Ion-Selective Transport Promotion Enabled by Angstrom-Scale Nanochannels in Dendrimer-Assembled Polyamide Nanofilm for Efficient Electrodialysis.

The ion permeability and selectivity of membranes are crucial in nanofluidic behavior, impacting industries ranging from traditional to advanced manufacturing. Herein, we demonstrate the engineering of ion-conductive membranes featuring angstrom-scale ion-transport channels by introducing ionic polyamidoamine (PAMAM) dendrimers for ion separation. The exterior quaternary ammonium-rich structure contributes to significant electrostatic charge exclusion due to enhanced local charge density; the interior protoplasmic channels of PAMAM dendrimer are assembled to provide additional degrees of free volume. This facilitates the monovalent ion transfer while maintaining continuity and efficient ion screening. The dendrimer-assembled hybrid membrane achieves high monovalent ion permeance of 2.81 mol m-2 h-1 (K+), reaching excellent mono/multivalent selectivity up to 20.1 (K+/Mg2+) and surpassing the permselectivities of state-of-the-art membranes. Both experimental results and simulating calculations suggest that the impressive ion selectivity arises from the significant disparity in transport energy barrier between mono/multivalent ions, induced by the "exterior-interior" synergistic effects of bifunctional membrane channels.

Nanochannel Stability of Chemically Converted Graphene Oxide Membranes.

Chemically converted graphene oxide laminate membranes, which exhibit stable interlayered nanochannels in aqueous environments, are receiving increasing attention owing to their potential for selective water and ion permeation. However, how the molecular properties of conversion agents influence the stabilization of nanochannels and how effectively nanochannels are stabilized have rarely been studied. In this study, mono-, di-, and tri-saccharide molecules of glucose (Glu), maltose (Glu2), and maltotriose (Glu3) are utilized, respectively, to chemically modify graphene oxide (GO). The aim is to create nanochannels with different levels of stability and investigate how these functional conversion agents affect the separation performance. The effects of the property differences between different conversion agents on nanochannel stabilization are demonstrated. An agent with efficient chemical reduction of GO and limited intercalation in the resulting nanochannel ensures satisfactory nanochannel stability during desalination. The stabilized membrane nanochannel exhibits a permeance of 0.69 L m-2 h-1 bar-1 and excellent Na2SO4 rejection of 96.42%. Furthermore, this optimized membrane nanochannel demonstrates enhanced stability under varying external conditions compared to the original GO. This study provides useful information for the design of chemical conversion agents for GO nanochannel stabilization and the development of nanochannel membranes for precise separation.

Side-Chain-Dependent Functional Intercalations in Graphene Oxide Membranes for Selective Water and Ion Transport.

Subnanometer interlayer space in graphene oxide (GO) laminates is desirable for use as permselective membrane nanochannels. Although the facile modification of the local structure of GO enables various nanochannel functionalizations, precisely controlling nanochannel space is still a challenge, and the roles of confined nanochannel chemistry in selective water/ion separation have not been clearly defined. In this study, macrocyclic molecules with consistent basal plane but varying side groups were used to conjunct with GO for modified nanochannels in laminates. We demonstrated the side-group dependence of both the angstrom-precision tunability for channel free space and the energy barrier setting for ion transport, which challenges the permeability-selectivity trade-off with a slightly decreased permeance from 1.1 to 0.9 L m-2 h-1 bar-1 but an increased salt rejection from 85% to 95%. This study provides insights into the functional-group-dependent intercalation modifications of GO laminates for understanding laminate structural control and nanochannel design.

Confined-Coordination Induced Intergrowth of Metal-Organic Frameworks into Precise Molecular Sieving Membranes.

