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An acid resistant nanofiltration membrane prepared from a precursor of poly(s-triazine-amine) by interfacial polymerization
Journal of Membrane Science 546 (2018) 225–233
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An acid resistant nanofiltration membrane prepared from a precursor of poly (s-triazine-amine) by interfacial polymerization
T
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Yanjun Zenga, Lihong Wangb, Lin Zhanga, , Jimmy Qiming Yuc a b c
Key Laboratory of Biomass Chemical Engineering, MOE, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China Department of Chemistry, Tangshan Normal College, Tangshan 063000, China Griffith School of Engineering, Griffith University, Nathan Campus, Brisbane, Queensland 4111, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyamine S-triazine amine Acid stability Interfacial polymerization Nanofiltration membrane
An acid resistant thin film composite nanofiltration (NF) membrane (named TPT-TMC/PSf membrane) was fabricated using interfacial polymerization. A precursor of poly(s-triazine-amine), 1,3,5-(tris-piperazine)-triazine (TPT), was synthesized and interfacially polymerized with trimesoyl chloride (TMC) to fabricate NF membrane on the surface of a polysulfone (PSf) ultrafiltration membrane. The chemical structure, surface charge and morphology of the TPT-TMC/PSf membrane were characterized by attenuated total reflection Fourier transform infrared spectrometer (ATR-FTIR), zeta potential measurement, and scanning electron microscope (SEM), respectively. The results indicated that the TPT-TMC/PSf membrane has a thinner skin layer and a smoother and more negatively charged surface than the conventional polypiperazine amide NF membrane. The newly fabricated TPT-TMC/PSf membrane exhibited excellent rejections against different aqueous salt solutions [Na2SO4 (98.6%) > MgSO4 (97.0%) > NaCl (40.5%)]. The membrane also exhibited excellent acid resistant behavior. After being immersed in the 0.05 M H2SO4 aqueous solution for 720 h, stable water permeance of about 8.68 L m−2 h−1 bar−1 and MgSO4 rejection above 94.2% still achieved.
1. Introduction Nanofiltration (NF) membranes have excellent capabilities to separate divalent metallic ions and low molecular weight organic compounds from monovalent ions. They are extensively used in wastewater treatment [1,2] and water purification operations in many industries such as chemical, biotechnology and pharmaceutical industries [3–5]. At present, aromatic polyamide thin film composite (TFC) membranes are the most commonly used commercial NF membranes due to their high water permeabilities and salt rejection coefficients. However, the amide bonds in these polyamide membranes are prone to hydrolysis under acidic conditions, which results in severe deteriorations in membrane performances. In particular, polypiperazine amide NF membranes are often observed to be less stable in pH extremes than the m-phenylenediamine (MPD) based amide membranes [6]. The acid instability of the polyamide membranes exclude its application from many processes that involve extreme acidic conditions, for examples, the processes of acid recovery or purification from drainage in electroplating industry [7,8], metal recovery from acid wastes in metal industry [9], nitrogenous compound removal from acid effluents in mining industry [10,11], and water recycling from acidic bleaching
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Corresponding author. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.memsci.2017.10.022 Received 5 May 2017; Received in revised form 5 October 2017; Accepted 9 October 2017 Available online 10 October 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
effluents in paper industry [12]. Acid resistant NF membranes are desired in these processes. Sulfonated aromatic polymers, such as sulfonated polysulfone and sulfonated poly(ether ketone), are most frequently investigated acid stable membrane materials [13–15]. Some commercially available acid stable membranes, including NP010, NP030, NF-PES-10 membranes made by Microdyn-Nadir [16,17] and NTR-7400 series by Hydranautics-Nitto Denko [18–20], are made of sulfonated aromatic polymers. However, most of these membranes have relatively low permeance and/or low salt rejections [13]. Polysulfonamide (PSA) is another acid stable membrane material, which has a similar chemical structure to polyamide. PSA has been reported to be more acid stable than polyamide due to the higher stability of the sulfonyl bond towards oxygen than carbonyl bond [6,21,22]. Liu et al. prepared an acid stable PSA NF membrane through interfacial polymerization (IP) of naphthalene-1,3,6- trisulfonylchloride (NTSC) and piperazine (PIP) and the resulting PSA membrane exhibited acid stability in 20% (w/v) H2SO4 for 2 months [23]. Aromatic diamine MPD also has been used in interfacial polymerization with benzene-1,3-disulfonyl chloride to synthesize acid-stable PSA membranes [22]. The disadvantage of PSA is that it is difficult to synthesize due to the weak reactivity of sulfonyl
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chemical structure and morphology of the TFC membrane were systematically studied by ATR-FTIR, Zeta potential, and SEM. The separation performance of the TFC membrane was tested by using a set of cross-flow filtration experiments. The acid resistant behavior of the TFC membranes was also evaluated and compared with conventional polypiperazine amide (PIP-TMC/PSf) NF membranes.
chloride, limiting its practical application in large scale [24]. New membrane materials with outstanding acid resistance as well as excellent water-salt separation properties still need to be developed. Recently, polymers with s-triazine rings as part of the polymer main chain have been used as a new type of acid stable membrane materials. The s-triazine ring consists of alternating nitrogen and carbon atoms that linked by alternating single and double bonds and has six unpaired electrons on the nitrogen atoms [25]. The stability of the conjugated bond structure of the s-triazine ring provides polymers with mechanical rigidity, thermal and chemical stabilities. Membranes based on s-triazine polyamine [poly(s-triazine-amine)] structure exhibited superior pH stability and have been prepared through interfacial polymerization between cyanuric chloride (CC) and amines [26–30]. Cadotte and Lee both obtained poly(s-triazine-amine) NF membranes by polymerizing cyanuric chloride with the large oligomer polyethyleneimine (PEI) interfacially [27,29] and the poly(s-triazine-amine) NF membranes (PEICC) exhibited excellent stability under extreme pH conditions ( pH 1 and 13) [29]. The commercially available BPT-NF serial membranes from Advanced Membrane System Technologies (AMS) consisted of an undisclosed poly(s-triazine-amine) structure also showed excellent resistance to 20% sulphuric acid for up to 4 months at 20 °C [31]. However, the interaction of cyanuric chloride with amines shows a clearly marked reactivity-stepwise behavior which ultimately lead to poor performance of the poly(s-triazine-amine) membranes. The reactivity of the first chlorine atom in cyanuric chloride is high and roughly equal to that of acid chlorides [32]. Nevertheless, replacement of the chlorine atoms in cyanuric chloride with electron-donors such as amines increases the electron density on the carbon atom of s-triazine, reducing the reactivity of the remaining chlorines. Thus, the second and/or the third chlorine in cyanuric chloride is more difficult to be replaced by amines, especially those with slight steric hindrance such as piperazine (PIP) and MPD. The incomplete substitution of the three chlorines in cyanuric chloride leads to a lower molecular weight of the polymer or a higher percentage of linear structures in the polymer. Frequently, rough, open and discontinuous layers are formed on the surface of the support membrane [30]. Even the well-defined poly(striazine-amine) (PEI-CC) composite membrane obtained only fivefold lower water permeance in a dead-end filtration setup as compared to the corresponding polyamide (PET-TMC) composite membrane [29]. Elevating the temperature can increase the reactivity of the second and third chlorines in cyanuric chloride. However, the temperature for membrane fabrication can only be raised up to around 60 °C considering the thermal stability of the supporting membrane and the operability of the IP process. A poly(s-triazine-amine) NF membrane has been prepared through interfacial polymerization between diethylene triamine (DETA) and cyanuric chloride under a temperature of 60 °C, but the membrane still exhibited a low water permeance and poor salt rejection [33]. This indicated that a temperature of 60 °C still not high enough to completely tri-substituted the chloride atoms in cyanuric chloride, which is the key point for obtaining high performance poly(s-triazine-amine) NF membranes. In this paper, in order to fabricate acid resistant membrane derive from cyanuric chloride based polymer and accomplish complete trisubstitution in cyanuric chloride, a precursor of poly(s-triazine-amine), 1,3,5-(tris-piperazine)-triazine (TPT), was first synthesized and then used as a monomer in the interfacial polymerization with trimesoyl chloride (TMC) to fabricate NF membrane. The precursor TPT was first synthesized through a nucleophilic substitution reaction between cyanuric chloride and N-(tert-butoxycarbonyl)- piperazine. As the nucleophilic substitution reaction was not restrained by the conditions of the IP process, all the three chlorine atoms in cyanuric chloride could be replaced by N-(tert-butixycarbonyl)- piperazine under a high reaction temperature of 75 °C with stirring. Then the precursor was used as an aqueous phase monomer to fabricate a thin film composite (TFC) membrane (named TPT-TMC/PSf membrane) via IP with TMC on the surface of a porous polysulfone (PSf) ultrafiltration membrane. The
2. Experimental 2.1. Materials and reagents Piperazine (PIP, 99%), cyanuric chloride (CC, 99%), trimesoyl chloride (TMC, 98%), N-(tert-butoxycarbonyl)-piperazine (98%), N, Ndiisopropylethylamine (DIPEA, 99%) were purchased from Aldrich Company. Isopar-G (Isoparaffin type hydrocarbon oil) was purchased from Exxon Mobil Chemical. Hydrochloric acid (HCl), tetrahydrofuran (THF), dichloromethane, chloroform, methanol, NaHSO4, NaCl, NaOH, Na2SO4, Na3PO4·12H2O, MgCl2, MgSO4 and molecular sieves (4 Å) were all of analytical reagent grade, and acquired from Sinopharm Chemical Reagent Co., Ltd. Molecular sieves (4 Å) were activated under 600 °C for 12 h in furnace before use. All other chemicals were used as received. The PSf ultrafiltration membranes with molecular weight cutoff around 30,000 were purchased from the Development Center for Water Treatment Technology, Hangzhou, China.
