Comprehensive insights into hydrothermal pretreatment of rice straw from physicochemical structure, organic matter transformation and hydrolysate reuse

Anaerobic digestion is an effective means of biomass utilization from solid waste, but the complex structure and low biodegradability of lignocellulose severely limit the anaerobic degradation conversion efficiency. In this study, hydrothermal pretreatment combined with chemicals was conducted to investigate physicochemical structure, chemical composition and changes, and enzymatic hydrolysis performance of rice straw under various combination hydrothermal pretreatment. The transformation mechanism of main components during alkaline-thermal pretreatment and the potential reuse of hydrolysate were further explored. Results showed that the total sugar concentration after hydrothermal pretreatment was 1.49 times higher than that after dissolution pretreatment. Hydrothermal-KOH pretreatment could destroy the waxy and siliceous layer on the straw surface. The roughness increased from 0.525 to 1.170 nm, and the average pore size 6.20 to 8.99 nm. Alkaline-thermal pretreatment could facilitate lignin dissolution and retained more cellulose and hemicellulose in the solid phase, and sugar concentration after pretreatment could reach 40.82 g/L. Porosity was affected by the dissolution of chemicals, and the pore structure also affected subsequent enzymatic hydrolysis. Dissolution of cellulose and hemicellulose resulted in a reduction in particle size and the formation of micropores, while re-aggregation and precipitation of lignin after dissolution led to the pore collapse and an increase in pore size. Furthermore, the hydrolysates contained nutrients, humic acids, plant hormones, and micronutrients, which had the potential to be developed into foliar fertilizers and reused. The results of this study will provide new insights and technical support for high-value utilization of straw.