Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives.

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

Polar Solvent-Induced Spontaneous Nanofoaming for Synthesizing Ultra-High-Performance Polyamide Nanofiltration Membranes.

Nanofiltration membranes with both high water permeance and selectivity are perpetually studied because of their applications in water purification. However, these two critical attributes are considered to be mutually exclusive. Here, we introduce a polar solvent, dichloromethane, in place of the apolar hexane used for decades as the organic phase for membrane interfacial polymerization synthesis to solve this dilemma. When a polar solvent as the organic phase is combined with a solvent-resistant aramid nanofibrous hydrogel film as the water phase, monomer enrichment in the reaction zone leads to a polyamide nanofiltration membrane with densely distributed nanobubble features, enhanced nanoporosity, and a loosened backbone. Benefiting from these structural features, the resulting membrane exhibits superior properties with a combination of high water permeance (52.7 L m-2 h-1 bar-1) and selectivity (water/Na2SO4, 36 bar-1; NaCl/Na2SO4, 357 bar-1), outperforming traditional nanofiltration membranes. We envision that this novel technology involving polar solvent systems and the water phase of nanofibrous hydrogel would provide new opportunities for membrane development for environmental engineering.

Superhydrophobic Nanoparticles: An Efficiently Selective Adsorbent for Surfactant-Like Contaminants from Complex Wastewater Matrices.

Surfactant-like contaminants (SLCs) with distinctive amphiphilic structures have become a global concern in wastewater due to their toxicity and persistency. Despite extensive efforts, achieving efficient and selective SLCs removal remains challenging because of their wide range of molecular weights and complex functional group compositions. Superhydrophobic nanoparticles can potentially tackle this challenge by targeting the long oleophilic chains of SLCs. However, conventional contact angle measurements hinder hydrophobicity characterization and corresponding selectivity research because of the powder morphology of nanoparticles. Herein, the authors offered information regarding the distribution of water molecular probes in surfaces and proposed a quantitative characterization approach based on low-field nuclear magnetic resonance. Through synthesizing superhydrophobic and hydrophilic polydopamine nanospheres with similar morphologies, the selective adsorption potential of superhydrophobic nanoparticles for SLCs is systematically demonstrated. As revealed by the interaction mechanisms, the superhydrophobic surface of nanospheres increased its affinity and selectivity for SLCs adsorption by enhancing hydrophobic interactions. Superhydrophobic modification achieved ten times the adsorption capacity of sodium dodecyl benzene sulfonate, an exemplified surfactant, compared with pristine nanoparticles. By regulated self-polymerization, the superhydrophobic nanospheres are coated onto the surface of a 3D sponge and enable efficient selective SLCs adsorption from highly polluted leachate matrices with long-term stability and reusability.

Engraving Polyamide Layers by In Situ Self-Etchable CaCO3 Nanoparticles Enhances Separation Properties and Antifouling Performance of Reverse Osmosis Membranes.

Nanovoids within a polyamide layer play an important role in the separation performance of thin-film composite (TFC) reverse osmosis (RO) membranes. To form more extensive nanovoids for enhanced performance, one commonly used method is to incorporate sacrificial nanofillers in the polyamide layer during the exothermic interfacial polymerization (IP) reaction, followed by some post-etching processes. However, these post-treatments could harm the membrane integrity, thereby leading to reduced selectivity. In this study, we applied in situ self-etchable sacrificial nanofillers by taking advantage of the strong acid and heat generated in IP. CaCO3 nanoparticles (nCaCO3) were used as the model nanofillers, which can be in situ etched by reacting with H+ to leave void nanostructures behind. This reaction can further degas CO2 nanobubbles assisted by heat in IP to form more nanovoids in the polyamide layer. These nanovoids can facilitate water transport by enlarging the effective surface filtration area of the polyamide and reducing hydraulic resistance to significantly enhance water permeance. The correlations between the nanovoid properties and membrane performance were systematically analyzed. We further demonstrate that the nCaCO3-tailored membrane can improve membrane antifouling propensity and rejections to boron and As(III) compared with the control. This study investigated a novel strategy of applying self-etchable gas precursors to engrave the polyamide layer for enhanced membrane performance, which provides new insights into the design and synthesis of TFC membranes.

