Nanoporous materials in polymeric membranes for desalination

Nanoporous materials in polymeric membranes for desalination

Available online at www.sciencedirect.com ScienceDirect Nanoporous materials in polymeric membranes for desalination Pinar Cay-Durgun1,2 and Mary Lau...

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Available online at www.sciencedirect.com

ScienceDirect Nanoporous materials in polymeric membranes for desalination Pinar Cay-Durgun1,2 and Mary Laura Lind1,2 Pressure-driven membrane desalination processes require new membranes that have increased energy-efficiency, permselectivity, resistance to chlorine, and resistance to fouling. Incorporation of nanoporous materials (e.g. zeolites, metal– organic frameworks, and graphene-based materials) into the state-of-the-art polyamide-based thin film composite (TFC) membranes is one strategy to address these challenges. This requires effectively incorporating nanomaterials into the polymer structure and understanding the true impact of the nanomaterials on membrane performances. Studies from 2015 to 2017 have revealed that thin-film nanocomposite (TFN) membranes with nanoporous materials (a sub-class of mixed matrix membranes), to some extent, address these desalination membrane challenges. Addresses 1 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, United States 2 Nanosystems Engineering Research Center for NanotechnologyEnabled Water Treatment, Arizona State University, Tempe, AZ 85287, United States Corresponding author: Lind, Mary Laura (mlli[email protected])

Current Opinion in Chemical Engineering 2018, 20:19–27 This review comes from a themed issue on Separation engineering Edited by Wei Zhang, WS Ho and Kang Li

https://doi.org/10.1016/j.coche.2018.01.001 2211-3398/ã 2018 Elsevier Ltd. All rights reserved.

Introduction The increasing global demand for fresh water drives extensive research to both improve existing water purification techniques and to discover new alternative purification methods. Among available water separation technologies, membrane-based desalination processes are promising to fulfill the increasing water demand because of their inherent high energy efficiency and efficacy and ability to extract water from saline sources [1]. Currently, reverse osmosis (RO) is the primary technology for seawater and brackish water desalination. RO is a pressuredriven membrane separation technique in which a semipermeable, dense membrane preferentially permeates water and primarily rejects dissolved salts. Nanofiltration www.sciencedirect.com

(NF) is a similar technique to RO; however, it uses more water permeable/less salt selective — often termed ‘loose’ — membranes under lower pressure. The stateof-the-art, commercially available RO/NF membrane is an asymmetric aromatic polyamide (PA) thin film composite (TFC) produced via interfacial polymerization (IP) on a microporous support membrane. More than four decades have passed since the initial development of the polyamide TFC membrane, however, methods to improve this unique membrane structure and to develop a fundamental understanding of the superior mechanism for mass transport are under ongoing investigation in membrane science [2–5]. Nevertheless, the permeability/selectivity trade-off, inherent in polymeric membranes, constrains the maximum performance of these membranes for desalination [6]. Additionally, RO/NF membranes still need improved fouling resistance and chlorine tolerance [7]. Thin film nanocomposite (TFN) membranes, which add nanoparticles into the thin polyamide film of the TFC membrane structure, have emerged to address these challenges over the last decade [8]. In addition to novel functionalities added by nanoparticles (e.g. antimicrobial, chemical resistance), it is hypothesized that nanoporous particles bring an additional transport mechanism and preferential flow paths through membranes [9]. Here, we review the recent literature from 2015 to 2017 of nanoporous materials incorporated into TFC polymeric membranes for RO/ NF desalination focusing on those that include zeolites, metal–organic frameworks, and graphene-based materials. Beyond the scope of this article, there are many nonporous nanoparticles that can be added to TFC membranes, as well as other types of membranes for desalination processes, which incorporate nanoparticles with specific functionalities (e.g. antimicrobial activity, chemical reactivity, and heat generation). For example, Dongare et al. recently developed porous, hydrophobic polymer membranes containing nanophotonic particles for membrane distillation desalination [10]. Furthermore, a recent review summarized membranes with stimuli-responsive nanoparticles for water purification [11]. Refer to Geise et al. for definitions of membrane performance parameters [12]. Commercial polyamide membranes have permeabilities for RO around 1–10 l m2 h1 bar1 (lmh bar1) and for NF around 10–20 lmh bar1 [7]. Cohen-Tanugi et al. modeled mass transport and fluid dynamics of an RO system to quantify the potential of ultra-permeable membranes [13]. They found that a threefold increase in permeability (1.5–4.5 lmh bar1) may Current Opinion in Chemical Engineering 2018, 20:19–27

