Ultrathin Polyamide Nanofiltration Membrane Fabricated on Brush

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Ultrathin Polyamide Nanofiltration Membrane Fabricated on BrushPainted Single-Walled Carbon Nanotube Network Support for Ion Sieving Shoujian Gao, Yuzhang Zhu, Yuqiong Gong, Zhenyi Wang, Wangxi Fang, and Jian Jin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09761 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Ultrathin Polyamide Nanofiltration Membrane Fabricated on Brush-Painted Single-Walled Carbon Nanotube Network Support for Ion Sieving

Shoujian Gao† ‡, Yuzhang Zhu‡, Yuqiong Gong§, Zhenyi Wang† ‡, Wangxi Fang† ‡ *, and Jian Jin† ‡ § *

†

School of Nano Technology and Nano Bionics, University of Science and Technology

of China, Hefei, 230026, China. ‡ i-Lab

and CAS Key Laboratory of Nano-Bio Interface, CAS Center for Excellence in

Nanoscience, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, 215123, China.

ACS Paragon Plus Environment 1

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§ College

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of Chemistry, Chemical Engineering and Materials Science, Soochow

University, Suzhou, 215123, China.

KEYWORDS: ultrathin polyamide membrane, ion sieving, high flux, single-walled carbon nanotube, brush painting.

ABSTRACT: Recently, ultrathin polyamide nanofiltration membranes fabricated on nanomaterial-based supports overcome the limitation of conventional supports and shows greatly improved separation performance. However, the feasibility of the nanomaterial-based supports for large-scale fabrication of the ultrathin polyamide membrane is still unclear. Herein, we report a controllable and saleable fabrication technique for single-walled carbon nanotube (SWCNT) network support via brush painting. The mechanical and chemical stability of the SWCNT network support were carefully examined, and an ultrathin polyamide membrane with thickness of ~15 nm was successfully fabricated based on such a support. The obtained thin-film composite (TFC) polyamide nanofiltration membranes exhibited extremely high water permeability of 40 L m–2 h–1 bar–1, high Na2SO4 rejection of 96.5%, high monovalent/divalent ion


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permeation selectivity, and maintained highly efficient ion sieving throughout a 48-hour testing. This work demonstrates a practical route towards the controllable large-scale fabrication of the TFC membrane with SWCNT network support for ion and molecule sieving. This work is also expected to boost the mass production and practical applications of the state-of-the-art membranes composed of one-dimensional and twodimensional nanomaterials, as well as the nanomaterial-supported TFC membranes.


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Most commercially available nanofiltration (NF) and reverse osmosis (RO) membranes possess a thin-film composite (TFC) structure: a dense polyamide selective skin layer and an underlying ultrafiltration support membrane. Despite the commercial success of TFC technique for NF/RO membrane production, overcoming the longstanding “tradeoff” relationship between permeability and selectivity of TFC membranes remains a challenge.1-3 Since the separation properties of TFC membranes are governed by the polyamide layer, extensive research efforts have been put on altering the layer formation chemistry or intrinsic nanostructure to simultaneously improve its permeability and selectivity.4-11 An ultrathin and defect-free polyamide layer is ideal to ensure highly efficient ion and molecule sieving with minimal water transport resistance. The formation of such a high-quality ultrathin polyamide layer requires a support membrane with carefully crafted pore structure and surface properties. However, the critical role of the porous support membrane on the formation of ultrathin polyamide layer was often overlooked. Conventional ultrafiltration membranes exhibit good mechanical stability and feasibility for large scale production, but their rough and low-porosity surfaces make it difficult to further reduce the polyamide layer thickness without generating defects.12-15


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Recently, several studies explored the formation of polyamide selective layer on porous supports constructed by nanomaterials, such as cadmium hydroxide nanostrand,16 carbon nanotube,17-19 cellulose nanocrystal,20 nanoscaffold,21 etc. For instance, Livingston et al. introduced a support layer of cadmium hydroxide nanostrand, and the subsequent interfacial polymerization gave rise to the formation of a polyamide layer with thickness below 10 nm.16 In our earlier studies, single-walled carbon nanotube (SWCNT) was adopted as the support layer to substitute the chemically unstable and toxic cadmium hydroxide nanostrand, and a polyamide layer with thickness of ~12 nm was successfully fabricated.17 Water permeability of the obtained NF membrane was greatly improved with divalent ion rejection above 95%.17,19 The high porosity, uniform small pore size and smooth surface of the nanomaterial-based supports enable homogeneous aqueous solution distribution and homogeneous monomer diffusion on the support surface during interfacial polymerization, and thus enable precise control of the polyamide layer formation.19 Such nanomaterial-based supports overcome the limitation of conventional support membranes, and thus provide


