New Insights into the Role of an Interlayer for the Fabrication of Highly

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New insights into the role of an interlayer for the fabrication of highly selective and permeable thin-film composite nanofiltration membrane Genghao Gong, Ping Wang, Zongyao Zhou, and Yunxia Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18719 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

New Insights into the Role of an Interlayer for the Fabrication of Highly Selective and Permeable Thin-film Composite Nanofiltration Membrane Genghao Gong,a Ping Wang,a Zongyao Zhou,a Yunxia Hu,a*

a State

Key Laboratory of Separation Membranes and Membrane Processes, School of Materials

Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, P. R. China *Corresponding author: Tel: +86-22-83955129, e-mail: [email protected]

ABSTRACT A triple-layered TFC nanofiltration (NF) membrane consisting of a polyamide (PA) top layer covered on a polyethersulfone microfiltration membrane with a carbon nanotube (CNT) interlayer was fabricated via interfacial polymerization. The structure and properties of PA active layer could be finely tailored by tuning the interfacial properties and pore structure of the CNT interlayer, including its surface pore size and thickness, and thus to improve its NF performance. This TFC NF membrane exhibited a high divalent salt rejection (the rejection of Na2SO4 and MgSO4 solution >98.3%) and dye rejection (the rejection of methyl violet (MV) > 99.5%) with a high pure water flux of around 21 L m-2 h-1 bar-1. Excitingly, this membrane also showed excellent selectivity to both mono-/divalent salt ion (the selectivity of Cl-/SO42- is as high as 85.5) and NaCl/dye solution (the selectivity of NaCl/MV is more than 123.5), which are much higher than most of other commercial and reported NF membranes. Moreover, this membrane also showed a good separation performance and long-term stability during a continuous NF process for a salt/dye mixture solution. This triple-layered TFC NF membrane showed a great promise for applications in both wastewater treatment and dyes recycling.

KEYWORDS: Polyamide, Carbon nanotube interlayer, Thin film composite membrane, Nanofiltration, Interfacial polymerization

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1. INTRODUCTION A shortage of clean water is a formidable and long-term challenge that mankind is confronting worldwide, and the pollution of clean water caused by the discharge of industrial wastewater is further exacerbating this threat.1-2 Among these water pollutions, the dyecontaining wastewater was considered as one of main industrial pollution sources as well as one of the intractable wastewater because of the complex components, high salinity and difficult biodegradation.3-5 Various methods, including adsorption, flocculation, biological treatment, advanced oxidation have been developed to treat dye wastewater.6-7 However, these conventional treatments mainly aimed to remove dyes and other contaminants as wastes. To further improve the sustainability of dyeing industry and reduce costs, dyes in the wastewater should be recycled and reused as resources rather than wastes.8-9 Membrane-based separation process, especially nanofiltration (NF) is an ideal candidate for this purpose to separate and recover dyes from others (mainly monovalent salt) in the wastewater due to high selectivity and a facile long-term and scalable operation process.10-11 NF membrane with a narrow pore distribution (0.5-2.0 nm) provides an excellent tool to recycle various dyes and multivalent salts from dyeing wastewater.9, 12 A typical NF membrane, the state-of-the-art thin film composite (TFC) membrane consists of a polyamide (PA) selective layer onto a porous sublayer.13 This thin PA top layer plays a crucial role in membrane performances including both water flux and selectivity, whereas porous substrates generally act as a mechanical support. Although TFC NF membranes have been commercialized and have widespread applications in water treatment, they still faced with the challenges in practical use because of their low selectivity and an intrinsic permeability-selectivity trade-off. In the past decades, extensive efforts have been focused on optimizing the structure of the PA layer to improve the perm-selectivity of the TFC membranes by tailoring the synthesis parameters of interfacial polymerization (IP), such as reaction time and temperature, concentration and diffusion rate of monomers, as well as cross-linking temperature.14-16 For example, Khorshidi et al. successfully developed a high-flux TFC NF membrane via tailoring the temperature of IP reaction.13 A thinner and smoother PA layer could be formed on the surface of a polyethersulfone (PES) support when synthesized at sub-zero temperature (-20°C), and this membrane exhibited up to 9-fold higher water flux than the membrane prepared at room


