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Development of nanofiltration membranes using mussel-inspired sulfonated dopamine for interfacial polymerization
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Development of nanofiltration membranes using mussel-inspired sulfonated dopamine for interfacial polymerization Jincheng Ding, Huiqing Wu **, Peiyi Wu * Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Center for Advanced LowDimension Materials, Donghua University, Shanghai, 201620, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sulfonation Dopamine Nanofiltration Interfacial polymerization
A mussel-inspired sulfonated dopamine (SDA) was synthesized and used as an aqueous monomer to cross-link with trimesoyl chloride (TMC) via interfacial polymerization to prepare nanofiltration membranes. The sul fonic acid groups of SDA were credited with the enhancement of membrane performance, as a result of restraining the process of interfacial polymerization and oxidative-polymerization. Moreover, it also endowed these nanofiltration membranes with more hydrophilic, negatively-charged and smoother surfaces. The opti mized nanofiltration membranes (NF-SDA-0.1) exhibited a water flux as high as 62.2 L∙m 2∙h 1 (0.6 MPa), while maintained applicable salt rejection of Na2SO4 (~90.0%) and dye rejections of Congo red, Methyl blue, Methylene blue and Brilliant green (all >99.9%). Thus, the sulfonated dopamine was considered as a promising candidate as aqueous phase monomer for the preparation of high-performance nanofiltration membranes.
1. Introduction Nanofiltration (NF), a pressure driven membrane technique between ultrafiltration (UF) and reverse osmosis (RO), is getting more and more attention as a tool of water purification and removal of hazardous sub stance [1–6]. As the pivotal components of NF, nanofiltration mem branes, with molecular weight cut off ranging from 200 to 1000 Da, are mostly prepared by an aqueous phase monomer reacting with an organic phase monomer on porous substrates via interfacial polymerization (IP) technique [7–9]. To date, the piperazine (PIP) or m-phenylenediamine (MPD) is commonly used aqueous phase monomer, and trimesoyl chloride (TMC) is always employed as organic phase monomer [10–13]. Nevertheless, these nanofiltration membranes prepared by the tradi tional PIP or MPD cross-linking with TMC usually suffer the disadvan tage of low water flux, which sits ill with their further development [14–16]. Therefore, more and more efforts have been devoted to exploring new monomers for the performance improvement of nano filtration membranes. Interfacial polymerization usually occurs by taking advantage of the reaction between amine groups and acyl chloride groups. Therefore, several multifunctional amine monomers were tried as aqueous mono mers to fabricate NF membranes. Jiang et al. [17] prepared a series of NF
membranes by using different amine monomers including diethylene triamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and piperazidine (PIP), and the results showed all prepared NF membranes exhibited good salt rejections, and the water flux was 15 L∙m 2∙h 1 at 0.2 MPa. Using 3,30 -diaminobenzidine (DAB) as an aqueous monomer by Huang and co-workers [18], the prepared NF membranes performed a Na2SO4 rejection of 84.2% and a water flux of 8.1 L∙m 2∙h 1 at 0.65 MPa. A composite NF membrane prepared by using cyclen and trimesoyl chloride was reported by Xu and co-workers [19]. The optimized membranes maintained a salt rejection of Na2SO4 (>90%) and water flux of 38.9 L∙m 2∙h 1 at 0.6 MPa. Zhang et al. [20] synthesized a new aromatic diamine monomer called 3,5-diaminoben zoylpiperazine (3,5-DABP) by a complicated seven steps reaction. The prepared NF membranes were representative negatively charged styles and the water flux was about 55 L∙m 2∙h 1 at 0.4 MPa. Li et al. [21] obtained the NF membranes by the cross-linked reaction between cyclohexane-1,4-diamine (CHD) and tannic acid (TA). These NF mem branes had a high rejection of Na2SO4 (97%) and a water flux of 35 L∙m 2∙h 1 at 1.0 MPa. As reported by He et al. [22], polyamido amine (PAMAM) was used as an aqueous monomer to prepare nano filtration membranes, and these optimized nanofiltration membranes had a potential application in acid systems with a rejection of Na2SO4
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Wu), [email protected] (P. Wu). https://doi.org/10.1016/j.memsci.2019.117658 Received 14 August 2019; Received in revised form 24 October 2019; Accepted 10 November 2019 Available online 12 November 2019 0376-7388/© 2019 Published by Elsevier B.V.
Please cite this article as: Jincheng Ding, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117658
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(88.3%) and water flux of 12.75 L∙m 2∙h 1 at 0.8 MPa. Besides amine monomers, the polyphenol and multifunctional hy droxyl monomers were also used to prepare nanofiltration membranes. Zhang et al. [23] obtained a polyester NF membrane by interfacial polymerization of pentaerythritol (PE) and trimesoyl chloride (TMC). The optimized membranes exhibited a rejection of Na2SO4 (98.1%), while the water flux was 6.1 L∙m 2∙h 1 at 0.5 MPa. Wu et al. [24,25] reported the preparation of NF membranes using triethanolamine (TEOA) or β-cyclodextrin (β-CD) and TEOA mixture as aqueous mono mers, and the obtained NF membranes exhibited good salt rejection and ant-fouling performance. Moreover, the introduction of hydrophilic groups is also a viable approach for new aqueous monomers. Wang et al. [1] obtained a sulfonated aromatic-diamine monomer potassium 2, 5-bis (4-aminophenoxy) benzenesulfonate (BAPBS), and used it as the aqueous phase monomer to react with TMC for NF membranes. A zwitterionic monomer N-aminoethyl piperazine propane sulfonate (AEPPS) was newly synthesized for aqueous phase monomer by An et al. [26]. Nevertheless, most of these new monomers are not easily obtained, and the water flux of prepared membranes still needs to be improved. Therefore, it is imperative to explore monomers which could be ob tained facilely and help to the membrane property. Dopamine (DA), which is famous as a mussel-inspired “bio-glue”, could form hydrophilic and compact polydopamine layers which could adhere to the surfaces of various substrates under aerobic and alkaline environment [27–29]. Meanwhile, the existence of catechol and amine groups in dopamine endows the reaction with thiols or nitrogen de rivatives by Michael addition or Schiff base reaction for modification [28,30–32]. The deposition process of raw dopamine is usually hetero geneous [33]. Su et al. [34] prepared nanofiltration membranes using dopamine as an aqueous monomer. Different conditions were conducted to explore the separation performance of these dopamine nanofiltration membranes. Nevertheless, the membranes with the rejection (more than 95%) of Orange GII and accepted rejection (70%) of Na2SO4 just had a water flux of 4.2 L∙m 2∙h 1 at 0.2 MPa. Dopamine is also used as a useful assistant in the preparation of nanofiltration membranes [10,35]. Furthermore, dopamine could also be sulfonated by a simple ring opening reaction of 1,3-PS with DA, and then used to modify boehmite nanoparticles to get high compatibility with water-based polymers [36]. In this study, sulfonated dopamine (SDA) was explored as an aqueous monomer by the interfacial polymerization to prepare nanofiltration membranes for the first time. Herein, the sulfonic acid groups of sulfo nated dopamine could contribute to the restraining of interfacial poly merization process as well as the hydrophilicity and charged property of the membranes. The nanofiltration performance of membranes was facilely controlled by changing the immersion time in SDA solution, TMC concentration in organic phase and reaction time as well as the concentration of sulfonated dopamine in aqueous phase. The structure and chemical composition, hydrophilicity, surface morphology and separation performance of these nanofiltration membranes were inves tigated exhaustively.
