 30 min and the pure water flux (Jw2) of the cleaned membrane was recorded again. Then the filtration cell was refilled with the BSA solution before carrying out the next testing cycle. Four indexes, including total flux decline ratio (DRt), irreversible flux decline ratio (DRir), reversible flux decline ratio (DRr) and flux recovery ratio (FRR) were introduced to evaluate the antifouling features of the membranes during each cycle and were calculated with the equations bellow [27, 28].
Jp DRt 1 100% J w1
(5)
J DRir 1 w2 100% J w1
(6)
J w2 J p DRr 100% J w1
(7)
J w2 100% J w1
(8)
FRR
The higher FRR and lower DRt values indicate the better antifouling property of the membrane. In addition, cross-flow filtration tests were also applied to better evaluate the antifouling performance of the membranes. The operation pressure was fixed at 0.1 MPa and the operation procedure was similar to that of the antifouling evaluation by dead-end filtration test. 2.5. Evaluation of the long-term operational pH stability and chlorine-resistant property of the TA-PVP/PES membranes To ensure the long-term stability of the TA-PVP layer, a 20-day water contact angle test of the 9
membrane surface was carried out. In details, the membrane sample was incubated in deionized water under stirring of 200 rpm at 25 oC. The water was refreshed every day and the membrane sample was taken out for water contact angle measurement every 5 days. Moreover, the long-term operational pH stability of the TA-PVP/PES membrane was further evaluated by 72-hour filtration tests using Orange GII solutions with different pH values (4.0, 9.0 and 10.0) as the feeding solution. The flux and rejections were measured every 1 h and the feeding solution were refreshed every 24 h. In addition, the chlorine-resistant property of the TA-PVP/PES membrane was evaluated by separation performance stability test after exposure to the sodium hypochlorite solution for 72 hours. In detail, a piece of prepared TA2-PVP/PES membrane was immersed in a sodium hypochlorite solution with the concentration of 1 g/L. The membrane was taken out every 1 hour and the separation performance was evaluated by filtration test of orange GII solution. After each filtration test, the membrane was re-immersed in the sodium hypochlorite solution. 3. Results and discussion 3.1. Mechanism of the formation of the TA-PVP/PES membranes
10
Fig. 1. Structural illustration of the TA-PVP complex.
The generation of TA-PVP layer was based on the interactions between the carbonyl oxygen (in the keto group, C=O) in PVP as hydrogen bonding acceptor and the phenolic hydroxyl group (OH) in TA as hydrogen bonding donor (Fig. 1), which have been reported in the literature [26, 29]. The hypothesized formation mechanism of the TA-PVP/PES membrane can be described as the in-situ assembly of TA and PVP molecules at the water-solvent interface during NIPS, as shown in Fig. 2. When the liquid membrane was immersed in the coagulation bath, the solvent exchange took place due to the steep gradients in water concentration near the water-solvent interface. Driven by the chemical potential, the PVP molecules diffused into the coagulation bath which contained TA. Then the PVP molecules were assembled with TA through hydrogen bonding interactions, generating a TA-PVP polymeric complex on the PES membrane surface. In order to confirm the in-situ membrane formation process, a miniature membrane formation experiment was carried out and the photographs were shown in Fig. S1 in Supplementary Information (SI). It can be seen that both PVP/DMF solution and TA aqueous solution were clear and tranparent (Fig. S1(a) and (b)). When two solutions were mixed, flocculation took place immediately and the mixture turned 11
turbid (Fig. S1(c)), indicating that PVP and TA can easily assemble in solution. However, when a glass rod covered by PES/PVP/DMF membrane casting solution was put in the TA solution, only the solidification of the membrane can be observed while the TA solution remained clear without any flocculation (Fig. S1(d)). Therefore, the assembling of PVP and TA took place within the membrane surface, rather than in solution. This was because the solidification of PES can anchor the generated TA-PVP complex on surface and form a continuous dense layer, restraining the further diffusion of PVP with a rather high molecular weight of 10000 Da.
Fig. 2. The hypothesized formation mechanism of the TA-PVP/PES membrane.