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temperature. Zhang et al. attempted to add polyvinyl alcohol into the aqueous phase to reduce the diffusion rate of the activator (piperazine molecule) during an IP process, thereby creating a new type of PA layer with bubble or tube structures for a large surface area, which was beneficial to enhance the water permeability without sacrificing its salt rejection property.17 It was well known that the structure and surface property of porous supports also significantly influenced the formation and performance of the PA layer for TFC membranes.1819

More recently, Livingston et al. fabricated a 10 nm thick PA film via the IP reaction onto a

porous interlayer consisting of cadmium hydroxide nanostrands that were covered on a UF membrane.20 The presence of this nanostrand layer with a uniform and smooth surface facilitated the formation of an ultrathin and defect-free PA top layer, dramatically improving solvent flux of membranes. Meanwhile, Xu and co-workers attempted to use the cellulose nanocrystal as an interlayer to fabricate a TFC NF membrane.21 This hydrophilic interlayer could store diamine solution and retard IP reaction, leading to the formation of a PA layer with low cross-linking degree. The resulting NF membrane showed an amazing water flux up to 34 L m-2 h-1 bar-1 and a Na2SO4 rejection of above 97%. Subsequently, Y. Tang et al. deposited a tannic acid-Fe nanoscaffold onto a polysulfone substrate to prepare a high performance TFC NF membrane.22 This nanoscaffold interlayer with a smaller surface pore size not only avoided the intrusion of PA into the substrate pores but also minimized the formation of defects in PA layer because of its smoother surface. Moreover, Jin et al. and our research group proposed the construction of a carbon nanotube (CNT) interlayer on porous substrates to fabricate highperformance PA membranes, which were applied to NF and forward osmosis (FO) processes, respectively, and both membranes exhibited an excellent water flux and comparable salt rejection23-24. However, the above works did not investigate how the changes in structure, morphology and property of the interlayers would affect the formation and transport properties of PA, which was still lack of a comprehensive understanding. In other words, no systemic study has been reported to investigate and understand the role of the interlayer for tailoring the structure and properties of PA. Therefore, in this work, we systematically investigated the impact of the interlayer on the formation of polyamide via IP. We first deposited a CNT thin layer onto a porous PES substrate as an interlayer, and then an ultrathin and defect-free PA active layer was synthesized on top of



the CNT interlayer via IP. Separation performance of the prepared PA TFC membranes was evaluated using a series of dyes and salt solutions via a NF process. Meanwhile, the effect of the changes in structure and morphology (pore size, thickness and surface roughness) of the CNT interlayers and their interfacial properties on the formation of PA active layer and transport performance were further investigated. Finally, by tuning and optimizing the structure and property of the CNT interlayer, we fabricated a high-performance TFC NF membrane with high selectivity in both the dye/NaCl and mono-/divalent ion, which showed a great promise for applications in both wastewater treatment and recycling of dyes.

2. EXPERIMENTTAL SECTION 2.1. Materials Microporous polyethersulfone (PES) microfiltration (MF) membranes (pore size: 0.22 μm) were supplied by YiBo Co. Ltd. (China). Single-walled carbon nanotube (CNT, diameter: < 2 nm, length: 5–30 μm, purity: > 95%) and Piperazine (PIP, 99%, anhydrous) were purchased from Aladdin Chemical Co. Ltd (China). Trimesoyl chloride (TMC, 99%) was obtained from Tokyo Chemical Industry. The other chemicals including 3-Hydroxytyramine hydrochloride, HCl-tris, and sodium dodecylbenzenesulfonate (SDS) were purchased from J＆K Chemical Co. Ltd (China). n-Hexane (99%), two cationic dyes including methylene blue (MB, MW=374 g/mol) and crystal violet (CV, MW=408 g/mol), and four anionic dyes including methyl orange (MO, MW=327 g/mol), methyl violet (MV, MW=394 g/mol), acid fuchsin (AF, MW=586 g/mol) and Congo red (CR, MW=697 g/mol), and several inorganic salts (NaCl, MgCl2, Na2SO4, MgSO4) were obtained from Kermel Chemical Reagent Co. Ltd. All chemicals were used as received.

