Reduced Graphene Oxide Membranes Research Articles

Metal cation crosslinked, partially reduced graphene oxide membranes with enhanced stability for high salinity, produced water treatment by pervaporative separation

Graphene oxide (GO)-based membranes hold significant promise for applications ranging from energy storage to protective coatings, to saline water and produced water treatment, owing to their chemical stability and unique barrier properties achieving a high selectivity for water permeation. However, unmodified GO membranes are not stable when submerged in liquid water, creating challenges with their commercial utilization in aqueous filtration and pervaporation applications. To mitigate this, we develop an approach to modify GO membranes through a combination of low temperature thermal reduction and metal cation crosslinking. We demonstrate that Zn2+–rGO and Fe3+–rGO membranes had the highest permeation flux of 8.3 ± 1.5 l m−2 h−1 and 7.0 ± 0.4 l m−2 h−1, for saline water separation, respectively, when thermally reduced after metal cross-linking; These membranes maintained a high flux of 7.5 ± 0.7 l m−2 h−1, and 5.5 ± 0.3 l m−2 h−1 for produced water separation, respectively. All the membranes had a salt rejection higher than 99%. Fe3+ crosslinked membranes presented the highest organic solute rejections for produced water of 69%. Moreover, long term pervaporation testing was done for the Zn2+–rGO membrane for 12 h, and only a minor drop of 6% in permeation flux was observed, while Zn2+–GO had a drop of 24%. Both modifiers significantly enhanced the stability with Fe3+–rGO membranes displaying the highest mechanical abrasion resistance of 95% compared to non-reduced and non-crosslinked GO. Improved stability for all samples also led to higher selectivity to water over organic contaminants and only slightly reduced water flux across the membrane.

Distinct ion transport behavior between graphene oxide and UV-irradiated reduced graphene oxide membranes

Graphene oxide (GO) membranes are promising materials for achieving selective ion permeation in aqueous solutions. However, the swelling of GO laminates, caused by the affinity of water molecules for oxygen-containing groups (OCGs) on GO, significantly limits the practical prospects of GO membranes for selective ion transport and separation applications. Selectively removing OCGs from GO has been shown to decrease the interlayer distance between individual GO sheets, thereby improving the ability of laminate membranes to discriminate between different ions based on a size-exclusion mechanism. In this work, GO reduced through exposure to ultraviolet light (UV-rGO) is shown to display ion transport behavior that raises questions about size-exclusion as the dominant mechanism determining selective ion transport in GO membranes. Notably, smaller monovalent Li+ cations are found to permeate more slowly through UV-rGO membranes than larger divalent metal cations such as Ca2+ and Mg2+. This unexpected behavior results in a 3 – 4 fold improvement in the separation selectivity between these representative cations with UV-rGO compared to pristine GO membranes. Through a joint experimental and theoretical study, we identified the factors influencing selective ion transport through the nanochannels in UV-rGO membranes. We demonstrate that the selective removal of hydroxyl (–OH) groups from the basal planes of GO enhances the interactions of metal cations with functional groups located at the edges of GO, thereby resulting in a lower ratio of free Li+ in solution, rather than stationary Li+ adsorbed onto the surface GO, compared to Ca2+ cations. Based on these results, we propose a separation mechanism analogous to chromatography, which emphasizes the crucial role of different OCGs on GO surfaces in controlling selective ion transport through GO membranes.

Building ordered graphene membranes by intermediate layer assisted stacking method for promoted desalination

Laminar separation membranes stacked by graphene and its derivatives hold the merit of controlled mass transfer behaviors. However, precise ion separation is challenged by the disordered assembly of nanosheets on macroporous substrates with complicated surface properties. Herein, we prepare ultrathin ordered graphene membranes by an intermediate layer assisted stacking method. Nanoporous covalent organic framework (COF) nanosheets are first vacuum-filtered on macroporous substrates, followed by deposition and thermal reduction of graphene oxide nanosheets to deliver ultrathin reduced graphene oxide (rGO) layers. The COF-decorated substrates provide a hydrophilic surface with reduced pores and roughness, facilitating the formation of ultrathin rGO layers with ordered stacking patterns and reduced microporous defects. Consequently, the obtained rGO/COF composite membranes can reject ∼90% of Na2SO4, ∼50% of NaCl, and >98% of various organic dyes, which are higher than pure rGO membranes. Meanwhile, the rGO/COF composite membranes can maintain good stability in solutions with different pH values and salt concentrations, and can be long-term operated for at least 10 days. This work is expected to deepen the understanding of the structure of laminar membranes and provides new perspectives to design ordered laminar membranes.

