Enhanced Water Flux through Graphitic Carbon Nitride Nanosheets

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1 Separations: Materials, Devices and Processes Enhanced Water Flux through Graphitic Carbon Nitride Nanosheets Membrane by Incorporating Polyacrylic Acid DOI /aic Yanjie Wang, Lingfei Liu, Jian Xue*, Jiamin Hou, Li Ding, Haihui Wang* School of Chemistry & Chemical Engineering, South China University of Technology No. 381 Wushan Road, Guangzhou , China *Corresponding authors:prof. Dr. Haihui Wang Dr. Jian Xue Abstract: Membranes assembled from two-dimensional (2D) layered materials have shown potential use in water purification. Recently, a 2D graphitic carbon nitride (g-c 3 N 4 ) nanosheets membrane exhibit considerable separation performance in water purification. In this study, to further improve this water separation performance, polyacrylic acid (PAA) was introduced to tune the nanochannels formed between the g-c 3 N 4 nanosheets. The fabricated g-c 3 N 4 -PAA hybrid membranes possessed higher water flux without sacrificing much rejection rate compared with that of the g-c 3 N 4 membrane; however noticeable fouling was observed upon addition of the PAA into the membrane composite structure. In addition, the effect of PAA on the morphology, surface hydrophilicity, separation performance, and antifouling properties of the g-c 3 N 4 membrane were examined in detail. Overall, incorporating PAA into the g-c 3 N 4 nanosheets membrane was an effective and convenient method to improve the water separation performance, which could promote the application of the 2D g-c 3 N 4 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: /aic American Institute of Chemical Engineers (AIChE) Received: Jul 04, 2017; Revised: Dec 27, 2017; Accepted: Dec 28, 2017

2 Page 2 of 32 nanosheets membrane in practical ultrafiltration processes. Keywords:g-C 3 N 4 nanosheets, membrane, water purification, modification Introduction In modern industrial chemical engineering, membrane technologies, such as microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF), have been widely applied for water purification due to their energy-saving, high-efficiency and low-cost properties. 1-4 The development of membranes is continually promoted by new materials and technologies. Recently, nanomaterial-based membranes have been extensively explored due to their unique chemical and physical properties Among them, two-dimensional (2D) nanosheets membranes have been widely studied due to their chemical inertness, high mechanical strength, high separation performance and flexible structures D graphene oxide (GO) nanosheets-based membranes are promising potential candidates for water purification. Since Geim and co-workers demonstrated that the interlayer spacing between GO nanosheets could act as molecular sieves, 5 GO-based membranes have been widely applied in gas separation and water purification However, the water flux through GO membranes via diffusion is quite low, limiting their application in pressure-driven membranes. Qiu et al. first found that the water flux through a thermally corrugated GO membrane could reach as high as 45 L m -2 h -1 bar -1, due to the formation of nanochannels between the corrugated GO nanosheets. 26 Furthermore, Peng and co-workers demonstrated that nanochannels caused by 2

3 Page 3 of 32 charged groups could also be formed in a GO membrane. 27 Therefore, nanochannels caused by irregular stacking are beneficial to pressure-driven water transport. Such nanochannels have also been found in other 2D-based membranes, such as WS 2 and MoS 2. 15, 16 Moreover, several modification methods can further improve the water flux of 2D-based membranes by creating additional space or channels. 16, For example, Chen et al. created reduced GO membranes with a high water flux by enlarging the interlayer spacing with carbon nanotubes. 28 Sun et al. fabricated a WS 2 -based membrane with a high water flux by creating additional nanochannels using nanostrands. 16 Wang et al. inserted carbon nanodots into the GO interspace to tune the spacing for obtaining a higher water flux. 30 Our group also found that Fe(OH) 3 nanoparticles could increase the interlayer spacing formed between MXene nanosheets, and the MXene membrane exhibited a higher water flux after the dissolution of the Fe(OH) 3 nanoparticles. 32 Currently, a graphene-like 2D material, layered 2D graphitic carbon nitride (g-c 3 N 4 ), has attracted much attention Compared with other 2D nanosheets, the preparation process of g-c 3 N 4 nanosheets is simple, and the raw materials are inexpensive. 15, 16, 33, 34, 38, 39 Recently, in our group, g-c 3 N 4 nanosheets were fabricated as a membrane for water purification, exhibiting considerable separation performance with a water flux of 29 L m -2 h -1 and rejection rate of 87% for Evans blue (EB) molecules. 40 To further improve the water flux, herein, we fabricated polyacrylic acid (PAA)-modified g-c 3 N 4 nanosheet membranes. The cheap commercial polymer PAA was inserted to tune the nanochannels formed between the 2D g-c 3 N 4 nanosheets 3