2.2. Synthesis and characterization of 1,3,5-(tris-piperazine)-triazine (TPT) TPT monomer was synthesized by a two-step process according to the literature [34]. In the first step, a sample of 5 g (27.1 mmol, 1.00 equiv) cyanuric chloride dissolved in 500 mL THF was injected into a 1 L three necked, round bottomed flask equipped with a mechanical stirrer, reflux condenser, and static nitrogen inlet. Then 17.0 g (91.5 mmol, 3.38 equiv) N-(tert-butoxycarbonyl)-piperazine was added into the mixture in small portions over 10 min. An acid acceptor N, Ndiisopropylethylamine (48.1 mL, 276 mmol, 10.2 equiv) was added and the reaction mixture was stirred at 0 °C for 1 h, then kept at the ambient temperature for 3 h, and finally kept at the temperature of 75 °C for 20 h. When the reaction was finished, the remaining THF was removed by using a rotary evaporator. The obtained light yellowish solid was dissolved in dichloromethane and washed with water, 10% NaHSO4 and NaCl saturated aqueous solution, respectively. The organic layer was dried over 4 Å molecular sieves, filtered and the organic solvent was removed by using a rotary evaporator. The intermediate product was named 1,3,5-[Tris-N-(tert-butoxycarbonyl)-piperazine] –triazine (TBPT). In the second step, TBPT (1.00 equiv) and methanol (160 equiv) were added into a 500 mL flask as previously. The solution was stirred at 0 °C for 30 min. Then hydrochloric acid (21 equiv) was added over 70 min while the temperature was kept at around 0 °C. The resulting slurry was stirred at 0 °C for 2 h. And then the reaction slurry was placed under ambient temperature for over 3 h, slowly heated to 40 °C for 12 h. The organic components were removed by using rotary evaporator. The remaining aqueous solution was cooled to 0 °C and its pH adjusted to 14.0 by using 10% (w/v) NaOH solution. The alkaline solution was extracted with chloroform (3 × 250 mL). The acquired organic phases were dried with sodium sulfate, filtrated and evaporated to obtain a final white solid product. The final product was named 1,3,5-(tris-piperazine)-triazine (TPT). Schematic illustration of the synthetic route is showed in Scheme 1. The reactivity of the precursor was high enough to form well-defined film with trimesoyl chloride (TMC) through interfacial polymerization within a few seconds as showed in the free-standing experiment (Fig. S.1). 226
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Scheme 1. Synthetic route for1,3,5-(tris-piperazine)triazine (TPT).
[35,36]. The trans-membrane pressure was set at 100 psi and the feed solution temperature was kept at 25 ± 1 °C with a constant temperature system. Inorganic salt solutions (NaCl, MgSO4, Na2SO4, 2000 ppm) were used as feed solutions and tested in terms of water permeance and salt rejections, respectively. Separation performances of the TFC membranes to different inorganic salt solutions were evaluated following the procedures described in literature [37]. Membranes were pre-pressured at 200 psi for stabilization with de-ionized water for 30 min before separation test. The water permeance (P) of the membranes were calculated by the following equation;
2.3. Preparation of TFC membranes The NF membranes were prepared through IP between TPT and TMC on the surface of a PSf support UF membrane. First, TPT and Na3PO4·12H2O (acid acceptor) were dissolved in deionized water to prepare aqueous solution. The organic phase solution was prepared by dissolving TMC monomer in Isopar-G solvent. Then, the aqueous solution was poured onto the surface of the flat sheet PSf membrane and immersed for about 2 min. The excessive solution was removed using a rubber roller. The organic solution was then poured upon membrane and let set for a fixed time. Any excessive organic solution was removed and the nascent membrane was heated in an oven at 70 °C. Finally, the prepared membrane was stored in deionized water until to be used. Optimal membrane fabrication conditions in terms of water permeance and salt rejection were obtained as follows: TPT concentration, 0.6% (w/v); TMC concentration, 0.1% (w/v); Na3PO4·12H2O, 1.5% (w/ v); time for IP process, 1 min; curing temperature, 70 °C; and curing time, 15 min. For comparison, conventional NF membranes obtained from PIP and TMC at the same optimal conditions (except for the concentration of PIP which was set at 0.232% (w/v) to ensure the number of amine groups in PIP-TMC system was the same as the number in TPT-TMC system) were also prepared by following the same procedures.
Permeance =
F V = ΔP At ΔP
where F is the permeate flux (L m−2 h−1), V is the volume of permeate (L) collected over a period of time t (h), A is the effective area of the membrane (m2) and ΔP is the trans-membrane pressure (bar). The salt rejection R was calculated as follows:
c R = ⎛1− 2 ⎞×100% ⎝ c1 ⎠ ⎜
⎟
where c1 and c2 are the salt concentration of feed and permeate solution, respectively. The concentrations of the feed and permeate were analyzed by measuring the conductivity using a conductivity meter (model DDS-11A, Neici Instrument, Shanghai, China).
2.4. Characterization of TPT monomer and TFC membranes 2.6. Long term acid stability tests The functional groups of TBPT and TPT were characterized by using Fourier transform infrared spectroscopy (FTIR) analyses on a Nicolet 5700 FTIR spectrometer. The elemental contents (C, N, H) of TBPT and TPT were quantified by an element analyzer (Elementar Analysen system GmbH, Vario Micro). 1H and 13C NMR spectra were measured on an Agilent DD2-600 MHz spectrometer. Test samples were prepared at room temperature by dissolving TBPT in D2O and TPT in CDCl3 solvent. Scanning electron microscope (SEM, SU-8010, Hitachi, Japan) with an accelerating voltage of 3 kV was used to observe the surface and cross sectional morphology of membranes. Before tests, the membranes were dried at 40 °C in a vacuum oven for 12 h and then cut into small stripes. For observing the cross-section morphology, stripe-shaped membrane samples were immersed in liquid nitrogen and broken through folding. Before SEM observation, all samples were sputtered with gold powder to enhance electric conductivity and to improve the resolution of SEM pictures. Surface charge of membranes were characterized by zeta potential measurements. Measurements were performed using a streaming potential instrument (SurPASS, Anton Paar) and conducted in a background electrolyte solution containing 1 mM KCl over a range of pH values. HCl and NaOH solutions were used to adjust the pH of the test solutions.
