Development of High-flux and Robust Reinforced Aliphatic Polyketone

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Development of High-flux and Robust Reinforced Aliphatic Polyketone Thin-film Composite Membranes for Osmotic Power Generation: The Role of Reinforcing Materials Yuchen Sun, Liang Cheng, Takuji Shintani, Yasuhiro Tanaka, Tomoki Takahashi, Takuya Itai, Shengyao Wang, Li-Feng Fang, and Hideto Matsuyama Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03392 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Industrial & Engineering Chemistry Research

Development of High-flux and Robust Reinforced Aliphatic Polyketone Thin-film Composite Membranes for Osmotic Power Generation: The Role of Reinforcing Materials Yuchen Sun, Liang Cheng, Takuji Shintani, Yasuhiro Tanaka, Tomoki Takahashi, Takuya Itai, Shengyao Wang, Lifeng Fang*, Hideto Matsuyama* Center for Membrane and Film Technology, Department of Chemical Science & Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan. KEYWORDS: Aliphatic polyketone; pressure retarded osmosis; reinforced membrane.

ABSTRACT

Membranes used for pressure-retarded osmosis (PRO) are required to be mechanically strong due to high external hydraulic pressures on the draw solution (DS) side. In this study, a series of non-woven fabrics of varying thickness, density, and hydrophilicity were used to fabricate reinforced aliphatic polyketone (PK) membranes with excellent mechanical strength and pressure resistance. Three suitable non-woven fabrics are selected based on the integrities of the PK layers formed on their surfaces. As a result, guidance for choosing suitable reinforcing materials

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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for PK membranes is provided. We show that a PK-based thin-film composite (TFC) membrane reinforced by an 11-µm-thick PET non-woven fabric provides a flux of 24.7 L m-2 h-1 (in “active layer facing feed solution”, AL-FS mode, 0.6 mol L-1 NaCl as the DS), while maintaining a pressure resistance of 28 bar. This membrane is shown to be capable of producing a power density of at least 6.1 W m-2 in PRO evaluation (with 0.6 mol L-1 NaCl). 1. Introduction The impending energy crisis, a result of the accelerating consumption of non-renewable resources, has seen significant progress in the development of renewable energy sources in recent years. In addition to “traditional” wind, solar, or hydraulic power generation methods, there is also interest in discovering new potential power resources. The mixing of two solutions with different salinities is known to release energy; consequently the mixing of fresh and saline waters, which regularly occurs in coastal areas, can be considered to be a vast energy treasury. It is estimated that the global power potentially available at river mouths exceeds one terawatt1,2. Therefore, the efficient use of this energy source may help meet the critical demand for power by modern society. A variety of methods have been developed for harvesting the energy released by an osmotic pressure difference, including reverse electrodialysis (RED)3,4, capacitive mixing5,6, and pressure-retarded osmosis (PRO), among which the PRO process has received more attention over the few past years due its higher efficiency, higher power density, and its greater suitability for extracting energy from high salinity gradients7. In the PRO process, a semi-permeable membrane is placed between two solutions of different salinity. The difference in osmotic pressure drives water from the low-concentration side to the pressurized high-concentration side,


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inducing a higher flow rate in the latter. The pressurized water at the high-concentration side then flows through a turbine to generate electricity. Since PRO-power generation was first conceived of by Loeb et al.8, there has been significant progress toward improving the efficiency of the PRO process. Mass-transport modeling, efficiency/cost evaluations9-16, and most importantly, robust, high-performance membranes exclusively designed for such processes, have been well investigated. The performance of the membrane used in the PRO process plays a critical role in the performance of the power-generation system. Yip et al.

17

tuned polysulfone-based (PSf-based)

thin-film composite (TFC) membranes by exposing them to NaClO solution. They found that a balance between permeability and selectivity is required to achieve the highest peak-power density. Arena et al.

18

reported the deposition of a polydopamine coating on the substrate layer

of a PSf-based TFC membrane; a significantly enhanced osmotic flux was observed when this membrane was operated in PRO mode, which was ascribed to an increase in “wetted porosity” originating from the polydopamine coating. Unfortunately, these reports mainly focus on improvements in performance (i.e., permeability and selectivity) and overlook the issue of pressure resistance, which is also an important property of a PRO membrane. One of the most important parameters used to assess PRO system performance is power density (PD), which is defined as the power extracted per unit area of the membrane, and can be expressed by the following equation19: ⋅ =

Eq. 1.

is the average water flux across the membrane (in PRO mode) and is the hydraulic Here,

pressure. To achieve a higher PD, the PRO process is usually performed in a pressurized module.


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Therefore, the membranes used in the PRO process are not only required to have high permeability and selectivity, but are also required to tolerate high pressures for more effective power generation. However, the lack of such membranes seriously limits the commercial application of PRO power generation. Recently, Straub et al.19 comprehensively reviewed a number of PRO-membrane reports that evaluate pressure resistance. The plot of structural parameter (S value) as a function of maximum operating pressure demonstrates a clear trade-off relationship, indicating that the fabrication of PRO membranes with both high fluxes and pressure resistances is difficult to achieve from a practical perspective20-24. Song et al. reported the fabrication of a series of thin-film nanofiber-composite PRO membranes25. Owing to the low tortuosity, high porosity, and interconnected structure of the nanofiber substrate, these membranes exhibited extremely low S values (i.e. < 150 µm) and a maximum experimental water flux of ~36 L m-2 h-1 at a transmembrane pressure of 15.2 bar, using “synthetic” sea water (1.06 M NaCl solution) as the draw solution; in addition, 15.2 bar was found to be the maximum operating pressure. Straub et al.26 used a commercial TFC membrane (manufactured by Hydration Technology Innovation, USA) embedded with a woven mesh for enhanced mechanical strength. The S value of this membrane was 564 µm and its maximum operating pressure was at least 48.3 bar. Elevated pressure and a concentrated draw solution were also confirmed to be beneficial for high PD. In the work of C. F. Wan et al.27, a strategy of adding calcium chloride (CaCl2) in PES doping solution was used to improve the mechanical strength of PES TFC hollow fibre membrane. The best result, as reported, was 38 W m-2 at 30 bar, with 1.2 M NaCl and DI water as DS and FS, respectively. W. Gai et al.28 employed carbon quantum dots as an additive in the aqueous phase of interfacial polymerization process, and successfully


