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According to the Freeman theory, increasing the backbone stiffness to some extent is an effective way to obtain the remarkable membrane with both high permeability and selectivity. In this work, a high performance nanofiltration (NF) membrane was prepared via tailoring the polyester/polyamide backbone stiffness. The flexible monomer-piperazine (PIP) and the rigid monomer-bisphenol F (BPF) were selected as the aqueous phase monomers, and they were interfacial reacted with trimesoyl chloride (TMC) on the polyethersulfone (PES) UF membrane to generate the polyester/polyamide layer. By mixing PIP with BPF, both of the polyester/polyamide backbone stiffness and microporosity degree enhanced. As a result, the PIP/BPF NF membrane possessed a 2.6-fold pure water flux (PWF) of the PIP NF membrane (the salt rejection remained at the same level), and 4.8-fold PWF of the BPF NF membrane. Our results revealed a novel and facile way to improve the NF membrane performance. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the temperature and time of the heat curing process have important effects on the nanofiltration (NF) membrane structure and properties. However, the mechanism of the relative humidity (RH), as one of the important factors in the heat curing of NF membranes, is still unclear. To analyze the specific effect of RH, we compared the heat curing results of the NF polyamide (PA) membranes fabricated with different monomer concentrations at various RH. The nanofiltration performance of the membranes appeared to be non-uniform when the RH was in the range of 40 %–60 %. Within the appropriate RH range, the rejection of Na 2 SO 4 was preserved at about 98.5 % for the manufactured NF membranes. At high concentrations, the increasing RH created a discontinuous condensate layer on the PA layer surface, which affected the morphology and physicochemical properties of the NF membrane causing a doubling of pure water permeability (PWP) to 8.33 L m−2 h−1·bar−1. At low concentrations, the PA layer was not significantly changed with the increase of RH, but the evaporation of water from the smaller capillary pores in its PES substrate was slowed down, which reduced the capillary stress on the substrate and prevented the collapse of the substrate pore size, resulting in a nearly 20 times increase in its PWP to 39.7 L m−2 h−1·bar−1. This work reveals the effect of RH during heat curing and gives significant guidance to the design and synthesis of high-performance NF membranes. [Display omitted] • The effect of relative humidity during heat curing on TFC nanofiltration membranes was investigated. • Capillary stress was introduced to reveal the mechanism of nanofiltration membranes flux variation. • The surface morphology of nanofiltration membranes changed with the relative humidity. • The relative humidity affects the nanofiltration membrane uniformity. [ABSTRACT FROM AUTHOR]

By causing the water molecule vibration to generate heat, microwave has become a rapid and efficient heating method which is widely used in industrial production. Herein, microwave heating assistant interfacial polymerization (IP) process was innovatively applied to fabricate polypiperazine-amide nanofiltration (NF) membrane for the first time. With microwave heating, a uniform and faultless polyamide (PA) layer could be generated with low monomer concentration. The pure water flux (PWF) of the NF membrane fabricated with microwave heating was 157.1 L m-2 h-1 with the Na 2 SO 4 rejection of 97.7% under 0.6 MPa. Meanwhile, such NF membrane showed extraordinary selectivity to Cl- and SO 4 2− for a series of mixing solution with different concentrations. This new insight into microwave heating assistant IP provides a feasible method for the high-permselectivity NF membranes fabrication. Image 1 • The microwave heating was applied for the IP process to fabricate TFC NF membrane for the first time. • The uniform and faultless PA layer could be generated with low monomer concentration with microwave heating. • The NF membrane showed good permeability and rejection and high selectivity to Cl- and SO 4 2−. [ABSTRACT FROM AUTHOR]