20 Separation engineering

reduce energy consumption 46% for brackish water (BW) RO and also result in use of 63% fewer pressure vessels [13]. However Cohen-Tanugi et al. calculate a further increase in BW RO permeability above >5 lmh bar1 will have a negligible change in energy consumption (because of inherent thermodynamic and process limits) but may continue to lower overall costs [13]. For instance, Werber et al. using their module-scale modeling study found that a permeability increase from 4 to 10 lmh bar1 may only reduce energy consumption for a single stage by 2.2%, but because of staging effects will result in a 12% reduction for a two-stage brackish water RO process [1]. Other opportunities to enhance the desalination membrane performance include: significantly improved efficiency with extremely high rejection, increased membrane performance and lifetime with improved fouling resistance, and lowered pretreatment costs with chlorine tolerance.

advantage of the embedded nanoparticles, they must have the appropriate size and internal structure, as well as surface properties compatible with the polyamide to ensure a suitable polymer–particle interface. Also, the concentration of the filler particles should not be too large to destroy the thin film integrity.

Thin film nanocomposite membranes

Zeolites

Nanomaterials with tunable mechanical and chemical properties and with robust porosity offer new possibilities for incorporation into desalination polymeric membranes without significantly altering the scalable, low-cost polymer synthesis process. TFN membrane synthesis involves incorporating nanoparticles into the TFC polyamide film before and/or during the IP reaction [14]. Ultimately, IP of polyamide is a complex, heterogeneous condensation reaction at the interface of a difunctional amine monomer in the aqueous phase and a trifunctional acid chloride monomer in the organic phase [15]. As a result of this reaction, extremely thin (20–200 nm), highly crosslinked, highly selective, negatively charged, low swelling, hydrophilic polyamide active layer forms on the support. In addition to this dense and thin layer, the final structure also contains voids [5]. Void size, content, and location have a significant impact on the effective water transport through the active layer [16]. Polymerization conditions such as phase type, concentration, and temperature; chemical contact time; membrane curing time; and curing temperature highly impact the resultant membrane morphology and separation performance [17,18].

Zeolites are inorganic, crystalline, rigid, hydrated aluminosilicates with a defined general formula and narrow pore distribution (3–8 A˚ pore size). Zeolites have been a popular nanoparticle included in TFN desalination membranes [8]. Of the 235 possible zeolite framework codes in the International Zeolite Association (IZA) database, only Linde type A (LTA), Faujasite (FAU), Linde type L (LTL), and Zeolite Socony Mobil Five (MFI) frameworks have been investigated to date for TFN membranes.

The most common TFN synthesis method is to add nanomaterials, referred to as ‘fillers’ into the casting solutions during the membrane casting process (different methods are described in the following sections and also are summarized in Figure 1). The addition of the fillers to the casting solution may affect the separation performance of the resulting polyamide phase by changing the crosslinking degree and overall reaction yield [19,20]. Moreover, the particle presence may alter the surface properties (roughness, charge density, hydrophilicity) and thus the overall membrane separation performance [21]. Furthermore, the performance of TFN membranes varies as a function of the type, size, and concentration of nanoparticles [19,22]. To fully take Current Opinion in Chemical Engineering 2018, 20:19–27

The exact mechanism for the particle–polymer interface and the structure for TFNs is not yet completely understood [8,23]. In addition to this unknown mechanism, other challenges can arise in TFN formation including particle aggregation in the polyamide layer; non-uniform particle dispersion in the structure; orientation difficulties, especially for non-three dimensional particles; and unknown particle stability during the reaction and during the high-pressure filtration [8].

