Fractionation of rice straw lignin with choline chloride acidic deep eutectic solvents: Structural characterization and multifunctional functional properties.

Lignin is largely wasted agro-industrial aromatic biomass. Green deep eutectic solvents (DES) fractionation optimizes its UV-blocking, antioxidant and antibacterial performance, outperforming traditional alkali extraction. This study compares NaOH and four choline chloride acidic DES to reveal their regulation effects on rice straw lignin structure and functionalities. The recovered lignin fractions were characterized by elemental analysis (EA), FTIR, UV-Vis spectroscopy, and gel permeation chromatography (GPC) to evaluate their elemental composition, chemical structure, molecular weight distribution, UV-shielding capacity, antioxidant activity, and antibacterial properties. Both alkaline and DES pretreatments effectively fractionated lignin from rice straw, yielding high-purity lignin fractions with well-preserved aromatic structures, low molecular weights, and narrow polydispersity. Compared with alkaline-extracted lignin, DES-derived lignin showed increased UV-shielding performance and a higher phenolic hydroxyl group content. These characteristics were associated with enhanced DPPH• radical scavenging activity, hydroxyl radical scavenging activity, and ferric ion reducing power. DES-derived lignin also displayed broad-spectrum antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Salmonella, and E. coli. Pearson correlation analysis revealed a structure-activity relationship in which reduced molecular weight was associated with increased phenolic hydroxyl content, leading to improved antioxidant capacity and antibacterial activity. These results provide a foundation for the use of DES-derived rice straw lignin in biodegradable biomaterials, natural antioxidant formulations, and bio-based antibacterial agents.