3.2. Characterizations of the TA-PVP/PES membranes
12
Fig. 3. Cross-sectional SEM images with different scales of ((a), (a1)) the PVP/PES membrane, ((b), (b1)) the TA1-PVP/PES membrane, ((c), (c1)) the TA2-PVP/PES membrane and ((d), (d1)) the TA3-PVP/PES membrane, respectively.
To observe the structures of the membranes, SEM and AFM were utilized and the results were shown in Fig. 3 below and Fig. S2 in the Supplementary Information (SI). From the SEM images of the membrane surfaces (Fig. S2(a)-(d)), it can be seen that both of the PVP/PES and TA-PVP/PES membranes presented rather smooth surfaces. AFM analysis further measured the surface morphology and topology of the membranes, presenting a little decrease of the surface mean roughness (Ra) with the increase of TA concentration (Fig. S2(a1)-(d1)). This was because the generated TA-PVP complex was rather homogeneous and flexible. For the cross-sectional morphology (Fig. 3), although all membranes presented similar finger-like pore structures (Fig. 3(a)-(d)), obvious differences can be observed under high magnification (Fig. 3(a1)-(d1)). The PVP/PES membrane showed a sponge-like structure in the near-surface area, while the TA-PVP/PES membranes presented an apparent dense layer at the top side, which was ascribed to the in-situ assembly between PVP and TA. The thickness of the TA-PVP layer was 13
gradually increased from ~450 nm to ~950 nm with the increase of TA concentration from 1.0 to 3.0 wt%.
Transmittance (%)
3436 cm-1
3422 cm-1
TA TA3-PVP/PES TA2-PVP/PES
3422 cm-1 3420 cm-1
TA1-PVP/PES PVP/PES
1725 cm-1
3500
3000
2000
1040 cm-1 1500
1000
-1
Wavenumber (cm ) Fig. 4. FTIR spectra of pure TA and the PVP/PES, TA1-PVP/PES, TA2-PVP/PES, TA3-PVP/PES membranes.
Fig. 4 presents the FTIR spectra of pure TA and the prepared membranes. Compared with the spectrum of the PVP/PES membrane, additional peaks can be found at 1040, 1725 and around 3400 cm-1 in the spectra of both TA and the TA-PVP/PES membranes, corresponding to C-O-C stretching vibrations, C=O stretching vibrations of ester groups and O-H stretching vibrations of phenol groups, respectively [30, 31]. Furthermore, the intensity of these peaks was increased with the increase of TA concentrations. The FTIR results indicated that TA was incorporated into the surface of the TA-PVP/PES membrane and the TA content in membrane was elevated with the increase of TA concentration in the coagulation bath. Moreover, compared with the spectrum of pure TA, the O-H stretching vibration peak in the spectrum of TA1-PVP/PES membrane exhibited an obvious red shift from 3436 to 3420 cm-1, ascribed to the formation of strong hydrogen bonding interactions between TA and PVP [32]. With the TA concentration further increased from 1.0 to 3.0 wt%, the O-H stretching vibration peak slightly shifted to 3422 cm-1, due to the 14
increase of residual free phenol groups of TA on the membrane surface. To further verify the existence of the hydrogen bond between TA and PVP, thermal properties of the membranes were analyzed by DSC, as shown in Fig. 5. Two endothermic relaxation peaks can be observed in each DSC curve, corresponding to the glass transition temperature (Tg) of PVP and PES, which existed in the separated phases [32, 33]. From Fig. 5(a), it can be calculated that of PVP in the PVP/PES membrane was 43.05oC. When TA concentration in the coagulation solution was 1.0 wt%, the Tg of PVP in the resultant TA-PVP/PES membrane was dramatically increased to 47.90oC. The change of Tg was caused by the hydrogen bonding interactions between TA and PVP, which lower the mobility of PVP molecules. Further increase of TA concentration to 3.0 wt% only led to little change of the Tg of PVP to 48.14oC, indicating the slight enhancement of the hydrogen bonding interaction. This was due to the gradual saturation of the coordinated TA molecules for each PVP molecule [32]. In addition, the Tg of PES was also changed with the change of TA concentration in the coagulation, as shown in Fig. 5(b). With the increase of TA concentration to 1.0 wt%, no obvious change of the Tg of PES was observed. This was because most of the TA molecules were coordinated with PVP as mentioned above. Further increase of TA concentration to 3.0 wt% resulted in apparent decrease of the Tg of PES from 222.28oC to 219.77oC. The reason was that the excess TA molecules were bonded to PES through hydrogen bonding interactions between the pyrogallol and ester groups [34]. This may lower the crystallinity of PES and decrease the Tg [33].