In 2007, Hoek’s group published a seminal work on TFN membranes for RO desalination in which NaA zeolites were added into the organic phase casting solution [9]. This highly hydrophilic zeolite with the LTA framework has 3-dimensional pores (4.2 A˚), which have a suitable pore size for selecting water (2.8 A˚) but rejecting hydrated ions (>6 A˚) and small organic molecules. In addition, the cubic structure of the zeolite pores ensures water access to the pore regardless of the orientation in the membrane. We studied the stability of hand-cast NaA-TFN membranes (particle addition into the organic phase) during a 4-month performance test with brackish waters (400 ppm TDS) at the United States Bureau of Reclamation’s Water Quality Improvement Center in Yuma, AZ. The water permeability of TFN membranes showed 80% higher water-salt perm-selectivity than the TFC membranes [29]. However, the membranes did not experience high fouling conditions, extreme pH conditions, cleaning, or chemical treatments. Because of dealumination from the framework, aluminum-rich zeolites, such as NaA, are less structurally stable when exposed to acidic solutions compared to silicon-rich structures, such as MFI [39]. Indeed, our group extensively studied acid environment stability of NaA zeolite and identified that it is not suitable for applications in low pH solutions (pH < 2) especially in presence of phosphate (pH < 5) [40]. www.sciencedirect.com

MOFs, COFs covalent organic frameworks Cay-Durgun and Lind 21

Figure 1

(a)

Method 1

Method 2

Method 3

Method 4

Thin film nonocomposite membrane

Thin film nonocomposite membrane

Thin film nonocomposite membrane

Thin film nonocomposite membrane

filler addition via “in-situ” method

filler addition into the aqueous casting phase

filler addition via “evaporation-controlled filler positioning” method

filler addition into the organic casting phase

Support casting typically by phase inversion Support treatments

Support saturation with diamine in aqueous

Excess solution removal

Saturated support immersion into acyl chloride in organic

IP reaction

Thin film composite membrane

(b)

COCI Polyamide

TMC in organic CIOC

COCI

Porous support membrane

H2 N

NH2

MPD in aqueous

IP Rxn

H

HN

CO HN

NHOC

CO NH

m

NHOC

CO

COOH

OH

n

NH

Current Opinion in Chemical Engineering

(a) Summary of TFN membrane casting procedures discussed in this review and (b) schematic of a polyamide interfacial polymerization reaction. (a) Method 1: filler embedded into the structure via ‘in situ method’ [24], method 2: fillers added into the aqueous membrane casting solution [20,25–27], method 3: fillers added through ‘evaporation-controlled filler positioning’ method [28], and method 4: fillers added into the organic casting solution [29–31]. Spherical particles represent either nanoporous zeolite particles, MOF particles, and graphene-based material nanosheets. However, nanosheets addition into the thin film structure occurred only through method 2 [32–37] and method 4 [38] during this review period. (b) The resulting polyamide has both crosslinked (m) and linear (n) structures. MPD: m-phenylenediamine and TMC: trimesoyl chloride (MPD and TMC are common monomers for polyamide RO membrane synthesis).

For BW RO Dong et al. studied TFN membranes synthesized by adding hydrophilic NaY zeolites (150 nm particle size) into the amine casting solution [20]. NaY has the FAU framework type with a larger pore size (7.4 A˚) than the NaA zeolite. The larger pore size may lead to a more preferential water pathway through www.sciencedirect.com

the zeolite but may weaken size exclusion effects. In the study, however, at the optimum zeolite loading, the membranes showed enhanced water permeation compared to the control TFC membranes and maintained the high salt selectivity [20]. Importantly, this study demonstrated that the reaction time necessary to form Current Opinion in Chemical Engineering 2018, 20:19–27

