The sustainable management of saline organic-rich wastewaters is crucial for a circular economy and net-zero carbon emissions. Current wastewater treatment focuses on contaminant removal, but a paradigm shift towards resource recovery (energy, nutrients, biomass) is urgently needed. One significant challenge is managing saline organic-rich waste streams from industries like textile processing, tanneries, and food processing. Effective fractionation of organic and inorganic salts (e.g., NaCl) is vital for resource recovery from these streams. Membrane-based separation technologies, such as nanofiltration, offer potential solutions. However, nanofiltration suffers from membrane fouling, cake-enhanced concentration polarization, and high pure water consumption, leading to reduced separation efficiency and productivity. Electrodialysis, an alternative, allows ion transfer through membranes under an electric field. Yet, organic compounds in wastewater can cause fouling on anion exchange membranes (AEMs), limiting anion transfer and fractionation efficiency. This study integrates the advantages of nanoporous membranes and electrodialysis, utilizing thin-film composite (TFC) nanoporous polyamide (PA) membranes as anion-conducting membranes (ACMs) in an electro-driven system to address these limitations. The inherent negative charge of these membranes impedes anion transfer. To enhance performance, a bio-inspired polydopamine (PDA)-polyethyleneimine (PEI) coating is employed to modify the membrane surface properties and intensify the charge shielding effect, improving anion transfer and selectivity while mitigating fouling.

The literature review highlights the limitations of existing technologies for the treatment of saline organic-rich wastewaters. Nanofiltration, while effective in sieving organics and salts based on size exclusion and electrostatic repulsion, suffers from membrane fouling, high osmotic pressure, and substantial water consumption, thereby reducing its overall efficiency. Electrodialysis, although energy-efficient, is hampered by membrane fouling caused by the electrostatic attraction of negatively charged organics to the anion exchange membranes (AEMs). Previous research has explored the use of polydopamine (PDA) coatings for improving membrane performance, but applications in electrodialysis with nanoporous membranes remain limited. The authors position their work within this context, aiming to create a superior membrane system that overcomes the drawbacks of both nanofiltration and conventional electrodialysis.

The study involved the fabrication of surface-engineered, highly anion-conducting, anti-fouling TFC nanoporous polyamide (PA) membranes. A loose poly(piperazine amide) TFC NPM with a molecular weight cut-off (MWCO) of 682 ± 17 Da served as the substrate. A bio-inspired coating was applied using co-deposition of dopamine and polyethyleneimine (PEI) at pH 8.5 for varying durations (6–36 hours). The resulting membranes (NPM-1 to NPM-6) were characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), optical surface analysis, electrokinetic analysis, and electrical resistance measurements to assess their morphology, chemical composition, surface properties (hydrophilicity, surface charge), and ion conductivity. Pressure-driven filtration experiments were performed using various antibiotics (ceftriaxone sodium, cefotaxime sodium, carbenicillin disodium, and ampicillin sodium) and NaCl solutions to evaluate the membranes' selectivity. Rejection and selectivity were calculated using standard equations. Constant-volume nanofiltration-based diafiltration was conducted to assess the fractionation capability under pressure. Electrodialytic separation performance was investigated using a custom-designed electrodialysis cell. The membranes served as ACMs replacing AEMs in a conventional electrodialysis setup. Desalination efficiency, energy consumption, and antibiotic recovery were evaluated in pure NaCl solutions and antibiotic/NaCl mixed solutions. Fouling propensity was investigated through an 18-cycle electrodialytic separation operation. Molecular dynamics (MD) simulations were conducted to explore the mass transport mechanism of the PDA/PEI-coated TFC NPMs in electrodialysis. Commercial AEMs were used for comparison.

Co-deposition of dopamine and PEI effectively tuned the surface properties of the TFC NPMs, increasing hydrophilicity and decreasing negative surface charge. This reduced the specific areal electric resistance, enhancing ion conductivity. The modified membranes exhibited high rejection of organics and decreased rejection of NaCl under pressure-driven filtration, leading to enhanced selectivity between organics and NaCl. Constant-volume nanofiltration-based diafiltration showed high desalination efficiency but moderate losses of antibiotics due to their permeation. Electrodialysis using the modified membranes as ACMs demonstrated exceptional desalination efficiency (>99.5%) for pure NaCl solutions and high desalination efficiency (>99.3%) with high antibiotic recovery (>99.1%) in antibiotic/NaCl mixed solutions. The membranes showed negligible fouling after 18 cycles of electrodialytic separation. MD simulations revealed that the applied electric field facilitated the passage of Cl- ions through the membrane while retaining ampicillin ions due to size exclusion. In comparison to commercial AEMs, the novel membrane showed significantly reduced fouling in long-term electrodialysis operation, with superior desalination and organic recovery.

The study successfully addressed the limitations of conventional nanofiltration and electrodialysis for treating saline organic-rich wastewaters. The novel PDA/PEI-coated TFC NPMs demonstrated significant improvements in both pressure-driven and electro-driven separation processes. The enhanced charge shielding effect, coupled with the nanoporous structure, allowed for fast anion transfer while effectively retaining organics, resulting in highly efficient fractionation. The negligible fouling propensity further underscores the practicality of this membrane for long-term operation. The MD simulations provided a mechanistic understanding of the ion transport behavior, supporting the experimental findings. The results demonstrate a significant advancement in membrane technology for resource recovery from complex waste streams.

This research presents a novel design strategy for high-performance anion-conducting membranes utilizing a bio-inspired PDA/PEI coating on TFC nanoporous membranes. The membranes exhibit unprecedented electrodialytic fractionation of organics and NaCl with negligible fouling, significantly outperforming commercial AEMs. Future research could focus on scaling up the membrane production process and exploring the application of this technology to various types of saline organic-rich wastewaters beyond those tested in this study.

The study focused on specific model antibiotics and NaCl. The generalizability of the findings to other organic compounds and salt mixtures needs further investigation. The long-term stability of the membrane under various operational conditions (e.g., different current densities, varying concentrations of organics) requires more extensive testing. The cost-effectiveness of the modified membranes compared to commercial AEMs needs further evaluation.