15
Fig. 5. DSC 2nd-heating thermograms within the (a) low temperature range and (b) high temperature range of the PVP/PES, TA1-PVP/PES, TA2-PVP/PES and TA3-PVP/PES membranes, respectively.
Aiming to analyze the surface chemical compositions of the membranes quantitatively, XPS was used and wide-scan spectra were shown in Fig. 6(a). From the spectra, it can be seen that the TA/PES membrane presented atomic signals of C1s, O1s, S2s and S2p in the spectrum, while the PVP/PES membrane presented an additional N1s signal. With the addition of TA in the coagulation solution, the intensity of N1s signal was increased, while that of S signals was decreased, simultaneously. The near-surface elemental compositions and Фcomponent were calculated and listed in Table 1. The XPS results showed that ФTA of the TA/PES membrane was only 1.4%. Moreover, the deconvolution of the high-resolution C1s peak showed two signals of *C-C and *C-O for the TA/PES membrane (Fig. 6(b)) but an additional *C=O peak for the PVP/PES and TA2-PVP/PES membranes (Fig. 6(c) and (d)), which was assigned to the ester groups of TA and pyrrolidone groups of PVP. The combined results verified that TA can hardly self-condense and deposit on PES membrane surface during NIPS without the presence of PVP. Without TA in the coagulation solution, the ФPVP value of the PVP/PES membrane was only 19.2%. This 16
was because the interaction between PVP and PES was weak and the majority of PVP molecules were leached into water during NIPS. When the concentration of TA was 1.0 wt%, ФPVP and ФTA were dramatically elevated to 50.88% and 11.84%, respectively, while ФPES was decreased down to 37.28%. The results indicated that the PVP molecules were assembled with TA and generated a TA-PVP layer on the membrane surface. When TA concentration was increased to 2.0 wt%, ФPVP and ФTA were further increased along with the decrease of ФPES. This was because higher TA concentration led to higher cross-linking degree of the TA-PVP layer, which anchored more TA and PVP molecules. However, although further increase of TA concentration to 3.0 wt% resulted in higher ФTA, ФPVP was no longer elevated but decreased a little bit, indicating that the in-situ assembly process within the near-surface region was completed. Moreover, even with the addition of 3.0 wt% TA, there still existed PES with the atomic coverage ratio of 18.72 wt% within the near-surface of the membrane [27]. As the XPS detection depth (~ 10 nm) was much smaller than the observed thickness of the generated TA-PVP layer (Fig. 3), it can be concluded that the TA-PVP layer was inserted inside the PES framework, not upon the membrane surface. This was caused by the “top-down” formation process of the TA-PVP layer. During the in-situ assembly process, a thin nascent TA-PVP layer was first generated, which restricted the penetration of PVP with a rather high molecular weight. Nevertheless, TA molecules, which are relatively small, can still penetrate through the nascent TA-PVP layer. Therefore, further assembly was carried out inside the bulk membrane and the TA-PVP layer was developed inwards.
17
(b)
(a) TA3-PVP/PES TA2-PVP/PES TA1-PVP/PES
*C-C
PVP/PES
*C-O
TA/PES
800
700
600
500
400
300
200
100 290
288
286
284
282
280
Binding energy (eV)
Binding energy(eV)
(d)
(c)
*C-C *C-O (*C-N)
*C-C *C-O (*C-N)
*C=O
290
*C=O
288
286
284
Binding energy(eV)
282
280
290
288
286
284
282
280
Binding energy (eV)
Fig. 6. (a) Wide-scan XPS spectra of the TA/PES, PVP/PES, TA1-PVP/PES, TA2-PVP/PES and TA3-PVP/PES membranes and high-resolution C1s XPS spectra of the (b) TA/PES, (c) PVP/PES and (d) TA2-PVP/PES membranes.