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a strategy on the formation of ultrathin and defect-free polyamide layer with very high separation performance. However, the mechanical and chemical stability of the nanomaterial-based supports, the influence of their characteristics on the formed polyamide layer, as well as the interaction with both top polyamide layer and underlying substrate have not been systematically investigated. More importantly, the feasibility of the nanomaterial-based supports for large-scale fabrication of the TFC membrane is still unclear, as most of the reported nanomaterial-based supports are prepared by a vacuum filtration method or a interfacial self-assembly method,16,17,19-22 and such preparation methods result in relatively small membrane area. The large-scale and highly controllable fabrication of the nanomaterial-based supports is the key of achieving the controllable mass production of the state-of-the-art nanomaterial-supported TFC membranes, as well as their practical use in water desalination, wastewater treatment, separation and purification in food and pharmaceutical industries, etc. Alternatively, ultrathin freestanding polyamide nanofilm can also be fabricated via interfacial polymerization at an aqueous-organic interface and transferred onto porous support as selective layer for


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desalination.23 Nevertheless, the difficulty in obtaining a large stable aqueous-organic interface and the process complexity of handling the free-standing nanofilm afterwards make the ultrathin polyamide nanofilm infeasible for fabrication scaling-up and practical applications. Herein, we report a scalable fabrication technique for SWCNT network membrane, where the membrane was fabricated via brush-painting a SWCNT dispersion onto a polymer microfiltration support. This traditional coating technique achieved simple and highly controllable fabrication of SWCNT network membrane at a much greater scale. The mechanical and chemical stability of the membrane were carefully examined. An ultrathin and defect-free polyamide layer was successfully fabricated with the SWCNT network membrane as the support, and exhibited very good performance for nanofiltration and ion sieving. The influence of the SWCNT network characteristics on formed polyamide layer, as well as its interaction with both the top polyamide layer and the underlying microfiltration substrate were also investigated. Results and Discussion


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Fabrication and characterization of SWCNT network membrane. As schematically shown in Figure 1a, the SWCNT network membrane was fabricated via painting a SWCNT dispersion on a polyethersulfone microfiltration (PES MF) membrane using a goat hair brush. Through the unidirectional movement of brush, the SWCNT dispersion stored in the goat hair matrix was transferred onto the surface of PES MF membrane to cover the whole membrane surface uniformly. The random deposition of SWCNTs during the SWCNT dispersion transfer process gave rise to the formation of the interpenetrating network structure of SWCNTs. The brush-painting was then repeated in the orthogonal direction on the PES MF membrane, so as to improve the uniformity of the formed SWCNT network structure. The two-step painting in two orthogonal directions formed one painting cycle, and the SWCNT network structure is stabilized after several painting cycles plus the sequential drying process. Figure 1b is the photograph of a SWCNT network membrane (size: 33  33 cm2) fabricated with 3 painting cycles, showing that the membrane is uniform with a black color. Figure 1c and 1d reveal the cross-sectional and top-view morphology of the membrane. A SWCNT network membrane is clearly observed on the top of PES MF membrane (Figure 1c),


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and the membrane appears a porous surface with interpenetrating network structure and intensively distributed nanometer-scale pores (Figure 1d). According to the atomic force microscopy (AFM) image of the membrane and the corresponding height information, thickness of the SWCNT layer is around 91 nm (Figure S1). Impacts of painting cycles on morphology and permeation flux of the SWCNT network membrane were investigated in detail. As shown in transmission electron microscopy (TEM) images of the SWCNT network membranes fabricated with 1-4 painting cycles (Figure 2a-2d), increased painting cycles leads to more densely packed SWCNT network, but the overall network remained a porous structure. Pore size of the SWCNT network membrane was determined by measuring the maximum diagonal distance of the pores according to the TEM images. Specifically, pore size of the network membranes decreases gradually with increasing the painting cycles (Table 1). As revealed in scanning electron microscopy (SEM) images of the membranes (Figure S2), SWCNTs gradually cover the micrometer-scale surface pores of PES MF membrane with increased painting cycles, and eventually formed a uniform SWCNT network membrane. It is also found that at least 3 painting cycles were applied before


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the surface pores of PES MF membrane were fully covered by SWCNTs, and a SWCNT network membrane was obtained. Moreover, thickness of the SWCNT network layer was obtained from AFM imaging and found to increase with the painting cycles (Table 1). The PES MF membrane exhibits a transient water contact angle (CA) of 30°, after loading the SWCNT network layer, water CA of the composite membrane increases gradually with increasing proportion of SWCNT covered surface. However, the composite membrane behaves hydrophilic all along the 1 to 4 painting cycles and is appropriate to work as the support for interfacial polymerization. Due to the increased layer thickness and decreased pore size, water flux of SWCNT network membrane decreased gradually. The above results indicate that the pore size, thickness and permeation flux of SWCNT network membrane are highly tunable by facilely adjusting the painting cycles. This is an advantage over traditional polymer or ceramic ultrafiltration membranes which usually take complicated measures to alter their morphology and permeation flux. Such an easily processable and scalable fabrication technique makes the brush-painted SWCNT network membrane promising for industrial mass production and practical applications.