Metal-organic framework (MOF) membranes with rich functionality and tunable pore system are promising for precise molecular separation; however, it remains a challenge to develop defect-free high-connectivity MOF membrane with high water stability owing to uncontrollable nucleation and growth rate during fabrication process. Herein, we report on a confined-coordination induced intergrowth strategy to fabricate lattice-defect-free Zr-MOF membrane towards precise molecular separation. The confined-coordination space properties (size and shape) and environment (water or DMF) were regulated to slow down the coordination reaction rate via controlling the counter-diffusion of MOF precursors (metal cluster and ligand), thereby inter-growing MOF crystals into integrated membrane. The resulting Zr-MOF membrane with angstrom-sized lattice apertures exhibits excellent separation performance both for gas separation and water desalination process. It was achieved H2 permeance of ~1200 GPU and H2/CO2 selectivity of ~67; water permeance of ~8 L ⋅ m-2 ⋅ h-1 ⋅ bar-1 and MgCl2 rejection of ~95 %, which are one to two orders of magnitude higher than those of state-of-the-art membranes. The molecular transport mechanism related to size-sieving effect and transition energy barrier differential of molecules and ions was revealed by density functional theory calculations. Our work provides a facile approach and fundamental insights towards developing precise molecular sieving membranes.

Two-Dimensional Interlayer Space Induced Horizontal Transformation of Metal-Organic Framework Nanosheets for Highly Permeable Nanofiltration Membranes.

Laminar membranes comprising graphene oxide (GO) and metal-organic framework (MOF) nanosheets benefit from the regular in-plane pores of MOF nanosheets and thus can support rapid water transport. However, the restacking and agglomeration of MOF nanosheets during typical vacuum filtration disturb the stacking of GO sheets, thus deteriorating the membrane selectivity. Therefore, to fabricate highly permeable MOF nanosheets/reduced GO (rGO) membranes, a two-step method is applied. First, using a facile solvothermal method, ZnO nanoparticles are introduced into the rGO laminate to stabilize and enlarge the interlayer spacing. Subsequently, the ZnO/rGO membrane is immersed in a solution of tetrakis(4-carboxyphenyl)porphyrin (H2 TCPP) to realize in situ transformation of ZnO into Zn-TCPP in the confined interlayer space of rGO. By optimizing the transformation time and mass loading of ZnO, the obtained Zn-TCPP/rGO laminar membrane exhibits preferential orientation of Zn-TCPP, which reduces the pathway tortuosity for small molecules. As a result, the composite membrane achieves a high water permeance of 19.0 L m-2  h-1  bar-1 and high anionic dye rejection (>99% for methyl blue).

Surface Mineralization of the TiO2-SiO2/PES Composite Membrane with Outstanding Separation Property via Facile Vapor-Ventilated In Situ Chemical Deposition.

Conventional polymeric membranes are broadly employed in water treatment processes; however, most of them suffer from relatively low water permeance and severe membrane fouling phenomena owing to their relatively hydrophobic nature. In this work, a novel class of inorganic-organic composite membranes was developed through a newly developed vapor-ventilated in situ chemical deposition method, where the Ti and Si precursors were first hydrolyzed and then conferred into metal oxides to form a continuous TiO2-SiO2 modification layer. Owing to the distinct physicochemical properties, the Ti and Si precursors were leveraged as quasi-molecular regulators to tune the membrane surface chemistry and pore aperture (within the nanoscale) to benefit highly efficient water purification by underpinning the rapid transport of water molecules and featuring an excellent fouling-resistant and fouling-releasing property against typical pollutants. The as-developed TiO2-SiO2/PES composite membrane showed a high water permeance of 187.4 L·m-2·h-1·bar-1, together with a relatively small mean pore aperture of 4.2 nm, showing an outstanding permeating efficiency among state-of-the-art membranes with a similar separation accuracy. This study provides a paradigm shift in membrane materials that could open avenues for developing high-performance inorganic-organic composite membranes for complex wastewater treatment.

Tough ion gels composed of coordinatively crosslinked polymer networks using ZIF-8 nanoparticles as multifunctional crosslinkers.

