Robust Multilayer Graphene-Organic Frameworks for Selective Separation of Monovalent Anions

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.

Ion Resource Recovery via Electrodialysis Fabricated with Poly(Arylene Ether Sulfone)-Based Anion Exchange Membrane in Organic Solvent System

Ion resource recovery from organic wastewater is beneficial for achieving emission peaks and carbon neutrality targets. Advanced organic solvent-resistant anion exchange membranes (AEMs) for treating organic wastewater via electrodialysis (ED) are of significant interest. Herein, a kind of 3D network AEM based on poly(arylene ether sulfone) cross-linked with a flexible cross-linker (DBH) for ion resource recovery via ED in organic solvent system is reported. Investigations demonstrate that the as-prepared AEMs show excellent dimensional stability in 60% DMSO (aq.), 60% ethanol (aq.), and 60% acetone (aq.), respectively. For example, the optimized AEM shows very low swelling ratios of 1.04-1.10% in the organic solvents. ED desalination ratio can reach 99.1% after exposure of the AEM to organic solvents for 30 days, and remain > 99% in a mixture solution containing organic solvents and 0.5 m NaCl. Additionally, at a current density of 2.5 mA cm-2, the optimized AEM soaked in organic solvents for 30 days shows a high perm-selectivity (Cl-/SO4 2-) of 133.09 (vs 13.11, Neosepta ACS). The superior ED performance is attributed to the stable continuous sub-nanochannels within AEM confirmed by SAXS, rotational energy barriers, etc. This work shows the potential application of cross-linked AEMs for resource recovery in organic wastewater.

Subnanometer Ion Channel Anion Exchange Membranes Having a Rigid Benzimidazole Structure for Selective Anion Separation

Ion-conductive polymers having a well-defined phase-separated structure show the potential application of separating mono- and bivalent ion separation. In this work, three side-chain-type poly(arylene ether sulfone)-based anion exchange membranes (AEMs) have been fabricated to investigate the effect of the stiffness of the polymer backbone within AEMs on the Cl^-/NO₃^- and Cl^-/SO₄^2- separation performance. Our investigations via small-angle X-ray scattering (SAXS), positron annihilation, and differential scanning calorimetry (DSC) demonstrate that the as-prepared AEM with a rigid benzimidazole structure in the backbone bears subnanometer ion channels resulting from the arrangement of the rigid polymer backbone. In particular, SAXS results demonstrate that the rigid benzimidazole-containing AEM in the wet state has an ion cluster size of 0.548 nm, which is smaller than that of an AEM with alkyl segments in the backbone (0.760 nm). Thus, in the electrodialysis (ED) process, the former exhibits a superior capacity of separating Cl^-/SO₄^2- ions relative to latter. Nevertheless, the benzimidazole-containing AEM shows an inability to separate the Cl^-/NO₃^- ions, which is possibly due to the similar ion size of the two. The higher rotational energy barrier (4.3 × 10^-3 Hartree) of benzimidazole units and the smaller polymer matrix free-volume (0.636%) in the AEM significantly contribute to the construction of smaller ion channels. As a result, it is believed that the rigid benzimidazole structure of this kind is a benefit to the construction of stable subnanometer ion channels in the AEM that can selectively separate ions with different sizes.

Effect of the bio-inspired modification of low-cost membranes with TiO2:ZnO as microbial fuel cell membranes

Microbial fuel cells (MFCs) are a novel technique for converting biodegradable materials into electricity. In this study, the efficiency of mixed crystal (TiO₂:ZnO) as a membrane modifier of a low-cost, antifouling and self-cleaning cation exchange membrane for MFCs was studied. The modification was prepared using polydopamine (PDA) as the bio-inspired glue, followed by gravity deposition of a mixture of catalyst nanoparticles (TiO₂:ZnO 0.03%, 1:1 ratio) as anti-biofouling agents. The effects of the membrane modification were evaluated in terms of power density, open circuit potential, coulombic efficiency, anti-biofouling properties and also color and COD removal efficiency. The results showed that the use of the PDA-modified membrane and a mixture of catalysts facilitated the transfer of cations released during the oxidation process in the anodic compartment of the MFC, which increased the power generation in the MFC by 2.5 times and 5.7 times the current compared to pristine and PDA pristine membranes, decreased the MFC operating cycle time from 5 to 3 days, doubled the lifetime of the membranes and demonstrated higher COD removal efficiency and color removal. Finally, SEM and AFM analysis showed that the modification significantly minimized surface fouling. The modified membranes in this study proved to be a potential alternative to the expensive membranes currently used in MFCs, furthermore, this modification could be an interesting alternative modification for other potential membranes for use in MFCs, due to the fact that the catalyst activation was only performed with visible light (artificial and solar), which could decrease operating costs.

