methylene blue composite membrane for dyes separation: Formation mechanism and separation performance

Journal Pre-proofs Full Length Article Graphene oxide/methylene blue composite membrane for dyes separation: Formation mechanism and separation performance Jingke Hou, Yingbo Chen, Wenxiong Shi, Chenlu Bao, Xiaoyu Hu PII: DOI: Reference:

S0169-4332(19)32961-7 https://doi.org/10.1016/j.apsusc.2019.144145 APSUSC 144145

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

8 June 2019 7 September 2019 21 September 2019

Please cite this article as: J. Hou, Y. Chen, W. Shi, C. Bao, X. Hu, Graphene oxide/methylene blue composite membrane for dyes separation: Formation mechanism and separation performance, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144145

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Graphene oxide/methylene blue composite membrane for dyes separation: Formation mechanism and separation performance Jingke Hou1, Yingbo Chen*1, Wenxiong Shi*1,2, Chenlu Bao1, and Xiaoyu Hu3 1

School of Materials Science and Engineering, State Key Laboratory of Separation

Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, P. R. China 2

Center for Programmable Materials, School of Materials Science and Engineering,

Nanyang Technological University, 639798, Singapore. 3

State Key Laboratory of Membrane Materials and Membrane Applications, Tianjin

Motimo Membrane Technology Co., Ltd., Tianjin 300042, P. R. China *Corresponding authors *E-mail: [email protected] (Yingbo Chen) *E-mail: [email protected] (Wenxiong Shi). ABSTRACT To improve the stability and rejection for dyes of graphene oxide (GO) membrane, we employed methylene blue (MB) as a modifier to prepare GO/MB composite membrane through vacuum filtration. The rejection of GO/MB membrane for different dyes (methyl orange, disperse black 9 and rhodamine B) reached 93.32%, 99.85% and 82.56%, respectively, and the pure water flux was 7.67 Lm-2h-1, indicating that GO/MB composite membranes have good application prospects. The interaction mechanism between GO and MB was described in detail. Moreover, the

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united-atomic models for molecular dynamics (MD) simulation was performed, through which we can trace the nanoscale events occurring over the interfacial behaviors and provided molecular information at nanoscale interfaces for our study. The effects of the interaction between dye molecules and composite membrane, charges of the dyes, molecular weight and molecular structure of dyes on nanofiltration performance were also discussed, which laid foundation of the research in field of water treatment.

KEYWORDS: Graphene oxide, Methylene blue, Membrane, - conjugation, Electrostatic interaction, Molecular dynamics simulation. 1. Introduction Water is the basics of all lives and the most precious resource for human beings [1]. Water shortages have been a serious problem due to the urbanization, industrialization and global warming become more and more serious with the rapid growth of population and economy [2]. Up to now, it can’t supply standard potable water [3-5] to satisfy local people in many regions, which makes human beings in trouble using water [6]. Membrane separation technology, an energy saving, reproducibility and cost-effectiveness technology, has become a rapidly developing field and is moving to the forefront of water purification [7-9]. Nanofiltration (NF) is a pressure-driven process [10] with good permeability and retention at low pressure [11, 12]. Recent 2

years, NF composite membranes based on nanomaterials including metal oxide nanoparticles, metal organic framework materials (MOFs), covalent organic framework materials (COFs) and carbon-based materials have drawn widespread concern [13-15]. Among them, carbon-based materials play an important role in various technical fields due to their unique chemical structure and physical properties. For instance, graphene and its derivatives such as graphene oxide (GO), is widely favored by researchers because of its high specific surface area, strong mechanical properties, low molecular weight and excellent chemical stability [16-21]. In addition, GO as a two-dimensional carbon nanomaterial can be easily compounded well with other materials due to it has affluent oxygen-containing functional groups (-O-, -COOH, -OH) [22-26], which not only endows GO remarkable hydrophilicity and negative charge, but also provides a rich chemical reaction site for further modification [27]. Compared with GO, the reduced GO (rGO) can decrease the structural damage caused by humidity [28], but its hydrophily is worse than GO. And it’s challenging to prepare uniform rGO membrane due to it agglomerates easily [29]. In addition, rGO displays less dispersibility, wettability, and mechanical and chemical stability compared to GO due to the reduced amount of oxygen functional groups. As a consequence, GO membranes display great application prospects in membrane separation with distinguished permeability [27, 30, 31]. The fabricating methods of GO membranes contain vacuum filtration [32, 33], layer-by-layer self-assembly [34, 35], spin coating [27, 36], electrostatic self-assembly [37], the combination of electrospinning and electrospraying technique [38-40] and dipping coating [30]

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methods. Vacuum filtration, a simple and the most widely used method to obtain membranes with uniform thickness, possess extensive pursuit. As a highly sought-after separation membrane, GO membrane is unstable in aqueous solution and easy to disintegrate [41-44], which caused by the deprotonation effect [45, 46] of –COOH. To improve the stability of GO membrane and put it into practical application, we must try to modify it on account of the stability plays a crucial role in the research. The main methods of modification include reduction [27, 47-50], chemical cross-linking [37, 51, 52], - conjugation [53-55] and electrostatic interaction [55]. Lou et al. [40] employed electrospinning/electrospraying technique to obtain a sandwich-structured polyamide 6 (PA 6@GO@PA 6) membrane. The water flux was 7.62 L m−2 h−1 bar−1 and the flux was improved to 13.77 L m−2 h−1 bar−1 when the membrane was intercalated by TiO2 nanoparticles. Moreover, it displayed good dye rejection (> 85% for Basic Fuchsin, > 92% for MB, > 99% for MO, and 99.85% for Evans Blue). Their another work [39] was a free-standing GO and nylon 6 (GO@nylon 6) membrane prepared by layer-by-layer assembly process. And the electrospinning/electrospraying technique was also employed. The GO@nylon 6 membrane with optimal properties demonstrated a pure water flux up to 11.15 L m−2 h −1

bar−1 with high dye rejection (> 95% for MB, and > 99% for MO). But the rejections

of Na2SO4 solution was only 56.5%. Although the sandwich membrane PA6@GO@PA6 they made displayed great performance, the experimental procedure took long time and the operation was relatively complicated. Chang et al. [50] obtained reduced pre-oxidized graphene membranes (rPGMs) at different reducing

