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Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance
Author’s Accepted Manuscript Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance Chuang Yu, Haiyan Li, Xiru Zhang, Zhenhua Lü, Sanchuan Yu, Meihong Liu, Congjie Gao www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(18)31994-X https://doi.org/10.1016/j.memsci.2018.09.012 MEMSCI16457
To appear in: Journal of Membrane Science Received date: 20 July 2018 Revised date: 30 August 2018 Accepted date: 2 September 2018 Cite this article as: Chuang Yu, Haiyan Li, Xiru Zhang, Zhenhua Lü, Sanchuan Yu, Meihong Liu and Congjie Gao, Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.09.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance
Chuang Yua, Haiyan Lia, Xiru Zhanga, Zhenhua Lüa, Sanchuan Yua, *, Meihong Liub, Congjie Gaoc
a
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
b
School of Civil Engineering and Architecture, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
c
The Development Center of Water Treatment Technology，SOA，Hangzhou 310012, People’s Republic of China
* Corresponding author: Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China. Tel.: +86-571-86843217; Fax: +86-571-86843217; E-mail: [email protected] (S. Yu).
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Abstract： In this study, the novel technique, namely interfacial polymerization (IP) coupled with surface amidation, was proposed to fabricate polyamide thin-film composite (TFC) membranes for improved reverse osmosis performance. TFC membrane with loose polyamide separation layer was firstly prepared through conventional IP process. Surface amidation was then performed through covering the surface of the loose polyamide layer with aqueous 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) solution. Amidation reaction between the free carboxylic and amino groups on membrane surface was confirmed through ATR-FTIR and XPS analyses. Permeation tests revealed that the step of surface amidation could effectively improve the ability of membrane rejection to solute, showing significant increases of NaCl, NaNO3 and glycerol rejections from about 96.4, 90.5 and 78.4% to 97.8, 93.7 and 91.3%, respectively. The water permeability of the membrane obtained via IP followed with surface amidation was at least 22.3% higher than the membrane of the same NaCl rejection obtained via conventional IP process. Thus, the method developed was attractive for the fabrication of TFC reverse osmosis membranes with improved permeability to water and rejection ability to solutes. Additionally, the step of surface amidation of present work can be potentially applied to the in-situ rejuvenation of used polyamide reverse osmosis membranes.
Keywords: Interfacial polymerization; Surface amidation; Polyamide thin-film composite membrane; Reverse osmosis; Membrane perm-selectivity
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1. Introduction Global water crisis is the most crucial problem for human beings. Reverse osmosis (RO) membrane process, which can provide high quality water through removing all the impurities in aqueous such as dissolved salts and organic compounds, has become the key technology to alleviate the water crisis [1-3]. However, the energy consumption of reverse osmosis technology, which is largely determined by the core component of reverse osmosis membrane, is still relatively high and the extensive application of reverse osmosis process is limited to a certain extent [4]. Therefore, it is of great practical significance to improve the separation efficiency of RO process through enhancing the water permeability, rejection ability as well as antifouling property of the semi-permeable membrane used [5, 6]. The major membrane currently used in RO process is the type of thin-film composite (TFC) membrane containing an active separation layer of fully aromatic polyamide (PA) for its better permeability to water, rejection ability to solutes and stability to chemicals in treating aqueous fluids compared with the type of asymmetric membrane made from cellulose acetate [7]. Thus, improvement of separation performance of the polyamide-based TFC RO membrane has been drawing more and more attentions of researchers in the past decades. The type of polyamide-based TFC RO membrane is typically fabricated through depositing a thin separation layer on the top of the substrate of porous polysulfone via the interfacial reaction between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) [8]. The permeation property of the TFC membrane is largely determined by the chemistry, network structure and thickness of the polyamide layer. Accordingly, efforts were firstly devoted to enhance membrane permeability to water through making the polyamide composite active layer
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thinner, since membrane permeability to water is inversely proportional to the thickness of the active separation layer [9]. However, the interfacial reaction between MPD and TMC, as a diffusion-controlled and self-limited process, has already produced a typical PA layer as thin as about 100 nm. Any further reduction of the thickness of the PA layer is restricted by the empirical property of the interfacial polymerization process, the generation of defect in mass production, as well as the reduction of the durability of the TFC membranes obtained [10]. Efforts have also been devoted to improve the permeation property of the polyamide-based TFC RO membrane through tuning the chemistry and network structure of the separation layer via using additive in MPD-aqueous or TMC-organic phase. Both organic additives such as acetone [11], amino-cyclodextrins (CDs) [12], alkyl phosphates [13] and surfactants [14], and inorganic additives such as silica [15], grapheme oxide (GO) [16, 17] and porous nanozeolites of NaA [18, 19] and NaX [20] have been proved effective in enhancing the water permeability of the formed TFC membrane. However, the use of additive usually generates a polyamide active layer with a relatively loose network structure and thus lowers the ability of membrane rejection to solutes. The above analysis reveals that, the conventional interfacial polymerization (IP) process, as the most popular and scalable technique for the mass production of polyamide-based TFC membrane, generally produces a separation layer with trade-off between its rejection ability to dissolved solutes and permeability to water [21, 22]. It is still of great challenge and difficulty to improve the permeation property of the polyamide-based TFC membrane through separately optimizing the chemistry, network structure, and thickness of the active layer formed via conventional IP process [23]. Additionally, as the possible
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routes to enhance membrane performance and antifouling properties, surface chemical modification mainly via grafting is relatively sophisticated and will alter membrane surface chemistry, and surface physical modification mainly via coating is of poor long-term stability [24-26]. Therefore, in this study, a novel two-step fabrication technique, namely interfacial polymerization (IP) followed by surface amidation (SA), was proposed to prepare the polyamide-based TFC RO membrane for enhanced salt rejection. As schematically illustrated in Fig. 1, conventional IP process was firstly adopted to generate a loose PA layer with relatively good permeability to water and poor rejection ability to solutes on the top of a porous support of polysulfone (PSF). Surface amidation was then performed to make the skin of the loose polyamide layer more crosslinked. In the experiment, PA layer with loose network structure was obtained through varying the MPD and TMC contents, surface amidation was facilely implemented through covering the surface of PA layer with an acidic aqueous solution containing 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) as activator. Different from the conventional IP process that generally produces a polyamide layer with a uniform cross-section structure of nearly the same density from top to bottom, the two-step fabrication process of interfacial polymerization followed with surface amidation will produce a polyamide layer with an asymmetric cross-section structure composed of an ultrathin dense skin on the top of a loose body. The PA layer with the asymmetric structure will endow the formed TFC membrane with high water permeability under the premise of high salt rejection. Additionally, the two-step fabrication process can be scaled up for mass production and the step of surface amidation can also be potentially applied to in-situ performance rejuvenation of the used polyamide-based TFC reverse osmosis
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membranes for prolonged membrane lifespan.
Fig. 1. Schematic for the preparation of TFC membrane via the two-step process of interfacial polymerization followed by surface amidation.
2. Experimental 2.1. Materials and reagents The substrate of flat-sheet polysulfone (PSF) ultrafiltration membrane for interfacial polymerization was friendly provided by Hangzhou Tianchuang Environmental Technology Co., LTD. (Hangzhou, China). The pure water permeability and molecular weight cut-off of the substrate were about 280 l/m2 h bar and 120,000 g/mol, respectively. Monomers such as m-phenylenediamine (MPD) for aqueous phase and trimesoyl chloride (TMC) for organic phase were of analytical grade and bought from Shanghai Amino-Chem. Co. Ltd. (Shanghai, China) and Sigma-Aldrich, respectively. Additives such as triethylamine (TEA, purity 99.5%) 6/33
and camphor sulfonic acid (CSA, purity 99.0%) for the aqueous phase were bought from Sigma-Aldrich. Isopar G, a type of isoparaffin fluid supplied by ExxonMobil Chemical, was used as the solvent for organic phase. 1-ethyl-(3-3-dimethylaminopropyl) carbodiimide hydrochloride (EDC.HCl purity 98.5%) purchased from Aladdin (Shanghai, China) was used as the activator for amidation reaction. All other analytical grade solvents and reagents were used as received.
