Electrospun Nanofibrous Membranes for Desalination

Electrospun Nanofibrous Membranes for Desalination

CHAPTER 4 Electrospun Nanofibrous Membranes for Desalination Hongyang Ma1,2 and Benjamin S. Hsiao2 1 State Key Laboratory of OrganicInorganic Compo...

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CHAPTER 4

Electrospun Nanofibrous Membranes for Desalination Hongyang Ma1,2 and Benjamin S. Hsiao2 1

State Key Laboratory of OrganicInorganic Composites, Beijing University of Chemical Technology, Beijing, P.R. China 2Department of Chemistry, Stony Brook University, Stony Brook, New York, NY, United States

1 Introduction In the 21st century, due to climate change and rapid population growth, the world is facing increasing pressure on energy and water shortage, where the scarcity of freshwater is one of the major problems directly impacting the human life [13]. Currently, about 40% (3 billion) of the world’s population lives in water-stressed areas. Water scarcity is particularly high in China, India, Africa, South America, and Australia (Fig. 1A). Furthermore, manmade water pollution has also become a prevalent problem that affects many people worldwide, where contaminated water kills millions people every year [4,5]. Only 0.01% of the water resources on earth are freshwater, but they are very unevenly distributed. On the other hand, 97% of water resources are the seawater in oceans [7]. Without question, one of the more direct approaches to address the water shortage will be through desalination from seawater. In seawater, the major salt component is sodium chloride (about 3.5 wt%, Fig. 1B). Therefore, for the varying desalination processes, the major goal is to remove salt ions of sodium chloride and others from water in the most energy-efficient manner [8]. Among the desalination methods, the reverse osmosis (RO) membrane operation has been regarded as the most efficient way to achieve this goal, as the operation can be run in a large industrial scale with good economic benefits [911]. In addition to the RO operations, there are two emerging membrane operations: Forward osmosis (FO) and membrane distillation (MD). We will first review the current state-of-the art membrane technologies for these operations: RO, FO, and MD.

Current Trends and Future Developments on (Bio-) Membranes. DOI: https://doi.org/10.1016/B978-0-12-813551-8.00004-8 © 2019 Elsevier Inc. All rights reserved.

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Fig. 1 (A) Water scarcity areas on earth and (B) the composition of seawater [6].

1.1 Reverse Osmosis Membranes There are two types of RO membranes for desalination today: the cellulose acetate (CA)based RO membranes and polyamide-based thin-film composite (TFC) membranes [1214]. The first CA membrane was demonstrated by Reid and Breton in 1959 for removal of salt ions from saline water using a pressure-driven process [15]. In these membranes, CA, cellulose diacetate, and cellulose triacetate (CTA), typically produced by acetylation of natural cellulose [16], have all been used to produce a dense asymmetric porous structure by the phase-inversion process [1721]. The best performing membrane, known as the LoebSourirajan membrane, was found to be the system of CA or CTA using acetone as solvent and water as nonsolvent. Fig. 2A illustrates the representative CA-based RO membrane structure, containing an asymmetrical porous layer supported by a nonwoven fibrous substrate. The total thickness of the porous layer is between 50 and 100 μm, while having a dense skin (thickness between 0.1 and 0.5 μm) that defines the pore size of the membrane. In one study, the dense skin was formed by the rapid evaporation of acetone (solvent) from the solution of CA and acetone [17]. As a result, the top skin layer was almost nonporous, which prevents hydrated sodium ions (the diameter is about 0.73 nm) and hydrated chloride ions (the diameter is about 0.69 nm) from passing through the membrane [22]. The pore size of the top skin layer could be adjusted by the hydrolysis of acetyl groups under basic conditions; as a result, a series of nanofiltration (NF) and ultrafiltration (UF) membranes with different pore sizes have been demonstrated [23,25]. The porous substrate layer could also be formed by solvent exchange, where the solvent, such as dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP), was leaching out, while the nonsolvent, such as water, was diffused into the CA matrix [19]. As a result, an asymmetrical porous finger structure (Fig. 2A) with porosity of 20%50% was created, providing good mechanical strength to the membrane [20,21]. Typically, the main mechanical strength of the membrane is due to the nonwoven polyester substrates.

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Fig. 2 Representative structures of (A) cellulose acetate-based RO membrane and (B) polyamide-based TFC RO membrane supported by a nonwoven substrate.