chloride (MgCl2) and magnesium sulfate (MgSO4) were all analytically grade and purchased from Shanghai Ling Feng Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water was used through all experiments. 2.2. Preparation of sulfonated dopamine (SDA) Sulfonated dopamine (SDA) was prepared as follows [36,37]. Dopamine (DA, 2.27 g) was firstly dissolved in 300 mL of anhydrous ethanol to form a homogenous solution in a round flask. Afterwards, 1.6 g of 1.3-propanesulfonate and 0.832 mL of ammonia solution were successively added into the DA/ethanol solution. And the resulting so lution was heated at 50 � C for 18 h. The final SDA (yield is 40%) was obtained by filtrating from the final solution and washing by ethanol bath several times, followed by drying at 40 � C in an oven for 24 h. The chemical reaction for preparing SDA is presented in Scheme 1. Furthermore, the chemical structure of SDA was characterized by FTIR and 1H NMR (Bruker 400 MHz) using D2O as the solvent. 2.3. The preparation process of nanofiltration membranes The hydrolyzed polyacrylonitrile (HPAN) membranes were used as the substrates to prepare NF membranes. The HPAN substrates were obtained by immersing PAN membranes in sodium hydroxide solution (2.0 mol/L) for 60 min at 50 � C, followed by thoroughly washing by DI water. Then the nanofiltration membranes were prepared by a tradi tional interfacial polymerization process as shown in Fig. 1. Firstly, the HPAN membranes were immersed in the aqueous phase (Tris-HCl buffer solution, pH ¼ 8.5) containing a certain amount of SDA for different times (5–60 min), then these membranes were cleaned and dried in air. Secondly, the interfacial polymerization was conducted by immersing these membranes into the organic phase (0.05–0.2% (w/v) TMC in cyclohexane solution) for a certain period of time (0.5–5 min). These membranes were then immediately placed in an oven for 30 min at a fixed temperature of 60 � C. The resulting membranes were denoted as NF-SDA-x (where x is the concentration of sulfonated dopamine in aqueous phase (w/v)). For example, NF-SDA-0.1 refers to the concen tration of sulfonated dopamine in aqueous phase is 0.1% (w/v), TMC Table 1 Properties of the commercial dyes used in this study. Dye name
Chemical structure
Molecular weight (g/mol)
Congo red
696
Methyl blue
799
Methylene blue
320
Brilliant green
483
2. Experimental 2.1. Materials and reagents Polyacrylonitrile (PAN) ultrafiltration (UF) membranes were sup plied by Ande Membrane Separation Technology Engineering (Beijing) Co. Ltd. Dopamine hydrochloride (DA), tris-(hydroxymethyl) amino methane (Tris), 1,3,5-benzenetricarbonyl trichloride (TMC), 1,3-pro panesulfonate (1,3-PS), ammonia solution, cyclohexane, anhydrous ethanol, polyethylene glycol (PEG, molecular weight 200, 400, 600, 800, 1000, 1500, 2000 and 4000 Da), bovine serum albumin (BSA, 99%), Congo red, Methyl blue, Methylene blue and Brilliant green were all obtained from Aladdin Reagent Co. Ltd (Shanghai, China). The basic characteristic properties of these commercial dyes were shown in Table 1. Sodium chloride (NaCl), sodium sulfate (Na2SO4), magnesium 2
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Scheme 1. Synthesis process of sulfonated dopamine.
Fig. 1. The preparation process of nanofiltration membranes (NF-SDA).
concentration in organic phase is 0.15% (w/v) and the immersion time in SDA and TMC solutions is 20 min and 1 min, respectively. The membrane named NF-DA was prepared via interfacial polymerization between trimesoyl chloride (TMC) and dopamine (DA) of the same concentration for comparison. All prepared membranes were kept in warm DI water for around 24 h before using.
testing, these membranes were pre-treated under 0.6 MPa pressure for 10 min. Then, the water flux was measured using DI water as feed at an operation pressure of 0.6 MPa. The water flux (Jwf , L⋅m 2⋅h 1) was calculated by the following equation:
2.4. Membrane characterization
where V (L) is the volume of permeated water; A (m2) is the effective membrane area and Δt (h) is the filtration time. In order to evaluate the rejection performance of these NF mem branes, four salt solutions (1 g/L) including Na2SO4, MgSO4, MgCl2 and NaCl and four typical dyes solutions (0.1 g/L) including Congo red, Methyl blue, Methylene blue and Brilliant green were used as feed so lutions. The permeation flux was calculated by equation (1) and rejec tion ratio (R) was defined as the following equation: � � Cp R¼ 1 � 100% (2) Cf
Jwf ¼
2.4.1. FTIR and XPS characterization Fourier transform infrared spectroscopy (FTIR, Nicolet-iS50) and Xray photoelectron spectroscopy (XPS, Escalab 250Xi) were conducted to study the chemical structure and composition of these NF membranes. Before testing, all membranes were clean and dried in a vacuum oven. 2.4.2. SEM and AFM observation The morphology and roughness of these NF membranes were observed by scanning electron microscopy (SEM, SU8010, Hitachi, Japan) and atomic force microscopy (AFM, Dimension Icon, Germany), respectively.
V A⋅Δt
(1)
where Cp and Cf were the concentration of permeate and feed solution, respectively. The concentrations of salt solutions were measured by an electrical conductivity (Mettler Toledo, FE38-Standard). Dye concen trations of Congo red, Methyl blue, Methylene blue and Brilliant green solutions were determined with a UV–vis spectrophotometer (Perki nElmer, Lambda 950) at a wavelength of 499 nm, 599 nm, 665 nm and 623 nm, respectively. At least three membrane samples were used for all separation performance measurements.
2.4.3. Contact angle and ζ-potential measurement The hydrophilicity of the membranes was characterized by using a contact angle goniometer (OCA-20, Data-physics, Germany). The sur face ζ-potential of these membranes was measured by an instrument (SurPASS3, AntonPaar, Austria) with 1 mM KCl as the electrolyte solu tion at pH 6.5. 2.5. Performance of nanofiltration membranes
2.6. Long-term stability measurements
The separation performance of these NF membranes was investi gated by a commercial laboratory scale cross-flow flat membrane module with an effective area of 10.2 cm2 at room temperature. Before
The long-term stability measurement of permeation flux and Na2SO4 rejection for the optimal membranes was conducted using 1 g/L Na2SO4 solution as feed at 0.6 MPa for more than 2500 min. 3
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3. Results and discussion
structure constriction and corresponding to a decrease in water flux. Therefore, it is reasonable to employ SDA rather than DA in interfacial polymerization on purpose to harvest NF membranes with high water permeability.
3.1. The synthesis and analysis of sulfonated dopamine (SDA) Sulfonated dopamine was firstly synthesized and characterized. The H NMR and FTIR spectra of SDA and raw DA are presented in Fig. 2. In the spectrum of DA, the proton signals at 6.90–7.86 (g1, h1, f1) and 2.5–3.5 (d1, e1) ppm could be assigned to the characteristic multihydrogen atoms of DA [37,38]. For the spectrum of SDA, the new pro ton signals which appear at 2.01 (b), 2.89 (a) and 3.19 (c) ppm are attributed to the hydrogen atoms of carbon chain (-CH2-CH2-CH2-SO3H) from the ring opening reaction of 1,3-PS with the primary amine groups of DA [37]. Moreover, from the FTIR spectrum of SDA (Fig. 2b), comparing with the raw DA, a new absorption band appears at 1054 cm 1, and it belongs to the symmetric stretching vibrations of –SO-3 groups [39,40]. All results above indicate the successful synthesis of SDA. Thanks to the existence of sulfonic acid groups in SDA, the oxidativepolymerization of SDA is quite different from DA. The UV–vis results following the oxidative-polymerization process of SDA and DA solutions in the Tris-HCl buffer solution (pH ¼ 8.5) are shown in Fig. 3. At the beginning (0 h), there is no absorption peak presented in the spectra of DA and SDA solution in the range of 350–450 nm (Fig. 3a). After 12 h, the UV absorption peaks (350–450 nm) of DA solution increase, which shows the enhancement of the conjugate structure via inter-/intra-mo lecular polymerization to form polydopamine [35,41,42]. By contrast, no peak appears for SDA solution. It indicates the existence of sulfonic acid groups in SDA restrains inter-/intra-molecular polymerization due to the electrostatic repulsion effect. From the photos of DA and SDA (Fig. 3b) after 12 h oxidation, it is obvious the SDA solution is still transparent without visible aggregation (faint yellow), while the orig inal DA solution presents black brown as a result of the higher reactivity of DA to form polydopamine. Furthermore, when the reactive time ex ceeds 24 h, the color of SDA solution becomes black brown but no visible aggregation could be observed, because of the existence of sulfonic acid groups in SDA, the self-aggregate speed of SDA is low due to the elec trostatic repulsion effect. Therefore, very small PSDA oligomers and other oxidation products could suspend in the solution, leading to the black brown but no visible aggregation of SDA solution. For interfacial polymerization using DA, after immersing in DA aqueous solution for 20 min, a polydopamine layer with a thickness of 2.5 nm would be formed on the surface of the substrate [43]. The polydopamine depo sition probably occurs within the membrane pore structure, leading to 1
3.2. Characterization of nanofiltration membranes 3.2.1. Chemical structure and composition of nanofiltration membranes Fig. 4 shows the FTIR spectra of PAN, HPAN, NF-DA-0.1 and NFSDA-0.1 membranes. For all membranes, the peak at 2242 cm 1 is assigned to the stretching vibration of -CN groups [44]. Compared with PAN membrane, a new peak at 1560 cm 1 which is associated with the stretching vibration of –COO- groups appears in the HPAN membrane, indicating the abundant hydrophilic carboxylate groups after hydro lyzation by NaOH on the HPAN membrane [45]. For NF-SDA-0.1 and NF-DA-0.1 membrane, compared with HPAN membrane, the new peak at 1116 cm 1 is ascribed to the stretching vi bration of C–O–C formed by the reaction between the hydroxyl groups of SDA or DA and acyl chloride groups of TMC. It indicates successful interfacial polymerization process between SDA or DA and TMC. The amine groups of SDA and DA could also react with the acyl chloride groups of TMC to produce amide groups. Nevertheless, the absorbance of stretching vibration of amide groups overlaps with the inherent stretching vibration at 1660 cm 1 of substrate membranes, leading to the increment of the intensity of NF-DA-0.1 and NF-SDA-0.1 at 1660 cm 1. Moreover, comparing with NF-DA-0.1 membranes, a new peak at 1043 cm 1 in the spectrum of NF-SDA-0.1 membrane is ascribed to the stretching vibration of –SO-3 groups [40], which suggests the ex istence of SDA. Meanwhile, the detailed spectra and element compositions (C, N, O, S) were measured by XPS as shown in Fig. 5 and Table S1 (in supple mental information), respectively. From the XPS results in Fig. 5, NFSDA-0.1 has the highest peak intensity of O1S at 531 eV, NF-DA-0.1 comes second, and the intensity of HPAN substrate is the lowest (The atom percentage of oxygen is ~22.6%, ~20.0% and ~13.5%, respec tively, from Table S1). Moreover, in the spectrum of NF-SDA-0.1 membrane, a new peak appears at 167 eV, which is ascribed to S2p. Those all indicates the existence of sulfonic acid groups and dopamine on the prepared membranes using SDA and DA as aqueous monomers. 3.2.2. Surface morphology of membranes The surface morphology SEM and three-dimensional AFM results of HPAN, NF-SDA-0.1 and NF-DA-0.1 membranes are shown in Fig. 6. After
Fig. 2. (a) 1H NMR spectra; (b) FTIR spectra of SDA and raw DA. 4
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Fig. 3. (a) UV–vis spectra of DA and SDA aqueous solution (pH ¼ 8.5) at reaction time of 0 h and 12 h; (b) The image of DA and SDA solution after the reaction time of 12 h.
