GO Membranes Research Articles

Flake size-dependent water transport through graphene oxide membranes and rejection of geosmin (GSM) and 2-methylisoborneol (MIB) from drinking water

Water transport through graphene oxide membranes (GOMs) and the rejection of inorganic and organic solutes have been extensively studied. However, the influence of the lateral size of GO flakes on membrane performance remains unclear. Here, we investigate the water permeation and separation performance of GOMs fabricated using various sizes of flakes. The membranes prepared with larger flakes showed higher water flux. Our experiment clearly shows that GOMs consist of both lamellae and void structures. Monte Carlo simulations indicate that water transport through the voids is faster than that through the lamellae in GO membranes. Moreover, the voids are more predominant for the membranes prepared with larger sizes of GO flakes and, hence, higher water flux for larger flake membranes. Additionally, the GOMs prepared with the large flakes effectively rejected more than 98 % of geosmin (GSM) and 2-methylisoborneol (MIB) with high reproducibility and a stable water flux of 1.49 LMH. Our results contribute to a better understanding of the complex structure of GOMs, where the rejection performance of membranes mainly depends on the interlayer space, but the water transport is governed by the voids. Our study also demonstrated the industrial potential of GOMs in drinking water purification technology.

Silk fibroin-graphene oxide composite membranes for selective separation of ions and molecules from water

Nanofiltration membranes with controlled pores are useful in many fields, such as pharmaceuticals, desalination, petroleum, and food processing. Recently, graphene oxide has emerged as a promising candidate for the fabrication of highly efficient and pressure-resistant nanofiltration membranes with excellent water permeability and solute rejection. However, it remains difficult to achieve increased permeability while maintaining excellent separation ability. Herein, we report a novel silk fibroin-graphene oxide (FGO) composite membrane prepared by using fibroin as a cross-link reagent, which is extracted from natural Bombyx mori silkworm cocoons. Such a FGO membrane shows a pure water permeance of ∼280 L m−2 h−1 bar−1, which is five times greater than previously reported GO-based membranes. Further, such a membrane also exhibits excellent separation efficiency (> 99 %) for different feed molecules. Additionally, these membranes exhibit high chemical stability in water, acidic and basic solutions for several days compared to pristine GO membranes.

Discrimination of Xylene Isomers by Precisely Tuning the Interlayer Spacing of Reduced Graphene Oxide Membrane.

Separating xylene isomers is a challenging task due to their similar physical and chemical properties. In this study, we developed a molecular sieve incorporating a reduced graphene oxide (rGO) membrane for the precise differentiation of xylene isomers. We fabricated GO membranes using a vacuum filtration technique followed by thermal-induced reduction to produce rGO membranes with precisely controllable interlayer spacing. Notably, we could finely tune the interlayer spacing of the rGO membrane from 8.0 to 5.0 Å by simply varying the thermal reduction temperature. We investigated the reverse osmosis separation ability of the rGO membranes for xylene isomers and found that the rGO membrane with an interlayer spacing of 6.1 Å showed a high single component permeance of 0.17 and 0.04 L m-2 h-1 bar-1 for para- and ortho-xylene, respectively, exhibiting clear permselectivity. The separation factor reached 3.4 and 2.8 when 90:10 and 50:50 feed mixtures were used, respectively, with permeance 1 order of magnitude higher than that of current state-of-the-art reverse osmosis membranes. Additionally, the membrane showed negligible permeance and selectivity decay even after continuous operation for more than 5 days, suggesting commendable membrane resistance to solvent swelling and operating pressure.

Novel strategy to enhance water flux and stability of low swelling GO/NaY/Nylon membrane to adsorb ciprofloxacin from water

To improve the water flux and stability of GO membrane, GO/NaY/Nylon membrane was prepared, in which zeolite were used to increase the layer spacing of GO nanosheets. And the interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) was employed to improve the stability of membrane. The GO/NaY/Nylon membrane with high content of nanoparticles was first used for antibiotic adsorption. The static thermodynamics, kinetics and isotherm for ciprofloxacin (CIP) were studied. The adsorption mechanism on CIP is chemisorption, conforming to the pseudo-second-order model and Langmuir model, and as the equilibrium concentration (ce) increases, the maximum adsorption capacity of GO/NaY/Nylon membrane first increases rapidly, then reaches a plateau, with a maximum adsorption capacity of 49.2 mg·g−1 at 328 K (ce:45.1 mg·L-1). The GO/NaY/Nylon membrane showed higher water flux and stability than the GO membranes, indicating great promising in removing CIP from aqueous solution.

