Improving Ion Rejection of Conductive Nanofiltration Membrane through Electrically Enhanced Surface Charge Density

Pulsatile Ion Transport in Nanofiltration Membranes Coupled with Electrically Tunable Pore and Hydroxyl Electrostatic Interactions

Pulsatile ion transport facilitates the adjusted transfer of substances, meeting the requirements for the gradient and timed separation of multiple components in membrane processes. Responsive nanofiltration membranes are thus currently receiving widespread attention but face limitations due to their narrow performance adjustment range. Herein, hydroxyl functional groups were introduced into electrically responsive nanofiltration membranes to broaden the adjustment range of separation performance through a combination of pore size sieving and functional group interactions, resulting in a greater change in rejection and flux compared to the original membrane. Membrane pore size is regulated by polypyrrole volume changes and becomes more variable when the cation's hydration radius is smaller. Although the hydroxyl group did not affect the charge transfer or volume change capacity of polypyrrole, it enhanced ion-pore interactions during ion transport, which was particularly pronounced in smaller nanochannels. The size effect of functional group interactions more strongly enhances the transmembrane energy barrier in the reduced state compared with the oxidized state, ultimately resulting in greater modulation of performance. This coupling strategy provides insights into the design of responsive membranes, offering the potential to achieve gradient separation of various solutes.

Comediation of voltage gating and ion charge in MXene membrane for controllable and selective monovalent cation separation

Artificial ion channels with controllable mono/monovalent cation separation fulfill important roles in biomedicine, ion separation, and energy conversion. However, it remains a daunting challenge to develop an artificial ion channel similar to biological ion channels due to ion-ion competitive transport and lack of ion-gating ability of channels. Here, we report a conductive MXene membrane with polydopamine-confined angstrom-scale channels and propose a voltage gating and ion charge comediation strategy to concurrently achieve gated and selective mono/monovalent cation separation. The membrane shows a highly switchable "on-off" ratio of ∼9.9 for K+ transport and an excellent K+/Li+ selectivity of 40.9, outperforming the ion selectivity of reported membranes with electrical gating (typically 1.5 to 6). Theoretical simulations reveal that the introduced high-charge cations such as Mg2+ enable the preferential distribution of target K+ over competing Li+ at the channel entrance, and the surface potential reduces the ionic transport energy barrier for allowing K+ to pass quickly through the channel.

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

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

Conductive nanofiltration membranes via in situ PEDOT-polymerization for electro-assisted membrane fouling mitigation

Nanofiltration (NF) membranes play a pivotal role in water treatment; however, the persistent challenge of membrane fouling hampers their stable application. This study introduces a novel approach to address this issue through the creation of a poly(3,4-ethylenedioxythiophene) (PEDOT)-based conductive membrane, achieved by synergistically coupling interfacial polymerization (IP) with in situ self-polymerization of EDOT. During the IP reaction, the concurrent generation of HCl triggers the protonation of EDOT, activating its self-polymerization into PEDOT. This interwoven structure integrates with the polyamide network to establish a stable selective layer, yielding a remarkable 90 % increase in permeability to 20.4 L m-2 h-1 bar-1. Leveraging the conductivity conferred by PEDOT doping, an electro-assisted cleaning strategy is devised, rapidly restoring the flux to 98.3 % within 5 min, outperforming the 30-minute pure water cleaning approach. Through simulations in an 8040 spiral-wound module and the utilization of the permeated salt solution for cleaning, the electro-assisted cleaning strategy emerges as an eco-friendly solution, significantly reducing water consumption and incurring only a marginal electricity cost of 0.055 $ per day. This work presents an innovative avenue for constructing conductive membranes and introduces an efficient and cost-effective electro-assisted cleaning strategy to effectively combat membrane fouling.

