RETRACTED ARTICLE: Surface modification of thin film composite forward osmosis membrane using graphene nanosheets for water desalination

In this study, the main motivation of this work is desalination of water for irrigation arid area such as Sidri- Baba basins- south Sinai, Egypt. Also, the novelty of this work is modification of TFC surface membrane by mix of HA, DA and GO to get high performance of FO technique. Interfacial polymerization was employed to modify a thin-film composite (TFC) membrane for forward osmosis (FO) applications; moreover, graphene oxide (GO) nanosheets (GONs), a dopamine solution (DA), and naturally accessible humic acid (HA) were modified on a polyethersulfone (PES) substrate. The effects of the different quantities of GO, HA, and DA on the membrane surfaces, as well as their various cross-sectional morphologies and FO-desalination capabilities, were investigated. The integrated TFC membrane containing appropriate GO, HA, and DA blends outperformed the control membrane, obtaining high water flux, and high salt rejection. Furthermore,.


R E T R
www.nature.com/scientificreports/ molecules transfer and more effective for salt ions rejection 19 . they present novel semi-permeable graphenebased membranes. Composite filters were designed and fabricated on polysulfone porous scaffolding using combinations of polycrystalline large-area High Strength Metallurgical Graphene (HSMG), graphene oxide, hydrazine and an in-situ interfacial polymerized polyamide. The prepared composites were proved to be semi-permeable membranes with great ions blocking efficiency (over 95%) and water flux only one order of magnitude lower than the commercial reverse osmosis membranes. The experiments' results demonstrated that the solutions proposed in this work indicate that graphene-based membranes can be used in water treatment technology 20 . suggested that surface hydrophilicity, pore size of membranes and oil-water separation performances was greatly affected by membrane shape. Owing to many advantages of HF membranes, this type of membrane has great potential for commercial applications.
Humic compounds contain polymeric carbon polymers with oxygen functional groups, e.g., hydroxyl and carboxylic acids, ketone, and quinone groups that are naturally available 21,22 . Moreover, humic acid (HA), which is a macromolecular material, exhibits a high dispersion capacity. Additionally, HA is an environmentally friendly material 23,24 revealed that HA can improve the pore size distribution of PES membranes, thereby increasing their permeability to pure water. Moreover, studies have demonstrated that the introduction of HA into GO enhanced the molecular diffusivity of water and the permeability of the GO membrane 25 .
Humic substances, natural and easily accessed polymeric carbon materials, are composed by a skeleton of aromatic units, which are cross-linked by oxygen-containing functional groups such as hydroxyl, carboxylic, ketone and quinone groups 26,27 . Compared with other carbon materials like graphene oxide or carbon nanotube, humic acid (HA) is prevalent in environments and much cheaper. Besides, HA, as a macromolecular substance, exhibits great dispersion capacity with polyethersulfone in the casting solution. In addition, HA itself is an environmentally friendly material 28 . In our previous work, we surprisingly found that HA could alter pore size distribution and then substantially improve pure water permeability of polyvinylidene fluoride membrane after doping it into the substrate 29 . Moreover, it has been proven that the introduction of HA into graphene oxide enhanced the water molecular diffusivity and permeability of graphene oxide membrane 23 . Inspired by these findings, we hypothesized that HA would be a candidate in improving support layer performance and further strengthening the permeability of FO membrane.
Also, a recently developed technique to impart a hydrophilic character onto microfiltration, ultrafiltration, and forward osmosis membrane selective layers for enhanced fouling resistance to oil/water emulsions and protein mixtures was reported by [30][31][32][33] using polydopamine (PDA). PDA is a polymer with a chemistry similar to the adhesive secretions of mussels [34][35][36] It is formed from the spontaneous polymerization of dopamine in an alkaline aqueous solution. A subsequent study by Arena examined the first use of PDA modified membranes for osmotically driven membrane process. This was done through the application of PDA to TFC membrane support layer(s). Significant improvements in the water flux of PDA modified TFC FO membranes were observed 37 . Others, such as 12 . adopted this technique prior to synthesis of the membrane.
