Permselective Membrane Research Articles

Fabrication of fluorinated polyamide seawater reverse osmosis membrane with enhanced boron removal

Developing a reverse osmosis membrane with desirable water/salt selectivity and boron removal has been recognized as crucial for high-efficient seawater desalination. Herein, fluorinated seawater reverse osmosis (SWRO) membranes were fabricated via fluorine-containing monomers-mediated interfacial polymerization or surface modification by grafting fluorine-containing monomers atop nascent polyamide (PA) layer via a second interfacial reaction. The fluorinated molecules, 5-(trifluoromethyl)-1.3-phenylenediamine (TFPD), 3,5-bis(trifluoromethyl) benzoyl chloride (TFBC) or 2,2,2-trifluoroethylamine (TFEA), were confirmedly integrated into the PA matrix presumably via strong chemical covalent bonds, which manipulated the physicochemical properties of the PA layer, including surface hydrophilicity, pore size distribution, cross-linking degree and interactions between membrane and solutes, thereby producing a high-selectivity thin-film composite (TFC) membrane. The optimized membranes, i.e., TFC-TFPD, TFC-TFBC and TFC-TFEA, presented significantly excellent separation performance than the control and commercial membrane, both in water/salt selectivity and boron removal capability, accompanied by a comparable water permeance. Among the three proposed incorporation method of fluorinated monomers, direct grafting of TFEA upon the nascent PA layer demonstrates the best efficiency in introducing fluorine elements in the TFC PA membranes, compared to the addition of TFPD and TFBC into aqueous and organic phases during IP process, respectively. The fluorine-incorporated strategy employed in this present work offered a new design of the PA layer containing fluorinated molecules to obtain the desired permselectivity of TFC SWRO membranes towards efficient removal of salt and boron acid in seawater desalination.

High-performance carbon molecular sieve membrane for C2H4/C2H6 separation: Molecular insight into the structure-property relationships

Carbon molecular sieve (CMS) membranes show attractive potential for energy-efficient separation of C2H4/C2H6 due to their unique slit-like pore structure. In this work, a novel precursor polymer of phenolphthalein-based poly(arylene ether ketone) (PEK-C) was introduced to prepare high-performance CMS membranes for C2H4/C2H6 separation. Molecular insight into the effect of the sp3/sp2 hybridized carbon ratio on the packing morphology of carbon layers, pore size and configuration, and C2H4/C2H6 diffusion and separation properties of CMS membranes was investigated via using in-depth characterization techniques combined with molecular simulations. Results show that the carbon layers with more sp3 hybridized carbons lead to forming a loose and disordered carbon structure and pore structure with large pore volume and micropore size (steric voids), resulting in low C2H4/C2H6 permselectivity of CMS membrane. With the reduction of the sp3/sp2 hybridized carbon ratio, a compact and graphite-like carbon structure with the two-dimensional slit-like ultramicropore structure is formed, and the C2H4/C2H6 permselectivity is obviously enhanced. The CMS membranes derived from the novel precursor of PEK-C exhibit high C2H4 permeability with superior C2H4/C2H6 selectivity, which have surpassed the trade-off bounds of both polymeric membrane and CMS membrane, and reveal a great potential in the industrial applications of C2H4/C2H6 separation.

Improving the permselectivity of asymmetric isoporous membranes by blending the micro-sized metal-organic frameworks(MOFs) crystals with block copolymer

Nanomaterials are widely used to improve the separation performance for asymmetric isoporous membranes. Different from those prevalent researches, MOFs crystals with particle sizes of 0.52–0.75 μm which were an order magnitude larger than the molecular size of block copolymers (BCPs) were loaded to casting solution, and their tailoring on membrane formation and permselectivity of isoporous membranes prepared via the SNIPS were carefully investigated. As far as we know, 3D particles with such big size have never been added to isoporous membranes before. Results from small angle X-ray scattering indicated that the microphase separation in solutions had been increased obviously although MOFs crystals were too large to co-assemble with BCPs. This interesting finding was assigned to the breathing effect from MOFs crystals that increased the effective concentrations of BCPs. Morphology observation indicated that MOFs crystals located on surface were fully covered by a thin layer of BCPs which also assembled into defect-free, isoporous structures. This result was observed even though the contents of MOFs in membranes reached 40 wt%. It was measured that the permeability and selectivity to protein mixtures improved obviously by the MOFs crystals blended to membranes. Finally, with the removal of MOFs from membranes by acid etching, the permselectivity of membranes improved further. Considering the easy synthesis and good dispersibility, micro-sized MOFs crystals can be used as more effective additives to improve the separation performances for isoporous membranes than that of nanomaterials. This finding will pave the ways for preparing the high-performance isoporous membranes by using MOFs crystals as the additives for block copolymers.