The membranes were exposed to solutions of 0.05 M H2SO4 for fixed periods of time. The solutions were kept at a constant temperature of 25 ± 1 °C with a constant temperature oscillation incubator (HZQF160, PeiYing laboratory equipment Co. Ltd., China). At the end of the exposure period, the membranes were taken out of the solutions and washed with deionized water until the pH value of the washing water was stable. The acid treated membranes were characterized for surface or permeation properties by following the procedures described above. 3. Results and discussion 3.1. Synthesis and characterization of TPT monomer The chemical composition and chemical structure of the intermediate product TBPT and the final monomer TPT (as shown in Scheme 1) were confirmed by FTIR, 1H NMR, 13C NMR, and Elemental analysis. The FTIR spectra of TBPT and TPT are presented in Fig. S2. Detailed characteristic peaks are listed as follows. TBPT: FTIR (KBr, cm−1): 1695, 1535, 1419, 1227, 998, 725. TPT: FTIR (KBr, cm−1): 3294, 2846, 1550, 1433, 1242, 1007, 806, 750. The peak at 1695 cm−1 appeared in the spectrum of TBPT was attributed to the protective functional group (t-tutyloxy carbonyl). The absence of the peak at 1695 cm−1 and the presence of the peak at 3294 cm−1 in the spectrum of TPT indicated the removal of the protective group and the existence of –NH group. The 13C NMR and 1H NMR spectrum are showed in Fig. S3. TBPT: 1 H NMR (600 MHz, CDCl3) δ: 1.46 (-CH3), 3.42 (CH2-C-N), 3.72 (NCH2-C); 13C NMR (600 MHz, CDCl3) δ: 31.02 (-CH3), 45.67 (N-C-C), 79.46 (solvent), 82.52 (-C-O), 157.44 (C˭O), 167.12（C˭N）. TPT: 1H
2.5. Evaluation of separation performance The separation performances of membranes were evaluated by using a customized flat-sheet cross-flow membrane system with an effective filtration area of 12.56 cm2, as reported in our previous work 227
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Fig. 2. Zeta potential of PIP-TMC/PSf TFC membrane and TPT-TMC/PSf TFC membrane measured under different pH values.
Fig. 1. ATR-FTIR spectra of PSf ultrafiltration membrane, PIP-TMC/PSf TFC membrane and TPT-TMC/PSf TFC membrane.
membrane [28,38]. The interfacial polymerization is a self-restraint process and the highly crosslinked nascent membrane blocks the further diffusion of TPT monomer into the organic phase. Furthermore, the molecular size of TPT is larger than that of PIP. Both the highly crosslinked nascent membrane and the larger molecular size of TPT make the diffusion of TPT monomer into the organic phase more difficult than PIP monomer does, resulting in increased hydrolysis of acyl chloride into carboxylic acid groups [37,39,40].
NMR (600 MHz, D2O) δ: 1.20 (-NH), 2.72 (CH2-C-N), 3.60 (N-CH2-C), 4.63 (solvent); 13C NMR (600 MHz, D2O) δ：45.7 (N-C-C), 47.0 (N-CC), 167.6（C˭N）. The elemental compositions of TBPT and TPT obtained from elemental content analyses were C, 55.71%; H, 8.19%; N, 19.14% and C, 54.32%; H, 8.20%; N, 37.05%, respectively. These closely compare with the theoretical elemental content for TBPT (C30H51N9O6) of C, 56.85%; H, 8.11%; N, 19.89%, and that for TPT (C15H27N9) of C, 54.03%; H, 8.16%; N, 37.81%. The results from the FTIR, NMR and elemental analysis all confirmed that the intermediate product TBPT and the final compound TPT were successfully synthesized.
3.2.3. Morphological characterization The morphologies of the NF membranes visualized by SEM are illustrated in Fig. 3. In the cross-sectional images, distinctive dense and continual layer can be seen on the top of the porous PSf support membrane. The top skin layer of TPT-TMC/PSf TFC membrane was observed thinner than that of PIP-TMC/PSf TFC membrane. Hung [41] also observed that the polyamide layer with a high polymerization degree exhibited a thin active layer thickness. The reduction in thickness could been ascribed to the smaller amount of the aqueous monomer diffusion into organic phase as discussed in Section 3.2.2. The surface image of the PIP-TMC/PSf TFC membrane showed a classical grainy topography while the TPT-TMC/PSf TFC membrane had a smooth membrane surface. According to Morgan's theory, membrane roughness comes from the inhomogeneity of further reactions after initial film formation of IP [42]. Because of the larger molecular size and the higher content of the crosslinking structure in the nascent membrane, further diffusion of TPT monomer into the organic phase was blocked [35]. Further reactions at the organic phase after the formation of nascent membrane were limited, and thus a smooth skin layer was obtained.
3.2. Characterization of TFC membrane 3.2.1. ATR-FTIR spectra The ATR-FTIR spectra of the PSf ultrafiltration support membrane, PIP-TMC/PSf and TPT-TMC/PSf TFC membranes are shown in Fig. 1. The absence of acyl chloride peak at 1760–1770 cm−1 and the presence of a strong band at 1618 cm−1 (C˭O) indicate the occurrence of IP. The peak at 1546 cm−1 in both TFC membranes corresponds to the stretching vibration of the C-N group in the piperazine-amide structure. However, the characteristic bands at 1503 cm−1 and 1362 cm−1 derived from the stretching vibration and deformation vibration of striazine ring overlapped with the peaks from the benzene ring of PSf UF membrane. 3.2.2. Surface charge evaluation The surface charge of the TFC membranes were evaluated by calculating the zeta potential from the streaming potential measurements. The results are shown in Fig. 2. The isoelectric point for PIP-TMC/PSf TFC membrane was found to be around 5.0 and around 3.5 for TPTTMC/PSf TFC membrane. Above the isoelectric points, the membranes were negatively charged due to the dissociation of the carboxylic acid groups, which were derived from the hydrolysis of unreacted acyl chloride groups. In contrast, the membranes were positively charged below the isoelectric point due to the protonation of the amine functional group. In the range of pH values measured in the test, TPT-TMC/ PSf TFC membranes were always more negatively charged than PIPTMC/PSf TFC membranes. This result indicated that the surface of TPTTMC/PSf TFC membranes contains more carboxylic acid groups than the surface of PIP-TMC/PSf TFC membranes. As demonstrated in Scheme 2, TPT-TMC/PSf TFC membrane was formed from 3 × 3 monomer pairs while PIP-TMC/PSf TFC membrane from 2 × 3 monomer pairs, the content of the crosslinking structure of TPT-TMC/ PSf TFC membrane should be higher than that of PIP-TMC/PSf TFC
3.2.4. Permeation properties of TFC membranes TPT -TMC/PSf TFC membrane with a high polymerization degree exhibited a small free volume diameter because the polymer chains in the cross-linked segment were densely arranged and the disturbance in the polymer chains was limited, resulting in a small free volume diameter and fraction [41]. Generally, the TPT -TMC/PSf TFC membrane with small free volume diameter and fraction would be highly selective and exhibited a low permeance. However, the water permeance of the TPT-TMC/PSf membrane for all solutions shown in Fig. 4 were higher than that of the PIP-TMC/PSf membrane. This was most likely due to the thinner skin layer of the TPT-TMC/PSf membrane. The rejection performances of the two membranes to different salts followed the same order of Na2SO4 > MgSO4 > NaCl, which is typical for negatively charged NF membrane. From the results of zeta potential measurements, in the pH range of 6.0–7.0, both membranes were negatively charged, and the rejections to charged solutes were affected by both the 228
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Scheme 2. (a) Crosslinking structure and linear structure of PIP-TMC/PSf membrane; (b) Crosslinking structure of TPT-TMC/PSf membrane.
showed that the TPT-TMC/PSf TFC membrane have higher rejections to Na2SO4 (98.6% versus 97.6%) and MgSO4 (97.0% versus 95.0%) salts and lower rejection to NaCl (40.5% versus 54.2%) salt as compared to PIP-TMC/PSf TFC membrane. This is in line with the fact that the TPTTMC/PSf TFC membrane is more negatively charged than the PIP-TMC/ PSf TFC membrane in the testing pH range. Enhanced Donnan exclusion effect between the TPT-TMC/PSf TFC membrane and the salt solutions
Donnan exclusion effect and the steric size exclusion effect. For the cations, although Mg2+ (0.428 nm) has a larger hydrated radius than that of Na+ (0.358 nm), Na2SO4 had a higher rejection than that of MgSO4 as a result of the stronger affinity between the higher charged cations and the negatively charged membrane surface. For anions, the sulfate ion was retained more than the chloride ion not only because of its larger radius but also because of its higher charge. The results also
Fig. 3. SEM images of membrane surface and crosssection of (a) (b) PIP-TMC/PSf TFC membrane and (c) (d) TPT-TMC/PSf TFC membrane.
