Insights on Tuning the Nanostructure of rGO Laminate Membranes for

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Insights on tuning the nanostructure of rGO laminate membranes for low pressure osmosis process Qiuze Wang, Cyril Aubry, Yaxin Chen, Huaihe Song, and Linda Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04803 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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ACS Applied Materials & Interfaces

Insights on tuning the nanostructure of rGO laminate membranes for low pressure osmosis process Qiuze Wang a, Cyril Aubry a, Yaxin Chenb, Huaihe Songb, Linda Zou a,*

a

Masdar Institute, Khalifa University of Science and Technology, Abu Dhabi, 54224, United

Arab Emirates b

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing, 100029, PR China

Abstract: In this research, rGO laminates were prepared by a controlled partial reduction step, aimed to avoid aggregation and tune the interlayer spacing (d) between the rGO layers. The mild reducing agent vitamin C (L-AA) and cross-linker polycarboxilic acids were used to improve the stability of the assembled rGO laminate membranes. AFM was used for the first time to further investigate the statistical size distribution of spacing between rGO layers. Topographical images of the edges of the rGO layers were obtained with an AFM instrument, interlayer spacing profiles were extracted and then the data was plotted and fitted with Gaussian curves. We confirmed that the differently sized spacing co-existed, and their size distribution was affected by the reduction degree of rGO. At greater levels of reduction, more interlayer spacing were formed in the smaller size range, while few large gaps were still present. The obtained rGO laminate composite membranes were evaluated in a low pressure osmosis process such as forward osmosis (FO). The water permeation was higher in the rGO membrane prepared with a medium reduction degree (1.2-R) than the sample prepared by higher reduction degree (2.0-R) due to 1

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well-balanced nanochannels in hydrophilic regions and hydrophobic walls for fast transport of water molecules. The solute flux of the FO membrane was inversely correlated to the reduction degree. These findings helped in developing future strategies for designing high water flux and low reverse solute flux rGO membranes that are ideal for a FO process. Keywords: reduced graphene oxide, membrane, interlayer spacing size distribution, low pressure osmosis process, hydrophilic and hydrophobic

1. Introduction Membrane filtration has been widely employed as a mainstream technology in separating various components from liquid or gas with superior performance than many other technologies. The semi-permeable membrane process is used as physical barrier and the membrane can be reused over longer period.1 The first generation polymeric membranes such as thin film composite membranes (TFC) 2 have been developed more than 50 years ago, and was used in pressure driven membrane process such as reverse osmosis (RO), more recently was employed in osmotic driven process such as forward osmosis (FO).4 In the past few decades, the fabrication, performance and energy consumption of these membranes have been greatly improved. Empirical evidence suggests that it is difficult to further increase the water permeability unless at the expense of the rejection.3 Moreover, most of these membranes still face the major challenges such as being easily affected by fouling and adversely affected by disinfection and chemical cleaning.4-6 This status prompts the researchers to search for better materials in advanced membrane development.

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Graphene and its derivatives have been viewed as ideal candidate materials for next generation separation membranes because of their extraordinary properties of being mechanically robust, chemically resistant and possessing atomic thickness.7-10 Graphene oxide (GO), with rich oxygen-containing groups, is an oxidized derivative of graphene and has attracted a lot of attention.11 Until now, pioneer studies have examined two potential strategies: nanoporous structure7, 12 and stacked laminate structure.13 Both can form a barrier to block the passage of even small molecular species.14-15 The primary mechanism for rejecting ions and molecules are quite different: for the former approach, it is to create many nanopores with narrow pore size distribution on single layer graphene sheets, while the latter is to stack the GO laminates and carefully control the formation of d-spacing between the laminates.16-18 19 Nair et al. found that sub-micrometer-thick membranes made from GO can be completely impermeable to liquids, vapors, and gas molecules, except water.20 Similarly, Mi et al. fabricated negatively charged GO nanosheets using a layer-by-layer assembly method. The water flux showed performance one order of magnitude higher than commercial FO membranes, and low solute flux for sucrose was achieved.10 Gao et al. investigated the reduced GO membrane and showed excellent performance for the retention of organic dyes, especially charged dyes, based on the mechanism of physical sieving and electrostatics.8 However, although GO has favorable characteristics, it is unstable and can become soluble in water. In addition, the interlayer spacing (d) of GO is too large to be suitable for the separation of salt ions from water.21 It was found that these type of rGO laminate membranes showed more sensitivity to hydraulic pressure than the conventional polymeric membranes. Its structure stability may be under stress when was used in high hydraulic pressure process such as RO. If rGO laminate 3