Critical selection of shear sheltering in electroconvective flow from chaotic to steady state

Ion and water are transported by electroconvection near permselective membranes, resulting in complex phenomena associated with the flow–fines interaction. Sheltering the flow chaos by the shear flow is a common strategy in plasma fluids and has recently been successfully applied to control ionic fluids. The paper herein reveals the critical selection of shear velocity regarding the fluid from a chaotic to a steady state through numerical and theoretical analyses. For the shear sheltering, the dimensionless Debye length ${\lambda _D}$ with varying channel height is introduced to achieve a comprehensive discussion of the factors and laws affecting the shear vortex state. Based on an analysis of the vortex driving mechanism, the scaling of the slip velocity ${u_s}\sim {(\lambda _D^{ - 1}\Delta {\phi ^4})^{1/3}}$ is recommended as the critical selection factor for the steady and chaotic state under a fixed shear flow velocity, which involves the dimensionless Debye length ${\lambda _D}$ and voltage difference $\Delta \phi $ . Furthermore, for ionic fluid control by shear flow, a critical shear velocity ${U_{HPC}}$ is proposed to distinguish the electroconvective flow from a chaotic state to a steady state. When the shear flow velocity ${U_{HP}} > {U_{HPC}}$ , the shear flow shelters chaos, and the scaling law is also recommended for the regulation of the critical shear flow velocity ${U_{HPC}}$ jointly by ${\lambda _D}$ and $\Delta \phi $ . The analysis is confirmed by direct numerical simulation and existing experimental data (J. Fluid Mech, vol. 813, 2017, pp. 799–823). This work provides a more comprehensive physical insight for shear sheltering and affects the design of electromembrane microfluidics.

Computational framework for resolving boundary layers in electrochemical systems using weak imposition of Dirichlet boundary conditions

We present a finite element based computational framework to model electrochemical systems. The electrochemical system is represented by the coupled Poisson–Nernst–Planck (PNP) and Navier–Stokes (NS) equations. The key quantity of interest in such simulations is the current (flux) at the system boundaries. Accurately computing the current flux is challenging due to the small critical dimension of the boundary layers (small Debye layer) that require fine mesh resolution at the boundaries. We present a numerical framework which resolves this challenge by utilizing a weak imposition of Dirichlet boundary conditions for Poisson–Nernst–Planck equations. In this numerical framework we utilize a block iterative strategy to solve NS and PNP equations. This allows us to efficiently and easily implement the weak imposition of Dirichlet boundary conditions. The results from our numerical framework shows excellent agreement when compared to strong imposition of boundary conditions (strong imposition requires a much finer mesh). Furthermore, we show that the weak imposition of the boundary conditions allows us to resolve the fluxes in the boundary layers with much coarser meshes compared to strong imposition. We also show that the method converges as we refine the mesh near the boundaries at a much faster rate compared to strong imposition of the boundary layer. We present multiple test cases with varying boundary layer thickness to illustrate the utility of the numerical framework. We illustrate the approach on canonical 3D problems that otherwise would have been computationally intractable to solve accurately. Lastly, we simulate electrokinetic instabilities near a perm-selective membrane with weakly imposed boundary conditions on the membrane. This approach substantially reduces the computational cost of modeling thin boundary layers in electrochemical systems.