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times. It was found the rPGMs displayed a spectacular rejection for methyl orange (>97.5%) and MgSO4 (71.2%), but a serious attenuation of flux from 22.2 Lm-2h-1bar-1 to 5.3 Lm-2h-1bar-1. Mi et al. [37] fabricated GO membrane cross-linked by 1,3,5-benzenetricarbonyl trichloride (TMC) via layer-by-layer deposition, which not only enhanced the stability to solve their disintegration in aqueous solution, but also tuned interlayer spacing of the GO sheets. It was discovered that the flux was roughly 4-10 times higher than commercial NF membranes. However, GO membrane exhibited an inferior rejection of salts (6-46%) and a low rejection of methylene blue (46−66%). In these two above reports, the modified membranes displayed better rejection but bad permeability or a good flux with inferior rejection, which were far from the industrial standard. The reduction and chemical cross-linking modification methods, however, were either time-consuming or inferior rejection or hard to repeat or not scalable, or all above, which made them not the best choice to be the modification method. Thus, an economic, time-saving, simplify and high repeatability GO nanofiltration membranes with excellent rejection for water purification application was desirable. Recently, the modification through - conjugation and electrostatic interaction has been widely concerned due to its effective and simplicity. Shi et al. [53] simply blended cationic 5, 10, 15, 20-tetrakis (1-methyl-4-pyridinio) porphyrin (TMPyP) and negativly charged chemically converted graphene (CCG) sheets with the interaction of electrostatic adsorption and - conjugation. As a consequence, the coordination reaction among TMPyP and Cd2+ ions was sped up from 20 h to 8 min because of the introducing of CCG sheets. Shen

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et al. [54] tuned the interlayer spacing of GO membranes by Solvent Green (SG) through dip-coating. The SG@GO membrane exhibited prominent stability due to the - stacking interactions between GO and SG. Simultaneously, the 2D channels were enlarged by the intensive electrostatic repulsion among GO sheets and showed excellent rejection of eriochrome black T (EBT). It is well-known that methylene blue (MB) has special structure, which can tune the 2D nanochannels of GO membrane by its benzene ring structure and positive charge. Their compatibility via - conjugation and electrostatic interaction has been researched in previous research [56]. In this study, we selected MB as a modifier and blended it with GO. In our previous work [57], we have explored the optimal conditions to prepare GO membrane with stable permeation performance, which laid the immense foundation for this research. The novel method for GO/MB composite membrane benefits for both regulating the interlayer spacing and improving the stability of GO/MB membrane. Interestingly, we can tune the rejection rate of different dyes by adjusting the content of modifier (MB). It demonstrated that the GO/MB composite membrane possessed potential application in separation of dyes. 2. Experimental 2.1. Materials. GO sheets were purchased from Nanjing Xianfeng Nano Material Technology Co., Ltd., China. Methyl orange (MO) and disperse black 9 (DB9) were purchased from Tianjin Sailboat Chemical Reagent Technology Co., Ltd., China. Rhodamine B (RB) and MB were obtained from Tianjin Guangfu Fine Chemical Research Institute. The

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mixed cellulose support membrane (pore size 0.45 m) was commercial products. The ultrapure water was laboratory-made (UP, 18.25 MΩ/cm). 2.2. Methods. 2.2.1. Preparation of GO and GO/MB dispersion. GO and GO/MB membranes were prepared by vacuum filtration. To obtain GO dispersion, the required GO sheets were ground in an agate mortar for a period of time firstly. Then, it was transferred to a flat-bottomed flask with UP water as a dispersant (5 mg·L-1). After that, GO dispersion was mechanically stirred for more than 1 h. Finally, the dispersion was stirred under ultrasound (400 W, room temperature) for 10-15 min. The dispersion was greatly stable and without precipitation for standing 7 days. GO/MB mixed solution was obtained by adding 10-110 L 0.2 g·L-1 MB solution into the GO dispersion. The mixed solution was stirred for more than 24 h to obtain the uniform dispersion, the experimental process was shown in Fig. S1a. It can be seen that GO dispersion was brownish yellow, which was consistent with previous study [58]. And the MB solution was blue. 2.2.2. Preparation of GO and GO/MB membranes. We employed vacuum filtration to prepare the GO and GO/MB membranes on mixed cellulose support membrane by filtrating 100 mL dispersion as shown in Fig. S1b. After the filtration, the membranes were dried at room temperature and stored in a refrigerator at 4 °C. The GO/MB membranes were called GO-10, GO-30, GO-50, GO-70, GO-90 and GO-110 based on the volume of the added MB solution. 2.3. Characterizations.