2.2. Fabrication of the TFC NF Membrane Polydopamine-modified CNT (PDA-CNT) dispersion was prepared following our reported procedure.24 Briefly, CNT (10 mg) and SDS (100 mg) were added to DI water (100 mL) and then sonicated at 270 W for 10 h, followed by 1 h centrifuge at 10,000 rpm to remove undispersed CNT. Upon the addition of dopamine hydrochloride (10 mg) and 0.1 m HCl–Tris


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(10 mL, pH=7.5) into the CNT dispersion, the mixed solution was stirred and reacted for more than 12 h at 40℃. Finally, the homogenous PDA/CNT dispersion was obtained after 30 min centrifugation at 10,000 rpm. The CNT interlayer was prepared by the vacuum filtration of a certain amount of PDA-CNT dispersion (0.5, 1.0, 2.0 and 3.0 mL) onto a PES MF membrane (effective surface area: 12.56 cm2) as shown in Figure 1. After that, the polyamide (PA) active layer was fabricated on the surface of this CNT interlayer covered PES (PES-CNT) substrate via interfacial polymerization (IP). First, the PES-CNT substrate was placed onto the glass plate, and then soaked in a certain amount of 0.05 wt% PIP aqueous solution for 30 s. Excess PIP solution was then removed from the PES-CNT substrate surface utilizing a rubber roller. Next, 0.02 wt% TMC hexane solution was introduced to the surface of the PES-CNT substrate for 30 s. After draining excess TMC solution, the obtained membrane was cured at 65°C for 5 min. After that, the TFC NF membrane of PES-CNT supported PA (PES-CNT-PA) was stored in DI water at 4°C for further use and characterization.

Figure 1. A schematic illustration of the fabrication process of a PES-CNT-PA nanofiltration membrane

2.3. Characterization and measurements The structures and morphologies of this TFC NF membrane were measured by field-emission scanning electron microscopy (Gemini SEM500, Japan) with an acceleration voltage of 10kV. The membrane surface topography was characterized using an atom force microscopy (AFM, Agilent-S5500, USA) under a tapping mode at atmospheric condition. The elemental composition of PA layer was analyzed by an X-ray photoelectron spectrometer (XPS, Thermofisher, K-alpha, USA) with Al Kα excitation radiation (1486.6 eV). Water contact angles of membrane surfaces were detected using a contact angle meter (Kruss, DAS25,



Germany) at room temperature. Surface zeta potential of the TFC NF membrane was measured by an electro-kinetic analyzer (SurPASS, Anton Paar, Austria) under 0.4 MPa operation pressure with KCl (1 mmol/L) solution as an electrolyte solution, and the pH dependence of membrane surface zeta potential was measured by adding HCl or NaOH solutions to titrate the pH of electrolyte solution.

2.4. Nanofiltration performance test NF experiments were conducted using a typical NF testing apparatus with a cross-flow permeation cell (effective membrane surface area: 0.002205 m2), as shown in Figure S1 in the Supporting Information (SI). Series of dye and inorganic salt solutions with a concentration of 0.1 g/L and 2000 ppm were used as feed solutions, respectively. The NF membrane was precompacted with DI water at 6 bar and 25°C for 1 h to achieve a steady flux before the filtration experiments. All the NF tests were carried out 25°C with a pressure of 5 bar. The pH of both the salt and dye solutions for separation was neutral (≈7). The permeate flux (J) and solute rejection (R) of the NF membrane were calculated by equation (1) and (2), respectively: 𝑉

(1)

𝐽 = 𝐴 × ∆𝑡

where V is the volume of the permeated solution (L); A is the effective filtration area (m2); ∆t is the permeation time (h).

