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.

Achieving Exceptional Volumetric Desalination Capacity Using Compact MoS2 Nanolaminates.

Capacitive deionization (CDI) has emerged as a promising technology for freshwater recovery from low-salinity brackish water. It is still inapplicable in specific scenarios (e.g., households, islands, or offshore platforms) due to too low volumetric adsorption capacities. In this study, a high-density semi-metallic molybdenum disulfide (1T'-MoS2) electrode with compact architecture obtained by restacking of exfoliated nanosheets, which achieve high capacitance up to ≈277.5 F cm-3 under an ultrahigh scan rate of 1000 mV s-1 with a lower charge-transfer resistance and nearly tenfold higher electrochemical active surface area than the 2H-MoS2 electrode, is reported. Furthermore, 1T'-MoS2 electrode demonstrates exceptional volumetric desalination capacity of 65.1 mgNaCl cm-3 in CDI experiments. Ex situ X-ray diffraction (XRD) reveal that the cation storage mechanism with the dynamic expansion of 1T'-MoS2 interlayer to accommodate cations such as Na+, K+, Ca2+, and Mg2+, which in turn enhances the capacity. Theoretical analysis unveils that 1T' phase is thermodynamically preferable over 2H phase, the ion hydration and channel confinement also play critical role in enhancing ion adsorption. Overall, this work provides a new method to design compact 2D-layered nanolaminates with high-volumetric performance for CDI desalination.

Enhancement of waterborne pathogen removal by functionalized biochar with ε-polylysine ″dynamic arms″: Potential application in ultrafiltration system.

Widespread outbreaks of threatening infections caused by unknown pathogens and water transmission have spawned the development of adsorption methods for pathogen elimination. We proposed a biochar functionalization strategy involving ε-polylysine (PLL), a bio-macromolecular poly(amino acid)s with variable folding conformations, as a "pathogen gripper" on biochar. PLL was successfully bridged onto biochar via polydopamine (PDA) crosslinking. The extension of electropositive side chains within PLL enables the capture of both nanoscale viruses and micrometer-scale bacteria in water, achieving excellent removal performances. This functionalized biochar was tentatively incorporated into ultrafiltration (UF) system, to achieve effective and controllable adsorption and retention of pathogens, and to realize the transfer of pathogens from membrane surface/pore to biochar surface as well as flushing water. The biochar-amended UF systems presents complete retention (∼7 LRV) and hydraulic elution of pathogens into membrane flushing water. Improvements in removal of organics and anti-fouling capability were observed, indicating the broken trade-off in UF pathogen removal dependent on irreversible fouling. Chemical characterizations revealed adsorption mechanisms encompassing electrostatic/hydrophobic interactions, pore filling, electron transfer, chemical bonding and secondary structure transitions. Microscopic and mechanical analyses validated the mechanisms for rapid adsorption and pathogen lysis. Low-concentration alkaline solution for used biochar regeneration, facilitated the deprotonation and transformation of PLL side chain to folded structures (α-helix/ß-sheet). Biochar regeneration process also promoted the effective detachment/inactivation of pathogens and protection of functional groups on biochar, corroborated by physicochemical inspection and molecular dynamics simulation. The foldability of poly(amino acid)s acting like dynamic arms, significantly contributed to pathogen capture/desorption/inactivation and biochar regeneration. This study also inspires future investigation for performances of UF systems amended by poly(amino acid)s-functionalized biochar under diverse pressure, temperature, reactive oxygen species of feeds and chemical cleaning solutions, with far-reaching implications for public health, environmental applications of biochar, and UF process improvement.

Energy harvesting from acid mine drainage using a highly proton/ion-selective thin polyamide film.