CA-based RO membranes exhibit high rejection ratio up to 99.5% for desalination at operating pressure of 15002000 psi, where the permeation flux could achieve from a value from 9 to 19 L/m2h [13]. The membrane should have good mechanical properties and its chlorine resistance should be higher than 5 ppm, typically generated from the sanitation of brackish water or seawater operations. Unfortunately, CA-based RO membranes have a very limited applicable pH range, usually from 4 to 6, due to the possible hydrolysis of acetyl groups [25,26], and do not have very good chlorine resistance. CA-based RO membranes have been commercialized in either the format of spiral wound module or hollow fiber module for desalination since 1963, and they dominated the desalination market for 10 years after their introduction. They have been extensively used in a wide range of desalination processes treating different water sources: Brackish water, seawater, and irrigation water [13]. Because CA-based RO membranes are easy to fabricate and cost-effective, their hollow fiber products are serving a broad range of desalination applications today [25]. TFC RO membrane was first introduced in 1963. One of the most representative products was the Filmtec FT30 membrane having a barrier layer based on cross-linked polyamide. Since then, a wide range of TFC membranes has been developed and commercialized rapidly [2730]. The barrier layer in the FT30 membrane was created by the amidation reaction between trimesoyl chloride (TMC) and m-phenyl diamine (MPD) using the interfacial polymerization approach. In the typical sample preparation scheme, an asymmetrical UF membrane (made of polysulfone or polyethersulfone (PES)) was employed as a substrate, which was first saturated by an MPD aqueous solution [31,32]. The organic phase was TMC in hexane, which was subsequently cast on the

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MPD infused UF substrate with a desired amount. As a result, a dense cross-linked polyamide was created immediately and functioned as a barrier to block the transportation of hydrated sodium and chloride ions, while allowing water molecules to pass through the membrane under the operating pressure. The typical structure of TFC RO membrane is shown in Fig. 2B. The typical thickness of the barrier layer in TFC membranes (Fig. 2B) is about 100 nm, while the reported range was from 8 to 200 nm [25]. This thickness could be adjusted by the concentration of the monomer, the diffusion rate of the monomers, the reaction time, and the properties of the interface between the aqueous and organic phases. For example, by adding additives into the aqueous phase or organic phase, the properties of the interface could be changed [3338]. Therefore, the thickness of the barrier layer could be fine-tuned correspondingly, and be used to control the water throughput (or flux) of the RO membrane. The supporting layer in the TFC RO membrane is typically a UF or even microfiltration (MF) membrane, fabricated by the phase-inversion method. As a result, the surface porosity of the support layer is from 20% to 50% [39]. The thickness of this supporting layer can be varied from 100 to 200 μm, where the surface pore size ranges from 10 to 100 nm. It is well known that the osmosis pressure of the salt aqueous solution mainly depends on the concentration. In seawater, the average sodium chloride content is about 3.5 wt% (the total salt concentration can be around 4 wt%), which would generate an osmosis pressure around 400 psi at 25 C. To overcome this osmosis pressure, the practical operating pressure will be as high as 800 psi [40]. Therefore, the requirement of mechanical properties of a RO membrane becomes very essential. One major advantage of polyamide-based TFC RO membranes over CA-based RO membranes appears to be the higher water permeability. The typical FT30 membrane exhibited flux of 15 L/m2h, which was higher than conventional CA-based RO membrane (around 8 L/m2h) for the feed solution of 35,000 ppm NaCl at the pressure of 800 psi, while maintaining the rejection ratio of 99.5% [25]. Another advantage of polyamide-based TFC RO membranes is the wide applicable pH range (from pH 5 2 to 12). However, there is a major concern for polyamide-based RO membranes, that is, the lower chlorine resistance than CA-based RO membrane [4144]. The reason for the poor chlorine resistance is because the amide bond can be attacked by free chlorine through two reactions: the Orton rearrangement and Hoffman degradation. These reactions would reduce the rejection ratio of the membrane against sodium chloride due to the damage of the barrier layer structure.

1.2 Forward Osmosis Membranes FO is an alternative desalination approach that using the osmosis pressure gradient as the mainly driving force extracts freshwater from brackish/seawater with an even more

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Fig. 3 Forward osmosis process using the sugar solution as the draw solution.

concentrated draw solution [4548]. Basically, pure water will pass through the FO membrane from the brackish/seawater side to the drawing solution side, which has a higher osmosis pressure. This process is illustrated in Fig. 3, using a high concentration sugar solution as the draw solution. However, it will take energy to separate water and recover the draw solution, so the total energy consumption may be higher than the RO membranebased operations. Recently, there have been great advances in the development of energyefficient draw solutions, such as ammoniacarbon dioxide, where both ingredients are readily dissociate into gases using heat and can be recovered and reused in a closed loop system [49]. The typical membranes used in the FO operation also have a TFC format, which has the same structure as that in the RO operation (Fig. 2B). The barrier (selective) layer is crosslinked aromatic polyamide. However, the support layer for the FO operation needs to be a highly porous substrate to minimize the issue of concentration polarization, reduce the resistance to mass transfer, and increase the water flux [50]. The first FO system was demonstrated in 1977 [51]. Since then, many different systems have been developed, in which the energy-saving potential is approaching the level of RO operations. The real breakthrough of the FO operation was made by the Yale group in 2007, where true energy-efficient drawing solutions and recovery operations were demonstrated to desalinate seawater [45,46]. For example, the draw solutions based on ammonium chloride or sugar can be further concentrated for other applications after the FO operation [49]. The major advantage of FO over RO is the potential of having a lower total energy consumption and better fouling resistance to the different pollutants [46]. Specifically, the

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FO operation does not require large energy (i.e., high pressure) to drive water from the feed side to the draw solution side, where a low pressure circulation system is sufficient. The low pressure characteristic of the FO process is beneficial to reduce the fouling tendency and lower the mechanical strength requirement of the membrane. However, FO has a greater issue about the concentration polarization across the membrane (both external and internal) than that of RO, due to the higher draw solution concentration, which needs to be addressed [52]. The external polarization comes from the formation of a concentrated salt layer on the surface of the FO membrane, while the internal polarization is due to the air in voids of the membrane that can dramatically decrease the permeation flux. To overcome these problems, high porous and high hydrophilic substrate, such as electrospun nanofibrous scaffold, appears to be ideally suited for the FO operation [53].

