High-performance and acid-resistant nanofiltration membranes prepared by solvent activation on polyamide reverse osmosis membranes

High-performance and acid-resistant nanofiltration membranes prepared by solvent activation on polyamide reverse osmosis membranes

Journal of Membrane Science xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: http://www.elsevi...

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Journal of Membrane Science xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci

High-performance and acid-resistant nanofiltration membranes prepared by solvent activation on polyamide reverse osmosis membranes Min Gyu Shin a, Soon Jin Kwon a, Hosik Park b, You-In Park b, Jung-Hyun Lee a, * a

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea Center for Membranes, Advanced Materials Division, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon, 34114, Republic of Korea

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Solvent activation Reverse osmosis Nanofiltration Interfacial polymerization Polyamide thin film composite membrane

We present a facile method for fabricating polyamide (PA) nanofiltration (NF) membranes exhibiting remarkable separation performance and high acid stability via solvent activation on PA reverse osmosis (RO) membranes with strong polar aprotic solvents (dimethyl sulfoxide (DMSO), dimethylformamide and N-methyl-2-pyrroli­ done). The solvents with strong solvency power for PA greatly swelled and deformed the dense RO PA layer, making the PA network more permeable and looser, which significantly improved the water permeance of the RO membrane while maintaining its high rejection to divalent salts. Consequently, the solvent-activated RO mem­ branes exhibited remarkable NF-grade separation performance, exceeding that of the commercial NF membrane (NF270, Dow Filmtec.). Particularly, the DMSO-activated membrane showed ~30% higher water permeance, higher salt rejection and ~6.8 times higher monovalent/divalent ion selectivity than NF270. This was attributed to the strongest solvency power of DMSO among the solvents used. Moreover, the solvent-activated membrane exhibited the superior acid stability to NF270 owing to the higher acid resistance of its fully-aromatic PA chemistry than that of the semi-aromatic PA. Our proposed method is a simple, effective and commercially viable strategy for fabricating high-performance and acid-resistant NF membranes that can expand the application spectrum of NF technology.

1. Introduction The application of nanofiltration (NF) technology has been rapidly expanding to desalination, water softening and removal or recovery of metal ions owing to its high retention to multivalent ions and small organic solutes (200–1000 Da) and higher flux than a reverse osmosis (RO) process [1–6]. RO and NF membranes typically have a thin film composite (TFC) configuration, where a polyamide (PA) selective layer made by interfacial polymerization (IP) of multifunctional amine and acyl chloride (i.e., trimesoyl chloride, TMC) monomers is adhered onto a porous support [7,8]. The amine monomer type and thus the resultant PA chemistry have been standardized for the target membrane grade. For example, the aromatic amine (m-phenylenediamine, MPD) mono­ mer is used to create a dense, fully-aromatic PA structure for RO, while aliphatic amine (piperazine, PIP) is used to produce a less dense, semi-aromatic PA network for NF. Although the NF membrane has less rejection to monovalent ions than the RO membrane, it exhibits much higher water permeance with a remarkable ability of discriminating

multivalent ions [4,9]. Despite remarkable advances in membrane technology, there is still an immense demand for NF membranes with improved separation performance to enhance the separation process efficiency [10–12]. Besides the concern on separation performance, the conventional PIP-based semi-aromatic PA NF membrane is known to be vulnerable to structural destruction under acidic conditions [4,9,10,13,14]. This constraint hinders the wide application of NF membranes to the metal, textile dyeing, pulp and mining industries where acidic effluents need to be treated [1,10,15]. Although some acid-durable NF membranes (e.g., SelRO series from Koch Membrane Systems and NTR 7450 from Nitto Denko) are commercially available, they suffer from a relatively low water flux [16,17]. To improve the acid resistance of NF membranes, substantial efforts have been dedicated to the tailoring of the selective layer chemistry through the introduction of polysulfonamide [5,14, 18–20] and poly(s-triazine-amine) [4,13,17]. The resultant NF mem­ branes exhibited enhanced acid stability imparted by the chemically stable sulfonyl bond of polysulfonamide [5,10,14,19] or the conjugated

* Corresponding author. E-mail address: [email protected] (J.-H. Lee). https://doi.org/10.1016/j.memsci.2019.117590 Received 17 June 2019; Received in revised form 14 September 2019; Accepted 18 October 2019 Available online 20 October 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Min Gyu Shin, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117590