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Raman spectra were analyzed by laser confocal scanning into phase Raman spectrometer (XploRA PLUS, Horiba). The scanning wave number range was 200-2500 cm-1 at room temperature and the laser selection was 532 nm. We employed an ultraviolet spectrophotometer (UV-1100, Shanghai Meipuda Instrument Co., Ltd.) to perform a qualitative spectral scan in the wavelength range of 200-800 cm-1 at room temperature. The interlayer spacing of all the membranes was tested by X-ray diffraction (XRD, BRUKER D8 ADVANCE) with a scan rate of 1 °·min -1 and the scan range of 2θ was 5°-50° under the Cu K radiation (λ=1.5406 nm, 40 Kv, 40 mA). Morphology and thickness of GO and GO/MB membranes were observed by the thermal field emission scanning electron microscopy (SEM, Gemini SEM500, Germany). The distribution of MB on GO membrane was analyzed by true color confocal microscopy (TCCM, Zeiss CSM700). The true color mode was selected, in which the acquisition speed was 7.5 fps. Additionally, atomic force microscopy (AFM, Agilent-S5500) was employed to view the three-dimensional morphology and surface roughness under tapping mode. The hydrophilicity of membranes was measured by water contact angle (WCA, DSA100, KRUSSA, Germany). The zeta potential of GO and GO/MB dispersion was tested by Zetasizer Nano Malvern instrument. 2.4. Membrane nanofiltration experiments. The flux and rejection of the membranes were evaluated by a self-assembled cross-flow device as showed in Fig. S2. Feed solutions were MO ( - , 327.33 Da), DB9 ( 0 , 300.36 Da) and RB ( + , 479.02 Da) with a concentration of 50 mg·L-1. In this work, the membranes should be pre-pressured for at least 30 min before the

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cross-flow test to obtain a stable flux [48]. The concentration of feed solution and permeation were measured by UV. To evaluate the permeation of the membranes, the flux (F) and dyes rejection (R) were got from the following equations: F

V tS

 C R  1  P  C f 

(1)    100%  

(2)

Where V (L) is the volume of permeation, t (s) is the permeation time and S (m2) is the effective area of the membrane cell. Cf and Cp are the concentrations of feed and osmotic dye solution respectively, which were tested by the UV. Each GO and GO/MB membranes should be tested at least three times to obtain more accurate data. 2.5. MD Simulations. Detailed simulation and calculation were provided in supporting information. 3. Results and discussion 3.1 The interaction mechanism between GO and MB 3.1.1 Raman spectra analysis

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Fig. 1. Raman spectra of MB, GO and GO/MB membrane (a-b); UV spectra of GO and GO/MB dispersion (c); XRD pattern of GO and GO/MB membranes (d).

Raman spectroscopy was applied to study the interaction mechanism between GO and MB firstly, as shown in Fig. 1a, which was partially enlarged to obtain Fig. 1b. As we can see that GO membrane displayed two characteristic peaks, which corresponded to D-bond and G-bond [59, 60]. However, the modified GO/MB membrane showed three more new peaks at 845 cm-1, 1288 cm-1 and 1746 cm-1 besides D peak and G peak, which were mainly due to the addition of MB. Furthermore, D peak of the GO/MB membrane was shifted to right, indicating the existing of - conjugation between GO and MB [61]. It was noteworthy that I(G)/I(D) of GO membrane was 1.35, but I'(G)/I'(D) of GO/MB membrane decreased from 1.35 to 1.02, which indicated the defect density of the modified GO/MB membrane was reductive. This was mainly caused by electrostatic interaction and - conjugation between GO and MB, through which the defective parts of GO sheets were repaired 10

by MB [62]. 3.1.2 UV spectra analysis UV full-spectrum scanning on GO and GO/MB mixed solution containing different amounts of MB was performed, from which we judged the interaction between GO and MB by different peak positions in the spectra. We can see from Fig. 1c that there were two peaks at 608 nm and 662 nm in the MB spectra, which we found shifted to 620 nm and 670 nm, respectively. The phenomenon was caused by GO/MB monolayer adsorption and - conjugation between GO and MB, respectively [55, 56]. In addition, there was another peak at 570 nm, which correspond to MB/GO/MB double-layer adsorption [56]. In this test, we diluted the mixed solution prepared in experimental part by 10 times as UV samples. We found there were only GO/MB single-layer adsorption and - conjugation interaction between GO and MB when the MB content was low. However, MB/GO/MB double-layer adsorption appeared when the volume of MB solution was more than 50 L. It demonstrated that GO adsorbed MB reached saturation when the volume of MB more than 50 L. 3.1.3 The d-spacing of GO and GO/MB membranes. The d-spacing variations of GO and GO/MB membranes were characterized by XRD. All the membrane samples were dried in the desiccator over 24 h before they were tested. We can see from Fig. 1d that a typical peak at around 11° of GO membrane, which was consistent with previous study in literature [50, 63-67]. The peaks at 20° and 30° were provided by the mixed cellulose support membrane. Besides, a right-deviation peak at 14° of membranes GO-10 and GO-30, which was

