Polytertiary amine nanofiltration membranes with exceptional alkali resistance for lignin/lye separation

Lignin, the second most abundant biomass resource, is widely utilized across various industries. Lye extraction is a prevalent approach for extracting lignin, but it needs to consume a lot of acid to neutralize the lye and precipitate the lignin products, resulting in a tremendous waste of acid and alkali. Direct separation of lignin and lye through nanofiltration technology can significantly reduce the consumption of acid and alkali. However, the development of nanofiltration membranes with sufficient alkali resistance for processing lignin and lye remains a challenge. Herein, we report the fabrication of polytertiary amine (PTA) nanofiltration membranes with exceptional alkali resistance via interfacial polymerization of piperazine and 1,3,5-tris-bromomethyl-benzene. PTA membranes maintained stable nanofiltration performance after immersion in alkaline solutions for 30 days and exhibited nearly unchanged lignin rejection even in NaOH solutions with concentrations of up to 5 mol L⁻¹. During the separation of lignin and lye, PTA membranes demonstrated high lignin rejection of 95 % and low NaOH rejection below 15 %, along with a high water permeance of 14 L m⁻² h⁻¹ bar⁻¹. Quantitative lignin/lye separation using the PTA membrane achieved 80 % recovery of the lye with a NaOH purity of 98.4 %, simultaneously concentrating the lignin sixfold. This work opens a new approach to developing durable nanofiltration membranes for sustainable lignin extraction and separation applications in alkaline environments.