Novel “loose” GO/MoS2 composites membranes with enhanced permeability for effective salts and dyes rejection at low pressure

Few-layer MoS2 co-assembly with GO to optimize defect channels and stability of GO membranes for high-performance organic-inorganic separation

The selective separation of organic compounds and inorganic salts is essential for wastewater recycling in fine chemical industries such as pharmaceuticals and pesticides. Membrane separation technology offers a promising solution. However, conventional organic membranes often face challenges related to precise separation and solvent resistance. While graphene oxide (GO) membranes exhibit excellent solvent resistance, their separation performance and structural stability require further improvement. In this study, we developed a GO/few-layer molybdenum disulfide (FLMoS2) membrane via co-assembly. The optimized GO/FLMoS2 membrane demonstrated a water permeability of 28.4 LMH/bar, approximately four times higher than conventional GO membranes, and achieved a separation factor exceeding 900 for organic/inorganic mixtures-among the highest reported for two-dimensional (2D) membranes. Comprehensive characterization, including low-field nuclear magnetic resonance (LF-NMR), revealed that this superior performance was attributed to controlled defect channels, enhanced interlayer cross-linking, and the intrinsic rigidity of FLMoS2, which provided high structural stability and minimal swelling. Moreover, mechanical strength assessments, including critical destructive load force and nanoindentation tests, confirmed significant improvement in structural robustness. As a result, the GO/FLMoS2 membrane maintained stable water permeability and separation efficiency over 100 hours of continuous operation and six chemical cleaning cycles, demonstrating its potential for sustainable wastewater treatment and resource recovery.

Sodium alginate-based nanocomposite hydrogel membrane for removal of heavy metal ions and dyes in water

Membrane filtration technology plays a crucial role in the field of water treatment because of its efficient separation and purification capability. However, it remains a major challenge to develop reusable membranes with high retention, stable filtration to remove heavy metal ions and dyes from water in the field. In this study, sodium alginate (SA), cellulose nanofibers (CNF) and metal-organic frame UiO-66 were innovatively used as raw materials to prepare SA/CNF/UiO-66 dual-network composite hydrogel membrane by unique cooking method. UiO-66 was homogeneously distributed within the SA matrix with the help of CNF, thereby achieving a synergistic interaction among the three components. As a result, the SA/CNF/UiO-66 composite hydrogel filter membrane not only showed excellent water flux and excellent mechanical properties, but also possessed excellent performance in improving the filtration efficiency and long-term filtration stability of heavy metal ions and dyes. The optimal removal rates of 200 mg/L Pb2+, Cu2+ and Cd2+ by SA/CNF/UiO-66 composite hydrogel membranes were as high as 99.9 %, 98.5 % and 96.5 %, respectively. At the same time, the filter membrane could maintain a flux of up to 71 Lm-2 h-1 and a permeability of 17.8 Lm-2h-1bar-1 at a working pressure of 0.4 MPa. The best removal rate for high concentration Congo red dyes could also reach >99.8 %. This study not only provides a new idea for the development of high-performance water treatment membranes, but also provides a practical and effective technical scheme for solving the problem of water resources pollution.

MXene Membranes Inserted with Tannic Acid Etched MOF Nanocrystals for Ultrafast Water Permeation: Elucidating the Water Transport Mechanism in Nanoconfined Interlaminar Channels

Utilizing pore and interlayer engineering within nanoconfined interlaminar channels represents an ingenious approach to design highly permselective MXene (Ti3C2TX) membranes. Herein, the tannic acid (TA) etched ZIF-8 (TZIF-8) nanocrystals with hollow structures were effectually inserted into the interlayer spacing of MXene membranes. First, the density functional theory (DFT) results demonstrated the reaction mechanism between TA and ZIF-8. Then, the underlying mechanism of enhanced water-adsorptive properties for MXene/TZIF-8 membrane was due to the higher binding energy of water/TZIF-8 system than that of water/ZIF-8 system, elucidated by molecular dynamic simulation. Furthermore, the low mass transfer resistance and abundant mass transfer pathways of the MXene/TZIF-8 membrane were comprehensively proved by various experimental conclusions, characterizations and simulation calculations. As a result, the optimal MXene/TZIF-8 membrane exhibited high water permeance and concurrently satisfactory separation efficacy toward various oil/water emulsions. This work is anticipated to deepen the comprehension of high-efficiency water transport along interbedded nanochannels in MXene membranes.

