Mechanism of Permselectivity Enhancement in Polyelectrolyte-Dense Nanofiltration Membranes via Surfactant-Assembly Intercalation

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.

Tailoring Polyelectrolyte Multilayer Nanofiltration Membranes by Aerosol-Assisted Printing: Insights into Membrane Formation Mechanisms

Polyelectrolyte multilayer (PEM) membranes, with advantageous features of versatile chemistry and structures, are driving the development of advanced nanofiltration (NF) membranes with exceptional performance. While developing a printing method holds great promise for the eventual mass production of these membranes, reports on the printing method and the underlying mechanisms of membrane formation are currently scarce. Herein, we develop an aerosol-assisted printing (AAP) system for fabricating PEM NF membranes with highly tunable separation characteristics. Our study unveils the three stages of membrane formation from assembly of polyethylenimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS): aerosol deposition, single PE layer formation, and PEM assembly. The droplet deposition is governed by inertial impaction, and the deposited PEs migrate/entangle to form a single PE layer. The thicknesses of the PE layer and PEM exhibit linear growth as the number of printing scan increases. Furthermore, PE interdigitation forms an effective polymeric network barrier, which increases the resistance to solute and water transport. By manipulating the PE deposition mass and layering, PEM membranes with tunable pore radii (0.40-0.56 nm) and water permeability (5-60 L·m-2·h-1·bar-1) were obtained for various water treatment applications, ranging from micropollutant removal to humic acid filtration. Our study offers valuable mechanistic insights into the PEM formation and precise structural adjustment via printing, thus facilitating scalable manufacturing and widespread applications of the PEM NF membranes.

Mechanistic Insights into Micelle-Enhanced Nanofiltration for Heavy Metal Removal: Transformation of Ion Transport and Fouling Phenomena

Toxic heavy metals are widely present in typical scenarios, such as mines and electroplating wastewater, presenting significant risks to biological and environmental safety. Membrane processes encounter a challenge in effectively intercepting heavy metals due to their small hydration radius. This research showcases the high efficiency of micelle-enhanced nanofiltration (MENF) in removing heavy metals. At the critical micelle concentration, sodium dodecyl sulfate demonstrated a high removal of Cu2+, Ni2+, Zn2+, and Cd2+ while maintaining substantial potential for complexation of heavy metals. The formation of micelles and the bonding of heavy metals with surfactants bolstered the resistance of heavy metal ions to transmembrane transport. The presence of heavy metals in ionic form in wastewater facilitated their complexation with surfactants or micelles. Notably, the valence state and concentration of interfering ions in the environment could slightly influence the removal of heavy metals by MENF. Additionally, MENF displayed remarkable antifouling properties. The loose gel layer created by surfactant molecules and the micelle enhanced the membrane permeability and reduced the scaling tendency of heavy metals. This study contributes to an improved understanding of the mechanisms involved in heavy metal rejection by using MENF.

In situ regulation of selectivity and permeability by electrically tuning pore size in trans-membrane ion process

Regulating ion transport behavior through pore size variation is greatly attractive for membrane to meet the need for precise separation, but fabricating nanofiltration (NF) membranes with tunable pore size remains a huge challenge. Herein, a NF membrane with electrically tunable pores was fabricated by intercalating polypyrrole into reduced graphene oxide interlayers. As the potential switches from reduction to oxidation, the membrane pore size shrinks by 11%, resulting in a 16.2% increase in salt rejection. The membrane pore size expands/contracts at redox potentials due to the polypyrrole volume swelling/shrinking caused by the insertion/desertion of cations, respectively. In terms of the inserted cation, Na+ and K+ induce larger pore-size stretching range for the membrane than Ca2+ due to greater binding energy and larger doping amount. Such an electrical response characteristic remained stable after multiple cycles and enabled application in ion selective separation; e.g., the Na+/Mg2+ separation factor in the reduced state is increased by 41% compared to that in the oxide state. This work provides electrically tunable nanochannels for high-precision separation applications such as valuable substance purification and resource recovery from wastewater.

Electrically Controlled Adsorptive Membranes with Tunable Affinity for Selective Chromium (VI) Separation from Water

Ionic contaminants such as Cr(VI) pose a challenge for water purification using membrane-based processes. However, existing membranes have low permeability and selectivity for Cr(VI). Therefore, in this study, we prepared an electrically controlled adsorptive membrane (ECAM-L) by coating a loose Cl--doped polypyrrole layer on a carbon nanotube substrate, and we evaluated the performance of ECAM-L for Cr(VI) separation from water. We also used electrochemical quartz crystal microbalance measurements and molecular dynamics and density functional theory calculations to investigate the separation mechanisms. The adsorption and desorption of Cr(VI) could be modulated by varying the electrostatic interactions between ECAM-L and Cr(VI) via potential control, enabling the cyclic use of the ECAM-L without additional additives. Consequently, the oxidized ECAM-L showed high Cr(VI) removal performance (<50 μg/L) and treatment capacity (>3500 L/m2) at a high water flux (283 L/m2/h), as well as reusability after the application of a potential. Our study demonstrates an efficient membrane design for water decontamination that can selectively separate Cr(VI) through a short electric stimulus.

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.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.