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Fig. 4. (a) Water permeance; (b) salt rejection of TPT-TMC/PSf TFC membrane and PIP-TMC/PSf TFC membrane to different inorganic salts. Tested conditions: trans-membrane pressure = 100 psi; temperature = 26 ± 1 °C.
was responsible for this phenomenon. 3.3. Acid stability of TFC membranes The acid stability of the TFC membranes was investigated by immersing membranes into 0.05 M H2SO4 aqueous solutions for fixed periods. The salt rejection and water permeance were measured as functions of time and the variations observed were used to evaluate the acid stability of the membranes. As results shown in Fig. 5, the performance of the PIP-TMC/PSf TFC membranes deteriorated significantly after being exposed to acid solutions for 720 h. The rejection to MgSO4 decreased from 95.0% to 61.8% and the corresponding water permeance increased from 7.11 L m−2 h−1 bar−1 to 15.16 L m−2 h−1 bar−1. This trend of performance deterioration is consistent with the results from a number of studies where polypiperazine amide membranes hydrolysis under acid treatment [39,40,43]. Fig. 6 shows the ATR-FTIR spectra of PIP-TMC/PSf TFC membrane before and after being exposed to the acidic solution, in which noticeable changes can be observed at the bands of 1735 cm−1, 1618 cm−1, 1546 cm−1 and 1441 cm−1. After acid treatment, the bands at 1618 cm−1, 1546 cm−1 and 1441 cm−1 were weakened significantly and a band at 1735 cm−1 that represents the carbonyl moiety of ester functional group was emerging, indicating the disintegration of the
Fig. 5. Normalized water permeance and rejection for TPT-TMC/PSf membrane and PIPTMC/PSf membrane after being immersed in 0.05 M H2SO4 solution for a fixed period.
Fig. 6. ATR-FTIR spectra of (a) PIP-TMC/PSf TFC membrane and (b) TPT-TMC/PSf TFC membrane before and after being exposed to acidic solution for 720 h.
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Fig. 7. SEM images of surfaces of (a) (b) PIP-TMC/ PSf membrane and (c) (d) TPT-TMC/PSf membrane before and after being exposed to acidic solution for 720 h.
Scheme 3. (a) Degradation mechanism of polypiperazine amide membrane under the acidic condition; (b) illustration of acid stability of the poly (amide-s-triazine-amine) membrane.
Table 1 A comparison of the nanofiltration membrane made in this work compared with other reported acid stable membranes. Membrane materials
Salt rejectiona
Water permeance (L m−2 h−1 bar−1)
Testing conditions
Ref.
Sulfonated poly (ether ether ketone) Polysulfonamide Poly (s-triazine-amine) Poly (s-triazine-amine) Poly(amide-s-triazine-amine)
85–95% 86.7% 75–95%b 85.2%b 98.6%
0.8 5.76 0.2–0.7 1.5 8.68
pH = 0 (HNO3); Room temperature; Dead-end filtration setup 20.0% (w/v) H2SO4; 25 °C； Cross-flow filtration setup 0.1 M HNO3; Room temperature; Dead-end filtration setup 0.1 M HNO3; Room temperature; Dead-end filtration setup 0.05 M H2SO4; 25 ± 1 °C；Cross-flow filtration setup
[13] [23] [29] [30] This work
a b
Separation performance to Na2SO4 solution. Separation performance to NaCl solution.
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Appendix A. Supporting information
amide bond. The corresponding SEM images are shown in Fig. 7, which exhibited significant changes in the membrane surface after being exposed to the acid solution for 720 h. The classical grainy morphology of the polypiperazine amide membrane was replaced by a collapsed and defective structure. The degradation mechanism of polypiperazine amide membrane are illustrated in Scheme 3. The oxygen in the amide bond is more electronegative than carbon, and thus electron density was drawn away from carbon, making carbon much more susceptible to nucleophilic attack with the presence of the acid catalyst [29]. In contrast, as seen in Fig. 5, the TPT-TMC/PSf TFC membrane showed no significant changes in salt rejection and water permeance after being exposed to the same acid solution for 720 h, indicating its excellent stability under acidic conditions. Nevertheless, during the initial period of acid treatment, there was a slight increase in salt rejection and a slight decrease in water permeance for TPT-TMC/PSf TFC membrane. This may due to a re-construction of the membrane structure when exposed to a highly acidic solution [44]. In the ATR-FTIR spectra and SEM images of the TPT-TMC/PSf TFC membrane before and after being exposed to acidic solution, no significant changes were observed. These results indicated that the incorporation of the s-triazine-amine structure into the polymer backbone could effectively enhance the acid stability of the composite membrane. The acid resistance of the poly(amide-s-triazine-amine) membrane derives from the presence of the s-triazine-amine structure that can alleviate the initial nucleophilic attacks to the membrane. Furthermore, the rigid conjugated bond structure of the s-triazine ring in the polymer backbone reinforced the acid stability. A comparison of the acid stability between the prepared membrane and other reported acid stable membranes has been given in Table 1. We can see that the acid stability of the prepared membrane is comparable to the poly(s-triazine) amine acid stable membranes and the separation performance is much better than other acid stable membranes.
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4. Conclusions An alternative approach for the fabrication of cyanuric chloride based acid resistant nanofiltration membrane was developed, in which a reactive s-triazine-amine precursor TPT was synthesized before the step of interfacial polymerization. The acid stable poly(amide-s-triazine-amine) thin film composite NF membrane (TPT-TMC/PSf TFC membrane) was successfully prepared by interfacial polymerization of the aqueous monomer TPT with TMC on PSf membrane surface. The characterization results indicated that the TPT-TMC/PSf TFC membrane had obtained a smooth and highly crosslinked skin layer with an isoelectric point around pH 3.5. The water permeance of TPT-TMC/PSf TFC membrane was about 8.68 L m−2 h−1 bar−1 and salt rejections over 97.0% for 2000 ppm divalent salt solutions under a pressure of 100 psi. The rejection to different salts followed the order of Na2SO4 > MgSO4 > NaCl, which is typical for the negatively charged membranes. After exposure to 0.05 M H2SO4 for 720 h, TPT-TMC/PSf TFC membrane retained the MgSO4 rejection above 94.2% with appropriate water permeance. No evident chemical and morphological changes were observed in the skin layer of the poly(amide-s-triazineamine) membrane after the long term acid treatment. Acknowledgements The authors acknowledge financial supports for this work from the National Natural Science Foundation of China (No. 51578485), the National Basic Research Program of China (No. 2015CB655303); the Research Fund for the Doctoral Program of Higher Education of China (No. 20130101110064), and the Zhejiang Provincial Collaborative Innovation Center Program 2011 (No. G1504126001900). 232
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Membr. Sci. 478 (2015) 75–84. [30] K.P. Lee, G. Bargeman, R. de Rooij, A.J.B. Kemperman, N.E. Benes, Interfacial polymerization of cyanuric chloride and monomeric amines: pH resistant thin film composite polyamine nanofiltration membranes, J. Membr. Sci. 523 (2017) 487–496. [31] S. Platt, M. Nyström, A. Bottino, G. Capannelli, Stability of NF membranes under extreme acidic conditions, J. Membr. Sci. 239 (2004) 91–103. [32] H.E. Fierz-David, M. Matter, Azo and anthraquinonoid dyes containing the cyanuric ring, J. Soc. Dyers Colour. 53 (1937) 424–436. [33] Y. Chih-Yao, Cyanuric Chloride Based Chlorine-resistant Nanofiltration Membrane, National Central Universtiy. [34] A. Chouai, V.J. Venditto, E.E. Simanek, Synthesis of 2‐[3,3′‐Di‐(Tert‐Butoxycarbonyl)‐Aminodipropylamine]‐ 4,6,‐Dichloro‐1,3,5‐Triazine as a monomer and 1,3,5‐[Tris‐Piperazine]‐Triazine as a core for the large scale synthesis of melamine (triazine) dendrimers, John Wiley & Sons, Inc, Organic syntheses, 2009. [35] J. Qin, S. Lin, S. Song, L. Zhang, H. Chen, 4-dimethylaminopyridine promoted interfacial polymerization between hyperbranched polyesteramide and trimesoyl chloride for preparing ultralow-pressure reverse osmosis composite membrane, ACS Appl. Mater. Interfaces 5 (2013) 6649–6656. [36] H. Huang, X. Qu, X. Ji, X. Gao, L. Zhang, H. Chen, L. Hou, Acid and multivalent ion resistance of thin film nanocomposite RO membranes loaded with silicalite-1 nanozeolites, J. Mater. Chem. A 1 (2013) 11343–11349.