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gradually weakened compared with GO membrane. Interestingly, the peak of GO/MB membrane was so feeble and shifted to the right further when the volume of MB solution was more than 50 L. There are two reasons why the peak was so weak. On the one hand, it was affected by the strong peak of mixed cellulose support membrane. On the other hand, the excess MB molecules we added to the membrane impeded the formation of the peak. All the changes indicated that MB narrowed the d-spacing of the membrane. According to Raman and UV spectra, electrostatic interaction and - conjugation existed between GO and MB, which attributed to the shrinking or even inconspicuous interlayer spacing among the GO/MB membranes. 3.2 Morphology analysis of GO and GO/MB membranes 3.2.1 AFM and TCCM analysis. The roughness of GO and GO/MB membranes was characterized by AFM, as shown in Fig. 2a and more detailed AFM diagrams were provided in Fig. S3. The roughness parameters were summarized in Table S1. We can see that the roughness of GO/MB composite membranes decreased firstly and then rose as the content of MB increased. As we all know, GO membrane was typical wrinkle structure [57], which determined the roughness of membrane. Nevertheless, the wrinkle structure of GO/MB membranes was less obvious than GO membrane, due to there were interaction among GO and MB so MB filled in the wrinkle of membrane that roughness was declined. In order to observe the distribution of MB on GO membrane, TCCM was utilized in true color mode, as shown in Fig. 2b and more detailed TCCM diagrams were provided in Fig. S4. It showed that GO membrane was yellow-brown

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and MB was blue. Combined with AFM, it can be clearly seen from TCCM images that MB distributed on GO membrane uniformly when a small amount of MB was added. GO and MB were stably combined due to there was adsorption interaction between GO and MB. The adsorption interaction includes adsorption ability of GO membrane and electrostatic interaction between positively charged MB dye molecule and negatively charged GO [39, 68]. However, MB began to be filled in the wrinkle structure of GO membrane when the adsorption interaction between GO and MB was saturated. Therefore, MB was trended to aggregate on the GO membrane when the addition of MB was excessive. Moreover, there existed its own agglomeration among MB dye molecules after the wrinkles of GO membrane were filled enough. So a large amount of agglomeration of MB on the membrane surface appeared at this time. As a consequence, the roughness of GO/MB membranes was recovered since the agglomeration of MB arose and even more rough contrast to GO membrane. It was noteworthy that MB agglomeration appeared when the volume of MB was higher than 70 L. Strangely, the roughness was bottom out at GO-30, but the aggregation was started at GO-90. This was mainly due to a mass of MB not only filled the winkles of GO membrane, but also inserted into the lamellas and support the folds, which made its peaks and valleys more obvious. Hence, the roughness was declined firstly and then increased from GO-50, not the GO-90. Combined with UV results, we can draw a conclusion that there were both - conjugation and GO/MB monolayer adsorption between GO and MB in the GO-10, GO-30 and GO-50. When MB content was up to 50 L, GO/MB monolayer adsorption reached saturation and MB/GO/MB

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double-layer adsorption began to appear. In addition, MB/GO/MB double layer adsorption was saturated when MB content reached 70 L and MB started to agglomerate if MB content was in excess of 70 L.

Fig. 2. AFM (a) and TCCM (b) of GO/MB membranes with different volume of MB solution. MD simulation of GO sheet with 9 -OH adsorbed 4 (c1, d1), 10 (c2, d2) and 20 (c3, d3) MB, the front view was (c1-c3) and side view was (d1-d3). The –OH were shown by red and white spheres.

3.2.2 MD simulations. To further explore the interaction mechanism between GO and MB, we employed molecular dynamics simulation (MD) to simulate MB adsorbed at GO sheets. Up to now, there is no clear ratio among various oxygen-containing functional groups due to 14

the complex structure of GO. Hence, the oxygen-containing functional groups of GO were designed to be -OH (Fig. 2c-d), -O- (Fig. S5) and -COOH (Fig. S6), respectively, through which we observed the mechanism of MB stabilized at the GO interface. We can see there were mainly two interactions between MB and GO, including electrostatic interaction (EI) and - conjugation. Clearer EI and - conjugation occurring over the interfacial behaviors were shown in Fig. 3a, which was obtained by part of magnification of the Fig. 2c2. More specifically, the EI played a leading role when small amount of MB was designed, and - conjugation was appeared gradually as the content of MB increased. For investigating the reason why EI was prior than - conjugation, we calculated the potential energy and coulomb energy of MB interacting with graphene (-) and -COOH (EI) in MD simulation system, as shown in Fig. 3b and Fig. 3c. According to principle of the lowest energy, it was easy to find that EI took precedence over - conjugation due to it made the system energy tend to be lower than - conjugation. Interestingly, the UV displayed that there were both EI and - conjugation when a bit of MB was added, whereas there was no - conjugation in MD simulation of a GO sheet adsorbed 4 MB, shown in Fig. 2c1, Fig. S5a and Fig. S6a. This was caused by the planar aromatic ring structure of MB, which would conjugate with GO besides the interaction of EI. As a consequence, the MD simulation results were consistent with the experimental characterization results.

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Fig. 3. Magnification of EI and - conjugation (a); The variety of potential energy (b) and coulomb energy with EI and - conjugation calculated by MD simulation; Schematic diagram of interaction between GO and MB (d-h).

Combined with the results of characterization and MD simulation, we got the interaction mechanism between GO and MB, as shown in Fig. 3d-g, which showed the way GO incorporated together with different amounts of MB. The blue globule illustrated MB was adsorbed in the front of GO sheet and the dark blue globule was in the back. Moreover, the two globules stuck together indicated the existence of MB agglomeration. There were only - conjugation and GO/MB monolayer adsorption when small amount of MB was added, shown in Fig. 3d-e. As MB content increased, Fig. 3f-g showed that MB/GO/MB double layer adsorption began to appear and MB agglomerated on GO membrane when the double-layer adsorption reached to saturation, which would affect the performance of the membranes. The mode of the interaction GO/MB and MB/GO/MB was shown in Fig. 3h. 3.2.3 SEM analysis and MD simulation.

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Fig. 4. SEM images of GO (a1) and GO/MB (b1) membrane; MD simulation of GO sheet with 9 -OH (a2-a3) adsorbed 13 MB (b2-b3), the front view was (a2, b2) and side view was (a3, b3).

