Polyamide Separation Layer Research Articles

Study on PPTA/PEMs/PA composite NF membranes for highly efficient desalination of high-saline Kevlar® brine and long-term anti-scaling performance

It produces a large number of organic brine during the polymerization process of Kevlar®, which contains N-(1-Methyl-2-pyrrolidinone) (NMP) and high concentrations of CaCl2 and NaCl. The wastewater will cause serious pollution of water and soil resources if discharged directly. The separation and recycling of inorganic salts have been an industrial difficulty. In this paper, based on the novel concept of green self-recycling, we used Poly (p-phenylene terephthamide) (PPTA, resin of Kevlar®) ultrafiltration membrane as the substrate, composited the loose and curled polyelectrolyte multilayers (PEMs) as interlayers and formed a homogeneous polyamide separation layer through interfacial polymerization, from which we obtained the homogeneous reinforcement aramid composite nanofiltration (NF) membrane. The composite NF membranes with different interlayer numbers showed different advantages. For single-component brine, the retentions of PEMs-1/PA membrane were 93 % and 98.4 % for 10 g/L CaCl2 and MgSO4 respectively. For mixed brine of 20 g/L CaCl2 and NaCl, the retentions of PEMs-3/PA membrane were 87.13 % and 1.05 % for CaCl2 and NaCl respectively with the separation factor as high as 82.95. In the test of long-term service stability, PEMs-3/PA maintained stable performance in 4 cycles total of 160 h. And the permeance could almost fully recover after pure water backwash of only 0.5 h. In addition, the structural stability and anti-scaling performance of the PEMs-3/PA membrane were further verified in high-temperature brine. Surprisingly, the obtained PPTA NF membrane in this work could effectively treat the high-saline brine in the Kevlar® polymerization process and would have more promising prospects in the treatment of industrial wastewater of brine.

Positively charged nanofiltration membranes with gradient structures for enhancing separation properties

Creating a gradient structure in a separation layer is an effective approach for enhancing separation performance of thin film composite membranes. Here we present the preparation of a positively charged nanofiltration membrane with a gradient structure. An inner layer consisting of a loose polyester separation layer with large pores was formed through the interfacial polymerization (IP) reaction between triethanolamine (TEA) and trimesoyl chloride (TMC). And a dense and thin top layer composed of a polyamide separation layer was obtained by secondary interfacial polymerization (SIP) reaction using polyethyleneimine (PEI) along with unreacted TMC and residual acyl chloride groups of IP reaction. The influences of TEA concentration, PEI concentration and SIP time on the physicochemical properties and separation performances of the membrane were systematically analyzed. Our results show that the presence of the gradient structure optimized mass transfer pathways and reduced mass transfer resistance, resulting in significantly improved permeability of 13.0 ± 0.5 L·m−2·h−1·bar−1 (over 3 times compared with control-PEI), high MgCl2 rejection of 91.7 ± 0.6 %, and an excellent stability and anti-fouling performance. Our work provides valuable insights and guidance for developing high-performance positively charged NF membrane with gradient structures.

Constructing high-performance nanofiltration membranes using nanofiber supports: Effects of structural stability and polyvinyl alcohol interlayers

Nanofiber supports are promising candidates for constructing high-performance nanofiltration (NF) membranes due to their unique interconnected pores and high porosity. Conventional nanofiber supports usually possess excessive surface roughness and low structural stabilities, influencing the formation of polyamide separation layers. However, research on the effects of nanofiber support properties on the NF membrane performances is relatively few. In this study, we systematically evaluated the effects of structural properties of the nanofiber supports on the performances of the fabricated thin-film nanofiber composite (TFNC) NF membranes. The polyimide (PI) support with heat-pressing post-treatments owns a smooth surface and high structural stability due to the welding and compaction of the PI nanofibers. This promoted the formation of a polyamide separation layer with high smoothness and low fouling tendencies. Consequently, the mechanical strength and long-term operation stability of the TFNC-PI NF membrane were much stronger than that of the membranes with polyacrylonitrile (PAN) and PVDF nanofiber supports. Membrane performance analyses illustrated that the TFNC NF membranes possess notable higher water permeance while slight lower salt rejection than the commercial NF membrane. Compared to TFNC-PAN NF membranes (18∼20 L m-2h−1 bar−1), the water permeance of the structurally stable TFNC-PI NF membrane was relatively low (15–16 L m-2h−1 bar−1) due to its smaller surface filtration area and membrane pore size. Further attempts illustrated that constructing interlayers with 2 wt% polyvinyl alcohol (PVA) coating solutions resulted in the formation of clear and regular Turing structures in the polyamide surface layer. It notably enhanced the water permeance of the membrane by ∼30 % without compromising the salt rejection capability of the membrane. This study provides fundamental insights into the effects of nanofiber support properties on NF performance and feasible pathways to construct high-performance TFNC NF membranes.

