Sulfonated polyaniline interlayer with controllable doping conditions for high-performance nanofiltration

Significant progress has been made in recent years in the fabrication of CO2 separation membranes using thin-film composite membranes (TFCs) prepared by the interfacial polymerization (IP) process. However, more advancement is still required to realize the full potential of IP-TFCs in highly permeable/selective gas separation membranes. Here, nitrogen-doped graphene oxide (N-GO) nanosheets and dicationic imidazolium ionic liquids (ILs) with two different aliphatic linkers (i.e., trimethylene and hexamethylene), C3IL and C6IL, respectively, are added in the polyamide (PA) selective layer of a TFC membrane, and the gas separation performance of the membranes is investigated. With the incorporation of the C3IL/N-GO additive, the CO2 permeance and CO2/CH4 and CO2/N2 selectivities of the TFC membranes are increased by 900, 2, and 6%, respectively. On the other hand, the corresponding increases in the CO2 permeance and CO2/gas selectivities, by introducing the C6IL/N-GO additive are 700, 9, and 16%, respectively. These enhancements are attributed to the role of IL molecules in improving the distribution of N-GO nanosheets in the PA selective layer of TFCs, tuning the N-GO sieving channels in favor of CO2 transport, and creating ultrasmall holes which are highly accessible to CO2 molecules. We believe that the incorporation of IL/GO additives into the selective layer of IP-TFCs will pave the way to improving the gas separation performance of thin-film composite membranes.

Independent control of the extrinsic and intrinsic properties of the polyamide (PA) selective layer is essential for designing thin-film composite (TFC) membranes with performance characteristics required for water purification applications besides seawater desalination. Current commercial TFC membranes fabricated via the well-established interfacial polymerization (IP) approach yield materials that are far from ideal because their layer thickness, surface roughness, polymer chemistry, and network structure cannot be separately tailored. In this work, tailor-made PA-based desalination membranes based on molecular layer-by-layer (mLbL) assembly are presented. The mLbL technique enables the construction of an ultrathin and highly cross-linked PA selective layer in a precisely and independently controlled manner. The mLbL-assembled TFC membranes exhibit significant enhancements in performance compared to their IP-assembled counterparts. A maximum sodium chloride rejection of 98.2% is achieved along with over 2.5 times higher water flux than the IP-assembled counterpart. More importantly, this work demonstrates the broad applicability of mLbL in fabricating a variety of PA-based TFC membranes with nanoscale control of the selective layer thickness and roughness independent of the specific polyamide chemistry.

Due to adverse health effects and the broad sources of per- and polyfluoroakyl substances (PFAS), PFAS removal is a critical research area in water purification. We demonstrate the functionalization of thin-film composite (TFC) hollow fiber nanofiltration (HFN) membranes by MXene nanosheets during the interfacial polymerization (IP) process for enhanced removal of perfluorooctane sulfonic acid (PFOS) from water. A MXene-polyamide (PA) selective layer was fabricated on top of a polysulfone (PSF) hollow fiber support via IP of trimesoyl chloride (TMC) and a mixture of piperazine (PIP) and MXene nanosheets to form MXene-PA thin-film nanocomposite (TFN) membranes. Incorporating MXene nanosheets during the IP process tuned the morphology and negative surface charge of the selective layer, resulting in enhanced PFOS rejection from 72% (bare TFC) to more than 96% (0.025 wt % MXene TFN), while the water permeability was also increased from 13.19 (bare TFC) to 29.26 LMH/bar (0.025 wt % MXene TFN). Our results demonstrate that both electrostatic interaction and size exclusion are the main factors governing the PFOS rejection, and both are determined by PA selective layer structural and chemical properties. The lamella structure and interlayer of MXene nanosheets inside the PA layer provided different transport mechanisms for water, ions, and PFAS molecules, resulting in enhanced water permeability and PFAS rejection due to traveling through the membrane by both diffusions through the PA layer and the MXene intralayer channels. MXene nanosheets showed very promising capability as a 2D additive for tuning the structural and chemical properties of the PA layer at the permeability-rejection tradeoff.

