Thin-film Composite Reverse Osmosis Membranes Research Articles

Leveraging tannic acid-ferric iron (TA/Fe3+) complexation in polyamide selective layer for synthesizing robust thin film composite reverse osmosis (TFC-RO) hollow fiber membranes for saline water treatment

The high efficiency and adaptability of reverse osmosis (RO) are two main reasons this separation technology is accepted worldwide, depending on its various uses, such as water purification, desalination, and concentration. The potential of RO can be enhanced by utilizing the hollow fiber (HF) mode, benefiting from higher packing density, ease of cleaning, and cost competitiveness. However, its suitability for RO applications becomes questionable due to its weaker and fragile mechanical resilience. To solve the abovementioned issue and improve the mechanical strength and burst/crush pressure, we propose optimizing the selective layer of the TFC-RO membrane using tannic acid complexation with FeCl3. The polyethersulfone (PES) support layer was initially fabricated to host the polyamide (PA) layer. Subsequently, the introduction of tannic acid-ferric chloride complexation was implemented as a sandwich layer between the aqueous and organic phase reactions to host a coherent and selective PA layer to further enhance its performance by controlling the kinetics of interfacial polymerization (IP). The surface morphology, physicochemical properties, and underlying reactions were analyzed using scanning electron microscopy (SEM), atomic force microscope (AFM), Fourier infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and water contact angle measurements. The TFC-RO membrane M_TA0.2/Fe7.5 (7.5 mM FeCl3 & 0.2 wt% tannic acid) showed significant improvement in pure water permeance and salt rejection along with 7.23 MPa of burst pressure and 68.41 % strain, which proves superior elasticity and mechanical robustness because of PA TA/Fe3+ complexes formed during the IP. Moreover, the proposed facile tannic acid modification is effective and scalable. It significantly improves the mechanical efficacy of the HF TFC-RO membrane, solving a great problem for its commercial adaptability for RO applications and paving the way for smoother industrial translation for direct human factor applications.

Prompted by surface-engineered honeycomb Turing nanostructures: Reflections on polyamide structure, growth and performance

The robust correlation between the performance of thin-film composite (TFC) reverse osmosis membranes and the polyamide (PA) active layer determines the necessity of precise design of the PA layer, and this regulation primarily involves the surface morphology and internal characteristics of the PA layer. In this study, we fabricated honeycomb Turing structure or nanovoid-rich TFC series membranes using polar/nonpolar organic phase additives. Characterization and simulation results confirmed that the enhancement of membrane performance relied more on the abundant nanovoids within the PA layer rather than the ordered honeycomb lattice surface. Notably, the TFC-nDCE membrane with multilayer nanovoids exhibited an approximately 89 % improvement in water permeability while maintaining satisfactory salt rejection (98.29 %). We emphasized the significant role of nanovoids in the diffusion process of amine monomers and investigated the potential relationship between nanovoids and the degree of surface cross-linking. Building upon this, a new insight into the formation of nanovoids induced by PA anisotropic growth was presented through a careful analysis of the amine monomer diffusion rate and the morphological evolution of nanovoids. The structure-property relationship between internal structures, surface morphologies, and performances was systematically and thoroughly investigated, pointing out the direction for the rational design of TFC membranes.

More resilient polyester membranes for high-performance reverse osmosis desalination.

Thin-film composite reverse osmosis membranes have remained the gold standard technology for desalination and water purification for nearly half a century. Polyamide films offer excellent water permeability and salt rejection but also suffer from poor chlorine resistance, high fouling propensity, and low boron rejection. We addressed these issues by molecularly designing a polyester thin-film composite reverse osmosis membrane using co-solvent-assisted interfacial polymerization to react 3,5-dihydroxy-4-methylbenzoic acid with trimesoyl chloride. This polyester membrane exhibits substantial water permeability, high rejection for sodium chloride and boron, and complete resistance toward chlorine. The ultrasmooth, low-energy surface of the membrane also prevents fouling and mineral scaling compared with polyamide membranes. These membranes could increasingly challenge polyamide membranes by further optimizing water-salt selectivity, offering a path to considerably reducing pretreatment steps in desalination.

