Study on the Transport Mechanism of a Freestanding Graphene Oxide Membrane for Forward Osmosis

Self-Assembly of Organic Semiconductors on Strained Graphene under Strain-Induced Pseudo-Electric Fields

Graphene is used as a growth template for van der Waals epitaxy of organic semiconductor (OSC) thin films. During the synthesis and transfer of chemical-vapor-deposited graphene on a target substrate, local inhomogeneities in the graphene-in particular, a nonuniform strain field in the graphene template-can easily form, causing poor morphology and crystallinity of the OSC thin films. Moreover, a strain field in graphene introduces a pseudo-electric field in the graphene. Here, the study investigates how the strain and strain-induced pseudo-electric field of a graphene template affect the self-assembly of π-conjugated organic molecules on it. Periodically strained graphene templates are fabricated by transferring graphene onto an array of nanospheres and then analyzed the growth and nucleation behavior of C60 thin films on the strained graphene templates. Both experiments and a numerical simulation demonstrated that strained graphene reduced the desorption energy between the graphene and the C60 molecules and thereby suppressed both nucleation and growth of the C60. A mechanism is proposed in which the strain-induced pseudo-electric field in graphene modulates the binding energy of organic molecules on the graphene.

Electrochemical Opening of Impermeable Nanochannels in Laminar Graphene Membranes for Ultrafast Nanofiltration

Reduced graphene oxide (rGO) could be theoretically used to construct highly permeable laminar membranes with nearly frictionless nanochannels for water treatment. However, their pristine (sp² C-C) regions usually restack into impermeable channels as a result of van der Waals interactions, resulting in a much low permeance. In this study, we demonstrate that the restacked regions could be electrochemically expanded to form ultrafast water transport nanochannels by providing a low positive potential (e.g., +1.00 V vs SCE) to the rGO membrane. Experimental investigations indicate that the structural expansion is attributed to the intercalation of water molecules into the restacked regions, driven by hydrogen bond interactions between water molecules and hydroxyl groups that are electrochemically produced on edges of rGO nanosheets. The structural expansion could be promoted by weakening the graphene-OH^- interactions through intermittent application of the potential. As a result of more ultrafast water transport nanochannels available, the electrochemically treated rGO membranes could have a permeance 2 orders of magnitude higher than that of the pristine one and ∼3 times higher than that of graphene oxide membranes. Because of their smaller average pore size, the rGO membranes also have a higher ionic/molecular rejection performance than graphene oxide membranes.

MXene Composite Membranes with Enhanced Ion Transport and Regulated Ion Selectivity

Two-dimensional (2D) material-based membranes are promising candidates for various separation applications. However, the further enhancement of membrane ion conductance is difficult, and the regulation of membrane ion selectivity remains a challenge. Here, we demonstrate the facile fabrication of MXene composite membranes by incorporating spacing agents that contain SO₃H groups into the MXene interlayers. The synthesized membrane shows enhanced ion conductance and ion selectivity. Subsequently, the membranes are utilized for salinity gradient power (SGP) generation and lithium-ion (Li⁺) recovery. The membrane containing poly(sodium 4-styrenesulfonate) (PSS) as the spacing agent shows a much higher power density for SGP generation as compared to the pristine MXene membrane. Using artificial seawater and river water, the power density reaches 1.57 W/m² with a testing area of 0.24 mm². Also, the same membrane shows Li⁺/Na⁺ and Li⁺/K⁺ selectivities of 2.5 and 3.2, respectively. The incorporation of PSS increases both the size and charge density of the nanochannels inside the membrane, which is beneficial for ion conduction. In addition, the density functional theory (DFT) calculation shows that the binding energy between Li⁺ and the SO₃H group is lower than other alkali ion metals, and this might be one major reason why the membrane possesses high Li⁺ selectivity. This study demonstrates that incorporating spacing agents into the 2D material matrix is a viable strategy to enhance the performance of the 2D material-based membranes. The results from this study can inspire new membrane designs for emerging applications including energy harvesting and monovalent ion recovery.

Influence of the Exclusion-Enrichment Effect on Ion Transport in Two-Dimensional Molybdenum Disulfide Membranes

Two-dimensional (2D) nanosheet membranes have been widely studied for water and wastewater treatment. However, mass transport inside 2D nanosheet membranes is far from being fully understood, and suitable applications of these membranes are yet to be identified. In this study, we investigate ion transport inside a 2D molybdenum disulfide (MoS₂) membrane by combining experimental results with numerical modeling. Specifically, we analyze the influence of the electrical double layer (EDL) extension on ion diffusion in the MoS₂ membrane, and a parameter called the exclusion-enrichment coefficient (β) is introduced to quantify how the electrostatic interaction between the coions and the EDL can affect the ion diffusion. Using the model developed in this study, the β values under different experimental conditions (feed solution concentration and applied hydraulic pressure) are calculated. The results show that coion diffusion inside the membrane can be retarded since β is smaller than one. Furthermore, the underlying mechanism is explored by theoretically estimating the radial ion concentration and electrical potential distributions across the membrane nanochannel. In addition, we find that convective mass transport can weaken the exclusion-enrichment effect by increasing β. Based on the results in this study, the potential applications and feasible membrane design strategies of 2D nanosheet membranes are discussed.