4 Page 4 of 32 (Figure 1a). Through the abundant carboxyl groups, the PAA bonded with the NH groups of the g-c 3 N 4 nanosheets via hydrogen bonding in the g-c 3 N 4 -PAA hybrid membranes (Figure 1b). The effects of PAA on the morphology, surface hydrophilicity, separation performance and antifouling properties of the g-c 3 N 4 membrane were investigated in detail. This study provides a convenient and cheap method to improve the water flux. Moreover, different 2D nanosheets and polymer molecules can be fabricated into hybrid membranes using this method for different applications. Experimental Section Materials Melamine, methyl blue (MB, M w = Da), methylene blue (MeB, M w = Da), rhodamine B (RB, M w = 479 Da), evans blue (EB, M w = Da), bovine serum albumin (BSA, M n = 65,000 Da), and PAA (M w = 1,800 Da) were purchased from Sigma. The polycarbonate (PC) membrane support was supplied by Whatman. All reagents were used as received Synthesis and exfoliation of the g-c 3 N 4 nanosheets The g-c 3 N 4 nanosheets were obtained through a thermal oxidation etching process. 34 Briefly, the bulk g-c 3 N 4 was obtained by heating melamine at 520 C for 3 h under ambient air conditions with a heating rate of approximately 3 C /min for both the heating and cooling processes. Then, the bulk g-c 3 N 4 was ground into a powder and heated at 500 C for 2 h in air. Subsequently, the resultant powder was dispersed 4

5 Page 5 of 32 into deionized water, followed by sonication for 1 h. The dispersed g-c 3 N 4 was centrifuged at 5000 rpm several times to remove unexfoliated aggregates and/or nanoparticles. Then, the g-c 3 N 4 nanosheets were left in the supernatant. Fabrication of the membranes and separation performance To fabricate the g-c 3 N 4 and g-c 3 N 4 -PAA x hybrid membranes, where x represents the mass ratio of PAA to the g-c 3 N 4 nanosheets, the g-c 3 N 4 nanosheet dispersion and PAA solution were mixed and filtered under vacuum on PC membranes with pore sizes of approximately 200 nm. The obtained g-c 3 N 4 and g-c 3 N 4 -PAA x hybrid membranes were dried under vacuum at room temperature for >12 h to remove residual water before the permeation tests. The pure water flux and retention measurements were performed on a homemade cross flow filtration device. 32 The effective area was 3.14 cm 2. The mass of water going through membrane was collected under 0.1 MPa and recorded by a balance. The corresponding volume was calculated through the mass. The water flux was calculated as introduced in other literatures. 30 For the retention experiments, BSA (1 g L -1 ) and the dyes, including MB, MeB, RB and EB, were applied for filtration. The concentrations of these dye solutions were 10 mg L -1. Each membrane was filtrated twice through solutions. The rejection rate after the first filtration is a little higher than that of the second filtration due to the adsorption effect of the dyes on the membranes To eliminate the adsorption effect of the dyes on the membranes, the permeate solution collected after the second filtration was used for analysis. The molecule concentrations were analyzed using UV-vis spectroscopy. The retention of each membrane was evaluated as introduced in other literatures. 30 5

