Facile and scalable flow-induced deposition of organosilica on porous

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Surfaces, Interfaces, and Applications

Facile and scalable flow-induced deposition of organosilica on porous polymer supports for reverse osmosis desalination Gong Genghao, Hiroki Nagasawa, Masakoto Kanezashi, and Toshinori Tsuru ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19075 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

Facile and scalable flow-induced deposition of organosilica on porous polymer supports for reverse osmosis desalination Genghao Gong,a Hiroki Nagasawa,b Masakoto Kanezashi,b and Toshinori Tsurub,*

a

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

and Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China b

Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

*Corresponding author: Tel.: +81 824 24 7714, Fax: +81 824 22 7191, e-mail: [email protected]

ABSTRACT The fabrication of a continuous and uniform organosilica membrane on a porous polymer substrate was achieved via a facile and technologically scalable flow-induced deposition (FD) approach. The uniformity of the thickness of an organosilica separation layer on a polymer surface with a large area was improved significantly via this two-step FD approach. Meanwhile, the optimal concentration of the organosilica used in membrane preparation was also investigated. This polymer-supported organosilica layered-hybrid membrane showed a high level of NaCl rejection (97.5－99%) in the reverse osmosis (RO) desalination of a 2,000 ppm NaCl solution at an operating pressure of 3MPa. This membrane also exhibited good stability and flexibility when rolled into a curvature radius of 11 mm.

KEYWORDS: Organosilica, Layered-hybrid, Scalable fabrication, Flow-induced, Desalination

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1. INTRODUCTION Owing to some prominent advantages, such as lower energy consumption, simplicity of operation and control, a smaller footprint, and reliable performance, membrane-based treatment technologies are becoming increasingly competitive in water purification and desalination compared with conventional water treatment processes.1-3 For example, membrane-based reverse osmosis (RO) desalination technologies only consume about one-fifth of the energy required in thermal desalination processes.4 Based on these advantages, membrane-based RO desalination plants currently command as much as 75% of global desalination capacity.5 Improvements in membrane performance have focused mainly on the choice and design of materials and on the methods used to fabricate membranes. A great diversity of membrane materials, both polymeric and inorganic, have been widely studied and applied. Polymeric membranes dominate the commercial market, largely due to their low cost, good performance and high processability.6-8 However, membranes made of polymeric materials such as cellulose acetate have generally displayed low chemical stability, and polyamide membranes have shown poor chlorine resistance with low levels of hydrothermal stability.9-11 Moreover, polymeric membranes have also shown limited levels of intrinsic permeability selectivity and a high propensity for biofouling.12 Inorganic molecular sieving membranes of zeolite and silica, have shown high selectivity and good levels of thermal and chemical stability, but their applications have been limited by the complex fabrication procedures required of zeolite membranes and the low hydrothermal stability of silica membranes.13,14 Organically bridged silica is a large emerging class of nanoporous materials composed of diverse organic bridging groups in silica networks. These organosilica materials have emerged as membrane candidates for gas and liquid separation owing to finely tunable bulk properties such


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as pore size, flexibility, surface wettability and chemical stability.15,17,18 Most work has focused on an organosilica material synthesized by the sol-gel condensation of the bridged precursor 1,2bis(triethoxysilyl)ethane (BTESE). For example, excellent hydrothermal stability has been reported for a BTESE-derived silica membrane that has shown stability for more than 1,000 days in the pervaporation dehydration of n-butanol at 150 °C.19 Subsequently, a highly permeable hydrogen BTESE membrane was successfully developed via a “spacer” technique proposed by Kanezashi et al. That membrane also showed improved hydrothermal stability in gas separation.20 Based on its exceptional hydrothermal stability and good molecular sieving ability, the application of this BTESE membrane has recently been expanded to water desalination. These membranes also exhibited good desalination performance and superior chlorine tolerance over a wide range of chlorine concentrations (35,000 ppm·h).21 Most of these organosilica membranes, however, are fabricated on flat or tubular ceramic supports.22 Expensive ceramic supports, complex fabrication processes, and poor reproducibility of membranes continue to restrict their large-scale fabrication. Compared with inorganic supports, polymer supports are easily available in large amounts and widely used due to their relatively low cost. Hence, the next step in the fabrication of advanced separation membranes will depend on organosilica separation layers deposited onto polymer supports. Several approaches to the fabrication of polymer-supported organosilica layered-hybrid membranes have been reported recently. Jang et al. deposited a modified mesoporous silica layer onto polymeric hollow fibers via a simple immersion and aging process, and the resultant membranes showed a high gas flux and improved CO2/N2 selectivity.23 Ngamou et al. and Gong et al. proposed techniques that use expanding thermal plasma chemical vapor deposition (ETPCVD) and sol-gel spin-coating, respectively, to deposit a BTESE-derived silica layer onto flat