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

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

The role of sheet-like TiO2 in polyamide reverse osmosis membrane for enhanced removal of endocrine disrupting chemicals

Thin film composite (TFC) reverse osmosis (RO) membrane shows good promise for treating wastewater containing endocrine disrupting chemical (EDC) pollutants. The incorporation of functional materials with exceptional structural and physico-chemical properties offers opportunities for the membranes preparation with enhanced permselectivity and better antifouling properties. The present study aims to improve the EDC removal efficiency of TFC RO membrane using two-dimensional titania nanosheets (TNS). RO membrane was prepared by incorporating TNS in the dense layer of polyamide (PA) layer to form thin film nanocomposite (TFN) membrane. The TNS loading was varied and the influences on membrane morphology, surface hydrophilicity, surface charge, as well as water permeability and rejection of EDC were investigated. The results revealed that the inclusion of TNS in the membrane resulted in the increase of water permeability and EDC rejection. When treating the mixture of bisphenol A (BPA) and caffeine at 100 ppm feed concentration, the TFN membrane incorporated with 0.05% TNS achieved water permeability of 1.45 L/m2·h·bar, which was 38.6% higher than that of unmodified TFC membrane, while maintaining satisfactory rejection of >97%. The enhancement of water permeability for TFN membrane can be attributed to their hydrophilic surface and unique nanochannel structure created by the nanoscale interlayer spacing via staking of TiO2 nanosheets. Furthermore, the 0.05TFN membrane exhibited excellent fouling resistance towards BPA and caffeine pollutants with almost 100% flux recovery for three cycles of operations.

Advanced reverse osmosis membranes prepared by counter-diffusion of organic phase monomers induced by polypyrrole additives

Highly permeable thin film composite (TFC) reverse osmosis (RO) membranes with abundant free volume are desirable for water treatment. However, the strategy of controlling the microstructure of the separation layer with high free volume rate is still a challenge. In this work, a modified strategy for the preparation of high-performance TFC RO membrane with inverse diffusible polypyrrole nanoparticles (PPYN) as organic phase additive was developed. The combination of PPYN and trimesoyl chloride (TMC) would diffuse backwards from the organic phase into the aqueous phase during the reaction process to disrupt the normal interfacial polymerization (IP) and thus introduce more free volume in the RO membrane due to the nanobubbles from violent reactions. Moreover, the thickness of the modified membrane decreased and the hydrophilicity increased based on the modulation of IP by PPYN, which facilitated the water molecules to transport. The prepared RO-PPYN membranes showed nearly three times the flux compared with the pristine RO membrane without sacrificing separation selectivity. Furthermore, the anti-biofouling property of the modified membrane was also improved. This work successfully demonstrated the potential use of PPYN as an organic filler to enhance the performance of seawater desalination in RO membranes. Our modification strategy provides an efficient and convenient way to prepare novel RO membranes with high permeability.

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.

Enhancing the antifouling and chlorine resistance capabilities of thin-film composite reverse osmosis via surface grafting of dipeptide

There has been constant progress in the development of innovative polyamide (PA) thin-film composite (TFC) reverse osmosis (RO) membranes to address limitations in fouling potential, chlorine resistance, salt rejection and water flux trade-offs. In this work, dipeptide of zwitterionic amino acid L-arginine, i.e., arginyl-arginine (Arg-Arg), was grafted on the surface of PA TFC RO membrane in order to simultaneously improve the antifouling properties, chlorine resistance, and separation performance. The interactions between the Arg-Arg dipeptide and PA layer were evidenced by Fourier transforms infrared spectroscopy and nuclear magnetic resonance spectroscopy, which have indicated the distinguished chemical grafting of Arg-Arg dipeptide on the RO membrane surface. The grafting of Arg-Arg dipeptide led to a thinner, smoother, more hydrophilic, and less negatively charged PA surface. The surface-modified membranes simultaneously exhibited enhanced desalination performances, antifouling and chlorine resistance properties. Compared to the neat TFC membrane, the water flux of Arg-Arg dipeptide-modified membrane was improved by 26.19%. The salt rejection also slightly increased from 96.25% to 98%. The roles of Arg-Arg dipeptide in altering the surface chemistry and the structure of the membranes are discussed to highlight the potential of surface-modified membranes in heightening the desalination performances of RO TFC membranes.

Treatment of radionuclide-containing wastewater using thin film composite reverse osmosis membrane with spray coating-assembled titania nanosheets