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boosted the maximum power density of PES TFC hollow fibre membrane to 34.20 W m-2 (23 bar, 1.0 M NaCl and DI water as DS and FS, respectively). In our previous study29-31, a new membrane material, namely an aliphatic polyketone (PK), was systematically investigated. Compared with conventional membrane materials, such as PSf, polyethersulfone (PES), and polyacetonitrile (PAN), PK is hydrophilic and resistant to multiple organic solvents. The structure of PK membranes can easily be regulated during fabrication, making them promising for use in osmosis-driven membranes. Nevertheless, PK membranes suffer from a serious problem – low mechanical strength; consequently they are not suitable for the PRO process. Therefore, in this study we aimed to develop a series of high-performance reinforced PK TFC membranes that are potentially applicable to the PRO process. Although woven/non-woven fabric materials have been widely used to reinforce osmosis-driven membranes, the introduction of these materials usually results in low osmotic fluxes and large S values32. Hence, we first screened non-woven fabrics with different densities, thicknesses, and hydrophilicity. TFC membranes were then prepared with defect-free substrate PK membranes. Forward osmosis (FO) and reverse osmosis (RO) experiments were carried out in order to evaluate water flux, reverse salt flux (salt leakage), water permeability (A value), salt permeability (B value), S value, and maximum hydraulic breaking pressure. The relationships between these parameters and the properties of the reinforcing non-woven fabric are discussed in detail. Finally, PRO experiments were carried out to evaluate the potential in power generation of the most balanced membrane. 2. Experimental 2.1 Materials and reagents


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Figure 1. Chemical structure of aliphatic polyketone (PK). Aliphatic polyketone powder (PK, Mw = 700,000 g mol-1, Fig. 1) was kindly provided by the Asahi Kasei Co. Ltd. (Nobeoka, Miyazaki, Japan). Resorcinol, methanol, acetone, hexane, sodium chloride, m-phenylenediamine (MPD), sodium dodecyl sulfate (SDS), and triethylamine (TEA) were purchased from the Wako Pure Chemical Co., Japan. Hexamethylphosphoric triamide (HMPA), (±)-10-camphorsulfonic acid (CSA), and 1,3,5-benzenetricarbonyl chloride (TMC) were purchased from the Tokyo Kasei Co., Japan. Milli-Q water was produced by a Millipore Milli-Q unit (Millipore, Bedford, MA, USA). All reagents were used as received. The non-woven reinforcing fabrics used in this research were supplied by the Awa Seishi and Hirose Seishi Co., Japan. Detailed fabric information is provided in Table 1. Each non-woven fabric was assigned a designatory code. For example “10HP” refers to a fabric that is approximately 10 µm thick and is made from high-density (H) non-woven polyester (PET, P). Similarly, “50LV” is approximately 50 µm thick and made from low-density vinylon (crosslinked polyvinyl alcohol). Table 1. Parameters of the non-woven supporting fabrics. Fabric

Density

Material

Chemical property

Thickness (µm)

10HP

High

Polyester

Hydrophobic

11


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90HP

High

Polyester

Hydrophobic

93

20LP

Low

Polyester

Hydrophobic

20

90LP

Low

Polyester

Hydrophobic

93

50LV

Low

Vinylon

Hydrophilic

49

90LV

Low

Vinylon

Hydrophilic

91

150LV

Low

Vinylon

Hydrophilic

153

A commercial cellulose acetate (CTA) FO membrane was used throughout this work for comparison of performance. This membrane was obtained from Fluid Technology Solutions, Ltd, and is referred to as FTS-CTA in further sections. 2.2 Preparation of the PK substrate membranes The doping solution was prepared by dissolving PK powder in a mixture of resorcinol and water (65/35, w/w) at 80 °C. The concentration of PK was 10% throughout this study. The mixture was vigorously stirred for 4–5 h, at which time a homogeneous pale-yellow solution had formed. The solution was then stored overnight in a 50 °C oven to remove gas bubbles. The membranes were either cast onto bare glass plates (PK-S200) or non-woven fabrics (reinforced PK membranes) taped to glass plates. The temperature of the doping solution was maintained at 50 °C throughout the casting process. The casting height was fixed at 200 µm. Immediately following casting, the glass plates were immersed in a coagulation bath (30% w/w methanol in water) for 20 min. The nascent membranes were then clipped by a pair of metal frames and transferred to an acetone