(

𝐶𝑃

)

𝑅 = 1 ― 𝐶𝑓 × 100%

(2)

where Cp and Cf are the concentrations of the permeate and feed solution, respectively. The concentration of dye in the permeate solution was measured using UV-Vis (Shimadzu, UV2700, Japan) and the salt concentration in the permeate solution was detected using the calibrated conductivity meter (Thermo, Eutech CON2700, USA). The concentrations of mono/divalent ion in the permeate solution were determined using ion chromatography (ICS2100, Thermo Fisher, USA). Selectivity of solute A to solute B, α, was calculated by the equation (3). 100 ― 𝑅𝐴

α = 100 ― 𝑅𝐵

(3)

where RA and RB represent the rejection of solute A and solute B, respectively. Molecular weight cutoff (MWCO) measurements were conducted using 1000 ppm PEG solutions with various molecular weights (200, 400, 600, 800 and 1000 Da). The concentration of PEG in the


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permeate solution was measured using TOC (Shimadzu, TOC-L, CPH, Japan).

3. RESULTS AND DISCUSSIONS 3.1 The surface and structure properties of CNT interlayer on the PES support The polydopamine-modified carbon nanotube (CNT) interlayer was deposited on the porous PES support through adopting the reported method of vacuum filtration23. The impact of CNT deposition on the surface morphology and properties of the PES membrane was investigated since it would play a crucial role of affecting the formation of polyamide (PA) active layer via interfacial polymerization and thus the properties of TFC NF membrane. Figure 2 shows the SEM images of the structure and morphology of CNT interlayer with various thickness on the PES supports. The surface pores of the PES support were partially covered with the increasing amount of CNT deposition and then totally disappeared when the deposition amount of CNT was 1 mL (PES-CNT-2), indicating a continuous and dense-compact CNT network was formed on the surface of the PES support. As Table 1 shows, with an increase of CNT deposition amount from 0.5 mL to 3 mL, the thickness of CNT interlayer on the PES support gradually increased from 63 nm to 248 nm, and its surface pore size decreased from 87.5 nm to 17.2 nm in diameter, with the decreased surface roughness from 29.8 to 16.7 nm. All these results illustrate that the CNT interlayer achieves a smoother and denser surface with more CNT deposition. Additionally, the PES-CNT membranes increased their water contact angles from 25.6±0.7° to 51.4±0.9° with the increasing deposition amount of CNT. This might because the CNT became densely packed and the pore size of the CNT network turned small with the increasing deposition amount of CNT, leading to an increase in water contact angle. These results strongly prove that the presence of CNT interlayer significantly changed the surface morphology of PES supports, and the deposition amount of CNT affected the compact of CNT network, thus reducing the surface pore size and surface roughness of the support.



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Figure 2. Surface and cross-sectional SEM images of the PES MF membrane, the CNTcovered PES membranes (PES-CNT) with various thickness of CNT interlayer (PES-CNT-1~4 refer to the deposition volume of CNT dispersion from 0.5, 1, 2 to 3 mL)

The pure water fluxes of the PES-CNT membranes were measured to see a significant decrease with the increasing thickness of CNT as Table 1 shows. Compared with the pristine PES support (pure water flux: 10,727.3 L m-2 h-1 bar-1), the pure water flux of the PES-CNT membrane decreased by 2~30 fold from 4,896.7 L m-2 h-1 bar-1to 350 L m-2 h-1 bar-1with the increasing deposition volume of CNT from 0.5 to 3 ml. This is mainly because the water transport resistance of the PES-CNT membrane increased from the decreased pore size and increased thickness of CNT interlayer with the increasing deposition amount of CNT. Notably, the PES-CNT membranes have a comparable water flux with the widely used ultrafiltration membranes (approximately 200–1500 L m-2 h-1 bar-1) as a TFC NF membrane support.25

Table 1. Structure parameters and pure water flux of the pristine PES membrane and the PESCNT membrane. CNT dispersion

Thickness of

Surface

Average pore

Water contact

Pure water flux

volume [mL]

CNT layer [nm]

Roughness [nm]

sizea [nm]

angle [°]

[L m-2 h-1 bar-1]

PES-CNT-0

0

—

78.3±1.2

234.50

20.35±0.9

10727.32±484

PES-CNT-1

0.5

63±3

29.8±0.3

87.50

25.6±0.7

4896.65±155

PES-CNT-2

1

84±2

27.5±0.2

48.91

30.5±1.0

1732.95±66

PES-CNT-3

2

154±5

18.7±0.6

29.36

39.1±1.1

750.99±65

PES-CNT-4

3

248±6

16.7±0.4

17.22

51.4±0.9

349.95±39

Samples

a: The membrane pore size was measured using a Capillary Flow Porometer in Figure S2 (Porolux 1000, IB-FT GmbH Berlin,


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German), and the experimental error of each measurement was less than 5%.