Angstrom-scale ion channels towards single-ion selectivity

Artificial ion channels with ion permeability and selectivity comparable to their biological counterparts are highly desired for efficient separation, biosensing, and energy conversion technologies. In the past two decades, both nanoscale and sub-nanoscale ion channels have been successfully fabricated to mimic biological ion channels. Although nanoscale ion channels have achieved intelligent gating and rectification properties, they cannot realize high ion selectivity, especially single-ion selectivity. Artificial angstrom-sized ion channels with narrow pore sizes <1 nm and well-defined pore structures mimicking biological channels have accomplished high ion conductivity and single-ion selectivity. This review comprehensively summarizes the research progress in the rational design and synthesis of artificial subnanometer-sized ion channels with zero-dimensional to three-dimensional pore structures. Then we discuss cation/anion, mono-/di-valent cation, mono-/di-valent anion, and single-ion selectivities of the synthetic ion channels and highlight their potential applications in high-efficiency ion separation, energy conversion, and biological therapeutics. The gaps of single-ion selectivity between artificial and natural channels and the connections between ion selectivity and permeability of synthetic ion channels are covered. Finally, the challenges that need to be addressed in this research field and the perspective of angstrom-scale ion channels are discussed.

Thin-Film Composite Membrane Prepared by Interfacial Polymerization on the Integrated ZIF-L Nanosheets Interface for Pervaporation Dehydration

Thin-film composite (TFC) membranes are attracting wide attention because their ultrathin selective layer usually corresponds to the higher membrane flux for pervaporation. However, the direct preparation of the TFC membranes on ceramic substrates confronted with the great difficulties because the larger pores on ceramic substrate surfaces are detrimental to the formation of an intact polyamide (PA) selective layer produced by interfacial polymerization (IP) reaction. Here, the integrated ZIF-L nanosheets were proposed to be used as an assistance interlayer for the first time to eliminate the existence of the pores of the ceramic support, and provides a better basis for the formation of an intact PA selective layer by IP reaction between TMC and ethylenediamine (EDA). The experimental data obtained in pervaporation (PV) show that the increased flux from 1.1 to 2.9 kg/m²h corresponds to the decreased separation factor from 396 to 110 when the feed concentration of ethanol decreases from 95 wt % to 80 wt % at 50 °C. In addition, the membrane flux increases from 0.8 to 2.5 kg/m²h with a change of the separation factor from 683 to 111 when the operational temperature varies from 30 to 60 °C. These results demonstrate the great potential of the fabricated TFC membranes in practical application for PV dehydration of organic solutions.

Designing Biomimic Two-Dimensional Ionic Transport Channels for Efficient Ion Sieving

Ion transport is crucial for biological systems and membrane-based technologies from both fundamental and practical aspects. Unlike biological ion channels, realizing efficient ion sieving by using membranes with artificial ion channels remains an extremely challenging task. Inspired by biological ion channels with proper steric containment of target ions within affinitive binding sites along the selective filter, herein we design a system of biomimic two-dimensional (2D) ionic transport channels based on a graphene oxide (GO) membrane, where the ionic imidazole group tunes the appropriate physical confinement of 2D ionic transport channels to mimic the confined cavity structures of the biological selectivity filter, and the ionic sulfonic group creates a favorable chemical environment of 2D ionic transport channels to mimic the affinitive binding sites of the biological selectivity filter. As a result, the as-fabricated ionic GO membrane demonstrates an exceptional K⁺ transport rate of ∼1.36 mol m^-2 h^-1 and competitive K⁺/Mg²⁺ selectivity of ∼9.11, outperforming state-of-the-art counterparts. Moreover, the semiquantitative studies of ion transport through 2D ionic transport channels suggest that efficient ion sieving with the ionic GO membrane is achieved by the high diffusion and partition coefficients of hydrated monovalent ions, as well as the large energy barrier and limited potential gradient of hydrated divalent ions encountered.

