Composite anti-scaling membrane made of interpenetrating networks of nanofibers for selective separation of lithium

High-Performance Monovalent Selective Cation Exchange Membranes with Ionically Cross-Linkable Side Chains: Effect of the Acidic Groups

Inspired by the charge-governed protein channels located in the cell membrane, a series of polyether ether ketone-based polymers with side chains containing ionically cross-linkable quaternary ammonium groups and acidic groups have been designed and synthesized to prepare monovalent cation-selective membranes (MCEMs). Three acidic groups (sulfonic acid, carboxylic acid, and phenolic hydroxyl) with different acid dissociation constant (pKa) were selected to form the ionic cross-linking structure with quaternary ammonium groups in the membranes. The ionic cross-linking induced the nanophase separation and constructed ionic channels, which resulted in excellent mechanical performance and high cation fluxes. Interesting, the cation flux of membranes increased as the ionization of acidic groups increase, but the selectivity of MCEMs did not follow the same trend, which was mainly dependent on the affinity between the functional groups and the cations. Carboxyl group-containing MCEMs exhibited the best selectivity (9.01 for Li+/Mg2+), which was higher than that of the commercial monovalent cation-selective CIMS membrane. Therefore, it is possible to prepare stable MCEMs through a simple process using ionically cross-linkable polymers, and tuning acidic groups in the membranes provided an attractive approach to improving the cation flux and selectivity of MCEMs.

Loose Nanofiltration Membrane Incorporating CeZnFe Layered Double Hydroxide with Enhanced Dye/Salt Separation Performance and Self-Cleaning Ability

The high-salinity wastewater from the textile industry faces a significant challenge in effectively separating dyes and salts. In this study, a CeZnFe-layered double hydroxide (LDH)-incorporated nanofiltration (LNF) membrane was fabricated using the conventional interfacial polymerization (IP) technique to fractionate dyes and salts within the wastewater. The impact of CeZnFe LDH on various aspects of membrane performance was examined, including water flux, dye removal efficiency, dye/salt separation capability, self-cleaning ability, and membrane integrity. The addition of LDHs resulted in improved membrane surface hydrophilicity, thereby enhancing water flux. The optimized TFN membrane (0.050 wt% LDH in PIP solution) significantly improved pure water flux, exceeding 150%. All TFN membranes exhibited excellent performance in dye and salt fractionation (93% for Congo red, 2.6% for NaCl, and 40.7% for Na2SO4). Also, excellent self-cleaning ability was observed for the optimized membrane, exhibiting a remarkable water flux recovery rate after three operation cycles. Moreover, including CeZnFe LDH in the optimized TFN membrane played a significant role in enhancing membrane integrity. This study provides new inspiration for fabricating self-cleaning loose NF membranes using CeZnFe LDH for effective dye/salt separation.

Constructing Mechanical Shuttles in a Three-dimensional (3D) Porous Architecture for Selective Transport of Lithium Ions

Lithium (Li) extraction from brines is a major barrier to the sustainable development of batteries and alloys; however, current separation technology suffers from a trade-off between ion selectivity and permeability. Herein, a crown ether mechanically interlocked 3D porous organic framework (Crown-POF) was prepared as the porous filler of thin-film nanocomposite membranes. Crown-POF with penta-coordinated (four O_crown atoms and one N_tert-amine atom) adsorption sites enables a special recognition for Li⁺ ion. Moreover, the four N_tert-amine atoms on each POF branch facilitate the flipping motion of Li⁺ ion along the skeletal thread, while retaining the specified binding pattern. Accordingly, the crown ether interlocked POF network displays an ultrafast ion transfer rate, over 10 times that of the conventional porous materials. Notably, the nanocomposite membrane gives high speed and selectivity for Li⁺ ion transport as compared with other porous solid-based mixed-matrix membranes.

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.

Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration-Bipolar Membrane Electrodialysis-Based Closed-Loop Process

Traditional technologies such as precipitation and coagulation have been adopted for fluoride-rich and silica-rich wastewater treatment, respectively, but waste solid generation and low wastewater processing efficiency are still the looming concern. Efficient resource recovery technologies for different wastewater treatments are scarce for environment and industry sustainability. Herein, a resource capture ultrafiltration-bipolar membrane electrodialysis (RCUF-BMED) system was designed into a closed-loop process for simultaneous capture and recovery of fluoride and silica as sodium silicofluoride (Na₂SiF₆) from mixed fluoride-rich and silica-rich wastewaters, as well as achieving zero liquid discharge. This RCUF-BMED system comprised two key parts: (1) capture of fluoride and silica from two wastewaters using acid, and recovery of the Na₂SiF₆ using base by UF and (2) UF permeate conversion for acid/base and freshwater generation by BMED. With the optimized RCUF-BMED system, fluoride and silica can be selectively captured from wastewater with removal efficiencies higher than 99%. The Na₂SiF₆ recovery was around 72% with a high purity of 99.1%. The aging and cyclic experiments demonstrated the high stability and recyclability of the RCUF-BMED system. This RCUF-BMED system has successfully achieved the conversion of toxic fluoride and silica into valuable Na₂SiF₆ from mixed wastewaters, which shows great application potential in the industry-resource-environment nexus.

Application and Visualization of Fluorescent-Tagged Antiscalants in Electrodialysis Processing of Aqueous Solutions Prone to Gypsum Scale Deposition

Membrane scaling is a serious problem in electrodialysis. A widely used technique for controlling scale deposition in water treatment technologies is the application of antiscalants (AS). The present study reports on gypsum scale inhibition in electrodialysis cell by the two novel ASs: fluorescent-tagged bisphosphonate 1-hydroxy-7-(6-methoxy-1,3-dioxo-1Hbenzo[de]isoquinolin-2(3H)-yl)heptane-1,1-diyl-bis(phosphonic acid), HEDP-F and fluorescein-tagged polyacrylate, PAA-F2 (molecular mass 4000 Da) monitored by chronopotentiometry and fluorescent microscopy. It was found that cation-exchange membrane MK-40 scaling is sufficiently reduced by both ASs, used in 10^-6 mol·dm^-3 concentrations. PAA-F2 at these concentrations was found to be more efficient than HEDP-F. At the same time, PAA-F2 reveals gypsum crystals' habit modification, while HEDP-F does not noticeably affect the crystal form of the deposit. The strong auto-luminescence of MK-40 hampers visualization of both PAA-F2 and HEDP-F on the membrane surface. Nevertheless, PAA-F2 is proved to localize partly on the surface of gypsum crystals as a molecular adsorption layer, and to change their crystal habit. Crystal surface coverage by PAA-F2 appears to be nonuniform. Alternatively, HEDP-F localizes on the surface of a deposit tentatively in the form of [Ca-HEDP-F]. The proposed mechanisms of action are formulated and discussed. The application of antiscalants in electrodialysis for membrane scaling mitigation is demonstrated to be very promising.

Vanadium recovery by electrodialysis using polymer inclusion membranes

Industrial applications and environmental awareness recently prompted vanadium recovery spell from secondary resources. In this work, a polymer inclusion membrane containing trioctylmethylammonium chloride as carrier was successfully employed in electrodialysis for vanadium recovery from acidic sulfate solutions. The permeability coefficient of V(V) increased from 0.29 µm·s^-1 (without electric field) to 4.10 µm·s^-1 (with the 20 mA·cm^-2 current density)_. The transport performance of VO₂SO₄^-, which was the predominant species containing V(V) in the acidic region (pH <3), was influenced by the aqueous pH value and sulfate concentration. Under an electric field, a low concentrated H₂SO₄ solution (0.2 M) effectively stripped V(V) from the membranes, avoiding the requirement of a highly concentrated H₂SO₄ without electric field. Under the optimum conditions, the permeability coefficient and flux reached 6.80 µm·s^-1 and 13.34 µmol·m^-2·s^-1, respectively. High selectivity was observed for the separation of V(V) and Mo(VI) from mixed solutions of Co (II), Ni (II), Mn (II), and Al (III). Additionally, the separation between Mo(VI) and V(V) was further improved by adjusting the acidity of the stripping solution. The V(V) selectivity for the resulting membrane was higher than that of commercial anion exchange membranes.