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bath for 20 min, followed by immersion in a hexane bath for another 20 min to remove the residual resorcinol and water from within the membrane in order to prevent membrane shrinkage. Finally, the membranes were dried overnight and stored for further use. The substrate membranes obtained in this manner are referred to as “PK-X”, where “X” refers to the non-woven fabric listed in Table 1 (e.g., PK-10HP is reinforced by the 10HP fabric). A standalone PK membrane, referred to as “PK-S200”, was also fabricated for comparison purposes. 2.3 Preparation of PK-based TFC membranes The polyamide active layer was formed by interfacial polymerization on the upper side of the PK supporting membrane following the procedure described in our previous report29. Briefly, two solutions were prepared. The aqueous phase was a solution of MPD (2.0 wt%), HMPA (3.0 wt%), TEA (1.1 wt%), CSA (2.3 wt%), and SDS (0.15 wt%) in water, and the organic phase was a solution of TMC (0.15 wt%) in hexane. During polymerization, the aqueous MPD solution was first introduced onto the upper side of the PK membrane. After 5 min, excess solution was removed from the membrane surface, and the wet membrane was allowed to stand vertically for 1 min, after which the TMC solution was introduced to the upper side of the wet membrane. After 2 min, excess solution was removed, and the membrane was allowed to stand vertically for 1 min. The TFC membrane obtained in this manner was finally cured at 90 °C for 10 min, followed by rinsing with water several times. The membrane was stored in Milli-Q water in the dark. The TFC membranes are referred to as “PK-X-TFC”, where the terminology follows that used for the corresponding substrate


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membranes (in which X represents the non-woven fabric used to reinforce the membrane, except for PK-S200-TFC, which represents the TFC membrane prepared on a standalone PK substrate). 2.4 Characterization of the PK substrate and TFC membranes The thicknesses of the non-woven fabrics and the PK membranes were measured using a micrometer (MCD130-25, Niigata Seiki Co., Japan). At least ten data points were collected from randomly chosen positions on the fabric/membrane. The morphologies of the non-woven PK membranes (both substrate and TFC) were examined by scanning electron microscopy (SEM) (JSM-7500F, JEOL Co. Ltd., Japan). The membrane samples were dried overnight in a freeze drier (FD-1000, EYELA, Japan) to remove moisture. Before microscopy, the samples were sputter-coated with a ~10-nm-thick layer of osmium with an osmium coater (Neoc-STB, MEIWAFOSIS Co. Ltd., Japan). The accelerator voltage was set to 7.0 kV. The water fluxes and reverse salt fluxes of the membranes were evaluated in a laboratory-scale forward osmosis (FO) crossflow filtration system previously reported29,30, with the exception that the effective membrane area was 4.1 cm2 in this work. Membrane performance was evaluated in both “active-layer facing feed solution” (AL-FS) mode and “active-layer facing draw solution” (AL-DS) mode. Milli-Q pure water and aqueous NaCl (0.3, 0.6, 0.9, and 1.2 mol L-1) were used as feed and draw solutions, respectively, which were circulated at a flow rate of 1500 mL min-1. Prior to each experiment, the mass of the feed solution was determined using an electric balance. The real-time mass and conductivity data of the feed solution were acquired by computer at constant time intervals, and were used to calculate the water flux and reverse salt flux of the FO


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membrane. Each measurement was allowed to last for at least 90 minutes for stability, and at least two replicates were measured. The water permeability (A value, L m-2 h-1 bar-1) and salt permeability (B value, L m-2 h-1) were evaluated using a stainless-steel RO apparatus, as described elsewhere33,34. The effective membrane area was 8.04 cm2. The membrane coupon was placed on a sintered metal plate and was sealed with a rubber O-ring. The hydraulic pressure was fixed at 10 bar. For the PK-S200 membrane, a piece of non-woven fabric was placed beneath the membrane, because the hydraulic breaking pressure of PK-S200 was only 6 bar. The following equations were used to calculate A and B:

A=

()

∆

B=

1 () (

!

=

Eq. 2, () − )exp (− ) ! ()

() ∆ ln $ × )1 − *+ %& − %'

Eq. 3,

Eq. 4.

Here () is the pure water flux, ∆ is the applied pressure, () is the salt water flux

(concentration of NaCl was 2000 ppm), !, is the mass-transport coefficient in the RO

experiment, and is the apparent salt rejection, which was calculated using the following equation: Ro = 1 −

-' .-&

Eq. 5,


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where cp and cb are the concentrations of NaCl in the feed and permeate, respectively, and %& and %' are the osmotic pressures of the feed and permeate, respectively.

The structural parameter (S, µm), which is related to the internal concentration polarization (ICP) of the substrate membrane, is a key membrane factor. By definition, S is determined by tortuosity (τ, a dimensionless quantity), thickness (t, µm), and porosity (ε, a dimensionless quantity)19 by:

S=

23 4

Eq. 6.

In this work, S was calculated by combining FO flux with the values of A and B, using the following equation35:

S=

6 + 8%9: ln ( ) 5 5 8%5: + 6 +

Eq. 7.

Here D is the bulk solution diffusivity of the draw solution, πDS and πFS are the osmotic pressures of the draw and feed solutions, respectively, and 5 is the water flux in AL-FS mode.

The hydraulic breaking pressures (Pmax) of the TFC membranes were evaluated with the apparatus used for the RO experiments and a procedure similar to that reported previously

29

.