Integrated polyamide thin-film nanofibrous composite membrane regulated by functionalized interlayer for efficient water/isopropanol separation

Thin-film composite (TFC) structure has been widely employed in polymeric membrane fabrication to achieve superior performance for desalination and water treatment. In particular, TFC membranes with a thin active polyamide (PA) selective layer are proven to offer improved permeability without compromising salt rejection. Several modifications to TFCs have been proposed over the years to enhance their performance by altering the selective, intermediate, or support layer. This study proposes the modification of the membrane support using nanozeolites prepared by a unique ball milling technique for tailoring the nanofiltration performance. TFC membranes were fabricated by the interfacial polymerization of Piperazine (PIP) and 1,3,5-Benzenetricarbonyl trichloride (TMC) on Polysulfone (PSf) supports modified with nanozeolites. The nanozeolite concentration in the casting solution varied from 0 to 0.2%. Supports prepared with different nanozeolite concentrations resulted in varied hydrophilicity, porosity, and permeability. Results showed that optimum membrane performance was obtained for supports modified with 0.1% nanozeolites where pure water permeance of 17.1 ± 2.1 Lm−2 h−1 bar−1 was observed with a salt rejection of 11.47%, 33.84%, 94%, and 95.1% for NaCl, MgCl2, MgSO4, and Na2SO4 respectively.

Reverse osmosis (RO) is considered as the most widely utilized technique worldwide for water treatment. However, the commercial thin-film composite (TFC) membranes, which are normally made of polyamide (PA) through interfacial polymerization (IP), still experience certain major issues in performance and fabrication. The spin assisted layer-by-layer (SA-LbL) technique was established for overcoming some drawbacks with commercially available PA membranes. Also, recent investigations have recognized the nanoparticle inclusion into the selective layer as a powerful technique for improving the membrane efficiency. Hence, two different methodologies are presented here to improve the membrane performance, i.e., (1) SA-LbL technique to fabricate TFC membrane by the deposition of alternate ultrathin layers of different polyelectrolytes on polysulfone (PSF) commercial ultrafiltration membrane and (2) the nanoclay incorporation into the membranes during IP process to develop TFC membrane. Two types of nanoclays, cloisite (CS)-15A and montmorillonite (MNT), were incorporated to enhance the separation efficiency. This SA-LbL is an innovative method for the RO membrane manufacture, and has not been described earlier to the best of our knowledge. In addition, this work validated for the first time, the efficiency of the two nanoclays at the PA selective layer in the RO membrane. The membrane performance was evaluated using sodium chloride solution in a cross-flow permeation-testing cell for salt rejection and water flux. The results show significant improvement in water flux and salt rejection. The permeation test of 120 bilayers of poly (allylaminehydrochloride)/poly (vinylsulfate) on PSF substrate showed water flux of 37 L/ (m2.h) and salt rejection of 53%, for a 2000-ppm salt solution feed. The highest water flux of 40 L/m2.h with 80% salt rejection, relative to the control membrane was obtained for the membranes containing nanoclays at 25oC temperature, 40.0 bar pressure and 2000 ppm feed concentration. Thus, our study demonstrated that these TFC membranes are promising, and these novel fabrication techniques are great tool to manufacture the RO membrane.

2D COFs interlayer manipulated interfacial polymerization for fabricating high performance reverse osmosis membrane

Thin film composite membranes containing intrinsic CD cavities in the selective layer