3.2 The impact of CNT interlayer on the formation of PA active layer PA active layer was fabricated on the PES-CNT support membrane with various CNT interlayer thickness via interfacial polymerization of PIP and TMC. The surface morphologies and structure properties of the formed PA were characterized intensively. Figure 3 shows the surface and cross-sectional SEM images of the TFC NF membrane without a CNT interlayer (TFC-0) and with various CNT thickness (TFC-1~4), and also the back surface SEM images of the CNT-PA films upon the removal of the PES support from the TFC NF membrane via solvent etching in DMAc. Very thin and smooth surfaces of PA from the TFC NF membranes were observed and the underlying CNT network or PES pores were faintly visible from their SEM images (top line) in Figure 3. Cross-sectional images of the TFC NF membrane clearly present the triple-layer structure of TFC NF membrane consisting of PA top layer, CNT interlayer and PES support. The thinner PA was formed on the CNT interlayer (Table 2) because the denser CNT interlayer prevented PIP solution from penetrating into the porous PES support through its smaller pore size during the IP process. This inference could be further confirmed by the SEM images of the back surface of the PA film as shown in Figure 3 (bottom line). Numerous granular particles could be observed on the back surface of the PA film formed on the PES support (TFC-0) without a CNT interlayer. This is because the meniscus of PIP aqueous solution was concave in the large pores of the PES support and polyamide was formed inside the PES pores, which was in agreement with previous results in literature.26 Less granular features were observed at the back surface of the PA film with the increasing thickness of the CNT interlayer. When the CNT interlayer reached to about 100 nm thick, no granular feature was observed, indicating no polyamide was formed on the back surface the CNT interlayer. All these results present that the thick CNT interlayer with small pores blocked the penetration of PIP aqueous solution into the PES support due to capillary force and thus restrained the intrusion of polyamide into the PES support, as shown in Figure 4.



Figure 3. Surface and cross-sectional SEM images of the TFC NF membranes with different thickness of CNT interlayer, and the back surface SEM images of the CNT-PA films released from their corresponding TFC NF membranes by dissolving the PES support in DMAc. (TFC1~4 refer to the filtrated volume of CNT dispersion from 0.5, 1, 2 to 3 mL)

Figure 4. Schematic diagram of TFC PA membrane with and without a CNT interlayer (top) and the cross-sectional SEM image of the PES-CNT-PA membranes (bottom).

Table 2 summarizes the structure parameters and pore sizes of PA active layer formed on PESCNT supports with different thickness of CNT interlayer. Results clearly present that all of PA formed on various CNT interlayers had similar low surface roughness (21~25 nm) as the PA


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formed on the PES support (TFC-0), indicating the surface topography of PA was not associated with the CNT interlayer. Importantly, the cross-linking degree of PA increased obviously with an increase of CNT interlayer thickness, and, as a result, the pore size and MWCO of these TFC NF membranes decreased, which were expected to increase their selectivity. This was probably because the generated heat from interfacial polymerization between PIP and TMC was not able to be dissipated immediately in the CNT interlayer with dense packing and small pore size, and led to an increase in the temperature in the interface at this instant, which further intensified the reaction and thereby increased the cross-linking degree of PA layer.27 Meanwhile, with increasing the cross-linking degree of PA layer, less hydrophilic carboxyl groups were left on the highly cross-linked PA, which may increase dynamic water contact angle of these TFC membranes. Among them, the TFC-4 membrane showed the largest water contact angle due to less carboxyl groups from its relatively high cross-linking degree of PA active layer (see Figure S3 in the SI), and the TFC-0 membrane presented the lowest water contact angle because of fruitful carboxyl groups from its relatively low cross-linking degree of PA active layer. Moreover, compared to TFC-1 and TFC-2 membranes, it is worth noting that TFC-0 membrane (without CNT interlayer) seems to show a lower MWCO value and smaller pore size, which is similar to those of TFC-3 membrane. As shown in Figure S6, however, PEG rejection of TFC-0 membrane is the lowest among all series of TFC membranes when the molecular weight of PEG is larger than 400 g/mol. This might because the formation of defects or pinholes in the PA layer deposited on microfiltration membrane surface. This provides evidence that it is difficult to form ultrathin and defect-free active layer on macropore microfiltration membrane supports with a rough surface. These results further suggest the presence of a CNT interlayer with a small pore size and smooth surface plays a crucial role in avoiding the formation of defects in the PA active layer.



