γ-Valerolactone/water system for lignin fractionation to enhance antibacterial and antioxidant capacities

In this study, γ-valerolactone, a green and recyclable organic solvent, was applied to fractionate organosolv lignin. The original lignin was dissolved in 60% γ-valerolactone/water solution, and sequentially sedimentated in 50%, 40%, 30%, and 1% γ-valerolactone aqueous solution with the addition of water afterwards, producing four fractions (L1, L2, L3, and L4). Multiple analytical methods were used to characterize the lignin fractions in detail. Results manifested that weight average molecular weight of lignin decreased gradually from 7900 (L1) to 1890 g/mol (L4), and the total phenolic hydroxyl content increased from 2.18 (L2) to 2.80 mmol/g (L4). Here, due to the increase of water content in the solvent system, the hydrogen bond capacity with lignin decreased and the solvent polarizability increased. Therefore, the solubility of high molecular weight lignin with low polar groups (hydroxyl and carboxyl) decreased, thus realizing the sequential fractionation of lignin. DPPH and agar diffusion test results indicated that low molecular weight lignin exhibited good antimicrobial/antioxidant activities, which was mainly due to the positive effects of the phenolic hydroxyl content and side chain structure of lignin. The mechanisms of improving antibacterial/antioxidant activities of lignin were proposed for potential practical applications.