1.3 Membranes for Membrane Distillation MD is a thermally driven separation process, in which separation is enabled due to the phase transition of the liquid (water), where the vapor phase (e.g., water vapor) passes through the hydrophobic membrane while the liquid phase cannot [5458]. The water vapor can then be condensed and coalesces to form liquid droplets, which can then be removed from the membrane. The driving force of the vapor/liquid transition process comes from the partial vapor pressure difference, commonly triggered by a temperature difference. MD can be used for water purification over a very broad range of applications. It is not merely a process that can offer great energy-saving potential in desalination, but also new opportunities for remediation of contaminated water, especially when there are substantially different contaminants, such as inorganics, organics, biological, and polymeric materials, in water [5962]. In principle, MD offers the following unique advantages, as compared with RO and FO operations: (1) MD is a lower pressure operation, requiring less stringent mechanical requirements of the membrane, and a very small system footprint; (2) MD is a concentration independent operation, where it can provide complete rejection of all nonvolatiles in water; and (3) MD’s main driving force is the temperature difference of the feed and permeate streams, where this driving force can be derived from solar heat or waste heat. A typical MD process is illustrated in Fig. 4 [63]. In this system, a temperature gradient is created across a microporous membrane that separates the vaporliquid (hot section) and liquidliquid (cold section) phases. There are still many advances that need to be made regarding the membrane system for this unique operation. In short, we summarize the following areas: (1) the membrane should have a highly porous structure, reducing the hydraulic resistance and thus the energy consumption, while maintaining a small average pore size (less than 0.2 μm); and (2) the membrane must be very hydrophobic, yet maintain low fouling (against varying organic contaminants) and low scaling (against the mineralization of varying metal ions) tendencies.

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Fig. 4 A membrane distillation system involving the hot section (red, black in print version) with the feed stream and the cold section (blue, gray in print version) with the purified water stream. Reprinted from Ke et al. (2016) [63], Copyright (2016), with permission from Elsevier.

2 Advances of Nanofibrous Composite Membrane for Desalination In this section, the current advances of this new class of nanofibrous membranes, based on electrospinning technology and its advantages over conventional polymeric membranes, for energy-saving desalination operations (i.e., RO, FO, and MD), are reviewed. Specifically, the unique characteristics of electrospun nanofibrous membranes that can enable the higher flux processing are emphasized.

2.1 The Adoption of Electrospun Nanofibrous Substrates for Desalination Electrospun nanofibers have offered a new opportunity to improve the filtration performance in varying water purification operations, due to the unique characteristics of high porosity, interconnected pores, high surface-to-volume ratio, and cost-effectiveness [39,6470]. This can be seen from a quick literature survey. There have been a great deal of R & D activities developed in the last decade (i.e., 200716) regarding the electrospinning technology development and applications (Fig. 5A) [72], where the use of electrospun nanofibrous membranes for desalination has just begun a rising trend in recent years (Fig. 5B). The physics, technology development, fabrication, and applications of electrospinning and the resulting products have been thoroughly described in other chapters (e.g., see Chapter 2: Carbon-Based Membranes for Desalination). In this chapter, we mainly focus on the advances of this technology in desalination. The initial realization of the benefit in using electrospun nanofibrous membranes for filtration was in the applications of MF and UF. This is because as electrospinning is essentially a nonwoven fiber spinning technology, the random deposition of the filament would lead to a substrate with high porosity usually in the range of 65%95% [64,68]. This porosity is substantially higher than those in conventional porous substrate (the mean bulk porosity usually around 50%) made by the

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Fig. 5 Statistical publications associated with (A) electrospun nanofibrous membrane and (B) the applications of electrospun nanofibrous membrane for desalination.

phase-inversion method. As higher porosity often results in higher permeability, the construct of the electrospun nanofibrous substrate was found to be ideal for high-flux filtration with the pore size in the MF and UF range. The pore size is directly related to the fiber diameter, which can be controlled by two sets of parameters in (1) materials, such as type of polymer, solvent, solution concentration, viscosity, conductivity, molecular weight, and molecular weight distribution of the polymer; and (2) processing, such as applied voltage, flow rate of polymer solution, distance between the spinneret and collector, temperature, humidity, etc. [68]. Besides higher porosity, electrospun nanofibrous membranes offer more advantages over conventional membranes in varying types of water purification [7275]. These advantages are mainly due to the geometrical characters of electrospun nanofibers, which include pore size, pore size distribution, fiber diameter, fiber diameter distribution, fiber geometry, surface-to-volume ratio, surface property, fiber core-shell structure, and surface functionality. It appears some geometrical characteristics of electrospun nanofibers are also suited to improve the desalination efficiency through different operations (i.e., RO, FO, and MD), and these are discussed next.

