Thin Film Nanocomposite Forward Osmosis Membranes on

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Functional Nanostructured Materials (including low-D carbon)

Thin Film Nanocomposite Forward Osmosis Membranes on Hydrophilic Microfiltration Support with an Intermediate Layer of Graphene Oxide and Multiwall Carbon Nanotube Wang Zhao, Huiyuan Liu, Yue Liu, Meipeng Jian, Li Gao, Huanting Wang, and Xiwang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10550 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Applied Materials & Interfaces

Thin Film Nanocomposite Forward Osmosis Membranes on Hydrophilic Microfiltration Support with an Intermediate Layer of Graphene Oxide and Multiwall Carbon Nanotube Wang Zhaoa, Huiyuan Liua, Yue Liua, Meipeng Jiana, Li Gaob, Huanting Wanga, Xiwang Zhanga* a

Department of Chemical Engineering, Monash University, Vic 3800, Australia

b

South East Water, PO Box 2268, Seaford, Victoria 3198, Australia

ABSTRACT A novel thin film nanocomposite forward osmosis (FO) membrane was fabricated on hydrophilic nylon microfiltration support (MF) by interfacial polymerization (IP) with the assistance of an intermediate layer of graphene oxide and multi-wall carbon nanotube (GO/MWCNT). The chemical composition, structure and surface properties of the synthesized FO membranes were studied using various characterization methods. It was found the GO/MWCNT composite layer not only provided ultrafast nanochannels for water transport but also reduced the thickness of the polyamide layer by up to 60%. As a result, the novel FO membrane exhibited a higher water flux and lower reverse salt flux compared with the membrane synthesized without GO/MWCNT intermediate layer. This method offers promising opportunities to fabricate thin film composite membranes on microfiltration substrates for FO application with inhibited concentration polarization phenomenon and expectant separation performance. KEYWORDS: Thin film composite; Graphene oxide; Multiwall carbon nanotubes, Forward osmosis; Interfacial polymerization

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1. INTRODUCTION Nowadays, global water scarcity and the increasing demand for fresh water have forced us to consider advanced technologies which can achieve water reuse and recovery in a more energy efficient and low-cost way 1. Among various technologies, forward osmosis (FO) process which uses osmotic pressure difference as the driving force is considered as a promising sustainable way to achieve the goal owing to its unique advantages, such as minimal hydraulic pressure operation, nearly complete rejection of most contaminants and potentially low membrane fouling tendency 2-5. FO process has therefore attracted considerable interests in various fields including wastewater treatment 6-7, seawater desalination 8 as well as power generation 9-10. However, one of the key challenges that hinders the implementation of FO processes is the lack of high-performance FO membranes. So far, the dominated FO membrane is thin film composite membrane (TFC) which is fabricated by forming a polyamide selective layer on a microporous support via an interfacial polymerization (IP) reaction

11

. Although TFC membranes exhibit

advantages of satisfying separation performance over a wide operating temperature and pH range as well as independently optimized selective and support layers 12, it is still facing the problem of internal concentration polarization (ICP). The ICP occurred in the porous support of TFC FO membranes can cause a dramatic loss in the osmotic driving force and cannot be eliminated by enhancing the cross-flow or turbulence along the membrane surface, which seriously limits membrane performance. Therefore, one of the major research topics in FO area is the modification of support substrates to mitigate the ICP effect 13. Meanwhile, it is also important to further optimize the selective layer of FO membranes to improve water permeance and solute selectivity

9, 14-16

. In this respect, numerous efforts have been devoted to tailoring the structure


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and properties of the support layer and selective layer independently to achieve desired FO performance. In terms of reducing the ICP in the support substrates of FO membranes, one strategy is to use supports with large pore size, such as microfiltration (MF) membranes for FO membrane synthesis. Large pores can significantly facilitate the diffusion of salts in the porous support so that ICP is alleviated 17-19. For example, Huang et al. applied a microfiltration nylon substrate as forward osmosis support and the prepared membrane possessed a higher flux and selectivity 18, 20. Zhao et al. also reported that supporting layer with larger pore size is beneficial to inhibit the ICP and improve FO membrane performance 17. Furthermore, Ren et al. investigated the relationship between osmosis performance with support layer pore size and found larger pores led to higher selective layer roughness and thus higher flux