18
Table 1 Near-surface elemental compositions and Фcomponent of the TA/PES, PVP/PES, TA1-PVP/PES, TA2-PVP/PES and TA3-PVP/PES membranes, respectively. TA concentration
Фcomponent (at%)
Near-surface atomic element contents (at%)
(wt%)
C
O
N
S
PES
PVP
TA
PES
69.65
24.19
-
6.16
98.6
-
1.4
0
74.35
18.2
2.4
5.05
80.8
19.2
0
1
74.8
16.51
6.36
2.33
37.28
50.88
11.84
2
82.86
9.04
6.92
1.18
18.88
55.36
25.76
3
81.84
10.61
6.38
1.17
18.72
51.04
30.24
The surface hydrophilicity and charge of the membrane were assessed by static water contact angle and zeta potential measurements and the results were shown in Fig. 7. Fig. 7(a) showed that the PVP/PES membrane exhibited a quite hydrophobic surface with the water contact angle of 62.7 ± 0.8o, ascribed to the large coverage of PES on the membrane surface. With the addition of TA in the coagulation solution, the membrane surface became hydrophilic. This could be explained by the generation of the TA-PVP layer with residual pyrogallol and pyrrolidone groups, which could strongly bind with water molecules. Moreover, the surface hydrophilicity was improved with the elevation of TA concentration from 1.0 to 2.0 wt%. This was because of the higher surface coverage of TA, leading to more bonding sites for water. With the further increase of TA concentration to 3.0 wt%, the water contact angle remained stable because of the saturation of TA on membrane surface. The zeta potential results in Fig. 7(b) showed that all membranes possessed negatively charged surfaces. With the increase of the TA concentration, the membrane surface
19
charge became more negative. This was caused by the increase of TA coverage, leading to more residual phenol groups on the membrane surface, which can generate negative charges after deprotonation. 80
TA concentration (wt%)
(a)
0
1.0
2.0
2.5
3.0
60 50
Zeta poteintial (mV)
Water contact angle (o)
70
0
40 30 20 10 0
0
1.0
1.5
2.0
2.5
-10
-20
-30
-40
3.0
(b)
TA concentration (wt%)
-50
Fig. 7. (a) Water contact angles and (b) zeta potential results of the PVP/PES, TA1.5-PVP/PES, TA2-PVP/PES, TA2.5-PVP/PES and TA3-PVP/PES membranes.
3.3. Separation and antifouling performances of the TA-PVP/PES membranes The effect of TA concentration on the separation performances of the prepared membranes was investigated systematically and the results were shown in Fig. 8(a). The PVP/PES membrane exhibited a pure water flux of 209.1 ± 5.1 Lm-2h-1 and relatively low dye rejections (34.4% for methyl orange and 60.1% for organic GII). This was because the size of the sponge-like pores was larger than those of dye molecules and could only reject some large dye aggregates. With TA concentration increased to 3.0 wt%, the dye rejections were dramatically increased to 82.5% for methyl orange and 97.4% for organic GII, while the pure water flux was first slightly elevated to 234.5 ± 6.8 Lm-2h-1 and then sharply down to 34.3 ± 1.5 Lm-2h-1. The variation of the separation performances was caused by the change of the physical (cross-linking degree and thickness) and chemical (surface hydrophilicity) structures of the TA-PVP layer. 20
The rejections were mainly affected by the cross-linking degree, while the pure water flux was influenced a “trade-off” effect. On one hand, with the increase of TA concentration, the cross-linking degree as well as the thickness of the TA-PVP layer was elevated, resulting higher resistance for permeation of both water and dye molecules [35, 36]. On the other hand, the surface hydrophilicity was enhanced simultaneously, which was beneficial for water absorption and penetration. Under a low TA concentration (1.0 wt%), the chemical structure of the TA-PVP layer dominated the water permeability, leading to a slight increase of the pure water flux. Under high TA concentrations (1.5 wt% to 3.0 wt%), the physical structure took the dominated influence, resulting in a dramatic drop of the pure water flux. After considering the separation performance comprehensively, 2.0 wt% was chosen as the optimized concentration.