2.2 New Support for Reverse Osmosis/Forward Osmosis Membranes In several recent studies, it was found that the replacement of conventional substrates, fabricated by the phase-inversion method, with electrospun nanofibrous substrate for fabrication of RO and FO membranes have led to significant flux increase [37,50,53,75,76]. The typical structure of these membranes using the electrospun nanofibrous substrate is shown in Fig. 6.

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Fig. 6 Representative structure of the RO/FO membrane using the electrospun nanofibrous membrane as the substrate.

In Fig. 6, the new membrane structure is termed thin-film nanofibrous composite (TFNC) format. This format consists of a three-tiered structure: the bottom layer is a nonwoven substrate (e.g., melt-blown or spun-bond), which mainly provides mechanical strength to the membrane; the middle layer is an electrospun nanofibrous scaffold deposited on the nonwoven substrate, and serves as a support to the barrier layer; the top barrier layer is a cross-linked polyamide layer or a nanocomposite layer containing the cross-linked polyamide matrix, created by interfacial polymerization [37,74,7779]. The barrier layer provides the most critical function of separating salt ions and water. The integrations of the top barrier layer and the middle support layer (electrospun scaffold), as well as the middle layer and the bottom layer (nonwoven substrate) are the essential element for fabrication of a functional RO and FO membrane system [69,80]. With unique properties of the nanofibrous substrate (midlayer), such as adjustable fiber diameter and pore size, high porosity and interconnected porous structure, and easy surface functionality, the TFNC format can offer many new opportunities for desalination either through RO, FO, or MD operations [39]. 2.2.1 Pore size and distribution Pore size and pore size distribution of an electrospun nanofibrous membrane can be determined by a capillary flow porometer based on the YoungLaplace equation as follows [81]: d5

4γ cos θ Δp

(1)

where Δp is the bubble point pressure, γ is the surface tension of the liquid (72 dynes/cm for water), θ is the wetting angle (for water, it is generally assumed to be zero), d is the diameter of the pore. The mean pore diameter of such a membrane can be experimentally

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determined by flow porometer using Eq. (1). In addition to the mean pore size, the maximum pore size is another major parameter, which can also be determined by flow parameter, for the membrane in water filtration. However, it should be noted that the term pore for the electrospun nanofibrous membrane is somewhat different from that in conventional membranes because the former is defined by the interaction of randomly deposited nanofibers, where a three-dimensional tortuous and interconnected channel structure is created [66]. Furthermore, the surface porosity of a nonwoven scaffold is close to its bulk porosity, whereas the surface porosity and the bulk porosity can be quite different in conventional membranes (due to the asymmetric structure). Pore size and its distribution are directly associated with the permeation flux (J) of the nanofibrous membrane, according to the Darcy’s law [25]: J 5 Kc

dp dx

(2)

where dp/dx is the pressure gradient in the membrane, c is the concentration of the component, and K is a coefficient reflecting the nature of the membrane, including the pore size, porosity, and membrane thickness. Assuming the membrane has tubular pores, the relationship between flux and pore size can be expressed by the HagenPoiseuille equation [70]: J5

r2 ΔPε 8μz

(3)

where J is the permeation flux, r represents the effective pore radius, ΔP/z is the differential pressure per unit thickness, μ is the solution viscosity, and ε is the porosity of the filtration media. It is clear that the permeation flux is proportional to the square of the pore size. Thus, the larger the pore size, the higher the flux. However, the solute retention also depends on the pore size, while large pore size leads to a low solute retention [72,8285]. Therefore, there is a trade-off between the flux and the solute retention. The pore size distribution is another important parameter related to the selectivity of the membrane for water filtration. Narrow pore size distribution usually offers high selectivity [82]. In principle, the pore size distribution can be obtained by accumulation of all pore size data; in reality, the pore size distribution can be determined by measurement using a capillary flow porometer [85]. Generally, the concept of the pore in an electrospun nanofibrous membrane is quite different from that in a conventional MF or UF membrane, as evidenced by the recent study involving the surface morphology [82]. It has been pointed out earlier that the surface porosity of an electrospun nanofibrous membrane is similar to its bulk porosity, but that is not the case for conventional membranes. Furthermore, the mean pore size of an

