Cross-Linked Graphene Oxide Membranes Research Articles

Impacts of mono/divalent cations on the lamellar structure of cross-linked GO layers and membrane filtration performance for different DOM fractions

This study evaluated the effects of co-existing cations (Na+ or Ca2+) on the lamellar structure of cross-linked graphene oxide (GO) layers and GO modified membrane performance in terms of their fouling behaviours and retention for single-model organic matter, namely, bull serum albumin (BSA), sodium alginate (SA), humic acid (HA) and tannic acid (TA). In the absence of co-existing cations, the GO layers mitigated membrane fouling for large molecules (SA, BSA, and HA) but led to severer pore blocking for small molecules (TA) compared with pristine membrane. Na+ and Ca2+ altered the performance impacts of the GO modified membrane due to different interactions with the cross-linked GO layers. Low concentrations of Na+ (<0.4 mM) enlarged interlayer spacing of the GO layers and caused a decrease in flux after physical cleaning, but the GO layers maintained the uniform lamellar structure. High concentrations of Na+ (>0.4 mM) promoted the aggregation of cross-linked GO layers through charge shielding and reduced the uniformity of lamellar structure, which weakened the antifouling performance for large molecules and promoted the passage of small molecules through the membrane. However, Ca2+ complexed with GO sheets and reinforced the uniform lamellar structure of the GO layers, leading to a better antifouling performance for the filtration of large molecules than the pristine membrane but aggravated TA fouling.

Preparation of novel cross-linked graphene oxide membrane for desalination applications using (EDC and NHS)-activated graphene oxide and PEI

A novel cross-linked graphene oxide (GO) membrane was prepared by vacuum filtration using a two-step method, including activation of GO carboxyl groups by a mixture of 1-Ethyl-3- (3-dimethylaminopropyl) carbodiimide (EDC) and N -Hydroxysuccinimide (NHS) and then cross-linking with polyethyleneimine (PEI). In the cross-linking process, the activated carboxyl groups of GO react with amine groups of PEI through a simple and efficient method. GO cross-linking with PEI significantly improves membrane dimensional stability and salt rejection. The rejection of the cross-linked GO membrane was evaluated by NaCl, Na2SO4 and MgCl2 solutions, and the obtained results followed the order of R (MgCl2) > R (NaCl) > R (Na2SO4). The prepared cross-linked GO membrane was characterized by FTIR, XRD, SEM, FESEM, EDS, WCA, AFM and BET analysis. Also, the effects of operating parameters such as N2 pressure, feed temperature and pH on the cross-linked GO membrane performance were investigated.

Enhanced covalent p-phenylenediamine crosslinked graphene oxide membranes: Towards superior contaminant removal from wastewaters and improved membrane reusability.

The increasing depletion of freshwater necessitates the re-use and purification of wastewaters. Among the existing separation membrane materials, graphene oxide (GO) is a promising candidate, owing to its tunable physicochemical properties. However, the widening of GO membranes pore gap in aqueous environments is a major limitation. Crosslinking agents can be incorporated to alleviate this problem. This study describes a comparative analysis of uncrosslinked and p-Phenylenediamine (PPD) crosslinked GO membranes' water purification performance. Dip-coating and dip-assisted layer-by-layer methods were used to fabricate the uncrosslinked and crosslinked membranes respectively. The covalent interaction between GO and PPD was confirmed by Fourier Transform Infra-Red and X-ray Photoelectron Spectroscopy. The excellent membrane topographical continuity and intactness was assessed by means of Scanning Electron Microscopy, while water contact angle measurements were undertaken to evaluate and confirm membrane hydrophilicity. The improvement impact of the crosslinker was manifested on the enhancement of the stability and performance of the membranes during nanofiltration tests of aqueous solutions of methylene blue in a homemade nanofiltration cell operated at 1 bar.

Interlocked Graphene Oxide Provides Narrow Channels for Effective Water Desalination through Forward Osmosis.