Utilization of graphene and rGO membranes for water and wastewater treatments

Utilization of graphene and rGO membranes for water and wastewater treatments

Highly ion-selective graphene-oxide-based membranes for nanofluidic osmotic energy conversion

Graphene oxide (GO) membranes have shown great potential for water treatment and osmotic energy conversion. However, the inherent challenge with GO membranes lies in their instability in water. While reduction techniques can improve stability, they simultaneously compromise selective ion transport properties due to the loss of functional groups. Here, we report preassembly modified GO (GOTA) membranes by using tannic acid (TA) as modification reagent. It is found that the TA molecules not only act as reducing agent, enhancing the aqueous stability of the membrane by generating stacked graphitic sp2 domains but also serve as intercalating agent, improving the selective ion transport properties by creating localized areas with high charge density and enlarged interlayer spacing. The cation selective number of GOTA membranes is ∼0.94–0.89 for different salinity gradients, much better than the thermally reduced GO membranes. In energy conversion system, the output power density approaches ∼0.32 W m−2 for the artificial seawater and river water with energy conversion efficiency of 41.7%. Meanwhile, the GOTA membranes exhibit excellent structural stability in various solution environments. This work demonstrates the importance of nanofluidic structure in improving the performance of ion-selective membranes and provides insights into the construction of advanced osmotic energy conversion system.

Porous rGO/networked cellulose composite membranes: Towards enhanced nanofiltration performance of rGO-based membranes

Graphene-based materials have been widely used for the fabrication of superior separation membranes for water treatment and purification. In particular, reduced graphene oxide (rGO), which has better water stability than graphene oxide (GO), has been demonstrated to be a good building block. However, rGO laminates are characterized by narrow interlayer spacing leading to dense packing and consequently lower flux limiting their direct utilization for membrane development. In this study, we report the fabrication of networked cellulose (NC)/porous rGO composite membranes with enhanced separation performance. We observed a synergistic effect due to the incorporation of NC and the presence of nano-pores on the rGO sheets. Firstly, in addition to the enhanced hydrophilicity and mechanical stability, the presence of NC with networked fibers reduced the horizontal transport resistance within the interlayer channels due to the formation of voids and wrinkles. Secondly, in-plane pores created by H2O2 oxidation of GO effectively contributed to additional transport channels providing shortened transport length for water molecules. Results showed that composite membranes maintained stable long-term performance with a pure water permeance up to 25.1 ± 2.2 Lm−2h−1bar−1 and salt rejection of 70.2 %, 63.08 %, 17.13 % and 25.62 %, for Na2SO4, MgSO4, MgCl2 and NaCl respectively. Unlike many previously reported rGO membranes that rely on ultra-thin selective layers, this work demonstrates an effective strategy for fabricating superior rGO membranes for water treatment and desalination.

Enhancing the seawater desalination performance of multilayer reduced graphene oxide membranes by introducing in-plane nanopores: a molecular dynamics simulation study.