Large-Area Clay Composite Membranes with Enhanced Permeability for Efficient Dye/Salt Separation

The escalating discharge of textile wastewater with plenty of dye and salt has resulted in serious environmental risks. Membranes assembled from two-dimensional (2D) nanomaterials with many tunable interlayer spacings are promising materials for dye/salt separation. However, the narrow layer spacing and tortuous interlayer transport channels of 2D-material-based membranes limit the processing capacity and the permeability of small salt ions for efficient dye/salt separation. In this work, a novel sepiolite/vermiculite membrane was fabricated using Meyer rod-coating and naturally occurring clay. The intercalation of sepiolite Nanofibers between vermiculite Nanosheets provides additional transport nanochannels and forms looser permeable networks, producing composite membranes with remarkably enhanced flux. As a result, the optimized membranes with 80% sepiolite exhibit remarkable flux as high as 78.12 LMH bar-1, outstanding dye rejection (Congo Red~98.26%), and excellent selectivity of dye/salt of 10.41. In addition, this novel all-clay composite membrane demonstrates stable separation performance under acidity, alkalinity and prolonged operation conditions. The large-scale sepiolite/vermiculite membranes made by the simple proposed method using low-cost materials provide new strategies for efficient and environmentally-friendly dye/salt separation.

Photocatalytically self-cleaning graphene oxide nanofiltration membranes reinforced with bismuth oxybromide for high-performance water purification

Graphene oxide (GO) membranes have emerged as promising candidates for water purification applications, owing to their unique physicochemical attributes. Nevertheless, the trade-off between permeability and selectivity, coupled with their vulnerability to membrane fouling, poses significant challenges to their widespread industrial deployment. In this study, we introduce an innovative in-situ growth and layer-by-layer assembly technique for fabricating multilayer GO membranes reinforced with bismuth oxybromide (BiOBr) on commonly employed Nylon substrates. This method allows for the creation of two-dimensional lamellar membranes capable of photocatalytic self-cleaning and tunable nanochannel dimensions. The synthesized GO/BiOBr composite membranes exhibit remarkable water permeance rates (approximately 493.9 LMH/bar) and high molecular rejection efficiency (>99 % for Victoria Blue B and Congo Red dyes). Notably, these membranes showcase an enhanced photocatalytic self-cleaning performance upon exposure to visible light. Our work provides a viable route for the fabrication of functionalized GO-based nanofiltration membranes with BiOBr inclusions, offering a synergistic combination of high water permeability, modifiable nanochannels, and effective self-cleaning capabilities through photocatalysis.

Recent advances in regulation methods for selective separation and precise control of two-dimensional (2D) lamellar membranes

Selective separation and precise control of the structure and surface characterization for two-dimensional (2D) membranes is the key technology that needs to be revealed for further development of the material in practical application. Current researches focus on the cross-linking and modification of single nanosheet to improve and manipulate the performance of 2D lamellar membranes. In this paper, the selectivity principles such as size exclusion, charge properties, and surface chemical affinity in the separation process of 2D membranes were comprehensively and systematically reviewed, as well as the preparation of hybrid membranes combining the advantages of various raw materials. We also analyzed the practical application of the separation principles in relevant researches and discussed the development directions of 2D membranes. These inductions have certain summary and guiding significance for the selective regulation and goal-oriented design of 2D membranes.

Polyethyleneimine Modified Two-Dimensional GO/MXene Composite Membranes with Enhanced Mg2+/Li+ Separation Performance for Salt Lake Brine