200
60
150
40
100
20
50
1.0
1.5
2.0
2.5
3.0
100 120
Rejection (%)
250
80
0
160
(b)
(a) Pure water flux (Lm-2h-1)
Rejection (%)
100
0
120
300
Methyl orange Orange GII Pure water flux
80
60
80
40 40 20
0
0
ge ran yl o h t e
TA concentration (wt%)
Flux (Lm-2h-1)
120
M
G ge an Or
II
0
lue yl b eth
ed
or ng Co
M
lue nb
ia Alc
Fig. 8. (a) Separation performances of the PVP/PES, TA1-PVP/PES, TA1.5-PVP/PES, TA2-PVP/PES, TA2.5-PVP/PES and TA3-PVP/PES membranes. (b) Separation performances of the TA2-PVP/PES membrane for different dyes.
To better verify the broad applicability of the resultant membranes for dye/water separation, the TA2-PVP/PES membrane was chosen to evaluate the separation performance for various dye solutions. From the results (Fig. 8(b)), the membrane exhibited satisfying rejections to organic dyes (73.6% for 21
methyl orange, 90.0% for organic GII, 99.4% for congo red, 99.9% for methyl blue and 99.8% for alcian blue). The permeation fluxes during filtration of dye solutions were a little bit lower than the pure water flux, owing to the concentration polarization and adsorption of dye molecules on the membrane surface. In addition, the separation performances of the TA-PVP/PES membranes were compared with the reported data in literatures, as shown in Fig. S3 with detailed data listed in Table S1 in SI. It can be seen that the TA-PVP/PES membranes exhibited high pure water permeance and dye rejections under a low operation pressure (1 bar) and outperformed the majority of the NF membranes reported in literatures. 300 PVP/PES TA1-PVP/PES
(a) 250
TA1.5-PVP/PES
Flux (Lm-2h-1)
TA2-PVP/PES TA2.5-PVP/PES
200
TA3-PVP/PES 150
100
50
0 0
50
100
150
200
250
300
Time (min)
70
1 2 3
DRir of each cycle
60
Percentage (%)
(b)
DRr of each cycle
50 40
1
1 2
30
3
2 3
2 3 1
2 3 1
20
1
10 0
0
1.0
1.5
2.0
TA content (wt%)
22
2.5
2 3
3.0
100
(c)
Cycle 1
Cycle 2
Cycle 3
FRR (%)
80
60
40
20
0
0
1.0
1.5
2.0
2.5
3.0
TA content (wt%)
Fig. 9. (a) Time-dependent fluxes, (b) DRt (sum of DRr, DRir) and (c) FRR values of the PVP/PES, TA1-PVP/PES, TA1.5-PVP/PES, TA2-PVP/PES, TA2.5-PVP/PES and TA3-PVP/PES membranes during the ternary-cycle filtration test using BSA as the model foulant.
Antifouling properties have been recognized as crucial features of NF membranes in long-term practical applications, such as wastewater purification [1, 37, 38]. In order to evaluate the antifouling performances of the prepared membranes, ternary-cycle filtration tests were carried out using BSA as the model foulant and the time-dependent flux curves were shown in Fig. 9(a). In each cycle, a common tendency can be observed that the permeation flux declined after exposure to BSA solution and recovered to a certain extent after water rinsing. Four indexes (DRt, DRr, DRir and FRR) were applied to evaluate the antifouling performances precisely. As shown in Fig. 9(b), membrane fouling (corresponding to DRt) can be divided into reversible fouling (corresponding to DRr) caused by concentration polarization and formation of the cake layer, and irreversible fouling (corresponding to DRir) caused by BSA adsorption, according to the resistance-in-series model [39, 40]. With the increase of TA concentration, the antifouling performances were obviously enhanced. Taking the first cycle as an example, both of the DRt and DRir values were dramatically decreased from 66.2% and 49.0% for the PVP/PES membrane down to 7.6% and 1.7% for the 23
TA3-PVP/PES membrane, while the FRR value were increased from 51.0% for the PVP/PES membrane up to 98.3% for the TA3-PVP/PES membrane. The antifouling properties are closely related to the chemical structure of the membrane surface. The generation of TA-PVP layer introduced hydrophilic groups to the membrane surface, which can bind with water molecules. Hence a continuous hydration layer was generated over the membrane surface, suppressing BSA adsorption [1]. In addition, as shown in Fig. S4, the TA2-PVP/PES membrane also exhibited a superior antifouling performance in the cross-flow antifouling test, compared with the PVP/PES membrane.
