Movatterモバイル変換

[0]ホーム
URL:

Next Article in Journal
Membrane Protein Stabilization Strategies for Structural and Functional Studies
Next Article in Special Issue
Theoretical Analysis of Constant Voltage Mode Membrane Capacitive Deionization for Water Softening
Previous Article in Journal
Recovery of Acid and Base from Sodium Sulfate Containing Lithium Carbonate Using Bipolar Membrane Electrodialysis
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Section
Special Issue
Volume
Issue
Number
Page
 
Logical OperatorOperator
Search Text
Search Type
 
add_circle_outline
remove_circle_outline
 
 
Journals
Membranes
Volume 11
Issue 2
10.3390/membranes11020153
first_page
Download PDF
settings
Order Article Reprints
Font Type:
ArialGeorgiaVerdana
Font Size:
AaAaAa
Line Spacing:
Column Width:
Background:
Article

Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes

1
School of the Environment and Safety Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, Jiangsu, China
2
Institute of Environmental Health and Ecological Security, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, Jiangsu, China
3
Singapore Membrane Technology Centre, Nanyang Technological University, 1 Cleantech Loop, Singapore 637141, Singapore
4
School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
*
Author to whom correspondence should be addressed.
Submission received: 15 January 2021 /Revised: 2 February 2021 /Accepted: 11 February 2021 /Published: 22 February 2021
(This article belongs to the Special IssueMembrane-based Technologies for Water and Energy Sustainability)

Abstract

:
The success of osmotically-driven membrane (OM) technology relies critically on high-performance membranes. Yet trade-off of membrane properties, often further complicated by the strongly non-linear dependence of OM performance on them, imposes important constraint on membrane performance. This work systematically characterized four typical commercial osmotic membranes in terms of intrinsic separation parameters, structure and surface properties. The osmotic separation performance and membrane scaling behavior of these membranes were evaluated to elucidate the interrelationship of these properties. Experimental results revealed that membranes with smaller structural parameter (S) and higher water/solute selectivity underwent lower internal concentration polarization (ICP) and exhibited higher forward osmosis (FO) efficiency (i.e., higher ratio of experimental water flux over theoretical water flux). Under the condition with low ICP, membrane water permeability (A) had dominant effect on water flux. In this case, the investigated thin film composite membrane (TFC,A = 2.56 L/(m2 h bar),S = 1.14 mm) achieved a water flux up to 82% higher than that of the asymmetric cellulose triacetate membrane (CTA-W(P),A = 1.06 L/(m2 h bar),S = 0.73 mm). In contrast, water flux became less dependent on theA value but was affected more by membrane structure under the condition with severe ICP, and the membrane exhibited lower FO efficiency. The ratio of water flux (Jv TFC/Jv CTA-W(P)) decreased to 0.55 when 0.5 M NaCl feed solution and 2 M NaCl draw solution were used. A framework was proposed to evaluate the governing factors under different conditions and to provide insights into the membrane optimization for targeted OM applications.

Graphical Abstract

    1. Introduction

    Osmotically-driven membrane (OM) technologies, e.g., forward osmosis (FO) and pressure-retarded osmosis (PRO), have received much attention, largely fueled by the development of commercial FO membranes with high water flux in the last decade. In an OM process, a semi-permeable membrane is used to separate a high concentration solution (draw solution, DS) and a low concentration solution (feed solution, FS) (Figure 1). Water flux from FS to DS is driven by the osmotic pressure difference (Δπ). Components in FS can be retained and concentrated, whereas water passing through the membrane can be recovered from the diluted DS by a re-concentration step if required. Niche applications using DSs without the need of regeneration (e.g., seawater [1], desalination brine [2], or fertilizer solutions [3]) are attractive due to the elimination of energy-intensive DS re-concentration step. OM and hybrid OM processes have been explored for diverse fields [4], such as wastewater reuse [5,6,7], desalination [8,9,10,11], osmotic membrane bioreactor [12], energy production [13,14,15], and food processing [16,17].
    High-performance membranes are crucial for OM technologies. Typical osmotic membranes consist of an active rejection layer and a porous support layer. The support layer, usually orders of magnitude thicker than the ultrathin active layer, hinders the mass transfer within membrane and causes internal concentration polarization (ICP) (Figure 1). Performance of membranes are often limited by ICP that reduces the effective osmotic pressure difference across the active layer [19,20]. Concentrative ICP of feed solutes occurs in the active-layer-facing-draw-solution (AL-DS) orientation, and dilutive ICP of draw solutes occurs in the active-layer-facing-feed-solution (AL-FS) orientation [21]. If the membranes have low rejection, reverse diffusion of draw solutes into the FS can further enhance ICP [22]. Due to the ICP phenomenon, membranes with a compact support layer (e.g., those used for reverse osmosis (RO) and nanofiltration (NF)) usually exhibit very low water flux in OM tests.
    Some promising membranes for OM process have been developed in recent years. Membranes for FO processes were first commercialized by Hydration Technology Innovations (HTI, formerly Osmotek, Albany, OR, USA) [23]. Their first generation of membranes were flat-sheet cellulose triacetate (CTA) integral asymmetric membranes [24]. These membranes showed superior FO water flux compared to commercial RO membranes, and they have been widely used in OM study. CTA hollow fiber FO and PRO membranes have been developed by Toyobo (Osaka, Japan) [25,26]. Commercial thin film composite (TFC) FO membranes were also provided by a few companies, including HTI [27], Oasys Water (Boston, MA, USA) [28], Toray Chemical Korea (Seoul, Korea) [29], Woongjin Chemical (Seoul, Korea) [30], Porifera Inc. (San Leandro, CA, USA) [31], Aquaporin (Kongens Lyngby, Denmark) [32,33], etc. In parallel, considerable studies of lab-scale membrane fabrication have been reported [34,35,36]. These membranes had different structures, e.g., integral asymmetric membrane [37,38], TFC membrane [39,40,41], double-skinned membrane [42], dual-layer membrane [43], etc. According to the water and salt permeability, these membranes can be classified into RO-like [39,40], NF-like [22,44,45], and ultrafiltration (UF)-like [46] membranes. Generally, the criteria of high-performance OM membranes include high water permeability (A), low solute permeability (B), and small structural parameter (S), as well as other properties, such as fouling resistance and mechanical strength. Nevertheless, improvement of OM performance is constrained by the strong trade-off among these parameters [47] (e.g.,A andB), and it is further complicated by its non-linear dependence on membrane properties and FS/DS concentrations [22]. Consequently, membrane optimization and selection shall be based on a set of comprehensive criteria, as well as understanding of the dominant mechanisms that govern the OM performance.
    The objectives of this work were to study the influence of membrane permeability and structure on OM performance and to develop efficient strategies for OM flux enhancement. Commercial FO membranes with different structure, separation property and chemical property were systematically characterized to study the trade-off of key membrane properties. According to the FS and DS sources in potential applications, a series of OM experiments were conducted to assess the advantages and drawbacks of these membranes. The OM performance especially the water flux and FO efficiency of membranes were evaluated and compared. Membranes reported in the FO literature were also investigated to validate the comprehensive influence of membrane permeability and structure on water flux. Based on this study, a framework was developed to identify the factors that govern OM flux under different testing conditions, and to provide implications for the evaluation and optimization of membranes in OM processes.

    2. Materials and Methods

    2.1. Membranes and Chemicals

    Four flat-sheet FO membranes, i.e., TFC, CTA-W(P), CTA-W, and CTA-NW were received from Hydration Technology Innovations (HTI, Albany, OR, USA), which are still commercially available from Fluid Technology Solutions, Inc. (FTSH2O, Albany, OR, USA) [48]. According to the manufacturer, the thin film composite membrane (referred to as TFC) had a mesh embedded in the substrate. Two of the integral asymmetric membranes (referred to as CTA-W(P) and CTA-W) had an embedded woven mesh. The integral asymmetric membrane with a nonwoven fabric was referred to as CTA-NW. All the membranes were stored in pure water before characterization.
    Sodium chloride (NaCl), sodium sulfate (Na2SO4) and calcium chloride (CaCl2) for OM tests were purchased from Merck (Darmstadt, Germany). All chemicals were analytical grade and were used as received. Ultrapure water with resistivity of 18.2 MΩ·cm was supplied by a Milli-Q water system (Milli-pore, Darmstadt, Germany).