SEM was employed to view the surface topography and cross-sectional structure of the GO and GO/MB membranes, as shown in Fig. 4. We prepared samples by 100 mL GO and GO-50 dispersion with a concentration of 50 mg·L-1 so that we can see the cross-section more clearly. The upper right corner of Fig. 4a1, b1 was the surface structure of GO and GO/MB membrane. We can see that there were no significant changes in the surface structure whether GO membrane was modified by MB or not. Nevertheless, the cross-sectional structure of GO/MB membrane was tighter in contrast to that of GO membrane, but still remained layered structure, which indicated that MB we drew into didn’t destroy the structure of GO membrane. We comprehensively analyzed both XRD and SEM, from which we obtained an interesting phenomenon. Theoretically, the thickness of GO/MB membrane should be thinner than GO membrane due to there was an interaction between GO and MB and 17

the interlayer spacing was less after modified by MB. Whereas, we found the thickness of GO/MB membrane was 2.03 m, which was thicker than GO membrane of 1.80 m. This phenomenon was mainly caused by the insertion of MB between the interlayer structures of GO membrane and increased the thickness eventually. The thickness of GO and GO/MB membranes were studied by MD simulation, as shown in Fig. 4a2-a3 and Fig. 4b2-b3. This was MD simulation of GO with -OH adsorbed MB and the two other MD simulations of GO with -O- and -COOH were shown in Fig. S7. We can see from all the results that the thickness of GO/MB membranes was increased almost twofold compared with GO membranes, which was not entirely consistent with the results of SEM image. The thickness of GO/MB membrane was enhanced both in MD simulation and SEM, although the degrees of increment were different. There were many reasons about why the simulation was so much different from SEM. First, the amount of MB added to GO in MD simulation was different from that in SEM samples. Moreover, there was hydrogen bond between actual GO layers, which would limit the freedom of oxygen-containing functional groups. The third was actual distribution of oxygen-containing functional groups on GO sheets, part of which densely packed with oxygen would support the d-spacing of GO layers as the burr effect. More importantly, there was electrostatic repulsion between adjacent GO sheets due to the deprotonation of -COOH actually. For simplicity and convenience, we didn’t take all the above factors into consideration in MD simulation experiments. All in all, the simulation further verified the correctness of the variation in thickness tested by SEM.

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3.3 Water contact angle and zeta potential analysis

Fig. 5. Water contact angle (a) and zeta potential test (b) of GO and GO/MB membranes; UV analysis of UP permeation (c) and the long-term stability test (d) for GO/MB membrane in MO solution.

Fig. 5a was WCA, which was employed to characterize hydrophilicity of the membranes. In this test, we chose the instantaneous contact angle when water was just dropped onto the membrane surface. As can be seen from Fig. 5a that WCA of GO membrane was about 60 °. Moreover, WCA of GO/MB membranes changed from 60 ° to 80 ° as the volume of MB varied from 10 L to 110 L, which indicated the hydrophilicity went down infinitely with MB content went up. It was noteworthy that both GO and MB were hydrophilic substance, but the hydrophilic property of GO/MB membranes was declined by introducing of MB. As is well-known, GO has a good hydrophilic property due to its affluent oxygen-containing functional groups. In 19

addition, the heterocyclic structure and charged group of MB endow MB excellent water solubility. While, the hydrophilicity of GO/MB composite membrane tended to decrease if we mixed the MB to GO. Wen et al. [69] reported that WCA was affected by lots of factors, such as surface charge, functional groups, roughness, porosity and so on. Although both GO and MB were hydrophilic, there existed EI and - conjugation, which encroached upon benzene ring on both sides of heterocyclic structure and made the heterocyclic structure lost its freedom. Moreover, the carboxyl group of GO was selectively trapped onto MB via EI. The hydrophilicity of GO and heterocyclic structure of MB were affected by the encroachment so that WCA of GO/MB membrane gradually increased as the content of MB augmented. Theoretically, the hydrophilicity of GO/MB membrane should be constant when the interaction between GO and MB reached saturation, whereas we find the hydrophilicity decreased linearly with the addition of MB. The reason was that the nitrogen element of MB and the oxygen-containing groups of GO would be bound with each other by hydrogen bonds. As a result, oxygen-containing functional groups were trapped over again so as to further decline of the hydrophilicity of GO/MB membranes even though MB was excessive. We evaluated the zeta potential of GO and GO/MB dispersion, as shown in Fig. 5b. We can see that the negative charge (the absolute value of the potential) of GO/MB mixed solution decreased relied on the supplementation of MB when the volume was lower than 70 L, indicating that EI occurred between MB and GO, which was consistent with the consequence of above characterization. In addition, we found that

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the electronegativity of the solution started to increase when MB content in GO/MB mixture was more than 70 L. This phenomenon maybe caused by the counter-ion Clcarried by MB when the interaction between MB and GO reached to saturation. The results manifested that the surface potential of GO/MB composite membrane was the weakest at the point VMB=70 L. The MD simulation was performed again to further verify our supposition. Fig. 6a1-a2 and Fig. 6b1-b2 were GO sheet with 5 -OH adsorbed superfluous MB with or without counter-ion Cl-. We can see that there were 5 EI and 9 - conjugations among GO and MB without counter-ion Cl-, but there were only 2 EI and 12 - conjugations with counter-ion Cl-, which indicated that the counter-ion Cl- had an obvious effect on the interaction between GO and MB. Moreover, GO sheet with -O- and -COOH adsorbed excess MB with or without counter-ion Cl- were shown in Fig. S8, which displayed the same phenomenon as Fig. 6. It was noteworthy that the interaction between the GO and MB was hardly affected by the counter-ion Cl- when a bit of MB was added, due to the concentration of counter-ion Cl- was comparatively dilute and Cl- moved irregularly in the aqueous solution system. Combined with the MD simulation and zeta potential, we obtained the minimum of zeta potential at the point VMB=70 L. 3.4 Stability.