19

. Although these previous studies showed that

MF substrates are good for the synthesis of FO membranes, unfortunately, it is still difficult to form a polyamide layer with low reverse salt flux and satisfactory membrane integrity on MF substrates by normal interfacial polymerization reaction 17-18, 21. On the other hand, recent studies have proved that incorporating various nanomaterials into the active layer could achieve enhanced membrane permeability and salt rejection 22-30. Among these nanomaterials, graphene oxide (GO) has gained great attention due to their unique nanostructures as well as desired physical and mechanical properties 31-34. A few studies have demonstrated that GO can be coated on the selective layer surface by layer-by-layer method linking method

39

, and embedded method

29, 40-43

35-38

, chemical cross-

. The formed membranes exhibited not only

enhanced water permeability but also better fouling resistance and chlorine resistance. However, the membranes prepared by above methods may suffer from a decline in water permeability because GO coating layer can interfere with water permeation


44

. Additionally, the narrow


interlayer spacing of GO films can limit the water flux. Therefore, researchers adjusted the GO interlayer space by carbon nanotubes (CNTS) to further improve membrane water flux. For example, Jin et al. 45 prepared a carbon nanotube (CNT)-interacted GO ultrathin laminar film by simply mixing CNT with GO and applying vacuum filtration. The nanochannels established by CNT in GO layer greatly improved water permeation compared with pure GO film without sacrificing the rejection. Additionally, Gao et al.

33

reported a CNT assembled graphene

nanofiltration membrane which exhibited 2 times higher water flux compared with neat graphene membrane. A reduced GO membrane intercalated by CNT was also fabricated by Fan et al. and the rGO-CNT hybrid membrane exhibited high retention towards dye and good permeability46. More recently, Hou et al.47 discovered a chemical cross-linked functional CNTS and functional GO hybrid membrane for the treatment of strontium containing wastewater and the membrane water flux was 4 times higher than the commercial NF membrane. In the above studies, the increased permeability is due to the uniform network formed by placing 1D CNTS with in 2D graphene sheets which provide many mass transfer channels. In addition, compared with using CNTS alone, the addition of GO facilitates the optimal dispersion of CNTS in the aqueous solution. A previous study showed that GO and acidic CNTS (CNTa) hybrid material can be more well dispersed in the aqueous solution than single CNTa, because of the surfactant effects of GO22. The results further showed the CNTa-GO hybrid material embedded membrane exhibited the higher water permeation than single CNTs or single GO embedded membranes. Therefore, CNTS and GO hybrid material will be a better candidate than GO alone or CNTS alone to incorporate with the active layer for forward osmosis application. Inspired by Andrew’s groups who pre-formed a nanowire intermediate layer on the porous substrate to precisely control the formation of polyamide layer, leading to a defect-free, ultrathin


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and uniform selective layer with outstanding permeability