Furthermore, the chemistry of dopamine (DA) or polydopamine (PDA) offers a new path toward producing high-performance membranes 38 . designed a novel method for performing surface modification with DA and polyethylene (PE) to significantly enhance the permeability of the membranes. Employing the IP technique, single DA in an aqueous solution successfully increased the structural and chemical stabilities of the membrane 39 . Recently 40 , fabricated three types of PDA-modified substrates (coated on top, bottom, and dual-surface substrates) before synthesizing the TFC membranes 41 . prepared TFC-FO membranes on the top surface of PDAmodified PES substrates, and the thickness of the PA layer decreased while the salt rejection increased on the membrane substrates at a short PDA coating time.
Lu et al. 42 reported that the constructed novel membrane exhibited improved water flux, following the addition of DA into a 1,3-metaphenylenediamine (MPD) solution, although it exhibited increased reverse salt flux and PA layer thickness. The thickness of the layer increased compared with that of the control membrane. Thus, the impact of the different DA concentrations of the aqueous phase that was paired with MPD on the properties and performance of the FO membrane must be investigated.
In this study, different concentrations of GO and HA were added to a PES substrate, and different concentrations of DA were added into MPD to improve the permeability-selectivity properties of a TFC-FO membrane. Further, the best concentrations of GO, HA, and DA were compared. Thereafter, membranes were modified by combining GO, HA, and DA. Thus, this study revealed an effective and ecologically acceptable substance, which could be doped into the TFC-FO membrane substrate, to alter the physicochemical properties of the membrane and improve its permeability-selectivity characteristics.

Methods. Preparation of GONs.
GONs were synthesized from graphite powders via the classical Hummer method [43][44][45] . Briefly, 1.0 g of graphite and 0.5 g of NaNO 3 were added into a flask containing 23.0 mL of a concentrated H 2 SO 4 solution and stirred in a cold water bath. Afterward, 3.0 g of KMnO 4 was added gradually for 2.0 h before the mixture was transferred into a 35.0 °C water bath and stirred for 0.5 h. Thereafter, 46.0 mL of DI water was added gradually, and the reactant was kept for 0.5 h at a temperature of 98.0 °C. Finally, 140.0 mL of the DI water was added to the mixture and stirred. Also, the resulting mixture was heated for 1 h at 95 °C without allowing it to boil. Elsewhere, the heater was switched off, and the mixture was allowed to cool by adding 500 mL of DI water with stirring for 1 h. Next, 40.0 mL of an H 2 O 2 solution was added to the mixture to terminate the reaction. To obtain the final product, the mixture was filtered and rinsed three times with DI water, after which the as-prepared GONs were washed three times with DI and methanol, successively, to remove the residues. Finally, the powders were baked for 12 h at 60 °C.
Preparation of the PES substrate. To prepare the casting solution from PES, a mixed solvent system containing PVP ( Thereafter, the substrate was dipped in a TMC solution (0.15 wt.% in hexane) for 1 min . Next, the substrate was removed from the hexane natural section and set vertically for 2 min to evaporate the residual solution. Afterward, the as-prepared substrate was vacuum dried for 5 min at 70 °C. Finally, the obtained composite membranes were washed and stored in DI water (4 °C) for the subsequent experiments.
FO performance tests. The FO membrane comprising a strong membrane (42 cm 2 ) was utilized. The temperatures of the feed and draw solutions were set at 25 °C. The membranes were examined in the FO mode (i.e., the active layer (AL) was passed through the FS). To compare the overall FO performance of the thin-film membranes, 2 M NaCl and DI water were utilized as the draw and feed solutions, respectively. The concentration of the extracted salt in DS was measured. Computer software was employed to record the mass of the permeated water (m) into the DS. FO was allowed to proceed for ~ 2 h to ensure precise measurements. The obtained values were averaged from the calculated data that were collected after 10 min of operation and replicated Three times. The FO flux (Jw, Lm −2 h −1 ) was calculated employing the following equation, wherein Q is the amount of permeate accumulated in "L, " A is the high area of the surface of the membrane in "m 2 , " and ∆t is the sampling time.