Fig. 4. FTIR spectra of (a) PAN; (b) HPAN; (c) NF-DA-0.1 and (d) NF-SDA-0.1 membranes.
aggregation of DA and crosslink with TMC. Compared with NF-DA-0.1, NF-SDA-0.1 membrane presents a smoother surface, as a result of moderate reaction rate of SDA. Observed from AFM images, compared with HPAN substrates (Ra ¼ 17.2 � 0.1 nm), both nanofiltration membranes have higher roughness. The lower roughness of NF-SDA-0.1 (18.2 �1.3 vs 27.9 �1.9 nm) membrane is related to the existence of sulfonic acid groups in SDA, which are useful to restrain oxidative-polymerization and the reaction with TMC, and correspondingly form homogeneous surface coating on the substrate during the process of interfacial poly merization. Furthermore, the results of AFM roughness measurement are consistent with SEM observation.
Fig. 5. XPS spectra HPAN membranes.
of
(a)
NF-SDA-0.1;
(b)
NF-DA-0.1
and
3.2.3. Surface hydrophilicity of membranes The hydrophilicity of membrane surfaces was characterized in terms of water contact angle as shown in Fig. 7. Compared with raw PAN membrane, HPAN substrate membrane has a high hydrophilicity with water contact angle of 38.1� , because of the abundant carboxylic acid groups on the surface. After surface coating, the water contact angle of NF-SDA-0.1 and NF-DA-0.1 membranes is 52.4� and 61.5� , respectively. Moreover, the NF-SDA-0.1 membranes have a lower water contact angle i.e. higher hydrophilicity than NF-DA-0.1 membranes, thanks to the hydrophilic sulfonic acid groups in SDA.
(c)
the interfacial polymerization, both the NF-SDA-0.1 and NF-DA-0.1 membranes have dense surfaces. The surface of NF-DA-0.1 membrane displays abundant “volcano” nodular, due to the heterogeneous 5
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Fig. 6. SEM and three-dimensional AFM images of (a, a1) HPAN; (b, b1) NF-SDA-0.1 and (c, c1) NF-DA-0.1.
Fig. 7. The water contact angle of PAN, HPAN, NF-SDA-0.1 and NF-DA0.1 membranes. Fig. 8. Effect of SDA concentration in aqueous phase on the separation per formance of NF-SDA-x membranes at 0.6 MPa.
3.3. Nanofiltration performance analysis 3.3.1. The effect of the concentration of sulfonated dopamine (SDA) in aqueous phase The NF membranes were prepared with different concentration of SDA in aqueous phase and the corresponding separation performance are presented in Fig. 8. Along with the increment of SDA concentration (0.05–0.6%), the water flux decreases from 151 to 27.5 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 45.9 to 93.5%. The higher con centration of SDA in aqueous phase brings more intensive cross-linking reaction with TMC during the interfacial polymerization process. The density of NF-SDA membranes is enhanced, resulting in the reduction of water flux and the increment of rejection. The rejection of Na2SO4 reaches up to 93.5% when the SDA concentration is 0.6% (w/v). Usually the existence of sulfonic acid groups may bring higher rejection of Na2SO4, as a result of the stronger Donnan’s effect of negatively charged bivalent sulfonic acid groups than monovalent carboxylic acid groups. Nevertheless, the inhibition effect to the cross-linking reaction of sul fonic acid groups in SDA because of stereo-hindrance effect and the low diffusion of SDA to organic phase [1], leading to weak negatively charged surfaces and loose interface polymerization layers to some extent [3]. The molecular weight cut-off of NF-SDA-0.1 was also measured and shown in Fig. S3 of supplemental information. The MWCO of NF-SDA-0.1 is around 2000 Da and the pore of NF-SDA-0.1 is calculated to be ~1.15 nm. Therefore, the sulfonic acid groups in SDA are favorable to the improvement of water flux. From the results, when
the concentration of SDA is 0.1%, the NF-SDA-0.1 membrane demon strates a high water flux of 62.2 L∙m 2∙h 1 and the Na2SO4 rejection of 90.0%, and it is considered as an optimal one for further characterization. 3.3.2. The effect of the immersion time in SDA solution The NF membranes with different immersion time in SDA solution were prepared and the corresponding separation properties were pre sented in Fig. 9. Obviously, with the increment of immersion time in SDA aqueous phase solution (5–20 min), the water flux decreases from 192 to 62.2 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 41.7% to 90.0%. It indicates the cross-linked reaction between SDA and TMC is enhanced, thanks to the adsorption of SDA on the surface of HPAN substrate is increased. When the immersion time continues to prolong (>20 min), the flux and rejection almost keep consistent, as a result of the saturated adsorption of SDA on the surface. An immersion time of 20 min is considered as the optimal parameter based on the balance of appropriate separation performance and time-saving operation. 3.3.3. The effect of the concentration of TMC in organic phase The NF separation performance of membranes prepared with various concentration of TMC in organic phase is presented in Fig. 10. With the increase of TMC concentration (0.05–0.2%), the water flux decreases 6
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Fig. 9. Effect of the immersion time in SDA solution on the separation per formance of NF-SDA-0.1 membranes at 0.6 MPa.