2.2. Fabrication of TFC membrane through conventional IP process TFC membranes were prepared through depositing a polyamide layer on the top of the support of porous polysulfone. Conventional IP process was adopted to produce the active separation layer following the procedure described in our previous works [27, 28]. The porous substrate of polysulphone ultrafiltraion membrane was firstly clamped with a frame of polytetrafluoroethylene (PTFE). The surface of the substrate was then soaked with a MPD-aqueous solution for 2.0 min. Any bubbles formed during the soaking process were eliminated through rolling the surface with a soft rubber roller. After draining off the excess MPD-aqueous solution from the surface and air-drying the MPD-soaked support under room temperature until there were no liquids on the surface, the interfacial polymerization was then implemented to laminate a PA layer on the top of the substrate through covering the surface of the MPD-saturated substrate with a TMC-organic solution for 30s. After removing the excess organic solution, a heat treatment of 5 min at 90℃ was performed with the membrane to make the structure of the formed polyamide layer more crosslinked. Finally, the composite membrane was sequentially soaked with aqueous Na2CO3 solution of 40.0 ℃ and pH 9.0 for
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5 min, rinsed thoroughly using hot de-ionized water of 40.0℃ for 10 min and kept wet for subsequent use. In this work, TEA was added into the MPD-aqueous solution as an acid acceptor at the content of 3.0 wt% and CSA was added to adjust the pH of the aqueous phase solution to about 9.6. PA layers with different solute rejection abilities were formed through mainly varying the contents of monomers MPD and TMC. The TFC membranes obtained through conventional IP process were denoted as IP-based membrane. The main parameters for the preparation of different IP-based PA TFC membranes are tabulated in Table 1. Table 1 Parameters for the preparation of IP-based PA TFC membranes. TFC membranes
MPD content (wt%)
TMC content (wt/v%)
pH of aqueous solution
M1
1.5
0.06
9.4
M2
1.7
0.06
9.4
M3
1.8
0.06
9.4
M4
2.0
0.07
9.5
M5
2.1
0.10
9.5
M6
2.5
0.12
9.7
2.3. Surface amidation of IP-based PA TFC membrane Surface amidation of the PA TFC membrane obtained through conventional IP process was facilely implemented under the activation by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). Fig. 2 schematically illustrates the reaction mechanism of surface amidation. As illustrated in the schematic, activating agent EDC reacts firstly with the free
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carboxyl groups of the PA layer of the TFC membrane to generate the highly reactive ester of O-acylisourea. The formed intermediate is unstable and can react with the adjacent amino groups to generate amide linkages [29]. In the experiment, surface amidation was carried out through covering the surface of the TFC membrane clamped in a PTFE frame with an aqueous phosphate-buffered solution (PBS) containing 50 mM EDC. The pH of the buffer solution was regulated between 3.0 and 6.5 through using aqueous HCl or NaOH solution. The membrane covered with EDC solution was then put into a sealed opaque container of room temperature for 12.0 h. Finally, the membrane was thoroughly rinsed using de-ionized water and kept wet for subsequent use. The IP-based PA TFC membrane after surface amidation was referred to as IP/SA-based membrane. COO-
COO-
NH2
COO-
NH2
N Cl
H+
N C N
N Cl
NH C N O
N Cl
+
H
O C
O
O
NH2
C
NH2
O COO-
NH2
H+
NH C N
COO-
H C
N
COO-
Fig.2. Schematic for the EDC-activated surface amidation of the polyamide-based TFC membrane. 9/33
2.4. Characterization of membrane physico-chemical property Membranes were characterized for their physico-chemical properties after being thoroughly rinsed with de-ionized water and vacuum dried. The chemical structure and composition of membrane surface were characterized through attenuated total reflectance infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. FTIR spectrometer of Nicolet Aratar 370 with a ZnSe crystal at an incidence angle of 45° was employed to perform the ATR-FTIR analysis. Perkin Elmer PHI 5000C ESCA System with the ultraviolet source of Mg/Al Dual Anode Hel/Hell was employed to carry out the XPS analysis. Membrane morphology observed through scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM images of the surface and cross-section of each membrane sample sputtered with gold were obtained with a Hitachi S-4800field emission scanning electron microscopy (Japan). The three-dimensional surface AFM images of 10.0μm×10.0μm were obtained with an atomic force microscopy of park instrument auto probe CT under the tapping mode in air. The root mean square roughness (RMS) of membrane surface was determined through using the height profile of the obtained AFM image [30]. Membrane surface hydrophilicity and charge were evaluated through measuring surface water contact angle (WCA) and zeta potential (ZP), respectively. DSA10-MK2 contact angle analyzer (KRUSS BmbH, Germany) was used to measure the surface water contact angle by adopting the method of sessile drop at room temperature [31]. The averaged WCA of each membrane sample was obtained through performing ten measurements on different positions. An electrokinetic analyzer (EKA) of Anton Paar GmbH (Austria) was employed to measure the streaming potential of membrane surface with 0.001 mol/l KCl
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electrolyte of 25.0 ℃ and pH 7.0. Helmholtz-Smoluchowski equation was adopted to determine the zeta potential of membrane surface by using the tested streaming potential [32]. For each type of membrane, at least five samples were tested and averaged.
2.5. Evaluation of reverse osmosis performance Membrane reverse osmosis performance was evaluated through cross-flow filtrations conducted on the permeation setup used previously [33]. Three identical test cells with an effective filtration area of 23.60 cm2 were parallelly installed in the permeation setup. Aqueous NaCl, NaNO3 and glycerol solutions were used as the feed solution separately to evaluate membrane reverse osmosis performance in terms of water permeation flux and solute rejection. In this study, if specified, the operating pressure was fixed as 5.0 bar and the solute concentration, temperature and pH of the feed solution were fixed as 500 mg/l, 25.0 °C and pH 7.0, respectively. Before the test, the membrane samples loaded in the test cells were pre-compacted to stabilize the water permeation flux by performing cross-flow filtration with de-ionized water under 10.0 bar for 3.0 h. The feed aqueous solution containing certain amount of solute was then pumped into the test cell and flowed across the side of the polyamide layer of each membrane sample. Both the concentrated and permeated streams from each test cell were circulated back to the feed container. The ratio of the flow rates of the permeated and concentrated streams was tuned at a very low value less than 1.0% to minimize the effects of concentration polarization on the transports of water and solute. The flow rate (F, l/h) of the permeated stream from each test cell with an effective filtration area of A (m2) was measured to determine the volumetric water permeation flux of
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the tested membrane (J, l/m2 h) according to the equation of J=F/A. The solute contents in the feed (Cf, mg/l) and permeated (Cp, mg/l) streams were measured to determine the observed solute rejection (Rs, %) according to the equation of Rs(%)=100×(Cf-Cp)/Cf. DDSJ-308A conductance meter (Cany Precision Instruments Co., Ltd. China) was employed to measure the electrical conductivity to obtain the solute content of the electrolyte through using the calibration curve drawn between conductivity and electrolyte content. GE Innov0x TOC analyzer was employed to measure the total organic carbon content to obtain the solute content of aqueous glycerol solution.
3. Results and discussion 3.1. Surface amidation of IP-based PA TFC membrane under different pHs Surface amidation was firstly performed with the loose PA TFC membrane using EDC aqueous solution under different pHs. The type of IP-based membrane M2 for surface amidation possesses an initial permeation property of water permeation flux of 42.0 l/m2 h and salt rejection of 94.2 % when filtrated with aqueous NaCl solution of 500 mg/l under 5.0 bar and 25.0 ℃. Fig. 3 depicts the salt rejections and water permeation fluxes of the membrane amidated under various pHs. It is evident from the graph that all the IP/SA-based PA TFC membranes obtained under the pHs within the scope of 3.0 to 6.5 exhibit an improved salt rejection and a declined water permeation flux compared with the base IP-based PA TFC membrane.
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Fig.3. Salt rejections and water permeation fluxes of the IP/SA-based PA TFC membranes amidated under different pHs.
The EDC-activated reaction between adjacent carboxyl and amino groups will generate new amide linkages and thus tighten the top skin of the loose polyamide active layer. As a result, the PA layer after surface amidation becomes less permeable to both solute and water, and the TFC membrane exhibits improved rejection ability to solute and declined permeation ability to water. Similar phenomena were also observed with the PA TFC membrane when its surface was treated with the cross-linking agent of aldehydes [34, 35]. However, the newly generated bonds through the process of aldehyde-involved cross-linking are less stable than the amide linkage. It can also be found from the figure that the pH of the EDC solution has a great influence on water permeation flux and salt rejection of the amidated membrane. The TFC membrane amidated at the pH between 3.5 and 4.0 exhibits the highest salt rejection. As illustrated in Fig. 2, the formation of the intermediate ester of
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amine-reactive O-acylisourea through the reaction between EDC and carboxyl groups on membrane surface is a nucleophilic addition reaction. Thus the activation of carboxyl groups by EDC takes place better at acidic conditions. However, if the pH is too low, the dissociation of carboxyl groups will decrease and thus the reaction of EDC with carboxyl group will be suppressed [36]. Therefore, there is an optimum pH for the activation of carboxyl group via using EDC to form amide bonds through the reaction with amino groups. The optimum pH for the surface amidation of IP-based PA TFC membrane via the activation of carboxyl group with EDC is around 4.0.