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Fig. 6. MD simulation of GO sheet with 5 -OH adsorbed 30 MB without (a1-a2) or with (b1-b2) counter-ion Cl-; Stability of GO (left) and GO/MB (right) membrane under ultrasound (c).

In order to verify the stability of MB combined with GO in GO/MB composite membrane during the permeation experiment, we select UP as feed solution to carry out the permeation experiment by GO/MB composite membrane. Then, we took the osmotic solution for UV analysis to check whether there was MB leakage in cross-flow filtration process. As can be seen from Fig. 5c, a typical absorption peak 22

appeared at 664 nm corresponding to the characteristic peak of MB and the permeate revealed no peak from 550 nm to 750 nm, which demonstrated that no MB leaked from GO/MB membrane in the test for up to 9 h, proving the great stability of MB combined with GO. Fig. 5d showed long-term stability of GO/MB membrane. In order to study the stability of GO/MB membrane more detailed in application process, we chose MO as feed solution and the permeability test was sustained up to 55 hours. The membrane was pre-pressed for half an hour before the test to ensure GO/MB membrane with a stable flux. We found that the flux and rejection for MO feed solution went down slightly within 16 hours and then maintained stability until to the end of the experiment for 55 hours. As the test time increased, the membrane flux declined mainly due to MO dye molecules blocked water transport channels. Although MO dye molecule is electronegative, the interception of GO/MB membrane is not 100%. Therefore, MO still can enter the channels of the membrane and hinder the transmission of water, so that the flux was attenuated. The traditional trade-off effect is that the rejection increases as the flux decreases. But we found that the rejection also gradually decreased with the water flux decreased in this study. This was mainly caused by the accretion on the surface of GO/MB membrane, resulting in a decrease in the electronegativity of the membrane surface. As a result, the repulsive force of the membrane to MO was debilitated so that the rejection was declined. However, negative effects of dye molecule on the membrane disappeared after a period of time, which displayed that GO/MB membrane had a good application prospect.

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In order to further study the stability of modified GO/MB composite membrane, we selected ultrasonic experiments to observe the decomposition of GO and GO/MB membrane under ultrasound. GO/MB membrane we chose was GO-70 membrane due to it had great separation for dye solution in the description below. We put the pre-pressed GO and GO/MB membrane into petri dishes containing ultrapure water and immersed the membranes firstly. Then, ultrasound was performed at 160 W to observe the rupturing time of GO and GO/MB membranes, as shown in Fig. 6c. We selected different ultrasound time to record the explode state of two membranes. The left was GO membrane and the right was GO/MB membrane. We found that GO membrane began to decompose within 30 s of ultrasound at 160 W and detached completely 15 min later. However, the modified GO/MB membrane initiated to show slight breakage after 5 min and even not completely decomposed after 30 min, which manifested the stability of GO/MB membrane has been significantly improved. This was mainly caused by the EI and - conjugation between GO and MB. For further study the stability of GO/MB membrane. The filtration performance was measured after the ultrasonic experiments. We selected the GO membrane with an ultrasonic time of one minute (GO1) as sample. Since the GO/MB composite membrane was not damaged within 5 minutes by ultrasound, we chose the GO/MB composite membrane with an ultrasonic time of 10 minutes (GO/MB10) to observe the permeability after the ultrasonic experiment. We found that the flux of both GO and GO/MB membranes increased sharply and the rejection went down to almost 0%, as shown in Figure S9b. The rejection went down to almost 0% for the sake of destroy of the membrane by

24

ultrasonic. Comparing these two membranes, it can be clearly seen that the flux of GO/MB10 was much lower than that of GO1 although the ultrasonic time of GO1 less than GO/MB10, which demonstrated the GO1 was destroyed more seriously than GO/MB10. This experiment further indicated that the modification method greatly improved the stability of the GO/MB membrane 3.5 Permeation and separation performance.

Fig. 7. Flux of UP water and different dyes feed solution (a); Dyes separation performance of GO and GO/MB membranes (The feed solution of b, c, d was MO, DB9, RB, respectively); Permeability of GO and GO/MB membranes in different feed solution (e); Rejection of GO and GO/MB membranes in feed different solution (f).

Fig. 7a showed permeability of GO and GO/MB membrane in UP, MO, DB9 and RB dye solution. Compared with the permeability of UP, there was not obvious change of GO/MB composite membrane both negative MO and neutral DB9 solution. On the contrary, the penetration of the positive RB was relatively lower than above of them. This was mainly by the absorption of RB on the negative GO/MB membrane, which resulted in a decline of flux. More specifically, the permeability of GO/MB