Unique two-dimensional water channels formed by stacked graphene oxide (GO) sheets that are "nonleachable" and nonswellable can show great potential for water remediation. The interlayer spacing controls the solute or ion sieving and plays a crucial role in water transport in GO-based membranes. Herein, the sub-nano-channels adjacent to the sheets are altered by either ionic or covalent cross-linking using magnesium hydroxide (Mg(OH)2) and graphene oxide quantum dots (GQDs) (named GOM and G-GQD), respectively. In aqueous solution, these cross-linkers prevent the GO sheets from swelling and precisely control the interlayer spacing required for water permeation. In addition, these narrowed GO sheets facilitate significant improvement in salt rejection of a divalent ion by forward osmosis and selective dye rejection and are resistive toward biofouling and bacterial growth. The cross-linked GO membranes are robust enough to withstand strong cross-flow velocity and aided in unimpeded water transport through the nanochannels. Among the membranes, the G-GQD membranes (G-GQD) show better antifouling characteristics, dye separation performance over 95-97% for various dyes, divalent ion rejection by 97%, and no cytotoxicity against HaCaT cells as compared with other GO membranes. Our findings on interlocking the domains of nanoslits of the GO structure by small ecofriendly molecules portray these materials as potential candidates for water separation applications.

Ultrahigh permeance of a chemical cross-linked graphene oxide nanofiltration membrane enhanced by cation-π interaction.

Cross-linking with large flexible molecules is a common method to improve the stability and control the interlayer spacing of graphene oxide (GO) membranes, but it still suffers from the limitation of low water flux. Herein, a novel high flux GO membrane was fabricated using a pressure-assisted filtration method, which involved a synergistic chemical cross-linking of divalent magnesium ions and 1,6-hexanediamine (HDA) on a polyethersulfone (PES) support. The membrane cross-linked with magnesium ions and HDA (GOHDA–Mg2+) exhibited a high water flux up to 144 L m−2 h−1 bar−1, about 7 times more than that of cross-linked GO membranes without adding magnesium ions (GOHDA), while keeping excellent rejection performance. The GOHDA–Mg2+ membrane also showed an outstanding stability in water for a long time. The effects of magnesium ions on the GOHDA–Mg2+ membrane were analyzed using several characterization methods, including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicated that magnesium ions not only promoted reasonable cross-linking, but also improved the stacking of GO sheets to give lower mass transfer resistance channels for water transport in the membranes, resulting in the ultrahigh permeance of the GO membranes.

Borate Inorganic Cross-Linked Durable Graphene Oxide Membrane Preparation and Membrane Fouling Control.

Graphene oxide (GO) membranes have the potential to be next-generation membranes. However, the GO layer easily swells in water and risks shedding during the long-term filtration. Organic GO interlayer organic cross-linking agent was not resistant to oxidation, which limits the application scope of GO membrane. In this study, an inorganic cross-linked GO membrane was prepared via the reaction of sodium tetraborate and GO hydroxyl groups, and a -B-O-C- cross-linking bond was detected by X-ray photoelectron spectroscopy (XPS). Additionally, a new atomic force microscope scratch method to evaluate the cross-linking force of a nanoscale GO layer was proposed. It showed that the critical destructive load of the inorganic cross-linked GO membrane increased from 8 to 80 nN, which was a 10-fold increase from that of the nonlinked sample. During the NaOH/sodium dodecyl sulfate (SDS) destructive wash tests, morphology, flux and retention rate of inorganic cross-linked GO remained stable while the comparative membranes showed significant destruction. At the same time, based on the better oxidation resistance, organic membrane fouling was effectively controlled by the introduction of trace ·OH radicals. This study provides a new perspective for GO membrane preparation, interlayer cross-linking force testing and membrane fouling control.

