Thin Film Composite Nanofiltration Membranes Research Articles

The effect of PEG-content and molecular weight of PEG-PPG-PEG block copolymers on the structure and performance of polyphenylsulfone ultra- and nanofiltration membranes

Thin-film composite (TFC) membranes obtained by forming a selective polyamide (PA) layer on a surface of a porous membrane-substrate via interfacial polymerization (IP) technique are the most effective membranes for nanofiltration (NF). The idea of this study is that addition of polyethylene glycol-polypropylene glycol-polyethylene glycol (PEG-PPG-PEG) block copolymers to the polyphenylsulfone (PPSU) casting solution tunes the pore structure (pore size and porosity), water contact angle and topology of the selective layer of ultrafiltration (UF) membranes. This influences the formation of PA layer via IP since membrane-substrate significantly effects the first stage of IP reaction. For the first time the effect of PEG-PPG-PEG copolymer molecular weight, content of PEG blocks and copolymer concentration in the PPSU casting solution on the structure, hydrophilicity and performance of ultrafiltration and TFC NF membranes was revealed. It was found that increase in PEG block content and PEG-PPG-PEG molecular weight led to the increase in pore size, porosity and hydrophilicity of selective layer of ultrafiltration membranes which results in the formation of thinner and more uniform PA layer with higher cross-linking degree of NF membranes via IP. It was revealed that NF membrane flux increased with the rise in the content of PEG units from 10 to 80 wt% and increase in molecular weight of PEG-PPG-PEG block copolymer. Modification of PPSU membrane-substrate yielded the increase in selectivity of the corresponding TFC NF membranes due to the formation of more uniform and denser defect-free PA layer attributed to the rise in hydrophilicity of membrane-substrate. It was found that membrane-substrate modification by PEG-PPG-PEG results in the enhancement of antifouling performance toward bovine serum albumin (flux recovery ratio is 99–100 %) and long-term stability during 48 h operation compared to the reference membrane. Modification of PPSU membrane-substrate by F38 PEG-PPG-PEG block copolymer (Mn = 5000 g mol−1, PEG block content of 80 wt%) was found to yield the TFC NF membranes with the best combination of permeation, separation and antifouling performance and long-term stability.

Highly permeable polyamide nanofiltration membranes fabricated via the construction of anionic covalent organic frameworks /polydopamine composite interlayer

Thin-film composite nanofiltration (TFC NF) membranes can separate neutral/charged solutes and play an essential role in water purification. However, the permeability of traditional NF membranes is severely limited by excessive mass transfer resistance due to the thick polyamide (PA) layer. Introducing porous nanomaterials as an interlayer into PA NF membranes has been recognized as an effective approach to improve permeability. Herein, a novel composite interlayer consisting of anionic covalent organic frameworks with abundant sulfonic acid groups (COF-SO3H) and polydopamine was designed to regulate interfacial polymerization to construct the highly permeable TFC NF membranes. The surface negative charge and hydrophilicity of the TFC NF membranes can be controlled by the abundant polar groups in the composite interlayer. Moreover, mass transfer resistance can be reduced by introducing COF-SO3H to create additional water transport channels. The results showed that the optimal membrane manifested attractive permeability (44.2 L⋅m−2⋅h−1⋅bar−1) and a high Na2SO4 rejection (98.7 %), almost 3-fold higher than that of the original TFC membrane. Furthermore, the optimal membrane exhibited favorable anti-fouling performance and stability. This work provides insight into strategies for fabricating highly permeable TFC NF membranes through the introduction of an interlayer.

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.

A sub-10 nm polyamide nanofiltration membrane from polyvinylpyrrolidone-mediated interfacial polymerization