Electrospun Nanofibrous Membranes for Desalination 91 electrospun nanofibrous membrane is strongly dependent on the membrane thickness, where the thicker membrane would result in a smaller mean pore size. However, this is also not the case for conventional membranes, where its mean surface pore size is not a function of the total membrane thickness. The surface morphology of the supporting layer can directly affect the thickness, topology, and uniformity of the barrier layer in RO/FO membranes [86]. In the case of RO membranes, the adoption of an interconnected network structure in the electrospun nanofibrous membrane can result in a nanocomposite barrier layer with the top part of the nanofibers embedded in the barrier layer matrix. For FO membranes, the interconnected pore structure in the electrospun nanofibrous membrane can result in a highly porous scaffold with reduced concentration polarization [50]. It has been well documented that high concentration polarization in the substrate of an FO membrane can drastically neutralize the osmosis pressure (or the main driving force) in FO operation and lower the permeation flux. This problem could be overcome with the use of an electrospun nanofibrous membrane, where the concentrated salt ions could diffuse and spread, and the polarization of ions could be released effectively [87]. 2.2.2 Fiber diameter and fiber geometry Fiber diameter, including the maximum fiber diameter, mean fiber diameter, and fiber diameter distribution, can all be measured using microscopic techniques, such as scanning electronic microscope (SEM) images [73]. Typically, the mean fiber diameter of electrospun nanofibers can be adjusted from 50 nm to 1 μm, where the suitable mean fiber diameter for the RO, FO, and MD scaffold is usually in the range of 150300 nm. This is because such mean fiber diameters can be easily achieved in a very consistent manner by the adjustment of concentration and viscosity of the spinning solution, as well as the spinning parameters, such as flow rate and electrical field strength [68]. How does the fiber diameter in the electrospun nanofibrous membrane affect the filtration efficiency? First of all, the fiber diameter can define the pore size of the membrane. In recent studies, there appears to be an empirical relationship between the fiber diameter and the pore size in electrospun nanofibrous membranes with optimum filtration performance (the highest flux and lowest rejection ratio), that is the mean pore size is about 3 6 1 times the mean fiber diameter, and the maximum pore size was about 10 6 2 times the mean fiber diameter, where the porosity of the membrane is around 80% [67,83]. Therefore, these two relations can be used as a guideline to adjust the pore size of the membrane by controlling the fiber diameter. For the nanofibrous scaffold to provide additional functionality, such as self-healing ability, control released ability, and adsorption ability, the topology and geometry of the nanofibers can also be designed and constructed by electrospinning. Specifically, electrospun nanofibers could be produced in the shape of round, ribbon, hollow, and core-shell, depending on the properties of spinning solution and the conditions of electrospinning setup

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Fig. 7 The porosity effect in the fibrous membrane on the water permeability.

[66,88]. For example, both hollow and core-shell nanofibers can be achieved by the coaxial electrospinning technique. Different morphologies of electrospun fibers are illustrated in Fig. 7 [89]. In addition, different assemblies of electrospun nanofibers can also be obtained, where the nanofibers can be deposited randomly as a nonwoven substrate, or aligned with directional construct using special collecting techniques. Finally, the surface roughness of the nanofibers, such as dents or spots on the fiber surface and grafted surface with nanobrushes of electrospun nanofibers, could also be implemented by electrospinning conditions, which would directly impact the surface hydrophobicity of the nanofibers [90]. For example, the most suitable membranes for MD would require superhydrophobic surface, high porosity, and narrow pore distribution (the average pore size less than 0.2 μm) [54], which could be achieved by the combined adoption of hydrophobic materials and suitable surface roughness adjustment. 2.2.3 Porosity The porosity of an electrospun nanofibrous membrane can be defined as the volume percentage of voids to that of the whole membrane. It can be calculated with the following equation [91]:     volume of fiber density of mat Porosity 5 1 2 3 100% 5 1 2 3 100% (4) total volume density of fiber The major effect of the membrane porosity on the filtration performance is the hydraulic resistance or the permeation flux. The porosity of the typical electrospun membrane

Electrospun Nanofibrous Membranes for Desalination 93 ranges from 60% to 95%, and is more commonly around 80%. This porosity is much higher than that of the conventional membrane fabricated by phase-inversion method, which is about usually 50% with significantly less on the surface (surface porosity around 20 %) [69]. Fig. 7 illustrates the porosity effect of the fibrous membrane on the water permeability. In this figure, an electrospun nanofibrous membrane with porosity of 80% was used as an example. As all the voids are interconnected, we can assume the flux of this membrane is related to the flux through the 80% space (voids) minus the hydraulic resistance mainly created by the fiber surface. In this case, the flux seems to be less sensitive to the total membrane thickness, as seen by the stacking of three layers of membranes with the same porosity. Indeed, this was seen experimentally [39]. The flux of the electrospun nanofibrous membrane was significantly higher than those of the conventional membrane, and its rate of decrease due to the membrane thickness in the electrospun nanofibrous membrane was much less than that of its counterpart. However, the advantage of using the highly porous substrate can only be maintained if the structure of this substrate can remain porous after compaction [9294]. As the typical operating pressure for RO desalination is between 150 psi (brackish water) and 800 psi (seawater), experiments have been carried out to correlate the porosity with operating pressure. It was found that porosity of the electrospun nanofibrous membrane did decrease with the increasing pressure. However, if the nanofibers of the membrane were chemically or physically cross-linked, the compaction effect became much less. This is because a pseudo-nanotruss structure could be obtained when the fiber crossing points were “soldered” together [66,95]. Some practical cross-linking approaches have been demonstrated to improve the compaction properties of the electrospun nanofibrous membrane. One approach was through the proper selection of solvent to electrospin the polymer. For example, the mechanical properties of the electrospun PES membrane could be significantly enhanced by using a solvent mixture of DMF and NMP (60:40 by weight ratio) to slow down the solvent evaporation rate during electrospinning [80]. Consequently, upon the deposition on the collector, some residual solvent in the PES nanofibers remained, whereby the fiber could partially fuse together before drying. Thus this approach can improve the bonding strength among the nanofibers and also improve the adhesion between the nanofibers and the polyethylene teraphthalate (PET) nonwoven substrate [66,95]. Another approach involved the dip coating of cross-linkable polymer on the surface of nanofibers, following by thermal cross-linking of the coated polymer to form a skin layer on the surface of nanofibers to join the fibers together [82]. For example, dual-vinyl and tri-vinyl monomers were used for this purpose. They were polymerized on the surface of the electrospun polyacrylonitrile (PAN) nanofibers through free-radical reaction, which significantly strengthened the mechanical properties (Young’s modulus and tensile strength) of the membrane [72].