Reverse osmosis (RO) process is a promising technology for treating radionuclide-containing wastewater owing to its reliability and effectiveness in ion separation. The long-term stability of RO membranes in terms of water flux and radionuclide rejection is a crucial factor for their practical application. In this study, thin film composite (TFC) RO membrane with high long-term stability and antifouling properties for radionuclide-containing wastewater treatment was developed by spray coating the surface of polyamide (PA) layer with oppositely charged titania nanosheets (TNS). Caesium ion (Cs+) as model radionuclide was used to assess the RO performance of the manufactured membranes, including permeability and radionuclide rejection The highest permeability of 1.53 L·m−2·h−1·bar−1, which was 45.8 % higher than that of neat TFC membrane, was achieved by thin film nanocomposite (TFN) membrane sprayed-coated with 15 ppm TNS bilayer. Compared to TFC membrane which exhibited CsCl rejection of 99.32 %, the rejection of all TNS spray-coated TFN membranes was improved to almost complete rejection. During the long-term testing, the optimum TFN/TNS(15) membrane demonstrated well-retained water flux and ion rejection. The roles of the TNS bilayer in improving the long-term stability and antifouling properties are discussed. The findings of this study highlight the potential of a tailored TFC RO membrane in treating radioanuclide-containing wastewater.

Effect of pore size distribution on the desalination performance of the selective layer of nanoporous atomically-thin membranes

Over the last decade, molecular simulations and experiments have shown that nanoporous atomically-thin membranes (NATMs) have the potential for high-permeance, high-selectivity separations, including seawater desalination. Realistic NATMs contain polydisperse pore sizes that could impede their performance, as large pores lead to salt leakage. This paper computationally examines the effect of pore size distributions (PSDs) on the desalination performance of the selective layer of NATMs by reverse osmosis (RO), considering size exclusion as the dominant selection mechanism. Analogous to thin-film composite RO membranes, the finite width of PSDs leads to a trade-off between water permeability and water/salt selectivity in NATMs. Tight PSDs with average pore sizes slightly below the size of the salt are needed to ensure high selectivity comparable to TFC membranes with high water permeance. Sealing of large, salt-permeable pores (e.g. by interfacial polymerization) limits salt leakage but substantially reduces water permeance. Introducing energy barriers that impede salt permeation by tuning nanopore structure and chemistry can make NATMs more robust to wider PSDs. In summary, some combination of control of PSDs, leakage mitigation, and pore functionalization is essential for NATMs to surpass the permeability/selectivity trade-off of polymeric RO membranes and achieve high water permeance with good salt rejection.

In-situ coating of Fe-TA complex on thin-film composite membranes for improved water permeance in reverse osmosis desalination

Post-modifications of thin-film composite (TFC) reverse osmosis (RO) membranes have been extensively studied to improve the overall separation performance. However, new approaches have yet to be explored not only to achieve better separation efficiency, but also to reduce the overall manufacturing cost. In this work, a ferric-tannic acid (Fe-TA) complex is post-coated onto the bare TFC membranes in an in-situ stepwise process. By optimizing the concentrations of Fe3+ and TA, the resulting membrane can achieve a water permeance of 3.58 L m−2 h−1 bar−1 (LMH bar−1) and a salt rejection of 98.37 % at 20 bar for brackish water desalination, owing to the improved surface hydrophilicity, roughness and additional water pathways. Moreover, the optimal membrane also exhibits comparable performance in seawater desalination and boron removal. This new approach of modification shows potentials to be integrated into the current roll-to-roll membrane fabrication process.

The effect of surficial function groups on the anti-fouling and anti-scaling performance of thin-film composite reverse osmosis membranes

Membrane fouling, including organic fouling and inorganic scaling, is one of the most prevalent problems in reverse osmosis (RO) membranes, which usually increases operating costs and reduces efficiency. In this study, various surface modified thin-film composite (TFC) RO membranes were developed to systematically investigate the effect of type of ionic groups (-COOH, –NH2, and zwitterions) on the membrane anti-fouling and anti-scaling performances. Compared with the pristine polyamide-based membrane, all the modified membranes maintained their surface topography with a "ridge-and-valley" structure, but their surface hydrophilicity were significantly improved and zeta potential values were changed. The results of fouling tests show that TFC-g-PEI membrane with positive zeta potential is beneficial to the anti-fouling performance against cationic foulants, but is very unfavorable against anionic foulants, although the surface hydrophilicity is very good. The zwitterion-modified membranes with different anions (carboxylate, sulfonate, and phosphate groups) have little difference in anti-fouling performance against organic foulants, but have obvious influence on anti-scaling performance. All the zwitterion-modified membranes exhibited good anti-fouling performances against charged and electroneutral foulant, owing to their remarkable hydrophilicity and neutrality. Interestingly, TFC-g-PEI-SO3H membrane exhibited the best anti-scaling performance because of the low pKsp value of calcium sulfate. In conclusion, the research of this study may provide the development direction for the preparation of anti-fouling and anti-scaling membranes.