and porous polymer supports.24-27 These polymer-supported organosilica membranes were applied to the pervaporative dehydration of an organic solvent and to water desalination, and they exhibited higher flux for better separation selectivity compared with that when using ceramic-supported membranes. However, the ETP-CVD process requires sophisticated devices and rigorous conditions, and almost 70% of ethane bridges (-CH2-CH2-) would be decomposed in BTESE networks, which could weaken the stability of a BTESE membrane. On the other hand, sol-gel processing shows the great advantage of retaining 100% of the bridging units. The sol-gel spin-coating process is not suitable for large-scale industrial utilization since it often is not scalable to a large-area polymer membrane such as that used in spiral-wound modules. In this work, we adopted a flow-induced deposition (FD) method toward a more scalable membrane platform for the fabrication of polymer-supported organosilica layered-hybrid membranes. FD techniques such as dip coating are well-known and widely used for the assembly of high-quality films or coatings with few defects.28,29 This deposition approach is driven by colloidal self-gravity and capillary forces and features fast solvent evaporation, which makes FD processing scalable and easily adapted to a continuous process. For example, Nagao et al. have reported the deposition of polystyrene and silica particles on glass substrates via a simple FD process (dip-coating).30 In this approach, the coating solution was dropped onto the substrate, and then the substrate was inclined vertically until the solution had dried. Although this process is very simple, the film formed on substrates has an increased thickness gradient along the vertical liquid flow direction because the concentration of the coating solution increases as the solvent evaporates.31 Here, to avoid this strong thickness gradient, we report the fabrication of organosilica membranes on porous polysulfone supports via a modified FD approach, and characterize both the structure and morphology of this membrane in detail. Reverse osmosis (RO)


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desalination testing was performed to explain the permeation behavior of layered-hybrid membranes. Moreover, the stability and flexibility of these membranes was also examined.

2. EXPERIMENTAL SECTION 2.1. Materials A sulfonated polyethersulfone (SPES) nanofiltration membrane (NTR-7430, manufactured by Nitto Denko Corporation in Japan) was used as a polymer support. The 1,2bis(triethoxysilyl)ethane (BTESE) precursor was purchased from Gelest, Inc. (Germany). Methanol, ethanol and 1-propanol were of analytical grade and were purchased from SigmaAldrich and Nacala Tesque, Inc. All chemicals were of reagent grade and used without further purification.

2.2. Synthesis of BTESE-derived sols 1,2-Bis(triethoxysilyl)ethane (BTESE) sol was synthesized via the hydrolysis and polymerization reactions of the precursor (EtO)3SiCH2-CH2Si-(OEt)3 in ethanol, as previously reported.32 Briefly, a required amount of BTESE was mixed with ethanol. Then, premixed water and HCl were added dropwise to this solution under vigorous stirring. The molar composition of the reactants was BTESE:H2O:HCl=1:120:0.1, and a 5.0 equivalent wt% of BTESE was maintained in the sol. The preparation of this polymer sol required 2 h at 60 °C before use. Finally, the BTESE sols were diluted with corresponding alcohol solvent to reach the concentration of BTESE.

2.3 Fabrication of organosilica membranes on polymer supports



Organosilica layers were deposited using BTESE sols on a flat and porous SPES substrate (NTR membrane) via a simple flow-induced deposition (FD) process, which is illustrated in Figure 1. First, the SPES substrate was fixed within a stainless steel frame (7.5x4 or 29x21cm) that could be scaled up so that only the top surface of the SPES support would contact the BTESE sol. A certain amount of BTESE sol was poured onto the surface of the SPES supports. After one minute, the substrate was inclined vertically from one side until the BTESE sol was drained off and dried (First-step coating). Then, the BTESE sol was poured on its surface again, and the substrate was inclined vertically from the opposite side (Second-step coating). The ambient temperature was maintained at 15 ± 3 °C during the entire process. After the sol had dried, the resultant SPES-supported BTESE (BTESE/SPES) membranes were then thermally treated in an oven at 120 °C for 10 min.