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2.2.4 Concept of directed water channels The concept of directed water channels is associated with discovery of the structure of aquaporin proteins coexisted with the phospholipid bilayer, which honored Nobel Prize winners in 2003. The size of the hourglass-shaped channel at the narrowest point is 0.30 nm [9698], therefore, water molecules (0.28 nm) can pass through these protein channels selectively, while other molecules or ions cannot. As an application, aquaporin proteins were incorporated into the barrier layer of a membrane that exhibited 801000 times higher water flux than conventional membranes [99]. Water channels could also be found in aligned carbon nanotube-based membranes, where 2 nm inner diameter of the nanotube can be used to get rid of contaminated issues from water with twice higher flux than those of commercially available membranes [100]. Other materials containing nanoporous structure, such as liquid crystals with specific phase [101], graphene oxide nanosheets [102], zeolites [103], and synthesized tubular molecules [104], also provide different options to integrate directed water channels with the barrier layer of a membrane. Unfortunately, all the abovementioned methods suffer great challenges for practical applications, considering processability, robustness, and fabrication cost. For instance, the density of directed water channels from aquaporin protein embedded in a barrier layer of a membrane was very limited, which greatly reduces the water permeability and the durability of the membrane. Also, carbon nanotubes have to be grown in situ on the substrate, followed by aligning and bonding together to prepare a membrane, which must increase the fabrication cost drastically and the risk of generating defects. Moreover, zeolites-based membrane with tortuous pore structure raises high hydraulic resistance; as a result, the water transportation rate will be increased limitedly. Especially, when graphene oxide was employed to prepare a barrier layer of a membrane for a water treatment process, additional immobilization was needed and therefore, the fabrication cost will be inevitably increased. Moreover, the formation conditions of a specific liquid crystal phase are critical and difficult to scale-up. Finally, the tubular molecules with water channels synthesized from multiple steps are cost-effective for the practical production. Therefore, it is expected to create tunable water channels facilely and extensively in the barrier layer of RO/FO membranes that address all of the abovementioned problems [105,106]. Our research team has expanded the concept of directed water channels and demonstrated a breakthrough on high-flux nanofibrous membranes. TFNC membranes with a nanocomposite barrier layer were fabricated, where the barrier layer was integrated with nanofibrous scaffolds and cross-linked polymer matrix. As mentioned earlier, electrospun nanofibers can be embedded into the polymer matrix and form an integrated top barrier layer of the membrane, where the fiber is surrounded by polymer chains [69,77,107]. Directed water channels are naturally formed by phase separation between an nanofibrous scaffold and a polymer matrix, where the gap between two phases can be used as the channels to

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Fig. 8 Representative structure of TFNC membrane where directed water channels formed by electrospun nanofibers and polymer matrix in the barrier layer. Reprinted from Ref. [78], Copyright (2014), with permission from Elsevier and reprinted with permission from (ACS Macro Lett., 2012, 1, 723726.). Copyright (2012) American Chemical Society.

discriminate water and other contaminant molecules, based on the mechanism of size exclusion [78,106]. The structure of TFNC membrane based on a three-layered structure with the top nanocomposite layer consisting of directed water channels is shown in Fig. 8. The circular diagram in Fig. 8 illustrates schematically the formation of directed water channels in the barrier layer of the TFNC membrane with the typical three-tier structure. In the diagram, (magnification part) cellulose nanofibers (yellow) work as one continuous phase to form three-dimensional skeleton, and polymer matrix (pink) surrounding the skeleton work as the other continuous phase. There are connected hollow cylindrical gaps between the two phases, which are regarded as directed water channels (blue). It should be noted that water can also transport through molecular cavities in the polymer matrix, as shown in Fig. 8, however, directed water channels are the preferred options due to the low hydraulic resistance. The advantages of directed water channels over molecular cavities of polymer matrix are obvious: (1) directed water channels, theoretically, can be formed between any bicontinuous phases by phase separation in the barrier layer, not limited by nanofibers and polymer matrix; (2) the size of directed water channels could be adjusted flexibly by tuning the phase separation between two phases, therefore, different types of membranes could be achieved for a variety of applications; (3) the surface properties of directed water channels could also be modified by simply decorate the surface of the gap with, for example, hydrophobic or hydrophilic, charged, or neutral species; and (4) the density of directed water channels could be controlled by the incorporation of nanofibrous scaffolds into the polymer matrix, which not only can provide directed paths for water transportation, but also improve the mechanical properties of the membrane and remain high porosity, therefore, the permeation flux should be increased drastically.