    2.2. Membrane Characterization

    2.2.1. Characterization of Membrane Structure, Morphology and Chemical Property

    Membrane structure and morphology were observed by field-emission scanning electron microscopy (FESEM, JSM-7600F, JEOL, Akishima, Japan). The membranes were freeze-dried before characterization. Samples were sputter coated a thin layer of platinum before FESEM scanning. The images were obtained with secondary electrons at 5.0 kV. Chemical compositions of membranes were characterized by Fourier transform infrared spectroscopy (FTIR, Spectrum 2000 FTIR Spectrometer, PerkinElmer, Boston, MA, USA) in attenuated total reflectance (ATR) mode. Porosity of membranes was evaluated by gravimetric method. Membrane samples were saturated with water and then thoroughly dried. The wet mass and dry mass were measured to calculate the porosity in accordance to previous work [24]. Surface roughness of membranes was characterized by atomic force microscopy (AFM, XE-100, Park Systems, Suwon, Korea). The root mean squared roughness (Rq) of membrane rejection layer was reported. Water contact angles of membranes were measured by static sessile drop method using a contact angle system (OCA, Dataphysics, Filderstadt, Germany). Mechanical strength of membranes was measured using a tensile testing system (Instron 5567, Norwood, MA, USA). For membranes with a mesh as reinforcing layer (TFC, CTA-W(P), and CTA-W), tensile testing was performed with tension in both the axial direction along filaments and the diagonal direction of mesh opening.

    2.2.2. Measurement of Membrane Permeability

    The water and salt permeabilities of membranes were determined in a cross-flow RO filtration setup (Figure S1). The temperature of feed was maintained at 23 ± 1 °C and the feed pressure was 5 bar for all tests. Water permeability coefficient (A) was measured using pure water as feed. To measure the salt rejection of membrane, 10 mM NaCl or 10 mM Na2SO4 solutions were used as feeds. The concentrations of feed and permeate were measured using a conductivity meter (UltraMeter II™ 4P, Myron L Company, Carlsbad, CA, USA) to calculate the salt rejection (R) and salt permeability coefficient (B) [22]. TheA,R, andB values of membranes were calculated according to:
    A=JΔP,
    R=Cf−CpCf×100%,
    B=1R−1×J,
    whereJ is the flux of permeate; ΔP is the transmembrane pressure;Cf andCp are the salt concentrations of feed and permeate, respectively.

    2.2.3. Evaluation of OM Performance

    FO performance of membranes were evaluated in a cross-flow FO setup in both AL-DS and AL-FS orientations in accordance to previous studies [22,24]. NaCl solutions with concentrations of 0, 10 mM, and 0.5 M were used as FS to simulate pure water, freshwater and seawater. NaCl solutions with concentrations of 0.2 M, 0.5 M, and 2 M were used as DS to simulate brackish water, seawater, and high-salinity brine. A cross-flow velocity of 23.2 cm/s was applied to both FS and DS channels. Water flux (Jv) and salt flux (Js) was calculated as the permeate rate of water and salt per unit of membrane area [22,24]. FO efficiency (Jv/Jv Theo) was calculated as the ratio of experimental FO water flux over the theoretical water flux (Jv Theo =AΔπ) that can be achieved when there was no concentration polarization. Mass transfer coefficient (Km) and structural parameter (S) of membranes were calculated according to the ICP model [49], as follows:
    AL−DS: Jv=Kmlnπdraw−JvA+BAπfeed+BA,
    AL−FS: Jv=Kmlnπdraw+BAπfeed+JvA+BA,
    Km=DS,
    whereπdraw andπfeed are the osmotic pressures of DS and FS, respectively;D is the diffusion coefficient of solute.
    FO scaling experiments were conducted using gypsum as a model scalant. The tests were performed in the AL-FS mode. This mode is preferred in FO applications with low scaling/fouling potential as it can eliminate the internal clogging of support layer and achieve more stable flux in long-term operation [50]. In the scaling tests, mixed solution containing Na2SO4 (72 mM), CaCl2 (26.1 mM), and NaCl (10 mM) was used as FS, of which the gypsum saturation index (SI) was 2.0 [51]. Baseline tests were performed using 0.163 M NaCl FS, of which the SI was 0 and the osmotic pressure (7.5 bar) was similar with that of scaling test. DS with concentration of 2–3 M NaCl was used to adjust initialJv to a moderate value of 15 L/(m2 h) for each test. A cross-flow velocity of 23.2 cm/s was applied to FS and DS channels.
    PRO tests were performed in a cross-flow PRO setup as in reference [52]. Membrane coupons were loaded in membrane cell with AL-DS orientation. Net-type spacers were placed on both the FS and DS sides of membrane. 1 M NaCl was used as DS to simulate seawater desalination brine, and was circulated by a high-pressure pump, while 10 mM NaCl FS was circulated without pressurization. Draw pressure (Pdraw) was increased in steps of approximately 3 bar, until the peak power density was reached. Pressure at FS side (Pfeed) was also monitored to calculate the pressure difference (ΔP =Pdraw −Pfeed). During PRO tests, pure water was dosed into FS to maintain a constant feed volume.Jv andJs at each pressure were measured as the water and salt permeate rate per unit of membrane area. Power density (W) of membrane was calculated as:
    W=Jv×ΔP.

    3. Results and Discussions

    3.1. Structure, Morphology, and Chemical Property of Membranes

    Morphology of the membranes was characterized using FESEM and AFM. TFC had a three-layer structure (Figure 2a): (1) a polyamide (PA) top layer featured with a ridge-and-valley surface morphology (Figure 2b,c) and characteristic FTIR peaks at 1659 cm−1 (amide I band), 1609 cm−1 (aromatic amide) and 1549 cm−1 (amide II band) [53] (Figure 3a), (2) a polysulfone (PSf) support layer with finger-like pore structure, and (3) a polyester (PET) mesh embedded at the bottom of PSf layer for mechanical reinforcement. The thin and porous support layer (thickness of 105 μm and porosity of 66%,Table 1) resulted in a smallS value (1.14 mm), which was an order of magnitude smaller than that of typical commercial TFC RO membranes [24].
    According to the FTIR results, CTA-W(P), CTA-W, and CTA-NW were all prepared with cellulose triacetate (Figure 3b). These membranes had smoother surfaces compared to the polyamide-based TFC. CTA-W(P) was an integral asymmetric membrane with a woven PET mesh embedded in the CTA layer (Figure 2d). Its CTA layer with finger-like pore structure had similar porosity (62%) but smaller thickness (52–71 μm) compared to TFC. Consequently, CTA-W(P) had a smallerS value of 0.73 mm. Structural property of CTA-W and CTA-NW had been reported in our previous study [24]. FESEM micrographs of their cross-sections can be found inAppendix A (Figure A1). CTA-W had a similar structure as CTA-W(P) with the same type of woven PET mesh embedded in support layer. However, CTA-W had a thinner and less porous CTA layer without macrovoid (Figure A1a). TheS value of CTA-W (0.82 mm) was slightly higher than that of CTA-W(P). Different from CTA-W(P) and CTA-W, CTA-NW was reinforced by a nonwoven fabric attached at the bottom of CTA layer (Figure A1b). To mitigate ICP, the nonwoven fabric of CTA-NW was much looser compared to those used in commercial RO membranes. The CTA layer of CTA-NW had similar thickness (~60 μm) but less porous finger-like pore structure compared to CTA-W(P). Due to the large total membrane thickness (129 μm), CTA-NW had the largestS value (1.77 mm) of the four membranes.
    Mechanical property of the membranes was characterized by tensile testing. Due to the porous structure, the mechanical strengths of PSf and CTA support layers were lower than the intrinsic values of bulk polymers. The membranes were strengthened by reinforcing fabric. The efficiency of fabric reinforcement depended on the orientation of fabric filaments. For membranes reinforced by plain woven mesh (TFC, CTA-W(P), and CTA-W), tensile tests were conducted at both axial and diagonal directions, as illustrated inFigure A2 inAppendix B. Membranes showed higher Young’s moduli and less strain at the point of break when tension was applied in the axial direction of mesh filaments (Table 1), implying a stronger reinforcement effect in this direction. For membrane reinforced by nonwoven fabric (CTA-NW), repeated measurements were conducted with tension at random directions. It showed isotropic mechanical property due to the randomly orientated filaments of nonwoven fabric. TFC membranes for pressure-driven process usually consist of a more compact nonwoven reinforcing fabric to reduce anisotropic stress and membrane deformation. The TFC FO membrane in this study showed compromised mechanical strength, as a result of the higher porosity and macrovoids in the PSf layer, as well as the mesh support (Figure 2a). Trade-off between mechanical strength andS value was also observed in the CTA membranes. For example, CTA-W with a dense support layer can stand higher stress at the point of break than CTA-W(P); however, it also resulted in a slightly largerS value.

