Controlling interlayer spacings of graphene oxide membranes with cationic for precise sieving of mono-/multi-valent ions

Reduction-enhanced water flux through layered graphene oxide (GO) membranes stabilized with H3O+ and OH- ions

Graphene oxide (GO) is one of the most promising candidates for next generation of atomically thin membranes. Nevertheless, one of the major issues for real world application of GO membranes is their undesirable swelling in an aqueous environment. Recently, we demonstrated that generation of H3O+ and OH- ions (e.g., with an external electric field) in the interlayer gallery could impart aqueous stability to the layered GO membranes (A. Gogoi, ACS Appl. Mater. Interfaces, 2022, 14, 34946). This, however, compromises the water flux through the membrane. In this study, we report on reducing the GO nanosheets as a solution to this issue. With the reduction of the GO nanosheets, the water flux through the layered GO membrane initially increases and then decreases again beyond a certain degree of reduction. Here, two key factors are at play. Firstly, the instability of the H-bond network between water molecules and the GO nanosheets, which increases the water flux. Secondly, the pore size reduction in the interlayer gallery of the membranes, which decreases the water flux. We also observe a significant improvement in the salt rejection of the membranes, due to the dissociation of water molecules in the interlayer gallery. In particular, for the case of 10% water dissociation, the water flux through the membranes can be enhanced without altering its selectivity. This is an encouraging observation as it breaks the traditional tradeoff between water flux and salt rejection of a membrane.

Two-Dimensional Sulfonate-Functionalized Metal-Organic Framework Membranes for Efficient Lithium-Ion Sieving

Two-dimensional (2D) membranes have shown promising potential for ion-selective separation but often suffer from the trade-off between permeability and selectivity. Herein, we report an ultrathin 2D sulfonate-functionalized metal-organic framework (MOF) membrane for efficient lithium-ion sieving. The narrow pores with angstrom precision in the MOF assist hydrated ions to partially remove the hydration shell, according to different hydration energies. The abundant sulfonate groups in the MOF channels serve as hopping sites for fast lithium-ion transport, contributing to a high Li-ion permeability. Then, the difference in affinity of the Li+, Na+, K+, and Mg2+ ions to the terminal sulfonate groups further enhances the Li-ion selectivity. The reported ultrathin MOF membrane overcomes the trade-off between permeability and selectivity and opens up a new avenue for highly permselective membranes.

Engineer Nanoscale Defects into Selective Channels: MOF-Enhanced Li+ Separation by Porous Layered Double Hydroxide Membrane

Two-dimensional (2D) membrane-based ion separation technology has been increasingly explored to address the problem of lithium resource shortage, yet it remains a sound challenge to design 2D membranes of high selectivity and permeability for ion separation applications. Zeolitic imidazolate framework functionalized modified layered double hydroxide (ZIF-8@MLDH) composite membranes with high lithium-ion (Li+) permeability and excellent operational stability were obtained in this work by in situ depositing functional ZIF-8 nanoparticles into the nanopores acting as framework defects in MLDH membranes. The defect-rich framework amplified the permeability of Li+, and the site-selective growth of ZIF-8 in the framework defects bettered its selectivity. Specifically speaking, the ZIF-8@MLDH membranes featured a high permeation rate of Li+ up to 1.73 mol m-2 h-1 and a desirable selectivity of Li+/Mg2+ up to 31.9. Simulations supported that the simultaneously enhanced selectivity and permeability of Li+ are attributed to changes in the type of mass transfer channels and the difference in the dehydration capacity of hydrated metal cations when they pass through nanochannels of ZIF-8. This study will inspire the ongoing research of high-performance 2D membranes through the engineering of defects.

Arresting Aqueous Swelling of Layered Graphene-Oxide Membranes with H3O+ and OH- Ions

Over the past decade, graphene oxide (GO) has emerged as a promising membrane material with superior separation performance and intriguing mechanical/chemical stability. However, its practical implementation remains very challenging primarily because of its undesirable swelling in an aqueous environment. Here, we demonstrated that dissociation of water molecules into H₃O⁺ and OH^- ions inside the interlayer gallery of a layered GO membrane can strongly affect its stability and performance. We reveal that H₃O⁺ and OH^- ions form clusters inside the GO laminates that impede the permeance of water and salt ions through the membrane. Dynamics of those clusters is sensitive to an external ac electric field, which can be used to tailor the membrane performance. The presence of H₃O⁺ and OH^- ions also leads to increased stability of the hydrogen bond (H-bond) network among the water molecules and the GO layers, which further reduces water permeance through the membrane, while crucially imparting stability to the layered GO membrane against undesirable swelling.