3.2. Surface amidation of IP-based PA TFC membranes with different original salt rejections Surface amidation was then performed with the IP-based PA TFC membranes with different original salt rejections using EDC aqueous solutions of pH 4.0. The IP-based PA TFC membranes of different initial salt rejections for surface amidation were obtained through tuning the TMC content in organic phase and the MPD content and pH of the aqueous phase [37]. The salt rejections and water permeation fluxes of the IP-based PA TFC membranes before and after surface amidation are tabulated in Table 2.
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Table 2 Salt rejections and water permeation fluxes of the IP-based PA TFC membranes before and after surface amidation. IP-based PA TFC
Water permeation flux* (l/m2 h)
Salt rejection* (%)
membranes
Before surface
After surface
Before surface
After surface
amidation
amidation
amidation
amidation
M1
45.8
37.8
93.60
95.92
M2
41.6
36.5
94.32
96.23
M3
38.5
33.8
95.02
96.82
M4
34.0
30.3
95.80
97.34
M5
29.5
27.8
96.52
97.73
M6
22.5
22.0
97.72
97.76
*
Filtrated with aqueous NaCl solution of 500 mg/l, 25.0 ℃ and pH 7.0 under 5.0 bar.
Of the six tested membranes, the IP-based membrane M6 has a relatively dense PA layer and exhibits the highest initial salt rejection, while the other five membranes have a looser PA layer and better water permeability. It can be found from the data that surface amidation can enhance the rejection ability of the IP-based PA TFC membrane with a relatively loose PA layer at the sacrifice of membrane permeability to water. IP-based membrane M1 with the highest water permeation flux and lowest salt rejection achieves the biggest improvement in its rejection ability after amidation, showing an increase of salt rejection from 93.60 to 95.92%. A higher salt rejection can be achieved after amidation for the TFC membranes with a relative higher initial salt rejection such as IP-based membranes M2,
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M3, M4 and M5. However, the surface amidation of present study has nearly no effect on improving the salt rejection and water permeability of the IP-based membrane M6, which has a relatively dense PA layer and exhibits a high initial salt rejection up to 97.72%. As depicted in Fig. 2, surface amidation is correlated with the surface contents of carboxyl and terminated amino groups, which are different for each IP-based PA TFC membrane. Therefore, the extent of the improvement of separation performance through surface amidation will be different for the IP-based polyamide-based TFC membranes of different initial permeation properties.
3.3. Reproducibility of surface amidation of IP-based PA TFC membrane Surface amidation was also performed to investigate its reproducibility through laboratory-scale trials by employing the type of IP-based PA TFC membrane M4 with an initial salt rejection around 95.8 % and a water permeation flux about 34.5 l/m2 h when filtrated with 500 mg/l aqueous NaCl solution of 25.0 ℃ under 5.0 bar. The results of the laboratory-scale trials of surface amidation of the IP-based PA TFC membrane using EDC aqueous solution of pH 4.0 are tabulated in Table 3.
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Table 3: Laboratory-scale trial results of surface amidation of IP-based PA TFC membrane. Water permeation flux* (l/m2 h)
Salt rejection* (%)
Before surface
After surface
Before surface
After surface
amidation
amidation
amidation
amidation
1
35.3
30.2
95.66
97.24
2
33.8
29.8
96.08
97.38
3
35.2
30.6
95.46
97.15
4
34.2
30.0
95.92
97.30
5
34.0
30.4
95.83
97.22
Average
34.5±0.7
30.2±0.3
95.79±0.24
97.26±0.09
Trial numbers
*
Filtrated with aqueous NaCl solution of 500 mg/l, 25.0 ℃ and pH 7.0 under 5.0 bar. It can be found from the data that the amidated membranes exhibit an average water
permeation flux of 30.2 l/m2 h and a salt rejection of 97.26 % with the standard deviations of only 0.3 and 0.09, respectively, which are much lower than those of 0.7 and 0.24 for the membranes before amidation. The low standard deviations of the results of laboratory-scale trials indicate that the process of surface amidation possesses good reproducibility.
3.4. Confirmation of the formation of amide bonds The formation of amide bonds through treating the IP-based PA TFC membrane with EDC aqueous solution was firstly ascertained through analyzing the chemical structure of membrane surface via ATR-FTIR. The FTIR spectra of the surface of the IP-based PA TFC
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membrane M1 before and after surface amidation are depicted in Fig. 4. The polyamide layer of the IP-based PA TFC membrane shows its characteristic absorptions of C=O stretching of amide I at about 1660 cm-1, hydrogen bonded C=O of amide I at about 1610 cm-1 and N–H in-plane bending of amide II near 1540 cm-1 [38]. No new absorption peak appears after surface amidation, since the step of surface amidation only generates amide bonds with the same characteristic absorption of the polyamide layer formed via IP process. However, as tabulated in Table 4, the strength ratios of the characteristic absorption peaks of the amide linkage of polyamide layer at 1660, 1610 and 1540 cm-1 to the characteristic absorption peak of benzene ring at 1490 cm-1 [39] I1660 /I1490, I1610/I1490 and I1540 /I1490 change appreciably after surface amidation. The augments of these peak strength ratios indicate the increase of the amount of amide bonds on the top of the IP-based PA TFC membrane after amidation. This confirms the occurrence of amidation reaction between adjacent carboxyl and amino groups when the surface of the IP-based PA TFC membrane was contacted with acidic aqueous EDC solution.
Fig.4. ATR-FTIR spectra of the IP-based PA TFC membrane before (a) and after (b) surface amidation. 18/33
Table 4 Strength ratios of the characteristic absorption peaks of the amide linkage to benzene ring. Membranes
I1660 /I1490
I1610/I1490
I1540 /I1490
IP-based membrane M1
0.6852
0.1541
0.5532
IP/SA-based membrane M1
0.7572
0.1736
0.6053
The formation of amide bonds was further ascertained through analyzing the chemical composition of membrane surface via XPS. The element contents of the IP-based PA TFC membrane before and after surface amidation are shown in Fig. 5. Both the O content and the ratio of O to N can be used as the indications of surface amidation, since the reaction between carboxyl and amino groups will reduce the O content on membrane surface and thus increase the ratio of O to N. It can be found from the graphs that, after surface amidation, the O content decreases appreciably from 17.63 to 16.18 mol% and the ratio of O to N decreases from 1.84 to 1.63. The declines of O content and O/N ratio further confirm the occurrence of amidation between adjacent carboxyl and amino groups when the surface of the IP-based PA TFC membrane was contacted with acidic aqueous EDC.
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Fig.5. XPS spectra of the IP-based PA TFC membrane before (a) and after (b) surface
amidation.
3.5. Surface roughness, charge and hydrophilcity of the amidated membrane Membrane surface roughness was characterized via AFM. Fig. 6 presents the surface AFM images of the IP-based TFC membranes M1, M2, M3, M4, M5 and M6 before and after surface amidation. The three-dimensional surface AFM graphics reveal the typical ridge-and-valley structure of the surface morphologies of the membranes before and after amidation. No evident change was observed with the morphology of membrane surface after surface amidation. The RMS values tabulated in Table 5 reflect a very slight decrease of surface roughness after amidation, which is possibly due to the densification effect of amidation on membrane surface.
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(a) Membrane M1
(b) Membrane M2
(c) Membrane M3
(d) Membrane M4
(e) Membrane M5
(f) Membrane M6 Fig.6. AFM images of the surfaces of the TFC membranes M1 (a), M2 (b), M3 (c), M4 (d), M5 (e) and M6 (f) before (left) and after (right) surface amidation.
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Table 5 Surface root mean square roughness (RMS), zeta potential and water contact angle values of the IP-based PA TFC membranes before and after surface amidation. IP-based
RMS (nm)
PA TFC
Before
Membrane
surface
Zeta potential a (mV)
Water contact angle b (°)
After
Before
After
Before
After
surface
surface
surface
surface
surface
amidation amidation
amidation amidation
amidation amidation
M1
69.4
67.1
-50.2±1.6
-42.6±1.3
34.8±0.9
36.5±1.1
M2
60.1
56.5
-47.5±1.5
-40.5±1.4
35.9±0.8
38.7±1.2
M3
67.2
63.7
-46.2±1.4
-40.3±1.3
36.5±0.9
38.9±1.0
M4
65.6
57.8
-48.6±1.3
-42.5±1.4
35.5±0.9
38.6±1.0
M5
67.1
63.3
-47.8±1.4
-40.6±1.2
38.5±0.8
40.8±0.9
M6
65.2
63.8
-45.8±1.3
-43.9±1.2
39.6±0.9
41.2±1.0
a
Evaluated using aqueous KCl solution of 0.001 mol/l, 25.0 ℃ and pH 7.0.
b
Evaluated using de-ionized of 25.0 ℃.