Constructing hydrophobic microenvironment in the separation layer by a small molecule for improving reverse osmosis membrane performance

The development of reverse osmosis (RO) membranes with both high permeability and selectivity is a key goal for RO technology, the leading water treatment technology. This work presents a simple and cost-effective method for improving RO membrane performance obviously via introducing a hydrophobic small molecule, benzoic acid (BA), in the polyamide (PA) separation layer. The introduction of BA is achieved by adding it in the oil phase of the interfacial polymerization technology because of its oil solubility, which is pretty simple. The hydrophobicity and self-assembly characteristic of BA could establish hydrophobic microenvironments within the PA separation layer. The hydrophobicity of this microenvironment can reduce the number of hydrogen bonds between water molecules, accelerating the rapid transport of water and thereby enhancing membrane performance. Concurrently, BA as a small molecule could moderately increase the space between PA chains and thus enlarge the free volume of hydrophobic microenvironment region, which is also in favour of the improvement of membrane flux. The as-developed RO membrane possessed both high rejection rate (with a NaCl rejection rate of 98.9 %) and high flux (91.7 L m−2 h−1) under the brackish water testing condition, surpassing the performance of many previously reported membranes, and noticeably, a flux increase of 113 % was achieved by utilizing BA. Besides, the performance showed a good long-term stability. It is worth mentioning that small molecules are generally purchasable and cheap, which is beneficial to the large-scale production. This work showcased the great potential of hydrophobic small molecules in improving PA water treatment membrane performance, paving the way for diverse small molecules to develop PA water treatment membrane with both high permeability and selectivity.

Small and wide angle X-ray scattering to reveal nanoscale structure evolution of the polyamide RO membranes triggered by ethyl acetate

The intrinsic structure of the polyamide separation layer determines the transport of water and salt through a thin film composite membrane. In this work, we probed nanoscale structure variation of the polyamide layers generated by ethyl acetate through small/wide angle X-ray scattering. The gyration radius and physical radius of the polyamide colloidal particles were calculated from the small angle X-ray scattering (SAXS) curves, and the results are confirmed by AFM and SEM. The form factor derived from SAXS curves and horizontal distance between the centers of adjacent benzene rings calculated from the wide angle X-ray scattering (WAXS) curves showed quantitative relationships with membrane performance including water permeability coefficient and salt permeability coefficient. The results derived from SAXS and WAXS curves provided evidence for the formation mechanism of polyamide layers.

Nanofiltration membrane with asymmetric polyamide dual-layer through interfacial polymerization for enhanced separation performance

Thickness reduction of polyamide separation layer played an important role in permeability enhancement of thin film composite (TFC) nanofiltration membrane. However, the practical application of TFC nanofiltration membrane with ultrathin polyamide layer is still a great challenge. Herein, we proposed a strategy for preparing nanofiltration membrane with asymmetric polyamide dual-layer through interfacial polymerization. The top polyamide layer is thin, smooth and dense. The bottom polyamide layer is relatively thick, rough and porous. The thin and dense top polyamide layer enabled membrane with relatively high water permeability (18.5 ± 1 L m−2 h−1 bar−1) and Na2SO4 rejection (97.2 ± 0.8 %) performance. The thick and porous bottom polyamide layer guaranteed membrane stability. This study offers a promising strategy for polyamide composite membrane fabrication with enhanced separation performance.

