Tailoring structures and performance of polyamide thin film composite (PA-TFC) desalination membranes via sublayers adjustment-a review

Desalination 417 (2017) 19–35

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Desalination journal homepage: www.elsevier.com/locate/desal

Engineering Advance

Tailoring structures and performance of polyamide thin film composite (PATFC) desalination membranes via sublayers adjustment-a review

MARK

Guo-Rong Xu⁎, Jian-Mei Xu, Hou-Jun Feng, He-Li Zhao, Shui-Bo Wu Institute of Seawater Desalination and Multipurpose Utilization, State Oceanic Administration (SOA), Tianjin 300192, China

A R T I C L E I N F O

A B S T R A C T

Keywords: PA-TFC membranes Desalination Sublayers adjustment Electrospun nanofibrous membranes (ENMs)

Polyamide thin film composite (PA-TFC) membranes are finding more and more popularity in desalination via both reverse osmosis (RO) and forward osmosis (FO) process, which can effectively alleviate worldwide freshwater crisis through translating seawater into potable water. Despite of huge progresses, great challenges still exist in trade-off between water flux and salt rejections, surface fouling, chlorination, and concentration polarization. This has encouraged tremendous research in tailoring structures and properties of virgin PA-TFC membranes. Nevertheless, it seems that major research has focused on the surface polyamide layer. It should be noticed that sublayer also significantly affect the PA-TFC membranes. Fortunately, researchers have realized this fact in recent years and have done some important and meaningful work. From this point of view, this paper reviews and discusses the state-of-the-art developments on achievements of sublayers adjustment for tailoring PA-TFC membranes used in RO and FO desalination. Traditional sublayers adjustment and novel electrospun nanofibrous membranes (ENMs) sublayers are highlighted. It also provides an insight for future research directions combined with comments and perspectives. It is sincerely expected that this review paper can provide some clues for further in-depth evaluation and research in exploring more advanced PA-TFC membranes by adjusting sublayers.

1. Introduction 1.1. Role of polyamide thin film composite (PA-TFC) membranes in reverse osmosis (RO) and forward osmosis (FO) desalination Fresh water shortage is an emergent issue with increasing attention all around the world [1]. Currently, 1 billion people are facing the threat of potable water crisis and this number will be 3 billion up to the year 2025 [2,3]. It has been well known that seawater constitutes 97.5% of total water resource on the earth [4]. As a result, desalination which can acquire freshwater from seawater provides an intriguing and effective strategy to meet this emergency and has been widely adopted. Presently, > 17 thousands desalination plants distributed over 150 countries have been built with a total daily capacity of > 90 million cubic meters freshwater and 300 million people are benefited [5]. The major desalination technologies currently in use are based on membrane separation via reverse osmosis (RO) and thermal distillation (multistage flash (MSF) and multi-effect distillation (MED)). Particularly, RO is finding an increasing role in desalination and accounts for over 50% of the installed capacity, especially with the fact that the specific energy of desalination by RO has been reduced from over 10

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Corresponding author. E-mail address: [email protected] (G.-R. Xu).

http://dx.doi.org/10.1016/j.desal.2017.05.011 Received 23 November 2016; Accepted 9 May 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.

kWh/m3 in the 1980s to below 4 kWh/m3 [6]. In addition, it is worthwhile to note that forward osmosis (FO) is an emerging technology which are grasping the attention of human beings. FO, which can gain fresh water from feed solution (low-osmotic-pressure) to draw solution (high-osmotic-pressure) by osmotic driving force through semi-permeable membranes, requires no applied hydraulic pressure and provides an effective and energy-saving method for desalination with lower membrane fouling propensity than RO [7]. Although the industrialization is still far from satisfaction, FO has attracted tremendous attention in desalination due to its great potential in energy-saving effectiveness especially after the introduction of ammonia-carbon dioxide solution in 2005 [8]. Since its invention in 1970s, the superior separation performance has enable polyamide thin film composite (PA-TFC) membranes overwhelmingly dominate the RO desalination area [9]. Typical PA-TFC RO membranes exhibit a three-layer structure in terms of non-woven fibrous mechanical support, polysulfone (PS) or polyethersulfone (PES) sublayer, and surface polyamide layer [10]. In spite of their excellent performance, PA-TFC RO membranes are still faced with three obstacles of trade-off between water flux and salt rejections [11], surface fouling [12], and chlorination [13], which hinder their fulfilled

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monomers. Due to the rapid hydrolysis of acyl chloride in aqueous phase and the asymmetric solubility, the mechanism is diffusioncontrolled and comprises three stages [36]. Initially, the polymers precipitate at the interface between two immiscible solvents and then the polymerization and the film formation simultaneously occur at this interface [37,38]. Followingly, amines in the aqueous phase diffuse to the organic phase and the film grows perpendicularly towards the organic phase [39]. Finally, increase in the thickness and density of the film inhibits the diffusion of the monomers and the IP [38]. The diffusion-controlled mechanism determines the dual structures and the depth heterogeneity of polyamide layer consisting a dense layer atop by a looser layer, which has been evidenced by both simulations [40–43] and experimental approaches [44–47]. The mechanism indicates the determined roles of amines diffusion. Thus, the sublayer on which amines spread would definitely affect IP process and thereby the structures of polyamide layer. It was reported that the thickness of the free-standing polyamide membranes is ten times higher than that formed on the porous support, which is ascribed to the impregnation of amines into the support [38]. It is believed that the impregnation of amines during IP reactions closely relates to the surface properties of support layer. In addition, occasionally, it is observed that the dense polyamide layer could penetrate into the porous support layer. This phenomenon can be determined by the pore structures of the support layer to some extent. Therefore, it can be confirmed that the sublayers structure has critical influences on the formation of polyamide layer and further affect the separation performances of PA-TFC membranes. Thereby it could be feasible method to tailor PA-TFC membranes by adjusting sublayers. Taking the aspects discussed above into account, one can expect that sublayers adjustment shows great potential in tailoring PA-TFC membranes. Moreover, the flexibility in PA-TFC membranes structure provides favorability for sublayers adjustment because both the surface active and support layers can be individually tailored for specific purpose (Fig. 1) Regrettably, compared with that in surface active layer, studies in sublayers are comparatively fewer. Rare review paper has involved research achievements for the sublayer investigations [48]. To the best of our knowledge, no dedicated review has been reported in elaborating the importance of sublayers adjustment in tailoring PA-TFC membranes and the corresponding research progress. At the same time, it is inspiring that researchers are beginning to focus on this issue and many interesting and important studies have been reported. Therefore, in this review, state-of-the-art research developments in this area was presented and commented with perspectives. The contents of this paper mainly focus on the adjustment of traditional sublayers (i.e., PSf and PES) and the most promising substitution of electrospun nanofibrous membranes (ENMs) that being used in PA-TFC membranes for both RO and FO desalinations. It is expected that this review paper can provide an insight and some useful clues for further in-depth research in exploring more advanced PA-TFC membranes via adjusting sublayers.