In conventional interfacial polymerization (IP) for thin-film composite nanofiltration membranes, the separation layer is typically thick (∼100 nm), which limits its water permeance. In this study, we successfully synthesized an ultrathin polyamide layer (∼8.6 nm) from polyvinylpyrrolidone (PVP)-mediated IP. With the surfactant concentration over its critical micelle concentration, PVP not only formed a monolayer at the water-heptane interface, but also stayed in the aqueous solution. The CO groups of PVP formed the N–H⋯O hydrogen bond interactions with N–H bonds of piperazine (PIP) monomers in the aqueous phase to slow down the diffusion of PIP monomers. Besides, PVP with amphiphilic groups minimized the interfacial tension at the water-heptane interface to regulate IP processes. By adjusting the concentration of PVP, we obtained a sub-10 nm polyamide nanofiltration membrane. Moreover, hydrophilic PVP macromolecules improved the membrane surface hydrophilicity. Due to the ultrathin thickness and improved hydrophilicity, the optimized polyamide membrane demonstrated a threefold increase in permeance compared to that of the pristine polyamide membrane without PVP addition, and maintained 98.4% rejection for 1000 ppm Na2SO4 solution. This study provides a new insight into fabricating sub-10 nm polyamide membranes for nanofiltration processes.

Application of a functionalized thin-film composite nanofiltration membrane in water desalination

To prepare a thin-film nanocomposite (TFN) nanofiltration (NF) membrane, characterized with high desalination rate and antifouling performance, carboxylated multi-walled magnetic 1,2,3,4-butanetetracarboxylic acid carbon nanotubes (hereafter, the C-MWCNT-MBTCA) with a choice of hydrophilic groups were initially synthesized in this study, and then incorporated into the polyamide (PA) layer via interfacial polymerization (IP). After that, the surface morphology and hydrophilicity of the fabricated NF membrane was examined by the field emission scanning electron microscopy (FESEM) and water contact angle measurements, respectively. To assess the membrane application, the solutions (2000 mg/L) of sodium sulfate (Na2SO4), magnesium sulfate (MgSO4), calcium chloride (CaCl2), and sodium chloride (NaCl) were consistently tested. On account of the ample carboxylic acid (R-COOH) groups in the modified TFN membrane, the study outcomes revealed that the given membrane could enhance the flux along with the desalination rate upon the addition of the C-MWCNT-MBTCA (0.04 wt%). Likewise, the pure water flux significantly grew from 58.8 to 110.5 Lm-2h−1 after the utilization of the C-MWCNT-MBTCA (0.04 wt%), and the Na2SO4 removal rate promoted from 87 to 98 %. Ultimately, it was established that the C-MWCNT-MBTCA could be used as an economical modifying agent to boost the performance of NF membranes for water desalination purposes.

Scalable nanofiltration membranes with sharpened pore distribution and enhanced negativity for mono/divalent anion separation

The petrochemical industry is an important producer of high salinity wastewater, necessitating the efficient separation of salts in zero discharge processes. This study reports the development of an industrially scalable thin film composite nanofiltration membranes that facilitate the effective separation of NaCl/Na2SO4. Employing a one-step functionalization process, the membrane pore distribution is sharpened, and the surface negativity is intensified. Branched polyethyleneimine with abundant reactive amine groups is employed as an aqueous co-monomer, leading to the generation of high density grafting active sites. Nucleophilic substitution reaction is employed to functionalize the membrane surface with negatively charged sulfonic acid groups. Simultaneously, large voids within the free volume are filled and the pore size distribution is effectively narrowed. The resultant membranes demonstrate an impressive rejection of 99.5 % for Na2SO4, a selectivity of 163.4 for NaCl/Na2SO4 and a selectivity of 488 for Cl−/SO42−, with a water flux of 45 LMH under 0.5 MPa. The corresponding 8″ × 40″ sized spiral wound membrane modules exhibit a NaCl/Na2SO4 selectivity of 100.4 and a Cl−/SO42− selectivity of 258.3, demonstrating high practical applicability. A pilot trial is planned to integrate these novel membrane products into zero discharge processes within the internal chemical enterprises of Sinopec.

A gelatin-zirconium phosphate nanoparticles composite interlayer for enhancing compaction resistance and antifouling performance of TFC NF membrane