Constructing crosslinked polymer networks via reversible interactions is a promising approach to recover the mechanical strength of damaged gels. In addition, by designing effective reversible crosslinks, the mechanical strength of the gel can be enhanced through energy dissipation based on the destruction of the crosslinks by an applied force. In this study, we introduced zeolitic imidazole framework-8 nanoparticles (ZIF-8 NPs), which acted as multifunctional crosslinkers, to provide multipoint coordination bonding with a poly(N,N-dimethylacrylamide)-based polymer network in a gel containing an ionic liquid. The mechanical strength of the gel increased with an increase in the content of ZIF-8 NPs up to 6 wt%. It was confirmed that the energy loaded onto the gel was dissipated through the desorption of the polymer network from the surface of the ZIF-8 NPs. Owing to the reversible destruction and reconstruction of the coordinative crosslinking between the polymer network and ZIF-8 NPs, the mechanical strength of the damaged gel was almost fully recovered through annealing.

Chemically Converted Graphene Nanosheets for the Construction of Ion-Exclusion Nanochannel Membranes.

Water and ion transport in nanochannels is an intriguing topic that has been extensively investigated in several energy- and environment-related research fields. Recently developed two-dimensional (2D) materials are ideal building blocks for constructing confined nanochannels by self-stacking. Among these, graphene oxide (GO) is the most frequently employed as the starting material because of its excellent solution processability. Since solvation of the GO nanostructure usually impairs the function of nanochannels, in this study, chemically converted graphene was prepared using a one-step method, to simultaneously acquire the desired stability and functionality of the nanochannels. The confined channels with high charge densities are capable of excluding ∼90% NaCl solutes from water in a pressure-driven filtration process. This surpasses the performance of most GO desalination membranes reported in the literature. Thus, this study provides useful information for the feasible development of ion-exclusion nanochannel membranes based on the proposed nanochannel-confined charge repulsion mechanism.

Zwitterionic Copolymer-Regulated Interfacial Polymerization for Highly Permselective Nanofiltration Membrane.

A highly permselective nanofiltration membrane was engineered via zwitterionic copolymer assembly regulated interfacial polymerization (IP). The copolymer was molecularly synthesized using single-step free-radical polymerization between 2-methacryloyloxyethyl phosphorylcholine (MPC) and 2-aminoethyl methacrylate hydrochloride (AEMA) (P[MPC-co-AEMA]). The dynamic network of P[MPC-co-AEMA] served as a regulator to precisely control the kinetics of the reaction by decelerating the transport of piperazine toward the water/hexane interface, forming a polyamide (PA) membrane with ultralow thickness of 70 nm, compared to that of the pristine PA (230 nm). Concomitantly, manipulating the phosphate moieties of P[MPC-co-AEMA] integrated into the PA matrix enabled the formation of ridge-shaped nanofilms with loose internal architecture exhibiting enhanced inner-pore interconnectivity. The resultant P[MPC-co-AEMA]-incorporated PA membrane exhibited a high water permeance of 15.7 L·m-2·h-1·bar-1 (more than 3-fold higher than that of the pristine PA [4.4 L·m-2·h-1·bar-1]), high divalent salt rejection of 98.3%, and competitive mono-/divalent ion selectivity of 52.9 among the state-of-the-art desalination membranes.

Molecular dynamics study on the elucidation of polyamide membrane fouling by nonionic surfactants and disaccharides.

Reverse osmosis (RO) is a widely used energy-efficient separation technology for water treatment. Polyamide (PA) membranes are the conventional choice for this process. Fouling is a serious problem for RO separation. This issue leads to significant decreases in the water permeability of PA membranes, and it has yet to be fully elucidated. In particular, the fouling behavior of a nonionic substance on the negatively charged surface of a PA membrane in an aqueous environment has not been previously studied. In this work, the mechanisms of nonionic substances such as polyoxyethylene octyl ether (PE5) and maltose (Mal) were investigated using molecular dynamics (MD) simulations. In a PA membrane in which the carboxyl group was not dissociated, the hydrophobic portion of the membrane was exposed due to the localization of water molecules around the carboxyl groups in the PA membrane. This caused hydrophobic interaction with the hydrophobic groups of PE5. In the case of an amine-modified PA membrane containing no carboxyl groups, water was not localized around the functional group, and the water orientation of the polyamide surface was also low. Due to this membrane property, the presence of stabilized water around PE5 reduced the number of hydrophobic interactions. In similar manner, a PA membrane with a slightly dissociated carboxyl group was hydrophilic, which reduced the PE5 adsorption. The presence of many dissociated carboxyl groups, however, enhanced the adsorption of PE5 due to the increase in interactions between the dissociated carboxyl groups and the hydrophilic groups of PE5. Therefore, PE5 exhibited an amphipathic adsorption wherein both hydrophilic and hydrophobic groups contributed to adsorption onto the PA membrane. Mal, on the other hand, was highly stable in every aqueous environment independent of the state of the functional groups of the PA membrane, and was not easily affected by the properties of the PA membrane.