Each membrane coupon was set inside the stainless-steel RO cell with the bottom side facing the sintered metal plate, and was subjected to a constant hydraulic pressure (beginning at 4 bar, maximum 40 bar). The flux was continuously monitored for 30 min. The membrane coupon was considered stable if the flux remained constant, and an additional 2 bar would be added and the flux monitoring repeated. Membrane breakage was characterized by a sudden increase in flux or


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decrease in transmembrane pressure. Three parallel experiments were carried out and the lowest value was reported. 2.5 PRO evaluation and modeling of PRO performance The PRO experiment was carried out using a designated PK TFC membrane, PK-10HP-TFC. The FS and DS were Milli-Q and 0.6 mol L-1 NaCl solutions, respectively. FS was circulated with a plunger pump (NP-KX-840, Nihon Seimitsu Kagaku) at 10.0 mL min-1, and DS with a diaphragm high-pressure pump (Hydra-Cell, Wanner Engineering Inc.) at 600 mL min-1. The round membrane coupons were fitted inside a modified Nitto Denko C-70F membrane cell with an effective membrane area of 38.15 cm2, whose internal structure has been given in Fig. 2. A metal plate with porous openings at the positions of cell entrances was inserted to minimize the rupture of membrane against cell wall. A tricot spacer (275 µm thick) was employed to create the flowing channel for FS. Hydraulic pressure was adjusted by a valve connected in the pipeline. The mass and conductivity of FS were continuously monitored and the data acquired were used to calculate the PRO water flux and reverse salt flux.

Figure 2. Side-view internal structure of the modified Nitto Denko C-70F membrane cell. The green and yellow arrows depict the flow direction of DS and FS, respectively.


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Based on the parameters measured during the FO and RO processes, water flux in PRO mode could also be predicted by the following equation19:

? > − %5,& exp = > ! = 8; − B 6 ? 1 + [exp = > − exp =− >] ! %9,& exp =−

Eq. 8.

Here ! is the draw-side mass-transport coefficient of the PRO cell and is the hydraulic

pressure applied during the PRO process, while ! is a parameter related to DS flow rate, the DS flow pattern, and the shape of the flow channel. A combination of Eq. 8 and Eq. 1 could also predict the potential PD for a certain membrane. As described by Achilli et al.13, k can be determined from the following equation:

! =

?ℎ DE

Eq. 9.

Here Sh is the Sherwood number and dh is the hydraulic diameter of the flow channel. Due to the insertion of mesh spacers, the draw-solution flow can become turbulent at a relatively low Reynolds number (e.g., Re < 50); consequently Sh can be determined by:

?ℎ = 0.2 I J.KL ?- J.MJ

Eq. 10,

where Sc is the Schmidt number. Hence, the specific value of ! is only dependent on the apparatus and the experimental conditions used, and is not related to the properties of the membrane. The kPRO value can be arbitrarily designated in modelling. 3. Results and discussion 3.1 Screening of suitable non-woven fabrics


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Figure 3. SEM micrographs depicting the top surfaces of the non-woven fabrics. All micrographs are at the same magnification (× 150).

Figure 4. Top-surface/cross-sectional SEM micrographs and thicknesses of the reinforced PK substrate membranes. The top-surface images are at the same magnification (× 150). The cross-


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sectional images are at magnifications required to present an overall view of the entire membrane. Thickness data are reported with errors of one standard deviation. Standalone membranes made from a variety of materials (e.g., polyimide, polysulfone, etc.23,36) have been used as support layers of FO and PRO membranes. Although Loeb et al.37 demonstrated that membrane permeability is reduced by the introduction of a porous supporting fabric, the weak mechanical strength of the standalone membranes are unfavorable for the PRO process; hence, an adequate supporting fabric is required. In addition, we previously showed that the polymer concentration in the doping solution, as well as the concentration of methanol in the coagulation bath are the two key factors that determine membrane structure29 and, as a consequence, they influence membrane permeability and pressure tolerance. Therefore, the polymer concentration and the composition of the coagulation bath were fixed throughout this study. To investigate the formation of the PK layers on various non-woven fabrics with different densities, thicknesses, and hydrophilicities, several non-woven reinforced PK substrate membranes were prepared and evaluated. Table 1 provides detailed information about the fabrics used in this research, and their corresponding SEM micrographs are displayed in Fig. 3; lowmagnification SEM images, as well as cross-sectional images are provided in Fig. 4. We found that PK was only compatible with specific non-woven fabrics; homogeneous, defect-free PK layers were formed on high-density PET fabrics (i.e., 10HP and 90HP), while defective PK layers were formed on low-density PET fabrics (i.e., 20LP and 90LP). Due to the penetration of the doping solution into the fabric38, the membranes were inhomogeneous and covered with multiple pinholes. Since the use of a hydrophilic non-woven fabric can partly reduce the effect of ICP during osmosis39, the PK doping solution was cast onto three hydrophilic vinylon-based


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non-woven fabrics (50LV, 90LV, and 150LV). However, the two thinner vinylon fabrics (50LV and 90LV) suffered from severe shrinkage in the methanol/water mixture, leading to deformed, wrinkled, and defective membranes (pinholes shown in Fig. 4). The thickest vinylon fabric (150LV) was mechanically robust and was resistant to deformation. Hence, we succeeded in preparing a defect-free PK-150LV membrane. To further investigate the compatibility of PK with the non-woven fabrics, membrane cross sections were also examined by SEM. The PK-S200 standalone membrane has a symmetric and dense structure throughout its entire cross-section, owing to delayed demixing resulting from a relatively high methanol concentration in the coagulation bath. According to our previous research29-30, PK membranes may demonstrate a void-free morphology if the methanol concentration in coagulation bath is higher than 30%; any lower methanol concentration results in the appearance of large voids. PK-10HP exhibits a “sandwich-like” cross-section; that is, the non-woven fabric is sandwiched between two PK layers, with the extra bottom PK layer facilitating higher pressure-resistance in the PK-10HP-TFC membrane. On the other hand, PK90HP and PK-150LV show no PK either under or inside the reinforcing fabric, clearly indicating that massive penetration did not occur during membrane preparation. In contrast, the defective membranes (i.e., PK-20LP, PK-90LP, PK-50LV, and PK-90LV) exhibit non-woven fabric fibers that are completely enwrapped by the PK polymer backbone and are significantly PK-penetrated. On the other hand, PK-50LV and PK-90LV display some large voids near their fibers that are ascribable to shrinkage of the vinylon fabric during membrane preparation and drying. Such structures are also not appropriate for improving pressure resistance.