Cross-Linked Graphene Oxide Membrane Functionalized with Self-Cross-Linkable and Bactericidal Cardanol for Oil/Water Separation

A cross-linked graphene oxide (GO) membrane was fabricated by a simple vacuum filtration, which was followed by a thermal treatment to functionalize GO with renewable cardanol (cardanol-GO). The cardanol-GO was fabricated through reactions between epoxy groups on the GO surface and phenolic moieties of cardanol with the participation of a catalyst. The prepared cardanol-GO membrane was then heated to form a cross-linked structure by reactions between the double bonds of cardanol. The cross-linked GO membrane exhibited outstanding dimensional stability and oil/water separation efficiency. Furthermore, the cross-linked cardanol-GO membrane was proved to contain a marked antibacterial property against Escherichia coli that originates from the cardanol moieties.

Dopamine crosslinked graphene oxide membrane for simultaneous removal of organic pollutants and trace heavy metals from aqueous solution

ABSTRACTA graphene oxide-polydopamine-(β-cyclodextrin) (GPC) ultrafiltration membrane was fabricated by using the method of drop-coating combined with vacuum filtration. The prepared GPC membrane was characterized with FTIR and XPS spectrophotometry and evaluated for its performances for the rejection of organic molecules (methylene blue) and adsorption of trace heavy metals (Pb2+) from aqueous solutions. The membrane exhibited an excellent rejection coefficient of 99.2% for methylene blue and the permeation flux was 12 L m−2 h−1 at 0.1 bar. The membrane also exhibited fast adsorption kinetics for Pb2+ and the adsorption capacity was 101.6 mg g−1 at a solution pH of 6. The performance of the membrane could almost be completely recovered after a simple clean and regeneration process. These results indicate that the GPC membrane has potential applications in treatment of complex industrial wastewater streams.

Trivalent metal cation cross-linked graphene oxide membranes for NOM removal in water treatment

This paper summarizes the development and testing of novel graphene oxide (GO) membranes for the removal of natural organic matter (NOM) from raw water sources used for drinking water supply. In this work, two trivalent cations, Al3+ and Fe3+, were employed as crosslinking agents to stack GO nanosheets layer by layer on a PVDF membrane support, in order to fabricate a suitable GO membrane. The trivalent cations greatly improve the bonding strength between the GO nanosheets through electrostatic forces and coordination bonds, and thus enhance the stability of the GO membrane; the integrity of the membrane in a range of solutions could be maintained for over a month. The initial interlayer spacing of GO nanosheets (0.80nm) could be increased up to 0.86–0.95nm by changing the Al3+ or Fe3+ ion concentration. It was found that the GO membrane flux ranged between 79 and 902 LMH/MPa when treating three representative NOM solutions and a real surface water. A relatively low thickness of the GO layer induced a higher flux of the GO membrane when prepared with the same cation concentration, while increasing the cation concentration resulted in a decline in flux. The flux of the Fe3+ cross-linked GO membrane was approximately 1.1–2.3 times that of the Al3+ cross-linked GO membrane, while both cation-modified GO membranes achieved a similar separation efficiency of the organic contaminants. The study has demonstrated a facile approach to the fabrication of a novel, stable GO membrane employing Al3+/Fe3+ ions as crosslinking species, in order to utilize the excellent properties of GO and produce a GO membrane with a high flux and organic removal performance for water treatment.

Cross-linked graphene oxide membrane having high ion selectivity and antibacterial activity prepared using tannic acid-functionalized graphene oxide and polyethyleneimine

A covalently cross-linked graphene oxide (GO) membrane was prepared by a facile vacuum filtration method followed by a cross-linking process using tannic acid-functionalized GO (TA-GO) and hyperbranched polyethyleneimine (PEI). The TA-GO was prepared by a simple and environmentally friendly method using GO and a renewable plant-based polyphenol, tannic acid (TA), and it was covalently cross-linked by PEI via the reaction between catechol groups in TA and amine groups in PEI to form covalent bonds in a weak alkaline environment producing the cross-linked GO membrane. The cross-linked GO membrane showed excellent dimensional stability and ions separation performance. In particular, the cross-linked GO membrane containing TA showed excellent antibacterial activity against Escherichia coli (E. coli) due to the intrinsic bactericidal properties of the TA moieties. Furthermore, the membrane exhibited higher water flux and rejections toward NaCl and MgSO4 than the commercial nanofiltration (NF) membranes.