efficiency. Therefore, it is reasonable to expect that future RO desalination will ideally have high water flux per unit of pressure applied, nearcomplete rejection of dissolved species, low fouling propensity, and tolerance to oxidants used in pretreatment for biofouling control. Thus PA-TFC RO membranes related research in recent years is mostly emphasized on these issues. Various strategies such as interfacial polymerization (IP) process controlling [14], monomers substitution [15,16],surface modifications [17–22], and hydrophilic additives and/ or functional nanoparticles incorporation [23–28], have been explored and adopted to solve these problems and further improve the performance of PA-TFC RO membranes. In FO desalination, the most widely used reference FO membrane in reported research is the asymmetric cellulose triacetate (CTA) membrane which is from Hydration Technology Innovations (HTI, Albany, OR) first and now the product of Fluid Technology Solutions (FTS) [29]. Although the hydrophilic nature of CTA favors osmotic transport, its susceptibility to hydrolysis, relatively low water permeations, and low salt rejection have limited their wider applications [30]. Recent reports indicated that the growing interest in FO desalination has inspired the exploration and application of PA-TFC FO membranes [31]. Likewisely, PA-TFC FO membranes also exhibit great flexibility in tailoring because both surface selective layer and support sublayer can be individually adjusted. However, a critical point that hinders the performance of PATFC FO membranes is internal concentration polarization (ICP), which could severely reduce the water permeations during the operations. ICP occurs in two ways [32]. In active layer facing feed solution (AL-FS) mode, permeate would dilute the draw solutions filled in the porous support layer. This will lower the concentration of the solute on active layer compared with that on the membrane surface and lead to dilutive ICP. In active layer facing draw solution (AL-DS) mode, solute of the feed would filled the porous support layer and accumulate on the surface of active layer. Under this condition, water chemical potential difference between two sides of the active would be lower than that of membrane surface and lead to concentrative ICP. In case of these two modes, the net osmotic driving force are reduced, which decrease the water flux dramatically. Tailoring in PA-TFC FO membranes are mainly focused on ICP decreasing. 1.2. Potential of sublayer in tailoring PA-TFC membranes It is always believed that the salt rejections and water permeability of the PA-TFC membranes are mainly dependent on the surface polyamide layer and the support layer mainly provide mechanical endurance during the operations. As a result, most currently used strategies in tailoring PA-TFC membranes (e.g., membrane modifications and optimization of synthetic materials) are applied on the surface selective layer. Many reviews emphasizing on the adjustment of surface active layer have also been reported [10,31,33]. Nevertheless, it is worthwhile to notice that the structures and properties of the sublayers (e.g., polysulfone (PSf) and polyethersulfone (PES)) which have their own characters are also in close relation to the performance of PA-TFC membranes. First of all, the characters of PA-TFC membrane are significantly different based on the structures underneath [34]. For instance, the physical mechanical durability, porous structure and hydrophilicity of the sublayers could affect the compaction during the high-pressure operations, the water flux and salt rejections of the membranes, respectively [35]. The mechanical strength of the sublayer is directly related to the endurability of PA-TFC membranes. The permeability of sublayer could also affect the total water flux of PA-TFC membranes. Specially, the sublayer plays more important roles in FO than that in RO because it can directly affect ICP. In addition, the sublayers provide the platform for the IP and their surface has direct effect on IP reactions. The mechanism for IP has been extensively researched and confirmed. In the fabrication of PA-TFC membranes, immiscible amines and acyl chlorides are used as IP

2. Sublayer materials for PA-TFC membranes 2.1. Traditional sublayer materials for PA-TFC membranes Porous materials are commonly explored as sublayers for the TFC membranes in pressure-driven processes because of their advantages in terms of water flux enhancement [49]. In addition, the polymeric materials used for sublayer making should exhibit high pH/thermal tolerance as well as solvent resistance. For PA-TFC RO membranes, PSf and PES are the mostly widely used sublayer materials because their pore sizes obtained by phase inversion synthesis methods are in the range of ultrafiltration-nanofiltration membranes [50]. PSf is used widely as sublayers for the commercial PA-TFC RO membranes. Meanwhile, high mechanical strength and chemical resistance, high structural polarity and flexibility, high heat-distortion temperature, strong 20

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Fig. 1. Flexibility in tailoring PA-TFC membranes by adjusting the surface active and support layers individually.

thicker sublayers and suppressed macrovoids formation could decrease the water flux and increase the salt rejections of PA-TFC RO membranes. It was also reported that PSf concentrations could affect the pure water flux of PA-TFC RO membranes by tuning pore volumes and water channels [71]. PA-TFC RO membranes are dramatically impacted by their surface properties in terms of surface roughness, hydrophilicity and charge, which are closely related to the fouling resistance [72,73]. Rougher surface is more prone to fouling than smoother one [74]. Higher hydrophilicity is always favor for fouling resistance [75]. The role of surface charge in alleviating the fouling depends on the types of foulants [76]. Meanwhile, these properties are also affected by the sublayers. For example, increasing the polymer solution concentration in fabricating sublayers would decrease the surface roughness while increase the hydrophobicity of thus prepared PA-TFC membranes [69]. The decreased roughness favors the fouling resistance but the increased hydrophobicity aggravates surface fouling. More charged sublayer surface might increase the thickness while decrease the density of polyamide layer, and thereby decrease the corresponding surface charge. The changed surface charge could affect the fouling resistance [77]. For PA-TFC FO membranes, besides the aspects mentioned above, the most important issue is the effect of sublayer on ICP, which could severely decreases the performance of FO membranes. It was reported that the optimized support layer structure for PSf supported PA-TFC membranes might consist of a mixed-structure where a thin sponge-like layer sits on tops of highly porous macrovoids when using 1 M NaCl and deionized water as draw and feed solutions [78]. As a result, the polymer solution concentration is the most important factor that influences the fabrication of sublayers and should be paid particular attention in order to optimize the separation properties and fouling resistance of PA-TFC membranes. Taking the important role of sublayers under consideration, it is possible to tailor the structures and separation properties of PA-TFC membranes by adjusting sublayers. For example, in very recent research, a novel kind of macro-porous isotropic PES based membrane was fabricated via combined processes of vapor- and non-solventinduced phase separation and was used as support layer of PA-TFC RO membranes [79]. The well-defined isotropic porous structures with optimized average pore diameter of ~100 nm, hydrophilic surface, and high water permeability endowed the novel support layer with superior properties. Compared with that supported by traditional PES and PSf layers, PA-TFC RO membranes supported by novel isotropic PES membranes exhibited significantly different film morphologies and

heat-aging resistance, as well as good environmental endurance make PES a good alternate to be used as the sublayer of PA-TFC RO membranes [51–53]. PSf and PES are not being overwhelmingly used as sublayers for PA-TFC FO membranes as that for RO membranes because their thick, dense and hydrophobic nature could aggravate internal concentration polarization (ICP) and reduce effective osmotic driving force, which would result in poor water flux [54]. Support layers being used in PA-TFC FO membranes should be with several characters [30]. They must be thin, highly porous, and have a low tortuosity. In addition, they should be hydrophilic so as to allow for complete wetting throughout the structure. Moreover, they might be chemically and thermally stable while retaining mechanical strength. Finally, they must adequately support the selective layer during both formation and operation. Taking these under consideration, various materials including modified PSf/PES have been explored as sublayers for fabricating PA-TFC FO membranes, such as hydrophilic membranes with fringe-like and finger-like macrovoids [55], PES/PSf substrate [56], hollow fiber membranes [57,58], sulfonated PSf [59], sulfonated polymer blends [60,61],polyacrylonitrile (PAN) [62], cellulose triacetate [63], cellulose acetate propionate [64], and microfiltration membranes [65,66]. Parameters of PSf/PES sublayers themselves affecting PA-TFC RO membranes mainly include porous structures (e.g., pore size and porosity), thickness and surface properties, which are determined by the synthesis conditions during phase inversion such as polymer concentrations, air humidity, and evaporation times. Particularly, polymer concentration is the most important factor that influences the structures of sublayers because it is critical in controlling the types of pores, pore distributions, porosity, roughness, and thickness of sublayers, and further affects the water flux and salt rejections of PATFC RO membranes. Polymer concentrations are in close relation to the pore sizes of the sublayers, which afterwards affect the properties of PATFC RO membranes. Higher pore size of sublayer can offer higher flux because additional resistance to mass transport is avoided, while lower pore size would decrease the flux but provide smooth transition to the top layer [67]. As a result, the sublayers have to make a compromise between them. Increasing the polymer concentrations results in a high viscosity and delays the solvent/water diffusion during the phase inversion process, which leads to the denser and thicker sublayers with decreased porosity and larger thickness of skin upper layer [68]. At the same time macrovoids formation is suppressed and the tendency to form a sponge-like structure is enhanced [69,70]. The denser and 21

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3. Sublayer adjustments for tailoring PA-TFC membranes

surface characteristics, displayed higher permeability without compromising salt rejections, and demonstrated better stability for the polyamide active layer. Recently, the influence of support layer pore size on the osmosis performance of PA-TFC membranes was systematically investigated using nylon 6,6 microfiltration membranes with similar physical and chemical properties but different pore sizes [30]. It was found that the pore size of microfiltration support layers could affect the IP process and the morphology, cross-linking degree, mechanical integrity of the resulted polyamide composite membranes. In addition, the osmotic water flux and salt rejections were also in part impacted. In summary, the structures and separation properties of PA-TFC RO/ FO membranes can be optimized by tailoring the sublayers. For one thing, superior performance of PA-TFC RO membranes is expected to be achieved by tailoring the sublayers. For another, PA-TFC FO membranes outperform commercial CTA membranes in chemical and mechanical stability [80,81], which enable them to endure severe circumstances during various operations. However, the dense and thick PSf/PES sublayers generate low flux and increase ICP in FO process. As mentioned above, the ideal sublayers for PA-TFC FO membranes should be thin, high porous, and have little tortuosity, and these characteristics can be achieved by tailoring the sublayers. All in all, sublayers adjustment is well expected to provide an effective strategy to optimize the structures and separation properties of PA-TFC FO membranes with special purpose.