Does Surface Roughness Necessarily Increase the Fouling Propensity of Polyamide Reverse Osmosis Membranes by Humic Acid?

Surface roughness has crucial influence on the fouling propensity of thin film composite (TFC) polyamide reverse osmosis (RO) membranes. A common wisdom is that rougher membranes tend to experience more severe fouling. In this study, we compared the fouling behaviors of a smooth polyamide membrane (RO-s) and a nanovoid-containing rough polyamide membrane (RO-r). Contrary to the traditional belief, we observed more severe fouling for RO-s, which can be ascribed to its uneven flux distribution caused by the "funnel effect". Additional tracer filtration tests using gold nanoparticles revealed a more patchlike particle deposition pattern, confirming the adverse impact of "funnel effect" on membrane water transport. In contrast, the experimentally observed lower fouling propensity of the nanovoid-containing rough membrane can be explained by: (1) the weakened "funnel effect" thanks to the presence of nanovoids, which can regulate the water transport pathway through the membrane and (2) the decreased average localized flux over the membrane surface due to the increased effective filtration area for the nanovoid-induced roughness features. The current study provides fundamental insights into the critical role of surface roughness in membrane fouling, which may have important implications for the future development of high-performance antifouling membranes.

Demystifying the Role of Surfactant in Tailoring Polyamide Morphology for Enhanced Reverse Osmosis Performance: Mechanistic Insights and Environmental Implications.

Surfactant-assisted interfacial polymerization (IP) has shown strong potential to improve the separation performance of thin film composite polyamide membranes. A common belief is that the enhanced performance is attributed to accelerated amine diffusion induced by the surfactant, which can promote the IP reaction. However, we show enhanced membrane performance for Tween 80 (a common surfactant), even though it decreased the amine diffusion. Indeed, the membrane performance is closely related to its polyamide roughness features with numerous nanovoids. Inspired by the nanofoaming theory that relates the roughness features to nanobubbles degassed during the IP reaction, we hypothesize that the surfactant can stabilize the generated nanobubbles to tailor the formation of nanovoids. Accordingly, we obtained enlarged nanovoids when the surfactant was added below its critical micelle concentration (CMC). In addition, both the membrane permeance and selectivity were enhanced, thanks to the enlarged nanovoids and reduced defects in the polyamide layer. Increasing the concentration above CMC resulted in shrunken nanovoids and deteriorated performance, which can be ascribed to the decreased stabilization effect caused by micelle formation. Interestingly, better antifouling performance was also observed for the surfactant-assisted membranes. Our current study provides mechanistic insights into the critical role of surfactant during the IP reaction, which may have important implications for more efficient membrane-based desalination and water reuse.

Inhibiting Polyamide Intrusion of Thin Film Composite Membranes: Strategies and Environmental Implications.

Thin film composite polyamide (TFC) nanofiltration (NF) membranes represent extensive applications at the water-energy-environment nexus, which motivates unremitting efforts to explore membranes with higher performance. Intrusion of polyamide into substrate pores greatly restricts the overall membrane permeance because of the excessive hydraulic resistance, while the effective inhibition of intrusion remains technically challenging. Herein, we propose a synergetic regulation strategy of pore size and surface chemical composition of the substrate to optimize selective layer structure, achieving the inhibition of polyamide intrusion effective for the membrane separation performance enhancement. Although reducing the pore size of the substrate prevented polyamide intrusion at the intrapore, the membrane permeance was adversely affected due to the exacerbated "funnel effect". Optimizing the polyamide structure via surface chemical modification of the substrate, where reactive amino sites were in situ introduced by the ammonolysis of polyethersulfone substrate, allowed for maximum membrane permeance without reducing the substrate pore size. The optimal membrane exhibited excellent water permeance, ion selectivity, and emerging contaminants removal capability. The accurate optimization of selective layer is anticipated to provide a new avenue for the state-of-the-art membrane fabrication, which opens opportunities for promoting more efficient membrane-based water treatment applications.

Cosolvent-Assisted Interfacial Polymerization toward Regulating the Morphology and Performance of Polyamide Reverse Osmosis Membranes: Increased m-Phenylenediamine Solubility or Enhanced Interfacial Vaporization?