    3.2. Intrinsic Separation Property of Membranes

    Water and salt permeabilities of membranes are shown inTable 1. The studied membranes had moderate to high rejections to NaCl and Na2SO4. CTA-W(P) had a water permeability of 1.06 L/(m2 h bar)), while lower water permeability of 0.41 L/(m2 h bar) and 0.53 L/(m2 h bar) were shown by CTA-W and CTA-NW, respectively. Compared to CTA-W(P), the dense structure of CTA-W was likely responsible for the increased hydraulic resistance. The low permeability of CTA-NW was mainly contributed by its thicker structure. Trade-off between membrane permeability and selectivity can be seen for the CTA membranes. HigherA value of CTA-W(P) was accompanied with a higherB/A value compared to CTA-W and CTA-NW. To overcome the limitation of CTA membrane, other types of membranes were also developed for OM process.Table A1 inAppendix C lists the intrinsic separation property of several commercial FO membranes reported. Most of these membranes were TFC membranes. Compared to the integral asymmetric CTA membranes, these TFC membranes had higherA values and smallerB/A values. As shown inTable 1,A value of TFC was 2.56 L/(m2 h bar), which was two times higher than that of CTA-W(P). The improvement could be attributed to the thinner and more hydrophilic polyamide layer (Table 1). Superior combination of water permeability and selectivity was also exhibited by TFC.BNaCl/A andBNa2SO4/A of TFC were 0.25 bar and 0.053 bar, respectively, both lower than those of CTA-W(P). As the salt flux/water flux (Js/Jv) ratio is proportional toB/A value of membrane in OM process, the solute reverse diffusion of TFC would be less severe. Nevertheless, OM water flux does not linearly increase with membrane permeability and it also depends on the support layer. In this work, CTA-W(P) was used as a benchmark to comprehensively evaluate the influence of membrane property on OM performance.

    3.3. Membrane Performance in Osmotically-Driven Processes

    3.3.1. FO Performance of Membranes

    The FO performance of TFC, CTA-W(P), CTA-W and CTA-NW were evaluated using 0.5 M NaCl DS and 10 mM NaCl FS. In AL-DS mode, TFC had the highestJv and smallJs/Jv (Figure A3a,b inAppendix D). For the integral asymmetric CTA membranes, CTA-W and CTA-NW had relatively lowJv that was approximately half ofJv CTA-W(P). It was mainly attributed to the lowA values of CTA-W and CTA-NW, which were only half ofACTA-W(P). CTA-W(P) also exhibited higherJv than CTA-W and CTA-NW when 2 M NaCl DS was used, in both AL-DS and AL-FS modes (Figure A3a,c). All the three CTA membranes in this study had moderateJs/Jv (<1 g/L) (Figure A3b,d). In general, CTA-W(P) was more promising for OM applications than the other two CTA membranes due to its higherA value, smallerS value and moderateB value. Thus, CTA-W(P) was used for further OM study in this work.
    As discussed in the above, TFC exhibited higherJv than CTA-W(P) in AL-DS mode when 0.5 M NaCl DS and 10 mM NaCl FS were used. However, when a more concentrated DS of 2 M NaCl was used,Jv CTA-W(P) was about 30% higher thanJv TFC in both AL-DS and AL-FS modes (Figure 4). In OM processes,Jv is mainly governed by membrane frictional resistance loss, ICP, and solute reverse diffusion. Due to the lowJs/Jv of TFC and CTA-W(P), membrane frictional resistance loss and ICP played more important roles onJv. ICP was less severe when FS and DS concentrations were low, leading to a greater importance ofA value forJv. Under such frictional-resistance-dominated conditions, the more permeable TFC achieved higherJv. When a more concentrated DS was used, however, ICP governedJv and CTA-W(P) with smallerS value performed better.
    To further elucidate the governing mechanisms, forward osmosis (FO) tests with different FS and DS concentrations were conducted on TFC and CTA-W(P).Jv of TFC and CTA-W(P) are compared inFigure 4. With a DI water FS and a low-concentration DS of 0.2 M NaCl (AL-DS), ICP was less severe, represented by a high FO efficiency of 0.70 for CTA-W(P). TFC achieved significantly higherJv than that of CTA-W(P) (Jv TFC/Jv CTA-W(P) = 1.82,Figure 4a).Jv TFC/Jv CTA-W(P) ratio was reduced as the concentrations of FS and DS increased.Jv CTA-W(P) was higher thanJv TFC when 2 M NaCl DS was used, especially when a concentrated FS of 0.5 M NaCl was used (Jv TFC/Jv CTA-W(P) = 0.55, AL-DS). This was attributed to the remarkably enhanced influence of ICP in this scenario, represented by a lower FO efficiency of 0.18 for CTA-W(P). In AL-FS mode,Jv TFC/Jv CTA-W(P) was 1.18 using 0.2 M NaCl DS and DI water FS (Figure 4b), which was significantly lower thanJv TFC/Jv CTA-W(P) in AL-DS mode. A low FO efficiency of 0.56 for CTA-W(P) implicated the more severe influence of dilutive ICP (AL-FS) than concentrative ICP (AL-DS) when identical FS and DS were used. Therefore, in AL-FS mode, more osmotic driving force was lost for TFC with largerS value. It can be seen from the above results that assessment of membranes in OM process should be based on membrane property and operational condition. Membranes with higherJv in one condition may show inferiorJv in another condition. In view of the non-linear dependence of OM performance on membrane properties and the trade-off of these properties, their combined influences in different conditions will be further discussed.

    3.3.2. Anti-Scaling Performance of Membranes

    OM applications can use different types of saline water, which may contain sparingly soluble ions and impurities with high concentrations. For example, in desalination-hybrid processes where seawater or seawater desalination brine is used, a great amount of scaling precursors like Ca2+, Mg2+, SO42−, and CO32− exist [54]. Sparingly soluble salts may precipitate and deposit on membrane during operation. In this study, FS containing Ca2+ and SO42− were used to investigate the impact of scaling on TFC and CTA-W(P). The membranes were tested in AL-FS mode with mitigated internal scaling [50]. Water flux of TFC and CTA-W(P) in the scaling tests are shown inFigure 5. CTA-W(P) exhibited stable performance during the scaling test of 18 h, without significant variation ofJv compared to the baseline. In contrast,Jv of TFC drastically declined after 3 h of scaling test. Severe leakage of draw solutes into FS took place after test of 5 h and resulted in the interruption of operation. Damage at the polyamide layer was observed when the tested membrane was later examined under FESEM (Figure S2). In the scaling test, Ca2+ and SO42− in FS were concentrated and caused precipitation of gypsum crystals. According to the FESEM and AFM images of membrane rejection layer (Figure 2), TFC had a rougher surface (roughnessRq = 69 nm) than that of CTA-W(P) (Rq = 5 nm) (Table 1). Moreover, the carboxylic groups of polyamide layer provided binding site for Ca2+ and led to surface crystallization of gypsum [55]. Therefore, faster deposition and growth of gypsum crystals were prone to take place on TFC. Crystallization beneath feed spacer filaments could also cause damage to membrane rejection layer, which tended to be more severe for the TFC membrane due to its thinner rejection layer [51]. Thus, CTA-W(P) had advantage in this condition because of its surface morphology, chemical property, and structure.

    3.3.3. PRO Performance of Membranes

    TFC and CTA-W(P) were tested in PRO process to estimate their efficiency of osmotic power production. Both membranes can withstand transmembrane pressure of 0–22 bar without burst. As ΔP gradually increased, water permeation through membrane was retarded.Jv of TFC declines less than that of CTA-W(P) upon pressurization (Figure 6a). TFC achieved a peak power density of 8.4 W/m2 at ΔP of 20 bar (Figure 6b). The peak power density of CTA-W(P) was 4.8 W/m2 at ΔP of 13 bar. In PRO process, the theoretically maximum power density can be achieved at ΔP = 1/2 Δπ, which was 22 bar in the current testing condition (10 mM NaCl FS and 1 M NaCl DS). As inFigure 6c,Js/Jv of CTA-W(P) sharply increased at ΔP > 10 bar. CTA-W(P) had relatively lower intrinsic salt rejection than TFC. Prior studies revealed that osmotic membranes are prone to rejection deterioration at high applied pressure due to the stretching of membranes [56]. CTA membrane with linear polymer network became loose after deformed and caused severe salt leakage. Therefore, the peak power density of CTA-W(P) was reached at a ΔP much lower than the theoretically optimal value. In contrast, TFC with superior salt rejection and a more rigid cross-linked polyamide rejection layer can minimize the influence of solute reverse diffusion and showed higher PRO performance.