From a practical point of view, the thin-film composite (TFC) nanofiltration (NF) membrane should not only have superior separation performance but also excellent compaction resistance and antifouling properties. Generally, polyamide (PA) separation layers of the TFC NF membrane prepared on the support with hydrophilic interlayer are crumpled, which may be detrimental to the compaction resistance and antifouling properties of TFC NF membranes. A flexible polymer-rigid nanoparticles composite interlayer was constructed on the pristine polysulfone (PSF) support in this work to design a TFC NF membrane with compaction resistance, antifouling, and excellent separation performance. Morphology and crosslinking degree of PA separation layers were investigated by scanning electron microscopy (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the composite interlayer can not only regulate the distribution and diffusion of piperazine (PIP) but also act as a template to induce the generation and growth of PA during the interfacial polymerization (IP) process, leading to the formation of the PA separation layer with a small nodular structure. The compaction resistance, antifouling properties, and separation performance of the TFC membrane were also investigated by nanoindentation, fouling experiment, and filtration test, respectively. The TFC NF membrane with the composite interlayer showed greatly improved mechanical properties with its hardness and elastic modulus approximately 1.5 and 1.6 times that of the reference, respectively. Furthermore, the flux recovery of the TFC NF membrane with the composite interlayer (93.8 %) was higher than that of the control (87.2 %), indicating that the TFC NF membrane with the composite interlayer had better antifouling properties. In addition, the obtained TFC NF membrane with the composite interlayer showed a water flux approximately twice as high as that of the reference with satisfactory rejection. This advancement will contribute to the development of more efficient and stable TFC NF membranes for industrial wastewater treatment.

Upcycled PVC support layer from waste PVC pipe for thin film composite nanofiltration membranes

Thin-film-composite (TFC) nanofiltration membranes represent the pinnacle of membrane technology in water treatment and desalination. These membranes typically consist of a polyamide (PA) selective layer and a membrane support layer, often constructed from commercially available materials like polyethersulfone (PES). However, there exists an alternative approach that involves the use of different polymers, preferably upcycled waste polymers, as a viable support layer for TFC membranes. Successfully implementing the upcycling of waste plastics into high-value support membranes can make a significant contribution to addressing the issue of plastic waste pollution. One of the primary challenges associated with utilizing upcycled polymers as support layers is the potential impact on the polyamide selective layer of TFC membranes, subsequently affecting their performance in terms of permeability, rejection, and antifouling properties. In this study, we demonstrate the successful fabrication of TFC membranes with a support layer crafted from upcycled waste PVC pipe. We conducted a comprehensive investigation into the effects of upcycled PVC on the structural and physicochemical properties of the polyamide layer. The ultrafiltration (UF) support membranes were fabricated from waste PVC pipe via the nonsolvent induced phase separation (NIPS) method. Subsequently, a polyamide layer was synthesized atop the Upcycled PVC membrane using interfacial polymerization (IP). The physicochemical properties and performance of the TFC membranes with upcycled PVC support layer were compared with membranes with research-grade (RG) PVC and commercial PES support layers. The results unveiled that the TFC membrane with upcycled PVC support layer exhibited higher water permeability (18.2 LMH/bar), in contrast to RG TFC (15.5 LMH/bar) and PES TFC membranes (13.7 LMH/bar). Furthermore, the salt rejection capabilities of the upcycled PVC TFC membrane were competitive and well within an acceptable range.

Investigation on the enhancement of solvent‐resistant nanofiltration membrane performance utilizing PDA‐UiO‐66@CNT as an interlayer

AbstractWith the growing complexity of separation systems, the application of thin film composite nanofiltration (TFN) membranes in organic solvent separation faces numerous challenges. To augment its solvent stability, an in‐situ constructed dopamine hydrogel doped with UiO‐66@CNT was developed as an intermediate layer on a polyetherimide (PEI) ultrafiltration membrane. Subsequent interfacial polymerization on this interlayer led to the formation of a solvent‐resistant nanofiltration membrane with a vast covalent bond structure, large specific surface area, and enhanced hydrophilicity. Our findings revealed that when the CNT loading in the UiO‐66@CNT composite nanoparticles was 2 wt%, the TFN‐U2C2 membrane exhibited a maximum pure water flux of 126.32 L/(m2·h) and a methanol flux of 45.45 L/(m2·h). The rejection rates for Congo red aqueous and methanol solutions were 96.88% and 92.14%, respectively. The membrane also demonstrated commendable anti‐fouling properties. Remarkably, even after 48 h of immersion in various organic solvents, the membrane retained its morphology and separation efficiency. Compared to the TFN‐U2 membrane without CNT addition, the enhancement in separation performance was considerably significant. Hence, this membrane has significant potential for application in treatment of wastewater containing organic solvents and is promising in related fields.

