A Facile and Scalable Fabrication Procedure for Thin-Film Composite Membranes: Integration of Phase Inversion and Interfacial Polymerization

Polydopamine-modified carboxylated cellulose nanocrystrals as functional fillers for polyethersulfone (PES) membranes to achieve superior dye/salt separation

This study presents the development of advanced tight polyethersulfone (PES) ultrafiltration membranes enhanced with polydopamine-coated carboxylated cellulose nanocrystrals (PDA@C-CNC) as functional fillers. The PDA@C-CNC fillers were synthesized via an in situ self-polymerization approach and employed as surface segregation agents during membrane preparation. Utilizing the non-solvent-induced phase separation (NIPS) technique, the highly hydrophilic PDA@C-CNC particles migrated to the interface between the polymer solution and the coagulation bath and tightly adhered to the polyethersulfone (PES) matrix through strong hydrogen bonding and π-π interactions, forming a dense, hydrophilic selective surface layer rich in polar functional groups (amino group (-NH2) and hydroxyl group(-OH)). Concurrently, the support layer developed a porous structure characterized by extended and widened cavities, facilitating enhanced mass transfer. The synergistic combination of a selective dense surface layer and an optimally structured support layer endowed the modified membranes with remarkable permeability and selectivity. Surprisingly, the water flux of the modified membrane with 0.2 % PDA@C-CNC (MPC0.2) achieved a remarkable 332 L·m-2·h-1·bar-1, which is 2.29 times higher than that of the unmodified membrane (M0). Additionally, MPC0.2 demonstrated exceptional dyes rejection rates (Congo red (CR) > 99.7 %, Eriochrome Black T (EBT) > 97.7 %) alongside minimal salt rejection (sodium chloride (NaCl): 0.2 %, sodium sulfate (Na2SO4): 1.7 %). These findings highlight the potential of PDA@C-CNC/PES composite membranes for efficient and selective removal of dyes and salts from textile wastewater.

Arrays of Bowl-Shaped Janus Particle Film with Structured Colors

Structural colors generated via total internal reflection (TIR) using nanostructure-free micro-concave shapes have garnered increasing attention. However, the application of large micro-concave structures for structural coloration remains limited. Herein, a flexibly tunable structural color film fabricated by casting polydimethylsiloxane (PDMS) on an array of large poly(glycidyl methacrylate) (PGMA) bowl-shaped particles is reported. The resultant film exhibits tunable red to green structural colors with changing observation angles. Moreover, the color can be further tailored by altering the shape of the film itself. The incorporation of the PDMS layer not only facilitates a shift in the locus of TIR from the bottom surface to the top concave surface of the particles, thereby enabling the generation of structural color, but also confers enhanced flexibility to the film. Further decoration with silver nanoparticles imparts antimicrobial properties, yielding a novel antimicrobial coating material with structural colors. The simple and cost-effective strategy for the production of structural color films provides potential applications in antimicrobial coatings, enabling accessible and customizable structural coloration using big-size micro-concave particles.

Distinct Efficacies of Interlayers in Tailoring Polyamide Nanofiltration Membrane Performance for Organic Micropollutant Removal: Dependent on Substrate Characteristics

Interlayered thin-film nanocomposite (TFN) membranes have shown the potential to boost nanofiltration performance for water treatment applications including the removal of organic micropollutants (OMPs). However, the effects of substrates have been overlooked when exploiting and evaluating the efficacy of certain kinds of interlayers in tailoring membrane performance. Herein, a series of TFN membranes were synthesized on different porous substrates with identical interlayers of metal-organic framework nanosheets. It was revealed that the interlayer introduction could narrow but not fully eliminate the difference in the properties among the polyamide layers formed on different substrates, and the membrane performance variation was prominent in distinct aspects. For substrates with small pore sizes exerting severe water transport hindrance, the introduced interlayer mainly enhanced membrane water permeance by affording the gutter effect, while it could be more effective in reducing membrane pore size by improving the interfacial polymerization platform and avoiding PA defects when using a large-pore-size substrate. By matching the selected substrates and interlayers well, superior TFN membranes were obtained with simultaneously higher water permeance and OMP rejections compared to three commercial membranes. This study helps us to objectively understand interlayer efficacies and attain performance breakthroughs of TFN membranes for more efficient water treatment.