48

, we believed that a GO/CNTS

intermediate layer would also benefit the formation of a high-quality polyamide layer on large pore support. Meanwhile, the GO/CNTS intermediate itself can supply additional nanochannels for water transport. In the present study, the GO/MWCNT layer was deposited on the nylon substrates first, which then served as a platform for the polyamide formation in the interfacial polymerization reaction. The role of the GO/MWCNT layer on the polyamide layer structure, morphology, and forward osmosis performance under various conditions were investigated systematically, which provides new insights into the development of high-performance FO membranes. 2. EXPERIMENTAL SECTION 2.1. Materials. Commercial GO was purchased from XFNANO CO. Ltd (Hummers method, diameter 500nm-5 , thickness 0.8-1.2nm, >99wt. %). Commercial MWCNT with outer diameter 8-15nm and length ~50 was obtained from Time Nano CO. Ltd. Polyvinylpyrrolidone (PVP, Mw=40,000g/mol, Sigma-Aldrich) was used to disperse MWCNTS in water. The nylon MF substrates with pore size 0.1m, 0.2 m, 0.45 m and 0.8 m were provided by Sterlitech. M-Phenylenediamine (MPD, 99%) and 1, 3, 5-benzenetricarbonyl trichloride (TMC, 98%) were purchased from Sigma-Aldrich. N-hexane (>99%) was supplied from Merck. Sodium Chloride (NaCl, Merck) was used as inorganic salt for forward osmosis performance test. 2.2. Fabrication of MWCNT/GO membranes. The overall membrane synthesis process was illustrated in Fig.1. 0.26 mg MWCNT (containing 30% PVP) and 0.05 mg GO were mixed with DI water and sonicated for 30 mins. Then, the GO/MWCNT suspension was filtered on the nylon MF membrane via vacuum-filtrating, forming a GO/MWCNT composite layer. Once the



composite layer was obtained, 30ml 4.5wt% MPD aqueous solution was immediately filtrated through the composite layer. Afterwards, the MPD saturated membrane was exposed to 30ml 0.15wt% TMC n-hexane solution for 1 min with vacuum pump off. The resultant membrane was then heated at 70 for 10 min for further cross-linking. The obtained membranes with GO/MWCNT layer were named as GPA-X where X (0.1, 0.2, 0.45, 0.8) represented the pore size of nylon substrates. Normal membranes without GO/MWCNT layer were also synthesized via similar procedure for comparison and named as PA-X, where X represented the pore size of nylon substrates.

Fig.1. Synthesis process of the thin film nanocomposite membranes with the GO/MWCNT intermediate layer. 2.3. Membrane characterization. Transmission electron microscopy (TEM, FEI Tecnai T20) was utilized to characterize the morphologies of GO nanosheets and MWCNT. The chemical functional groups of the membranes were analyzed by Fourier transform infrared spectroscopy (FTIR, PerkinElmer, Australia). The surface and cross-sectional morphologies of the membranes were observed by a field emission scanning electron microscope (FESEM; Magellan 400 and Nova NanoSEM 450, FEI, USA). The thickness of the active layer of each membrane was calculated based on the averaged value of 10 measurements from three samples. The error bars


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represented the standard derivatives. Atomic force microscopy (AFM, Bruker Dimension Icon) was used to analyze the surface morphology and roughness of the prepared membranes. At least four measurements were taken to get the average roughness and the error bar represented the standard deviation. 2.4. Forward osmosis performance test. The membranes were evaluated in a house-made FO cross-flow membrane system as described in our previous study 49. DI water was used as the feed and 0.5-1.5M NaCl solutions were used as the draw solutions. The membranes were tested in two different operational modes: active layer facing feed solution (ALFS) and active layer facing draw solution (ALDS). During the FO tests, the membranes with an effective area of 0.238 cm2 were attached on a plastic sheet via a carbon tape (pre-heat under 80 for 30min), according to the method described in our previous study

49

. Each membrane sample was tested for at least

three times to obtain the average water flux and reverse salt flux, and the error bar represented the standard deviation. The water flux Jw was determined by measuring the weight changes of the feed solution according to Eq. (1). And the reverse salt flux Js was evaluated by applying Eq. (2) =

=

∆

∆

(1)

∆( )

∆

(2)

Where Jw is the permeate flux (Lm-2 h-1, LMH), A is the effective membrane area (0.238 cm2), ∆ is the data collection time (h), and ∆ is the volume change of feed solution (L). is the reverse salt flux (gm-2h-1), Ct is the feed salt concentration at a given time (t) which can be obtained by measuring the conductivity through conductivity meter, and Vt is the volume of the feed solution at a given time (t).