Energy dissipation via the internal fracture of the silica particle network in inorganic/organic double network ion gels.

Inorganic/organic double network (DN) ion gels, which are composed of an inorganic silica particle network, an organic poly(N,N-dimethylacrylamide) (PDMAAm) network, and a large amount of ionic liquid, showed excellent mechanical strength of over 25 MPa compression fracture stress at an 80 wt% ionic liquid content. The excellent mechanical strength of these inorganic/organic DN ion gels was attributed to the energy dissipation of the inorganic/organic DN structure. It has been considered that the energy dissipation in inorganic/organic DN ion gels is caused by the internal fracture of the silica particle network, which is preferentially fractured by deformation. However, no studies aiming to investigate the internal fracture of the silica particle network in inorganic/organic DN ion gels have been conducted by direct approaches. In this study, the internal fracture of the silica particle network in the inorganic/organic DN ion gel was directly evaluated by a small angle X-ray scattering (SAXS) technique. The synchrotron SAXS measurements conducted under a uniaxial loading-unloading process demonstrated that the aggregation size of the silica particle network irreversibly decreased with uniaxial stretch. Based on these results, it was clarified that the energy dissipation of the inorganic/organic DN ion gels was attributed to the internal fracture of the silica particle network.

Change of foulant concentration in an anaerobic membrane bioreactor.

Anaerobic membrane bioreactors (AnMBRs) have many advantages, such as producing methane gas for energy generation and little excess sludge. However, membrane fouling is a serious problem because the foulant, which causes the membrane to foul, may get rejected by the membrane and accumulate in the reactor, resulting in an acceleration of membrane fouling. However, there is no information related to a change in the foulant concentration in an AnMBR. Therefore, we examined the changes in the foulant concentration in the reactor, related to membrane fouling in an AnMBR. For the influent, reactor solution, and effluent, the concentration of each component of the foulant was analyzed by using a liquid chromatography-organic carbon detector (LC-OCD). It was found that fouling in the AnMBR was closely related to the components in the reactor, and the main foulant of the ultrafiltration (UF) membrane was biopolymers (BPs). BP accumulated in the reactor because of a high rejection by the UF membrane. However, once the BP accumulated in the reactor was biodegraded, the concentration of BP decreased with time even under a high organic loading rate of 1.9kg TOC/m3/day.

Electrostatic Adsorption Behavior of Zwitterionic Copolymers on Negatively Charged Surfaces.

To investigate the effect of the surface properties and the coating layer properties on surface modification via electrostatic adsorption, the electrostatic adsorption behavior of zwitterionic copolymers on negatively charged surfaces was studied. A series of positively charged zwitterionic copolymers and a series of negatively charged surfaces, including porous substrates and dense films, were fabricated. The electrostatic adsorption behavior of the zwitterionic copolymers on the negatively charged porous substrates was confirmed using the contact angles and fluorescently labeled protein adsorption experiments. The adsorption behavior of the zwitterionic copolymers on the negatively charged dense films was confirmed using quartz crystal microbalance determination and a fluorescently labeled protein adsorption experiment. The results indicated that a lower charge density on the zwitterionic copolymer brings about a higher adsorption mass on the charged surface, whereas an extremely low charge density on the coating layer results in a lower adsorption mass on the charged surface, due to weak interaction. A high density of the film surface charge is beneficial for surface adsorption, whereas an extremely high density of the film surface charge leads to low surface adsorption due to steric hindrance of the negatively charged sites. This work provides an insight into the best strategy for surface modification via electrostatic adsorption.

Improving Water Permeability of Hydrophilic PVDF Membrane Prepared via Blending with Organic and Inorganic Additives for Humic Acid Separation.