High-performance polyethyleneimine based reverse osmosis membrane fabricated via spin-coating technology

Conventional thin-film composite (TFC) reverse osmosis (RO) membranes prepared by ultrafast and uncontrolled interfacial polymerization (IP) generally have unsatisfactory permeability and high fouling propensity, which distinctly impedes their development and wide application. Controllable thickness and roughness offer an opportunity to optimize the permeability and fouling resistance of RO membranes. In this work, we propose spin-coating assisted multilayer interfacial polymerization (SMIP) as a promising approach to fabricating high-performance RO membranes using polyethyleneimine (PEI) and 1, 3, 5-benzoyl chloride (TMC). SMIP integrates the high efficiency and uniformity of spin-coating technology, the nano-level and individual properties control of multilayer interfacial polymerization, and the simplicity of the IP process. Combining the strong hydrophilicity with low diffusion efficiency and positive charge of PEI, a thin, intrinsically hydrophilic, smoother and almost electro-neutral polyamide (PA) nanofilm was obtained. The optimal RO membrane with a controlled thickness of the selective layer (≈61 nm) has a water permeance of 3.12 L m−2 h−1 bar−1 and an acceptable NaCl rejection of 97.5%, while the strong hydrophilicity, minimal surface roughness and almost neutral surface charge afford it excellent fouling resistance. SMIP technology not only provides a new approach to preparing highly permeable and low-fouling RO membranes using macromolecular monomers, but also has great application potential in other separation membranes due to its precise control of the selective layer.

Synergetic effects of COFs interlayer regulation and surface modification on thin-film nanocomposite reverse osmosis membrane with high performance

Reverse osmosis (RO) membrane technology is of great significance to solve the increasingly serious fresh water supply problem. Researchers have found that substrate and polyamide layer properties influence the separation performance of RO membranes greatly. In this work, an interlayer of ultra-thin and hydrophilic two-dimensional Covalent organic frameworks (COFs) nanomaterial was in-situ constructed on commercial polysulfone (PSf) substrate surface via interfacial reaction between 1,4-phenylenediamine-2-sulfonic acid (Pa-SO3H) and 1,3,5-triformylphloroglucinol (Tp), both with ultra-low concentration. This COFs interlayer successfully adjusted the interfacial polymerization (IP) process between m-phenylenediamine (MPD) and trimesoyl chloride (TMC), and thus helped to fabricate a kind of thin-film nanocomposites (TFN) RO membrane with excellent performance. Compared with the baseline thin-film composite (TFC) RO membrane without COFs interlayer, the water permeance of the TFN RO membrane was increased from 17.0 to 26.3 L m−2 h−1 MPa−1, accompanied with a NaCl rejection remaining about 99.3 %. The TFN RO membrane was further modified by ultra-low concentration Tp aqueous solution to reduce the surface defects and improve the separation performance. The formed TFN-Tp RO membrane has a further increased water permeance from 26.3 to 31.1 L m−2 h−1 MPa−1, and an increased NaCl rejection from 99.3 % to 99.5 % as compared with the TFN RO membrane.

Permeability improvement of reverse osmosis membranes by addition of dimethyl sulfoxide in the interfacial polymerization media

In this study, it is presented a solvent addition method to significantly increase the performance of reverse osmosis (RO) membranes. Polyamide (PA) thin film composite RO membranes were prepared by interfacial polymerization (IP) method with a mixture containing dimethyl sulfoxide (DMSO) in different weight percentages. With the addition of DMSO, the selective PA layer was successfully activated, increasing its water flux to 41.7 L/m2 h while preserving a NaCl rejection of up to 95.7%. Morphological characteristics and properties of the model PA film under the influence of solvent with different weight percentages of DMSO showed the structural deformation caused by the activation of the PA layer, which affects the pore size and separation properties of the membrane. The research provides an expandable way to fabricate high-performance membranes in terms of flux and desalination with good resistance to fouling agents of polymeric membranes.

Artificial water channels engineered thin-film nanocomposite membranes for high-efficient application in water treatment

Inspired by biological aquaporins, artificial water channels (AWCs) can achieve high-performance for thin-film composite (TFC) reverse osmosis (RO) membranes. In our work, two types of liposomes (C18lyso and D5A liposomes) with superior permeability serving as AWCs were designed by regulating the molecular structures of lipids, in which D5A liposomes have a better ability to water transport water molecules. Herein, liposome-based RO membranes (TFN-C18lyso and TFN-D5A) were fabricated via in situ interfacial polymerizations (IP). The resulting TFN-C18lyso shows an increase in water permeability of ∼ 51.4 % (up to 3.83 L m−2 h−1 bar−1) while TFN-D5A displays an increase in water permeability of ∼ 67.2 % (up to 4.23 L m−2 h−1 bar−1), and both membranes keep high rejection of ∼ 98.6 %. Moreover, both TFN-C18lyso and TFN-D5A present good separation stability, chemical resistance, and good desalination. Our work provides a valid and versatile foundation for developing advanced biomimetic membranes in water treatment.