Highly efficient monovalent ion transport enabled by ionic crosslinking‐induced nanochannels

AbstractCharge‐governed ion transport is of significant importance to industrial development, and advanced membrane materials with fast and selective ionic transport are essential components. In cell membranes, ionic transport is mainly determined by the charge‐governed protein channels, representing an architecture with functional differentiation. Inspired by this, a novel class of membranes was developed by ionically crosslinking sulfonated (poly[ether ether ketone]) and quaternized poly(2,6‐dimethyl‐1,4‐phenylene oxide) to construct the cationic conductive biomimetic nanochannels. Ionic crosslinking was tailored to realize nanophase separation and efficient ion transport mainly based on surface chemistry without altering the scaffold feature of polymeric pore channels. The best‐performing ionic crosslinking membrane exhibited a high ionic permeation (2.23 mol·m−2·h−1 for K+) and high cationic selectivity (7.91 for K+/Mg2+), which were comparable with the commercial monovalent cation permselective CIMS membrane, owing to the negligent surface resistance toward monovalent cations but strong positively charged repulsion against divalent cations.

The Cation Role in Ion Transport Selectivity of Gold-Plated Nanotubes

Ion permselectivity is an important ionic transport property exhibited in nanopores and nanotubes. Permselectivity develops due to the strong interactions between ions in solution and the nanopore/nanotube surfaces. Here, we study the ion permselectivity of gold-plated nanotube membranes. Gold-plated membranes were obtained by using an electroless gold plating method to deposit gold nanotubes into the pores and on the faces of track-etched polycarbonate membranes. These membranes have the potential to display cation permselectivity due to the excess surface charge density present on the membrane from chemisorbed Cl- anions. The cation permselectivity of membranes with 9.9 + 0.6 nm diameter nanotubes was studied potentiometrically by using a concentration cell. A new Debye-sphere based model was presented here to describe the extent of permselectivity. However, some ion specific effects with the gold-plated membrane are observed that cannot be interpreted by Debye theory. These ion-specific surface interactions lead to a reduction in the negative surface charge, causing no permselectivity for some cations as demonstrated by their lower than ideal cation transference numbers. Quantification of the surface charge density by XPS indicated the gold-plated membranes are highly charged due to Cl- chemisorption with a surface charge density of ~ 5.3 uC per cm2 corresponding to 3.3 x 1013 Cl--ions per cm2.

Electrokinetic Preconcentration and Label-Free Electrical Detection of Sars-Cov-2 RNA at a Packed Bed of Bioconjugated Microspheres

In diagnostics, point-of-care (POC) devices, such as lateral flow assays, play an important role due to their ease of use, practical application, and fast response. However, one of the major limitations of POC devices is low sensitivity. To address this limitation, electrokinetic techniques have been utilized to enrich the target analyte prior to detection. In one such technique, ion concentration polarization (ICP), an ion depleted zone (IDZ) is formed near an ion permselective membrane or electrode following the application of an external electric field. The low conductivity of the IDZ supports the formation of an extended electric field gradient. Enrichment arises from focusing of a charged analyte along this electric field gradient in the presence of opposing fluid flow. ICP focusing is a reliable technique for the enrichment and mobility-based separation of charged species, such as DNA, RNA, or proteins. However, ICP focusing is negatively impacted by electroconvective vortices that drive unwanted mixing.We utilize an out-of-plane faradaic ion concentration polarization (fICP) technique that employs a 3D porous electrode and an adjacent packed bed of insulating microbeads to improve the stability of the enriched plug. In this work, we exhibit the detection of SARS-CoV-2 RNA, extracted from lysed virus, at subpicomolar concentrations in this microfluidic device. Here, the insulating beads are bioconjugated with an uncharged peptide nucleic acid (PNA) oligoprobe that hybridizes with the target RNA. The resulting increase in negative charge on the bead surface leads to a shift in a current-voltage curve between the 3D electrode and a driving electrode at the microchannel inlet. This shift in conductivity serves as a non-optical sensing mechanism for detection of the hybridized SARS-CoV-2 RNA. Most notably, this enrichment and sensing scheme utilizes off-the-shelf streptavidin-modified beads, making it highly versatile, while the reagent-free approach and low driving voltage render it amenable to POC implementation.This work is supported by a grant from the Roy J. Carver Charitable Trust.