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An acid resistant nanofiltration membrane prepared from a precursor of poly (s-triazine-amine) by interfacial polymerization
T
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Yanjun Zenga, Lihong Wangb, Lin Zhanga, , Jimmy Qiming Yuc a b c
Key Laboratory of Biomass Chemical Engineering, MOE, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China Department of Chemistry, Tangshan Normal College, Tangshan 063000, China Griffith School of Engineering, Griffith University, Nathan Campus, Brisbane, Queensland 4111, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyamine S-triazine amine Acid stability Interfacial polymerization Nanofiltration membrane
An acid resistant thin film composite nanofiltration (NF) membrane (named TPT-TMC/PSf membrane) was fabricated using interfacial polymerization. A precursor of poly(s-triazine-amine), 1,3,5-(tris-piperazine)-triazine (TPT), was synthesized and interfacially polymerized with trimesoyl chloride (TMC) to fabricate NF membrane on the surface of a polysulfone (PSf) ultrafiltration membrane. The chemical structure, surface charge and morphology of the TPT-TMC/PSf membrane were characterized by attenuated total reflection Fourier transform infrared spectrometer (ATR-FTIR), zeta potential measurement, and scanning electron microscope (SEM), respectively. The results indicated that the TPT-TMC/PSf membrane has a thinner skin layer and a smoother and more negatively charged surface than the conventional polypiperazine amide NF membrane. The newly fabricated TPT-TMC/PSf membrane exhibited excellent rejections against different aqueous salt solutions [Na2SO4 (98.6%) > MgSO4 (97.0%) > NaCl (40.5%)]. The membrane also exhibited excellent acid resistant behavior. After being immersed in the 0.05 M H2SO4 aqueous solution for 720 h, stable water permeance of about 8.68 L m−2 h−1 bar−1 and MgSO4 rejection above 94.2% still achieved.
1. Introduction Nanofiltration (NF) membranes have excellent capabilities to separate divalent metallic ions and low molecular weight organic compounds from monovalent ions. They are extensively used in wastewater treatment [1,2] and water purification operations in many industries such as chemical, biotechnology and pharmaceutical industries [3–5]. At present, aromatic polyamide thin film composite (TFC) membranes are the most commonly used commercial NF membranes due to their high water permeabilities and salt rejection coefficients. However, the amide bonds in these polyamide membranes are prone to hydrolysis under acidic conditions, which results in severe deteriorations in membrane performances. In particular, polypiperazine amide NF membranes are often observed to be less stable in pH extremes than the m-phenylenediamine (MPD) based amide membranes [6]. The acid instability of the polyamide membranes exclude its application from many processes that involve extreme acidic conditions, for examples, the processes of acid recovery or purification from drainage in electroplating industry [7,8], metal recovery from acid wastes in metal industry [9], nitrogenous compound removal from acid effluents in mining industry [10,11], and water recycling from acidic bleaching
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Corresponding author. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.memsci.2017.10.022 Received 5 May 2017; Received in revised form 5 October 2017; Accepted 9 October 2017 Available online 10 October 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
effluents in paper industry [12]. Acid resistant NF membranes are desired in these processes. Sulfonated aromatic polymers, such as sulfonated polysulfone and sulfonated poly(ether ketone), are most frequently investigated acid stable membrane materials [13–15]. Some commercially available acid stable membranes, including NP010, NP030, NF-PES-10 membranes made by Microdyn-Nadir [16,17] and NTR-7400 series by Hydranautics-Nitto Denko [18–20], are made of sulfonated aromatic polymers. However, most of these membranes have relatively low permeance and/or low salt rejections [13]. Polysulfonamide (PSA) is another acid stable membrane material, which has a similar chemical structure to polyamide. PSA has been reported to be more acid stable than polyamide due to the higher stability of the sulfonyl bond towards oxygen than carbonyl bond [6,21,22]. Liu et al. prepared an acid stable PSA NF membrane through interfacial polymerization (IP) of naphthalene-1,3,6- trisulfonylchloride (NTSC) and piperazine (PIP) and the resulting PSA membrane exhibited acid stability in 20% (w/v) H2SO4 for 2 months [23]. Aromatic diamine MPD also has been used in interfacial polymerization with benzene-1,3-disulfonyl chloride to synthesize acid-stable PSA membranes [22]. The disadvantage of PSA is that it is difficult to synthesize due to the weak reactivity of sulfonyl
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chemical structure and morphology of the TFC membrane were systematically studied by ATR-FTIR, Zeta potential, and SEM. The separation performance of the TFC membrane was tested by using a set of cross-flow filtration experiments. The acid resistant behavior of the TFC membranes was also evaluated and compared with conventional polypiperazine amide (PIP-TMC/PSf) NF membranes.