Zwitterionic loose polyamide nanofiltration membrane with good antifouling properties for efficient removal of humic acid and dyes

Loose nanofiltration membrane exhibits significant advantages in removing organic pollutants due to its high selectivity at a lower operating pressure. Herein, a novel zwitterionic loose polyamide separation layer was designed and formed on the modified polyethersulfone substrate to obtain the zwitterionic loose nanofiltration (ZLNF) membrane with high water permeability and good antifouling properties. The BAPP-TMC polyamide layer was fabricated by the interfacial polymerization between 1,4-bis(3-aminopropyl)piperazine (BAPP) and trimesoyl chloride (TMC). Afterward, the BAPP-TMC polyamide layer was grafted with zwitterions by a facile surface modification. Owing to the fabrication of the relatively loose polyamide structure with zwitterions, the prepared ZLNF-0.2 membrane presented an ultra-high pure water permeability of 148.5 L m−2 h−1 bar−1 while maintaining relatively good removal capacities for organic pollutants. The removal rates for humic acid (HA, 100 ppm) and Congo Red (CR, 100 ppm) were 98.1 % and 97.6 %, respectively. Furthermore, the good hydrophilicity and strong surface hydration effect of zwitterions endowed the ZLNF membranes with excellent anti-organic fouling and antibacterial adhesion properties. The good antibacterial abilities of ZLNF membranes were also proved by the fluorescence staining results. Due to the effective separation performance and good antifouling properties, the fabricated ZLNF membranes present great application potential in removing natural organic matter and dyes.

Fabrication of polyamide/zwitterion functionalized mesoporous silica nanofiltration membrane with highly enhanced antifouling performance

BackgroundThe nanofiltration membrane plays an important role in wastewater treatment and desalination. Fabricating the hydrophilic separation layer is a good way to construct nanofiltration membranes with both high water flux and antifouling performance. MethodsThe chemical and physical structure of the membrane surface are optimized simultaneously to enhance its performance. Three kinds of zwitterions bearing different positively and negatively charged units are grated onto mesoporous silica (MS), then the functionalized MS nanoparticles are introduced into polyamide separation layer on porous polyethersulfone substrate to construct hybrid nanofiltration membranes. The influence of zwitterion functionalized MS on the structure and comprehensive performance of hybrid membranes is thoroughly investigated by experimental and computational means. Significant findingsThe existence of zwitterion functionalized MS overcomes the tradeoff effect between water flux and salt rejection. The hybrid membrane shows a high water flux of 334.0 L m − 2h − 1 MPa−1 and a superior flux recovery ratio of 94.1% against bovine serum albumin. The corrected zero-point binding energies of three kinds of zwitterions with water molecules are computed for the first time, the carboxybetaine methacrylate shows the highest energy of 36.99 kJ mol−1, explaining why the as-prepared hybrid membranes possess outstanding antifouling performance.

Polyelectrolyte interlayer assisted interfacial polymerization fabrication of a dual-charged composite nanofiltration membrane on ceramic substrate with high performance