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Figure 1. Schematic diagram for preparing BTESE/SPES layered-hybrid membranes via a flowinduced deposition method.

2.4 Characterization The size distribution of the sols was determined via dynamic light scattering (DLS) using a Malvern Zetasizer Nano-ZS (ZEN3600) instrument at 25 °C. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT/IR-4100, JASCO, Japan) was performed to confirm the functional groups and chemical structures of the membrane surface. Hybrid membrane morphology and thickness was examined using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) with an acceleration voltage of 5.0 kV.

2.5 Membrane performance test Reverse osmosis (RO) desalination experiments were carried out using a NaCl aqueous solution in concentrations that ranged from 0.2 to 3.5 wt% via a typical RO test system, as previously described.33 A series of neutral solutes (ethanol, isopropanol, glucose, maltose, raffinose and cyclodextrin) was used to determine the molecular weight cutoff (MWCO) of BTESE/SPES membranes. The concentrations of NaCl and neutral solutes in the samples were determined using a conductivity meter (HORIBA, ES-51) and a total organic carbon analyzer (Shimadzu, TOC-VE), respectively. RO tests were conducted at 25 °C with a feed pressure of 1.5 MPa unless otherwise specified. According to the solution-diffusion (SD) model in the RO process, the permeate flux of water (Jv) and salt (Js) through a membrane can be expressed as follows:

= (∆P − ∆π)

(1)



= B( − )

(2)

Here, Lp and B are the water permeability and salt permeability, respectively. ∆P and ∆π are the applied pressure and osmotic pressure differences, respectively. Cf and Cp are the salt concentrations in the feed and permeate, respectively. The osmotic pressure difference (∆π) for NaCl was determined using van’t Hoff Eq. (3)

∆π= 2RT( − )

(3)

where R is the gas constant and T is the absolute temperature. The osmotic coefficient was assumed to equal 1 due to the dilute solution. The observed rejection, Robs, was evaluated using Eq. (4).

= (1 − ⁄ ) × 100%

(4)

In this work, the effect of concentration polarization was assumed to be negligible due to the low permeate flux. In addition, each RO test was initially run for at least 5 h before collecting feed and permeate solutions in order to insure stability.

3. RESULTS AND DISCUSSIONS 3.1 The effect of sol concentration on the formation of a BTESE layer on a polymer surface In a flow-derived deposition (FD) process, the concentration of a BTESE sol is an important factor for the formation of an organosilica coating layer on a porous polymer surface. Therefore,


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the effect of the concentration of BTESE sols was first investigated. Figure 2 shows SEM images of cross-sections of BTESE/SPES membranes prepared with different concentrations of BTESE sols via a two-step FD approach. Compared with the SPES support, an organosilica top layer clearly formed on the SPES surface after coating with BTESE. Some spots, or holes, however, were not fully covered on the SPES surface when using a low concentration (0.5 wt%) of BTESE sol (Figure 2b), which led to a discontinuous and incomplete organosilica layer. On the other hand, some penetration of BTESE sol into the SPES support was inevitable during the membrane preparation process due to its porous structure, this also resulted in the formation of spots and holes in the surface of the membrane. Subsequently, complete and uniform coatings were formed on the SPES surface with a thickness that gradually increased with increases in the concentration of the BTESE sol, as shown in Figures 2c-f. This suggested that the minimum effective concentration for a BTESE sol was 1.0 wt% when used in the formation of a complete organosilica layer on a porous SPES surface via this FD approach.



Figure 2. SEM images of the cross-section of BTESE/SPES membranes as a function of different concentrations of BTESE sols (0.0 (a), 0.5 (b), 1.0 (c), 2.0 (d), 3.0 (e), and 4.0 wt% (f).