chloride, limiting its practical application in large scale [24]. New membrane materials with outstanding acid resistance as well as excellent water-salt separation properties still need to be developed. Recently, polymers with s-triazine rings as part of the polymer main chain have been used as a new type of acid stable membrane materials. The s-triazine ring consists of alternating nitrogen and carbon atoms that linked by alternating single and double bonds and has six unpaired electrons on the nitrogen atoms [25]. The stability of the conjugated bond structure of the s-triazine ring provides polymers with mechanical rigidity, thermal and chemical stabilities. Membranes based on s-triazine polyamine [poly(s-triazine-amine)] structure exhibited superior pH stability and have been prepared through interfacial polymerization between cyanuric chloride (CC) and amines [26–30]. Cadotte and Lee both obtained poly(s-triazine-amine) NF membranes by polymerizing cyanuric chloride with the large oligomer polyethyleneimine (PEI) interfacially [27,29] and the poly(s-triazine-amine) NF membranes (PEICC) exhibited excellent stability under extreme pH conditions ( pH 1 and 13) [29]. The commercially available BPT-NF serial membranes from Advanced Membrane System Technologies (AMS) consisted of an undisclosed poly(s-triazine-amine) structure also showed excellent resistance to 20% sulphuric acid for up to 4 months at 20 °C [31]. However, the interaction of cyanuric chloride with amines shows a clearly marked reactivity-stepwise behavior which ultimately lead to poor performance of the poly(s-triazine-amine) membranes. The reactivity of the first chlorine atom in cyanuric chloride is high and roughly equal to that of acid chlorides [32]. Nevertheless, replacement of the chlorine atoms in cyanuric chloride with electron-donors such as amines increases the electron density on the carbon atom of s-triazine, reducing the reactivity of the remaining chlorines. Thus, the second and/or the third chlorine in cyanuric chloride is more difficult to be replaced by amines, especially those with slight steric hindrance such as piperazine (PIP) and MPD. The incomplete substitution of the three chlorines in cyanuric chloride leads to a lower molecular weight of the polymer or a higher percentage of linear structures in the polymer. Frequently, rough, open and discontinuous layers are formed on the surface of the support membrane [30]. Even the well-defined poly(striazine-amine) (PEI-CC) composite membrane obtained only fivefold lower water permeance in a dead-end filtration setup as compared to the corresponding polyamide (PET-TMC) composite membrane [29]. Elevating the temperature can increase the reactivity of the second and third chlorines in cyanuric chloride. However, the temperature for membrane fabrication can only be raised up to around 60 °C considering the thermal stability of the supporting membrane and the operability of the IP process. A poly(s-triazine-amine) NF membrane has been prepared through interfacial polymerization between diethylene triamine (DETA) and cyanuric chloride under a temperature of 60 °C, but the membrane still exhibited a low water permeance and poor salt rejection [33]. This indicated that a temperature of 60 °C still not high enough to completely tri-substituted the chloride atoms in cyanuric chloride, which is the key point for obtaining high performance poly(s-triazine-amine) NF membranes. In this paper, in order to fabricate acid resistant membrane derive from cyanuric chloride based polymer and accomplish complete trisubstitution in cyanuric chloride, a precursor of poly(s-triazine-amine), 1,3,5-(tris-piperazine)-triazine (TPT), was first synthesized and then used as a monomer in the interfacial polymerization with trimesoyl chloride (TMC) to fabricate NF membrane. The precursor TPT was first synthesized through a nucleophilic substitution reaction between cyanuric chloride and N-(tert-butoxycarbonyl)- piperazine. As the nucleophilic substitution reaction was not restrained by the conditions of the IP process, all the three chlorine atoms in cyanuric chloride could be replaced by N-(tert-butixycarbonyl)- piperazine under a high reaction temperature of 75 °C with stirring. Then the precursor was used as an aqueous phase monomer to fabricate a thin film composite (TFC) membrane (named TPT-TMC/PSf membrane) via IP with TMC on the surface of a porous polysulfone (PSf) ultrafiltration membrane. The
2. Experimental 2.1. Materials and reagents Piperazine (PIP, 99%), cyanuric chloride (CC, 99%), trimesoyl chloride (TMC, 98%), N-(tert-butoxycarbonyl)-piperazine (98%), N, Ndiisopropylethylamine (DIPEA, 99%) were purchased from Aldrich Company. Isopar-G (Isoparaffin type hydrocarbon oil) was purchased from Exxon Mobil Chemical. Hydrochloric acid (HCl), tetrahydrofuran (THF), dichloromethane, chloroform, methanol, NaHSO4, NaCl, NaOH, Na2SO4, Na3PO4·12H2O, MgCl2, MgSO4 and molecular sieves (4 Å) were all of analytical reagent grade, and acquired from Sinopharm Chemical Reagent Co., Ltd. Molecular sieves (4 Å) were activated under 600 °C for 12 h in furnace before use. All other chemicals were used as received. The PSf ultrafiltration membranes with molecular weight cutoff around 30,000 were purchased from the Development Center for Water Treatment Technology, Hangzhou, China.
2.2. Synthesis and characterization of 1,3,5-(tris-piperazine)-triazine (TPT) TPT monomer was synthesized by a two-step process according to the literature [34]. In the first step, a sample of 5 g (27.1 mmol, 1.00 equiv) cyanuric chloride dissolved in 500 mL THF was injected into a 1 L three necked, round bottomed flask equipped with a mechanical stirrer, reflux condenser, and static nitrogen inlet. Then 17.0 g (91.5 mmol, 3.38 equiv) N-(tert-butoxycarbonyl)-piperazine was added into the mixture in small portions over 10 min. An acid acceptor N, Ndiisopropylethylamine (48.1 mL, 276 mmol, 10.2 equiv) was added and the reaction mixture was stirred at 0 °C for 1 h, then kept at the ambient temperature for 3 h, and finally kept at the temperature of 75 °C for 20 h. When the reaction was finished, the remaining THF was removed by using a rotary evaporator. The obtained light yellowish solid was dissolved in dichloromethane and washed with water, 10% NaHSO4 and NaCl saturated aqueous solution, respectively. The organic layer was dried over 4 Å molecular sieves, filtered and the organic solvent was removed by using a rotary evaporator. The intermediate product was named 1,3,5-[Tris-N-(tert-butoxycarbonyl)-piperazine] –triazine (TBPT). In the second step, TBPT (1.00 equiv) and methanol (160 equiv) were added into a 500 mL flask as previously. The solution was stirred at 0 °C for 30 min. Then hydrochloric acid (21 equiv) was added over 70 min while the temperature was kept at around 0 °C. The resulting slurry was stirred at 0 °C for 2 h. And then the reaction slurry was placed under ambient temperature for over 3 h, slowly heated to 40 °C for 12 h. The organic components were removed by using rotary evaporator. The remaining aqueous solution was cooled to 0 °C and its pH adjusted to 14.0 by using 10% (w/v) NaOH solution. The alkaline solution was extracted with chloroform (3 × 250 mL). The acquired organic phases were dried with sodium sulfate, filtrated and evaporated to obtain a final white solid product. The final product was named 1,3,5-(tris-piperazine)-triazine (TPT). Schematic illustration of the synthetic route is showed in Scheme 1. The reactivity of the precursor was high enough to form well-defined film with trimesoyl chloride (TMC) through interfacial polymerization within a few seconds as showed in the free-standing experiment (Fig. S.1). 226
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Scheme 1. Synthetic route for1,3,5-(tris-piperazine)triazine (TPT).
[35,36]. The trans-membrane pressure was set at 100 psi and the feed solution temperature was kept at 25 ± 1 °C with a constant temperature system. Inorganic salt solutions (NaCl, MgSO4, Na2SO4, 2000 ppm) were used as feed solutions and tested in terms of water permeance and salt rejections, respectively. Separation performances of the TFC membranes to different inorganic salt solutions were evaluated following the procedures described in literature [37]. Membranes were pre-pressured at 200 psi for stabilization with de-ionized water for 30 min before separation test. The water permeance (P) of the membranes were calculated by the following equation;
2.3. Preparation of TFC membranes The NF membranes were prepared through IP between TPT and TMC on the surface of a PSf support UF membrane. First, TPT and Na3PO4·12H2O (acid acceptor) were dissolved in deionized water to prepare aqueous solution. The organic phase solution was prepared by dissolving TMC monomer in Isopar-G solvent. Then, the aqueous solution was poured onto the surface of the flat sheet PSf membrane and immersed for about 2 min. The excessive solution was removed using a rubber roller. The organic solution was then poured upon membrane and let set for a fixed time. Any excessive organic solution was removed and the nascent membrane was heated in an oven at 70 °C. Finally, the prepared membrane was stored in deionized water until to be used. Optimal membrane fabrication conditions in terms of water permeance and salt rejection were obtained as follows: TPT concentration, 0.6% (w/v); TMC concentration, 0.1% (w/v); Na3PO4·12H2O, 1.5% (w/ v); time for IP process, 1 min; curing temperature, 70 °C; and curing time, 15 min. For comparison, conventional NF membranes obtained from PIP and TMC at the same optimal conditions (except for the concentration of PIP which was set at 0.232% (w/v) to ensure the number of amine groups in PIP-TMC system was the same as the number in TPT-TMC system) were also prepared by following the same procedures.
Permeance =
F V = ΔP At ΔP
where F is the permeate flux (L m−2 h−1), V is the volume of permeate (L) collected over a period of time t (h), A is the effective area of the membrane (m2) and ΔP is the trans-membrane pressure (bar). The salt rejection R was calculated as follows:
c R = ⎛1− 2 ⎞×100% ⎝ c1 ⎠ ⎜
⎟
where c1 and c2 are the salt concentration of feed and permeate solution, respectively. The concentrations of the feed and permeate were analyzed by measuring the conductivity using a conductivity meter (model DDS-11A, Neici Instrument, Shanghai, China).