Guanidinium-based covalent organic framework membrane for single-acid recovery

Acids are extensively used in contemporary industries. However, time-consuming and environmentally unfriendly processes hinder single-acid recovery from wastes containing various ionic species. Although membrane technology can overcome these challenges by efficiently extracting analytes of interest, the associated processes typically exhibit inadequate ion-specific selectivity. In this regard, we rationally designed a membrane with uniform angstrom-sized pore channels and built-in charge-assisted hydrogen bond donors that preferentially conducted HCl while exhibiting negligible conductance for other compounds. The selectivity originates from the size-screening ability of angstrom-sized channels between protons and other hydrated cations. The built-in charge-assisted hydrogen bond donor enables the screening of acids by exerting host-guest interactions to varying extents, thus acting as an anion filter. The resulting membrane exhibited exceptional permeation for protons over other cations and for Cl^- over SO₄^2- and H_nPO₄^(3-n)- with selectivities up to 4334 and 183, respectively, demonstrating prospects for HCl extraction from waste streams. These findings will aid in designing advanced multifunctional membranes for sophisticated separation.

Modelling the Performance of Electrically Conductive Nanofiltration Membranes

Electrically conductive membranes are a class of stimuli-responsive materials, which allow the adjustment of selectivity for and the rejection of charged species by varying the surface potential. The electrical assistance provides a powerful tool for overcoming the selectivity-permeability trade-off due to its interaction with charged solutes, allowing the passage of neutral solvent molecules. In this work, a mathematical model for the nanofiltration of binary aqueous electrolytes by an electrically conductive membrane is proposed. The model takes into account the steric as well as Donnan exclusion of charged species due to the simultaneous presence of chemical and electronic surface charges. It is shown that the rejection reaches its minimum at the potential of zero charge (PZC), where the electronic and chemical charges compensate for each other. The rejection increases when the surface potential varies in positive and negative directions with respect to the PZC. The proposed model is successfully applied to a description of experimental data on the rejection of salts and anionic dyes by PANi-PSS/CNT and MXene/CNT nanofiltration membranes. The results provide new insights into the selectivity mechanisms of conductive membranes and can be employed to describe electrically enhanced nanofiltration processes.

Electroactive membrane with the electroactive layer beneath the separation layer to eliminate the interference of humic acid in the oxidation of antibiotics

Removing harmful antibiotics is essential to reclaiming water from municipal secondary effluent. Electroactive membranes are effective in the removal of antibiotics but challenged by the abundant coexisting macromolecular organic pollutants in municipal secondary effluent. To eliminate the interference of macromolecular organic pollutants in the removal of antibiotics, we propose a novel electroactive membrane with a top polyacrylonitrile (PAN) ultrafiltration layer and a bottom electroactive layer composed of carbon nanotubes (CNTs) and polyaniline (PANi). When filtering the mixture of tetracycline (TC, a typical antibiotic) and humic acid (HA, a typical macromolecular organic pollutant), the PAN-CNT/PANi membrane performed sequential removal. It retained HA at the PAN layer (by ∼96%) and allowed TC to reach the electroactive layer where it was electrochemically oxidized (e.g., by ∼92% at 1.5 V). The TC removal of the PAN-CNT/PANi membrane was marginally affected by HA, unlike that of the control membrane with the electroactive layer on the top that showed decreased TC removal after the addition of HA (e.g., decreased by 13.2% at 1 V). The decreased TC removal of the control membrane was attributed to the attachment (but not competitive oxidation) of HA on the electroactive layer that impaired the electrochemical reactivity. The HA removal prior to TC degradation realized by the PAN-CNT/PANi membrane avoided the attachment of HA and guaranteed TC removal on the electroactive layer. Long-term filtration for 9 h revealed the stability of the PAN-CNT/PANi membrane, and its advantageous structural design was conformed in the context of real secondary effluents.