3. RESULTS AND DISCUSSION 3.1. Characterization of GO/MWCNT composite layer. Figs.2 (a) and (b) show the TEM images of typical MWCNT and GO used in the experiments. The MWCNT have a length of several micrometers and a diameter of 13.66 ± 0.78 nm, while the GO nanosheets give a typical wrinkled and exfoliated structure with a lateral size of a few micrometers. As displayed in Figs.2 (c) and (d), in comparison to the bare nylon substrate, after the filtration of GO/MWCNT dispersion, a uniform GO/MWCNT composite layer is formed on the nylon substrate. Additionally, it is obvious that the visible micron-sized pores on nylon support are covered well by GO/MWCNT layer.


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Fig.2. (a) TEM of MWCNT (b) TEM of GO nanosheets (c) SEM image of the nylon substrate (0.45µm) (d) SEM image of the GO/MWCNT composite layer on nylon substrate (0.45µm) (mag. 50000X). 3.2. Characterization of the PA and GPA Membranes. After the preparation of GO/MWCNT composite layer, the polyamide layer was fabricated on it via interfacial polymerization between MPD and TMC. The FTIR spectra of the nylon substrate, PA-0.45 membrane, and GPA-0.45 membranes are presented in Fig.3. The increased peak intensity of GPA membrane at 1450cm-1 (corresponding to the C=O stretching and O-H bending of carboxylic) indicates the presence of GO/MWCNT layer

50-51

. The existence of polyamide characteristic peaks such as 1660 cm-1

(C=O amide I), 1610cm-1 (N-H amide II) and 1540 cm-1 (C-N amide II) suggests the successful formation of polyamide layer in the GPA membrane. Furthermore, the enhancement in absorbance at 1540cm-1 and 1610 cm-1 can be found for GPA membrane compared with PA membrane, which is probably due to the interaction between GO nanosheets and MPD monomers, leading to the formation of additional amide linkages 52. In addition, compared with PA membrane, GPA membrane shows a reduced peak intensity at 3300cm-1, 2935cm-1, and 2850cm-1 (belonging to nylon substrate), indicating a better coverage of the polyamide layer on the substrate is achieved in the GPA membrane.



Fig.3. FTIR spectra of nylon substrate, PA-0.45 and GPA-0.45 membranes. The cross-sectional SEM images of the membranes are shown in Fig.4. As can be seen in Fig.4, continuous polyamide films are formed for both PA and GPA membranes on the nylon substrates with different pore sizes except the 0.8 µm substrate. On the 0.8µm nylon support, the collapse of the active layer and inside formation of polyamide can be found. This observation is due to the extremely large pores of the 0.8 µm substrate and insufficient thickness of the intermediate layer. Comparing the thickness of the active layer of PA and GPA membranes, shown in Fig.5, it is clear that the incorporation of GO/MWCNT composite layer can significantly reduce the thickness of the active layer by 20-60%.


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Fig.4. Cross-sectional SEM images of the PA and GPA membranes fabricated on nylon substrates with different pore sizes from 0.1 µm to 0.8µm. (a) PA-0.1, (b) GPA-0.1, (c) PA-0.2, (d) GPA-0.2, (e) PA-0.45, (f) GPA-0.45, (g) PA-0.8 and (h) GPA-0.8 membranes.



Fig.5. The top layer thickness of the PA and GPA membranes fabricated on nylon substrates with different pore sizes from 0.1 µm to 0.8µm.

Fig.6. Proposed models for the polyamide formation process with the incorporation of GO/MWCNT composite layer (a) PA membrane formation process (b) GPA membrane formation process. The reduced thickness of the polyamide layer of GPA membranes can be attributed to several factors: firstly, the GO/MWCNT composite layer creates and controls the IP reaction interface.


