Assisted Single-Step Acid Pretreatment Process for Enhanced Delignification of Rice Straw for Bioethanol Production

Abstract

Dilute acid pretreatment of lignocellulosic biomass at higher temperatures (>160 °C) solubilizes/removes hemicelluloses (xylan, arabinan, mannan, galactan) and acid-soluble lignin (ASL), but, it does not remove acid-insoluble lignin (AIL). During acid pretreatment, the condensation and redeposition of coalesced lignin over cellulose fibers reduces the access of cellulose to cellulases. For higher delignification, various multistage pretreatments are available, however, all these are energy/chemical intensive methods. Therefore, an effective pretreatment which provides increased cellulose accessibility by enhanced removal of hemicelluloses and lignin in a single-step would be preferred. Our investigation reports a novel assisted single-step acid pretreatment (ASAP) process for enhanced delignification of biomass under acidic conditions. Pretreatment of rice straw (particle size of 20 mm) with H2SO4 (0.75% v/v) + boric acid (1% w/v) + glycerol (0.5% v/v) (solid/liquid (S/L), 1:5) at 150 °C for 20 min removed hemicelluloses completely, 44% of the lignin, and ∼48.5% of the silica leaving a solid consisting of 69 ± 1.5% glucan, 0.7 ± 0.06% ASL, 20 ± 2.0% AIL, and 12 ± 1.5% silica. The C/L (cellulose/lignin) ratio of solids resulted from ASAP was found to be > 3.00, while it was < 2.00 for acid only and untreated solids. Enzymatic hydrolysis of ASAP treated biomass with enzyme loadings of 20 FPU g–1 at 15% (w/v) solids concentration gave about 72% glucan conversion to glucose. This amount of glucose was around 2.6 times higher than obtained with enzymatic hydrolysis of acid-only-pretreated solids and 4.2 times higher than untreated rice straw (control). Therefore, the assisted-acid pretreatment dramatically enhanced delignification of rice straw and thereby glucan-to-glucose conversion.