Asymmetric Cellulose/Carbon Nanotubes Membrane with Interconnected Pores Fabricated by Droplet Method for Solar-Driven Interfacial Evaporation and Desalination

Solar-driven interfacial water purification and desalination have attracted much attention in environmentally friendly water treatment field. The structure design of the photothermal materials is still a critical factor to improve the evaporation performance such as evaporation rate and energy conversion efficiency. Herein, an asymmetric cellulose/carbon nanotubes membrane was designed as the photothermal membrane via a modified droplet method. Under 1 sun irradiation, the evaporation rate and energy efficiency of pure water can reach up to 1.6 kg m−2 h−1 and 89%, respectively. Moreover, stable reusability and desalination performance made the cellulose/carbon nanotubes membrane a promising photothermal membrane which can be used for solar-driven desalination.

Fabrication of Polyamide-6 Membranes—The Effect of Gelation Time towards Their Morphological, Physical, and Transport Properties

Porous polyamide-6 membranes were fabricated via a non-solvent induced phase inversion method, and the influence of gelation time on the properties of the membranes was investigated. Membrane samples with various gelation times were prepared. The evaluation of the membranes’ properties was carried out by various analyses and tests, such as scanning electron microscopy, atomic force microscopy, contact angle, wet and dry thickness, mean pore size measurements, porosity, water uptake, mechanical resistance, hydrodynamic water fluxes, membrane hydrodynamic permeability, and retention testing. The scanning electron microscopy images (both surface and cross-section) demonstrated that the increase in gelation time from 0 (M₀) to 10 (M₁₀) min led to the morphological change of membranes from isotropic (M₀) to anisotropic (M₁₀). The wet and dry thickness of the membranes showed a downward tendency with increasing gelation time. The M₀ membrane exhibited the lowest bubble contact angle of 60 ± 4° and the lowest average surface roughness of 124 ± 22 nm. The highest values of mean pore size and porosity were observed for the M₀ sample (0.710 ± 0.06 µm and 72 ± 2%, respectively), whereas the M₁₀ membrane demonstrated the highest tensile strength of 4.1 MPa. The membrane water uptake was diminished from 62 to 39% by increasing the gelation time from 0 to 10 min. The M₀ membrane also showed the highest hydrodynamic water flux among the prepared membranes, equal to 28.6 L m⁻² h⁻¹ (at Δp = 2 bar).

Ionic Control of Functional Zeolitic Imidazolate Framework-Based Membrane for Tailoring Selectivity toward Target Ions

Ion exchange membranes with strong ionic separation performance have strategic importance for resource recovery and water purification, but the current state-of-the-art membranes suffer from inadequate ion selective transport for the target ions. This work proposes a new class of zeolitic imidazolate framework (ZIF)-based anion exchange membranes (named as S@ZIF-AMX) with suppressed multivalent anion mobility and enhanced target ion transport via an ionic control strategy under alternating current driven assembly. In electrodialysis with an initial concentration of 50 mM of NaBr, NaCl, Na₂SO₄, and Na₃PO₄ (mixed feed) and a current density of 10 mA cm^-2, the S@ZIF-AMX membrane demonstrated an excellent transport of the target ion (Cl^-) based on the synergy between the Cl^- regulated ZIF cavity and the electrostatic interaction with sulfonic groups. The separation efficiency and permselectivity of PO₄^3-/Cl^- through S@ZIF-AMX largely increased to 83% and 32, respectively, compared to 42% and 4.0 of the pristine AMX membrane (a commercial anion exchange membrane), respectively. Furthermore, the separation between SO₄^2- and Cl^- was also enhanced, the separation efficiency and permselectivity of SO₄^2-/Cl^- increased from 11% and 1.4 to 45% and 4.3, respectively. In addition, the combined strategy developed in the S@ZIF-AMX membrane was proven effective in promoting Cl^- transport by shifting the separation equilibrium of the ion pair Br^-/Cl^-, which is known to be extremely challenging. This work provides a new design strategy toward pushing the limits of current ion exchange membranes for target ion separation in water, resource, and energy applications.