In this study, using MD simulation, the effect of creating in-plane nanopores in a reduced graphene oxide (rGO) membrane and the formation of a reduced nanoporous graphene oxide (rNPGO) membrane is proposed to increase salt rejection and water flux. To this end, the desalination performance of r1NPGO, r2NPGO and r4NPGO membranes, which have 1, 2 and 4 pore(s), respectively, with a diameter of 0.9 nm and the r1NPGO-3 nm membrane, which has 1 pore with an approximate diameter of 3.0 nm, was investigated and compared from a molecular point of view. The simulation results show that in the rNPGO membranes, by increasing the number of pores from 1 to 4, water flux increases by ∼6 times compared to the rGO membrane. Meanwhile, upon increasing the pore size from 0.9 to 3.0 nm, water flux is enhanced by ∼16 times compared to the rGO membrane. The simulation results also demonstrate that the rGO membrane has two paths for water penetration, which are called the interlayer pathway and in-slit pathway. Moreover, pores in the rNPGO membranes provide another additional path for water transfer by shortening the lateral size of the membranes. This path is referred to as the in-pore pathway. By increasing the size of the pore in the r1NPGO-3 nm membrane, the contribution of the in-pore pathway increases and plays an important role. Furthermore, the simulation results show that in all rGO and rNPGO membranes, the interlayer space acts as a barrier for ions. Therefore, complete salt rejection is observed. Interestingly, by increasing the pore size in the r1NPGO-3 nm membrane, this membrane still maintains complete salt rejection. The observed phenomenon can be a result of very high water flux in this membrane. By increasing water flux, the presence of water molecules around Na+ and Cl- ions decreases. As a result, the formation of Na+Cl- ionic clusters is strengthened in such a way that these clusters do not have the ability to pass through large pores of 3.0 nm.

Development in desalination and wastewater treatment: state of the art challenges, role of solar energy, and recommendations

Abstract Water purification is crucial to ensure the availability of safe and clean water for protecting human health and helping in a sustainable environment. This review explicitly provides an overview of the major developments related to seawater desalination and wastewater treatments as a unit. Anticipating these traditional independent facilities will operate together in the future mainly for economic benefits. The advances in seawater desalination are primarily focused on the production of membranes using different types of nanoparticles (CeO2, UiO-66, CS-NPs) and carboxylated multi-walled CNTs into a polyamide that provides high salt rejection (as high as 90%) with excellent selectivity and permeability. Further, the use of nanoporous reduced graphene oxide (rGO) membranes provided 27.7–62.6% desalination rates and up to 96.8% salt rejection, along with protection against membrane fouling. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) showed excellent rejection of divalent ions during desalination. It is anticipated that the use of solar energy for membrane and thermal desalination may be promising since it reduces the reliance on fossil energy and significantly minimizes greenhouse gas emissions (up to 94%). Solar energy can integrate with the reverse osmosis (RO) process bearing the cost of energy-intensive compressors, making the RO economically attractive.

Reduced graphene oxide membrane confined bimetallic organic framework boosts Fenton-like process to ultrafast contaminants degradation

Fenton-like processes generate reactive oxygen species (ROS) to achieve degradation of organic pollutants, however, practical water treatment processes suffer from low ROS utilization owing to its ultra-short lifetime and mass transfer limitations. To solve these problems, a nano-confined reaction space was designed to facilitate a fast mass transfer process for efficient removal of organic pollutants. Herein, the CuCo-based bimetallic organic framework (CuCo-MOF) was loaded in the water-transporting interlayer nanochannels of reduced graphene oxide (rGO) membrane. Membrane-confined CuCo-MOF with multiple exposed reaction sites activate peroxymonosulfate (PMS), generating ROS for ultrafast removal of oxytetracycline (OTC) at a water fluxes of 87.3 L·m−2·h−1. Notably, this membrane can continuously and stably degrade OTC (100 %) within 30 h. In addition, free radical capture experiments and EPR analysis indicated that the main ROS of the reaction system were singlet oxygen (1O2) and sulfate radical (SO4−). Based on Fukui index results and LC-MS analysis, reaction sites of OTC attacked by ROS were identified and it’s possible degradation pathways were proposed.