The effects of surface amidation on membrane surface hydrophilicty and charge were also evaluated through performing the measurements of surface water contact angle and streaming potential, respectively. The data tabulated in Table 5 clearly reveal that the surface of the polyamide-based TFC membrane becomes less negatively charged at neutral pH after amidation. The dissociation of carboxyl group on membrane surface is responsible for the surface negative charge. The amidation between the carboxyl and amino groups on membrane surface will reduce the amount of free carboxyl groups on membrane surface for dissociation,
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thus leading to an applicable decrease of membrane surface negative charge at neutral pH. The conversion of part of the surface carboxyl and amino groups into amide linkages is the reason for the slight change surface hydrophilicity.
3.6. Permeation property of the IP/SA-based PA TFC membrane The permeation property of IP-based PA TFC membrane before and after surface amidation was firstly investigated through measuring membrane pure water permeability (PWP) and rejections to different solutes including the neutral solute of glycerol and the electrolytes of NaNO3 and NaCl. Considering the salt rejection performance of commercial polyamide-based TFC reverse osmosis membrane, the IP/SA-based membrane M5 with relatively high salt rejection was employed to investigate the influence of the step of surface amidation on the permeation property of the IP-based PA TFC membrane through comparing with the IP-based membrane M5. For comparison, the permeation properties of IP-based membrane M6 and the commercial membrane BW30 of Dow were also evaluated. The data presented in Table 6 clearly illustrate that the step of surface amidation can effectively improve the ability of membrane rejection to both electrolyte and neutral solute. Comparing the permeation data of IP/SA-based membrane M5 and IP-based membrane M6 with nearly the same rejection ability in filtration of aqueous NaCl solution of 500 mg/l at the pressure of 5.0 bar, the PA TFC membrane prepared through IP followed with surface amidation exhibits higher rejections to NaNO3 and glycerol and better permeability to pure water compared with the PA TFC membrane fabricated via conventional IP process, indicating the superiority of the membrane obtained via the two-step technique in water
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permeability and rejection ability. Furthermore, the IP/SA-based PA TFC membrane M5 even possesses a comparable rejection ability and better water permeability compared with the commercial membrane BW30.
Table 6 Pure water permeability and solute rejection values of the tested membranes. Membranes
Pure water
NaCl
NaNO3
Glycerol
permeability a
rejection b
rejection c
rejection d
(l/m2 h bar )
(%)
IP-based membrane M5
6.02±0.13
96.4±0.3
90.5±0.2
78.4±0.6
IP/SA-based membrane M5
5.60±0.07
97.8±0.2
93.7±0.3
91.3±0.5
IP-based membrane M6
4.58±0.12
97.7±0.3
92.6±0.2
89.5±0.5
Membrane BW30 (Dow)
4.16±0.10
97.7±0.2
93.5±0.3
90.9±0.6
(%)
(%)
a
Obtained through permeation test using de-ionized water under 25.0 ℃.
b
Filtrated with aqueous NaCl solution of 500 mg/l, pH 7.0 and 25.0 ℃ under 5.0 bar.
c
Filtrated with aqueous NaNO3 solution of 2000 mg/l, pH 7.0 and 25.0 ℃ under 5.0 bar.
d
Filtrated with aqueous glycerol solution of 500 mg/l, 25.0 ℃ under 4.0 bar.
As illustrated in Fig. 7, IP/SA-based membrane M5 and IP-based membrane M6 possess a polyamide active layer with nearly the same thickness. Therefore, the superiority of water permeability of IP/SA-based membrane M5 compared with the IP-based membrane M6 is mainly because of the structural difference of the polyamide active layer between the two types of TFC membranes. As shown schematically in Fig. 1, the thin-film composite
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membrane prepared via the two-step process of IP followed by surface amidation contains a PA layer composed of a dense top skin and a loose bulk, while the TFC membrane prepared from conventional IP process possesses a PA layer of the same density from top to bottom. Therefore, of the IP-based and IP/SA-based TFC membranes with a PA layer of nearly the same thickness and rejection ability to 500 mg/l NaCl aqueous solution, the water permeation resistance across the TFC membrane with the polyamide layer prepared via conventional IP process will be higher than that of the TFC membrane with the polyamide layer prepared via IP followed by surface amidation.
Fig.7: SEM images of the cross sections of IP/SA-based membrane M5 (right) and IP-based membrane M6 (left). Additionally, as presented in table 5, the surface zeta potentials of IP -based membrane M5, IP/SA-based membrane M5 and IP-based membrane M6 are -47.8±1.4, -40.6±1.2 and -45.8±1.3 mV, respectively. The electrostatic repulsion between the negatively charged solute and the surface of IP/SA-based membrane M5 is the lowest of the three membranes. Thus the improved rejections to electrolytes NaNO3 and NaCl of the IP/SA-based membrane M5 are mainly resulted from the enhancement of size exclusion effect. The improved size exclusion reveals that the step of surface amidation can make the skin of the PA layer denser. This is the reason why the IP/SA-based membrane M5 exhibited improved rejection to neutral solute after surface amidation. Furthermore, the higher glycerol rejection reveals that the pore size of 25/33
the polyamide skin layer of the IP/SA-based membrane M5 is even smaller than that of the IP-based membrane M6. Therefore, the technique of IP followed with surface amidation can produce PA layer with high perm-selectivity and thus TFC membrane with improved reverse osmosis performance. The performance advantage of TFC membrane prepared via the two-step process of IP followed with surface amidation was further illustrated through investigating the change of salt rejection with feed concentration under the fixed operation pressure of 5.0 bar. As depicted in Fig. 8, of the three tested membranes, IP/SA-based membrane M5 exhibits the lowest decrease of NaCl rejection when the feed NaCl concentration ascends from 500 to 2000 mg/l, indicating that the superiority of the two-step membrane in rejection ability is more remarkable under high feed salt concentration. This is also mainly due to the improved size exclusion effect of the two-step membrane.
Fig.8. Changes of salt rejection with feed salt concentration for IP-based membrane M5, IP/SA-based membrane M5 and IP-based membrane M6 filtrated with aqueous NaCl solution of pH 7.0 and 25 ℃ at 5.0 bar. 26/33
The performance stability of the PA TFC membrane prepared via the two-step process of IP followed with surface amidation was also evaluated through a 30-day long-term permeation test by employing the IP/SA-based membrane M5. The data illustrated in Fig. 9 clearly reflect the slight changes of water flux and salt rejection during the whole filtration period, indicating the good performance stability of the amidated membrane.
Fig.9. Changes of water permeation flux (●) and salt rejection (▲) with filtration time for the IP/SA-based membrane M5 filtrated with aqueous NaCl solution of pH 7.0 and 25 ℃ at 5.0 bar. 4. Conclusions The fabrication of polyamide TFC membranes for improved reverse osmosis performance via the novel technique of interfacial polymerization followed by surface amidation has been demonstrated in this work. The amidation between adjacent carboxyl and amino groups was facilely implemented through covering the surface of the polyamide-based TFC membrane with an acidic aqueous EDC solution. The increase of amide bonds on membrane surface after surface amidation has been properly confirmed through the analyses
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of ATR-FTIR and XPS. Surface amidation was found to make the surface of the TFC membrane less negatively charged under neutral pH through reducing the amount of surface carboxyl groups. Surface amidation was demonstrated effective in improving the ability of membrane rejection to both electrolyte and neutral solute at a sacrifice of water permeability. The TFC membrane prepared through IP followed with surface amidation possessed higher rejections to solutes NaNO3 and glycerol and better permeation ability to pure water compared to the TFC membrane of the same rejection to NaCl prepared via conventional IP process. Polyamide-based TFC membrane with improved perm-selectivity could be fabricated through the two-step technique of IP followed with surface amidation. Furthermore, the step of surface amidation of present work can be potentially applied to in-situ performance rejuvenation of used polyamide reverse osmosis membranes for prolonged lifespan.
Acknowledgments The authors gratefully thank the National Natural Science Foundation of China (NNSFC) for financial support (Grant no 21476213 and 21676256).
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Research Highlights: ►TFC membrane was fabricated via interfacial polymerization and surface amidation. ►Surface amidation was facilely implemented via using acidic EDC aqueous solution. ►PA selective layer composed of a top dense skin on loose bulk was obtained. ►PA TFC membrane with improved reverse osmosis performance could be obtained.