Membrane bioreactors for hospital wastewater treatment: recent advancements in membranes and processes

Discharged hospital wastewater contains various pathogenic microorganisms, antibiotic groups, toxic organic compounds, radioactive elements, and ionic pollutants. These contaminants harm the environment and human health causing the spread of disease. Thus, effective treatment of hospital wastewater is an urgent task for sustainable development. Membranes, with controllable porous and nonporous structures, have been rapidly developed for molecular separations. In particular, membrane bioreactor (MBR) technology demonstrated high removal efficiency toward organic compounds and low waste sludge production. To further enhance the separation efficiency and achieve material recovery from hospital waste streams, novel concepts of MBRs and their applications are rapidly evolved through hybridizing novel membranes (non hydrophilic ultrafiltration/microfiltration) into the MBR units (hybrid MBRs) or the MBR as a pretreatment step and integrating other membrane processes as subsequent secondary purification step (integrated MBR-membrane systems). However, there is a lack of reviews on the latest advancement in MBR technologies for hospital wastewater treatment, and analysis on its major challenges and future trends. This review started with an overview of main pollutants in common hospital waste-water, followed by an understanding on the key performance indicators/criteria in MBR membranes (i.e., solute selectivity) and processes (e.g., fouling). Then, an in-depth analysis was provided into the recent development of hybrid MBR and integrated MBR-membrane system concepts, and applications correlated with wastewater sources, with a particular focus on hospital wastewaters. It is anticipated that this review will shed light on the knowledge gaps in the field, highlighting the potential contribution of hybrid MBRs and integrated MBR-membrane systems toward global epidemic prevention.

De Novo Ion-Exchange Membranes Based on Nanofibers

The unique functions of nanofibers (NFs) are based on their nanoscale cross-section, high specific surface area, and high molecular orientation, and/or their confined polymer chains inside the fibers. The introduction of ion-exchange (IEX) groups on the surface and/or inside the NFs provides de novo ion-exchangers. In particular, the combination of large surface areas and ionizable groups in the IEX-NFs improves their performance through indices such as extremely rapid ion-exchange kinetics and high ion-exchange capacities. In reality, the membranes based on ion-exchange NFs exhibit superior properties such as high catalytic efficiency, high ion-exchange and adsorption capacities, and high ionic conductivities. The present review highlights the fundamental aspects of IEX-NFs (i.e., their unique size-dependent properties), scalable production methods, and the recent advancements in their applications in catalysis, separation/adsorption processes, and fuel cells, as well as the future perspectives and endeavors of NF-based IEMs.

Recycling of High-Value-Added Aramid Nanofibers from Waste Aramid Resources via a Feasible and Cost-Effective Approach

High-performance aramid fibers are extensively applied in the civil and military fields. A great deal of waste aramid resources originating from the manufacturing process, spare parts, or end of life cycle are wrongly disposed (i.e., landfill, smash, fibrillation), causing a waste of valuable resources as well as severe environmental pollution. Although aramid nanofibers (ANFs) have recently been recently reported as one of the most promising building blocks due to their excellent properties, they suffer from an extremely high production expenditure, thereby greatly hindering their scale-up application. Herein, in this paper, from a resources-saving and cost-reductional perspective, we present a feasible top-down approach to recycle high value-added ANFs with an affordable cost from various waste aramid resources. The results indicate that although the reclaimed ANFs have a molecular weight reduction of 8.1% compared with the recycled aramid fibers, they still exhibit a molecular weight of 43.0 kg·mol^-1 that represents the highest value compared to other methods. It is noteworthy that the fabrication cost of ANFs is significantly reduced (∼7 times) due to the reclamation of waste aramid fibers instead of the expensive virgin aramid fibers. The obtained ANFs show impressive tensile strength (149.2 MPa) and toughness (10.43 MJ·m^-3), excellent thermal stabilities (T_d of 542 °C), and a high specific surface area (65.2 m²·g^-1), which endows them to be promising candidates for constructing advanced materials. Compared to the aramid pulp obtained by the traditional recycling method, ANFs show significant advantages in dimensional homogeneity, aspect ratio, dispersibility, film-forming property, and especially the excellent properties of the ANF film. In addition, the scale-up preparation of ANFs from the recycled waste aramid fibers is carried out, demonstrating it is highly economically viable. Therefore, this work provides a highly feasible and cost-effective recycle system to reclaim the waste aramid resources together with significantly reducing the preparation cost of ANFs.