22 Separation engineering

a dense zeolite-polyamide layer in the TFN was increased compared to the TFC, suggesting the particle presence was limiting amine diffusion during the IP reaction. Additionally, glycerol–salt solution post-treatment further improves the TFN permeability performance by 16% without any sacrifice in selectivity [20]. Dong et al. developed a TFN ‘in situ’ method (TFN-I) in which LTL zeolites (7.0 A˚ pore size) were added to the support prior to the IP process yielding NF performance [24]. During the phase inversion preparation of the polysulfone support membrane, they dispersed 80-nm diameter LTL particles into the water bath solution, calling it an ‘in situ’ method. They report this resulted in 49% particle coverage ratio of the support membrane surface without particle aggregation. Based on their previous study [41], Dong et al. stated the particles are strongly attached to the support. Compared to TFN and TFC control membranes, TFN-I membranes, had a significantly higher surface area difference because of the initially higher support surface area. These TFN-I membranes demonstrated significantly enhanced water permeation but decreased monovalent and divalent salt selectivity (possibly indicating the presence of membrane defects). Natural zeolites could be a low-cost and environmentalfriendly alternative to synthetic zeolites. These materials require treatments to enhance uniformity, purity, hydrothermal stability, and to alter separation characteristics. Unfortunately, treatment methods may also impair the desired characteristics of zeolites. The hydrophilic clinoptilolite zeolite, which is the most abundant naturally occurring zeolite in the world [42], loses its crystallinity under high temperature and steam treatment [43]. Safarpour et al. studied non-thermal glow discharge plasma treated clinoptilolite zeolite-TFN for RO desalination and fouling resistance [25]. According to X-ray diffraction pattern, the zeolites retained their crystallinity after the plasma treatment. However, based on the scanning electron microscope images, instead of fine particles, after plasma treatment, the zeolites looked like a big chunk. The TFN membranes were synthesized with 0.01 wt% plasma-treated zeolites incorporated into the aqueous phase IP casting solution. These demonstrate improvement on hydrophilicity, antifouling property, water permeability, and salt selectivity compared to the controls. Two-dimensional zeolite nanosheets are an emerging material for desalination which offer a shorter diffusion path-length than 3-D materials. Non-equilibrium molecular dynamic simulations by Jamali et al. identified possible hydrophobic zeolite nanosheets which might have 100% rejection with a pore limited diameter <5.5 A˚ and estimated 54 lmh bar1 permeability (for 100 nm thick membrane) [44]. However, there are no experimental Current Opinion in Chemical Engineering 2018, 20:19–27

reports on two-dimensional zeolite membranes or zeolite nanosheets in TFN membranes.

Metal–organic frameworks Metal–organic frameworks (MOFs) are hybrid organic– inorganic materials consisting of inorganic metal centers and organic linkers which create crystalline, flexible, and highly tunable pore structures (pore windows 3 A˚ to 100 A˚). MOFs offer a great variety of nanomaterials for membrane separation applications [45]. Although the structural diversity and highly tunable hybrid structure make MOFs promising fillers for aqueous separations, the majority of the MOF structures degrade upon exposure to ambient moisture [45]. Wang et al. have reviewed the water stable MOFs with potential applications in membranes [46]. Zeolitic imidazolate (Im) frameworks (ZIFs), a well-known subclass of MOFs, have received increasing attention because of their excellent chemical stability. ZIF-8 is a hydrophobic 3-D material with a Zn(MeIM)2 composition and 3.4 A˚ nominal aperture size and is the most extensively studied ZIF material in membrane applications. Zhang et al.’s study reveals that ZIF-8 might provide a molecular sieve in 4–6 A˚ range, because of the flexible framework [47]. This range makes the material a suitable candidate for desalination. Although Zn releases from the structure when exposed to seawater, the material maintains the crystal structure [48]. Duan et al. state the potential advantages as first, the hydrophobicity of the material, which may provide fast water transport and second, the organic linker may enhance the particle compatibility in the nanocomposite [30]. In two studies, nanosized ZIF-8 TFNs, with 80 and 200 nm particles incorporated through the organic phase, demonstrate increased permeability and maintain RO rejection at same particle loading [30,31]. Van Goethem’s report on ZIF-8 TFN membranes introduces a new TFN synthesis method called ‘evaporationcontrolled filler positioning (EFP)’ [28]. They developed a two-step process to pre-position the particles at the aqueous/organic interface during the polymerization reaction. First, particles dispersed in an organic solvent are coated on the amine saturated support. Second, the organic solvent was evaporated before submerging the coated support into the organic phase for the polymerization reaction. Van Goethem hypothesizes this method may address TFN particle orientation challenges and reduce expensive particle usage by eliminating waste. Interestingly, at a ZIF-8 loading of 0.005 (w/v%), the filler particle size, whether 75 nm or 150 nm, has no significant effect on the membrane performance. A performance difference is expected based on the difference in the diffusion path-lengths of the particles and the actual number of the particles present (8 times more small particles than large ones). This indicates that performance enhancement may be related to the polymer www.sciencedirect.com