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Journal of Membrane Science xxx (xxxx) xxx

structure of the triazine ring [3,4,17]. However, the low polymerization reaction rate [5,14,17–19], low membrane performance [5,13,14, 17–19] and required additional steps for monomer synthesis [4,5,14] limit their large-scale practical application. Although acid-resistant membranes have also been fabricated using other materials, including graphene oxide, polybenzimidazole, poly(epoxyether) and poly(vinyli­ dene difluoride) [21–26], multiple treatments are required [22,23] or their high NF performance for water treatment has not been demon­ strated [21,24–26]. These considerations necessitate the development of new NF membranes with high acid stability as well as excellent sepa­ ration performance. To address this issue, we recalled two important scientific discov­ eries: (1) The MPD-based fully-aromatic PA chemistry has superior acid resistance to the PIP-based semi-aromatic PA [4,9,10] and (2) activation on the MPD-based RO membrane with an appropriate solvent can deform its selective layer structure and thus effectively tune its separa­ tion performance [27,28], as also demonstrated in other membrane systems [29–32]. Here, for the first time, combining these two aspects, we demonstrate that the NF membrane with high acid resistance and excellent separation performance can be fabricated through simple solvent activation on the MPD-based PA RO membrane. To demonstrate our strategy, we first fabricated the RO-grade, MPD-based PA TFC membrane on a porous polyethylene (PE) support whose excellent solvent resistance can broaden the range of the organic solvents used for activation [33]. To activate the membrane, polar aprotic solvents having strong solvating power for the fully-aromatic PA, dimethyl sulfoxide (DMSO), dime­ thylformamide (DMF) and N-methyl-2-pyrrolidone (NMP), were used because these solvents can effectively alter membrane performance by substantially deforming the PA network [34–36]. The separation per­ formance (water permeance and salt rejection) and acid stability of the solvent-activated PE-supported TFC membranes were evaluated and compared with those of the pristine counterpart and commercial NF membrane (NF270, Dow Filmtec.). The structures and properties of the solvent-activated membranes were also comprehensively characterized to elucidate the underlying solvent activation mechanism and the membrane structure-property-performance relationship.

oxygen-containing functional groups on the support, which was believed to strengthen the PA layer-support interface by forming ionic and hydrogen bonds as well as to improve the support water wettability [37]. The plasma-modified PE support was impregnated with the MPD (3 wt%)/SDS (0.05 wt%) aqueous solution for 5 min, and then rubbed with a roller to remove the excess solution. The use of SDS helped to further improve the support water wettability and accelerate MPD migration toward the organic phase, which hence facilitated the IP process [37–39]. Next, the support was contacted with a TMC (0.15 wt %) organic (n-hexane) solution for 3 min. The membrane was subse­ quently rinsed with n-hexane and dried at 70 � C for 5 min. This drying condition has been optimized in our previous report [37]. The fabricated PE-supported TFC RO membrane was designated as PE-TFC. To activate the PE-TFC membrane, the membrane surface was contacted with a polar aprotic organic solvent (DMSO, DMF or NMP) for the fixed time duration (1 min 4 h), and then thoroughly rinsed with a copious amount of DI water. To examine the effect of the PA chemistry on the acid resistance of the membrane, the PIP-based PA TFC membrane was also made on the PE support through the aforementioned IP process using the PIP (2.5 wt%)/SDS (0.05 wt%) aqueous and TMC (0.15 wt%) organic (n-hexane) solutions. The prepared PIP-based PA TFC mem­ brane was designated as PE-TFCPIP. All the prepared membranes were kept in DI water before the test. 2.3. Membrane characterization The membrane structures were characterized using scanning elec­ tron microscopy (SEM, FEI Inspect F50). Nine different regions of each membrane sample were examined to determine the average PA layer thickness. Atomic force microscopy (AFM, Innova, Bruker) was employed to quantify the root-mean-square (rms) roughness of the membrane surfaces by imaging their topographies of 5 μm � 5 μm area in a tapping mode with the stiff tip (NCHR, Nanoworld, spring con­ stant ¼ 42 N m 1). Five different positions were imaged to obtain the average rms data. X-ray photoelectron spectroscopy (XPS, X-tool spec­ trometer, ULVAC-PHI) and Fourier transform infrared spectroscopy (FTIR, Spectrum Two spectrometer, PerkinElmer) were performed to analyze the chemical compositions and functional groups of the mem­ branes. The water contact angle (θ) of the membranes was quantified with a contact angle analyzer (Pheonix-300, SEO Corp.). The intrinsic hydrophilicity of the membrane surface was characterized with the solid-liquid interfacial free energy ( ΔGSL), which reflects the influence of the surface roughness on the contact angle,

2. Experimental 2.1. Materials MPD (Tokyo Chemical Industry), PIP (Daejung Chemical), sodium dodecyl sulfate (SDS, Sigma-Aldrich), TMC (Tokyo Chemical Industry), sodium sulfate (Na2SO4, Junsei Chemical), magnesium sulfate (MgSO4, Junsei Chemical), magnesium chloride (MgCl2, Junsei Chemical), cal­ cium chloride (CaCl2, Junsei Chemical), sodium chloride (NaCl, Junsei Chemical), polyethylene glycol (PEG, weight-average molecular weights (Mw): 200 8000 g mol 1, Sigma-Aldrich), NMP (Daejung Chemical), DMF (Daejung Chemical), DMSO (Daejung Chemical) and n-hexane (Daejung Chemical) were used as purchased. De-ionized (DI, 18.2 MΩ cm) water was produced using the Millipore Milli-Q system. The ~20 μm-thick PE support membrane (SK Innovation) and commercial RO (SWC4þ, Nitto Denko) and NF (NF270, Dow Filmtec) membranes were used as purchased. The mean pore size of the PE support has been re­ ported to be 35 � 2 nm [33].