In this part, research developments on sublayers adjustment for tailoring PA-TFC RO/FO membranes are discussed. For PA-TFC RO membranes, hydrophilic polymer additives incorporations and inorganic nanomaterials modifications are focused. For PA-TFC FO membranes, surface modifications and nanomaterials incorporations are emphasized. Last but not least, novel ENMs sublayers supported PA-TFC RO/FO membranes are analyzed. 3.1. PSf/PES sublayers adjustment for PA-TFC RO membranes 3.1.1. Sublayers adjustment by hydrophilic polymeric additives Hydrophilic polymers incorporation into the sublayers can significantly alter their structures and further tune the separation properties of PA-TFC RO membranes, which has been evidenced by various research. Pore structures, pore sizes, porosities, and surface hydrophilicity of the sublayers are the critical factors that affected by the hydrophilic additives [96]. The pristine PES/PSf sublayers usually exhibit finger-like and sponge-like pore structures with low and high polymer solutions concentrations, respectively [71]. PEG and PVP are the most usual hydrophilic polymeric additives that being used to be incorporated into the sublayers, which exhibit different effects on the structures and properties of PA-TFC RO membranes. While PEG additives can increase the number and size of macrovoids and decrease the thickness of porous PES sublayer (Fig. 3a and b) [97,98], PVP additives decrease the pore size and increase the thickness to the contrary (Fig. 3c and d) [99]. Moreover, for the effect of PVP additives on the pore structures, controversial results have been reported. PVP incorporation can suppress the formation of macrovoids of the sublayer in PES/N-methylpyrrolidone (NMP)/PVP solution, while the result was contrary in PSf/Dimethylformamide (DMF)/PVP solutions [96,100]. The different effects of PEG and PVP might be attributed to their viscosities. PEG additives can enhance the permeability of PA-TFC RO membranes. However, the influence of PVP additives on the separation properties of PA-TFC RO membranes is more complex compared with that of PEG, which will be discussed in following contents. In recent research, hydrophilic PEG 600 and PVP k90 were used as hydrophilic additives to be added into the PES sublayers and their further effects on PA-TFC RO membrane were investigated by Mahdi et al. [101] Experiment results indicated that the contact angles of PES sublayers were respectively increased to 64.5° and decreased to 58.5°from original 62.5° by incorporation of 5 wt% PEG and PVP [101]. The pore size of PES, PES/PEG, and PES/PVP were 25.42, 24.86, and 9.06 nm, respectively [101]. It should be noted that this result is opposite with that mentioned above, during which PEG addition increased the pore sizes. Thus it can be concluded that variability exists in researching effect of hydrophilic additives incorporations and more systematical research should be carried out. PEG incorporation could increase the water flux of porous PES sublayer from 70.74 to 284.97 L/m2·h, while that of PVP dramatically decreased the water flux to 6.93 L/m2·h conversely [101]. The different water flux changes induced by PEG and PVP were ascribed to their different effects on the sublayer structures. For PEG addition, although higher surface hydrophilicity was favorable for the water flux, the surface porosity of support layer affected the contact angle/hydrophilicity [59], and herein more porous structure (macrovoids) induced by addition of PEG played more important role. Because the pore size was decreased, it could be concluded that the pore numbers must be increased to ensure the increased porosity. For PVP addition, besides the significantly decreased pore size, more factors would contribute to the dramatically decreased water flux, which will be discussed in the following. PEG and PVP incorporation in PES sublayers greatly affected the thickness of polyamide layer of PA-TFC RO membranes. In the research above, the thickness of polyamide layer of PA-TFC RO membranes supported by PES, PES/5wt%PEG, and PES/5 wt% PVP

2.2. Novel ENMs sublayers for PA-TFC RO/FO membranes Fascinating features such as excellent mechanical behaviors and large surface-to-volume ratios have made nanofibrous membranes very attractive in various areas [82,83]. Electrospinning involving a process during which polymer is stretched into nanofibers from their solutions under the applied electrostatic effect has proved to be a simple, versatile, and multifunctional technique to fabricate randomly oriented polymeric nanofibrous membranes [84]. ENMs exhibit lots of eminent characteristics. Among which the most intriguing one is that they exhibit interconnected pores with uniform pore sizes in the range of 2.7–0.17 μm and porosities of 20–80%, which enable them to be very promising in filtration areas [85]. These characteristics have also expanded their applicability to a variety of areas, such as energy storage [86],catalysis [87], drug delivery [88], filtration [89], and biomedicine applications [90]. In particular, ENMs have found their potential in aqueous separation processes and tremendous research progress has been reported on the usage of ENMs on ultrafiltration (UF) and nanofiltration (NF) [91–94]. Moreover, it is worthwhile to note that PA-TFC membranes supported by ENMs (i.e., thin film nanocomposite (TFNC) membranes) have shown promising applications in and monovalent ions (e.g., NaCl) rejections, indicating their feasibility in desalination [95]. As shown in Fig. 2a and b, in the fabrication of ENMs supported PA-TFC membranes, PSf/PES layers usually prepared by phase inversion are substituted by ENMs to be used as the sublayers for surface active polyamide layer. Compared with PSf/PES layer, ENMs exhibit prominent advantages in terms of high porosity (Fig. 2c and d). This unique property can be adopted to explore novel PA-TFC membranes with superior performances, which will be discussed in following. Likewise, the structures and separation properties of ENMs supported PA-TFC membranes are also closely related to the sublayers of ENMs, including fiber kinds and diameters, pore sizes, and porosity. Fortunately, the characteristics of ENMs could be easily controlled by tuning the electrospinning procedures. For example, the nanofibers diameters, pore sizes, and porosities can be tuned by altering electrospun procedures including polymer types and concentrations, applied voltages, collecting distances, etc. This enables us to realize the performance tailoring of ENMs supported PA-TFC membranes by adjusting sublayers easily. 22

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Fig. 2. (a) Typical SEM images of ENMs, (The inset shows the typical electrospinning process) (b) illustration for the fabrication of ENMs supported PA-TFC membranes, typical crosssectional SEM images for (c) nanofibrous membrane and (d) PSf/PES substrate supported PA-TFC membranes.

further evaluated. Moreover, it is worthy to be noted that the apparently decreased thickness of polyamide layer after PEG and PVP incorporations did not increase the water flux. Thus, it can be concluded that the water flux changes depended on both direct effect on sublayers and indirect effect on polyamide layer by PEG and PVP additives. Nevertheless, it is interesting to find that different results were obtained by the same research group in the same experimental system [98]. Under the experimental conditions, although the pore sizes changes induced by the two used additives were similar to their former research, controversial results were obtained for their effects on

were 446 nm, 277 nm, and 185 nm, respectively [101]. The separation experiments indicated that incorporation of PEG and PVP into the sublayers all decreased the water flux and increased the salt rejections of PA-TFC RO membranes and the magnitude for PVP incorporation was larger. It is easy to understand the effect of PVP incorporation. Decreased pore size and increased thickness of PES sublayer densified polyamide structure, thereby decreased the water flux and increased the salt rejection. However, although PEG incorporation had contrary effects on the PES sublayers, the same effect on the separation properties of PA-TFC RO membranes was obtained, which needed to be

Fig. 3. Cross-sectional SEM images for PES sublayers incorporated with PEG and/or PVP hydrophilic additives: (a) 15 wt% PES, (b) 15 wt% PES + 5 wt% PEG, (c) 15 wt% PES + 5 wt% PVP, (d) 15 wt% PES + 2.5 wt% PEG + 2.5 wt% PVP [98].(Copyright ©2014 Taylor & Francis).