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Generally, the IP reaction occurs when the aqueous MPD solution is in contact with the TMC hexane solution. The polyamide film grows in the organic phase since the solubility of MPD diamine in hexane is higher than that of TMC in water

19

. For PA membrane, the IP reaction

interface is along the rough surface of nylon support, while for the GPA membrane, the reaction interface will be along the GO/MWCNT intermediate layer since the intermediate layer is in direct contact with the organic phase. The difference of the IP reaction interface causes the difference in the initial nuclei formation of the polyamide materials. As illustrated in Fig.6, when the MPD saturated substrate is in contact with TMC organic solution, initial nuclei of polyamide material is formed upon the pore opening region of the substrate for PA membranes. Given the pore size of the microporous nylon substrate is large, the large amount of MPD are stored in these pores, which leads to a rapid and violent MPD eruption and the formation of large initial polyamide oligomers. By contrast, for GPA membrane, the initial nuclei of polyamide material are formed homogeneously along and inside the GO/MWCNT layer due to its presence, which limits the growth of initial oligomers. This controlled nucleation could be the reason for the formation of thin polyamide layer. Secondly, the presence of GO/MWCNT layer can affect the migration of the MPD monomers to the interfacial polymerization interface. Basically, after forming initial polyamide thin layer, MPD monomers will continuously erupt from the pores and partition into the organic phase48, 53. This rapid migration of the MPD monomers twists and crumples the initial thin film and leads to the ridge and valley structures. Since the GO/MWCNT layer contains a lot of functional groups which interacts with MPD monomers through covalent bonds and hydrogen bonds, the diffusion of MPD monomers through the initial layer is limited. This limited and controlled MPD diffusion should also contribute to the formation of the thinner polyamide active layer of GPA membranes

44

. Thirdly, the GO/MWCNT composite layer



inhibits the polyamide from forming deeper inside substrate pores. As illustrated in Fig.6, for the PA membrane, due to the hydrophilic nature of the support substrate, a concave surface of MPD solution is favored at the open surface pore area. In this situation, the TMC is likely to diffuse into substrate pores and form polyamide deeper inside the pores of the support substrate and thus the membrane thickness is relatively high

54-55

. However, for the GPA membrane, the presence

of GO/MWCNT layer confines the polyamide formation just along the intermediate layer but not into substrate pores since the intermediate layer will retard the diffusion of TMC monomer into the substrate pores

56

. In this case, the polyamide growth direction is controlled, consequently

leading to a thinner polyamide layer.

Fig.7. Surface morphologies of PA and GPA membranes after peeling off the polyamide layer from the support layer by carbon tape (a) illustration of the peeling off process, (b) i-PA, (c) iGPA, (d) ii-PA, (e) ii-GPA. “i” represents positon which is the back of active layer and “ii” represents position which is the top layer after peeling off the active layer. To verify the proposed models for polyamide formation process in Fig.6, we peeled off the polyamide active layer from the nylon substrate using a carbon tape and investigated the


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nanostructure at the interface between the polyamide active layer and the nylon support. As shown in Fig.7, for PA membrane, a holey structure is observed at the back surface of the polyamide layer (Fig.7b) which can also be found from the top surface of the active layer (Fig.7d). This holey structure is formed because the MPD monomer rapidly and directly erupts from substrate pores, twisting the initial thin polyamide film and forming cavities. Apart from the holey structure, an uneven and rough surface is observed at Fig.7b. This is because polyamide layer is formed along the rough nylon support substrate. These findings confirm the PA membrane formation process in Fig.6a, where the polyamide layer of PA membrane is formed upon substrate opening regions and along the support substrate. The PA membrane formation process can be further verified by the cross-sectional SEM images (Fig.4), where the polyamide layer directly grows along the nylon support. By contrast, for GPA membrane, a smooth GO/MWCNT layer can be found on the back surface of the polyamide layer (Fig.7c). The absence of holey structure indicates the MPD eruption is no longer directly from substrate pores but through GO/MWCNT intermediate layer. The cross-sectional image in Fig.4 is in good agreement with this observation that polyamide layers of GPA membranes grow on the intermediate layer rather than directly on the support substrate. These observations confirm our proposed model in Fig.6b, where the presence of the GO/MWCNT layer creates and controls the interfacial polymerization interface and works as a platform for polyamide growth.