Design of Monovalent Ion Selective Membranes for Reducing the Impacts of Multivalent Ions in Reverse Electrodialysis

Reverse electrodialysis (RED) represents one of the most promising membrane-based technologies for clean and renewable energy production from mixing water solutions. However, the presence of multivalent ions in natural water drastically reduces system performance, in particular, the open-circuit voltage (OCV) and the output power. This effect is largely described by the “uphill transport” phenomenon, in which multivalent ions are transported against the concentration gradient. In this work, recent advances in the investigation of the impact of multivalent ions on power generation by RED are systematically reviewed along with possible strategies to overcome this challenge. In particular, the use of monovalent ion-selective membranes represents a promising alternative to reduce the negative impact of multivalent ions given the availability of low-cost materials and an easy route of membrane synthesis. A thorough assessment of the materials and methodologies used to prepare monovalent selective ion exchange membranes (both cation and anion exchange membranes) for applications in (reverse) electrodialysis is performed. Moreover, transport mechanisms under conditions of extreme salinity gradient are analyzed and compared for a better understanding of the design criteria. The ultimate goal of the present work is to propose a prospective research direction on the development of new membrane materials for effective implementation of RED under natural feed conditions.

A facile approach to prepare crosslinked polysulfone-based anion exchange membranes with enhanced alkali resistance and dimensional stability

Novel anion exchange membranes with enhanced ion exchange capacity, dimensional stability and alkali stability were prepared by a facile synthesis method. Internal crosslinking networks in the resulting membranes were achieved by reacting chloromethylated polysulfone with 4,4′-trimethylene bis(1-methylpiperidine) (BMP), where BMP was used as both a quaternization reagent and crosslinker without requirement of post-functionalization. In order to evaluate the alkali resistance and dimension stability performance of the resulting membranes, the molar ratio of BMP in the resulting membranes was fixed at four different contents: 40%, 60%, 80% and 100%. The obtained membranes were accordingly denoted as CAPSF-N, in which N = 40, 60, 80 and 100, respectively. Due to the dense internal network structure and spatial conformation of the six-membered rings, the resulting CAPSF-N AEMs showed enhanced dimensional structures (at 60 °C, the water uptakes and swelling ratios of CAPSF-N were 8.42% to 14.84% and 2.32% to 5.93%, respectively, whereas those for the commercial AEM Neosepta AMX were 44.23% and 4.22%, respectively). In addition, after soaking in 1 M KOH solution at 60 °C for 15 days, the modified membranes exhibited excellent alkaline stability. The CAPSF-100 membrane showed the highest alkali stability (retained 85% of its original ion exchange capacity and 84% of its original OH⁻ conduction after the alkaline stability test), whereas the non-crosslinked APSF broke into pieces. Additionally, compared to the commercial Neosepta AMX membrane under the same test conditions, the desalination efficiency of CAPSF-100 was enhanced, and the energy consumption was lower.

Novel anion exchange membranes with enhanced ion exchange capacity, dimensional stability and alkali stability were prepared by a facile synthesis method.

Technology-driven layer-by-layer assembly of a membrane for selective separation of monovalent anions and antifouling

Selective separation of monovalent anions with reduced fouling is one of the major challenges for anion exchange membranes (AEM) in electrodialysis (ED). In this research, an alternating current layer-by-layer (AC∼LbL) assembly technology was first proposed and then applied to the construction of a durable multilayer with the selective separation of monovalent anions with reduced fouling. Under an alternating current (AC) electric field, the hydrophilic poly(4-styrenesulfonic acid-co-maleic acid) sodium salt and 2-hydroxypropyltrimethyl ammonium chloride chitosan were homogenized and rapidly assembled on a commercial original AEM and then crosslinked using 1,4-bis(2',3'-epoxypropyl) perfluoro-1-butane. In ED, the permselectivity and the selective separation efficiency [separation parameter between sulfate (SO42-) and chloride (Cl-) ions] of the resulting membrane (AC∼LbL#7.5 AEM) were 4.87 and 62%, respectively, whereas the original AEM had corresponding parameters of 0.81 and -8%, respectively. Furthermore, the AC∼LbL#7.5 AEM still retained a permselectivity of 4.52 and a selective separation efficiency for Cl- of 57% after 96 h of ED operation. In addition, the AC∼LbL#7.5 AEM showed an excellent antifouling property when three types of organic fouling materials: sodium dodecylbenzenesulfonate, bovine serum albumin and humic acid were used as model foulants.