The salt rejection that was retained in the membrane (R, %) was calculated, as follows: where Cd is the salt concentration of the DS after a given time, as obtained via the widespread curve technique employing a conductivity meter and Vd is the volume of the DS. Cf and Vf are the initial concentration and volume of the FS, respectively.
Antiorganic fouling performance evaluation. The anti-fouling performance of the FO membrane was evaluated by adopting sodium alginate as module pollutant. The feed solution was composed of 250 mg L − 1 sodium alginate, 0.45 mM KH 2 PO 4 , 9.2 mM NaCl, 0.61 mM MgCl 2 Because it is binary salts., 0.5 mM NaHCO 3 , 0.5 mM CaCl 2 and 0.93 mM NH 4 Cl 46 . 2 M NaCl was selected as the draw solution. The cumulative volume of draw solution was recorded every 2 min by a digital balance connected to a computer via a Hyperterminal Software. The entire anti-fouling evaluation process lasted 600 min.
Characterizations of the synthesized membranes. The membranes were characterized by Fouriertransform infrared (FTIR), scanning electron microscopy (SEM), and contact angle measurements. A Genesis Unicam spectrophotometer was employed for the FTIR test. The contact angle was measured by a Cam-Micro contact angle meter (Tantec Inc.) to estimate the hydrophilicity of the membrane. A water droplet (DI water) was dropped onto the surface of the air-dried membrane at 25 °C employing a numerical micro-syringe. The water contact corresponding to a median of five measurements was acquired for every membrane pattern at selected points. Ramírez et al. 50 Reported that the HA-changed PES substrate exhibited a new peak at 1486 cm −1 , and this could be ascribed to the deformation of O-H and C = O from alcoholic and phenolic -OH or -COO uneven stretching of HA 51 . Moreover, compared with the substrate, AL exhibited a high peak, which could be ascribed to the C-N stretching in amide II, at 1577 cm −1 , indicating the formation of an amide. A new peak at 1610 cm −1 originated from the relaxation of the aromatic ring (PA) 52 . According to 53 , the maxima of GONs at 3400 and 1322 cm −1 were due to O-H stretching and deformation, respectively. The C = O stretching vibrations within the carboxyl group of GO were evident around 1653 cm −1 , while the peak at 1011 cm −1 was caused by the C-C stretching of the epoxy and alkoxy groups. The peaks around 1669 and 1486 cm −1 were attributed to the C = O stretching of carbonyls and the deformation of the C-H bond, respectively. The peaks at 1322 and 1242 cm −1 corresponded to the stretching vibration of the C-H bond. The strongest band around 1072 cm −1 indicated the ether C-O-C functional group; concurrently, the bands at 872 and 1105 cm −1 indicated the modes of saccharide 54 . Moreover, the band revealed a large spectrum at 1577 cm −1 that was associated with the carbonyl (C = O) stretching vibration of the molecules in the membrane structure 55 . The peak corresponded to hydroxyl group was detected in HA, GO and DA samples in Fig. 1 but less visible in HA + GO + DA sample this is due to accumulation between HA, GO and DA.  Fig. 2. The contact angle is the most significant component for determining the hydrophilicity or hydrophobicity of membrane surfaces. Generally, a small contact angle (0° < θ < 90°) corresponds to the high hydrophilicity of the membrane, while a higher contact angle (90° < θ < 180°) corresponds to its high hydrophobicity. Figure 2 shows that the average contact angles of the control, HA/TFC, GO/TFC, DA/TFC, and HA, GO, DA/TFC-modified membranes were 69° ± 2°, 53.6° ± 1.2°, 57° ± 1.2°, 64.3° ± 1.6°, and 35.9° ± 6°, respectively, demonstrating that the membranes exhibited hydrophilic surfaces. The presence of the OH and COOH groups was due to the high hydrophilicity of the membrane. The DA/ TFC-modified membrane exhibited a lower contact angle around 64° at 25 °C, signifying that its hydrophilic surfaces improved with the incorporation of DA into the polymer casting mixture. The increased hydrophilicity of the DA-modified TFC membrane could be attributed to the stronger attraction of the H 2 O molecules in DA. Following the addition of DA, the contact angle was reduced, thereby increasing the surface energy of the membrane. This can allow the easy movement of H 2 O over the surface of the membrane to enhance the capacity of the hydrophilic pores to absorb H 2 O through their capillary properties. When 0.25 wt. GO/TFC membrane was added, the contact angle was reduced to 57°, increasing the hydrophilicity of the membrane by incorporating GO mainly as a pore-generating agent. Moreover, the contact angle was further reduced to 53°, following the incorporation of the TFC membrane with HA. These findings corroborated previous research on improving the hydrophilicity of PES and TFC films via the incorporation of GO and HA 56 . The incorporation of hydrophilic GO and HA slightly increased the hydrophilization of the membrane, as observed by a decrease in the contact angle. Further, the incorporation of the hydroxyl, carboxylate, and epoxy moieties would enhance the hydrophilicity of the HA, GO, and DA/TFC-modified membrane.