As global demand for renewable energy and electric vehicles increases, the need for lithium has surged significantly. Extracting lithium from salt lake brine has become a cutting-edge technology in lithium resource production. In this study, two-dimensional (2D) GO/MXene composite membranes were fabricated using pressure-assisted filtration with a polyethyleneimine (PEI) coating, resulting in positively charged PEI-GO/MXene membranes. These innovative membranes, taking advantage of the synergistic effects of interlayer channel sieving and the Donnan effect, demonstrated excellent performance in Mg2+/Li+ separation with a mass ratio of 20 (Mg2+ rejection = 85.3%, Li+ rejection = 16.7%, SLi,Mg = 5.7) in simulated saline lake brine. Testing on actual salt lake brine in Tibet, China, confirmed the composite membrane's potential for effective Mg2+/Li+ separation. In the actual brine test with high concentration, Mg2+/Li+ after membrane separation is 2.2, which indicates that the membrane can significantly reduce the concentration of Mg2+ in the brine. Additionally, the PEI-GO/MXene composite membrane demonstrated strong anti-swelling properties and effective divalent ion rejection. This research presents an innovative approach to advance the development of 2D membranes for the selective removal of Mg2+ and Li+ from salt lake brine.

Construction of PDA-PEI/ZIF-L@PE tight ultra-filtration (TUF) membranes on porous polyethylene (PE) substrates for efficient dye/salt separation

Tight ultra-filtration (TUF) membranes were constructed by in situ growing zinc imidazole frameworks micro-crystalline leaves (ZIF-L) in polyethylene imine (PEI) and polydopamine (PDA) deposit layers on porous polyethylene (PE) substrates. The effects of preparation conditions on the surface physical and chemical structures as well as on the dye/salt separation performance of the formed TUF membranes were systematically investigated. By inserting selective water permeation channels and increasing contacting surface areas, in situ-grown ZIF-L arrays tightly cross-linked in the coating matrix greatly increased water permeation without trading off dye/salt retention selectivity. The morphology of the included ZIF-L particles could be varied by adjusting the ligand/Zn molar ratio (α) in the preparation processes. Optimized PDA-PEI/ZIF-L@PE TUF membranes containing ZIF-L of cross-cross block morphology showed very high pure water permeability of 180 ± 20 L·m-2·h-1·bar-1 (LMHB) and retention selectivity (SCR/Na2SO4 and SMB/Na2SO4) of 267 and 43, respectively, as well as excellent stability and anti-fouling properties.

Efficient separation of dyes using two-dimensional heterogeneous composite membranes

Two-dimensional materials are widely used in membrane separation, but the loose distribution and severe expansion between graphene oxide (GO) nanosheets limit its application. Here, we introduce a two-dimensional MOF material into the GO membrane to enhance its water permeance and separation performance. The MOF/GO composite membrane was prepared by vacuum filtration. The MOF and GO nanosheets were tightly stacked through the π-π effect, and the shortened transmission path and enhanced pore structure greatly improved the water permeance of the composite membrane. The MOF/GO membrane exhibited a high water permeance of 56.94 L m-2 h-1 bar-1. The rejection rates of methylene blue and was as methyl orange dyes were as high as 99.79% and 99.11%, respectively. At increased dye concentration, the rejection rate of methylene blue was still maintained greater than 99%. Dye rejection after 18 h of continuous operation remains above 90%. This work provides new ideas for improving membrane separation materials. The combination of two-dimensional heterogeneous materials can result in synergistic advantages for the development of composite membranes with high water permeance and high rejection rate.

Construction of a Two-Dimensional GO/Ti3C2TX Composite Membrane and Investigation of Mg2+/Li+ Separation Performance

Graphene oxide (GO) two-dimensional (2D) membranes with unique layer structures and tunable layer spacing have special advantages and great potential in the field of water treatment. However, GO membranes face the issues of weak anti-swelling ability as well as poor permeability. We prepared GO/Ti3C2TX 2D composite membranes with 2D/2D structures by intercalating Ti3C2TX nanosheets with slightly smaller sizes into GO membranes. Ti3C2TX intercalation can effectively expand the layer spacing of GO, thereby substantially enhancing the flux of the composite membrane (2.82 to 6.35 L·m-2·h-1). Moreover, the GO/Ti3C2TX composite membrane exhibited a good Mg2+/Li+ separation capability. For the simulated brine, the separation factor of M2 was 3.81, and the salt solution flux was as high as 5.26 L·m-2·h-1. Meanwhile, the incorporation of Ti3C2TX nanosheets significantly improved the stability of GO/Ti3C2TX membranes in different pH environments. This study provides a unique insight into the preparation of highly permeable and ion-selective GO membranes.