Forward Solute Transport in Forward Osmosis Using a Freestanding Graphene Oxide Membrane

A graphene oxide membrane (GOM) has the potential to be used in forward osmosis (FO) because it has a high water permeability and low reverse salt flux. To explore suitable applications, we initiated the investigation of the forward solute transport through a freestanding GOM in FO. Both uncharged solutes (PEG 200 and PEG 1000) and charged solutes (NaCl, MgSO₄, and MgCl₂) were investigated, and the forward solute flux in FO was tested. The Donnan steric pore model (DSPM) was utilized to calculate the forward solute flux of the freestanding GOM in FO when discussing diffusion, convection, and electromigration. Our results showed that the freestanding GOM has a better separation performance for multivalent ions than the monovalent ions in the FO mode. We found an information gap between the calculated and experimental forward solute flux values, especially when charged solutes were used in the feed solution and the electrical double layer (EDL) was thick. We propose that the EDL inside the GOM has a screening effect on the forward ion transport during FO, even in the presence of relatively high water flux. According to our analysis, the forward solute transport for charged solutes is governed by steric exclusion and interfacial Donnan exclusion as well as EDL screening along the nanochannels inside the membrane. Our study provides guidance for the future use of the freestanding GOM during FO for water and wastewater treatment.

Separation, anti-fouling, and chlorine resistance of the polyamide reverse osmosis membrane: From mechanisms to mitigation strategies

Membrane technology has been widely used in the wastewater treatment and seawater desalination. In recent years, the reverse osmosis (RO) membrane represented by polyamide (PA) has made great progress because of its excellent properties. However, the conventional PA RO membranes still have some scientific problems, such as membrane fouling, easy degradation after chlorination, and unclear mechanisms of salt retention and water flux, which seriously impede the widespread use of RO membrane technology. This paper reviews the progress in the research and development of the RO membrane, with key focus on the mechanisms and strategies of the contemporary separation, anti-fouling and chlorine resistance of the PA RO membrane. This review seeks to provide state-of-the-art insights into the mitigation strategies and basic mechanisms for some of the key challenges. Under the guidance of the fundamental understanding of each mechanism, operation and modification strategies are discussed, and reasonable analysis is carried out, which can address some key technical challenges. The last section of the review focuses on the technical issues, challenges, and future perspective of these mechanisms and strategies. Advances in synergistic mechanisms and strategies of the PA RO membranes have been rarely reviewed; thus, this review can serve as a guide for new entrants to the field of membrane water treatment and established researchers.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Power Harvesting

Salinity gradient power (SGP) has been identified as a promising renewable energy source. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two membrane-based technologies for SGP harvesting. Developing nanopores and nanofluidic membranes with excellent water and/or ion transport properties for applications in those two membrane-based technologies is considered viable for improving power generation performance. Despite recent efforts to advance power generation by designing a variety of nanopores and nanofluidic membranes to enhance power density, the valid pathways toward large-scale power generation remain uncertain. In this review, we introduce the features of ion and water transport in nanofluidics that are potentially beneficial to power generation. Subsequently, we survey previous efforts on nanofluidic membrane synthesis to obtain high power density. We also discuss how the various membrane properties influence the power density in RED and PRO before moving on to other important aspects of the technologies, i.e., system energy efficiency and membrane fouling. We analyze the importance of system energy efficiency and illustrate how the delicately designed nanofluidic membranes can potentially enhance energy efficiency. Previous studies are reviewed on fabricating antifouling and antimicrobial membrane for power generation, and opportunities are presented that can lead to the design of nanofluidic membranes with superior antifouling properties using various materials. Finally, future research directions are presented on advancing membrane performance and scaling-up the system. We conclude this review by emphasizing the fact that SGP has the potential to become an important renewable energy source and that high-performance nanofluidic membranes can transform SGP harvesting from conceptual to large-scale applications.

Cross-linked Graphene Oxide Framework Membranes with Robust Nano-Channels for Enhanced Sieving Ability

It remains challenging for graphene oxide (GO) membranes to achieve highly efficient performance and sufficient stability for aqueous molecule/ion precise separations. Herein, a molecular-level rational structure design protocol was proposed to develop ceramic-based graphene oxide framework (GOF) membranes with significantly enhanced sieving performance and stability for efficient removal of salts and micropollutants. Via a molecular cross-linking strategy, interlayered nanochannels between GO nanosheets can be rationally designed, featuring precisely tailorable channel size, promising surface chemistries and remarkably robust stability suitable for aqueous separation. Due to a significantly decreased nanochannel size, cross-linking of TU (thiourea) molecule significantly improved monovalent salt rejection (95.6% for NaCl), outperforming existing state-of-the-art GO-based, commercial organic nanofiltration and emerging two-dimensional MoS₂ membranes, while moderately decreasing water permeability. In comparison, the GOF membranes cross-linked with MPD (m-phenylenediamine) exhibited a simultaneous increase in permeability and rejection for both salts and micropollutants (21.0% and 53.3% enhancement for chloramphenicol (CAP) solution), breaking their conventional trade-off issue. Cross-linking mechanism indicates that more robust nanochannels were formed by stronger covalent bonds via dehydration condensation between amine (TU/MPD) and carboxyl groups (GO), and nucleophilic addition between amine (TU/MPD) and epoxy groups (GO). Molecule/ion separation mechanism involved size sieving (steric hindrance), electrostatic interaction, Donnan effect, and partial dehydration effect. This work provides a novel protocol for rationally designing size and surface chemistry of highly robust GO nanochannels at a subnanometer level to construct water-stable functional GOF membranes with enhanced sieving performance for water treatment applications.