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MMMs could be tailored with variable characters such as permeability and selectivity, chemical, thermal and mechanical stability, crystallinity, and antibacterial ability [104–106]. Various inorganic nanomaterials have been incorporated into PA-TFC RO membranes to optimize their structures and separation properties, such as zeolites [23,107–109], graphene oxide (GO) [28,110,111], carbon nanotubes (CNTs) [112,113], silver [114], silica [26,115], alumina [116], and titania [117,118]. Nevertheless, most of these incorporations were executed on the active polyamide layer and comparatively less analogue research has been focused on the sublayers. Moreover, the pore structures of sublayer could be tuned by inorganic nanomaterials incorporations because voids may be created between both organic and inorganic phases when the polymer chains do not completely interact with the filler particles [119]. Therefore, it is speculated that PA-TFC RO membranes can be tailored by sublayers adjustment through inorganic nanometerials incorporations. Recent research has also indicated the possiblities and suitabilities of PES/PSf sublayes adjustment by some unqiue inorganic nanoparticles such as zeolites, CNTs, GO, and TiO2. Zeolites are aluminosilicate minerals with a microstructure composed of 0.3–0.8 nm pores (e.g., NaA zeolite (0.42 nm), MFI type zeolite (0.56 nm)), which are between the diameters of hydrated salt ions (e.g., Na+ (aq):0.72 nm diameter) and water molecules (0.27) nm [120]. Moreover, they are always with high porosity of 30–40%. Thus, they can effectively reject the salt ions while approving the permeation of water molecules (Fig. 4a). Taking advantage of these benefits, lots of research has focused on the incorporation of zeolite nanoparticles (e.g., NaX [108], NaY [23,121], and NaA [115]) into polyamide layer to fabricate thin film nanocomposite (TFN) reverse osmosis membranes or directly fabrication of zeolite inorganic desalination membranes such as MFI type membrane [122]. In recent years, NaA zeolite nanoparticles have also seen their applicability in adjusting the sublayers of PA-TFC RO membranes. NaA zeolite nanoparticles were incorporated into the PSf sublayer of PA-TFC RO membranes to investigate their functionality in tailoring the membrane structures and properties [123]. Incorporation of NaA zeolite nanoparticles made PSf support structure rougher, more hydrophilic, and mechanically more robust. Additionally, more negatively charged, hydrophilic, smooth, permeable, and selective PA-TFC RO membrane was obtained. The evaluation of separation properties indicated that the water permeability coefficient increased 6-fold for the PA-TFC membranes with incorporation of NaA zeolite nanoparticles in PSf support layer. However, the passage for salt ions was also enhanced. It was found that when NaA zeolite was incorporated into the polyamide layer, the enhanced water permeations were companied by decreased salt rejections. Thus the surface active layer played the critical role in the salt ions removal. Therefore, NaA zeolite incorporated into the support layer could enhance the permeability while did not influence the salt ions removal. In addition, the unique characters of NaA zeolite which allowed the passage of water and excluded large solute particles due to the molecular sieving effect endowed support layer with more selective capability. Besides their effects on the membrane structures and performances, NaA zeolite nanoparticles incorporation enhanced the compaction resistance during operations, which was verified by time dependent RO experiments. Graphene is a two-dimensional single-layered materials consist of sp2-hybridized carbon atoms (Fig. 4b). Graphene oxide (GO) and CNTs can be derived from graphene. GO is single layer of graphene functionalized with oxygen-rich groups including carboxyl, hydroxyl, ether and epoxy groups (Fig. 4b) [127]. CNTs are tiny hexagonal tubes with several nanometers by rolling of graphene sheets [126]. CNTs are always being functionalized by carboxyl and hydroxyl groups to enhance their hydrophilicity. Excellent unique mechanical, physical, and chemical properties have made graphene-based two-dimensional nanomaterials (graphene and GO) attract unprecedented attention in separation areas [128]. In desalination, graphene based nanomaterials

the separation properties of PA-TFC RO membranes. Unlike their early research in which PEG and PVP additives all decreased the water flux and increased the salt rejections, water flux and salt rejections herein were all increased. The contradictory results might be induced by the varied experiment conditions and methods even in the same system. Thus, consistent research results are needed to provide more indicative reference for the peer research. Moreover, the effect of hydrophilic addition in sublayers on PA-TFC RO membranes seems to be discussed on a macro level. Deeper research on their effects on IP process (e.g., diamines diffusion) should be also done in order to reveal it in essence. Effect of PVP additives on the structures and separation properties of sublayers and PA-TFC RO membranes is more intricate, which has been specially researched by Ding et al. [71] In this research, the effect of PVP concentrations on their effect on PSf sublayers of PA-TFC RO membranes were investigated. With PVP concentration of 0% and 3%, the surface support layers were quite porous. Increasing the concentration would make the surface denser and less porous, which is attributed to the decreased diffusion rate caused by the comparatively high viscosity. Correspondingly, the water flux of support layer was first increased and then decreased with PVP concentration increasing. As PVP concentration increased, the water contact angle of the sublayers decreased from 83.6 ± 4.5°to 34.5 ± 10.8° [71], indicating an increased surface hydrophilicity. As PVP concentration increased, the water flux of PA-TFC RO membranes decreased gradually, which was ascribed to the significantly enhanced hydrophilicity. The enhanced hydrophilicity could impede the diffusion of MPD during interfacial polymerization and promote the TMC into the pores of support layers, and thereby enhance the thickness of PA layer [77]. On the premise of mechanical strength, the support layer for PA-TFC RO membranes should be as porous as possible with suitable pore size and pore numbers. Because the salt rejections are dominated by the surface polyamide layer, more porous support layer could effectively enhance the permeability of the PA-TFC RO membranes. There is also an optimized rather than precise value for the hydrophilicity of support layer. The support layer should be with high hydrophilicity to ensure the interfacial polymerization, but the excessive high hydrophilicity might be detrimental to the permeability [102]. The research discussed above indicated that support layers of PA-TFC membranes could be tuned by hydrophilic additives to the optimized condition. However, it should be noticed that incorporation of hydrophilic polymeric additives into the sublayers is such a complicated process and their effects on the PA-TFC RO membranes needed to be investigated more deeply under more precise experiment conditions in order to obtain more persuasive results. In addition, comprehensive factors should be taken into considerations. Moreover, it is worthwhile to note that effect of sublayers adjustment sometimes depends on the applied technologies. For example, Kim et al. reported that increasing the hydrophilicity of sublayer by surface plasma increased the water flux of PA-TFC RO membranes [102]. Nevertheless, Ghosh et al. reported that hydrogen bonding between MPD and hydrophilic additives limited the diffusion of MPD inside the support pores and some TMC may diffuse into the pores and form PA deeply within the pores, which actually increased the thickness of polyamide layer for water permeation [77]. Similar results were also reported by Ding et al. [71] Overall, hydrophilic additives incorporation definitely provides an effective strategy to adjust the sublayers of PA-TFC RO membranes and thereby tailor their performance. 3.1.2. Sublayers adjustment by inorganic nanomaterials Mixed matrix membranes (MMMs) fabricated by incorporation of inorganic nanomaterials into polymer matrix have brought about a new field in membrane science [103]. For MMMs, synergistic effect of both polymers (e.g., high separation efficiency, low cost, and easy processing) and inorganic nanomaterials (e.g., unique optical, thermal physiochemical activities) can be achieved to realize the functionality maximization. Meanwhile, more parameters are adjustable and the 24

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Fig. 4. (a) Pore structure of zeolite [120], (Copyright 2004, John Wiley and Sons) (b) schematic illustration of graphene, GO, and CNTs, (c) graphene desalination membrane [124], (Copyright (2015), with permission from Elsevier) (d and e) SEM images of stacked GO and ultrafast water flows through nanochannels between GO laminates [125], (Copyright © 2012, American Association for the Advancement of Science) (f) CNTs in desalination by providing excellent water passage while effectively rejecting salt ions [126].(Copyright (2014), with permission from Elsevier).

speed of water flow in vertically aligned CNTs with diameter of < 2 nm was three times faster than the theoretical values [140]. These unique characters have stimulated tremendous applications of GO and CNTs in desalination. They have been widely incorporated into active layer or deposited onto surface of PA-TFC RO membranes to improve their properties [141–146]. CNTs and GO have also seen potential in functionalizing the sublayers of PA-TFC RO membranes and further tailoring their properties recently. For example, Kim et al. reported that incorporation of acid modified multi-walled carbon nanotubes (MWCNTs) in sublayers could enhance the pure water permeability of PA-TFC RO membranes by 23% [114]. Recently, a kind of PA-TFC RO membrane with functionalized carbon nanotubes (fCNT) blended support PES layer (fTFC) was successfully synthesized by phase inversion and IP (Fig. 5a), and the effect of fCNT incorporation were systematically investigated [147]. Incorporation of fCNT changed the structures of PES support layers obviously. After the fCNT blending, the contact angle of PES support layer decreased from 54.76° to 51.03° [147], while that of PA-TFC RO membranes were similar (Fig. 5b). The average pore width, total pore area, and porosity were increased from 10.32 to 14.55 nm, from 75.19 to 83.96 m2/g, and from 76.69% to 82.05%, respectively (Fig. 5c) [147]. Meanwhile, incorporation of fCNT decreased surface zeta potential of PA-TFC RO membranes from −33.97 to − 37.88 mV (Fig. 5d) [147]. The surface hydrophilicity and the porous structure of sublayer changes could be attributed to the intrinsic characters of fCNTs and effect of fCNTs on the network structure. Experimental results indicated that the fTFC membrane showed a 10–20% and 90% enhanced water flux in seawater reverse osmosis (SWRO) and brackish water desalination osmosis (SWRO) at the operating pressure of 50 and