A novel wholly-Tröger's Base-based polyamide nanofiltration membrane constructed for the separation application in strong acidic condition via acid-induced segregation

In this study, a new kind of wholly-Tröger's base-based polyamide (wTB-PA) thin film composite (TFC) nanofiltration (NF) membrane was successfully fabricated for the separation application in strong acidic condition via the acid-induced segregation (AIS) effect. Firstly, the self-synthesized Tröger's base diamine monomer — 2,8-diamino-4,10-dimethyl-6H,12H-5,11-methanodibenzo [1,5]-diazocine (TBDA) was added in the polyethersulfone (PES) casting solution to prepare the TBDA-based PES substrate by synchronously coupling the non-solvent-induced phase transformation (NIPs) and AIS, and then the TBDA molecules on the PES substrate surface further reacted with trimesoyl chloride (TMC) via interfacial polymerization (IP) to obtain the wTB-PA TFC NF membranes. Meanwhile, the acid-induced segregation effect of on the microstructure and separation performance of the wTB-PA TFC NF membrane were systematically investigated. As compared with the conventional poly(piperazine amide) NF membrane (MPIP-TMC), the wTB-PA NF membranes exhibited thinner, smoother but more hydrophobic surface as well as lower isoelectric point, and especially showed much higher tolerance to 20 wt% H2SO4 after soaking 63 days and favorable dye rejection (～99.9 %). Thereinto, the acid segregation agent of citric acid monohydrate (CA) endowed the wTB-PA TFC NF membrane with relatively optimal acid resistance and SO42−/Cl− separation selectivity (about 11.8) than glacial acetic acid (AC), nitric acid (HNO3) and hydrochloric acid (HCl). Moreover, this wTB-PA NF membrane based on CA-induced segregation had higher recovery of the positively charged dye of methyl orange (MO) (＞98.0 %) in acidic condition. Therefore, the wTB-PA TFC NF membranes have great potential for the separation application in the strong acidic condition in the future.

A novel in-situ hydrolysis approach to prepare ultra-selective and ultrasmooth nanofiltration membrane for efficient and sustainable ion separation

Nanofiltration (NF) membranes are increasingly recognized for their proficiency in monovalent/divalent ion separation, making them highly suitable for application such as water softening and resource recovery. This study introduces an innovative in-situ hydrolysis method to enhance the ion selectivity of thin-film composite nanofiltration (TFC-PA NF) membranes. In this method, the reactive monomer in the oil phase was pre-hydrolyzed by contacting with water, producing a partially hydrolyzed monomer that remained soluble in the oil phase. This in-situ hydrolysis process introduced more carboxylic groups to the membrane. That could lead to a proper enlargement of the pore size, subsequently facilitating the separation of Cl−/SO42−. Additionally, the hydrolysis altered the polarity of the acyl chloride molecule, prompting its accumulation at the oil/water interface and accelerating the polymerization reaction rate. This resulted in a thin and exceptionally smooth PA layer. The resulting membrane exhibited a remarkable Cl−/SO42−separation factor of ∼316 under moderate feed concentration and ∼478 under high feed concentration, while maintaining decent water permeance (14.4 L m−2 h−1 bar−1). Moreover, its maintained surface carboxylic group density, combined with ultra-low surface roughness, offered drastically enhanced fouling resistance. Our study illuminates the pathway for a straightforward and scalable method to fabricate highly selective membranes with superior fouling resistance.

Acetone extraction induced piperazine diffusion reaction for regulating thin film composite nanofiltration membrane

The regulation of monomer diffusion in interfacial polymerization played a vital role in manipulating structure and enhancing separation performance of nanofiltration (NF) membrane. The co-solvent strategy has been recognized to be valid and applicable for improving membrane performance. However, the underlying mechanism for its effect on the diffusion progress of amine monomers was still unclear and contradictory. In this study, we found that the added co-solvent (acetone) induced piperazine diffusion flood through interfacial co-solvent extraction effect due to the interfacial enrichment of acetone and innovatively proposed a concept of “acetone extraction effect”. The mechanism and molecular dynamics simulation (MD) were used to systematically explain the effect of diffusion on the morphology and properties of the membranes. The prepared NF membranes exhibited improved separation performance for Calcium (CaCl2, 71.5 ± 0.6 %) and Magnesium (MgCl2, 83.1 ± 0.8 %) ions, which were better than many of the recorded membranes. This study provided a new idea for the co-solvent effect in interfacial polymerization and promising guidance for the preparation of superior NF membranes.