Regulation of molecular transport in polymer membranes with voltage-controlled pore size at the angstrom scale

Polymer membranes have been used extensively for Angstrom-scale separation of solutes and molecules. However, the pore size of most polymer membranes has been considered an intrinsic membrane property that cannot be adjusted in operation by applied stimuli. In this work, we show that the pore size of an electrically conductive polyamide membrane can be modulated by an applied voltage in the presence of electrolyte via a mechanism called electrically induced osmotic swelling. Under applied voltage, the highly charged polyamide layer concentrates counter ions in the polymer network via Donnan equilibrium and creates a sizeable osmotic pressure to enlarge the free volume and the effective pore size. The relation between membrane potential and pore size can be quantitatively described using the extended Flory-Rehner theory with Donnan equilibrium. The ability to regulate pore size via applied voltage enables operando modulation of precise molecular separation in-situ. This study demonstrates the amazing capability of electro-regulation of membrane pore size at the Angstrom scale and unveils an important but previously overlooked mechanism of membrane-water-solute interactions.

Improving the Performance of the Lamellar Reduced Graphene Oxide/Molybdenum Sulfide Nanofiltration Membrane through Accelerated Water-Transport Channels and Capacitively Enhanced Charge Density

Graphene is promising in the construction of next-generation nanofiltration membranes for wastewater treatment and water purification. However, the application of graphene-based membranes has still been prohibited by their deficiencies in permeability and ion rejection. Herein, regulating the 2D channel and enhancing the charge density are co-adopted for simultaneous enhancement of the water flux and salt rejection of reduced graphene oxide (rGO) membranes through the intercalation of molybdenum sulfide (MoS₂) nanosheets and external electrical assistance. The fabricated rGO/MoS₂ membranes possess expanded nanochannels with less friction and a higher water molecule transport velocity gradient (from 8.57 to 14.07 s^-1) than those of rGO membranes. Consequently, their water permeance increases from 0.92 to 34.9 L m^-2 h^-1 bar^-1. Meanwhile, benefiting from the high capacitance and negative potential of -1.1 V versus the saturated calomel electrode given to the membranes, their rejection rates toward NaCl reach 87.2% and those toward Na₂SO₄ reach 93.7%. The Donnan steric pore model analysis indicates that the capacitively and electrically increased surface charge density make great contributions to the higher ion rejection rate. This work gives new insights into membrane design for high water flux and salt rejection efficiency.

Incorporating Ag@RF core-shell nanomaterials into the thin film nanocomposite membrane to improve permeability and long-term antibacterial properties for nanofiltration

Ag@resorcinol-formaldehyde resin (Ag@RF) core-shell nanomaterials were prepared by Stöber method, and introduced into polyamide (PA) selective layer of thin-film nanocomposite (TFN) membranes through the interfacial polymerization (IP) process. Due to the abundant hydroxyl groups on the surface and suitable particle size, Ag@RF nanoparticles (Ag@RFs) could be uniformly dispersed in the piperazine aqueous solution and participate in the IP process to precisely regulate the microstructure of the PA selective layer. The resulting "crater structure" and irregular granular structure enlarged the permeable area and contributed to the surface hydrophilicity. For the nanofiltration application, the water flux of TFN membrane modified by Ag@RFs to Na₂SO₄ solution reached 150 L·m^-2·h^-1 which was 87.5% greater than TFC, and salt rejection was maintained. The antibacterial efficiency of the prepared TFN membrane on E. coli reached 99.6% in the antibacterial experiment. In addition, due to the special structure of Ag@RFs, the TFN membrane also showed an expected slow-release capability of Ag⁺, allowing for long-term anti-biofouling properties. This work demonstrates that Ag@RF core-shell nanoparticles with high compatibility of organic nanoparticles and antibacterial properties of Ag nanoparticles could be used as promising nanofillers for designing functional nanofiltration TFN membranes.

