Free nitrous acid–ultrasound pretreatment enhances cellulose conversion by complementary physical and chemical deconstruction

Biorefining of cellulosic biomass has emerged as a promising strategy to address the global energy crisis and advance second-generation biofuel production. To overcome the inherent structural recalcitrance of cellulose, this study proposes a novel combined pretreatment strategy using free nitrous acid (FNA) and ultrasound (US) and systematically elucidates its underlying mechanisms. While US and FNA pretreatment individually improved substrate digestibility, their combination (FNA + US) produced the greatest enhancement, with the final total organic carbon concentration and cellulose conversion after 48 h reaching 2.6- and 2.4-fold those of the untreated control, respectively. Multiscale characterization indicated complementary physical and chemical changes under the combined treatment: US was associated with pronounced physical disruption and structural loosening, whereas FNA was associated with surface chemical modification and hydrogen-bond disruption. These combined structural changes were accompanied by macromolecular chain cleavage and crystalline reorganization, with an increased proportion of cellulose II. Validation with sugarcane bagasse indicated disruption of lignin- and hemicellulose-associated barriers and marked acceleration of hydrolysis kinetics, achieving 86.3% of the final 40-h saccharification yield within only 8 h. Compared with conventional harsh processes, this mild pretreatment strategy has the potential to reduce energy demand and limit inhibitory byproduct formation, offering a highly efficient and sustainable pathway for the high-value valorization of lignocellulosic waste.