ΔGSL ¼ γLð1 þ cos θ = rÞ

(1)

where γ L is the surface tension of pure water at 25 � C (72.8 mJ m 2) and r is the ratio of the real surface area to the projected area of the mem­ brane [40]. The surface zeta potentials of the membranes were measured using a zeta potential analyzer (ELSZ-2000, Otsuka Elec­ tronics) at pH 5.8. The water contact angle and zeta potential data at seven different positions were averaged. The solvating power of the organic solvent for PA was evaluated by characterizing the size and amount of the leached materials during solvent activation of the PA layer [27,28,35]. The pristine PE-TFC membrane of ~15 cm2 in area was dipped in 5 mL of an activating solvent for 12 h. The separated super­ natant solution was dried with a rotary evaporator, and then the mo­ lecular weights of the collected materials were quantified using gel permeation chromatography (JASCO SEC System). However, the quantity of the materials leached out from the PE-TFC membrane was too small to be accurately quantified. To address this issue, a large amount of the unsupported PA film was synthesized through IP at the free interface [28,41] using the identical monomer composition. The MPD (3 wt%)/SDS (0.05 wt%) aqueous solution was filled in a glass bath. Subsequently, the TMC (0.15 wt%) organic (n-hexane) solution was gently spread on the MPD solution and allowed to stay for 3 min.

2.2. Membrane fabrication TFC RO membranes were fabricated through the IP process of MPD and TMC using PE supports by following our reported protocol [33,37, 38]. The excellent solvent resistance of the PE support allows us to use the wide spectrum of solvents for activation [33]. To improve the water wettability of the hydrophobic PE support, we hydrophilized the PE support through oxygen plasma treatment (plasma power: 20 W, expo­ sure time: 20 s) prior to IP. The plasma treatment generated 2

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The formed PA film was separated and rinsed with n-hexane and DI water, followed by drying with a rotary evaporator. 5 g of the collected PA film was soaked in 50 mL of an activating solvent (DMSO, DMF or NMP). After 12 h, centrifugation (8000 rpm for 10 min) was applied to separate the supernatant solution, which was subsequently dried with a rotary evaporator. The weight of the final product was measured to quantify the amount of the dissolved materials during solvent activation. Three different samples per each activating solvent were tested.

to have strong solvating power for the PA material based on their sol­ ubility parameters as presented in Table 1. The Hildebrand solubility parameter (HSP, δ) has been widely used for estimating the mutual miscibility of two materials [45]: A smaller difference in the HSP values of two materials implies a higher miscibility. Although this approach reasonably described the mixing behavior of non-polar systems, it often resulted in an unsatisfactory prediction for polar materials [46]. In principle, HSP consists of three Hansen solubility contributions corre­ sponding to the dispersion (δd), polar (δp) and hydrogen bonding (δH) interactions, as expressed by δ ¼ (δ2d þ δ2p þ δ2H)1/2 [47]. The Hansen solubility parameter difference in the three-dimensional space between two materials (Ra), as defined by Ra ¼ [4(δd2 δd1)2 þ (δp2 δp1)2 þ (δH2 δH1)2]1/2, has been successfully used for assessing their mutual affinity even for polar systems [47]. A lower Ra value indicates a higher mutual miscibility. Recently, we demonstrated that the Ra approach is more appropriate for evaluating the solvency power (activating effi­ ciency) of the organic solvent for the PA RO membrane rather than the HSP difference [28]. As summarized in Table 1, all the selected organic solvents have significantly low HSP difference (δ δPA) and Ra values for PA, indicating their strong solvating abilities for PA. In addition, both δ δPA and Ra values decrease in the order of DMSO > DMF > NMP, predicting that the solvency power of the solvent increases in the order of DMSO < DMF < NMP.

2.4. Membrane performance Separation performance of the membranes was assessed using a cross-flow filtration system (operating pressure: 10 bar, flow rate: 1.0 L min 1, temperature: 25 � C) where a salt (Na2SO4, MgSO4, MgCl2, CaCl2 or NaCl, 1000 ppm) aqueous solution was permeated through the membrane with an area (am) of 14.5 cm2. Prior to data collection, DI water was filtrated at a pressure of 10 bar for 24 h to compact the membrane. The permeate volume (ΔV) collected for a certain time period (Δt) was measured to calculate water flux (Jw, L m 2 h 1 bar 1, LMH), (2)

Jw ¼ ΔV=am Δt 1

Water permeance (A, LMH bar ) was determined by dividing the water flux by the applied (Δp) and osmotic pressure (Δπ) difference, A ¼ Jw =ðΔp

3.2. Membrane structures and properties

(3)

Δπ Þ

The salt concentrations of the feed (Cf) and permeate (Cp) were also determined using a conductivity meter to estimate salt rejection (Rs, %), � Rs ¼ ð1 Cp Cf Þ � 100 ​ (4)

