Graphene Oxide Membranes and Membrane Cascades for Lignin Separation and Fractionation from Biorefinery Streams

Abstract Reduced graphene oxide (rGO) membranes are increasingly considered promising for energy-efficient and cost-effective fractionation and separation of diverse and complex biomass mixture resource streams, including kraft black liquor and other biorefinery feedstock/byproduct streams. We have fabricated rGO membranes with a range of molecular weight cutoffs (MWCOs) from 300 Da to 700 Da, and employed them in two-stage membrane cascades to produce lignin fractions of different molecular weight (Mw) distributions from liquors generated from kraft and soda pulping of softwood and grass, respectively. We present a detailed exploration of the lignin separation characteristics of tailored rGO membranes, as well as a detailed analysis of the resulting lignin fractions, including the molecular weight distributions, lignin recovery, and other parameters. A higher-MWCO membrane in the first stage resulted in a retentate product rich in high-Mw lignin (>1.4 kDa), while a lower-MWCO membrane in the second stage, using the first-stage permeate, produced two products: medium-Mw lignin (0.6−1.4 kDa) and a low-Mw lignin (