CO2/CH4 mixed-gas separation through carbon nitride membrane: A molecular dynamics simulation

We performed molecular dynamics simulations to study the performance of a pristine carbon nitride (C2N) membrane for separation of CO2/CH4 gas mixture. The influences of system temperature (298, 323, and 348 K), applied pressure difference (up to 2000 kPa), and feed composition on the CO2/CH4 separation were also explored. The permselectivity of the C2N membrane for separation of CO2/CH4 mixture was theoretically determined. The highest CO2 permeance of 1.32 × 106 GPU in the absence of applied external pressure was achieved. It is demonstrated that CO2 molecules can easily diffuse through the membrane, while no CH4 molecule was seen on the permeate side. Thus, an ultrahigh selectivity of CO2 over CH4 was achieved. The membrane indicated a superior performance, which easily exceeds the Robeson’s upper bound. The investigation of the gas permeation mechanism revealed that the preferential adsorption of CO2 and suitable size of this molecule owing to proper orientation are two important factors for efficient separation of CO2 from CH4 by the C2N membrane. In fact, CO2 molecules tend to reorient themselves close to the membrane nanopores to diffuse perpendicularly through. To this end, the density map and the potential of the mean force analysis were also conducted, which confirmed the simulation results.

Zirconia-Doped Methylated Silica Membranes via Sol-Gel Process: Microstructure and Hydrogen Permselectivity

In order to obtain a steam-stable hydrogen permselectivity membrane, with tetraethylorthosilicate (TEOS) as the silicon source, zirconium nitrate pentahydrate (Zr(NO3)4·5H2O) as the zirconium source, and methyltriethoxysilane (MTES) as the hydrophobic modifier, the methyl-modified ZrO2-SiO2 (ZrO2-MSiO2) membranes were prepared via the sol-gel method. The microstructure and gas permeance of the ZrO2-MSiO2 membranes were studied. The physical-chemical properties of the membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), and N2 adsorption–desorption analysis. The hydrogen permselectivity of ZrO2-MSiO2 membranes was evaluated with Zr content, temperature, pressure difference, drying control chemical additive (glycerol) content, and hydrothermal stability as the inferred factors. XRD and pore structure analysis revealed that, as nZr increased, the MSiO2 peak gradually shifted to a higher 2θ value, and the intensity gradually decreased. The study found that the permeation mechanism of H2 and other gases is mainly based on the activation–diffusion mechanism. The separation of H2 is facilitated by an increase in temperature. The ZrO2-MSiO2 membrane with nZr = 0.15 has a better pore structure and a suitable ratio of micropores to mesopores, which improved the gas permselectivities. At 200 °C, the H2 permeance of MSiO2 and ZrO2-MSiO2 membranes was 3.66 × 10−6 and 6.46 × 10−6 mol·m−2·s−1·Pa−1, respectively. Compared with the MSiO2 membrane, the H2/CO2 and H2/N2 permselectivities of the ZrO2-MSiO2 membrane were improved by 79.18% and 26.75%, respectively. The added amount of glycerol as the drying control chemical additive increased from 20% to 30%, the permeance of H2 decreased by 11.55%, and the permselectivities of H2/CO2 and H2/N2 rose by 2.14% and 0.28%, respectively. The final results demonstrate that the ZrO2-MSiO2 membrane possesses excellent hydrothermal stability and regeneration capability.

Ethanol dehydration performance of three types of commercial-grade zeolite permselective membranes.

Many organic solvents form difficult-to-separate mixtures with water and have an affinity for water, making drying a potential reuse prerequisite. Pervaporation (PV) and vapor permeation (VP) membrane technologies hold promise for energy-efficient solvent drying. Several water-selective membrane materials are commercially available, but performance data is limited, particularly for two recently commercialized membrane materials: chabazite (CHA) and T-type zeolites. In this work, commercial-grade samples of CHA and T-type membranes, along with a NaA zeolite membrane, were evaluated for the removal of water from ethanol. The CHA sample had the highest initial PV water permeance (6820 GPU) and water permselectivity (3430) with 5 wt% water in ethanol at 50 °C. Initial NaA membrane performance was slightly lower (6060 GPU and 3260), while the T-type membrane had the lowest initial permeance and selectivity (4260 GPU and 1090). Performance declined over time, most notably for the NaA membrane, for which water permeance fell over 50% through 39 days of testing. The T-type membrane exhibited the steadiest PV water permeance, but the most variable ethanol permeance. The PV performance of the three membranes largely overlapped the predicted range for T-type membranes. That performance generally exceeds the anticipated ethanol drying performance of non-zeolitic PV membranes but is less than that predicted for NaA and CHA membranes. The present CHA membrane results, along with other recent reports, refine earlier predictions of the ethanol dehydration performance of that type of zeolite. The changing performance with time should be understood to properly design a solvent dehydration system.

