Lignin extraction and fractionation from rice straw biorefinery residues

Crops straw contains many valuable natural compounds. Cellulose and hemicellulose in agricultural wastes have been widely used in biofuel production. However, the large-scale application of residue lignin from biorefinery is still an unsolved problem. Various impurities and heterogeneous lignin with a very complicated molecular weight distribution were generally found in the recovered raw solid products. In this study, brown solid residue from rice straw biorefinery was processed using membrane separation in combination with the soda-ethylene glycol (Soda-EG) process to extract high-purity lignin. At first, wet brown solid was mixed into an aqueous solution of sodium hydroxide and ethylene glycol for lignin fragmentation, depolymerization, and solubilization at elevated temperatures. Afterwards, microfiltration (MF) was applied to recover the dissolved lignin in the filtrate from the extracts. As high as 90% of the lignin in the brown solid could be recovered after acid precipitation of the filtrate. The highest lignin content in the precipitate product could be 90% (dry basis). The precipitate was re-dissolved at high pH aqueous solutions for further molecular weight fractionation by ultrafiltration (UF). The best rejection of high-molecular-weight (>1 × 105 g/mol) lignin was 77%, while low-molecular-weight (<3 × 103 g/mol) lignin was <13% with a MWCO of 10 kDa polysulfone UF membrane in a dead-end filtration system. The cross-flow operation with the same UF membrane showed that >80% lignin could be recovered from the retentate and the flux of permeate could be maintained after three operation-cleaning cycles. Overall, the combined MF-UF operation could be a viable method for obtaining high-value-added lignin products with a specific molecular weight range.