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composite membrane can achieve the desired water permeation and salt separation, it could be a good candidate for low pressure osmosis process, such as FO process. In this contribution, we focused on manipulating the properties of rGO laminates by functionalizing followed by partial reduction, so the resultant graphene laminates had finely tuned residual functional groups and formed graphene layers with suitable interlayer spacing as part of the FO membrane. A mild and non-toxic reducing agent L-AA was used; in addition, polycarboxilic acids (adipic acid and succinic acid), were employed as cross-linkers to form ester bonds between the two graphene layers. Furthermore, the cause-effect relationship between the reduction degrees and distribution of the interlayer spacing ranges on FO membranes were explored and understood. Figure. 1(a) illustrates the concept of the stacked rGO laminate membranes with spacing formed depending on the reducing conditions. The water and salt permeation performance were conducted by using monovalent salts NaCl as draw solute in FO process. Monovalnet draw solute offered high osmotic pressure, at the same time also posed challenge to minimize the reverse solute permeation.

2. Experimental 2.1 Materials and chemicals Graphite flakes were obtained from Bay Carbon Ltd., USA. All the chemicals employed in the experiment were analytical grade and were purchased from Sigma-Aldrich. Polyvinylidene fluoride (PVDF) membrane (Durapore 0.45µm) was provided by Millipore.

2.2 Fabrication of rGO The schematic of assembly of rGO membrane process is illustrated in Fig. 1(b). It is divided into several parts. 4


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1) Fabrication of GO solution: firstly, a modified Hummers’ method was applied to obtain GO solution; then the sulfonation reaction was employed to graft oxygen-containing sulfonic functional groups onto the GO sheets which were described in our previous studies.22, 25 2) Preparation of rGO solution: in order to investigate the cause-effect relation of reduction of GO and d-spacing, we fabricated membranes of three different reduction degrees. Different dosages of L-AA were added to the 250 ml sulfonated GO (0.1mol/l) solution, followed by stirring for 24 hours to form a mild and homogeneous solution under ambient conditions. Aliquots of adipic acid and succinic acid were added as cross-linkers to each of the above solutions, then stirred for another two hours. 3) Assembly of rGO membrane: rGO solutions were diluted and filtered on a Polyvinylidene fluoride (PVDF) membrane as a substrate to obtain rGO membranes. An aliquot of hydrochloric acid was filtered through the rGO membrane, and another was added onto the membrane and kept at 80°C for 30min to catalyze the esterification reaction. The resulting crosslinking reaction aims to enhance the electrostatic interaction, hydrophobic interaction and hydrogen bonding between the rGO layers. Finally, the obtained membranes were washed with DI water several times to eliminate residual acid, and dried at room temperature. These membranes were named according to the ratio of L-AA/GO as 0.4-R, 1.2-R, and 2.0-R, respectively.

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(b)

Figure 1. a) Concept of the stacked rGO laminate membranes with d-spacing b) Schematic of the fabrication procedure of rGO membrane.

2.3 Evaluation of rGO membranes in FO process The performance of the fabricated rGO membrane was evaluated via a lab-scale FO unit, as described throughout the literature.

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This FO system consists of two pumps for feed and

draw, a permeate cell with a circular channel on each side of the membrane (surface area approx. 6


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3.31x10-3 m2), a digital balance which can be automatically recorded using data acquisition software to monitor the permeate water flux, and a conductivity meter to detect the conductivity in feed water. Each membrane was evaluated under two different modes: AL-DS, where the draw solution faces the active coating layer, and AL-FS, where the feed solution faces the active layer. All the tests used DI water as feed solution and 0.3M NaCl solution as draw solution, and all tests were conducted for 30 minute duration. The water flux (Jw) and solute flux (Js) were calculated from the change of the feed or draw solution as follows: =

∆

(1)

=

(2)

Where ∆VF (L) is the volume of water that has permeated across the membrane in a predetermined time T (h) during the test. A is the effective membrane surface area (m2). CF and VF are the final NaCl concentration and the volume of the feed solution, respectively. The salt concentration was measured by a conductivity meter. The dilution of the draw solution was ignored, because the ratio of water permeation flux to the volume of the draw solution was less than 1%.

2.4 Characterization techniques The size distribution of spacing between two layers of graphene and the surface roughness were measured by AFM in a novel systematic approach. Interlayer spacing or gap was defined as the difference in height between layers (G1-G4 in the figure 2 below). Topography images of the edge of the rGO layers were obtained with an AFM instrument in non-contact mode using ACT model probes from AppNano (k = 40 N/m, f = 300 kHz) 1024x1024 pixels resolution and 2x2 µm scanned area topography images were acquired and then treated with Gwyddion software. 7



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Small portions of the topography were cropped and height profiles were extracted. Then distribution of measurements was plotted and fitted with Gaussian curves. First, select the Gaussian function as follows, then included the data for fitting the function.