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PII: DOI: Reference:
S0376-7388(18)31994-X https://doi.org/10.1016/j.memsci.2018.09.012 MEMSCI16457
To appear in: Journal of Membrane Science Received date: 20 July 2018 Revised date: 30 August 2018 Accepted date: 2 September 2018 Cite this article as: Chuang Yu, Haiyan Li, Xiru Zhang, Zhenhua Lü, Sanchuan Yu, Meihong Liu and Congjie Gao, Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.09.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyamide thin-film composite membrane fabricated through interfacial polymerization coupled with surface amidation for improved reverse osmosis performance
Chuang Yua, Haiyan Lia, Xiru Zhanga, Zhenhua Lüa, Sanchuan Yua, *, Meihong Liub, Congjie Gaoc
a
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
b
School of Civil Engineering and Architecture, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China
c
The Development Center of Water Treatment Technology，SOA，Hangzhou 310012, People’s Republic of China
* Corresponding author: Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China. Tel.: +86-571-86843217; Fax: +86-571-86843217; E-mail: [email protected] (S. Yu).
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Abstract： In this study, the novel technique, namely interfacial polymerization (IP) coupled with surface amidation, was proposed to fabricate polyamide thin-film composite (TFC) membranes for improved reverse osmosis performance. TFC membrane with loose polyamide separation layer was firstly prepared through conventional IP process. Surface amidation was then performed through covering the surface of the loose polyamide layer with aqueous 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) solution. Amidation reaction between the free carboxylic and amino groups on membrane surface was confirmed through ATR-FTIR and XPS analyses. Permeation tests revealed that the step of surface amidation could effectively improve the ability of membrane rejection to solute, showing significant increases of NaCl, NaNO3 and glycerol rejections from about 96.4, 90.5 and 78.4% to 97.8, 93.7 and 91.3%, respectively. The water permeability of the membrane obtained via IP followed with surface amidation was at least 22.3% higher than the membrane of the same NaCl rejection obtained via conventional IP process. Thus, the method developed was attractive for the fabrication of TFC reverse osmosis membranes with improved permeability to water and rejection ability to solutes. Additionally, the step of surface amidation of present work can be potentially applied to the in-situ rejuvenation of used polyamide reverse osmosis membranes.
Keywords: Interfacial polymerization; Surface amidation; Polyamide thin-film composite membrane; Reverse osmosis; Membrane perm-selectivity
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1. Introduction Global water crisis is the most crucial problem for human beings. Reverse osmosis (RO) membrane process, which can provide high quality water through removing all the impurities in aqueous such as dissolved salts and organic compounds, has become the key technology to alleviate the water crisis [1-3]. However, the energy consumption of reverse osmosis technology, which is largely determined by the core component of reverse osmosis membrane, is still relatively high and the extensive application of reverse osmosis process is limited to a certain extent [4]. Therefore, it is of great practical significance to improve the separation efficiency of RO process through enhancing the water permeability, rejection ability as well as antifouling property of the semi-permeable membrane used [5, 6]. The major membrane currently used in RO process is the type of thin-film composite (TFC) membrane containing an active separation layer of fully aromatic polyamide (PA) for its better permeability to water, rejection ability to solutes and stability to chemicals in treating aqueous fluids compared with the type of asymmetric membrane made from cellulose acetate [7]. Thus, improvement of separation performance of the polyamide-based TFC RO membrane has been drawing more and more attentions of researchers in the past decades. The type of polyamide-based TFC RO membrane is typically fabricated through depositing a thin separation layer on the top of the substrate of porous polysulfone via the interfacial reaction between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) [8]. The permeation property of the TFC membrane is largely determined by the chemistry, network structure and thickness of the polyamide layer. Accordingly, efforts were firstly devoted to enhance membrane permeability to water through making the polyamide composite active layer
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thinner, since membrane permeability to water is inversely proportional to the thickness of the active separation layer [9]. However, the interfacial reaction between MPD and TMC, as a diffusion-controlled and self-limited process, has already produced a typical PA layer as thin as about 100 nm. Any further reduction of the thickness of the PA layer is restricted by the empirical property of the interfacial polymerization process, the generation of defect in mass production, as well as the reduction of the durability of the TFC membranes obtained [10]. Efforts have also been devoted to improve the permeation property of the polyamide-based TFC RO membrane through tuning the chemistry and network structure of the separation layer via using additive in MPD-aqueous or TMC-organic phase. Both organic additives such as acetone [11], amino-cyclodextrins (CDs) [12], alkyl phosphates [13] and surfactants [14], and inorganic additives such as silica [15], grapheme oxide (GO) [16, 17] and porous nanozeolites of NaA [18, 19] and NaX [20] have been proved effective in enhancing the water permeability of the formed TFC membrane. However, the use of additive usually generates a polyamide active layer with a relatively loose network structure and thus lowers the ability of membrane rejection to solutes. The above analysis reveals that, the conventional interfacial polymerization (IP) process, as the most popular and scalable technique for the mass production of polyamide-based TFC membrane, generally produces a separation layer with trade-off between its rejection ability to dissolved solutes and permeability to water [21, 22]. It is still of great challenge and difficulty to improve the permeation property of the polyamide-based TFC membrane through separately optimizing the chemistry, network structure, and thickness of the active layer formed via conventional IP process [23]. Additionally, as the possible
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routes to enhance membrane performance and antifouling properties, surface chemical modification mainly via grafting is relatively sophisticated and will alter membrane surface chemistry, and surface physical modification mainly via coating is of poor long-term stability [24-26]. Therefore, in this study, a novel two-step fabrication technique, namely interfacial polymerization (IP) followed by surface amidation (SA), was proposed to prepare the polyamide-based TFC RO membrane for enhanced salt rejection. As schematically illustrated in Fig. 1, conventional IP process was firstly adopted to generate a loose PA layer with relatively good permeability to water and poor rejection ability to solutes on the top of a porous support of polysulfone (PSF). Surface amidation was then performed to make the skin of the loose polyamide layer more crosslinked. In the experiment, PA layer with loose network structure was obtained through varying the MPD and TMC contents, surface amidation was facilely implemented through covering the surface of PA layer with an acidic aqueous solution containing 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) as activator. Different from the conventional IP process that generally produces a polyamide layer with a uniform cross-section structure of nearly the same density from top to bottom, the two-step fabrication process of interfacial polymerization followed with surface amidation will produce a polyamide layer with an asymmetric cross-section structure composed of an ultrathin dense skin on the top of a loose body. The PA layer with the asymmetric structure will endow the formed TFC membrane with high water permeability under the premise of high salt rejection. Additionally, the two-step fabrication process can be scaled up for mass production and the step of surface amidation can also be potentially applied to in-situ performance rejuvenation of the used polyamide-based TFC reverse osmosis
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membranes for prolonged membrane lifespan.
Fig. 1. Schematic for the preparation of TFC membrane via the two-step process of interfacial polymerization followed by surface amidation.
2. Experimental 2.1. Materials and reagents The substrate of flat-sheet polysulfone (PSF) ultrafiltration membrane for interfacial polymerization was friendly provided by Hangzhou Tianchuang Environmental Technology Co., LTD. (Hangzhou, China). The pure water permeability and molecular weight cut-off of the substrate were about 280 l/m2 h bar and 120,000 g/mol, respectively. Monomers such as m-phenylenediamine (MPD) for aqueous phase and trimesoyl chloride (TMC) for organic phase were of analytical grade and bought from Shanghai Amino-Chem. Co. Ltd. (Shanghai, China) and Sigma-Aldrich, respectively. Additives such as triethylamine (TEA, purity 99.5%) 6/33
and camphor sulfonic acid (CSA, purity 99.0%) for the aqueous phase were bought from Sigma-Aldrich. Isopar G, a type of isoparaffin fluid supplied by ExxonMobil Chemical, was used as the solvent for organic phase. 1-ethyl-(3-3-dimethylaminopropyl) carbodiimide hydrochloride (EDC.HCl purity 98.5%) purchased from Aladdin (Shanghai, China) was used as the activator for amidation reaction. All other analytical grade solvents and reagents were used as received.