have also shown great potential and have been extensively investigated [124,128–130]. Monolayer graphene membranes can be designed for desalination with permeability of much higher than that of current RO membranes (Fig. 4c) [124]. However, difficulties in fabricating a large area of monolayer graphene and in generating high-density nanopores with controllable and relatively uniform sizes on the graphene sheets are severely restricting their practical applications [131,132]. Comparatively, graphene oxide (GO) provides better suitability due to its high hydrophilicity except the pristine properties of graphene. A unique feature of GO is that the planar nanochannels exist between the stacked GO layers can provide ultrafast pathway with a speed of several orders of magnitude faster than usual diffusion for water molecules, which is attributed to the size exclusion and a large slip length (low friction) (Fig. 4c-e) [133,134]. In light of the size of hydrated Na+ (with a hydrated diameter of 0.72 nm) and water molecule (with diameter of 0.27 nm), the space of the GO nanochannels should be < 0.7 nm in order to reject the salt ions effectively while allow the passage of water [135]. This can be achieved by reducing GO to decrease hydrated functional groups sizes or by covalently bonding the stacked GO nanosheets with small-size molecules to overcome the hydration force [128]. CNTs have also emerged as an intriguing nanomaterials in water purifications owing to their extraordinary feature of fast water transport and easy functionalization (Fig. 4f) [136,137]. Water molecules flow through the inside of CNTs can induce the formation of ordered hydrogen bonds [138]. In addition, the hydrophobic and smooth inner walls of CNTs make the interactions between the water molecules and the inwall of the CNTs very weak [139]. The weak interactions and ordered hydrogen bonds can lead to ultrafast water transport, especially for vertically aligned CNTs. Holt et al. experimentally reported that the

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Fig. 5. (a) Schematic illustration for the incorporation of fCNT in PES support layer of PA-TFC membrane, (b) contact angles of PES support layer and PA-TFC membranes before and after fCNT blending, (c) and (d) average pore width, total pore area, and porosity of PES support layer, and surface zeta potential of PA-TFC membranes before and after fCNT blending [147]. (Copyright (2015), with.

exhibited a water flux of 1.6–4 times of magnitude higher than that of commercial RO membranes [148]. Thus, incorporation of exfoliated GO into the sublayer was estimated to be an effective method to enhance the water flux of PA-TFC RO membranes while not compromising mechanical strength and salt rejections. GO is superior to CNTs in sublayer incorporations. GO nanosheets without tubular structures might have less effect on the porous structure of sublayers. Moreover, the well dispersion of GO nanosheets can be favorable for their better functionalization. However, the effect of numbers of GO layers should be discussed because it is critical for the ultrafast water passage provided by GO laminates. TiO2 is another kind of functional inorganic nanomaterials that being used to tune the sublayers and further the PA-TFC RO membranes because of their unique properties such as hydrophilicity and antifouling properties. Recent research suggested that incorporation of TiO2 nanoparticles in polymer solutions during the phase inverse process had an apparent effect on the sublayer structure. TiO2-entrapped sublayer showed more open structure with more larger macrovoids inside, which was ascribed to the hindrance effect of nanoparticles during the phase inversion [150]. The separation experiment indicated that dispersion of TiO2 nanoparticles in sublayers of PA-TFC membranes significantly increased the water flux while the rejection of monovalent salt was less affected with little decrease. In another research, TiO2 nanoparticles were first coated on the PSF sublayer surface and then the IP using MPD and TMC as monomers was executed to fabricate PA-TFC RO NF membranes (Fig. 6a) [151]. The most fascinating thing was that defect-free PA-TFC membranes were fabricated on TiO2 coated PSf supporting sublayers, indicating that the sandwich layer of TiO2 did not interfere formation of polyamide layer. TiO2 nanoparticles coating could smoothen PSf sublayers, which was attributed to the nanoparticles densifications and settlements in the valleys of the sublayer surface. The same effect was also found on the PA-TFC membranes. The contact angles measurement in Fig. 6b displayed that the sandwich layer of TiO2 nanoparticles significantly increased the surface hydrophilicity of the obtained PA-TFC RO membranes, which could enhance the anti-

30–40 bar, respectively. The increased water flux was ascribed to the enhanced hydrophilicity and porous properties of support layer. However, it is suspected that other two aspects could also play some roles. The first one was that the ultrafast water channels of fCNTs could enhance the water permeability. Besides, considering unchanged surface hydrophilicity and zeta potential of PA-TFC RO membrane, fCNTs incorporated into the sublayer did not directly effect the polyamide layer. Taking the IP mechanism into account, it could be attributed to the effect of changed sublayer surface on the IP process. The network structure of polyamide layer might be changed, which contributed to the significantly enhanced water flux. In addition, the increased surface zeta potential retained the salt rejections and enhanced the fouling resistance due to the enhanced repulsion force between the membrane surface and negatively charged foulant. Thus, it is an effective way to solve the problem of trade-off between water flux and salt rejections which are always encountered in tailoring PA-TFC RO membranes. permission from Elsevier). Although the water flux of PA-TFC RO membranes can be enhanced by decreasing polymer concentrations during the preparation of sublayers by phase inversion, the mechanical strength would be seriously weakened because the surface pores are decreased. In order to overcome this problem, well-exfoliated GO platelets with mean thickness of about 1.5 nm were incorporated into the sublayers of PA-TFC RO membranes to enhance the water flux without compromising mechanical strength [148]. The key point in this research was the exfoliation of GO, which was more effective in enhancing the mechanical strength due to the larger surface area and the better dispersibility. Under the optimized condition with 0.9 wt% GO loading, 90% of the mechanical strength compared with that of pristine sublayer was retained. Moreover, GO addition had little effect on the structure of surface active layer [148]. The roughness and thickness of surface polyamide layer, and the nitrogen/oxygen (N/O) ratio which is an indicator of crosslinking degree [149], were not significantly influenced. The maintained surface active layer structures helped to retain the high salt rejections. In addition, GO embedded sublayers supported PA-TFC RO membranes 26

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Fig. 6. PA-TFC membranes synthesized on TiO2 nanoparticles coated PSf sublayers (a) fabrication illustrations, (b) contact angles of the membranes with and without sandwich layer of TiO2 nanoparticles (NF1: 0.1 wt%, NF2: 0.25 wt%, NF3: 0.5 wt%) [151]. (Copyright (2014), with permission from Elsevier).

between the inorganic nanoparticles and organic phase.

fouling properties. In addition, it was also found that sandwich layer of TiO2 nanoparticles could increase the thickness of the active polyamide layer because TiO2 nanoparticles coatings on the PSf sublayers were more favorable for attaching MPD monomers. The permeability was enhanced and NaCl rejection was increased from 70% to 84% with 0.5 wt% TiO2 coating [151]. The increased permeability and salt rejection were attributed to the enhanced surface hydrophilicity and enlarged active layer thickness respectively. Besides, antibacterial property was also improved as a result of photocatalytic characteristic of TiO2 nanoparticles. Again, it is believed that the effect of TiO2 nanoparticles modifications on sublayers, and especially surface deposition, on the IP process should be investigated. At the same time, during the inorganic nanoparticles deposition on the membrane surface, one important problem needed to be paid particular attention is the agglomeration. The dispersion is an important issue in the applications of nanoparticles. Perhaps it is the most intriguing and promising method to tailor PATFC membranes by controlling the surface properties of sublayers through hydrophilic coatings. As mentioned above, the surface characteristics of sublayers closely relate to the IP process, which determines the network structures of polyamide active layer. The pore size, pore distribution, and hydrophilicity of sublayer affect the adsorption of monomers and influence the formation of polyamide layer [77]. By taking advantage of unique surficial properties of sublayers, we can tailor PA-TFC RO membranes through controlling the diffusion of MPD monomers in the usual first step of IP. Actually, similar methodologies involving MPD monomers diffusion controlling have been applied in former research, such as addition of surfactants into MPD aqueous solutions [14,152]. IP adjustment has also been proved to be an effective strategy to optimize the PA-TFC membranes, such as molecular layer-by-layer assembly process during with MPD and TMC monomers are sequentially adsorbed on the support to synthesize polyamide layer [153–156]. Therefore, it is expected and believed that more interesting work would be done focusing on PA-TFC RO membranes adjustment by optimizing the properties of interface between sublayers and active layers via sublayer surface modifications. In summary, as shown in Fig. 7, besides the hydrophilic polymeric additives, inorganic nanomaterials modification including incorporations and surface coating provides a promising alternate to tune PA-TFC RO membranes through adjusting sublayers. Furthermore, inorganic nanoparticles adjustment provides more flexibility due to their unique physical and chemical properties. Research developments indicate that the corresponding study has mainly focused on the structures and separation properties change induced by the inorganic nanoparticles modifications. It is recommended that further research might pay more attention on the mechanisms, especially the interface investigation