A huge chemical potential difference exists between the acid mine drainage (AMD) and the alkaline neutralization solution, which is wasted in the traditional AMD neutralization process. This study reports, for the first time, the harvest of this chemical potential energy through a controlled neutralization of AMD using H+-conductive films. Polyamide films with controllable thickness achieved much higher H+ conductance than a commercially available cation exchange membrane (CEM). Meanwhile, the optimal polyamide film had an excellent H+/Ca2+ selectivity of 63.7, over two orders of magnitude higher than that of the CEM (0.3). The combined advantages of fast proton transport and high proton/ion selectivity greatly enhanced the power generation of the AMD battery. The power density was 3.1 W m-2, which is over one order of magnitude higher than that of the commercial CEM (0.2 W m-2). Our study provides a new sustainable solution to address the environmental issues of AMD while simultaneously enabling clean energy production.

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.

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.

Enhanced water treatment performance of ceramic-based forward osmosis membranes via MOF interlayer.

Forward osmosis (FO) membrane processes could operate without hydraulic pressures, enabling the efficient treatment of wastewaters with mitigated membrane fouling and enhanced efficiency. Designing a high-performance polyamide (PA) layer on ceramic substrates remains a challenge for FO desalination applications. Herein, we report the enhanced water treatment performance of thin-film nanocomposite ceramic-based FO membranes via an in situ grown Zr-MOF (UiO-66-NH2) interlayer. With the Zr-MOF interlayer, the ceramic-based FO membranes exhibit lower thickness, higher cross-linking degree, and increased surface roughness, leading to higher water flux of 27.38 L m-2 h-1 and lower reverse salt flux of 3.45 g m-2 h-1. The ceramic-based FO membranes with Zr-MOF interlayer not only have an application potential in harsh environments such as acidic solution (pH 3) and alkaline solution (pH 11), but also exhibit promising water and reverse salt transport properties, which are better than most MOF-incorporated PA membranes. Furthermore, the membranes could reject major species (ions, oil and organics) with rejections ＞94 % and water flux of 22.62-14.35 L m-2 h-1 in the treatment of actual alkaline industrial wastewater (pH 8.6). This rational design proposed in this study is not only applicable for the development of a high-quality ceramic-based FO membrane with enhanced performance but also can be potentially extended to more challenging water treatment applications.

Enhanced metronidazole removal in seawater using a single-chamber bioelectrochemical system.

The aim of this study was to investigate the removal of metronidazole (MNZ) from seawater using a bioelectrochemical system (BES). Single-chamber BES (i.e., S-BES) and dual-chamber BES (i.e., D-BES) were constructed with carbon brush as the anode and cathode. With the inoculum of sea mud and 2 g/L of glucose as the substrate in seawater, S-BES and D-BES were acclimated to test the MNZ removal. Results showed that S-BES could remove almost 100 % of 200 mg/L MNZ within 120 h and remain stable within 10 cycles of operation (∼50 d) under the applied voltage of 0.8 V. The MNZ removal reached ∼100 % and 60.2 % in the cathodic and anodic chambers of D-BES fed by 100 mg/L MNZ under 0.8 V, respectively. The MNZ concentration of 200 mg/L significantly inhibited the sulfur metabolism, decreased the ratio of live to dead cells in the electrode biofilms, and thus reduced the SO42- removal in the S-BES. The MNZ degradation and S2- oxidation was mainly attributed to the cathodic and anodic biofilms of S-BES, respectively. Three degradation pathways of MNZ were proposed based on the identified intermediates and results of density functional theory calculations. The synergies among different genus species in the bacterial communities of biofilms, and between anodic and cathodic reactions could be responsible for the high performance of S-BES. Results from this study should be not only useful for the MNZ removal but also for effective MNZ inhibition of sulfate-reducing bacteria induced microbiologically influenced corrosion in seawater.

New directions on membranes for removal and degradation of emerging pollutants in aqueous systems.

Emerging pollutants (EPs) refer to a group of non-regulated chemical or biological substances that have been recently introduced or detected in the environment. These pollutants tend to exhibit resistance to conventional treatment methods and can persist in the environment for prolonged periods, posing potential adverse effects on ecosystems and human health. As we enter a new era of managing these pollutants, membrane-based technologies hold significant promise in mitigating impact of EPs on the environment and safeguarding human health due to their high selectivity, efficiency, cost-effectiveness and capability for simultaneous separation and degradation. Moreover, these technologies continue to evolve rapidly with the development of new membrane materials and functionalities, advanced treatment strategies, and analyses for effectively treating EPs of more recent concerns. The objective of this review is to present the latest directions and advancements in membrane-based technologies for addressing EPs. By highlighting the progress in this field, we aim to share valuable perspectives with researchers and contribute to the development of future directions in sustainable treatments for EPs.