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Table 2. Surface structural properties of PA active layer of TFC NF membranes with various thickness of CNT interlayer. Samples

CNT layer thickness [nm]

PA layer thickness [nm]a

Degree of Network Cross-linking of PA b [%]

PA Roughness c [nm]

MWCO d [Da]

TFC-0

—

53±5

28.14

25.9±0.8

345

0.433

TFC-1

63±3

27±2

31.66

26.5±1.2

370

0.450

TFC-2

84±2

29±2

34.38

21.6±0.6

353

0.439

TFC-3

154±5

32±3

42.60

22.0±0.8

337

0.428

TFC-4

248±6

33±3

47.21

24.0±2.5

317

0.414

Pore radius [nm]e

a: The thickness of PA layer was obtained according to the AMF method (a detailed description in Figure S4 in the SI). b: The cross-linking degree of PA was calculated from XPS results (see Table S1 in the SI). c: The polyamide surface roughness of TFC NF membranes was measured by AFM (see Figure S5 in the SI). d: MWCOs of TFC NF membranes were calculated from Figure S6 in the SI. e: Stokes radius (ds) of TFC NF membrane were calculated on the basis of its molecular weight cut-off28: 𝑑𝑠 = 16.73 × 10 ―12 × M0.557 PEG

(4)

Where ds and MPEG is the stokes radius and the molecular weight of PEG solutes, respectively.

Figure 5 illustrates the surface zeta potential of PA active layers of TFC NF membranes with (TFC-1~4) and without (TFC-0) a CNT interlayer as a function of pH. The PA layer from the TFC-0 membrane presented negative charges over the entire pH ranging from 3 to 10, and its surface zeta potential markedly decreased from the increasing deprotonation of carboxylic acid groups with the increasing pH. Interestingly, the addition of CNT interlayer onto PES support significantly affects the surface zeta potential value of PA layer, which changes gradually from negative to positive with an increase in the thickness of the CNT interlayer. Among them, TFC2 membrane exhibited an isoelectric point (IEP) at acidic pH value of approximately 5.5, which is basically in agreement with the other literature23. Above the IEP, TFC-2 also showed negatively charged character, but the PA from the TFC-2 membrane had less negative charge than that of PA from the TFC-0 membrane, mainly because of the less carboxylic acid groups from the more highly cross-linked PA on the CNT interlayer. This result suggests that the presence of CNT interlayer affects the zeta potential of PA layer to some extent, which is of great importance for adjusting the rejection behavior of monovalent or multivalent ions.29


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Figure 5. Polyamide surface zeta potential of TFC-0~4 membranes.

3.3 Nanofiltration performance of the fabricated TFC polyamide membranes The permeate fluxes and solute rejections of the TFC NF membranes for different dye or salt solutions were evaluated using a cross-flow membrane module, and the results are shown in Figure 6. Figure 6A shows the water flux and methyl orange (MO) rejection of the TFC NF membranes with various CNT thickness. Obviously, all the as-prepared TFC NF membranes with the CNT interlayer (TFC-1~4) showed the higher permeate flux than the one without CNT interlayer (TFC-0). This was because the CNT interlayer prevented the intrusion of polyamide into the PES substrate, resulting in the formation of a thinner PA layer as shown in Figure 4, which gave rise to reduce the transport resistance of membranes.18,