, ε, =

√.

ε

ε

(3)

Where s: size; s0: mean size; ε: standard deviation; A: amplitude. Parameters are A, ε and s0 are fitted by the software.

Figure 2. Schematic depiction of topographical imaging of the edges of rGO layers by AFM X-ray photoelectron spectroscopy (XPS) were performed with an Omicron ESCA probe using monochromatic Al Ka radiation (hm=1486.6 eV). The morphological structure of the rGO laminate layers was observed directly without coating by scanning electron microscope (FEI Nova NanoSEM 650). The PANalytical Empyrean X-ray diffractometer (XRD) in BragBrentano geometry (theta and 2 theta coupled) was used to confirm the d-spacing gaps between the laminate sheets of the membrane. The interlayer spacing was determined by directly measuring the gaps on the edge of each layer by an atomic force microscope machine (AFM, Nano-Observer, CSI, France). A FTIR (Fourier transform infrared spectroscopy) spectroscope over the range 5004000cm-1 in the attenuated total reflectance (ATR) mode and Raman (Witec Alpha 300ra. Laser 8


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532) were used to identify the functional groups in the rGO membrane. The membrane surface hydrophilic properties were determined by water contact angle by using the sessile drop method (FAMAS, DM-501, Kyowa Interface Science). The membrane surface streaming potential was measured by a Surpass electrokinetic Analyser (Anton Paar, Austria).

3. Results and Discussion 3.1 Structures and properties of rGO laminates The rGO laminate layers were deposited on the silicon wafer substrate for SEM imaging as shown in Fig 3(a,b,c). The images of sample 0.4-R showed more visible wrinkles in top views; the surface morphology became smoother for sample 1.2-R, and sample 2.0-R hardly had any visible wrinkles. These results can be confirmed by the surface roughness measured by AFM. According to the above AFM images, the roughness of the membrane surface was reduced when the reduction degree was increased. At the low reduction (0.4-R) the Sa was 289nm and the Sq was 364nm. When the reduction increased to 1.2-R, both Sa and Sq decreased more than three times (86.8nm and105nm respectively). For the sample with high reduction degrees, the Sa and Sq values of sample 2.0-R decreased less, as half of values for 1.2-R. In general, the sample with less reduction degree has higher surface roughness, whereas the higher reduction degree (both 1.2-R and 2.0-R) resulted more smooth surface and the reduction degree had less effect on the surface roughness. (Sa is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Sq is the root mean square average of the profile height deviations from the mean line, recorded within the evaluation length). These observations are complementary to the results of interlayer spacing formation obtained by other characterization techniques. rGO layers were coated on the PVDF substrate as selective layer to enhance the desalting performance. The thickness of these coated rGO layers is around 25-30nm which has been reported in our previous report.22 The cross-sectional images (Fig 3 d) shows clearly an ordered multiple layered structure of rGO laminates.

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Figure 3 (a,b,c) The SEM images of the sample (0.4-R, 1.2-R, 2.0-R) at top view. (d) The cross-section view of multilayers of rGO laminates (without the substrate)

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(e)

Figure 4. (a-d) The distribution of the gap and Gaussian curves fitted of three samples; (e) Illustration of different d-spacing gap in multilayer rGO

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The size distribution of spacing between two layers of graphene were measured by AFM in a novel systematic approach. For each rGO laminate sample, many different profiles across 510 layers on the surface were chosen. Around 40 spacing measurements were conducted for each sample and the obtained results were fitted using Gaussian equation for size distribution. The overall size distribution was the combined fitting of 3 Gaussian curves with standard deviations. It was found that the spacing formed between layered graphene laminates varied in size and were non-homogeneous. In the sample with lower reduction degree (0.4-R), it showed a bimodal size distribution with the main peak at 0.5 nm, and a second peak at 1.1nm. The average interlayer space for sample 0.4-R was 0.78 ± 0.11 nm. The sample with medium degree reduction (1.2-R) demonstrated a trimodal size distribution, a peak at smaller size appeared at 0.3nm together with other peaks at 0.6nm and 1.0nm. An average interlayer spacing of 0.65 ± 0.09 nm was calculated for sample 1.2-R. The size distribution peaks of the sample with high degree reduction (2.0-R) shifted to the smaller size with two main peaks at 0.3nm and 0.5nm, while a minor peak was found at around 1.1nm, its average interlayer spacing was 0.61 ± 0.04nm. As revealed by the above information in Fig. 4(a-d), when the reduction degree was increased, the peak shifted towards a smaller size, although a few larger sized d-spacing still existed. This finding suggests that interlayer spacing formation was heavily affected by the rGO reducing condition, i.e., the reducing degree is correlated to the size distribution of the spacing: the higher the reducing degree, the smaller the d-spacing size. The drawing in Fig. 4(e) illustrates the cause-effect correlations between amount and type of functional groups that remained on the rGO sheets and the size of spacing. The gaps were formed due to the existence of oxygencontaining functional groups; when those functional groups were removed by reduction, smaller gaps were formed. However, it was also confirmed that the interlayer spacing was formed 12


