Application of new expansion pretreatment method on agricultural waste. Part I: Influence of pretreatment on the properties of lignin

Abstract

In the present work, a new expansion pretreatment was used to fractionate agricultural wastes (rice straw and rice husk) into lignin and carbohydrates. The expansion pretreatment takes the advantages of extrusion and milder steam explosion. The lignin fractions obtained from expanded rice straw and rice husk by mild alkaline extraction were characterized by gel permeation chromatography (GPC), sugar analysis, 2D NMR and derivatization followed by reductive cleavage (DFRC). Results showed that the expansion pretreatment resulted in higher extraction yields of lignin from pretreated rice straw than that from untreated control. The results from molecular weight measurement and DFRC analysis suggested that the condensation reaction could take place between lignin units during the expansion processing. 2D NMR spectra showed that guaiacyl (G) and syringyl (S) units were predominant in the lignin fractions, and a small amount of p-hydroxyphenyl (H) unit was also detected. It was also found that the flavones-type structure tricin was apparently present in lignin fractions from rice straw but not in rice husk. Considering the high extraction yields and purity of lignins from pretreated biomass, the expansion pretreatment could be an attractive method for fractionation of agricultural wastes.