30

Moreover, the MO

rejection of these TFC NF membranes increased slightly with increasing the CNT interlayer thickness, mainly because of the decreasing pore size of membranes from their increasing crosslinking degree of PA active layers gradually with increasing the CNT thickness (Table 2). For the TFC-2~4 membranes, it is due to the slowly reduced pore size, their water fluxes decreased gradually. Compared to TFC-2 membrane, however, TFC-1 membrane showed a relatively low water flux. This is because some large pores of PES support surface were not fully covered by the CNT interlayer due to a limited deposition amount of CNT (from SEM images of the PES-NCT-1 sample in Figure 2) and some granular structures of PA were formed inside the PES support (Figure 3) for the increased transport resistance of the TFC-1 membrane. Therefore, the water fluxes of these TFC membranes (TFC-0~4) increased and then decreased



with an increase in the thickness of the CNT interlayer. As a result, TFC-2 membrane exhibited the highest permeate flux of 17.4 L m-2 h-1 bar-1 and a high MO rejection of 91.5%. Subsequently, the permeate fluxes and dye rejections of the TFC-2 membrane for various dye (different molecular weight and electric charge) solutions were investigated, and the results were shown in Figure 6B. TFC-2 membrane showed similar, stable and high permeate fluxes (>16L m-2 h-1 bar-1) for almost all the dye solutions, but different rejection behavior. This membrane exhibited a relatively high rejection (> 98%) for dyes with larger molecular weight (MW) such as CV, AF and CR, the MWs of which are larger than the MWCO value (365 Da) of the TFC-2 membrane, indicating that the size exclusion effect dominates the rejection behavior of TFC-2 membrane for these dyes in the NF process.30 It should be noted that a great rejection difference was observed for the positively charged MB and negatively charged MV, despite both having the similar MWs. This could be explained by the donnan effect.31 TFC-2 membrane had a negatively charged membrane surface, which led to a higher rejection (99.5%) for the anionic dye (MV) due to electrostatic repulsions but a lower rejection (86.4%) for cationic dye (MB) because of electrostatic attraction. Therefore, TFC-2 membrane showed a relatively high rejection (92.5%) for the negatively charged MO, even though its MW is lower than the MWCO of this membrane.


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Figure 6. The permeate flux and rejection rate of the TFC NF membranes with various CNT thickness: (A) the methyl orange solution was used as a feed solution; (B) TFC-2 membrane was selected to test for the rejection of different organic dyes and diverse inorganic salt solutions (C); long-term performance of the TFC-2 membrane for the separation of a dye/salt mixture solution having 2000 ppm NaCl and 0.1 g/L MV (D). All measurements were tested in the cross-flow NF set-up with the tested effective membrane surface area of 22.05 cm2 at 25°C under the pressure of 5 bar, and the experimental error of each measurement was less than 3%.

To further explore the separation properties of membranes, the NF performances of the TFC2 membranes for different salt (monovalent and divalent ions) solutions were examined, and the results were presented in Figure 6C. TFC-2 membrane showed the salt rejection sequence as follows: Na2SO4 (98.5%) ≈ MgSO4 (98.3%) > MgCl2 (93.2%) > NaCl (18.8%). The high rejection of the divalent ions (SO42- and Mg2+) is possible due to the combination of the sieving effect and the donnan effect32-33. The hydrated ionic radius of SO42- and Mg2+ is larger than that of Cl- and Na+, thus divalent ions would more likely suffer from a greater transfer resistance than monovalent ions because of steric hindrance effect. Meanwhile, donnan effect also plays an important role in the rejection behavior. The negatively charged membrane surface endowed stronger electrostatic repulsion to SO42- than that to Cl-, leading to a higher Na2SO4 and MgSO4 rejection. Interestingly, this membrane showed a low NaCl rejection of 18.8%, this could have been caused by the weak negative charge of membrane surface as confirmed by surface zeta potential test. This weakening effect was more pronounced for the monovalent ion (NaCl) than divalent ions with a larger size (Na2SO4, MgSO4, MgCl2). Therefore, the above results suggested that TFC-2 membrane had a high ion selectivity between monovalent and divalent ions, or to monovalent salt ion and various dye molecules. Figure 6D showed the long-term time course for the NF performances of TFC-2 membrane for a dye/salt (MV/NaCl) mixture solution. This membrane showed a high and constant MV rejection (around 99.4%) and a relatively low NaCl rejection (about 24.7%) during this continuous NF process. Permeate flux of membrane imperceptibly decreased from 17.6 to16.7 L m-2 h-1 bar-1after 24 h. These results suggested the TFC-2 membrane had a good NF performance and long-term stability, as well as a high selectivity to dye (MV) and NaCl.



