Self-supported hydrogel loose nanofiltration membrane for dye/salt separation

A novel self-supported hydrogel loose nanofiltration (LNF) membrane was prepared by using sodium alginate as matrix, large molecular weight PEG as pore-making agent, cellulose nanofibers (CNF) and carboxylated multi-walled carbon nanotubes (MWCNT-COOH) as additives, in which free water evaporation ("cooking") technique followed by ionic cross-linking method was utilized. The "cooking" technique and the addition of CNF and MWCNT-COOH guaranteed the mechanical strength of the gel, and the synergistic effect of large molecular weight PEG and MWCNT-COOH made the gel form a loose structure. The resulting composite membrane possessed high water permeability of 51.6 Lm−2h−1bar−1. Meanwhile, based on the porous structure and the synergistic action of multi-ionic groups, the membrane exhibited a high dye rejection: Crystal violet (99.8%), Congo red (98.9%), Tartrazine (93.1%), Methylene blue (85.7%), while maintaining a rather low retention of salts: MgSO4 (11.6%), Na2SO4 (10.5%), MgCl2 (9.1%), NaCl (7.3%). Furthermore, the membrane exhibited outstanding separation efficiency towards the mixed dye/salt feed solution with special combination. In addition, the membrane not only showed excellent hydrophilicity and stain resistance, but also had good recycling effect. This study provides a simple and feasible method for the preparation of dye desalination membrane.