Oxygen vacancy in CoO/reduced graphene oxide composite for enhancing long-term effectiveness of photocatalytic CO2 reduction via mediating exciton

Photocatalytic reduction of carbon dioxide (CO2) has been expected to be an effective way to reduce carbon emissions. Designing photocatalytic materials with long-term effectiveness is the key of photocatalytic technology. In this work, CoO nanoparticles loaded on the surface of reduced graphene oxide (rGO) membranes on silicon substrate were in-situ fabricated by one-step method. The resulting materials can convert CO2 into carbon monoxide (CO) up to 70 h at a steady rate of ∼185 ± 30 µmol g−1 h−1 with a selectivity of nearly 100%. This material system contained rich oxygen vacancies and generated new oxygen vacancies during the photocatalytic process. Oxygen vacancies mediate the interactions with excitons: (i) promoting the dissociation of free excitons; (ii) leading to form bound excitons under the coupling effect with phonons, inhibiting the recombination of photogenerated electrons and holes as well as enhancing the long-term effectiveness of photocatalytic CO2 reduction. We hope this work can provide valuable insights for the design and optimization of photocatalytic materials.

Facilitated water transport in composite reduced graphene oxide pervaporation membranes for ethanol upgrading

High purity ethanol is one of the most sought-after renewable energy sources. However, standard production methods yield ethanol of insufficient quality. Membrane processes such as pervaporation are recognized as a viable method for upgrading ethanol. Their performance and selectivity depend solely on membrane employed. Hydrophilic polyvinyl alcohol (PVA) membranes are used industrially for this purpose, but there is a trade-off between selectivity and permeability. Among other materials, chemically converted graphene attracts particular attention due to its exceptional water transport properties, however its application is limited by the fabrication of free-standing membranes. In this study, a composite reduced PVA/graphene oxide (rGO) membranes with different rGO content (up to 49 wt%) was synthesized. Polyvinyl alcohol acted as a mediator to improve the mechanical stability of membrane layers by crosslinking rGO flakes with hydrogen bonds. The resulting membranes were fully characterized by scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, water contact angle and mechanical tests. Pervaporation tests with water/ethanol mixtures (10/90 wt%) at temperatures between 20 and 50 °C demonstrated an excellent selectivity (over 12000) of membranes and satisfactory flux, even at high temperatures. The total permeate flux for membranes varied slightly as a function of operating temperature, demonstrating a good thermostability of the reduced graphene oxide-based membranes. The pervaporation separation index (PSI) of synthesized membrane exceed 5000 and surpassed majority of rGO containing membranes reported in the literature. Results indicate that rGO membranes noncovalently strengthened with PVA are a promising material for selective ethanol dehydration via pervaporation.

Thermally Reduced Graphene Oxide Membranes From Local Kazakhstan Graphite “Ognevsky”

AbstractThe properties of pure graphene and reduced graphene oxide (RGO) are close, so thermal annealing is a simple, safe and more cost‐effective alternative, in comparison to other methods to produce graphene oxide (GO) on a significant scale. In this work, for the first time, for the production of GO and its membrane with subsequent thermal reduction, local Kazakhstan “Ognevsky” graphite was used as the initial raw material. The GO membrane was obtained by vacuum filtration and subjected to thermal annealing at various temperatures in a hydrogen atmosphere. It was noticed, that after the thermal recovery of GO membranes, their physical and chemical properties changed. This changes show the possibility of using GO new membranes as a potential material for sensitive elements, such kind of gas and humidity sensors. In this regard, GO and RGO membranes were studied by: SEM, EDX, FTIR, XRD, TGA and Raman spectroscopy. This work demonstrates a new and efficient strategy for manufacturing high performance RGO membranes from local raw materials.