The Preparation and Performance Study of Polyamide Film Based on PDA@MWCNTs/PVDF Porous Support Layer

It is urgent to develop a polyamide (PA) thin-film composite (TFC) membrane with a new method in this study by designing and constructing a new nanomaterial support layer instead of the conventional support layer. Polydopamine-wrapped single-walled carbon nanotubes (PDA@MWCNTs) as the place of the polymerization reaction can optimize the PA film structure and performance. The resulting composite membrane presents a higher water flux of 15.8 L·m-2·h-1·bar-1 and a rejection rate of 97% to Na2SO4, simultaneously maintaining this high separation performance in 300 min. It is a new ideal to construct novel support layer by using inorganic nanoparticles and organic polymer nanofiber membranes.

Energy-efficient trehalose-based polyester nanofiltration membranes for zero-discharge textile wastewater treatment

Recovery of water, salts, and hazardous dye from complex saline textile wastewater faces obstacles in separating dissolved ionic substances and recovering organic components during desalination. This study realized the simultaneous fractionation, desalination, and dye removal/recovery treatment of textile wastewater by using trehalose (Tre) as an aqueous monomer to prepare polyester loose nanofiltration (LNF) membrane with fine control microstructure via interfacial polymerization. Outperforming the NF270 commercial membrane, the Tre-1.05/TMC optimized membrane achieves zero-discharge textile wastewater treatment, cutting energy consumption by 295% and reducing water consumption by 42.8%. This efficiency surge results from remarkable water permeability (130.83 L m-2 h-1 bar-1) and impressive dye desalination (NaCl/ Direct Red 23 separation factor of 275) of the Tre-1.05/TMC membrane. For a deeper comprehension of filtration performance, the sieving mechanism of polyester LNF membranes was systematically elucidated. This strategic approach offers significant prospects for energy conservation, carbon emission mitigation, and enhanced feasibility of membrane-based wastewater treatment systems.

Optimized Polymeric Membranes for Water Treatment: Fabrication, Morphology, and Performance

Conventional polymers, endowed with specific functionalities, are extensively utilized for filtering and extracting a diverse set of chemicals, notably metals, from solutions. The main structure of a polymer is an integral part for designing an efficient separating system. However, its chemical functionality further contributes to the selectivity, fabrication process, and resulting product morphology. One example would be a membrane that can be employed to selectively remove a targeted metal ion or chemical from a solution, leaving behind the useful components of the solution. Such membranes or products are highly sought after for purifying polluted water contaminated with toxic and heavy metals. An efficient water-purifying membrane must fulfill several requirements, including a specific morphology attained by the material with a specific chemical functionality and facile fabrication for integration into a purifying module Therefore, the selection of an appropriate polymer and its functionalization become crucial and determining steps. This review highlights the attempts made in functionalizing various polymers (including natural ones) or copolymers with chemical groups decisive for membranes to act as water purifiers. Among these recently developed membrane systems, some of the materials incorporating other macromolecules, e.g., MOFs, COFs, and graphene, have displayed their competence for water treatment. Furthermore, it also summarizes the self-assembly and resulting morphology of the membrane materials as critical for driving the purification mechanism. This comprehensive overview aims to provide readers with a concise and conclusive understanding of these materials for water purification, as well as elucidating further perspectives and challenges.

Ice-confined synthesis of highly ionized 3D-quasilayered polyamide nanofiltration membranes

Existing polyamide (PA) membrane synthesis protocols are underpinned by controlling diffusion-dominant liquid-phase reactions that yield subpar spatial architectures and ionization behavior. We report an ice-confined interfacial polymerization strategy to enable the effective kinetic control of the interfacial reaction and thermodynamic manipulation of the hexagonal polytype (Ih) ice phase containing monomers to rationally synthesize a three-dimensional quasilayered PA membrane for nanofiltration. Experiments and molecular simulations confirmed the underlying membrane formation mechanism. Our ice-confined PA nanofiltration membrane features high-density ionized structure and exceptional transport channels, realizing superior water permeance and excellent ion selectivity.

Enhancing Nanofiltration Selectivity of Metal-Organic Framework Membranes via a Confined Interfacial Polymerization Strategy

Development of well-constructed metal-organic framework (MOF) membranes can bring about breakthroughs in nanofiltration (NF) performance for water treatment applications, while the relatively loose structures and inevitable defects usually cause low rejection capacity of MOF membranes. Herein, a confined interfacial polymerization (CIP) method is showcased to synthesize polyamide (PA)-modified NF membranes with MOF nanosheets as the building blocks, yielding a stepwise transition from two-dimensional (2D) MOF membranes to polyamide NF membranes. The CIP process was regulated by adjusting the loading amount of piperazine (PIP)-grafted MOF nanosheets on substrates and the additional content of free PIP monomers distributed among the nanosheets, followed by the reaction with trimesoyl chloride in the organic phase. The prepared optimal membrane exhibited a high Na2SO4 rejection of 98.4% with a satisfactory water permeance of 37.4 L·m-2·h-1·bar-1, which could be achieved by neither the pristine 2D MOF membranes nor the PA membranes containing the MOF nanosheets as the conventional interlayer. The PA-modified MOF membrane also displayed superior stability and enhanced antifouling ability. This CIP strategy provides a novel avenue to develop efficient MOF-based NF membranes with high ion-sieving separation performance for water treatment.