Understanding butanol recovery and coupling effects in pervaporation of Acetone-Butanol-Ethanol (ABE) solutions: A modelling and experimental study

The sustainable downstream recovery of bio-butanol from multicomponent fermentation broths through organophilic pervaporation is still challenged, in part, by a limited knowledge on the effect of by-products ethanol and acetone on final butanol recovery. This study therefore involves an experimental and theoretical inquisition on extraction of butanol and coupling effects involved between the various components. A commercial polydimethylsiloxane membrane was used for pervaporation of binary and ternary model solutions. Modified solution-diffusion based models were then customized to emulate temperature and concentration dependence of permeation. Membrane performance and coupling were found to be functions of the concentration of both organic components involved as well as temperature of the feed. The membrane was observed to be most intrinsically selective towards butanol in all systems. In ternary mixtures, the presence of second organic component positively enhanced coupling effects in favor of butanol recovery but lead to a decreased membrane plasticization by butanol itself. Ultimately permeance based coupling factors revealed that membrane permselectivity towards butanol was best benefited by coupling at lower butanol to ethanol/acetone concentration ratios in feed. In contrast, minimal coupling was experienced at equimolar conditions and multicomponent pervaporation in such cases could be satisfactorily modelled as binary mixture separation.

Reconstructing polyamide with nucleophilic catalyst for enhancing reverse osmosis membrane performance

In this work, the permselectivity of PA RO membrane was significantly improved by treating the PA layer with 4-dimethylaminopyridine (DMAP) aqueous solution. With the assistance of DMAP, more residues in PA layer released during water soaking, unblocking the water channel in the PA layer. DMAP could also catalyze the reaction between the low activity amino groups and acyl chloride groups in PA, thereby reconstructing the resultant PA layer of RO membranes, generating a highly hydrophilic and negatively charged neonatal PA layer in-situ, and enhancing PA main part crosslinking. These factors were beneficial to membrane permselectivity. Consequently, the selectivity of the DMAP modified membrane could be increased by 62% at most (21594) with a 90% increase in permeance (3.37 L·m−2·h−1·bar−1) compared with those of the virgin membrane (13,294, 1.78 L·m−2·h−1·bar−1). This work provides a facile and low-cost strategy for preparing advanced RO membrane with outstanding permselectivity, exhibiting the potential for industrial application.

Hydrophilic montmorillonite in tailoring the structure and selectivity of polyamide membrane

2D nanomaterials have been extensively studied for their superiority in desalination applications. For instance, the incorporation of Graphene-Family nanomaterials has been reported to enhance membrane separation performance significantly, yet the other important low-cost 2D nanomaterial, montmorillonite (MMT), is less studied. For the first time, we systematically studied the formation and performance of polyamide membranes with the modification of exfoliated MMT. The exfoliated MMT has overcome the classical membrane permeability-selectivity trade-off attributed to its uniformity, excellent dispersion and compatibility with monomer m-phenylenediamine (MPD). The effects of MMT charges on the perm-selectivity of resultant TFN membranes were also studied symmetrically. MMT can tailor the surface morphology, roughness, hydrophilicity, and crosslink density of polyamide due to tunable charges and ionic strength. In addition, it was found that MMT exhibited a positive effect on the chlorine resistance of the membrane. Taken together, our work provides fundamental insights into the formation and transport mechanisms in thin-film nanocomposite (TFN) membranes.