Fig. 8. Surface morphologies of PA and GPA membranes fabricated on the nylon substrates with


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different pore sizes from 0.1 µm to 0.8µm (a) PA-0.1, (b) GPA-0.1, (c) PA-0.2, (d) GPA-0.2, (e) PA-0.45, (f) GPA-0.45, (g) PA-0.8 and (h) GPA-0.8 membranes.

Fig.9. AFM images of PA and GPA membranes fabricated on nylon substrates with different pore sizes from 0.1 µm to 0.8µm (a) PA-0.1, (b) PA-0.2, (c) PA-0.45, (d) PA-0.8, (e) GPA-0.1, (f) GPA-0.2, (g) GPA-0.45 and (h) GPA-0.8 membranes. The top surface morphology of the membranes was observed using Scanning Electron Microscopy (SEM) and the images are shown in Fig. 8. Both PA and GPA membranes show classical ridge and valley polyamide films with a rough surface containing nodules and “leaf-like” folds 41. As can be found in these images, the polyamide features turn from relatively small and worm-like structure to “leaf-like” structure with the increase of the substrate pore size. A similar phenomenon was also reported in a previous study which found that large pores produced more “leaf-like” structures19. The nylon substrates of the pore size from 0.1µm to 0.45µm are found to be covered well with polyamide layer for both PA and GPA membranes without an obvious difference. However, when the pore size of nylon substrates increases to 0.8 µm, GPA membrane



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shows a better coverage of polyamide layer with less visible micron-sized pores, which further proves our proposed model that the presence of GO/MWCNTS intermediate layer facilitates the polyamide layer growth on large pores, leading to a defect-less polyamide layer. Fig.9 shows the two-dimensional AFM surface morphology of PA and GPA membranes with a scan size of 10 µm ×10 µm. The root mean surface roughness (Rq) values of the two membranes are calculated from AFM images and summarized in Fig. 10. It should be noted that the Rq values increase with the increasing pores size of the nylon support for both PA and GPA membranes. This result is in a great agreement with previous studies, which reported that larger support pores produced rougher polyamide layer

19, 57

. There is no obvious difference between the roughness of PA and

GPA membranes. They have the similar Rq values. The possible reason is that the incorporation of CNTS into polyamide layer increases membrane roughness

28, 58

while the GO would make

surface smoother 44, 59-61. As a result, the GPA membrane still shows similar roughness as the PA membrane.

Fig.10. Root mean surface roughness (Rq) values of PA and GPA membranes on nylon substrates with different pore sizes from 0.1 µm to 0.8µm.


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3.3. Forward osmosis performance. Fig.11 presents the osmotic performance of the PA and GPA membranes fabricated on 0.1 µm, 0.2 µm, 0.45 µm and 0.8 µm substrates. The membranes were evaluated under ALFS mode, with 1M NaCl as draw solution and DI water as the feed. It could be found that with increasing pore size from 0.1 µm to 0.45 µm, the water fluxes of PA and GPA membranes increased by 103% and 175%, respectively. This observation is expected because the increased substrates pore size accelerates the diffusion of salt ions and lessens the mass transfer resistance for water, consequently alleviating ICP 17. Moreover, the rougher active layer on the large pore size substrate (see Fig.10) also enhances surface area for water transport, resulting in an improvement in water flux

62

. However, further increasing substrate pore size to

0.8 µm, both PA and GPA membranes show a sharp decline in water flux. This significant drop in water flux could be attributed to the formation of polyamide inside the support pores (see Fig.4), which increases the overall pathway for water transport. Apart from that, the defective and discontinuous active layer on 0.8 µm substrate also increases the salt crossover from the draw solution, which induces ICP and lowers the osmotic pressure difference and thus leads to a decrease in water flux. As for reverse salt flux, it is noted that with the increase of support pore size, the salt fluxes gradually increase for both PA and GPA membranes, shown in Fig.11b. The increased reverse salt flux value is most likely because the loose polyamide layer is formed on the large pore size substrate. As can be found in Fig.4, when interfacial polymerization occurs on the large pores, the cavities in the polyamide layer tend to enlarge and polyamide layers become loose. The relatively loose active layer can reduce the salt transfer resistance, consequently leading to an increase in reverse salt flux. A similar trend was found in the previous study where polyamide layers on larger pore substrate exhibited higher reverse salt flux 18. It is interesting to note that on 0.8 µm nylon substrate, reverse salt fluxes of both PA and GPA membranes are