Tuning the interlayer spacing of graphene oxide membrane via surfactant intercalation for ultrafast nanofiltration

A facile method has been developed to fabricate lamellar composite graphene oxide (CGO) membranes by incorporating positively charged surfactants – cetyltrimethylammonium bromide (CTAB). The interlayer spacing of GO nanosheets was regulated by the sum of both vertical and horizontal alignment of CTAB on the GO surface via electrostatic interaction. The CTAB intercalated GO membrane exhibited an internal layer spacing of 16.97 Å compared to 8.26 Å for the GO membrane. The obtained CGO lamellar membrane with well-defined nanochannels showed ultrafast pure water flux of ∼1112 L h−1 m−2 bar−1 and an excellent separation efficiency of >99 % towards negatively charged organic dye molecules at a high permeation flux of ∼932 L h−1 m−2 bar−1, which was nearly 2.5-fold enhancement compared with that of the pristine GO membrane (∼400 L h−1 m−2 bar−1). The electrostatic and π–π interaction forces between the CGO membrane and organic dye molecules played a major role in the overall dye separation mechanism. Furthermore, the CGO membrane demonstrated excellent stability with no loss in separation performance (>96 % organic molecules rejection) after exposure to various pH solutions and deionized water (DI) water. The current work provides a straightforward approach for the formation of highly tunable and stable CTAB-intercalated GO membranes for ultrafast molecular separation.

Metal-organic framework-intercalated graphene oxide nanofiltration membranes for enhanced treatment of wastewater effluents

Graphene oxide membranes (GO) hold immense potential in the field of water purification. However, when applied directly to real wastewater effluents, pure GO membranes suffer from drawbacks such as fouling sensitivity and limited stability. To address these challenges and unlock the full potential of GO membranes, novel nanocomposite membranes have been developed by the intercalation of GO with nanoparticles of ZIF-8 (a type of zeolitic imidazolate framework). The prepared GO/ZIF-8 (GZ) nanocomposite membranes have exhibited enhanced hydrophilicity and exceptional water purification capabilities. Specifically, the GZ membranes have demonstrated a permeance enhancement of over two-fold when compared to the pristine GO reference membrane. This enhancement is coupled with anti-fouling performance and competitive rejection rates for both salts and organic pollutants. GZ membranes have been effectively employed for the purification by cross-flow filtration of 3 industrial wastewater effluents. They have shown improved separation performance compared to the pristine GO reference membrane, and high stability under cross-flow conditions. The origin of the high performances of the GZ membrane has been clarified using structural and morphological analyses. This work highlights the significant progress made in the field of water treatment using graphene-based membranes.

Role of tea polyphenols in enhancing the performance, sustainability, and catalytic cleaning capability of membrane separation for water-soluble pollutant removal

Tea polyphenols (TPs), like green tea polyphenol (GTP) and black tea polyphenol (BTP), with phenolic hydroxyl structures, form coordination and hydrogen bonds, making them effective for bridging inorganic catalysts and membranes. Here, TPs were employed as interface agents for the preparation of TPs-modified needle-clustered NiCo-layered double hydroxide/graphene oxide membranes (NiCo-LDH-TPs/GO). The incorporation of porous guest material, NiCo-LDH-TPs, facilitated water channel expansion, enhancing membrane permeability and resulting in the development of high-performance, sustainable catalytic cleaning membranes. The introduction of TPs through coordination weakened the surface electronegativity of NiCo-LDH, promoting a uniform mixed dispersion with GO and facilitating membrane self-assembly. NiCo-LDH-GTP/GO-5 and NiCo-LDH-BTP/GO-5 membranes demonstrated permeances of 85.98 and 90.76 L m−2 h−1 bar−1, respectively, with rejections of 98.73% and 99.54% for methylene blue (MB). Notably, the NiCo-LDH-BTP/GO-5 membrane maintained a high rejection of 97.11% even after 18 cycles in the catalytic cleaning process. Furthermore, the modification of GTP and BTP enhanced MB degradation through PMS activation, resulting in a 0.33% and 0.35% increase in the reaction rate constants of NiCo-LDH, respectively, while reducing metal ion spillover. These findings highlighted the potential of TPs in enhancing the efficiency and sustainability of catalytic cleaning GO membranes for water purification and separation processes.