3.4. Membrane stability The long-term stability of the TA-PVP layer was evaluated by measuring the change of the water contact angle of the membrane surface, as shown in Fig. 10. According to the structure verified by SEM and XPS results, the disassembly of the TA-PVP layer may lead to further exposure of the PES matrix and the decrease of surface hydrophilicity. During 20 days, the water contact angle of the PVP/PES membrane was gradually increased. The change of the surface hydrophilicity was caused by the leaching of the PVP molecules to water, which increased the surface coverage of hydrophobic PES matrix. However, the water contact angle of the TA2-PVP/PES membrane remained relatively unchanged. On one hand, the hydrogen bonds between TA and PVP were strong and stable in water [26, 29]. On the other hand, its highly cross-linked structure made it physically anchored to the PES matrix. In addition, the hydrogen bonds between PES and the residual TA molecules of the TA-PVP layer provided additional interactions to maintain the structural stability [34].
24
Normalized water contact angle
1.6
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Time (d) Fig. 10. Water contact angle variation of the PVP/PES and TA2-PVP/PES membrane during 20 days.
In addition, the long-term operational pH stability of the membrane was evaluated and the results were shown in Fig. 11(a)-(c). From Fig. 11(a) and (b), it can be seen that the separation performance of the TA2-PVP/PES membrane remained relatively stable with the pH value at both 4.0 and 9.0. When the pH value was further increased to 10.0, the pure water flux of the membrane was increased while the dye rejections were decreased, indicating that the TA-PVP layer was partially destroyed. This was because under high pH value, the phenol groups of TA were more likely to be deprotonated, decreasing the number of the hydrogen bonding donors. However, even after operation at pH of 10.0 for 72 h, the rejection to orange GII still remained as high as 70%. Therefore, the TA-PVP/PES membrane exhibited high structural stability under a wide range of pH conditions. Considering that the operation pH for NF is around 6.0 [41], the TA-PVP/PES membrane can remain stable during real applications. Chlorine-resistant property is important for NF membranes, due to the demand for chemical cleaning of membranes in real applications. The results in Fig. 11(d) showed that after exposure to the sodium hypochlorite solution (1 g/L) for 72 h, the rejection to orange GII remained stable at about 90%, while the 25
flux was slightly increased. The excellent chlorine-resistant property of the TA-PVP/PES membrane could be ascribed to the existence of radical scavenging phenol groups in the active layer [42]. 300
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Fig. 11. (a)-(c) Long-term operational stability of the TA2-PVP/PES membrane under the pH of 4.0, 9.0 and 10.0, respectively and (d) the time-dependent separation performance of the TA2-PVP/PES membrane after immersion by the sodium hypochlorite solution with a concentration of 1 g/L. 4. Conclusions In summary, a novel TA-PVP/PES TFC membrane was prepared through a facile polyphenol-assisted in-situ assembly approach during phase separation. PVP, the pore-forming agent, was in-situ cross-linked by TA in the coagulation bath through hydrogen bonds, generating a hydrophilic TA-PVP selective layer. The chemical composition and the physical structure of the TA-PVP layer can be facilely tailored by 26
altering TA concentration. Under the optimized condition, the TA-PVP/PES membrane exhibited a high pure water flux (152.9 ± 3.2 Lm-2h-1) with high rejections to various dyes. Compared with the PVP/PES membrane, the TA-PVP/PES membrane showed higher surface hydrophilicity and superior antifouling property. Moreover, the cross-linked structure and strong hydrogen bonding interactions rendered the TA-PVP/PES membrane high stability. Considering the convenience and controllability of the procedure, this approach shows great potential for large-scale commercial membrane preparations. Acknowledgements This research was financially supported by National Natural Science Foundation of China (21621004) and National Key Research and Development Program of China (No. 2016YFB0600503). Appendix. Supplementary Information Supplementary data associated with this article can be found in the online version at…
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Highlights 1.
Nanofiltration membranes were fabricated by polyphenol-assisted in situ assembly.
2.
The assembly process was mediated by hydrogen bonds between TA and PVP.
3.
Both structure and surface composition of the selective layer were manipulated.
4.
The membrane showed high fluxes, dyes rejections and antifouling performance.
5.
The membrane showed satisfying pH and long-term operational stability.
30