almost 1 fold higher than that of membranes on 0.45 µm substrate, indicating that some defects formed in the polyamide layers, which provide a direct pathway for salts. Comparing the osmotic water flux performance of both PA and GPA membranes formed on the same pore size substrates, it is obvious that GPA membranes yield a higher water flux. This enhancement of water flux can be explained by a few factors: firstly, the membrane thickness is significantly reduced by introducing GO/MWCNT layer, as illustrated in Fig. 5. The reduced thickness of the polyamide top layer can reduce the water transfer resistance which in return promote the water permeability to some extent. Secondly, the ultrafast channels existing among MWCNT and GO composite layer can deliver the ultrafast water transport path and thus enhances water flux

63-64

. Thirdly, the better coverage of the polyamide on the substrate is

achieved with the assistance of GO/MWCNT layer, demonstrated by Fig.3 and Fig.8. The polyamide and GO/MWCNT layer together create an effective solute barrier, reducing the salt permeation and thus increasing the effective osmotic driving force and water flux. In terms of reverse salt flux, it is found the GPA membranes outperform the PA membranes, especially on 0.8 µm substrate where the GPA membrane achieved approximately 49% lower salt flux than PA membrane. The lower reverse salt flux of GPA membranes indicates that fewer defects in the polyamide layer. This further confirms our model that GO/MWCNT intermediate layer creates and controls the interfacial polymerization interface and works as a platform for polyamide growth, which reduces the potential of the collapse of the active layer and defects formed on large substrate pores.


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Fig. 11. Membrane performance (a) water flux (b) reverse salt flux of PA and GPA membranes formed on nylon substrate with different pore sizes. (1M NaCl as draw solution in ALFS mode).

Fig. 12. Performance comparison of PA and GPA membranes formed on nylon substrate with a pore size of 0.45 µm. (a) Water flux (b) Reverse salt flux. Fig. 12 shows the FO performances of PA and GPA membranes formed on 0.45 µm nylon substrate as a function of draw solution concentrations in both ALFS and ALDS modes. It is noted that both water flux and reverse salt flux of PA and GPA membranes exhibit similar increasing trends with the increase of draw solution concentration regardless of membrane



orientations. The increase in water flux is attributed to the larger osmotic driving force supplied by the higher draw solution concentration, while the increase in reverse salt flux is ascribed to the increase in the salt concentration gradient. For both PA and GPA membranes, the water flux and reverse salt flux are higher in ALDS mode than those in ALFS mode because of less ICP in the ALDS mode. It should be noted that even large pores can reduce the ICP effect, the relatively thick commercial support substrate (~150 µm) can still generate ICP. As can be seen from Fig.12, GPA membrane exhibits higher water flux and lower reverse salt flux than PA membrane regardless of membrane orientation and over the entire range of draw solution concentrations. Especially, under ALDS orientation and with 1.5M NaCl as DS, the GPA-0.45 membrane exhibits the highest water flux of 30 LMH, which is 26% higher than PA-0.45 membrane and 60% higher than commercial HTI membrane

49

. Meanwhile, the reverse salt flux of GPA-0.45

membrane is only 5.02g/m2h, which is 20% lower than that of PA-0.45 membrane, and ~50% lower than commercial HTI membrane49. In addition, we fabricated membranes with MWCNT alone (MWCNT-FO) and GO alone (GOFO) on the 0.45 µm nylon substrate by the same method using 0.26 mg MWCNTS (containing 30% PVP) and 0.05mg GO, respectively. Compared to PA-0.45 membrane, both water flux (26.87 LMH) and reverse salt flux (8.49 g/m2h ) are increased for MWCNT-FO membrane at ALDS model with 1M NaCl as draw solution. While for GO-FO membrane, the reverse salt flux is reduced to 4.23 g/m2h and the water flux does not show apparent change (20.58 LMH compared with 20.05 LMH of PA-0.45 membrane). These results suggest that MWCNT has a significant impact on improving membrane water flux while GO is beneficial for reducing the reverse salt flux. As a result, the combination of MWCNT and GO helps to improve the water flux and reduce the reverse salt flux simultaneously.