A critical review on classifications, characteristics, and applications of electrically conductive membranes for toxic pollutant removal from water: Comparison between composite and inorganic electrically conductive membranes

Research efforts have recently been directed at developing electrically conductive membranes (EMs) for pressure-driven membrane separation processes to remove effectively the highly toxic pollutants from water. EMs serve as both the filter and the electrode during filtration. With the assistance of a power supply, EMs can considerably improve the toxic pollutant removal efficiency and even realize chemical degradation to reduce their toxicity. Organic-inorganic composite EMs and inorganic EMs show remarkable differences in characteristics, removal mechanisms, and application situations. Understanding their differences is highly important to guide the future design of EMs for specific pollutant removal from water. However, reviews concerning the differences between composite and inorganic EMs are still lacking. In this review, we summarize the classifications, fabrication techniques, and characteristics of composite and inorganic EMs. We also elaborate on the removal mechanisms and performances of EMs toward recalcitrant organic pollutants and toxic inorganic ions in water. The comparison between composite and inorganic EMs is emphasized particularly in terms of the membrane characteristics (pore size, permeability, and electrical conductivity), application situations, and underlying removal mechanisms. Finally, the energy consumption and durability of EMs are evaluated, and future perspectives are presented.

Highly Permeable Sulfonated Graphene-Based Composite Membranes for Electrochemically Enhanced Nanofiltration

A sulfophenyl-functionalized reduced graphene oxide (SrGO) membrane is prepared. The SrGO membranes have a high charge density in water and could provide many atomically smooth nanochannels, because of their strong ionized-SO3H groups and low oxygen content. Therefore, the SrGO membranes have an excellent performance in terms of high permeance and high rejection ability. The permeance of SrGO membranes could be up to 118.2 L m−2 h−1 bar−1, which is 7.6 times higher than that of GO membrane (15.5 L m−2 h−1 bar−1). Benefiting from their good electrical conductivity, the SrGO membranes could also function as an electrode and demonstrate a significantly increased rejection toward negatively charged molecules and positively charged heavy metal ions such as Cu2+, Cr3+ and Cd2+, if given an appropriate negative potential. The rejection ratios of these metal ions can be increased from <20% at 0 V to >99% at 2.0 V. This is attributed to the enhanced electrostatic repulsion between the SrGO membrane and the like-charged molecules, and the increased electrostatic adsorption and electrochemical reduction in these heavy metal ions on the membranes. This study is expected to contribute to efficient water treatment and the advance of graphene-based membranes.

Atomic-scale understanding of the Na and Cl trapping on the Mo1.33C(OH)2-MXene

Drinking water scarcity in arid and semi-arid regions is a reality that may turn into a global healthcare problem in the next few years. The scientific community is always looking for new materials to achieve effective sea and brackish water desalination to reduce water scarcity. Commonly, theoretical, and experimental methods make a synergy to better understand and explain the chemical and physical processes in water desalination electrodes. In this way, experimental evidence pointed Mo1.33CTx MXene as an efficient ion intercalation material, in which both Na+ and Cl- are removed. However, the atomic scale understanding of the physicochemical processes due to the Na and Cl interaction with the MXene is still unknown. We report the Na0 and Cl0 interaction with an OH functionalized Mo1.33C monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through hydrogen bonds to the functional groups in the MXene, these bonds have two energy contributions: electrostatic and charge transfer, which increases its adsorption energy. Electrostatic potential isosurfaces, Bader charge analysis, and non-covalent interactions index help clarifying the way Na0 and Cl0 attach to the MXene layer. Oxygen atoms have an affinity for the electropositive Na0 atoms, which after interaction oxidizes to Na+, whereas hydrogen atoms-of the hydroxyl groups-interact with the electronegative Cl0 atoms, which upon adsorption reduce to Cl-. Our findings explain why OH-functionalized Mo1.33C can efficiently remove both Na and Cl atoms based on their affinities with the functional groups present in the MXene layer.

Based On Confined Polymerization: In Situ Synthesis of PANI/PEEK Composite Film in One-Step

Confined polymerization is an effective method for precise synthesis, which can further control the micro-nano structure inside the composite material. Polyaniline (PANI)-based composites are usually prepared by blending and original growth methods. However, due to the strong rigidity and hydrogen bonding of PANI, the content of PANI composites is low and easy to agglomerate. Here, based on confined polymerization, it is reported that polyaniline /polyether ether ketone (PANI/PEEK) film with high PANI content is synthesized in situ by a one-step method. The micro-nano structure of the two polymers in the confined space is further explored and it is found that PANI grows in the free volume of the PEEK chain, making the arrangement of the PEEK chain more orderly. Under the best experimental conditions, the prepared 16 µm-PANI/PEEK film has a dielectric constant of 205.4 (dielectric loss 0.401), the 75 µm-PANI/PEEK film has a conductivity of 3.01×10-4 S m-1 . The prepared PANI/PEEK composite film can be further used as electronic packaging materials, conductive materials, and other fields, which has potential application prospects in anti-static, electromagnetic shielding materials, corrosion resistance, and other fields.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Nanofiltration for drinking water treatment: a review