Fig. 11. Effect of the immersion time in TMC solution on the separation per formance of NF-SDA -0.1membranes at 0.6 MPa.
from 290 to 37.0 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 35.1 to 94.1% due to more intensive cross-linking reaction between TMC in organic phase and SDA during the interfacial polymerization process. The density of NF-SDA membranes is enhanced, resulting in the reduc tion of water flux and the increment of rejection. According to the water flux and rejection, the TMC concentration of 0.15% is selected as the optimal value for other characterization.
sulfonic acid groups in SDA restrain the reaction with TMC and endow NF-SDA-0.1 with a looser and hydrophilic interface polymerization layer for the increment of water flux. Besides, NF-SDA-0.1 membrane is also compared with some commercial and lab-scale NF membranes in terms of the water flux and salt rejection in Table 2. Apparently, for all commercial NF membranes or modified new NF membranes, the water flux is in the range of 1.22–9.20 L∙m 2∙h 1. It should be noted that the NF-SDA-0.1 membranes in this work show an excellent water flux that is up to 10.4 L∙m 2∙h 1∙bar 1, while the rejection is also acceptable. This proves sulfonated dopamine is a promising aqueous monomer for the preparation of high-performance NF membranes. The rejection and permeation flux of different salts of NF-SDA-0.1 membranes are shown in Fig. 12b. The NF-SDA-0.1 membranes exhibit a high permeation flux for different salts, and a salt rejection sequence of Na2SO4 > MgSO4 > NaCl > MgCl2 which conforms to the typically negatively charged NF membranes from previous works [1,46] and also matches with the zeta potential value ( 4.76 mV at pH ¼ 6.5, Fig. S1 in supplemental information). Furthermore, the rejection of NaCl is higher than MgCl2 in NF-SDA-0.1 membranes. On one hand, the ex istence of sulfonic acid groups in SDA restrain the reaction with TMC and lead to a looser and hydrophilic interface polymerization layer of NF-SDA-0.1, on the other hand, the negatively charged membrane sur face would provide stronger electrostatic attraction for bivalent Mg ions than monovalent Na ions [46]. Interestingly, the selectivity of different salts of two membranes is shown in Fig. 12c. Comparing NF-DA-0.1 membranes, the NF-SDA-0.1 membranes have higher salt selectivity (SNa2 SO4 =MgCl2 ¼ 11:3) than that of NF-DA-0.1 membranes (SNa2 SO4 =MgCl2 ¼ 2:02). Moreover, the salt selectivity (SNa2 SO4 =MgCl2 ) of NF-SDA-0.1 membranes is competitive by calculating the selectivity from the previous works [47–49], which can be used to separate MgCl2 from the mixed solution of MgCl2 and Na2SO4.
3.3.4. The effect of the immersion time in TMC solution The separation performance of NF membranes prepared with different immersion time in TMC organic phase solution is presented in Fig. 11. The results of separation performance emerge that water flux decreases and rejection increases. Along with the increment of immer sion time in TMC solution (0.5–5 min), the water flux decreases from 101 to 41.0 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 75.3 to 92.0%. It indicates the increased cross-linking reaction of aqueous and organic phase along with the increment of immersion time in TMC. In terms of overall separation performance, 1.0 min is considered as the optimal immersion time. 3.3.5. The performance of sulfonated dopamine nanofiltration membranes The separation performance of NF-SDA-0.1 membranes is presented in Fig. 12. From Fig. 12a, obviously, the water flux of NF-SDA-0.1 (62.2 L∙m 2∙h 1) is much higher than NF-DA-0.1 membranes (10.8 L∙m 2∙h 1), while both NF-DA-0.1 and NF-SDA-0.1 membranes have the similar high rejection of Na2SO4 (>90%). So it is advisable that
3.3.6. Analysis of dyes rejection The dye separation performance were tested for the optimal NF-SDA0.1 membranes using four representative dyes (negatively-charged Congo red and Methyl blue; positively-charged Methylene blue and Brilliant green). From Fig. 13, it is observed that the NF-SDA-0.1 membrane exhibits excellent rejection (>99.0%) to all these dyes. Moreover, after the filtration of negatively-charged dyes (Congo red and Methyl blue), there are few dyes left on the membrane surfaces. It is indicated the good ability of the membrane to resist the fouling of negatively charged dyes, as a result of the special chemical structure of SDA. The sulfonic acid groups and secondary amine groups in the SDA is helpful to the hydrophilicity of membrane and form hydration layer on the surface, and negatively-charged sulfonic acid groups also offer electrostatic repulsion effect. Therefore, the NF-SDA-0.1 membranes
Fig. 10. Effect of TMC concentration in organic phase on the separation per formance of NF-SDA-0.1 membranes at 0.6 MPa. 7
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Fig. 12. (a) Nanofiltration performance of NF-DA-0.1 and NF-SDA-0.1 membranes at 0.6 MPa; (b) Permeation flux and rejection of NF-SDA-0.1 membrane to different salts; (c) Salt selectivity of NF-DA-0.1 and NF-SDA-0.1 membranes. Table 2 The comparison of NF-SDA-0.1 membranes with commercial and lab-scale NF membranes. NaCl rejection (%)
Na2SO4 rejection (%)
Reference
1.22 6.48
49.7 10.1
98.1 91.4
[23] [19]
7.10 9.20 2.15 5.05 10.4
83.5 51.0 45.0 50.0 31.5
/ 92.0 95.0 78.0 90.0
[50] [51] [51] [52] This work
Membrane
Water flux (L∙m 2∙h 1∙bar
PE-TMC CyclenTMC NF-90 NTR-7450 NF-40 NS-300 SDA-TMC
1
)
could have a good antifouling ability of negatively-charged dyes and BSA (shown in Fig. S2 of supplemental information). 3.4. The long-time stability measurement A long-time operation was conducted on NF-SDA-0.1 for separating Na2SO4 solution (1 g/L) at 0.6 MPa. The normalized permeation flux (Jt/ J0) and the normalized rejection (Rt/R0) are presented in Fig. 14. Obviously, both permeation flux and salt rejection are quite stable during testing period, suggesting the NF-SDA-0.1membranes has a good structure and performance stability.
Fig. 13. (a) The rejection of NF-SDA-0.1 membranes to Congo red, Methyl blue, Methylene blue and Brilliant green; (b) The photo images of NF-SDA-0.1 membranes after the filtration of Congo red and Methyl blue. (For interpreta tion of the references to color in this figure legend, the reader is referred to the Web version of this article).
4. Conclusions
membranes. The hydrophilic sulfonic acid groups of SDA could restrain inter-/intra-molecular polymerization and the reaction with TMC. So SDA could endow nanofiltration membranes hydrophilic and smooth
Sulfonated dopamine (SDA) was prepared by the ring opening re action of 1,3-PS with DA and then used as a aqueous monomer in interfacial polymerization for the preparation of nanofiltration 8
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Fig. 14. The long-time operation stability of NF-SDA-0.1 membrane.
surfaces. The optimal NF-SDA-0.1 membranes performed a high water flux of 62.2 L∙m 2∙h 1, five times higher than that of NF-DA-0.1 membranes and a considerable rejection of 90.0% to Na2SO4 at 0.6 MPa. Meanwhile, the NF-SDA-0.1 membranes had excellent re jections (>99.9%) to four representative dyes including Congo red, Methyl blue, Methylene blue and Brilliant green. Moreover, NF-SDA-0.1 membranes also presented a long-time operational stability. This work provides enlightenment for preparing high-flux nanofiltration mem branes using sulfonated monomers and explaining the roles sulfonated monomers play in the membrane performance. Declaration of competing interest There is no conflict of interest. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 21704085) and the Fundamental Research Funds for the Central Universities (No. 2232019G-04). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117658. References [1] Z. Lv, J. Hu, J. Zheng, X. Zhang, L. Wang, Antifouling and high flux sulfonated polyamide thin-film composite membrane for nanofiltration, Ind. Eng. Chem. Res. 55 (2016) 4726–4733. [2] M. Paul, S.D. Jons, Chemistry and fabrication of polymeric nanofiltration membranes: a review, Polymer 103 (2016) 417–456. [3] Z. Zhang, G. Kang, H. Yu, Y. Jin, Y. Cao, Fabrication of a highly permeable composite nanofiltration membrane via interfacial polymerization by adding a novel acyl chloride monomer with an anhydride group, J. Membr. Sci. 570–571 (2019) 403–409. [4] R. Wang, X. Shi, A. Xiao, W. Zhou, Y. Wang, Interfacial polymerization of covalent organic frameworks (COFs) on polymeric substrates for molecular separations, J. Membr. Sci. 566 (2018) 197–204. [5] L. Shan, J. Gu, H. Fan, S. Ji, G. Zhang, Microphase diffusion-controlled interfacial polymerization for an ultrahigh permeability nanofiltration membrane, ACS Appl. Mater. Interfaces 9 (2017) 44820–44827. [6] P. Xu, J.E. Drewes, T.U. Kim, C. Bellona, G. Amy, Effect of membrane fouling on transport of organic contaminants in NF/RO membrane applications, J. Membr. Sci. 279 (2006) 165–175. [7] V. Freger, Nanoscale heterogeneity of polyamide membranes formed by interfacial polymerization, Langmuir 19 (2003) 4791–4797. [8] J.R. Werber, C.O. Osuji, M. Elimelech, Materials for next-generation desalination and water purification membranes, Nat. Rev. Mater. 1 (2016) 16018–16034. [9] A.W. Mohammad, Y.H. Teow, W.L. Ang, Y.T. Chung, D.L. Oatley-Radcliffe, N. Hilal, Nanofiltration membranes review: recent advances and future prospects, Desalination 356 (2015) 226–254.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Development of nanofiltration membranes using mussel-inspired sulfonated dopamine for interfacial polymerization Jincheng Ding, Huiqing Wu **, Peiyi Wu * Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Center for Advanced LowDimension Materials, Donghua University, Shanghai, 201620, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sulfonation Dopamine Nanofiltration Interfacial polymerization
A mussel-inspired sulfonated dopamine (SDA) was synthesized and used as an aqueous monomer to cross-link with trimesoyl chloride (TMC) via interfacial polymerization to prepare nanofiltration membranes. The sul fonic acid groups of SDA were credited with the enhancement of membrane performance, as a result of restraining the process of interfacial polymerization and oxidative-polymerization. Moreover, it also endowed these nanofiltration membranes with more hydrophilic, negatively-charged and smoother surfaces. The opti mized nanofiltration membranes (NF-SDA-0.1) exhibited a water flux as high as 62.2 L∙m 2∙h 1 (0.6 MPa), while maintained applicable salt rejection of Na2SO4 (~90.0%) and dye rejections of Congo red, Methyl blue, Methylene blue and Brilliant green (all >99.9%). Thus, the sulfonated dopamine was considered as a promising candidate as aqueous phase monomer for the preparation of high-performance nanofiltration membranes.
