Pretreatment of bamboo with choline chloride-lactic acid integrated with calcium chloride hydrates deep eutectic solvent to boost bioconversion for ethanol production

Abstract

Deep eutectic solvent has been recognized as a green and nascent solvent to overcome the recalcitrance of lignocellulosic biomass for effective valorization. In this study, choline chloride (ChCl) and lactic acid (LA) at molar ratio of 1:2 integrating with 1–10 wt% calcium chloride hydrates (CaCl2·6H2O) deep eutectic solvents (DESs) were applied to pretreat bamboo. With increasing of calcium chloride hydrates dosage, maximum removals of lignin and xylan were obtained as 84.7% and 91.0%, respectively. DESs pretreatments also induced degradation of cellulose, declining CrI, DP and hydrophobicity of cellulosic samples. With change of structural features, enzyme accessibility and affinity of substrates increased and prompted bioconversion of bamboo. Feasible bioconversion was achieved by pretreating bamboo with ChCl-LA-5% CaCl2∙6 H2O at 120 ºC for 3 h. 0.149 g/g glucose and 0.128 g/g ethanol were produced from the substrate via enzymatic hydrolysis and simultaneous saccharification and fermentation, respectively. Pearson correlation coefficients (PCCs) indicated that lignin and xylan removal, enzyme accessibility, DP and hydrophobicity showed greatly strong relationship between bioconversion of substrate (all PCCs>0.8). Ratio of crystalline index to cellulose content showed strong relationship between bioconversion with − 0.71 and − 0.72 PCCs for enzymatic digestibility and ethanol production, respectively. Relationship between enzyme adsorption and bioconversion of substrates was moderate strength (0.57 and 0.42 for enzymatic digestibility and ethanol production, respectively). The recycling experiment indicated that ChCl-LA-5% CaCl2∙6 H2O retained acceptable pretreatment performance after twice recycles. Taking all cycles of ChCl-LA-5% CaCl2∙6 H2O into consideration, the solid to liquid ratio reduced from 1:20–1:7.35.