Cosolvent-assisted interfacial polymerization (IP) can effectively enhance the separation performance of thin film composite (TFC) reverse osmosis (RO) membranes. However, the underlying mechanisms regulating the formation of their polyamide (PA) rejection films remain controversial. The current study reveals two essential roles of cosolvents in the IP reaction: (1) directly promoting interfacial vaporization with their lower boiling points and (2) increasing the solubility of m-phenylenediamine (MPD) in the organic phase, thereby indirectly promoting the IP reaction. Using a series of systematically chosen cosolvents (i.e., diethyl ether, acetone, methanol, and toluene) with different boiling points and MPD solubilities, we show that the surface morphologies of TFC RO membranes were regulated by the combined direct and indirect effects. A cosolvent favoring interfacial vaporization (e.g., lower boiling point, greater MPD solubility, and/or higher concentration) tends to create greater apparent thickness of the rejection layer, larger nanovoids within the layer, and more extensive exterior PA layers, leading to significantly improved membrane water permeance. We further demonstrate the potential to achieve better antifouling performance for the cosolvent-assisted TFC membranes. The current study provides mechanistic insights into the critical roles of cosolvents in IP reactions, providing new tools for tailoring membrane morphology and separation properties toward more efficient desalination and water reuse.

Carbon Nanotube Interlayer Enhances Water Permeance and Antifouling Performance of Nanofiltration Membranes: Mechanisms and Experimental Evidence.

Interlayered thin-film nanocomposite (TFNi) membranes have been shown to achieve enhanced water permeance as a result of the gutter effect. Nevertheless, some studies report impaired separation performance after the inclusion of an interlayer. In this study, we resolve the competing mechanisms of water transport in the transverse direction vs that in the normal direction. To enable easy comparison, carbon nanotube (CNT)-incorporated TFNi membranes with an identical polyamide rejection layer but different interlayer thicknesses were investigated. While increasing the thickness of the CNT interlayer facilitates water transport in the transverse direction (therefore improving the gutter effect), it simultaneously increases its hydraulic resistance in the normal direction. An optimal water permeance of 13.0 ± 0.7 L m-2 h-1 bar-1, which was more than doubled over the control membrane of 6.1 ± 0.7 L m-2 h-1 bar-1, was realized at a moderate interlayer thickness, resulting from the trade-off between these two competing mechanisms. In this study, we demonstrate reduced membrane fouling and improved fouling reversibility for a TFNi membrane over its control without an interlayer, which can be attributed to its more uniform water flux distribution. The fundamental mechanisms revealed in this study lay a solid foundation for the future development of TFNi membranes toward enhanced separation properties and antifouling ability.

Understanding Selectivity in Solute-Solute Separation: Definitions, Measurements, and Comparability.

The development of membranes capable of precise solute-solute separation is still in its burgeoning stage without a standardized protocol for evaluating selectivity. Three types of membrane processes with different driving forces, including pressure-driven filtration, concentration difference-driven diffusion, and electric field-driven ion migration, have been applied in this study to characterize solute-solute selectivity of a commercial nanofiltration membrane. Our results demonstrated that selectivity values measured using different methods, or even different conditions with the same method, are generally not comparable. The cross-method incomparability is true for both apparent selectivity, defined as the ratio between concentration-normalized fluxes, and the more intrinsic selectivity, defined as the ratio between the permeabilities of solutes through the active separation layer. The difference in selectivity measured using different methods possibly stems from the fundamental differences in the driving force of ion transport, the effect of water transport, and the interaction between cations and anions. We further demonstrated the difference in selectivity measured using feed solutions containing single-salt species and that containing mixed salts. A consistent protocol with standardized testing conditions to facilitate fair performance comparison between studies is proposed.

Electro-Enhanced Separation of Microsized Oil-in-Water Emulsions via Metallic Membranes: Performance and Mechanistic Insights.