This study investigated the chemical stability differences between dry and wet nanofiltration membranes when immersed in strong acid and base environments. Systematic experimental studies were conducted on both types of nanofiltration (NF) membranes, supplemented by Molecular Dynamics (MD) simulations and Density Functional Theory (DFT) calculations. The results indicate that wet NF membranes, soaked in deionized water for a week, exhibit higher chemical stability compared to dry membranes treated only with air. Traditional air heating causes membrane pore contraction and excessive crosslinking. However, after a week of immersion in water, the polyamide (PA) layer gradually becomes a loose and well-extendable PA layer. MD simulations revealed that H+ and OH− ions possess lower diffusion coefficients and extended residence times in dry NF membranes, making them more prone to damage. Additionally, DFT calculations disclosed that the Gibbs free energy (ΔG) for amide bond hydrolysis in wet NF membranes, in the presence of water molecules, is elevated, significantly delaying their hydrolysis rate under extreme conditions. This study highlights the pivotal role of membrane wetness in the chemical durability of NF membranes and offers a novel perspective for improving membrane lifespan and efficiency. [Display omitted] • Wet NF membranes show markedly higher chemical stability in acidic and basic environments. • MD simulations indicate lower ion diffusion and longer residence times in dry NF membranes. • DFT calculations reveal slower amide bond hydrolysis in wet membranes, enhancing durability. • Study offers new insights for extending NF membrane lifespan and improving filtration efficiency. [ABSTRACT FROM AUTHOR]

Recent research has focused on the acid resistance of polyamide nanofiltration (NF) membranes, a critical aspect for their stability during practical applications. However, the impact of different acids, particularly the role of various anions, on NF membrane acid resistance remains unexplored. Our research systematically evaluates NF membrane performance and property alterations under various acids at identical pH levels, supplemented by density functional theory (DFT) simulations. Results reveal that membranes treated with H₃PO₄ retain desirable NF performance, in stark contrast to those treated with H₂SO₄ and HCl, which undergo significant degradation. Specifically, H₃PO₄-treated membranes showed only a 4.2 % decline in Na₂SO₄ rejection, compared to a dramatic 83.3 % decline for HCl-treated membranes under identical pH. This degradation is linked to the acids infiltrating the polyamide structure. The membrane's enhanced resistance to H₃PO₄ is primarily due to the strong negative charge of phosphate ions, influenced by the Donnan effect, which prevents deep penetration into the membrane. This is further supported by DFT simulations, which reveal that the interaction energy between the membrane surface and phosphate ions is over twice as high as with chlorine ions, highlighting the importance of ion-specific interactions. This research highlights the crucial role of anions in acidic environments for the maintenance and operation of NF membranes in industrial applications. It provides key insights for enhancing membrane efficiency and longevity under acidic conditions. [Display omitted] • Filled the research gap in polyamide membranes resistance to different acids. • Revealed the variance in acid resistance for anionic charge-induced Donnan effect. • Provides insights for optimizing acid utilization in industrial processes. [ABSTRACT FROM AUTHOR]

Nanofiltration (NF) membranes with both high permeability and salt rejection are in dire need for water treatment. Generally, heat treatment is an essential step for fabricating polyamide NF membranes. Conventional heat treatment in an oven causes membrane pore shrinkage and over cross-linking, which often leads to a severe loss of permeation performance. Herein, we report a single-step method by heat-treating membranes in non-neutral solutions to avoid pore shrinkage and generate graded hydrolysis in membrane selective layer. As confirmed by the XPS result, the polyamide NF membrane fabricated via controlled hydrolysis possesses a gradient cross-linking degree vertically along the membrane surface. This optimal polyamide NF membrane had a superior pure water permeability (PWP) of 27.5 L m−2 h−1 bar−1 and high Na 2 SO 4 rejection of 98.5%. Such controlled hydrolysis approach provides novel strategy for fabricating high performance polyamide membranes. • Heat treatment in water was employed to avoid pore shrink and over cross-linking of the polyamide active layer. • The performance of NF membrane with gradient cross-linked structure can be easily tuned by adjusting the pH value of water. • The optimal membrane owns a superior pure water permeability of 27.5 L m−2 h−1 bar−1 and a high Na 2 SO 4 rejection of 98.5%. [ABSTRACT FROM AUTHOR]