1. Introduction Nanofiltration (NF), a pressure driven membrane technique between ultrafiltration (UF) and reverse osmosis (RO), is getting more and more attention as a tool of water purification and removal of hazardous sub stance [1–6]. As the pivotal components of NF, nanofiltration mem branes, with molecular weight cut off ranging from 200 to 1000 Da, are mostly prepared by an aqueous phase monomer reacting with an organic phase monomer on porous substrates via interfacial polymerization (IP) technique [7–9]. To date, the piperazine (PIP) or m-phenylenediamine (MPD) is commonly used aqueous phase monomer, and trimesoyl chloride (TMC) is always employed as organic phase monomer [10–13]. Nevertheless, these nanofiltration membranes prepared by the tradi tional PIP or MPD cross-linking with TMC usually suffer the disadvan tage of low water flux, which sits ill with their further development [14–16]. Therefore, more and more efforts have been devoted to exploring new monomers for the performance improvement of nano filtration membranes. Interfacial polymerization usually occurs by taking advantage of the reaction between amine groups and acyl chloride groups. Therefore, several multifunctional amine monomers were tried as aqueous mono mers to fabricate NF membranes. Jiang et al. [17] prepared a series of NF
membranes by using different amine monomers including diethylene triamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and piperazidine (PIP), and the results showed all prepared NF membranes exhibited good salt rejections, and the water flux was 15 L∙m 2∙h 1 at 0.2 MPa. Using 3,30 -diaminobenzidine (DAB) as an aqueous monomer by Huang and co-workers [18], the prepared NF membranes performed a Na2SO4 rejection of 84.2% and a water flux of 8.1 L∙m 2∙h 1 at 0.65 MPa. A composite NF membrane prepared by using cyclen and trimesoyl chloride was reported by Xu and co-workers [19]. The optimized membranes maintained a salt rejection of Na2SO4 (>90%) and water flux of 38.9 L∙m 2∙h 1 at 0.6 MPa. Zhang et al. [20] synthesized a new aromatic diamine monomer called 3,5-diaminoben zoylpiperazine (3,5-DABP) by a complicated seven steps reaction. The prepared NF membranes were representative negatively charged styles and the water flux was about 55 L∙m 2∙h 1 at 0.4 MPa. Li et al. [21] obtained the NF membranes by the cross-linked reaction between cyclohexane-1,4-diamine (CHD) and tannic acid (TA). These NF mem branes had a high rejection of Na2SO4 (97%) and a water flux of 35 L∙m 2∙h 1 at 1.0 MPa. As reported by He et al. [22], polyamido amine (PAMAM) was used as an aqueous monomer to prepare nano filtration membranes, and these optimized nanofiltration membranes had a potential application in acid systems with a rejection of Na2SO4
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Wu), [email protected] (P. Wu). https://doi.org/10.1016/j.memsci.2019.117658 Received 14 August 2019; Received in revised form 24 October 2019; Accepted 10 November 2019 Available online 12 November 2019 0376-7388/© 2019 Published by Elsevier B.V.
Please cite this article as: Jincheng Ding, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117658
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(88.3%) and water flux of 12.75 L∙m 2∙h 1 at 0.8 MPa. Besides amine monomers, the polyphenol and multifunctional hy droxyl monomers were also used to prepare nanofiltration membranes. Zhang et al. [23] obtained a polyester NF membrane by interfacial polymerization of pentaerythritol (PE) and trimesoyl chloride (TMC). The optimized membranes exhibited a rejection of Na2SO4 (98.1%), while the water flux was 6.1 L∙m 2∙h 1 at 0.5 MPa. Wu et al. [24,25] reported the preparation of NF membranes using triethanolamine (TEOA) or β-cyclodextrin (β-CD) and TEOA mixture as aqueous mono mers, and the obtained NF membranes exhibited good salt rejection and ant-fouling performance. Moreover, the introduction of hydrophilic groups is also a viable approach for new aqueous monomers. Wang et al. [1] obtained a sulfonated aromatic-diamine monomer potassium 2, 5-bis (4-aminophenoxy) benzenesulfonate (BAPBS), and used it as the aqueous phase monomer to react with TMC for NF membranes. A zwitterionic monomer N-aminoethyl piperazine propane sulfonate (AEPPS) was newly synthesized for aqueous phase monomer by An et al. [26]. Nevertheless, most of these new monomers are not easily obtained, and the water flux of prepared membranes still needs to be improved. Therefore, it is imperative to explore monomers which could be ob tained facilely and help to the membrane property. Dopamine (DA), which is famous as a mussel-inspired “bio-glue”, could form hydrophilic and compact polydopamine layers which could adhere to the surfaces of various substrates under aerobic and alkaline environment [27–29]. Meanwhile, the existence of catechol and amine groups in dopamine endows the reaction with thiols or nitrogen de rivatives by Michael addition or Schiff base reaction for modification [28,30–32]. The deposition process of raw dopamine is usually hetero geneous [33]. Su et al. [34] prepared nanofiltration membranes using dopamine as an aqueous monomer. Different conditions were conducted to explore the separation performance of these dopamine nanofiltration membranes. Nevertheless, the membranes with the rejection (more than 95%) of Orange GII and accepted rejection (70%) of Na2SO4 just had a water flux of 4.2 L∙m 2∙h 1 at 0.2 MPa. Dopamine is also used as a useful assistant in the preparation of nanofiltration membranes [10,35]. Furthermore, dopamine could also be sulfonated by a simple ring opening reaction of 1,3-PS with DA, and then used to modify boehmite nanoparticles to get high compatibility with water-based polymers [36]. In this study, sulfonated dopamine (SDA) was explored as an aqueous monomer by the interfacial polymerization to prepare nanofiltration membranes for the first time. Herein, the sulfonic acid groups of sulfo nated dopamine could contribute to the restraining of interfacial poly merization process as well as the hydrophilicity and charged property of the membranes. The nanofiltration performance of membranes was facilely controlled by changing the immersion time in SDA solution, TMC concentration in organic phase and reaction time as well as the concentration of sulfonated dopamine in aqueous phase. The structure and chemical composition, hydrophilicity, surface morphology and separation performance of these nanofiltration membranes were inves tigated exhaustively.