Enhanced Separation Performance of Radioactive Cesium and Cobalt in Graphene Oxide Membrane via Cationic Control

The great applications of nuclear power for the most promising clean energy sources have been challenged by a large amount of radioactive wastewater generated, specifically the Cs⁺/Co²⁺ separation for nuclear waste storage, retreatment or recycling of radioactive wastewater, because of their wide difference in half-life and high heat release. In this work, graphene oxide membranes (GOMs) with interlayer spacing controlled by cations were used to separate mixed Cs⁺/Co²⁺ ions. The separation factors of Cs⁺/Co²⁺ for K⁺-controlled graphene oxide membranes (K-GOMs) was 2∼3 times higher than that of GOMs without treatment. In addition, the separation factors of Cs⁺/Co²⁺ for K-GOMs can be further enhanced with the increase of membranes thickness and change the initial ratios of the two ions. Typically, the separation factors of K-GOMs with a thickness of ∼300 nm reached up to 73.7 ± 3.9. Moreover, the K-GOM showed outstanding stability of the separation performance under long-term operation within 7 days. First-principles calculation revealed that the enhanced ionic selectivity of controlled GOM is induced by the difference of adsorption energies between the hydrated cations and aromatic rings, resulting in a significant increase in the mobility differences between Cs⁺ and Co²⁺ through a fixed narrow interlayer spacing. This study demonstrated excellent separation performances of GO-based membranes based on their size-exclusion effect rather than electrostatic repulsion effect, and we believe this work can enable potential efficient treatment technologies for radioactive wastewater needed urgently.

Critical Flux and Fouling Analysis of PVDF-Mixed Matrix Membranes for Reclamation of Refinery-Produced Wastewater: Effect of Mixed Liquor Suspended Solids Concentration and Aeration

Fouling tends to cause a significant increase in hydraulic resistance, decreased permeate flux, or increased transmembrane pressure (TMP) when a process is operated under constant TMP or constant flux conditions. To control membrane fouling and maintain sustainable operation, the concept of critical flux has been discussed by several researchers. Various fouling mechanisms, such as macromolecule adsorption, pore plugging, or cake build-up, as well as hydrodynamic conditions, for example aeration, can take place at the membrane surface. This study aimed to investigate the effects of mixed liquor suspended solid (MLSS) concentration and air bubble flow rate (ABFR) on the critical flux and fouling behavior, when treating refinery-produced wastewater. To determine the critical flux values, the experimental flux-steps were the following: (1) the filtration began with a 30 min step duration at a low flux (10 to 20 L/m²h); (2) at the end of this step (after 30 min), the permeate flux was increased, (3) this step was repeated until the TMP did not remain constant at the constant permeate flux, (4) the critical flux was then achieved. A critical flux model with an R² of 0.9 was, therefore, derived, which indicates that the particle properties were regulated by the suspended solids. The increase of MLSS concentration, from 3 mg/L to 4.5 mg/L, resulted in a decrease of the permeate flux by 18%. Moreover, an increase in ABFR, from 1.2 mL/min to 2.4 mL/min, increased the permeate flux, but this decreased with a greater flow rate of aeration. To assess the stability and reversibility of fouling during critical flux (J_c) determination using a mixed matrix membrane, flux-step methods were utilized. A step height of 14.3 L/m²h and 30 min duration were arbitrarily chosen. The flux increased to 32.5 L/m²h with a slight increase of trans membrane pressure (TMP), while the rate of increase became significant at a higher flux of 143.6 L/m²h, due to fouling. Overall, this study proved that the response of MLSS concentration and aeration affected the membrane performance, based on the critical flux and fouling behavior.

Fast Reduced Graphene-Based Membranes with High Desalination Performance

Graphene-oxide (GO) membrane with notable ions sieving properties has attracted significant attention for many applications. However, because of the water swelling of GO membrane, the rejection of monovalent metal cations is generally low. In this work, we developed a fast and facile method to fabricate a kind of reduced GO membranes using the thermal treatment method at 160 °C for only one minute, which denoted as fast reduced GO membrane (FRGO). Surprising, the FRGO membrane represents high ion sieving ability and ultrahigh water/ions selectivity, compared with other reduced GO membranes with similar average interlayer spacings, and even superior to most of GO-based membranes reported in literature. Building on these findings, we provide a new light on fabricating of energy- and environment-related high desalination performance of GO-based membranes as well as a new insight into the transport mechanism within 2D laminar nanochannels.

A Lamellar MXene (Ti3 C2 Tx )/PSS Composite Membrane for Fast and Selective Lithium-Ion Separation

A two-dimensional (2D) laminar membrane with Li⁺ selective transport channels is obtained by stacking MXene nanosheets with the introduction of poly(sodium 4-styrene sulfonate) (PSS) with active sulfonate sites, which exhibits excellent Li⁺ selectivity from ionic mixture solutions of Na⁺ , K⁺ , and Mg²⁺ . The Li⁺ permeation rate through the MXene@PSS composite membrane is as high as 0.08 mol m^-2  h^-1 , while the Li⁺ /Mg²⁺ , Li⁺ /Na⁺ , and Li⁺ /K⁺ selectivities are 28, 15.5, and 12.7, respectively. Combining the simulation and experimental results, we further confirm that the highly selective rapid transport of partially dehydrated Li⁺ within subnanochannels can be attributed to the precisely controlled interlayer spacing and the relatively weaker ion-terminal (-SO₃ ^- ) interaction. This study deepens the understanding of ion-selective permeation in confined channels and provides a general membrane design concept.

Ultrahigh water permeance of a reduced graphene oxide nanofiltration membrane for multivalent metal ion rejection

We develop a kind of pure rGO membrane using amino-hydrothermal reduction that exhibits an ultrahigh water permeance of 142.5 L m-2 h-1 bar-1 while still maintaining a high rejection rate of 91.6% for multivalent metal ions. The prepared rGO membranes have two types of spacing: larger hydrophilic spacing and smaller hydrophobic spacing, resulting in superior filtration performance. This provides a new avenue for multivalent metal ion separation using pure rGO membranes.