High-Flux lamellar MoSe2 membranes for efficient dye/salt separation

Membrane-based technology is emerging as an efficient technique for wastewater treatment in recent years. Membranes made up of two-dimensional materials provide high selectivity and water flux compared to conventional polymeric membranes. Herein, we report the synthesis and use of MoSe₂ membrane for dye and drug separation in wastewater, mainly from textile and pharmaceutical industries. The as-prepared MoSe₂ membrane shows ∼ 100% rejection for organic dyes and ciprofloxacin drug with a water flux reaching up to ∼ 900 Lm^-2h^-1bar^-1. Further, the MoSe₂ membrane shows lower NaCl rejection of ∼ 1.9% for the dye/salt mixture. The interlayer spacing in the MoSe₂ membrane allows the water molecules and ions from the salt to pass through freely but restricts the movement of large contaminants. The membrane is stable against the bovine albumin serum fouling with a flux recovery rate of 96%. It also shows good performance even in harsh environments (pH 3-10). To the best of our knowledge, the MoSe₂ membranes were fabricated for the first time for wastewater treatment application. The dye/salt separation performance of the MoSe₂ membrane is significantly better than several other membranes. This work highlights the promising potential for using two-dimensional materials for textile and pharmaceutical wastewater treatment.

A critical review on graphene oxide membrane for industrial wastewater treatment

An important way to promote the environmental industry's goal of carbon reduction is to promote the recycling of resources. Membrane separation technology has unique advantages in resource recovery and advanced treatment of industrial wastewater. However, the great promise of traditional organic membrane is hampered by challenges associated with organic solvent tolerance, lack of oxidation resistance, and serious membrane fouling control. Moreover, the high concentrations of organic matter and inorganic salts in the membrane filtration concentrate also hinder the wider application of the membrane separation technology. The emerging cost-effective graphene oxide (GO)-based membrane with excellent resistance to organic solvents and oxidants, more hydrophilicity, lower membrane fouling, better separation performance has been expected to contribute more in industrial wastewater treatment. Herein, we provide comprehensive insights into the preparation and characteristic of GO membranes, as well as current research status and problems related to its future application in industrial wastewater treatment. Finally, concluding remarks and future perspectives have been deduced and recommended for the GO membrane separation technology application for industrial wastewater treatment, which leads to realizing sustainable wastewater recycling and a nearly "zero discharge" water treatment process.

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 SO₄^2- 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.

Low-pressure thin-film composite nanofiltration membranes with enhanced selectivity and antifouling property for effective dye/salt separation

For better sustainable resource recovery and elevating the separation efficiency of dye/salt mixture, it is essential to develop an appropriate nanofiltration membrane for the treatment of textile dyeing wastewater containing relatively smaller molecule dyes. In this work, a novel composite polyamide-polyester nanofiltration membrane was fabricated by tailoring amino functionalized quantum dots (NGQDs) and β-cyclodextrin (CD). An in-situ interfacial polymerization occurred between the synthesized NGQDs-CD and trimesoyl chloride (TMC) on the modified multi-carbon nanotubes (MWCNTs) substrate. The incorporation of NGQDs significantly elevated the rejection (increased by ∼ 45.08%) of the resultant membrane for small molecular dye (Methyl orange, MO) compared to the pristine CD membrane at low pressure (1.5 bar). The newly developed NGQDs-CD-MWCNTs membrane exhibited enhanced water permeability without compromising the dye rejection compared to the pure NGQDs membrane. The improved performance of the membrane was primarily attributed to the synergistic effect of functionalized NGQDs and the special hollow-bowl structure of CD. The optimal NGQDs-CD-MWCNTs-5 membrane expressed pure water permeability of 12.35 L m^-2h^-1 bar^-1 at the pressure of 1.5 bar. Noteworthily, the NGQDs-CD-MWCNTs-5 membrane not only showed high rejection for the larger molecular dye of Congo Red (CR, 99.50%) but also for the smaller molecular dye of MO (96.01%) and Brilliant Green (BG, 95.60%) with the permeability of 8.81, 11.40, and 6.37 L m^-2h^-1 bar^-1, respectively at low pressure (1.5 bar). The rejection of inorganic salts by the NGQDs-CD-MWCNTs-5 membrane was 17.20% for sodium chloride (NaCl), 14.30% for magnesium chloride (MgCl₂), 24.63% for magnesium sulfate (MgSO₄), and 54.58% for sodium sulfate (Na₂SO₄), respectively. The great rejection of dyes remained in the dye/salt binary mixed system (higher than 99% for BG and CR, <21% for NaCl). Importantly, the NGQDs-CD-MWCNTs-5 membrane exhibited favorable antifouling performance and potential good operation stability performance. Consequently, the fabricated NGQDs-CD-MWCNTs-5 membrane suggested a prospective application for the reuse of salts and water in textile wastewater treatment owing to the effective selective separation performance.

