A novel polyester-amide loose composite nanofiltration membrane for effective dye/salt separation: The effect of long molecule on the interfacial polymerization

Fabrication of hollow fiber composite membranes via opposite transmission reaction method for dye/salt separation

The separation layer prepared by the conventional coating-crosslinking method is typically thick and prone to forming defective macropores, significantly affecting the water permeability and dye/salt separation performance of membranes. This work presented a novel method to prepare hollow fiber composite membranes for dye/salt separation based on the opposite transmission reaction of crosslinker. In this method, the macromolecule in situ reacted with a small-molecule crosslinker at the openings of membrane pore channels, forming a separation layer with discontinuous sheet-like and granular structure. Compared to the conventional forward coating-crosslinking method, the separation layer prepared by the opposite transmission reaction method exhibited an ultra-thin thickness of 29.1 nm. Consequently, the composite membrane exhibited a high water permeability of 72.7 L·m-2·h-1·bar-1, which was 2.3 times higher than that of conventional methods. Moreover, the prepared composite membrane presented a more uniformed pore structure, completely retaining the VBB (100%) with a low Na2SO4 rejection of 4.3%, demonstrating excellent dye/salt separation performance. Additionally, the prepared composite membrane exhibited superior anti-fouling properties compared to that prepared by the conventional method. Therefore, the opposite transmission reaction method proposed in this study held promising applications in the preparation of hollow fiber composite membranes for efficient dye/salt separation.

An Innovative Surface Modification Technique for Antifouling Polyamide Nanofiltration Membranes

In this study, we developed a novel surface coating technique to modify the surface chemistry of thin film composite (TFC) nanofiltration (NF) membranes, aiming to mitigate organic fouling while maintaining the membrane's permselectivity. We formed a spot-like polyester (PE) coating on top of a polyamide (PA) TFC membrane using mist-based interfacial polymerization. This process involved exposing the membrane surface to tiny droplets carrying different concentrations of sulfonated kraft lignin (SKL, 3, 5, and 7 wt %) and trimesoyl chloride (TMC, 0.2 wt %). The main advantages of this surface coating technique are minimal solvent consumption (less than 0.05 mL/cm2) and precise control over interfacial polymerization. Zeta potential measurements of the coated membranes exhibited enhancements in negative charge compared to the control membrane. This enhancement is attributed to the unreacted carboxyl functional groups of the SKL and TMC monomers, as well as the presence of sulfonate groups (SO3) in the structure of SKL. AFM results showed a notable decrease in membrane surface roughness after polyester coating due to the slower diffusion of SKL to the interface and a milder reaction with TMC. In terms of fouling resistance, the membrane coated with a polyester composed of 7 wt % SKL showed a 90% flux recovery ratio (FRR) during Bovine Serum Albumin (BSA) filtration, showing a 15% improvement compared to the control membrane (PA). PE-coated membranes provided stable separation performance over 40 h of filtration. The sodium chloride rejection and water flux displayed minimal variations, indicating the robustness of the coating layer. The final section of the presented study focuses on assessing the feasibility of scaling up and the cost-effectiveness of the proposed technique. The demonstrated ease of scalability and a notable reduction in chemical consumption establish this method as a viable, environmentally friendly, and sustainable solution for surface modification.

Low Fouling Polyelectrolyte Layer-by-Layer Self-Assembled Membrane for High Performance Dye/Salt Fractionation: Sequence Effect of Self-Assembly

Layer-by-layer (LbL) self-assembly of oppositely charged polyelectrolytes (PEs) is usually performed on a conventional ultrafiltration base substrate (negative zeta potential) by depositing a cationic PE as a first layer. Herein, we report the facile and fast formation of high performance molecular selective membrane by the nonelectrostatic adsorption of anionic PE on the polyvinylidene fluoride (PVDF, zeta potential -17 mV) substrate followed by the electrostatic LbL assembly. Loose nanofiltration membranes have been prepared via both concentration-polarization (CP-LbL, under applied pressure) driven and conventional (C-LbL, dipping) LbL self-assembly. When the first layer is poly(styrene sodium) sulfonic acid, the LbL assembled membrane contains free -SO3- groups and exhibits higher rejection of Na2SO4 and lower rejection of MgCl2. The reversal of salt rejection occurs when the first layer is quaternized polyvinyl imidazole (PVIm-Me). The membrane (five layers) prepared by first depositing PStSO3Na shows higher rejection of several dyes (97.9 to >99.9%), higher NaCl to dye separation factor (52-1800), and higher dye antifouling performance as compared to the membrane prepared by first depositing PVIm-Me (97.5-99.5% dye rejection, separation factor ∼40-200). However, the C-LbL membrane requires a longer time of self-assembly or higher PE concentration to reach a performance close to the CP-LbL membranes. The membranes exhibit excellent pressure, pH (3-12), and salt (60 g L-1) stability. This work provides an insight for the construction of low fouling and high-performance membranes for the fractionation of dye and salt based on the LbL self-assembly sequence.