MOFs, COFs covalent organic frameworks Cay-Durgun and Lind 23

but not the particle incorporation. However, at higher loadings, the permeability increases with the particle size and decreases with the loading. Another TFN synthesis method introduces ZIF-8 particles into the aqueous/amine casting solution with an anionic, water-soluble polymer, Poly(sodium 4-styrenesulfonate) (PSS) [26]. Zhu et al. hypothesized that the PSS modifies the ZIF-8 particle surfaces which will improve their dispersion into the aqueous solution. Thus, the final membrane structure may have less particle aggregation. The method involves a similar evaporation step to Van Goethem’s to position the particles before IP. The modified ZIF-8 TFN-NF membranes demonstrated increased hydrophilicity, roughness, and negative charge. They also demonstrated double water permeation compared to control membranes and slightly decreased divalent ion retention [26]. Another hydrostable MOF structure is hydrophilic chromium(III) terephthalate Materials Institute Lavoisier (MIL-101(Cr)) with 12 A˚ pentagonal pore size. Xu et al. embedded 200 nm MIL-101(Cr) particles in TFN membranes for desalination [27,49]. They compared TFNs synthesized with particles dispersed in the aqueous IP casting phase and TFNs with particles incorporated in the organic IP casting phase [27,49]. In the latter study, the TFN membranes prepared with MIL-101(Cr) particles in organic solution showed enhanced, stable RO separation performance, increased roughness and hydrophilicity compared to the TFC membrane [27]. Two-dimensional MOF nanosheets have been recently assembled with a polycationic polymer for nanofiltration. The membranes exhibited a very high permeability of 4000 lmh bar1 (for 48 nm thick membrane) and high selectivity to organic dyes, but only 20–40% salt rejection [50]. However, there are no reports on the MOF nanosheets incorporated into TFN membranes.

Graphene-based materials Graphene is a monolayer of carbon atoms with a flexible, hydrophobic, and nonporous structure. Perforated graphene has attracted increasing attention because it has ideal membrane features: it can be as thin as 3.4 A˚, has a robust structure under harsh conditions, and has a theoretically near perfect molecular sieving mechanism [51]. Pore creation in graphene experimentally demonstrated a potential for desalination applications [52,53]. For instance, O’Hern et al. synthesized a defect-free, centimeter-scale graphene membrane on a polycarbonate track etch support [53]. Their creative method included two defect sealing procedures: first, they filled small defects with Hafnia using atomic layer deposition and second, they sealed large defects with nylon-6,6 via interfacial polymerization [53]. However, an efficient, scalable synthesis creating a narrow pore distribution of a defect-free graphene membrane on an economical support is a significant challenge [51]. Graphene oxide (GO) (graphene with hydrophilic functional groups) membranes have water transport through the tunable interlayer spacing and defects within the sheets [54]. Although recent work showed a great desalination potential of GO membranes, more research is necessary for stability and scalability challenges [55]. Abraham et al. experimentally showed the salt rejection by channel confinement in 100 mm thick, stacked GO-graphene laminates using epoxy. Their method enables control of layer spacing to dimensions <10 A˚ resulting in less swelling in water. However, the membrane displayed much lower water permeability than the predictions (experimental: 0.5–1 lmh bar1 vs prediction: 208–1042 lmh bar1 [56]) because of the difficulty of scale-up of the thin structure [54]. Therefore, adding GO nanosheets into polyamide, currently, can be more economical and convenient approach than trying to synthesize pure GO membranes. Adding graphene oxide in TFN membranes (addition through the aqueous phase) improved permeability with a

