All-Lignin Polyelectrolyte Multilayers as Renewable and Biodegradable Nanofiltration Membranes

Polyelectrolyte multilayers have proven to be versatile materials for the fabrication of nanofiltration membranes with a wide range of properties and applications. A problem of these membranes is that they are made of nonrenewable and nonbiodegradable fossil resources, rendering them unsustainable for the future. To solve this, we present lignin as a renewable and biodegradable alternative for the fabrication of polyelectrolyte multilayer membranes (PEMMs). Here, lignosulfonate was used as a polyanion in combination with modified Kraft lignin as a polycation in a layer-by-layer self-assembly process to coat hollow fiber support membranes to obtain so-called all-lignin PEMMs. The PEMMs showed loose nanofiltration properties (molecular weight cutoff > 1 kDa, MgSO₄ retention 20%) that could easily be fine-tuned by changing the ionic strength of the coating solutions. Furthermore, the lignin PEMMs have excellent stability in saline solutions of up to 5 M NaCl and were stable in a pH range from 1 to 11. Additionally, the lignin retained its biodegradable properties in the presence of laccase enzymes after forming a PEMM. Our results indicate that lignins are a suitable candidate for replacing fossil-based polyelectrolytes for the fabrication of chemically stable, renewable, and biodegradable PEMMs.