To examine the mono-/divalent ion selectivity of the TFC-2 membrane, the NF experiment was carried out using two mixed-salt solutions including NaCl/Na2SO4 and NaCl/MgCl2 mixture solutions, and the results were presented in Figure 7. The water permeability and the mono-/divalent ion selectivity of the TFC-2 membrane and other NF membranes were plotted in Figure 7A. Compared with most of other membranes, TFC-2 membrane showed an excellent selectivity for the Cl-/SO42- ion pair, which is as high as 85.5, as well as the highest water permeability for the NaCl/Na2SO4 mixture solution. Meanwhile, this membrane also exhibited the highest water permeability and a competitive selectivity for the Na+/Mg2+ ion pair. Both high mono-/divalent ion selectivity and water permeability are comparable to that of NF membranes reported in the literature.34 Figure 7B showed the water permeability and the dye/salt selectivity of the TFC-2 membrane and other commercial and lab-made membranes.35 Compared with other NF membranes, TFC-2 membrane showed excellent separation performances in both water permeability (17 L m-2 h-1 bar-1) and NaCl/MV selectivity (123.5) for the dye/NaCl mixture solution. On the other hand, TFC-2 membrane with a CNT interlayer exhibited much better separation performances in both water permeability and selectivity (including mono-/divalent ion and dye/salt selectivity) than the original TFC-0 membrane without a CNT interlayer (see Tables S2 and S3 in the SI), suggesting its great potential for the purification and recycling of dyes from the diverse dyes/NaCl mixture solutions, which is a big challenge to recover dyes from salt solutions in dye manufactures and textile dying industry.

Figure 7. The trade-off of the NF membrane towards mono-divalent selectivity vs water permeability (A) and towards NaCl/dye selectivity vs water permeability (B) from this work and literatures (references are listed in Table S2 and S3 in the Supporting Information, and the


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selected NF membrane was TFC-2 from this work)

4. CONCLUSIONS A triple-layered TFC nanofiltration (NF) membrane consisting of a polyamide (PA) active layer, a CNT interlayer and a PES porous support layer was fabricated via interfacial polymerization. The structure and properties of PA active layer can be tuned by tailoring the properties of CNT interlayer, including its thickness and surface pore size, thereby achieving a high nanofiltration performance. This TFC NF membrane exhibited a high divalent salt rejection (the rejection of Na2SO4 and MgSO4 solution >98.3%) and dye rejection (the rejection of methyl violet (MV) > 99.5%) with a high pure water flux of around 21 L m-2 h-1 bar-1. Meanwhile, they also showed excellent selectivity to both mono-/divalent salt ion (the selectivity of Cl-/SO42- is as high as 85.5) and NaCl/dye solution (the selectivity of NaCl/MV is more than 123.5), which are much higher than most of other reported NF membranes. Moreover, this TFC NF membrane also showed a good separation performance and long-term stability during a continuous NF process for a salt/dye mixture solution lasting more than 24 h, suggesting its great potential for the purification and recycling of dyes from diverse dye/NaCl mixture solutions.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors gratefully acknowledge the funding support from National Natural Science Foundation of China (No. 21476249, No. 51708408), Chang-jiang Scholars and Innovative Research Team in the University of Ministry of Education, China (No. IRT17R80) and Program for Innovative Research Team in University of Tianjin (No. TD13-5044) and the Science and Technology Plans of Tianjin (No. 17PTSYJC00060 and 18PTSYJC00170) and the Natural Science Foundation of Tianjin (No. 18JCYBJC43300).

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Table of Contents Graphic


New Insights into the Role of an Interlayer for the Fabrication of Highly

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