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Figure 5. Schematic cartoons illustrating the formation of the reinforced PK substrate membranes, highlighting different polymer-penetration behavior, the generation of defects and large voids that form through fabric shrinkage. By considering the SEM results together with the properties of the non-woven fabrics, the formation of the PK layers on the various fabrics can be described as follows (Fig. 5). No defects are formed for the high-density non-woven fabrics, as observed for PK-10HP and PK-90HP. When the PK doping solution is cast onto 10HP or 90HP, which are devoid of large randomly distributed openings, the casting solution steadily penetrates into the fabric, leading to the


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formation of a homogeneous, defect-free membrane. On the other hand, the casting solution passes through the thin fabrics prior to solidification, while penetration only reaches the top sections of the thick fabrics prior to solidification of the polymer solution in the coagulation bath. However, the low-density non-woven fabrics are penetrated differently to the high-density nonwoven fabrics; defective PK layers are formed more easily on low-density substrates. Due to the existence of large openings on the fabric surface (Fig. 3), the casting solution penetrates into the fabric at different rates at different positions; i.e., the casting solution quickly passes through the fabric by way of the larger openings where there is less resistance, while penetration at other positions is slower, leading to the formation of rough liquid films with pinholes prior to phase separation; obvious macro-scale defects are formed as a consequence (Fig. 4). More severely, the low-density hydrophilic substrates (50LV and 90LV) become soft after immersion in the coagulation bath and cannot resist shrinkage during the transformation of the liquid film into the solid membrane. However, PK-150LV is an exception; the doping solution cannot cross through the thick fabric in the limited time available. In addition, due to specific interactions between the water in the doping solution and the hydrophilic vinylon fibers, the rate of penetration is also suppressed. Hence, we successfully prepared defect-free PK-150LV membranes. These results are expected to provide criteria for choosing suitable fabrics for reinforced PK membranes. 3.2 Membrane morphology


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Figure 6. SEM micrographs showing morphologies of various defect-free PK substrate membranes. a) Top surfaces; b) bottom surfaces; and c) top surfaces following interfacial polymerization (except for FTS-CTA membrane). Following fabric screening, the morphologies of defect-free PK membranes (PK-10HP, PK90HP, PK-150LV, and PK-S200), TFC membranes (PK-10HP-TFC, PK-90HP-TFC, PK150LV-TFC, and PK-S200-TFC) and FTS-CTA membrane were examined in detail (Fig. 6). Firstly, for PK substrate membranes, no obvious differences in pore size and density were observed by comparing the top surfaces of the four membranes (Fig. 6a). This is a reasonable observation since all membranes were fabricated using the same recipe, and the incorporated non-woven fabrics should not affect the phase separations at the top surfaces. Image analysis using the ImageJ software reveals that the average surface pore size was about 91 nm. Generally, during the preparation of TFC membranes, substrate membranes with smaller pore radii are preferred (about 20–50 nm)40; however, the special affinity between the PK polymer and the MPD molecules enables good rejection layers to be prepared on the PK membranes, even though


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large surface pores are present29. On the other hand, the FTS-CTA membrane demonstrated a dense surface without obvious pore, even in a higher magnification of x50,000. The bottom surfaces of the membranes were also examined (Fig. 6b). The PK-S200 standalone membrane exhibits a porous structure formed by interconnected polymer fibers, with considerably larger pores at the bottom surface than at the top surface. For the fabric-reinforced PK-10HP, the bottom surface is composed of an incomplete polymer layer, with partly exposed non-woven fabric due to the partial penetration of the PK doping solution. The structure of the polymeric component is similar to that of PK-S200. For PK-90HP and PK-150LV, only fibers were observed, confirming that significant penetration to the bottom surface did not occur during membrane preparation. For the FTS-CTA membrane, apart from the embedded woven mesh, a rough bottom surface with small pores, whose size was several to several tens of nanometers, was observed. Polyamide rejection layers are formed on the PK membranes after interfacial polymerization. Fig. 6c reveals that all membranes exhibit typical and similar valley-ridge morphologies. Therefore, we successfully fabricated non-woven-fabric-reinforced PK supporting membranes and reinforced-TFC membranes; TFC membrane performance is discussed in the following sections. 3.3 Osmotic performance of the reinforced TFC membranes Table 2 lists the water permeability (A value), salt permeability (B value), and salt-rejection values of each membrane. First, with the exception of PK-150LV-TFC, all PK-based membranes exhibited similar values. As discussed earlier, the PK substrate membranes have identical surface morphologies (Fig. 6), which leads to similar polyamide-layer performance, as well as similar A