    4. Implications

    The study of TFC and CTA-W(P) showed the non-linear dependence of OM water flux on membrane property. It was complicated with the trade-off of critical membrane properties, e.g.,A,B/A,S, and mechanical strength, and increased the complexity of membrane optimization. FO membranes in the literature were investigated to study the comprehensive influence of membrane permeability and structure onJv. Experimental data of the membranes in the current work and the literature (Table A1,Tables S1 and S2) were plotted according to two potential applications: (1) dehydration of low salinity solutions using seawater as DS (Figure 7a,b), and (2) seawater desalination using high concentration DS (Figure 7c,d). AsJv is positively related to bothA andKm,Km was plotted instead ofS for better illustration and understanding. The isolines present datapoints with the sameJv, which were achieved by membranes with differentA andKm. HigherJv were mostly exhibited by membranes with higherA andKm, if the solute reverse diffusion was low. Meanwhile, there would be a governing factor in a specific case, depending on the combination ofA andKm. For example, whenKm was smaller than 20 L/(m2 h) (i.e., S > 0.3 mm), the increase ofJv withKm was more remarkable while increasingA had less effect onJv if the membranes had moderateA value. It means that improving membrane structure can significantly improveJv. On the other hand, for membranes with optimized structure (e.g.,Km > 20 L/(m2 h)) but low water permeability (e.g., A < 2 L/(m2 h bar)), the increase ofJv withA was more significant, suggesting thatA was the governing factor onJv. The membrane property dominating OM performance also varied by testing condition. Taking CTA-W(P) (A = 1.06 L/(m2 h bar),Km = 7.9 L/(m2 h)) as an example, the constraint of its low water permeability onJv was crucial in the test with low concentration FS and AL-DS mode (Figure 7a). As this condition was frictional-resistance-dominated, improving theA value of CTA-W(P) can effectively increaseJv. In contrast, the constraint ofKm onJv became more important when seawater was used as FS (Figure 7c,d), i.e.,Jv varied more obviously withKm instead ofA value, due to the dominant effect of ICP. Comparison of the above membranes showed that the dominant factor and mechanism for OM flux varied by membrane property and OM conditions.
    A framework was proposed to illustrate the mechanisms dominating flux performance, and to guide the optimization of membranes for targeted OM applications. A few potential modes of OM applications have been studied inSection 3.3. The FO efficiency (Jv/Jv Theo) of CTA-W(P) varied with the FS, DS, and membrane orientation due to the comprehensive influence of concentration polarization, frictional resistance loss and solute reverse diffusion (Figure 8). With CTA-W(P) as a benchmark, pros and cons of TFC were assessed using this framework (Figure 8). When 10 mM NaCl FS and 0.5 M NaCl DS were used, moderate FO efficiency around 0.5 was showed by CTA-W(P). Both ICP and frictional resistance loss constrained the performance of CTA-W(P). TFC had comparableJv as CTA-W(P) under this condition, and using TFC as an alternative might not significantly enhanceJv, but its excellent salt rejection would be beneficial to reduce the solute reverse diffusion. Higher FO efficiency is shown at the right side of the diagram. The corresponding OM conditions (e.g., tests with AL-DS mode, dilute FS and DS) had less ICP, andJv was likely to be governed by frictional resistance loss mechanism.Jv TFC/Jv CTA-W(P) sharply increased in the conditions with high FO efficiency. In an ideal case without ICP (FO efficiency = 1),Jv will be linearly proportional toA. InFigure 8,Jv TFC/Jv CTA-W(P) tended to approximateATFC/ACTA-W(P) at the right side of the diagram. For applications with less ICP, indicated by high FO efficiency, the influence of frictional resistance loss is critical.Jv can be significantly increased by using membranes with higherA value (e.g., TFC).
    Lower FO efficiency is shown at the left side of the diagram (Figure 8). Results of tests with high ICP potential, e.g., tests with AL-FS mode, concentrated FS or DS, are mostly in this area. TFC exhibited lowerJv thanJv CTA-W(P) here despite of the highATFC, andKm was the governing factor. In harsh condition like FO desalination with seawater as FS and high concentration DS, Equations (4) and (5) can be derived asJv =Kmln(πdraw/πfeed) [49].Jv will be almost linearly proportional toKm asπdraw/πfeed is constant. The performance of TFC and CTA-W(P) with 0.5 M FS and 2 M DS agreed with this equation.Jv TFC/Jv CTA-W(P) tended to approximateKm TFC/Km CTA-W(P) in both AL-DS and AL-FS modes (Figure 8). IncreasingA value of membrane would have minor effect onJv but might compromise the membrane selectivity. Therefore, in applications dominated by ICP, indicated by low FO efficiency of the benchmark membrane, optimization or selection of membrane should be targeted to have improved mass transfer coefficient. Moreover, active layer with higher salt rejection, and lower fouling propensity for systems with high saturation index, would be important to reduce the salt leakage andJs-induced ICP.

    5. Conclusions

    In this work, the influence of membrane permeability and structure on OM performance was studied by systematically characterizing and evaluating the OM performance of commercial osmotic membranes. The polyamide-based TFC membrane with superior water permeability and salt rejection had high water flux in the FO conditions with low ICP, as well as in PRO tests with high hydraulic pressure. In FO tests with more severe ICP, however, an integral asymmetric CTA membrane (CTA-W(P)) with optimized structure of support layer showed higher water flux, despite of its lower water permeability coefficient. In OM process, flux is highly non-linear with respect to membrane property. The trade-off of membrane properties imposes limitations on membrane optimization. This study established a framework based on the FO efficiency to identify the factors governing water flux in different conditions. High FO efficiency indicates the dominant effect of frictional resistance loss, and improvement of the membrane water permeability would be more effective to achieve higher water flux. On the other hand, low FO efficiency is usually exhibited in the ICP-dominated condition, and membranes with smaller structural parameter should be used to enhance OM performance.

    Supplementary Materials

    The following are available online athttps://www.mdpi.com/2077-0375/11/2/153/s1, S1. Setup for Membrane Permeability Measurement, S2. Surface Morphology of TFC Membrane after Scaling Test, S3. Property and Performance of Lab-Scale FO Membranes in the Literature.

    Author Contributions

    Conceptualization, J.W. and Q.S.; Methodology, J.W.; Investigation, J.W., Q.S. and X.L.; Writing—Original Draft Preparation, J.W.; Writing—Review & Editing, J.W., Q.S. and X.L.; Supervision, Q.S. All authors have read and agreed to the published version of the manuscript.

    Funding

    This research was funded by Singapore National Research Foundation under its Environment and Water Industry Program Office (EWI) (1102-IRIS-07-01), National Natural Science Foundation of China (51808264), Collaborative Innovation Center of Water Treatment Technology & Material (XTCXSZ2020-4) and Senior Talent Funding of Jiangsu University (18JDG031).

    Institutional Review Board Statement

    Not applicable.

    Informed Consent Statement

    Not applicable.

    Conflicts of Interest

    The authors declare no conflict of interest.

    Appendix A. Structure of CTA-W and CTA-NW Membranes

    Cross-sections of CTA-W and CTA-NW are shown inFigure A1. CTA-W had a similar structure as CTA-W(P), consisting of a CTA rejection layer and a mesh embedded in the CTA layer. Compared to CTA-W(P), the CTA layer of CTA-W was thinner and denser without macrovoid, which resulted in a slightly largerS value of 0.82 mm. CTA-NW comprised a CTA layer and a nonwoven fabric (thickness of ~80 μm) supported on the bottom. Finger-like pore structure was shown in the CTA layer. Due to the thick and dense nonwoven fabric support, CTA-NW had a smaller porosity of 55% and thicker cross-section (129 mm) than CTA-W(P), as well as had a largeS value of 1.77 mm.
    Membranes 11 00153 g0a1 550
    Figure A1. FESEM micrographs of the cross-sections (at 500×) of (a) CTA-W and (b) CTA-NW.
    Figure A1. FESEM micrographs of the cross-sections (at 500×) of (a) CTA-W and (b) CTA-NW.
    Membranes 11 00153 g0a1

    Appendix B. Tensile Strength Tests

    Mechanical property of the mesh-reinforced TFC, CTA-W(P), and CTA-W membranes was measured with tension in the axial direction along filaments, and the diagonal direction of mesh opening, as shown inFigure A2.
    Membranes 11 00153 g0a2 550
    Figure A2. Illustration of tensile testing with tension in (a) axial direction and (b) diagonal direction.
    Figure A2. Illustration of tensile testing with tension in (a) axial direction and (b) diagonal direction.
    Membranes 11 00153 g0a2