6 Page 6 of 32 Antifouling properties of the membranes A protein solution (BSA, 1 g/l, ph = 7) was used to evaluate the antifouling properties of the membranes. The measurement was performed on a dead-end filtration device (Millpore Corp., Billerica, USA). The detailed process was performed as follows: (1) the pure water flux (J 1 ) was measured at 0.1 MPa and recorded for 30 min; (2) a 1 g/l BSA solution was used to replace the pure water, and the flux for the protein solution (J B ) was recorded for another 60 min; (3) the protein-fouled membranes were washed with water, and then, the flux for pure water (J 2 ) was measured again. The flux recovery ratio (FRR) was calculated by: J FRR = 2 100% (1) J 1 A higher FRR value indicates that the membrane possesses better antifouling properties. To analyze the antifouling properties of the membranes in detail, the total fouling ratio (R t ), irreversible fouling ratio (R ir ) and reversible fouling ratio (R r ) were calculated as follows: J B J 1 R t = (1- ) 100% (2) J2 R ir = (1- ) 100% (3) J 1 J 2 J B R r = ( ) 100% (4) J 1 Characterization The g-c 3 N 4 nanosheets dispersions were analyzed by UV-vis spectroscopy (Shimadzu UV-1601). The crystalline structures of the g-c 3 N 4 nanosheets were 6

7 Page 7 of 32 investigated by X-ray diffraction (XRD, Bruker-D8 ADVANCE, Cu Kα radiation) in the 2θ range of with intervals of The chemical structures of the g-c 3 N 4 nanosheets, PAA and g-c 3 N 4 -PAA were characterized by Fourier transform infrared (FT-IR) spectroscopy (Nicolet 6700) with a scan range of cm -1. The morphologies and structures of the membranes were observed using scanning electron microscopy (SEM, Quanta 450) under high vacuum with an accelerating voltage of 5 kv. Atomic force microscopy (AFM) images were obtained on a Ntegra Solaris under semicontact mode. The contact angles of the membranes were measured using a contact angle goniometer (Attension Theta Optical Tensiometer). The tensile strength of membranes were measured on Electronic Universal Testing Machine (Shimadzu AG-IC50kN). The zeta potential of membrane surface was obtained on a Zeta Nanosizer (Malvern Nano ZS90). Results and Discussion Characterization of the g-c 3 N 4 nanosheets and PAA Figure 2a shows the SEM image of g-c 3 N 4 after thermal oxidation etching. A loose and laminar morphology of the g-c 3 N 4 nanosheets was observed. The crystal structure of the obtained g-c 3 N 4 nanosheets was investigated by XRD. As shown in Figure 2b, two characteristic peaks of the g-c 3 N 4 nanosheets were observed. The weak peak at 12.7, corresponding to the (1 0 0) plane, originated from the in-plane structural packing motif of the tri-s-triazine units. In addition, the strong peak at 27.9, corresponding to the (0 0 2) plane, was attributed to the interlayer distance of nm. 41 To further understand the thickness and morphology of the g-c 3 N 4 nanosheets, AFM measurements were performed. The AFM image (Figure 2c) revealed that the g-c 3 N 4 nanosheets were several micrometers in size. In addition, the thickness was 7

8 Page 8 of 32 approximately 10 ~ 30 nm (Figure 2d). Figure S1a displays the UV-vis spectra of the g-c 3 N 4 nanosheets dispersed in water. A strong peak at 310 nm was observed. To determine the concentration of the g-c 3 N 4 nanosheet solution, prepared standard g-c 3 N 4 nanosheets solutions were measured by UV-vis spectroscopy. The absorbance showed a highly linear relationship with the g-c 3 N 4 concentration (Figure S1b). Therefore, the mass of the filtrated g-c 3 N 4 nanosheets was calculated through the UV-vis absorption. The interfacial interactions between the g-c 3 N 4 nanosheets and PAA were determined by FT-IR spectroscopy (Figure 3). For the g-c 3 N 4 nanosheets, a sharp peak at 806 cm -1 is attributed to the heptazine ring system. 42 The peaks in the range of cm -1 correspond to the typical stretching modes of g-c 3 N 4 heterocycles. 42 In addition, the broad peaks between 3080 and 3250 cm -1 are ascribed to the terminal N-H bands at the defect positions of the aromatic rings, such as NH and/or NH 2 groups. 43 For PAA, the strong peak at 1695 cm -1 is ascribed to C=O of the COOH groups, while the broad peak at cm -1 corresponds to -OH stretching mode of COOH. 44 After PAA was added into the g-c 3 N 4 nanosheets, the broad peak corresponding to the -OH groups in the mixture shifted to a lower wavenumber compared with that in the PAA (Figure 3b), which was highlighted by the dash line. This result suggests that the g-c 3 N 4 nanosheets and PAA formed intermolecular H-bonds through the NH groups of the g-c 3 N 4 nanosheets and the COOH groups of PAA. Therefore, PAA combined with the g-c 3 N 4 nanosheets via H-bonding interactions. Morphology and surface hydrophilicity of the hybrid membranes Then, the g-c 3 N 4 and g-c 3 N 4 -PAA x (0.1 x 0.5) hybrid membranes were 8