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randomly and cannot be controlled homogeneously. This was because a large number of rGO laminates with different shapes were used, and so it was not possible to precisely control the formation.

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Figure 5. (a,b,c) The C1s XPS spectrum of three degree samples; (d) XRD diffraction curves of three samples; (e) FTIR spectra of rGO membrane; (f) The Raman spectrums of sample 0.4-R, 1.2-R and 2.0-R; (g) The zeta potential of three samples. From the above XPS spectra in Fig 5 (a,b,c), it can be seen that sample 0.4-R with the low reduction degree has more oxygen contained groups than other two samples. When the reduction degree was increased in sample 1.2-R and 2.0-R, we found that the intensity of oxygen containing functional groups were further reduced but not decreased in a liner trend as expected. This implied that once medium level reduction was achieved, the continuing increase of the reductant dosage alone will not remove more functional groups, instead it increased the reaction kinetic, ie removed completely the functional groups of some parts of the graphene sheets. This action resulted in the re-stacking of the reduced graphene sheets and form small interlayer spaces. The XRD measurement helped to further confirm the different reducing degrees as well as the d-spacing. The peak position for GO was at 10°. In our samples, peaks were present at around 10° and 25° 2 theta, representing the GO and rGO peaks respectively. As depicted in Fig. 5(d), with a higher degree of reduction, the first peak at 10° became weak and broad, while the second peak shifted slightly with higher 2 theta angles; this suggests that there was a decrease in the average interlayer d-spacing of rGO laminates when the ratio of rGO/GO increased from 0.4 to 1.2 and 2.0. The FT-IR spectra in Fig 5.(e) showed that, for the rGO sample (1.2-R) the peaks observed at 1615, 1504,1412,1222, 1124, 1041 cm-1 were associated with the stretching vibrations of OC=O, C=O, C=C, C-O-C, OH, S=O, respectively. The result demonstrated that the mildly reduced graphene oxide still has many oxygen containing functional groups attached, and also 14


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confirmed that the sulphonation of rGO was successful and led to the partial reduction of graphene oxide. To further investigate the structural properties of GO and rGO with different reduction degrees, Raman spectroscopy was employed. It is known that two characteristic bands located around 1350 and 1590 were associated to D band and G band, respectively (Fig 4.(d)).The D band gives evidence of the presence of structural defects associated with vacancies, grain boundaries, and amorphous carbon species.27 The G band is related to the sp2 graphitic domains (non-oxide regions).28 Raman spectra of all three samples showed significant D band peaks which were expected for the partially reduced graphene oxide material. It was also found that the intensity ratio of D band to G band of GO was increased when the reduction degree increased. It confirmed that more structural defects were present on the highly reduced rGO sample than the slightly reduced sample. Additionally, the spectra revealed that more non-oxide regions 29 and more edges were formed in the rGO.30 A zeta potential investigation was conducted to study the association between the reduction degree and membrane surface charge; the obtained results are shown in Fig 5. (e). It can be seen that all three rGO samples have a strong negative charge on the membrane surface over a pH range of 2-5; and with greater reduction, the negative charge receded as the more charged oxygen containing groups were removed. It is suggested in the literature that charged functional groups can help to block ions, but at the cost of reduced water diffusion, mostly due to steric effects.31-32 The surface hydrophobicity of the rGO laminate layer samples were measured by a contact angle analyser. The contact angles were measured as 69.7° (0.4-R), 78.9 ° (1.2-R) and 99.4° (2.0-R), respectively. Based on the results, it is obvious that when the reduction degrees were increased, the hydrophilicity also increased. This result is consistent with the theory that 15