Correlations between conditions of the polyaniline (PANI) interlayer formation on the surface of a polysulfone (PSF) porous membrane substrate and the structure and performance of thin-film composite (TFC) membranes for nanofiltration with a polyamide (PA) selective layer prepared via interfacial polymerization (IP) were studied. It was shown that application of the PANI layer significantly enhanced hydrophilicity (the water contact angle decreased from 55 ± 2° down to 26-49 ± 2°), decreased pore size and porosity, and increased the surface roughness of the selective layer surface of porous PSF/PANI membrane substrates due to the formation of bigger PANI globules, which affect the formation of the PA layer of TFC membranes via IP. It was shown that the application of the PANI intermediate layer yielded the formation of a thinner PA selective layer, a decline in surface roughness, and an increase in hydrophilicity (the water contact angle declined from 28 to <10°) and crosslinking degree of the selective layer of TFC NF membranes. The developed approach allows us to enhance the water permeation up to 45-64 L·m-2·h-1 at ΔP = 0.5 MPa and improve membrane selectivity (rejection coefficient of MgSO4->99.99%; LiCl-5-25%; sulfadimetoxine-80-95%) and also ensure enhanced long-term operational stability of TFC nanofiltration membranes with a PANI interlayer. Moreover, Mg2+/Li+ separation factor values were found to increase to 37 and 58 for PANI-modified membranes compared to 9 and 8 for the reference NF-PSF membranes.

Interfacial polymerization of thin film selective membrane layers: Effect of polyketone substrates

Nanofiltration (NF) is a pressure-driven membrane process that separates small solutes through nanometer-scale pores, governed mainly by steric hindrance, Donnan exclusion, and dielectric effects. NF has become increasingly important for removing multivalent ions and low-molecular-weight organics, supporting applications ranging from desalination and wastewater treatment to biomedical and pharmaceutical processes. In water treatment, it is especially valued for softening and removing natural organic matter. However, most commercial NF membranes are negatively charged, limiting their ability to remove divalent and multivalent cations. Positively charged membranes are crucial for water softening, heavy-metal removal, and Mg2+/Li+ separation, yet their development remains challenging. To address these gaps, this thesis focuses on fabricating high-performance, positively charged thin-film composite (TFC) NF membranes using interfacial polymerization (IP) and post-modification. This work first develops a positively charged hollow-fiber NF membrane through a twice-IP strategy. Polyethyleneimine (PEI) and piperazine (PIP) react with trimesoyl chloride (TMC) to form the primary layer on polyethersulfone (PES) hollow fiber substrate, followed by tris(2-aminoethyl) amine (TAEA) modification to increase positive surface charge density. The resulting membrane reaches 97.6% MgCl2 rejection with a pure water permeability (PWP) of 16.0 L m−2 h−1 bar−1 and achieves >99% rejection of Cu2+ and Zn2+, demonstrating an efficient route for softening and heavy-metal removal (Chapter 3). Next, acid-resistant NF membranes were designed for industrial wastewater treatment. Using PEI and cyanuric chloride (CC) for the first IP on the PES hollow fiber substrate and TAEA for secondary modification, the optimized membrane delivers a PWP of 25.2 L m−2 h−1 bar−1 and ~90% MgCl2 rejection. In synthetic electroplating rinse water at pH 1, it maintains ~85% rejections of Cu2+ and Ni2+ across seven days, showing strong potential for acidic wastewater applications (Chapter 4). To achieve selective ion separation, particularly Mg²⁺/Li⁺ fractionation for lithium extraction, a new NF membrane was fabricated by adding 3-bromopropyl trimethylammonium bromide (BPTAB) into the aqueous phase during IP process, and its performance was compared with a TAEA-TMC control membrane on a PES substrate. The incorporation of BPTAB increased the positive charge density on both surfaces of the selective layer and generated smaller, more uniform pores, significantly enhancing MgCl2 rejection and Mg2+/Li+ selectivity (SLi/Mg). The resulting membrane achieved a high PWP of 23.5 L m−2 h−1 bar−1 and achieved a SLi/Mg of 24.8 in a 2000 ppm mixed solution with a Mg2+/Li+ ratio of 20, far surpassing the control membrane (SLi/Mg = 5.2), demonstrating strong potential for lithium-brine applications (Chapter 5). Overall, this thesis advances NF membrane design by enabling tunable, positively charged selective layers with improved ion-separation performance. The developed membranes address critical needs in water softening, heavy-metal removal, and Mg2+/Li+ separation. This work provides a foundation for next-generation NF technologies and suggests future efforts should focus on scaling production and enhancing long-term operational stability for industrial adoption.

