Bioinspired lignin-based loose nanofiltration membrane with excellent acid, fouling, and chlorine resistances toward dye/salt separation

Exploring green, renewable, and low-cost raw materials has great significance on developing high-performance loose nanofiltration (LNF) membranes using a scalable fabrication approach. Lignin is the second most abundant biomacromolecule in the botanic field. In this report, a lignin-based LNF membrane is successfully fabricated using alkaline lignin and sodium ligninsulfonate as raw materials and dopamine (DA) as a surface modifier by a brush assisted layer-by-layer assembly. The driving forces for the assembly are hydrogen bonds between hydrophilic groups and π-π stacking in phenyl groups. The experimental results show that the membrane possesses ultrahigh pure water permeability (65.2 LMH bar−1), and high dye rejections (100% for brilliant blue R (BBR), 96.1% for congo red (CR)) and salt permeations (85.1% for NaCl, 96.7% for MgCl2), exhibiting satisfactory dye/salt separation performance. The separation process is dominated by the synergistic action of the size exclusion and Donna repulsion between charged dye clusters and the membrane surface. The separation selectivity of the membrane can keep stable accompanied by a small permeate-flux fluctuation during immersed in HCl solution for 7 days. The membrane also shows low flux decline and excellent flux recoverability during a three-cycle CR fouling process. Moreover, the separation capability and permeate flux only show small fluctuations under 5500 ppm·h chlorination strength (pH = 5). The DA surface modification can significantly enhance the membrane permeability, fouling and chlorine resistances. The membrane possesses satisfactory performance stability on a long-term running process for the CR/NaCl separation.