Stable Graphene-Based Membrane with pH-Responsive Gates for Advanced Molecular Separation

Nanoconfined electrostatic interaction for efficient anion sieving in graphene oxide membranes

Controllable ion transport and precise ion sieving are crucial for sustainable water treatment and resource recovery. 2D materials, including graphene oxide (GO) with tunable nanochannels, are emerging as ideal material platforms to develop ion sieving membranes. However, accurate ion sieving remains challenging due to the swollen and enlarged interlayer spacing of GO membranes in aqueous solution, resulting in the non-selective of small ions. Here, we reformed the GO nanosheets by physical reduction method and modified them with negatively charged molecule chains. The nanochannel sizes and electronegativity of the stacked 2D membranes were precisely controlled simultaneously. As a result, 2D nanochannel membrane with fast permeability, high efficiency and accurate Cl-/SO42- separation was constructed. The characterization and performance analysis further proved that the interlayer spacing and electrification of 2D nanochannels are strongly related to ion sieving. By precisely adjusting the synergy between the two, Cl-/SO42- selectivity up to 91.83 with Cl- permeation rate of 1.03 mol m-2h-1 was achieved, which is superior to state-of-the-art ion sieving membranes. Our study provides new insights into understanding ion separation mechanisms within nanochannels and enables the development for the precise construction of nanochannels to manipulate the selective transport of ions.

Reconstructing Electrically Conductive Nanofiltration Membranes with an Aniline-Functionalized Carbon Nanotubes Interlayer for Highly Effective Toxic Organic Treatment

Conductive nanofiltration (CNF) membranes hold great promise for removing small organic pollutants from water through enhanced Donnan exclusion and electrocatalytic degradation. However, current CNF membranes face limitations in conductivity, structural stability, and nanochannel control strategies. This work addresses these challenges by introducing aniline-functionalized carbon nanotubes (NH2-CNTs) as an interlayer. NH2-CNTs enhance the dispersibility and adhesion of pristine carbon nanotubes, leading to a more conductive and stable composite nanofiltration membrane. The redesigned NH2-CNTs interlayered conductive nanofiltration (NICNF) membrane exhibits a 10-fold increase in conductivity and a high response degree (80%) with excellent cyclic stability, surpassing existing CNF membranes. The synergistic effects of enhanced Donnan exclusion, voltage switching, and electrocatalysis enable the NICNF membrane to achieve selective recovery of mixed dyes, 98.97% removal of residual wastewater toxicity, and a 5.2-fold increase in permeance compared to the commercial NF270 membrane. This research paves the way for next-generation multifunctional membranes capable of the efficient recovery and degradation of toxic organic pollutants in wastewater.

Advanced stimuli-responsive membranes for smart separation

Membranes have been extensively studied and applied in various fields owing to their high energy efficiency and small environmental impact. Further conferring membranes with stimuli responsiveness can allow them to dynamically tune their pore structure and/or surface properties for efficient separation performance. This review summarizes and discusses important developments and achievements in stimuli-responsive membranes. The most commonly utilized stimuli, including light, pH, temperature, ions, and electric and magnetic fields, are discussed in detail. Special attention is given to stimuli-responsive control of membrane pore structure (pore size and porosity/connectivity) and surface properties (wettability, surface topology, and surface charge), from the perspective of determining the appropriate membrane properties and microstructures. This review also focuses on strategies to prepare stimuli-responsive membranes, including blending, casting, polymerization, self-assembly, and electrospinning. Smart applications for separations are also reviewed as well as a discussion of remaining challenges and future prospects in this exciting field. This review offers critical insights for the membrane and broader materials science communities regarding the on-demand and dynamic control of membrane structures and properties. We hope that this review will inspire the design of novel stimuli-responsive membranes to promote sustainable development and make progress toward commercialization.