Cross-Linked and Doped Graphene Oxide Membranes with Excellent Antifouling Capacity for Rejection of Antibiotics and Salts

Graphene oxide (GO) membranes have suffered from the instability of water permeability and low rejection of pollutant separation. In this paper, a reasonable modification protocol for GO nanosheets at the molecular level was proposed. A molecular cross-linking strategy was adopted to regulate the interlayer spacing of GO nanosheets, and nanofiltration membranes with high water stability and excellent antifouling capacity were prepared, which could effectively reject antibiotics and salts. The GO₁-MPD_0.5 (the mass ratio of GO nanosheets to MPD is 1:0.5) and GO/GO₁-MPD_0.5-0.25 (the doping ratio of GO₁-MPD_0.5 is 25%) membranes had stable water permeability of 4.22 ± 0.06 and 3.65 ± 0.11 L m^-2 h^-1 bar^-1, and the rejection rates for ciprofloxacin (CIP) and ofloxacin (OFX) were 93.35 ± 3.62 and 95.48 ± 2.97 and 85.89 ± 6.52 and 88.21 ± 3.67%, respectively. Molecular dynamics simulations well explained the high water stability of membranes, and the cross-linked hydrophobic benzene ring played a role in the rejection of pollutant molecules. Moreover, the GO₁-MPD_0.5 membrane showed excellent antifouling capacity and the flux recovery ratio (FRR) was more than 98%. This paper provides a new idea for the design of nanofiltration membranes with high stability and good rejection permeability at the molecular level and provides a prospect for the application of nanofiltration membranes in practical water treatment and water purification.

Effect of polypyrrole surface modification on antifouling performance of PTFE microfiltration membrane

In this study, polypyrrole was prepared by in situ chemical oxidation polymerization and deposited on the surface of the PTFE membrane. The surface morphology of the membrane shown that the membrane fouling degree of the modified membrane was much lower than that of the original membrane. Besides, the contact angle value decreased from 107.20° to 72.62°, and its hydrophilicity was significantly enhanced. It took humic acid (HA) as a typical representative membrane foulants, and static and dynamic HA adsorption experiments were carried out on the membranes before and after modification. In the static adsorption experiment of HA, the adsorption capacity of an original membrane was 1.28 times that of a modified membrane. In the dynamic antifouling experiment of HA, the rejection of the modified membrane to HA was 62.99%, while that of the original membrane was only 39.82%. In addition, the experimental results showed that the modified membrane had a higher flux recovery rate, which was 1.18 times that of the original membrane. This study proves that the modified membrane has an extraordinary antifouling effect.

Rapid growth of MXene-based membranes for sustainable environmental pollution remediation

Water consumption has grown in recent years due to rising urbanization and industry. As a result, global water stocks are steadily depleting. As a result, it is critical to seek strategies for removing harmful elements from wastewater once it has been cleaned. In recent years, many studies have been conducted to develop new materials and innovative pathways for water purification and environmental remediation. Due to low energy consumption, low operating cost, and integrated facilities, membrane separation has gained significant attention as a potential technique for water treatment. In these directions, MXene which is the advanced 2D material has been explored and many applications were reported. However, research on MXene-based membranes is still in its early stages and reported applications are scatter. This review provides a broad overview of MXenes and their perspectives, including their synthesis, surface chemistry, interlayer tuning, membrane construction, and uses for water purification. Application of MXene based membrane for extracting pollutants such as heavy metals, organic contaminants, and radionuclides from the aqueous water bodies were briefly discussed. Furthermore, the performance of MXene-based separation membranes is compared to that of other nano-based membranes, and outcomes are very promising. In order to shed more light on the advancement of MXene-based membranes and their operational separation applications, significant advances in the fabrication of MXene-based membranes is also encapsulated. Finally, future prospects of MXene-based materials for diverse applications were discussed.