3.2. Sublayers adjustment for tailoring PA-TFC FO membranes Both dilutive and concentrative ICP are closely related to the structures of sublayers. A suitable sublayer should be able to provide as high as permeations while with well mechanical properties. In light of this, sublayers adjustment is expected to be an effective and promising method to alleviate ICP of PA-TFC FO membranes. Actually, this strategy has been widely investigated in recent years and the mainly used methods include surface modifications and inorganic nanomaterials incorporations. 3.2.1. Surface modifications of sublayers Surface modification is an effective method that being used to adjust the membrane surface for the aimed purpose, which has been extensively applied in modifying PA-TFC membrane. Like that being used in tailoring sublayers of PA-TFC RO membranes, this strategy can also be used in adjusting the sublayers to tailor PA-TFC FO membranes. However, the focused point is different. In RO membranes, the enhanced water flux was most important, accompanied with membrane performance such as antifoulinsg capability. In FO membranes, besides water flux, ICP was simultaneously emphasized. Surface modifications by unique hydrophilic polymers can significantly improve the hydrophilicity of the sublayers. For instance, the significantly improved substrate surface and inside hydrophilicity can effectively alleviate the concentrative ICP which was induced by the accumulation of salt ions inside the pores of sublayers [157]. Recently, the PSf substrate surface of PA-TFC FO membrane was modified by a bio-inspired polymer polydopamine (PDA) through the oxidation induced PDA polymerization [158]. Sublayer surface modification by PDA firstly produced a hydrophilic smooth surface with smaller pores and narrower pore size distribution for IP reaction and improved the hydrophilicity of the pore wall inside the support layer. With increasing the coating times from 0 to 5 h, the contact angles of the sublayers decreased from 85o to 54o, which was similar to that of pristine PDA [159]. The increased hydrophilicity significantly affected the permeations of sublayer. With increasing coating times from 0 to 5 h, the permeation firstly increased from 961 to 1011 L/m2·bar·h and then decreased to 651.7 L/m2·bar·h [158]. The increase was ascribed to the increased surface hydrophilicity and the decrease was attributed to the increased resistance. The change of sublayer by surface modification will further influence the synthesis of polyamide layer. The modification could actively interact with TMC monomers during IP process, which favored the formation of better polyamide layer with higher salt rejections. With PDA coating times of 1, 3, and 5 h, the 27

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Fig. 7. Comparision of separation performance improvements of PA-TFC RO membranes induced by sublayers adjustments via inorganic nanomaterials. (The values of unmodified membranes were defined as 1.00).

attributed to the higher surface porosity, better hydrophilicity, decreased thickness and additional water pathways through porous particles. Moreover, reverse osmosis test indicated that the incorporation of zeolite nanoparticles into the sublayer could increase the water permeability and decrease the salt rejection of the PA-TFC membranes, indicating that the sublayer changes tuned the IP process, which might decrease the crosslinking of polyamide layer. In addition, zeolite incorporation effectively decreased the ICP by reducing the substrate structural parameter of S (the direct indicator of ICP) from 0.96 nm for TFC to 0.34 nm [160]. Under this condition, the water flux of the sublayer was increased from 128 to 461 L/m2•h [160]. Thus fabricated PA-TFC FO membrane exhibited water flux of 80 and 40 L/m2·h in ALDS and AL-FS mode respectively using 2 M NaCl as draw solution and 0–0.01 M NaCl as feed solution [160]. The increased water flux in FO was attributed to the significantly improved permeability of sublayer and the decreased ICP. PSf substrate incorporated with TiO2 nanoparticles (PSf-TiO2) was used as sublayer for PA-TFC FO membranes [161,162]. TiO2 incorporation increased the hydrophilicity, porosity of the substrate accompanied with the formation of finger-like macrovoids. Under the optimized TiO2 loading (0.6 wt%), the ICP was significantly decreased and the structure parameter of S decreased from 0.91 to 0.39 mm [161], which contributed to the enhanced water flux. When the incorporation content was very high (0.90 wt%), water permeability was increased and salt rejection was decreased for the PA-TFC membrane, indicating that the crosslinking of polyamide was changed. Therefore, TiO2 incorporation in sublayer tuned the IP process again. In FO test, PSfTiO2 supported PA-TFC FO membrane displayed water flux of 18.81 L/ m2·h [161] using DI water and 0.5 M NaCl as feed and draw solutions in AL-FS mode, which was 97% higher than that of controlled membrane with no significantly changed reverse solute flux. When using seawater and 2 M NaCl as feed and draw solution, incorporation of TiO2 in PSf substrate also increased water flux from 4.2 to 8.1 L/m2·h in AL-FS mode and from 6.9 to 13.8 L/m2·h [161] in AL-DS mode. The significantly increased water flux was attributed to the enhanced permeability of sublayer and the decreased ICP. In their parallel research, hydrophilic halloysite nanotubes (HNTs) was incorporated into the PSf sublayers of PA-TFC FO membranes [163]. Under the optimized condition, HNTs incorporation decreased structure parameter from 0.95 to 0.37 mm [161]. The water flux was significantly increased from 13.3 to 27.7 L/m2·h [161] using 0.5 M and 2 M NaCl as feed and draw solutions. Carbon-based nanomaterials including CNTs and GO have also been

thickness of polyamide layer were 150, 185, and 280 nm [158], which were much lower than that of pristine one (380 nm). The modification could also smoothen the polyamide layer. With PDA coating times of 1, 3, and 5 h, the roughness of polyamide layer were 60.13, 61.13, and 67.79 nm, [158] which were much lower than that of pristine one (94.69 nm). With controlled coating times, PDA modification could simultaneously increased the water permeability and salt rejection of PA-TFC FO membranes. With coating time of 1 h, the water permeability and salt rejection were 0.60 L/m2·bar·h and 85%, which were higher than that of pristine membrane with the values of 0.51 L/ m2·bar·h and 80% [158]. Thus obtained PA-TFC FO membranes exhibited a Jw/Js of about 20 L/g by using 2 M NaCl as the draw solution and deionized water as the feed solution in AL-DS mode. In addition, the increased water flux could be favorable for decreasing ICP. In this case, hydrophilic surface modification of sublayer could enhance the water permeations while not changing their pore structures and retained the mechanical strength. Moreover, the increased salt rejections and decreased polyamide layer thickness all indicated that the surface coating of sublayer significantly influenced the IP process and increased the crosslinking of polyamide network structures. The decreased thickness might be attributed to the decreased diamines diffusion because of increased hydrogen bonding between diamines and the hydrophilic sublayer surface. However, the increased crosslinking of polyamide needs further investigation.

3.2.2. Inorganic nanomaterials incorporations Inorganic nanomaterials have been incorporated into FO membranes to improve their performance. For example, functionalized multi-walled carbon nanotubes (fCNT) were blended into cellulose acetate to fabricate inorganic-organic composite FO membranes [29]. It demonstrated that addition of fCNT effectively enhanced the antifouling properties of the membranes due to the enhanced electrostatic repulsion between the membrane and the alginate foulant. In addition, fCNT endowed the membrane surface with more hydrophilicity, which contributed to the significantly increased water permeations. As mentioned above, inorganic nanomaterials incorporation into the sublayers of PA-TFC membranes could be an effective alternative to tailor their properties, which has also been proved in PA-TFC FO membranes. In recent research, zeolite nanoparticles incorporation into PSf was applied to fabricate the nanocomposite substrate, which was used as the sublayer for PA-TFC FO membrane [160]. With loading content of 0.5 wt%, zeolite incorporation in the sublayer could increase the water flux of PA-TFC FO membranes by two times, which was 28

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undesired. From this point of view, inorganic nanomaterials incorporation provides a more feasible strategy. Inorganic nanomaterials can significantly enhance the permeations by increasing the hydrophilicity and by imparting the unique inorganic-organic structures. Besides, the mechanical properties are not influenced and sometimes can even be enhanced.