The removal of impurities from water or wastewater by the membrane filtration process has become more reliable due to good hydraulic performance and high permeate quality. The filterability of the membrane can be improved by having a material with a specific pore structure and good hydrophilic properties. This work aims at preparing a polyvinylidene fluoride (PVDF) membrane incorporated with phospholipid in the form of a 2-methacryloyloxyethyl phosphorylcholine, polymeric additive in the form of polyvinylpyrrolidone, and its combination with inorganic nanosilica from a renewable source derived from bagasse. The resulting membrane morphologies were analyzed by using scanning electron microscopy. Furthermore, atomic force microscopy was performed to analyze the membrane surface roughness. The chemical compositions of the resulting membranes were identified using Fourier transform infrared. A lab-scale cross-flow filtration system module was used to evaluate the membrane's hydraulic and separation performance by the filtration of humic acid (HA) solution as the model contaminant. Results showed that the additives improved the membrane surface hydrophilicity. All modified membranes also showed up to five times higher water permeability than the pristine PVDF, thanks to the improved structure. Additionally, all membrane samples showed HA rejections of 75-90%.

Inorganic/Organic Double-Network Ion Gels with Partially Developed Silica-Particle Network.

Tough inorganic/organic composite network gels consisting of a partially developed silica-particle network and a large amount of an ionic liquid, named micro-double-network (µ-DN) ion gel, are fabricated via two methods. One is a one-pot/one-step process conducted using a simultaneous network formation via sol-gel reaction of tetraethyl orthosilicate and free radical polymerization of N, N-dimethylacrylamide in an ionic liquid. When the network formation rates of the inorganic and organic networks are almost the same, the µ-DN structure is formed. The second method is simpler and involved the use of silica nanoparticles as the starting material. By controlling the dispersion state of the silica nanoparticles in an ionic liquid, the µ-DN structure is formed. In both µ-DN ion gels, silica nanoparticles partially aggregate and form network-like clusters. When a large deformation is induced in the µ-DN ion gels, the silica-particle clusters rupture and dissipate the loaded energy. The fracture stress and Young's modulus of the µ-DN ion gel increase as the size of the silica nanoparticles decreases. The increment in the mechanical strength would have been caused by the increase in the total van der Waals attraction forces and the total number of hydrogen bonding in the silica-particle networks.

Sucrose purification and repeated ethanol production from sugars remaining in sweet sorghum juice subjected to a membrane separation process.

The juice from sweet sorghum cultivar SIL-05 (harvested at physiological maturity) was extracted, and the component sucrose and reducing sugars (such as glucose and fructose) were subjected to a membrane separation process to purify the sucrose for subsequent sugar refining and to obtain a feedstock for repeated bioethanol production. Nanofiltration (NF) of an ultrafiltration (UF) permeate using an NTR-7450 membrane (Nitto Denko Corporation, Osaka, Japan) concentrated the juice and produced a sucrose-rich fraction (143.2 g L-1 sucrose, 8.5 g L-1 glucose, and 4.5 g L-1 fructose). In addition, the above NF permeate was concentrated using an ESNA3 NF membrane to provide concentrated permeated sugars (227.9 g L-1) and capture various amino acids in the juice, enabling subsequent ethanol fermentation without the addition of an exogenous nitrogen source. Sequential batch fermentation using the ESNA3 membrane concentrate provided an ethanol titer and theoretical ethanol yield of 102.5-109.5 g L-1 and 84.4-89.6%, respectively, throughout the five-cycle batch fermentation by Saccharomyces cerevisiae BY4741. Our results demonstrate that a membrane process using UF and two types of NF membranes has the potential to allow sucrose purification and repeated bioethanol production.

Permeation of concentrated oil-in-water emulsions through a membrane pore: numerical simulation using a coupled level set and the volume-of-fluid method.