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Chemical and mechanical stability of SWCNT network membrane. As the support layer of TFC membrane, its chemical and mechanical stability is of great significance to guarantee the separation performance of TFC membrane in diverse operating conditions and complex environments. Chemical stability of the brush-painted SWCNT network membrane was examined by monitoring its morphology and water flux while keeping the membrane in acidic, neutral, alkaline environments and in organic solvent. Three pieces of SWCNT network membranes fabricated with 3 painting cycles were immersed in water with pH of 1, 7, 13, and another one was immersed in h-hexane. The immersion test lasted for 30 days, and all the four membranes showed no damage and no SWCNT membranes peeled off from the underlying PES MF membrane throughout the test (Figure 3a). SEM images of the four membranes also confirm that the SWCNT network structure is well-kept after the test (Figure S3). Moreover, as shown in Figure 3b, water flux of the four membranes after the immersion test is in the range of 16161650 L m–2 h–1 bar–1, which is similar to that of a fresh membrane before the test (1610 L m–2 h–1 bar–1). These results prove that the SWCNT network membrane has good acid, alkali and organic solvent resistance.


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Mechanical stability of the SWCNT network membrane is evaluated by its resistance to water flushing and external abrasion. For flushing resistance test, water permeation flux of a SWCNT network membrane was recorded during a continuous cross-flow filtration process with an averaged cross-flow velocity of ~0.2 m s-1 for 48 hours. As shown in Figure 3c, the membrane exhibits a stable water flux around 1608 L m–2 h– 1

bar–1 for the whole test period, indicating that the shear force of the high-speed water

flow doesn’t damage the SWCNT network membrane. SEM image and TEM image of the membrane show that the membrane maintains its network structure and pore size after the test (Figure 3d), demonstrating the good stability of SWCNT network membrane towards high-speed flushing of water flow. For abrasion resistance test, a SWCNT network membrane was firstly wetted by water and then manually rolled by a rubber roller, horizontally for 10 times and vertically for 10 times. Moreover, a P2000 sandpaper loading a weight of 20 g was used to scrub the water-wetted SWCNT membrane back and forth for 5 times. As shown in Figure 4a-b and Movie S1-S2, the membrane shows an untouched appearance during the test. The SEM images before and after abrasion test confirm that the abrasion of rubber roller and sandpaper doesn’t


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change the surface morphology of the SWCNT membrane (Figure 4c). Water flux of the membrane before and after the abrasion test are almost the same (Figure 4d), further revealing the stability of SWCNT network membranes against external mechanical abrasion. We believe that the high mechanical strength of SWCNTs and the highly interpenetrating network structure of the membrane together lead to the good mechanical stability of the membrane. Due to high specific surface area of SWCNT network membrane, it can attach to the PES MF membrane firmly especially after drying treatment. Polyamide membrane fabricated on SWCNT network support. The polyamide membrane was fabricated via interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) to form a highly crosslinked active layer. Different from polymer ultrafiltration membranes used in conventional interfacial polymerization processes, SWCNT network membrane with high porosity and uniform small pore size was adopted as the support here. The SWCNT network support helps the formation of an ultrathin and defect-free polyamide membrane through enabling more homogeneous distribution of aqueous phase and thus benefiting PIP monomer diffusion on the support


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surface during the interfacial polymerization process.19 Low concentrations of PIP (0.125%, w/v) and TMC (0.1%, w/v) and a short polymerization time of 30 seconds were adopted in order to achieve ultrathin membrane thickness. As a contrast, a blank PES MF membrane without loading SWCNT network layer was also adopted as the support to fabricate polyamide membrane under the same experimental conditions. Figure 5a-e are the SEM images of the as-fabricated polyamide membranes on PES MF support and on SWCNT network supports with 1-4 painting cycles. The polyamide membrane formed on the PES MF support surface was discontinuous and not covering all surface pores of the support (Figure 5a), but formed inside some internal pores (marked by red arrows). Apparently, the MF membrane failed to provide a flat surface (the surface roughness is 122 nm as shown in Figure S4) with proper pore size for constructing a stable interface between the two immiscible phases, and thus homogeneous monomer interfacial diffusion and reaction did not occur. Contrastively, the polyamide membrane formed on SWCNT network supports became increasingly continuous and complete with the increase of painting cycles. For SWCNT network supports fabricated with 1-2 painting cycles and pore size in the range of 40-75 nm, the