A review on polyester and polyester-amide thin film composite nanofiltration membranes: Synthesis, characteristics and applications

Nanofiltration (NF) membranes have been widely used in various fields including water treatment and other separation processes, while conventional thin film composite (TFC) membranes with polyamide (PA) selective layers suffer the problems of fouling and chlorine intolerance. Due to the abundant hydrophilic hydroxyl groups and ester bonds free from chlorine attack, the TFC membranes composed of polyester (PE) or polyester-amide (PEA) selective layers have been proven to possess enhanced anti-fouling properties and superior chlorine resistance. In this review, the research progress of PE and PEA nanofiltration membranes is systematically summarized according to the variety of hydroxyl-containing monomers for membrane fabrication by the interfacial polymerization (IP) reaction. The synthesis strategies as well as the mechanisms for tailoring properties and performance of PE and PEA membranes are analyzed, and the membrane application advantages are demonstrated. Moreover, current challenges and future perspectives of the development of PE and PEA nanofiltration membranes are proposed. This review can offer guidance for designing high-performance PE and PEA membranes, thereby further promoting the efficacy of nanofiltration.

Recycling the High-Salinity Textile Wastewater by Quercetin-Based Nanofiltration Membranes with Minimal Water and Energy Consumption

Effective recovery of dyes and salts from textile wastewater by nanofiltration (NF) remains a serious challenge due to the high consumption of water and energy caused by the limited performance of the available membranes. Herein, a novel strategy is described to prepare loose polyester NF membranes by using renewable quercetin as the aqueous monomer for fractionation of high salinity textile wastewater with minimal water and energy consumption. Compared with NF270, taken as the reference membrane, the QE-0.2/TMC-0.2 membrane significantly improved the efficiency for dye/salt fractionation by 288%. The water consumption was also decreased by 42.9%. The efficiency is attributed to an ultrahigh water permeance of 198 ± 2.1 L^-1 m^-2 h^-1 bar^-1 with a high selectivity of 123 (extremely low NaCl rejection of 1.6% and high Congo red rejection of 99.2%). The optimal quercetin-based membrane had an ultrathin separation layer of about 39 ± 1.2 nm with good hydrophilicity and negative charge density. Moreover, this work includes a novel method of comparison with a theoretically ideal membrane, which shows that both the energy and water consumption are near their theoretical minimum. This strategy is expected to save energy and minimize carbon emissions for membrane-based wastewater treatment systems.

Efficient capture of endocrine-disrupting compounds by a high-performance nanofiltration membrane for wastewater treatment

Conventional polyamide (PA) nanofiltration (NF) membranes can readily adsorb aromatic compounds, such as endocrine disrupting compounds (EDCs). Therefore, these substances can easily be transported across the membrane by solution-diffusion, resulting in a poor EDC-rejection. In this work, a novel thin film nanocomposite (TFN) membrane was fabricated by incorporating covalent organic frameworks (COFs) into the PA layer via an interfacial polymerization reaction. COFs with functional groups can provide abundant active binding sites for highly efficient EDC-capture. The rejection of the optimal TFN-COF membrane for bisphenol A, bisphenol AF, and sodium 2-biphenylate was 98.3%, 99.1%, and 99.3%, respectively, which was much higher than of the rejection of the pristine NF-membrane (82.4%, 95.5%, and 96.4%, respectively). Additionally, the TFN-COF membrane could be regenerated fast and efficiently by washing with ethanol for some minutes. COF nanofillers with porous structures provide additional water channels, making it possible to overcome the permeability-selectivity trade-off of NF membranes. The water permeance (17.1 L m^-2 h^-1 bar^-1) of the optimal membrane was about two times higher than for the pristine NF-membrane (8.7 L m^-2 h^-1 bar^-1). In addition, the TFN-COF membrane with a COF-loading of 0.05% w/v had an excellent Na₂SO₄ rejection (95.2%) due to size exclusion and strong Donnan effect. This work combines traditional NF membranes and adsorption materials to achieve efficient capture and rapid release of EDCs without sacrificing salt rejections, which opens the door to develop fit-for-purpose adsorptive NF membranes.