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stronger reduction removed more oxygen-containing functional groups, and caused the surface to become less hydrophilic and more hydrophobic. The highly reduced rGO laminate layers had less hydrophilic regions to form the interconnected nanochannels between rGO sheets: this may seriously reduce the chance for water molecules to pass through. For reducing the GO, two strategies were employed to achieve the desired outcomes: 1) to avoid aggregation. When the GO was directly reduced, the reduced graphene sheets usually formed heavy aggregates and precipitated easily from the reaction medium because the recovered graphite domain increased the sheet’s hydrophobic property and corresponding stacking interactions.31 Sulfonation of the GO before reduction prevented the aggregation and improved the dispersion of the reduced grapehene nanosheets, making them more suitable to prepare the rGO laminates. Good results in both water flux and salt rejection were achieved.22 2) To form suitable interlayer nanochanels and d-spacing. Because the formation of the interlayer nanochannels as well as d-spacing of the rGO laminates was affected by the oxygen-rich hydrophilic sections, whereas, the fast transport of water molecules was dictated by the presence of small slit type of hydrophobic channels between of rGO laminates, as a result, both hydrophilic and hydrophobic sections were needed on the rGO laminates to facilitate water transport, and this can be achieved by balancing the hydrophilic and hydrophobic regions distribution.23-24 The rGO membrane’s desalination performance in a FO process was evaluated in terms of water permeation (water flux) and solute flux. The ideal FO membrane should possess very high water permeation and very low solute flux. As shown in Fig 6.(a), at the lower reduction degree (0.4 ratio) the obtained water flux was 30.14 L m-2 h-1 (AL-DS) and 33.75 L m-2 h-1 (AL-FS) respectively; when reduction degree was raised to 1.2 ratio, the water flux increased slightly to 16


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27.33 L m-2 h-1 (AL-DS) and 28.42 L m-2 h-1 (AL-FS). The water flux sharply decreased to 1.45 L m-2 h-1 (AL-DS), 1.68 L m-2 h-1 (AL-FS) at the highest reduction degree of 2.0 ratio. In the stacked rGO laminate membranes, the hydrophilic sections with oxygen-containing groups were responsible for the formation of the interlayer nanochannels; in addition, water molecules were rapidly transported in the formed interconnected nanochannels with hydrophobic walls. In the low reduction degree sample (0.4-R), more nanochannels were formed due to the large amount of attached hydrophilic groups. However, when the reduction degree was increased to very high for sample 2.0-R, some GO sheets were completely reduced and only a small amount of nanochannels were formed due to very few remaining hydrophilic groups, this was supported by the almost disappeared GO peak at 2 Theta 10 degree of XRD diffraction curves in Fig 5 (d) as well as the XPS spectra in Fig 5 (a,b,c). Most of the regions of the samples were tightly packed with smaller interlayer spacing and not allowing the easy passage of water, which caused significant decrease in the water permeation as shown in Fig 6.(a). On the other hand, it was interesting to observe that sample with the medium reduction degree (1.2-R) had the higher water flux compared to sample 2.0-R, this may be because this sample (1.2-R) had both sufficient nanochannels and plenty of hydrophobic walls in favor of fast transport of water molecules. As a result, water could easily enter the nanochannels and their transport in the channels would be facilitated by the hydrophobic walls across the rGO laminate layers. This discovery confirmed that both hydrophilic and hydrophobic regions were important for achieving the high water permeation. Previous studies have reported that solute flux could be inversely correlated with water flux in the FO process.33-34 Results in Fig 6. (b) showed that, the solute flux experienced a linear decreasing trend with the increasing of the reduction degree. The lowest ion permeation was 1.13 17



mol m-2 h-1(AL-DS) 1.54 mol m-2 h-1 (AL-FS), receptivity. This phenomenon can be described by two mechanisms: the first mechanism being size exclusion, which played an important role in blocking salt ions while allowing water flow through the membrane. When the reduction degree was increased, the size distribution of the d-spacing shifted to a smaller range (Fig 4.a), and the average d-space became smaller: this enhanced the rejection of salt ions, for example the average interlayer spacing decreased from 0.78 nm in sample 0.4-R to 0.61 nm in sample 2.0-R. The hydrated ionic sizes of Na+ and Cl- were 0.71nm and 0.66 nm respectively, 35 whereas the size of water molecule is 0.27 nm. The size exclusion mechanism was supported by the experimental results in Fig 6. (b), where the salt rejection flux. This confirmed that when reduction degree was increased, the size distribution of the d-spacing shifted to the smaller range, and more salt ions were rejected by size exclusion mechanism. The second mechanism was the net surface charge donnan exclusion. This mechanism is particularly important for determining the ionic selectivity in a solution with a mixture of ions, but is less significant in a single salt solution system in this

(b)

AL-DS AL-FS

35 30 25 20 15 10 5 0

Figure 6.