Analysis of the morphology of the membranes. SEM analysis was performed to examine the forms and dispersions of HA, DA, and GONs within the composite membranes. Figures 3a-f show the cross-sectional SEM images of the PES, control, HA/TFC, DA/TFC, GO/TFC, and HA + DA + GO/TFC-modified membranes, respectively.The prepared membranes exhibited an asymmetric structure comprising a porous support/sublayer with cellular morphologies and a thin dense skin layer. The membranes exhibited straight finger-like microvoids, which expanded and were bent toward the center and further elongated until they reached the bottom of the membrane, upon GO, HA, and DA incorporations. Further, the cellular pores were broadened, and the thin walls interacted. The microvoid structures of the composite GO, HA, and DA membranes expanded and the pores increased, following the addition of the hydrophilic materials owing to the instantaneous demixing of the membrane material in the solvent. Further, the thin layer of the composite PES membranes became thinner than that of the control PES membrane. The transportation of the water molecules across the membrane via narrow interspaces enhanced it significantly. Moreover, besides improving the permeation flux, a very hydrophilic membrane could also minimize fouling on the membrane surface 57 .

Results and discussions
Membrane desalination performance. Optimum conditions of the TFC control. Figure 4 shows the water fluxes of the FO membranes when different doses of MPD (0.5, 1, 2, and 3 g/100 mL) and TMC (0.05, 0.1, 0.15, and 0.2 g/100 mL) were utilized. Additionally, different times for preparing MPD (1, 2, and 3 min) and TMC (0.5, 1, and 2 min), as well as different membrane thicknesses (30,35, and 40 µ) were exploited to obtain the optimum condition for preparing the control membrane. The DS and FS modes employed 2.0 M NaCl and DI water as DS and FS, respectively. The water flows of the optimum TFC membrane were recorded at 2 g/100 mL MPD; the TMC concentration was 0.15 g/100 mL, and the MPD and TMC preparation times were 2 and 1 min, respectively.
Effect of the different concentration of HA. The pure water permeability of the TFC membrane is shown in Fig. 5. Compared with the control membrane substrate, the HA-modified membrane substrate exhibited essentially www.nature.com/scientificreports/ pure water flux. It was observed that employing 0.3 wt% HA achieved the best result. The water flux increased to ~ 27.26 L/m 2 .h, while the salt rejection reached 82%. This result is consistent with the findings that large macrovoids 58 , which rendered the membrane more hydrophilic, were formed after HA doping (Fig. 6). Based on these observations, we believed that the addition of HA to the mixture reduced the hydrophilic resistance and improved the permeability of the substrate. Moreover, additional COOH functional groups were added to the PA layer to achieve a more hydrophilic surface .