2.4. Characterization of TPT monomer and TFC membranes 2.6. Long term acid stability tests The functional groups of TBPT and TPT were characterized by using Fourier transform infrared spectroscopy (FTIR) analyses on a Nicolet 5700 FTIR spectrometer. The elemental contents (C, N, H) of TBPT and TPT were quantified by an element analyzer (Elementar Analysen system GmbH, Vario Micro). 1H and 13C NMR spectra were measured on an Agilent DD2-600 MHz spectrometer. Test samples were prepared at room temperature by dissolving TBPT in D2O and TPT in CDCl3 solvent. Scanning electron microscope (SEM, SU-8010, Hitachi, Japan) with an accelerating voltage of 3 kV was used to observe the surface and cross sectional morphology of membranes. Before tests, the membranes were dried at 40 °C in a vacuum oven for 12 h and then cut into small stripes. For observing the cross-section morphology, stripe-shaped membrane samples were immersed in liquid nitrogen and broken through folding. Before SEM observation, all samples were sputtered with gold powder to enhance electric conductivity and to improve the resolution of SEM pictures. Surface charge of membranes were characterized by zeta potential measurements. Measurements were performed using a streaming potential instrument (SurPASS, Anton Paar) and conducted in a background electrolyte solution containing 1 mM KCl over a range of pH values. HCl and NaOH solutions were used to adjust the pH of the test solutions.
The membranes were exposed to solutions of 0.05 M H2SO4 for fixed periods of time. The solutions were kept at a constant temperature of 25 ± 1 °C with a constant temperature oscillation incubator (HZQF160, PeiYing laboratory equipment Co. Ltd., China). At the end of the exposure period, the membranes were taken out of the solutions and washed with deionized water until the pH value of the washing water was stable. The acid treated membranes were characterized for surface or permeation properties by following the procedures described above. 3. Results and discussion 3.1. Synthesis and characterization of TPT monomer The chemical composition and chemical structure of the intermediate product TBPT and the final monomer TPT (as shown in Scheme 1) were confirmed by FTIR, 1H NMR, 13C NMR, and Elemental analysis. The FTIR spectra of TBPT and TPT are presented in Fig. S2. Detailed characteristic peaks are listed as follows. TBPT: FTIR (KBr, cm−1): 1695, 1535, 1419, 1227, 998, 725. TPT: FTIR (KBr, cm−1): 3294, 2846, 1550, 1433, 1242, 1007, 806, 750. The peak at 1695 cm−1 appeared in the spectrum of TBPT was attributed to the protective functional group (t-tutyloxy carbonyl). The absence of the peak at 1695 cm−1 and the presence of the peak at 3294 cm−1 in the spectrum of TPT indicated the removal of the protective group and the existence of –NH group. The 13C NMR and 1H NMR spectrum are showed in Fig. S3. TBPT: 1 H NMR (600 MHz, CDCl3) δ: 1.46 (-CH3), 3.42 (CH2-C-N), 3.72 (NCH2-C); 13C NMR (600 MHz, CDCl3) δ: 31.02 (-CH3), 45.67 (N-C-C), 79.46 (solvent), 82.52 (-C-O), 157.44 (C˭O), 167.12（C˭N）. TPT: 1H
2.5. Evaluation of separation performance The separation performances of membranes were evaluated by using a customized flat-sheet cross-flow membrane system with an effective filtration area of 12.56 cm2, as reported in our previous work 227
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Fig. 2. Zeta potential of PIP-TMC/PSf TFC membrane and TPT-TMC/PSf TFC membrane measured under different pH values.
Fig. 1. ATR-FTIR spectra of PSf ultrafiltration membrane, PIP-TMC/PSf TFC membrane and TPT-TMC/PSf TFC membrane.
membrane [28,38]. The interfacial polymerization is a self-restraint process and the highly crosslinked nascent membrane blocks the further diffusion of TPT monomer into the organic phase. Furthermore, the molecular size of TPT is larger than that of PIP. Both the highly crosslinked nascent membrane and the larger molecular size of TPT make the diffusion of TPT monomer into the organic phase more difficult than PIP monomer does, resulting in increased hydrolysis of acyl chloride into carboxylic acid groups [37,39,40].
NMR (600 MHz, D2O) δ: 1.20 (-NH), 2.72 (CH2-C-N), 3.60 (N-CH2-C), 4.63 (solvent); 13C NMR (600 MHz, D2O) δ：45.7 (N-C-C), 47.0 (N-CC), 167.6（C˭N）. The elemental compositions of TBPT and TPT obtained from elemental content analyses were C, 55.71%; H, 8.19%; N, 19.14% and C, 54.32%; H, 8.20%; N, 37.05%, respectively. These closely compare with the theoretical elemental content for TBPT (C30H51N9O6) of C, 56.85%; H, 8.11%; N, 19.89%, and that for TPT (C15H27N9) of C, 54.03%; H, 8.16%; N, 37.81%. The results from the FTIR, NMR and elemental analysis all confirmed that the intermediate product TBPT and the final compound TPT were successfully synthesized.
3.2.3. Morphological characterization The morphologies of the NF membranes visualized by SEM are illustrated in Fig. 3. In the cross-sectional images, distinctive dense and continual layer can be seen on the top of the porous PSf support membrane. The top skin layer of TPT-TMC/PSf TFC membrane was observed thinner than that of PIP-TMC/PSf TFC membrane. Hung [41] also observed that the polyamide layer with a high polymerization degree exhibited a thin active layer thickness. The reduction in thickness could been ascribed to the smaller amount of the aqueous monomer diffusion into organic phase as discussed in Section 3.2.2. The surface image of the PIP-TMC/PSf TFC membrane showed a classical grainy topography while the TPT-TMC/PSf TFC membrane had a smooth membrane surface. According to Morgan's theory, membrane roughness comes from the inhomogeneity of further reactions after initial film formation of IP [42]. Because of the larger molecular size and the higher content of the crosslinking structure in the nascent membrane, further diffusion of TPT monomer into the organic phase was blocked [35]. Further reactions at the organic phase after the formation of nascent membrane were limited, and thus a smooth skin layer was obtained.
3.2. Characterization of TFC membrane 3.2.1. ATR-FTIR spectra The ATR-FTIR spectra of the PSf ultrafiltration support membrane, PIP-TMC/PSf and TPT-TMC/PSf TFC membranes are shown in Fig. 1. The absence of acyl chloride peak at 1760–1770 cm−1 and the presence of a strong band at 1618 cm−1 (C˭O) indicate the occurrence of IP. The peak at 1546 cm−1 in both TFC membranes corresponds to the stretching vibration of the C-N group in the piperazine-amide structure. However, the characteristic bands at 1503 cm−1 and 1362 cm−1 derived from the stretching vibration and deformation vibration of striazine ring overlapped with the peaks from the benzene ring of PSf UF membrane. 3.2.2. Surface charge evaluation The surface charge of the TFC membranes were evaluated by calculating the zeta potential from the streaming potential measurements. The results are shown in Fig. 2. The isoelectric point for PIP-TMC/PSf TFC membrane was found to be around 5.0 and around 3.5 for TPTTMC/PSf TFC membrane. Above the isoelectric points, the membranes were negatively charged due to the dissociation of the carboxylic acid groups, which were derived from the hydrolysis of unreacted acyl chloride groups. In contrast, the membranes were positively charged below the isoelectric point due to the protonation of the amine functional group. In the range of pH values measured in the test, TPT-TMC/ PSf TFC membranes were always more negatively charged than PIPTMC/PSf TFC membranes. This result indicated that the surface of TPTTMC/PSf TFC membranes contains more carboxylic acid groups than the surface of PIP-TMC/PSf TFC membranes. As demonstrated in Scheme 2, TPT-TMC/PSf TFC membrane was formed from 3 × 3 monomer pairs while PIP-TMC/PSf TFC membrane from 2 × 3 monomer pairs, the content of the crosslinking structure of TPT-TMC/ PSf TFC membrane should be higher than that of PIP-TMC/PSf TFC
3.2.4. Permeation properties of TFC membranes TPT -TMC/PSf TFC membrane with a high polymerization degree exhibited a small free volume diameter because the polymer chains in the cross-linked segment were densely arranged and the disturbance in the polymer chains was limited, resulting in a small free volume diameter and fraction [41]. Generally, the TPT -TMC/PSf TFC membrane with small free volume diameter and fraction would be highly selective and exhibited a low permeance. However, the water permeance of the TPT-TMC/PSf membrane for all solutions shown in Fig. 4 were higher than that of the PIP-TMC/PSf membrane. This was most likely due to the thinner skin layer of the TPT-TMC/PSf membrane. The rejection performances of the two membranes to different salts followed the same order of Na2SO4 > MgSO4 > NaCl, which is typical for negatively charged NF membrane. From the results of zeta potential measurements, in the pH range of 6.0–7.0, both membranes were negatively charged, and the rejections to charged solutes were affected by both the 228
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Scheme 2. (a) Crosslinking structure and linear structure of PIP-TMC/PSf membrane; (b) Crosslinking structure of TPT-TMC/PSf membrane.