0.4-R

1.2-R

2.0-R

-1

40

-2

(a)

Reverse solute flux (mol m h )

study.

Pure water flux (LMH)

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14

AL-DS AL-FS

12 10 8 6 4 2 0 0.4-R

1.2-R

Pure water flux (a) and solute flux of NaCl (b) with three samples

18


2.0-R

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For the conventional FO membrane with dense salt rejection layer and porous support layer, osmotic pressure ∆π across membrane of the AL-DS mode is higher than the AL-FS mode, as the dilutive ICP is absent, so normally the water flux of AL-DS is higher than AL-FS.36 However, results of our rGO laminate membranes suggested that water flux of AL-FS were consistently higher than the AL-DS. It was also found that the less degree of reduction, the bigger the difference, the dilutive ICP still exists in the AL-DS mode, so there must be an additional cause of this results. Because of the rGO layers were assembled by vacuum filtrationinduced directional flow, so the rGO laminate layers may have directional effects in performance, ie its water flux was affected by the direction of the flow. In AL-FS, the water flowed in the same direction as the rGO layers were assembled, ie from top to bottom, so the nanochannels are more open for water passage; in AL-DS mode, the water flows in reversed direction as the rGO layers were assembled, so the nanochannels within the layers were less open and created more resistance. It seems this difference caused by flow direction is quite significant to overcome the difference of the dilutive ICP. Among the other GO membrane research, some have reported to use GO-assisted membranes in FO process. It was found that the GO membranes prepared using LBL assembly via electrostatic interaction 10 reported only the divalent ionic solute MgCl2 flux and other larger molecules, so did the hydrogel/GO nanocomposite membrane designed for shale gas wastewater which also reported the divalent ionic solute Na2SO4 flux.37 Whereas in this work, monovalent ionic solute NaCl was used as draw, it is more challenging to reduce the monovalent ionic solute flux. One literature reported free-standing GO membrane with different degree of cross-linking without reduction, the resultant interlayer d-spacing is greater than 0.8 nm which is too large for efficient exclusion of monovalent salt ions,38 whereas another literature reported that after the 19



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reduction by hydriodic acid,39 an interlayer d-space of 0.35nm was obtained which was too small for water molecule to fill the channels as suggested.20 Comparing the performance of these rGO membranes with other membranes used for FO process, sample 0.4-R and 1.2-R have water flux that are better than the commercial FO membrane, however, their reverse salt flux are higher than the commercial FO membrane. The promising results obtained from our research has revealed the inter-relations between key performance indicators of the membrane such as pure water flux and the monovalent ions rejection and the rGO lanimate structures, and the deciding role played by the controlled reduction process.

4. Conclusions We prepared the graphene laminates by controlled partial reduction using a mild and nontoxic reducing agent, vitamin C (L-AA), as well as polycarboxilic acids as crosslinking agents to form ester bonds between graphene layers. This strategy aimed to finely tune the interlayer dspacing between the rGO laminate layers. The obtained composite membrane, consisting of rGO laminate layers on a porous polymeric substrate, was evaluated using a FO membrane process. AFM was used for the first time to further investigate the statistical size distribution of interlayer spacing, allowing the cause-effect relationship between reduction and distribution of the dspacing ranges on FO membranes to be further understood. It was found that interlayer d-spacing formations were heavily influenced by the rGO reducing condition: the higher the reduction degree, the more small sized gaps were formed. Nevertheless, we also confirmed the random nature of the interlayer spacing formations. Rather than having evenly sized interlayer gaps homogeneously across the membrane, different sizes of gaps co-existed. Water permeation was 20


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most favored by the rGO membrane prepared from medium reduction degree, as not only were sufficient nanochannels presented, but also it possessed enough hydrophobic walls for promoting the fast transport of water molecules. The salt flux was inversely correlated to the reduction degree; the highest salt rejection was achieved by highly reduced rGO membrane. These findings helped to develop future strategies for designing high water flux and low solute flux rGO membranes that are ideal for low hydraulic pressure osmosis process, such as FO processes. In order to maintain a desirable level of water permeation, achieving very low solute flux cannot solely rely on increasing the reduction degree; other strategies that can help to enhance the ionic selective barrier properties have been under development and will be employed to improve further of the performance of such membranes.