Conventional separation membranes suffer from evitable fouling and flux decrease for water treatment applications. Herein, a novel protocol of electro-enhanced membrane separation is proposed for the efficient treatment of microsized emulsions (∼1 µm) by rationally designing robust electroresponsive copper metallic membranes, which could mitigate oil fouling and coenhance permeance (from ∼1026 to ∼2516 L·m-2·h-1·bar-1) and rejection (from ∼87 to ∼98%). High-flux Cu membranes exhibit superior ductility and electrical conductivity, enabling promising electroactivity. Separation performance and the fouling mechanism were studied under different electrical potentials and ionic strengths. Application of negative polarization into a large-pore (∼2.1 µm) Cu membrane is favorable to not only almost completely reject smaller-sized oil droplets (∼1 µm) but also achieve antifouling and anticorrosion functions. Moreover, surfactants around oil droplets might be redistributed due to electrostatic repulsion, which effectively enhances the steric hindrance effect between neighboring oil droplets, mitigating oil coalescence and consequently membrane fouling. Furthermore, due to the screening effect of surfactants, the presence of low-concentration salts increases the adsorption of surfactants at the oil-water interface, thus preventing oil coalescence via decreasing oil-water interfacial tension. However, under high ionic strengths, the fouling mechanism converts from cake filtration to a complete blocking model due to the reduced electrostatic repulsion between the Cu membrane and oil droplets. This work would provide mechanistic insights into electro-enhanced antifouling for not only oil emulsion separation but also more water treatment applications using rationally designed novel electroresponsive membranes.

Superhydrophobic Carbon Nanotube Network Membranes for Membrane Distillation: High-Throughput Performance and Transport Mechanism.

Despite increasing sustainable water purification, current desalination membranes still suffer from insufficient permeability and treatment efficiency, greatly hindering extensive practical applications. In this work, we provide a new membrane design protocol and molecule-level mechanistic understanding of vapor transport for the treatment of hypersaline waters via a membrane distillation process by rationally fabricating more robust metal-based carbon nanotube (CNT) network membranes, featuring a superhydrophobic superporous surface (80.0 ± 2.3% surface porosity). With highly permeable ductile metal hollow fibers as substrates, the construction of a superhydrophobic (water contact angle ∼170°) CNT network layer endows the membranes with not only almost perfect salt rejection (over 99.9%) but a promising water flux (43.6 L·m-2·h-1), which outperforms most existing inorganic distillation membranes. Both experimental and molecular dynamics simulation results indicate that such an enhanced water flux can be ascribed to an ultra-low liquid-solid contact interface (∼3.23%), allowing water vapor to rapidly transport across the membrane structure via a combined mechanism of Knudsen diffusion (more dominant) and viscous flow while efficiently repelling high-salinity feed via forming a Cassie-Baxter state. A more hydrophobic surface is more in favor of not only water desorption from the CNT outer surface but superfast and frictionless water vapor transport. By constructing a new superhydrophobic triple-phase interface, the conceptional design strategy proposed in this work can be expected to be extended to other membrane material systems as well as more water treatment applications.

Unveiling the Growth of Polyamide Nanofilms at Water/Organic Free Interfaces: Toward Enhanced Water/Salt Selectivity.

The permeance and selectivity of a reverse osmosis (RO) membrane are governed by its ultrathin polyamide film, yet the growth of this critical film during interfacial polymerization (IP) has not been fully understood. This study investigates the evolution of a polyamide nanofilm at the aqueous/organic interface over time. Despite its thickness remaining largely constant (∼15 nm) for the IP reaction time ranging from 0.5 to 60 min, the density of the polyamide nanofilm increased from 1.25 to 1.36 g cm-3 due to the continued reaction between diffused m-phenylenediamine and dangling acyl chloride groups within the formed polyamide film. This continued growth of the polyamide nanofilm led to a simultaneous increase in its crosslinking degree (from 50.1 to 94.3%) and the healing of nanosized defects, resulting in a greatly enhanced rejection of 99.2% for NaCl without sacrificing water permeance. Using humic acid as a molecular probe for sealing membrane defects, the relative contributions of the increased crosslinking and reduced defects toward better membrane selectivity were resolved, which supports our conceptual model involving both enhanced size exclusion and healed defects. The fundamental insights into the growth mechanisms and the structure-property relationship of the polyamide nanofilm provide crucial guidance for the further development and optimization of high-performance RO membranes.

Unraveling the Kinetics and Mechanism of Surfactant-Induced Wetting in Membrane Distillation: An In Situ Observation with Optical Coherence Tomography.