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formed polyamide membrane doesn’t completely cover all the support surface pores (Figure 5b and c), and there are still some defects on the polyamide membranes (marked by blue arrows). For SWCNT network supports fabricated with 3 and 4 painting cycles and pore size in the range of 12-25 nm, a defect-free polyamide membrane is obtained in both cases (Figure 5d and 5e). Moreover, even with the formation of a polyamide membrane on top of the SWCNT network support, the interpenetrating network structure of SWCNTs is clearly seen from the SEM images, especially in the case of 3 SWCNT painting cycles (Figure 5d), indicating that the fabricated polyamide membrane is extremely thin. Nanofiltration performance of the fabricated polyamide membranes was evaluated preliminarily by separating a Na2SO4 solution as shown in Figure 5f (salt concentration: 1000 ppm, applied pressure: 6 bar). The polyamide membrane fabricated on PES MF support gives water permeation flux of 522 ± 20 L m–2 h–1 and Na2SO4 rejection of 22.9 ± 3.3%, indicating very low ion separation performance. As for the polyamide membrane fabricated on SWCNT network supports, the water flux reduces gradually with the increase of SWCNT painting cycles, from 370 ± 15 L m–2 h–1 for 1 painting cycle


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to 279 ± 10 L m–2 h–1 for 2 cycles, 242 ± 8 L m–2 h–1 for 3 cycles and 171 ± 5 L m–2 h–1 for 4 cycles. Meanwhile, Na2SO4 rejection of the polyamide membrane is enhanced with increasing the painting cycles, from 61.9 ± 2% for 1 painting cycle to 84.7 ± 1.5% for 2 painting cycles, 96.5 ± 0.7% for 3 painting cycles and 97 ± 0.5% for 4 painting cycles, respectively. Among all these polyamide membranes, the polyamide membrane fabricated on the 3-painting-cycle SWCNT network support exhibits the best performance in consideration of both water flux and ion rejection. We think it is due to the appropriate pore size and porosity of the 3-painting-cycle SWCNT network support in comparison with other SWCNT supports. We therefore choose the polyamide membrane fabricated on 3-painting-cycle SWCNT network support for the subsequent characterization and performance test in detail. The AFM image of SWCNT network support shows an interpenetrating network structure (Figure 6a). After fabricating the polyamide membrane on the support surface, the outline of SWCNTs is still visible (Figure 6b), indicating an extremely thin thickness of the polyamide membrane. Surface roughness changes from 13.4 nm for the SWCNT network support to 11.2 nm for the polyamide membrane. The TEM images further


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confirm that all the pores of SWCNT network support (insert in Figure 6c) are covered by the polyamide membrane after interfacial polymerization (Figure 6c). The ultrathin polyamide membrane presents a high transparency, and the interpenetrating network structure of SWCNTs is clearly observed in the shown TEM image. To investigate the interfacial structure between SWCNT network support and top polyamide layer, and underlying PES MF support, and as well as to obtain accurate measurement of the polyamide layer thickness, the cross-sectional TEM image of polyamide membrane with SWCNT network support was captured. Before capturing TEM image, the membrane was embedded in an epoxy resin and vertically cut into thin sheets. As shown in Figure 6d, the major portion of the polyamide layer is tightly connected on the SWCNT network support surface to form a bilayer structure, but there are still cavities observed between the polyamide membrane and the support (marked by red arrow). The SWCNT network support is also tightly attached on the underlying PES MF support. The thickness of SWCNT network support layer is measured to be ~90 nm, which is consistent with the thickness obtained from the AFM image (Figure S1). The thickness of polyamide membrane is measured to be as thin as ~15 nm. Additionally, X-ray photoelectron


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spectroscopy (XPS) survey spectrum of the polyamide membrane was also measured to investigate its crosslinking degree. Based on the characteristic peak areas in consideration of the sensitivity factors (Figure S5), the atom percentage of C, O, N is calculated to be 78.28%, 11.75%, 9.97%, respectively. The O/N ratio is 1.18, and the crosslinking degree is calculated to be 75.42%, suggesting a highly crosslinked polyamide layer.16,19,24,25 The high crosslinking degree of a polyamide membrane usually leads to small pore size and high solute rejection. To evaluate the pore size of the ultrathin polyamide membrane, its rejection to polyethylene glycol (PEG) with different molecular weights was measured, and the rejection curve was plotted by nonlinear fitting (Figure 7a). As obtained from the rejection curve, the molecular weight cutoff (MWCO) of the ultrathin polyamide membrane is 397 Da, which is defined as the molecular weight at rejection of 90%. Stokes radius of the PEG molecule was calculated to be 0.468 nm. Based on the rejection curve and a probability density function between rejection and Stokes radius,26,27 a log-normal model of pore radius distribution is obtained as shown in the insert of Figure 7a. It reveals that the mean pore radius of the ultrathin polyamide


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membrane is 0.283 nm with a narrow pore size distribution. In addition to membrane pore size, membrane surface charge is a very important factor affecting the ion sieving performance of a NF membrane, especially for separating ions with different valence. According to the result of surface zeta potential measurement, the ultrathin polyamide membrane shows a negative surface zeta potential when the pH is above 3.1 (Figure 7b). Therefore, the polyamide membrane is negatively charged in neutral condition, which is in favor of the rejection to divalent or multivalent anions via the Donnan effect, whereas the monovalent ions are easier to transport through the membrane.28-30 Concentration gradient-driven ion diffusion across the polyamide membrane. Selective separation between monovalent and multivalent ions is crucial for NF membranes. For potential applications like lithium extraction, high selectivity between ions with different valence assures highly efficient removal of multivalent ions without significant loss of target monovalent ions. Whereas for water softening and desalination pretreatment, low NaCl rejection prevents unnecessary build-up of osmotic pressure difference across the membrane. Therefore, a NF membrane with high ion selectivity and high water permeability would bring about high-quality separation product with low energy