Interfacially polymerized thin-film composite polyamide membranes: Effects of annealing processes on pervaporative dehydration of aqueous alcohol solutions

Poly(N-isopropylacrylamide) (PNIPAAm) was introduced into a polyethylene terephthalate (PET) nonwoven fabric to develop novel support for polyamide (PA) thin-film composite (TFC) membranes without using a microporous support layer. First, temperature-responsive PNIPAAm hydrogel was prepared by reactive pore-filling to adjust the pore size of non-woven fabric, creating hydrophilic support. The developed PET-based support was then used to fabricate PA TFC membranes via interfacial polymerization. SEM–EDX and AFM results confirmed the successful fabrication of hydrogel-integrated non-woven fabric and PA TFC membranes. The newly developed PA TFC membrane demonstrated an average water permeability of 1 L/m2 h bar, and an NaCl rejection of 47.0% at a low operating pressure of 1 bar. The thermo-responsive property of the prepared membrane was studied by measuring the water contact angle (WCA) below and above the lower critical solution temperature (LCST) of the PNIPAAm hydrogel. Results proved the thermo-responsive behavior of the prepared hydrogel-filled PET-supported PA TFC membrane and the ability to tune the membrane flux by changing the operating temperature was confirmed. Overall, this study provides a novel method to fabricate TFC membranes and helps to better understand the influence of the support layer on the separation performance of TFC membranes.

Nanofiltration membranes (NFMs) with superior permeability and high rejection of both divalent anions and cations are highly desirable to meet the increasing separation demands of complex systems. Herein, we propose a three-in-one strategy to develop a state-of-the-art dually charged thin-film composite (TFC) nanofiltration membrane consisting of a positively charged electrospun nanofiber substrate (NFS) with surface mineralization and a negatively charged polyamide (PA) selective layer prepared by interfacial polymerization (IP). The highly hydrophilic mineralized nanofiber substrate not only effectively reduces the thickness of the PA selective layer but also crumples its structures by the abundant zirconia nanoparticles on the substrate surface, resulting in excellent water flux (15.0 L m-2 h-1 bar-1) for the TFC NFMs. The relationship between the thickness of the selective layer and substrate is further investigated using dissipative particle dynamics (DPD) simulations. Meanwhile, the dually charged NFM exhibits relatively high rejection for both anions (97.1% for Na2SO4 and 97.9% for MgSO4) and cations (87.9% for MgCl2) in aqueous solutions compared with single-charged membranes, which is attributed to the dual-repulsion effect of the selective layer and the substrate surface bearing opposite charges. Moreover, the prepared NFMs exhibit good stability and excellent antifouling performance. This work may pave the way for the development of highly efficient nanofiltration membranes for the practical separation of comprehensively charged solutes.

Thin-film composite (TFC) membranes are favored for precise molecular sieving in liquid-phase separations; they possess high permeability due to the minimal thickness of the active layer and the high porosity of the support layer. However, current TFC membrane fabrication techniques are limited by the available materials for the selective layer and do not demonstrate the level of structural control needed to substantially advance organic solvent nanofiltration (OSN) membrane technology. In this work, we employ the newly developed thin-film lift-off (T-FLO) technique to fabricate polybenzimidazole (PBI) TFC membranes with porous support layers uniquely tailored to OSN. The drop-cast dense PBI selective layers endow the membranes with an almost complete rejection of common small dye molecules. The polymeric support layer is optimized by a combinatorial approach using four different monomers that alter the cross-linking density and polymer chain flexibility of the final composite. These two properties substantially affect the porogen holding capacity of the reticular polymer network, leading to the formation of different macropore structures. With a 150 nm thick PBI selective layer and fine-tuning of the support layer, the resulting membrane achieves stable and superior permeance of 14.0, 11.7, 16.4, 11.4, 17.1, and 19.7 L m-2 h-1 bar-1 for water, ethanol, methanol, isopropanol, tetrahydrofuran (THF), and acetonitrile, respectively.