Graphene oxide membrane chemically modified by electron-transfer diazonium chemistry for efficient dye separation

By chemical modification of the graphene oxide (GO) surface via diazonium chemistry, we introduce nitrobenzene groups as new interlayer pillars to GO memebranes like the surface oxygen-containing functional groups. The larger pillar can finely enlarge the interlayer space of the GO membrane. The filtration performance of modified GO membranes with different mass ratios of nitrobenzene diazonium tetrafluoroborate (NDT) were tested for EB, DR81, and MB. Notably, when the GO : NDT ratio is 1 : 1, it is found that the water flux can be enhanced by more than twice and by nearly 1.4 times its value for EB and DR81, respectively, while maintaining a high rejection (92% for EB and 95% for DR81). In conclusion, the chemical modification of GO through the dediazonization reaction of NDT can indeed improve the separation efficiency of the dye.

Self-assembly of stable and high-performance molecular cage-crosslinked graphene oxide membranes for contaminant removal

Membrane separation is regarded as efficient technology to alleviate global water crisis. Two-dimensional membranes are promising for contaminant removal from wastewaters, but their uncontrollable transport pathway and instability hinder the further development. In this study, the high-performance and stable two-dimensional framework membranes are self-assembled by graphene oxide (GO) nanosheets and amino-appended metal-organic polyhedrons (MOPs) for water purification and remediation. The MOP molecular cages are uniformly intercalated between GO nanosheets and enriched at defects/edges, and can crosslink membranes, to provide in-plane selective channels, refine vertical passageways, and fix out-of-plane interlayer spaces. The prepared GO/MOP framework membranes have improved stability and nanofiltration performance under cross-flow condition, can keep performance in water after 50 h filtration, and show high rejections over 92% for Na₂SO₄ and 99% for antibiotic and dye contaminants with molecular weights over 280 g mol^-1, and sixfold permeance as that of GO membranes. Our molecular cage-intercalated and crosslinked two-dimensional frameworks offer an alternative route to design robust membranes for efficient removal of contaminants in wastewaters.

Graphene Oxide-Based Membranes Intercalated with an Aromatic Crosslinker for Low-Pressure Nanofiltration

Graphene oxide (GO), a carbonaceous 2D nanomaterial, has received significant interest as a next-generation membrane building block. To fabricate high-performance membranes, an effective strategy involves stacking GO nanosheets in laminated structures, thereby creating unique nanochannel galleries. One outstanding merit of laminar GO membranes is that their permselectivity is readily tunable by tailoring the size of the nanochannels. Here, a high-performance GO-based nanofiltration membrane was developed by intercalating an aromatic crosslinker, α,α/-dichloro-p-xylene (DCX), between the layers in laminated GO nanosheets. Owing to the formation of strong covalent bonds between the crosslinker and the GO, the resulting GO laminate membrane exhibited outstanding structural stability. Furthermore, due to the precisely controlled and enlarged interlayer spacing distance of the developed DCX-intercalated GO membrane, it achieved an over two-fold enhancement in water permeability (11 ± 2 LMH bar^-1) without sacrificing the rejection performance for divalent ions, contrary to the case with a pristine GO membrane.

Ganzoury M, Zhang N, de Lannoy CF. Nanocomposite Polymeric Membranes for Organic Micropollutant Removal: A Critical Review

The prevalence of organic micropollutants (OMPs) and their persistence in water supplies have raised serious concerns for drinking water safety and public health. Conventional water treatment technologies, including adsorption and biological treatment, are known to be insufficient in treating OMPs and have demonstrated poor selectivity toward a wide range of OMPs. Pressure-driven membrane filtration has the potential to remove many OMPs detected in water with high selectivity as a membrane's molecular weight cutoff (MWCO), surface charge, and hydrophilicity can be easily tailored to a targeted OMP's size, charge and octanol-water partition coefficient (Kow). Over the past 10 years, polymeric (nano)composite microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) membranes have been extensively synthesized and studied for their ability to remove OMPs. This review discusses the fate and transport of emerging OMPs in water, an assessment of conventional membrane-based technologies (NF, reverse osmosis (RO), forward osmosis (FO), membrane distillation (MD) and UF membrane-based hybrid processes) for their removal, and a comparison to the state-of-the-art nanoenabled membranes with enhanced selectivity toward specific OMPs in water. Nanoenabled membranes for OMP treatment are further discussed with respect to their permeabilities, enhanced properties, limitations, and future improvements.