Solvent activation can alter the surface morphology of the membrane by deforming the PA structure [27,35]. Fig. 1 exhibits the surfaces and cross-sections of the pristine and solvent-activated PE-TFC membranes. The pristine PE-TFC membrane displayed the ridge-and-valley surface morphology, which has been typically observed for the IP-assembled (Fig. 1a), MPD-based fully-aromatic PA layer [43]. It is noteworthy that all the solvent-activated PE-TFC membranes exhibited similarly deformed surface structures with less ridges and more pronounced nodules compared to the pristine counterpart (Fig. 1b–d), which was consistent with our previous observation [28]. The strong activating solvent would significantly swell the PA matrix and even partially etch the relatively looser upmost PA layer, resulting in the reduction of the ridge-like features [27,28,35]. As shown in Fig. 1e–h, no discernable variation in the PA layer thickness was observed after solvent activation. We speculated that solvent activation was not strong enough to induce the noticeable thickness variation in the PA layer with the highly rough and heterogeneous structure [48]. Regardless of the activating solvent type, solvent activation distinctly reduced the surface roughness (rms value) of the pristine membrane to a similar extent, as shown in Fig. 2. This result was consistent with our SEM data and supported the hypothesis that mem­ brane activation with the strong solvent can result in the partial etching of the PA surface [27,28,35]. It should be noted that all the solvent-activated PE-TFC membranes displayed the similar surface and cross-sectional structures, indicating that the morphological deforma­ tion of the membrane by solvent activation was not sensitive enough to distinguish the solvency power of the solvent.

Salt permeance (B, LMH) was quantified by following the equation [38], B ¼ Að1 = Rs

1ÞðΔp

Δπ Þ

(5)

The intrinsic salt selectivity of the membranes can be expressed as the ratio of A to B (A/B), which is referred to as permselectivity [28,42, 43]. PEG rejection of the membranes was determined by filtrating PEG (Mw: 200 8000 g mol 1, 1000 ppm) aqueous solutions in a cross-flow system (operating pressure: 10 bar, flow rate: 1.0 L min 1, tempera­ ture: 25 � C). A total organic carbon analyzer (Sievers 900, GE Analytical Instruments) was used to measure the PEG concentrations of the feed and permeate. The molecular weight cut-off (MWCO) of the membrane was determined by the PEG Mw value whose rejection by the membrane is 90%. Performance data were averaged from the measurements of three different samples per each membrane and error bars denote standard deviations. 2.5. Acid resistance assessment The acid stability of the membranes was assessed by monitoring the variations in their performance after soaking them into the acid (15 wt% H2SO4) aqueous solution for 4 weeks [2,5,44]. The physical and chemical structures of the membranes before and after acid exposure were also characterized using SEM and FT-IR, respectively. The acid stability of the commercial RO (SWC4þ) and NF (NF270) membranes, known as the MPD-based fully-aromatic and PIP-based semi-aromatic PA chemistries, respectively, was also assessed for comparison.

Table 1 Solubility parameters (MPa1/2) of the PA [34] and organic solvents [47]. Solubility parameter PA NMP DMF DMSO

3. Results and discussion 3.1. Solubility parameters of activating solvents

δ

δ

δd

δp

δH

23.0 23.0 24.8 26.7

18.0 18.0 17.4 18.4

11.9 12.3 13.7 16.4

7.9 7.2 11.3 10.2

– 0 1.8 3.7

δPA

Ra

– 0.8 4.0 5.1

HSP (δ) is represented by δ ¼ (δ2d þ δ2p þ δ2H)1/2. δPA represents δ of PA. Ra is defined by Ra ¼ [4(δd2 δd1)2 þ (δp2 δp1)2 þ (δH2 δH1)2]1/2, where the sub­ scripts (1) and (2) denote a solvent and PA, respectively.

Here, a series of polar aprotic solvents (DMSO, DMF and NMP) were selected for activating PE-TFC RO membranes since they were expected 3

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Fig. 1. (a–d) Surface and (e–h) cross-sectional SEM images of the (a, e) pristine, (b, f) NMP-activated, (c, g) DMF-activated and (d, h) DMSO-activated PE-TFC membranes (activation time ¼ 10 min).

Fig. 2. AFM surface topographies and roughness of the (a) pristine, (b) NMP-activated, (c) DMF-activated and (d) DMSO-activated PE-TFC membranes (activa­ tion time ¼ 10 min).