chloride (MgCl2) and magnesium sulfate (MgSO4) were all analytically grade and purchased from Shanghai Ling Feng Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water was used through all experiments. 2.2. Preparation of sulfonated dopamine (SDA) Sulfonated dopamine (SDA) was prepared as follows [36,37]. Dopamine (DA, 2.27 g) was firstly dissolved in 300 mL of anhydrous ethanol to form a homogenous solution in a round flask. Afterwards, 1.6 g of 1.3-propanesulfonate and 0.832 mL of ammonia solution were successively added into the DA/ethanol solution. And the resulting so lution was heated at 50 � C for 18 h. The final SDA (yield is 40%) was obtained by filtrating from the final solution and washing by ethanol bath several times, followed by drying at 40 � C in an oven for 24 h. The chemical reaction for preparing SDA is presented in Scheme 1. Furthermore, the chemical structure of SDA was characterized by FTIR and 1H NMR (Bruker 400 MHz) using D2O as the solvent. 2.3. The preparation process of nanofiltration membranes The hydrolyzed polyacrylonitrile (HPAN) membranes were used as the substrates to prepare NF membranes. The HPAN substrates were obtained by immersing PAN membranes in sodium hydroxide solution (2.0 mol/L) for 60 min at 50 � C, followed by thoroughly washing by DI water. Then the nanofiltration membranes were prepared by a tradi tional interfacial polymerization process as shown in Fig. 1. Firstly, the HPAN membranes were immersed in the aqueous phase (Tris-HCl buffer solution, pH ¼ 8.5) containing a certain amount of SDA for different times (5–60 min), then these membranes were cleaned and dried in air. Secondly, the interfacial polymerization was conducted by immersing these membranes into the organic phase (0.05–0.2% (w/v) TMC in cyclohexane solution) for a certain period of time (0.5–5 min). These membranes were then immediately placed in an oven for 30 min at a fixed temperature of 60 � C. The resulting membranes were denoted as NF-SDA-x (where x is the concentration of sulfonated dopamine in aqueous phase (w/v)). For example, NF-SDA-0.1 refers to the concen tration of sulfonated dopamine in aqueous phase is 0.1% (w/v), TMC Table 1 Properties of the commercial dyes used in this study. Dye name
Chemical structure
Molecular weight (g/mol)
Congo red
696
Methyl blue
799
Methylene blue
320
Brilliant green
483
2. Experimental 2.1. Materials and reagents Polyacrylonitrile (PAN) ultrafiltration (UF) membranes were sup plied by Ande Membrane Separation Technology Engineering (Beijing) Co. Ltd. Dopamine hydrochloride (DA), tris-(hydroxymethyl) amino methane (Tris), 1,3,5-benzenetricarbonyl trichloride (TMC), 1,3-pro panesulfonate (1,3-PS), ammonia solution, cyclohexane, anhydrous ethanol, polyethylene glycol (PEG, molecular weight 200, 400, 600, 800, 1000, 1500, 2000 and 4000 Da), bovine serum albumin (BSA, 99%), Congo red, Methyl blue, Methylene blue and Brilliant green were all obtained from Aladdin Reagent Co. Ltd (Shanghai, China). The basic characteristic properties of these commercial dyes were shown in Table 1. Sodium chloride (NaCl), sodium sulfate (Na2SO4), magnesium 2
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Scheme 1. Synthesis process of sulfonated dopamine.
Fig. 1. The preparation process of nanofiltration membranes (NF-SDA).
concentration in organic phase is 0.15% (w/v) and the immersion time in SDA and TMC solutions is 20 min and 1 min, respectively. The membrane named NF-DA was prepared via interfacial polymerization between trimesoyl chloride (TMC) and dopamine (DA) of the same concentration for comparison. All prepared membranes were kept in warm DI water for around 24 h before using.
testing, these membranes were pre-treated under 0.6 MPa pressure for 10 min. Then, the water flux was measured using DI water as feed at an operation pressure of 0.6 MPa. The water flux (Jwf , L⋅m 2⋅h 1) was calculated by the following equation:
2.4. Membrane characterization
where V (L) is the volume of permeated water; A (m2) is the effective membrane area and Δt (h) is the filtration time. In order to evaluate the rejection performance of these NF mem branes, four salt solutions (1 g/L) including Na2SO4, MgSO4, MgCl2 and NaCl and four typical dyes solutions (0.1 g/L) including Congo red, Methyl blue, Methylene blue and Brilliant green were used as feed so lutions. The permeation flux was calculated by equation (1) and rejec tion ratio (R) was defined as the following equation: � � Cp R¼ 1 � 100% (2) Cf
Jwf ¼
2.4.1. FTIR and XPS characterization Fourier transform infrared spectroscopy (FTIR, Nicolet-iS50) and Xray photoelectron spectroscopy (XPS, Escalab 250Xi) were conducted to study the chemical structure and composition of these NF membranes. Before testing, all membranes were clean and dried in a vacuum oven. 2.4.2. SEM and AFM observation The morphology and roughness of these NF membranes were observed by scanning electron microscopy (SEM, SU8010, Hitachi, Japan) and atomic force microscopy (AFM, Dimension Icon, Germany), respectively.
V A⋅Δt
(1)
where Cp and Cf were the concentration of permeate and feed solution, respectively. The concentrations of salt solutions were measured by an electrical conductivity (Mettler Toledo, FE38-Standard). Dye concen trations of Congo red, Methyl blue, Methylene blue and Brilliant green solutions were determined with a UV–vis spectrophotometer (Perki nElmer, Lambda 950) at a wavelength of 499 nm, 599 nm, 665 nm and 623 nm, respectively. At least three membrane samples were used for all separation performance measurements.
2.4.3. Contact angle and ζ-potential measurement The hydrophilicity of the membranes was characterized by using a contact angle goniometer (OCA-20, Data-physics, Germany). The sur face ζ-potential of these membranes was measured by an instrument (SurPASS3, AntonPaar, Austria) with 1 mM KCl as the electrolyte solu tion at pH 6.5. 2.5. Performance of nanofiltration membranes
2.6. Long-term stability measurements
The separation performance of these NF membranes was investi gated by a commercial laboratory scale cross-flow flat membrane module with an effective area of 10.2 cm2 at room temperature. Before
The long-term stability measurement of permeation flux and Na2SO4 rejection for the optimal membranes was conducted using 1 g/L Na2SO4 solution as feed at 0.6 MPa for more than 2500 min. 3
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3. Results and discussion
structure constriction and corresponding to a decrease in water flux. Therefore, it is reasonable to employ SDA rather than DA in interfacial polymerization on purpose to harvest NF membranes with high water permeability.
3.1. The synthesis and analysis of sulfonated dopamine (SDA) Sulfonated dopamine was firstly synthesized and characterized. The H NMR and FTIR spectra of SDA and raw DA are presented in Fig. 2. In the spectrum of DA, the proton signals at 6.90–7.86 (g1, h1, f1) and 2.5–3.5 (d1, e1) ppm could be assigned to the characteristic multihydrogen atoms of DA [37,38]. For the spectrum of SDA, the new pro ton signals which appear at 2.01 (b), 2.89 (a) and 3.19 (c) ppm are attributed to the hydrogen atoms of carbon chain (-CH2-CH2-CH2-SO3H) from the ring opening reaction of 1,3-PS with the primary amine groups of DA [37]. Moreover, from the FTIR spectrum of SDA (Fig. 2b), comparing with the raw DA, a new absorption band appears at 1054 cm 1, and it belongs to the symmetric stretching vibrations of –SO-3 groups [39,40]. All results above indicate the successful synthesis of SDA. Thanks to the existence of sulfonic acid groups in SDA, the oxidativepolymerization of SDA is quite different from DA. The UV–vis results following the oxidative-polymerization process of SDA and DA solutions in the Tris-HCl buffer solution (pH ¼ 8.5) are shown in Fig. 3. At the beginning (0 h), there is no absorption peak presented in the spectra of DA and SDA solution in the range of 350–450 nm (Fig. 3a). After 12 h, the UV absorption peaks (350–450 nm) of DA solution increase, which shows the enhancement of the conjugate structure via inter-/intra-mo lecular polymerization to form polydopamine [35,41,42]. By contrast, no peak appears for SDA solution. It indicates the existence of sulfonic acid groups in SDA restrains inter-/intra-molecular polymerization due to the electrostatic repulsion effect. From the photos of DA and SDA (Fig. 3b) after 12 h oxidation, it is obvious the SDA solution is still transparent without visible aggregation (faint yellow), while the orig inal DA solution presents black brown as a result of the higher reactivity of DA to form polydopamine. Furthermore, when the reactive time ex ceeds 24 h, the color of SDA solution becomes black brown but no visible aggregation could be observed, because of the existence of sulfonic acid groups in SDA, the self-aggregate speed of SDA is low due to the elec trostatic repulsion effect. Therefore, very small PSDA oligomers and other oxidation products could suspend in the solution, leading to the black brown but no visible aggregation of SDA solution. For interfacial polymerization using DA, after immersing in DA aqueous solution for 20 min, a polydopamine layer with a thickness of 2.5 nm would be formed on the surface of the substrate [43]. The polydopamine depo sition probably occurs within the membrane pore structure, leading to 1
3.2. Characterization of nanofiltration membranes 3.2.1. Chemical structure and composition of nanofiltration membranes Fig. 4 shows the FTIR spectra of PAN, HPAN, NF-DA-0.1 and NFSDA-0.1 membranes. For all membranes, the peak at 2242 cm 1 is assigned to the stretching vibration of -CN groups [44]. Compared with PAN membrane, a new peak at 1560 cm 1 which is associated with the stretching vibration of –COO- groups appears in the HPAN membrane, indicating the abundant hydrophilic carboxylate groups after hydro lyzation by NaOH on the HPAN membrane [45]. For NF-SDA-0.1 and NF-DA-0.1 membrane, compared with HPAN membrane, the new peak at 1116 cm 1 is ascribed to the stretching vi bration of C–O–C formed by the reaction between the hydroxyl groups of SDA or DA and acyl chloride groups of TMC. It indicates successful interfacial polymerization process between SDA or DA and TMC. The amine groups of SDA and DA could also react with the acyl chloride groups of TMC to produce amide groups. Nevertheless, the absorbance of stretching vibration of amide groups overlaps with the inherent stretching vibration at 1660 cm 1 of substrate membranes, leading to the increment of the intensity of NF-DA-0.1 and NF-SDA-0.1 at 1660 cm 1. Moreover, comparing with NF-DA-0.1 membranes, a new peak at 1043 cm 1 in the spectrum of NF-SDA-0.1 membrane is ascribed to the stretching vibration of –SO-3 groups [40], which suggests the ex istence of SDA. Meanwhile, the detailed spectra and element compositions (C, N, O, S) were measured by XPS as shown in Fig. 5 and Table S1 (in supple mental information), respectively. From the XPS results in Fig. 5, NFSDA-0.1 has the highest peak intensity of O1S at 531 eV, NF-DA-0.1 comes second, and the intensity of HPAN substrate is the lowest (The atom percentage of oxygen is ~22.6%, ~20.0% and ~13.5%, respec tively, from Table S1). Moreover, in the spectrum of NF-SDA-0.1 membrane, a new peak appears at 167 eV, which is ascribed to S2p. Those all indicates the existence of sulfonic acid groups and dopamine on the prepared membranes using SDA and DA as aqueous monomers. 3.2.2. Surface morphology of membranes The surface morphology SEM and three-dimensional AFM results of HPAN, NF-SDA-0.1 and NF-DA-0.1 membranes are shown in Fig. 6. After
Fig. 2. (a) 1H NMR spectra; (b) FTIR spectra of SDA and raw DA. 4
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Fig. 3. (a) UV–vis spectra of DA and SDA aqueous solution (pH ¼ 8.5) at reaction time of 0 h and 12 h; (b) The image of DA and SDA solution after the reaction time of 12 h.