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consumption. The ion diffusion selectivity of the ultrathin polyamide membrane was firstly investigated by the concentration gradient-driven ion diffusion experiments using a homemade U-shaped glass apparatus (Figure S6). The anion and cation diffusion results are illustrated in Figure 8a-b and 8c-d, respectively. With the same concentration (0.1 M) adopted for all salts, various ions were diffused through the membrane under the same concentration gradient, and such diffusion was observed at a nearly constant rate for each salt across the 3-hour experiment. Figure 8a and c show that the diffusion of salt with divalent ions is significantly slower than that with monovalent ions only, and salts with trivalent ions diffused even slower. It is found that the diffusion rate of monovalent ions can be 5 times higher than that of trivalent ions, indicating good ion diffusion selectivity of the membrane. Since the membrane is negatively charged, the diffusion rate difference for anions can be well explained by the Donnan principle, whereas diffusion behavior of cations conflicts with the principle. Instead, the effect of membrane pore size dominates the diffusion of cations. Based on the pore size distribution shown in Figure 7a, most pores of the membrane ranges from 0.2 to 0.4 nm in radius. As shown in Figure 8b and d, all the tested divalent and trivalent cations


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possess hydrated radius larger than 0.4 nm, so that the steric hindrance plays a key role in the reduction of the diffusion rate of multivalent cations, and the decrease of ion permeance is in good accordance with the increase of ion size. Therefore, the combination effect of membrane charge and pore size contributed to good diffusion selectivity of the membrane between monovalent and multivalent ions. Nanofiltration and ion sieving performance of the polyamide membrane. The ion sieving performance of the ultrathin polyamide membrane was examined by separating five salt solutions including Na2SO4, MgSO4, MgCl2, CaCl2 and NaCl in a cross-flow mode nanofiltration testing. The salt concentration was 1000 ppm and the applied pressure was 6 bar. As shown in Figure 9a, the polyamide membrane shows extremely high water flux above 230 L m–2 h–1 when separating all the five salt solutions (242 ± 8 L m– 2

h–1 for Na2SO4, 237 ± 5 L m–2 h–1 for MgSO4, 246 ± 8 L m–2 h–1 for MgCl2, 250 ± 5 L m–

2

h–1 for CaCl2, 265 ± 10 L m–2 h–1 for NaCl). It is worth to note that pure water flux of the

polyamide membrane is as high as 302 ± 10 L m–2 h–1 at this pressure. The high flux is mainly ascribed to the ultrathin thickness of the polyamide membrane. Meanwhile, the polyamide membrane shows very high rejection to Na2SO4 and MgSO4 of 96.5 ± 0.7%


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and 95.3 ± 0.2%. The rejection of MgCl2, CaCl2 and NaCl is 64.9 ± 1.7%, 63.4 ± 1.3% and 13.4 ± 0.8%, which is much lower than that of the sulfate salts. The rejection profile of the polyamide membrane further confirms that the ion sieving mechanism of the membrane is a synergistic effect of size sieving and strong Donnan exclusion.19 Water flux of the polyamide membrane varies basically in direct proportion to the transmembrane pressure in the range of 1 bar to 6 bar (Figure S7). Meanwhile, the membrane

exhibits

nearly

unchanged

Na2SO4

rejection

under

the

different

transmembrane pressures. Compared with a commercial Dow NF270 membrane, our membrane exhibits a nearly 3-fold increase of water flux and similar Na2SO4 rejection. As supplementary optical images shown in Figure S8, the dynamic spreading and permeating behaviors of a water droplet on our membrane and a NF270 membrane also visually reveal the water permeating speed of our membrane is much faster than the NF270 membrane. When further compare our membrane with various NF membranes reported in the literatures (Figure 9b), it can be found that our membrane has faster water permeation than other TFC membranes with various monomers (marked by orange rhombus),31-36 TFC membranes with various interlayers (marked by


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black squares),17,20,21,37-39 thin-film nanocomposite (TFN) membranes (marked by green circles),40-43 2D nanomaterials-constructed laminar membranes (marked by purple triangles),44-47 as well as commercial NF membrane (e.g., NF270), and the Na2SO4 rejection of our membrane is also on the high side. To further demonstrate the ion sieving capability of the ultrathin polyamide membrane, the permeance of different salts across the membrane during the abovementioned pressure-driven NF experiment is illustrated in Figure 9c. The salt permeance is calculated to be around 10 g m-2 h-1 for Na2SO4 and MgSO4, 90 g m-2 h-1 for MgCl2 and CaCl2, and 230 g m-2 h-1 for NaCl, respectively. Based on the salt permeance, the permeation selectivity of the membrane could be 23 times for mono/divalent anions (NaCl to Na2SO4 and MgSO4). The ion transport mechanism through a nanofiltration membrane involves both diffusion and convection.28-30 The concentration gradient-driven ion diffusion experiment only measures the ion transport

via diffusion. During the NF permeation experiment, monovalent ions with smaller size and lower rejection permeate through the membrane along with the convective flow, while larger divalent ions get hindered and transport mainly via diffusion. The