Reduced graphene oxide/TiO2(B) immobilized on nylon membrane with enhanced photocatalytic performance

Taking advantage of the unique properties of reduced graphene oxide (rGO) and monoclinic crystalline titanium dioxide (TiO₂(B)) nanomaterials, a novel rGO-TiO₂(B) composite membrane (MrGO-TiO₂(B)) was constructed by UV-light-assisted self-assembly of rGO and TiO₂ on a nylon membrane. The structure of MrGO-TiO₂(B) was characterized by scanning electron microscopy, transmission electron microscopy, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analysis. Through 2D/2D self-assembly, rGO and TiO₂(B) were more tightly combined, and then MrGO-TiO₂(B) exhibited outstanding photocatalytic activity and an excellent methylene blue (MB) removal rate. MB was completely removed in 60 min at a constant rate of 0.042 min^-1 by the MrGO-TiO₂(B)/H₂O₂/MB system upon solar simulating Xe lamp irradiation. The synergistic effect of rGO and TiO₂(B) facilitated the photocatalytic degradation of MB. TiO₂(B) was excited and generated electrons and holes upon irradiation. Some electrons migrated to the surface of TiO₂(B) to react with H₂O₂ to produce hydroxyl radicals (OH), while the other electrons migrated to the surface of rGO to react with H₂O₂, producing OH. In addition, a number of superoxide radicals (O₂^-) was detected. The holes in the valence band of TiO₂(B) directly oxidized MB. The catalytic activity of MrGO-TiO₂(B) toward MB degradation remained stable after four rounds of reuse. Therefore, the surface modification of a nylon membrane with TiO₂(B) and rGO can serve as a promising route to fabricate photocatalytic membranes for use in the water treatment industry.

High-Flux pH-Responsive Ultrafiltration Membrane for Efficient Nanoparticle Fractionation

Fractionation of nanoparticles with different sizes from the mixture by using a single membrane would reduce the membrane cost and enhance the efficiency. In this study, an amphiphilic pH-responsive copolymer was prepared by grafting a pH-responsive hydrophilic polymethacrylic acid (PMAA) side chain from a hydrophobic poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-CTFE) backbone. Subsequently, the isoporous pH-responsive membranes (PPMs) were prepared from the functional copolymers with different PMAA chain lengths. PPM indicated reversible pore size decreasing with the increasing pH of the feed. Moreover, the membrane pore size variation range was further extended by adjusting the PMAA side chain length of the copolymer to reach a wide range from 10.2 to 34.5 nm. Owning to the amphiphilic nature of the copolymer, PPM showed a narrow pore size distribution which is responsible for the much higher pure water flux of PPM than the conventional UF membrane with similar retention capability. In the fractionation test, the mixed 20 and 30 nm polystyrene nanoparticles were penetrating PPM at pH 11 and 3, respectively. The pH-responsive PPM indicated great potential for nanoparticle fractionation, while the uniform pores of PPM further enhanced the membrane performance in terms of permeability and selectivity.

Planning of smart gating membranes for water treatment

The use of membranes in desalination and water treatment has been intensively studied in recent years. The conventional membranes however have various problems such as uncontrollable pore size and membrane properties, which prevents membranes from quickly responding to alteration of operating and environmental conditions. As a result the membranes are fouled, and their separation performance is lowered. The preparation of smart gating membranes inspired by cell membranes is a new method to face these challenges. Introducing stimuli-responsive functional materials into traditional porous membranes and use of hydrogels and microgels can change surface properties and membrane pore sizes under different conditions. This review shows potential of smart gating membranes in water treatment. Various types of stimuli-response such as those of thermo-, pH-, ion-, molecule-, UV light-, magnetic-, redox- and electro-responsive gating membranes along with various gel types such as those of polyelectrolyte, PNIPAM-based, self-healing hydrogels and microgel based-smart gating membranes are discussed. Design strategies, separation mechanisms and challenges in fabrication of smart gating membranes in water treatment are also presented. It is demonstrated that experimental and modeling and simulation results have to be utilized effectively to produce smart gating membranes.