25

membrane appeared to gradually decrease at first and then slowly rose as MB added. On the one hand, MB was inserted into the interlayer of GO membrane and trapped onto it, which hindered the transmission channel of water molecules, resulting in attenuation of flux. On the other hand, MB filled into the wrinkle structure of GO membrane, which made the roughness of GO/MB membranes go down. Meanwhile, the specific surface area of membranes decreased, which was another reason for the attenuation of the flux when we introduced MB to GO membrane. In addition, MB weakened the hydrophilicity of GO/MB composite membrane compared with GO membrane. Interestingly, MB would agglomerate in the interlayer spacing of GO/MB membranes when the supplementation of MB was excessive (V≥70 L), due to which - conjugation and EI between GO and MB were saturated. Moreover, the agglomeration of MB made the membrane structure looser so that the flux of GO/MB membranes recovered since the volume of MB was more than 70 L. We selected three kinds of dyes with different charge, different molecular weight and different structure to study separation property of GO/MB composite membrane. Fig. 7b was the rejection performance of GO and GO/MB membrane for the negative dye MO. When GO membrane modified by MB, the optimal rejection increased from 84.27% to 93.32%, mainly caused by the reduced interlayer spacing of GO/MB composite membranes and the block of transport channel. We analyzed a cut-off test of the neutral dye DB9, which can be seen from Fig. 7c that the rejection of GO membrane was 99.60% and the rejection of GO/MB composite membrane for DB9 dye was as high as 99.85%. Although the boost of rejection was not obvious, it truly

26

demonstrated GO/MB composite membrane has a potential application prospect. Fig. 7d showed the rejection for positive RB. We found GO membranes was rejected RB badly, which was due to electrostatic adsorption between GO and RB during cross-flow filtration. Surprisingly, the retention of RB by the modified GO/MB composite membrane was increased from 46.40% to 82.56%, almost doubled in contrast to that of GO membrane, indicating it was modified successfully. Compared with the rejection of MO, it was found the modification of GO membrane had more significant improvement on the retention of RB. This was mainly affected by the different charge of MO and RB. Combined with the zeta potential analysis in Fig. 5b, there was EI between GO and MB, which lead the electronegativity of the membrane surface to decrease. As a consequence, the repulsive force between GO/MB composite membrane and MO was weakened, which was unfavorable for its rejection. Meanwhile, the adsorption among the positive charged RB and GO was subdued which was beneficial to the retention of RB. Therefore, the rejection of RB by GO/MB composite membranes was more obviously improved than that of MO. Theoretically, molecular weight of MO was larger than DB9 and there existed electrostatic repulsion between GO and MO, which should be more easily rejected by GO/MB composite membrane [70]. In fact, we found that the retention of MO was less than DB9. On the one hand, the result was related to the molecular structure, wherein the molecular structure of MO was linear, while the two hydroxyl functional groups of DB9 molecule were similar to the fork structure. Hence, DB9 was more difficult to pass through the membrane. On the other hand, DB9

27

molecule with -NH2, N=N and -OH, easily interacted with oxygen-containing functional groups on the GO sheets, including hydrogen-bond interaction and the condensation polymerization reaction with the not deprotonated carboxyl groups. So there was great resistance in the transmission of DB9, resulting in higher rejection. In summary, the rejection of dye solution was not only related to the molecular weight, charge and molecular structure of dye molecules, but the interaction between dyes and membrane also played an important role in the wastewater treatment. It was noteworthy that we can control the rejection of different dyes by changing the amount of MB. The performance of GO/MB membrane reported in this study was compared with other works in Table 1. The results indicated that GO/MB membrane exhibited relatively high rejection. It's worth noting that the performance of our membranes is inferior to some works of them but better than others we listed in the Table 1. That’s mainly caused by the different choices of membrane functional materials and the completely different preparation methods. Compared to their works, our work is much more simple and we can save a large part of the process time. Low time consumption, simple operation method and high rejection endow GO/MB membrane superiority in water treatment applications. It is worth stressing that the flux of GO/MB membrane decreased with the increase of rejection, which affects its effectiveness in the application. More importantly, we obtained the interaction mechanism between GO and MB by MD simulation and other characterization methods. In addition, the effects of various factors of dye molecules on the rejection of membrane during the

28

de-dyeing process were summarized clearly, which would lay the foundation for other studies. Table 1 Comparison of the permeability and rejection in different membranes. Membrane

Dye

molecular weight

Rejection (%)

Flux (L·m-2·bar-1)

Ref

rPGMs

MO

327.33

97.5

5.3

[50]

SG@GO

EBT

461.38

>90

33

[54]

TMC@GO

MB

319.85

44-46

8-27.6

[37]

GO@PA6

MO

327.33

99

11.15

[39]

PA6@GO@PA6

MO

327.33

99.47

13.77

[40]

GO/MB

MO

327.33

96.37

GO/MB

DB9

300.36

99.77

GO/MB

RB

479.01

82.56

GO/MB

Na2SO

-

>70

This study 3.83

This study This study

14.1

This study

4

(EBT: Eriochrome black T) 3.6 Effect of salt solution on performance of GO and GO/MB membranes In order to study the effect of salt solution on permeability of GO and GO/MB membrane (GO/MB mixed solution was obtained by adding 50 L MB of 0.2 gL-1 to 100 ml GO solution of 5 mg·L-1), we employed UP, RB solution, Na2SO4 solution and RB/Na2SO4 mixture to evaluate the penetrability and rejection, as shown in Fig. 7e and Fig. 7f. From the figure we can see that the permeability performance: Na2SO4 > RB/Na2SO4 > UP > RB. And we found that the rejection of salt or dye for the salt/dye mixed solution was lower than that in pure salt solution and dye solution with the same concentration (RNa2SO4 :Na2SO4 > RB/Na2SO4; RRB: RB > RB/Na2SO4). For the rejection of Na2SO4, the rejection in Na2SO4 solution was higher than that in the RB/Na2SO4 mixed solution mainly due to the GO is negatively charged and positively RB can be adsorbed on the GO sheets. Therefore, the electronegativity of the membrane was decreased and weakened the electrostatic repulsion to SO42-. For the 29