Layer-by-layer assembly of cation exchange membrane for highly efficient monovalent ion selectivity

The development of cation exchange membranes (CEMs) with high monovalent/divalent cation selectivity has great potential for enhanced salt separation for wastewater reclamation, and layer-by-layer (LbL) self-assembly of polyelectrolyte films with high monovalent selectivities among cations has expanded the applications of such membranes. However, the structure–property relationship for constructing multilayered polyelectrolyte films with enhanced separation performance remains poorly understood. This study reports a novel CEM engineered by coating a [polyethyleneimine/polystyrene sulfonate (PEI/PSS)]n alternate electro-deposition multilayer on a polyacrylate substrate membrane. The newly developed CEM exhibited a monovalent selectivity of 4.59 for Li+/Mg2+ with an optimal permeation of 0.16 mmol·cm−2·h−1 for Li+, demonstrating a competitive separation property compared to state-of-the-art permselective membranes. A detailed quantitative investigation provides the underlying insights into the monovalent selectivity for each polyelectrolyte layer. The results indicate negligible interface impedance toward monovalent cations but strengthened repulsion against divalent cations, thereby promoting a quick passage for monovalent cations while imposing temporal retardation on divalent cations during the electrodialysis (ED) process. Therefore, this study elucidates the characteristic reason for the enhanced monovalent ion permselectivity of LbL polyelectrolyte membranes in electro-driven separation processes.

Highly efficient Li+/Mg2+ separation of monovalent cation permselective membrane enhanced by 2D metal organic framework nanosheets

Monovalent cation permselective membranes (MCPMs) manifest remarkable effectiveness and potential in lithium extraction whereas facing the challenge to achieve high permselectivity and high Li+ permeability simultaneously. In this work, Zn-TCPP, a kind of two-dimensional nanosheet with abundant negative charges, was successfully synthesized via surfactant-assisted method and introduced into surface cross-linked SPES membrane to fabricate MCPMs. The physicochemical properties, electrochemical properties and Li+/Mg2+ separation performance of resulted MCPMs were systematically estimated. As ions passing through the membrane, dense PVA/GA crosslinked layer separated Li+ and Mg2+ by pore-size sieving effect. Afterwards, uniformly distributed Zn-TCPP nanosheets with rich negative charges and physically adsorbed water molecules elevated the charge density and water content in the membrane and thus expedited Li+ transporting. Therefore, high Li+/Mg2+ permselectivity (PMg2+Li+= 8.99) and Li+ permeation flux (JLi+= 9.12 × 10−9 mol cm−2∙s−1) were obtained. Furthermore, membrane surface resistance was reduced from 108.99 Ω cm2 to 24.12 Ω∙cm2. This study points out the feasibility of the new method for accelerating monovalent ion transport and alleviating the restriction between ion permeation and permselectivity in selective ion exchange membranes.

Impact of Cation–Ligand Interactions on the Permselectivity of Ligand-Functionalized Polymer Membranes in Single and Mixed Salt Systems

We conduct molecular dynamics simulations to study the effects of cation–ligand interactions on salt permeation in ligand-functionalized polymer membranes. Our results indicate that salt partitioning into the membrane increases while the salt diffusivity decreases with enhancement in the cation–ligand interaction strength. Moreover, the increase in partitioning dominates the decline in salt diffusivity, leading to a net enhancement in salt permeation at higher cation–ligand interaction strengths. Equimolar binary mixed salt systems (having a common anion) generally exhibit much more selective partitioning of salts into the membrane as compared to their single salt counterparts. However, single salt systems exhibit much higher ratio of salt diffusivities than their mixed salt analogues. The permselectivity data closely resemble the trends in selective partitioning for both single and mixed salt systems, indicating that salt diffusivity selectivity plays a weaker role than salt partitioning selectivity in dictating the selective permeation of salts across the membrane.