    Appendix C. Property and Performance of Commercial FO Membranes

    The separation property and FO performance of membranes in this study was compared with other commercial FO membranes reported in the literature. TFC membranes generally showed higher water permeability and selectivity than integral asymmetric membranes (Table A1). With support layer tailored to have small structural parameter, the TFC membranes achieved high water flux in OM process.
    Table
    Table A1. Property and FO performance of commercial membranes.
    Table A1. Property and FO performance of commercial membranes.
    Membrane IDManufacturerMembrane StructureMembrane MaterialMembrane PropertyFO Performance and Testing ConditionsReference
    A
    (L/(m2 h bar))    		BNaCl
    (L/(m2 h))    		BNaCl/A
    (bar)    		S
    (mm)    		Jv
    (L/(m2 h))    		Js
    (g/(m2 h))    		Js/Jv
    (g/L)    		Membrane OrientationFeed SolutionDraw Solution
    1TFCHTITFC flat-sheet membrane with meshPA/PSf/PET2.56 ± 0.080.63 ± 0.210.25 ± 0.081.14 ± 0.1215.82 ± 2.014.54 ± 2.440.28 ± 0.13AL-DS10 mM NaCl0.5 M NaClCurrent work
    8.51 ± 0.252.74 ± 0.990.32 ± 0.11AL-FS10 mM NaCl0.5 M NaCl
    8.07 ± 1.81N.A.N.A.AL-DS0.5 M NaCl2 M NaCl
    7.35 ± 0.83N.A.N.A.AL-FS0.5 M NaCl2 M NaCl
    2CTA-W(P)HTIAsymmetric flat-sheet membrane with meshCTA/PET1.06 ± 0.070.67 ± 0.240.65 ± 0.270.73 ± 0.3114.85 ± 0.3412.80 ± 1.880.86 ± 0.14AL-DS10 mM NaCl0.5 M NaCl
    8.88 ± 0.417.05 ± 0.220.80 ± 0.06AL-FS10 mM NaCl0.5 M NaCl
    14.65 ± 0.64N.A.N.A.AL-DS0.5 M NaCl2 M NaCl
    11.90 ± 0.69N.A.N.A.AL-FS0.5 M NaCl2 M NaCl
    3CTA-WHTIAsymmetric flat-sheet membrane with meshCTA/PET0.41 ± 0.010.15 ± 0.010.38 ± 0.020.82 ± 0.397.07 ± 0.323.78 ± 0.620.54 ± 0.10AL-DS10 mM NaCl0.5 M NaCl
    5.16 ± 0.171.94 ± 1.240.37 ± 0.24AL-FS10 mM NaCl0.5 M NaCl
    9.71 ± 0.85N.A.N.A.AL-DS0.5 M NaCl2 M NaCl
    8.32 ± 0.05N.A.N.A.AL-FS0.5 M NaCl2 M NaCl
    4CTA-NWHTIAsymmetric flat-sheet membrane with nonwoven fabricCTA/PET0.53 ± 0.030.16 ± 0.040.31 ± 0.061.77 ± 0.207.03 ± 0.372.37 ± 10.35 ± 0.01AL-DS10 mM NaCl0.5 M NaCl
    3.54 ± 0.211.03 ± 0.270.29 ± 0.09AL-FS10 mM NaCl0.5 M NaCl
    4.41 ± 0.48N.A.N.A.AL-DS0.5 M NaCl2 M NaCl
    4.14 ± 0.23N.A.N.A.AL-FS0.5 M NaCl2 M NaCl
    5OasysOasys WaterTFC flat-sheet membrane with fabricPA/PSf/PET4.25 ± 0.040.38 ± 0.110.089a0.483 ± 0.079~32b~7b0.22aAL-DSDI water~0.5 M NaCl[28]
    ~15b~2b0.13aAL-FSDI water~0.5 M NaCl
    6PoriferaPoriferaTFC flat-sheet membrane with meshPA-based2.20.500.23a0.34415.6 ± 0.510.30a0.66 ± 0.02AL-FSDI water0.5M NaCl[31]
    7WJ-FOWoongjin ChemicalTFC flat-sheet membranePA-based2.7 ± 1.120.83 ± 0.470.3220.55 ± 0.21~11b16.5a1.5AL-FSDI water0.5 M NaCl[30]
    8TCK-NToray Chemical KoreaTFC flat-sheet membrane with nonwoven fabricPA/PSf/PET6.6011.190.18a0.461~56b~34b0.61aAL-DSDI water1 M NaCl[29]
    ~28b~15b0.54aAL-FSDI water1 M NaCl
    9TCK-WToray Chemical KoreaTFC flat-sheet membrane with meshPA/PSf/PET5.3620.950.18a0.266~64b~25b0.39aAL-DSDI water1 M NaCl
    ~38b~12b0.32aAL-FSDI water1 M NaCl
    10Aquaporin InsideAquaporin A/S, DenmarkTFC flat-sheet membraneAquaporin-PA-based0.52 ± 0.060.090.17a0.825~8b~3b~0.4bAL-DSDI water0.5 M NaCl[32]
    ~5b~1.5b~0.32bAL-FSDI water0.5 M NaCl
    11HF-AToyoboAsymmetric hollow fiber membraneCTA0.2660.080.301.024~6bN.A.N.A.AL-DSDI water0.6 M NaCl[25]
    ~3bN.A.N.A.AL-FSDI water0.6 M NaCl
    12HF-BToyoboAsymmetric hollow fiber membraneCTA0.2910.020.0690.724~6bN.A.N.A.AL-DSDI water0.6 M NaCl
    ~4bN.A.N.A.AL-FSDI water0.6 M NaCl
    13HF-CToyoboAsymmetric hollow fiber membraneCTA0.5450.040.0730.639~10bN.A.N.A.AL-DSDI water0.6 M NaCl
    ~6bN.A.N.A.AL-FSDI water0.6 M NaCl
    a The value was calculated based on the data provided in references.b Data were obtained from the figures in the references.

    Appendix D. FO Performance of Membranes

    FO performance of the membranes were tested with DS of 0.5 M and 2 M NaCl, and FS of 10 mM NaCl (Figure A3). ComparableJv were shown by TFC and CTA-W(P) with 0.5 M NaCl DS. TFC had remarkably lowerJs/Jv due to the superior salt rejection of polyamide rejection layer. Nevertheless, TFC had lowerJv than CTA-W(P) when 2 M DS was used. Due to the constraint of lowA values, CTA-W and CTA-NW had lowJv in the tests. CTA-NW with largeS value encountered more severe ICP and showed the lowestJv of all the membranes.
    Membranes 11 00153 g0a3 550
    Figure A3. FO water flux and salt flux/water flux of the TFC, CTA-W(P), CTA-W, and CTA-NW membranes. Testing conditions: 10 mM NaCl as FS, and 0.5 M or 2 M NaCl as DS.
    Figure A3. FO water flux and salt flux/water flux of the TFC, CTA-W(P), CTA-W, and CTA-NW membranes. Testing conditions: 10 mM NaCl as FS, and 0.5 M or 2 M NaCl as DS.
    Membranes 11 00153 g0a3