Effect of the different concentrations of GONs. Figure 6 depicts the water flows of the FO membranes that were doped with different concentrations of GONs employing 2.0 M NaCl as DS. It was observed that as the concentration of GONs increased, the water fluxes increased initially before eventually reaching an ideal value (0.25 wt%). Thus, the ideal concentration of GONs was set at 0.25 wt%. The water flux increased by ~ 16.43 L/m 2 .h, while the salt rejection reached 89% probably because of the improvement of the hydrophilic characteristics of www.nature.com/scientificreports/ GO-incorporated membranes (the higher the hydrophilic characteristics of the membranes, the easier it is for water molecules to move through them) [59][60][61] . Furthermore, as the thickness of the PA AL of the GO-incorporated membrane reduced, the FO water flux increased. When the concentration of GONs increased dramatically, the membrane water flux decreased.
Effects of the different concentrations of DA:. Figure 7 shows the different self-polymerizations of DA in the FO mode using DI water and 2 M NaCl as FS and DS, respectively. It was observed that 0.3 wt% is the ideal concen- www.nature.com/scientificreports/ DA. Comparing the water fluxes of the HA, GO, DA, and mixed membranes, it was discovered that the mixed membrane achieved the best results. The water flux increased to ~ 33.21 L/m 2 .h, while the salt rejection reached 98%. Improved water flux via mixed HA,DA,GO membrane may be owing to enhancing the hydrophilicity and resaons negatively charged reactive group on to the membrane surfaces 63 . After ceratine time , the mixed membrane content is aggregated which subsequently leads to some pores of the membrane surfaces that could be plugged possessing to the mixed membrane aggregation resulting in water flux decay 64 . It is concluded that membrane structure and morphology are greatest possibly diverse where the higher hydrophilic CA membrane may cause a further swollen and flexible skin layer, thus the membrane permeability is high.Thus, the mixed membrane was the best because of its high porosity and hydrophilicity.
Evaluation of the antiorganic fouling performance of the membrane. The membrane with mix of HA, DA and GO exhibited superiority in permeability-selectivity trade-off, as analyzed above. On the other hand, the antifouling ability of FO is another crucial aspect for its practical application. Thus, SA was selected as a model polysaccharide to examine the flux variations under 2 M NaCl draw solution in FO mode. As shown in Figs. 9, the flux exhibited a sharp decline from 0 to 200 min and a relative stable flux profile. Similar phenomenon was reported in previous studies 3, 38,65 . The sharp decrease in flux in the starting time could be ascribed to the bridging and gel forming of sodium alginate (SA) by Ca 2+ ions. Besides, the initial high permeation drag force also contributed to the sharp decline of flux, which promoted substantial hydraulic resistance and thus a severe flux decline. After 200 min, slow flux decline was observed, suggesting that further accumulation of SA became insignificant. This result could be explained by the classical "critical flux concept", which suggests that the fouling will become negligible once reaching critical flux 58 . At the end of fouling experiments, the water flux of mix HA,  www.nature.com/scientificreports/ DA and GO-doped membrane exhibited a slower water flux decline ratio than that of the others membranes, indicating that the mix HA, DA and GO -modified membrane exhibited a higher anti-organic fouling performance. Previous studies have demonstrated a more permeable support layer would transport more water across the membrane surface and induce a membrane surface with a less fouling propensity 66,67 . The mix HA, DA and GO -modified support layer exhibited higher water permeability and smoother surface, leading to a higher resistance to organic foulant. Therefore, a higher anti-fouling performance could be realized after modifying FO membrane with mix HA, DA and GO membrane.

Conclusion
In this work, we demonstrate that, after blending mix of GO, HA, and DA into the PES substrate of FO membrane, the porous structure and surface chemistry of membrane were altered, and then the pure water flux of substrate was further improved. The pore structure of the membrane with mix of GO, HA, and DA doped had a higher porosity than that of the others membranes. More OH functional groups were introduced onto the polyamide layer by embedding with mix of GO, HA, and DA, leading to a more hydrophilic surface. These results endowed the membrane a better permeability-selectivity property. In addition, the anti-fouling performance for the mix of GO, HA, and DA -modified membrane was superior over the others membranes. Furthermore, the performance of the mix of GO, HA, and DA doped membrane in this work was comparable to those commercial membranes in terms of permeability and selectivity. Thereby, our results suggest that mix of GO, HA, and DA could act as an efficient and cost-effective additive and would be used for membrane fabrication.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.