Table 1 Comparison of the nanoporous filler materials discussed in this review. Zeolites are microporous inorganic materials, metal–organic frameworks are microporous hybrid organic–inorganic materials, and tunable interlayer spacing graphene-based materials are nonporous graphene with functional groups. LTA, ZIF-8, and GO accessible pore structures are given as examples of their groups

Structural diversity Pore robustness Flexibility Production cost

Zeolites

Metal–organic frameworks

Graphene-based

LTA

ZIF-8

GO

Medium High Low High

High Medium Medium Medium

Low Low High Low

This qualitative comparison methodology is adapted from [45]. ZIF-8 pore structure modified from [48].

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Current Opinion in Chemical Engineering 2018, 20:19–27

Overview of filler materials in TFN membranes discussed in this review. ", #, $ indicate an increase, decrease, and remain same, respectively. Rejection of RO membranes in a NaCl test and rejection of NF membrane in a MgSO4 test except [29] in a brackish water test and [26,38] in a Na2SO4 test. The comparison behaviors are based on the reported test results, calculated as values of jTFC  TFNj/TFC Filler material

Zeolites

Metal–organic frameworks

Graphene-based

Filler size (nm)

b c

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d e

RO/NF

Filler addition (amount and phase)

Permeability (lmh bar1)

Rejection (%)

Compared to their corresponding TFC membrane Per. b

Rej. c

F.R. d

C.R. e

NaA [29] NaY [20] LTL [24] Clinoptilolite [25] ZIF-8 [30]

100 150 80 n/a 200

4.2 7.4 7.0 n/a 3.4

RO RO NF RO RO

0.30 wt% in organic 0.15 wt% in aqueous In situ to the support 0.01 wt% in aqueous 0.04 wt% in organic

1.9 4.8 8.0 2.7 3.4

97.9 98.8 93.4 97.1 98.5

44% " 84% " 120% " 39% " 162% "

0.5% " 0.4% " 3% # 2% " 0.4% "

n/a n/a n/a " n/a

n/a n/a n/a n/a n/a

ZIF-8 [31] ZIF-8 [28] m-ZIF-8 [26] MIL-101(Cr) [27]

80 150 59 200

3.4 3.4 3.4 12

RO RO NF RO

0.04 wt% in organic 0.005 wt% in EFP a 0.10 wt% in aqueous 0.05 wt% in aqueous

1.7 2.7 14.9 3.2

99.4 85 95 99

53% " 185% " 115% " 44% "

1% " $ $ $

n/a n/a n/a n/a

n/a n/a n/a n/a

GO [32] GO [33] GO [34] rGO/TiO2 [36] cGO [35] pTA-f-GO [37] GO-COCl [38]

Sheet size (nm) 70-140 n/a n/a n/a n/a n/a 500-5000

Layer space (A˚) n/a 8.8 n/a n/a n/a n/a n/a

RO RO NF RO NF NF NF RO

38 ppm in aqueous 100 ppm in aqueous 0.2 wt% in aqueous 0.02 wt% in aqueous 100 ppm in aqueous 19 ppm in aqueous 0.002 wt% in organic

1.1 2.0 1.5 3.4 11.7 1.8 3.8 1–10

99.3 97.5 97 99.5 99.2 94.8 97.1 98–99.8%

80% " 39% " 1120% " 50% " 185% " 69% " 95% "

$ 1% # 1% # 2% " 11% " 13% " 2% "

" " " " " " "

" " n/a " n/a " n/a

10–20

98–99%

Commercial TFC membranes [7] Commercial TFC membranes [7] a

Pore size (A˚)

EFP = evaporation-controlled filler positioning. Per. = permeability. Rej. = rejection. F.R. = fouling resistance. C.R. = chlorine resistance.