Metal-polyphenol cross-linked titanium carbide membranes with stable interlayer spacing for efficient wastewater treatment

Membranes based on transition metal carbides/nitrides (MXenes) have significant water treatment potential because of their unique molecular sieving properties and excellent permeation performance. However, hydrophilic MXenes swell upon water immersion, and improving their stability remains challenging. In this study, a Fe³⁺-tannic acid (TA) complex was used as a cross-linker and surface modifier to prepare high-performance titanium carbide (Ti₃C₂T_x) MXene laminar membranes. Fe³⁺-TA formation on the nanosheets increased the interlayer spacing and stabilized the laminar structure. The membrane with the highest performance among the as-prepared membranes exhibited a high water permeance of 90.5 L/m^-2(-|-)h^-1 bar^-1 (which is twice that of the pristine Ti₃C₂T_x membrane) and good separation efficiency (methyl blue rejection rate: ∼99.8 %; Na₂SO₄ rejection rate: ∼5.0 %). Furthermore, the Fe³⁺-TA complex enhanced the membrane hydrophilicity, resulting in excellent antifouling properties. This study provides an environmentally friendly and facile method for fabricating two-dimensional loose nanofiltration membranes for textile wastewater treatment.

Recent Development in Laminar Transition Metal Dichalcogenides-based Membranes Towards Water Desalination: A Review

Transition metal dichalcogenides (TMDCs)-based laminar membranes have gained significant interest in energy storage, fuel cell, gas separation, wastewater treatment, and desalination applications due to single layer structure, good functionality, high mechanical strength, and chemical resistivity. Herein, we review the recent efforts and development on TMDCs-based laminar membranes, and focus is given on their fabrication strategies. Further, TMDCs-based laminar membranes for water purification and seawater desalination are discussed in detail. Finally, present their merits, limits and future challenges needed in this area.

Controlled Synthesis of Perforated Oxide Nanosheets with High Density Nanopores Showing Superior Water Purification Performance

A method for creating genuine nanopores in high area density on monolayer two-dimensional (2D) metallic oxides has been developed. By use of the strong reduction capability of hydroiodic acid, active metal ions, such as Fe^III and Co^III, in 2D oxide nanosheets can be reduced to a divalent charge state (2+). The selective removal of FeO₂ and CoO₂ metal oxide units from the framework can be tuned to produce pores in a range of 1-4 nm. By monitoring of the redox reaction kinetics, the pore area density can be also tuned from ∼0.9 × 10⁴ to ∼3.3 × 10⁵ μm^-2. The universality of this method to produce much smaller pores and higher area density than the previously reported ones has been proven in different oxide nanosheets. To demonstrate their potential applications, ultrasmall metal organic framework particles were grown inside the pores of perforated titania oxide nanosheets. The optimized hybrid film showed ∼100% rejection of methylene blue (MB) from the water. Its water permeance reached 4260 L m^-2 h^-1 bar^-1, which is 1-3 orders of that for reported 2D membranes with good MB rejections.

Optimizing graphene oxide membranes for effective removal of dyes by modulating the reduction degree and doped nitrogen

The excellent mechanical and chemical characteristics of graphene oxide (GO) enable their potential application in the realm of membrane separation. However, the expansion and instability of GO nanosheets in water limit its application. In this work, nitrogen-doped GO (NGO) was obtained by a harmless hydrothermal reduction method. The obtained NGO films were attached to a polyvinylidene fluoride support membrane by vacuum filtration. By changing the hydrothermal reaction temperature, the reduction degree of GO and doping amount of nitrogen was adjusted to control the inter-layer structure and permeability of NGO. The defect of NGO nanosheets and the reduction of oxygen-containing functional groups could accelerate the transportation of water molecules through the inter-layer space of the hydrophobic graphene sheets. Significantly, the polarization and high adsorption energy of pyridine-N serve as a supplement to the exclusion mechanism of the inter-layer spacing. NGO membranes have better permeability than the initial GO membranes without sacrificing the rejection rate. The optimized NGO film has a significant rejection rate of above 99% for various dyes, such as methylene blue, Congo red and methyl blue.