In this study, we performed a direct contact membrane distillation and successfully demonstrated the non-invasive imaging of surfactant-induced wetting using optical coherence tomography. This method enabled us to investigate the wetting kinetics, which was found to follow a "three-region" relationship between the wetting rate and surfactant concentration: the (i) nonwetted region, (ii) concentration-dependent region, and (iii) concentration-independent region at low, intermediate, and high surfactant concentrations, respectively. This wetting behavior was explained by the "autophilic effect", i.e., the wetting was caused by the transfer of surfactants from the water-vapor interface to the unwetted membrane and rendered this membrane hydrophilic, and then the wetting frontier moved forward under capillary forces. At region-(i), the surfactant concentration in the water-vapor interface (Clv) was too low to make the unwetted membrane sufficiently hydrophilic; thereby, the membrane could not be wetted. At region-(ii), due to the fast adsorption of the surfactant on the newly wetted membrane, the wetting rate was determined by the advection/diffusion of surfactants from the feed stream. Consequently, the wetting rate increased with the increases in the water flux and surfactant concentration. At region-(iii), the advection/diffusion provided excess surfactants for adsorption, and thus Clv reached its upper limit (maximum surface excess) and the wetting rate leveled off.

Nanofiltration Membranes with Crumpled Polyamide Films: A Critical Review on Mechanisms, Performances, and Environmental Applications.

Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance ( Science 2018, 360 (6388), 518-521), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.

Interlayered Forward Osmosis Membranes with Ti3C2Tx MXene and Carbon Nanotubes for Enhanced Municipal Wastewater Concentration.

Forward osmosis (FO) hybrid systems have the potential to simultaneously recover nutrients and water from wastewater. However, the lack of membranes with high permeability and selectivity has limited the development and scale-up of these hybrid systems. In this study, we fabricated a novel thin-film nanocomposite membrane featuring an interlayer of Ti3C2Tx MXene intercalated with carbon nanotubes (M/C-TFNi). Owing to the enhanced confinement effect on interfacial degassing and increased amine monomer sorption by the interlayer, the resulting M/C-TFNi FO membrane has a greater degree of cross-linking and roughness. In comparison with the thin-film composite (TFC) membrane without an interlayered structure, the M/C-TFNi membrane attained a water flux that was four times higher and a lower specific salt flux. Notably, the M/C-TFNi membrane exhibited excellent concentration efficiency for real municipal wastewater and enhanced rejection of ammonia nitrogen, which breaks the permeability-selectivity upper bound. This study provides a new avenue for the rational design and development of high-performance FO membranes for environmental applications.

Stable Zr-Based Metal-Organic Framework Nanoporous Membrane for Efficient Desalination of Hypersaline Water.

Treatment of hypersaline waters is a critical environmental challenge. Pervaporation (PV) desalination is a promising technique to address this challenge, but current PV membranes still suffer from challenging issues such as low flux and insufficient stability. Herein, we propose in situ nanoseeding followed by a secondary growth strategy to fabricate a high-quality stable metal-organic framework (MOF) thin membrane (UiO-66) for high-performance pervaporation desalination of hypersaline waters. To address the issue of membrane quality, a TiO2 nano-interlayer was introduced on coarse mullite substrates to favor the growth of a UiO-66 nanoseed layer, on which a well-intergrown UiO-66 selective membrane layer with thickness as low as 1 µm was finally produced via subsequent secondary growth. The PV separation performance for hypersaline waters was systematically investigated at different salt concentrations, feed temperatures, and long-term operation in different extreme chemical environments. Besides having nearly complete rejection (99.9%), the UiO-66 membrane exhibited high flux (37.4 L·m-2·h-1) for hypersaline waters, outperforming current existing zeolite and MOF membranes. The membrane also demonstrated superior long-term operational stability under various harsh environments (hypersaline, hot, and acidic/alkaline feed water) and mild fouling behavior. The rational design proposed in this study is not only applicable for the development of a high-quality UiO-66 membrane enabling harsh hypersaline water treatment but can also be potentially extended to other next-generation nanoporous MOF membranes for more environmental applications.

Degradation of Polyamide Nanofiltration Membranes by Bromine: Changes of Physiochemical Properties and Filtration Performance.