Bioinspired cell membrane-based nanofiltration membranes with hierarchical pore channels for fast molecular separation

Owing to intrinsic perm-selectivity, cell membranes have attracted considerable attention in the field of biomimetic membranes. Herein, largest possible cell membranes of E.coli were prepared and then used as two-dimensional (2D) materials to fabricate high-performance nanofiltration (NF) membranes. The structure parameters of the selective layer of the E.coli-NF membrane could be finely tailored through the loading mass of cell membranes and the applied pressure during vacuum filtration. The E.coli-NF membrane exhibited a high rejection towards dyes (98.5%, brilliant blue R), antibiotics (91.1%, erythromycin), and divalent ions (64.8%, sodium sulfate). The membrane effective pore size was found to be significantly smaller than the interlayer spacing because of the small intrinsic nanochannels inside the E.coli cell membranes, which is distinct from GO and MXene-based 2D membranes. Moreover, the membrane water permeance was 52.6 L m−2 h−1 bar−1, higher than that of most of the reported 2D materials-based NF membranes. The E.coli-NF membrane exhibited the typical separation behavior of the negatively charged NF membranes governed by Donnan effect and size exclusion. This work gives some insights into the fabrication of the selective membranes using cell membranes as 2D materials with intrinsic nanochannels and fruitful choices in nature.

Role of water in vapor permeation through graphene oxide membranes

This study demonstrates that GO membranes enable efficient separation of aqueous and alcoholic vapor mixtures and discusses the critical vapor-permeation mechanism.

High-performance asparagine-modified graphene oxide membranes for organic dyes and heavy metal ion separation

Asparagine-modified GO membranes are prepared to efficiently reject rhodamine B dye up to 99.9%, along with a water permeability of ∼1740 ± 10 L m−2 h−1 bar−1. Further, such membranes are stable in water up to 34 days.

Ionization‐engineered two‐dimensional confined channels in GO membranes for highly efficient OH− separation

AbstractOH− sieving membranes provide a valuable means for treating alkaline effluents but their development presents several challenges. In this study, the concept of ionization engineering is applied to two‐dimensional (2D) laminated membranes to facilitate OH− separation. This concept is demonstrated through the stacking of the self‐designed sulfonated graphene oxide (SGO) nanosheets to fabricate 2D membranes. The SGO membranes enhance synergistically the OH− dialysis coefficients and separation factors when treating simulated NaOH/Na2WO4 alkaline wastewater, compared with the performance of 2D GO membranes and commonly adopted polymeric cation exchange membranes. Furthermore, the separation factor achieved by the SGO membranes is considerably higher than that of most of the existing alkali recovery membranes. Results of molecular dynamics simulations indicate that the high efficiency of OH− separation in the 2D SGO confined channels is attributable to dehydration effects and intensified electrostatic repulsion. Overall, this research provides an alternative strategy for achieving rapid OH− sieving through the ionization of 2D confined channels.

Tailoring the interlayer channel structure of graphene oxide membrane with conjugated cationic dyes for butanol dehydration

For the design of high-performance GO membrane, the structure of intercalation material is a key consideration due to its decisive impacts on nanosheet assembly and mass transfer channel structure. Herein, conjugated cationic dyes were incorporated into GO interlayer channels. The large π-conjugated structure and delocalized positive charge can form π-π and electrostatic interactions with the non-oxidized region and negatively-charged group on GO nanosheet, respectively. Through interlayer cross-linking, the nanosheet orientation and interlayer bonding can be improved, conferring the membrane with higher anti-swelling capacity, more ordered channel structure, and controllably tailored interlayer d-spacing. The electron cloud density and planar structure of cationic dye molecules are evidenced to be critical in influencing membrane structure and separation performance. After incorporating conjugated cationic dyes, the separation factor for water/butanol separation can reach 5.3 times of that of pristine GO membrane, while the permeate flux also increases by 20%. This work provides a category of promising intercalation materials for GO membranes with good potential application in butanol dehydration.