Interfacial polymerization has been applied to prepare polyamide membrane for wastewater treatment. It faces challenges in simultaneously rejection of divalent anions/cations. Moreover, weak interfacial bonding appears when the polyamide layer is formed on the ceramic substrate. Herein, a dual-charged composite nanofiltration (NF) membrane (CM-(CS/SA)1-PA) on ceramic ultrafiltration substrate, comprising a positive charged polyelectrolyte interlayer and a negatively charged polyamide separation layer, was obtained by layer-by-layer (LBL) self-assembly of chitosan/sodium alginate and subsequently interfacial polymerization. The interlayer with positive charge and excess amino group, can not only react to form covalent bonds with the polyamide layer, but also promote the divalent anions/cations rejection. The prepared PA layer is thin and dense with a thickness of ∼20 nm. The MWCO of the composite membrane is 527 Da with a pure water flux of 60.41 L m−2 h−1 at 0.4 MPa. The prepared composite membrane maintains good rejection for divalent anions/cations (MgSO4 88.11%, Na2SO4 89.11%, MgCl2 81.65%, CaCl2 80.45%) and multiple charged dyes (mthylene blue 99.83%, congo red 98.72%, active blue 19 97.7%, methyl orange 93.92%, rhodamine B 93.1%). During the 40 h constant filtration, the 126 h sustained immersion in ethanol and the 24 h immersion in solutions with pH from 6 to 10, the composite membrane shows excellent stability. Moreover, the unique dual-charged structure and the hydrophilic/smooth membrane surface promote the anti-fouling property in multiple charged dyes separation. In conclusion, the construction of polyelectrolyte interlayer provides a new approach for the preparation of a dual-charged composite NF membrane on ceramic substrate with excellent interfacial stability and anti-fouling property. It has potential applications in wastewater treatment, such as desalination and dyes removal.

Electro-nanofiltration membranes with high Li+/Mg2+ selectivity prepared via sequential interfacial polymerization

With the rapid development of batteries and other industries, the demand for lithium has exploded. As about 70 wt% of the lithium resources exist in brines, the extraction of lithium by selective electrodialysis (S-ED) has attracted extensive attention. However, the similar hydrated ionic radius of Li+ and Mg2+ caused many challenges. Herein, a simple sequential interfacial polymerization (SIP) method was developed to adjust the surface charge and pore size of the electro-nanofiltration membranes (ENFMs), and the ENFMs were endowed with excellent selectivity due to the electrostatic repulsion and pore-size sieving effect. Polyethyleneimine (PEI) with different molecular weights was used for the SIP process based on the nascent polyamide separation layer prepared by interfacial polymerization. With the increase of the PEI molecular weight, the zeta potential of the membrane surface changed from negative to positive, while the pore size of the membranes also decreased. The optimal ENFMs achieved outstanding selectivity for Li+/Mg2+ (16.55) and high Li+ flux (3.08 × 10−8 mol·cm−2·s−1) at a current density of 10 mA·cm−2. Moreover, the optimal ENFMs exhibited high limiting current density (50.6 mA·cm−2) and low membrane electrical resistance (2.42 Ω·cm2), which has a great potential to be used for practical applications.

N-methylglucamine modified poly (vinyl chloride) support assists the construction of uniform dually charged nanofiltration membrane via interfacial polymerization

Support membrane plays a crucial role in interfacial polymerization (IP) reaction for nanofiltration membranes (NFMs) preparation and dramatically affects the properties of NFMs. This paper reported NFMs with improved uniformity and dually charged separation layer, formed on properly modified poly(vinyl chloride) (PVC) substrate by N-methylglucamine (NMG) molecules. The effect of NMG grafting on the structure and performance of PVC UF membranes and NFMs was studied. The results showed that the appropriate grafting of NMG molecules on the surface of PVC UF membrane significantly improved the hydrophilicity of PVC UF membrane without affecting the pore size. It assisted the convenient formation of uniform and defect-free dually charged polyamide separation layer by regulating the adsorption, storage and dispersion of diamine monomers. Simultaneous high rejection for both divalent anions and cations was obtained. This work provides a convenient and effective strategy to construct uniform dually charged NFMs.

Electrosprayed thin film nanocomposite polyamide nanofiltration with homogeneous distribution of nanoparticles for enhanced separation performance

The water transport channel brought about by nanoparticles enables to improve the permeability of thin film nanocomposite (TFN) membranes. For the TFN nanofiltration membranes fabricated by conventional interfacial polymerization, most nanoparticles are concentrated and agglomerate in the bottom of the separation layer. Therefore, the efficient utilization of nanoparticles for homogeneous water channel formation remains a challenge. In this study, we propose to prepare TFN polyamide nanofiltration membrane by nanoparticles involved electrosprayed interfacial polymerization. TiO2 nanoparticles with particle size <10 nm were dispersed in the aqueous phase for electrosprayed polymerization, and the nanoparticles were evenly distributed inside the polyamide separation layer, which regulated the uniform distribution of newly formed interfacial water transport channels around the nanoparticles in the separation layer. With adopting a low concentration of 0.015 wt% TiO2, the water flux of TFN membrane increased by >60 %, achieving a water flux of 101.5 L m−2 h−1 under 5 bar. Meanwhile, rejection for Na2SO4 was still kept as high as 91.1 %. This study provides a simple and controllable strategy for homogeneous distribution of water transport channels inside polyamide separation layer.