Complexation of tannic acid/silver nanoparticles on polyamide thin film composite reverse osmosis membrane for enhanced chlorine resistance and anti-biofouling properties

Reverse osmosis (RO) is a well-established and reliable desalination method that has been widely implemented on a commercial scale to resolve water scarcity issues. Constant efforts have been made to develop thin film composite membranes that can strike a balance between selectivity and permeability, while maintaining high durability towards fouling and chlorine attack. In this work, the surface of thin film composite RO membrane was modified by complexing tannic acid (TA) and silver nanoparticles (Ag NPs). TA was used for enhancing the chlorine resistance while Ag NPs act as a biocide to mitigate biofouling. Upon the formation of PA layer through interfacial polymerization, TA was chemically grafted, proceeded by in-situ reduction of Ag NPs. Transmission electron microscopy (TEM) image revealed that Ag NPs were deposited on top of the TA-PA interlayer and reduced as discrete spherical Ag NPs with diameter <15 nm. The greatest flux of 20.57 L/m2·h was produced by TFC-TA/Ag1, representing a 60 % improvement as compared to neat TFC membrane. The NaCl rejection of TFC-TA membrane was 97.83 %, which was slightly increased compared to neat TFC membrane with rejection of 95.14 %. Improved chlorine resistance and improved antibacterial properties against Gram-positive bacteria (S. aureus) and Gram-negative bacteria (E. coli) were witnessed for the TFC-TA/Ag1 membranes. This study demonstrates a facile chemical grafting and deposition to introduce multiple functionalities for improving the physical and separation properties of TFC RO membrane.

Distinct impact of substrate hydrophilicity on performance and structure of TFC NF and RO polyamide membranes

Substrate properties have profound impacts on the structure and performance of both thin-film composite (TFC) nanofiltration (NF) and reverse osmosis (RO) polyamide (PA) membranes. Some studies have previously investigated the impact of substrate hydrophilicity on PA formation and TFC membrane performance. However, the observed phenomena and explanations remain contradictory in literature. Herein, we performed interfacial polymerization (IP) reactions of both piperazine (PIP)-trimesoyl chloride (TMC) and m-phenylenediamine (MPD)-TMC systems on substrates with different hydrophilicity. We found that the TFC RO membrane showed higher water permeance and NaCl rejection on the relatively hydrophobic substrate, while the TFC NF membrane favored the relatively hydrophilic substrate. The critical importance of interfacial degassing and local monomer concentration was highlighted to dissect the distinct impact of substrate hydrophilicity. For the MPD-TMC system, interfacial nanobubble generation was inhibited because of the decreased local MPD concentration and heat production for the more hydrophilic substrates, resulting in a decrease in the roughness feature and compromised water permeance of RO membranes. In contrast, interfacial degassing was not a dominant mechanism in the PIP-TMC system due to the slower reaction rate of PIP-TMC than MPD-TMC. Consequently, the PA layer of NF membrane became thinner and looser when the substrate became more hydrophilic, resulting from the diluted local PIP concentration. Our study unveils the fundamental relationship among substrate hydrophilicity, PA structure, and separation performance of both TFC NF and RO PA membranes, providing important guides on their design and synthesis.

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

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

High performance polyamide TFC reverse osmosis membrane fabricated on co-deposition hydrophilic modified polyethylene substrate

The separation performance of polyamide (PA) thin film composite (TFC) membrane is critically influenced by the substrates. Numerical models and experimental studies suggest that substrates with high porosity comprising small pores can effectively improve membrane performance. Here we report that a commercial polyethylene (PE) micro-porous membrane with high porosity was chosen as substrate for fabricating PA TFC membrane. The PE substrate was modified by co-depositing the copolymers of gallic acid and γ-aminopropyltriethoxysilane to improve its bulk hydrophilicity. The porosity, pore size, morphology and chemical properties of the modified PE substrates were characterized. The resultant PA TFC-PE membrane had a favorable water permeance of 2.33 ± 0.06 L m−2 h−1 bar−1 as well as a high NaCl rejection of 99.0 ± 0.4 % because of the suitable porosity, pore size and hydrophilicity for interfacial polymerization reaction. Our work is expected to provide an efficient way for the modification of hydrophobic porous membrane for fabricating PA TFC membrane.