The potential coexistence and interaction of bromine and polyamide membranes during membrane-based water treatment prompts us to investigate the effect of bromine on membrane performance. For fully aromatic polyamide membrane NF90 exposed under a mild bromination condition (10 mg/L), bromine incorporation resulted in more negatively charged (-13 vs -25 mV) and hydrophobic (55.2 vs 58.9°) surfaces and narrower pore channels (0.3 vs 0.29 nm). The permeabilities of water and neutral solutes were reduced by 64 and 69-87%, respectively, which was attributed to the decreased effective pore radius and hydrophilicity. NaCl permeability was reduced by 90% as a synergistic result of enhanced size exclusion and charge repulsion. The further exposure (100 and 500 mg/L bromine) resulted in a more hydrophobic surface (61.7 and 65.5°) and the minor further reduction for water and solute permeabilities (1-9%). Compared with chlorine, the different incorporation efficiency and properties (e.g., atomic size, hydrophilicity) of bromine resulted in opposite trends and/or different degrees for the variation of physicochemical properties and filtration performance of membranes. The bromine incorporation, the shift and disappearance of three characteristic bands, and the increased O/N ratio and calcium content indicated the degradation pathways of N-bromination and bromination-promoted hydrolysis under mild bromination conditions (480 mg/L·h). The further ring-bromination occurred after severe bromine exposure (4800-24,000 mg/L·h). The semi-aromatic polyamide membrane NF270 underwent a similar but less significant deteriorated filtration performance compared with NF90, which requires a different explanation.

Dissecting the Role of Substrate on the Morphology and Separation Properties of Thin Film Composite Polyamide Membranes: Seeing Is Believing.

Recent studies show that the surface morphology of a thin film composite (TFC) polyamide membrane depends strongly on its porous substrate. Nevertheless, the underlining mechanisms and the effects on membrane separation performance remain controversial. To dissect the exact role of pore properties, we synthesized TFC polyamide membranes on polycarbonate substrates with cylindrical track-etched pores (PCTE) of well-defined pore size ranging from 10 to 800 nm. Leaf-like roughness features were most prominent for polyamide films formed on substrates of intermediate pore sizes (80 and 100 nm). Smaller pores inhibited leaf-like features as a result of insufficient storage of m-phenylenediamine (MPD) monomers for the interfacial reaction, whereas larger pores resulted in diminished surface roughness due to the lack of confinement to the interfacially degassed nanobubbles. Substrate porosity plays a critical role on membrane water permeability, while smaller pores with greater pore density are favored to improve membrane rejection. TFC polyamide membranes prepared on sponge-like poly(ether sulfone) and polysulfone substrates exhibit better water permeability and salt rejection compared to the PCTE-TFC membranes thanks to the simultaneously enhanced confinement and MPD storage effects. The mechanistic insights gained in this study reveal the huge potential of substrate design toward high-performance TFC RO membranes.

Dually Charged MOF-Based Thin-Film Nanocomposite Nanofiltration Membrane for Enhanced Removal of Charged Pharmaceutically Active Compounds.

Removal of pharmaceutically active compounds (PhACs) is of great importance in wastewater reclamation due to their potent negative impacts on human health. Typical polyamide nanofiltration (NF) membranes are negatively charged, which compromises their rejection rate of positively charged PhACs. Herein, we propose to rationally design a novel thin-film nanocomposite (TFN) NF membrane featuring a dually charged metal organic framework (MOF) to effectively remove both positively and negatively charged PhACs. Ethylenediamine (ED) was grafted to the coordinately unsaturated metal sites inside the MIL-101(Cr). The resulting ED-MIL-101(Cr) contained both strong positively charged amine groups inside its channels and negatively charged carboxyl groups at its surface. This dually charged nature of the MOF nanoparticles enabled the ED-MIL-101(Cr)-containing TFN membrane to achieve high rejection rates (mostly >90%) for both positively (terbutaline, atenolol, fluoxetine) and negatively charged PhACs (ketoprofen, diclofenac, bezafibrate). At the same time, the ED-MIL-101(Cr) TFN membrane had greatly improved water permeance (140% over the control membrane with MOF loading). Calculations based on density functional theory further confirmed the large energy barrier for the migration of both negatively and positively charged PhACs across the nanochannels of ED-MIL-101(Cr). This study highlights a promising potential of dually charged MOF-TFN membranes for efficient removal of trace organic contaminants in wastewater reclamation.