Efficient removal of lignin from vegetable wastes by ultrasonic and microwave-assisted treatment with ternary deep eutectic solvent

Abstract

A low-cost and green biorefinery will increase the economy and revenue from cellulose food waste biomass. Ultrasound and microwave-assisted Ternary deep eutectic solvents (TDES) pretreatment were developed to deconstruct the recalcitrant structure of garlic skin (GS) and green onion root (GOR) for further lignocellulose fractionation and cellulose enzymatic hydrolysis. In this study, the TDES consists of choline chloride (ChCl) as hydrogen bond acceptor, glycerin, oxalic acid, urea and metal chloride as hydrogen bond donor. The results showed that ultrasound (room temperature, 30 min) and microwave (80 °C, 20 min)-assisted TDES pretreatment resulted in notable removal of lignin. Under the optimum conditions (ultrasound frequency: 20+28+40 kHz; TDES: ChCl-Gly-AlCl3·6H2O (1:2:0.2); heating method: ultrasound microwave), the lignin removal rate of GS and GOR reached 90.14 and 92.34 %, respectively. Simultaneously, the cellulose content of GS and GOR was increased by 48.52 and 50.28 %, respectively, which was significantly enhanced by 1.94 and 1.45 times, respectively, compared to that of the original biomass without pretreatment. Besides, the content of the reducing sugar in the enzymatic hydrolysate of GS and GOR was 2.63 and 3.11 %, respectively. All these structural characteristics (FT-IR, XRD and SEM) confirmed the efficient fractionation of GS and GOR by ultrasonic and microwave pretreatment. Overall, this work demonstrated a novel and high-performance pretreatment method for eff ;ective lignocellulose deconstruction and cellulose digestibility.