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permeation rate difference between mono/divalent ions is thus enlarged, which contributes to additional enhancement in ion permeation selectivity. The very high water permeability of the nanomaterial-supported TFC membrane also further facilitates the monovalent ion movement and strengthens the membrane ion sieving capability. Figure 9d summarizes the NaCl/MgSO4 permeation selectivity and water permeability of laboratory-prepared (marked by black circles) and commercial (marked by red squares) NF membranes, which is adapted from a recent work by Tang et al.21 They introduced a salt permeation selectivity/water permeability upper bound of conventional NF membranes in the paper, and several TFC membranes with different nanomaterialbased interlayers broke the upper bound (marked by blue circles).20,21,36 It is clearly seen from the figure that our membrane outperforms all the conventional membranes and other TFC membranes with interlayers in terms of both selectivity and permeability. In addition, continuous nanofiltration test was conducted with water flux and Na2SO4 rejection monitored for 48 hours to examine the performance stability of the ultrathin polyamide membrane (Figure S9). The membrane exhibited stable water flux around 240 L m–2 h–1 and nearly unchanged rejection above 96% throughout the whole test,


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demonstrating that the high performance of the ultrathin polyamide membrane is stable over time. Conclusions In summary, we have developed a facile and easily scalable fabrication technique for SWCNT network membrane based on brush-painting a SWCNT dispersion on a polymer MF membrane. The SWCNT network membrane had well-controlled pore size and thickness at a nanometer scale depending on the painting cycles, and was mechanically and chemically robust to be adopted as the support to fabricate ultrathin and defect-free polyamide membrane via interfacial polymerization. A polyamide membrane with thickness of ~15 nm was thus successfully fabricated. The TFC membrane with ultrathin polyamide selective layer and SWCNT network support layer exhibited extremely high water permeability of 40 L m–2 h–1 bar–1, very high Na2SO4 rejection of 96.5%, high monovalent/divalent ion permeation selectivity, and stable ion sieving performance during NF test. Our ultrathin polyamide membrane on the SWCNT network support showed greatly improved permeability compared to polyamide membranes on conventional UF supports. What’s more, the brush-painting technique is


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radically different from the widely reported membrane-forming techniques of the nanomaterial-based membranes via vacuum filtration or chemical vapor deposition which have critical limitations either in membrane area scalability, in equipment accessibility or in fabrication cost-effectiveness.16,17,19,48-55 The scalable fabrication technique of SWCNT network membrane is expected to boost the mass production of the state-of-the-art membranes composed of one-dimensional and two-dimensional nanomaterials and their practical applications in water/wastewater treatment,48,49,51,53,54 gas separation,50,52 organic nanofiltration,55 etc., as well as the nanomaterial-supported TFC membranes.16,17,19 Methods Materials: The SWCNT powder (OD: 1-2 nm, Length: 5-30 µm, Purity: > 95%) used in this work is a carboxylated product from Nanjing XFNANO Materials Tech Co.,Ltd (Nanjing, China). PES MF membrane (effective pore size: 0.45 μm) was commercially available from Yibo Co.,Ltd. (Haining, China). Anhydrous PIP (99%) and TMC (99%) were commercially available from Aladdin Co. Ltd. (Shanghai, China). All the other chemicals were analytical grade, commercially available from Sinopharm Chemical


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Reagent Co.,Ltd (Shanghai, China) and used as received. Goat hair brush was a commercial product from supermarket. Sandpaper (P2000) is a commercial product from the Minnesota Mining and Manufacturing Company. All the water used in the experiment is deionized water. Fabrication of SWCNT network membrane: A SWCNT dispersion was prepared by adding and sonicating 0.25 g SWCNT powder in 500 mL 1 g L-1 sodium dodecyl benzene sulfonate (SDBS) water solution using a Fisher Scientific ultrasonic cell crusher under the power of 10 W for 5 hours. To improve the dispersion degree, the SWCNT dispersion was centrifuged at 10000 rpm for 30 minutes and the supernatant dispersion was collected. The final concentration of SWCNT in the dispersion was 0.35 g L-1. SWCNT network membrane was then fabricated via painting the dispersion onto a PES MF membrane using a goat hair brush for several painting cycles. The SWCNT dispersion was first painted on the PES membrane surface via unidirectional movement of the brush, and the brush-painting was repeated in the orthogonal direction subsequently after the whole surface was covered by the dispersion. The two-step painting in two orthogonal directions formed one painting cycle. After each painting, the