Modificationof ceramic-supported graphene oxide composite membranes to dedicate them the dry state CO2 permselectivity

Graphene oxide composite membranes have revealed great potential for gas separation tasks. These membranes' structural integrity and stability and also separation performance considerably depend on their support characteristics. This work studies the effect of modification of the GO membranes’ support by coating them with some γ-alumina interlayers to obtain defect-free graphene oxide composite membranes by filtration method. Also, graphene oxide nanosheets were functionalized with ethylenediamine to improve their CO2 permselectivity via the promotion of the adsorption selectivity portion. The morphology and microstructure of the synthesized materials and the prepared membranes were characterized by FTIR, SEM and FESEM, AFM, XRD, DLS, and BET analysis. The results confirmed successful GO nanosheet preparation. CO2 and CH4 permeances through the prepared membranes were measured at 30 °C and a pressure range of 0.5–3 barg. Ideal CO2/CH4 selectivity and CO2 permeance of the functionalized graphene oxide coated on the modified support at 2.5 barg and 30 °C under the dry state, were inverted to 1.33 in favor of CO2 and increased by nearly three times, respectively, compared with those of graphene oxide composite membrane coated on the unmodified support.

Scalable aqueous-phase fabrication of reduced graphene oxide nanofiltration membranes by an integrated roll-to-roll (R2R) process

Economic fractionation of multicomponent biomass-derived streams is a key challenge in biobased fuels and chemicals production. Kraft black liquor (BL) is generated at ∼1 billion tons/yr globally from biomass pulping and contains ∼15 wt% total solids including lignin, hemicellulose fragments, and inorganics. Membrane-based BL concentration is attractive but challenged by low solute rejections and poor stability in BL, which combines alkaline pH (∼13), high dissolved solids content (15+ wt%) and high temperature (70–85 °C). Here we discuss the scaled fabrication and characterization of reduced graphene oxide (rGO) nanofiltration membranes by slot die coating on a roll-to-roll (R2R) with integrated vacuum filtration. We obtain high-quality 90 × 30 cm rGO membranes (referred to as “R2R-rGO membranes” in this work) with ∼100 nm average thickness supported on porous poly(ethersulfone) (PES) sheets, and an effective area of 2700 cm2. We present characterizations of their microstructure and uniformity as mapped over large areas by scanning electron microscopy (SEM), ellipsometry, adhesion test and X-ray photoelectron spectroscopy (XPS). The constructed ellipsometry model for the rGO membrane/PES substrate composite was successfully applied to 48 different locations of the R2R-rGO membranes. The R2R-rGO membranes showed >98% lignin rejection with a steady state flux of ∼12 LMH at 50 bar in a 15.7 wt% total solids (TS) kraft BL stream at 72 °C and 2–3 gal/min crossflow rates. Slot die coating on a R2R platform with integrated vacuum filtration can enable rapid fabrication of high-quality GO membranes at scale without organic solvents or volatile organic compounds.

Fundamental Understanding of Ultrathin, Highly Stable Self-Assembled Liquid Crystalline Graphene Oxide Membranes Leading to Precise Molecular Sieving through Non-equilibrium Molecular Dynamics

Self-assembled graphene oxide lyotropic liquid crystal (GO LLC) structures are mostly formed in aqueous medium; however, most GO derivatives are water insoluble, so processing GO LLCs in water poses a practical limitation. The use of polar aprotic solvent (like dimethyl sulfoxide) for the formation of GO LLC structures would be interesting, because it would allow incorporating additives, like photoinitiators or cross-linkers, or blending with polymers that are insoluble in water, which hence would expand its scope. The well-balanced electrostatic interaction between DMSO and GO can promote and stabilize the GO nanosheets' alignment even at lower concentrations. With this in mind, herein we report mechanically robust, chlorine-tolerant, self-assembled nanostructured GO membranes for precise molecular sieving. Small-angle X-ray scattering and polarized optical microscopy confirmed the alignment of the modified GO nanosheets in polar aprotic solvent, and the LLC structure was effectively preserved even after cross-linking under UV light. We found that the modified GO membranes exhibited considerably improved salt rejection for monovalent ions (99%) and water flux (120 LMH) as compared to the shear-aligned GO membrane, which is well supported by forward osmosis simulation studies. Additionally, our simulation studies indicated that water molecules traveled a longer path while permeating through the GO membrane compared to the GO LLC membrane. Consequently, salt ions permeate slowly across the GO LLC membrane, yielding higher salt rejection than the GO membrane. This begins to suggest strong electrostatic repulsion with the salt ions, causing higher salt rejection in the GO LLC membrane. We foresee that the ordered cross-linked GO sheets contributed to excellent mechanical stability under a high-pressure, cross-flow, chlorine environment. Overall, these membranes are easily scalable, exhibit good mechanical stability, and represent a breakthrough for the potential use of polymerized GO LLC membranes in practical water remediation applications.