NF

24 Separation engineering

Current Opinion in Chemical Engineering 2018, 20:19–27

Table 2

MOFs, COFs covalent organic frameworks Cay-Durgun and Lind 25

slight decrease in selectivity for RO [32,33] and for NF [34]. Besides the improved permeability, the membranes exhibited lower contact angle, enhanced anti-fouling properties [32–34], and more importantly improved chlorine resistance [32,33]. It is hypothesized that the GO nanosheets prevented chlorine attack of the underlying polyamide. Horizontal GO orientation in the structure is attributed to the Langmuir–Blodget film formation, which commonly results in smooth membrane surfaces [32,35]. Recently, Chae et al. further improved the membrane permeability and antifouling properties by embedding GO both the active layer and the support layer [57]. Safarpour et al. observed a synergetic effect in RO TFN membranes made with reduced GO and titanium dioxide (rGO/TiO2) [36]. The membranes showed better separation performances, anti-fouling, and chlorine resistance behavior when embedded with both fillers. Moreover, rGO and TiO2 dispersed well in water but not in the organic phase, thus particle addition to TFN structure was through the aqueous phase. Carboxylated graphene oxide (cGO) has increased hydrophilicity compared to GO with carboxylic groups added; cGO has been studied for better filler dispersion in the aqueous phase for NF membranes [35]. Compared to control untreated GOTFN membranes, the cGO membranes have 22% lower contact angle, 7% higher divalent salt rejection, and 87% higher permeability. However, the cGO-TFN membranes have 58% lower monovalent salt rejection than GO-TFN membranes [35]. TFN-RO membranes with functionalized GO nanosheets (polyethyleneimine (PEI) and tannic acid (TA), pTA-f-GO) showed improved permeability and selectivity, chlorine resistance, and antibacterial properties compared to TFC [37]. In this low cross-linked membrane, TA tightly binds GO and PEI allows crosslinking of amine to TA and PA [37]. Wen et al. added acyl chloride-functionalized graphene oxide (GO-COCl) (homogenously dispersed in ethanol) into the organic IP casting phase for the synthesis of TFNNF membranes [38]. Although the resulting membranes displayed enhanced separation performances, using ethanol as a dispersion phase made the membranes rougher and more susceptible to fouling [38]. Table 1 presents a comparison of the nanoporous filler material groups in TFN membranes and Table 2 summarizes the recent development of TFN membranes based on the porous filler types and their membrane performances and key features.

Conclusion There is no uniform perm-selectivity result from adding nanoporous materials (zeolites, MOFs, and graphenebased materials) to polyamide-based osmotic membranes. In some rare cases both the membrane permeability and selectivity are improved significantly, however, in most cases the selectivity only changes from the control TFC www.sciencedirect.com

membranes by 2%. Additionally, many of the novel reported TFN membranes do not have performance outside of the range of commercially available TFC membranes. This results from the inherent complexity of the interfacial polymerization reaction, which is compounded by the various researchers’ slight change in IP steps, and actual presence of nanoporous materials during the reaction. To realize the full potential of the nanoporous material in transport, TFN membrane synthesis may need to be optimized vigorously independently of adding particles into previously optimized TFC membrane synthesis protocols. In mixed matrix membrane for gas separation there has been extensive research modeling and predicting transport properties, there is significantly less in TFN osmotic membranes. The field may benefit from theoretical TFN modeling studies. Additionally, there is significant room for exploration in the emerging area of 2-D materials. Furthermore, TFN membranes exhibit great promise for the addition of new features such a chlorine resistance and fouling resistance.

Acknowledgements We gratefully acknowledge support from the National Science Foundation CAREER award CBET-1254215 and National Science Foundation Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment EEC-1449500.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

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2.

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7. 

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