Synthesizing high performance robust PSf loose nanofiltration membranes through nanobubble-assisted pore-forming

Abstract

Loose nanofiltration (LNF) membranes have extensive applications in precision separation processes, however, synthesizing highly permselective LNF membranes is still a considerable obstacle. Nanobubble-assisted pore-forming method was adapted to prepare a highly perm-selective LNF membrane for the first time in this work, using polysulfone (PSf) as the membrane-forming material, azodicarbonamide (AC) as the reactive pore-forming agents, and NaOH solution as the coagulation and immersion baths. By leveraging nano-bubbles produced through the reaction between AC and NaOH without adding other pore-forming agents, the membrane-forming process of PSf was regulated. It was found that the PSf LNF membranes prepared in NaOH-solution coagulation and immersion baths exhibited significantly enhanced permeation performance without compromising the rejection. These results suggested that the coagulation and immersion post-treatment processes under NaOH solution had a great effect upon the structure and the property of the membranes. The LNF membrane prepared from 20 wt% PSf casting solution with 8 wt% AC achieved a permeation flux of 188.7 L m−2 h−1, and a Congo red (CR, Mw =696 Da) rejection of over 99 %, while a rejection of NaCl below 7 %, showing a good separation for the CR/NaCl mixed solution. Additionally, the membrane exhibited favorable mechanical properties, with a tensile strength as high as 6.5 MPa and a fracture elongation reaching as much as 32.3 %. This work provided a novel approach for producing high-performance LNF membranes in treating dye wastewater.