The potential of microalgae Dunaliella salina to treat shrimp pond wastewater in a PAN/GO membrane bioreactor

Graphene has attracted a significant amount of attention because to its excellent mechanical, electrical, thermal, and optical characteristics. In this work, a membrane bioreactor with hollow fibre PAN/GO nanocomposite was studied for the treatment of Persian Gulf shrimp pond wastewater. Dunaliella salina microalgae have been used for better treatment and the formation of sludge mass in a shorter period of treatment in the MBR system. Additionally, GO nanoparticles were used in order to improve the hydrophilicity of the membranes. Various tests, such as Pure water permeate (PWP), X-ray diffraction spectroscopy (XRD), Atomic force microscopy (AFM), Dynamic light scattering (DLS), Contact angle (CA), Scanning electronic microscopy (SEM), Fourier transform infrared substances (FTIR) were used to characterize the synthesized membranes. To evaluate the treated wastewater, several factors were evaluated, including: TP, TN, TSS, NTU, BOD, COD, EC. The contact angle was reduced by the inclusion of GO nanoparticles from 53.8° for PAN-0 to 45.4° for PAN-3. The results of FTIR analysis confirmed the synthesis of GO and showed the formation of different deposits as fouling on the surface of the prepared membranes after MBR process. Also, the removal percentage of COD and BOD5 was over 90% for membranes with graphene oxide nanoparticles. The turbidity for all fabricated membranes were removed ∼98%. Also, very little fouling occurred in the membranes constructed with GO membranes and the maximum concentration of GO let to maximum performance regarding to the high potential of fouling control. In addition, the growth of Microalgae Dunaliella salina with shrimp wastewater was observed successfully. In conclusion, the finding of this work not only proposed a promising solution for controlling fouling in an MBR but also resulted in a benefit product, i.e. microalgae Dunaliella salina.

Sorption of Polar Sorbents into GO Powders and Membranes.

The comparative study of sorption of polar substances acetonitrile and water into powders and membranes (>10 μm thick) of modified Hummers (HGO) and Brodie (BGO) graphite oxides was performed using isopiestic method (IM) and differential scanning calorimetry (DSC). Additional sorption data were obtained for pyridine and 1-octanol. Sorption measurements were accompanied by conventional XRD and XPS control. Electron paramagnetic resonance (EPR) was additionally used to characterize ordering of the membranes. The impact on sorption of synthetic procedure (Brodie or Hummers), method of making membranes, chemical nature of the sorbent, and method of sorption was systematically examined. It was demonstrated that variations in synthetic procedures within both Hummers and Brodie methods did not lead to changes in the sorption properties of the corresponding powders. Sorption of acetonitrile and pyridine was reduced by approximately half when switching from powders to membranes at ambient temperature. DSC measurements at a lower temperature gave equal sorption of acetonitrile into HGO powder and membranes. Water has demonstrated unique sorption properties. Equal sorption of water was measured for HGO membranes and powders at T = 298 K and at T = 273 K. It was demonstrated that lowering the orientational alignment of the membranes led to the increase of sorption. In practice this could allow one to tune sorption/swelling and transport properties of the GO membranes directly by adjusting their internal ordering without the use of any composite materials.

Functionalized graphene oxide-based lamellar membranes for organic solvent nanofiltration applications.

In this study, two-dimensional graphene oxide-based novel membranes were fabricated by modifying the surface of graphene oxide nanosheets with six-armed poly(ethylene glycol) (PEG) at room conditions. The as-modified PEGylated graphene oxide (PGO) membranes with unique layered structures and large interlayer spacing (∼1.12 nm) were utilized for organic solvent nanofiltration applications. The as-prepared 350 nm-thick PGO membrane offers a superior separation (>99%) against evans blue, methylene blue and rhodamine B dyes along with high methanol permeance ∼ 155 ± 10 L m-2 h-1, which is 10-100 times high compared to pristine GO membranes. Additionally, these membranes are stable for up to 20 days in organic solvent. Hence the results suggested that the as-synthesized PGO membranes with superior separation efficiency for dye molecules in organic solvent can be used in future for organic solvent nanofiltration application.