Fig. 3 presents the FT-IR data of the pristine and solvent-activated PE-TFC membranes. The pristine PE-TFC membrane had the peaks at – O bond stretching), 1610 cm 1 (H bonded C– – O bond 1668 cm 1 (C– stretching), 1542 cm 1 (N–H bond in-plane bending) and 1490 cm 1 (C–C bond stretching of benzene ring) [37,38], which correspond to the characteristic peaks of the MPD-based fully-aromatic PA chemistry. All the solvent-activated PE-TFC membranes preserved the characteristic PA spectra of the pristine membrane, implying that

solvent activation did not alter the PA chemical structure while deforming the PA physical structure [28]. If solvent activation had caused the partial surface etching of the PA layer, the chemical composition and physical properties of the PA sur­ face would have been altered. To verify this, the pristine and solventactivated PE-TFC membranes were analyzed using XPS (Table 2). The atomic ratio of oxygen to nitrogen (O/N ratio) of the pristine PE-TFC membrane was 1.56, which was close to that of the PE-supported TFC membrane reported in our previous study [28]. The PA surface is generally identified as a carboxyl group-rich surface because a large amount of the unreacted surface acyl chloride groups are converted to carboxyl groups by hydrolysis [8]. It has been claimed that the partial surface etching of the PA membrane caused by solvent activation with a strong solvent reduces the surface oxygen contents by eliminating the oxygen-rich carboxyl groups [27,28,35]. In fact, all the solvent-activated PE-TFC membranes exhibited a smaller O/N ratio than the pristine membrane. A stronger solvent was expected to result in a Table 2 Surface chemical composition (O/N ratio) and physical properties of the pristine and solvent-activated (act.) PE-TFC membranes (activation time ¼ 10 min).

Pristine NMP-act. DMF-act. DMSOact.

Fig. 3. FT-IR data of the pristine and solvent-activated (act.) PE-TFC mem­ branes (activation time ¼ 10 min). 4

O/N ratio

Water contact angle (� )

ΔGSL (mJ m 2)

Zeta potential (mV)

1.59 � 0.10 1.40 � 0.03 1.30 � 0.04 1.23 � 0.05

73.7 � 1.5 80.3 � 2.7 81.5 � 1.3 88.2 � 2.0

87.4 � 3.1 82.2 � 4.5 81.0 � 3.0 74.6 � 6.6

18.2 � 3.3 13.5 � 2.3 12.7 � 3.7 8.2 � 3.1

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more reduction in the O/N ratio. However, unexpectedly, the reduction rate of the O/N ratio increased in the order of NMP < DMF < DMSO, which was contradictory to the solubility parameter-predicted solvating power of the solvent. This tendency was consistently observed in the surface hydrophilicity and charge properties of the solvent-activated PE-TFC membranes. For example, it has been reported that the solvent-activation-induced PA partial etching reduces the hydrophilicity and negative charge of the membrane surface by reducing the number of the hydrophilic and negatively charged carboxyl groups [27,28]. Simi­ larly, solvent activation with the polar aprotic solvents used in this study resulted in a decrease in both the surface hydrophilicity ( ΔGSL) and negative zeta potential of the membrane (Table 2). Consistent with the XPS data, among the solvents used, DMSO caused the largest reduction in the surface hydrophilicity and negative charge of the membrane. This experimental observation also contradicted the theoretical expectation of the solvating power of the solvent. To confirm the observed discrepancy between the experimental data and theoretical prediction, we evaluated the solvating (swelling) ability of the activating solvent by characterizing the size and amount of the materials dissolved out from the PA layer during solvent activation (Table 3). It can be reasonably postulated that activation with a stronger solvent would swell more the PA matrix and hence dissolve out larger and higher amount of PA fragments [27,28]. The results showed that the molecular weight and amount of the PA fragments dissolved out by solvent activation increased in the order of NMP < DMF < DMSO, which was in good agreement with our experimentally determined solvency power of the solvent. We made a further attempt to verify the correctness of our experi­ mental observations by monitoring the variations in the intrinsic sepa­ ration properties (water permeance and permselectivity) of the membranes as a function of operating (compaction) time, as shown in Fig. 4. In the early stage of operation, all the solvent-activated PE-TFC membranes exhibited remarkably enhanced (12 18 times higher) water permeance together with the dramatically reduced permse­ lectivity compared to the pristine control, which was attributed to the significantly loosened PA structure caused by solvent activation. As operating time elapsed until the stabilized state was attained, water permeance remarkably decreased by 50–60%, while the permselectivity increased by two- or three-fold due to the significant chain compaction of the solvent-activated, loosened PA network under the operating pressure. The pristine PE-TFC exhibited a significantly less compactioninduced performance variation (10% reduction in water permeation and 60% increase in the permselectivity) owing to its denser PA structure. A stronger solvent would produce a looser and more open PA structure, which would result in a greater enhancement in water permeation and a higher reduction in the permselectivity of the membrane [28,43]. Importantly, the DMSO-activated PE-TFC membrane exhibited the highest initial and stabilized water permeance, followed by the DMFand NMP-activated membranes. In addition, DMSO activation resulted in the largest reduction in both the initial and stabilized permselectivity, followed by activation with DMF and NMP. These results also indicated that the solvating power of the solvent increased in the order of NMP < DMF < DMSO, which was consistent with the above experi­ mental observations. We suspected that the observed discrepancy between our experi­ mental data and the theoretical prediction for the solvency power of the solvent stems from the inaccuracy of the reference solubility parameters

