Utilization of walnut shell by deep eutectic solvents: enzymatic digestion of cellulose and preparation of lignin nanoparticles

Studies have been devoted to deep eutectic solvent (DES) pretreatment of biomass to improve its enzymatic efficiency, while subsequent processing of regenerated lignin was absent. Nanosizing is one of the common approaches to the value-addition of lignin, based on this, we reported a facile strategy to prepare lignin nanoparticles (LNPs) from walnut shells (WS), combining DES pretreatment and antisolvent precipitation. Three DESs (choline chloride (ChCl): oxalic acid (OA): ethylene glycol (EG) = 1:1:1, ChCl: p-toluenesulfonic acid (TsOH): EG (1:1:1), and ChCl: TsOH: EG (1:1:2)) were assessed for enzymatic hydrolysis efficiency of cellulose solids (CSs) and evaluated the yield and functional properties of LNPs. After pretreatment of ChCl: TsOH: EG (1:1: 2) at 90 ℃ for 2 h, 42.72% of lignin in WS was converted into LNPs with a particle size of 157.71 nm and ζ-potential of − 24.85 mV under dropping speed of 0.15 mL/min and reaction pH of 6, meanwhile, the pretreatment resulted in 2.6 times enzymatic efficiency relative to untreated WS. LNPs prepared from ChCl: OA: EG (1:1:1) exhibited excellent stability (ζ-potential of −31.97 mV) and antioxidant properties (radical scavenging rate of 61.43%), and the lignin molecules possessed high content of functional groups. Our study is expected to provide implications for the chemical deep processing of biomass and the preparation of LNPs.