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and B values. However, the NaCl-rejection and B values for PK-150LV-TFC are relatively low. The uneven penetration of the PK doping solution in 150LV may have left some grooves on the surface of the substrate membrane, which interfered with the formation of a perfect rejection layer. Similar phenomena were observed when high nanoparticle dosages, that notably increased surface roughness, were used as additives in the substrate layers of TFC membranes41,42. Moreover, it should be noted that the apparent salt rejection values of the PK-based TFC membranes are slightly lower compared to those in previous reports17,20,32. According to a previous report43, the addition of HMPA to the aqueous MPD solution results in increased flux, but it also compromises salt rejection. Contrarily, the commercial FTS-CTA membrane demonstrated very low A and B values compared with PK TFC membranes (0.42 L m-2 h-1 bar-1 and 0.06 L m-2 h-1, respectively), which means that the active layer in FTS-CTA membrane is less permeable than PK. In fact, this dense active layer was shown to induce a larger detrimental impact on the osmotic performance than the support layer, as can be seen in the FO results presented later. Table 2. A, B, B/A, and NaCl rejection values of the PK-based TFC membranes. All data are the averages of at least three replicates. Errors are expressed as single standard deviations. Water permeability Membrane

-2

(A value, L m h

Salt permeability

B/A

Apparent NaCl

(B value, L m-2 h-1)

(×10-2 bar)

rejection (%)

-1

-1

bar ) PK-S200-TFC

2.69 ± 0.04

0.32 ± 0.01

11.9

94.0 ± 0.1

PK-10HP-TFC

3.08 ± 0.30

0.37 ± 0.08

12.0

94.4 ± 2.5


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PK-90HP-TFC

3.10 ± 0.07

0.35 ± 0.09

11.3

95.7 ± 1.2

PK-150LV-TFC

2.51 ± 0.15

0.60 ± 0.14

23.9

88.7 ± 3.4

FTS-CTA

0.42 ± 0.01

0.06 ± 0.02

15.3

96.9 ± 0.1


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Figure 7. FO performance of the TFC (or CTA) membranes in both AL-FS and AL-DS modes. (a) FO water flux at four different DS (NaCl) concentrations. (b) Reverse salt fluxes at 0.6 mol L-1 NaCl. The data in the figure are averages of at least two replicates, and the error bars represent single standard deviations. To evaluate the influence of the non-woven fabric on osmotic performance, as well as to compare the performance of PK based TFC membrane to commercial CTA membrane, FO experiments were conducted in both AL-FS and AL-DS modes. Fig. 7 displays the relation of FO water fluxes to draw-solution NaCl concentration (0.3, 0.6, 0.9, and 1.2 mol L-1), and reverse salt fluxes using 0.6 mol L-1 NaCl as the DS. The PK-S200 standalone membrane exhibits the highest FO flux (27.3 L m-2 h-1 in AL-FS mode and 35.8 L m-2 h-1 in AL-DS mode, with 0.6 mol L-1 NaCl as DS). As described in our previous papers29-30, PK forms hydrophilic and highly porous substrate membranes; both of these properties are beneficial for lower resistances in osmotic processes. It is also widely known that in AL-DS mode, the ICP effect is less pronounced and hence the FO flux showed an obvious increase. However, the introduction of non-woven fabrics into the PK substrate membranes results in decreased FO flux. PK-10HP-TFC exhibits a high flux (24.7 L m-2 h-1 in AL-FS and 34.9 L m-2 h-1 in AL-DS), which is the highest among the three non-woven-fabric-reinforced TFC membranes; its reverse salt flux is not significantly different to that of PK-S200-TFC. Therefore, the 10HP fabric can be considered to have a minimal negative effect. PK-150LV-TFC exhibits a moderately high water flux (18.3 L m-2 h-1 in AL-FS, 22.4 L m-2 h-1 in AL-DS); its reverse salt flux, however, is almost twice that of PK-S200-TFC, which is consistent with the low salt-rejection and high B values observed in the RO experiments. PK-90HP-TFC exhibits the lowest FO flux (5.6 L m-2 h-1 in AL-FS, 6.8 L m-2 h-1 in AL-DS), as well as an extremely low reverse salt flux, indicating that this membrane is


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seriously affected by the ICP phenomenon following introduction of the thick, hydrophobic nonwoven fabric. Lastly, the FO flux of commercial FTS-CTA membrane lies in between the PK150LV-TFC and PK-90HP-TFC membranes (7.1 L m-2 h-1 in AL-FS, 10.3 L m-2 h-1 in AL-DS). This low flux is in agreement with our conclusion in previous section that a low water permeability (A value) in the active layer is not beneficial for a high performance in FO.

Figure 8. Structural parameters (S values) of the membranes. To quantitatively compare the ICP effects inside the TFC membranes, the S values were calculated using Eq. 7, the results of which are displayed Fig. 8. The S value is a commonly used criterion for evaluating the resistance of a substrate membrane to solute diffusion, and should be as low as possible in order to minimize the effect of ICP and to ensure a high flux. The S value of