of PA. Strictly speaking, the solubility parameters of the crosslinked fully-aromatic PA of the RO membrane have not been experimentally or theoretically estimated, presumably due to its insoluble and complex structural nature. Rather, the solubility parameters of the uncrosslinked fully-aromatic PA, poly(p-phenylene terephthalamide) (PPT), have been used as the reference solubility parameters of the RO PA material [49, 50]. Notwithstanding the structural similarity, the crosslinked and meta-oriented structure of the MPD/TMC-based RO PA would cause its spatial chain alignment and functional group composition and density to differ from those of the uncrosslinked and para-oriented PPT. This structural dissimilarity in turn would cause a difference in the type and level of the intermolecular interactions and thus the solubility parame­ ters between the RO PA and reference PPT. This can account for the contradiction between our experimental observation and the theoretical prediction. In this regard, more reliable solubility parameters for the PA chemistry used in membranes need to be estimated to accurately interpret and predict the solvent activation effect. 3.3. Membrane performance To study the kinetics of solvent activation, the variation in the sta­ bilized performance of the PE-TFC membrane was assessed as a function of solvent activation time (Fig. 5). As activation time increased, the water permeance of the membrane drastically increased. Meanwhile, its NaCl rejection significantly decreased due to the formation of the looser PA network by solvent activation [27,28]. The solvent-activation-induced performance change was so rapid to reach the plateau within 10 min. We believe that the strong solvency power and small molecular size (kinetic diameters: < 0.6 nm) of the solvents enabled the rapid penetration of the solvent molecules to the PA network to cause its structural deformation [27,28]. As expected, a stronger solvent induced a larger performance change in the order of NMP < DMF < DMSO. Based on the results, solvent activation time was fixed to 10 min for the subsequent study on the solvent-activated PE-TFC membranes. Table 4 summarizes the water permeance and rejection to various salts of the pristine and solvent-activated PE-TFC and commercial NF (NF270) membranes. The pristine PE-TFC membrane had RO-grade performance with high rejection (�99.6%) to all the salts, indicating the successful fabrication of the highly selective and dense PA layer using our IP protocol. Solvent activation significantly increased (6 8.5 times higher) the water permeance of the PE-TFC membrane, while maintaining or reducing its salt rejection depending on the salt type. For example, after solvent activation, high rejection (~99.9%) to divalent anion salts (Na2SO4 and MgSO4) remained unchanged, while rejection to monovalent anion salts (MgCl2, CaCl2 and NaCl) decreased noticeably with the reduction rate increasing in the order of MgCl2 < CaCl2 < NaCl. Importantly, all the solvent-activated PE-TFC membranes exhibited much higher rejection to all the salts with the similar or even higher water permeance compared to the commercial NF270 membrane, demonstrating their excellent NF performance. In addition to water permeance and salt rejection, a higher monovalent/divalent ion selec­ tivity is desirable for precisely separating specific ion species in various NF applications including metal ion recovery, biomass extraction and food industry [1,11,12]. RO membranes typically have low ion selec­ tivity since their dense PA networks effectively reject both monovalent and divalent ions [6], as evidenced by the very low ion selectivity value (~4) of the pristine PE-TFC membrane. The solvent-activated, loosened PE-TFC membranes exhibited the significantly higher ion selectivity than the pristine PE-TFC and even commercial NF270 membranes, which further highlighted their beneficial features. Salt rejection of the membrane is determined by the combination of the Donnan exclusion and size exclusion mechanisms [4,51,52]. For example, NF270 membrane exhibited salt rejection that decreased in the order of Na2SO4 > MgSO4 > MgCl2 or CaCl2. This tendency can result from the Donnan exclusion mechanism that the negatively charged

Table 3 Molecular weights (weight-average: Mw, number-average: Mn) and concentra­ tions (C) of the dissolved materials during PA activation with organic solvents. Solvent

Mw (g mol

NMP DMF DMSO

3870 � 190 4302 � 210 5171 � 280

1

)

Mn (g mol 1)

Mw/Mn

C (g L 1)

2917 � 160 3153 � 155 3350 � 200

1.32 1.36 1.54

4.8 � 0.5 5.6 � 0.4 6.6 � 0.5

5

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Fig. 4. Relative (a) water permeance (A) and (b) water/salt permselectivity (A/B) as a function of operation time and (c) relative initial and stabilized A and A/B data of the pristine and solvent-activated (act.) PE-TFC membranes (activation time ¼ 10 min). Relative A and A/B values were obtained by dividing the estimated A and A/B values by the respective initial values of the pristine PE-TFC membrane.