In addition, the chemical structures of a BTESE/SPES membrane surfaces were analyzed via ATR-FTIR. Figure 3 shows the ATR-FTIR spectra of membranes with different concentrations of BTESE. The ATR-FTIR spectra of porous SPES (0.0wt%) supports were characterized by the appearance of absorption bands such as ─SO2─ stretching (1080 cm−1), aromatic ether stretching (1,237 and 1,015 cm−1), and benzene rings (1,585 and 1,490 cm−1).34 Compared with the use of SPES supports, the ATR-FTIR spectra of these BTESE/SPES membranes were similar to those reported in our previous study.26 In all the spectra of BTESE/SPES membranes, two new peaks that appeared at 1,000–1,200 cm−1 and 900–950 cm−1 were assigned to the Si–O–Si and Si–OH groups, respectively, which was ascribed to the organosilica networks of BTESE on SPES support surfaces. Moreover, the absorbance of Si-OH groups appeared to increase with increasing concentrations of BTESE, while the absorbance of the benzene ring groups was decreased. These results also indicated an increased thickness of the BTESE layer deposited onto the SPES surface.


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Figure 3. ATR-FTIR spectra of membranes with different concentrations of BTESE (0.0, 0.5, 1.0, 2.0, 3.0, and 4.0 wt%).

3.2 Thickness uniformity of BTESE coating layers on a polymer surface As mentioned above, the thicknesses of the BTESE coating layers deposited onto polymer surfaces was not uniform throughout the one-step FD process, as shown in Figure 4. As the Figure clearly shows, the thickness of the BTESE coating layer on the B side (location B, approximately 300 nm) of the membrane was greater than that on the A side (location A, approximately 800 nm). This was because the BTESE sol flowed slowly along the vertical surface from the A side to the B side, and therefore the liquid film thickness of the BTESE sol gradually increased in the direction of the flow. Simultaneously, the concentration of the BTESE sol increased as the solvent evaporated, which led to a non-uniform increase in the thickness of the BTESE coating layer on both sides of the membrane.



Figure 4. Cross-sectional SEM images of the BTESE/SPES membranes show different locations of the membrane surface (A and B). The membrane was prepared via a one-step FD process with a BTESE concentration of 3 wt%.

To avoid the strong thickness gradient and confirm the improvement in thickness uniformity using the two-step FD approach, a simple method was established to assess the thickness uniformity of a BTESE layer on porous polymer supports. As mentioned in the discussions illustrated by Figure 3, the absorbance of Si-OH groups was increased with increases in the thickness of the BTESE layer while that of the benzene ring (C6H6) groups was reduced. Therefore, the ratio of the absorbance of Si-OH to that of C6H6 can be used as an indicator of the thickness of an organosilica layer. To discuss the thickness uniformity of the BTESE layer on a SPES support, we defined a parameter, α, as follows:

α=

! "# !#

(5)


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where ISI-OH and IC6H6 are the peak intensities (absorbance) at 908 cm-1 and 1,585 cm-1, respectively. Therefore, the α value for a certain location of a membrane surface can be measured via the ATR-FTIR technique and calculated using Eq. (5). Figure 5a shows changes in the α (the center of membrane surface) of BTESE/SPES membranes at different concentrations of BTESE. The increase in α with increases in the BTESE concentration corresponded to increases in the thickness of the BTESE layer, as observed by SEM. This indicated that a certain linear relationship was confirmed between α and the thickness of the BTESE layer. Therefore, the thickness uniformity of a BTESE coating layer on a polymer surface could be evaluated by measuring the α value for different locations of a membrane surface. First, the membrane surface was plotted by multiple vertical and horizontal dotted lines, as shown in Figure 5b. The FTIR spectrum of the membrane corresponding to every intersection (black dot) was measured using the ATR-FTIR technique, and then the α value of every intersection on the membrane surface could be obtained using Eq. 5. The changes in the α of the BTESE/SPES membrane prepared by the one- and two-step FD approaches are shown in Figures 5c and d, respectively. For the one-step FD process, although all the values for α that corresponded to the digital coordinates on any set of letter coordinates (A-F) were basically the same, it was obvious that the α value gradually increased from locations A to F on the membrane surface, which indicated that the thickness of the BTESE layer gradually increased in the direction of the BTESE sol flow (from the A side to B). This result was consistent with that shown in Figure 4. It is worth noting that the α values for the two-step FD processes all changed in a much smaller range, indicating the thickness distribution of the BTESE layer on the polymer surface was generally consistent. These results suggested that the thickness uniformity of a BTESE coating layer on a polymer surface could be improved significantly with a two-step



FD approach. Moreover, twice coating cycles (two-step FD process) had also filled the surface porosity associated with previously deposited layers, this is beneficial to avoid the formation of defects in the SPES-supported BTESE layer, thereby further improving the separation performance of the membrane.