    References

    1. Xue, W.; Tobino, T.; Nakajima, F.; Yamamoto, K. Seawater-driven forward osmosis for enriching nitrogen and phosphorous in treated municipal wastewater: Effect of membrane properties and feed solution chemistry.Water Res.2015,69, 120–130. [Google Scholar] [CrossRef]
    2. Liyanaarachchi, S.; Jegatheesan, V.; Muthukumaran, S.; Gray, S.; Shu, L. Mass balance for a novel RO/FO hybrid system in seawater desalination.J. Membr. Sci.2016,501, 199–208. [Google Scholar] [CrossRef]
    3. Phuntsho, S.; Shon, H.K.; Hong, S.; Lee, S.; Vigneswaran, S. A novel low energy fertilizer driven forward osmosis desalination for direct fertigation: Evaluating the performance of fertilizer draw solutions.J. Membr. Sci.2011,375, 172–181. [Google Scholar] [CrossRef]
    4. Blandin, G.; Ferrari, F.; Lesage, G.; Le-Clech, P.; Héran, M.; Martinez-Lladó, X. Forward Osmosis as Concentration Process: Review of Opportunities and Challenges.Membranes2020,10, 284. [Google Scholar] [CrossRef]
    5. Coday, B.D.; Xu, P.; Beaudry, E.G.; Herron, J.; Lampi, K.; Hancock, N.T.; Cath, T.Y. The sweet spot of forward osmosis: Treatment of produced water, drilling wastewater, and other complex and difficult liquid streams.Desalination2014,333, 23–35. [Google Scholar] [CrossRef]
    6. Valladares Linares, R.; Li, Z.; Sarp, S.; Bucs, S.; Amy, G.; Vrouwenvelder, J.S. Forward osmosis niches in seawater desalination and wastewater reuse.Water Res.2014,66, 122–139. [Google Scholar] [CrossRef]
    7. Ferrari, F.; Pijuan, M.; Rodriguez-Roda, I.; Blandin, G. Exploring Submerged Forward Osmosis for Water Recovery and Pre-Concentration of Wastewater before Anaerobic Digestion: A Pilot Scale Study.Membranes2019,9, 97. [Google Scholar] [CrossRef] [PubMed] [Green Version]
    8. Phuntsho, S.; Hong, S.; Elimelech, M.; Shon, H.K. Forward osmosis desalination of brackish groundwater: Meeting water quality requirements for fertigation by integrating nanofiltration.J. Membr. Sci.2013,436, 1–15. [Google Scholar] [CrossRef]
    9. Blandin, G.; Verliefde, A.R.D.; Tang, C.Y.; Le-Clech, P. Opportunities to reach economic sustainability in forward osmosis–reverse osmosis hybrids for seawater desalination.Desalination2015,363, 26–36. [Google Scholar] [CrossRef]
    10. Altaee, A.; Braytee, A.; Millar, G.J.; Naji, O. Energy efficiency of hollow fibre membrane module in the forward osmosis seawater desalination process.J. Membr. Sci.2019,587, 117165. [Google Scholar] [CrossRef]
    11. Choi, B.G.; Zhan, M.; Shin, K.; Lee, S.; Hong, S. Pilot-scale evaluation of FO-RO osmotic dilution process for treating wastewater from coal-fired power plant integrated with seawater desalination.J. Membr. Sci.2017,540, 78–87. [Google Scholar] [CrossRef]
    12. Wang, X.; Chang, V.W.C.; Tang, C.Y. Osmotic membrane bioreactor (OMBR) technology for wastewater treatment and reclamation: Advances, challenges, and prospects for the future.J. Membr. Sci.2016,504, 113–132. [Google Scholar] [CrossRef]
    13. Werner, C.M.; Logan, B.E.; Saikaly, P.E.; Amy, G.L. Wastewater treatment, energy recovery and desalination using a forward osmosis membrane in an air-cathode microbial osmotic fuel cell.J. Membr. Sci.2013,428, 116–122. [Google Scholar] [CrossRef]
    14. Helfer, F.; Lemckert, C.; Anissimov, Y.G. Osmotic power with Pressure Retarded Osmosis: Theory, performance and trends—A review.J. Membr. Sci.2014,453, 337–358. [Google Scholar] [CrossRef] [Green Version]
    15. Lu, Y.; Jia, J.; Miao, H.; Ruan, W.; Wang, X. Performance Improvement and Biofouling Mitigation in Osmotic Microbial Fuel Cells via In Situ Formation of Silver Nanoparticles on Forward Osmosis Membrane.Membranes2020,10, 122. [Google Scholar] [CrossRef]
    16. Rastogi, N.K. Opportunities and Challenges in Application of Forward Osmosis in Food Processing.Crit. Rev. Food Sci.2016,56, 266–291. [Google Scholar] [CrossRef] [PubMed]
    17. Sant’Anna, V.; Marczak, L.D.F.; Tessaro, I.C. Membrane concentration of liquid foods by forward osmosis: Process and quality view.J. Food Eng.2012,111, 483–489. [Google Scholar] [CrossRef]
    18. She, Q.; Wang, R.; Fane, A.G.; Tang, C.Y. Membrane fouling in osmotically driven membrane processes: A review.J. Membr. Sci.2016,499, 201–233. [Google Scholar] [CrossRef]
    19. Tang, C.Y.; She, Q.; Lay, W.C.L.; Wang, R.; Fane, A.G. Coupled effects of internal concentration polarization and fouling on flux behavior of forward osmosis membranes during humic acid filtration.J. Membr. Sci.2010,354, 123–133. [Google Scholar] [CrossRef]
    20. Tang, C.Y.; She, Q.; Lay, W.C.L.; Wang, R.; Field, R.; Fane, A.G. Modeling double-skinned FO membranes.Desalination2011,283, 178–186. [Google Scholar] [CrossRef]
    21. McCutcheon, J.R.; Elimelech, M. Influence of concentrative and dilutive internal concentration polarization on flux behavior in forward osmosis.J. Membr. Sci.2006,284, 237–247. [Google Scholar] [CrossRef]
    22. Wei, J.; Qiu, C.; Wang, Y.N.; Wang, R.; Tang, C.Y. Comparison of NF-like and RO-like thin film composite osmotically-driven membranes-Implications for membrane selection and process optimization.J. Membr. Sci.2013,427, 460–471. [Google Scholar] [CrossRef]
    23. Cath, T.Y.; Childress, A.E.; Elimelech, M. Forward osmosis: Principles, applications, and recent developments.J. Membr. Sci.2006,281, 70–87. [Google Scholar] [CrossRef]
    24. Wei, J.; Qiu, C.; Tang, C.Y.; Wang, R.; Fane, A.G. Synthesis and characterization of flat-sheet thin film composite forward osmosis membranes.J. Membr. Sci.2011,372, 292–302. [Google Scholar] [CrossRef]
    25. Shibuya, M.; Yasukawa, M.; Takahashi, T.; Miyoshi, T.; Higa, M.; Matsuyama, H. Effect of operating conditions on osmotic-driven membrane performances of cellulose triacetate forward osmosis hollow fiber membrane.Desalination2015,362, 34–42. [Google Scholar] [CrossRef]
    26. Kumano, A.; Marui, K.; Terashima, Y. Hollow fiber type PRO module and its characteristics.Desalination2016,389, 149–154. [Google Scholar] [CrossRef]
    27. Ren, J.; McCutcheon, J.R. A new commercial thin film composite membrane for forward osmosis.Desalination2014,343, 187–193. [Google Scholar] [CrossRef]
    28. Arena, J.T.; Manickam, S.S.; Reimund, K.K.; Brodskiy, P.; McCutcheon, J.R. Characterization and Performance Relationships for a Commercial Thin Film Composite Membrane in Forward Osmosis Desalination and Pressure Retarded Osmosis.Ind. Eng. Chem. Res.2015,54, 11393–11403. [Google Scholar] [CrossRef]
    29. Nguyen, T.P.N.; Jun, B.M.; Lee, J.H.; Kwon, Y.N. Comparison of integrally asymmetric and thin film composite structures for a desirable fashion of forward osmosis membranes.J. Membr. Sci.2015,495, 457–470. [Google Scholar] [CrossRef]
    30. Sahebi, S.; Phuntsho, S.; Tijing, L.; Han, G.; Han, D.S.; Abdel-Wahab, A.; Shon, H.K. Thin-film composite membrane on a compacted woven backing fabric for pressure assisted osmosis.Desalination2017,406, 98–108. [Google Scholar] [CrossRef] [Green Version]
    31. Blandin, G.; Vervoort, H.; D’Haese, A.; Schoutteten, K.; Bussche, J.V.; Vanhaecke, L.; Myat, D.T.; Le-Clech, P.; Verliefde, A.R.D. Impact of hydraulic pressure on membrane deformation and trace organic contaminants rejection in pressure assisted osmosis (PAO).Process Saf. Environ.2016,102, 316–327. [Google Scholar] [CrossRef]
    32. Xia, L.L.; Andersen, M.F.; Helix-Nielsen, C.; McCutcheon, J.R. Novel Commercial Aquaporin Flat-Sheet Membrane for Forward Osmosis.Ind. Eng. Chem. Res.2017,56, 11919–11925. [Google Scholar] [CrossRef]
    33. Ye, W.; Lin, J.; Tækker Madsen, H.; Gydesen Søgaard, E.; Hélix-Nielsen, C.; Luis, P.; Van der Bruggen, B. Enhanced performance of a biomimetic membrane for Na2CO3 crystallization in the scenario of CO2 capture.J. Membr. Sci.2016,498, 75–85. [Google Scholar] [CrossRef] [Green Version]
    34. Yadav, S.; Saleem, H.; Ibrar, I.; Naji, O.; Hawari, A.A.; Alanezi, A.A.; Zaidi, S.J.; Altaee, A.; Zhou, J. Recent developments in forward osmosis membranes using carbon-based nanomaterials.Desalination2020,482, 114375. [Google Scholar] [CrossRef]
    35. Wu, W.; Shi, Y.; Liu, G.; Fan, X.; Yu, Y. Recent development of graphene oxide based forward osmosis membrane for water treatment: A critical review.Desalination2020,491, 114452. [Google Scholar] [CrossRef]
    36. Xu, W.; Chen, Q.; Ge, Q. Recent advances in forward osmosis (FO) membrane: Chemical modifications on membranes for FO processes.Desalination2017,419, 101–116. [Google Scholar] [CrossRef]
    37. Zhang, S.; Wang, K.Y.; Chung, T.S.; Jean, Y.C.; Chen, H. Molecular design of the cellulose ester-based forward osmosis membranes for desalination.Chem. Eng. J.2011,66, 2008–2018. [Google Scholar] [CrossRef]
    38. Su, J.; Yang, Q.; Teo, J.F.; Chung, T.S. Cellulose acetate nanofiltration hollow fiber membranes for forward osmosis processes.J. Membr. Sci.2010,355, 36–44. [Google Scholar] [CrossRef]
    39. Wang, R.; Shi, L.; Tang, C.Y.; Chou, S.; Qiu, C.; Fane, A.G. Characterization of novel forward osmosis hollow fiber membranes.J. Membr. Sci.2010,355, 158–167. [Google Scholar] [CrossRef]
    40. Yip, N.Y.; Tiraferri, A.; Phillip, W.A.; Schiffman, J.D.; Elimelech, M. High performance thin-film composite forward osmosis membrane.Environ. Sci. Technol.2010,44, 3812–3818. [Google Scholar] [CrossRef] [PubMed]
    41. Bui, N.N.; McCutcheon, J.R. Hydrophilic nanofibers as new supports for thin film composite membranes for engineered osmosis.Environ. Sci. Technol.2013,47, 1761–1769. [Google Scholar] [CrossRef] [PubMed]
    42. Fang, W.X.; Liu, C.; Shi, L.; Wang, R. Composite forward osmosis hollow fiber membranes: Integration of RO- and NF-like selective layers for enhanced organic fouling resistance.J. Membr. Sci.2015,492, 147–155. [Google Scholar] [CrossRef]
    43. Setiawan, L.; Wang, R.; Shi, L.; Li, K.; Fane, A.G. Novel dual-layer hollow fiber membranes applied for forward osmosis process.J. Membr. Sci.2012,421, 238–246. [Google Scholar] [CrossRef]
    44. Saren, Q.; Qiu, C.Q.; Tang, C.Y. Synthesis and characterization of novel forward osmosis membranes based on layer-by-layer assembly.Environ. Sci. Technol.2011,45, 5201–5208. [Google Scholar] [CrossRef]
    45. Wang, K.Y.; Chung, T.-S.; Qin, J.-J. Polybenzimidazole (PBI) nanofiltration hollow fiber membranes applied in forward osmosis process.J. Membr. Sci.2007,300, 6–12. [Google Scholar] [CrossRef]
    46. Qi, S.R.; Li, Y.; Zhao, Y.; Li, W.Y.; Tang, C.Y.Y. Highly Efficient Forward Osmosis Based on Porous Membranes-Applications and Implications.Environ. Sci. Technol.2015,49, 4690–4695. [Google Scholar] [CrossRef] [PubMed]
    47. Akther, N.; Phuntsho, S.; Chen, Y.; Ghaffour, N.; Shon, H.K. Recent advances in nanomaterial-modified polyamide thin-film composite membranes for forward osmosis processes.J. Membr. Sci.2019,584, 20–45. [Google Scholar] [CrossRef]