9 Page 9 of 32 fabricated on porous supports through vacuum filtration. To investigate the effect of PAA on the hybrid membrane structure, the surface and cross-section of the membranes were observed with SEM. Figure S2a-d show the thicknesses of the g-c 3 N 4, g-c 3 N 4 -PAA 0.1, g-c 3 N 4 -PAA 0.25, and g-c 3 N 4 -PAA 0.5 membranes, which are approximately 405 nm, 490 nm, 540 nm, and 680 nm, respectively. The increased thickness indicates that PAA was successfully inserted into the g-c 3 N 4 membrane, resulting in an increased layer distance. Thus, the simple mixture and filtration method effectively introduced PAA molecules into the membranes. Figure S3a reveals that the porous PC substrate is smooth and has a uniform pore size of approximately 200 nm. Even though filtrating PAA solution, the surface characteristic of the substrate is still the same (Figure S3b). This result indicates that the PAA couldn t form a layer on the PC substrate by filtrating. After the g-c 3 N 4 nanosheets were loaded onto the substrate, the surface of the g-c 3 N 4 membrane became rough (Figure S3c), which was also observed for the g-c 3 N 4 -PAA x hybrid membranes (Figure S3d-f). Some pores caused by unordered stacking were also observed on the surfaces of these membranes. Furthermore, for complementary information, AFM was applied to investigate the morphologies of these hybrid membranes (Figure S4). The AFM information was obtained in 20*20 µm 2. According to the AFM images, the g-c 3 N 4 membrane possessed the roughest surface (the average roughness R a = 64 nm), and the roughness decreased from 63 nm to 50 nm with an increasing addition of PAA from 0.1 to 0.5. The decreased roughness is likely due to the small PAA molecules filling in defects. 9

10 Page 10 of 32 To evaluate the surface hydrophilicity of the membranes, their contact angles were tested. Generally, a smaller contact angle results in a greater surface hydrophilicity. As shown in Figure S5, the contact angle of the g-c 3 N 4 membrane was In addition, the contact angle decreased from 37.7 to 32.9 with an increasing addition of PAA from 0.1 to 0.5. The decrease in the contact angle of the g-c 3 N 4 -PAA x hybrid membranes is attributed to the introduction of the abundant hydrophilic carboxyl groups of PAA. This result indicates that the surface hydrophilicity of the hybrid membrane can be increased by embedding hydrophilic additives into the g-c 3 N 4 membrane. Filtration performance and antifouling properties of the membranes The effect of PAA on the water flux of the membrane was measured. As shown in Figure 4a, the pure water flux through the g-c 3 N 4 membrane was 47.5 L m -2 h -1. In addition, the water flux through the g-c 3 N 4 -PAA x hybrid membranes increased from 117 to L m -2 h -1 with an increasing addition of PAA from 0.1 to 0.5. The enhanced water flux indicates that the PAA molecules create additional space in the hybrid membrane. The larger number of hydrophilic carboxyl groups of PAA may also provide elevated water flux through the hybrid membranes. As shown in Figure 4a, notably, the water flux through the g-c 3 N 4 -PAA 0.5 membrane was only 22% higher than that through the g-c 3 N 4 -PAA 0. 1 membrane; however, the addition of PAA increased the water flux by 4 times. Therefore, over-loading PAA provides little contribution to the enhancement of the water flux. Then, EB molecules were used to measure the separation performance of the g-c 3 N 4 and g-c 3 N 4 -PAA x hybrid membranes. As revealed in Figure 4b, the g-c 3 N 4 membrane showed a rejection rate of 85% for EB molecules, and the rejection rate 10