Figure 5. (a) Changes in the α values for BTESE/SPES membranes prepared with different concentrations of BTESE (0.0, 1.0, 2.0, 3.0, and 4.0 wt%) via a two-step FD approach (all α values were obtained from the center of the membrane surface); (b) Sketch map of the ATRFTIR measurement of different locations (intersection area) on the membrane surface; The changes in the α of BTESE/SPES membranes prepared via one-step (c) and two-step (d) FD approaches. (All membranes were prepared with a BTESE concentration of 3.0%, and the experimental error of each measurement was less than 5%).


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3.3 The effect of the BTESE sol concentration on the desalination performance of membranes The RO performances of BTESE/SPES membranes prepared using different concentrations of BTESE sol are shown in Figure 6a. Compared with SPES support (0.0% BTESE), water permeability was gradually reduced while the NaCl rejection significantly increased with increases in the concentration of BTESE sol. When comparing BTESE concentrations, NaCl rejection was further increased with increases from 1.0 to 2.0 wt%, while water permeability was reduced. For concentrations of 3.0 and 4.0 wt%, the rate of NaCl rejection showed almost no change, but at a concentration of 4.0 wt% water permeability was lowered significantly. This showed that a defect-free BTESE separation layer deposited on a porous SPES support via the two-step FD process using 3.0 wt% BTESE sol achieved a high rate of NaCl rejection (98.4%), whereas a higher concentration of BTESE sol only led to an increase in the thickness of the BTESE layer, which caused a reduction in water permeability. To further explore the pore sizes of BTESE/SPES membranes, the molecular weight cut-off (MWCO) curves of membranes prepared with different concentrations of BTESE sols were plotted, as shown in Figure 6b, by measuring the rejections of a series of neutral solutes. Membranes prepared with both 3.0 and 4.0 wt% BTESE sols showed a high level of rejection for substances with a low molecular weight (Isopropanol, 71.8% (3 wt% BTESE) and 81.3% (4.0 wt% BTESE); glucose, >98.5%) of approximately the same MWCO as seawater desalination membranes. However, glucose rejections of membranes prepared with 1.0 and 2.0 wt% BTESE sols were less than 95.5%, suggesting the formation of defects or pinholes in the BTESE separation layer, which lowered the level of NaCl rejection by the membranes.



Figure 6. RO desalination performances (a) and MWCO curves (b) of BTESE/SPES membranes with different concentrations of BTESE sols (levels of operating pressure and temperature were maintained at 1.5 MPa and 25 ˚C, respectively).

3.4 Effects of operating pressure and feed concentration for BTESE/SPES membranes The RO experiment was carried out using a 2,000 ppm NaCl solution by adjusting the operating pressure (in the range of 1.0, 1.5, 2.0, 2.5, 3.0, and 4.0 MPa). Figure 7 shows the effect of feed pressure on the permeate flux of water and NaCl. With an increase in operating pressure (from 1.0 to 3.0 MPa), both the water flux (Jv) and NaCl rejection (R) of membranes increased gradually, as shown in Figure 7a. It should be noted that the water flux sharply increased while NaCl rejection began to decrease at a feed pressure of 4.0 MPa. Neither the BTESE layer nor the SPES support can withstand a pressure as high as 4.0 MPa, which could have damaged either the BTESE layer or the SPES supports due to the further deformation of SPES support under such high pressure. Hence, the membrane performance was only examined within a pressure range of from 1.0 to 3.0 MPa. Using Eqs. 1 and 2, the water (Lp) and salt permeabilities (B) of the membranes were calculated and are shown in Figure 7b. Both were approximately constant within a feed pressure that ranged from 1.0 to 3.0 MPa. Based on simplifications of Eqs. 1-3 in a


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typical solution-diffusion (SD) model, water transport through a membrane is driven by the transmembrane pressure difference (∆p-∆π), thus, a linear increase in water flux while increasing the feed pressure should result in a constant level of water permeability within this pressure range. Based on Eq. 2, the permeation of salt ions across the membrane was driven by the transmembrane concentration gradient rather than by the operating pressure, thereby maintaining a basically constant level of salt permeability (B). Therefore, the increase in water flux with no change in salt flux and an increase in the operating pressure resulted in an increase in NaCl rejection. Moreover, according to the relationships between water and salt permeability in the SD model, a theoretical level for salt rejection could be obtained using Eq. 6:35

R=$

$% (∆&'∆() % (∆&'∆())*

× 100%

(6)

Therefore, based on Eqs. 1 and 6, two dotted curves showing theoretical water flux and salt rejection were plotted in Figure 7a using membrane averages for Lp and B. These results are basically in accord with the experimental values.



