Multistage treatment of bamboo powder waste biomass: Highly efficient and selective isolation of lignin components

Bamboo pulp and papermaking produce a lot of bamboo powder waste, and its resource utilization is of great significance for biomass refining and environmental protection. Here, we propose an integrated approach involving mechanical activation, hydrothermal extraction, and deep eutectic solvents (DESs) multiple delignification for the efficient separation of bamboo powder. Among seven carboxylic acids based DESs, choline chloride (ChCl)-lactic acid (La) DES (1:1) is the most effective, with over 78.0% lignin removal and 88.9% cellulose retained after mechanical-hydrothermal (180 °C, 5 h)-DES (110 °C, 12 h) treatment. Notably, 84.7% of delignification is achieved after three times of ChCl-La DES treatment at 70, 90, and 110 °C respectively. The delignification rate is negatively correlated with the amount of carboxyl group in the DESs. The lower the pKa value, the higher the delignification rate. Additionally, the selectivity for lignin is improved with decreasing solvent polarity. DES treatment effectively degrades the guaiacyl unit lignin fractions and disrupts several β-aryl-ether bonds (e.g., β-O-4, β-β, and β-5). Furthermore, DESs exhibit good recyclability, with less than 10% reduction in delignification after three cycles. Theory calculations confirm that ChCl-carboxylic acid DESs could compete with lignin to break hydrogen bonds in lignocellulosic biomass by providing their chloride, hydroxyl, and carboxyl groups. Overall, this study demonstrates the practical significance of multistage treatment for the effective fractionation of biomass into its three components.