showed that the TPT-TMC/PSf TFC membrane have higher rejections to Na2SO4 (98.6% versus 97.6%) and MgSO4 (97.0% versus 95.0%) salts and lower rejection to NaCl (40.5% versus 54.2%) salt as compared to PIP-TMC/PSf TFC membrane. This is in line with the fact that the TPTTMC/PSf TFC membrane is more negatively charged than the PIP-TMC/ PSf TFC membrane in the testing pH range. Enhanced Donnan exclusion effect between the TPT-TMC/PSf TFC membrane and the salt solutions
Donnan exclusion effect and the steric size exclusion effect. For the cations, although Mg2+ (0.428 nm) has a larger hydrated radius than that of Na+ (0.358 nm), Na2SO4 had a higher rejection than that of MgSO4 as a result of the stronger affinity between the higher charged cations and the negatively charged membrane surface. For anions, the sulfate ion was retained more than the chloride ion not only because of its larger radius but also because of its higher charge. The results also
Fig. 3. SEM images of membrane surface and crosssection of (a) (b) PIP-TMC/PSf TFC membrane and (c) (d) TPT-TMC/PSf TFC membrane.
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Fig. 4. (a) Water permeance; (b) salt rejection of TPT-TMC/PSf TFC membrane and PIP-TMC/PSf TFC membrane to different inorganic salts. Tested conditions: trans-membrane pressure = 100 psi; temperature = 26 ± 1 °C.
was responsible for this phenomenon. 3.3. Acid stability of TFC membranes The acid stability of the TFC membranes was investigated by immersing membranes into 0.05 M H2SO4 aqueous solutions for fixed periods. The salt rejection and water permeance were measured as functions of time and the variations observed were used to evaluate the acid stability of the membranes. As results shown in Fig. 5, the performance of the PIP-TMC/PSf TFC membranes deteriorated significantly after being exposed to acid solutions for 720 h. The rejection to MgSO4 decreased from 95.0% to 61.8% and the corresponding water permeance increased from 7.11 L m−2 h−1 bar−1 to 15.16 L m−2 h−1 bar−1. This trend of performance deterioration is consistent with the results from a number of studies where polypiperazine amide membranes hydrolysis under acid treatment [39,40,43]. Fig. 6 shows the ATR-FTIR spectra of PIP-TMC/PSf TFC membrane before and after being exposed to the acidic solution, in which noticeable changes can be observed at the bands of 1735 cm−1, 1618 cm−1, 1546 cm−1 and 1441 cm−1. After acid treatment, the bands at 1618 cm−1, 1546 cm−1 and 1441 cm−1 were weakened significantly and a band at 1735 cm−1 that represents the carbonyl moiety of ester functional group was emerging, indicating the disintegration of the
Fig. 5. Normalized water permeance and rejection for TPT-TMC/PSf membrane and PIPTMC/PSf membrane after being immersed in 0.05 M H2SO4 solution for a fixed period.
Fig. 6. ATR-FTIR spectra of (a) PIP-TMC/PSf TFC membrane and (b) TPT-TMC/PSf TFC membrane before and after being exposed to acidic solution for 720 h.
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Fig. 7. SEM images of surfaces of (a) (b) PIP-TMC/ PSf membrane and (c) (d) TPT-TMC/PSf membrane before and after being exposed to acidic solution for 720 h.
Scheme 3. (a) Degradation mechanism of polypiperazine amide membrane under the acidic condition; (b) illustration of acid stability of the poly (amide-s-triazine-amine) membrane.
Table 1 A comparison of the nanofiltration membrane made in this work compared with other reported acid stable membranes. Membrane materials
Salt rejectiona
Water permeance (L m−2 h−1 bar−1)
Testing conditions
Ref.
Sulfonated poly (ether ether ketone) Polysulfonamide Poly (s-triazine-amine) Poly (s-triazine-amine) Poly(amide-s-triazine-amine)
85–95% 86.7% 75–95%b 85.2%b 98.6%
0.8 5.76 0.2–0.7 1.5 8.68
pH = 0 (HNO3); Room temperature; Dead-end filtration setup 20.0% (w/v) H2SO4; 25 °C； Cross-flow filtration setup 0.1 M HNO3; Room temperature; Dead-end filtration setup 0.1 M HNO3; Room temperature; Dead-end filtration setup 0.05 M H2SO4; 25 ± 1 °C；Cross-flow filtration setup
[13] [23] [29] [30] This work
a b
Separation performance to Na2SO4 solution. Separation performance to NaCl solution.
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Appendix A. Supporting information
amide bond. The corresponding SEM images are shown in Fig. 7, which exhibited significant changes in the membrane surface after being exposed to the acid solution for 720 h. The classical grainy morphology of the polypiperazine amide membrane was replaced by a collapsed and defective structure. The degradation mechanism of polypiperazine amide membrane are illustrated in Scheme 3. The oxygen in the amide bond is more electronegative than carbon, and thus electron density was drawn away from carbon, making carbon much more susceptible to nucleophilic attack with the presence of the acid catalyst [29]. In contrast, as seen in Fig. 5, the TPT-TMC/PSf TFC membrane showed no significant changes in salt rejection and water permeance after being exposed to the same acid solution for 720 h, indicating its excellent stability under acidic conditions. Nevertheless, during the initial period of acid treatment, there was a slight increase in salt rejection and a slight decrease in water permeance for TPT-TMC/PSf TFC membrane. This may due to a re-construction of the membrane structure when exposed to a highly acidic solution [44]. In the ATR-FTIR spectra and SEM images of the TPT-TMC/PSf TFC membrane before and after being exposed to acidic solution, no significant changes were observed. These results indicated that the incorporation of the s-triazine-amine structure into the polymer backbone could effectively enhance the acid stability of the composite membrane. The acid resistance of the poly(amide-s-triazine-amine) membrane derives from the presence of the s-triazine-amine structure that can alleviate the initial nucleophilic attacks to the membrane. Furthermore, the rigid conjugated bond structure of the s-triazine ring in the polymer backbone reinforced the acid stability. A comparison of the acid stability between the prepared membrane and other reported acid stable membranes has been given in Table 1. We can see that the acid stability of the prepared membrane is comparable to the poly(s-triazine) amine acid stable membranes and the separation performance is much better than other acid stable membranes.
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4. Conclusions An alternative approach for the fabrication of cyanuric chloride based acid resistant nanofiltration membrane was developed, in which a reactive s-triazine-amine precursor TPT was synthesized before the step of interfacial polymerization. The acid stable poly(amide-s-triazine-amine) thin film composite NF membrane (TPT-TMC/PSf TFC membrane) was successfully prepared by interfacial polymerization of the aqueous monomer TPT with TMC on PSf membrane surface. The characterization results indicated that the TPT-TMC/PSf TFC membrane had obtained a smooth and highly crosslinked skin layer with an isoelectric point around pH 3.5. The water permeance of TPT-TMC/PSf TFC membrane was about 8.68 L m−2 h−1 bar−1 and salt rejections over 97.0% for 2000 ppm divalent salt solutions under a pressure of 100 psi. The rejection to different salts followed the order of Na2SO4 > MgSO4 > NaCl, which is typical for the negatively charged membranes. After exposure to 0.05 M H2SO4 for 720 h, TPT-TMC/PSf TFC membrane retained the MgSO4 rejection above 94.2% with appropriate water permeance. No evident chemical and morphological changes were observed in the skin layer of the poly(amide-s-triazineamine) membrane after the long term acid treatment. Acknowledgements The authors acknowledge financial supports for this work from the National Natural Science Foundation of China (No. 51578485), the National Basic Research Program of China (No. 2015CB655303); the Research Fund for the Doctoral Program of Higher Education of China (No. 20130101110064), and the Zhejiang Provincial Collaborative Innovation Center Program 2011 (No. G1504126001900). 232
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