Fig. 4. FTIR spectra of (a) PAN; (b) HPAN; (c) NF-DA-0.1 and (d) NF-SDA-0.1 membranes.
aggregation of DA and crosslink with TMC. Compared with NF-DA-0.1, NF-SDA-0.1 membrane presents a smoother surface, as a result of moderate reaction rate of SDA. Observed from AFM images, compared with HPAN substrates (Ra ¼ 17.2 � 0.1 nm), both nanofiltration membranes have higher roughness. The lower roughness of NF-SDA-0.1 (18.2 �1.3 vs 27.9 �1.9 nm) membrane is related to the existence of sulfonic acid groups in SDA, which are useful to restrain oxidative-polymerization and the reaction with TMC, and correspondingly form homogeneous surface coating on the substrate during the process of interfacial poly merization. Furthermore, the results of AFM roughness measurement are consistent with SEM observation.
Fig. 5. XPS spectra HPAN membranes.
of
(a)
NF-SDA-0.1;
(b)
NF-DA-0.1
and
3.2.3. Surface hydrophilicity of membranes The hydrophilicity of membrane surfaces was characterized in terms of water contact angle as shown in Fig. 7. Compared with raw PAN membrane, HPAN substrate membrane has a high hydrophilicity with water contact angle of 38.1� , because of the abundant carboxylic acid groups on the surface. After surface coating, the water contact angle of NF-SDA-0.1 and NF-DA-0.1 membranes is 52.4� and 61.5� , respectively. Moreover, the NF-SDA-0.1 membranes have a lower water contact angle i.e. higher hydrophilicity than NF-DA-0.1 membranes, thanks to the hydrophilic sulfonic acid groups in SDA.
(c)
the interfacial polymerization, both the NF-SDA-0.1 and NF-DA-0.1 membranes have dense surfaces. The surface of NF-DA-0.1 membrane displays abundant “volcano” nodular, due to the heterogeneous 5
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Fig. 6. SEM and three-dimensional AFM images of (a, a1) HPAN; (b, b1) NF-SDA-0.1 and (c, c1) NF-DA-0.1.
Fig. 7. The water contact angle of PAN, HPAN, NF-SDA-0.1 and NF-DA0.1 membranes. Fig. 8. Effect of SDA concentration in aqueous phase on the separation per formance of NF-SDA-x membranes at 0.6 MPa.
3.3. Nanofiltration performance analysis 3.3.1. The effect of the concentration of sulfonated dopamine (SDA) in aqueous phase The NF membranes were prepared with different concentration of SDA in aqueous phase and the corresponding separation performance are presented in Fig. 8. Along with the increment of SDA concentration (0.05–0.6%), the water flux decreases from 151 to 27.5 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 45.9 to 93.5%. The higher con centration of SDA in aqueous phase brings more intensive cross-linking reaction with TMC during the interfacial polymerization process. The density of NF-SDA membranes is enhanced, resulting in the reduction of water flux and the increment of rejection. The rejection of Na2SO4 reaches up to 93.5% when the SDA concentration is 0.6% (w/v). Usually the existence of sulfonic acid groups may bring higher rejection of Na2SO4, as a result of the stronger Donnan’s effect of negatively charged bivalent sulfonic acid groups than monovalent carboxylic acid groups. Nevertheless, the inhibition effect to the cross-linking reaction of sul fonic acid groups in SDA because of stereo-hindrance effect and the low diffusion of SDA to organic phase [1], leading to weak negatively charged surfaces and loose interface polymerization layers to some extent [3]. The molecular weight cut-off of NF-SDA-0.1 was also measured and shown in Fig. S3 of supplemental information. The MWCO of NF-SDA-0.1 is around 2000 Da and the pore of NF-SDA-0.1 is calculated to be ~1.15 nm. Therefore, the sulfonic acid groups in SDA are favorable to the improvement of water flux. From the results, when
the concentration of SDA is 0.1%, the NF-SDA-0.1 membrane demon strates a high water flux of 62.2 L∙m 2∙h 1 and the Na2SO4 rejection of 90.0%, and it is considered as an optimal one for further characterization. 3.3.2. The effect of the immersion time in SDA solution The NF membranes with different immersion time in SDA solution were prepared and the corresponding separation properties were pre sented in Fig. 9. Obviously, with the increment of immersion time in SDA aqueous phase solution (5–20 min), the water flux decreases from 192 to 62.2 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 41.7% to 90.0%. It indicates the cross-linked reaction between SDA and TMC is enhanced, thanks to the adsorption of SDA on the surface of HPAN substrate is increased. When the immersion time continues to prolong (>20 min), the flux and rejection almost keep consistent, as a result of the saturated adsorption of SDA on the surface. An immersion time of 20 min is considered as the optimal parameter based on the balance of appropriate separation performance and time-saving operation. 3.3.3. The effect of the concentration of TMC in organic phase The NF separation performance of membranes prepared with various concentration of TMC in organic phase is presented in Fig. 10. With the increase of TMC concentration (0.05–0.2%), the water flux decreases 6
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Fig. 9. Effect of the immersion time in SDA solution on the separation per formance of NF-SDA-0.1 membranes at 0.6 MPa.