Integrating graphene oxide into layers of PVDF/PVDF@cross-linked sodium alginate/polyamide membrane for efficiently enhancing desalination performances

The membrane modules of the water treatment system are faced costly damages; thereby executing pre-desalination units based on Nanofiltration (NF) could prevent these suffers, and improve the permeated water flux (PWF) and salt rejection (SR). Hence, we focused on the construction of a novel ternary-layer NF membrane through “electrospinning Polyvinylidene Fluoride (PVDF) (as bottom layer)”, “generating middle layer by electrospinning PVDF along with, the implementation cross-linking after electrospraying Sodium Alginate”, and “synthesizing Polyamide (as top layer) through interfacial polymerization”. More importantly, it anticipated that the Taguchi statistical method can expeditiously optimize the effects of Graphene Oxide nano-sheets (GOns) on water-dependent properties, such as PWF and SR. Astonishingly, the desalination capabilities significantly improved, when the top, middle, and bottom layers simultaneously had 1, 0.1, and 0.1 wt.% of GOns, respectively. Overall, comparing the performances between the optimized sample containing low-dosage and without GOns demonstrated the PWF ameliorated from 6.68 to 20.36 L/m² h; also, the SR ability remained on an incremental basis as NaCl < MgCl₂ < MgSO₄ under 6 bar pressure. Manifestly, these authentic results denoted promising, innovative, and large-scaling insights when effectual PWF and SR be necessary.

Fabrication of Thin Film Composite Membranes on Nanozeolite Modified Support Layer for Tailored Nanofiltration Performance

Thin-film composite (TFC) structure has been widely employed in polymeric membrane fabrication to achieve superior performance for desalination and water treatment. In particular, TFC membranes with a thin active polyamide (PA) selective layer are proven to offer improved permeability without compromising salt rejection. Several modifications to TFCs have been proposed over the years to enhance their performance by altering the selective, intermediate, or support layer. This study proposes the modification of the membrane support using nanozeolites prepared by a unique ball milling technique for tailoring the nanofiltration performance. TFC membranes were fabricated by the interfacial polymerization of Piperazine (PIP) and 1,3,5-Benzenetricarbonyl trichloride (TMC) on Polysulfone (PSf) supports modified with nanozeolites. The nanozeolite concentration in the casting solution varied from 0 to 0.2%. Supports prepared with different nanozeolite concentrations resulted in varied hydrophilicity, porosity, and permeability. Results showed that optimum membrane performance was obtained for supports modified with 0.1% nanozeolites where pure water permeance of 17.1 ± 2.1 Lm^-2 h^-1 bar^-1 was observed with a salt rejection of 11.47%, 33.84%, 94%, and 95.1% for NaCl, MgCl₂, MgSO_4, and Na₂SO₄ respectively.

Fabrication of Loose Nanofiltration Membranes with High Rejection Selectivity between Natural Organic Matter and Salts for Drinking Water Treatment

Loose nanofiltration (LNF) membranes with a molecular weight cut-off (MWCO) of about 1000 Da and high surface negative charge density have great application potential for drinking water treatment pursuing high rejection selectivity between natural organic matter (NOM) and mineral salts. This study was conducted to exploit the novel method coupling non-solvent induced phase separation (NIPS) and interfacial polymerization (IP) for the preparation of high-performance LNF membranes. A number of LNF membranes were synthesized by varying the polyethersulfone (PES) and piperazine (PIP) concentrations in the cast solution for the PES support layer preparation. Results showed that these two conditions could greatly affect the membrane water permeance, MWCO and surface charge. One LNF membrane, with a water permeance as high as 23.0 ± 1.8 L/m²/h/bar, when used for the filtration of conventional process-treated natural water, demonstrated a rejection of NOM higher than 70% and a low rejection of mineral salts at about 20%. Both the mineral salts/NOM selectivity and permselectivity were superior to the currently available LNF membranes as far as the authors know. This study demonstrated the great advantage of the NIPS-IP method for the fabrication of LNF membranes, particularly for the advanced treatment of drinking water.

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.

