Loose nanofiltration-based electrodialysis for highly efficient textile wastewater treatment

Abstract

Effective fractionation of dyes and salts (NaCl) potentially paves a new path to sustainable treatment of textile wastewater. Herein, a loose nanofiltration (NF) membrane was innovatively used as the anion conductive membrane in electrodialysis (ED) for dye and salt separation. The NF membrane with a molecular weight cut-off of 678 Da showed a sufficient retention for reactive dyes with small molecular weights (<627 Da) predominantly based on synergistic effect of steric repulsion and Donnan repulsion. Simultaneously, the NF membrane had a considerably fast transport for anions (i.e., Cl−), as its loose surface structure created extra nano-channels for ion transfer. The anion conductive loose NF membrane based ED process yielded excellent separation performance in a one-step dye/NaCl fractionation. Specifically, a 98.9% desalination efficiency was obtained by the loose NF-based ED process along with 99.4% dye recovery. Additionally, an extremely low fouling propensity was observed during a 5-cycle ED operation, as the negative surface charge of the loose NF membrane served as an intrinsic barrier to prevent the dye species from entering the membrane pores through electrostatic repulsion. This study demonstrated a desirable potential of loose NF membranes as anion conductive membranes in ED for resource recovery from highly-saline wastewater.