Radioactive strontium ions sieving through reduced graphene oxide membrane

Partitioning of 90Sr from other radionuclides in radioactive liquid waste is crucial for treating high-level liquid waste (HLLW) generated during spent fuel reprocessing, as the unique high heat-generating and long half-life of 90Sr pose a significant challenge for the long-term storage of nuclear waste. However, it remains difficult to sieve 90Sr for membrane based on size-selective effect, as the radioactive liquid waste contains numerous typical divalent ions, such as 60Co, 63Ni, and 65Zn, which have the same valence and similar hydration diameters to 90Sr. Here, we experimentally demonstrate a rapid and efficient separation of radioactive 90Sr from other radioactive ions by a reduced graphene oxide membrane, which prepared by the amino-hydrothermal method from graphene oxide suspensions. For SrCl2/CoCl2, SrCl2/NiCl2, and SrCl2/ZnCl2 binary mixed solutions, SrCl2 can pass through efficiently, while the rejection rates for CoCl2, NiCl2, and ZnCl2 reached 99.9 %. The separation factors reached 3232, 4254, and 3314, which are about one or two orders of magnitude higher than the highest reported separation factors for membranes sieving divalent ions. Our findings offer a simple avenue for the precise treatment of radioactive liquid waste, which is vital for the advancement of nuclear energy technology.

Solvent Effect on the Nanotextural Formation of Reduced Graphene Oxide Membranes.

Solvent is involved in many wet-chemical synthesis and bottom-up assembly processes. Understanding its influence on the nanotextural formation of the resultant assemblies is essential for the design and control of the properties for targeted applications. With wet chemically reduced graphene oxide (rGO) membranes as a materials platform, this study investigates the solvent effect on nanotexture formation in 2D nanomaterial-based membranes through light scattering and electrochemical characterization. Our finding indicates that the nanotexture of the resultant rGO membrane is largely correlated to the dielectric constant of the solvent. Specifically, solvents with higher dielectric constants yield rGO membranes with more wrinkled, loosely stacked, and less graphitized structures. In contrast, solvents with a lower dielectric constant tend to yield densely stacked structures with larger graphitized domains. Our finding underscores the important role of solvents in wet processing and nanoengineering of 2D nanomaterial-based membranes and provides valuable insights for their controlled synthesis and application.

Tunable nano-wrinked channels of reduced graphene oxide membranes for molecular sieving gas separation

Graphene oxide (GO) lamellar membranes have exhibited excellent performance in watertreatment, however there has been far less progress in the application of gas purification. Actually, exposure of the underlying graphitic framework of GO basically determines the gas permeableness of the GO lamellar membranes, which is usually neglected in the fabrication of GO lamellar membranes. Herein, we report nano-wrinkled lamellar reduced GO (rGO) sheets as gas separation membranes with oxygen-rich domains as face-to-face spacer for CO2 blockage and oxygen-poor domains for fast H2 permeation. Thanks to the excellent size-sieving mechanism of the nano-wrinkled channels, the rGO membranes exhibit high H2 permeance of 6.12 × 10−7 mol·m−2·s−1·pa−1 with exceptional selectivity of 529.9 for H2/CO2 gas mixture, surpassing the state-of-the-art ones. The membranes also show excellent stability in separation performance for H2/CO2 gas mixture under a long-term operation. Therefore, such lamellar membranes with well-tuned reduced GO nanosheets hold great potential in gas separation applications.

Chemical modification of reduced graphene oxide membranes: Enhanced desalination performance and structural properties for forward osmosis

Graphene oxide (GO) nanosheets can offer a breakthrough into the development of novel and efficient laminated forward osmosis membranes, with high hydrophilicity and outstanding desalination performance. However, fabrication of desirable GO-based nanocomposite membrane which can overcome the on-going trade-off between forward osmosis (FO) performance and chemical/mechanical stability, remains to be challenging in practical application. In this study, polyethyleneimine (PEI) was used to reduce GO (rGO) and crosslink and surface modify membrane's GO sheets for the performance enhancement. The crosslinked rGO was then deposited on a hydrophilic nylon support layer (modified by polydopamine, pDA, for structural stability) using vacuum filtration method. The resulting PEI:rGO/pDA membrane showed excellent water flux (25.5 LMH), high sodium chloride (NaCl) salt rejection (98.9%), and low reverse solute flux (0.91 gMH) in lab-scale tests. The combination of crosslinking GO with PEI and modifying the support layer with polydopamine (pDA) enhanced the membranes hydrophilicity and structural stability. The incorporation of PEI into GO resulted in compacted nanochannels, improving molecular/ions separation. Moreover, the pDA coating enhanced desalination performance and reduced internal concentration polarisation effects, further improving the membrane durability and integrity through the formation of numerous binding sites for firmly anchoring with the PEI:rGO nanocomposite structure.