    48. She, Q.; Zhang, L.; Wang, R.; Krantz, W.B.; Fane, A.G. Pressure-retarded osmosis with wastewater concentrate feed: Fouling process considerations.J. Membr. Sci.2017,542, 233–244. [Google Scholar] [CrossRef]
    49. Loeb, S.; Titelman, L.; Korngold, E.; Freiman, J. Effect of porous support fabric on osmosis through a Loeb-Sourirajan type asymmetric membrane.J. Membr. Sci.1997,129, 243–249. [Google Scholar] [CrossRef]
    50. Zhang, M.; Shan, J.; Tang, C.Y. Gypsum scaling during forward osmosis process—A direct microscopic observation study.Desalination Water Treat.2016,57, 3317–3327. [Google Scholar] [CrossRef]
    51. Wang, Y.N.; Järvelä, E.; Wei, J.; Zhang, M.; Kyllönen, H.; Wang, R.; Tang, C.Y. Gypsum scaling and membrane integrity of osmotically driven membranes: The effect of membrane materials and operating conditions.Desalination2016,377, 1–10. [Google Scholar] [CrossRef]
    52. She, Q.; Jin, X.; Tang, C.Y. Osmotic power production from salinity gradient resource by pressure retarded osmosis: Effects of operating conditions and reverse solute diffusion.J. Membr. Sci.2012,401, 262–273. [Google Scholar] [CrossRef]
    53. Tang, C.Y.; Kwon, Y.-N.; Leckie, J.O. Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry.Desalination2009,242, 149–167. [Google Scholar] [CrossRef]
    54. Giagnorio, M.; Ricceri, F.; Tagliabue, M.; Zaninetta, L.; Tiraferri, A. Hybrid Forward Osmosis–Nanofiltration for Wastewater Reuse: System Design.Membranes2019,9, 61. [Google Scholar] [CrossRef] [PubMed] [Green Version]
    55. Xie, M.; Gray, S.R. Gypsum scaling in forward osmosis: Role of membrane surface chemistry.J. Membr. Sci.2016,513, 250–259. [Google Scholar] [CrossRef]
    56. She, Q.; Hou, D.; Liu, J.; Tan, K.H.; Tang, C.Y. Effect of feed spacer induced membrane deformation on the performance of pressure retarded osmosis (PRO): Implications for PRO process operation.J. Membr. Sci.2013,445, 170–182. [Google Scholar] [CrossRef]
    Membranes 11 00153 g001 550
    Figure 1. Schematic illustration of osmotic pressure profile across the membrane in osmotically-driven membrane processes. (a) Active-layer-facing-draw-solution (AL-DS) mode. (b) Active-layer-facing-feed-solution (AL-FS) mode. Δπeffective is the effective osmotic pressure difference. ΔπCICP, ΔπDICP, ΔπCECP, and ΔπDECP are the osmotic pressure difference loss caused by concentrative internal concentration polarization (ICP), dilutive ICP, concentrative external concentration polarization (ECP), and dilutive ECP [18].
    Figure 1. Schematic illustration of osmotic pressure profile across the membrane in osmotically-driven membrane processes. (a) Active-layer-facing-draw-solution (AL-DS) mode. (b) Active-layer-facing-feed-solution (AL-FS) mode. Δπeffective is the effective osmotic pressure difference. ΔπCICP, ΔπDICP, ΔπCECP, and ΔπDECP are the osmotic pressure difference loss caused by concentrative internal concentration polarization (ICP), dilutive ICP, concentrative external concentration polarization (ECP), and dilutive ECP [18].
    Membranes 11 00153 g001
    Membranes 11 00153 g002a 550Membranes 11 00153 g002b 550
    Figure 2. Structure and surface morphology of the thin film composite membrane (TFC) and cellulose triacetate membrane (CTA-W(P)). (a) Field-emission scanning electron microscopy (FESEM) micrograph of TFC cross-section at 500×. (b) FESEM micrograph of the surface of TFC active layer at 100,000×. (c) Atomic force microscopy (AFM) image of the surface of TFC active layer. (d) FESEM micrograph of CTA-W(P) cross-section at 500×. (e) FESEM micrograph of the surface of CTA-W(P) active layer at 100,000×. (f) AFM image of the surface of CTA-W(P) active layer.
    Figure 2. Structure and surface morphology of the thin film composite membrane (TFC) and cellulose triacetate membrane (CTA-W(P)). (a) Field-emission scanning electron microscopy (FESEM) micrograph of TFC cross-section at 500×. (b) FESEM micrograph of the surface of TFC active layer at 100,000×. (c) Atomic force microscopy (AFM) image of the surface of TFC active layer. (d) FESEM micrograph of CTA-W(P) cross-section at 500×. (e) FESEM micrograph of the surface of CTA-W(P) active layer at 100,000×. (f) AFM image of the surface of CTA-W(P) active layer.
    Membranes 11 00153 g002aMembranes 11 00153 g002b
    Membranes 11 00153 g003a 550Membranes 11 00153 g003b 550
    Figure 3. Fourier transform infrared spectroscopy (FTIR) spectra of (a) TFC, and (b) CTA-W(P), CTA-woven (W), and CTA-nonwoven (NW) membranes.
    Figure 3. Fourier transform infrared spectroscopy (FTIR) spectra of (a) TFC, and (b) CTA-W(P), CTA-woven (W), and CTA-nonwoven (NW) membranes.
    Membranes 11 00153 g003aMembranes 11 00153 g003b
    Membranes 11 00153 g004 550
    Figure 4. Forward osmosis (FO) water flux of TFC and CTA-W(P) membranes with different feed and draw solutions. The tests were performed in (a) AL-DS mode, and (b) AL-FS mode. Pure water, 10 mM and 0.5 M NaCl were used as feed solution (FS), which are denoted as DI, 0.01 M and 0.5 M. 0.2 M, 0.5 M, and 2 M NaCl were used as draw solution (DS), which are denoted as 0.2 M, 0.5 M, and 2 M.
    Figure 4. Forward osmosis (FO) water flux of TFC and CTA-W(P) membranes with different feed and draw solutions. The tests were performed in (a) AL-DS mode, and (b) AL-FS mode. Pure water, 10 mM and 0.5 M NaCl were used as feed solution (FS), which are denoted as DI, 0.01 M and 0.5 M. 0.2 M, 0.5 M, and 2 M NaCl were used as draw solution (DS), which are denoted as 0.2 M, 0.5 M, and 2 M.
    Membranes 11 00153 g004
    Membranes 11 00153 g005 550
    Figure 5. Water flux of TFC and CTA-W(P) in FO scaling tests. Testing conditions: FS for baseline tests was 0.163 M NaCl; FS for scaling tests was a mixed solution containing 72 mM Na2SO4, 26.1 mM CaCl2, and 10 mM NaCl; crossflow velocities of FS and DS on membrane were 23.2 cm/s; 2–3 M NaCl were used as DS to achieve initial water flux of 15 ± 1 L/(m2 h).
    Figure 5. Water flux of TFC and CTA-W(P) in FO scaling tests. Testing conditions: FS for baseline tests was 0.163 M NaCl; FS for scaling tests was a mixed solution containing 72 mM Na2SO4, 26.1 mM CaCl2, and 10 mM NaCl; crossflow velocities of FS and DS on membrane were 23.2 cm/s; 2–3 M NaCl were used as DS to achieve initial water flux of 15 ± 1 L/(m2 h).
    Membranes 11 00153 g005
    Membranes 11 00153 g006 550
    Figure 6. Pressure-retarded osmosis (PRO) performance of TFC and CTA-W(P) membranes: (a) water flux, (b) power density, and (c) salt flux/water flux ratio. Testing conditions: 10 mM NaCl as FS, 1 M NaCl as DS, and active-layer-facing-draw-solution (AL-DS) mode.
    Figure 6. Pressure-retarded osmosis (PRO) performance of TFC and CTA-W(P) membranes: (a) water flux, (b) power density, and (c) salt flux/water flux ratio. Testing conditions: 10 mM NaCl as FS, 1 M NaCl as DS, and active-layer-facing-draw-solution (AL-DS) mode.
    Membranes 11 00153 g006
    Membranes 11 00153 g007 550
    Figure 7. FO water flux of membranes with different water permeability (A) and mass diffusion coefficient (Km). The data was obtained from the experiments in the current work and the literature, and the membrane properties are presented inTable A1 andTable S1. Data in (a,b) were evaluated with FS of 0–10 mM NaCl and DS of 0.5–0.75 M NaCl. Data in (c,d) were evaluated with FS of 0.5–0.6 M NaCl and DS of 1.5–2 M NaCl.
    Figure 7. FO water flux of membranes with different water permeability (A) and mass diffusion coefficient (Km). The data was obtained from the experiments in the current work and the literature, and the membrane properties are presented inTable A1 andTable S1. Data in (a,b) were evaluated with FS of 0–10 mM NaCl and DS of 0.5–0.75 M NaCl. Data in (c,d) were evaluated with FS of 0.5–0.6 M NaCl and DS of 1.5–2 M NaCl.
    Membranes 11 00153 g007
    Membranes 11 00153 g008 550
    Figure 8. The FO efficiency of benchmark membrane (CTA-W(P)) andJv TFC/Jv CTA-W(P) in different testing conditions. DI water, 0.01 M and 0.5 M NaCl were used as FS; 0.2 M, 0.5 M, and 2 M NaCl were used as DS, as in the legend.
    Figure 8. The FO efficiency of benchmark membrane (CTA-W(P)) andJv TFC/Jv CTA-W(P) in different testing conditions. DI water, 0.01 M and 0.5 M NaCl were used as FS; 0.2 M, 0.5 M, and 2 M NaCl were used as DS, as in the legend.
    Membranes 11 00153 g008
    Table
    Table 1. The chemical property, structural property, and intrinsic separation property of membranes.
    Table 1. The chemical property, structural property, and intrinsic separation property of membranes.
    PropertyTFCCTA-W(P)CTA-WCTA-NW
    Membrane materialPA/PSf/PETCTA/PETCTA/PETCTA/PET
    Structural propertyThickness (µm)105 ± 471 ± 7a61 ± 1a
    (44.7 ± 14.1)b    		129 ± 6
    (144 ± 24)b
    Porosity (%)66 ± 162 ± 150 ± 1
    (46 ± 1)b    		55 ± 1
    (50 ± 2)b
    Structural parameter,S (mm)1.14 ± 0.120.73 ± 0.310.82 ± 0.39
    (1.00 ± 0.54)b    		1.77 ± 0.20
    (1.38 ± 0.26)b
    Surface propertyRoughness,Rq (nm)69 ± 155 ± 15 ± 133 ± 18
    Contact angle (°)48 ± 363 ± 167 ± 2
    (73 ± 2)b    		63 ± 4
    (64 ± 2)b
    Mechanical propertyYoung’s modulus,E (MPa)
    With tension in axial direction158 ± 31427 ± 34384 ± 33261 ± 70
    (Isotropic)
    With tension in diagonal direction38 ± 3152 ± 13264 ± 14
    Tensile strain at break (%)
    With tension in axial direction67 ± 255 ± 465 ± 777 ± 11
    (Isotropic)
    With tension in diagonal direction151 ± 14175 ± 16150 ± 12
    Tensile stress at break (MPa)
    With tension in axial direction54 ± 376 ± 992 ± 826 ± 5
    (Isotropic)
    With tension in diagonal direction32 ± 176 ± 295 ± 4
    Intrinsic separation propertyWater permeability,A (L/(m2 h bar))2.56 ± 0.081.06 ± 0.070.41 ± 0.01
    (0.33 ± 0.04)b    		0.53 ± 0.03
    (0.46 ± 0.07)b
    NaCl rejection (%)96.5 ± 0.988.1 ± 4.392.5 ± 0.3
    (81.9)b,c    		93.9 ± 1.2
    (92.4)b,c
    NaCl permeability,BNaCl (L/(m2 h))0.63 ± 0.210.67 ± 0.240.15 ± 0.01
    (0.14 ± 0.03)b    		0.16 ± 0.04
    (0.10 ± 0.01)b
    BNaCl/A (bar)0.25 ± 0.080.65 ± 0.270.38 ± 0.02
    (0.47 ± 0.12)b    		0.31 ± 0.06
    (0.22 ± 0.03)b
    Na2SO4 rejection (%)98.9 ± 0.296.4 ± 1.899.0 ± 0.0498.8 ± 0.01
    Na2SO4 permeability,BNa2SO4 (L/(m2 h))0.13 ± 0.020.17 ± 0.080.019 ± 0.00020.029 ± 0.001
    BNa2SO4/A (bar)0.053 ± 0.0080.17 ± 0.090.047 ± 0.0010.055 ± 0.001
    a The cross-sections of CTA-W(P) and CTA-W were non-uniform due to the embedded mesh. Thickness at the thinnest parts were 52 ± 7 µm and 25 ± 1 µm for CTA-W(P) and CTA-W, respectively.b Values in parentheses were reported in previous study [24]. The values in current study were slightly different, likely due to variations of different membrane batches.c Salt rejection was tested at 3.75 bar, using 20 mM NaCl as feed in previous study [24].
    Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

    © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

    Share and Cite

    MDPI and ACS Style

    Wei, J.; She, Q.; Liu, X. Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes.Membranes2021,11, 153. https://doi.org/10.3390/membranes11020153

    AMA Style

    Wei J, She Q, Liu X. Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes.Membranes. 2021; 11(2):153. https://doi.org/10.3390/membranes11020153

    Chicago/Turabian Style

    Wei, Jing, Qianhong She, and Xin Liu. 2021. "Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes"Membranes 11, no. 2: 153. https://doi.org/10.3390/membranes11020153

    APA Style

    Wei, J., She, Q., & Liu, X. (2021). Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes.Membranes,11(2), 153. https://doi.org/10.3390/membranes11020153

    Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further detailshere.

    Article Metrics

    No
    No

    Article Access Statistics

    For more information on the journal statistics, clickhere.
    Multiple requests from the same IP address are counted as one view.
    Membranes, EISSN 2077-0375, Published by MDPI
    RSSContent Alert

    Further Information

    Article Processing Charges Pay an Invoice Open Access Policy Contact MDPI Jobs at MDPI

    Guidelines

    For Authors For Reviewers For Editors For Librarians For Publishers For Societies For Conference Organizers

    MDPI Initiatives

    Sciforum MDPI Books Preprints.org Scilit SciProfiles Encyclopedia JAMS Proceedings Series

    Follow MDPI

    LinkedIn Facebook X
    MDPI

    Subscribe to receive issue release notifications and newsletters from MDPI journals

    © 1996-2025 MDPI (Basel, Switzerland) unless otherwise stated
    Terms and Conditions Privacy Policy
    We use cookies on our website to ensure you get the best experience.
    Read more about our cookieshere.
    Accept
    Back to TopTop
    [8]ページ先頭
    ©2009-2025 Movatter.jp