Fig. 11. Effect of the immersion time in TMC solution on the separation per formance of NF-SDA -0.1membranes at 0.6 MPa.
from 290 to 37.0 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 35.1 to 94.1% due to more intensive cross-linking reaction between TMC in organic phase and SDA during the interfacial polymerization process. The density of NF-SDA membranes is enhanced, resulting in the reduc tion of water flux and the increment of rejection. According to the water flux and rejection, the TMC concentration of 0.15% is selected as the optimal value for other characterization.
sulfonic acid groups in SDA restrain the reaction with TMC and endow NF-SDA-0.1 with a looser and hydrophilic interface polymerization layer for the increment of water flux. Besides, NF-SDA-0.1 membrane is also compared with some commercial and lab-scale NF membranes in terms of the water flux and salt rejection in Table 2. Apparently, for all commercial NF membranes or modified new NF membranes, the water flux is in the range of 1.22–9.20 L∙m 2∙h 1. It should be noted that the NF-SDA-0.1 membranes in this work show an excellent water flux that is up to 10.4 L∙m 2∙h 1∙bar 1, while the rejection is also acceptable. This proves sulfonated dopamine is a promising aqueous monomer for the preparation of high-performance NF membranes. The rejection and permeation flux of different salts of NF-SDA-0.1 membranes are shown in Fig. 12b. The NF-SDA-0.1 membranes exhibit a high permeation flux for different salts, and a salt rejection sequence of Na2SO4 > MgSO4 > NaCl > MgCl2 which conforms to the typically negatively charged NF membranes from previous works [1,46] and also matches with the zeta potential value ( 4.76 mV at pH ¼ 6.5, Fig. S1 in supplemental information). Furthermore, the rejection of NaCl is higher than MgCl2 in NF-SDA-0.1 membranes. On one hand, the ex istence of sulfonic acid groups in SDA restrain the reaction with TMC and lead to a looser and hydrophilic interface polymerization layer of NF-SDA-0.1, on the other hand, the negatively charged membrane sur face would provide stronger electrostatic attraction for bivalent Mg ions than monovalent Na ions [46]. Interestingly, the selectivity of different salts of two membranes is shown in Fig. 12c. Comparing NF-DA-0.1 membranes, the NF-SDA-0.1 membranes have higher salt selectivity (SNa2 SO4 =MgCl2 ¼ 11:3) than that of NF-DA-0.1 membranes (SNa2 SO4 =MgCl2 ¼ 2:02). Moreover, the salt selectivity (SNa2 SO4 =MgCl2 ) of NF-SDA-0.1 membranes is competitive by calculating the selectivity from the previous works [47–49], which can be used to separate MgCl2 from the mixed solution of MgCl2 and Na2SO4.
3.3.4. The effect of the immersion time in TMC solution The separation performance of NF membranes prepared with different immersion time in TMC organic phase solution is presented in Fig. 11. The results of separation performance emerge that water flux decreases and rejection increases. Along with the increment of immer sion time in TMC solution (0.5–5 min), the water flux decreases from 101 to 41.0 L∙m 2∙h 1 and the rejection of Na2SO4 increases from 75.3 to 92.0%. It indicates the increased cross-linking reaction of aqueous and organic phase along with the increment of immersion time in TMC. In terms of overall separation performance, 1.0 min is considered as the optimal immersion time. 3.3.5. The performance of sulfonated dopamine nanofiltration membranes The separation performance of NF-SDA-0.1 membranes is presented in Fig. 12. From Fig. 12a, obviously, the water flux of NF-SDA-0.1 (62.2 L∙m 2∙h 1) is much higher than NF-DA-0.1 membranes (10.8 L∙m 2∙h 1), while both NF-DA-0.1 and NF-SDA-0.1 membranes have the similar high rejection of Na2SO4 (>90%). So it is advisable that
3.3.6. Analysis of dyes rejection The dye separation performance were tested for the optimal NF-SDA0.1 membranes using four representative dyes (negatively-charged Congo red and Methyl blue; positively-charged Methylene blue and Brilliant green). From Fig. 13, it is observed that the NF-SDA-0.1 membrane exhibits excellent rejection (>99.0%) to all these dyes. Moreover, after the filtration of negatively-charged dyes (Congo red and Methyl blue), there are few dyes left on the membrane surfaces. It is indicated the good ability of the membrane to resist the fouling of negatively charged dyes, as a result of the special chemical structure of SDA. The sulfonic acid groups and secondary amine groups in the SDA is helpful to the hydrophilicity of membrane and form hydration layer on the surface, and negatively-charged sulfonic acid groups also offer electrostatic repulsion effect. Therefore, the NF-SDA-0.1 membranes
Fig. 10. Effect of TMC concentration in organic phase on the separation per formance of NF-SDA-0.1 membranes at 0.6 MPa. 7
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Fig. 12. (a) Nanofiltration performance of NF-DA-0.1 and NF-SDA-0.1 membranes at 0.6 MPa; (b) Permeation flux and rejection of NF-SDA-0.1 membrane to different salts; (c) Salt selectivity of NF-DA-0.1 and NF-SDA-0.1 membranes. Table 2 The comparison of NF-SDA-0.1 membranes with commercial and lab-scale NF membranes. NaCl rejection (%)
Na2SO4 rejection (%)
Reference
1.22 6.48
49.7 10.1
98.1 91.4
[23] [19]
7.10 9.20 2.15 5.05 10.4
83.5 51.0 45.0 50.0 31.5
/ 92.0 95.0 78.0 90.0
[50] [51] [51] [52] This work
Membrane
Water flux (L∙m 2∙h 1∙bar
PE-TMC CyclenTMC NF-90 NTR-7450 NF-40 NS-300 SDA-TMC
1
)
could have a good antifouling ability of negatively-charged dyes and BSA (shown in Fig. S2 of supplemental information). 3.4. The long-time stability measurement A long-time operation was conducted on NF-SDA-0.1 for separating Na2SO4 solution (1 g/L) at 0.6 MPa. The normalized permeation flux (Jt/ J0) and the normalized rejection (Rt/R0) are presented in Fig. 14. Obviously, both permeation flux and salt rejection are quite stable during testing period, suggesting the NF-SDA-0.1membranes has a good structure and performance stability.
Fig. 13. (a) The rejection of NF-SDA-0.1 membranes to Congo red, Methyl blue, Methylene blue and Brilliant green; (b) The photo images of NF-SDA-0.1 membranes after the filtration of Congo red and Methyl blue. (For interpreta tion of the references to color in this figure legend, the reader is referred to the Web version of this article).
4. Conclusions
membranes. The hydrophilic sulfonic acid groups of SDA could restrain inter-/intra-molecular polymerization and the reaction with TMC. So SDA could endow nanofiltration membranes hydrophilic and smooth
Sulfonated dopamine (SDA) was prepared by the ring opening re action of 1,3-PS with DA and then used as a aqueous monomer in interfacial polymerization for the preparation of nanofiltration 8
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Fig. 14. The long-time operation stability of NF-SDA-0.1 membrane.
surfaces. The optimal NF-SDA-0.1 membranes performed a high water flux of 62.2 L∙m 2∙h 1, five times higher than that of NF-DA-0.1 membranes and a considerable rejection of 90.0% to Na2SO4 at 0.6 MPa. Meanwhile, the NF-SDA-0.1 membranes had excellent re jections (>99.9%) to four representative dyes including Congo red, Methyl blue, Methylene blue and Brilliant green. Moreover, NF-SDA-0.1 membranes also presented a long-time operational stability. This work provides enlightenment for preparing high-flux nanofiltration mem branes using sulfonated monomers and explaining the roles sulfonated monomers play in the membrane performance. Declaration of competing interest There is no conflict of interest. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 21704085) and the Fundamental Research Funds for the Central Universities (No. 2232019G-04). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117658. References [1] Z. Lv, J. Hu, J. Zheng, X. Zhang, L. Wang, Antifouling and high flux sulfonated polyamide thin-film composite membrane for nanofiltration, Ind. Eng. Chem. Res. 55 (2016) 4726–4733. [2] M. Paul, S.D. Jons, Chemistry and fabrication of polymeric nanofiltration membranes: a review, Polymer 103 (2016) 417–456. [3] Z. Zhang, G. Kang, H. Yu, Y. Jin, Y. Cao, Fabrication of a highly permeable composite nanofiltration membrane via interfacial polymerization by adding a novel acyl chloride monomer with an anhydride group, J. Membr. Sci. 570–571 (2019) 403–409. [4] R. Wang, X. Shi, A. Xiao, W. Zhou, Y. Wang, Interfacial polymerization of covalent organic frameworks (COFs) on polymeric substrates for molecular separations, J. Membr. Sci. 566 (2018) 197–204. [5] L. Shan, J. Gu, H. Fan, S. Ji, G. Zhang, Microphase diffusion-controlled interfacial polymerization for an ultrahigh permeability nanofiltration membrane, ACS Appl. Mater. Interfaces 9 (2017) 44820–44827. [6] P. Xu, J.E. Drewes, T.U. Kim, C. Bellona, G. Amy, Effect of membrane fouling on transport of organic contaminants in NF/RO membrane applications, J. Membr. Sci. 279 (2006) 165–175. [7] V. Freger, Nanoscale heterogeneity of polyamide membranes formed by interfacial polymerization, Langmuir 19 (2003) 4791–4797. [8] J.R. Werber, C.O. Osuji, M. Elimelech, Materials for next-generation desalination and water purification membranes, Nat. Rev. Mater. 1 (2016) 16018–16034. [9] A.W. Mohammad, Y.H. Teow, W.L. Ang, Y.T. Chung, D.L. Oatley-Radcliffe, N. Hilal, Nanofiltration membranes review: recent advances and future prospects, Desalination 356 (2015) 226–254.
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