used to tune the sublayers of PA-TFC FO membranes. Nanocomposite of PES/MWCNT substrate was used as the sublayer for PA-TFC FO membranes [164]. During the phase inversion, –OH and –COOH groups of MWNCTs led to higher affinity towards water, which could lead to quicker formation of skin layer and create additional resistance to mass transfer. Thus, the exchange between the solvent and non-solvent was increased, leading to more open structure [165]. With the optimized incorporation content (2 wt%), MWCNT addition in sublayer increased the porosity from 85.6 ± 0.35% to 87.1 ± 0.55% and decreased the pore size from 11.38 ± 0.16 nm to 9.77 ± 0.07 nm [164]. It is worthwhile to note that MWCNTs were all wrapped in the polymer matric and thus had little effect on the sublayer surface hydrophilicity. The higher porosity of sublayer favored the adsorption of diamines and led to thicker polyamide layer, while smaller pore size limited the diffusion of MPD and decreased the polyamide layer thickness and roughness. However, the smaller pores of the substrate could increase the crosslinking of polyamide layer and enhanced the salt selectivity [101]. Overall, more open interior pore structure, smoother polyamide layer, and higher salt rejections could effectively decrease ICP in FO process and thus increased the water flux. Moreover, the mechanical strength was also effectively improved. In another research, GO was embedded into the PSf to obtain PSf/GO composite membrane support layer, which was then used to fabricate novel kind of PA-TFC FO membranes [166]. GO incorporation could increase the porosity and pore size of the sublayer through similar way as that in MWCNTs incorporation discussed above because of the similar hydroxyl groups [165]. The water permeability of the sublayer was increased from 100 to 720 L/m2·h·bar and the contact angle was decreased from 75° to 65°, [166] indicating that GO was not only dispersed in the polymer matrix. Besides, the polyamide layer with reduced thickness and increased crosslinking provided increased salt rejections. Under the optimized condition, the incorporation of GO (0.25 wt%) in the sublayer increased the water flux from 6.08 to 19.77 L·m− 2·h− 1 and the reverse flux selectivity from 5.75 to 3.36 L·g− 1 in AL-FS mode [166]. Besides, the structural parameter (S) was effectively reduced, indicating the alleviated ICP. Comparison of separation performances improvements of FO membranes by sublayers adjustments via different materials and methods is shown in Fig. 8. In summary, ICP of PA-TFC FO membranes closely relate to the structure of sublayers. Generally, sublayers with more opened structure are favorable for alleviating the ICP because the higher permeations can decrease the dilution of draw solutions in AL-FS mode and the aggregation of salt ions in AL-DS mode. However, more opened structure always decrease the mechanical strength, which is

3.3. Novel ENMs supported PA-TFC membranes 3.3.1. ENMs supported PA-TFC RO membranes Different from polymeric membranes, ultrathin ENMs possess unique properties such as high surface-to-volume ratio and high porosity, which can be controlled by altering nanofibers diameters [167]. This speciality has enabled ENMs to be used in various areas, including nanocomposite, functional coating and high-performance air filters [85,89,168]. Particularly, ENMs are most desirable in filtration because they provide dramatically increased permeations. The separation efficiency of ENMs based filters depends on the membrane pores, which can be altered by varying the nanofibers diameters. Recently, they have also been explored for their viability in liquid filtration and classified as microfilters [169,170]. For the thin film composite membranes, the porous support layer should be biologically, chemically, mechanically, and thermally stable. And it should also be noticed that the morphology and chemistry of support layer can influence the formation of the ultrathin polyamide layer. Fortunately, the particularity of ENMs well meet these demands. By optimizing fabrication procedures, the mechanical properties of ENMs can be well designed for different purposes [171]. Moreover, advanced nanotechnologies make it easy to realize the surface functionalization of ENMs [84]. The water fluxes of TFC membranes supported by ENMs were evidently higher than that of commercial ones, which was ascribed to the creation of “water channel” through the unique interfacial region between the nanofibers and the barrier layer [172]. All these advantages have motivated the applications of ENMs in desalination and ENMs supported PA-TFC membranes have displayed great potential in salt ions removal. Compared with that supported by PSf/PES, PA-TFC membranes using ENMs as sublayers exhibited extraordinarily higher water flux but with the compromising salt rejections [173]. ENMs was firstly used as support layers of NF membranes to investigate their potential in desalination. In NF applications, permeation of electrospun PAN nanofibrous scaffold supported PA-TFC NF membranes was 38% higher than that of commercial NF membranes (NF270, DOW Filmtec) while the salt rejection maintained similar [174]. Followingly, monovalent salt (NaCl) rejections of ENMs sup-

Fig. 8. Comparison of separation performances improvements of PA-TFC FO membranes by sublayers adjustments via different materials and methods. (The values of unmodified membranes were defined as 1.00).

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salt rejections. Under these considerations, ENMs supported PA-TFC RO membranes with high water flux and low operating pressures might be more preferable. Large pore size and porosity endow the ENMs sublayer with high permeations, which subsequently enhance the fluxes of the PA-TFC membranes. However, the high permeations sacrifice the salt rejections. In addition, although ENMs sublayer can provide extremely high permeations, their structures are usually loose and the pressure resistance of ENMs supported PA-TFC RO membranes during desalination process, especially under high operating pressures, should be enhanced. Thus, more advanced technologies might be developed. For example, the hot-pressing of ENMs before the IP is an alternative to alter the characteristics of ENMs [176]. Fig. 10a-d shows the SEM images of PAN ENMs treated by hot pressing at 87 °C under pressures of 0, 0.14, 0.28, and 0.41 MPa. Hot pressing significantly densified the interconnected structure of ENMs, enhanced the adhesion between the nanofibers, and decreased pore size and porosity, which would subsequently decrease their pure water flux. Fig. 10e and f exhibit the effect of hot pressing on the pore structures and thickness of ENMs. As anticipated, hot pressing significantly decreased the pore sizes and thicknesses of ENMs sublayers, and thereby dramatically decreased the pure water flux. The changed properties of ENMs sublayers would definitely influence the separation performance of PA-TFC membranes. The capability of manipulating the separation properties of ENMs supported PA-TFC membranes by altering the pore-size and thickness of sublayers through easy strategies (e.g., hot pressing) as desired is highly advantageous because no other additives are needed and more cost-effectiveness can be achieved.

ported PA-TFC RO membranes were investigated. ENMs obtained from PAN solutions with different concentrations supported interfacial polymerized PA-TFC membranes were fabricated and their desalination properties were evaluated [175]. With increasing solution concentrations, the stretching of nanofibers by the electrostatic force during electrospinning could be limited and thereby increased the nanofibers diameters and the pore sizes of ENMs, leading to the increased water flux. Under the optimized condition with solution concentration of 8 wt %, ENMs supported PA-TFC membrane exhibited rejection of 54% and flux of 200 L/m2·h with NaCl concentration of 2000 mg/L and operating pressure of 190 psi. The extremely more open structure of ENMs compared with that of PSF/PES provided a totally different platform for the IP, which made more MPD immersion into the sublayer. In addition, the very large pores of ENMs would decrease the crosslinking of polyamide layer. Thus, adoption of ENMs could significantly enhance the water flux but decrease the salt rejections. Almost all the reported ENMs supported PA-TFC NF/RO membranes were fabricated using piperazine (PIP) and trimesoyl chloride (TMC) as monomers during the IP. However, it should be noticed that these two monomers are always used to fabricate NF membranes while RO membranes are usually synthesized using m-Phenylenediamine (MPD) and TMC as reactive monomers. Taking this fact under consideration, in our former research, we executed the feasibility study on the synthesis of ENMs supported PA-TFC RO membrane using MPD and TMC as monomers [152]. It was found that polyamide layer cannot be successfully formed directly on the ENMs surface. Thus, we improved this strategy. Instead of direct IP on ENMs surface, prior chitosan coating was done first. By using sodium dodecyl sulfonate (SDS) as additives in aqueous phase, polyamide layer with unique ridge-andvalley structures were obtained (Fig. 9a). Interestingly, it is evidenced that addition of SDS played the critical role in the success fabrication of polyamide layer on chitosan coated ENMs, as shown in Fig. 9b-e. Under the optimized condition, the membrane exhibited a water flux of 16.5 L/m2·h and rejection of 94.4% using 2000 mg/L NaCl as the feed solution at the operating pressure of 0.8 MPa. Coating of chitosan could reduce the difference between the ENMs and usual PSF/PES sublayers and increase the salt rejections of polyamide layer while retaining the advantage of ENMs. Despite of comparatively unsatisfactory salt especially monovalent salt rejections, ENMs supported PA-TFC RO membranes can still find their applications in desalination due to their extremely high water flux and low operating pressures, which favors the energy-effectiveness. For example, in some unique desalination processes, energy-saving and economy-effectiveness are more important points compared with high

3.3.2. ENMs supported PA-TFC FO membranes ENMs can also be used as support layers for PA-TFC FO membranes. Initially, the only research is that ENMs were used as support for PSf layer which was used as sublayer for polyamide active layer [177]. The support layer of electropsun polyethylene terephthalate nanofibrous membranes effectively enhanced the resistance to delamination at high cross-flow velocities, which was always applied to reduce the external concentration polarization. The effectiveness was attributed to the enmeshment of ENMs into the PSf layer. Afterwards, Song et al. initiated the application of nanofibers sublayers for PA-TFC FO membranes [178]. Given the fact that the ICP of PA-TFC FO membranes is mainly caused by the dense and thick porous support, and an ideal support layer should have low tortuosity, be highly porous, and have a very thin structure, they introduced nanofibers membranes into support layers to fabricated a novel kind of

Fig. 9. (a) PA-TFC membrane fabricated on chitosan coated PAN nanofibrous membranes, ENMs supported PA-TFC membranes without (b-d) and with (e) addition of SDS in aqueous phase during IP [152]. (Copyright (2013), with permission from Elsevier).