In recent decades, nanofiltration (NF) is considered as a promising separation technique to produce drinking water from different types of water source. In this paper, we comprehensively reviewed the progress of NF-based drinking water treatment, through summarizing the development of materials/fabrication and applications of NF membranes in various scenarios including surface water treatment, groundwater treatment, water reuse, brackish water treatment, and point of use applications. We not only summarized the removal of target major pollutants (e.g., hardness, pathogen, and natural organic matter), but also paid attention to the removal of micropollutants of major concern (e.g., disinfection byproducts, per- and polyfluoroalkyl substances, and arsenic). We highlighted that, for different applications, fit-for-purpose design is needed to improve the separation capability for target compounds of NF membranes in addition to their removal of salts. Outlook and perspectives on membrane fouling control, chlorine resistance, integrity, and selectivity are also discussed to provide potential insights for future development of high-efficiency NF membranes for stable and reliable drinking water treatment.

Space charge modulation and ion current rectification of a cylindrical nanopore functionalized with polyelectrolyte brushes subject to an applied pH-gradient

Considering versatile potential applications of bioinspired membranes, we simulate the electrokinetic behavior of a cylindrical nanopore, surface modified by a polyelectrolyte (PE) layer. Taking account of the effect of electroosmotic flow and an additionally applied pH gradient, the influences of the strength of the pH gradient, the PE layer thickness, the length of the nanopore and its radius on its conductance and ion current rectification (ICR) performance are assessed. We show that if pH_U (the pH at the higher pH end of the nanopore) is fixed at 11 and pH_L (the pH at the lower pH end of the nanopore) varies from 3 to 11, the rectification factor R_f has a local maximum occurring in 6 < pH_L <8; the greater the magnitude of the applied potential bias |V| the smaller the pH_L at which the local maximum occurs. The influence of the PE layer thickness on the nanopore rectification performance is important only if 5 < pH_L <8, and the optimum performance is reached at a medium thick PE layer (ca. 3 nm). Possible mechanisms associated with the ion transport phenomenon under consideration are proposed and discussed in detail.

Nanopore-based desalination subject to simultaneously applied pressure gradient and gating potential

The performance of a dielectric membrane in desalting is assessed by considering a cylindrical nanopore, surface modified by a dielectric layer, subject to simultaneously applied pressure gradient and gating potential. The charged conditions of the nanopore can be tuned by modulating the applied gating potential so that it can be used for rejecting different types of salt. In general, the thinner the dielectric layer and/or the larger its dielectric constant the better the salt rejection performance. For example, if the thickness of the dielectric layer is 10 nm with a relative dielectric constant of 25, applying a pressure difference of 5 MPa and gating potential of 1 V yields 49% rejection. However, it declines to 9% if the relative dielectric constant is lowered to 5 with other parameters fixed, and 23% if that thickness is 50 nm with other parameters fixed. The results of numerical simulation based on various types of single salt and mixture salts with ions of different valences reveal that the type of ions which need be filtrated can be selected effectively through regulating the gating potential.

Recent Progress in One- and Two-Dimensional Nanomaterial-Based Electro-Responsive Membranes: Versatile and Smart Applications from Fouling Mitigation to Tuning Mass Transport

Membrane technologies are playing an ever-important role in the field of water treatment since water reuse and desalination were put in place as alternative water resources to alleviate the global water crisis. Recently, membranes are becoming more versatile and powerful with upgraded electroconductive capabilities, owing to the development of novel materials (e.g., carbon nanotubes and graphene) with dual properties for assembling into membranes and exerting electrochemical activities. Novel nanomaterial-based electrically responsive membranes have been employed with promising results for mitigating membrane fouling, enhancing membrane separation performance and self-cleaning ability, controlling membrane wettability, etc. In this article, recent progress in novel-nanomaterial-based electrically responsive membranes for application in the field of water purification are provided. Thereafter, several critical drawbacks and future outlooks are discussed.