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membrane was dried in a drum wind drying oven at 60 °C for 5 min. After finishing all the painting cycles, the membrane was washed in ethyl alcohol for 30 min and then dried before further use. Fabrication of polyamide membrane on SWCNT network support: The as-fabricated SWCNT network membrane was used as the support to carry out the interfacial polymerization reaction for the polyamide layer fabrication. The reaction temperature was kept at 25°C and air humidity was 60%. A PIP water solution with concentration of 0.125% (w/v) and a TMC n-hexane solution with concentration of 0.1% (w/v) were prepared beforehand. The SWCNT membrane was trapped on a glass plate and the PIP solution was dripped to cover the SWCNT membrane surface for 1 minutes. Excess PIP solution was removed using a rubber roller. Then, the TMC solution was introduced on the SWCNT membrane surface for 30 seconds. Excess TMC solution was removed afterwards, and the SWCNT network membrane was washed in n-hexane for 30 seconds and dried at 60 °C for 30 minutes. The fabricated polyamide membrane was stored in water at 4 °C.


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Concentration

gradient-driven

ion

diffusion

experiment:

To

conduct

the

ion

transmembrane diffusion experiment, a homemade U-shaped glass apparatus with a leak hole (effective area: 3.3 cm2) in the center were used as shown in Figure S6. The as-prepared polyamide membrane was fixed at the leak hole to separate the two solutions. The salt solution with concentration of 0.1M was filled up and sealed in the apparatus facing the polyamide selective layer. On the other side of apparatus, pure water (volume: 105 mL) was filled up to the same height (8.5 cm) of the salt solution. One magnetic stirrer was placed in each side to ensure homogenous distribution of salt ions. The concentration of diffused salts in the permeate side was monitored according to the solution conductivity for 3 hours. The molar quantity of diffused salts across the membrane was calculated according to the salt concentration and solution volume of the permeate side. The curve of diffused salts quantity as a function of time was plotted and linearly fitted. The salt diffusion rate was obtained according to the slope of the curve. Membrane performance test: Nanofiltration and ion sieving performance of the asfabricated polyamide membrane was tested on a cross-flow NF apparatus with an


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effective separation area of 7.065 cm2 at temperature of 25 °C. Five salt solutions with concentration of 1000 ppm were separated by the membrane. Cross-flow flux was 10 L h-1 and hydraulic transmembrane pressure was 6 bar. Water flux Jw (L m–2 h–1) was determined by recording the weight of filtrated water over time and calculated according to Equation (1). Where m (kg) was the weight increase of filtrated water during the separation time t (h),  was the density of filtrated water (consider to be 1 kg L−1), A (m2) was the effective separation area.

Jw = ∆m/A∆t

(1)

Salt rejection R (%) was determined by recording the salt concentration in filtrated water Cf (g L-1) and feed solution Cp (g L-1), and calculated according to Equation (2).

R = (1 - Cp/Cf ) × 100%

(2)

Salt permeance Ps (g m-2 h-1) during the NF process was calculated according to Equation (3).

Ps = JwCf (1 - R)

(3)

The permeation selectivity α of NaCl to MgSO4 was calculated according to Equation (4).


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α = PNaCl/PMgSO4

(4)

Characterization: SEM images were captured on a FEI Quanta FEG 250 scanning electron microscope. TEM images were captured on a Tecnai G2 F20 S-Twin fieldemission transmission electron microscope. AFM images were captured on a Bruker Dimension Icon atomic force microscope and the membrane was transferred onto a fresh mica sheet beforehand. To capture the top-view TEM images and AFM images of SWCNT network membrane and polyamide membrane, the freestanding membrane was obtained by dissolving and removing the underlying PES MF membrane using N,NDimethylformamide in advance. Membrane thickness was calculated according to the height information near the membrane edge from the AFM image. Surface zeta potential was measured on an Anton Paar Surpass solid surface analysis. XPS was measured on a Thermo Fisher Scientific ESCALAB 250Xi XPS. Contact angles were measured on Data-physics OCA20 machine. Salt concentration was measured according to the conductivity of salt solution by a METTLER TOLEDO FE30K conductivity meter. Membrane pore size information was obtained according to the


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rejection of PEG with different molecular weights (200, 400, 600 and 1000 Da). The rejection was calculated according to the PEG concentration of filtrated water and feed solution as measured by an Aurora 1030w TOC analyzer (O.I. Analytical). The PEG rejection curve was plotted by nonlinear fitting. The MWCO which is defined as the molecular weight at rejection of 90% was obtained from the rejection curve. Stokes radius rp (nm) of PEG was calculated based on its average molecular weight M according to Equation (5):

rp = 16.73 × 10-12 × M 0.557

(5)

A log-normal model of pore radius distribution was obtained according to the rejection curve and a probability density function between rejection and Stokes radius.25,26

Associated Content

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. AFM images and thickness of SWCNT network membrane fabricated with 3 painting cycles; SEM images of SWCNT network membranes fabricated with


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different painting cycles; SEM images showing the stability of SWCNT network membrane in different pH and in hexane; AFM image and surface roughness of the PES MF membrane; XPS survey spectrum of polyamide membrane fabricated on SWCNT network support; U-shaped glass apparatus for concentration gradient-driven ion diffusion experiment; Water flux and Na2SO4 rejection of the polyamide membrane under different transmembrane pressures; dynamic spreading and permeating behaviors of a water droplet on the polyamide membrane and commercial NF270 membrane; 48-hour separation performance of the polyamide membrane (PDF). Videos showing abrasion resistance test of a water-wetted SWCNT network membrane (AVI).