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Fig. 10. (a-d), SEM images, (e) Pore size and (f) Cross-section thickness and pure water flux of ENMs treated by hot pressing at 87 °C at different pressures (0, 0.14, 0.28, and 0.41 Mpa for ENM-control, ENM-1, ENM-2, and ENM-3) [176]. (Copyright (2011), with permission from Elsevier).

Tian et al. fabricated a kind of PA-TFC FO membrane supported by electrospun polyetherimide (PEI) nanofibrous substrate functionalized by multi-walled carbon nanotubes (f-CNTs) [180]. Functionalization by f-CNTs extended the superior inter-connected porous structure of nanofibrous membranes in terms of increasing the porosity by 18%, reducing membrane structural parameter by 30%, and improving the substrate tensile modulus by 53% [180]. The high mechanical strength made it possible to further increase the substrate pore size and porosity, which would be effective to mitigate the ICP. The obtained membranes exhibited water fluxes of 61 and 33 L/m2·h [170] in AL-DS and AL-FS mode respectively using 1.0 M NaCl as draw solution and DI as feed solution in FO process. Recently, hydrophobic/hydrophilic composite nanofibrous membranes (HHCNF) owing interpenetrating network structures were used as support layer for PA-TFC FO membranes [181]. HHCNF support layer was constituted by electrospun hydrophobic polyethylene terephthalate (PET)/hydrophilic polyvinyl alcohol (PVA) composite nanofibrous membranes. Under the optimized experimental condition which was determined by PET/PVA ratios, HHCNFs supported PA-TFC FO membranes displayed extremely high water flux of 47.2 L/m2·h and low salt leakage of 9.5 gMH [181] using 0.5 M NaCl as draw solution and DI water as feed solution in AL-DS mode. The improved water flux was attributed to the wetting behavior of support layer and the watertransferring function. In addition, the unique structure of HHCNFs formed between PET and PVA nanofibers could reduce the ICP effectively.

nanocomposite FO (NC-FO) membranes. The used electrospun PES nanofibrous membranes possessed extraordinary advantages over conventional support layer obtained by phase inversion (PI) method such as low tortuosity, high porosity, and very thin thickness, which was promising in alleviating the ICP. As shown in Fig. 11, PES support layer prepared by phase inversion showed typical sponge-like structures on finger-like structures, while the nanofibrous support layer exhibited a unique, scaffold-like, porous structure with interconnected pores. Using DI water and salt solutions with different concentrations as feed and draw solutions under AL-FS mode, NC-FO membranes exhibited water flux three times higher than that of PI-FO membranes while the salt rejections were similar. The porosity, tortuosity, and structure parameter of NC-FO membranes were 83 ± 1%, 1.33, and 80 ± 6, while that of PI-FO membranes were 81 ± 1%, 3.65, and 450 ± 50, respectively [178]. Therefore, the dramatically decreased ICP of NCFO was attributed to the lower tortuosity and S value, which effectively favored the salt diffusion in the support layer and got high net osmotic driving force in FO process. Afterwards, many kinds of ENMs were explored to be used as support layers for PA-TFC FO membranes, such as crosslinked PVA [179]. The exceptionally porous structure of PVA ENMs with low tortuosity and remarkable hydrophilicity effectively reduced ICP and increased the water flux. The obtained membranes showed a flux of 7–8 times higher (22.74 L/m2·h) [179] than that of commercial HTI-NW FO membrane (3.69 L/m2·h) using 0.5 M NaCl and DI water as draw and feed solutions in AL-FS mode. 31

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first place, effect of hydrophilic additive and inorganic nanomaterials on the formation mechanism (phase inversion) of sublayer should be investigated, as that discussed in CNTs incorporation [165]. In other words, research on how the additives and nanomaterials incorporation influence the pore structures of sublayer by tuning the phase inversion processes should be carried out. In the second place, considering the fact that the sublayer can directly influence the IP and thereby the thickness and crosslinking of polyamide layer, the effect of sublayer changes induced by these two strategies on the polyamide formation should also be researched. For example, how the changed sublayers control the MPD diffusion during IP process. (2) Novel ENMs Sublayers. Although ENMs supported PA-TFC membranes exhibit significantly enhanced water permeability, they are challenged by comparatively lower salt rejections. Strategies to modify the ENMs to increase the salt rejections of ENMs supported PA-TFC membranes should be executed. Moreover, their mechanical strength should also be focused. In another aspect, considering more practical applications of ENMs in FO process which always does not need high operating pressures, further directions should be in exploring breakthrough to develop corresponding industrial products. (3) Other Aspect. Novel sublayer altering methods should be explored. For example, surface modifications of sublayers can be achieved through versatile methods except that have been reported (e.g., surface plasma [102]). Although up to now ENMs are the only promising alternative that have been used to substitute PES/PSf in the fabrication of PA-TFC membranes, it is expected that the ongoing research and nanotechnology developments will help to find more advanced ones. Overall, sublayers adjustment provides an effective method to tailor the structures and separation properties of PA-TFC membranes. In spite of the progresses and achievements that have been made, more research is still necessary. Moreover, it should be noted that sublayers adjustment is a kind of multi-disciplinary study that involves physics, chemistry, and materials science. Therefore, researchers with different backgrounds from different regions should collaborate closely. It is believed that breakthrough will be made in this area, which will do favor to motivate more advanced tailoring strategies for PA-TFC membranes and water treatment technologies and further benefit human beings.

Fig. 11. SEM images for (a) PI PES sublayer, (b) NC PES sublayer, surface and crosssection for (c, e) NC-FO and (d, f) PI-FO membranes [178]. (Copyright 2011, John Wiley and Sons).

In summary, ENMs were identified to be an effective alternative to be used as support layer for PA-TFC FO membranes, which can control the ICP and improve the water flux due to the unique inter-connected porous structures. Compared with that being used in RO desalination, ENMs provide more feasibility in FO process because less mechanical impact is imposed on the sublayer. Therefore, it is believed that ENMs will find more and more applications in PA-TFC FO membranes.

Acknowledgment 4. Conclusions and perspectives

This study was supported by Science and Technology Assistance Project for Developing Countries: Research and Applications of High Concentrated Reverse Osmosis Seawater Desalination Technology for Tropical Islands (KY201602001). Central-Level Research Institutes Fundamental Research Team Project: Research on Anti-scaling Technologies of High-pressure Seawater Desalination Membranes (KJBYMF-2015-T02).

In conclusion, the sublayers closely relate to the structures and separation properties of PA-TFC membranes. Given this fact, it is expected to realize the PA-TFC membranes tailoring by adjusting the sublayers. The adjustment includes traditional PSf/PES sublayers tuning and exploration of novel sublayer typically in terms of ENMs. Although lots of interesting and meaningful developments combined with inspiring results have been made in recent years, great challenges still exist in the deep understanding and improvement of existing strategies and in seeking more advanced and effective adjustment technologies. It is expected that the points needed to be paid particular attention should emphasize on the following aspects. (1) Traditional Sublayers Altering: First of all, the present research on the effect of hydrophilic additives (e.g., PEG and PVP) on the sublayers always goes to different results, which might be induced by the varied experiment conditions [98,101]. Therefore, more systematical and precise investigation might be executed to explore more persuasive conclusions. For the inorganic nanomaterials modifications, more functional and innovative nanomaterials should be explored, which is the development impetus in material science. In addition, more research on mechanism should be studied. In the

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