Water scarcity and pollution necessitate advanced water treatment technologies. Nanofiltration (NF), a low-energy process, uses membranes to remove divalent ions based on size and charge exclusion. Thin-film composite (TFC) NF membranes, prepared via interfacial polymerization, are central to NF technology. However, a trade-off between permeability and selectivity limits their performance. Incorporating nanomaterials into the PA layer to create thin-film nanocomposite (TFN) membranes has emerged as a strategy to enhance water permeance and selectivity. Previous studies have explored various nanomaterials, including graphene oxide, MoS₂, and quantum dots, demonstrating improvements in water flux and antifouling properties. However, challenges remain, such as aggregation of inorganic nanofillers and compatibility issues with the PA layer. Covalent organic frameworks (COFs), a class of 2D nanoporous materials, offer potential advantages due to their organic structure, inherent porosity, and compatibility with the PA layer. Several studies have shown that incorporating COFs enhances membrane performance. This work aims to improve upon previous COF-based TFN membranes by synthesizing sulfonated COFs (S-CONs), which possess enhanced hydrophilicity and electronegativity compared to unmodified COFs. The hypothesis is that the incorporation of S-CONs will further enhance the performance and chlorine resistance of TFN-NF membranes.

The existing literature highlights the challenges and advancements in nanofiltration membrane technology. The inherent trade-off between water permeance and salt rejection in traditional TFC membranes has driven research into thin-film nanocomposite (TFN) membranes. The incorporation of various nanomaterials, such as zeolites, graphene oxide, MoS₂, and quantum dots, has been shown to improve membrane performance by increasing the effective filtration area and reducing the thickness of the selective layer. However, the use of inorganic nanomaterials often faces challenges related to aggregation and poor compatibility with the polyamide layer. The emergence of two-dimensional (2D) covalent organic frameworks (COFs) as nanofillers presents a promising alternative. COFs offer advantages such as tunable porosity, high surface area, and good compatibility with the polyamide layer. Previous studies have reported the successful incorporation of COFs into TFN membranes, resulting in improved water permeance and salt rejection. However, optimizing the functionality of COFs to further enhance membrane properties, particularly hydrophilicity and chlorine resistance, remains an area of active research. This study builds upon this existing knowledge base by focusing on sulfonated COFs to achieve superior performance characteristics.

The study involved the synthesis of sulfonated covalent organic framework nanosheets (S-CONs) using a solvothermal method. Specifically, 1,4-Phenylenediamine-2-sulfonic acid (Pa-SO₃H) and 2,4,6-Triformylphloroglucinol (Tp) were used as monomers, resulting in TpPa-SO₃H COFs (S-COFs). Mechanical grinding exfoliated the S-COFs into S-CONs. The morphology and properties of S-CONs were characterized using SEM, TEM, EDX, ATR-FTIR, XPS, P-XRD, AFM, and ZetaPALS. The reaction between S-CONs and 1,3,5-Trichlorobenzoylchloride (TMC) was confirmed using FT-IR and XPS. TFN membranes were fabricated by incorporating varying amounts of S-CONs (0.004–0.010 g) into the organic phase (n-hexane) during the interfacial polymerization of piperazine (PIP) and TMC on a porous polysulfone (PSf) ultrafiltration substrate. Membrane characterization included FE-SEM, TEM, AFM, XPS, contact angle measurement, and zeta potential analysis. Separation performance was evaluated by measuring pure water permeance and salt rejection using Na₂SO₄, MgSO₄, NaCl, and MgCl₂ solutions. Molecular weight cut-off (MWCO) was determined using polyethylene glycols (PEGs) of different molecular weights. Anti-fouling performance was assessed using bovine serum albumin (BSA) solutions. Long-term operational stability was evaluated over 51 days using Na₂SO₄ solution. Chlorine resistance was tested at different pH values (5 and 10) and chlorine concentrations. Molecular dynamics (MD) simulations were employed to study the free volume and water density distribution within the membrane.

The synthesized S-CONs exhibited a lamellar structure with a rough surface. AFM analysis revealed a thickness of 1.35–1.57 nm, significantly thinner than the original S-COFs. The S-CONs demonstrated strong electronegativity (-45.20 mV at pH 7.00). FT-IR and XPS confirmed the formation of amide bonds between S-CONs and TMC during interfacial polymerization. Incorporation of S-CONs into the TFN membranes resulted in a significant increase in pure water permeance, reaching a maximum of 8.84 L·m⁻²·h⁻¹·bar⁻¹ at an optimal S-CONs addition of 0.006 g (1.75 times higher than the TFC membrane). This was attributed to the reduced PA layer thickness (~100–128 nm compared to ~178–198 nm in the TFC membrane), increased surface roughness, enhanced hydrophilicity (WCA decreased from 62.98° to 45.14°), and increased electronegativity (-44.41 mV at pH 7.00 for the optimal membrane). High Na₂SO₄ rejection (98.97%) was also achieved at the optimal S-CONs concentration. The improved anti-fouling performance was attributed to the enhanced electronegativity and hydrophilicity of the membrane surface. The TFN membranes showed excellent long-term stability, with only a 13.9% decrease in permeate flux and 0.94% decrease in Na₂SO₄ rejection after 51 days. Importantly, the covalent bonding between S-CONs and the PA layer significantly enhanced chlorine resistance, particularly at pH 10. MD simulations supported the experimental findings, revealing higher free volume in the TFN membrane compared to the TFC membrane. The effective pore diameter of the optimal TFN membrane was determined to be 0.36 nm.

The results demonstrate the effectiveness of incorporating sulfonated covalent organic framework nanosheets (S-CONs) as nanofillers to enhance the performance of TFN nanofiltration membranes. The significant improvement in pure water permeance and salt rejection is attributed to the synergistic effects of reduced membrane thickness, enhanced hydrophilicity, and increased surface electronegativity. The formation of covalent bonds between the S-CONs and the polyamide layer contributes to the improved long-term stability and chlorine resistance. These findings confirm the potential of functionalized 2D COFs as a promising class of nanofillers for modifying TFC membranes. The observed optimal S-CON concentration suggests a balance between enhancing permeability and preventing aggregation. The superior anti-fouling and chlorine resistance properties highlight the practical advantages of this approach for real-world applications. Future research could explore other functional groups on COFs and investigate the effect of different COF structures on membrane performance.

This study successfully fabricated high-performance TFN nanofiltration membranes by incorporating sulfonated covalent organic framework nanosheets (S-CONs). The S-CONs significantly enhanced membrane properties, leading to improved pure water permeance, salt rejection, anti-fouling performance, and chlorine resistance. The findings demonstrate the potential of using functionalized 2D COFs to address the limitations of traditional TFC membranes. Future research could explore the use of other functionalized COFs and investigate the long-term performance and scalability of this technology for various water purification applications.

While this study demonstrates significant improvements in membrane performance, several limitations should be acknowledged. The study focuses on a specific type of COF (TpPa-SO3H), and the results may not be directly generalizable to other COF structures. The long-term stability testing was conducted under specific conditions (Na₂SO₄ solution), and the membrane performance under different feed solutions and operational conditions should be further investigated. The cost-effectiveness of large-scale production of S-CONs and their incorporation into the membrane fabrication process remains to be explored. Finally, the MD simulations, while providing valuable insights, are simplified representations of the complex membrane system.