Fig. 5. Stabilized (a) water permeance (A) and (b) NaCl rejection of the solvent-activated (act.) PE-TFC membranes as a function of solvent activation time. Activation time of 0 h corresponds to the pristine PE-TFC membrane. Table 4 Water permeance (A), salt rejection and ion selectivity of the pristine and solvent-activated (act.) PE-TFC and commercial NF (NF270) membranes (activation time ¼ 10 min). Sample Pristine NMP-act. DMF-act. DMSO-act. NF270

A (LMH bar 1) 1.7 � 0.1 10.9 � 1.1 12.1 � 1.5 14.5 � 0.9 11.3 � 0.7

Ion selectivity ¼ (100

Salt rejection (%)

Ion selectivity (NaCl/Na2SO4)

Na2SO4

MgSO4

MgCl2

CaCl2

NaCl

99.9 � 0.1 99.9 � 0.1 99.9 � 0.1 99.9 � 0.1 98.1 � 0.3

99.9 � 0.1 99.9 � 0.1 99.9 � 0.1 99.9 � 0.1 97.1 � 0.7

99.8 � 0.1 99.8 � 0.3 99.5 � 0.1 99.1 � 0.2 75.3 � 0.4

99.8 � 0.1 99.6 � 0.2 99.5 � 0.1 98.5 � 0.8 65.5 � 0.3

99.6 � 0.2 94.3 � 0.8 91.4 � 0.5 85.1 � 0.9 59.1 � 0.8

NaCl rejection)/(100

4 57 86 149 22

Na2SO4 rejection).

membrane surface more effectively repels anions with higher valency, while more strongly attracting cations with higher valency through electrostatic interaction [52]. On the other hand, the pristine PE-TFC RO membrane exhibited high rejection to all the salts due to the dominant

size exclusion resulting from its dense PA structure [53]. Moreover, the observation that rejection to NaCl was lower than that to the other salts for all the membranes could be attributable mainly to the size exclusion mechanism where smaller ion species more readily permeates through 6

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the PA network [53]. Solvent activation on the pristine PE-TFC mem­ brane reduced the structural density of the PA layer in conjunction with the reduction in its negative surface charge, which thus would suppress both Donnan and size exclusion mechanisms. The noticeable decrease in NaCl rejection of the PE-TFC membrane after solvent activation could be due primarily to the suppressed size exclusion mechanism resulting from the loosened PA structure, considering the fact that NaCl rejection is governed predominantly by size exclusion [53]. To verify our claim, the MWCO values of the membranes were esti­ mated by measuring their PEG rejection as a function of the PEG Mw values (Fig. 6) since the MWCO value is directly related with the PA crosslinking density [52]. The MWCO values of the solvent-activated PE-TFC membranes (170 240 g mol 1) were larger than that of the pristine counterpart (~60 g mol 1). This result supported our claim that solvent activation created the more open and less crosslinked PA network, which facilitated permeation of small Naþ and Cl ions through the membrane and thus reduced NaCl rejection [27,54]. Solvent activation increased the MWCO value in the order of NMP < DMF < DMSO, following the experimentally estimated solvency power of the activating solvent, which well matched with the tendency of the reduction rate of NaCl rejection. Although solvent activation on the PE-TFC membrane could reduce rejection to divalent anion salts by suppressing Donnan exclusion, un­ changed high rejection (~99.9%) to divalent anion salts suggested that size exclusion prevailed over Donna exclusion in determining salt rejection. In addition, the dominant size exclusion mechanism is responsible for higher rejection to divalent anion salts of the solventactivated PE-TFC membranes than that of NF270 with a higher nega­ tive surface charge ( 38.7 � 3.7 mV) but a lower molecular density (higher MWCO). The less distinct values of rejection to different divalent salts of the solvent-activated PE-TFC membranes compared to those of NF270 can also be explained by their prevailing size exclusion mecha­ nism. This is because the solvent-activated PA network is still tight enough to effectively reject divalent salts. Hence, it is reasonable to postulate that activation on the PE-TFC membrane with the proposed polar aprotic solvents created the highly water-permeable and properly loosened PA network that permits the passage of the small NaCl salt to some extent while highly rejecting divalent salts. This would account for the significantly enhanced ion selectivity of the solvent-activated PETFC membranes. Based on the performance results, DMSO was found to be the most desirable solvent for activation among the polar aprotic solvents used since the DMSO-activated membrane showed the highest NF perfor­ mance: It exhibited ~30% higher water permeance, higher salt rejection and 6.8 times higher ion selectivity compared to the commercial NF270