Diamines cross-linked graphene oxide free-standing membranes for ion dialysis separation

Design and tuning of the permeation channels of graphene oxide (GO) membranes are crucial for separation. Herein, GO membranes were covalently cross-linked by a series of diamines utilizing two cross-linking methods (carboxyl activation/amidation, catalytic amidation) for the first time. The effects of cross-linking parameters (temperature, diamines concentration, diamines molecular structure) on the elastic modulus, contact angle, swelling degree and ion dialysis separation of GO membranes were systematically studied. It was found that the carboxyl activation/amidation cross-linking method does not favor the fabrication of free-standing GO membranes. In the catalytic amidation cross-linking, with the increase in chain length of aliphatic diamines, the elastic modulus and the contact angle with water generally increase, while the swelling degree in water decreases. The cross-linked GO membranes display higher fluxes and K+/Mg2+ separation factor than those of the pristine membrane. The p-phenylenediamine (PPD) cross-linked membranes shows excellent elastic modulus (10.5402GPa) and high K+/Mg2+ selectivity factor (7.15) along with low swelling degree due to its rigid structure, demonstrating its great potential in ion dialysis separation.

Tailoring permeation channels of graphene oxide membranes for precise ion separation

The design and tailoring of membrane permeation channels is crucial for precise separation of ions or molecules. Herein, a method for tuning the permeation channels of graphene oxide (GO) membranes is developed, in which the basal planes and edges of GO flakes are simultaneously crosslinked by dicarboxylic acid and diamine. By altering the chain length of the crosslinkers, the size, structure and properties of the permeation channels are successfully tuned. In the permeation of single metal salt solution, the fluxes of metal ions are related to the swelling degree of GO membranes. In the separation of mixed salt solutions, the fluxes are determined by the radii of hydrated cations, and the crosslinked GO membranes display outstanding size-selectivity. This is the first report about the distinct orders of permeation fluxes of single salt solution and mixed salt solution. With the increasing hydrophobicity of diamines, the permeation fluxes of aqueous solution decrease. The optimized crosslinked GO membranes display excellent fluxes and separation factor, which are more than 2 times and 3 times that of the pristine membranes, respectively. The elastic modulus of the crosslinked GO membranes is double that of the pristine one, and the swelling degree in water is 75 times lower.

Dicarboxylic acids crosslinked graphene oxide membranes for salt solution permeation

Abstract Graphene oxide (GO) membranes were crosslinked by dicarboxylic acids in order to improve the mechanical strength, stability and permeation performance. The effects of crosslinking conditions (temperature, catalyst concentration, time, crosslinker concentration) on the mechanical performance, along with the influences of crosslinker molecular structure on the thermal stability and swelling degree of GO membranes, were studied. The thermal pyrolysis temperature of crosslinked GO membranes increased about 2–8 °C while the swelling degree averagely decreased about 66% in DI water and 40% in 0.05 mol L−1 of KCl solution in comparison with those of pristine one. In the permeation of single salt solutions, the fluxes are positively correlated to the swelling degree of GO membranes. The effects of swelling degree on the permeation fluxes were firstly noticed. The crosslinked GO membranes show high permeation fluxes (more than 2 times) along with low swelling degree, demonstrating its potential application in ion dialysis.

Covalently crosslinked graphene oxide membranes by esterification reactions for ions separation

We present a method to prepare covalently crosslinked graphene oxide (GO) membranes with adjustable intersheet spacing by esterification reactions, using dicarboxylic acids, diols or polyols as the crosslinker and hydrochloric acid as the catalyst.