Ionic covalent organic framework engineered super-charged thin-film composite polyamide membrane for desalination

The development of highly charged polyamide PA membranes to enhance the separation performance of nanofiltration membranes is an area of active research. In this study, negatively charged covalent organic framework (COF) nanosheets with a superior density of sulfonic groups and piperazine-2-carboxylic acid (CPIP) were utilized to elevate the surface charge density of the thin-film composite PA membrane. A super-charged membrane surface with a high negative charge density of-6.4 mC m- 2 was obtained, exhibiting the lowest reported isoelectric point of 1.89. Moreover, the COF nanosheets effectively manipulated the storage and diffusion of CPIP monomers, resulting in an enhanced crosslinking degree of the PA membrane. Consequently, both the Donnan effect and size sieving effect were intensified, endowing the resultant membrane with Na2SO4 rejection of 95.3 % and water permeance up to 71.0 L m-2h- 1 bar- 1, surpassing most of the state-of-the-art nanofiltration membranes. This study inspires the rational design of super-charged nanofiltration membranes for efficient desalination.