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The formation of directed water channels in the barrier layer of a membrane are not limited by electrospun nanofibers, any other nanoscale materials, such as carbon nanotubes [105,107] or cellulose nanofibers [106,108110], can also be used to create directed water channels. This new membrane design is particularly suited for NF, RO, and FO applications. All membranes containing directed water channels could exhibit 25 times higher permeation flux than the corresponding membrane without water channels [39].

2.3 New Chemical Properties for Membrane Distillation Membranes Electrospun nanofibrous membranes are very suitable for MD operations [111113]. The basic requirement for the membrane in MD is that only the vapor (no liquid water) can go through the porous membrane, which guarantees the high rejection ratio against any contaminants including salt ions. Therefore, the surface of the membrane for MD operations must be hydrophobic [114]. As a result, polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) are common hydrophobic materials used for the fabrication of membranes for MD [55]. The water contact angle of typical MD membranes is around 120 . To avoid water clogging the pores, the requirement of water sliding angle of the membrane for MD should be less than 10 . Moreover, conventional MD membranes have symmetrically porous structure with the average pore size between 0.1 and 1.0 μm, which are in the range of MF. As electrospun nanofibrous membrane exhibits adjustable pore size, high porosity, interconnected pores, and high surface-to-volume ratio, they are ideal candidates to improve the MD operations. To design an optimized membrane for MD using electrospun nanofibrous membrane, two essential parameters need to be considered: mass transfer rate and heat transfer rate. These parameters directly impact the separation efficiency of a MD membrane [115120]. According to the evaluation of mean pore size of the membrane and mean free path of vapor molecules, the mass transfer rate can be described by two equations: Knudsen law and Poiseuille law [115117,120]. If the pore size is smaller than the mean free path, the collision rate between the molecule and the nanofiber will be higher than the intermolecular collision rate. In this case, the relationship between the mass transfer rate and the structural parameters can be described by the Knudsen equation:   2rε 8RT 0:5 M ðP1 2 P2 Þ (5) Nk 5 3χδ πM RT where Nk is mass transfer rate, r is mean pore radius, ε is the porosity of the membrane, χ is the tortuosity factor, δ is the thickness of the membrane, T is the average of temperature on both sides of the membrane, M is the molecular weight, P1 and P2 are the vapor pressures on both sides, respectively, and R is ideal gas constant. On the other hand,

Electrospun Nanofibrous Membranes for Desalination 97 if the pore size is larger than the mean free path, and the rate of intermolecular collision is higher than that between the molecule and the nanofiber, then the correlation between mass transfer rate and the structural parameters of the membrane follows the Poiseuille law, where η is the viscosity of media. Np 5

r2 ε MP ðP1 2 P2 Þ 3 8χδ ηRT

(6)

It is clear that mass transfer rate Np is proportional to the mean pore radius of the membrane, no matter which mass transfer equation is used [115,116]. However, to guarantee an efficient MD processing, the liquid entry pressure should be sufficiently high to avoid the liquid penetration into the membrane. The liquid entry pressure (ΔP) can be calculated from Eq. (7): ΔP 5 Pf 2 Pp 5

2 2Bγcosθ rmax

(7)

where Pf and Pp are liquid pressure on the feeding side and vapor pressure in the pores of the membrane, respectively, B is the geometrical pore coefficient, γ is the liquid surface tension, θ is water contact angle, and rmax is the maximum pore radius of the membrane. Based on Eq. (7), the maximum pore radius is reversely proportional to the liquid entry pressure. To guarantee the liquid entry pressure, the maximum pore radius should be sufficiently small, which however, lowers the mass transfer rate. Therefore, there is trade-off between the mass transfer rate and the liquid entry pressure, which could be balanced by adjusting the pore size of the membrane, that is, by controlling the fiber diameter in the electrospun nanofibrous membrane. The porosity and surface-to-volume are another two essential parameters affecting the mass transfer rate [121,122]. According to the Knudsen or Poiseuille equations, the mass transfer rate is proportional to the porosity, that is, the mass transfer rate increases with increasing porosity. Therefore, the membrane for MD should exhibit porosity as high as possible. Electrospun nanofibrous membrane exhibited high porosity ranging from 80% to 95%, which is ideal for the MD operations. The heat transfer rate must also be considered for the design of membranes for MD, where this rate can be described as follows [115,118,119]: q 5 UA ΔTLM

(8)

where q is heat transfer rate, U is thermal conductivity coefficient, A is the conducting area, and ΔTLM is logarithmic mean temperature difference. It is clear that the heat transfer rate

98

Chapter 4

is proportional to the conducting area (i.e., surface-to-volume ratio for unit volume). A surface-to-volume ratio of nanofibers can be calculated by the following expression [123]: S5