Effects of acid acceptor and pH on the structure and filtration performance of nanofiltration membrane

The thin film composite nanofiltration (TFC-NF) membranes with various properties were prepared by changing type and concentration of acid acceptors in the piperazine (PIP) solution. The influences of acid acceptors on interfacial polymerization (IP) process depend on the pH of PIP solution and the properties of acid acceptors. Na2CO3 and TEA provide positive effects at a pH range of 11.2–12.0. Especially for Na2CO3, water permeability and ion selectivity of the membrane simultaneously enhanced. The pH values reached above 12.8 with NaOH, and negative effects on permeance and selectivity were detected. The pH values ranged 10–11 after adding Na3PO4 and NaHCO3. The rejections of both Cl− and SO42− were high, especially for Cl− rejection increasing to above 73%. The pH values got to around 7 with NaH2PO4. The IP reaction was retarded and defects were detected on the membrane with quite low SO42− rejection. Protonation of PIP happens at low pH, while hydrolysis of acyl chloride of trimesoyl chloride happens at high pH. These changes will slow down the deprotonated PIP diffusion and the IP reaction. It is a feasible way to fabricate tailored NF membranes by changing acid acceptors and adjusting pH value of the PIP solution.

Hydrophilic carbon-carbon covalent linkage network structure for strong acid/alkali resistant and antifouling nanofiltration membrane

The nanofiltration (NF) membrane plays a crucial role in wastewater treatment by separating ions and molecules. However, traditional polyamide (PA) thin-film composite (TFC) NF membranes have limitations in durability under acidic or alkaline conditions and are prone to fouling, which limits their applications and increases costs. In this study, we have successfully developed a new nanofiltration membrane with an active layer composed of hydrophilic carbon-carbon covalent bond linkage network through UV irradiation-initiated polymerization of a kind of sulfobetaine - [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (DMAPS) - or acrylic acid (AA) on the surface of poly (ether sulfone) (PES) substrate. The carbon-carbon covalent bond (C–C) chemistry used in the preparation of the NF membrane results in exceptional durability towards acid and base, enabling it to maintain its ion separation efficiency (up to 94.2 % and 90.3 % of the initial Na2SO4 rejections) under strong acidic (20 % (w/v) H2SO4) and strong alkaline (2 M NaOH) conditions. Additionally, the water-attracting nature of polyDMAPS and PAA effectively prevents the fouling of protein and bacteria on the NF membrane surface. This breakthrough in manufacturing NF membranes significantly improves their environmental durability, making them suitable for use in highly acidic or alkaline environments (e.g., pH < 0 and pH > 14), including heavily contaminated ones.

Regenerable planting of multifunctional amine: Regeneration-enhanced antifouling/antiscaling properties and performance of thin film composite nanofiltration membrane

The problems of poly(piperazineamide)-based thin film composite (TFC) nanofiltration (NF) membrane are the fouling, scaling and poor separation performance of uncharged organics and heavy metal ions. Permanent surface modification lowers the fouling/scaling problems. However, these problems persist as the separation progresses. We address these issues by the on-demand regenerable planting of polyethyleneimine (PEI) into the polyamide (PA) layer of TFC NF membrane. The regenerable planting is based on the electrostatic interaction among the protonated amine groups of low molecular weight PEI and ionized carboxylic acid groups situated at the relatively inner sites (low dielectric constant) of the PA network. The incorporated PEI chains are stable at the pH range of 3–9. The PEI incorporated membrane exhibits significantly enhanced antifouling/antiscaling property, rejection of heavy metal ions (98–99%) and bisphenol A (BPA, 95%) without much negatively affecting the bivalent anion rejection (Na2SO4, 97.5%) and water permeance (11 L m−2 h−1 bar−1) as compared to the pristine and commercial membranes (55–82%, 50–76% and 98–99% rejections of heavy metal ions, BPA and Na2SO4). Advantageously, PEI chains are removable at pH ≤ 2 or pH ≥ 9.5 without affecting the main PA layer. The multiple removal and re-generation of the membrane have been achieved. The flux recovery of the membrane after antifouling/antiscaling tests significantly improved upon intentional removal and reincorporation of PEI. Inherent properties of the modified membrane channels couple with the on-demand regenerable ability show immense promises of the membrane in the separation applications.

