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In this study, we utilized the NaOH solution as a heat-treating medium to replace the conventional oven-heating process after interfacial polymerization for the RO membrane fabrication. XPS result confirmed that the nascent RO membrane possessed a gradient cross-linking density along the polyamide layer depth, which is relatively loose on top and dense facing the substrate. The top loose polyamide could be exfoliated under NaOH treatment. The TFC membrane treated in a 1.5 mol L-1 NaOH solution exhibited a pure water flux nearly 3 times higher than that heat-treated in a water bath and also maintained a high NaCl rejection. In consideration of the practical operation process, membrane stability was evaluated, and the results showed that the membrane heat-treated by 1.5 mol L-1 NaOH solution possessed good long-term and pH stability. Thus, the facile heat-treating strategy provides a new path for fabricating high flux TFC-RO membranes. • Novel heat-treating medium of NaOH solution was utilized to fabricate the novel RO membrane. • The top loose layer of the nascent polyamide membrane could be exfoliated under NaOH treatment. • The modified heat-treating strategy showed great superiority over traditional oven-heating process. [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]

In this study, the impacts of heat curing conditions on the morphology and permeability of polyamide nanofiltration (NF) membranes were comprehensively investigated. The surface chemical composition, morphology, charge property, hydrophilicity and NF performance of the resulting membranes were studied. In order to preserve the instantaneous polyamide layer morphology, freeze drying process was employed after heat curing. Results showed that the surface roughness of the polyamide selective layer increased first and then decreased with curing time. The tunable morphology of polyamide NF membranes can be acquired by adjusting the curing temperature and time. The nascent membrane without heat curing exhibited excellent pure water permeability of 16.7 L m−2 h−1 bar−1 and maintained Na 2 SO 4 rejection of 97.4%. Increasing the curing time, the permeability of the NF membranes was greatly diminished. This work provided novel viewpoint about the effects of heat curing on morphology and permeability for high-performance polyamide NF membrane. • Interfacial polymerization is terminated by liquid nitrogen to preserve the instantaneous polyamide layer morphology. • Polyamide layer roughness increases first and then decreases as a function of the heat curing time. • The nascent membrane exhibited excellent pure water permeability of 16.7 L m−2 h−1 bar−1 and Na 2 SO 4 rejection of 97.4%. [ABSTRACT FROM AUTHOR]