Energy-efficient trehalose-based polyester nanofiltration membranes for zero-discharge textile wastewater treatment

Recovery of water, salts, and hazardous dye from complex saline textile wastewater faces obstacles in separating dissolved ionic substances and recovering organic components during desalination. This study realized the simultaneous fractionation, desalination, and dye removal/recovery treatment of textile wastewater by using trehalose (Tre) as an aqueous monomer to prepare polyester loose nanofiltration (LNF) membrane with fine control microstructure via interfacial polymerization. Outperforming the NF270 commercial membrane, the Tre-1.05/TMC optimized membrane achieves zero-discharge textile wastewater treatment, cutting energy consumption by 295% and reducing water consumption by 42.8%. This efficiency surge results from remarkable water permeability (130.83 L m-2 h-1 bar-1) and impressive dye desalination (NaCl/ Direct Red 23 separation factor of 275) of the Tre-1.05/TMC membrane. For a deeper comprehension of filtration performance, the sieving mechanism of polyester LNF membranes was systematically elucidated. This strategic approach offers significant prospects for energy conservation, carbon emission mitigation, and enhanced feasibility of membrane-based wastewater treatment systems.

Interfacial Polymerization of Highly Active Thiolated Cyclodextrin for the Fabrication of a Loose Nanofiltration Membrane with a Chlorine-Resistant Poly(thioester) Linkage

Cyclodextrins have been frequently used to fabricate membranes via interfacial polymerization (IP). However, the relatively low reactivity of pristine cyclodextrins often induces a lower cross-linking density and unsatisfactory separation performance. In this work, to introduce a highly active thiolated β-cyclodextrin (CD-SH) monomer into IP progress, we constructed a dense and porous poly(thioester) linkage on a commercial membrane surface with loose nanofiltration by IP of CD-SH and trimesoyl trichloride (TMC) as the monomer in an aqueous phase and organic phase separately for the first time. Furthermore, the reactivity of CD-SH has been fully demonstrated by the two-phase IP aiming at unmodified β-CD, a CD-SH/TMC freestanding membrane with a thicker interfacial layer and a smoother surface, and a PAN/CD-SH membrane with a narrow porous distribution. The composite membrane possessed superior separation performance for a high rejection (83.1-99.6%) of different anionic dyes and a low rejection (<20%) of salts, as well as a high-efficiency sieving ability of dye/dye and dye/salt mixtures. The membrane with a poly(thioester) selective layer could steadily operate in a long-term filtration test and exhibit great stability, chloride-resistance performance, and recyclability.

Loose Nanofiltration Membrane Incorporating CeZnFe Layered Double Hydroxide with Enhanced Dye/Salt Separation Performance and Self-Cleaning Ability

The high-salinity wastewater from the textile industry faces a significant challenge in effectively separating dyes and salts. In this study, a CeZnFe-layered double hydroxide (LDH)-incorporated nanofiltration (LNF) membrane was fabricated using the conventional interfacial polymerization (IP) technique to fractionate dyes and salts within the wastewater. The impact of CeZnFe LDH on various aspects of membrane performance was examined, including water flux, dye removal efficiency, dye/salt separation capability, self-cleaning ability, and membrane integrity. The addition of LDHs resulted in improved membrane surface hydrophilicity, thereby enhancing water flux. The optimized TFN membrane (0.050 wt% LDH in PIP solution) significantly improved pure water flux, exceeding 150%. All TFN membranes exhibited excellent performance in dye and salt fractionation (93% for Congo red, 2.6% for NaCl, and 40.7% for Na2SO4). Also, excellent self-cleaning ability was observed for the optimized membrane, exhibiting a remarkable water flux recovery rate after three operation cycles. Moreover, including CeZnFe LDH in the optimized TFN membrane played a significant role in enhancing membrane integrity. This study provides new inspiration for fabricating self-cleaning loose NF membranes using CeZnFe LDH for effective dye/salt separation.