Interlayered thin-film nanocomposite membrane with synergetic effect of COFs interlayer and GQDs incorporation for organic solvent nanofiltration

The separation performance of current organic solvent nanofiltration (OSN) membranes are still not satisfied. In this work, an ultra-thin TpHz covalent organic frameworks (COFs) interlayer was in situ constructed in advance on polyimide substrate, then an ultra-thin polyamide separation layer incorporated with amino-functionalized graphene quantum dots (af-GQDs) was constructed via interfacial polymerization (IP). A high performance interlayered thin-film nanocomposite (iTFN) OSN membrane with integrally covalent bonding structure, ultra-thin and ultra-smooth surface was obtained after post crosslinking and activation. The effect of ultra-low concentrations for the polyamide reaction monomers, the COFs monomer Tp and the doped af-GQDs nanoparticle reveals that they could help to form a very thin (7–8 nm in thickness) and defect-free skin layer with very smooth surface (about 2.7 nm in roughness). The iTFN OSN membrane shows excellent solvent permeance and solute selectivity, with an ethanol permeance reaching 97.6 L m−2 h−1 MPa−1 and an RDB rejection higher than 99%. After a 180 h long-term filtration with 100 mg L−1 RB/DMF solution, it remained an RB rejection of higher than 99%, demonstrating its superior resistance to strong polar solvent. Our work paves a unique way for the fabrication of novel OSN membrane with both in situ constructed nanomaterials interlayer and nanomaterials incorporated in skin layer.

Imidazole assisted molecular-scale regulation of polyamide for preparing reverse osmosis membrane with enhanced water permeance

The development of reverse osmosis (RO) technology is still hindered by many obstacles, especially the low water permeance of RO membrane. Herein, an interfacial polymerization regulated by N-acyl imidazole chemistry was proposed to fabricate high permeance RO membranes. The improvement of membrane performance was achieved by simply adding 2-Methylimidazole (Hmim) into aqueous phase. More angstrom-scale free volume elements, which acted as water molecule transport channels, were formed in the polyamide separation layer due to the reaction and hydrolysis of Hmim. As a result, a substantial increase in water permeance was achieved while maintaining high NaCl rejection. In addition, more carboxylic acid groups were formed on the membrane surface due to the hydrolysis of N-acyl imidazole, thereby imparting higher hydrophilicity and anti-fouling properties to the membrane. With the addition of 1.5 w/w% Hmim, the water permeance of the prepared RO membrane was improved to 4.4 L m−2 h−1 bar−1, nearly 4.5 times compared to the control group, while maintaining a high NaCl rejection of above 98 %, thus successfully overcoming the permeability-selectivity trade-off limit. This work paves a new idea for the development of additives in the preparation of RO membranes.

Fabrication of organic solvent nanofiltration membrane using commercial PVDF substrate via interfacial polymerization on top of metal-organic frameworks interlayer