Thin film composite polyamide nanofiltration membranes with interlayer constructed with core-shell structured polystyrene-polyacrylamide nanospheres for antibiotics separation

Nanofiltration (NF) is an efficient and economic separation technology which has great application potential in solving worldwide water shortage. Constructing interlayer is an effective approach to fabricate thin film composite (TFC) polyamide (PA) NF membranes with better performance. Herein, the polystyrene-polyacrylamide nanospheres with core-shell structure were synthesized using an emulsion polymerization method. The PA NF membranes were prepared via interfacial polymerization (IP) on the mixed cellulose esters microfiltration (MCE MF) membranes which modified with the core-shell structured nanospheres. The fabricated PA NF membranes showed good performance with a pure water permeability of 18.9 L·m−2·h−1·bar−1 and Na2SO4 rejection of 97.4 %. The NF membranes also showed good antibiotics separation performance with rejection to levofloxacin (LEV), penicillin G (PG), tetracycline (TC), and erythromycin (ERY) of 97.1 %, 86.9 %, 92.2 %, and 96.4 %, respectively and efficient separation to antibiotics and NaCl. This work provides a feasible strategy to construct an interlayer of PA NF membranes with the core-shell structured polystyrene-polyacrylamide nanospheres.

Preparation and performance of thin film composite solvent nanofiltration membrane based on fluorinated diamine monomers

The high perm-selectivity and long-term solvent resistance of organic solvent nanofiltration (OSN) membranes in various solvents pose significant challenges for their development. In this study, ultra-low content mixture of fluorinated diamine (5-trifluoromethyl-1,3-phenylenediamine or 4-trifluoromethyl-1,2-phenylenediamine) and m-phenylenediamine (MPD) as aqueous phase monomers and ultra-low content trimesoyl chloride (TMC) in n-hexane as organic phase monomer were used to perform interfacial polymerization (IP), then followed by crosslinking and solvent activation. By this way, two kinds of ultra-thin and ultra-smooth fluorinated polyamide OSN membranes were achieved. Under optimized conditions, these two kinds of OSN membranes, TFC-5F2,II and TFC-4F1,II, have surface roughness of 2.5 and 3.0 nm, respectively. They show ethanol permeance of 73.9 and 60.9 L m−2 h−1 MPa−1, and RDB rejection of 98.5% and 99.2%, respectively, using 100 mg L−1 rhodamine B / ethanol solution as feed. They show ethyl acetate permeance of 209.4 and 117.0 L m−2 h−1 MPa−1, respectively, and catalyst rejection of 98.2% and 98.8%, respectively, using 50 mg L−1 Jacobsen catalyst/ethyl acetate solution as feed. Meanwhile, they have pure n-hexane permeance of 32.5 and 36.8 L m−2 h−1 MPa−1, respectively. Moreover, they show excellent stability in highly polar solvents, thus have good application prospects in the separation and purification of various organic solvent systems.

Sustainable β- and γ-cyclodextrins for development of highly permeable thin film composite nanofiltration membranes

The use of macromolecules, such as cyclodextrins (CDs), given their porosity and compatibility with organic materials, are very attractive for the progress of polymeric membranes in nanofiltration (NF) of aqueous dye solutions. Two types of membranes were fabricated. Firstly, β- and γ-CDs were incorporated to polyamide (PA) thin film composite (TFC) membranes. These membranes, named CD/MPD-TMC, were prepared by conventional interfacial polymerization —i.e., an aqueous solution of m-phenylenediamine (MPD) followed by a solution of trimesoyl chloride (TMC) in hexane— by replacing a certain amount of the MPD monomer with varying proportions of CDs. Secondly, two-step double-IP (TD-IP) membranes were fabricated as follows: CDs were deposited atop the polymer support, followed by addition of TMC —forming a polyester (PE) layer—, and subsequently the membrane surface was modified with MPD molecules. In both CD/MPD-TMC and TD-IP membranes, the presence of CDs improved the membrane performance, the latter configuration leading to much higher water permeances. The TD-IP membrane using β-CD achieved the highest water permeance of 10 L m−2 h−1·bar−1 (LMH bar−1) and a rejection of acridine orange dye (265 Da) greater than 98 %. This represents a 6-fold increase in water permeance compared to bare PA (1.6 LMH bar−1). The enhanced water transport was mainly attributed to the more hydrophilic nature of the membranes and the presence of a PE layer with a porous structure and high pore connectivity. High dye rejection resulted from electrostatic and Donnan repulsions due to surface modification with MPD molecules.