Conflict of Interest The authors declare no competing financial interest.

Author Information

Corresponding Authors * E-mail (J. Jin): [email protected]. * E-mail (W. Fang): [email protected].


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ORCID Jian Jin: 0000-0003-0429-300X Wangxi Fang: 0000-0003-1046-8590

Acknowledgment This work was supported by the National Natural Science Funds for Distinguished Young Scholar (51625306), the Key Project of National Natural Science Foundation of China (21433012), the Natural Science Foundation of Jiangsu Province (BK20180259), Joint Research Fund for Overseas Chinese, Hong Kong and Macao Scholars (21728602), and the National Natural Science Foundation of China (51603229). Funding support from the CAS Pioneer Hundred Talents Program is grateful appreciated as well.

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TOC graphic


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Figure 1. a) A schematic showing the fabrication of SWCNT network membrane via painting a SWCNT dispersion on a PES MF membrane and the fabrication of ultrathin polyamide (PA) membrane on the SWCNT network support. b) Digital photograph, c) cross-sectional SEM image and d) top-view SEM image of SWCNT network membrane fabricated with 3 painting cycles.


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Figure 2. TEM images of brush-painted SWCNT network membranes fabricated with different painting cycles.


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Figure 3. Chemical and mechanical stability of SWCNT network membrane. a) Digital photographs and b) water flux of the membrane after it was kept in water with pH of 1, 7, 13 and in n-hexane for 30 consecutive days. c) Water flux of the membrane in a 48-hour continuous cross-flow filtration test with an averaged cross-flow linar velocity of 0.2 m s-1 (applied pressure: 1 bar). d) SEM and TEM images (the insert) of the membrane after the cross-flow filtration test.


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Figure 4. Abrasion resistance of a water-wetted SWCNT network membrane. Digital photographs showing the implementing processes of a) rubber roller rolling and b) sandpaper scrubbing on the membrane. A weight of 20 grams was loaded on the P2000 sandpaper in b). c) SEM images and d) water flux of the membrane before and after the abrasion test.


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Figure 5. SEM images of polyamide membranes fabricated on supports of a) PES MF membrane, b-e) SWCNT network membranes. f) Water flux and rejection of the polyamide membranes corresponding to a-e) for separating a Na2SO4 solution (concentration: 1000 ppm, applied pressure: 6 bar).


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Figure 6. AFM images of a) SWCNT network membrane and b) polyamide membrane fabricated on SWCNT network membrane. c) Top-view TEM images of the polyamide membrane and pure SWCNT network membrane (the insert). d) Cross-sectional TEM image showing the ultrathin polyamide membrane on the SWCNT network support.


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Figure 7. a) Rejection curve of the polyamide membrane for PEG with different molecular weights. The insert in a) is the pore radius distribution calculated based on the rejection curve and a probability density function. b) Surface zeta potential of the membrane under different pH.


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Figure 8. Concentration gradient-driven ion diffusion across the polyamide membrane. a) and c) Variation of diffused salts as a function of diffusion time. Variation of b) anion and d) cation diffusion rate as a function of ion hydrated radius.


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Figure 9. Nanofiltration and ion sieving performance of the polyamide membrane. a) Water flux and rejection of the membrane for separating different salt solutions (concentration: 1000 ppm, applied pressure: 6 bar). b) Summary of Na2SO4 rejection and water permeability of the state-of-the-art NF membranes reported in literatures. c) Salt permeance of the polyamide membrane during the NF test. d) Trade-off between water permeability and NaCl/MgSO4 permeation selectivity of NF membranes.21 Adapted with permission from reference 21. Copyright 2018 American Chemical Society. The red star in b) and d) represent our membrane.


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Table 1. Pore size, thickness, water CA and pure water flux of the brush-painted SWCNT network membranes with different painting cycles.

Cycle

Pore size

Thickness

Water CA

Water flux

(nm)

(nm)

(°)

(L m-2 h-1 bar-1)

1

75 ± 25

42 ± 2

39

3300 ± 100

2

40 ± 15

65 ± 3

46

2480 ± 50

3

25 ± 10

91 ± 3

58

1610 ± 30

4

12 ± 4

116 ± 5

71

1030 ± 20


Ultrathin Polyamide Nanofiltration Membrane Fabricated on Brush

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