2.2. Fabrication of TFC membrane through conventional IP process TFC membranes were prepared through depositing a polyamide layer on the top of the support of porous polysulfone. Conventional IP process was adopted to produce the active separation layer following the procedure described in our previous works [27, 28]. The porous substrate of polysulphone ultrafiltraion membrane was firstly clamped with a frame of polytetrafluoroethylene (PTFE). The surface of the substrate was then soaked with a MPD-aqueous solution for 2.0 min. Any bubbles formed during the soaking process were eliminated through rolling the surface with a soft rubber roller. After draining off the excess MPD-aqueous solution from the surface and air-drying the MPD-soaked support under room temperature until there were no liquids on the surface, the interfacial polymerization was then implemented to laminate a PA layer on the top of the substrate through covering the surface of the MPD-saturated substrate with a TMC-organic solution for 30s. After removing the excess organic solution, a heat treatment of 5 min at 90℃ was performed with the membrane to make the structure of the formed polyamide layer more crosslinked. Finally, the composite membrane was sequentially soaked with aqueous Na2CO3 solution of 40.0 ℃ and pH 9.0 for
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5 min, rinsed thoroughly using hot de-ionized water of 40.0℃ for 10 min and kept wet for subsequent use. In this work, TEA was added into the MPD-aqueous solution as an acid acceptor at the content of 3.0 wt% and CSA was added to adjust the pH of the aqueous phase solution to about 9.6. PA layers with different solute rejection abilities were formed through mainly varying the contents of monomers MPD and TMC. The TFC membranes obtained through conventional IP process were denoted as IP-based membrane. The main parameters for the preparation of different IP-based PA TFC membranes are tabulated in Table 1. Table 1 Parameters for the preparation of IP-based PA TFC membranes. TFC membranes
MPD content (wt%)
TMC content (wt/v%)
pH of aqueous solution
M1
1.5
0.06
9.4
M2
1.7
0.06
9.4
M3
1.8
0.06
9.4
M4
2.0
0.07
9.5
M5
2.1
0.10
9.5
M6
2.5
0.12
9.7
2.3. Surface amidation of IP-based PA TFC membrane Surface amidation of the PA TFC membrane obtained through conventional IP process was facilely implemented under the activation by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). Fig. 2 schematically illustrates the reaction mechanism of surface amidation. As illustrated in the schematic, activating agent EDC reacts firstly with the free
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carboxyl groups of the PA layer of the TFC membrane to generate the highly reactive ester of O-acylisourea. The formed intermediate is unstable and can react with the adjacent amino groups to generate amide linkages [29]. In the experiment, surface amidation was carried out through covering the surface of the TFC membrane clamped in a PTFE frame with an aqueous phosphate-buffered solution (PBS) containing 50 mM EDC. The pH of the buffer solution was regulated between 3.0 and 6.5 through using aqueous HCl or NaOH solution. The membrane covered with EDC solution was then put into a sealed opaque container of room temperature for 12.0 h. Finally, the membrane was thoroughly rinsed using de-ionized water and kept wet for subsequent use. The IP-based PA TFC membrane after surface amidation was referred to as IP/SA-based membrane. COO-
COO-
NH2
COO-
NH2
N Cl
H+
N C N
N Cl
NH C N O
N Cl
+
H
O C
O
O
NH2
C
NH2
O COO-
NH2
H+
NH C N
COO-
H C
N
COO-
Fig.2. Schematic for the EDC-activated surface amidation of the polyamide-based TFC membrane. 9/33
2.4. Characterization of membrane physico-chemical property Membranes were characterized for their physico-chemical properties after being thoroughly rinsed with de-ionized water and vacuum dried. The chemical structure and composition of membrane surface were characterized through attenuated total reflectance infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. FTIR spectrometer of Nicolet Aratar 370 with a ZnSe crystal at an incidence angle of 45° was employed to perform the ATR-FTIR analysis. Perkin Elmer PHI 5000C ESCA System with the ultraviolet source of Mg/Al Dual Anode Hel/Hell was employed to carry out the XPS analysis. Membrane morphology observed through scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM images of the surface and cross-section of each membrane sample sputtered with gold were obtained with a Hitachi S-4800field emission scanning electron microscopy (Japan). The three-dimensional surface AFM images of 10.0μm×10.0μm were obtained with an atomic force microscopy of park instrument auto probe CT under the tapping mode in air. The root mean square roughness (RMS) of membrane surface was determined through using the height profile of the obtained AFM image [30]. Membrane surface hydrophilicity and charge were evaluated through measuring surface water contact angle (WCA) and zeta potential (ZP), respectively. DSA10-MK2 contact angle analyzer (KRUSS BmbH, Germany) was used to measure the surface water contact angle by adopting the method of sessile drop at room temperature [31]. The averaged WCA of each membrane sample was obtained through performing ten measurements on different positions. An electrokinetic analyzer (EKA) of Anton Paar GmbH (Austria) was employed to measure the streaming potential of membrane surface with 0.001 mol/l KCl
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electrolyte of 25.0 ℃ and pH 7.0. Helmholtz-Smoluchowski equation was adopted to determine the zeta potential of membrane surface by using the tested streaming potential [32]. For each type of membrane, at least five samples were tested and averaged.
2.5. Evaluation of reverse osmosis performance Membrane reverse osmosis performance was evaluated through cross-flow filtrations conducted on the permeation setup used previously [33]. Three identical test cells with an effective filtration area of 23.60 cm2 were parallelly installed in the permeation setup. Aqueous NaCl, NaNO3 and glycerol solutions were used as the feed solution separately to evaluate membrane reverse osmosis performance in terms of water permeation flux and solute rejection. In this study, if specified, the operating pressure was fixed as 5.0 bar and the solute concentration, temperature and pH of the feed solution were fixed as 500 mg/l, 25.0 °C and pH 7.0, respectively. Before the test, the membrane samples loaded in the test cells were pre-compacted to stabilize the water permeation flux by performing cross-flow filtration with de-ionized water under 10.0 bar for 3.0 h. The feed aqueous solution containing certain amount of solute was then pumped into the test cell and flowed across the side of the polyamide layer of each membrane sample. Both the concentrated and permeated streams from each test cell were circulated back to the feed container. The ratio of the flow rates of the permeated and concentrated streams was tuned at a very low value less than 1.0% to minimize the effects of concentration polarization on the transports of water and solute. The flow rate (F, l/h) of the permeated stream from each test cell with an effective filtration area of A (m2) was measured to determine the volumetric water permeation flux of
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the tested membrane (J, l/m2 h) according to the equation of J=F/A. The solute contents in the feed (Cf, mg/l) and permeated (Cp, mg/l) streams were measured to determine the observed solute rejection (Rs, %) according to the equation of Rs(%)=100×(Cf-Cp)/Cf. DDSJ-308A conductance meter (Cany Precision Instruments Co., Ltd. China) was employed to measure the electrical conductivity to obtain the solute content of the electrolyte through using the calibration curve drawn between conductivity and electrolyte content. GE Innov0x TOC analyzer was employed to measure the total organic carbon content to obtain the solute content of aqueous glycerol solution.
3. Results and discussion 3.1. Surface amidation of IP-based PA TFC membrane under different pHs Surface amidation was firstly performed with the loose PA TFC membrane using EDC aqueous solution under different pHs. The type of IP-based membrane M2 for surface amidation possesses an initial permeation property of water permeation flux of 42.0 l/m2 h and salt rejection of 94.2 % when filtrated with aqueous NaCl solution of 500 mg/l under 5.0 bar and 25.0 ℃. Fig. 3 depicts the salt rejections and water permeation fluxes of the membrane amidated under various pHs. It is evident from the graph that all the IP/SA-based PA TFC membranes obtained under the pHs within the scope of 3.0 to 6.5 exhibit an improved salt rejection and a declined water permeation flux compared with the base IP-based PA TFC membrane.
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Fig.3. Salt rejections and water permeation fluxes of the IP/SA-based PA TFC membranes amidated under different pHs.
The EDC-activated reaction between adjacent carboxyl and amino groups will generate new amide linkages and thus tighten the top skin of the loose polyamide active layer. As a result, the PA layer after surface amidation becomes less permeable to both solute and water, and the TFC membrane exhibits improved rejection ability to solute and declined permeation ability to water. Similar phenomena were also observed with the PA TFC membrane when its surface was treated with the cross-linking agent of aldehydes [34, 35]. However, the newly generated bonds through the process of aldehyde-involved cross-linking are less stable than the amide linkage. It can also be found from the figure that the pH of the EDC solution has a great influence on water permeation flux and salt rejection of the amidated membrane. The TFC membrane amidated at the pH between 3.5 and 4.0 exhibits the highest salt rejection. As illustrated in Fig. 2, the formation of the intermediate ester of
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amine-reactive O-acylisourea through the reaction between EDC and carboxyl groups on membrane surface is a nucleophilic addition reaction. Thus the activation of carboxyl groups by EDC takes place better at acidic conditions. However, if the pH is too low, the dissociation of carboxyl groups will decrease and thus the reaction of EDC with carboxyl group will be suppressed [36]. Therefore, there is an optimum pH for the activation of carboxyl group via using EDC to form amide bonds through the reaction with amino groups. The optimum pH for the surface amidation of IP-based PA TFC membrane via the activation of carboxyl group with EDC is around 4.0.