Enhanced Water Permeability and Antifouling Property of Coffee-Ring-Textured Polyamide Membranes by In Situ Incorporation of a Zwitterionic Metal-Organic Framework

Modulation of the polyamide structure is critically important for the reverse-osmosis performance of thin-film composite (TFC) membranes in the field of water reuse and desalination. Herein, zwitterionic nanoparticles of zeolitic imidazolate framework-8 (PZ@ZIF-8) were fabricated and incorporated into the polyamide active layer through the interfacial polymerization method. A hydrophilic, zwitterionic coffee-ring structure was formed on the surface of polyamide thin-film nanocomposite (TFN) membranes due to the adjusted diffusion rate of m-phenylenediamine (MPD) from the aqueous phase into the organic phase during the interfacial polymerization process. Surface characterization demonstrated that the coffee-ring structure increased the amounts of water transport channels on the membrane surface and the intrinsic pores of PZ@ZIF-8 maintained the salt rejection. Antifouling and bactericidal activities of TFN membranes were enhanced remarkably owing to the bacterial-"defending" and bacterial-"attacking" behaviors of hydrophilic and zwitterionic groups from PZ@ZIF-8 nanoparticles. This work would provide a promising method for the application of MOFs to enhance the bio-/organic-fouling resistance of TFN membranes with high water permeation and salt rejection.

Functionalized electrospun nanofiber membranes for water treatment: A review

Electrospun nanofiber membranes (ENMs) have high porosity, high specific surface area and unique interconnected structure. It has huge advantages and potential in the treatment and recycling of wastewater. In addition, ENMs can be easily functionalized by combining multifunctional materials to achieve different water treatment effects. Based on this, this review summarizes the preparation of functionalized ENMs and its detailed application in the field of water treatment. First, the process and influence factors of electrospinning process are introduced. ENMs with high porosity, thin and small fiber diameter have better performance. Secondly, the modification methods of ENMs are analyzed. Pre-electrospinning and post-electrospinning modification technology can prepare specific functionalized ENMs. Subsequently, functionalized ENMs show water treatment capabilities such as separation, adsorption, photocatalysis, and antimicrobial. Subsequently, the application of functionalized ENMs in water treatment capabilities such as separation, adsorption, photocatalysis, and antimicrobial capabilities were listed. Finally, we also made some predictions about the future development direction of ENMs in water treatment, and hope this article can provide some clues and guidance for the research of ENMs in water treatment.

Performance Evaluation of 1-Cyclohexylpiperidine as a Draw Solute for Forward Osmosis Water Separation and CO2 Recovery

Membrane-based technologies, such as forward osmosis (FO), offer the advantage of treating water through a spontaneous process that requires minimal energy input while achieving favorable water permeability and selectivity. However, the FO process still has some challenges that need to be solved or improved to become entirely feasible. The main impediment for this technology is the recovery of the draw solute used to generate the osmotic potential in the process. In this paper, we discuss the use of a switchable polarity solvent, 1-cyclohexylpiperidine (CHP), as a draw solute that responds to external stimuli. Specifically, the miscibility of CHP can be switched by the presence of carbon dioxide (CO₂) and is reversible by applying heat. Thus, in this study, the hydrophobic CHP is first converted to the hydrophilic ammonium salt (CHPH⁺), and its capability as a draw solution (DS) is thoroughly evaluated against the typical osmotic agent, sodium chloride (NaCl). Our results show that the water permeability across the thin film composite membrane increases by 69% when CHPH⁺ is used as the DS. Also, the water permeability when using different feed solutions: aqueous solutions of (a) urea and (b) NaCl were evaluated. In both cases, the CHPH⁺ generates water fluxes in the range of 65 ± 4 LMH and 69 ± 2 LMH, respectively. We then separate the diluted DS by applying 75 °C to the solution to recover the pure CHP and water. The results of this work provide a proof-of-concept of a CHP wastewater and desalination method via an FO process.

Tailoring Charged Nanofiltration Membrane Based on Non-Aromatic Tris(3-aminopropyl)amine for Effective Water Softening

High-performance positively-charged nanofiltration (NF) membranes have a profound significance for water softening. In this work, a novel monomer, tris(3-aminopropyl)amine (TAEA), with one tertiary amine group and three primary amine groups, was blended with trace amounts of piperazine (PIP) in aqueous solution to fabricate a positively-charged NF membrane with tunable performance. As the molecular structures of TAEA and PIP are totally different, the chemical composition and structure of the polyamine selective layer could be tailored via varying the PIP content. The resulting optimal membrane exhibited an excellent water permeability of 10.2 LMH bar⁻¹ and a high rejection of MgCl₂ (92.4%), due to the incorporation of TAEA/PIP. In addition, this TAEA NF membrane has a superior long-term stability. Thus, this work provides a facile way to prepare a positively charged membrane with an efficient water softening ability.