Fine-tuning the architecture of loose nanofiltration membrane for improved water flux, dye rejection and dye/salt selective separation

Abstract

Herein, loose nanofiltration membranes were prepared using non-solvent induced phase separation (NIPS) method. In NIPS, the non-solvent controls the morphology of the skin layer of the asymmetric membranes formed, while the solvent tunes the morphology of the lower layer. The membranes were made from polyethersulfone in n-methyl pyrrolidone (NMP) with the addition of dicarboxylic acids (glutaric acid, azelaic acid). Aliphatic alcohols (1-butanol, 1-octanol) were added to the solvent to tailor the skin layer pore morphology. Cross-sectional electron microscope images revealed the acids created nano-voids and shorter micro-voids. The addition of alcohols extended the micro-voids and improved symmetry. Pore size distribution results showed that the addition of the alcohols did indeed reduce the pore size as hypothesised. The performance of the modified polyethersulfone membranes was greatly improved in terms of pure water flux and salt passage without sacrificing the dye rejection. The loose nanofiltration membranes prepared in this study showed good dye/salt separation capability compared to most of the results reported in literature. The membranes were compacted with de-ionised water at an applied pressure of 4 bar for 5 h and rejections tests were done at 2 bar. It is hereby reported that the membranes achieved a high salt passage (Na2SO4 rejection varying between <3% and ~12%), as well as sustainable high Congo red rejection of 97%.