Photogenerated outer electric field induced electrophoresis of organic nanocrystals for effective solid-solid photocatalysis.

Rapid mass transfer in solid-solid reactions is crucial for catalysis. Although phoretic nanoparticles offer potential for increased collision efficiency between solids, their implementation is hindered by limited interaction ranges. Here, we present a self-driven long-range electrophoresis of organic nanocrystals facilitated by a rationally designed photogenerated outer electric field (OEF) on their surface. Employing perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecular nanocrystals as a model, we demonstrate that a directional OEF with an intensity of 13.6-0.4 kV m-1 across a range of 25-200 µm. This OEF-driven targeted electrophoresis of PTCDA nanocrystals onto the microplastic surface enhances the activity for subsequent decomposition of microplastics (196.8 mg h-1) into CO2 by solid-solid catalysis. As supported by operando characterizations and theoretical calculations, the OEF surrounds PTCDA nanocrystals initially, directing from the electron-rich (0 1 1) to the hole-rich [Formula: see text] surface. Upon surface charge modulation, the direction of OEF changes toward the solid substrate. The OEF-driven electrophoretic effect in organic nanocrystals with anisotropic charge enrichment characteristics indicates potential advancements in realizing effective solid-solid photocatalysis.

Impact of nanoplastics on membrane scaling and fouling in reverse osmosis desalination process.

Nanoplastics (NPs) are a prevalent type of emerging pollutant in marine environment. However, their fouling behavior and impact on reverse osmosis (RO) membrane performance remain unexplored. We investigated the relationship between polystyrene (PS), one of the most abundant NPs, with silica scaling and humic acid (HA) fouling in RO. The results demonstrated that the surface potential of NPs played an important role in the combined scaling and fouling process. Compared with the negatively charged NPs (original PS and carboxyl group modified PS, PS-COOH), the amino-functionalized PS (PS-NH2) with positive surface charge significantly accelerated membrane scaling/fouling and induced a synergistic water flux decline, due to the strong electrostatic attraction between PS-NH2, foulants, and the membrane surface. The amino groups acted as binding sites, which promoted the heterogeneous nucleation of silica and adsorption of HA, then formed stable composite pollutants. Thermodynamic analysis via isothermal titration calorimetry (ITC) further confirmed the spontaneous formation of stable complexes between PS-NH2 and silicates/HA. Our study provides new insights into the combined NPs fouling with other scalants or foulants, and offers guidance for the accurate prediction of RO performance in the presence of NPs.

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.

Engineering In Situ Catalytic Cleaning Membrane Via Prebiotic-Chemistry-Inspired Mineralization.

Pressure-driven membrane separation promises a sustainable energy-water nexus but is hindered by ubiquitous fouling. Natural systems evolved from prebiotic chemistry offer a glimpse of creative solutions. Herein, a prebiotic-chemistry-inspired aminomalononitrile (AMN)/Mn2+ -mediated mineralization method is reported for universally engineering a superhydrophilic hierarchical MnO2 nanocoating to endow hydrophobic polymeric membranes with exceptional catalytic cleaning ability. Green hydrogen peroxide catalytically triggered in-situ cleaning of the mineralized membrane and enabled operando flux recovery to reach 99.8%. The mineralized membrane exhibited a 9-fold higher recovery compared to the unmineralized membrane, which is attributed to active catalytic antifouling coupled with passive hydration antifouling. Electron density differences derived from the precursor interaction during mediated mineralization unveiled an electron-rich bell-like structure with an inner electron-deficient Mn core. This work paves the way to construct multifunctional engineered materials for energy-efficient water treatment as well as for diverse promising applications in catalysis, solar steam generation, biomedicine, and beyond.