24

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PK-S200-TFC was determined to be only 206 µm. For the aforementioned reasons, including hydrophilicity, high porosity, and low thickness (74.4 µm), PK-S200-TFC exhibits a significantly reduced ICP effect. On the other hand, the S values of the non-woven-fabricreinforced PK membranes increase in the order: PK-10HP-TFC < PK-150LV-TFC < PK-90HPTFC, with values of 305 µm, 512 µm, and 2987 µm, respectively. There are numerous reasons for these observed increases. Firstly, the reinforcing fabric increases the overall thickness of the entire composite membrane (see Fig. 4), which results in a higher diffusion resistance and a lowering of the osmotic water flux. Although a hydrophilic vinylon fabric, such as 150LV, can be completely wetted by water, which is beneficial for ion diffusion, the tortuosity and thickness of PK-150LV are larger than those of PK-S200. Since the total thickness of PK-150LV (t1) is twice that of PK-S200 (t2) and the porosity of PK-S200 (ε2) is ~80%30, the ratio of thickness to porosity for PK-150LV (t2/ε2) is much higher than that of PK-S200 (t1/ε1). Consequently, the S value of PK-150LV is higher than that of PK-S200. Secondly, the hydrophobic non-woven PET fabric may further increase diffusion resistance. McCutcheon et al.39 found that the FO flux of a cellulose membrane dramatically increased following removal of the reinforcing PET-fabric layer. By introducing the thick and dense 90HP fabric, the performance of the PK-90HP reinforced membrane is lowered, and the S value is the largest among the membranes studied. Although a hydrophobic PET fabric is used in the PK-10HP-TFC membrane, its thickness does not increase significantly (i.e., it is similar to that of PK-S200), resulting in an S value comparable to that of PK-S200-TFC. The commercial FTS-CTA membrane has an S value of 459 µm; this is interesting because the low FO flux and such S value seem to be in contradiction. In fact, this is again the adverse effect of active layer, because such an S value is representative


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of the relatively small ICP effect inside the support layer; thus the low FO flux could only be attributed to the dense, less permeable structure of active layer. In conclusion, although the implementation of a non-woven fabric into a PK membrane increases its S value, the increase can be minimized through the judicious choice of non-woven fabric. 3.4 Maximum hydraulic breaking pressure

Figure 9. a) Maximum hydraulic breaking pressures. Three parallel tests were carried out, with the lowest values reported. Note that PK-90-TFC, PK-150LV-TFC and FTS-CTA did not break under the testing conditions; consequently their breaking pressures are recorded as “> 40 bar”. b) Stress at break values of the substrate membranes. At least three tests were carried out. The error bars represent single standard deviation ranges. The value for FTS-CTA was not shown because the membrane did not brake under our testing condition.


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To evaluate the maximum hydraulic breaking pressures of the reinforced TFC membranes, each membrane was set against a sintered plate in RO cell while a gradually increasing hydraulic pressure was applied to the active-layer side. Fig. 9 (a) reveals that the standalone PK-S200-TFC membrane tolerates a low hydraulic pressure of 6 bar, while the maximum hydraulic breaking pressures of the reinforced membranes are much higher. PK-10HP-TFC can withstand a hydraulic pressure of 28 bar, while PK-90HP-TFC, PK-150LV-TFC and FTS-CTA can resist maximum pressures in excess of 40 bar, which are beyond the measurement capabilities of our laboratory system. Higher mechanical strength result in membranes that are more difficult to break and can tolerate higher pressures. Inspection of Fig 9 (b) reveals that the mechanical strengths of the membranes correlate very well with their breaking pressures. We would also like to mention that the hydraulic breaking pressure measured here is only for comparing the strength of the membranes. In RO and PRO apparatus, the membranes are supported by sintered metal plate and tricot mesh, respectively. The rupture of membrane is different in these two cases, so a direct use of the breaking pressure measured here in PRO should be avoided. Due to relatively low breaking pressure or FO fluxes, PK-S200-TFC, PK-90HP-TFC and PK150LV-TFC are not good candidates for large-scale PRO process and are not further discussed in the following section. 3.5 PRO evaluation


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Figure 10. Water flux, PD and selectivity (Js/Jw) of PK-10HP-TFC and FTS-CTA membranes, measured at transmembrane pressures from 0 to 20 bar, using Milli-Q as feed and 0.6 mol L-1 NaCl as draw solution. The data is represented as an average of two parallel experiments. The aforementioned evaluations have proved that PK-10HP-TFC has the most balanced performance of all reinforced PK TFC membranes. This membrane, along with the commercial


28

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FTS-CTA membrane, were used in PRO experiments. For easier comparison, we selected 0.6 mol L-1 NaCl as the draw solution, because this concentration is similar to that of sea water. The measured water flux, PD and selectivity are presented in Fig. 10. The maximum PD for PK10HP-TFC membrane is reached at a transmembrane pressure of 14 bar, topping at 6.1 W m-2, and the corresponding water flux is 15.8 L m-2 h-1. Meanwhile, FTS-CTA membrane reached its maximum power generation performance at 16 bar, with a PD of only 2.2 W m-2 and the corresponding flux being 4.9 L m-2 h-1. For an economical viable PRO system, the threshold of power density is generally considered to be 5 W m-2 (when using river water and sea water as FS and DS, respectively)19; since the salinity of river water is relatively low, we could consider that PK-10HP-TFC successfully meets such requirement and is a promising candidate in the PRO application. However, the maximum PD of PK-10HP-TFC is still bound by several limiting factors in the PRO system, such as loss of selectivity at high hydraulic pressure (as seen in Fig. 10), external concentration polarization (ECP)44 effect and spacer shadow effect45. The effect of these factors cannot be overlooked. With a combination of Eq. 1 and Eq. 8, it is possible to predict the performance of membranes in ideal PRO mode using A, B and S values and an arbitrarily designated kPRO value (a quantity used to describe the extent of ECP effect). Such a modelling shows that PK-10HP-TFC is capable of generating an even higher PD of 9.7 W m-2 in ideal conditions (with the corresponding pressure and flux being 14 bar and 25.0 L m-2 h-1), assuming kPRO = 100 L m-2 h-1 and no loss of selectivity occurs. Contrarily, in the current cell configuration, kPRO is ~50 L m-2 h-1. This means the operating condition of our PRO system should be further improved in the future. 3.6 Concluding remarks


