Multimode ultrasound and ternary deep eutectic solvent sequential pretreatments enhanced the enzymatic saccharification of corncob biomass

Abstract

Corncob (CB) is a valuable agricultural waste in high abundance. It is mainly composed of cellulose, hemicellulose and lignin that link in a complex interlocking matrix. The key to its utilization depends on separating cellulose from lignin and hemicellulose using an environment-friendly and effective pretreatment method to improve glucose yield. The study aimed to investigate the effect of sequential pretreatment of CB with ultrasound (US) in conjunction with ternary deep eutectic solvent (TDES). The US, type and molar ratio of TDES, temperature, and time were optimized. The results showed that under the optimum conditions, US (40 +60 kHz, 60 min) + TDES (ChCl-Gly-LA, 1:1:2, 120 ℃, 3 h), the removal of lignin and hemicellulose were 87.54 % and 67.00 %, respectively, and the recovery of cellulose was 82.18 %. After 72 h of enzymatic hydrolysis , the yield of glucose and xylose were 76.94 % and 41.26 %, respectively. Comparatively, pretreated CB yielded 61.58 % glucose and 26.90 % xylose higher than the untreated. Scanning electron microscopy revealed that the surface of the CB became cracked and rugged. X-ray diffraction showed that the CrI increased significantly from 36 % to 59 %. Fourier transform mid-infrared spectroscopy indicated significant changes in the characteristic functional groups of lignin and hemicellulose. Increases in Tonset, Tendset, Tp, Vmax and decreases in Char at 600 °C all corresponded to increases in thermal stability. These results further confirmed that TDES combined with ultrasonic pretreatment effectively removed lignin and hemicellulose. In conclusion, this approach could greenly and efficiently pretreat CB as a new method for biomass pretreatment. • Double hydrogen bond acceptor (donor) ternary pretreatment DES was studied. • The lignin removal efficiency of ternary DES was higher than binary DES. • TDES containing lactic acid reduced significantly the lignin content. • TDES combined with ultrasonic pretreatment increased lignin removal rate. • TDES combined with ultrasonic pretreatment enhanced sugar yields.