In this paper, we investigated the demulsification behavior of oil-in-water (O/W) emulsions during membrane permeation in the oil-water separation process using a numerical simulation approach. To accurately deal with the large deformation of the oil-water interface by coalescence and wetting, and to estimate the volume of the coalesced oil droplet, the coupled level set and volume-of-fluid method was used as the interface capturing method. We applied the simulation model to the permeation of O/W emulsions through a membrane pore, and then investigated the effects of the wettability of the membrane surface, filtration flux, and pore size on the demulsification efficiency. The results showed that oil droplets were likely to coalesce on the outlet membrane surface. High wettability on the membrane surface and low fluid velocity inside the pore increased the demulsification efficiency. This is the first work to numerically simulate the demulsification behavior of emulsions through membranes in the oil-water separation process.

High-Degree Concentration Organic Solvent Forward Osmosis for Pharmaceutical Pre-Concentration.

Over half of the pharmaceutical industry's capital investments are related to the purification of active pharmaceutical ingredients (APIs). Thus, a cost-effective purification process with a highly concentrated solution is urgently required. In addition, the purification process should be nonthermal because most APIs and their intermediates are temperature-sensitive. This study investigated a high-degree concentration organic solvent forward osmosis (OSFO) membrane process. A polyketone-based thin-film composite hollow fiber membrane with a polyamide selective layer on the bore surface was used as the OSFO membrane to achieve a high tolerance for organic solvents and an effective concentration. MeOH, sucrose octaacetate (SoA), and 2M polyethylene glycol 400 (PEG-400)/MeOH solution were used as the solvent, model API, and a draw solution (DS), respectively. OSFO was performed at room temperature (23 ± 3 °C). Consequently, the 11 wt% SoA/MeOH solution was concentrated to 52 wt% without any SoA leakage into the DS. To our knowledge, there are no studies in which up to a 5 wt% concentration by OSFO has been demonstrated. However, the final feed solution contained 17 wt% PEG-400. This study demonstrates the promising potential of OSFO for pharmaceutical pre-concentration and the technical problems that need to be solved for social implementation.

Development of an ion gel-based CO2 separation membrane composed of Pebax 1657 and a CO2-philic ionic liquid.

A tough ion gel membrane containing a CO2-philic ionic liquid, 1-ethyl-3-methylimidazolium tricyanomethanide ([Emim][C(CN)3]), was developed, and its CO2 permeation properties were evaluated under humid conditions at elevated temperatures. Pebax 1657, which is a diblock copolymer composed of a polyamide block and a polyethylene oxide block, was used as the gel network of the ion gel membrane to prepare a tough ion gel with good ionic liquid-holding properties. The polyamide block formed a semicrystalline structure in [Emim][C(CN)3] to toughen the ion gel membrane via an energy dissipation mechanism. The polyethylene oxide block exhibited good compatibility with [Emim][C(CN)3] and contributed to the retention of the ionic liquid in the ion gel. The developed ion gel membrane showed a good CO2 separation performance of 1677 barrer CO2 permeability and 37 CO2/N2 permselectivity under humid conditions of 75% relative humidity at an elevated temperature of 50 °C, which corresponds to an exhaust gas from a coal-fired power plant.

Energy-efficient trehalose-based polyester nanofiltration membranes for zero-discharge textile wastewater treatment.

Recovery of water, salts, and hazardous dye from complex saline textile wastewater faces obstacles in separating dissolved ionic substances and recovering organic components during desalination. This study realized the simultaneous fractionation, desalination, and dye removal/recovery treatment of textile wastewater by using trehalose (Tre) as an aqueous monomer to prepare polyester loose nanofiltration (LNF) membrane with fine control microstructure via interfacial polymerization. Outperforming the NF270 commercial membrane, the Tre-1.05/TMC optimized membrane achieves zero-discharge textile wastewater treatment, cutting energy consumption by 295% and reducing water consumption by 42.8%. This efficiency surge results from remarkable water permeability (130.83 L m-2 h-1 bar-1) and impressive dye desalination (NaCl/ Direct Red 23 separation factor of 275) of the Tre-1.05/TMC membrane. For a deeper comprehension of filtration performance, the sieving mechanism of polyester LNF membranes was systematically elucidated. This strategic approach offers significant prospects for energy conservation, carbon emission mitigation, and enhanced feasibility of membrane-based wastewater treatment systems.