membrane. This could be attributed to DMSO’s strongest solvency power, which enabled the production of the most permeable and appropriately loosened PA structure. In addition, DMSO is known to be harmless to humans than the other polar aprotic solvents used [35, 55–57]. Together with the remarkable tunability of membrane perfor­ mance, the low toxicity of DMSO can increase its commercial viability in the membrane fabrication industry, as previously claimed by other re­ searchers [35]. Hence, we focused on the DMSO-activated PE-TFC membrane in the subsequent study. 3.4. Acid resistance assessment The acid stability of the membranes was evaluated by monitoring their performance variations as a function of exposure time to the H2SO4 (15 wt%) aqueous solution (Fig. 7). The commercial NF270 membrane suffered from a significant reduction in salt (NaCl and MgSO4) rejection together with a remarkable increase in water permeance, particularly after 2 weeks. Similarly, the PE-supported, PIP-based PA TFC (PETFCPIP) membrane also showed the drastic performance deterioration upon acid exposure. This can be explained by the fact that the semiaromatic PA is vulnerable to structural damage by acid-catalyzed hy­ drolysis of the amide bond under acidic conditions [9,44]. In contrast, the pristine and DMSO-activated PE-TFC and commercial RO (SWC4þ) membranes did not experience any performance change during the long-term exposure to the acidic environment for 4 weeks. This was attributed to the excellent acid resistance of their MPD-based full­ y-aromatic PA chemistries [4,58]. Consistent with the performance results, SEM images (Fig. 8) revealed that the PA layers of the pristine and DMSO-activated PE-TFC and SWC4þ membranes remained intact, while the PA layers of the NF270 and PE-TFCPIP membranes were severely destroyed (Fig. 8i) or even detached (Fig. 8j) after the long-term acid exposure. The FT-IR data of the membranes before and after the long-term acid exposure were compared to further confirm their acid stability (Fig. 9). The pristine and DMSO-activated PE-TFC and SWC4þ membranes retained their characteristic fully-aromatic PA peaks at 1668, 1610, 1542 and 1490 cm 1 after acid exposure for 4 weeks. In contrast, for the NF270 and PE-TFCPIP membranes, their characteristic semi-aromatic PA – O stretching, amide I) [9] drastically decreased peak at 1634 cm 1 (C– in the intensity (NF270) and disappeared (PE-TFCPIP) after acid expo­ sure, indicating the significant destruction of their PA layers, which was consistent with the performance and SEM results. It has been well documented that the fully-aromatic PA has higher acid resistance than the semi-aromatic PA owing to its more planar amide structure that can increase the energy barrier of the hydrolysis reaction by stabilizing the resonance of the amide bond [9,59]. As a result, the DMSO-activated PE-TFC membrane exhibited a better acid resistance as well as supe­ rior NF performance compared to the commercial NF270 membrane due to its fully-aromatic PA chemistry. Although PA has been recognized to be more susceptible to hydrolysis than polysulfonamides and poly (s-triazine-amines) chemistries under extreme acidic conditions [4,10, 14,19], our solvent-activated, fully-aromatic PA PE-TFC membrane can guarantee its durable operation under moderately extreme acidic con­ ditions at room temperature, as demonstrated above. 4. Conclusions We demonstrated that solvent activation on the RO membrane with strong polar aprotic solvents (DMSO, DMF and NMP) can create the high-performance NF membrane with the excellent acid stability. The polar aprotic solvent effectively loosened the dense fully-aromatic PA structure of the RO membrane by severely swelling it. Thereby, the PA network became more permeable and less tight, which enabled the transformation of membrane performance from RO to NF-grade. Among the polar aprotic solvents used, DMSO activated the RO membrane most effectively, resulting in the superior NF performance (higher water

Fig. 6. PEG rejection as a function of the PEG Mw values and corresponding MWCO values of the pristine and solvent-activated (act.) PE-TFC and com­ mercial NF270 membranes (activation time ¼ 10 min). 7

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Fig. 7. (a) Relative water permeance (A), (b) NaCl rejection and (c) MgSO4 rejection of the pristine and DMSO-activated (act.) PE-TFC, commercial SWC4þ and NF270 and PE-supported, PIP-based PA TFC (PE-TFCPIP) membranes as a function of exposure time to the H2SO4 (15 wt%) aqueous solution. Relative A values were obtained by dividing the estimated A values by the A values of the membranes before acid exposure.

Fig. 8. Surface SEM images of the (a, f) pristine and (b, g) DMSO-activated PE-TFC and commercial (c, h) SWC4þ and (d, i) NF270 and (e, j) PE-supported, PIP-based PA TFC (PE-TFCPIP) membranes (a–e) before and (f–j) after exposure to the H2SO4 (15 wt%) aqueous solution for 4 weeks.

Fig. 9. FT-IR data of the (a) pristine and DMSO-activated (act.) PE-TFC and commercial SWC4þ membranes and the (b) PE-supported, PIP-based PA TFC (PE-TFCPIP) and commercial NF270 membranes (solid line) before and (dashed line) after exposure to the H2SO4 (15 wt%) aqueous solution for 4 weeks.

permeance, salt rejection and ion selectivity) to the commercial NF membrane owing to its strongest solvency power. In addition, the sol­ vent (DMSO)-activated membrane exhibited higher acid resistance than the commercial NF membrane due to the higher acid stability imparted by its fully-aromatic PA chemistry. From a fundamental perspective, our study further clarified our understanding of the underlying solvent

activation mechanism that can tune the structure, properties and per­ formance of the membrane. From a practical perspective, solvent acti­ vation is a facile and versatile technique for tailoring and improving membrane separation performance. In particular, the least toxic and benign nature of DMSO as an activating solvent can enhance the com­ mercial viability of the solvent activation technique. 8

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Declaration of competing interest

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