Enhancing the removal of organic micropollutants by nanofiltration membrane with Fe (III)-tannic acid interlayer: Mechanisms and environmental implications.

Nanofiltration technology has been applied in a variety of water treatment scenarios. However, conventional thin-film composite (TFC) membranes fail to remove emerging organic micropollutants (OMPs) efficiently. Here we applied thin-film nanocomposite membrane with an interlayer (TFNi) of Fe (III)-tannic acid to remove various types of OMPs, such as endocrine disrupting chemicals (EDCs), pharmaceutically active compounds (PhACs), and perfluoroalkyl substances (PFASs). Compared to the pristine TFC membrane, TFNi membrane exhibited crumpled morphology and its rejection layer was denser, better cross-linked and possessed smaller average pore size with narrower distribution. Significant enhancement in water-OMPs selectivity of PhACs and PFASs was observed. The mechanism lies in the effects of interlayer in improving the membrane permeance to water and meanwhile reducing the permeance to some OMPs by enhancing size exclusion effects. This work confirms the effectiveness of using TFNi membrane to simultaneously enhance the OMPs rejection and water permeance. The unraveled mechanism might inspire the future development of high-performance nanofiltration membranes targeting OMPs removal.

Gas station in blood vessels: An endothelium mimicking, self-sustainable nitric oxide fueling stent coating for prevention of thrombosis and restenosis.

Stenting is the primary treatment for vascular obstruction-related cardiovascular diseases, but it inevitably causes endothelial injury which may lead to severe thrombosis and restenosis. Maintaining nitric oxide (NO, a vasoactive mediator) production and grafting endothelial glycocalyx such as heparin (Hep) onto the surface of cardiovascular stents could effectively reconstruct the damaged endothelium. However, insufficient endogenous NO donors may impede NO catalytic generation and fail to sustain cardiovascular homeostasis. Here, a dopamine-copper (DA-Cu) network-based coating armed with NO precursor L-arginine (Arg) and Hep (DA-Cu-Arg-Hep) is prepared using an organic solvent-free dipping technique to form a nanometer-thin coating onto the cardiovascular stents. The DA-Cu network adheres tightly to the surface of stents and confers excellent NO catalytic activity in the presence of endogenous NO donors. The immobilized Arg functions as a NO fuel to generate NO via endothelial nitric oxide synthase (eNOS), while Hep works as eNOS booster to increase the level of eNOS to decompose Arg into NO, ensuring a sufficient supply of NO even when endogenous donors are insufficient. The synergistic interaction between Cu and Arg is analogous to a gas station to fuel NO production to compensate for the insufficient endogenous NO donor in vivo. Consequently, it promotes the reconstruction of natural endothelium, inhibits smooth muscle cell (SMC) migration, and suppresses cascading platelet adhesion, preventing stent thrombosis and restenosis. We anticipate that our DA-Cu-Arg-Hep coating will improve the quality of life of cardiovascular patients through improved surgical follow-up, increased safety, and decreased medication, as well as revitalize the stenting industry through durable designs.

Robust ceramic-based graphene membrane for challenging water treatment with enhanced fouling and scaling resistance.

Membrane fouling and scaling are two challenges for efficient treatment of hypersaline wastewater, greatly hindering separation performance and operation stability of desalination membranes. In this work, we report a smooth ceramic-based graphene desalination membrane, exhibiting enhanced anti-fouling and anti-scaling ability and operational performance for efficient treatment of both synthetic and real industrial wastewaters, outperforming polypropylene (PP) membrane. For treatment of hypersaline waters containing organic or inorganic substance, we demonstrate that the graphene membrane exhibits more stable water flux and almost complete salt rejection (>99.9%) during constant operation. Enhanced anti-fouling and desalination performance of graphene membrane could be attributed to the lower attractive interaction force with foulant (-4.65 mJ m-2), lower surface roughness (Ra = 2.2 ± 0.1 nm) and higher affinity with water than PP membrane. Furthermore, an anti-scaling mechanism enabled by graphene membrane is evidenced, with a highlight on the roles of smooth graphene surface with lower roughness, less nucleation sites and lower binding force with scaling crystals. Importantly, even for industrial petrochemical wastewater, such a graphene membrane also exhibits relatively more stable water flux and promising oil and ions rejection during long-term operation, outperforming PP membrane. This study further confirms a promising practical application potential of robust ceramic-based graphene membrane for efficient treatment of more challenging hypersaline wastewater with complicated compositions, which is not feasible by conventional desalination membranes.