Author information Corresponding Author E-mail: [email protected]. Tel: 971 2 810 9304.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgement 21



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The authors acknowledge the financial support of Masdar Institute of Science and Technology, and Abu Dhabi Education Council Award for Research Excellence (A2E) program. The authors also thank Mr Haoran Liang at Masdar Institute for his help on this project.

Reference 1. Fane, A. G.; Wang, R.; Hu, M. X. Synthetic Membranes for Water Purification: Status and Future. Angew. Chem. Int. Ed. 2015, 54 (11), 3368-3386. 2. Nguyen, T. P. N.; Jun, B.-M.; Kwon, Y.-N. The Chlorination Mechanism of Integrally Asymmetric Cellulose Triacetate (CTA)-based and Thin Film Composite Polyamide-based Forward Osmosis Membrane. J.Membr.Sci.2017, 523,111-121. 3. Elimelech, M.; Phillip, W. A. The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333 (6043), 712-717. 4. Lay, W. C.; Chong, T. H.; Tang, C. Y.; Fane, A. G.; Zhang, J.; Liu, Y. Fouling Propensity of Forward Osmosis: Investigation of the Slower Flux Decline Phenomenon. Water Sci.Technol. 2010, 61 (4), 927-936. 5. She, Q.; Wang, R.; Fane, A. G.; Tang, C. Y. Membrane Fouling in Osmotically Driven Membrane Processes: A Review. J.Membr.Sci. 2016, 499, 201-233. 6. Lee, J.-W.; Jung, J.; Cho, Y. H.; Yadav, S. K.; Baek, K. Y.; Park, H. B.; Hong, S. M.; Koo, C. M. Fouling-tolerant Nanofibrous Polymer Membranes for Water Treatment. ACS Appl. Mater. Interfaces. 2014, 6 (16), 14600-14607. 7. Cohen-Tanugi, D.; Grossman, J. C. Water Desalination Across Nanoporous Graphene. Nano Lett. 2012, 12 (7), 3602-3608. 8. Han, Y.; Xu, Z.; Gao, C. Ultrathin Graphene Nanofiltration Membrane for Water Purification. Adv. Funct. Mater. 2013, 23 (29), 3693-3700. 9. Park, M. J.; Phuntsho, S.; He, T.; Nisola, G. M.; Tijing, L. D.; Li, X.-M.; Chen, G.; Chung, W.-J.; Shon, H. K. Graphene Oxide Incorporated Polysulfone Substrate for the Fabrication of Flat-sheet Thin-film Composite Forward Osmosis Membranes. J.Membr.Sci. 2015, 493, 496-507. 10. Hu, M.; Mi, B. Layer-by-layer Assembly of Graphene Oxide Membranes via Electrostatic Interaction. J.Membr.Sci. 2014, 469, 80-87. 11. Hegab, H. M.; Zou, L. Graphene Oxide-assisted Membranes: Fabrication and Potential Applications in Desalination and Water Purification. J.Membr.Sci. 2015, 484, 95-106. 12. Joshi, R.; Carbone, P.; Wang, F.-C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H.; Geim, A. K.; Nair, R. R. Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes. Science 2014, 343 (6172), 752-754. 22


Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Hu, M.; Mi, B. Enabling Graphene Oxide Nanosheets as Water Separation Membranes. Environ. Sci.Technol. 2013, 47 (8), 3715-3723. 14. Werber, J. R.; Osuji, C. O.; Elimelech, M. Materials for Next-generation Desalination and Water Purification Membranes. Nat. Rev. Mater. 2016, 1, 16018. 15. Qiu, L.; Zhang, X.; Yang, W.; Wang, Y.; Simon, G. P.; Li, D. Controllable Corrugation of Chemically Converted Graphene Sheets in Water and Potential Application for Nanofiltration. Chem. Commun. 2011, 47 (20), 5810-5812. 16. Koenig, S. P.; Wang, L.; Pellegrino, J.; Bunch, J. S. Selective Molecular Sieving Through Porous Graphene. Nat. nanotechnol. 2012, 7 (11), 728-732. 17. Li, H.; Song, Z.; Zhang, X.; Huang, Y.; Li, S.; Mao, Y.; Ploehn, H. J.; Bao, Y.; Yu, M. Ultrathin, Molecular-sieving Graphene Oxide Membranes for Selective Hydrogen Separation. Science 2013, 342 (6154), 95-98. 18. Kim, H. W.; Yoon, H. W.; Yoon, S.-M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S. Selective Gas Transport Through Few-layered Graphene and Graphene Oxide Membranes. Science 2013, 342 (6154), 91-95. 19. Perreault, F.; de Faria, A. F.; Elimelech, M. Environmental Applications of Graphenebased Nanomaterials. Chem. Soc. Rev. 2015, 44 (16), 5861-5896. 20. Nair, R.; Wu, H.; Jayaram, P.; Grigorieva, I.; Geim, A. Unimpeded Permeation of Water Through Helium-leak–tight Graphene-based Membranes. Science 2012, 335 (6067), 442-444. 21. Yeh, C.-N.; Raidongia, K.; Shao, J.; Yang, Q.-H.; Huang, J. On the Origin of the Stability of Graphene Oxide Membranes in Water. Nat. Chem. 2015, 7 (2), 166-170. 22. Zhang, Z.; Zou, L.; Aubry, C.; Jouiad, M.; Hao, Z. Chemically Crosslinked RGO Laminate Film as an Ion Selective Barrier of Composite Membrane. J.Membr.Sci. 2016, 515, 204-211. 23. Lin, L.-C.; Grossman, J. C. Atomistic Understandings of Reduced Graphene Oxide as an Ultrathin-film Nanoporous Membrane for Separations. Nat. Commun. 2015, 6. 24. Mi, B. Graphene Oxide Membranes for Ionic and Molecular Sieving. Science 2014, 343 (6172), 740-742. 25. Jia, B.; Zou, L. Graphene Nanosheets Reduced by a Multi-step Process as Highperformance Electrode Material for Capacitive Deionisation. Carbon 2012, 50 (6), 2315-2321. 26. Sun, Y.; Tian, J.; Zhao, Z.; Shi, W.; Liu, D.; Cui, F. Membrane Fouling of Forward Osmosis (FO) Membrane for Municipal Wastewater Treatment: A Comparison Between Direct FO and OMBR. Water Res. 2016, 104, 330-339. 27. Zhou, Y.; Bao, Q.; Tang, L. A. L.; Zhong, Y.; Loh, K. P. Hydrothermal Dehydration for the “Green” Reduction of Exfoliated Graphene Oxide to Graphene and Demonstration of Tunable Optical Limiting Properties. Chem. Mater. 2009, 21 (13), 2950-2956. 28. Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prud'Homme, R. K.; Aksay, I. A.; Car, R. Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets. Nano Lett.2008, 8 (1), 36-41. 29. Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45 (7), 1558-1565. 30. Khan, U.; O'Neill, A.; Lotya, M.; De, S.; Coleman, J. N. High‐Concentration Solvent Exfoliation of Graphene. Small 2010, 6 (7), 864-871. 31. Corry, B. Water and Ion Transport Through Functionalised Carbon Nanotubes: Implications for Desalination Technology. Energy Environ. Sci. 2011, 4 (3), 751-759. 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

32. Konatham, D.; Yu, J.; Ho, T. A.; Striolo, A. Simulation Insights for Graphene-based Water Desalination Membranes. Langmuir 2013, 29 (38), 11884-11897. 33. Geise, G. M.; Park, H. B.; Sagle, A. C.; Freeman, B. D.; McGrath, J. E. Water Permeability and Water/salt Selectivity Tradeoff in Polymers for Desalination. J.Membr.Sci. 2011, 369 (1), 130-138. 34. Zhang, S.; Chung, T.-S. Minimizing the Instant and Accumulative Effects of Salt Permeability to Sustain Ultrahigh Osmotic Power Density. Environ. Sci. & Technol. 2013, 47 (17), 10085-10092. 35. Conway, B. E. Ionic Hydration in Chemistry and Biophysics. Elsevier Science Ltd: 1981; Vol. 12. 36. Duong, P. H.; Zuo, J.; Chung, T.-S. Highly Crosslinked Layer-by-layer Polyelectrolyte FO Membranes: Understanding Effects of Salt Concentration and Deposition Time on FO Performance. J.Membr.Sci.2013, 427, 411-421. 37. Qin, D.; Liu, Z.; Sun, D. D.; Song, X.; Bai, H. A New Nanocomposite Forward Osmosis Membrane Custom-designed for Treating Shale Gas Wastewater. Sci. Rep. 2015, 5,15304. 38. Yan, F.; Yu, C.; Zhang, B.; Zou, T.; Zhao, H.; Li, J. Preparation of Freestanding Graphene-based Laminar Membrane for Clean-water Intake via Forward Osmosis Process. RSC Adv. 2017, 7 (3), 1326-1335. 39. Liu, H.; Wang, H.; Zhang, X. Facile Fabrication of Freestanding Ultrathin Reduced Graphene Oxide Membranes for Water Purification. Adv. Mater. 2015, 27 (2), 249-254.

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