3.2. Surface amidation of IP-based PA TFC membranes with different original salt rejections Surface amidation was then performed with the IP-based PA TFC membranes with different original salt rejections using EDC aqueous solutions of pH 4.0. The IP-based PA TFC membranes of different initial salt rejections for surface amidation were obtained through tuning the TMC content in organic phase and the MPD content and pH of the aqueous phase [37]. The salt rejections and water permeation fluxes of the IP-based PA TFC membranes before and after surface amidation are tabulated in Table 2.
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Table 2 Salt rejections and water permeation fluxes of the IP-based PA TFC membranes before and after surface amidation. IP-based PA TFC
Water permeation flux* (l/m2 h)
Salt rejection* (%)
membranes
Before surface
After surface
Before surface
After surface
amidation
amidation
amidation
amidation
M1
45.8
37.8
93.60
95.92
M2
41.6
36.5
94.32
96.23
M3
38.5
33.8
95.02
96.82
M4
34.0
30.3
95.80
97.34
M5
29.5
27.8
96.52
97.73
M6
22.5
22.0
97.72
97.76
*
Filtrated with aqueous NaCl solution of 500 mg/l, 25.0 ℃ and pH 7.0 under 5.0 bar.
Of the six tested membranes, the IP-based membrane M6 has a relatively dense PA layer and exhibits the highest initial salt rejection, while the other five membranes have a looser PA layer and better water permeability. It can be found from the data that surface amidation can enhance the rejection ability of the IP-based PA TFC membrane with a relatively loose PA layer at the sacrifice of membrane permeability to water. IP-based membrane M1 with the highest water permeation flux and lowest salt rejection achieves the biggest improvement in its rejection ability after amidation, showing an increase of salt rejection from 93.60 to 95.92%. A higher salt rejection can be achieved after amidation for the TFC membranes with a relative higher initial salt rejection such as IP-based membranes M2,
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M3, M4 and M5. However, the surface amidation of present study has nearly no effect on improving the salt rejection and water permeability of the IP-based membrane M6, which has a relatively dense PA layer and exhibits a high initial salt rejection up to 97.72%. As depicted in Fig. 2, surface amidation is correlated with the surface contents of carboxyl and terminated amino groups, which are different for each IP-based PA TFC membrane. Therefore, the extent of the improvement of separation performance through surface amidation will be different for the IP-based polyamide-based TFC membranes of different initial permeation properties.
3.3. Reproducibility of surface amidation of IP-based PA TFC membrane Surface amidation was also performed to investigate its reproducibility through laboratory-scale trials by employing the type of IP-based PA TFC membrane M4 with an initial salt rejection around 95.8 % and a water permeation flux about 34.5 l/m2 h when filtrated with 500 mg/l aqueous NaCl solution of 25.0 ℃ under 5.0 bar. The results of the laboratory-scale trials of surface amidation of the IP-based PA TFC membrane using EDC aqueous solution of pH 4.0 are tabulated in Table 3.
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Table 3: Laboratory-scale trial results of surface amidation of IP-based PA TFC membrane. Water permeation flux* (l/m2 h)
Salt rejection* (%)
Before surface
After surface
Before surface
After surface
amidation
amidation
amidation
amidation
1
35.3
30.2
95.66
97.24
2
33.8
29.8
96.08
97.38
3
35.2
30.6
95.46
97.15
4
34.2
30.0
95.92
97.30
5
34.0
30.4
95.83
97.22
Average
34.5±0.7
30.2±0.3
95.79±0.24
97.26±0.09
Trial numbers
*
Filtrated with aqueous NaCl solution of 500 mg/l, 25.0 ℃ and pH 7.0 under 5.0 bar. It can be found from the data that the amidated membranes exhibit an average water
permeation flux of 30.2 l/m2 h and a salt rejection of 97.26 % with the standard deviations of only 0.3 and 0.09, respectively, which are much lower than those of 0.7 and 0.24 for the membranes before amidation. The low standard deviations of the results of laboratory-scale trials indicate that the process of surface amidation possesses good reproducibility.
3.4. Confirmation of the formation of amide bonds The formation of amide bonds through treating the IP-based PA TFC membrane with EDC aqueous solution was firstly ascertained through analyzing the chemical structure of membrane surface via ATR-FTIR. The FTIR spectra of the surface of the IP-based PA TFC
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membrane M1 before and after surface amidation are depicted in Fig. 4. The polyamide layer of the IP-based PA TFC membrane shows its characteristic absorptions of C=O stretching of amide I at about 1660 cm-1, hydrogen bonded C=O of amide I at about 1610 cm-1 and N–H in-plane bending of amide II near 1540 cm-1 [38]. No new absorption peak appears after surface amidation, since the step of surface amidation only generates amide bonds with the same characteristic absorption of the polyamide layer formed via IP process. However, as tabulated in Table 4, the strength ratios of the characteristic absorption peaks of the amide linkage of polyamide layer at 1660, 1610 and 1540 cm-1 to the characteristic absorption peak of benzene ring at 1490 cm-1 [39] I1660 /I1490, I1610/I1490 and I1540 /I1490 change appreciably after surface amidation. The augments of these peak strength ratios indicate the increase of the amount of amide bonds on the top of the IP-based PA TFC membrane after amidation. This confirms the occurrence of amidation reaction between adjacent carboxyl and amino groups when the surface of the IP-based PA TFC membrane was contacted with acidic aqueous EDC solution.
Fig.4. ATR-FTIR spectra of the IP-based PA TFC membrane before (a) and after (b) surface amidation. 18/33
Table 4 Strength ratios of the characteristic absorption peaks of the amide linkage to benzene ring. Membranes
I1660 /I1490
I1610/I1490
I1540 /I1490
IP-based membrane M1
0.6852
0.1541
0.5532
IP/SA-based membrane M1
0.7572
0.1736
0.6053
The formation of amide bonds was further ascertained through analyzing the chemical composition of membrane surface via XPS. The element contents of the IP-based PA TFC membrane before and after surface amidation are shown in Fig. 5. Both the O content and the ratio of O to N can be used as the indications of surface amidation, since the reaction between carboxyl and amino groups will reduce the O content on membrane surface and thus increase the ratio of O to N. It can be found from the graphs that, after surface amidation, the O content decreases appreciably from 17.63 to 16.18 mol% and the ratio of O to N decreases from 1.84 to 1.63. The declines of O content and O/N ratio further confirm the occurrence of amidation between adjacent carboxyl and amino groups when the surface of the IP-based PA TFC membrane was contacted with acidic aqueous EDC.
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Fig.5. XPS spectra of the IP-based PA TFC membrane before (a) and after (b) surface
amidation.
3.5. Surface roughness, charge and hydrophilcity of the amidated membrane Membrane surface roughness was characterized via AFM. Fig. 6 presents the surface AFM images of the IP-based TFC membranes M1, M2, M3, M4, M5 and M6 before and after surface amidation. The three-dimensional surface AFM graphics reveal the typical ridge-and-valley structure of the surface morphologies of the membranes before and after amidation. No evident change was observed with the morphology of membrane surface after surface amidation. The RMS values tabulated in Table 5 reflect a very slight decrease of surface roughness after amidation, which is possibly due to the densification effect of amidation on membrane surface.
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(a) Membrane M1
(b) Membrane M2
(c) Membrane M3
(d) Membrane M4
(e) Membrane M5
(f) Membrane M6 Fig.6. AFM images of the surfaces of the TFC membranes M1 (a), M2 (b), M3 (c), M4 (d), M5 (e) and M6 (f) before (left) and after (right) surface amidation.
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Table 5 Surface root mean square roughness (RMS), zeta potential and water contact angle values of the IP-based PA TFC membranes before and after surface amidation. IP-based
RMS (nm)
PA TFC
Before
Membrane
surface
Zeta potential a (mV)
Water contact angle b (°)
After
Before
After
Before
After
surface
surface
surface
surface
surface
amidation amidation
amidation amidation
amidation amidation
M1
69.4
67.1
-50.2±1.6
-42.6±1.3
34.8±0.9
36.5±1.1
M2
60.1
56.5
-47.5±1.5
-40.5±1.4
35.9±0.8
38.7±1.2
M3
67.2
63.7
-46.2±1.4
-40.3±1.3
36.5±0.9
38.9±1.0
M4
65.6
57.8
-48.6±1.3
-42.5±1.4
35.5±0.9
38.6±1.0
M5
67.1
63.3
-47.8±1.4
-40.6±1.2
38.5±0.8
40.8±0.9
M6
65.2
63.8
-45.8±1.3
-43.9±1.2
39.6±0.9
41.2±1.0
a
Evaluated using aqueous KCl solution of 0.001 mol/l, 25.0 ℃ and pH 7.0.
b
Evaluated using de-ionized of 25.0 ℃.