A 2πrL 2 4 5 2 5 5 V πr L r df

(9)

where S is the surface-to-volume ratio and df is the fiber diameter. Thus, the smaller the fiber diameter, the higher the surface-to-volume ratio. Based on the above mass and heat transfer rate considerations, we can conclude that the key structural parameters to design a nanofibrous membrane with the best separation efficiency for MD operations are (1) fiber diameter and (2) membrane porosity. As mentioned earlier, conventional hydrophobic materials, such as PE, PP, PVDF, and PTFE, are used for fabrication of the MD membrane [115118]. However, the difficult processing conditions have often limited the cost-effectiveness to achieve the best separation efficiency. With the successful adoption of electrospinning, varying electrospun nanofibrous membranes with high hydrophobicity have been demonstrated for MD processing in desalination. For example, hydrophobic polymers, such as polystyrene [63,124], PVDF [125,126], fluorinated polytriazole (FPTA) [127], and PVDF-HFP [128] were used to prepare electrospun nanofibrous membranes, and they are very suitable for MD operations. Recently, a new class of superhydrophobic electrospun nanofibrous membranes, such as polyhedral oligomeric silsesquioxane (POSS)-modified polymethylmethacrylate (PMMA) nanofibers [129], POSSPVDFHFP composite nanofibrous membrane [130], fluorinated silicon nanofibrous membrane [131], and bioinspired superhydrophobic membranes [132,133], have also been demonstrated. We expect them to have even higher mass transfer rate and good rejection ratio to further improve the energy efficiency of MD processing. Considering the further improvement in the electrospun nanofibrous MD membrane, especially with regard to the mass transfer rate and antifouling properties, a composite structure containing a hydrophobichydrophilic asymmetrical arrangement may be very beneficial. This composite structure is schematically illustrated in Fig. 9A, which consists of a three-tier structure. The layer near the cold water side should be hydrophilic, which can change the curvity of the liquidvapor interface to improve the mass transition rate. The middle layer is the selective hydrophobic layer to transfer the water vapor, where the hydrophilic layer near the hot water side can reduce the fouling and scaling of the feed solution to the membrane [134]. Meanwhile, the hydrophobic middle layer can be designed to have a symmetric sandwich structure (Fig. 9B) [135]. In this midlayer, the fine fibrous layers on both sides could ensure the membrane with high liquid entry pressure, while the coarse fibrous layer (e.g., nonwoven substrate) in the middle provides large pore radius to provide high mass transfer rate as well as mechanical strength. Using such composite design, this nanofibrous membrane system can reach very high efficiency for MD desalination.

Electrospun Nanofibrous Membranes for Desalination 99

Fig. 9 (A) Asymmetrical structure of a composite membrane for MD and (B) finely structured design of the hydrophobic layer.

3 Conclusions and Future Trends Electrospun nanofibrous membranes can be used in desalination either as a highly porous substrate for fabrication of RO and FO membranes, or directly for MD. These unique nanofibrous membranes demonstrated excellent performance over conventional membranes due to high porosity, adjustable pore size, and functionalized surface of the nanofibrous scaffold. In other words, electrospun nanofibrous membranes can be easily tailored designed to target specific desalination applications, that is, desalination of brackish water or seawater. High permeation flux and high rejection ratio based on the usage of electrospun nanofibrous membrane have been achieved, which indicates the great potential of this technology. However, there are still many challenges, including the enhanced mechanical properties and large-scale production rate of electrospun nanofibrous membranes specifically for desalination purposes. We believe all these challenges will be overcome in the future, where the nanofibrous membrane technology will become an important element in varying desalination processes.

Acknowledgments This work was supported by a SusChEM award from the National Science Foundation in the United States (DMR-1409507), the National Natural Science Foundation of China (51673011), the State Key Laboratory of OrganicInorganic Composites at Beijing University of Chemical Technology (oic-201503004), and the Fundamental Research Funds for the Central Universities (buctrc201501).

Lists of Acronyms and Symbols RO FO MD MF

reverse osmosis forward osmosis membrane distillation microfiltration

100 Chapter 4 UF NF TFC TFNC CA CTA TMC MPD PES PE PP PVDF PTFE POSS Δp γ θ d J dp/dx c K r ΔP/z μ ε N χ δ T M P R η ΔP B q U A ΔTLM S df

ultrafiltration nanofiltration thin-film composite thin-film nanofibrous composite cellulose acetate cellulose triacetate trimesoyl chloride m-phenyl diamine polyethersulfone polyethylene polypropylene polyvinylidene fluoride polytetrafluoroethylene polyhedral oligomeric silsesquioxane bubble point pressure surface tension of liquid wetting angle diameter of pore permeation flux pressure gradient in membrane concentration coefficient reflecting the nature of the membrane effective pore radius differential pressure per unit thickness solution viscosity porosity of filtration media mass transfer rate tortuosity factor thickness of membrane average of temperature molecular weight vapor pressures ideal gas constant viscosity of media liquid entry pressure geometrical pore coefficient heat transfer rate thermal conductivity coefficient conducting area logarithmic mean temperature difference surface-to-volume ratio fiber diameter

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