High-Yield Synthesis of Sodium Chlorides of Unconventional Stoichiometries.

Abnormal salt crystals with unconventional stoichiometries, such as Na2 Cl, Na3 Cl, K2 Cl, and CaCl crystals that have been explored in reduced graphene oxide membranes (rGOMs) or diamond anvil cells, hold great promise in applications due to their unique electronic, magnetic, and optical properties predicted in theory. However, the low content of these crystals, only <1% in rGOM, limits their research interest and utility in applications. Here, a high-yield synthesis of 2D abnormal crystals with unconventional stoichiometries is reported, which is achieved by applying negative potential on rGOM. A more than tenfold increase in the abnormal Na2 Cl crystals is obtained using a potential of -0.6V, resulting in an atomic content of 13.4±4.7% for Na on rGOM. Direct observations by transmission electron microscopy and piezoresponse force microscopy demonstrates a unique piezoelectric behavior arising from 2D Na2 Cl crystals with square structure. The output voltage increases from 0 to ≈180mV in the broad 0-150° bending angle regime, which meets the voltage requirement of most nanodevices in realistic applications. Density functional theory calculations reveal that the applied negative potential of the graphene surface can strengthen the effect of the Na+ -π interaction and reduce the electrostatic repulsion between cations, making more Na2 Cl crystals formed.

Effects of graphene oxide membrane thickness reduction on microstructure and crossflow separation performance in kraft black liquor dewatering

Reduced graphene oxide (rGO) membranes are emerging as viable candidates for energy-efficient fractionation of multicomponent biomass streams, such as kraft black liquor (BL). Increased fluxes in rGO membranes under crossflow conditions – while maintaining high solute rejections – are highly desirable for improving the economics of such processes. We have conducted a detailed experimental study of a series of rGO membranes with progressively lower thicknesses (from 130 nm to 40 nm). The influence of the membrane thickness and the membrane microstructure is analyzed in terms of its effects on water permeation flux, divalent salt rejection, and molecular weight cutoff (MWCO) measurements using dye molecules. A selected rGO membrane (70 nm) showing both high fluxes and rejections was scaled up to a 650 cm2 sheet and used for extended crossflow operation under real BL streams for more than 22 days. Techno-economic calculations reveal significant positive effects of membrane thickness reduction on the economics of BL dewatering.

Thermally Reduced Graphene Oxide Membranes Revealed Selective Adsorption of Gold Ions from Mixed Ionic Solutions.

The recovery of gold from water is an important research area. Recent reports have highlighted the ultrahigh capacity and selective extraction of gold from electronic waste using reduced graphene oxide (rGO). Here, we made a further attempt with the thermal rGO membranes and found that the thermal rGO membranes also had a similarly high adsorption efficiency (1.79 g gold per gram of rGO membranes at 1000 ppm). Furthermore, we paid special attention to the detailed selectivity between Au3+ and other ions by rGO membranes. The maximum adsorption capacity for Au3+ ions was about 16 times that of Cu2+ ions and 10 times that of Fe3+ ions in a mixture solution with equal proportions of Au3+/Cu2+ and Au3+/Fe3+. In a mixed-ion solution containing Au3+:Cu2+:Na+:Fe3+:Mg2+ of printed circuit board (PCB), the mass of Au3+:Cu2+:Na+:Fe3+:Mg2+ in rGO membranes is four orders of magnitude higher than the initial mass ratio. A theoretical analysis indicates that this selectivity may be attributed to the difference in the adsorption energy between the metal ions and the rGO membrane. The results are conducive to the usage of rGO membranes as adsorbents for Au capture from secondary metal resources in the industrial sector.

Synthesis and characterization of graphene oxide and reduced graphene oxide membranes for water purification applications

Synthesis and characterization of graphene oxide and reduced graphene oxide membranes for water purification applications