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.

NaHCO3 addition enhances water permeance and Ca/haloacetic acids selectivity of nanofiltration membranes for drinking water treatment.

The existence of disinfection by-products such as haloacetic acids (HAAs) in drinking water severely threatens water safety and public health. Nanofiltration (NF) is a promising strategy to remove HAAs for clean water production. However, NF often possesses overhigh rejection of essential minerals such as calcium. Herein, we developed highly selective NF membranes with tailored surface charge and pore size for efficient rejection of HAAs and high passage of minerals. The NF membranes were fabricated through interfacial polymerization (IP) with NaHCO3 as an additive. The NaHCO3-tailored NF membranes exhibited high water permeance up to ∼24.0 L m - 2 h - 1 bar-1 (more than doubled compared with the control membrane) thanks to the formation of stripe-like features and enlarged pore size. Meanwhile, the tailored membranes showed enhanced negative charge, which benefitted their rejection of HAAs and passage of Ca and Mg. The higher rejection of HAAs (e.g., > 90%) with the lower rejection of minerals (e.g., < 30% for Ca) allowed the NF membranes to achieve higher minerals/HAAs selectivity, which was significantly higher than those of commercially available NF membranes. The simultaneously enhanced membrane performance and higher minerals/HAAs selectivity would greatly boost water production efficiency and water quality. Our findings provide a novel insight to tailor the minerals/micropollutants selectivity of NF membranes for highly selective separation in membrane-based water treatment.

Relating critical and limiting fluxes to metastable and long-term stable fluxes in colloidal membrane filtration through collision-attachment theory.

In membrane technology for water/wastewater treatment, the concepts of critical flux (JC) and limiting flux (JL) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of JC, JL to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J0), foulant-clean-membrane energy barrier (Ef-m), and foulant-fouled-membrane energy barrier (Ef-f). When J0 is below JL, water flux remains stable over a long time even for the case of J0 over JC, thanks to the strongly repulsive Ef-f. At J0 > JL and J0 > JC, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to JL as the long-term stable flux. Under the condition of JL < J0 ≤ JC, an initial metastable flux appears owing to the high Ef-m, with longer metastable period observed at lower J0 and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak Ef-f, and the water flux eventually degenerates to JL in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.

Making waves: Why do we need ultra-permeable nanofiltration membranes for water treatment?

Over the last few decades, developing ultra-permeable nanofiltration (UPNF) membranes has been a focus research area to support NF-based water treatment. Nevertheless, there have been ongoing debates and doubts on the need for UPNF membranes. In this work, we share our perspectives on why UPNF membranes are desired for water treatment. We analyze the specific energy consumption (SEC) of NF processes under various application scenarios, which reveals the potential of UPNF membranes for reducing SEC by 1/3 to 2/3 depending on the prevailing transmembrane osmotic pressure difference. Furthermore, UPNF membranes could potentially enable new process opportunities. Vacuum-driven submerged NF-modules could be retrofitted to existing water/wastewater treatment plants, offering lower SEC and lower cost compared to conventional NF systems. Their use in submerged membrane bioreactors (NF-MBR) can recycle wastewater into high-quality permeate water, which enables energy-efficient water reuse in a single treatment step. The ability for retaining soluble organics may further extend the application of NF-MBR for anaerobic treatment of dilute municipal wastewater. Critical analysis of membrane development reveals huge rooms for UPNF membranes to attain improved selectivity and antifouling performance. Our perspective paper offers important insights for the future development of NF-based water treatment technology, which could potentially lead to a paradigm shift in this burgeoning field.