A loose polyethersulfone hybrid nanofiltration membrane incorporated with polyethyleneimine-decorated silica nanoparticles for highly-efficient dye/salt separation

Loose nanofiltration (LNF) is a newly-developing technology for desalting dye-containing saline effluents, but suffers from membrane materials with low permeance and poor dye/salt separation selectivity. Herein, a hybrid polyethersulfone (PES) LNF membrane for dye/salt separation was prepared via a nonsolvent-induced phase inversion method, which was carried out using polyethyleneimine (PEI)-modified silica nanoparticles as a dopant. This design not only featured a hybrid PES LNF membrane with high porosity and small mean pore size, but also partially neutralized its charge and enhanced its hydrophilicity. Hybrid PES LNF membrane from a 21- wt% PES solution with 3- wt% PEI-decorated silica nanoparticles addition was demonstrated to achieve high water permeance (92.0 L m−2h−1 bar−1) and dyes retention ratio (98.4 % for Congo Red, 98.8 % for Nile Blue A, and 95.1 % for Methyl Blue) accompanied with fully-pass for various salts (Na2SO4, NaCl, MgCl2 and CaCl2). In addition, the hybrid membrane exerted excellent resistances to strong acidic, oxidative and hot water (90 °C) environments and exhibited a pH-sensitive permeability. Multicycle test revealed that the hybrid membrane had high permeance recovery ratio (>92.8 %) and maintained its high dye retention and complete salt permeation even after 10 cycles, manifesting its outstanding fouling resistance and recyclability. Besides, the hybrid membrane repelled 92.6 % of dyes from a simulated wastewater consisting of multiple industry-applied direct dyes and salts after 12 h consecutive test, but allow 100 % of mixed salts passing through the membrane, proving its practical applicability to industrial dyeing wastewater. This study provided a simple and economically feasible route to high-performance LNF membrane production for dye/salt separation, thus popularizing LNF process in dyeing wastewater treatment, organic products purification, and resource recovery.