Hydrophilic-hydrophobic bilayer interlayer for high performance thin-film composite reverse osmosis membranes

The strategy of modifying substrates with hydrophilic interlayers has been widely used to improve the performance of thin-film composite (TFC) nanofiltration (NF) membranes. However, the preparation of TFC reverse osmosis (RO) membranes with excellent performance on hydrophilic substrates through conventional interfacial polymerization (IP) method has proven to be very challenging. In this work, a hydrophilic polycarboxylic acid – hydrophobic ZIF-8 nanoparticles bilayer interlayer was developed to enhance the performance of TFC RO membranes. Based on this, the interlayer was designed to improve the performance of TFC RO membranes by creating a synergistic effect between the hydrophilic polycarboxylic acid and the hydrophobic ZIF-8 nanoparticles. The hydrophilic polycarboxylic acid interlayer was used to heal defects in the PSF substrate and provide growth sites for the in-situ growth of ZIF-8 nanoparticles. Hydrophobic ZIF-8 nanoparticles weakened the inhibition of the hydrophilic polycarboxylic acid interlayer on the diffusion of MPD, thus promoting the IP reaction and forming a dense PA separation layer (NaCl rejection 98.8%). The modulation of MPD erupted from the pores of the modified substrate by the bilayer interlayer results in the formation of an asymmetric PA layer, which has a dense PA in the upper region and loose PA encapsulating ZIF-8 nanoparticles in the lower region. ZIF-8 nanoparticles encapsulated by PA not only provided a low-resistance channel for water to pass through the separation layer but also enhanced the compaction resistance of the RO membrane, thus improving the permeance of the TFC RO membrane (4.2 L m−2 h−1·bar−1). In addition, thanks to the good compatibility between ZIF-8 nanoparticles and the PA matrix, as well as the smoother PA layer, TFC RO membranes with the bilayer interlayer presented excellent long-term stability and antifouling ability. This work presents a new idea to improve the performance of TFC RO membranes using interlayer strategies.

Ternary deep eutectic solvent enhanced the diffusion control of amine monomer for highly permeable and anti-fouling polyamide nanofiltration membranes

Thin-film composite nanofiltration (TFC NF) membranes have been widely used in water treatment, while the common active polyamide (PA) separation layer suffers the problems of excessive mass transfer resistance and fouling. In this work, a novel ternary deep eutectic solvent (DES) of citric acid/choline chloride/glycol as an additive in the aqueous phase was focused on regulating the formation of PA layer. The ternary DES can interact with piperazine (PIP) via hydrogen bonding and electrostatic interaction, synergistically enhancing the diffusion control of PIP with viscosity modulation during interfacial polymerization. Notably, a thinner and rougher separation layer with a cellular-like structure was obtained by optimizing the ratio of citric acid in ternary DES. Accordingly, the optimized TFC10%-3 membrane maintained an excellent Na2SO4 rejection (98.8 %) and a 104.7 % increase in pure water permeance (39.5 L·m−2·h−1·bar−1) compared to the original membrane, outperforming commercial membranes and the most NF membranes reported in the literature. Besides, the introduction of citric acid in PA layer increased the surface hydrophilicity and electronegativity of TFC10%-3 membrane, resulting in a higher flux recovery rate (FRR = 93.2 %) of TFC10%-3 membrane than that of TFC10%-0 membrane (FRR = 83.9 %). This study provides a feasible design strategy for the preparation of highly permeable and anti-fouling TFC NF membranes and enrichs the application of ternary DES in water treatment.