Fabrication of water permeation-favored nanofiltration membrane with incorporation of phosphonic acid groups and the application for sulfate retention

Despite significant advancements in nanofiltration (NF) technology, achieving a balance between high salt rejection and sufficient water permeability remains a critical challenge, thereby triggering tremendous efforts to develop performance-enhanced NF membranes. In this study, an improved interfacial polymerization method was used to introduce ethylenediamine tetramethylphosphoric acid (EDTMPA) into the aqueous phase and tetrabutyl orthotitanate (TBOT) into the organic phase. Consequently, the hydrophilic and negatively charged phosphonic acid groups were incorporated into the ultrathin separation layer of the fabricated NF membrane (NF-E-T) via the chemical bonding reaction between TBOT and EDTMPA, for the aim of enhancing salt rejection and water permeability. The physicochemical properties of the prepared membrane were analyzed using various characterization techniques, and molecular dynamic simulations were executed to validate the mechanisms underlying structural and performance changes of the membrane. The fabricated NF-E-T membrane exhibits a reduced separation layer thickness, enhanced hydrophilicity and slightly larger pore sizes, contributing to the improvement of membrane permeability; the increased negative charge in membrane surface and uniform pore distribution maintain the high salt rejection. Molecular dynamics simulations confirm that the incorporation of EDTMPA significantly enhances the diffusion rate of PIP and facilitates the formation of a thinner polyamide layer. The EDTMPA-TBOT complex promotes the diffusion of water molecules within the membrane matrix, thereby providing a favorable condition for the improvement of water permeability. Moreover, this membrane shows significant improvements in anti-fouling performance, oxidation stability, and long-term operation stability, achieving the favorable treatments for two types of industrial wastewater. This work will provide a firm guideline for developing high-performance nanofiltration membranes.