11 Page 11 of 32 decreased with an increased PAA loading. When 10 wt% PAA was incorporated into the g-c 3 N 4 membrane, the rejection rate decreased to 83%, while the g-c 3 N 4 -PAA 0.5 membrane only showed a rejection rate of 70% for EB molecules. The decreased rejection rate implies that larger nanochannels were formed due to the embedding of PAA. In addition, compared with the g-c 3 N 4 membrane, the g-c 3 N 4 -PAA 0.1 membrane showed a 147% water flux enhancement while sacrificing the rejection rate by 2%. However, for the g-c 3 N 4 -PAA 0.5 membrane, the water flux enhancement and rejection rate degradation were 201% and 23%, respectively. Therefore, the above results indicate that over-loaded PAA leads to the formation of nanochannels with larger sizes. Next, the antifouling properties of these membranes were examined. As revealed in Figure 5, the flux of the BSA solution was lower than the flux of pure water due to protein fouling. After the membrane was cleaned, the water flux still did not completely recover, likely due to the BSA molecules restrained in the nanochannels blocking the water channels. To further explore the antifouling properties of these membranes, other parameters, including FRR, R t, R r and R ir, were calculated and analyzed. As revealed in Table 1, the FRR for the g-c 3 N 4 membrane (79.5%) was similar to that for the g-c 3 N 4 -PAA 0.1 membrane (80%), and both membranes were higher than those of the g-c 3 N 4 -PAA 0.25 membrane (48.3%) and g-c 3 N 4 -PAA 0.5 membrane (47.5%). In addition, upon increasing the addition of PAA from 0.1 to 0.5, the R t and R ir increased from 73.4% and 20% to 84.0% and 52.5%, respectively. The dramatic enhancement of the irreversible fouling ratio implies that the g-c 3 N 4 -PAA 0.5 hybrid membrane was seriously fouled. From the abovementioned results, we conclude that the larger nanochannels from the addition of over-loaded PAA were easily blocked by the BSA molecules, which decreased the antifouling properties of 11

12 Page 12 of 32 the hybrid membrane. Therefore, considering the water flux, rejection rate and antifouling properties, the g-c 3 N 4 -PAA 0.1 membrane was selected for further measurements. Besides, the Youngs modulus of the PC substrate deposited by g-c 3 N 4 -PAA 0.1 hybrid membrane is 34,490 Mpa, while the Youngs modulus of the PC substrate is 27,523 Mpa. This result indicates that the hybrid film possesses a stronger tensile strength than PC substrate. Performance of the g-c 3 N 4 -PAA 0.1 membrane The g-c 3 N 4 -PAA 0.1 hybrid membranes with different thicknesses were obtained by controlling the total mass loading of the g-c 3 N 4 nanosheets and PAA. EB molecules were used to evaluate the effect of the membrane thickness on the separation performance of the hybrid membrane. As shown in Figure 6, with an increased thickness from 150 to 610 nm, the water flux through the hybrid membrane decreased from 170 to 82 L m -2 h -1, and the rejection rate for EB molecules increased from 21% to 85%. This result implies that the defects in the hybrid membrane were effectively reduced by increasing the thickness. In addition, Figure 6 further shows that, with an increased thickness from 490 to 610 nm, the increase of the rejection rate slows down, indicating that the thickness effect is saturated. The effect of the EB concentration on the separation performance of the g-c 3 N 4 -PAA 0.1 hybrid membrane was measured. As revealed in Figure S6a, the rejection rate of the g-c 3 N 4 -PAA 0.1 hybrid membrane slightly declined with an increasing EB concentration from 2 mg/l to 100 mg/l in the feed. For an intuitive comparison, the UV-vis absorbance spectra of the concentrate, feed and permeated solutions are shown in Figure S6b. The retention properties for molecules with different molecular weights were also investigated. The retention curve was calculated 12