retention of RB, we can see that the retention of RB in pour RB solution was better than in RB/Na2SO4 mixed solution, which is radically affected by salt ions. And the more detailed reasons will be discussed below. It was well known that GO membrane generally swelled slightly in water, which was mainly due to water molecules inserted into the interlayer structure during filtration process. The diameter of water molecule, hydrated diameters of Na+ and SO42- are 0.4 nm, 0.72 nm and 0.76 nm, respectively, from which we can see both Na+ and SO42- are larger than water molecule. Moreover, GO and GO/MB membrane can reject about 70% of 1 g·L-1 Na2SO4 solution which obtained from Fig. S9a. So part of Na+ and SO42- will be inserted into the zigzag channel of the membranes during the permeation. As we all know, GO sheets swell in aqueous solution, so the salt cations can combine easily with the deprotonated oxygen-containing functional groups of GO in the permeation test of feed solution containing Na2SO4, which made electronegativity of GO surface decrease so that the GO sheets agglomerated more seriously. Moreover, the agglomeration of GO lead to the inability of GO sheets to stretch freely, which kept the interlayer spacing of GO sheets lager [71] and the flux was improved. That's one of the reasons why the effect of salt solution on permeability of GO membrane was so obvious. The hydrogen-bonds existed in GO membrane were caused by the oxygen-containing functional groups of GO sheets. However, the hydrogen-bond has negative impact on the transport of water molecules. When we added Na2SO4 to the feed solution, the ionic strength became stronger and it made GO sheets swell further so that the hydrogen-bonds between -COOH and -OH resolved, as shown in Fig. 8a. The charge

30

density of membrane was increased with the salt added, which was caused by adsorption of Na+ from the feed solution onto GO membrane surface. Therefore, the counterions concentration inside interlayer will be greater and it compensated the surface charge. This effect could lead to membrane swell further due to stronger electrostatic repulsion between counterions [72]. That’s why the hydrogen-bond was destroyed to some extent. And this phenomenon endowed the membrane better rejection of dyes in pure dye solution than that in salt/dye mixed solution. It has been reported in the literature [73] that the -COOH resists the transport of water due to its relatively large steric geometric structure. On the contrary, -OH promotes water transport due to its relatively stronger interaction with water. When the hydrogen-bond was opened, it changed to -COOH and -OH. On the one hand, the newly formed -COOH has a smaller steric hindrance than the original hydrogen-bond. On the other hand, There was more -OH in the interlayer structure of GO. These are the other two reasons that made the flux improve further. Therefore, the permeability of Na2SO4 solution was much higher than UP. By the same token, permeability of RB/Na2SO4 mixed solution by GO and GO/MB membranes was superior to RB solution. In addition, the reason why the flux of UP higher than RB and the Na2SO4 solution higher than RB/Na2SO4 mixed solution can be obtained from previous description. In order to investigate if it can keep a high rejection for a long time after the adsorption of ions get saturation, we took a test to observe the permeability and rejection of GO/MB membranes for a long time in Na2SO4 feed solution. We found that the flux and rejection of the membrane decreased with time, which was mainly

31

due to the concentration polarization [74, 75]. However, when the velocity of the salt ions flowing toward the GO/MB membrane surface was equivalent to the diffusion rate of the salt ions to the bulk solution caused by the concentration gradient, a boundary layer had been formed on the membrane surface. After that, the attenuation of the flux and rejection became slowly and the retention of GO/MB membrane still remained at 65%.

Fig. 8. Schematic diagram of the effect of salt ions on hydrogen-bond (a); The permeability and rejection test of GO/MB membranes for a long time (b).

4. Conclusions In summary, an effective strategy to prepare GO/MB composite membrane was described in detail. GO membrane was modified by MB through electrostatic interaction and - conjugation. The interlayer spacing of GO/MB membrane was shrank due to the interaction between GO and MB. Meanwhile, the stability of GO/MB composite membrane was enhanced obviously and it showed an outstanding separation performance for different dyes solution. The study of interaction mechanism between GO and MB in GO/MB composite membrane was discussed clearly. In addition, we utilized MD simulations to understand the mechanism of MB

32

stabilized at the GO interface, which enabled us to trace the nanoscale events occurring over the interfacial behaviors and provided molecular information at nanoscale interfaces. Furthermore, the effect of various factors including molecular weight, molecular structure and molecular charge on the dyes rejection was summarized, which had far-reaching significance in the field of dye wastewater treatment.

Associated content Supporting information Detailed MD simulation process and calculation method, schematic diagram of vacuum filtration device and cross-flow device, AFM, roughness parameters, TCCM, MD simulation of detailed interaction mechanism and thickness variety of GO and GO/MB membranes, the effect of counter-ion Cl- on the adsorption between GO and MB, the flux and rejection of Na2SO4 solution by GO and GO/MB membranes. Acknowledgments The authors acknowledge the financial support from the Science and Technology Plans of Tianjin (16YFZCSF00330). The work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 （A）. The computational work for this article was partially performed on resources of the National Supercomputing Centre, Singapore (https://www.nscc.sg).

33

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Highlights  The stability of graphene oxide/methylene blue (GO/MB) membrane was enhanced obviously.  An effective strategy to prepare GO/MB membrane with outstanding separation performance.  The effect of various factors on dyes rejection was summarized, which had far-reaching significance in wastewater treatment.  Molecular dynamics simulations were utilized to trace nanoscale events occurring over interfacial behaviors.