Interfacial polymerization (IP) is a rapid and convenient approach to fabricating thin-film composite (TFC) membranes with high permeance and satisfactory rejection for organic solvent nanofiltration (OSN). However, it is normally difficult to directly conduct the IP approach on the commercial membrane substrate (particularly, in the microfiltration range) because the monomers on the substrate are unevenly distributed during the IP reaction. In the current work, Cu-TCPP nanosheets were in-situ grown on commercial PVDF microfiltration substrate (pore size of 0.45 μm) by using preinstalled Cu2O nanoparticles as a copper ion source. The Cu-TCPP nanosheets covered the large pores and penetrated the surface layer of the PVDF substrate. The obtained PVDF/i-Cu-TCPP was then used as a substrate to conduct the IP and to fabricate the PVDF/i-Cu-TCPP/PA membrane. It was found that i-Cu-TCPP which serves as an interlayer could store the aqueous solution of the amine and was critical to the formation of a thin and defect-free polyamide separation layer. In comparison, loading of the Cu-TCPP on the PVDF substrate by filtration led to the formation of PVDF/e-Cu-TCPP/PA, which however exhibited inferior separation performance as compared to the PVDF/i-Cu-TCPP/PA membrane. More importantly, the PVDF/i-Cu-TCPP/PA membrane exhibited higher tensile strength (6.0 vs. 2.8 MPa) compared to the PVDF/e-Cu-TCPP/PA membrane, due to the enhanced interaction (adhesion) between the PVDF substrate and the i-Cu-TCPP interlayer. Under optimal synthetic conditions, the permeance of the PVDF/i-Cu-TCPP/PA membrane was almost 4 times that of the PVDF/PA membrane (1.93 vs. 0.49 L m-2 h-1 bar-1) without compromising rejection (>96%). The optimal TFC membrane also showed excellent OSN performance towards various organic solvents and good long-term stability to VB12/ethanol mixtures (50 h). Furthermore, the concentration test of erythromycin in methanol solution indicated that the PVDF/i-Cu-TCPP/PA membrane has great potential in the pharmaceutical industry for drug concentration and organic solvent recovery.

High Flux and Antifouling Nanofiltration Membrane Modified by Ag@UiO-66-NH2 and Its Application for Biphenol A Removal

Owing to the specific porous structure which could provide additional passage channel for some molecules, metal organic frameworks are attractive candidates for enhancing permeability and selectivity of membranes in pervaporation, reverse osmosis, and gas separation. In this experiment, Ag@UiO-66-NH2 was introduced into polyamide separation layer by interfacial polymerization of triethylenetetramine and 1,3,5-benzenetricarboxylic acid chloride for nanofiltration. The results indicated that Ag@UiO-66-NH2 nanoparticles did endow the membranes with rapid diffusion pathways for water molecules. When the content of Ag@UiO-66-NH2 was 0.03 g, the prepared membrane (NF-Ag-3) showed high flux about 47.3 L·m-2·h-1 at 0.6 MPa, which is about 2-fold higher than that of polyamide membrane without Ag@UiO-66-NH2, while the MgSO4 rejection rate remained about 87.4%. The membrane also showed excellent antifouling properties, and the water flux recovery ratio was 95.6% after filtration BSA solution. When it was applied for 50 mg/L bisphenol A removal, the rejection rate reached 94.6%, and the flux is about 49.1 L·m-2·h-1. Moreover, Ag particles on UiO-66-NH2 rendered the membrane with good inhibition for Escherichia coli. The antibacterial rate of the membranes is above 95% when the loading of Ag@UiO-66-NH2 is more than 0.03 g.

Polyamide membranes enabled by covalent organic framework nanofibers for efficient reverse osmosis

AbstractPolyamide reverse osmosis membranes are indispensable for the worldwide clean water supply. Incorporation of nanomaterials into polyamide separation layer provides an evident methodology to enhance the water flux. However, the desalination performance would be compromised by the inadequate compatibility between the nanomaterial and the polyamide matrix. In this work, we propose a straightforward strategy to use covalent organic frameworks (COF) nanofibers as the efficient nanofiller for the fabrication of high flux polyamide membranes. The COF nanofibers featuring crystalline structures, large surface area and well‐defined micropores are synthesized by a surfactant‐mediated solvothermal method. It is found that the COF nanofibers are able to mediate the interfacial polymerization process via the controlled release of amine molecules. This creates the reduced thickness of the polyamide separation layer. Moreover, the rigid and permanent nanopores of COF could serve as additional channels for water permeation. Benefitting from the low thickness and the additional channels, thus fabricated membranes exhibit a significantly enhanced water flux, while maintaining a slightly improved NaCl rejection. Our work is expected to provide a new insight for the design and fabrication of advanced polyamide reverse osmosis membranes.