13 Page 13 of 32 and is shown in Figure 7a, showing that the rejection rate increased with an increased molecular weight. The above results demonstrate that the molecules are effectively rejected by the nanochannels in the g-c 3 N 4 -PAA 0.1 hybrid membrane. To test the chemical stability of the membrane, the water with different ph was filtrated through the membrane and the flux was obtained. The effect of ph on permeated flux mainly involves two parts: osmotic pressure between feed and permeate, membrane permeability. 45 When adding acid/alkali into solution to adjust the ph conditions, these two factors may change. In this study, the osmotic pressure between feed and permeate could be negligible, as the hybrid membrane hardly block the acid/alkali (HCl/NaOH). Besides, PAA contains abundant acidic carboxyl groups and the pk a value of PAA-water solution ranges from 5.78 to Thus the PAA may be charged and uncharged at different phs, which would affect the membrane permeability. Figure 7b shows that the flux is stable in ph range between 2 and 12. This result implies that the change of PAA have a little influence on membrane permeability. This may be attributed to the followed reasons. Firstly, the additive PAA is only 10%. Secondly, the hydrogen bonding formed between carboxyl groups and NH groups would reduce the environmental influence. Thirdly, the possible scale change of PAA under different ph is small compared to the pore size. Combine abovementioned factors, the membrane exhibits relative stable flux under different ph. To highlight the excellent performance of the hybrid membrane, the separation performance of commercial polymeric membranes (10 kda, 30 kda polyethersulfone (PES) ultrafiltration membranes, Sepro Company) were also measured for comparison. As shown in Table 2, PES 10kDa and PES 30kDa membranes have a contrast trend when filtrating RB and EB solutions. The paradoxical result could be attributed to the 13

14 Page 14 of 32 charge interaction and sieving. In solution, the RB is positive charged, while the EB is negative charged. 47 The zeta potential of the PES membrane surface is approximately -20 mv. For PES 10kDa membrane, the negative charged PES membrane absorb the positive charged RB. The absorbed RB clog water transport pathway, and decreases the water flux. Besides, the molecular weight of the RB (479 g/mol) is smaller than that of the EB (960.1 g/mol). And the smaller RB could easily pass through the clogged pores. For PES 30 kda membrane, the pore size is larger enough, thus the charge interaction make little contribution to rejection rate. And rejection rate could mainly be attributed to sieving. Overall, the g-c 3 N 4 -PAA 0.1 hybrid membrane showed both higher water permeance and higher rejection than those of commercial membranes under the same conditions. Conclusions The g-c 3 N 4 -PAA hybrid membranes were successfully prepared. The introduced PAA combined with the g-c 3 N 4 nanosheets through H-bonding interactions between the NH groups of the g-c 3 N 4 nanosheets and the COOH groups of PAA, as demonstrated by FT-IR spectroscopy. SEM results confirmed that PAA was successfully inserted into the g-c 3 N 4 membrane. The g-c 3 N 4 -PAA 0.1 hybrid membrane exhibited a water flux of 117 L m -2 h -1 and rejection rate of 83% for EB molecules. The water flux was enhanced by 147% without considerably sacrificing the rejection rate and antifouling properties compared with that of the g-c 3 N 4 membrane. Moreover, the g-c 3 N 4 -PAA 0.1 hybrid membrane also showed better separation performance than commercial membranes. An increased addition of PAA 14

15 Page 15 of 32 led to an enhancement of the surface hydrophilicity and water flux. However, over-loading PAA (> 25 wt%) resulted in larger nanochannels, which decreased the retention performance and antifouling properties. Overall, the addition of an appropriate amount of PAA (low cost and uniform size) effectively improved the water flux of the g-c 3 N 4 membrane, which will promote the application of 2D-based membranes in the water purification industry. Acknowledgments We gratefully acknowledge the funding from the Natural Science Foundation of China ( , , ), the Natural Science Foundation of the Guangdong Province (2014A , 2017A ), China Postdoctoral Science Foundation (No. 2017M612665) and Fundamental Research Funds for the Central Universities (No. 2017BQ016). Literature Cited 1. Fane AG, Wang R, Hu MX. Synthetic membranes for water purification: status and future. Angew Chem Int Ed. 2015;54: Su J, Ong RC, Wang P, Chung TS, Helmer BJ, de Wit JS. Advanced FO membranes from newly synthesized CAP polymer for wastewater reclamation through an integrated FO-MD hybrid system. AlChE J. 2013;59: Werber JR, Osuji CO, Elimelech M. Materials for next-generation desalination and water purification membranes. Nat Rev Mater. 2016;1:

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22 Page 22 of 32 Figure Captions Figure 1. (a) Schematic illustration of the separation process of the g-c 3 N 4 -PAA hybrid membrane. (b) Molecular structure of PAA and the interaction between g-c 3 N 4 and PAA. Figure 2. (a) SEM image of g-c 3 N 4 after thermal oxidation etching. (b) XRD pattern of the g-c 3 N 4 nanosheets after thermal oxidation etching. (c) AFM image of the g-c 3 N 4 nanosheets. (d) Corresponding height curve of the g-c 3 N 4 nanosheets along the yellow line in Figure 2c. Figure 3. FT-IR spectra of the g-c 3 N 4 nanosheets, PAA and g-c 3 N 4 -PAA in the frequency ranges of (a) cm -1 and (b) cm -1 (magnified view). Figure 4. Effect of the PAA content in the g-c 3 N 4 -PAA x hybrid membranes on (a) the pure water flux and (b) the rejection of EB molecules. Figure 5. Relative water flux profiles of the g-c 3 N 4 -PAA x hybrid membranes during the filtration of pure water and BSA solution (the points at 0 minute and at 65 minutes are the permeate flux of pure water). Figure 6. Thickness-dependent separation performance of the g-c 3 N 4 -PAA 0.1 hybrid membrane. Figure 7. (a) Separation performance as a function of the rejected molecular weight. (b) The relative flux through the g-c 3 N 4 -PAA 0.1 hybrid membrane under different ph (the water flux under ph=7 is normalized as 1). Table 1. Comparison of FRR, R t, R r, R ir of g-c 3 N 4 and g-c 3 N 4 -PAA x hybrid membranes under BSA solution fouling tests.

23 Page 23 of 32 Table 2. Separation performance of different membranes.

24 Page 24 of 32 Figure 1. (a) Schematic illustration of the separation process of the g-c 3 N 4 -PAA hybrid membrane. (b) Molecular structure of PAA and the interaction between g-c 3 N 4 and PAA.

25 Page 25 of 32 Figure 2. (a) SEM image of g-c 3 N 4 after thermal oxidation etching. (b) XRD pattern of the g-c 3 N 4 nanosheets after thermal oxidation etching. (c) AFM image of the g-c 3 N 4 nanosheets. (d) Corresponding height curve of the g-c 3 N 4 nanosheets along the yellow line in Figure 2c.

26 Page 26 of 32 Figure 3. FT-IR spectra of the g-c 3 N 4 nanosheets, PAA and g-c 3 N 4 -PAA in the frequency ranges of (a) cm -1 and (b) cm -1 (magnified view).

27 Page 27 of 32 Figure 4. Effect of the PAA content in the g-c 3 N 4 -PAA x hybrid membranes on (a) the pure water flux and (b) the rejection rate of EB molecules.

28 Page 28 of 32 Figure 5. Relative water flux profiles of the g-c 3 N 4 -PAA x hybrid membranes during the filtration of pure water and BSA solution (the points at 0 minute and at 65 minutes are the permeate flux of pure water).

29 Page 29 of 32 Figure 6. Thickness-dependent separation performance of the g-c 3 N 4 -PAA 0.1 hybrid membrane.

30 Page 30 of 32 Figure 7. (a) Separation performance as a function of the rejected molecular weight. (b) The relative flux through the g-c 3 N 4 -PAA 0.1 hybrid membrane under different ph (the water flux under ph=7 is normalized as 1).

31 Page 31 of 32 Table 1. Comparison of FRR, R t, R r, R ir of g-c 3 N 4 and g-c 3 N 4 -PAA x hybrid membranes under BSA solution fouling tests. Membrane FRR (%) R t (%) R r (%) R ir (%) g-c 3 N g-c 3 N 4 -PAA g-c 3 N 4 -PAA g-c 3 N 4 -PAA

32 Page 32 of 32 Table 2. Separation performance of different membranes. g-c 3 N 4 -PAA 0.1 (490 nm) PES (10 kda, 190 µm) PES (30 kda, 190 µm) Molecule Permeance Rejection Permeance Rejection Permeance Rejection or ion (L m -2 h -1 bar -1 ) (%) (L m -2 h -1 bar -1 ) (%) (L m -2 h -1 bar -1 ) (%) RB EB BSA

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