Figure 7. Effect of feed pressure on membrane permeate flux and salt rejection (a), and on water and salt membrane permeability (b) for a 2,000 ppm NaCl solution. (Dotted curves are calculated based on Eqs. 1-3 and 6 with membrane averages of Lp and B). The experimental error of each measurement was less than 3%.

RO performance of membranes was also evaluated using NaCl aqueous solutions in concentrations ranging from 2,000 to 35,000 ppm, with operating pressure maintained at 3 MPa. Figure 8 shows the effect that the NaCl concentration in the feed solution exerted on the RO desalination performance of the membranes. An increase in osmotic pressure was apparent when the NaCl concentration was increased, and this led to a decrease in the transmembrane pressure difference (∆p-∆π). Therefore, the water flux of the membranes was reduced in a linear fashion with increases in the NaCl concentration under a constant operating pressure, whereas the water permeability was not significantly changed. Meanwhile, NaCl rejection decreased as the


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concentration increased because salt passage through the membrane increased due to an increase in the concentration gradient. In addition, although both salt rejection and water flux decreased as the concentration increased, both returned to their initial level (water flux and salt rejection was 6.4x10-7 and 98.1%, respectively) when the NaCl concentration was decreased to the initial level of 2,000 ppm. This suggests that the BTESE/SPES layered-hybrid membranes are stable when highly concentrated NaCl solutions are applied.

Figure 8. The effect of the feed concentration on the permeation performance of membranes at 25 ˚C and 3.0 MPa (Dotted curves are calculated based on Eqs. 1-3 and 6 with averages for Lp and B of this membrane); Open and closed cycles (red) represent the NaCl rejection and water flux of membranes, respectively, after the concentration of the NaCl solution was increased from



2,000 to 35,000 ppm and then decreased to the initial concentration (2,000 ppm NaCl solution). The experimental error of each measurement was less than 3%.

Interestingly, increasing the feed concentration from 2,000 to 20,000 ppm only resulted in a slight decrease in the salt rejection of from 98.9 to 93.5%, whereas using 35,000 ppm of NaCl concentration led to a sharp decline in salt rejection (79.2%). The permeation of salt ions across a membrane is driven by the transmembrane concentration gradient, and increasing the salt concentration is known to enlarge the concentration difference and lead to an increase in the salt flux. However, with an increase in salt concentration (from 2,000 to 20,000 ppm), the salt permeability remained relatively constant, as shown in Figure 8. This is why the salt rejection decreased only slightly as the concentration increased from 2,000 to 20,000 ppm. It is noteworthy that the salt rejection declined sharply at a salt concentration of 35,000 ppm. This could have been caused by the charge effect (Donnan), which plays a major role in the rejection behavior of BTESE-derived membranes for the rejection of salt with a low level of ionic strength, which has been observed and reported for many nanofiltration membranes.36,37 This was also why both water flux and salt rejection could recover to the starting level when the concentration of the NaCl solution was returned from 35,000 to 2,000 ppm. These results suggest that the transport behavior of solutes for layered-hybrid membranes is affected by both a steric (sieving) effect and a charge (Donnan) effect when ionic strength is low and that the steric (sieving) effect gradually becomes dominant as the salt concentration is increased.