Double-charged self-assembled rGO/g-C3N4 membrane prepared by “functional group substitution” for heavy metal ions rejection at low pressure

Heavy metals are still the critical pollutants in industrial wastewater and there is an urgent need for efficient and environmentally friendly treatment technologies. Reduced graphene oxide (rGO) is widely used for preparations of nanofiltration (NF) membranes but suffers from poor hydrophilicity and electronegativity. In this work, a double-charged rGO/g-C3N4-P membrane was prepared for removal of heavy metals at low pressure. Graphitic carbon nitride (g-C3N4) assisted reduction of GO membranes under ultraviolet (UV) irradiation, and the modification of functional groups with high polarity improved the hydrophilicity of membrane surface. The filtration performance for heavy metals of rGO/g-C3N4-P membrane was evaluated under low pressure (1–2 bar). The rejection rates of Cu2+, Cr3+, Mn2+, Cd2+, and Pb2+ by membranes reduced by UV for 18 h (rGO/g-C3N4–18-P membrane) reached 94.72 %, 98.05 %, 82.32 %, 88.2 % and 77.15 %, respectively. In the experiment of mixed simulated wastewater, the rejection rates of NO3− and SO42− both reached >95 %. Outstanding rejection rates were attributed to the interaction and the synergy effect of double-charged layers as well as steric effects. Meanwhile, the water flux of rGO/g-C3N4–18-P membrane was as high as 37.14–50.16 L m−2h−1bar−1 (1–2 bar). The high flux was due to the reduced degree of oxidation so that water molecules transported between GO nanochannels more smoothly and the transport path was shortened through the nanopores of g-C3N4. Obviously, flux and heavy metal rejection of rGO/g-C3N4–18-P membrane were simultaneously improved, breaking “trade-off” effect. Furthermore, rGO/g-C3N4–18-P membrane showed excellent antifouling ability and the potential for heavy metal wastewater filtration in comparison with other NF membranes reported in literature.

Effect of etching and reduction time on the structure, performance, and stability of GO-based membrane for water purification

Holey graphene oxide (HGO) and reduced HGO membranes (rHGO) can overcome high tortuosity and poor stability of GO-based membranes, respectively. In this study, HGO and rHGO membranes were fabricated to try to quantitatively elucidate the etching and reduction time on structure, stability and rejection performance for slat and pharmaceuticals. Water flux of HGO membrane linearly increased (Jv=2.07t + 1.02, R2=0.9931) and Na2SO4 rejection linearly decreased (RNa2SO4=−4.37t + 88.26, R2=0.9511), while the membranes stability substantially increased with etching time from 1 to 5 h (water flux variation from 22% to 5% and rejection ratio variation from 17% to 7%) under long-term vibration . Subsequently, water flux of rHGO membranes exponentially decreased (Jv=exp(2.89–0.12t + 0.0039t2), R2= 0.9996) with reduction time, while slightly improved salt rejection (RNa2SO4=exp(2.89–0.12t + 0.0039t2), R2= 0.9971) were observed. Effective length of actual pathways for GO, HGO4 and rHGO4–5 membranes calculated by the mass transfer-based model was 7.37 ± 0.47, 4.48 ± 0.68 and 7.23 ± 1.29 µm, respectively, which were much higher than that geometric thickness of the GO membranes. These results indicated that d-spacing, tortuosity and porosity should be comprehensively considered for membrane design and fabrication. With a series of saccharides as probes, average pore size slightly decreased for HGO4 (rp=0.36 nm, Sp=0.34) and rHGO4–5 (rp=0.33 nm, Sp=0.28) membrane compared with GO membranes (rp=0.38 nm, Sp=0.3), demonstrating that etching and following reduction marginally affected the pore size distribution. H2O2 etching increased both water permeability (A) and its selectivity (A/B) for three pharmaceuticals, while the thermal reduction substantially decreased the water flux. Moderate and low rejection ratios for the three pharmaceuticals also demonstrated the importance of trade-off between etching and reduction time and the urgent need for narrowing pore-size distribution for GO-based membranes for water purification.