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We fabricated reinforced PK TFC membranes with non-woven fabrics of different density, thickness, and hydrophilicity. Only the high-density PET fabrics and the thickest vinylon fabric were able to support the formation of defect-free PK substrate membranes. Evaluation of the osmotic performance and pressure resistance of the corresponding TFC membranes revealed that, through the use of a thin, dense PET fabric, PK-10HP-TFC exhibited the least compromised FO water flux (24.73 L m-2 h-1, AL-FS, 0.6 mol L-1 NaCl as draw solution) and a significant pressure tolerance (28 bar). The use of a thick non-woven PET fabric, as in PK-90HP-TFC, could be useful when operating at even higher pressures (> 40 bar); however a poor FO water flux was observed due to a serious ICP effect. Finally, due to the high hydrophilicity and mechanical strength of the fabric layer, PK-150LV-TFC concurrently exhibited a moderately high water flux and excellent pressure resistance, but suffered from lack of selectivity (high reverse salt flux and B value). PRO evaluation demonstrates that PK-10HP-TFC can produce a power density of at least 6.1 W m-2 (0.6 mol L-1 NaCl solution as DS and DI water as FS, similar to a fresh water – seawater PRO system). With the findings described in this study, we confirm that it is possible to combine mechanically weak PK with a suitable non-woven fabric to obtain tough reinforced composite membranes, with only marginally compromised osmotic properties. We believe that these findings will pave the way for the development of new generation PRO membranes with improved performance. AUTHOR INFORMATION Corresponding Author Tel/Fax: +81 78 803 6180; E-mail: [email protected] (L. Fang), [email protected] (H. Matsuyama).


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We gratefully thank Grants-in-Aid from the Special Coordination Funds for Promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction, Kobe) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, for the financial support. The authors would like to thank Mr. Masanao Okamoto for his kind assistance in PRO measurement. REFERENCES (1) Olume V; Renberth K.E.E.T. Estimates of Freshwater Discharge from Continents: Latitudinal and Seasonal Variations J. Hydrometeorol 2002, 3, 660–687. (2) La Mantia F; Pasta M; Deshazer H.D; Logan B.E; Cui Y. Batteries for Efficient Energy Extraction from a Water Salinity Difference, Nano Lett., 2011, 11, 1810–1813. (3) Tedesco M; Scalici C; Vaccari D; Cipollina A; Tamburini A; Micale G. Performance of the First Reverse Electrodialysis Pilot Plant for Power Production from Saline Waters and Concentrated Brines, J. Memb. Sci. 2016, 500, 33–45. (4) Tedesco M; Cipollina A; Tamburini A; Micale G. Towards 1 kW Power Production in a Reverse Electrodialysis Pilot Plant with Saline Waters and Concentrated Brines, J. Memb. Sci. 2017, 522, 226–236.


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(22) Li Y; Wang R; Qi S; Tang C.Y. Structural Stability and Mass Transfer Properties of Pressure Retarded Osmosis (PRO) Membrane under High Operating Pressures, J. Membr. Sci. 2015, 488, 143–153. (23) Li X; Zhang S; Fu F; Chung T. Deformation and Reinforcement of Thin-film Composite (TFC) Polyamide-imide (PAI) Membranes for Osmotic Power Generation, J. Memb. Sci. 2013, 434, 204–217. (24) Bason S; Oren Y; Freger V; Ion Transport in the Polyamide Layer of RO Membranes: Composite Membranes and Free-standing Films, J. Memb. Sci. 2011, 367, 119–126. (25) Song X; Liu Z; Sun D.D. Energy Recovery from Concentrated Seawater Brine by Thin-film Nanofiber Composite Pressure Retarded Osmosis Membranes with High Power Density, Energy Environ. Sci. 2013, 6, 1199–1210. (26) Straub A.P; Yip N.Y; Elimelech M. Raising the Bar: Increased Hydraulic Pressure Allows Unprecedented High Power Densities in Pressure-Retarded Osmosis, Environ. Sci. Technol. 2014, 1, 55-59. (27) Wan C.F; Yang T; Gai W; Lee Y. D; Chung T. Thin-film composite hollow fiber membrane with inorganic salt additives for high mechanical strength and high power density for pressureretarded osmosis. J. Membr. Sci. 2018, 555, 388-397. (28) Gai W; Zhao D L; Chung T. Novel thin film composite hollow fiber membranes incorporated with carbon quantum dots for osmotic power generation. J. Membr. Sci. 2018, 551, 94-102.


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(42) Ma N; Wei J; Qi S; Zhao Y; Gao Y; Tang C.Y. Nanocomposite substrates for controlling internal concentration polarization in forward osmosis membranes, J. Membr. Sci., 2013, 441, 54-62. (43) Duan M; Wang Z; Xu J; Wang J; Wang S. Influence of Hexamethyl Phosphoramide on Polyamide Composite Reverse Osmosis Membrane Performance, Sep. Purif. Technol. 2010, 75, 145–155. (44) Yip N.Y; Elimelech M. Performance Limiting Effects in Power Generation from Salinity Gradients by Pressure Retarded Osmosis, Environ. Sci. Technol. 2011, 45, 10273–10282. (45) She Q; Wei J; Ma N; Sim V; Fane A. G; Wang R; Tang C.Y. Fabrication and characterization of fabric-reinforced pressure retarded osmosis membranes for osmotic power harvesting, J. Memb. Sci. 2016, 504, 75–88.

Table of Contents Graphic


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Development of High-flux and Robust Reinforced Aliphatic Polyketone

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