Hollow fiber nanofiltration membranes for surface water treatment: Performance evaluation at the pilot scale

Four types of hollow fiber nanofiltration membranes were fabricated and tested during pilot scale treatment of surface water taken from Istanbul Ömerli Lake Reservoir. Membrane types included: polymer thin film composite, reinforced thin film composite, TiO2-polymer thin film nanocomposite, and Aquaporin-polymer reinforced thin film composite. Interfacial polymerization procedure was used to form the polyamide separation layer for all membrane types. The amount of permeate water collected from membranes was controlled by membrane fouling. In terms of productivity, the membranes were ranked as follows: reinforced Aquaporin membrane (6.30 L/m2/h) > TiO2 incorporated membrane (5.69 L/m2/h) > reinforced thin film composite membrane (4.76 L/m2/h) > polymeric thin film composite membrane (3.24 L/m2/h). TiO2-incorporated membranes had the highest initial flux while reinforced thin film composite membranes had the highest removals for each water constituents tested (MgCl2, tannic acid, synthetic lake water, pre-filtered lake water). The study points to the practicability and promise of hollow fiber nanofiltration membranes for surface water treatment applications.

Emerging sandwich-like reverse osmosis membrane with interfacial assembled covalent organic frameworks interlayer for highly-efficient desalination

Covalent organic frameworks (COFs) have emerged as promising candidate for the construction of advanced separation membranes due to their well-defined pore structure and highly ordered porous channels. Nevertheless, the implementation of high-efficient COFs-incorporated membranes still suffers from harsh synthetic conditions and possible agglomeration of COFs. Herein, a kind of sandwich-like thin film composite reverse osmosis (RO) membranes was fabricated to overcome such challenges via interfacial assembly of a COFs interlayer on polysulfone substrate, then followed by the formation of a polyamide skin layer via traditional interfacial polymerization (IP) process. The COFs interlayer not only tailored the pore structure and interfacial properties of the substrate, but also manipulated the uptake and release of the amine monomer during the IP process, which would contribute to forming a more ordered polyamide separation layer. The resultant COFs-interlayered RO membrane exhibited a nearly 33.8% higher water permeance (16.78 L m−2 h−1 MPa−1) along with a slightly increased NaCl rejection (99.2%) compared with the pristine RO membrane. More importantly, the emerging RO membrane presented a superior chemical (critic acid and NaOH) resistance and desirable long-term filtration stability, which paved a new pathway to construct high-performance RO membranes for desalination.

Intrinsic Nanoscale Structure of Thin Film Composite Polyamide Membranes: Connectivity, Defects, and Structure-Property Correlation.

Transport of water, solutes, and contaminants through a thin film composite (TFC) membrane is governed by the intrinsic structure of its polyamide separation layer. In this work, we systematically characterized the nanoscale polyamide structure of four commercial TFC membranes to reveal the underlying structure-property relationship. For all the membranes, their polyamide layers have an intrinsic thickness in the range of 10-20 nm, which is an order of magnitude smaller than the more frequently reported apparent thickness of the roughness protuberances due to the ubiquitous presence of nanovoids within the rejection layers. Tracer filtration tests confirmed that these nanovoids are well connected to the pores in the substrates via the honeycomb-like opening of the backside of the polyamide layers such that the actual separation takes place at the frontside of the polyamide layer. Compared to SW30HR and BW30, loose membranes XLE and NF90 have thinner intrinsic thickness and greater effective filtration area (e.g., by the creation of secondary roughness features) for their polyamide layers, which correlates well to their significantly higher water permeability and lower salt rejection. With the aid of scanning electron microscopy, transmission electron microscopy, and tracer tests, the current study reveals the presence of nanosized defects in a polyamide film, which is possibly promoted by excessive interfacial degassing. The presence of such defects not only impairs the salt rejection but also has major implications for the removal of pathogens and micropollutants.