3.5 Membrane scale-up fabrication


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As previously mentioned, the FD fabrication approach for layered-hybrid membranes might be facile and easy to scale up. Therefore, a larger stainless steel frame (A4 size: 29x21 cm) was used to prepare a membrane on a large scale, as shown in Figure 9a. Meanwhile, Figures 9b and c show the thickness of the organosilica layer for membranes prepared in a small size and in an A4 size, respectively, using the same preparation conditions. No significant difference was observed and the thickness of the organosilica layers in both types of membranes was uniform at approximately 800 nm. The RO performance of small- and large-scale membranes was also investigated. Figure 9d shows the trade-offs between NaCl rejection and water permeability for different sizes of layered-hybrid membranes. The RO performances of large-scale membranes prepared via FD (A4 size) were similar to that of smaller versions, and these RO performances were basically reproducible. We noted slight differences in the RO performances of the membranes (both small and large) at different surface locations. Apart from experimental error, this could have been caused by the rough surface of the SPES supports, and it led to a locally thick inhomogeneity on the surface of the BTESE layer during the FD process. However, the error values of the RO performance for these membranes was less than 3%, which is consistent with the previous experimental results shown in Figure 5(d). These results indicated that the FD approach is also applicable to the large-scale fabrication of layered-hybrid membranes. Meanwhile, this simple and rapid fabrication of polymer-supported organosilica membranes also highlighted another advantage over ceramic-supported organosilica membranes that require a much longer processing time.



Figure 9. Photographs of membrane scale-up fabrication (a) and the cross-sectional SEM images of small membranes (b) and A4-sized (c); (d) the trade-offs between NaCl rejection and water permeability for layered-hybrid membranes prepared in different sizes (the trade-offs for the A4sized membrane are illustrated by the RO performance for different areas on the same membrane).

3.6 Membrane flexibility and stability In order to further assess the feasibility of using a layered-hybrid membrane for large-scale applications such as spiral-wound modules, the BTESE/SPES membranes were purposefully bent in a known radius (rolling test), as shown in Figures 10a and b. The RO performances of BTESE/SPES membranes before and after rolling were evaluated and compared, as shown in Figure 11a. The rate of NaCl rejection by membranes rolled through a radius of 7.5 mm were decreased compared with the original version, and these curved membranes showed a large fluctuation (93-96%). This was probably due to defects and cracks on the membrane surface of the membranes rolled through this radius, as shown in Figure 11c, which resulted in a decreased desalination performance. It is encouraging that when the membranes were rolled (Figure 10c) using a cylinder roller with a radius of 11 mm, neither cracks nor defects could be observed on


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the membrane surface (Figure 11b), and these membranes continued to show similar and reproducible desalination performances compared with the original state before rolling. This indicated that this BTESE/SPES layered-hybrid membrane has good stability and flexibility in a wide range of bending radii (radius of curvature: ≥ 11 mm), which is likely the result of an organosilica top layer that is flexible irrespective of the high rate of rejection for a rigid silica network. In addition to this, we previously reported that an interlocked structure was formed at the interface between the porous BTESE layer and the polymeric support due to the penetration of small amounts of BTESE sols into the polymeric support. This interlocked structure helped avoid the formation of defects in the BTESE layer and also enhanced the interfacial adhesion between them, which gave rise to further improved flexibility and stability of the membranes.27

Figure 10. (a) Schematic and photograph (b) of the rolling experiment (cylinder outer radius, R= 7.5 or 11 mm) and (c) photograph of the layered-hybrid membrane after rolling.

Figure 11. (a) The trade-offs between NaCl rejection and water permeability for layered-hybrid membranes before and after rolling through cylinder rollers with different curvature radii; The



SEM images of the membrane surface after rolling using cylinder rollers with radii of (b) 11 mm and (c) 7.5 mm.

4. CONCLUSIONS The fabrication of continuous and uniform organosilica membranes on a porous polymer substrate via a facile and technologically scalable flow-induced deposition (FD) approach was demonstrated. The thickness uniformity of the BTESE separation layer on a polymer surface was improved significantly via a two-step FD approach. This BTESE/SPES layered-hybrid membrane showed a high level of NaCl rejection (97.5－99%) during the reverse osmosis (RO) desalination of a 2,000 ppm NaCl solution at an operating pressure of 3 MPa. Moreover, this membrane also exhibited good stability and flexibility in a wide range of bending radii, thereby suggesting the viability of large-scale application for separations with organosilica layeredhybrid membranes such as spiral-wound modules.

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

ACKNOWLEDGMENTS This work was supported by Core Research for Evolutional Science and Technology (CREST) Program of Japan Science and Technology Agency (JST).


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Facile and scalable flow-induced deposition of organosilica on porous

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