The effects of surface amidation on membrane surface hydrophilicty and charge were also evaluated through performing the measurements of surface water contact angle and streaming potential, respectively. The data tabulated in Table 5 clearly reveal that the surface of the polyamide-based TFC membrane becomes less negatively charged at neutral pH after amidation. The dissociation of carboxyl group on membrane surface is responsible for the surface negative charge. The amidation between the carboxyl and amino groups on membrane surface will reduce the amount of free carboxyl groups on membrane surface for dissociation,
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thus leading to an applicable decrease of membrane surface negative charge at neutral pH. The conversion of part of the surface carboxyl and amino groups into amide linkages is the reason for the slight change surface hydrophilicity.
3.6. Permeation property of the IP/SA-based PA TFC membrane The permeation property of IP-based PA TFC membrane before and after surface amidation was firstly investigated through measuring membrane pure water permeability (PWP) and rejections to different solutes including the neutral solute of glycerol and the electrolytes of NaNO3 and NaCl. Considering the salt rejection performance of commercial polyamide-based TFC reverse osmosis membrane, the IP/SA-based membrane M5 with relatively high salt rejection was employed to investigate the influence of the step of surface amidation on the permeation property of the IP-based PA TFC membrane through comparing with the IP-based membrane M5. For comparison, the permeation properties of IP-based membrane M6 and the commercial membrane BW30 of Dow were also evaluated. The data presented in Table 6 clearly illustrate that the step of surface amidation can effectively improve the ability of membrane rejection to both electrolyte and neutral solute. Comparing the permeation data of IP/SA-based membrane M5 and IP-based membrane M6 with nearly the same rejection ability in filtration of aqueous NaCl solution of 500 mg/l at the pressure of 5.0 bar, the PA TFC membrane prepared through IP followed with surface amidation exhibits higher rejections to NaNO3 and glycerol and better permeability to pure water compared with the PA TFC membrane fabricated via conventional IP process, indicating the superiority of the membrane obtained via the two-step technique in water
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permeability and rejection ability. Furthermore, the IP/SA-based PA TFC membrane M5 even possesses a comparable rejection ability and better water permeability compared with the commercial membrane BW30.
Table 6 Pure water permeability and solute rejection values of the tested membranes. Membranes
Pure water
NaCl
NaNO3
Glycerol
permeability a
rejection b
rejection c
rejection d
(l/m2 h bar )
(%)
IP-based membrane M5
6.02±0.13
96.4±0.3
90.5±0.2
78.4±0.6
IP/SA-based membrane M5
5.60±0.07
97.8±0.2
93.7±0.3
91.3±0.5
IP-based membrane M6
4.58±0.12
97.7±0.3
92.6±0.2
89.5±0.5
Membrane BW30 (Dow)
4.16±0.10
97.7±0.2
93.5±0.3
90.9±0.6
(%)
(%)
a
Obtained through permeation test using de-ionized water under 25.0 ℃.
b
Filtrated with aqueous NaCl solution of 500 mg/l, pH 7.0 and 25.0 ℃ under 5.0 bar.
c
Filtrated with aqueous NaNO3 solution of 2000 mg/l, pH 7.0 and 25.0 ℃ under 5.0 bar.
d
Filtrated with aqueous glycerol solution of 500 mg/l, 25.0 ℃ under 4.0 bar.
As illustrated in Fig. 7, IP/SA-based membrane M5 and IP-based membrane M6 possess a polyamide active layer with nearly the same thickness. Therefore, the superiority of water permeability of IP/SA-based membrane M5 compared with the IP-based membrane M6 is mainly because of the structural difference of the polyamide active layer between the two types of TFC membranes. As shown schematically in Fig. 1, the thin-film composite
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membrane prepared via the two-step process of IP followed by surface amidation contains a PA layer composed of a dense top skin and a loose bulk, while the TFC membrane prepared from conventional IP process possesses a PA layer of the same density from top to bottom. Therefore, of the IP-based and IP/SA-based TFC membranes with a PA layer of nearly the same thickness and rejection ability to 500 mg/l NaCl aqueous solution, the water permeation resistance across the TFC membrane with the polyamide layer prepared via conventional IP process will be higher than that of the TFC membrane with the polyamide layer prepared via IP followed by surface amidation.
Fig.7: SEM images of the cross sections of IP/SA-based membrane M5 (right) and IP-based membrane M6 (left). Additionally, as presented in table 5, the surface zeta potentials of IP -based membrane M5, IP/SA-based membrane M5 and IP-based membrane M6 are -47.8±1.4, -40.6±1.2 and -45.8±1.3 mV, respectively. The electrostatic repulsion between the negatively charged solute and the surface of IP/SA-based membrane M5 is the lowest of the three membranes. Thus the improved rejections to electrolytes NaNO3 and NaCl of the IP/SA-based membrane M5 are mainly resulted from the enhancement of size exclusion effect. The improved size exclusion reveals that the step of surface amidation can make the skin of the PA layer denser. This is the reason why the IP/SA-based membrane M5 exhibited improved rejection to neutral solute after surface amidation. Furthermore, the higher glycerol rejection reveals that the pore size of 25/33
the polyamide skin layer of the IP/SA-based membrane M5 is even smaller than that of the IP-based membrane M6. Therefore, the technique of IP followed with surface amidation can produce PA layer with high perm-selectivity and thus TFC membrane with improved reverse osmosis performance. The performance advantage of TFC membrane prepared via the two-step process of IP followed with surface amidation was further illustrated through investigating the change of salt rejection with feed concentration under the fixed operation pressure of 5.0 bar. As depicted in Fig. 8, of the three tested membranes, IP/SA-based membrane M5 exhibits the lowest decrease of NaCl rejection when the feed NaCl concentration ascends from 500 to 2000 mg/l, indicating that the superiority of the two-step membrane in rejection ability is more remarkable under high feed salt concentration. This is also mainly due to the improved size exclusion effect of the two-step membrane.
Fig.8. Changes of salt rejection with feed salt concentration for IP-based membrane M5, IP/SA-based membrane M5 and IP-based membrane M6 filtrated with aqueous NaCl solution of pH 7.0 and 25 ℃ at 5.0 bar. 26/33
The performance stability of the PA TFC membrane prepared via the two-step process of IP followed with surface amidation was also evaluated through a 30-day long-term permeation test by employing the IP/SA-based membrane M5. The data illustrated in Fig. 9 clearly reflect the slight changes of water flux and salt rejection during the whole filtration period, indicating the good performance stability of the amidated membrane.
Fig.9. Changes of water permeation flux (●) and salt rejection (▲) with filtration time for the IP/SA-based membrane M5 filtrated with aqueous NaCl solution of pH 7.0 and 25 ℃ at 5.0 bar. 4. Conclusions The fabrication of polyamide TFC membranes for improved reverse osmosis performance via the novel technique of interfacial polymerization followed by surface amidation has been demonstrated in this work. The amidation between adjacent carboxyl and amino groups was facilely implemented through covering the surface of the polyamide-based TFC membrane with an acidic aqueous EDC solution. The increase of amide bonds on membrane surface after surface amidation has been properly confirmed through the analyses
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of ATR-FTIR and XPS. Surface amidation was found to make the surface of the TFC membrane less negatively charged under neutral pH through reducing the amount of surface carboxyl groups. Surface amidation was demonstrated effective in improving the ability of membrane rejection to both electrolyte and neutral solute at a sacrifice of water permeability. The TFC membrane prepared through IP followed with surface amidation possessed higher rejections to solutes NaNO3 and glycerol and better permeation ability to pure water compared to the TFC membrane of the same rejection to NaCl prepared via conventional IP process. Polyamide-based TFC membrane with improved perm-selectivity could be fabricated through the two-step technique of IP followed with surface amidation. Furthermore, the step of surface amidation of present work can be potentially applied to in-situ performance rejuvenation of used polyamide reverse osmosis membranes for prolonged lifespan.
Acknowledgments The authors gratefully thank the National Natural Science Foundation of China (NNSFC) for financial support (Grant no 21476213 and 21676256).
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Research Highlights: ►TFC membrane was fabricated via interfacial polymerization and surface amidation. ►Surface amidation was facilely implemented via using acidic EDC aqueous solution. ►PA selective layer composed of a top dense skin on loose bulk was obtained. ►PA TFC membrane with improved reverse osmosis performance could be obtained.
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