Electrokinetic Enhancement of Water Flux and Ion Rejection through Graphene Oxide/Carbon Nanotube Membrane

Graphene oxide (GO) is promising for constructing next-generation high-performance membranes for water treatment and desalination. However, GO-based membranes are still subjected to low ion rejection or limited water flux. Herein, the electrokinetic effect is employed as a new strategy for the coenhancement of water flux and ion rejection through an ethylenediamine-polystyrenesulfonate intercalated graphene oxide/carbon nanotube (GO&EDA-PSS/CNT) asymmetric membrane. Benefiting from the external voltage applied across the GO&EDA-PSS layer, the electrokinetically driven water transport velocity is significantly increased from 0 to 23.7 μm s^-1 with increasing the voltage from 0 to 3.0 V. As a result, the water flux is improved from 9.1 to 17.4 L m^-2 h^-1 under a transmembrane pressure of 1 bar. Simultaneously, the rejection rate for NaCl is increased from 52.4% to 78.3%. Numerical analysis reveals that the increased rejection rate is attributed to the electrokinetic enhancements of water transport through the membrane and ion partitioning between the membrane and bulk solution. These results indicate that the assistance of the electrokinetic effect is an effective means to improve membrane filtration performance, which provides a new perspective on the design of advanced membranes for achieving high water flux and rejection efficiency.

Enhancing Polyvalent Cation Rejection Using Perfluorophenylazide-Grafted-Copolymer Membrane Coatings

Surface modification offers a straightforward means to alter and enhance the properties and performance of materials, such as nanofiltration membranes for water softening. Herein, we demonstrate how a membrane's surface charge can be altered by grafting different electrostatically varying copolymers onto commercial membrane surfaces using perfluorophenylazide (PFPA) photochemistry for enhanced ion separation performance. The native membrane's performance-i.e., in terms of divalent cation separation-with copolymer coatings containing a positively charged quaternary ammonium (-N(Me)₃⁺), a negatively charged sulfonate (-SO₃^-), and an essentially neutral zwitterion (sulfobetaine, -N(Me)₂R₂⁺, and -SO₃^-), respectively, indicates that: (a) the sulfonated polymer induces robust Coulombic exclusion of divalent anions as compared to the negatively charged native membrane surface on account of its higher negative charge; (b) the positively charged ammonium coating induces exclusion of cations more effectively than the native membrane; and significantly, (c) the zwitterion polymer coating, which reduces the surface roughness and improves wettability, in spite of its near-neutral charge enhances exclusion of both divalent cations and anions on account of aperture sieving by the compact zwitterion polymer that arises from its ability to limit the size of ions that transport through the polymer along with dielectric exclusion. The outcomes thereby inform new pathways to achieve size- and charge-based exclusion of ionic, molecular, and other species contained in liquid streams.

Sharpening Nanofiltration: Strategies for Enhanced Membrane Selectivity

Nanofiltration plays an increasingly large role in many industrial applications, such as water treatment (e.g., desalination, water softening, and fluoride removal) and resource recovery (e.g., alkaline earth metals). Energy consumption and benefits of nanofiltration processes are directly determined by the selectivity of the nanofiltration membranes, which is largely governed by pore-size distribution and Donnan effects. During operation, the separation performance of unmodified nanofiltration membranes will also be impacted (deleteriously) upon unavoidable membrane fouling. Many efforts, therefore, have been directed toward enhancing the selectivity of nanofiltration membranes, which can be classified into membrane fabrication method improvement and process intensification. This review summarizes recent developments in the field and provides guidance for potential future approaches to improve the selectivity of nanofiltration membranes.