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Table 1 lists the FO performance of membrane prepared in this work and other GO incorporated membranes in the previous studies. It can be seen that the GPA membrane in our study exhibited better water flux and lower reverse salt flux compared with other studies using single GO nanosheets as the additive. The additional water pathway established by MWCNT in GO layer 45 could be one of the reasons for the improvement in the FO performance. Tab. 1. Performance comparison between the GPA-0.45 membrane in this work and GO incorporated FO membranes reported in the literature. Nanomaterial

Draw solution

Mode

Water flux(LMH)

Reverse salt flux (g/m2h)

Ref.

GO/MWCNT

1M NaCl

ALDS

26.72

3.86

This study

GO/MWCNT

1M NaCl

ALFS

17.24

3.73

This study

GO

1M NaCl

ALFS

28

5.8

65

GO

1M NaCl

ALFS

9.2

3.8

66

GO

1M NaCl

ALDS

18

6.4

66

GO

0.5M TSC

ALDS

18.8

13.45

63

GO

1M NaCl

ALDS

27.5

7.5

29

GO

2M NaCl

ALDS

16.4

26

40

PVP/GO

2M NaCl

ALDS

33.5

36.8

40

GO

1M NaCl

ALFS

14.5

2.6

67

4. CONCLUSION In this study, a novel thin film nanocomposite FO membrane was successfully synthesized on hydrophilic microfiltration support by interfacial polymerization using a GO/MWCNT intermediate layer. The membrane characterization revealed that the incorporation of



GO/MWCNT layer can control the IP reaction and work as a platform for polyamide growth, thus leading to a thinner and less-defect polyamide layer. As a result, the GPA membrane containing GO/MWCNT intermediate layer possessed higher water flux and lower reverse salt flux than the PA membranes without GO/MWCNT. The study indicated that GO/MWCNT intermediate layer provided a promising way to fabricate high quality polyamide layer on hydrophilic microporous support. The strategy could be useful for the synthesis of other osmotic membranes. 5. AUTHOR INFORMATION *Corresponding author: [email protected]; Tel. +61 3 9905 1867 6. ACKNOWLEDGMENTS The authors gratefully acknowledge funding support by Australian Research Council (H170100009 and DP14103535). Xiwang Zhang especially thanks Australian Research Council and Monash University for his ARF and Larkins fellowships. The authors also sincerely thank Monash University and the Monash Center for Atomically Thin film materials (MCATM) for the scholarships. The authors acknowledge Monash Centre for Electron Microscopy and Melbourne Centre for Nanofabrication for membrane characteristic instruments. 7. REFERENCES 1. Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Mariñas, B. J.; Mayes, A. M., Science and technology for water purification in the coming decades. Nature 2008, 452 (7185), 301-310. 2. Aaberg, R. J., Osmotic power: A new and powerful renewable energy source? Refocus 2003, 4 (6), 48-50. 3. Achilli, A.; Cath, T. Y.; Childress, A. E., Power generation with pressure retarded osmosis: An experimental and theoretical investigation. Journal of membrane science 2009, 343 (1), 42-52. 4. Bamaga, O.; Yokochi, A.; Zabara, B.; Babaqi, A., Hybrid FO/RO desalination system: Preliminary assessment of osmotic energy recovery and designs of new FO membrane module configurations. Desalination 2011, 268 (1), 163-169.


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Thin Film Nanocomposite Forward Osmosis Membranes on

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