D-spacing controllable GO membrane intercalated by sodium tetraborate pentahydrate for dye contamination wastewater treatment

Sodium tetraborate pentahydrate (STB) was intercalated into graphene oxide (GO) nanosheets to form a nanocomposite (STB@GO). Subsequently, it was self-assembled on a substrate membrane to prepare STB@GO nanofiltration membrane. The properties of the STB@GO powder samples and the nanofiltration membrane were studied using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), contact angle (CA), and zeta potential. When the STB concentration was 1.0 g/L in the cross-linking reaction, the membrane was described as the STB₂@GO membrane and exhibited a large interlayer space (d-spacing = 1.347 nm), high hydrophilicity (CA = 22.2°), and high negative potential (zeta = -18.0 mV). Meanwhile, the pure water flux of the membrane was significantly increased by 56.60% than that of the GO membrane. In addition, the STB₂@GO membrane exhibited a favorable capability for dye rejection,98.52% for Evans blue (EB), 99.26% for Victoria blue B (VB), 91.94% for Alizarin yellow (AY), and 93.21% for Neutral red (NR). Furthermore, the STB₂@GO membrane performed better in dye separation under various types and concentrations of dye, pH values, and ions in solution. Thus, this study provides a promising method for preparing laminated GO nanofiltration membranes for dye wastewater treatment.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Amorphous TiO2 Bridges Stabilized WS2 Membranes with Excellent Filtration Stability and Photocatalysis-Driving Self-Cleaning Ability

Two-dimensional (2D) membranes as a new type of water filtration membrane have shown great potential in water separation and purification. However, their long-term stability under cross-flow conditions and their antifouling property are two main concerns for practical separation and purification processes. In this work, a strategy of nanoparticle bridges based on amorphous TiO₂ is developed to link adjacent WS₂ nanosheets on a WS₂ membrane surface, leading to a strong membrane surface with excellent stability during 204 h of continuous cross-flow filtration. Moreover, the amorphous TiO₂ bridges also form a TiO₂/WS₂ heterojunction on the WS₂ membrane surface, exhibiting an impressive photocatalysis-driving self-cleaning property by pollutant photodegradation. And the flux recovery ratio (FRR) exceeds 95% after three cycles of separation experiments. The excellent long-term stability and photocatalysis-driving self-cleaning property of the WS₂/TiO₂ membrane provide a new approach to construct robust 2D membranes.

Chemically Cross-Linked Graphene Oxide as a Selective Layer on Electrospun Polyvinyl Alcohol Nanofiber Membrane for Nanofiltration Application

Graphene oxide (GO) nanosheets were utilized as a selective layer on a highly porous polyvinyl alcohol (PVA) nanofiber support via a pressure-assisted self-assembly technique to synthesize composite nanofiltration membranes. The GO layer was rendered stable by cross-linking the nanosheets (GO-to-GO) and by linking them onto the support surface (GO-to-PVA) using glutaraldehyde (GA). The amounts of GO and GA deposited on the PVA substrate were varied to determine the optimum nanofiltration membrane both in terms of water flux and salt rejection performances. The successful GA cross-linking of GO interlayers and GO-PVA via acetalization was confirmed by FTIR and XPS analyses, which corroborated with other characterization results from contact angle and zeta potential measurements. Morphologies of the most effective membrane (CGOPVA-50) featured a defect-free GA cross-linked GO layer with a thickness of ~67 nm. The best solute rejections of the CGOPVA-50 membrane were 91.01% for Na2SO4 (20 mM), 98.12% for Eosin Y (10 mg/L), 76.92% for Methylene blue (10 mg/L), and 49.62% for NaCl (20 mM). These findings may provide one of the promising approaches in synthesizing mechanically stable GO-based thin-film composite membranes that are effective for solute separation via nanofiltration.