Influence of membrane structure-dependent water transport on conductivity-permselectivity trade-off and salt/water selectivity in electrodialysis: Implications for osmotic electrodialysis using porous ion exchange membranes

Water transport, including osmosis and electroosmosis, is an important phenomenon in electrodialysis (ED) but is generally ignored in many studies on the analysis of ion transport. Having a fundamental understanding of the influence of water transport on ion transport during ED is of particular help for the optimization of the design of ion exchange membranes (IEMs) and the development of novel ED-related processes. In this study we systematically investigated the effect of membrane structure-dependent water transport on ED performance and explored the potential of using porous IEMs in ED. A 1-D model, which is based on the extended Nernst–Planck equation and duly considers the membrane structure-dependent water flux, was developed to analyze mass (ion and water) transport in IEMs during ED. Results show that the electroosmotic flux increased with increasing the water volume fraction and the pore size of IEMs and decreasing the thickness of IEMs, which further leads to the increase in counterion flux and the decrease in co-ion flux. Osmotic water flux exhibited a similar trend to electroosmotic flux with changing membrane structural properties. The existence of osmotic water transport further enhanced the change of counterion and co-ion fluxes, which leads to the increase in both membrane conductivity and permselectivity. This suggests that osmotic water transport can break the trade-off between conductivity and permselectivity of IEMs during ED. On the other hand, osmotic water transport leads to the decrease in specific salt flux (i.e., the ratio of salt flux over water flux) despite the increase in salt flux, suggesting the decrease in salt/water selectivity. The effect of osmotic water transport is more significant for porous IEMs due to higher osmotic water flux compared to dense IEMs. This work provides new insights into the use of porous IEMs in osmotic electrodialysis for potential applications. • Electroosmosis and osmosis are dependent on membrane structural properties and change in a similar trend in ED. • The increase in electroosmosis leads to the increase in counter-ion flux and the decrease in co-ion flux. • The existence of osmotic water transport leads to the increase in both conductivity and permselectivity. • Increasing water transport increases salt flux but decreases salt/water selectivity. • Osmotic electrodialysis using porous ion exchange membranes was proposed.

Mathematical Modeling of Monovalent Permselectivity of a Bilayer Ion-Exchange Membrane as a Function of Current Density.

Modification of an ion-exchange membrane with a thin layer, the charge of which is opposite to the charge of the substrate membrane, has proven to be an effective approach to obtaining a composite membrane with permselectivity towards monovalent ions. However, the mechanism of permselectivity is not clear enough. We report a 1D model based on the Nernst–Planck–Poisson equation system. Unlike other similar models, we introduce activity coefficients, which change when passing from one layer of the membrane to another. This makes it possible to accurately take into account the fact that the substrate membranes usually selectively sorb multiply charged counterions. We show that the main cause for the change in the permselectivity coefficient, P1/2, with increasing current density, j, is the change in the membrane/solution layer, which controls the fluxes of the competing mono- and divalent ions. At low current densities, counterion fluxes are controlled by transfer through the substrate membrane, which causes selective divalent ion transfer. When the current increases, the kinetic control goes first to the modification layer (which leads to the predominant transfer of monovalent ions) and then, at currents close to the limiting current, to the depleted diffusion layer (which results in a complete loss of the permselectivity). Thus, the dependence P1/2 − j passes through a maximum. An analytical solution is obtained for approximate assessment of the maximum value of P1/2 and the corresponding fluxes of the competing ions. The maximum P1/2 values, plotted as a function of the Na+ ion current density at which this maximum is reached, gives the theoretical trade-off curve between the membrane permselectivity and permeability of the bilayer monovalent selective ion-exchange membrane under consideration.

Electroosmosis of the Second Kind on Flat Charged Surfaces - a Direct Numerical Simulation Study

While the electroosmosis of the first kind (equilibrium) is accepted widely, the electroosmosis of the second kind (nonequilibrium) is still controversial. In this work, the theory of electroosmosis slip, of either the first kind or of the second kind at electrolyte membrane system is revisited via our direct numerical simulation. The obtained results show that above a certain voltage threshold, the basic conduction state becomes electroconvectively unstable. This instability provides a mechanism for explaining the over-limiting conductance in concentration polarization at a permselective membrane. The most important work in our study is to examine the famous electroosmosis of the second kind formula suggested by Rubinstein and Zaltzman in 1999. Although their formula has been presented for a long time, there has been no work to validate its accuracy experimentally or numerically due to the difficulty in pinpointing exactly the extended space charge layer in their formula. By using direct numerical simulation, we could solve this problem and inspect the application range of their formula. This also helps to strongly confirm the relationship between the electroosmosis of the second kind and the instability in concentration polarization at electrodialysis membranes.